Effect of Bi2O3 addition on the microstructure and electrical properties of the SnO2.CoO.Nb2O5 varistor system

Abstracts are not published in this journal     

Electrical properties of SnO2·CuO·Ta2O5 varistor system

The CuO and Ta₂O₅-doped SnO₂ system was obtained by conventional ceramics processing. The ceramic phases of samples was analyzed by X-ray diffraction (XRD) and the microstructure by means of scanning electron microscopy (SEM). The electrical field of a single phase of CuO and Ta₂O₅-doped SnO₂ system was studied as a function of current density behavior and compared with the CoO and Ta₂O₅-doped SnO₂ system. A high nonlinear coefficient α=37.9 was obtained. To illustrate the grain–boundary barrier formation of CuO and Ta₂O₅-doped SnO₂-based varistors, a modified defect barrier model is introduced.     

Influence of the rare-earths oxides doped on the SnO2CoOMnO2Ta2O5 varistor system

The tin dioxide is an n-type semiconductor, which exhibits varistor behavior with high capacity of absorption of energy, whose function is to restrict transitory over-voltages without being destroyed, when it is doped with some oxides. Varistors are used in alternated current fields as well as in continuous current, and it can be applied in great interval of voltages or in great interval of currents. The electric properties of the varistor depend on the defects that happen at the grain boundaries and the adsorption of oxygen. The (98.90–x)%SnO₂·0.25%CoO+0.75%MnO₂+0.05%Ta₂O₅+0.05%Tr₂O₃ systems, in which Tr=La or Nd. Current–voltage measurements were accomplished for determination of the non-linear coefficient were studied. SEM microstructure analysis was made to evaluate the microstructural characteristics of the systems. The results showed that the rare-earth oxides have influenced the electrical behavior presented by the system.     

Na对SnO2-CoO-Nb2O5系压敏材料电学性能的影响

研究了Na对新型(Co，Nb)掺杂SnO2压敏材料微观结构和电学性质的影响。当Na2CO3的含量从0增加到1.2mol％时，(Co，Nb)掺杂SnO2压敏电阻的击穿电压从275V／mm增到919V／mm。样品的微观结构分析发现，当Na2CO3的含量从0增加到1．2mol％时，SnO2的晶粒尺寸明显的变小。晶界势垒高度测量揭示，SnO2的晶粒尺寸的迅速减小是压敏电压急剧增高的原因。对Na含量增加引起SnO2晶粒减小的根源进行了解释。掺杂0．4mol％Na2CO3的SnO2压敏电阻非线性系数达28．4．击穿电压为755V／mm，掺杂1.2mol％Na2CO3的SnO2压敏电阻非线性系数为11．5，击穿电压高达919V／mm，它们在中高压保护领域会有很好的应用前景。本文并指出替代Sn的受主离子Na不应处于SnO2晶格中，而是处于间界上，从而进一步解释了压敏电压急剧增高的原因。     

Ta2O5薄膜的结构和电学性能研究

采用紫外光诱导热丝CVD沉积技术制备Ta2O5薄膜和Al/Ta2O2/SiMOS电容。利用XRD，AFM测试分析方法研究了紫外光源功率对Ta2O5薄膜结构的影响；通过C-V和，I-V测试对Ta2O5薄膜的介电常数，击穿场强和漏电流等电学性能进行了研究，结果表明：紫外光源的功率越大，Ta2O5薄膜的结晶性越好，介电常数越大，最大值为29，但紫外光功率对击穿场强和漏电流没有明显改善。     

Conduction processes in the Ta(-)/Ta2O5-electrolyte system

Thin tantalum oxide (Ta₂O₅) films of controlled thickness d (from 1 to 80 nm) were obtained by the anodization of tantalum in a phosphate buffer at pH 7. Experimental results for the d.c. conduction characteristics of amorphous highly imperfect Ta₂O₅ films are presented. Semiquantitative agreement is observed between the experiment and theory. For electrodes covered with very thin films, direct electron exchange between the metal and the electrolyte predominates. In the high field region, for films thicker than 30 nm, the conduction mechanism is space charge limited. The effective electron mobility is 10^-13 m² V^-1 s^-1 for ε = 13. In the low field region, the data can best be interpreted in terms of an ohmic mechanism with a resistivity of approximately 5 × 10¹⁴ Ω cm.The band gap for these films was determined from the spectral distribution of the photoconductivity and amounts to 4 eV. The flat-band potential is approximately - 1 V vs. the Ag/AgCl electrode.     

Nonlinear Electrical Properties of SnO_2-Li_2O-Nb_2O_5 Varistor System

The electrical properties of (Nb, Li)-doped SnO2 ceramics as a new varistor material were investigated. The sample 97.95%SnO2·0.50%Li2O·0.05%Nb2O5 (mol fraction) sintered at 1450= possess the highest density (ρ=6.77 g/cm3) and nonlinear electrical coefficient (α=11.6). The substitution of Sn4+ with Li+ increases the concentration of oxygen vacancies, together with the formation of solid solution, which will increase the sintering rate greatly and decrease the optimized sintering temperature. The substitution of Sn4+ with Li+ and the variation of temperature play very important effects on the densities, dielectric constant, nonlinear electrical properties and other characteristics of the samples. The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature. A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.     

Nonlinear Electrical Properties of SnO2—LiO—Nb2O5 Varistor System

The electrical properties of (Nb,Li)-doped SnO2 ceramics as a new varistor material were investigated.The sample 97.95% SnO20.50%Li2O0.05%Nb2O5( mol fraction)sintered at 1450℃ possess the highest density(p=6.77g/cm^3) and nonlinear electrical coefficient(α=11.6).The substitution of Sn^4 with Li^ increases the concentration of oxygen vacancies,together with the formation of solid solution ,which will increase the sintering rate greatly and decrease the optimized sintering temperature.The substitution of Sn^4 with Li^ and the variation of temperature play very important effects on the densities,dielectric constant,nonlinear electrical properties and other characteristics of the samples.The properties of the grain boundary barrier and the microstructural characteristics were investigated to ensure the effect of the dopants and the temperature.A grain boundary defect barrier model was used to illustrate the grain boundary barriers formation in SnO2-Li2O-Nb2O5 varistors.     

Cr2O3对(Co,Ta)掺杂的SnO2压敏电阻电学特性的影响

研究了Cr对(Co,Ta)掺杂的SnO2压敏材料电学性质的影响.当Cr2O3的含量从0增加到0.15mol%时,(Co,Ta)掺杂SnO2压敏电阻的击穿电压从206V/mm增加到493V/mm;1kHz时的相对介电常数从1968猛降至498;晶界势垒高度分析表明,SnO2晶粒尺寸的迅速减小是样品击穿电压增高、相对介电常数急剧降低和电阻率迅速增大的主要原因.对Cr含量增加引起SnO2晶粒减小的原因进行了解释.掺杂0.15mol% Cr2O3的SnO2压敏电阻非线性系数为24,击穿电压达498V/mm,在高压保护领域有很好的应用前景.     

Co2O3掺杂对SnO2-Ni2O3-Nb2O5系压敏材料性能的影响

研究并分析了Ni^2＋掺杂和Co^2＋掺杂对SnO2压敏电阻致密度和电学非线性性能的影响。研究了掺Co^3＋对Sno2-Ni2O3-Nb2O5压敏材料性能的影响。实验结果表明，Co2O3在高温下可转变为CoO。Co^2＋的掺入不仅能够增大Sno2-Ni2O3-Nb2O5材料的质量密度，而且在线性系数，在较大程度上提高了Sno2-Ni2O3-Nb2O5压敏材料的性能。     

Oxygen gas-sensing behaviour of V2O5–SnO–TeO2 glass

The d.c. conductivity, σ, and the oxygen gas-sensing behaviour of V₂O₅–SnO–TeO₂ glass prepared by press-quenching were studied in argon and oxygen gas atmospheres at temperatures ranging from 303–473 K. The glass of 50V₂O₅·20SnO·30TeO₂ (mol %) was n-type semiconducting. The high-temperature conductivity was lower in oxygen and higher in argon than that in air. This was explained by the V⁴⁺ ions in the glass being oxidized by oxygen which had diffused into the glass, resulting in an increase in V⁵⁺ with time. The experimental relationship between σ and oxygen partial pressure, P _O2, agreed quantitatively with the theoretical relation σ ∝ P _O2 ^-1/4 . Changes in conductivity by switching the atmospheres between oxygen and argon gases were found to be reproducible. From the data of these dynamic changes, the oxygen gas sensitivity, S, at 473 K was obtained to be 1.3 in oxygen atmosphere. The dynamic changes could be quantitatively explained by an oxygen diffusion model. Throughout these discussions, the present tellurite glass was found to possess a potential applicability as an oxygen gas sensor. This revised version was published online in November 2006 with corrections to the Cover Date.     

New low loss microwave dielectric ceramics in the BaO-TiO2-Nb2O5Ta2O5 system

Microwave ceramic dielectric resonator materials in the BaO-TiO₂-Nb₂O₅Ta₂O₅ system such as BaTiNb₄O₁₃, BaTiTa₂Nb₂O₁₃, BaTiTa₄O₁₃, Ba₃Ti₄Nb₄O₂₁, Ba₃Ti₄Ta₄O₂₁, Ba₃Ti₅Nb₆O₂₈, Ba₃Ti₅Ta₆O₂₈ and Ba₃Ti₅Nb₃Ta₃O₂₈ have been prepared by the conventional solid state ceramic route. They have relatively high dielectric constant and high quality factor. The resonator materials are characterized by X-ray diffraction and scanning electron microscopy (SEM) methods. The BaTiNb₄O₁₃Ba₃Ti₅Nb₆O₂₈ and Ba₃Ti₅Nb₃Ta₃O₂₈ have small temperature variation of the resonant frequency and are possible microwave dielectric resonator materials for practical applications.     

Non-ohmic behaviour of the binary ZnO-Nb2O5 system

Studies have been made of the non-ohmic behaviour of the system ZnO-Nb₂O₅, as a function of composition, in the range 0.1 to 0.5 wt % of Nb₂O₅ and sintering temperature varying from 900 to 1300° C. It is found that the non-linearity coefficient varies with composition and sintering temperature. The maximum value of ( 8) is achieved for the samples containing 0.2 wt% Nb₂O₅, sintered at 1100°C. These results are interpreted in terms of the variation of barrier height with composition.     

Frequency-dependent electrical properties in the system SnO2-TiO2

The a.c. impedance response of polycrystalline ceramics Sn_xTi_-x1O₂ was studied in the air in the temperature range 298–1073 K. The impedance spectra were analyzed in terms of two proposed equivalent circuits involving both resistor and non-Debye constant phase element (CPE). The presence of CPE elements may be interpreted by diffusion phenomena. The results indicate a negligible contribution of grain boundary resistance to the total resistivity of the studied materials. The determined activation energy of the bulk conductivity of TiO₂ was compared with literature data. © 2001 Kluwer Academic Publishers     

球形Ta2O5/Nb2O5制备方法的研究

研究了用氨沉淀氟钽(或铌)酸溶液制备球形Ta2O5/Na2O5工艺.通过实验探讨了沉淀条件、溶液浓度、沉淀时间、焙烧温度等工艺参数的最佳组合.结果表明,用氨沉淀氟钽(或铌)酸溶液制备球形的Ta2O5/Nb2O5,必须首先得到不规则的Ta(OH)5/Nb(OH)5.要得到不规则Ta(OH)5/Nb(OH)5,反应时搅拌强度要大,搅拌速率应控制在800～1 200 r/min,反应结束后继续搅拌10～20 min,终点pH值控制为8.5;氟钽(或铌)酸溶液浓度为60～80 g/L沉淀10 min,浓度为120～140 g/L沉淀15 min,之后可得到不规则Ta(OH)5/Nb(OH)5.再在800～850℃,焙烧6 h可得到球形Ta2O5/Nb2O5.     

Formation of monodispersed Ta2O5 powders

The Ta₂O₅ powders synthesized by the hydrolysis of tantalum pentaethoxide, Ta(OC₂H₅)₅ in alcoholic solution were monodispersed fine oxide particles, which were a uniform, spherical shape, non-agglomerate, and had a narrow size distribution. They grew to 1.2m after ageing for 1 h after hydrolysis. Powder X-ray diffraction and differential thermal analysisthermogravimetric analysis showed the particles were amorphous and hydrated. These particles lost the water at 290° C and gave well-crystalline Ta₂O at 740° C. Throughout these thermal processes, the particle morphology was kept almost the same.