Study On The Synthesis And Properties Of Titanium Oxides Based Middle And High Permittivity Dielectric Ceramics

Nowadays,as communication technology develops,the efficient,small and highly stabilized communication device is highly needed.As one of the key materials to be used in electronic device fabrication,dielectric ceramic materials are also faced with big challenges.The dielectric ceramics are used in capacitors,dielectric resonator,dielectric substrate and dielectric filters.In recent years,as the rapid expansion of the electronic industry in home appliances,auto cars,and military,more dielectric materials are needed.Furthermore,the popularity of 5G communication and the outlook of 6G communication,a high quality of dielectric materials has been put forward.CaCu₃Ti₄O₁₂?CCTO?ceramics have a huge dielectric constant,good temperature stabilization,and stabilization of dielectric constant in frequency between 10²Hz to10⁵Hz.So that,we firstly studied the CCTO ceramics in this dissertation.However,the dielectric properties of CCTO decrease rapidly when using frequency beyond 1MHz.In order to meet the development of communication at higher frequency,we have made a further study on NiTiNb₂O₈ ceramic,which have a higher dielectric constant in microwave frequency.The exact studies are as follows:We used the La³⁺ion to substitute on the Ca²⁺site or Cu²⁺site of CCTO ceramics,respectively.Then,we studied the influence of substitution on dielectric properties of CCTO ceramics.The substitution of La³⁺ion on the Ca²⁺site can make the sintering temperature lower than that on the Cu²⁺site.Which makes Ca²⁺site substitution of CCTO ceramic have a higher dielectric constant than that of Cu²⁺site substitution.Then we used the Sr²⁺doped on CCTO ceramic to study the effect of semi conductive grain and microstructure on the dielectric properties of CaCu₃Ti₄O₁₂ ceramics,the band gap of CCTO increased with Sr²⁺doping.According to the IBLC model,which indicates that Schottky-type barrier forms between the grain boundaries,the dielectric constant should increase with Sr²⁺doping,and the resistance of grain boundaries should increase together.However,the microstructure of Ca_1-xSr_xCu₃Ti₄O₁₂2 contributed to the dielectric properties were researched,the grain size became larger with Sr²⁺doping,thus the dielectric constants increased and grain boundary resistance decreased with Sr²⁺substitution.The results showed that Schottky-type barriers in the GBs were the factors to form the special dielectric properties of CCTO,and microstructure influence the dielectric properties notably.We used F^-ion to substitute on the O^2-site of CCTO ceramic to study the influence of oxygen deficiency on dielectric properties of CCTO ceramic.After the substitution,the grain size decreased and resistance of grain boundaries increased.According to IBLC theory,higher resistance of grain boundaries leads to lower dielectric loss tangent in CCTO related ceramics.Using the Complex electric modulus,we have calculated the activation energies for the relaxation behaviors,which increased with F^-ion substitution.The activation energies are correlated to barriers height,and F^-ion substitution increases the barriers height,thus the resistance of grain boundaries increased.The dielectric loss tangent of CaCu₃Ti₄O_12-xF_x ceramic was about 0.05 when x=0.8.The low loss Ni_1-xZn_xTiNb₂O₈?x=0.2,0.4,0.6,0.7,0.8?ceramics were prepared by the solid-state synthesis methods.The increase of Zn²⁺doping content lead to the ZnTiNb₂O₈ phase content increased.At the same time,both dielectric constant and temperature coefficient of resonant frequency??_f?of Ni_1-xZn_xTiNb₂O₈ ceramics were reduced.In order to lower the sintering temperature of Ni_0.3Zn_0.7TiNb₂O₈ ceramics,we studied the influence of B₂O₃-ZnO glass as the sintering aids added to Ni_0.3Zn_0.7TiNb₂O₈ ceramics.Typically,the highly dense and temperature stabled Ni_0.3Zn_0.7TiNb₂O₈ ceramics were obtained with 3wt%B₂O₃-ZnO glass addition,and a fairly good microwave dielectric properties with?_r=34.3,Q×f=20255 GHz and a near-zero?_f=-4.14 ppm/?were shown when the compounds was sintered at 950?.In order to further improve the microwave dielectric properties of Ni_0.4Zn_0.6TiNb₂O₈ceramics??_r=51.23,Q×f=32114GHz,?_f=38.1ppm/??,the substitution of Zr⁴⁺ion at Ti site of Ni_0.4Zn_0.6TiNb₂O₈ ceramic was researched.The substitution of Zr⁴⁺ions at Ti sites changed the component phase of ZnTiNb₂O₈ ceramics.As the increase of Zr⁴⁺ion substitution,NiTiNb₂O₈ phase was decreased,ZnTiNb₂O₈ phase increased and relative density increased at the same time.Finally,under the substitution of Zr⁴⁺ion on Ni_0.4Zn_0.6TiNb₂O₈ ceramic,we have got a good temperature stabilization and good dielectric properties Ni_0.4Zn_0.6Ti_0.9Zr_0.1Nb₂O₈ ceramic??_r=35.3,Q×f=45733GHz,?_f=3ppm/??.Meanwhile Ni_0.4Zn_0.6Ti_0.7Zr_0.3Nb₂O₈ sintered at 1120?showed very low dielectric loss??_r=29.7,Q×f=92078GHz,?_f=-38ppm/??.The influence of ion polarization in NbO₆ octahedral on the dielectric properties of NiTiNb₂O₈ ceramic was studied,which used Ta⁵⁺as the substitution of Nb⁵⁺site of NiTiNb_2-2xTa_2xO₈ ceramics.Four longer Nb-O bonds were shortened,and two shorter Nb-O bonds were prolonged with Ta⁵⁺substitution.According to Clausius-Mostti equations,the longer bond lengths determines the intensity of polarization in oxygen octahedron.As the Ta⁵⁺content increased gradually,four longer bonds were shortened,which means polarization in oxygen octahedron was restricted.This tendency lead to the decrease of dielectric constant.As the Nb-O bond valence of NiTiNb₂O₈ ceramic unit cell increased with Ta substitution,the Q×f values increased.we calculated the values of distortions of Nb-O bonds(?_Nb-O).The?_Nb-O decreased with increased Ta⁵⁺content increased,so that,the?_f values were decreased with Ta additions.Finally,the optimal microwave dielectric properties of?_r=48.5,Q×f=17500 GHz and?_f=88.6ppm/?were obtained for NiTiNb_1.6Ta_0.4O₈ sintered at 1140?.We studied the NiTiNb₂O₈ ceramic applied in LTCC technology,in which the sintering temperature must be lower than 960?.The NiTiNb₂O₈+xwt%BiVO₄?2.5<x<10?ceramics were prepared through the solid-state synthesis methods,the addition of BiVO₄ to NiTiNb₂O₈ ceramics sintered at lower temperatures contained three phases:NiTiNb₂O₈,BiVO₄ and a small amount of NiNb₂O₆ phase.The sintering temperature of NiTiNb₂O₈ ceramic reduced with the addition of BiVO₄.Furthermore,the dielectric constants and Q×f values increased with BiVO₄ contents increased.The additions of BiVO₄ ceramics decreased the?_f values because of the negative?_f values of BiVO₄ phase and NiNb₂O₆ phase.Finally,the additions of BiVO₄ ceramics to Ni_0.5Ti_0.5NbO₄ ceramics sintered at 900?obtained dielectric properties of?_r=56.7,Q×f=7062GHz,?_f=55.59ppm/?.