Zang Chuan-Yi

In-Situ Measurement of Electrical Character of PbTe at High Pressure and High Temperature

There is a widespread interest in lead telluride (PbTe) as a good thermoelectric material. We report the temperature dependence of thermopower S(T) and resistance R(T) for PbTe at the different pressures of from 1.8 GPa to 5 GPa obtained by using the cubic anvil high pressure apparatus. With increasing pressure, R(T) and S(T) decrease. The effect of pressure on R(T) is larger than that on S(T). The power factor that is determined by thermopower and resistivity increases with increasing pressure. This method is an efficient tool for synthesizing good thermoelectric materials at high pressure and high temperature.

Grow Large High-Quality Diamonds with Different Seed Surfaces

Large high-quality type Ib diamond crystals have been grown with different seed surfaces by temperature gradient method at 5.5 GPa, 1500–1600 K, with NiMnCo alloy as the metal solvent. Compared with {100} as the growth surface, the growth region of large high-quality diamond crystals with {111} as the growth surface at a higher growth rate shifts markedly from lower temperatures (suitable for {100}-facet growth) to higher temperatures (suitable for {111}-facet growth). However, regardless of different growth surfaces, {100} or {111}, the grown crystals of sheet-shaped shape are most difficult for metal inclusions to be trapped into, and whether or not matched growth between the seed surfaces and the growth temperatures determines the crystal shapes. In view of the growth rates, large high-quality diamond crystals of sheet-shaped shapes can be grown at a growth rate of above 2.5 mg/h, while the growth rate of large high-quality diamond crystals should not be beyond 1.5 mg/h for tower-shaped crystals.

Effects of Al and Ti/Cu on Synthesis of Type-IIa Diamond Crystals in Ni70Mn25Co5-C System at HPHT

High-quality type-Ma gem diamond crystals are successfully synthesized in a Ni70Mn25 C05-C system by temperature gradient method (TGM) at about 5.5 GPa and 1560 K. Al and Ti/Cu are used as nitrogen getters respectively. While nitrogen getter Al or Ti/Cu is added into the synthesis system, some inclusions and caves tend to be introduced into the crystals. When Al is added into the solvent alloy, we would hardly gain high-quality type-IIa diamond crystals with nitrogen concentration Nc < 1 ppm because of the reversible reaction of Al and N at high pressure and high temperature (HPHT). However, when Ti/Cu is added into the solvent alloy, high-quality type-IIa diamond crystals with Nc < 1 ppm can be grown by decreasing the growth rate of diamonds.

Finite Element Analysis of Convection in Growth Cell for Diamond Growth Using Ni-Based Solvent

Thermal-electrical-fluid coupled finite element analyses are performed in the model of the growth cell in a high- pressure and high-temperature (HPHT) cubic apparatus in which the large diamond crystal can be grown by using Ni-based solvent with temperature gradient method (TGM). The convection in the Ni-based solvent with different thicknesses at 1700–1800K is simulated by finite element method (FEM). The experiments of diamond crystal growth are also carried out by using Ni-based solvent at 5.7GPa and 1700–1800K in a China-type cubic high pressure apparatus (CHPA). The simulation results show that the Rayleigh number in the solvent is enhanced obviously with the increasing solvent thickness. Good quality diamond single crystal cannot be grown if the Rayleigh number in the solvent is too high.

Growth of Large High-Quality Type-II a Diamond Crystals

Large high-quality type-II a diamond crystals in size of about 4.0 mm have been grown under the condition of 5.5 GPa and 1200–1300 degrees C by using the temperature gradient method in a domestic cubic anvil high-pressure apparatus. The Fe55Co16Ni25 alloy (KOV) is used as the solvent metal, and Ti with the content 1.5 wt.% of the solvent metal is selected as the nitrogen getter to reduce the impurity of nitrogen in the diamond crystal. To avoid the impurities and cave in the crystal, the growth rate of the initial stage of the growing process is controlled within 0.45 mg/h and the ring carbon source of the size Φ8 mm–Φ6 mm×3 mm is used to grow large diamond crystals.

HPHT Synthesis of Micron Grade Boron-Doped Diamond Single Crystal in Fe-Ni-C-B Systems

Micron grade boron-doped diamond crystals with octahedral morphology are successfully synthesized in a Fe—Ni—C—B system under high pressure and high temperature (HPHT). The effects of the additive boron on synthesis conditions, nucleation and growth, crystal morphology of diamond are studied. The synthesized micron grade diamond crystals were characterized by optical microscope (OM), scanning electron microscope (SEM), x-ray diffraction (XRD) and Raman spectroscopy. The research results show that the V-shaped section of synthetic diamond moves downwards to the utmost extent due to 0.3a wt% (a is a constant.) boron added in the synthesis system. The crystal colour is black, and the average crystal size is about 25 μm. The crystal faces of synthetic diamond are mainly {111} face. The synthesis of this kind of diamond is few reported, and it will have important and widely applications.

Dependence of Crystal Quality and β Value on Synthesis Temperature in Growing Gem Diamond Crystals

High quality Ib gem diamond single crystals were synthesized in cubic anvil high-pressure apparatus (SPD-6 × 1200) under 5.4 GPa and 1230°C-1280°C. The (100) face of seed crystal was used as growth face, and Ni70Mn25Co5 alloy was used as solvent/catalyst. The dependence of crystal quality and β value (the ratio of height to diameter of diamond crystal) on synthesis temperature was studied. When the synthesis temperature is between 1230°C and 1280°C, the β value of the synthetic high-quality gem diamond crystals is between 0.4 and 0.6. The results show that when the β value is between 0.4 and 0.45, the synthetic diamonds are sheet-shape crystals; however, when the β value is between 0.45 and 0.6, the synthetic diamonds are tower-shape crystals. In addition, when the β value is less than 0.4, skeleton crystals will appear. When the β value is more than 0.6, most of the synthetic diamond crystals are inferior crystals.

Surface Structure of Large Synthetic Diamonds by High Temperature and High Pressure

With NiMnCo and FeCoNi alloys as solvent metals, large single-crystal diamonds of about 3 mm across are grown by temperature gradient method (TGM) under high temperature and high pressure (HPHT). Although both {100} and {111} surfaces are developed by a layer growth mechanism, some different characteristic patterns are seen clearly on the different surfaces, no matter whether NiMnCo or FeCoNi alloys are taken as the solvent metals. For {100} surface, it seems to have been melted or etched greatly, no dendritic patterns to be found, and only a large number of growth hillocks are dispersed net-likely; while for {111} surface, it often seems to be more smooth-faced, no etched or melted traces are present even when a lot of depressed trigonal growth layers. This distinct difference between {111} and {100} surfaces is considered to be related to the difference of surface-atom distribution of different surfaces, and {111} surfaces should be more difficult to be etched and more steady than {100} surfaces.

Effect of Regrown Graphite on the Growth of Large Gem Diamonds by Temperature Gradient Method

Generally, when growing high-quality large gem diamond crystals by temperature gradient method under high pressure and high temperature, the crystal growth rate is only determined by the temperature gradient. However, we find that the seed crystal cannot completely absorb all the diffused carbon sources, when growing gem diamonds under a higher temperature gradient. Other influence factors appear, and the growth rate of growing diamonds is partly dependent on the crystalline form of superfluous unabsorbed carbon source, flaky regrown graphite or small diamond crystals nucleated spontaneously. The present form is determined by the growth temperature if the pressure is fixed. Different from spontaneous diamond nuclei, the appearance of regrown graphite in the diamond-stable region can retard the growth rate of gem diamonds substantially, even if the temperature gradient keeps unchanged. On the other hand, the formation mechanism of metastable regrown graphite in the diamond-stable region is also explained.

HPHT Synthesis of Different Shape Coarse-Grain Diamond Single Crystals

Synthesis of coarse-grain diamond crystals is studied in a China-type SPD6×1670T cubic high-pressure apparatus with high exact control system. To synthesize high quality coarse-grain diamond crystals, advanced indirect heat assembly, powder catalyst technology and optimized synthesis craft are used. At last, three kinds of coarsegrain diamond (about 0.85 mm) single crystals with hexahedron, hex-octahedron and octahedron are synthesized successfully under HPHT (about 5.4 GPa, 1300–1450°C). The growth characters of different shape crystals are discussed. The results and techniques might be useful for the production of coarse-grain diamonds.