Crystal growth,structure and optical properties of Pr^(3+)-doped yttria-stabilized zirconia single crystals

The development of blue semiconductor light-emitting diodes(LEDs)has produced potential applications for Prdoped materials that can absorb blue light,especially crystals,and we now report structure and optical properties for high-quality Pr-doped single crystals of yttria-stabilized zirconia(YSZ)grown by the optical floating zone(FZ)method.X-ray diffraction(XRD)and Raman spectroscopy showed that all of the single crystal samples were in the cubic phase,whereas the corresponding ceramic samples contained a mixture of monoclinic and cubic phases.X-ray photoelectron spectroscopy(XPS)and electron paramagnetic resonance(EPR)spectroscopy showed that Pr was present as the Pr^(3+)ion in ceramic rods and single crystals after heating to high temperatures.The absorption and photoluminescence excitation(PLE)spectra of the Pr-doped YSZ crystals measured at room temperature showed strong absorption of blue light,while their photoluminescence(PL)spectra showed five emission peaks at 565 nm,588 nm,614 nm,638 nm,and 716 nm under450 nm excitation.The optimum luminescence properties were obtained with the crystal prepared using 0.15 mol%Pr_(6)O_(11),and those with higher concentrations showed evidence of quenching of the luminescence properties.In addition,the color purity of Pr-doped YSZ single crystal reached 98.9%in the orange–red region.

Densification behavior of yttria-stabilized zirconia powders for solid oxide fuel cell electrolytes

Yttria-stabilized zirconia(YSZ) is the most common electrolyte material for solid oxide fuel cells. Herein, we conducted a comparative study on the densification behavior of three different kinds of commercial 8 mol% YSZ powders:(i) TZ-8Y(Tosoh, Japan),(ii) MELox 8Y(MEL Chemicals, UK), and(iii) YSZ-HT(Huatsing Power, China). The comparison was made on both the selfsupporting pellets and thin-film electrolytes coated onto a NiO–YSZ anode support. For the pellets, MELox 8Y showed the highest densification at lower sintering temperatures with 93% and 96% of the theoretical density at 1250 and 1300 ℃, respectively. Although YSZ-HT showed a higher sintering rate than TZ-8Y, a sintering temperature of 1350 ℃ was required for both the powders to reach 95% of the theoretical density. For the thin-film electrolytes, on the other hand, YSZ-HT showed the highest sintering rate with a dense microstructure at a co-sintering temperature of 1250 ℃. Our results indicate that besides the average particle size, other factors such as particle size distribution and post-processing play a significant role in determining the sintering rate and densification behavior of the YSZ powders. Additionally, a close match in the sintering shrinkage of the electrolyte and anode support is important for facilitating the densification of the thin-film electrolytes.

YSZ超细粉末的溶胶——凝胶法制备

应用溶胶－凝胶方法合成了ＹＳＺ（Ｚｒ０２＋９ｍｏｌ％Ｙ３Ｏ３）超细粉末。对凝胶形成超细粉末的热分解及晶化过程用ｘ射线衍射（ＸＲＤ）和热重一差热（ＴＧ－ＤＴＡ）等分析方法进行了研究。试验表明，溶胶一凝胶法制备ＹＳＺ超细粉末的反应过程分为：水份和有机溶剂的挥发；有机基团的燃烧和无定形Ｚｒ０２的形成；无定形Ｚｒ０２向立方相Ｚｒ０２的转变三个阶段。在４７０℃时，凝胶转变为ＹＳＺ固溶体，形成立方相Ｚｒ０２结构。

送粉位置对等离子喷涂YSZ粒子熔化状态的影响

研究了等离子喷枪采用3个送粉位置时距离喷嘴90mm处YSZ粒子的熔化状态(包括粒子温度及速度、粒子粒度分布、粒子撞击基体后的扁平形貌),其中送粉位置1和2为枪外送粉,送粉孔轴线距离喷嘴端面分别为9mm和3.5mm,送粉位置3为枪内送粉,送粉孔轴线距离喷嘴端面5mm。结果表明,送粉位置沿等离子射流逆流方向移动时(分别对应送粉位置1、2和3),粒子熔化效果明显增强,粒子扁平化程度提高;涂层中未熔颗粒和孔隙百分比由33.25%减小至10.13%,涂层结合强度由16.25MPa增加至37.25MPa,涂层表面剥落20%时经历的热震次数由35次增加至130次;采用送粉位置3时,喷涂功率33.60kW,低于其他送粉位置的电弧功率(46.75kW),实现了低功率制备高性能YSZ涂层。

Fabrication,microstructure,and properties of 8 mol%yttria-stabilized zirconia(8YSZ)transparent ceramics

Fine grained 8 mol%yttria-stabilized zirconia(8YSZ)transparent ceramics with high optical and mechanical properties were fabricated by air pre-sintering and hot isostatic pressing(HIP)using commercial 8YSZ powders as the raw material.The pre-sintered ceramics with fine grains and appropriate relative density play a key role to achieve high transparency and suppressed grain size after HIP post-treatment at relatively low temperatures.With the increase of HIP temperature from 1350 to 1550℃,the in-line transmittance of 8YSZ ceramics at 600 nm increases from 56.9%to 71.5%(2.5 mm in thickness),and the average grain size increases from 2.4 to 16.3μm.The corresponding bending strength of 8YSZ transparent ceramics decreases from 328±20 to 289±19 MPa,the hardness(H)decreases from 12.9±0.1 to 12.5±0.2 GPa,and the fracture toughness(KIC)decreases from 1.30±0.02 to 1.26±0.03 MPa·m^(1/2).Systematical investigations were carried out to study the combination of high optical transparency and excellent mechanical properties in 8YSZ ceramics.

Influence of characteristics of stabilized zirconia electrolyte on performance of cermet supported tubular SOFCs

Ni-Al_(2)O_(3)cermet supported tubular SOFC was fabricated by thermal spraying.Flame-sprayed Al_(2)O_(3)-Ni cermet coating plays dual roles of a support tube and an anode current collector.4.5mol.%yttria-stabilized zirconia(YSZ)and 10mol.%scandia-stabilized zirconia(ScSZ)coatings were deposited by atmospheric plasma spraying(APS)as the electrolyte in present study.The electrical conductivity of electrolyte was measured using DC method.The post treatment was employed using nitrate solution infiltration to densify APS electrolyte layer for improvement of gas permeability.The electrical conductivity of electrolyte and the performance of single cell were investigated to optimize SOFC performance.The electrical conductivity of the as-sprayed YSZ and ScSZ coating is about 0.03 and 0.07 S·cm^(-1)at 1000℃,respectively.The ohmic polarization significantly influences the performance of SOFC.The maximum output power density at 1000℃increases from 0.47 to 0.76 W·cm^(-2)as the YSZ electrolyte thickness reduces from 100μm to 40μm.Using APS ScSZ coating of about 40μm as the electrolyte,the test cell presents a maximum power output density of over 0.89 W·cm^(-2)at 1000℃.

Investigation on the thermo-chemical reaction mechanism between yttria-stabilized zirconia （YSZ） and calciummagnesium-alumino-silicate （CMAS）

Thermal barrier coatings （TBCs） with Y2O3-stabilized ZrO2 （YSZ） top coat play a very important role in advanced turbine blades by considerably increasing the engine efficiency and improving the performance of highly loaded blades. However, at high temperatures, environment factors result in the failure of TBCs. The influence of calcium-magnesium-alumino-silicate （CMAS） is one of environment factors. Although thermo-physical effect is being paid attention to, the thermo-chemical reaction becomes the hot-spot in the research area of TBCs affected by CMAS. In this paper, traditional twolayered structured TBCs were prepared by electron beam physical vapor deposition （EB- PVD） as the object of study. TBCs coated with CMAS were heated at 1240℃ for 3 h. Additionally, 15 wt.% simulated molten CMAS powder and YSZ powder were mixed and heated at 1240℃ or 1350℃ for 48 h. SEM and EDS were adopted to detect morphology and elements distribution. According to XRD and TEM results, it was revealed that CMAS react with YSZ at high temperature and form ZrSiO4, Ca0.2Zr0.8O1.8 and Ca0.15Zr0.85O1.85 after reaction, as a result, leading to the failure of TBCs and decreasing the TBC lifetime.

The first observation of Ni nanoparticle exsolution from YSZ and its application for dry reforming of methane

Ni nanocatalysts produced through exsolution have shown strong resistance to particle sintering and carbon coking in a beneficial dry reforming of methane(DRM)reaction utilizing greenhouse gases such as CH_(4)and CO_(2).However,most of the existing oxide supports for exsolution have been limited to perovskite oxide,while studies on fluorite support have been rarely conducted due to the limited solubility despite its excellent redox stability.Here we demonstrate that 3 mol%Ni can be successfully dissolved into the yttria-stabilized zirconia(YSZ)lattice and be further exsolved to the surface in a reducing atmosphere.The YSZ decorated with exsolved Ni nanoparticles shows enhanced catalytic activity for DRM reaction compared to the conventional cermet type of bulk Ni-YSZ.Moreover,the catalytic activity is extremely stable for about 300 h without significant degradation.Overall results suggest that the YSZ-based fluorite structure can be utilized as one of the support oxides for exsolution.

Thermal Transport in Nanoporous Yttria-Stabilized Zirconia by Molecular Dynamics Simulation

Yttria-stabilized zirconia(YSZ) is widely used as thermal barrier coatings(TBCs) to reduce heat transfer between hot gases and metallic components in gas-turbine engines. Porous structure can generally reduce the lattice thermal conductivity of bulk material, so porous YSZ can be potentially used as TBCs with better thermal performance. In this work, we investigate the thermal conductivity of nanoporous YSZ using the nonequilibrium molecular dynamics(NEMD) simulation, and comprehensively discuss the effects of cross-sectional area, pore size, structure length, porosity, Y_2O_3 concentration and temperature on the thermal conductivity. To compare with the results of the NEMD simulation, we solve the heat diffusion equation and the gray Boltzmann transport equation(BTE) to calculate the thermal conductivity of the same porous structure. From the results,we find that the thermal conductivity of YSZ has a weak dependence on the structure length at the length range from 10 to 26 nm, which indicates that the majority of heat carriers have very short mean free path(MFP) but there exists small percentage(about 3%) of phonons with longer MFP(larger than 10 nm) contributing to the thermal conductivity. The thermal conductivity predicted by NEMD simulation is smaller than that of solving heat diffusion equation(diffusive limit) with the same porous structure. It shows that the presence of pores affects phonon scattering and further affects the thermal conductivity of nanoporous YSZ. The results agree well with the solution of gray BTE with a average MFP of 0.6 nm. The thermal conductivity of nanoporous YSZ weakly depends on the Y_2O_3 concentration and temperature, which shows the phonons with very short MFP play the major contribution to the thermal conductivity. The results help to better understand the heat transfer in porous YSZ structure and develop better TBCs.

Sol－Gel法制备ZrO_2超微粉末的反应机理研究

以自制Ｚｒ（ＯＣ３Ｈ７）４和Ｙ（ＣＨ３ＣＯＯ）３为原料，应用溶胶－凝胶法（Ｓｏｌ－Ｇｅｌ）制备了ＹＳＺ（ＺｒＯ２－９ｍｏｌ％Ｙ２Ｏ３）凝胶和超细粉末．研究了Ｓｏｌ－Ｇｅｌ形成机理以及ｐＨ值对胶凝时间和凝胶结构的影响．通过ＴＧ－ＤＴＡ、ＸＲＤ等分析手段，研究了从干凝胶到ＹＳＺ超细粉末过程中的反应机理．实验证明：反应可分为３个阶段。

用内凝胶法制备球形Y_2O_3-ZrO_2等离子喷涂粉料

以无机物为前驱物，用内凝胶法制备了适合于等离子喷涂的、粒径在10～50μm范围内的、用氧化钇稳定的氧化锆（YSZ）球形粉末，实验结果表明，通过改变乳化条件可以控制这种粉末的形貌、粒径和粒度分布．利用SEM和光学显微镜观察发现，用该方法得到的粉末主要是球形的，并且符合粒径分布的要求（5～50μm）．致密的球形粉末显示出良好的流动性．差热分析表明，用47mol％氧化钇稳定的氧化锆（YSZ）粉末在468℃有一放热峰，是YSZ粉末的相转变温度．XRD分析表明，上述粉末在800℃条件下煅烧可以得到100％非平衡四方相（t’）的YSZ粉末，没有单斜相存在．

Gas-discharge induced flash sintering of YSZ ceramics at room temperature

This is the first study to conduct the flash sintering of 3 mol%yttria-stabilized zirconia(3YSZ)ceramics at room temperature(25℃)under a strong electric field,larger than 1 kV/cm.At the standard atmospheric pressure(101 kPa),the probability of successful sintering is approximately half of that at low atmospheric pressure,lower than 80 kPa.The success of the proposed flash sintering process was determined based on the high electric arc performance at different atmospheric pressures ranging from 20 to 100 kPa.The 3YSZ samples achieved a maximum relative density of 99.5%with a grain size of~200 nm.The results showed that as the atmospheric pressure decreases,the onset electric field of flash sintering decreases,corresponding to the empirical formula of the flashover voltage.Moreover,flash sintering was found to be triggered by the surface flashover of ceramic samples,and the electric arc on the sample surfaces floated upward before complete flash sintering at overly high pressures,resulting in the failure of flash sintering.This study reveals a new method for the facile preparation of flash-sintered ceramics at room temperature,which will promote the application of flash sintering in the ceramic industry.