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.

In this work, the effect of catalyst viscosity on diamond growth was studied by simulating and analyzing the catalyst convection field in a diamond synthesis chamber under different viscous catalyst...

In this work, diamond crystals were synthesized with different doping proportions of N-H-O at 5.5-7.1GPa and 1370-1450℃. With the increase of N-H-O doping ratio, the crystal growth rate decreases, a more serve temperature and pressure condition are required for diamond nucleation process and the crystalline process for diamond are affected. [111] becomes dominant plane of diamonds, and the surface morphology is blocked, more growth texture, stacking fault and etch pit were appeared. Diamond crystals own a two-dimensional growth habit. Increasing the doping concentration also leading more nitrogen enter into diamond crystals, which is confirmed by FTIR. However, the quality of the crystal gradually deteriorates which could be ascertained by the red shift of Raman peak position and the widening of Raman full width at half maximum. With the increase of doping ratio, the photoluminescence property of diamond crystal also drastically changed. The intensity of NV center of diamond crystal changes, and several Ni-related defect center such as NE1 center and NE3 center appear. Synthesis of diamond in N-H-O bearing fluid provides important information for a deeper understanding of the growth characteristics of diamond in complex system and the formation mechanism of natural diamond as nature diamonds are almost nitrogen rich and full of various defect centers. Meanwhile, this work proves that the type of defect center in diamond crystal could be regulated by controlling the N-H-O impurities content in the synthesis system.