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This paper reported the synthesis of high quality sheet cubic diamond crystals by temperature gradient method under high pressure and high temperature conditions with FeNi alloy as solvent catalyst. The growth mechanisms were discussed by finite element simulation. The synthetic sheet cubic diamond crystals had a higher growth rate in radial direction than that in axial direction. Raman spectrum and Fourier transform infrared spectrum indicated that synthesized sheet cubic diamonds had fewer crystal lattice distortions and nitrogen impurities. Finite element simulations indicated that the solvent metal convection field in the radial direction was stronger than that in the axial direction. A mechanism of sheet cubic diamond crystal growth was proposed and discussed.

In the paper, under 5.6 GPa and 1250−1450 ℃, the Ib-ype diamond single crystals chosen as the seed crystals with different sizes, are synthesized in a cubic anvil at high pressure and high temperature. High-purity Fe-Ni-Co solvents are chosen as the catalysts. High-purity graphite powder (99.99%, purity) is selected as the carbon source. Hexahedral abrasive grade high-quality diamonds of 0.8 mm, 1.5 mm or 2.2 mm in diameter are chosen as seed crystals. The effects of seed crystal size on the growth of gem-diamond single crystal are studied in detail. Firstly, the influence of the change of seed size on the cracking of diamond single crystal is investigated. The crystal growth law of increasing the probability of cracking crystal with larger seed crystal is obtained. It can be attributed to the following two points. i) The residual cross section at the separation of the main crystal from the larger seed crystal is too large, thus reducing the overall compressive strength of the crystal. ii) The growth rate of the diamond crystal synthesized by larger seed crystal is too fast, which leads to the increase of impurities and defects and the decrease of compressive strength of the crystal. The decrease of crystal compressive strength leads to cracks in diamond crystals during cooling and depressurizing. Secondly, in the growth time of 25 hours, the relationships between the growth time and the limit growth rate of the diamond single crystals synthesized by choosing three sizes of seed crystals are investigated. The results show that the high-quality single crystal synthesis efficiency can be improved and the synthesis period can be shortened by selecting large seed crystals. This is because the size of the seed crystal becomes larger at each stage of crystal growth, resulting in the enhancement of the ability of diamond single crystal to receive carbon, so that high-quality diamond single crystals can be grown at a faster growth rate. Thirdly, with the help of scanning electron microscope or optical microscope, we calibrate the surface morphologies of diamond single crystals grown with different-size seed crystals. Using the seed crystals of 0.8 mm, 1.5 mm or 2.2 mm in diameter, high-quality diamond single crystals with smooth surfaces can be synthesized. However, with the increase of seed crystal in size, the surface flatness of the grown crystals tends to decrease and the possibility with which surface defects occur and string inclusions increase. The growth rate of high-quality diamond single crystals grown with larger seed crystals must be strictly controlled. Finally, the N impurity content values of diamond single crystals grown with different seed crystals in size are characterized by Fourier transform infrared measurement. The results show that the N impurity content of the crystal increases with the diamond growing rapidly by selecting larger seed crystal.

In the paper, under 5.6 GPa and 1200-1400℃, the type Ib diamond single crystals on defect-free[111] -oriented seed crystals are synthesized in a cubic anvil under high pressure and high temperature when the crack problem of diamond single crystal appears frequently. Highpurity Fe-Ni-Co solvents are chosen as the catalysts. Highpurity graphite powder (99.99%, purity) is selected as a carbon source. The effects of cooling process on the qualities of Gem-diamond single crystals are studied carefully. First, in order to study the common crack defects of diamond single crystals, using scanning electron microscope (SEM), the surface morphologies of high quality diamond single crystals and crack crystals are obtained respectively. Our SEM test results show that the surfaces of the crack crystals and the high quality crystals are all very smooth. Therefore, the crack crystal problem is not directly caused by the unordered accumulation of carbon. Second, the concentrations of nitrogen in the high quality diamonds and crack crystals are measured by Fourier transform infrared. In our studies, the nitrogen content of the diamond single crystal with crack is similar to the nitrogen content of high quality single crystal, so the appearance of crystal crack is not caused by high impurity content. According to the test results and the regularity of the occurrence of crack crystals, the reasons for the occurrence of crack crystals are analyzed seriously. When the weather conditions such as seasonal change, wind, rain or snowfall are not very stable, the probability of crack crystal problem to appear will increase greatly. In our opinion, the decrease of diamond crystal quality caused by the fluctuation of external growth conditions is the internal cause of crack crystal problem appearing. After growing diamond crystals, choosing the traditional power failure mode and slowing cooling process respectively, the effect of cooling process on the quality of diamond single crystal is investigated. In the season of the crack problem occurring frequently, choosing power failure cooling process, cracks appear in both diamond crystals with 1.3 mm or 6.0 mm in diameter. With the slow cooling process, the synthetic diamond crystals with 1.2 mm or 5.8 mm in diameter are all high-quality single crystals with no cracks inside. The research results show that the slow cooling process can effectively restrain the occurrence of crack crystal problems. In addition, the mechanism problems of crack crystals and the mechanisms of the effects of slow cooling process on diamond crystal qualities are discussed in detail. We believe that the slow cooling process is effective in solving the crack crystal problem, which is mainly attributed to the following two aspects:on the one hand, the slow cooling makes the internal stress of diamond single crystal growing effectively released, which improves the compressive strength of the crystal and the crystal quality as well; on the other hand, the slow cooling makes the solidification process of the catalyst melt slowly, which provides enough time for the crystal to balance the external stress of the catalyst and the equipment, so that the crystals, which are not affected by the unbalanced external stress, are not cracked.

The shape controlled growth of diamond is beneficial to its subsequent processing. The shape controlled growth for abrasive grade diamond, whose particle size is less than 1 mm, has been studied extensively, while the shape controlled growth of large diamond crystals, which have important commercial and scientific applications, has not been investigated in detail. Therefore, it is necessary to do further researches. In this study, we synthesize large type Ib diamond crystals and investigate their growth shapes at pressures of 5.3-5.9 GPa and temperatures of 1200-1370℃, by using Fe64Ni36 alloy as the catalyst and (100) or (111) face of seed as growth face. Experimental results show that for the diamond crystals grown along the (100) face, the crystal shapes presents plate shape at 1206-1215℃, tower shape at 1216-1260℃, and tower steeple shape at 1261-1360℃; in sequence while for those grown along the (111) face, the crystal shape is of tower at 1233-1238℃ and becomes plate at 1239-1364℃. The ratio of height to diameter, which can provide a standard to quantify the shape of a diamond, is used to describe the crystal shape in detail. For large diamond crystals growing along the (100) face, under a high pressure of 5.6 GPa, the ratio of height to diameter increases with temperature increasing but the ratio of height to diameter, when growing along the (111) face, decreases. The shape distributions of large diamond crystals in the V-shaped region can be determined in the experiments of large diamond crystal synthesis at different temperatures (1200-1370℃) and pressures (5.3 GPa, 5.6 GPa, 5.9 GPa). The lower limit temperature of large diamond crystal growing along the (111) face in the V-shape region is obviously higher than that growing along the (100) face, but the difference between the higher limit temperatures for growing along these two faces is not obvious. The difference between the lower temperature limits of large diamond crystals growing along the (100) and (111) face can be explained by the different energies of the crystal surface and diamond/graphite equilibrium line in the phase diagram of carbon/alloy. Therefore, it has been realized that the shapes for type Ib large diamond crystals are controlled.

This paper reports that diamond single crystals were synthesized from sulfur-added Ni70Mn25Co5+C system under high pressure and high temperature (HPHT). It was found that additive sulfur had inhibited the nucleation and growth of diamond to some extent. X-ray diffraction of the collected sample indicated that under the synthesis conditions, a new compound MnS had been formed through the reaction of additive sulfur with manganese in the catalyst. The MnS has a fcc structure, and its average crystal size was about 30 nm. By scanning electron microscope, the {111} surface of diamond was found to be flat, while there was usually a large depression on the central region of {100}. Further observation showed that there were many small upside-down pyramidal pits in the expression. The results of x-ray photoelectron spectroscopy shows that MnS can only be detected in the depression in the range of detection precision. It was inferred that MnS had been dissolved in the melted alloy during the growth experiment, and precipitated in the sequent quenching process.

Synthesis of diamond single crystals in Fe80Ni20-C system was carried out in a cubic anvil high-pressure and high-temperature apparatus. This paper reports that the surface morphology and inclusion distribution of the grown diamonds had been observed. It finds that the inclusions in cubic and octahedral diamonds radiated along certain crystal orientation, while the inclusion distribution in cubo-octahedral diamond seemed independent of crystal orientation. By using scanning electron microscope, the surface morphology of the three shapes of diamonds was observed. The results of Mossbauer spectrum indicated that there were iron-inclusions Fe3C and Fe–Ni alloy in the diamonds. According to the Fe–C phase diagram, Fe3C should have formed during the quenching process. Nickel might have an inhibitory effect on the formation of Fe3 C.

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.

We report a new diamond synthesis process in which cubic boron nitride single crystals are used as seeds, Fe80Ni20 alloy powder is used as catalyst/solvent and natural flake-like graphite is used as the carbon source. The samples are investigated using laser Raman spectra and x-ray diffraction (XRD). Morphology of the sample is observed by a scanning electron microscope (SEM). Based on the measurement results, we conclude that diamond single crystals have grown on the cBN crystal seeds under the conditions of high temperature 1230°C and high pressure 4.8 GPa. This work provides an original method for synthesis of high quality hetero-semiconductor with cBN and diamond single crystals, and paves the way for future development.

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.

Very rich nitrogen concentration with the dominant C centres and some A centres are found in diamonds grown from a Fe90Ni10-C-high-content NaN3 additive system. The concentrations of C centres rapidly increase with increasing content of NaN3 additive, while the concentrations of A centres increase slowly. The total nitrogen concentration tends to increase rapidly with increasing content of NaN3 additive when the content of NaN3 is below 0.7 wt%. However, the total concentration of nitrogen in the diamonds increases slowly when the content of NaN3 is further increased up to 1.0 wt%, and the total nitrogen average concentration are calculated to be around 2230 ppm for most of the analysed synthetic diamonds. Furthermore, the nitrogen impurities in different crystal sectors of the diamonds are inhomogeneously distributed. The nitrogen impurities in the diamonds in [111] zones are incorporated more easily than that in [100].

Using three kinds of graphites with different graphitization degrees as carbon source and Fe–Ni alloy powder as catalyst, the synthesis of diamond crystals is performed in a cubic anvil high-pressure and high-temperature apparatus (SPD-6×1200). Diamond crystals with perfect hexoctahedron shape are successfully synthesized at pressure from 5.0 to 5.5 GPa and at temperature from 1570 to 1770 K. The synthetic conditions, nucleation, morphology, inclusion and granularity of diamond crystals are studied. The temperature and pressure increase with the increase of the graphitization degree of graphite. The quantity of nucleation and granularity of diamonds decreases with the increase of graphitization degree of graphite under the same synthesis conditions. Moreover, according to the results of the Mossbauer spectrum, the composition of inclusions is mainly Fe3C and Fe-Ni alloy phases in diamond crystals synthesized with three kinds of graphites.

Nitrogen is successfully doped in diamond by adding sodium azide (NaN3) as the source of nitrogen to the graphite and iron powders. The diamond crystals with high nitrogen concentration, 1000–2200 ppm, which contain the same concentrations of nitrogen with natural diamond, have been synthesized by using the system of iron-carbon-additive NaN3. The nitrogen concentrations in diamond increase with the increasing content of NaN3. When the content of NaN3 is increased to 0.7–1.3 wt.%, the nitrogen concentration in the diamond almost remains in a nitrogen concentration range from 1250 ppm to 2200 ppm, which is the highest value and several times higher than that in the diamond synthesized by a conventional method without additive NaN3 under high pressure and high temperature (HPHT) conditions.

采用微波辅助水热法一步合成尺寸约为5 μm的Zn 2 GeO 4 微米球。实验研究了微波水热的反应温度、反应时间、乙酸锌与氧化锗的摩尔比等因素对合成Zn 2 GeO 4 微米球的影响。采用FE-SEM、TEM、XRD和UV-Vis对合成的微米球进行表征。结果表明, 当乙酸锌：氧化锗为6:2, 微波辐射温度为170℃, 反应时间10 min, 尿素用量3.604 g, 制备的Zn 2 GeO 4 微米球具有良好的光催化效果。实验测试Zn 2 GeO 4 微米球比表面积为13 m 2 /g, 在紫外光辐射下, 在甲醇体系中的光解水产氢速率可达到3.76 mmol/(h·g)。该方法缩短反应时间, 增强了光催化活性。

Micron grade boron-doped diamond crystals with octahedral morphology are successfully synthesized in a Fe—Ni—C—B system under high pressure and high temperature (HPHT). The effects of the additive boron on synthesis conditions, nucleation and growth, crystal morphology of diamond are studied. The synthesized micron grade diamond crystals were characterized by optical microscope (OM), scanning electron microscope (SEM), x-ray diffraction (XRD) and Raman spectroscopy. The research results show that the V-shaped section of synthetic diamond moves downwards to the utmost extent due to 0.3a wt% (a is a constant.) boron added in the synthesis system. The crystal colour is black, and the average crystal size is about 25 μm. The crystal faces of synthetic diamond are mainly {111} face. The synthesis of this kind of diamond is few reported, and it will have important and widely applications.

High quality Ib gem diamond single crystals were synthesized in cubic anvil high-pressure apparatus (SPD-6 × 1200) under 5.4 GPa and 1230°C-1280°C. The (100) face of seed crystal was used as growth face, and Ni70Mn25Co5 alloy was used as solvent/catalyst. The dependence of crystal quality and β value (the ratio of height to diameter of diamond crystal) on synthesis temperature was studied. When the synthesis temperature is between 1230°C and 1280°C, the β value of the synthetic high-quality gem diamond crystals is between 0.4 and 0.6. The results show that when the β value is between 0.4 and 0.45, the synthetic diamonds are sheet-shape crystals; however, when the β value is between 0.45 and 0.6, the synthetic diamonds are tower-shape crystals. In addition, when the β value is less than 0.4, skeleton crystals will appear. When the β value is more than 0.6, most of the synthetic diamond crystals are inferior crystals.

With NiMnCo and FeCoNi alloys as solvent metals, large single-crystal diamonds of about 3 mm across are grown by temperature gradient method (TGM) under high temperature and high pressure (HPHT). Although both {100} and {111} surfaces are developed by a layer growth mechanism, some different characteristic patterns are seen clearly on the different surfaces, no matter whether NiMnCo or FeCoNi alloys are taken as the solvent metals. For {100} surface, it seems to have been melted or etched greatly, no dendritic patterns to be found, and only a large number of growth hillocks are dispersed net-likely; while for {111} surface, it often seems to be more smooth-faced, no etched or melted traces are present even when a lot of depressed trigonal growth layers. This distinct difference between {111} and {100} surfaces is considered to be related to the difference of surface-atom distribution of different surfaces, and {111} surfaces should be more difficult to be etched and more steady than {100} surfaces.

The growth rate of diamond has been investigated for a long time and researchers have been attempting to enhance the growth rate of high-quality gem diamond infinitely. However, it has been found according to previous research results that the quality of diamond is debased with the increase of growth rate. Thus, under specific conditions, the growth rate of high-quality diamond cannot exceed a limited value that is called the limited growth rate of diamond. We synthesize a series of type Ib gem diamonds by temperature gradient method under high pressure and high temperature (HPHT) using the as-grown {100} face. The dependence of limited growth rate on growth conditions is studied. The results show that the limited growth rate increases when synthetic temperature decreases, also when growth time is prolonged.

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.

Generally, when growing high-quality large gem diamond crystals by temperature gradient method under high pressure and high temperature, the crystal growth rate is only determined by the temperature gradient. However, we find that the seed crystal cannot completely absorb all the diffused carbon sources, when growing gem diamonds under a higher temperature gradient. Other influence factors appear, and the growth rate of growing diamonds is partly dependent on the crystalline form of superfluous unabsorbed carbon source, flaky regrown graphite or small diamond crystals nucleated spontaneously. The present form is determined by the growth temperature if the pressure is fixed. Different from spontaneous diamond nuclei, the appearance of regrown graphite in the diamond-stable region can retard the growth rate of gem diamonds substantially, even if the temperature gradient keeps unchanged. On the other hand, the formation mechanism of metastable regrown graphite in the diamond-stable region is also explained.

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.

Large high-quality type-II a diamond crystals in size of about 4.0 mm have been grown under the condition of 5.5 GPa and 1200–1300 degrees C by using the temperature gradient method in a domestic cubic anvil high-pressure apparatus. The Fe55Co16Ni25 alloy (KOV) is used as the solvent metal, and Ti with the content 1.5 wt.% of the solvent metal is selected as the nitrogen getter to reduce the impurity of nitrogen in the diamond crystal. To avoid the impurities and cave in the crystal, the growth rate of the initial stage of the growing process is controlled within 0.45 mg/h and the ring carbon source of the size Φ8 mm–Φ6 mm×3 mm is used to grow large diamond crystals.

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.

Using the temperature gradient method under high pressure and high temperature, we investigate the dependence of growing high-quality gem diamond crystals on the growth rates. It is found that the lower the growth rate of gem diamond crystals, the larger the temperature range of growing high-quality gem diamond crystals, and the easier the control of temperature. In particular, when growing gem diamonds under a very-low growth rate, the temperature range of growing high-quality gem diamonds can extend from a low-temperature pure {100} growth region to {100} +{111} growth regions, and finally to a high-temperature only-{111}-growth region. When growing gem diamonds under a high growth rate, some metal inclusions in the growing diamonds always exist near the seeds, no matter whether the growth temperature is high or low. This result is not in agreement with the result of Sumitomo Electric Corporation in Japan.

The synthesis of diamond single crystal in the Fe64Ni36-C system with h-BN additive is investigated at pressure 6.5 GPa and temperature range of 1300–1400°C. The color of the obtained diamond crystals translates from yellow to dark green with increasing the h-BN addition. Fourier-transform infrared (FTIR) results indicate that sp2 hybridization B-N-B and B-N structures generate when the additive content reaches a certain value in the system. The two peaks are located at 745 and 1425 cm−1, respectively. Furthermore, the FTIR characteristic peak resulting from nitrogen pairs is noticed and it tends to vanish when the h-BN addition reaches 1.1 wt%. Furthermore, Raman peak of the synthesized diamond shifts down to a lower wavenumber with increasing the h-BN addition content in the synthesis system.