One Step Process for Producing Dense Aluminum Nitride and Composites Thereof

The present invention is directed to a process whereby dense parts of aluminum nitride and/or cermets and composites thereof can be formed by combusting aluminum powder in nitrogen gas under high pressure. The process basically comprises igniting pure aluminum powder in a nitrogen atmosphere at a pressure in the range of about 500 to about 1000 atmospheres or higher. The minimum nitrogen pressure below which ignition and subsequent combustion will not take place is determined by the particle size and type of the aluminum powder used. This minimum nitrogen pressure is also determined or influenced by the presence of other inert diluents. Therefore, prior to the combustion synthesis process, this minimum required pressure must first be established based on the particle size and type of aluminum powder used and the presence, if any, of other inert diluents in the synthesis process. More specifically, the process comprises pouring aluminum powder into a suitable container; placing an igniter such as a cold-pressed pellet of Ti B.sub.1.5 on top of the container; loading the container into a high pressure autoclave; pressurizing the autoclave with nitrogen gas to a preselected pressure; igniting the igniter pellet by any suitable means, such as, for example, heated tungsten coil, electric match, laser and the like; allowing the combustion to proceed to completion; cooling and depressurizing the autoclave; and removing the product formed.

Typically, by way of example, dense aluminum nitride or a cermet or composite thereof was prepared as follows: aluminum powder or a mixture of aluminum powder and an inert diluent such as, for example, silicon nitride, boron nitride, titanium nitride, hafnium nitride and zirconium nitride, was poured into a quartz container about 1.9cm in diameter and 2.5cm high. A cold-pressed pellet of Ti B.sub.1.5 was placed near the top of the container, to act as an igniter. A flat tungsten coil was located just above the top surface of the igniter pellet. This assembly of electrical leads, quartz container with the aluminum powder and tungsten coil was loaded into a high pressure autoclave. The autoclave was pressurized with nitrogen gas to a preselected pressure. The reaction was then started by ignition of the starter or igniter pellet with a resistively heated tungsten coil. The calculated adiabatic temperature (maximum temperature) for the combustion of aluminum in nitrogen is 2600.degree. C. without allowances for the heat of fusion. Once ignition occurred, the reaction was completed with the spontaneous movement of a combustion wave through the reactants. The high temperatures are self-generated and the reaction is completed within one to two seconds.

Whether the aluminum combusts or not and whether simple phase aluminum nitride on a Al-AIN cermet is formed or not depends on the pressure of nitrogen gas. For combustion to occur, the nitrogen pressure must be generally higher than about 100 atmospheres. When the pressure was about 680 atmospheres, the product consisted of a cermet of AlN in an Al metal matrix. At a pressure of about 1000 atmospheres nitrogen, the product formed was a completely converted AlN compact that was densified to about 92% of the theoretical densification value. The spherical shape of the AlN formed indicates that its formation was by a fusion process under the high temperature and pressure conditions. Therefore, depending upon the predetermined nitrogen pressure (based on the particle size and type of the aluminum nitride powder used), the final product was either a single phase AlN or a cermet of Al--AlN, in which the matrix phase is aluminum; and, when a mixture of aluminum and an inert diluent was used as the starting material, the resulting product is a dense composite of AlN and the inert diluent. For example, when silicon nitride was used as the inert diluent, the resulting composite was AlN--Si.sub.3 N.sub.4. Similarly, if aluminum nitride, boron nitride, hafnium nitride, zirconium nitride or titanium nitride are used as the inert diluents, the corresponding composites with AlN are formed.

The purity of the combusted product is directly determined by the degree of purity of the nitrogen and the aluminum powder used. AlN of high purity with a low oxygen content (oxygen being a contaminant in the product) can be synthesized by using high purity nitrogen gas and aluminum powder of sufficient size to reduce the amount of oxygen present. The temperature of the combustion process can be controlled within certain limits by the addition of inert diluents such as AlN to the reactant aluminum powder. The amount of aluminum nitride added to the aluminum reactant powder also controls the nitrogen pressure that is required for the densification process. Nitride composites may also be produced by using aluminum nitride or a different nitride such as silicon nitride, boron nitride, hafnium nitride, zirconium nitride and titanium nitride, as the inert diluent. When Si.sub.3 N.sub.4 was used as the inert diluent, the composite formed was AlN--Si.sub.3 N.sub.4.

The experiments showed that it was possible to obtain dense Al--AlN cermet material or a dense AlN product by controlling the pressure of nitrogen. In both cases, the product formed took the shape of the container. The size of the parts is determined by the size and volume of the autoclave. Although quartz containers were used in th examples, other materials such as other refractory materials, for example, boron nitride and the like. Since there may be reactions between the refractory material and the combustion product at the high prevailing temperatures and pressure, the selection of the container material must be carefully considered depending on the product and the degree of its purity desired.

It has thus been shown that the subject invention provides a one step combustion method for the synthesis of aluminum nitride and composites of aluminum nitride. The method is very efficient and economic and produces high purity, dense aluminum nitride, cermets and dense composites thereof. The aluminum nitride, cermets and composites thus formed are very useful as electronic substrates and for other high temperature applications due to their excellent thermal conductivity, electrical resistivity, high mechanical strength, oxidation and thermal shock resistance and relatively low density.

The above embodiments were chosen and described in order to explain best the principles and the practical application of the subject invention thereby to enable those skilled in the art to utilize the invention in various other embodiments and various modifications as are suitable for the particular use contemplated. The foregoing description of a preferred embodiment of the invention has been presented therefore for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.