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Diamond possesses a unique combination of excellent optical, thermal, and mechanical properties, and is therefore an ideal transparent ceramic material for harsh and extreme environments. Due to its important applications in technology, transparent diamond ceramic (TDC) has been explored and prepared by chemical vapor deposition (CVD) or direct conversions of non-diamond carbon precursors at high pressure and high temperature (HPHT), but the preparation of large-size TDC with high mechanical strength remains a challenge. Here, we report for the first time, a transparent polycrystalline diamond ceramic from diamond powder with a transmittance of ∼60 % at wavelengths of 400–1600 nm. The analyses of phase composition, residual stress and microstructure evolution of the sintered samples with different sintering conditions indicate that compression at high temperatures (>2000 ⁰C) facilitates the deformation of diamond grains, allowing for densification and diamond-diamond bonding formation. The sintering pressure of the diamond powders with an optimized particle size distribution was dramatically reduced from 16 GPa to 10 GPa. Our results, based on successfully preparing centimeter-sized TDC, set the standard and the precedent for the large-scale preparation of larger TDC in proximity to industrial conditions. [ABSTRACT FROM AUTHOR]

Polycrystalline diamond in the presence of silicon with different sintering times was investigated under high pressure and high temperature of 5.0 GPa and 1400 °C, respectively, using a multi-anvil apparatus. In this research, phase analysis demonstrated that the content of SiC increased and the amount of Si decreased clearly with the sintering time extending by X-ray diffraction (XRD). The characterization of the sintered body demonstrated that the electrical resistance decreased as the reaction time rose. Specimens with good conductivity of 17 Ω·cm−1 sintered for 20 min at 1400 °C under 5.0 GPa were successfully obtained. The measured Vickers hardness and thermal stability of the synthesized diamond-SiC composites in the present study were >40 GPa and 1450 K, respectively. • Diamond–SiC composites were investigated by sintering different times in the range of 1–20 min at 5.0 GPa and 1400 °C. • SiC was densely packed between the diamond's grain boundaries, preventing the contact between the oxygen and diamond. • Extending the sintering time, and the graphitization of diamond occurred in low pressure zone are gradually in process. [ABSTRACT FROM AUTHOR]

Pure diamond powder and mixed powder of diamond and sodium chloride were treated at high pressures (5 GPa and 10 GPa) and high temperatures (1000–2700 °C), and then the graphitization behavior of diamond powder under non-hydrostatic (pure diamond powder) and quasi-hydrostatic (mixed powder of diamond and sodium chloride) compression was quantitatively studied by analyzing the X-ray diffraction peaks' area of treated samples. It is found that the graphitization under non-hydrostatic compression is much more easily triggered by heating due to the heterogeneous stress distribution within the diamond grains. Under the quasi-hydrostatic compression of 5 GPa, the starting graphitization temperature is about 1600 °C, which is 300 °C higher than that under non-hydrostatic compression. Under a pressure of 10 GPa, graphitization could be observed after pure diamond powder was treated at 1400 °C, but no graphitization occurred in the mixed powder even after a high temperature treatment at 2300 °C. Optimizing the stress distribution among diamond grains by the mixing of grain-size can significantly reduce the graphitization at high temperature even under non-hydrostatic compression. [Display omitted] • Graphitization of diamond was quantitatively studied by x-ray diffraction peak areas. • The graphitization behavior of diamond is affected by stress state. • Higher temperature of graphitization is under quasi-hydrostatic compression of 5 GPa. • No graphitization occurs under quasi-hydrostatic compression of 10 GPa at 2300 °C. • The graphitization degree is greatly weakened by the mixing of grain-size. [ABSTRACT FROM AUTHOR]

The thickness of the silicon carbide layer around diamond grains in the reaction sintering process of diamond/SiC composites starting from diamond and silicon was investigated at 1100–1400 °C and 5 GPa using two different sample assemblies. Dense silicon carbide layers with thicknesses from 60 nm to 350 nm were observed using energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) analysis. This demonstrated that the thickness of the silicon carbide layer increased with increasing sintering temperature and time. The surface morphology of diamond grains/single-crystal diamond wafers eroded by liquid silicon was analysed by field emission scanning electron microscopy (FESEM), and it was found that SiC growth was controlled by the diffusion of silicon and carbon atoms through the existing layer of SiC. [Display omitted] • The thickness of silicon carbide layer around diamond grains was measured at 1100–1400 °C and 5 GPa. Dense silicon carbide layers with thicknesses from 60 nm to 350 nm were observed using EDS and XRD analysis. • The thickness of silicon carbide layer increased with the increase of temperature and SiC growth was controlled by the diffusion of silicon and carbon atoms through the existing layer of SiC. • The quantitative data of the thickness of silicon carbide layer around diamond under high pressure is reported for the first time. [ABSTRACT FROM AUTHOR]

The stability and solubility of diamond in cobalt at a pressure of 5 GPa was investigated. At temperatures ranging from 1000 °C and 1350 °C, diamond can be dissolved in cobalt without graphitization. Meanwhile, the solubility of single diamond crystals in cobalt was measured at 5 GPa and temperatures of up to 1300 °C through weight analysis, and it was found to increase with the increase in temperature. The morphological changes of the diamond crystals after high-pressure and high-temperature (HPHT) treatments were observed using a scanning electron microscope. The resulting quantitative data and morphology analysis enabled the construction of a scheme for the morphological evolution of diamond crystals during their dissolution in cobalt at HPHT. The diamond formation process in cobalt solvent is discussed in relation to the dynamic equilibrium. Unlabelled Image • The solubility of diamond in cobalt was measured at 5.0 GPa and high temperatures up to 1300 °C. The temperatures region of diamond growth at 5 GPa is determined. • The solubility of diamond in cobalt increased with the increase of temperature and the diamond formation in metal solvents is a process of the dissolution and precipitation of carbon atoms. • The quantitative data of solubility of diamond in cobalt under high pressure is reported for the first time, which provides favorable evidence of solvent theory. [ABSTRACT FROM AUTHOR]