Science of Sintering, 42 (2010) 133-141 ________________________________________________________________________

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*) Corresponding author: zbbjh@sdut.edu.cn

doi: 10.2298/SOS1002133B UDK 622.785:661.846 Solution Combustion Synthesis and Sintering Behavior of Porous MgAl2O4 Powders J.H. Bai*), J.C. Liu School of Materials Science and Engineering, Shandong University of Technology, Zibo, 255049, Shandong, P.R. China Abstract: Porous MgAl2O4 powders were synthesized with a solution combustion route using a mixture of glycine and urea with the glycine/urea molar ratio of 2/9 as fuel. For the comparison purpose, denser powders were also combustion synthesized using urea as fuel. The porous/denser structure, characteristics (e.g. crystallite size and specific surface areas) and sinterability of the two powders were investigated in detail. Experimental results disclosed that the as-prepared porous powders exhibited a much lower degree of agglomeration, smaller average agglomerate particle size and larger surface areas and thus far higher sintering behavior than the denser powders. Key words: Solution combustion synthesis; Porous powders; MgAl2O4; Sintering behavior

Introduction

MgAl2O4 spinel has attracted increasing attention owing to its wide applications in

many industrial areas, such as refractory materials [1], humidity sensors [2] and transparent ceramics [3]. The synthesis of high purity and high reactivity MgAl2O4 is influenced by the production method, so many novel chemical routes, e.g. the citrate-nitrate route [4], sol-gel technique [5], Pechini [6] and coprecipitation method [7], have been extensively investigated. Among the various wet chemical routes, the solution combustion technique has been regarded as one of the effective and economic methods due to its convenient processing, simple experimental setup and significant time-saving and high purity products [8, 9, 10]. Therefore, it is widely employed to synthesize MgAl2O4 powders [11, 12, 13, 14]. For example, high reactivity MgAl2O4 powders were combustion synthesized using starch and triethylenetetramine as fuel, respectively, and then were used to produce well sintered ceramics [10, 15]. On the other hand, Mimani reported that glycine fuel could lead to the formation of porous combustion synthesized spinel powders [16]. However, to our best knowledge, the effect of porous structure on the sintering behavior of MgAl2O4 powders synthesized using the solution combustion technique has not been reported to date. Therefore, it was proposed to synthesize porous MgAl2O4 powders via a solution combustion technique using a fuel mixture with the glycine/urea molar ratio of 2/9 and to investigate its sintering behavior. In addition, for the purpose of comparison, denser MgAl2O4 powders were also combustion synthesized and their sinterability was also studied.

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2. Experimental 2.1. MgAl2O4 synthesis MgAl2O4 powders were combustion synthesized using analytical purity Al(NO3)3·9H2O, Mg(NO3)2·6H2O, urea (U, CH4N2O) and glycine (G, C2H5NO2) as starting materials. First, Al(NO3)3·9H2O and Mg(NO3)2·6H2O were dissolved into 10 ml deionized water. Then the solutions were heated to 70 °C and kept for 1 h under magnetic stirring, followed by adding fuel(s) into them with the glycine /urea molar ratio of 0/1 and 2/9, respectively. Here the main reasons that the two fuels were used as starting materials were as follows: First, a denser and porous structure of the MgAl2O4 powders can be formed by using the fuel mixture with the glycine/urea molar ratio of 0/1 and 2/9, respectively. In addition, there was no remarkable difference in the average crystallite size of the MgAl2O4 spinel in two powders. Furthermore, MgAl2O4 spinel could be directly synthesized with the solution combustion route using the two fuel mixture without further heat treatment [13, 14, 15]. In two cases, a stoichiometric metal nitrates/fuels molar ratio was used [17] and 12.07 g of spinel was designed to be produced. After magnetically stirred at 70 °C till viscous gels were formed, the two redox mixtures contained in china crucibles were quickly put into a muffle furnace preheated to 500 °C, respectively. After several minutes, the solution precursors boiled, swelled, evolved a large amount of gases and finally were ignited, followed by yielding voluminous products. After soaking in a furnace for 10 min, the combustion products were pulled out quickly. It should be noted that the product from the fuel mixture was much more voluminous that that from the single fuel of urea. Finally, the products were dry ball milled for 1 h. Thereinafter, the combustion product from the fuel mixture with the glycine /urea molar ratio 2/9 and from urea were referred to as product A and B, respectively. 2.2. Ceramics fabrication The as-prepared powders were granulated in a agate mortar with PVA solution (5 wt%) as binder. Then the powders were bidirectionally pressed into rectangular compacts of Φ10.0 mm × 2.0 mm under the pressure of 20 MPa. The powder compacts were heated up to 1650 oC in air and soaked for 2 h, followed by natural cooling in the furnace. 2.3. Characterization

The phase compositions of the two powders were determined using an X-ray diffractometer (XRD, D8 Advance, Brucker, Germany), equipped with a Ni-filtered Cu Kα radiation source (λ=0.154178 nm). The average crystallite size was calculated according to XRD patterns using the peaks corresponding to the (311), (400) and (440) plane and the following Scherrer formula:

θ

λcosFWHH

9.0•

=XRDD (1)

where DXRD is the crystallite size (nm); λ is the radiation wavelength (0.15406 nm); FWHH is the full-width at half-maxima (radian); θ is the Bragg angle (degree).

The lattice parameter was calculated by the following equation: )( 222 lkhda hkl ++•= (2)

where a is the average lattice parameter (Å), d is the interplanar distance (Å), h k l are the Miller indices (the same as used for DXRD calculation).

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The specific surface area of the two powders was determined by BET measurement on N2 adsorption at 77k (BELmax 00035, Ankersmid, Netherlands) after heat treatment at 523 K for 2 h. The morphology of the powders and the microstructures of polished surfaces of the as-fabricated ceramics from the two products were characterized with a field emission scanning electron microscope (FESEM, Sirion 2000, FEI, Netherlands). The two powders were also characterized by TEM (H-800, HITACHI, Japan).The ignition losses (IOL) of the two powders were determined after annealing them at 900 oC for 2 h .The particle size distribution analysis was conducted using a laser particle size analyzer (BN-2009, Bona Ltd, China). Bulk density and apparent porosity of the as-prepared ceramics were determined by the Archimedes method using distilled water as liquid media. The chemical compositions of the two products were measured with inductively coupled plasma emission spectrometry (ICP, Optima 2000, Inc. Perkin Elmer). The flame temperature was determined with an IR pyrometer (Cyclops100, Keno Ltd, China). Tab. I Relevant thermodynamics data [15, 19]

Compound ≥Hf (293K), Kcal mol-1 Cp( cal (mol K)-1)a

Al(NO3)3·9H2O (c) -897.57 - Mg(NO3)2·6H2O(c) -622.21 - CO(NH2)2(c) -79.71 - NH2CH2COOH(c) -126.34 - MgAl2O4(c) -547.38 36.80+0.0064T CO2(g) -94.05 10.34+0.00274T H2O(g) -57.80 7.20+0.0036T N2(g) 0 6.5+0.0010T

(c): Crystallite (g): gas T: absolute temperature. a Calculated from the discrete alues.

Moreover, the enthalpies of combustion reactions and theoretical adiabatic flame temperature as a function of the fuel compositions were approximately calculated using the data listed in the tab. I and the following equation: ∫ ∑−=∫ ∑−=∑−∑= Ta p

TaT prp dTnCdTnCHnHnH 298 )()(0ΔΔΔ (3)

where ≥Hp and ≥Hr are the enthalpies of formation of the products and reactants, respectively; Ta is the theoretical adiabatic flame temperature, T0 is the room temperature of 298 K; Cp is the molar heat capacity of products at constant pressure. 3. Results and discussion 3.1. Synthesis and characterization

Fig.1 shows the XRD patterns of the two powders. As shown in Fig. 1, the main crystal phase of the two products was MgAl2O4 spinel. However, there was a little periclase in product A. In spite, the peaks of product A were far more intensive and sharper than those of product B, so it was believed that the MgAl2O4 crystals in powders A were much better crystallized. According to the Scherrer formula, the crystallite size of the powders A, B was calculated as 39.2 and 33.1 nm, respectively, which was mainly associated with the combustion temperature reached during combustion. As listed in tab. II, the combustion temperature of product A was much higher than that of product B, as a result, the former exhibited a larger average crystallite size.

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Fig. 1 XRD patterns of the combustion synthesized powders. (a) The product A from a fuel mixture with the urea/glycine molar ratio of 9/2, (b): The product B from urea.

Fig. 2 displays the FESEM micrographs of the two powders. As shown in Fig. 3(a1) and Fig. 3(b1), the powders B exhibited much denser agglomeration, while the powders A mainly consisted of much more porous aggregates. In addition, platy agglomerates were mostly formed in the product A, of which the cross section normal to the surfaces of the platy agglomerates looked like a honeycomb structure as shown in the Fig. 2(a1) and Fig. 2(a2), while there were mainly granular particles in the product B. Moreover, the average size of the agglomerates was significantly decreased with increasing of the glycine/urea molar ratio from 0/1 to 2/9.

Fig. 2 FESEM micrographs of the combustion synthesized MgAl2O4 powders. (a1) and (a2): The product A, (b1) and (b2): The product B

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Fig. 3(a2) and Fig. 3(b2) present the typical magnified image of product A and B, respectively. The two images further confirmed that the powders B were of denser sintered agglomeration, while the powders A were much more porous. Furthermore, many walls between pores in the product A were packed with nanoparticles ranging from 30 nm to 60 nm, in agreement with its crystallite size calculated using the Scherrer formula, revealing that nanoscale particles was formed in the samples. To confirm the difference of the two powders in average crystallite size, their TEM images are shown in Fig. 3. The product A exhibited a much lower degree of agglomeration, i.e. were better dispersed and its crystallite size in the range of 40 to 70 nm could be readily determined according to the image, in an agreement with that shown in Fig. 1. However, the product B exhibited harder agglomeration and only a few nanoparticles were observed in the TEM images as shown in the Fig. 3(b), so it was hard to determine its average crystallite size. The difference in the degree of agglomeration of the two products could be confirmed by their particle size distribution.

Fig. 3 TEM images of (a): the porous product A and (b): the denser product B

Fig. 4 presented the particles size distribution of the two products. As shown in Fig. 4,

the particle size distribution of the porous product A was much narrower than that of the product B. Their medium diameters were 3.78 and 6.88 μm, respectively, indicating that the porous structure resulting from the fuel mixture was helpful to decrease the average size of agglomerates and increase specific surface area.

Fig. 4 Particle size distribution of the porous product A and the denser product B

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To further clarify the difference in properties of the two combustion products, the characteristics of the two products were listed in Tab. II. As can be seen from Tab. II, the two products were both white and their ignition losses were less than 1 wt%, indicating that the combustion reactions were complete and there was no residual carbon or other charred organic materials in them. Meanwhile, more energy was emitted and a higher flame temperature was reached during combustion for product A, which was confirmed by the fact that the combustion in the product A was visibly more vigorous and faster than that of the product B, thus crystallization of MgAl2O4 spinel in product A was significantly promoted. In addition, the BET surface area of product A was much larger than that of product B. However, as listed in Tab. II, the combustion temperature of product A was higher than that of product B, and the crystallite size of the former was slightly larger than the latter, which both usually lead to lower BET surface area. Therefore, it was believed that the porous powder structure of product A was, at least partly, responsible for its lower degree of agglomeration and higher BET specific surface.

Tab. II Characteristics of the two products

Product Combustuion reaction Color IOL

(wt %)

Reaction enthalpy

(Kcal mol-1)

Adiabatic temperature a

(K)

IR temperature

(K)

A

Flame combustion, 31.2 s,

bright glowing incandescence

white 0.97 -396.0 1480 1540±100

B

Flame combustion, 39.5 s,

Glowing incandescence

white 0.86 -372.3 1419 1400±100

Specific surface area (g/cm-3) DXRD (nm)

α (Å) Chemical composition (wt%)

16.3 39.2 8.073 Al2O3: 71.33 MgO: 28.08 Na2O: 0.16 K2O: 0.12 CaO: 0.20 SiO2: 0.11

6.1 33.1 8.076 Al2O3: 71.23 MgO: 28.13 Na2O: 0.17 K2O: 0.14. CaO: 0.18 SiO2: 0.15

To explain the difference in porous/denser structure and degree of dispersion of the two

products, the amount of gases escaped during the spinel formation and the evolution rate of gases were investigated. Based on the propellant chemistry, it was assumed that the combustion reactions amongst metal nitrates and fuels were complete and N2, O2 and H2O (g) evolved during combustion [17]. Thus, the amount of gases in the two cases could be determined as follows:

Therefore, theoretically, 29.56 and 30.67 mol gases were evolved, respectively, when 1 mole MgAl2O4 spinel was formed in cases A and B. It was reported by. Ganesh et al that, generally, the specific surface area and fineness of the powders were a function of the amount of gases evolved during combustion [15]. However, in this present work, it was not true.

(4) O(g)115/9H(g)65/9CO(g)86/9N)(OMgAl NOH10/9CON5CH)Mg(NO)2Al(NO : (A)

22242252242333

+++→+++

s

(5) O(g)40/3H(g)20/3CO(g)32/3N)(OMgAl ON20/3CH)Mg(NO)2Al(NO :(B)

22242242333

+++→++

s

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Therefore, the gas evolution rate of the two products was taken into consideration. Theoretically, the gas evolution rate of products A and B was 0.95 mol/s and 0.77 mol/s, respectively. Noted that product A were much more voluminous than product B, revealing that the powders A were more dissipated by the gases escaped during combustion. Since 12.07 g of MgAl2O4 powders was both designed to be produced in the two products, a larger volume implied that there was more vacant space in the powders A. i.e. more pores (higher porosity) was formed in the powders A. Therefore, a much larger volume of the product A was mainly attributed to its higher evolution rate of the gases, revealing that the gas evolution rate during combustion played an important role in formation of the porous structure and increasing specific surface area.

3.2. Sintering behavior

To investigate the effect of the porous/denser structure on sintering behavior, Water absorption (W.A), apparent porosity (A.P), bulk density and relative density (R.D) of the MgAl2O4 ceramics sintered at 1650 °C for 2 h from the powders A, B were summarized in Tab. III. As can be seen from Tab. III, water absorption and apparent porosity of the sintered bodies in powders A was far less than that of those in powders B, and the relative sintered density of former was about 20 % larger than that of the latter, i.e. the powders A exhibited far better sintering behavior than the powders B. Since there was no remarked difference in the chemical compositions of the two powders, the better sintering behavior of product A could be mainly attributed to the smaller average agglomerate size and larger surface specific area. So it was believed that the porous powder structure, lower degree agglomeration and larger surface area of product A were mainly responsible for its higher sinterability as compared with product A. It was worth noting that the bulk density of the ceramics from powders A was 3.09 g cm-3, lower than that (3.44 g cm-3, 96.0 % of theoretical) reported by Ganesh [14]. However, the green density in our work was no larger than 1.2 g cm-3, far smaller than that (1.9 g cm-3 or so) produced by Ganesh [14]. At the same time, bulk density is one of the key factors affecting the sintered density of the spinel ceramics [18]. Therefore, it is believed that if higher pressing pressure is applied, the sintered density of product A could be significantly increased.

Tab. 3 Sintering behavior of the two products

Product W.A (%) A.P (%) B.D (g cm-

3) R.D (%)

A 1.59 4.90 3.07 85.73

B 15.92 36.4 2.29 63.90

To confirm the structure (porous or denser) dependent sintering behavior of the

MgAl2O4 powders, the microstructures of the polished surfaces of the sintered bodies from the two products were observed with FESEM and shown in Fig. 5. As shown in Fig. 5, the porosity of the ceramics from product A (ceramics A) was much less and its average pore size was remarkably smaller than that of the product B (ceramics B). However, the average grain size of the ceramics A appeared to be smaller than that of the ceramics B. The difference in microstructure of the two ceramics may be, at least partly, related to the porous/denser structure. In short, the densification of the ceramics A was significantly improved owing mainly to its porous structure, smaller average aggregate size and larger surface area. At the

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same time, as there were many pores in the agglomerates of product A, growth of some grains may be depressed during sintering. On the other hand, the agglomerates of product B were much denser, so it was believed that the grain growth in the denser agglomerates may be accelerated during sintering and thus larger average grain size resulted.

Fig. 5 FESEM micrographs of the MgAl2O4 ceramics from (a, c): the product A and (b, d): the product B

Finally, it should be noted that the sintered body from powders A was not dense

enough, so in our further work, highly porous, better dispersed and higher reactivity MgAl2O4 powders would be synthesized and much denser ceramics would be fabricated.

4. Conclusions

Porous MgAl2O4 powders were successfully synthesized with the solution combustion route using glycine and urea as fuel. The porous powders A exhibited much lower agglomeration, higher specific area and thus far higher sinterability than the denser powders B. The porous structure of the MgAl2O4 powders played a key role in decreasing agglomeration, increasing surface area and improving sintering behavior. Acknowledgement The author gratefully acknowledges the financial support by the Science & Technology Plan of the Education Department of Shandong Province (Grant No: J08LA10).

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Садржај: Синтетисани су порозни прахови MgAl2O4 коришћењем сагоревања мешавине глицина и урее са моларним односом 2/9 као горива. У сврху поређења, гушћи прахови су такође синтетисани саогервањем коришћењем урее као горива. Детаљно су прочене порозна/густа структура, карактеристике (величина кристалита, специфична површина) и синтерабилност ова два праха. Експериментални резултати су показали да су припремљени порозни прахови имали мањи степен агломерације, мању просечну величину честица агломерата и веће површине честица и стога бољу синтерабилност од гушћих прахова. Кључне речи: Синтеза сагоревањем раствора, порозни прахови, MgAl2O4, синтеровање.