In this paper, the synthesis of ceramic spinel MgAl2O4 by starch assisted sol - gel process is presented. The gel mixture was prepared by using Mg(NO3)2.6H2O, Al(NO3)3.9H2O as precursors and starch as a gel agent. The study focused on relevant components of gel mixture and the calcinating temperature. The results showed that suitable molar proportion of Mg2+/Al3+; starch/(Mg2+ + Al3+) and H2O/starch were 0.5, 0.4 and 60, respectively. The calcination at               1100 oC for 60 minutes was suitable for forming phase of spinel. The product was a single phase of spinel MgAl2O4 with excellent crystallinity and uniform size in the range of 300 to 400 nm

Hai, Ho Van Minh

Ngoc, Tran Minh

Quyen, Nguyen Duc Vu

Tin, Dang Xuan

Tuyen, Tran Ngoc

Vietnam Academy of Science and Technology: Journals Online

  
Tạp chí Khoa học và Công nghệ 53 (4) (2015) 433-440 
DOI: 10.15625/0866-708X/53/4/5777 
A STUDY ON THE SYNTHESIS OF MgAl2O4 SPINEL                                
BY STARCH ASSISTED SOL-GEL PROCESS 
Tran Ngoc Tuyen*, Nguyen Duc Vu Quyen, Tran Minh Ngoc, Dang Xuan Tin,                 
Ho Van Minh Hai 
Department of Chemistry, Hue College of Sciences, 77 Nguyen Hue Str., Hue  
*Email: trntuyen@gmail.com 
Received: 22 December 2014, Accepted: 12 May 2015 
ABSTRACT 
In this paper, the synthesis of ceramic spinel MgAl2O4 by starch assisted sol - gel process is 
presented. The gel mixture was prepared by using Mg(NO3)2.6H2O, Al(NO3)3.9H2O as 
precursors and starch as a gel agent. The study focused on relevant components of gel mixture 
and the calcinating temperature. The results showed that suitable molar proportion of Mg2+/Al3+; 
starch/(Mg2+ + Al3+) and H2O/starch were 0.5, 0.4 and 60, respectively. The calcination at               
1100 oC for 60 minutes was suitable for forming phase of spinel. The product was a single phase 
of spinel MgAl2O4 with excellent crystallinity and uniform size in the range of 300 to 400 nm. 
Keywords: spinel MgAl2O4, ceramic material, metal-starch precursor. 
1. INTRODUCTION 
Spinel MgAl2O4 is stable compound consisted of MgO and Al2O3 constituents and has the 
face-centred cubic (FCC) lattice formed by oxygen ions. While Mg2+ cations occupy at centre of 
tetrahedron holes and Al3+ cations occupy at the centre of octahedron holes. Spinel MgAl2O4 has 
many wonderful chemical and physical properties such as mechanical endurance, chemical 
stability, thermal and electrical durability. In the past, spinel MgAl2O4 was an important ceramic 
material used for lining unburnable material, as substrate for thermal durable pigments in 
ceramic industry [1, 2].  
Nowadays, spinel has many applications in manufacturing of filter-super ceramic 
membrane, electrical insulator and optical material. So, many studies have reported chemical 
and physical properties and applications of MgAl2O4 material paying special attention on its 
synthesis methods. There are some different ways to synthesize this material such as traditional 
ceramic method, coprecipitation, sol-gel and Pechini method [2, 3, 4, 5, 6, 9]. Among them, 
starch assisted sol-gel method exhibits such important advantage that Al3+, Mg2+ cations are 
dispersed highly thanks to the formation of complex between these cations and -OH group from 
starch, and as a result, the spinel phase formation will take place at low temperature. This paper 
presents the synthesis of spinel MgAl2O4 by starch assisted sol-gel process.  
 
  
Tran Ngoc Tuyen et al.  
434 
2. EXPERIMENTS 
The chemicals used in this work were Mg(NO3)2.6H2O, Al(NO3)3.9H2O and dissolved 
starch (QuangZu, China). The molar ratio of Mg2+/Al3+ in the synthesized mixture was fixed at 
0.5. Two other ratios of starch/(Mg2+ + Al3+) and H2O/starch would be changed in the survey. 
The heating process included two stages: the first step was at 60 – 70 oC for 1 hour with the aim 
of hydrolyzing starch, and the second one was at 80 – 90 oC for 3 - 4 hours and stirred in order to  
evaporate water slowly until the mixture became viscous and gel of metal-starch precursor was 
formed. The gel was dried at 105oC till stable mass. For the purpose of completely burning starch, 
the gel after drying was preheated. After that, the obtained mixture was grinded and pressed into 
cylinder shape with the diameter of 30 mm and the thickness of 5 mm by hydraulic press with the 
compression of 300 kG.cm-2 (Danir, Denmark). Then, the sample was sintered in furnace 
(Lennton, England) at suitable temperature. 
The synthesis of pigment was processed as shown in Schema 1. 
 
Schema 1. The diagram of the synthesis of pigment by starch assisted sol-gel process. 
The obtained material was characterized by thermal analysis (TG-DSC) using Labsys 
TG/DSC Setaram (France) in ambient atmosphere with the maximum temperature of 800 oC, 
heating rate of 10 oC.min-1.  
Crystal phase of the sample was determined by X-ray diffraction (XRD) using Brucker D8 
Advance with λCuKα = 1,5406 Å.  
The crystallite size, D of spinel was calculated by Scherrer’s equation [6]:  
Magnetic stirring              
60oC/1.0 h 
Metals-starch complex 
precursor 
Magnetic stirring              
80oC/4.0 h 
Metals-starch gel 
precursor 
Mixture of oxides     
(MgO + Al2O3) 
Preheating 
Spinel MgAl2O4 
Heating 
Reaction medium  
Mg(NO3)2.6H2O Al(NO3)3.9H2O Starch H2O 
  
Synthesis of CoxMg1-xAl2O4 as cobalt blue ceramic pigments by starch assisted sol-gel process 
435 
0,9D
FWHM cos
λ
=
× θ
 
where λ is the X wavelength (Å), θ is the diffraction angle (rad) of the (311) peak with highest 
intensity and FWHM is the abbreviation of full width at half maximum of diffraction peak.  
IR spectrum of the sample was analyzed on IRPrestige-21 (Shimadzu, Japan). Morphology 
of obtained sample was observed by scanning electron microscope (SEM) using Hitachi S4800 
(Japan). 
3. RESULTS AND DISCUSSION 
As the ratio of H2O/starch was too high the gel would be dilute resulting a long time for 
evaporation of water, reverse the gel would be too viscous to stir.  
Four samples with the molar ratio of Mg2+/Al3+ = 0.5; starch/(Mg2+ + Al3+) = 0.4 and 
H2O/starch increased from 50 to 80 corresponding with the notations from A1 to A4 were 
prepared as shown in Table 1. 
Table 1. The initial composition of samples with different molar ratio of H2O/starch. 
Notation Starch/Mn+ H2O/starch 
Percentage composition (w/w) 
Mg(NO3)2.6H2O Al(NO3)3.9H2O Starch H2O  
A1 0.4 50 11.2 32.9 8.5 11.2 
A2 0.4 60 10.3 30.0 7.8 10.3 
A3 0.4 70 9.5 27.7 7.2 9.5 
A4 0.4 80 8.8 25.6 6.6 8.8 
(Mn+ = Mg2+ + Al3+). 
The results showed that as the molar ratio of H2O/starch was 50, the obtained gel was too 
viscous to stir, whereas it was rather dilute if this ratio increased to 70 and 80, so, it took a long 
time to evaporate water. Therefore, the molar ratio of H2O/starch = 60 was  to select for further 
surveys. 
Table 2. The initial composition of samples with different molar ratio of starch/(Mg2+ + Al3+). 
Notation Starch/Mn+ H2O/starch 
Percentage composition (w/w) 
Mg(NO3)2.6H2O Al(NO3)3.9H2O Starch H2O  
B1 0.3 60 12.1 35.3 6.9 45.8 
B2 0.4 60 10.3 30.0 7.8 51.9 
B3 0.5 60 8.9 26.1 8.5 56.5 
B4 0.6 60 7.9 23.1 9.0 60.0 
B5 0.7 60 7.1 20.8 9.4 62.7 
  
Tran Ngoc Tuyen et al.  
436 
The molar ratio of starch/(Mg2+ + Al3+) could effect on the formation of precursor. So, in 
order to determine the most suitable value, five samples possessed the molar ratio of Mg2+/Al3+ = 
0.5, H2O/starch = 60 and starch/(Mg2+ + Al3+) increased from 0.3 to 0.7 corresponding with the 
notations from B1 to B5 were prepared. The initial compositions of the samples are shown in 
Table 2. 
The images of samples after preheating at 500 oC shown in Figure 2 indicate that the 
samples of B3, B4 and B5 were gray. The reason is because starch was burned uncompletely, so 
there was a large amount of starch existed in precursor. On the contrary, samples of B1 and B2 
after being sintered at 1100 oC were light white. However, we chose the molar ratio of 
starch/Mn+ = 0.4 corresponding with B2 sample because the gel from this sample was more 
viscous than B1 sample. 
     
Figure 2. Samples images after preheating at 500 oC for 1 hour. 
0 100 200 300 400 500 600 700 800
-90
-60
-30
0
30
60
16,6 %
31,9 %
23,2 %
751oC
333oC
399oC
238oC
161oC
DS
C 
(µV
)
TG
 
(%
)
Temperature (oC)
-80
-60
-40
-20
0
20
 
Figure 3. TG-DSC diagram of metals-starch gel precursor. 
DSC-TG diagram of B2 sample is presented in Figure 3. As can see, there was a strong 
endothermic peak in the range from room temperature to 160 oC with the weight loss of 23.2 %. 
This corresponded with the physical-water evaporation and the water seperation from -OH 
groups on the surface of material [7, 9]. Next, there were endothermic peaks at 238oC and 333oC 
and exothermic peak at 399 oC with the total weight loss of 32 %. This might be originated from 
the decomposition of nitrate salts, the burning of precursor and abundant starch, respectively. In 
the temperature range from 450 oC to 800 oC, we could see an exothermic peak at 751 oC 
B5 B4 B3 B2 B1 
  
Synthesis of CoxMg1-xAl2O4 as cobalt blue ceramic pigments by starch assisted sol-gel process 
437 
corresponding with spinel formation from MgO and Al2O3 creating from the decomposition of 
precursor. The weight loss in this period was negligible, about 2.4 % and we predict that nitrate 
salt continued to decompose, precursor and abundant starch continued to combust completely. 
From above-mentioned, we chose the preheating and spinel forming temperatures to be of                
500 oC and 800 oC. 
With the aim of determination of suitable sintering temperature, we calcinated the sample 
at various temperatures 900, 1000 and 1100 oC for 1.5 hours with the heating rate of 10 oC.min-1. 
The notations of samples were PTB900, PTB1000 and PTB1100. XRD patterns are shown in 
Figure 4. The characteristic peaks of spinel at 2θ = 19.03o (d111); 31.32o (d220); 36.87o (d113); 
44.83o (d400); 59.43o (d115); 65.29o (d440) were observed with high intensity at 900oC while 
characteristic peaks of MgO and Al2O3 were not appeared, indicating that the formation of spinel 
took place completely. The values of FWHM, I and D of samples are shown in Table 3. As the 
calcinated temperature increases from 900 to 1100 oC the intensity of diffraction peaks of spinel 
increases significantly, the value of FWHM reduced rapidly from 0.652o to 0.175o and the value 
of D increased promptly from 13 to 47 nm.  
 
 
Figure 4. XRD diagram of spinel MgAl2O4. 
Table 3. The value of FWHM, diffraction intensity (I) and crystallite size (D) of spinel samples. 
Notations FWHM  (o) I     (cps) D    (nm) 
PTB900 0.652 268 13 
PTB1000 0.311 493 27 
PTB1100 0.175 743 47 
When increasing the temperature from 900 to 1100 oC, the crystalization of spinel took 
place strongly, Mg2+ and Al3+ cations diffused contrariwise through spinel layer according to 
Wagner mechanism [1]. At 1100 oC, the single phase of spinel MgAl2O4 was obtained with high 
crystalinity. 
20 30 40 50 60 70
S: MgAl2O4
S
S
S
S
S
SS
200 cps
PTB900
PTB1000
PTB1100
Li
n 
(cp
s)
2 Theta Scale (o)
  
Tran Ngoc Tuyen et al.  
438 
 
Figure 5. The structure of metals-starch gel precursor [9]. 
According to traditional ceramic method in study of Kong and coworkers [5], the initial 
mixture contained MgO and Al2O3 were sintered at the temperature from 900 to 1300 oC for 120 
minutes, the obtained material was multi-phase including spinel and corundum. Only with the 
sinterned temperature higher than 1400oC, the sample exhibited  single phase of spinel. 
 
Figure 6. FTIR spectrum and SEM image of spinel MgAl2O4.  
As can see, by starch assisted sol-gel method, solid phase reaction between MgO and Al2O3 
took place favourably, the formation of spinel phase occured at low temperature. Hence, the 
sintering temperature could be lower in comparison with that by traditional ceramic method. 
This could be explained  by the fact that dissolved state (amilose) of starch had kinky polymer 
chain structure with 3 –OH group for each link and they played three main roles: (i) ligand of the 
complex formation bewteen OH group and metal cations, (ii) structure directing agent and (iii) 
formed gel when heating because it could dissolve in water completely. This helped metal 
4000 3500 3000 2500 2000 1500 1000 500
0
20
40
60
80
100
523686
1111
1627
3441
T%
1/cm
  
Synthesis of CoxMg1-xAl2O4 as cobalt blue ceramic pigments by starch assisted sol-gel process 
439 
cations disperse highly and the surface area bewteen MgO and Al2O3 was increased resulting 
solid phase reaction took place favourably (Figure 5) [4, 6, 7, 9].  
IR spectrum  of spinel MgAl2O4 is demonstrated in Figure 6. The band around wave 
numbers of 3441 and 1627 cm-1 corresponds to stretching and bending variations of water 
molecule adsorped on the surface of sample. Besides, the bands around wave number of 686 and 
523 cm-1 corresponding with symmetrical valence variation inside octahedrons [AlO6] and 
lattice variation of Mg-O bond extension. This met well with the study of Nassar and coworkers 
[6]. SEM image of MgAl2O4 is shown in Figure 6. That particles were agglomerated and formed 
large particle with the size in the range of 300 - 400 nm. This size is larger used in comparision 
with crystallite size calculated from Scherrer’s equation (about 47 nm) suggesting that particles 
on SEM image may consists of many crystallite particles.  
4. CONCLUSION 
Spinel MgAl2O4 material was synthesized successfully by starch assisted sol-gel method. 
Main advantage of this way was the sintering temperature was much lower than that used in 
traditional ceramic method. The suitable conditions were the molar ratio of Mg2+/Al3+ = 0.5; 
starch/(Mg2++Al3+) = 0.4; H2O/starch = 60; hydrolyzing temperature was 60 – 70 oC; gelation 
temperature was 80 – 90 oC. The gel was preheated at 500 oC for 1.0 hour and sintered at                
1100 oC for 1.0 hour. The obtained product was characterized by modern method such as XRD, 
TG-DSC, FTIR and SEM. The results showed that spinel MgAl2O4 material consists of particles 
in nanoscale. The obtained spinel is of single phase with high crystallinity. 
Acknowledgements. This research was financially supported by the project of Hue University: "Study on 
synthesis of some pigments using for ceramic on spinel MgAl2O4", Code DHH2014-01-70. 
REFFERENCES 
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gel-combustion method, Ceramics International 41 (2015) 3178-3185. 
3. Alvar E. N., Rezaei M., Alvar H. N. - Synthesis of mesoporous nanocrystalline MgAl2O4 
spinel via surfactant assisted precipitation route, Powder Technology 198 (2010) 275-278. 
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of nanoscale MgAl2O4 powders, Advanced Powder Technology 22 (2011) 72-76. 
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high-energy ball milling process, Materials Letters 56 (2002) 238-243. 
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nanoparticles using sol–gel auto combustion method and their photocatalytic properties, 
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8. George D. G., David L. D. - Modeling the X-ray diffraction pattern of opal-Cf, American 
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TÓM TẮT 
NGHIÊN CỨU TỔNG HỢP VẬT LIỆU GỐM SPINEL MgAl2O4 BẰNG PHƯƠNG PHÁP 
SOL-GEL CÓ SỰ HỖ TRỢ CỦA TINH BỘT 
Trần Ngọc Tuyền*, Nguyễn Đức Vũ Quyên, Trần Minh Ngọc, Đặng Xuân Tín,                                       
Hồ Văn Minh Hải 
Khoa Hóa, Trường Đại học Khoa học Huế, 77 Nguyễn Huệ, Huế 
*Email: trntuyen@gmail.com 
Bài báo trình bày kết quả tổng hợp vật liệu gốm spinel MgAl2O4 bằng phương pháp sol-gel 
có sự hỗ trợ của tinh bột. Hỗn hợp gel được chuẩn bị từ Mg(NO3)2.6H2O, Al(NO3)3.9H2O và 
tinh bột tan. Các điều kiện tổng hợp bao gồm: tỷ lệ mol H2O/tinh bột; tỷ lệ mol tinh bột/(Mg2+ + 
Al3+) và nhiệt độ nung đã được khảo sát. Kết quả cho thấy với thành phần phối liệu có các tỷ lệ 
mol Mg2+/Al3+ = 0,5; tinh bột/(Mg2+ + Al3+) = 0,4; H2O/tinh bột = 60 và nhiệt độ nung thiêu kết 
1100 oC trong 60 phút, sản phẩm thu được hoàn toàn đơn pha tinh thể spinel MgAl2O4, kích 
thước hạt cở 300 - 400 nm. 
Từ khóa: spinel MgAl2O4, vật liệu gốm, tiền chất kim loại - tinh bột. 
 
 
 


A STUDY ON THE SYNTHESIS OF MgAl2O4 SPINEL BY STARCH ASSISTED SOL-GEL PROCESS

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A STUDY ON THE SYNTHESIS OF MgAl2O4 SPINEL BY STARCH ASSISTED SOL-GEL PROCESS

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