| In this dissertation, a laser scanning method was developed to metalize the surface of alumina substrate with copper. This method can be proceeded at room temperature rapidly and economically. Transparent ceramic microspheres of Gd2O3, YAG:Ce, and Al2O3 have been successfully fabricated via laser heat treatment. Their microstructure, optical and luminescent properties have been studied, and the main results are listed as follows.A laser scanning method was developed to metalize the surface of alumina substrate with copper. Micro-morphology and phase structure were investigated systematically. The bonding strength between alumina substrate and Cu layer was also measured. Laser heating undergoes melting and solidification processes. A temperature-depth model on the relationship between the temperature and the depth was defined. A higher temperature was found at a less depth. In this process, the laser energy was first absorbed by alumina and then transferred to the Cu powder, which results in melting. When the power was off, the temperature dropped rapidly to the room temperature. The surface of metalized alumina substrate is composed of copper spheres (about 30-50 μm in diameter), which embedded into the alumina substrate, and at the interval of the copper sheres a thin layer of CuAlO2 exists on the alumina surface. The modeling of heating and cooling rates of the samples during the laser scanning process indicates that the metallization process involves rapid melting, solidification of Al2O3 and Cu, and evaporation and deposition of some of the Cu. Material splashes due to the rapid melting leads to the splashing of Cu particles (about 200 nm) into the alumina substrate, forming a 4 μm thick splashing layer. So the ways of mass transfer can be divided into three categories:at the low temperature, diffusion takes charge; at high temperature, splashing becomes the key mechanism; at very high temperature, evaporation and deposition prevails.The welding performance of copper metalized alumina was investigated. The metalized alumina surface can be easily soldered to a Cu plate using a solder tin at 300℃. The copper-plate/solder-tin/metalized-alumina sample has well bonded interfaces, and no pores and other defects were observed, indicating that the solder tin can wet metalized alumina substrate at the soldering temperature. The good wetting of solder tin to the metalized alumina substrate is attributed to the existence of copper spheres and a thin layer of CuAlO2 on the surface of the alumina substrate. We also investigate the influence of scanning times, scanning speed, the degree of oxidation and the containment of second phase on the microstructure and welding shear strength of the samples. As the scanning times increase, the size of the surface cracks and the oxygen content on the surface of copper spheres increase, and the interfacial shear strength decreases,.As the scanning speed increase, the coverage of copper, the content of oxygen on the surface of copper spheres and the binding force between copper and ceramics decrese, the shear strength increases at the beginning and then decreases, and the max shear strength is about 26.1MPa.In fracture analysis, there are three basic fracture characteristics:the fractures in ceramics, the fractures between ceramics and solder, and fractures between ceramics and copper spheres. The microstructure of the samples fracture is the mixure of these three basic fracture characteristics. The propotion of these fracture changes as the scanning speed changes. The interphase of Cu-Sn interface include Cu6Sn5 and Cu3Sn which enhance the binding force of copper and solder.The thermal conductivity performance of copper metalized alumina is also investigated. The copper-plate/solder-tin/metalized-alumina sample has a similar thermal conductivity coefficient value to that of alumina substrate, which is about 8 times that of rubber bonded alumina substrate, indicating an excellent thermal conduction performance. Therefore, this method provides a convenient way for bonding ceramic and metal.Transparent ceramic with chemical composition of monoclinic Gd2O3 is successfully fabricated by laser melting method. The fabrication of Gd2O3 transparent microspheres consists of three stages:melting, pore elimination and cooling solidification. Simple models were used to describe these stages, and high-temperature monoclinic phase can be obtained at room temperature mainly due to the fast cooling rate. Because the difference between ordinary refractive index no and extraordinary refractive index ne is slight, the volume size of the spheres is small, and the light refraction loss is also slight, the pores-free monoclinic Gd2O3 is transparent and the in-line transmittance of transparent Gd2O3 microspheres is up to 44% in visible light range. A scintillator array like film can be obtained by mixing microspheres and water glue, and then dried in air. This film can be wildly used in the application of scintillator devices.The YAG:Ce3+ and Al2O3 microspheres were fabricated by laser heat method, and the macro-/micro-morphology and phase structure were systematically investigated. The transparency can be varied by change the model. The employed microspheres is about 30-60μm in diameter. YAG:Ce3+ microspheres are polycrystalline and composed of randomly aggregated crystallites. The average size of the crystallites determined from the SEM image is about 2.5um. Calcing YAG:Ce3+microspheres at 1200℃ for 2h can optimized their crystallization and luminescence property. Upon the excitation at 470 nm, the emission intensity of YAG:Ce3+ at 535nm. A higher emission intensity was found for the sample calcined at 1200℃ for 2h, owing to the perfect crystallization. The luminescent quenching concentration of Ce3+ was found to be 2 at% for YAG:Ce3+. |
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