Huang Guo-Feng

FEM simulations and experimental studies of the temperature field in a large diamond crystal growth cell

We investigate the temperature field variation in the growth region of a diamond crystal in a sealed cell during the whole process of crystal growth by using the temperature gradient method (TGM) at high pressure and high temperature (HPHT). We employ both the finite element method (FEM) and in situ experiments. Simulation results show that the temperature in the center area of the growth cell continues to decrease during the process of large diamond crystal growth. These results are in good agreement with our experimental data, which demonstrates that the finite element model can successfully predict the temperature field variations in the growth cell. The FEM simulation will be useful to grow larger high-quality diamond crystal by using the TGM. Furthermore, this method will be helpful in designing better cells and improving the growth process of gem-quality diamond crystal.

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.

Influence of annealing treatment on as-grown Ib-type diamond crystal at a high temperature and high pressure

In this paper, we report on the influence of annealing treatment on as-grown Ib-type diamond crystal under high pressure and high temperature in a china-type cubic anvil high-pressure apparatus. Experiments are carried out at a pressure of 7.0 GPa and temperatures ranging from 1700 °C to 1900 °C for 1 h. Annealing treatment of the diamond crystal shows that the aggregation rate constant of nitrogen atoms in the as-grown Ib-type diamond crystal strongly depends on diamond morphology and annealing temperature. The aggregation rate constant of nitrogen remarkably increases with the increase of annealing temperature and its value in octahedral diamond is much higher than that in cubic diamond annealed at the same temperature. The colour of octahedral diamond crystal is obviously reduced from yellow to nearly colorless after annealing treatment for 1 h at 1900 °C, which is induced by nitrogen aggregation in a diamond lattice. The extent of nitrogen aggregation in an annealed diamond could approach approximately 98% indicated from the infrared absorption spectra. The micro-Raman spectrum reveals that the annealing treatment can improve the crystalline quality of Ib-type diamond characterized by a half width at full maximum at first order Raman peak, and therefore the annealed diamond crystals exhibit nearly the same properties as the natural IaA-type diamond stones of high quality in the Raman measurements.

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.

Transformation of Nitrogen State in High-Level N-Doped Gem-Quality Diamond Crystal Annealed at High Temperature and High Pressure

The colourless IaA-type gem-quality diamond crystals containing a high concentration of nitrogen (1500–1700 ppm) were successfully prepared by annealing the as-grown Ib-type N-doped diamonds at a high temperature and high pressure in China-type cubic anvil high-pressure apparatus. Experiments were carried out at pressures of 6.5–7.0 GPa and temperatures from 1900 K to 2100 K. Annealing treatment on high-level N-doped diamond crystals shows that the colour of the diamond crystals is obviously reduced from green to colourless after annealing treatment within 1 h at a higher temperature, which is induced by nitrogen aggregation in the diamond lattice indicated by infrared (IR) spectroscopy. It is further revealed that active energy of the nitrogen atom transforming from the dispersed form to the aggregated form is much lower than that in the standard Ib-type diamond crystals with nitrogen concentration less than 300 ppm. The colourless IaA-type diamond crystal prepared by annealing at 2100 K displays the same properties in IR spectra as the high-quality natural diamonds which are classified into the IaA type.

Reaction Mechanism of Al and N in Diamond Growth from a FeNiCo-C System

It is significant to investigate the action of nitrogen getters, which are used to synthesize type-IIa large diamond single crystals under high pressure and high temperature (HPHT). The reaction mechanism of Al as a nitrogen getter and N in the HPHT alloy solvent is still indeterminate at present. In order to investigate the reaction of Al and N in the HPHT alloy solvent, Al and AlN are respectively added to the system of Fe55Ni29Co16-C (wt%, abbr. FeNiCo-C) for the synthesis of diamonds at about 5.5 GPa and 1600K. The concentration of nitrogen in the diamonds is characterized by a micro Fourier transform infrared (micro-FTIR) spectrometer. The experimental results show that cN decreases when Al is added to the FeNiCo-C system. However, it increases when AlN is added. A reversible reaction confirms that Al and N can react and form AlN, simultaneously AlN can be decomposed into Al and N in the HPHT alloy solvent. Therefore the mechanism of eliminating the nitrogen of nitrogen getter Al is realized in detail.

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.

Dependence of Limited Growth Rate of High-Quality Gem Diamond on Growth Conditions

The growth rate of diamond has been investigated for a long time and researchers have been attempting to enhance the growth rate of high-quality gem diamond infinitely. However, it has been found according to previous research results that the quality of diamond is debased with the increase of growth rate. Thus, under specific conditions, the growth rate of high-quality diamond cannot exceed a limited value that is called the limited growth rate of diamond. We synthesize a series of type Ib gem diamonds by temperature gradient method under high pressure and high temperature (HPHT) using the as-grown {100} face. The dependence of limited growth rate on growth conditions is studied. The results show that the limited growth rate increases when synthetic temperature decreases, also when growth time is prolonged.