WO2014149007A1

WO2014149007A1 - Composite material with a homogeneous distribution of carbon nanotubes and a method for production of thereof j

Info

Publication number: WO2014149007A1
Application number: PCT/SK2014/000007
Authority: WO
Inventors: Jaroslav SEDLÁČEK; Pavol ŠAJGALÍK; Ondrej HANZEL
Original assignee: Ustav Anorganickej Chemie Sav; Centrum Vedecko-Technickych Informacii Sr (Cvti Sr)
Priority date: 2013-03-18
Filing date: 2014-03-13
Publication date: 2014-09-25
Also published as: SK292013A3

Abstract

A composite material with a homogeneous distribution of carbon nanotubes, the material is being made of aluminium oxide, corundum or zirconium dioxide or silicon nitride or silicon carbide or aluminium with a volume representation of 90 to 99.5 %, and carbon nanotubes with a volume representation of 0.5 to 10 %. The composite material is showing a high compactness of at least 99 % of theoretical density, with the homogeneous distribution of carbon nanotubes preserved along with ensuring a fine grained microstructure of the alumina matrix with an average size of particles less than 1 μm.

Description

Composite material with a homogeneous distribution of carbon nanotubes and a method for production of thereof

Field of the Invention

The technical solution is in the field of carbon nanotube-based composite materials and the technologies used in the production of these materials.

Description of the Related Art

Achievement of a homogeneous distribution of carbon nanotubes in a ceramic or metallic matrix is difficult due to the tendency of carbon nanotubes (hereinafter CNTs) to create agglomerates in the form of bonds as a consequence of van der Waals forces as well as large specific surface area and diameter to length ratio of CNTs. A large volume of CNTs in composite materials leads to their agglomeration and thus negatively influences the required physical and mechanical properties of the resulting composite materials.

The ways of achieving homogeneous distribution of discrete (individual) CNT in a material with a ceramic or metallic matrix have been intensively studied. Different approaches of CNTs dispersion in a matrix were proposed, e.g. method of conventional mixing of powders by using ultrasound or by ball milling; or a colloidal process with added dispersants, surfactants or by acid treatment; sol-gel techniques of capturing CNTs nets of the resulting gel; as well as in-situ synthesis (growth) of CNTs using the technique of chemical vapour deposition (CVD).

Conventional mixing of powders, used widely in the dispersion of the secondary phase in composites, does not have effectiveness high enough in the homogenization of CNTs due to possible mechanical damage to CNTs, which limits the usage of high energy mixing. The application of conventional mixing was observed on different materials, such as aluminium oxide, silicon nitride, etc. [1-5]:

[1] Zhan G.D., Kuntz J.D:, Wan J., Mukherjee A.K. Nat. Mater. 2003, 2, 38-42

[2] Zhan G.D., Kuntz J.D:, Garay J.E., Mukherjee A.K. Appl. Phys. Lett. 2003, 83, 1228-1230

[3] Wang J., Kou H.M., Liu X.J., Pan Y.B., Guo J.K. Ceram. Int. 2007, 33, 719-722

[4] Balaszi C, Shen Z., Konia Z., Kasztovski Z., Weber R, Vertesy Z., et al. Compos. Sci.

Technol. 2005, 65, 727-733

[5] Balaszi C, Sedlackova K., Czigany Z. Compos. Sci. Technol. 2008, 68, 1596-1599

The colloid process uses a modification or a treatment of the CNT surface chemistry, which results in their stabilization in suspension with ceramic or metallic particles. The treatment of CNTs results in functionalization of synthesized CNTs due to their low surface charge after synthesis and removing of contaminants as amorphous carbon as well as the rests of metallic catalyst particles. Acid treatment thus leads to cleaning of the CNTs as well as

functionalization by over-absorption or creation of functional groups (mostly with negative charge), especially carboxylic and oxygen-containing groups. Functionalization also results in changing of the hydrophobic nature of CNTs to hydrophilic, which significantly facilitates further processing, especially using polar liquid as medium for suspension, [6-9]:

[6] Liu J., Rinzler AG, Dai H, Hafher JH, Bradley RK, Boul PJ, et al. Fullerene pipes. Science

1998; 280:1253-6.

[7] Esumi K, Ishigami M, Nakajima A, Awada KS, Hodna H. Chemical treatment of carbon nanotubes. Carbon 1996;34:279-81.

[8] Kim B, Park H, Sigmund W. Electrostatic Interactions between shortened multiwalled carbon nanotubes and polyelectrolytes. Langmuir 2003;19:2525-7.

[9] Hu H, Yu A, Kim E, Zhao B, Itkis ME, Bekyarova E, et al. Influence of the zeta potential on the dispensability and purification of single-walled carbon nanotubes. J Phys Chem B 2005;109:11520-4

The sol-gel process - as an alternative way of CNTs homogenization - uses the mixing of CNTs with the respective sol without negative phenomena which can occur within the conventional mixing, such as destruction of the CNTs and separation of solid phases as a result of different densities between the ceramic or metallic particles and CNTs. The sol-CNT blend transforms into gel as a result of gelling agents or by changing the process conditions (pH, temperature, humidity). Increasing viscosity during gelling prevents formation of CNTs aggregates. Finaly, gel-CNT system can be easily converted by thermal treatment into a homogeneous mixture of powders and CNTs. The disadvantage of such a procedure is the ability to only prepare composite materials with a ceramic matrix. Another negative factor is economic difficulty of the production and high price of the input materials, especially the ceramic precursors.

Presently, , method of preparation of composite materials with a homogeneous distribution of CNTs in a ceramic or metallic matrix is an in-situ growth of CNTs through a gaseous phase (CVD technique). However, using this method the homogeneity of resulting CNTs

distribution in the composite material depends on a homogeneous distribution of catalysts on which the CNTs growth takes place. Another factor that limits a wider use of this procedure is its economic difficults and the necessity of using a matrix with a relatively high open porosity. This in fact increase costs related to the further densification step which require using of higher sintering temperatures, longer sintering times and/or usinge of pressure. Moreover, this results also in increasing of environmental loads besides the economical (higher energy consumption during the procedure).

Alumina-based ceramics represent important materials used especially as a construction material for various machines, equipment and parts. Widespread using of alumina ceramics is due to the properties, such as low density, hardness, wear and corrosion resistance as well as thermal and chemical stability even at high temperatures. However, alumina ceramics has low fracture toughness and low thermal shock resistance, which inhibit their wider usability. Addition of CNTs into the alumina matrix, can significantly improve a fracture toughness and moreover, improve a functional properties, especially thermal and electrical conductivity. To achieve improved properties of alumina ceramic composites with CNTs it is necessary to ensure a homogeneous distribution of both phases in the entire volume of composite material. From the point of preparation of alumina-based composites the main was achieving the homogeneous distribution of CNTs in an alumina matrix, and subsequent densification.

Existing solutions were mostly focused on using the knowledge and methods of CNTs stabilization using dispersing agents, surfactants, CNTs surface functionalization, and use of the sol-gel method as well as in-situ growth of CNTs in a matrix procedure.

The brittleness of alumina ceramics is the greatest disadvantage from the point of view of its application prospects. An important property, which characterizes the crack formation and propagation is fracture toughness.In attempt to improve toughness of such a materials, the majority of published works focused on the improvements of fracture toughness of AI2O₃- based composites, where CNTs are used as a strengthening element in ceramic matrix, improving thus fracture toughness, hardness and flexural strength [1, 2]. However, previous results in this area are inconsistent. Some authors indicate an increase of the AI2O3-CNT nanocomposite fracture toughness. Wnag et al. [3] achieved fracture toughness of 6.4 ± 0.3 MPa.m^1/2 for AI2O₃-SWCNTS composite, which is twice as much as with a monocrystalline AI2O3. Zhan et al. achieved almost threefold improvement in fracture toughness for composite with 10 vol. % of SWCNT compared to monocrystalline AI2O3 ceramics; Siegel et al. observed an increase up to 24 % in alumina composite with 10 vol.% of multi- walled carbon nanotubes (hereinafter MWCNTs) [4], Balaszi et al. [5] achieved 79% increase of the fracture toughness for A1₂0₃-MWCNT composite with 3 vol. % of MWCNTs in comparison of nanocrystalline ceramics. Other authors, Laurent et al. [6], Wang et al. and Sun et al. have not observed any or only marginal improvement of fracture toughness compared to monocrystalline alumina ceramics [4, 7]. Flexural strength is rise up with a increasing amount of CNTs, until reaching a certain level, at which a higher probability of agglomerate formation as well as higher porosity occurs [8]. As far as hardness of these composites is concerned, it is usually lower compared to monocrystalline AI2O3, probably due to the weak bonds between CNTs and AI2O3 grains, the lubricating nature of CNTs, as well as the presence of a relatively soft phase (the hardness of multi-walled nanotubes in radial direction is 6 - 10 GPa) on the grain boundaries [9].

Mechanical properties of as prepared materials do not meet a required criteria, due to the problems occurring during the production of AI2O3-CNT nanocomposites, especially in achieving of homogeneous distribution of CNTs in ceramic matrix. To achieve uniform distribution, several procedures were suggested. They involve mixing on molecular level, which consists of a reaction between functionalized CNTs and metallic ions in a solution ([10] CHA, S. I. et al.: In Scripta Materialia, 2005, vol. 53, p. 793-797), or in-situ synthesis of CNTs in a ceramic matric, preparation of dispersed AI2O3 and CNTs suspensions using ultrasound or grinding in an Atritor with a surface active substance ([11] ZHANG, S. C. et al.: In J. Eur. Ceram. Soc, 2010, vol. 30, p. 1373-1380), preparation of A1₂0₃-MWCNT using precursors ([12] YAMAMOTO, G. et al.: In Diamond and Related Materials, 2008, vol. 17, p. 1554-1557), coating the nanotubes with a layer of A1₂0₃ ([13] ESTILI, M., KAWASAKI, A.: In Scripta Materialia, 2008, vol. 58, p. 906-909), and more.

A uniform distribution of CNTs inside the matrix by itself is not enough to improve the properties of a material. Another problem is weak phase compatibility caused by chemical inhomogeneity of these two phases due to the different nature of bonds between CNTs and the matrix, and the strong tendency of CNTs to form clusters that are bound by attractive van der Waals forces [13], ([14] LIU, S. et al.: In Carbon, 2011, vol. 49, p. 3698-3704), ([15]

SARKAR, S., DAS, P. K.: In Ceramics International, 2012, vol. 38, p. 423-432). It is also necessary to achieve fully dense composites and also to optimize the sintering process without CNTs degradation. Exposing the nanotubes to high temperatures (above 1250 °C) can destroy the structural integrity of carbon nanotubes, which leads to the failure of many of their toughness-improving mechanisms [8]. To densify A1₂0₃-CNT ceramic composites, different methods of sintering were used, such as pressureless sintering, hot pressing, and the SPS method. The most promising one seems to be the SPS method, which makes it possible to achieve better densification at lower temperatures and shorter holding period compared to conventional methods, which decreases the probability of damaging the CNTs structure, and inhibits grain growth [4, 5, 10, 15]. The presence of carbon nanotubes prevents the grain growth of matrix grains, and refinedf microstructure generally has a positive influence on the improvement of its rigidity, hardness, wear and thermal shock resistance . In addition, the refinement of microstructure improves fracture toughness of many materials [9]. Toughness- improving mechanisms in composites of this type include crack deflection, crack bridging, nanotube pull-out [3, 14] and ([16] HE, C. N. et al.: In Journal of Alloys and Compounds, 2009, vol. 478, p. 816-819).

Further research focused on the influence of CNTs on functional properties of corundum ceramics. Alumina ceramics is an insulator with electrical conductivity of 10^'10 - 10^"12 S/m. The addition of CNTs, as a component with high conductivity, can significantly increase electrical conductivity. However, similarly to their effect on mechanical properties, a homogeneous distribution of CNTs in a matrix and strong phase interface between the nanotubes and A1₂0₃ is very important. Electrical properties of the nanocomposite are then dependent mostly on the amount of CNTs, ceramic compactness and sintering conditions. Zhan et al. achieved an electrical conductivity of 3345 S/m for alumina-SWCNT with 15 vol. % of SWCNTs , sintered using the SPS method. ([17] KUMARI, L. et al.: In Ceramics International, 2009, vol. 35, p. 1775-1781). Authors ([18] INAM, F. et al.: In J Eur. Ceram. Soc, 2010, vol. 30, p. 153-157) have achieved electrical conductivity of 576 S/m for A1₂0₃- 5vol.% MWCNT where SPS method was also used for sintering, since it preserves structural integrity of the CNTs, which is very important in improving electrical conductivity of the nanocomposite. The electrical conductivity of these nanocomposites also increases along with the increasing size of grains, which was caused by increasing amounts of conductive paths on grain boundaries, where the nanotubes are placed [18].

To improve thermal conductivity of a material, homogeneous distribution of nanotubes in the matrix and hindering of agglomerate formation is similarly important, because thermal conductivity of CNT bundles is three orders of magnitude lower than the conductivity of individual CNTs,e.g. disordered CNT films have conductivity of about 30 W/m.K and magnetically ordered CNT films have thermal conductivity of about 200 W/m.K. CNT defects similarly lower their thermal conductivity [19] BAKSHI, S. R. et al.: In Computational Materials Science, 2010, vol. 50, p. 419-428). Adding CNTs to a matrix (metallic or ceramic) shows great potential for improving thermal conductivity. Shi et al. have shown that the addition of 10 vol. % of CNTs coated with copper to a W-Cu powder increased the thermal conductivity of hot-pressed samples from 180.97 to 640.53 W/m.K ([20] SHI, X. L. et al.: In Materials Science and Engineering A, 2007, vol. 457, p. 18-23). Authors ([21] KUMARI, L. et al.: In Composites Science and Technology, 2008, vol. 68, p. 2178-2183) achieved the highest thermal conductivity of 90.4 W/m.K, measured at 100 °C for an A1₂0₃-CNT nanocomposite with 7.39 wt. % of CNTs, i.e. increasing up to 229% compared to pure A1₂0₃. This composite was sintered at a temperature of 1550 °C using the SPS method. Authors attribute this improvement to several factors: scattering of phonons in CNT agglomerates bound by van der Waals forces occurs more frequently, which leads to a decrease of thermal conductivity, therefore a homogeneous distribution of nanotubes is essential for increasing the thermal conductivity, higher sintering temperatures during the SPS process led to increasing the thermal conductivity, which is probably caused by increased compactness and therefore lower porosity [21].

Summary of the Invention

The negatives of existing solutions or approaches are mostly eliminated by a composite material with a homogeneous distribution of carbon nanotubes formed in a manner described in this invention. The described solution deals with a composite material containing carbon nanotubes and the manner of its production, which allows for a significant change of material properties by increasing its fracture toughness, electrical conductivity and thermal conductivity. The invention solves the problem of achieving the homogeneous distribution of carbon nanotubes in particular. A composite material with a homogeneous distribution of carbon nanotubes is formed with aluminium oxide (corundum) or zirconium dioxide or silicon nitride or silicon carbide or aluminium with a volume representation of 90 to 99.5 % and carbon nanotubes with a volume representation of 0.5 to 10 %. Carbon nanotubes are for instance distributed in an alumina matrix, and the resulting composite material shows a high density of at least 99% of theoretical density, while securing homogeneous distribution of carbon nanotubes and a fine grained microstructure of the alumina matrix with an average size of particles less than Ιμπι.

The approach of preparation of a composite material according to this solution comprises the combination of methods of stabilization of CNT suspensions with ceramic or metallic particles and the use of„freezing" the suspension after spraying it into liquid nitrogen with resulting freeze-drying (lyophilization).

The technology of preparation is in the first step based on an excitation of CNT clusters using acids, specifically sulfuric and nitric acid in a volume ratio of 3 : 1. In this step of the preparation of the material, a disturbance of van der Waals forces between individual nanotubes and removal of remains of the catalyst (after the CNTs creation itself) as well as of free amorphous carbon (undesirable because of its degrading properties) occurs. To increase the efficiency of the preparation it is appropriate to use concurrent influence of ultrasound, which aids in the separation of individual nanotubes. Subsequently modified CNTs are filtered out and washed in water to remove acids. In the second step, the modified CNTs are mixed into a water suspension (in literature, other types of liquid medium are routinely used - isopropanol or other higher alcohols, hexane) with an appropriate material (ceramic or metallic particles, for instance), with an additive of an appropriate steric stabilizer (sodium dodecyl sulphate, dimethylformamide). During this step, homogenization between powder particles and nanotubes occurs, while - due to the organic molecules of the stabilizer - stabilization of the suspension (blend) itself occurs. Homogenization of the suspension can occur by mixing using milling bodies (ball milling, attrition milling). In this step, other additives that fill the role of granulation additives (binder agents) can be used. In the third step, spraying the stable suspension into liquid nitrogen occurs, where at a temperature of -196°C immediate freezing of the carrier medium (water), and therefore freezing of the blend of ceramic or metallic powder and CNT occurs. To remove the presence of water or to inhibit its phase transition back to liquid state (which could lead to separation of individual phases from each other due to their different densities), it is necessary to remove the water in its solid phase by sublimation (lyophilization under reduced pressure).

The outcome of this technological process is a blend in the form of a powder or a granulate, which is then further modified for other purposes by removing large particles by sieving to a desired fraction. Mixtures prepared in this way are then densified by sintering at high temperatures and possibly pressures (e.g., hot pressing) under an inert atmosphere (to prevent oxidation - CNT burning out). The resulting composite material can be created with aluminium oxide (corundum), or zirconium dioxide, or silicon nitride, or silicon carbide, or aluminium with a volume representation of 90 to 99.5 % and (multi-walled) carbon nanotubes with a volume representation of 0.5 to 10 %.

Composite materials prepared according to this solution show properties listed in the following table:

Aluminium 230 ± 0.5 36.7 ± 0.6 0.041 ± 0.002 261 ± 1.8

Detailed Description of the Invention

Example 1:

Multi-walled carbon nanotubes (hereinafter MWCNTs) in an amount of 5 g were treated and functionalized by mixing in a 400 ml mixture of sulfuric and nitric acid with a volume ratio of 3:1 for 5 hours. After each hour of mixing, the mixture was exposed to ultrasound for 20 minutes to ensure a more intensive separation of individual nanotubes from each other. After the process of mixing the mixture was diluted by distilled water and MWCNTs were filtered out through a 0.1 μπι nylon filtration membrane and washed with distilled water until a filtrate pH of 7 was achieved. The MWCNTs were subsequently dried for 10 hours at a temperature of 100°C. MWCNTs modified in this manner were dispersed using ultrasound in a solution of sodium dodecyl sulfate (SDS from this point), with a weight ratio of SDS : MWCNTs being 1.5 : 1. The dispersion of de-agglomerated carbon nanotubes prepared in this way was then used further for preparation of composite materials.

A ceramic powder of aluminium oxide with an average particle size of 400 nm was dispersed using ultrasound and subsequently mixed with the MWCNTs dispersion prepared according to the aforementioned procedure, with pH modified to the value of 10.9 using an ammonium hydroxide solution. The mixing was intensified using corundum milling bodies with a milling time of 24 hours. After the homogenization, the milling bodies were separated out by pouring through a sieve, and the homogeneous mixture was sprayed into liquid nitrogen. The spraying speed was controlled with the dispersion flow, which was kept at a constant rate of 20 ml/min and the pressure of the propellant gas at 10 kPa. Under these conditions, granules of the mixture of aluminium oxide, MWCNT and frozen water were created by rapid supercooling of the sprayed dispersion at a temperature of -196°C. The water was removed with

lyophilization in the next step, in a reduced pressure of 100 Pa, cold trap temperature of -56°C, and exposure time of 24 hours. The resulting granulate of aluminium oxide with a homogenously distributed MWCNT phase was subsequently cold-pressed under a pressure of 100 MPa, and this pressed intermediate product was sintered using hot pressing. Hot pressing took place at a temperature of 1550°C and a pressure of 30 MPa in an atmosphere of inert gas (argon) for the duration of 1 hour.

The outcome of this preparation procedure is a composite ceramic material with carbon nanotubes homogeneously distributed in an aluminium oxide matrix, which show high density (at least 99% of theoretical density), with not only the homogeneous CNTs distribution preserved, but also the fine-grained microstructure of the corundum matrix preserved (narrow distribution of the size of corundum grains), with an average size of particles lower than 1 μπι.

Example 2: 5 g of multi-walled carbon nanotubes (MWCNTs) were treated and functionalized by mixing in a 400 ml mixture of sulfuric and nitric acid with a volume ratio of 3 : 1 for 5 hours. After each hour of mixing, the mixture was exposed to ultrasound for 20 minutes to ensure a more intensive separation of individual nanotubes from each other. After the process of mixing the mixture was diluted by distilled water and MWCNTs were filtered out through a 0.1 μιη nylon filtration membrane and washed with distilled water until a filtrate pH of 7 was achieved. The MWCNTs were subsequently dried for 10 hours at a temperature of 100°C. MWCNTs modified in this manner were dispersed using ultrasound in a solution of sodium dodecyl sulfate (SDS from this point), with a weight ratio of SDS:MWCNTs being 1.5:1. The dispersion of de-agglomerated carbon nanotubes prepared in this way was then used further for preparation of composite materials.

An aluminium powder with an average particle size of 150 nm was dispersed using ultrasound and subsequently mixed with the MWCNT dispersion prepared according to the

aforementioned procedure, with pH modified to the value of 10.9 using an ammonium hydroxide solution. The mixing was intensified using corundum milling bodies with a milling time of 24 hours. After the homogenization, the milling bodies were separated out by pouring through a sieve, and the homogeneous mixture was sprayed into the liquid nitrogen. The spraying speed was controlled with the dispersion flow, which was kept at a constant rate of 20 ml/min and the pressure of the propellant gas at 10 kPa. Under these conditions, granules of the mixture of aluminium, MWCNTs and frozen water were created by rapid supercooling of the sprayed dispersion at a temperature of -196°C. The water was removed with

lyophilization in the next step, in a reduced pressure of 100 Pa, cold trap temperature of -56°C, and exposure time of 24 hours. The resulting granulate of aluminium with a

homogenously distributed MWCNTs phase was subsequently cold-pressed under a pressure of 100 MPa, and this pressed intermediate product was sintered using hot pressing. Hot pressing took place at a temperature of 550°C and a pressure of 30 MPa under reduced pressure of 100 Pa for 5 minutes.

Industrial Applicability

The invention introduces a convenient alternative to already known solutions considering the technological procedure as it ensures lower input costs of the production process and eliminates the negative ecological impact of currently used techniques.

Composite materials with carbon nanotubes prepared according to this approach are suitable for the use as construction elements, with two main areas of application:

1. defence industry, engineering and automotive industry (improvement of mechanical properties, fracture toughness in particular)

2. electrical engineering - the use of improved electrical properties of these materials, especially low resistivity, as well as the use of their multifunctional character (in the case of ceramics), e.g. combination of resistance to aggressive environments and mechanical and electrical properties. In electrical engineering it is also possible to use the ability of these materials to conduct heat due to their increased thermal conductivity, for instance as substrates.

Claims

PATENT CLAIMS

1. A composite material with a homogeneous distribution of carbon nanotubes, with the material being made of aluminium oxide, corundum or zirconium dioxide or silicon nitride or silicon carbide or aluminium with a volume representation of 90 to 99.5 %, and carbon nanotubes with a volume representation of 0.5 to 10 %.

2. A composite material with a homogeneous distribution of carbon nanotubes according to claim 1 , characterized by carbon nanotubes distributed in an aluminium oxide, corundum matrix, and the composite material showing a high compactness of at least 99 % of theoretical density, with the homogeneous distribution of carbon nanotubes preserved along with ensuring a fine grained microstructure of the alumina matrix with an average size of particles less than Ιμπι.

3. A method of production of a composite material with a homogeneous distribution of carbon nanotubes according to claims 1 and 2, characterized by (i) deaglomeration of carbon■ nanotubes bundles using acids, best sulfuric and nitric acid in a volume ratio of 3: 1, which' causes an elimination of van der Waals forces between individual nanotubes, as well as the removal of the rest of remaining catalyst particles and free amorphous carbon; (ii) simultaneous exposure to ultrasound, which enhance the separation of individual nanotubes and is used to increase efficiency of the preparation; (iii) filtering the carbon nanotubes and washing by distilled water to remove the acids; carbon nanotubes are then mixed into a water suspension with a ceramic or metallic powder with an additive of an appropriate steric stabilizer, best sodium dodecyl sulphate or dimethylformamide, which causes homogenization between powder particles and nanotubes, stabilization of the suspension occurs as a result of the influence of organic molecules of the stabilizer, and further homogenization of the suspension takes place, with or without the presence of milling bodies, with the addition of granulation additives; the stable suspension is then sprayed into liquid nitrogen, where at a temperature of -196°C immediate freezing of the carrier medium - water - and freezing of the powder and carbon nanotube mixture occurs, which ensures the elimination of separation of individual phases from each other, and the resulting mixture in the form of a powder or a granulate is modified for other purposes by removing big particles by sieving to a desired fraction, and mixtures prepared in this manner are then compacted by sintering at high temperatures, or also by applying pressure by hot pressing in an inert atmosphere.