WO1997005081A1

WO1997005081A1 - Manufacture of composites of silicon carbide and aluminium nitride

Info

Publication number: WO1997005081A1
Application number: PCT/NO1996/000174
Authority: WO
Inventors: Jorulf Brynestad; Harald A. ØYE; Ketil Motzfeldt; Laure Delmas
Original assignee: Nyfotek A/S
Priority date: 1995-07-26
Filing date: 1996-07-11
Publication date: 1997-02-13
Also published as: NO952955L; AU6535996A; NO952955D0

Abstract

Method for the preparation of composites of aluminium nitride and silicon carbide, whereby aluminium carbide and silicon nitride and/or silicon together with nitrogen are heated to a temperature of at least 1400 °C in the presence of one or more co-reactants and possibly substances which are bonded together by the reaction product to a composite. Composites of aluminium nitride and silicon carbide are made by reacting aluminium carbide (Al4C3) with silicon nitride (Si3N4) or silicon (Si). The reaction is carried out in an atmosphere of nitrogen. Oxidic contaminants and sintering aids, such as SiO2 and Al2O3 react with Al4C3 and nitrogen to form SiC and AlN, respectively.

Description

MANUFACTURE OF COMPOSITES OF SILICON CARBIDE AND

ALUMINIUM NITRIDE

This invention concerns the manufacture of silicon carbide materials, aluminium nitride materials and silicon carbide/aluminium nitride composite materials, by using aluminium carbide (A1 C₃) as a key reactant.

The system SiC-AIN is well known, and among others described in GB patent application No. 2 271 577, US Patent No. 5 371 049, Textures in AIN-SiC composite ceramics, Sandlin, Michael S., Bowman, Keith J., Mater. Res. Soc. Symp. Proc. 1994, 327 263-8; Solid Solutions in the SiC/AIN system, Zangvil, Avigdor, Ruh, Robert, Inst. Phys. Conf. Ser. 1994, 137 (Silicon Carbide and Related Materials), 385-8; Pressureless sintering and high-temperature strength of SiC-AIN ceramics; Li, Jing-Feng, Watanabe, Ryuzc, J. Ceram. Soc. Jpn. 1994, 102 (Aug.), 727-31; Fracture toughness of ceramics in the AIN-SiC system, Katz, R. N., Grendahl, S., Cho, K., Bar-On, I, Rafaniello, W; Ceram. Eng. Sci. Proc. 1994, 15(5), 877-84; A new compound Si Al₄N₄C with the wurtzite structure in the system Si₃N₄-Al₄C₃, K. Tsukuma, M. Shimada, M. Koizumi, Journal of Materials Science Letters 1 (1982) 9.

GB patent application No. 2 271 577 describes mixing silicon nitride, aluminium and carbon to make a mixture, heating of the mixture to make an AlN-SiN composite and/or a solid solution. The reaction is:

Si₃N₄ + 4 Al + 3 C = 4 AIN + 3 SiC.

It is not necessary to use a silicon nitride with high purity. Sintering aids, such as

CaO or CaCO₃ can be used. Oxide contaminations in the form of SiO₂ are converted into aluminium oxide and silicon carbide. The ground reactants are hot pressed in a die at about 1700°C in approximately 1 hour. It seems that the reaction takes place without a nitrogen atmosphere.

The above mentioned article of K. Tskuma, M. Shimada and M. Koizumi also describes reaction of A1₄C₃ with Si₃N . In this case it is worked in an atmosphere of argon, which leads to loss of nitrogen, and products obtained will contain oxides if pure materials are not used for the reaction.

The closest known art seems to be US Patent No. 3 649 310. In this patent A1₄C₃ is used together with Si₃N as a reactant, with hot pressing up to about 2000°C under vacuum. The method is also very similar to other methods where AIN and SiC are sintered together directly by hot pressing at high temperatures, and a such temperatures A1 C₃ and Si₃N₄ are not necessary starting materials. At lower temperatures and without hot pressing AIN and SiC will not react with each other. According to the invention it is provided a method for producing composites of aluminium nitride and silicon carbide, and the method is characterized in that aluminium carbide (A1₄C₃) and silicon nitride (Si₃N ) and/or silicon (Si) are heated until reaction together with nitrogen at a temperature of at least 1400°C, possibly in the presence of one or more co-reactants and possibly materials which are bound together by the reaction product to a composite. In the present method suitable mixtures of aluminium carbide, silicon nitride and/or elementary silicon are used, together with nitrogen and possibly sintering aids such as aluminium oxide, silicon dioxide, oxides of rare earths or a combination of these, as precursors or for reaction binding, for preparation of an aluminium nitride/silicon carbide. In the method the unique properties of aluminium carbide which will be described in the following are used for preparation of silicon carbide materials, aluminium nitride materials and aluminium nitride/silicon carbide composites, and other heat resistant materials using aluminium nitride/silicon carbide as a binding agent. The materials are formed by reaction binding from mixtures of aluminium carbide, silicon nitride and/or silicon, with or without sintering aids and possible contaminations such as aluminium oxide, silicon dioxide, oxides of rare earth and/or clay materials, in a nitrogen atmosphere.

Aluminium carbide reacts with silicon nitride according to the (simplified) reaction scheme

A1₄C₃ + Si₃ N₄ = 4 AIN + 3 SiC (1) The free energy of reaction for Equation (1) is strongly negative, and we have calculated that by 2000 K is

ΔGι° = -400.4 kJ (JANAF Thermochemical Tables). Even though free nitrogen is no part ofthe equation, the presence of N₂ is necessary to make up for the loss of N₂ which occurs during the heating. Aluminium nitride and silicon carbide form solid solutions with each other at high temperatures. Thus, at temperatures about 2000°C they form solid solutions across the whole SiC-AlN-system, whereas a miscibility gap appears below ~2000°C, with an "A1N"- phase, (AlN)ι._x (SiC)_x and a Δ "SiC'-phase, (SiC),._y(AlN)_y. Aluminium carbide reacts with silicon and nitrogen according to the reaction scheme

Al₄C₃ + 3 Si + 2 N₂ = 4 AlN + 3 SiC (2)

We have calculated the free energy of reaction for Equation (2), and this is strongly negative, at 2000 K is

ΔG₂° = -448.0 kJ and at 1700 K is

ΔG₂° = -597.0 kJ An advantage of this particular reaction is that it can proceed at temperatures even slightly above the melting point of silicon (1412 ± 3°C). This is desirable for some manufacturing processes.

Aluminium carbide reacts with silicon dioxide and nitrogen according to the reaction scheme

Al₄C₃ + SiO₂ + 2 N₂ = 4 AlN + SiC + 2 CO (3) We have calculated the free energy of reaction for Equation (3) as strongly negative, at 2000 K is

ΔG₃° = -374.2 kJ This property of aluminium carbide can be used to remove the silicon dioxide contamination in the industrial grade silicon, silicon carbide and silicon nitride used as raw materials in manufacturing the above mentioned compounds.

Aluminium carbide reacts with aluminium oxide according to the reaction scheme

Al₄ C₃ + Al₂ O₃ + 3 N₂ = 6 AlN + 3 CO (4)

We have calculated the free energy of reaction for Equation (4) to be strongly negative, at 2000 K is ΔG₄ ° = -320.0 kJ

This property of aluminium carbide can be used to remove the aluminium oxide contamination in industrial grade aluminium nitride used in manufacturing the composites.

It also can be utilised for converting aluminium oxide used as sintering aid, into aluminium nitride. Analogous reactions take place when using lanthanide oxides, such as yttrium oxide, as sintering aid.

The unique properties of aluminium carbide described above make it possible to use industrial grade raw materials, i.e., industrial grade aluminium carbide, aluminium nitride, silicon, silicon carbide, silicon nitride, silicon dioxide, aluminium oxide, lanthanide oxides and clay materials, and obtain ceramic composites virtually free of oxide contamination in binder phase or in the grain boundaries. By proper adjustment of the amount of aluminium carbide employed, virtually all oxide can be removed during the processing. An excess of A1₄C₃ is therefore always used. How much this excess should be, is among others depending on the contents of the materials which are reacted in the reactions

(3) and (4).

The method is carried out in the presence of particles of SiC and/or Al O₃ which thereby are bound together by the primary formed composite to a composite product. Especially suitable is the use of SiC particles which are bound together with a reaction product made of A1₄C₃, Si₃N and/or Si, N₂ and possible oxides.

Examples

Experiment No. 1: Synthesis of a 4 AIN»3 SiC composite 15.4 g of aluminium carbide powder (99.9 % pure), 15.0 g of silicon nitride powder

(99.9 % pure) were well mixed in a glove box. 1.5 g stearic acid was dissolved in dry diethyl ether and added to the mixture. After removal of the ether, the powder was transferred to a die with 30 mm diameter, and pressed into a pellet using a pressure of 15 tons. The pellet was transferred to a furnace with a graphite heating element and controlled atmosphere (including vacuum). After removal ofthe stearic acid in vacuum at a moderate temperature, the furnace was heated in nitrogen at 0.6 atm. and kept at 1800°C for 1 hour.

The sintered pellet was analyzed by XRD and SEM. It consisted of AIN and SiC with no detectable A1₄C₃, Si₃N or oxides. The porosity was -20 %.

Experiment No. 2: Synthesis of a SiC pellet reaction bonded with 4 AIN»3 SiC.

24.0 g of an industrial grade SiC, 0.2-0.5 mm powder, was mixed with 4.0 g A1₄C₃,

3.0 g Si₃N₄ and 1 g stearic acid/ether with the same procedure as in experiment No. 1. The pellet was kept at ~1820°C in 0.6 atm. N for 1.5 hours. The sintered pellet was analyzed with XRD and SEM. The material consisted of coarse SiC grains embedded in an AIN-SiC binder, with no detectable oxides or free carbon. The porosity was -20 %. Experiment No.3: Synthesis of a SiC pellet reaction bonded with 4 A1N«3 SiC

24.0 g of an industrial grade SiC, 0.2-0.5 mm powder was mixed with 4.0 g A1₄C₃, 1.8 g Si powder (10 micron) and 1 g stearic acid, with the same procedure as above. The pellet was kept at ~1420°C in 0.6 atm. N₂ for 1/2 hour whereupon it was raised to 1800°C and kept for 1 hour. The sintered pellet was analysed with XRD and SEM. The material consisted of coarse SiC grains embedded in an AIN-SiC binder, with no detectable oxides of free silicon or carbon. The porosity was -20 %.

Claims

1. Method for the preparation of composites of aluminium nitride and silicon carbide, characterized in that aluminium carbide (A1₄C₃), silicon nitride (Si₃N ) and/or silicon (Si) are heated to reaction together with nitrogen to a temperature of at least 1400°C, possibly in the presence of one or more co-reactants and possibly substances which are bonded together by the reaction product to a composite.

2. Method according to claim 1, characterized in that the co-reactants are selected from sintering aids and contaminants comprising Al₂O₃, SiO and/or oxides of rare earths.

3. Method according to claim 1 or 2, characterized in that the raw materials are of industrial grade and contain contaminants in the form of oxides.

4. Method according to claim 2, characterized in that an excess of aluminium carbide is used to react oxides - which can be present as contaminants or added as sintering aid - with nitrogen and aluminium carbide for the preparation of aluminium nitride (equation 4) and possibly silicon carbide (equation 3).

5. Method according to claims 1 - 4, characterized in that it is carried out in the presence of particles of SiC and/or AIN which are bounded together to a composite product by the primary composite formed by the reaction.

6. Method according to claim 5, characterized in that SiC and an oxidic sintering aid are used together with at least the stoichiometric amount of A1₄C₃ and Si₃N and/or Si, in a nitrogen athmosphere, to make a composite of SiC bound together by the reaction product of A1₄C₃, Si₃N and/or Si, N and oxides which are present.