Performance Adjustment And Mechanism Investigation In The High-Q Li₂TiO₃-based Microwave Dielectric Ceramics

Ultra-high-speed（20 Gbit/s）communication indicators has been proposed on the development of 5G technology.With the increasing of corresponding communication frequency,attenuation in the electromagnetic wave by air and steam becomes more obvious during transmission process,which lead to the shortened signal transmission distance and expanded base stations construction.Moreover,the application of scale antenna array technology requires communication systems with lower energy consumption.For communication base stations,the loss of microwave passive components（such as dielectric resonators,filters and antennas）can account for over40%of the energy consumption of systems.Hence,reducing the loss of microwave passive components is crucial to lowering the energy consumption of communication systems.As the core of microwave passive devices,microwave dielectric ceramic directly determines the loss magnitude of the devices by quality factor（Q）.Recently,extensive attentions have been concentrated on the lithium titanate（Li₂Ti O₃）based ceramic systems,which not only have a high Q value,but moderate dielectric constant of～20,and low density（～3.1 g/cm³）are reported,facilitating the miniaturization,integration,and light weighting of the new generation communication systems.This thesis concentrates on the performance regulation and mechanism of the Li₂Ti O₃ system,with emphasis on the physical mechanism of the dielectric loss（1/Q）regulation,revealing the laws of affecting the microwave dielectric properties from different perspectives,designing and simulatiing based on the modified material system,and providing the theoretical and practical guidance for the development of novel ultra-low loss dielectric ceramics and devices.This thesis is aimed at the study of microwave dielectric properties of Li₂Ti O₃system and focused on the influence mechanism of intrinsic and extrinsic factors（ordering degree,lattice vibration,microstructure,lithium-ion diffusion and oxygen vacancies）by performing structural analysis methods such as X-ray diffraction spectroscopy（XRD）,Raman scattering spectroscopy（Raman）,scanning electron microscope（SEM）,and EDS energy spectroscopy etc.,combining with defect characterization methods such as X-ray photoelectron spectroscopy（XPS）,leakage current,and AC impedance spectroscopy etc..Meanwhile,prototype devices such as the layered dielectric resonator,substrate filter,and antenna are studied based on Ansoft HFSS three-dimensional electromagnetic simulation software,expanding the practical prospect of the modified Li₂Ti O₃ system in the communication system.The principal research work is summarized as follows:1.Establishing the concept of high-Q designation and performance optimization for the Li₂Ti O₃-based microwave dielectric ceramics.The(Mg_1/3Nb_2/3)⁴⁺compensate ion cluster was introduced to jointly substitute the Ti^4+atB-site,making an improvement of Q×f values by 125%;The temperature coefficient of resonant frequency near zero（25～85°C）was achieved in the designed Li₂Ti_0.85(Mg_1/3Nb_2/3)_0.15O₃-Mg Ti O₃-Li₂Ti_0.85(Mg_1/3Nb_2/3)_0.15O₃ layered structure with Q×f values increased by～46%compared to the traditional two-phase composition method.The principal progresses are as follow:（1）Comprehensively utilizing analytical methods like XRD and SEM etc.,the influence of ion substitution（single substitution and co-substitution）on the crystal and microstructure of systems were intensively studied;（2）Based on the leakage current and AC impedance spectral measurements,correlation mechanism between the Li⁺diffusion and dielectric loss were discovered.Deeper mechanisms of the Q×f value improved by Mg²⁺/Nb⁵⁺co-substitution were clarified;（3）The distribution of electromagnetic field in different dielectric layers were systematically analyzed by HFSS software;（4）The dielectric performance of systems were fitted using the parallel distribution models,and the influential mechanism on the dielectric loss of layered composite ceramics were explored from electromagnetic field distribution.Ultimately,the samples of Li₂Ti_0.85(Mg_1/3Nb_2/3)_0.15O₃ ceramics withε_r=19.01,Q×f=113,774 GHz,τ_f=13.80 ppm/°C（25～85°C）,and Li₂Ti_0.85(Mg_1/3Nb_2/3)_0.15O₃-Mg Ti O₃-Li₂Ti_0.85(Mg_1/3Nb_2/3)_0.15O₃ layered composite ceramics withε_r=19.05,Q×f=76,705GHz,τ_f=-0.99 ppm/°C（25～85°C）were prepared,providing the theoretical guidance on the modification of ionic co-substitution and layered structure performance.2.Proving the mechanism of solid solution modification and characteristics of AC impedance response of Li₂Ti O₃-based microwave dielectric ceramics.Li₂CO₃,Li F,Mg O,Mg O&Li F,and Mg F₂ additives were intoduced to modify the microwave dielectric performance of Li₂Ti O₃ system,among which 210%improvement in Q×f values by Mg F₂ was achieved.The principal progresses are as follow:（1）Based on the combination of XRD,SEM,and Raman measurement methods,modification mechanisms of various additives were characterized,clarifying a co-influence of Mg O and Li F by Mg F₂ additive;（2）The influence of structural parameters such as bond valence and oxygen octahedral distortion on the microwave dielectric properties were comprehensively analyzed through XRD full spectrum fitting data;（3）Based on the AC impedance spectroscopy analysis,in-depth mechanism of Mg F₂ single additive was intensively studied,and the correlation mechanism of conductivity activation energy and Q×f values was established.Ultimately,samples of 0.966Li₂Ti O₃-0.034Mg F₂（LTMF）ceramics withε_r=24.91,Q×f=156,398 GHz,τ_f=15.98 ppm/°C（25～85°C）were prepared,which further provides a fresh way for the design of high-Q systems.3.Expanding the operating temperature interval of Li₂Ti O₃-based microwave dielectric ceramics to meet the requirements of wide temperature-stability.Based on the of developed solid-solution modification mechanism,solid solution of Li₂Mg₃Ti_0.92(Zn_1/3Nb_2/3)_0.08O₆（LMTZN）and 0.966Li₂Ti O₃-0.034Mg F₂（LTMF）was formed.The temperature coefficient of the resonant frequency is close to zero,and the operating temperature interval was widened from 25～85℃to-40～120℃.The principal progresses are as follow:（1）Based on the XPS fitting results of O 1s,intrinsic relationship between oxygen vacancy and dielectric loss in the system was explored;（2）Based on the XRD full-spectrum fitting data,regulation mechanism of structural changes on the temperature stability was systematically analyzed;（3）A hairpin ceramic-based microstrip line filter was designed and the simulation results showed that a lower insertion loss of 0.06 d B in the passband was acheived.The insertion loss in pass band is less than 1 d B owing to the high Q value.Ultimately,samples of0.892LTMF-0.108LMTZN ceramics withε_r=17.98,Q×f=145,916 GHz,τ_f=0.29ppm/°C（-40～120°C）were prepared,which is expected to meet the requirements of low loss microwave devices working in harsh environments.4.Refining the low dielectric loss theory of Li₂Ti O₃-based microwave dielectric ceramics,realizing low-temperature co-fired of the systems.Based on the mechanism of substitution and solid solution,Nb₂O₅ additive was introduced to Li₂Ti_0.98Mg_0.02O_2.96F_0.04 system,increasing Q×f value by 267%.Furthermore,benefiting from their low melting point,B₂O₃ and Cu O oxides were carried out to suppress the sintering temperature from 1120℃to 790℃in the system.The principal progresses are as follows:（1）Based on the XPS fitting results of Ti 2p and the chemical bond theory of complex crystals,correlation mechanism between lattice vibration,lattice defects,and dielectric loss was comprehensively analyzed;（2）Based on XRD pattern and EDS element surface distribution pattern,co-fired matching characteristics of ceramics and Ag were analyzed;（3）A LTCC patch antenna was designed and the simulation results showed that a high gain of 4.634 d B can be obtained at the center frequency of 2.49 GHz.Ultimately,samples of Li₂Ti_0.98Mg_0.02O_2.96F_0.04-1wt%Nb₂O₅ceramics withε_r=19.01,Q×f=185,262 GHz,τ_f=12.23 ppm/°C（25～85°C）,and Li₂Ti_0.98Mg_0.02O_2.96F_0.04-1 wt%Nb₂O₅-1 wt%（x B₂O₃-y Cu O）（mass ratio x:y=1:1）sintered at 790°C withε_r=23.50,Q×f=60,574 GHz,τ_f=22.8 ppm/°C（25～85°C）were prepared,which possess potential applications benefits in high-performance microwave integrated devices.