Growth of Ferroelectric Pb(Ta1 – xNbx)2O6 Single Crystals

The liquidus relations in the Pb₂V₂O₇–PbNb₂O₆–PbTa₂O₆ system were studied by direct visual observation of melting and x-ray diffraction analysis. Pb(Ta_{1 – x} Nb _x )₂O₆ crystals, both ferroelectric (x= 0.3, 0.5, 0.7; tetragonal potassium tungsten bronze structure type) and nonferroelectric (rhombohedral structure), were grown by precipitation from Pb₂V₂O₇ fluxes. The Curie temperature and peak permittivity of the crystals were found to increase systematically with Nb content.     

Phases Resulting from the Reduction of Manganese Metaniobate by Niobium and Tantalum

The sequence of reactions in the reduction of manganese metaniobate with Nb, NbO, and Ta is studied. The reduction of MnNb₂O₆ by Nb or NbO differs in some respects from that by Ta. No Mn(Nb_{2 – x} Ta _x )O_4.33 or Mn(Nb_{2 – x} Ta _x )O_3.67 solid solutions were detected in the reduction products. Tantalum converts directly into manganese metatantalate, without oxidation according to the scheme Ta–TaO–TaO₂–Ta₂O₅. A reaction scheme is suggested for the reduction of MnNb₂O₆ by niobium and tantalum. The unit-cell parameters of the orthorhombic phase MnNb₂O_4.33 resulting from the reduction of MnNb₂O₆ with Nb or Ta are determined.     

Microstructural evolution of FINEMET type alloys with chromium: An electron microscopy study

Microstructural changes in Fe_73.5–xCr_xCu₁Nb₃Si_13.5B₉ (0x5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe(Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is enhanced with the Cr content of the alloy. In a second stage, around 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type unknown crystal phase with a lattice parameter of a=1.52 nm, and recrystallization of the previous Fe(Si) nanophase also occurs. No qualitative differences were found between dynamic and isothermal crystallization. The size effect for thin samples is limited to a lowering of crystallization temperatures. For isothermal nanocrystallization in the temperature range 775–900 K, the mean grain size of the nanocrystals increases for short annealing times to stabilize at a constant value of about 10–15 nm for long annealing times. The stabilized grain size increases with increasing annealing temperature and slightly decreases with the Cr content of the alloy.     

Preparation and characterization of Zn3P2-Cd3P2 solid solutions

Solid solutions of Zn₃P₂-Cd₃P₂ systems of the type (Zn _x Cd_1–x )₃P₂ have been prepared by direct reaction of the constituent elements (Zn, Cd, P) for values of x equal to 0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0. X-ray diffraction data indicate that all the systems crystallize in tetragonal () phase only, exhibiting preferred orientation along the (220) and (224) directions. The lattice parameters, a and c, and the interplanar spacing, d, vary linearly with x, obeying Vegard's law. The systems show minimum conductivity at room temperature for composition corresponding to x values in the range 0.4–0.6. Electrical conductivity for all systems is measured in the temperature range 100–450 K. In view of the estimated (low) values of the activation energy the conduction process in the different temperature regions has been attributed to the presence of shallow trapping levels in the systems.     

Structural, spectroscopic and microwave characterizations of (Sm0.5Y0.5)Ti(Nb1?xTax)O6 ceramics

Microwave dielectric ceramics of the type (Sm_0.5Y_0.5)Ti(Nb_1−x Ta _x )O₆ were prepared for x = 0, 0.2, 0.4, 0.6, 0.8 and 1 through the conventional solid state ceramic route. The ball-milled compositions were calcined at 1,250 °C. The cylindrical pellets were sintered at temperatures between 1,385 and 1,450 °C. The densities were determined by Archimedes method. The structure was analyzed using X-ray diffraction method. The microwave dielectric properties of the polished pellets were measured using cavity resonator method. The morphological studies were done using Scanning Electron Microscopy and Transmission Electron Microscopy. The Raman spectra were recorded and analyzed to confirm the structural change. The photoluminescence spectra were also taken and the emission lines were identified. A correlation study was done among the measured properties and parameters. Most of the samples have high dielectric constant, high quality factor and low temperature coefficient of resonant frequency and hence suitable for microwave applications.     

SAXS study on crystallization of an amorphous Pd76Au6Si18 alloy

The crystallization behaviour of an amorphous Pd₇₆Au₆Si₁₈ alloy has been investigated mainly by means of small angle X-ray scattering measurements. The amorphous alloy crystallized to form MS-I phase with composition Pd₇₈Au₂₂ in the amorphous matrix, which later crystallized as MS-II. The crystallization kinetics of MS-I phase were analysed in the framework of conventional nucleation theory. It was suggested that the considerable number of nuclei of MS-I phase had been already formed in the as-quenched specimen and the remaining nucleation attained within the short period of isothermal ageing. The remarkable increase of the scattering intensity corresponded to the growth of MS-I phase, the kinetics of which were found to be controlled by the diffusion mechanism. The interdiffusion constant was obtained fromD=6.0×10¹⁵ exp (–420 kJ mol^–1/RT) m² sec^–1, which agreed fairly with the reported values.     

Crystallization of amorphous Ni60 Nb40−x Cr x alloys

Crystallization behaviour of amorphous Ni₆₀ Nb_40-x Cr _x (x = 0, 5, 10 and 13 at%) alloys was studied by differential Scanning calorimetry and X-ray diffraction measurements. It is shown that the addition of chromium reduces the crystallization temperature, stages of crystallization and activation energies associated with the crystallization sages of the Ni₆₀Nb₄₀ glass. Crystallization of the Ni₆₀Nb₄₀ glass occurred in three stages; in the initial stage a metastable M-phase formed in the amorphous matrix as reported earlier [1] . However, contrary to earlier observation [1], M -phase was not very stable and transformed together with some amorphous phase to the equilibrium Ni₃Nb phase in the second stage of crystallization. In the third stage, the remaining amorphous matrix transformed to the equilibrium NiNb phase. On addition of chromium the formation/stability of the M-phase was found to be suppressed and equilibrium NbCr₂ phase precipitated preferentially in the first stage. The second stage, corresponding to the formation of Ni₃Nb phase, remained almost unaltered. The third stage corresponding to the crystallization of NiNb phase disappeared completely at 13 at% Cr. In the fully crystallized samples the proportion of the NiNb phase decreased and that of NbCr₂ phase increased continuously with chromium concentration.     

High dielectric constant ceramics in the PbSc0.5Ta0.5O3-PbZrO3 and PbSc0.5Ta0.5O3-PbTiO3 systems

The properties of (1-x) PbSc_0.5Ta_0.5O₃-(x) PbZrO₃ ceramics (0.075 <x < 0.2), have been investigated using three different mixed oxide preparation routes. These were (i) a single-stage calcination (at 900°C) between lead, scandium, tantalum and zirconium oxides, (ii) the reaction between single phase PbSc_0.5Ta_0.5O₃ and PbZrO₃ powders (also at 900°C), and (iii) the formation of zirconium-modified scandium tantalate (via a high-temperature prereaction stage) prior to calcination with lead oxide. It was found that this third preparation route results in transparent, 100% dense, single-phase perovskite ceramics which exhibit particularly sharp first-order ferroelectric to paraelectric phase transitions. Material with a maximum relative permittivity of 36 × 103 at 64°C, and a dielectric loss of 7.5 × 10^–4, was obtained for anx value of 0.125. Results are compared with those for ceramics in a similar range of compositions in the (1-x) PbSc_0.5Ta_0.5O₃-(x)PbTiO₃ system; these were prepared using only one of the above routes, and involved calcining PbSc_0.5Ta_0.5O₃ with PbTiO₃ powder.     

Etude cristallochimique,dielectrique et pyroelectrique de la solution solide Na1−xLix(Nb0,6Ta0,4)O3 (0,02 ⩽ x ⩽ 0,20)

A solid solution with formulation Na_1?xLi_x(Nb_0.6Ta_0.4)O₃ (0.02 ? x ? 0.20) and a perovskite related structure has been prepared by substituting sodium by lithium in Na(Nb_0.6Ta_0.4)O₃. The ferroelectric Curie temperature increases with x from 308 to 428 K. The pyroelectric properties have been investigated. The high performances may lead to appropriate applications.     

Structural investigation of amorphous iron-nickel-boron alloys

Accurate X-ray diffraction investigation has been made of ten amorphous alloys having the compositions (Fe_0.6Ni_0.4)_100–x B _x with x=14 to 24 and (Fe_100–y Ni _y )₈₀B₂₀ with y=30 to 70 which were obtained by rapid quenching from the melt. Using the common Fourier analysis, the radial distribution function was calculated from which the distance and its number of the near neighbour atoms were derived. The present results indicate that the atomic distribution of metallic glasses with low boron content differs slightly from that observed previously for a number of metallic glasses of transition metalmetalloid type. The compositional effect, in particular the boron concentration effect, on the structure and characteristic structural features of amorphous Fe-Ni-B alloy glasses is discussed together with the mean atomic volume and the partial atomic volume of metalloid elements using the measured density data.     

Phase relations in the Nb-Cr-Sb and Nb-Mn-Sb systems

The 970-K phase relations in the Nb-Cr-Sb and Nb-Mn-Sb systems have been studied by X-ray diffraction. The systems contain no ternary compounds. There are substitutional solid solutions based on binary antimonides. The Nb₅Sb₄-based solid solution (Ti₅Te₄ structure) extends to the composition Nb_4.1Mn_0.9Sb₄; the Nb₃Sb-based solid-solution series (Cr₃Si structure) is bounded by the compositions Nb_2.6Cr_0.4Sb and Nb_2.4Mn_0.6Sb; the Cr_{1.04 − x} Sb_{0.96 + x} -based solution with x = 0.08 (NiAs structure) exists between the compositions Cr_0.94Nb_0.10Sb_0.96 and Cr_0.70Nb_0.30Sb_1.00; and the Mn_{1.00 + x} Sb_{1.00 − x} -based solution with x = 0.1 extends to the composition Mn_0.9Nb_0.2Sb_0.9. In addition, there is an interstitial solid solution (Ni₂In structure), which extends to MnNb_0.25Sb. The 970-K isothermal sections of the Nb-Cr-Sb and Nb-Mn-Sb phase diagrams are mapped out in the region 0–80 at % Sb.     

Phase formation and characterization of [Fe, Mg]NbO4 as a new precursor for the PMN–PFN system

An X-ray diffraction (XRD) and scanning electron microscopy (SEM) study of the phase composition and microstructure characteristics of the Mg_{(1 –x)/3}Nb_{(4 –x)/6}Fe _x/2O₂ (x = 0.5) chemical compound is presented. The samples were prepared by the conventional ceramic method and subjected to different heat treatments. Columbite (MgNb₂O₆) and iron niobium oxide (FeNbO₄, Wolframite) were identified as intermediate compounds in the reaction. A new single phase precursor for the (1 –x)Pb(Mg_1/3Nb_2/3)O₃-xPb(Fe_1/2Nb_1/2)O₃ (PMN–PFN) system identified as [Fe, Mg]NbO₄, was obtained, isostructural with the FeNbO₄ where Fe and Mg ions occupy the same crystal site (space group P1 2/a 1). From the Rietveld refinement method the cell parameters of the monoclinic structure were determined. The microstructure analysis indicates that the particles are irregular in shape and the grain size tends to increase with the calcination temperature.     

Properties of LixNa1 ? xTa0.1Nb0.9O3 ferroelectric ceramic solid solutions

We have studied the electrical and elastic properties of Li _x Na_{1 − x} Ta_0.1Nb_0.9O₃ (x = 0–0.16) ferroelectric ceramic solid solutions. The results demonstrate that these properties of the Li _x Na_{1 − x} Ta_0.1Nb_0.9O₃ solid solutions are rather sensitive to the lattice distortions resulting from compositional structural phase transitions. The elastic constants determined by acoustic and contact techniques agree well.     

Properties of sol-gel derived strontium barium niobate ceramics and the effect of V2O5 additive

Strontium barium niobate, Sr _x Ba_1–x Nb₂O₆, (SBN) ceramics with a range of Sr/Ba ratios were fabricated using a solution chemistry approach with alkoxides dissolved in acetic acid. Powders obtained from dried gels were calcined at 800°C for three hours, a heat treatment that produced fully crystalline powders. After preparing bulk ceramics from these powders by sintering at 1200°C for 1–24 hours, the tetragonal tungsten bronze (TTB) phase was present in all compositions between 0 x 1, although not predicted by the phase diagram for BaNb₂O₆and SrNb₂O₆. Vanadium pentoxide (V₂O₅) additions increased the densification, the amount of tungsten bronze phase, and the dielectric constant for all compositions except for Sr_0.65Ba_0.35Nb₂O₆(SBN65) ceramics.     

Infra-red transmitting glass-ceramics of the As-Ge-Se system nucleated by zirconium selenide

The aim of this study was to produce an infra-red transmitting chalcogenide glass-ceramic nucleated by an appropriate nucleant. In order to crystallize chalcogenide glasses in a controlled manner a preliminary series of experiments, in which the metals of group IVB of the periodic table of elements were used as nucleants, showed that zirconium is an effective additive. Studies of glass compositionsxZrSe₂-(100–x) [As_0.1Ge_0.3Se_0.6] indicated that for values ofx 0.25 and 0.50 mol%, glass-ceramics with high thermomechanical properties were produced. By heat treating at 400° C for 15 h, the glass As_0.1Ge_0.3Se_0.6 with 0.25 mol% ZrSe₂ was transformed into a glass-ceramic with a fracture toughness of 0.848 MN m^–3/2, a sag point of 530° C and satisfactory infra-red transmission. X-ray diffraction analysis revealed that the precipitated crystal phase was germanium selenide (GeSe₂). The parasite absorption band with a maximum around 800cm^–1 generally present in the infra-red spectra was eliminated by adding 0.1 wt% metallic zirconium. The infra-red transmision and thermomechanical properties for glass-ceramics of compositions with 0.50 mol% ZrSe₂ were poorer than for those with 0.25 mol% ZrSe₂, for the same heat treatment condtions. The kinetics and activation energies of crystallization were studied by means of electron microscopy. The mechanism of controlled crystallization proceeds by the precipitation of crystalline GeSe₂ on the ZrSe₂ nuclei formed by heat treatment of the initial chalcogenide glass supersaturated in ZrSe₂.     

Ga as an additive in the As2Te3 glass

Results of measurements of the mean atomic volume (V), the glass transition temperature (T _g), the activation energy for glass transition (E _t) and the d. c. electrical conductivity () are reported and discussed for ten glass compositions of the Ga–As–Te system. The glasses studied can be represented as Ga _x (As_0.4Te_0.6)_100–x glasses, with the additive Ga ranging from 0 to 12 atomic percent (at.%) in the parent As₂Te₃glass. In the Ga _x (As_0.4Te_0.6)_100–x glasses, changes in slope are observed in the V, T _g, E _t, and other electronic properties, at the composition with a Ga content of 2 at.%. The results are compared with those obtained on introduction of Ag and Cu to the As₂Te₃and the [0.5As₂Te₃–0.5As₂Se₃] glasses. Analysis of the data suggest formation of GaAs, Ga₂Te₃and excess Te structural units (s.u.) in lieu of some of the original As₂Te₃s.u., for addition of Ga up to 2 at.% to the parent As₂Te₃glass; for higher Ga contents, formation of GaAs, GaTe and excess Te s.u. are indicated.     

Synthesis of Sr- and Fe-doped LaGaO3 perovskites by the modified citrate method

Fine particle strontium and iron substituted lanthanum gallates La_1–x Sr _x Ga_1–y Fe _y O_3–, where x = 0.2, 0.4, and 0.6; y = 0.2, 0.4, 0.6, and 0.8, have been synthesized by a modified citrate method. The formation of these powders was confirmed by the X-ray powder diffraction (XRD) and the fine particle of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was investigated by scanning electron microscopy (SEM), and particle size analysis. The single phase of La_0.8Sr_0.2Ga_0.4Fe_0.6O_3–, La_0.6Sr_0.4Ga_0.2Fe_0.8O_3–, and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– powders could be obtained both with and without calcination. The amount of the secondary phase increased when the amount of Sr in La_1–x Sr _x Fe_0.6Ga_0.4O_3– was more than 0.2 (x > 0.2) and the amount of Fe in La_0.6Sr_0.4Ga_1–y Fe _y O_3– and La_0.4Sr_0.6Ga_0.2Fe_0.8O_3– was less than 0.8 (y < 0.8). The results indicated that in the pH range of 1.36–9.27, the single phase of La_0.6Sr_0.4Ga_0.2Fe_0.8O_3– was formed without calcination and the pH had negligible effects on the structure and lattice parameter. The fine particle of these calcined powders (<4 m) was obtained with the average particle size 1.70 m at pH = 1.36 and with the average particle size between 0.56–0.60 m at pH range between 3.39–9.27, and with a lattice parameter about 3.9 Å.     

Elaboration of mixed tantalum and niobium carbides from tantalite mineral (Fe,Mn)(Ta1−x Nbx)2O6

This paper presents an original method of synthesis of mixed tantalum and niobium carbides from an oxalic precursor elaborated from the tantalite mineral (Fe,Mn)(Ta_1–x Nb _x )₂O₆. The process of elaboration consists initially in melting the ore with potassium pyrosulphate, then performing a lixiviation with a concentrated hydrochloric acid solution. During the latter reaction, iron and manganese oxides are converted into water-soluble chlorides while the fused mass containing potassium, niobium and tantalum is not dissolved. Traces of chlorides are eliminated by filtration with boiling aqueous solution. Then, the solid is dissolved slowly in hot concentrated sulphuric acid solution. Adding ammonium hydroxide up to a pH of 8 completes the precipitation of niobium and tantalum hydroxides. The precipitate (Ta_1–x Nb _x )(OH)₅ is washed and submitted to a reaction of complexation with oxalate ions in an aqueous environment. This reaction involves the formation of a water-soluble oxalic complex containing tantalum and niobium. The excess of water is eliminated by evaporation at 333 K. The tantalum and niobium carbides are obtained by submitting the oxalic complex of chemical formula (NH₄)₃(Ta_1–x Nb _x )O(C₂O₄)_3· nH₂O to a gas-solid reaction in a methane-hydrogen atmosphere at 1273 K. The oxalic precursor and their resultant mixed carbides are characterized by atomic absorption and Infra-Red spectroscopies, thermogravimetric-differential thermal analysis coupled, laser granulometry, X-ray diffraction and transmission electronic microscopy.     

Possibilities for formation of the 110 K phase 2223 in Sb or V doped Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O materials

Superconducting oxide materials with nominal composition Bi_2–x Sb _x Sr₂Ca₂Cu₃O_y (x=0.05–0.3) and Bi_1.6Pb_0.4–x V _x Sr₂Ca₂Cu₃O_y (x=0.1 0.4) were synthesized. It was found that after prolonged synthesis, formation of the 2223 phase in the Bi-Sb-Sr-Ca-Cu-O system is possible. However, the critical temperatures of the samples are around 90 K and are lower than those of the superconducting materials from the Bi-Pb-Sr-Ca-Cu-O and Bi-Pb(Sb)-Sr-Ca-Cu-O systems. It was also found that V inhibits the formation of the phase 2223, raises the resistance of the samples and has a negative effect on theT _c and the phase composition of the Bi-Pb(V)-Sr-Ca-Cu-O materials.     

Phase Diagram and Dielectric Properties of Pb[Zr1 – x(Fe0.5Nb0.5)x]O3 (0 < x < 0.2) Solid Solutions

The structural, dielectric, and ferroelectric properties of Pb[Zr_{1 – x} (Fe_0.5Nb_0.5) _x ]O₃ (0 < x < 0.2) solid solutions were investigated. The results were used to assess in detail the phase relations near the morphotropic boundary.