Anvil cell, sapphire

Prompted by the success of the DAC, opposed-anvil cells equipped with large, normally sapphire, anvils have been used in a number of high-resolution diffraction studies that have used classical four-circle diffractometers [187-189] to perform high quality studies to above 2 GPa. The quality of the data is excellent, particularly if collected using small area detectors which became available in the late 1980s [190], and the use of which is now widespread. 

Raman spectroscopy with high pressure windowed cell (Sum et al., 1997 Thieu et al., 2000) P, T and hydrate phase Yes P, P, hydrate phase vs. time (mins) Typically for sapphire window < 10,000 psi (for capillary tubes <60,000 psi diamond anvil cell GPa s) Guest occupancy ratios, structure, structural transitions... 

The use of relatively large sapphire anvil cells has also been reported for PL measurements at LHeT in the near IR. This allows chamber volume for the sample about one order of magnitude larger than the one of DACs at reasonable cost, at the expense of a smaller hydrostatic stress [42]. 

The tunneling modes in a-MnH are found to be suppressed by elastic stresses [128]. In fact, the INS peak due to tunneling disappears at a pressure of 0.8 GPa in a quasi-hydrostatic regime of a sapphire anvil cell. On the other hand, the appli-... 

DACs above this value (sapphire-anvil cells may be an alternative for work in the u.v. or the near i.r., and for moderate (P < 10 GPa) pressures). 

This subsection deals with the specific problems associated with DACs and sapphire-anvil cells (which are analogous but are usable over a more modest... 

Diamond is the hardest material known to man. The hardness of diamond on the Mohs hardness scale is 10. The second hardest material is corundum (sapphire [AI2O3]), which has a hardness of 9. Corundum, as well as diamond, is transparent to visible radiation. The real difference in hardness is quite large because the absolute hardness of diamond is 7,000 Knoop and of the corundum is 2,000 Knoop [24]. Bulk modulus of diamond is 4420 kbar [25] but is only 2400 kbar for synthetic sapphire. However, because the cost of sapphire is much lower than that of the diamond and also sapphire is more transparent to UV light, sapphire anvil cells (SAC) are often used for an optical cell for pressure <100 kbar. Sometimes cubic zirconia anvils are also used for pressure <30 kbar [26]. 

In the case, the experiments of in situ Raman were planned to understand the mechanism of exfoliation of the nanoparticles in the contact. They were performed in collaboration with the Laboratoire des Sciences de la Terre of Ecole Normale Sup6rieure de Lyon. The tribometer using a fiat made of sapphire was placed directly under the microscope of a Raman spectrometer. The originality of the experimental device used involved the presence of a diamond anvil cell to carry ont tests with very high hydrostatic pressures (35 GPa) (Figure 2.71). 

Figure 2.71 Experimental device used for the tribo-Raman study. This device is composed of a pin-on-flat tribometer with a sapphire flat, a microscope of the Raman spectrometer and also a diamond anvil cell...

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

Fig. 33. Polarized emission spectra of single-crystal Ba[Pt(CN)4] 4 H20 at different hydrostatic pressures (T = 295 K)133). The emission intensities at different pressures cannot be compared. The excitation wavelength was varied with pressure to fit approximately the maximum of the E c polarized reflectance. For the high pressure investigations a modified sapphire cell of Bridgman s opposed anvil type was used. The pressure was determined by the amount of red-shift of the Rt- and R2-lines167) of ruby crystals placed around the sample...

The DAC is unsuitable for an ordinary neutron experiment, because the latter requires much larger samples. Alternative means of compression, such as liquid- and gas-pressure cells, " or even sapphire anvils (specially designed for use with He cryostats), are but poor substitutes in the magnitudes of pressure. However, encouraging results have been obtained by using DACs in conjunction with high-power neutron sources and optimizing the diffraction instrument itself... 

The diamond anvils are typically gemstone quahty and weigh about 0.2 carat. Since pressure is simply force per unit area and the upper force hmit is determined by the strength limits of the components of the cell, the diameter of the diamond culet ultimately determines the maximum achievable pressure. No definitive relationship between the culet diameter and maximum pressure is available, but with a well-designed cell one can typically expect to reach pressures of 100 kbar with 700 pm culets, 500 kbar with 400 pm culets, and 1 Mbar with 200 pm culets [77]. Bevelled diamonds are recommended for pressures above 1 Mbar [77,80-82]. Sapphire [83,84] and cubic zirconia [85] are less expensive alternatives to diamond, but are not as strong and can be used only to 100 and 30 kbar, respectively. 

Takano KJ, Wakatsuki M (1991) An optical high pressure cell with sphtaical sapphire anvils. Rev Sci Instrum 62 1576... 

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