WO1991005747A1

WO1991005747A1 - Method of heat-treating unstable ceramics by microwave heating and susceptors used therefor

Info

Publication number: WO1991005747A1
Application number: PCT/CA1990/000358
Authority: WO
Inventors: Prasad Shrikrishna APTÉ; Robert Murray Kimber; Mark Christopher Lawrence Patterson; Raymond Yves Roy; David Nelson Mitchell
Original assignee: Alcan International Limited
Priority date: 1989-10-19
Filing date: 1990-10-19
Publication date: 1991-05-02
Also published as: JPH05500940A; CA2001062A1; JP2628104B2; CN1054579A; AU6540090A; EP0495850A1

Abstract

A method of sintering thermally unstable (usually non-oxyde) ceramics by microwave heating. In the method, a body of the ceramic is at least partially surrounded with a protective bed of particulate material and then the body and the bed (34) are subjected to microwave irradiation to cause heating and sintering of the body. In order to allow the method to be carried out in an oxygen-containing gas such as air and at low pressure, the bed contains effective amounts of: (a) a microwave susceptor, if the unstable ceramic is not already a susceptor, (b) a protective material capable of generating a localized atmosphere which reduces decomposition and/or oxidation of the unstable ceramic, (c) an oxygen getter and (d) a material of sufficiently good thermal conductivity to prevent excessive localized heating in the bed. The process can be combined with a hot isostatic pressing step carried out simultaneously or subsequently. The invention also relates to the bed of particulate material used in the method and to a process for joining bodies of unstable ceramics by microwave irradiation using the protective bed.

Description

Method of heat-treating unstable ceramics by microwave heating and susceptors used therefor Technical Field

This invention relates to heat-treating ceramic powders. More particularly, the invention relates to a method of heat-treating thermally unstable ceramic powders utilizing microwaves to generate the required heat, to a susceptor composition used in the method and to the sintered products thus produced. Background Art

Although it is already known to use microwaves to heat and, if necessary, to sinter ceramic powders (i.e. powders comprising compounds of metals and/or non- metals) , such procedures are best suited to heating and sintering stable metal oxide ceramics because other ceramics often do not absorb microwave energy to the necessary extent (at least at ambient temperatures) or they decompose or oxidize before heat-treatment temperatures are reached. The problem posed by poor microwave absorption can be solved by surrounding a "^■green" body of a non- susceptor ceramic with a powder made of a material that does absorb microwaves adequately, i.e. a microwave susceptor. The body is then heated by conduction, convection and/or radiation from the susceptor powder either until the desired treatment temperature is reached or until the ceramic reaches a temperature at which it does absorb microwave energy sufficiently for further direct heating to the desired temperature. The problem posed by the thermal instability or reactivity of certain ceramics can also be overcome by providing a suitable protective atmosphere for the green body during the heating procedure. For example, the decomposition and oxidation of silicon nitride can be substantially prevented by heating the material in an atmosphere containing a suitable partial pressure of nitrogen. The partial pressure required in any particular case depends on the time of the procedure and on the desired treatment temperature. However, at high temperatures (which are required to achieve sintered products of high density) , decomposition can occur even under pure nitrogen at one atmosphere. The procedure must consequently be carried out at high pressures and this is disadvantageous because it requires the use of atmosphere-controlled, high temperature and high pressure furnaces which are expensive and inconvenient to use. There is accordingly a need for a method of heat- treating unstable ceramics that makes use of microwaves for heating the material but which avoids the need for controlled atmospheres and high pressures. An object of the present invention is to satisfy this need. Disclosure of Invention

According to one aspect of the present invention, there is provided a method of heat-treating a body of thermally unstable ceramic material, which comprises at least partially surrounding said body with a bed of particulate material and irradiating said bed with microwave energy in an oxygen-containing gas, said bed comprising effective amounts of: (a) a microwave susceptor if said unstable ceramic is not itself a microwave susceptor; (b) a protective material capable of generating a localized protective atmosphere which reduces decomposition and/or oxidation of the ceramic material; (c) an oxygen getter; and (d) a material having sufficiently good thermal conductivity to prevent excessive localized heating in said bed.

According to another aspect of the present invention, there is provided a particulate material capable of being used as a protective bed for heat- treating a body of an unstable .ceramic material with microwaves in an oxygen-containing gas, said material comprising effective amounts of: (a) a microwave susceptor, if said ceramic material is a non-susceptor; (b) a protective material capable of generating a localized atmosphere which discourages decomposition of the unstable ceramic; (c) an oxygen getter; and (d) a material having sufficiently good thermal conductivity to prevent excessive localized heating in said particulate material during microwave irradiation thereof.

According to yet another aspect of the invention, there is provided a process for joining bodies made of thermally unstable ceramics, which comprises: bringing said bodies into contact; surrounding said bodies in the area of contact with a protective bed of particulate material; irradiating said protective bed in an oxygen- containing gas with microwave^' radiation sufficiently to heat said bed and bodies to cause joining of the latter; and allowing said bodies to cool; wherein said bed of particulate material comprises effective amounts of

(a) a microwave susceptor, if said ceramic material is a non-susceptor;

(b) a protective material capable of generating a localized atmosphere which discourages decomposition of the unstable ceramic;

(c) an oxygen getter; and

(d) a material having sufficiently good thermal conductivity to prevent excessive localized heating in said particulate material during microwave irradiation thereof.

By using a susceptor bed of particulate material (powder base) of the above kind for the heat-treating procedure, the method can be carried out in a low pressure oxygen-containing atmosphere, i.e. in a conventional microwave furnace open to atmospheric air. Brief Description of Drawings

Figure 1 is a side elevational view of a hot isostatic press used to carry oμt a preferred embodiment of one form of the present invention;

Figure 2 is a view similar to Figure 1 except that the press is in the pressing position; Figure 3 is a resonant cavity, shown on an enlarged scale, suitable for the simultaneous heating and pressing of a product;

Figure 4 is a cross-section of apparatus used to carry out a preferred embodiment of another form of the present invention;

Figure 5 is a graph showing the average grain size across the diameter of a silicon nitride sintered product produced according to Example 5 below; and Figure 6 is a photomicrograph of the sintered product of Example 5. Best Mode For Carrying Out Invention

The powder bed used in the present invention may be a mixture of different ingredients each one of which provides one only of the properties (a) to .(d) mentioned above. However, a single ingredient may provide two or more, or possibly even all, of the stated functions so the number of ingredients of the powder bed should be reduced. The number of ingredients should preferably be chosen on the basis that the desired properties should be optimised, even if this means providing a separate ingredient for each property. The various properties required in the powder bed are explained in more detail below. Unless the unstable ceramic is itself an adequate microwave susceptor, the powder bed should contain at least one microwave susceptor.

A microwave susceptor is a material that absorbs energy rom microwaves at a rate faster than the rate at which it loses energy. The microwave susceptor used in the powder bed of the present invention can be any material that is capable of absorbing (coupling with) microwave radiation to the extent necessary to raise the temperature of the ceramic body at least partially buried in the bed to the desired temperature (at which the body itself absorbs sufficient energy directly from the microwaves) . Silicon carbide, for example, acts as a microwave susceptor and has the advantage of also being an oxygen getter (see below) . Other carbides and carbon may replace silicon carbide in this function and other suitable microwave susceptors include porcelain, soda- lime glass and barium titanate

The nature of the protective material used in the susceptor bed depends on the nature of the unstable ceramic used for the body to be heat-treated. However, the protective material generally functions by generating an atmosphere around the body to be heated that protects the body against decomposition and/or oxidation. For example, when the ceramic body is made of (or contains) silicon nitride, the protective material may itself be silicon nitride. This decomposes (Si₃N₄ → 3Si + 2N₂) and forms an atmosphere having a high nitrogen partial pressure localized around the body which protects the body against further decomposition. The decomposition of the silicon nitride begins in the bed (rather than the body) since the bed heats up first and is initially hotter than the body. When the body itself begins to heat, the protective nitrogen atmosphere has already been established.

In contrast, when carbon is used to protect carbides, it protects the carbides primarily from oxidation, e.g.

SiC + 0₂ + C → SiC + C0₂. Whenever possible, it is desirable to use a protective material which is the same as the unstable ceramic in the body to be heated because there is then less contamination of the body as a result of the heating method.

Further examples of protective materials for particular non-oxide ceramics are:

(a) MoS₂ which decomposes when heated to Mo and S-(g). useful for protecting bodies comprising MoS₂ and/or MoS₃;

(b) silicon nitride for protecting oxynitrides; (c) silicon carbide for protecting borides, e.g. TiB₂; and

(d) lead based ceramics, (e.g. lead zirconate titanate and lead lanthanate zirconate titanate) used to protect bodies comprising the same materials. These materials produce lead-based protective atmospheres, have adequate conductivity and are microwave susceptors. An oxygen getter is a material which at least partially eliminates oxygen from the localized atmosphere around the body to be heated, at least at the temper¬ atures at which the unstable ceramics are vulnerable to oxidation. Getters function by reacting chemically with oxygen to lower the partial pressure .of the oxygen to such an extent that oxygen is minimized. Metal carbides generally, and silicon carbide in particular, are effective oxygen getters, as are carbon and oxidizable metals such as Zr, Ca, Al, Ti, W, Mo, Ta and Cu.

The material having good thermal conductivity can be any material that allows uniform heating in the bed and transference of the heat to the body. This prevents the development of hot spots (large temperature gradients) in the bed leading possibly to thermal runaways as well as instabilities in electrical and thermal distribution. Such a material is often required in the bed because the other ingredients are often ceramics of low thermal conductivity. Boron nitride is a particularly suitable material for this, but aluminum nitride for example, could alternatively be used. Moreover, a high metal content in the bed (for example, when metals are used as oxygen getters) also provides high thermal conductivity. It is difficult to give preferred conductivity ranges because optimum values vary according to bed size and composition.

The ingredients of the powder bed should each be present in an "effective" amount, i.e. the amount necessary to exert their desired effect (heating, protection, oxygen removal and conductivity) . The minimum amount of each material depends on the nature of the material employed. For example, when the powder bed is a mixture of SiC, Si₃N₄ and BN, the minimum amounts are respectively 25 wt%, 20 wt% and 10 wt%. The preferred relative proportions are about 40:30:30 in wt% respectively.

The method of the invention can be used to protect a variety of unstable ceramics especially non-oxide ceramics, in addition to those mentioned above, e.g. superconductors I, II and III using a super-conductor powder bed.

The method of the invention can be used for virtually any type of heat-treating of ceramic bodies including sintering, annealing, recrystallization of glassy phases, other thermal treatments and joining together of ceramic bodies. The joining technique will be described more fully below.

In some cases, when sintering is being carried out, sintering aids may have to be incorporated into the bodies to be heated in order to facilitate sintering. Sintering aids are sometimes microwave susceptors, in which case they contribute to the heating of the body.

The powder bed can be prepared simply by mixing the powdered ingredients in the normal way (e.g. stirring, shaking, tumbling etc.) and the particle sizes of the different ingredients may be the same or different. The average particle size is not particularly critical, although smaller average particle sizes are preferred in order to confine the protective atmosphere with the bed and to limit the penetration of air from outside. A particularly preferred type of bed can be prepared by the following procedure using particles of different sizes.

First of all the body to be treated can be buried in a bed of the ingredient having;the largest particles. The ingredient having the second largest particles can then be poured on top of the bed and the bed gently shaken or vibrated until the smaller particles flow down into the interstices between the larger particles. This is continued until the ingredient having the smallest particles is then poured on top of the bed and the bed gently shaken or vibrated until the small particles flow down and fill the voids remaining in the bed. Naturally, the relative particle sizes must be such that such void filling can take place.

This preferred procedure produces a dense bed, usually having a void component of less than 30% by volume, which is easy to construct. The small remaining void size reduces oxygen permeability and the fact that the particles have considerably different sizes means that the components are easy to separate from each other after use. Desirably, the protective material is used to form the smallest particles because the resulting large surface to volume ratio ensures thorough evolution of the protective gases and the size of the particles ensures a dense and uniform layer adjacent to the surface of the green body.

Rather than making the susceptor bed uniform throughout, in some cases it may be desirable to vary the composition of the susceptor bed from place to place to take into account different shapes, thicknesses and/or compositions of different parts of the green body and different microwave powers at different positions within the resonant microwave cavity of the furnace. For example, the bed can be arranged in layers of higher or lower thermal conductivity, or areas of higher or lower microwave absorption (i.e. heating) . Similar compensatory effects can be made by specifically designing the size and/or shape of the bed itself.

The process of the invention can also be combined with a hot isostatic pressing technique carried out as the ceramic material is being heated by the microwaves. This involves positioning the ceramic body to be treated and the powder bed mentioned above in a high-pressure resistant chamber capable of acting as a resonant cavity for microwave, irradiating the body and bed with microwave energy for a time sufficient to raise the temperature of the bed and body to a required treatment temperature, and either subsequently or simultaneously raising the fluid pressure in the cavity to pressurize the body isostatically.

If the heating and pressing steps are carried out together, provision must be made to introduce the microwaves into the pressurized cavity. This can be done by equipping the chamber with a high pressure-resistant microwave transparent window or a solid microwave antenna passing through the vessel wall in a pressure tight arrangement. However, it is generally more economical to carry out the heating and pressing steps sequentially; for example, a wave guide can be introduced into an opening in the chamber during the heating step and then, for the pressing step, the waveguide can be removed and replaced by a high-pressure resistant closure device. The pressure inside the cavity can be elevated to the desired extent (usually 40,000-50,000 psi) during the pressing step by introducing a gas from a pressurized container or a high pressure pump. If the heating and pressing steps are to be carried out simultaneously, the cavity can be sealed during heating and the body subjected to autogenously-generated pressure.

This procedure can make use of conventional hot isostatic pressing equipment modified for the generation, introduction and resonance of the microwave energy. The pressure chamber, normally made of steel, should be designed as a single or multi-mode cavity resonator for a particular wavelength. The pressure chamber of a conventional press can be modified to achieve this, for example by inserting conducting cylinders of predetermined diameters into the chamber so that only the required cavity modes will resonate. As an example, at a frequency of 2.45 GHz, a cavity diameter of between 7.2 and 9.4 cm is required to excite the dominant mode in a cylindrical cavity. Depending on the shape of the pre¬ form body, other modes may have to be excited and this may be achieved by making the chamber larger. For extensive production runs involving the same pre-form configuration, the pressure vessel itself may be specifically designed to ensure resonance of the desired modes.

Conventional magnetrons can be employed to generate the microwaves, e.g. magnetrons capable of generating microwaves at 2.45 GHz or 915 MHz at power levels of about 500-600 watts. Magnetrons of this type can generate sufficient heat within the body to raise the temperature rapidly to the sintering level, e.g. up to about 2100*C in as little as 5 to 10 minutes.

A preferred example of a hot isostatic press according to the present invention is shown in Figs. 1 and 2. The press 10 comprises an open rectangular framework 11 supporting a rigid "C-shaped^M beam 12. A hydraulic ram 13 is supported by the lower framework members and is laterally slidable between the positions shown respectively in Figs. 1 and 2. The piston 14 of the ram supports a pressure vessel 15, the interior of which forms a chamber designed to act as a resonant cavity for microwaves. The chamber contains a pre-form body 16 (shown in broken lines) to be treated supported in a powder bed in accordance with the invention (not shown) . The upper end of the pressure vessel has an open collar 17. An engaging flange 18 is rigidly attached to the beam 12 at one side and has a central hole which receives a waveguide 19. The waveguide is connected to a magnetron (not shown) which generates microwaves of the desired frequency. These microwaves are conveyed by the waveguide in the direction of the arrow into the interior of the pressure vessel 17.

The equipment is used as follows. Firstly, the pre¬ form 16 is subjected to a heating cycle. For this, the apparatus is arranged in the manner shown in Figure 1. The hydraulic ram 13 presses the pressure vessel 15 against the flange 18. The magnetron is operated to direct microwave energy along the waveguide 19, through the collar 17 and into the chamber where the energy is absorbed by the powder bed so that the temperature of the body rises rapidly. After a length of time suitable to raise the temperature of the body to the sintering range, the magnetron is switched off and the ram is lowered. A high pressure-sealing lid 20 is placed on the pressure vessel and the assembly moves to the right hand position as shown in Figure 2 whereupon the ram is actuated to force the upper end of the pressure vessel and lid 20 against the upper end of the C-shaped beam 12 while the opposite end of the ram is braced against the lower end of the beam. Gas at high pressure is introduced into the pressure vessel through inlet 21. The pressurized gas introduced into the vessel during this part of the cycle may have a pressure as high as 20,000 psi or more. The pressure has the same effect on the ceramic product as conventional hot isostatic pressing. The body is maintained at a suitable temperature during this step by virtue of the powder bed in which it is embedded by virtue of its own thermal mass. After the pressurizing step is complete, the pressure in the pressure vessel is relieved and the product is allowed to cool down whereupon it may be removed from the pressure vessel and powder bed.

Although not shown in Figures 1 and 2, the apparatus may comprise two or more presses of the illustrated design, each being connected to a single magnetron via their waveguides 19. Switching means are provided in the waveguides so that microwave energy may be conveyed to only one of the presses at a time. In this way, when microwave energy is no longer required for one press

(i.e. during the pressing or cooling cycles) , it can be switched to the other (or one of the other presses) . The capital costs can thereby be reduced and productivity increased.

Figure 3 shows an alternative pressure vessel 30. As in the previous embodiment, the interior of the pressure vessel forms a chamber which acts as a resonant cavity. In this case, the microwave radiation is introduced into the resonant cavity by means of antenna 31 which extends into the cavity through a high pressure seal 32. A pressure head 33 completes the sealing of the cavity and a pressurized gas is introduced through inlet 35. Because this equipment is capable of both pressurizing and irradiating a sample 34 at the same time, if desired, there is no need to separate the heating step and the pressurizing step as in the previous embodiment and both can be carried out with the press in the position shown in Figure 2, i.e. with the pressure vessel clamped between the ram and the C-shaped beam.

Although it is not shown in the drawings, alternative apparatus may comprise a chamber sealed by a lid having a microwave-transparent window and a high pressure shutter behind the window to protect the window from the high pressure in the chamber during the pressing cycle. Using this arrangement, microwaves can be introduced through the window during the heating cycle and then, after closure of the shutter, pressurized gas may be introduced into the chamber for the pressing cycle. Using such apparatus, it may not be necessary to move the vessel between two different stations for the heating and pressing steps. In the case of microwave energy at 2.45 GHz up to 5kw, the window may be made of quartz and should have an area of approximately 2 inches.

The body to be heat-treated by the method of the present invention, either with ₍or without hot isostatic pressing, is often a compact of a single powdered material but may also be a mixture of different powders or a mixture of a powder with other solid bodies, e.g. fibres. Furthermore, the body may consist solely of one or more unstable ceramics or may be a mixture of one or more unstable ceramics with one or more stable (e.g. oxide) ceramics. The latter may be, for example, sintering aids and are usually present in the minority. The method of the present invention is particularly suitable for sintering nitrides, e.g. silicon nitride, aluminum nitride, etc. and also composites such as A^O^SiC (particulates and whiskers) and Al_jO_j/TiC to high densities without degradation of the non-oxide phase. The invention is particularly suited for the heat treatment and sintering of such additional materials as aluminum nitride containing up to 5 wt.% of Y₂0₃ as a sintering aid, which has been sintered to densities in excess of 99% of theoretical in a powder bed comprising A1N and SiC in various proportions, e.g. 80% A1N and 20% SiC. The SiC acts as a susceptor and an oxygen getter, while the A1N acts as a good thermal conductor. The SiC may contaminate the product and if this is undesired its content is maintained as low as possible down to 5 wt.%. Alternatively, the A1N body may be surrounded with A1N powder held in a thin-walled alumina crucible itself embedded in a Sic powder bed so that there is no direct contact between the body and the SiC powder. This makes it clear that the components of the powder bed are not necessarily intimately mixed and may be confined to different zones which may be separated from each other. An overpressure is not so important when sintering A1N as it is when sintering silicon nitride, so the use of silicon nitride in the powder bed is not always necessary.

Titanium diboride has also been sintered to a density of 87% of theoretical using the process of the invention using a powder bed of alumina and silicon carbide. The SiC provides high thermal conductivity and an oxygen getter ability. Less of the thermal conductor is necessary in this case because (a) the titanium diboride has a very high melting point of around 2900*C and (b) titanium diboride becomes an electrical conductor at high temperatures, thereby limiting the amount of field it is capable of absorbing. The sintered silicon nitride-containing materials produced by the process of the invention, such as sintered alpha and beta silicon nitride, silicon nitride mixtures with other ceramics, e.g. alumina and yttria, and sialons (compounds of silicon, aluminum, oxygen and nitrogen) are, in particular, are believed to be a novel and unique forms of these materials and, as such, form part of this invention. The grain size of these, and most likely other, products of the invention is significantly finer than that of conventionally sintered silicon nitride-containing materials. The grain size of the conventional materials is typically greater than one micron and often as course as 3-4 microns. The grains forming the product of the present invention are usually elongated in shape, typically having an aspect ratio of around 10:1 and this makes the measurement of grain size somewhat difficult because sections cut for examination through a product sample intersect the grains at various inclinations between 0 and 90* to the long axis. The maximum grain size is especially difficult to measure because this requires a sufficient number of grains to be intersected in a plane parallel to their long axes, an unlikely event. Hence the grain size of the product of the invention is expressed as the average minimum grain size measured by microscopic examination. The average minimum grain size of the products of the invention is 0.5 microns or less. This is found to be true for sintered alpha or beta silicon nitride, silicon nitride mixed with other ceramics (such as alumina and yttria) , sialons, etc. We have found that finer grain materials have improved wear properties and are thus more desirable than the equivalent conventional materials.

As briefly mentioned above, the process of the present invention may be used for joining bodies of unstable ceramics as well as heat treating or sintering ceramics. This can be achieved by placing (and preferably pressing) together the bodies to be joined, if desired, with an intermediary interposed between the bodies. The bodies are then surrounded with a bed according to the invention and the joint area is irradiated with microwaves to raise the temperature of the materials in the vicinity of the joint so that partial melting or sintering takes place and, upon cooling, a satisfactory joint is formed.

A suitable procedure is explained with reference to Fig. 4 which shows equipment 40 for joining two ceramic bodies 41 and 42 via an intermediary body 43. The intermediary body 43 is preferably a compressed sinterable green body made of the same material as the bodies 41 and 42 to be joined. The resulting joint area is surrounded by a protective powder bed 44. The bodies 41, 42 and 43 are pressed together by means of a load 45. The powder bed is contained within a housing 46 which has microwave-transparent windows 47, 48 on opposite sides of the joint. Waveguides 49 and 50 are aligned with the windows 47, 48. A magnetron (not shown) is connected to waveguide 49 and a grounded movable short 51 is positioned in waveguide 50. The apparatus is cooled by water pipes 52 and a linear vertical differential transducer 53 is used to measure movements of the load 45.

The bodies are joined by operating the magnetron and adjusting the position of the short 51 to create standing waves 54 and 55 in the waveguides so that the apparatus forms a resonant cavity with the joint area positioned for maximum microwave absorption.

The protective bed 44 increases in temperature and raises the temperature of the bodies 41, 42 and 43. The transducer 53 first registers an upward movement of the load 45 as the bodies expand and then a downward movement as the body 43 compacts, densifies and sinters. When there is no further movement of the load 45, the pro¬ cedure is complete and the bodies can be allowed to cool. The use of a protective bed in accordance with the invention ensures that there is little or no decomposition of the bodies 41, 42 and 43 in the joint area in the final product.

The present invention is illustrated further by the following Examples, but should not be construed as limited thereto.

EXAMPLE 1

A silicon carbide whisker reinforced alumina body (20% by volume) was sintered to approximately 85% of the theoretical density (10% of whiskers by volume sintered to 97%) by burying the body in a packed powder bed consisting of 30% by wt silicon carbide (susceptor and oxygen getter) , 30% by wt boron nitride (good thermal conductor) and 40% by wt silicon nitride (protective material) and heating the body and bed with microwaves in air. The heating time was 40 minutes and the microwave energy was 500 watts.

The sintered sample was analyzed for oxidation products to investigate whether reaction with the external atmosphere had taken place. No such products were observed. EXAMPLE 2

A composite body consisting of titanium carbide and alumina (27% by weight - 11% TiC by volume sintered to 95%) was sintered to a density of greater than 95% theoretical using a packed powder bed similar to that of Example 1 and a similar heating time, power and conditions.

No oxidation products were observed in the product. EXAMPLE 3 Silicon nitride was sintered to a density of 97% theoretical using a powder bed and procedure the same as that of Example 1.

No oxidation products were observed in the product. EXAMPLE 4 Aluminium nitride was sintered to a density of 97% theoretical using a powder bed of

A1N 30% (good thermal conductor) Si₃N₄ 30% (protective material) SiC 40% (susceptor and oxygen getter) and the same procedure as in Example 1. EXAMPLE 5

A powder mixture of silicon nitride containing 5 wt.% of A1₂0₃ and 5 wt.% of Y₂0₃ in a powder bed having the same composition as the bed of Example 4 was sintered by exposure to microwave irradiation and subsequently immediately hot isostatically pressed. The product was sectioned and 30 frames were taken across a horizontal section and 17 frames taken across a vertical section. The data thus obtained is shown in the following tables:

HORIZONTAL DATA

VERTICAL DATA

The variance in D.max is quite high because of the high aspect ratio discussed earlier. As can be seen, the average minimum grain size is less than 0.5μm.

The average minimum grain size across the diameter of the sintered product was calculated and the results are shown in Figure 5.

Figure 6 is a photomicrograph of an etched section of the product at a magnification of 10,000 diameters. Industrial Applicability The invention can be applied to the production of heat-treated and sintered bodies of ceramic materials used, for example, as electronic substrates, components of tools, electrical insulators and the like.

Claims

CLAIMS :

1. A method of heat-treating a body of thermally unstable ceramic material, which comprises at least partially surrounding said body with a bed of particulate material and irradiating said bed with microwave energy in an oxygen-containing gas, characterized in that said bed comprising effective amounts of:

(a) a microwave susceptor if said unstable ceramic is not itself a microwave susceptor; (b) a protective material capable of generating a localized protective atmosphere which reduces decomposition and/or oxidation of the ceramic material;

(c) an oxygen getter; and

(d) a material having sufficiently good thermal conductivity to prevent excessive localized heating in said bed.

2. A method according to Claim 1 characterized in that said ceramic material is a non-oxide ceramic.

3. A method according to Claim 1 characterized in that said ceramic material is a non-susceptor.

4. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said bed contains at least one component having at least two of the properties (a) to (d).

5. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said body comprises a material selected from a nitride and an oxynitride and wherein said protective material comprises silicon nitride.

6. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said body comprises a material selected from a carbide or a boride and said protective material comprises a material selected from the group consisting of metal carbides and carbon.

7. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said body comprises a material selected from a carbide or a boride and said protective material comprises silicon carbide.

8. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said body comprises a material selected from MoS₂ and MoS₃ and said protective material is MoS₂.

9. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said body comprises a lead based ceramic and said protective material is a lead based ceramic.

10. A method according to Claim 9 characterized in that said lead based ceramic of said body and/or said protective material is selected from the group consisting of lead zirconate titanate and lead lanthanate zirconate titanate.

11. A method according to Claim l. Claim 2 or Claim 3 characterized in that said body comprises a superconductor selected from the group consisting of superconductors I, II and III, and said protective material is also a superconductor selected from the group consisting of superconductor I, II and III.

12. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said oxygen getter is selected from the group consisting of metal carbides, carbon and oxidizable metals.

13. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said oxygen getter is silicon carbide.

14. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said material having good thermal conductivity is selected from the group consisting of boron nitride, aluminum nitride and metals.

15. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said material having good thermal conductivity is boron nitride.

16. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said microwave susceptor is selected from the group consisting of metal carbides, carbon, porcelain, soda-lime glass and barium titanate.

17. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said susceptor is silicon carbide.

18. A method according to Claim 1, Claim 2 or Claim 3 characterized in that the protective material is the same as the material used for said body.

19. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said bed is prepared from components of different particle sizes by at least partially burying said body in a component of said bed having the largest particles, placing said component of said bed having the next largest particles on said bed of largest particles and shaking or vibrating the resulting bed structure until the smaller particles flow down into the interstices formed between the larger particles and, if there are additional components of said bed, continuing said procedure of placing components of the next smaller particles followed by shaking or vibrating the resulting structure until the bed is complete.

20. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said oxygen containing gas is air.

21. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said oxygen-containing gas is at about atmospheric pressure.

22. A method according to Claim 1, Claim 2 or Claim 3 characterized in that said body is subjected to hot isostatic pressing simultaneously or subsequently to said heat-treating of said body.

23. A particulate material capable of being used as a protective bed for heat-treating a body of an unstable ceramic material with microwaves in an oxygen-containing gas, characterized in that said material comprises effective amounts of:

(a) a microwave susceptor, if said ceramic material is a non-susceptor; (b) a protective material capable of generating a localized atmosphere which discourages decomposition of the unstable ceramic; (c) an oxygen getter; and

24. A particulate material according to Claim 23 characterized in that said bed contains at least one component having at least two of the properties (a) to (d).

25. A particulate material according to Claim 23 characterized in that said susceptor is selected from the group consisting of metal carbides, carbon, porcelain, soda-lime glass and barium titanate.

26. A particulate material according to Claim 23 characterized in that said protective material is selected from the group consisting of silicon nitride, metal carbides, carbon, MoS₂, lead based ceramics and superconductors I, II and III.

27. A particulate material according to Claim 23 characterized in that said oxygen getter is selected from the group consisting of metal carbides, carbon and oxidizable metals.

28. A particulate material according to Claim 23 characterized in that said material having good thermal conductivity is selected from the group consisting of boron nitride, aluminum nitride and metals.

29. A particulate material according to Claim 23 characterized by comprising silicon carbide, silicon nitride and boron nitride.

30. A particulate material according to Claim 29 characterized by comprising at least 25 wt % of said silicon carbide, at least 20 wt % of said silicon nitride and at least 10 wt % of said boron nitride.

31. A particulate material according to Claim 23 characterized by comprising a mixture of particles of different sizes, said bed having been prepared by at least partially burying said body in a component of said bed having the largest particles, placing said component of said bed having the next largest particles on said bed of largest particles and shaking or vibrating the resulting bed structure until the smaller particles flow down into the interstices formed between the larger particles and, if there are additional components of said bed, continuing said procedure of placing components of the next smaller particles followed by shaking or vibrating the resulting structure until the bed is complete.

32. A process for joining bodies made of thermally unstable ceramics, which comprises: bringing said bodies (41,42) into contact; surrounding said bodies in the area of contact with a bed (44) of particulate material; irradiating said protective bed in an oxygen- containing gas with microwave radiation sufficiently to heat said bed and bodies to cause joining of the latter; and allowing said bodies to cool; characterized in that said bed (44) of particulate material comprises effective amounts of

(a) a microwave susceptor, if said ceramic material is a non-susceptor; (b) a protective material capable of generating a localized atmosphere which discourages decomposition of the unstable ceramic;

(c) an oxygen getter; and

33. A sintered product characterized in that said product is produced by a process which comprises at least partially surrounding a body of thermally unstable ceramic material with a bed of particulate material and irradiating said bed with microwave energy in em oxygen- containing gas, said bed comprising effective amounts of:

(a) a microwave susceptor if said unstable ceramic is not itself a microwave susceptor;

(b) a protective material capable of generating a localized protective atmosphere which reduces decomposition and/or oxidation of the ceramic material;

(c) an oxygen getter; and

34. A product according to claim 33 characterized by comprising silicon nitride.

35. A product according to claim 33 characterized by comprising aluminum nitride.

36. A product according to claim 33 characterized by comprising titanium diboride.

37. A sintered ceramic material selected from alpha silicon nitride, beta silicon nitride, mixtures of silicon nitride and other ceramic materials, and sialons, characterized in that said sintered material has an average minimum grain size of o.5 microns or less.

38. A material according to claim 37 characterized by comprising a mixture of silicon nitride and up to 5 wt.% of alumina and up to 5 wt.% of yttria.

39. A process of hot isostatically pressing a thermally unstable ceramic material, which comprises at least partially surrounding a body of said material with a bed of particulate material, irradiating said bed with microwave energy in an oxygen-containing gas to raise the temperature of said body to a treatment temperature, and simultaneously or subsequently subjecting said body to a fluid under pressure while said material is at said treatment temperature, characterized in that said bed comprises effective amounts of: (a) a microwave susceptor if said unstable ceramic is not itself a microwave susceptor;

(c) an oxygen getter; and