Characterization and dielectric properties of sodium fluoride doped talc

Clay Minerals, (2014) 49, 551–558

Characterization and dielectric properties of sodium fluoride doped talc ˘ L U 2 , E . E . Y A L Ç I N K A Y A 1 , * ¨ SEOG ¨ MU ¨ S T A S 1 , K . K O S. GU 1

AND

M. BALCAN1

Ege University, Faculty of Science, Chemistry Department, Bornova/Izmir, 35100, Turkey, and 2 Ege University, Vocational School, Ceramic Department, Bornova/Izmir, 35100, Turkey

(Received 11 February 2014; revised 9 August 2014; Editor: George Christidis)

AB ST R ACT : The purpose of this paper is to determine the effect of NaF and firing temperature on the dielectric properties (dielectric constant and dielectric loss) of talc, which is used in the electrical and electronic industries as a circuit element. A detailed characterization of the samples was made by XRD, FTIR, SEM and TG-DTG methods. Dielectric measurements were performed in the frequency range from 1 MHz to 80 MHz at room temperature. The dielectric constant value increased with an increase in firing temperature due to the removal of polarizable compounds from the talc structure. The higher dielectric constant values were obtained by addition of NaF. The dielectric loss of NaF doped talc decreased with the increase of firing temperature and increased with the increase of the amount of NaF.

KEYWORDS: dielectric properties, talc, firing temperature, NaF doping. Talc is a trioctahedral magnesium layer silicate mineral, usually present in the form of small crystals, which can be easily deformed. It usually occurs in low-grade metamorphic rocks that originate from ultrabasic to basic igneous precursors. Rocks that are composed almost entirely of talc have a greasy feel and are referred to as soapstone or steatite. Talc has a perfect cleavage that follows planes between weakly bonded sheets. These sheets are held together only by van der Waals bonds which allows them to slip past one another easily. At high temperature it recrystallizes into different forms of enstatite (anhydrous magnesium silicate) (Grim, 1968). The electric and mechanical properties of talc depend on the crystal structure and the type of impurities present. Shrinkage which takes place during sintering of the dielectric materials is observed also in talc and affects the dielectric

* E-mail: [email protected] DOI: 10.1180/claymin.2014.049.4.05

constant and dielectric loss. Hence, the firing temperature, the frequency and the impurities may affect the dielectric properties of talc. Talc is used in many industries such as paper production, plastics, paints and coatings, rubber, food, electrical-electronic industries, pharmaceuticals, cosmetics, ceramics, and many other applications. It is often used in electrical switchboards because of its resistance to heat, electricity, and acid attack; and also because it has a relatively low dielectric constant. The dielectric properties of talc have been studied extensively in the past (Hausner & Gunzenhauser, 1948; Mehrotia & Giannelis, 1992; Kaviratna et al., 1996; Kumar & Sirdeshmukh, 1996). Many different dopants such as KF, MgO, Mg(NO3)2, B2O3, Fe2O3, CeO2 and TiO2 have been used in these studies (Kırak et al., 1999; Zıpkın et al., 2007). The dielectric properties were affected by the dopant charge, dopant radius and structural defects of talc. It has been observed that the electrical properties of alkali fluoride glasses depend on the concentration and the nature of alkali ions (Bobe et al., 1997; Reau et al., 1990a,b, 1997; Sural & Ghosh, 1998, 1999).

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S. Gu¨mu¨s tas et al.

However, studies on the effects of the frequency and firing temperature on the dielectric properties of NaF added to talc have not been carried out yet. The aim of this study was to determine the effect of NaF and firing temperature on the dielectric properties of talc. NaF was selected as a dopant compound due to its low cost and abundance. The investigation of the effect of NaF on the dielectric properties of talc during sinterting is of particular importance because the water is rich in Na+ ions. Also, F ions are released from talc in the form of SiF4 at higher temperatures. Hence the inflence of F could be determined clearly by selection of NaF as a dopant material. The detailed characterization of the samples was performed by means of XRD, FTIR, SEM and TG-DTG methods. Dielectric constants (er) and dielectric loss (tan d) for talc and doped talc were calculated and given as a function of the frequency, firing temperature and NaF concentration.

MATERIAL AND METHODS Materials The talc sample was obtained from the Sivas area, Turkey (Yalçın & Bozkaya, 2006). NaF was purchased from Merck. The samples were prepared from powders of talc and NaF. Talc was mixed with distilled water and then NaF was added in concentrations ranging from 0.025 to 25 mmol (TNaF1: 0.025 mmol (0.001 g NaF); TNaF2: 0.25 mmol (0.01 g NaF); TNaF3: 2.5 mmol (0.11 g NaF); TNaF4: 12.5 mmol (0.55 g NaF); TNaF5: 25 mmol (1.1 g NaF), where T = talc). All samples were mixed for 12 h. The mixtures were dried at 105ºC in air, and were then ground in an agate mortar. The samples were prepared in disk form with 20 mm diameter and 2 mm thickness. The disks were fired at different temperatures (300– 500–650–900–1000ºC) for 2 h and were then cooled at room temperature in a desiccator.

Methods The chemical composition of talc was obtained by X-ray fluoscence spectrometry (XRF, Spectro IQ2 XRF spectrometer) as follows: SiO2 63 wt.%, MgO 29 wt.%, CaO 2 wt.%, Fe2O3 1 wt.% and LOI 5 wt.%. The characterization of undoped talc and NaF doped talc was performed by means of X-ray diffraction (XRD, Philips E’xpert Pro; Cu-Ka1

radiation). Fourier transform infrared (FTIR) spectra were obtained with a Perkin Elmer Spectrum 100 FTIR Spectrometer with the KBr method. Thermogravimetric analysis (TGA/DTG) of the samples was performed with a Perkin Elmer Pyris thermal analyser, by heating 5 mg powder samples in ceramic crucibles from room temperature to 1000ºC at a heating rate of 10ºC/min under a nitrogen flow. Broken surfaces of the samples were studied by scanning electron microscopy (SEM) using a Philips XL-30S FEG instrument. Dielectric measurements, such as capacitance and dielectric loss (tan d) were performed with the LCR (Inductance Capacitance Resistance) Dielectric Meter (Hioki 4284A) at various frequencies, ranging from 1 MHz to 100 MHz at 25ºC. The dielectric constants (er) were calculated using the formula C = er eo A/d; where C is the observed capacitance, d is the film thickness, A is the gold electrode area, and eo is the vacuum permittivity as 8.85610 12 F m 1. Fluoride measurements were carried out with a Thermo-Orion 4 Star Plus pH/Conductivity Meter. Before measurements, the electrode was calibrated using a series of fluoride standard solutions that were diluted with TISAB (Total Ionic Strength Adjustment Buffer) in plastic beaker. The fluoride content of the samples was measured with a fluoride electrode using a calibration plot. The calibration plot was obtained by using NaF at 2.5 20 ppm concentration range (y = 60.2x + 86.921; R2 = 0.9978).

RESULTS AND DISCUSSION The typical powder X-ray diffraction (XRD) patterns of the original sample and after firing at different temperatures (Fig. 1) are compatible with previous work (Grim, 1968). The original sample consists of talc, dolomite and magnesite. No significant change was observed in the XRD pattern of talc after firing at temperatures lower than 650ºC. However, for higher firing temperatures (900ºC and 1000ºC), the specific talc reflections disappeared and enstatite (MgSiO3) with diffraction ˚ and 2.52 A ˚ formed. The maxima at about 4.08 A structure changed almost completely and new MgSiO3 peaks appeared at high firing temperatures (MacKenzie & Meinhold, 1994). The comparison between the spectra of unfired talc and TNaF1 fired at different temperatures is given in Fig. 2. The FTIR spectra of the original talc and TNaF1 fired up to 650ºC were identical.

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FIG. 2. FTIR spectra of talc and TNaF1 fired at different temperatures.

FIG. 1. XRD patterns of talc and fired talc at different temperatures (D: dolomite, E: enstatite, M: magnesite, T: talc).

The OH-stretching frequency of the octahedral Mg-O-H unit in talc occurred at 3677 cm 1 (Takahashi et al., 1994). The only one band in the spectrum obtained was caused by high site symmetries of the trioctahedral talc structure which resulted in degeneracy of the OH sites (Schroeder, 2002). The observed strong band at around 1040 cm 1 is assigned to the out-of-plane symmetric stretching of Si–O–Si groups of talc (Farmer, 1974; Wang & Somasundaran, 2005). The sharp band at 670 cm 1 is due to Mg3OH-bending in the talc structure. Changes in the IR spectra occurred for some characteristic bands of the doped talc fired at 900 and 1000ºC. As a result of firing at

high temperature, the stretching vibration of talc hydroxyls at 3677 cm 1 disappeared. The asymmetric Si-O-Si absorption bands at 1047 cm 1 shifted and the sharp OH- bending band at 670 cm 1 disappeared. This could be explained by the formation of enstatite. Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of undoped talc and talc doped with different amounts of NaF and fired at 300ºC are shown in Figs 3 and 4, respectively. Three stages of weight loss were observed at ~500, 700 and 850ºC (Wesołowski, 1984; Bertin et al., 1998; Ersoy et al., 2013). The original sample contains mainly talc, dolomite and magnesite. The weight loss at ~500ºC and ~700ºC is attributed to the decomposition of magnesite and dolomite respectively (Gunasekaran & Anbalagan, 2007). The event at 500ºC might also be due to the dehydration of talc (Ewell et al., 1935; Wesołowski, 1984), which is not accompanied by the breakdown of the crystal structure of the mineral, since the

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FIG. 3. TG thermograms for talc and doped talc with different amounts of NaF fired at 300ºC.

FIG. 4. DTG thermograms for talc and doped talc with different amounts of NaF fired at 300ºC.

XRD patterns showed that the crystal structure of talc has not been affected. Upon heating at 850oC, talc starts to transform to ensteatite and probably to amorphous silica according to reaction (1) (Piga et al., 1992; Takahashi et al., 1994). These results were supported by XRD and FTIR data.

residual weight of TNaF5 is lower than that of the undoped talc according to the data obtained from the TG results, suggesting that the weight loss increases with addition of NaF. The surface morphologies of the talc samples were compared by SEM analysis (Fig. 5). The layered structure was preserved at low temperatures. The talc particles approached each other with increasing temperature and the number of pores decreased. These textural changes are related to the release of volatile SiF4 compound. The probable reactions between the NaF and the talc samples are shown below:

4MgO·5SiO2·H2O ? 4(MgO·SiO2) +SiO2+H2O (1) The talc sample has 7.5 wt.% weight loss due to volatile compounds, which is higher than LOI of talc and this is due to the presence of dolomite and magnesite. If the added NaF is not released during heating, the residual solids will be 92.62 wt.%. The

FIG. 5. SEM images of talc (a) and NaF doped talc at 1000ºC (b).


SiO2+4NaF+2H2O ? SiF4+4NaOH MgO+2NaF+H2O ? MgF2+2NaOH CaO+2NaF+H2O ? CaF2+2NaOH

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(2) (3) (4)

The concentration of F measured by the potentiometric method are plotted as a function of the fired temperatures in Fig. 6. The added NaF might be converted into SiF4, MgF2 or CaF2 according to the reactions 4 6. The reaction products, except SiF4, will remain in the solid phase because they are not volatile. When the talc fired at different temperatures was mixed with water, fluorine was detected, which was probably derived from MgF2 and CaF2. If the F ions present in the samples had not been removed as SiF4, the concentration of F determined at different temperatures should have been constant. However, the concentration of F in the solution decreased as the firing temperature increased by forming volatile Si-F compounds at temperatures as low as 500ºC. Hence, defects and pores form when Si is released from the structure, causing an increase in spacecharge polarization and dielectric constant. For all firing temperatures, the dielectric constants decreased with an increase of frequency in the lower frequency region (Fig. 7). Also, the dielectric constants of the fired talc were lower than that of the original sample. From evaluation of previous studies, the decrease of the dielectric constant with increase in frequency at room temperature could be explained by space-charge polarization. The structural defects in talc could cause this change in the space-charge polarization (Kırak et al., 1999).

FIG. 6. The fluoride concentration of NaF doped talc as a function of the firing temperatures.

FIG. 7. The dielectric constant of talc as a function of frequency at different firing temperatures.

Dielectric polarization arises from the electrical response of individual molecules of a medium and may be classified as electronic, atomic orientation and space-charge or interfacial polarization, according to the mechanism involved. The spacecharge polarization occurs due to orientation of charge, along the field direction, thereby causing redistribution of charges in the dielectrics. This process always results in a distortion of the macroscopic field and is important only at low frequencies. The dielectric constant due to the space-charge polarization decreases with increasing frequency (Sawada, 2008). Similar behaviour was obtained for the dielectric constant of NaF doped talc at different NaF concentrations and firing temperatures (Fig. 8). The dielectric constant of NaF doped talc mostly exhibited higher values than that of the undoped talc. The dielectric constant and the loss tangent of talc are small and are mainly governed by minor impurities and defects. Addition of some alkali ions affects the space-charge polarization of the mineral. The dielectric constant of NaF doped talc decreased as the firing temperature increased. The dielectric constant of doped talc was 6.1 at 300ºC (5 MHz) and decreased to 5.3 at 1000ºC for the same doping amount. The effect of NaF concentrations on the dielectric constant of NaF doped talc was similar at 5 MHz, 50 MHz and 100 MHz frequencies. The greatest variation in the dielectric constant of NaF doped talc was obtained at 5 MHz and the results are shown in Fig. 9. The dielectric constant increased

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FIG. 8. The dielectric constant of NaF doped talc as a function of frequency at different concentrations and firing temperatures.

significantly with increasing NaF concentration. The effect of NaF decreased with increasing firing temperature. The crystal order of doped and undoped talc ceramics increased with firing temperature up to 400ºC. In the crystallized talc ceramics, a large number of defects may exist including vacancies, interstitial ions and dislocations (Kırak et al., 1999). When the NaF doped talc is heated further, water and HF are formed at about 400ºC (Sadler, 1965). HF dissolves the talc and Si is released in the form of SiF4. Due to the high volatility of SiF4, the SiF4

FIG. 9. The dielectric constant of NaF doped talc as a function of NaF concentrations at different firing temperatures (5 MHz).

was released from the structure forming defects, which may account for the build up of space-charge polarization. Electronic, ionic, dipolar and spacecharge polarization contributes to the nature of the dielectric constant, especially when the latter space charge polarization depends on the purity and perfection of the ceramics (Kırak et al., 1999). Figure 10 shows the effect of the firing temperatures on the dielectric constant of NaF doped talc at constant frequency 5 MHz. Similar results were obtained at 50 MHz and 100 MHz frequencies. The dielectric constant of NaF doped talc decreased with increasing firing temperature regardless of NaF concentrations. The highest dielectric constant values were obtained at high NaF concentrations for all firing temperatures. The tan d of undoped talc and NaF doped talc (Fig. 11) followed a similar trend to the dielectric constant. The tan d of NaF doped talc decreased with increasing firing temperature. The loss tangent values increased with increase of the NaF concentration.

CONCLUSIONS The dielectric properties of NaF doped talc at various temperatures were examined. When the firing temperature increased, the conductance decreased at all NaF concentrations. The rate of decrease was fast up to 650ºC, becoming slower at higher temperatures. Talc was converted to enstatite and the addition of NaF facilitated this conversion.

FIG. 10. The dielectric constant of talc as function of the firing temperatures at different NaF concentrations (5 MHz).


FIG. 11. The dielectric loss of talc as function of the firing temperatures at different NaF concentrations (5 MHz).

The dielectric constant increased with increasing firing temperature because of the removal of polarizable compounds from the talc structure. The dielectric constant of doped talc was higher than that of the original talc due to the structure defects. These defects resulted from the removal of Si from the talc in the form of SiF4 and changes in space-charge polarization. The dielectric constant varied with the measuring frequency. It decreased at 10 MHz, enhancing slightly thereafter and increasing with a higher rate at nearly 100 MHz. The lowest dielectric constant value was obtained at 10 MHz at all firing temperatures. The dielectric loss of NaF doped talc decreased with increasing firing temperature. The dielectric loss values increased with increasing NaF concentration. The added F caused space-charge polarization because of pores and defects in the structure at higher firing temperatures. REFERENCES

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