Darstellung von VO2-einkristallen der oberen und unteren phasengrenze durch chemischen transport

The chemical transport of VO₂ with TeCl₄ was investigated in the temperature range of about 1000°C and 600°C. In the first case there is TeO₂ existing in the gaseous phase, in the second case the TeO₂ has no noticeable vapour pressure. The transport takes place from the hot to the cool zone. In both cases we obtained high transport rates of 10 to 20 mg h^-1. The composition of the gaseous phase is calculated and the transport mechanisms are discussed. It is possible to obtain VO₂ with the upper and lower phase boundary by variation of the initial compositions. Starting with compositions lower than VO_1.99 we obtained near 1000°C in the deposition zone V₈O₁₅ first and after a time VO₂ with the lower phase boundary. Compositions higher than VO_2.00 give vanadium dioxide with the upper phase boundary. In the temperature range of about 600°C however we obtained the phase with the higher oxygen content first. The results are compared with those, which have been obtained by the transport of VO₂ with HCl and Cl₂ — which takes place from the cool to the hot zone.     

Effect of In and Sb Vacancies on Temperature Dependence of InSb Lattice Parameter at High Temperatures

The temperature dependence of the thermal expansion coefficient for InSb has been determined in the range 25–515 °C by precision measurements of the lattice parameter of InSb single crystals. It is shown that beginning from 460 °C the temperature dependence of the lattice parameter for InSb single crystals grown from melts of different compositions is essentially affected by vacancies in the indium and antimony sublattices. The vacancy concentration and possible deviation of InSb from the stoichiometric composition are estimated from a relative decrease in the lattice parameter.     

Synthesis of AgI?AgCl?CuI solid solutions for ice nucleation studies

Lattice parameter calculation and the nucleation temperature of AgI AgCl CuI system are reported in this study. By vacuum fusion three samples of different molar compositions were prepared. The formation of the cubic solid solution of the above system was confirmed by X-ray analysis. It is found that the 50:25:25 mole% composition of AgI AgCl CuI has the highest nucleation temperature of −1.0 °C, which is far greater than −2.53 °C, −4.02 °C and −2.26 °C for pure AgI, AgCl, and CuI, respectively.     

Thermal balancing via distributed inert-gas streams for high-meniscus ribbon crystal growth

Superior melt growth of ribbon crystals by thermal profile control that is continuously variable, inert, and capable of heat extraction has been achieved. In the device developed, multiple inert-gas jets are directed at various parts of the hot zone, each with individually controlled flow adjustment. In a furnace at 1200°C, the temperature at one edge of a 38-mm-wide ribbon growth zone can be varied at a rate of 6.8°C per liter per minute of argon flow. Asymmetrical temperature profiles in the zone can be flattened to within ±1°C or symmetrically tailored. Silicon ribbons having uniform widths of 38 and 50 mm and grown with a high melt meniscus have been produced by the capillary action shaping technique with inert-gas thermal balancing. Such crystals show minority carrier lifetime values of up to about 300 μs.     

An X‐ray powder diffraction study of the microstructural evolution on heating 3:2 and 2:1 mullite single‐phase gels

Single‐phase gels with compositions 3Al₂O₃·2SiO₂ and 2Al₂O₃·SiO₂ were prepared by gelling mixtures of aluminium nitrate and tetraethylorthosilicate. Gels were fast heated at different temperatures between 900°C and 1600°C. The phase transformation and microstructural changes of both mullite precursor gels over the temperature range were followed by X‐ray powder diffraction (XRD), lattice parameter determination (LP), and scanning and transmission electron microscopies (SEM and TEM). The distribution of crystallite sizes and strains were determined by linewidth refinements of X‐ray diffraction patterns using the integral breadth method of Langford and the Warren‐Averbach analysis. XRD of both heated gels showed the formation of crystalline mullite single phase. Some amount of glassy phase coexisted with mullites at low temperatures, i. e. below 900°C. The compositional range of mullites formed on heating gels at temperatures between 900°C and 1600°C was dependent on the starting nominal composition of gels. SEM and TEM micrographs of both heated gels below 1200°C showed the formation of small, discrete, prismatic, well‐shaped nanocrystals in a very ordered arrangement. The size of these nanocrystals was dependant on the nominal composition of gels and increased on rising the heating temperature of gel precursors. The microstructural features obtained from linewidth refinement results of X‐ray diffraction patterns also allowed to suggest the formation of prismatic a little elongated nanocrystals at temperatures below 1200°C. Microstrain values were small and only displayed a relatively significant value for mullites processed at 900°C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization of copper sulphate

Results of the studies of copper sulphate pentahydrate batch crystallization in the temperature range from 70 down to 30 °C are presented. In some of the experiments, free sulphuric acid has been added. The metastable zone width, nucleation and growth kinetic parameters, as well as the system kinetic constant were determined and compared with the data published in the literature.     

Crystal Growth in Gels at Elevated Pressures: The Upper Limit of Temperature for Metastable Formation of Aragonite

Both rapid precipitation and diffusion controlled gel growth were applied to crystallize calcium carbonate at temperatures in the range of 100 °C to 270 °C. The amount of aragonite was determined by means of X-ray diffraction data. The morphology of the aragonite crystals are described. Metastable formation of aragonite was observed only at temperatures below 270 °C.     

Diffusionsuntersuchungen am System Cu–Ti

As a result of diffusion studies in the binary system of Cu–Ti within a temperature range of 750°C and 850°C the intermetallic phases Ti₂Cu, TiCu, Ti₃Cu₄, Ti₂Cu₃ and Ti₂Cu₇ have been detected by means of electron microprobe analysis. Up to now according to the equilibrium diagram of Zwicker the intermetallic phase Ti₃Cu₄ was reported to be instable only. By the present investigation a time dependence of the width of the total diffusion zone as well as of each of the phases, d∼ √t, has been verified, i.e. volume diffusion is dominating. The chemical diffusion coefficients of all phases have been measured for 750°C, 800°C and 850°C. The partial diffusion coefficients for Cu and Ti in Ti₂Cu₇ for 850°C have also been determined. The chemical diffusion coefficients follow the Arrhenius law, the activation energy Q and the frequence constant D₀ could be calculated.     

Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

Crystallization of glaserite from aqueous solutions

Crystallization of glaserite (double salt of potassium and sodium sulphate) from aqueous solutions was studied by determining solubility, metastable zone width, growth and dissolution kinetics. Solubility measurements confirmed the presence of a large range of mother liquor compositions from which the precipitation of glaserite at about 73 – 76 wt% K₂SO₄ is obtained: inside this region the solubility lines resulted curvilinear. The metastability zone is quite large, ranging from 14 to 23 °C, and mainly increases by increasing the Na₂SO₄ content in the mother liquor. The growth rate of glaserite is second order, slightly size dependent and primarily controlled by diffusion as for K₂SO₄ but both growth and dissolution constants are quite lower than those for K₂SO₄.     

Thermal Analysis of LiGaO2 and NaGaO2

The high-temperature thermal properties of the ternary oxides LiGaO₂ and NaGaO₂ are studied by simultaneous differential thermal analysis and thermogravimetry between room temperature and about 1700 °C. For the melting temperature of LiGaO₂ a value of 1595 ± 10 °C is determined. NaGaO₂ undergoes a solid state phase transition at 1280 ± 10 °C and melts at 1395 ± 10 °C.     

Hopping conduction in Cr-doped amorphous gallium arsenide films

Amorphous Cr-doped GaAs has been prepared by rf sputtering and the dc conductivity measured at fields up to 10⁷ V/m and temperatures down to 80 K. The results are explained in terms of field-assisted hopping. Effects of isothermal annealing at temperatures up to 200°C are studied. The studies are repeated and the results compared with undoped a-GaAs. Room temperature resistivities of a-GaAs: Cr, a-GaAs and polycrystalline GaAs are also compared. The recrystallization temperature for a-GaAs films is determined to be between 320°C and 340°C.     

Investigation of InxGa1−xAs films grown from thin solution layer

In_xGa_1−xAs films with x⩽0.12 were grown from a thin solution layer between substrates. The calculation of final film thickness as a function of liquid phase composition, based on supposition of film deposition only on the substrate, is in a good agreement with experimental results. The dependences between compositions of liquid and solid phases at 800°, 750° and 700°C were determined. The morphology of the film surface was investigated as a function of liquid phase composition and (100), (111) A, (111) B substrate orientations. Dislocation density increases from 10⁴ cm⁻² to 10⁷ cm⁻² with change of x from 0 to 0.12.     

Neutron diffraction studies of the orthorhombic lattice distortions and magnetic properties of lanthanum manganite obtained under different technological regimes

Neutron diffraction studies have been carried out on two series of lanthanum manganite compositions obtained under different technological regimes. Based on the plot of angle dependence β° characterizing the degree of monoclinic distortions on “tolerance factor” t Mn³⁺ concentration is determined in the compositions obtained by us. With partial oxygen pressure increasing in annealing atmosphere the crystalline transition (orthorhomb-cube) temperature lowers from 583 ± 10°K to 508 ± 10°K. The correlation between magnetic properties of the compositions obtained and the degree of orthorhombic distortions γ′ = \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm b}\sqrt {{\rm 2/c}} $\end{document}\end{document} is established. Neutron diffraction studies of magnetic properties of LaMnO₃ compositions show that with decrease of orthorhombic distortion degree the decrease of antiferromagnetic component and lowering of the Néel point occur, while the ferromagnetic contribution increases and the Curie point rises.     

Growth and dielectric properties of Ta2O5 single crystal by the floating zone method

Large Ta₂O₅ single crystal with high‐dielectric permittivity was successfully grown by floating zone (FZ) method under air atmosphere. The grown crystal that has been obtained was typically about 8 mm in diameter and 90 mm in length. The crystal growth parameters were optimized. The crystal symmetry, characterized by means of X‐ray diffraction (XRD), was found to be tetragonal. The relative permittivity and loss tangent along growth and [001] direction were measured in the temperature range between ‐200 °C and 200 °C, which showed a strong dielectric anisotropy. At a frequency of 1 MHz and 20 °C, the dielectric permittivity along the growth direction and [001] direction are 81.17 and 25.04 respectively. The stabilization of high‐temperature phase can explain the dielectric enhancement.     

Solubility and metastable zone width of DL‐tartaric acid in aqueous solution

Solubility and metastable zone width (MSZW) of DL‐tartaric acid (DL‐TA) in aqueous solution have been determined. Solubility of DL‐TA was measured in the temperature range from 0 to 50 °C at atmospheric pressure by means of the conventional polythermal method using Turbidity Monitoring Technique, which was verified by a gravimetric method. The dissolution enthalpy and entropy of DL‐TA were then calculated from the solubility data using van't Hoff equation. Two approaches was used to estimate the nucleation kinetics from the measured metastable zone width data, the self‐consistent approach and the approach based on 3D nucleation. In addition, the metastable zone width slightly decreases with increasing agitation rate and was independent of working volume.     

Novel Cinnamate Esters—Synthesis and Mesomorphic Properties in Relation to Molecular Structure

A novel homologous series 4-[4′-n-alkoxy cinnamoyloxy] benzyl cinnamates consisting of 11 homologs was synthesized and studied with a view to understanding and establishing the relation between molecular structure and mesomorphic behavior of a substance. Mesomorphic behavior of the series commences from the third homolog and then continues until the last hexadecyloxy homolog. The first and second members of a series are non-mesomorphic. Nematogenic mesophase formation is observed from the propoxy homolog to the hexadecyloxy homolog, but the smectogenic mesophase formation is observed from the hexyloxy homolog to the tetradecyloxy homolog. All mesomorphic transitions are enantiotropic in nature. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type and that of the smectic mesophases are focal conic fan shaped of the smectic A or smectic C type. A phase diagram of the series shows a normal behavior of the transition curves with a minor abnormality of the last three homologs for the nematic-isotropic transition curve. The average thermal stability for smectic and nematic mesophases is 176.0°C and 219.3°C, respectively. Analytical and spectral data agree with the molecular structures of homologs. Mesomorphic phase length varies between 22°C and 76°C. Smectic phase length varies from 6°C to 33°C and nematic phase length varies from 22°C to 54°C. Thus, the novel present series is predominantly nematogenic and partly smectogenic. Mesomorphic properties of the novel series are compared with structurally similar other known homologous series.     

Synthesis by organic molten salt method and characterization of chalcopyrite AgInS2 nanorods

In this paper, chalcopyrite AgInS₂ nanorods were synthesized for the first time by a one‐step, ambient pressure, environment friendly organic molten salt (OMS) method at 200 °C. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The XRD results reveal that the as‐synthesized products at 120–160 °C under ambient pressure contain AgIn₅S₈ which will decrease with the increase of growth temperature. A sample containing only the chalcopyrite AgInS₂ phase is successfully obtained at 200 °C. Furthermore, the elemental compositions are found to become increasingly stoichiometric with increasing temperature. UV‐Vis and photoluminescence (PL) spectra are utilized to investigate the optical properties of AgInS₂ nanorods. By testing on UV‐Vis spectra, it is concluded that the limiting wavelength of the AgInS₂ nanorods is 661 nm and the band gap is 1.88 eV. A broad red emission band peak centered at about 1.874 eV (662 nm) is clearly observed at room temperature, and the intensity of the emission increases with excitation wavelength. In addition, the photoluminescence quantum yield (PLQY) of the nanocrystals at the excitation wavelength of 250 nm was determined to be 13.2%. A possible growth mechanism of AgInS₂ nanorods was discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization of amorphous germanium films

Kinetics of isothermal crystallization has been studied in the temperature range from 375 to 525°C. The kinetic curves are obtained and the rate of isothermal transformation of amorphous films into crystalline ones has been determined. Using experimentally determined kinetic curves the stability diagram of the amorphous films has been plotted in the temperature range from 400 to 525°C. The value of effective activation energy has been defined.     

Electronic states of SnO-ZnO–P2O5 glasses and photoluminescence properties

SnO–ZnO–P₂O₅ glasses with 30 and 40 mol% P₂O₅ were prepared by a melting process in an air atmosphere. The glass transition temperature, refractive index, and photoluminescence of the glasses were investigated. The electronic states of Sn(II) and Sn(IV) were determined by Mössbauer spectroscopy. The PO₄ units were investigated by Raman spectroscopy. The glass transition temperature was lower than 450 °C, and decreased as the Sn concentrations increased, so that the minimum was about 250 °C. The refractive index increased as the Sn concentration increased. The emission spectra of the glasses peaked at around 2.0–3.0 eV and depended on the glass compositions.