PDF《INTRODUCTION》

Khare Rendon, &

Iglesias 1976;

Serna, 1982;

There are also measurements of other modi- Gervais 1991). ?cations of crystalline alumina, e.g., an arti?cial c-Al

2 O

3 , product with cubic symmetry et al. and of (Koike 1995) amorphous alumina et al. et al. (Eriksson 1981;

Chu 1988). All of these alumina samples exhibit bands coinciding nearly but not exactly with the peak position of the observed

13 km band. The widths of the bands vary over a wide range. As for crystalline grains, one has to keep in mind that the calculated band position and pro?le is con- siderably in?uenced by surface modes. From condensation theory, it can be expected that imme- diately after their condensation, alumina grains should be amorphous rather than crystalline The (Sedlmayr 1989). data of amorphous alumina available in the literature are not sufficient for an adequate description of the optical properties of such grains. The amorphous by c-Al

2 O

3 et al. is not representative for amorphous Eriksson (1981) alumina. The data set provided by et al. su?ers Chu (1988) from the weakness of the absorption signal measured of the ?lm on a sapphire substrate. Therefore, we feel that measuring representative optical data of amorphous is very important for a general Al

2 O

3 discussion of the possible contribution of alumina to the IR opacity in the circumstellar envelopes around oxygen-rich late-type stars, the understanding of the origin of the

13 km band, and the solid-state structure of condensed circumstel- lar grains. Al

2 O

3 In this paper, which continues our e?orts on oxides of possible astrophysical signi?cance (see et al. Henning 1995), we wish to discuss the optical properties of amorphous and crystalline alumina particles and their relevance to the iden- ti?cation of the

13 km band. describes the prep- Section

2 aration of amorphous alumina particles by sol-gel reactions, the phase transitions to crystalline modi?cations, and the derivation of the optical constants of amorphous alumina. presents the extraction of the pro?le of Section

3 the

13 km band from IRAS LRS spectra, the comparison of all optical data available of with this pro?le, and Al

2 O

3 alternative suggestions for the carrier of the

13 km band. 2. OPTICAL PROPERTIES OF AMORPHOUS ALUMINA 2.1. Preparation and Analytics The techniques of melting and quenching normally used in our laboratory for preparing amorphous refractory oxides (see, e.g., et al. fails in the case of Begemann 1995) aluminum oxide. Therefore, we applied the sol-gel tech- nique commonly used in the material sciences for producing homogeneous amorphous solids (for details see, e.g., &

Scherer This technique, which is totally Brinker 1990). di?erent from the condensation process of in astro- Al

2 O

3 physical environments, is exclusively used as an experimen- tal trick to realize the totally amorphous structure. Since the astrophysical processes leading to the formation of solids are hardly understood in their details, in many cases the prepara........