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Effect of Excess Carbon Content of Boron Carbide and
Temperature on Synthesis of Calcium Hexaboride Powders
Abstract
Calcium hexaboride (CaB6) was synthesized by the
boroncarbide(B4C) method via reaction of calcium
carbonate(CaCO3), B4C and carbon(C). B4C is an
expensive raw material which is commercially
synthesized by the carbothermal reduction at a high
temperature. In this study, B4C powder which provided
from our previous study had been synthesized by the
carbothermal reduction of precursor obtained from boric
acid ± polyol mixture. B4C powder had been synthesized
via low temperature method and it was used as both raw
material and carbon source due to its excess C amount.
B4C powder was mixed with CaCO3 powder and B4CCaCO3 mixtures were prepared. In this study, effect of
boro/carbothermal reduction (BCTR) temperature and
excess C amount have been studied using X-Ray
diffraction and the optiumum product morphologies
have been investigated via scanning electron
microscopy. CaB6 is formed by a solid state process
which is carried out by the transitional phases that occur
due to the interaction between CaCO3-B4C-C.
1.
Introduction
CaB6 is one of the most popular ceramic materials with
its unique properties such as high melting point, high
chemical stability, high hardness, and the ability of
neutron radiaton absorbance of its composites[1-2]. It is
also a bright candidate for using as a cathode
materials[3] and electronic materials[4]. In recent years,
CaB6 is commonly used for its antioxidant and
deoxidant effect on refractory industry and copper, steel
production industry due to its ineffectiveness on
electrical conductivity [5-6].
The one of the most common synthesis method for
boride production is boro/carbothermal reduction
(BCTR). Boron carbide, carbon and metal oxide are
used as a raw materials and the process is also a proper
method to yield mass production of CaB6 [7-8].
In this study, we synthesized CaB6 powder via the
boro/carbothermal reduction of calcium oxide (CaO)
formed from the calcination of CaCO3 during the
heating process that was followed by BCTR. Effect of
excess C content of B4C dispersed along B4C that we
synthesized in our previous study and temperature of
Duygu Yılmaz Çakta¹, Nurşen Koç¹, Servet Turan²
¹Eskişehir Osmangazi University, ²Anadolu University - Türkiye
BCTR process on CaB6 formation are investigated in the
present paper, systematically.
2.
Experimental Procedure
Raw materials used in this study were CaCO3 (Merck,
%99) and B4C with excess carbon that was synthesized
in our previous study. B4C powder with excess carbon
content was synthesized via boro/carbothermal
reduction method with using H3BO3 (ETITM
Mine,%99.5) and D(-)-Mannitol (C6H14O6, MerckTM,
%99). C/B2O3 ratios were chosen to obtain both
carbothermic reduction ratio (C/B2O3:3,5-CB35) and
esterification ratio of raw materials(C/B2O3:6-CB60).
Then, calcium carbonate(CaCO3:B2O3 = 1:3) and boron
carbide were mixed with ethanol and milled for 24 hours
in a ball mill. Ethanol±powders mixture was dried to
obtain as-blended powders and pressed into a pellets
with hydraulic press. Pellets were placed into a graphite
crucible and heated at 1300-Û&IRUKLQDQ$UIORZ
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Phase analysis of resulting products were conducted by
using x-ray diffractometry (XRD-Rigaku, MiniFlex600,
Rigaku Co., Ltd., Tokyo, Japan) which is operated at 40
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Precursors for boron carbide were pressed into pellets by
mixing of KBr powders to investigate functional groups
by using Fourier Transform Infrared Spectroscopy
(Bruker-Tensor 27 FTIR) in transmission mode.
Thermal analysis of the precursors for boron carbide
was studied to investigate the thermal characteristics of
precursors by using simultaneous thermal analyzer
(STA)-thermogravimetric analysis (TG)/differential
thermal analysis (DTA) (Netzsch 449F3) and heating
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in an Ar atmosphere.
Morphology of the resulting powder was performed via
using scanning electron microscopy (SEM- Zeiss Supra
50 VP) operated at 15 kV. Samples were coated with
Au-Pd before SEM imaging.
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IMMC 2016
95
UCTEA Chamber of Metallurgical & Materials Engineers
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Proceedings Book
Results And Discussion
The FTIR spectrum of precursor for B4C synthesis is
shown in Fig. 1. For the precursor CB60, there was no
absorption peak at 1195 cm-1 which is related to
deformation band of B-O-H [9] therefore, it can be said
that no unreacted boric acid left in the precursor.
However, there were some boric acid left in the
precursor CB35. Absorption peaks at 1130 indicated
that B-O-C bonds were formed successfully and these
peaks were especially sharp for CB60 [9-11].
Fig.2. TGA/ DTA curves of precursor(CB35).
Fig.3. TGA/ DTA curves of precursor (CB60).
Fig.1. FTIR spectra of precursor for B4C.
XRD patterns of synthesized B4C powders are given in
Fig. 4. It can be seen that in precursor CB60, there was
significant excess C in comparison with precursor
CB35. XRD patterns of CB35-CaCO3 and CB60-CaCO3
REWDLQHGDWƒ&IRUKRXUVDUHJLYHQLQ)LJ,Wis
obvious that BCTR was carried out a transitional phases
and 1200ƒC was not sufficient to form CaB6 over CB35CaCO3 mixture.
To investigate the thermal characteristics of the
precursors, TG/DTA analysis were carried out (Fig. 23). Significiant decomposition peak was observed
approximately at 425Û& for precursor CB60 and there
was no peak indicated of unreacted boric acid. For
precursor CB35, decomposition peak was observed
approx. at 415ƒC and there was an endothermic peak
indicated of melting point of boric acid. These results
showed that B-O-C bonds were formed homogeneously
in precursor CB60.
Fig. 4. XRD patterns of B4C powders.
96
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18 th International Metallurgy & Materials Congress
Bildiriler Kitabı
TMMOB Metalurji ve Malzeme Mühendisleri Odası
Fig. 5. XRD patterns of CB35-CaCO3 and CB60-CaCO3
mixtures DWƒ&IRUKRXUV
Fig. 7. XRD patterns of CB60-CaCO3 mixtures at 13001450ƒ&IRUKRXUV
In Fig. 6., XRD patterns of powders obtained by BCTR
of CB35-CaCO3 mixture at 1300-1500ƒC for 6 hours
under an Ar flow are given. It can be seen that, some
transitional phase such as CaC2, Ca3B2O6 and Ca2B2O4
obtained at 1200ƒC. When the temperature increased,
reduction yield was also increased with consuming of
transitional phases. When the temperature was set at
1500ƒC, there were only B4C and CaB6 in the system.
In Fig. 8.. SEM images of powders obtained by BCTR
at 1400ƒC for 6 h can be seen at both high and low
magnifications. Particle size of powders was a few
micrometer and particles were dispersed as a micron and
submicron particles.
Fig. 6. XRD patterns of BCTR of CB35-CaCO3
mixtures at 1300-1500ƒ&IRUKRXUV
In Fig. 7., XRD patterns of powders obtained by BCTR
of CaB60-CaCO3 mixture at 1300-1450ƒC for 6 hours
under an Ar flow are given. In this system, it can be seen
that formation and consuming of transitional phases
were balanced in comparison with CB35-CaCO3
mixture. There were only CaB6 and B4C phases present
in the system. At 1400ƒC, CaB6 powders were
successfully synthesized from CB60-CaCO3 mixture.
Fig. 8. SEM images of CB60-CaCO3 mixtures at
1450ƒ&IRUKRXUV at high and low mag.
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UCTEA Chamber of Metallurgical & Materials Engineers
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Proceedings Book
nanometer CaCO3 as reactant, Key Engineering
Materials, 326-328 (2006) 369-372.
Conclusions
In this study, CaB6 powders were synthesized
successfully via BCTR method with using B4C included
excess carbon. With using of CB60-CaCO3 mixtures,
CaB6 powders were synthesized as a single phase at
1450ƒC for 6 hours. However, with using of CB35CaCO3ƒ&ZDVQRWVXIILFient to synthesize CaB6
as a single phase.
Acknowledgement
This research was funded and supported by TUBITAK.
(Project No:114M931)
References
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