Different qualities are from different technologies.
Currently the preparation methods of high-purity alumina are a great many, mainly including the sol-gel method, hot decompounding method, Bayer method, etc.In China, the Technology of most companies in hebei,Shandong,Guangzhou provinces is alumina powder and water hydrolysis, and some companies add bilineurine in the process, which has been popular in Japan,Germany,America 20 years ago. Specific process of the technology likes this: cut metal alumina bark or block into powder with metal knives, then mix the powder with high purity water, in the process of reaction, some inorganic compounds such as aluminium hydroxides, ferric hydroxides, nickel hydroxides, zirconium hydroxides are unable to be filtered and removed.So, in this way, the purity of alumina can’t reach to 99.99%, 99.9% in fact. According to the test by Aluminum Research Institute of the Chinese Science Academy, the purity is 99.97%.Therefore; the method has much limitation on enhancing purity though its cost is low.
Now, the developed countries adopt aluminum iso-propoxide method, which is higher about 1～2 orders than the methods of alumina powder and water hydrolysis, decomposing ammonium aluminium Sulfate by Heating, Bayer. Specific process like this:
Firstly, as the reaction of block and alcohol is very fast ,use high purity alumina block and isopropyl alcohol or butanol or ethyl hydrate as materials instead of cutting metal alumina bark or block into powder with metal knives ,which will avoid ferric pollution taken by knives.
Secondly, to refine or purify, especially by distillation at 230-250℃ from aluminum iso-propoxide can collect middle high purity aluminum iso-propoxide liquid, with getting rid of former and bottom distillation impurities, which is the most important procedure to distinct from other methods. The liquid is got by condensing of aluminum iso-propoxide steam. The method will leave impurities such as Fe, Ni, Si, and Cu in the bottom of the reactor.
Thirdly, after distillation, to refine or purify further by ceramic membrane(which is made by high purity alumina of 99.999% purity) at 230℃can remove other impurities like K,Na,Zn.
Finally, use the high purity aluminium iso propoxide and high purity water as materials, after reaction, purifying by ceramic membrane at 150-160℃ will remove other impurities further. Next wash and clean it with water.
Why so many developed countries such as Japan(Sumitomo),America,Germany,China(Xuan cheng jing rui new material co.,ltd) adopt the method though it is dangerous and high-cost ?The only answer is high purity, which can make sure that the high purity alumina will growing sapphire wafer with high quality. It’s well known that impurities of 5N high purity alumina for growing LED sapphire wafer must be controlled in the range (Fe <6ppm,Si<5ppm,Zr <1ppm,Cu <1ppm,K <1ppm,Na <1ppm).Actually, the method is well controlled impurity content .Presently, Sumitomo,in Japan, produces the largest quantity of 5N (99.999%)alumina with the way. Their capacity is 1000Tons per year and price is 450-500 per kilogram. Their product is popular worldwide owing to high purity and quality. In China, xuan cheng jing rui new material co.,ltd,Hang Zhou wan jing new material co.,ltd adopt the advanced method since 1996 to produce high purity alumina. In the way, though the companies take a lot of detours and the purity is unqualified at first, they constantly make effort to explore technology and innovate. It takes nearly 6 years to resolve the purity, technique, equipment.Now their technology is rather mature, and own their independent intellectual property rights. Practice proves that their product of VK-L100G,VK-L100K can grow big and superior sapphire wafer.
Why 5N alumina is better than 4N （99.99%）in growing sapphire wafer? As we knows, alumina for sapphire wafer should be high—purity and bubble-free, bright in the process of growing wafers. Besides，the wafers must be lattice matching.Otherwise,the product can only be used as industrial jewel. So whether the wafers are in good quality depends on the purity of alumina. By experience, 4N (99.99%) alumina can’t grow sapphire wafers in high quality. Specific reasons are as follows:
1. The transparency of wafer is not white(always mix pink, yellow and so on) when some customers use 4N or 3N alumina powder to grow sapphire wafers. It is because too much impurities such as Fe, Si, Na, K, Ni can’t be got rid of.
2.Wafers are crystal and line defect, which will cause line defect of gallium arsenide when start epitaxial processing, subsequently, the lightening rate of the LED chips is only about 30%.On the other hand, the lightening rate of the LED chips is over 90% when the sapphire wafers grow from 5N high purity alumina.
3. In the view of profit, 1 kg crystalline bar grown from 5N high purity alumina can sell at $2380-$6349 per kilogram. While 1 kg crystalline bar grown from 4N high purity alumina can only sell at $65-$160 per kilogram.