F. P. Bundy

Direct Transformation of Hexagonal Boron Nitride to Denser Forms

Hexagonal, graphitelike boron nitride may be changed directly into the zincblende cubic form reported earlier or into a new wurtzite form by the application of static high pressures. No catalyst appears to be necessary. At high temperatures, between about 2500° and 4000°K, the zincblende form is favored; at lower temperatures, down to 300°K, the wurtzite form is favored; frequently both forms appear together. The minimum pressure required for the transformations is about 115 kbar at 2000°K; somewhat higher pressures, of the order of 130 kbar, suffice at higher and lower temperatures. The crystallites of the dense phases are small but give good x‐ray diffraction patterns from which the crystal structures can be determined.

Direct Conversion of Graphite to Diamond in Static Pressure Apparatus

Static pressure apparatus has been developed which is capable of pressures up to 200 kbar and transient temperatures up to about 5000°K, using an electric flash‐heating technique. At pressures above about 125 kbar and temperatures in the 3000°K range it is found that graphite spontaneously collapses completely to polycrystalline diamond which may be retrieved quantitatively. The threshold temperature of the transformation is several hundred degrees lower than the melting temperature of graphite. The diamond/graphite/liquid triple point is found to be located at about 125 (+10, —0) kbar and 4100 (±200)°K. The shock compression results of DeCarli and Jamieson, and of Alder and Christian, are linked with the present results to construct a phase diagram of carbon extending to 5000°K and 800 kbar.

Diamond‐Graphite Equilibrium Line from Growth and Graphitization of Diamond

Diamond growth occurs at high temperatures and pressures in the presence of certain molten metals which serve as solvent catalysts. The zones of pressure and temperature in which diamond growth occurs have been determined for a number of metals. These zones are bounded on the low‐temperature side by the melting point of the metal‐carbon eutectic at pressure. They are bounded on the high‐temperature side by the diamond‐graphite equilibrium line. This experimentally determined equilibrium line agrees very closely with the theoretical extrapolation of the thermodynamically calculated line proposed by Berman and Simon, viz., P(kbar)=7.1+0.027T(∘K).

Direct transitions among the allotropic forms of boron nitride at high pressures and temperatures

The direct transition behavior among the graphitic (hBN), wurtzitic (wBN), and zincblende (zBN) crystal forms of boron nitride is investigated as a function of temperature for pressures up to 130 kbar. At pressures in the 45 to 70 kbar range, direct transformation of both the hBN and wBN forms to the zincblende form are observed and at higher pressures (85 kbar and above) direct transformation of hBN to wBN is also observed. A pressure/temperature phase diagram is presented for pressures up to 130 kbar. In this pressure range, the thermodynamically stable solid phases are hBN and zBN, the experimental behavior indicating that wBN is not thermodynamically stable over this range. From temperature/time data, the activation energy for both the hBN to zBN and wBN to zBN transitions is estimated to be about 200 kcal/mole. From these high activation energies it is concluded that the direct conversion processes essentially require disruption of the hBN and wBN lattices before the atoms can re‐form into the zincblende structure.

Melting of Graphite at Very High Pressure

A method of flash‐heating small rods of graphite inside a superpressure cell has been developed. The heating energy was inserted in less than 7 msec from a bank of electrolytic capacitors. This quick heating and cooling allowed fusion and freezing of the graphite to occur without serious melting or reaction of the surrounding wall material. Electrical data were recorded with oscillographs and cameras. The start of melting was found to be indicated by an abrupt downward trend of resistance. Polished cross sections of the samples showed clearly the part which melted. Melting temperatures increased from about 4100°K at 9 kbar to a maximum of about 4600°K in the region of 70 kbar, then decreased to about 4100°K at 125 kbar. A value of 25 kcal/mole for the heat of fusion at 48 kbar was determined. The graphite/diamond/liquid triple point is shown to be at about 4000 to 4200°K and 125 to 130 kbar.

Pressure—Temperature Phase Diagram of Iron to 200 kbar, 900°C

The resistance changes in iron during the transitions between the α(bcc), γ(fcc), and e(hcp) phases have been studied quantitatively in the high‐compression belt apparatus. The samples were monitored by thermocouples and voltage probes. The P,T locations of the (α,γ) (α,e), and (e,γ) phase lines, and the (α,γ,e) triple point were determined. The latter is placed at 110 ±3 kbar and 490 ±10°C. The changes of molar volume at the triple point are deduced to be −0.275, +0.135, and +0.140 cc/mole for the (α,e), (e,γ), and (γ,α) transitions, respectively. The resistivity of the α phase follows a quadratic relationship with temperature, whereas the γ and e forms vary linearly with temperature.

Phase Diagrams of Silicon and Germanium to 200 kbar, 1000°C

The phase diagrams of Si and Ge have been investigated experimentally over a P, T range of about 200 kbar and 1000°C by observing electrical resistance behavior. For Si the boundary between the diamond cubic form and the metallic form extends from about 120 kbar at room temperature to about 150 kbar, 810°C, where melting occurs at the triple point. For Ge the corresponding boundary extends from about 115 kbar at room temperature to about 103 kbar, 600°C. A line drawn through the triple points for Sn, Ge, and Si, and extended, suggests that the diamond—metal—liquid triple point for carbon may be around 500 kbar, 2400°C. When the metallic forms are decompressed at room temperature they transform back to semiconducting forms different and more dense than the original diamond cubic forms. Upon heating these denser forms to a few hundred degrees at room pressure they transform to the usual diamond cubic forms. The absolute resistivity and temperature coefficient of resistance of metallic Ge has been determined.

Flat Panel Vacuum Thermal Insulation

Evacuated mats of glass fiber made up of fibers of proper size and orientation are capable of supporting a compressive mechanical loading of at least one atmosphere and yet maintain a thermal conductivity of less than 10 microcalories/cm°C sec. The use of such a glass fiber mat as a filler makes possible an evacuated flat‐panel thermal insulation which is comparable to a Dewar flask in insulation efficiency. The rate of heat transfer through a Dewar flask wall was reduced several‐fold at liquid nitrogen temperatures and below by adding a 2‐cm‐thick layer of orientated and evacuated glass fiber mat to the outer surface.This investigation showed that in evacuated glass fiber mats, supporting external atmospheric loading, the fiber to fiber contact area is less than 10−4 the mat area, making the contact pressure about 15 000 kg/cm2. The effective length of the thermal conduction paths along the fibers is about four times the mat thickness. The mean pore size for gas molecule motion in the mat was found to be...

Electrical behavior of Se and Te to pressures of about 500 kbar

Using sintered diamond tipped carbide piston apparatus, and techniques of temperature‐cycling the specimens while under pressure, the electrical resistance behavior of various kinds of Se specimens and of Te specimens has been explored as a function of pressure up to about 500 kbar. In general, earlier reported behavior to about 160 kbar was confirmed. New information includes: (i) Any type of initial Se goes to a stable metallic form above about 250 kbar. (ii) The ’’130 kbar metallic phase’’ of Se is probably the same as the ’’250 kbar metallic phase’’. (iii) The resistivity of metallic Se is very pressure sensitive, much more so than metallic Te. (iv) The activation energy of electrical conduction vs pressure results supports the earlier proposal that above 140 kbar Se goes into a high pressure semiconducting phase, which at about 250 kbar transforms to a metallic phase.

Ultra-high pressure apparatus

Abstract This review applies to static pressure apparatus capable of developing pressures over about 25 kbar for purposes of scientific measurements of the physical and chemical behavior of matter, and in some cases for the high-pressure, high-temperature, synthesis of materials like diamond and cubic boron nitride. A brief history is presented, and major emphasis is given to geometry and stress/strain analysis and the properties of materials that are useful in ultra-high pressure (UHP) apparatuses. Examples are given, and analyzed, of various kinds of UHP apparatuses which have been used extensively in actual service. Finally there is an assessment of the future possibilities for realizing pressures greater than those that have been attained to date.