Chemistry Letters Vol.32, No.10 (2003)
959
In the present route, no CaB6 could be detected in the prod-
uct if the temperature is below 450 ꢀC. Heating at a higher tem-
perature such as 600 ꢀC will result in an obvious increase of the
crystalline CaB6 sizes. An optimum temperature for ultrafine
CaB6 is about 500 ꢀC. A reaction time at 500 ꢀC in the range
of 3–10 h did not significantly affect the crystallite size. If the
time is shorter than 2 h, the reaction becomes very incomplete
and the crystallinity is very poor due to too short reaction time.
One feature of this synthesis route is the high pressure in the au-
toclave, coming from the produced H2 (about 30 kg/cm2 esti-
mated by the ideal gas law). The high pressure makes the crys-
talline CaB6 form at a relatively low temperature.
800
1200
1600
2000
Raman shift / cm-1
Figure 3. Raman spectra of the CaB6 sample at room
temperature.
In summary, the ultrafine CaB6 powder was successfully
synthesized using CaCl2 and NaBH4 as the reactants at a low
temperature of 500 ꢀC. The products mainly consisted of cubic
particles with an average size of 180 nm. This route allows for
the facile formation of CaB6 at a low temperature, and may be
extended to synthesis other borides.
150
125
100
75
30
0
-30
-60
-90
-120
This work was supported by the National Natural Science
Foundation of China and the 973 Projects of China.
0
200
400
600
800
1000
Temperature / oC
References
1
2
3
4
J. Matsushita, K. Mori, Y. Nishi, and Y. Sawada, J. Mater.
Synth. Process., 6, 407 (1998).
T. Rymon-Lipinsk, B. Schmeizer, and S. Ulitzka, Steel Res.,
65, 234 (1994).
N. Tsushinsha, in ‘‘Handbook on High-Melting-point Com-
posites (Japanese Edition),’’ Nisso Tsushinsha (1977).
D. P. Young, D. Hall, M. E. Torelli, Z. Fisk, J. L. Sarrao, J.
D. Thompson, H.-R. Ott, S. B. Oseroff, R. G. Goodrichk,
and R. Zysler, Nature, 397, 412 (1999).
Figure 4. TGA and DTA curves for CaB6 sample.
sample at room temperature. Three peaks at 754.3, 1121.8, and
1246.9 cmÀ1 could be observed clearly, which is attributed to
Raman active modes A1g, Eg, and T2g for CaB6, respectively.12
The Raman peaks of the as-prepared CaB6 sample are broader
than that of the CaB6 single crystal grown by floating-zone
method, which may be due to disorders induced by the exis-
tence of defects and strains in ultrafine materials.12
Figure 4 shows the TGA and DTA curves of the CaB6. It
can be seen that a pronounced weight gain step occurred in
the temperature range of 526 to 995 ꢀC, which can be attributed
to the oxidation of CaB6. Two exothermic peaks at 608 and
753 ꢀC can be observed in the DTA curve. This suggests that
the oxidation of CaB6 is composed of two steps, which has been
reported for CaB6.1 A small weight loss step can also be ob-
served around 100 ꢀC, which may arise from the evaporation
of absorbed water on the surface of the sample. The TGA and
DTA data reveal that the initial oxidation temperature for ultra-
fine CaB6 is 526 ꢀC, which is much lower than that of CaB6
with larger size (median particle size = 8 mm).1 The decrease
of oxidation resistance for ultrafine CaB6 may be due to its
small grain size. The ratio of the surface to volume increases re-
markably when the particle size decreases. This will lead to
more defects and strains exposed on the crystal surface, which
is not beneficial to oxidation resistance.
5
6
7
8
M. E. Zhitomirsky, T. M. Rice, and V. I. Anisimov, Nature,
402, 251 (1999).
H. J. Tromp, P. van Gelderen, P. J. Kelly, G. Rocks, and P.
A. Bobbert, Phys. Rev. Lett., 87, 016401 (2001).
T. Moria and S. Otania, Solid State Commun., 123, 287
(2002).
R. R. Urbanoa, C. Rettori, G. E. Barberisa, M. Torelli, A.
Bianchi, Z. Fisk, P. G. Pagliuso, A. Malinowsk, M. F.
Hundley, J. L. Sarrao, and S. B. Oseroff, Physica B, 320,
419 (2002).
R. W. Johnson and A. H. Daane, J. Chem. Phys., 38, 425
(1963).
9
10 S. Otani, J. Cryst. Growth., 192, 346 (1998).
11 S. Zheng, G. Min, Z. Zou, H. Yu, and J. Han, J. Am. Ceram.
Soc., 84, 2725 (2001).
12 N. Ogitaa, S. Nagai, N. Okamoto, F. Iga, S. Kunii, J.
Akimitsu, and M. Udagawa, Physica B, 328, 131 (2003).
Published on the web (Advance View) September 22, 2003; DOI 10.1246/cl.2003.958