Flame structure of HMX/GAP propellant at high pressure

The chemical and thermal structure of a HMX/GAP propellant flame was investigated at a pressure of 0.5 MPa using molecular beam mass spectrometry and a microthermocouple technique. The pressure dependence of the burning rate was measured in the pressure range of 0.5–2 MPa. The mass spectrometric probing technique developed for flames of energetic materials was updated to study the chemical structure of HMX/GAP flames at high pressures. Eleven species, including HMX vapor, were identified, and their concentrations were measured in a zone adjacent to the burning surface at pressures of 0.5 and 1 MPa. Temperature profiles in the propellant combustion wave were measured at pressures of 0.5 and 1 MPa. Species concentration profiles were measured at 0.5 MPa. Two main zones of chemical reactions in the flame were found. The data obtained can be used to develop and validate combustion models for HMX/GAP propellants.     

Investigation of Nd2CuO4 crystal structure at high pressure by neutron diffraction

Abstract

The crystal structure of Nd₂CuO₄ has been studied by neutron diffraction at pressure up to 5 Gpa. The volume compressibility value was determined as 5·6·10^?3/Gpa. The decrease of positional parameter of neodymium at high pressure has been observed. This structural change is explained by pressure induced neodymium ions charge increase.     

Crystal structures of silicon-rich lithium silicides at high pressure

Silicon (Si) is one of the most essential elements, as it is indispensable for modern electronic technology. The standard Si structure at ambient conditions is the cubic diamond structure, and it has an indirect band gap, which prevents it from being considered as a next-generation platform for semiconductor technologies. Therefore, the search for new allotropes of silicon has attracted great attention. Herein, first principles swarm-intelligence structure searches coupled with density-functional theory were performed to explore the stable high-pressure phases of silicon-rich lithium containing compounds, LiSi_x ($x=4\text{–}8$). The LiSi₄ stoichiometry was predicted to be stable, and it was found to assume one of the following space groups, P4/mnc, Cmmm, and C2/m within the pressure range of 0 to 50 GPa. By removing the Li atoms from these compounds, three silicon allotropes were obtained that were metastable at ambient pressures. Our work illustrates how novel silicon allotropes can be predicted using the CALYPSO method.     

Density measurements of liquids at high pressure: Modifications to the sink/float method by using composite spheres,and application to Fe-10wt%S

Abstract

We describe a new modification of the sink/float method of determining the density of molten materials under high pressure, and apply it to liquid Fe-10wt%hS. The modification is based on the use of composite spheres made of a dense, metallic core and a sapphire or ruby mantle. Composite spheres permit tailored densities for use in a variety of liquid compositions. The inert mantle also prevents chemical reaction with molten metallic samples. The pressure dependence of the density of liquid Fe-10wt%S at 1773 K, determined by bracketing, using composite spheres, is compared with the equation of state of Fe-10wt%S obtained by Sanloup et al. (2000), using X-ray absorption.     

Spectroscopy of II–VI nanocrystals at high pressure and high temperature

We review experimental measurements of CdS, CdSe, and CdS_xSe_{1 − x} nanocrystals which address the properties and phase stability of both powder and embedded nanoparticles under extreme conditions. We address the high pressure phase transition of wurtzite or zincblende phase to the rock salt structure; the high temperature solid to liquid phase transition; and the homogeneous nucleation of nanoparticles in glass from dissolved reactants. We also review the use of high pressure optical measurements to study electronic states.     

Single crystal EXAFS at high pressure

Abstract

We present a new technique for structure characterization under high pressure conditions. The use of an undulator beam of the third-generation ESRF source of synchrotron radiation has enabled the first single crystal EXAFS experiments at high pressure using a diamond anvil cell as pressure generator. Taking advantage of the linear polarization of X-rays the technique becomes an orientation-selective probe of the local structure of materials. We describe the principle of the technique and some applications.     

Structure and high pressure behaviour of organic crystals based on aromatically substituted 1,3,4-oxadiazoles

The crystalline structure of a new compound containing the 1,3,4-oxadiazole moiety, 4-(5-methyl-1,3,4-oxadiazole-2yl-)-N,N′-dimethyl-phenylamine (MODPA) was determined. It shows a monoclinic structure with space group P2₁/c and lattice parameters: a=1.02997(6), b=0.64840(4), c=1.58117(10) nm and β=99.4820(10)°. To study the intermolecular interactions in oxadiazole containing organic crystals, X-ray studies on MODPA and 2,5-diphenyl-1,3,4-oxadiazole (DPO) were performed up to 5 GPa at room temperature. The Murnaghan equation of state is used to describe the compression behaviour of both substances. From these results, the bulk modulus and its pressure derivative were determined. The values obtained are: K₀=6.3 GPa and K′₀=6.8 for MODPA and K₀=7.3 GPa and K′₀=6.7 for DPO. Additionally, measurements under increasing temperature at ambient pressure were carried out to evaluate the thermal expansion coefficient: α=1.8×10⁻⁴ K⁻¹ for MODPA and α=1.9×10⁻⁴ K⁻¹ for DPO.     

Fabrication of composite structure of carbon fibers and high density SiC nanowires

Composite structure of carbon fibers and SiC nanowires was fabricated by a simple chemical vapor deposition process, using commercial silicon dioxide and graphite powders as raw materials. The analysis of scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction indicates that the synthesized SiC nanowires distribute uniformly with high density in the used carbon fiber preform, which are perpendicular to and around each carbon fiber in a radial array. The SiC nanowires located at the interface of advanced composites is very favorable to the interfacial bonding between composites matrix and carbon fibers, thereby increasing the strength of composites greatly.     

Structure and dynamics of liquid formamide at high pressure and high temperature: comparison of X-ray diffraction and molecular dynamics results

New molecular dynamics simulations with optimised potentials for liquid simulation are presented for liquid formamide at high pressures and high temperatures. The structural results are compared to those found by X-ray diffraction measurements, and the H-bonding structure is analysed in detail. While it is ambiguous from purely experimental data, the simulation results support the idea that pressure increase can enhance ring dimer formation at the expense of linear chain structure, but increasing temperature results in an opposite effect. Dynamical results (reorientational correlation times and lifetimes of hydrogen bonds) are in agreement with these findings.     

Structural, electronic and vibrational properties of InN under high pressure

The structural, electronic and vibrational properties of InN under pressures up to 20 GPa have been investigated using the pseudo-potential plane wave method (PP-PW). The generalized-gradient approximation (GGA) in the frame of density functional theory (DFT) approach has been adopted. It is found that the transition from wurtzite (B4) to rocksalt (B1) phase occurs at a pressure of approximately 12.7 GPa. In addition, a change from a direct to an indirect band gap is observed. The mechanism of these changes is discussed. The phonon frequencies and densities of states (DOS) are derived using the linear response approach and density functional perturbation theory (DFPT). The properties of phonons are described by the harmonic approximation method. Our results show that phonons play an important role in the mechanism of phase transition and in the instability of B4 (wurtzite) just before the pressure of transition. At zero pressure our data agree well with recently reported experimental results.     

Structural study of ND4Br at high pressure

Abstract

A structure of ND₄Br has been studied at pressures up to 9 GPa by means of time-of-flight neutron diffraction. A phase transition to the high pressure phase V was observed at P=8·2(5)GPa. It was found that the phase V has a tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm which is in strong resemblance with low temperature modification ND₄Br(III). Deuterium positional parameter as a function of pressure was obtained.     

Electronic structure of platinum at ultrahigh pressure

Abstract

Platinum is studied, theoretically, under very high compression. The calculated equation of state is found to agree well with the recent experimental data. At V/V₀ = 0.4, where V₀ is the experimental equilibrium volume, we find a transition from the face centered cubic structure (fcc), found at ambient pressure, to the body centered cubic structure (bcc). The calculated transition pressure is 26 Mbar. The stabilization of the bcc structure is explained by the eigen value sum.     

Flame structure and stabilization of lean-premixed sprays in a counterflow with low-volatility fuel

An experimental study was performed on the combustion of lean-premixed spays in a counterflow. n-Decane was used as a liquid fuel with low volatility. The flame structure and stabilization were discussed based on the flame-spread mechanism of a droplet array with a low-volatility fuel. The spray flame consisted of a blue region and a yellow luminous region. The flame spread among droplets and group-flame formation through the droplet interaction were observed on the premixed spray side, while envelope flames were also observed on the opposing airflow side. The blue-flame region consisted of premixed flames propagating in the mixture layer around each droplet, the envelope diffusion flames around each droplet, the lower parts of the group diffusion flame surrounding each droplet cluster, and the envelope flame around droplets passing through the group flame. The flame was stabilized within a specific range of the mean droplet diameter via a balance between the droplet velocity and the flame-spread rate of the premixed spray.     

Burning velocity correlation of methane/air turbulent premixed flames at high pressure and high temperature

Turbulent burning velocities for methane/air mixtures at pressures ranging from atmospheric pressure up to 1.0 MPa and mixture temperatures of 300 and 573 K were measured, which covers the typical operating conditions of premixed-type gas-turbine combustors. A bunsen-type flame stabilized in a high-pressure chamber was used, and OH-PLIF visualization was performed with the pressure and mixture temperature being kept constant. In addition to a burner with an outlet diameter of 20 mm for the high-pressure experiments, a large-scale burner with an outlet diameter of 60 mm was used at atmospheric pressure to extend the turbulence Reynolds number based on the Taylor microscale, R_λ, as a common parameter to compare the pressure and temperature effects. It was confirmed that R_λ over 100 could be attained and that u′/S_L could be extended even at atmospheric pressure. Based on the contours of the mean progress variable c = 0.1 determined using OH-PLIF images, turbulent burning velocity was measured. S_T/S_L was also found to be greatly affected by pressure for preheated mixtures at 573 K. The bending tendency of the S_T/S_L curves with u′/S_L was seen regardless of pressure and mixture temperature and the R_λ region where the bending occurs corresponded well to the region where the smallest scale of flame wrinkling measured as a fractal inner-cutoff approaches the characteristic flame instability scale and becomes almost constant. A power law of S_T/S_L with (P/P₀)(u′/S_L) was clearly seen when S_T was determined using c = 0.1 contours, and the exponent was close to 0.4, indicating agreement with the previous results using the mean flame cone method and the significant pressure effects on turbulent burning velocity.     

A high pressure raman study of ThO2 to 40 GPa and pressure-induced phase transition from fluorite structure

The pressure dependence of the first-order Raman peak and two second-order Raman features of ThO₂ crystallizing in the fluorite-type structure is investigated using a diamond anvil cell, up to 40GPa. A phase transition from the fluorite phase is observed near 30 GPa as evidenced by the appearance of seven new Raman peaks. The high pressure phases of ThO₂ and CeO₂ exhibit similar Raman features and from this it is believed that the two structures are the same, and have the PbCl₂-type structure. The pressure dependence dω/dP of the observed phonons and their mode Grüneisen parameters are similar to the isostructural CeO₂. The observed second-order Raman features are also identified from the calculated phonon dispersion curves for ThO₂.     

Behavior of decomposed ammonia borane at high pressure

High pressure behavior of ammonia borane after thermal decomposition was studied by Raman spectroscopy at high pressure up to 10 GPa using diamond anvil cell (DAC). The ammonia borane was decomposed at around 140 °C under the pressure at ∼0.7 GPa. Raman spectra show the hydrogen was desorbed within 1 h. The hydrogen was sealed in DAC well and cooled down to room temperature. Applying higher pressure up to ∼10 GPa indicates interactions between the products and loss of dihydrogen bonding. No rehydrogenation was detected in the pressure range investigated.     

Decomposition of binary sulphate KHSO4 at high pressure

Abstract

Pressure induced decomposition (PID) is known to occur only at elevated temperatures. Here we report its first occurrence at ambient temperature in a binary sulphate, KHSO₄ at 4kbar. Raman spectroscopy is used for identifying the decomposition products as K₃H(SO_{2 4}) and H₂SO₄ from their characteristic spectra. One of the decomposition products being a liquid is argued to be the reason for the occurrence of the phenomenon at ambient temperature. Other possible decomposition routes are also examined.     

Raman spectra for hydrogen hydrate under high pressure: Intermolecular interactions in filled ice Ic structure

Raman studies of a high-pressure structure of hydrogen hydrate, a filled ice Ic structure, were performed using a diamond anvil cell in the pressure range 3.2-44.1 GPa. The Raman spectra of a vibron revealed that extraction of hydrogen molecules from the filled ice Ic structure occurred above 20 GPa. In addition, the Raman spectra of a roton revealed that a rotation of hydrogen molecules in the filled ice Ic structure was suppressed at around 20 GPa and then the rotation recovered, and the rotation of hydrogen molecules was suppressed again above 35.5 GPa. These results indicate that intermolecular interactions increased between guest hydrogen molecules and host water molecules at around 20 and 35.5 GPa. These intermolecular interactions were considered to be induced to stabilize the filled ice Ic structure. Above 40 GPa, symmetrization of hydrogen bond was considered to contribute to the stability of hydrogen hydrate.     

基于拉曼频移的白宝石压腔无压标系统高温高压实验标定

在高压实验科学中, 各类宝石压腔是最为常见的高压设备之一, 其样品腔中压力的精确标定是实验的关键. 目前, 人们主要通过加入红宝石等压标物质来进行定压, 但压标物质的加入会增加实验的装样难度, 改变样品腔中的物理化学环境, 甚至直接与实验样品发生反应, 从而对实验结果产生影响. 在0–6.3 GPa和300–573 K下, 利用共聚焦拉曼显微镜, 根据白宝石压砧砧面的ν₁₂ 拉曼频移与温度和压力的变化关系, 建立了一套适用于高温高压水热体系的无压标白宝石压腔系统. 实验结果表明: 白宝石砧面的ν₁₂ 峰随着压力的升高发生线性蓝移, 而随着温度升高则发生线性红移, 且温度和压力对拉曼频移的影响存在耦合效应. 利用本实验结果, 可在高温高压下根据白宝石砧面的拉曼频移计算出样品腔的压力P=(Δλ-0.01913×ΔT)/(1.9158-0.00105×ΔT), 在物理学、材料学和地球科学等领域具有重要应用.