WO2018197612A1

WO2018197612A1 - Preparation of powders of nitrided inorganic materials

Info

Publication number: WO2018197612A1
Application number: PCT/EP2018/060718
Authority: WO
Inventors: Sumohan MISRA; Silwia J. TOMASIAK; Bernard Reesink; Lian Zhang
Original assignee: Basf Se
Priority date: 2017-04-27
Filing date: 2018-04-26
Publication date: 2018-11-01

Abstract

Described are powders of nitrided inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases, and pro- cesses for obtaining such materials.

Description

Preparation of powders of nitrided inorganic materials

The present application relates to powders of nitrided inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases, and to processes for obtaining such materials.

In several technical fields it is of importance that inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases are provided in the form of powders having well-defined and adjusted particle size and shape. For example, in reactors, heat exchangers and other apparatuses comprising a packed bed usually powders exhibiting substantially uniform particle shape and high specific particle surface area are needed. An example of such application is a magneto- caloric heat exchanger comprising a packed bed of particles of a magnetocaloric material designated for flow-through of a heat transfer fluid. Such a heat exchanger is disclosed e.g. in WO 201 1/018348 A2.

Processes for preparing powders of inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases are known in the art, see e.g. US 2004/0231338 A1. By appropriately choosing the powder preparation technique and adjusting the process parameters, it is possible to control size and shape of the particles of the obtained powder. Related prior art is also

US 2004/0141870 A1

DE 24 00 286 A1

CN 105 609 224 A. However, inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases for the above-mentioned applications often need to be subjected to a heat treatment at temperatures in a range close to the melting point of the inorganic material, in order to cure crystal defects and to homogenize the crystal structure and chemical composition of said material. Unfortunately, during said heat treatment, inevitable sintering of the initial powder typically results in formation of a solid block of material. Once such solid block is formed, the only practicable technique of fragmenting such block is grinding, which results in formation of irregular shaped granules and remarkable material loss in the form of fines and dust. Moreover, control of particle size and shape during grinding is not feasible, so usually particle size adjustment can be achieved only by screening the particles resulting from grinding, which may require discarding significant amounts of the obtained particles, due to their inappropriate size.

Accordingly, there is a need to prevent undesired sintering of powders of inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases during heat treatment of such powders, in order to retain the shape and size of the powder particles once adjusted during powder preparation.

Surprisingly, it has been found that said undesired sintering of powders of inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases is substantially prevented by subjecting the powder to a nitriding treatment, i.e. exposing said powder of said inorganic material to a gas flow comprising, preferably consisting of, nitrogen at a temperature in the range of from 800 °C to 1000 °C at a nitrogen flow rate of 1 to 5 l/hour and gram of said powder of said inorganic material. This nitriding treatment either replaces the usual heat treatment (in cases the temperature range of 800 °C to 1000 °C is sufficient to achieve the desired curing of crystal defects and homogenizing of the crystal structure and chemical composition), or is followed by the usual heat treatment under an inert gas atmosphere not comprising nitrogen at temperatures higher than the temperatures applied during the nitriding treatment (in cases where higher temperatures than those applied during the nitriding treatment are needed to achieve the desired curing of crystal defects and homogenizing of the crystal structure and chemical composition).

Without wishing to be bound by any theory, it is presently assumed that during the nitrid- ing treatment nitrogen slowly diffuses into the particles of the powder. Due to the slow diffusion, the penetration depth of the nitrogen is significantly smaller than the particle size, and at the surfaces of said particles nitrides of elements present in the inorganic material of the particles are formed as additional phases, while the composition of the bulk of the particle remains substantially unchanged. Thus, the nitriding treatment accord- ing to the present invention is surface-limited, like in commonly known processes for nitriding of iron and steel. The nitrides formed at the surfaces of the particles are assumed to have higher melting temperatures than the phases selected from the group consisting of intermetallic phases and alloy phases present in the bulk of the particles, and thereby sintering is inhibited. According to the present invention, there is provided a process for obtaining a powder of a nitrided inorganic material and a powder of a nitrided inorganic material obtainable by said process. As used herein, the term "nitrided inorganic material" denotes an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases.

Further aspects, details and advantages of the present invention are disclosed in the following description and the examples in combination with the figures. All figures are schematically and not drawn to scale.

According to one aspect of the present application there is provided a process for obtain- ing a powder of a nitrided inorganic material, said nitrided inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases. Said process comprises the steps of

(a) preparing or providing a powder of an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases

(b) exposing said powder of said inorganic material to a gas flow comprising nitrogen at a temperature in the range of from 800 °C to 1000 °C at a nitrogen flow rate of 1 to 5 l/hour and gram of said powder of said inorganic material, thereby forming a powder of a nitrided inorganic material, said nitrided inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases.

By means of the above-defined process according to the present invention, a powder of a nitrided inorganic material is obtained. Said nitrided inorganic material generally comprises one or more phases selected from the group consisting of intermetallic phases and alloy phases, and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases. Said nitrides are additional phases which are not present in the powder prepared or provided in step (a). Thus, in another aspect, the present invention relates to a powder of a nitrided inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases obtainable by the above-defined process according to the present invention.

In the process according to the present invention, the powder provided in step (a) is a powder of an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases. Preferably said inorganic material consists of one or more phases selected from the group consisting of intermetallic phases and alloy phases. Preparation of such powders is achieved by means of any suitable technique. Techniques for preparation of said powders are known in the art, see e.g. US 2004/0231338 A1 . The powder prepared or provided in step (a) is herein referred to as the "initial powder".

Step (b) of the process according to the invention is herein referred to as a "nitriding treatment". In step (b), the powder prepared or provided in step (a) is exposed to a gas flow comprising nitrogen, preferably to a gas flow consisting of nitrogen at a temperature in the range of from 800 °C to 1000 °C. The nitriding treatment of the present invention differs from the nitriding treatment established as hardening technique in the field of surface engineering of iron and steels, due to the significantly higher temperature range used and to the fact that nitrogen gas is used.

In surface engineering of iron and steels, application of nitrogen as the nitriding agent is usually not feasible due to the presence of a significant amount of the a-Fe phase which shows low solubility of nitrogen. Accordingly, it is preferred that the powder of an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases which is prepared or provided in step (a) does not contain a significant amount of the a-Fe phase, and preferably is substantially free of the a-Fe phase. Preferably, in the powder prepared or provided in step (a) the weight fraction of the a-Fe phase is 10 % or less, as determined by means of X-ray diffraction.

Process step (b) includes heating the powder prepared or provided in step (a) to a target temperature in the range of from 800 °C to 1000 °C under continuous nitrogen flow at a nitrogen flow rate of 1 to 5 l/hour and gram, keeping the powder under continuous nitrogen flow at this target temperature for a dwelling time preferably of from 1 to 1 .5 hours and thereafter allowing the powder of to cool under continuous nitrogen flow. Surprisingly it has been found that the amount of nitride phase formed by a nitriding treatment includ- ing a dwelling time of 1 to 1 .5 hours at the target temperature is sufficient to achieve the desired effect of retaining the shape and size of the particles after a heat treatment step. A longer dwelling time may be disadvantageous from the viewpoint of process efficiency, and may result in conversion of an undesirably high fraction of the initial material into nitrides, thereby undesirably changing important properties of the initial powder. In certain cases (for details see below), the nitriding treatment according to step (b) as defined above is followed by a heat treatment (herein referred to as step (c)):

c) heat treating said powder of said nitrided inorganic material formed in step (b) under an inert gas atmosphere not comprising nitrogen, at temperatures higher than the temperatures applied in step (b). The inert gas atmosphere applied in step (c) preferably comprises noble gasses, preferably argon. Preferably the inert gas atmosphere consists of noble gasses, preferably of argon. In the context of the present invention, nitrogen is not considered as an inert gas, because of its capability to form nitrides with elements present in the inorganic material prepared or provided in step (a). A heat treatment according to step (c) is typically carried out for those materials where the temperatures applied during the nitriding treatment according to step (b) are not sufficient to achieve the desired curing of crystal defects and homogenizing of the crystal structure and chemical composition.

Said heat treatment according to step (c) is finished either by quenching or by slow cool- ing, e.g. furnace cooling. In step (a), preparing a powder of an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases preferably comprises

preparing or providing said inorganic material in the liquid state, fragmenting the liquid inorganic material and allowing the obtained fragments to solidify

or

preparing or providing said inorganic material in the liquid state, transferring the liquid inorganic material into the solid state and fragmenting the obtained solid inorganic material.

Herein, an inorganic material prepared or provided in the liquid state is typically in the molten state. The liquid inorganic material is either first fragmented, and the obtained fragments are then allowed to solidify, or the liquid inorganic material is first transferred into the solid state, and the obtained solid inorganic material is then fragmented.

Fragmenting an inorganic material which is in the liquid state typically comprises formation of droplets, i.e. the obtained fragments are in the form of droplets. Fragmentation of the molten material into droplets is achieved e.g. by means of liquid jets (preferably water or oil jets), gas jets (preferably argon jets), centrifugal force or ultrasonic energy acting on the material in the liquid state. These techniques of fragmentation are often referred to as "atomization". The obtained drops are allowed to solidify e.g. by allowing them to fall down in a tower, or to fall into a liquid bath (e.g. an oil bath or a bath of liquid argon). The particles of a powder obtained in this manner typically have spherical or spheroidal particle shape. The particle size is determined by the process parameters applied during atomization.

Transfer of a material which is in the liquid state into the solid state is preferably achieved by means of melt-spinning, resulting in ribbons or flakes (depending on the brittleness of the material). Very brittle materials tend to form flakes rather than ribbons. The process of melt-spinning is known in the art. Fragmenting the obtained solid inorganic material is achieved e.g. by grinding the melt-spun ribbons or flakes into powder. The shape of the particles of a powder obtained in this manner typically generally deviates from spherical shape. Nevertheless, this technique of powder generation allows for controlling the particle size since one dimension of the particles is preset by the thickness of the ribbons or flakes, which in turn is determined by the process parameters applied during melt- spinning. Further techniques for obtaining inorganic materials comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases in powder form include

melting of an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases by means of a plasma jet, spraying the molten material into droplets and allowing the spray droplets to solidify (this technique is also referred to as "plasma spheroidization")

forming an inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases into an electrode, rotating said electrode while supplying it with a current by plasma arc discharge, thereby melting the electrode surface and fragmenting the melt into droplets by centrifugal force, and allowing the droplets to solidify.

For further details, see e.g. US 2004/0231338 A1 .

Step (b) of the process according to the present invention (as defined above) is preferably carried out in a fluidized bed reactor or in a rotary kiln furnace, in order to ensure uniform distribution of nitrogen throughout the powder of the material to be nitrided. Preferably, in step (b) the powder of said inorganic material is exposed to a gas flow comprising nitrogen for a duration of from 0.5 to 6 hours, preferably of from 1 to 1.5 hours at a temperature in the range of from 800 °C to 1000 °C, preceded by heating the powder up to the desired temperature under continuous nitrogen flow and followed by allowing the obtained powder of the nitrided inorganic material to cool under continuous nitrogen flow.

In certain preferred processes according to the present invention, the powder prepared or provided in step (a) consists of particles having a spherical shape or at least a shape only slightly deviating from spherical shape, because for many technical applications involving powders it is advantageous that the particles of the applied powders have a uniform, regular shape, as close to ideally spherical shape as possible. Examples of such technical applications are packed beds for reactors and heat exchangers, and 3D-printing/rapid prototyping processes.

Spherical particle shape is often beneficial with regard to flowability, thereby facilitating transport and handling of powders. Flowability means that the powder does not consolidate during storage and transport and flows out of a device like a silo or a hopper due to the force of gravity alone so that no flow promoting devices are required. Flowability of a bulk solid is characterized by its unconfined yield strength, o_c, as a function of the consol- idation stress, σι , and the storage period, t. Usually the flow function coefficient ffc which is defined as the ratio of consolidation stress, σι , to unconfined yield strength, o_c, is used to characterize flowability numerically.

Furthermore, spherical particles exhibit an advantageous balance between the specific particle surface area and the pressure drop of a fluid (e.g. a heat transfer fluid in the case of a heat exchanger) flowing through a packed bed of said particles.

As mentioned above, techniques for generating powders having substantially spherical particle shape are generally known in the art, however the spherical particle shape and the advantages conferred by it are often lost when the initial powder needs to be subject- ed to a heat treatment. Thus, it is an important benefit of the present invention that as a result of the nitriding treatment of the initial powder, the spherical particle shape of the particles of the initial powder is retained even at temperatures where the initial powder usually sinters into a block.

In preferred processes according to the present invention, the obtained powder of a nitrided inorganic material comprises particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95% of the particles of a representative sample of said particles differ by not more than 25%, preferably by not more than 15%. Such particles which exhibit a shape close to spherical shape are often referred to as spheroids (see e.g. US 2004/0231338 A1 ). More preferably, the obtained powder of a nitrided inorganic material consists of particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95% of a representative sample of said particles differ by not more than 25%, preferably by not more than 15%.

As used herein, the term diameter denotes a straight line connecting two points at the surface of the particle which passes through the volume center of the particle.

The minimum and maximum particle diameter can be obtained by means of image analysis from microscopic images, e.g. from SEM images like those in figures 6 and 8.

Preferably, said maximum diameter is in the range of from 45 μιη to 450 μιη, preferably in the range of from 50 μιη to 150 μιη, in accordance with the requirements of typical tech- nical applications, which will be described below in more detail. Since the spherical shape of the particles of the initial powder is retained during thermal treatment as a result of the nitriding treatment according to the present invention, easy flowability of such initial powder is retained, too. In preferred processes according to the present invention, the obtained powder of a nitrided inorganic material has an instantaneous powder flow function coefficient ffc of 10 or more at a consolidation stress of oi = 2615 Pa. The flow function coefficient is determined by means of a ring shear tester, as it is known in the art. "Instantaneous powder flow function coefficient" means that the unconfined yield strength is measured directly after consolidation.

In specific processes according to the first aspect of the present invention said inorganic material comprising one or more phases selected from the group consisting of intermetal- lic phases and alloy phases prepared or provided in step (a) is selected from the group consisting of magnetocaloric materials. Magnetocaloric materials and their preparation are known in the art. Preferably said magnetocaloric materials are selected from the group consisting of magnetocaloric materials according to any of formulae (I) - (XIII), namely:

compounds of the general formula (I)

FeaMnbAcPxSiyZz (I)

wherein

A represents one or more elements selected from the group consisting of Co, Cr and Ni

0 < a < 2.2, 0 < b < 2.2, 0 < c < 0.3 and 1.8 < (a + b + c) < 2.2, preferably 1.95≤(a + b + c) < 2.05

Z represents one or more elements selected from the group consisting of B, C, Ge, Ga, Sn, N, As and Sb

0 < x < 1.05, 0 < y < 1.05, 0 < z < 0.35 and 0.95 < (x + y + z) < 1.05

compounds of one the general formulae (II), (III), (IV) and (V)

La(FexAh-x)i₃H_y or La(Fe_xSii-x)i₃H_y (II)

where

x is a number from 0.7 to 0.95, y is a number from 0 to 3; La(Fe_xAlyCoz)i3 or La(Fe_xSi_yCo_z)i3 (III)

where

x is a number from 0.7 to 0.95,

y is a number from 0.05 to 1 - x,

z is a number from 0.005 to 0.5;

LaMn_xFe₂-xGe (IV)

where

x is a number from 1.7 to 1.95

(Lai-zCez)(Fei-x-_yMnySi_x)i₃Hn (V)

where

x is a number from 0.08 to 0.15 and

y is a number from 0 to 0.05,

z is a number from 0 to 0.3,

n is a number from 1.5 to 3

compounds of the general formula (VI)

Gd₅(SixGe₁-x)₄ (VI)

where x is a number from 0.2 to 1 ,

compounds of one of the general formulae (VII) and (VIII)

Tb₅(Si₄-xGex) (VII)

where x = 0, 1 , 2, 3, 4,

XTiGe (VIII)

where X selected from the group consisting of Dy, Ho, Tm,

compounds of one of the general formulae (IX) - (XII)

Mn₂-xZxSb (IX)

Mn₂ZxSbi-_x (X)

where

Z is selected from the group consisting of Cr, Cu, Zn, Co, V, As, Ge, x is from 0.01 to 0.5, Mri2-xZxAs (XI)

MrteZxAsi-x (XII)

where

Z is selected from the group consisting of Cr, Cu, Zn, Co, V, Ge,

x is from 0.01 to 0.5

Heusler alloys of the MnT∑X type (XIII)

where T is a transition metal and X is a p-doping metal having an electron count per atom e/a in the range from 7 to 8.5,

wherein T is selected from the group consisting of Ni, Cu wherein X is selected from the group consisting of Al, Ga.

Said magnetocaloric materials comprise one or more phases selected from the group consisting of intermetallic phases and alloy phases.

Most preferred are magnetocaloric materials according to formula (I). A magnetocaloric material according to formula (I) typically comprises a main phase having a Fe2P- structure. Usually said main phase occupies 90 % or more of the volume of said compound of general formula (I). Magnetocaloric materials according to general formula (I) and methods for preparation thereof are known in the art. Such materials and processes for their preparation are generally described in WO 2004/068512. Magnetocaloric materials according to formula (I) which contain manganese, iron, silicon and phosphorus, and methods for their preparation are disclosed in WO 201 1/083446 and US 201 1/0220838. Magnetocaloric materials according to formula (I) which contain manganese, iron, silicon, phosphorus and boron, and methods for their preparation are disclosed in WO 2015/018610, WO 2015/018705 and WO 2015/018678. Magnetocaloric materials according to formula (I) which contain manganese, iron, silicon, phosphorus, nitrogen and optionally boron, and methods for their preparation are disclosed in WO 2017/072334. Magnetocaloric materials according to formula (I) which contain manganese, iron, silicon, phosphorus, carbon, and optionally one or both of boron and nitrogen, and methods for their preparation are disclosed in WO 2017/21 1921 . Magnetocaloric materials according to formula (I) which contain manganese, iron, one or both of nickel and cobalt, silicon, phosphorus, and boron, and methods for their preparation are disclosed in WO 2018/060217. In certain cases, it is preferred that the magnetocaloric material to be prepared or provided in step (a) is a magnetocaloric material according to formula (I) which does not contain nitrogen.

Especially preferred materials of formula (I) are those according to formula (Γ):

FeaMnbPxSiy (Ι')

wherein 0.68 < a < 0.86, 1.10 < b < 1.28, 0.42 < x < 0.51 , 0.49 < y < 0.58

and 1.8 < (a + b) < 2.2, preferably 1 .95 < (a + b) < 2.05

and 0.95 < (x + y) < 1.05, preferably 0.98 < (x + y) < 1.02.

A preferred specific process according to the present invention comprises the steps of

(a) preparing or providing a powder of a magnetocaloric material having a composition according to formula (I),

(b) exposing said powder of said magnetocaloric material to a gas flow comprising nitrogen at temperatures in the range of from 890 °C to 950 °C at a nitrogen flow rate of 1 to 5 l/hour and gram of said powder of said magnetocaloric material, thereby forming a powder of a nitrided magnetocaloric material,

(c) heat treating the powder of the nitrided magnetocaloric material obtained in step (b) at temperatures in the range of from 1000 °C to 1200 °C for a duration of from 1 to 20 hours under an inert gas atmosphere.

In said process, the powder provided in step (a) preferably consists of a magnetocaloric material having a composition according to formula (I). Step (b) of the process according to the invention is a "nitriding treatment" as defined above. In step (b), the powder prepared or provided in step (a) is exposed to a gas flow comprising, preferably consisting of nitrogen. The nitriding treatment according to step (b) is followed by a heat treatment according to step (c) as defined above.

The inert gas atmosphere applied in step (c) preferably comprises noble gasses, preferably argon. Preferably the inert gas atmosphere consists of noble gasses, preferably of argon. During the heat treatment according to step (c) curing of crystal defects and homogenizing of the crystal structure and chemical composition is achieved, which results in an improvement of the magnetocaloric properties. Said heat treatment according to step (c) is finished either by quenching or by slow cooling, e.g. furnace cooling. Surprisingly it has been found that magnetocalonc materials, especially those according to formula (I), which have been subjected to a nitriding treatment according to step (b), exhibit similar magnetocalonc properties like magnetocalonc materials which have the same initial composition, but have been prepared in the conventional manner e.g. as described in the above-cited prior art documents, i.e. without a nitriding treatment according to step (b).

In the above-defined preferred specific process of the invention, step (a) preferably comprises

(a-1 ) providing a mixture of precursors comprising atoms of the elements

manganese and iron,

optionally one or more of chromium, nickel and cobalt,

phosphorus and silicon, and

optionally one or more of boron, carbon, germanium, gallium, tin, nitrogen, arsenic and antimony,

(a-2) melting together and reacting said mixture of precursors under inert gas atmosphere to obtain a liquid product having a composition according to formula (I)

(a-3) preparing a powder having a composition according to formula (I) by

fragmenting the liquid product obtained in step (a-2) and solidifying the obtained fragments

or

transferring the liquid product obtained in step (a-2) into the solid phase to obtain a solid product, and fragmenting the obtained solid product.

Preferably, said mixture of precursors provided in step (a-1 ) comprises one more substances selected from the group consisting of elemental manganese, elemental iron, elemental cobalt, elemental nickel, elemental phosphorus, elemental silicon, elemental boron, elemental carbon, carbonizable organic compounds, phosphides of iron, phosphides of manganese, borides of iron, borides of manganese, carbides of iron, carbides of manganese, nitrides of iron, nitrides of manganese, alloys of silicon and manganese.

The inert gas atmosphere applied in step (a-2) preferably comprises noble gasses, preferably argon. Preferably the inert gas atmosphere consists of noble gasses, preferably of argon. Preferably, in step (a-2) melting is achieved by induction-heating the mixture of precursors to a temperature in the range of from 1400 °C to 1500 °C.

Preferably, in step (a-3)

the liquid product obtained in step (a-2) is fragmented into droplets and the ob- tained droplets are allowed to solidify

or

the liquid product obtained in step (a-2) is transferred into the solid phase by means of melt-spinning of said liquid product, and the melt-spun solid product is fragmented by means of grinding it into powder. Herein, the liquid product obtained in step (a-2) is typically in the molten state. For further details and preferred features, see above.

Step (b) of the preferred specific process according to the present invention (as defined above) is preferably carried out in a fluidized bed reactor or in a rotary kiln furnace. Preferably, in step (b) the powder of said magnetocaloric material is exposed to a gas flow comprising nitrogen for a duration of from 0.5 to 6 hours, preferably of from 1 to 1.5 hours. Preferably, in step (b) the powder of said inorganic material is exposed to a gas flow comprising nitrogen for a duration of from 0.5 to 6 hours, preferably of from 1 to 1.5 hours at a temperature in the range of from 890 °C to 950 °C, preceded by heating the powder up to the desired temperature under continuous nitrogen flow and followed by allowing the obtained powder of the nitrided inorganic material to cool under continuous nitrogen flow.

Step (c) of the preferred specific process according to the present invention (as defined above) is preferably carried out at temperatures in the range of from 1020 °C to 1 150 °C for a duration of from 10 hours to 20 hours. During step (c), the nitrided powder is con- fined under inert gas atmosphere, for instance in a sealed vessel.

In preferred cases, in step (c) the heat treatment is finished by quenching the heat treated powder of the nitrided magnetocaloric material wherein preferably quenching of the heat- treated powder is carried out at a quenching rate of 25 K/s or more, preferably 100 K/s or more. Particularly preferably, quenching is carried out by means of contacting the vessel comprising the heat treated powder with oil or water or aqueous liquids, for example cooled water or ice/water mixtures. For example, the vessel containing heat treated powder is allowed to fall into ice-cooled water, or the heat treated powder is quenched with sub- cooled gases such as liquid nitrogen or liquid argon.

According to a second aspect of the present invention, there is provided a powder of a nitrided inorganic material. As explained above, said nitrided inorganic material comprises one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases. As described above in the context of the first aspect of the invention, said inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases is preferably selected from the group consisting of magnetocaloric materials according to any of formulae (l)-(XIII) as defined above. Thus, in the second aspect of the present invention, preferably said powder of a nitrided inorganic material comprises particles of a nitrided inorganic material, wherein said nitrided inorganic material comprises a magnetocaloric material according to any of formulae (l)-(XIII) as defined above and one or more nitrides of elements of said magnetocaloric material. Most preferred magnetocaloric materials are those according to formula (I), especially formula (Γ) as defined above.

Accordingly, said powder according to the second aspect of the present invention comprises, preferably consist of, particles of a nitrided inorganic material wherein each particle comprises one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases. Said powder according to the present invention

is obtainable by a process according to the above-described first aspect of the present invention, preferably by a process according to the above-described first aspect of the present invention which has one or more of the above-described preferred features

and/or

comprises particles of a nitrided inorganic material, said nitrided inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases, wherein in each of said particles along a straight line connecting the surface of the particle with the volume center of said particle the fraction of nitrogen atoms has its maximum at a position located at a distance from the particle surface which is less than 50 % of the distance between the particle surface and the volume center of said particle.

Said powder according to the present invention comprises particles of a nitrided inorganic material, said nitrided inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases. In each of said particles along a straight line connecting the surface of the particle with the volume center of said particle the fraction of nitrogen atoms has its maximum at a position having a distance from the particle surface which is less than 50 % of the distance between the particle surface and the volume center of said particle. Herein, the maximum of the fraction of nitrogen atoms corresponds to the highest value the fraction of nitrogen atoms exhibits along said straight line connecting the surface of said particle with the volume center of said particle.

Preferably said powder according to the present invention consists of particles of a nitrided inorganic material, said nitrided inorganic material comprising one or more phases selected from the group consisting of intermetallic phases and alloy phases and one or more nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases. In each of said particles along a straight line connecting the surface of the particle with the volume center of said particle the fraction of nitrogen atoms has its maximum at a position having a distance from the particle surface which is less than 50 % of the distance between the particle surface and the volume center of said particle. Herein, the maximum of the fraction of nitrogen atoms corresponds to the highest value the fraction of nitrogen atoms exhibits along said straight line connecting the surface of said particle with the volume center of said particle.

The fraction of nitrogen atoms as used herein refers to the percentage of nitrogen atoms relative to the total amount of atoms. Locally resolved determination of the fraction of atoms of individual elements can be performed e.g. by means of scanning electron microscopy (SEM) combined with energy-dispersive X-ray analysis (EDX). The change of the fraction of nitrogen atoms with increasing distance from the particle surface is studied by means of scanning electron microscopy (SEM) of a cross section of the particle (as shown for an exemplary particle in figure 1 1 ) in combination with energy-dispersive X-ray analysis (EDX). In a particle of a powder according to the present invention, along a straight line connecting the surface of a particle with the volume center of said particle the fraction of nitrogen atoms exhibits a maximum. The maximum of the fraction of nitrogen atoms corresponds to the highest value the fraction of nitrogen atoms exhibits along said straight line con- necting the surface of said particle with the volume center of said particle.

When the fraction of nitrogen atoms along said straight line connecting the surface of said particle with the volume center of said particle is plotted against the distance from the particle surface to the volume center of said particle, the fraction of nitrogen atoms has a maximum at a position which is closer to the surface of the particle than to the volume center of said particle. Accordingly, said maximum is located at a position having a distance from the particle surface which is less than 50 %, preferably less than 40 %, further preferably less than 30 %, further preferably less than 20 %, most preferably less than 10 % of the distance between the particle surface and the volume center of said particle. The presence of the maximum of the fraction of nitrogen atoms at a position which is closer to the surface of the particle than to the volume center of said particle is a result of the limited diffusion of nitrogen into the bulk of said particle during the nitriding treatment according to step (b) of the process according to the present invention.

In spherical particles, said straight line connecting the surface of a particle with the volume center of said particle substantially corresponds to the radial direction. Thus, when the fraction of nitrogen atoms is plotted against a straight line corresponding to the particle radius, the fraction of nitrogen atoms has a maximum at a position which is closer to the surface of the particle than to the volume center of said particle. Accordingly, said maximum is located at a position having a distance from the particle surface which is less than 50 %, preferably less than 40 %, further preferably less than 30 %, further preferably less than 20 %, most preferably less than 10 % of the particle radius.

Such spherical particle is shown schematically in the upper part of figure 1. In spherical particle 1 , the straight line A-B extends in radial direction and connects point A at the surface of particle 1 with point B corresponding to the volume center of particle 1. In the lower part of figure 1 , the fraction of nitrogen atoms is plotted against the percentage of the distance between the surface (0 %) of particle 1 and the volume center (100 %) of particle 1 along straight line A-B. Along this line, the fraction of nitrogen atoms has a maximum M at a position which is closer to the surface of particle 1 than to the volume center of said particle. Said maximum M is located at a position having a distance from the particle surface which is less than 50 % of the particle radius. It is noted that figure 1 has schematic character and does not intend to show values of the fraction of nitrogen atoms of a real sample, but illustrates in a qualitative manner the change of the fraction of nitrogen atoms with the distance from the particle surface along straight line A-B.

It has to be noted that in a particle of a powder according to the present invention not necessary along each straight line which could be drawn to connect the surface of said particle with the volume center of said particle the fraction of nitrogen atoms exhibits a maximum. There may exist one or more of such straight lines along which no maximum of the fraction of nitrogen atoms can be determined, as a result of non-uniform distribution of nitrogen over the particle surface, resulting in non-uniform diffusion of nitrogen into said particle during the nitriding treatment of step (b), and/or because the composition of the outer region comprising the nitrides formed in step (b) has changed during the heat treatment according to step (c) following nitriding step (b). The latter may result from differences between the thermal expansion coefficients of the nitrides in the outer region and the phases selected from the group consisting of intermetallic phases and alloy phases in the inner region of the particles. Thus, when particles obtained in step (b) having an outer region substantially consisting of nitrides are subjected to the heat treatment according to step (c), the underlying phases in the inner region expand differently compared to the nitrides in the outer region, and finally become exposed at the particle surface.

In specific cases, a powder according to the present invention comprises particles where- in in each of said particles the fraction of nitrogen atoms decreases along a straight line connecting the surface of said particle with the volume center of said particle in the direction from the surface of said particle towards the volume center of said particle.

In a particle of such powder according to the present invention, the fraction of nitrogen atoms decreases along a straight line connecting the surface of said particle with the volume center of said particle in the direction from the surface of said particle towards the volume center of said particle. In spherical or spheroid particles, the fraction of nitrogen atoms decreases in the radial direction from the surface of said particle towards the volume center of said particle. Accordingly, the fraction of nitrogen atoms has its maximum (as defined above) substantially at the particle surface. Such spherical particle is shown schematically in the upper part of figure 2. In spherical particle 2, the straight line A-B extends in radial direction and connects point A at the surface of particle 2 with point B corresponding to the volume center of particle 2. In the lower part of figure 2, the fraction of nitrogen atoms is plotted against the percentage of the distance between the surface (0 %) of particle 1 and the volume center (100 %) of particle 2 along the straight line A-B. The fraction of nitrogen atom decreases in the radial direction from point A at the surface of particle 2 towards point B corresponding to the volume center of said particle. It is noted that figure 2 has schematic character and does not intend to show values of the fraction of nitrogen atoms of a real sample, but illustrates in a qualitative manner the change of the fraction of nitrogen atoms with the distance from the particle surface along straight line A-B.

As mentioned above, the powders obtainable by a preferred process according to the first aspect of the present invention have advantageous flowing behavior. A preferred powder according to the present invention has an instantaneous powder flow function coefficient ff c of 10 or more at a consolidation stress of σι = 2615 Pa.

It is preferred that the particles of a powder according to the invention have a uniform, regular shape, as close to ideally spherical shape as possible. Preferably, a powder according to the present invention comprises particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95% of the particles of a representative sample of said particles differ by not more than 25%, preferably by not more than 15%. Such particles which exhibit a shape close to spherical shape are often referred to as spheroids (see e.g. US 2004/0231338 A1 ). More preferably, a powder according to the present invention consists of particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95% of a representative sample of said particles differ by not more than 25%, preferably by not more than 15%.

Preferably, said maximum diameter is in the range of from 45 μιη to 450 μιη, preferably in the range of from 50 μιη to 150 μιη, in accordance with the requirements of typical technical applications, which will be described below in more detail. Preferably a powder according to the present invention comprises particles

wherein each of said particles consists of

an inner region extending around the volume center of the particle

an outer region extending around said inner region,

said outer region having a thickness corresponding to 50 % or less of the distance between the surface of said particle and the volume center of said particle, wherein in said outer region the average fraction of nitrogen atoms exceeds the average fraction of nitrogen atoms of the total particle by a factor of 2 or more. The terms "inner region" and "outer region" merely geometrically (based on the distance between the surface of said particle and the volume center of said particle) define certain portions of the particle. The inner region extends around the volume center of the particle. The outer region extends around said inner region and has a thickness corresponding to 50 % or less of the distance between the surface of said particle and the volume center of said particle. Accordingly, said outer region completely surrounds said inner region and extends up to the particle surface. In said outer region, the average fraction of nitrogen atoms (averaged over the total volume of the outer region) exceeds the average fraction of nitrogen atoms of the total particle (averaged over the total volume of the particle, i.e. over the inner and the outer region) by a factor of 2 or more. The higher average fraction of nitrogen atoms in said outer region, compared to the average fraction of nitrogen atoms of the total particle, is a result of the limited depth of diffusion of nitrogen into the bulk of said particle during the nitriding treatment according to step (b) of the process according to the present invention. The average fraction of nitrogen atoms of the total particle (averaged over the total volume of the particle, i.e. over the inner and the outer region) can be determined by X-ray diffraction analysis of a sample of crushed particles of a powder according to the invention.

Preferably said outer region has a thickness corresponding to 40 % or less, further preferably 30 % or less, further preferably 20 % or less, most preferably 10 % or less of the distance between the surface of said particle and the volume center of said particle, and in said outer region the average fraction of nitrogen atoms exceeds the average fraction of nitrogen atoms of the total particle by a factor of 5 or more, preferably by a factor 10 or more. Herein, it is understood that generally the smaller the thickness of the outer region is defined, the larger is the ratio between the average fraction of nitrogen atoms of said outer region and the average fraction of nitrogen atoms of the total particle, resulting from the limited diffusion of nitrogen into the bulk of said particle during the nitriding treatment according to step (b) of the process according to the present invention.

In spherical or spheroid particles, said inner region is substantially spherical, and said outer region has the shape of a spherical shell completely surrounding the spherical the inner region, wherein said shell has a thickness corresponding to 50 % or less of the particle radius.

Such spherical particle is shown schematically in the upper part of figure 3. Spherical particle 3 consists of a spherical inner region 3i extending around the volume center of particle 3 and an outer region 3o extending around said inner region 3i. Outer region 3o has the shape of a spherical shell completely surrounding inner region 3i. The outer region 3o has a thickness corresponding to 50 % of the radius of spherical particle 3. The lower part of figure 3 is a bar diagram showing the average fraction of nitrogen atoms in the outer region 3o (averaged over the total volume of the outer region) and the average fraction of nitrogen atoms of the total particle 3 (averaged over the total volume of particle 3, i.e. over the inner region 3i and the outer region 3o). The average fraction of nitrogen atoms in the outer region 3i exceeds the average fraction of nitrogen atoms of the total particle 3 by a factor of 2. It is noted that figure 3 has schematic character and does not intend to show values of the fraction of nitrogen atoms of a real sample, but illustrates in a qualitative manner the difference between the average fraction of nitrogen atoms in the outer region 3o (averaged over the total volume of the outer region 3o) and the average fraction of nitrogen atoms of the total particle 3 (averaged over the total volume of particle 3, i.e. over the inner region 3i and the outer region 3o).

In a particle as defined above, i.e. a particle consisting of an inner region extending around the volume center of said particle and an outer region extending around said inner region, wherein said outer region has a thickness corresponding to 50 % or less of the distance between the surface of said particle and the volume center of said particle, preferably

said inner region comprises one or more phases selected from the group consisting of intermetallic phases and alloy phases,

and

said outer region comprises nitrides of one or more elements of said phases selected from the group consisting of intermetallic phases and alloy phases present in the inner region of said particle.

Preferably, said inner region consists of one or more phases selected from the group consisting of intermetallic phases and alloy phases.

Said outer region does not necessarily completely consist of nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases present in the inner region of said particles. In certain cases, especially when preparing of the powder of the nitrided inorganic material includes a heat treatment step (c) as de- scribed above, the outer region is only partially formed of nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases present in the inner region. This may result from differences between the thermal expansion coefficients of the nitrides in the outer region and the phases selected from the group consisting of intermetallic phases and alloy phases in the inner region. Thus, when parti- cles obtained in step (b) having an outer region substantially consisting of nitrides are subjected to the heat treatment according to step (c), the underlying phases in the inner region expand differently compared to the nitrides in the outer region, and finally become exposed at the particle surface. Therefore, the outer region, which is defined merely geometrically based on the distance between the surface of said particle and the volume center of said particle, comprises nitrides of one or more elements of said phases selected from the group consisting of intermetallic phases and alloy phases present in the inner region of said particle, and beside said nitrides said outer region also comprises such phases selected from the group consisting of intermetallic phases and alloy phases which are present in the inner region.

Figure 4 shows a spherical particle 4 which consists of a spherical inner region 4i extending around the volume center of particle 4 and an outer region 4o extending around said inner region 4i. The inner region 4i consists of one or more phases selected from the group consisting of intermetallic phases and alloy phases. The outer region 4o has the shape of a spherical shell completely surrounding inner region 4i. The outer region 4o has a thickness corresponding to 50 % of the radius of spherical particle 4. Some portions 4n of outer region 4o are formed of nitrides of elements of said phases selected from the group consisting of intermetallic phases and alloy phases present in the inner region 4i of particle 4. The remaining portions of outer region 4o are formed of such phases selected from the group consisting of intermetallic phases and alloy phases present in the inner region 4i of particle 4. It is noted that figure 4 has schematic character and does not intend to show the local distribution of nitrides of a real sample, but illustrates in a qualitative manner the presence of portions 4n formed of nitrides in the outer region 4o. Preferably, the thickness of said outer region is 10 μιη or less, preferably 5 μιη or less, more preferably 1 μιη or less, even more preferably 0.5 μιη or less, wherein in said outer region the average fraction of nitrogen atoms exceeds the average fraction of nitrogen atoms of the total particle by a factor of 5 or more, preferably by a factor 10 or more. Herein it is understood that the thickness of the outer region of a particle of a powder according to the present invention is to be defined in consideration of the distance between the surface of said particle and the volume center of said particle resp. in case of spherical or spheroidal particles in consideration of the particle radius. In any case, the thickness of the outer region must not exceed 50 % of the distance between the particle surface and the volume center of the particle, resp. in case of spherical or spheroidal particles must not exceed 50 % of the particle radius. Specific powders according to the second aspect of the present invention comprise or consist of particles consisting of an inner region extending around the volume center of said particle and an outer region extending around said inner region, wherein said outer region has a thickness corresponding to 50 % or less of the of the distance between the surface of said particle and the volume center of said particle, wherein

said inner region comprises, preferably consist of, a magnetocaloric material, and

said outer region comprises nitrides of one or more elements of said magnetocaloric material which is present in said inner region. Such a powder is herein referred to as powder of a nitrided magnetocaloric material.

Preferably in the particles of said powder of a nitrided magnetocaloric material, said inner region comprises a magnetocaloric material according to any of formulae (l)-(XIII) as defined above. More preferably said inner region consists of a magnetocaloric material according to any of formulae (l)-(XIII) as defined above. In the particles of said powder of a nitrided magnetocaloric material, said outer region does not necessarily completely consist of nitrides of elements of said magnetocaloric material which is present in said inner region. In certain cases, especially when preparing of the powder includes a heat treatment step (c) as described above, the outer region is only partially formed of nitrides of elements of said magnetocaloric material which is present in said inner region.

Preferred powders of nitrided magnetocaloric materials according to the invention comprise or consist of particles consisting of an inner region extending around the volume center of said particle, and an outer region extending around said inner region wherein said outer region has a thickness corresponding to 50 % or less of the of the distance between the surface of said particle and the volume center of said particle, wherein

said inner region comprises a magnetocaloric material according to formula (I) as defined above

and

said outer region comprises nitrides of one or more of the elements of the magne- tocaloric material according to formula (I) which is present in said inner region. Preferably said inner region consists of a magnetocaloric material according to formula (I) as defined above. It is understood that when the material according to formula (I) comprises nitrogen, said outer region comprises nitrides of elements of the magnetocaloric material according to formula (I) with the exception of nitrogen. In certain cases, it is preferred that the magnetocaloric material according to formula (I) which is present in said inner region does not contain nitrogen.

Said outer region preferably has a thickness of 10 μιη or less, preferably 5 μιη or less, further preferably 1 μιη or less, even more preferably 0.5 μιη or less, wherein in said outer region the average fraction of nitrogen atoms exceeds the average fraction of nitrogen atoms of the total particle by a factor of 5 or more, preferably by a factor 10 or more. Herein, it is understood that the thickness of the outer region of a particle of a powder according to the present invention is to be defined in consideration of the distance between the surface of said particle and the volume center of said particle resp. in case of spherical or spheroidal particles in consideration of the particle radius. In any case, the thickness of the outer region must not exceed 50 % of the distance between the particle surface and the volume center of the particle, resp. in case of spherical or spheroidal particles must not exceed 50 % of the particle radius.

A particularly preferred powder of a nitrided magnetocaloric material according to the present invention comprises, preferably consists of particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95% of a representative sample of said particles differ by not more than 25%, preferably by not more than 15%, wherein said maximum diameter is in the range of from 50 μιη to 150 μιη,

wherein each of said particles consists of

- an inner region extending around the volume center of the particle, said inner region consisting of a magnetocaloric material according to formula (I) as defined above,

an outer region extending around said inner region, said outer region comprising nitrides of one or more of the elements of the magnetocaloric material according to formula (I) which is present in said inner region, wherein

said outer region has a thickness of 10 μιη or less, preferably 5 μιη or less, further preferably 1 μιη or less, even more preferably 0.5 μιη or less in said outer region the average fraction of nitrogen atoms exceeds the average fraction of nitrogen atoms of the total particle by a factor of 5 or more, preferably by a factor 10 or more,

wherein said powder has an instantaneous powder flow function coefficient ffc of 10 or more at a consolidation stress,

= 2615 Pa.

According to a third aspect of the present invention, there is provided a packed heat exchanger bed comprising a powder of a nitrided magnetocaloric material as defined above in the context of the second aspect of the present invention.

Regarding specific and preferred characteristics of said powder of said nitrided magneto- caloric material, reference is made to the disclosure provided above in the context of the second aspect of the present invention. Preferably, said powder of a nitrided magnetocaloric material is selected from the preferred ones described in the context of the second aspect of the present invention.

Said powder of a nitrided magnetocaloric material in said packed bed preferably compris- es particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95 % of a representative sample of said particles differ by not more than 25 %, preferably not more than 15 %, wherein said maximum diameter is preferably in the range of from 100 μιη to 150 μιη.

Preferably, said powder of a nitrided magnetocaloric material in said packed bed consists of particles each having a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95 % of a representative sample of said particles differ by not more than 25 %, preferably not more than 15 %, wherein said maximum diameter is preferably in the range of from 100 μιη to 150 μιη.

It has been found that when said maximum diameter is preferably in the range of from 100 μιη to 150 μιη, there is optimum balance between surface area of the particles and pressure drop of a heat transfer fluid flowing through the packed bed. A high surface area of the particles is desirable to facilitate heat transfer, while sufficient void fraction between the particles is needed in order to reduce the pressure drop of a heat transfer fluid flowing through the packed bed. Control of the particle size can be achieved during step (a) by appropriate adjustment of the process parameters. Additionally, the size distribution of the particles for the packed bed can be further adjusted by screening or sieving. In preferred heat exchanger beds according to the present invention, the particles of the powder of the nitrided magnetocaloric material are coated with a polymer layer, in order to avoid corrosion which may occur when the magnetocaloric material is in direct contact with the heat transfer fluid. Preferably said polymer is an epoxy polymer

According to a fourth aspect of the present invention, there is provided a process for preparing a heat exchanger bed, said process comprising the steps of

preparing a powder of a nitrided magnetocaloric material by the above-defined specific process according to the first aspect of the present invention or providing a powder of a nitrided magnetocaloric material according to the second aspect of present invention

preparing a packed heat exchanger bed comprising said powder.

Regarding specific and preferred characteristics of said powder of said nitrided magnetocaloric material resp. of said process for preparing said powder, reference is made to the disclosure provided above in the context of the second resp. first aspect of the present invention. Preferably, said powder of a nitrided magnetocaloric material is selected from the preferred ones described in the context of the second aspect of the present invention. Preferably, said process for preparing said powder has one or more of the preferred features described in the context of the first aspect of the present invention.

Techniques for preparing packed beds from a material which is in powder form are generally known in the art. Basically, preparing such packed bed comprises disposing said powder in a suitable vessel where it is allowed to settle to form a packed bed. For example, the powder is poured in the vessel, and even distribution of the particles and settling of the bed is facilitated by shaking the vessel and/or by subjecting the powder in the vessel to compression. In order to prevent the powder from being carried away by a fluid flowing through the packed bed, the powder is confined in any suitable manner, e.g. by means of a mesh cage.

A preferred process for preparing a heat exchanger bed comprises the step of coating the particles of the powder of the nitrided magnetocaloric material with a polymer layer before preparing a packed heat exchanger bed comprising said powder. Preferably said polymer is an epoxy polymer. For applying the coating, the powder of the nitrided magnetocaloric material and a solution of the polymer (or of its precursor in combination with a curing agent) are thoroughly mixed, the mixture is spread and the solvent is allowed to evapo- rate. Curing of the polymer coating (if applicable) is carried out in the packed bed, typically be subjecting the packed bed to temperatures in the range of from 100 °C to 200 °C.

According to a further aspect of the present invention, there is provided a device selected from the group consisting of cooling systems, refrigeration systems, heat exchangers, heat pumps, thermomagnetic power generators, climate control units and air conditioning devices wherein said device comprises a powder of a nitrided magnetocaloric material as defined above in the context of the second aspect of the present invention or a packed bed as defined above in the context of the third aspect of the present invention.

Regarding specific and preferred characteristics of said powder of a nitrided magnetoca- loric material resp. said packed bed, reference is made to the disclosure provided above in the context of the second resp. third aspect of the present invention. Preferably, said powder of a nitrided magnetocaloric material is selected from the preferred ones described in the context of the second aspect of the present invention. Preferably, said packed bed has one or more of the preferred features described in the context of the third aspect of the present invention.

Cooling systems, refrigeration systems, heat exchangers, heat pumps, climate control units and air conditioning devices are generally known in the art. A thermomagnetic power generator is a device which converts heat to electricity by means of the magnetocaloric effect. By heating and cooling a magnetocaloric material, the magnetization of the material changes. The changing magnetization can be converted to electricity by exposing said changing magnetization to a coil, thereby inducting an electrical current in said coil.

The invention is now illustrated further by the following examples.

Examples

1. Preparation of powders of nitrided maqnetocaloric materials according to the invention and non-nitrided maqnetocaloric materials for comparison tests

Step (a): preparation of powders of magnetocaloric materials having a composition ac- cording to formula Fe_aMnbPxSi_y (Γ)

A precursor mixture consisting of iron phosphide, manganese, silicon, and - depending on the target composition - one of manganese phosphide and iron, each in a weight fraction as per the proportions of Fe, Mn, P and Si of the target composition (see below) was provided. All precursors were in the form of chunks or chips with the exception of iron which was in the form of powder. The mixture of precursors was melted in a ceramic/graphite crucible by means of an induction furnace at temperatures in a range of from 1400 °C to 1500 °C. The entire melting process was carried out in argon atmosphere. The obtained liquid product was then poured through the nozzle of an atomization device to form a liquid stream that was subsequently fragmented (atomized) by several argon jets leading to the formation of liquid droplets. The droplets were cooled very fast as they fell down the atomization tower and solidified into spherical powder, which was collected at the bottom of the tower. The powder prepared in this way is herein referred to as the "initial powder" or the "initial magnetocaloric material".

Powders of magnetocaloric materials of the following composition were prepared:

Mn1.i8Fe0.73P0.48Si0.52, Mn1.14Fe0.77P0.48Si0.52, Mn1.i8Fe0.72P0.47Si0.53.

The obtained solid product was sieved to obtain a size fraction of 100 μιη to 150 μιη (i.e. passing the sieve having 150 μιη mesh width, but not passing the sieve with 100 μιη mesh width), using laboratory test sieves of metal wire cloth. This size fraction was chosen because packed heat exchanger beds using this size fraction have an optimum balance of particle surface area and pressure drop of the heat transfer fluid flowing through the heat exchanger bed.

The obtained particles had a shape close to spherical and easy flowability. Step (b): nitriding treatment

The powder of the magnetocaloric powder obtained in step (a) was exposed to a gas flow consisting of nitrogen in a fluidized bed reactor. A schematic illustration of the fluidized bed reactor (FBR) is shown in figure 5. The entire assembly is made of fused silica to allow for high reaction temperatures (> 800°C). The fluidized bed reactor comprises a vertically oriented reactor tube, a gas inlet tube at the bottom and a top piece with two outlets: one for exhaust gas and a second outlet connected to a bubbler. The arrows in figure 5 indicate the direction of the nitrogen flow. A thermocouple to monitor the internal temperature is incorporated via the first outlet. In the bottom zone of the reactor tube, a fixed bed of quartz beads having a diameter of about 2 mm is sandwiched between two porous distributor plates (made of quartz) having different porosities. In the direction of the nitrogen flow, the first distributor plate has coarser pores (designated as "coarse filter" in figure 5) while the second one has finer pores (designated as "fine filter" in figure 5). This assembly supports uniform axial distribution of the nitrogen gas across the entire width of the reactor tube and holds back the solid particles of the powder of the magneto- caloric material which when the nitrogen flow has started form a fluidized bed above the second distributor plate.

The nitrogen flow was adjusted using a digital flow control unit. A typical nitrogen flow rate of 100 l/h for an amount of ca. 40 g of powder of the magnetocaloric material was used. The reactor tube is placed in a Carbolite vertical split tube furnace containing three heating zones to ensure uniform heating. The nitriding treatment includes heating the powder of the magnetocaloric material obtained in step (a) to a target temperature in the range of from 900 °C or 950 °C at a rate of 5 °C/minute under flowing nitrogen, followed by a dwelling time of 1 to 6 hours at the target temperature. At the end of the nitriding treatment, the heating is switched off and the material is allowed to furnace cool to room temperature under continuous nitrogen flow.

Step (c): heat treatment

The powder of the nitrided magnetocaloric material obtained in step (b) was evacuated and then sealed in fused silica ampoules under -200 mbar of argon pressure. The am- poules were then heat treated in a Carbolite chamber furnace at temperatures in the range of 1050 °C to 1 100°C, at a heating rate of 5 °C/minute, with a dwelling time of up to 20 hours at the target temperature. At the end of the heat treatment, the ampoules were taken out of the furnace and were quickly quenched in a bucket of cold water. Non-nitrided magnetocaloric materials for comparison tests

For obtaining non-nitrided magnetocaloric materials for comparison tests, samples of the initial powder prepared in step (a) as described above were subjected to the heat treatment according to step (c) as described above without previous nitriding treatment ac- cording to step (b).

2. Characterization of the nitrided magnetocaloric materials according to the invention

2.1 Behavior during heat treatment according to step (c)

The seguence of optical and SEM images in figure 6 demonstrates how the consistency of the initial powder of a magnetocaloric material which has not been subject to a nitriding treatment (comparison sample) changes during heat treatment at 1 100°C for 20 hours finished by water guenching (step (c)). The initial powder obtained according to step (a) as described above exhibits easy flowability, and the particle shape is close to spherical, see upper left hand image of figure 6. The high temperatures applied in the heat treatment according to step (c) promote undesirable sintering of the easy flowing initial pow- der to result in formation of a solid block of material, see lower left hand image of figure 6. Once such solid block is formed, the only practicable technigue of re-comminuting the heat-treated magnetocaloric material is grinding, which results in formation of irregular shaped granules (see right image of figure 6).

The optical images in figure 7 show a powder of a nitrided magnetocaloric material pre- pared according to steps (a) and (b) as described before (left hand image) and after (right hand image) heat treatment at 1 100 °C for 20 hours followed by water guenching (step (c)). The powder of the nitrided magnetocaloric material obtained according to step (b) as described above exhibits easy flowability, and the particle shape is close to spherical, see left hand image of figure 7, thus retaining the consistency of the initial powder obtained in step (a) (see upper left hand image of figure 6). The right hand image of figure 7 shows that after heat treatment according to step (c), the consistency of the nitrided magnetocaloric material has not significantly changed, i.e. it is still in the form of a powder with a particle shape close to spherical, and easy flowability is retained, too.

The SEM images obtained in back-scattered electron (BSE) mode in figure 8 show a particle of a nitrided magnetocaloric material prepared according to steps (a) and (b) as described above without subseguent heat treatment step (c) (left hand image) and a particle of a nitrided magnetocaloric material prepared according to steps (a) and (b) as described above after heat treatment at 1 100 °C for 20 hours finished by water quenching (step (c)). The left image displays uniform contrast across the entire surface of the spherical particle, which points to the presence of only a single phase on the surface. The right image shows two-tone contrast on the surface of the spherical particle, which points to the existence of at least two different phases on the surface of the spherical particles. The brighter regions correspond to the composition of initial magnetocaloric material formed in step (a) without the presence of nitrogen, whereas the darker regions correspond to the nitride phase. It is assumed that the thermal expansion coefficient for the magnetocaloric material phase and the nitride phase are different. Hence, when the particles of the nitrided magnetocaloric material which have a smooth nitride surface layer (left image of figure 8) are exposed to a heat treatment at 1 100°C in step (c), the underlying magnetocaloric phase present in the inner region (bulk) of the particle expands differently compared to the nitride phase at the outer region (at and slightly beneath the sur- face). This exposes the underlying bulk phase at the expense of the surface nitride layer.

2.2 Chemical composition of the nitrided magnetocaloric material formed in step (b)

The left part of figure 9 shows the development of the X-ray diffraction pattern of a magnetocaloric material having the initial composition Mn1.i6Feo.75Po.48Sio.52 (referred to as "base material") which has not been subject to a nitriding treatment (comparison sample) after different durations of thermal treatment at 900 °C in a fused silica ampoule sealed under 200 mbar of argon pressure, while the right part of figure 9 shows the development of the X-ray diffraction pattern after different durations of thermal treatment at 900 °C under nitrogen flow in a fluidized bed reactor (see figure 5), i.e. after different durations of a nitriding treatment according to the above-described step (b) without any heat treatment according to step (c). Comparison of the regions of 2Θ from about 27.5° to about 38° of both series of diffraction patterns reveals the formation of three additional diffraction peaks at positions of 33.8°, 35.1 ° and 37.4° during the nitriding treatment. These peaks correspond to a metal (i.e. iron and/or manganese)-silicon nitride phase. No such peaks are present in the XRD patterns of the comparison samples. This confirms that during the nitriding treatment according to step (b) nitrogen reacts with the magnetocaloric material obtained in step (a) to form an additional crystalline phase. Comparison of the XRD patterns before ("base material") and after one hour of nitriding treatment show that a significant change in the region of 2Θ from about 27.5° to about 38° occurs already after the first hour of the nitriding treatment at the target temperature, and it turned out that the amount of nitride phase formed by a nitriding treatment including a dwelling time of 1 to 1.5 hours at the target temperature is sufficient to achieve the desired effect of avoiding sintering during subsequent heat treatment according to step (c), i.e. to retain the shape and size of the particles after heat treatment step. Thus, for the sake of process efficiency the duration of the nitriding treatment at a temperature in the range of from 890 °C to 950 °C is preferably limited to about 1 to 1.5 hours. Rietveld refinement of the XRD pattern was carried out (not shown) to obtain further information on the phase formed during the nitriding treatment. The XRD pattern of the sample subjected to a nitriding treatment for 3 hours at 900 °C was used for this purpose, because due to the longer nitriding treatment the peaks in the region from about 27.5° to about 38° (see fig. 9) are more pronounced than after 1 hour of nitriding treatment at 900 °C, thus facilitating Rietveld refinement. From this analysis, the nitride phase was identified to exhibit an orthorhombic crystal structure (space group Pna2i), similar to MnSiN₂.

For probing whether the formation of this phase is limited to a region close to the particle surface or extends over the whole volume of the particle, the XRD pattern of as-obtained particles of a powder of a nitrided magnetocaloric material prepared according to steps (a) and (b) as described above (3 hours nitriding treatment at 900 °C) and the XRD pattern of crushed particles of said powder are compared. The penetration of X-rays into the particle bulk is limited by certain factors (energy, angle Θ, mass attenuation coefficient, density of the phase into which the X-ray penetrate) to a depth significantly smaller than the particle size (maximum diameter in the range of from 100 to 150 μιη, see above). Thus, the XRD patterns of the uncrushed particles provide information on the chemical composition in the near-surface-region of the particles (a region extending from the surface over a thickness corresponding to the penetration depth), while the XRD- patterns of the crushed particles provide information on the average chemical composi- tion of the total particle volume. Thus, if formation of the nitride phase is limited to a near surface region having a thickness equal to or smaller than the penetration depth of the X- rays, the fraction x of the nitride phase determined from the XRD pattern of the uncrushed particles should be significantly larger than the fraction x of the nitride phase determined from the XRD pattern of the crushed particles. Alternatively, if nitride for- mation occurs in the whole volume of the particle there should be no substantial difference between the fraction x of the nitride phase determined from the XRD pattern of the uncrushed particles and the fraction x of the nitride phase determined from the XRD pattern of the uncrushed particles. This relationship is illustrated in figure 10.

From the XRD pattern of the uncrushed particles a nitride fraction of 12.7 mol % was determined, while from the XRD pattern of the crushed particles a nitride fraction of about 1 mol % was determined. Accordingly, nitride formation is limited to a near surface region having a thickness equal to or smaller than the penetration depth of the X-rays. Thus, the particles of the powder of the nitrided magnetocaloric material have an outer region in which the average fraction of nitrogen atoms (averaged over the total volume of said outer region) exceeds the average fraction of nitrogen atoms of the total particle (averaged over the total volume of the particle, i.e. over the inner and the outer region).

The change of the fraction of nitrogen atoms with increasing distance from the particle surface was studied by means of scanning electron microscopy (SEM) in combination with energy-dispersive X-ray analysis (EDX). Figure 1 1 shows the cross section of a particle of a nitrided magnetocaloric material prepared according to steps (a) and (b) (6 hours nitriding treatment at 950 °C) as described above without subsequent heat treatment step (c). The fraction of nitrogen atoms was determined by means of EDX at ten positions P01-P10 along a straight line connecting the surface of the particle with the volume center of said particle. The first position P01 is at the surface of the particle, while the tenth position P10 has a distance to the surface of about 4 μιη. The fraction of nitrogen atoms tends to decrease from position P01 to P10, see figure 12. Considering that the particle has a maximum diameter in the range of from 100 μιη to 150 μιη, the position of the maximum of the nitrogen concentration occurs at a distance from the particle surface which is less than 50 % of the distance between the particle surface and the volume center of said particle.

Images obtained from SEM combined with focused ion beam (FIB) cut analysis (figure 13) show that a nitride layer is formed as the result of the nitriding treatment according to step (b) (1 hour nitriding treatment at 900 °C). Before preparing the cut, the surface was coated with a protection layer of platinum (bright layer in the figures). The nitride layer (thin dark layer beneath the bright protection layer) extends continuously over the particle surface but has non-uniform thickness, probably mimicking some surface roughness of the particles of the initial powder. The maximum thickness of the nitride layer is about 400 nm.

It is noted that some of the above-described experiments regarding the chemical compo- sition of the nitrided magnetocaloric material formed in step (b) and the local distribution of the nitride in the particles of said nitrided magnetocaloric material have been done with samples which were exposed to a nitriding treatment including a rather long dwelling time (> 1. 5 to 6 hours) at the target temperature in order to increase the amount of nitride phase formed, for the sake of obtaining more reliable results. Nevertheless, even after a quite long dwelling time like 6 hours, the nitride formation is limited to a near surface region having a thickness equal to or smaller than the penetration depth of the X-rays, so that it can be reasonably concluded that the same holds for nitrided magnetocaloric materials obtained by a nitriding treatment including a dwelling time of only 1 to 1 .5 hours.

2.3 Magnetocaloric properties In order to evaluate the influence of the nitriding treatment (step (b)) on the magnetocaloric properties, magnetocaloric parameters of samples of several nitrided magnetocaloric materials according to the invention as well as of comparison samples in each case obtained from the same initial powder were determined. For details of the preparation of the nitrided magnetocaloric materials and the comparison materials, see above. The Curie temperature upon heating (T_c), the highest value of the heat capacity peak upon heating (C_P), full width at half maximum for the heat capacity peak upon heating (FWHM) and the thermal hysteresis (AThys) were all determined from differential scanning calorimetry (DSC) zero field measurements. Thermal hysteresis is the difference between the positions of T_c upon heating vs. cooling. Table 1 : Magnetocaloric properties of Mn1.i8Feo.73Po.48Sio.52 after heat treatment (step (c)) at 1 100°C for 20 hours followed by water quenching

Tc [K] Cp [J/g^*K] FWHM [K] AThys [K]

No nitriding treatment before

heat treatment 284.0 2.25 3.57 3.6 (comparison sample)

Nitriding treatment (step (b))

at 900 °C for 1 hour 289.6 2.07 3.76 3.5 before heat treatment

Table 2: Magnetocaloric properties of Mn1.14Feo.77Po.48Sio.52 after heat treatment (step (c)) at 1 100°C for 20 hours followed by water quenching.

The data shown in tables 1 to 3 shows that the nitriding treatment does not significantly influence the magnetocaloric behavior.

Claims

Claims:

1. Process comprising the steps of

(a) preparing or providing a powder of an inorganic material selected from the group consisting of magnetocaloric materials according to any of formulae (I) - (XIII):

FeaMnbAcPxSiyZz (I)

wherein

0 < a < 2.2, 0 < b < 2.2, 0 < c < 0.3 and 1 .8 < (a + b + c) < 2.2, preferably 1.95≤(a + b + c) < 2.05

0 < x < 1.05, 0 < y < 1.05, 0 < z < 0.35 and 0.95 < (x + y + z) < 1.05

La(FexAh-x)i₃H_y or La(Fe_xSii-x)i₃H_y (II)

where

x is a number from 0.7 to 0.95,

y is a number from 0 to 3;

La(Fe_xAlyCoz)i3 or La(Fe_xSi_yCo_z)i3 (III)

where

x is a number from 0.7 to 0.95,

y is a number from 0.05 to 1 - x,

z is a number from 0.005 to 0.5;

LaMn_xFe₂-xGe (IV)

where

x is a number from 1.7 to 1.95

(Lai-zCez)(Fei-x-_yMn_ySix)i3Hn (V)

where

x is a number from 0.08 to 0.15 and y is a number from 0 to 0.05,

z is a number from 0 to 0.3,

n is a number from 1 .5 to 3 and

Gd5(SixGei-x)4 (VI)

where x is a number from 0.2 to 1 ,

Tb₅(Si₄-xGex) (VI I)

where x = 0, 1 , 2, 3, 4,

XTiGe (VII I)

where X selected from the group consisting of Dy, Ho, Tm,

Mn₂-xZxSb (IX)

where

Z is selected from the group consisting of Cr, Cu, Zn, Co, V, As, Ge, x is from 0.01 to 0.5,

Mn₂-xZxAs (XI)

MrteZxAsi-x (XI I)

where

Z is selected from the group consisting of Cr, Cu, Zn, Co, V, Ge, x is from 0.01 to 0.5

Heusler alloys of the ΜΠΤΣΧ type (XI II)

wherein T is selected from the group consisting of Ni, Cu wherein X is selected from the group consisting of Al, Ga

exposing said powder of said inorganic material to a gas flow comprising nitrogen at a temperature in the range of from 800 °C to 1000 °C at a nitrogen flow rate of 1 to 5 l/hour and gram of said powder of said inorganic material, thereby forming a powder of a nitrided inorganic material, said nitrided inorganic material comprising a magnetocaloric material according to any of formulae (I) - (XIII) and one or more nitrides of elements of said magnetocaloric material.

2. Process according to claim 1 , wherein

in step (a) preparing said powder of an inorganic material comprises

preparing or providing said inorganic material in the liquid state, fragmenting the liquid inorganic material and allowing the obtained fragments to solidify or

3. Process according to any preceding claim, wherein step (b) is carried out in a fluidized bed reactor or in a rotary kiln furnace.

4. Process according to any of claims 1 to 3, comprising

(c) heat treating the powder of the nitrided magnetocaloric material formed in step (b) at temperatures in the range of from 1000 °C to 1200 °C for a duration of from 1 to 20 hours under an inert gas atmosphere.

5. Process according to claim 4, wherein

step (c) is carried out at temperatures in the range of from 1020 °C to 1 150 °C for a duration of from 10 hours to 20 hours.

6. Powder,

obtainable by a process according to any of claims 1 to 5

and/or comprising particles of a nitrided inorganic material, said nitrided inorganic material comprising a magnetocaloric material according to any of formulae (l)-(XIII) defined in claim 1 and one or more nitrides of elements of said magnetocaloric material,

wherein in each of said particles along a straight line connecting the surface of said particle with the volume center of said particle the fraction of nitrogen atoms has its maximum at a position located at a distance from the particle surface which is less than 50 % of the distance between the particle surface and the volume center of said particle.

7. Powder according to of claim 6, wherein said particles each have a maximum diameter and a minimum diameter, wherein said maximum diameter and said minimum diameter of at least 95% of a representative sample of said particles differ by not more than 25%, preferably by not more than 15%.

8. Powder according to any claims 6 or 7,

wherein each of said particles consists of

an inner region extending around the volume center of the particle an outer region extending around said inner region,

said outer region having a thickness corresponding to 50 % or less of the distance between the surface of said particle and the volume center of said particle,

wherein in said outer region the average fraction of nitrogen atoms exceeds the average fraction of nitrogen atoms of the total particle by a factor of 2 or more.

9. Powder according to claim 8, wherein

said inner region comprises a magnetocaloric material according to any of formulae (l)-(XIII) defined in claim 1 , and

said outer region comprises nitrides of one or more elements of said magnetocaloric material present in the inner region of said particle.

10 Powder according to claim 8 or 9, wherein in said particles, said inner region comprises a magnetocaloric material according to formula (I) defined in claim 1 ,

and

said outer region comprises nitrides of one or more of the elements forming the composition according to formula (I) which is present in said inner region.

1 1. Packed heat exchanger bed comprising a powder of a nitrided magnetocaloric material as defined in any of claims 6 to 10.

12. Device selected from the group consisting of cooling systems, refrigeration systems, heat exchangers, heat pumps, thermomagnetic power generators, climate control units and air conditioning devices wherein said device comprises a powder of a nitrided magnetocaloric material as defined in any of claims 6 to 10, or a packed heat exchanger bed as defined in claim 1 1.