Electrical conductivity and dielectric behaviour of modified sepiolite clay

Applied Clay Science 25 (2004) 17 – 22 www.elsevier.com/locate/clay

Electrical conductivity and dielectric behaviour of modified sepiolite clay Kadir Esmer * Department of Physics, Faculty of Science and Arts, Kocaeli University, 41300, Kocaeli, Turkey Received 22 July 2002; accepted 24 June 2003

Abstract Electrical and dielectric properties of modified sepiolite mineral taken from the Anatolia region have been investigated at room temperature. Sepiolite mineral dispersed by ultrasonic methods and saturated with Fe2 + by ion exchange has been oriented in a 10-K Gauss magnetic field. It has been observed that sepiolite shows a paramagnetic property in the EPR spectrum. Then, the current – voltage changes have been investigated in parallel and perpendicular directions to the magnetic field. It has been observed that while the conductivity in magnetic field direction being increases, it decreases slightly in the perpendicular direction. The capacitive measurements have been done between 10 kHz and 10 MHz, and the dielectric permittivities and the loss tangent and dielectric conductivity have been calculated. The dielectric conductivity of Fe-sepiolite is found to increase having a permanent orientation polarization due to the magnetic field. In the natural sepiolite, relaxation of O ! H dipole moment polarization depending on insufficient amounts of Fe2 + and Fe3 + has increased with the effect of the electric field. Thus, overall polarization at high frequencies has been observed a very rapid decrease. The changes for both samples have been observed at 100 kHz. D 2003 Elsevier B.V. All rights reserved. Keywords: Sepiolite; Electrical conductivity; Dielectric; Magnetic field

1. Introduction It is known that doped iron oxide glasses and different oxide ceramic-type materials can show quite good conducting properties when modified. Studies carried out in recent years on the structural, conductivity and dielectric properties of ceramic composites and simple or complex oxides are significant. As a result of these studies, quite a number of active

* Fax: +90-262-324-99-09. E-mail address: [email protected] (K. Esmer). 0169-1317/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-1317(03)00159-5

and passive applications have been developed in the electronics industry and computer technology (Choudhary et al., 2000; Sharma et al., 1999; Pe´rez Martı´nez et al., 2001; Franco et al., 1999; Sharma and Choudhary, 1999; Bera and Choudhary, 1998; Rukmini et al., 1998). Clay minerals take place within the ceramic-type materials group, and are natural raw materials used for various purposes in industry (Thomas and Teocharis, 1991). Studies on the electrical properties of clay matrixes can be investigated under two groups. The first is the study of conductivity in aquatic surroundings. In this group, the conductivity is

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K. Esmer / Applied Clay Science 25 (2004) 17–22

Table 1 Chemical analysis of sepiolite (in wt.%) SiO2

dielectrical behaviour and the orientation of the iron atoms in the sepiolite structure.

Al2O3 Fe2O3 MgO CaO Na2O K2O H2O Au-L Total

51.94 2.12 51.45* 7.01

1.59 7.46

20.14 0.20 0.20 0.13 23.67 – 99.99 16.46 – 0.20 0.13 5.36 11.93 100

2. Experimental

* Data of elemental analysis (Au-L is used for the contacts).

mainly reliant upon the proton movement between the clay layers and the polymerization of organics in the structure (Hitzky et al., 1996; Saarenketo, 1998; Theng, 1974). The second one consists of studies in modifications and in ceramics such as KYHP3O10 compound, Nb-doped BaTiO3, iron sodium phosphate glasses. In this group, studies on the conductivity and dielectric properties are mainly carried out on polycrystal compounds and alloyed thin film (Zouari et al., 1999; Parvanov et al., 1999; Mohamed, 2000; El-Desoky et al., 2001; Kowalski et al., 2001). Sepiolite is a fiklosilicate mineral of crystalline structure made up of two tetrahedral (T) sheets of SiO4 and one octahedral (O) sheet of Al(O,OH)6 units. A ribbon-like structure that is discontinuous along the y- and z-crystallographic axes and continuous along the x-direction is formed from the inversed two T sheets and the O sheet between them (Grim, 1968). This structural order is responsible for the fibrous nature of sepiolite with rectangular channels that are continuous along the xdirection (Esmer and Yeniyol, 1999; Gonza´lez et al., 2000). These channels may also contain changeable cations such as Ca2 +, Mg2 + and Na+. The dimensions of the fibres change between 0.2 and 2 ˚ width and 50– 100 A ˚ Am in length, 100– 300 A thickness (Martin Vivaldi and Robertson, 1971). The linear array of Fe cations in the structure should confer a paramagnetic property to sepiolite and would increase the magnetic property of sepiolite, making orientation easier. Previous studies on the bentonite and sepiolite type clays have shown that their electrical conductivity increases significantly when modified (Esmer and Yeniyol, 1999; Esmer, 1998). In this study, the current– voltage changes have been investigated in parallel and perpendicular directions to the magnetic field for the determination of the anisotropic behaviour of the orientation in the magnetic field. In addition, it is also aimed to analyse the

Chemical analysis, cation exchange and orientation processes have been presented in detail by Esmer and Yeniyol (1999). Contacts were taken in a vacuum with Edwards Coating Unit Model 6E using an evaporation method. Contacts in the direction of and perpendicular to the magnetic field were determined using a Keitley 487 Picoammeter/Voltage source for the current–voltage measurements. Capacitive measurements were performed with a Hewlett Packard 4192 Impedance Analyzer using the evaporation technique on bulk contacts taken from both surfaces of the samples. For electrical measurements, distance between contacts for all samples (d), cross-sectional area (A), bulk thickness (dV) and contact area (AV) were measured, and found to be 46  10 2 cm, 62.8  10 2 cm2, 45.5  10 2 cm and 9.9  10 2 cm2, respectively. EPR spectrum was measured using Bruker MMX model x band spectrometer, employing magnetic field modulation to record field-derivative of the absorbed power, which is carried out FMR measurements. All measurements were done at room temperature.

3. Results During process of taking contact the loss of zeolitic water is 18.31%. The data obtained from elemental analysis have shown that iron ratio, which is 1.59% in natural sepiolite, rises up to 7.46% as a result of cation exchange (Table 1). Table 2 Resistance and resistivity values of natural and ion-exchanged sepiolite Samples

Resistance (V)

Resistivity (V cm)

Natural sepiolite (O) Fe-sepiolite (O) Natural sepiolite (?) Fe-sepiolite (?)

2.00  107 1.02  107 3.05  107 8.84  107

2.76  105 1.40  105 4.20  106 1.22  106

Parallel (O) and perpendicular (?) to the magnetic field.

K. Esmer / Applied Clay Science 25 (2004) 17–22

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Table 3 Data was calculated according to + 5 V (T = 300 jK) F (kHz)

10 25 50 75 100 250 500 750 1000

C (pF)

tand  10 5

eV(pF/m)

r  10 6

eW(pF/m)

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

74.00 56.00 48.00 44.50 40.50 20.80 9.70 6.87 5.74

41.60 30.50 23.60 21.50 20.60 17.30 15.50 14.41 13.70

1.98 3.74 5.06 5.39 5.30 3.79 2.98 3.01 3.23

4.30 5.19 6.04 6.28 6.41 6.89 7.16 7.30 7.38

3291.23 2490.66 2134.85 1979.18 1801.28 925.10 431.42 305.55 255.29

1850.20 1356.52 1049.64 956.24 916.21 769.44 689.38 640.90 609.32

65.24 93.19 107.98 106.68 95.43 35.05 12.86 9.19 8.24

79.63 70.39 63.37 60.01 58.70 53.01 49.38 46.78 44.99

4.10 14.63 33.91 50.24 59.93 55.02 40.38 43.29 51.75

5.00 11.05 19.90 28.27 36.86 83.23 155.05 220.36 282.53

3.1. Results of the current –voltage The changes in the current– voltage in the direction of the magnetic field are in good agreement with the previous study (Esmer and Yeniyol, 1999). Current – voltage changes perpendicular and parallel to the magnetic field have been analysed for anisotropic behaviour (Fig. 1). The resistivity has been calculated for both situations from the current– voltage changes which are given in Table 2.

Although dielectric permittivities of Fe-sepiolite are found to be similar to those of natural one the decrease at high frequencies is seen faster for natural sepiolite. This difference is a typical dielectric behaviour, and is in response to the decrease in total polarization at high frequencies (Figs. 2 and 3). eW/ eVchanges for Fe- and natural sepiolite are shown in Figs. 4 and 5, respectively. Changes in tand according to frequency is shown in Fig. 6.

3.2. Dielectric behaviour

4. Discussion

Capacitive measurements were taken between frequencies of 10 kHZ –10 MHz and F 10 V. Dielectric permittivities (eVand eW), loss tangent (tand) and dielectric conductivity (r), calculated for F 5 V using C = e0eA/d, tand = eW/eV, r = xeW (e0: permittivity of space and x: angular frequency) and other dielectric equations, are given in Tables 3 and 4.

4.1. The change of the current –voltage As can be seen from Fig. 1, the conductivity perpendicular to the magnetic field has decreased to some degree. This situation probably arises from the decrease of the electrical field between the mineral surfaces of the water molecules covering sepiolite

Table 4 Data was calculated according to  5 V (T = 300 jK) F (kHz)

10 25 50 75 100 250 500 750 1000

C (pF)

tand  10 5

eV(pF/m)

r  10 6

eW(pF/m)

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

Natural sepiolite

Fe-sepiolite

46.30 45.50 43.00 40.40 36.80 19.00 9.30 6.81 5.69

40.60 30.90 23.30 21.90 20.80 17.40 15.40 14.35 13.73

1.43 3.47 4.99 5.35 5.27 3.86 3.14 3.25 3.47

4.37 5.12 6.11 6.21 6.36 6.86 7.16 7.32 7.37

2059.24 2023.66 1912.47 1796.83 1636.72 845.05 413.63 302.88 253.07

1805.73 1374.31 1036.29 974.03 925.10 773.88 684.93 638.23 610.66

29.38 70.29 95.34 96.11 86.31 32.61 13.00 9.86 8.79

78.91 70.31 63.28 60.48 58.82 53.12 49.06 46.70 45.03

1.85 11.04 29.94 45.27 54.20 51.21 40.83 46.42 55.18

4.96 11.04 19.87 28.49 36.94 83.40 154.03 219.94 282.81

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K. Esmer / Applied Clay Science 25 (2004) 17–22

Fig. 1. Current – voltage characteristics of the oriented sepiolite samples: parallel (O) and perpendicular (?) to the magnetic field.

Fig. 3. Variation of imaginary part of dielectric permittivity of sepiolite as a function of frequency at room temperature.

The change in eVaccording to the thickness of the samples can give more detailed information. Sample thicknesses could not be measured accurately. Due to the low density of the fibres in the sepiolite suspensions, it is possible that the thickness is not dispersed homogeneously and variations in thickness have occurred on the glass during the orientation and drying

process. During the study carried out on dielectric films, it has been observed that the structure becomes more porous through a similar decrease in thickness (Wearver, 1965). The anomaly observed at 100 kHz in natural sepiolite due to frequency changes of eWand eV may arise from the presence of different polarization mechanisms (Figs. 2 and 3). Because when eW/ eV changes for natural and Fe-sepiolite are taken into account, the increase of eWat high frequencies, especially in Fe-sepiolite, must arise from the presence of a polarization with a short relaxation period (Fig. 4). This is due to the fact that the phase difference between the orientation polarization formed by the effect of the magnetic field and the frequency of the applied electrical field in Fe-sepiolite become smaller.

Fig. 2. Variation of reel part of dielectric permittivity of sepiolite as a function of frequency at room temperature.

Fig. 4. Variation of complex dielectric permittivity of the Fesepiolite.

fibres and displaceable ions in the solution. Kitayama et al. (1997) similarly observed that increasing I2 content the electrical conductivity increases from their work on polymerization of pyrrole in intracrystalline tunnels of sepiolite. 4.2. Dielectric behaviour

K. Esmer / Applied Clay Science 25 (2004) 17–22

On the other hand, natural sepiolite has an ionic polarization with a long relaxation period (Fig. 5) because of insufficient Fe ions. Thus, these ions emphasizes the effect of polar OH groups in the water molecules, which surround the fibres where these ions are situated. Therefore, the formation of a polarization induced along the magnetic field by the randomly oriented (distortion) O ! H dipole moments in accordance with the electrical field frequency causes the duration of the relaxation to be longer. The anomaly observed in Fig. 5 is at 100 kHz, and can be interpreted as the maximum increase of the dielectric permittivity with the polarization in the structure, in relation to the total polarization (oriented + induced). In the study of Lian et al. (1994) on the relaxation for polyaniline/polyvinylalcohol mixture, they found that the orientation of O ! H dipole moment increases relaxation with the applied electrical field. A change of loss tangent according to frequency, both the natural and Fe-sepiolite show an increase up to 100 kHz (Fig. 6). At higher frequencies, the active (ohmic) and reactive (capacitive) components of the current are harmonious in Fe-sepiolite and cause a partial saturation. In natural sepiolite, on the other hand, the active component of the current decreases faster than the reactive component at 100 kHz, and the reactive component changes relatively to the increasing frequency. Hence, tan d decreases frequencies higher than 100 kHz. This result is similar to the studies of Choudhary et al. (2000) on the dielectric and electrical properties of cadmium tungstate.

Fig. 5. Variation of complex dielectric permittivity of the natural sepiolite.

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Fig. 6. Variation of dielectric loss (tand) of sepiolite as a function of frequency.

In conclusion, dielectrical conductivity increases with the effect of the orientation polarization in Fesepiolite in connection with the orientation within the magnetic field. The peak obtained in EPR spectrum at the value 3485 Oe belongs to the Fe3 + arrangement and confirms the paramagnetic properties of sepiolite (Fig. 7). In such samples, it can be thought that the contribution of magnetic anisotropy, in addition to the Fe2 +, Fe3 + ions, are also important. The closeness of the values for + 5 and  5 V supports the confirmation of this anisotropic behaviour (Tables 2 and 3). In addition, the analysis of magnetic anisotropy and the current– voltage changes at low temperatures can give important results. Also, the determination of the role of surface or bulk structures in ionic transport could explain the rela-

Fig. 7. EPR spectrum of the Fe-sepiolite.

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tionship of conductivity and relaxation. Therefore, trying to make Hall measurements on such samples can be important. In addition to ceramic composites, which have been studied extensively in recent years, and which have multipurpose application ranges, modified clays can contribute similarly to industrial usage.

Acknowledgements I would like to thank Professor M. Yeniyol for supplying the sepiolite minerals, and Mr. Y. Kaya for his experimental works. I also acknowledge Professor B. Aktas and his PhD student F. Yildiz for the ERP measurements.

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