Science of Sintering, 36 (2004)73-79
___________________________________________________________________________
UDK 621.926.085:661.183.8:622.785
Influence of Mechanical Activation of Al2O3 on Synthesis of
Magnesium Aluminate Spinel
Zhang Zhihui*, Li Nan
Hubei Province Key Lab of Refractories and Ceramics, Wuhan University of Sci. &
Tec, Wuhan, Hubei, P.R.China, 430081.
Abstract:
Magnesium aluminate (MA) spinel is synthesized by reaction sintering from alumina
and magnesia. The effects of mechanical activation of Al2O3 on reaction sintering were
investigated. Non-milled α - Al2O3 and α - Al2O3 high-energy ball milled for 12h, 24h and
36h were mixed with a MgO analytical reagent according to the stoichiometric MA ratio,
respectively and pressed into billets with diameters of 20mm and height of 15mm. The greenbody billets were then sintered at high temperature in an air atmosphere. The results show
that bulk density, relative content of MA and grain size of MA increase with increasing highenergy ball milling time of Al2O3. However prolonged milling time over 24h has a small
beneficial effect on the densification of MA. Bulk density and grain size of a sample of
α3
Al2O3 milled for 24h are 3.30g/cm and 4-5 µm, respectively.
Keywords: Mechanical activation; Magnesium aluminate spinel; Sintering; Densification
1. Introduction
Magnesium aluminate spinel (MA), which is the only stable compound in the MgOAl2O3 system, possesses a high melting point (2135oC), good mechanical strength and
excellent chemical resistance etc. [1-3]. It is becoming more and more important in
refractories and ceramics [4]. Especially in recent years, as the hazardous character of chrome
bearing materials is exposed, work and use of magnesium aluminate spinel are very important
[5]. The major application areas of MA refractories are transition and burning zones of
cement rotary kilns, side walls and bottom of steel teeming ladles and checker work of glass
tank of furnace regenerators instead of MgO-Cr2O3 refractories. However the reaction of
MgO and Al2O3 to form MA is accompanied by a volume expansion of approximately 7%,
making it difficulty to obtain a dense reaction sintered body [6]. Hence a two stage firing
process is employed, the first one is to synthesize MA. In the second stage the MA
synthesized is ground, pressed and sintered in order to be densified, which results in
increasing cost of products. To obtain a dense reaction sintered body, researchers have studied
several other processes such as freeze drying [7], sol-gel of metal alkoxides or inorganic [8],
hydroxide coprecipitation [9] etc. However these chemical processes also have some intrinsic
disadvantages. For instance, coprecipitation usually uses aluminum and magnesium chloride
or nitrate salts, making repeated washing necessary to remove the anions, which will alter the
composition designed and need a lot of water [10].
The effect of fineness of magnesia and alumina on the formation and densification of
MA has been studied a lot. There are numerous studies of the influence of raw material
_____________________________
*)
Corresponding author: [email protected], [email protected]
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characteristics on the sintering of MA. Ritwik Sarkar et al [11] studied the effect of the
calcination temperature of Al2O3 on the densification of rich magnesium spinel (MgO 34
wt.%). The results showed that sintered density does not change greatly with the increase in
calcination temperature of alumina up to 1200oC, but calcination at 1600oC resulted in
reduced sinterability due to greater agglomeration of alumina particles, leading to larger
particle size and smaller surface area of alumina particles. Kostic and others [12] found that
the grinding energy increases the surface area and structural imperfections and observed MA
formation at lower temperature after prolonged grinding of starting materials. We studied the
effect of polymorphism of Al2O3 on the synthesis and densification in reaction sintering of
MA and found that γ- Al2O3 as a raw material instead of α- Al2O3 can improve synthesis and
densification of MA [13].
Mechanical activation processing or high-energy ball milling, which was initially
invented for ceramic strengthened alloys [14], can produce grain size down to the nanoscale,
together with structure defects. Energetic lattice defects, combined with short diffusion
distances, are the driving forces of faster solid-state alloying and chemical reactions at low
temperatures [15]. Some authors report that some systems can react during high-energy ball
milling [16]. At present, this method has been successfully used to synthesize a wide range of
nano-sized ceramic powders and has become one of the most promising methods of
promotion of multi-component system reactions. In the present work, we will study the
synthesis and densification of MA from MgO and Al2O3 that was treated by a high-energy
ball milling process for different times.
2. Experimental procedure
The starting materials used in this study were α- Al2O3 and an MgO analytical
reagent. α- Al2O3 was the production of gibbsite heated at 1400oC for 4h. Its X-ray patterns
are shown in Fig.1. The composition of the raw material is given in table 1. From Fig.1 we
can see that α- Al2O3 is the dominant crystal phase and there is no other phase in the XRD
pattern, which means gibbsite has already converted to α- Al2O3 at 1400oC.
Table I Compositions(mass %), particle size (µm) and surface area(m2/g) of raw
materials
Particle Surface
Type
Al2O3 SiO2 Fe2O3 TiO2 K2O Na2O IL MgO
size
area
97.86 0.88 0.07 0.02 0.08 0.45
5.06
43.36
aAl2O3
MgO
0.01 0.14 0.08 0.07 0.09 0.08
99.30
5.00
46.03
Note: IL is ignition loss. The operation temperature is 1000-1050oC for 1h.
α- Al2O3 was milled in a QM-SB type planetary ball milling system with a stainless
steel pot in air at room temperature for 12h, 24h and 36h, respectively. The stainless steel ball
to powder weight ratio in the pot was 10:1. Every pot was filled with 50g - Al2O3 and 70g
ethanol. The milling speed was 220 rpm. Non-milled α- Al2O3 and α- Al2O3 milled in a highenergy ball for different times were mixed with a MgO analytical reagent according to the
stoichiometric MA ratio, respectively. Green-body billets 20mm in diameter and height of
15mm were prepared by cold-pressing under a pressure of 200MPa, in a stainless-steel die.
The billets were then heated at 1400oC and 1600oC for 3h in an air, respectively.
XRD was performed (D/MAXβB) using Ni filtered Cu Kα under the following
conditions: scanning speed of 2°min-1 and temperature of 16oC. The density and porosity of
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the sintered specimens was measured by the Archimedes method. The microstructure and
grain size of sintered samples were examined via SEM (Philip 3lx). In order to study the
influence of milling time on the amount of MA formed, the relative content of MA of four
samples sintered at 1000oC were measured with a standard-free qualitative method proposed
by Zevin [17].
Fig.1 X-ray patterns of Al(OH)3
sintered at 1400oC for 4h
This is not an accurate method to measure the phase content in samples, but it can be
used to compare the phase content of Al2O3 samples in the same synthesis conditions with
different milling times. The phase content can be calculated from the following formula:
⎧ n ⎡⎛ I i J
⎪∑ ⎢⎜⎜1 −
⎪ i =1 ⎣⎝ I iK
⎨
⎪n
⎪∑ xiK = 1
⎩ i =1
⎤
⎞
⎟⎟ xiK u mi ⎥ = 0
⎠
⎦
In the formula: IiJ and IiK is the intensity of i phase of sample J and K respectively; xiK is the
content of i phase of sample K.In order to minimize errors, we used polynomial linear
regression.
3. Results and discussion
Fig. 2, Fig. 3 and Fig. 4 give the relationships between bulk density, apparent porosity
and linear changes of samples and milling time of α- Al2O3 at 1400oC and 1600 oC,
respectively. The milling time of α- Al2O3 had a great effect on the densification of MA.
Milling sharply increased the bulk density and linear changes and decreased apparent porosity
of MA but prolonged milling time over 24h had little effect on the densification of MA.
Fig. 5 gives the relative content of MA in the samples made from α- Al2O3 milled for
0, 12, 24 and 36h, respectively, and heated at 1000 oC. It is found that the MA content
increases with increasing milling time of Al2O3 but prolonged milling time over 24h had
almost no effect on the content of MA.
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Fig. 2 Effect of milling time on the bulk
density of samples
Fig. 3 Effect of milling time on the apparent
porosity of samples
Scanning electron photomicrographs of four samples sintered at 1600 oC for 3h are
shown in Fig.6. It is noted in Fig.6 that at the same sintering temperature, the grain size of
MA in the samples is different. The MA in the sample with non-milled Al2O3 has the smallest
grain size (≤2um). The grain size of MA increases with increasing milling time of Al2O3 but
prolonged milling time over 24h had little beneficial effect on the growth of MA. The sample
of Al2O3 milled for 24h has the largest grain size (4-5 µm). All grain sizes were measured
using the linear intercept method [18].
Fig. 4 Effect of milling time on the linear
changes of samples
Fig. 5 Effect of milling time on the relative
content of spinel of samples sintered at 1000oC
Formation of magnesium aluminate spinel from its constituent oxides is a counter
diffusion process of Al3+ and Mg2+ ions [19]. In the MA formation based on the Wagner
mechanism [20], oxygen ions remain at the initial sites. In order to keep electroneutrality,
3Mg2+ diffuse towards the alumina side and 2Al3+ diffuses towards the magnesia side, so four
MgO change to one MA at the MgO side and four Al2O3 change to three MA.
Reduction of particle size can decrease the distance between vacancy sites (or that of
grain boundaries) and enhance the vacancy diffusion to external surface and thus help
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formation of MA and grain growth. Fig.7 shows X-ray diffraction patterns of α-- Al2O3
milled for 12, 24 and 36h. It can be seen from Fig.7 that the diffraction peak of α-- Al2O3
becomes broad and smooth with increasing of high-energy ball milling time.
A
B
C
D
Fig. 6 SEM photographs of the samples sintered at 1600o for 3h: (A) 0h, (B) 12h (C)24h
(D)36h
Fig. 7 X-ray diffraction patterns of high-energy ball milled for 12h, 24h and 36h α- Al2O3
That implies an increase of distortion and defects of α- Al2O3 crystals, which improve
reactivity and sinterability of α- Al2O3. This is one of the factors resulting in increasing of
MA at 1000oC and higher bulk density of samples heated at 1400 and 1600 oC. When the
milling time is longer than 24h it has little effects on sintering.
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There may be two reasons for this. One is that the particle size of Al2O3 only changes
a little when the milling time increases from 24h to 36h. The other may be that a very fine
particle size may have a small negative effect on sintering because of agglomeration of fine
particles [21].
4. Conclusion
Reaction sintering of MgO and Al2O3 is significantly enhanced by mechanical
activation of Al2O3. Bulk density, relative content of MA and grain size of samples increase
with increasing milling time of Al2O3. However when prolonged milling time is more than
24h, the effects of milling time on bulk density of samples and grain size of MA in the
samples are small.
References
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2. M.Sindel, N.A.Travitzky, N.Claussen, Influence of magnesium-aluminum spinel on
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3. G.Baudin, R.Martinez, P.Pena, High-temperature mechanical behavior of
stoichiometric magnesium spinel, Journal of the American Ceramic Society. 78(7)
(1995) 1857-1862
4. W.Bakkar, J.G.Lindsay, Reactive magnesia spinel, preparation and properties,
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9. G.Gusmano, P.Nunziante, E.Traversa, G.Chiozzini, The mechanism of MgAl2O4
spinel formation from the thermal decomposition of coprecipitated hydroxides,
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activated by a high-energy ball milling process, Materials Letter 56 (2002) 238-243
11. Ritwik Sarkar, Samir Kumar Das, Goutam Banerjee, Effect of attritor milling on the
densification of magnesium aluminate spinel, Ceramics International 25 (1999) 485489
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temperature processes in solids, Proceeding of the 4th European Ceramic Society
Conference 2 (1995) 319-326
13. Zhihui Zhang, Nan Li, Effect of polymorphism of Al2O3 on the synthesis of
magnesium aluminate spinel, Accepted by Ceramics International.
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Резюме: Шпинель алюмината магния (МА) синтезировали реакционным спеканием
глинозема и магния. Изучали воздейсвие механической активации Al2O3 на реакционное
спекание. Неизмельченный α-Al2O3 и α-Al2O3 предварительно измельченный в
высокоэнергетической шаровой мельнице в течение 12,24 и 36 часов, перемешивали с
аналитическим реагентом МgO в соответствии с стехиометрическим соотношением
МА и получали прессовки, диаметр которых составлял 20 мм, высота 15 мм. Спекание
прессовок проводили при высоких температурах на воздухе. Из полученных
результатов можно сделать вывод, что с продолжением времени измельчения Al2O3.
плотность, относительное содержание МА и размер зерен МА увеличиваются.
Однако, с продлинением времени измельчения свыше 24 часов на уплотнение МА
положительного влияния не имеет.Плотность и размер зерен α-Al2O3, измельченного в
течение 24 часа, 3,30 г/3 и 4 – 5 мкм, соответственно.
Ключевые слова: механическая активация; шпинель алюмината магния; спекание;
уплотнение.
Садржај: Магнезијум-алуминатни (МА) спинел синтетисан је реакционим
синтеровањем алумине и магнезијума. Проучени су ефекти механичке активације Al2O3
на реакционо синтеровање. Немлевен α-Al2O3 и α-Al2O3 млевен у кугличном млину
велике енергије у току 12, 24 и 36 сати мешани су са аналитичким реагенсом MgO у
складу са стехиометријским односом МА и пресовани у испреске пречника 20 mm и
висине 15 mm. Испресци су затим синтеровани на високим темературама у
атмосфери ваздуха. Резултати показују да се густина, релативни садржај МА и
величина зрна МА увећавају са продужењем времена млевења Al2O3.. Међутим,
продужено време више од 24 ч. нема повољан утицај на згушњавање МА. Густина и
величина зрна узорка α-Al2O3 млевеног 24 ч. износе 3,30 g/cm3 и 4-5 µm, респективно.
Кључне речи: механичка активација; магнезијум- алуминатни спинел; синтеровање;
згушњавање.