High pressures dynamic-static

Compaction is generally carried out at high pressure in static or dynamic modes. In static compression, the sample is subjected to a period of consfanf pressure, and in d5mamic compression, a pressure pulse is applied and fhen fhe pressure wave passes through the sample to effect compaction. The direction of pressure in both methods can be one-way, two-way, axial, or... 

New metliods appear regularly. The principal challenges to the ingenuity of the spectroscopist are availability of appropriate radiation sources, absorption or distortion of the radiation by the windows and other components of the high-pressure cells, and small samples. Lasers and synchrotron radiation sources are especially valuable, and use of beryllium gaskets for diamond-anvil cells will open new applications. Impulse-stimulated Brillouin [75], coherent anti-Stokes Raman [76, 77], picosecond kinetics of shocked materials [78], visible circular and x-ray magnetic circular dicliroism [79, 80] and x-ray emission [72] are but a few recent spectroscopic developments in static and dynamic high-pressure research. 

The wurt2ite form differs only slightly from the cubic form, but it is not quite as stable. It is most easily obtained by static or dynamic compression of hBN or rBN at high pressures (17). In the presence of a Hquid catalyst at high pressures, the wurt2ite form changes rapidly to the cubic form. The change occurs more slowly without a catalyst above 6 GPa (60 kbar) (18). 

In this process, diamond forms from graphite without a catalyst. The refractory nature of carbon demands a fairly high temperature (2500—3000 K) for sufficient atomic mobiUty for the transformation, and the high temperature in turn demands a high pressure (above 12 GPa 120 kbar) for diamond stabihty. The combination of high temperature and pressure may be achieved statically or dynamically. During the course of experimentation on this process a new form of diamond with a hexagonal (wurtzitic) stmcture was discovered (25). 

High-pressure static and dynamic light scattering were used to closely examine the behavior of block copolymers of poly(vinyl acetate) (PVAc) and poly(l,l,2,2-tetrahydroperfluoroalkyl acrylate (PTAN) as a function of C02 density (Buhler et al., 1998). The phase diagram for PVAc-6-TAN shows three distinct phases as a function of polymer concentration and C02 density at a fixed temperature of 45 °C (see Figure 9.1). The block copolymer forms a precipitated phase at low C02 densities, spherical micelles at intermediate C02 densities, and unimers, or free polymer chains in solution, at high densities. The micelles-to-unimer transition was found to be very... 

Studies at extremely high static pressure can be carried out in diamond anvil cells. Studies of chemical dynamics in gem anvil cells are beginning to be carried out, and some unique chemical synthesis can also occur at these high pressures. For example, a high pressure polymerized phase of carbon monoxide was recently reported to be synthesized using visible laser light to irradiate a carbon monoxide sample at pressures of over 5 GPa in a diamond anvil cell. ... 

Geophysicists have spent considerable efforts to develop experimental techniques to determine the phase diagram and physical properties of iron at high pressure and temperature since the 1950s. Much of our initial knowledge on the density and phase transformation of iron at high pressures is from dynamic shock wave experiments (e.g., Bancroft et al, 1956 McQueen and Marsh, 1966 Barker and Hollenbach, 1974 Brown and McQueen, 1986). However, structural information for high-pressure polymorphs of iron was obtained from static compression experiments combined with in situ X-ray diffraction measurements (e.g., Jamieson and Lawson, 1962 ... 

The outer core is in a liquid state. Unfortunately, there are very limited data on the physical properties of liquid iron at high pressures and temperatures. Shock-wave studies provide data on the density of liquid iron at very high pressures (>243 GPa) along the Hugoniot (Brown and McQueen, 1986). Static data on the structure and density of liquid iron are limited to pressures less than 5 GPa (e.g., Sanloup et ai, 2000a,b Balog et al., 2001). Anderson and Ahrens (1994) derived an EOS for liquid iron based on available experimental data. There is no immediate solution to hll the pressure gap between static and dynamic experiments. For the moment, we have to rely on theoretical calculations of the stmcture of liquid iron under pressure (Stixrude and Brown, 1998 Alfe et ai, 2000c). 

Extraction with an aqueous or organic solvent at a high pressure and/or temperature can be made by using the static mode, the dynamic mode (the solvent is circulated in a continuous manner through the sample) or a combination of both. 

The use of different names for the technique can lead to confusion as it may leave the impression that the names refer to different techniques rather than a single one involving extraction with solvent at a high pressure and temperature. However, authors tend to use different names for the static and dynamic modes, even though the names provide no clue as to which mode was used. 

In many cases, the extraction process includes an additional, static extraction step. To this end, the outlet valve (OV in Fig. 6.10) is supplemented by an inlet valve (IV in Fig. 6.10) between the high-pressure pump and the extractor. Once the system has been pressurized, the inlet valve is closed, the high-pressure pump stopped and the oven temperature raised. After the desired temperature is reached, the system is maintained under a static regime with both valves closed, and then the valves are opened and the pump restarted to allow the solvent to flow over the dynamic extraction period. Several studies [147,150,153] have shown that a combination of both extraction modes can result in substantially improved extraction and shorter extraction times. This is commented on in greater detail in discussing specific applications below. 

Processes without catalysts are only of minor industrial importance, since they provide only gray graphite-contaminated diamond powder with a maximum crystal size of ca, 50 Xm and require significantly higher pressures of 120 to 300 kbar. In the dynamic process operated by DuPont the pressure and temperature are produced for a few microseconds in a shock wave apparatus. The starting material is also graphite, which should be as crystalline as possible. Static high pressure synthesis processes without catalysts are industrially unimportant. 

Wurtzite (wBN) can be prepared by various static and dynamic compression methods , depending on the relative amounts of hBN and rBN in the initial sample, and the T-P conditions of the consolidation. Phase stability data for wBN is available . hBN can be converted to cBN and wBN at pressures from 12 to 40 GPa and temperatures between 300 and 1200 K , also with the use of Mg as a catalyst. A mixture of hBN, H2O, and an alkaline solution (e.g., NaOH) may be subjected to a shock wave at or above 10 GPa to prepare high quality wBN . wBN is transformed to zBN by shock compression at pressure above 100 GPa , while static high pressure transforms zBN to wBN . However, the reverse transformations are also possible". Additional references on wBN are given in physics abstracts. ... 

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