Interaction between a titanium-containing molten salt and an alumina plate

P. Baumli1, J. Sytchev1, Zs. H. Göndör2, G. Kaptay3

Dep. Physical Chemistry, University of Miskolc, Egyetemváros, Hungary-3515

E-mail: peter.baumli@freemail.hu Keywords: alumina, titanium, molten salt, interaction

Abstract. In the present paper the chemical interaction between an alumina plate and a one-phase NaCl-KCl-K2TiF6 as well as two-phase NaCl-KCl-K2TiF6/Ti system has been studied experimentally at 700 oC during 2 hours, as function of K2TiF6 content of the melt. In all experiments the reaction product TiO2 of an exchange reaction has been found at the surface of the alumina plate. Its thickness changes through a maximum point as function of the K2TiF6-content of the melt, as a result of the competition of two processes: the exchange reaction leading to the formation of the TiO2 layer, and its dissolution in the melt. Introduction

Ceramic particles reinforced metal matrix composites with desired microstructure and

properties can be produced only if the liquid metal wets the surface of the solid particles [1-5]. As ceramic particles are usually not perfectly wetted by liquid aluminum [6-9], the wettability should be improved. One of the ways to improve wettability of the particles is to coat them with a thin layer of metal [10-11] or metal-like ceramics [11-12], or to dissolve the oxide layer on the surface of liquid aluminum by a salt K2ZrF6 [13-14].

Molten salts are ideal media for transferring [15] and coating particles with transition metals, as they are compatible with liquid metals. One of the most perspective coatings is Ti, as it is known as a very good wetting promoter [9]. Nevertheless, till now only a few examples of this technique have been published [16-21]. In these works, usually the “classical” molten salt system NaCl-KCl-K2TiF6 is used, with a piece of titanium metal in it. The interaction between Ti+4 ion and Ti metal provides low valence Ti+2 or Ti+3 ions in the melt, which are expected to disproportionate at suitable heterogeneous nucleation sites, such as solid oxide particles. This phenomenon was observed about 40 years ago, on glass and quartz surfaces [22-24]. The goal of the present work is to study the chemical interaction between an alumina plate and the molten salt, with and without Ti metal phase. Experimental

The chemical interaction has been studied between an alumina plate (10.2 * 10.2 * 1.2

mm in size, hung into the molten salt on a Mo wire) and the NaCl-KCl-K2TiF6 molten salt mixture at 700 °C during 2 hours in a glassy carbon crucible, under an argon atmosphere. In the first series of three different experiments, the melt contained 5, 10 and 20 wt. % of K2TiF6. In the second series of experiments a piece of titanium metal (with a weight of about 10 g) was also introduced into the melt (at the bottom of the crucible with no straight contact with the alumina plate). The alumina plate was introduced an hour later, after the melt (with, or without Ti metal) reached the working temperature of 700oC. After 2 hours of reaction time, the alumina plate was drawn out of the melt and the system was left to cool down spontaneously.

Materials Science Forum Vols. 473-474 (2005) pp. 39-44online at http://www.scientific.net© 2005 Trans Tech Publications, Switzerland

Licensed to Kaptay (fkmkap@gold.uni-miskolc.hu) - University of Miskolc - HungaryAll rights reserved. No part of the contents of this paper may be reproduced or transmitted in any form or by any means without thewritten permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 193.6.15.94-23/09/04,14:32:48)

The expected chemical processes

In case the molten salt is used without titanium metal, we expect the following

exchange chemical reaction to take place:

2 Al2O3 + 3 TiF4 = 4 AlF3 + 3 TiO2 (∆rGo973 K = -502 kJ [25]) (1)

The reaction product, AlF3 is expected to dissolve in the melt in the form of a chloro-

fluoride complex, while TiO2 is expected to stick to the surface of alumina, although its partial dissolution is also expected, intensifying with the fluoride content of the melt. When a piece of titanium metal is placed in the melt, in addition to reaction (1), also the following chemical reactions are expected to occur:

Ti4+ + Ti (s) = 2 Ti2+ (2)

2 Ti2+ = Ti4+ + Ti (possibly on the surface of the alumina plate) (3)

If reaction (1) takes place as well, then the following reactions are also possible:

Ti + 3 TiO2 = 2 Ti2O3 (∆rGo973 K = -194.5 kJ [25]) (4)

Ti + Ti2O3 = 3 TiO (∆rG

o973 K = -98.0 kJ [25]) (5)

It should be understood that if the coating is Ti, it could interact with the melt (see

reaction (2)). On the other hand, whatever oxide is formed on the alumina surface (TiO2, TiO, Ti2O3), it can be dissolved in the melt, especially if it contains high amount of K2TiF6. Experimental results

Phases obtained in the molten salt / alumina system. In Figs. 1-3, SEM images and

EDS spectra taken from the surface of the alumina plates are shown, after they have been treated in the molten salt containing 5, 10 and 20 wt. % of K2TiF6, respectively. In Table 1, the most probable phases formed on the surface of the alumina plate, determined by the X-ray analysis, are given. As one can see, there is a strong Ti-peak in each of the EDS spectra. From the results of the X-ray analysis it turned out that this titanium is in the form of a titanium dioxide (rutile) coating, formed on the alumina plate surface. This is in perfect agreement with our expectations (see reaction (1)).

Phases obtained in the molten salt / titanium / alumina system. SEM images and EDS spectra of alumina plates obtained in these experiments are shown in Figs. 4-6. In Table 2, the most probable phases formed on the surface of the alumina plate, determined by the X-ray analysis, are given. One can see that the results are very similar to those obtained in the case when no titanium metal was present in the system.

Thickness of the TiO2 coating. The TiO2 coating thickness was measured using the cross-sectional views (SEM) of the samples. The dependence of the coating thickness on the concentration of K2TiF6 in the melt (with no Ti metal in the system) is shown in Fig. 7. Although it is not shown in Fig. 7, when Ti metal was present in the system, the thickness of the TiO2 phase was somewhat smaller.

Materials Science, Testing and Informatics IV40

Table 1. The value of parameter R* of the most probable phases, found by X-ray analysis on the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 melt

5 wt. % K2TiF6 10 wt. % K2TiF6 20 wt. % K2TiF6 Corundum 1.017 - 1.014

Rutile 1.012 1.006 1.018 Brookite - 1.209 1.654

* The existence of the given phase is most probable if R = 1.

1.

2.

Fig. 1. SEM image of the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 (5

wt. %) melt (magnification x500) and its EDS spectrum at point 2

Fig. 2. SEM image of the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 (10

wt. %) melt (magnification x1000) and its EDS spectrum at point 1

Fig. 3. SEM image of the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 (20

wt. %) melt (magnification x1000) and its EDS spectrum

1

Materials Science Forum Vols. 473-474 41

Table 2. The value of parameter R of the most probable phases, found by X-ray analysis on the surface of an alumina plate treated in the NaCl-KCl-K2TiF6/Ti system

5 wt. % K2TiF6 10 wt. % K2TiF6 20 wt. % K2TiF6 Corundum 1.019 1.036 1.007 Rutile 1.018 1.057 1.000 Brookite - - 1.264

Fig. 4. SEM image of the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 (5

wt. %)/Ti system (magnification x500) and its EDS spectrum

Fig. 5. SEM image of the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 (10

wt. %)/Ti system (magnification x500) and its EDS spectrum

Fig. 6. SEM image of the surface of an alumina plate treated in the NaCl-KCl-K2TiF6 (20

wt. %)/Ti system (magnification x500) and its EDS spectrum

1

Materials Science, Testing and Informatics IV42

0

2

4

6

8

10

12

0 5 10 20

Concentration of K2TiF6, wt%

Coa

ting

thic

knes

s, µ

m

Fig. 7. Dependence of the thickness of the coating obtained on the alumina surface in the

molten salt without Ti phase as function of the K2TiF6 concentration in the melt Discussion of the results

When the alumina plate was kept at 700oC in the NaCl-KCl-K2TiF6 system, the only

expected reaction between them is the exchange reaction (1) leading to the formation of a TiO2 coating. As follows from Fig-s 1-3 and Table 1, indeed, this is the only reaction product. As follows from Fig. 7, the thickness of the TiO2 layer strongly increases up to about 10 wt. % of K2TiF6 in the melt and at higher concentrations it slightly decreases.

Kinetically, the exchange reaction (1) can be divided at least into the following steps: i. diffusion of Ti+4 in the molten salt to the surface of the alumina plate, ii. diffusion of Al+3 and Ti+4 through the TiO2 layer, iii. heterogeneous chemical reaction (1), iv. diffusion of Al+3 ions into the bulk of the melt. Theoretically, the rate of the exchange reaction (1) may be limited by the rate of either of the steps i. through iv., or by the rate of all the four steps (in case their rates are of the same order of magnitude). In the series of experiments without titanium metal in the melt we were unable to determine the rate-limiting step of the process.

The solubility of oxides in molten chloro-fluoride systems is known to increase with increasing the concentration of fluorides [26]. Therefore, the thickness of the TiO2 coating is expected to decrease with the K2TiF6 content of the melt due to its faster dissolution. The maximum point appearing in Fig. 7 is a result of the two competing processes: the exchange reaction (1) and the dissolution of its reaction product, TiO2.

When Ti metal is present in the melt, Ti-coating or, at least, the formation of a titanium oxide with lower valency of Ti (TiO or Ti2O3) is expected. However, experimentally none of these phases has been found on the alumina plate. The reason is that the rate of transfer of Ti through the melt in the form of Ti+2 ions is probably slower than the rate of dissolution of these coatings. These findings contradict with those achieved by Wei et al [16], who found Ti2O and not TiO2 in similar experiments. Conclusions

TiO2 coatings have been obtained on the surface of alumina plates as the result of 2-

hour treatment in the NaCl-KCl-K2TiF6 and NaCl-KCl-K2TiF6/Ti systems at 700°C.

Materials Science Forum Vols. 473-474 43

Titanium metal or lower titanium oxides were not detected on the surface of alumina even in the case when there was a piece of titanium metal present in the system. The reason is probably the fast dissolution of these phases in the molten salt. The thickness of the TiO2 layer changes through a maximum point as function of the K2TiF6-content of the melt, as a result of the competition of two processes: the exchange reaction leading to the formation of the TiO2 layer, and its dissolution in the melt. Acknowledgement

The authors are grateful to Á. Kovács for SEM and EDS analyses and to J. Sólyom for

the X-ray diffraction data. This work has been financed by OTKA T 037626. References

1. F. Dellanay, L. Froyen, A. Deruyttere – J.Mater.Sci., 22 (1987) 1-16. 2. A. Mortensen – Mater. Sci. Eng., A135 (1991) 1-11. 3. R. Asthana, S.N. Tewari – Composite Manufacturing, 4 (1993) 3-25. 4. G. Kaptay – Mater. Sci. Forum, 215-216 (1996) 459-466. 5. G. Kaptay – in: Affordable Metal-Matrix Composites for High Performance Applica

tions, ed. By A.B. Pandley, K.L. Kending and T.J. Watson, TMS, 2001, 72-99. 6. Y.V. Naidich – Progr. In Surface and Membrane Science, 14 (1981) 353-492. 7. G. Kaptay, Mater Sci. Forum, vol. 77 (1991) 315-330. 8. G. Kaptay, E. Báder, L. Bolyán – Materials Science Forum, 329-330 (2000) 151-156. 9. N. Eustathopoulos, M.G. Nicholas, B. Drevet: Wettability at high temperatures,

Pergamon Press, Amsterdam, 1999. 10. P.K. Rohatgi, R. Asthana – in Cast Reinforced Metal Composites, ed. By S.G. Fishman

and A.K. Dhingra, ASSM, 1988, 61-66. 11. T.P.D. Rajan, R.M. Pillai, B.C. Pasi – J. Mater. Sci., 33 (1998) 327-337. 12. C. Körner, W. Schaff, M. Ottmüller, R.F. Singer – Adv. Eng. Mater, 2 (2000) 327-337. 13. J. Procher, J.M. Quenisset, R. Naslain – J.Mater. Sci., 24 (1989) 2697-2703. 14. A. Magyar, Z. Gácsi, G. Kaptay, I. Szalai – in: Proc. of MICROCAD 2000, Materials

Technology, University of Miskolc, 2000, 183-188. 15. A. R. Kennedy, A. E. Karantzalis - Mater Science and Eng. A, 264 (1999) 122-129. 16. P. Wei, H. Qiliang, C. Jian, C. Juan, Y. Huang: Materials Letters, 31 (1997) 317-320. 17. J. Chen, P. Wei, Q. Huang, Y. Huang – Materials Letters, 35 (1998) 90-94. 18. P. Wei, J. Li, J. Chen – Thin Solid Films, 422 (2002) 126-129. 19. G. Kaptay, J. Sytchev, Zs.H. Göndör - Proc. 6th Int. Symp. on Molten Salt Chemistry

and Technology, ed. by C. Nianyi, Q. Zhiyu, Shanghai University Press, 2001, 178-181. 20. J. Sytchev, Zs.H. Göndör. P. Baumli P., Z. Gácsi, Á. Kovács, J. Sólyom, G. Kaptay -

Proc. of MicroCAD 2002, Section: Chemistry, University of Miskolc, 2002, 131-136. 21. P. Baumli, Zs.H. Göndör, J. Sytchev, G. Kaptay: Proc. of MicroCAD 2003 Conference,

Section Chemistry, University of Miskolc, 2003, 1-4. 22. V.A. Sukhodskii, V.G. Gopienko – Trudi VAMI, 46 (1960) 146-158. 23. O.V. Baimakov, M.M. Vetyukov: Electrolysis of molten salts (in Russian), Moscow,

Metallurgiya, 1966. 24. V.G. Gopienko, L.A. Pavlova – Trudi VAMI, 63 (1968) 108-114. 25. I. Barin: Thermochemical Data of Pure Substances – VCH, Weinheim, 1993. 26. K. Grjotheim, C. Krohn, M. Malinovsky, K. Matiasovsky, J. Thonstad: Aluminium

Electrolysis, 2nd Ed., Aluminium-Verlag, Düsseldorf, 1982.

Materials Science, Testing and Informatics IV44