SCIENTIFIC RESEARCH AND DEVELOPMENT

EFFECT OF WATER 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. 1, pp. 24 – 27, January 2014.

Original article submitted September 5, 2013.

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

3from dehydrated

alumogel. It is shown that water removal from an original precursor has a marked effect on subsequent phase

transformations in the synthesis temperature range. By comparison with the crystallization temperature for

�-Al2O

3precipitated from aqueous solution, for gel this is reduced by 300°C from 1200 to 900°C.

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

Structural applications are an extensive field for use of

ceramic based on Al2O3. High specifications are laid down

for ceramic of this designation with respect to mechanical

strength. Particles of submicron size powder are used in or-

der to provide this property in manufacture of objects. In-

deed, this specification should provided technology for pre-

paring powder raw material used for these purposes. The ba-

sis of the majority of contemporary industrial technology for

preparing powder raw material for oxide structural ceramics

is a liquid phase synthesis method, which includes synthesis

of hydroxides followed by high-temperature treatment trans-

formation into oxides. The firing temperature for hydroxide

precursors mainly determines fineness and other less impor-

tant properties of the raw material powder obtained, for ex-

ample presence within it of strong interparticle formations

(agglomerates). A high homologous treatment temperature in

synthesizing powder raw material from hydroxide precursors

unavoidably leads to a reduction in product fineness. This is

demonstrated by comparing the homologous synthesis tem-

perature for powders of two widely used structural oxide ce-

ramic materials, i.e., particles of stabilized zirconium dioxide

and aluminum oxide. Whereas for zirconium dioxide powder

it is 500/2750 = 0.18 (synthesis temperature in the numera-

tor, powder material melting temperature in the denomina-

tor), for corundum ceramic powder this value is much higher,

i.e., 1200/2044 = 0.59. Therefore in preparing ultrafine pow-

der raw material for zirconia structural ceramic the process-

ing problems are less than in preparing �-Al2O3 powder.

A reduction in synthesis temperature for powder raw mate-

rial is a direct way of increasing its fineness.

The basis of a liquid phase synthesis method for �-Al2O3

powder is a process of preparing aluminum hydroxides

(AlOOH, Al(OH)3) in different crystalline form (boehmite,

hydrargillite, gibbsite, bayerite, etc.) [1]. The concluding

stage of this technology is thermal removal of OH– hydroxyl

groups, followed by an increase in temperature to a field

where there is �-Al2O3 crystallization. The sequence of

phase transformations for �-Al2O3 hydroxide precursors has

been studied quite well. It is demonstrated in Fig. 1 [3]. It is

seen that the temperature required for �-Al2O3 crystalliza-

tion from those precursors containing OH– groups is quite

high, i.e., 1200°C [2]. An exception is diaspore (�-AlOOH),

which is encountered in bauxite ores in extremely small

amounts. The technology of its synthesis is complicated and

so far it has not found industrial implementation [4].

The �-Al2O3, crystallization temperature (1200°C)

shown in Fig. 1 is the temperature for initializing crystalliza-

tion. For complete transformation in alumna manufacture it

is normal to use a higher temperature level, i.e., up to 1450°C

[1]. Attempts are made by different ways to reduce the

�-Al2O3 synthesis temperature. One of them involves intro-

ducing �-Al2O3 seeding crystals into hydroxide precursors.

Kinetics for transformation in boehmite powder (�-AlOOH

according to the classification in [2]) with introduction of

�-Al2O3 seeding has been studied [5]. Seeding was intro-

duced into a boehmite aqueous sol, which was then dried and

heat treated at 500°C for formation of a uniform

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

17

1083-4877�14�05501-0017 © 2014 Springer Science+Business Media New York

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

the Russian Academy of Sciences2

E-mail: aleksander.galakhov@yandex.ru.

�-Al2O3 mixture with introduced seeding. In this case the ef-

fect of seeding on the crystallization temperature itself for

�-Al2O3 � �-Al2O3 was insignificant. Crystallization of

�-Al2O3 in powder obtained from pure boehmite started at

1050°C, whereas in powder with seeding the � � � transfor-

mation commenced at 1000°C. However, with introduction

of seeding the incubation period for transformation was re-

duced, and its rate increased many times. A similar study was

carried out by the authors of an article in [6] on a precursor

obtained by precipitation from aqueous aluminum nitrate so-

lution. In this work 5 wt.% of �-Al2O3 seeding powder was

added to aluminum nitrate solution, i.e., on the basis of a salt

solution an �-Al2O3 sol was prepared, from which by addi-

tion of aqueous ammonia (direct precipitation) a

hydroxide precipitate was obtained with intro-

duced seeding. The precipitate dried at 400°C con-

sisted entirely of boehmite. A series of thermal fir-

ings with monitoring of phase composition

showed that introduction of seeding markedly re-

duces the temperature for initializing and the tem-

perature for total � � � transformation, which for

precursor without seeding does not differ strongly

from the known temperature, i.e., 1200°C. Addi-

tion of seeding crystals reduced it by 300°C

(900°C). The “seeding” method for reducing the

� � � transformation temperature is not limited

to use of �-Al2O3 seeding crystals. The effect has

been achieved due to application of diaspore as a

seeding powder [7]. Seeding was carried out by

the well-known procedure [6], i.e., a diaspore sol

was prepared based on aluminum nitrate solution.

Aluminum-containing precursor was precipitated

by the same scheme with aqueous ammonia solu-

tion. Phase analysis of precipitates, calcined at

different temperature, showed that �-Al2O3 forms

at 600°C.

In all of the work in question synthesis com-

mences with preparation of very fine precursors

followed by thermal destruction. At the same time

there are methods of “dehydration” for starting

solutions. For example, use of evaporation of so-

lutions or distillation of water under vacuum us-

ing a vacuum rotary evaporator. The authors of

the present article attempted to evaluate the effect

of water in original precursors on the sequence of

phase transformations during heat treatment. For

this a 1 M aqueous solution of aluminum nitrate

(specially pure Al(NO3)3·9H20) was prepared.

One part of the solution was “evaporated” in a

vacuum rotary evaporator to cessation of water

vapor liberation, The product obtained was a

transparent viscous gel (subsequently called

“gel”). The other part of the solution was used for

preparing a hydroxide precipitate by a traditional

method, i.e., precipitation with ammonia solution

with liquid separation on a filter under vacuum. For complete

precipitation three moles of NH4OH were added for one

mole of aluminum nitrate, (the gel is subsequently called the

“precipitate”). Then the gel and precipitate were heat treated

at different temperatures with monitoring of phase composi-

tion. Recording of specimens was performed in a DRON-3

x-ray diffractometer in monochromatized Cu K�-radiation.

The Rietveld method was used [8] in order to determine the

quantitative ratio of phases, realized in a software program

[9]. Results are shown in Fig. 2 and provided in Table 1. The

chain of phase transformations with an increase in tempera-

ture for precursors of different origin (gel and precipitate)

differs markedly. Whereas precipitate undergoes the

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

Fig. 1. Sequence of phase transformations during heat treatment of �-Al2O3 hy-

droxide precursors [3].

Fig. 2. X-ray patterns of precipitate (a) and gel (b) after series of two-hour firings at

different temperatures: �) �-AlOOH (boehmite); �) �-Al2O3; �) �-Al2O3,

�) �-Al2O3.

well-known transformation sequence �-AlOOH

(boehmite) � �-Al2O3 � �-Al2O3 � �-Al2O3, which ceases

with crystallization of �-phase at 1200°C, anhydrous gel re-

tains an amorphous structure to quite a high temperature, i.e.,

500°C. Then at 750°C there is formation of poorly crystal-

line �-Al2O3, which is transformed into �-Al2O3 entirely at

900°C. This is 300°C lower than with crystallization from

precipitate. One feature should also be noted, coming to light

in analyzing results presented in Fig. 1 and Table 1. This is

coexistence of several different phases at different tempera-

tures, which indirectly points to local inhomogeneity of hy-

droxide precipitate structure. Whereas in local areas of phase

formation has already ceased, in others it has not started. In

contrast to a precipitate, within a gel transformation proceeds

uniformly throughout the whole volume. The latter is con-

firmed by data of quantitative phase analysis, which is pre-

sented in Table 1.

In addition, differences in the degree of precipitate and

gel uniformity may be a reason for the difference in transfor-

mation mechanism in these structures. Whereas in an

inhomogeneous structure of precipitate phase transforma-

tions proceed by a “slow” diffusion mechanism, within the

uniform structure of a gel this transformation may proceed

by a “rapid” martensitic type. This transformation mecha-

nism cannot be entirely excluded since all modifications of

oxygen compounds of aluminum, i.e., dense packing of an-

ions O (–2) in tetra- and octapores some Al (+3) cations are

located. In fact, it is impossible to accomplish redistribution

of these cations through pores by pure displacement. How-

ever, short distances (~10 Å), required for rebuilding an ele-

mentary cell, should facilitate to a considerable extent accel-

eration of this “jump” with participation of diffusion.

In a practical respect it is interesting to compare some

properties of �-Al2O3 powders prepared by a classical

scheme (a precipitate) and from anhydrous gel, in particular

specific surface specifying powder fineness. The specific

surface of particles was evaluated by a BET-method. Mea-

surements were made in a ASAP 2020 instrument from

Micrometrics, USA. For �-Al2O3 obtained from a precipi-

tate, calcined at 1200°C, it was 3.56 m2/g, whereas �-Al2O3

prepared from anhydrous gel calcined at 900°C it was

12.84 m2/g. The size of particles corresponding to this spe-

cific surface for powder from a precipitate was 0.34 �m, but

for that prepared from anhydrous gel it was 0.09 �m.

The results presented indicate that a reduction in water

(OH—group) content in an original precursor markedly

changes the sequence of phase transformations in alumogel.

It excludes development of some intermediate phases: alumi-

num hydroxides and transformation between �- and �-mono-

clinic �-phase. The �-Al2O3 crystallization temperature is re-

duced by 300°C. This last situation, together with an increase

in product fineness, makes use of the scheme for preparing

�-Al2O3 from anhydrous gel attractive both in laboratory

practice and in implementing manufacturing technology.

REFERENCES

1. A. I. Lainer, N. I. Eremin, Yu. A. Lianer, and I. Z. Pevzner, Alu-

mina Production [in Russian], Metallurgiya, Moscow (1978).

2. V. P. Chalyi, Metal Hydroxides [in Russian], Naukova Dumka,

Kiev (1972).

3. W. H. Gitzen (editor), Alumina as a Ceramic Material, ACerS,

Columbus OH (1970).

4. L. Loffler and W. Mader, “Transformation mechanism of the de-

hydration of diaspore,” J. Amer. Ceram. Soc., 86(4), 534 – 540

(2003).

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

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

line Solids, 82, 277 – 285 (1986).

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

nanocrystalline �-alumina powders,” Acta Mater., 48,

3103 – 3112 (2000).

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

processing,” Nanostructured Materials, 11, No. 8, 1141 – 1153

(1999).

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

peaks for structure refinement,” Acta Crystallographica, 22,

151 – 152 (1967).

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

sis of polycrystals,” Metal Science and Heat Treatment, 42(8),

309 – 313 (2000).

Effect of Water on á-Al2O3 Crystallization in Alumogels 19

TABLE 1. Precursor Phase Composition as a Function of Firing

Temperature, wt.%

Temperature,

°C

Precipitate Gel

boehmite � � � boehmite � � �

350 31.2 68.8 — — — — — —

500 — 100 — — — — — —

750 — 31.5 68.5 — — 100 — —

900 — 78 19.5 2.5 — — — 100

1000 — 54.3 38.7 7 — — — —

1200 — — — 100 — — — —