Research On Silicate And Phosphate LTCC Materials And Their Applications

The high speed,low delay,wide coverage and low power consumption of the fifth generation of mobile communication technology（5G for short）,on the one hand,has led to a dramatic increase in the number of communication base stations,and on the other hand,it has put forward higher requirements for the performance of the important microwave devices therein-filters.LTCC filters based on microwave dielectric ceramic materials with low dielectric constant and low loss are not only in line with the development trend of device miniaturization and integration,but also conducive to reducing the delay of the signal in the transmission process and the power consumption of the device,which has a greater application advantage in 5G and future 6G communications.Therefore,in this dissertation,two ceramic systems with low dielectric constants,phosphate and silicate,are taken as the research objects to explore the intrinsic characterization（chemical bond characteristics,lattice vibration and ion polarizability,etc.）and extrinsic（secondary phase,porosity and densification,etc.）factors on the microwave dielectric properties of ceramics by means of X-ray diffraction,Raman scattering spectroscopy,infrared transmission spectroscopy,X-ray photoelectron spectroscopy,scanning electron microscopy and other characterization means,combined with theoretical calculations such as the Rietveld refinement,the First-Principles,and the dielectric theory of the chemical bonding of the complex crystals—P-V-L theory,providing some guidance and reference value for further regulation of microwave dielectric properties.In addition,based on the optimization and improvement of the microwave dielectric properties of ceramics,the glass-free low-temperature sintering of ceramics is realized,and finally an LTCC low-pass filter is designed and fabricated based on the ceramics with optimal performance.The specific research contents and results are as follows:（1）The microwave dielectric properties of Ca Mg P₂O₇ ceramics,which can be densely sintered at 950℃,were investigated:ε_r=7.8,Q×f=13165 GHz,andτ_f=-85.04 ppm/℃.The crystal structure of the ceramics belongs to the triclinic crystal system,which was determined by Rietveld refinement.P-V-L theoretical calculations show that the chemical bond characteristics of Ca Mg P₂O₇ ceramics are mainly dominated by the P1-O and P2-O bonds,and thus these two chemical bonds have a large influence on the microwave dielectric properties.Co²⁺and Zn²⁺were utilized to replace Mg²⁺in Ca Mg P₂O₇,respectively,and Co²⁺was able to reduce the sintering temperature to 925°C without damaging the properties of the ceramics,whereas Zn²⁺not only reduced the densification temperature to 875°C,but also further improved the Q×f value,and Ca Mg_0.84Zn_0.16P₂O₇ presented the best microwave dielectric properties:ε_r=8.05,Q×f=20670 GHz,τ_f=-87.59 ppm/℃.（2）Ba Zn P₂O₇ ceramics with a triclinic crystal system were prepared,and the microwave dielectric properties of single-phase Ba Zn P₂O₇ sintered at 950°C were obtained to beε_r=7.48、Q×f=37228 GHz和τ_f=-62.5 ppm/℃.The Q×f value of Ba Zn P₂O₇ was increased by the equivalent substitution of Nb⁵⁺for P⁵⁺,and excellent microwave dielectric properties ofε_r=7.98、Q×f=60615 GHz和τ_f=-52.93 ppm/℃were obtained for Ba Zn P_1.98Nb_0.02O₇ samples sintered at 925°C.Based on the experimental measurements and first-principles calculations of the Raman spectra and the FTIR spectroscopy,the vibrational properties of Ba Zn P₂O₇ ceramics and their relationship with the microwave dielectric properties were studied,and the variation of Q×f was found to be determined by a combination of the density and the FWHM of the Raman peaks.In order to minimize the effect of the secondary phase on the properties,an equivalent substitution of Zn²⁺by Co²⁺with a similar radius was employed.The W-H plots show that a small amount of Co²⁺has little effect on the crystalline size and internal stresses of Ba Zn_1-xCo_xP₂O₇,and that the ionic substitution does not cause excessive changes in the lattice parameters.The substitution of Zn²⁺by Co²⁺further reduced the sintering temperature of the ceramics compared to the substitution of Nb⁵⁺for P⁵⁺,and the Ba Zn_0.99Co_0.01P₂O₇ ceramics sintered at 875°C had a Q×f value comparable to Ba Zn P_1.98Nb_0.02O₇:60045 GHz.Neither of the above schemes significantly improved the Q value,and Ni²⁺was chosen to replace Zn²⁺in order to achieve this goal.When sintered at 900°C,Ba Zn_0.94Ni_0.06P₂O₇ exhibited the most excellent microwave dielectric properties:ε_r=7.43,Q×f=102538 GHz,τ_f=-64 ppm/℃.The lattice energy and energy band gap were calculated according to the P-V-L theory and DFT,respectively,and it was demonstrated that the optimal Q×f values of Ba Zn_0.94Ni_0.06P₂O₇ ceramic originated from the higher lattice energy of the P1-O and Zn-O bonds and the larger energy band gap of Ba Zn_0.94Ni_0.06P₂O₇ ceramics.（3）Since both phosphate ceramics in（1）and（2）react with Ag,the goal of the study was shifted to the exploration of property modulation and low-temperature sintering of Ca Mg Si₂O₆ ceramics by different ion substitution.The substitution of Si⁴⁺in Ca Mg Si₂O₆ by Ti⁴⁺with a larger radius improves the Q×f value of the ceramics and promotes a positive shift of its negative temperature coefficient of resonance frequency,and the Ca Mg Si_1.95Ti_0.05O₆ sintered at 1275°C have excellent dielectric properties:Q×f=80774 GHz,ε_r=7.9,τ_f=-58.56 ppm/°C.Non-equivalent substitution of Mg²⁺by Na⁺reduced the sintering temperature of the Ca Mg Si₂O₆ ceramics to 1100°C,but its Q×f value was also reduced.Subsequent non-equivalent substitution of Mg²⁺by Li⁺,with the addition of 0.4wt.%LBSCA（Li₂CO₃-B₂O₃-Si O₂-Ca CO₃-Al₂O₃）glass as a sintering aid,the Ca Mg_1-xLi_2xSi₂O₆ ceramics could be densely sintered at 900°C,and exhibited the best microwave dielectric properties when x=0.05:ε_r=7.44,Q×f=41017 GHz,τ_f=-59.3 ppm/℃.Lower activation energy due to oxygen vacancies arising from the lower valence of substituting ions and lattice distortion caused by ion substitution are important reasons for the lower sintering temperature.Finally,on the basis of Li⁺ion substitution,Zn²⁺was used for further substitution of Mg²⁺to achieve glass-free low-temperature sintering of Ca Mg Si₂O₆ ceramics at 900°C.The results of the Rietveld refinement show that the substitution of Zn²⁺for Mg²⁺promotes the densification process of the ceramics,and greatly influences the structural characteristics and the phase composition.The microwave dielectric properties of the samples were:ε_r=7.51,Q×f=68190 GHz,τ_f=-68 ppm/°C,when x=0.28.The negativeτ_f value of Ca Mg_0.62Li_0.2Zn_0.28Si₂O₆（CMLZS for short）ceramic was adjusted to near zero by composite with Li₂Ti O₃ to obtain 0.68CMLZS-0.32Li₂Ti O₃+1wt.%LBSCA ceramics with a good chemical compatibility with Ag when sintered at 900°C,and the microwave dielectric properties are:ε_r=9.33,Q×f=32568 GHz,τ_f=+7.57 ppm/°C.（4）A low-pass filter with a cutoff frequency of 3.6 GHz was designed and fabricated based on 0.68CMLZS-0.32Li₂Ti O₃+1 wt.%LBSCA ceramics and LTCC technology,and the test results of the specimen and the simulation results of the model show good agreement,which verifies the material’s utility in the field of LTCC microwave devices and its applicability to 5G low-band communication.