Optimization of dielectric constant temperature coefficient of pyrochlores containing bismuth

1　INTRODUCTIONDielectric ceramics are widely used with ad vances in microelectronic technologies. Bismuth based pyrochlores exhibit well known high fre quency dielectric properties for multilayer capaci tor. Group of pyrochlore structured compounds isof general formula A2B2O7(where A is trivalent orbivalent cations and B is quadrivalent and quinque valent cations). As a member of the pyrochlorefamily of compound, bismuth zinc niobate is a typi cal high frequenc…     

基于MEMS超级电容器的高介电常数CaCu3Ti4O12薄膜制备及电学特性研究

相对于电化学超级电容器,静电式电容器具有更高的功率密度和可靠性,但能量密度过低。本文提出制备基于高介电常数薄膜CaCu₃Ti₄O₁₂(CCTO)的高能量密度MEMS静电式超级电容器。首先,以硅片为基底,通过溶胶-凝胶法在不同烧结温度(700℃、800℃、900℃)下制备CCTO薄膜,分别采用X射线衍射仪(XRD)和场发射扫描电镜(FE-SEM)对薄膜的形貌、组分和结晶状况进行表征,发现在800℃烧结温度下CCTO薄膜结晶状况最佳。然后,利用金属-绝缘层-半导体结构测试其I-V和C-V特性,计算得到薄膜的最大阈值电压和能量密度分别为47V和3.2J/cm₃。同时,首次对高介电常数介质膜中存在的介质充电现象进行了研究,并分析了介质充电对静电式超级电容器性能的影响。     

Simulations of the dielectric constant of bonding materials and field emission properties of CNT cathodes

The effect of the dielectric constant (k) of bonding materials in a screen-printed carbon nanotube (CNT) cathode on the field enhancement factor was investigated for high-efficiency CNT cathodes using the ANSYS software. The values obtained by a simulation study were compared to the experimental results obtained for screen-printed CNT cathodes. The field enhancement factor increased as the dielectric constant decreased, reaching a maximum value at a dielectric constant of 1, the value for a vacuum. The findings indicate that the larger sheet resistance of the bonding materials, after the firing process, can be attributed to the larger emission current of the CNT cathode. From these results, it was concluded that the best bonding materials for screen-printed CNT cathodes should have a low dielectric constant and a high sheet resistance. This finding can be used as criteria for selecting bonding materials for use in CNT pastes for highly efficient CNT cathodes.     

Origin of giant dielectric constant in Ba[(Fe1−xCox)1/2Nb1/2]O3

The X-ray diffraction Rietveld refinement of Ba[(Fe_1−xCo_x)_1/2Nb_1/2]O₃ with 0 ≤ X ≤ 1 shows cubic structure formation with space group P_m3_m. No distinct tilting of oxygen octahedron is observed. The dielectric measurement of such a cubic system exhibited giant values (?′ > 10⁴) in the temperature range of 298-483 K and frequency range of 10²-10⁵ Hz. An analysis of the permittivity, electric modulus, and electrical conductivity properties in these systems confirmed the presence of oxygen vacancies induced dipolar relaxation. Our investigations show that the observed extremely high dielectric constant values are predominantly the result of oxygen vacancies induced dipoles produced at the grain boundaries. Additional significant intrinsic contributions to the permittivity comes from the directly doped electrons at the unit cell, as indicated by the enhancement in the observed values of the permittivity on replacement of Fe³⁺ (3d⁵) by Co³⁺ (3d⁶). The contributions of the doped free charges and the oxygen vacancy induced dipoles are separated using the Jump Relaxation Model.     

宽带隙n-型半导体CuIn5Se8的热电性能研究

利用放电等离子烧结技术制备了宽带隙三元半导体化合物CuIn5Se8,并对其热电性能进行了研究。物相分析表明,化合物为单相CuIn5Se8,带隙宽度为1.13eV,比In2Se3合金的低。电学性能测试结果表明,随温度升高Seebeck系数绝对值从370.0μV·K1降低到263.0μV·K1,而电导率则随温度迅速增大。在818K时,其电导率达到最大值2.921031·m1,热导率为0.50W·K1·m1,最高热电优值ZT值达到0.33。     

添加Li2O-MgO-B2O3玻璃的Li2MgTi3O8陶瓷的微波介电性能及其与Ag共烧相容性

研究添加Li2O-MgO-B2O3玻璃对Li2MgTi3O8陶瓷的烧结特性、相纯度、微观组织和微波介电性能的影响。结果表明：添加少量的玻璃能有效地将陶瓷的烧结温度从1025℃降低到875℃,且没有恶化陶瓷的微波介电性能。添加1.5%玻璃的陶瓷在875℃烧结4 h后具有优良的微波介电性能能,其介电常数εr=25.9,品质因数Q×f=45403 GHz,谐振频率温度系数τf≈0。陶瓷和Ag电极共烧几乎不发生化学反应,表现为良好的化学相容性。所制备的陶瓷可望用于低温烧结的多层微波器件。     

Crystallization and dielectric properties of lead-free glass-ceramic composites with Gd2O3 addition

Lead-free glass-ceramic composites in barium sodium niobate silica system with Gd2O3 addition were synthesized through melt-casting fol-lowed by controlled crystallization technique. Crystallization and dielectric properties of the Gd2O3 adding glass-ceramic composites were investigated. With the increase in the concentration of Gd2O3, the glass transition temperature and the crystallization temperature of the pre-cursor glass shift towards the higher temperature. The crystallization behavior that occurred during the heat treatment procedure leads to the enhancement of dielectric constant. All the three compositions of glass-ceramic composites exhibit ferroelectricity when tested at room tem-perature. Both the values of the remanent polarization and coercive field are enhanced regularly with the gradual increase in the concentration of Gd2O3 additive under the same testing field.     

Structural characteristics and dielectric properties of glass-ceramic nanocomposites of (Pb,Sr)Nb2O6-NaNbO3-SiO2

The structure and dielectric properties of(Pb,Sr)Nb2O6-NaNbO3-SiO2 glass-ceramics with different Pb and Sr contents were investigated.The XRD pattern of glass-ceramics without Sr substitution is different from that with Sr substitution,which indicates the existence of orthorhombic phase in the latter ones.TEM bright field observation shows nanosized microstructures,while for samples with Sr,typical eutectic microstructure with separated crystallized bands is found in the glass matrix.Dielectric properties measurement of the samples indicates an obvious improvement of dielectric constant,dielectric loss,DC field and temperature dependence of dielectric constant when the molar ratio of Sr to Pb is 4:6.     

p型宽带隙半导体CuAl_(1-x)Fe_xO_2的光电性能研究

采用固相反应合成了CuAl1-xFeO2单相多晶材料,系统报道了该系列样品的X射线衍射(XRD)、紫外吸收光谱以及电学性能的测量.结果表明,Fe3+取代CuAlO2中的Al3+,不改变材料的晶体结构.随着掺杂量的增加,材料的光学带隙宽度逐渐减小,导电性能明显提高.当x=0.10时,样品的室温电导率达到3.38×10-1 S·cm-1;所有掺杂样品的电导率随温度变化曲线在近室温区,很好地符合Arrhenius关系,其热激活能为20～32 meV;Hall系数均为正值,表明所有样品都为P型半导体.     

Microwave dielectric properties of (1 − x)ZnMoO4-xTiO2 composite ceramics

(1 − x)ZnMoO₄-xTiO₂ (x = 0.0, 0.05, 0.158, 0.25, and 0.35) composite ceramics were synthesized by the conventional solid state reaction process. The sintering behavior, phase composition, chemical compatibility with silver, and microwave dielectric properties were investigated. All the specimens can be well densified below 950 °C. From the X-ray diffraction analysis, it indicates that the triclinic wolframite ZnMoO₄ phase coexists with the tetragonal rutile TiO₂ phase, and it is easy for silver to react with ZnMoO₄ to form Ag₂Zn₂(MoO₄)₃ phase and hard to react with TiO₂. When the volume fraction of TiO₂ (x value) increasing from 0 to 0.35, the microwave dielectric permittivity of the (1 − x)ZnMoO₄-xTiO₂ composite ceramics increases from 8.0 to 25.2, the Q_f value changes in the range of 32,300-43,300 GHz, and the temperature coefficient τ_f value varies from −128.9 to 157.4 ppm/°C. At x = 0.158, the mixture exhibits good microwave dielectric properties with a ?_r = 13.9, a Q_f = 40,400 GHz, and a τ_f = +2.0 ppm/°C.     

Production of nanoscale Ba(Zn1/3Nb2/3)O3 microwave dielectric ceramics by polymerised complex method

Ba(Zn_1/3Nb_2/3)O₃ nanoparticles have been synthesized by a polymerised complex method by using precursor materials of barium nitrate, zinc acetate, niobium oxide, hydrofluoric acid and citric acid. Thermal decomposition characteristics and crystallization behavior of the powders were investigated by the thermogravimetric and differential thermal analysis, X-ray diffractometer and Fourier transform infrared spectroscopy. Ba(Zn_1/3Nb_2/3)O₃ phase started to form at low temperature of 400 °C and, single phase Ba(Zn_1/3Nb_2/3)O₃ perovskite structure was obtained at 1000 °C. Microstructural investigation revealed that the major particle size of Ba(Zn_1/3Nb_2/3)O₃ nanoparticles were in the range of 80–110 nm with spherical morphology and homogeneous size distribution. But the powders also contained some agglomeration.     

Microwave dielectric properties and its compatibility with silver electrode of Li2MgTi3O8 ceramics

The effects of BaCu(B₂O₅) (BCB) additions on the sintering temperature and microwave dielectric properties of Li₂MgTi₃O₈ ceramic have been investigated. The pure Li₂MgTi₃O₈ ceramic shows a relative high sintering temperature (∼1000 °C) and good microwave dielectric properties as Q × f of 40,000 GHz, ?_r of 27.2, τ_f of 2.6 ppm/°C. It was found that the addition of a small amount of BCB can effectively lower the sintering temperature of Li₂MgTi₃O₈ ceramics from 1025 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 0.5 wt% BCB added Li₂MgTi₃O₈ ceramic sintered at 900 °C for 2 h exhibited good microwave dielectric properties of Q × f = 36,200 GHz (f = 7.31 GHz), ?_r = 26 and τ_f = −2 ppm/°C. Compatibility with Ag electrode indicates this material can be applied to low temperature-cofired ceramics (LTCC) devices.     

Densification, microstructural evolution, and dielectric properties of hexagonal Ba(Ti1−xMnx)O3 ceramics sintered with fluxes

Recently, doped hexagonal BaTiO₃ (6h-BaTiO₃) ceramics have been reported as potential candidates used in microwave dielectric resonators. However, similar to other common microwave ceramics, doped 6h-BaTiO₃ ceramics require a high sintering temperature, greater than 1300 °C. In this study, the effect of sintering aids, including Bi₂O₃, B₂O₃, BaSiO₃, Li₂CO₃, CuO, V₂O₅, 5ZnO·2B₂O₃, and 5ZnO·2SiO₂, on the densification, microstructural evolution, and microwave properties of the 6h-Ba(Ti_0.85Mn_0.15)O₃ ceramics was examined. Results indicate that among the fluxes studied, Bi₂O₃, B₂O₃, and Li₂CO₃ could effectively reduce the sintering temperature of 6h-Ba(Ti_0.85Mn_0.15)O₃ ceramics through liquid phase sintering, while retaining the hexagonal structure and the microwave dielectric properties. The best results were obtained for the 6h-Ba(Ti_0.85Mn_0.15)O₃ with the additions of 5 wt% Bi₂O₃ sintered at 900 °C (_r: 54.7, Qf_r: 1323, and τ_f:183.3 ppm/°C), 10 wt% B₂O₃ sintered at 1100 °C (_r: 54.4, Qf_r: 3448, and τ_f: 254.5 ppm/°C), and 5 wt% Li₂CO₃ sintered at 950 °C (_r: 43.7, Qf_r: 2501, and τ_f: −29.8 ppm/°C).     

Electromagnetic wave absorbing properties of Fe<Subscript>73</Subscript>Si<Subscript>16</Subscript>B<Subscript>7</Subscript>Nb<Subscript>3</Subscript>Cu<Subscript>1</Subscript> base P/M sheets mixed with dielectric fine particles

Fe-based nanocrystalline powder sheets with dielectric TiO₂ powder additives were investigated to improve the characteristics of electromagnetic (EM) wave absorption. The amorphous ribbons of Fe₇₃Si₁₆B₇Nb₃Cu₁ (at.%) alloys were prepared by a planar flow casting (PFC) process, and the ribbons were pulverized using an attrition mill. Fe-based flake powder crystallized at 550°C for 1h was mixed with a nano-sized and a micro-sized TiO₂ powder. The powder mixtures were then tape-cast with binders to become EM wave-absorbing sheets. The absorbing properties of the fabricated sheet sample, such as complex permittivity and permeability, were measured by a network analyzer. The properties of EM wave absorption improved with the increase of TiO₂ powder in the mixture. The mixture with micro-sized TiO₂ powder was a little more effective in causing power loss of EM waves than the mixture with nano-sized TiO₂ powder.     

Low temperature sintering and microwave dielectric properties of (Mg0.7Zn0.3)0.95Co0.05TiO3 ceramics with BaCu(B2O5) additions

The effects of BaCu(B₂O₅) additives on the sintering temperature and microwave dielectric properties of (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were investigated. The (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics were not able to be sintered below 1000 °C. However, when BaCu(B₂O₅) were added, they were sintered below 1000 °C and had the good microwave dielectric properties. It was suggested that a liquid phase with the composition of BaCu(B₂O₅) was formed during the sintering and assisted the densification of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics at low temperature. BaCu(B₂O₅) powders were produced and used to reduce the sintering temperature of the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics. Good microwave dielectric properties of Q × f = 35,000 GHz, ?_r = 18.5.0 and τ_f = −51 ppm/°C were obtained for the (Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃ ceramics containing 7 wt.% mol% BaCu(B₂O₅) sintered at 950 °C for 4 h.