Thermodynamic Properties of LaCrO4, La2CrO6, and La2Cr3O12, and Subsolidus Phase Relations in the System Lanthanum-Chromium-Oxygen

In the system La-Cr-O, there are three ternary oxides (LaCrO4, La2Cr3O12, and La2CrO6) that contain Cr in higher valence states (V or VI). On heating, LaCrO4 decomposes to LaCrO3, La2Cr3O12 to a mixture of LaCrO4 and Cr2O3, and La2CrO6 to LaCrO3 and La2O3 with loss of oxygen. The oxygen potentials corresponding to these decomposition reactions are determined as a function of temperature using solid-state cells incorporating yttria-stabilized zirconia as the electrolyte. Measurements are made from 840K to the decomposition temperature of the ternary oxides in pure oxygen. The standard Gibbs energies of formation of the three ternary oxides are derived from the reversible electromotive force (EMF) of the three cells. The standard enthalpy of formation and standard entropy of the three ternary oxides at 298.15K are estimated. Subsolidus phase relations in the system La-Cr-O are computed from thermodynamic data and displayed as isothermal sections at several temperature intervals. The decomposition temperatures in air are 880 (+/- 3)K for La2Cr3O12, 936 (+/- 3)K for LaCrO4, and 1056 (+/- 4)K for La2CrO6.

Gd1.96-xYxEu0.04O3 (x=0.0, 0.49, 0.98, 1.47, 1.96 mol%) nanophosphors: Propellant combustion synthesis, structural and luminescence studies

Gd1.96-xYxEu0.04O3 (x = 0.0, 0.49, 0.98, 1.47, 1.96 mol%) nanophosphors were synthesized by propellant combustion method at low temperature (400 degrees C). The powder X-ray diffraction patterns of as formed Gd1.96Eu0.04O3 showed monoclinic phase, however with the addition of yttria it transforms from monoclinic to pure cubic phase. The porous nature increases with increase of yttria content. The particle size was estimated from Scherrer's and W-H plots which was found to be in the range 30-40 nm. These results were in well agreement with transmission electron microscopy studies. The optical band gap energies estimated were found to be in the range 5.32-5.49 eV. PL emission was recorded under 305 nm excitation show an intense emission peak at 611 nm along with other emission peaks at 582, 641 nm. These emission peaks were attributed to the transition of D-5(0) —> F-7(J) (J = 0, 1, 2, 3) of Eu3+ ions. It was observed that PL intensity increases with increase of Y content up to x = 0.98 and thereafter intensity decreases. CIE color co-ordinates indicates that at x = 1.47 an intense red bright color can be achieved, which could find a promising application in flat panel displays. The cubic and monoclinic phases show different thermoluminescence glow peak values measured under identical conditions. The response of the cubic phase to the applied dose showed good linearity, negligible fading, and simple glow curve structure than monoclinic phase indicating that suitability of this phosphor in dosimetric applications. (C) 2014 Elsevier B.V. All rights reserved.

Creep in nanocrystalline zirconia

Grain boundary sliding remains a dominant deformation process during creep in both nanocrystalline and submicron-grained zirconia. The level of segregation of Y to grain boundaries is reduced by a factor of similar to 2 in nanocrystals. However, a scaling relationship for compression creep was valid in a 3 mol.% yttria-stabilized tetragonal zirconia with grain sizes in the range of similar to 65-400 nm, indicating the same deformation mechanism over this range of grain sizes. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Oxygen potentials and phase equilibria in the system Ca-Co-O and thermodynamic properties of Ca3Co2O6 and Ca3Co4O9.163

Oxygen potentials established by the equilibrium between three condensed phases, CaOss+CoOss+ Ca3Co2O6 and CoOss+Ca3Co2O6+Ca3CO3.93+O-alpha(9.36-delta), are measured as a function of temperature using solid-state electrochemical cells incorporating yttria-stabilized zirconia as the electrolyte and pure oxygen as the reference electrode. Cation non-stoichiometry and oxygen non-stoichiometry in Ca3Co3.93+alpha O9.36-delta are determined using different techniques under defined conditions. Decomposition temperatures and thermodynamic properties of Ca3Co2O6 and Ca3Co4O9.163 are calculated from the results. The standard entropy and enthalpy of formation of Ca3Co2O6 at 298.15 K are evaluated. Using thermodynamic data from this study and auxiliary information from the literature, phase diagram for the ternary system Ca-Co-O is computed. Isothermal sections at representative temperatures are displayed to demonstrate the evolution of phase relations with temperature. (C) 2014 Elsevier Inc. All rights reserved.

Fabrication and properties of TBCCO films via air-atomising spray deposition of thallium-free precursors

The high-field properties of polycrystalline superconducting TlBaCaCuO films fabricated by the incorporation of thallium vapour into air-atomised BaCaCuO precursors are described. Thick films with Tc values in the range 106-111 K have been prepared on polycrystalline yttria-stabilised zirconia substrates. The surface morphology, crystal structure and composition of the films are related to their high-field transport and magnetisation properties. Typical 10 mm × 9 mm films show Jc values > 1×104 A/cm2 at 77 K (0 T). The best film has a Jc=1.3×104 A/cm2 (Ic=3.6 A) at 77 K (0 T). Films prepared on 26 mm×9 mm substrates show typical large-area Jc values > 0.5×104 A/cm2 (77 K, 0 T). A square planar specimen of dimensions 4.3 mm ×4.3 mm exhibited magnetisation Jc values=1.2×105 A/cm2 at 4.2 K (0.1 T), 9.3×104 A/cm2 at 10 K (0.1 T), 3.3×104 A/ cm2 at 4 K (8 T), and 1.6×104 A/cm2 at 10 K (8 T). © 1994.

Correlation of transport and magnetic critical currents in melt-processed YBa2Cu3O7-δ thick films

Transport critical current measurements have been carried out on melt-processed thick films of YBa2Cu3O7-δ on yttria-stabilized zirconia in fields of up to 8 T both within grains and across grain boundaries. These measurements yield Jc values of ∼3000 A cm-2 at 4.2 K and zero magnetic field and 400 A cm -2 at 77 K and zero magnetic field, taking the entire sample width as the definitive dimension. Optical and scanning electron microscopy reveals that the thick-film grains consist typically of a central "hub" region ∼50 μm in diameter, which is well connected to radial subgrains or "spokes" which extend ∼1 mm to define the complete grain structure. Attempts have been made to correlate the transport measurements of inter- and intra-hub-and-spoke (H-S) critical current with values of this parameter derived previously from magnetization measurements. Analysis of the transport measurements indicates that current flow through H-S grains is constrained to paths along the spokes via the grain hub. Taking the size of the hub as the definitive dimension yields an intra-H-S grain Jc of ∼60 000 A cm-2 at 4.2 K and 0 T, which is in reasonable agreement with the magnetization data. Experiments in which the hub is removed from individual grains confirm that this feature determines critically the J c of the film.

Distribuição de tensões em próteses parciais fixas posteriores livres de metal com retentores intracoronários: análise em elementos finitos

Para reabilitar a ausência de um elemento dentário posterior, as próteses parciais fixas (PPF) com retentores intracoronários são uma alternativa aos implantes osseointegrados. O objetivo deste estudo foi avaliar a distribuição de tensões nessas próteses com três combinações de materiais: cerâmica de zircônia parcialmente estabilizada por ítria (ZPEI) revestida por cerâmica de fluorapatita (α), cerâmica de dissilicato de lítio (β) ou compósito fibrorreforçado (γ). Na composição α, foram analisadas a presença ou ausência da cerâmica de revestimento na parede cervical das caixas proximais e três variações na área total da seção transversal dos conectores (4 mm de largura x 3,2, 4,2 ou 5,2 mm de altura). Em 8 modelos bidimensionais de elementos finitos, uma carga vertical de 500 N foi aplicada na fossa central do pôntico e as tensões principais máximas (tração) e mínimas (compressão) foram apontadas em MPa. Inicialmente foram avaliados os 6 modelos com PPF de ZPEI e suas variações. Os maiores valores das tensões de tração foram encontrados no terço cervical dos conectores. Quando presente nestas regiões, a cerâmica de revestimento recebeu tensões acima do limite de sua resistência à flexão. Na comparação entre os modelos sem cerâmica de revestimento na parede cervical das caixas proximais, mesmo aquele com conectores de 3,2 x 4 mm, cuja infraestrutura apresentava 2,5 x 3 mm, poderia ser recomendado para uso clínico. Altos valores de tensões de compressão foram registrados entre o terço oclusal e médio dos conectores, correspondente à união entre as cerâmicas, o que poderia ocasionar, devido à flexão, falhas adesivas. Posteriormente, o modelo de ZPEI com a cerâmica de fluorapatita ausente da parede cervical das caixas proximais e área total dos conectores de 4,2 x 4 mm foi comparado aos dois outros materiais com conectores de mesma área. Na PPF de dissilicato de lítio, os valores representaram uma provável violação do limite de sua resistência à flexão. A PPF de compósito fibrorreforçado apresentou tensões bem abaixo do limite de resistência à flexão de sua infraestrutura, mas, como no modelo de ZPEI, tensões compressivas se concentraram com alto valor entre o terço oclusal e médio dos conectores, local de união entre a resina composta e a infraestrutura de fibras. Os resultados mostraram que a cerâmica de dissilicato de lítio e a presença da cerâmica de fluorapatita na parede cervical das caixas proximais deveriam ser contraindicadas para a condição proposta. Parece viável uma área de conectores na infraestrutura de ZPEI com no mínimo 2,5 x 3 mm. A PPF de compósito fibrorreforçado apresenta resistência estrutural para a situação estudada, mas, como também aquelas compostas de ZPEI, aparenta ter como pontos fracos a adesão entre a infraestrutura e o material de cobertura e a própria resistência deste último.

沉积温度对氧化钇稳定氧化锆薄膜残余应力的影响

采用自制掺摩尔分数12%的Y2O3的ZrO2混合颗粒料为原料,在不同的沉积温度下用电子束蒸发方法沉积氧化钇稳定氧化锆(YSZ)薄膜样品。利用ZYGOMarkⅢ-GPI数字波面干涉仪对氧化钇稳定氧化锆薄膜的残余应力进行了研究,讨论了沉积温度对残余应力的影响。实验结果表明:随沉积温度升高,氧化钇稳定氧化锆薄膜中残余应力状态由张应力变为压应力,且压应力值随着沉积温度升高而增大;用X射线衍射仪表征了不同沉积温度下氧化钇稳定氧化锆薄膜的微观结构,探讨了薄膜微观结构与其应力的对应关系,并对比了纯ZrO2薄膜表现出的应力状态。

CHANNELING ALIGNMENT FOR EPITAXIAL LIGHT-MASS FILM ON HEAVY-MASS SUBSTRATE

A simple procedure for obtaining a background-free backscattering spectrum of a light-mass film on a heavy-mass substrate by a normal incidence/grazing exit geometry has been described. Using this method such films can be aligned rapidly and accurately, and the impurity or defect information on the films can be obtained without need for realignment. Example is given from MeV Li-3+ analysis of a deposited film of Si on a single crystal substrate of yttria-stabilized, cubic zirconia.

Failure of the plasma-sprayed coating of lanthanum hexaluminate

Lanthanum magnesium hexaluminate (LaMgAl11O19, LMA) is an attractive material for thermal barrier coatings (TBCs), and the failure of its coating was studied in this work by thermal cycling, X-ray diffraction, dilatometric measurement and thermal gravimetric-differential thermal analysis. The dilatometric measurement indicates that even though the bulk material of LMA has a higher sintering-resistance than the typical TBC material, i.e. yttria-stabilized zirconia (YSZ), the plasma sprayed coating of LMA has two serious contractions due to the re-crystallization of LMA and phase transitions of alumina.

La-2(Zr0.7Ce0.3)(2)O-7 - A new oxide ceramic material with high sintering-resistance

Oxide ceramics with high sintering-resistance above 1473 K have very important applications in thermal barrier coatings (TBCs), catalytic combustion and high-temperature structural materials. Lanthanum zirconate (La2Zr2O7, LZ) is an attractive TBC material which has higher sintering-resistance than yttria stabilized zirconia (YSZ), and this property could be further improved by the proper addition of ceria.

Phase stability and thermophysical properties of neodymium cerium composite oxide

Nd2CexO3+2x (x = 2.25, 2.5, 2.75, 3.0) were synthesized by solid-state reaction, and their phase stabilities and thermophysical properties were investigated. The X-ray diffraction (XRD) results indicated that Nd2CexO3+2x with fluorite structure were stable after long-term annealing at 1673 K. They have higher thermal expansion coefficients (TECs) than yttria-stabilized zirconia (YSZ) which is the typical thermal barrier coating (TBC) material, especially the thermal expansion as a function of temperature is parallel to that of the nickel-based superalloy.

Double-ceramic-layer thermal barrier coatings of La2Zr2O7/YSZ deposited by electron beam-physical vapor deposition

Double-ceramic-layer(DCL) thermal barrier coatings (TBCs) of La2Zr2O7 (LZ) and yttria stabilized zirconia (YSZ) were deposited by electron beam-physical vapor deposition (EB-PVD). The composition, crystal structure, surface and cross-sectional morphologies and cyclic oxidation behavior of the DCL coating were studied. Both the X-ray diffraction (XRD) and thermogravimetric-differential thermal analysis (TG-DTA) prove that LZ and YSZ have good chemical applicability to form a DCL coating. The thermal cycling test at 1373 K in an air furnace indicates the DCL coating has a much longer lifetime than the single layer LZ coating. and even longer than that of the single layer YSZ coating. The failure of the DCL coating is a result of both the bond coat oxidation and the thermal strain between bond coat and ceramic layer generated by the thermal expansion mismatch.

Adhesive strength of new thermal barrier coatings of rare earth zirconates

La2Zr2O7 (LZ) and La-2(Zr0.7Ce0.3)(2)O-7 (LZ7C3) as novel candidate materials for thermal barrier coatings (TBCs) were prepared by electron beam-physical vapor deposition (EB-PVD). The adhesive strength of the as-deposited LZ and LZ7C3 coatings were evaluated by transverse scratch test. Meanwhile, the factors affecting the critical load value were also investigated. The critical load value of LZ7C3 coating is larger than that of LZ coating, whereas both values of these two coatings are lower than that of the traditional coating material, i.e. 8 wt% yttria stabilized zirconia (8YSZ). The micro-cracks formed in the scratch channel can partially release the stress in the coating and then enhance the adhesive strength of the coating. The width of the scratch channel and the surface spallation after transverse scratch test are effective factors to evaluate the adhesive strength of LZ and LZ7C3 coatings.

Sol-gel derived Y2O3 as an efficient bluish-white phosphor without metal activator ions

In this paper, Y2O3 powder phosphors without metal activators were successfully prepared by the sol-gel method. The obtained sample shows an intense bluish-white emission (ranging from 350 to 600 nm, centered at 416 nm) under a wide range of UV light excitation (235-400 nm). The chromaticity coordinates of the sample are x = 0.159, y = 0.097, and the quantum yield is as high as 64.6%, which is a high value among the phosphor family without metal activators. The luminescent mechanisms have been ascribed to the carbon impurities in the Y2O3 host.