Synthesis of Calcium Hexaboride Powder via the Reaction of Calcium Carbonate with Boron Carbide and Carbon

Article abstract of DOI:10.1111/j.1151-2916.2001.tb01083.x

The synthesis of calcium hexaboride (CaB₆) powder via the reaction of calcium carbonate (CaCO₃) with boron carbide (B₄C) and carbon has been investigated systematically in the present study. The influences of heating temperature and holding time on the reaction products have been studied using X-ray diffractometry, and the morphologies of CaB₆ obtained at various temperatures and holding times have been investigated via scanning electron microscopy. The interaction in the CaCO₃-B₄C-carbon system by which CaB₆ is formed is a solid-phase process that passes through the transition phases Ca₃B₂O₆ and CaB₂C₂. The optimal conditions for CaB₆ synthesis are a holding time of 2.5 h at a temperature of 1673 K, under vacuum (a pressure of 10^-1 Pa). CaB₆ powder has the same morphology as B₄C, and the properties and the shape of CaB₆ powders can be improved by choosing good-quality raw materials.

Full text of DOI:10.1111/j.1151-2916.2001.tb01083.x

J. Am. Ceram. Soc., 84 [11] 2725–27 (2001)
journal
Synthesis of Calcium Hexaboride Powder via the Reaction of
Calcium Carbonate with Boron Carbide and Carbon
Shuqi Zheng, Guanghui Min, Zengda Zou, Huashun Yu, and Jiande Han
College of Materials Science and Engineering, Shandong University, Jinan 250061, China
The synthesis of calcium hexaboride (CaB ) powder via the
II. Experimental Procedures
6
reaction of calcium carbonate (CaCO ) with boron carbide
3
(
B C) and carbon has been investigated systematically in the
The starting powders in the present study were CaCO , B C,
3 4
and carbon powders. The characteristics of those powders are
listed in Table I.
4
present study. The influences of heating temperature and
holding time on the reaction products have been studied using
The CaCO , B C, and carbon powders were combined in the
X-ray diffractometry, and the morphologies of CaB obtained
3
4
6
desired molar ratio (CaCO :B C:C ϭ 2:3:1). Then, the combined
at various temperatures and holding times have been investi-
gated via scanning electron microscopy. The interaction in the
CaCO –B C–carbon system by which CaB is formed is a
3
4
powders were mixed with methanol and attrition-milled in an agate
container at 600 rpm for 1 h, using agate ball media. The milled
powder was dried and pressed into pellets 25 mm in diameter and
3
4
6
solid-phase process that passes through the transition phases
Ca B O and CaB C . The optimal conditions for CaB syn-
5
0 mm long. Next, seven or eight pellets were loaded into a boron
3
2
6
2
2
6
nitride (BN) crucible 110 mm in diameter. The crucible was placed
inside a vacuum resistance furnace (Model FVPHP-R-5 FRET-20,
Fujidempa Kogyo Co., Ltd., Osaka, Japan), which was evacuated
thesis are a holding time of 2.5 h at a temperature of 1673 K,
؊2
under vacuum (a pressure of 10 Pa). CaB powder has the
6
same morphology as B C, and the properties and the shape of
4
Ϫ2
to a pressure of 10 Pa. The furnace was heated at a rate of 25
CaB powders can be improved by choosing good-quality raw
6
K/min and held for various times at different temperatures, then
cooled by water circulation. The valve of the furnace was opened
when the inner temperature was Ͻ373 K. The cooled pellets were
removed from the crucible, and some of the pellets were ground to
a powder for phase analysis via X-ray diffractometry (XRD)
materials.
I. Introduction
(
Model D/max-RB, Rigaku Co., Ltd., Tokyo, Japan). The powder
ARE-EARTH alkali-earth metal borides belong to a group of
morphology was observed using scanning electron microscopy
R
refractory, non-oxide-type, metal-like compounds that are
(SEM) (Model X-650EDAX-100, Hitachi Co. Ltd., Tokyo, Japan).
characterized by a high melting point, high strength, and high
chemical stability, as well as other special peculiarities, such as a
low electronic work function, stable specific resistance, a low
expansion coefficient in some temperature ranges, diverse mag-
netic orders, and high neutron absorbability. All these outstanding
properties result in a wide range of applications for such materials
in modern equipment.
III. Results and Discussion
Different heating temperatures and holding times were used to
determine the optimal synthesizing conditions for CaB powder.
These parameters are shown in Table II.
6
1
–4
Many countries have devoted research to these materials,
XRD patterns of the samples that were held for 2.5 h at various
heating temperatures are shown in Fig. 1. The XRD pattern of the
raw materials, which are used as a reference, is shown at the
bottom of the illustration. The compositions of the raw materials
remained unchanged after ball milling; however, the diffraction
peaks of the carbon powder were not detected, because it is an
amorphous phase. The product that was obtained at 1073 K
consisted of CaO, B C, Ca B O , and CaB C , which is an
indication that CaCO had decomposed to CaO and CO and the
CaO then had reacted with B C and carbon, or that CaCO had
reacted directly with B C and carbon. In either case, the transition
phases Ca B O and CaB C were formed.
and from that research, the special properties of calcium hexabo-
ride (CaB ) and the resistance of its composites to neutron
6
radiation in the nuclear industry have attracted widespread atten-
tion. In addition, a composite composed of boron carbide (B C)
4
and CaB6 can be used as abrasives, as well as in tool and
5
structural-ceramic applications. Matsushita et al. investigated the
oxidation of CaB and a composite of titanium diboride (TiB ) and
6
2
4
3
2
6
2 2
CaB ; a theoretical foundation for their use in oxidation-resistant
6
6
7
3
2
materials has been established. Paderno et al. studied the
4
3
emission characteristics of CaB6 and CaB –TiB composites.
6
2
4
However, few papers have systematically described the synthesis
3
2
6
2 2
of CaB powder. Therefore, in this paper, the conditions that are
6
When the reaction temperature was 1273 K, the main phases
appropriate for synthesizing CaB6 powder via the reaction of
calcium carbonate (CaCO ) with B C and carbon are described, as
were the same as those obtained at 1073 K, but CaB diffraction
peaks already existed, although they were very weak. This result
indicates that CaB can be formed at 1273 K. When the reaction
temperature was 1473 K, the CaB C transition phase disappeared
6
3
4
well as the effects of heating temperature and holding time on the
6
synthesis of CaB powder.
6
2
2
and the intensities of the B C, CaO, and Ca B O peaks decreased;
4
3 2 6
accordingly, the intensity of the CaB peak increased. When the
6
heating temperature reached 1673 K, only one phase—CaB —was
6
present. Thus, only the CaB phase can be obtained at 1673 K.
J. J. Petrovic—contributing editor
6
During the formation of CaB , the raw materials, transition phases,
6
and final product were all solid, so that the interaction by which
CaB formed in the CaCO –B C–carbon system was a solid-phase
6
3
4
Manuscript No. 188090. Received December 13, 2001; approved June 20, 2001.
process.
2
725

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726
Communications of the American Ceramic Society
Vol. 84, No. 11
Table I. Characteristics of Starting Powders
Substance
Particle size (␮m)
Purity (%)
Impurities present and quantity
CaCO3
4
C
0.5
25–35
Ͻ0.5
99.5
97.0
Na O (0.20%), Mg (0.05%), K (0.01%), Fe (0.001%), Pb (0.002%)
2
B C
Si (1.58%), Fe (0.22%)
Zn (0.1%), sulfate (0.15%), Pb (0.01%), Fe (0.10%)
Table II. Basic Technical Parameters of Synthesizing
CaB Powder
6
Sample
Holding time (h)
Reaction temperature (K)
S1 (raw materials)
S2
S3
S4
S5
S6
S7
S8
2.5
2.5
0.5
1.5
2.5
6
1073
1273
1473
1473
1473
1473
1673
2.5
Fig. 2. XRD patterns of several samples reacted at 1473 K for various
times ((a) 0.5, (b) 1.5, (c) 2.5, and (d) 6 h).
Table III. XRD Analysis Results of Powder Obtained under
Various Conditions
Sample
Phase composition
S1 (raw materials)
CaCO , B C, CaO
3 4
4 2 2 3 2 6
S2
S3
B C, CaO, CaB C , Ca B O
B C, CaO, Ca B O , CaB C ,
4
3
2
6
2
2
CaB (trace)
6
S4
S5
S6
S7
S8
B C, CaO, Ca B O , CaB
4 3 2 6
6
6
B C, CaO, Ca B O , CaB
4
3 2 6
B C, CaO, Ca B O (trace), CaB
4
3
2
6
6
Fig. 1. XRD patterns of several samples reacted for 2.5 h at various
CaO (trace), B C (trace), CaB
4
6
temperatures ((a) starting mixture, (b) 1073 K, (c) 1273 K, (d) 1473 K, and
CaB6
(e) 1673 K).
Although CaB powder can be obtained at a temperature of 1273
size during powder preparation. The results indicated that the
powders obtained at 1473 and 1673 K had the same particle
6
K, its synthesis is slow, because the reaction is a solid-phase process.
To determine the optimum conditions for synthesis, various holding
times at 1473 K were studied. Figure 2 shows XRD patterns of the
samples reacted at 1473 K for various times. The intensities of the
morphologies as those of the B C raw material; the results also
4
show that the powder was dispersed without agglomeration (Figs.
3(a) and (c)). The CaB powder obtained at 1673 K (Fig. 3(d)) was
6
CaB diffraction peaks in the mixture held for 0.5 h at 1473 K were
much coarser than that obtained at 1473 K (Fig. 3(b)), and many
6
stronger than those of the sample held for 2.5 h at 1273 K, and the
CaB C transition phase disappeared. This finding indicates that the
of the CaB powders exhibited stratifications (Fig. 3(f)). The flat
6
surface of the B C was smooth, but the surface of the powder
2
2
4
CaB phase formed violently at 1273–1473 K.
obtained at 1473 K was coarse, because of the formation of
6
With a prolonged holding time, the intensities of the CaO, B C,
transition phases, and the surface of the CaB was much coarser,
4
6
and Ca B O diffraction peaks gradually decreased and the inten-
although it maintained the particle morphology of the B C raw
3
2
6
4
sities of the CaB diffraction peaks increased. A limited amount of
material. The CaB powder was dispersed, with many pores inside
6
6
the Ca B O transition phase formed after a holding time of 2.5 h.
(Fig. 3(e)), but the faces of the stratified CaB powder particles
3
2
6
6
After holding for 6 h, the main phase was CaB , although some
were much smoother.
6
B C and CaO phases still were present. All the transition phases
The reaction of CaCO (particle size of Ͻ0.5 ␮m) with B C
4
3
4
disappeared in the temperature range of 1273–1473 K; however,
complete reaction required ϾϾ6 h. In economic terms, this proce-
dure was not feasible. The phase compositions obtained under the
various conditions are listed in Table III.
(particle size of Ͼ25–35 ␮m) was the same as the reaction of a
small sphere on a large plane. The reaction began from the neck
that formed between the sphere and the plane. With prolonged
holding time and increased heating temperature, the reaction
To analyze the formation mechanism of CaB6 powder, the
powder obtained after holding at 1473 K for 1.5 h, after which a
moved continuously inside while the CaB grains formed on the
6
surface became larger. The reaction reached completion when
given sufficient synthesis time. A diffuse structure was formed for
gas release during the reaction.
certain amount of CaB powder has been formed, is shown in Figs.
6
3
(a) and (b). Figures 3(d)–(f) show SEM micrographs of as-
received CaB powder. As described previously, the processing
B C and CaB have different crystal structures; thus, the crystal
6
4
6
temperature and holding time influences the purity and particle
structures change during the synthesis of the CaB powder. When
6

November 2001
Communications of the American Ceramic Society
2727
Fig. 3. SEM micrographs of samples obtained under various conditions ((a) and (b) 1473 K, holding time of 1.5 h and (c)–(f) 1673 K, holding time of 2.5 h).
the stress caused by the transformation of the crystal lattice is
endurable, the CaB maintains the B C morphology; however, if
B C is one of the most important ways to improve the
4
properties of a CaB powder.
6
4
6
the stress is beyond the endurance limit, the CaB6 becomes
stratified. Thus, the morphology of the B C powders determines
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Ⅺ
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4