Atomic critical potentials and structure of non-single-well potentials in glasses

The microscopic nature and structure of atomic local non-single-well potentials in amorphous materials are considered. It is shown that the nature of such potentials is related to the local potential instability in an appropriate critical mode of the atomic motion. The occurrence of critical potential with unusually small spring constants is predicted. Some simple microscopic models are discussed in detail. It is shown that the non-single-well potentials practically reduce to critical double-well potentials in real amorphous materials. Two different types of the double-well potentials in glasses are revealed. Some predictions are given for the distribution densities of the parameters describing tunneling states in the critical double-well potentials and for some effects in amorphous materials.     

Seebeck coefficient in amorphous chalcogenide films

The Seebeck coefficient determined for a number of amorphous chalcogenide films is linear in 1/T. Slopes were typically 0.1 to 0.2 eV less than the conductivity activation energies. A parameter A in the expression for thermopower appears to be a measure of the disorder, being large for highly disordered materials and small for annealed stoichiometric materials.     

Determination of the density of states of semiconductors from steady-state photoconductivity measurements

We discuss a method to obtain the density of states of photoconductive semiconductors from the light-intensity-dependence of the steady-state photoconductivity. Considering a material having different species of gap states – i.e., with different capture coefficients – we deduce a simple expression relating the defect density to measurable quantities. We show that the relevant capture coefficient appearing into the formula is that of the states that control the recombination. We check the validity of the approximations and the applicability of the final expression from numerical calculations. We demonstrate the usefulness of the method by performing measurements on a standard hydrogenated amorphous silicon sample.     

Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5 μm grown on silicon can be taken before and after etching a thin surface layer of 20–30 nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2 μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.     

Role of the silica source on the geopolymerization rate

The synthesis of geopolymer-silica composites was achieved at room temperature to determine the role of the silica source (quartz or amorphous silica) on the polycondensation rate and the mechanical properties of synthesized materials. Then, samples with a composition range from 100% quartz to 100% amorphous silica were formed, compared and characterized by XRD, infrared spectroscopy, thermal analysis, SEM, and compression tests. The results give evidence that the increase of amorphous silica in the mixture favors the polycondensation reaction (i.e., “geopolymerization”) to form consolidated materials whereas quartz led to heterogeneous materials without cohesion. These facts are explained by the modification of the Si/Al ratio in the geopolymer matrix due to the increase of quartz in the mixture.     

Kohlrausch and Arrhenius analysis for some solid and liquid materials

Time- and temperature-dependent kinetics are analyzed for four materials: a single crystal, two liquids and an amorphous structure. While all these materials have fractal time dependence, fractal temperature dependence is found in some liquids and amorphous materials. In the single crystal case, the temperature dependency follows the regular Arrhenius equation. Some new results are found on the time and temperature dependence from the analysis and will be discussed.     

Crystallization of amorphous selenium and As0.005Se0.995

The effect of temperature on the crystallization kinetics of bulk amorphous selenium and the alloy As_0.005Se_0.995 has been studied using differential scanning calorimetry. A time-temperature-transformation (TTT) curve has been plotted from isothermal results over the temperature range 320 to 490 K, and shows the presence of two minima for both materials. Using the empirical Avrami expression for solid-state transformations, a number of kinetic parameters has been determined.     

Trap-limited diffusion of hydrogen in precursor derived amorphous Si–B–C–N-ceramics

The tracer diffusion of hydrogen is studied in precursor derived amorphous Si–C–N and Si–B–C–N ceramics using deuterium as a tracer and secondary ion mass spectrometry (SIMS). Since the amorphous ceramics are separated in carbon rich phases (amorphous carbon and amorphous C(BN)_x, respectively) and silicon rich phases (amorphous Si₃N₄ and amorphous Si_3+(1/4)xC_xN_4−x, respectively) we additionally measured the diffusivities of hydrogen in amorphous carbon, in amorphous SiC and in amorphous C–B–N films. The silicon rich phases are identified as diffusion paths for hydrogen in the precursor derived ceramics. Diffusion of hydrogen in these materials is explained with a trap limited diffusion mechanism with a single trap level. We found activation enthalpies of about 2 eV for the precursor derived ceramics, where the activation enthalpy is the sum of a migration enthalpy and a binding enthalpy. The low values for the pre-exponential factors of less than 10⁻⁷ m²/s can be explained with an appropriate expression for the entropy factor.     

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.     

Crystallization of the amorphous phase and martensitic transformations in multicomponent (Ti,Hf,Zr)(Ni,Cu)-based alloys

Shape memory materials with specific microstructure can be obtained from amorphous precursors. Rapidly solidified amorphous and crystalline–amorphous ribbons have been produced by planar flow casting for a number of multicomponent TiNi-based alloys of pseudobinary 50:50 and 55:45 compositions with substitutions of Zr, Hf, Nb for Ti and Cu, Co, Pd, Ag, Al for Ni. The glass transition and crystallization behavior of the amorphous phase as well as the martensitic transformations in the crystallized materials have been studied by differential scanning calorimetry. To control the crystal structure at different stages of crystallization, X-ray diffractometry and transmission electron microscopy were used. The effects of crystallization conditions and the resulting microstructural state on the martensitic transformation characteristics have been investigated. The transformation temperature intervals are shown to depend on the mean size of the parent phase crystals.     

Threefold coordinated model structure of amorphous GeS,GeSe and GeTe

The black P structure is presented as a model for the structures of amorphous GeS, GeSe and GeTe. It is shown that the short interatomic distances, low near neighbor coordinations and high covalencies of the amorphous materials, relative to the crystalline, can be rationalized with the model. When scaled to the near neighbor interatomic distances in the amorphous materials, the model yields satisfactory agreement with the observed position and area of the second neighbor X-ray radial distribution function peaks. The model predicts: (a) A first neighbor peak area for GeS which is significantly different from that predicted by the random covalent model and (b) phase separation in certain composition regions which, for the Ge-S system, should be observable by means of transmission electron microscopy. An explanation of why phase separation is not likely to be observable through transmission electron microscopy studies of amorphous GeTe and GeSe is also presented.     

Trap-controlled transient current injection in amorphous materials

A theory for trap-controlled transient current injection is presented. The basic equations governing a carrier transport in amorphous materials are solved analytically for an arbitrary energy distribution of traps in the gap. Two special models, opposite with respect to each other, are treated: (1) the model of monoenergetic trapping level; and (2) the model of trapping levels distributed uniformly over a wide energy range. The theoretical results obtained are in agreement with existing transient current injection data on some amorphous materials.     

Effects of Ge addition on the optical and electrical properties of eutectic Sb70Te30 films

The aim of this work is to study the influence of Ge addition on the optical and electrical properties in eutectic SbTe thin films (with the compositions: Sb₇₀Te₃₀, Ge₂Sb₇₀Te₂₈, Ge₅Sb₇₀Te₂₅ and Ge₁₀Sb₆₅Te₂₅) using visible optical reflectance, ellipsometry measurements, near infrared transmittance spectra, and four probe electrical resistivity measurements. From near infrared transmittance measurements the optical band gap was determined using Tauc’s expression for amorphous materials, a value of about 0.47 eV was obtained without any clear dependence on the Ge content. All amorphous films have approximately the same reflectance value, however the contrast ratio between the crystalline and amorphous phases decrease with increase of Ge. Using in situ four probe measurements versus temperature the dependence of the activation energy of conductance and the onset of the crystallization temperature have been determined for different materials. Four probe measurements have shown that the resistivity of amorphous films increases with increase of Ge. The results obtained have shown that optical and electrical properties of SbTe films with near eutectic composition change with the Ge content and depending on the application, in either optical or electrical memory devices, the most suitable composition needs to be determined.     

An investigation of the atomic bonding present in amorphous chalcogenide materials by electron spectroscopy

The atomic bonding in amorphous switching materials based on tellurium and germanium has been shown to be largely dependent on the germanium atoms. The results of examination by electron spectroscopy show that germanium atoms can possess five different forms of chemical bonding. These are elemental, oxidised, ‘glassy’, and two states associated with the interatomic compound GeTe. The ‘glassy’ and compound forms of germanium have been found in the amorphous materials. Tellurium, however, shows no significant different in bonding from its elemental state in these materials.     

Observation of band structure on amorphous silicon surface and performance of solar cells with controlled band offsets on hetero-junctions

Control of band offsets on hetero-junctions is important to achieve a higher efficiency amorphous silicon-based solar cell. It is expected to improve cell's properties of Voc and FF by suppressing band offsets on the junction. The configuration of conduction band level (Ec), valence band level (Ev) and Fermi level (Ef) based on vacuum level governs the formation of band offset on the junction. We show that the configuration of the band structure on the surface of amorphous Si thin film is deposited by plasma-CVD equipment with a VHF power supply. Ec and Ev were obtained from ionization energy and energy band gap observed by ultraviolet photoemission spectroscopy and optical measurement, respectively. Ef was measured as work function by means of Kelvin probe method. As a result, it is found that shifts of energy levels originate from change of bonding characteristics in the amorphous Si network. This phenomenon is peculiar to amorphous Si-based materials. The results indicate that the upper shift of Ec and Ev are effective in suppressing band offsets on the hetero-junction. We show the performance of solar cells with controlled band offsets on hetero-junctions and discuss the possibility of higher Voc and FF by employing amorphous Si-based materials.     

The structure of nanovolumes of amorphous materials

In this paper we demonstrate reduced density function (G(r)) analysis from amorphous volumes as small as 2 nm in width. Obtaining the G(r) from a diffraction pattern is a common starting point in the characterisation of amorphous materials. The use of electrons to form the diffraction pattern allows very small volumes of amorphous material to be investigated.     

Thermoelectric power of amorphous compound semiconductors

Thermoelectric power measurements as a function of temperature have been made on four V–VI amorphous chalcogenides, ten IV–VI amorphous chalcogenides of the Ge_xTe_1?x system, and five amorphous IV–V materials. In the IV–V and the intermediate composition IV–VI materials, the charge transport cannot be described on the basis of conduction at only one energy level. The former exhibit characteristics of conduction both in extended states and in localized states at the Fermi level. Transport in the V–VI materials can be formally described in terms of conduction at one level in either the chaotic band or the small polaron models, but the use of a two-channel model with transport simultaneously in both p-type extended and localized states seems the most promising.     

Structural studies of amorphous solids by small-angle neutron scattering

Sophisticated new methods of preparation developed over the last decade have considerably extended the range and nature of amorphous materials, many of which have a microstructure on a scale of 10–1000 Å. Whilst in some cases this is due to the composition, in others it is a direct result of the preparation technique. This paper reviews the present state of small-angle scattering as a tool to study the microstructure in the four main classes of amorphous materials: bulk glass, metal ribbons, thin films and inorganic gels. The techniques specific to neutron scattering, which have been developed to separate the various components of the scattering curves, are reviewed.     

锂离子电池正极材料FePO3

通过控制溶液的pH值用FeCl3和Na3PO4溶液共沉淀法制备出了无定型FePO4·1.3H2O,通过成分析,热分析,X射线衍射和扫描电镜分析对材料进行了表征.该材料在0.2 mA·cm-2的电流密度下起始容量达到130mA·h·g-1同时具有良好的循环性能,表现出一个良好的锂离子电池候选材料.球磨可以提高材料的电化学性质,可能是因为其中活性成份含量提高的原因.当其加热到700℃成为晶态的FePO4时则容量变低,这种高温下容量的损失的机理可能是与高温下形成非活性相有关.     

Study of characteristics of plastic deformation in amorphous Fe-Cr-Mo-V-B-Si alloy by the method of local deformation

Characteristics of plastic deformation in the amorphous Fe-Cr-Mo-V-B-Si alloy have been studied by the method of local deformation. It is established that plastic deformation in the rapidly quenched iron-based alloy proceeds by two channels. The model for interpreting the energy aspects of plastic deformation in amorphous materials is suggested.