High pressure effects on the photoluminescence intensity of sexithiophene single crystals

We report on the influence of interchain interactions on the light emission properties of a sexithiophene single crystal. The strength of the intermolecular interactions is controlled by applying hydrostatic pressure. The combined use of both steady-state and time-resolved photoluminescence techniques permits to show that the pressure-induced quenching of the photoluminescence is caused by a reduction of the radiative recombination rate.     

High pressure study of the intramolecular vibrational modes in sexithiophene single crystals

We report on the effects of a high hydrostatic pressure on the intra-molecular modes of a sexithiophene (T6) single crystal. Raman experiments were performed at low temperature up to a pressure of 80 kbar. We find that intermolecular interaction gives rise to a sizeable redistribution of the intensities of the strongest intra-molecular Raman lines. These changes are due to a slight deformation occurring in the terminal rings of the sexithiophene molecule. The frequencies of the bending modes involving sulfur atoms are the most affected by pressure. A Davydov doublet and a mixed inter/intra-molecular mode are observed. These results confirm the role played by the sulfur atoms in modulating the electronic overlap between chains.     

Magnetic properties of ErMn2 single crystals

Single crystals of the ErMn₂ compound were grown by the Czochralski method from a levitated melt. The thermal dependence of the lattice parameters exhibits a magnetovolume effect along the a axis, a contraction for both the c axis and unit cell volume below 20 K. The thermal dependence of the electrical resistivity exhibits a magnetic transition at 23 K. A second transition occurs at 13 K. Moreover, for ρ(T) a negative curvature above the ordering temperature and a saturation effect above 200 K occurs. The magnetization in strong magnetic fields, the d.c. and a.c. magnetic susceptibilities measured along the principal crystallographic directions show ordering of the erbium moments to a spin-canted magnetic structure at about 15 K and a spin reorientation about 10 K.     

Phase-field analysis of fracture-induced twinning in single crystals

Deformation twinning at the tip of a straight crack or notch is analyzed using a phase-field method that seeks equilibrium twin morphologies via direct minimization of a free energy functional. For isotropic solids, the tendency to twin under mode I or mode II loading is found to depend weakly on Poisson’s ratio and elastic nonlinearity and strongly on surface energy and twinning shear (i.e. eigenstrain). Depending on the coherent twin boundary energy, anisotropy of surface energy is important for mode I loading but less so for mode II. Model predictions for several single crystals are in agreement with experimental observations. Calcite demonstrates a preference for mode I cleavage crack extension over crack tip twinning. Magnesium shows a likelihood for tensile twinning from a pre-existing crack on the basal plane. In sapphire, a preference for rhombohedral twins over basal twins is apparent, with the latter thinner in shape than the former.     

Characterization of MLEC GaSb Crystal

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Stoichiometry control and mechanical properties of NiAl single crystals

Stoichiometric NiAl crystals have been prepared by a modified Bridgman method. Chemical analysis methods have been elaborated for accurate composition analyses with a total error of 0.1 at.% Ni. The NiAl samples exhibit high room-temperature ductility in the as-grown state.     

Synthesis and properties of Mo5Si3 single crystals

The ultra-high temperature structural intermetallic Mo₅Si₃ has been studied for alloy processing, physical properties, and mechanical behavior. High purity single crystals of Mo₅Si₃ have been synthesized by both optical floating zone and Czochralski methods. Structural, thermal, and elastic properties of Mo₅Si₃ single crystals were measured by X-ray powder diffraction, thermal mechanical analysis, and resonant ultrasound spectroscopy, respectively. Results show that the thermal expansion of Mo₅Si₃, a tetragonal structure with I4/mcm symmetry, is strongly anisotropic along the a and c directions with α_c/α_a=2.2. Single crystal elastic moduli of Mo₅Si₃ indicate that it has less elastic anisotropy and lower shear modulus than most transition metal disilicides. The impacts of these physical properties on alloy processing and mechanical behavior are discussed. Room temperature Vickers indentation tests on the (100) and (001) planes have been performed for different orientations of the indenter diagonal and the corresponding hardness, fracture toughness, and deformation behavior have been obtained as a function of the crystallography. Finally, the physical properties and mechanical behavior of Mo₅Si₃ are compared with those of other high temperature structural silicides, e.g. MoSi₂.     

On thermal and vibrational properties of LaGaO3 single crystals

The thermal expansion and vibrational properties of [1 0 0] and [0 0 1] LaGaO₃ single crystals have been studied by thermal mechanical analysis and micro-Raman spectroscopy. A first-order orthorhombic to rhombohedral phase transition has been confirmed by both techniques, as well as by in situ heating using optical microscopy. The appearance of a metastable intermediate phase, tentatively assigned as monoclinic, has been detected both by optical microscopy and Raman spectroscopy upon heating of the [1 0 0] and [0 0 1] LaGaO₃ single crystals. Not only temperature, but the stress-induced orthorhombic to rhombohedral phase transition has also been detected by Raman mapping of the residual impression made by Vickers indentation. The position map of bands belonging to the lower-temperature/pressure orthorhombic and the higher-temperature/pressure rhombohedral phase show that the rhombohedral phase is located inside the impression, where the applied indentation stresses are the highest, whereas no rhombohedral phase is detected outside the impression, where the surface has not been altered by contact stresses.     

Spectroscopic and crystal field analysis of absorption and photoluminescence properties of red phosphor CaAl12O19:Mn modified by MgO

The spectroscopic properties of a series of red phosphors with general composition of CaAl₁₂O₁₉:yMn⁴⁺ and (Ca_1−xAl₁₂O₁₉, xMgO):yMn⁴⁺ (x = 0-1, y = 0.001-1.5%) synthesized by a modified solid state method in air have been investigated in detail. Addition of MgO is necessary for Mn⁴⁺ charge compensation and leads to the formation of separate crystal phases of Al₂O₃ and MgAl₂O₄, which was confirmed by the XRD studies. Enhancement of Mn⁴⁺ luminescence with increasing content of MgO was observed and a mechanism for explanation of this phenomenon is suggested. For an analysis of the crystal phases and luminescent efficiency of the phosphors in the prepared series, crystal field calculations of the Mn⁴⁺ energy levels have been performed. This theoretical approach allowed for assigning the observed excitation and emission spectra. Red shift of the Mn⁴⁺ luminescence with increasing concentration of Mg ions is explained from the point of view of enhanced nephelauxetic effect after doping.     

Effect of dysprosium active ions on spectral properties of KGW single crystals

Dysprosium (Dy) doped potassium gadolinium tungstate single crystals were grown by top seeded solution growth using K₂WO₄ as solvent. Two different concentrations of laser active ions were doped in the crystal. Crystals grown with lower growth rate using K₂WO₄ as solvent yielded better quality than that grown by K₂W₂O₇ as solvent. Growth conditions, like cooling rate and the rotation rate were optimized. Absorption studies confirm dominant absorption peaks in the visible as well as in near infrared regions. Photoluminescence reveals a strong yellow emission for the doped samples and the emission intensity of 3% Dy doped sample is 1.4 times higher than that of 1% Dy concentration in KGW. Raman spectra indicate the characteristic vibrational modes corresponding to the tungstate anion. The intensity of peaks in Raman spectra are increased with increase in concentration of Dy, in addition to a slight shift in Raman frequency.     

Effects of two-step annealing on properties of Cd1-xZnxTe single crystals

The Cd1-xZnxTe（CZT） single crystals were annealed by a two-step method including a vapor-environment step and a liquid-environment step in sequencel The effects of annealing on the properties of CZT were analyzed in detail. IR transmission measurement results show that IR transmission of CZT is improved dramatically after annealing. X-ray rocking curves indicate that the annealing treatment ameliorates crystal quality obviously, which is ascribed to the release of residual stress and the reduction of point defects. Photoluminescence（PL） spectra reveal that the full width at half maximum（FWHM） of the donor-bound exciton （D^0, X） peak is reduced obviously, and the free exciton emission is weakened after annealing. Meanwhile, the intensity of the donor-acceptor pair（DAP） peak decreases to a great degree, which implies that the impurities are removed from CZT wafers. In addition, the deep defect-related emission band Dcomplex disappears after annealing, which mean that Cd vacancies are well-compensated. The results confirm that the two-step annealing is an effective approach to improve the qualities of CZT single crystals.     

In situ micro-cantilever tests to study fracture properties of NiAl single crystals

In situ micro-cantilever tests were carried out to determine the anisotropic fracture toughness of NiAl single crystals. Notched micro-cantilever beams with a beam length of 8 μm, 1.5 μm thickness and 1.8 μm width were milled in so-called “hard” and “soft” orientations of NiAl using a focused ion beam. These cantilevers were loaded in situ with the help of a cantilever-based nanoindenter mounted inside a scanning electron microscope. A fracture toughness of 3.52 ± 0.29 MPa m^1/2 was obtained for the “soft” orientation and 5.12 ± 0.50 MPa m^1/2 for the “hard” orientation, which is in good agreement with literature values on the fracture toughness of macroscopic NiAl specimens. Furthermore, nanoindentations were performed for studying the size effects occurring at small length scales for both orientations. The applicability of the small sample geometries for testing the fracture toughness is finally discussed in terms of size effects in the flow stress of the material due to dislocation nucleation and strain gradients at the crack tip.     

High-pressure ultrasonic properties of spin-density-wave Cr–Re alloy single crystals

The pressure dependence of the elastic constants (c_ij) of Cr–Re alloy single crystals, containing 0.3 and 0.5 at.% Re, are reported. For 0.3 at.% Re the concentration (c) is below the triple point concentration (c_t) on the (c–T) magnetic phase diagram. This crystal therefore remains in the incommensurate (I) spin-density-wave (SDW) antiferromagnetic phase at all temperatures below the ISDW–paramagnetic (P) Néel transition temperature (T_N). The pressure derivatives of the elastic constants, dc_ij/dp, were measured for the Cr+0.3 at.% Re crystal as a function of temperature through T_N. The Cr+0.5 at.% Re crystal has c>c_t and exhibits an incommensurate–commensurate (I–C) SDW phase transition at a temperature T_IC<T_N on heating. In the case of this crystal dc_ij/dp was studied at temperatures close to and above T_IC. The acoustic mode Grüneisen parameters (γ_n), which quantify the lattice vibrational anharmonicity, were calculated as a function of temperature from the dc_ij/dp data for each crystal. These calculations indicate much larger coupling of the SDW to the long-wavelength longitudinal phonons than to the shear modes for both Cr–Re crystals. Longitudinal mode softening under applied pressure, due to strong magnetoelastic interactions between the SDW and the longitudinal acoustic phonons, in Cr+0.3 at.% Re at T<T_N, gives nearly discontinuously way to extremely large mode stiffening as the crystal is heated through T_N. γ_n in the CSDW and pressure-induced ISDW phases of the Cr+0.5 at.% Re crystal shows unusual behaviour with temperature. This is particularly so for the shear mode Grüneisen parameters which are relatively large and negative just above T_IC, implying significant shear mode softening under applied pressure. γ_n (shear) then increases on further increasing the temperature of this crystal. In comparison, γ_n (shear) for both the CSDW and pressure-induced ISDW phases of other Cr alloys, with c>c_t, for instance Cr–Ru and Cr–Ir alloys, is near zero, slightly positive, and remains constant with temperature. The measurements on the Cr+0.5 at.% Re crystal suggest the existence of a new phase line, separating the ISDW phase present at atmospheric pressure from that induced from the CSDW phase by applying high pressure, on the pressure–temperature magnetic phase diagrams of dilute Cr alloys with c>c_t. More experimentation, particularly high pressure neutron diffraction studies, are needed to verify this point.     

Effect of magnetic Gd impurities on the electrical properties of PbTe single crystals

Under investigation are temperature dependences of electric conductivity, free carrier concentration and mobility in single-crystalline PbTe:Gd samples with varied impurity content. The features of electron transport in PbTe:Gd may be caused by a variable gadolinium valence. The striking result from the Seebeck coefficient measurements is that the thermoelectric power factor increases dramatically. Measurements of the magnetic susceptibility at low temperatures permit us to suggest that Gd ions may exist in different charge states.     

Magnetic properties and magnetocaloric effect in the aluminide RENiAl2 (RE = Ho and Er) compounds

Two single-phased aluminide RENiAl₂ (RE = Ho and Er) compounds have been synthesized by an arc-melting method. The magnetic properties and magnetocaloric effect (MCE) of HoNiAl₂ and ErNiAl₂ have been studied by magnetization measurements. A second-order paramagnetic (PM) to ferromagnetic (FM) phase transition together with a considerable reversible MCE were observed around the Curie temperature T_C ∼7.5 K and 5.0 K for HoNiAl₂ and ErNiAl₂, respectively. Under the magnetic field change (ΔH) of 0–5 T, the maximal values of magnetic entropy change, refrigerant capacity and relative cooling power are 14.0 J/kg K, 171 J/kg, 213 J/kg for HoNiAl₂ and 21.2 J/kg K, 267 J/kg, 357 J/kg for ErNiAl₂, respectively.     

Acoustical properties of Ni3Al single crystals alloyed with cobalt and niobium

High-temperature nickel-base alloys (superalloys) are widely adopted in industry as materials for structural elements of devices operating upon high temperatures (aerospace turbine blades, etc.). Successful application of these alloys is based on their high mechanical properties. Today, the range of service conditions expands toward progressively higher temperatures. Thereupon, the investigation of mechanical and thermal properties of nickel high-temperature alloys appears to be a topical question.     

Electrical properties and electrical field in depletion layer for CZT crystals

Current--voltage （I--V） and capacitance-voltage （C--V） characteristics of Au/p-CZT contacts with different surface treatments on cadmium zinc telluride （CZT） wafer＇s surface were measured with Agilent 4339B high resistance meter and Agilent 4294A precision impedance analyzer, respectively. The Schottky barrier height was 0.85 ＋0.05, 0.96＋ 0.05 eV for non-passivated and passivated CZT crystals by I-V measurement. By C-- V measurement, the Schottl~ barrier height was 1.39＋ 0.05, 1.51 ＋ 0.05 eV for non-passivated and passivated CZT crystals. The results show that the passivation treatment can increase the barrier height of the Au/p-CZT contact and decrease the leakage current. The main reason is that the higher barrier height of Au/p-CZT contacts can decrease the possibility for electrons to pass through the native TeO2 film. Most of the applied voltage appears on the depleted layer and there is only a negligible voltage drops across the nearly undepleted region. Furthermore, the electric field in the depleted layer is not uniform and can be calculated by the depletion approximation. The maximum electric field of CZT crystals is Eml= 133 V/cm at x=0 for non-passivated CZT crystal and Em2=55 V/cm for passivated CZT crystal, respectively.     

湿度对恒电场下BaTiO3单晶中畴变的影响

通过偏振光显微镜观察了在恒电场作用下湿空气对BaTiO3单晶畴变的影响.结果表明,在恒电场下,湿度对不同的畴结构有不同的影响.较低的湿度对a-b和b-c畴结构没有影响.而在较高的湿度下,随着放置时间的增加,b-c畴结构发生了变化,b畴逐渐向c畴转变;但是较高的湿度对a-b畴结构无影响.可用水分子在a(b)畴和c畴上的吸附差别解释上述现象.     

Methods for obtaining two way memory effect and stressed two way memory effect of CuAlNi single crystals

1　ＩＮＴＲＯＤＵＣＴＩＯＮＴｗｏｗａｙｍｅｍｏｒｙｅｆｆｅｃｔ (ＴＷＭＥ)ｒｅｆｅｒｓｔｏｔｈｅｒｅｖｅｒｓｉｂｌｅｓｐｏｎｔａｎｅｏｕｓｓｈａｐｅｃｈａｎｇｅｂｙｈｅａｔｉｎｇａｎｄｃｏｏｌｉｎｇｂｅｔｗｅｅｎａ“ｈｏｔ”ａｕｓｔｅｎｉｔｅｓｈａｐｅａｎｄａ“ｃｏｌｄ”ｍａｒｔｅｎｓｉｔｅｓｈａｐｅｗｉｔｈｏｕｔａｎｙｍａｃｒｏ ｓｔｒｅｓｓａｐｐｌｉｃａ ｔｉｏｎ[1] .Ｉｎｏｔｈｅｒｗｏｒｄｓ ,ｔｈｉｓｅｆｆｅｃｔｉｓｉｎｔｒｉｎｓｉｃｗｈｉｃｈｉｓｄｉｓｔｉｎｇｕｉｓｈｅｄｆｒｏｍｓｔｒｅｓ…