Study On The Changes Of Defects In Large Diamond Crystals Under High Temperature And High Pressure

With many exceptional physical and chemical properties,diamond has been widely applied in many fields such as industry,scientific research,national defense,medical treatment.High-quality large gem diamond crystal can maximize the ultimate property of diamond and there is a great demand for it in the market.Our country is not rich in natural diamond and a large number of high-quality large diamond crystals can only be obtained by artificial synthesis.At present,the synthesis level of large diamond crystal is still relatively low and there are often various defects in synthesized diamond crystals in our country.These defects have a serious influence on the properties of large diamond crystals.In this paper,large diamond crystals with several common defects were treated to study on the changes of these defects,which aimed to explore the method of improving and repairing common defects in large diamond crystals and to improve the quality of large diamond crystals,using China-type cubic high-pressure apparatus under high pressure and high temperature(HPHT).Three types of defects including metal inclusions,flocculent inclusions and black hue in the central area of large diamond crystal were mainly studied in this paper.It is found that,under the experimental conditions of pressure 5.4 GPa and temperatures from 1500 K to 1650 K,when temperature changes,the morphology of metal inclusions in large diamond crystal will change,while the morphology of flocculent inclusions and black hue in the central area of large diamond crystal will not change.The morphological changes of metal inclusions are caused by the recrystallization after they react with diamonds.The temperature range of metal inclusions' morphological changes is consistent with the temperature range of large diamond crystal growth,which is in the range from about 1500 K to 1650 K.With the increase of temperatures from 1525 K to 1625 K,the morphology of every part of metal inclusions trends to sphere and the morphology will reach a steady state with the extension of treatment time.The steady morphologies of metal inclusions differ at different temperatures,which are also related with the morphologies of metal inclusions before treatments,but the steady morphologies formed under high temperatures areirreversible.In addition,it is found that before and after treatments,the main existing form of nitrogen impurity as isolated nitrogen in large diamond crystal is not changed through IR spectrum.Under the experimental conditions of pressure 6.3 GPa and temperatures from1950 K to 2100 K,it is possible to make the color of large diamond crystal change from yellow to colorless or near colorless at appropriate temperatures.At the same time,the black hue in the central area of large diamond crystal has been improved significantly,for the former black area becomes colorless or near colorless after the black hue disappears.However,flocculent inclusions are still stable in large diamond crystal and the morphology of flocculent inclusions does not change as temperature changing.In addition,it is found that before and after treatments,the main existing form of nitrogen impurity in large diamond crystal is transformed from isolated nitrogen to nitrogen atom pair through IR spectrum.