Aluminum nitride (AlN) ceramics have good thermal, electrical, mechanical and chemical stability, and are now more ideal substrate materials and electronic packaging materials. The excellent performance of AlN ceramics is directly related to the performance of raw material powder, and the high performance AlN powder is the key to prepare high thermal conductivity AlN ceramics.
In foreign countries, the manufacturing process of AlN powder has been quite mature and the commercialization degree is also very high. Previously, according to the research data of China Power 43 Research Institute, the world’s production of AlN powder reached 3,000 tons per year in 2015. However, there are not many manufacturers mastering the production technology of high performance AlN powder, mainly distributed in Japan, Germany and the United States. Then, what are the preparation methods of aluminum nitride powder? Where are the technical difficulties?
Al2O3 powder carbon reduction method
Al2O3 powder thermal reduction method is to mix ultrafine Al2O3 powder and high-purity carbon powder ball mill. In the atmosphere of nitrogen, at a certain temperature (1400 ~ 1800℃), alumina is reduced by carbon and nitrogen to form AlN powder. The reaction formula is Al2O3+3C+N2→2AlN+3CO.
The main difficulties of this method are as follows: the raw material requirements of alumina and carbon are relatively high, the raw material is difficult to mix uniformly, the nitridation temperature is high, the synthesis time is long, and the excess carbon needs to be decarburized, the process is complex, and the preparation cost is high.
Direct nitriding of Al powder
Direct nitrification of Al powder is the earliest method to prepare AlN powder. This method is to heat aluminum powder in nitrogen. At high temperature (800 ~ 1200℃), chemical reaction occurs between aluminum powder and nitrogen directly to produce aluminum nitrite powder.
The disadvantages of this method are obvious. In the early stage of the reaction, nitride film will gradually form on the surface of aluminum powder particles, making it difficult for nitrogen to penetrate further and hindering nitrogen reaction, resulting in a low yield. In addition, the reaction between aluminum and nitrogen is a strong exothermic reaction with a high speed, resulting in self-sintering of AlN powder, formation of agglomeration, and coarsening of powder particles.
Self propagating high temperature synthesis
Self-propagating high-temperature synthesis method, also known as combustion synthesis method, USES the heat generated by the reaction of aluminum powder and nitrogen to maintain the spontaneous reaction until the end of the reaction after the aluminum powder is ignited in high-pressure nitrogen by relevant means. The reaction formula is the same as the direct nitrification method of Al powder, namely, 2Al+N2→2AlN.
The defects of this method lie in that, on the one hand, the reaction process is not easy to be controlled because of the rapid reaction; On the other hand, AlN powder prepared by this method has large particle size, low purity, and low thermal conductivity. AlN ceramics are often used as additives in steel, rubber and plastic industries.
Chemical vapor deposition
Chemical vapor deposition (CVD) is an effective method to prepare ultrafine nitride powder with high purity, high specific surface area and uniform particle size distribution. The core principle of preparing AlN powder by this method is that the volatile compounds of aluminum (aluminum halide or alkyl aluminum) are brought into the reaction chamber from N2 to react with NH3, and the AlN grains are deposited from the gas phase.
This process has high requirements on equipment and low production efficiency. Adopting alkyl aluminum as raw material will lead to high cost, while adopting inorganic aluminum as raw material will generate corrosive gas, so it is still difficult to carry out large-scale industrial production at present.
Plasma method is an advanced technology for the synthesis of nanoscale AlN powder. The principle is that the aluminum powder is fed into the plasma reactor through gas. Under the high temperature plasma environment, the aluminum powder is rapidly heated up, fused and gasified to form aluminum vapor, and then the aluminum vapor reacts with high energy nitrogen ions to form AlN nanoparticles.
This process has many problems, such as high equipment requirements, low product yield, irregular morphology of single particle, and difficulty in obtaining low oxygen content powder, so the preparation of AlN powder by this process is still in the laboratory stage.
Development trend of aluminum nitride preparation technology
AlN powder is a kind of powder raw material with excellent performance. Researchers at home and abroad solve the technical problems existing in the existing process through constant scientific and technological innovation, and at the same time, they are constantly exploring new and more efficient preparation technology. In the synthesis of micron-scale AlN powder, the most important processes at present are still carbon thermal reduction and direct nitridation. These two processes are characterized by mature technology, simple equipment and good product quality, which have been widely used in industry.
The development direction of AlN preparation technology is to obtain high-performance powder with higher purity, controllable particle size and uniform dispersion morphology. A variety of powder specifications should be developed for different application fields to meet the raw material requirements of AlN powder in thermal conductive ceramic substrate, AlN single crystal semiconductor, high purity target material, thermal conductive filler and other fields. At the same time, continuous optimization of existing technologies and equipment is also needed in production to further improve batch stability of products, increase output efficiency and reduce production cost.
Domestic aluminum nitride powder industry development
In China, there are also a lot of manufacturers carrying out the research and production of AlN powder, mainly including the following enterprises:
The 43rd Research Institute of China Electronics Technology Corporation is a well-known supplier of LTCC/ Aln substrate, power drive circuit, signal circuit, electronic kiln and other products in China. It has obvious comprehensive advantages in the field of hybrid integrated circuit and microassembly technology and leads the industry development. As early as in the “Seventh five-year plan” period, Al2O3 powder carbon reduction method was used to prepare high-purity ultra-fine AlN powder, and the basic production method of AlN powder was mastered. The performance of AlN powder is in the leading level in China. However, whether the mass production has been realized or not has not been reported.
National porcelain material: high-end, high-purity, ultra-fine aluminum nitrite powder material has been unanimously approved by domestic customers through mass production.
Xiamen Juporcelain Technology Co., LTD. : Founded in Xiang ‘an District, Xiamen city on December 27, 2016, it is an innovative high-tech enterprise dedicated to the research, development, production and sales of high-grade aluminum nitrite powder and ceramic products. Based on the research achievements in the field of aluminum nitride in University of Science and Technology Beijing, the company has won many honors, such as the first prize of Science and Technology of China nonferrous Metal Industry (Invention) and the first prize of New Materials Industry in China Innovation and Entrepreneurship Competition (the first place in China). The company’s main products are three series of high-purity micro aluminum nitride powder, spherical aluminum nitride particles and complex precision aluminum nitride ceramics by injection molding, which have been recognized and praised by customers at home and abroad.
With other well-known enterprises have: yantai high-tech new materials co., LTD., liaoning DE sheng special ceramics manufacturing co., LTD., shandong inati on ceramic new material technology co., LTD., sanhe yanjiao dextrys high-tech ceramic material co., LTD., fujian ShiNuoRui new materials co., LTD., jinjiang fresh material technology co., LTD., etc.
At present, our country’s high performance AlN powder basically depends on import, which is not only expensive, but also has the risk of raw material supply being cut off at any time. Therefore, it is urgent to realize the localization of high performance AlN powder manufacturing technology.