Surface electrification of rocks and charge trapping centers
Introduction
It is well known that fracturing and frictional slipping of materials such as metals, polymers and minerals cause electrification, the emission of electrons, ions and neutral species, visible light (luminescence) and/or electromagnetic radiation over a wide wave length region (e.g. Deryagin et al., 1978; Dickinson, 1990 and references therein). Many laboratory studies of these phenomena, using rock samples, have been conducted to investigate the mechanisms of various seismo-electromagnetic phenomena such as earthquake luminosity, earth potential changes, electromagnetic radiation and ionosphere disturbances (e.g. Nitsan, 1977; Ogawa et al., 1985; Brady and Rowell, 1986; Enomoto and Hashimoto, 1990; Hadjicontis and Mavromatou, 1994, Hadjicontis and Mavromatou, 1995; Freund, 2000, Freund, 2002; Takeuchi and Nagahama, 2001, Takeuchi and Nagahama, 2002a, Takeuchi and Nagahama, 2002b). For centuries seismo-electromagnetic phenomena have been observed before, during and after earthquakes (e.g. Park et al., 1993; Hayakawa, 1999; Hayakawa and Molchanov, 2002 and references therein).
As possible sources of seismo-electromagnetic phenomena, piezoelectricity (e.g. Finkelstein et al., 1973; Nitsan, 1977) and streaming potentials (e.g. Mizutani et al., 1976) have been considered. However, piezoelectricity and streaming potentials require, respectively the presence of piezoelectric minerals such as quartz and water flowing through porous rocks. That is, piezoelectricity and streaming potentials could lead to seismo-electromagnetic phenomena only under limited conditions. On the other hand, all minerals, even of high chemical purity and carefully cleaned, generally contain charge trapping centers (e.g. Kittel, 1953; Varotsos and Alexopoulos, 1986). For example, in quartz crystals Al3+ substituting for Si4+ acts as a hole trapping center, while Ge4+ and Ti4+ at Si4+ sites act as electron trapping centers. The E′ center is a dangling bond at an Si4+ site, i.e. a lone electron at a three-coordinated Si4+. Takeuchi and Nagahama, 2002a, Takeuchi and Nagahama, 2002b found that the surface charge density on fractured or frictionally slipped quartz or granite is of the order of 10−4–10−2 C/m2 which is consistent with the surface charge density estimated from the density of unpaired electron on fractured quartz and in oxide films of silicon wafers. Based on this consistence, the authors pointed out that charge trapping centers should be sources for surface electrification induced by fracturing or frictional slipping of rocks.
To explain the generation of electric currents induced by impacts on rocks, Freund, 2000, Freund, 2002 considered positive hole pairs (PHPs): O3X/OO/XO3, where X=Si4+, Al3+, Fe3+ and where the oxygens in the peroxy link (–O–O–) has converted to the 2− to the 1− oxidation state. The PHP defects can dissociate: O3X/O• •O/XO3, where • represent holes associated with the O− state, also known as “positive holes”. After dissociation, one hole remains at the defect site, while the other can propagate as an electronic charge carrier through the O 2p-dominated valence band. This process does not involve O diffusion but only the movement of the electronic charge associated with the O− state. When positive holes reach the surface of rocks, they become be trapped and form a thin charge layer. The attendant electric field can be high enough to cause dielectric breakdown of the surrounding air (Freund, 2000, Freund, 2002). Because PHPs are believed to be ubiquitous in common rock-forming minerals, they may explain the electrification induced by fracturing or frictional slipping of rocks. However, minerals contain not only PHPs but also other kinds of charge trapping and charge releasing centers. Therefore, to elucidate the mechanisms of electrification, we investigate here such charge trapping and charge releasing centers in quartz before and after fracturing or frictional slipping.
Early studies of the surfaces of ground or milled quartz grains found that the near-surface layers are generally amorphous and/or highly disturbed (e.g. Gibb et al., 1953; Rieck and Koopmans, 1964; Burton, 1966). This layer is often called the surface disordered layer and its layer is of the order of 101–102 nm (e.g. Moody and Hundley-Goff, 1980; Yund et al., 1990). Arends et al. (1963) measured electron spin resonance (ESR) signals from freshly crushed powders of high purity quartz. They found that the ESR signal from E′ centers per unit mass increased in intensity with decreasing grain size, strongly suggesting most of E′ centers are formed on or near the surface of the quartz grains. In other words, the surface disordered layers contain most of E′ centers in quartz grains.
Quartz generally acquires not only E′ centers and PHPs, but also Al-hole centers (e.g. Weil, 1975). Al-hole centers are thought to be one of the sources for thermoluminescence (TL) which is the thermally stimulated emission of light from insulator or semiconductor (e.g. McKeever, 1985; Chen and McKeever, 1997 and references therein). TL measurements can be used to investigate the states and properties of charge trapping centers in materials. A schematic diagram of the TL generation mechanism is shown in Fig. 1. When electrons trapped in the forbidden band are thermally excited to the conduction band, they behave as quasi-free electrons. Some of these electrons return to lower trapping states, while others recombine with trapped holes in the forbidden band. Any retrapping or recombination releases energy as radiation corresponding to the energy difference of transition states involved. When the energy difference is small, the transition leads to the emission of phonons that is of heat. When the energy of the emitted radiation is in or near the region of visible light, TL is observed. For example, TL from quartz above room temperature is usually blue and/or red, called blue-TL and red-TL, respectively (e.g. Hashimoto et al., 1986; Krbetschek et al., 1997). The TL intensity is related to the density (number) of charge trapping centers.
Early measurements of TL, especially blue-TL, from quartz or silica above room temperature suggested Ge and/or Ti as electron sources and Al-hole centers as a hole source although these issues are still being discussed (e.g. Arnold, 1960, Arnold, 1973; Schlesinger, 1964; Durrani et al., 1977; Marfunin, 1979; McKeever, 1985; Martini et al., 1995; Hashimoto et al., 2000). Yamaguchi et al. (2003) recognized that the TL color of quartz shifted from blue to red after a 1100 °C for 100 h annealing treatment, and pointed out that deformed Al-hole centers were the hole sources of red-TL. In spite of intensive investigations (e.g. Hashimoto et al., 1986, Hashimoto et al., 1996, Hashimoto et al., 2001; Fattahi and Stokes, 2000), the details of the red-TL sources are not yet fully understood.
While there are extensive studies dealing with the TL of quartz crystals, little attention has been paid to surface disordered layers. If (deformed) Al-hole centers are related to surface electrification induced by fracturing or frictional slipping of quartz, there should be differences between the TL signature of quartz crystals and of their surface disordered layers. In the present investigation, TL from quartz grains with and without surface disordered layers are measured (Section 2) to study Al-hole centers in surface disordered layers due to milling (Section 3) and to discuss generation mechanisms of surface electrification induced by fracturing or frictional slipping (Section 4).
Section snippets
Methods
Natural pegmatite quartz from Marumori, Miyagi prefecture, Japan was used in the present TL measurements. This quartz emits mainly red-TL during heating to 50–450 °C. It was crushed to fragments with diameter less than 1 mm by a hammer. Those fragments were treated with 36% HCl for 30 min and then with 6N NaOH for 30 min in an ultrasonic syringe to remove impurities from the fragment surfaces. Half of treated fragments were annealed at 450 °C for 30 min. Another half were dried in an oven at 50
Results
Typical growth curves are shown in Fig. 4. The unannealed samples (QXE and QXX) exhibit the red-TL with a pronounced peak at approximately 400 °C, while the annealed samples (QAE and QAX) give very weak peaks around 400 °C. For QXE and QXX the TL intensities decreased with decreasing the grain diameter d as shown in Fig. 5 for the red-TL intensity IRTL where IRTL is the integral counts of red-TL from 370 to 430 °C. At d=10–150 μm at least, log–log plots of both samples are like to be linear
Discussion
Milling treatment produces quartz grains with surface disordered layers and generates numerous E′ centers in these layers (e.g. Gibb et al., 1953; Arends et al., 1963; Rieck and Koopmans, 1964; Burton, 1966; Fukuchi, 1993, Fukuchi, 1996). However, Al-hole centers in the surface disordered layer can also be eliminated during formations of surface disordered layers. Al–O bonds in this layer can be expanded or contracted due to disorder of around lattice structure. The energy states of these
Conclusions
We measured TL from quartz grains with and without surface disordered layers formed during the milling procedure. The results indicate apparent changes in the state of Al-hole centers in the surface disordered layers. Broken Al-hole centers release holes, while newly formed E′ centers release electrons. These holes and electrons are disturbed in surface disordered layers during the milling procedure. Some charges form surface electrification on grains and others generate TL due to
Acknowledgements
We would like to thank Prof. Otsuki and Dr Nakamura of Tohoku University for their valuable comments for our manuscript. We express our deep graduate to Prof. Freund of NASA Ames Research Center and Prof. Enomoto of Shinshu University for their helpful comments and discussions. One of the authors (A. Takeuchi) is supported by Research Fellowships of the Japan Society for the Promotion of Science (JSPS) for Young Scientists.
References (64)
Defects in natural and synthetic quartz
J. Phys. Chem. Solids
(1960)- et al.
Exoelectron emission: possible relation to seismic geo-electromagnetic activities as a microscopic aspect in geotribology
Wear
(1993) - et al.
Extending the time range of luminescence dating using red TL (RTL) from volcanic quartz
Radiat. Meas.
(2000) Charge generation and propagation in igneous rocks
J. Geodyn.
(2002)Vacancy-associated type ESR centers observed in natural silica and their application to geology
Appl. Radiat. Isotopes
(1993)- et al.
Thermoluminescence and thermally stimulated currents in quartz
J. Lumin.
(1970) - et al.
Red and blue colouration of thermoluminescence from natural quartz sands
Nucl. Tracks Radiat. Meas.
(1986) - et al.
Changes in luminescence colour images from quartz slices with thermal annealing treatments
Radiat. Meas.
(1996) - et al.
Effects of atomic hydrogen and annealing temperatures on some radiation-induced phenomena in differently originated quartz
Radiat. Meas.
(2001) - et al.
Spectral information from minerals relevant for luminescence dating
Radiat. Meas.
(1997)
On the generation mechanism of ULF seismogenic electromagnetic emissions
Phys. Earth Planet. Inter.
Microscopic characteristics of orthoquartzite from sliding friction experiments. II Gouge
Tectonophys.
Thermoluminescence in aluminium-containing quartz
Phys. Lett.
Interpretation of charging on fracture or frictional slip surface of rocks
Phys. Earth Planet. Inter.
Color centers in quartz produced by crushing
Phys. Stat. Sol.
Ion-implantation effects in noncrystalline SiO2
IEEE Trans. Nucl. Sci.
Chemistry in noninteger dimensions between two and three. II. Fractal surfaces of adsorbents
J. Chem. Phys.
Laboratory investigation of the electrodynamics of rock fracture
Nature
Changes in the state of solids due to milling processes
Trans. Inst. Chem. Eng.
Theory of Thermoluminescence and Related Phenomena
Adhesion of Solids
Fracto-emission
Thermally stimulated currents in rocks II
Tectonophys.
Thermally stimulated currents in rocks
J. Geophys.
Porosity dependence and mechanism of brittle fracture in sandstones
J. Geophys. Res.
The dependence of the thermoluminescence sensitivity upon the temperature of irradiation in quartz
J. Phys. D
Emission of charged particles from indentation fracture of rocks
Nature
The piezoelectric theory of earthquake lightning
J. Geophys. Res.
Time-resolved study of charge generation and propagation in igneous rocks
J. Geophys. Res. B
Highly mobile oxygen hole-type charge carriers in fused silica
J. Mater. Res.
Critical review of electrical conductivity measurements and charge distribution analysis of magnesium oxide
J. Geophys. Res. B
A mechanism of the formation of E′ and peroxy centers in natural deformed quartz
Appl. Radiat. Isotopes
Cited by (27)
A review of volcanic electrification of the atmosphere and volcanic lightning
2022, Journal of Volcanology and Geothermal ResearchSpatial-temporal infrared radiation precursors of coal failure under uniaxial compressive loading
2018, Infrared Physics and TechnologyCitation Excerpt :There have been countless reports of observations of infrared radiation (IR) related to rock failures in mines and earthquakes [3]. Hypotheses and models, such as piezoelectric potentials of quartz [4], streaming potentials by moving ground water [5], emanation of special gases [6], tribological electromagnetics [7], moving dislocations [8], surface oscillating dipoles [9,10] and P-hole effects [11] have been proposed successively in the scientific community in an attempt to explain the IR phenomena and characteristics in the process of rock fractures and failures. Many scholars have conducted IR observations of rocks (concrete) under different loading conditions, finding that the IR precursors of fractures and failures are related to rock properties and the responses to stress.
Ionospheric disturbances associated with the 2015 M7.8 Nepal earthquake
2017, Geodesy and GeodynamicsCitation Excerpt :How is this electric field generated in seismogenic zone? Some opinions have been proposed for the generation of this electric field, such as the electric field effect of geochemical process [17,58], the piezoelectric potential effect of rock [24–27], the flow potential effects of groundwater [59], and the friction electromagnetic effects [28]. Among these opinions, the first two gained more attention.
Background thermal noise correction methodology for average infrared radiation temperature of coal under uniaxial loading
2017, Infrared Physics and TechnologyIdentification of intrinsic electron trapping sites in bulk amorphous silica from ab initio calculations
2013, Microelectronic EngineeringCitation Excerpt :However, little is still known regarding the possibility of intrinsic electron trapping in the a-SiO2 network. There have been suggestions that electrons can be trapped in the bulk and at surfaces of silica [15] but new models of electron trapping centres started to appear only recently. It has been suggested by Bersuker et al., who used molecular models, that electrons can be trapped by Si–O bonds in a-SiO2 leading to their weakening and thus facilitating Si–O bond dissociation [16].