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Application of autoignition technique for synthesis of magnesium aluminate spinel in nano scale: Influence of starting solution pH on physico-chemical characteristics of particles
Materials Chemistry and Physics xxx (2015) 1e8
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Application of autoignition technique for synthesis of magnesium aluminate spinel in nano scale: Influence of starting solution pH on physico-chemical characteristics of particles Shiva Salem Chemical Engineering Group, Urmia University of Technology, Urmia, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Autoignition technique was used for synthesis of nano-sized magnesium aluminate. The effect of pH on powder characteristics was studied. The specific surface area variation was related to pH and crystal size. The obtained result was confirmed by TEM observation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2014 Received in revised form 12 January 2015 Accepted 31 January 2015 Available online xxx
The present investigation is a follow-up of studies on glycine-nitrate autoignition technique for production of magnesium aluminate nano-particles. The technical characteristics of powders were studied in relation to a wide range of pH, 2.5e10.5. The effect of pH on particle formation was particularly emphasized to achieve a powder in nano-scale. In preparation of precursors the Mg/Al mole ratio was considered to be 0.50 and the level of fuel/NO3 was adjusted at 0.75. The thermal behavior of precursors was evaluated by DTA-TG technique. Also, the obtained powders were calcined at 600, 800, 1000 and 1200 C and characterized by XRD, BET, TEM and elemental analysis methods. The results indicated that the physico-chemical characteristics of products are strongly influenced by the pH of synthesis environment. The precursor prepared in pH of 2.5 exhibits flaming type ignition, while the precursors synthesized at neutral and alkali environments show underwent smoldering type combustion. In conclusion, the optimum characteristics were achieved when precursor is manufactured at neutral condition. © 2015 Elsevier B.V. All rights reserved.
Keywords: Oxides Chemical synthesis Nanostructures Surfaces
1. Introduction The nano-sized magnesium aluminate spinel (MAS), which is formulated as MgAl2O4, is one of the ceramic components. This
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material is used as catalyst support [1,2], refractory [3,4], optical devices [5,6], etc. MAS is a magnesium-based ceramic which is employed at metallurgy, electrochemical fields, dentistry applications, reinforcing fibers, and inorganic pigments [5] for over 10 years because of its excellent properties such as high melting point (2135 C), relatively low density (3.58 g cm3), well transmittance in the wavelength of 0.25e5.0 mm, high strength (180 MPa) and
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Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
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Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
hardness (16 GPa), inert in acidic environments, low thermal expansion coefficient (9 106 C1 between 30 and 1400 C) and high thermal shock resistance [7]. The nano-sized MAS powder has been widely used in the fabrication of humidity sensors, transparent armor and visible-infrared windows [5,6]. Moreover, MAS is currently used for an effective refractory material for cement rotary kilns and steel ladles. The spinel also has been employed in manufacturing photo-luminescent materials and pressure vessels. The structure of magnesium aluminate spinel is cubic [8] and oxygen atoms are arranged in a close-packed cubic lattice. Mg and Al atoms occupy tetrahedral (1/8 of the available tetrahedral sites) and octahedral (1/2 of the available octahedral sites) positions, respectively. This arrangement is considered to be a good host capable for holding a large number of two and trivalent cations and forming a solid state solution. The synthetic MAS powder is produced by a variety of ceramic processing routes, including the wet-chemical process [5], precipitation [9], solegel [10e12], polymerized complex method [13], microwave [14,15] and pyrolysis [16]. Unfortunately many of these processing techniques are inherently complex to apply in the industrial scale and rigid control should be carried out over chemical compounds and morphology of particles. The products of mentioned techniques are affected by pH, time and temperature. The variations in these factors lead to some differences in the morphology and crystallographic structures. The deviation from stoichiometry and requirement of expensive precursors are the main problems in development above techniques in industrial scale. The dry grinding of magnesium and aluminum hydroxides, in the form of powder with a planetary ball mill, followed by heating is one of the oldest techniques for preparation of MAS in microscale [17]. The microwave assisted high-energy ball milling technique was applied to prepare nano-sized MAS, 28e149 nm [18]. According to the reported results, it is impossible to achieve the nano-sized pure spinel with calcination of Mg(OH)2 and Al(OH)3 mixture. In order to manufacture the particles smaller than 20 nm, the other techniques should be employed, consequently. The fuelnitrate autoignition technique shows a great potential in the preparation of ceramic components [19e21]. For the last two decades, this method has been attractive for many scientists to devote their attentions in studying the key factors of process [22e24]. In order to evaluate the combustion process, it is important to produce a solegel system, through changing the preparation conditions [25]. The formation of transparent gel usually indicates that a homogeneous system is obtained and the chemical reactions are well controlled under optimal conditions. This step is critical in the manufacture of corresponding material [26]. Among the publications about combustion process, nitrate systems are extensively studied and the main interest is focused on the good gel formation. In the autoignition synthesis method, a thermally induced redox reaction takes place between an oxidant and a fuel. In general, the nitrate salts, acting as cation sources, are used as oxidants, whereas organic compositions are employed as fuels [22e24]. From the application point of view, the fuel content is very important since it can be widely influence the crystal size of spinel [25,26]. The gel preparation should be taken to account to provide nano-particles with appropriate morphology. The gel formation by different fuels through the nitrate solution has been the subject of numerous investigations [27]. However, specific attempts were made to study the nano-sized MAS formation as a function of fuel composition. The nitrate transparent gel was formed using ureaeformaldehyde and the pH of solution was adjusted at 8.5 [22]. The nano particles in the range of 10e30 nm were produced after calcination at 850 C. The use of triethylenetetramine as a fuel yields a spinel powder with an average crystallite size of 4.9 nm
after annealing at 700 C, whilst urea and triethylenetetramine mixture leads to a powder with an average crystallite size of 30.8 nm after calcination at the same temperature [23]. It was demonstrated that urea, glycine, sugar, and mixtures of these materials are suitable fuels for synthesis of MAS from a stoichiometric mixture of aluminum and magnesium nitrates by the autoignition method. The single-phase MAS powder with crystallite size of 18.4 nm was obtained when sugar-urea was used as a fuel [24]. Nano-scale powders were successfully synthesized via combustion process using various mixtures of urea, glycine and starch. The addition of starch alters the ignition type from flame to smoldering combustion and lowers the temperature [22]. The nature of fuel used in the autoignition process greatly influences the crystal size. The longer chain of fuel molecule causes the larger crystallites [10]. Nano-crystalline MAS was synthesized by an autoignition process using citric acid, metal nitrates, and ammonia at relatively low temperatures. The pH of solution was controlled at level of 6. The fully crystallized MAS appeared at temperatures above 900 C [28]. In comparison to complex methods, the autoignition technique, based on the principles of propellant chemistry, has many advantages such as uniform composition, low cost and short reaction time. Therefore, it was known as attractive process for the synthesis of spinel powders, recently. The success of combustion is fundamentally related to non-violently reaction of fuel. The amino acids are well known to act as a complexing agent for a number of metal ions because this group of organic materials has a carboxylic acid at one end and amino at the other molecule section. Glycine, which is one of the amino acids, can effectively maintain compositional homogeneity among the constituents [29,30]. Therefore, glycinenitrate system has been a very effective method for the preparation of metal spinel. Although the kinetics of the reaction between fuel and nitrate salts has been widely investigated as above explanation however, to author knowledge, there are a few publications about the effect of solution pH on technical characteristics of magnesium aluminate particles. It seems that preparation condition has a considerable influence on the properties of MAS. Since solution pH is an important factor in combustion process, the effect of acidic, neutral and alkali conditions on the MAS characteristics is discussed in detail in present work.
2. Materials and methods 2.1. Synthesis process The magnesium aluminate precursors were prepared by solegel method using high purity grade of magnesium nitrate, Mg(NO3)2.3H2O, and aluminum nitrate, Al(NO3)3.9H2O, both manufactured by SigmaeAdrich Company, as starting materials. A solution with molar ratio of 0.50 was obtained through the dissolution of nitrate salts in de-ionized water. The mixture was vigorously stirred to form a transparent solution and its temperature was maintained at 25 C. Then glycine, NH2CH2COOH (SigmaeAdrich, 99.5 wt.% as received), was slowly added drop-wise to the nitrate solution and stirred to achieve a homogenous mixture. The glycine content was set above the stoichiometric amount at fuel/NO3 of 0.75 which can be calculated according to propellant chemistry [26,31]. The resulted solution was divided into three parts and the pH of samples was controlled by adding nitric acid and ammonia solutions at three levels of 2.5, 7.0 and 10.5. In the next step, water has been removed from the solutions through the heating at 110 C by a hot plate. The sol converted to a viscous gel by evaporation of water and the reaction between the magnesium and aluminum nitrates, leading to the formation of hydrate components have been identified as:
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
MgðNO3 Þ2 $3H2 O þ AlðNO3 Þ3 $9H2 O þ 5NH4 ðOHÞ/MgðOHÞ2 þ AlðOHÞ3 þ 5NH4 NO3 þ 12H2 O (1) Afterwards, the viscous gels were transferred to a furnace preheated at 300 C and ignited spontaneously with rapid evolution of gases, producing foamy and voluminous powders. The flame combustion was extinguished after 1 min, while the smolder combustion lasted more than 7 min. The autoignition of precursor synthesized at pH of 2.5 was flaming type, whilst the precursors prepared at neutral and alkali pHs underwent smoldering type ignition. The spinel formation can be explained as:
MgðOHÞ2 þ 2AlðOHÞ3 /MgAl2 O4 þ 4H2 O
(2)
Because the time for autoignition is rather short, to remove traces of un-decomposed glycine, nitrates and their decomposition products, the powders should be further heated. In order to recrystallize the MAS particles, the resulting powders were also calcined at different temperatures, 600, 800, 1000 and 1200 C, for 1 h. 2.2. Precursor and powder characterizations The autoignition of precursors resulting from the gel formation process was evaluated by simultaneous differential thermal analysis and thermogravimetric test (DTA-TG, Model 409, Netzsch, Germany) at a heating rate of 10 C min1 in air atmosphere. Approximately, 10 mg of each precursor was placed in a ceramic pan and temperature was increased from 25 to 600 C, steadily. In order to better evaluate the ignition of fuel after calcination, the residual hydrogen, carbon and nitrogen amounts were determined by elemental analysis (Carlo Erba, Model EA 1110, Italy). The phase transformation during heat treatment and crystallite size evolutions were carried out by X-ray diffraction analysis using a Philips diffractometer (X'PERT PRO, Philips Research Laboratories, The Netherland) in the diffraction angle range of 10e80 with CuKa radiation. The Scherer's equation was used to calculate the crystallite size, D [32].
D¼
0:9l b cos q
(3)
where b is the breadth of observed diffraction peak at its half-
Fig. 1. The DTA curves of precursors synthesized at different pH.
3
intensity, q is the Bragg angle, and l is the radiation wavelength, 0.15406 nm. The specific surface areas of the resulting powders were determined with low temperature nitrogen adsorption isotherms, using Brunauer, Emmett and Teller (BET) technique (Gemini 2360 Apparatus, Micromeritics, Norcross, GA, USA). Finally, transmission electron microscopy (TEM, JEM 2010, JEOL, Tokyo, Japan) was used in characterizing the particle morphology. For this purpose, particles were deposited onto cupper grids, which support a holey carbon film. The particles were placed onto the support grids by deposition from a dilute suspension. The particle shapes and sizes were characterized by high resolution imaging technique. 3. Results and discussion 3.1. Effect of pH on autoignition The DTA curves of precursors synthesized at different pHs are shown in Fig. 1. Only the thermal effect related to ignition of glycine are observable at 275 C. For precursors prepared at the acidic, neutral and alkali environments the additional exothermic peaks did not appear. With increase in pH, the position of exothermic peak is negligibly shifted to lower temperatures. The TG curves, Fig. 2, of the precursors show three well-defined weight loss regions due to the loss of physically absorbed water below 250 C, ignition of glycine in narrow temperature range, 250e280 C, and decomposition of remained organic materials at higher temperatures, above 280 C. In the present study, the low temperature weight loss can be assigned to the water absorbed by gels, whereas the weight loss about 10.0e15.0 wt.% is observed. On comparing the TG profiles, it is seen that pH of synthesis environment cannot affect the dehydration reaction of gels, considerably. However, in the high temperature, the weight loss is extraordinarily high in all studied cases and 60.0e65.0 wt.% of mass is removed due to autoignition reaction between the fuel and nitrate. The weight losses and associated thermal effects at narrow temperature range of 250e280 C are due to the thermal decomposition of glycine. Between 280 and 600 C, a part of the weight loss, 10.0e15.0 wt.% is due to the burning of the residual glycine remained between the particles. According to the principle of propellant chemistry for redox reaction between a fuel and an oxidizer, the following reaction can be considered for autoignition:
Fig. 2. The TG curves of precursors synthesized at different pH.
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
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Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
9 5 1 H2 NCH2 COOH þ O2 /2CO2 þ H2 O þ N2 4 2 2
(4)
It should be emphasized that glycine can react with ammonia:
H2 NCH2 COOH þ 4:5NH4 NO3 /2CO2 þ 11:5H2 O þ 5N2
(5)
For glycine-nitrate ignition, primarily N2, CO2 and H2O are evolved as the gaseous products. When the glycine content is larger than stoichiometric value, oxygen would be involved to ensure the complete combustion of glycine in fuel-rich condition. Thus, the combustion reaction can be represented as a follows:
MgðNO3 Þ2 $3H2 O þ 2AlðNO3 Þ3 $9H2 O þ 8NH4 OH þ 2H2 NCH2 COOH þ
1 O /MgAl2 O4 þ 4CO2 þ 46H2 O þ 9N2 2 2 (6)
In this case, the total weight loss for the manufacturing one mol MgAl2O4 is about 89.0e93.0 wt.%, and it is near to the theoretical value of 89.8 wt.%. These results of thermal analysis give a strong evidence for the adoption of 275 C as the initiation temperature to start the combustion reaction. Different ignition phenomena resulting from different synthesis conditions have been observed. The incandescent flame was governed after autoignition of precursor prepared at pH of 2.5 whilst the smoldering combustion is observed for precursors manufactured at pHs of 7.0 and 10.5. The optimum calcination temperature should be determined to ensure the complete decomposition of precursors. In overall, the variations of carbon, nitrogen and hydrogen are similar to each other, with differences in the amounts, Table 1. However, the amounts of organic elements are dramatically different when calcination is carried out at temperatures lower than 800 C. In general, the calcination of powders prepared at different environments, above 800 C, cusses the significant reduction of fuel content. The removal of glycine by calcination is independent of pH. The term of combustion covers the flaming, gas phase, smoldering, solidegas, reactions. The effect of pH on autoignition is a direct consequence of different prevailing glycine and hydroxides species. NHþ 3 CH2COOH and NH2CH2COO are the dominant species for acidic and alkali conditions, respectively. Also, NHþ 3 CH2COO is dominant species at neutral environment [33]. In the acidic environment, the rate of protonation and dissolution of hydroxides is high as a result, the interaction between NHþ 3 CH2COOH and ions are limited. The absorbed glycine is attached with hydroxides via hydrogen bounds. Therefore, the flame type combustion occurs in this case. In the neutral and alkali environments, the interaction
Table 1 The elemental analyses of calcined powders synthesized at different pH. Element (wt.%)
pH: 2.5 Carbon Nitrogen Hydrogen pH: 7.0 Carbon Nitrogen Hydrogen pH: 10.5 Carbon Nitrogen Hydrogen
between the ions of hydroxides and glycine occurs via -COO and NH2 in NH2CH2COO, respectively. These interactions are beneficial for dispersion of ions and facilities the reaction between the solid and gas, consequently. Therefore, the autoignition decomposition needs higher energy and the smoldering type combustion is observable in these conditions.
3.2. Phase evolution Fig. 3 represents the XRD patterns for powders calcined at 600 C after synthesis at different pHs. The powders show a strong amorphous background and contain a negligible amount of poorly crystalline MAS. With decrease in pH a gradual improvement in crystallinity is observed, in terms of peak sharpness of MAS phase (JCPDS 01-075-1796). These changes continued until calcination at 800 C in which the powders are predominantly contain crystalline phase, Fig. 4. After calcination, considerable improvement in reflection intensity is seen in the powder synthesized at acidic condition. MgO (JCPDS 00-045-0946) seems to be the only impurity phase in the powders synthesized at neutral and alkali conditions when the materials are calcined at 1000 C, Fig. 5. When the synthesis is carried out at these conditions, the negligible amounts of Mg and Al cannot well arranged in a close-packed cubic lattice and remain free. These cations are re-crystallized at higher calcination temperatures as oxides. The MgAl2O4 formation by solidesolid reaction of MgO and Al2O3 involves with diffusion of Mg2þ onto alumina particle cores. The intensities of reflections related to MgO and Al2O3 increase concomitantly with increment in calcination temperature. Moreover, Al2O3 usually does not show the sharp peek in the presence of MgAl2O4 and it is hardly distinguishable when calcination is performed at temperatures lower than 1000 C. These observations suggest that the critical condition is essentially required to achieve pure MAS through the proposed route. Consequently, for those powders subject to different calcination temperature, MgO was formed due to incomplete reaction. However, further examination of XRD diagrams in calcination range of 600e1000 C shows different crystallinity. These crystalline phases imply the complexity of the real reaction pathways, which are certainly much more complex than that suggested by Eq. (6). According to Figs. 3e5, MgeAl intermediate transforms to crystalline MAS at elevated temperatures. The higher calcination temperature results in even stronger crystalinity, with little or no amorphous phase as illustrated in Fig. 6. Two rather reflection lines at 2q of 42.99 and 62.26 are displayed in these patterns, suggesting that the Eq. (6) is not completed when solution preparation is performed in pH range of 7.0e10.5. A comparative inspection of Figs. 4e6 indicates that the MgO can evolve from the transformation of the amorphous MgeAl intermediate if the solution preparation is carried out at neutral and alkali conditions. Al2O3 (JCPDS 01-081-
Calcination temperature ( C) As obtained
600
800
1000
1200
8.74 3.77 1.06
2.77 1.20 1.11
0.64 0.15 0.50
0.50 0.12 0.19
0.38 0.11 0.0
7.80 3.65 2.02
2.62 0.89 1.38
0.89 0.12 0.91
0.56 0.0 0.47
0.25 0.0 0.0
8.59 3.85 2.15
2.01 0.72 1.40
0.76 0.10 0.83
0.58 0.0 0.28
0.49 0.0 0.16
Fig. 3. The XRD patterns of precursors synthesized at different pHs and calcined in 600 C.
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
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Fig. 4. The XRD patterns of precursors synthesized at different pHs and calcined in 800 C.
Fig. 6. The XRD patterns of precursors synthesized at different pHs and calcined in 1200 C.
Fig. 5. The XRD patterns of precursors synthesized at different pHs and calcined in 1000 C.
performed at neutral condition. However, the spinel is better crystallized in other cases. The crystal size is more sensitive to
2266) was formed at high calcination temperature, i.e. 1200 C, when initial solution pH is controlled at level of 7.0. According to XRD patterns, it is absolutely impossible to perform the solid state reaction at temperature lower than 1200 C to form spinel. As postulated above, the XRD diagrams suggest that progress of the reaction towards the right side of Eq. (6) leads to the formation of full crystalline MAS if pH of solution is adjusted at level of 2.5. Eq. (6) is approximately completed to form the MgeAl intermediate and the most of Mg and Al are consolidated into the spinel structure after calcination in temperature range of 800e1200 C. It can be concluded that the pure magnesium aluminate is obtained by the initial preparation of solution in acidic environment. The effects of pH and calcination temperature on crystal size of MAS obtained at various processing conditions are presented in Fig. 7. Altering the solution condition and calcination temperature can influence the crystal size of powders. Raising the pH of the initial solution yields a finely crystalline product with crystal size of 12 nm. The thermal treatment of the products at 1000 C results in re-crystallization of finely crystals. As temperature rises, the crystal size increases from 22 to less than 50 nm when synthesis is Fig. 7. The crystal size of MAS as function of solution pH and calcination temperature.
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
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Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
temperature in comparison to pH. The grain growth in the powder obtained at neutral condition occurs slowly. This means that the crystal size of the powder prepared at neutral pH is smaller than the other studied cases. On the contrary, a shift towards greater sizes at higher calcination temperature takes place if preparation of solution is performed in the acidic or alkali environments. In conclusion, the calcination temperature affects predominantly the crystalline phase fraction and crystallite size. The most influential parameter is temperature and pH only affects the crystal size slightly. The effect of pH and calcination temperature on specific surface area was also investigated and the results are presented in graphical form in Fig. 8. It appears that the powders manufactured at acidic condition have the lowest specific values. Though, the surface area increases up to 19 m2 g1 due to calcination at 800 C, no significant change is observed when the preparation is carried out in acidic environment. The results presented in Fig. 8 show that the synthesis at neutral condition yields the highest surface area with a more pronounced effect for the powders prepared at pH of 10.5. Clearly, heating decreases the surface area and low calcination temperature favors to achieve large value. The synthesis at neutral pH and maturation at lower temperatures is the optimum procedure to obtain fine MAS particles. An alternative process to achieve fine particles with higher purity is the preparation of solution in the alkali condition. Basically, similar behavior is observed for powder synthesized at pH of 10.5. Regardless the formation of impurities, the preparation in the pH of 7.0 is interesting because the producer is easy to synthesize the nano-sized particles. The specific surface area of all materials manufactured at acidic and alkali pH remains constant when calcination is carried out at temperature lower than 600 C and a progressive increase is seen up to 800 C, which tends to reach areas near 19 and 55 m2 g1, respectively. The identical results were observed with increment in temperature for powder prepared at pH of 7.0. The heat treatment at 600 C causes the sintering of amorphous particles therefore the relatively reduction occurs in surface area. An increase in the pH leads to a marked change in surface area but the more increase in pH causes a slight decrease in surface area for powders calcined between 800 and 1200 C. When pH is considered to be 7.0 and/or 10.5, the results are in accordance with the data regarding crystallite size, as determined from XRD data. The specific surface areas of the
powders synthesized at the acidic condition are quite low and in disagreement with crystal size data. This phenomenon confirms the agglomeration of particles which took place from the beginning of synthesis. Regardless the crystal size of powders, the purity of MAS produced by the autoignition system under acidic condition is high but this way is unacceptable from the practical and engineering point of view. The synthesis of powder at the neutral pH allows obtaining the best results. The TEM micrograph presented in Fig. 9 shows the formation of nano-sized particles of MAS which was produced in neutral condition and calcined at 800 C. It is obvious that rather uniform particles, with an average size about 15 nm, were formed. However, this observation is in agreement with the result obtained by XRD. In the autoignition synthesis, the structure and texture of product are mainly determined by the combustion temperature. It is well known that product with large particle size, small surface area and high degree of crystallinity is obtained at high temperatures. Since the ignition is not an adiabatic process, for a given combustion reaction, the particle size and morphology are related to the following factors: combustion enthalpy, duration and evolved gas content. Since the fuel amount was kept constant in all of studied cases, the heat released by the combustion reaction was constant. Based on the thermodynamic data, the enthalpy of combustion is equal to 4680 kJ mol1 [34]. The shorter duration of combustion causes the higher temperature. In a short time, the most of the combustion heat is imposed on heating the products. Because the duration of combustion is mainly affected by the fuel content, the same condition can be considered for studied cases, consequently. The gaseous evolved during the combustion dissipates the heat of combustion and impedes the rise of temperature. The amount of evolved gases changes with the variation of fuel content. Furthermore, the gas evolution also helps in limiting the inter-particle contact and hinders particle growth. Apparently, the particle size is determined by competitive results of these factors. At the same time, it can be seen that all these factors are connected with the fuel content. The autoignition system described in this investigation forces on spinel formation at different environments by creating the same combustion conditions. The basic character of starting solution is the main reason for the arrangement of elements for manufacturing nano-sized powder. In order to better understand the relationship between the particle formation and pH, the value of specific surface area is plotted versus crystal size in Fig. 10. It is absolutely impossible to
Fig. 8. The specific surface area of synthesized powders as function of calcination and solution pH.
Fig. 9. The TEM micrograph of powder synthesized in neutral pH and calcined at 800 C.
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
Fig. 10. The variation of specific surface area with the crystal size.
achieve nano-sized MAS at acidic condition, though the pH of 2.5 is favor for formation of pure product. The neutral created condition in the reactor is high enough to produce nano-sized MAS particles after calcination below 1000 C. On the other hand, Fig. 10 indicates that the pH play a small role in the crystal growth mechanism and the crystal size distribution depends fundamentally on calcination temperature. The critical agglomeration in the precursor prepared in acidic condition and calcined below 1000 C, causes the low specific surface area. Finally, it is also important to point out that, the negligible changes is observed in the composition of products fabricated at pHs 7.0 and 10.5 if calcination is carried out below 1000 C. The predominate phase is MAS in the mentioned cases but the average crystallite size determined from X-ray diffraction peaks varies from 12 to 26 nm as the solution pH is raised from 7.0 to 10.5. 4. Conclusions The pH effect of starting solution on formation of magnesium aluminate particles by autoignition process was systematically studied by calcination of obtained precursors at different temperatures. A phase evolution map based on the optimal conditions for spinel formation is proposed. Although, the acidic environment is required to form pure phase, it is absolutely impossible to achieve the nano-sized particles. The impurities, such as MgO and Al2O3, evolve if the pH and calcination temperature are not properly controlled. Raising the initial solution pH and synthesis within the limits under study gives nano crystals. Interestingly, the nano-sized magnesium aluminate powder with negligible amounts of impurities can be synthesized in the neutral environment if calcination is carried out at temperatures lower than 1000 C. An optimum pH of the solution was determined for nano-sized MgAl2O4 formation, providing a useful guideline for the process control purposes. The obtained data make it possible to forecast conditions for synthesis of MgAl2O4 with specified mineralogical composition and crystalline structure. Acknowledgment The author thanks Prof. F. Bondioli for her helpful suggestions. References [1] J. Guo, H. Lou, H. Zhao, X. Wang, X. Zheng, Novel synthesis of high surface area MgAl2O4 spinel as catalyst support, Mater. Lett. 58 (2004) 1920e1923.
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Sinha, Low temperature synthesis of spinel (MgA12O4), Mater. Lett. 31 (1997) 281e285. [13] P.Y. Lee, H. Suematsu, T. Yano, K. Yatsui, Synthesis and characterization of nanocrystalline MgAl2O4 spinel by polymerized complex method, J. Nanoparticle Res. 8 (2006) 911e917. [14] I. Ganesh, B. Srinivas, R. Johnson, B.P. Saha, Y.R. Mahajan, Microwave assisted solid state reaction synthesis of MgAl2O4 spinel powders, J. Eur. Ceram. Soc. 24 (2004) 201e207. [15] I. Ganesh, R. Johnson, G.V.N. Rao, Y.R. Mahajan, S.S. Madavendra, B.M. Reddy, Microwave-assisted combustion synthesis of nanocrystalline MgAl2O4 spinel powder, Ceram. Int. 31 (2005) 67e74. [16] S. Tripathy, D. Bhattacharya, Rapid synthesis and characterization of mesoporous nanocrystalline MgAl2O4 via flash pyrolysis route, J. Asian Ceram. Soc. 1 (4) (2013) 328e332. [17] W. Kim, F. Saito, Effect of grinding on synthesis of MgAl O spinel from a powder mixture of Mg(OH)2 and Al(OH)3, Powder Technol. 113 (2000) 109e113. [18] H. Barzegar Bafrooei, T. Ebadzadeh, MgAl2O4 nanopowder synthesis by microwave assisted high energy ball-milling, Ceram. Int. 39 (8) (2013) 8933e8940. [19] C. Wang, S. Liu, L. Liu, X. Bai, Synthesis of cobalt-aluminate spinels via glycine chelated precursors, Mater. Chem. Phys. 96 (2006) 361e370. [20] J. Li, Y. Pan, F. Qiu, Y. Wu, J. Guo, Nanostructured Nd: YAG powders via gel combustion: the influence of citrate-to-nitrate ratio, Ceram. Int. 34 (2008) 141e149. [21] P.P. Hankare, V.T. Vader, U.B. Sankpal, R.P. Patil, A.V. Jadhav, I.S. Mulla, Synthesis and characterization of cobalt substituted zinc ferri-chromites prepared by solegel auto-combustion method, J. Mater. Sci. Mater. Electron. 22 (2011) 1109e1115. [22] K. Prabhakaran, D.S. Patil, R. Dayal, N.M. Gokhale, S.C. Sharma, Synthesis of nanocrystalline magnesium aluminate (MgAl2O4) spinel powder by the ureaeformaldehyde polymer gel combustion route, Mater. Res. Bull. 44 (2009) 613e618. [23] R. Ianos, R. Lazau, Combustion synthesis, characterization and sintering behavior of magnesium aluminate (MgAl2O4) powders, Mater. Chem. Phys. 115 (2009) 645e648. [24] O.G. Gromov, E.L. Tikhomirova, E.P. Lokshin, V.T. Kalinnikov, Synthesis of a magnesium aluminum spinel by the burning method, Russ. J. Appl. Chem. 85 (1) (2012) 16e19. [25] Sh Salem, Cobalt-aluminate nano pigment synthesized by combustion method: effect of calcination temperature on the colour behaviour, J. Ind. Eng. Chem. 20 (2014) 818e823. [26] Sh Salem, Relationship between gel rheology and specific surface area of nano-sized CoAl2O4 powder manufactured by autoignition technique, Mater. Lett. 139 (2015) 498e500. [27] J. Bai, J. Liu, C. Li, G. Li, Q. Du, Mixture of fuels approach for solution combustion synthesis of nanoscale MgAl2O4 powders, Adv. Powder Technol. 22 (2011) 72e76. [28] A. Saberi, F. Golestani-Fard, H. Sarpoolaky, M. Willert-Porada, T. Gerdes, R. Simon, Chemical synthesis of nanocrystalline magnesium aluminate spinel via nitrateecitrate combustion route, J. Alloys Compd. 462 (2008) 142e146. [29] J.C. Toniolo, M.D. Lima, A.S. Takimi, C.P. Bergmann, Synthesis of alumina powders by the glycineenitrate combustion process, Mater. Res. Bull. 40 (2005) 561e571. [30] R.D. Purohit, B.P. Sharma, K.T. Pillai, A.K. Tyagi, Ultrafine ceria powders via
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glycine nitrate combustion, Mater. Res. Bull. 36 (2001) 2711e2721. [31] Sh Salem, S.H. Jazayeri, F. Bondioli, A. Allahverdi, M. Shirvani, Effect of the synthesis conditions of cobalt aluminate nano-sized powder on the melting behaviour of transparent glaze, J. Ceram. Sci. Tech. 2 (3) (2011) 169e178. [32] G. Cambaz, M. Timucin, Compositional modifications in humidity sensing MgAl2O4 ceramics, Key Eng. Mater. 264e268 (2004) 1265e1268.
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Application of autoignition technique for synthesis of magnesium aluminate spinel in nano scale: Influence of starting solution pH on physico-chemical characteristics of particles Shiva Salem Chemical Engineering Group, Urmia University of Technology, Urmia, Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Autoignition technique was used for synthesis of nano-sized magnesium aluminate. The effect of pH on powder characteristics was studied. The specific surface area variation was related to pH and crystal size. The obtained result was confirmed by TEM observation.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 September 2014 Received in revised form 12 January 2015 Accepted 31 January 2015 Available online xxx
The present investigation is a follow-up of studies on glycine-nitrate autoignition technique for production of magnesium aluminate nano-particles. The technical characteristics of powders were studied in relation to a wide range of pH, 2.5e10.5. The effect of pH on particle formation was particularly emphasized to achieve a powder in nano-scale. In preparation of precursors the Mg/Al mole ratio was considered to be 0.50 and the level of fuel/NO3 was adjusted at 0.75. The thermal behavior of precursors was evaluated by DTA-TG technique. Also, the obtained powders were calcined at 600, 800, 1000 and 1200 C and characterized by XRD, BET, TEM and elemental analysis methods. The results indicated that the physico-chemical characteristics of products are strongly influenced by the pH of synthesis environment. The precursor prepared in pH of 2.5 exhibits flaming type ignition, while the precursors synthesized at neutral and alkali environments show underwent smoldering type combustion. In conclusion, the optimum characteristics were achieved when precursor is manufactured at neutral condition. © 2015 Elsevier B.V. All rights reserved.
Keywords: Oxides Chemical synthesis Nanostructures Surfaces
1. Introduction The nano-sized magnesium aluminate spinel (MAS), which is formulated as MgAl2O4, is one of the ceramic components. This
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material is used as catalyst support [1,2], refractory [3,4], optical devices [5,6], etc. MAS is a magnesium-based ceramic which is employed at metallurgy, electrochemical fields, dentistry applications, reinforcing fibers, and inorganic pigments [5] for over 10 years because of its excellent properties such as high melting point (2135 C), relatively low density (3.58 g cm3), well transmittance in the wavelength of 0.25e5.0 mm, high strength (180 MPa) and
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Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
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hardness (16 GPa), inert in acidic environments, low thermal expansion coefficient (9 106 C1 between 30 and 1400 C) and high thermal shock resistance [7]. The nano-sized MAS powder has been widely used in the fabrication of humidity sensors, transparent armor and visible-infrared windows [5,6]. Moreover, MAS is currently used for an effective refractory material for cement rotary kilns and steel ladles. The spinel also has been employed in manufacturing photo-luminescent materials and pressure vessels. The structure of magnesium aluminate spinel is cubic [8] and oxygen atoms are arranged in a close-packed cubic lattice. Mg and Al atoms occupy tetrahedral (1/8 of the available tetrahedral sites) and octahedral (1/2 of the available octahedral sites) positions, respectively. This arrangement is considered to be a good host capable for holding a large number of two and trivalent cations and forming a solid state solution. The synthetic MAS powder is produced by a variety of ceramic processing routes, including the wet-chemical process [5], precipitation [9], solegel [10e12], polymerized complex method [13], microwave [14,15] and pyrolysis [16]. Unfortunately many of these processing techniques are inherently complex to apply in the industrial scale and rigid control should be carried out over chemical compounds and morphology of particles. The products of mentioned techniques are affected by pH, time and temperature. The variations in these factors lead to some differences in the morphology and crystallographic structures. The deviation from stoichiometry and requirement of expensive precursors are the main problems in development above techniques in industrial scale. The dry grinding of magnesium and aluminum hydroxides, in the form of powder with a planetary ball mill, followed by heating is one of the oldest techniques for preparation of MAS in microscale [17]. The microwave assisted high-energy ball milling technique was applied to prepare nano-sized MAS, 28e149 nm [18]. According to the reported results, it is impossible to achieve the nano-sized pure spinel with calcination of Mg(OH)2 and Al(OH)3 mixture. In order to manufacture the particles smaller than 20 nm, the other techniques should be employed, consequently. The fuelnitrate autoignition technique shows a great potential in the preparation of ceramic components [19e21]. For the last two decades, this method has been attractive for many scientists to devote their attentions in studying the key factors of process [22e24]. In order to evaluate the combustion process, it is important to produce a solegel system, through changing the preparation conditions [25]. The formation of transparent gel usually indicates that a homogeneous system is obtained and the chemical reactions are well controlled under optimal conditions. This step is critical in the manufacture of corresponding material [26]. Among the publications about combustion process, nitrate systems are extensively studied and the main interest is focused on the good gel formation. In the autoignition synthesis method, a thermally induced redox reaction takes place between an oxidant and a fuel. In general, the nitrate salts, acting as cation sources, are used as oxidants, whereas organic compositions are employed as fuels [22e24]. From the application point of view, the fuel content is very important since it can be widely influence the crystal size of spinel [25,26]. The gel preparation should be taken to account to provide nano-particles with appropriate morphology. The gel formation by different fuels through the nitrate solution has been the subject of numerous investigations [27]. However, specific attempts were made to study the nano-sized MAS formation as a function of fuel composition. The nitrate transparent gel was formed using ureaeformaldehyde and the pH of solution was adjusted at 8.5 [22]. The nano particles in the range of 10e30 nm were produced after calcination at 850 C. The use of triethylenetetramine as a fuel yields a spinel powder with an average crystallite size of 4.9 nm
after annealing at 700 C, whilst urea and triethylenetetramine mixture leads to a powder with an average crystallite size of 30.8 nm after calcination at the same temperature [23]. It was demonstrated that urea, glycine, sugar, and mixtures of these materials are suitable fuels for synthesis of MAS from a stoichiometric mixture of aluminum and magnesium nitrates by the autoignition method. The single-phase MAS powder with crystallite size of 18.4 nm was obtained when sugar-urea was used as a fuel [24]. Nano-scale powders were successfully synthesized via combustion process using various mixtures of urea, glycine and starch. The addition of starch alters the ignition type from flame to smoldering combustion and lowers the temperature [22]. The nature of fuel used in the autoignition process greatly influences the crystal size. The longer chain of fuel molecule causes the larger crystallites [10]. Nano-crystalline MAS was synthesized by an autoignition process using citric acid, metal nitrates, and ammonia at relatively low temperatures. The pH of solution was controlled at level of 6. The fully crystallized MAS appeared at temperatures above 900 C [28]. In comparison to complex methods, the autoignition technique, based on the principles of propellant chemistry, has many advantages such as uniform composition, low cost and short reaction time. Therefore, it was known as attractive process for the synthesis of spinel powders, recently. The success of combustion is fundamentally related to non-violently reaction of fuel. The amino acids are well known to act as a complexing agent for a number of metal ions because this group of organic materials has a carboxylic acid at one end and amino at the other molecule section. Glycine, which is one of the amino acids, can effectively maintain compositional homogeneity among the constituents [29,30]. Therefore, glycinenitrate system has been a very effective method for the preparation of metal spinel. Although the kinetics of the reaction between fuel and nitrate salts has been widely investigated as above explanation however, to author knowledge, there are a few publications about the effect of solution pH on technical characteristics of magnesium aluminate particles. It seems that preparation condition has a considerable influence on the properties of MAS. Since solution pH is an important factor in combustion process, the effect of acidic, neutral and alkali conditions on the MAS characteristics is discussed in detail in present work.
2. Materials and methods 2.1. Synthesis process The magnesium aluminate precursors were prepared by solegel method using high purity grade of magnesium nitrate, Mg(NO3)2.3H2O, and aluminum nitrate, Al(NO3)3.9H2O, both manufactured by SigmaeAdrich Company, as starting materials. A solution with molar ratio of 0.50 was obtained through the dissolution of nitrate salts in de-ionized water. The mixture was vigorously stirred to form a transparent solution and its temperature was maintained at 25 C. Then glycine, NH2CH2COOH (SigmaeAdrich, 99.5 wt.% as received), was slowly added drop-wise to the nitrate solution and stirred to achieve a homogenous mixture. The glycine content was set above the stoichiometric amount at fuel/NO3 of 0.75 which can be calculated according to propellant chemistry [26,31]. The resulted solution was divided into three parts and the pH of samples was controlled by adding nitric acid and ammonia solutions at three levels of 2.5, 7.0 and 10.5. In the next step, water has been removed from the solutions through the heating at 110 C by a hot plate. The sol converted to a viscous gel by evaporation of water and the reaction between the magnesium and aluminum nitrates, leading to the formation of hydrate components have been identified as:
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
MgðNO3 Þ2 $3H2 O þ AlðNO3 Þ3 $9H2 O þ 5NH4 ðOHÞ/MgðOHÞ2 þ AlðOHÞ3 þ 5NH4 NO3 þ 12H2 O (1) Afterwards, the viscous gels were transferred to a furnace preheated at 300 C and ignited spontaneously with rapid evolution of gases, producing foamy and voluminous powders. The flame combustion was extinguished after 1 min, while the smolder combustion lasted more than 7 min. The autoignition of precursor synthesized at pH of 2.5 was flaming type, whilst the precursors prepared at neutral and alkali pHs underwent smoldering type ignition. The spinel formation can be explained as:
MgðOHÞ2 þ 2AlðOHÞ3 /MgAl2 O4 þ 4H2 O
(2)
Because the time for autoignition is rather short, to remove traces of un-decomposed glycine, nitrates and their decomposition products, the powders should be further heated. In order to recrystallize the MAS particles, the resulting powders were also calcined at different temperatures, 600, 800, 1000 and 1200 C, for 1 h. 2.2. Precursor and powder characterizations The autoignition of precursors resulting from the gel formation process was evaluated by simultaneous differential thermal analysis and thermogravimetric test (DTA-TG, Model 409, Netzsch, Germany) at a heating rate of 10 C min1 in air atmosphere. Approximately, 10 mg of each precursor was placed in a ceramic pan and temperature was increased from 25 to 600 C, steadily. In order to better evaluate the ignition of fuel after calcination, the residual hydrogen, carbon and nitrogen amounts were determined by elemental analysis (Carlo Erba, Model EA 1110, Italy). The phase transformation during heat treatment and crystallite size evolutions were carried out by X-ray diffraction analysis using a Philips diffractometer (X'PERT PRO, Philips Research Laboratories, The Netherland) in the diffraction angle range of 10e80 with CuKa radiation. The Scherer's equation was used to calculate the crystallite size, D [32].
D¼
0:9l b cos q
(3)
where b is the breadth of observed diffraction peak at its half-
Fig. 1. The DTA curves of precursors synthesized at different pH.
3
intensity, q is the Bragg angle, and l is the radiation wavelength, 0.15406 nm. The specific surface areas of the resulting powders were determined with low temperature nitrogen adsorption isotherms, using Brunauer, Emmett and Teller (BET) technique (Gemini 2360 Apparatus, Micromeritics, Norcross, GA, USA). Finally, transmission electron microscopy (TEM, JEM 2010, JEOL, Tokyo, Japan) was used in characterizing the particle morphology. For this purpose, particles were deposited onto cupper grids, which support a holey carbon film. The particles were placed onto the support grids by deposition from a dilute suspension. The particle shapes and sizes were characterized by high resolution imaging technique. 3. Results and discussion 3.1. Effect of pH on autoignition The DTA curves of precursors synthesized at different pHs are shown in Fig. 1. Only the thermal effect related to ignition of glycine are observable at 275 C. For precursors prepared at the acidic, neutral and alkali environments the additional exothermic peaks did not appear. With increase in pH, the position of exothermic peak is negligibly shifted to lower temperatures. The TG curves, Fig. 2, of the precursors show three well-defined weight loss regions due to the loss of physically absorbed water below 250 C, ignition of glycine in narrow temperature range, 250e280 C, and decomposition of remained organic materials at higher temperatures, above 280 C. In the present study, the low temperature weight loss can be assigned to the water absorbed by gels, whereas the weight loss about 10.0e15.0 wt.% is observed. On comparing the TG profiles, it is seen that pH of synthesis environment cannot affect the dehydration reaction of gels, considerably. However, in the high temperature, the weight loss is extraordinarily high in all studied cases and 60.0e65.0 wt.% of mass is removed due to autoignition reaction between the fuel and nitrate. The weight losses and associated thermal effects at narrow temperature range of 250e280 C are due to the thermal decomposition of glycine. Between 280 and 600 C, a part of the weight loss, 10.0e15.0 wt.% is due to the burning of the residual glycine remained between the particles. According to the principle of propellant chemistry for redox reaction between a fuel and an oxidizer, the following reaction can be considered for autoignition:
Fig. 2. The TG curves of precursors synthesized at different pH.
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9 5 1 H2 NCH2 COOH þ O2 /2CO2 þ H2 O þ N2 4 2 2
(4)
It should be emphasized that glycine can react with ammonia:
H2 NCH2 COOH þ 4:5NH4 NO3 /2CO2 þ 11:5H2 O þ 5N2
(5)
For glycine-nitrate ignition, primarily N2, CO2 and H2O are evolved as the gaseous products. When the glycine content is larger than stoichiometric value, oxygen would be involved to ensure the complete combustion of glycine in fuel-rich condition. Thus, the combustion reaction can be represented as a follows:
MgðNO3 Þ2 $3H2 O þ 2AlðNO3 Þ3 $9H2 O þ 8NH4 OH þ 2H2 NCH2 COOH þ
1 O /MgAl2 O4 þ 4CO2 þ 46H2 O þ 9N2 2 2 (6)
In this case, the total weight loss for the manufacturing one mol MgAl2O4 is about 89.0e93.0 wt.%, and it is near to the theoretical value of 89.8 wt.%. These results of thermal analysis give a strong evidence for the adoption of 275 C as the initiation temperature to start the combustion reaction. Different ignition phenomena resulting from different synthesis conditions have been observed. The incandescent flame was governed after autoignition of precursor prepared at pH of 2.5 whilst the smoldering combustion is observed for precursors manufactured at pHs of 7.0 and 10.5. The optimum calcination temperature should be determined to ensure the complete decomposition of precursors. In overall, the variations of carbon, nitrogen and hydrogen are similar to each other, with differences in the amounts, Table 1. However, the amounts of organic elements are dramatically different when calcination is carried out at temperatures lower than 800 C. In general, the calcination of powders prepared at different environments, above 800 C, cusses the significant reduction of fuel content. The removal of glycine by calcination is independent of pH. The term of combustion covers the flaming, gas phase, smoldering, solidegas, reactions. The effect of pH on autoignition is a direct consequence of different prevailing glycine and hydroxides species. NHþ 3 CH2COOH and NH2CH2COO are the dominant species for acidic and alkali conditions, respectively. Also, NHþ 3 CH2COO is dominant species at neutral environment [33]. In the acidic environment, the rate of protonation and dissolution of hydroxides is high as a result, the interaction between NHþ 3 CH2COOH and ions are limited. The absorbed glycine is attached with hydroxides via hydrogen bounds. Therefore, the flame type combustion occurs in this case. In the neutral and alkali environments, the interaction
Table 1 The elemental analyses of calcined powders synthesized at different pH. Element (wt.%)
pH: 2.5 Carbon Nitrogen Hydrogen pH: 7.0 Carbon Nitrogen Hydrogen pH: 10.5 Carbon Nitrogen Hydrogen
between the ions of hydroxides and glycine occurs via -COO and NH2 in NH2CH2COO, respectively. These interactions are beneficial for dispersion of ions and facilities the reaction between the solid and gas, consequently. Therefore, the autoignition decomposition needs higher energy and the smoldering type combustion is observable in these conditions.
3.2. Phase evolution Fig. 3 represents the XRD patterns for powders calcined at 600 C after synthesis at different pHs. The powders show a strong amorphous background and contain a negligible amount of poorly crystalline MAS. With decrease in pH a gradual improvement in crystallinity is observed, in terms of peak sharpness of MAS phase (JCPDS 01-075-1796). These changes continued until calcination at 800 C in which the powders are predominantly contain crystalline phase, Fig. 4. After calcination, considerable improvement in reflection intensity is seen in the powder synthesized at acidic condition. MgO (JCPDS 00-045-0946) seems to be the only impurity phase in the powders synthesized at neutral and alkali conditions when the materials are calcined at 1000 C, Fig. 5. When the synthesis is carried out at these conditions, the negligible amounts of Mg and Al cannot well arranged in a close-packed cubic lattice and remain free. These cations are re-crystallized at higher calcination temperatures as oxides. The MgAl2O4 formation by solidesolid reaction of MgO and Al2O3 involves with diffusion of Mg2þ onto alumina particle cores. The intensities of reflections related to MgO and Al2O3 increase concomitantly with increment in calcination temperature. Moreover, Al2O3 usually does not show the sharp peek in the presence of MgAl2O4 and it is hardly distinguishable when calcination is performed at temperatures lower than 1000 C. These observations suggest that the critical condition is essentially required to achieve pure MAS through the proposed route. Consequently, for those powders subject to different calcination temperature, MgO was formed due to incomplete reaction. However, further examination of XRD diagrams in calcination range of 600e1000 C shows different crystallinity. These crystalline phases imply the complexity of the real reaction pathways, which are certainly much more complex than that suggested by Eq. (6). According to Figs. 3e5, MgeAl intermediate transforms to crystalline MAS at elevated temperatures. The higher calcination temperature results in even stronger crystalinity, with little or no amorphous phase as illustrated in Fig. 6. Two rather reflection lines at 2q of 42.99 and 62.26 are displayed in these patterns, suggesting that the Eq. (6) is not completed when solution preparation is performed in pH range of 7.0e10.5. A comparative inspection of Figs. 4e6 indicates that the MgO can evolve from the transformation of the amorphous MgeAl intermediate if the solution preparation is carried out at neutral and alkali conditions. Al2O3 (JCPDS 01-081-
Calcination temperature ( C) As obtained
600
800
1000
1200
8.74 3.77 1.06
2.77 1.20 1.11
0.64 0.15 0.50
0.50 0.12 0.19
0.38 0.11 0.0
7.80 3.65 2.02
2.62 0.89 1.38
0.89 0.12 0.91
0.56 0.0 0.47
0.25 0.0 0.0
8.59 3.85 2.15
2.01 0.72 1.40
0.76 0.10 0.83
0.58 0.0 0.28
0.49 0.0 0.16
Fig. 3. The XRD patterns of precursors synthesized at different pHs and calcined in 600 C.
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Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
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Fig. 4. The XRD patterns of precursors synthesized at different pHs and calcined in 800 C.
Fig. 6. The XRD patterns of precursors synthesized at different pHs and calcined in 1200 C.
Fig. 5. The XRD patterns of precursors synthesized at different pHs and calcined in 1000 C.
performed at neutral condition. However, the spinel is better crystallized in other cases. The crystal size is more sensitive to
2266) was formed at high calcination temperature, i.e. 1200 C, when initial solution pH is controlled at level of 7.0. According to XRD patterns, it is absolutely impossible to perform the solid state reaction at temperature lower than 1200 C to form spinel. As postulated above, the XRD diagrams suggest that progress of the reaction towards the right side of Eq. (6) leads to the formation of full crystalline MAS if pH of solution is adjusted at level of 2.5. Eq. (6) is approximately completed to form the MgeAl intermediate and the most of Mg and Al are consolidated into the spinel structure after calcination in temperature range of 800e1200 C. It can be concluded that the pure magnesium aluminate is obtained by the initial preparation of solution in acidic environment. The effects of pH and calcination temperature on crystal size of MAS obtained at various processing conditions are presented in Fig. 7. Altering the solution condition and calcination temperature can influence the crystal size of powders. Raising the pH of the initial solution yields a finely crystalline product with crystal size of 12 nm. The thermal treatment of the products at 1000 C results in re-crystallization of finely crystals. As temperature rises, the crystal size increases from 22 to less than 50 nm when synthesis is Fig. 7. The crystal size of MAS as function of solution pH and calcination temperature.
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temperature in comparison to pH. The grain growth in the powder obtained at neutral condition occurs slowly. This means that the crystal size of the powder prepared at neutral pH is smaller than the other studied cases. On the contrary, a shift towards greater sizes at higher calcination temperature takes place if preparation of solution is performed in the acidic or alkali environments. In conclusion, the calcination temperature affects predominantly the crystalline phase fraction and crystallite size. The most influential parameter is temperature and pH only affects the crystal size slightly. The effect of pH and calcination temperature on specific surface area was also investigated and the results are presented in graphical form in Fig. 8. It appears that the powders manufactured at acidic condition have the lowest specific values. Though, the surface area increases up to 19 m2 g1 due to calcination at 800 C, no significant change is observed when the preparation is carried out in acidic environment. The results presented in Fig. 8 show that the synthesis at neutral condition yields the highest surface area with a more pronounced effect for the powders prepared at pH of 10.5. Clearly, heating decreases the surface area and low calcination temperature favors to achieve large value. The synthesis at neutral pH and maturation at lower temperatures is the optimum procedure to obtain fine MAS particles. An alternative process to achieve fine particles with higher purity is the preparation of solution in the alkali condition. Basically, similar behavior is observed for powder synthesized at pH of 10.5. Regardless the formation of impurities, the preparation in the pH of 7.0 is interesting because the producer is easy to synthesize the nano-sized particles. The specific surface area of all materials manufactured at acidic and alkali pH remains constant when calcination is carried out at temperature lower than 600 C and a progressive increase is seen up to 800 C, which tends to reach areas near 19 and 55 m2 g1, respectively. The identical results were observed with increment in temperature for powder prepared at pH of 7.0. The heat treatment at 600 C causes the sintering of amorphous particles therefore the relatively reduction occurs in surface area. An increase in the pH leads to a marked change in surface area but the more increase in pH causes a slight decrease in surface area for powders calcined between 800 and 1200 C. When pH is considered to be 7.0 and/or 10.5, the results are in accordance with the data regarding crystallite size, as determined from XRD data. The specific surface areas of the
powders synthesized at the acidic condition are quite low and in disagreement with crystal size data. This phenomenon confirms the agglomeration of particles which took place from the beginning of synthesis. Regardless the crystal size of powders, the purity of MAS produced by the autoignition system under acidic condition is high but this way is unacceptable from the practical and engineering point of view. The synthesis of powder at the neutral pH allows obtaining the best results. The TEM micrograph presented in Fig. 9 shows the formation of nano-sized particles of MAS which was produced in neutral condition and calcined at 800 C. It is obvious that rather uniform particles, with an average size about 15 nm, were formed. However, this observation is in agreement with the result obtained by XRD. In the autoignition synthesis, the structure and texture of product are mainly determined by the combustion temperature. It is well known that product with large particle size, small surface area and high degree of crystallinity is obtained at high temperatures. Since the ignition is not an adiabatic process, for a given combustion reaction, the particle size and morphology are related to the following factors: combustion enthalpy, duration and evolved gas content. Since the fuel amount was kept constant in all of studied cases, the heat released by the combustion reaction was constant. Based on the thermodynamic data, the enthalpy of combustion is equal to 4680 kJ mol1 [34]. The shorter duration of combustion causes the higher temperature. In a short time, the most of the combustion heat is imposed on heating the products. Because the duration of combustion is mainly affected by the fuel content, the same condition can be considered for studied cases, consequently. The gaseous evolved during the combustion dissipates the heat of combustion and impedes the rise of temperature. The amount of evolved gases changes with the variation of fuel content. Furthermore, the gas evolution also helps in limiting the inter-particle contact and hinders particle growth. Apparently, the particle size is determined by competitive results of these factors. At the same time, it can be seen that all these factors are connected with the fuel content. The autoignition system described in this investigation forces on spinel formation at different environments by creating the same combustion conditions. The basic character of starting solution is the main reason for the arrangement of elements for manufacturing nano-sized powder. In order to better understand the relationship between the particle formation and pH, the value of specific surface area is plotted versus crystal size in Fig. 10. It is absolutely impossible to
Fig. 8. The specific surface area of synthesized powders as function of calcination and solution pH.
Fig. 9. The TEM micrograph of powder synthesized in neutral pH and calcined at 800 C.
Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066
Sh. Salem / Materials Chemistry and Physics xxx (2015) 1e8
Fig. 10. The variation of specific surface area with the crystal size.
achieve nano-sized MAS at acidic condition, though the pH of 2.5 is favor for formation of pure product. The neutral created condition in the reactor is high enough to produce nano-sized MAS particles after calcination below 1000 C. On the other hand, Fig. 10 indicates that the pH play a small role in the crystal growth mechanism and the crystal size distribution depends fundamentally on calcination temperature. The critical agglomeration in the precursor prepared in acidic condition and calcined below 1000 C, causes the low specific surface area. Finally, it is also important to point out that, the negligible changes is observed in the composition of products fabricated at pHs 7.0 and 10.5 if calcination is carried out below 1000 C. The predominate phase is MAS in the mentioned cases but the average crystallite size determined from X-ray diffraction peaks varies from 12 to 26 nm as the solution pH is raised from 7.0 to 10.5. 4. Conclusions The pH effect of starting solution on formation of magnesium aluminate particles by autoignition process was systematically studied by calcination of obtained precursors at different temperatures. A phase evolution map based on the optimal conditions for spinel formation is proposed. Although, the acidic environment is required to form pure phase, it is absolutely impossible to achieve the nano-sized particles. The impurities, such as MgO and Al2O3, evolve if the pH and calcination temperature are not properly controlled. Raising the initial solution pH and synthesis within the limits under study gives nano crystals. Interestingly, the nano-sized magnesium aluminate powder with negligible amounts of impurities can be synthesized in the neutral environment if calcination is carried out at temperatures lower than 1000 C. An optimum pH of the solution was determined for nano-sized MgAl2O4 formation, providing a useful guideline for the process control purposes. The obtained data make it possible to forecast conditions for synthesis of MgAl2O4 with specified mineralogical composition and crystalline structure. Acknowledgment The author thanks Prof. F. Bondioli for her helpful suggestions. References [1] J. Guo, H. Lou, H. Zhao, X. Wang, X. Zheng, Novel synthesis of high surface area MgAl2O4 spinel as catalyst support, Mater. Lett. 58 (2004) 1920e1923.
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Please cite this article in press as: Sh. 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, Materials Chemistry and Physics (2015), http://dx.doi.org/ 10.1016/j.matchemphys.2015.01.066