Effect of Polymorphism of Al 2 O 3 on Sintering and Grain Growth of Magnesia Aluminate Spinel

The effect of polymorphism of Al2O3 on sintering and grain growth of magnesia aluminate spinel was studied. γAl2O3 and αAl2O3 were mixed with MgO according to the stoichiometric MA ratio, respectively, and pressed into billets with a 20mm diameter and 15mm height, and then were sintered at temperature from 1250C to 1400C in air atmosphere. Bulk density, apparent porosity and grain size were measured. The results indicated that the grain size of MA with γ-Al2O3 is larger than the grain size of MA with αAl2O3. This is because the activation energy of grain growth of magnesia aluminate spinel prepared by γ-Al2O3 is lower than that by α-Al2O3, the former is 159KJ/mol and the latter is 217KJ/mol.


Introduction
Magnesium aluminate spinel (MA) is widely used in refractories because of its high melting point (2135 о C), good mechanical strength and excellent chemical resistance et al [1][2][3].Especially in recent years, as the hazardous character of chrome bearing materials is exposed, MA becomes more important [4].At present, people usually use Alpha aluminum (α-Al 2 O 3 ) as a raw material in the formation of MA because it is a readily available industrial material.However the production temperature of α-Al 2 O 3 is high, usually up to 1400 o C in industry [5], which results in a decrease of its activity and an increase in energy consumption.At the same time, the process of α-Al 2 O 3 transformation to MA is accompanied by a large volume expansion, because of its higher density, making it difficult to obtain a dense sintered body [6].
In order to avoid these disadvantages, some researches used various additives to enhance the MA formation and to densify the material at lower temperatures.Ritwik Sarkar et al [7][8] studied the effect of various oxides additives on the densification of reaction sintered and presynthesised stoichiometric MA.They found TiO 2 showed the greatest beneficial effect and that Cr 2 O 3 showed some benefit on densification of all the presynthesised and reaction sintered MA.However, V 2 O 5 and B 2 O 3 showed detrimental effects on the densification behavior of presynthesised and reaction sintered MA.Kostic et al reported [9] that the fluorine ion from AlF 3 or CaF 2 increased the solid-state reaction synthesis of magnesium aluminate spinel by increasing the cation vacancy.Although these additives can improve the densification of MA to a certain extent, they introduce impurities in the MA.Other researches improved the densification of MA using high-energy ball milling of MgO, Al 2 O 3 , or their mixture [10][11].However, one of the drawbacks attributed to this technique is the lack of reproducibility and the difficulty in comparing results obtained by different authors using different mills [12].On the other hand, high energy ball mills create a high energy consumption.
Gamma aluminum (γ-Al 2 O 3 ) has a lower density than α-Al 2 O 3 , and when γ-Al 2 O 3 is used as a raw material to synthesize MA, the volume expansion is lower in the formation of MA.Also, the temperature at which boehmite transfers to γ-Al 2 O 3 is rather low, resulting in γ-Al 2 O 3 powder with higher activity and a low energy consumption [5].Furthermore, γ-Al 2 O 3 has the same crystal structure as MA.They are a cubic system.But α-Al 2 O 3 is a rhombohedral system.In our former research, we found that γ-Al 2 O 3 is beneficial for synthesizing MA [13].In this paper, the effect of polymorphism of Al 2 O 3 on the grain growth of MA was studied.

Experimental procedures
The starting materials used in this study are γ-Al 2 O 3 , α-Al 2 O 3 and an analytical reagent grade of MgO.γ-Al 2 O 3 was produced from boehmite heated at 700 o C for 4h, and α-Al 2 O 3 was produced from gibbsite heated at 1400 o C for 4h.Composition, particle size and surface area of the two kinds of Al 2 O 3 and MgO used in this study are given in tab.I and their X-ray patterns shown in Fig. 1.
Tab.I Compositions (mass %), particle size (μm) and surface area (m 2 /g) of raw materials used to make MA  γ-Al 2 O 3 , α-Al 2 O 3 and MgO were mixed in a stoichiometric ratio of MA by ballmilling for 5h, using highly pure Al 2 O 3 balls as a milling medium.The mixtures were pressed at 200MPa using a stainless-steel die to make the billets 20mm in diameter and 15mm in height.The billets were then sintered at 1250, 1300, 1350 and 1400 o C for 4h with a heating rate about 3 o C per min, respectively, in an air atmosphere.The first sample was taken from the furnace when the temperature in it reached the scheduled temperature and was marked as 0h.Additional samples were taken at 1h, 2h, 3h and 4h from time zero.The bulk density and apparent porosity of the sintered specimens was measured by the Archimedes method using water as the liquid media.XRD crystalline phase analysis was conducted using Ni filtered Cu K α at a scanning speed of 2°min -1 and a temperature of 16 o C. The MA grain size was measured by the half-peak breadth of the MA {3 1 1} peak using the Warren-Scherrer foumula [14]: In this formula: D is the grain size (nm); β is the half-peak breadth (rad); θ is the diffraction angle(°); λ is the X-ray wave length, 0.154178nm; and 1 = κ .

Effect of the sintering temperature and soaking time on sample densification
The bulk density and apparent porosity as a function of sintering temperature and soaking time are shown in Fig. 2 and Fig. 3 respectively.It is noted that the effect of temperature and soaking time on sample densification is related to the kind of Al 2 O 3 used in making a sample.The apparent porosity of samples with γ-Al 2 O 3 decreased and bulk density increased with increasing sintering temperature and soaking time.And at every sintering temperature, the relationships between apparent porosity, bulk density and soaking time were linear.At same time, the slope of these straight lines increased with increasing sintering temperature.The MA formation reaction occurs with additional time at each temperature, along with a volume expansion.This volume expansion canceled out the volume shrinkage from sintering.In the samples heated at 1400 o C, unreacted α-Al 2 O 3 and MgO are small, with soaking time causing the porosity to decrease and bulk density to increase rapidly.From Fig. 2 and Fig. 3, we also found that samples with γ-Al 2 O 3 had higher bulk density and lower apparent porosity than samples made with α-Al 2 O 3 .This result may originate from two causes; one was the volume change from the formation of MA using different Al 2 O 3 sources.The other may be the starting crystal structure of γ-Al 2 O 3 , which is similar to MA.

Effect of the sintering temperature and soaking time on the grain growth of MA
Fig. 5 gives the relationship between the grain size of MA and the soaking time for the different types of Al 2 O 3 when heated at different temperature.It is found that no matter what kind of Al 2 O 3 raw material was used, the grain size of MA increased with increasing sintering temperature and soaking time.When the soaking time was less than 1h, the grain size rapidly increased with increasing soaking time.When the soaking time was greater than 1h, the grain size increased slowly with increasing soaking time.This maybe because when the sintering temperature just arrived at the set temperature, the grain size of the new product was very small, specific surface area was large, and the crystal lattice had many deficiencies [15], causing the grain size of MA to rapidly increase with increasing soaking time for the first hour.With the soaking time increasing, the grain size of the MA became large, the specific surface area became smaller, and the crystal lattice was prone to integrity, so the grain growth of the MA decreased.From fig. 5 we also found that the grain size of MA in the samples withγ-Al 2 O 3 was larger than that of MA from α-Al 2 O 3 .This can be explained by the following analysis of activation energy of grain growth for MA.According to Brook's kinetic model of grain growth [16], grain growth can be showed as follows: (2) Here G and G 0 are the grain size of hour t = t and t = 0 of time t respectively, and n and k are constants.The value of n is between 1 and 3 [17].When grain growth is non-normal, n = 1; when grain growth is normal, n = 2; when grain growth is limited by the impurity or porosity around the grain boundary, n = 3.In the formula (3): A is a proportional constant that is related to the transition of ions.And according to Wagner's mechanism [18], in this experiment, the transition ions are mainly Al 3+ and Mg 2+ ions.Q is the apparent activation energy of grain growth, R is the gas constant, and T is the Kelvin temperature.
When we analyze the grain growth of a certain material by formula (2), we must define the value of n according to experimental conditions.In our experiment, we take n=3 because of impurities and high porosity of the samples.Thus the formula (2) can be expressed as follow: (4) And, taking formula (3) into formula (4), can get the following formula ( 5): (5) After both sides of formula (5) were divided by t and then logarithms were taken, we obtained formula (6): 3 − ln vs. 10 3 /T is linear, as shown in Fig. 6, which shows a plot of these two parameters.The average rate of the slope of these straight lines in fig.6 (A) and fig.6 (B) is 19.11 and 26.13 respectively.According to the result of linear regression analysis, the activation energy of grain growth of MA made using γ-Al 2 O 3 is 159KJ/mol, and the activation energy of grain growth of MA made using α-Al 2 O 3 is 217KJ/mol.From these results, it is obvious that the activation energy of grain growth for MA with γ-Al 2 O 3 is lower than that of MA with α-Al 2 O 3 .This lower activation maybe due to the similar crystal structure of γ-Al 2 O 3 to MA structure.

Conclusions
1.For the same sintering temperature and soaking time, MA samples made using γ-Al 2 O 3 had higher bulk density and lower apparent porosity than samples made using α-Al 2 O 3 .2. The grain size of MA in the samples made using γ-Al 2 O 3 is larger than the grain size of MA in the samples made using α-Al 2 O 3 .
3. At the same time, the activation energy grain growth for MA prepared using γ-Al 2 O 3 is lower than that of MA prepared using α-Al 2 O 3 , the former is 159KJ/mol and the latter is 217KJ/mol.The reason for this difference maybe is that the crystal structure of γ-Al 2 O 3 is similar to MA.

From Fig. 1 (
A) and Fig.1(B), we find that γ-Al 2 O 3 and α-Al 2 O 3 are the dominate crystal phases, which means at 700 o C and 1400 o C , boehmite and gibbsite have already converted to γ-Al 2 O 3 and α-Al 2 O 3 , respectively.

Fig. 1 X
Fig. 1 X-ray patterns of (A) boehmite sintered at 700 o C for 4 h, and (B) Al(OH) 3 sintered at 1400 o C for 4h

Fig. 5
Fig.5 The effect of sintering temperature and time on the grain size of spinel:(A) γ-Al 2 O 3 , (B) α-Al 2 O 3 IL is ignition loss.The operation temperature is 1000-1050 0 C for 1h.