Photo-EPR of deformation-produced defects in GaAs

Deformation-produced paramagnetic centres have been studied in GaAs crystals uniaxially compressed at 400° using EPR technique (X band). The effect of light with photon energies from 0.4 to 1.5 eV on the populations of the centres has been investigated. On the basis of the experimental results a model is proposed which locates the energy levels of two of the centres at E_c − 1.05 eV and E_v + 0.75 eV.     

GeO2/SiO2-glasses from gels to increase the oxidation resistance of porous silicon containing ceramics

GeO₂/SiO₂-glasses with relatively high expansion coefficients (between 5 and 7.3 × 10^?6 °C^?1) were prepared by hydrolysis of metal alkoxides (tetramethoxy silane+tetraethoxy germane as well as mixtures of the silane with GeO₂-powder) with subsequent heat treatment up to 1000°C. It is shown that the preparation of thin films on glasses and ceramics is possible but also that this technique can be used for liquid infiltration of porous SiC- and Si₃N₄-materials to increase the oxidation behaviour in the temperature range 1000 to 1400°C. In the case of molybdenumdisilicide as a ceramic material it is shown that the catastrophic inner oxidation in the low temperature range (600 to 800°C) of porous specimens (tested with plasma-sprayed MoSi₂-layers on refractory metals) can be hindered if these materials are infiltrated and heat treated.     

Oxidation Kinetics of Amorphous,Polycrystalline, and Nanocrystalline Co33Zr67 Alloys

The oxidation kinetics of amorphous, polycrystalline, and nanocrystalline CoZr₂ alloys have been studied under isothermal conditions in air atmosphere with a microbalance. DSC and X-ray diffraction were used to study the influence of oxidation on crystallization. The three alloys under investigation which have the same composition but differ in the microstructure showed noticeable differences in their oxidation behaviour: the oxidation of the amorphous alloy is characterized by a very fast initial oxygen absorption step, completed in less than 1–2 min, followed by a slow oxide growth obeying a logarithmic time law at 400 °C. The polycrystalline thermodynamically stable tetragonal alloy absorbs small amounts of oxygen and obeys a parabolic rate law in the temperature range 500–800 °C. The activation energy of oxygen diffusion in tetragonal CoZr₂ was determined from the temperature dependence of the parabolic rate constant. The intermediate nano-crystalline phase with a local structure more closely packed and ordered than that of the precursor amorphous phase, shows an oxidation behaviour similar to that of the tetragonal phase, but with a different time exponent.     

Silica and silica-titania glasses prepared by the sol-gel process

Clear monolithic samples of silica and silica-titania glasses were prepared by the sol-gel process from alkoxides as starting materials. The effects of the composition of the initial alcoholic solutions on the gelation of the silica materials and the effects of using different titanium compounds on the formation of silica-titania gels and glasses were investigated. DTA and TGA revealed losses of water and organic volatiles during heat treatment of the gels at lower temperatures (up to 400°C) and the glass transformation and crystallization behaviour at higher temperatures (up to 1500°C). The effects of using atmospheres with varying oxygen contents on the DTA peaks caused by oxidation reactions were also studied.Structural changes occurring during heat treatment were monitored by infra-red spectroscopy which indicated that the water contents of the glasses after heat treatment to 900°C were about 1000 ppm. Transmission electron microscopy of ion beam thinned foils of a 80 SiO₂20TiO₂ composition showed a microstructure of extremely fine pores for heat treatments up to 1000°C. However, after extended heat treatments above 950–1000°C, the porosity appeared to decrease and a high concentration of fine crystallites of anatase (approximately 100 Å in diameter) embedded in a silica-rich glass matrix were obtained.     

Irradiation plus annealing-induced 1.20 eV emission band in p-type GaAs

The effect of 2.2 MeV electron irradiation and subsequent annealings on the luminescence of tellurium-compensated p-type GaAs crystals is studied and analyzed. A comparatively strong emission band peaked at hv_m near 1.20 eV (induced by radiative electron recombination in Te_AsV_Ga pairs) appears in irradiated annealed (at T ≧ 250 °C) compensated p-type GaAs. The data obtained testify about the following: a) electron irradiation of GaAs creates stable (at T ≦ 200 °C) defects not only in the arsenic sublattice of GaAs, but in its gallium sublattice too; b) radiation plus annealing (r.p.a.)-induced Te_AsV_Ga pairs are characterized by a rather high (compared to that for grown-in Te_As V_Ga pairs) probability of electron radiative transitions in them. The electrical characterization of the r.p. a.-induced 1.20 eV radiative centres (Te_AsV_Ga pairs) is given.     

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.     

Synthesis and crystal structure of anhydrous copper suberate: [Cu<Subscript>2</Subscript>(OOC-(CH<Subscript>2</Subscript>)<Subscript>6</Subscript> -COO)<Subscript>2</Subscript>]

The title compound [Cu₂(OOC-(CH₂)₆-COO)₂] was synthesized and its crystal structure has been determined by single crystal X-ray diffraction methods. The complex crystallizes in the triclinic space group P-1 with a=5.1077(5) ?, b=8.362(2) ?, c=11.378(2) ?, α=93.773(6)°, β=97.587(9)°, γ=90.493(’9)° and D _cal=1.629 mg/m³ for Z=1.  The structure is polymeric and consists of discrete anhydrous centrosymmetric binuclear units [Cu₂(OOC-(CH₂)₆-COO)₂]. The two copper(II) centres bridged by the suberate groups in a syn-syn conformation, are in pentacoordinated distorted square-pyramidal coordination environment, with an intramolecular Cu–Cu distance of 2.5793(10) ?. Each binuclear unit, related to the next through μ_oxo bridges with a Cuμ_oxo–Cuμ_oxo separation of 3.2326(10)?, defines infinite chains of one-edge sharing CuO₅ square pyramid.     

XPS and AFM studies of surface chemistry and morphology of In2O3 ultrathin films deposited by rheotaxial growth and vacuum oxidation after air exposure

In this paper we present the results of an XPS study of the surface chemistry of In₂O₃ (IO) ultrathin deposited by rheotaxial growth and vacuum oxidation after their air exposure and subsequent UHV annealing. For the freshly deposited RGVO IO ultrathin films the relative [O]/[In] concentration is equal to 1.17 ±0.05. After exposure to air it increases to 1.25 ±0.05. However, a strong C contamination appears, confirmed by the relative [C]/[In] concentration that is equal to 1.07 ±0.05. After subsequent UHV annealing at 200 °C the relative [O]/[In] concentration remains and only slightly increases to 1.30 ±0.05 after subsequent UHV annealing at 350 °C. After this process the relative [C]/[In] concentration decreases to 0.43 ±0.05. The additional AFM experiments showed that the RGVO IO ultrathin films exhibit the granular‐type surface morphology, with an average grain height and width in the range of 2–5 nm, and 10–50 nm, respectively, and an average roughness of 1.1 nm.     

Dehydration of Selenite Cleavage Faces

Initial dehydration of selenite cleavage surfaces start at temperature 86 °C. The dislocations are revealed by distilled water. Dislocations are mobile at dehydration temperature. Impurity centres may act as obstacles for dislocation movement. Dehydration figures are in probability formed at impurity centres.     

Oxidation of solid aluminium silicon alloys in air

The kinetics of oxidation of solid Al–Si alloys in air was studied at temperature interval between 490 °C and 532 °C. The initial time intervals in which oxidation is governed by a parabolic quadratic low were found form the weight time-gain curves. At temperature higher than 512 °C the exponent of the parabolic ratio differs from 2. It was shown the influence of the alloying element Si on the oxidation and the effective activation energies of oxidation of Al Si alloys were calculated. They are considerably higher than the effective activation energies for pure aluminium.     

Surface Chemical Bonding States and Ferroelectricity of Pb(Zr0.52Ti0.48)O3 Thin Films

The surface chemical bonding states and the ferroelectric properties of sol‐gel deposited lead zirconate titanate [Pb(Zr_0.52Ti_0.48)O₃, PZT] thin films coated on (111)Pt/Ti/SiO₂/Si substrates were investigated. X‐ray photoelectron spectroscopy (XPS) was used to determine the oxidation state of the surface and the chemical composition as a function of depth in ferroelectric PZT thin layers. Values for the dielectric constant and dissipation factor at 1 kHz for the 300 nm‐thick film were 1214 and 0.014 for the film annealed at 520 °C, and 881 and 0.015 for a film annealed at 670 °C. Measured values for the remanent polarization (P_r) and coercive field (E_c), from polarization‐electric field (P‐E) hysteresis loops biased at 10 V at a frequency of 100 Hz, were 16.7, 14.4 μC/cm² and 60, 41.7 kV/cm for 520 °C and 670 °C. The leakage current density (J) was 72 and 96 nA/cm² at an applied field of 100 kV/cm. It was found that the bonding states of lead and oxygen in the surface regions could be correlated with the ferroelectric properties of the integrated thin layers.     

Synthesis,structural and thermal studies of tetrathioureacopper(I) chloride crystals

Tetrathioureacopper(I) chloride, hereafter abbreviated as TCC, was synthesised and single crystals were obtained from saturated aqueous solution by slow evaporation (solution growth) method at room temperature. The crystals obtained are bright, colourless and transparent having well defined external faces. The grown crystals were characterized through elemental analysis, single crystal X‐ray diffraction study, thermal analysis, electron spin resonance spectroscopy and Fourier Transform infrared spectroscopy. The elemental analysis confirms the stoichiometry of the compound. The single crystal diffraction studies indicate that TCC crystallises in the tetragonal lattice and the unit cell parameters are a = b = 13.4082 Å, c = 13.8074 Å, V = 2482.29 ų, α = β = γ = 90°. Space group and the number of molecules per unit cell (Z) are found to be P4₁2₁2 and 8 respectively. The TG curve of the sample shows a prolonged decomposition from 210 to 628.3 °C, from which the decomposition pattern has been formulated. The endothermic peaks in the DTA curve indicate melting and decomposition of the compound at 165.2 and 633.8 °C respectively. An exothermic peak in high temperature DSC indicates a phase transition in the compound at 274.8 °C. Thermal anomalies observed in the low temperature DSC at –163.3, –152.0, –141.5, –108.3, 1.0 and 12.1 °C in the heating run and –157.1 and –153.9 °C in the cooling run reveal first order phase transitions in the crystal. The peaks observed at –146.2 °C in both the heating and cooling runs suggest occurrence of a second order phase transition in this compound. The IR spectroscopic data were used to assign the characteristic vibrational frequencies of various groups present in the compound. The ESR study confirms that the copper is in the +1 oxidation state in the complex. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Elektrisch aktive Zentren in thermisch behandelten p-Si-Substraten und Si-Homoepitaxieschichten

P-Si-substrates with and without deposited homoepitaxial Si layers were annealed in vacuum at 800 < T < 1000°C and then quenched down to room temperature. The energy levels, the spatial distribution and the annealing behaviour of the electric centres, formed by this procedure, were investigated. The investigated centres are interpreted as complexes of point defects containing vacancies.     

Morphology and crystal structure of A1-doped TiO2 nanoparticles synthesized by vapor phase oxidation of titanium tetrachloride

Al-doped titanium dioxide nanoparticles with precisely controlled characteristics were synthesized in an aerosol reactor between 900 °C and 1500 °C by vapor-phase oxidation of titanium tetrachloride. The effect of process variables (reactor temperature, initial TiCl₄ concentration, residence time and feeding temperature of oxygen) on particle morphology and phase characteristics was investigated using TEM, XRD, EDS, ICP and XPS, etc. The average particle size increased with decreasing oxygen feeding temperature and increasing reaction temperature, residence time and TiCl₄ concentration. The presence of aluminum during gas phase reaction increased the rate of phase transformation from anatase to rutile and altered the particle morphology from polyhedral to irregular crystals. TiO₂ and Al₂O₃ co-precipitated during particle formation which lead to the aluminum solid solution in titania. α-Al₂O₃ and Al₂TiO₅ were observed at AlCl₃/TiCl₄ ratios higher than 1.1 and reactor temperatures in excess of 1400 °C. The rutile content, which increased with increasing Al/Ti ratio and residence time, was at a maximum at about 1200 °C and decreased at both lower and higher reactor temperatures.     

Investigation of the formation of nanowires from silicon whiskers

Nanowires have been prepared by the high-temperature oxidation of Si whiskers. The dependences of the nanowire formation on the oxidation parameters have been investigated. The oxidation rate is shown to depend on the whisker diameter. Oxidation in dry oxygen at temperatures no higher than 950°C results in self-stopping; i.e., the nanowire diameter is stabilized. Stabilization is not observed at oxidation temperatures above 950°C or at oxidation in wet oxygen. Oxidation at higher temperatures made it possible to obtain nanowires ≤5 nm in diameter in relatively thick (up to 200 nm in diameter) whiskers.     

Crystal structure of a new mineral lahnsteinite Zn4(SO4)(OH)6 · 3H2O

A sample of a new mineral from the oxidation zone of the hydrothermal Pb, Zn, Ag Friedrichssegen deposit (Rhineland-Palatinate, Germany) was studied by single-crystal X-ray diffraction. The parameters of the triclinic (pseudoorthorhombic) unit cell are found to be a = 8.312(1) ?, b = 14.545(1) ?, c = 18.504(2) ?, ?? = 89.71 (1)°, ?? = 90.05(1)°, ?? = 90.13(1)°, and V = 2237.3(3) ?³. The structure is solved by direct methods into the sp. gr. P1 and refined to an R factor of 10.7% using 3788 reflections with |F| > 3??(F) in the isotropic-anisotropic approximation. The crystallochemical formula of lahnsteinite (Z = 8) is [(Zn_2.6Fe_0.3Cu_0.1)^VI(OH)₃][Zn^IV(OH)₃(H₂O)][SO₄] · 2H₂O, where the compositions of the layer composed of Zn octahedra and isolated Zn and S tetrahedra are given in square brackets. The mineral under study is chemically and structurally similar to namuwite and is a natural analog of synthetic zinc hydroxide sulfate trihydrate.     

Effect of Neutron Irradiation and Annealing on the Intensity of the Copper Related 1.01 eV Emission Band in n-type GaAs

Effect of neutron irradiation (E = 2 MeV, ϕ ≤ 10¹⁵ n/cm²) and subsequent annealing (T ≤ 700 °C, t = 30 min) on the intensity of the copper-related peaked at hvm =1.01 eV emission band in n-type GaAs (n₀ = 2 × 10¹⁸ cm⁻³) is studied. A strong irradiation-induced increase of the above emission intensity was observed testifying about the irradiation-stimulated growth in the concentration of copper-related 1.01 eV radiative centres (Cu_GaV_As pairs). A model is presented to explain this effect.     

Dark-degradation of reactive brilliant blue X-BR in aqueous solution using α-Fe2O3

The precursor of Fe₂O₃ was prepared by chemical precipitation method with sodium hydroxide (NaOH) and iron chloride hexahydrate (FeCl₃ · 6H₂O). The samples annealed at different temperature were characterized by means of X-ray diffraction (XRD) and infrared absorption spectroscopy (IR). Degradation of reactive dye in aqueous solution was used to evaluate the catalytic performance of the Fe₂O₃. The experiments of degradation had been done in dark place. The experimental results indicated that the catalytic property of the precursor of α-Fe₂O₃ was excellent. The mechanism of the degradation of reactive dye in aqueous solution was investigated by comparing the data in the absence and presence of oxygen. The precursor of α-Fe₂O₃ annealed at 300 °C was the best. The degradation rate of reactive brilliant blue X-BR could exceed 95% in 8 min at 25 °C when the concentration of the precursor of α-Fe₂O₃ was 0.1 g/L. The mechanism of the catalytic oxidation reaction was investigated by comparing the X-BR catalytic oxidation data in the absence and presence of oxygen.     

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.     

On the influence of the pre-ageing temperature on the reversion behaviour of an Al-4.5 at.% Zn-2.0 at.% Mg alloy

The reversion behaviour of an Al-Zn(4.5)-Mg(2.0) alloy was investigated by SAXS in dependence of the pre-ageing temperature, T_pre, (ranging between 60°C and 100°C) and the reversion temperature, T_rev, (120°C till 200°C) starting with precipitates having a radius of (1.2 ± 0.1) nm and (1.5 ± 0.1) nm, respectively. During the reversion treatment applied up to T_rev = 160°C three stages could be distinctly distinguished, namely the dissolution of unstable zones, the growth of the stable zones on the expense of the dissolved one, and at last the coarsening of the precipitates by the OSTWALD-ripening process, where the structure changes become independent of the pre-history (start radius). The change of T_pre from 60°C to 100°C does not influence the structure changes going on at T_rev, that means between 60°C and 100°C the same type of G.P. zones grows.