Structures of vapour-deposited amorphous films of arsenic chalcogenides

The structures of vapour-deposited amorphous films of composition As_xSe_1?x′, with x between 0 and 1, have been investigated by X-ray diffraction. In all cases the structure of the freshly deposited film differs very considerably from that of the corresponding bulk glass. For the elemental amorphous films the structure is highly disordered and contains voids which for Se cannot be annealed out below the crystallization temperature of 70°C or, for As, below the temperature ($\text{?130°}\text{C}$) where the As film re-evaporated. Annealing of the arsenic selenide films at temperatures below T_g causes the structures to relax towards those of the bulk, with a distribution of activation energies around 25 kJ mol^?1. The composition of the vapour has been examined by mass spectroscopy and it is concluded that even if some molecular identity is retained on condensation there must be considerable cross-linking to give the observed structural behaviour.     

Fabrication of nano-gratings in arsenic sulphide films

Chalcogenide glasses, generally soluble in alkaline solvents, become more resistant to etching in amine-based solvents after exposure to high energy electrons (30 keV). We have used this characteristic for fabricating structures that are suitable for nanolithography. Using a scanning electron microscope equipped with a lithography system, nano-gratings are written digitally with varying point spacing, line pitch, and electron dose. The spot size of the electron beam is ∼1 nm, but the finest structures that have been created in our arsenic sulphide films have lines with widths of 27 nm, separation of 7 nm, and heights from 80 to 250 nm. As the line pitch decreases, the best resolved spacing between the lines decreases until the lines blend into each other, much before the e-beam overlap. A discussion of the optimization of fabrication parameters is included.     

Stability of Ni-based bulk metallic glasses

Several ternary (Ni_xNb_ySn_z) refractory alloy glasses (RAGs) were studied at elevated temperatures in order to assess the stability of the amorphous state, i.e. devitrification, and to identify subsequent phase transformations in these materials. differential scanning calorimetry (DSC) experiments indicated a complex phase transformation sequence with several distinct crystallization and melting events being recorded above the glass transition temperature, T_g. Below T_g the RAG samples were studied with an in situ environmental X-ray furnace facility, which allowed step-wise isothermal ramping experiments commencing at a temperature below the reduced temperature of T/T_g ≈ 0.80. Distinct crystalline phases were observed when T/T_g ≈ 0.84 for ternary RAG alloys, while similar experiments on Zr-based Vit 106 glass alloys did not reveal any apparent phase separation until T/T_g ≈ 0.96. The phase separation kinetics followed an Arrhenius type of relationship with Ni₃Sn, and Nb₂O₅ being the principle crystalline precipitates.     

Possible differences between the surface and bulk structure of glasses

Changes in cation co-ordination at glass surfaces (indicating an increase of the oxygen-to-cation ratio within the topmost superficial layers) are evidenced by comparing data acquired by spectroscopic techniques of different excitation depth. Based on these experimental results, a theoretical model is developed that predicts structural discrepancies between surface and bulk structure. Such reconstructed surfaces are of great importance for all kind of surface-related crystallization phenomena.     

Elastic properties of some bulk metallic glasses

The relationships between the elastic moduli, glass forming ability and response to deformation of bulk metallic glasses are investigated. Five bulk metallic glasses are prepared from high purity elements via suction casting. The results confirm that there exists a correlation between energy absorbed to failure during compression testing and the bulk to shear modulus ratio. This finding is developed such that it corresponds only to the elastic component of energy absorption, and that the bulk modulus dominates this. Plastic deformation appears to be favored by a reduced shear modulus, although it shows greater dependence on structural features that are frozen in during the glass transition, and so may well be dependent on the liquid fragility.     

Thermal analysis of bulk amorphous arsenic triselenide

Through the use of differential scanning calorimetry, the heat capacity and crystallization parameters of amorphous As₂Se₃ have been studied. It is found that the heat capacity above the glass transition temperature (T_g) exceeds, by a factor of 1.6, the Dulong-Petit limiting value found below T_g. The crystallization process is found to obey first-order kinetics with an activation enthalpy of 1.24 eV and an enthalpy of crystallization of 0.36 eV · molecule^?1. The effect of sample age on the rate constant is also reported.     

Formation and thermal stability of Pd-based bulk metallic glasses

The glass-forming ability and thermal stability of bulk glassy Pd₇₉Cu₆Si₁₀P₅ alloy were studied by substitution of Cu with Ag and with Au + Ag from 0 to 6 at.%. The results indicated that the small addition of Ag strongly affects the thermal stability and glass-forming ability of the Pd₇₉Cu₆Si₁₀P₅ alloy. The alloy doped with 4 at.% Ag (Pd₇₉Cu₂Ag₄Si₁₀P₅) exhibits the largest glass-forming ability among the Pd₇₉Cu_6−xAg_xSi₁₀P₅ (x = 0–6 at.%) alloys. The critical diameter for glass formation of this alloy reaches as large as 7 mm by copper mold casting. On the other hand, the multi-addition of Au + Ag does not increase the glass forming ability though the Ag and Au are similar in atomic size. The largest glass forming ability is obtained at 1 at.% Au + 2 at.% Ag among the Pd₇₉Cu_6−x−yAu_xAg_ySi₁₀P₅ (x = 1–4 at.%, y = 1–3 at.%) alloys. The critical diameter of this alloy is 5 mm by copper mold casting.     

The structure of vapour-deposited arsenic sulphides prepared using a cooled (78 K) substrate

Amorphous molecular As₄S₄ and non-stoichiometric “As₂S₃” evaporated films have been prepared by condensing onto a substrate at liquid nitrogen temperature in situ in the X-ray diffractometer. The structures of these films are shown to be similar, thus confirming the importance of As₄S₄ (realgar-type) molecules in amorphous films prepared by evaporation of “As₂S₃” glass. The local molecular packing in the as-deposited amorphous As₄S₄ film shows a preference for a packing arrangement similar to that found in the α-crystalline phase. Both as-deposited films contain gross structural defects such as molecular clusters and/or voids which irreversibly reduce in importance as the film structures relax on warming to ambient temperatures. Also, the As₄S₄ film partially devitrifies to the high-temperature β-crystalline phase on warming to ambient, but no devitrification occurs in the non-stoichiometeric film due to the presence of more than one molecular type.     

Behavior of Ag photodoping in sulfide bulk glasses

The relationship between Ag photodoping behavior and glass structure has been studied using various sulfide bulk glasses. The transmission measurement revealed that the photodoping rate increased with an increase of S concentration in Ge–S glasses and GeS₂–SbS₂ glasses, while the disappearance of Ag film was not be observed in Ge₄₀S₆₀ and Ga₂S₃-based glasses, which contain modifier ions. The correlation between photodoping rate and S–S bond content in a host glass was confirmed. From the point of view of the behavior of Ag photodoping, sulfide glasses were classified into two groups: (1) a covalent glasses containing S–S bonds and (2) an ionic glasses containing modified ions.     

Electrical switching and thermal studies on bulk Ge-Te-Bi glasses

Bulk, melt quenched Ge₁₈Te_82-xBi_x glasses (1 ≤ x ≤ 4) have been found to exhibit memory type electrical switching behavior, which is in agreement with the lower thermal diffusivity values of Ge-Te-Bi samples. A linear variation in switching voltages (V_th) has been found in these samples with increase in thickness which is consistent with the memory type electrical switching. Also, the switching voltages have been found to decrease with an increase in temperature which happens due to the decrease in the activation energy for crystallization at higher temperatures. Further, V_th of Ge₁₈Te_82-xBi_x glasses have been found to decrease with the increase in Bi content, indicating that in the Ge-Te-Bi system, the resistivity of the additive has a stronger role to play in the composition dependence of V_th, in comparison with the network connectivity and rigidity factors. In addition, the composition dependence of crystallization activation energy has been found to show a decrease with an increase in Bi content, which is consistent with the observed decrease in the switching voltages. X-ray diffraction studies on thermally crystallized samples reveal the presence of hexagonal Te, GeTe, Bi₂Te₃ phases, suggesting that bismuth is not taking part in network formation to a greater extent, as reflected in the variation of switching voltages with the addition of Bi. SEM studies on switched and un-switched regions of Ge-Te-Bi samples indicate that there are morphological changes in the switched region, which can be attributed to the formation of the crystalline channel between two electrodes during switching.     

Lutetium and thulium based rare earth bulk metallic glasses

We report the formation of lutetium and thulium based bulk metallic glasses based on the correlations between the thermodynamic, kinetic, elastic and other properties of metallic glasses. The two novel rare earth based bulk metallic glasses (REBMGs) exhibit excellent glass formation ability, high elastic moduli, considerable smaller Poisson’s ratio, high thermal stability, and even higher mechanical strength than that of typical high strength Zr-based BMGs. The reasons for the properties of the Lu- and Tm-based and other REBMGs are discussed.     

Properties of as-cast and structurally relaxed Zr-based bulk metallic glasses

We have studied the influence of the arc melting procedure of Nb-containing Zr-based bulk metallic glasses (BMGs) on their thermal and mechanical properties. We found that the strength and plastic strain to failure, determined at room temperature, increases by the addition of a few percentage of Nb into the Zr-based BMGs. High-resolution transmission-electron microscopy results show that the addition of 2 at.% Nb introduces the formation of as-cast nanocrystals. At the same time, thermal analyses indicate an increase in glass forming ability with the small addition of Nb. Contrary to the reported results in other amorphous alloys, we found that the plastic strain increases further after heat-induced structural relaxation in the Zr-based BMGs.     

Chemical annealing of oxygen hole centers in bulk glasses

Chemical annealing of the oxygen hole centers in bulk glasses by using hydrogen (or deuterium) has been extensively reported in the literature. Hydrogen chemically anneals these defect centers by reacting with them to form hydroxyl species. We have here presented a reaction-diffusion model to predict the chemical annealing rates of these defect centers in bulk glasses. The model considers diffusion of hydrogen (or its isotopes, e.g., deuterium) through the glass and its simultaneous reaction with the oxygen hole defect centers, resulting in the formation of the hydroxyl species in the glass. The predictions from the model are in excellent agreement with the experimental data presented in [J. Appl. Phys. 60 (12) (1986) 4325]. The model also establishes the precise connection between the dependence of effective hydrogen diffusivity on its intrinsic diffusivity, temperature, hydrogen and defect concentration. We also discuss methods for understanding chemical annealing behavior at high temperatures, where hydrogen not only reacts with the oxygen hole centers, but also with the silica matrix. For the case where hydrogen interaction with the silica matrix is the dominant reaction, a reaction-diffusion model is presented and the equilibrium constant (between 500 and 900 °C) for the reaction is estimated by comparing the predictions from the model with the hydroxyl profiles reported in [Appl. Phys. Lett. 47 (3) (1985) 328; J. Appl. Phys. 61 (12) (1987) 5447].     

Inhomogeneous structure and glass-forming ability in Zr-based bulk metallic glasses

Recently, a series of quaternary Zr-based bulk metallic glasses (BMGs), i.e., Zr₅₃Cu_18.7Ni₁₂Al_16.3, Zr_51.9Cu_23.3Ni_10.5Al_14.3 and Zr_50.7Cu₂₈Ni₉Al_12.3, have been developed and their glass-forming ability (GFA) increases with Cu concentration. In this work, atomic structures of the three BMGs were rebuilt by reverse Monte Carlo simulations based on the reduced pair distribution functions measured by high energy X-ray diffraction. The results show that a certain amount of substitution of short Zr-Cu, Cu-Cu pairs with long Zr-Zr and Zr-Al pairs enhances the local denser packing of Kasper polyhedral centered by Zr and Al atoms. A cell sub-divided method is proposed to describe the fluctuation of local number density which is found to follow the normal distribution function. The largest possible free volume in the three alloys is found to approaches to 3.8 Å. For the three alloys, the enhancement of GFA with increasing Cu content is due to the increase in the fluctuation degree of local density instead of the dense packing.     

Study of microindentation cracks in bismuth-doped arsenic selenide glasses

The formation of radial cracks from Vickers indentations on the surfaces of both unirradiated and irradiated samples of (As_1 ? xBi_x)₂Se₃ glasses with x ≤ 0.1 is investigated as a function of indentation load P. It was found that radial cracks around indentations are produced on the surfaces of all samples at loads exceeding a particular applied load P_c^app but this threshold load is higher by a factor of 15 to 30 from the value of P_c calculated from the theoretical dependence of ratio c/d of crack length c measured from the center of indentation impression to indentation impression diagonal d on applied load P. By fitting the experimental P(c^3/2) data in an expression relating c and P according to expanding cavity for elastic/plastic solid and from a theoretical expression relating load-independent hardness H and threshold load P_c for crack formation estimated from the experimental data of the dependence of c/d on P, the value of indentation fracture toughness K_C of As₂Se₃ glasses, irrespective of Bi content in the samples and their irradiation, was found to be about 0.52 MPa?m^1/2.     

Thermal stability and glass-forming ability of new Ti-based bulk metallic glasses

New Ti-based metallic glass (Ti₅₃Cu₁₅Ni_18.5Al₇Si₃M₃B_0.5 (M = Sc, Hf, Ta, Nb)) alloys were prepared by melt spinning and copper mold casting. The effects of foreign atoms on the thermal stability and the glass-forming ability were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The results show that the values of T_rgs are 0.619, 0.609, 0.606, 0.597 K and ΔT_xs are 58, 54, 85, 78 K, respectively. The foreign atom’s size factor (∥R_F − R_Ti∣/R_Ti − 0.12∣) and mixing heat factor (∑x_iH_iF/∑x_iW_i) are proposed to predict the glass-forming ability and thermal stability, respectively.     

Effect of antimony addition on the thermal and electrical-switching behavior of bulk Se-Te glasses

The thermal properties and electrical-switching behavior of semiconducting chalcogenide Sb_xSe_55−xTe₄₅ (2 ? x ? 9) glasses have been investigated by alternating differential scanning calorimetry and electrical-switching experiments, respectively. The addition of Sb is found to enhance the glass forming tendency and stability as revealed by the decrease in non-reversing enthalpy ΔH_nr, and an increase in the glass-transition width ΔT_g. Further, the glass-transition temperature of Sb_xSe_55−xTe₄₅ glasses, which is a measure of network connectivity, exhibits a subtle increase, suggesting a meager network growth with the addition of Sb. The crystallization temperature is also observed to increase with Sb content. The Sb_xSe_55−xTe₄₅ glasses (2 ? x ? 9) are found to exhibit memory type of electrical switching, which can be attributed to the polymeric nature of network and high devitrifying ability. The metallicity factor has been found to dominate over the network connectivity and rigidity in the compositional dependence of switching voltage, which shows a profound decrease with the addition of Sb.     

Characterization of the elasticity and anelasticity of bulk glasses by dynamical subresonant and resonant techniques

The determination of the elastic and anelastic characteristics by means of original non-destructive techniques has been applied to glasses and glass composites in order to link together the macroscopic data with structural aspects. The dynamical Young’s modulus determined by a free resonance technique allows a good accuracy measurement. Some examples concerning oxides, Ge(As)Se or metallic glasses are presented: the abrupt drop of the modulus in the range of the glass-transition temperature T_g is a general observation, which leads us to an attempt at normalization of the curves E versus T on master curves E/E(T_g) versus T/T_g. To study the viscoelastic properties, a low frequency torsional spectrometer is preferentially used to measure the damping due to viscous movements at a microscopic scale. A study of MgSiAlON glasses allows us to show that the intrinsic activation energy is much smaller than the one measured by creep or relaxation tests and that the glassy transition is characterized by a smooth change from vitreous solid (highly correlated) to quasi-liquid behavior; this has been confirmed on a metallic glass.     

Local order and non-linear optical properties in bulk nanostructured niobiosilicate glasses

This communication presents and discusses an experimental proof of the correlation among local structure and second harmonic generation (SHG) in bulk nanostructured potassium niobiosilicate (KNS) glasses. In particular, SHG shows a maximum in correspondence of the early stages of nanostructuring, that are characterized by the segregation within the amorphous matrix of nanosized inhomogeneities. EXAFS experiments indicate that these inhomogeneities are determined by the maximum size reached by the niobium second coordination shell combined with the sudden inclusion of potassium atoms in it. Such rearrangement at the local level determines the maximum fluctuation of the bulk glass refractive index and in turn its maximum SHG activity.     

Tensile properties of ZrCu-based bulk metallic glasses at ambient and cryogenic temperatures

The tensile behaviors of a series of (Zr_47.5Cu_47.5Al₅)_1 ? x(Zr₈₀Nb₂₀)_x (x = 0, 0.05, 0.10, 0.15) bulk metallic glasses were studied at ambient and cryogenic (77 K) temperatures. It is found that the tensile strength of the alloys increases as the temperature decreases from 298 K to 77 K. The maximum enhancement is 15.7%, and the toughness of these alloys does not deteriorate at low temperatures. We demonstrate that the higher energy required to raise the temperature in the shear bands from the cryogenic temperature to glass transition temperature is the origin of the tensile strength enhancement at low temperatures.