IE61432B1 - Powders for ceramics of metal carbides and nitrides made by carbothermal reduction and process for their manufacture - Google Patents

Abstract

Nitride and carbide powders by a carbothermal reduction route and, if appropriate, nitration of metal oxides capable of being employed especially for the manufacture of ceramics and having a specific surface of between 30 and 250 m<2>/g. Process for the manufacture of powders comprising the following stages: a) from metal oxide or oxides with a particle size of less than 10 mu m, a binder generating carbon, not later than during the carbothermal reduction reaction and, if appropriate, additional carbon, manufacture of granules with a controlled pore volume, b) carbothermal reduction reaction of the granules obtained under a, if appropriate in the presence of an atmosphere containing nitrogen, c) decarbonisation, d) obtaining powders. s

Description

The invention relates to metal nitride and carbide powders usable especially for the manufacture of ceramics, the said powders being obtained by the route known as carbothermal reduction of metal oxides. The invention also relates to a process for the manufacture of such powders.

Various processes for the manufacture of carbides and nitrides for ceramics are known. Independently of th® laboratory processes such as the process making use of laser" or plasma-induced reactions, the literature describes essentially three access! routes, especially to silicon nitrides and carbides,, namely the gas phase reaction starting with halosilane and methane or ammonia, direct carbiding or nitriding starting with silicon and carbon or nitrogen, and carbothermal reduction in the presence of a neutral or nitrogenous atmosphere, starting with silica and carbon (F-K. Van Dijen, R- Metselaar & C.A.M. Siskens. Spechs&al Vol. 117 No- 7, 1984 pages 6279)» Is contrast to the laser or plasma techniques referred to above, which can result in carbides or « <9 nitrides of high specific surface, that is to say capable of reaching or exceeding 100 or 150 a’/g (see, for example, Y. Kizaki, T. Kandori and Y- Fujitani in the Japanese Journal of Applied Physics vol. 24, No. 7, July 1985, pp, 800-805 or Y. Suyama, Robert M. Marra, John S. Haggerty and Ξ- Kent Bowen in An. Ceram. Soc- Bull. 64 (10) 1356-59 1985), the carbothermal reduction route results ia powders of low specific surface, that is to say lower than 20 mF/g. (See, for example, F.K. Van Dijen et al-, op. cit. page 628, who introduce the value of ΙΟΙ 5 aF/g as the maximum value which can be expected from an optimized reaction,? David L- Segal in Chemistry & Industry, 19 August 1985, pp. 544-545, who gives the value of 5 aF/g? Shi Chang Zhang and W. Roger Cannon in the Journal of American Ceramic Society, Vol. 67 No- 10, pp. 691-5, who indicate 10-3 sF/g).

Three processes for the preparation of ultrafine silicon carbide powders starting from carbon, a carbon™ aceous binder and silica are to fee found described ia - 2 OS Patent 4,529#575. According to this reference carbon of high specific surface is employed necessarily mixed with an organic liquid (hydrocarbon# ester# ether# ketone - first process - or with water containing a surfaceactive agent - second process -, or else carbon particles heated in an oxidizing atmosphere - third process. In each of these processes a silica of particle size of *· between 20 and 170 μχη is employed (col. 4# lines 51-52) and it is possible to obtain silicon carbide whose specific surface can range from 14-3 to 38»7 m2/g (Table 2# col. 13-14)..

The invention proposes a new class of carbide and nitride powders by the carbothenaal reduction route and# if appropriate, nitriding route# these powders having a high specific surface.

Th® invention also relates to a process for the manufacture of such powders and the specific means used is this process.

Another subject of the invention is the ceramics obtained from the powders in - accordance with the invention. . , lit These new powders consist of metal carbides with the exception of silicon carbide and nitrides and are characterized ia that thev have a snecific surface of e» w. between 30 and 250 m’/g.

The invention relates most especially t© such powders which are in the form of agglomerates of average dimensions smaller than 5 μη and consisting of elementary particles whose average dimensions are between and 50 nsa* The invention relates specifically to the powders in which at least 30 % have a crystalline structure.

Within the meaning of the invention the specific surface is measured by application ©f the B.E.T. method -« according to Brunauer# Ssraaett £ Teller, J.A.C.S.# 309# 1938* Similarly# the crystallized structure is assessed by X-ray diffraction according to the method described by C.F. Gazzara & D.R. Messier in Bull. Asa. Ceram. Soc. 5S# 777-80, 1977.

The invention relates very particularly to the powdered carbides and nitrides of metals chosen from the group consisting of silicon, aluminium, titanium, 5 zirconium, hafnium and boron with the exception of silicon carbide.. Xt relates more specifically to silicon nitride and aluminium nitride powders.

Another subject of the invention is a process for the preparation of the abovementioned powders and more 10 precisely a process for the manufacture of powder® with a high and controlled specific surface, the said process comprising the following stages: a) Manufacture of granulates with a controlled pore volume froa the metal oxide or oxides of particle 15 size smaller than 10 jsim, from a binder generating carbon, in the conditions of the carbothermal reduction reaction and, if appropriate, carbon. bl Carbothermal reduction reaction of fche granulates obtained under a, if appropriate in the presence of 20 an atmosphere containing nitrogen. cj Decarbonization. * · θ t3 d| Obtaining powders.

Stage a consists in forming granulates with a controlled pore volume. One or more metal oxides and one ©r more compounds or elements contributing carbon and acting as a binder are employed in. this operationThe metal oxide may be chosen especially from this group «consisting of SiOa, AlaOs, TiOa, ZrOa, a£Qa and BaOs.

Is general, the particle size of the metal oxide 30 is smaller than 10 pm and more precisely of the order of 0.1 to 5 μβ.

Xa the process ia accordance with the invention it is possible to use «either, on the one hand, carbon and, on the other hand, a binder, or a binder capable of 35 providing the contribution of carbon necessary for the reaction.

When carbon and a binder are employed at the same time, the carbon may be chosen from the different varieties of carbon, and especially vegetable black, thermal black, acetylene black,, coke, channel black and graphite® In general, the particle size of the carbon is smaller than 10 μη and more precisely of the order of 0.1 to 5 μη.

The binder, employed by itself or in combination » with carbon, may b© chosen from a very large class of natural or synthetic substances which are converted into k carbon not later than during the carbothermal reduction reaction and are capable of promoting the agglomeration of the oxides and, where appropriate, of the carbon with a view to forming granulates. Among th® products which correspond to this definition there will be mentioned coal tars, resins or polymers, especially heat-curable ones such as phenolic resins, for example phenol» formaldehyde, epoxy resins, polyimides, polyureas and polycarbonates.

In general, the binder within the meaning given above, by itself or used in combination with carbon, is employed in a quantity such that the carbon/oxide molar ratio is higher than 1 and preferably between 2 and 60/1. fthen carbon and a binder are employed at the same time «9 the quantity of binder represents at least 2 % of the weight of the mixture of oxide + carbon.

The oxide or the oxides, the binder and optionally the carbon are subjected to a mixing and blending operation. The operation may be performed over a wide range of temperature which can rum, for example, from ambient temperature to 200®C, the choice of th® temperature being, inter alia, determined by the use of the binder.

At the end of this operation the paste formed is advantageously shaped, ©specially by extrusion, the particles obtained, for example after cutting or grinding of the reed formed by the extruder, being advantageously ·* heated to a temperature which makes it possible to obtain the drying and/or the curing and/or the polymerization of the binder, a temperature which may be situated between and 25©*C, for example, it being possible for the said particles to be next agglomerated into the form of granulates, in the process in accordance with the invention the pore volume of the granulates which are subsequently subjected to the carbothenaal reduction can be adjusted precisely during the manufacture of the granulates by modifying the quantity of the binder used and/or by modifying the temperature and/or pressure conditions (for example between 2 .and 200 bars).

According to an alternative form the binder may be coked, for example between 350 and 500eC before the carbothenaal reduction reaction.

These granulates with a controlled pore volume, and more precisely such granulates comprising an agglomerate mixture of metal oxide (s), of carbon-generating binder as defined above and, if appropriate, the complementary carbon or the coked binder, generally have a pore volume whose value may be chosen between 0.1 and 3 cm’/g (measurement performed with a mercury porosxmeter in the region 0 to 2000 bars).

These granulates may be ia the form of tablets, cylinders or, more generally, of particles or regular or irregular shape» In general, th® largest dimension of these granulates is greater than 0.5 mm aad preferably between 1 and 3© am, these values being given merely by way of indication. Such granulates thus defined form another subject of the said invention, by way cf specific means of process for the manufacture of the ultrafine powders in accordance with the invention.

In stage fa of the process, the oxide(s), the binder and, if appropriate, the carbon, which are mixed or agglomerated in the form of granulates according to a above, are subjected to a carbothenaal reduction reaction. In general, this operation may be performed at a temperature which may be between 1300' and 1600*C. This reaction takes place in the presence of an atmosphere containing nitrogen or releasing nitrogen in the conditions of the reaction, when the intention is to manufacture nitrides or in an atmosphere which is neutral in the conditions of the reaction in the case of the carbides. Oa this second assumption it will be possible - S to employ an atmosphere of hydrogen or of a rare gas such as argon. In the particular case of an atmosnhere taking part in the reaction (nitrogen) an excess of nitrogen which can run from 2 to 10 times the stoichiometry of the reaction is advantageously employed, these values having to be considered as forming an order of magnitude. Since the carbothermal reduction reaction releases oxygen (especially in the form of CO), the continuation of the reaction until all the metal oxide has beers converted may be followed by monitoring the CO emission.

Stage a Is a decarbonizafcion operation, with the aim of removing the excess of carbon contributed by the binder and, optionally, directly in the form of carbon. This operation say be advantageously performed at a temperature of between 500 and 8©0*C. It will preferably be continued until all the carbon has been consumed, it being possible for the said consumption to be followed by monitoring the emission of combustion gases, that Is to say CO and CO-,.

At the end of the decarbonizafcion operation the «0 carbides or nitrides ia accordance with the invention are collected (stage d), that is to say in the form of particles or of agglomerates of particles which have as exceptional specific surface. If appropriate, It is possible to proceed to a deagglomeration operation or, more precisely, making the average dimension of the agglomerates uniform, for example by grinding and screening.

As has been stated, the carbide and nitride powders In accordance with, the invention constitute a new class of carbide and nitride powders by the carbothermal reduction route and, optionally, nitriding, insofar as such powders exhibit a reproducibly available specific surface incommensurate with the specific surfaces of the powders using carbothermal reduction and described hitherto. These new powders for®, materials of choice for the manufacture of ceramics, their properties permitting especially an easy sintering and the production of highdensity sintered articles. Ί The following examples, given purely by way of guidance, illustrate the invention.

EXAMPLE 1 115 g of a silica powder whose characteristics 5 are as follows: - Specific area: 180 a»2 g1 - Median diameter: 2 μ® - Loss on ignition: 11.46 % - Si content: 40.53 % are mixed in a blender with 300 g of ex-acetylene carbon black whose characteristics -are: - Specific area: 64 sa g1 - Median diameter: < 3 μΜ - Purity: > 99 % The blender is provided with a heating jacket is which a heat transfer agent heated to about 80 °C is circulated. 660 g of pine pitch are added progressively to the powder mixture which is kept in motion, to obtain an extrudable mass» shea the paste thus formed exhibits . ί> a homogeneous appearance and a stable rheology, it is extruded in the form of reeds 5 sm in diameter in an apparatus which allows a pressure of approximately 80 bars to be applied.

These extrudates are first of all dried progres25 sively in air between the ambient temperature and 200*C. Then, when they have acquired a solid consistency, they are coked by heat treatment at 400°C in a stream of nitrogen. At the end of these operations of dryingcoking, the loss in mass is 46.6 %. 398 g of these hard extrudates are next subjected to a carbothermal reduction in a nitrogen atmosphere. To d© this, the extrudates, whose pore volume is 0.73 ©a3 g, are placed in a cylindrical reactor through which a stress of nitrogen passes at a flow rate of 0.300 m’ h*1. A temperature increase programme is applied, which allows the charge to be heated to the· temperature of 1400°C in Ih 30 min. This temperature is maintained for 5 hours and then the heating is switched off and the product is allowed to cool under nitrogen blanketing. It is then found that the product is still in the form of black granulates which offer a good mechanical strength. 314 g thereof are collected.

These extrudates are heated to a temperature ,·, which is progressively raised to 700°C under a stream of air and are maintained at this temperature for 15 hours. ς After cooling, 48 g of a slightly beige powder are collected, the X-ray diffraction analysis of which shows that it does not contain any detectable quantity of crystallized silica (quartz or cristobalite) any more than it contains ox residual carbon.

Oa the other hand, the presence of the 2 crystalline varieties of silicon nitride is detected, as well as an amorphous phase. An estimate of the respective contents of these various constituents was performed according to the method described by C.?. CS&zzara and D.R. Messier, Bull. Am. Ceram. Soc. 56, 777-80, 1977.

The following are founds . Amorphous Si^ - 20 % s SijH« - 45 % P Si3N^ - 35 % The specific surface of this powder, determined by the 3ΞΤ method is 88.8 a? g*. ssysaa 150 g of silica are mixed with 400 g of exacetylene carbon black (products from Example 1) in a two-bladed blender similar to that employed in Bxample 1.

An aqueous solution of phenolic resin (Fen-O-Fen trademark) is then added progressively; 126 g of pure resin and 24 g of polymerization catalyst are thus incorporated into the mixture before it is given the adequate consistency. The bottom of the blending vessel comprises aa extrusion screw with which the paste is spun ia the form of extrudates 6 raw in diameter.

The latter are then dried at 150eC in a vacuum oven. During this treatment the resin polymerizes and the extrudates harden. Their pore volume, determined by mercury porosyneaetry, is 1,46 cm' g_1.

They are then reimpregnated with the same phenolic resin in the form of a ©ethanolic solution containing 20 % by weight of polymer. 30.9 % thereof, of the weight of the granules measured after oven drying is incorporated in this way. The pore volume of the granules, dried at 120*C, is thus returned to 0.97 cm3 g* *. An aliquot of 10 g of these extrudates is placed in a vertical reactor through which flows a stream of nitrogen «hose flow rate is approximately 20 litres per hour.

The granules are then heated to 1400°C over 2 hours and 10 min. and then maintained at this temperature for 5 hours. After cooling, 7.97 g of product, still in the for® of granules, are collected.

The excess carbon is next removed by burning in air. The operation is carried out as in Example 1. The final residue weighs 1.22 g.

Its analysis, which is carried out using X-ray diffraction, shows that all the silica has been attacked, that the carbon has been completely removed and that bo oxynitride Si,MaO has formed.

A The following are determined by applying the method described above: - 55 % of a Si3K4. - 15 % of β SijS4. - 30 % of amorphous Si3M4The silicon nitride thus prepared exhibits a specific; surface of 59 m3 g"x.

EXAMPLE 3 Th® operation is carried out as in Example 2. However, the content of phenolic resin introduced by successive impregnations with the methanolic solution is raised to 104 % of the mass of the initial granules. The pore volume of the granules is 0.48 cm3 g’1. A 10 g aliquot is subjected fo the carbothenaal reduction in the same conditions as in Example 2. The reaction product weighs 5.46 g. It contains a large excess of carbon, which is removed by burning using the same procedure as previously. Th© final residue weighs 0.74 g.

Phase analysis makes it possible to ascertain that the silica ha® been completely attacked and that the carbon has completely disappeared. The only detectable crystalline phases are the & and β varieties of Si,ft4; '< they are accompanied by a predominant amorphous partApproximately 40 % of a Si3fl4 and 10 % of β Si,SI4 are in * fact determined. The specific area of this silicon nitride is 92 ras cf1» EXAMPLE 4 The metal oxide is gamma alumina which exhibits the following characteristics: - Specific area « 100 g"l - Median diameter: < 4 - Loss on ignitions 6.9 % - Purity: > 99.95 % g of this product are mixed with 300 g of exacetylene carbon black from Bxample 1. The homogenization is carried out in a heating blender. When the mixture looks homogeneous, 660 g of pine pitch are incorporated progressively to convert the charge into an extrudable paste. The extrusion takes place at 80®C and the product is again found in fche form of laces 5 mm in diameter.

The latter are next dried at 150*C for 2 hours.

The loss ia mass during this operation amounts to 22 %.

The consolidated extrudates are taken up to be coked under nitrogen blanketing. To do this, they are heated to 450®C and maintained at this temperature for 25 min., which results in an additional loss of 27 % of the dried mass. The pore volume is 0.23 c®3 g". 580 g of these dried and coked extrudates are next subjected to the carbonitriding operation in the conditions of Example 1» The operation take© place at the saaws temperature of 140OeC at which the extrudates are maintained for 4 hours, nitrogen being passed through the oven at a flow rate of 0.300 »s h% After cooling, 480.2 g of black granules are collected, which contain especially the excess carbon involved in the reaction.

These granules are ground in an impeller-disc mill. The powder collected is placed in a rotary cylindrical reactor.

After the vessel has been purged with nitrogen# the powder is heated to 500®C and air is introduced into the oven# in which the oxygen content of the effluent gases is monitored. The air flow rate is adjusted so that the oxygen content at the exit does not exceed 2 % by volume. Towards the end of the operation the temperature is raised to 650*C in order to complete the combustion of the residual carbon. 67.2 g of beige-grey coloured powder are finally collected.

Phase analysis performed by X-ray diffraction shows the complete absence of alumina and carbon. The only crystalline phase detected is AIN (52 %) accompanied by an amorphous, phase.

The specific surface of this aluminium nitride is 34 s2 ga. gWigLBjS / Mixing of 85 g of gamma alumina identical with that described in Example 4# with 300 g-of exacetylene carbon black from Example 1 and 1.5 1 of aa aqueous solution containing 125 g of Fen-O-Fen phenolic resin and g of polymerization catalyst is carried out in a blender.

This charge is extruded in the fora of laces 6 mm in diameter.

The latter ax’® first of all dried at 130®C ia a 30 vacuum oven.

After this consolidation the extrudates are impregnated with a methanolic resin solution identical with that used in Example 2. Sy repeating this operation, 64 % of resin are successfully incorporated in addition fco the total mass of the extrudates.

The granules# whose pore volume is 1.0 cm3 g~S are then subjected to the carbothenaal reduction in the conditions of Example 4. This results is a mass loss of X2 Similarly, they are next decarbonized by the procedure used in Sxample 4. The excess carbon removed represents 37.45 % of the total mass before treatment. A powder is finally collected in which alumina and carbon are absent. This final product consists of aluminium nitride whose specific area is 76 m3 g*1, the crystalline phase representing 35 %.

Claims (6)

1. - Nitride and carbide powders with th© exception of silicon carbide by a carbothermal reduction route and, If appropriate,» nitriding of metal oxides, usable especially 5 for the manufacture of ceramics, characterized in that they have a specific surface of between 30 and 250 m*/g.

2. Powders according to Claim 1, characterized in that they are in the form of agglomerates of average dimensions smaller than 5 pm and consisting of elementary 10 particles whose average dimensions are between 10 and 50 nm

3. Powders according to either of Claims 1 and 2, characterized in that at least 30 % of the said powders have a crystalline structure. 15

4. Powders according to one of Claims 1 to 3, characterised in that they are chosen frosa powdered carbide® and nitrides of metals chosen from the group consisting of silicon, aluminium, titanium, zirconium, hafnium and boron. 20 5- Powders according to one of Claims 1 to 4,· • characterized in Ihat they are chosen from the group consisting of silicon nitrides and aluminium nitrides. S- Process for the manufacture of the powders according to one of Claims 1 to 5, characterized in that 25 it comprises the following stages: a| Manufacture of granulates with a controlled pore volume from the metal oxide or oxides of particle size smaller than 10 pm, from a binder generating carbon, not later than during the carbothermal reduction reaction 30 and, if appropriate, additional carbon. fe) Carbothermal reduction reaction of the granulates obtained under a, if appropriate in the presence of aa atmosphere containing nitrogen. e) Decarbonization35 d) Obtaining powders. 7. Process according to Claim 6, characterized in that the metal oxide or oxides is (are) chosen from the group consisting of Si0 a , Al a 0 3 , TiO a , Zr0 a , HfO a and EUO^» 8. Process according to Claim 6, characterised la that carbon and a binder are used at the same time. 9» Process according to Claim 8, characterized in that the carbon is chosen from the group consisting of vegetable black, thermal black, acetylene black, coke, 5 channel black and graphite. 10» Process according to Claim 6, characterized in that only a binder generating carbon is used. 11» Process according to any one of Claims 6 to 10, characterized in that the binder is chosen from natural 10 or synthetic substances which are converted into carbon under the conditions ox the carbothermal reduction reaction and are capable of promoting the agglomeration of the oxide®. 12. Process according to Claim 11, characterized in 15 that the binder is chosen from the group consisting of coal tars, resins or polymers, ©specially heat-curable ones such as phenolic resins, for example phenol-formaldehyde, epoxy resins, polyimides, polyureas and polycarbonates 20 13. Process according to asy one of Claims 6 to 12, characterized in. that the binder, to which carbon is added if appropriate, is employed in a quantity sucb that the carbon/oxide molar ratio is higher than 1 and preferably between 2 and 60/1. 25 14. Process according to either of Claims 8 and 9, characterized in that the quantity of binder represents at least 2 % of the weight of the mixture of oxide + carbon. 15. As specific means for implementing the process 30 according to Claim 6, granulates consisting of an agglomerate mixture of metal oxide(s}, of binder generating carbon and, if appropriate, of carbon, the said granulates having a pore volume whose value is chosen between 0.1 and 3 cm 3 /g. 35 16. Granulates according to Claim 15, characterized ia that they are in the form of tablets, cylinders ox particles of regular or irregular shape whose largest dxiaeusxoK! is greater than 0.5 sm. 17. Granulates according tc either of Claims 15 and 16, characterized in that their largest dimension is between 1 and 30 wa. 18. Application of the powders according to any one of Claims 1 to 5 to the manufacture of ceramics.

5. 19. a nitride or carbide powder according to claim 1, substantially as hereinbefore described and exemplified. 20. A process according to claim 6 for the manufacture of a powder, substantially as hereinbefore described and exemplified.

6. 10 21. a powder whenever manufactured by a process claimed in any ©ne of claims 6-14 or claim 20» 22,.. Granulates according to claim 15, substantially as hereinbefore described and exemplified.