Jia Xiao-peng

Growth and annealing study of hydrogen-doped single diamond crystals under high pressure and high temperature

A series of diamond crystals doped with hydrogen is successfully synthesized using LiH as the hydrogen source in a catalyst-carbon system at a pressure of 6.0 GPa and temperature ranging from 1255 °C to 1350 °C. It is shown that the high temperature plays a key role in the incorporation of hydrogen atoms during diamond crystallization. Fourier transform infrared micro-spectroscopy reveals that most of the hydrogen atoms in the synthesized diamond are incorporated into the crystal structure as sp3 -CH2-symmetric (2850 cm−1) and sp3 CH2-antisymmetric vibrations (2920 cm−1). The intensities of these peaks increase gradually with an increase in the content of the hydrogen source in the catalyst. The incorporation of hydrogen impurity leads to a significant shift towards higher frequencies of the Raman peak from 1332.06 cm−1 to 1333.05 cm−1 and gives rise to some compressive stress in the diamond crystal lattice. Furthermore, hydrogen to carbon bonds are evident in the annealed diamond, indicating that the bonds that remain throughout the annealing process and the vibration frequencies centred at 2850 and 2920 cm−1 have no observable shift. Therefore, we suggest that the sp3 C-H bond is rather stable in diamond crystals.

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.

High Thermoelectric Properties of PbTe Doped with Bi2Te3 and Sb2Te3

The composition-dependent thermoelectric properties of lead telluride (PbTe) doped with bismuth telluride (Bi2Te3), antimony telluride (Sb2Te3) and (BiSb)2Te3 have been studied at room temperature. All the samples exhibit small thermal conductivity. The figures of merit, 7.63, 1.03 and 8.97×10−4, have been obtained in PbTe with these dopants, respectively. These values are several times higher than those of PbTe containing other dopants with small grain sizes. The high thermoelectric performance is explained by electronic topological transition induced by alloying. The results indicate that these dopants are effective to enhance the thermoelectric performance of PbTe.

In-Situ Measurement of Electrical Character of PbTe at High Pressure and High Temperature

There is a widespread interest in lead telluride (PbTe) as a good thermoelectric material. We report the temperature dependence of thermopower S(T) and resistance R(T) for PbTe at the different pressures of from 1.8 GPa to 5 GPa obtained by using the cubic anvil high pressure apparatus. With increasing pressure, R(T) and S(T) decrease. The effect of pressure on R(T) is larger than that on S(T). The power factor that is determined by thermopower and resistivity increases with increasing pressure. This method is an efficient tool for synthesizing good thermoelectric materials at high pressure and high temperature.

Grow Large High-Quality Diamonds with Different Seed Surfaces

Large high-quality type Ib diamond crystals have been grown with different seed surfaces by temperature gradient method at 5.5 GPa, 1500–1600 K, with NiMnCo alloy as the metal solvent. Compared with {100} as the growth surface, the growth region of large high-quality diamond crystals with {111} as the growth surface at a higher growth rate shifts markedly from lower temperatures (suitable for {100}-facet growth) to higher temperatures (suitable for {111}-facet growth). However, regardless of different growth surfaces, {100} or {111}, the grown crystals of sheet-shaped shape are most difficult for metal inclusions to be trapped into, and whether or not matched growth between the seed surfaces and the growth temperatures determines the crystal shapes. In view of the growth rates, large high-quality diamond crystals of sheet-shaped shapes can be grown at a growth rate of above 2.5 mg/h, while the growth rate of large high-quality diamond crystals should not be beyond 1.5 mg/h for tower-shaped crystals.

Reactive Mechanical Alloying Synthesis of Nanocrystalline Cubic Zirconium Nitride

Zirconium nitride powders with rock salt structure (γ-ZrNx) are prepared by mechanical milling of a mixture of Zirconium and hexagonal boron nitride (h-BN) powders. The products are analysed by x-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy (RS). The formation mechanism of γ-ZrNx by ball milling technique is investigated in detail. N atoms diffuse from amorphous BN (α-BN) into Zr to form Zr(N) solid solution alloy then the Zr(N) solid solution alloy decomposes into γ-ZrNx. No ZrB2 is observed in the as-milled samples or the samples annealed at 1050° C for 2h.

Synthesis and Magnetotransport Properties of CrO2-TiO2 Composites

CrO2-TiO2 composites are synthesized by using a high temperature and high pressure method using CrO3 and H2TiO3 as precursors. The composites consist of large rod-like CrO2 crystals separated by small TiO2 grains. The CrO2 in the composites is very pure and its saturation magnetization is very close to the theoretical value (i.e., 2μB per formula unit). The composites exhibit a large negative magnetoresistance (MR) at 5K. The MR in CrO2-TiO2 composites is mainly attributed to spin-polarized tunneling between CrO2 crystals. The conductivity of the composites is best described by a fluctuation-induced tunneling model below 230K.

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.

An Effective Solution for the Best Set of Beveling Parameters of the Cubic High-Pressure Tungsten Carbide Anvil

Determining the best set of beveling parameters is an advantageous characteristic of the geometrical conditions for a cubic high-pressure tungsten carbide (WC) anvil, but it is almost impossible to deduce experimentally (much affected by defects in the material). In order to remove the affection of defects in materials, we investigate computational stress analyses in different beveling parameters of WC anvils by the finite element method. The results indicate that the rate of cell pressure transmitting and failure crack in the WC anvil monotonically increases with the bevel angle from 42° to 45°. Furthermore, there are two groups of actual users of beveled anvils, one group preferring 41.5°, which can decrease the rate of failure crack in WC anvil, the other group preferring 42°, which can increase the rate of cell pressure transmitting. This work would give an effective solution to solve the problem of the design of a cubic high-pressure WC anvil experimentally and will greatly help to improve the cubic high-pressure WC anvil type high pressure techniques.

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.