Influence of Mechanical Activation of Al 2 O 3 on Synthesis of Magnesium Aluminate Spinel

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 αAl2O3 milled for 24h are 3.30g/cm and 4-5 μm, respectively.


Introduction
Magnesium aluminate spinel (MA), which is the only stable compound in the MgO-Al 2 O 3 system, possesses a high melting point (2135 o C), good mechanical strength and excellent chemical resistance etc. [1][2][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-Cr 2 O 3 refractories.However the reaction of MgO and Al 2 O 3 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]._____________________________ *) Corresponding author: zhangzhihui99@sohu.com,linanref@public.wh.hb.cn 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 characteristics on the sintering of MA.Ritwik Sarkar et al [11] studied the effect of the calcination temperature of Al 2 O 3 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 1200 o C, but calcination at 1600 o C 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 Al 2 O 3 on the synthesis and densification in reaction sintering of MA and found that γ-Al 2 O 3 as a raw material instead of α-Al 2 O 3 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 Al 2 O 3 that was treated by a high-energy ball milling process for different times.

Experimental procedure
The starting materials used in this study were α-Al 2 O 3 and an MgO analytical reagent.α-Al 2 O 3 was the production of gibbsite heated at 1400 o C 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-Al 2 O 3 is the dominant crystal phase and there is no other phase in the XRD pattern, which means gibbsite has already converted to α-Al 2 O 3 at 1400 o C. α-Al 2 O 3 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 -Al 2 O 3 and 70g ethanol.The milling speed was 220 rpm.Non-milled α-Al 2 O 3 and α-Al 2 O 3 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 1400 o C and 1600 o C for 3h in an air, respectively.
XRD was performed (D/MAXβB) using Ni filtered Cu K α under the following conditions: scanning speed of 2°min -1 and temperature of 16 o C. The density and porosity of 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 1000 o C were measured with a standard-free qualitative method proposed by Zevin [17].In the formula: I iJ and I iK is the intensity of i phase of sample J and K respectively; x iK is the content of i phase of sample K.In order to minimize errors, we used polynomial linear regression.

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-Al 2 O 3 at 1400 o C and 1600 o C, respectively.The milling time of α-Al 2 O 3 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-Al 2 O 3 milled for 0, 12, 24 and 36h, respectively, and heated at 1000 o C. It is found that the MA content increases with increasing milling time of Al 2 O 3 but prolonged milling time over 24h had almost no effect on the content of MA.Formation of magnesium aluminate spinel from its constituent oxides is a counter diffusion process of Al 3+ and Mg 2+ ions [19].In the MA formation based on the Wagner mechanism [20], oxygen ions remain at the initial sites.In order to keep electroneutrality, 3Mg 2+ diffuse towards the alumina side and 2Al 3+ diffuses towards the magnesia side, so four MgO change to one MA at the MgO side and four Al 2 O 3 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

Z. Zhihui et al. /Science of Sintering, 36 (2004) 73-79 ___________________________________________________________________________ 78
There may be two reasons for this.One is that the particle size of Al 2 O 3 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].

Fig. 1 X
Fig.1 X-ray patterns of Al(OH) 3 sintered at 1400 o C for 4h

Fig. 3 Fig. 2
Fig. 3 Effect of milling time on the apparent porosity of samples

Fig. 5 Fig. 4
Fig. 5 Effect of milling time on the relative content of spinel of samples sintered at 1000 o C

Z
. Zhihui et al. /Science of Sintering, 36 (2004) 73-79 ___________________________________________________________________________ 77 formation of MA and grain growth.Fig.7 shows X-ray diffraction patterns of α--Al 2 O 3 milled for 12, 24 and 36h.It can be seen from Fig.7 that the diffraction peak of α--Al 2 O 3 becomes broad and smooth with increasing of high-energy ball milling time.

Table I
Compositions(mass %), particle size (µm) and surface area(m 2 /g) of raw materials Note: IL is ignition loss.The operation temperature is 1000-1050 o C for 1h.