Peculiarities of unusual electret state in porous zeolite microstructure

Highlights

• Peculiarities of unusual electret state (ES) in nanoporous zeolite microstructure is studied.

• The I-V characteristic with hysteresis loop in the wide pressure range is described.

• It was found the influence of electric field on the stability of ES and dielectric response value.

• The dielectric response has long-term dynamics, which indicates on the electret state.

Abstract

This study explores, for the first time to our knowledge, the influence of electret state on the dielectric parameters of porous zeolite of frequencies up to 10⁶ Hz, at room temperature and normal atmospheric pressure. The I-V characteristics with unusual hysteresis loop in the wide pressure range were measured on zeolite plates having three different material characteristics: A) plate, cut out from a monoblock natural zeolite; B) plate, modified by silver ions, and C) plate containing silver nanoparticles. Zeolite samples were exposed to an electric field (E_p = 2 kV cm⁻¹) in an air atmosphere for 240 min. It is established, that the stability of electret state and value of the dielectric response ε′ and ε″ for clinoptilolite Ag nanoparticles containing plates changed under influence of DC electric field. During some time the dielectric response is restored and observed unique phenomenon indicates on the electret behavior of clinoptilolite samples. These changes are observed within a few days consequently, there is long-term dynamics of the dielectric response changes. This interpretation is based on the assumption that due influence of the dc electric field part of the silver atoms in nanoparticles decays into ions and electrons.

Introduction

The interest to electrically charge storage in solid dielectric materials has existed for a long time ago [1]. The electrets are the substances having effect of memory in relation to the electric field. Such dielectrics keep the polarized state for a long time after removal of electric field. If the electric field of residual polarization in the volume of the dielectric is directed along the forming field, then such an electret is created by homo-charges (charges injected into the dielectric from the electrodes). At the same time, if electric field of the residual polarization has the opposite direction to the forming field, in this case electret occurs as a result hetero-charges (the polarizing charges of this dielectric substance). The electret has thermodynamically a no equilibrium state. Therefore in both cases storage time of an electret is defined by time of emission of carriers from traps, or other mechanism of transition to balance. This phenomenon was initially of interest only to the domain of solid state physics. However, these materials became significantly important for optoelectronic applications and became popularly known as electrets. The specific examples of an application of these unique materials are electret transducers (e.g. microphones and loudspeakers), generators, air filters [2], electret floppy disc [3] and dosimeters. Polymeric and nonorganic electrets [4] are widely used as they exhibit efficient mechanical properties, effective operate in aggressive atmospheres [5] and at different temperatures.

Zeolites represent themselves as nanoporous host materials for NPs with their well-defined and three dimensional characteristic micropores with a range of 3–20 Å, cavities and channels with respect to their framework structures. The size of their micropores gives them size-selective screening properties enabling the selective extraction of the some gas molecules from mixtures. This feature leads them to be referred as molecular sieves. Zeolite framework comprises a three-dimensional network of AlO₄⁻ and SiO₄ tetrahedral linked through bridging oxygen atoms. To balance the negative charge created as a result of isomorphous substitution of Si⁴⁺ by Al³⁺, the cations are needed in the zeolite framework, which are also required to stabilize the zeolite framework. The mobility of zeolite cations is primarily responsible for its unique features: adsorption capacity, hydrophobicity, ion exchange capacity, and catalytic activity [6]. The band gap of zeolites is also known to be in the range of 2.3–12 eV [7], [8], [9]. The above mentioned properties of zeolites make them good candidates for their use in the possible applications like microelectronics [10], energy storage devices [11], absorbance [12], catalysts [13], optical materials [14], lower energy consumption to obtain a plasma light source [15]. Influence of the external electric field (E) on the cations movement causes the electric charge transport and dielectric relaxation. Zeolite is dielectric material because it has the possibility to store energy during application of the E.

There are many approaches of the studies of dielectric spectra (DS) of zeolites under different conditions. In Ref. [16] authors studied the influence of the type of primary ion (i.e. the ion controlling the ion migration polarization phenomen) on the DSs of the zeolite. In the above article authors discussed the temperature dependence of the DS at various frequencies. It is reported that the measured results are completely match the results of the model of zeolite - air pores system. The effect of humidity and zeolite water on the DS of Ca modified-clinoptilolite is investigated in Ref. [17]. The authors measured the dependence of the DS on the relaxation of water with different concentrations of the clinoptilolite-water system. It is found that water, bounded in the pores of zeolite and the water in the freely surrounding the zeolite have a different impact the DSs. In Ref. [18] it was found, that modification of the porous clinoptilolite by some type ions increases its DS. Thus, ion modification of zeolite - reduces of the dielectric response with respect to the un-modified sample [19], [20], [21], [22].

In Ref. [23] a model of an inhomogeneous structure is proposed which explains the anomalously a large (up to thousands) values of the dielectric permittivity's at low frequencies in the silver-modified zeolite porous microstructure. At low frequencies the DS is determined by a narrow dielectric gap, large capacity which is a consequence of its small thickness, and efficient redistribution of the E from the conductive clinoptilolite (CL) in the bulk to a narrow dielectric gap. Furthermore, dielectric permittivity's and stability of electret state, depend on the structure, phase composition and polarization condition (i.e. electrical field strength, atmosphere, temperature, etc.) [24].

Thus, in this work measurements of above mentioned parameters on natural and Ag-modified zeolite plates are performed. The silver-modified nanozeolites are used in optoelectronics, medicine and biotechnology [25], [26], [27]. The object of our study involved the 3 different CL samples. The knowledge of a frequency dependence of capacitance and resistance (from which possible calculate the parameters of the dielectric permittivity) is enough to deduce the capable information about the element of circuit. Thus, the primary concern of this research is to examine the influence of the DC electric field and time storage on the dielectric response and stability of electret state (i.e. dependence on time) of these zeolite samples. At the same time, the results of studying the changes of dynamic I-V characteristics, current-voltage loops connected with electret state at different pressure (i.e. humidity) for the nanoporous zeolite samples are presented.