Thermal stability of Al3BC3

Article abstract of DOI:10.1111/j.1551-2916.2009.03171.x

The thermal stability of Al₃BC₃ powder was analyzed. Nearly X-ray-pure Al₃BC₃ powder was obtained through the calcination of the aluminum, B₄C, and carbon mixture at 1800°C in Ar. In contrast to the f

Full text of DOI:10.1111/j.1551-2916.2009.03171.x

J. Am. Ceram. Soc., 92 [9] 2172–2174 (2009)
DOI: 10.1111/j.1551-2916.2009.03171.x
r 2009 The American Ceramic Society
ournal
J
Thermal Stability of Al3BC3
Sea-Hoon Leew
Korea Institute of Materials Science (KIMS), Changwon, Gyeongnam 641-831, Republic of Korea
Hidehiko Tanaka
National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
The thermal stability of Al3BC3 powder was analyzed. Nearly
X-ray-pure Al3BC3 powder was obtained through the calcina-
tion of the aluminum, B4C, and carbon mixture at 18001C in Ar.
In contrast to the former investigations, which reported the
melting of so called ‘‘Al8B4C7’’ at 18001C, Al3BC3 did not melt
up to 21001C. Instead, it decomposed by the vaporization of
aluminum. The decomposition occurred distinctly at 14001 and
19001C in flowing Ar and a sealed carbon crucible, respectively.
The results indicated that the decomposition temperature
depended on the partial pressure of aluminum vapour in the
atmosphere.
In the present investigation, the thermal stability of Al3BC3
was investigated in vacuum or an argon atmosphere.
II. Experimental Procedure
Metal aluminum (Reagent grade, Koso Chemical Inc., Tokyo,
Japan), B4C (Grade HD20, H. C. Starck, Goslar, Germany),
and carbon (carbon black, MA-600B, Mitsubishi Chem.,
Tokyo, Japan) were used for the synthesis of Al3BC3. The raw
powders were mixed (molar ratio between Al:B:C5 3:1:3) in
ethyl alcohol using an ultrasonifier (US-1200T, Nissei, Tokyo,
Japan. The mixed slurries were dried with stirring using a hot
plate. 3.5 g of the dried powder mixture was placed in a carbon
mold and capped. The gap between the mold and the cap was
sealed using a BN paste (Fig. 1). Then, the raw powder mixture
was calcined at 18001C for 2 h in flowing Ar (heating rate: 301C/
min).
The thermal stability of Al3BC3 was analyzed using thermo-
gravimetric analysis (TGA) in flowing Ar (100 mL/min, heating
rate: 251C/min) up to 19501C (STA 409CD, Netzsch, Selb, Ger-
many) or by heating the powder at 14001–21001C for 30 min in
vacuum or Ar (heating rate: 301C/min). Aluminum, boron, car-
bon, and oxygen contents of the synthesized or heated powders
were measured using an inductively coupled plasma atomic
emission spectrometer (analyzed components: aluminum and
boron, ICP-AES, Optima 3300DV, Perkin Elmer, Wellesley,
MA), the infrared absorption method (analyzed component:
carbon, CS 444-LS, Leco, St. Joseph, MI), and an inert gas car-
rying melting-infrared absorptiometer (analyzed component:
oxygen, TC-600, Leco), respectively. The phases and morphol-
ogy of the heated Al3BC3 powder were measured using X-ray
powder diffraction (XRD, JDX-3500, JEOL, Tokyo, Japan)
with CuKa radiation and scanning electron microscopy (SEM,
JSM-6700F, JEOL).
I. Introduction
HE Al3BC3 has been considered as a promising candidate for
structural applications in cases where the mass of the com-
T
ponents becomes an important factor, such as in aerospace ma-
terials, because of its low density (2.66 g/cm3) and rather high
Young’s modulus (163 GPa).1 Several reports have described
the physical and chemical properties of Al3BC3.1–4 The com-
pound is not sensitive to hydrolysis and is stable up to 6001C in
air.5
Inoue et al.6 synthesized an aluminum-rich aluminum boro-
carbide and reported its molecular formula as ‘‘Al8B4C7’’ at
19801C. The material was reported to melt above 18001–
18301C.7 In several publications, the melting of ‘‘Al8B4C7’’
and consequent formation of a liquid phase have been consid-
ered as one of the reasons for the low-temperature sintering of
SiC when using the mixture of Al, B (or B4C), and C (hereafter
termed Al–B–C) as a sintering additive.8–10
Later on, Hillebrecht and Meyer reported that the correct
formula of the compound is Al3BC3.5 However, systematic
studies about the thermal stability of Al3BC3 have been scarcely
reported. Solozhenko et al.3 reported that Al3BC3 did not show
structural transformation up to 15271C under 2.5–5.3 GPa pres-
sure.
III. Results and Discussion
The Al–B–C has been considered as an important sintering
additive of SiC because it reduces the densification temperature
and improves the fracture toughness of SiC.8,9 For the further
application of the additive, it is important to precisely under-
stand the sintering mechanism of the Al–B–C/SiC system. Ac-
cordingly, it is required to analyze the thermal properties of
Al3BC3, which is formed as a secondary phase during the sinte-
ring of the Al–B–C/SiC.
The weight loss of Al3BC3 became distinct above 14001C in
flowing Ar (Fig. 2), which was mainly caused by the loss of
aluminum (Table I). As a result, a black layer (thickness: 2–5
mm) containing an excess amount of boron and carbon was
formed at the surface area of the calcined Al3BC3 powder (gray-
ish green color, Fig. 1) by the thermal decomposition of the
compound during calcination at 18001C.
The vapor pressure of molten aluminum was reported to be
291 Pa at 15271C.11 The vapor pressures of aluminum from
aluminum-based compounds are lower than that of pure alu-
minum (e.g., Al4C3, B100 Pa at 18001C), and the values are
affected by the bonding property between aluminum and the
neighboring atoms.12 The partial pressure of aluminum, boron,
and carbon gas generated from Al3BC3 could not be calculated
due to the lack of thermodynamic data. However, Wang et al.12
Y. Zhou—contributing editor
Manuscript No. 25785. Received January 22, 2009; approved April 26, 2009.
wAuthor to whom the correspondence should be addressed. e-mail: seahoon1
@kims.re.kr
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September 2009
Communications of the American Ceramic Society
2173
Fig. 1. Schematic of the sealed carbon mold applied for the calcination of Al3BC3 at 18001C and the subsequent heating at and above 20001C.
reported the equilibrium partial pressure of the gas phases in the
Al–B–C system, showing that the value of aluminum gas was at
least four orders of magnitude higher than those of boron and
carbon gas at 15501–18001C when using an Al4C3 and B4C mix-
ture as the source material. The results indicated that most of the
weight loss in the Al–B–C system was presumably originated
from the vaporization of aluminum. In addition, AlC gas was
reported to be generated together with aluminum and boron
gas. The strong weight loss of Al3BC3 at 19501C (80.6%, Fig. 2)
was presumably caused by the consumption of carbon together
with aluminum and boron by the formation of aluminum, bo-
ron, and AlC gas at high temperature.
The thermal stability of Al3BC3 was tested in vacuum
(1Â 10À2 Pa) and flowing Ar using the sealed carbon mold. In
contrast to the TGA results measured in an open atmosphere,
the decomposition of Al3BC3 could be effectively suppressed up
to 18001C when using the carbon crucible sealed with BN pow-
der (Fig. 1). The aluminum content of the synthesized Al3BC3
powder was only slightly lower than the stoichiometric value
when using the sealed carbon mold (Table I). In contrast, the
value decreased strongly after heating Al3BC3 again at 14001–
15001C in vacuum (Table I). In the Al–B–C system, the equi-
librium partial pressure of aluminum was reported to be 102 Pa
at 14001C, while that of boron and carbon was 3 Â 10À4 and
3 Â 10À7 Pa, respectively.12 Accordingly, compared with the va-
porization of aluminum, that of boron and carbon is expected to
be negligible at 14001–15001C. The results indicated that the
decomposition of the ternary compound strongly depends on
the partial pressure of aluminum in the atmosphere, and the
decomposition occurred at 14001C in vacuum (10À2 Pa).
The decomposition of Al3BC3 became distinct at 19001C in
Ar when using the sealed carbon mold. However, the XRD data
of the central area of the heated specimen did not change up to
20001C (Fig. 3(b)), informing that the decomposition mainly
occurred at the surface up to 20001C. The color of the as-syn-
thesized Al3BC3 was grayish green, which changed into gray and
subsequently black, as decomposition proceeded, because of the
formation of excess carbon (Fig. 1). The formation of metal
aluminum at 20501C (Figs. 3(c) and (d)) was caused by the de-
composition inside the specimen (Fig. 1). Aluminum gas con-
densed into droplets on the surface of the specimens at and
above 20501C (Figs. 1 and 4). EDS analysis indicated that the
droplets were mainly composed of aluminum with a small
amount of carbon (data not shown). The formation of alumi-
num and carbon were clearly identified by XRD in the specimen
heated at 20751C (Fig. 3(d)).
Fig. 2. Thermo-gravimetric analysis data of Al3BC3 in flowing Ar.
(heating rate, 251C/min; gas flow rate, 100 mL/min).
Table I. Chemical Composition of Al3BC3 (wt%) Before and
After Heating for 30 min
Al
B
C
O
Stoichiometric Al3BC3
Synthesized Al3BC3
63.3
63.1
8.5 28.2 —
8.5 28.0 0.4
Heating at 14001C in vacuum
Heating at 15001C in vacuum
Heating at 21001C in
60.9 10.3 28.5 0.3
58.1 10.2 31.0 0.7
Fig. 3. X-ray diffraction data of (a) the as-synthesized Al3BC3 powder,
and after heating for 0.5 h in Ar at (b) 20001C, (c) 20501C, (d) 20751C,
and (e) 21001C. The data were obtained from the middle area of the
heated specimens shown in Fig. 1 (., aluminum; ꢀ, graphite; & ,
Al3BC3).
47.2
9.4 42.8 0.6
Ar using a sealed carbon mold

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Vol. 92, No. 9
sition depended on the partial pressure of aluminum gas in the
atmosphere. The decomposition occurred at 14001C in flowing
Ar and at 19001C in a sealed carbon mold. The melting of
‘‘Al8B4C7’’ can be hypothesized to be affected by a liquid phase
composed of aluminum and boron.
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&