Shishuai Sun

New assembly design suitable for tower-shaped large size single-crystal diamond growth under high pressure and high temperature

During the synthesis of tower-shaped large size single crystal diamonds, a concave growth defect always occurs on the top surface of the diamonds when using the temperature gradient growth (TGG) method under high pressure and high temperature (HPHT) conditions. To explain the formation of these defects, we have analyzed experimental results, and performed theoretical calculations. Then, we designed a new assembly that is suitable for tower-shaped diamond growth. Compared to the traditional assembly, it can effectively eliminate the growth defects and provide a more suitable convection field for tower diamond growth. Simulation and experimental results confirm the effectiveness of the proposed design. The new assembly can be widely used in the industrial and commercial production of synthetic diamonds.

Influence of carbon convection field on high quality large single crystal diamonds morphology under high pressure and high temperature

The temperature and convection fields of a catalyst with three different heights were simulated in a temperature gradient growth (TGG) system under high pressure and high temperature (HPHT) conditions. Temperature fields were simulated to rule out the influence of temperature on the crystal morphology. The features of the calculated convection field could predict the particular situations of diamond growth systems very well, and could explain the change of diamond morphology accompanying the growth process. According to the calculated results, we predict that the morphology of a diamond crystal changes from cubic crystal to cub-octahedral. A good agreement has been obtained between the calculated results and the observed experimental data. The morphology and structural properties of the synthesized samples are characterized by optical microscopy and a Raman spectrum. The results illustrate that the synthesized diamond crystals have less lattice distortion and with high quality.

Studying the effect of hydrogen on diamond growth by adding C10H10Fe under high pressures and high temperatures

ABSTRACT In this paper, hydrogen-doped industrial diamonds and gem diamonds were synthesized in the Fe–Ni–C system with C10H10Fe additive, high pressures and high temperatures range of 5.2–6.2 GPa and 1250–1460°C. Experimental results indicate similar effect of hydrogen on these two types of diamonds: with the increasing content of C10H10Fe added in diamond growth environment, temperature is a crucial factor that sensitively affects the hydrogen-doped diamond crystallization. The temperature region for high-quality diamond growth becomes higher and the morphology of diamond crystal changes from cube-octahedral to octahedral. The defects on the {100} surfaces of diamond are more than those on the {111} surfaces. Fourier transform infrared spectroscopy (FTIR) results indicate that the hydrogen atoms enter into the diamond crystal lattice from {100} faces more easily. Most interestingly, under low temperature, nitrogen atoms can also easily enter into the diamond crystal lattice from {100} faces cooperated with hydrogen atoms.

Method to eliminate the surface growth defects of large single crystal diamonds: an effective solution to improve the utilization rate for commercial production

In this work, a growth defect with bowl shaped pits has been found to form during the growth of a diamond, synthesized over a long period of time using the temperature gradient growth (TGG) method under high pressure and high temperature (HPHT) conditions. The experimental results show that the defect arises during the later growth stage of the diamond synthetic process. In order to explain the formation of the defect, the temperature and convection fields in the later growth state of the catalyst have been analyzed using the finite element method (FEM). The formation mechanism of the growth defect on the diamond crystal has been explained accurately by the simulated results and a good agreement has been obtained between the calculated results and the observed experimental data. Exhilaratingly, we propose a simple and efficient method to eliminate growth defects by adjusting the catalyst thickness. This method not only can improve the quality of large single crystal diamonds, but also may be helpful in reducing the cost of diamond cutting in the commercial market.

Synthesis and characterization of HPHT large single-crystal diamonds under the simultaneous influence of oxygen and hydrogen

In this paper, we report the influence of oxygen and hydrogen additives in the metal melt on the growth process, morphology, and defect-and-impurity structure of large single-crystal diamonds. The main experiments were performed in the Ni70Mn25Co5–C system at pressure ranging from 5.5 GPa to 6.5 GPa, temperature from 1250 °C to 1550 °C and C6H8O7 concentration from 0 to 1.0 wt%. It was found that the synthesis conditions (temperature and pressure) increase with the increasing quantity of C6H8O7 additive. As the C6H8O7 content was increased from 0 to 0.8 wt%, the presence of oxygen and hydrogen was more likely to cause defects in the {100} surface. A further increase in the C6H8O7 concentration above approximately 0.9 wt% completely terminated the nucleation and growth of the diamonds and led to the crystallization of graphite alone. FTIR, Raman and XPS test results showed that oxygen and hydrogen coexist in the diamond crystal.

Synthesis of diamonds in Fe–C systems using nitrogen and hydrogen co-doped impurities under HPHT*

In this study, we investigate the effect of nitrogen and hydrogen impurities on colors, morphologies, impurity structures and synthesis conditions of diamond crystals in Fe–C systems with C3N6H6 additives at pressures in the range 5.0–6.5 GPa and temperatures of 1400–1700 °C in detail. Our results reveal that the octahedron diamond nucleation in a Fe–C system is evidently inhibited by co-doped N–H elements, thereby resulting in the increase of minimum pressure and temperature of diamond synthesis by spontaneous nucleation. The octahedron diamond crystals synthesized from a pure Fe–C system are colorless, while they become green in the system with C3N6H6 additive. The surface defects of diamond will deteriorate when the nitrogen and hydrogen atoms simultaneously incorporate in the diamond growth environment in the Fe–C system. We believe that this study will provide some important information and be beneficial for the deep understanding of the crystallization of diamonds from different component systems.

Ultrathin SnO2 nanorod/reduced graphene oxide nanosheet composites for electrochemical supercapacitor applications with excellent cyclic stability

Abstract Composites of ultrathin SnO2 nanorods, ∼20 nm in diameter and ∼100 nm in length, intercalated with reduced graphene oxide nanosheets were synthesized by a simple one-step hydrothermal process. The electrochemical performance of the composites as electrode materials for supercapacitors was studied in 1 M Na2SO4 electrolyte. The experimental results indicated that a maximum specific capacitance of 184.6 F g−1 could be obtained from composites at a current density of 100 mA g−1, which was much higher than that of pure SnO2 (62.4 F g−1). Furthermore, the composite exhibited excellent cycling stability (the specific capacitance still retained 98% after 6000 cycles when the scan rate was 50 mV s−1). The excellent electrochemical performance of the composites was attributed to the synergistic effect of SnO2 nanorods and reduced graphene oxide, which makes up for the shortcomings of the individual components. These results indicated that the prepared composites are excellent candidates as electrode materials for high performance energy storage devices.