# Wetting ofAl2O3 by Molten Aluminum:  The Influence of BaSO4 Additions

Authors: Joaquin Aguilar-Santillan
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

The effects ofBaSO4 additions on the wetting of alumina by molten aluminum were studied by the sessile drop technique. To study the effect of BaSO4 decomposition (1100–1150∘C), the additions were treated at two temperatures 700∘C (973 K) and 1450∘C (1723 K), respectively. BaSO4 additions at low and high temperatures did not improve the nonwetting character of these compositions. However, at higher firing temperature, the formation of BA6(BaO•6Al2O3) has a nonwetting trend with increasing its content. To address the BA6 specifically a pure BaO•6Al2O3 was produced and tested. It was more nonwetting than the pure alumina. After the analysis of the contact angles for the BaSO4 and the BA6(BaO•6Al2O3), 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 BaSO4 additions to alumina refractories are discussed.


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## Body

## 1. Introduction

The wetting
of molten metals on solid ceramics such as described  by the liquid/solid
contact angle (ϕ) 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% [2–13].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: (1)BaSO4⟶BaO+SO2+12O2. 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: (2)3BaSO4+8Al(l)⟶3BaAl2O4+Al2S3,ΔG933K=−3215kJ/moleΔV=10%,3BaSO4+8Al(l)⟶3BaO+Al2S3+3Al2O3,ΔG933K=−2870kJ/moleΔV=−2%,BaSO4+2Al(l)⟶BaAl2O4+S(g),ΔG933K=−596kJ/moleΔV=15%. 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).
Q (kJ/mol)SampleAtmosphereTemperature (°C)Reference415.8±0.2PowderVacuum1124[3]407.6CrystalVacuum1154383.5±8.4CrystalVacuum1147–1267[4]409.7CrystalVacuum1177The data in
Table1 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 (BaO•6Al2O3)
or the monoaluminate spinel (BaO•Al2O3),
as illustrated in the two following reactions: (3)BaO+6Al2O3⟶BaO•6Al2O3,ΔG1373K=−1133.3kJ/moleΔV=5%,BaO+Al2O3⟶BaO•Al2O3,ΔG1373K=−132.88kJ/moleΔV=15.5%.  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 [5–9].
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: (4)BaSO4(s)+Al(l)+O2(g)⟶Baaluminates(s)+Al2(SO4)3(s). 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 [5–9].
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,ϕ or θ, depending on the particular angular definition. The wetting
relationship of the surface tensions of the solid-vapor interface (γsv), the solid-liquid interface (γsl), and the liquid-vapor interface or surface tension (γlv) in a sessile drop configuration is usually expressed by Young equation [15].
It can be written either as (5)cosϕ=γsv−γslγlv or (6)cosθ=γsl−γsvγlv. Referring
to Figure 1, the ϕ definition of the wetting contact angle is
applied throughout this thesis. Because ϕ+θ=π, it naturally allows the
interchangeable application of both definitions for the wetting angle. Some
researchers use θ.Figure 1
Surface energy balance and the contact
angle equilibrium definition of the Young equation. Note the two different
definitions of the contact angles,ϕ and θ.Figure2 illustrates the definition of (5) using ϕ as the contact angle. When ϕ is equal to 180°, no wetting
occurs, and the droplet sits over the solid in point contact. When ϕ is >90°, a partial nonwetting
condition prevails. Partial wetting occurs when ϕ 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.Wetting stages resulting from the interaction of a solid and a liquid.
(a)(b)(c)(d)


### 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% 0.5μm alumina powder with 1, 3, and 5%
of a 99.9% BaSO4 (1μm). 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 (45μm) 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 BaO•6Al2O3 phase was produced using same experimental set up and used the theoretical
composition of BaO•6Al2O3 by mixing 27.6 wt% BaSO4 with 72.4 wt% Al2O3.
To produce the pure BA6 (BaO•6Al2O3)
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 Cuλ=0.15406 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 Table2. 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%).
SiFeCuMnMgCrNiZnTiCaNaGaothers0.150.200.020.030.120.0010.0050.0030.010.0060.0030.013<0.01


### 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 [16–25]. Measurements in the sessile drop
furnace were under argon gas at ~1520 Torr. The partial pressures of several
gas species were PH2=10−16,PH2O=10−19,PN2/CO=10−19, and PO2=10−17 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 Figure4. 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 [16–25] to form gas
by the reaction: (7)Al2O3(film)+4Al(l)=3Al2O(g). Although
this equation is not thermodynamically favorable (ΔG1173K=1,040.7 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 (ϕ).
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].Typical droplet acquisition data to obtain
the contact angle (ϕ) by image
analysis and optimization of the Laplace equation
of capillarity.
(a)(b)(c)


## 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% 0.5μm alumina powder with 1, 3, and 5%
of a 99.9% BaSO4 (1μm). 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 (45μm) 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 BaO•6Al2O3 phase was produced using same experimental set up and used the theoretical
composition of BaO•6Al2O3 by mixing 27.6 wt% BaSO4 with 72.4 wt% Al2O3.
To produce the pure BA6 (BaO•6Al2O3)
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 Cuλ=0.15406 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 Table2. 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%).
SiFeCuMnMgCrNiZnTiCaNaGaothers0.150.200.020.030.120.0010.0050.0030.010.0060.0030.013<0.01


## 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 [16–25]. Measurements in the sessile drop
furnace were under argon gas at ~1520 Torr. The partial pressures of several
gas species were PH2=10−16,PH2O=10−19,PN2/CO=10−19, and PO2=10−17 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 Figure4. 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 [16–25] to form gas
by the reaction: (7)Al2O3(film)+4Al(l)=3Al2O(g). Although
this equation is not thermodynamically favorable (ΔG1173K=1,040.7 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 (ϕ).
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].Typical droplet acquisition data to obtain
the contact angle (ϕ) by image
analysis and optimization of the Laplace equation
of capillarity.
(a)(b)(c)


## 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.Figure8 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 ϕo-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.Wetting contact angle (ϕ)
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.
(a)(b)(c)Additions of
BaSO4 affect the wetting contact angles (ϕ)
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 (ϕ) 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.Figure9
illustrates the contact angle (ϕo)
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 (ϕo) 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
theϕo-values with temperature can be expressed
as (8)ϕo=ϕo′−(∂ϕo∂T)tT, where ϕo′
is the value at zero Kelvin and the slope (∂ϕo/∂T)t 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 ϕ<90° 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 BaO•6Al2O3 (3). This is confirmed by the XRD in Figure 10, where the two phases
coexist. The amounts of BaO•6Al2O3, 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 BaO•6Al2O3 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
BaO•6Al2O3.
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-BaO•6Al2O3 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 BaO•6Al2O3 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-BaO•6Al2O3 specimens after firing 1450°C (1723 K)
for one hour.The wetting
contact angle (ϕ)
variations with BaO•6Al2O3 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-BaO•6Al2O3 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 (ϕo)
at t=0 are 125° at 800°C, 121° at 1000°C, and 118° at 1200°C.Wetting contact angle (ϕ) variation with time  at
different temperatures for the  Al2O3-BaO•6Al2O3 system fired at 1450°C
(1723 K) for one hour.
(a)(b)(c)The BaO•6Al2O3 effects on the contact angles (ϕ) are
similar for the three temperatures. At the lowest temperature, 800°C (1073 K),
the 3.1% BaO•6Al2O3 decreases the wetting contact angle to 95º, about 30° less than that for the pure
alumina. The addition of BaO•6Al2O3 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 BaO•6Al2O3 the wetting contact angles decrease to 105° and 107°,
respectively. This suggests that by increasing the BaO•6Al2O3 phase in the microstructure the contact angles will increase toward less
wetting. One can speculate that further increases of BaO•6Al2O3 phase content may increase the wetting contact angles beyond that obtained for
the pure alumina.The effect of
temperature decreases the wetting contact angles (ϕ) for all
of the Al2O3-BaO•6Al2O3 specimens. From 800°C (1073 K) to 1200°C (1473 K) contact angles are reduced
for all levels of BaO•6Al2O3 additions. The result is that at all three temperatures, the pure alumina is
the most difficult to wet. The presence of BaO•6Al2O3 consistently decreases the tendency toward wetting. It must be concluded
that the amounts of BaO•6Al2O3 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 BaO•6Al2O3 additions.Figure 14
Variation of the wetting contact angle (ϕo)
with temperature for the BaO•6Al2O3 additions to alumina and after firing at 1450°C (1723 K) for one hour.Figure14 shows a linear tendency of the decrease of the wetting contact angle (ϕo)
with temperature. This describes the effects of temperature on the contact
angle variation. With BaO•6Al2O3 additions to alumina, it is observed that the contact angles are increased from
those obtained with pure alumina. However, the resultant slopes (∂ϕo/∂T)t are similar to those for the pure alumina.The decrease ofϕo with
increasing temperature is described by (8). It is probable that the BaO•6Al2O3 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 (BaO•6Al2O3)
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 BaO•6Al2O3 Phase

Davis et al. [26, 27]
suggest different processing routes for the synthesis of pure BaO•6Al2O3 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 (BaO•6Al2O3),
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 (BaO•6Al2O3)
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 (BaO•6Al2O3)
and Al2O3 was present in the microstructure. However,
when firing the natural minerals at 1700°C (1973 K), a mix between BA6 (BaO•6Al2O3),
Al2O3, and BAS2 (BaO•Al2O3•2SiO2)
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).Figure15
illustrates the reaction of the Al2O3-BaSO4 specimens to form BA6 (BaO•6Al2O3)
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 (BaO•6Al2O3)
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 BaO•6Al2O3 at 1700°C (1973 K) for one hour.The Al wetting
contact angle (ϕ) 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 BaO•6Al2O3 phase is consistently more nonwetting than pure alumina.Figure 17
Wetting contact angle (ϕ) with
time at constant temperatures. Note the Al contact angles obtained with the
pure BA6 (BaO•6Al2O3)
are larger than those obtained with the pure Al2O3.The Al contact
angles (ϕo)
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 ϕos
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 (BaO•6Al2O3)
has larger wetting contact angle than that for the pure alumina for all
temperatures. Both have similar slopes, (∂ϕo/∂T)t.
This clearly illustrates that the pure BA6 (BaO•6Al2O3)
phase is less wetting to the molten aluminum than pure alumina.Figure 18
Variation ofϕo with temperature for the couple systems of BaO•6Al2O3/Al
and Al2O3/Al. Note that the BaO•6Al2O3 phase is more nonwetting than pure alumina.


### 3.4. The Pure Alumina Wetting Contact Angle

In the
assessment of the effect of BaSO4 and BaO•6Al2O3 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].


## 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.Figure8 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 ϕo-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.Wetting contact angle (ϕ)
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.
(a)(b)(c)Additions of
BaSO4 affect the wetting contact angles (ϕ)
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 (ϕ) 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.Figure9
illustrates the contact angle (ϕo)
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 (ϕo) 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
theϕo-values with temperature can be expressed
as (8)ϕo=ϕo′−(∂ϕo∂T)tT, where ϕo′
is the value at zero Kelvin and the slope (∂ϕo/∂T)t 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 ϕ<90° 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 BaO•6Al2O3 (3). This is confirmed by the XRD in Figure 10, where the two phases
coexist. The amounts of BaO•6Al2O3, 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 BaO•6Al2O3 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
BaO•6Al2O3.
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-BaO•6Al2O3 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 BaO•6Al2O3 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-BaO•6Al2O3 specimens after firing 1450°C (1723 K)
for one hour.The wetting
contact angle (ϕ)
variations with BaO•6Al2O3 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-BaO•6Al2O3 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 (ϕo)
at t=0 are 125° at 800°C, 121° at 1000°C, and 118° at 1200°C.Wetting contact angle (ϕ) variation with time  at
different temperatures for the  Al2O3-BaO•6Al2O3 system fired at 1450°C
(1723 K) for one hour.
(a)(b)(c)The BaO•6Al2O3 effects on the contact angles (ϕ) are
similar for the three temperatures. At the lowest temperature, 800°C (1073 K),
the 3.1% BaO•6Al2O3 decreases the wetting contact angle to 95º, about 30° less than that for the pure
alumina. The addition of BaO•6Al2O3 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 BaO•6Al2O3 the wetting contact angles decrease to 105° and 107°,
respectively. This suggests that by increasing the BaO•6Al2O3 phase in the microstructure the contact angles will increase toward less
wetting. One can speculate that further increases of BaO•6Al2O3 phase content may increase the wetting contact angles beyond that obtained for
the pure alumina.The effect of
temperature decreases the wetting contact angles (ϕ) for all
of the Al2O3-BaO•6Al2O3 specimens. From 800°C (1073 K) to 1200°C (1473 K) contact angles are reduced
for all levels of BaO•6Al2O3 additions. The result is that at all three temperatures, the pure alumina is
the most difficult to wet. The presence of BaO•6Al2O3 consistently decreases the tendency toward wetting. It must be concluded
that the amounts of BaO•6Al2O3 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 BaO•6Al2O3 additions.Figure 14
Variation of the wetting contact angle (ϕo)
with temperature for the BaO•6Al2O3 additions to alumina and after firing at 1450°C (1723 K) for one hour.Figure14 shows a linear tendency of the decrease of the wetting contact angle (ϕo)
with temperature. This describes the effects of temperature on the contact
angle variation. With BaO•6Al2O3 additions to alumina, it is observed that the contact angles are increased from
those obtained with pure alumina. However, the resultant slopes (∂ϕo/∂T)t are similar to those for the pure alumina.The decrease ofϕo with
increasing temperature is described by (8). It is probable that the BaO•6Al2O3 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 (BaO•6Al2O3)
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 BaO•6Al2O3 Phase

Davis et al. [26, 27]
suggest different processing routes for the synthesis of pure BaO•6Al2O3 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 (BaO•6Al2O3),
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 (BaO•6Al2O3)
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 (BaO•6Al2O3)
and Al2O3 was present in the microstructure. However,
when firing the natural minerals at 1700°C (1973 K), a mix between BA6 (BaO•6Al2O3),
Al2O3, and BAS2 (BaO•Al2O3•2SiO2)
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).Figure15
illustrates the reaction of the Al2O3-BaSO4 specimens to form BA6 (BaO•6Al2O3)
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 (BaO•6Al2O3)
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 BaO•6Al2O3 at 1700°C (1973 K) for one hour.The Al wetting
contact angle (ϕ) 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 BaO•6Al2O3 phase is consistently more nonwetting than pure alumina.Figure 17
Wetting contact angle (ϕ) with
time at constant temperatures. Note the Al contact angles obtained with the
pure BA6 (BaO•6Al2O3)
are larger than those obtained with the pure Al2O3.The Al contact
angles (ϕo)
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 ϕos
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 (BaO•6Al2O3)
has larger wetting contact angle than that for the pure alumina for all
temperatures. Both have similar slopes, (∂ϕo/∂T)t.
This clearly illustrates that the pure BA6 (BaO•6Al2O3)
phase is less wetting to the molten aluminum than pure alumina.Figure 18
Variation ofϕo with temperature for the couple systems of BaO•6Al2O3/Al
and Al2O3/Al. Note that the BaO•6Al2O3 phase is more nonwetting than pure alumina.


## 3.4. The Pure Alumina Wetting Contact Angle

In the
assessment of the effect of BaSO4 and BaO•6Al2O3 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 T<1124°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 T>1450°C (1723 K) cause the decomposition of
the BaSO4 (2). The reaction results in a stable compound BA6 (BaO•6Al2O3).
The amount of BaO•6Al2O3 is proportional to the content of BaO from the sulfate decomposition
reaction in (1). The BA6 (BaO•6Al2O3)
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-BaO•6Al2O3 mixtures are decreased below that of the pure alumina. They do increase
proportionally with the amount of BA6 (BaO•6Al2O3)
presents in the system. This suggests that increments of BA6 (BaO•6Al2O3)
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 BaO•6Al2O3 ceramic by molten aluminum yielded wetting contact angles greater than those
for pure Al2O3. This indicates that BaO•6Al2O3 is less wetting toward molten aluminum than pure alumina. However, the contact
angles for BaO•6Al2O3 ceramic do not approach the ϕ=180° 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 (ϕ)
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 BaO•6Al2O3 or BaO•Al2O3 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.



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*Source: 629185-2008-08-03.xml*