Low temperature electrical conductivity ...

Low temperature electrical conductivity measurements under high pressure up to 10 GPa Yadunath Singh Citation: AIP Conference Proceedings 1728, 020693 (2016); doi: 10.1063/1.4946744 View online: http://dx.doi.org/10.1063/1.4946744 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1728?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Phase transition of [ C n -mim ] [ PF 6 ] under high pressure up to 1.0 GPa J. Chem. Phys. 130, 184503 (2009); 10.1063/1.3127363 Electric conductivity of liquid argon, krypton and xenon under shock compression up to pressure of 90 GPa AIP Conf. Proc. 505, 983 (2000); 10.1063/1.1303633 Sintered diamond anvil high‐pressure cell for electrical resistance measurements at low temperatures up to 50 GPa Rev. Sci. Instrum. 64, 1979 (1993); 10.1063/1.1143985 High pressure clamp for electrical measurements up to 8 GPa and temperature down to 77 K Rev. Sci. Instrum. 51, 136 (1980); 10.1063/1.1136041 High pressure–low temperature apparatus for electrical conductivity measurements Rev. Sci. Instrum. 46, 1025 (1975); 10.1063/1.1134400

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Low Temperature Electrical Conductivity Measurements under High Pressure up to 10 GPa Yadunath Singh Department of Physics, Government College of Engineering & Technology, Bikaner (Rajasthan)-India E-mail ID: [email protected] Abstract. This paper report about a modified version of Fuji high pressure cell and other necessary instrumentation required for the calibration of the high pressure cell and electrical resistivity measurement under high pressure and very low temperature environment.

INTRODUCTION Research in high pressure is very much dependent upon techniques, and every advance in the latter has resulted in a phenomenal expansion in our knowledge concerning the behavior of matter at high pressure. The study of materials at high pressure has been a long-standing goal in high pressure research. Studies of materials properties at extreme pressure-temperature conditions have a major impact on problems in physics, chemistry, geo-science, planetary science and material science[1,3]. The effect of pressure on materials brings about changes in the physical properties due to lattice compression and the electronic structural changes. The decrease in the inter-atomic distances or increase in the density leads to metal-insulator transition[4], inter band electron and valence transition[5,6], change in topology of the Fermi surface[7] and so forth. As a consequence their physical properties such as electronic specific heat, superconductivity and magnetism undergo changes. The advancement in high pressure techniques has enabled investigations of matter at pressure exceeding 500 GPa using diamond anvil cell (DAC) and synchrotron radiation sources[8]. It is of great importance in material science and geophysics, because information on the structure of materials provide a basis for investigating numerous macroscopic physical properties such as viscosity and self-diffusion[9,10], electrical resistivity[11,13], compressibility[14,16], and thermal expansion[17]. Here, we describe the clamp type high pressure cell, pressure calibration, sample mounting, sample assembly for four probe method and optimization of gasket thickness etc. to determine electrical conductivity of quasi one dimensional samples up to a pressure of 10 GPa.

HIGH PRESSURE CELL The idea of high pressure cell is very simple. As pressure is directly proportional to the applied force and inversely proportional to the area over which the force is applied. The pressure can be increased either by increasing the force or decreasing the area. The modified version of opposite anvils high pressure cell of Fuji et al[18] can be used in temperature-pressure electrical conductivity measurements. This cell can be pressurized by hydraulic press at room temperature and then it can be clamped at any required pressure. After clamping, it can be transferred to a cryostat and cool down to any temperature down to liquid helium for low temperature measurements. The high pressure cell consists of two opposite central anvils of 1 cm diameter made of tungsten carbide with 4 mm tip. The working area has micron finished and tapered to an angle of 12°. EN24 steel binding rings provide mechanical support for the anvils with 1% interference and 1° wedge half angle. This pair of anvils is placed in a metal case which properly aligns the anvils and provides further support also. A small hole is made in the outer case for the electrical leads. A small pin is located between the interfaces of anvils, when the cell is clamped. The maximum generated pressure depends on the material of the anvils, working area, tapering angle, external support and external load on the anvils.

International Conference on Condensed Matter and Applied Physics (ICC 2015) AIP Conf. Proc. 1728, 020693-1–020693-4; doi: 10.1063/1.4946744 Published by AIP Publishing. 978-0-7354-1375-7/$30.00

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SAMPLE MOUNTING In the used high pressure cell, anvils tip has a diameter of 4 mm and micron level finishing. Circular ring shaped pyrophyllite gaskets of a suitable thickness 0.15 mm, outer and inner diameters of 4 and 2 mm respectively were used. Before using, the gaskets were heated to 600°C for two hours to increase their hardness. Heat treated those gaskets were ground carefully on fine emery paper till a required thickness of 0.15 mm is reached. The gasket holds the hydrostatic pressure in the sample region. The steatite powder was used as a pressure transmitting medium, which provides a homogeneous hydrostatic pressure to the sample. The powder is pressed in a die and cut into a disc of 2 mm diameter. This disc was also ground using a fine emery paper to a thickness of 0.15 mm and ten placed in the pyrophyllite gasket. The gasket along with steatite disc is placed on the tip of lower anvil. A thin coating of ferric chloride in quick-fix is applied to the tip of anvil and gasket to give friction between anvil surface and gasket. The sample having typical dimensions of 1×0.01×0.01 mm3 is placed on the steatite disc. Four copper wires in parallel (electrical leads) are stretched on the sample and stuck initially on the anvil face with a cellophane tape. Now, the second gasket with steatite disc prepared in the same way is placed above to the first one. Further, the anvil with whole arrangement is slowly inserted into the hollow outer case of pressure cell. After taking out the electrical leads from the small slot provided on the outer case, these are soldered separately on the PCB fixed at outside of the outer case. Now, the second anvil is inserted carefully inside the case, that whole assembly should not be disturbed and placed above the first. Pressure is exerted on the sample via piston using a hydraulic press. Electrical contacts between the leads and sample are established on applying load to the anvil. In general, two methods are used to determine electrical conductivity, four probes method in case of low resistive and two probes for high resistive samples. Therefore, in case of four probes method, four copper wires of 40 SWG and two copper wires of 25 SWG were used in two probes method. The whole arrangement[13], high-pressure clamp type apparatus, placing of the sample in between anvils and sample assembly for four-probe method is shown schematically in the following figure 1.

FIGURE 1. Whole arrangement, high-pressure clamp type apparatus, placing of the sample in between anvils and sample assembly for four-probe method

CLAMPING OF THE HIGH PRESSURE CELL In order to study the electrical properties of the material at low temperatures under high pressure, a proper clamping is needed. Clamping of the high pressure cell means, locking of the desired pressure on the anvils till the observations are taken. To clamp the high pressure cell at the desired pressure, a nut and screw arrangement is given in the outer case. The procedure is to first apply the pressure on the high pressure cell to the desired value and then the nut is tightened. A pin is inserted in the hole made at the interface of the anvil and the outer cell. This prevents twisting of the anvil, electrical leads and damage of the sample. Now, the high pressure cell is taken out from the hydraulic press after tightening the nut. The resistance of the sample is monitored for some time to note any change. For a perfect clamping, change in the resistance value would be negligibly small. Usually, the clamping is carried out at roughly 5% higher pressure to the desired value and in all cases the final checking of pressure is done by measuring the resistance of the sample.

PRESSURE CALIBRATION It is not possible to measure pressure by a primary standard in the Bridgman anvil cell. Therefore, a secondary method is used and the applied oil pressure is standardized against some predetermined fixed point[19]. Since the cell is designed for the electrical resistivity measurement, therefore the calibration of the cell is carried out against resistance discontinuities[20,21]. The calibration is carried out in two ways as follows:


Pressure Calibration at Room Temperature The calibration of the high pressure cell is done at room temperature with the well known phase transitions of bismuth[22,23], ytterbium[24] and thallium[25]. These compounds undergo phase transitions and exhibiting resistance discontinuities when subjected to pressure. These transitions are now standardized [19]. In the calibration process, the electrical resistivity is determined for all the said compounds and then normalized resistivity is plotted against the oil pressure. The internationally accepted standardized pressure points corresponding to the resistance discontinuity of various materials were determined from these curves. Now these standardized pressure points were plotted against the oil pressure, which is linear variation. The slope of this curve is used as the calibration factor, from which actual pressure in GPa in the sample region can be determined.

Pressure Calibration at Low Temperatures To calibrate a clamped type high pressure cell for the low temperatures, a relaxation factor is taken into account due to thermal contraction and stiffening of the material used. To obtain this, a superconductivity manometer has been suggested by Smith[26]. Below a pressure of 1 GPa, The change in transition temperature with pressure can be fitted into a polynomial shown in equation (1) as below: (1) ο ൌ ሺሻ െ ሺሻ Where, P denotes the pressure and is expressed in GPa. To calibrate the clamped Bridgman anvil cell in the temperature range of 77 to 300K, lead material is used. At room temperature, resistance of lead decreases with pressure and temperature dependence of resistivity shows a linear relationship even at higher pressure of 10 GPa[27]. Resistance of lead can be expressed mathematically as a function of pressure and temperature and shown in equation (2). ሺǡ ሻ ൌ ൅ ൅ ʹ ሺ ൅ ሻ ሺʹሻ Where, A, B, C, D and E are constants and is the actual pressure at temperature T in Kelvin. On cooling the cell to the temperature T, the pressure relaxes from the clamped to , which can be expressed as, (3) ൌ ൅ ο ሺሻ Where, is the clamped pressure at room temperature and ο ሺሻ is the change in pressure at temperature T. On considering the volume expansion behavior between room and low temperature, the equation (3) becomes, (4) ൎ ൅ ሼሺ െ ሻ ൅ ሺ െ ሻʹ ሽ Where L and M are constants and is the temperature at which the cell is clamped. Knowing the values of all constants from experimental data actual pressure can be determined from relation (4). Relaxation in pressure is a function of clamped pressure and temperature difference between the monitoring temperature and room temperature at which clamping is performed. It is notable that during these experiments, on cooling the high pressure cell, the temperature remains almost same at side of the cell and tip of the anvil.

OPTIMIZATION OF GASKET THICKNESS The geometrical dimensions of the gaskets seriously affect the stress distribution. Particularly, the thickness of the gaskets is the most important factor. A study about the nature of the distribution of pressure as well as to standardize the gasket thickness, the effect of the gasket thickness on the load has been done, assuming the polymorphic phase transition points of bismuth as the fixed points[27]. In this work a relation between normalized resistivity of bismuth against load for different gasket thicknesses was studied. A variation of load and pressure at tip of anvils against gasket thickness is also discussed. Wakatsukii et al[28] have also reported about the redistribution of stress as a function of gasket thickness. A number of researchers[29-31] have reported different studies about gasket thickness and concluded that some factors like inner diameter of the gasket, material of the gasket and thickness etc. play an important role to optimize the suitable gasket. On taking all care as suggested in the previous studies, it has been practical observed that in the electrical resistivity measurement under high pressure up to 10 GPa, the pyrophyllite gasket thickness of 0.15 mm was most suitable and rate of failure for this thickness is less than 50%.

EFFECT OF THE SAMPLE THICKNESS To see the effect of the sample thickness on the distribution of pressure, a high pressure study[27] has been done on considering different thicknesses of bismuth sample. It has been concluded from the study that there is no remarkable change in load and hydrostatic stress of the medium due to the change of the sample thickness.


CONCLUSIONS AND OUTLOOK The high pressure technique can provide a lot of information either impossible or difficult to obtain in other ways and this is the main reason for still increasing interest in such a kind of research into the various fields of science. The full list of applications of high pressure technique has by no means been exhausted. The present paper gives the review about the understanding as well as some selection of interesting results and physical phenomena which can be observed using this technique. This is due in part to the large compression that can be reached on molecular materials; with new techniques, volume compression in excess of an order of magnitude can be realized relative to ambient pressure (atmospheric pressure), corresponding to pressure in the multimegabar range (several hundred gigapascals). Moreover, the application of pressure provides an ideal means to carefully tune electronic, magnetic, structural and vibrational properties for testing fundamental theory and a range of applications. Important implications are found throughout the physical, geo-science, planetary science, material science and even biological sciences.

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