New Phase in Fluid Hydrogen at Room Temperature

The presence of phase-transition in hydrogen (H2) at around 560 megapascal (MPa) and room temperature was clarified by Raman and x-ray diffraction studies on both pure H2 and graphite-H2 mixture. H2 is intercalated into the nano-space of graphite, which lowers the transition pressure and temporally expands the size of the honeycomb lattice of graphite under pressure up to 600 MPa. It is supposed that is caused by a gas-liquid phase-transition. According to the peak analysis for Q1(J) mode, the ortho-para conversion of H2 gradually begins to appear after the phase-transition pressure even at room temperature, while peak separation is difficult to achieve under pressure above 1.6 gigapascal (GPa) because of significant overlapping of the peak intensities. Because we have missed the ortho-para conversion which could be observed in only such a small pressure range, the fluid phase at room temperature was full of mystery.

A New Macro-Model of Gas Flow and Parameter Extraction for a CMOS-MEMS Vacuum Sensor

When using a MEMS sensor to measure the vacuum of a medium, the transition flow between the viscous flow and molar flow is usually used to describe the gas convection due to the physical principle, which is difficult to study through analysis and simulation. In this study, the description of gas flow under different pressures in a CMOS-MEMS vacuum sensors has been incorporated into a new behavioral ANSYS model. The proposed model was built and the characteristic parameters in the model were obtained based on a CMOS-MEMS thermopile patterned with circular symmetry and an embedded heater as a heat source. It contains a characteristic length to describe the effective distance of heat dissipation to the silicon substrate, and the corresponding transition pressure to describe the symmetrical phenomenon of gas heat conduction. The macro-model is based on the description of the physical characteristics of heat transfer and the characteristic parameters of the CMOS-MEMS vacuum sensor. The characteristic length of 49 μm and the corresponding transition pressure of 2396 mTorr for the thermoelectric-type vacuum sensor were extracted and verified successfully. The results show that the average error for the prediction of vacuum sensing by the macro-model we proposed is about 1.11%. This approach provides more applications for vacuum analysis. It can reduce the complexity of simulation and analysis and provide better simulation effects, including gas conduction mechanisms.

The high-pressure study of nanocrystalline CeO2

The structure and properties of the nanosized [Formula: see text] under high pressure have been investigated by synchrotron X-ray diffraction in the diamond anvil cell combined with the first-principle methods based on density functional theory. The experimental data demonstrate that nanosized [Formula: see text] is highly stable upto 31.3 GPa and bulk modulus is [Formula: see text] GPa. The calculated results by [Formula: see text] indicate that [Formula: see text] undergoes structural transition from cubic fluorite-type structure to orthorhombic [Formula: see text]-[Formula: see text]-type structure at 20 GPa. The indirect band gap [Formula: see text] is 1.89 eV and increases with the increasing pressure while it suddenly reduces to 1.62 eV at transition pressure. The transition pressure calculated by GGA is 40.3 GPa.

On the role of U/ThO8 polyhedral distortions in controlling the high-pressure zircon→reidite type transition in UxTh1-xO4

<p><strong>On the role of U/ThO<sub>8 </sub>polyhedral distortions in controlling the high-pressure zircon→reidite type transition in U<sub>x</sub>Th<sub>1-x</sub>O<sub>4</sub></strong></p><p> </p><p>Sudip Kumar Mondal<sup>1<strong>,</strong>2</sup>, Pratik Kr Das<sup>2<strong>,</strong>3</sup>, Nibir Mandal<sup>2</sup> and Ashok Arya<sup>4</sup><br>1 Department of Physics, Jadavpur University, Kolkata 700032, India<br>2 Faculty of Science, High Pressure and Temperature Laboratory, Jadavpur University, Kolkata 700032, India<br>3 The Centre for Earth Evolution and Dynamics, University of Oslo, Oslo, N-0315, Norway<br>4 Material Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India</p><p> </p><p>Coffinite (USiO<sub>4</sub>) and thorite (ThSiO<sub>4</sub>) are conspicuous radiogenic silicates in the geonomy. They form U<sub>1</sub><sub>-</sub><sub>x</sub>Th<sub>x</sub>SiO<sub>4</sub> (uranothorite) solid solutions in zircon-type phase. Investigating the phase-evolution of these minerals is of utmost significance in realizing their applicability in the front-as well as at the back-end of nuclear industries and also from geological perspective, such as geochronology. We carried out a systematic study of zircon- to reidite-type (tetragonal I41/amd to I41/a) structural transitions of U<sub>1</sub><sub>-</sub><sub>x</sub>Th<sub>x</sub>SiO<sub>4 </sub>solid solution, and investigated their mechanical behaviour. Our ab-initio calculations revealed a unique interconnection of phase transition pressure (p<sub>t</sub>) with the change in U-Th concentration in the solid solution. The transition pressure is found to be minimum (6.82 GPa) for x = 0.5 whereas for the endmembers coffinite and thorite p<sub>t</sub>’s are 8.52 and 8.68 GPa, respectively. We developed a novel method to estimate the longitudinal and angular distortions of the highly irregular U/ThO<sub>8</sub>-triangular dodecahedra (snub-disphenoids). We have parameterized two new factors: δ (longitudinal distortions) and σ<sup>2</sup> (angular distortions) to quantify the polyhedral distortions. A detailed analysis of the snub-disphenoidal distortions demonstrates that the difference in angular distortion of UO<sub>8</sub> and ThO<sub>8</sub> polyhedra (i.e. σ<sub>U</sub><sup>2 </sup>and σ<sub>Th</sub><sup>2</sup>) between zircon- and  reidite-type phases becomes minimum when U and Th percentage are equal, leading to the structural phase transition at the minimum hydrostatic pressure for the unique chemical composition: U<sub>0.5</sub>Th<sub>0.5</sub>SiO<sub>4</sub>. Our result is also substantiated by the minimum compressibility observed for the zircon-type U<sub>0.5</sub>Th<sub>0.5</sub>SiO<sub>4</sub>. It is worthwhile to note that the distortions parameters, δ and σ<sup>2</sup> are defined without any attribute to external parameters. They are also independent to the elements occupying the polyhedra. Thus, we propose that these parameters: δ and σ<sup>2</sup> can also be used to calculate the distortions of similar AB<sub>8</sub>-type snub-disphenoids observed in zircon-, reidite-, fergusonite- and wolframite-type mineral phases.</p>

Judging the phase transition pressure of the unknown parent phase if the resulting state is known

Under the condition that the parent phase is unknown, we have directly judged the phase transition points from the new phase.