Preparation And Dielectric Properties Of Bi_1.5MgNb_1.5O₇ Multilayer Dielectric Thin Films By Sol-gel Method

With the rapid development of 5G and Internet of Things communication technology,microwave devices such as dielectric voltage controlled varactor and related materials have great market potential in the military and civil markets in the future.It is a tendency to develop new large tunable dielectric materials with low loss.Bismuth based pyrochlore structured Bi_1.5Mg Nb_1.5O₇（BMN）thin film materials have attracted considerable attention due to their low dielectric loss,appropriate dielectric constants,and high dielectric tuning rates.Therefore,this thesis starts with the preparation process conditions of sol-gel method,and aims to improve the dielectric properties of BMN thin films by regulating process parameters,non-stoichiometric and ion doping substitution.The main research contents are as follows:（1）In this thesis,BMN thin films were prepared by sol-gel method.The properties were optimized by adjusting the citric acid content,water bath heating temperature,water bath heating time,different p H values and annealing temperatures.The complexation mechanism of precursor solution was analyzed by infrared spectroscopy.The relationship between the microstructure and dielectric properties of BMN films was studied.The results showed that the optimum conditions for preparation of clear precursor solution were as follows:Nb⁵⁺:CA=1:6,heated at 70℃for 20min.Stable BMN sol can be prepared at p H=7 with moderate dielectric constants（ε=114）and minimal dielectric loss（tan（?）=0.0077）at 1MHz frequency.At an annealing temperature of 650°C,the BMN thin films have a significant（222）growth orientation and high crystallinity with moderate dielectric constant（ε=112）and low dielectric loss（tan（?）=0.00763）.（2）Based on the determined optimal process parameters,the effects of the microstructure and dielectric properties of non-stoichiometric Bi_1.5Mg_xNb_1.5O₇（BM_xN）thin films were studied in detail.By changing the content of Mg²⁺ions,the filling mechanism of Mg²⁺ions in BM_xN thin films was discussed,and it was clear that Mg²⁺ions preferentially replaced the B-position and then replaced the A-position.Research showed that increasing the content of Mg²⁺could effectively improve the dielectric constant,but excessive Mg²⁺ions had a negative effect on reducing dielectric loss.The smallest dielectric loss（tan（?）=0.00972）,and the largest tuning rate（n=8.1%,E=0.8MV/cm）were obtained with x=1.0.（3）To further improve the dielectric properties of BMN thin films,Zn²⁺with larger polarizabilities and ionic radii was selected as a dopant to prepare Bi_1.5Zn_xMg_1-xNb_1.5O₇(BZ_xM_1-xN)materials.The effects of the doping of Zn²⁺ions on the structure and properties of the BMN thin films were investigated,and the ion radius differences and bond-bond vibrational states were explained in terms of changes in the internal structure of the lattice as the reason for the abnormal reduction in cell volume.It is shown that doping with Zn²⁺ions of high polarizability can increase the dielectric constant of BMN films and improve the dielectric tuning properties of the films,but the presence of a second phase significantly affects the dielectric loss of the films.The smallest dielectric loss（tan（?）=0.00687）and the largest tuning rate（n=11.6%,E=1.02 MV/cm）were achieved with x=0.4.