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Journal of Nanomaterials
Volume 2008 (2008), Article ID 629185, 12 pages
Research Article

Wetting of by Molten Aluminum: The Influence of Additions

Department of Metallurgical and Materials Engineering, The University of Alabama, P.O. Box 870202, Tuscaloosa, AL 35487, USA

Received 24 October 2007; Accepted 2 April 2008

Academic Editor: S. Mathur

Copyright © 2008 Joaquin Aguilar-Santillan. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The effects of additions on the wetting of alumina by molten aluminum were studied by the sessile drop technique. To study the effect of decomposition , the additions were treated at two temperatures (973 K) and (1723 K), respectively. additions at low and high temperatures did not improve the nonwetting character of these compositions. However, at higher firing temperature, the formation of has a nonwetting trend with increasing its content. To address the specifically a pure was produced and tested. It was more nonwetting than the pure alumina. After the analysis of the contact angles for the and the , it was concluded that these additions to alumina do not inhibit wetting by molten aluminum. In fact, at the addition levels common for refractories, the wetting tendency of molten aluminum is enhanced. Alternative explanations for the effectiveness of additions to alumina refractories are discussed.

1. Introduction

The wetting of molten metals on solid ceramics such as described by the liquid/solid contact angle () directly affects whether it is possible to use a specific ceramic for many different engineering applications. Potential uses include composites, brazed components, the bonding of ceramics with metals, and the use of ceramics as refractories to contain molten metals [1].

Molten aluminum containment during its production process has used certain chemical additives to alumina refractories starting from late 1970s. These have been suggested to decrease the wetting of the refractory by molten aluminum and thereby reduce the aluminum penetration into the monolithic refractory products. These chemical additives include zircon (ZrSiO4), barium sulfate (BaSO4), calcium fluoride (CaF2), and some glass frits as well. The levels of these additions to the refractories are usually only from 0.5 to about 6 wt% [213].

The Handbook of Chemistry and Physics [14] reports that BaSO4 melts at 1580°C, however prior to melting, it decomposes. The decomposition reaction occurs from about 1100°C to 1150°C: This reaction defines two temperature regimes for the effects of BaSO4 additions to Al2O3. One is below 1100°C and the other is above 1150°C, the latter after the sulfate has decomposed to the oxide, BaO.

Brosnan [2], in his review of refractory corrosion, has suggested that the BaSO4 additives to aluminum refractories may act to provide a mechanism for the oxidation of the molten aluminum. This is important if the metal penetrates the exposed surfaces of the refractory. Oxidation could occur through reactions of the type: These reactions suggest that BaSO4 can react with molten Al above its melting point 660°C (~933 K) and form stable phases like BaAl2O4 (spinel), BaO, Al2S3, and Al2O3.

At temperatures above that of the Al melting point, barium aluminates can be formed after the BaSO4 decomposes. Although BaSO4 is reported to melt at 1580°C, the study by L’vov and Ugolkov [3] suggests that the decomposition reaction is kinetically favorable from 1124°C (1397 K) to 1154°C (1450 K). Mohazzabi and Searcy [4] applied mass spectrometry to determine if SO3 was present in the products of (1). They concluded that the decomposition process is correctly described by (1). Table 1 summarizes some of the kinetic results from these studies [3, 4].

Table 1: Activation energies (Q) reported for (2).

The data in Table 1 suggests that the BaSO4 decomposition occurs over the temperature range from 1124°C (1397 K) to 1267°C (1540 K). After the BaSO4 decomposition, the barium oxide may react with alumina to form stable oxide phases. These may include the barium hexaluminate (BaO6Al2O3) or the monoaluminate spinel (BaOAl2O3), as illustrated in the two following reactions: As these reactions are highly exothermic, extreme care must be used during experiments to observe or to utilize these reactions.

Teiken and Krietz [5] have reported that the addition of 6%-BaSO4 reduces the penetration of molten aluminum in conventional castables and in low-cement castables. A comparison of these refractories gives the advantage in penetration resistance to the conventional castable. Teiken and Krietz also have suggested that alumina or aluminosilicate refractories are mostly improved for the lining of aluminum-melting furnaces and other refractory containers just from 980°C to 1090°C.

Afshar and Allaire [6] have also studied the additions of BaSO4 and those of CaF2 to aluminosilicate castable refractories to reduce their corrosion. They reported that an addition of 7%-BaSO4 improved the corrosion resistance of refractory samples for firing temperatures at <1000°C (1273 K). For additions of CaF2, they reported less efficient corrosion protection after firing below 1000°C. The use of 3.5% CaF2 actually provoked the formation of large cracks in the refractory, facilitating metal-penetration and enhancing corrosion. These studies suggest that these additives may be limited to temperatures <1000°C (1273 K).

The addition of BaSO4 to phosphate bonded, or phosphate-free plastics, ramming mixes, brick, mortar, or castables has been confirmed to reduce molten aluminum penetration and significantly reduces dross and molten aluminum adherence [59]. The principal additives effects studied by these researchers were their reactions with molten aluminum and its alloys. Investigators have also suggested that the additives may fill the voids in the refractories and inhibit penetration by reducing porosity at the interfacial contact region between molten Al and additives.

The above empirical corrosion tests were all completed at <1090°C (1363 K) in air or by ASTM guidelines with the cup test. The barium sulfate also can be affected by the partial pressure of oxygen in the presence of molten aluminum as follows: This reaction is even more exothermic than the previous ones as they are analyzed without any oxygen (2). The Gibbs free energy changes at 1000°C (1363 K) for the formation of barium monoaluminate (BaAl2O4) and hexaaluminate (BaAl12O19) are 4866 kJ/mol and 6620 kJ/mol, respectively. These energies are high and are likely possible to occur in the dross zone of a refractory lining, which contains the reactants for the completion of (4).

The above observations suggest that additions of BaSO4 to alumina refractories may have two temperature regimes of protection for these refractories. One is below the temperature of the BaSO4 decomposition reaction at ~1100°C and the second is above the decomposition reaction: >1250°C. From previous studies, it was confirmed that additions of compounds, such as BaSO4 to refractories for molten aluminum containment, reduced their corrosion [59]. However, the mechanism of this corrosion reduction is not clear. It could be either decreased molten Al wetting of the refractory, or a pore channel blocking effect by Ba-compound formation. To address the wetting aspect of this controversy, this molten aluminum wetting study of the effects of BaSO4 additions to an alumina was undertaken.

2. Experimental Procedures

Consider a flat, undeformable, perfectly smooth, and chemically homogeneous solid surface in contact with a nonreactive liquid in the presence of a vapor phase. If the liquid does not completely cover the solid, the liquid surface will intersect the solid surface at a contact angle, or , depending on the particular angular definition. The wetting relationship of the surface tensions of the solid-vapor interface (), the solid-liquid interface (), and the liquid-vapor interface or surface tension () in a sessile drop configuration is usually expressed by Young equation [15]. It can be written either as or Referring to Figure 1, the definition of the wetting contact angle is applied throughout this thesis. Because , it naturally allows the interchangeable application of both definitions for the wetting angle. Some researchers use .

Figure 1: Surface energy balance and the contact angle equilibrium definition of the Young equation. Note the two different definitions of the contact angles, and .

Figure 2 illustrates the definition of (5) using as the contact angle. When is equal to 180°, no wetting occurs, and the droplet sits over the solid in point contact. When is >90°, a partial nonwetting condition prevails. Partial wetting occurs when is <90°. However, when the contact angle is equal to 0°, it is understood that complete wetting and droplet spreading occurs over the solid surface.

Figure 2: Wetting stages resulting from the interaction of a solid and a liquid.
2.1. Specimen Preparation

The effect of BaSO4 on the wettability of alumina by molten aluminum was studied for several Al2O3-BaSO4 compositions. One was pure alumina and other three had BaSO4 additions at the 1, 3, and 5% levels. The specimen preparation consisted of mixing a 99.96% 0.5m alumina powder with 1, 3, and 5% of a 99.9% BaSO4 (1m). Individual 100 g batches were prepared by ball mill mixing for 2 hours using a cylindrical container and spherical alumina balls with a ratio of balls to mix of 3 : 1. The ball-mixing fluid medium was isopropanol. A mix speed of 100 rpm was used. After mixing, the powder was dried at 70°C (343 K) for 48 hours.

The dried powder was then passed through a 325 mesh sieve (45m) to break up the agglomerates. The resultant powder was weighed for 5 g specimens for axial pressing of ~1 inch diameter discs at 150 MPa. Sintering schedules were using a heating rate of 5°C/min to 700°C (973 K) and to 1450°C (1723 K) for one hour hold each in air.

In addition, the pure BaO6Al2O3 phase was produced using same experimental set up and used the theoretical composition of BaO6Al2O3 by mixing 27.6 wt% BaSO4 with 72.4 wt% Al2O3. To produce the pure BA6 (BaO6Al2O3) phase, the Al2O3-BaSO4 specimens were sintered using a heating rate of 5°C/min to 1700°C (1973 K) for one hour hold in air.

The BaSO4-Al2O3 specimens at different firing temperature were subsequently analyzed by density using the Archimedes method and by X-ray diffraction measurements (35 kV, 25 mA, and  nm). The surfaces of all ceramic specimens prepared for the wetting studies were semipolished with an SiC # 325 mesh sandpaper to achieve a constant roughness effect.

Aluminum metal specimens were fabricated from a commercial 1020 aluminum ingot provided by Alcoa. The chemical analysis of the Al is shown in Table 2. Impurities were Si, Fe, and Mg, each at the <0.20% level. The Al test specimens were 0.5 cm diameter cylinders with a height of 0.5 cm. To minimize oxidation during the experiments, the Al specimens were first semipolished with an SiC # 400 mesh sandpaper then ultrasonically cleaned in isopropanol.

Table 2: Chemical analysis of the 1020 aluminum (wt%).
2.2. Sessile Drop Experiments

Sessile drop tests of the molten 1020 aluminum on the three different types of BaSO4-Al2O3 specimens were completed using a commercial sessile drop apparatus manufactured by Centorr Associates, Inc. (Suncook, NH, USA). It is schematically illustrated in Figure 3. The experiment consisted of placing the aluminum metal cylinder on the refractory substrate in the center of the furnace. After closing, the furnace was evacuated to less than 50 millitorrs. An ultrahigh purity argon (~99.999%) was introduced to a pressure of 1000 millitorrs. These vacuum/argon steps were repeated four times to reduce the experimental chamber oxygen level. The sessile drop experiments were then completed in the argon atmosphere to avoid any of the oxide skin effects that have been observed on Al droplets and reported by other investigators [1625]. Measurements in the sessile drop furnace were under argon gas at ~1520 Torr. The partial pressures of several gas species were , and Torr as determined by a gas analyzer (MKS Vision 100-B).

Figure 3: Schematic of the sessile drop apparatus.

Heating of the test assembly was manually controlled by applying current (mA) to the heating element until the desired temperature was achieved. The actual heating profile is illustrated in Figure 4. It had isothermal temperature holds from 800°C (1073 K) to 1200°C (1473 K) at intervals of 200°C (473 K). Once the solid aluminum cylinder melted during heating, it immediately formed a droplet. A camera lens and a video monitor (black and white) connected with a VCR recorded the experiments. The droplet shape was monitored for 20 minutes at each of the holding temperatures. This length of time, ~20 minutes, was the same as that reported by Li [18].

Figure 4: The heating schedule followed in the sessile drop measurements.

A principal difficulty when studying the wetting of the molten Al and a solid Al2O3 system couple is the presence of a high partial pressure of oxygen. Aluminum metal may react with oxygen at high temperatures [1625] to form gas by the reaction: Although this equation is not thermodynamically favorable ( kJ), it has been reported to occur during experimental observations and by spectrographic analysis [18, 21].

Droplet shape analysis is required to accurately determine the contact angle (). Videotapes of the sessile drops were transferred to a PC by using the DFG/LC1 frame grabber with Grab and View 2.1 software. This allowed the direct observation of individual frames at any time during the duration of the experiment. It has an efficiency reading of 0.01 second. Figure 5 illustrates a droplet and its representation. Analysis determined the droplet profiles and fitted them to theoretical shapes obtained from the solution of the Laplace equation of capillarity [22].

Figure 5: Typical droplet acquisition data to obtain the contact angle () by image analysis and optimization of the Laplace equation of capillarity.

3. Results

3.1. Wetting of the Al2O3-BaSO4 System Fired at 700°C (973 K)

After firing in air at 700°C (973 K), the BaSO4 content in the alumina was confirmed by microchemical analysis in a scanning electron microscope (SEM) using the EDS technique (35 kV and 25 mA) getting a 99.85% of reading accuracy. It was also checked by X-ray diffraction (XRD) as shown in Figure 6 (35 kV and 25 mA) using Rietveld measurements. The EDS results were for an average of five specimens. The BaSO4 contents were found to be 0.98%, 2.97%, and 4.98%. Firing at a temperature of 700°C does not affect the BaSO4. The densities of the Al2O3-BaSO4 specimens fired at 700°C (973 K) are illustrated in Figure 7. Open porosity increased as the amount of BaSO4 increased. The open porosity changed from 34.0 to 40.0%, and the density decreased from 2.4 to 1.9 g/cm3 for 1.0 and 5.0% of BaSO4, respectively. These Al2O3-BaSO4 specimens were quite porous as well as the pure aluminas.

Figure 6: X-ray diffraction patterns of the Al2O3-BaSO4 specimens fired at 700°C (973 K) for one hour. Observe that BaSO4 is present in each sample and its intensity increases proportionally to the amount of BaSO4 added.
Figure 7: Density measurements by Archimedes method for Al2O3-BaSO4 specimens at different additive amounts of BaSO4, sintered at 700°C (973 K) for one hour.

Figure 8 illustrates the Al wetting contact angles as a function of time for the Al2O3-BaSO4 specimens at 800°C (1073 K), 900°C (1173 K), and 1000°C (1273 K). These measurements were at temperatures above the sintering temperature of the mixes. The wetting of the BaSO4-free alumina is slightly temperature dependent. The wetting contact angles, the -values at zero time, are 134° at 800°C (1073 K), 132° at 900°C (1173 K), and 128° at 1000°C (1273 K). The contact angle is >90° at all temperatures, a nonwetting condition as previously illustrated in Figure 2.

Figure 8: Wetting contact angle () variation with time at different temperatures for the Al2O3-BaSO4 systems fired at 700°C (973 K) for one hour. The regression equations are listed on the figures.

Additions of BaSO4 affect the wetting contact angles () differently at the three different temperatures. At the lowest temperature, 800°C (1073 K), the 1% addition of BaSO4 actually decreases the contact angle to 105º, nearly 30° less than that for pure alumina. At 800°C, the 3% addition of BaSO4 also increases the wetting tendency of the alumina by the molten aluminum. The 5% BaSO4 addition has nearly the same contact angle profile as that for the pure alumina, but the 3% addition decreases the contact angle () from 134° to 126°.

The effects of higher temperatures on the wetting of the Al2O3-BaSO4 specimens are always that of increasing the wetting tendency. At 800°C (1073 K), only the 5% BaSO4 addition appeared to decrease the wetting tendency of pure alumina, all the wetting contact angles decreased, but those for the BaSO4 additions decreased more rapidly than those for pure alumina. The result is that from 900°C (1173 K) to 1000°C (1273 K), the pure alumina is more difficult to wet than the alumina with the BaSO4 additions.

Figure 9 illustrates the contact angle (o) as a function of temperature. These results show that by adding BaSO4 to alumina, the wetting contact angles are less than those for pure alumina. Although the contact angles remain >90º, their decreases indicate an increased wetting tendency with the BaSO4 additions. The 5%-BaSO4 specimen has a slightly larger contact angle than alumina at 800°C (1073 K). However, at higher temperatures the contact angle becomes less than that for the pure alumina.

Figure 9: Variation of the wetting contact angle () at time zero with temperature for the pure alumina and BaSO4 additions of 1%, 3%, and 5% after firing at 700°C (973 K).

Variation of the -values with temperature can be expressed as where is the value at zero Kelvin and the slope represents the decrease of the wetting contact angle with increasing temperature. These two parameters increase with the BaSO4 content as illustrated in Figure 9. The BaSO4 addition effects in alumina at temperatures from 800°C (1073 K) to 1000°C (1273 K) are generally to increase the wetting tendency by molten aluminum. However, at no times the contact angles were reduced to less than 90º. As is the criteria for partial wetting, it must be concluded that no wetting process was observed, but the additions of BaSO4 to Al2O3 increased the wetting tendency.

3.2. Wetting of the Al2O3-BaSO4 System after Firing at 1450°C (1723 K)

After firing the Al2O3-BaSO4 specimens at 1450°C, the BaO content in the alumina specimens was analyzed by microchemical analysis in an SEM (EDS) (35 kV and 25 mA) getting a 99.85% of reading accuracy and by X-ray diffraction (XRD). The EDS results represent an average value for five specimens. Only Ba, Al, and O were detected. Sulfur was absent. The amounts of BaO increased in the order of 0.64, 1.92, and 3.25% for the initial introductions of 1.0, 3.0, and 5.0% of BaSO4. From (1), the reaction for BaSO4 decomposition, the theoretical amounts of BaO are 0.66, 1.97, and 3.28% for additions of 1.0, 3.0, and 5.0% BaSO4. These values are nearly the same as those obtained by EDS microanalysis. This confirms that the BaSO4, which was introduced to the alumina and then fired at 1450°C (1723 K), decomposed as described by (1). The BaO obtained then reacted with the Al2O3 to form BaO6Al2O3 (3). This is confirmed by the XRD in Figure 10, where the two phases coexist. The amounts of BaO6Al2O3, obtained by X-ray diffraction analysis, are of 3.10, 9.50, and 16.20%, respectively. These values are in agreement with the theoretical ones of 3.28, 9.85, and 16.42%, respectively.

Figure 10: X-ray diffraction patterns of Al2O3-BaSO4 specimens fired at 1450°C (1723 K) for one hour. Observe that the BaSO4 has decomposed and reacted with the Al2O3, forming the stable BaO6Al2O3 phase.

The equilibrium binary phase diagram of Al2O3 and BaO illustrated in Figure 11 can be used to explain the above results. It is shown that when adding BaO to Al2O3 the first stable compound formation is BaO6Al2O3. This compound has about 20 wt% BaO and 80 wt% Al2O3. It is congruently melting at ~1915°C (2188 K) and it is a highly refractory material.

Figure 11: Binary phase diagram of the BaO-Al2O3 system [23].

The Al2O3-BaO6Al2O3 specimen densities, after firing at 1450°C/1hour (1723 K), are presented in Figure 12. The densities are much higher and the porosities are much lower than the previous series of Al2O3-BaSO4 specimens. The results show an increase of porosity as the amount of BaO6Al2O3 increases. The total porosity changes from 12.0% to 17.0% for the 1.0% and 5.0% additions of BaSO4, respectively.

Figure 12: Density measurements by the Archimedes method in Al2O3-BaO6Al2O3 specimens after firing 1450°C (1723 K) for one hour.

The wetting contact angle () variations with BaO6Al2O3 content and their changes with temperature are illustrated in Figure 13. Large contact angle differences are obtained from the BaSO4 additives after firing at 1450°C. The temperature effect is evident. It normally decreases the contact angle. Figure 13 illustrates the aluminum contact angles of the Al2O3-BaO6Al2O3 specimens at the three temperatures, 800°C (1073 K), 1000°C (1273 K), and 1200°C (1473 K). The wetting of the alumina without BaSO4 additions after firing at 1450°C is only slightly temperature dependent. Its contact angles () at are 125° at 800°C, 121° at 1000°C, and 118° at 1200°C.

Figure 13: Wetting contact angle () variation with time at different temperatures for the Al2O3-BaO6Al2O3 system fired at 1450°C (1723 K) for one hour.

The BaO6Al2O3 effects on the contact angles () are similar for the three temperatures. At the lowest temperature, 800°C (1073 K), the 3.1% BaO6Al2O3 decreases the wetting contact angle to 95º, about 30° less than that for the pure alumina. The addition of BaO6Al2O3 has the effect of increasing the wetting tendency of the alumina by molten aluminum. At 800°C (1073 K) the 9.5% and 16.2% of BaO6Al2O3 the wetting contact angles decrease to 105° and 107°, respectively. This suggests that by increasing the BaO6Al2O3 phase in the microstructure the contact angles will increase toward less wetting. One can speculate that further increases of BaO6Al2O3 phase content may increase the wetting contact angles beyond that obtained for the pure alumina.

The effect of temperature decreases the wetting contact angles () for all of the Al2O3-BaO6Al2O3 specimens. From 800°C (1073 K) to 1200°C (1473 K) contact angles are reduced for all levels of BaO6Al2O3 additions. The result is that at all three temperatures, the pure alumina is the most difficult to wet. The presence of BaO6Al2O3 consistently decreases the tendency toward wetting. It must be concluded that the amounts of BaO6Al2O3 additions to alumina tested at temperatures from 1000°C (1273 K) to 1200°C (1473 K) do not reduce or inhibit the wetting of pure alumina by molten aluminum. Figure 14 summarizes the effects of temperature on the wetting contact angles for the addition of BaO6Al2O3 additions.

Figure 14: Variation of the wetting contact angle (o) with temperature for the BaO6Al2O3 additions to alumina and after firing at 1450°C (1723 K) for one hour.

Figure 14 shows a linear tendency of the decrease of the wetting contact angle () with temperature. This describes the effects of temperature on the contact angle variation. With BaO6Al2O3 additions to alumina, it is observed that the contact angles are increased from those obtained with pure alumina. However, the resultant slopes are similar to those for the pure alumina.

The decrease of with increasing temperature is described by (8). It is probable that the BaO6Al2O3 phase may increase the resultant contact angle of molten aluminum beyond that for pure alumina. The trend in Figure 14 certainly suggests that larger additions of AB6 (BaO6Al2O3) may achieve contact angles above from those of the pure alumina and actually decrease the wetting by molten Al.

3.3. Wetting of the Pure BaO6Al2O3 Phase

Davis et al. [26, 27] suggest different processing routes for the synthesis of pure BaO6Al2O3 and establish that this barium compound is of interest as a refractory. Barium monoaluminate BA (BaAl2O4) hydrates, m.p. ~1800°C (2073 K), and could be the basis of cement or grouting material. The hexaluminate BA6 (BaO6Al2O3), however, is the more likely engineering ceramic, m.p. ~1925°C (2198 K) [26, 27].

The only study in the literature reporting properties of the barium hexaaluminate BA6 (BaO6Al2O3) is that by Zawrah and Khalil [28]. They prepared the compound from both low and high purity materials by sintering at 1700°C (1973 K). The first was prepared from high purity BaSO4 and Al2O3, while the second was prepared from natural minerals, Egyptian barite and Chinese bauxite. They found that by firing the pure materials at 1700°C (1973 K), a mix of BA6 (BaO6Al2O3) and Al2O3 was present in the microstructure. However, when firing the natural minerals at 1700°C (1973 K), a mix between BA6 (BaO6Al2O3), Al2O3, and BAS2 (BaOAl2O32SiO2) was present in the microstructure. They identified these phases by X-ray diffraction. The X-ray diffraction results for their pure materials are in agreement with those presented in Figure 15 for the present study.

Figure 15: X-ray diffraction patterns of the Al2O3-BaSO4 specimens fired from 1200°C (1473 K) to 1700°C (1973 K).

Figure 15 illustrates the reaction of the Al2O3-BaSO4 specimens to form BA6 (BaO6Al2O3) as a function of the firing temperature. After firing at 1200°C (1473 K) there remains a large amount of BaSO4 that has not reacted with the Al2O3. However, on firing at 1400°C (1673 K) is observed that the reaction process has initiated and as a result there is a formation of BA6. After firing for one hour at 1700°C (1973 K), the formation of the BA6 phase is complete.

The densities of the BA6 (BaO6Al2O3) specimens are illustrated in Figure 16. There is a reduction of open porosity as the temperature increases. The open porosity is 6.5% after firing at 1700°C (1973 K). The density is 3.97 [g /cm3]. The pure alumina fired at the same conditions had 5% open porosity and a density of 3.52 [g /cm3].

Figure 16: Density and open porosity variation with temperature after firing BaO6Al2O3 at 1700°C (1973 K) for one hour.

The Al wetting contact angle () for the BA6 phase is illustrated in Figure 17. The results for the pure alumina fired at 1700°C (1973 K) are also presented. Distinct differences with respect to the pure alumina are obtained for all of the temperatures. This result was predicted by Figures 13 and 14 that show increases of the Al contact angles proportional to the BA6 phase content. The BA6 phase actually increases the wetting angle trend of molten aluminum beyond that for pure alumina. The BaO6Al2O3 phase is consistently more nonwetting than pure alumina.

Figure 17: Wetting contact angle () with time at constant temperatures. Note the Al contact angles obtained with the pure BA6 (BaO6Al2O3) are larger than those obtained with the pure Al2O3.

The Al contact angles (o) at time zero for the BA6 phase changed from 135.3° for 800°C (1073 K) to 127.4° for 1200°C (1473 K). These os are above those obtained for the pure alumina of 122.7° and 115.3° at the same temperatures. These represent a nearly 10% increase of the contact angles, respectively. Figure 18 illustrates this variation of contact angle with temperature. It is observed that the BA6 (BaO6Al2O3) has larger wetting contact angle than that for the pure alumina for all temperatures. Both have similar slopes, . This clearly illustrates that the pure BA6 (BaO6Al2O3) phase is less wetting to the molten aluminum than pure alumina.

Figure 18: Variation of o with temperature for the couple systems of BaO6Al2O3/Al and Al2O3/Al. Note that the BaO6Al2O3 phase is more nonwetting than pure alumina.
3.4. The Pure Alumina Wetting Contact Angle

In the assessment of the effect of BaSO4 and BaO6Al2O3 additions, the fired refractory ceramics were compared with the pure alumina fired under the same conditions. The contact angles of the three aluminas were presented in Figures 8, 13, and 17 at the three different temperatures. The molten aluminum wetting contact angles are different for the three different alumina firing conditions. These three different firing schedules for the pure alumina produced three different levels of porosity, 34.5%—700°C (973 K), 11%—1450°C (1673 K), and 5%—1700°C (1973 K), respectively. This shows the densification process of alumina with temperature and coupled with the microstructural differences, this naturally created different surfaces roughnesses. The reason for the different contact angles for these pure aluminas is their different structural surface roughnesses [28].

4. Summary and Conclusions

The additions of BaSO4 to alumina have two different Al wetting regimes as the firing temperature is increased to produce refractory ceramic bodies. The decomposition temperature of BaSO4 divides these regimes. For C (1397 K) (700°C (973 K) firing was used in this study) the BaSO4 does not decompose. It remains as the sulfate mixed with alumina. It is observed that the resultant molten Al contact angles are less for the Al2O3-BaSO4 mixtures than those obtained with the pure alumina. Specimens do not show higher contact angles than the pure alumina. The BaSO4 does not reduce the wetting of alumina to molten aluminum. In fact, the BaSO4 reduces the wetting contact angle toward more wetting conditions.

The Al2O3-BaSO4 specimens fired at (1723 K) cause the decomposition of the BaSO4 (2). The reaction results in a stable compound BA6 (BaO6Al2O3). The amount of BaO6Al2O3 is proportional to the content of BaO from the sulfate decomposition reaction in (1). The BA6 (BaO6Al2O3) amounts obtained by SEM (EDS) analysis from 1%, 3%, and 5% of BaSO4 additions are 3.1%, 9.5%, and 16.2%. The Al wetting angles of the Al2O3-BaO6Al2O3 mixtures are decreased below that of the pure alumina. They do increase proportionally with the amount of BA6 (BaO6Al2O3) presents in the system. This suggests that increments of BA6 (BaO6Al2O3) in the system would increase the Al wetting contact angle, possibly significant at higher BA6 phase levels.

The investigation of the wetting of the pure BaO6Al2O3 ceramic by molten aluminum yielded wetting contact angles greater than those for pure Al2O3. This indicates that BaO6Al2O3 is less wetting toward molten aluminum than pure alumina. However, the contact angles for BaO6Al2O3 ceramic do not approach the mark indicative of an absolute nonwetting condition.

All of the contact angles decreased with increasing temperature. Higher temperatures promote wetting. They also might be expected to enhance refractory penetration and increased corrosion at higher temperatures.

The contact angles () resulting from these refractory systems and 1020 Al decrease linearly with increasing temperature even in the presence of the additives. Measurements of the Al contact angles on the BaSO4 additive containing alumina do not confirm that this additive inhibits wetting. In fact, in most cases the contact angles were reduced toward more wetting, suggesting that the BaSO4 addition may actually promote wetting characteristics. However, BaSO4 addition did not decrease the contact angles below 90º, the criterion for the transition from nonwetting to wetting. One must conclude that the nonwetting theory for the improvement of aluminum refractories by BaSO4 additions does not appear to be the correct explanation.

There remain several other alternatives for the obvious corrosion improvements of aluminum containing refractories by BaSO4 additions. One is the formation of compounds such as BaO6Al2O3 or BaOAl2O3 that fill the refractory pores and channels reducing porosity and the penetration of molten aluminum. Another is the suggestion by Brosnan [2] in his review, namely, that the sulfate participates in a reaction, or a series of reactions to promote the oxidation of molten aluminum if it does penetrate the refractory structure. Both of these are viable explanations.

Finally, it must be noted that this study was for the additions of BaSO4 and its derivative compounds on the wetting of a pure alumina by a pure aluminum metal. These results may not directly translate to the wetting of other refractories such as the different aluminosilicates and cement-containing refractories by more complex highly alloyed aluminum metal.


The author is grateful to R. C. Bradt for his guidance and study of refractories materials. Special thanks are addressed to A. Saavedra and D. Stewart from Alcoa for the supply of the 1020 aluminum and to L. Krietz of Plibrico Co. (Chicago, Ill, USA) for the supply of the BaSO4. The author would like to thank the Consejo Nacional de Ciencia y Tecnologia (CONACYT-Mexico) for its financial support of the graduate studies at the University of Alabama, Tuscaloosa, Ala, USA.


  1. W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, Wiley-Interscience, New York, NY, USA, 1976.
  2. D. A. Brosnan, “Refractories for aluminum,” Class Notes from the Center for Professional Advancement, Piscataway, NJ, USA, 1991.
  3. B. V. L'vov and V. L. Ugolkov, “Kinetics of free-surface decomposition of magnesium and barium sulfates analyzed thermogravimetrically by the third-law method,” Thermochimica Acta, vol. 411, no. 1, pp. 73–79, 2004. View at Publisher · View at Google Scholar
  4. P. Mohazzabi and A. W. Searcy, “Kinetics and thermodynamics of decomposition of barium sulphate,” Journal of the Chemical Society, Faraday Transactions 1, vol. 72, pp. 290–295, 1976. View at Publisher · View at Google Scholar
  5. J. Teiken and L. Krietz, “Additive effects on the physical properties of castable systems for aluminium contact applications,” in Proceedings of the 9th Symposium on Refractories for the Aluminum Industry, G. Oprea, Ed., pp. 5–22, The Minerals, Metals, and Materials Society, Warrendale, Pa, USA, 2000.
  6. S. Afshar and C. Allaire, “Furnaces: improving low cement castables by non-wetting additives,” Journal of the Minerals, Metals and Materials Society, vol. 53, no. 8, pp. 24–27, 2001. View at Publisher · View at Google Scholar
  7. O. J. Siljan, G. Rian, D. T. Pettersen, A. Solheim, and C. Schoning, “Refractories for molten aluminum contact—part I: thermodynamics and kinetics,” in Proceedings of the Unified International Technical Conference on Refractories (UNITECR-ALAFAR '01), pp. 531–550, Cancun, Mexico, November 2001.
  8. O.-J. Siljan and C. Schoning, “Refractories for molten aluminum contact—part II: influence of pore size on aluminum penetration,” Refractories Applications and News, vol. 8, no. 1, pp. 21–29, 2003. View at Google Scholar
  9. J. G. Lindsay, W. T. Bakker, and E. W. Dewing, “Chemical resistance of refractories to AI and AI-Mg alloys,” Journal of the American Ceramic Society, vol. 47, no. 2, pp. 90–94, 1964. View at Publisher · View at Google Scholar
  10. N. Yu. Taranets and Y. V. Naidich, “Forecasting capillary characteristics for aluminum nitride ceramic—liquid metal systems,” Powder Metallurgy and Metal Ceramics, vol. 41, no. 3-4, pp. 177–182, 2002. View at Publisher · View at Google Scholar
  11. J. G. Li, “Role of electron density of liquid metals and bandgap energy of solid ceramics on the work of adhesion and wettability of metal-ceramic systems,” Journal of Materials Science Letters, vol. 11, no. 13, pp. 903–905, 1992. View at Publisher · View at Google Scholar
  12. R. Pelletier, C. Allaire, O.-J. Siljan, and A. Tabereaux, “The corrosion of potlining refractories: a unified approach,” Journal of the Minerals, Metals and Materials Society, vol. 53, no. 8, pp. 18–22, 2001. View at Publisher · View at Google Scholar
  13. Y. V. Naidich, “Wettability of halides with molten metals. Physico-chemical and practical aspects,” Powder Metallurgy and Metal Ceramics, vol. 39, no. 7-8, pp. 355–362, 2000. View at Publisher · View at Google Scholar
  14. D. R. Lide, Handbook of Chemistry and Physics, CRC Press, Cleveland, Ohio, USA, 84th edition, 2003.
  15. T. Young, “An essay on the cohesion of fluids,” Philosophical Transactions of the Royal Society of London A, vol. 95, pp. 65–87, 1805. View at Publisher · View at Google Scholar
  16. A. Passerone, E. Biagini, and V. Lorenzelli, “Wetting of barium hexaferrite by molten metals,” Ceramurgia International, vol. 1, no. 1, pp. 23–27, 1975. View at Publisher · View at Google Scholar
  17. J. M. Charig, “Low-energy electron diffraction observations of α-alumina,” Applied Physics Letters, vol. 10, no. 5, pp. 139–140, 1967. View at Publisher · View at Google Scholar
  18. J.-G. Li, “Wetting of ceramic materials by liquid silicon, aluminium and metallic melts containing titanium and other reactive elements: a review,” Ceramics International, vol. 20, no. 6, pp. 391–412, 1994. View at Publisher · View at Google Scholar
  19. H. Fujii and H. Nakae, “Equilibrium contact angle in the magnesium oxide/aluminium system,” Acta Materialia, vol. 44, no. 9, pp. 3567–3573, 1996. View at Publisher · View at Google Scholar
  20. J. A. Champion, B. J. Keene, and J. M. Sillwood, “Wetting of aluminium oxide by molten aluminium and other metals,” Journal of Materials Science, vol. 4, no. 1, pp. 39–49, 1969. View at Publisher · View at Google Scholar
  21. G. Levi and W. D. Kaplan, “Aluminium-alumina interface morphology and thermodynamics from dewetting experiments,” Acta Materialia, vol. 51, no. 10, pp. 2793–2802, 2003. View at Publisher · View at Google Scholar
  22. J. Aguilar-Santillan, submitted to Langmuir.
  23. N. A. Toropov and F. Ya. Galakhov, “Diagramma sostoyaniya sistemy BaO-Al2O3,” Doklady Akademii Nauk SSSR, vol. 82, no. 1, pp. 69–70, 1952. View at Google Scholar
  24. V. Laurent, D. Chatain, C. Chatillon, and N. Eustathopoulos, “Wettability of monocrystalline alumina by aluminium between its melting point and 1273 K,” Acta Metallurgica, vol. 36, no. 7, pp. 1797–1803, 1988. View at Publisher · View at Google Scholar
  25. X. B. Zhou and J. Th. M. De Hosson, “Reactive wetting of liquid metals on ceramic substrates,” Acta Materialia, vol. 44, no. 2, pp. 421–426, 1996. View at Publisher · View at Google Scholar
  26. T. J. Davies, W. A. Al-Douri, Q.-G. Chen, and H. G. Emblem, “Aluminium-barium oxides as binders for refractory grain mixes,” Journal of Materials Science Letters, vol. 16, no. 20, pp. 1673–1674, 1997. View at Publisher · View at Google Scholar
  27. T. J. Davies, H. G. Emblem, M. Biedermann, Q.-G. Chen, and W. A. Al-Douri, “Preparation and properties of barium mono- and hexaaluminate,” Journal of Materials Science Letters, vol. 17, no. 21, pp. 1845–1847, 1998. View at Publisher · View at Google Scholar
  28. M. F. M. Zawrah and N. M. Khalil, “Preparation and characterization of barium containing refractory materials,” Ceramics International, vol. 27, no. 3, pp. 309–314, 2001. View at Publisher · View at Google Scholar