Tetrahedral anvil

Figure 3.2 Anvil designs (a) opposed anvil (b) tetrahedral anvil and (c) cubic anvil.

The high pressure apparatus used in this work is a tetrahedral anvil apparatus. Details of the equipment, the operating procedure, and the sample assembly have been described previously (2) and will not be repeated here. Briefly, the powdered oxide samples are wrapped in platinum or in gold foil to isolate them, as much as possible, from reaction with materials other than the reactant mixture. We shall see that on occasion the oxide samples will react with the platinum foil container to give lower valence platinum oxides. As a rule, three oxide samples, which have been wrapped in foil and pressed into pellets, are loaded into the sample cavity ( —1 cc volume) of a pyrophyllite tetrahedron for each high pressure run. 

Iso isoelectronic with graphite is B2O, prepared by reducing B2O3 with B or Li at high temperatures under high pressure. (In the tetrahedral anvil pressures of 50-75 kbar and temperatures of 1200-1800°C are reached.) The B and 0 atoms were not definitely located in the graphite-like stmcture of 820. ... 

Figure 4.9 Schematic diagram of opposed tetrahedral anvil device...

A second device (actually the first to be publicly disclosed) with the same pressure, temperature, and diamond-synthesizing capabilities as the Belt was developed at Brigham Young University during 1956-1957. It is called the Tetrahedral Anvil Press and is currently finding considerable use as a research tool. ... 

Beyond 10 kbar, mechanical means must be used diamond anvils. The diamond anvil method uses industrial diamonds, shaped to define a small volume, and tetrahedrally mounted pistons and anvils compress the two diamonds together to achieve very high ultimate pressures (200 kbar). Beyond such pressures, explosive charges can be used to achieve very high pressures for a very short time. The maximum pressures measured are around 1 Mbar. 

Camera design involves the difficult problem of applying large forces to the specimen and simultaneously getting x-ray beams into and out of it. Hydrostatic pressures up to about 5 kbars have been achieved in cameras operated under gas pressure. Higher pressures are obtained by compressing the specimen between anvils, either uniaxially or tetrahedrally (along directions, in cubic notation, of the form <111 . 

Beyond this range, the pressures are normally created by static pressure devices, such as piston-cylinder apparatus, solid state presses or anvil devices (opposed, tetrahedral and cubic anvil), when the sample is simply squeezed to create pressure up to 150 kbar (15 GPa) (Figure 4.9). These methods have allowed the synthesis of compounds which could not be created in any other way. However, only very small samples are produced (50 mg) in these expensive reactions, and so industrial applications of the materials produced are unlikely. 

Pressures and temperatures equivalent to that at the base of the upper mantle and top of the lower mantle can be achieved with multi-anvil devices. Hall (1958) described a tetrahedral multianvil device used to generate pressures high enough to synthesize diamond. Further developments led to the octahedral multi-anvil device (Kawai and Endo, 1970), refined by many Japanese scientists (e.g., Akaogi and Akimoto,1977 Onodera, 1987 Ohtani, 1987). Walker et al. 

The principle of most multiple-anvil systems is to compress a sample volume, in the shape of a regular polyhedron, by a variable number of identically shaped pistons which advance towards each other. Thus, tetrahedral, cubic, and octahedral compression have successfully been achieved. In this type of compression, the thrust is strictly along the axis of the pistons. Other geometries which introduce nonaxial (shear) components, such as hexahedral geometries, have proven to be remarkably unsuccessful and have been abandoned. Sliding-... 

---