Crystallization processes of Ag–Ge–Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge₂₅Se₇₅)_100−yAg_y with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag₈GeSe₆ and the secondary phase GeSe₂ was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag₈GeSe₆ phase in all the (Ge₂₅Se₇₅)_100−yAg_y samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.     

Crystallization of silicon thin films by current‐induced joule heating using a tungsten overcoat

A modified crystallization process using current‐induced joule heating under vacuum is presented. A thin layer of high temperature resistant tungsten was sputtered on the amorphous silicon as the conducting and annealing medium. The thin film thickness was measured by α‐stepper. The high current density provided effective means in crystallizing the amorphous silicon layer. The crystalline morphology was studied by scanning electron microscopy (SEM) after Secco‐etch, transmission electron microscopy (TEM), and x‐ray diffraction (XRD), under different annealing conditions. The grain size was controlled in the range of 0.1‐0.5 μm and could be increased with annealing time. No tungsten silicide was found. Some defects were formed due to electron‐migration effect near the electrodes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The influence of titania content on structure and properties of spherical mixed silica/titania particles

Spherical mixed silica/titania particles are prepared from silica/titania sol by an ultrasonic vibrator. The titania content of the samples varies from pure silica to a titania mole fraction of 0.31. Narrow size distributions with most frequent particle diameter of about 1 μm are obtained. Specific surface area and pore volume, mean pore radius as well as the meso pore size distribution are influenced by titania content. The spheres are composed of both silica and titania homogeneous distributed as it is indicated by scanning electron microscopy. X-ray diffraction studies show that the particles must be considered amorphous. Structural modifications of the framework are detected. Samples with varying refractive index may be prepared.     

Defect-induced luminescence in Co(II)-doped anatase TiO2 prepared by the sol-gel method

The defect luminescence properties of titania sol-gel materials doped with Co²⁺ ions are reported. The laser irradiation of Co-doped titania samples produces a fluorescence, which is reduced as the continuous irradiation time is increased. The fluorescence decay depends on irradiation time as a double exponential function; the characteristic times involved for the luminescence quenching are in the range of seconds. We have associated the suppression of luminescence to the effect of the local heating produced by the laser beam on the reduction of physical defects such as grain boundaries, surface states and chemical defects originating from oxygen vacancies.     

TEM,chemical etching and FTIR characterization of ZnTe grown by physical vapor transport

ZnTe ingots were obtained by the physical transport method, using an in‐house designed and built tubular furnace. The growth of ZnTe subsequently to the growth of a seed of this material allowed obtaining an ingot formed by only one large and single crystalline grain. TEM was used for the characterization of the as‐grown ZnTe single crystal ingots and commercial single‐crystalline wafers of the same material but grown by a higher temperature and more expensive technique, the Bridgman method. Both materials show very good crystalline microstructure, although some stacking faults were found in the commercial one. The infrared transmittance spectra of both materials were measured by FTIR and some differences, most likely due to differences in raw materials and growth methods, were found. The effectiveness and convenience of several chemical etchants to obtain the dislocation density and the minimal misorientation between adjacent subgrains in the as‐grown ZnTe wafers were checked. It has been found as the most advantageous the chemical solution that does not produce over‐etching. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microscopic Theory of Tensosensibility of Multi‐Layer Polycrystalline Films

The microscopic theory for the coefficient of longitudinal tensosensibility of multi‐layer polycrystalline metal films has been developed. The theory takes into consideration as the scattering of conductivity electrons on external and internal surfaces of separate layers and crystalline boundaries, so as the dependence of specular reflection coefficient and coefficient of transmission of crystalline boundaries and boundaries between separate layers on deformation. The experimental verification of the theory done on three‐layer film systems Cr/Co/Cr, Cr/Cu/Cr and Ni/Co/Cr given as the quality correspondence.     

Manifestation of microcrystalline structure in transport properties of p‐quinquophenyl films

The electrical conductivity measurements of the p‐quinquephenyl films possessing microcrystallite structure are done at temperature between 10 K–300 K. The investigated film thickness varies within 0.2–2.5 µm. The measurements were performed with Al(Au) electrodes for the films deposited by spin coating in argon atmosphere on glass substrates. The molecular dynamics simulations as well as quantum chemical calculations have shown that three different mechanisms contribute to the observed electrical feature, which are: the conductivity originated from proper crystalline states; this of the amorphous‐like inter‐grain region and the one created in the grain boarder interfaces. Comparing the experimental data and theoretically calculated dependencies, we have shown that the main contribution to the observed DC conductivity of this material results from the interfaces located at the borders of polycrystalline grains, although they possess a relatively low part of the total film volume. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the oxidation mechanism of microcrystalline silicon thin films studied by Fourier transform infrared spectroscopy

Insight into the oxidation mechanism of microcrystalline silicon thin films has been obtained by means of Fourier transform infrared spectroscopy. The films were deposited by using the expanding thermal plasma and their oxidation upon air exposure was followed in time. Transmission spectra were recorded directly after deposition and at regular intervals up to 8 months after deposition. The interpretation of the spectra is focused on the Si-H_x stretching (2000-2100 cm⁻¹), Si-O-Si (1000-1200 cm⁻¹), and O_xSi-H_y modes (2130-2250 cm⁻¹). A short time scale (< 3 months) oxidation of the crystalline grain boundaries is observed, while at longer time scales, the oxidation of the amorphous tissue and the formation of O-H groups on the grain boundary surfaces play a role. The implications of this study on the quality of microcrystalline silicon exhibiting no post-deposition oxidation are discussed: it is not sufficient to merely passivate the surface of the crystalline grains and fill the gap between the grains with amorphous silicon. Instead, the quality of the amorphous silicon tissue should also be taken into account, since this oxidation can affect the passivating properties of the amorphous tissue on the surface of the crystalline silicon grains.     

Preparation, characterization and catalytic properties of singly and doubly titanium-modified mesoporous silica gel

A series of titanium-modified mesoporous silica gel have been synthesized using tetrabutyl titanate. The samples were characterized by nitrogen adsorption-desorption, FT-IR and Raman spectroscopy, as well as by solid state diffuse reflectance UV-VIS spectroscopy. Physicochemical characterization of the materials showed that Ti atoms were part of the framework of silica gel, and it was probably in a tetrahedral coordination for low Ti contents. The resulting titania modified the inner walls of the mesoporous silica gel after hydrolysis and calcination. Actually, the titanium precursor reacted and condensed with the active silanol groups on silica gel via Si-O-Ti bonds. In addition, the titanium-modified mesoporous silica gel showed distinct activity behavior in the catalytic oxidation of cyclohexene and styrene with hydrogen peroxide as oxidant.     

The structure and properties of nanocrystalline Fe78B13Si9 alloy

A new method has been employed to prepare a nanocrystalline alloy from amorphous ribbons and some of the structure-dependent properties of this alloy have been studied and compared with those of coarse-grained crystalline and amorphous FeBSi alloys. The new method of preparing nanocrystalline alloys involves annealing isothermally an amorphous ribbon of the same composition. By comparing the structure-dependent properties, it can be concluded that the novel properties of the nanocrystalline alloy reflect the unusual nature of its grain boundaries.     

Coordination of aluminum in amorphous sodium aluminosilicate films

Amorphous sodium aluminosilicate thin films containing large amounts of Al₂O₃ were deposited on fused silica substrates by rf-sputtering, and their aluminum K-band X-ray emission spectra were measured by using an electron probe X-ray microanalyser in order to determine the coordination number of aluminum ions in the amorphous thin films.

The chemical shifts for the amorphous films with Al₂O₃/Na₂O<1 were almost identical with those of tetrahedrally coordinated aluminum ions in microline. On the other hand, for the amorphous films with Al₂O₃/Na₂O>1, the chemical shifts increased with increasing Al₂O₃/NA₂OF ratio, approaching that of amorphous alumina. From the comparison with the chemical shifts of -Al₂O₃ and mullite, the coordination state of aluminum ions in amorphous alumina was found to be about 5, and its structure was found similar to be that in crystalline Al₂O₃ with spinel-type structure. These results indicate that in amorphous sodium aluminosilicate thin films aluminum ions exist in the tetrahedrally coordinated state when the Al₂O₃/Na₂O ratio is nearly equal to or less than unity. However, when the Al₂O₃/Na₂O ratio exceeds unity, some of the aluminum ions begin to assume the octahedrally coordinated state and increase in number with increasing Al₂O₃/Na₂O ratio.     

Poole‐Frenkel electrical conduction in europium oxide films deposited on Si(100)

Thin Eu₂O₃ films were prepared on Si (P) substrates to form MOS devices. The oxide crystal structure was determined by X‐ray diffraction (XRD). The electrical transport properties of the devices with amorphous and crystalline Eu oxide were investigated. The current‐voltage and current‐temperature characteristics suggest a Poole‐Frenkel (PF) type mechanism of carrier transport through the device when the applied field is more than 105 V/cm. A deviation from PF leakage current course was found and attributed to the current carrier trapping. We have also observed that, the dielectric spectra of MOS structure are different when the insulator is an amorphous or crystalline thin film. From which we calculate the relaxation time (τ) of the interface (insulator/semiconductor) dipoles. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

高铝陶瓷显微结构及制备工艺对金属化层性能影响的研究

通过配方及烧成工艺的改变,得到两种不同显微结构的高铝陶瓷材料,其一为晶粒大小均匀,边界清晰,晶粒堆砌结构;其二为晶粒大小不一,晶粒边界不清,甚至晶粒被包裹在玻璃相中,形成玻璃相堆积结构.测定了材料的抗折强度、体积电阻率、抗击穿强度、比重、金属化层抗张强度和气密性.分析了两种显微结构的成因及不同微观结构对陶瓷材料性能和金属化层性能的影响.实验证明:采用微粉氧化铝配料,利于获得金属化性能优良的细晶结构的高铝陶瓷材料;用粗粉氧化铝配料,并加人复合的添加剂,也可以获得细小晶粒堆砌结构的高铝陶瓷材料,这类结构高铝陶瓷材料的金属化层性能好.     

Photoelectrical properties of crystalline titanium dioxide thin films after thermo‐annealing

This paper reports the photoelectrical properties of sol gel derived titanium dioxide (TiO₂) thin films annealed at different temperatures (425‐900°C). The structure of the as‐grown film was found to be amorphous and it transforms to crystalline upon annealing. The trap levels are studied by thermally stimulated current (TSC) measurements. A single trap level with activation energy of 1.5 eV was identified. The steady state and transient photocurrent was measured and the results are discussed on the basis of structural transformation. The photocurrent was found to be maximum for the films annealed at 425°C and further it decreases with annealing at higher temperatures. The photoconduction parameters such as carrier lifetime, lifetime decay constant and photosensitivity were calculated and the results are discussed as a function of annealing temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An X-ray scattering study of hydrogenated nanocrystalline silicon with varying crystalline fraction

Using X-ray diffraction (XRD) and small angle X-ray scattering (SAXS), we probed the nanostructural features of several PECVD grown nc-Si:H thin films with varying crystalline volume fraction. XRD results of a mixed phase film, 70% a-Si:H and 30% c-Si:H, show these crystallites have a preferred [220] orientation in the growth direction. Another film with approximately 90% c-Si also shows elongated grains, but with a preferred [111] orientation. The SAXS results also show an increase in scattering intensity when compared to the mixed phase material. In the mixed phase material, models show that the electron density fluctuations between the amorphous and crystalline phases are not enough to explain the measured SAXS scattering. Hydrogen clustered at the crystallite boundaries and in void regions of the a-Si phase must be included as well.     

K X-ray spectra of amorphous and crystalline silicon

K X-ray emission and absorption spectra of crystalline and amorphous silicon have been analysed and compared to band structure calculations. The experimental results confirm the influence of rings and bound-angle fluctuations on different regions of the valence and conduction band of amorphous silicon.     

Phase Formation and Solid State Structure on Calcination of a Nickel Ferrite Acetate Precursor

The thermal decomposition of a freeze dried Ni‐Fe‐μ‐oxo‐acetate results in the formation of an amorphous nickel ferrite. Crystallization of the spinel NiFe₂O₄ takes place between 300 and 500°C. High disperse amorphous (or less crystalline) products with specific surfaces up to 500 m²/g can be obtained by carful decomposition between 260 and 300°C. Raising the calcination temperature and calcination time effects a decrease of porosity and of the specific surface. The primary nanocrystalline spinel powder is strongly agglomerated. During an annealing process between 300 and 500°C the crystallite size is increased but the mean diameter of agglomerates of about 15 μm does not change. A combination of milling and ultrasonic dispersion allows the adjustment of grain sizes < 1 μm with a narrow grain size distribution.     

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.     

Structure and morphology of nickel-alumina/silica solar thermal selective absorbers

Nickel-alumina/silica thin film materials for the use in solar thermal absorbers have been investigated using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Elastic Recoil Detection Analysis (ERDA). The TEM images revealed that all layers have a very small thickness variation and that the layers are completely homogenous. High resolution images showed 5-10 nm (poly) crystalline nickel nano-particles. ERDA showed that both the silica and alumina compositions contain more oxygen than 2:1 and 3:2 respectively. SEM showed the surface morphology and characteristics of the top silica anti-reflection layer. Hybrid-silica has showed to generate a smoother surface with less cracking compared to pure silica. The final curing temperature revealed to be of importance for the formation of cracks and the surface morphology.     

Liquid phase separation in immiscible Ag-Ni-Nb alloy and formation of crystalline/amorphous composite

Phase decomposition in liquid miscibility gap presents a unique opportunity for designing amorphous/crystalline composites with a multilayer structure. One of typical monotectic systems, Ag-Ni alloy, is selected, and element Nb is added in the Ag-Ni alloy. The new ternary Ag-Ni-Nb monotectic alloys are rapidly quenched by using melt spinning technique. It is revealed that the ternary alloy undergoes a liquid-liquid phase decomposition and forms two immiscible Ag-rich and NiNb-rich liquids. Subsequently, the immiscible liquids solidify into Ag-rich crystalline and NiNb-based amorphous phases, respectively. Self-assembled amorphous/crystalline composites with double layer and sandwich type structures have been developed. The structure of the alloy ribbons with a dependence on the composition changes from a particle-dispersed structure to double layer structure and to sandwich type structure. The phase formation and thermal property of the quenched alloys are investigated. The formation mechanism of the crystalline/amorphous composite structures has been discussed.