Dielectric properties of halloysite and halloysite-formamide intercalate M. Adamczyk, M. Rok, A. Wolny, and K. Orzechowski Citation: Journal of Applied Physics 115, 024101 (2014); doi: 10.1063/1.4857015 View online: http://dx.doi.org/10.1063/1.4857015 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/115/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Microwave absorption properties of conducting polymer composite with barium ferrite nanoparticles in 12.4 – 18 GHz Appl. Phys. Lett. 93, 053114 (2008); 10.1063/1.2969400 Dielectric constant engineering with polymethylmethacrylate-graphite metastate composites in the terahertz region J. Appl. Phys. 99, 066103 (2006); 10.1063/1.2178389 Optimization of two-layer electromagnetic wave absorbers composed of magnetic and dielectric materials in gigahertz frequency band J. Appl. Phys. 98, 084903 (2005); 10.1063/1.2108158 Magnetic, dielectric, and microwave absorbing properties of iron particles dispersed in rubber matrix in gigahertz frequencies J. Appl. Phys. 97, 10F905 (2005); 10.1063/1.1852371 Electromagnetic and absorption properties of some microwave absorbers J. Appl. Phys. 92, 876 (2002); 10.1063/1.1489092 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 156.17.103.92 On: Tue, 16 Sep 2014 07:38:42 JOURNAL OF APPLIED PHYSICS 115, 024101 (2014) Dielectric properties of halloysite and halloysite-formamide intercalate M. Adamczyk,a) M. Rok, A. Wolny, and K. Orzechowski Faculty of Chemistry, University of Wroclaw, Wroclaw 50-383, Poland (Received 30 July 2013; accepted 9 December 2013; published online 8 January 2014) Due to a high increase in electromagnetic pollution, the protection from non-ionizing electromagnetic radiation (EMR) represents an important problem of contemporary environmental science. We are searching for natural materials with the potential for EMR screening. We have discovered that hydrohalloysite has interesting properties as an EMR absorber. Unfortunately, it is a very unstable material. Drying it for even a short period of time leads to the loss of desired properties. In the paper, we have demonstrated that the intercalation of halloysite (the process of introducing guest molecules into the mineral structure) makes it possible to recover the ability to absorb an electromagnetic wave and C 2014 AIP Publishing LLC. obtain a promising material for electromagnetic field shielding applications. V [http://dx.doi.org/10.1063/1.4857015] I. INTRODUCTION The protection from non-ionizing electromagnetic radiation (EMR) has recently become a very important issue. Both, radio and television transmitter frequencies and a rapid growth in mobile telephony have caused an enormous increase in electromagnetic pollution.1 The impact of electromagnetic radiation on animals, flora, and humans has been broadly discussed,1–4 but no clear evidence was found that EMR has an adverse effect on human health. However, it should be taken into consideration that during the process of evolution, people were not influenced by strong electromagnetic radiation and, therefore, the result is highly unpredictable.2 Searching for electromagnetic wave absorbers is a significant objective of contemporary material science and shielding applications. In many cases, the materials are based on polymer composites filled with carbonaceous particles,5 strontium ferrite—carbon black—nitrile rubber composites,6 graphene,7 and nickel/carbon hybrid nanostructures.8 In this study, we prove that halloysite, a chemically modified clay mineral, has the ability for electromagnetic wave screening. Halloysite is a layered aluminosilicate and belongs to the kaolin group. Each layer contains silicate (Si2O5) and gibbsite (Al2(OH)4) sheets. The layers are bound by hydrogen bonds between the tetrahedral oxygens of the silicate sheet and the outer hydroxyl groups of the gibbsite sheet (from the adjacent layer). This structure is comparable to the structure of kaolinite, however, the replacement of Al3þ ions by Fe3þ ions in the gibbsite sheet is frequently observed together with the presence of interlayer water molecules. Halloysite usually adopts the shape of an elongated tubule, nonetheless, short-tubular, spherical, or platy morphologies have also been recognised.9 The natural halloysite may exist as the mixture of hydro-halloysite (interlayer distance d001 ¼ 10 6 0.2 Å, called halloysite 10 Å), containing weakly bonded interlayer water molecules, and halloysite (interlayer distance d001 ¼ 7 6 0.2 Å, called halloysite 7 Å) without interlaying water. a) Author to whom correspondence should be addressed. Electronic mail: mariusz.adamczyk@chem.uni.wroc.pl. 0021-8979/2014/115(2)/024101/5/$30.00 Halloysite may interact with ionic or polar molecules by intercalation. Guest molecules enter the interlamellar space producing a nanomaterial with diverse properties. Guest molecules cause an increase in the interlayer distance (d001), which can liberate the movement of molecules or elements of molecules and resulting in increasing of e0 and e00 . Taking into consideration such assumption, we have decided to transform halloysite into an intercalated material (by placing formamide molecules between the layers of the mineral). In this paper, we have demonstrated that hydrohalloysite has the potential electromagnetic wave screening applications. This is associated with the presence of weakly bound water (absorption of the electric component of an electromagnetic wave) and the content of Fe3þ, caused in an increase in magnetic permeability (absorption of the magnetic component of an electromagnetic wave). We have also established that the intercalation of halloysite enables to obtain a promising material for electromagnetic field shielding applications. II. MATERIALS AND METHODS A. Sample preparation The halloysite described in the experiment was used in the form as received. The mineral was ground and investigated directly afterwards. The halloysite presented herein was obtained from the “Dunino” mine, Lower Silesia, Poland. At first, the intercalate was obtained by shaking the mineral together with the mixture of formamide and anhydrous ethanol (1:1) for a time period of seven days. The suspension was left at room temperature for a time period of 13 days and then, centrifuged. The intercalate was then washed with ethanol and dried in the air afterwards. B. X-ray experiment The X-ray experiment was performed with the D8 ADVANCE powder diffractometer, using the CuKa radiation, operating at 40 kV and 40 mA. The sample was measured in step-scan mode from 5 to 30 2h with steps of 0.05 . 115, 024101-1 C 2014 AIP Publishing LLC V [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 156.17.103.92 On: Tue, 16 Sep 2014 07:38:42 024101-2 Adamczyk et al. J. Appl. Phys. 115, 024101 (2014) C. Dielectric experiments Electric permittivity (e0 and e00 ) measurements were performed on powdered samples in the temperature range from 5 to 80  C and the frequency range from 0.1 kHz to 1 MHz, using the HP-4284A Precision LCR Meter in a two-electrode configuration. The capacitor was made of stainless steel and Plexiglas and consisted of two circular flat electrodes separated by a Plexiglas ring. The ring was filled with the powdered material, and the electrodes were pushed into the sample. Live capacity was calculated on the basis of the capacitor geometry, lead capacity, on the basis of measurements conducted for an empty capacitor. Since the investigated samples were composed of a granular material, the electric permittivity obtained directly from the measurements was an apparent value, depending on the packing of grains. In order to recalculate the apparent permittivity (eapp0 ) to the material value (e0 ), we applied the formula originally proposed by Robinson10 and verified by us in the previous paper11 e0 ¼ e0app  eair ð1  uÞ ; u e00 ¼ e00app u ; (1) where eair is the relative permittivity of air (eair ¼ 1), u ¼ dx/dc is the volume fraction of the solid content in a powder sample, calculated as the ratio of the sample density (dx obtained from the mass and geometry) and the crystallographic density (dc) of the material. For halloysite 7 Å, the crystallographic density of 2.57 g/cm3 was obtained from the literature.12 For the purpose of intercalate, the density dc was calculated as follows: dc ¼ dh  ð1  wXRD Þ þ dh   For dry halloysite, we obtained dx ¼ 1.07 g/cm3, dc ¼ dh, and, hence, u ¼ 0.42. We were unable to estimate the “u” factor for natural halloysite, because the mass of the interlayer water was unknown. The relative error of permittivity measurements was of the order of 5%, however, the repeatability was at the level of 30% only. III. RESULTS In Fig. 1, the curve signed as “natural halloysite” presents the XRD powder patterns of the freshly ground sample. It is evident that the material consists of hydrohalloysite (reflection related to d001 ¼ 9.8 Å) and halloysite (d001 ¼ 7.1 Å). By comparing the intensities it may be estimated that the freshly grounded material consists of 70% hydro-halloysite and 30% halloysite. The curve labelled as “dry halloysite” in Fig. 1 presents the XRD powder patterns of halloysite after washing it with water and afterwards with ethanol and drying at room temperature. The reflection characteristic for hydro-halloysite (9.8 Å) is practically invisible. XRD experiments performed in the intercalate (Fig. 1) allowed us to observe a reflection at 2h ¼ 8.57 , which was associated with the expanded layers. The obtained distance d001 was equal to 10.3 Å, which was consistent with literature findings for the halloysite-formamide intercalate.13 Apart from the reflection at 2h ¼ 8.57 , also a reflection at 2h ¼ 12.14 was visible, related to the presence of  wXRD d001 ðhalÞ  wXRD ;  wXRD  wTGA d001 ðintÞ (2) where dh is the crystallographic density of halloysite (2.57 g/cm3),12 d001(hal) is the interlayer distance in halloysite, d001(int) is the interlayer distance in the intercalate, wTGA is the ratio of the mass of the guest molecule (formamide) placed between the halloysite layers and the mass of the sample, obtained from the thermogravimetric analysis diagram, wXRD represents the fraction describing the effectiveness of intercalation, estimated from the intensities of d001 reflection assigned to the unchanged mineral I(d001hal) and the one expanded by intercalation I(d001int) wXRD ¼ Iðd001 intÞ : Iðd001 intÞ þ Iðd001 halÞ (3) For the investigated material, intercalated with formamide, we obtained dx ¼ 0.81 g/cm3, wXRD ¼ 0.79, wTGA ¼ 0.096, dc ¼ 2.13 g/cm3, and finally, the volume fraction of the solid material in the powdered sample was determined as u ¼ 0.38. FIG. 1. X-ray diffraction patterns of investigated materials: natural halloysite, dry halloysite, and halloysite intercalated with formamide. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 156.17.103.92 On: Tue, 16 Sep 2014 07:38:42 024101-3 Adamczyk et al. J. Appl. Phys. 115, 024101 (2014) Unfortunately, natural halloysite is highly unstable material. Drying it for a short period of time in the atmosphere of low humidity leads to the loss of water and the irreversible transformation into halloysite 7 Å. Removing of interlaying water and, undoubtedly, water adsorbed on the surface, leads to a considerable decrease in permittivity. This property impedes the use of natural halloysite as an electromagnetic wave absorbing material. Figures 3 and 4 present the outcome of dielectric measurements in the halloysite intercalated with formamide in comparison to that obtained in dry natural halloysite. The data were obtained in powdered samples, and afterwards the apparent permittivity was transformed into the material quantity using Eq. (1). It is evident that intercalation led to the restoration of dielectric absorption. Electric permittivity measured as a function of temperature increases in the low temperature range, and then decreasing. At temperatures exceeding 60  C, the material begins to decompose. Dispersion and absorption curves (e0 (f) and e00 (f), Fig. 3) show a clear relaxation process within the investigated frequency range. FIG. 2. Apparent electric permittivity obtained at 25.5  C in powdered samples of natural and dry halloysite. unmodified halloysite. The efficiency of intercalation (estimated from the intensities of both reflections) was found to be 79%. Fig. 2 presents the apparent electric permittivity investigated in the natural material and in the dried halloysite. The data obtained for the powdered samples were not corrected due to the packing. The real component of apparent permittivity attains large values at low frequencies, but it decreases strongly with an increase in frequency. A negative imaginary component of apparent permittivity (e00 app ¼ Im(eapp)) evidences the existence of a relaxation process located at the kHz region. We suspect that the dispersion and absorption of eapp observed in natural halloysite could be related to both, the adsorbed water molecules, encaged in the micro-pore structure and the interlayer water molecules in hydro-halloysite. IV. DISCUSSION At this stage of research, we may propose three processes possibly responsible for the observed relaxation. The first one is related to the dynamics of guest molecules. If the highly polar formamide molecules have some possibility of moving, the absorption and dispersion of e should be observed. The expansion of the interlayer distance after the intercalation with formamide is approximately 3.2 Å. It correlates well with the mean diameter of the guest molecule, which may be estimated from the crystal structure.12 The formation of the halloysite-formamide intercalate is frequently used to distinguish between halloysite and kaolinite.14,15 However, according to our best knowledge, the structure of halloysite-formamide complexes has not been thoroughly examined yet, unlike the structure of the kaolinite-formamide intercalate, which has been extensively investigated.16,17 It has been found that the formamide molecule is bound to the adjacent layers by two hydrogen bonds, FIG. 3. Experimental data of e0 and e00 as a function of frequency in the halloysite-formamide intercalate. [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 156.17.103.92 On: Tue, 16 Sep 2014 07:38:42 024101-4 Adamczyk et al. J. Appl. Phys. 115, 024101 (2014) FIG. 4. Experimental data of e0 and e00 as a function of temperature in the dry halloysite and in the halloysiteformamide intercalate. which may result in the rigidity of the structure. Presumably, it is further the case of the formamide—halloysite intercalate. The two bonds between formamide and the host structure may result in the stiffening of the structure, which, however, does not exclude some movements, observed applying dielectric spectroscopy. The O  H    O bond to the gibbsite sheet and the N  H    O bond to the silicate sheet are nearly co-linear and, hence, large-angle axial librations activated by the interaction with an external electric field cannot be excluded. Another possible explanation for dielectric absorption is linked to the movement of fragments of host structure. Intercalation leads to the breaking of hydrogen bonds between the OH groups of the gibbsite sheet and the oxygen atoms of the silicate sheet and, then, to the formation of new bonds between the atoms of the sheets and the guest molecules. Despite the formation of new hydrogen bonds, some of the OH groups of the gibbsite sheet may preserve a tendency to large-angle movements, being the reason for dielectric absorption. The intercalate was obtained by long-term shaking with the mixture of formamide and ethanol, followed by washing with ethanol, and finally drying in the air. Ethanol does not form intercalates with halloysite, however, it cannot be excluded that alcohol molecules enter the interlayer area, “propped” by formamide molecules. The movement of ethanol molecules encaged in the interlayer area is the third possible mechanism of the observed relaxation. Well shaped absorption and dispersion curves enabled us to fit the equation describing the dielectric relaxation. We found that the experimental points could be described by the Davidson-Cole equation, supplemented by the DC conductivity term and electrode polarization term, in the form originally proposed by Johnson and Cole18 e ¼ C eo  e1 r ; þ e1 þ j 2pev f f2 ð1  j  ð2pfsÞÞb (4) where eo is the static electric permittivity, e1, the high frequency limit of permittivity, f, the frequency, r, the specific DC conductivity, ev, the electric permittivity of vacuum, b, the parameter describing the distribution of relaxation times, and C, the adjustable parameter related to electrode polarization. Table I presents the results of the fittings. TABLE I. Results of the fitting of Eq. (4) to the measurements of electric permittivity performed in the halloysite-formamide intercalate, as a function of frequency (0.1–1000 kHz) and temperature (14–75  C). The quality of fittings was controlled using the v2 test. T[  C] 6 0.5 14.4 19.6 25.2 29.3 35.0 39.0 44.8 50.2 54.0 59.5 64.9 70.0 75.2 C[m/s2] 6 0.05 r[lS/m] 6 0.005 e0 6 5 e1 6 0.5 s [ls] b 6 0.02 v2 0.1 0.2 0.2 0.3 0.4 0.4 0.5 0.5 0.6 0.6 0.6 0.5 0.6 0.31 0.41 0.51 0.60 0.72 0.82 0.93 0.98 0.99 0.94 0.81 0.69 0.62 387 403 419 428 437 441 437 420 400 365 328 320 310 5 6 6 6 6 6 6 5 4 3 (3) (3) (3) 1230 6 120 880 6 80 660 6 60 540 6 50 420 6 40 340 6 30 270 6 20 210 6 20 170 6 10 130 6 10 100 6 10 87 6 5 76 6 4 0.50 0.50 0.51 0.51 0.51 0.51 0.51 0.50 0.49 0.47 0.46 0.46 0.45 0.49 0.46 0.51 0.58 0.73 0.91 1.17 1.53 1.85 2.19 2.55 2.93 3.20 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 156.17.103.92 On: Tue, 16 Sep 2014 07:38:42 024101-5 Adamczyk et al. J. Appl. Phys. 115, 024101 (2014) intercalated with formamide is considerably higher than the one in the dry halloysite (Fig. 5). V. CONCLUSIONS We have demonstrated dielectric measurements performed in the natural halloysite, dry halloysite, and halloysite intercalated with formamide. We are searching for low-cost materials capable to absorb an electromagnetic wave. Natural halloysite could be a good candidate, as the material is slightly magnetic and effectively absorbs the electric component of an electromagnetic wave. Unfortunately, halloysite easily loses these properties, just after grinding and drying. We have proved that the desired properties can be easily restored by intercalation with formamide. The material is stable at room and elevated temperatures (up to 60  C) and shows dielectric absorption in the kilohertz region. Several mechanisms possibly responsible for the observed absorption have been proposed and discussed. ACKNOWLEDGMENTS FIG. 5. Attenuation constant at 25.2  C in the halloysite-formamide intercalate and in the dry halloysite. The temperature dependence of the relaxation time was analyzed according to the Eyring equation.18 The calculated activation enthalpy was 36 6 1 kJ/mol. This relatively large value of activation enthalpy seems to exclude the mechanism of dielectric relaxation related to the movement of unbounded OH groups. Unfortunately, it is beyond the bounds of possibility to agree which of the other mechanisms is applicable. The large value of activation enthalpy appears to match the axial movements of double-bonded formamide. However, the movement of ethanol molecules encaged between the halloysite layers may not be excluded either. The strongly hindered movement of ethanol molecules may also need a large activation energy. The attenuation constant, being the real part of the propagation factor, can be used to evaluate the ability to absorb electromagnetic waves.19,20 The attenuation constant (in Np/m) for non-magnetic materials could be presented as follows:5 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi x 0 e þ ðe02 þ e002 Þ; (5) a ¼ pffiffiffi 2c where a is the attenuation constant, x, circular frequency, and c, speed of light in vacuum. According to Eq. (5), the most suitable condition requires a large value of e00 and a low value of e0 . However, in real dielectrics, large e00 is usually associated with large e0 which decreases the attenuation constant. It is also the case of the investigated material. Nevertheless, the attenuation constant in the halloysite The authors thank for the financial support of the Wroclaw University, Grant No. 2254/M/WCH/12. We are especially grateful to INTERMARK, Poland, for the kind supply of halloysite samples. 1 A. Balmori, Pathophysiology 16, 191 (2009). S. J. Genuis and C. T. Lipp, Sci. Total Environ. 414, 103 (2012). 3 M. Otto and K. E. von M€ uhlendahl, Int. J. Hygiene Environ. Health 210, 635 (2007). 4 S. J. Genuis, Public Health 122, 113 (2008). 5 F. Qin and C. Brosseau, J. Appl. Phys. 111, 061301 (2012). 6 S. Vinayasree, M. A. Soloman, V. Sunny, P. Mohanan, P. Kurian, and M. R. Anantharaman, Compos. Sci. Technol. 82, 69 (2013). 7 X. Sun, J. He, G. Li, J. Tang, T. Wang, Y. Guo, and H. Xue, J. Mater. Chem. C 1, 765 (2013). 8 V. Sunny, D. S. Kumar, P. Mohanan, and M. R. Anantharaman, Mater. Lett. 64, 1130 (2010). 9 E. Joussein, S. Petit, J. Churchman, B. Theng, D. Right, and B. Delvaux, Clay Miner. 40, 383 (2005). 10 D. A. Robinson and S. P. Friedman, J. Non-Cryst. Solids 305, 261 (2002). 11 K. Leluk, K. Orzechowski, K. Jerie, A. Baranowki, T. Słonka, and J. Głowi nski, J. Phys. Chem. Solids 71, 827 (2010). 12 G. B. Mitra and S. Bhattacherjee, Acta Crystallogr. 31, 2851 (1975). 13 E. Joussein, S. Petit, and B. Delvaux, Appl. Clay Sci. 35, 17 (2007). 14 G. J. Churchman, Clays Clay Miner. 38, 591 (1990). 15 G. J. Churchman, J. S. Whitton, G. G. C. Claridge, and B. K. G. Theng, Clays. Clay Miner. 32(4), 241 (1984). 16 R. L. Frost, D. A. Lack, G. N. Paroz, and Thu Ha T. Tran, Clays Clay Miner. 47(3), 297 (1999). 17 S. Olejnik, A. M. Posner, and J. P. Quirk, Clays. Clay Miner. 19, 83 (1971). 18 N. E. Hill, W. E. Vaughan, A. H. Price, and M. Davies, Dielectric Properties and Molecular Behaviour (Van Nostrand Reinhold Company, London, 1969), pp. 71 and 285. 19 K. J. Vinoy and R. M. Jha, Radar Absorbing Materials—From Theory to Design and Characterization (Kluwer Academic Publishers, Boston, MA, 1996). 20 S. J. Yan, L. Zhen, C. Y. Xu, J. T. Jiang, and W. Z. Shao, J. Phys. D: Appl. Phys. 43, 245003 (2010). 2 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 156.17.103.92 On: Tue, 16 Sep 2014 07:38:42