Polar nanodomains and relaxor behaviour in (1 − x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 crystals with x = 0.3–0.5

Phase transitions and dielectric properties of the (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ crystals with x = 0.3–0.5 are studied. The solid solutions in this composition range are shown to be relaxor ferroelectrics. The crystals with low x demonstrate a diffused maximum in the temperature dependences of the dielectric permittivity at T_m. T_m varies with frequency according to the Vogel–Fulcher law. The polarizing microscopy investigations reveal a first-order phase transition from the relaxor phase to the low-temperature ferroelectric phase at T_C, which is several degrees below T_m. The permittivity peak in the crystals with x = 0.5 is sharp, and T_m is equal to T_C and does not depend on frequency, as is typical of the transition from a ferroelectric to an ordinary paraelectric phase. Nevertheless, the relaxor, but not the paraelectric, phase is observed at T > T_m. This conclusion is confirmed by the observation of the temperature behaviour of complex dielectric permittivity at T > T_m, which is typical of relaxors and related to the existence of polar nanodomains.     

Effect of ferroic domain pattern changes on the Raman spectra of some ferroic crystals

The influence of changes in the pattern of ferroic domain structure on the Raman spectra of β-LiNH₄SO₄ and (NH₄)₃H(SO₄)₂ single crystals were studied. It was shown that the Raman spectra of β-LiNH₄SO₄ passed from the ferroelastic phase differ from those of “as-grown” crystal and those of the crystal, which was in the paraelectric phase. Significant changes could be observed in the Raman bands related to triply degenerated ν₃ and ν₄ vibrations of the SO₄ tetrahedron. Detailed temperature studies of the Raman spectra of β-LiNH₄SO₄ close to the paraelectric–ferroelectric phase transition, exhibit anomaly of some internal vibrations of SO₄ in the temperature range where a regular large-scale structure is observed. Different types of evolution of the ferroelastic domain structure and temperature behaviour of the donor and acceptor vibrations were shown while heating and cooling the (NH₄)₃H(SO₄)₂ crystal. Different values of temperature hysteresis were found in temperature studies of the ferroelastic domain structure (ΔT_S  3–5 K) and in Raman spectra studies (ΔT_S  12 K). No changes were observed in the pattern of ferroelastic domain structure at the temperature T_II–III  265 K, at which C2/c → P2/n structural phase transition takes place. On the other hand, at T_III–IV  135 K additional domains with W′-type of domain wall orientation were found.     

A model for predicting fatigue crack growth behaviour of a low alloy steel at low temperatures

In the present study, a model to predict the fatigue crack growth (FCG) behaviour at low temperatures is proposed for a low alloy steel (16 Mn). The experimental results indicate that fatigue ductile-brittle transition (FDBT) occurs in 16 Mn steel and the FDBT temperature (T_FDBT) is about 130 K. When T > T_FDBT, the FCG mechanism in the intermediate region is the formation of ductile striation and the FCG rates decrease with decreasing temperature. When T ≈ T_FDBT, the FCG mechanism changes into microcleavage and the fatigue fracture toughness K_fc of the steel decreases sharply. The FCG rates tend to increase as the temperature is further reduced. The test data of the FCG rates are well fitted by the formula developed by Zheng and Hirt. An approximate method to predict ΔK_th of the steel at low temperatures is proposed and then a general expression of the FCG rates is given at temperatures ranging from room temperature to T_FDBT. By means of the expressions proposed in this paper, the FCG rates at low temperatures can be predicted from the tensile properties if the endurance limit σ₋₁ and δk_th, at room temperature are known. Finally, a model for FDBT is tentatively proposed. Using this model, one can predict T_FDBT from the ductile-brittle transition curve determined from impact or slow bending tests of cracked Charpy specimens.     

Epitaxial growth of AB type compounds under quasi-equilibrium conditions

Single-crystal films of CdS, CdSe and CdTe have been grown in vacuum on mica (fluophlogopite and muscovite) under isothermal conditions, i.e. with T_ev ≈ T_ep ≈ T_gr where T_ev and T_ep are the evaporation and epitaxial temperatures respectively and T_gr is the growth temperature. The synthesis was carried out in the temperature range 430°–800°C in the case of CdS, 300°–650°C for CdSe and 270°–550°C for CdTe. It is shown that the growth rate of single crystal layers (V_gr) depends exponentially on the growth temperature: V_gr (Å/sec) = D exp (−E/RT_gr) Perfect epitaxial CdS, CdSe and CdTe films have a wide range of electrophysical properties. Co-evaporation of CdS and sulphur and of CdSe and selenium allowed high-resistance films of cadmium sulphide and cadmium selenide of both n- and p-types to be obtained.     

Electron temperature measurements in the TBR-1 tokamak by the two-foil absorbing method

The electron temperature of the plasma confined by the TBR-1 tokamak was measured using the method of soft-X-ray absorbing foils. The radiation flux was detected by pairs of surface barrier detectors after being passed through beryllium foils of two different thicknesses, 2 and 5 kÅ. Local electron temperatures were obtained, giving T_e(r = 0) ≈ 20.5 eV and T_e(r = ± 3.8 cm) ≈ 85 eV.     

Effect of Ca2PbO4 on the formation of the (2223) phase in the Bi---Pb---Sr---Ca---Cu---O system

The effect of excess Ca₂PbO₄ on the superconducting properties of Bi_1.7Pb_0.3Sr₂Ca₂Cu₃O_yB_x (B ≡ Ca₂PbO₄) is investigated through X-ray diffraction, resistivity and a.c. magnetic susceptibility measurements. The Xpray diffraction results show that the volume friction of the low T_c (221s) phase decreases and that of (2223) phase increases with the addition of Ca₂PbO₄. For x = 0.6 and 1.5, only the (2223) phase exists. At higher Ca₂PbO₄ concentrations, i.e. for x = 3.0, the low T_c phase again appears along with the high T_c phase. Diffraction peaks appear at d = 2.02, 2.814 and 2.85, which are characteristic peaks of Ca₂PbO₄, and their intensities increase with increasing x, indicating that Ca₂PbO₄ exists in the lattice in the same form. However, the T_c(0) values decreased gradually from 107 to 98 K with increasing Ca₂PbO₄ concentration. Compared with the undoped sample, the width of the transition δT_c is broad for impurity-added samples and reaches a maximum value for x = 3.0. Pure and Ca₂PbO₄-doped samples showed two peaks in ξ″ vs. T curves and two steps in ξ′ vs. T curves. The first peak is close to the transition temperature and corresponds to the midpoint of the first step in the ξ′ curve. The second peak appears below 98 K and this peak maximum corresponds to the midpoint of the second step. The position of both sets of peaks remained almost the same up to x = 1.5 and shifted towards lower temperatures thereafter. The role of Ca₂PbO₄ in the growth of the (2223) phase and the mechanism of energy loss are discussed.     

Magnetoacoustic measurements on pure and Zn doped YBCO crystals

Investigations on the temperature dependence in the range from 4 to 100 K of the ultrasonic attenuation (T, B) and the relative change Δv(T,B)/v of the sound velocity for various applied magnetic fields have been made. The ultrasonic attenuation was measured by the pulse-echo technique and the relative change of the sound velocity Δv/v by a phase-comparison method. The measurements serve for the examination of the elastic behaviour of the flux line lattice (FLL) of pure and Zn doped YBCO crystals and YBCO/Ag composites. These crystals are important for applications as permanent magnets. In the framework of the thermally assisted flux flow model (TAFF) the activation energies U(B) were deduced from the ultrasonic data.     

Transient phenomena and their effect on the insulator barrier in Al-(Al oxide)-Al structures: T = 300°K

The application of a constant voltage (≈1 V) to an Al-(Al oxide, 35 Å)-Al tunnelling structure at T≈300°K results in a current which initially decreases with time for the base electrode positive, but increases with time when the base electrode is negative. It is shown that these changes are connected with an increase or a decrease, respectively, of the insulator barrier. This paper reports investigations which are analogous to those at T100°K described recently. A model which involves electron trapping and ionic processes was proposed for explaining the transient phenomena below about 100°K, and this model also applies for the present experiments.     

熔融纺丝过程优化制备细直径碳化硅纤维及其对力学性能的影响

强度、模量和柔顺性作为碳化硅(SiC)纤维重要的力学性能受到纤维直径大小的影响, 而制备工艺中的熔融纺丝过程对纤维直径起决定作用。本工作研究了纺丝温度、纺丝压力和卷绕速度对聚碳硅烷(Polycarbosilane, PCS)原纤维直径的影响, 分析了纺丝过程中纤维断裂的原因, 并初步探究了SiC纤维直径与力学性能的关系。结果表明, 在一定范围内降低纺丝温度、降低纺丝压力和提高卷绕速度均能显著减小原纤维的直径。在连续纺丝的前提下, 最优纺丝工艺下得到的PCS原纤维直径为13.5 μm。随着PCS纤维直径由18.3 μm减小至13.5 μm, SiC纤维直径则由13.8 μm减小至9.5 μm, 而SiC纤维的强度与模量分别由1.7、181 GPa提高至2.9、233 GPa, 强度分布更为集中, 柔顺性得到显著提高。     

(1-x)K0.48Na0.52NbO3-xBi0.45Nd0.05(Na0.92Li0.08)0.5ZrO3无铅压电陶瓷相结构及压电性能

为了在获得较高压电性能的同时又不大大降低陶瓷的居里温度(T_C), 设计和制备了Bi_0.45Nd_0.05(Na_0.92Li_0.08)_0.5ZrO₃改性的K_0.48Na_0.52NbO₃系无铅压电陶瓷((1-x)KNN-xBNNLZ), 研究了BNNLZ含量对KNN基无铅压电陶瓷相结构和电学性能的影响。研究结果表明, 所有陶瓷样品均具有较高的居里温度T_C(>300℃)。随着BNNLZ含量的增加, 陶瓷的正交-四方相变温度(T_O-T)不断向低温方向移动, 而三方-正交相变温度(T_R-O)不断向高温方向移动, 最终在陶瓷中形成了三方-四方(R-T)共存相, R-T共存相处于0.05<x<0.07范围。BNNLZ的加入引起陶瓷相结构的演化改变导致压电常数(d₃₃ )、介电常数(ε_r )、剩余极化强度 (P_r )和机电耦合系数(k_p )都先增大后减小, 当x=0.06时陶瓷具有最佳压电性能: d₃₃=313 pC/N, k_p=42%, P_r=25.48 μC/cm², ε_r=1353, tanδ=2.5%, T_C=327℃。     

Vapour-liquid equilibrium of ternary mixtures of the refrigerants R32, R125 and R134a

Ternary mixtures of R32, R125 and R134a of different compositions are recommended for replacing refrigerants R22 and R502. As a prerequisite for reliably calculating vapour pressure and phase equilibria of ternary mixtures within the relevant range of temperature and composition, VLE data of the three binary systems R32/R134a, R125/R134a and R32/R125 have been measured from −70°C up to the critical temperature. The real mixing behaviour of these binary systems is described by simple cubic equations of state, based only on precise experimental data of the critical properties and one value of the vapour pressure at T/T_c ≈ 0.7 for one mixture of nearly equimolar composition, respectively. Besides the properties of the pure substances, these data are sufficient to calculate the saturation pressure as well as the composition of the coexisting phases with rather high accuracy for both the binary and the ternary mixtures. This has been proved by comparison with experimental data for binary mixtures and for three ternary mixtures of different compositions.     

Photoinduced changes in the optical constants of Ge–Se–AgI thin films

The photoinduced changes in the complex refractive index n=n − ik of thin films from the Ge–Se–AgI system with constant ratio Ge/Se=1/4 and concentrations of AgI of 0, 5 and 10 mol% are studied by real time measurements of reflectance (R) and transmittance (T) of the films. The phase delay (δ) between the components of the transmitted wave, which is proportional to the birefringence of the film, is measured in real time as well. The changes in the average value of the refractive index (Δn), and in the average value of the absorption index (Δk) in the imaginary part of n as well as the induced optical anisotropy are estimated by solving the inverse optical problem. It is found that involving small quantities of AgI into a Ge–Se matrix increases the sensitivity of the films, but the anisotropic effects are comparatively weakly in them. The maximum values of changes in the average refractive index (Δn=0.025) and in the average absorption index (Δk=−0.03) are obtained in the films, containing 5 and 10 mol% AgI, respectively. Considerable values of the photoinduced anisotropy – birefringence (Δn_a≈−0.002) and dichroism (ΔD≈0.05, which corresponds to Δk_a≈0.035) are observed in films without addition of AgI (“pure” Ge–Se film).     

Co掺杂钙钛矿锰氧化物La0.8Sr0.2MnO3电磁特性研究

采用传统的高温固相反应法制备了La_0.8Sr_0.2Mn_1-xCo_xO₃(x = 0, 0.1, 0.3)多晶样品。系统研究了Co掺杂量对La_0.8Sr_0.2MnO₃(LSMO)多晶样品的类Griffiths相、磁熵变、临界行为和电输运性质的影响。研究结果表明: 制备的多晶样品均具有菱形对称结构; 三样品在低温磁转变温度(T_C2)以上均存在类Griffiths相; La_0.8Sr_0.2Mn_1-xCo_xO₃(x = 0, 0.1, 0.3)样品外加磁场为7 T的最大磁熵变ΔS_max分别为-2.28、-2.05和-2.75 J/(kg·K), Co元素的掺杂使得ΔS_max先减小后增大; 母相的临界行为与平均场模型拟合得最好, 掺杂后样品的临界行为和3D海森伯模型拟合最好; 母相为半导体材料, Co元素掺杂量达到0.1时在低温磁转变温度(T_C2)附近出现金属绝缘体转变; 高温区三样品的导电方式均满足小极化子模型。     

MBE-growth of heteropolytypic low-dimensional structures of SiC

The controlled growth of SiC heteropolytypic structures consisting of hexagonal (-) and cubic (3C-) polytypes has been performed by solid-source molecular beam epitaxy. On on-axis substrates, 4H/3C/4H-SiC(0001) and 6H/3C/6H-SiC(0001) structures were obtained by first growing some nanometers thick 3C-SiC layer at lower temperatures (1550 K) and Si-rich conditions, and subsequent growth of -SiC on top of the 3C-SiC layer at higher temperatures (1600 K) under more C-rich conditions. On off-axis substrates, multi-heterostructures consisting of 4H/3C- or 6H/3C-stacking sequences were also obtained by first nucleating selectively wire-like 3C-SiC nuclei on the terraces of well-prepared off-axis -SiC(0001) substrates at low T (<1500 K). Next, SiC was grown further in a step-flow growth mode at higher T and Si-rich condition. After the growth many wire-like regions consisting of 3C-SiC were found within the hexagonal SiC layer material matrix indicating a simultaneous step-flow growth of both the cubic and the hexagonal SiC material.     

AC Conductivity and Dielectric Response of Polycrystalline and Amorphous C70

The AC conductivity and dielectric constant of polycrystalline and amorphous C₇₀ samples were measured in the 75-300 K temperature range and in the 100 Hz to 1 MHz frequency range. For polycrystalline samples, we observe effects caused by O₂ intercalation due to prolonged exposure to ambient air. The conductivity σ of these samples around 300 K depends on the measuring frequency ν as a σ ∼ νⁿ with n ≈ 0.88, implying a strong reduction of DC conductivity to less than 10^-12 S/cm. The dielectric constant of polycrystalline samples shows an anomaly at 285 K which is interpreted as due to the transition from its intermediate rhombohedral phase into its monoclinic low-temperature phase. In contrast with the polycrystalline samples, the amorphous C₇₀ samples prepared by sublimation do not contain interstitial 0₂, their conductivity at 300 K is of about 10^-6 S/cm, is independent of frequency, and is well described by the hopping mechanism (Davis-Mott T^1/4 law) in the 200-300 K range. All evidence of phase transitions disappears in the amorphous samples.     

Molecular dynamics simulations of oxygen ion diffusion and superionic conduction in ytterbia-stabilized zirconia

Superionic conduction of oxygen ions in 10 mol% ytterbia-stabilized zirconia (YbSZ) at different temperatures is studied employing molecular dynamics simulations. Eventhough discrete hopping of one or two oxygen ions starts at about 675 K, onset of superionic conduction occurs at about 1200 K when almost all the oxygen ions participate in the hopping process. The activation energy for oxygen ion diffusion is found to be 53.25 kJ mol⁻¹. At this temperature and above, oxygen ionic conductivity exceeds 0.1 Ω⁻¹ cm⁻¹ thereby confirming superionic conduction in the YbSZ material. For 675 K < T < 1200 K, the material acts as a normal ionic conductor. The ionic conductivity values, obtained through our simulation compare well with experimental results. But activation energy for oxygen ion conduction, found from the Arrhenius plot of our simulation is 52.71 kJ mol⁻¹ which is 35% less than experimental value. Radial distribution functions, g(r) show that there is no sharp structural phase transition and no oxygen ion sub-lattice melting in YbSZ material at superionic transition. However, the reduction in, broadening and shifting of the peaks in g(r) for all ionic pairs, at higher temperatures, indicate a volume expansion of the crystal.     

First principles calculations of thermal equations of state and thermodynamical properties of MgH2 at finite temperatures

We present the first principles calculations of the thermodynamical properties of magnesium hydride (MgH₂) over a temperature range of 0–1000 K. The phonon dispersions are determined within the density functional framework and are used to calculate the free energy of MgH₂ within the quasiharmonic approximation (QHA) at each cell volume and temperature T. Using the free energies the thermal equation of state (EOS) is derived at several temperatures. From the thermal EOS structural parameters such as the equilibrium cell volume (V₀) and elastic properties, namely, bulk modulus (K₀) and its pressure derivative are computed. The free energies are also used to calculate various thermodynamical properties within QHA. These include internal energy E, entropy S, specific heat capacity at constant pressure C_P, thermal pressure P_thermal(V, T) and volume thermal expansion ΔV/V (%). The good agreement of calculated values of S and C_P with experimental data exhibits that QHA can be used as a tool for calculating the thermodynamical properties of MgH₂ over a wide temperature range. P_thermal(V,T) increases strongly with T at all the volumes but it is a slowly varying function of volume for T = 298–500 K. According to Karki [B.B. Karki, Am. Miner. 85 (2000) 1447] such volume based variations can be neglected and so it is possible to estimate the thermal EOS only with the knowledge of the measured P_thermal(V, T) versus temperature at ambient pressure and isothermal compression data at ambient temperature. Temperature dependence of ΔV/V(%) shows that V₀ increased with increase in temperature. However, the percentage decrease in K₀ superseded this percentage increase in V₀ even at temperatures moderately higher than 298 K. Therefore, we suggest application of temperature (T > 298 K) as an approach to enhance the hydrogen storage capacity of MgH₂ because of its better compressibility at these temperatures.     

Preparation and crystallization of Ti53Cu27Ni12Zr3Al7Si3B1 bulk metallic glass with wide supercooled liquid region

Ti-based bulk metallic glass (BMG) alloy with the composition of Ti₅₃Cu₂₇Ni₁₂Zr₃Al₇Si₃B₁ was prepared by copper molder casting method and ribbon sample was prepared by melt spinning to compare. The thermal instability of this glass phase was examined by using differential scanning calorimetry (DSC) and differential thermal analysis (DTA). The results revealed that the supercooled liquid region (ΔT_x), glass transition temperature (T_g) and reduced glass transition temperature (T_g/T_m) of the glassy alloy are detected to be 69, 685 and 0.62 K, respectively. The crystallization behavior of the Ti-based glass phase was also investigated by annealing the glass phase at series temperatures above T_g. The annealed microstructures were examined by means of X-ray diffraction experiments. The crystallization process of the BMG can be characterized by metastable crystalline phases at the first crystallization step and further transition to stable crystalline phases at high temperature through metastable crystalline phase.     

Heat pump cycles based on the AF–F transition in Fe–Rh alloys induced by tensile stress

The heat-pumping scheme based on the 1st order antiferromagnetism–ferromagnetism transition induced in FeRh alloy by one-dimensional tensile stress is proposed. Using the model S–T diagram for this alloy, the heat-pump cycles are drawn up based both on the transition latent heat absorption and emission when the transition is induced isothermally and on the change in alloy's temperature when the transition is induced adiabatically by applying tensile stress. The calculated values of heat coefficient φ for the cycles are 30 at ΔT=5 К and 20 at ΔT=10 К, where ΔT is the difference between the temperature surrounding and that of the heat receiver. These values are achieved using the tensile stress of 1·10⁹ Pa. The high values of φ make it possible to consider Fe–Rh alloys near the equiatomic composition as an effective refrigerant for elastocaloric heat-pumping near the room temperature.     

A dynamic measure of order in structural glasses

Although some patterns of physical behavior are common in the glass transition and in the properties of supercooled liquids and glasses (characteristic viscoelasticity, temperature dependence of viscosity and relaxation times, property evolution through “physical aging”, difficulties in performing equilibrium measurements or simulations, etc.), it is difficult to arrive at a definition of the glass transition which distinguishes it from other phenomena exhibiting similar features. The present paper addresses this problem by defining a dynamical measure of order involving the average “shape” of particle trajectories in supercooled liquids. This dynamic order parameter should provide a measure of “closeness” to the glass transition and some indirect insights into the physical nature of supercooled liquids and glasses. Arguments are given that the proposed dynamic measure of order [“generalized capacity”, C(T)] is related to the temperature-dependent “effective hydrodynamic radius” R_H(T) measured in supercooled liquids and model numerical calculations are given to support this view. Some consequences of the intermittent particle motion at low temperatures for stress relaxation are also discussed.