Thermal analysis of the (Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1−x</Subscript>(Y<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>x</Subscript> pigments

The synthesis of new compounds based on Bi₂O₃ is investigated because they can be used as new ecological inorganic pigments. Chemical compounds of the (Bi₂O₃)_1−x(Y₂O₃)_x type were synthesized. The host lattice of these pigments is Bi₂O₃ that is doped by Y³⁺ ions. The incorporation of doped ions provides the interesting colours and contributes to a growth of the thermal stability of these compounds. The simultaneous TG-DTA measurements were used for determination of the temperature region of the pigment formation and thermal stability of pigments. This paper also contains the results of the pigment characterization by X-ray powder diffraction and their colour properties.     

Electrophysical properties of Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>2 − δ</Subscript> + <Emphasis Type="Italic">x</Emphasis> mol % Al<Subscript>2</Subscript>O<Subscript>3</Subscript> solid composite electrolytes

The influence of aluminum and cobalt oxides on the microstructure and the mechanical, electrical, and optical properties of the Ce_0.8Gd_0.2O_2−δ solid electrolyte was studied. Using nanosized Al₂O₃ in the synthesis allowed it to be uniformly distributed in the composite electrolyte and improved the microhardness of the resulting ceramics. The introduction of cobalt oxide made it possible to lower the synthesis temperature during the preparation of gasproof ceramics and hindered, to a certain extent, the formation of the GdAlO₃ phase during synthesis. Using the standard set of investigation methods allowed us to find solutions to several problems in using this electrolyte for solid-oxide fuel cells (SOFCs).     

Characterisation of electroplated Bi<Subscript>2</Subscript>(Te<Subscript>1−x</Subscript>Se<Subscript>x</Subscript>)<Subscript>3</Subscript> alloys

Composition modulated Bi₂(Te_1−xSe_x)₃ thin films were prepared on stainless steel substrates by cathodic electrodeposition. The composition was dependent on the deposition conditions. It was possible to obtain, in the same electrolyte, films with either an excess or a deficiency of bismuth in relation to stoichiometric Bi₂(Te_0.9Se_0.1)₃ by changing the deposition potential or the applied current density. The excess of bismuth was reached at the highest cathodic conditions. The variation of the crystallographic axis and the morphology with a granular structure were correlated with the presence of the Bi enrichment in the ternary. The crystallographic texture of bismuth telluride films was studied according to the electrodeposition conditions. The films presented a fibre texture, and a main orientation {11.0} was observed. Electrical and thermoelectric properties of a Bi_1.98Te_2.67Se_0.39 film were measured and showed an n-type behaviour.     

Synthesis and properties of high-density protonic solid electrolyte BaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3 − α</Subscript>

The described method for synthesis of proton-conducting ceramics BaZr_0.9Y_0.1O_{3 ? α} at the temperature of 1850°C from mixture of barium carbonate and zirconium-yttrium hydroxides enabled to prepare high-density translucent specimens. According to powder diffraction data, the synthesized ceramics consists of single phase and has the cubic perovskite structure with the elementary cell parameter a = 0.42009 ± 0.00004 nm. The electrophysical properties of the synthesized ceramics were measured. The conductivity of such high-density specimens was shown to excel by an order of magnitude the conductivity of porous (80% relative density) material.     

Synthesis and morphology of Ba(Zr<Subscript>0.20</Subscript>Ti<Subscript>0.80</Subscript>)O<Subscript>3</Subscript> powders obtained by sol–gel method

Barium zirconate titanate, Ba(Zr_0.20Ti_0.80)O₃ (BZT) powders were prepared by sol–gel method. These powders were characterized by thermogravimetric and differential thermogravimetric analyses (TG-DTA), X-ray diffraction (XRD) and microcopy electron transmission (TEM). The decomposition of the precursors was monitored by TG-DTA. XRD patterns reveal that BZT powders heat treated at 800 °C present single phase with perovskite-type cubic structure. TEM micrographs were employed to estimate the average particle size of the BZT powders (≈ 20 nm). The results indicate that the particle size of the BZT powders increases with the increasing of the holding time and aging temperature. The low aging temperature can reduce the agglomeration of the nanopowders. Three polyalcohols were employed as surfactants in sol–gel method: butanol (BTOL), polyethylene glycol (PEG) and polyvinyl alcohol (PVA). It is noted that PEG has a better effect on reducing agglomeration of BZT powders than that of the BTOL and PVA.     

Study of dielectric characteristics of graded Ba<Subscript>1−x</Subscript>Ca<Subscript>x</Subscript>Zr<Subscript>0.05</Subscript>Ti<Subscript>0.95</Subscript>O<Subscript>3</Subscript> thin films grown by a sol-gel process

The compositionally graded Ba_1−xCa_xZr_0.05Ti_0.95O₃ (x = 0, 0.05, 0.10) (BCZT) thin films with compositional gradient from BaZr_0.05Ti_0.95O₃ to Ba_0.90Ca_0.10Zr_0.05Ti_0.95O₃ were deposited on Pt/Ti/SiO₂/Si substrates by sol-gel processing. The crystal structure of the thin films was determined by X-ray diffraction. Field emission scanning electron microscopy (FESEM) was used to examine crystallite size and morphology of compositionally graded thin films. The dielectric properties of compositionally graded thin films were characterized by measuring the dielectric constant and dielectric loss as a function of temperature, applied electric field and frequency. As a result, compositionally graded thin films with weak temperature dependence were realized. Dielectric constant peaks, common to a ferroelectric transition, were not observed in the temperature range from 298 to 413 K. The compositionally graded BCZT thin films with weak temperature dependence of tunability could be attractive materials for frequency and phase agile tunable microwave components such as tunable filters, tunable oscillators, and phase shifters for application in phased array antennas.     

Na<Subscript>x−y</Subscript>H<Subscript>y</Subscript>Ti<Subscript>2−x</Subscript>Fe<Subscript>x</Subscript>O<Subscript>4</Subscript>·nH<Subscript>2</Subscript>O nanosheets with lepidocrocite-like layered structure synthesized by hydrothermal treatment of ilmenite sand

Na_x−yH_yTi_2−xFe_xO₄·nH₂O nanosheets with lepidocrocite-like layered structure were produced through alkaline hydrothermal treatment at very low temperatures (130°C) from ilmenite sand. The crystal structure, morphology and optical properties were investigated by X-Ray diffraction, transmission electron microscopy, selected area electron diffraction, energy dispersive spectroscopy and UV-Vis spectroscopy. The product shows leaf-like nanosheet morphology with thickness <30 nm and lengths <1 μm. Three lepidocrocite-like titanates (Imm2 space group) with similar a and c lattice parameters but different interlayer distances (b/2) were identified. This appears to be the first preparation of lepidocrocite-like layered nanosheets by a simple, energy efficient (low temperature) and low cost (starting from mineral sand) procedure.     

Peculiarities of electrical transfer and isotopic effects H/D in the proton-conducting oxide BaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3 − δ</Subscript>

Electrical conduction of the oxide BaZr_0.9Y_0.1O_{3 − δ} is studied by electrochemical impedance spectroscopy as a function of temperature (300–600°C) and the oxygen partial pressure in gas phase saturated with H₂O or D₂O vapor. The full electrical conduction is separated into components, in particular, the bulk and grain boundary conduction. It is shown that at P _O2> 1 Pa the BaZr_0.9Y_0.1O_{3 − δ} conduction is contributed significantly by electron holes whose transference number in air atmosphere may be as high as 0.5–0.6. In reductive conditions, the electrical transfer involves proton (deuteron) charge carriers. Isotopic effect H/D in the conduction in the bulk and along the grain boundaries is determined. Isotopic effect H/D in the hole conduction is also revealed. It is shown that this effect comes out of different solubility of deuterons and protons in the oxide (the thermodynamic isotopic effect).     

Chemical synthesis and granulometric composition of CaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3–δ</Subscript> powders

CaZr_0.9Y_0.1O_3–δ powders were synthesized by chemical solution methods: modified Pechini method and from solutions of inorganic salts in water and ethanol. The structure crystallizes into the orthorhombic type upon annealing at 1000°C for powders prepared by the Pechini method and from solution of salts in water. It was shown that CaZr_0.9Y_0.1O_3–δ powders synthesized by various methods have different dispersities. The results obtained in a study of the granulometric composition by the sedimentation method and microscopic analysis enable fabrication of dense and mechanically strong electrolyte films and ceramics.     

Synthesis of Y<Subscript>3−x</Subscript>Lu<Subscript>x</Subscript>Al<Subscript>3</Subscript>MgSiO<Subscript>12</Subscript> garnet powders by sol–gel method

Y_3−xLu_xAl₃MgSiO₁₂ (x = 0–3) garnet powders were synthesized by an aqueous sol–gel method based on metal chelates with 1,2-ethanediol in aqueous media. Target samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy and reflection spectra. XRD analysis revealed that sintering of polycrystalline Y_3−xLu_xAl₃MgSiO₁₂ powders at 1,600 °C results in single-phase garnet materials.     

Glass-forming ability and thermal stability of Se<Subscript>100−x</Subscript>(Ge<Subscript>2</Subscript>Sb<Subscript>2</Subscript>Te<Subscript>5</Subscript>)<Subscript>x</Subscript> glassy alloys

Glassy Se_100?x(Ge₂Sb₂Te₅)_x (x?=?5, 10, 15 and 20) bulk alloys were prepared by melt-quenched technique and studied by using differential scanning calorimetry at different heating rates under non-isothermal condition. The detailed thermal analysis shows that the glass transition temperature (T_g) depends on heating rates and x content. In particular, it is found that the glass-forming ability, thermal stability (T_c???T_g) and crystallization activation energy (E_c) increase with increased x content in amorphous Se, whereas glass transition activation energy (E_g) and fragility index (F) decrease with increased x contents. Variation in these parameters can be explained on the basis of network-forming ability of Se and bonding arrangement among the constituent atoms of alloys.     

Impedance Study of the Conductivity of Solid Oxide Electrolyte Films SrZr<Subscript>0.95</Subscript>Y<Subscript>0.05</Subscript>O<Subscript>3–δ</Subscript> and CaZr<Subscript>0.9</Subscript>Y<Subscript>0.1</Subscript>O<Subscript>3–δ</Subscript>

Information on the across-plane conductivity of films of solid-oxide electrolytes SrZr_0.95Y_0.05O_3–δ and CaZr_0.9Y_0.1O_3–δ deposited on ion-conducting supports is acquired by the impedance method. It is shown that the support/film interface and the intergrain boundaries considerably affect the across-plane charge transfer in the film. The effect of the crystallographic orientation of the YSZ support on the microstructure and conductivity of the CaZr_0.9Y_0.1O_3–δ electrolyte film is demonstrated.     

Oxygen ionic and electronic transport in Gd<Subscript>2−x</Subscript>Ca<Subscript>x</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript><Subscript>−δ</Subscript> pyrochlores

Oxygen ion transference numbers for Gd_2−xCa_xTi₂O_{7 −δ} (x=0.10–0.14) pyrochlore ceramics were determined at 973–1223 K by the modified e.m.f. and faradaic efficiency techniques, taking into account electrode polarization, and from the results on oxygen permeation. The ion transference numbers vary in the range 0.95–0.98 in air, increasing when the temperature or oxygen partial pressure decreases. The activation energies for the ionic and p-type electronic transport in air are 74–77 and 87–91 kJ/mol, respectively. The p-type conductivity and oxygen permeability of Gd₂Ti₂O₇-based pyrochlores can be adequately described by relationships common for other solid electrolytes. At temperatures below 1273 K under a gradient of 10%H₂+90%N₂/air, average ion transference numbers for doped gadolinium titanate are not less than 0.97. Thermal expansion coefficients for Gd_2−xCa_xTi₂O_{7 −δ} ceramics, calculated from dilatometric data in air, are in the range (10.4–10.6)×10⁻⁶ K⁻¹ at 400–1300 K.     

Thermal analysis of the Ce<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Tb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>2</Subscript> pigments

Compounds based on CeO₂ were synthesized as high-temperature environment-friendly inorganic pigments with interesting hues. The pigments have been synthesized by using the solid state reaction in the temperature range from 1,300 to 1,600 °C. The host lattice of these pigments is CeO₂ that is doped by terbium ions. The incorporation of doped ions provides interesting orange colours after application into ceramic glaze. The goal was to develop conditions for the synthesis of these compounds and to determine the influence of calcination temperature on their colouring effects. The simultaneous TG-DTA measurements were used for determination of the temperature region of the pigment formation and thermal stability of pigments. The pigments were also evaluated from the standpoint of their structure and particle sizes.     

Structure,microstructure and physicochemical properties of BaW<Subscript>1−x</Subscript>Nb<Subscript>x</Subscript>O<Subscript>4−δ</Subscript> materials

Nb-doped BaWO₄ with the assumed formula BaW_1?xNb_xO_4?δ (x = 0, 0.005, 0.01, 0.02 and 0.05) were prepared by solid-state reaction method. Crystal structure and phase composition were determined by X-ray diffraction method. Scanning electron microscopy (SEM) coupled with energy-dispersive spectrometry (EDS) was used to describe microstructure and chemical composition of synthesised materials. It was found that solubility limit of niobium in the BaWO₄ structure is the range 0.5–1 mol%, as formation of second phase—Ba₅Nb₄O₁₅—was observed for samples with higher dopant content. For evaluation of the chemical stability of synthesized materials, the comparative CO₂/H₂O exposure test was performed. Samples were exposed to carbon dioxide- and water vapour-rich atmosphere (7% CO₂ in air, 100% RH) at 298 K for 700 h. During this exposition, the chemical reactions between the samples and the surrounding gaseous atmosphere resulting in formation of barium hydroxide and/or barium carbonate can process. Thermogravimetry (TG) method was used for chemical stability evaluation. The comparison of samples before and after the CO₂/H₂O exposure test was performed. To support the interpretation of TG results, the analysis of gaseous products evolved during thermal treatment of the samples was done using mass spectrometer. The effect of dopant on the BaWO₄ chemical stability improvement was observed. In order to determine the electrical properties of obtained materials, the DC resistance measurements in synthetic air atmosphere were taken. It was shown that niobium doping and the presence of second phase—Ba₅Nb₄O₁₅—leads to an increase in the total conductivity of synthesised materials.     

Formation of solid solutions of multiferroics in the Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>–Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>–Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> system

The sequence of phases appearance during the formation of Bi_1–xNd_xFeO₃ solid solutions in powder oxides mixtures of bismuth, neodymium, and iron has been determined. It has been shown that the closeness of the reaction mixture composition to that of the individual compound (BiFeO₃ or NdFeO₃) is essential for the realization of the series of phase transformations yielding solid solutions of multiferroics Bi_1–xNd_xFeO₃ as the final product, due to the prevalence of various interphase contacts in the starting reaction zone.     

Influence of synthesis route on morphology and conduction behavior of BaCe<Subscript>0.8</Subscript>Y<Subscript>0.2</Subscript>O<Subscript>3−δ</Subscript>

In this article, the role of the preparation route and calcinations temperature on the thermal expansion and conductivity of BaCe_0.8Y_0.2O_3−δ (BCY) has been studied. In particular, the samples were synthesized by means of the solid-state reaction and by a sol–gel route. BCY has been suggested as proton conducting electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Proton conductivity strongly depends on the densification of the material as well as the crystal structure, which is generally influenced by the preparation procedure. It was found that a single phase material could be achieved at 1000 °C for the samples prepared through the sol–gel route with ~96% packing density. In case of ceramic route, single phase could be obtained at higher temperatures (1200 °C) and does not lead to good density values. The ceramic synthesis produces BCY material in cubic symmetry where as the gel–citrate complexation route leads to homogenous orthorhombic BCY. The conductivity measurements of sample synthesized by two different routes were investigated by means of impedance spectroscopy and electron microscopy. A comparative study of thermal expansion behavior of BCY synthesized by different route was carried out.     

Synthesis and properties of γ-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> solid solutions

The textural and structural properties of mixed oxides Ga₂O₃–Al₂O₃, obtained via impregnating γ-Al₂O₃ with a solution of Ga(NO₃)₃ and subsequent heat treatment, are studied. According to the results from X-ray powder diffraction, gallium ions are incorporated into the structure of aluminum oxide to form a solid solution of spinel-type γ-Ga₂O₃–Al₂O₃ up to a Ga₂O₃ content of 50 wt % of the total weight of the sample, accompanied by a reduction in the specific surface area, volume, and average pore diameter. It is concluded that when the Ga₂O₃ content exceeds 50 wt %, the β-Ga₂O₃ phase is observed along with γ-Ga₂O₃–Al₂O₃ solid solution. ⁷¹Ga and ²⁷Al NMR spectroscopy shows that gallium replaces aluminum atoms from the tetrahedral position to the octahedral coordination in the structure of γ-Ga₂O₃–Al₂O₃.     

Heat capacity of GdBaSr(Cu<Subscript>3−x</Subscript><Emphasis Type="Italic">M</Emphasis><Subscript>x</Subscript>)O<Subscript>7−δ</Subscript> superconductor (<Emphasis Type="Italic">M</Emphasis>=Zn and Ni)

In this study, GdBaSr(Cu_3−x M _x)O_7−δ bulk samples (M=Zn and Ni; 0≤x≤0.1) were prepared via solid-state reaction. Specific heat measurement (measured with thermal relaxation technique using PPMS) shows an obvious specific heat jump around the T _c for GdBaSrCu₃O_7−δ sample as observed in most of the high temperature superconductors. It shifts towards lower temperature with increasing of both Zn and Ni doping contents, whose tendency is similar to the decreasing of T _c. Debye temperature, Θ_D (derived from specific heat measurements) calculated at around 10 K is found to be directly proportional to the T _c.     

Cellulose-precursor synthesis of nanocrystalline Ce<Subscript>0.8</Subscript>Gd<Subscript>0.2</Subscript>O<Subscript>2−δ</Subscript> for SOFC anodes

Developments of intermediate-temperature solid oxide fuel cells (IT SOFCs) require novel anode materials with a high electrochemical activity at 800–1070 K. The polarization of cermet anodes, made of nickel, ceria and yttria-stabilized zirconia (YSZ) and applied onto a YSZ solid electrolyte, can be significantly reduced by catalytically active ceria additions, the relative role of which increases with decreasing temperature. Further improvement is observed when using Ce_0.8Gd_0.2O_2– (CGO) having a high oxygen ionic conductivity instead of undoped ceria, owing to enlargement of the electrochemical reaction zone. Nanocrystalline CGO powders with grain sizes of 8–35 nm were thus synthesized via the cellulose-precursor technique and introduced into Ni–CGO–YSZ cermets, and tested in contact with a (La_0.9Sr_0.1)_0.98Ga_0.8Mg_0.2O_3– (LSGM) electrolyte at 873–1073 K. The results showed that the anode performance can be enhanced by additional surface activation, in particular by impregnation with a Ce-containing solution, and also by incorporation of YSZ, which probably acts as a cermet-stabilizing component. The overpotential of the surface-modified Ni–CGO (25 wt%–75 wt%) anode in a 10% H₂/90% N₂ atmosphere was approximately 110 mV at 1073 K with a current density of 200 mA/cm².Presented at the OSSEP Workshop Ionic and Mixed Conductors: Methods and Processes, Aveiro, Portugal, 10–12 April 2003