Synthesis of fine-grained .alpha.-silicon nitride by a combustion process

A combustion synthesis process for the preparation of .alpha.-silicon nitride and composites thereof is disclosed. Preparation of the .alpha.-silicon nitride comprises the steps of dry mixing silicon powder with an alkali metal azide, such as sodium azide, cold-pressing the mixture into any desired shape, or loading the mixture into a fused, quartz crucible, loading the crucible into a combustion chamber, pressurizing the chamber with nitrogen and igniting the mixture using an igniter pellet. The method for the preparation of the composites comprises dry mixing silicon powder (Si) or SiO.sub.2, with a metal or metal oxide, adding a small amount of an alkali metal azide such as sodium azide, introducing the mixture into a suitable combustion chamber, pressurizing the combustion chamber with nitrogen, igniting the mixture within the combustion chamber, and isolating the .alpha.-silicon nitride formed as a reaction product.

BACKGROUND OF THE INVENTION 
This invention relates generally to combustion synthesis, more specifically 
to self-propagating high-temperature synthesis (SHS) and still more 
specifically to the synthesis of .alpha.-Si.sub.3 N.sub.4 by combustion 
synthesis. 
The United States Government has rights in this invention pursuant to 
Contract No. W-7405-ENG-48 between the U.S. Department of Energy and the 
University of California, for the operation of Lawrence Livermore National 
Laboratory. 
Refractory materials, including ceramics, are used in many applications 
which have specific requirements such as increased resistance to 
corrosion, greater tolerance to high temperatures, superior mechanical 
properties, and special electrical characteristics. Some of these 
applications include electronic devices, cutting tools, industrial 
machinery, containers for high temperature reactions and the like. 
Combustion synthesis is a process which has been known for quite some time, 
especially in the Soviet Union but has not been widely applied elsewhere. 
In combustion synthesis, strong exothermic chemical reactions produce heat 
that causes combustion waves to propagate spontaneously through the 
reactants, converting them to the desired products, without requiring the 
addition of thermal energy from an external source. Some of the attractive 
and desirable characteristics of combustion synthesis for application to 
ceramics and refractories include: (1) temperatures in the range of 
2000.degree.-3500.degree. C. generated without the addition of external 
energy; (2) the combustion wave which moves rapidly (1.1-10 cm/sec); (3) 
the high rate of heating at the combustion front; and (4) the 
volatilization of cation impurities at the combustion front, which creates 
products that are purer than the reactants. It would be most desirable, 
therefore, to have available a combustion synthesis route for the 
production .alpha.-Si.sub.3 N.sub.4. 
U.S. Pat. No. 4,161,512 "Process for Preparing Titanium Carbide" issued 
July 17, 1979 to A. G. Merzhanov et al, discloses a combustion synthesis 
route for the preparation of titanium carbide. 
U.S. Pat. No. 4,337,463 "Controlled Atmosphere Processing of TiB.sub.2 
/Carbon Composites", issued Mar. 22, 1983 to L. A. Joo, describes a 
process for preparing TiB.sub.2 -carbon composites by mixing titanium 
boride and carbon, pitch and other reactants, pressing the mixture into 
the desired shape and heating the shaped article to 2100.degree. C. in a 
nitrogen atmosphere and a noble gas atmosphere above 2100.degree. C. 
A. G. Merzhanov and co-workers of the USSR claimed that they synthesized 
Si.sub.3 N.sub.4 by a combustion process. A. Merzhanov, "Self-Propagating 
High Temperature Synthesis", Fizica Khimii Soverm. Problemy, pp 6-45 
(1983); A. Merzhanov, "From Academic Idea To Industrial Production", Nauk 
SSR, vol. 1981, No. 10, pp 30-36 (1981). 
Transition metal nitrides (TiN, ZrN, HfN and YN) and composites with 
aluminum oxide (Al.sub.2 O.sub.3) have been synthesized by the combustion 
of the metal with sodium azide (NaN.sub.3) which is a solid source of 
nitrogen. The general reaction is given by equation 1. 
EQU 3Me+NaN.sub.3 .fwdarw.3MeN (1) 
where Me is either Zr, Ti, or Hf metal powder. The combustion is carried 
out in 1 atmosphere of nitrogen with 100% conversion. These combustion 
processes are described in U.S. Pat. Nos. 4,446,242 issued May 1, 1984 to 
J. B. Holt; and 4,459,363 issued July 10, 1984 to J. B. Holt. 
Si.sub.3 N.sub.4 is an advanced ceramic material which is important because 
of its wear, corrosion and thermal shock resistance at high temperatures. 
It would be useful for applications in the construction of heat engines, 
heat exchangers, cutting tools, radar windows, high temperature bearings 
and the like. There are three conventional ways of synthesizing Si.sub.3 
N.sub.4. These three methods are illustrated by reactions shown in the 
following equations: 
EQU 3Si+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 ( 2) 
EQU 3SiO.sub.2 +6C+2N.sub.2 .fwdarw.Si.sub.3 N.sub.4 +6CO (3) 
EQU 3SiCl.sub.4 -4NH.sub.3 Si.sub.3 N.sub.4 -12 HCl (4) 
The first method is the direct nitration of silicon powder in a nitrogen 
atmosphere. The second is the carbothermic reduction of silica by carbon 
in a nitrogen atmosphere. Vapor phase reaction of SiCl.sub.4 and NH.sub.4 
(ammonia) are shown by equation 4. Some of these methods of the 
preparation of silicon nitride are exemplified by the following patents: 
U.S. Pat. No. 4,117,095 "Method of Making .alpha.-Type Silicon Nitride 
Powder", issued Sept. 26, 1978 to K. Komeya et al, discloses a method for 
the preparation of high strength .alpha.-silicon nitride using 
.alpha.-silicon nitride powder as the starting material and including 
therein additives such as magnesium and yttrium oxide. 
U.S. Pat. No. 4,414,190, "Method of Preparing Silicon Nitride", issued Nov. 
8, 1983 to M. Seimiya et al, describes a method of preparing silicon 
nitride by heating a wet process carbon in the presence of a carbon 
source, such as a hydrocarbon or solid carbon and a nitrogen source such 
as nitrogen gas or ammonia. 
U.S. Pat. No. 4,590,053 "Method for Producing .alpha.-Form Silicon Nitride 
Fine Powders", issued May 20, 1986 to T. Hashimoto et al, relates to a 
method for producing .alpha.-form silicon nitride powder by heat-treating 
in a nitrogen atmosphere, a mixture of silicon nitride powder, carbon, 
magnesium or calcium and/or compounds thereof. 
U.S. Pat. No. 3,839,541 "Silicon Nitride Products", issued Oct. 1, 1974 to 
R. J. Lumby et al, describes a process for the preparation of silicon 
nitride powder with a nitriding atmosphere at elevated temperatures below 
the melting point of the silicon powder. 
U.S. Pat. No. 4,399,115 "Synthesis of Silicon Nitride", issued Aug. 16, 
1983 to K. Sato et al, describes a process for synthesizing silicon 
nitride by reacting a silicon halide and ammonia at a high temperature. 
However, none of the above-described methods is completely satisfactory 
because of incomplete reaction, the presence of carbon, or because of high 
material costs and high cost of production. 
Combustion of silicon powder even with the use of a solid source of 
nitrogen such as NaN.sub.3 is very unlikely at or near 1 atmosphere of 
nitrogen pressure normally employed for the reaction. The Si and N.sub.2 
combustion reaction does not proceed at low nitrogen pressures (1 atm.) 
because of the high decomposition pressure of Si.sub.3 N.sub.4. When 
compared to the dissociation pressure of the transition metal nitrides, it 
is higher at all temperatures. For example, the decomposition pressure for 
silicon nitride is approximately 100 atmospheres at 2500.degree. C. The 
experiments to study the combustion of silicon powder (3 .mu. average 
diameter), as a function of nitrogen pressure, indicates that the powder 
will not ignite until a pressure of about 450 atmospheres is reached, and 
even then there is only partial combustion. Only above approximately 680 
atmospheres will the silicon powder completely burn with a 92% yield. The 
yield may be increased to 96% by the addition of up to 20 wt% of Si.sub.3 
N.sub.4 powder to the silicon powder prior to ignition. However, the 
powder product formed is 88-90% .beta.-phase Si.sub.3 N.sub.4. For 
sintering purposes, the .alpha.-form is preferable because of enhanced 
sinterability due to the .alpha.-.beta. phase transition. Also, operating 
at lower pressures would make the combustion process more economical. 
A cost-effective process for the production of .alpha.-Si.sub.3 N.sub.4 
powder should, therefore, greatly increase its use in high technology 
applications. 
It is, therefore, an object of the present invention to provide a cost 
effective method for the preparation of .alpha.-silicon nitride powder. 
Another object is to provide a combustion synthesis process for the 
preparation of .alpha.-silicon nitride. 
Yet another object is to synthesize .alpha.-silicon nitride at relatively 
low nitrogen pressures. 
Still another object is to provide pure .alpha.-silicon nitride powder and 
composites thereof. 
Yet another object is to provide .alpha.-silicon nitride in relatively pure 
form and in high yields. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art upon examination of the 
following or may be learned by practice of the invention. The objects and 
advantages of the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
SUMMARY OF THE INVENTION 
To achieve the foregoing and other objects and in accordance with the 
purpose of the present invention as embodied and broadly described herein, 
the present invention is directed to a process for the synthesis of 
.alpha.-silicon nitride (Si.sub.3 N.sub.4) in relatively pure form and in 
greater yields and composites thereof. The method basically comprises dry 
mixing silicon powder (Si), with an alkali metal azide, such as sodium 
azide, introducing the mixture into a suitable combustion chamber, 
pressurizing the combustion chamber with nitrogen, igniting the mixture 
within the combustion chamber, and isolating the .alpha.-silicon nitride 
formed as a reaction product. More specifically, the method comprises 
powder by the combustion of silicon (Si) powder mixed with sodium azide 
(NaN.sub.3) powder in a nitrogen atmosphere of about 50 atmospheres. The 
powder mixture is ignited and upon ignition, a combustion wave rapidly 
self-propagates through the reactants transforming them into a fine 
grained .alpha.-Si.sub.3 N.sub.4 powder. The sodium from the NaN.sub.3 is 
volatilized and consequently is not present in the Si.sub.3 N.sub.4 
powder. Exothermic heat from the chemical reaction provides the high 
temperature of synthesis so that a furnace is not required for the 
production of .alpha.-Si.sub.3 N.sub.4. 
Another embodiment of the present invention provides .alpha.-silicon 
nitride composites, such as .alpha.-Si.sub.3 N.sub.4 -Al.sub.2 O.sub.3, by 
the combustion of the reactants shown in equation 5. 
EQU 9SiO.sub.2 +12Al+4NaN.sub.3 .fwdarw.3Si.sub.3 N.sub.4 +6Al.sub.2 O.sub.3 
+4Na (5) 
The pressure of nitrogen required for the reaction may be lowered by the 
addition of a diluent of either Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is a process for the synthesis of .alpha.-silicon 
nitride (Si.sub.3 N.sub.4) by the combustion of silicon (Si) mixed with an 
alkali metal azide such as sodium azide (NaN.sub.3), in a nitrogen 
atmosphere in the range of about 50 to 100 atmospheres, preferably about 
50 atmospheres. The mixture which, optionally in the powdered form, is 
ignited using any convenient ignition source such as a coil of tungsten 
wire, or an electric arc or graphite strip and the like. Upon ignition, a 
combustion wave rapidly self-propagates through the reactants transforming 
them into the desired product, a fine grained .alpha.-Si.sub.3 N.sub.4 
powder. The sodium from the NaN.sub.3 is volatilized and consequently is 
not present in the Si.sub.3 N.sub.4 powder. Exothermic heat from the 
chemical reaction provides the high temperature for synthesis so that a 
furnace is not required for the production of .alpha.-Si.sub.3 N.sub.4. 
The optimum procedure for synthesizing .alpha.-Si.sub.3 N.sub.4 with a 
combustion of silicon powder is described below. Silicon (average particle 
size of 3 .mu.) is dry mixed with NaN.sub.3 according to the ratio given 
in equation 6. 
EQU 9Si+4NaN.sub.3 .fwdarw.3Si.sub.3 N.sub.4 +4Na (6) 
The mixed powder is cold-pressed into a cylindrical form or poured into a 
quartz crucible. An igniter pellet of Ti-1.5B is placed on top. The 
samples are loaded on a platen which is attached by the electrode to the 
top flange of the combustion chamber. The chamber is pressurized with 
nitrogen from about 50-100 atm. A heated tungsten coil ignites the pellet 
which ignites the silicon-NaN.sub.3 mixture and the combustion is 
completed within a few seconds. X-ray analysis of the powder confirms the 
product to be 98%-100% .alpha.-Si.sub.3 N.sub.4. The use of the NaN.sub.3 
is important because it enables the combustion to be completed at 
relatively low nitrogen pressures. The exact over-pressure of nitrogen 
required can be adjusted somewhat by the addition of Si.sub.3 N.sub.4 as 
diluent. This means that the over pressure of 50 atmospheres can be 
lowered by the addition of the Si.sub.3 N.sub.4 dilutent to the powder 
mixture of silicon and NaN.sub.3. The average particle size of Si.sub.3 
N.sub.4 is about 1 .mu. or below depending on the pressure and amount of 
diluent. 
Composite materials such as silicon nitride-metal oxide composites, such 
as, for example, silicon nitride-aluminum oxide (Si.sub.3 N.sub.4 
-Al.sub.2 O.sub.3) may be formed by the combustion of the reactants shown 
in equation 5. 
EQU 9SiO.sub.2 +12Al+4NaN.sub.3 .fwdarw.3Si.sub.3 N.sub.4 +6Al.sub.2 O.sub.3 
+4Na (5) 
The starting materials for the composites may be silica or silicon oxide 
and a metal oxide or the metal depending on whether silicon or silicon 
dioxide is the starter material. Composites using other metals oxides such 
as the oxides of Ti, Zr, Hf may also be similarly prepared. Again, the 
over-pressure of nitrogen may be adjusted by the addition of a diluent of 
either Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4. 
For the preparation of the composites, silicon dioxide-aluminum composites 
being exemplary thereof, 7.21 gm of SiO.sub.2, 4.32 gm of aluminum, 3.47 
gm of NaN.sub.3 were mixed and thoroughly blended (in a blender) for about 
5 min. 15 gms of the powder mixture was poured into a fused quartz 
crucible, about 1.9 cm in diameter and 6.4 cm high. An igniter pellet of 
Ti.sub.1.5 B was placed on top of the powder such that the top of the 
pellet was located just below the tungsten coil, which is heated to ignite 
the pellet, which in turn ignited the mixture. The crucible was mounted on 
a platform attached to a seal ring. The crucible-platform assembly was 
loaded into a combustion chamber. The chamber was then evacuated, 
pressurized with nitrogen and then ignited by heating the tungsten coil. 
The resulting composite powder was removed and either sintered or cold 
pressed into desired shapes. 
Without the presence of sodium azide, at nitrogen pressures of less than 
about 340 atmospheres, there was no ignition of the reaction mixture. 
Between, about 340 and about 600 atmospheres, there was a partial burn, 
i.e., the combustion wave did not propagate throughout the reaction 
mixture to produce the desired product. 
At about 680 atmospheres of nitrogen pressure, and with no sodium azide 
present, only 12% of .alpha.-silicon nitride was produced. 
At about 850 atmospheres of N.sub.2 pressure and no NaN.sub.3, only 8.5% of 
the product formed was .alpha.-silicon nitride. 
At about 1000 atmospheres of N.sub.2 pressure and no NaN.sub.3, only 7% of 
the produce was .alpha.-silicon nitride. At 3000 atmospheres of N.sub.2, 
the yield of a-Si.sub.3 N.sub.4 was about 2%. With increasing nitrogen 
pressure, the yield of a-silicon nitride dropped. 
Using sodium azide and silicon powder in the reaction mixture produced 
.alpha.-silicon nitride in appreciable amounts even at 50 to 100 
atmospheres of nitrogen pressure. 
Use of sodium azide and silicon powder also lowered the combustion 
temperature sufficiently even in the pressure range of 50-100 atmospheres. 
With the addition of sodium azide in an approximate ratio of 1:2 
(azide:silicon), greater than about 98% of the product formed was 
a-silicon nitride, at a nitrogen pressure of about 280 atmospheres. 
This invention permits the synthesis of .alpha.-Si.sub.3 N.sub.4 at 
relatively low pressures without the use of a high temperature furnace and 
at rapid reaction time (within 3-5 seconds for small samples). These 
procedures provide a very economical process for the synthesis of 
.alpha.-Si.sub.3 N.sub.4 -metal oxide composite materials. Preferred metal 
oxides include but are not limited to aluminum oxide, zirconium oxide, 
titanium oxide and hafnium oxide. 
While a particular embodiment of the invention and specific materials and 
parameters have been illustrated and described, the invention is not 
limited to the particular illustrations or embodiments so described. 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 preferred 
embodiments of the invention have 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. 
It is intended that the scope of the invention be defined by the claims 
appended hereto.