Crystal Structure And Microwave Dielectric Properties Of Novel Low-Permittivity Li₂O-Al₂O₃ Ceramics For LTCC Application

In order to meet the requirements of increasingly high-performance devices for communication technology,the research on microwave dielectric ceramics,which is the key material of microwave devices,has been extremely active.In this paper,a novel type of low-permittivity Li₂O-Al₂O₃ series microwave dielectric ceramics is used as the research materials.The ion substitution modification,two-phase combination,and adding of low melting point oxides materials methods were used to obtain good properties of LiAl₅O₈ based microwave dielectric ceramics and LiAlO₂ based low temperature co-fired ceramic(LTCC).Moreover,through X-ray diffraction,scanning electron microscopy,Raman spectroscopy,impedance spectroscopy,network analyzer and other testing methods,a large number of systematic work and innovative research have been made between the material's crystal structure,phase composition,micro morphology,and microwave dielectric properties.LiAl_5-xZn_xO_8-0.5x and LiAl_5-xNi_xO_8-0.5x microwave dielectric ceramics were prepared through solid-state reaction method.It was found that when the substitution content was x?0.3,the phase structure of the material changed from P4₃32 to Fd-3m space group.In addition,as the substitution content increased,the lattice constant took on an increasing tendency.Both Zn²⁺ion and Ni²⁺ion acceptor substitution promoted the sintering density of LiAl₅O₈ materials,and it was accompanied by the tendency of grain growth.The calculation using the Clausius–Mossotti equation showed that the substitution of Zn²⁺ion and Ni²⁺ion led to a decrease in the dielectric constant,which is consistent with the experimental results.Partial substitution of Al³⁺ions by Zn²⁺ion and Ni²⁺ion acceptors in LiAl₅O₈ promoted the transformation of the lattice structure.But at the same time,the increased oxygen vacancies and the decreased ordering degree in LiAl_5-xZn_xO_8-0.5x(0.3?x?0.5)materials had a negative impact on the quality factor value.Both the Zn²⁺ion and Ni²⁺ion acceptor substitution of LiAl₅O₈ materials led to the increase of temperature coefficient of resonant frequency in the negative direction.The sintering temperature of LiAl₅O₈ materials substituted by Zn²⁺ion and Ni²⁺ion was still high,so this thesis continued to study LiAlO₂ microwave dielectric ceramics with relatively low sintering temperature in Li₂O-Al₂O₃ series materials.The material was prepared at sintering temperature range from 1100°C to 1300°C,but it had shortcoming of poor density and water absorption.Dense ceramic materials were obtained by replacing the Zn²⁺ions at the B site and sintering at 1450°C,which could inhibit the water absorption of LiAlO₂ materials.When x=0.06,the dielectric constant reached a maximum of 6.45.However,the substitution of Zn²⁺ions caused oxygen vacancies in LiAlO₂ materials,and the conductance loss was also increased which was testified by impedance spectroscopy analysis.Due to the combined effect of compactness and oxygen vacancies,the optimal quality factor of LiAl_1-xZn_xO_2-0.5x microwave dielectric ceramics was obtained at x=0.02 of 27987 GHz.The temperature coefficient of resonance frequency is related to the bond energy.With the increase of Zn²⁺ion substitution content,the bond energy of LiAl_1-xZn_xO_2-0.5x microwave dielectric ceramics decreases,so the thermal stability of the material decreases,and the temperature coefficient of resonant frequency showed a trend of increase in the negative direction.In order to further reduce the sintering temperature of LiAlO₂ microwave dielectric ceramics and oxygen vacancies introduced by the substitution of divalent Zn²⁺ions,the trivalent[Zn_0.5Si_0.5]³⁺ion and[Mg_0.5Si_0.5]³⁺ion doping methods were used to continue to improve the LiAlO₂ material.Compared with the Zn²⁺ions doped LiAlO₂ material,the sintering temperature was continuously reduced from 1450°C to 1300°C,and the quality factor was also improved.The results showed that the quality factor of the[Zn_0.5Si_0.5]³⁺ion substitution material was better than that of[Mg_0.5Si_0.5]³⁺ion substitution.For the doping of[Zn_0.5Si_0.5]³⁺ions,the best performance was obtained at x=0.02,and the microwave dielectric properties were?_r=6.12,Q×f=56 986 GHz,and?_f=-122 ppm/°C.As the Ca TiO₃ material had the characteristic of positive temperature coefficient of resonant frequency,adding Ca TiO₃ material in the LiAl_0.98(Zn_0.5Si_0.5)_0.02O₂ microwave dielectric ceramic not only adjusted the temperature coefficient of resonant frequency to near zero,but also reduced the sintering temperature to1150°C.In order to further reduce the sintering temperature,adding a low melting point HBO₃material with a mass fraction of 7 wt.%reduced the sintering temperature to 900°C.There was no chemical reaction when the material was co-fired with the silver electrode,and the microwave dielectric properties were?_r=9.39,Q×f=10 224 GHz,?_f=-7.8 ppm/°C.Thus,a novel low-permittivity microwave dielectric ceramic material meeting the requirements of LTCC was prepared.