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Study of ground and unground leached vermiculite II. Thermal behaviour of ground acid-treated vermiculite
Applied Clay Science 51 (2011) 274–282
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Study of ground and unground leached vermiculite II. Thermal behaviour of ground acid-treated vermiculite J.L. Perez-Rodriguez a,⁎, C. Maqueda b, N. Murafa c, J. Šubrt c, V. Balek c, P. Pulišová c, A. Lančok c a b c
Instituto de Ciencia de Materiales de Sevilla (UNSE-CSIC) Americo Vespucio s/n, 41098 Sevilla, Spain Instituto de Recursos Naturales y Agrobiología (CSIC) Apdo 1052, 41080-Sevilla, Spain Institute of Inorganic Chemistry of the AS CR, v.v.i., 250 68 Husinec-Řež, Czech Republic
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Article history: Received 25 September 2010 Received in revised form 22 November 2010 Accepted 24 November 2010 Available online 4 December 2010 Keywords: Vermiculite Annealing Amorphous silica β-FeOOH α-Fe2O3 ε-Fe2O3
a b s t r a c t In this study, we examined the annealing effect on the material obtained after acid treatment of ground vermiculite which constituted amorphous silica and β-FeOOH. The XRD patterns of the starting sample measured at temperatures from 30 to 1200 °C showed that the crystalline phase was present until ~ 300 °C; whereas the sample heated between 300 and 800 °C was practically amorphous. This is in agreement with previous observations that β-FeOOH decomposes to amorphous or poorly crystalline phase, β-Fe2O3, and transforms only slowly to crystalline α-Fe2O3. At 850 °C the sample showed the first signs of a crystalline phase which was fully developed at 1050 °C. The XRD, HRTEM and Mössbauer spectroscopy showed, after heating at 1050 °C, the presence of crystalline phase, consisting of quartz, cristobalite, α-Fe2O3 and ε-Fe2O3. This effect showed in fact that well crystallized iron oxide nanoparticles embedded into the silica matrix are usually formed at relative high temperatures (~ 1000 °C), which is in contrast to silica-free material. Element mapping of one particle of the composite obtained by annealing the sample at the highest temperature showed well-separated Fe2O3 and SiO2 particles in a composite material. Impurities of Al and Mg (from the original vermiculite) accompanied the silica components and TiO2 associated with Fe2O3 grains was also detected. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The preparation of porous materials from clay minerals including vermiculite by selective leaching with acid has been studied by several authors (Maqueda et al., 2008; Okada et al., 2006; Temuujin et al., 2003). Recently the results of leaching of ground vermiculite from Santa Olalla (Spain) with 1 M HCl (Maqueda et al., 2009) have been published. It was observed that the formation of a material of high specific surface area constituted amorphous silica and β-FeOOH (akaganeite) microcrystals containing a small amount of Ti4+ and Cl−. Porosity studies showed that the high specific surface area of this residue may be attributed to the presence of iron from the structural iron of the vermiculite. It is well known that the β-FeOOH microcrystals are typically uniform rod-like shapes consisting of an oriented bundle of loosely packed rods, also in an orthogonal array, wherein the repeating distance is about 60 Å (Cai et al., 2002; Watson et al., 1962). The β-FeOOH is a typical hydrolytic product in solutions containing small amounts of anions (usually Cl− or SO2− 4 , according to the synthetic procedure used)
⁎ Corresponding author. Tel.: + 34 954489532; fax: + 34 954460665. E-mail address: [email protected] (J.L. Perez-Rodriguez). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.11.031
which are essential for the stabilization of its structure (Cornell and Schwertmann, 1996; Ishikawa and Inouye, 1975; Music et al., 2004; Ohyabu and Ujihira, 1981; Saric et al., 1998; Shao et al., 2005). Thermal decomposition of β-FeOOH microcrystals of various origins was described earlier. Hematite (α-Fe2O3) or magnetite (Fe3O4) is the most common reaction products (Babcan and Kristin, 1971; Gonzalez-Calbet and Franco, 1982; Ishikawa and Inouye, 1975; Merono et al., 1985; Music et al., 2004; Naono et al., 1982; Paterson et al., 1982). Transformation of akaganeite to rhombus hematite was often observed; with the size, shape and porosity of the annealing product particles dependent predominantly on the conditions of synthesis of the starting material as well as on the annealing conditions (Hu et al., 2008; Zhang et al., 2007). Thermal decomposition of particular modifications of hydrated iron oxides is often of topotactic and pseudomorphic in character, with the shape and size of the initial particles being preserved up to temperatures of about 800 °C. Above this temperature stepwise sintering and growth of isotropic-shaped iron oxide particles occur (Balek and Subrt, 1994; Cudennec and Lecerf, 2005; Perez-Maqueda et al., 2002; Solcova et al., 1980; Subrt et al., 1981, 1982). Annealing affects the potential application (catalysis, magnetic properties, porosity, pigments, etc.) of the material obtained after acid treatment of ground vermiculite from Santa Olalla. However, the annealing of the material which constituted amorphous silica and
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akaganeite has not been reported. The aim of this study was to describe the changes which occur in the material obtained by leaching ground vermiculite with HCl on controlled heating at temperatures up to 1050 °C. 2. Materials and methods 2.1. Samples Vermiculite from Santa Olalla (Huelva, Spain) was used as starting material. Its half-unit cell composition is (Si2.64Al1.36)(Mg2.48Fe3+0.32 Fe2+0.04Al0.14Ti0.01Mn0.01)O10(OH)2Mg0.44 (Perez-Maqueda et al., 2001; Wiewiora et al., 2003). Vermiculite particles less than 80 μm in size were obtained with a knife-mill (Retsch ultracentrifuge mill, model 255 M-1 equipped with a suitable sieve). 2.2. Sample preparation Grinding experiments were carried out in batches of 10 g of vermiculite using a vibratory mill (Herzog HSM-100). The vibratory mill ground by friction and impact and was operated at 1500 rpm for grinding times of 0.5, 1, 2, 3, 4, 5 and 10 min. The properties of these samples were described in the first part of this study (Maqueda et al., 2009). The sample ground for 3 min was used for subsequent acid treatment. This sample had the relatively highest surface area value before acid treatment; the samples ground for 2 and 4 min showed similarly high surface area values. Vermiculite sample (b80 μm) ground for 3 min was treated with 1 M HCl solution at a solid/acid mass ratio of 1:20. The suspension was maintained at 80 °C and stirred for 24 h. The sample was then cooled and washed with distilled water until the supernatant was free of acid. The sample was dried at 60 °C overnight and used for the heat treatment. It had the lowest content of acid leachable elements (about 1% of MgO and Al2O3) as well as high surface area and porosity. The sample was heated to 850, 950 and 1050 °C, respectively by using a laboratory furnace (003 LP with regulator Ht Ceramic). The heating rate was 10 °C min−1, and lasted for 2 h. The samples were subsequently cooled to room temperature in air. The sample was also treated in a high temperature chamber (Anton Paar) from 30 to 1200 °C, with a heating rate of 10 °C min−1.
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electron microscope with a LaB6 electron source and equipped with Semi STEM and EDX. The samples were prepared from a dispersion in ethanol that was sonicated for 4 min. The Cu grid was immersed into the dispersion, followed by drying at room temperature in air. EDX measurements were performed with an Oxfords Instruments EDX detector. Mössbauer spectrum measurements were carried out at 293 K in order to determine the phase composition of iron oxides in the final nanocomposites present in the samples. The Mössbauer spectrum measurements were carried out in the transmission mode with 57Co diffused into an Rh matrix as a source moving with constant acceleration. The spectrometer (Wissel) was calibrated by means of a standard α-Fe foil, and the isomer shift was expressed with respect to this standard at 293 K. The fitting of the spectra was performed with the help of the NORMOS program. The ETA–DTA measurements were performed on samples under a flow of dry air at a heating rate of 6 K min−1, using a modified NETZSCH DTA 404 instrument. During the ETA–DTA measurements the labelled sample of 0.1 g was situated in a corundum crucible with a constant overflow of dry air (flow-rate of 40 ml min−1), which carried the radon released from the sample into the measuring chamber of radon radioactivity. TG measurements were carried out under the same experimental conditions as the ETA–DTA measurements. Emanation thermal analysis (ETA) (Balek and Tölgyessy, 1984; Balek et al., 2002) involved the measurement of radon release rate from previously labelled samples. The samples for ETA were labelled by soaking in an acetone solution containing traces of 228Th and 224Ra nitrates. The specific activity of a sample after labelling was 105 Bq/g. Atoms of 220Rn were formed by the spontaneous alpha decay of 228Th and 224Ra. The 224Ra and 220Rn atoms were incorporated into the sample to a maximum depth of 80 nm due to the recoil energy (85 keV/atom), which the atoms gain by the spontaneous α-decay. The maximum depth of 220Rn penetration was 80 nm as calculated with the TRIM code (Ziegler et al., 1985). Evolved gas analysis was carried out by heating the samples in synthetic air by using Quadrupole mass spectrometry (QMS) on the NETZSCH STA (QMS) 409/429–403 equipment. The amount of the sample used for this analysis was 0.05 g. 3. Results and discussion
2.3. Sample characterization X-ray diffraction patterns (XRD) were recorded with a PANalytical X' pert Pro equipped with an X' celerator. CuKα or CoKα radiations were used. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, The Netherlands, version 3.0) and JCPDS PDF-2 database. For quantitative analysis and the degree of crystallinite were used Diffract-Plus Topas (Brucker AXS, Germany, version 4.2) with structural model based on ICSD database.PANalytical X' pert Pro equipped with an X' celerator. CuKα or CoKα radiations were used. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, The Netherlands, version 3.0) and JCPDS PDF-2 database. For quantitative analysis and the degree of crystallinite were used Diffract-Plus Topas (Brucker AXS, Germany, version 4.2) with structural model based on ICSD database. In the high temperature chamber, the XRD diagrams of the sample were collected every 50 °C. Surface area and porosity values were determined by using the BET and BJH methods, respectively. The adsorption of N2 was measured with Multi-Station High Speed Gas Sorption Analyzer (Quantachrome Instruments NOVA 4200). The samples were outgassed by heating at 200 °C under a flow of air for 12 h. Data were recorded at values p/p0 between 0.0005 and 0.99. High resolution transmission electron microscopy (HRTEM) measurements were carried out using a 300 kV JEOL JEM 300 UHR
Quantitative estimation from XRD analysis of the material obtained after acid treatment with 1 M HCl showed that the sample consisted of 13% crystalline material and 87% amorphous material. The crystalline portion consisted of 91.2% of β-FeOOH and 8.8% nanocrystalline SiO2, as was previously described (Maqueda et al.,
Fig. 1. X-ray difraction patterns of the ground vermiculite after acid treatment collected between 30 and 1200 °C. Some selected XRD are shown in the figure. A = akaganeite.
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2009). As shown by electron microscopy, the acid leached materials consisted of spindle-shape particles approximately 0.1 × 0.5 μm in size, assigned to β-FeOOH, and irregular aggregates of amorphous hydrated silica nanoparticles a few nm in size. The XRD patterns of the starting sample measured at temperatures from 30 to 1200 °C are shown in Fig. 1. The XRD diagrams carried out at 30 °C showed diffraction peaks corresponding to d values of 0.3796 nm, 0.337 nm, 0.253 nm, etc. attributed to akaganeite, together with a high background produced by amorphous silica. The initial partially crystalline mixture turned to an amorphous form above ~ 300 °C; formations of new crystalline phases were observed above 850 °C. The XRD patterns of samples heated to 850, 950, and 1050 °C are shown in Fig. 2. Whereas the starting ground vermiculite treated with 1 M HCl was partially crystalline on account of the presence of crystalline β-FeOOH, the sample heated to 850 °C was practically amorphous. This is in agreement with the observations that β-FeOOH decomposes into an amorphous or poorly crystalline phase, β-Fe2O3, which transforms only slowly to crystalline α-Fe2O3 (Music et al., 2004; Zhang et al., 2007). At 950 °C the sample showed the first signs of crystallinity which was fully developed at 1050 °C. At this temperature, the sample was crystalline, consisting of quartz, cristobalite, α-Fe2O3 and ε-Fe2O3. It has been demonstrated that in the case of nanocrystalline iron oxides, namely ε-Fe2O3, Mössbauer spectroscopy has better detection limits for the iron oxide phases compared to powder XRD (Brazda et al., 2009; Popovici et al., 2004; Tronc et al., 1998). Previous results have shown that the crystal structure of ε-Fe2O3 has four different Fe sites. Three of these have an octahedral coordination of oxygen atoms and one has a tetrahedral coordination. Thus, the Mössbauer spectrum measured on ε-Fe2O3 has four contributions from four different Fe sites. The values of hyperfine field (Bhf) and the chemical shift of two of these sites are so close to each other that they are fitted by one sextet (Bhf= 45 T). The third octahedral Fe site as subspectrum 2 shows that Bhf = 39 T. The fourth Fe-site (Bhf= 26 T) has a tetrahedral coordination (Brazda et al., 2009). The Mössbauer spectrum of the sample which annealed at 1050 °C is given in Fig. 3. Each measured spectrum was fitted by these three independent sextets corresponding to a magnetically ordered phase and one doublet corresponding to a magnetically non-ordered phase which can be either nanoparticles of iron oxide in the superparamagnetic state or isolated iron cations (Brazda et al., 2009; Cannas et al., 2001). Two sextets with HT 50.75 and 49.17 T can be attributed to nanocrystalline α-Fe2O3 (the determined value of BH
Fig. 3. Mössbauer spectra of the ground vermiculite after acid treatment taken at 293 K.
49.17–50.75 was slightly smaller than in well-developed α-Fe2O3 crystals; see Table 1). The remaining sextets showed acceptable matching with the data found in the literature for ε-Fe2O3 (Brazda et al., 2009). In accordance with the X-ray diffraction analysis, it can be concluded that the sample heated at 1050 °C contained quartz, cristobalite, and small particles of α-Fe2O3 and ε-Fe2O3. Nevertheless, a significant part (about 25 wt.%) of the total amount of iron present in the sample was superparamagnetic, forming a doublet in the spectrum (see Table 1). Fig. 4 shows the results of TG/DTG, DTA, ETA, and EGA results of the sample measured on heating up to 1200 °C in air. The evolved gas analysis (QMS) was used for the detection of the release of H2O and HCl. The starting sample consisted of amorphous hydrated silica and crystalline β-FeOOH particles. The first phenomenon observed by the TG, DTA and EGA (H2O) in the temperature range up to 200 °C could be assigned to the loss of physically bound water. After this effect associated with a sudden mass loss and endothermic effect on the DTA curve, the loss of mass continues slowly up to temperatures ~800 °C. In a temperature range of 400–800 °C the continuous mass loss shown in the TG curve was connected with the release of HCl and the small release of water as shown in the EGA curve, resulting from decomposition of β-FeOOH (where Cl− is an essential component
Fig. 2. X-ray difraction patterns of the ground vermiculite after acid treatment heated at 850, 950 and 1050 °C. Q = quartz; C = cristobalite; Fe-α = α-Fe2O3; Fe-ε = ε-Fe2O3.
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Table 1 Mössbauer spectroscopy characteristics of the sample annealed at 1050 °C. S = 6-line-component. D = 2-line-component. Component
Area fraction of total
Area fraction of component
Isomer shift [mm/s]
S1
0.209 ± 0.013 0.134 ± 0.011 0.046 ± 0.009 0.284 ± 0.019 0.096 ± 0.010 0.769 ± 0.009 0.231 ± 0.009 0.231 ± 0.009
0.271 ± 0.017 0.175 ± 0.015 0.060 ± 0.012 0.370 ± 0.023 0.125 ± 0.013
0.369 −0.207 ± 0.005 ±0.009 0.368 −0.222 ± 0.008 ±0.016 0.323 −0.101 ± 0.000 ±0.001 0.172 −0.164 ± 0.000 ±0.000 0.292 −0.192 ± 0.000 ±0.000 Mean hyperfine induction
1.000 ± 0.000
0.191 ± 0.003
S2 S3 S4 S5 S D1 D
Quadrupole shift/splitting [mm/s]
0.382 ±0.004
for the stabilization of its structure) (Cornell and Schwertmann, 1996; Ishikawa and Inouye, 1975; Music et al., 2004; Ohyabu and Ujihira, 1981; Saric et al., 1998; Shao et al., 2005). At temperatures above 800 °C, the sample started to crystallize as was supported by XRD
Hyperfine induction [T]
Area ratio line 2 to 1
Area ratio line 3 to 1
Linewidth 1 (and 6) [mm/s]
50.75 ± 0.05 49.17 ± 0.08 45.81 ± 0.00 26.33 ± 0.00 39.00 ± 0.00 39.69 ± 0.50
0.667 ± 0.000 0.667 ± 0.000 0.667 ± 0.000 0.667 ± 0.000 0.667 ± 0.000
0.333 ±0.000 0.333 ±0.000 0.333 ±0.000 0.333 ±0.000 0.333 ±0.000
0.305 ± 0.016 0.332 ± 0.007 0.749 ± 0.000 6.120 ± 0.000 0.884 ± 0.000
1.000 ± 0.000
Linewidth 2 (to 5) [mm/s]
Phase
α-Fe2O3 α-Fe2O3 ε-Fe2O3 ε-Fe2O3 ε-Fe2O3
0.305 ±0.007
analysis. The emanation thermal analysis made it possible to characterise the transport properties of the material obtained after acid treatment of the vermiculite sample. The development of microstructure in these samples has been characterized by
Fig. 4. DTA,TG, EGA and ETA curves of ground vermiculite after acid treatment.
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Fig. 5. Nitrogen adsorption–desorption isotherm of ground vermiculite after acid treatment anneded at 850 °C.
Fig. 6. Pore distribution of ground vermiculite after acid treatment annealed at 850 °C.
changes in the measured radon release rate (Balek et al., 2002, 2007; Perez-Maqueda et al., 2003; Poyato et al., 2002). The crystallization of amorphous SiO2 was reflected on the ETA curve as a decrease of the radon release rate in the temperature range from 950 to 1050 °C. The results of porosity measurements of the samples obtained by heating the starting sample at 850 °C are shown in Figs. 5 and 6. The observed hysteresis loop matched well to the H2 type, which corresponded to the pores with narrow and wide sections and possible interconnecting channels. The pore distribution is shown in
Table 2 Surface area and porosity values of the annealed samples to temperatures 850, 950 and 1050 °C. Temperature [°C]
2
BET surface area [m /g] BJH pore volume [cm3/g] BJH pore diameter [nm] Average pore size [nm] Total pore volume [cm3/g]
850
950
1050
85.70 0.133 3.65 6.06 0.129
9.02 0.057 3.82 21.53 0.049
1.91 0.020 3.86 33.39 0.018
Fig. 6. The results of surface area and porosity measurements of samples obtained by heating the starting sample at 850, 950, and 1050 °C are given in Table 2. Significant changes were observed in surface area and porosity of the samples heated to these temperatures. The sample prepared at 850 °C had a surface area 85.7 m2/g with pronounced porosity, while the sample obtained at 1050 °C was more compact with surface area only 1.9 m2/g and negligible porosity. The character of the hysteresis loop remained the same for all three samples corresponding well to the H2 hysteresis loop type. The morphology of the starting sample is shown in Fig. 7. On the micrographs, it was observed that spindles of β-FeOOH were embedded in nanoparticulate SiO2 material. A detailed description of the microstructure of this sample has been previously published (Maqueda et al., 2009). The morphology of the sample annealed at 950 °C is given in Fig. 8. The sample showed darker acicular particles of iron oxide, still amorphous, in the SiO2 material (Fig. 8a and b), which started to show signs of crystallization on the HRTEM images (Fig. 8c and d). The signs of SiO2 crystallization were also visible on the XRD pattern of the sample annealed at 950 °C (see Figs. 1 and 2). The EDX analysis confirmed the presence of Fe in the darker particles and Si as the main
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Fig. 7. TEM micrographs of ground vermiculite after acid treatment: (a) typical cluster of amorphous silica and akaganeite particles; (b) isolated akaganeite particle; (c) and (d) akaganeite particle embedded in silica materials.
Fig. 8. TEM micrographs of ground vermiculite after acid treatment annealed at 950 °C.
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Fig. 9. TEM micrographs of ground vermiculite after acid treatment annealed at 1050 °C.
component in the amorphous materials. The particles which did not show the presence of darker needle were formed exclusively of SiO2, whereas the remaining regions in which darker needles were present contained, except for Si, mainly Fe. It was also observed in the EDX spectra that a minor mixture of Ti accompanied Fe in all the particles and it is not present in the Si-rich particles. The Cl− anion was not present in this sample. This element escaped completely during annealing of the sample at about 950 °C. After annealing at 1050 °C, the sample was well crystallized, as demonstrated by the XRD pattern (see Figs. 1 and 2). Fig. 9 shows the morphology of the sample annealed at 1050 °C. The micrograph showed dark Fe2O3 particles with shapes from almost spherical to acicular, or irregular, embedded into homogeneous crystalline SiO2 (Fig. 9a and b). The well-developed crystallinity of both SiO2 and Fe2O3 was well visible on the HRTEM images (Fig. 9c and d, respectively). Element mapping of a single particle of the composite obtained by annealing the sample at 1050 °C (see Fig. 10) showed well-separated Fe2O3 and SiO2 particles in a composite material. From Fig. 10, it followed that impurities of Al and Mg (from the original vermiculite) accompanied the silica components. TiO2 was associated with Fe2O3 grains. It is known that nanoparticles of amorphous Fe2O3 usually crystallize into nanocrystalline maghemite (γ-Fe2O3) and this transformation is normally characterized by an exothermic peak at 280 °C. However, it has been reported that Fe2O3–SiO2 amorphous nanoparticulate systems show a major difference from the case of pure amorphous nanoparticles of Fe2O3, in spite of the fact that the same transformation phase product (i.e., γ-Fe2O3) was observed. The difference appears in the shifting of the transformation temperature toward a higher crystallization temperature (ca. 700 °C) (Khalil et al., 2008). The shifting of the crystallization temperature toward a higher
temperature and the consequent stabilization of the amorphous Fe2O3 nanophase are attributed to what is called the preventive role of the silica matrix. Consequently, well-crystallized iron oxide nanoparticles embedded into the silica matrix are formed usually in the range of 600–1000 °C (Bhaumik et al., 2005; Bourlinos et al., 2001; Braileanu et al., 2006; Cannas et al., 2001, 2002; Ennas et al., 1998; Moreno et al., 2002; Niznansky et al., 1995; Popovici et al., 2005; Savii et al., 2000; Solinas et al., 2001). This temperature range is higher in comparison to silica-free iron oxides (Balek and Subrt, 1994; Perez-Maqueda et al., 1999, 2002). As one can conclude from the published data, the formation of ε-Fe2O3 is rather frequent in the presence of amorphous or nanocrystalline silica, up to relatively high temperatures (Brazda et al., 2009; Lancok et al., 2007; Mori et al., 2008; Sakurai et al., 2008; Tadic et al., 2008). Recently, the εFe2O3 phase has attracted much attention due to its enormous roomtemperature coercivity of about 2 T, and the coupling of magnetic and dielectric properties, the so-called magneto-electric response (Gich et al., 2006; Kurmoo et al., 2005). 4. Conclusions The material obtained after the leaching of ground vermiculite consisted of amorphous silica and akaganeite. The heating of this mixture transformed the crystalline akaganeite to an amorphous phase about 300 °C. New crystalline phases appeared above 800 °C, and were fully developed at 1050 °C, consisting of quartz, crystobalite and iron oxides. The shifting of the crystallization temperature of iron oxides and the consequent stabilization of the amorphous Fe2O3 nanophase were attributed to what is called the preventive role in the silica matrix. The HRTEM showed that the annealing of ground and acid treated vermiculite resulted in a composite consisting of Fe2O3 and SiO2. The composite obtained at 1050 °C consisted of separate
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Fig. 10. Elemental mapping of a single particle of ground vermiculite annealed at 1050 °C.
grains of Fe2O3 embedded into a SiO2 matrix. The crystalline SiO2 material was formed by hexagonal quartz and tetragonal cristobalite, whereas the crystalline Fe2O3 grains consist of ε-Fe2O3 and α-Fe2O3 (hematite), as was confirmed by XRD and Mösbauer spectroscopy. The detectable impurities of Al and Mg accompanied the silica component, whereas TiO2 was associated with Fe2O3 grains. The composite materials obtained in this wok by simple and cheap route from natural raw materials could be of interest from a practical point of view. Acknowledgments The authors wish to thank the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) for the financial support under the project no. AGL-2005-0164, Ministry of Education, Youth and Sport of the Czech Republic (project no. LC 523), and the Exchange Program between the Czech Academic and the Consejo Superior de Investigaciones Científicas (CSIC, Spain).
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Study of ground and unground leached vermiculite II. Thermal behaviour of ground acid-treated vermiculite J.L. Perez-Rodriguez a,⁎, C. Maqueda b, N. Murafa c, J. Šubrt c, V. Balek c, P. Pulišová c, A. Lančok c a b c
Instituto de Ciencia de Materiales de Sevilla (UNSE-CSIC) Americo Vespucio s/n, 41098 Sevilla, Spain Instituto de Recursos Naturales y Agrobiología (CSIC) Apdo 1052, 41080-Sevilla, Spain Institute of Inorganic Chemistry of the AS CR, v.v.i., 250 68 Husinec-Řež, Czech Republic
a r t i c l e
i n f o
Article history: Received 25 September 2010 Received in revised form 22 November 2010 Accepted 24 November 2010 Available online 4 December 2010 Keywords: Vermiculite Annealing Amorphous silica β-FeOOH α-Fe2O3 ε-Fe2O3
a b s t r a c t In this study, we examined the annealing effect on the material obtained after acid treatment of ground vermiculite which constituted amorphous silica and β-FeOOH. The XRD patterns of the starting sample measured at temperatures from 30 to 1200 °C showed that the crystalline phase was present until ~ 300 °C; whereas the sample heated between 300 and 800 °C was practically amorphous. This is in agreement with previous observations that β-FeOOH decomposes to amorphous or poorly crystalline phase, β-Fe2O3, and transforms only slowly to crystalline α-Fe2O3. At 850 °C the sample showed the first signs of a crystalline phase which was fully developed at 1050 °C. The XRD, HRTEM and Mössbauer spectroscopy showed, after heating at 1050 °C, the presence of crystalline phase, consisting of quartz, cristobalite, α-Fe2O3 and ε-Fe2O3. This effect showed in fact that well crystallized iron oxide nanoparticles embedded into the silica matrix are usually formed at relative high temperatures (~ 1000 °C), which is in contrast to silica-free material. Element mapping of one particle of the composite obtained by annealing the sample at the highest temperature showed well-separated Fe2O3 and SiO2 particles in a composite material. Impurities of Al and Mg (from the original vermiculite) accompanied the silica components and TiO2 associated with Fe2O3 grains was also detected. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The preparation of porous materials from clay minerals including vermiculite by selective leaching with acid has been studied by several authors (Maqueda et al., 2008; Okada et al., 2006; Temuujin et al., 2003). Recently the results of leaching of ground vermiculite from Santa Olalla (Spain) with 1 M HCl (Maqueda et al., 2009) have been published. It was observed that the formation of a material of high specific surface area constituted amorphous silica and β-FeOOH (akaganeite) microcrystals containing a small amount of Ti4+ and Cl−. Porosity studies showed that the high specific surface area of this residue may be attributed to the presence of iron from the structural iron of the vermiculite. It is well known that the β-FeOOH microcrystals are typically uniform rod-like shapes consisting of an oriented bundle of loosely packed rods, also in an orthogonal array, wherein the repeating distance is about 60 Å (Cai et al., 2002; Watson et al., 1962). The β-FeOOH is a typical hydrolytic product in solutions containing small amounts of anions (usually Cl− or SO2− 4 , according to the synthetic procedure used)
⁎ Corresponding author. Tel.: + 34 954489532; fax: + 34 954460665. E-mail address: [email protected] (J.L. Perez-Rodriguez). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.11.031
which are essential for the stabilization of its structure (Cornell and Schwertmann, 1996; Ishikawa and Inouye, 1975; Music et al., 2004; Ohyabu and Ujihira, 1981; Saric et al., 1998; Shao et al., 2005). Thermal decomposition of β-FeOOH microcrystals of various origins was described earlier. Hematite (α-Fe2O3) or magnetite (Fe3O4) is the most common reaction products (Babcan and Kristin, 1971; Gonzalez-Calbet and Franco, 1982; Ishikawa and Inouye, 1975; Merono et al., 1985; Music et al., 2004; Naono et al., 1982; Paterson et al., 1982). Transformation of akaganeite to rhombus hematite was often observed; with the size, shape and porosity of the annealing product particles dependent predominantly on the conditions of synthesis of the starting material as well as on the annealing conditions (Hu et al., 2008; Zhang et al., 2007). Thermal decomposition of particular modifications of hydrated iron oxides is often of topotactic and pseudomorphic in character, with the shape and size of the initial particles being preserved up to temperatures of about 800 °C. Above this temperature stepwise sintering and growth of isotropic-shaped iron oxide particles occur (Balek and Subrt, 1994; Cudennec and Lecerf, 2005; Perez-Maqueda et al., 2002; Solcova et al., 1980; Subrt et al., 1981, 1982). Annealing affects the potential application (catalysis, magnetic properties, porosity, pigments, etc.) of the material obtained after acid treatment of ground vermiculite from Santa Olalla. However, the annealing of the material which constituted amorphous silica and
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akaganeite has not been reported. The aim of this study was to describe the changes which occur in the material obtained by leaching ground vermiculite with HCl on controlled heating at temperatures up to 1050 °C. 2. Materials and methods 2.1. Samples Vermiculite from Santa Olalla (Huelva, Spain) was used as starting material. Its half-unit cell composition is (Si2.64Al1.36)(Mg2.48Fe3+0.32 Fe2+0.04Al0.14Ti0.01Mn0.01)O10(OH)2Mg0.44 (Perez-Maqueda et al., 2001; Wiewiora et al., 2003). Vermiculite particles less than 80 μm in size were obtained with a knife-mill (Retsch ultracentrifuge mill, model 255 M-1 equipped with a suitable sieve). 2.2. Sample preparation Grinding experiments were carried out in batches of 10 g of vermiculite using a vibratory mill (Herzog HSM-100). The vibratory mill ground by friction and impact and was operated at 1500 rpm for grinding times of 0.5, 1, 2, 3, 4, 5 and 10 min. The properties of these samples were described in the first part of this study (Maqueda et al., 2009). The sample ground for 3 min was used for subsequent acid treatment. This sample had the relatively highest surface area value before acid treatment; the samples ground for 2 and 4 min showed similarly high surface area values. Vermiculite sample (b80 μm) ground for 3 min was treated with 1 M HCl solution at a solid/acid mass ratio of 1:20. The suspension was maintained at 80 °C and stirred for 24 h. The sample was then cooled and washed with distilled water until the supernatant was free of acid. The sample was dried at 60 °C overnight and used for the heat treatment. It had the lowest content of acid leachable elements (about 1% of MgO and Al2O3) as well as high surface area and porosity. The sample was heated to 850, 950 and 1050 °C, respectively by using a laboratory furnace (003 LP with regulator Ht Ceramic). The heating rate was 10 °C min−1, and lasted for 2 h. The samples were subsequently cooled to room temperature in air. The sample was also treated in a high temperature chamber (Anton Paar) from 30 to 1200 °C, with a heating rate of 10 °C min−1.
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electron microscope with a LaB6 electron source and equipped with Semi STEM and EDX. The samples were prepared from a dispersion in ethanol that was sonicated for 4 min. The Cu grid was immersed into the dispersion, followed by drying at room temperature in air. EDX measurements were performed with an Oxfords Instruments EDX detector. Mössbauer spectrum measurements were carried out at 293 K in order to determine the phase composition of iron oxides in the final nanocomposites present in the samples. The Mössbauer spectrum measurements were carried out in the transmission mode with 57Co diffused into an Rh matrix as a source moving with constant acceleration. The spectrometer (Wissel) was calibrated by means of a standard α-Fe foil, and the isomer shift was expressed with respect to this standard at 293 K. The fitting of the spectra was performed with the help of the NORMOS program. The ETA–DTA measurements were performed on samples under a flow of dry air at a heating rate of 6 K min−1, using a modified NETZSCH DTA 404 instrument. During the ETA–DTA measurements the labelled sample of 0.1 g was situated in a corundum crucible with a constant overflow of dry air (flow-rate of 40 ml min−1), which carried the radon released from the sample into the measuring chamber of radon radioactivity. TG measurements were carried out under the same experimental conditions as the ETA–DTA measurements. Emanation thermal analysis (ETA) (Balek and Tölgyessy, 1984; Balek et al., 2002) involved the measurement of radon release rate from previously labelled samples. The samples for ETA were labelled by soaking in an acetone solution containing traces of 228Th and 224Ra nitrates. The specific activity of a sample after labelling was 105 Bq/g. Atoms of 220Rn were formed by the spontaneous alpha decay of 228Th and 224Ra. The 224Ra and 220Rn atoms were incorporated into the sample to a maximum depth of 80 nm due to the recoil energy (85 keV/atom), which the atoms gain by the spontaneous α-decay. The maximum depth of 220Rn penetration was 80 nm as calculated with the TRIM code (Ziegler et al., 1985). Evolved gas analysis was carried out by heating the samples in synthetic air by using Quadrupole mass spectrometry (QMS) on the NETZSCH STA (QMS) 409/429–403 equipment. The amount of the sample used for this analysis was 0.05 g. 3. Results and discussion
2.3. Sample characterization X-ray diffraction patterns (XRD) were recorded with a PANalytical X' pert Pro equipped with an X' celerator. CuKα or CoKα radiations were used. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, The Netherlands, version 3.0) and JCPDS PDF-2 database. For quantitative analysis and the degree of crystallinite were used Diffract-Plus Topas (Brucker AXS, Germany, version 4.2) with structural model based on ICSD database.PANalytical X' pert Pro equipped with an X' celerator. CuKα or CoKα radiations were used. Qualitative analysis was performed with the HighScorePlus software package (PANalytical, The Netherlands, version 3.0) and JCPDS PDF-2 database. For quantitative analysis and the degree of crystallinite were used Diffract-Plus Topas (Brucker AXS, Germany, version 4.2) with structural model based on ICSD database. In the high temperature chamber, the XRD diagrams of the sample were collected every 50 °C. Surface area and porosity values were determined by using the BET and BJH methods, respectively. The adsorption of N2 was measured with Multi-Station High Speed Gas Sorption Analyzer (Quantachrome Instruments NOVA 4200). The samples were outgassed by heating at 200 °C under a flow of air for 12 h. Data were recorded at values p/p0 between 0.0005 and 0.99. High resolution transmission electron microscopy (HRTEM) measurements were carried out using a 300 kV JEOL JEM 300 UHR
Quantitative estimation from XRD analysis of the material obtained after acid treatment with 1 M HCl showed that the sample consisted of 13% crystalline material and 87% amorphous material. The crystalline portion consisted of 91.2% of β-FeOOH and 8.8% nanocrystalline SiO2, as was previously described (Maqueda et al.,
Fig. 1. X-ray difraction patterns of the ground vermiculite after acid treatment collected between 30 and 1200 °C. Some selected XRD are shown in the figure. A = akaganeite.
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2009). As shown by electron microscopy, the acid leached materials consisted of spindle-shape particles approximately 0.1 × 0.5 μm in size, assigned to β-FeOOH, and irregular aggregates of amorphous hydrated silica nanoparticles a few nm in size. The XRD patterns of the starting sample measured at temperatures from 30 to 1200 °C are shown in Fig. 1. The XRD diagrams carried out at 30 °C showed diffraction peaks corresponding to d values of 0.3796 nm, 0.337 nm, 0.253 nm, etc. attributed to akaganeite, together with a high background produced by amorphous silica. The initial partially crystalline mixture turned to an amorphous form above ~ 300 °C; formations of new crystalline phases were observed above 850 °C. The XRD patterns of samples heated to 850, 950, and 1050 °C are shown in Fig. 2. Whereas the starting ground vermiculite treated with 1 M HCl was partially crystalline on account of the presence of crystalline β-FeOOH, the sample heated to 850 °C was practically amorphous. This is in agreement with the observations that β-FeOOH decomposes into an amorphous or poorly crystalline phase, β-Fe2O3, which transforms only slowly to crystalline α-Fe2O3 (Music et al., 2004; Zhang et al., 2007). At 950 °C the sample showed the first signs of crystallinity which was fully developed at 1050 °C. At this temperature, the sample was crystalline, consisting of quartz, cristobalite, α-Fe2O3 and ε-Fe2O3. It has been demonstrated that in the case of nanocrystalline iron oxides, namely ε-Fe2O3, Mössbauer spectroscopy has better detection limits for the iron oxide phases compared to powder XRD (Brazda et al., 2009; Popovici et al., 2004; Tronc et al., 1998). Previous results have shown that the crystal structure of ε-Fe2O3 has four different Fe sites. Three of these have an octahedral coordination of oxygen atoms and one has a tetrahedral coordination. Thus, the Mössbauer spectrum measured on ε-Fe2O3 has four contributions from four different Fe sites. The values of hyperfine field (Bhf) and the chemical shift of two of these sites are so close to each other that they are fitted by one sextet (Bhf= 45 T). The third octahedral Fe site as subspectrum 2 shows that Bhf = 39 T. The fourth Fe-site (Bhf= 26 T) has a tetrahedral coordination (Brazda et al., 2009). The Mössbauer spectrum of the sample which annealed at 1050 °C is given in Fig. 3. Each measured spectrum was fitted by these three independent sextets corresponding to a magnetically ordered phase and one doublet corresponding to a magnetically non-ordered phase which can be either nanoparticles of iron oxide in the superparamagnetic state or isolated iron cations (Brazda et al., 2009; Cannas et al., 2001). Two sextets with HT 50.75 and 49.17 T can be attributed to nanocrystalline α-Fe2O3 (the determined value of BH
Fig. 3. Mössbauer spectra of the ground vermiculite after acid treatment taken at 293 K.
49.17–50.75 was slightly smaller than in well-developed α-Fe2O3 crystals; see Table 1). The remaining sextets showed acceptable matching with the data found in the literature for ε-Fe2O3 (Brazda et al., 2009). In accordance with the X-ray diffraction analysis, it can be concluded that the sample heated at 1050 °C contained quartz, cristobalite, and small particles of α-Fe2O3 and ε-Fe2O3. Nevertheless, a significant part (about 25 wt.%) of the total amount of iron present in the sample was superparamagnetic, forming a doublet in the spectrum (see Table 1). Fig. 4 shows the results of TG/DTG, DTA, ETA, and EGA results of the sample measured on heating up to 1200 °C in air. The evolved gas analysis (QMS) was used for the detection of the release of H2O and HCl. The starting sample consisted of amorphous hydrated silica and crystalline β-FeOOH particles. The first phenomenon observed by the TG, DTA and EGA (H2O) in the temperature range up to 200 °C could be assigned to the loss of physically bound water. After this effect associated with a sudden mass loss and endothermic effect on the DTA curve, the loss of mass continues slowly up to temperatures ~800 °C. In a temperature range of 400–800 °C the continuous mass loss shown in the TG curve was connected with the release of HCl and the small release of water as shown in the EGA curve, resulting from decomposition of β-FeOOH (where Cl− is an essential component
Fig. 2. X-ray difraction patterns of the ground vermiculite after acid treatment heated at 850, 950 and 1050 °C. Q = quartz; C = cristobalite; Fe-α = α-Fe2O3; Fe-ε = ε-Fe2O3.
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Table 1 Mössbauer spectroscopy characteristics of the sample annealed at 1050 °C. S = 6-line-component. D = 2-line-component. Component
Area fraction of total
Area fraction of component
Isomer shift [mm/s]
S1
0.209 ± 0.013 0.134 ± 0.011 0.046 ± 0.009 0.284 ± 0.019 0.096 ± 0.010 0.769 ± 0.009 0.231 ± 0.009 0.231 ± 0.009
0.271 ± 0.017 0.175 ± 0.015 0.060 ± 0.012 0.370 ± 0.023 0.125 ± 0.013
0.369 −0.207 ± 0.005 ±0.009 0.368 −0.222 ± 0.008 ±0.016 0.323 −0.101 ± 0.000 ±0.001 0.172 −0.164 ± 0.000 ±0.000 0.292 −0.192 ± 0.000 ±0.000 Mean hyperfine induction
1.000 ± 0.000
0.191 ± 0.003
S2 S3 S4 S5 S D1 D
Quadrupole shift/splitting [mm/s]
0.382 ±0.004
for the stabilization of its structure) (Cornell and Schwertmann, 1996; Ishikawa and Inouye, 1975; Music et al., 2004; Ohyabu and Ujihira, 1981; Saric et al., 1998; Shao et al., 2005). At temperatures above 800 °C, the sample started to crystallize as was supported by XRD
Hyperfine induction [T]
Area ratio line 2 to 1
Area ratio line 3 to 1
Linewidth 1 (and 6) [mm/s]
50.75 ± 0.05 49.17 ± 0.08 45.81 ± 0.00 26.33 ± 0.00 39.00 ± 0.00 39.69 ± 0.50
0.667 ± 0.000 0.667 ± 0.000 0.667 ± 0.000 0.667 ± 0.000 0.667 ± 0.000
0.333 ±0.000 0.333 ±0.000 0.333 ±0.000 0.333 ±0.000 0.333 ±0.000
0.305 ± 0.016 0.332 ± 0.007 0.749 ± 0.000 6.120 ± 0.000 0.884 ± 0.000
1.000 ± 0.000
Linewidth 2 (to 5) [mm/s]
Phase
α-Fe2O3 α-Fe2O3 ε-Fe2O3 ε-Fe2O3 ε-Fe2O3
0.305 ±0.007
analysis. The emanation thermal analysis made it possible to characterise the transport properties of the material obtained after acid treatment of the vermiculite sample. The development of microstructure in these samples has been characterized by
Fig. 4. DTA,TG, EGA and ETA curves of ground vermiculite after acid treatment.
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Fig. 5. Nitrogen adsorption–desorption isotherm of ground vermiculite after acid treatment anneded at 850 °C.
Fig. 6. Pore distribution of ground vermiculite after acid treatment annealed at 850 °C.
changes in the measured radon release rate (Balek et al., 2002, 2007; Perez-Maqueda et al., 2003; Poyato et al., 2002). The crystallization of amorphous SiO2 was reflected on the ETA curve as a decrease of the radon release rate in the temperature range from 950 to 1050 °C. The results of porosity measurements of the samples obtained by heating the starting sample at 850 °C are shown in Figs. 5 and 6. The observed hysteresis loop matched well to the H2 type, which corresponded to the pores with narrow and wide sections and possible interconnecting channels. The pore distribution is shown in
Table 2 Surface area and porosity values of the annealed samples to temperatures 850, 950 and 1050 °C. Temperature [°C]
2
BET surface area [m /g] BJH pore volume [cm3/g] BJH pore diameter [nm] Average pore size [nm] Total pore volume [cm3/g]
850
950
1050
85.70 0.133 3.65 6.06 0.129
9.02 0.057 3.82 21.53 0.049
1.91 0.020 3.86 33.39 0.018
Fig. 6. The results of surface area and porosity measurements of samples obtained by heating the starting sample at 850, 950, and 1050 °C are given in Table 2. Significant changes were observed in surface area and porosity of the samples heated to these temperatures. The sample prepared at 850 °C had a surface area 85.7 m2/g with pronounced porosity, while the sample obtained at 1050 °C was more compact with surface area only 1.9 m2/g and negligible porosity. The character of the hysteresis loop remained the same for all three samples corresponding well to the H2 hysteresis loop type. The morphology of the starting sample is shown in Fig. 7. On the micrographs, it was observed that spindles of β-FeOOH were embedded in nanoparticulate SiO2 material. A detailed description of the microstructure of this sample has been previously published (Maqueda et al., 2009). The morphology of the sample annealed at 950 °C is given in Fig. 8. The sample showed darker acicular particles of iron oxide, still amorphous, in the SiO2 material (Fig. 8a and b), which started to show signs of crystallization on the HRTEM images (Fig. 8c and d). The signs of SiO2 crystallization were also visible on the XRD pattern of the sample annealed at 950 °C (see Figs. 1 and 2). The EDX analysis confirmed the presence of Fe in the darker particles and Si as the main
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Fig. 7. TEM micrographs of ground vermiculite after acid treatment: (a) typical cluster of amorphous silica and akaganeite particles; (b) isolated akaganeite particle; (c) and (d) akaganeite particle embedded in silica materials.
Fig. 8. TEM micrographs of ground vermiculite after acid treatment annealed at 950 °C.
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Fig. 9. TEM micrographs of ground vermiculite after acid treatment annealed at 1050 °C.
component in the amorphous materials. The particles which did not show the presence of darker needle were formed exclusively of SiO2, whereas the remaining regions in which darker needles were present contained, except for Si, mainly Fe. It was also observed in the EDX spectra that a minor mixture of Ti accompanied Fe in all the particles and it is not present in the Si-rich particles. The Cl− anion was not present in this sample. This element escaped completely during annealing of the sample at about 950 °C. After annealing at 1050 °C, the sample was well crystallized, as demonstrated by the XRD pattern (see Figs. 1 and 2). Fig. 9 shows the morphology of the sample annealed at 1050 °C. The micrograph showed dark Fe2O3 particles with shapes from almost spherical to acicular, or irregular, embedded into homogeneous crystalline SiO2 (Fig. 9a and b). The well-developed crystallinity of both SiO2 and Fe2O3 was well visible on the HRTEM images (Fig. 9c and d, respectively). Element mapping of a single particle of the composite obtained by annealing the sample at 1050 °C (see Fig. 10) showed well-separated Fe2O3 and SiO2 particles in a composite material. From Fig. 10, it followed that impurities of Al and Mg (from the original vermiculite) accompanied the silica components. TiO2 was associated with Fe2O3 grains. It is known that nanoparticles of amorphous Fe2O3 usually crystallize into nanocrystalline maghemite (γ-Fe2O3) and this transformation is normally characterized by an exothermic peak at 280 °C. However, it has been reported that Fe2O3–SiO2 amorphous nanoparticulate systems show a major difference from the case of pure amorphous nanoparticles of Fe2O3, in spite of the fact that the same transformation phase product (i.e., γ-Fe2O3) was observed. The difference appears in the shifting of the transformation temperature toward a higher crystallization temperature (ca. 700 °C) (Khalil et al., 2008). The shifting of the crystallization temperature toward a higher
temperature and the consequent stabilization of the amorphous Fe2O3 nanophase are attributed to what is called the preventive role of the silica matrix. Consequently, well-crystallized iron oxide nanoparticles embedded into the silica matrix are formed usually in the range of 600–1000 °C (Bhaumik et al., 2005; Bourlinos et al., 2001; Braileanu et al., 2006; Cannas et al., 2001, 2002; Ennas et al., 1998; Moreno et al., 2002; Niznansky et al., 1995; Popovici et al., 2005; Savii et al., 2000; Solinas et al., 2001). This temperature range is higher in comparison to silica-free iron oxides (Balek and Subrt, 1994; Perez-Maqueda et al., 1999, 2002). As one can conclude from the published data, the formation of ε-Fe2O3 is rather frequent in the presence of amorphous or nanocrystalline silica, up to relatively high temperatures (Brazda et al., 2009; Lancok et al., 2007; Mori et al., 2008; Sakurai et al., 2008; Tadic et al., 2008). Recently, the εFe2O3 phase has attracted much attention due to its enormous roomtemperature coercivity of about 2 T, and the coupling of magnetic and dielectric properties, the so-called magneto-electric response (Gich et al., 2006; Kurmoo et al., 2005). 4. Conclusions The material obtained after the leaching of ground vermiculite consisted of amorphous silica and akaganeite. The heating of this mixture transformed the crystalline akaganeite to an amorphous phase about 300 °C. New crystalline phases appeared above 800 °C, and were fully developed at 1050 °C, consisting of quartz, crystobalite and iron oxides. The shifting of the crystallization temperature of iron oxides and the consequent stabilization of the amorphous Fe2O3 nanophase were attributed to what is called the preventive role in the silica matrix. The HRTEM showed that the annealing of ground and acid treated vermiculite resulted in a composite consisting of Fe2O3 and SiO2. The composite obtained at 1050 °C consisted of separate
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Fig. 10. Elemental mapping of a single particle of ground vermiculite annealed at 1050 °C.
grains of Fe2O3 embedded into a SiO2 matrix. The crystalline SiO2 material was formed by hexagonal quartz and tetragonal cristobalite, whereas the crystalline Fe2O3 grains consist of ε-Fe2O3 and α-Fe2O3 (hematite), as was confirmed by XRD and Mösbauer spectroscopy. The detectable impurities of Al and Mg accompanied the silica component, whereas TiO2 was associated with Fe2O3 grains. The composite materials obtained in this wok by simple and cheap route from natural raw materials could be of interest from a practical point of view. Acknowledgments The authors wish to thank the Spanish Comisión Interministerial de Ciencia y Tecnología (CICYT) for the financial support under the project no. AGL-2005-0164, Ministry of Education, Youth and Sport of the Czech Republic (project no. LC 523), and the Exchange Program between the Czech Academic and the Consejo Superior de Investigaciones Científicas (CSIC, Spain).
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