Crystal structure and microwave dielectric properties of a new A4B3O12-type cation-deficient perovskite Ba3LaTa3O12

A new microwave dielectric ceramic Ba₃LaTa₃O₁₂ has been prepared by conventional solid-state ceramic route. Through Rietveld refinement of X-ray powder diffraction data, the compound is identified as an A₄B₃O₁₂-type B-site cation-deficient perovskite with space group and lattice constants a = 5.7573(2) Å, c = 28.2386(2) Å, V = 810.62(4) Å³ and Z = 3. The microwave dielectric properties of Ba₃LaTa₃O₁₂ were investigated. The compound exhibits a relative dielectric constant (_r) of 36.8, a quality factor Q_u × f of 21,965 at 6.4040 GHz and a small negative temperature coefficient of resonant frequency (τ_f) of −49.6 ppm/°C.     

Preparation and thermal conductivities of Gd2Ce2O7 and (Gd0.9Ca0.1)2Ce2O6.9 ceramics for thermal barrier coatings

Two kinds of rare earth cerium oxides Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 were prepared by solid state reaction method at 1600 °C for 10 h. The phase compositions, microstructures, and their thermal conductivities of these materials were investigated. XRD results revealed that single phase Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 with fluorite structure were synthesized. Results of SEM and EDS showed that the microstructures of these materials were dense and no other phases existed among grains. Because of phonon scattering resulted by the oxygen vacancies and difference in atomic mass between substitutional atoms and host atoms, thermal conductivities of Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 are lower than that of 8YSZ at 800 °C, and thermal conductivity of (Gd_0.9Ca_0.1)₂Ce₂O_6.9 is lower than that of Gd₂Ce₂O₇. These results imply the Gd₂Ce₂O₇ and (Gd_0.9Ca_0.1)₂Ce₂O_6.9 can be used as novel candidate materials for thermal barrier coatings in the future.     

Effects of Y2O3 addition on the phase evolution and thermophysical properties of lanthanum zirconate

Lanthanum zirconate (La₂Zr₂O₇, LZ) powders with the addition of various Y₂O₃ contents for potential thermal barrier coatings (TBCs) application were synthesized by solid-state reaction. The structure evolution, sintering-resistance and thermophysical properties of the synthesized powders and sintered ceramics were systematically studied. X-ray diffraction (XRD) results indicate that LZ containing 3–12 wt.% Y₂O₃ mainly keeps a pyrochlore-type structure, and two new phases of LaYO₃ and Y_0.18Zr_0.82O_1.91 are also detected. Raman spectra confirm that the higher the Y₂O₃ content, the easier is the formation of LaYO₃. With the increase of Y₂O₃ content, the linear thermal expansion coefficients (TEC) of different La₂O₃–ZrO₂–Y₂O₃ ceramics may gradually decrease. Additionally, LZ ceramics doped with 3 wt.% Y₂O₃ have the lowest relative density and the highest sintering-resistance among all samples, implying that it is the best candidate for TBCs among those samples in this investigation.     

等离子喷涂Gd2Zr2O7-SrZrO3复合陶瓷涂层的微观结构和性能

通过固相反应法合成了Gd2Zr2O7-SrZrO3 (GZSZ,Gd2Zr2O7:SrZrO3=7:3)复合陶瓷粉末,并采用喷雾造粒法和大气等离子喷涂法分别制备了适合等离子喷涂使用的相应喷涂粉末及涂层。使用X射线衍射、扫描电子显微镜对粉末和涂层的相组成、显微结构进行分析。借助激光热导仪、高温热膨胀仪对涂层的热扩散系数和热膨胀系数、烧结系数进行了表征。结果表明,制备的GZSZ复合陶瓷粉末和涂层都由Gd2Zr2O7和SrZrO3两相组成,粉末中的Gd2Zr2O7为烧绿石结构,而涂层中的Gd2Zr2O7为萤石结构,SrZrO3都为钙钛矿结构。制备态GZSZ涂层的孔隙率为~14%。GZSZ涂层1400℃热处理5 h后的热膨胀系数为(9.8~11.2)×10-6 K-1。制备态GZSZ涂层的热导率为~0.8 W·m-1·K-1,与制备态SrZrO3涂层的热导率~1.0 W·m-1·K-1相比降低~20%。1400℃热处理360 h后GZSZ涂层的热导率增加到~1.5 W·m-1.K-1。综上表明,GZSZ涂层是一种很有前景的复合陶瓷热障涂层材料。     

Preparation and luminescent properties of europium-activated YInGe2O7 phosphors

Effects of precursor milling on phase evolution and morphology of mullite (3Al₂O₃·2SiO₂) processed by solid-state reaction have been investigated. Alumina and silica powders were used as starting materials and milling was taken place in a medium energy conventional ball mill and a high-energy planetary ball mill. Milling in a conventional ball mill although decreases mullite formation temperature by 200 °C, but does not considerably change mullite phase morphology. Use of a planetary ball mill after 40 h of milling showed to be much more effective in activating the oxide precursors, and mullitization temperature was reduced to below 900 °C. Whisker like mullite was formed after sintering at 1450 °C for 2 h and volume fraction of this structure was increased by increasing the milling time. XRD results showed that samples mechanically activated for 20 h in the planetary ball mill were fully transformed to mullite after sintering at 1450 °C, whereas Al₂O₃ and SiO₂ phases were still detected in the samples milled in the conventional ball mill for 20 h and then sintered at the same conditions.     

Influence of B2O3 additives on microstructure and electrical properties of ZnO-Bi2O3-Sb2O3-based varistors

B₂O₃-doped ZnO-Bi₂O₃-Sb₂O₃-based varistors were fabricated by conventional ceramic technique. The microstructure and electrical properties were investigated by SEM, XRD and electrical measurements. With the addition of B₂O₃, the liquid-assisted sintering based on Bi₂O₃ was improved, and the Bi₂O₃-B₂O₃ glass and Zn₃(BO₃)₂ phase were formed on the grain boundaries. The doping of B₂O₃ markedly improved the varistor performance of the ZnO-Bi₂O₃-Sb₂O₃-based varistors. The nonlinear coefficient of the sample with 3.5 mol% B₂O₃ sintered at 1100 °C reached 56 and the leakage current was only 0.3 μA.     

Thermal cycle behavior of plasma sprayed La2O3, Y2O3 stabilized ZrO2 coatings

The effects of La₂O₃ addition on thermal conductivity, phase stability and thermal cycle life of Y₂O₃ stabilized ZrO₂ plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La₂O₃ addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life.     

Optical and thermal properties of P2O5-Na2O-CaO-Al2O3:CoO glasses doped with transition metals

The aim of this paper is to report the optical and thermal properties of V₂O₅ and CuO doped P₂O₅-Na₂O-CaO-Al₂O₃:CoO glasses so as to investigate their possible use in solar collection applications. The optical absorption spectra of the glasses at room temperature were in the spectral range of 200-1100 nm. The optical band gaps of the glass samples were determined for direct and indirect transitions. When transition metal ions doped to the base glass, the optical band gap decreased. Changes in the refractive indices vs. wavelength for all the specimens were also examined by spectroscopic ellipsometry. By measuring the heat capacities and thermal diffusion coefficients of the specimens at varying temperatures, their thermal conductivities were calculated to be in the 320-620 K temperature range. The obtained glasses seem to be promising materials and can be used in solar collector applications.     

Sintering effect on electrical properties of ZnO-V2O5-MnO2-Nb2O5 ceramics

The microstructure and electrical properties of quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics were investigated at different sintering temperature (875-950 °C). The average grain size increased from 4.4 μm to 9.6 μm with increasing sintering temperature. The breakdown field decreased from 6991 V/cm to 943 V/cm with increasing sintering temperature. Proper sintering for quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics led to surprisingly high nonlinear coefficient (50). The donor concentration increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with increasing sintering temperature and the barrier height exhibited the maximum value (1.07 eV) at 900 °C.     

A2Zr2O7型稀土锆酸盐热障涂层研究进展

综述了国内外稀土锆酸盐热障涂层在制备技术,纳米涂层,涂层结构及涂层的热物理性能、力学性能及热腐蚀性能等方面的研究成果,讨论了稀土锆酸盐热障涂层在每个方面研究存在的不足。指出未来应该进一步改善稀土锆酸盐涂层的制备工艺及后处理工艺,提高涂层的结合强度,延长涂层的服役寿命,改善涂层耐腐蚀、抗烧结等性能;开发新的涂层制备工艺,重点研究各类纳米稀土锆酸盐涂层的性能;进一步提高涂层的隔热效果、服役温度及工作寿命。     

Gd2-xNdxZr2O7烧绿石的高温高压合成及其相组成研究

为探索高温高压合成条件对Gd2-xNdkxZr2O7固溶体相组成、相结构以及相转变的影响,在不同合成压力(3～4 GPa)和温度(1 573～1 673 K)下制备了Gd2-xNdxZr2O7(x=0.2,0.4)系列样品,并利用X射线衍射仪、扫描电镜对样品进行分析.结果表明:所有样品均获得了以立方烧绿石(Gd,Nd)2Zr2O7为主相的混合相,未能生成单一的立方烧绿石相,其原因可能是合成时间过短和压力诱导相变所致;在高温高压合成条件下,不同掺杂量的Nd在极短合成时间内可全部固溶进入陶瓷基材的晶格中;Nd掺杂量降低,高温+高压将促进烧绿石相转变,其有序化程度也逐渐下降;Nd掺杂量升高,高温+高压则是形成烧绿石相的理想条件;在高温高压合成条件下,样品晶粒细小,分布均匀且致密性好.     

Physical properties of Gd7NiPd2 single crystal

Investigations of X-ray diffraction, electronic structure, dc-magnetization M(T), ac-magnetic susceptibility χ_ac(T) and magnetocaloric properties for the Gd₇NiPd₂ single crystal were performed. A single crystal of Gd₇NiPd₂ was grown by the Czochralski method from a levitating melt. Anomalies in χ_ac(T) and M(T)-curves establish that Gd₇NiPd₂ undergoes a long-range ferromagnetic-type ordering at T_C = 298 K, followed by a spin-reorientation below 135 K. The magnetization data indicate that there is an excess magnetic moment calculated per Gd³⁺ ions. The measured XPS valence band indicates the hybridization effect between Gd 5d, Ni 3d and Pd 4d states. The calculated values of entropy change ΔSm for the examined compound amount to −6.85 J/K kg for the a-axis and −6.49 for the c-axis at 7 T.     

Influence of microstructure on the thermophysical properties of sintered SiC ceramics

The specific heat, thermal diffusivity and thermal conductivity of porous SiC ceramics sintered using two kinds of SiC powders (fine and coarse) have been investigated for sintering temperatures in the range 1700–2000 °C. Sintered SiC has a porous structure with approximately 30–40 vol.% porosity. Thermal diffusivity was measured by the laser flash method. The thermal diffusivities and thermal conductivities of sintered SiC ceramics increased with increasing sintering temperature. The specific heat decreased slightly with increasing sintering temperature. The thermal diffusivities and thermal conductivities of SiC sintered from coarse powder were higher than those of SiC sintered from fine powder. The thermal conductivity of samples increased markedly with increasing grain size.     

(Gd0.8Ca0.2)2Ce2O6.6陶瓷的制备及热导率

以Gd2O3、CeO2和CaO为原料,采用固相反应法制备(Gd0.8Ca0.2)2Ce2O6.6陶瓷材料。用X射线衍射(XRD)技术和扫描电镜(SEM)研究样品的相组成和微观组织,用激光脉冲法测试样品的热扩散系数。结果表明,所制备的样品具有单一的萤石结构,组织致密且晶界清晰,其在800℃的热导率仅为YSZ陶瓷的89%,较低热导率表明该材料有潜力用作新型热障涂层用陶瓷材料。     

YSZ/Sm_2Zr_2O_7复合粉体制备及性能

采用球磨造粒法制备Sm_2Zr_2O_7(SZO)和Y_2O_3部分稳定ZrO_2(YSZ)复合粉体,对造粒团聚体的尺寸、形貌及相结构进行了表征。并采用大气等离子制备SZO/YSZ复合涂层,研究等离子喷涂过程对复合粉体相结构和相稳定性的影响。用扫描电子显微镜(SEM)和X射线衍射(XRD)分析了YSZ/SZO复合粉体和涂层的微观组织和相结构,并利用差示扫描量热法(DSC)研究复合粉体稳定性。结果表明:SZO和YSZ复合团聚粉体表面光滑致密,室温呈混合相结构。在室温～1200°C范围内无相变发生,说明YSZ/SZO粉末在该范围内较为稳定。等离子喷涂YSZ/SZO涂层呈典型层状组织结构,涂层成分和组织分布较为均匀,与单一SZO涂层相比,复合陶瓷涂层结合强度得到了提高。但等离子喷涂过程中SZO与YSZ发生离子扩散,粉体稳定性下降,SZO发生有序-无序转变,涂层呈现单一萤石结构。     

Effect of LiSbO3 on the phase structure, microstructure and electric properties of Sr0.53Ba0.47Nb2O6 ceramics

LiSbO₃ doped Sr_0.53Ba_0.47Nb₂O₆ ceramics were synthesized by conventional mixed-oxide method. The phase structure, microstructure, dielectric and ferroelectric properties of obtained ceramics were investigated. Pure tungsten bronze structure could be obtained in all ceramics and LiSbO₃ additive could promote densification and reduce the sintering temperature. The dielectric characteristics showed diffuse phase transition phenomena for all samples, which was proved by linear fitting of the modified Curie-Weiss law with γ value varying between 1.65 and 1.92. With increasing LiSbO₃ content, the transition temperature T_c decreased gradually to near room temperature. Normal ferroelectric hysteresis loops could be observed in all compositions, but the remnant polarization (P_r) and coercive field (E_c) all decreased gradually. Besides, the underlying mechanism for variations of the electrical properties caused by LiSbO₃ doping was explained in this work.     

表面陶瓷层厚度对Sm2Zr2O7/8YSZ热障涂层热冲击性能的影响

稀土锆酸盐与8YSZ所组成的双陶瓷层涂层是目前热障涂层领域研究的热点,而陶瓷层厚度对其热冲击性能有着显著影响。采用有限元软件ANSYS研究了表层厚度对Sm2Zr2O7/8YSZ热障涂层淬冲击热应力的影响,并与单一Sm2Zr2O7涂层进行了比较。结果表明,在Sm2Zr2O7/8YSZ涂层的表面处具有最大的径向热冲击应力,最大轴向应力则存在于陶瓷层/金属粘结层界面处,涂层各处剪应力基本相当。涂层表面及两陶瓷层界面处的径向热应力随表层厚度的增加而减小,陶瓷层/粘结层界面处径向应力则随表层厚度增加而增大。每个界面处的轴向应力随表层厚度增加而降低,而剪应力绝对值则随表层厚度增加而增大。与单一Sm2Zr2O7涂层相比,Sm2Zr2O7/8YSZ涂层的热应力明显偏小,说明增加涂层的层数有利益改善涂层的抗热冲击性能。     

Effect of Sm2O3 content on microstructure and thermal conductivity of Spark Plasma Sintered AlN ceramics

Dense aluminum nitride ceramics were prepared by Spark Plasma Sintering at a lower sintering temperature of 1700 °C with Sm₂O₃ as sintering additives. The effect of Sm₂O₃ content on the density, phase composition, microstructure and thermal conductivity of AlN ceramics was investigated. The results showed that Spark Plasma Sintering could fabricate dense AlN ceramics with superior thermal properties in a very short time. Sm₂O₃ not only facilitated the densification via the liquid-phase sintering mechanism but also improved thermal conductivity by decreasing oxygen impurity. Thermal conductivity decreased with increasing amount of Sm₂O₃ and the highest thermal conductivity was obtained for the AlN ceramics with 2 wt.% Sm₂O₃ content. During Spark Plasma Sintering process, only 2–3 wt.% sintering additives was enough to fabricate dense AlN ceramics, and the microstructures played a key role in controlling the thermal conductivity of AlN ceramics.     

Preparation, structure and electrical conductivity of the pyrochlore-type phase Sm2Zr2O7 codoped with bivalent magnesium and trivalent dysprosium cations

The pyrochlore-type phases with the compositions of SmDy_1−xMg_xZr₂O_7−x/2 (0 ≤ x ≤ 0.20) have been prepared by pressureless-sintering method for the first time as possible solid electrolytes. The structure and electrical conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics have been studied by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy measurements. SmDy_1−xMg_xZr₂O_7−x/2 (x = 0, 0.05, 0.10) ceramics exhibit a single phase of pyrochlore-type structure, and SmDy_1−xMg_xZr₂O_7−x/2 (x = 0.15, 0.20) ceramics consist of pyrochlore phase and a small amount of the second phase magnesia. The total conductivity of SmDy_1−xMg_xZr₂O_7−x/2 ceramics obeys the Arrhenius relation, and the total conductivity of each composition increases with increasing temperature from 673 to 1173 K. SmDy_1−xMg_xZr₂O_7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10⁻⁴ to 1.0 atm at all test temperature levels. The highest total conductivity value is about 8 × 10⁻³ S cm⁻¹ at 1173 K for SmDy_1−xMg_xZr₂O_7−x/2 ceramics.     

Thermal conductivity and thermal cycle life of La2O3 and HfO2 doped ZrO2-Y2O3 coatings produced by EB-PVD

The effects of La₂O₃ and HfO₂ addition on thermal conductivity and thermal cycle life of EB-PVD YSZ coatings were investigated. La₂O₃ and HfO₂ were selected as additives, because they significantly suppress the sintering of YSZ. The developed coating showed low thermal conductivity as well as high resistance to sintering. Burner rig tests confirmed that the developed coating have a superior thermal insulating effect and have a longer life than that of a coating with conventional composition.