Comparison of the properties of Nextel 720/SiC composites at room temperature and 1300°C

Continuous Nextel 720 fibers reinforced SiC composites with PyC interface are fabricated by LPCVI at 1000°C for 200 h using SiCH₃Cl₃ as precursor. The mechanical properties at RT and 1300°C are measured by three-point bending. The microstructures of the interface are characterized by TEM. The results indicate the composites have the metal-like behavior of fracture, whether they are at RT or high temperature. The RT and 1300°C strengths are 310 MPa and 140 MPa, respectively. The RT and 1300°C strains are 0.32% and 0.12%, respectively. The loss of flexural strength and strain of the Nextel 720/SiC composites at high temperature result from stronger residual thermal stress caused by the mismatch of CTE between fibers and matrix. A gap appears between fibers and PyC interface after the 1300°C test, which could be resulted from 7.7% compressive strain of PyC interface caused by the residual thermal stress and 0.1% sintering shrinkage of Nextel 720 fiber.     

Spatial and temporal thermal analysis of acousto-optic deflectors using finite element analysis model

Thermal effects greatly influence the optical properties of the acousto-optic deflectors (AODs). Thermal analysis plays an important role in modern AOD design. However, the lack of an effective method of analysis limits the prediction in the thermal performance. In this paper, we propose a finite element analysis model to analyze the thermal effects of a TeO₂-based AOD. Both transducer heating and acoustic absorption are considered as thermal sources. The anisotropy of sound propagation is taken into account for determining the acoustic absorption. Based on this model, a transient thermal analysis is employed using ANSYS software. The spatial temperature distributions in the crystal and the temperature changes over time are acquired. The simulation results are validated by experimental results. The effect of heat source and heat convection on temperature distribution is discussed. This numerical model and analytical method of thermal analysis would be helpful in the thermal design and practical applications of AODs.     

Thermal characterization of nanocrystalline porous CePO4 ceramics

A laser-induced photoacoustic technique was employed to investigate thermal transport through nanocrystalline CePO₄ samples prepared via the sol–gel route. Evaluation of thermal diffusivity was carried out using the one-dimensional model of Rosencwaig and Gersho for the reflection configuration of the photoacoustic method. Structural analyses of samples revealed that they are nanoporous in nature, possessing micron-sized grains. Analysis of results shows that thermal diffusivity value varies with sintering temperature. Results are explained in terms of the variation in porosity with sintering temperature and the effects of various scattering mechanisms on the propagation of phonons through the nanoporous ceramic matrix. Further analyses confirm that apart from porosity, grain boundary resistance and interface thermal resistance influence the effective value of thermal diffusivity of the samples under investigation.     

Quick preparation and thermal transport properties of nanostructured β-FeSi2 bulk material

This paper reports that the nanostructured β-FeSi2 bulk materials are prepared by a new synthesis process by combining melt spinning(MS) and subsequent spark plasma sintering(SPS).It investigates the influence of linear speed of the rolling copper wheel,injection pressure and SPS regime on microstructure and phase composition of the rapidly solidified ribbons after MS and bulk production respectively,and discusses the effects of the microstructure on thermal transport properties.There are two crystalline phases(α-Fe2Si5 and ε-FeSi) in the rapidly solidified ribbons;the crystal grains become smaller when the cooling rate increases(the 20 nm minimum crystal of ε-FeSi is obtained).Having been sintered for 1 min above 1123 K and annealed for 5 min at 923 K,the single-phase nanostructured βFeSi2 bulk materials with 200-500 nm grain size and 98% relative density are obtained.The microstructure of β-FeSi2 has great effect on thermal transport properties.With decreasing sintering temperature,the grain size decreases,the thermal conductivity of β-FeSi2 is reduced remarkably.The thermal conductivity of β-FeSi2 decreases notably(reduced 72% at room temperature) in comparison with the β-FeSi2 prepared by traditional casting method.     

Transition from antiferromagnetic to ferromagnetic state of systems LaMnO3+λ and La1−x Srx(Mn1−x/2Nb x/2)O3

The crystal structure and magnetic and elastic properties of the system LaMnO_3+λ are investigated for various concentrations of oxygen. Upon an increase in the oxygen concentration, the orbital-ordered phase is transformed into an orbital-disordered phase via a two-phase crystal-structure state in the interval 0.04<λ<0.06. The transition is accompanied by a jumplike increase in the Curie temperature and spontaneous magnetization. An analysis of the magnetic properties in weak fields and of the temperature dependence of the Young modulus reveals the properties typical of the orbital-ordered antiferromagnetic phase up to λ=0.08. It is proposed that the two-phase state is associated with the martensite type of the orbital order-disorder phase transformation. The system La_1?x Sr_x(Mn_{1 ?x/2}Nb _x/2)O₃ in which all manganese ions are in the trivalent state exhibits a sequence of antiferromagnetic-ferromagnetic (x>0.2) and ferromagnetic-spin glass (x>0.4) transitions. In both systems, the orbital-disordered phases are ferromagnetic, indicating the crucial role of orbital ordering in the formation of magnetic properties.     

Structural and spectroscopic studies of Lu2O3/Eu nanocrystallites embedded in SiO2 sol-gel ceramics

Preparation and optical properties of Lu₂O₃/Eu³⁺ nanocrystals embedded in silica gel glass are described. The structure of the obtained ceramics was determined by XRD and transmission electron microscopy. It was found that with increasing sintering temperature the Lu₂O₃/Eu³⁺ nanocrystallites underwent transformation into monoclinic Lu₂Si₂O₇ nanocrystallites. The fluorescence properties were investigated as a function of thermal treatment of the samples. It was found that after sintering at 1250 °C the fluorescence lifetime increased to 1.5 ms at room temperature. The material studied in this work exhibits a very high intensity of fluorescence with potentials for display application.     

Anisotropy of mechanical and thermal properties of perovskite LaYbO3: first-principles calculations

LaYbO₃ ceramic material with a perovskite structure has the advantages of a high melting point, sintering resistance and high-temperature phase stability. It is a promising candidate for a structurally and functionally integrated material. However, the anisotropy of physical properties of LaYbO₃ has rarely been studied. Herein, the anisotropy of the mechanical and thermal properties of LaYbO₃ was studied by first-principles calculations. The elastic coefficients, Young’s modulus, Poisson’s ratio and minimum thermal conductivity of LaYbO₃ were found to exhibit anisotropic characteristics. In particular, the sound velocity of the longitudinal wave was nearly twice that of the transverse wave. Especially, the minimum thermal conductivity of LaYbO₃ at high temperature was found to be as low as 0.88?W/m?K, indicating that the compound has potential for use in thermal insulation applications.     

Enhancing magnetoelectric properties of 0–3 particulate composite ceramics by introducing nano-sized sintering aids via self-combustion method

The 0.55Pb(Ni_1/3Nb_2/3)O₃-0.05PbHfO₃-0.4PbTiO₃/Ni_0.875Zn_0.125Fe₂O₄ (PNNHT/NZF) 0–3 particulate composite ceramics were prepared by the conventional solid-state reaction method via introducing the nano-sized WO₃ and CuO sintering aids by the self-combustion method. Due to the introducing of nano-sized sintering aids, densified PNNHT/NZF composite ceramics are synthesized sintered at low sintering temperature. Furthermore, the nano-sized sintering aids act effectively as blocking layers, leading to compact composite ceramics with homogenously dispersed and isolated microstructure morphology, inhibiting inter-phase diffusion, avoiding interfacial chemical reaction, and achieving strong inter-phase coupling. Therefore, the synthesized PNNHT/NZF composite ceramics exhibit enhanced magnetoelectric properties.     

Phase transition and high temperature thermoelectric properties of copper selenide Cu2-x Se （0 ≤ x ≤ 0.25）

With good electrical properties and an inherently complex crystal structure, Cu_2-xSe is a potential “phonon glass electron crystal” thermoelectric material that has previously not attracted much interest. In this study, Cu_2-xSe (0 ≤ x ≤ 0.25) compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu_2-xSe were investigated in the temperature range of 300 K—750 K. The results of X-ray diffraction at room temperature show that Cu_2-xSe compounds possess a cubic structure with a space group of Fm3m (#225) when 0.15 < x le 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤ x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu₂Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.     

Effect of BiFeO3 addition on Bi2O3–ZnO–Nb2O5 based ceramics

In this paper, low temperature sintering of the Bi₂(Zn_1/3Nb_2/3)₂O₇ (β-BZN) dielectric ceramics was studied with the use of BiFeO₃ as a sintering aid. The effects of BiFeO₃ contents and the sintering temperature on the phase structure, density and dielectric properties were investigated. The results showed that the sintering temperature could be decreased and the dielectric properties could be retained by the addition of BiFeO₃. The structure of BiFeO₃ doped β-BZN was still the monoclinic pyrochlore phase. The sintering temperature of BiFeO₃ doped β-BZN ceramics was reduced from 1000 °C to 920 °C. In the case of 0.15 wt.% BiFeO₃ addition, the β-BZN ceramics sintered at 920 °C exhibited good dielectric properties, which were listed as follows: ε_r = 79 and tan δ = 0.00086 at a frequency of 1 MHz. The obtained properties make this composition to be a good candidate for the LTCC application.     

Microstructure and dielectric properties of Li2CO3 doped 0.7(Ba,Sr)TiO3–0.3MgO ceramics

Microstructure and dielectric properties of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃–0.3MgO ceramics for the low temperature sintering and microwave applications will be presented. In these days, low temperature sintering process has been widely spread out for the integrated electronic modules for the communication systems such as front-end modules, antenna modules, and switching modules. We have added Li₂CO₃ and MgO to (Ba,Sr)TiO₃ material to reduce the sintering temperature and improve dielectric properties such as loss tangent, and frequency dispersion.In this paper, we have discussed the crystalline properties, dielectric properties, and the microstructures of Li₂CO₃ doped 0.7(Ba,Sr)TiO₃–0.3MgO ceramics. No pyro phase was observed in the X-ray diffraction method. Very weak frequency dispersion (<0.7%) of dielectric permittivity was observed from the 1 kHz to 1 MHz range. We found that the grain size of BST is around 2 μm, while the grain size of Li₂CO₃ dope 0.7BST–0.3MgO is around 4 μm from the SEM analysis.     

Influence of sintering conditions on structural,thermal, electric and ferroelectric properties of Na0.5Bi0.5TiO3 ceramics

Na_0.5Bi_0.5TiO₃ ceramics were prepared by a conventional solid-state reaction method and by a hot-pressing route. The influence of sintering conditions on structural, thermal, dielectric and ferroelectric properties of these ceramics was investigated. All obtained samples exhibited a single perovskite phase. It was shown that the sintering conditions significantly influence the properties under investigation. This includes changes in the value of the electric permittivity ? and dielectric loss tanδ, a shift of T_m and T_d and change of the ferroelectric properties. These effects are mainly related to volatility of the Na and Bi components during the sintering process along with formation of their compensating charge defects, which leads to local structure change.     

Laser-quality oxide Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript> ceramics. Comparative studies of its basic characteristics and laser ceramics of a known manufacturer

The effect of sintering aids of SiO₂, ZrO₂, B₂O₃, and MgO oxides on the optimum sintering temperature, ceramics grain growth, total volume of residual pores, and optical quality of obtained ceramics is studied. The best combinations of sintering aids are found; as a result, YAG:Nd (1 at%) samples of ceramics of high optical quality are obtained. An original method for characterizing laser properties of ceramics is developed. Comparative measurements of main laser characteristics of the obtained ceramics and ceramics of the Konoshima Chemical Corp. Ltd wellknown in the world practice, are performed.     

Thermal properties and ionic conductivity of Li<Subscript>1,3</Subscript>Ti<Subscript>1,7</Subscript>Al<Subscript>0,3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> solid electrolytes sintered by field-assisted sintering

Li_1,3Ti_0,7Al_0,3(PO₄)₃ (LATP) powder was obtained by a conventional melt-quenching method and consolidated by field-assisted sintering technology (FAST) at different temperatures. Using this technique, the samples could be sintered to relative densities in the range of 93 to 99 % depending on the sintering conditions. Ionic and thermal conductivity were measured and the results are discussed under consideration of XRD and SEM analyses. Thermal conductivity values of 2 W/mK and ionic conductivities of 4?×?10^?4 Scm^?1 at room temperature were obtained using relatively large particles and a sintering temperature of 1000 °C at an applied uniaxial pressure of 50 MPa.     

Deposition of ZnO Nanocrystals on Fe3O4 Nanocubes and Their Special Luminescent and Magnetic Properties

With increasing miniaturization, it is extremely important to maintain the magnetization stability at small scale. Herein, more efforts and interests focus on the interface of magnetic core and semiconductor shell to obtain desired magnetic and/or luminescent properties. Here, Fe₃O₄ nanocubes are synthesized via a thermal decomposition followed by coating ZnO nanocrystals. To create a large interface, large Fe₃O₄ nanocubes with 78 ± 3 nm average side‐length are synthesized through adjusting the ratio of iron precursor to stabilizer. The average diameter of the particular ZnO nanostructures coated on the nanocubic Fe₃O₄ is around 10 ± 2 nm. In addition to the photoluminescent properties of the ZnO‐coated nanostructures, core‐shell Fe₃O₄@ZnO nanostructures demonstrate enhanced UV absorption at 360 nm, which has a 20 nm blueshift compared to bulk ZnO. The superparamagnetic properties of Fe₃O₄@ZnO core–shell hybrid nanocrystals at room temperature are dominated by the ferromagnetic properties when the temperature is lower than the Blocking temperature, 235.7 K. The observed exchange bias and temperature‐dependent magnetization can result from the interfacial interphase between ZnO and Fe₃O₄. The anisotropy contributed by the interfacial interphase allows the nanostructures to maintain stable magnetization in miniaturized devices.     

Preparation and properties of Nd-doped BCTH lead-free ceramics by solid-phase twin crystal method

0.5 mol% Nd-doped (Ba_0.85Ca_0.15)(Ti_0.9Hf_0.1)O₃ (BCTH-Nd) lead-free ceramics were prepared by a solid-phase twin crystal method, where the effects of sintering condition on structure, electrical and optical properties were studied. All the sintered BCTH-Nd ceramics exhibit pure perovskite structure, dense microstructure with several micron grain size, which tends to increase with elevating sintering temperature. All synthesized ceramics have complex dielectric behavior, which presents normal ferroelectrics characteristic with slight dispersion phenomenon. The BCTH-Nd ceramics exhibit excellent piezoelectric and ferroelectric properties and acceptable dielectric performance when sintered at 1480 °C for 2 h. Under 269 nm light excitation, several fluorescent emission peaks are excited with a whole indigo fluorescence, where the strongest emission peak is emitted at 473 nm, corresponding to the ⁴G_3/2 → ⁴I_9/2 energy level transition of Nd³⁺. Multifunctional performance is fulfilled in the lead-free BCTH ceramics via rare earth doping, which can broaden the application fields of piezoelectric-based materials.     

p型硅MOS结构Si/SiO2界面及其附近的深能级与界面态

用深能级瞬态谱(DLTS)技术系统研究了Si/SiO₂界面附近的深能级和界面态。结果表明,在热氧化形成的Si/SiO₂界面及其附近经常存在一个浓度很高的深能级,它具有若干有趣的特殊性质,例如它的DLTS峰高度强烈地依赖于温度,以及当栅偏压使费密能级与界面处硅价带顶的距离明显小于深能级与价带顶的距离时,仍然可以观测到一个很强的DLTS峰。另外,用最新方法测量的Si/SiO₂界面连续态的空穴俘获截面与温度有关,而与能量位置无明显关系,DLTS测 关键词：     

Synthesis of submicron CaZrO<Subscript>3</Subscript> in combustion reactions

Submicron CaZrO₃ powder is obtained in combustion reactions (solution combustion synthesis—SCS) with glycine. It is found that SCS reduces the sintering temperature of CaZrO₃ powders. The dielectric properties of calcium zirconate ceramics are studied by the electrochemical impedance method. It is shown that a ceramics of powders obtained by the SCS method has high dielectric characteristics.     

3ω法加热/测温膜中温度波解析及其在微/纳米薄膜导热系数测量中的应用

给出了3ω法测试系统中描述薄膜表面加热/测温膜中温度波动的级数形式解，并将复数温度波动的实部和虚部分开表示.利用该解分析了交流加热频率、加热膜宽度和材料热物性的组合参数对加热膜温度波动幅度的影响.并根据此解对测量原理的数学模型进行了修正，建立了相应的3ω测试系统，首先测定了厚度为500 nm SiO₂薄膜的导热系数，验证了实验系统的合理性.加大了测试频率，利用级数模型在高频段直接得到SiO₂薄膜的导热系数，结合低频段的数据同时确定了Si基体的导热系数.利用级数解分析测试了激光晶体Nd:YAG〈111〉面上多层ZrO₂/SiO₂增透膜的导热系数，测试的ZrO₂薄膜的导热系数比体材料小.进行了不确定度分析.结果表明，提出的分析方法可以有效研究微器件表面薄膜结构的导热性能. 关键词： ω法')" href="#">3ω法 微/纳米薄膜 导热系数 微尺度加热膜     

Synthesis and magnetic properties of hard magnetic (CoFe2O4)-soft magnetic (Fe3O4) nano-composite ceramics by SPS technology

CoFe₂O₄/Fe₃O₄ nano-composite ceramics were synthesized by Spark Plasma Sintering. The X-ray diffraction patterns show that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases when the sintering temperature is below 900 °C. It is found that the magnetic properties strongly depend on the sintering temperature. The two-step hysteresis loops for samples sintered below 500 °C are observed, but when sintering temperature reaches 500 °C, the step disappears, which indicates that the CoFe₂O₄ and Fe₃O₄ are well exchange coupled. As the sintering temperature increases from 500 to 800 °C, the results of X-ray diffractometer indicate the constriction of crystalline regions due to the ion diffusion at the interfaces of CoFe₂O₄/Fe₃O₄ phases, which have great impact on the magnetic properties.