High‐pressure torsion of SiO₂ quartz sand: Phase transformation, optical properties, and significance in geology

Abstract

Phase transformation and optical properties of silica (silicon dioxide, SiO2) quartz sand under high pressure/temperature has been of interest in geology and optical physics for many years. In this study, besides high pressure/temperature, high plastic strain is simultaneously applied to the quartz sand by high‐pressure torsion (HPT) processing. The material shows oxygen vacancy formation and transformation to (a) a denser nanocrystalline quartz phase, (b) a high‐temperature amorphous phase and (c) a high‐pressure coesite phase. These structural and microstructural changes lead to light absorbance, electron spin resonance, photoluminscence and photocatalytic activity, while these changes are enhanced by increasing strain. This study introduces a possible pressure‐temperature‐strain‐based mechanism for the formation of naturally observed vacancies and coesite phase in SiO2‐based minerals and sands.

Microstructure Study of Phase Transformation of Quartz in Potassium Silicate Glass at 900 °C and 1000 °C

“…For quartz, there is no phase transition when it is kept in air at 900 or 1000 • C for a long period of time, which is consistent with other reports [21]. However, when it is mixed with potassium silicate glass, as shown in the broken line diagrams in Figures 4, 7, 10 and 11, obvious phase transition occurs: quartz particles first transform into cristobalite, and then into tridymite.…”

“…The schematic illustration of phase transformation of quartz-potassium silicate glass composite is shown in Figure 12. The phase transition of composites will be discussed in the following two aspects: For quartz, there is no phase transition when it is kept in air at 900 or 1000 °C for a long period of time, which is consistent with other reports [21]. However, when it is mixed with potassium silicate glass, as shown in the broken line diagrams in Figures 4, 7, 10 and…”

Advanced Photocatalysts for CO2 Conversion by Severe Plastic Deformation (SPD)

“…The inducing ultra-SPD [ 51 ] together with fast dynamic diffusion [ 52 , 53 ] introduces the HPT method as a unique path to mechanically synthesize new materials even from immiscible systems [ 54 , 55 ]. Due to these unique features of HPT, the method was even used for the process and synthesis of hard and brittle ceramics, but the number of publications on ceramics is quite limited despite the high potential of these materials for various applications [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 ]. Published studies regarding ceramics processed or synthesized by HPT are presented in Table 1 , although there are other classic publications on HPT processing of ceramics mainly by physicists and geolog...…”

“…The formation of high-pressure phases is one of the HPT effects that can occur for some ceramics, such as TiO 2 [ 65 ], ZrO 2 [ 58 ], ZnO [ 26 ], SiO 2 [ 34 ], VO 2 [ 78 ], Y 2 O 3 [ 66 ], BaTiO 3 [ 64 ], Al 2 O 3 [ 27 ], and BN [ 68 ]. It was observed that these high-pressure phases contain defects, such as oxygen vacancies and dislocations, and have nanosized grains, which makes them attractive for photocatalytic applications.…”

“…Therefore, finding new strategies to improve the photocatalytic activity and suppress the recombination rate of electrons and holes is a key issue. In this regard, severe plastic deformation (SPD) through the high-pressure torsion (HPT) method, which is mainly used for nanostructuring of metallic materials, has been used as a new tool to develop active photocatalysts for water splitting [ 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ], dye degradation [ 31 , 32 , 33 , 34 ], and especially CO 2 conversion [ 35 , 36 , 37 , 38 ]. This method not only does not increase the recombination rate of electrons and holes but also effectively suppresses it and improves some other optical properties.…”