| With the development of the infrared technology, infrared window materials demand more excellent performance on various aspects including optical properties, mechanical performance and thermal stability. On one hand, high infrared transmittance is required; on the other hand, good mechanical performance is essential. Y2O3-MgO composites possess relative high infrared transmittance within 3～7μm wavelength range, and excellent mechanical performance, impact resistance and erosion resistance, which can be utilized as a potential infrared window material.In the present work, Y2O3-MgO nanopowders were synthesized by microwave-induced combustion method. The effects of binary and ternary raw material system on the particle size, particle distribution and morphology of the powders were investigated. High-performance Y2O3-MgO ceramics were prepared by two-step microwave sintering process using as-synthesized Y2O3-MgO nanopowders as raw materials. The influences of heating rate, sintering temperature and holding time on the performance of microwave sintered Y2O3-MgO ceramics were studied and compared with those prepared by conventional sintering technology. The main research contents are as follows:Yttrium nitrate, magnesium nitrate, magnesium acetate and yttrium acetate were used as raw materials. One yttrium salt and one magnesium salt were chosen to form xerogel via sol-gel method. The xerogel was calcined at 600℃ for 2 h to prepare Y2O3-MgO nanopowders. The results show that the powders prepared using only acetate system as precursor were inhomogeneous with mean crystalline grain size of over 80 nm. The powders prepared using only nitrate system as precursor were seriously agglomerated with mean crystalline grain size of around 50 nm. The powders prepared using nitrate- acetate system as precursor were uniform with mean crystalline grain size of around 40 nm. Using yttrium nitrate and magnesium nitrate as oxidant and magnesium acetate as reductant, the effect of the reductant /oxidant ratio on the particle size, morphology and phase composition was investigated. The results present that the powders had a mean crystalline grain size of 30 nm and disk-shape agglomerates appeared when the reductant /oxidant ratio was 0.8:1; the powders were uniformly spherical-like and well-crystallized as pure phase with mean crystalline grain size of 35 nm when the reductant /oxidant ratio was 0.9:1; the mean crystalline grain size of the powders was slightly increased to be 40 nm when the reductant /oxidant ratio was 1:1; there were residual organics in the powders when the reductant /oxidant ratio was 1.1:1. All the results indicate that it is beneficial for the improvement of uniformity and reduction of grain size when the oxidant amount is slightly higher than reductant amount in the precursor. Using the system of reductant /oxidant ratio of 0.9:1 as the precursor solution, Y2O3-MgO nanopowders were synthesized by microwave-induced combustion method. The results show that the Y2O3-MgO nanopowders prepared by microwave-induced combustion method was homogeneous with grain size of less than 30 nm and specific surface area of 35.6 m2/g. Compared with sol-gel method, the powders prepared by microwave-induced combustion method had higher activity and better distribution. Besides, microwave-induced combustion method is time-saving and energy-saving.Y2O3-MgO green bodies were prepared using Y2O3-MgO nanopowders synthesized via microwave-induced combustion method. The green bodies were put in tube furnace and microwave furnace respectively and heated to different temperatures with heating rate of 10℃/min, and then cooled along with furnace. The relative density, grain growth exponent and sintering activation energy were calculated, and the mechanism of conventional sintering and microwave sintering was analyzed. In addition, the influence of heating rate, sintering temperature and holding time on microwave sintered Y2O3-MgO ceramics were studied. The research indicates that when the sintering temperature is below 1300℃, grain boundary diffusion is the main migration mechanism for microwave sintering, while volume diffusion is the main migration mechanism for conventional sintering. As a result, microwave sintering is advantageous to obtain fine crystallite at the same sintering temperatures. The calculated sintering activation energy of microwave sintered samples (108.22 kJ/mol) is much lower than that of conventional sintered ones (160.42 kJ/mol), indicating that microwave sintering process can reduce the sintering temperature of Y2O3-MgO ceramics. The results also reveal that excessively fast heating rate led to the appearance of pores and was bad for the densification of ceramics; while too slow heating rate led to the waste of energy. Too high sintering temperature and too long holding time both likely caused abnormal grain growth. The optimal sintering parameters were determined to be heating rate of 10℃/min, sintering temperature of 1400℃ and holding time of 2 h. The relative density of as-prepared Y2O3-MgO ceramics can reach up to 99.6%.Based on the study of densification process for microwave sintering of Y2O3-MgO ceramics, two-step sintering parameters were determined:in the first step, the sintering temperature was 1375℃ for lmin with heating rate of 10℃/min; in the second step, the sintering temperature was sharply cut to 1300℃ with holding time of 3 h. The as-prepared Y2O3-MgO ceramics possess average grain size of 800 nm, relative density of 99.7%, Vickers hardness of 11.7±0.3GPa and infrared transmittance of 50-60% within 3-5 μm wavelength.In this study, infrared window material, Y2O3-MgO ceramics were prepared by microwave sintering. The preparation technology is simple, low cost, time-saving and energy-saving, which has potential application prospects in the infrared window field. |
PDF Full Text Request