TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

9518. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Eff ect of Excess Carbon Content of Boron Carbide and Temperature on Synthesis of Calcium Hexaboride Powders

Duygu Yılmaz Çakta¹, Nurşen Koç¹, Servet Turan²

¹Eskişehir Osmangazi University, ²Anadolu University - Türkiye

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 B4C-CaCO3 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

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-

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

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

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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3. 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.1. FTIR spectra of precursor for B4C.

To investigate the thermal characteristics of the precursors, TG/DTA analysis were carried out (Fig. 2-3). Significiant decomposition peak was observed approximately at 425 for precursor CB60 and there was no peak indicated of unreacted boric acid. For precursor CB35, decomposition peak was observed approx. at 415 C 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.2. TGA/ DTA curves of precursor(CB35).

Fig.3. TGA/ DTA curves of precursor (CB60).

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

is obvious that BCTR was carried out a transitional phases and 1200 C was not sufficient to form CaB6 over CB35-CaCO3 mixture.

Fig. 4. XRD patterns of B4C powders.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

9718. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

Fig. 5. XRD patterns of CB35-CaCO3 and CB60-CaCO3 mixtures

In Fig. 6., XRD patterns of powders obtained by BCTR of CB35-CaCO3 mixture at 1300-1500 C 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 1200 C. When the temperature increased, reduction yield was also increased with consuming of transitional phases. When the temperature was set at 1500 C, there were only B4C and CaB6 in the system.

Fig. 6. XRD patterns of BCTR of CB35-CaCO3 mixtures at 1300-1500

In Fig. 7., XRD patterns of powders obtained by BCTR of CaB60-CaCO3 mixture at 1300-1450 C 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 1400 C, CaB6 powders were successfully synthesized from CB60-CaCO3 mixture.

Fig. 7. XRD patterns of CB60-CaCO3 mixtures at 1300-1450

In Fig. 8.. SEM images of powders obtained by BCTR at 1400 C 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. 8. SEM images of CB60-CaCO3 mixtures at 1450 at high and low mag.

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4. 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 1450 C for 6 hours. However, with using of CB35-CaCO3 ient to synthesize CaB6 as a single phase.

Acknowledgement

This research was funded and supported by TUBITAK. (Project No:114M931)

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