Electrochemical characterization of nanostructured zirconias

Redox stability of cubic nanostructured zirconia ceramics, free of any secondary phases, has been investigated experimentally as a function of grain size. Pure 8 mol% Y₂O₃-doped ZrO₂ powders were synthesized by a spray pyrolysis process and then compacted by uniaxial pressing, followed by cold isostatic pressing. Using appropriate thermal treatments, average grain sizes ranging from 25 to 242 nm and relative densities from 71% up to 98% were obtained. An electrochemical characterization was performed with comparison on ceramics of 3.2 and 7.6 μm and 98% of theoretical density starting from commercial YSZ powder.     

A study of the structure and properties of nanostructured ZrO<Subscript>2</Subscript>-Y<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite powders

The correlation between temperature treatment conditions and the ratio of components in nanostructured fibrous powders with a composition of ZrO₂-Y₂O₃-Al₂O₃ and their porous crystal structure and physicochemical properties is studied. The dependences of the ratio between zirconia tetragonal and monoclynic phases on the treatment temperature and the alumina content are found to have a nonmonotonic character. The growth of zirconia crystallite size is suppressed by introduced nanocrystalline alumina in a temperature range of 600–1200°C, which is caused by the processes of ternary solid solution formation. The bulk and picnometric density values of materials are proportional to the temperature of heat treatment. The temperature dependence of the specific surface and the size of oxide grain particles has an inversely proportional character. With increasing alumina content in the powders, the specific surface increases, while the picnometric and bulk densities decrease.     

Suspension plasma sprayed composite coating using amorphous powder feedstock

Al₂O₃-ZrO₂ composite coatings were deposited by the suspension plasma spray process using molecularly mixed amorphous powders. X-ray diffraction (XRD) analysis shows that the as-sprayed coating is composed of α-Al₂O₃ and tetragonal ZrO₂ phases with grain sizes of 26 nm and 18 nm, respectively. The as-sprayed coating has 93% density with a hardness of 9.9 GPa. Heat treatment of the as-sprayed coating reveals that the Al₂O₃ and ZrO₂ phases are homogeneously distributed in the composite coating.     

Size-dependent luminescence in Y2Si2O7 nanoparticles doped with Ce3+ ions

Powders and thin films of nanocrystalline yttrium disilicate (Y₂Si₂O₇) doped with Ce³⁺ have been prepared by a sol–gel method. Structure and morphology of the synthesised samples have been determined and spectroscopic properties compared. The triclinic α-Y₂Si₂O₇ form (space group P 1-) for the powders annealed between 1000°C and 1200°C has been found. A total conversion into a thortveitite-type monoclinic β-Y₂Si₂O₇ polymorph after annealing of powder samples at 1400°C (space group C2/m) has been observed. In the case of films even at 1300°C the basically pure triclinic α-Y₂Si₂O₇ phase was observed with luminescent spectroscopy. The influence of grain size, controlled by thermal treatment, on the structure and luminescence properties of the fabricated materials are presented and discussed.     

Processing of coked Pt-Re/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts with high-concentration ozone dissolved in supercritical carbon dioxide

The potentialities of low-temperature (50–150°C) regeneration of coked Pt-Re/γ-Al₂O₃ bimetallic catalysts of reforming with high-concentration ozone dissolved in supercritical carbon dioxide (SC-CO₂) are examined. Raman spectroscopy, thermogravimetric, and differential microcalorimetric studies show that solid organic deposit (coke), a by-product of the reforming process, can be partially removed from the mesoporous surface of a coked catalyst by processing with an O₃/SC-CO₂ mixture without any loss of the active components (Pt, Re). Treatment of coked Pt-Re/γ-Al₂O₃ catalysts with an O₃/SC-CO₂ mixture causes changes in the parameters (extent of conversion, selectivity) of the catalytic reforming of n-heptane.     

X-ray diffraction and calorimetric studies of powder nanocrystalline systems based on ZrO<Subscript>2</Subscript>(Y) and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with second insoluble component

Four nanocrystalline ZrO₂(Y)- and Al₂O₃-based powders synthesized by plasma spray pyrolysis have been studied. It has been shown that the ZrO₂(3Y)-based system with second component Al₂O₃ forms a nonequilibrium solid solution ZrO₂(3Y, Al) with the tetragonal structure. It has been found that the existence of an component (ZrO₂(Y)) insoluble in the coarse-grained state in Al₂O₃-based systems causes the delay of the γ → α transformation and decreases the size of the coherently scattering domains of formed nanosized modifications of Al₂O₃.     

Specific physical properties of nanocrystalline (La<Subscript>0.65</Subscript>Sr<Subscript>0.35</Subscript>)<Subscript>0.8</Subscript>Mn<Subscript>1.2</Subscript>O<Subscript>3 ± Δ</Subscript> samples obtained by cold isostatic pressing

Single-phase powders of manganites (La_0.65Sr_0.35)_0.8Mn_1.2O_{3 ± Δ} with average crystallite sizes of 30, 50, and 500 nm were produced by co-precipitation. The samples studied were obtained by cold isostatic pressing of powders at a pressure of 1 GPa without subsequent sintering. It is shown that the size of particles has a significant effect on the electromagnetic properties of the manganite samples. As the crystallite size decreases, the electrical resistance and coercive force increase and the tunneling magnetoresistance of the samples and the Curie temperature decrease.     

Electronic structure of an oxygen vacancy in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from the results of Ab Initio quantum-chemical calculations and photoluminescence experiments

The electronic structure of an oxygen vacancy in α-Al₂O₃ and γ-Al₂O₃ is calculated. The calculation predicts an absorption peak at an energy of 6.4 and 6.3 eV in α-Al₂O₃ and γ-Al₂O₃, respectively. The luminescence and luminescence excitation spectra of amorphous Al₂O₃ are measured using synchrotron radiation. The presence of a luminescence band at 2.9 eV and a peak at 6.2 eV in the luminescence excitation spectrum indicates the presence of oxygen vacancies in amorphous Al₂O₃.     

Phonon kinetics in solid solutions of ceramics in the ZrO2-Y2O3-MgO-Al2O3 system

The propagation of weakly nonequilibrium phonons is studied in ceramics based on ZrO₂-Y₂O₃-MgO-Al₂O₃. Under the assumption of a large dispersion in the ceramic grain sizes, the characteristics of phonon scattering are studied as a function of the temperature and duration of isothermal annealing of the samples and the contribution of the intergrain boundaries to the phonon kinetics is isolated. Fiz. Tverd. Tela (St. Petersburg) 39, 1091–1093 (June 1997)     

Studies on the magnetic viscosity and the magnetic anisotropy of γ-Fe2O3 powders

2 O₃ powders, acicular γ-Fe₂O₃, and CoFe–γ-Fe₂O₃ powders are prepared by different methods. Particle shapes and mean particle sizes of samples are determined by transmission electron microscopy (TEM). Magnetic parameters are measured by a vibrating sample magnetometer (VSM) at different temperatures. Effective magnetic anisotropy constants K_E of granular γ-Fe₂O₃ powders at different temperatures are obtained by using the law of approach to saturation (LATS). K_E values of acicular γ-Fe₂O₃ and CoFe–γ-Fe₂O₃ powders are measured by a magnetotorquemeter. It is found for the first time that the variation tendency of K_E with temperature for granular γ-Fe₂O₃ is about the same as that of shape magnetic anisotropy K_sh. Fluctuation field H_f and activation volume V_f of samples are measured. A theoretical expression of V_f is derived. For granular γ-Fe₂O₃ powders, calculated activation volumes are consistent with experimental ones at different temperatures. But as for acicular γ-Fe₂O₃ powders, calculated activation volumes are larger than experimental ones. Experimental results show that magnetization reversal of granular γ-Fe₂O₃ at different temperatures is close to homogeneous rotation. Received: 27 November 1996/Accepted: 18 April 1997     

Preparation of alumina nanowires, nanorods, and nanowalls by chemical etching

Alumina (Al₂O₃) nanowires, nanorods, and nanowalls have been prepared from anodic aluminum oxide (AAO) templates by chemical etching in NaOH solution. Heating the template prior to etching is crucial to the morphology of subsequent prepared alumina nanostructures, which greatly depend on the phases of the AAO templates. It has been found that the templates with amorphous Al₂O₃, γ-Al₂O₃, and α+γ-Al₂O₃ phases will grow nanowires, nanorods, and nanowalls, respectively. A possible mechanism for forming different alumina nanostructures is proposed.     

Specific features in the kinetics of thermally stimulated luminescence of α-Al2O3 crystals containing defects

A study of specific features in the kinetics of thermoluminescence (TL) of defective α-Al₂O₃ crystals is reported. The TL properties are experimentally shown to be related to the presence of oxygen vacancies of thermochemical or radiation origin. It is established that the differences in the TL kinetics in α-Al₂O₃ samples grown under different conditions can be accounted for by the hypothesis that the energy spectra of the trapping levels have different widths. As is demonstrated with a titanium impurity, this broadening of the energy spectrum can originate from the presence of traces of native impurities in the α-Al₂O₃ lattice. Fiz. Tverd. Tela (St. Petersburg) 39, 1538–1543 (September 1997)     

Crystallite size and magnetic properties of La0.7Mn1.3O3 ± Δ

The results of investigation into nanocrystalline lanthanum manganites La_0.7Mn_1.3O_{3 ± Δ} produced by repeated cold isostatic pressing of a charge material are reported. A powder compact with a crystallite size of 5–7 nm exhibits no magnetic properties, unlike a coarse-grained (20 nm) powder compact.     

Phase composition and low-frequency internal friction of ZrO2 + 4 mol % Y2O3 ceramics

Low-frequency internal friction was studied at a torsional-vibration frequency of 24 Hz in polycrystalline samples of ZrO₂ + 4 mol % Y₂O₃ ceramics. The samples were prepared using cold isostatic pressing of a powder followed by sintering in air, hot isostatic pressing of the sample in an argon atmosphere, subsequent mechanical deformation of the surface, and further calcination in air. The x-ray phase analysis method is used to study the variation in the phase composition after mechanical deformation of the sample surface followed by calcination in air. The internal friction was measured in the temperature range 280–380 K on a sample subjected to calcination in air. At 315 K, an internal-friction peak was revealed, which is explained by stress relaxation at the boundaries of the T’-phase domains.     

Phase composition and structure of nanocrystalline powders based on ZrO<Subscript>2</Subscript>(Y) and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by plasma spray pyrolysis

It is shown that a nonequilibrium solid solution ZrO₂(3Y, Al) with tetragonal structure is formed in systems based on ZrO₂(3Y) with Al₂O₃ as a second component. A delay in the γ → α Al₂O₃ transformation and a reduction in the size of the coherently scattering domain of modifications are observed in systems based on Al₂O₃ with ZrO₂(Y) as a second component.     

Studies of characteristics of adsorption and radiolysis of methane on an aluminum-oxide surface by IR spectroscopy in diffusely scattered light

The method of IR spectroscopy in diffuse scattered light was used to study adsorption and heterogeneous radiation-induced transformations of methane (deuteromethane) on an aluminum γ-oxide surface. It is found that under the action of γ-quanta the CH₄ adsorption centers appear on the γ-Al₂O₃ surface at room temperature, following the molecular and dissociative mechanisms. It is shown that hydrocarbon decomposition products, aluminum hydrates, and surface hydroxyl groups are produced in radiolysis. A mechanism of radiolytic processes in γ-Al₂O₃+ ads. CH₄(CD₄) systems is suggested on the basis of the present experimental results. Here and in what follows γ-forms of Al₂O₃ are meant. Sector of Radiological Studies of the Academy of Sciences of the Azerbaidzhan Republic, 31a, G. Dzhavid Ave., Baku, 37143, Azerbaidzhan. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 34–39, January–February, 1998.     

Er3+及Er3+/Yb3+掺杂ZrO2-Al2O3的制备及发光性质

采用共沉淀法制备了纳米晶ZrO₂-Al₂O₃∶Er³⁺发光粉体.所制备的粉体室温下具有Er3+离子特征荧光发射,主发射在绿光,其中位于547 nm、560 nm的绿光最强,并得出稀土离子与基质之间有能量传递.对不同煅烧温度下的样品研究表明:因不同温度下所制得的样品晶相不同.研究了纳米晶ZrO2-Al2O3∶Er3+及ZrO₂-Al₂O₃∶Er³⁺/Yb³⁺的上转换发光,并分析了上转换的跃迁机制.发现ZrO₂-Al₂O₃∶Er³⁺的绿光为双光子过程,而ZrO₂-Al₂O₃∶Er³⁺、Yb³⁺的上转换光谱中,红光和绿光也为双光子过程,而极弱的蓝光为三光子过程.讨论了Er³⁺的浓度猝灭现象.最适宜掺杂浓度的原子分数为2%（Er³⁺/Zr⁴⁺）.     

Characteristics of γ-Al2O3 nanoparticles generated by continuous-wave laser ablation in liquid

Laser ablation in liquid is one of the most widely investigated methods for generating various nanoparticles (NPs) that are difficult to produce using other means. In this paper, we report the generation of Al-oxide NPs by continuous-wave (CW) fibre laser ablation of corundum (α-Al₂O₃) target submerged in deionised water. The effects of CW fibre laser power and radiation time have been investigated. Characterisation of the NPs generated, in terms of size, size distribution, shape, chemical composition, and phase structure, was carried out by means of high-resolution transmission electron microscopy (HR-TEM), high angle annular dark field (HAADF) in scanning-transmission (STEM) mode, energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The results show that the average size of Al-oxide NPs, in the range of 17 to 29 nm, increased with increasing the laser power and laser exposure time, and the NPs are dominated by stoichiometric γ-Al₂O₃ with a minor phase of α-Al₂O₃. The mechanism involved in the CWLAL is also discussed.     

High-pressure properties of high-TC superconductor samples produced by hot isostatic pressing

Abstract

The electrical resistance of dense YBa₂Cu₃O_x and YBa₂Cu₄O_y produced by hot isostatic pressing has been measured vs. T and p. At 295 K we find d (ln R)/dp ? -0.12 and -0.09 GPa^?1, respectively, with no systematic dependence on initial density. For 1-2-4, dT_C/dp ? 5.1 K/GPa, which is ten times that of 1-2-3.     

K X-ray emission spectra from ν-Al2O3

Summary η-Al₂O₃ sample was bombarded with 12.5 keV electron.K X-ray spectra of this sample were compared with those of pure Al and α-Al₂O₃.