Zur Struktur mechanisch erzeugter amorpher Quarzschichten

By mechanical treatment of quartz in a vibration mill an amorphization is reached connected with the loss of distance order. The near order of quartz in the range r < 0.65 nm is kept on to 50%. There is a difference in the structure between the near order of mechanical treated quartz and of amorphous silica. The formation of oxygen defect electron centres — they are only formed in presence of O₂ — is discussed in connection with the special structure of quartz, the order of which is mechanically destroyed.     

EPR of Mo5+ in amorphous MoO3 thin films

Mo⁵⁺ centres in amorphous thin films of MoO₃ have been studied as a function of optical density using EPR techniques. Measurements were made for over an order of magnitude change in optical density and down to helium temperatures. A direct correlation between the optical density and the number of Mo⁵⁺ centres is found for low optical densities but further colouration does not continue to increase the number of Mo⁵⁺ centres. A model is proposed to explain the results.     

The application of small polaron theory to transition metal oxide glasses

A review of the experimental data on a range of transition metal oxide glasses shows a strong relationship between activation energy and site spacing for glasses based on V₂O₅. The behaviour confirms that transport in amorphous V₂O₅ and vanadate glasses is by adiabatic hopping with an appreciable polaron bandwidth, while non-adiabatic hopping between isolated centres is apropriate for glasses based on most other transition metal oxides. The behaviour of vanadium is linked to the retention of a well-defined VO^5? structural unit in the glass network and to optimisation of the packing of this unit within the glass.     

Investigation of the interaction of quartz with H2 under supply of mechanical energy (I)

The gas molecules absorbed during mechanical activation of quartz in a H₂ atmosphere are situated as well as the formed E′ centres in the amorphous ranges. The concentrations of the E′ centres are about one order of magnitude less after activation in H₂ than after an activation in Ar. Hence and because of the existence of a proton doublet (splitting of about 73 G) in the EPR spectra a chemical interaction of a part of the absorbed H₂ molecules with quartz is concluded.     

Electrical conduction in amorphous WO3 films

Measurements are reported on the dc and ac conductivity and dielectric constant for vacuum-deposited films of tungsten trioxide. The behaviour is consistent with that expected for hopping conduction in an amorphous solid containing long-range potential centres associated with oxygen vacancies. A new method of calculating the density of states gives N(E) ～ 5.0 × 10²⁰ eV^?1 cm^?3.     

Oxidation kinetics of BaF2 single crystals

Oxidation of BaF₂ single-crystals as a result of annealing in air in the temperature range 600° to 900 °C leads to mechanical strengthening. The process kinetics was investigated. It is shown that oxidation takes place in the bulk of crystals owing to oxygen diffusion by formation and growth of oxidation centres. These centres are precipitates of a BaO phase which were revealed by chemical etching.     

Optical properties and X-ray diffraction of amorphous systems

By appealing to the persistence of short range order in amorphous materials a simple estimate of the form of the static structure factor can be obtained. It is also possible to derive an expression for the optical absorption of amorphous materials involving a convolution of the corresponding crystalline expression and a simple function.     

Colour centre studies in MoO3 films

The growth of defects in vacuum-evaporated MoO₃ films has been studied by optical absorption in the present work. The as-grown films were subjected to different kinds of treatment such as thermal annealing, UV and X-ray irradiation, and their optical absorption spectra were recorded between 300 and 1500 nm at room temperature as well as at liquid N₂ temperature The films were found to be amorphous from X-ray and electron-microscopic studies. From these studies it is observed that in addition to point defects, defect aggregates similar to the colloidal centres in silver and alkali halides, are present in these films.     

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.     

Raman scattering and structural order in liquid GeSe2

High temperature polarized and depolarized Raman spectra have been obtained in liquid GeSe₂ near the melting point of 770 C. The spectra indicate a number of similarities to amorphous GeSe₂, including the presence of the “companion” line which has been associated with intermediate range order in the amorphous state. The Raman spectra, as well as previous low angle diffraction studies, indicate that intermediate range structural order is also present in liquid GeSe₂.     

Preparation of amorphous carbon tetrachloride and characterization by neutron diffraction

A 5 g sample of carbon tetrachloride (CCl₄) deposited from the vapour on to a cadmium substrate at 5 K is shown to be amorphous by the absence of Bragg peaks in its neutron diffraction spectrum. Simple analysis shows that the range of correlations in amorphous CCl₄ extends only to next-nearest neighbours.     

Complex permeability and electromagnetic wave absorption properties of amorphous alloy-epoxy composites

Electromagnetic wave absorption properties of amorphous alloy-epoxy composites have been investigated with various amorphous alloy particle sizes and fractions in the 45 MHz to 10 GHz frequency range. The fraction of amorphous alloy in amorphous alloy-epoxy composites varied from 30 to 60 vol.% at a fixed amorphous alloy particle size and the size of amorphous alloy particles was varied from several μm to 125 μm at a fixed amorphous alloy particle fraction. Complex permeability (μ), permittivity (ε) of composites and reflection loss were measured by the reflection/transmission technique. The composites with small sized amorphous alloy particles (<26 μm) and a small amount of particles (<50%) had good reflection loss values less than −20 dB with thin thickness (<1 cm). The decreasing amorphous alloy particle size and fraction in amorphous alloy-epoxy composites resulted in a high minimum reflection loss frequency and small minimum reflection loss thickness. Variations of minimum reflection loss frequency and thickness of composites resulted from variations of materials constants such complex permeability, permittivity and resonance frequency. Changes in materials variables were due to the demagnetization effect and the eddy current effect, which operate differently in composites according to amorphous alloy particle size and fraction.     

The influence of water steam on the direct phase transformation of zeolite NaA to nepheline by thermal treatment

Heating of zeolite NaA leads to the consecutive formation of amorphous phase, carnegieite and nepheline in the high temperature region. These phase transformations are influenced not only by temperature but also by the presence of water (inside of the crystals). Water causes destruction of zeolite framework by hydrolytic splitting of Si-O-Al-bonds in addition to thermal effects. Thus, the amorphous phase as the first product of phase transformation occurs in the presence of water steam at a lower temperature. The formation of the crystalline phases carnegieite and nepheline was observed in a temperature range, 300 K lower than usually, and without occurrence of the amorphous phase.     

A new vanadium pentoxide amorphous phase

Hydration of amorphous V₂O₅ under controlled water vapor pressure yields a new amorphous phase that appears to be composed of entangled polymeric chains. Thermal analysis shows that upon heating crystallization is followed by the breaking of these chains and leads to a non-stoichiometric oxide. ESR experiments indicate that short range order around vanadium ions is modified by adsorption of water. The V⁴⁺ sites appear to be solvated by strongly bonded water molecules. Hydration leads to a swelling of the polymeric amorphous oxide. It can even produce a gel if enough water is added.     

Acoustic study of low-energy activation processes in amorphous Se,Ge and SeGe compounds

The study of the elastic properties of amorphous selenium over a wide range of frequency (30 kHz−100 MHz) reveal a single internal friction peak below the glass transition temperature.This one involves low activation energies ranging down to very small values and is reduced when the chains are crosslinked. In amorphous germanium, there is no evidence for the existence of activation processes exhibiting very small values. We suggest that the underlying microscopic mechanisms responsible for the peak in a-Se are closely related to the polymeric structure and so should exist in any amorphous polymers. We also suggest that the numerous other peaks existing in most polymers are not intrinsically related to structure in chains.     

Extended and localized electronic states for the undulatorily-graded band-gap model of amorphous semiconductors

A novel approach is taken regarding the theoretical basis for the electronic structure of amorphous semiconductors. For these materials we propose a model displaying intermediate range order with undulations in composition or structure on a scale within the range of validity of the graded band-gap concept. Electronic states well within the allowed bands are determined by a perturbative analysis starting with Bloch states; states near the band edges, by a modification of an effective mass analysis previously developed for monotonically graded mixed crystals. Modulated extended states are deduced from the first analysis; localized states, by the second. We identify the band edges as the loci of the classical turning points of the effective mass functions for the localized states. Thermal activation from the band edge troughs and tunneling between troughs are considered. From the model and analysis it appears possible to explain field-dependent conductivity, to account for the persistence of the optical band-gap on conversion from crystalline to amorphous semiconductors, and to predict sensitivity of electronic properties to doping in some amorphous semiconductors.     

X-ray scattering studies of polymeric zirconium species in aqueous xerogels

X-ray scattering in the small angle range was used to investigate the structure and size of the polymeric species present in zirconium acetate (ZrAc)-based gel powders (GP) and thin films (TF). The formation of amorphous zirconia aggregates/linear polymers was studied by measuring their gyration radii and their correlation and hydrodynamic lengths via Guinier, `longrods', Zimm and Porod plots. Scattering data obtained in this investigation suggest that both amorphous ZrAc-based GP and TF contain `tetrameric' units, arranged in a cylindrical fashion, that polymerize upon thermal treatment. Housdorff fractal dimensionality, D, of amorphous zirconia GP (D∼2.25) and TF (D∼2.5) indicates a diffusion-limited aggregation mechanism. Tricyclic cluster aggregates (Porod D∼1.85) with a M_w∼12 000 were found within amorphous ZrAc-based GP. This structure is preserved over the entire curing range. In contrast, a linear polymeric chain (Porod D∼1.52), with a M_w∼20 000 below 165 °C and ∼13 000 at higher temperatures, was suggested for amorphous ZrAc-based TF.     

Raman study of gallium arsenide thin films

The influence of deposition parameters on the structure of the gallium arsenide thin films was investigated by Raman scattering. The study was based on the analysis of the first-order Raman spectra which allows for a differentiation between the amorphous component and crystallites of various sizes. The amorphous and crystalline volume fractions were calculated from the integrated intensities of the deconvoluted peaks. It was demonstrated that a transition occurs from μ-GaAs to a-GaAs for particular plasma conditions and substrate temperature. As a function of the deposition parameters the entire range from mostly microcrystalline to completely amorphous films can be obtained. These properties were consistent with the results obtained on the same samples by transmission high-energy electron diffraction and conventional transmission electron microscopy.     

Crystallization kinetics and phase transformations of zinc- and cadmium diphosphide films

The kinetics of isothermal crystallization of amorphous ZnP₂ films in the temperature range from 730 to 810 K and amorphous CdP₂ films between 630–770 K has been investigated. The kinetic curves of isothermal transformation of amorphous films into crystalline ones have been obtained. For ZnP₂ the transformation occurs as follows: amorphous films → α-ZnP₂ (tetragonal modification) → β-ZnP₂ (monoclinic modification). For CdP₂ – amorphous films → γ-CdP₂ (cubic modification) → β-CdP₂ (tetragonal modification). From the experimentally determined kinetic curves the stability diagrams for ZnP₂ and CdP₂ films have been plotted. The most stable modification in the investigated temperature range are β-ZnP₂ and β-CdP₂ other ones are metastable. The values of effective energy of activation of transformation have been determined.