Ab initio calculations of the thermodynamics and phase diagram of ScSb

The hydrostatic pressure-induced transition phase of ScSb from the sixfold-coordinated NaCl-type (B1) to the eightfold-coordinated CsCl-type (B2) is investigated within the Perdew–Burke–Ernzerhof (PBE) form of generalized gradient approximation. It is found that the transition pressure from the B1 to B2 phase is at 42.8 GPa which agrees with experiment. Through the quasi-harmonic Debye model, the dependences of the relative volume V on the pressure P, the thermal expansion, the Grüneisen parameter ratio, (γ − γ ₀)/γ ₀, the Debye temperature Θ, and heat capacity C _V on the pressure P and temperature T are estimated. The quasi-harmonic Debye model for the first time is used to predict phase diagram of B1 → B2.     

Charge Order in (TMTTF)2PF6 Investigated by Infrared Spectroscopy

To explore novel physical phenomena related to strong π-d interaction, we measured the resistivity (ρ) and magnetoresistance of the first organic ferrimagnetic π-d system, (EDT-TTFVO)₂FeBr₄ under high pressures up to 8 GPa, where EDT-TTFVO denotes ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide. At ambient pressure, ρ(T) exhibits resistivity minimum near T_min ∼ 170 K followed by a gradual increase below it. With increasing pressure, T_min abruptly decreases till 4 GPa, beyond which it slightly increases. The increase ofT _min above 4 GPa is discussed in terms of the enhancement of the π-d interaction by applying pressure.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

Phase transitions and hard magnetic properties for rapidly solidified MnAl alloys doped with C,B, and rare earth elements

MnAl alloys are attractive candidates to potentially replace rare earth hard magnets because of their superior mechanical strength, reasonable magnetic properties, and low cost. In this study, the phase transitions and magnetic properties of melt spun Mn₅₅Al₄₅ based alloys doped with C, B, and rare earth (RE) elements were investigated. As-spun Mn–Al, Mn–Al–C, and Mn–Al–C–RE ribbons possessed a hexagonal ε crystal structure. Phase transformations between the ε and the L1₀ (τ) phase are of interest. The ε → τ transformation occurred at ~500 °C and the reversed τ → ε transformation was observed at ~800 °C. Moderate carbon addition promoted the formation of the desired hard magnetic L1₀ τ-phase and improved the hard magnetic properties. The Curie temperature T _C of the τ phase is very sensitive to the C concentration. Dy or Pr doping in MnAlC alloy had no significant effect on T _C. Pr addition can slightly improve the magnetic properties of MnAlC alloy, especially J_S. Doping B could not enhance the magnetic properties of MnAl alloy since B is not able to stabilize either the ε phase or the L1₀ hard magnetic τ phase.     

Mathematical Model for Transitional Processes in Josephson Cryotrons Based on Tunnel Junctions

In this work, we consider controlling of logical states of Josephson memory cells (cryotrons) based on superconductor-insulator-superconductor (SIS) Josephson tunnel junctions by external current impulses. A mathematical model for the transitional processes that take place during direct logical transitions “0” → “1” and inverse logical transitions “1” → “0” is proposed. By means of mathematical modeling, we investigate transitional processes in cryotrons during the change of their logical state and obtain their transitional characteristics for operational temperatures T ₁=11.6 K and T ₂=81.2 K, close to the boiling temperatures of helium and nitrogen, respectively. It is shown that such memory cells can efficiently operate under the temperature T ₂=81.2 K. The behavior of the Josephson cryotrons as well as their operational stability is explored.     

Surface form memory by indentation and planarization of NiTi: displacements and mechanical energy density during constrained recovery

Indentation-induced two-way shape memory leads to pronounced temperature dependence of the depth of spherical indents made in martensitic NiTi shape-memory alloys. They are shallower when austenitic, and depth varies during both M → A and A → M transformations. If the impression is planarized, by metallographic grinding at T < M _f , a protrusion rises at the site when warmed past A _f . If cooled again this “exdent” retreats, restoring optical flatness. The cycle is repeatable, and exdent heights can exceed 15% of prior indent depth. Since it maps between macroscopically distinguishable topographies, or forms, at orders greater length scale than the surface roughness, we call the effect “surface form memory”—SFM. Notable regarding potential applications is that, when loaded in compression by planar contact with a strong base metal, exdents exert sufficient pressure to indent the latter, suggesting that subsurface transformational mechanisms operate at volumetric work-energy densities >10⁷ J/m³, fully ~10% of the M → A enthalpy.     

Pressure induced structural behaviour in f-electron based AB, AB2 and AB3 intermetallics

The rare-earth and actinide based compounds are endowed with several exotic physical and chemical properties due to the presence of f-electrons. Under pressure, the nature of f-electrons can be changed from localized to itinerant, leading to significant changes in their structural, physical and chemical properties. The present review on these f-electron based binary intermetallics compounds is an outcome of a detailed literature survey as well as our own research` during the last one decade. It attempts to bring out the structural sequences observed among the various homologues and their correlations with their electronic structure. It is seen that the majority of the AB type compounds show the NaCl to CsCl type structural transformation; whereas the AB₃ type compounds stabilizing in cubic structure at STP, remain stable over a wide pressure range. However, the AB₂ type compounds exhibit a variety of structural transitions, which broadly fall into the following sequence: MgCu₂ →.......... → CeCu₂ → AlB₂ → ZrSi₂ → ThSi₂ → SmSb₂ →.......... Further, the structural transitions, the transition pressures and bulk modulii values in any homologous series are seen to follow a systematic trend with respect to the atomic numbers of their constituent elements.     

Nonlinear electrical properties and aging characteristics of (NiO, MgO, Cr2O3)-doped Zn-Pr-Co-R (R = Y, Er) oxide-based varistors

The electrical properties and stability of the varistors, which composed of (NiO, MgO, Cr₂O₃)-doped Zn-Pr-Co-R (Y, Er) oxide-based ceramics, were investigated for different additives. The breakdown voltage of the varistors increased in order of NiO→undoped→MgO→Cr₂O₃: 1200→1551→1691→1959 V/cm for ZPCY system and undoped→NiO→MgO→Cr₂O₃: 1024→1041→1500→1668 V/cm for ZPCE system, respectively. The nonlinear coefficient value increased in order of undoped→NiO→MgO→Cr₂O₃: 21→25→38→50 in ZPCY system and NiO→undoped→MgO→Cr₂O₃: 27→32→35→38 in ZPCE system, respectively. In ZPCY and ZPCE systems, the Cr₂O₃-additives most greatly improved the nonlinear properties. In Cr₂O₃-doped system, ZPCY system exhibited higher nonlinear properties than that of ZPCE system. The stability against d.c. accelerated aging stress was higher in Cr₂O₃-additives than in NiO- and MgO-additives for ZPCY system and was higher in NiO-additives than in MgO- and Cr₂O₃-additives for ZPCE system.     

Two-step sol–gel method for preparing MgO–MnO2–B2O3–Li2O co-added core-shell structural Ba0.55Sr0.40Ca0.05TiO3 powders and the dielectric properties of the composite ceramics

The Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO (BSCT–MgO) composite powders possessing core-shell structure, small particle size and high specific surface area were prepared by the novel two-step sol–gel method which was based on the citric acid (CA)–ethylene glycol (EG) system. In this experiment, the Ba_0.55Sr_0.40Ca_0.05TiO₃ (BSCT) powders were firstly synthesized by chemical co-precipitation method. MgO [(MgO/BSCT)_mass = 1] and MnO₂ [(MnO₂/BSCT)_mole = 0.01] were added into the BSCT powders in the first sol–gel step. 1.5 wt% B₂O₃–Li₂O as sintering aids was added into the composite powders during the second sol–gel step. The BSCT–MgO composite powders were detected to have a perfect core-shell structure which was detected by the transmission electron microscope. Perovskite BSCT and periclase MgO were confirmed to exist in the BSCT–MgO composite powders according to the X-ray diffraction patterns. The Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO (BSCT–MgO) composite ceramics that synthesized with the core-shell powders were sintered at 1,000 °C for 2 h. According to the scanning electron microscope images, the grain sizes of BSCT and MgO in the BSCT–MgO composite ceramics were in the range of 0.5–2.0 μm and 1.0–2.5 μm, respectively. The dielectric constant of the BSCT–MgO composite ceramics was 210 and the dielectric loss was 0.0012 when tested at 1 MHz, room temperature. The BSCT–MgO composite ceramics were expected to be a promising candidate for applying as phase shifters or tunable components, etc, in the microwave field.     

Effect of aging on martensitic transformation behavior of Ti48.8Ni50.8V0.4 alloy

In this article, the effect of aging on martensitic transformation of Ti_48.8Ni_50.8V_0.4 alloy was investigated. The results show that the martensitic transformation of the solution-treated shape memory alloy is a typical single-stage transformation process. The transformation temperatures of the samples aged at different temperatures for 0.5 h were lower as compared to that of the solution-treated alloy. With the increase of aging temperature, the transformation temperatures increase. After aging at 500 °C, the samples exhibit a multiple-stage transformation. The samples after aging at 500 °C for more than 5 h resulted in the transformation sequence of A → R→M1 and A → M2 upon cooling and M2 → A and M1 → A upon heating.     

Thermal equation of state of natural chromium spinel up to 26.8 GPa and 628 K

A pressure–volume–temperature data set has been obtained for natural chromium spinel, using synchrotron X-ray diffraction with a resistance heated diamond-anvil cell (RHDAC). The unit cell parameter of the chromium spinel was measured by energy dispersive X-ray diffraction up to pressures of 26.8 GPa and temperatures of 628 K. No phase change has been observed. The observed P–V–T data were fit to the high-temperature Birch-Murnaghan equation of state, with V ₀ fixed at its experimental value, yields K ₀ = 209 ± 9 GPa, (∂K/∂T)_P = −0.056 ± 0.035 GPa K⁻¹, and α₀ = 7±1 × 10⁻⁵ K⁻¹. The temperature derivative of the bulk modulus (∂K/∂T)_P of chromium spinel is determined here for the first time. The obtained K ₀ is slightly higher than the previous results of synthetic spinel. We suggest that Fe²⁺–Mg²⁺ substitution is responsible for the high bulk modulus of chromium spinel.     

Coherent elastic energy calculation of ω particles in β matrix for Zr–Nb alloys

The misfit and coherent elastic energy caused by ω particles in β matrix is quantitatively calculated in this study. First, the coherent strain matrixes for four ω variants are established including the misfit parameters based on Khachaturyan’s theory. Then, the misfit and coherent elastic energy in athermal β → ω transition, and isothermal β → ω transition are calculated, respectively. The calculation results indicate that the coherent elastic energy gets maximum value when x _Nb = 0.08 (Nb content) and gets minimum value when x _Nb = 0.1518 in quenching Zr–Nb alloys, which are in fair agreement with experimental results. For isothermal β → ω transition, the misfit and coherent elastic energy depend on composition and aging temperature. The misfit caused by isothermal ω phase is much larger than the one caused by athermal ω phase. This results in larger coherent elastic energy in isothermal β → ω transition. In addition, the misfit is found as an approximate linear function of temperature and composition for Zr–Nb alloys, and the coherent elastic energy is revealed as an increasing function of |v _F –v _S | for the two kinds of transition.     

Microstructure of TiNi shape-memory alloy synthesized by explosive shock-wave compression of Ti–Ni powder mixture

Cylinders of TiNi shape-memory alloy were synthesized from mixtures of equiatomic fine irregular titanium and nickel powders by explosive-wave compression with a detonation velocity of about 6500 m s^-1. B2 type parent phase, R phase, B19′ type martensite, Ti₂Ni, Ti₃Ni₄ and Ti₂Ni₃ phases were observed in this as-synthesized material. In the B2 matrix high density dislocations existed. The Burgers vectors of many dislocations were determined to be parallel to directions. The R phase variants formed (0 0 1) B2 twinning structure. The substructure of the B19′ martensite was (0 0 1) B19′ type I twin and stacking faults on the (0 0 1) B19′ plane. When increasing the temperature of the as-synthesized material in a differential scanning calorimeter, no B19 ′ → R → B2 transitions were observed on the temperature range −50 to 100 °C. However, B2 → B19′(R) transitions occurred during the cooling cycle. After heat treating the specimen at 800 °C for 1 h and then ageing at 400 °C for 10 min, both B2 → R → B19′ and B19′(R) → B2 phase transitions were observed. This revised version was published online in November 2006 with corrections to the Cover Date.     

First report on observation of abnormal creep in a Zr–2.5wt.%Nb alloy at low stresses

Low-stress creep behaviour of a two-phase Zr–2.5%Nb alloy, differently heat treated, has been investigated using helical test specimens. The phase diagram of the α (hcp) + β (bcc) alloy is characterized by the monotectoid reaction at 893 ± 10 K: β ₁ → (α + β ₂) where β ₁ (Zr–20Nb) and β ₂ (Zr–80Nb) have widely differing compositions. At the creep testing temperature, 818 K, which is close to but below the monotectoid temperature, the creep rate for samples with the equilibrium α + β ₂ structure has been found to be considerably higher, over an order of magnitude, than that in samples with the metastable α + β ₁ structure. Microstructural changes accompanying the markedly enhanced creep rate for the α + β ₂ structure at stresses as low as 1–4 MPa have been explained in terms of the relative stability of the β ₁ and the β ₂ phases during the creep process. An attempt has been made to elicit the likely mechanism underlying the observed enhancement of creep rate and the changes in morphology, composition and volume fraction of the β phase.     

Luminescence properties and electronic structure of Sm<Superscript>3+</Superscript>-doped YAl<Subscript>3</Subscript>B<Subscript>4</Subscript>O<Subscript>12</Subscript>

The luminescence properties of Sm³⁺ ions in YAl₃B₄O₁₂ were studied upon synchrotron excitation in the 3.8–11 eV region. In addition to the 4f → 4f excitation bands, the excitation spectra of the Sm³⁺ emission contain broad bands at 6.1 and ~7.0 eV. These bands are attributed to charge transfer transition in Sm³⁺–O²⁻ complexes and 4f → 5d transition of Sm³⁺ ions, respectively. The optical absorption edge of YAl₃B₄O₁₂ was determined at 7.3 eV. A comparison with the results of electronic structure calculations on YAl₃B₄O₁₂ is also made.     

Synthesis and characterization of BaTiO<Subscript>3</Subscript>-based X9R ceramics

The effects of (Na_0.5Bi_0.5)TiO₃ (NBT) and MgO addition on the dielectric properties and microstructures of BaTiO₃ (BT) ceramics were investigated. NBT was first added to Nb₂O₅-doped BT system. As NBT content increases from 0 to 0.2 mol, the Curie temperature of the systems shifts to high temperatures and dielectric constant peak at T _c is suppressed evidently. The variation of capacity (ΔC/C _20 °C (%)) of the system at 200 °C decreases with increasing NBT content from 0.1 to 0.2 mol, but that of −55 and 125 °C increases monotonously. The stable temperature characteristics of the dielectric properties improved by NBT doping would be connected with the distortion and deformation of the structure induced by substitution of Na⁺ and Bi³⁺ into Ba sites. MgO was employed to further flatten the ΔC/C _20 °C–T curve. It is very helpful for this ceramic system to satisfy the requirement of EIA-X9R specification on ΔC/C _20 °C and still keep a satisfied dielectric constant. The addition of MgO improved effectively the temperature stability of the dielectric properties. Changes of the crystalline structure and microstructure induced by MgO doping might contribute to these improvements.     

Fabrication of conductive SrRuO<Subscript>3</Subscript> thin film and Ba<Subscript>0.60</Subscript>Sr<Subscript>0.40</Subscript>TiO<Subscript>3</Subscript>/SrRuO<Subscript>3</Subscript> bilayer films on MgO substrate

Conductive SrRuO₃ (SRO) thin films have been grown on (100) MgO substrates by pulsed laser deposition (PLD) technique. Effects of oxygen pressure and deposition temperature on the orientation of SRO thin film were investigated. X-ray diffraction (XRD) θ/2θ patterns and the temperature dependent resistivity measurements indicated that oxygen pressure of 30 Pa and deposition temperature of 700 °C were the optimized deposition parameters. A parallel-plate capacitor structure was prepared with the SRO films deposited under optimized condition as an electrode layer and Ba_0.60Sr_0.40TiO₃ (BST) thin film as the dielectric layer. XRD Φ scans indicated a epitaxial relationship between BST and SRO on MgO substrate. The dielectric constant and loss tangent measured at 10 kHz and 300 K was 427 and 0.099 under 0 V bias, and 215 and 0.062 under 8 V bias, respectively. A tunability of 49.6% has been achieved with DC bias as low as 8 V. The C–V hysteresis curve and the P–E hysteresis loop suggested that the BST films epitaxially grown on SRO/MgO have ferroelectricity at room temperature. The induced ferroelectricity was believed to originate from the compressive strain between the epitaxial BST and SRO thin films. These results show the potential application of the BST/SRO heterostructures in microelectronic devices.     

Vacuum arc deposition of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–ZrO<Subscript>2</Subscript> coatings: arc behavior and coating characteristics

Al₂O₃–ZrO₂ coatings were deposited using a vacuum arc deposition system equipped with two co-planar cathodes. The plasma was injected into a cylindrical magnetic duct through annular anode apertures toward a substrate or an electrostatic ion current probe positioned on the duct axis, in vacuum and in a low-pressure oxygen or argon + oxygen background. Ion current and arc voltage measurements and visual observation of the cathode spots were used to find stable arcing conditions, using a straight plasma duct configuration. The cathode spot operation and transport of the plasma beam in the duct were studied as a function of arc current (I _arc = 25–200 A) and oxygen or oxygen + argon pressures (P = 0.1–1.5 Pa). Coatings were fabricated by exposing Si or WC–Co substrates simultaneously to Al and Zr plasmas using a 1/8 torus filter configuration in O₂ + Ar pressures. The coating composition, structure, microhardness, adhesion, and wear behavior were studied as functions of the deposition parameters. Favorable conditions for stable arcing were obtained with I _arc = 75 and 100 A for Al and Zr plasmas, respectively. The ion current decreased, and the arc voltage increased with the oxygen pressure. Behavior of the ion current and arc voltage suggested that cathode poisoning started at P = 0.5 Pa. Deposition rates were 0.3-0.6 μm/min, depending on the substrate position. All coatings were “Zr rich”, i.e., the Zr:Al ratio was in the range of 1.2–5.6 depending on the substrate position and deposition conditions. The coatings with higher ZrO₂ concentration were harder and had better resistance to wear. The coating’s hardness reached a maximum of ~22–24 GPa at a deposition temperature of 500 °C or a negative bias voltage of 75–100 V.     

The π<Subscript>v</Subscript>-index: a new indicator to characterize the impact of journals

For determining the eminence of scientific journals, a new indicator stressing the importance of papers in the “elite set” (i.e., highly cited papers) is suggested. The number of papers in the elite set (P _πv) is calculated with the equation: (10 log P) − 10, where P is the total number of papers in the set. The one-hundredth of citations (C) obtained by P _πv papers is regarded as the π_v-index which is field and time dependent. The π_v-index is closely correlated with the citedness (C/P) of P _πv papers, and it is also correlated with the Hirsch-index. Three types of Hirsch-sets are distinguished, depending on the relation of the number of citations received by the Hirsch-paper (ranked as h) and the paper next in rank (h + 1) by citation. The h-index of an Anomalous Hirsch-set (AH) may be increased by a single citation to a paper outside the Hirsch-core. (A set of papers may be regarded as AH, where the number of citations to the Hirsch-paper is higher than the h-index and the next paper in rank shows as many citations as the value of the h-index.)     

Fabrication of epitaxial conductive LaNiO3 films on different substrates by pulsed laser ablation

LaNiO₃ (LNO) thin films were deposited on (1 0 0) MgO, SrTiO₃ (STO) and LaAlO₃ (LAO) crystal substrates by pulsed laser deposition (PLD) under 20 Pa oxygen pressure at different substrate temperatures from 450 to 750 °C. X-ray diffraction (XRD), ex situ reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM) were employed to characterize the crystal structure of LNO films. LNO films deposited on STO and LAO at a temperature range from 450 to 700 °C exhibit high (0 0 l) orientation. XRD ψ scans and RHEED observations indicate that LNO films could be epitaxially grown on these two substrates with cubic-on-cubic arrangement at a wide temperature range. LNO films deposited at 700 °C on MgO (1 0 0) substrate have the (l l 0) orientation, which was identified to be bicrystalline epitaxial growth. La₂NiO₄ phase appears in LNO films deposited at 750 °C on three substrates. The epitaxial LNO films were tested to be good metallic conductive layers by four-probe method.