Study On Microwave Dielectric Properties And Low-temperature Sintering Of Novel Wolframite-type Ceramics

Developing new ultra-low loss microwave dielectric ceramic material has always been a hot research topic in the field of material science. Recently, great attention has been paid to AZrNb2O8, a new class of materials with wolframite crystal structure due to their excellent microwave dielectric properties. However, the sintering temperature of AZrNb2O8 ceramics are above 1200℃, making them unavailable for the application of microwave devices. Low-temperature sintering and microwave dielectric properties of AZrNb2O8-based ceramics have not yet been reported so far.Firstly, the sintering process, crystal structure and microwave dielectric properties of AZrNb2O8 （A:Mg, Zn, Co） ceramics were studied. Secondly, the influence of ions substitution, low temperature sintering and the chemical compatibility between ceramics and Ag （Ag/Pd） electrodes were discussed. The important results achieved in this dissertation are listed as follows:（1） The sintering processes of MgZrNb2O8, ZnZrNb2O8 and CoZrNb2O8 ceramics were optimized and the relationship between microstructure and microwave dielectric properties was thoroughly investigated. MgZrNb2O8 ceramics with the optimal performance of εr25.67, Q×f=109400GHz, Ï„f=-48.78ppm/℃ can be achieved when sintered at 1325℃; ZnZrNb2O8 ceramics sintered at 1200℃: î–¸=28.8, Q×f=81520GHz, Ï„f=-52.95ppm/℃ and CoZrNbO8 ceramics sintered at 1225℃:εr=26.41, Q×f=44766GHz, Ï„f=-44.28ppm/℃. The crystal structure of these materials was determined by Rietveld refinement. We found that the εr, Qf and Ï„f of these compounds are correlated with the ionic polarizability, packing fraction and octahedral distortions, respectively.（2） Mgi-xZnxZrNb2O8 （x=0.1,0.3,0.5,0.7,0.9） ceramics were prepared and we found that ZnO dopant can effectively lower the sintering temperature. Single phase Mg0.5Zn0.5ZrNb2O8 can be obtained and it shows a wolframite crystal structure. Mg0.5Zn0.5ZrNb2O8 ceramics have a wider singtering temperature ranged between 1175℃ and 1275℃, and the samples sintered at 1200℃ exhibited excellent microwave dielectric properties of εr=27.24,Qf=75399GHz, Ï„f=-63.88ppm/℃. The crystal structure and crystallographic parameters of Mg0.5Zn0.5ZrNb2O8 were achieved by Rieteveld refinement. The influence of Mg/Zn molar ratio on the crystal structure, microstructure and microwave dielectric properties of these compounds was aslo studied.（3） Zn1-xCoxZrNb2O8 （x=0.1,0.3,0.5,0.7,0.9） ceramics were prepared. For all the composition, dense ceramics with relative densities above 94% can be obtained when sintered at 1175℃-1250℃. The crystal characteristics of all the compounds were revealed by Rieteveld refinement. The influence of Zn/Co molar ratio on the microstructure and microwave dielectric properties of these compounds was investigated. Due to the variation in the ionic polarizability and packing fraction of Zni-xCoxZrNb2O8 ceramics, both the εr and Qf values reduce with the Co concentration. The well-sintered Zn1-xCoxZrNb2O8 ceramics shows highly promising microwave dielectric properties of these compounds was investigated. Due to the variation in the ionic polarizability and packing fraction of ZmxCoxZrNbiO₂ ceramics, both the and of values reduce with the Co concentration. The well-sintered Zni-xCoxZrNbaOg ceramics shows highly promising microwave dielectric properties: Sr=26.5926.1; 45146⁶5290GHz; 1.13^-44.28ppm/&-174C.（4） The effects of BaCu（B205） doping on the phase structure, sintering characteristics and microwave dielectric properties of ZnZrNbsOs ceramics were investigated. A single phase ZnZrNbaOg is formed when the BaCu（B205） content is<3wt%. A small amount of BasNbsOii and BaNbaOe second phases occurred with further increase of BaCu（B205） content. The BaCu（B205） addition can effectively lower the sintering temperature of ZnZrNbzOs ceramics from 1200°C to 950°C due to liquid phase sintering. ZnZrNbsOs ceramics with 3wt% BaCu（B205） sintered at 950°C for 4h could reach dense structure and exhibited good microwave dielectric properties（Sr=28.4,Q x/=56720GHz, and xf =-53ppm/°C）. In addition,the ceramics are compatible with Ag electrodes, and therefore, suitable for LTCC application.（5） The effects of BaCu（B205） doping on sintering characteristics, phase constitute,and dielectric properties of MgZrNbaOg ceramics were investigated. Results indicated that a small amount of BaCuCBiOs) can effectively lower the sintering temperature of MgZrNb208 ceramics from 1300。C to 1100。C due to liquid phase sintering effect.However, the second phase of BasNbgOai appeared when the addition of BaCu（B205）exceeds 2wt%. Typically, 2wt% BaCu（B205）-added MgZrNbaOs ceramics sintered at1100°C for 4h could reach dense structure and exhibited microwave dielectric properties of ï¿¡>=25.96,Q x/=65064GH2（f=8.68GHz）, and r/=-47ppm/°C. Moreover,this ceramic has a chemical compatibility with Ag70/Pd30 electrodes.