Abstract
Corundum, α-Al 2 O 3 , is the thermodynamically stable phase of coarsely crystalline aluminum oxide, but syntheses of nanocrystalline Al 2 O 3 usually result in γ-Al 2 O 3 . Adsorption microcalorimetry, thermogravimetric analyses, and Brunauer-Emmett-Teller adsorption experiments, coupled with recently reported high-temperature solution calorimetry data, prove that γ-Al 2 O 3 has a lower surface energy than α-Al 2 O 3 and becomes energetically stable at surface areas greater than 125 square meters per gram and thermodynamically stable at even smaller surface areas (for example, 75 square meters per gram at 800 kelvin). The results are in agreement with recent molecular dynamics simulations and provide conclusive experimental evidence that differences in surface energy can favor the formation of a particular polymorph.
References
34
Referenced
835
- For reviews see H. Gleiter Nanostruct. Mater. 6 3 (1995); R. W. Siegel ibid. 4 121 (1994); H. Gleiter Prog. Mater. Sci. 33 223 (1989); R. Freer Ed. Nanoceramics (British Ceramic Proceedings 51 Institute of Materials London 1993); R. W Siegel MRS Bull. 5 60 (1990).
-
Flaschen S. S., J. Am. Chem. Soc. 77, 6194 (1955).
(
10.1021/ja01628a030) / J. Am. Chem. Soc. by Flaschen S. S. (1955) -
Garvie R. C., J. Phys. Chem. 82, 218 (1978).
(
10.1021/j100491a016) / J. Phys. Chem. by Garvie R. C. (1978) -
G. Skandan et al. Nanostruct. Mater. 1 313 (1992).
(
10.1016/0965-9773(92)90038-Y) - A. W. Adamson Physical Chemistry of Surfaces (Wiley New York 1990) pp. 313–318.
- See for example
-
Johnston G. P., Muenchausen R., Smith D. M., Fahrenholtz W., Foltyn S. J., J. Am. Ceram. Soc. 75, 3293 (1992);
(
10.1111/j.1151-2916.1992.tb04424.x) / J. Am. Ceram. Soc. by Johnston G. P. (1992) -
; P. M. Kumar et al. Mater. Chem. Phys. 36 354 (1994).
(
10.1016/0254-0584(94)90054-X) - K. Wefers and C. Misra Alcoa Tech. Pap. 19 (ALCOA Alcoa Center PA 1987).
-
Blonski S., Garofalini S. H., Surf. Sci. 295, 263 (1993).
(
10.1016/0039-6028(93)90202-U) / Surf. Sci. by Blonski S. (1993) - The surface energies for α-Al 2 O 3 in (8) are in fairly good agreement with the results of other simulations. See for example
-
Mackrodt W. C., Davey R. J., Black S. N., J. Cryst. Growth 80, 441 (1987);
(
10.1016/0022-0248(87)90093-5) / J. Cryst. Growth by Mackrodt W. C. (1987) - ; P. W. Tasker in Surfaces of Magnesia and Alumina W. D. Kingery Ed. (American Ceramic Society Columbus OH 1984) pp. 176–189;
-
Manassidis I., Gillan M. J., J. Am. Ceram. Soc. 77, 335 (1994);
(
10.1111/j.1151-2916.1994.tb07000.x) / J. Am. Ceram. Soc. by Manassidis I. (1994) - . No other data are available for γ-Al 2 O 3 surfaces.
- At high temperatures the T Δ S term in the free energy may dominate and a transition from coarse α-Al 2 O 3 to γ-Al 2 O 3 can be expected at a high enough temperature. No such transition has been detected up to the melting point of α-Al 2 O 3 2327 K [
-
Schneider S., Pure Appl. Chem. 21, 115 (1970);
(
10.1351/pac197021010115) / Pure Appl. Chem. by Schneider S. (1970) - ] but the liquid structure of alumina has been shown to have γ-Al 2 O 3 character [that is tetrahedrally coordinated Al 3+ see
10.1103/PhysRevLett.78.464- ]. The lowest temperature at which the α-Al 2 O 3 to γ-Al 2 O 3 transition could occur would then be the melting point so the change in Gibbs free energy Δ G of the α-Al 2 O 3 to γ-Al 2 O 3 transition would be zero at 2327 K. Taking the enthalpy of the α-Al 2 O 3 to γ-Al 2 O 3 transition as 13.4 kJ mol −1 (11) Δ S α → γ is 5.7 J K −1 mol −1 .
-
J. M. McHale A. Navrotsky A. J. Perrotta J. Phys. Chem. B 101 603 (1997).
(
10.1021/jp9627584) -
Cerofolini G. F., Surf. Sci. 51, 333 (1975).
(
10.1016/0039-6028(75)90260-5) / Surf. Sci. by Cerofolini G. F. (1975) - A. J. Perrotta et al. unpublished material.
-
Zhou R.-S., Snyder R. L., Acta Crystallogr. B 47, 617 (1991).
(
10.1107/S0108768191002719) / Acta Crystallogr. B by Zhou R.-S. (1991) -
Coster D. J., Fripiat J. J., Muscas M., Auroux A., Langmuir 11, 2615 (1995).
(
10.1021/la00007a047) / Langmuir by Coster D. J. (1995) -
Gervasini A., Auroux A., J. Phys. Chem. 97, 2628 (1993).
(
10.1021/j100113a026) / J. Phys. Chem. by Gervasini A. (1993) - Hysteresis in adsorption isotherms which implies a difference between the heats of adsorption and desorption has been observed [see for example
-
Bailey A., et al., Trans. Faraday Soc. 67, 231 (1971);
(
10.1039/tf9716700231) / Trans. Faraday Soc. by Bailey A. (1971) - ]. However these phenomena are usually associated with porous adsorbates where irreversible changes in the pore structure occur upon adsorption. For the relatively nonporous aluminas considered here the assumption that the heat of adsorption is the negative of the heat of desorption should be valid.
-
A. Navrotsky Phys. Chem. Miner. 2 89 (1977).
(
10.1007/BF00307526) - This value was also proved to be accurate for the physisorbed H 2 O in our previous study (11).
-
Begg B. D., Vance E. R., Nowotny J., J. Am. Ceram. Soc. 77, 3186 (1994).
(
10.1111/j.1151-2916.1994.tb04568.x) / J. Am. Ceram. Soc. by Begg B. D. (1994) -
Holmes H. F., Fuller E. L., Gammage R. B., J. Phys. Chem. 76, 1497 (1972).
(
10.1021/j100654a023) / J. Phys. Chem. by Holmes H. F. (1972) - This work was supported by the Aluminum Company of America and NSF grants DMR 95-00812 and DMR 92-15802. We thank I. Aksay for the use of his BET surface-area–measuring equipment.
@article{McHale_1997, title={Surface Energies and Thermodynamic Phase Stability in Nanocrystalline Aluminas}, volume={277}, ISSN={1095-9203}, url={http://dx.doi.org/10.1126/science.277.5327.788}, DOI={10.1126/science.277.5327.788}, number={5327}, journal={Science}, publisher={American Association for the Advancement of Science (AAAS)}, author={McHale, J. M. and Auroux, A. and Perrotta, A. J. and Navrotsky, A.}, year={1997}, month=aug, pages={788–791} }