Microwave Drying of Spontaneous-Coagulation-Cast Wet Alumina Green Body

doi:10.15541/jim20220469

Abstract

Abstract:

To solve moisture gradients in the conventional drying with controlled temperature and relative humidity, microwave heating was employed to dry wet alumina green bodies shaped by spontaneous coagulation casting. The weight loss, linear shrinkage, surface temperature, and moisture distribution of the green bodies by conventional drying (temperature: 40 ℃; humidity: 60%) and microwave drying were investigated. The time for no further weight loss and shrinkage of the body by microwave drying (power: 250 W) were respectively shortened to 1/6.8 and 1/6 of those by conventional drying. Surface temperature of the green body during the microwave drying increased firstly and then decreased with time, which was strongly correlated with the internal moisture, while the temperature in the conventional drying keeping at 40 ℃. Low-field nuclear-magnetic-resonance (NMR) imaging revealed that the moisture distribution in the green bodies dried by microwave drying was more uniform than that by conventional drying, indicating that drying stress in the former was lower than that in the latter. After sintering at 1550 ℃ for 6 h, alumina ceramics from microwave drying had a higher flexural strength with a smaller deviation than that from conventional drying.

Key words: microwave drying, low-field nuclear-magnetic-resonance imaging, moisture distribution, alumina, coagulation

CLC Number:

TQ174

LIU Wenlong, ZHAO Jin, LIU Juan, MAO Xiaojian, ZHANG Jian, WANG Shiwei. Microwave Drying of Spontaneous-Coagulation-Cast Wet Alumina Green Body[J]. Journal of Inorganic Materials, 2023, 38(4): 461-468.

Figures/Tables 10

Fig. 1 Effects of (a) drying methods and (b) microwave power on the volume fraction of residual water in wet alumina green bodies

Table 1 CRP and residual water (after CRP and drying) of samples

Drying method	Microwave power/W	CRP/ h	Residual water after CRP/% (in vol.)	Residual water after drying/% (in vol.)
Conventional	—	10	(20.4±1.5)	0.8
Microwave	250	1.8	(11.2±1.4)	0.6
	400	1.3	(9.4±1.4)	0.5
	550	1.0	(7.7±0.7)	0.4

Fig. 2 Effects of (a) drying methods and (b) microwave power on shrinkage behavior of wet alumina green bodies

Table 2 Shrinkage stop time and shrinkage of samples

Drying method	Microwave power/W	Shrinkage stop time/h	Shrinkage/%
Conventional	—	6.0	(4.9±0.1)
Microwave	250	1.0	(5.2±0.1)
	400	0.7	(5.1±0.1)
	550	0.5	(4.8±0.1)

Fig. 3 Effect of microwave power on the surface temperature of wet alumina green bodies

Fig. 4 Photos of alumina green bodies dried by (a, e) conventional drying and microwave drying ((b, f) 250 W, (c, g) 400 W, and (d, h) 550 W)

Fig. 5 Effects of (a) drying methods and (b) microwave power on pore-size distribution of alumina green bodies

Fig. 6 Fracture surfaces of alumina green bodies (a) Microwave drying (250 W); (b) Conventional drying

Fig. 7 Moisture distributions of wet alumina green bodies in the drying process (a) Conventional drying for 0, 2, 4, 8, and 15 h; (b) 250-W microwave drying for 0, 0.3, 0.6, 1.2, and 2 h

Fig. 8 Effect of drying methods on flexural strength of alumina green bodies and ceramics

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