氧化硅基陶瓷的湿化学制备与微波介电性能研究

【作者】 胡成西；

【导师】 刘鹏；

【作者基本信息】 陕西师范大学 ， 声学， 2014， 博士

【摘要】 当前,用于基板的低介电常数类微波介质材料引起了极大的关注。微波基板材料要求低的介电常数(εr),高Q·f值以及近零谐振频率温度系数(τf)。二氧化硅陶瓷是一种高性能微波基板材料,介电常数低、介电损耗低、勺小,机械强度高、与银化学兼容性好、无毒、原料来源丰富、成本低廉,作为微波基板和LTCC设备具有广阔的应用前景。但存在其烧结温度过高、谐振频率温度系数不近零、出现微裂纹等问题。本文利用溶胶-凝胶(Sol-Gel)、水热(Hydro thermal method)、溶剂热法(Solvent thermal method)等湿化学方法制备出SiO2微纳米粉体,并用这种粉体制备出二氧化硅基微波介质陶瓷。研究了SiO2陶瓷烧结特性、相结构的演化、微观显微组织的变化、密度变化以及微波介电性能随烧结温度和测量温度的变化关系。Si02单相陶瓷的烧结温度从1675℃降到了1550℃,将其Q·f从80400GHz提高到92400GHz,发现了Si02陶瓷的Q·f值对温度的敏感性并用复合介电模型给出了解释；利用SiO2-TiO2核壳结构将陶瓷的烧结温度降到了1200℃,调节了τf值近零,同时保持了较高的品质因数；利用Li2Ti03添加获得了鳞石英相的微波介电性能,将陶瓷的烧结温度降到了1050℃,调节了τf值近零；硼酸添加将0.9SiO2-0.1TiO2陶瓷的烧结温度降到950℃,调节了τf值近零,同时将Q·f值提高到78000GHz,实现了与Ag共烧；研究了Si02陶瓷的老化、高低温激光拉曼光谱和工艺改进。本文主要研究成果分如下五个部分。第一部分,用Sol-Gel法制备SiO2陶瓷以及微波介电性能研究,通过正硅酸乙酯(TEOS)的水解-缩聚反应,分别在酸性溶液和碱性溶液环境下利用乙醇和水的混合溶液在室温下制备出了单分散高比表面积非晶态二氧化硅纳米块和纳米球,并且用制备出的二氧化硅纳米块和纳米球为原料制备了SiO2陶瓷。用二氧化硅纳米块制备的Si02陶瓷在1250℃以上均为方石英相,在1600℃烧结的陶瓷片的密度为2.03g/cm3(相对密度85%),Q·f值68000GHz,介电常数3.13,谐振频率温度系数-14.2ppm/℃。另外,发现样品的Q·f值随测量温度的升高而明显增加。采用碱性溶液环境中制备的Si02粉体制备的陶瓷的致密度和微波介电性能发生明显改善,在1550℃烧结3小时的样品得到了优异的微波介电性能。Si02陶瓷的Q·f值在室温下达到92400GHz,高于固相法制备的性能。研究发现,陶瓷的Q·f值在从80℃降温到室温后的测量值明显高于原先在室温下测量的值,但是小于80℃下的测量值。Q·f值对温度的敏感性用复合介电模型给出了解释。第二部分,以前一部分介绍的单分散高比表面积非晶态二氧化硅纳米球为核分别利用静电吸附沉积法和溶胶凝胶-溶剂热法在二氧化硅表面包覆一层二氧化钛,合成单分散、高比表面积SiO2@TiO2复合微球,研究用复合微球制备的SiO2@TiO2复相陶瓷的微波介电性能。用静电吸附沉积法得到的复合粉体包覆效果不好,均匀性较差,用此粉体制备的陶瓷的微波性能都比较低。溶胶凝胶-溶剂热法可以得到非常均匀的复合粉体,分散性好,包覆效果比较好,十到几十纳米的二氧化钛纳米颗粒均匀包裹在直径为500-600纳米的二氧化硅微球上,得到了核壳结构。用这种具有核壳结构的粉体制备了(1-x) SiO2-xTiO2复相微波介电陶瓷,得到了谐振频率温度系数近零的陶瓷,Q·f=4O5OO GHz,并且将烧结温度从1550℃降到了1200℃。第三部分,分别利用钛酸锂和硼酸对前述Si02陶瓷进行掺杂改性进一步降低烧结温度以实现与金属电极共烧之目的。实验结果表明钛酸锂的添加不仅把烧结温度从1200℃降到了1050℃,而且调节了二氧化硅的τf值。(1-x) SiO2-xLi2TiO3陶瓷在1050℃烧结具有较好的微波性能：εr=3.22, τf=0.2ppm/℃, Q·f=10280GHz;硼酸的添加不仅把0.9SiO2-0.1TiO2复相陶瓷的烧结温度降到了950℃,而且使Q·f值从40500GHz提高到78000GHz,同时保持了τf值近零。Ag粉与0.9SiO2-0.1TiO2+10wtH3BO3在950℃共烧实验并没有检测到第二相出现,说明与金属Ag电极的化学兼容性好。第四部分,研究了二氧化硅微波介电陶瓷的介电频谱、老化及高低温激光拉曼光谱,同时优化了制备工艺。研究表明：Si02微波介电陶瓷的介电常数在低频下几乎为常数,大约4.2,比微波频段的介电常数稍大；SiO2微波介电陶瓷的微波性能随着岁月流逝不但没有降低反而明显改善,搁置2年后的SiO2方石英相微波介电陶瓷经退火处理后其微波介电性能下降。第五部分,研究了镁橄榄石粉体的纯相制备方法,首次用一种简单方法制备了纯相镁橄榄石粉体,用此纯相粉体制备了硅酸镁微波介电陶瓷。以纯相硅酸镁为基础,利用静电吸附沉积法和热溶剂法制备了Mg2Si04@CaTi03复合结构,研究了Mg2Si04@CaTi03复相陶瓷的微波介电性能。得到勺近零的Mg2SiO4@CaTiO3复合陶瓷,1300℃烧结得到的Q·f=19000GHz。更多还原

【Abstract】 Nowadays, low permittivity microwave materials have attracted much attention for their applications as substrate in microwave integrated circuits. The microwave substrate material should have a low permittivity (εr), high Q·f value and near zero temperature coefficient of resonant frequency (τf). Among the microwave substrate materials, SiO2-based composite was used in the field of microwave substrates due to its excellent chemical and thermal stabilities. It’s attractive advantages such as low dielectric constant, low dielectric loss, low τf, high mechanical strength, chemical compatibility with Ag metal electrodes, no toxicity, and abundant raw materials source and low cost constituents make the SiO2ceramic as potential candidates for substrate and LTCC devices. However, the main disadvantages of SiO2ceramics are its high sintering temperature, not near zero τf and microscopic cracks limiting its practical application.SiO2nanospheres were synthesized by wet chemical methods, such as the Sol-Gel, hydrothermal method, and solvent thermal method, and then the powders were used as raw material to fabricate the SiO2based ceramics. The phase-formation and transformation, sintering behavior, microstructures, and dielectric properties of SiO2based ceramics were investigated in detail. The SiO2ceramics sintered at1550℃exhibited excellent microwave properties of εr～3.52, Q·f·92400GHz and τf～-14.5ppm/℃. The optimum sintering temperature of SiO2ceramics was reduced125℃by aqueous Sol-Gel process compared to a conventional solid-state method. The Q·f and εr values of SiO2ceramics are sensitive to the measured temperatures, which was explained by a simple composite dielectric model. TiO2coated SiO2powders were prepared by a SGS and were used as raw material to fabricate the (1-x) SiO2-xTiO2(0.1≤x≤0.3) ceramics. The0.85SiO2-0.15TiO2ceramics sintered at1200℃possess excellent microwave properties of εr～5.4, Q·f～40500GHz and τf near to0ppm/℃. The optimum sintering temperature of ceramics was reduced475℃compared to a conventional solid-state method. The sintering temperature of0.9SiO2-0.1TiO2ceramic is lowered from1200℃/3h to950℃16h by the addition of H3BO3.0.9SiO2-0.1TiO2composite ceramics mixed with10wt%H3BO3sintered at950℃/16h posses a excellent microwave dielectric properties of εr～4.67, Q·f～78000GHz and τf -0.7ppm/℃. From the X-ray diffraction and EDS analysis of cofired ceramics, the H3BO3added0.9SiO2-0.1TiO2powders does not react with Ag at950℃. The aging property, high-low temperature Raman spectra and the improvement for preparation of SiO2ceramics were investigated. The principal expariment results were shown as follows.In the first part, SiO2nanoblocks and nanospheres were synthesized using a Stober method by hydrolysis and condensation of TEOS (tetraethylorthosilicate Si(OC2H5)4) at room temperature under the acidic and alkaline conditions respectively. The powders used as raw materials to fabricate SiO2ceramics. The SiO2ceramics using nanoblock sintered at above1250℃are to be the cristobalite phase, and when the sintering temperature reaches to1600℃, have the density of2.03g/cm3(relative density of85%).The ceramics exhibited microwave dielectric properties of εr～3.13, Q·f～68000GHz and τf～-14.2ppm/℃measured at room temperature. In addition, it was found that the Q·f values of specimens increased significantly with the increase of sintering temperature. The density and microwave dielectric properties of fabricated ceramics using SiO2powder prepared in alkaline environment got improved apparently, and reached up to the optimized values of microwave dielectric properties(εr·3.52, Q·f～92400GHz and τf～-14.5ppm/℃) for specimens sintered at1550℃for3h. The optimum sintering temperature of SiO2ceramics was reduced (125℃) by aqueous sol-gel process compared to a conventional solid-state method. Microwave dielectric properties of SiO2ceramics were firstly measured at varying temperatures from room temperature to80℃.We found that the Q-f values are sensitive to the measurement temperature, due to contribution from silica medium, void volume and adsorbed water. The Q·f values measured at room temperatures after cooling down from80℃are obviously higher than those measured at original room temperatures, but lower than the ones measured at80℃.In the second part, SiO2@TiO2composite microspheres were prepared using electrostatic adsorption deposition method and sol-gel process under solvothermal conditions (SGS). Fabricatinging composites with electrostatic adsorption deposition method exhibited a not good coating effect, and then led a poor uniformity and also relatively lower microwave dielectric properties. TiO2coated SiO2powders with homogeneous coating were prepared by a Sol-Gel process under solvothermal conditions and were used as raw material to fabricate the (1-x) SiO2-xTiO2(0.1≤x≤0.3) ceramics. The0.85SiO2-0.15TiO2ceramics sintered at1200℃possess excellent microwave dielectric properties of εr～5.4, Q·f～40500GHz and τf near to0ppm/℃. The optimum sintering temperature was reduced475℃compared to that by a conventional solid-state method.In the third part, using a conventional solid-state reaction method, SiO2ceramics with addition of Li2TiO3were synthesized and their phase evolution, chemical reactions, microstructure and microwave dielectric properties were studied. The microwave dielectric properties of SiO2ceramics with addition of Li2TiO3strongly depend on the formation and the decomposed of Li2(TiO)(SiO4) intermediate phase. The tridymite phase was formed for all samples sintered above1150℃and its microwave dielectric properties were reported for the first time (εr=2.58,Q·f=258OO GHz, τf=-17.8ppm/℃). The promising ceramics of0.945SiO2-0.055Li2TiO3sintered at1050℃were found to possess good microwave dielectric properties:εr=3.21,Q·f=10180GHz and τf～0.17ppm/℃. The sintering temperature of0.9SiO2-0.1TiO2ceramic is lowered from1200℃/3h to950℃/6h by the addition of H3BO3.0.9SiO2-0.1TiO2ceramics with10wt%H3BO3sintered at950℃/6h possesses excellent microwave dielectric properties of εr～4.67, Q·f～78000GHz and τf～0.7ppm/℃. From the X-ray diffraction and EDS analysis of cofired ceramics, the H3BO3added0.9SiO2-0.1TiO2ceramic does not react with Ag at950℃.In the fourth part, the aging property, high-low temperature Raman spectra, and the improvement for preparation process of SiO2ceramics were investigated. Experimental results indicated that the εr value of SiO2ceramics was almost constant approximately equal to4.2at low frequency, slightly bigger than that of the former. After two years aging, microwave dielectric properties of SiO2ceramic did not decrease, nevertheless improved apparently compared to those as-sintered samples, but the annealing led to a decline in Q·fvalue.In the fifth part, the forsterite powders of pure phase were synthesized with a simple method for the first time, and were used as raw materials to fabricate the Mg2SiO4ceramics.The Mg2SiO4@CaTiO3composite ceramics were prepared with electrostatic adsorption-deposition and solvent thermal methods, and their microwave dielectric properties were also investigated. Although the τfof Mg2SiO4@CaTiO3could reach to near zero, its sintering temperature was very high (1300℃), and the Q·f value was equal to19000GHz.更多还原