Though the average sizes of these t

Though the average sizes of these two particles do not make a big difference, Fig. 1 shows that the classified powder compact can achieve almost full density but the as-received one cannot, which indicates the aggregation in the as-received powder. Moreover, the microstructural evolution in the intermediate stage of sintering of the compacts of these powders has been detailed in the previous work[10], which implies that the unsinterable large pores exists in the as-received powder compact. Therefore, if the extent of densification at low temperature is less for as-received powder compact than classified one, the influence of the low heating rate on the as-received powder compacts will be reduced, as discussed above. Though the effect of the low heating rate becomes less significant for as-received powder compact, the evaluated activation energy values using nonisothermal sintering are still high. These values close to the activation energy of the lattice diffusion of the aluminum ion may be fortuitous.