Structural studies on Na0.75CoO2 thermoelectric material at high pressures

The crystal structure of Na_0.75CoO₂ was studied at ambient and low temperatures down to 10 K at pressures up to 40 GPa using synchrotron x-rays and a diamond cell in angle dispersion geometry. A reduction in the c/a ratio was observed at both conditions with the application of pressure. An increase in Co–O bond lengths and a decrease in Na–O bond lengths were observed above 10 GPa. The results of the density functional calculations performed agree well with the pressure induced bond length changes. The anomalous change in the c/a ratio and bond lengths indicate a pressure induced isostructural phase transition above 10 GPa. Bulk modulus calculations show this compound is less compressible than its hydrated analogues.     

Structural and electrical properties evolution in Ba1−xSrxRuO3 synthesized under high pressure

The 6H and 6M Ba_1−xSr_xRuO₃ at x?0.6 with the normal and distorted hexagonal BaTiO₃ structures were synthesized by using high-pressure and high-temperature method. It is found that the unit cell volume deviates from Vegard's law between 0.3 and 0.4 for the solid solutions due to the increasing distortion degree of crystal structure. With the increasing x, the electrical resistivity at the same temperature is increasing. With the substitution of Sr for Ba ion, the 6H BaRuO₃ transforms to a Fermi-liquid metal at x=0.25 from the primal non-Fermi-liquid metal, and then becomes a semiconductor at low temperature when x is larger than 0.4.     

High-pressure synthesis,structure and magnetic properties of LaCrO3 ceramics

Single-phase LaCrO₃ ceramics was synthesized successfully by the solid state sintering method under high pressure (5 GPa). X-ray diffraction measurements suggest that the high-pressure synthesized sample is well-crystallized, but the cell volume is larger than that of the sample synthesized under ambient pressure, which can be ascribed to the occurrence of Cr²⁺ converted form part of Cr³⁺. Such mixed valent state with coexistence of Cr²⁺ and Cr³⁺ could be confirmed by X-ray photoelectron spectroscopy measurements. Magnetic measurement results indicate that in addition to antiferromagnetic superexchange interaction of Cr³⁺-O-Cr³⁺ network accompanied by weak ferromagnetism at high temperature, another kind of ferromagnetic behavior can be observed at low temperature (below 30 K), which could be attributed to double exchange interaction of Cr²⁺-O-Cr³⁺ networks.     

Influence of sintering atmosphere and thermobaric treatment (TBT) on dielectric behaviors of CaCu3Ti4O12 ceramics

Copper titanate (CaCu₃Ti₄O₁₂, or CCTO) ceramics sintered in oxygen and vacuum at 1100°C for 12 h have been treated by thermobaric treatment (TBT) at 9 GPa and 1000°C for 10 min and then quenched in liquid nitrogen (LN₂). Pure cubic body-centered perovskite-related structure was confirmed from XRD results. Besides, after TBT and quenching, additional minor peaks of TiO₂ and Cu were indexed, as well as a little cell volume expansion. A little reduction of the grain size as well as fuzzy grain boundaries can be observed in FE-SEM after TBT. The dielectric constant ε' of CCTO ceramics sintered in oxygen (～4600 @ 100 Hz) were a half of those treated by TBT and quenching (～9600 @ 100 Hz), whereas the value decreased almost by an order of magnitude (from 10⁵ to 10⁴) after TBT and quenching for the samples sintered in vacuum. The complex impedance spectra at high temperature showed three semicircles so a three RC model was used to explain the different relaxation regions consisting of grain, grain boundary and domain boundary. Besides, two relaxation peaks appeared in the frequency dependence of the imaginary part of the electric modulus formalism $M^{?}$ and the complex impedance $Z^{?}$. Moreover, the contributions of grain boundaries can be figured out by the relaxation activation energy $E_{re}$ (338–629 meV) and the conduction one $E_{dc}$ (396–823 meV) fitted by $M^{?}$ and $Z^{?}$ plots separately. As a result, the disorder and heterogeneity typical of grain boundaries should be responsible for the giant dielectric relaxation characteristics of CCTO ceramics, which can be affected by the sintering atmosphere as well as the extreme treating conditions.     

Pressure-sensitive Transistor Fabricated from an OrganicSemiconductor 1,1'-Dibutyl-4,4'-bipyridinium Diiodide

Although organic semiconductors have attracted extensive interest and been utilized to fabricate a variety of optoelectronic devices, their electrical transportation characteristics under high pressure have rarely been investigated. However, the weak intermolecular interaction of organic semiconductors endows them with a pre- ssure-sensitive crystal structure and electrical transportation performance, especially the latter. Herein, a new pre- ssure-sensitive transistor was fabricated from an organic semiconductor 1,1'-dibutyl-4,4'-bipyridinium diiodide. It was found that this transistor exhibited increasing resistance as the pressure gradually increased and that it eventually shut off under a pressure of 288 MPa. Such a characteristic makes this organic semiconductor a potential candidate for the use in the fabrication of pressure-sensitive switches and regulators. In addition, these results shed light on the electrical performance of flexible organic optoelectronic devices working under high pressure levels resulted from the bending force.     

Soot formation and temperature field structure in co-flow laminar methane-air diffusion flames at pressures from 10 to 60 atm

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane-air laminar diffusion flames over a wide pressure range, from 10 to 60 atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100 atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60 atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.     

Pressure variation of Luttinger liquid parameters in single wall carbon nanotubes networks

We measure electrical transport on networks of single wall nanotube of different origin as a function of temperature T, voltage V and pressure P. We observe Luttinger liquid (LL) behavior, a conductance ∝T^α and a dynamic conductance ∝V^α. We observe a sample dependent P variation of the α parameters, interpreted as Fermi level changes due to pressure induced charge transfer. We show how, through standard four-leads and crossed configuration methods, it is possible to determine α_bulk and α_end, respectively. We study and discuss the pressure and doping level dependences of the number of channels N, the LL parameter g and the intra-rope tube-tube coupling constant U within a phenomenological model.     

高压下CO_2-乙醇二元系统的气液平衡

在带有可视石英窗的可变体积高压釜内 ,在 60－ 180℃温度范围、 4.0－ 14.5 MPa压力范围下测定了 CO2和乙醇二元系统的相平衡数据 .根据实验结果预测了 pc,Tc,xc.用 Peng-Robinson方程和 Wong-Sandler混合规则进行了拟合 ,所得结果与实验数据基本相符 .     

Phase transitions in 1,3,4-oxadiazole crystals under high pressure

Crystalline 2,5-di(4-nitrophenyl)-1,3,4-oxadiazole (DNO) has been investigated at pressures up to 5 GPa using Raman and optical spectroscopy as well as energy dispersive X-ray techniques. At ambient pressure DNO shows an orthorhombic unit cell (a=0.5448 nm, b=1.2758 nm, c=1.9720 nm, density 1.513 g cm⁻³) with an appropriate space group Pbcn. From Raman spectroscopic investigations three phase transitions have been detected at 0.88, 1.28, and 2.2 GPa, respectively. These transitions have also been confirmed by absorption spectroscopy and X-ray measurements. Molecular modeling simulations have considerably contributed to the interpretation of the X-ray diffractograms. In general, the nearly flat structure of the oxadiazole molecule is preserved during the transitions. All subsequent structures are characterized by a stack-like arrangement of the DNO molecules. Only the mutual position of these molecular stacks changes due to the transformations so that this process may be described as a topotactical reaction. Phases II and III show a monoclinic symmetry with space group P2₁/c with cell parameters a=1.990 nm, b=0.500 nm, c=1.240 nm, β=91.7°, density 1.681 g cm⁻³ (phase II, determined at 1.1 GPa) and a=1.890 nm, b=0.510 nm, c=1.242 nm, β=89.0°, density 1.733 g cm⁻³ (phase III, determined at 2.0 GPa), respectively. The high-pressure phase IV stable at least up to 5 GPa shows again an orthorhombic structure with space group Pccn with corresponding cell parameters at 2.9 GPa: a=0.465 nm, b=1.920 nm, c=1.230 nm and density 1.857 g cm⁻³. For the first phase a blue pressure shift of the onset of absorption by about 0.032 eV GPa⁻¹ has been observed that may be explained by pressure influences on the electronic conjugation of the molecule. In the intermediate and high-pressure phases II–IV the onset of absorption shifts to increased wavelengths due to larger intermolecular interactions and enhanced excitation delocalization with decreasing intermolecular spacing.     

Single-crystal growth of Tl2Ru2O7 pyrochlore using high-pressure and flux method

Single crystals of the thallium ruthenium pyrochlore have been grown by flux method under high oxygen pressure. The growth conditions were determined by direct observations using in situ powder X-ray diffraction (XRD) method under high pressure and high temperature. The crystals were grown using NaCl-KCl flux at 1350 °C and B₂O₃ flux at 1150 °C. High growth temperature of 1350 °C for the NaCl-KCl flux caused Pt contamination from the crucible and oxygen deficiency for the crystals obtained. The crystal growth using B₂O₃ flux proceeded at lower temperature by grain growth with material transfer through B₂O₃. The crystal obtained was characterized by single-crystal XRD method, and was found to have a stoichiometric composition, Tl₂Ru₂O_7−δ (δ=0), with a structural phase transition around 120 K. The grain growth technique with B₂O₃ is efficient for high-temperature single-crystal growth under high pressure.