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Peculiarities of unusual electret state in porous zeolite microstructure
Superlattices and Microstructures xxx (2017) 1e8
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Peculiarities of unusual electret state in porous zeolite microstructure U. Bunyatova a, V.I. Orbukh c, G.M. Eyvazova d, N.N. Lebedeva c, Z.A. Agamaliev d, I.C. Koçum a, M. Ozer b, B.G. Salamov b, e, * a
Department of Biomedical Engineering, Faculty of Engineering, Baskent University, Baglıca, Ankara 06810, Turkey Physics Department, Faculty of Sciences, Gazi University, Besevler 06500 Ankara, Turkey c Baku State University, Institute for Physical Problems, Baku AZ 1148, Azerbaijan d Baku State University, Faculty of Physics, Nano Centre, Baku AZ 1148, Azerbaijan e Azerbaijan Academy of Science, Institute of Physics, AZ0143 Baku, Azerbaijan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2017 Received in revised form 8 August 2017 Accepted 8 August 2017 Available online xxx
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 (Ep ¼ 2 kV cm1) in an air atmosphere for 240 min. It is established, that the stability of electret state and value of the dielectric response ε0 and ε00 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. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Electret state Porous zeolite Hysteresis loop Nanoparticles Dielectric spectra
1. 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.
* Corresponding author. Physics Department, Faculty of Sciences, Gazi University, Besevler 06500 Ankara, Turkey. E-mail address: [email protected] (B.G. Salamov). http://dx.doi.org/10.1016/j.spmi.2017.08.024 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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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 3e20 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 AlO 4 and SiO4 tetrahedral linked through bridging oxygen atoms. To balance the negative charge created as a result of isomorphous substitution of Si4þ by Al3þ, 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 2.3e12 eV [7e9]. 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 [19e22]. 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 [25e27]. 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. 2. Experimental 2.1. Characterization of zeolite materials The samples of dielectric-clinoptilolite electronic material considered in this study, as mentioned earlier,20 is a skeleton aluminosilicate, the structure of which has following composition: of SiO2 65e72%, Al2O3 10e12%, CaO 2.4e3.7%, K2O 2.5e3.8%, Fe2O3 0.7e1.9%, MgO 0.9e1.2%, Na2O 0.1e0.5%, MnO 0e0.08%, Cr2O3 0e0.01%, P2O5 0.02e0.03%, SiO2/Al2O3 5.4e7.2%. The composition of the used dielectric-clinoptilolite can be described by the following formula: (Ca, K2, Na2, Mg)4Al8Si40O96.24 H2O. The unmodified zeolite cathodes (ZC) was cut for the purpose of obtaining the targeted shape (i.e. diameter D ¼ 22 mm with a thickness of 2 mm). Detailed information regarding location, preparation and silver modification method of the ZC sample is given in our earlier work.21 The chemical composition of the unmodified ZC and silver-modified Ag0-ZC samples (resistivity ~ 1011-106 U cm) were given in Table 1. We measmed three different types of CL and the natural ZC plates were denoted as A. In order to get the silver-modified ZC samples we have used two different approaches. This Ag ion-modified form of Ag(þ)- ZC was denoted as B. The third sample modification was conducted via chemical reduction of Ag ions to (Ag0) nanoparticles into Ag ion-modified ZC plates and was denoted as C. Phase identification of the ZC plates was done by XRD using diffractometer (i.e. Rigaku Mini Flex 600 c using a Cu (Ka) radiation). Morphology and surface distribution of the elemental compositional analyses for ZC paltes were analysed by field emission SEM (JEOL JSM7600F) modulated energy dispersive X-ray spectroscopy (SEM/EDX). Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Table 1 The SEM-EDX determined chemical composition of natural ZC (A) silver ion-exchanged zeolite (Agþ) (B) and silver-modified zeolite Ag0 plate (C). Elements
O Mg Al Si K Ca Fe Cu Zn Ag Na Si/Al
Sample A
Sample B
Sample C
Wt.%
Wt.%
Wt.%
46.33 0.70 7.38 39.10 2.31 9.93 1.15 0.00 1.00 0.00 0.09 5.30
48.21 1.77 6.05 26.23 1.50 1.92 0.98 0.30 0.00 5.94 1.77 4.30
47.06 0.37 5.08 19.15 1.77 0.25 0.91 0.07 0.18 19.33 0.00 3.76
2.2. Measurement method As we mentioned in Refs. [28,29] the conductivity characteristics of the ZC plates were experimentally determined by an impedance spectroscopy. For the experimental measurement, the ZC plates were placed in a chamber between a glass disc with a semi-transparent conductive SnO2 layer and a polished metallic Cu disc. By using the spanner nuts of the Cu disc, the ZC plate is sandwiched in a chamber. The chamber connected to the LRC and the impedance meter E7-22. The resonance phenomenon and dielectric responses of zeolite were examined in the wide frequency range of 200 Hze1 MHz. The measurements were performed at room temperature and the atmospheric pressure an air humidity of 0.85%. Since the crystals of zeolites are dielectrics, electrical conductivity measurements were carried out according to the procedure described in Ref. [32], using a special electrodes (see Fig. 2 in Ref. [22]), which allows separating surface currents and determining only its volume component. The dimensions of the electrodes, as well as the samples themselves, were selected in accordance with the recommendations proposed in Refs. [30e32]. According to Ref. [22], the volumetric electrical conductivity of a parallel plane sample was calculated from the formula:
sv ¼
4l
pd2 Rv
(1)
where l - is the thickness of the sample, Rv - is the bulk resistance of the sample, and d is the value determined as:
d¼
d1 þ d2 2
(2)
where d1 is the diameter of the measuring electrode, d2 is the internal diameter of the special electrode (see Fig.2 in Ref. [22]). 3. Results and discussion 3.1. Characterization of zeolite The measured X-ray powder diffraction characteristics for C Ag0-modified zeolite sample is shown in Fig. 1. The investigated plate is Ca-Clinoptilolite with following unit cell parameters: a ¼ 1.7627 nm, b ¼ 1.7955 nm, c ¼ 0.7399 nm, b ¼ 116 monoclinic and therefore has a monoclinic system. Fig. 1 showed reflections with values of additional peaks in the spectra at angles 2q ¼ 65.49 and 78.74 , corresponding to Miller indices (220) and (331) which is in good agreement with ICDD database. Thus, the modification does not change the position of the maxımums diffraction peaks of the CL, at the same time we observed reduction in the relative intensity of the peaks. Moreover additional peaks observed in the range of 600e900 (2q ¼ 65,680 and 78.580), that is confirmation the presence of Ag in the ZC plate. The average crystal size for the investigated CL plate was 8e10 nm. Studies of the elemental mapping and the spectra of EDS for investigated CL plates demonstrate that presence of the Ag ions and nanoparticles (NPs) in the sample depends on the ion modification condition, whether physical or chemical. EDS spectra results a show that the natural ZC sample A has the greatest concentration value (9.93%) of Ca cations. In the Agmodified B and C samples, the content of Ca depends on the surface distribution. This concent is between 2% and 0.25% (see Table 1). The elemental mapping and the spectra of EDS distribution images of unmodified zeolite plate A and Ag-modified zeolite samples B and C are shown in Fig. 2. An analysis of EDS spectra demonstrated that the Ag ion exchange with the Ca ion in the pores of the ZC (see Fig. 3). Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 1. Diffractogram of Ag0 -modified porous zeolite plates.
Fig. 2. Elemental mapping of the surface of natural ZC (a) silver ion-exchanged zeolite (Agþ) (b) and silver-modified zeolite Ag0 plate (c).
Fig. 3. EDS spectra of silver-modified zeolite Ag0 plate.
3.2. Capacitance and resistance measurements Thus, obtained samples were also exposed to an electric field. For this purpose they were placed in the cassette with the two push electrodes through which dc power supply was applied up 300 V over 4 h. After this procedure we again measured the values of dielectric response across different time intervals (e.g. when an external electric field Ep ¼ 2 kV cm1 is applied through the 1 h; after 1 day; after 3 days and after 15 days). The study showed that the DC electric field has influence only on the dielectric properties of the silver nanoparticle doped zeolite Ag0 samples. When we applied external electric field (E ¼ 2 kV cm1) to the silver-modified zeolite Ag0 samples the dielectric response increased. Moreover, this growth depends on the amount of time after turning off the DC electric field. Fig. 4 shows the frequency dependence of the capacitance (а), resistance (b), real ε’ (c) and imaginary ε” parts (d) of the dielectric permittivity for silver-modified zeolite Ag0 plate after turning off the DC electric field. Curve 1 corresponds to the measurements on the sample that is not subjected to the effects of electric field and the other curves correspond to measurements on the sample after the field is switched off, respectively, 1 h, 1 day, 3 days and 15 days. When we applied external electric field to the silver-modified zeolite Ag0 samples, the dielectric response first increases and then return, to the state close to the initial. Immediately after turning off the electric field, the dielectric response has changed insignificantly (Fig. 4 curves 1 and 2), no more than 20%. In the future it increases substantially (in 2 times, as can be seen from the Fig. 4, curves 2,3), after which time, it returns to the initial values. This process takes place over 15 days (curve 5 Fig. 4). Such long-term dynamics indicate transformations occurring with Ag nanoparticles. The measured results illustrated in Fig. 4 demonstrate the following peculiarities: Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 4. Frequency dependence of the capacitance (а), resistance (b), real ε’ (c) and imaginary ε” parts (d) of the dielectric permittivity for silver-modified zeolite Ag0 without electric field and after turn off the DC electric field.
Due to the high conductivity of silver nanoparticles the electric field from them is displaced into the space between them. Thus from the moment nanoparticle begins injecting electrons, which leave the pore (and transfer into the electrode, leaving the pore as charged), the number of free silver ions in the pore is not yet changed. Therefore immediately after removal of the electric field the dielectric response is almost unchanged. Moreover, silver nanoparticles transfer into an unstable state. This is due the small size of nanoparticles energy, which have Coulomb repulsion of charged ions on their surface which is comparable to the binding energy of silver atoms constituting the nanoparticle. Due to the Coulomb repulsion between ions of silver on the surface, silver is gradually decay and from neutral nanoparticles and free ions of silver.
3.3. Electret state and theoretical model A similar mechanism of oxidation of the silver atoms in an electric field was considered by Refs. [33,34]. For the application of such mechanism to consideration of our experimental results it must be assumed that this decay occurs during a few days. This explains the increase of the dielectric response in subsequent days (see Fig. 5), because the dielectric response is determined by the concentration of the ions in the pore. This concentration gradually increases due to the decay of charged nanoparticles in to neutral nanoparticle and positive ions. However, this electret state when the pore have positive charge but electrode has negative charge the electret state is not in equilibrium. It is therefore in the future the system transfers in equilibrium and returns to its initial state due the injection of electrons from the electrode in the pores and neutralization of the additional ions. As a result, the dielectric response becomes the same as before application of the electric field. Mechanism for the restoration of equilibrium is mutual attraction between the positive charge, accumulated in the pores of the zeolite, and the negative charge on the electrodes. In other words, the electrons from the electrodes are returned to the pore and neutralise its charge, localized at ions of silver. It is important to note that this electron transfer process from nanoparticles to the electrodes can-not occur by itself, without, performing work on the system. In our case, the charge was carried by an external power supply. The above-mentioned decay process of charged nanoparticles can be described by a simple formula with a decay time t:
t N ¼ N0 1 exp
t
(3)
where N0 is the initial number of charged nanoparticles and N is the number of nanoparticles that have decayed after time t. Formula (1) shows that all nanoparticles decayed over long period of time t. It should be noted that the neutralization process of newly formed positive silver ions is determined by the slow drift of ions in a weak electric field created by the negatively charged electrode. The difference between of restore time and dielectric response time it is results of differences in the decay mechanisms of charged nanoparticles and neutralization of the positive ions. We suggests that the change in the I-V loops behavior is due the volume of mesoporous in the majority of samples have been reduced during the modification. This fact may be explained by the partial blockage of the pores by nanoparticles (NPs), Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 5. Time dependence of the dielectric response for silver-modified zeolite Ag0 after turning off the DC electric field.
that are located on surfaces of the polycristall. It should be noted that Ag nanoparticules are located on the surface of polycrystals. It is seen that during the modification most of the nanoparticules which have charged surface aren't assembled or built into the structure of the crystal [35]. Therefore due to the gravity of Van der Waals's forces they are attached and kept on the surface of the zeolite. It is unlikely that any of the modificator particules would be able to penetrate into the zeolite channels due to the nanoparticles size (from several nm up to 200 nm) significantly exceeding the diameter of porous of the zeolite. The existence of a “hump” on I-V curves is characteristic for both type ZC samples during charge transfer process [36] is due differences in the decay mechanisms of charged nanoparticles and neutralization of the positive ions [37]. It should be noted that in the addition to above mentioned reasons of polarization it is possible to inject the charge into samples with the rather high electrical field applied us a result gas discharge gap between electrode and sample. It leads to formation of homocharge, which has the same polarity with the nearest electrode during the polarization. Besides, in the sample it is possible to displace of charge due to influence of internal electrical field of electret [35]. The studying of zeolite electret shows that at electrical flied strength of the order of 105 V/m and above there forms homocharge caused by breakdown of the gas discharge gap between electrode and sample of the gas discharge electronic device (GDED) (see Fig. 1a in Ref. [36]). The value of homocharge depends on applied voltage so it may be linked with increasing of ion quantity in the gas gap. At the small applied voltage the homocharge value increases with the polarization time. In this case the number of ions occurs in gas gap and settling on sample surface increases. The decreasing of pressure in presence of not too high value of potential difference also leads to increasing homocharge [36] due to the increasing of mean free path. The relaxation time is too large during the 2 h of measurement we did not noted any change. However, the polarity of homocharge changed with the change of applied voltage polarity operating only 10 s. This phenomena can be explained by fact that homocharge was fixed on surface of zeolite sample [28]. With the time due to thermal motion the residual polarization decreases, and free homocharge caused ohmic conductivity changes and in the field of zeolite electret occurs slowly established polarization [35e37]. For consideration the effect of pressure on the resistivity of CL, the IeV characteristics of GDED were measured respectively for Ag0-ZC and ZC plates from atmospheric pressure to 44 Torr (Fig. 6b). With the forward increment and reverse decrement feeding voltages, measured dates for natural CL demonstrated the dramatic increase and extended range (DU0 ¼ 430 V) of the hysteresis loop in device at AP under applied voltage from ±1000 V to ±1430 V (see Fig. 6a). In the case of Ag0-ZC the range of hysteresis loop considerable depends on the applied voltages and pressure. When the applied voltages U0 were ±510, ±590 and ± 660 V, the corresponding width of hysteresis loop in its IeV curves are about 1, 35 and 65 V, respectively (see Fig. 6b). Moreover, as we mentioned in Ref. [38], the large hysteresis loop to the localized nature of the surface state by charging and discharging these surface states, one can change the ZC conductivity.
Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 6. a The hysteresis behavior of the GDED cell with silver-modified zeolite Ag0 -ZC and un-modified ZC dependence of the I-V characteristic at atmospheric pressure. The system parameters are gas discharge gap d ¼ 50 mm and diameter of ZC D ¼ 22 mm b The hysteresis behavior of the GDED cell with Ag0 -ZC dependence of the I-V characteristic at different pressures. The system parameters are gas discharge gap d ¼ 50 mm and diameter of ZC D ¼ 22 mm.
4. Conclusion The distinguishing/properties of microporous zeolites [38], including ion-exchange properties [29], adsorption, catalysis, and conductivity have been exploited in order to improve the performance of optoelectronic devices [39]. In this study the introduction of silver metals into the natural zeolite framework by ion exchange allowed us to achieve a dramatic change in the conductivity and lifetime of electret state. All of the observed peculiarities of the electret state shows that zeolite-like mixed conductors show considerable promise as are promising as electrically conducting frameworks for electrical energy storage. But at the same time the zeolite frameworks can be envisioned in energy applications including ion-exchange membrane technologies and catalysis. Using the method for chemical reduction of silver ions to silver atoms, we transferred part of the silver ions into nanoparticles, which were confirmed by a decrease in the dielectric response of the zeolite sample with nanoparticles. It has been established that above mentioned system (silver-modified zeolite) demonstrates the long-term dynamics of the dielectric response after exposure to an electric field. The lifetime of such an electret state in our samples is observed during 10e15 days. In the proposed model the electret state arises as a result of ionization of neutral nanoparticles with the subsequent formation of additional free silver ions due to the silver nanoparticles. This explains the increase in the value of dielectric response immediately after the switching off of the external electric field. In other words, there is observed influence of DC electric field on the nanoparticles; they decay into ions and that leads to an increase in the value of dielectric response. The restoration of nanoparticles to their neutral state, explains the decrease in the dielectric response to the initial value. In summary, there is considerable promise for zeolites to influence the dielectric response through a variety of novel pathways, many of which have been articulated in this experimental article.
Acknowledgments The authors thank the Turkish Scientific and Technological Research Council (TUBITAK) for the financial support of this work through BIDEB-2221. Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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References [1] M. Eguchi, Philos. Mag. 49 (1925) 178e192. [2] G.M. Sessler, J.E. West, Applications, in: G.M. Sessler (Ed.), Electrets, second ed., Springer- Verlag, 1987, pp. 347e453. Berlin/Heidelberg/New York/ London/Paris/Tokyo. [3] H. Amjadi, C.-P. Franz, J. Electrost. 50 (2001) 265e277. [4] A.N. Gubkin, G.I. Skanavi, Solid State Phys. 22 (1958) 330e342 (in Russian). [5] A.N. Gubkin, G.I. Skanavi, Solid State Phys. 3 (1961) 297e304 (in Russian). [6] A. Yin, X. Guo, W.L. Dai, K. Fan, J. Phys. Chem. C 114 (2010) 8523e8532. [7] L.H. Tjeng, M.B.J. Meinders, J.V. Elp, J. Ghijsen, G.A. Sawatzky, Phys. Rev. B 41 (1990) 3190e3199. [8] H.H. Kim, Y. Teramoto, T. Sano, N. Negishi, A. Ogata, Appl. Catal. B Environ. 166 (2015) 9e17. [9] H.H. Kim, Y. Teramoto, N. Negishi, A. Ogata, Catal. Today 256 (2015) 13e22. € [10] M. Alvaro, J.F. Cabeza, D. Fabuel, H. Garcia, E. Guijarro, J.L. Martinez de Juan, Chem. Mater. 18 (2006) 26e33. [11] S. Il Ahn, S.E. Lee, S. Ho Kim, K.H. Cho, K.C. Choi, Appl. Phys. Lett. 93 (2008) 071507. [12] T. Motsi, N.A. Rowson, M.J.H. Simmons, Int. J. Min. Process. 92 (2009) 42e48. [13] D.J. Wales, J. Grand, V.P. Ting, R.D. Burke, K.J. Edler, C.R. Bowen, S. Mintova, A.D. Burrows, Chem. Soc. Rev. 44 (2015) 4167e4474. [14] Z. Bai, M. Fujii, T. Hasegawa, S. Kitano, K. Imakita, M. Mizuhata, S. Hayashi, Opt. Mater. 34 (2012) 821e825. € [15] S. Ozturk, M. Ozer, E. Bulur, B.G. Salamov, IEEE Trans. Plasma Sci. 43 (2015) 1839e1844. [16] K. Kazutshi, J. Takeuchi, J. Zeit, Krystal 145 (4) (1977) 216. [17] S.V. Baryshnikov, S.V. Lankin, E.V. Stukova, V.V. Yurkov, Sovremenniye Naukoemkiye Tehnologii, 2004, pp. 26e27. [18] L.I. Gafarova, D.V. Sheds, I.V. Lunev, Y. Gusev, Strukt. i Din. Mol. Syst. 10 (1) (2003) 354e357. [19] A. Yin, X. Guo, W.L. Dai, K. Fan, J. Phys. Chem. C 114 (2010) 8523e8532. [20] V.I. Orbukh, G.М. Eyvazova, А.H. Мuradov, N.N. Lеbеdeva, B.G. Sаlаmоv, Trans. ANAS Sci. Phys. Astron. 33 (5) (2013) 49e55. [21] A.X. Muradov, V.I. Orbuch, G.M. Eyvazova, N.N. Lebedeva, ChG. Akhundov, A.M. Mamedov, Int. J. Sci. Techn. Res. 1 (9) (2015) 55e59. ISSN 2422e8702. [22] S. Ozturk Koc, V.I. Orbukh, G.M. Eyvazova, N.N. Lebedeva, B.G. Salamov, Superlattices Microstruct. 9 (2016) 269e277. [23] V.I. Orbukh, G.М. Eyvazova, N.N. Lebedeva, М.B. Мuradov, B.G. Salamov, Tpansactions ANAS Sci. Phys. Astron 35 (5) (2015) 51e54. [24] C. Jovalekic, M. Pavlovic, P. Osmokrovic, M. Zdujic, IEEE Trans. Electr. Insulation 2 (3) (1995) 409e412. [25] I. Tiscornia, S. Irusta, P. Pradanos, C. Teller, I. Coronos, J. Santamaria, J. Phys. Chem. C 111 (2007) 4702e4709. [26] Z. Cai, T.L. Coordrich, K.S. Ziemer, J. Warrywocla, A.Jr Sacco, Appl. Catal. B Environ. 102 (2011) 323e333. [27] S. Ozturk, M. Ozer, E. Bulur, B.G. Salamov, IEEE Trans. Plazma Sci. 43 (2015) 1330e1344. [28] V.I. Orbukh, N.N. Lebedeva, S. Ozturk, B.G. Salamov, Superlattice. Microstruct. 54 (2013) 16e25. [29] S. Ozturk Koc, K. Koseoglu, S. Galioglu, B. Akata, B.G. Salamov, Micropor. Mesopor. Mater. 223 (2016) 18e26. [30] Y. Sadiq, M. Ozer, B.G. Salamov, J. Phys. D. Appl. Phys. 41 (2008) 045204. [31] Y.P. Raizer, Gas Discharge Physics, second ed., Springer-Verlag, Berlin, 1997, pp. 128e130. [32] U. Bunyatova, S. Ozturk Koc, V.I. Orbukh, G.M. Eyvazova, Z.A. Agamaliev, N.N. Lebedeva, I.C. Koçum, B.G. Salamov, M. Ozer, Superlattice. Microstruct. 98 (2016) 295e305. [33] O. Deparis, P.G. Kazansky, A. Abdolvand, A. Podlipensky, G. Seifert, H. Graener, Appl. Phys. Lett. 85 (6) (2004) 872e874. [34] A. Podlipensky, A. Abdolvand, G. Seifert, H. Graener, O. Deparis, P.G. Kazansky, Phys. Chem. B 108 (46) (2004), 17699e1770. [35] S.V. Lankin, NTP, Phisica 2 (2011) 90854. [36] J.C. Burfoot, Ferroelectrics. An Introduction to the Physical Principles, Princeton, New Jersey e Toronto, 1967. [37] M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New-York, 1970. [38] S. Oztürk, K. Koseoglu, M. Ozer, B.G. Salamov, Mod. Phys. Lett. B 29 (N31) (2015) 1550195 (16 pages). € [39] K. Koseoglu, M. Ozer, S. Ozturk, B.G. Salamov, Jpn. J.Appl.Phys. 53 (N8) (2014) 086203.
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Peculiarities of unusual electret state in porous zeolite microstructure U. Bunyatova a, V.I. Orbukh c, G.M. Eyvazova d, N.N. Lebedeva c, Z.A. Agamaliev d, I.C. Koçum a, M. Ozer b, B.G. Salamov b, e, * a
Department of Biomedical Engineering, Faculty of Engineering, Baskent University, Baglıca, Ankara 06810, Turkey Physics Department, Faculty of Sciences, Gazi University, Besevler 06500 Ankara, Turkey c Baku State University, Institute for Physical Problems, Baku AZ 1148, Azerbaijan d Baku State University, Faculty of Physics, Nano Centre, Baku AZ 1148, Azerbaijan e Azerbaijan Academy of Science, Institute of Physics, AZ0143 Baku, Azerbaijan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 July 2017 Received in revised form 8 August 2017 Accepted 8 August 2017 Available online xxx
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 (Ep ¼ 2 kV cm1) in an air atmosphere for 240 min. It is established, that the stability of electret state and value of the dielectric response ε0 and ε00 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. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Electret state Porous zeolite Hysteresis loop Nanoparticles Dielectric spectra
1. 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.
* Corresponding author. Physics Department, Faculty of Sciences, Gazi University, Besevler 06500 Ankara, Turkey. E-mail address: [email protected] (B.G. Salamov). http://dx.doi.org/10.1016/j.spmi.2017.08.024 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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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 3e20 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 AlO 4 and SiO4 tetrahedral linked through bridging oxygen atoms. To balance the negative charge created as a result of isomorphous substitution of Si4þ by Al3þ, 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 2.3e12 eV [7e9]. 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 [19e22]. 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 [25e27]. 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. 2. Experimental 2.1. Characterization of zeolite materials The samples of dielectric-clinoptilolite electronic material considered in this study, as mentioned earlier,20 is a skeleton aluminosilicate, the structure of which has following composition: of SiO2 65e72%, Al2O3 10e12%, CaO 2.4e3.7%, K2O 2.5e3.8%, Fe2O3 0.7e1.9%, MgO 0.9e1.2%, Na2O 0.1e0.5%, MnO 0e0.08%, Cr2O3 0e0.01%, P2O5 0.02e0.03%, SiO2/Al2O3 5.4e7.2%. The composition of the used dielectric-clinoptilolite can be described by the following formula: (Ca, K2, Na2, Mg)4Al8Si40O96.24 H2O. The unmodified zeolite cathodes (ZC) was cut for the purpose of obtaining the targeted shape (i.e. diameter D ¼ 22 mm with a thickness of 2 mm). Detailed information regarding location, preparation and silver modification method of the ZC sample is given in our earlier work.21 The chemical composition of the unmodified ZC and silver-modified Ag0-ZC samples (resistivity ~ 1011-106 U cm) were given in Table 1. We measmed three different types of CL and the natural ZC plates were denoted as A. In order to get the silver-modified ZC samples we have used two different approaches. This Ag ion-modified form of Ag(þ)- ZC was denoted as B. The third sample modification was conducted via chemical reduction of Ag ions to (Ag0) nanoparticles into Ag ion-modified ZC plates and was denoted as C. Phase identification of the ZC plates was done by XRD using diffractometer (i.e. Rigaku Mini Flex 600 c using a Cu (Ka) radiation). Morphology and surface distribution of the elemental compositional analyses for ZC paltes were analysed by field emission SEM (JEOL JSM7600F) modulated energy dispersive X-ray spectroscopy (SEM/EDX). Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Table 1 The SEM-EDX determined chemical composition of natural ZC (A) silver ion-exchanged zeolite (Agþ) (B) and silver-modified zeolite Ag0 plate (C). Elements
O Mg Al Si K Ca Fe Cu Zn Ag Na Si/Al
Sample A
Sample B
Sample C
Wt.%
Wt.%
Wt.%
46.33 0.70 7.38 39.10 2.31 9.93 1.15 0.00 1.00 0.00 0.09 5.30
48.21 1.77 6.05 26.23 1.50 1.92 0.98 0.30 0.00 5.94 1.77 4.30
47.06 0.37 5.08 19.15 1.77 0.25 0.91 0.07 0.18 19.33 0.00 3.76
2.2. Measurement method As we mentioned in Refs. [28,29] the conductivity characteristics of the ZC plates were experimentally determined by an impedance spectroscopy. For the experimental measurement, the ZC plates were placed in a chamber between a glass disc with a semi-transparent conductive SnO2 layer and a polished metallic Cu disc. By using the spanner nuts of the Cu disc, the ZC plate is sandwiched in a chamber. The chamber connected to the LRC and the impedance meter E7-22. The resonance phenomenon and dielectric responses of zeolite were examined in the wide frequency range of 200 Hze1 MHz. The measurements were performed at room temperature and the atmospheric pressure an air humidity of 0.85%. Since the crystals of zeolites are dielectrics, electrical conductivity measurements were carried out according to the procedure described in Ref. [32], using a special electrodes (see Fig. 2 in Ref. [22]), which allows separating surface currents and determining only its volume component. The dimensions of the electrodes, as well as the samples themselves, were selected in accordance with the recommendations proposed in Refs. [30e32]. According to Ref. [22], the volumetric electrical conductivity of a parallel plane sample was calculated from the formula:
sv ¼
4l
pd2 Rv
(1)
where l - is the thickness of the sample, Rv - is the bulk resistance of the sample, and d is the value determined as:
d¼
d1 þ d2 2
(2)
where d1 is the diameter of the measuring electrode, d2 is the internal diameter of the special electrode (see Fig.2 in Ref. [22]). 3. Results and discussion 3.1. Characterization of zeolite The measured X-ray powder diffraction characteristics for C Ag0-modified zeolite sample is shown in Fig. 1. The investigated plate is Ca-Clinoptilolite with following unit cell parameters: a ¼ 1.7627 nm, b ¼ 1.7955 nm, c ¼ 0.7399 nm, b ¼ 116 monoclinic and therefore has a monoclinic system. Fig. 1 showed reflections with values of additional peaks in the spectra at angles 2q ¼ 65.49 and 78.74 , corresponding to Miller indices (220) and (331) which is in good agreement with ICDD database. Thus, the modification does not change the position of the maxımums diffraction peaks of the CL, at the same time we observed reduction in the relative intensity of the peaks. Moreover additional peaks observed in the range of 600e900 (2q ¼ 65,680 and 78.580), that is confirmation the presence of Ag in the ZC plate. The average crystal size for the investigated CL plate was 8e10 nm. Studies of the elemental mapping and the spectra of EDS for investigated CL plates demonstrate that presence of the Ag ions and nanoparticles (NPs) in the sample depends on the ion modification condition, whether physical or chemical. EDS spectra results a show that the natural ZC sample A has the greatest concentration value (9.93%) of Ca cations. In the Agmodified B and C samples, the content of Ca depends on the surface distribution. This concent is between 2% and 0.25% (see Table 1). The elemental mapping and the spectra of EDS distribution images of unmodified zeolite plate A and Ag-modified zeolite samples B and C are shown in Fig. 2. An analysis of EDS spectra demonstrated that the Ag ion exchange with the Ca ion in the pores of the ZC (see Fig. 3). Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 1. Diffractogram of Ag0 -modified porous zeolite plates.
Fig. 2. Elemental mapping of the surface of natural ZC (a) silver ion-exchanged zeolite (Agþ) (b) and silver-modified zeolite Ag0 plate (c).
Fig. 3. EDS spectra of silver-modified zeolite Ag0 plate.
3.2. Capacitance and resistance measurements Thus, obtained samples were also exposed to an electric field. For this purpose they were placed in the cassette with the two push electrodes through which dc power supply was applied up 300 V over 4 h. After this procedure we again measured the values of dielectric response across different time intervals (e.g. when an external electric field Ep ¼ 2 kV cm1 is applied through the 1 h; after 1 day; after 3 days and after 15 days). The study showed that the DC electric field has influence only on the dielectric properties of the silver nanoparticle doped zeolite Ag0 samples. When we applied external electric field (E ¼ 2 kV cm1) to the silver-modified zeolite Ag0 samples the dielectric response increased. Moreover, this growth depends on the amount of time after turning off the DC electric field. Fig. 4 shows the frequency dependence of the capacitance (а), resistance (b), real ε’ (c) and imaginary ε” parts (d) of the dielectric permittivity for silver-modified zeolite Ag0 plate after turning off the DC electric field. Curve 1 corresponds to the measurements on the sample that is not subjected to the effects of electric field and the other curves correspond to measurements on the sample after the field is switched off, respectively, 1 h, 1 day, 3 days and 15 days. When we applied external electric field to the silver-modified zeolite Ag0 samples, the dielectric response first increases and then return, to the state close to the initial. Immediately after turning off the electric field, the dielectric response has changed insignificantly (Fig. 4 curves 1 and 2), no more than 20%. In the future it increases substantially (in 2 times, as can be seen from the Fig. 4, curves 2,3), after which time, it returns to the initial values. This process takes place over 15 days (curve 5 Fig. 4). Such long-term dynamics indicate transformations occurring with Ag nanoparticles. The measured results illustrated in Fig. 4 demonstrate the following peculiarities: Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 4. Frequency dependence of the capacitance (а), resistance (b), real ε’ (c) and imaginary ε” parts (d) of the dielectric permittivity for silver-modified zeolite Ag0 without electric field and after turn off the DC electric field.
Due to the high conductivity of silver nanoparticles the electric field from them is displaced into the space between them. Thus from the moment nanoparticle begins injecting electrons, which leave the pore (and transfer into the electrode, leaving the pore as charged), the number of free silver ions in the pore is not yet changed. Therefore immediately after removal of the electric field the dielectric response is almost unchanged. Moreover, silver nanoparticles transfer into an unstable state. This is due the small size of nanoparticles energy, which have Coulomb repulsion of charged ions on their surface which is comparable to the binding energy of silver atoms constituting the nanoparticle. Due to the Coulomb repulsion between ions of silver on the surface, silver is gradually decay and from neutral nanoparticles and free ions of silver.
3.3. Electret state and theoretical model A similar mechanism of oxidation of the silver atoms in an electric field was considered by Refs. [33,34]. For the application of such mechanism to consideration of our experimental results it must be assumed that this decay occurs during a few days. This explains the increase of the dielectric response in subsequent days (see Fig. 5), because the dielectric response is determined by the concentration of the ions in the pore. This concentration gradually increases due to the decay of charged nanoparticles in to neutral nanoparticle and positive ions. However, this electret state when the pore have positive charge but electrode has negative charge the electret state is not in equilibrium. It is therefore in the future the system transfers in equilibrium and returns to its initial state due the injection of electrons from the electrode in the pores and neutralization of the additional ions. As a result, the dielectric response becomes the same as before application of the electric field. Mechanism for the restoration of equilibrium is mutual attraction between the positive charge, accumulated in the pores of the zeolite, and the negative charge on the electrodes. In other words, the electrons from the electrodes are returned to the pore and neutralise its charge, localized at ions of silver. It is important to note that this electron transfer process from nanoparticles to the electrodes can-not occur by itself, without, performing work on the system. In our case, the charge was carried by an external power supply. The above-mentioned decay process of charged nanoparticles can be described by a simple formula with a decay time t:
t N ¼ N0 1 exp
t
(3)
where N0 is the initial number of charged nanoparticles and N is the number of nanoparticles that have decayed after time t. Formula (1) shows that all nanoparticles decayed over long period of time t. It should be noted that the neutralization process of newly formed positive silver ions is determined by the slow drift of ions in a weak electric field created by the negatively charged electrode. The difference between of restore time and dielectric response time it is results of differences in the decay mechanisms of charged nanoparticles and neutralization of the positive ions. We suggests that the change in the I-V loops behavior is due the volume of mesoporous in the majority of samples have been reduced during the modification. This fact may be explained by the partial blockage of the pores by nanoparticles (NPs), Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Fig. 5. Time dependence of the dielectric response for silver-modified zeolite Ag0 after turning off the DC electric field.
that are located on surfaces of the polycristall. It should be noted that Ag nanoparticules are located on the surface of polycrystals. It is seen that during the modification most of the nanoparticules which have charged surface aren't assembled or built into the structure of the crystal [35]. Therefore due to the gravity of Van der Waals's forces they are attached and kept on the surface of the zeolite. It is unlikely that any of the modificator particules would be able to penetrate into the zeolite channels due to the nanoparticles size (from several nm up to 200 nm) significantly exceeding the diameter of porous of the zeolite. The existence of a “hump” on I-V curves is characteristic for both type ZC samples during charge transfer process [36] is due differences in the decay mechanisms of charged nanoparticles and neutralization of the positive ions [37]. It should be noted that in the addition to above mentioned reasons of polarization it is possible to inject the charge into samples with the rather high electrical field applied us a result gas discharge gap between electrode and sample. It leads to formation of homocharge, which has the same polarity with the nearest electrode during the polarization. Besides, in the sample it is possible to displace of charge due to influence of internal electrical field of electret [35]. The studying of zeolite electret shows that at electrical flied strength of the order of 105 V/m and above there forms homocharge caused by breakdown of the gas discharge gap between electrode and sample of the gas discharge electronic device (GDED) (see Fig. 1a in Ref. [36]). The value of homocharge depends on applied voltage so it may be linked with increasing of ion quantity in the gas gap. At the small applied voltage the homocharge value increases with the polarization time. In this case the number of ions occurs in gas gap and settling on sample surface increases. The decreasing of pressure in presence of not too high value of potential difference also leads to increasing homocharge [36] due to the increasing of mean free path. The relaxation time is too large during the 2 h of measurement we did not noted any change. However, the polarity of homocharge changed with the change of applied voltage polarity operating only 10 s. This phenomena can be explained by fact that homocharge was fixed on surface of zeolite sample [28]. With the time due to thermal motion the residual polarization decreases, and free homocharge caused ohmic conductivity changes and in the field of zeolite electret occurs slowly established polarization [35e37]. For consideration the effect of pressure on the resistivity of CL, the IeV characteristics of GDED were measured respectively for Ag0-ZC and ZC plates from atmospheric pressure to 44 Torr (Fig. 6b). With the forward increment and reverse decrement feeding voltages, measured dates for natural CL demonstrated the dramatic increase and extended range (DU0 ¼ 430 V) of the hysteresis loop in device at AP under applied voltage from ±1000 V to ±1430 V (see Fig. 6a). In the case of Ag0-ZC the range of hysteresis loop considerable depends on the applied voltages and pressure. When the applied voltages U0 were ±510, ±590 and ± 660 V, the corresponding width of hysteresis loop in its IeV curves are about 1, 35 and 65 V, respectively (see Fig. 6b). Moreover, as we mentioned in Ref. [38], the large hysteresis loop to the localized nature of the surface state by charging and discharging these surface states, one can change the ZC conductivity.
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Fig. 6. a The hysteresis behavior of the GDED cell with silver-modified zeolite Ag0 -ZC and un-modified ZC dependence of the I-V characteristic at atmospheric pressure. The system parameters are gas discharge gap d ¼ 50 mm and diameter of ZC D ¼ 22 mm b The hysteresis behavior of the GDED cell with Ag0 -ZC dependence of the I-V characteristic at different pressures. The system parameters are gas discharge gap d ¼ 50 mm and diameter of ZC D ¼ 22 mm.
4. Conclusion The distinguishing/properties of microporous zeolites [38], including ion-exchange properties [29], adsorption, catalysis, and conductivity have been exploited in order to improve the performance of optoelectronic devices [39]. In this study the introduction of silver metals into the natural zeolite framework by ion exchange allowed us to achieve a dramatic change in the conductivity and lifetime of electret state. All of the observed peculiarities of the electret state shows that zeolite-like mixed conductors show considerable promise as are promising as electrically conducting frameworks for electrical energy storage. But at the same time the zeolite frameworks can be envisioned in energy applications including ion-exchange membrane technologies and catalysis. Using the method for chemical reduction of silver ions to silver atoms, we transferred part of the silver ions into nanoparticles, which were confirmed by a decrease in the dielectric response of the zeolite sample with nanoparticles. It has been established that above mentioned system (silver-modified zeolite) demonstrates the long-term dynamics of the dielectric response after exposure to an electric field. The lifetime of such an electret state in our samples is observed during 10e15 days. In the proposed model the electret state arises as a result of ionization of neutral nanoparticles with the subsequent formation of additional free silver ions due to the silver nanoparticles. This explains the increase in the value of dielectric response immediately after the switching off of the external electric field. In other words, there is observed influence of DC electric field on the nanoparticles; they decay into ions and that leads to an increase in the value of dielectric response. The restoration of nanoparticles to their neutral state, explains the decrease in the dielectric response to the initial value. In summary, there is considerable promise for zeolites to influence the dielectric response through a variety of novel pathways, many of which have been articulated in this experimental article.
Acknowledgments The authors thank the Turkish Scientific and Technological Research Council (TUBITAK) for the financial support of this work through BIDEB-2221. Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024
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Please cite this article in press as: U. Bunyatova et al., Peculiarities of unusual electret state in porous zeolite microstructure, Superlattices and Microstructures (2017), http://dx.doi.org/10.1016/j.spmi.2017.08.024