IE870755L - Silicon nitride powders for ceramics - Google Patents

Abstract

Silicon nitride Si3N4 powders by a silica carbothermal reduction and nitriding route, capable of being employed especially for the manufacture of ceramics and exhibiting a specific surface of at least 30 m<2>/g. Process for the manufacture of the powders comprising the following stages: a) From silica, from a binder generating carbon not later than during the carbothermal reduction reaction and, if appropriate, from additional carbon, manufacture of granules with a controlled pore volume. b) Carbothermal reduction reaction of the granules obtained under a, in the presence of an atmosphere containing nitrogen. c) Decarburisation. d) Obtaining powders. h

Description

The invention relates to silicon nitride (Si3N«) powders which can be used especially for the manufacture of ceramics, the said powders being obtained by means of the so-called carbothennic reduction of silica. The invention also relates to a process for the manufacture of such powders.

Various processes for the manufacture of silicon nitrides for ceramics are known. Independently from laboratory processes such as the process which makes use of reactions induced by laser or by plasma, the literature describes essentially three approaches, i.e., the reaction in the gaseous phase, using halosilane and ammonia, the direct nitridation, using silicon and nitrogen and the carbothermic reduction in the presence of a nitrogenous atmosphere, using silica and carbon (F.K. Van Dijen, R. Metselaar & C.A.M. Siskens. SPECHSAAL Vol. 117 No. 7, 1984 pages 627-9). Unlike the techniques using laser or plasma mentioned above, which may lead to nitrides of a high specific surface area, that is to say, reaching or exceeding 100 or 150 m2/g (see for example, Y. Kizaki, T. Kandori and Y. Fujitani in Japanese Journal of Applied Physics Vol. 24, No. 7, July 1985, pp 800-805), the carbothermic reduction method leads to powders with low specific surface area, that is to say, less than 20 m2/g. (See for example David L. Segal in Chemistry & Industry, 19 August 1985, pp 544-545 who gives a value cf 5 m2/g; Shi Chang Zhang and W. Roger Cannon in Journal of American Ceramic Society, Vol. 67, No. 10, pp 691-5 who mention 10.3 m2/g).

In British Patent 1,028,977, a process is proposed for the manufacture of Si3N4, consisting in heating silicon to a temperature of between 1000 and 1900°C in an atmosphere containing nitrogen in the presence of more than 5% by weight of a metal chosen from alkali metals, alkaline-earth metals and those having an atomic number ranging from 21 to 30, from 39 to 48 and from 57 to 79. This process can be accomplished in the presence of a reducing agent such as carbon.

The invention proposes a new family of silicon nitride powders obtained by carbothermic reduction and nitridation, these powders having a high specific surface area.

The invention also relates to a process for the manufacture of such powders and the specific means employed in this process.

The invention also relates to ceramics obtained from the powders according to the invention. 10 These new powders consist of silicon nitride and are characterised in that they have a specific surface area of at least 30 m2/g.

The invention relates especially to such powders, which are in the form of agglomerates of mean dimensions 15 of less than 5 and consist of elementary particles the mean dimensions of which are between 10 and 50 nm.

The invention specifically relates to powders of which at least 50% have a crystalline structure. The invention also relates to powders with a specific surface 20 area of between 40 and 250 m2/g.

The invention also relates to Si3N4 powders, 2 to 40% of which are made up of a £ phase.

As meant in the invention, the specific surface area is measured using the B.E.T. method according to 25 Brunauer, Emmett and Teller, J.A.C.S. 60, 309, 1938.

Also, the crystalline structure is determined by X-ray diffraction according to the method described by C.P. Gazzara and D. R. Messier - Bull. Am. Ceram. Soc. 56, 777-80, 1977.

The determination of the p (and a) phases is carried out by X-ray diffractometry (C.P. Gazzara op. Cit.).

The invention also relates to a process for the preparation of the above-mentioned powders, and more 35 precisely, to a process for the manufacture of powders with a high specific surface area which is controlled, the said process consisting in the following stages: a) Preparation of granules of controlled pore volume, using silica, a binder which produces carbon and, where appropriate, additional carbon. b) Carbothermic reduction reaction of the granules obtained in a, in the presence of an atmosphere containing nitrogen. c) Decarbonisation. d) Obtaining the powders.

Stage a consists in preparing granules of controlled pore volume. In this operation, silica (Si02) and one or more compounds or elements contributing carbon and 10 having the role of a binder are employed.

In general, the particle size of the silica is less than 10 ^m and more precisely, of the order of 0.1 to 5 pm.

In the process according to the invention, either 15 carbon on the one hand and a binder on the other, or a binder capable of ensuring the adhesion of the carbon required for the reaction may be employed.

When both carbon and a binder are used, the carbon may be chosen from the different varieties of 20 carbon, and especially from vegetable carbon, thermal black, acetylene black, coke, lampblack and graphite. In general, the particle size of the carbon is less than 10 fj.m and, more precisely, of the order of 0.1 to 5 The binder, used alone or in combination with the 25 carbon, may be chosen from a very large family of natural or synthetic substances which are transformed into carbon not later than during the carbothermic reduction reaction and are capable of promoting the agglomeration of the silica and, where appropriate, of the carbon in order to 30 produce the granules. Among the products corresponding to this definition, coal tars, resins or polymers, especially thermosetting, such as phenolic resins, for example phenolformol, epoxy resins, polyimides, polyureas and polycarbonates will be mentioned. 35 In general, the carbon and/or the binder in the sense given above, are used in a quantity such that the molar ratio carbon/Si02 is greater than 1 and, preferably between 2 and 60/1.

When both carbon and a binder are used, the quantity of binder represents at least 2% of the weight of the silica + carbon mixture.

The silica, the carbon and/or the binder undergo a mixing and malaxing operation. The operation may be 5 carried out over a wide temperature range, which may range, for example, from room temperature to 200°C, the choice of the temperature being dependent, among other things, on the use of the binder.

Following this operation, the paste formed is 10 advantageously shaped, especially by extrusion, the particles obtained, for example after cutting or grinding the rod formed by the extruder, being advantageously brought to a temperature which enables the drying and/or the hardening and/or the polymerisation of the binder to 15 be obtained, it being possible for the temperature to be located between 50 and 250°C for example, the said particles then being capable of being agglomerated in the form of granules. In the process according to the invention, the pore volume of the granules which are subse-20 quently subjected to carbothermic reduction can be precisely controlled during the manufacture of the granules by adjusting the quantity of the binder employed and/or by adjusting the conditions of temperature and/or pressure (for example, between 2 and 200 bars). 25 According to a variation, the binder may be coked, for example, between 350 and 500°C before the carbothermic reduction reaction.

These controlled pore volume granules, and more precisely such granules which contain an agglomerated 30 mixture of silica, of binder which produces carbon as defined above and, where appropriate, the additional carbon or of coked binder, have a pore volume the value of which is chosen between 0.1 and 3 cm3/g (determination carried out with a mercury porometer in the range 0 to 35 2,000 bars).

These granules may be in the form of pastilles, cylinders or, more generally, particles of regular or irregular shape. In general, the largest dimension of these granules is greater, than 0.5 mm and, preferably between 1 and 30 mm, these values being given only by way of indication. Such granules defined in this way form another subject of the said invention, by way of specific means of the process for the manufacture of ultrafine 5 powders according to the invention. ' In stage b of the process, the silica, the binder and, where appropriate the carbon, which are mixed and agglomerated in the form of granules according to a above, are subjected to a carbothermic reduction reac-10 tion. In general, this operation may be carried out at a temperature which may be between 1,300 and 1,500°C. This reaction is carried out in the presence of an atmosphere which contains nitrogen or which releases nitrogen under the conditions of the reaction. An excess of nitrogen, 15 which may range from 2 to 10 times the quantity stoichio- metrically required for the reaction is advantageously used; these values should be regarded as forming an order of magnitude.

As the carbothermic reduction reaction releases 20 oxygen (especially in the form of CO), the progress of the reaction until the transformation of the entire silica may be followed by monitoring the CO emission.

Stage c is a decarbonisation operation aimed at removing the excess of carbon supplied by the binder and, 25 where appropriate, directly in the form of carbon. This operation may advantageously be carried out at a temperature of between 500 and 800°C. It will preferably be continued until the consumption of the entire carbon; the said consumption may be followed by monitoring the 30 emission of the combustion gases, that is to say, CO and C02.

At the end of the decarbonisation operation, the nitrides according to the invention, that is to say in r the form of particles or agglomerates of particles having 35 an exceptionally high specific surface area, are col- lected (stage d). Where appropriate, this may be followed by a disagglomeration operation or, more precisely, a standardisation of the mean dimensions of the agglomerates, for example by grinding and sieving.

As mentioned, the silicon nitride powders according to the invention constitute a new family of Si3N4 powders obtained by carbothermic reduction and nitrida-tion, insofar as such powders have a specific surface 5 area, which can be controlled in a reproducible manner, and which has no comparison with the specific surface areas of Si3N4 powders obtained by carbothermic reduction described to date. These new powders constitute choice materials for the manufacture of ceramics, their proper-10 ties especially enabling an easy fritting and the obten- tion of high density fritted objects.

The following examples which are given purely by way of indication, illustrate the invention.

EXAMPLE 1 115 g of a silica powder the characteristics of which are as follows: specific area : 180 m2.g_1 median diameter : 2 ^m loss on ignition : 11.46% 20 Si content : 40.53% are mixed with 300 g of carbon black ex. acetylene having the following characteristics: specific area : 64 n^.g"1 median diameter : < 3 pm 25 purity : > 99% in a malaxator.

The malaxator is provided with a heating jacket in which a heat exchanger heated to approximately 80°C is circulated. In order to obtain an extrudable mass, 660 g 30 of pine tar are gradually added to the mixture of powders which are kept stirred. When the paste formed in this way exhibits a homogeneous appearance and stable flow properties, it is extruded in the form of 5 mm diameter rods in an apparatus which makes it possible to apply a pressure 35 of approximately 80 bars.

First of all, these extrudates are gradually dried in air between room temperature and 200°C. Then, when they have reached a solid consistency, they are coked by heat treatment at 400°C in a stream of nitrogen.

At the end of these drying-coking operations, the mass loss is 46.6%. 398 g of these hard extrudates are then subjected to a carbothennic reduction in a nitrogen atmosphere. For 5 doing this, the extrudates having a pore volume of 0.73 cm'.g"1, are placed in a cylindrical reactor in which a stream of nitrogen is circulated at a rate of 0.300 m3.h_1. A temperature rise schedule which makes it possible to raise the temperature of the load to 1,400°C 10 in 1 h 30 min is applied. This temperature is maintained for 5 hours, then the heating is discontinued and the product is allowed to cool in an atmosphere swept with nitrogen. It is then ascertained that the product is still in the form of black granules having good mechani-15 cal properties. 314 g are collected.

These extrudates are gradually heated to a temperature of 700°C under a stream of air and maintained at this temperature for 15 hours. After cooling, 48 g of a lightly beige coloured powder, the X-ray diffraction 20 analysis of which reveals that it does not contain a detectable quantity of crystalline silica (quartz or crystoballite) or of residual carbon, are collected.

In contrast, the presence of 2 crystalline varieties of silicon nitride and an amorphous phase is 25 detected. A determination of the respective amounts of these different constituents has been carried out according to the method described by C.P. Gazzara and D.R. Messier, Bull. Am. Ceram, Soc. 56., 777-80, 1977. The results were as follows: 30 amorphous Si3N4 = 20% q Si3N4 - 45% p Si3N4 - 35% The specific surface area of this powder, determined by the BET method, is 88.8 ma.g'1. 35 EXAMPLE 2 150 g of silica are mixed with 400 g of carbon black ex. acetylene (products of Example 1) in a two-armed malaxator, similar to that used in Example 1. An aqueous solution of phenolic resin (registered trademark Fen-O-Fen) is then gradually added: 126 g of pure resin and 24 g of polymerisation catalyst are incorporated into the mixture in this way in order to give it a satisfac-, tory consistency. The bottom of the malaxating vessel contains an extruding screw with which the paste is drawn } in the form of 6 mm diameter extrudates.

These are then dried at 150°C in a vacuum oven. During this treatment, the resin polymerises and the extrudates harden. Their pore volume determined by 10 mercury porometry is 1.46 cm'.g"1.

They are then reimpregnated with the same phenolic resin in the form of a methanol solution (20% by weight of polymer). The quantity incorporated thereby amounts to 30.9% of the weight of the granules measured 15 after drying in the oven. The pore volume of the granules dried at 120°C is thus brought to 0.97 cm3.g_1. A 10 g aliquot of these extrudates is placed in a vertical reactor in which a stream of nitrogen is circulated at a flow rate of approximately 20 litres per hour. 20 The granules are then heated to 1,400°C over 2 hours and 10 min, then maintained at this temperature for 5 hours. After cooling, 7.97 g of the product, still in the form of granules, are collected.

The excess carbon is then removed by combustion 25 in air. The operation is carried out as in Example 1. The final residue weighs 1.22 g.

The X-ray diffraction analysis thereof reveals that the entire silica has been acted on, that the carbon has been entirely removed and that oxynitride Si2N20 has 30 not been formed.

Applying the method described previously, the analysis gives: - 55% of a Si3N4. - 15% of /3 Si3N4. - 30% of amorphous Si3N4.

The silicon nitride prepared in this way has a specific surface area of 59 m'.g"1.

EXAMPLE 3 The procedure is as in Example 2.

But, the content of phenolic resin introduced by successive impregnations with the methanol solution is brought 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 to the carbothermic reduction under the same conditions as in Example 2. The reaction product weighs 6.46 g. It contains a significant excess of carbon which is removed by combustion according to the same procedure as before. The final residue weighs 0.74 g.

Phase analysis makes it possible to ascertain that the silica has been entirely acted on and that the carbon has completely disappeared. The only crystalline phases detectable are the a and p varieties of Si3N4; they are accompanied by a major amorphous part. In fact, the analysis gives approximately 40% of a Si3N4 and 10% of p Si3N4. The specific area of this silicon nitride is 92 m2.g"1.

Claims (22)

1. Silicon nitride Si3N4 powders obtained by carbothermic reduction and nitridation of silica which can be used especially for the manufacture of ceramics, charac- 5 terised in that they have a specific surface area of at least 30 m2/g.

2. Powders according to Claim 1, characterised in that they are in the form of agglomerates of mean dimensions of less than 5 nm and consist of elementary 10 particles the mean dimensions of which are between 10 and 50 nm.

3. Powders according to either of Claims 1 or 2, characterised in that at least 50 % of the said powders have a crystalline structure. 15

4. Powders according to any one of Claims 1 to 3, characterised in that their specific surface area is between 40 and 250 m2/g.

5. Powders according to any one of Claims 1 to 4, characterised in that 2 to 40 % of them are made up of a 20 p phase.

6. Process for the manufacture of the powders according to one of Claims 1 to 5, characterised in that it consists in the following stages: a) Preparation of granules having a pore volume 25 of between 0.1 and 3 cmJ/g by determination with a mercury porometer in the range 0 to 2000 bars, using silica, a binder which produces carbon not later than during the carbothermic reduction reaction and, where appropriate, additional carbon. 30 b) Carbothermic reduction reaction of the granules obtained in a, in the presence of an atmosphere containing nitrogen. c) Decarbonisation. d) Obtaining the powders. 35

7. Process according to Claim 6, characterised in that the particle size of the silica is less than 10 nm.

8. Process according to either of Claims 6 and 7, characterised in that both carbon and a binder are employed. - 11 -

9. Process according to Claim 8, characterised in that the carbon is chosen from the group consisting of vegetable carbon, thermal black, acetylene black, coke, lampblack and graphite. 5

10. Process according to either of Claims 6 and 7, characterised in that only a binder producing carbon is employed.

11. Process according to any one of Claims 6 to 10, characterised in that the binder is chosen from natural 10 or synthetic substances which are transformed into carbon under the conditions of the carbothermic reduction reaction and are capable of promoting the agglomeration of the silica.

12. Process according to Claim 11, characterised in 15 that the binder is chosen from the group consisting of coal tars, resins or polymers, especially thermosetting, such as phenolic resins, for example phenolformol, epoxy resins, polyimides, polyureas and polycarbonates.

13. Process according to any one of Claims 6 to 20 12, characterised in that the binder, with added carbon when appropriate, is used in a quantity such that the molar ratio carbon/Si02 is greater than 1 and, preferably, between 2 and 60/1.

14. Process according to either of Claims 8 and 9, 25 characterised in that the quantity of binder represents at least 2 % of the weight of the silica + carbon mixture.

15. By way of specific means for the accomplishment of the process according to Claim 6, granules consisting 30 of an agglomerated mixture of silica, binder which produces carbon and, where appropriate, carbon, the said granules having a pore volume the value of which is chosen between 0.1 and 3 cm3/g.

16. Granules according to Claim 15, characterised in 35 that they are in the form of pastilles, cylinders or particles of regular or irregular shape the largest dimension of which is greater than 0.5 mm.

17. Granules according to either of Claims 15 and 16, characterised in that their largest dimension is between - 12 - 1 and 30 mm.

18. Application of Si3N4 powders according to any one v of Claims 1 to 5 to the manufacture of ceramics.

19.^ 19. A silicon nitride powder according to claim 1, 5 substantially as hereinbefore described and exemplified.

20. A process according to claim 6 for the manufacture of a silicon nitride powder, substantially as hereinbefore described and exemplified.

21. A silicon nitride powder whenever prepared by a 10 process claimed in any one of claims 6-14 or claim 20.

22. Granules according to claim 15, substantially as hereinbefore described and exemplified. F. R. KELLY & CO., AGENTS FOR THE APPLICANTS. >