Modi fi ed-starch Consolidation of Alumina Ceramics
JU Chenhui 1, WANG Yanmin 1, YE Jiandong 1*, HUANG Yun 2
(1. Key Laboratory of Specially Functional Materials of Ministry of Education and School of Materials Science and Engineering,
South China University of Technology, Guangzhou 510641, China; 2. School of Light Industry and Food Science, South China
瑞典美食
University of Technology, Guangzhou 510641, China)
Abstract: The alumina ceramics with the homogeneous microstructure and the higher density were fabricated via the modified-starch consolidation process by 1.0 wt% of a modified starch as a consolidator/binder. The swelling behavior of the modi fi ed oxidized tapioca starch was analyzed by optical microscope, and two other corn starches (common corn starch and high amylo corn starch) were also analyzed for comparison. The modi fi ed starch ud as a binder for the consolidation swelled at about 55 ℃, began to gelatinize at 65 ℃ and then was completely gelatinized at 75 ℃. But the corn starches could not be completely gelatinized even at 80 ℃ for 1 h. The high-strength green bo
dies (10.6 MPa) with the complex shapes were produced. The green bodies were sintered without any binder burnout procedure at 1 700 ℃ and a relative density of 95.3% was obtained for the sintered bodies, which is similar to that of the sintered sample formed by conventional slip casting. In addition, the effect of temperature on the apparent viscosity of the starch/alumina slurry in the process was investigated, and the corresponding mechanism for the starch consolidation was discusd.
Key words: oxidized starch; gelation; alumina ceramic; starch consolidation
DOI 10.1007/s11595-006-4558-0
1 Introduction
Since starch has some favorable characteristics such as good thickening, stabilizing, membrane-forming and gelling properties, a new ceramic formation technique, starch consolidation, has been developed for ceramic forming [1]. Starch consolidation is a promising near-net shaping method in water due to the environmental and economic benefits. Recent studies on the processing of various ceramics with starch have been performed [2-14]. For instance, starch has been ud as a pore-forming agent for the production of porous structural ceramics [2-4], porous electronic ceramics [5-8]
and porous bioactive ceramics [9-14]. In general, the full density cannot be obtained by this technique becau the pores are generated during the burn out of starch granules [1,15].
Native starch can be chemically, enzymatically or physically modi fi ed to induce the novel characteristics for the subquent u. If the starch granules could be readily gelatinized to become starch pastes at a lower temperature, they can be ud as a binder additive at lower contents in the forming process of the high
density ceramic, instead of only producing the porous ceramics.
The objective of this work is to investigate the starch consolidation of the high-density ceramic components using a modi fi ed starch ud as a binder. The swelling behavior of the modified starch and the other corn starches and the effect of temperature on the apparent viscosity of the starch/alumina slurry were studied. In addition, the mechanism of the starch consolidation for the formation of high density alumina ceramics was discusd.
2 Experimental
三腔共鸣An oxidized modified tapioca starch named OX-5 was prepared for the study. The other two native st
arches, a common corn starch and a high amylo corn starch (Yangming biochemistry Inc., Guangxi, China) were ud for comparison. Two alumina powders, named A4N2 (Gongrong Co., Dalian, China) and FC-C12 (Fuwei Co., Zhengzhou, China), were mixed to prepare a test material with a median particle size of 1.3 μm [16]. The dispersant ud was a four-member copolymer HC-323 (Wujin Hongguang Chemicals Factory, China). Distilled water was ud in this study to prepare the ceramic slurries.
An alumina suspension of 58 vol% was prepared with distilled water in the prence of the dispersant of 0.7 wt% in mass. The suspension was kept stirred for 30 min and then ground in a planet ball mill with zirconia grinding media for 8 h. The slurry was further
milled for 1 h after the 1.0 wt% OX-5 starch was
jdye@scut.edu
Funded by the Foundation of National Defence Science and Technology of China(No. 51412020203
JW1608)
added.
The rheological measurement of the starch/alumina slurry was conducted using a Brook fi eld R/S rheometer (Brookfield Viscometers Ltd., USA). The evaporation was minimized by coating the surface of the samples with silicon oil during the measurement. The progressive swelling and fragmentation of the starch granules in excess water during heating at an incread rate of the elevated temperature of 2 ℃/min were visualized with an Axioskop 40 POL optical microscope (ZEISS Ltd, Germany). All the obrvations were performed using (×500) magni fi cation. The swelling performance was recorded every 30 s.
壁纸猫After milling and degassing, the slurry was poured into the non-porous moulds and the moulds with slurry were heated in water at 60-80 ℃ for 1 h. The moulds were airproof to forbid the evaporation of water before and during the solidification to prevent gregation phenomena and so an uneven shrinkage and subquent deformations during sintering. Fig.1 shows the formation process.
The bending strength of the dried green bodies was tested by the three-point bending method with a universal testing machine (5567, Instron Co., USA). Microstructural obrvation was performed on the fracture surface of the green specimen with a scanning electron microscope (1530VP, LEO Co., Germany). The green bodies were sintered at 1 700 ℃ for 6 h. The bulk density of the sintered bodies was determined by the immersion in water. The bulk density of the
sintered samples formed through slip casting was also
determined for comparison.
3 Results and Discussion
3.1 Gelation process of starches
Compared with the starches ud both as a binder and a pore-forming agent for the preparation of porous ceramics [2-14], the starch ud just as a binder
Fig.1 The fl
ow chart of the processing route
Fig.2 Micrographs of swelling behavior of oxidized tapioca starch globules (×500) in excess water during heating at a temperature rate of 2
℃ /min
in the production of high density ceramics is required to be gelatinized more readily at low temperatures. Therefore, it is necessary to investigate the gelling properties of the starches.
不要浪费时间Fig.2 shows the swelling behavior of the oxidized tapioca starch globules with respect to the effect of temperature, in which the OX-5 starch globules swells by water uptake at 60 ℃, and begin to gelatinize at 65 ℃. The polarized cross of starch gradually disappears in the range of 65-75 ℃, indicating that the crystal structure is destroyed. As the starch globules all disappear at 75 ℃, the OX-5 starch had been totally gelatinized. Compared to the OX-5 starch, the common corn starch could not be fully gelatinized even at 80 ℃ for 25 min (in Fig.3(a)), and the high amylo corn starch does not show any gelation phenomena at 80 ℃
for 25 min (Fig.3(b)). Also, all the suspensions show a turbid state at 25 ℃. After heating at 80 ℃ for 1 h, the state of the suspensions varied: the OX-5 starch suspension becomes a transparent liquid, reprenting full gelation of the starch; the common corn starch suspension appears mitransparen
t, indicating incomplete gelation; the high amyla corn starch suspension keeps a turbid state, showing no apparent gelation phenomena occurred. Clearly, the OX-5 starch is suitable for u as a binder to produce high density ceramics.
Fig.4 shows the effect of temperature on the apparent viscosity of den alumina suspensions (58 vol%) with and without OX-5 starch at a constant shear rate of 50 s -1. It is clear that the viscosity of alumina/starch slurry is higher than that of the alumina slurry. The viscosity of the both slurries slightly decread with the increa of temperature below 50 ℃, and then remained nearly constant from 50 ℃ to 55 ℃. The viscosity of the alumina slurry without starch keeps constant, and the viscosity of the alumina/starch slurry incread when the temperature is higher than 55 ℃. The results could be mainly explained by the propod mechanism of consolidation process, as shown in Fig.5. The starch granules begin to swell by absorbing water at 55-60 ℃ during heating the ceramic suspension (Figs.5(a) and 5(b)), and then the starch granules are gelatinized gradually and the molecule chains extend at > 60 ℃ (Fig.5(c)). In fact, the swelling and gelatinization of the starch granules by absorbing water in-situ reduces the free water in the ceramic suspension and the consolidation occurs, forming the green bodies (Fig.5(d)). When the starch granules absorb water and swell, they will create an
(b) High amylo corn starch
(a) Common corn starch Fig.3 Micrographs of swelling starch globules in excess water at 80
℃
for 25 min(×500)
Fig.4 Apparent viscosity vs. temperature curves of 58 vol% alumina
suspension without starch (1) and with 1 wt% OX-5 starch (2)
Fig.5 The schematic of the starch consolidation mechanism
Fig.6 Specimen formed by starch consolidation
Fig.7 SEM micrograph of alumina green body prepared by starch
consolidation with 58 vol% suspension
extruding force among the ceramic particles, leading to the ceramic particles to contact. With the gelatinization of the starch granules, the molecule chains extend in a great extent and permeate into the clearance among the alumina particles and combine the particles. This will promote the consolidation of the ceramic suspension and increa the strength of the green bodies.
In starch consolidation, OX-5 starch rves as a body-forming agent or a binder. However, the other two corn starches ud (Fig.3) will swell by absorbing water as increasing the temperature. As a result, the swelled starch granules leave in the green body during the consolidation (Figs.5(a) and 5(b)). Therefore, tho corn starches could be only ud as pore-forming agents.
3.2 Properties of dried green bodies and sintered bodies
Fig.6 shows a de-molded specimen formed by starch consolidation. The green part after easily removed from the mold posss a good shape with a smooth surface. A rapid drying of the gelled bodies can cau the non-uniform shrinkage, resulting in cracks or warpage. To minimize the development of flaws, the wet green part was firstly dried at room temperature (~25 ℃) and then in a blast air oven at 120 ℃. The average linear shrinkage during drying period was 1.4% and the bending strength of the dried green body was 10.6 MPa. Fig.7 shows the microstructure of the dried green bodies. Clearly, the coar particles disperd in the fine particles matrix, and the particles packed denly.
The green bodies were sintered without the burnout of the binder due to just small amount of modifi ed starch (1 wt%) ud as an organic binder, as tho were produced by the gelation with small amount of gelatine[17]. The relative density of the sintered body was 95.3% (The theoretical density of pure alumina is assumed to be 3.98 g/cm3). In comparison, the relative density of the sintered body prepared by slip casting was 95%-96%. Clearly, there is no apparent difference between the sintering densities obtained with the two forming techniques. This illustrates that the modified starch with a lower content ud in the consolidation could be totally gelatinized, and no starch granules left in the green bodies afterwards. It is assumed that the alumina ceramics with a full density could be p
roduced through the starch consolidation. However, this should be examined via the experiments with a more sinterable alumina powder or under more proper sintering conditions.飞机发明时间
4 Conclusion
High-performance alumina ceramics with homogeneous microstructure and high density were formed using 1.0 wt% of the oxidized starch as a consolidator or binder via the starch consolidation process in this study. The modified starch in the alumina/starch slurry begins to swell at about 55 ℃, and be gelatinized at > 60 ℃, resulting in a rapid increa of the slurry viscosity. The strength of the green body reached 10.6 MPa, and the relative density of the sintered sample was 95.3%, which is similar to the relative density of the ceramics produced by the conventional slip casting.
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