Hui Ming Ji

Effects of Sol Network Modifiers on Preparation, Morphology and Phase Composition of Zirconium Diboride Nano-Powders by Novel Sol-Gel Technology

Zirconium diboride nano-powders were synthesized by novel sol-gel technology using zirconium oxychloride, boric acid and sucrose as row materials. Different sol network modifiers, such as propylene oxide (PO) and citric acid (CA), were used to control the formation of zirconia precursor sol, respectively. PO could stabilize the sol by protonation and ring-opening reactions, and CA could form the sol network by chelation, then transformed to gel network. Gel was dried, ground, and exposed to carbonthermal reduction heat treatment (1500°C, 2h, flowing 95%argon mix with 5% hydrogen) to prepare ZrB2 nano-powders. In this study, effects of sol network modifiers on preparation procedure, powder characteristics were performed by using Infrared spectroscopy (FTIR), X-ray diffraction (XRD) and Transmission electron microscopy (TEM). The purity of ZrB2 will improve by increasing the molar ratio of B/Zr, the photomicrograph revealed nearly pure ZrB2 nano-powders with spherical shape of 50-200nm and well dispersed were obtained with a molar ratio of B/Zr of 3.0 at 1500°C for 2h using PO as chemical modifier.

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.

SrTiO3 Based Dual-Functional Ceramics with Nano-Additives

The electrical properties and microstructures of SrTiO3 based voltage-sensing and dielectric dual-functional ceramics with nanometer donor and acceptor additives were studied. The La2O3 nanopowders and MnO-SiO2-Al2O3 (or CuO-SiO2-Al2O3) nano-composite powders were incorporated into SrTiO3 as donor, acceptor and liquid-phase sintering aids. Then semiconducting SrTiO3-based ceramics were sintered at 1360-1440oC for 2 h in a reducing atmosphere. The effects of the nanometer donor and acceptor additives and the sintering temperature on the electrical properties and microstructures of materials were discussed. The results showed that SrTiO3-based varistor ceramics with 1.1 mol% La2O3 and 0.1 mol% MnO nano-additives sintered at 1360-1420oC in graphite and N2 reducing atmosphere have excellent voltage-sensing and dielectric characteristics. The varistor voltage ranges from 2.3 to 5.3 V/mm, the nonlinear coefficient from 3.0 to 3.8, and the dielectric constant from 215,600 to 413,000.

Preparation and Acetone Sensitive Characteristics of Pure and Cu2+-Doped LaFeO3 Semiconductor Thin Films

Pure and Cu2+-doped LaFeO3 thin films were prepared on Al2O3 substrate by polymer complex method using La(NO3)3⋅6H2O, FeCl3⋅6H2O and Cu(NO3)2⋅3H2O as raw materials, citric acid (CA) as chelating agent and ethylene glycol (EG) as cross-linking agent. As-growth thin films were well-crystallized and the grain size was about 40nm after being annealed at 650°C. The sensitivities of pure and Cu2+- doped LaFeO3 thin films to 80 ppm acetone gas measured at 350°C were 315 and 30, respectively. Partial substitution of Fe3+ in LaFeO3 with Cu2+ could widen testing temperature of the materials.

Preparation and CO2 Gas Sensitive Properties of CuO-SrTiO3-Based Semiconductor Thin Films

CuO-SrTiO3-based thin films were prepared by novel sol-gel technology on Al2O3 substrates using Cu(NO3)2, SrCl2 and TiCl4 as the starting materials, critic acid and ethylene glycol as chelating agents. CO2 sensing properties of the films were investigated. Structure characteristics of the sol and asgrown thin films were analyzed by FT-IR spectrum, X-ray diffraction and SEM. The results reveal that the films consisted of CuO phase and SrTiO3 phase have nanocrystalline microstructure at 750°C for 40 min. The modified CuO-SrTiO3 thin films exhibit good resistance-temperature and gas sensitivity properties in a wide range of temperature. The films exposed to 6% CO2 show that sensitivity are 32, and response and recover time are within 2 s at 250 °C operating temperature.

A Metastable Ion (III) Oxide Phase (ε-Fe2O3): Preparation and Gas Sensing Properties

ε-Fe2O3 is a rare and metastable iron (III) oxide phase. ε-Fe2O3/SiO2 composites were prepared by combining the reverse-micelle and sol-gel methods. An appropriate amount of Ba2+ was needed in this system to promote the formation of ε-Fe2O3 nanorods in SiO2. The size of nanorods varied with different Ba2+ addition amount and sintering procedure. Then pure ε-Fe2O3 nanorods were obtained after stripping SiO2 by etching due to NaOH aqueous solution. The as-synthesized ε-Fe2O3 nanorods were discussed using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). Interestingly, metastable ε-Fe2O3 nanorods showed a promising performance for the response for ethanol, compared with the stable phases of α-Fe2O3 and γ-Fe2O3. It indicates that nanostructure ε-Fe2O3 (including ε-Fe2O3 nanorods) could be a valuable material for the fabrication of advanced sensing devices.

Preparation of Zirconium Diboride Nanopowders Using Different Zirconium Sources via Sol-Gel Method

Zirconium diboride nanopowders were synthesized by sol–gel method using different zirconium sources of zirconium carbonate and zirconium nitrate based on different gelling processing. Both of zirconium source can be applied in the synthesis of good performance ZrB2 powders while the in-suit sol-gel using zirconium carbonate tend to form the spherical ZrB2 powders about 50nm and the traditional sol-gel using zirconium nitrate prefer to form worm ZrB2 powders about 200nm. The influences of B/Zr molar ration of zirconium carbonate and zirconium nitrate of sol-gel method on the phase constitution was investigated. And the gel mechanism was discussed to explain the different phase constitution, morphology of final products.

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.

Effect of Mn2+ Additive on the Sinterability and Dielectric Properties of Ba0.55Sr0.4Ca0.05TiO3-MgO Composite Ceramics

Ba0.55Sr0.4Ca0.05TiO3-MgO (BSCTM) with Mn2+ additive composite ceramics were prepared by the conventional solid state method with BaCO3, SrCO3, CaCO3, MgO and MnCO3 as raw materials and B2O3 and Li2CO3 as sintering agent. The influence of the MnCO3 additive on the sinterability and dielectric properties of BSCTM composite ceramics are investigated. The morphologies were analysized by scanning electron microscopy (SEM). The dielectric constant and dielectric loss of ceramics were measured by LCR meter at 1 kHz and 1 MHz. The temperature dependences of dielectric constant were measured by high-low temperature incubator tank and LCR meter at 1 MHz and a temperature range from-55 to 125 °C. The dielectric tunabilities were tested by C-T-V converter and LCR meter at 1 MHz and room temperature (25 °C). SEM results showed that BSCTM ceramics became more compact with the increase of Mn2+ content, because Mn2+ helped to form more oxygen vacancies for accelerating densification process. The addition of Mn2+ largely reduced the dielectric loss while had little effect on dielectric constant. The dielectric constant and dielectric loss of 0.5mol% Mn2+ doped BSCTM ceramics were 222 and 0.001, respectively. The dielectric tunability increased at first and then decreased with the increase of Mn2+ content. The tunability of 0.5mol% Mn2+ doped BSCTM ceramics were 4.04% under a DC electric field of 8.2 kV/cm.