Electrical resistance behaviour of Fe-24 wt.%Mn alloy under high pressure

Electrical resistance behaviour of Fe-24 wt.%Mn SMA was studied up to a pressure of ∼6 GPa by using an opposed anvil high pressure device. The system shows a steep rise in resistance up to ∼1 GPa and thereafter a monotonic decrease up to ∼6 GPa during the forward cycle, whereas it shows a monotonic increase during the return cycle. XRD studies of the as-prepared and pressure quenched samples show a mixed α, γ and ε phase in the former and a predominantly ε phase in the latter, indicative of a possible structural transition at ∼1 GPa, as evidenced from the resistance maximum. The decrease in the transition pressure, when compared with alloys of lower Mn concentration, provides a clue that it should be possible to further reduce the transition pressure to the predominantly ε phase by alloying with suitable elements, which may have positive effect on the shape memory of the alloy.     

High-pressure phases of lead chalcogenides

Ab initio electronic structures have been carried out to find the pressure-induced structural phase transitions of lead chalcogenides (PbS, PbSe and PbTe) compounds. The zinc-blende, wurtzite, rocksalt, CsCl, GeS, TlI and orthorhombic Pnma phases are considered. Results show that the intermediate phase transition for these compounds is not the GeS nor the TlI type structures, as previously reported, but the orthorhombic Pnma phase. All these compounds are predicted to undergo a structural phase transition from the rocksalt to Pnma phase at about 8.13, 7.45 and 5.40 GPa for PbS, PbSe and PbTe respectively. Moreover, further structural phase transitions from this intermediate phase to the CsCl phase have been predicted at about 25.3, 18.76 and 15.43 GPa for PbS, PbSe and PbTe respectively.     

Selenium nanowires and nanotubes synthesized via a facile template-free solution method

Trigonal selenium (t-Se) nanowires and nanotubes were successfully prepared on a large scale via an environment-friendly synthetic process, in which no templates or surfactants were employed. These t-Se nanowires having a width of 70-100 nm and length up to tens of micrometers were synthesized in absolute ethanol at room temperature, while t-Se nanotubes with outer diameter ranging from 180 to 350 nm were obtained at 85 °C in water system. SEM and TEM analyses of the samples obtained at different stages indicated that the formation of these t-Se 1D nanostructures was governed by a “solid-solution-solid” growth process. The amorphous Se (a-Se) nanoparticles were initially generated and then would transform into crystal seeds for the subsequent growth of nanowires or nanotubes. Detailed experiments found that temperature and solvents as well as concentrations of starting materials were crucial to the formation of final morphology.     

Low temperature growth of single crystalline germanium nanowires

Ge nanowires have been prepared at a low temperature by a simple hydrothermal deposition process using Ge and GeO₂ powders as the starting materials. These as-prepared Ge nanowires are single crystalline with the diameter ranging from 150 nm to 600 nm and length of several dozens of micrometers. The photoluminescence spectrum under excitation at 330 nm shows a strong blue light emission at 441 nm. The results of the pressure and GeO₂ content dependences on the formation and growth of Ge nanowires show that the hydrothermal pressure and GeO₂ content play an essential role on the formation and growth of Ge nanowires under hydrothermal deposition conditions. The growth of Ge nanowires is proposed as a solid state growth mechanism.     

Preparation of VO2 nanowires and their electric characterization

Large-scale VO₂ nanowires have been synthesized by two-step method. First, we have been obtained (NH₄)_0.5V₂O₅ nanowire precursors by hydrothermal treatment of ammonium metavanadate solution at 170 °C. Secondly, the precursors have been sealed in quartz tube in vacuum and annealed to form VO₂ nanowires at 570 °C. Scanning electron microscope and transmission electron microscope analysis show that the nanowires have self-assembling nanostructure with the diameter of about 80-200 nm, length up to125 μm. Electrical transport measurements show that it is semiconductor with conduction activate energy of 0.128 eV. A metal-semiconductor transition can be observed around 341 K.     

Structural,thermodynamic, electronic,and optical properties of NaH from first-principles calculations

The structural stability, thermodynamic, electronic, and optical properties of NaH with rock salt (B1) structure and cesium chloride (B2) structure under high pressure are investigated by first-principles calculations using norm-conserving pseudopotential applying a generalized gradient approximation (GGA) for exchange and correlation. Through the analysis of energy–volume variation, we find the phase transition of NaH from B1 to B2 structure occurs at 32.3 GPa, which in good agreement with the diamond-anvil-cell high-pressure experimental value of 29.3 ± 0.9 GPa [Phys. Rev. B 36 (1987) 7664]. By using the quasi-harmonic Debye model, the thermodynamic properties including the Debye temperature Θ_D, heat capacity C_V, thermal expansion coefficient α, and Grüneisen parameter γ are successfully obtained in the temperature range from 0 to 700 K and pressure ranges from 0 to 32 GPa and 33 to 100 GPa for NaH B1 and B2 phases, respectively. Analysis of band structures suggests that the NaH has an indirect band gap that the valence band maximum is at the W point and the conduction minimum locates at L point. The calculated energy gaps is very close to that value obtained in recent full potential augmented plane wave calculations. The optical properties including dielectric function ?(ω), absorption coefficient α(ω), reflectivity coefficient R(ω), and refractive index n(ω) are also calculated and analyzed.     

Fabrication, structural and electrical characterization of VO2 nanowires

The structural and electrical properties of VO₂ nanowires synthesized on Si₃N₄/Si substrates or molybdenum grids by a catalyst-free vapour transport method were investigated. The grown VO₂ nanowires are single crystalline and rectangular-shaped with a preferential axial growth direction of [1 0 0], as examined with various structural analyses such as transmission electron microscopy, electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, it was found that growing VO₂ nanowires directly on Si₃N₄ deposited molybdenum transmission electron microscopy grids is advantageous for direct transmission electron microscopy and electron diffraction characterizations, because it does not involve a nanowire-detachment step from the substrates that may cause chemical residue contamination. In addition to structural analyses, VO₂ nanowires were also fabricated into field effect transistor devices to characterize their electrical properties. The transistor characteristics and metal-insulator transition effects of VO₂ nanowires were investigated.     

Elastic properties and Vickers hardness of optically transparent glass-ceramics with fresnoite Ba2TiSi2O8 nanocrystals

Optically transparent glass-ceramics (40BaO-20TiO₂-40SiO₂ (mol%)) consisting of nonlinear optical fresnoite Ba₂TiSi₂O₈ (BTS) nanocrystals (diameter: 100-200 nm) are fabricated, and their elastic properties and deformation behavior are examined as a function of the volume fraction (f) of BTS nanocrystals using cube resonance and Vickers indentation techniques. The elastic properties such as Young's modulus (E) increases linearly with increasing the volume fraction of nanocrystals, e.g., E = 84 GPa for f = 0% (glass) to E = 107 GPa for f = 54.5%. The Vickers hardness (H_v) and indentation fracture toughness (K_c) increase from 5.0 to 6.0 GPa for H_v and 0.48 to 1.05 MPa m^−1/2 for K_c with increasing the volume fraction (from f = 0% to f = 54.5%), but they do not change linearly against the volume fraction of nanocrystals. It is suggested that BTS nanocrystals themselves induce a high resistance against deformation during Vickers indenter loadings.     

First-principles study of phase transition and elastic properties of XBi (X = Ce, Pr) compounds

The pressure-induced phase transitions of CeBi and PrBi compounds were investigated by using full-potential linearized augmented plane-wave (FP-LAPW) method. The calculations indicate that the transition pressure for CeBi compound from the NaCl-type (B1) structure to the body centered tetragonal (BCT) structure are 11.53 GPa from total energy (E)-volume (V) data and 6.48 GPa from equal Gibbs free energy (G). For PrBi compound, the same phase transition sequence occurred at 10.94 GPa obtained from the slope of the common tangent of E-V curves and 6.04 GPa from the equal G. The detailed structural changes during the phase transition were analyzed. From the elastic constants at zero pressure, we can conclude that B1 phase of XBi (X = Ce, Pr) compounds are mechanical stable, consistent with the experimental observations.     

Neutron diffraction study of magnetic ordering of the manganese bismuth chloro-sulfide: MnBiS2Cl

In quaternary compounds of Mn²⁺PnQ₂X (Pn = Sb, Bi; Q = S, Se; X = Cl, Br, I), Mn atoms in octahedral coordination (4 Q and 2 X) form waved layers separated by Pn atoms. The magnetic structure of the manganese bismuth chloro-sulfide MnBiS₂Cl has been determined by neutron powder diffraction, revealing a magnetic ordering with an incommensurate wave-vector along b-axis (k = [0, 0.3978, 0]) at 1.6 K. Two modulation models, sinusoidal and helicoidal, give quite equivalent magnetic reliability factors (R_mag = 0.0450 and 0.0481, respectively). The magnetic moment decreases with increasing temperature, to zero at T_N = 32 K. The evolution of the propagation wave-vector shows an irregularity at about 28 K. It could evidence two-phase transitions in agreement with the specific heat measurements. These results are compared to those of manganese antimony chloro-sulfide MnSbS₂Cl, isotypic with MnBiS₂Cl.     

High pressure Raman spectroscopy of ferrite MgFe2O4

An in situ Raman spectroscopic study was conducted to explore the pressure-induced phase transformation of ferrite MgFe₂O₄ to 51.6 GPa. Results indicate that MgFe₂O₄ transforms to a high pressure polymorphism at a pressure of 27.7 GPa, which was assigned to an orthorhombic structure. Upon release of pressure to ambient conditions, this high pressure polymorphism of MgFe₂O₄ remains stable. The crystallization of high pressure phase of MgFe₂O₄ is dominated by a diffusionless crystallizing mechanism.     

Nanoindentation study of magnetron-sputtered CrN and CrSiN coatings

CrN and CrSiN coatings were deposited on stainless steel substrate by reactive magnetron sputtering. The coatings were characterized for phases, chemical composition, microstructure, and mechanical properties by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM)/energy dispersive spectroscopy (EDS), atomic force microscopy (AFM), and nanoindentation technique, respectively. The cubic phase was the only phase observed in both the coatings as observed in XRD results. A dense morphology was observed in these coatings deposited with high nitrogen and Si contents, 50:50 and 18.65 at.%, respectively. Nanoindentation measurement of CrN coatings, with Ar + N₂ proportions of 60:40, showed maximum hardness (H) and modulus (E) of 21 ± 0.85 GPa and 276 ± 13 GPa, respectively. The CrN coatings deposited in pure N₂ atmosphere showed H and E values of 27 ± 1.62 and 241 ± 10 GPa, respectively. The measured H and E values of CrSiN coatings were found to be 28 ± 1.40 GPa and 246 ± 10 GPa, respectively. The improved hardness in both the coatings was attributed mainly to a reduction in crystallite size, decrease in surface roughness, and dense morphology. The incorporation of Si into the CrN coatings has improved both hardness and Young’s modulus.     

Control of growth orientation and shape for epitaxially grown In2O3 nanowires on a-plane sapphire

Vertically aligned indium oxide nanowires were grown on a-plane sapphire substrate by the method of catalyst-assisted carbothermal reduction. The morphology and crystal structure of the nanowires are determined by X-ray diffraction, transmission electron microscopy and field-emission scanning electron microscopy. Two types of In₂O₃ nanowires were found by controlling the growth conditions. The nanowires with a hexagonal cross-section were shown to grow in [1 1 1] direction, whereas those with a square cross-section grow in [0 0 1] direction. In addition to the temperature effects, the concept of supersaturation in Au catalyst is proposed to explain the formation of these two types of nanowires. Besides, tapering, which is explained with the interplay between the vapor-liquid-solid and vapor-solid growth mechanisms, is observed in the nanowires.     

Influence of oxygen pressure on elastic strain and excitonic transition energy of ZnO epilayers prepared by pulsed laser deposition

High quality ZnO epilayers (χ_min ∼ 10%) were prepared on Al₂O₃ (0 0 0 1) substrates at a temperature of 750 °C by pulsed laser deposition (PLD) with oxygen pressure of 0.015, 0.15, 1.5, and 15 Pa. The best crystalline quality and strongest intensity of UV photoluminescence were observed on ZnO layer with oxygen pressure of 15 Pa. It is probable due to the higher oxygen pressure lessens oxygen deficiency in the film. The tetragonal distortion e_T, which is caused by elastic strain in the epilayer, was determined by Rutherford backscattering/channeling. It reduces as a whole (from 0.93 to 0.65%) with the increase of oxygen pressure from 0.015 to 15 Pa and the excitonic transition energy simultaneously shows a weak blue shift.     

Nanoindentation of laser micromachined 3C-SiC thin film micro-cantilevers

Single crystalline thin films of 3C-SiC with a thickness of 1.7 ± 0.2 μm were deposited on Si (100) substrate using atmospheric chemical vapor deposition technique. A Q-switched Nd:YAG laser in the fundamental wavelength with a pulse duration of 100 ns and average power of 1 W was then used to pattern 50 μm wide and 150 μm long cantilever beams in direct-writing mode. Following laser patterning, wet chemical etching using KOH anisotropic etchant was carried out to remove the underlying Si and form free-standing 3C-SiC cantilever beams. The cantilevers were subjected to nanoindentation test to obtain deflection versus load curves. The average Young’s modulus and fracture strength were determined to be 423 GPa and 1.5 GPa respectively which are comparable to those obtained by the reactive ion etching. Laser patterning thus offers nearly identical properties as that of ion etching with the added benefit of much higher etch rates.     

Synthesis of aluminium borate nanowires by sol-gel method

A sol-gel process followed by annealing was employed to fabricate single crystal aluminium borate (Al₄B₂O₉ and Al₁₈B₄O₃₃) nanowires. The diameter of Al₄B₂O₉ nanowires synthesized at 750 °C annealing is ranging from 7 to 17 nm, and that of Al₁₈B₄O₃₃ nanowires synthesized at 1050 °C annealing is about 38 nm. Instead of the well-known vapor-liquid-solid (VLS) mechanism, self-catalytic mechanism was used to explain the growth of the nanowires.     

Research on the novel GeSe2-In2Se3-KBr chalcohalide optic glasses

(1 − x)(0.75GeSe₂-0.25In₂Se₃) − x(1/3In₂Se₃-2/3KBr) (x = 0, 0.15, 0.3, 0.45, 0.525) chalcohalide glasses were prepared by traditional melt-quenching method and its glass-forming region was determined. The physical, thermal and optical properties of the GeSe₂-In₂Se₃-KBr ternary glass system are reported. The results show that the GeSe₂-In₂Se₃-KBr glass system has relatively high glass transition temperature (T_g = 286-335 °C) and good thermal stability. With increasing KBr content from 0 to 35 mol%, a blue-shift from 750 to 620 nm at the visible absorbing cutting-off edge is observed. The red-shifting of the transmission cutting-off edge at the long-wave IR band occurs linearly with increasing KBr. The allowed direct transition and indirect transition of samples were calculated according to the classical Tauc equation. The direct optical band gaps and indirect optical band gaps were in the range from 1.649 to 1.931 eV and 1.513 to 1.863 eV, respectively.     

Supercritical hydrothermal synthesis of Cu2O(SeO3): Structural characterization, thermal, spectroscopic and magnetic studies

Cu₂O(SeO₃) has been synthesized in supercritical hydrothermal conditions, using an externally heated steel reactor with coupled hydraulic pump for the application of high pressure. The compound crystallizes in the P2₁3 cubic space group. The unit cell parameter is a = 9.930(1) Å with Z = 12. The crystal structure has been refined by the Rietveld method. The limit of thermal stability is, approximately, 490 °C. Above this temperature the compound decomposes to SeO₂(g) and CuO(s). The IR spectrum shows the characteristic bands of the (SeO₃)²⁻ oxoanion. In the diffuse reflectance spectrum two intense absorptions characteristic of the Cu(II) cations in five-coordination are observed. The ESR spectra are isotropic from room temperature to 5 K, with g = 2.11(2). The thermal evolution of the intensity and line width of the signals suggest a ferromagnetic transition in the 50-45 K range. Magnetic measurements, at low temperatures, confirm the existence of a ferromagnetic transition with a critical temperature of 55 K.     

Synthesis and high-pressure sintering of (W0.5Al0.5)C

A new solid solution of Al in WC, which can be expressed by the chemical formula (W_0.5Al_0.5)C, has been synthesized directly by reaction milling (RM) of a W_0.5Al_0.5 alloy and the proper amount of carbon. The total reaction time is about 50 h. The ESEM photograph shows that the prepared (W_0.5Al_0.5)C powders are spherical, and the average particle size is about 40 nm. (W_0.5Al_0.5)C has been identified to crystallize in the hexagonal space group P-6m2 (No.187) and belongs to the WC structure type. The lattice parameter of (W_0.5Al_0.5)C is calculated to be a = 2.908(1) Å, c = 2.836(1) Å. This nanocrystalline powder can be well sintered at the high temperature (1600 °C) under the high pressure (4.5 GPa), and the relative density reaches 99.1%. The hardness of the sintered (W_0.5Al_0.5)C is tested to be 1500 ± 50 kg mm⁻², while the density is about 9.417 ± 0.003 g cm⁻³, which is far lower than that of WC.     

High-pressure phase transition in Ho2O3

High-pressure X-ray diffraction and Raman studies on holmium sesquioxide (Ho₂O₃) have been carried out up to a pressure of ∼17 GPa in a diamond-anvil cell at room temperature. Holmium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase transition at around 9.5 GPa. The high-pressure phase has been identified to be low symmetry monoclinic type. The two phases coexist to up to about 16 GPa, above which the parent phase disappears. The high-pressure laser-Raman studies have revealed that the prominent Raman band ∼370 cm⁻¹ disappears around the similar transition pressure. The bulk modulus of the parent phase is reported.