Xiao Lei Li

The Effect of Aging Temperature and pH Value of Aging Solution on the Microstructure of ZrO2-SiO2 Aerogels

ZrO2-SiO2 gels were prepared by prehydrolysis method with sol-gel process. Then,the wet gels were aged at different temperatures (60,110,170 °C) in ethanol or alkaline solution (pH=8). Finally, the monolithic ZrO2-SiO2 aerogels were obtained by supercritical fluid drying. N2 adsorption-desorption, scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the microstructure of aerogels. The aim of this research is to study the effect of aging temperature and pH value of aging solution on the microstructure of ZrO2-SiO2 aerogels. The results show that the specific surface area and pore volume of ZrO2-SiO2 aerogels aging in alkaline solution (pH=8) is lower than that of aging in the ethanol. And there is a shift of the pore size distribution towards larger values. This is because the cross-linking reaction in alkaline solution enlarges the particle size. As the aging temperature increases, the specific surface area and pore volume decrease and the particle size distribution of ZrO2-SiO2 aerogels is more uniform.

Improvement of Thermal Stability of ZrO2-SiO2 Aerogel Modified by Ca(II) Cations

Ca (II) modified ZrO2-SiO2 aerogel (CaZSA) with excellent thermal stability at 1000 °C was prepared by aging the ZrO2-SiO2 wet gel in calcium nitrate (Ca (NO3)2) ethanol solution followed with alcohol supercritical fluid drying method. The reaction between surface hydroxyl groups on the aerogel and Ca (II) ions played an important role in reducing the high temperature activity and inhibiting the particle growth caused by the condensation of hydroxyl groups of aerogel. Moreover, tiny secondary-phase particles, Ca (II) ions, introduced along grain boundaries serve as the pinning particles to inhibit the crystallization of ZrO2-SiO2 aerogel (ZSA). Thus the high-temperature stability of ZSA was significantly improved by surface modification with Ca (II). The specific surface area of CaZSA still maintained 223 m2·g-1 after 1000 °C calcination, which was 52.7% higher than that of ZSA at the same treatment condition.

The Effect of pH of the Aging Solution on Microstructure of Silica/Zirconia Aerogel

ZrO2 wet gel was prepared using zirconium n-butoxide as precursor and 1,2-propylene oxide (PO) as a gelation agent in sol-gel process. Then, tetraethoxysilane (TEOS) was used to strengthen the wet gel in aging process. Finally, the monolithic silica/zirconia aerogel was obtained by supercritical fluid drying. N2 adsorption-desorption method, scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize microstructure of the aerogel. The influence of pH of the aging solution on microstructure of silica/zirconia aerogel was studied. The results indicate that the particle size distribution of SiO2/ZrO2 aerogel was more uniform and less aggregate when aging in acid solution. However, the average pore diameter and particle size of SiO2/ZrO2 aerogel was increased by aging in alkaline solution. Aging in alkaline condition was useful for largening the pore size of SiO2/ZrO2 aerogel, and the maximum specific surface area and pore volume were obtained at pH=9. Meanwhile, different reactions were produced between the acid/alkaline solution and the hydroxyl of SiO2/ZrO2 aerogel, and the ester group/amino group was obtained relatively which can modify the network of aerogel and decrease the shrinkage of aerogel during the supercritical fluid drying.

Effect of Al2O3 and MgO Additives on Dielectric Properties of BaZrxTi1-xO3-Based Ceramics

BaZrxTi1-xO3-based (BZT) ceramics with Al2O3 and MgO addtives were prepared by the conventional solid state method with BaCO3, ZrO2, TiO2, Al2O3 and MgO as raw materials and B2O3 and Li2CO3 as sintering additive. The morphologies were analysized by scanning electron microscopy (SEM). The dielectric constant and dielectric loss of ceramics were measured by LCR meter. The temperature dependences of dielectric constant were measured by high-low temperature incubator tank and LCR meter at 1 MHz and a temperature range-55 to 125 °C. The tunabilities were tested by C-T-V converter and LCR meter at 1 MHz at room temperature. The results show that with the increase of Zr/Ti, BZT ceramic dielectric constant increases, the loss increases, the Curie temperature moves to a lower temperature, and dielectric bias field coordination is relatively lower. The SEM images show that the grain size reaches about 1-2 μm when the sintering temperature is 1100 °C, and the addition of Al2O3 and MgO promote the grain growth and densification of the composite ceramics. The Curie peaks are broadened and depressed with the addition of Al2O3 and MgO. The tunability is improved to 9.59% under a DC electric field of 7.0 kV/cm after the addition of Al2O3. The dielectric constant and dielectric loss of BaZr0.25Ti0.75O3-30wt%Al2O3 and BaZr0.25Ti0.75O3-30wt%MgO are 586, 0.011 and 486, 0.003, respectively. The optimistic dielectric properties make it a promising candidate for the application of tunable capacitors and phase shifters.

Synthesis and Characteristics of MgO Coated BST-Mg2TiO4 Composite Ceramics

The (Ba, Sr)TiO3 (hereafter BST) ceramics are promising candidate for applying in tunable devices. MgO coated BST-Mg2TiO4 (BSTM-MT) composite ceramics were prepared to obtain the low dielectric constant, low dielectric loss, good dielectric constant temperature stability, and high tunability of BST ceramics. The Ba0.55Sr0.40Ca0.05TiO3 nanoparticles were coated with MgO using the precipitation method and then mixed with Mg2TiO4 powders to fabricate BSTM-MT composite ceramics. The morphologies, phases, elements, and dielectric properties of the sintered ceramics were investigated. The core-shell structure of BST powder wrapped with MgO was clearly observed from the TEM image. After sintered at 1100 °C for 2 h, the composite ceramics expressed dense microstructures from SEM images and two main phases BST and Mg2TiO4 were detected in the XRD patterns. The dielectric constant and loss tangent were both reduced after the coating. The reduced dielectric constant and loss tangent of BSTM-MT were 190, 0.0011 (2MHz), respectively. The ceramics exhibited the diffuse phase transition near the Curie temperature and the Curie temperature shifted from 10 °C to 5 °C after the coating. Since the continuous Ti-O bonds were disconnected with the MgO coating, the tunability was reduced to 15.14 % under a DC bias field of 1.1 kV/mm. The optimistic dielectric properties made it useful for the application of tunable capacitors and phase shifters.

(Ba,Sr,Pb) TiO3-BaPbO3 System Compound PTC Ceramic

This paper made a thorough investigation on (Ba,Sr,Pb) TiO3-BaPbO3 system compound PTC ceramic. The optimum compound method is determined by lots of experiments in different compound schemes. Also the formula and process are optimized and improved. The best properties acquired are: ρroom-temperature=18.8Ω·cm, ρmax/ρmin >103. In the end, the phenomena in the experiment are illustrated with electron trap model.

Study of Graphite/Bakelite/BaTiO3-Based PTC Ceramics Composite

The influence of grahtite and bakelite content of Graphite/Bakelite/BaTiO3-based PTC ceramic composite on their properties was studied. It was found that the composites room- temperature resistivity was decreased and the PTC intensity was assured. This method offered a new way to solving the problem of high room-temperature resistivity of the BaTiO3-based PTC ceramic.

Effects of Glass-Additives on the Properties of Ni/Ceramic PTC Composites

The effects of glass additives on the sintering and properties of Ni/(Ba0.92Sr0.08)TiO3 composites were investigated. Due to the addition of glass additives, Ni/ceramic composites with low room-temperature resistivity and obvious PTC effect were obtained at a low sintering temperature. It was shown that glass-additives could form liquid phase that aided the solution and diffusion of solid atoms, acting as sintering aids to accelerate the sintering and lower the sintering temperature. The room-temperature resistivity decreased first and increased later with the increasing content of glassadditives, which was explained by two functions of glass-additives, decreasing interface contact resistance as sintering aids and adding volume resistance as insulators. Moreover, a suitable amount of glass-additives could enhance the PTC effect unexpectedly, which was attributed to the decrease of the contact resistance existing at the ceramic/ceramic interface.

Ni/Graphite/BaTiO3 PTCR Composites

Nickel/graphite/BaTiO3 PTC composites with lower room-temperature resistivity and higher resistance jump were fabricated by adding graphite and nickel powders into BaTiO3 PTC ceramics. Characteristics and conductive mechanism of the composites were discussed based on the experimental results.

Preparation of Ni/BaTiO3 Composites and PTC Effect

Ni/BaTiO3 composite was prepared by decomposition of NiC2O4·2H2O/BaTiO3 precursor, which was prepared by precipitating of nickel in the form of oxalate into the BaTiO3 slurry. The composite must be sintered in reducing atmosphere. Otherwise NTC effect would be introduced. The prepared composite almost had no PTC effect. But PTC effect of the Ni/BaTiO3 composite can be effectively renewed by heat-treatment in air. Under a proper composition and method, the composite shows low room-temperature resistivity (ρRT=6.0 Ω·cm) and obvious PTC effect (ρmax/ρmin=102).