Compressibility measurements of β- and β″ -alumina

The compressibilities of the a- and c-axes for sodium - and -aluminas were determined up to 10 GPa from the pressure dependence of powder X-ray diffraction measured at room temperature using synchrotron radiation as an X-ray source. Powders of sodium - and -aluminas which were prepared from grinding synthesized single crystals were used as the specimens for X-ray diffraction. The compressibilities of - and -aluminas are 1.5 ± 0.2 ×10^–12 and 1.7 ± 0.2 × 10^–12 Pa^–1 for the a-axis and 2.9 ± 0.2x10^–12 and 1.6 ± 0.2 ×10^–12 Pa^–1 for the c-axis, respectively. For the c-axis, the compressibility of -alumina is larger than that of -alumina. This experimental fact is explained by the different stacking of oxygen layers and the different content in sodium ion between - and -aluminas.     

Thermodynamic investigations of liquid Bi-Na and Sn-Na alloys by coulometric titration using β″-alumina

The following double galvanic cell was assembled and the thermodynamic properties of liquid Bi-Na and Sn-Na alloys, and the ion selectivity of -alumina during coulometric titration, were investigated. Mo, Na(I)¦-alumina¦M-Na(I), Mo [I] M-Na(I)¦-alumina¦Au + Au₂Na, Mo [II] (M = Bi or Sn) where M-Na(1) and Au + Au₂Na were used as the common electrode and reference electrode, respectively. Sodium was coulometrically titrated through the -alumina electrolyte of cell I both ways, and the EMFs were measured. It was found that no ion-exchange reaction occurs between the liquid alloys and the -alumina, and only Na was transferred in the -alumina during coulometric titrations. The thermodynamic properties of liquid Sn-Na and Bi-Na alloys were found to be in agreement with the literature.     

Strength improvement in beta″ alumina by incorporation of zirconia

Beta alumina ceramic electrolytes for use in Na/S batteries are inherently weaker than most engineering ceramics due to the presence of weakly-bonded conduction planes in the crystal structure and to difficulties in controlling grain growth during firing. Substantial improvement in microstructural control is obtained by incorporation of monoclinic zirconia (m-ZrO₂) or partially stabilized zirconia (PSZ) resulting in increases in strength and fracture toughness to around 350 MPa and 4 MPam^1/2, respectively. PSZ may adversely influence the electrical resistivity of the ceramic owing to the presence of impurities. With most zirconia powders a high level of retention of tetragonal zirconia (t-ZrO₂) is obtained at levels of addition up to 15% by weight ZrO₂. At these levels ZrO₂/-Al₂O₃ ceramics show low resistivity and stable resistance in Na/S cells.     

Optical properties of zinc–vanadium glasses doped with samarium trioxide

Zinc–vanadium glasses doped with samarium oxide having the chemical composition Sm₂O_3(x) ZnO_(40?x) V₂O₅₍₆₀₎(where x = 0?1–0?5 mol%) were prepared by melt quenching method. The density of these glasses was measured by Archimedes method; the corresponding molar volumes have also been calculated. The values of density range from 3?7512 to 5?0535 gm/cm³ and those of molar volume range from 28?3004 to 37?6415 cm^?3. The optical absorbance studies were carried out on these glasses to measure their energy bandgaps. The absorption spectra of these glasses were recorded in UV–Visible region. No sharp edges were found in the optical spectra, which verify the amorphous nature of these glasses. The calculated optical bandgap energies of these glasses were found to be in the range of 0?3173–0?6640 eV. The refractive index and polarizability of oxide ion have been calculated by using Lorentz–Lorentz relations. The values of refractive index range from 1?1762 to 1?2901 and those of polarizability of oxide ion range from 1?6906 × 10^?24 to 2?2379 × 10 ^?24cm³.     

Structural and optical properties of γ-alumina thin films prepared by pulsed laser deposition

Preparation of γ-alumina thin films by pulsed laser deposition from a sintered α-alumina target is investigated. The films were deposited on (100) silicon substrates at 973 K with varying oxygen partial pressures in the range 2.0 × 10⁻⁵-3.5 × 10^− 1 mbar. X-ray diffraction results indicated that the films were polycrystalline γ-Al₂O₃ with cubic structure. The films prepared in the oxygen partial pressure range 2.0 × 10^− 5-3.5 × 10^− 2 mbar contained nanocrystals of sizes in the range 10-16 nm, and became amorphous at pressures > 3.5 × 10^− 1 mbar. Topography of the films was examined by atomic force microscopy using contact mode and it showed the formation of nanostructures. The root-mean square surface roughness of the film prepared at 2.0 × 10^− 5 mbar and 3.5 × 10^− 1 mbar were 1.4 nm and 3.5 nm, respectively. The thickness and optical properties were studied using ellipsometry in the energy range 1.5-5.5 eV for three different angles of incidence. The refractive index was found to decrease from 1.81 to 1.73 with the increase of oxygen partial pressures from 2.0 × 10^− 5 to 3.5 × 10^− 2 mbar. The variation in the refractive index has been found to be influenced by the microstructure of the films obtained as a function of oxygen partial pressure.     

Synthesis of Fe–Co alloy and cobalt–magnetite composites doped with Nd3+ by using iron disproportionation

Iron–cobalt alloy and cobalt–magnetite composites doped with Nd³⁺ (Co _x Fe_1?x /Co _y Fe_1?y Nd _z Fe_2?z O₄) in which the Fe alloy has either a bcc or a fcc structure and the oxide is a spinel phase, have been synthesized by using the disproportionation of Fe(OH)₂ and the reduction of Co(II) by Fe⁰ in a concentrated KOH solution. Powder X-ray diffraction, scanning electron microscope and vibrating sample magnetometer were employed to characterize the crystallite sizes, structure, morphology and magnetic properties of the composites. And the effect of the Co(II)/Fe(II) ratio (0 ≤ Co/Fe ≤ 1), concentration of KOH, reaction time and substitution Fe³⁺ ions by Nd³⁺ ions on structure, magnetic properties of the composites were investigated. From the perspective of thermodynamics, we explain the postulated mechanism of the disproportionation reaction.     

Structural properties and Mössbauer spectroscopy of Finemet™ doped with Ge

Structural and magnetic properties of amorphous and crystalline alloys Fe_73.5Cu₁Nb₃B₉Si_13?xGe_{x=1;5;10;13.5} were studied by means of energy dispersive X-rays (SEM + EDX), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Mössbauer spectroscopy (MS), as-quenched (a-q) and after annealing (a). EDX show agreement between the nominal and the measured atomic contents for all alloys. DSC provided three phase transition temperatures of the materials, MS spectra reveal amorphous structure of as-quenched alloys and a mixture of amorphous and crystalline structures of annealed alloys. The XRD spectra of annealed alloys allow us to resolve three crystalline phases of Fe.     

FTIR and Raman spectroscopic investigation of some strontium–borate glasses doped with iron ions

Structural investigation of xFe₂O₃·(100 − x)[3B₂O₃·SrO] glass system, with 0 ≤ x ≤ 40 mol%, was performed by means of X-Ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopies. The purpose of this work was to investigate the structural changes that appear in the 3B₂O₃·SrO glass matrix with the addition and increasing of iron ions content. The XRD pattern for the prepared samples shows that vitreous phase is present only for x ≤ 40 mol%. For sample containing 50 mol% Fe₂O₃ was evidenced the presence of a unique crystalline phase, Fe₂O₃, embedded in an amorphous matrix. FTIR data show that BO₃ and BO₄ are the main structural units of the glass system and the iron ions are located in the network. The Raman spectrum of glass matrix indicates a structure with several borate groups (di-, meta-, pyro-borate, etc.). In higher concentrations the iron ions break the regulate glass network and determines the appearance of BO₄ isolated units.     

Adsorption properties of carbonized polyacrylonitrile deposited on γ-alumina and silica gel by precipitation polymerization

The precipitation polymerization method was used for the deposition of various contents of polyacrylonitrile on two oxide-type supports (γ-alumina and silica gel). The synthesized materials were characterized by thermal analysis performed in inert and oxidizing atmospheres. The mechanism of polyacrylonitrile decomposition was proposed. In order to gain effective adsorbents of volatile organic compounds the polyacrylonitrile/support composites were carbonized at elevated temperatures. The texture and morphology of the calcined materials were examined by low-temperature sorption of N₂ and scanning electron microscopy, respectively. An influence of thermal treatment conditions and carbonaceous species loading on adsorption capacity of methyl-ethyl ketone vapour was also determined. Attenuated total reflection Fourier transform infrared spectroscopy measurements revealed that the ladder-type polyacrylonitrile species formed above 250 °C and stable up to about 350–400 °C are the most effective sites for methyl-ethyl ketone sorption. The carbonaceous species dispersion was found to be an additional factor influencing the adsorption capacity of the carbonized polyacrylonitrile/support composites.     

Elastic properties of polycrystalline (La,Sr)2CuO4 and YBa2Cu3O7−δ doped with iron

The elastic modulus of polycrystalline (La, Sr)₂CuO₄ and YBa₂Cu₃O_7– with copper replaced by iron has been measured in the temperature range from room temperature to 4.2 K using the cube resonance method. For the former system, 0.5% Cu is displaced by iron and the content of strontium is varied. The incorporation of iron gives rise to an apparent coexistence of magnetic order and superconducting order in samples La_2–x Sr _x (Cu, Fe)O₄ withx=0.15 and 0.3, which is reflected in the temperature dependence of Young's modulus. For samples withx=0 and 0.05, an anomalous behaviour of Young's modulus, which is thought to correspond to orthorhombic-monoclinic transition, is observed. For the YBa₂(Cu_1–Fe)₃O_7– system, softening and a thermal hysteresis loop are observed in the temperature dependence of shear modulus of the sample with=0. The softening also appears in the temperature variation of Young's modulus. The degree of softening becomes smaller as the iron content which displaces copper is increased. These phenomena are discussed on the basis of the microstructure of the samples and the change of crystal structure with the iron content.     

Electrical and optical properties of single crystalline α-Al2O3 doped with nickel

Nickel, substituting for aluminium in α-Al₂O₃ acts as an acceptor with a level ∼2.46 eV above the conduction band if a large polaron model applies, ≅ 2.57eV−H(μ _h) above the band if a small polaron model applies. It is present as Ni³⁺ at high, and as Ni²⁺ at low, oxygen pressures, the concentration of Ni³⁺ being reduced to one-half of its high value at =1 Pa. Analysis of the data provides proof that the native defect compensating the charge of Ni²⁺ (=ni _al ^′ ) is V _O ^2. , Al _i ^3. being a minority species; H _F,Al−1/2H _s=121 kJ mol⁻¹.     

Electrical conductivity and creep of polycrystalline α-Al2O3 doped with titanium or iron plus titanium

Creep and conductivity were measured for polycrystalline -Al₂O₃ doped with titanium or iron plus titanium. Both series of samples show a transition from acceptor domination to donor domination at the point where the concentrations of titanium and acceptors are approximately equal. Parameters in the expression for the creep rate S ⁿd-^PP^r _{O 2} exp (-QlkT) (whereS is the applied stress,d the grain size,Q an activation energy andn, p, r are constants characterizing the creep mechanism) indicate limitation by bulk diffusion in the acceptor-dominated samples and in the donor-dominated samples at low [Ti]. In the latter samples the ratelimiting species isO ⁱ at highP _{O 2}, VA ^Al at lowP _{O 2}, with involvement of e at mediumP _{O 2}. At higher titanium concentrations the creep rate is limited by generation/recombination of defects at grain boundaries. Iron appears to increase the rate of these grain-boundary reactions. When second-phase particles of Al₂TiO₅ are present, is decreased but ionic conductivity is increased.     

Structural and magnetic properties of Ce3+doped Mg-Co ferrite prepared by sol–gel method

Journal of Materials Science: Materials in Electronics - Ce3+-doped Mg-Co ferrite powder can be prepared by sol–gel spontaneous combustion method. The chemical formula for...     

The nonlinear optical, magnetic, and Mössbauer spectral properties of some iron(III) doped silica xerogels

Iron(III) species dispersed in silica have been synthesized with a sol-gel process. The iron(III) was introduced as the acetylacetonate complex into a solution of tetraethoxysilane to yield, after evaporative drying, pellets or monoliths. Two gels were dried very slowly over a period of five months in order to prepare a defect free monolith useful for nonlinear optical studies. Z-scan experimental studies on the resulting, transparent, monolithic, doped solid revealed an optical Kerr effect, a third order nonlinear optical phenomenon showing a linear dependence of the refractive index on the irradiance with a nonlinear refractive index, n ₂ , of ?1.95 × 10^?11 cm²/W. Magnetic susceptibility studies between 4.2 and 295 K revealed paramagnetic behavior with a Curie constant of 4.433 (emu/mol)K and a Weiss temperature of ?7.1 K. Magnetization studies at 5 K and at applied fields of up to 4 T and Mössbauer spectral studies between 4.2 and 295 K revealed a 50:50 mixture of paramagnetic species and nanoparticles with an average particle radius of 1.3 ± 0.2 nm. A blocking temperature of 70 K and a magnetic anisotropy energy of 2.4 × 10⁵ J/m³ are derived from the Mössbauer spectra.     

Structural and electrochemical properties of α-MnO<Subscript>2</Subscript> doped with cobalt

In this study, co-doped and undoped MnO₂ samples were prepared by redox reaction in acidic medium. X-ray diffraction patterns of pure and doped samples revealed that they crystallized in the α-form with a (2 × 2) tunnel structure. Chemical analysis indicated an increase in the concentration of K⁺ cations upon Co doping resulting in higher thermal stability. Magnetic properties show the coexistence of Mn⁴⁺ and Mn³⁺ valence states, with reduced amount of Mn³⁺ ions after the doping process. Co-doped MnO₂ has better rechargeability and capacity retention than pure α-MnO₂, but the reduction of the capacity fading occurs at the expense of the initial capacity.     

Interaction of acetylacetone with HZSM-5: Visualization of “n.m.r. invisible” aluminum

²⁷Al MAS n.m.r. spectroscopy, ¹³C MAS n.m.r. spectroscopy, in situ infrared spectroscopy, and thermogravimetric analysis were employed to study acetylacetone adsorbed on HZSM-5 samples of varying acid-site concentration and extralattice aluminum content. Three types of adsorbed acetylacetone were observed: (1) physisorbed acetylacetone, (2) acetylacetone interacting strongly via the carbonyl groups with acid sites within the zeolite structure, and (3) acetylacetone weakly adsorbed on extralattice aluminum and giving rise to a sharp octahedral aluminum signal observed in the ²⁷Al MAS n.m.r. spectra. The extralattice aluminum acetylacetone species decomposes slowly under ambient laboratory conditions. The implications of these observations to the use of acetylacetone as a means of visualizing “n.m.r. invisible” aluminum are discussed.     

Effect of temperature on dielectric and electrical properties of Co–Zr doped barium hexaferrites prepared by sol–gel method

In the present research, temperature dependence of dielectric properties of cobalt–zirconium substituted barium hexaferrites, fabricated using citric acid sol gel method, has been reported. The dielectric constant, loss tangent and A.C. conductivity were investigated on the circular pellets in temperature range 30–350 °C and frequency range 10 kHz–1 MHz using impedance analyzer. This paper also presents impedance (Z*) and electric modulus (M*) analysis of all the samples. The single semi-circular arcs, observed in impedance Nyquist plots, suggest the dominance of grain boundaries in the conduction process. Dielectric constant and dielectric loss tangent show very small variation up to 200–250 °C temperature and abrupt increase afterwards up to 350 °C. Thus, these ferrites can be successfully implemented in the practical applications like capacitors, microwave devices etc. up to 250 °C, without any significant change in properties.     

Subsecond measurements of the optical properties in the spectral range 0.4–1.1 μm near high-temperature transformations of samarium oxide

A new dynamic method is described for measuring the spectral directional-hemispherical reflectivity in the spectral range 0.4–1.1 m near high-temperature phase transformations of refractory materials. The use of diffuse sample irradiation enables one to exclude the influence of an abrupt variation of the angular reflection distribution of sample surface at the phase transitions. The experimental results on samarium oxide showed that light pulses are emitted by the sample during the high-temperature transformations.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Deposition of SixC1−x:H films by reactive r.f. sputtering

The preparation of thin Si_xC_{1 − x}:H films at low temperatures by reactive r.f. sputtering in acetylene-argon gas using pure silicon wafers as target material was investigated. Optimum conditions for SiC deposition were selected by variation of r.f. power density, r.f. peak voltage and the acetylene partial pressure. The deposited Si_xC_{1 − x}:H films were characterized by density measurements, transmission high energy electron diffraction, transmission electron microscopy and UV and IR transmission spectroscopy as well as by electron probe microanalysis and X-ray photoelectron spectroscopy analysis. It is seen that the chemical composition and the structure of reactively sputtered Si_xC_{1 − x}:H films are strongly influenced by the ratio of the acetylene content in the plasma to the power density used. For acetylene contents smaller than 7.5 vol.% in the glow discharge, polycrystalline β-SiC films can be prepared if the r.f. power density is limited to 4.6 W cm⁻².