Haiying Hu

Epitaxial growth and structural property of graphene on Pt(111)

We report on epitaxial growth of graphene on Pt(111) surface. It was found out that the proportion of different rotational domains varies with growth temperature and the graphene quality can be improved by adjusting both the growth temperature and ethylene exposure. Rippled and unrippled domains of high quality graphene are observed. The adhesive energy and electronic structure of two models, representing rippled and unrippled graphene, are obtained with density functional theory calculation, which shows that the interaction between graphene and Pt(111) surface is very weak and the electronic structure is nearly the same as that of a free standing graphene.

Half-metallic ferromagnetism in hypothetical semi-Heusler alloys NiVM (M=P, As, Sb, S, Se, and Te)

A theoretical study based on first-principle band-structure calculations is carried out for the hypothetical semi-Heusler alloys NiVM (M=P, As, Sb, S, Se, and Te). For all compounds it is found that the ferromagnetic state is more favorable than the paramagnetic state. NiVAs is predicted to be a half-metallic ferromagnet with a small half-metallic gap of 0.07 eV and an integer magnetic moment of 2 μB, and NiVP and NiVSb are so-called nearly half-metallic ferromagnets. Furthermore, we find a clear indication that the substitution on the sp atoms influences the hybridization between Ni and V atoms, and can even destroy the half metallicity.

The electrical conductivity of dry polycrystalline olivine compacts at high temperatures and pressures

Abstract The electrical conductivity of dry polycrystalline olivine compacts (hot-pressed and sintered pellets) was measured at pressures of 1.0-4.0 GPa, at temperatures of 1073-1423 K, and at different oxygen fugacities via the use of a YJ-3000t multi-anvil press. Oxygen fugacity was controlled successfully by means of five solid buffers: Fe3O4-Fe2O3, Ni-NiO, Fe-Fe3O4, Fe-FeO and Mo-MoO2. Within the selected frequency range of 102-106 Hz, the experimental results indicate that the grain interior conduction mechanism is characterized by a semi-circular curve on an impedance diagram. As a function of increasing pressure, the electrical conductivity of polycrystalline olivine compacts decreases, whereas the activation enthalpy and the temperature-independent pre-exponential factors increase slightly. The activation energy and activation volume of polycrystalline olivine compacts were determined to be 141.02±2.53 kJ/mol and 0.25±0.05 cm3/mol, respectively. At a pressure of 4.0 GPa, electrical conductivity was observed to increase as a function of increasing oxygen fugacity, and the relationship between electrical conductivity and oxygen fugacity can be described as log10 (σ) = (2.47±0.085) + (0.096±0.023) × log10 fO₂ + (-0.55±0.011)/T, which presents the exponential factor q (~0.096). Our observations demonstrate that the primary conduction mechanism for polycrystalline olivine compacts is a small polaron.

Electrical conductivity of albite at high temperatures and high pressures

Abstract The electrical conductivity of low albite has been measured using a complex impedance spectroscopic technique at 1.0-3.0 GPa and 773-1073 K in the frequency range of 10−1 to 106 Hz in a YJ-3000t multi-anvil press. Within this frequency range, the complex impedance plane displays a semi-circular arc that represents a grain interior conduction mechanism. The electrical conductivity of albite increases with increasing temperature, and the relationship between electrical conductivity and temperature fits the Arrhenius formula. Pressure has a weak effect on the electrical conductivity of albite in the experimental pressure-temperature (P-T) range in the present work. The pre-exponential factors decrease, and the activation enthalpy increases slightly with increasing pressure. The activation energy and activation volume of albite are 0.82 ± 0.04 eV and 1.45 ± 0.28 cm3/mol, respectively. Comparison with previous results with respect to albite indicates that our data are similar to previous data within the same temperature range. The dominant conduction mechanism in albite is suggested to be ionic conduction, where loosely bonded sodium cations, the dominant charge carriers, migrate into interstitial sites within the feldspar aluminosilicate framework. The Na diffusivity inferred from electrical conductivity of albite in this study using the Nernst-Einstein relation is consistent with that of previous studies on natural albite.

Martensitic transformation and magnetic properties of Co-Ni-Al shape memory alloy ribbons

We report on the magnetic and martensitic transformation properties of Co39Ni33Al28 ferromagnetic shape memory alloy ribbons. We found that the phase formation of Co39Ni33Al28 is strongly dependent on the method of preparation. The conventional as-cast ingot sample contains a large amount of the γ phase embedded in the primary β phase, while melt-spun ribbons contain the pure β phase. Co39Ni33Al28 ribbons exhibit a perfect thermoelastic martensitic transformation from a cubic to a tetragonal structure at 240 K during cooling. The martensite structure can be well described by the L10 lattice, similar to that of Ni–Al alloys. The martensitic phase at 5 K exhibits a saturation magnetization of 48.67 emu g −1 and saturates at about 8000 Oe. A large increase in coercive force after ageing at 500uC for 1 h has been found due to the change in atomic chemical ordering. The temperature dependence of the saturation magnetization indicates that magnetization can be well interpreted by spin-wave theory at temperatures lower than 200 K. The material shows a recoverable strain of 500 ppm upon the martensitic transformation.

Influence of temperature, pressure, and chemical composition on the electrical conductivity of granite

Abstract The electrical conductivities of granites with different chemical compositions [XA = (Na2O + K2O + CaO)/SiO2 = 0.10, 0.13, 0.14, and 0.16 in weight percent] were measured at 623-1173 K and 0.5 GPa in a multi-anvil high-pressure apparatus using a Solartron-1260 Impedance/Gain Phase analyzer within a frequency range of 10-1-106 Hz. The conductivity of the granite sample with XA = 0.13 was also measured at 0.5-1.5 GPa. The results indicate that pressure has a very weak influence on the electrical conductivity in the stability field of granite, whereas increases in temperature and the value of XA produce dramatic increases in the electrical conductivity. For the granite samples with XA = 0.16 and 0.13, the activation enthalpies are 1.0 eV above 773 K and 0.5 eV below 773 K, suggesting that impurity conduction is the dominant conduction mechanism in the lower-temperature region. For the granites with XA = 0.14 and 0.10, the activation enthalpy is 1.0 eV over the whole temperature range, suggesting that only one conduction mechanism dominates the conductivity. Based on the value of activation enthalpy (~1.0 eV) and the dependence of electrical conductivity and activation enthalpy on XA at high temperatures, we propose that intrinsic conduction is the dominant conduction mechanism in all samples, and that K+, Na+, and Ca2+ in feldspar are the probable charge carriers controlling the conductivity. All conductivity data at high temperatures can be fitted to the general formula where σ0 is the pre-exponential factor; α, β, and γ are constants; ΔH0 is the activation enthalpy at very small values of XA; k is the Boltzmann constant; and T is the temperature. The present results suggest that the granite with various chemical compositions is unable to account for the high conductivity anomalies under stable mid- to lower-crust and southern Tibet.

Novel technique to control oxygen fugacity during high-pressure measurements of grain boundary conductivities of rocks.

This paper describes the development and application of a novel method for the measurement of grain boundary electrical conductivity of rock at high temperature and pressure. In this method, the metal electrodes, the corresponding metal shielding case, and the sleeves are altered in order to appropriately adjust and monitor the oxygen fugacity in a sample cavity in a high-pressure apparatus. As an example, a series of oxygen buffers including Fe(3)O(4) + Fe(2)O(3), Ni + NiO, Fe + Fe(3)O(4), Fe + FeO, and Mo + MoO(2) was selected and tested, and the oxygen fugacity was confirmed as adjusted during the process of electrical conductivity measurements. Application of this method provides a powerful means of restricting specific thermodynamic conditions at high temperature and pressure.

Evidence of the pressure-induced conductivity switching of yttrium-doped SrTiO3

The electrical transport properties of undoped and yttrium-doped strontium titanate (Sr(Ti1 - x Y x )O3 - δ , x  =  0, 0.02) under high pressure were investigated with in situ impedance spectroscopy measurements. A pressure-induced conductivity switching for undoped and 2 mole% Y-doped strontium titanate is observed at around ~10.0 and 7.0 GPa respectively, which are caused by a cubic to tetragonal I4/mcm phase transition. The decrease of the phase transition point of 2 mole% Y-doped strontium titanate can be attributed to larger Y(3+) atoms occupying the B-site and the creation of more oxygen vacancies, which lead to octahedra tilting and symmetry breaking. The results of the voltage-bias dependence of grain-boundary impedance of undoped and 2 mole% Y-doped strontium titanate at different pressures revealed that Schottky-type potential barriers formed at grain boundaries are the key factor for the accumulation of oxygen vacancy at the interface under pressure.

Pressure-induced permanent metallization with reversible structural transition in molybdenum disulfide

This report presents a pressure-induced permanent metallization for MoS2 under non-hydrostatic conditions. Impedance and Raman spectra were measured to study the pressure-induced structural and electronic transformations of MoS2 at up to ∼25 GPa in diamond anvil cells under both non-hydrostatic and hydrostatic conditions. The results show evidence for isostructural hexagonal distortion from 2Hc to 2Ha and metallization at ∼17 GPa and ∼20 GPa under non-hydrostatic and hydrostatic conditions, respectively. Interestingly, the metallization is irreversible only under non-hydrostatic compression. We attribute this phenomenon to the incorporation of molecules of pressure medium between layers, which mitigate compressed stress and reduce interlayer interaction.

Pressure-induced phase-transition and improvement of the microdielectric properties in yttrium-doped SrZrO3

In this study, the effect of pressure on undoped and 5% yttrium-doped SrZrO3 (SZY0 and SZY5) were conducted from the ambient condition to with a diamond anvil cell. The comparison of the high-pressure Raman spectra of SZY0 and SZY5 indicate that SZY0 displays a rigid structure without any structural modification, whereas for SZY5 a structural transition at is revealed. Some characteristic physical parameters such as bulk conductivities, grain boundary conductivities, Warburg diffusion coefficient, transference number and bulk relaxation frequency were determined by the high-pressure impedance spectroscopy data. An obvious discontinuous inflexion point (at ) for SZY5 sample is observable which was also verified the phase-transition of the Raman spectroscopy results. A mixed conduction mechanism for both SZY0 and SZY5 are coexisting for both SZY0 and SZY5 in a wide pressure range. The pressure-induced phase-transition of SZY5 would result in an inversion of conduction mechanism that is characterized by the dominant charge carriers transformation from electron to ion. The Maxwell-Wagner relaxation arising at the interfaces of grain and grain boundary indicates that Y-doping and pressure could make the ions diffusion much easier through the boundaries and finally enhance the dielectric performance of the sample. It is suggested that pressure could be a useful tool to manipulate the microstructure and dielectric performance of polycrystal through altering the grain boundary distribution.