Chemistry of inorganic compounds – Nitrogen or compound thereof – Binary compound
Reexamination Certificate
2000-11-14
2002-11-19
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Nitrogen or compound thereof
Binary compound
Reexamination Certificate
active
06482384
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method for preparing aluminum nitride (AlN), and more particularly to a combustion synthesis method for preparing the powdery aluminum nitride.
BACKGROUND OF THE INVENTION
Having a high thermal conductivity, a high electrical resistivity, a good mechanical strength, and a good oxidation and thermal-shock resistance, AlN becomes a very important ceramic material in industrial applications. It can be used for high-performance electronic substrate material, optical lenses, cutting tools, heat sinks, and many high-temperature structure materials.
The manufacturing methods for AlN include:
1) the gas phase reaction method, e.g.,
2) the direct nitridation method, e.g.,
3) the carbothermal reduction-nitridation method, e.g.,
4) the combustion synthesis method.
The gas reaction method is not suitable for mass production of AlN in industry because of the high cost and low productivity involved.
The direct nitridation of Al and the carbothermal reduction-nitridation of powdery Al
2
O
3
methods in commercial form require a process executed under a high temperature and a long period of time, e.g., 5 hours, to fully complete the reaction, which can thus result in common disadvantages including a greater energy consumption and a slow manufacturing rate.
In comparison with other methods, the combustion synthesis method is a new method used to synthesize ceramic materials by self-propagation combustion reactions. The advantages achieved thereby include that it has a fast reaction rate, a less energy consumption and a simple manufacturing process and that it can be used for mass production.
Prior arts for the synthesis of aluminum nitride by the combustion synthesis method are described hereinbelow:
(1) Japan Patent Application Laid-Open No. 63-274605
Mix aluminum, aluminum nitride, and one powder selected from the group consisting of calcium carbonate (CaCO
3
), calcium nitrate (Ca(NO
3
)
2
), yttrium oxide (Y
2
O
3
), barium carbonate (BaCO
3
), barium nitrate (Ba(NO
3
)
2
), yttrium nitrate (Y(NO
3
)
3
), cerium oxide (CeO
2
), and yttrium oxalate hydrate (Y
2
(C
2
O
4
)
2
. 8H
2
O), according to an appropriate proportion, and then press the mixture into a suitable shape. Burn the cake by using an electric plate in a nitrogen atmosphere at 50 atm to form an aluminum nitride powder.
(2) Japan Patent Application Laid-Open No. 64-76906
Mix aluminum with aluminum nitride according to an appropriate ratio to be loaded in a porous refractory container. The assembly is heated by using an electric wire to form an aluminum nitride powder.
(3) Japan Patent Application Laid-Open No. 64-76905
Powders of aluminum and sodium nitride (NaN
3
) or other nitrogen-containing solid compound (such as potassium nitride (KN
3
), barium nitride (Ba
3
N
2
), etc.) are mixed according to an appropriate ratio and then placed in a refractory container. An igniting agent is placed on the powder mixture, and the refractory container is then placed into an electric oven which will be soon filled with nitrogen gas at a pressure of 10 kg/cm
2
. Prior to the igniting the reaction, the reactants are heated in the electric oven and then ignited with an electric wire to carry out the combustion reaction and form an aluminum nitride powder.
(4) U.S. Pat. No. 5,460,794
An aluminum powder and a solid nitrogen-containing compound are used as the raw material and molded after mixing. The blank is fully wrapped with an igniting agent and placed in an enclosed container filled with nitrogen which is ignited by heating the igniting agent with heating elements to form an aluminum nitride powder.
(5) U.S. Pat. No. 5,453,407
An aluminum powder and a solid-state nitrogen source are used as the raw material which is added with an ammonium halide powder. The mixture of the three is molded and fully wrapped with an igniting agent. The blank is placed in an enclosed container filled with nitrogen which is ignited by heating the compact by passing an electric current through the heating coil to form an aluminum nitride powder.
(6) U.S. Pat. No. 5,846,508
An aluminum powder and an ammonium halide salt are used as the raw material which are molded into a tablet or placed in an open or porous refractory container and then placed in an enclosed container filled with nitrogen. The combustion reaction is ignited to form an aluminum nitride powder.
(7) U.S. Pat. No. 5,649,278
An aluminum powder is used as a reactant and diluted with 20-80 wt % (based on the total weight of the aluminum powder and the aluminum nitride powder) of an aluminum nitride powder. The resultant mixture is poured into a graphite crucible or a refractory container made of oxide, ceramic, etc. The powder mixture has a bulk density of from 0.5-1.5 g/cm
3
. The container is placed in a reactor filled with nitrogen at 0.75-30 atm and the combustion reaction is ignited by directly heating the powder to form an aluminum nitride powder.
The key factors of synthesizing the aluminum nitride powder by combustion reaction are: (1) how to provide a sufficient amount of nitrogen; (2) how to prevent agglomeration of molten aluminum powder; and (3) how to achieve a complete reaction.
If a nitrogen gas is used as the nitrogen source, according to M. Costantino and C. Firpo. (J. Mater. Res. 6 : 2397 (1991)), the pressure needs to be upto 1000 atmosphere before the reaction can take place. The process used in the above-mentioned JP Laid-Open No. 63-274605 uses a pressure of 50 atm which is rather high and will cause an increase in the costs in facilities and operation thereby increasing the complexity and danger in the operation thereof.
If liquid nitrogen is used as the nitrogen source (as mentioned in the above JP 64-76906) without the need of using high pressure, the cost and the complexity of operation of the facilities will be increased because the boiling point of the liquid nitrogen is too low.
If a solid-sate nitrogen source is used as the nitrogen source (such as the above-mentioned JP 64-76905, U.S. Pat. No. 5,460,794 and U.S. Pat. No. 5,453,407), even without the use of high pressure, said solid-state nitrogen source needs to be a compound that is easily thermally decomposable for the combustion reaction to advance. Under such a circumstance, the reaction setup needs to be designed properly (such as wrapped with an igniting agent) so that the nitrogen gas generated by the thermal decomposition of the solid nitrogen source can promptly react with the aluminum powder. Otherwise, the escape of nitrogen gas will occur and hinder the performance of the reaction.
If an ammonium halide salt is added into the aluminum powder (e.g. the above-mentioned U.S. Pat. No. 5,846,508), the reaction process will generate by-products such as HCl, NH
3
, NH
4
Cl, H
2
or Cl
2
, etc., and increase the complexity and operation costs of the downstream processes even though a high yield can be achieved at a low nitrogen pressure.
If the aluminum powder is added with a diluent such as aluminum nitride, etc. and the density of the mixture powder is 0.5-1.5 g/cm
3
(such as mentioned in U.S. Pat. No. 5,649,278), the process requires a homogeneous mixing of the aluminum powder and the aluminum nitride and the content of the diluent needs to be upto 30 wt % even though said process can prevent the agglomeration of the aluminum powder, maintain a proper circulation of the nitrogen gas, and reach a high conversion rate. Such a process also has a high complexity, a high operation cost, and a reduced production efficiency (based on the amount of the aluminum nitride that can be synthesized by the feed per unit weight). Furthermore, the density of the powdered feed needs to be 0.5-1.5 g/cm
3
, which is not suitable for a packing density higher than (>1.5 g/cm
3
) or lower than (<0.5 g/cm
3
) of the aluminum powder (and the aluminum nitride powder), and limits the selection scope of the raw material.
SUMMARY OF THE INVENTION
The present invention provides an efficient new technology to synthesize an aluminum nitride of excellent properties by
Chung Shyan-Lung
Lin Chun-Nan
Lin Zheng-Xiuo
Jackson Walker L.L.P.
Langel Wayne A.
National Science Council
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