Miniature ceramic-anvil high-pressure cell for magnetic measurements in a commercial superconducting quantum interference device magnetometer

A miniature opposed-anvil high-pressure cell has been developed for magnetic measurement in a commercial superconducting quantum interference device magnetometer. Non-magnetic anvils made of composite ceramic material were used to generate high-pressure with a Cu-Be gasket. We have examined anvils with different culet sizes (1.8, 1.6, 1.4, 1.2, 1.0, 0.8, and 0.6 mm). The pressure generated at low temperature was determined by the pressure dependence of the superconducting transition of lead (Pb). The maximum pressure P(max) depends on the culet size of the anvil: the values of P(max) are 2.4 and 7.6 GPa for 1.8 and 0.6 mm culet anvils, respectively. We revealed that the composite ceramic anvil has potential to generate high-pressure above 5 GPa. The background magnetization of the Cu-Be gasket is generally two orders of magnitude smaller than the Ni-Cr-Al gasket for the indenter cell. The present cell can be used not only with ferromagnetic and superconducting materials with large magnetization but also with antiferromagnetic compounds with smaller magnetization. The production cost of the present pressure cell is about one tenth of that of a diamond anvil cell. The anvil alignment mechanism is not necessary in the present pressure cell because of the strong fracture toughness (6.5 MPa?m(1∕2)) of the composite ceramic anvil. The simplified pressure cell is easy-to-use for researchers who are not familiar with high-pressure technology. Representative results on the magnetization of superconducting MgB(2) and antiferromagnet CePd(5)Al(2) are reported.     

Application of a new composite cubic-boron nitride gasket assembly for high pressure inelastic x-ray scattering studies of carbon related materials

We have developed a new composite cubic-boron nitride (c-BN) gasket assembly for high pressure diamond anvil cell studies, and applied it to inelastic x-ray scattering (IXS) studies of carbon related materials in order to maintain a larger sample thickness and avoid the interference from the diamond anvils. The gap size between the two diamond anvils remained ~80 μm at 48.0 GPa with this new composite c-BN gasket assembly. The sample can be located at the center of the gap, ~20 μm away from the surface of both diamond anvils, which provides ample distance to separate the sample signal from the diamond anvils. The high pressure IXS of a solvated C(60) sample was studied up to 48 GPa, and a pressure induced bonding transition from sp(2) to sp(3) was observed at 27 GPa.     

Determination of the thickness of a gasket in a diamond anvil high-pressure cell

A method for determining the thickness of a gasket in a diamond anvil high-pressure cell is described. This method is based on measuring the relative position of the anvils. The gasket thickness is determined by measuring the distance between two marks, viz., virtual points “locked” to side surfaces of the anvils. The distance between the marks is determined by digital processing of microphotographs. The error of measuring the gasket thickness disregarding the deformations of the anvils is ∼1 μm. The results of measuring the gasket thickness, the aperture area, and the optical length between the working planes of the anvils for a helium-filled cell are presented. The pressure dependences of the refractive index and the volume of helium, which is compressed to a pressure of 16 GPa, that are calculated using these data are in good agreement with the published data.     

Note: high-pressure generation using nano-polycrystalline diamonds as anvil materials

Nano-polycrystalline diamonds (NPDs) consist of nanosized diamond grains oriented in random directions. They have high toughness and isotropic mechanical properties. A NPD has neither the cleavage feature nor the anisotropy of hardness peculiar to single-crystal diamonds. Therefore, it is thought to be useful as a diamond anvil. We previously reported the usefulness of a NPD as an anvil for high-pressure development. In this study, some additional high-pressure generating tests using diamond anvils of various shapes prepared from NPDs were conducted to investigate the advantage of using NPDs for anvil applications. The results revealed that the achievable pressure value of a NPD anvil with a culet size of more than 300 μm is about 1.5 to 2 times higher than that of single-crystal diamond anvils, indicating that NPD anvils have considerable potential for large-volume diamond anvils with large culet sizes.     

Fabrication and efficiency evaluation of a hybrid NiCrAl pressure cell up to 4 GPa

A hybrid NiCrAl pressure cell was fabricated to measure magnetic quantities under high pressure above 3 GPa. A pressure of 4.0 GPa was achieved and the pressure cell was found to be reusable even after a pressurizing trial up to 4.0 GPa. Pressure was monitored using (63)Cu nuclear quadrupole resonance of Cu(2)O and ruby fluorescence. The pressure efficiency of a fresh cell was maintained at 96%, and no appreciable deformation was observed at pressures below 3 GPa; on the other hand, the efficiency after pressurizing trials decreased gradually and reached 75% at 4 GPa accompanied by a maximum expansion inside the cylinder of 2%.     

Note: An anvil-preformed gasket system to extend the pressure range for large volume cubic presses

An anvil-preformed gasket system has been developed to extend the pressure range for the widely used large volume cubic press without sacrificing the sample volume. The relationship of the sample chamber pressure versus press load for this system was calibrated at room temperature using transitions in Bi, Tl, and Ba. With similar sample volumes (8-11 mm in diameter and 8 mm in length), the anvil-preformed gasket system can generate pressures up to about 8.5 GPa, significantly higher than 6 GPa, which was generally the maximum pressure for the conventional anvil-gasket system. The details on the optimized design for the anvil-preformed gasket system are given in this note.     

Density measurements of noncrystalline materials at high pressure with diamond anvil cell

We describe an x-ray absorption method for in situ density measurement of non-crystalline materials in the diamond anvil cell using a monochromatic synchrotron x-ray microbeam. Sample thickness, which is indispensable in the absorption method, can be determined precisely by extrapolating the thickness profile of the gasket obtained by x-ray absorption and diffraction measurements. Diamond deformation across the sample chamber becomes noticeable at high pressures above 10 GPa, which can be monitored with a precision better than 1%, as demonstrated by measurements on crystalline Ag. We have applied the developed method to measure densities of the classic network-forming GeO(2) glass in octahedral form at pressures up to 56 GPa. The fit to the pressure-volume data with the Birch-Murnaghan equation from 13 to 56 GPa gives parameters of V(0)=23.2+/-0.4 cm(3)mol and K=35.8+/-3.0 GPa, assuming that K(')=4. This method could be applicable for in situ determination of the density of liquids and other noncrystalline materials using a diamond anvil cell up to ultrahigh pressures.     

Adaptation of the Bridgman anvil cell to liquid pressure mediums

The advantage of Bridgman anvil pressure cells is their wide pressure range and the large number of wires which can be introduced into the pressure chamber. In these pressure cells, soft solid pressure mediums such as steatite are used. We have succeeded in adapting the Bridgman cell to liquid pressure mediums. With this breakthrough, it is now possible to measure in very good hydrostatic pressure conditions up to 7 GPa, which is about twice the pressure attainable in piston-cylinder cells. The pressure gradient in the cell, estimated from the superconducting transition width of lead, is reduced by a factor of 5 in the liquid medium with respect to steatite. By using nonmagnetic materials for the anvils and the clamp and due to the small dimensions of the latter, our device is specially suitable for magnetotransport measurements in dilution fridges. This pressure cell has been developed to measure very fragile and brittle samples such as organic conductors. Resistivity measurements of (TMTTF)(2)BF(4) performed in a solid and a liquid pressure medium demonstrate the necessity of hydrostatic pressure conditions for the study of organic conductors at high pressures.     

High-pressure behavior of toroidal CVT fluid for automobile

Practical toroidal-type continuously variable transmission (t-CVT) is subjected to extremely severe operating conditions, viz., maximum contact pressure is very high at 4 GPa, rolling speed exceeds over 30 m/s and lubricant temperature goes up over 140 °C. Traction measurements were recently made at wide range of contact pressures, oil temperatures and rolling speeds by USCAR Group. The objective of this research is to investigate high-pressure rheology of the traction characteristics at extremely severe EHL contact conditions. The effects of contact pressure and temperature on maximum traction coefficient were evaluated using the phase diagram and the liquid/solid transition lubrication diagram.     

A miniature cubic anvil apparatus for optical measurement under high pressure

A miniature cubic anvil apparatus was developed for optical measurement on materials in a high-pressure space of 8-10 mm(3) under high pressure, and preliminary experiments were conducted to 3.6 GPa at room temperature with optical visual observation and ruby fluorescence measurement. In the apparatus, a cubic pressure medium was squeezed with six tungsten carbide anvils, which are driven with a pair of guide blocks by tightening four loading screws. Optical access on the sample was made through holes in axial anvils and the guide blocks as well as optical windows made of Al(2)O(3) single crystals embedded in the pressure medium. The apparatus is compact and light, ~53 mm in diameter and height and ~530 g in weight, and the features of the apparatus benefit easy application of the apparatus to various types of standard optical measurement systems. The optical measurement on the sample in the high-pressure space of 8-10 mm(3) should greatly contribute to advancements of studies relevant to high-pressure behaviors of materials.     

Nonmagnetic indenter-type high-pressure cell for magnetic measurements

An indenter-type high-pressure cell has been developed for electric and magnetic measurements in low-temperature and high-magnetic-field environments. The maximum pressure achieved at low temperatures is more than 4.5 GPa, which is higher than that of a conventional piston-cylinder cell. The typical sample space at maximum pressure is 1.6 mm in diameter and approximately 0.7 mm in depth, and magnetic measurements such as ac-susceptibility and nuclear magnetic resonance can be performed using a miniature coil. All the components of the indenter cell are made of nonmagnetic materials that have enough thermal conductivity for low-temperature experiments using a 3He/4He dilution refrigerator. Another indenter-type cell designed for a commercial superconducting quantum interference device magnetometer is also reported.     

Gas loading apparatus for the Paris-Edinburgh press

We describe the design and operation of an apparatus for loading gases into the sample volume of the Paris-Edinburgh press at room temperature and high pressure. The system can be used for studies of samples loaded as pure or mixed gases as well as for loading gases as pressure-transmitting media in neutron-scattering experiments. The apparatus consists of a high-pressure vessel and an anvil holder with a clamp mechanism. The vessel, designed to operate at gas pressures of up to 150 MPa, is used for applying the load onto the anvils located inside the clamp. This initial load is sufficient for sealing the pressurized gas inside the sample containing gasket. The clamp containing the anvils and the sample is then transferred into the Paris-Edinburgh press by which further load can be applied to the sample. The clamp has apertures for scattered neutron beams and remains in the press for the duration of the experiment. The performance of the gas loading system is illustrated with the results of neutron-diffraction experiments on compressed nitrogen.     

Absolute pressure measurements and analysis of diamonds subjected to maximum static pressures of 1.3-1.7 Mbar

Force per unit area measurements made in the megabar pressure cell, independently of other pressure calibration systems, are consistent with the ruby R1 scale of Mao, Bell, Shaner, and Steinberg and its extrapolation to 1.4 Mbar. Physical analysis of diamond anvils removed after experiments to maximum pressures of 1.3-1.7 Mbar suggests that the nitrogen platelet concentration may be related to the strength of the diamonds. The pressure face of one of the diamonds from the 1.7-Mbar experiment was deformed plastically by a macroscopic amount.     

High pressure ellipsometry: a novel method for measuring the optical properties and electronic structure of materials in diamond anvil cells

High pressure ellipsometry (HPE) method was developed for determining the index of refraction of opaque materials in a diamond anvil cell (DAC). A main difficulty in DAC-based HPE, namely, the pressure-induced birefringence developed in the diamond, was overcome enabling the extraction of the ellipsometric parameters of the sample. The method used was based on the fact that an unpolarized light is unaffected by a retarding optical element and thus reduces the number of unknown parameters in the problem. Because of technical difficulties in using unpolarized light, a linear combination of orthogonal polarizations was applied. In the experimental procedure, multiangle measurements of the ellipsometric parameter ψ are collected at each pressure and the data is fitted together with a measurement of the near normal reflectivity, in order to extract the complex index of refraction. As a test case, this procedure was used to measure the high pressure index of refraction of iron up to 30 GPa for light with wavelengths of 532 and 633 nm. From the index of refraction as a function of pressure the diamond-iron interface emissivity for different pressures was derived and from which the phase transition α → ε could be identified and characterized. The emissivity increases with pressure both at the α (0-9 GPa) and the ε phase (21-30 GPa) however decreases at the mixed α - ε (9-21 GPa) range. From the imaginary part of the index of refraction the pressure dependence of the energy skin depth of iron was extracted. It was found that the energy skin depth increases by an order of magnitude at 30 GPa relative to ambient conditions.     

Simultaneous structure and elastic wave velocity measurement of SiO2 glass at high pressures and high temperatures in a Paris-Edinburgh cell

An integration of multi-angle energy-dispersive x-ray diffraction and ultrasonic elastic wave velocity measurements in a Paris-Edinburgh cell enabled us to simultaneously investigate the structures and elastic wave velocities of amorphous materials at high pressure and high temperature conditions. We report the first simultaneous structure and elastic wave velocity measurement for SiO(2) glass at pressures up to 6.8 GPa at around 500°C. The first sharp diffraction peak (FSDP) in the structure factor S(Q) evidently shifted to higher Q with increasing pressure, reflecting the shrinking of intermediate-range order, while the Si-O bond distance was almost unchanged up to 6.8 GPa. In correlation with the shift of FSDP position, compressional wave velocity (Vp) and Poisson's ratio increased markedly with increasing pressure. In contrast, shear wave velocity (Vs) changed only at pressures below 4 GPa, and then remained unchanged at ~4.0-6.8 GPa. These observations indicate a strong correlation between the intermediate range order variations and Vp or Poisson's ratio, but a complicated behavior for Vs. The result demonstrates a new capability of simultaneous measurement of structures and elastic wave velocities at high pressure and high temperature conditions to provide direct link between microscopic structure and macroscopic elastic properties of amorphous materials.     

Patterned anvils for high pressure measurements at low temperature

Multiprobe high pressure measurements require electrical leads in the sample chamber. Compared to conventional wire-based techniques, metallic tracks patterned onto the anvil surface improve reliability and ease of use, and enable novel and more demanding measurements under high pressure. We have developed new anvil designs based on sputter-deposited tracks on alumina and moissanite anvils. These anvils allow convenient and reliable measurements of electrical transport properties or of the magnetic susceptibility under hydrostatic conditions, as demonstrated by test measurements on Pb and Ca(3)Ru(2)O(7).     

Yield strength of molybdenum at high pressures

In the diamond anvil cell technology, the pressure gradient approach is one of the three major methods in determining the yield strength for various materials at high pressures. In the present work, by in situ measuring the thickness of the sample foil, we have improved the traditional technique in this method. Based on this modification, the yield strength of molybdenum at pressures has been measured. Our main experimental conclusions are as follows: (1) The measured yield strength data for three samples with different initial thickness (100, 250, and 500 microm) are in good agreement above a peak pressure of 10 GPa. (2) The measured yield strength can be fitted into a linear formula Y=0.48(+/-0.19)+0.14(+/-0.01)P (Y and P denote the yield strength and local pressure, respectively, both of them are in gigapascals) in the local pressure range of 8-21 GPa. This result is in good agreement with both Y=0.46+0.13P determined in the pressure range of 5-24 GPa measured by the radial x-ray diffraction technique and the previous shock wave data below 10 GPa. (3) The zero-pressure yield strength of Mo is 0.5 GPa when we extrapolate our experimental data into the ambient pressure. It is close to the tensile strength of 0.7 GPa determined by Bridgman [Phys. Rev. 48, 825 (1934)] previously. The modified method described in this article therefore provides the confidence in determination of the yield strength at high pressures.     

Thickness measurement of sample in diamond anvil cell

We report on an original method that measures sample thickness in a diamond anvil cell under high pressures. The method is based on two hypotheses: completely plastic deformation on the gasket and completely elastic deformation of the diamonds. This method can further eliminate the effect of diamond deformation on the thickness measurement of a sample, which permits us to measure the thickness of alumina up to 41.4 GPa.     

Development of high-pressure, high-field and multifrequency electron spin resonance system

The electron spin resonance (ESR) system which covers the magnetic field region up to 16 T, the quasicontinuous frequency region from 60 to 700 GHz, the temperature region from 1.8 to 4.2 K, and the hydrostatic pressure region up to 1.1 GPa has been developed. This is the first pulsed high-field and multifrequency ESR system with the pressure region over 1 GPa as far as we know. Transmission ESR spectra under hydrostatic pressure can be obtained by combining a piston-cylinder-type pressure cell and the pulsed magnetic field ESR apparatus. The pressure cell consists of a NiCrAl cylinder and sapphire or zirconia inner parts. The use of sapphire or zirconia as inner parts enables us to observe ESR under pressure because these inner parts have high transmittance for the electromagnetic wave with millimeter and submillimeter wavelengths. We have successfully applied this system for the pressure dependence measurements of an isolated spin system NiSnCl(6)6H(2)O up to 1.1 GPa. It was found that the single ion anisotropy parameter D of this compound strongly depends on pressure. The parameter D is approximately proportional to the pressure up to 0.75 GPa, and the relation between D and the pressure can be used for the pressure calibration of this high-field and high-pressure ESR system.     

Construction of a low-temperature thermodynamic measurement system for single crystal of molecular compounds under pressures

An apparatus to obtain low-temperature thermodynamic information under high pressures for a tiny single crystal of molecular compounds was developed based on the ac technique. To detect small temperature oscillation of a sample inside the cramp-type pressure cell, we have used a small ruthenium oxide chip sensor as a thermometer. The adoption of the four-terminal method by the ac resistance bridge has made high-resolution detection of thermal anomaly possible in the low-temperature region. The constructed high-pressure thermodynamic system was mounted on a 3He refrigerator and we have succeeded to detect the thermal anomaly in relevant to magnetic order of single crystal sample of Mn4-cluster complex up to 1.05 GPa. A distinct peak of the heat capacity and its upward shift with increasing pressures was observed using a tiny crystal of about 100 microg. The high-pressure behavior of the discontinuity of heat capacity at the superconductive transition of 6 mg of metal indium has also been detected by this apparatus. The details and performance of the technique are reported.