Metal induced effects on some physical properties of Se0.8Te0.2 amorphous system

Thermally evaporated selenium and tellurium-based thim film glasses were carefully characterized to establish the interdependence between their chemical composition and some physical parameters, such as density and optical energy gap.The effects of different metallic additions and annealing at a temperature below the glass transition temperature T_g on the optical characteristics was explored. The optical energy gap E_opt was found to decrease with the addition of metal, the extent of the diminution dependings on the chemical character of the added metal.     

Stabilization of γ-La2S3 by alkali metal ion doping

La₃S₄ is known to crystallize in a defect cubic Th₃P₄ structure. The effect of alkali metal ion doping in the lacunar La_3−xS₄ structure has been studied. Compounds with molecular formula La_3−xA_xS₄ (A = Li, Na and K) with varying A/La ratios (0.1, 0.15, 0.2, 0.25 and 0.3) have been synthesized by gas-solid reaction method. The band gap increases with increase in dopant concentration. The optical properties show an increase in yellowness of the doped compositions when compared to the parent phase.     

Single step synthesis of chalcogenide nanoparticles Cu2ZnSnS4, Cu2FeSnS4 by thermal decomposition of metal precursors

Cu₂ZnSnS₄ (CZTS) and Cu₂FeSnS₄ (CFTS) nanoparticles were synthesized by thermal decomposition of metal precursors. Dispersion of the precursors in the solvent prior to reaction significantly lowered the temperature and time required for the reaction. Extensive characterization of the synthesized nanoparticles was done. Materials characteristics of the synthesized nanoparticles such as elemental composition, band gap and morphology were found to be similar to the nanoparticles prepared by conventional synthesis techniques.     

Calorimetric and optical study of amorphous Se85 − xTe15Bix glassy alloy

Bulk samples of Se_85 − xTe₁₅Bi_x (where x = 0, 1, 2, 3, 4, 5) glassy alloys are obtained by melt quenching technique. Differential scanning calorimetric (DSC) technique has been applied to determine the thermal properties of Se-rich Se_85 − xTe₁₅Bi_x glassy alloys in the glass transition and crystallization regions at four heating rates (5, 10, 15, 20 K min^− 1). The glass transition temperature (T_g) and peak crystallization temperature(T_p) are found to shift to a higher temperature with increasing heating rate. With Bi addition, the value of (T_g)increases. (T_p) is found to increase as Bi is introduced to the Se-Te host, however further increase in Bi concentration is responsible for the reduction of. Thin film of bulk samples are deposited on glass substrate using thermal evaporation technique under vacuum for optical characterization. Optical band gap is estimated using Tauc's extrapolation and is found to decrease from 1.46 to 1.24 eV with the Bi addition.     

Magnetic properties of binary Cu50–(Ni, Fe and Co)50 alloys prepared by ball milling and isothermal annealing

The magnetic properties of Cu–Ni, Cu–Fe and Cu–Co binary alloys prepared by ball milling and subsequent isothermal annealing have been investigated systematically. A detailed microstructural characterization by X-ray diffraction (XRD) and high-resolution transmission electron microscopic (HRTEM) analysis shows that single phase Cu–Ni solid solution formed by isothermal annealing of ball-milled Cu–Ni powder blend deteriorates the magnetic properties. In contrast, isothermal annealing of Cu–Fe and Cu–Co powder blends resulted into significant improvement of magnetic properties due to precipitation of Fe and Co from the respective supersaturated solid solution. Dispersion of Co nanoparticle in Cu matrix yielded the most attractive magnetic properties.     

Structural transition and atomic ordering of Ni49.8Mn28.5Ga21.7 ferromagnetic shape memory alloy powders prepared by ball milling

The Ni_49.8Mn_28.5Ga_21.7 powders of micro-scale irregular equiaxial particles are prepared by ball milling method, and characterized by XRD, DSC and SEM techniques. The powders are found to contain disordered fct structure. Upon heating to high temperatures, the crystal structure of the as-milled powder is found to evolve from disordered fct to disordered bcc and then to a Heusler-type structure sequentially. The critical temperature for the transition from the bcc phase to the Heusler phase is 360 °C. This phase transition process is also a disorder–order transition. An atomic ordering model similar to the grain nucleation and growth is established to explain the annealing temperature dependence of the phase transformation temperature.     

Melting of iron nanoparticles embedded in silica prepared by mechanical milling

For decades, experimental studies on the size-dependent melting of metals are regretfully limited to some eight archetypal examples. In this work, to expand this slim range of materials, the melting behavior of Fe nanoparticles embedded in SiO₂ prepared by using mechanical milling are investigated. Effects of factors in sample preparation on the size, isolation and thermal stability of Fe nanoparticles are systematically studied. On this basis, the size-dependent melting of Fe is successfully traced: for Fe nanoparticles with a diameter of about 15 nm, the melting point depression is 30 °C in comparison with bulk Fe, in accordance with our recent theoretical prediction.     

Phase transformations of NbCr2 intermetallics produced by mechanical alloying followed by hot-pressing consolidation

Mechanical alloying followed by hot-pressing consolidation has been used to produce NbCr₂ intermetallics under different conditions. High-purity Nb and Cr crystalline powders, in the relative (molar) ratio of 13:1, were milled for periods up to 100 h. This powder was vacuum-sintered at temperatures ranging from 1423 to 1573 K for 0.5 h under a pressure of 45 MPa. The phase transformations of the NbCr₂ were investigated by X-ray diffraction and scanning electron microscopy; several different phase transformations were observed. Increasing the milling time up to 100 h transforms into a mixture of C14, Nb, Cr and C15. The experimental results show that new evidence based on X-ray diffraction measurements further establishes the existence of a high-temperature C14 Laves polytype; an intermediate C36 structure for NbCr₂, reported in the literature, was not detected in this study. The relationship between the various phase transformations, based on the atomic radii and different preparation techniques, is discussed.     

Synthesis and catalytic performance of antimony trioxide nanoparticles by ultrasonic-assisted solid-liquid reaction ball milling

In this study, the cubic antimony trioxide (Sb₂O₃) nanoparticles were successfully synthesized by ultrasonic-assisted solid-liquid reaction ball milling technique. And the synthetic process, using Sb powder as raw material, was conducted with an atmosphere of acetic acid aqueous solution at low temperature (?100 °C). Some controlled trials, including without the assistance of ultrasonic wave, different reaction solutions and diverse ultrasonic frequencies, were performed. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the Sb₂O₃ nanoparticles with uniform and ultrafine particle size were obtained within short reaction time under the coaction of both ultrasonic wave and ball milling. Furthermore, the catalytic capacity of the Sb₂O₃ nanoparticles for decomposition of H₂O₂ was measured during the oxidative decomposition of methylene blue (MB). The test exhibited rapid and efficient color removal in the degradation of MB.     

Characterization of Ti-HA composite fabricated by mechanical alloying

Titanium (Ti) and its alloys possess suitable mechanical characteristics for utilization in orthopedic implants. However, their poor integrity with native tissues is a major challenge in their clinical application. Composite structures of Ti and hydroxyapatite (HA) can be used to promote the bone ingrowth and integration of the implant with the surrounding tissue. Here, we report the fabrication of Ti-HA nanocomposite powders using a high energy planetary ball mill. We investigate the effects of fabrication parameters including HA content (10–30% w/w), milling time (20 and 50 h), and HA particle size (50 nm and 15 μm) on the characteristics of the fabricated composites. In particular, we determine the samples hardness, sintering density, surface roughness and topography for different conditions. The results show that the addition of HA to Ti decreases the sintering density and enhances the surface hardness. Also, we observe a direct relationship between HA concentration in the Ti matrix and the surface roughness.     

Effects of annealing temperatures on optical and electrical properties of vacuum evaporated Ga15Se77In8 chalcogenide thin films

The optical constants (absorption coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary parts of dielectric constants) of amorphous and thermally annealed thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses with thickness 4000 Å have been investigated from absorption and reflection spectra as a function of photon energy in the wave length region 400-800 nm. Thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses were thermally annealed for 2 h at three different annealing temperatures 333 K, 348 K and 363 K, which are in between the glass transition and crystallization temperature of Ga₁₅Se₇₇In₈ glasses. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It was found that the optical band gap decreases with increasing annealing temperature. It has been observed that the value of absorption coefficient and extinction coefficient increases while the values of refractive index decrease with increasing annealing temperature. The decrease in optical band gap is explained on the basis of the change in nature of films, from amorphous to crystalline state. The dc conductivity of amorphous and thermally annealed thin films of Ga₁₅Se₇₇In₈ chalcogenide glasses is also reported for the temperature range 298-393 K. It has been observed that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges. The dc conductivity was observed to increase with the corresponding decrease in activation energy on increasing annealing temperature in the present system. These results were analyzed in terms of the Davis-Mott model.     

Nanoindentation behavior of bulk metastable Al65Cu20Ti15 alloy prepared by consolidation of the ball milled powder

Present study concerns assessment of nanomechanical property in the bulk Al₆₅Cu₂₀Ti₁₅ alloy with varying microstructure synthesized by consolidation of mechanically alloyed powder at different temperature. The microstructure after consolidation at room temperature and 500 °C exhibits completely amorphous and nanocrystalline states respectively. Nanoindentation experiments suggest that the maximum strength and hardness values are achieved in the sample sintered at 450 °C. The corresponding microstructure revealed dispersion of nanocrystalline intermetallic phase (<50 nm) in the amorphous matrix.     

Nanocrystalline Al/Al12(Fe,V)3Si alloy prepared by mechanical alloying: Synthesis and thermodynamic analysis

Al–Al₁₂(Fe,V)₃Si nanocrystalline alloy was fabricated by mechanical alloying (MA) of Al–11.6Fe–1.3V–2.3Si (wt.%) powder mixture followed by a suitable subsequent annealing process. Structural changes of powder particles during the MA were investigated by X-ray diffraction (XRD). Microstructure of powder particles were characterized using scanning electron microscopy (SEM). Differential scanning calorimeter (DSC) was used to study thermal behavior of the as-milled product. A thermodynamic analysis of the process was performed using the extended Miedema model. This analysis showed that in the Al–11.6Fe–1.3V–2.3Si powder mixture, the thermodynamic driving force for solid solution formation is greater than that for amorphous phase formation. XRD results showed that no intermetallic phase is formed by MA alone. Microstructure of the powder after 60 h of MA consisted of a nanostructured Al-based solid solution, with a crystallite size of 19 nm. After annealing of the as-milled powder at 550 °C for 30 min, the Al₁₂(Fe,V)₃Si intermetallic phase precipitated in the Al matrix. The final alloy obtained by MA and subsequent annealing had a crystallite size of 49 nm and showed a high microhardness value of 249 HV which is higher than that reported for similar alloy obtained by melt spinning and subsequent milling.     

Hydrothermal synthesis and thermoelectric properties of nanostructured Bi0.5Sb1.5Te3 compounds

Single-phase Bi_0.5Sb_1.5Te₃ compounds have been prepared by hydrothermal synthesis at 150 °C for 24 h using SbCl₃, BiCl₃ and tellurium powder as precursors. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) have been applied to analyze the phase distributions, microstructures and grain sizes of the as-grown Bi_0.5Sb_1.5Te₃ products. It is found that the hydrothermally synthesized Bi_0.5Sb_1.5Te₃ nanopowders have a morphology dominated by irregular hexagonal sheets due to the anisotropic growth of the crystals. The Bi_0.5Sb_1.5Te₃ nanosheets are parallelly stacked in certain direction to form sheet-agglomerates attribute to the temperature gradients in the solution.     

Thermoelectric properties of the tetradymite-type Bi2Te2S-Sb2Te2S solid solution

The thermoelectric properties of the tetradymite-type Bi_2−xSb_xTe₂S solid solution (0 ≤ x ≤ 2) are reported for the temperature range 5-300 K. The properties of non-stoichiometric, Cl and Sn doped n- and p-type variants are reported as well. The Seebeck coefficients for these materials range from −170 to +270 μV K⁻¹ while the resistivities range from those of semimetals, 2 mΩ cm, to semiconductors, >1000 mΩ cm. Thermal conductivities were low for most compositions, typically 1.5 W m⁻¹ K⁻¹. Nominally undoped Bi₂Te₂S shows the highest thermoelectric efficiency amongst the tested materials with a ZT = 0.26 at 300 K that decreased to 0.04 at 100 K. The crystal structure of Sb₂Te₂S, a novel tetradymite-type material, is also reported.     

Influence of substrate absorption on accuracy of determination of refractive index and thickness of uniform thin chalcogenide Cu1[As2(S0.5Se0.5)3]99 film

The envelope method is a commonly used method for determination of some important optical constants, by using the envelopes of the transmittance T(λ) and/or reflectance R(λ) spectrum of the thin film deposited on transparent substrate. Two envelope methods were carried out in this paper: standard—method which assumes that substrate is absolutely transparent and modified—method which takes substrate absorption into account.The investigated sample is a uniform thin chalcogenide Cu₁[As₂(S_0.5Se_0.5)₃]₉₉ film, deposited onto two kinds of a weakly absorbing substrates that differ in thickness.It was shown that the degree of accuracy in determination of chosen optical parameters for both investigated samples is notably improved when the absorbance of the bare substrates is considered in the expressions for the envelopes.     

Study on the optical property and crystallinity of Ge90Te10 films deposited by electron beam evaporation

Optical property and crystallinity of Ge₉₀Te₁₀ films prepared by electron beam evaporation have been studied. The films grown at different substrate temperatures (Ts) and deposition rates (R) have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and spectroscopic ellipsometry. The polycrystalline film was obtained at Ts = 300 °C, while the amorphous film was obtained when Ts ≦ 200 °C. However, the film showed the columnar structure when Ts ≦ 100 °C. It was found that Ts had the stronger effect on the crystallinity of the film rather than R. The optical constant in the infrared region was determined. All the film exhibited no absorption, but the refractive index was varied with the change of Ts and R. The relationship between optical constant, the film structure and the deposition parameters were also discussed. In addition, the optimum deposition condition of Ge₉₀Te₁₀ film was found.     

Structure and microstructure of In4Te3 nanopowders prepared by solid state reaction

In this paper, we investigated the structure and microstructure of In₄Te₃ nanopowders obtained by mechanically alloying an In₇₅Te₂₅ powder mixture. Structural, chemical, thermal and vibrational studies of the In₇₅Te₂₅ powder mixture were carried out using X-ray diffraction, energy dispersive spectroscopy, transmission electron microscopy, differential scanning calorimetry and Raman spectroscopy. The orthorhombic In₄Te₃ phase (In₃Se₄-type) was nucleated in 2 h of synthesis, although non-reacted tetragonal indium (In) was still present at that time. Small amounts of cubic In₂O₃ phase were observed after 31 h of synthesis. Rietveld analyses allowed the measurement of mean crystallites sizes and phase fraction variations when milling times were increased. These analyses showed that, after 31 h of synthesis, about 65 wt% of In₄Te₃ phase contained mean crystallite sizes smaller than 27 nm and microstrains greater than 1.5%. The crystallite and interfacial components sizes were determined by high resolution transmission electron microscopy. Differential scanning calorimetry measurements showed the influence of nanometric crystallite sizes on the melting of the In₄Te₃ and non-reacted In phases. Raman measurements showed that the trigonal Te and α-TeO₂ modes, observed for the precursor Te powder, are absent for the sample milled for 31 h. The structural stability of the nanocrystalline phases of the In₇₅Te₂₅ sample milled for 31 h was attested by X-ray diffraction measurements performed twelve months after its production.     

Optical characterization of vacuum evaporated a-Se80Te20−xCux thin films

Thin films of a-Se₈₀Te_20−xCu_x (where x=2, 6, 8 and 10) were deposited on glass substrates by vacuum evaporation technique. The absorbance, reflectance and transmittance of as-deposited thin films were measured in the wavelength region 400-1000 nm. The optical band gap and optical constants of amorphous thin films have been studied as a function of photon energy. The optical band gap increases on incorporation of copper in Se₈₀Te_20−xCu_x system. The value of refractive index (n) decreases while the value of the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states.     

Medium-term thermal stability of amorphous Ge2Sb2Te5 flash-evaporated thin films with regards to change in structure and optical properties

The stability of flash-evaporated amorphous Ge₂Sb₂Te₅ thin films has been studied under medium-term temperature treatment (30 - 80 °C, with a step of 10 °C) in ten subsequent heating and cooling cycles. The significant changes in structure and optical properties are reported. The temperature cycling of the films resulted in formation of an isolated 5 - 7 nm nano-crystalline phase in the amorphous phase. The corresponding increase in refractive index and change in optical bandgap energy and sheet resistance are also presented. The formation of Ge₂Sb₂Te₅ nano-crystals (~ 5 - 7 nm) even under temperature below 80 °C could contribute to the explanation of mechanism of resistivity fluctuation (drift) of the “amorphous phase” films. We also show that the optical and electrical properties of flash evaporated Ge₂Sb₂Te₅ thin films are very similar to those reported for sputtered films.