Study On Real-time Dynamic Acoustic Emission Detection And Growth Mechanism Of Diamond Under HPHT

It is very difficult to obtain directly the experimental data of diamond growth process, because the diamond crystals are synthesized in the airtight cavity under HPHT. Until now, the study on the growth mechanism of diamond is based on the static observation and characterization of diamond and related phases at atmospheric pressure and room temperature, or the process simulation and analysis of diamond synthesis under HPHT in the specific experimental conditions. And the real information about diamond growth under HPHT is not directly collected. Thus, finding a detection method to reflect real-time dynamic information about diamond growth under high pressure and high temperature is of great scientific significance and application value to figure out that how graphite become diamond, how the diamond grow up, and how the iron-based catalyst work.In the paper, the diamond was synthesized under HPHT using graphite as the carbon source and iron-based metallic catalyst produced by powder metallurgy. And the real-time dynamic detection system of diamond growth under high pressure and high temperature was established using acoustic emission instrument and diamond synthesis equipment, which was used to collect dynamically acoustic emission signals of diamond growth process. The morphology and microstructure of diamond and related phases were studied at atmospheric pressure and room temperature by means of metallographic microscope, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM). Meanwhile, the acoustic emission signals detected in the diamond growth process were processed and analyzed using parametric analysis, wavelet analysis and other methods. With the results of the static detection and characterization, the relevance and consistency of the variation of acoustic emission signals of diamond growth process were studied, and comprehensive analysis of the growth mechanism of diamond was carried out. According to the analysis on the fracture morphologies of the synthesis block, granularity distribution of diamond grains, production quantities of the diamonds and morphologies of the inside surface of iron-based metallic film in the different synthetic time, the growth rate of diamond growth process is faster in the earlier stage and slower in the later stage. SEM and FESEM study indicates that the nanoscale pyramid cone shapes exist on the metallic film interface (100), while serrate steps are found on the metallic film interface (111). The diamond growth units have different accumulation mode on different diamond interfaces, which leads to different morphologies of diamond corresponding metallic film interfaces. According to XRD and TEM analysis, the carbon source for diamond growth with iron-based metallic catalyst under HPHT comes from the carbon atomics separated from Fe3C, and y-(Fe,Ni) plays a role of catalysis phase in the process.Through introduction and design of the waveguides and installation of the sensors, the acquisition area of acoustic emission signals is limited within the synthesis block, which makes the acoustic emission signals effectively transmitted through the waveguide to the acoustic emission instrument. And it is sure that the acoustic emission signals collected are caused by the change in the synthesis cavity under high pressure and high temperature. The space location of eight sensors by the method of antisymmetric distribution and reasonable settings of acoustic emission testing parameters could remove the most of the interference noise, and ensure the acoustic emission signals authenticity, rationality and reliability. Meanwhile, according to the characteristics of acoustic emission signals, the detected acoustic emission signals are processed using parameters analysis and wavelet analysis. Through these methods, a new real-time detection system of diamond growth under HPHT is established, which could provide an effective experimental method for growth mechanism research of diamond.Based on wavelet transform, the de-noising treatment of acoustic emission signals collected is carried out, in which the selection of wavelet bases, the choice of de-noising methods and so on are analyzed. De-noising results show that the acoustic emission signals after de-noising treatment filter out most noise, and there is no damage to the characteristics of acoustic emission signals, which could more accurately characterize the characteristics of acoustic emission signals of diamond crystals growth.According to the contrastive analysis on acoustic emission signals with or without diamond growth, it shows that the high amplitude signals are produced correspondingly with diamond crystal growth, and the cumulative value and the average value of rise time, event count rate, ring-down counts, energy counts and duration time of recorded acoustic emission signals with diamond growth have increased. When there is no diamond crystals growth, the acoustic emission signals are mainly in the low frequency band below 80 kHz; When diamond crystals grow, there are many new frequency peaks in the frequency range that is higher than 80 kHz. Comparative analysis shows that these new frequency peaks are caused from the growth process of diamond crystals.By analyzing parameters of acoustic emission signals from diamond growth process, it shows that the characteristic parameters of acoustic emission signals, such as energy counts, amplitude and rise time increase firstly and then decrease with the synthesis time. With observation of the fracture morphologies of the synthesis block and parameter analysis results, it shows that the variation of the characteristic parameters of acoustic emission signals fit in with the diamond crystal growth process. In the earlier stage, diamond growth rate gradually increases with the extension of synthesis time, and the acoustic emission source activity correspondingly increases. When the growth rate decreases after reaching the maximum rate, the activity of acoustic emission source gradually becomes weak.According to the analysis on acoustic emission signals from diamond crystals growth based on wavelet packet energy algorithm, the value of the signal energy and peak in different frequency bands are compared along with the diamond growth process. And it has reflected the dynamic changes in the time domain of the acoustic emission signals stimulated by the different acoustic emission sources. It also shows that the frequency of acoustic emission signals could be used as the effective means to distinguish the different acoustic emission sources in the diamond growth process.