Yukai Zhuang

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.

Crystallization of ZrTiCuNiBe bulk metallic glasses

Crystallization behavior and kinetics, and microstructural change of ZrTiCuNiBe bulk metallic glass (BMG) were systematically investigated in ambient, preannealed, addition, phase separation, and high pressure annealed conditions by means of differential scanning calorimetry (DSC), x-ray diffraction (XRD), neutron scattering, in situ synchrotron XRD, density and acoustic measurements. The effective activation energy of the crystallization of the BMGs was determined by Kissinger method. It was found that the crystallization as well as the apparent activation energy of the crystallization was sensitive to the external conditions. Pressure annealing in the supercooled liquid region produces a composite with dispersion of very fine nanocrystallites in the amorphous-matrix. The pressure also controls the phase selection during the crystallization. The glass forming ability of the glass forming alloys has been discussed in view of the crystallization kinetics.

Pressure-induced improvement of grain boundary properties in yttrium-doped BaZrO3

Yttrium-doped BaZrO3 (BZY) is a promising electrolyte for intermediate-temperature protonic ceramic fuel cells. However, BZY exhibits a high resistance because of the blocking effect of the grain boundaries. In this study, the effect of pressure on undoped and 5% yttrium-doped BaZrO3 (BZY0 and BZY5) were investigated at 0.45–24.01 GPa and 273–673 K with a diamond anvil cell. Their bulk, grain boundary, and total electrical conductivities were determined by impedance spectroscopy and direct-current resistance measurement. Both samples tended to show increasing electrical conductivity with increasing pressure, although each showed a discontinuous inflexion point (at ~14.54 GPa for BZY0 and at ~11.11 GPa for BZY5) indicating a phase transition from a cubic to a tetragonal structure. The samples showed a 3.43 GPa difference in the onset pressure of the structure change. Characteristic parameters, including space charge potential, relaxation frequency, and transport activation energy, were obtained before and after the phase transition. The results suggest that pressure significantly improves oxygen ion conduction in acceptor-doped perovskites oxides.

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.

Anomalous phase transition of Bi-doped Zn2GeO4 investigated by electrical conductivity and Raman spectroscopy under high pressure

We present a pressure-induced amorphization for pure and 0.5 mol. % Bi3+-doped Zn2GeO4 samples, measured by high pressure Raman spectroscopy and high resolution transmission electron microscopy. Pressure-induced conductivity switching phenomena were discovered for both samples at around ∼7.01 GPa and ∼11.11 GPa, respectively, which closely correlated with the crystalline-to-amorphous transformation. The detailed conduction mechanism and the defect reaction process at high pressure indicate that the application of pressure could efficiently manipulate the microstructure and electrical performance of rare-earth doped polycrystalline materials, and therefore holds great promise for numerous applications in the future.

Pressure-induced irreversible amorphization and metallization with a structural phase transition in arsenic telluride

The structural, vibrational and electronic properties of α-As2Te3 in different pressure environments were investigated using a diamond-anvil cell (DAC) in conjunction with AC impedance spectroscopy, Raman spectroscopy, atomic force microscopy and high-resolution transmission electron microscopy up to ∼25 GPa. Under non-hydrostatic conditions, α-As2Te3 endured a structural phase transition at ∼6 GPa, and a ∼2 GPa delay in the transition point was observed under hydrostatic conditions. With increasing pressure, amorphization and metallization simultaneously appeared at ∼11 GPa, as characterized by the Raman spectra and temperature-dependent conductivity results. We found that both amorphization and metallization were irreversible after decompression under non-hydrostatic conditions. However, under hydrostatic conditions, both amorphization and metallization were reversible. The unique properties displayed by α-As2Te3 in different pressure environments may be attributed to the effects of deviatoric stresses and the interlayer interaction constrained by the pressure medium.

Ultrasonic attenuation in Zr41Ti14Cu12.5Ni10−xBe22.5Cx(x=0,1) bulk metallic glasses under high pressure

The pressure dependence of ultrasonic attenuation in Zr41Ti14Cu12.5Ni10−xBe22.5Cx (x=0,1) bulk metallic glasses has been studied up to 0.5 GPa by using a pulse echo overlap method. The effect of carbon addition on the attenuation is also investigated. Some unique characteristics of the ultrasonic attenuation are found and compared with those of other glasses. The origin of the anomalous attenuation behavior is discussed.

SURFACE SCATTERING OF X-RAYS FROM INP(001) WAFERS

We have studied the surface scattering of x rays from mechanical‐chemical polished InP (001) wafers with sulfur and/or iron doping. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model was proposed to explain the experimental data. The results were also compared with those obtained from crystal truncation rod measurements.

X-RAY-SCATTERING FROM A ROUGH-SURFACE AND DAMAGED LAYER OF POLISHED WAFERS

Theoretical and experimental investigations were performed to show the application of x-ray crystal truncation rod scattering combined with x-ray reflectivity in the measurements of surface roughness and near-surface damage of mechanochemically polished wafers. By fitting the measured crystal truncation rod curves it has been shown that polished wafers are divided into three parts -irregular steps on the surface, a damaged thin layer beneath the surface and a perfect bulk. The results show that the root mean square of the surface roughness of mechanochemically polished Fe-doped and/or S-doped InP wafers is one to two atom layers, and that the lateral correlation length of the surface roughness is about 3000-7500 Angstrom. The thickness of the damaged region is found to be about 1000 atom layers.