untitled European Journal of Chemistry 6 (3) (2015) 314‐318 European Journal of Chemistry ISSN 2153‐2249 (Print) / ISSN 2153‐2257 (Online)  2015 Atlanta Publishing House LLC ‐ All rights reserved ‐ Printed in the USA http://dx.doi.org/10.5155/eurjchem.6.3.314‐318.1276 European Journal of Chemistry Journal webpage: www.eurjchem.com Synthesis, characterization and ionic conductivity of MgAl2O4 Abdelhakim Elmhamdi and Kais Nahdi * Laboratoire d'Application de la Chimie aux Ressources et Substances Naturelles et à l'Environnement (LACReSNE), Faculté des Sciences de Bizerte, Université de Carthage, 7021 Bizerte, Tunisie * Corresponding author at: Laboratoire d'Application de la Chimie aux Ressources et Substances Naturelles et à l'Environnement (LACReSNE), Faculté des Sciences de Bizerte, Université de Carthage, 7021 Bizerte, Tunisie. Tel.: +216.72.590717. Fax: +216.72.590566. E‐mail address: k_nahdi@yahoo.fr (K. Nahdi). ARTICLE INFORMATION ABSTRACT DOI: 10.5155/eurjchem.6.3.314‐318.1276 Received: 01 June 2015 Accepted: 28 June 2015 Published online: 30 September 2015 Printed: 30 September 2015 Magnesium aluminate nanoparticles have been synthesized by a citrate sol‐gel route. X‐ray diffraction and IR spectroscopy study confirmed the formation of MgAl2O4 cubic spinel structure without presence of any secondary phase. Crystallite size of the synthesized nanoparticles was found to be equal to 24 nm. The ac conductivity of MgAl2O4 was studied using complex impedance spectroscopy technique in the frequency range from 5 Hz to 13 MHz and temperature range from 673 to 798 K. The temperature and frequency dependence of ac conductivity were highlighted and the activation energies of, respectively, ac conduction and relaxation processes were also calculated. KEYWORDS MgAl2O4 Jonscher law Citrate sol‐gel Ac conductivity Activation energy Impedance spectroscopy Cite this: Eur. J. Chem. 2015, 6(3), 314‐318 1. Introduction The spinel phase is a mixed oxide with general formula of AB2O4. It is an association between one divalent cation (A2+), and two trivalent cations (B3+). The unit cell consists of a face‐ centered cubic arrangement of oxygen ions containing 32 O ions, 64 tetrahedral and 32 octahedral sites which are occupied with A2+ and B3+ cations [1,2]. When one half of the octahedral interstices are occupied by the B3+ cations and one‐ eighth of the tetrahedral sites are occupied by the A2+ cations the spinel phase is called “normal”. Whereas, when tetrahedral sites are occupied by half of the B3+ and octahedral sites are occupied by the other half and A2+ the spinel phase is called “inverse”. Spinel phase with chemical formula MgAl2O4 is the mineral type of this oxide family. Thanks to its high thermal stability (melting point at 2135 °C), high hardness (16 GPa), high mechanical resistance, high resistance against chemical attack, wide band gap energy, high electrical resistivity, relatively low thermal expansion coefficient (9.10‐6 1/°C) between 30 and 1400 °C, low dielectric constant (ε = 7.5), low density 3.58 g/cm3, high thermal shock resistance, hydrophobicity and low surface acidity, MgAl2O4 has been widely used in various applications such as in metallurgical, electrochemical, radiotechnical and chemical industrial fields [3‐10]. Several studies have focused on the preparation of MgAl2O4 phase using different methods such as, conventional solid‐state‐reaction, sol‐gel, spray drying (atomization), co‐ precipitation, microwave, hydroxide co‐precipitation, hydro‐ thermal, modified pechinic process, and organic gel‐assisted citrate complexation [5,6,11‐42]. The preparation method affects not only the purity and the reactivity of the obtained MgAl2O4 but also the physical properties such as the electrical conductivity. Few works have focused on the investigation of the electrical properties of MgAl2O4 [43‐47]. The first work [43] have studied the electrical properties of both thin film and bulk MgAl2O4 at 313 K and in environments at different relative humidity values between 2 and 95%, using ac impedance spectroscopy in 0.1 Hz ‐ 10 kHz frequency range. The second work [44] deals with the study of conductivity and dielectric properties of magnesium aluminate nanoparticles in the temperature range 303‐450 K and frequency range 200 Hz ‐ 4 MHz. Recently the dielectric properties [45‐47], DC conduc‐ tivity [45] and AC conductivity [47] of MgAl2O4 were investi‐ gated. The experimental conditions and the principal results of the above mentioned works are summarized in Table 1. Our work is a contribution to the investigation of the electrical properties of MgAl2O4 carried out in a larger frequency range from 5 Hz to 13 MHz and in higher tempe‐ rature range from 673 to 798 K. Hakim and Nahdi / European Journal of Chemistry 6 (3) (2015) 314‐318 315 Table 1. Literature data for electrical conductivity study of MgAl2O4. Frequency Temperature (K) Ea (eV) Reference 0.1 Hz ‐ 10 kHz 313 ‐ [43] 200 Hz ‐ 4 MHz 303‐450 ‐ [44] 1 khz ‐ 1 MHz 293‐673 0.770 [45] 200 Hz ‐ 100 kHz Room temperature ‐ [46] 0.01 Hz ‐ 10 MHz 423‐573 E1 = 0.1828 (T < 503 K), E2 = 0.8791 (T > 503 K) [47] The AC conductivity was calculated. The temperature and frequency dependence of ac conductivity were highlighted and the activation energies of, respectively, ac conduction and relaxation processes were also calculated. 2. Experimental 2.1. Instrumentations X‐ray diffraction (XRD) patterns have been recorded with a Bruker D8‐Advance diffractometer using CuKα radiation (λ = 1.5406 Å). XRD data have been collected over the 15‐70 ° 2θ range, at the scan speed of 1.2 °/min and 0.02 ° step. The crystalline phases have been identified using the International Centre for Diffraction Data (ICDD) powder diffraction files. Fourier transformed infrared (FT‐IR) spectrum has been obtained with a Bruker spectrometer, in the 4000‐400 cm−1 range, using the KBr pellet technique. Electrochemical Impedance Spectra (EIS) were obtained using a Hewlett‐ Packard HP 4192 analyser. The impedance measurements were taken in an open circuit using two electrode configurations with signal amplitude of 50 mV and a frequency band ranging from 5 Hz to 13 MHz. Powder have been pressed under 5 tons/cm2 and sintered at 1523K to prepare the pellet. Both pellet surfaces were coated with silver pastes electrodes while the platinum wires attached to the electrodes were used as current collectors. Electrodes have been then heated at 1028 K to ensure good electrical contacts. All these measurements were performed at equilibrium potential at high temperature ranging between 673 and 798 K in air atmosphere. In order to obtain the bulk, grain boundary and total ionic conductivities, the resulting data was analysed using the equivalent circuit of the Zview software [48]. Conductivity values have been estimated with help of the following relation: (1) where R is the resistance deduced from impedance diagrams and S and l are the area and the thickness of pellets, respectively. The temperature dependence of conductivity was calculated using the following Arrhenius Equation (2), (2) where A is the pre‐exponential factor (which is related to the effective number of mobile species), Ea the activation energy of the conduction process, K Boltzmann’s constant and T the absolute temperature. 2.2. Synthesis Magnesium chloride (MgCl2.6H2O), aluminum chloride (AlCl3) and citric acid ((HOCO‐CH2)2C(OH)‐COOH) were used as starting chemicals for the preparation of MgAl2O4spinel phase particles by citric assisted gel‐combustion method. Magnesium and aluminum chlorides were dissolved, in appropriate molar ratio, in distilled water then an excess of citric acid, initially dissolved in water, was added. The mixture was kept under stirring at 80 °C until complete evaporation of the liquid phase. The resulting gel was dried overnight at 100 °C then calcined at 900 °C for 4 h. 3. Results and discussion 3.1. X‐ray diffraction studies Figure 1 is represented the powder X‐ray diffraction pattern of the prepared MgAl2O4 obtained after heat treatment of the citric precursor at 900 °C. The observed diffraction peaks centered at 4.66, 2.84, 2.42, 2.01, 1.65, 1.55, 1.43 and 1.36 Å corresponds to the standard JCPDS Card No77‐0435 data of a pure crystalline phase of MgAl2O4 spinel structure. The calculated lattice parameter of MgAl2O4 is found to be 8.0831 Å which is close to the reported value of the magnesium aluminate. The space group Fd‐3m is so confirmed. The average crystallite size of the prepared annealed sample was determined from X‐ray line broadening using the Scherrer formula and it is found to be 24 nm. Figure 1. X‐ray diffraction pattern of prepared MgAl2O4. The observed diffraction peaks at 4.66, 2.84, 2.42, 2.01, 1.65, 1.55, 1.43 and 1.36 Å are attributed to the lattice plane (111), (220), (311), (400), (422), (511), (440) and (531), respectively, and these are well matched with the reported values [40,41,49]. 3.2. Infrared spectroscopy (FT‐IR) Figure 2 is represented the IR spectrum of the prepared MgAl2O4 obtained after heat treatment of the citric precursor at 900 °C. It exhibits two absorption bands centered at 512 and 685 cm‐1. These bands are attributed to the stretching vibration of Mg‐O in MgO4 tetrahedral and of Al‐O in AlO6 octahedral groups, respectively, in the MgAl2O4 spinel structure [40‐42]. 3.3. Electrical conductivity 3.3.1. Complex electrical impedance analysis Typical complex impedance spectra (Nyquist plots: ‐Z'' vs Z') of MgAl2O4, in the frequency range from 13 MHz to 5 Hz 316 Hakim and Nahdi / European Journal of Chemistry 6 (3) (2015) 314‐318 and temperature range from 673 to 798 K, are shown in Figure 3. Figure 2. FT‐IR spectrum of prepared MgAl2O4. Figure 3. Nyquist plots of MgAl2O4 at different temperature. The impedance spectra of the pellet prepared sample, at different temperature, are characterized by a complete single semicircular arc. The absence of a second semicircle at higher frequency in complex impedance plots suggests the dominance of bulk over the grain boundary contribution in the studied frequency range. The decreases of curvature radius when temperature increases, observed in Figure 3, can be attributed to the presence of single electrical relaxation phenomena in the material under investigation [50], the bulk resistance of MgAl2O4 decreases and hence the electrical conductivity increases. The analysis results of the impedance spectra are shown in Table 2. Here, the bulk resistance was calculated from the intercept of the semicircle with the real axis and the total conductivity of MgAl2O4 pellet was calculated using Equation (1). The conductivity of the studied sample reaches the value of 8.44970×10‐6 S/cm at 798 K. Table 2. Resistance and conductivity of MgAl2O4 at different temperatures. Temperature (K) Resistance () Conductivity (.10‐6, S.cm‐1) 673 1.2486×105 0.0755865 698 93118 1.01301 723 52443 1.80005 748 29118 3.24198 773 17046 5.53796 798 11172 8.44970 3.3.2. AC Conductivity AC conductivity measurement is an important tool for studying the ionic transport properties of materials. When an ac electric current passes through the solid electrolyte, processes like ion motion through bulk of the electrolyte, charge transfer across the electrode‐electrolyte interface, etc., take place. 3.3.2.1. Temperature dependence of AC conductivity Figure 4 shows the Arrhenius plot of Ln(T) vs 1000/T. The activation energy calculated from the slope of the Arrhenius plot was found equal to 1.11 eV. The goodness of the fit (r = 0.9992) all along the studied temperature range indicates the existence of a unique conduction mechanism due to the transfer of electrons between Al3+ and Mg2+ ions through hopping conduction mechanism. Figure 4. Arrhenius plot of the electronic conductivity of MgAl2O4. 3.3.2.2. Frequency dependence of AC conductivity Figure 5 illustrates the frequency () dependence of ac conductivity (ac) at different temperatures of MgAl2O4 sample. The corresponding curves exhibit two distinctly frequency regions which can be separated by a change in the slope for each temperature. Figure 5. Frequency dependence of AC conductivity at different tempera‐ tures. The frequency at which the slope of ac conductivity changes is known as the “hopping frequency, ωp”. It is Hakim and Nahdi / European Journal of Chemistry 6 (3) (2015) 314‐318 317 important to note that the values of hopping frequencies are observed to go up with the increase in temperature. Figure 5 shows that σac increases with increasing frequency and becomes independent of frequency after a certain value. At higher frequencies, the values of ac electrical conductivity become closer, and are temperature‐independent. The frequency dependence of conductivity displays two different regimes that can be described using the Jonscher’s universal power law [51]: 0 (3) where (0) is the dc conductivity obtained from the extrapolation to zero frequency of ac(), A is a constant and s is the frequency exponent describing ion‐ion interactions in hopping process; i.e., the exponent n would be 0 for random ion hopping (absence of interactions) whereas it tends to1 for fully correlated ion motions [52,53]. Figure 6 shows the dependence of the imaginary part of impedance (‐Z'') of MgAl2O4 on frequency at different temperatures. Each curve shows a maximum that shifts to higher frequencies when temperature increases. Figure 6. Frequency dependence of the imaginary part (‐Z''). The appearance of this maximum at a particular frequency (fmax) is an indication of a thermally activated electrical relaxation process in the material when temperature increases. The broadening of peaks on increasing temperature confirms the existence of temperature dependent relaxation phenomena in MgAl2O4. The impedance data are used to evaluate the relaxation time (τ) of the electrical phenomena in MgAl2O4 using the following relation: (4) where fmax is the relaxation frequency. The variation of τ with temperature is shown in Figure 7. This plot shows a steady increase in the relaxation time with rising temperature. This result suggests the presence of temperature‐dependent electrical relaxation phenomena in the material, possibly due to the migration of species/defects. The curve was found to follow the Arrhenius relation: (5) where τ0 is a pre‐exponential factor, Eaτ is the activation energy of the relaxation phenomena, K is the Boltzmann constant and T is the absolute temperature. Activation energies (Eaτ) estimated from the slope of the ln(τ) against 1000/T curve is found equal to 0.98 eV. Figure 7. Dependence of the relaxation time with inverse of temperature in MgAl2O4. 4. Conclusion MgAl2O4 of high purity and 24 nm grain size has been prepared by citric assisted gel‐combustion method. The characterization of the obtained powder at 900 °C by XRD and FT‐IR techniques confirms the obtaining of a pure nano‐ crystalline of MgAl2O4 with spinel structure. The electrical properties of MgAl2O4 were studied using complex impedance spectroscopy. The analysis of the impedance data showed that (i) The calculated conductivity value of nanocrystalline MgAl2O4 pellet at 798 K is found to be 8.4497×10‐6 S/cm. (ii) The corresponding activation energy follows the Arrhenius law, which was found equal to 1.11 eV. (iii) The relaxation frequencies shift to a higher frequency side with increasing temperature. (iv) The activation energy of the relaxation process is equal to 0.98 eV and (v) The frequency dependence of ac conductivity data follows the ‘universal’ power Jonscher's law in MgAl2O4. Acknowledgements The authors thank Dr. Abdelwahab Inoubli and Dr. Massoud Kahlaoui (Laboratoire de Physique des Matériaux, Faculté des Sciences de Bizerte‐Université de Carthage) for the conductivity measurements. The authors would also like to thank the language expert Samar Amri Epouse Dridi for proofreading the manuscript. References [1]. Sickafus, K. E.; Wills, J. M. J. Am. Ceram. Soc. 1999, 82(12), 3279‐3292. [2]. Wyckoff, R. W. G. Wiley Interscience, New York, 1964. [3]. Yamakawa, A.; Hashiba, M.; Nurishi, Y. J. Mater. Sci. 1989, 24, 1491‐ 1498. [4]. Ganesh, I.; Srinivas, B.; Johnson, R.; Saha, B. P.; Mahajan, Y. R. Br. Ceram. Trans. 2002, 101, 247‐254. [5]. Ganesh, I.; Johnson, R.; Rao, G. V. N.; Mahajan, Y. R.; Madavendra, S. S.; Reddy, B. M. Ceram. Int. 2005, 31, 67‐74. [6]. Adak, A. K.; Saha, S. K.; Pramanik, P. J. Mater. Sci. Lett. 1997, 16, 234‐ 235. [7]. Ganesh, I.; Bhattacharjee, S.; Saha, B. P.; Johnson, R.; Rajeshwari, K.; Sengupta, R.; Ramanarao, M. V.; Mahajan, Y. R. Ceram. Int. 2002, 28(3), 245‐253. [8]. Bhaduri, S.; Bhaduri, S. B. Ceram. Int. 2002, 28(2), 153‐158. [9]. Ji‐Guang Lee, T. I.; Lee, J. H.; Mori, T. J. Am. Ceram. Soc. 2000, 83(11), 2866‐2868. [10]. Baudin, C.; Martinez, R.; Pena, P. J. Am. Ceram. Soc. 1995, 78, 1857‐ 1862. [11]. Ryshekewitch, E. Oxide Ceramics, Academic Press, New York 1960, pp. 257. [12]. Kriegel, W. W.; Palmer III, H.; Choi, D. M. Spec. Ceram. 1964, 3, 167‐ 186. [13]. Bakker, W. T.; Lindsay, L. G. Am. Ceram. Soc. Bull. 1967, 46, 1094‐ 1097. 318 Hakim and Nahdi / European Journal of Chemistry 6 (3) (2015) 314‐318 [14]. Bailey, J. T.; Russell, R. Am. Ceram. Soc. Bull. 1968, 47, 1025‐1029. [15]. Hamono, K.; Kanzaki, S. J. Ceram. Soc. Jpn. 1977, 85, 225‐230. [16]. Kostic, E.; Momcilovic, L. Ceramurgia Int. 1977, 3, 57‐60. [17]. Teoreanu, I.; Ciocea, N. Interceram. 1987, 4, 19‐21. [18]. Park, H. C.; Lee, Y. B.; Oh, K. D.; Riley, F. L. J. Mater. Sci. Lett. 1997, 16, 1841‐1844. [19]. Sarkar, R.; Das, S. K.; Banerjee, G. Ceram. Int. 1999, 25, 485‐489. [20]. Sarkar, R.; Banerjee, G. J. Eur. Ceram. Soc. 1999, 19, 2893‐2900. [21]. Zografou, C.; Reynen, P.; Van Mallinckrodt, D. Interceram. 1983, 38, 37‐39. [22]. Zografou, C.; Reynen, P.; Van Mallinckrodt, D. Interceram. 1983, 38, 40‐43. [23]. Serry, M. A.; Hammad, S. M.; Zawrah, M. F. Br. Ceram. Trans. 1998, 97, 275‐282. [24]. Domanski, D.; Urretavizcaya, G.; Castro, F. J.; Gennan, F. C. J. Am. Ceram. Soc. 2004, 87, 2020‐2024. [25]. Angappan, S.; Berchmans, L. J.; Augustin, C. O. Mater. Lett. 2004, 58, 2283‐2289. [26]. Chen, S. K.; Cheng, M. Y.; Lin, S. J. J. Am. Ceram. Soc. 2002, 85, 540‐544. [27]. Ghosh, A.; Das, S. K.; Biswas, J. R.; Tripathi, H. S.; Banerjee, G. Ceram. Int. 2000, 26, 605‐608. [28]. Ganes, I.; Bhattacharjee, S.; Saha, B. P.; Johnson, R.; Mahajan, Y. R. Ceram. Int. 2001, 27, 773‐779. [29]. Li, J. G.; Ikegami, T.; Lee, J. H.; Mori, T. J. Am. Ceram. Soc. 2000, 83, 2866‐2868. [30]. Yang, N.; Chang, L. Mater. Lett. 1992, 15, 84‐88. [31]. Behera, S. K.; Barpanda, P.; Pratihar, S. K.; Bhattacharyya, S. Mater. Lett. 2004, 58, 1451‐1455. [32]. Zhang, H.; Jia, X.; Liu, Z.; Li, Z. Mater. Lett. 2004, 58, 1625‐1628. [33]. Debsikdar, J. C. J. Mater. Sci. 1985, 20, 4454‐4458. [34]. Naskar, M. K.; Chatterjee, M. J. Am. Ceram. Soc. 2005, 88, 38‐44. [35]. Guo, J.; Lou, H.; Zhao, H.; Wang, X.; Zheng, X. Mater. Lett. 2004, 58, 1920‐1923. [36]. Bratton, R. J. Ceram. Bull. 1969, 48(8), 759‐762. [37]. Ayman, A. A.; Mahmud, R. A.; Teymur, M. I. Casp. J. Appl. Sci. Res. 2013, 2(2), 85‐90. [38]. Kakooei, S.; Rouchi, J.; Mohammad, E.; Alimanesh, M.; Dehzangi, A. Casp. J. Appl. Sci. Res. 2012, 1(13), 16‐22. [39]. Pacurariu, C.; Lazau, I.; Ecsedi, Z.; Lazau, R.; Barvinschi, P.; Marginean, G. J. Eur. Ceram. Soc. 2007, 27, 707‐710. [40]. Saberi, A.; Golestani‐Fard, F.; Sarpoolaky, H.; Willert‐Porada, M.; Gerdes, T.; Simon, R. J. Alloy. Compd. 2008, 462, 142‐146. [41]. Marakkar, P. V. K.; Subrata, D. Ceram. Int. 2013, 39, 7891‐7894. [42]. Alvar, E. N.; Rezaei, M.; Alvar, H. N. Powder Technol. 2010, 198, 275‐ 278. [43]. Gualtiero, G.; Giampiero, M.; Enrico, T. J. Am. Ceram. Soc. 1993, 76, 743‐750 [44]. Kurien, S.; Sebastian, S.; Mathew, J.; George, K. C. Indian J. Pure Appl. Phys. 2004, 42, 926‐933. [45]. Iqbal, M. J.; Ismail, B. J. J. Alloys Compd. 2009, 472, 434‐440. [46]. Saha, S.; Das, B.; Mazumder, N.; Bharati, A.; Chattopadhyay, K. K. J. Sol‐ Gel Sci. Technol. 2012, 61, 518‐526. [47]. Padmaraj, O.; Venkateswarlu, M.; Satyanarayana, N. Ceram. Int. 2015, 41, 3178‐3185. [48]. Massoud, K.; Sami, C.; Abdelwahab, I.; Adel, M.; Chaabane, C. Ceram. Inter. 2013, 39, 3873–3879 [49]. Pacurariu, C.; Lazau, I.; Ecsedi, Z.; Lazau, R.; Barvinschi, P.; Marginean, G. J. Eur. Ceram. Soc. 2007, 27, 707‐710. [50]. Molak, A.; Paluch, M.; Pawlus, S.; Klimontko, J.; Ujma, Z.; Gruszka, I. J. Phys. D Appl. Phys. 2005, 38, 1450‐1460. [51]. Jonscher, A. K. Nature 1977, 267, 673‐679. [52]. Funke, K. J. Non‐Cryst. Solids 1994, 1215, 172‐174. [53]. Ngai, K. L.; Tsang, K. Y. Phys. Rev. E 1999, 60, 4511‐4517.