Pressure Transmitting Media

A NMR probe for very high temperature and high pressure was published by Jonas in 1979 [5]. The probe had a relatively low spectral resolution but allowed NMR relaxation measurements at temperatures up to 700 °C. The pressure vessel was made from the IMI-680 titanium alloy and could withstand pressures up to 200 MPa. Because of the high temperatures attained with the probe, gaseous argon was used as the pressure transmitting medium. A first NMR relaxation study of supercritical water performed with this probe was published in 1981 [25]. 

The sample is loaded into the gasket hole, and surrounded by a pressure transmitting fluid - which might be an alcohokwater mixture (which provides a quasi-hydrostatic medium to 10-16 GPa) or a crystalline gas such as Ar, Ne or He (loaded either at high pressures or cryogenically as a liquid) which is... 

Fig. 2. Basic principle of the diamond cell. Pressure is generated in the gasket hole when the diamonds are pushed against one another. The sample and a small chip of ruby for pressure calibration are placed in the hole and the latter is filled with a pressure-transmitting medium. The purpose of the gasket is to provide containment for the pressure medium as well as support the diamond Dais. Suitable apertures in the diamond support Mocks provide access to optical, x-ray. and other radiation...

Fig. 1. Principle of the pressuregenerating method with opposed diamond anvils. The enlargement on the right side shows the central hole of the gasket filled with rubies, a sample, and a pressure-transmitting medium.

The principle of pressure-generation with opposed diamond anvils is shown in fig. 1. A gasket with a central hole is placed between the two diamond tips to provide a sample chamber which can be filled with a pressure-transmitting medium to maintain hydrostatic conditions. Furthermore, the sample chamber is loaded with the sample and some pressure sensor. 

NaCl, which are sometimes used as the pressure transmitting medium. The unknown magnitude of the deviant stresses in such systems has often cast serious doubts on the interpretation of the measurements. 

Isostatic compaction Powder encased in flexible mold is pressed equally from all directions by pressure transmitted to the mold by a liquid medium. Isostatic molding, isostatic pressing Hydrostatic molding chambers, automatic isostatic presses... 

The basic principle of the DAC is extremely simple [6,7] an anvil is made of a brilliant cut diamond with the tip cut to form a small flat part, the culet. In a DAC, two such anvils are placed in front of each other, with the culets parallel. The experimental volume is a cylindrical hole drilled in a metallic foil, the gasket. In that volume are the sample, a pressure gauge, normally a ruby chip, whose luminescence is calibrated versus pressure, and a pressure transmitting medium whose function is to ensure the stresses on the sample are as homogeneous as possible (FIGURE 2). All DACs work with the same principle. They differ only in the way the force is applied on the diamonds. 

Pressures from 1 bar to 1 kbar can be attained by using a hand-operated hydraulic piston, similar to what is used in an automobile repair shop. Above 1 kbar, pressure intensifiers can boost these pressures tenfold, reaching about 10 kbar. Both hydraulic pistons and pressure intensifiers require a hydraulic fluid (heavy oil at room temperature, n-pentane down to 77 K or so) which can be compressed isotropically for some pressures, talcum powder can act as an almost isotropic pressure-transmitting medium. If higher pressures are needed, the demand for isotropic compression must be abandoned, and anisotropies creep in. 

Silicon oil was used as the pressure transmitting medium. The pressure perephery is conventional and a discription can be omitted. The gauge used is of 0.1 class and hence accuracy of the pressure measurement is around 1 bar. 

For structure studies under high pressure at low temperature (HP + LT) the pressure cell must fit in a cryostat and the pressure transmitting medium must not freeze or, if it does, must not produce a large uniaxial additional component. Several combinations of devices described in Sections IV.D.l and IV.D.2 are possible. While four-circle diffractometers have been often used for room-temperature and LT structural studies of organic conductors,... 

Figure 12 Clamp made of maragin steel with Fluorinert FC 75 as pressure-transmitting medium. Inside diameter 6 mm, height 10 mm, wall thickness 25 mm, absorption 50% for a neutron wavelength of 1.175 A. (From Ref. 102.)...

Fig. 6. Schemes for SHS densification. I-sample 2-press die 3-pressure-transmitting medium 4-pressing body 5-ignitor 6-metal container 7-massive piston 8-explosive 9-electric fuse 10-glass containers ll- chemical furnace mixture.

The experiments were similar to those of Niederwald and Schwoerer except that the crystal was mounted in a high pressure cell operated with water as pressure transmitting medium and that measurements were done at ambient temperature only Figure 14 shows a series of absorption transients associated with intermediate B... 

The simplest case is the first-order phase transition of semicrystalline polymers in the presence of a chemically inert fluid as a pressure-transmitting medium. Mercury was preferred due to its chemical inertness and due to its convenient and well-known thermomechanical coefficients ( p= 1.80X10 " K" and k j- = 0.40X10 MPa ). The polymer sample was always in intimate contact with the pressure-transmitting fluid. 

Within the cylindrical hole serving as a sample chamber, a suitable, normally liquid, pressure transmitting medium is used to provide hydrostatic pressure conditions. However, soft salts like CsCl with low shear strength have also been employed in some experiments. One of the conditions for a suitable pressure medium is that even under irradiation by X-rays it does not undergo chemical reactions with the investigated sample. 

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