Complex Oxide Ceramics Research Articles

Synthesis of Complex Oxide Ceramics in a Fast Electron Beam

Synthesis of Complex Oxide Ceramics in a Fast Electron Beam

Room‐Temperature Synthesis of a Compositionally Complex Rare‐Earth Carbonate Hydroxide and its Conversion into a Bixbyite‐Type High‐Entropy Sesquioxide

AbstractIn the present work, the solvent‐deficient synthesis of the high‐entropy rare‐earth carbonate hydroxide RE(CO3)(OH) (RE=La, Ce, Pr, Nd, Sm, and Gd) and its thermal conversion into bixbyite‐type sesquioxide RE2O3 are reported for the first time. The high‐entropy rare earth carbonate hydroxide was prepared via mechanochemical reaction of the corresponding metal nitrate hydrates with ammonium hydrogen carbonate followed by the removal of the NH4NO3 by‐product. Calcination of the carbonate hydroxide precursor in ambient atmosphere at temperatures in the range from 500 to 1000 °C led to the high‐entropy rare earth sesquioxide which exhibited a bixbyite‐type structure ( ) independent of the calcination temperature. Transmission electron microscopy (TEM) investigation revealed the homogeneous distribution of all six rare earth cations in the high‐entropy sesquioxide lattice, however, with some compositional variation between individual grains. The bixbyite‐type structure may be considered as the result of heavy doping of the fluorite‐type CeO2 lattice with the other rare earth cations, which leads to a high concentration of oxygen vacancies, as revealed by electron diffraction and Raman spectroscopy data. The solvent‐deficient synthesis method used in the present study is considered as a valuable, straightforward and easily up‐scalable method to synthesize compositionally complex oxide ceramics.

Influence of baric and thermobaric effects on dielectric properties of complex oxide ceramics La1.8Sr0.2Ni0.8Co0.2O4+δ

With a purpose to clarify the reasons for the manifestation of high dielectric permittivity (ε) of oxides with K2NiF4-type structure, the influences of pressure (up to 50 GPa) and thermobaric effects (T = 1273 K, P = 2.5 GPa) on the electrical properties of the appropriate La1.8Sr0.2Ni0.8Co0.2O4+δ oxide have been analyzed.Ceramic samples have been synthesized by the “solution combustion” method with diammonium citrate and subsequent sintering of pyrolysis products at 1473 К and thermobaric treatment (TBT). At room temperature, the dielectric constant of thermobarically treated ceramics was found to be an order of magnitude greater than ε of sample obtained by sintering. Complex studies of electrical properties in broad ranges of pressure, temperatures and electric field frequencies have shown that the permittivity values of La1.8Sr0.2Ni0.8Co0.2O4+δ ceramics are not only due to external causes determined Maxwell-Wagner effects on grain boundaries and heterogeneity. Internal causes, the main of which are associated with bulk processes of charge polarization and distortion of polyhedra when the structure deviates from ideal, as well as with polarization processes caused by polaron carriers, are discussed. The baric behavior of thermo-electromotive (thermo-emf) force for some number of cycles of increasing and subsequent decreasing pressure makes it possible to use the examined material as a conductive medium capable of operating with controlled thermo-emf values under conditions of pressure change.

Синтез сложно-оксидной керамики в пучке быстрых электронов

The synthesis of ceramic materials in a fast electrons beam for the production of complex oxide ceramics is considered. Powder reagents, in addition to irradiation, are exposed to air currents that prevent gases and particles from entering the accelerator. To keep powder mixtures of ultrafine powders in the irradiation zone, they were granulated. Two methods of granulation of an ultrafine powder of the composition (in mass %) 80 % Al2O3 + 20 % (ZrO2 – 3 Y2O3) were used. The first method involves moistening, drying and then sifting through a coarse sieve. In the second method, a binding additive was introduced into the powder mixture, which gave the sample a stable volumetric shape. For the granulation methods used, the features of short-term heating of oxide powders in air with a powerful beam of fast electrons with an energy of 2 MeV and the synthesis of zirconia corundum under these conditions were studied. Granulation of the ultrafine powder made it possible to minimize the loss of its mass during irradiation. During irradiation of the powder mass, its local melting took place, which was accompanied by intense gas release processes leading to the formation of hollow ceramic droplets. X-ray phase analysis has shown that mutual dissolution of oxides in their walls does not occur, and recrystallization processes are accompanied by the formation of cubic aluminum oxide microcrystallites and the transition of aluminum oxide in them from the monoclinic to the corundum phase. The presence of microcrystallites of evenly distributed small particles of zirconia dioxide in the interboundary space indicates the production of zirconia corundum under irradiation. At the same time, the phase composition of zirconium dioxide after irradiation does not change in comparison with the initial powder.

Correlated evolution of dual-phase microstructures, mutual solubilities and oxygen vacancies in transparent La2-xLuxZr2O7 ceramics

The A2B2O7 series of ternary oxides are derivatives of fluorite structure over a wide range of rA/rB. Competing by two rare-earths the A-site, La2-xLuxZr2O7 ceramics were found transparent only in pore-free microstructures with similar grain sizes of pyrochlore (PY) and defective fluorite (DF) phases. Mutual solubilities of Lu and La in both phases were found by imaging and energy-dispersive spectroscopy analysis in scanning electron microscope. The dual-phase microstructures were developed with liquid-phase resulted from the exothermal reactions, creating a miscibility gap between two structures to moderate their competing grain growth. Change in grain growth behaviors in liquid-phase is described by a nucleation line in La2Zr2O7‒Lu2Zr2O7 phase diagram. Variations of solution levels in DF grains and co-existing of dual-phase grain clusters in common orientation were revealed in transparent ceramics by electron backscattered diffraction, resulted by epitaxial relation of two phases promoted by the liquid-phase. Oxygen vacancies and various hole states common in both phases were revealed by characteristic cathodoluminescence peaks. The collective effects of pores, phase and grain boundaries, oxygen vacancies on scattering or absorption of visible light enables to establish a hierarchical microstructure‒transparency relationship in such complex oxide ceramics, which could be tailored or further optimized by controllable sintering.

A complete computational route to predict reduction of thermal conductivities of complex oxide ceramics by doping: A case study of La2Zr2O7

Rare-earth (RE) zirconate pyrochlores, of low thermal conductivity and high temperature stability, are widely considered as a candidate material group for thermal barrier coatings (TBCs). Doping of a RE zirconate pyrochlore has been shown to be one possible method towards additional reduction in the thermal conductivity. Focusing on lanthanum zirconate (La2Zr2O7) as a representative RE zirconate pyrochlore, we systematically investigated the effect of various doping elements, Gd, Y, Yb, In, Sc, Ce and Hf, on the thermal conductivity. A complete computational route, based on first-principles calculations, to predict the reduction of thermal conductivity by doping, has been developed. Employing first-principles calculations combined with thermodynamic modeling, the defects resulted from doping are explicitly clarified, with their concentrations determined considering the chemical environment. The phonon-defect scattering is then evaluated and the resultant reduction of thermal conductivity is determined from single-mode relaxation-time approximation. Good agreement has been achieved between our model predictions and available experimental data. The present study established a complete computational route, based on first-principles calculations, to predict the reduction of thermal conductivity by doping, not only for RE zirconate pyrochlores, but also for other complex oxide ceramics.

Difference between transition metal cation substitution and Nonstoichiometric addition on nanostructure and thermoelectric performance of complex oxide ceramics

This work presents the impact of different methodologies in introducing the transition metal dopants into the thermoelectric Ca3Co4O9 ceramics and their resultant nanostructure and electrical transport properties. The polycrystalline ceramics samples are with designed nominal composition of cation substitution of Ca3Co4-xCuxO9 (x = 0, 0.01, 0.05, 0.1) and cation non-stoichiometric addition of Ca3Co4CuyO9 (y = 0, 0.01, 0.05, 0.1), respectively. At low-temperature regime, the electrical resistivity decreased to the lowest values for each set of samples, upon the minute Cu doping with x = 0.01, and the electrical resistivity increase with Cu doping level. The Seebeck coefficient presents little change among Ca3Co4-xCuxO9 samples. By contrast, the Seebeck coefficient was significantly increased especially in the low-temperature regime in samples of Ca3CuyCo4O9, when the Cu addition level is over y = 0.05. While the grain size and crystal texture status are almost unchanged among Ca3Co4-xCuxO9 samples, non-stoichiometric Cu addition substantially triggered the grain growth, and essentially change the misfit relationship between the Ca2CoO3 and CoO2 layers within the unit cell. The present study demonstrates a novel approach for hierarchically modifying the structure of complex oxide ceramics to tune their thermoelectric properties through cation non-stoichiometric addition.

Reactive flash sintering: MgO and α‐Al2O3 transform and sinter into single‐phase polycrystals of MgAl2O4

AbstractWe show that flash experiments with three phase mixed‐powders of yttria‐stabilized zirconia (8YSZ), MgO, and α‐Al2O3 not only produce polycrystals of high density, but also the transformation of magnesia and alumina into single‐phase spinel. The presence of zirconia facilitates the onset of the flash. The sintering experiments in the laboratory were extended to live experiments at the National Synchrotron Light Source II at Brookhaven National Laboratory in order to measure the time‐dependent evolution of single‐phase spinel. The phase transformation occurred in <3 seconds during Stage II. Later, during Stage III the cubic zirconia transformed partly into the monoclinic phase, which reverted back to the cubic phase when the flash was extinguished by turning off the current to the specimen. The results underpin a recent report on the synthesis of single‐phase bismuth ferrite from constituent oxides in reactive flash experiments, raising the specter of flash as a method for synthesis as well as sintering of complex oxide ceramics. The role of zirconia in catalyzing the flash in the present study is discussed.

Facile route to prepare grain-oriented multiferroic Bi7Fe3−xCoxTi3O21 ceramics

A facile route was developed to fabricate Bi7Fe3−xCoxTi3O21 (BFCTO) ceramics in which the micrometer sized grains were highly [001] oriented. The preparation involved dry pressing nanoplates with high aspect ratio under low axial pressure (10MPa), and subsequently sintering the green compact at high temperature without applied pressure. The formation mechanism and the effects of Co doping on the orientation were investigated. The results indicated that the orientation degree of BFCTO grains was depended on the aspect ratio of nanoplate powder, which increased with the increase of doping amount of Co. The degree of orientation was achieved as high as 0.91 in Bi7Fe2CoTi3O21 ceramic. The saturation magnetization (2Ms) values were larger when surfaces of the oriented grains were perpendicular to the direction of applied magnetic field, indicating obvious anisotropic magnetism. This fabrication method provided an economical yet convenient approach to manufacture grain-oriented complex oxide ceramics with improved magnetic property.

Interface Analysis of Complex Oxide Ceramics in Electrolyte Supported Solid Oxide Fuel Cell

Interface Analysis of Complex Oxide Ceramics in Electrolyte Supported Solid Oxide Fuel Cell

X-Ray Diffraction Analysis of Nano Crystalline Ceramic PbBaTiO<sub>3</sub>

X-ray crystallography is concerned with discovering and describing the crystal structure. High-temperature superconductivity in ceramic oxides is a new technology in which advances are occurring at a rapid pace. Here, the author describes some properties of a new nano crystalline ceramic type II superconductor, PbBaTiO. Type II superconductors are usually made of metal alloys or complex oxide ceramics. The PBT perovskite phase structure was prepared by the conventional solid state reaction technique. The sample was analyzed by X-ray Diffraction (XRD), Particle size determination, SEM and EDX. The comparison of XRD results with JCPDS files confirmed the tetragonal structure of the sample with a = b ≠ c and α = β = γ = 90°. Scanning electron microscopy (SEM) studies revealed that its particle size was in the nanometer range. It also confirmed the calculated value of particle size from Debye Scherrer’s formula. EDX spectrum shows the elements of the sample. X-ray instrumental peak broadening analysis was used to evaluate the size and lattice strain by the Williamson-Hall Plot method.

XRD Studies on Nano Crystalline Ceramic Superconductor PbSrCaCuO at Different Treating Temperatures

High-temperature superconductivity in ceramic oxides is a new technology in which advances are occurring at a rapid pace. Here, the author describes some properties of a new nano crystalline ceramic Type II superconductor, PbSrCaCuO. Type II superconductors are usually made of metal alloys or complex oxide ceramics. The PSCCO perovskite phase structure was prepared by the conventional solid state reaction technique. In order to show the viability of the proposed method, super-conducting powder was prepared in special furnace. The sample was analyzed by X-ray Diffraction (XRD), Particle size determination, SEM and EDX. The comparison of XRD results with JCPDS files confirmed the orthorhombic structure of the sample with a ≠ b ≠ c and α = β = γ = 90°. Scanning electron microscopy (SEM) studies revealed that its particle size is in the nanometer range. It also confirmed the calculated value of particle size from Debye Scherrer’s formula. EDX spectrum shows the elements of the sample. X-ray instrumental peak broadening analysis was used to evaluate the size and lattice strain by the Williamson-Hall Plot method.

Nanopowders for preparing a new generation of oxide ceramics

Future application is presented for ultra- and nanodispersed powders in the technology of simple and complex oxide ceramics. Ceramic materials based on these powders are considered relating to different functional classes, their properties, features of their preparation and fields of application are described in detail. The development of ceramic materials with high operating properties will make it possible to resolve important scientific and technical problems connected with developing new technology

Thermoelectric properties of Bi3+ substituted Co-based misfit-layered oxides

Highly densified (Ca1−xBix)3Co4O9 thermoelectric ceramics with a layered structure were prepared by a sol–gel method followed by spark plasma sintering (SPS). Thermoelectric (TE) properties of the complex oxide ceramics were measured from room temperature to 700 °C. The results show that Bi3+ substitution leads to an increase in both electrical conductivity and Seebeck coefficient simultaneously. Bi3+ doped samples also show a lower thermal conductivity than undoped samples. The dimensional figure of merit ZT value of (Ca0.95 Bi0.05)3Co4O9 samples is 0.25 at 700 °C.

Thermoelectric Oxides and Oxynitrides with Perovskite-type Structure

AbstractTransition metal oxides and oxynitrides with perovskite-type structures are potential high temperature stable thermoelectric materials. With innovative soft chemistry synthesis procedures the crystallite size in complex oxide ceramics with perovskite-type structure can be decreased compared to classical synthesis routes and allows to lower the heat conductivity dramatically while a large power factor can be maintained at the same time. Submicron crystallites are beneficial to lower the lattice thermal conductivity without influencing the electron mobility.In this work, perovskite-type cobaltates, molybdates and manganates are studied. The influence of heterostructures and nanocrystalline domains in perovskite oxides/oxynitrides and the correlation with transport properties and Seebeck coefficient will be discussed.

Direct Writing of Dielectric Ceramics and Base Metal Electrodes

Three‐dimensional structures were assembled by direct writing of colloidal gel‐based inks of electro‐ceramic and metal electrode compositions. The ternary mixtures of BaTiO3, BaZrO3, and SrTiO3 were explored via an active mixing nozzle print strategy that allowed for rapid screening of complex oxide ceramics. In a second experiment, Ni and BaTiO3 inks were developed from commercially available powders in the form of concentrated, aqueous colloidal gels having shear thinning with yield stress rheology. The different surface chemistry of Ni and BaTiO3 was accommodated by adsorbing a bi‐layer of oppositely charged polyelectrolytes on the BaTiO3 particles. Alternating layers of metal and ceramic were printed to build a multilayer structure. The multilayer Ni–BaTiO3 structures were sintered in a reducing atmosphere and the capacitance was measured. These two examples illustrate the use of discrete or graded deposition capabilities of direct writing.

Fine structure EELS analysis of glasses and glass composites

EELS is a powerful technique for the study of complex oxide ceramics, especially when exploiting the ELNES sensitivity to cation valency and coordination. The same applies for oxide glasses, with the additional benefit of gaining information about glass order parameters and structural units. The major benefit of ELNES in comparison to Raman, NMR and X-ray alternatives is the high resolution of measurement which makes it particularly suitable for nanocomposites. For doped glasses with some of the extra cations exceeding the solubility limit, we have also analysed the systematic change of oxidation state in transit from the glass matrix into precipitates using the example of CeO2 and the Ce-M-edge pair of spinorbit split lines.

Synthesis and Thermoelectric Properties of (NaxCa1‐x)3Co4O9 Ceramics.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis and thermoelectric properties of (NaxCa1−x)3Co4O9 ceramics

(NaxCa1� x)3Co4O9 (x=0.05� 0.2) ceramics with a layered crystal structure were prepared by a sol–gel method followed by a lowtemperature sintering procedure. The electrical conductivity and Seebeck coefficient of the complex oxide ceramics were measured from 400 to 900 � C. Their electrical conductivity and power factor increase with increasing temperature, while the thermal conductivity is very weakly dependant on the temperature. Na dopant amount has a remarkable effect on electrical and thermal transport properties. The figure of merit in the ceramic samples is smaller than that of traditional thermoelectric alloys. # 2002 Elsevier Science Ltd. All rights reserved.

The influence of processing parameters on luminescent oxides produced by combustion synthesis

Rare-earth activated oxide phosphors have application in high energy photoluminescent (plasma panels) and cathodoluminescent (field emission devices) flat panel displays. These phosphors are composed of a highly insulating host lattice with fluorescence arising from the 3d! 3d, 5d! 4f or 4f! 4f transitions in transition metal or rare earth ions. Fabrication of complex host compositions Y 2SiO5 ,Y 3Al5O12, Y2O3, and BaMgAl10O27 along with controlled amounts of the activators (Cr 31 ,M n 21 ,C e 31 ,E u 21 ,E u 31 ,T b 31 ,T m 31 ) represent a challenge to the materials synthesis community. High purity, compositionally uniform, single phase, small and uniform particle size powders are required for high resolution and high luminous efficiency in the new flat panel display developments. This paper will review the synthesis techniques and present physical and luminescent data on the resulting materials. q 1999 Published by Elsevier Science B.V. All rights reserved.