Researches Of Yellow Spots Defect On ZnS-SiO₂ Ceramic Target Material

ZnS-SiO₂ ceramic is the target material that is widly used in rewritable CD-RW, DVD+/-RW discs. In the surface of ZnS-SiO₂ targets which made with hot isostatic pressing technology, yellow spots defect alwayse occured.The optical micrograph of the spots area was not different from normal area. The content of impurities was too low to be checked by the analysis technic such as EDS. In this paper, the reasons yellow spots occured and the methods to be avoided were probed from two aspects, one was high purity ZnS powder was manufactured by myself, another was the impurities of commercial ZnS powder were reduced.Single phase nanometer ZnS powder was manufactured with together-deposition method.This powder was contrasted with the purified commercial powder in optical micrograph, phases composing ,thermal stability. The density, microhardness and fracture toughness of sintered ZnS-SiO₂ ceramics that respectively used two kind of ZnS powder were contrasted .The homemade ZnS powder is α-ZnS. It is difficult to control the reaction speed, due to experiment conditions. So, the size of ZnS powder was asymmetric. Because the cleanout of ZnS powder was inadequate, there were some Na₂S and ZnCl₂ remained wich made its thermal stability was not well.The size of ZnS powder was excessively small,therefore,it is easy to reuniteduring the course of dryness. After isostatic pressing,the microstructure was rough and asymmetric, so its actual density,microhardness and fracture toughness were relatively lower than the commercial ZnS powder's sample. It is showed that more researches should be done to gain the ZnS powder product which has a preferable size , equality structure, high purity and easily controlled production conditions.Two methods were taken to purify the commercial ZnS powder,one was roasting at a high tempreture, another was cleaning with hydrochloric acid which ph equaled to 3.After roasting at high temperature, the content of sulphur and SO42" in this ZnS powder were both reduced. Industry production proved that the amount of yellow spot in the surface of ZnS^SiO2 targets was greatly decreased after roasting the raw ZnS powder by cleaning the commercial ZnS powder with hydrochloric acid and distilled water,the amount of impurities such as ferrum had a little decreasing.As the process of cleaning was very complex, and needs a great deal of hydrochloric acid and distilled water, it should not be adopted in producing.The result shows, the main reason of yellow spots appeared is pure S and SO/'remained in the raw ZnS powder. Adopting to roast the raw material at high temperature can greatly reduce pure S and S042^content to reach the goal of reducing or avoiding the yellow spots.