Preparation of nanocomposite materials containing WS2, δ-WN,Fe3O4, or Fe9S10 in a silica aerogel host

Nanocomposite materials based on silica aerogel hosts have been produced using chemical vapour infiltration/decomposition methods and characterized by X-ray diffraction and electron microscopy. Amorphous tungsten in SiO₂ aerogel was formed by the decomposition of W(CO)₆ at 250 °C. Alternatively, reaction of this material with sulphur at 700 °C produced needle-shaped WS₂ crystals with lengths ranging from 25–230 nm. Reaction of the W/SiO₂ composite with anhydrous NH₃ formed crystals of -WN with diameters of 1–5 nm. Fe(CO)₅ is readily absorbed into the silica aerogel, forming an amorphous iron oxide/SiO₂ composite after slow oxidation in air. Treatment of this material with additional Fe(CO)₅ produced an Fe₃O₄/SiO₂ aerogel composite. Fe₃O₄ particle sizes were 20–55 nm. After additional heat treatment, this composite exhibited soft ferromagnetic behaviour with a coercivity of 170 Oe. Fe₉S₁₀ crystals with diameters of 30–90 nm were formed by the reaction of the amorphous iron oxide/SiO₂ composite with H₂S at 900 °C.     

Spinel growth in the interface of δ-Al2O3 fibre reinforced aluminium piston alloys

Spinel growth is studied by analytical electron microscopy in the fibre-matrix interface of as-processed and in thermal treated -Al₂O₃ fibre reinforced AlSi12CuMgNi alloys. In the as-processed composites a few nanometer large spinel crystals are observed in the fibre-matrix interface. They are interpreted as the reaction product of remnants of the SiO₂ binder, with magnesium segregating in the interface. In the thermally treated material, large spinel crystals of a few tenths of a micrometer grow on the fibre, preferentially at positions where -Al₂O₃ crystals reach the fibre surface. The spinel grows into the aluminium grain of the matrix, as well as into -Al₂O₃ of the fibre. The orientational relationship of the spinel and the alumina is studied and the results are discussed in relation to the fibre wetting, adhesion and fibre-matrix reaction.     

Optical Characteristics of C60 Single Crystals Grown in Microgravity Conditions

This work is devoted to the growing and characterization of perfect C ₆₀ single crystals with the aim of further understanding of the physical properties of this material related to the low energy excited states which determine in a considerable degree its electronic properties, which, in turn, are important for its possible application. Here we present several characterization techniques based on optical properties of C ₆₀ crystals and the first results of the investigation of the C ₆₀ samples grown at the orbital space station MIR.     

Localized High-Tc Superconductivity on the Surface of Na-Doped WO3

WO₃ crystals were doped with Na on the surface to 7% nominal atomic concentration. Scanning tunneling microscopy and spectroscopy were employed, together with magnetization measurements, in the study of these crystals. Tunneling experiments reveal superconducting islands, 20–150 nm in size, covering about 10% of the surface, the rest of which is insulating. Magnetization measurements show that the superconducting phase formed at the surface has a critical temperature of 91 K, while tunneling spectroscopy yields superconducting gaps having values up to 2/kT _c 4. Presumably most of the sodium is concentrated in these islands and therefore they are metallic in nature, above 30% in atomic concentration. This material is therefore a noncuprate superconductor with a high critical temperature.     

Growth and characterization of Bi2Sr2Ca1Cu2O8+x single crystals

High-quality single crystals are well suited to the investigation of some intrinsic material properties. A modified Bridgman method using a sharp temperature gradient (300°C/cm) was used to grow Bi₂Sr₂Ca₁Cu₂O_8+x single crystals. Although the samples were contained in alumina ampoules, no aluminium contamination of the samples was detected. Blade-shaped crystals up to 7–8 mm length and 3–4 mm width could be grown by this method, although extraction from the matrix was difficult. Electron diffraction patterns of the [001] zone axis revealed a high degree of crystallinity. The narrowness of the superconducting transition temperature, as determined by ac susceptibility, also suggests the existence of well-formed crystalline domains. In order to determine the relative orientation of the crystalline domains, electron channeling patterns were recorded from several consecutive growth steps from a fracture surface. The poor contrast of these and Kikuchi patterns suggests the presence of a stacking structure. The results showed a [100] growth direction and (001) cleavage plane. Reversible oxygen loss at the peritectic decomposition temperature of 863°C was observed. Knoop indentation measurements showed that the crystals were quite soft, having a microhardness of 0.44 GPa.     

Structural changes of glass-ceramics of the Cu2O-Al2O3-SiO2 system on heating in air

The behaviour of copper ions in low thermal expansion glass-ceramics prepared from Cu₂O · Al₂O₃ ·_nSiO₂ glasses, or in-spodumene type Cu₂O · Ae₂O₃ ·nSiO₂ crystals included in the glass-ceramics on heating in air was investigated. On hewing at 300 to 500° C, the copper ions behaved as in the corresponding glasses. Cuprous ions in the glass-ceramics or-spondumene type crystals were oxidized into the cupric state, and at the same time an equal amount of cuprous ions to those oxidized were expelled out of the specimen for the requirement of electrical charge neutrality and then reacted with oxygen to form CuO on the surface. The oxidation of cuprous ions and the decrease of the copper content of the-spondumene type crystals brought about a considerable decrease in the lattice spooings.     

Synthesis, Crystal Growth, and Acoustophotovoltaic Effect of TlIn1 – x Pr x Se2 and TlIn1 – x Pr x Te2

Procedures were developed for growing perfect TlIn_{1 – x} Pr _x Se₂ and TlIn_{1 – x} Pr _x Te₂ (0 x 0.08) single crystals. The growth charges were synthesized by a process involving partial cooling of a tilted, rotating tube. The crystals grown by the floating zone technique were found to readily cleave in two, mutually perpendicular mirror planes to give rectangular parallelepipeds. The crystals were shown to exhibit a sizeable acoustophotovoltaic effect: a combined action of radiation and sound waves produced an emf between the electrodes or a short-circuit current.     

Electron spin resonance in single crystal yttria stabilized zirconia

Electron spin resonance has been observed at 35 GHz in 8 and 12 mol % yttria stabilized zirconia single crystals before and after blackening by current passage. Measurements were made between 293 and 77K. In both as-grown and blackened crystals the spectra showed anisotropic lines (type A) characterized by g =2.003±0.002 and g =1.880 ±0.002 with respect to a [1 1 1] symmetry axis and a broader, slightly anisotropic line (type B) centred near g=1.993. In current blackened crystals a weak isotropic line (type C) was also found near g=1.986. The type A lines are attributed to a charged complex formed by an electron trapped at an oxygen vacancy and associated with an yttrium ion. Tentative models are suggested to explain the type-B and type-C lines.     

Magnetic properties of (Dy1−xGdx)3Ga5O12 garnet single crystals

Magnetic properties of Dy_1–x Gd _x )₃Ga₅O₁₂ (DGGG) garnet single crystals were calculated using the Weiss molecular field theory and also measured using the vibrating sample magnetometer in the temperature range 4.2–40 K in the effective magnetic field from 0–7 T. The magnetic properties of DGGG single crystals are distributed between those of Dy₃Ga₅O₁₂ (DGG) and Gd₃Ga₅O₁₂ (GGG) single crystals, but are considerably closer to those of DGG. Based on the magnetic properties, the magnetic entropy change, S_M, was evaluated in the temperature range below 15 K. DGGG single crystals are a prospective material for magnetic refrigeration below 15 K.     

Homogeneity of high-resistivity <Emphasis Type="Italic">p</Emphasis>-Si<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>〈Au〉 crystals

p-Si_{1 ? x} Ge _x crystals have been diffusion-doped with gold. Gold diffusion in the p-Si_{1 ? x} Ge _x 〈Au〉 samples and their electrical properties have been studied. The results demonstrate that the highest gold concentration in the crystals can be achieved in the temperature range 1000–1050°C. An expression has been derived which indicates that, all other factors being the same, compensation with Au, an amphoteric impurity, insures better homogeneity compared to codoping with acceptor and donor impurities. The hole concentration homogeneity in gold-compensated samples is at the same level as or even better than that in the uncompensated material.     

Dielectric studies of Ca x Sr1−x F2 mixed crystals

The dielectric constant (K) and loss (K) of CaF₂, SrF₂ and their mixed crystals as a function of frequency in the range 10² to 10⁵ Hz at room temperature were measured. Except at 10² Hz, the value of the dielectric constant was found to be independent of frequency. At any frequency the value ofK for mixed crystals was found to vary non-linearly as a function of composition, showing a maximum at about equimolar composition. The value ofK was less in mixed crystals compared to the end members. An attempt is made to explain the results.     

DTA measurements of normal-incommensurate phase transition in (NH4)2ZnCl4 and K2ZnCl4 crystals

Differential thermal analysis (DTA) was applied to determine the changes in enthalpy and entropy of (NH₄)₂ZnCl₄ and K₂ZnCl₄ crystals at their phase transition from the orthorhombic normal phase to the incommensurate phase. The temperature of this transition, T _i , is 406 K for (NH₄)₂ZnCl₄ and 555 K for K₂ZnCl₄ and the entropy changes (S/R) are 0.053 and 0.035, respectively. The low value obtained for S/R is characteristic of incommensurate phase transitions. The results were compared with the data reported for other crystals of the A₂BX₄ family. Thermal properties of the crystals of the A₂ZnCl₄ subgroup were found to the correlated with the length of A-Cl bonds.     

Optical differentiation of orthorhombic superconductors such as YBa2Cu3O7 from their tetragonal precursor material

The orthorhombic superconducting phase of YBa₂Cu₃O₇ is shown to be differentiable from its tetragonal precursor by optical microscopy. Individual grains 1 m in diameter or larger are found to either exhibit the color of the compensator in reflected polarized light (tetragonal material) or to change colors from the compensator (orthorhombic material). By using color differences, retained tetragonal material and the orthorhombic nature of untwinned areas of grains are identified. The advantage to producing better superconducting single crystals and sintered polycrystalline materials is discussed.     

Growth and characterization of SnSe2

Tin diselenide single crystals were grown by direct synthesis using the Bridgman technique and were subsequently purified by successive recrystallizations. An X-ray powder diffraction study verified that the crystals obtained belong to the D _3d ³ — P¯3m1 space group with lattice parameters a=0.3811 nm and c=0.6137 nm. A stoichiometric analysis gave a maximum deviation of their nominal SnSe₂ composition of 4%. Study of their morphology by scanning electron microscopy reveals the layered-type structure of the obtained material. Finally, the microhardness parallel and perpendicular to the layer planes was found to be strongly anisotropic, as expected from the layered structure of SnSe₂.     

Nucleation and crystallization kinetics of CaO-Al2O3-2SiO2 in powdered anorthite glass

The nucleation and crystallization kinetics of CaO-Al₂O₃-2SiO₂ crystals in powdered anorthite glass with particle size <44 m in which CaO-Al₂O₃-2SiO₂ crystals were found to crystallize in the heating process of the glass, were studied by nonisothermal measurements using differential thermal analysis (DTA). The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy and kinetic parameters were simultaneously calculated from the DTA data using previously reported kinetic models. The crystallization process of a sample heat treated at the temperature of the maximum nucleation rate was fitted to kinetic equations with an Avrami constant, n2 and a dimensionality of crystal growth, m2, indicating that the constant number of nuclei of CaO-Al₂O₃-2SiO₂ precipitated in a glass matrix grew two-dimensionally with an activation energy taken as an average of the values calculated by the Kissinger and also the Augis and Bennett method of 679±4 kJ mol^–1.     

Laser Damage Threshold and UV Absorption of Doped Potassium Dihydrogen Phosphate Crystals

The effects of Pb, Ca, and Si impurities on the laser damage threshold and UV absorption of KH₂PO₄ crystals were studied. The laser damage threshold was found to be 6 × 10¹¹ W/cm² in crystals containing 5 × 10^–5 wt % and to decrease nonlinearly at higher impurity contents.     

High temperature solution growth of yttrium aluminium garnet crystals using accelerated crucible rotation

The value of ACRT stirring has been confirmed in the case of Y₃Al₅O₁₂ crystal growth, its main advantage being to suppress nucleation in the bulk solution and increase the yield of crystals. Restriction of nucleation to 1 to 5 crystals has been achieved, particularly by close control of temperature stability and by the provision of an anchor to prevent crystals from becoming detached due to the stirring action.     

On the preparation and electrical properties of PtSb2

The preparation and the Czochralski growth of single crystals of PtSb₂ by a liquid encapsulation technique is described. Conductivity and Hall effect measurements have been made in the temperature range 20 to 300° K. It is found that the material is n-type, with a carrier concentration of about 6 × 10¹⁶/cm³, the ratio of electron to hole mobility, _e/ _h, is 0.57 at 110° K, and the electron mobility has a maximum value of 2100 cm²/V sec at 50° K. The thermal energy gap is 0.112 eV.     

Thermal properties and radiation damage in NaNH4XO4·2H2O single crystals (XO4=SO4,SeO4)

Differential thermal analysis (DTA) studies of NaNH₄SO₄·2H₂O, NaND₄SO₄·2H₂O, and NaNH₄SeO₄·2H₂O single crystals were performed in the temperature range of the ferroelectric-paraelectric phase transition and of the dehydration process. The first-order phase transition at the Curie point was confirmed for NaNH₄SO₄·2H₂O, whereas the transition in the isomorphous NaNH₄SeO₄·2H₂O at the Curie temperature was found to be similar to a contineous (second order) with considerable higher transition enthalpy. Dehydration process of the crystals studied was found to proceed in two stages, with maximum dehydration rates at 355 and 400K. A linear decrease in thermal energy related to the long-range ordering in NaNH₄SO₄·2H₂O crystals was observed with the dose of -cobalt-60 irradiation and discussed with respect to the radiation induced decrease in spontaneous polarization.     

Growth of colorless transparent GaN single crystals on prismatic GaN seeds using a Ga melt and Na vapor

Growth of GaN on seeds of GaN prismatic single crystals was carried out at 900 °C and N₂ pressure (PN₂) of 0.8-7.0 MPa for 72 h by the Na flux method using premixed Na-Ga melts or Ga melt and Na vapor. Black GaN crystals, having some pits and striations on the facets, grew on the seeds when the premixed Na-Ga melts were used. A full-width at half maximum (FWHM) of the X-ray rocking curve measured for the (m plane) of the grown crystals was over 360 arcsec. Colorless and transparent GaN crystals with smooth facets were grown on the m plane of the seed crystals by using a Ga melt and Na vapor. The FWHM measured for the m plane of the colorless crystals was 112-204 arcsec. Cathodoluminescence (CL) spectra from the m plane of the crystals were measured at room temperature. Besides a near-band-edge (NBE) emission at 361-363 nm, the specimens grown with Ga melt and Na vapor at higher PN₂ had a broad deep emission peak at 617 nm, while the specimens grown at lower PN₂ had a shallow-level emission peak at 380 nm and a broad deep emission peak at 550 nm.