Alumina Sintering at 1400C
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Transcript of Alumina Sintering at 1400C
On the High Pure Alumina Composite powder for Sintering at 1400oC,
A Preliminary Investigation
Pei Ching Yu(1) and Fu Su Yen(2)
Department of Resources Engineering
National Cheng Kung University
Tainan, Taiwan (1) [email protected] (2) [email protected]
Key words: Alumina powders, Nano materials, Sintering, Phase transformation
Abstract. Pure alumina composite powders for low-temperature sintering purpose are
examined. θ- and α- phase alumina particles with crystallite sizes 20 to 50 nm and 50 to 200
nm, respectively were mixed in an appropriate ratio. The composite powder can be easily
sintered at temperatures as low as 1400oC, preparing high density and high pure alumina
ceramics with grain size finer than 500 nm. Except the higher surface energy provided by the
finer particle sizes, the performance of the powder can be attributed to the critical size
phenomena occur during θ- and α- alumina phase transformation and the Furnas model that
induced higher pacing density of the compact.
1. Introduction
Preparation of high purity alumina ceramics with sub-micrometer grain size at temperatures
lower than 1600oC without HP or HIP techniques has been the dream of ceramicist for many years.
Extensive studies demonstrated that reducing the particle size and avoiding agglomerates of the
powder may effectively result in lowering sintering temperatures [1,2]. Inference has been made for
sintering alumina ceramics at temperatures as low as 1000oC [3] if α-Al2O3 powders with particle
sizes smaller than 20 nm could be used. Recently, Particulate Materials Center of Cheng Kung
University released one composite alumina powder (designated as T1400) for sintering at 1400oC.
The powder is composed of crystallites of θ- and α- phases with particle (= crystallite) sizes 20-50
and 50-200 nm, respectively, that mixed based on Furnas packing model [4]. Further, the presence
of θ- with α- may result in the suppression of growth of α- crystallite (by the neighbored θ-grains)
[5] before the θ to α- phase transformation takes place. In this study, a preliminary investigation on
the sintering behavior of the composite powder was made.
2. Experimental procedures
Starting powder. The starting powder (T1400) was prepared by Particulate Materials Research
Center of Cheng Kung University. Table 1 lists some basic physical properties of the powder. After
homogenized by ball milling the powder was de-watered and pulverized into -150 µm sizes. Particle
size distribution of the powder in slurry state was measured using laser light scattering methods
(Zetasizer 1000, Malvern). Microstructure was examined using TEM techniques (TEM, HF-2000,
Key Engineering Materials Vol. 313 (2006) pp. 59-62online at http://www.scientific.net© (2006) Trans Tech Publications, Switzerland
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 140.116.34.73-18/07/07,11:37:05)
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� α-Al2O3
� θ-Al2O3
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100 nm
(a)
100 nm
(b)
Hitachi).
Green compacts. Green compacts (10 mm in diameter and 3 mm in thickness) were fabricated by
dry uniaxial pressing (125 MPa). A dilatometer (DIL420 C, Natzsch) was employed to examine the
thermal shrinkage behavior before the compacts were thermal treated at 1300o – 1400
oC for 2 – 8
hrs.
Bulk densities of the green and the sintered compacts were obtained using the Archimeds
method. Microstructures and grain sizes (Feret diameter) of the sintered samples were examined
using SEM techniques (SEM, S4100, Hitachi).
Table 1�Basic physical properties of starting powders.
α - phase θ-phase BET, Sample name
Content [wt%] *Xtallite size [nm] *Xtallite size [nm] [m2/g]
A75 >75 51 25 25
*�XRD-Scherrer formula.
3. Results and discussion
Characteristics of the starting
powder.
The XRD spectrum shows that the
powder is composed of θ- and α- Al2O3
phases (Fig. 1). TEM micrographs
demonstrate that there are two different
crystallite sizes for the powder (Fig. 2).
The particle size measurement reveals
that the bimodal distribution is obscure,
although two peaks at 70 and 90 nm are
observed. However, it is obvious that 90
% of the powder is finer than 200 nm in
particle sizes (Fig. 3).
Characteristics of the green compact.
The relative density of the green
compact pressed uniaxially was 52%
T.D., indicating that the bimodal size
distribution increases the compact
density. The dilatometric curve shows
that the thermal shrinkage started at
temperature ~1100oC with a maximum
Fig. 1 XRD phase identification of starting powders
Fig. 2 TEM micrographs of starting powders
Composite Materials IV60
0.1 1 10 100 10000
5
10
% in volume
Diameter, nm
0 2 4 6 860
65
70
75
80
85
90
95
100
Relative density, %
Time, h
1400_Pure
T1400
T1350
T1300
occurring at ~1350oC (Fig. 4). The lower
starting shrinkage temperature may result from θ- to α- phase transformation.
Density and grain size. Fig. 5 displays the variations of relative density and grain size of the
sintered body with different thermal treatment conditions (1300o to 1400
oC for 2 to 8 hrs). It is
found the relative density larger than 95% T. D. can be achieved after the compact was heated at
1400oC for 2hrs. The holding duration is shorter than that needed for the densification of pure
α-Al2O3 powder, being 6 hrs at the same temperature.
The sintered body also exhibits smaller grain sizes when it is examined using SEM techniques,
being finer than 0.5 µm for 1400oC/2 – 4 hrs (Fig. 6). It is thus evidence that the composite alumina
powder composed of θ- and α- phases can be easily sintered at 1400oC to obtain high density
alumina ceramics with finer grain sizes.
Fig. 5 Relative densities (a) and grain sizes (b) of sintered samples heated at temperature rang of
1300-1400 oC for 2-8 hours
Fig. 3 Particle size distribution of starting
powders
Fig. 4 Thermal shrinkage curve of the green
compact prepared by starting powders
0 200 400 600 800 1000 1200 1400
-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
2
dL/(L0 dt),%
DIL(δL/L
0,%)
Temperature, 0C
-2
-1
0
1
0 2 4 6 80.0
0.2
0.4
0.6
0.8
Grain size, µm
Time, h
(a) (b)
Key Engineering Materials Vol. 313 61
Fig. 6. SEM micrographs of the sintered samples at 1400 oC for 2 (a, b) and 4 (c, d) hours. (a) and (c)
are polished surfaces observation, (b) and (d) are fracture sections observation
4. Conclusions
The composite alumina powder formed by mixing of θ- and α- Al2O3 powders at an appropriate
ratio can be sintered at 1400oC/2 hr to fabricate high-density fine-grained alumina ceramics. The
performance can result from the Furnas packing model for bimodal powder mixtures and θ- to α-
phase transformation of θ-phase crystallite during thermal treatments.
(Acknowledgement: The authors wish to thank Mr. M. S. Tzeng for assistance in laboratory
experiments. This study was supported by the National Science Council, ROC under Contract No.
NSC92-2216-E-006-161 and Ministry of Economic Affair, ROC under Contract No.92-EC-17–A-
08-S1-023.)
References
[1] E. A. Barringer and H.K. Bowen: J. Am. Ceram. Soc. 65 (1982), p. C-199-C-201
[2] M. P. Yan and W. W. Rhodes: Mater. Sci. Eng. 61 (1983), p. 59-66.
[3] G. L. Messing and M. Kumagai: Am. Ceram. Soc. Bull. 73 [10] (1994), p. 88-91.
[4] J. S. Reed: Principles of Ceramics Processing, 2nd (Wiley, 1995), p. 219-222.
[5] P. L. Chang, F. S. Yen, K. C. Cheng, and H. L. Wen: Nano-Letters 1 (2001), p. 253-261.
2 �m
2 �m 2 �m
2 �m
(a)
(b)
(c)
(d)
Composite Materials IV62