EFFECT OF SEEDING ON á-Al2O3 CRYSTALLIZATION IN ALUMOGELS

A. V. Galakhov,1,2 V. A. Zelenskii,1 E. V. Shelekhov,1 and L. V. Kovalenko1

Translated from Novye Ogneupory, No. 2, pp. 48 – 50, February 2014.

Original article submitted October 17, 2013.

Results are presented for a study of phase transformations during synthesis of �-Al2O

3from dehydrated

alumogel. It is shown that introduction of �-Al2O

3seeding crystals into gel does not affect the temperature for

initiation of transformation of �-Al2O

3into �-phase. However, presence of seeding markedly increases the

transformation rate, which under certain conditions leads to a reduction in �-phase total recrystallization tem-

perature.

Keywords: �-Al2O3, aluminum oxide, phase transformations, alumogel, seeding.

The thermal destruction method for alumogel is often

used in order to prepare �-Al2O3 powder both within indus-

trial and laboratory practice. In a preceding publication [1]

we considered the difference in crystallization of �-Al2O3

from alumogel, containing aluminum hydroxide compounds

and alumogel with an amorphous structure. It was estab-

lished that the crystallization temperature for �-Al2O3 from

dehydrated amorphous gel is significantly lower (by 300°C)

than the crystallization temperature of this phase from alumi-

num hydroxide. At the same time, water removal from

alumogel is effective but not the only way of reducing the

�-Al2O3 transformation temperature into �-phase. A so-

called “nucleation” method is used with great popularity,

which involves accelerating �-Al2O3 crystallization in nucle-

ated �-crystals, introduced into aluminum-containing precur-

sor before thermal decomposition. Introduction of nucleating

crystals of �-phase makes it possible to provide artificially

occurrence of the first stage of �-Al2O3 phase transformation

into �-phase (or �-Al2O3 to �-phase), i.e., nucleus formation,

with a further marked acceleration of crystal growth under

conditions of a high concentration of growth centers. Many

publications have been devoted to using this method, al-

though research results have been obtained for alumogels

containing aluminum hydroxide compounds. In this study we

have attempted to evaluate the effect of nucleating �-Al2O3

crystals on crystallization of �-phase in dehydrated amor-

phous alumogel.

Amorphous gel was prepared by the method described in

[1]. For this purpose 1M aqueous solution of ammonium ni-

trate Al(NO3)3·9H2O was dehydrated in a vacuum rotary

evaporator. In parallel for the same batch of solution a

so-called “pure” portion of gel was prepared, and in the orig-

inal solution for another portion 1 wt.% of seeding �-Al2O3

crystals was added. Seeding crystals were prepared using

part of the “pure” gel, which after dehydration and drying

was fired at 900°C for 3 h. This is sufficient for total crystal-

lization of �-phase [1]. Gel was prepared for comparison

containing aluminum hydroxide (in the form of boehmite), in

order to compare the temperature sequence of crystallization

from solution with similar gel properties, subsequently called

deposits. A deposit was prepared by precipitation of alumi-

num hydroxide from solution of concentrated (3M) aqueous

ammonia solution. As in the case of anhydrous gel, a “pure”

deposit and a deposit containing 1 wt.% �-Al2O3 crystals

were prepared.

Then the gel and deposit were heat treated, including a

series of firings at different temperatures, with monitoring of

phase composition. Recording of specimens was performed

in a DRON-3 x-ray diffractometer in monochromatic Cu

K�-radiation. The Rietveld method [5] was used in order to

determine the quantitative ratio of phases, implemented in a

software program [6]. X-ray patterns are shown in Fig. 1 as

an illustration for “pure” deposit without seeding and gel

with seeding. The sequence of x-ray patterns provided is a

good illustration of the difference in course of phase transfor-

mations in a hydroxide deposit and in amorphous alumogel

[1]. Crystallization of boehmite deposit commences at 350°C

with formation of �-Al2O3. Crystallization of �-Al2O3 com-

mences in amorphous gel much later, i.e., only at 750°C. In

Refractories and Industrial Ceramics Vol. 55, No. 1, May, 2014

20

1083-4877�14�05501-0020 © 2014 Springer Science+Business Media New York

1A. A. Baikov Institute of Metallurgy and Materials Sciences of

the Russian Academy of Sciences.2

E-mail: aleksander.galakhov@yandex.ru

amorphous gel �-Al2O3 forms immediately from

�-Al2O3. In contrast to amorphous gel, in the

course of transformation of boehmite deposit di-

rect transformations of �-Al2O3 into �-phase is

excluded, and �-Al2O3 crystallizes from an inter-

mediate monoclinic �-phase. The temperature for

total �-phase crystallization for dehydrated amor-

phous gel is significantly lower, i.e., by 300°C.

A more complete representation of the effect

of seeding crystals on the course of phases trans-

formations is given by analysis of data provided

in Tables 1 and 2. Introduction of seeding crystals

into deposit (see Table 1) markedly increases tem-

perature for the start of crystallization of interme-

diate �-phase from �-Al2O3. Whereas in “pure”

deposit it forms at 750°C, the temperature for the

start forming �-crystals in deposit with seeding

shifts to a higher temperature region and corre-

sponds to 900°C. In “pure” deposit this phase is

retained in a considerable amount up to 1100°C,

but in deposit with seeding only up to 1000°C,

i.e., addition of seeding �-crystals markedly re-

duces the temperature range for existence of

�-Al2O3. In deposit without seeding this region

occupies a “broad” temperature range from 1100

to 750°C (350°C). It is not surprising that within

this range a temperature exists (900°C) at which

all of the volume of deposit is in �-form. In de-

posit with seeding the “extended” �-phase exis-

tence is considerably reduced to 100°C from 1000

to 900°C. Reduction in this range is also accom-

panied by a sharp increase the rate of

� � �-transformation, caused by a reduction in

total recrystallization of �-phase from 1200°C for

“pure” deposit to 1100°C for deposit with seed-

ing.

In contrast to boehmite deposit, in amorphous

gel three hours of exposure at different tempera-

tures does not generate marked differences in the

order of occurrence of phase transformations in

“pure” gel with seeding (see Table 2). In another

case formation of intermediate �-phase is ex-

cluded, transformation of �-Al2O3 into �-Al2O3 is

initiated at 750°C, and is completed by total syn-

thesis of �-Al2O3 at 900°C. We draw attention

only to marked growth of �-phase crystallization

rate in gel with seeding crystals, prepared with

“pure” gel in the temperature range 750 – 900°C.

The latter touches on the idea that an increase in firing dura-

tion should lead to more complete formation of �-phase with

a lower temperature in transformation range 750 – 900°C.

This is confirmed by data for phase composition of gel with

seeding, fired at lower temperature (850°C), but with an in-

crease in soaking time. The content of �-Al2O3 in specimens

of amorphous gel after firing at 850°C as a function of soak-

ing time :

Soaking time at 850°C, h. . . . . 3 6 9 12

�-Al2O3 content, wt.% . . . . . . 32.0 72.3 91.5 98.8

The results provided may be treated as a reduction in

�-Al2O3 crystallization temperature to 850°C. It is not ex-

Effect of Seeding on á-Al2O3 Crystallization in Alumogels 21

Fig. 1. X-ray patterns of deposit (a) and gel (b ) after series of three-hour firings at

different temperatures:�) boehmite;�) �-Al2O3;�) �-Al2O3,�) �-Al2O3.

cluded that with an increase in soaking time to some foresee-

able limits it is possible to achieve total transformation of

�-Al2O3 into �-phase at 800°C or lower temperature within

the limits of the transformation temperature “corridor”

(750 – 900°C).

CONCLUSION

Analysis of results obtained in this work make it possible

to draw the following conclusions. Introduction of seeding

�-Al2O3 crystals both into alumogel, containing aluminum

hydroxide in the form of boehmite, and into amorphous gel

obtained from a dehydrated precursor, does not affect the

temperature for the start of �-phase formation. At the same

time, presence of seeding �-Al2O3 crystals markedly in-

creases the rate of �-phase growth in gels in the second stage

of transformation, where the factor of nucleus formation pu-

rity plays such a marked role. This makes it possible to re-

duce the time for completion of total transformation of

�-Al2O3 into �-phase, and under some conditions to reduce

temperature for completion of �-Al2O3 crystallization. How-

ever, the effect of this “nucleation” method for reducing

�-Al2O3 crystallization temperature is not so marked

(50 – 100°C) compared with the effect achieved with dehy-

dration of an original precursor (300°C). [1].

REFERENCES

1. A. V. Galakhov, V. A. Zelenskii, E. V. Shelekhov, et al., “Effect of

water on �-Al2O3 crystallization in alumogels,” Novye

Ogneupory, No. 1, 24 – 27 (2014).

2. R. A. Shelleman, G. L. Messing, and M. Kumagai, “Alpha alu-

mina transformation in seeded boehmite gels,” J. Non-crystalline

Solids, 82, 277 – 285 (1986).

3. J. G. Li and X. Sun, “Synthesis and sintering behavior of a

nanocrystalline á-alumina powders,” Acta Mater., 48, 3103 –

3112 (2000).

4. A. Krell and H. Ma, “Nanocorundum — advanced synthesis and

processing,” NanoStructured Materials, 11(8), 1141 – 1153

(1999).

5. H. M. Rietveld, “Line profiles of neutron powder diffraction

peaks for structure refinement,” Acta Crystallographica, 22,

151 – 152 (1967).

6. E. V. Shelekhov and T. A. Sviridova, “Programs for x-ray analy-

sis of polycrystals,” Metal Science and Heat Treatment, 42,

No. 8, 309 – 313 (2000).

22 A. V. Galakhov, V. A. Zelenskii, E. V. Shelekhov, and L. V. Kovalenko

TABLE 1. Phase Composition of Boehmite Deposit After Three-

Hour Firing at Different Temperatures

Firing

tempera-

ture, °C

Crystallization phases

in “pure” deposit in deposit with seeding

boehmite � � � boehmite � � �

350 31.2 68.8 — — 98.8 — — 0.2

500 — 100 — — — 99 — 1.0

750 — 31.2 68.8 — — 99.1 — 0.9

900 — — 100 — — 33.0 66.0 1.0

1000 — — 96.4 3.6 — 13.2 89.5 17.1

1100 — — 19.4 80.6 — — — 100.0

1200 — — — 100 — — — —

TABLE 2. Amorphous Gel Phase Composition After Three-Hour

Firing at Different Temperatures

Firing

temperature,

°C

Crystallization phases

in “pure” gel in gel with seeding

� � � �

350 — — — 0.9

500 — — — 1.0

750 100 — 98.2 1.8

800 97.3 2.7 97.3 2.7

850 93.5 6.5 67.4 32.7

900 4.2 95.8 — 100