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The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials
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The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials Katerina Zaharieva, Maya Shopska, Ilyana Yordanova, Sonia Damyanova
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Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium aluminate spinel Milling Thermal treatment Characterization
Magnesium aluminate-based materials were prepared by applying different methods: (i) mechanochemical milling of the initial mixture of magnesium and aluminium nitrate powders (in appropriate stoichiometric amounts) followed by heat treatment at temperatures of 650 °C and 850 °C and (ii) melting of the mixture of nitrate precursors at 240 °C followed by thermal treatment at 650 °C, 750 °C and 850 °C. The effect of synthesis method on the structure and morphology of the obtained solids was studied by using various techniques such as: nitrogen adsorption-desorption isotherms, powder XRD, IR spectroscopy and SEM. It was shown that the mechanochemical milling performed before calcination procedure leads to obtaining of nanocrystalline magnesium aluminate spinel phase at lower temperature of 650 °C in comparison with the method using thermal treatment only (at 750 °C). The obtained nanomaterials exhibit mesoporous structure.
1. Introduction Nanostructured materials of an average crystalline size of a few nanometers have been of a great interest in the last decades. These materials exhibit increased strength, hardness and specific heat, as well as improved ductility and reduced density and elastic modulus, etc. The magnesium aluminate spinel (MgAl2O4) is one of the well-known and widely used material. Its solid-state synthesis from magnesia and alumina is revealed by inter-diffusion of cations (3Mg2+ ⇔ 2Al3+) through the product layer between the oxide particles at high temperatures (> 1400 °C) [1]. It possesses fcc structure of oxygen ions with eight molecules per unit cell, in which there are 64 tetrahedral and 32 octahedral sites [2]. The aluminium ions occupy the 16 octahedral sites and the magnesium ions – eight tetrahedral sites in the ideal case [2]. This ceramic material has been found application in the chemistry, metallurgy and electrochemistry because of its refractory properties, good thermal shock resistance and mechanic resistance, high melting point (2135 °C), high chemical inertia, low thermal expansion coefficient, low density and excellent optical and dielectric properties [3,4]. Many researchers have been synthesized the magnesium aluminate by various synthesis methods, for example, such as self-heat-sustained (SHS) technique [5], surfactant assisted precipitation [6], autoignition technique [7], a wet-chemical process [8], co-precipitation [9,10], molten-salt method [11], reactive sintering using bauxite and magnesite [12], nitrate–citrate combustion route [13], sol–gel auto combustion method [14], microwave-assisted combustion synthesis [15], co-
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crystallizing and decomposing of aluminium and magnesium nitrates mixture [2,16], etc. Among the methods for obtaining of nanostructured materials, mechanochemical process is very popular due to its simplicity and low process cost. Synthesis method by mechanochemical activation is an effective way to improve the interaction and contact of the reactants by milling process, which leads to enhancement of the chemical homogeneity of product and to decrease of the severity of thermal treatment [17]. The mechanochemical milling can accelerate the reaction in a multi-component system by significantly decrease of the temperatures of thermal treatment [18]. Many researchers have been used various mechanochemical synthesis methods at different conditions for obtaining magnesium aluminate spinel [1,2,18–25]. Different precursors have been used for obtaining of nanocrystalline magnesium aluminate spinel by mechanochemical activation followed by annealing such as Al2O3 and NgCO3 [1,18,19] or MgCl2 and AlCl3 in the presence of NaO together with a small amount of water [20], etc. Due to the high chemical inertness in the acidic and basic environments, as well as, due to the high thermal resistance at elevated temperatures, the magnesium aluminate spinel has been found wide application in heterogeneous catalysis as carrier of different supported metal oxide catalysts for clean energy production and environmental production. It was shown that MgAl2O4 obtained by co-precipitation of the solutions of the Al and Mg precursor salts, followed by calcination at high temperatures, leads to obtaining of materials with a high surface area suitable as supports for catalysts [26–29]. In the present paper, it was attempted to synthesize magnesium
Corresponding author. E-mail address: [email protected] (S. Damyanova).
http://dx.doi.org/10.1016/j.ceramint.2017.09.176 Received 8 September 2017; Received in revised form 20 September 2017; Accepted 21 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zaharieva, K., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.176
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Fig. 2. Powder X-ray diffractograms of magnesium aluminate samples obtained by Method 2. Fig. 1. Powder X-ray diffractograms of magnesium aluminate samples obtained by Method 1.
molar ratio of 1:2) in agate mortar for 6 h followed by thermal treatment at 240 °C in air for 4 h to obtain a melt. After that, the sample powder was annealed at different temperatures of 650 °C, 750 °C and 850 °C in air for 2 h at each temperature.
aluminate by different methods and to study the effect of synthesis method on the physicochemical properties of the obtained spinel materials. It was studied the influence of the prior segregation of the phase selection and the evolution in thermally decomposed samples synthesized from nitrate precursors. For this purpose, the magnesium aluminate was prepared by mechanochemical treatment for various periods or by melting of the mixture of the salts of magnesium and aluminium nitrates in appropriate amounts with subsequent annealing. The powder X-ray diffraction analysis (PXRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and N2 adsorption-desorption isotherms were performed in order to characterize the synthesized samples.
2.2. Characterization The powder XRD patterns were recorded on a Bruker D2 Phaser diffractometer within the range of 2θ values between 4° and 75° using Cu Kα radiation (λ = 0.154056 nm) at 40 kV (step size of 0.05° and time per step of 1 s). The phases were identified using of JCPDS database (Powder Diffraction Files, Joint Committee on Powder Diffraction Standards, Philadelphia PA, USA, 1997). Infrared spectra of the magnesium aluminate samples were performed on a Nicolet 6700 FTIR spectrometer (Thermo Electron Corporation, USA) in KBr pellet (0.5% studied substance). The spectra were collected in the middle IR region (400–4000 cm−1) using 50 scans at a resolution of 4 cm−1 (data spacing 1.928 cm−1). The SEM investigations were carried out by the scanning electron microscope JSM-5510 at acceleration voltage of 10 kV and different magnifications of 10,000 and 20,000. The samples were prepared by dispersing the powders in acetone. Ultrasonic oscillation for 1 h was
2. Materials and methods 2.1. Sample preparation Magnesium aluminate samples were prepared by two different synthesis methods. The first method labeled as Method 1 was used mechanochemical milling of the initial mixture of powders precursors of Al(NO3)3·9H2O (99% purity, Merck) and Mg(NO3)2·6H2O (99% purity, Merck) in an appropriate stoichiometric amount of 2:1, followed by heat treatment procedure at 650 °C and 850 °C in air. The samples were milled at different times of 1 h, 2 h, 3.30 h and 5.30 h using a high-energy ball milling by planetary ball mill type PM 100, Retsch, Germany. The mechanochemical milling process was performed in air atmosphere at room temperature in a stainless steel milling container of 250 ml and with a rotation speed 390 rpm. The mass ratio between balls and powder was 17:1. The second preparation method denoted as Method 2 was a mixing of initial materials of magnesium and aluminium nitrates powders (in a
Table 1 Calculated values of average crystallite size (D), lattice strain (ε) and unite cell parameter (a) of the magnesium aluminate phase. Sample MgAl2O4-650-Method MgAl2O4-850-Method MgAl2O4-750-Method MgAl2O4-850-Method
2
1 1 2 2
D (nm)
ε × 10−3 (a.u.)
A (Å)
8.3 12.5 7.6 15.0
3.5 2.1 3.6 3.4
8.11 8.09 8.07 8.08
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mixture of nitrates after ball milling for 1 and 2 h. Increasing the milling time up to 3.30 h and 5.30 h leads to change of the XRD spectra of the mixture that could be due to the formation of intermediate compounds with unidentifiable peaks in all the compositions during the mechanical activation process. Since these phases did not exist at temperature treatment ≥ 650 °C, it was presumed that they are complex hydrated nitrates. Peaks of magnesium aluminate spinel phase (PDF-77-1193) at temperature treatments of 650 °C and 850 °C (Fig. 1) are well visible in the XRD of the previously mechanically activated samples. The diffraction lines at 31.4°, 36.9°, 44.8°, 59.5° and at 65.3° of the spinel phase correspond to the planes (220), (311), (400), (511) and (400) reflection, respectively. In addition, it should be noted that traces of iron phase (PDF-870721) are found during the mechanical activation (Fig. 1). The presence of iron phase could be due to the impurities under the strong and continuous contact between the sample and stainless-steel balls and container during the mechanochemical milling. The iron-containing contamination obtained during the mechanical activation was also established by Adhami et al. [30]. The authors have been determined that the incorporation of the iron impurity into the mixture during the milling process leads to formation of some MgFe0.6Al1.4O4 spinel phase [30]. According to the literature [31] spinel powder has been prepared by solid-state reaction, which needs a very high annealing temperature of 1400–1600 °C. However, in the present work it can be seen that the magnesium aluminate spinel can be formed by annealing the 6 h ball milled powder at 650 °C for 2 h due to the higher reactivity and contact surface area of the initial powders. The XRD patterns of co-molted mixture of the magnesium and aluminium nitrates at 240 °C, previously mixed in the agate mortar for 4 h, show unidentified phases (Fig. 2). The heat treatment at 650 °C
Table 2 Textural characteristics of the magnesium aluminate samples obtained by Methods 1 and 2: specific surface area (SBET), total pore volume (Vp) and average pore diameter (Dp). Sample MgAl2O4-650-Method MgAl2O4-850-Method MgAl2O4-750-Method MgAl2O4-850-Method
1 1 2 2
SBET (m2/g)
Vp (cm3/g)
Dp (nm)
92 21 98 51
0.12 0.06 0.13 0.10
5.4 12.0 5.3 7.8
introduced to decrease the aggregation followed by placing a drop of the suspension on holey carbon film supported on copper grids. Specific surface area (SBET), total pore volume (Vpore) and average pore size diameter (Dpore) of the samples were determined by N2 adsorption-desorption isotherms at 77 K with a Micromeritics TriStar 3000 apparatus. The samples were previously out-gassed under vacuum at 150 °C for 24 h. The specific surface area was measured by the method of Brunauer-Emmett-Teller (SBET), and the distribution of pore size was calculated by the method of Barrett-Joyner-Halenda (BJH) by desorption of curves/branches of the isotherms.
3. Results and discussion The powder XRD patterns of the initially salts Al(NO3)3·9H2O and Mg(NO3)2·6H2O, as well as of the samples mixture after mechanical activation for various periods at room temperature with subsequent annealing at 650 °C and 850 °C for 2 h (Method 1), are shown in Fig. 1. The XRD of the co-melted samples at 240 °C for 4 h, followed by the heat treatment at 650 °C, 750 °C and 850 °C (Method 2) are displayed in Fig. 2. From the Fig. 1 it is obvious a structural similarity of the starting
Fig. 3. SEM images at different magnifications of magnesium aluminate obtained by Method 1 at thermal treatments: 650 °C (A) and 850 °C (B).
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Fig. 4. SEM images at different magnification of magnesium aluminate obtained by Method 2 at thermal treatments: 750 °C (A) and 850 °C (B).
precursors before the heat treatment procedure (Method 1) leads to synthesis of magnesium aluminate materials at lower temperature (650 °C) in relation to that observed for samples synthesized by Method 2 (750 °C). The crystallinity of magnesium aluminate samples synthesized by the both methods was established at increasing the calcination temperature. The mean crystallite size (D), lattice strain (ε) and unite cell parameter (a) of magnesium aluminate phase are listed in Table 1. These parameters are calculated by PowderCell 2.4 program [32] and using Williamson–Hall equation [33]:
leads to formation of an amorphous phase of magnesium aluminate spinel like structure. However, increasing the temperature of the heat treatment up to 750 °C and 850 °C leads to formation of well-defined peaks of MgAl2O4 spinel phase (PDF-77-1193). The peaks broadening of the spinel is due to the small grain size formed at 650 °C. Because of the decrease of the grain size, the number of the planes in a crystal that diffract at the same angle is decreased that causes the peak broadening. It can be concluded that the mechanochemical milling of the oxide
β cos θ = 0.9 λ/ D + 4ε sin θ
(1)
where ε is the value of internal strain, β is the full-width half-maximum (FWHM) of diffraction, θ is the Bragg's angle, λ is the wavelength of Xray beam used and D is the average crystallite size of the phase under study. The results in Table 1 show that the synthesized magnesium aluminates by Methods 1 and 2 possess an average crystallite size in the range of 7.6–15.0 nm. As can be seen from the Table 1 the magnesium aluminate samples obtained at lower heat treatment temperatures of 650 °C (Method 1) and at 750 °C (Method 2) have lower mean crystallite size of 8.3 and 7.6 nm, respectively, than those of the materials heated at 850 °C (12.5 nm and 15.0 nm for samples obtained by Methods 1 and 2, respectively). It suggests that there is a gradual increase in the crystallite size of the both series samples with increasing the temperature treatment. The same effect of thermal treatment on the particle size increase was established by Govha et al. [34] and Ewais et al. [35]. The researchers were determined that the particle size and crystallinity of the magnesium aluminate spinel increase with increasing the calcination temperature [34,35]. This reciprocal action can be related to the nucleation, the growth rates of produced powders, as well as to the high crystallinity of the powder spinel phase with the
Fig. 5. Infrared spectra of the both series of magnesium aluminate samples.
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Fig. 6. The nitrogen adsorption-desorption isotherms of magnesium aluminate samples obtained by: Method 1 (a) at temperature treatments of 650 °C and 850 °C; Method 2 (b) at temperature treatments of 750 °C and 850 °C.
water absorption during the compact of powder samples with KBr. It should be noted that at lower heat treatment of the both samples (at 650 °C and 750 °C for Methods 1 and 2, respectively) no characteristic band of nitrate ions at 1464 cm1 is observed in the IR spectra, which means a complete decomposition of the magnesium and aluminium nitrates. Nitrogen adsorption-desorption isotherms and pore size distribution of magnesium aluminate spinel materials synthesized by Methods 1 and 2 are shown in Figs. 6 and 7, respectively. According to the IUPAC the nitrogen adsorption/desorption isotherms can be classified as a type IV isotherm, typical of mesoporous materials. The nitrogen adsorptiondesorption isotherms of prepared samples exhibit different hysteresis loops. The magnesium aluminate spinel materials prepared by Method 1 at 650 °C and 850 °C show hysteresis loops types H4 and H3, respectively. The sample synthesized by Method 2 at 750 °C demonstrates hysteresis loop type H2, whereas the material obtained at 850 °C shows between H2 and H4 type. The H2 loop is associated with pores with narrow necks and wide bodies (ʻink bottleʼ pores) [38]. The H3 loop is characteristic for aggregates of plate-like particles giving rise to slitshaped pores. The H4 loop corresponds to narrow slit-like pores [38]. The specific surface area (SBET), total pore volume (Vp) and average pore diameter (Dp) of magnesium aluminate samples are listed in Table 2. The results in Table 2 show that the samples thermally treated at 850 °C have lower surface areas and pore volumes than those of the samples calcined at lower temperatures. Magnesium aluminate spinel phase with the highest specific surface area of 98 m2/g having the smallest crystallite size of 7.6 nm is obtained by Method 2 at 750 °C. From the Fig. 6 it is observed that the higher values of the adsorbed volume for magnesium aluminate samples prepared by Method 2 could
increase of temperature treatment. The synthesis of magnesium aluminate spinel by Method 2 leads to formation of material with the smallest crystallite size (7.6 nm) at 750 °C (Table 2). Therefore, the advantage of our work was to obtain ultrafine nanocrystalline magnesium aluminate with a high surface area (see below). Fig. 3(A, B) and 4(A, B) show the SEM micrographs of the materials at different magnifications, synthesized by Methods 1 and 2, respectively. Aggregated nanoparticles with irregular and spherical shapes are observed in the SEM images of magnesium aluminate samples synthesized by the both methods. It should be noted, that the particles with irregular shapes are dominant in the samples obtained by Method 2 (Fig. 4(A, B)). There is agglomeration of particles with increasing the temperature treatment. The results obtained by Scanning electron microscopy are in agreement with the data derived by the powder XRD analysis. The IR spectra of the synthesized magnesium aluminate materials by the both methods are displayed in Fig. 5. The presence of oxide can be indicated by the absorption bands observed below 1000 cm−1 [36]. The bands positioned at the ca. 508/515 cm−1 and at ca. 692 cm−1 are attributed to the spinel structure of magnesium aluminate [35]. These bands are due to the vibration of AlO6 groups, which built up magnesium aluminate spinel structure [35,36]. The registered IR band at ca. 3442 cm−1 corresponds to the stretching vibration (O-H) of hydroxyl groups. The area and intensity of this band decreases with rising the temperature of heat treatment of materials. The band at ca. 1628 cm−1 can be assigned to the deformation vibration of physically adsorbed H2O molecules on the surface of samples [35,37]. The appearance of this band at high temperature treatment, most probably is due to the 5
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Fig. 7. The pore size distribution of magnesium aluminate samples obtained by: Method 1 (a) at temperature treatments of 650 °C and 850 °C; Method 2 (b) at temperature treatments of 750 °C and 850 °C.
by the presence of iron impurities in the synthesized powders.
be associated with the higher specific surface area (SBET) and with the smaller average pore diameter (Dp) in relation to those of the materials obtained by Method 1. The pore size distribution, calculated by the BJH method from the desorption branch of the nitrogen isotherm, is displayed in Fig. 7. The pore size distribution curves of the samples heated at lower temperatures of 650 °C (Method 1) and 750 °C (Method 2) exhibit one maximum at pore diameters of 3.4 and 3.8 nm, respectively. It means that the samples contain small mesopores. Increasing the temperature treatment to 850 °C causes the appearing of new maxima at 3.7 nm and 4.2 nm for the samples prepared by Methods 1 and 2, respectively. It has been established by Mosayebi et al. [6] that the higher temperature of sample treatment influences the pore size distribution, which become broader at higher calcination temperature.
Acknowledgements The financial support of National Science Fund at the Ministry of Education and Science of Bulgaria by Contract DFNI-Е02/16/2014 is appreciated. References [1] K.J.D. Mackenzie, J. Temuujin, T.S. Jadambaa, M.E. Smith, P. Angerer, Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel, J. Mater. Sci. 35 (2000) 5529–5535. [2] M.F.M. Zawrah, A.A.E. Kheshen, Synthesis and characterisation of nanocrystalline MgAl2O4 ceramic powders by use of molten salts, Br. Ceram. Trans. 101 (2) (2002) 71–74. [3] P. Orosco, L. Barbosa, María del Carmen Ruiz, Synthesis of magnesium aluminate spinel by periclase and alumina chlorination, Mater. Res. Bull. 59 (2014) 337–340. [4] K. Prabhakaran, D.S. Patil, R. Dayal, N.M. Gokhale, S.C. Sharma, Synthesis of nanocrystalline magnesium aluminate (MgAl2O4) spinel powder by the urea formaldehyde polymer gel combustion route, Mater. Res. Bull. 44 (2009) 613–618. [5] L.R. Ping, A.M. Azad, T.W. Dung, Magnesium aluminate (MgAl2O4) spinel produced via self-heat-sustained (SHS) technique, Mater. Res. Bull. 36 (2001) 1417–1430. [6] Z. Mosayebi, M. Rezaei, N. Hadian, Fazlollah Zareie Kordshuli, F. Meshkani, Low temperature synthesis of nanocrystalline magnesium aluminate with high surface area by surfactant assisted precipitation method: effect of preparation conditions, Mater. Res. Bull. 47 (2012) 2154–2160. [7] S. Salem, Application of autoignition technique for synthesis of magnesium aluminate spinel in nano scale: influence of starting solution pH on physico-chemical characteristics of particles, Mater. Chem. Phys. 155 (2015) 59–66. [8] Ji-Guang Li, T. Ikegami, Jong-Heun Lee, T. Mori, Y. Yajima, A wet-chemical process yielding reactive magnesium aluminate spinel (MgAl2O4) powder, Ceram. Int. 27 (2001) 481–489. [9] M.F. Zawrah, H. Hamaad, S. Meky, Synthesis and characterization of nano MgAl2O4 spinel by the co-precipitated method, Ceram. Int. 33 (2007) 969–978. [10] E.M.M. Ewais, D.H.A. Besisa, A.A.M. El-Amir, S.M. El-Sheikh, Di.E. Rayan, Optical properties of nanocrystalline magnesium aluminate spinel synthesized from industrial wastes, J. Alloy. Compd. 649 (2015) 159–166.
4. Conclusions The magnesium aluminate materials were prepared by two different approaches: mechanochemical treatment of the mixture of aluminium and magnesium nitrates followed by calcination (Method 1) and by thermal treatment only, without milling, (Method 2) at different temperatures. The influence of synthesis method on the physicochemical properties was established. The temperature rise leads to increasing the magnesium aluminate nanoparticle size. The obtained magnesium aluminate materials are characterized by mesoporous structure. Magnesium aluminate spinel phase with the highest specific surface area of 98 m2/g having the smallest crystallite size of 7.6 nm was obtained by Method 2 at 750 °C. That material with high surface area and nanocrystalline size could be a suitable carrier for supported catalysts. Although the prolonged high energy ball milling of spinel precursors leads to synthesis of the magnesium aluminate spinel at low temperature of 650 °C, this method could increase the cost of production caused 6
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The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials Katerina Zaharieva, Maya Shopska, Ilyana Yordanova, Sonia Damyanova
⁎
Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium aluminate spinel Milling Thermal treatment Characterization
Magnesium aluminate-based materials were prepared by applying different methods: (i) mechanochemical milling of the initial mixture of magnesium and aluminium nitrate powders (in appropriate stoichiometric amounts) followed by heat treatment at temperatures of 650 °C and 850 °C and (ii) melting of the mixture of nitrate precursors at 240 °C followed by thermal treatment at 650 °C, 750 °C and 850 °C. The effect of synthesis method on the structure and morphology of the obtained solids was studied by using various techniques such as: nitrogen adsorption-desorption isotherms, powder XRD, IR spectroscopy and SEM. It was shown that the mechanochemical milling performed before calcination procedure leads to obtaining of nanocrystalline magnesium aluminate spinel phase at lower temperature of 650 °C in comparison with the method using thermal treatment only (at 750 °C). The obtained nanomaterials exhibit mesoporous structure.
1. Introduction Nanostructured materials of an average crystalline size of a few nanometers have been of a great interest in the last decades. These materials exhibit increased strength, hardness and specific heat, as well as improved ductility and reduced density and elastic modulus, etc. The magnesium aluminate spinel (MgAl2O4) is one of the well-known and widely used material. Its solid-state synthesis from magnesia and alumina is revealed by inter-diffusion of cations (3Mg2+ ⇔ 2Al3+) through the product layer between the oxide particles at high temperatures (> 1400 °C) [1]. It possesses fcc structure of oxygen ions with eight molecules per unit cell, in which there are 64 tetrahedral and 32 octahedral sites [2]. The aluminium ions occupy the 16 octahedral sites and the magnesium ions – eight tetrahedral sites in the ideal case [2]. This ceramic material has been found application in the chemistry, metallurgy and electrochemistry because of its refractory properties, good thermal shock resistance and mechanic resistance, high melting point (2135 °C), high chemical inertia, low thermal expansion coefficient, low density and excellent optical and dielectric properties [3,4]. Many researchers have been synthesized the magnesium aluminate by various synthesis methods, for example, such as self-heat-sustained (SHS) technique [5], surfactant assisted precipitation [6], autoignition technique [7], a wet-chemical process [8], co-precipitation [9,10], molten-salt method [11], reactive sintering using bauxite and magnesite [12], nitrate–citrate combustion route [13], sol–gel auto combustion method [14], microwave-assisted combustion synthesis [15], co-
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crystallizing and decomposing of aluminium and magnesium nitrates mixture [2,16], etc. Among the methods for obtaining of nanostructured materials, mechanochemical process is very popular due to its simplicity and low process cost. Synthesis method by mechanochemical activation is an effective way to improve the interaction and contact of the reactants by milling process, which leads to enhancement of the chemical homogeneity of product and to decrease of the severity of thermal treatment [17]. The mechanochemical milling can accelerate the reaction in a multi-component system by significantly decrease of the temperatures of thermal treatment [18]. Many researchers have been used various mechanochemical synthesis methods at different conditions for obtaining magnesium aluminate spinel [1,2,18–25]. Different precursors have been used for obtaining of nanocrystalline magnesium aluminate spinel by mechanochemical activation followed by annealing such as Al2O3 and NgCO3 [1,18,19] or MgCl2 and AlCl3 in the presence of NaO together with a small amount of water [20], etc. Due to the high chemical inertness in the acidic and basic environments, as well as, due to the high thermal resistance at elevated temperatures, the magnesium aluminate spinel has been found wide application in heterogeneous catalysis as carrier of different supported metal oxide catalysts for clean energy production and environmental production. It was shown that MgAl2O4 obtained by co-precipitation of the solutions of the Al and Mg precursor salts, followed by calcination at high temperatures, leads to obtaining of materials with a high surface area suitable as supports for catalysts [26–29]. In the present paper, it was attempted to synthesize magnesium
Corresponding author. E-mail address: [email protected] (S. Damyanova).
http://dx.doi.org/10.1016/j.ceramint.2017.09.176 Received 8 September 2017; Received in revised form 20 September 2017; Accepted 21 September 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Zaharieva, K., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.09.176
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Fig. 2. Powder X-ray diffractograms of magnesium aluminate samples obtained by Method 2. Fig. 1. Powder X-ray diffractograms of magnesium aluminate samples obtained by Method 1.
molar ratio of 1:2) in agate mortar for 6 h followed by thermal treatment at 240 °C in air for 4 h to obtain a melt. After that, the sample powder was annealed at different temperatures of 650 °C, 750 °C and 850 °C in air for 2 h at each temperature.
aluminate by different methods and to study the effect of synthesis method on the physicochemical properties of the obtained spinel materials. It was studied the influence of the prior segregation of the phase selection and the evolution in thermally decomposed samples synthesized from nitrate precursors. For this purpose, the magnesium aluminate was prepared by mechanochemical treatment for various periods or by melting of the mixture of the salts of magnesium and aluminium nitrates in appropriate amounts with subsequent annealing. The powder X-ray diffraction analysis (PXRD), infrared spectroscopy (IR), scanning electron microscopy (SEM) and N2 adsorption-desorption isotherms were performed in order to characterize the synthesized samples.
2.2. Characterization The powder XRD patterns were recorded on a Bruker D2 Phaser diffractometer within the range of 2θ values between 4° and 75° using Cu Kα radiation (λ = 0.154056 nm) at 40 kV (step size of 0.05° and time per step of 1 s). The phases were identified using of JCPDS database (Powder Diffraction Files, Joint Committee on Powder Diffraction Standards, Philadelphia PA, USA, 1997). Infrared spectra of the magnesium aluminate samples were performed on a Nicolet 6700 FTIR spectrometer (Thermo Electron Corporation, USA) in KBr pellet (0.5% studied substance). The spectra were collected in the middle IR region (400–4000 cm−1) using 50 scans at a resolution of 4 cm−1 (data spacing 1.928 cm−1). The SEM investigations were carried out by the scanning electron microscope JSM-5510 at acceleration voltage of 10 kV and different magnifications of 10,000 and 20,000. The samples were prepared by dispersing the powders in acetone. Ultrasonic oscillation for 1 h was
2. Materials and methods 2.1. Sample preparation Magnesium aluminate samples were prepared by two different synthesis methods. The first method labeled as Method 1 was used mechanochemical milling of the initial mixture of powders precursors of Al(NO3)3·9H2O (99% purity, Merck) and Mg(NO3)2·6H2O (99% purity, Merck) in an appropriate stoichiometric amount of 2:1, followed by heat treatment procedure at 650 °C and 850 °C in air. The samples were milled at different times of 1 h, 2 h, 3.30 h and 5.30 h using a high-energy ball milling by planetary ball mill type PM 100, Retsch, Germany. The mechanochemical milling process was performed in air atmosphere at room temperature in a stainless steel milling container of 250 ml and with a rotation speed 390 rpm. The mass ratio between balls and powder was 17:1. The second preparation method denoted as Method 2 was a mixing of initial materials of magnesium and aluminium nitrates powders (in a
Table 1 Calculated values of average crystallite size (D), lattice strain (ε) and unite cell parameter (a) of the magnesium aluminate phase. Sample MgAl2O4-650-Method MgAl2O4-850-Method MgAl2O4-750-Method MgAl2O4-850-Method
2
1 1 2 2
D (nm)
ε × 10−3 (a.u.)
A (Å)
8.3 12.5 7.6 15.0
3.5 2.1 3.6 3.4
8.11 8.09 8.07 8.08
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mixture of nitrates after ball milling for 1 and 2 h. Increasing the milling time up to 3.30 h and 5.30 h leads to change of the XRD spectra of the mixture that could be due to the formation of intermediate compounds with unidentifiable peaks in all the compositions during the mechanical activation process. Since these phases did not exist at temperature treatment ≥ 650 °C, it was presumed that they are complex hydrated nitrates. Peaks of magnesium aluminate spinel phase (PDF-77-1193) at temperature treatments of 650 °C and 850 °C (Fig. 1) are well visible in the XRD of the previously mechanically activated samples. The diffraction lines at 31.4°, 36.9°, 44.8°, 59.5° and at 65.3° of the spinel phase correspond to the planes (220), (311), (400), (511) and (400) reflection, respectively. In addition, it should be noted that traces of iron phase (PDF-870721) are found during the mechanical activation (Fig. 1). The presence of iron phase could be due to the impurities under the strong and continuous contact between the sample and stainless-steel balls and container during the mechanochemical milling. The iron-containing contamination obtained during the mechanical activation was also established by Adhami et al. [30]. The authors have been determined that the incorporation of the iron impurity into the mixture during the milling process leads to formation of some MgFe0.6Al1.4O4 spinel phase [30]. According to the literature [31] spinel powder has been prepared by solid-state reaction, which needs a very high annealing temperature of 1400–1600 °C. However, in the present work it can be seen that the magnesium aluminate spinel can be formed by annealing the 6 h ball milled powder at 650 °C for 2 h due to the higher reactivity and contact surface area of the initial powders. The XRD patterns of co-molted mixture of the magnesium and aluminium nitrates at 240 °C, previously mixed in the agate mortar for 4 h, show unidentified phases (Fig. 2). The heat treatment at 650 °C
Table 2 Textural characteristics of the magnesium aluminate samples obtained by Methods 1 and 2: specific surface area (SBET), total pore volume (Vp) and average pore diameter (Dp). Sample MgAl2O4-650-Method MgAl2O4-850-Method MgAl2O4-750-Method MgAl2O4-850-Method
1 1 2 2
SBET (m2/g)
Vp (cm3/g)
Dp (nm)
92 21 98 51
0.12 0.06 0.13 0.10
5.4 12.0 5.3 7.8
introduced to decrease the aggregation followed by placing a drop of the suspension on holey carbon film supported on copper grids. Specific surface area (SBET), total pore volume (Vpore) and average pore size diameter (Dpore) of the samples were determined by N2 adsorption-desorption isotherms at 77 K with a Micromeritics TriStar 3000 apparatus. The samples were previously out-gassed under vacuum at 150 °C for 24 h. The specific surface area was measured by the method of Brunauer-Emmett-Teller (SBET), and the distribution of pore size was calculated by the method of Barrett-Joyner-Halenda (BJH) by desorption of curves/branches of the isotherms.
3. Results and discussion The powder XRD patterns of the initially salts Al(NO3)3·9H2O and Mg(NO3)2·6H2O, as well as of the samples mixture after mechanical activation for various periods at room temperature with subsequent annealing at 650 °C and 850 °C for 2 h (Method 1), are shown in Fig. 1. The XRD of the co-melted samples at 240 °C for 4 h, followed by the heat treatment at 650 °C, 750 °C and 850 °C (Method 2) are displayed in Fig. 2. From the Fig. 1 it is obvious a structural similarity of the starting
Fig. 3. SEM images at different magnifications of magnesium aluminate obtained by Method 1 at thermal treatments: 650 °C (A) and 850 °C (B).
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Fig. 4. SEM images at different magnification of magnesium aluminate obtained by Method 2 at thermal treatments: 750 °C (A) and 850 °C (B).
precursors before the heat treatment procedure (Method 1) leads to synthesis of magnesium aluminate materials at lower temperature (650 °C) in relation to that observed for samples synthesized by Method 2 (750 °C). The crystallinity of magnesium aluminate samples synthesized by the both methods was established at increasing the calcination temperature. The mean crystallite size (D), lattice strain (ε) and unite cell parameter (a) of magnesium aluminate phase are listed in Table 1. These parameters are calculated by PowderCell 2.4 program [32] and using Williamson–Hall equation [33]:
leads to formation of an amorphous phase of magnesium aluminate spinel like structure. However, increasing the temperature of the heat treatment up to 750 °C and 850 °C leads to formation of well-defined peaks of MgAl2O4 spinel phase (PDF-77-1193). The peaks broadening of the spinel is due to the small grain size formed at 650 °C. Because of the decrease of the grain size, the number of the planes in a crystal that diffract at the same angle is decreased that causes the peak broadening. It can be concluded that the mechanochemical milling of the oxide
β cos θ = 0.9 λ/ D + 4ε sin θ
(1)
where ε is the value of internal strain, β is the full-width half-maximum (FWHM) of diffraction, θ is the Bragg's angle, λ is the wavelength of Xray beam used and D is the average crystallite size of the phase under study. The results in Table 1 show that the synthesized magnesium aluminates by Methods 1 and 2 possess an average crystallite size in the range of 7.6–15.0 nm. As can be seen from the Table 1 the magnesium aluminate samples obtained at lower heat treatment temperatures of 650 °C (Method 1) and at 750 °C (Method 2) have lower mean crystallite size of 8.3 and 7.6 nm, respectively, than those of the materials heated at 850 °C (12.5 nm and 15.0 nm for samples obtained by Methods 1 and 2, respectively). It suggests that there is a gradual increase in the crystallite size of the both series samples with increasing the temperature treatment. The same effect of thermal treatment on the particle size increase was established by Govha et al. [34] and Ewais et al. [35]. The researchers were determined that the particle size and crystallinity of the magnesium aluminate spinel increase with increasing the calcination temperature [34,35]. This reciprocal action can be related to the nucleation, the growth rates of produced powders, as well as to the high crystallinity of the powder spinel phase with the
Fig. 5. Infrared spectra of the both series of magnesium aluminate samples.
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Fig. 6. The nitrogen adsorption-desorption isotherms of magnesium aluminate samples obtained by: Method 1 (a) at temperature treatments of 650 °C and 850 °C; Method 2 (b) at temperature treatments of 750 °C and 850 °C.
water absorption during the compact of powder samples with KBr. It should be noted that at lower heat treatment of the both samples (at 650 °C and 750 °C for Methods 1 and 2, respectively) no characteristic band of nitrate ions at 1464 cm1 is observed in the IR spectra, which means a complete decomposition of the magnesium and aluminium nitrates. Nitrogen adsorption-desorption isotherms and pore size distribution of magnesium aluminate spinel materials synthesized by Methods 1 and 2 are shown in Figs. 6 and 7, respectively. According to the IUPAC the nitrogen adsorption/desorption isotherms can be classified as a type IV isotherm, typical of mesoporous materials. The nitrogen adsorptiondesorption isotherms of prepared samples exhibit different hysteresis loops. The magnesium aluminate spinel materials prepared by Method 1 at 650 °C and 850 °C show hysteresis loops types H4 and H3, respectively. The sample synthesized by Method 2 at 750 °C demonstrates hysteresis loop type H2, whereas the material obtained at 850 °C shows between H2 and H4 type. The H2 loop is associated with pores with narrow necks and wide bodies (ʻink bottleʼ pores) [38]. The H3 loop is characteristic for aggregates of plate-like particles giving rise to slitshaped pores. The H4 loop corresponds to narrow slit-like pores [38]. The specific surface area (SBET), total pore volume (Vp) and average pore diameter (Dp) of magnesium aluminate samples are listed in Table 2. The results in Table 2 show that the samples thermally treated at 850 °C have lower surface areas and pore volumes than those of the samples calcined at lower temperatures. Magnesium aluminate spinel phase with the highest specific surface area of 98 m2/g having the smallest crystallite size of 7.6 nm is obtained by Method 2 at 750 °C. From the Fig. 6 it is observed that the higher values of the adsorbed volume for magnesium aluminate samples prepared by Method 2 could
increase of temperature treatment. The synthesis of magnesium aluminate spinel by Method 2 leads to formation of material with the smallest crystallite size (7.6 nm) at 750 °C (Table 2). Therefore, the advantage of our work was to obtain ultrafine nanocrystalline magnesium aluminate with a high surface area (see below). Fig. 3(A, B) and 4(A, B) show the SEM micrographs of the materials at different magnifications, synthesized by Methods 1 and 2, respectively. Aggregated nanoparticles with irregular and spherical shapes are observed in the SEM images of magnesium aluminate samples synthesized by the both methods. It should be noted, that the particles with irregular shapes are dominant in the samples obtained by Method 2 (Fig. 4(A, B)). There is agglomeration of particles with increasing the temperature treatment. The results obtained by Scanning electron microscopy are in agreement with the data derived by the powder XRD analysis. The IR spectra of the synthesized magnesium aluminate materials by the both methods are displayed in Fig. 5. The presence of oxide can be indicated by the absorption bands observed below 1000 cm−1 [36]. The bands positioned at the ca. 508/515 cm−1 and at ca. 692 cm−1 are attributed to the spinel structure of magnesium aluminate [35]. These bands are due to the vibration of AlO6 groups, which built up magnesium aluminate spinel structure [35,36]. The registered IR band at ca. 3442 cm−1 corresponds to the stretching vibration (O-H) of hydroxyl groups. The area and intensity of this band decreases with rising the temperature of heat treatment of materials. The band at ca. 1628 cm−1 can be assigned to the deformation vibration of physically adsorbed H2O molecules on the surface of samples [35,37]. The appearance of this band at high temperature treatment, most probably is due to the 5
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Fig. 7. The pore size distribution of magnesium aluminate samples obtained by: Method 1 (a) at temperature treatments of 650 °C and 850 °C; Method 2 (b) at temperature treatments of 750 °C and 850 °C.
by the presence of iron impurities in the synthesized powders.
be associated with the higher specific surface area (SBET) and with the smaller average pore diameter (Dp) in relation to those of the materials obtained by Method 1. The pore size distribution, calculated by the BJH method from the desorption branch of the nitrogen isotherm, is displayed in Fig. 7. The pore size distribution curves of the samples heated at lower temperatures of 650 °C (Method 1) and 750 °C (Method 2) exhibit one maximum at pore diameters of 3.4 and 3.8 nm, respectively. It means that the samples contain small mesopores. Increasing the temperature treatment to 850 °C causes the appearing of new maxima at 3.7 nm and 4.2 nm for the samples prepared by Methods 1 and 2, respectively. It has been established by Mosayebi et al. [6] that the higher temperature of sample treatment influences the pore size distribution, which become broader at higher calcination temperature.
Acknowledgements The financial support of National Science Fund at the Ministry of Education and Science of Bulgaria by Contract DFNI-Е02/16/2014 is appreciated. References [1] K.J.D. Mackenzie, J. Temuujin, T.S. Jadambaa, M.E. Smith, P. Angerer, Mechanochemical synthesis and sintering behaviour of magnesium aluminate spinel, J. Mater. Sci. 35 (2000) 5529–5535. [2] M.F.M. Zawrah, A.A.E. Kheshen, Synthesis and characterisation of nanocrystalline MgAl2O4 ceramic powders by use of molten salts, Br. Ceram. Trans. 101 (2) (2002) 71–74. [3] P. Orosco, L. Barbosa, María del Carmen Ruiz, Synthesis of magnesium aluminate spinel by periclase and alumina chlorination, Mater. Res. Bull. 59 (2014) 337–340. [4] K. Prabhakaran, D.S. Patil, R. Dayal, N.M. Gokhale, S.C. Sharma, Synthesis of nanocrystalline magnesium aluminate (MgAl2O4) spinel powder by the urea formaldehyde polymer gel combustion route, Mater. Res. Bull. 44 (2009) 613–618. [5] L.R. Ping, A.M. Azad, T.W. Dung, Magnesium aluminate (MgAl2O4) spinel produced via self-heat-sustained (SHS) technique, Mater. Res. Bull. 36 (2001) 1417–1430. [6] Z. Mosayebi, M. Rezaei, N. Hadian, Fazlollah Zareie Kordshuli, F. Meshkani, Low temperature synthesis of nanocrystalline magnesium aluminate with high surface area by surfactant assisted precipitation method: effect of preparation conditions, Mater. Res. Bull. 47 (2012) 2154–2160. [7] S. Salem, Application of autoignition technique for synthesis of magnesium aluminate spinel in nano scale: influence of starting solution pH on physico-chemical characteristics of particles, Mater. Chem. Phys. 155 (2015) 59–66. [8] Ji-Guang Li, T. Ikegami, Jong-Heun Lee, T. Mori, Y. Yajima, A wet-chemical process yielding reactive magnesium aluminate spinel (MgAl2O4) powder, Ceram. Int. 27 (2001) 481–489. [9] M.F. Zawrah, H. Hamaad, S. Meky, Synthesis and characterization of nano MgAl2O4 spinel by the co-precipitated method, Ceram. Int. 33 (2007) 969–978. [10] E.M.M. Ewais, D.H.A. Besisa, A.A.M. El-Amir, S.M. El-Sheikh, Di.E. Rayan, Optical properties of nanocrystalline magnesium aluminate spinel synthesized from industrial wastes, J. Alloy. Compd. 649 (2015) 159–166.
4. Conclusions The magnesium aluminate materials were prepared by two different approaches: mechanochemical treatment of the mixture of aluminium and magnesium nitrates followed by calcination (Method 1) and by thermal treatment only, without milling, (Method 2) at different temperatures. The influence of synthesis method on the physicochemical properties was established. The temperature rise leads to increasing the magnesium aluminate nanoparticle size. The obtained magnesium aluminate materials are characterized by mesoporous structure. Magnesium aluminate spinel phase with the highest specific surface area of 98 m2/g having the smallest crystallite size of 7.6 nm was obtained by Method 2 at 750 °C. That material with high surface area and nanocrystalline size could be a suitable carrier for supported catalysts. Although the prolonged high energy ball milling of spinel precursors leads to synthesis of the magnesium aluminate spinel at low temperature of 650 °C, this method could increase the cost of production caused 6
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