High thermal stability of diamond-cBN-B_4C-Si composites

Improving the thermal stability of diamond and other superhard materials has great significance in various applications. Here, we report the synthesis and characterization of bulk diamond–cBN–B4C–Si composites sintered at high pressure and high temperature（HPHT, 5.2 GPa, 1620–1680 K for 3–5 min）. The results show that the diamond, cBN, B4C,BxSiC, SiO2 and amorphous carbon or a little surplus Si are present in the sintered samples. The onset oxidation temperature of 1673 K in the as-synthesized sample is much higher than that of diamond, cBN, and B4C. The high thermal stability is ascribed to the covalent bonds of B–C, C–N, and the solid-solution of BxSiC formed during the sintering process. The results obtained in this work may be useful in preparing superhard materials with high thermal stability.

Inclusions in large diamond single crystals at different temperatures of synthesis

The inclusions in large diamond single crystals have effects on its ultimate performance, which restricts its industrial applications to a great extent. Therefore, it is necessary to study the inclusions systematically. In this paper, large diamond single crystals with different content values of inclusions are synthesized along the(100) surface by the temperature gradient method(TGM) under 5.6 GPa at different temperatures. With the synthetic temperature changing from 1200?C to 1270?C,the shapes of diamonds change from plate to low tower, to high tower, even to steeple. From the microscopic photographs of the diamond samples, it can be observed that with the shapes of the samples changing at different temperatures, the content values of inclusions in diamonds become zero, a little, much and most, correspondingly. Consequently, with the temperature growing from low to high, the content values of inclusions in crystals increase. The origin of inclusions is explained by the difference in growth rate between diamond crystal and its surface. The content values of inclusions in diamond samples are quantitatively calculated by testing the densities of diamond samples. And the composition and inclusion content are analyzed by energy dispersive spectroscopy(EDS) and x-ray diffraction(XRD). From contrasting scanning electron microscopy(SEM) photographs, it can be found that the more the inclusions in diamond, the more imperfect the diamond surface is.

Synthesis and characterizations of boron and nitrogen co-doped high pressure and high temperature large single-crystal diamonds with increased mobility

We synthesized and investigated the boron-doped and boron/nitrogen co-doped large single-crystal diamonds grown under high pressure and high temperature(HPHT) conditions(5.9 GPa and 1290℃). The optical and electrical properties and surface characterization of the synthetic diamonds were observed and studied. Incorporation of nitrogen significantly changed the growth trace on surface of boron-containing diamonds. X-ray photoelectron spectroscopy(XPS) measurements showed good evident that nitrogen atoms successfully incorporate into the boron-rich diamond lattice and bond with carbon atoms. Raman spectra showed differences on the as-grown surfaces and interior between boron-doped and boron/nitrogen co-doped diamonds. Fourier transform infrared spectroscopy(FTIR) measurements indicated that the nitrogen incorporation significantly decreases the boron acceptor concentration in diamonds. Hall measurements at room temperature showed that the carriers concentration of the co-doped diamonds decreases, and the mobility increases obviously. The highest hole mobility of sample BNDD-1 reached 980 cm^(2)·V^(-1)·s^(-1), possible reasons were discussed in the paper.

Effect of FeS doping on large diamond synthesis in FeNi-C system

The large single-crystal diamond with FeS doping along the （111） face is synthesized from the FeNi-C system by the temperature gradient method （TGM） under high-pressure and high-temperature （HPHT）. the effects of different FeS additive content on the shape, color, and quality of diamond are investigated. It is found that the （111） face of diamond is dominated and the （100） face of diamond disappears gradually with the increase of the FeS content. At the same time, the color of the diamond crystal changes from light yellow to gray-green and even gray-yellow. The stripes and pits corrosion on the diamond surface are observed to turn worse. The effects of FeS doping on the shape and surface morphology of diamond crystal are explained by the number of hang bonds in different surfaces of diamond. It can be shown from the test results of the Fourier transform infrared （FTIR） spectrum that there exists an S element in the obtained diamond. The N element content values in different additive amounts of diamond are calculated. The XPS spectrum results demonstrate that our obtained diamond contains S elements that exist in S-C and S-C-O forms in a diamond lattice. This work contributes to the further understanding and research of FeS-doped large single-crystal diamond characterization.

Synthesis and Characteristics of Type Ib Diamond Doped with NiS as an Additive

Large diamond single crystals doped with NiS are synthesized under high pressure and high temperature. It is found that the effects on the surface and shape of the synthesized diamond crystals are gradually enhanced by increasing the NiS additive amount. It is noted that the synthesis temperature is necessarily raised to 1280℃ to realize the diamond growth when the additive amount reaches 3.5% in the synthesis system. The results of Fourier transform infrared spectroscopy(FTIR) demonstrate that S is incorporated into the diamond lattice and exists in the form of C–S bond. Based on the FTIR results, it is found that N concentration in diamond is significantly increased, which are ascribed to the NiS additive. The analysis of x-ray photoelectron spectroscopy shows that S is present in states of C–S, S–O and C–S–O bonds. The relative concentration of S compared to C continuously increases in the synthesized diamonds as the amount of additive NiS increases. Additionally,the electrical properties can be used to characterize the obtained diamond crystals and the results show that diamonds doped with NiS crystals behave as n-type semiconductors.

Characteristics of urea under high pressure and high temperature

The properties of urea under high pressure and high temperature(HPHT) are studied using a China-type large volume cubic high-presentation apparatus(CHPA)(SPD-6 × 600).The samples are characterized by scanning electron microscopy(SEM), x-ray diffraction(XRD), and Raman spectroscopy.By directly observing the macroscopic morphology of urea with SEM, it is confirmed that the melting point of urea rises with the increase of pressure.The XRD patterns of urea residues derived under different pressures show that the thermal stability of urea also increases with the increase of pressure.The XRD pattern of the urea residue confirms the presence of C3H5N5O(ammeline) in the residue.A new peak emerges at 21.80°, which is different from any peak of all urea pyrolysis products under normal pressure.A more pronounced peak appears at 708 cm^-1 in the Raman spectrum, which is produced by C-H off-plane bending.It is determined that the urea will produce a new substance with a C-H bond under HPHT, and the assessment of this substance requires further experiments.

Regulation mechanism of catalyst structure on diamond crystal morphology under HPHT process

To elucidate the regulation mechanism of catalyst geometry structure to diamond growth,we establish three catalyst modes with different structures.The simulation results show that with the decrease of the protruding height of the catalyst,the low-temperature region gradually moves toward the center of the catalyst,which causes the distribution characteristics of the temperature and convection field in the catalyst to change.The temperature difference in vertical direction of the catalyst decreases gradually and increases in the horizontal direction,while the catalyst convection velocity has the same variation regularity in the corresponding directions.The variation of temperature difference and convection velocity lead the crystal growth rate in different crystal orientations to change,which directly affects the crystal morphology of the synthetic diamond.The simulation results are consistent with the experimental results,which shows the correctness of the theoretical rational analysis.This work is expected to be able to facilitate the understanding of catalyst structure regulation mechanism on diamond morphology and the providing of an important theoretical basis for the controllable growth of special crystal shape diamond under HPHT process.

Effects of MgSiO3 on the crystal growth and characteristics of type-Ib gem quality diamond in Fe-Ni-C system

We report the effects of MgSiO3 addition on the crystal growth and characteristics of type-Ib diamonds synthesized in Fe–Ni–C system. The experiments were carried out with pressure at 5.5 GPa, temperature at 1385℃–1405℃, and duration of 23.1 h. As MgSiO3 increases from 0.0 wt% to 3.0 wt%, the diamond growth temperature increases from1385℃ to 1405℃, the addition of MgSiO3 and the movement of P–T diagram toward the higher temperature direction result in a series of effects to the Fe–Ni–C system and crystal growth. Firstly, it increases the content of metastable recrystallized graphite and accelerates the competition with the carbon source needed for diamond growth, thus causing the decreased crystal growth rate. Diamond crystals exhibit the combination form of {111}, {100}, {113}, and {110}sectors, the decreased {100} and {113} sectors, dominated {111} sector are all attributed to the higher growth rate in [100]direction caused by the synergy of MgSiO3 and the movement of P–T diagram. The higher growth rate in [100] direction also increases the metal catalyst and graphite inclusions and leads to the increase of residual tensile stress on the crystal surface. Accompanying with the high growth rate, a higher dissolution rate along [100] and [113] directions than [111]direction occurs at the microstructure and forms the significantly developed(111) stepped growth layer. In addition to the movement of P–T diagram, the addition of MgSiO3 poisons the catalyst and increases the nitrogen content of diamond from 120 ppm to 227 ppm.