3.1 Weight loss and drying time of gelling tube
Figure 4 showed the weight loss percentages during the drying of gelling Al2O3 tubes immersed in acetone at different periods of time. Apparently, the conventional air-drying tube displayed the tendency of gradually decreasing weight loss and spent 840 min (top abscissa) completing the drying. However, the tendency of weight loss for the acetone-immersing tubes indicated the marked decreases within early 40 min and then a slight decreases. The early marked decreases during drying implied that there was the sudden evaporation of acetone contained in the gelling Al2O3 tubes. Moreover, it was found that over the progressive drying the gelling Al2O3 tubes were cool confirming that there was the continuous evaporation of acetone.
The inset of Fig. 4 displayed the weight-loss percentages of early 100 min for all the gelcast tubes. It was able to divide the tendency of weight losses into 3 characteristics according to the periods of immersing time: 1h, 5–30 h, and 40–50 h. Firstly, after 1 h of immersion the tendency of weight loss gradually decreased, rather similar to that of conventional air-drying tube. Secondly, the immersing time of 5–30 h demonstrated the weight loss of 27–28% at the early 30-min drying. Thirdly, at 30-min drying for the immersing time of 40 and 50 h, the weight losses were rather rapidly steady at 26 and 24.5%, respectively. The reduction in the weight losses from 28 to 24.5% was attributed to the calculation of weight loss percentages based on initial weight of gelling tubes containing only water inside. However, since the lost weights during rapid drying rooted from the acetone evaporation and the density of water (0.997 g/ml) is more than that of acetone (0.791 g/ml), the longer periods of acetone-immersing time led to more acetone replacement for water resulting in the lesser weight losses.
Figure 5 indicated the percentages of acetone replacing water in the gelled Al2O3 tubes at the different periods of immersing time obtaining from calculation based on the initial weight of gelling tubes just after removing from acetone. According to Fig. 4, the percentages of acetone replacement for water suggested the highly possible reasons for the weight loss divided into 3 characteristics. In comparison to Fig. 5, the acetone replacement percentages were also able to be divided into 3 ranges: < 30% for 1-h immersion, 30–50 % for 5- to 30-h immersion, and > 50% for 40- to 50-h immersion. When the acetone replacement was in the range of < 30%, the weight loss tendency was mainly dominated by the existence of water, i.e. the weight of gelling tube gradually decreased comparing with the other two ranges and then begin to be steady after 360-min drying. However, when the acetone replacement was in the range between 30 and 50%, the combined tendencies of weight loss from acetone and water were observed, noticeably for the initial 25 min, and then the period of 25–200 min, respectively. In other words, this range showed the two behaviors of weight loss in which rapid and then slow from acetone and water, respectively. For the acetone replacement in the range of > 50%, the rapidly acetone evaporation greatly dominated over the slowly water evaporation.
From Figs. 4 and 5, the immersion of agar-gelling tubes in acetone facilitated more rapid drying stage. The results of the behaviors of weight losses during drying confirmed the acetone replacement for water during the immersion of gelling tubes in acetone. The percentages of acetone replacement for water had the profound effect on the period of drying time.
3.2 Roundness of Al2O3 tubes
The comparison of the shape of green Al2O3 tubes between no immersion and 5-h immersion in acetone was shown in Fig. 6 (a) and (b), respectively. The cross section of gelling tube dried from no immersion noticeably deformed. Its cross section changed from circle to oval as well as the distortion along its length as shown in Fig. 6(a). On the other hand, after 5-h immersion its cross section remained unchanged as shown in Fig. 6(b). The unchanged cross-section proved the beneficial effect of immersion in acetone.
Figure 7 indicated the cross section of sintered tubes having the immersion in acetone at the different periods of time from 0 to 50 h. In order to be able to study the effect of acetone-assisted drying on the tube shape, the roundness of sintered-tube cross section was analyzed. The ratios of horizontal-to-vertical diameters of cross section was displayed in Table 1. It was noticeable that no immersion produced the tube with the smallest roundness. However, their roundness was considered to be rather equal for all the tubes with the immersion. As a result, the step of the immersion were highly beneficial for the drying of the agar gelcasting tubes with near-net shape.
Table 1
Roundness of sintered gelcast tubes having different periods of acetone-immersing time
Dimension (mm) | Acetone-immersing time (h) |
0 | 1 | 5 | 10 | 15 | 20 | 30 | 40 | 50 |
a | 12.24 | 12.679 | 13.226 | 12.975 | 12.627 | 12.991 | 12.884 | 13.000 | 12.830 |
b | 11.39 | 12.785 | 13.226 | 12.871 | 12.754 | 12.928 | 12.946 | 12.872 | 12.766 |
Roundness (a/b ratio) | 1.08 | 0.992 | 1.000 | 1.008 | 0.990 | 1.005 | 0.995 | 1.010 | 1.005 |
Note: a = Horizontal, outer diameter, b = Vertical, outer diameter |
In addition, It was found that the cycle number of immersion affected the performance of acetone replacement for water. In other words, the acetone replacement during the immersion was less and less when the acetone was repeatedly used. Obviously, for 500-ml acetone and only one gelling tube it was suitable for only 4-time immersion. For the immersion more than 4 times, it appeared to not allow the gelling tubes to be dried rapidly as well as the fresh acetone. The > 4-time immersion produced the tubes with unacceptable distortion. The distortion might resulted from the acetone-water concentration gradient decreased when the acetone was reused. Therefore, the acetone was usable for a limited number of immersion. The results suggested that the water staying inside the gelling tubes diffused into the surrounding acetone during the immersing time, and vice versa. The interdiffusion between water and acetone was responsible for the dilution of acetone. Thus, the diffusion of acetone into the gelling tubes ended when their too low concentration gradient was approached. When the too low replacement of acetone for water occurred, the drying of gelling tubes was replaced by most of water evaporation.
Figure 8 showed the SEM images of both the surface and cross section of gelcast tubes with 5-h acetone immersion after both drying (a and a1) and sintering (b and b1). After drying, it was found that the alignment of Al2O3 particles at the surface occurred as shown in Fig. 8(a). However, there was no aligned Al2O3 particles for the cross section as shown in Fig. 8(a1). Moreover, both the microstructures after drying was the same as those after sintering as shown in Fig. 8(b) and (b1). At the surface, the alignment of Al2O3 particles being in all the same direction was attributable to behavior of particles pulled out of the glass mold. The step of de-molding of gelling tubes was similar to the extrusion process of ceramic tubes. The de-molding resulted in the preferred orientation of Al2O3 particles at the surface of Al2O3 tubes [1, 26–27]. Whereas, at the cross section the alignment of Al2O3 particles was apparently random to the surface of glass mold. However, it was worth reminding that the large Al2O3 particles of A-321 had the shape of platelet, thereby able to be arranged in a certain direction.
Furthermore, the microstructures of dried tubes seemed denser than those of sintered tubes. The denser microstructures could result from the existence of dried agar strongly bound to the Al2O3 particles. The dried agar functioning as a binder offered the advantage of high green strength. After sintering, the microstructures of sintered tubes indicated the porous structure due to sintering effect as shown in Fig. 8(b) and (b1).
3.3 Shrinkage, porosity and pore size of gelcast Al2O3 tubes
The percentages of linear shrinkage during drying of the gelcast tubes as a function of elapsed time were shown in Fig. 9(a); while, the total linear shrinkage as a function of the acetone-immersing time was shown in Fig. 9(b). The tendency of linear shrinkage between the conventional air-drying and the acetone-assisted drying clearly different. The air drying spent 420 min approaching no shrinkage gradually. In contrast, the acetone-assisted drying spent approximately 20–50 min approaching no shrinkage. Moreover, the longer the acetone-immersing time, the shorter the spending time in approaching no shrinkage. The results of shrinkage confirmed the benefit of acetone-assisted drying leading to the rapidly dried tubes with near-net shape. Furthermore, the total drying shrinkage significantly decreased from 12.1 to 4.7% for the air drying and the drying having the step of immersing in acetone for 50h, respectively. The total firing shrinkage slightly increased from 2 to 2.6% from no immersion to 50h immersion in acetone. All the results of shrinkage suggested that the microstructure and pore size between no immersion and the immersion were likely to differ with each other due to the significant effect of drying shrinkage.
It was interesting to compare the drying shrinkage of agar-based gelcasting in this work with the other polymer-based gelcasting system as shown in Table 2. In this work, although the agar gelcasting with 50 vol.% solid loading and 0.5 wt.% of agar was used, the air-drying shrinkage was the highest (12.1%). The highest shrinkage was attributable to both the characteristics of agar macromolecules during their gel formation creating the network of wall and internal cavity and its high molecular weight. Moreover, the high shrinkage resulted from agar acting as flocculating agent or bridging flocculation [7, 29]. In other words, agar was adsorbed on each adjacent particles of Al2O3 forming polymer bridges. The polymer bridges acted as a barrier preventing the close contact of Al2O3 particles. In addition to the agar properties, the shape and size of Al2O3 particles had significant effect on asymmetric shrinkage of the gelcast Al2O3 tubes as well [30].
Table 2
Comparison of shrinkage percentages among various kinds of gelling agent in gelcasting system
Gelling agent (as % of solid loading) | Molecular weight (g/mol) | Solid loading (vol. %) | Drying shrinkage (%) | Drying method | References |
Agar and galactomannan = 80:20 (0.2–0.6 wt%) | 336.33 and 504.4 | 40 50 60 | 3.9–6.1 0.9–2.6 0.4–1.3 | Air drying | [7] |
AM/MBAM (17 wt%) | 71.08 and 154.17 | 55 | 2.8 | 95% relative humidity | [8] |
AM/MBAM (17 wt%) | 60 | 0.53 |
AM/MBAM (not shown) | 40 | 6.0 | PEG 1000 (60 wt% in water) | [22] |
Agarose (0.5 wt%) | 306.267 | 39 | 6.0 | Air drying | [28] |
Agar (0.5 wt%) | 336.33 | 50 | 12.1 | Conventional air-drying | This work |
4.7 | Acetone-assisted drying for 50h |
Figure 10 showed the apparent porosity, water absorption and bulk density of the sintered gelcast tubes as a function of the periods of acetone-immersing time. The apparent porosity of gelcast tubes increased from 40 to 44% when the acetone-immersing time increased from 0 to 5 h. After the immersing time of ≥ 5 h, their porosity was constant at 44% on average. The water absorption indicated the same trend as the porosity. The water absorption increased from 17 to 20% while the immersing time reached 5 h. Moreover, the bulk density decreased from 2.35 to 2.15 g/cm3 during the early 5 h of immersion. Therefore, those tendencies were divided into 2 main ranges on the basis of immersing time: the immersion periods of ≤ 5 h and > 5 h. Additionally, it was found that those tendencies were the same as the linear drying shrinkage.
Figure 11 displayed the pore size distribution within the sintered Al2O3 tubes characterized with MIP. No immersion in acetone displayed the feature of narrow pore size distribution between 7.5 and 10 µm; whereas the immersion characterized broad pore size distribution between 0.04 and 4.5 µm. The different results of pore size distribution were closely related to the markedly different evaporation rate of acetone and water during drying at room temperature. The evidence from the measurement of pore size distribution suggested that the average pore sizes were in the range of microfiltration (MF) membrane.
3.4 Microstructure of gelcast tubes after sintering
The SEM images of sintered tubes with the variation in acetone- immersing time of 0, 1, 5 and 50 h at outer surface and cross section were shown in Figs. 12 and 13, respectively. Noticeably, the gradual evolution of the microstructure of sintered tubes appeared in the way to the more open structure at both surface and cross section when the periods of acetone-immersing time increased. The more open structure corresponded with the decreasing density and increasing porosity as shown in Fig. 10. Therefore, the rapid drying from the high acetone volatility tended to offer the more open structure than the conventional air drying. It suggested that if the denser structure of gelcast tubes was required, it needed the higher sintering temperature than the usual.