WETTING OF PURE ALUMINIUM ON FILTER MATERIALS GRAPHITE, AlF 3 AND Al 2 O 3 Sarina Bao 1 , Anne Kvithyld 2 , Sean Gaal 2 , Thorvald Abel Engh 1 , Merete Tangstad 1 1 Norwegian University of Science and Technology, Trondheim, NO-7491, Norway, Tel: +47--73594891, Fax: +47--73550203, Email: sarina.bao@material.ntnu.no, 2 SINTEF Materials and Chemistry, Trondheim, N-7465, Norway Keywords: Wettability, Contact angle, Aluminium, Alumina, Graphite, AlF 3 Abstract The wettability of pure aluminium on filter materials is believed to be an important factor affecting the filtration of aluminium. The contact angle of molten aluminium on graphite, alumina and AlF 3 has been measured in 1 atmosphere of argon with an oxygen partial pressure of about 10 -17 Pa in the temperature range of 1000- 1800ºC. Improved techniques to remove the aluminium oxide layer from a molten aluminium drop have been employed in a horizontal graphite tube furnace. The wettability of aluminium on graphite, alumina and AlF 3 has been discussed and compared with the aim to obtain “new” and more effective filter materials. Introduction The contact angle of a liquid is the angle a liquid-vapour interface makes against a solid surface, as Figure 1. It is determined by the interaction between all three interfaces. If the liquid is strongly attracted to a solid surface, then the liquid will spread out and the contact angle will be less than 90º, possibly as small as 5º. If the liquid is not attracted to the solid surface, then the droplet will have a contact angle greater than 90º. Figure 1 A sessile drop on a solid substrate When filtering aluminium to remove inclusions, the wetting behaviour of liquid aluminium on filter materials is believed to strongly influence the efficiency. The common filter material is alumina. The wetting of molten aluminium on alumina has been investigated by many researchers [1-12], as shown in Figure 2. Contact angles of 88-167º have been reported at 700ºC, and it has been found that they tend to decrease with increasing temperature. The results are discordant because they were performed with various aluminium and alumina substrates in different vacuum. The effect of total pressure on the contact angle was discussed in [13]. A petrol coke filter had been used by Swiss Aluminium Ltd. [14]. The contact angle of aluminium on vitreous carbon was measured by K. Landry et al [15] as 139-160º at 917ºC under a dynamic vacuum of 10 -3 Pa. The wettability of aluminium on AlF 3 has not been reported, but it has been shown to be an effective material to remove alkali metals, such as Na and Ca [16, 17]. The wettability of aluminium on graphite, alumina and AlF 3 is studied with the sessile drop set-up in the interest of new filter materials in this paper. New filter materials are expected to be effective in filtration. Experiments with wetting materials should throw the light on filtration mechanisms. 600 800 1000 1200 50 75 100 125 150 175 John.1986 Klinter.2008 Wang.1994 Naidich.1983 Ksiazek.2002 Shen.2003 Wolf.1966 Carnahan.1958 Brennan.1968 Nicholas.1968 Laurent.1988 Contact angle/ o Temperature/ o C Figure 2 Contact angles of Al on Al 2 O 3 from literature [1-12] Experimental procedure The contact angle between molten aluminium and various materials was measured using the sessile drop method in a furnace sketched in Figure 3. The apparatus essentially consists of a graphite heater surrounded by graphite radiation shields, located in a water-cooled vacuum chamber. The chamber was fitted with windows to allow a digital video camera to record the shape of the droplet. A pyrometer was used to measure the temperature. The contact angles and linear dimensions of the images were measured directly from the image of the drop using FTA Video Drop Shape Analysis software. The experiments were performed with a substrate of either alumina, graphite (ISO-88) or AlF 3 and 99.999% pure aluminium. The aluminium was cut into small 3mm cubes, then ground with 320 mesh SiC paper and cleaned in ethanol to minimize the oxide layer. AlF 3 powder was pressed to form a 10mm diameter x 3mm high tablet. Argon gas was cleaned using an All-pure gas purifier from Alltech, followed by Mg turnings at 450 o C. Inside the vacuum chamber there was also a small getter furnace which contained Ti sponge at 650 o C. The oxygen level in the gas leaving the furnace was measured with a Rapidox 2100 from Cambridge Sensotec. When it was sufficiently low, the sample was quickly heated to 950 o C in about 80s, then 50 o C/min to 1000 o C after which it was held for 1 hour. In these experiments, the contact angle and dimensions of the drop were studied during the isothermal period at 1000 o C. 767 Light Metals 2009 Edited by: Geoff Bearne TMS (The Minerals, Metals & Materials Society), 2009 Light Metals 2009 Edited by: Geoff Bearne TMS (The Minerals, Metals & Materials Society), 2009