Structural studies of amorphous and crystalline Tl-As-Se compounds

X-ray diffraction and microstructure studies for amorphous samples of composition (As₂Se₃)_1–x Tl _x , wherex varies between 0 and 0.7, show that up tox=0.25 the short-range order is similar to that of As₂Se₃. A change in the structure is observed asx increases. Diffraction from samples crystallized by annealing confirms that a thallium content up tox=0.25 is not enough to alter a characteristic matrix similar to crystalline As₂Se₃; however, it results in a decrease of crystallization tendency. Forx>0.25 up to 0.7 the structural matrix features are almost similar to those of crystalline TlAsSe₂. The observed structural changes could rationalize reported changes in the electrical and thermal transport properties of the amorphous ternary system compounds.     

Structural transformations in amorphous As x Se1−x (0 ≤ x ≤ 0.20) films

Structural transformations are examined by the employment of Raman scattering measurements for amorphous Se-rich As _x Se_1−x (0 ≤ x ≤ 0.2) alloys. It is found that the molecular structure of amorphous Se (a-Se) on the scale of medium-range order differs from the structure of most inorganic glasses and may be placed between 3-dimensional network glasses and polymeric ones. Further experiments show the existence of successive phases in laser-induced glass—crystalline transition with pronounced threshold behavior. By comparing peak width, peak location and Raman intensity in the range of bond modes it is derived that the changes occur not monotonically with increasing As content. The composition-induced changes of the spectra are explained by cross-linking of Se chains. Under laser irradiation, the changes in the optical transmission, holographic recording properties and Raman spectra of amorphous As _x Se_1−x films with 0 < x ≤ 0.2 have been examined. The dependence of the transmissivity and diffraction efficiency on the irradiation energy density shows two qualitatively different regions. Below the energy density threshold, E _th, only small changes in the local structure of the system can be detected. In the low-energy region, transient changes in transmissivity are observed. Qualitative explanation of this behavior may be based on associating such with alternating of deep defect states. Above E _th, the changes were attributed to crystallization transformation. The corresponding Raman spectra reveal transformation of the system from amorphous into the crystalline phase under laser irradiation. Although several articles and texts have provided reviews on various properties and applications of chalcogenide glasses, there is no thorough study of local atomic structure and its modification for Se-rich amorphous As _x Se_1−x . The present paper is concerned with this problem.     

Nanocrystalline microstructures in Fe93 − x Zr7B x alloys

The crystallization behaviour of amorphous Fe_{93 – x} Zr₇B _x (x = 3, 6, 12 at.%) alloys, the microstructures of the primary crystallization products of stable and metastable phases and the subsequent transformations, have been studied using a combination of differential scanning calorimetry, differential thermal analysis, X-ray diffraction and transmission electron microscopy, including microdiffraction. It has been found that, for x = 3 and 6 at.%, the sole product of primary crystallization is the bcc -Fe phase and the average grain sizes of the crystalline phase were 14 nm and 12 nm for the two alloys, respectively. However, when x = 12 at.%, primary crystallization results in more than one crystalline phase, and a metastable phase with the cubic Fe₁₂Si₂ZrB structure is the major crystallization product after the primary crystallization reaction, accompanied by the -Fe phase. The average grain size of this metastable phase was 35 nm for the alloy heated to 883 K at 20 K/min. Isothermal heat treatments at 873 K and 973 K confirm that after being heated for 240 h, this metastable phase transforms into equilibrium phases: bcc -Fe, hcp ZrB₂ and probably hcp Fe₂Zr. The apparent activation energies for the primary crystallization reaction during continuous heating for these three alloys are 4.4 ± 0.2 eV, 3.5 ± 0.2 eV and 6.9 ± 0.3 eV, respectively.     

Mössbauer spectroscopic studies of the crystallization of amorphous Fe80B20−x Si x (x=0, 2, and 8) alloys

The crystallization process of amorphous Fe₈₀B_20–x Si _x (x=0, 2, and 8) ferromagnetic alloys has been studied by using ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction studies. Results for samples heat treated at different temperatures for different times show that the crystallization of Fe₈₀B_20–x Si _x samples having x=0 and 2 leads to -Fe and t-Fe₃B, while for x=8, it leads to -Fe, t-Fe₂B, and perhaps Fe-Si. It is further observed that the addition of silicon to the Fe-B system improves the thermal stability of the system.     

Crystal Growth and Properties of Cd1 – x Zn x As2 Solid Solutions

Cd_{1 – x} Zn _x As₂ (x = 0.03, 0.05, 0.06) single crystals are grown by the Bridgman method, and their optical absorption spectra are measured. The introduction of Zn is shown to increase the band gap of CdAs₂, by up to 14 meV at x = 0.06. The highest content of ZnAs₂ incorporated into CdAs₂ is 6 mol %.     

Thermal induced transformations in glassy chalcogenides SixTe60−xAs30Ge10

Temperature-induced transformations are considered to be interesting characteristic properties of amorphous materials including the Si _x Te_60–x As₃₀Ge₁₀ system, withx=5, 10, 12 and 20. Density (), X-ray diffraction and differential thermal analysis (DTA) were used to characterize the compositions. DTA traces of each glass composition at different heating rates from 5 to 30° C min^–1 were obtained and interpreted. Fast and slow cooling cycles were used to determine the rate of structure formation. Cycling studies of materials show no memory effect but only ovonic switching action. The compositional dependence of the crystallization activation energy (E) and the coefficient of glass-forming tendency (K _gl) have been calculated. The thermal transition temperatures and associated changes in specific heat have been examined as a function of the Te/Si ratio by differential scanning calorimetry. It was found that andE increase linearly with increasing tellurium content, while the heat capacity (c _p) andK _gl, decrease with increasing tellurium content.E=1.54 eV andc _p=0.246 J g^–1 K^–1 forx=20 whileE=2.74eV andc _p=0.22 J g^–1 K^–1 forx=5.     

Viscous flow behaviour of NixZr100−x metallic glasses from Ni30Zr70 to Ni64Zr36

The compositional dependence of viscous flow in the Ni _x Zr_100–x amorphous system was investigated under non-isothermal conditions at a heating rate of 10 K min^–1in the compositional range fromx=30 tox=64 at % with the aid of a Hereaus TMA 500 dilatometer. The crystallization behaviour of the same glassy alloys under the same non-isothermal conditions was studied with a Perkin-Elmer DSC 7 differential scanning calorimeter. The characteristic crystallization and viscous flow parameters (the onset temperature,T _x, of crystallization; the temperature,T _m, of maximum heat evolution of the first crystallization step; the enthalpy, H _x, of crystallization; the activation energy,Q _x, of crystallization; the glass transition temperature,T _g; the viscosity values (T _g) and _min; and the activation energy for viscous flowQ (T>T _g), were shown to be dependent on composition. This dependence was examined on the basis of the equilibrium phase diagram of the Ni-Zr-system, and it is shown that glassy alloys possessing eutectic compositions manifest the greatest thermal stability because of the long-range atomic diffusion needed for crystallization to occur. Glassy alloys with nearly peritectoid compositions show low thermal stability, as no long-range diffusion is needed for the formation of the stable crystallization end-products NiZr₂ and NiZr. In all cases, the crystallization process is governed by viscosity flow of these glassy alloys.     

Study of the system FePO4-FeVO4 prepared from the solution

The samples of the pseudobinary system (FePO₄)_1–x -(FeVO₄) _x with x=0–1 were prepared by precipitation from a solution of iron nitrate and ammonium phosphate/vanadate, and the evolution of their structure during the heating of the precipitate within the temperature range of 20–1000 °C was studied by thermal analysis, X-ray diffraction and infrared spectroscopy. The samples dried at 120°C were amorphous having a high specific surface area of 160–222 m²g^–1 and their crystallization took place at 440–560 °C. The regions of solid solutions between the two boundary phases were smaller than 10 mol % of the second phase and a new crystalline phase was formed in the system at the composition of FePO₄FeVO₄=11.     

Formation of amorphous Ni-Zr alloy powder by mechanical alloying of intermetallic powder mixtures and mixtures of nickel or zirconium with intermetallics

Mechanical alloying was used to synthesize Ni_xZr_1–x alloys from mixtures of intermetallic compound powders, and also from mixtures of intermetallic compound powders and pure elemental powders. The mechanically alloyed powders were amorphous in the range 0.24 x 0.85. This range is larger than amorphous alloys produced by the melt-spinning technique and mechanical alloying of elemental crystalline powders. Two-phase mixtures of the amorphous phase and the corresponding crystalline terminal solid solution were formed in the range 0.10 x 0.22, and x=0.90. It is found that the morphological development during mechanical alloying of these powders is different from mechanical alloying using only pure ductile crystalline elemental powders. The thermal stability has been investigated. The enthalpy and activation energy of crystallization for Ni-Zr amorphous powders prepared by mechanical alloying are lower than those for melt-spun samples of the same composition. The crystallization temperature of the mechanically alloyed Ni-Zr amorphous powders is higher than that of meltspun samples in the composition range Ni₂₀Zr₈₀ to Ni₃₃Zr₆₇ and Ni₄₀Zr₆₀ to Ni₆₀Zr₄₀. The presence of tiny crystallites as nucleation centres and high oxygen levels in the mechanically alloyed amorphous alloys might be responsible for the differences in crystallization behaviour. A new crystalline metastable phase was observed during crystallization studies of Ni₂₄Zr₇₆ amorphous powder.     

AC conductivity of Se-Ge-As glassy system in relation to rigidity percolation

Measurements of conductivity (ac & dc) and dielectric constant () have been made for amorphous alloyed samples of Se_0.75Ge_0.25–x As _x with x= 0.05, 0.10, 0.15 and 0.20 at different temperature (289 to 389 K) and various frequencies (10²to 10⁵Hz). The conductivity and the dielectric constant of these glasses have been explained on the basis of the correlated barrier hopping (CBH) model. Recent progress in applying percolation theory to explain properties and glass forming ability of chalcogenide glasses is critically reviewed. Percolation theory is shown to be relevant to the liquid-state behavior of glass-forming ability of the Se_0.75Ge_0.25–x As _x chalcogenide system. The relationship between the optical gap (E _g) and chemical composition is also discussed in terms of the average heat of atomization (H _s) and the average coordination number (r). These findings provide to some extent an important link between experimental and theoretical results.     

The morphology of α-Fe crystals which grow in the amorphous Fe80(C1−xBx)20 alloys

Crystallization behaviour of amorphous Fe₈₀(C_1–x B _x )₂₀ alloys, obtained by splat-cooling technique, for x values ranging from zero to unity has been investigated mainly by transmission electron microscopy. The crystalline phase which first appeared in the amorphous matrix was -Fe for all alloys studied. However, the morphology of -Fe phase changed from a spherical shape for low x values to a watch-glass shape for intermediate x values and to dendritic for large x values. The nucleation of -Fe crystals was homogeneous for low x samples while preferred nucleation on edges and surfaces was noted for samples with higher x values. The final volume fraction of -Fe phase before the appearance of the second crystalline phase increased with the increase in x.     

Dielectric and crystallization properties of massive amorphous selenium

The dielectric constant of amorphous selenium at 330kHz has been measured under vacuum as a function of time on different isothermsT _a. After a normal heating period, depending onT _a, the value of increases abruptly from 5.8 to a maximum (9.6 to 11) followed by a decrease to reach a constant value. The behaviour of the curves=f(t) is discussed in terms of the structural transformation in the amorphous matrix. The morphological changes during the spherulite growth of selenium are also discussed. The dielectric loss tan has been calculated during the crystallization stages using the mesurements of time-dependence of resistivityr(t). The crystallization kinetic parameters have also been computed from the variation of during the growth stage. A value of 1.2eV is obtained for the activation energy of the radial growth of selenium in the temperature range 90 to 140° C.     

Ultrasonic spray pyrolysis of a chelated precursor into spherical YBa2Cu3O7−x high temperature superconductor powders

YBa₂Cu₃O_7–x powders have been prepared directly by ultrasonic spray pyrolysis using nitrate salts as precursors and citric acid and ethylene glycol as chelating agents. This method consists of ultrasonically atomizing a precursor solution into droplets, thermally chelating, drying, decomposing and solid state reacting these droplets in a carrier gas flowing through a tube furnace, forming a well characterized powder. The chelated precursor adjusted to pH 8 forms bidentate bonding between the cations and the chelating agents. Thermal analysis and infrared spectroscopy identify the decomposition steps of the precursor. The dry gel of the chelated precursor is nearly amorphous indicating intimate mixing on the atomic level. X-ray diffraction suggests the mechanism of forming the 123 crystalline phase. Spherical powders are produced with diameters ranging from 0.2 to 0.8 m depending on the ultrasonic frequency and the solution concentration. The spherical particles are hollow or solid depending on the precursor type and the furnace temperature. The primary crystallite size is about 10–50 nm. X-ray diffraction data and infrared spectra show that the spray pyrolysed powder from the chelating precursor forms the YBa₂Cu₃O_7–x phase at 800 °C, which is 100 °C lower than that formed from unchelated precursors.     

The structure of lead silicate glasses determined by vibrational spectroscopy

The polarized Raman spectra and infrared powder absorption spectra for PbO ·xSiO₂ glasses, with 0.5x2, were measured and interpreted in terms of the structure of the glasses. Comparison of the spectra of the glasses with the spectra of numerous stable and metastable crystalline phases was also made. It was found that the glasses are composed of a number of silicate species. In glasses for whichx is less than 1, the presence of ionic oxygen (oxygen not bonded to silicon) is confidently inferred from the spectra. A new way of deconvoluting the spectra is also reported.Also affiliated with the Department of Geosciences.     

Preparation and study of Ca1 − xMgx(PO3)2 glassy and crystalline phases

The structure of Ca_{1 – x} Mg _x (PO₃)₂ crystalline and glassy samples was investigated in the whole concentration region of x = 0–1.0. From X-ray diffraction data it was found that, in the crystalline samples, solid solutions are formed for x < 0.3 on the calcium-side of the system, with the structure of -Ca(PO₃)₂, and for x > 0.6 on its magnesium-side, with the structure of Mg₂P₄O₁₂. Similar results were obtained from the study of their infrared and Raman spectra. In the glassy state, homogeneous glasses were formed within the whole concentration region. The values of their transformation temperatures, T _g, and crystallization temperature, T _c, change slightly with the composition and lie within the region of T _g = 529–544 °C and T _c = 631–677 °C.     

Crystallization of Fe-Si-B metallic glasses studied by X-ray synchrotron radiation

We have studied the crystallization kinetics of Fe_90-x Si _x B₁₀ amorphous alloys withx ranging from 7 to 21, by synchrotron X-ray radiation. Using energy- dispersive X-ray diffraction, the kinetics of the different crystalline phases evolving during isothermal annealing were followed. These crystalline phases were identified as precipitation of-Fe(Si) and/or Fe₃Si in the amorphous matrix. At a later time or at a higher temperature, Fe₂B starts to crystallize (x < 21 ). Only at low iron concentration (x = 21) was the second phase different, namely Fe₅SiB₂ The hypo- and hyper-eutectic Fe-Si-B glasses were found to crystallize differently. The crystallization processes are discussed in some detail.     

Crystallization kinetics of Mo-N

Thin film samples of molybdenum were electron-beam evaporated at 80 K in a 6.6×10^–4 Pa partial pressure of nitrogen and subsequently analysed for structure, superconductivity and crystallization behaviour. X-ray diffraction of as-deposited samples showed a typical amorphous structure and fcc and bcc phases upon heating. A superconducting transformation at 7.0 to 7.2 K was detected by the resistivity technique. Isothermal DSC scans obtained for a range of temperatures were calibrated and integrated to produce transformed volume (x)-time data. All plots of In [-In (1–x)] against Int have two linear regions with slopes ofn ₁3.6 for 0<x<0.5 andn ₂ 2.0 for 0.7<x<1; wheren is the principal parameter in the Johnson-Mehl-Avrami theory of the crystallization transformation. The relationship of this binary pattern of crystallization to the observation of two apparent crystallization peaks in dynamic DSC scans of several samples of electron-beam deposited Mo-N and to the appearance of both Mo-bcc and Mo₂N-fcc phases in the X-ray patterns of the crystallized samples is discussed. Possible patterns of simultaneous or successive modes of nucleation and growth, including combinations of varying nucleation rates, different dimensionals of growth, and interfacial and diffusion controlled growth, are considered.     

The effect of Fe on the crystallization behavior of Al–Mm–Ni–Fe amorphous alloys

The crystallization behavior and thermal stability of Al₈₆Mm₄Ni_10–x Fe _x alloys were investigated as a function of Fe content. Alloys, produced by a single roll melt-spinner at a circumferential speed of 52 m/s, revealed fully amorphous structures. The thermal stability of the present amorphous alloys increased with the increase of Fe content. The activation energy for crystallization of -Al increased as the Fe content increased. This increase of activation energy resulted in the simultaneous precipitation of -Al and intermetallic phase observed especially in Al₈₆Mm₄Ni₅Fe₅ and Al₈₆Mm₄Ni₂Fe₈ alloys. The glass transition was observed in DSC thermogram only after proper annealing treatment. The effect of alloy composition on the thermal stability could be explained in terms of the atomic structure of the amorphous alloy.     

Non-isothermal crystallization kinetics of Ag-As2S3 semiconductor glasses

Using a differential thermal analysis technique to give single scan thermograms, the transformation mode,n, and the activation energy of crystallization,E, were determined for the chalcogenide glasses of the system Ag _x (As₂S₃)_100–x' withx=6, 15 and 25. Bothn andE were found to be compositionally dependent. The value ofn varies between 2.1 and 3.6, and that ofE between 2.4 and 3.6 eV. The crystallization kinetic data were explained using X-ray diffraction results of amorphous and crystalline structures.     

Crystallization behavior of bulk amorphous Se-Sb-In system

In the present work crystallization behavior of the amorphous Se_90–x Sb₁₀In _x (0 x 15) system has been investigated using Differential Scanning Calorimetry (DSC). The DSC data was taken at different heating rates varying from 1°C/min to 90°C/min. From the heating rate dependence of the crystallization temperature, the activation energy for crystallization has been determined using the Kissinger equation and Matusitas equation for non-isothermal crystallization of materials. The effect of addition of In to the Se-Sb system on the dimensionality of crystal growth has been investigated.