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The Synthesis of Low Nitrogen Industrial Diamond in Fe-Ni-C System at HPHT

Author: HanWei
Tutor: JiaXiaoPeng
School: Jilin University
Course: Condensed Matter Physics
Keywords: low nitrogen concentration Ti additive(or Al additive) HPHT
CLC: TQ164
Type: Master's thesis
Year: 2009
Downloads: 46
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Abstract

Since diamond was first synthesized from a metal catalyst/solvent and graphite mixture by using the hydrostatic high-pressure and high-temperature (HPHT) method in 1954, numerous impurities were found in the diamonds synthesized by the HPHT method, part of which can enter diamond to form substitutes or inclusions. Different impurities have different influences on the properties of diamonds. Nitrogen is one of the most common impurities in both natural and synthetic diamonds. Nitrogen impurity has significant influences on the physical properties of diamonds, including mechanical, electrical,thermal properties, and especially optical properties, which directly determine the potential applications of diamonds in high-tech fields and other industries. Therefore, many researchers have focused on the synthesis of low nitrogen concentration diamond with Ti, Al, etc. as nitrogen getters. It was reported that large high-quality type IIa diamond crystals without impurities of 7–8 ct (about 10 mm across) can be grown at a high growth rate of 6–7 mg/h with an adequate selection of solvent metal and additives by use of temperature gradient method (TGM). In the synthesis process, Ti was added to the solvent as the nitrogen getter, and Cu was also added for minimizing the formation of TiC. Industrial diamonds can be synthesized by film growth method (FGM). As is known, there are fundamental differences between FGM and TGM. However, so far, the influence of Ti (Al) additive on the synthesis of industrial diamonds at low nitrogen concentration has been seldom reported.In this work, high purity Ti as nitrogen getter was added to the Fe-Ni-C system to synthesize industrial diamonds by FGM. The effects of Ti (Al) additive on the synthetic conditions, crystal color, crystal shape, surface morphology and nitrogen impurity concentration etc. were investigated in detail. The research results should be useful for future study on high purity diamond.Firstly,Diamond crystals with low nitrogen concentration were synthesized from the Fe-Ni-C system with Ti additive at high-pressure and high-temperature (HPHT) in a china-type cubic high pressure apparatus (CHPA). The synthesis pressure range was 4.8GPa– 5.2 GPa, and the temperature range was 1420 K–1600 K. The lowest synthesis pressure for diamond fell first and then rose with the increase of Ti additive. The color, shape, surface morphology and nitrogen impurity concentration of the synthesized diamond crystals were characterized by using optical microscopy (OM), scanning electron microscopy (SEM) and micro Fourier transform infrared (FTIR) spectrometry. The results show that the Ti additive has significant effects on color, growth rate, crystal shape, surface morphology and nitrogen impurity concentration of the synthesized diamond crystals. The color of diamond crystals synthesized without Ti additive is yellow, while that with Ti additive becomes light and nearly colorless. The growth rate without Ti additive is higher than that with Ti additive. The crystal shapes of as-grown diamond crystals vary with the increase of Ti additive. The {111} crystal faces become dominant and some {311} crystal faces appear with the increase of Ti additive. The concentration of nitrogen impurity in diamond crystals without Ti additive is higher than that with Ti additive.(1) It is found that the minimum synthesis pressure and temperature for diamonds synthesized with Ti additive are lower than those without Ti additive when Ti additive is less than 0.8 wt%, but become higher when Ti additive is more than 0.8 wt%.(2) It is found that the color of diamond crystals synthesized without Ti additive is yellow, however, that with Ti additive becomes light and nearly colorless. With the increase of Ti additive, the color of crystals is almost colorless, and the crystals tend to be imperfect even on the conditions which are supposed to grow high-quality pure diamonds.(3) The high-quality crystals when without Ti doped are hexahedron, cubo-octahedral, octahedral, with the increase of Ti, crystal size tends to uniformity, and the crystal shape of high-quality crystals is octahedral.(4) Some inclusions and pits appear when Ti is added to the Fe-Ni-C system.(5) With the increase of Ti, the rate of decline in the integrity of crystal, polycrystalline worsened.And then, diamond crystals with low nitrogen concentration were synthesized from the Fe-Ni-C system with Al additive at HPHT to research the growth characteristics of diamond. Al as nitrogen getters was compared with Ti for the synthesis of low nitrogen nitrogen industrial diamond.(1) The V-shape region for the diamond growth.of the Fe-Ni-C system with Al additive moves downwards obviously compare with without Al additive. When the synthesis pressure is 5.1Gpa, the temperature decrease 60K as the content of Al additive from 0.6% to 2.0%.(2) It is found that the color of diamond crystals synthesized without Al additive is yellow, however, that with Al additive becomes light and nearly colorless. With the increase of Al additive, the color of crystals becomes dark, and inclusions in the synthetic diamond are diffusion-like.(3) Some inclusions and pits appear when Al is added to the Fe-Ni-C system. The number of inclusions and pits increases with the increase of Al. The result of low nitrogen concentration diamond crystals synthesized with different nitrogen getters are follows:(1) In the Fe-Ni-C system, the synthesis pressure and temperature for diamonds synthesized with Ti additive (or Al additive) changed Obviously, that is to say the movement of the V-shape region for the diamond growth.(2) It is found that the color of diamond crystals synthesized with Ti additive (or Al additive) becomes light and nearly colorless when the content of additive is 0.6%.(3) Some inclusions and pits appear when Ti (or Al) is added to the Fe-Ni-C system. The number of inclusions and pits increases with the increase of Ti (or Al).(4) No absorption peaks are detected either at 1130 or at 1344 cm?1. It indicates that the nitrogen concentration in the synthetic diamond with addition of Ti decreases significantly and carbon–nitrogen peaks are not detectable.(5)With the increase of these nitrogen getters, the rate of decline in the integrity of crystal, polycrystalline worsened.

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