In the high pressure method of growing diamonds with a metallic flux, two phases of carbon, diamond and graphite, or a stable and a metastable phases, can exist and take part in the reaction, simultaneously. This gives a variety to the nature of the driving force for the crystallization of diamond, which can, in turn, be utilized to control the growing process according to the purpose. Behavior of the nucleation process is explained by a reaction model in which the nucleation of diamond and recrystallization of graphite competes. The model can deduce a pressure-temperature cycle for sinthesizing well-crystallized particles suitable to the metal-bond type abrasive use. The model also deduces a new method of growing single crystals using seeds, which is different from the temperature-gradient method.
Various morphologies of synthetic diamonds grown at high temperatures and pressure are described. The morphology changes with effect of metal solvent, H_2O present as an impurity in the growth cell, supersaturation or growth temperature. Surface microtopographs of as-grown surfaces are also demonstrated, although surfaces of synthetic diamond are usually covered with dendritic patterns which are formed during quenching the metal solution.
Crystal growth of cubic boron nitride (cBN) at high pressure is briefly reviewed. Recent success in the growth of large cBN crystals by using the temperature difference solvent method at high pressure made it possible to fabricate a functioonable cBN electronic device at high pressure. Since cBN is a good potential candidate as an wide-gap semiconductor material, further improvements, in the growth techniques at high pressure to a grade of the semiconductor technology is greatly desired.
Synthesrs of single crystals under high pressure and high-temperature conditions was described for geophysically important silicates. Crystal growth is generally enhanced at high temperatures very close to the phase boundary, which is effective to obtain crystals of up to 200 μm in siz for β, γ-Mg_2SiO_4, garnet-, and ilmenite-types of MgSiO_3. In crystal growth of SiO_2 stishovite and perovskite-type of MgSiO_3, however, fluxes such as H_2O and Li_2WO_4 have been found to play important roles. Significance of the large single crystals of high pressure silicates was pointed out from the geophysical point of view.
In order to study the crystal growth under pressure, we have developed a novel diamond anvil cell (DAC) which is thin enough to perform microscope observation using objective lenses with working distances greater than 6 mm. The cell permits one to apply the standard microscope techniques to investigation of phenomena taking place under high pressure We demonstrate the performance of the cell by showlng the observation of the movement of thin growth steps on a surface of an ADP crystal at 5.1 kbar. Aqueous solution growth of NH_4Cl crystals under presure is also reported as an example in which pressure has a remarkable effect on the growth behavior. In this case, facet formation was realized under high pressure, whereas dendritic growth was driven by pressure jump. It is indicated that, for the study of crystal growth, the DAC is a useful tool and the pressure is a significant parameter.
Structure and nature of air-hydrate crystals found in deep ice cores from Dye 3, Greenland are summarized. Those crystals were formed by a reaction between ice and air under high overburden pressure in the ice sheet. The crystallographic structure is the Stackelberg's structure II, which is one of the two distinct structures of clathrate hydrates. In the structure, individual gas molecules are encaged in polyhedral cages formed by water molecules. Growth process of the air-hydrate crystals in the ice sheet is well understood by a theoretical consideration of nature and behavior of air molecules in ice and in clathrate hydrate. The consideration is made on the basis of the data on the distribution and shape of the natural crystals in the ice sheet as well as artificial crystals grown in a high pressure vessel. Some numerical calculations are also made for estimating a growth period of the crystals.
Development of hydrothermal growth of quartz crystals in Japan is reviewed. Close relationship between physical properties of quartz, its Q-value estimated by infrared absorption method and electrical characteristics is observed. The fact that inclusions, dislocations and impurites are deeply related to the quality of quartz crystals is also shown. Quartz crystals grown in NaCl, KCl solutions and pure water are proved to be practically equivalent to natural quartz.
Natural Convection under supercritical pressure in autoclave for growing the quartz crystal take place by force balance when buoyancy by temperature increasing at heating section has much advantage than density increasing by dissolving the silica into alkalline solution. Additional friction forces consisting of bafflc placed in center position and nutrient, seed materials should be considered. Experiments using water under normal pressure could simulate natural convection. The linear relation between logarithm of heat transfer from heating to cooling section and temperature gradient indicated the equation below, Q=A・ΔAT^m (m=1.55, 1.90) Experiments using water under supercritical pressure also indicated the linear relation between log Q and log ΔT. The index (m) of 1.7 in case of baffle closure more than 88% was similar to that of under normal pressure.
Artificial growth of calcite single crystals has been investigated by flux and hydrothermal methods under high pressure conditions. Natural calcite crystal called lceland spar is considered to occur hydrothermally. Recently the authors found that nitrate solutions were effective solvents for the growth of calcite single crystals under low temperature and pressure conditions. In this paper a study on hydrothermal growth of calcite single crystal and dissolution behavior in this solvent would be presented.
A new process for manufacturing magnetic γ-Fe_2O_3 has been established in commercial scale. This process consists of following two steps. The first step is preparation of acicular α-Fe_O_3 particles by hydrothermal reaction with crystal control agents from amorphous Fe(OH)_3, and the second step is conversion to γ-Fe_2O_3 by calcination using those α-Fe_2O_3 for precusor instead conventional acicular FeOOH. The γFe_2O_3 from this process has good magnetic properties and excellent electoro-acoustic properties on magnetic tapes due to good particle shape and non-pore the particles. The outline of this process and properties on products are described.