PSI - Issue 65

Igor Zh. Bunin et al. / Procedia Structural Integrity 65 (2024) 32–38 Igor Zh. Bunin et al. / Structural Integrity Procedia 00 (2024) 000–000

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Fig. 2. SEM-micrographs of (a) sandstone, and (b) granite surface after HPEMP treatment ( t treat. = 300 s). The scales are (a) 10, and (b) 100  m.

It should be noted that, according to SEM data, numerous cracks on the surface of coal samples were formed after exposure to HPEMP. However, according to the results of computer microtomography of the bulk structure of the same samples, no microcracks were found. Thus, the formation of microcracks occurred mainly in the surface layers of the coal samples. A mixed mechanism of fracture was likely to be observed in coal samples. In the sandstone (Figure 2a) and granite (Figure 2b) samples, we observed the formation and propagation of microcracks, mainly along grain boundaries (intergranular fracture), as a result of the HPEMP effect ( t treat. = 300 s). The electrical breakdown microchannels were also formed on the coal (Figure 1d) and sandstone (Figure 2a) samples surface. The crystal lattice of quartz is characterized by their silicon–oxygen structural motifs, and an ionic-covalent type bonding, see, e.g., Emlin et al. (2009). Low-temperature (  -quartz) and high-temperature (  -quartz) modifications of the mineral differ negligibly in their crystal structure; the widths of the band gaps are ~8.8 and 8.82 eV for  - and  - quartz, and those of the lower conduction band are ~9 and 11 eV, respectively, see, e.g., Emlin et al. (2009). In our paper, see, Bunin et al (2019), the Infrared Spectrum (FTIR) of the vein quartz samples was found to have lines typical of  -quartz. Analysis of IR spectra, see, Bunin et al (2019), showed that the nonthermal effect of nanosecond electromagnetic pulses generally altered the hydrated cover state of mineral surface; namely, we observed a general increase in the hydrated surface of mineral particles. Apparently, the silicon atoms of the most deformed and hydrated surface SiO 4  tetrahedra were the primary centers of adsorption of water molecules. It is very interesting to note that, according to the results of DFT-calculations, see, Makarov et al. (2020), the band structure of kaolinite (Al 2 [Si 2 O 5 ](OH) 4 , wt.%: 39.5 Al 2 O 3 , 46.5 SiO 2 and 14 H 2 O) under the influence of microwave fields ( E ~10 5 V  m  1 ) may change very insignificantly. In the microwave field, the change in the free energy of the unit cell of kaolinite is only a few kJ/mol, that is, in this case the breakage of covalent bonds does cannot occur. When exposed to microwave fields, kaolinite clays release only interplanar and adsorption water, which can pass into the gaseous phase. At a certain critical internal pressure of water vapor inside the mineral there will be destruction of mineral substance of explosive nature, see, Makarov et al. (2020). Nevertheless, according to the our microscopy experimental data (CLSM), the irregular microdefects < 5  m in size formed on quartz surface upon increasing the high-voltage treatment times of the polished samples. The smoothing of the surface roughness occurred, so that we observed a decrease in the roughness parameters: arithmetic mean Ra and mean square Rq deviation of the samples surface profile reduced from 1.3 and 1.5 μm (in the initial state) to 1.1 and 1.2 μm ( t treat. = 30–50 s), respectively. The pulsed impacts caused a significant softening of the quartz surface. With increasing HPEMP treatment time ( t treat. = 10–150 s), a monotonic decrease in mineral microhardness occurred. The maximum relative change (decrease) of microhardness ( HV , %) was fixed at t treat. = 150 s and has made ~30% (from 1435 to 1010 kgf/mm 2 ). At the same time, as a result of short-term pulse treatment ( t treat. = 10–30 s), the HV of quartz was 6  14%.