Study On Spark Plasma Sintering Of Zinc Aluminate Ceramics And Its Properties

ZnAl₂O₄ spinel has many advantages,such as low thermal expansion coefficient,good thermal conductivity,high chemical stability,good hydrophobicity,strong mechanical properties,high quantum yield and low surface acidity.What's more,it is a good microwave dielectric material and is widely used in the field of microwave communications,especially as antennas,dielectric resonators and microwave substrates.In the paper,firstly,high-purity ZnAl₂O₄ nanopowders were synthesized by using high-temperature calcination method.Zinc sulfate heptahydrate?ZnSO₄·7H₂O?and aluminum ammonium dodecahydrate?AlNH₄?SO₄?₂·12H₂O?were used with a molar ratio of 1:2 to prepare the ZnAl₂O₄ nanopowders.Then X-ray diffraction?XRD?,scanning electron microscope?SEM?and transmission electron microscope?TEM?were used to characterize ZnAl₂O₄ powders.The results showed that the ZnAl₂O₄ nanopowders synthesized by high-temperature calcination method were pure ZnAl₂O₄ spinel phase with good dispersibility and had a mean particle size of about 200 nm,and the shape of the powders was elliptical and spherical.The laws of densification and grain growth of ZnAl₂O₄ ceramics were studied by spark plasma sintering?SPS?.At first,the ZnAl₂O₄ ceramics was sintered in the range of1000°C to 1350°C with two different heating rates of 10°C/min and 100°C/min and no holding time.The relative density,grain growth and micromorphology of ZnAl₂O₄ceramics were characterized using Archimedes method and SEM to study the laws of densification and grain growth of ZnAl₂O₄ ceramics in the sintering process.The results showed that when the sintering temperature was lower than 1200?,the rapid heating rate?100°C/min?was conducive to the densification of ceramics;while grains grew faster at low heating rate?10°C/min?when the sintering temperature was higher than 1200?.Then,ZnAl₂O₄ ceramics were respectively sintered at 1100°C and 1300°C for 5 minutes,10 minutes,15 minutes and 20 minutes with two different heating rates.The SEM was used to investigate the changes in grain size during the holding stage.When the sintering temperature was lower than 1200°C,the main sintering mechanism was surface diffusion;when the sintering temperature was higher than 1200°C,the grain boundary migration dominated.Finally,ZnAl₂O₄ transparent ceramics were prepared by SPS method.The effects of sintering temperature on the microstructure,microwave dielectric properties and mechanical properties of ZnAl₂O₄ transparent ceramics were investigated.With the sintering temperature rose from 1240°C to 1280°C,the average grain size increased from0.35?m to 0.76?m,and the relative density increased first and then decreased in the range of 98.54%to 99.81%.When the sintering temperature was 1260°C,the highest relative density was 99.81%.As the temperature went up,the values of Q×f and permittivity increased first to the maximum and then decreased,and when the sintering temperature was 1260°C,the optimal microwave dielectric properties were?_r=8.71,Q×f=64,395 GHz,and?_f=-68 ppm/?.The value of Vickers hardness was affected by relative density and grain size,and reached a maximum of 16.3 GPa at 1260°C.ZnAl₂O₄transparent ceramics with optimal optical properties,microwave dielectric properties and mechanical properties will be a new generation of optoelectronic integrated materials.