Effect of V2O5 additive to 0.4SrTiO3–0.6La(Mg0.5Ti0.5)O3 ceramics on sintering behavior and microwave dielectric properties

The effect of V₂O₅ addition on the microwave dielectric properties and the microstructures of 0.4SrTiO₃–0.6La(Mg_0.5Ti_0.5)O₃ ceramics sintered for 5 h at different sintering temperature were investigated systematically. It was found that the sintering temperature was effectively lowered about 200 °C by increasing V₂O₅ addition content. The grain sizes, bulk density as well as microwave dielectric properties were greatly dependent on sintering temperature and V₂O₅ content. The 4ST–6LMT ceramics with 0.25% V₂O₅ sintered at 1400 °C for 5 h in air exhibited optimum microwave dielectric properties of ɛ_r = 50.7, Q × f = 15049.6 GHz, T_f = −1.7 ppm/°C.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

La掺杂对Sr2Bi4Ti5O18铁电陶瓷性能的影响

利用传统的固相烧结工艺制备了Sr2LaxBi1-xTi5O18(x=0．00、0．05、0．1、0．25、0．5、0．75、1．00)陶瓷样品。用X射线衍射对其微结构进行了分析．并测量了其铁电、介电性能。X射线衍射结果表明La掺杂对Sr2Bi4TixO18的晶体结构几乎没有影响。样品的铁电、介电结果表明。随着La掺杂量的增加，样品的剩余极化(2Pr)和矫颈场(Ec)逐渐减小。这是由于离子半径较大的La取代类钙钛矿层A位Bi离子。使得样品晶格畸变变小，从而导致2Pr降低。晶格畸变的减小也使得沿着外电场方向氧八面体中的阳离子更易运动。导致Ec减小。样品的相变温度L随着La含量的增加而降低。这也与样品晶格畸变有关。     

Magnetoimpedance effect in semiconducting La0.4Sr0.6MnO3

In the present work, it was found that for La_0.4Sr_0.6MnO₃, the dc resistance decreases with increasing temperature, from 77 to 280 K. Different from the case of metallic La_0.65Sr_0.35MnO₃, the ac impedance of the semiconducting La_0.4Sr_0.6MnO₃ at room temperature decreases with increasing frequency, from 100 kHz to 12 MHz. The magnetoimpedance effect was observed in La_0.4Sr_0.6MnO₃ at room temperature. The value of impedance ratio (Z(0)−Z(H))/Z(0) at H=0.8 kOe reaches 5% at a frequency of 500 kHz.     

Influence of CuO addition to BaSm2Ti4O12 microwave ceramics on sintering behavior and dielectric properties

Microwave dielectric ceramics of tungsten–bronze-type BaSm₂Ti₄O₁₂ were prepared by doping CuO (up to 2 wt.%) as the liquid-phase sintering aid. The effects of CuO additive on the densification, micro structure and dielectric properties were investigated. Due to the liquid-phase effect, the sintering temperature of BaSm₂Ti₄O₁₂ ceramics with 1 wt.% CuO addition can be effectively reduced to 1160 °C, about 200 °C lower than that of pure BaSm₂Ti₄O₁₂ ceramics, while good microwave dielectric properties of ɛ_r = 75.8, Q*f = 4914.6 GHz and τ_f = −7.65 ppm/°C were still achieved.     

La0.6Sr0.4Co0.8Fe0.22O3粉末的合成与性能表征

La0.6Sr0.4Co0.8Fe0.22O3钙钛矿型氧化物具有良好的电子和离子混合导电性能．可作为中温固体氧化物燃料电池（ITSOFC）的阴极材料。本文中采用EDTA-柠檬酸法合成La0.6Sr0.4Co0.8Fe0.22O3（LSCF）粉末，借助差热-热重（DTA／TG）、X射线衍射（XRD）、扫描电镜（SEM）等分析手段．对阴极材料的合成、掺杂机理、与电解质（YSZ）间的高温化学相容性进行了研究和讨论。结果表明：干凝胶的最佳煅烧温度为800℃。所合成的粉末具有单一的钙钛矿结构，平均粒径约为40nm；掺杂后的LSCF仍保持菱方结构；当SOFC的运行温度低于800℃时，LSCF与YSZ具有良好的化学相容性。     

La_(0．6)Sr_(0．4)Co_(0．8)Fe_(0．2)O_3粉末的合成与性能表征

La_(1-x)Sr_xCo_(1-y)Fe_yO_3钙钛矿型氧化物具有良好的电子和离子混合导电性能,可作为中温固体氧化物燃料电池(ITSOFC)的阴极材料。本文中采用EDTA-柠檬酸法合成La_(0．6)Sr_(0．4)Co_(0．8)Fe_(0．2)O_3(LSCF)粉末,借助差热-热重(DTA-TG)、X射线衍射(XRD)、扫描电镜(SEM)等分析手段,对阴极材料的合成、掺杂机理、与电解质(YSZ)间的高温化学相容性进行了研究和讨论。结果表明：干凝胶的最佳煅烧温度为800℃,所合成的粉末具有单一的钙钛矿结构,平均粒径约为40 nm；掺杂后的LSCF仍保持菱方结构；当SOFC的运行温度低于800℃时,LSCF与YSZ具有良好的化学相容性。     

La0.4Ca0.6Mn0.98Ru0.02O3中的电致电阻效应研究

单相多晶锰氧化物样品La_0.4Ca_0.6Mn_0.98Ru_0.02O₃具有典型的电子相分离基态, 即低温下铁磁金属相与电荷有序绝缘相共存并竞争, 这为我们研究电致电阻效应提供了很好的模型。本论文基于此目的, 采用固相合成法制备了La_0.4Ca_0.6Mn_0.98Ru_0.02O₃的多晶陶瓷样品, 用X射线衍射证明了样品是单相, 并详细测量了样品的电致电阻效应。实验结果表明, 该体系的电阻率能够在大范围内被输入电流调控, 获得的电致电阻效应高达80%以上。此外, 低温下该样品的电阻态可以通过改变电流输入强度进行连续转换, 反映了样品基态在电流的激发下发生了改变。通过讨论, 我们认为这些观测到的物理效应起源于输入电流提升了样品中的铁磁金属相的稳定性。     

Microwave dielectric properties of Ca4La2Ti5O17–LaAlO3 system ceramic materials

Compositions in the (1 − x) Ca₄La₂Ti₅O₁₇–xLaAlO₃ system were prepared in order to modify the positive temperature coefficient of the resonant frequency (τ _f ) of Ca₄La₂Ti₅O₁₇. The microwave dielectric properties and phase composition of this system ceramics were investigated. X-ray powder diffraction results showed that Ca₄La₂Ti₅O₁₇ and LaAlO₃ formed a solid solution only when x ≤ 0.2. The τ _f values showed a near linear decrease with increasing additions of LaAlO₃. It was observed that near zero τ _f value can be achieved as x = 0.64. The permittivity (ε_r) and the quality factor (Q × f) values exhibited a non-linear behavior with LaAlO₃ additions. The microwave dielectric properties were strongly correlated with composition, secondary phases and grain sizes. For practical application, a permittivity (ε_r) of 42, a quality factor (Q × f value) of 12,450 GHz and a temperature coefficient of resonant frequency (τ _f ) of ~0 ppm/°C for 0.36 Ca₄La₂Ti₅O₁₇−0.64 LaAlO₃ were proposed.     

La0.6Sr0.4Co1-yFeyO3系阴极材料的电性能

La1-xSrxCo1-yFeyO3系(LSCF)钙钛矿结构氧化物是一类性能优异的离子-电子混合导体,同时具有良好的化学稳定性和催化活性,有望作为中低温下工作的固体氧化物燃料电池阴极材料的候选材料.本文考察了La0.6Sr0.4Co1-yFeyO3(y=0.2～0.8)系材料的导电机制.XPS光电子能谱分析表明,除了存在小极化子导电机制外,同时还可能存在以下几种机理:1)热激发引起Co3 离子的电荷歧化;2)高温下氧空位生成导致的离子补偿;3)Fe4 较Co4 优先进行电荷补偿.     

Charge Stripes and Superconductivity in La2−x Ba x CuO4 and La1.6−x Nd0.4Sr x CuO4

We report x-ray scattering study of charge density wave (stripe) order in La_2−x Ba _x CuO₄ (LBCO) and La_1.6−x Nd_0.4Sr _x CuO₄ (LNSCO). The doping dependence of the charge stripe ordering temperature and the low-temperature correlation length is compared with the bulk superconducting transition temperature. We find that the charge stripe ordering temperature and correlation length are quite different in these two families of cuprates, while there seems to be inverse correlation between the bulk superconducting temperature and the charge stripe correlation length.     

多孔 La0.6Sr0.4Co0.2Fe0.8O3-的制备及表征

多孔La0.6Sr0.4Co0.2Fe0.8O3-陶瓷具有一定的强度、良好的透气和电传导性能,可用于中温SOFC阴极支撑体和氧分离膜活性支撑体. 本文用固相反应法制备了多孔La0.6Sr0.4Co0.2Fe0.8O3-陶瓷. 考察了烧结条件、成型压力和有机添加剂量对孔隙率和孔径的影响. 研究发现气体渗透率随孔隙率线性增长,电导率随孔隙率的增大而下降,并满足关系式=0(1-P)3.1.     

La0.6Sr0.4Co1-yFeyO3系阴极材料制备及表征

La1-xSrxCo1-yFeyO3系(LSCF)钙钛矿结构氧化物是一类性能优异的离子-电子混合导体,同时具有良好的化学稳定性和催化活性,有望作为中低温条件下工作的固体氧化物燃料电池阴极材料的候选材料.本文采用固相反应法制备了多孔La0.6Sr0.4Co1-yFeyO3(y=0.2～0.8)氧化物陶瓷,通过XRD,TG/DTA等测试技术研究了LSCF的结构与制备过程,采用直流四探针法测试了样品的电导率,同时考察了这类组成样品的热膨胀系数.结果表明,这类组成的电导率都有着相同的变化趋势,即先随温度升高而增大,至极大值后又开始降低.分析表明,除了存在小极化子导电机制外,同时还可能存在以下几种机理:1)热激发引起Co3 离子的电荷歧化;2)高温下氧空位生成导致的离子补偿;3)Fe4 较Co4 优先进行电子补偿.     

Silver addition in La2CuO4 ceramics

The effects of Ag addition on the microstructural and electrical properties of sintered La₂CuO₄ have been studied. Unexpectedly, Ag-doped sample shows strong limitation of grain growth. The resistivity behavior of the variable range hopping was observed in the low temperature range (13–41 K) and the ordinary thermal activation process was observed in the high temperature range (41–90 K). A model was proposed to deduce the accurate transition temperature and transition range. The experimental results indicate that the addition of Ag not only increases the hole density but also creates sufficient disorder in the sample.     

Critical Behavior in the La0.6Ca0.4MnO3 Perovskite Manganite

Polycrystalline La_0.6Ca_0.4MnO₃ sample has been prepared by sol-gel method and has an orthorhombic perovskite structure with Pnma space group. Close to magnetic temperature transition, the order transition and the critical behavior are investigated by dc magnetization measurements. Using various techniques such as modified Arrott plots, Kouvel–Fisher method, and Widom scaling relation, the values of T _C ferromagnetic transition temperature, as well as the β, γ, and δ critical exponents, are estimated. The values of critical exponents, derived from the magnetic data, yield 0356 ≤ β ≤ 0382, 1273 ≤ γ ≤ 1291, and 4.37 ≤ δ ≤ 457. The critical exponents reported in this study obey the Widom scaling relation δ = 1 + γ/ β, implying that the obtained values of β and γ are reliable. We conclude that La_0.6Ca_0.4MnO₃ belongs to the 3D-Heisenberg class with short-range interaction.