Influence of Parameters of High-energy Ball Milling on the Synthesis and Densification of Magnesium Aluminate Spinel

This study investigated the effect of planetary ball mill parameters on the particle size of a powder mixture of alumina and magnesia, the composition of synthetic magnesium aluminate spinel (MAS), and the subsequent densification of MAS. The results show that the particle size of the milled powders decreases gradually from 32.39 μm to 11.69 μm with the increase of milling time from 1 h to 7 h at milling speed of 120 rpm. The particle size of the milled powders decreases gradually from 42.92 μm to 9.00 μm as the milling speed was increased from 60 rpm to 240 rpm at a milling time of 3 h. Only a spinel phase can be obtained from these calcined powders when the starting powders of the calcined powders are milled for more than 1 hour or at above 60 rpm. The relative density and flexural strength of the sintered products exceed 86.67% and 105 MPa, respectively.


Introduction
Magnesium aluminate spinel (MAS) is a ceramic material that displays favorable thermal, chemical, and mechanical properties [1][2][3][4].Therefore, spinel is an excellent refractory material [5,6].However, the high cost of sintered spinel is a core problem in synthesizing refractory ceramic materials through a double-stage firing process [7].High formation temperature was previously used for spinel synthesis [8].This condition decreases the volume expansion effect in the subsequent densification.However, the production cost of MAS has also been increased.In addition, the synthetic temperature of MAS should be maintained at the lowest possible temperature to avoid the formation of large agglomerates, which inhibit sintering activity in the subsequent densification [9].Hence, a low temperature is used for the spinel formation.However, oxides at a low temperature react at a slow rate to form spinel because of their poor reactivity.Therefore, numerous researchers attempted to improve the reactivity of particles by decreasing the particle size of the initial powders.The particle size and reactivity of the initial powders can also affect the chemical composition of mixtures due to the difference of the synthesis speed of spinel.Furthermore, the chemical composition of presynthesized spinel mixtures influences the subsequent densification because of the volume expansion effect.
High-energy ball milling is a simple and cost effective method for the large-scale production of fine powders [10][11][12].Planetary ball mill is a commonly used high-energy ball mill.Kim and Saito [13] found that MgAl 2 O 4 spinel could be obtained from planetary ball milled mixtures of Mg(OH) 2 and Al(OH) 3 after over 15 min of calcination at 900 °C.Kong et al. [14] milled calcined powders prepared from the first firing stage for 2, 4, and 6 h.The result showed that a maximum of 99.5% of theoretical density was obtained in 6 h milling without carrying any presynthesization of spinel.However, the effect of milling on the particle size of the oxide raw materials has yet to be systematically researched.Moreover, the influence of the particle size of starting powders on the synthesis and subsequent densification of MAS in double stage firing remains unclear.
In the present work, a powder mixture of alumina and magnesia was subjected to planetary ball milling under different conditions.The influence of high-energy milling time and milling speed on particle size was investigated.Furthermore, the chemical composition and densification of the calcined powders were investigated.The milled powders were calcined at 1200 °C for 2 h and then characterized via phase analysis.The calcined powders were sintered at 1700 °C for 2 h and then characterized in terms of bulk density, microstructural evaluation, and flexural strength.

Experimental procedure
The starting materials were high purity α-Al 2 O 3 (97.3%)and MgO (99.9%) powders.To make stoichiometric spinel powder from the mixture of MgO and α-Al 2 O 3 (molar ratio 1:1), conventional powder mixing and planetary ball mill mixing were used.Conventional powder mixing, identifying with unmilled mixing, was carried out by the threedimensional blender for 20 min.The oxide powders were mixed by a planetary ball mill (Retsch PM 100) in ethanol media.Zirconia pot and zirconia balls were utilized.The milling was performed for different durations of 1, 3, 5 and 7 h, fixing the speed at 120 rpm, and at the speeds of 60, 120, 180 and 240 rpm, each for 3 h.After drying the mixed powders were calcined at 1200 °C for 2 h with a heating rate of 3 °C/min, which results in the formation of different composition powders.The calcined powders were uniaxially pressed at 200 MPa for 3 min to make cylindrical samples (Φ20 mm × 8 mm) and cuboid samples (45 mm × 6 mm × 3 mm).The sintering of samples was carried out at 1700 °C for 2 h with a heating rate of 3 °C/min.
The particle sizes of mixed powders were analyzed by the applying of laser particle size analyzer (LS-POP (6)).Phase analysis was done by X-ray diffraction technique (by Rigaku3014).The diffraction patterns of the finely powdered samples were obtained in a Xray diffractometer using nickel filtered Cu-Ka radiation.Bulk density of the sintered products was measured by the conventional liquid displacement method using Archimedes' principle.Microstructure evaluation of the sintered products was observed by scanning electron microscope (SEM, Nova NanoSEM 230) using sputtered gold coating on the polished surface after thermal etching.Flexural strength was measured by three-point bending method using cuboid samples (6 mm × 6 mm × 6 mm) at room temperature.The flexural strength was measured using a CSS-44100 type universal tester with a loading rate of 0.5 mm/min.Flexural strength (σ) was calculated from the following equation.where P, L, b and h represent maximum load of sample damage (N), the length of the support span (mm), width (mm) and thickness (mm), respectively.

Particle size of the starting powders
The variations in the particle size distribution (a) and the medium diameter of milled powders (b) as a function of milling time at a speed of 120 rpm are shown in Fig. 1.Fig. 1(a) shows that the particle size distribution intervals decrease with increasing milling time.The bimodal distribution of the particle size is still presented but dominant fraction is fraction of smaller particles, which is consequence of mechanical treatment.Fig. 1(b) indicates that high energy milling can significantly decrease the medium diameter (D 0.5 ) of particles compared with the unmilled mixture.Results also indicate that the D 0.5 gradually decreases in from 32.39 µm to 11.69 µm when the milling time was increased from 1 h to 7 h.In addition, the D 0.5 of the starting powders markedly decreased for up to 3 h of milling time.Beyond 3 h, the D 0.5 continued to decrease, but the particle refinement rate also decreased.The decrease in particle size with milling time could be a complicated process.At the beginning of the ball mill process, the powder is always refined first because of the collision and friction of the ball.The surface energy of the powder increases with decreasing powder particle size.Thus, the system energy increases.Meanwhile, the particles also have a tendency to agglomerate over time.In the final stage of the milling process, the milling energy is expended mainly on breaking the agglomerated tiny particles.When particle refinement and agglomeration establish a balance, the reduction of particle size tends to reach a constant value.
The changes in the particle size distribution (a) and the D 0.5 (b) as a function of speed for 3 h of milling time are shown in Fig. 2.There is a similar phenomenon of Fig. 2(a) with that of Fig. 1(a) that the particle size distribution intervals decrease with increasing milling speed.Fig. 2(b) indicates that the particle size gradually decreased from 42.92 µm to 9.00 µm when the milling speed was increased from 60 rpm to 240 rpm.The particle size D of the powders on the milling speed for a fixed milling time is expressed as [15]: where D is their particle size, ω is the angular rotation speed, k is a constant parameter of the milling system, a and b are some constants at a certain milling time, respectively.
Eq. ( 2) describes how an increase in the angular rotation speed of the ball mill for a fixed milling time leads to a gradual decrease in the particle size D of the milled powder to a saturation value.The milling energy increases with increasing milling speed, which favors cleavage of particles to produce new, finer particles.However, in the final stage of the milling process, fine particles tend to agglomerate.As a result, the reduction in particle size will reach a limit because of the interplay between particle refinement and agglomeration.This hypothesis is strongly supported by the observed decrease in particle size with the increase in milling speed, as shown in Fig. 2. The energy supplied by the planetary ball milling is used in the rupture of interatomic bonds in the crystal and in the formation of additional surface as a result of cleavage of crystalline grains [16].Based on experimental results, different process parameters and conditions, such as milling time and milling speed, significantly affect the particle size of the powders.

XRD study
Fig. 3 shows the XRD patterns of the mixture milled at various milling times and calcined at 1200 °C.The milling times of the starting powder are 0, 1, 3, 5, and 7 h at a milling speed of 120 rpm.These calcined powders are referred to as P 00 , P 1120 , P 3120 , P 5120 , and P 7120 , respectively.Fig. 4 shows the XRD patterns of the mixtures milled at various milling speeds and calcined at 1200 °C.The milling speeds of the starting powder are 0, 60, 120, 180, and 240 rpm at a milling time of 3 h.These calcined powders are referred to as P 00 , P 360 , P 3120 , P 3180 , and P 3240 , respectively.The pattern of P 00 is shown as a reference.The results show that the P 00 , P 1120 , and P 360 powders contain three phases, namely, spinel (MgAl 2 O 4 ), periclase (MgO), and corundum (α-Al 2 O 3 ).This result indicates that spinel formation is incomplete, with the presence of small amounts of unreacted α-Al 2 O 3 and MgO.The P 3120 , P 5120 , P 7120 , P 3180 , and P 3240 powders contain only the spinel phase because MgO almost completely reacts with α-Al 2 O 3 to form spinel.
The schematic diagram (Fig. 5) clearly shows the counter diffusion distance (d) of the cations for spinels with different particle sizes.The formation of spinel should proceed via counter diffusion of the cations through the product layer, where oxygen ions remain at the initial sites [7].The increase in milling time and milling speed significantly decreases the particle size of the powders.A decrease in the particle size will lead to a decrease in the counter diffusion distance (d) of the cations.Hence, oxides react at a fast rate to form spinel.In addition, the microstrain produced in the particles increases with the decrease in particle size.Both these processes lead to instability in the powder particles.Consequently, the reactivity of the powders increases.Thus, spinel is formed easily.

Densification and Flexural strength
The bulk density, relative density, and flexural strength of the sintered samples obtained under different conditions are shown in Tab.I. Cylindrical samples prepared using the calcined powders P 00 , P 1120 , P 3120 , P 5120 , P 7120 , P 360 , P 3180 , and P 3240 were sintered at 1700 °C for 2 h.These sintered samples are referred to as S 00 , S 1120 , S 3120 , S 5120 , S 7120 , S 360 , S 3180 , and S 3240 , respectively.The bulk density of the samples increases from 2.678 g/cm 3 to 3.181 g/cm 3  when the milling time of the starting powders was increased from 1 h to 7 h, and the corresponding flexural strength increases from 28 MPa to 112 MPa.The bulk density, relative density, and flexural strength of the samples significantly increase up to 3 h of milling time.Beyond 3 h, the increase in bulk density, relative density, and flexural strength is unremarkable.The bulk density of the samples increases from 2.399 g/cm 3 to 3.255 g/cm 3  when the milling speed of the starting powders was increased from 60 rpm to 240 rpm, and the corresponding flexural strength increases from 20 MPa to 110 MPa.The bulk density, relative density, and flexural strength of the samples significantly increase up to a milling speed of 120 rpm.Beyond 120 rpm, the increase in bulk density, relative density, and flexural strength is unremarkable.Fig. 6.Variation in flexural strength as a function of relative density under the studied conditions (▲-the sintered samples S 00 , S 1120 , S 3120 , S 5120 and S 7120 ; ▼-the sintered samples S 00 , S 360 , S 3120 , S 3180 and S 3240 ).
Tab.I Bulk density, relative density, and flexural strength of sintered samples obtained under the studied conditions.The chemical reaction between Al 2 O 3 and MgO is accompanied by 8% volume expansion [17][18][19].A volume expansion effect occurs during the densification of MAS when unreacted Al 2 O 3 and MgO are present in the system.For samples S 00 , S 1120 , and S 360 , the effect of spinellization on densification is harmful because of the associated volume expansion.For samples S 3120 , S 5120 , S 7120 , S 3180 , and S 3240 , only the densification process of spinel was observed.However, variation in sintered density for these samples may be due to the differences in particle size [20].To obtain a better visualization and understanding of the results presented in Tab.I, the variation in flexural strength as a function of relative density under the studied conditions is shown in Fig. 6.The flexural strength increases with the increase in relative density because of the decrease in porosity.Pores cause stress concentration, accelerate crack propagation, and even lead to material failure.Dense sintered samples prepared using starting mixtures milled at higher speeds (≥120 rpm) or longer times (≥3 h) can be obtained via the two-stage firing processes.Based on the experimental results, there is a strong correlation between flexural strength and relative density.

Microstructure evolution
The SEM images of sintered sample S 00 , S 1120 , S 3120 , S 5120 , S 7120 , S 360 , S 3180 , and S 3240 are presented in Fig. 7, 8, and 9. Fig. 7 shows the SEM image of the sintered sample S 00 in which the granules adhered and sintering necks were formed.The observed microstructure is a typical characteristic of a ceramic in the initial stage of the sintering process.Fig. 8 shows the SEM images of samples S 1120 , S 3120 , S 5120 , and S 7120 in which sintering necks grew and the connected network of pores was formed.The pore pinning phenomenon causes continuous densification without remarkable grain growth.As a result, a more compact microstructure with a small number of pores was obtained.Fig. 9 shows SEM images of samples S 360 , S 3120 , S 3180 , and S 3240 which are similar to those in Fig. 8.These results show that the microstructure of these samples is more compact than that of the reference sample (S 00 ).The raw materials P 360 , and P 1120 of S 360 , and S 1120 contain numerous unreacted MgO and Al 2 O 3 .MgO reacts with Al 2 O 3 to form spinel during the sintering process.The effect of spinellization on densification in the sintering process is harmful because of the associated volume expansion.As a result, a low relative density with a number of pores was achieved.For S 3120 , S 5120 , S 7120 , S 3180 , and S 3240, only the densification process occurs without spinellization.Hence, the microstructure of sintered products is more compact with a small number of pores when the spinel content of the mixture is high.

Conclusions
A stoichiometric mixture of alumina and magnesia with different particle sizes has been prepared by high-energy ball milling.The particle size of the milled powders decreases gradually from 32.39 µm to 11.69 µm as the milling time increases from 1 h to 7 h at a fixed speed of 120 rpm.With increasing milling speed of the ball mill from 60 rpm to 240 rpm at a fixed milling time of 3 h, the particle size of the milled powders decreases gradually 42.92 µm to 9.00 µm.Reduction of particle size has a significant impact on the formation of spinel.Bulk density, relative density and flexural strength of the sintered samples increase with the increase in spinel content of the mixture.The microstructure of sintered products is more compact with a small number of pores when the spinel content of the mixture is high.

Fig. 1 .
Fig. 1.Variations in the particle size distribution (a) and the D 0.5 (b) as a function of milling time at a speed of 120 rpm.

Fig. 2 .
Fig. 2. Variations in the particle size distribution (a) and the D 0.5 (b) as a function of speed for 3 h of milling time.

Fig. 5 .
Fig. 5. Counter diffusion distance (d) of the cations for spinels with different particle sizes.