Dependence of Limited Growth Rate of High-Quality Gem Diamond on Growth Conditions

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.     

Effects of Al and Ti/Cu on Synthesis of Type-IIa Diamond Crystals in Ni70Mn25Co5-C System at HPHT

High-quality type-Ⅱa gem diamond crystals are successfully synthesized in a NiToMn25Co5-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）. Piowever, 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.     

HPHT Synthesis of Different Shape Coarse-Grain Diamond Single Crystals

Synthesis of coarse-grain diamond crystals is studied in a China-type SPD6× 1670T cubic high-pressure apparatus with high exact control system. To synthesize high quality coarse-grain diamond crystals, advanced indirect heat assembly, powder catalyst technology and optimized synthesis craft are used. At last, three kinds of coarse- grain diamond （about 0.85 mm） single crystals with hexahedron, hex-octahedron and octahedron are synthesized successfully under HPHT （about 5.4 GPa, 1300-1450℃）. The growth characters of different shape crystals are discussed. The results and techniques might be useful for the production of coarse-grain diamonds.     

Effect of Carbon Source with Different Graphitization Degrees on the Synthesis of Diamond

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 5.5GPa and at temperature from 1570 to 1770K. 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 ofdiamonds decreases with the increase of graphitization degree of graphite under the same synthesis conditions. Moreover, according to the results of the M6ssbauer spectrum, the composition of inclusions is mainly Fe3 C and Fe-Ni alloy phases in diamond crystals synthesized with three kinds of graphites.     

HPHT Synthesis of High-Quality Diamond Single Crystals with Micron Grain Size

High-quality diamond single crystals with micron grain size are synthesized with a new high-pressure and high-temperature （HPHT） synthesis technique in a cubic anvil high pressure apparatus. Morphology of the synthesized diamonds is observed by a scanning electron microscope （SEM）. The samples are characterized using laser Raman spectra. The results show that the new synthesis technique improves the nucleation of diamond greatly, and diamond single crystals with perfect morphology and micron grain size are successfully synthesized, with the average grain size of about 6μm. This work provides a new synthesis technique to implement industrialization of high-quality diamond single crystals with super-fine grain size, and paves the way for future development.     

A New Dopant of NaN3 for High-Concentration-Nitrogen Diamond Synthesized by HPHT

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, 1000-2200ppm, 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 1250ppm to 2200ppm, 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.     

In Situ Observation of Skeletal Shape Transition during BaB2O4 Crystal Growth in High-Temperature Solution

The transition from a fiat solid-liquid interface to a skeletal shape during BaB2O4 （BBO） single crystal growth in Li2B4O7 flux is observed in real time by an optical high-temperature in-situ observation system. The movement of crystal step is also investigated. The observation results demonstrate that the steps propagate along and parallel to the fiat interface when the crystal size is small. Nevertheless, they will ‘bend＇ close to the face centre if the crystal size becomes greater. Atomic force microscopy reveals that more deposition places near the face centre give rise to the bending of advancing steps and thus the formation of a vicinal interface structure. Measurements of step velocity show that the velocity keeps nearly constant at different moments for one specific step, whereas the step on a newly formed layer advanced faster than that on a previously formed one when the crystal size is larger than 210μm or so. Thus interracial morphological instability occurs and a skeletal interface is obtained.     

Effects of NaN3 Added in Fe-C System on Inclusion and Impurity of Diamond Synthesized at High Pressure and High Temperature

Effects of NaN3 added in Fe-C system to synthesize nitric diamond at high pressure and high temperature are investigated. Diamond crystals with high nitrogen concentration are synthesized by the system of Fe-C and NaN3 additive at pressure 5.8 GPa and at temperatures 1750-1780 K for 15 min. The synthetic diamond crystals have a cubo-octahedral or octahedral shape with yellowish green or green colour. Some disfigurements are observed on the surfaces of most diamond crystals. The composition and content of inclusions formed by iron in diamond are changed and iron nitride is detected in diamond crystals synthesized with Fe-C-NaN3 additive. As the amount of NaN3 additive increases, Fe3C decreases and iron nitride increases with α-Fe being nearly constant. Moreover, the nitrogen concentrations in diamond crystals synthesized with 1.5 wt% NaN3 additive is up to 2250ppm in substitutional form.     

Formation of Mixture of A and C Centres in Diamond Synthesized with Fe90Ni10-C-High-Content Additive NaN3 by HPHT

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 2230ppm for most of the analysed synthetic diamonds. Eurthermore, 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].     

Dependence of Crystal Quality and β Value on Synthesis Temperature in Growing Gem Diamond Crystals

High quality Ib gem diamond single crystals were synthesized in cubic anvil high-pressure apparatus （SPD- 6 × 1200） under 5.4GPa and 1230℃-1280℃. The （100） face of seed crystal was used as growth face, and Ni70Mn25Co5 alloy was used as solvent/catalyst. The dependence of crystal quality andβ value （the ratio of height to diameter of diamond crystal） on synthesis temperature was studied. When the synthesis temperature is between 1230℃ and 1280℃, theβ value of the synthetic high-quality gem diamond crystals is between 0.4 and 0.6. The results show that when theβ value is between 0.4 and O. 45, the synthetic diamonds are sheet-shape crystals; however, when theβ value is between 0.45 and 0.6, the synthetic diamonds are tower-shape crystals. In addition, when theβ value is less than 0.4, skeleton crystals will appear. When theβ value is more than 0.6, most of the synthetic diamond crystals are inferior crystals.     

Surface Structure of Large Synthetic Diamonds by High Temperature and High Pressure

With NiMnCo and FeCoNi alloys as solvent metals, large single-crystal diamonds of about 3mm 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.     

Chiral Symmetry Breaking During Growing Process of NaClO3 Crystal under Direct-Current Electric Field

We investigate the influence of dc electric field on chiral symmetry breaking during the growing process of NaClO3 crystal. Nucleation and growth of NaClO3 are completed from an aqueous solution by a fast cooling temperature technology. A pair of polarization microscopes are used to identify a distribution of chiral crystals. Experimental results indicate that the dc electric field has an effect on distribution of chirality, but the direction of the dc electric field is not sensitive to the chiral autocatalysis and selectivity, i.e. the nature convection driving by the gravity does not play an important role on a thin layer of NaClO3 solution. The experimental phenomena may be elucidated by the ECSN mechanism.     

HPHT Synthesis of Micron Grade Boron-Doped Diamond Single Crystal in Fe-Ni-C-B Systems

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 25μm. 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.     

Undercooling and Unidirectional Solidification of CuNi Alloy Melts

Cylinder-shaped CusoNi20 alloy melt is undercooled and solidified by the combination of the electromagnetic levitation technique and the flux treatment method. Nearly constant temperature gradient of 8-10 K/cm is realized for the cylindrical melts with differen~ undercooling levels at the bottom ends. The experimental results reveal that with the increase of the undercoo]ing of the melts from 35 to 220 K, the microstructures undergo transition from coarse dendrites to granular grains, unidirectional dendrites, and finally to equiaxed grains.     

Hetero-Epitaxial Diamond Single Crystal Growth on Surface of cBN Single Crystals at High Pressure and High Temperature

We report a new diamond synthesis process in which cubic boron nitride single crystals are used as seeds, FesoNi20 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℃ and high pressure 4.8 GPa. This work provides an original method for synthesis of high quality hereto-semiconductor with cBN and diamond single crystals, and paves the way for future development.     

Effect of Crucibles on Qualities of Self-Seeded Aluminium Nitride Crystals Grown by Sublimation

Self-seeded aluminium nitride （AlN） crystals are grown in tungsten and hot pressed boron nitride （HPBN） crucibles with different shapes by a sublimation method. The qualities of the AlN crystals are characterized by high-resolution transmission electronic microscopy （HRTEM）, scanning electron microscopy （SEM） and Micro-Rarnan spectroscopy. The results indicate that the better quality crystals can be collected in conical tungsten crucible.     

pH and temperature dependence of particle size in Pb1−xFexS nanoparticle films

The growth mechanism of Pb_1−xFe_xS nanoparticle films in chemical deposition is discussed. Grain growth for the Pb_1−xFe_xS films with increasing temperature of the chemical bath is observed to be due to the phenomenon of coalescence and formation of bigger particles at higher pH is possibly due to aggregation.     

Efficiency Enhancement of Dye-Sensitized Solar Cells： Using Salt CuI as an Additive in an Ionic Liquid

Energy conversion efficiency of the dye-sensitized solar cell is improved from 3.5% to 4.5% by adding a small amount of CuI into an ionic liquid electrolyte. It is found that other copper-I salts, for example, CuBr, have the same effect for the dye-sensitized solar cell. Experimental results show that no Cu^2＋ ions exist in this electrolyte. It is suggested that this improvement is caused by the adsorption of Cu^＋ onto the TiO2 porous film.     

Lower limit for single-wall carbon nanotube diameters from hydrocarbons and fullerenes

A theoretical model for the growth of single-wall carbon nanotubes produced by metal-catalyzed decomposition of hydrocarbons and fullerenes is presented. The growth process is treated as a thermodynamic equilibrium between carbon in the gas phase and carbon in the nanotube. The minimum possible nanotube diameters based on several published experimental conditions are calculated by combining the free energy of the reaction with an equation derived from elastic theory. The model predicts the possibility of generating nanotubes with extremely small diameters that are smaller than in the corresponding experiments. Received: 18 July 2001 / Accepted: 19 November 2001 / Published online: 4 March 2002     

A facile one-pot solvothermal route to tubular forms of luminescent polymeric networks [(C3N3)2(NH)3]n

A facile one-pot solvothermal route has been developed for the synthesis of tubular luminescent polymeric networks [(C₃N₃)₂(NH)₃]_n, structurally related to the proposed g-C₃N₄. XRD patterns showed a characteristic 002 basal plane diffractions, indicating an interlayer d spacing of 3.23 Å. XPS spectra show that the C1s and N1s have a symmetric peak and an asymmetric peak at 288.10 and 399.00 eV, respectively. The bulk composition C₆N_8.9H_4.5 determined by elemental analysis is comparable to the calculated value C₆N₉H₃ for this proposed polymer. FTIR spectra indicated the presence of s-triazine ring, which was further supported by the luminescent and UV-vis absorption characteristics probably depending on π→π^* electronic transition. The tubular structure has been studied by TEM, SAED, and HREM.