X-ray absorption spectroscopy under high pressures in diamond anvil cells

Abstract

Although potentially extremely important for understanding the high-pressure microscopic behaviour of materials, over the years the area of high-pressure EXAFS in particular using diamond anvil cells has proved to be technically difficult. This has significantly hampered its development. The interference of X-ray dimaction from the diamonds in the diamond anvil cell with the absorption signal has proved to be a challenging problem to tackle, restricting the use of high-pressure EXAFS to energies below about 11 key Below 11 keV however the technique is also limited due to absorption of incident X-rays by the diamonds making it virtually impossible to conduct X-ray absorption experiments below about 9keV In this paper we describe in detail the methodology for scanriirig high-pressure EXAFS in diamond anvil cells and examine the origins of the associated problems and ways of dealing with them. We also demonstrate that it is possible to extend the useful range of studied absorption edges from 7keV up to at least 30keV This brings about new opportunities for high pressure EXAFS using diamond anvil cells.     

High pressure atomic structure of Zr–Cu metallic glass via EXAFS spectroscopy and molecular dynamics simulations

ABSTRACT

In this work, we use extended X-ray absorption fine structure (EXAFS) data collected using nano-polycrystalline diamond anvil cell to study the atomic arrangement in Zr–Cu metallic glass in high pressure (HP) conditions. To reveal the microscopic details of stress accommodation mechanism, we performed molecular dynamics (MD) simulations of the HP atomic arrangement. By comparing the experimental and the calculated Zr and Cu K-edge EXAFS signal we prove the realistic character of the computer simulations. A detailed geometrical analysis of the simulated atomic configurations shows that with increasing hydrostatic pressure the local structure of Zr–Cu amorphous alloy becomes gradually dominated by Cu-centred icosahedral structural motifs involving fivefold symmetry incompatible with crystalline ordering. The variation of the short-range order is attributed to preferential straining of mechanically soft bonds between zirconium atoms.     

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.     

Electrooptical effect in the plasmon structure glass–In<Subscript>2</Subscript>O<Subscript>3</Subscript>: Sn–ferroelectric–Al with a subwavelength grating

We have constructed the equations of state for crystalline boron carbide B₁₁C (C–B–C) and its melt under high dynamic and static pressures. A kink on the shock adiabat for boron carbide has been revealed in the pressure range near 100 GPa, and the melting curve with negative curvature in the pressure range 0–120 GPa has been calculated. The results have been used for interpreting the kinks on the shock adiabat for boron carbide in the pressure range of 0–400 GPa.     

The properties of boron carbide/silicon heterojunction diodes fabricated by Plasma-Enhanced Chemical Vapor Deposition

p-n heterojunction diodes have been fabricated from boron carbide (B_1–x C _x ) and n-type Si(111). Boron carbide thin films were deposited on Si(111) using Plasma-Enhanced Chemical Vapor Deposition (PECVD) from nido-pentaborane (B₅H₉) and methane (CH₄). Composition of boron carbide thin films was controlled by changing the relative partial pressure ratio between nido-pentaborane and methane. The properties of the diodes were strongly affected by the composition and thickness of boron carbide layer and operation temperatures. Boron carbide/silicon heterojunction diodes show rectifying properties at temperatures below 300° C. The temperature dependence of reverse current is strongly dependent upon the energy of the band gap of the boron carbide films.     

Convergence acceleration of molecular dynamics methods for shocked materials using velocity scaling

ABSTRACT

In this work, a convergence acceleration method applicable to extended system molecular dynamics techniques for shock simulations of materials is presented. The method uses velocity scaling to reduce the instantaneous value of the Rankine–Hugoniot conservation of energy constraint used in extended system molecular dynamics methods to more rapidly drive the system towards a converged Hugoniot state. When used in conjunction with the constant stress Hugoniostat method, the velocity scaled trajectories show faster convergence to the final Hugoniot state with little difference observed in the converged Hugoniot energy, pressure, volume and temperature. A derivation of the scale factor is presented and the performance of the technique is demonstrated using the boron carbide armour ceramic as a test material. It is shown that simulation of boron carbide Hugoniot states, from 5 to 20 GPa, using both a classical Tersoff potential and an ab initio density functional, are more rapidly convergent when the velocity scaling algorithm is applied. The accelerated convergence afforded by the current algorithm enables more rapid determination of Hugoniot states thus reducing the computational demand of such studies when using expensive ab initio or classical potentials.     

Deformation twinning in boron carbide particles within nanostructured Al 5083/B4C metal matrix composites

The presence of deformation twins is documented in boron carbide reinforcement particles within a nanostructured Al 5083/B₄C metal matrix composite. High resolution transmission electron microscopy analysis suggests that these are (0001) twins. This work discusses the mechanisms responsible for their formation based on crystallographic analysis and mechanical loading conditions. Specifically, we propose that there are two potential models that can be used to describe twin formation in boron carbide particles. The structural models involve slip in the 1/3[1100] (0110) or 1/3[0110] (0110) planes of C–C–C chains and the appropriate reconfiguration of B–C bonds. Analysis of the loading conditions experienced by the boron carbide particles indicates that local high stress intensity and the presence of a high shear force around the boron carbide particles are two factors that contribute to twin formation.     

Equation of state of turbostratic boron nitride

Abstract

The lattice parameters of turbostratic boron nitride (tBN) have been measured to 6.1 GPa at room temperature using energy-dispersive powder diffraction with synchrotron radiation. A fit to the experimental p-V data using Birch-Murnaghan equation of state gives values of the tBN bulk modulus 17.5(8) GPa and its pressure derivative ll.4(5). These values point to significantly higher compressibility of turbostratic BN as compared to three-dimensionally ordered graphite-like hexagonal and rhombohedral boron nitride.     

Pressure Dependence of Wurtzite ZnO Structure

From simultaneous X-ray diffraction and EXAFS experiments, structural properties in the Wurtzite phase of ZnO have been measured up to the transition pressure (9 GPa). These results are compared with first-principles calculations and discussed in terms of wurtzite mechanical stability at high pressure.     

A new cell for X-ray absorption spectroscopy study under high pressure

X-ray absorption fine structure （XAFS） spectroscopy is a powerful technique for the investigation of the local environment around selected atoms in condensed matter. XAFS under pressure is an important method for the synchrotron source. We design a cell for a high pressure XAFS experiment. Sintered boron carbide is used as the anvils of this high pressure cell in order to obtain a full XAFS spectrum free from diffraction peaks. In addition, a hydraulic pump was adopted to make in-suit pressure modulation. High quality XAFS spectra of ZrH2 under high pressure （up to 13 GPa） were obtained by this cell.     

Raman spectroscopy of the BC3 phase obtained under high pressure and high temperature

A direct transformation of the g‐BC₃ phase to a new diamond‐like d‐BC₃ phase was observed in a diamond‐anvil cell (DAC) at high temperature, 2033 ± 241 K, and high pressure, 50 GPa. Analysis of the peak positions of the d‐BC₃, B₄C, α‐boron, and the boron‐doped diamond leads to the conclusion that the positions of the peaks of the d‐BC₃ are more similar to the peak pattern of the boron‐doped diamond rather than that of boron carbide, α‐boron. Copyright © 2007 John Wiley & Sons, Ltd.     

High pressure generation in the Kawai-type multianvil apparatus equipped with sintered diamond anvils

ABSTRACT

Generation of high pressure is a key to the investigation of the interior of the Earth. The Kawai-type multianvil apparatus (KMA) has been widely used in the fields of Earth science and material science. In conventional KMA, tungsten carbide (WC) has been used as the second stage anvil material. However, attainable pressure is limited to ～70?GPa even if newly developed WC is used. Recently, on the other hand, second stage anvils of sintered diamond that is much harder than WC have enabled us to extend the pressure range of KMA up to 120?GPa, corresponding to the pressure of the D” layer in the Earth’s mantle. It is evident that our development of pressure generation facilitates the investigation of the structure and dynamics of the deep mantle of the Earth.     

Pressure study of monoclinic ReO2 up to 1.2 GPa using X‐ray absorption spectroscopy and X‐ray diffraction

The crystal and local atomic structure of monoclinic ReO₂ (α‐ReO₂) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X‐ray absorption spectroscopy and high‐resolution synchrotron X‐ray powder diffraction and a home‐built B₄C anvil pressure cell developed for this purpose. Extended X‐ray absorption fine‐structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re—Re distances and a distorted ReO₆ octahedron in the α‐ReO₂ structure. X‐ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the α‐phase, demonstrating a modulation of the Re—Re distances. The relatively small portion of the diffraction pattern accessed in the pressure‐dependent measurements does not allow for a detailed study of the crystal structure of α‐ReO₂ under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re—Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that α‐ReO₂ is a possible inner pressure gauge for future experiments up to 1.2 GPa.     

Theoretical band structure calculation and superconductivity of NbC under pressure

Abstract

We report a theoretical calculation of the band structure and superconductivity of niobium carbide in the NaCl structure under pressure. The effect of pressure on the band structure is obtained by means of the self-consistent linear muffin-tin orbital method. The parameters necessary to calculate the superconducting transition temperature (T_c) are taken from our band structure results. The dependence of total energy on volume is calculated and is in good agreement with other earlier works. The calculated value of the cell parameter is in agreement with the experimental value (8.45 a.u). McMillan formula is used to calculate the value of T_c The calculated values of T_c are compared with the available experimental data.     

Possibilities of the formation of carbides and borides by ion implantation

In the system of boron and carbon, the formation of boron carbide was investigated after ion implantation of 25 keV B ions into carbon or of 25 keV C ions into boron and subsequent annealing. TEM and electron diffraction studies showed that the crystallization of boron carbide begins only at temperatures above 1050°C. By implantation of 20 keV C ions into iron (ion dose 10¹⁷ C ions/cm²) only the metastable ε-Fe₂O will be generated, which at above 220°C transforms into the stable cementite Fe₃C. After implantation of 20 keV B ions into iron, no formation of iron boride could be found. These experimental facts can be understood qualitatively with the help of the thermal-spike model. The energy density or the temperature in the thermal spikes is not sufficient for the generation of cementite iron boride or boron carbide.     

In situ observation of Ni–Mo–S phase formed on NiMo/Al2O3 catalyst sulfided at high pressure by means of Ni and Mo K‐edge EXAFS spectroscopy

To obtain direct evidence of the formation of the Ni–Mo–S phase on NiMo/Al₂O₃ catalysts under high‐pressure hydrodesulfurization conditions, a high‐pressure EXAFS chamber has been constructed and used to investigate the coordination structure of Ni and Mo species on the catalysts sulfided at high pressure. The high‐pressure chamber was designed to have a low dead volume and was equipped with polybenzimidazole X‐ray windows. Ni K‐edge k³χ(k) spectra with high signal‐to‐noise ratio were obtained using this high‐pressure chamber for the NiMo/Al₂O₃ catalyst sulfided at 613 K and 1.1 MPa over a wide k range (39.5–146 nm^?1). The formation of Ni–Mo and Mo–Ni coordination shells was successfully proved by Ni and Mo K‐edge EXAFS measurement using this chamber. Interatomic distances of these coordination shells were almost identical to those calculated from Ni K‐edge EXAFS of NiMo/C catalysts sulfided at atmospheric pressure. These results support the hypothesis that the Ni–Mo–S phase is formed on the Al₂O₃‐supported NiMo catalyst sulfided under high‐pressure hydrodesulfurization conditions.     

A new cell for X-ray absorption spectroscopy study under high pressure

X-ray absorption fine structure (XAFS) spectroscopy is a powerful technique for the investigation of the local environment around selected atoms in condensed matter. XAFS under pressure is an important method for the synchrotron source. We design a cell for a high pressure XAFS experiment. Sintered boron carbide is used as the anvils of this high pressure cell in order to obtain a full XAFS spectrum free from diffraction peaks. In addition, a hydraulic pump was adopted to make in-suit pressure modulation. High quality XAFS spectra of ZrH2 under high pressure (up to 13 Gpa) were obtained by this cell.     

High-hardness ceramics based on boron carbide fullerite derivatives

A new type of ceramics based on the phases of fullerite derivatives and boron carbide B₄C is obtained. The material is synthesized at a temperature of 1500 K and a relatively low pressure of 4 GPa; it has a high hardness of 45 GPa and fracture toughness of 15 MPa m^1/2.     

Technology and equipment for hydrostatic treatment of materials

Abstract

The main requirements of the quality of products are the homogeneity of technological and service properties, the improved characteristics of strength and plasticity, endurance and brittle failure resistance. In recent years many investigations into developing the High Hydrostatic Pressure Technologies (HHPT) and equipment to meet these requirements have been carried out by scientific schools of Academicians, L. F. Vereshchagin, G. V. Kurdyumov, A. I. Tselikov and A. A. Galkin in the USSR. The physical basis of these technologies is represented by the plastic deformation effects under high pressure: material plasticization’, strain ageing², decrease of a carbide inhomogeneity³, use of a liquid as a machining tool⁴ etc. HHPT are widely spread and especially promising in metallurgy as the ferrous metals account for more than 96% of the total structural material consumption in machine building, while assortment of products is about 5000 profile sizes of bars, 30 000 profile sizes of tubes, 50 000 types of metalware.