Microstructures And Properties Of ZnAl₂O₄-Based Low-Permittivity Microwave Dielectric Ceramics

Based on the development, application and research actuality of microwave dielectric ceramics, ZnAl₂O₄-based ceramics are chosen as research objective in this dissertation. The relationships among preparing methods (including solid-state reaction and molten salt method) and processes, phase compositions, microstructures and microwave dielectric properties have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), electron-probe microanalysis (EPMA) and network analyzer, etc.Sintering characteristic, phase compositions and microwave dielectric properties of (1-x)ZnAl₂O_4-xTiO₂ ceramics had been studied. It shows that TiO2 can effectively improve the sintering characteristic and control the temperature coefficient of resonant frequency (Ï„f). (1-x)ZnAl₂O_4-xTiO₂ (x=0.21) ceramics with near-zeroÏ„f value includes smaller ZnAl₂O₄ spinel grains and larger rutile grains. And then, the sintering processes such as calcining temperature, sintering temperature and heating rate of the (1-x)ZnAl₂O_4-xTiO₂ (x=0.21) system have been researched systematically, and the optimal sintering process parameters have been determined which are 1150℃, 1500℃and 5℃/min, respectively, in which sintering conditions the microwave dielectric properties are as follows:εr=11.6, Q·f=74000GHz,Ï„f=-0.4ppm/℃.Sintering characteristic, microstructures and microwave dielectric properties of (1-x)ZnAl₂O_4-xTiO₂ ceramics synthesized by molten salt method had been explored. In contrast to the solid-state reaction, smaller size, more homogenous and higher activity powder can be synthesized by LiCl and ZnCl₂ molten salt, which lowers the sintering temperature, at the same time, also deteriorates the density and microwave dielectric properties of the (1-x)ZnAl₂O_4-xTiO₂ ceramics. (1-x)ZnAl₂O_4-xTiO₂ (x=0.21) calcined at 900℃in LiCl molten salt can be fully densified at 1425℃, and the microwave dielectric properties withεr=10.0, Q·f=39970GHz andÏ„f=-21.2ppm/℃can be obtained. When x is equal to 0.25, the densification temperature reduces to 1300℃, andÏ„f value is adjusted to -7.1ppm/℃and theεr value nearly doesn't change, however, the Q·f value reduces to 27000GHz. (1-x)ZnAl₂O_4-xTiO₂ (x=0.25) calcined at 900℃in ZnCl2 molten salt can be fully densified at 1350℃and the ceramics exhibits the microwave dielectric properties withε_r=10.0, Q·f=56440GHz andÏ„f=-25.4ppm/℃.When MO(M=Co, Mg, Mn) and TiO₂ were added to ZnAl₂O₄, the phase compositions, microstructures and microwave dielectric properties of (1-x)ZnAl₂O_4-xM₂TiO₄ (x=0.21) ceramics were compared with those of (1-x)ZnAl₂O_4-xTiO₂ (x=0.21) ceramics. It is found that the single phase solid solution appears in the (1-x)ZnAl₂O_4-xCo₂TiO₄ (x=0.21) ceramics because Co²⁺ ion can promote Ti⁴⁺ ion to diffuse into the ZnAl₂O₄ crystal lattice which increases the Q·f value from 74000 to 94000GHz. However, the MgTi₂O₅ and ZnMn₃O₇ phase can be observed in the (1-x)ZnAl₂O_4-xMg₂TiO₄ (x=0.21) and (1-x)ZnAl₂O_4-xMn₂TiO₄ (x=0.21) ceramics, whose Q·f value is 188540GHz and 23530GHz, respectively. Theεr andÏ„f values of the (1-x)ZnAl₂O_4-xM₂TiO₄ (x=0.21) are lower than those of (1-x)ZnAl₂O_4-xTiO₂ (x=0.21) ceramics, whereas theεr andÏ„f values are weakly dependent on the M element and are equal to about 9.7 and -65ppm/℃, respectively.Phase transition and microwave dielectric properties of (1-x)ZnAl₂O_4-xM₂TiO₄ (M=Co, Mg) ceramics had been further studied. In the (1-x)ZnAl₂O_4-xMg₂TiO₄ system, ZnAl₂O₄ can form a spinel solid solution with Mg2TiO₄ for x=0.1, while the MgTi2O5 and MgTiO₃ second phase exists in the systems for 0.21≤x≤0.8 and 0.9≤x≤1.0, respectively. In the (1-x)ZnAl₂O_4-xCo₂TiO₄ system, ZnAl₂O₄ can also form a spinel solid solution with Co2TiO₄ for 0.1≤x≤0.3, however, the Co2TiO₄ second phase appears in the ceramics for x=0.4. When x is equal to 0.5, Co2TiO₄ and (Zn, Co)Al₂O₄ phase coexist and grow alternately in the (1-x)ZnAl₂O_4-xCo₂TiO₄ matrix. As the content of M2TiO₄ increases in the (1-x)ZnAl₂O_4-xM₂TiO₄ system, theεr value takes on a tendency of linear increase, and the Q·f value increases initially and then decreases, while theÏ„f value only changes slightly.The effects of TiO₂, CaTiO₃ and SrTiO₃ additions on theÏ„f value of the 0.79ZnAl₂O₄-0.21M2TiO₄ (M=Mg, Co) ceramics had been investigated. TiO₂ cannot control theÏ„f value of the 0.79ZnAl₂O₄-0.21M2TiO₄ (M=Mg, Co) ceramics due to the MTi2O5, formed by the reaction of TiO₂ with the matrix, with relative high negativeÏ„f value, however, CaTiO₃ and SrTiO₃ can adjust effectively theÏ„f value. In general, with the increasing amount of CaTiO₃ (or SrTiO₃), bothεr andÏ„f values increase gradually, while, the Q?f value has a tendency of decrease.The influences of ZnB₂O₄ and B₂O₃ sintering aids on the sintering characteristic, phase compositions, microwave dielectric properties of (1-x)(0.79ZnAl₂O₄-0.21Co2TiO₄)- xCaTiO₃(x=0.08)(ZCC) and (1-x)(0.79ZnAl₂O₄-0.21Mg₂TiO₄)-xCaTiO₃(x=0.06)(ZMC) ceramics had been studied. When ZnB₂O₄ is added to ZCC system, the sintering temperature is reduced to 1100℃, which is 300℃lower than the sintering temperature of undoped ZCC. When B₂O₃ is added to ZCC system, or ZnB₂O₄ (or B₂O₃ ) is added to ZMC system, the sintering temperature only lowers 50～100℃. In comparison with the undoped ZCC (or ZMC), theεr values of the doped system all reduce, and the Q-f andÏ„f values are dependent on the category and amount of additions. On the whole, ZnB₂O₄ addition greatly reduces the Q-f value of the ZCC (or ZMC) ceramics, while B₂O₃ addition does not affect obviously on the Q-f value.