| In order to broaden the application field of transparent ceramics, and obtain transparent ceramics with high transmittance in visible-and IR-wavelength even extending to microwave ranges, the ceramics with spinel structure including MgAl2O4Mg1-xZnxAl2O4and ZnAlO4were chosen as the research objective. Not only the synthesis process and structure characteristics of raw powders, but also the effects of Spark Plasma Sintering (SPS) processes parameters and annealing process on phase compositions, microstructures, optical properties and microwave dielectric properties of spinel-based transparent ceramics have been investigated by X-ray diffraction (XRD), thermogravimetric and differential thermal analysis (TG-DTA), scanning electron microscopy (SEM), transmission electron microscope(TEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), positron annihilation lifetime spectroscopy (PALS), Ultraviolet-visible spectroscopy (Uv-Vis), Fourier Translation Infrared Spectroscopy (FT-IR) and network analyzer, etc. The main contents include:At first, the reverse microemulsion method, hydrothermal method and high-temperature calcination method had been employed respectively to studty the best preparation conditions of nanosized MgAl2O4powders which could meet the requirements of the preparation of transparent ceramics. The results showed that the reverse microemulsion method has dramatically lowered the calcination temperature of MgAl2O4comparing to the classical solid-state reaction sintering. The surfactant and titration process have effects on the powder morphology and particle size:The MgAl2O4nanoparticles obtained from SPAN-80/Triton X-100compound emulsifier using forward-titration process have spherical shape, narrow size distribution, particle size of2～3μm and serious agglomeration, while reverse-titration process is difficult to control the shape of nanoparticles. The phase composition, powder morphology and particle size of MgAl2O4powders obtained from hydrothermal method are affected by the morphology control agent. Using oleic acid as morphology control agent, the powders are the single spinel phase with the uniform particle size and the average value is about4μm.MgAl2O4powders are the sea urchin in shape. The MgAl2O4powders prepared by the simple high-temperature calcination method powders have spherical shape. A mean particle size is about45nm. MgAl2O4powders exhibit good dispersion, narrow size distribution and little agglomeration, which can meet the sintering requirement of transparent ceramics.The MgAl2O4powders obtained from the above three methods had been used to fabricate transparent ceramics by spark plasma sintering. However, only the MgAl2O4ceramics using the powders from high-temperature calcination method were transparent. And then, the influences of sintering processes such as sintering temperature, heating rate, sintering time and sintering pressure on the microstructure, optical properties and microwave dielectric properties of MgAl2O4transparent ceramics had been studied systematically. The results showed that the in-line transmittance of MgAl2O4transparent ceramics can be as high as70%at the wavelength of550nm and82%at the wavelength of2000nm, respectively when the sintering temperature was1275℃with a heating rate of10℃-min-1and a soaking time of20min under a pressure of80MPa, and the results of the microwave dielectric properties were er=8.42, Qxf=20,000GHz and if=-76ppm/℃. While the optimal microwave dielectric properties (εr=8.38, Qxf=54000GHz, Tf=-74ppm/℃) were achieved for transparent MgAl2O4ceramics produced by spark plasma sintering at1325℃.The MgAl2O4transparent ceramics had been annealed to study the effects of annealing temperature on not only the concentration change of carbon and oxygen vacancies but also the optical and microwave dielectric properties. The results showed that with the annealing temperature increasing from800℃to1300"C, the concentration of carbon decreases, oxygen vacancies combine and their relative concentration declines, while the defect volume increases. With the increasing of the annealing temperature, the in-line transmittance at the wavelength of550nm at first increases then declines. The temperature of900℃is a critical annealing temperature for good optical properties with the high in-line transmittance74.9%. The Qxf values are enhanced after annealing and reached the highest value of52,640GHz at1200℃, The dielectric constants (εr) showed a downward trend with the annealing temperature from800℃to1300℃and the value decreased from8.27to8.06.The nano-sized (Mg1-xZnx)Al2O4powders were fabricated by high-temperature calcination method at first, and then (Mg1-xZnx)Al2O4transparent ceramics were sintered by spark plasma sintering. The effects of Zn content on the optical and microwave dielectric properties have been studied. With the increasing of Zn content from0.5at%to3.0at%, the in-line transmittance at λ-550nm first increases and then decreases, which reachs70%when x=0.02. A small addition of Zn (about2.0at%) could simultaneously improve the optical and microwave dielectric properties of MgAl2O4transparent ceramics. The optimized microwave dielectric properties (εr=8.54, Q×f=66000GHz and τf=-65.5ppm/℃) could be obtained at x=0.02.The ZnAl2O4nanosized powders have been prepared by high-temperature calcination method. For the first time, the Z11AI2O4transparent ceramics have been fabricated by Spark Plasma Sintering technology. The influences of sintering temperature and the additive content of TEOS on the optical properties of ZnAl2O4transparent ceramics have been discussed. It showed that with the increasing of the sintering temperature, the in-line transmittance of ZnAl2O4ceramics at the wavelength of550nm first increases and then decreases, and is only23%at the sintering temperature of1260℃. The addition of0.5wt%TEOS significantly improves the in-line transmittance to60%,64%and47%at the sintering temperature of1250℃,1260℃and1270℃, respectively. The in-line transmittance at the wavelength of2500nm is85%. With the content of TEOS increasing (0.3wt%,0.5wt%and1.0wt%), the in-line transmittance at550nm increase first, then decrease, and reaches the highest value of64%when the sintering temperature is1260℃and the TEOS content is0.5wt%. |
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