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

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� � � � � �

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� α-Al2O3

� θ-Al2O3

�

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