Preparation Of SiO 2 And Ceramics By Microwave Dielectric Properties

As one of the low dielectric constant microwave dielectric materials, silica has a very low dielectric constant (εr=3.8), thus it has great development prospect in the dielectric substrate materials and high frequency microwave components fields. However, high sintering temperature, large resonant frequency temperature coefficient, and plenty of micro cracks have been seriously restricted the commercial application of the SiO2 ceramics.Based on SiO2 nanospheres prepared by sol-gel method, this paper has firstly studied the effects of precursor powder particle size and sintering process on the microwave dielectric properties of SiO2 ceramics, in order to determine the optimum preparation parameters. Secondly, rutile and anatase TiO2 have been used to prepare SiO2-TiO2 composite ceramics, respectively, trying to adjust the temperature coefficient of resonator frequency to nearly zero. Then the sintering additive B2O3 was added to the above composite ceramics, reducing the sintering temperature to 95℃0 to meet the requirements of LTCC technology. The specific research contents and results are as follows:(1)The SiO2 nanoparticles were synthesized through sol-gel method at 50℃ thermostatic water bath. The SiO2 nanospheres and the commercial high purity SiO2 powders were used as raw material to prepare SiO2 ceramics, respectively. Then the phase composition, microstructures and microwave dielectric properties were studied. The results show that ceramics derived from SiO2 nanospheres has optimal microwave dielectric properties of εr= 3.85, Qxf= 63,000GHz, τf=-18.86, while the ceramics prepared using commercial SiO2 powders has the microwave dielectric properties of εr= 2.9, Q×f= 29,500GHz, τf=-16.87. Obviously, the more uniform and smaller of the raw powders, the better properties of the SiO2 ceramics.The effects of cooling rate on phase composition, microstructure, and microwave dielectric properties of SiO2 ceramics have been investigated. Cracks on the surface of the ceramics have significantly decreased after controlling the cooling rate, indicating that when through the phase transition temperature range with appropriate cooling rate, the volume shrinkage caused by phase transition will greatly reduced. The Q×f value of S2 is only about 20,000 GHz when sintered at 1150℃, while the S1 sintered at 1150℃ for 3 h gets the better microwave dielectric properties:εr=3.82, Q×f= 53,000 GHz, τf =-13.6ppm/℃.(2)The (1-x)SiO2-xTiO2(RST x=0,0.08,0.12,0.16,0.20) composite ceramics were prepared by a conventional solid-state route with the raw powders SiO2 nanospheres and Rutile TiO2. The microstructures, phase compositions and microwave dielectric properties of the RST composite ceramics were investigated. XRD revealed that the phases of SiO2 and TiO2 were co-existed as a two-phase system. As TiO2 content x increased, the εr and τf values increased gradually, while the Q×f values declined inversely. The optimal microwave dielectric properties are achieved in 0.84SiO2-0.16TiO2 sintered at 1275℃ for 3 h with εr= 5.91, Q×f= 39,680 GHz, and τf =-4.53ppm/℃.When adding Anatase TiO2 to the SiO2 nanospheres with the same proportion, the phase compositions of AST composite ceramics are very complicated, the main phase is quartz, accompanied by a small amount (SiO2)x phase, at the same time several titanium oxide Ti5O9, Ti9O17 and TiO2 are also detected. The densities, dielectric constant, and τf values are increased with increasing TiO2 content, mainly for the density and dielectric constant of TiO2 are greater than that of SiO2. Compared with the RST composite ceramic, AST ceramics in composite 0.88SiO2-0.12TiO2 obtained better properties:εr= 5.41,Q×f= 52,000 GHz, τf=+2.1 ppm/℃.(3)B2O3 was selected as single sintering additives to lower the sintering temperature of RST composite ceramics, and the phase composition, microstructures and microwave dielectric properties were studied. After adding B2O3, the sintering temperature was reduced to below 1000℃, the dielectric constant and the Q×f value firstly increase and then decrease with the increase of sintering temperature.10wt% B2O3 added 0.84 SiO2-0.16 TiO2 (RST) ceramics sintered at 950℃ for 5 h gained good microwave dielectric properties:the εr= 4.4, Q×f= 20,000 GHz, τf=+1.45 ppm/℃, thus can meet the requirements of the low temperature co-firied ceramics technology.Different mass fraction of B2O3 (5wt%-12.5wt%) were added to 0.88SiO2-0.12TiO2 (AST), the microstructures, phase compositions and microwave dielectric properties of the RST composite ceramics were investigated. XRD shows that main phases of the ceramics are cristobalite and rutile phase, and accompanied by a small amount of tridymite phase. The εr and Q×f values firstly increase and then decline with increasing the sintering temperature. Compared with the B2O3 added RST composite ceramics,the better microwave dielectric properties are obtained in 10 wt% B203 added 0.88Si02-0.12TiO2(AST)ceramics sintered at 950℃:εr=4.68,Q×f= 28,000 GHz,τf=-0.24 ppm/℃.