| Yttrium aluminium garnet(Y3Al5O12,YAG) and Yttrium oxide(Y2O3) ceramics, who have excellent optical properties, have been widely studied and reported to be the important laser matrix materials. However, the researches about the microwave dielectric properties of the YAG and Y2O3 ceramics have not been reported. In this dissertation, in order to expand the application areas of the transparent ceramics and obtain the ceramics with good both optical and microwave dielectric properties, the YAG and Y2O3 materials are chosen to be as the experimental objects. Firstly, morphology characteristics of YAG and Y2O3 nanopowders prepared by improved processes were discussed. Then, the YAG and Y2O3 ceramics with good optical and microwave dielectric properties were prepared by using the spark plasma sintering(SPS). On this basis, the effects of powder microstructure, sintering parameters (heating rate, sintering temperature, soaking time and pressure way), sintering aid(LiF) and doped Nd3+on optical, microwave dielectric and mechanical properties were analyzed systematically. Some mechanisms, involved during the powder synthesis and ceramic preparation, were studied deeply. Impact factors on microwave dielectric properties of the samples were discussed in detail. Finally, the interrelationships among the preparation-microstructure-dielectric properties-optical properties were established. The experimental foundation for exploiting the microwave dielectric properties of the transparent ceramics was offered. The specific research Content as follows:(1)YAG powders were prepared by thermal decomposition method. The morphology characteristics of YAG powders with different raw materials were analyzed systematically. When using aluminum nitrate, aluminum sulfate and yttrium nitrate(1.5:2:3) as raw materials, YAG powders were synthesized at 1000?. And YAG powders showed good dispersion and semispherical shape. Alumina obtained by the decomposition of aluminum nitrate can be used as seed crystal to reduce the synthesis temperature. Sulfate radicals obtained by the decomposition of aluminum sulfate can decrease the elements diffusion between particles and then increase the dispersibility of the powders. This method is simple for operating and can avoid the loss of a small amount of doping ions.YAG powders were prepared by co-precipitation methods, using ammonium hydroxide as as precipitant. Powders with narrow size distribution in 50-120nm, clear grain boundaries, semispherical shape, and good dispersion were prepared at 1000?. With the increase of temperature, the phenomenon of sintering neck and reunion obviously increased. The operations of ethanol soak and adding dispersant were both effective to improve the dispersion of powders. YAG powders with large particle size and uniform particle size distribution were prepared by ball milling method.(2) The microwave dielectric properties of YAG ceramics prepared via thermal decomposition methods and subsequently muffle furnace sintering were tested, as early-stage preparation for transparent microwave ceramics. With the increase of sintering temperature, the quality factor Qxf values of YAG ceramics increased and then decreased. At 1600?, the maximum Qxf value was 59 682 GHz. TEOS addition promoted the sintering of ceramics. The Q×f value of YAG-TEOS ceramic at 1520? was 83 278 GHz. The TiO2 can optimize Tf value, but make Q×f value decrease.(3) YAG powders as the raw materials obtained by three different methods were used to prepare YAG transparent ceramics. At the same time, we systematically studied the influences of SPS sintering processs (sintering temperature, holding time, heating rate and pressure mode) and sintering aid on the microstructure, optical performance and microwave performance of ceramics. Among them, the transparent ceramics obtained by coprecipitation showed high transmittance. And thus, the best sintering process was determined. The in-line transmittance of YAG transparent ceramics(h?3mm) was as high as 80.1% at the wavelength of 1064 nm, when the sintering temperature was 1330? with a heating rate of 15?/min and a soaking time of 20 min under two steps pressure mode(80MPa).And the optimal microwave dielectric and optical properties for YAG transparent ceramics(h?5 mm) produced by spark plasma sintering at 1360? were Qxf=89 810 GHz, ??=10.63, ?f=-51.4 ppm/?, and 82% at 1064nm. The values of Q×f and the transmittance at the wavelength of 1064 nm of 1 at.%Nd:YAG ceramics were 65 644 GHz and 77.1% at 1360?. The laser activity of Nd:YAG transparent ceramic (3mmx 3mmx3mm) was studied by using of LD end pumped experiment platform. When absorption power was 2 W, the values of output power and optical to optical conversion efficiency were about 0.22 W and 11%. The values of vickers hardness of the transparent YAG ceramics sintered at 1330? and 1350? were 14.5 GPa and 14.6 GPa.(4) Y2O3 powders were prepared by coprecipitation, using NH4HCO3 as precipitan. The dispersion should go up if the temperature goes up. Y2O3 powders with good dispersion, and highly crystalline were calcined at 1000?. The average particle size was about 80 nm.Y2O3 powders were prepared by using combustion method with polyacrylic acid(PAA) as fuel at 400?. The influences of fuel content and sintering temperatures on the morphology were analyzed. When yttrium nitrate:PAA(YN:PAA)=7:2 as starting materials, the powders calcined at 500? showed good dispersion.(5) Y2O3 ceramics obtained by coprecipitation combined with muffle furnace sintering. The maximum of Qxf value of Y2O3 ceramics prepared at 1500? was 82 188 GHz. The Qxf values of 2at.% and 5 at.%Nd:Y2O3 ceramics sintered at 1500? were 93 111 GHz and 66 270 GH, respectively. Appropriate neodymium ions can improve microwave dielectric properties, but excessive doping concentration can cause the decrease of microwave performance parameters.6) Y2O3 transparent ceramics were prepared at 1300? for 20 min by precipitation combined with SPS sintering technique. The transmittance in 1100 nm was 69.7%. Samples sintered at 1350? showed many pores and low relative density, so the optical transmittance was decreased. Y2O3 transparent ceramics also showed good light penetration in infrared region. The cutoff wavelength is around 8.0?m. The Q×f of Nd:Y2O3 transparent ceramics sintered at 1330? was 72 220 GHz, using SPS. |
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