Oxidation kinetics of LaB6 in oxygen rich conditions

doi:10.1016/j.jeurceramsoc.2003.11.016

Journal of the European Ceramic Society

Volume 24, Issues 10–11, September 2004, Pages 3235-3243

https://doi.org/10.1016/j.jeurceramsoc.2003.11.016 Get rights and content

Abstract

This paper investigated the oxidation behavior of LaB₆ in the temperature range of 700–1185 °C. The results obtained by thermogravimetric analysis (TGA) showed that the initial oxidation has parabolic kinetics in the temperatures between 800–945 °C. The oxidation process exhibited temperature dependence with an activation energy of 195±15 kJ/mol in air and in pure oxygen atmosphere. Two possible reactions, either the evaporation of B₂O₃ species (occurring ⩾945 °C) or the oxidation of surface LaB₆ (⩽800 °C), influence the oxidation mechanism to be interfacial reaction control, which results in mass loss deviating from parabolic behavior. The details of the microstructure evolution of the formation of glass and crystalline La(BO₂)₃ grains are investigated. The effects of porosity, the melting of La(BO₂)₃, coarsening of La(BO₂)₃ grains are reported and discussed.

Introduction

LaB₆ is an electron-offering material, which has been extensively used as the material of electronic guns for thermal emission electron microscope. The covalent bonds between boron atoms cause high melting temperature, and can be useful for the application in the high temperature environment. Similar to the borides of the fourth, fifth, and sixth periodic groups, LaB₆ has metallic appearance and properties, which are evidenced by their high electrical conductivity, low working function, and positive temperature coefficient of electrical resistance.¹

Previous literature has studied mainly on ZrB₂ or the other rare earth borides, but not on the LaB₆ material. Berkowitz-Mattuck² reported that the oxygen-consumed law of ZrB₂ between 927–1727 °C was only parabolic relation was observed. For the temperatures greater than 1127 °C, the activation energy of the oxidation of ZrB₂ is 321 kJ/mol. Below 1127 °C the activation energy is much lower about 105 kJ/mol. The summary of the test results is shown in Table 1. The mechanisms of the oxidation of ZrB₂ are different and may be influenced by the presence of oxidized product B₂O₃, which would vaporize at high temperature. When the temperature is lower than 1027 °C, the parabolic rate constant appears increasing with oxygen partial pressure.³

Similarly, Tripp and Graham⁴ reported the oxidation test of ZrB₂ in the range of 800–1500 °C. When the temperature is below 1000 °C, the activation energy is about 105 kJ/mol, and activation energy is 196.5 kJ/mol when higher than 1000 °C. The oxidation results showed an increasing deviation from parabolic law with increasing temperature, which might also concern with the vaporization of B₂O₃. They reported when the temperature was greater than 1000 °C, B₂O₃ had an appreciable vapor pressure, since the B₂O₃ melted above 450 °C,⁴ at which ZrO₂ was produced and mixed with the liquid B₂O₃.

The oxidation of TiB₂ was also investigated5, 6 The TiB₂ starts to oxidize from 400 °C, and the oxidation resulted in crystalline TiO₂ and amorphous B₂O₃ at about 1000 °C⁷ under high oxygen partial pressure. Table 1 also shows the data of obtained from various borides, e.g. HfB₂ and LaB₆–ZrB₂ composite. It is noted that the activation energy of the LaB₆–ZrB₂ composite is determined to be 130 kJ/mol which is identical to that of LaB₆ at the temperature range of 912–1053 °C.⁸

The objectives of this research are to confer the oxidation kinetic behavior of LaB₆ at high temperature, and report the temperature dependent behavior. We also investigate the oxidizing microstructure by SEM and TEM, to realize the effects of oxidation temperature, atmosphere, and porosity in bulky LaB_6.

Section snippets

Experimental procedure

LaB₆ samples are 99.5% in purity and have a sintered density of 3.61g/cm³, which is correspondent to 13 vol.% open porosity. Sample surfaces were ground by No. 1000 mesh SiC sandpaper, and cleared in a supersonic bath for 15 min. Each pure LaB₆ sample blocks in about 1×1×0.5 cm³ dimensions were prepared and measured by precise micro-meter after cutting and polishing the surface. Fig. 1 shows the surface morphology of as-polished LaB₆ bulky sample. The pore size on the surface is about 1–3 μm.

Oxidation kinetics of LaB₆

The mass change of the samples tested in the temperatures ranging from 700 °C to 1188 °C is shown in Fig. 2. The mass change data were continuously collected for a period of time (less than 32 min) at each temperature. The mass change as a function of time in the range of temperatures 700–945 °C behaves different than that recorded at temperature higher than 945 °C. In the temperature range of 1040–1138 °C [Fig. 2(b)], the mass gain of every curve stops within 2 min. The same behaviour was also

Conclusion

From the results of TGA, TEM and SEM analysis, LaB₆ reacts with oxygen above 700 °C in air. The oxidation behavior obeys parabolic law in the range of 800–945 °C. The parabolic rate constant k_p, gives an oxidation activity energy Q=195±15 kJ/mol in airflow and in pure oxygen atmosphere. Below 850 °C and higher than 995 °C, the oxidation behavior is partially influenced by the interfacial reactions, either the oxidation of surface LaB₆ or the evaporation of B₂O₃ glass. The oxidation mass gain

Acknowledgements

The authors are grateful to thank the use of TGA offered by CSIST and the funding supported by National Science Council in Taiwan under the contract NSC92-2216-E-002-026

References (14)

C.M. Chen et al.
Microstructure performance and oxidation mechanism of boride in situ composites
Composites Science and Technology
(2001)
C.-M. Chen et al.
High temperature oxidation of lab₆-zrb₂ eutectic in situ composite
Acta Mater.
(1999)
Campbell, I. E., Sherwood, E. M. In High-Temperature Materials and Technology, Part III, 10. John Wiley and Sons, Inc.,...
J.B. Berkowitz-Mattuck
High-temperature oxidation III
zirconium and hafnium diborides. J. Electrochem. Soc.
(1966)
A.K. Kuriakose et al.
The oxidation kinetics of zirconium diboride and zirconium carbide at high temperatures
J. Electrochem. Soc.
(1964)
W.C. Tripp et al.
Thermogravimetric study of the oxidation of ZrB₂ in the temperature range of 800 °C to 1500 °C
J. Electrochem. Soc.
(1971)
A. Kulpa et al.
Oxidation of TiB₂ powders below 900 °C
J. Am. Ceram. Soc.
(1996)

There are more references available in the full text version of this article.

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Oxidation kinetics of LaB6 in oxygen rich conditions

Abstract

Introduction

Section snippets

Experimental procedure