Surface electrification of rocks and charge trapping centers

Abstract

In order to elucidate mechanisms of seismo-electromagnetic phenomena such as earthquake luminosity, earth potential changes, electromagnetic radiation and ionosphere disturbances, numerous fracture or frictional slip tests using rock samples have been conducted. Fracturing or frictional slipping generates electrification on the affected surfaces. The surface structures and gouges thus generated are generally disordered containing many charge trapping centers, which are important to understand surface electrification. To investigate the state changes of charge trapping centers in surface disordered layers, we measured thermoluminescence (TL) from milled quartz grains with and without surface disordered layers. The TL intensity per unit mass of the quartz decreased with decreasing grain diameter. Moreover, the TL intensity of samples with surface disordered layers decreased with grain diameter more rapidly than of those without such layers. This diameter-dependence can be explained by disturbances of TLs on the grain surfaces and by deformations of Al-hole centers in the surface disordered layers. Al-hole centers release holes as well as the newly formed E^′ centers in surface disordered layers release electrons. Charges released from charge trapping centers are disturbed in the surface disordered layers during milling. Like milling, fracturing or frictional slipping could generate charge. These charges express themselves on the fractured or frictionally slipped surfaces as surface electrification. On the geological scale, fault zones are characterized by fracturing and frictional slipping. Therefore, the releases of charges from charge trapping centers in surface disordered layers within faults might provide a mechanism to explain seismo-electromagnetic phenomena.

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).