DISCUSSION

4SS eea eea 1eaa 1200 uae 1639

TWO 9 TWO 9

4.3 DISCUSSION

Three distinct types of beta alumina products were identified from the powder X-ray diffraction patterns. These products were derived by distinctly different reaction pathways. The evolution of each type of beta alumina powder product is dis- cussed separately.

Distinct two-phase mixtures of ~ plus ~ "-Al 203 were p reduced when a-Al 20 3 was reacted with sodiun and lithium salts at 1200 °C. At this tefll)erature the proportion of ~ "-Al20 3 was

about 40%. a-Al 203 has a hexagonal close packed oxygen lat- tice with an [ ••• AB AB AB ••• ] stacking of the oxygen ions.

The formation of ~ and ~'~Al2⁰3 from this precursor requires a reconstructive transformation of the oxygen sublattice.

The endothermic event at approximately 800

°

C is related to the transformation of a portion of the a:-Al20 3 to Y-NaAl0 2 which subsequently reacts with the residual a:-Al 203 to form a two-phase mixture of ~ and ~ "-Al 20 3• [}Jring these reactions the hexagonal close-packed sublattice of the corundum struc- ture is transformed to a close-packed cubic sublattice.

4.3.2 Pure ~"-Al2⁰3 ^formation

Hydrothermal boehmite and bayerite produced a single phase, distinctly crystalline ~ "-Al 203 when reacted \'iith sodium and lithium carbonates at 1200 °C. ~ "-Al20 3 was in both cases produced directly, with no traces of sodium aluminate or

~-Al203 intermediates observed. The direct fGrmation of

~ "-Al 203 from boehmite was first reported by Duncan (1983) amd Barrow et al. (1986). These authors made an important contributiun to the understanding of the direct formation of

~ "-Al203 from hydrothermal boehmite. Barrcw (1986) pointed out that pure ~"-Al2^D3 is only formed from boehmite which exhibits a long range order of the oxygen sublattice.

The crystal structures of monohydrate hydrothermal boehmite and trihydrate bayerite differ substantially. Bayerite con- sists of densely packed layers with the aluminil.JTI ions ar- ranged on the octahedrally co-ordinated lattice sites. The layers exhibit a hexagonal close-packed sequence with an [ ••• AB AB AB •••

J

oxygen sublattice stacking. Boehmite also has a layered structure in which the Al ions are located in octahedral sites. However, in the case of the boehmite the OH ions are arranged on the outside of the layers and ensure the cohesion of the layers by hydrogen bonding.

The differences in structure are responsible for the differ- ent dehydroxylation behaviour observed for these materials.

Dehydroxylation of bayerite occurs at an onset temperature of 240

°

C while the hydrogen bonds in boehmite require higher temperatures to be broken. This results in an onset tempera- ture of 500

°

C for the dehydroxylation of boehmite.

Structural changes in the materials as a result of dehydroxy- lation were observed in both mate~ials upon heating. Bayerite produced n-Al₂0_{3 ,} which has a cubic spinel-like crystal lat- tice with a lattice constant a

=

^{7,92 A. Y-~1}₂⁰₃ ^{was formed}

when boehmite was dehydroxylated. This spinel-like lattice is tetragcnally deformed but is in all other cases similar to n-Al₂0₃• The tetragonal deformation of the y-Al₂0 ₃ compound is illustrated by the splitting of the 440 and 400 lines of the spinel lattice as indicated in Fig. 4.11. The tetragonal deformation can be explained by the observation Jf Krishner (1971) who reported that the Al occupation in the boehmite is inherited by the y-Al₂0_{3 •} Cunsequently the alumini ^lJTI ions occupy octahedral sites not usually occupied in spinel struc- tures.

The cubic n-Al₂0₃ formed by the dehydroxylation of bayerite, does not retain the octahedral occupation of the aluminium, resulting in the cubic spinel lattice.

-

en a.

-

(.)

_{>- ..,_}

en z

LtJ

..,_

z -

Si Standard

400 I

y

AJ₂0₃

lt

~

7] Al₂0₃

40 45 50 5~ 60 6~

TWO 8

FIGURE 4.11: Comparative XRD trace of Y-Alz03 and ~-Al2⁰3 ^indicat- ing tetragonal distortion

Apart from the occupation of the octahedral sites, y- and ~­

Al20 ₃ are very similar. Si gr.ificantly, both compounds have an [ ••• ABC ABC ••• ] oxygen sub lattice which is maintained in- tact until the shear ~rensformati~n to a-Al20

3, which occurs at about 1200 °C.

A (1120) section through the idealized structure of ~"-Al

2

⁰

3

(Fig. 4.12) shows that the compound consists of spinel blocks containing oxygen ions in the close-packed cubi~ arrangement.

X-ray intensity calculations by Whittingham and Huggins (1971) supported by work of Le Cars et al. (1972) suggested that the most lH:ely arrangement of the oxygen is as follows:

[ ••• ABCABCABCABCABCABCAB •••

J

- - - -

The underlined oxygen ion layer represents the conduction plane, which contains a third of the oxygen ions of the close-packed layers. The simila~ity in the stacking order of the oxygen sublattice of y-end n-Al₂0₃ with the

~ "-Al₂0₃ oxygen sublattice suggests a possible mechanism by which ~"-Al2⁰3 is formed directly from these transition aluminas when lithium and sodium salts are present.

A B

c

A.

CONDUCTION SLAB

c

A B

c

CONDUCTION SLAB

8

c

A B

•

® 0

Aluminium Sodium Oxygen

{3^{11 -} Alumina

FIGURE 4.12: A (1120) section through the idealized structure of 13 "-Al203

After the dehydroxylation of boehmite or bayerite, lithium diffuses through a three-dimensional network of face-shared tetrahedra and octahedra of the close-packed cubic oxygen lattice of the y- and ~-alumina structures &nd ion exchanges with the residual hydrogen ione. The lithium ions stabilize the close-packed cubic oxygen sublattice. It has been demon- strated by DTA that small amounts of lithium (~2 mol percent) considerably in~rease the tra~sformation temperature of the transition alumina to a-Al₂0_{3 •} This increase in transforma- tion temperature can be ascribed to the formation of a lithium-stabilized, close-pac~ed cubic transition alumina.

In the proposed mechanism, the function of the lithium is to 'pin' the cubic oxygen sublattice, which is formed upon d€hy- droxylation. The sodium ions, which are larger than the oxy- gen ions, are not accommodated in the crystal lattice at low temperatures. It is suggested that the sodi urn ions reside on the surface of the transition alumina grains as an amorphous soda phAse until a temperature is reached which makes it thermodynamically feasible for the sodium to diffuse into the oxygen sublattice to form the ~"-Al2⁰3 conduction layers.

When lithium is absent from the reaction mixture, the crys- tallization pathway is substantially different and the reac- tion product at 1200 °C contains a-Alz03, Y-NaAl0_{2 ,} ~"-Al

2

⁰

3

and ~-Al2⁰3^• Consistent with the previous observations, the dehydroxylation of bayerite and boehmite produces Tl- and y-alumina respectively. When only sodiu;n is present, two simultaneous reactions take place during the heating of the reaction mixture. A portion of the transition alumina trans- forms to a-Al₂0 3 as no lithium is present to stabilize the close-packed cubic oxygen sublattice. The rest of the tran- sition alumina proceeds to an unstabilized ~ "-Al₂03 by the

reaction of sodium with the material. The a-Al

20 3 reacts with the sodium to form ~- and ~ "-Al₂03 mixtures via y-NaAl02

intermediate.

At 1600

°

C, the reaction product of boehmite and soJa is similar to that of a material derived from the ~-Al203 pre- cursor.

The reaction products of the materials derived from boeh~ite

and bayerite in the absence of the lithium, demonstrate that two important criteria must be satisfied to allow pure

~ "-Al 203 to be formed. Firstly, a regular close-packed cubic oxygen sublattice must be formed upon dehydroxylation of the hydroxide material and secondly the sublattice must be main- tained or stabilized to prevent the transformation to ^~-Al2⁰3 before the reaction with soda can occur.

4.3.3 Intergrown ~/~"-Al2⁰3 ^Polytypes

Disordered pseudoboehmite and gibbsite produced a reaction prcduct which was broadly classified as an intergrown ~/~"­

Al203 polytype. This material is characterized by the simul- taneous presence of broad and sharp XRD peaks as discussed in Sections 2.5 and 4.2.3.

Poulieff et al. (1978) and M:Jrgan (1976) reported these mate- rials and suggested that they were formed from disordered precursors. According to Poulieff et al. (1978) intergrown

~~~ "-Al 203 was formed as a consequence of the retention of the precursor stacking faults. Hodge (1983) postulated the epitaxial nucleation of ~ and ~"-Al2⁰3 on the disordered sub- strate followed by the subsequent growth of the phases. In none of the above cases, was the role of the lithium or an initial close-packed cubic oxide lattice alluded to.

The formation of the intergrown phases can be explained by a modification of the model of Poulieff et 91. (1978). It has been demonstrated that both pseudoboehmite and gibbsite dehydroxylate to poorly ordered transition alumina phases.

Pseudoboehmite produces a virtually amorphous transition

alumina product while gibbsite produces poorly ordered x-Al₂0₃ after dehydroxylation. The hignly mobile lithium ions diffuse into the structure after dehydroxyletion, exchange for the hydrogen ions end stabilize the faulted cubic oxygen sublattice. As in the case of they- and

n-

_{Al 203 ,} sodium is accommodated on the surface of the grain. Because of the higher density of faults and irregularities in the transition alumina derived from pseudoboehmite and gibbsite, sodium dif-

fuses into the structure at lower temperatures to form beta alumina polytypes, which retain the original disordered oxygen sub lattice. The disordered sub lattice is partially maintained by lithium occupation of tetrahedral and octahe- dral sites. The disordered ~/~"-Al2⁰3 poly types are very stable once formed and the structures are retained even at 1600 °C.

When pseudoboehmite and gibbsite are reacted with sodium in the absence of lithium, a different reaction product and crystallization mechanism is observed. With no lithium pres- ent to maintain the cubic oxygen sublattice, the disordered

transition alumina converts to a:-Al 20^3• The a:-Al 203 reacts with soda to form a distinctly two-phase mixture of ^~ and

~ "-Al 203 via a sodium aluminate intermediate. As in the c&se of boehmite and bayerite, the 1 600 °C product of pseudoboeh- mite and gibbsite in the absence of lithium resembles that of an a:-Al 203 derived material.