SYNTHESIS OF ALUMINUM
BORATE WHISKERS FOR METAL MATRIX COMPOSITES
I. Erkin Gonenli and Doc. Dr. A. Cuneyt Tas
Department of Metallurgical and Materials
Engineering,
* A.E. Uzgur, I.E.
Gonenli and A.C.
TAS, “Synthesis of Aluminum Borate Whiskers for Metal Matrix
Composites,” III. Ceramic Congress, Proceedings Book, Vol. 2, pp. 85-91,
Istanbul,
* I.E. Gonenli
and A.C. TAS, "Chemical Preparation of Aluminum Borate
Whiskers," Powder Diffraction (
Abstract
Aluminum Borate (Al18B4O33
or 9Al2O3.2B2O3) is a refractory
compound with a melting point of 1440°C. It has a low density, 2.94 g/cm3,
a Mohs hardness of 7, a tensile strength of 8 GPa, a high Young’s Modulus of 400 GPa,
and a low thermal expansion coefficient of 1.9x10-6 K-1. It tends to
form needle-shaped crystals. Such quite extraordinary physical properties of
this ceramic material make it a candidate as a reinforcement for metals. These
properties are competitive with those of a silicon carbide whisker which is now
recognized as the best whisker for Al-based composite materials. The superior
feature of such whiskers is that its price would be far below that of the SiC whisker. Al18B4O33 whiskers were synthesized in potassium
sulphate flux from aluminum sulphate
and boric acid. The B/Al and K2SO4/(Al+B) ratios in raw material mixtures (which were achieved
by dissolving the starting materials in water, followed by evaporation to
dryness), and heating temperatures studied did have a significant effect on the
resultant whisker morphology. The synthesis temperature of 1050°C was found to
be the optimum with respect to whisker morphology. The product purity, phase
composition, and whisker morphology were investigated by XRD, EDXS, and SEM,
respectively.
Introduction
Al18B4O33 is a refractory compound with a melting point of 1440°C [1]. It has a low density, 2.94 g/cm3 [1], and a low coefficient of thermal expansion [2]. It tends to form needle-shaped crystals [3]. These characteristics suggest that Al18B4O33 could be an alternative material for whiskers to reinforce light metals like aluminum alloys [4]. Whiskers of Al18B4O33 have been synthesized [5] by a vapor phase method and by a flux method using borates. However, the product yields were found to be very low, 10% or less. The low yields in the flux method are attributed to glass forming between the flux and aluminum compounds [3].
The aluminum borate whisker has been established by a Japanese chemical company (Shikoku Chemical Co. Ltd.) in cooperation with the Government Industrial Research Institute of Shikoku since April 1989 and is commercially available from the above-mentioned company, under the name of ALBOREX-G [4, 6]. The low price of the whisker is one of the incentives to adopt whiskers for metal matrix composites in industrial uses such as automobile engine components.
The present study attempts to determine the experimental parameters and conditions of the synthesis of Al18B4O33 (aluminum borate) whiskers. The starting point for this work has been taken as the experimental synthesis parameters described in Ref. 3, and the whisker synthesis conditions investigated were 1) B/Al ratio in the starting mixtures, and 2) heating temperature.
The below XRD diagram shows the
effect of different B/Al ratios studied on the Al-borate formation at a single
temperature. At all compositions studied, the only phase present was 9Al2O3.2B2O3.
A Tentative XRD Pattern for the 9Al2O3.2B2O3 Whiskers
(Cu-K-alpha radiation, Step: 0.02°, Count: 5 sec)
Lattice Parameters: a = 15.0077
Å, b = 7.6850 Å, c = 5.3088 Å, V = 612.2883 Å3
(Orthorhombic, Amam)
hkl 2
theta obs 2 theta calc d (Å)
2 0 0
11.783 11.784 7.5044
2 1 0
16.496 16.498 5.3696
0 1 1
20.315 20.315 4.3680
0 2 0
23.129 23.129 3.8424
4 0 0
23.693 23.695 3.7522
2 2 0
26.029 26.032 3.4206
4 1 0
26.414 26.414 3.3716
4 1 1
31.403 31.406 2.8464
4 2 0
33.348 33.351 2.6847
2 0 2
35.853 35.855 2.5026
2 3 0
37.050 37.053 2.4245
0 3 1
39.009 39.009 2.3071
3 3 0
39.492 39.490 2.2800
0 2 2
41.309 41.306 2.1838
4 0 2
41.646 41.646 2.1669
4 3 0
42.703 42.705 2.1157
3 3 1
43.144 43.145 2.0951
3 2 2
45.265 45.269 2.0017
5 3 0
46.573 46.573 1.9485
0 4 0
47.275 47.273 1.9212
8 0 0
48.483 48.487 1.8761
2 4 0
48.891 48.896 1.8614
8 1 0
50.008 50.007 1.8224
6 3 0
50.990 50.989 1.7896
6 1 2
51.546 51.552 1.7716
0 1 3
53.065 53.062 1.7244
4 4 0
53.534 53.545 1.7104
8 2 0
54.379 54.379 1.6858
1 2 3 57.637
57.629 1.5980
4 1 3
58.897 58.893 1.5668
0 4 2
59.334 59.331 1.5563
8 0 2
60.363 60.372 1.5322
8 3 0
61.193 61.187 1.5134
0 5 1
62.903 62.900 1.4763
6 4 1
63.464 63.467 1.4646
1 3 3
64.223 64.219 1.4491
5 2 3
65.860 65.862 1.4170
10 2 0 66.877
66.875 1.3979
5 5 0
68.533 68.535 1.3681
4 3 3
69.155 69.152 1.3573
8 4 0
70.034 70.045 1.3424
0 0 4
70.950 70.956 1.3273
10 0 2 72.262
72.261 1.3064
10 3 0 73.013
73.006 1.2948
0 6 0
73.943 73.941 1.2808
6 5 1
74.589 74.581 1.2713
2 6 0
75.192 75.194 1.2626
4 5 2
75.827 75.823 1.2536
4 1 4
77.185 77.180 1.2349
4 4 3
77.572 77.571 1.2297
2 6 1
77.669 77.676 1.2284
9 4 1
77.910 77.900 1.2252
7 5 1
78.621 78.617 1.2159
8 5 0
80.760 80.767 1.1890
5 6 0
81.673 81.668 1.1780
0 6 2
83.794 83.789 1.1535
2 6 2
84.999 85.002 1.1402
6 6 1 87.425
87.430 1.1147
0 4 4
89.724 89.726 1.0920
Conclusions
Al18B4O33 whiskers, of superior
mechanical properties (previously reported), were synthesized by using a
flux-growth method from the starting materials of high-purity aluminum sulphate, boric acid, and potassium sulphate.
Three different B/Al ratios and two synthesis temperatures were investigated
for their influence on the whisker growth patterns. All of the
composition-temperature combinations studied in this work did yield the XRD
patterns of “single-phase” Al18B4O33 of orthorhombic structure. The
thickness and orientation of
the grown whiskers were found to depend on the B/Al atomic ratio utilized
during synthesis.
References
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(1942).
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