CANALES DE DISTRIBUCIÓN

In addition to the importance of alumina for its applications and fundamental studies, alumina is also important from the point of view that it has many metastable polymorphs such as κ-, γ-, δ-, θ-Al2O3 besides the thermodynamically stable α-Al2O3.

Among the several metastable polymorphs of alumina (transition alumina phases), γ- alumina is important due to its application in industry as catalyst, adsorbents, coatings, and soft abrasives. The metastable phases are generally divided into two structural categories depending on the O anion arrangement: face centered cubic (fcc) and hexagonal close packed (hcp). Metastable phases of γ, η (cubic), δ (either tetragonal or orthorhombic), and θ (monoclinic) with ABCABC stacking sequence have the fcc structure while metastable phase of κ (orthorhombic), χ (hexagonal) and thermodynamically stable phase of α (trigonal)with ABAB stacking sequence have the hcp structure [7].

Table 1 summarizes the processing routes to produce different metastable Al2O3

phases. All these phases can be observed at room temperature but the sequence of transformation is not reversible. It can be observed from Table 1, that gibbsite is the only aluminum hydroxide that produces χ-alumina crystals by thermal dehydroxilation [11]. κ-Al2O3 can form from heating χ-Al2O3 at about 700 °C or by thermal dehydroxilation of

tohite at 700-800 °C [7]. The κ-Al2O3 phase can be also produced from amorphized

sapphire by megatron sputtering and from thermal chemical vapor deposition (CVD) [12]. In all most cases κ-Al2O3 can be converted into α-alumina by further heating. It is

Table 1 Common processing routes for producing different Al2O3 polymorphs and the sequences of phase transformations

toward α-Al2O3 with approximate hcp and fcc packing of oxygen for the metastable Al2O3 structures[7, 12, 13]

Approximate packing of oxygen for the metastable Al2O3 structures

(hcp)

Approximate packing of oxygen for the metastable Al2O3 structures (fcc)

α-AlOOH (diaspore) α-Al2O3

γ-AlOOH (boehmite) γ δ θ α- Al2O3

γ-Al (OH) 3(gibbsite) χ κ α-Al2O3 _{α-Al (OH)}

3 (bayerite) η θ α- Al2O3

5Al2O3.H2O (tohdite) ′ κ α- Al2O3 _Melt α- Al 2O3

Vapor (thermal CVD) α-Al2O3

Cathodic arc

Vapor (thermal CVD) α-Al2O3

Amorphous (plasma enhanced CVD) α-Al2O3

Amorphous (magnetron sputtering)

α-Al2O3

Table 1 show that γ-Al2O3 can be produced by several different routes: (a)

dehydroxilation of monohydroxide boehmite in air, at 300 °C, (b) from amorphous alumina anodic films, (c) from melt (d) from cathodic arc, (e) from amorphous by plasma enhanced CVD, and (f) from amorphous alumina produced by ion implantation. According to Zywitzki et al. and Schneider et al. [14-17], γ-Al2O3 can be formed by

heating κ-Al2O3 produced by magnetron sputtering. Cao et al. reported the formation of

γ-Al2O3 directly from pulsed-laser irradiated sapphire. They believe the γ-Al2O3 was

formed because laser irradiation is a rapid heating and cooling process in which thin surface layer is heated and melted and evaporated during a 41-ns pulse duration [18]. Also δ-Al2O3 is produced only from γ-Al2O3; indicating that the transformation is

pseudomorphic either from boehmite or from amorphous and melt. The metastable phase of θ-Al2O3 may be produced by three routes: (a) from δ- Al2O3; (b) from η- Al2O3; (c)

from γ- Al2O3. Metastable phase of η- Al2O3 may be produced from bayerite. The

presence of δ- Al2O3 in the transformation sequence of γ- Al2O3 to θ-Al2O3 has been

speculated from studies of Zhou [19] and Gan [20]. Based on NMR and IR data, Pecharroman has suggested that the δ- Al2O3 detected in the transformation sequence of

γ- Al2O3 to θ-Al2O3 actually is not a single δ- Al2O3 phase but a heterogeneous mixture of

well crystallized γ- Al2O3 and θ-Al2O3 metastable phases [21].

Santos et al. have reported that η- Al2O3 can be produced from gelatinous

boehmite. No information exists on pseudomorphism from these crystals [11]. A brief summary on the structures and properties of common alumina polymorphs is presented in Table 2 [7, 22, 23].

Table 2 Structure and properties of alumina polymorphs Phase Structure (arrangement of oxygen) Space group Density [g/cm3] Cations/unit cell Lattice parameter [Å] α hcp R- c hR10 3.99 10 30 5.128 a = 4.789 c = 12.991 γ fcc m 3.65-3.67 64/3 7.9 δ fcc m2 3.60-3.65 64 a=7.9 b=15.8 c=11.85 θ fcc C2/m 3.60-3.65 8 a=11.85 b=2.793 c=5.586 κ hcp Pna21 3.98 8 a=4.834 b=8.310 c=8.93 .

Although the exact structures and properties of some metastable aluminas are still not well understood, the transition aluminas have found significant attention for use in industry. For instance κ-Al2O3 has application in wear resistance coatings on cement

carbide cutting tools. The metastable phase, δ- Al2O3, is used in bioactive bone cement

composites as an alternative to α- Al2O3 due to a greater osteoblastic activity for in-vivo

bone formation. Ultra-high purity polycrystalline α- Al2O3 can be produced from θ-Al2O3.

adsorbents, etc. [24]. The γ- Al2O3 phase is a very fine-grained material making it very

difficult to impossible to sinter into a dense body. Therefore γ- Al2O3 powder has a very

high specific surface area ≈ 100 m2_{/g compare with the α-Al}

2O3 with the surface area of

≈5 m2

/g.

1.4.1 γ- Al2O3

The structure of γ-alumina has been described as a defect spinel (Fd m) with the formula Al21+1/3

□

□

distributed over the fcc packing, while 64/3 Al ions and vacancies are distributed over octahedral (16d) and tetrahedral (8a) positions of the ideal spinel structure [25]. The deviations from an ideal spinel arise from the assumption that the 2+2/3 aluminum vacancies are randomly distributed over the tetrahedral or octahedral sites. Different researchers have reported different percentage of aluminum ion occupancy in the octahedral and tetrahedral positions. Shirasuka et al. and John et al. respectively have reported that the aluminum ions occupancy of the two 16-fold octahedral sites are 62.5% and 65% and they assumed the remaining aluminum ions are distributed equally over the eightfold and the 48-fold tetrahedral sites [26, 27]. These percentages are reported to be 63% and 70% respectively in the work of Shelberg et al. and Lee et al. [28, 29]. A similar disagreement can be found in case of the aluminum vacancy distribution. The studies show that the aluminum vacancies are situated entirely in octahedral positions [30-32].

A study by Jayaram [33] concluded that these aluminum vacant sites are entirely in tetrahedral positions. Lippens et al. believe whether the vacant sites will occur in octahedral or in tetrahedral positions, the greatest disorder is to be expected for the atoms

in tetrahedral positions [34]. Most of density functional theory (DFT) studies along with experimental works indicate the aluminum vacancies to reside at both tetrahedral and octahedral sites [35-38].