Determination of pressure effect on thermocouple electromotive force using multi-anvil apparatus

We developed a method to determine the absolute pressure effect on thermocouple electromotive force (EMF), based on a single wire method using Kawai-type multi-anvil apparatus. In this method, pressure conditions along the wires were evaluated based on in situ X-ray diffraction using synchrotron X-ray radiation. The pressure effect of the Seebeck coefficients of chromel and alumel was determined up to 7?GPa and 600°C by the analyses of single wire EMFs and pressure-temperature profiles along the wires. The temperature correction for the type K thermocouple was calculated to be from 0°C to –3°C in the studied conditions. Since the multi-anvil apparatus is capable of achieving much higher pressure and temperature, the method presented in this study promises to reveal absolute temperature correction for thermocouples over a wide range of pressure and temperature conditions.     

Behavior of thermocouples under high pressure in a multi-anvil apparatus

We have conducted experiments to study the behavior of W5%Re–W26%Re (type C) and Pt10%Rh–Pt (type S) thermocouples under high pressure in a multi-anvil apparatus. The electromotive force (emf) between four different or three identical thermocouple wires was measured up to 15?GPa and 2100?°C. Mechanical and chemical stability of the thermocouples was examined during and after the experiments. Due to the effect of pressure on the emf/temperature relation, the temperature reading of the type C minus that of the type S thermocouple rises to +5?°C then falls to ?15?°C between room temperature and 1500?°C at 5?GPa, and to +25?°C and then ?35?°C between room temperature and 1800?°C at 15?GPa. In addition, we observed variations in the emf/temperature relation caused by uncertainties in the position and geometry of hot junctions in a steep temperature gradient, and by variable distribution of pressure gradient and non-hydrostatic stress on the thermocouple wires. These errors are estimated at 1.6% for the type S thermocouple up to 1700?°C, and 0.8% for the type C thermocouple up to 2100?°C. Self-diffusion and chemical contamination of the thermocouples by high-purity insulating ceramics appear negligible for the type S thermocouple at 1700?°C for one hour, and for the type C thermocouple at 2100?°C for half an hour. In contrast, large-scale displacement of the hot junction due to dislocation of the type C thermocouple wires and plastic deformation of the type S thermocouple wires may lead to large errors in temperature measurement (±200?°C).     

High–pressure electrical conductivity and Raman spectroscopy of chalcanthite

Abstract

The phase transitions and dehydration of chalcanthite were investigated by electrical conductivity and Raman spectroscopy at 1.0–24.0?GPa and 293–673?K in a diamond anvil cell. At ambient temperature, two secondary phase transitions were observed according to discontinuous changes in the slope of Raman shifts, full width at half maximum and electrical conductivities at ～7.3 and ～10.3?GPa. The dehydration temperatures were determined by the splitting of Raman peaks and changes in electrical conductivity as ～350 and ～500?K at respective ～3.0 and ～6.0?GPa. A positive relationship for chalcanthite between dehydration temperature and pressure is established.     

A XANES study of the Cu K-edge in A2CuCl4 perovskite layers under pressure. Influence of antiferrodistortive structure

Abstract

The temperature dependence of ice resistivity R has been investigated in the diamond anvil cell (DAC) of “rounded cone-plane” type. Mean pressure is about 20 GPa, temperature interval is 4,2–273 K. Measuring R it has been detected that ice changes to metallic conductivity type state between 220–260 K. At 25 K the temperature dependence of R shows the transport current dependence.     

Pressure–volume–temperature equations of state of Au and Pt up to 300 GPa and 3000 K: internally consistent pressure scales

By using a Mie–Grüneisen-type analysis method, the pressure–volume–temperature equations of state (P–V–T EOSs) of Au and Pt have been determined up to 300?GPa and 3000?K based on the experimental shock Hugoniot and thermodynamic data. The calculated results of Au and Pt show an excellent agreement with available experimental volume compression data over a wide range of pressures and temperatures. A comparison of our results with previous theoretical investigations has also been done. In addition, we have further examined the consistency of our results and the P–V–T EOS of MgO [K. Jin, X.Z. Li, Q. Wu, H.Y. Geng, L.C. Cai, X.M. Zhou, and F.Q. Jing, The pressure–volume–temperature equation of state of MgO derived from shock Hugoniot data and its application as a pressure scale, J. Appl. Phys. 107 (2010), pp. 113518] using simultaneous volume measurements of Au, Pt, and MgO at various temperatures. The good agreement among the P–V–T EOSs of Au, Pt, and MgO implies that these EOSs can be used as the reliable pressure scales in high pressure–temperature diamond anvil cell experiments.     

Direct measurement of magnetization isotherms of Fe64Ni36 Invar alloy in a diamond anvil cell

Magnetization isotherms of the Fe₆₄Ni₃₆ Invar alloy have been measured under pressure up to 5.3?GPa in magnetic field up to 5?T using a diamond anvil cell and SQUID magnetometer. The unambiguous change of the pressure parameter dlnM_S/dP (from ?9 to ?13×10^?2?GPa^?1) has been observed in a narrow pressure interval from 2.5 to 3.5?GPa at all temperatures in the range from 5 to 300?K. The pressure interval, where the sharp decrease in magnetization was observed, coincides with the critical pressures of the pressure-induced decrease in Fe-moment that were determined by the X-ray Magnetic Circular Dichroism and the X-ray Emission Spectra studies, recently. The pronounced decrease in the Curie temperature of the Fe₆₄Ni₃₆ alloy under pressure, dT_C/dP = ?44 ±2?K/GPa, has been confirmed.     

Synthesis of nano-polycrystalline diamond from glassy carbon at pressures up to 25 GPa

ABSTRACT

Nano-polycrystalline diamond (NPD) with various grain sizes has been synthesized from glassy carbon at pressures 15–25?GPa and temperatures 1700–2300°C using multianvil apparatus. The minimum temperature for the synthesis of pure NPD, below which a small amount of compressed graphite was formed, significantly increased with pressure from ～1700°C at 15?GPa to ～1900°C at 25?GPa. The NPD having grain sizes less than ～50?nm was synthesized at temperatures below ～2000°C at 15?GPa and ～2300°C at 25?GPa, above which significant grain growth was observed. The grain size of NPD decreases with increasing pressure and decreasing temperature, and the pure NPD with grain sizes less than 10?nm is obtained in a limited temperature range around 1800–2000°C, depending on pressure. The pure NPD from glassy carbon is highly transparent and exhibits a granular nano-texture, whose grain size is tunable by selecting adequate pressure and temperature conditions.     

Thermophysical properties of the polymorphic modifications of lithium hydride in the megabar shock pressure range

The region of a high electrical conductivity of lithium hydride is experimentally determined in the pressure range 100–150 GPa and the temperature range 2000–3000 K of multiple shock compression. This result is used to construct thermodynamic potentials for the two polymorphic modifications of lithium hydride (B1, B2), and these potentials make it possible to calculate its thermophysical properties in the shock pressure range 80–1200 GPa. The calculated and experimental results are analyzed to determine the B1 ? B2 equilibrium line for the polymorphic modifications of lithium hydride at pressures up to 300 GPa and temperatures up to 2000 K.     

Calibration of the R ruby fluorescence lines in the pressure range [0-1 GPa] and the temperature range [250-300 K]

Abstract

The pressure range [&1 GPa] and the temperature range [250–300 K] are commonly used in many science fields like biology, agro-chemistry, pharmacology, or geology. In this paper, the calibration of the ruby R lines of fluorescence is performed in these pressure and temperature ranges, using the melting curve of pure water. The linear shifts of ruby peaks are equal to ?0.140cm^?1/K and ?0.768cm^?1/kbar with R₁, and to ?0.137cm^?1/K and ?0.779 cm^?1/kbar with R₂. The accuracy of pressure measurements can be as good as ± 10MPa if the temperature is known with ±0.5 K. Such a precision is achieved if: (1) the position of each peak is determined using an inversion method; (2) daily shifts of the spectrometer are corrected before each acquisition; (3) peak positions of each ruby are known at ambient pressure and temperature.     

Magnetic Transitions in Fe 3 Pt Invar Alloy Under High Pressure and Temperature Studied by Inelastic X-ray Scattering

The magnetic properties of the Invar alloy Fe 72 Pt 28 have been investigated by X-ray emission spectroscopy as a function of temperature up to 900 K and pressure up to 25 GPa. With increasing temperature and pressure, the amplitude of the Fe local moment, deduced from the Fe K g line satellite intensity, is reduced. Both the temperature and pressure dependence are interpreted in terms of transitions from a high-spin state to a low-spin state. This behavior provides a microscopic picture of the Fe magnetism in Invar alloys in agreement with the 2 n state model.     

Sulphur electric properties at superhigh pressure around the polymerization temperatures

Abstract

The electrical resistivity temperature dependence of S has been studied in the diamond anvil cell (DAC) of the “rounded cone-plane” type. The pressure is about 50 GPa. The experimental data analysis shows the polymerization between 380–410 K. The electrical resistivity temperature dependence is distorted by a number of maxima, some of them being shifted along the temperature axis towards the lower temperature and disappear with increasing transport current.     

Determination of the phase boundary of GaP using in situ high pressure and high-temperature X-ray diffraction

The high pressure behavior of gallium phosphide, GaP, has been examined using the synchrotron X-ray diffraction technique in a diamond anvil cell up to 27?GPa and 900?K. The transition from a semiconducting to a metallic phase was observed. This transition occurred at 22.2?GPa and room temperature, and a negative dependence of temperature of this transition was found. The transition boundary was determined to be P (GPa)?=?22.6???0.0014?×?T (K).     

Variations of lattice constants and thermal expansion coefficients of indium at high pressure and high temperature

ABSTRACT

The effects of pressure and temperature on the lattice constants and thermal expansion coefficients of Indium were studied up to 18.6?GPa and 506?K based on in situ X-ray diffraction method with an externally heated diamond anvil cell. The results show that the measured axial ratio (c/a) decreases with increasing temperature and its temperature dependence decreases with increasing pressure. The thermal expansion coefficient of the a-axis decreases with increasing pressure up to 7?GPa and remains almost constant above 7?GPa, whereas that of the c-axis increases monotonously with pressure and changes from negative to positive at around 7?GPa. The observed behavior suggests that temperature reduces the tetragonal distortion on the lattice, and its effect is dominant below 7?GPa; in contrast, pressure enhances lattice distortion, and tends to have a stronger effect above 7?GPa.     

Decreasing electrical resistivity of gold along the melting boundary up to 5 GPa

ABSTRACT

The electrical resistivity of gold was experimentally measured at high pressures from 2 to 5?GPa and temperatures ～300?K above melting. The resistivity decreased as a function of pressure and increased as a function of temperature as expected. The temperature dependence of resistivity in the solid and liquid phases are comparable to 1?atm results. The observed melting temperatures at each pressure agree well with previous experimental and theoretical studies. The essential result of this study is that resistivity decreases along the pressure-dependent melting boundary, conflicting with a prediction of invariant behavior as reported in the literature. This result is discussed in terms of the interaction between s and d-bands as both pressure and temperature increase along the melting boundary. The thermal conductivity of gold was calculated from the measured electrical resistivity using the Wiedemann-Franz law. The temperature-induced effect on the thermal conductivity at high temperatures is as expected in both the solid and liquid phase while the pressure-effect shows some variability.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Phase transitions in uranium dioxide at high pressures and temperatures

The melting curve is plotted for uranium dioxide with fluorite structure in a pressure range from ?2.5 to +100 GPa. This curve has a peak at the point 3348 K, 6 GPa, and has a negative derivative at high pressures. The pressure corresponding to a polymorphic transition of uranium dioxide (37 GPa) at a temperature of 1015 K is determined. The slope of the equilibrium curve of the polymorphic transition in UO₂ in the temperature range 300–1000 K is ? 56 K/GPa.     

Thermo-elastic property of Ca(OH)2 portlandite

An equation of state (EoS) for Ca(OH)₂ portlandite has been obtained through measurements of pressure and temperature dependence of volume by means of in-situ X-ray observation. The bulk modulus and its pressure derivative at zero pressure calculated using third-order Birch-Murnaghan's equation of state is 33.1 GPa and 4.2 at 300 K, respectively. The unit cell parameters and the volumes have been also determined at 573 K and 673 K. Temperature derivatives of the bulk modulus and its pressure derivative have been calculated to be ?0.022 GPa/K and 0.0072 K^?1, respectively. Thermal expansion coefficient of portlandite has been calculated from the EoS. The pressure dependence of entropy has been obtained from the present thermo-elastic parameters.     

TiC-MgO composite: an X-ray transparent and machinable heating element in a multi-anvil high pressure apparatus

ABSTRACT

TiC-MgO composite was developed as a heating element for X-ray study in the multi-anvil high pressure apparatus. We synthesized TiC-MgO blocks (50–70 wt.% of TiC) by compression in a cold isostatic press followed by baking in a gas flow furnace. Heaters of tubular shape were manufactured from the synthesized blocks either by lathe or numerically controlled milling machine. The so-produced heating elements have been proved to generate temperatures up to 2250?K at 10?GPa, condition where classical graphite heaters are not suitable anymore due to graphite-diamond transition. These new heaters have been successfully used for in situ X-ray radiography and diffraction measurements on liquid Fe alloys, exploiting excellent X-ray transparency.     

Electron Temperature Measurement in a Premixed Flat Flame Using the Double Probe Method

The electron temperatures T_e were measured using a double probe in a premixed methane flame produced by a calibration burner according to Hartung et al. The experiment was performed at atmospheric pressure. In contrast to other authors, we have managed to find typical nonlinearities corresponding to the retarding electron current region and to calculate electron temperatures using a suitable fit on the basis of the measured characteristics. A Pt‐Rh thermocouple was used to measure temperatures T_h corresponding to “heavy” species. Our results indicate that the flame plasma can be considered to be weakly non‐isothermic — T_e = (2400–4000) K, T_h = (1400–1600) K. On the basis of measurement of the saturated ion current, the number density of the charged particles was estimated at (0.3–3.8) · 10¹⁷ m^‐3. The trends in T_e and T_h in dependence on the positions of the probes and thermocouple in the flame differ substantially; this fact has not yet been explained (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Distribution of light alkanes in the reaction of graphite hydrogenation at pressure of 0.1–7.8 GPa and temperatures of 1000–1350°C

ABSTRACT

We studied the effect of pressure and temperature on the hydrocarbon (HC) chain length distribution and total amount of HCs in the reaction of direct graphite hydrogenation at pressures of 0.1–7.8?GPa and temperatures of 1000–1350°C. An increase in pressure was found to lead both to an increase in the absolute yield of HCs due to direct graphite hydrogenation and to chain elongation of HC products. Light alkanes predominate among HCs in the entire studied range of P–T parameters. However, their concentration in quenched fluids increases as pressure is elevated, from less than 10?rel.% at 0.1?GPa to more than 40–50?rel.% at P?≥?3.8?GPa. Methane is actually the only light alkane among reaction products at 0.1?GPa and 1000°C, while it is a minor component at 7.8?GPa and 1350°C. The most stable alkane at pressures above 3.8?GPa is ethane (C₂H₆).