Sintering behaviour of alumina powder
Sintering is a process in which the compacted powders are heated to a certain temperature so that the the particles can adhere to each other. Sintering process depends on the mass flow of material . The microstructure evolution during sintering includes the changes in size and shape of grains and pores . All these changes relate to the products' properties. Porosity, as an important evaluation criterion of a ceramic's quality, can influence both the mechanical and electrical properties of a ceramic. For structural ceramics, pores are desirable to be eliminated as much as possible .
Alumina is among the very first investigated ceramic materials. Alumina ceramics possesses some good properties such as high strength and insulativity. The cost of alumina is also relatively lower over other ceramic materials. That's why alumina is considered to be a useful material and have already been widely used in various applications . Many theories have been built for alumina sintering. There has been a mature sintering system for alumina. These days, surface effect on microstructure evolution during sintering process has attracted researchers' attention. Many assumptions have been made and some of them have already been approved. In this review, the focus is on one of the surface effects, which is the increase of surface energy brought by mechanical activation.
Activated powders are referred to those powders of which the surface energy is relatively higher than normal powders. Both decreasing in size and surface damage can result in a larger surface energy than initial state of the particles . Attrition milling, which is a common method for grinding large particles, are employed as a way to induce surface damage . All the theories and possible factors mentioned above will be further discussed hereinafter.
2. Sintering theories
2.1 Traditional sintering theories
During sintering, compacted materials are heated at gradually increasing temperatures until a certain point. The properties of the sintered compound are changed . The changes during firing process are quite complex . Decomposition and phase transformation are both possible during sintering .
According to Rahaman, there are several stages of sintering: pore consolidating, pore removal, components' shrinkage and grain growth . Basically, the changes during sintering process can be catalogued into densification and coarsening . With the evaporation and deposition of surface atoms, cohesive necks firstly grow at the particle contacts  and the pores will occur between these necks. Some of the pores can be exhausted while some of them will remain in the bulk forever . As the porosity have a direct influence on the properties of ceramics, the elimination and exhausting of the pores deserve much attention [1,2]. With the diffusion and migration of surface atoms of particles, the compact has a tendency to shrink to achieve a smaller surface area, say, the surface energy is lowered . Pore drag, which has a crucial influence on grain growth, is partly decided by the rate of densification . The densification and coarsening cannot be divided as separate period, instead, they have a close interaction and interference . The densification can be directly studied and the coarsening can be indicated by densification ratio .
Rahaman pointed out that there were three possible driving forces for sintering: the curvature of the particle surface, an externally applied pressure and a chemical reaction . The principal driving force is the reduction of surface area, with a replacement of surface energy by continuous solid . Driven by the forces, diffusion and deposition occur with the lowing of particles surface energy [1,3]. Obviously, particles with a higher surface energy show a better sintering performance . The surface energy of a particle mostly depends on the surface area(s) and the specific surface energy (γ). Most of existing theories about sintering focus on the reduction in particle size, which is the minimization in the surface area (s) .
The sintering behaviour correspond to the particle size and the shape of the compact . Particles of small dimension have a larger surface energy, which shows a larger tendency to sinter than coarse particles . However, fine powders are easier to agglomerate, which must be avoided in sintering . The influence of green body is also decisive: the green density and green porosity are prior to sintering . 1 is a photograph of the real porous alumina which is made by Dı́az et al.
Possibly, the most important parameter involved in sintering is the density, which is related to the sintering kinetics and many properties of sintered materials . The degree of sintering can be measured by microscopic analysis of surface area, the relative neck size ration X/D(X is neck diameter and D is particle diameter), densification parameter ψ and the porosity .
The porosity can be measured by mercury porosimetry. It measures the porosity and void size distribution inside the sintered ceramics . Combined with other characterisation methods such as magnetic resonance imaging (MRI), mercury porosimetry is able to obtain details on the whole void space for macroscopically (>10μm) heterogeneous materials . Mercury is a non-wetting liquid which can be imposed into the pores and the external pressure change can be tracked as a evidence of the porosity . The dispersion of pores can also be obtained using mercury porosimetry .
2.2 Sintering of alumina
Alumina is extremely useful for many current technologies for its high hardness, good resistance to corrosion, high insulation and ease of processing. High density alumina often attained by sintering compacted alumina powder over 1700oC . Both by doping and decreasing the particle size can reduce the sintering temperature, which has been proved by many researchers [1,2,6,12].
Additions of 3 wt% MnO2 + (0.5, 1.5, 3.0) wt% TiO2 additive combinations was doped into alumina and a significant decrease in sintering temperature (1250 oC) was attained . The densification also rose to 98% of the theoretical densification and the sintered alumina obtained better mechanical properties .
The sintering temperature can be lowered by using very fine powder made by advanced techniques like sol-gel processing, in-flight oxidation of plasma reactivated nano-sized alumina [10,11,15]. Long time milling of alumina powders also help decrease the sintering temperature [4,13].
An apparent decrease of sintering temperature by long time milling of the powders was also reported . A commercial alumina (AKP-53) was milled for 6hours in a SPEX 8000 Mixer Mill, using a vial and hardened steel balls (10mm diameter), with a ball-to-powder mass ratio of 4:1 . The contamination of the iron balls was eliminated and the sintering behaviour was observed .The samples for sintering were prepared by cold-isostatic pressing under the pressure of 200MPa and sintered in air atmosphere from 15oC to 1500oC . The shrinkage began around 650oC and became slow around 1000oC . The maximum of linear shrinkage rate stopped at the temperature of 1285oC, which is much smaller than 1700oC .
Abnormal grain growth (AGG) may occur on some grains during the sintering of alumina . AGG is harmful to the sintering of alumina and the causes of AGG have been investigated a lot by researchers. AGG may occur in the high concentrated impurity region . The evidence that AGG was arouse by the chemical inhomogeneity in the alumina powders was provided . Four Bayer-processed commercial alumina powders were used and three of them were doped with MgO . These powders were made into different compact with different coarse proportions and the sintering behaviour were investigated . The sintered samples were tested by TEM and XRD and it was found that the proportion of AGG in the sample which was more chemical inhomogeneous was larger . The result of the experiment showed that AGG could be influenced by different chemical inhomogeneity . The AGG also correspond to the surface energy. Mullins et al used the linear bubble mode to study the AGG and found that higher surface energy of unequal boundaries relative to equal boundaries had an influence on AGG . The AGG also appears in the sintering of doped alumina . In TiO2 doped alumina, some grains were found to grow abnormally above 0.6% wt TiO2 . The AGG may lower the products' quality and should be avoid as much as possible.
3. Activation of alumina powders
It has been proved that fine powder has a lower sintering temperature and the ceramic sintered using fine powder shows a lower porosity [6,10,11,15]. It is partially raised by the dramatically expanding of surface area, which makes the surface energy higher. In ceramic processing, milling has been used to reduce the particle sizes, to change the particle size distribution and the particle shape and the surface activity , agglomerates also can be dispersed by milling . There is a possibility that the particle size will not decrease any more upon a certain point. Therefore, another possible theory has been put forward, which still needs to be proved. Wu et al thought that the specific surface energy (γ) was elevated in the mechanical reduction of the particles .
3.1 Activation by attrition milling
The particle size can be reduced and the morphology can be changed by attrition milling. Particles' surface energy will rise with the decreasing of sizes, say, the powders will be activated. After certain times of milling, particle size will not reduce any more. Instead, the surface morphology of the spheres will experience a definitiveness change. 3 shows the change of particle size of different grinding time .
Instead of the size, the morphology of the surface will change . A subsurface layer of high dislocation density introduced by grinding and polishing was observed using TEM . Wu et al found out some subsurface damage of alumina and alumina/SiC particles after longtime grinding which is shown in 4 .
The layer observed varied with different components and conditions, which indicated that the subsurface damage is induced by these external factors . With these high density dislocations, the surface tends to be amorphous and have a higher specific surface energy . The destroyed surface can be seen in 5 .
However, the influence of the activation on the sintering behaviour of alumina powders has not been studied a lot. It is still a mystery whether the dramatically change of surface morphology is good or not for the sintering and whether it is more decisive compared with the increasing surface area.
3.3 Possible influencing factors
There are many parameters involved in the attrition milling process which can influence the milling result, such as milling liquid, milling temperature, milling ball material.
The choice of milling liquid is of great importance to the milling process. As two kind of typical solutions, acetone and water are widely used in attrition milling. Acetone is the organic compound with the formula C3H6O. This colorless, mobile, flammable liquid is the simplest example of the ketones. Acetone is a typical example of nonpolar solutions. Water, a typical polar solution, is a ubiquitous chemical substance that is composed of hydrogen and oxygen. For alumina, positive and negative surfaces charges are determined by potential determining ions H+ and OH− . During the milling process, the ions on the surface of alumina spheres may react with the ion in the solution. Water has a larger chemical activity and Al(OH)3, which is extremely harmful to the sintering, is likely to form. Since acetone has a lower surface tension than water, lager pores are likely to form in the compact dispersed in water during milling .
The milling efficiency can be raised by applying balls of different sizes [25,26]. It was found that the finest mean particle sizes were obtained with 40:1 ball–powder weight ratio in the studied range using vibratory horizontal attritor . The possibly induced contamination should also be taken into consideration. ZrO2 balls produce the least contamination to alumina .
The properties of alumina powder may vary with the elevation of milling temperature . The surface energy has a rising tendency along with the temperature. However, the temperature is hard to control. In real operation, cool water are applied to stabilize the temperature of the milling system.
3.4 X-ray diffraction peak broadening analysis
The morphology change of the surface can be directly observed using TEM. X-ray peak broadening offer another aspect to study the milling behaviour of alumina powders . Peak broadening of the X-ray diffraction may be caused both by crystallite size and micro-strains in the crystal . The enhanced activity can then be well measured . The uncorrected relative changes of peak width ( 6) are obvious especially at high 2θ-angles .
It can be seen that the milling process causes the diffraction peaks to broaden . The peak width of only the alfa-component at half of the peak maximum, which called beta, are calculated to reveal the distortion on the surface of the crystals  . When plotted as shown in 7, the slope of which is a relative measure of average lattice distortions .
The comparison of the calculated changes in crystallite size and distortions identifies the theory that the increase of the stored energy comes from both the decrease in sizes and the lattice distortions.
3 Project plan and current progress
The project's aim is to investigate the sintering behaviour of activated alumina powder. Experiments are designed to identify the assumptions that the grinding process can mechanical activate the surface of powders and investigate the influence brought by this activation.
Alumina powders will be attrition-milled for various times using ball milling machine ( 8). The set times are 2 hours, 4 hours and more. The milling balls used here is made of pure zirconia and the milling liquid is acetone. All the milled and unmilled powders will be tested by XRD, SEM and TEM to reveal the surface and subsurface change to these powders during attrition milling process. The ground powders will also be compacted by isostatic pressing into plates and sintered at certain temperatures. The sintered sample will be tested by mercury porosymitry to investigate the porosity and the dispersion of pores. The density of the green and sintered samples of the same powder will be compared so the densification ratio will be achieved. Currently, three kind of powders (Unmilled, milled for 2 hours and milled for 4 hours) have been prepared. The following job is to characterize these powders using XRD, SEM and TEM.
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