Influence of Zn 2+ or Cu 2+ on Reduction and Recalcination Behavior of Fe 2 TiO 5

Transcription

1 J. Mater. Sci. Technol., Vol.23 No.5, Influence of Zn 2+ or Cu 2+ on Reduction and Recalcination Behavior of Fe 2 TiO 5 M.Bahgat 1), M.H.Khedr 2) and H.S.Abdelmaksoud 2) 1) CMRDI, P.O.Box 87-Helwan, Cairo, Egypt 2) Chemistry Department, Faculty of Science, Benisuef University, Egypt [Manuscript received November 21, 2006, in revised form March 7, 2007] Fe 2 O 3, TiO 2, CuO and ZnO powders were mixed according to the formula of (1-x)TiO 2 xcuo-fe 2 O 3 or (1-x)TiO 2 xzno-fe 2 O 3 (x=0, , 0.6, 0.8, 1), and well ball-milled with H 2 O for 3 h to ensure homogeneity of the powdered solids, then fired at 1200 C for 4 h. The fired samples were reduced at 500 C with hydrogen gas. The reduced samples were subjected to recalcination at 500 C in CO 2 atmosphere. Both of fired, reduced and calcined samples were characterized by X-ray diffraction, vibrating sample magnetometry, reflected light microscopy and scanning electron microscopy. Different phases were formed after firing of Cu +2 or Zn 2+ substituted Fe 2 TiO 5. Magnetization (B s ) of the formed phases after firing are very low corresponding to diluted magnetic semiconductors (DMS) and increases with increasing the substituted cations (Cu +2 or Zn 2+ ). The reduction of the fired samples enhanced the B s values whereas the reducibility increases with increasing the Cu +2 or Zn 2+ content. Samples show different tendency toward CO 2 decomposition which is very important for environmental minimization for CO 2. KEY WORDS: Nanosized ferrite; Titanate; Reduction; Magnetic properties; Reduction-recalcination; CO 2 decomposition 1. Introduction The polycrystalline ferrites have been studied for several years due to their wide use as magnetic materials for telecommunications, audio and video power transformers and many other applications. The crystallographic, electrical and magnetic properties depend strongly on the stoichiometry, preparation condition and particle size [1 12]. Several synthesis methods have been developed to obtain fine particles with controlled shapes and sizes [13,14] and particular magnetic properties [14 16]. Solid-solid interaction between Fe 2 O 3 and transition metal oxides to produce the corresponding ferrites is much influenced by the prehistory of parent oxide, their ratio and effect of additives [17 20]. These solid-solid interactions are controlled by thermal diffusion of reaction cation through the whole mass of each solid [7 16]. Soft ferrites in the CuZnTi system, having the formula Cu 1 x Zn x Ti y Fe 2 y O 4, with 0.50 x 0.60 and 0.00 y 0.05 were investigated as a function of composition, sintering temperature and cooling speed, in order to obtain materials with controlled T C and very high change rate of permeability with temperature around their T [21] C. The effect of Zn and Ti additions was a drastic change of the Curie temperature with about C for each atomic percent of Zn and Ti introduced into the spinel lattice while the cooling speed changed the behaviour of magnetic permeability with temperature around the Curie point. An application using such magnetic temperature sensors for a thermostat is presented. Rahman et al. [22] studied the effect of copper substitution on microstructure and composition related magnetic properties in a series of nominal composition Ni (0.8 x) Zn 0.2 Cu x Fe 2 O 4 (x=0.05, 0.15, 0.15, 0.25, 0.42, 0.45, 0.5, 0.55, 0.6). To whom correspondence should be addressed, m bahgat70@yahoo.com. X-ray diffraction (XRD) patterns of powder samples showed the cubic spinel phase up to x=0.25 and a second phase of composition CuO above x>0.25. The crystallite sizes of as-dried samples when calcined at high temperatures changed from 10 to 50 nm at 1000 C. The partial substitution of Ni 2+ with divalent cations Cu 2+ influenced the magnetic parameters due to the modification by cation distribution. Kim et al. [23,24] reported that Ni x Zn 1 x Fe 2 O 4 δ showed a higher CO 2 decomposition efficiency than NiFe 2 O 4 δ. Recently, Hawang and Wang [25] studied the decomposition of CO 2 on nano-sized Mn-Zn ferrites and Mn-Ni ferrites. The nano-sized Mn-Zn ferrites catalysts showed a better CO 2 decomposition than Mn-Ni ferrites catalysts for the samples with the same Mn content. Shin et al. [26] studied the redox mechanism of (Ni x Cu 1 x )Fe 2 O 4 solid solution by the thermogravimetric analysis and XRD experiments. The rate of reaction was accelerated by a kind of substitution with divalent metals, such as Ni and Cu, where increasing Ni content, samples showed excellent CO 2 decomposition. Despite the great development of studying preparation and characterization of transition metal ferrites, a limited number of investigators studied its reduction and reoxidation behavior and their effect on the magnetic properties. The present work reports a study on the reduction reoxidation behavior of CuO and ZnO substituted Fe 2 O 3 /TiO 2 system at 1200 C and its effect on microstructure and magnetic properties. 2. Experimental Fe 2 O 3, TiO 2, CuO and ZnO powder were mixed according to the formula of (1-x)TiO 2 xcuo-fe 2 O 3 or (1-x)TiO 2 xzno-fe 2 O 3 (x=0, , 0.6, 0.8, 1) and well ball-milled with H 2 O for 3 h to ensure homogeneity of the powdered solids. The mixed powder were

2 698 J. Mater. Sci. Technol., Vol.23 No.5, 2007 Table 1 Phase identification of the mixed oxide with different Cu 2+ concentrations after firing, reduction and reoxidation process as observed in their XRD pattern Cu/Ti Fired Reduced Reoxidized 0:1.0 Fe 2 TiO 5 Fe 3 O 4, Ti 3 O 5, Fe 2 TiO 5 Fe 2TiO 5, Fe 2.25Ti 0.75O 4 0.2:0.8 Fe 2TiO 5, Fe 2O 3, Fe 5CuO 8 Cu, Fe 2TiO 4, FeTiO 3, Fe 2TiO 5, Fe 5CuO 8 Fe 2.75Ti 0.25O 4 0.4:0.6 Fe 2 TiO 5, Fe 2 O 3, Fe 5 CuO 8 Fe, Cu, Fe 2 TiO 4, Cu 0.67 Fe 2.33 O 4 Fe 2 TiO 5, Fe 5 CuO 8 0.6:0.4 Fe 2 TiO 5, Fe 2 O 3, Fe 5 CuO 8, Fe, Cu, Fe 2 TiO 4, Cu 0.67 Fe 2.33 O 4 Fe 2 TiO 4, Fe 2 O 3, Fe 5 CuO 8, Cu 2 TiO 3, Cu Ti Fe O 3 0.8:0.2 Fe 2 O 3, Fe 5 CuO 8, Cu 2 TiO 3, Fe, Cu, Fe 2.75 Ti 0.25 O 4, Fe 2 O 3, Fe 5 CuO 8 Cu 0.667Ti 0.667Fe 0.667O 3 Cu 0.67Fe 2.33O 4 1.0:0 Fe 5CuO 8, CuFeO 2 Fe, Cu, Cu 0.67Fe 2.33O 4 Fe 2O 3, Fe 5CuO 8 the gas starvation. The reduced samples were reoxidized at 500 C using CO 2 gas. The thermogravimetric technique was used to follow up the reoxidation process. 3. Results and Discussion Fig.1 Photomicrograph of sample containing (a) 0.4 mol Cu +2, (b) 1.0 mol Cu +2 compacted into briquettes at 250 kg/cm 2 then fired in a muffle furnace at 1200 C for 4 h. XRD results was obtained at room temperature using Philips PW (1790) with CuKα radiation at 40 kv and 30 ma. The magnetic hysteresis of samples was measured using magnetometer model 9600 LDJ-VSM. The microstructure of the sample was studied by optical microscopy MEIJI 9300-Japan with video camera and scanning electron microscopy (SEM, JEOL-JSM- 5410). The crystallite size was determined from line broadening of reflection using Scherrer s equation, D=Kλ/Bcosθ, where D is the diameter in Å, K is a constant (shape factor), B is the half-maximum line width and λ is the wavelength of the X-rays. This was calculated using software TOPAS 2-Release 2. Isothermal reduction experiment was carried out at 500 C using thermogravimetric balance [PRECISA-SWISS] equipped with vertical tube furnace (heating rate 10 C/min). A gas purification system is used to obtain 99.99% purity hydrogen gas [27]. Preliminary reduction experiments showed that the most suitable hydrogen flow rate required is 11/min to ensure an adequate supply of gas and overcome the gas boundary layer diffusion resistance, thus avoiding 3.1 CuO-substituted Fe 2 O 3 /TiO 2 system Phase identification The XRD diffraction analysis of various solid mixtures after firing, reduction and reoxidation process are shown in Table 1. The results indicate that heating different molar mixture of iron (III), titanium (IV) oxide and copper (II) oxide results in the formation of Fe 2 TiO 5, Fe 5 CuO 8, Cu 2 TiO 3 and CuFeO 2 phases via several solid-state reactions. The average crystallite size of calcined powder as calculated from corresponding pattern using Scherrer formula was nm. It can be observed that the concentration of Fe 5 CuO 8 phase gradually increased at the expense of Fe 2 TiO 5 phase with the increase of CuO addition up to x=1.0. The photomicrographs for polished samples containing 0.4 and 1 mol Cu +2 are shown in Fig.1(a) and (b), respectively. Figure 1(a) shows the formation of two phases. Light gray represents Fe 2 TiO 5 and dark gray phase indicates Fe 5 CuO 8. On the other han Fig.1(b) shows the formation of a composite of two oxides Fe 5 CuO 8 and CuFeO 2 whereas the minor is CuFeO 2 (triangular shape). The fired samples were isothermally reduced at 500 C by using hydrogen gas. The reduction curves are shown in Fig.2. It can be seen that the reduction rate and the reduction extent increases with increasing the added Cu 2+ mole (0 1) due to the presence of easily reduced copper iron oxide phases. It was found that during the reduction process the ferrite phase decomposed to different oxide whereas their subsequent reduction to the metallic phase increased with increasing Cu content due to the easier reduction of copper ferrite phases. The reduced samples with different molar ratios of Cu/Ti were subjected to CO 2 atmosphere at 500 C. Reduced phases were partially and completely reoxidized and carbon is deposited. The reoxidation curves are shown in Fig.3. It was observed that the maximum achieved oxidation extent was about 70% (x=1) while the minimum one was about 15% (x=0). It can be seen that the reoxidation rate and the reoxidation extent increases with increasing the added Cu 2+ meaning higher reoxidation takes place in highly reduced samples. It was found that Fe 2 TiO 5, Fe 5 CuO 8, and Fe 2 O 3 phases were formed after the recalcination.

3 J. Mater. Sci. Technol., Vol.23 No.5, Table 2 Variation of magnetic properties with Cu +2 -substituted Fe 2 TiO 5 after firing, reduction and recalcination Cu +2 H c /Oe B s /(emu/g) B r /(emu/g) F R Reox F R Reox F R Reox Notes: F fired, R reduced, Reox reoxidized Fig.2 Reduction curves of synthesized samples in presence of different molar ratio of CuO (x=cu +2 mol) Fig.4 Photomicrographs for the reoxidized compacts with different Zn 2+ /Ti 4+ molar ratio: (a) x=1, (b) x=0.4 Fig.3 Reoxidation curves of titanium copper Cu ferrite samples with different Cu 2+ /Ti 4+ molar ratio at 500 C with H 2 It was observed that almost all the synthesized phases in the fired samples were reproduced again in limited number after the reoxidation of its reduced products. The photomicrograph of reoxidized samples are shown in Fig.4(a) and (b) for samples containing CuO with molar ratio x=1.0 and x=0.4. Comparatively (Figs.1 and 4) it was observed that reoxidation led to the formation of same phases with sharp grain boundary, and carbon deposition is observed Magnetic properties The comparative magnetization measurements for Cu 2+ substituted Fe 2 TiO 5 samples fired, H 2 - reduced and then reoxidized with CO 2 are summarized in Table 2. It can be seen that as-prepared samples shows very weak magnetic properties. Saturation magnetic flux density (B s ) for fired samples increases from to 38.6 emu/g with the increase of Cu 2+ from 0 to 1 mole. Upon reduction in hydrogen at 500 C, B s shows a considerable change to 1.72 emu/g for Cu 2+ =0 and a drastic increase for reduced samples with the increase of Cu +2. At Cu 2+ =0.6, fired sample shows B s =5.15 emu/g while the reduced samples give emu/g. For samples containing Cu 2+ =1, B s increases from 38.6 for fired to 82.6 emu/g for reduced. This could be attributed to the reduction and formation of metallic phases Fe and Cu as shown in Table 1. After calcinations in CO 2, the reduced samples react with oxygen to form oxides, ferrites and titanate again. B s shows a drastic increase upon oxidation at Cu 2+ =0, it increases from for fired samples to 15.6 emu/g for samples reoxidized after reduction. The same trend is shown as Cu 2+ increases (Table 2), which could be attributed to the formation of more perfect magnetic domains that leads to an increase in magnetization Fig.4. Figure 5 shows coercivities (H c ) of samples before, after H 2 -reduction and after reoxidation. Reduced samples show a sharp increase in the values of H c and the increase becomes more pronounced as Cu 2+ =0.2. H c increases from for fired to 408 Oe for reduced, while at Cu 2+ =1, it increases from to 261 Oe. Reoxidized samples show a less appreciable increase than reduced samples. Also the chart in Fig.6 indicates the change in re-

4 700 J. Mater. Sci. Technol., Vol.23 No.5, 2007 Table 3 Phase identification of the mixed oxide with different Zn 2+ concentrations after firing, reduction and reoxidation process as observed in their XRD pattern Zn/Ti Fired Reduced Reoxidized 0:1.0 Fe 2 TiO 5 Ti 3 O 5, Fe 2 TiO 5, Fe 2.25 Ti 0.75 O 4 Fe 2 TiO 5 0.2:0.8 Fe 2 ZnTi 3 O 10, Fe 3 O 4, Fe 2 TiO 5, ZnFe 2 O 4, FeZnTiO 4, Ti 2O 3 (Zn 0.984Fe 0.015)Fe 1.953O FeZnTiO 4, Fe 2TiO 5 0.4:0.6 ZnTiO 3, Fe 2ZnTi 3O 10, TiO 2, Fe 2TiO 5, ZnFe 2O 4, FeZnTiO 4, Fe 2TiO 5 FeZnTiO 4, Ti 2 O 3 (Zn Fe )Fe O :0.4 Fe 2 ZnTi 3 O 10, Fe 4 Zn 9, TiO 2, Fe 2 TiO 5, (Ti 0.92 Zn 0.08 ) (Zn 1.92 Ti 0.08 )O 4, ZnFe 2 O 4, FeZnTiO 4, Fe 2 TiO 5 FeZnTiO 4, ZnFe 2 O 4 (Zn Fe )Fe O :0.2 Fe 2 ZnTi 3 O 10, Fe 4 Zn 9, Fe 2 TiO 5, (Ti 0.92 Zn 0.08 ) (Zn 1.92 Ti 0.08 )O 4, FeZnTiO 4, ZnFe 2 O 4 FeZnTiO 4, ZnFe 2O 4 (Zn 0.984Fe 0.015)Fe 1.953O :0 ZnFe 2O 4 Fe 4Zn 9, Fe 3O 4, ZnFe 2O 4 ZnFe 2O 4 Fig.5 Coercivity values for Cu 2+ substituted Fe 2 TiO 5 samples after firing (F), reduction (R) and reoxidation (Reox) Fig.7 SEM micrograph for the fired samples with different Zn +2 concentrations: (a) x=0.0 (b) x=0.4 Fig.6 Remanent magnetization values for Cu 2+ substituted Fe 2 TiO 5 samples after firing (F), reduction (R) and reoxidation (Reox) manence magnetic flux density (B r ) for all samples. B r for fired samples are ranged from 0.09 to emu/g as Cu 2+ increases. These values increases to up to 13.6 emu/g for reduced samples, which can be explained by the formation of metallic phases upon reduction with H 2 gas. B r shows a drastic jump by reoxidation and it ranges from 3.37 to 37.4 emu/g. It can also be explained by the formation of the highly perfect magnetic domains. 3.2 ZnO-substituted Fe 2 O 3 /TiO 2 system Phase identification Various solid mixtures of Fe 2 O 3 with different ZnO/TiO 2 molar ratios were fired at 1200 C. The formation of Fe 2 TiO 5, FeZnTiO 4, Fe 2 ZnTi 3 O 10, ZnTiO 3, and ZnFe 2 O 4 phases via several solid-state reactions was detected by XRD as shown in Table 3. The SEM images for the synthesized phases at ZnO ratio x=0.0 and 0.4 are shown in Fig.7(a) and (b), respectively. It was observed that grains (2 6 µm) with different sizes and irregular crystalline shapes are formed in a slightly porous structure (Fig.7(a)). By the presence of ZnO in the fired sample, the average grain size increased (5 6 µm) in more dense structure whereas the macropores decreased obviously (Fig.7(b)). The fired samples with different molar ratios of Zn/Ti were isothermally reduced at 500 C using hydrogen gas. The reduction curves are shown in Fig.8. It can be seen that for all samples, incomplete reduction was achieved whereas the rate of reduction was highest at early stage and gradually decreased with time till the end of the experiment. Also the reduction rate and the reduction extent increase with increasing the added Zn 2+ mole (0 1) whereas the maximum reduction extent (27%) was detected for pure zinc ferrite (ZnFe 2 O 4 ) at Zn 2+ =1 and the minimum reduction extent (13%) was detected at pure iron ti-

5 J. Mater. Sci. Technol., Vol.23 No.5, Fig.8 Reduction curves of Ti Zn ferrite samples with different Zn 2+ /Ti 4+ molar ratio at 500 C with H 2 Fig.9 Photomicrographs for the reduced compacts with different Zn 2+ /Ti 4+ molar ratio: (a) x=0.6, (b) x=1.0 tanate (Fe 2 TiO 5 ) at Zn 2+ =0. This means that the reducibility increases gradually with increasing the Zn content in the ferrite phase due to the formation of easily reduced zinc ferrite phase. It was found that during the reduction process the ferrite and titanate phases decomposed to different metal oxide (TiO 2, Ti 3 O 4, Fe 2 O 3, Fe 3 O 4, ZnO) and other un-reducible ferrite and titanate ((Zn Fe )Fe O 3.938, (Ti 0.92 Zn 0.08 ) (Zn 1.92 Ti 0.08 )O 4, Fe 2.25 Ti 0.75 O 4 ) phases at current reduction temperature. The subsequent reduction of these metal oxides to the metallic phase (Fe, Zn) increased with increasing Zn content due to the easier reduction of zinc ferrite phase as shown in Fig.9(a) and (b) where white area represents the metallic phases formed as a results of reduction. The reduced samples with different molar ratios of Zn/Ti were recalcined at 500 C in CO 2 atmosphere. Fig.10 Reoxidation curves of Ti Zn ferrite samples with different Zn 2+ /Ti 4+ molar ratio (x=0 1) at 500 C in air Partially reduced and metallic phases were oxidized and the reoxidation curves are shown in Fig.10. It was observed that incomplete reoxidation was detected in all calcined samples whereas the maximum attained oxidation extent was about 34% (x=1). Also the reoxidation rate and the reoxidation extent increases with increasing the added Zn +2 whereas the maximum reoxidation extent (34%) was detected at Zn 2+ =1 while the minimum reduction extent (18%) was detected at Zn 2+ =0. This means that the reoxidation extent increases gradually with increasing the reduction extent of the fired samples. It was found that ZnFe 2 O 4, FeZnTiO 4, Fe 2 TiO 5 phases were formed after the recalcination Magnetic properties The measured B s (saturation magnetic flux density), H c (coercive forces ) and B r (remnance magnetic flux density) for Zn 2+ substituted Fe 2 TiO 5 before reduction, after reduction and after reoxidation are shown in Fig.11 and summarized in Table 4. B s shows a very small range values from to 0.72 emu/g which are close to diluted magnetic materials. These decrease in values of B s are attributed to the presence of low magnetic materials such as Fe 2 TiO 5, Fe 2 ZnTi 3 O 10 and TiO 2 which cause a discontinuety of magnetic domain (Fig.7 and Table 4). Reduced samples show an increase in the value of B s which can be explained by the formation of magnetite (Fe 3 O 4 ) and metallic phase (Fe 4 Zn 9 ). By oxidation of the reduced samples by CO 2 a sharp increase in the values of B s is observed from to 15.6 emu/g at Zn 2+ =0 and 0.72 to emu/g at Zn 2+ =1, respectively. This is explained by the disappearance of the less magnetic phase Fe 2 ZnTiO 3 O 10 in the reoxidized samples. The change in remanent magnetization and coercive force for all samples after reoxidation are detected. It was found that H c and B s values are much greater than unreduced samples, and range from 3.37 to 1.28 emu/g compared to up to emu/g for unreduced samples. Reduced and reoxidized samples show much difference composing with fired samples containing Zn , which could be explained by the presence of more reducible phases Fe 4 Zn 9 and Fe 3 O 4 in reduced and ZnFe 2 O 4, FeZnTiO 4 in reoxidized samples.

6 702 J. Mater. Sci. Technol., Vol.23 No.5, 2007 Table 4 Variation of magnetic properties with Zn 2+ substituted Fe 2 TiO 5 after firing, reduction and recalcination Zn 2+ H c /Oe B s /(emu/g) B r /(emu/g) F R Reox F R Reox F R Reox Notes: F fired, R reduced, Reox reoxidized REFERENCES Fig.11 B-H hysterisis loops for samples doped with different molar ratios of Zn +2 ions after reduction with hydrogen at 500 C Fig.12 Photomicrograph for Fe 2TiO 5 sample doped with Zn +2 =0.2 and reduced with hydrogen at 500 C 4. Conclusion Ti-Cu and Ti-Zn mixed ferrite samples were prepared by replacing TiO 2 with calculated amounts of CuO and ZnO (x=0.2, 0.4, 0.6, 0.8 and 1 mole). Fired samples show very low magnetic properties corresponding to diluted magnetic semiconductors (DMS). The reduction rate and reduction extent increases with increasing the added Cu 2+ or Zn 2+ due to the formation of easily reduced copper iron oxide or zinc iron oxide phases. Magnetization increased for reduced samples due to the formation of magnetite (Fe 3 O 4 ) and metallic phases Fe, Cu or Zn. Recalcination led to a better structure than fired and improved magnetic properties due to the disappearance of the less magnetic phases and the formation of more perfect magnetic domains that leads to an increase in magnetization (H c and B r ). CO 2 is catalytically decomposed over the reduced samples which could be considered as a valuable method for the removal of CO 2. [1 ] B.S.Randhawa, S.Kaur and P.S.Bassi: Indian J. Chem. A, 1998, 28, 463. [2 ] L.Dong, Z.Liu, Y.Hu, B.Xu and Y.Chen: J. Chem. Soc. Faraday Trans., 1998, 94(19), [3 ] M.H.Khedr a,., A.A.Omarb and S.A.Abdel-Moaty: Colloids Surf. A: Physicochem. Eng. Aspects, 2006, 281, 8. [4 ] P.Lahiri and S.Sengupta: J. Chem. Soc. Faraday Trans., 1995, 91(19), [5 ] N.K.Singh, S.K.Tiwari, K.I.Anitha and R.N.Singh: J. Chem. Soc. Faraday Trans., 1996, 92(13), [6 ] M.H.Khedr, A.A.Omar, M.I.Nasr and E.K.Sedeek: J. Anal. Appl. Pyrolysis, 2006, 76, 203. [7 ] C.G.Ramoukatty and S.Sugunan: J. Applied Catalysis A, 2001, 218, 39. [8 ] N.J.Jebarthinan and V.Krishnaswamy: Catalysis- Present and Future, eds. P.kanta Rao and B.S.Benwal Wiley, Newdelhi, 1995, 228. [9 ] N.R.E.Radwan and H.G.El-Shobaky: Thermochim. Acta, 2000, 360, 147. [10] G.A.El-Shobaky, N.R.E.Radwan and E.M.Radwan: Thermochim. Acta, 2001, 380, 27. [11] V.M.Bujoreanu and E.Segal: J. Therm. Anal. Calorim., 2000, 61, 967. [12] T.Sato, K.Haneda, M.Seki and T.lijima: Proceedings of the Int. Symposium on Physics of Magnetic Materials, World Scientific, Singapore, 1987, 210. [13] P.Mollard, A.Collomb, J.Devenyl and A.Rouset: J. Paris, IEEE Trans. Mag., 1975, 11, 894. [14] T.Sato, I.Iijima, M.Seki and N.Inagaki: J. Magn. Magn. Mater., 1987, 65, 252. [15] J.A.Toledo, M.A.Valenzuela, P.Bosch, H.Armendariz, A.Monloya, N.Nava and A.Vazquez: Appl. Catal. A, 2000, 198, 235. [16] T.Konvicka, P.Mosner and Z.Sole: J. Therm. Anal. Calorim., 2000, 60, 629. [17] G.A.Kolta. S.Z.El-Tawil, A.A.Ibrahim and N.S.Felix: Thermochim. Acta, 1980, 39, 359. [18] M.H.Khedr: J. Anal. Appl. Pyrolysis, 2005, 73, 123. [19] G.A.El-Shobaky, A.N.Al-Noaimi, A.Abd El-Aal and A.M.Ghozza: Mater. Lett., 1995, 2, 39. [20] G.A.El-Shobaky, M.A.Shouman and M.N.Alaya: Adsorpt. Sci. Technol., 2000, 18, 243. [21] C.Micleaa, C.Tanasoiua, C.Micleaa and V.Tanasoiua: J. Magn. Magn. Mater., 2005, , [22] I.Z.Rahman and T.T.Ahmed: J. Magn. Magn. Mater., 2005, , [23] J.S.Kim and J.R.Ahn: J. Mater. Sci., 2001, 36, [24] J.S.Kim, J.R.Ahn, C.W.Lee, Y.Murakami and D.Shindo: J. Mater. Chem., 2001, 11, [25] C.Hwang and N.Wang: Mater. Chem. Phys., 2004, 88, 258. [26] H.C.Shin, J.H.Oh, J.C.Lee, S.H.Han and S.C.Choi: J. Phys. Stat. Sol., 2002, 189(3), 741. [27] A.A.El-Geassy: J. Mater. Sci., 1986, 21, 3889.