High-Pressure Structures and Superconductivity of Barium Iodide

Generally, pressure is a useful tool to modify the behavior of physical properties of materials due to the change in distance between atoms or molecules in the lattice. Barium iodide (BaI2), as one of the simplest and most prototypical iodine compounds, has substantial high pressure investigation value. In this work, we explored the crystal structures of BaI2 at a wide pressure range of 0–200 GPa using a global structure search methodology. A thermodynamical structure with tetragonal I4/mmm symmetry of BaI2 was predicted to be stable at 17.1 GPa. Further electronic calculations indicated that I4/mmm BaI2 exhibits the metallic feature via an indirect band gap closure under moderate pressure. We also found that the superconductivity of BaI2 at 30 GPa is much lower than that of CsI at 180 GPa based on our electron–phonon coupling simulations. Our current simulations provide a step toward the further understanding of the high-pressure behavior of iodine compounds at extreme conditions.

Crystallization of Isotactic Polypropylene Nanocomposites with Fibrillated Poly(tetrafluoroethylene) under Elevated Pressure

Nanocomposites of isotactic polypropylene with 1–5 wt.% of fibrillated PTFE (PP/T) were prepared, and their crystallization during cooling under elevated pressure, in a wide pressure range, up to 300 MPa, as well as the resulting structure, were examined. The crystallization peak temperatures of PP/T, especially with 3 and 5 wt.% of PTFE, exceeded by up to 13 °C those of neat PP. Moreover, a fine-grain structure was formed in PP/T in the entire pressure range, which proved the ability of the fibrillated PTFE to nucleate crystallization of PP in the γ-form under elevated pressure. This also resulted in a higher crystallinity level developed in the γ-domain, before the temperature range of the α-domain was reached during cooling. Hence, the γ-content increased in comparison to that in neat PP, under the pressure up to 200 MPa, especially under 50–100 MPa.

Density functional prediction of the structural, elastic, electronic, and thermodynamic properties of the cubic and hexagonal (c, h)-Fe2Hf

Using density functional theory (DFT), the structural, elastic, electronic, and thermodynamic properties of Fe2Hf in the cubic and hexagonal solid phases with Fd-3m and P63/mmc are reported with generalized gradient approximations (GGA). To achieve energy convergence, we report the k-point mesh density and plane-wave energy cut-offs. The calculated equilibrium parameters are in good agreement with the available theoretical data. A complete elastic tensor and crystal anisotropies of the ultra-incompressible Fe2Hf are determined in the wide pressure range. Finally, by using the quasi-harmonic Debye Model, the isothermal and adiabatic bulk modulus and heat capacity of Fe2Hf are also successfully obtained in the present work. By the elastic stability criteria, it is predicted that Fd-3m and P63/mmc structures of Fe2Hf are stable in the pressure range studied, respectively.

Gas discharge sustained by powerful THz and sub-THz gyrotrons in the mixtures of noble gases with nitrogen

The discharge propagation velocity towards electromagnetic radiation of sub-THz and THz bands was measured in various noble gases (argon, krypton) mixtures with nitrogen in the wide pressure range (0.1 – 2 atm) for various field intensities into the focal spot (from dozen of kW/cm2 to several MW/cm2). In the experimental setups two different gyrotrons were used. In case of 263 GHz it was CW gyrotron with power up 1 kW, in case of 670 GHz – pulsed gyrotron (20 μs) with power up to 40 kW. In both cases the focusing system provided the size of the focal spot of (2–3)·λ, which ensured the investigation of discharge phenomena in a wide pressure range (0.1 – 2 atm). In both cases discharge appeared in the focal spot spread towards heating radiation into the area with the field intensity much less than one in the focal spot. Velocity of the discharge propagation was measured by using photos from speed camera with small exposure (down to 20 ns) and streak camera. It was demonstrated that discharge velocity increase along with pressure decrease and drops with electric field decrease as it moves away from the focal spot.

Nanosecond breakdown in a pulsed open discharge

The paper presents the results of studies of open discharge breakdown characteristics at nanosecond supply pulse in helium, neon, argon working medium in a wide pressure range. It is shown that the open discharge exists in the range of p He = 20–100 Torr, pNe = 1.5–25 Torr, p Ar = 0.5–3.7 Torr with used experimental conditions. With increasing pressure, the role of separate elementary processes increases which can lead to a change of the discharge form to a presumably avalanche or streamer discharge. However, in this case, due to the photoemissive nature of the open discharge initiation, the similarity law Ep = f(pι) does not coincide with that for a nanosecond avalanche discharge.

Temperature Compensated Wide-Range Micro Pressure Sensor with Polyimide Anticorrosive Coating for Harsh Environment Applications

In this work, a MEMS piezoresistive micro pressure sensor (1.5 × 1.5 × 0.82 mm) is designed and fabricated with SOI-based micromachining technology and assembled using anodic bonding technology. In order to optimize the linearity and sensitivity over a wide effective pressure range (0–5 MPa) and temperature range (25–125 °C), the diaphragm thickness and the insulation of piezoresistors are precisely controlled by an optimized micromachining process. The consistency of the four piezoresistors is greatly improved by optimizing the structure of the ohmic contact pads. Furthermore, the probability of piezoresistive breakdown during anodic bonding is greatly reduced by conducting the top and bottom silicon of the SOI. At room temperature, the pressure sensor with 40 µm diaphragm demonstrates reliable linearity (0.48% F.S.) and sensitivity (33.04 mV/MPa) over a wide pressure range of 0–5.0 MPa. In addition, a polyimide protection layer is fabricated on the top surface of the sensor to prevent it from corrosion by a moist marine environment. To overcome the linearity drift due to temperature variation in practice, a digital temperature compensation system is developed for the pressure sensor, which shows a maximum error of 0.43% F.S. in a temperature range of 25–125 °C.