Their cutting mechanism is a scrapping or “machining” action that is best used to drill rock formations that fail structurally in a plastic mode (e.g., firm and medium-hard, nonabrasive rock formations). These studs in turn are embedded in a steel body that is made up to the bottom of the drill string. These drill bits have specially designed diamond cutter elements bonded to small tungsten carbide studs. The most recent development in drag bit technology is the polycrystalline diamond compact (PDC) bit. This type of drag bit evolved into diamond drill bits that had natural diamonds embedded in a tungsten carbide matrix body that is made up to the bottom of the drill string. Later natural diamonds were used as the cutter elements were attached to the steel blade structures. The original drag bits used in early use of rotary drilling have fixed hardened steel cutter blades or elements that were integral to the body of the bit. The PDC bits are a modern form of the old drag bit. Tom Weller, in Air and Gas Drilling Manual (Fourth Edition), 2021 PDC Bits As the starting graphite contains virtually no nitrogen, the small amount of nitrogen found in NPD might originate from the air present in the pores/grain boundaries of the graphite polycrystals. The fact that the absence of the absorption band near 1130 cm −1, which is characteristic of the isolated form of nitrogen seen in natural Ia-type diamond, also confirms that the nitrogen in NPD is in aggregated forms ( Sumiya et al., 2009). The absorption band near 1220 cm −1 may be attributed to that of a small amount of 6H-type hexagonal diamond ( Wu, 2007), which is diminished in the NPD sample where the conversion to cubic diamond is completed at higher temperatures. NPD has absorption bands in a range around 1750-2750 cm −1 due to multiphonon absorption similar to those of single-crystal diamonds. In fact, photoluminescence measurements demonstrate the presence of N 3 and H 3 color centers, indicative of the aggregated forms of nitrogen in NPD ( Sumiya et al., 2009).įigure 9(b) shows the corresponding IR spectra for the diamonds shown in Figure 9(a). This suggests that the nitrogen in NPD is distributed in aggregated forms. The characteristic absorption due to the presence of an isolated form of nitrogen around 500 nm is not confirmed in NPD, in contrast to Ib-type synthetic diamond, although secondary ion mass spectrometry analyses revealed that NPD contains 50-100 ppm nitrogen atoms. On the other hand, the sharp decrease in the transmittance of NPD below about 500 nm is caused by the absorption due to the electron transition from A color center to the conduction band, as is the case for natural Ia-type diamond. (b) The corresponding transmittance of the various diamonds in the infrared regions. (a) Transmittance of the light around the ultraviolet-visible regions for some synthetic and natural single crystals and for NPD. Optical and IR spectra of NPD (after Sumiya et al., 2009).
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