Annealing is likely to occur in astrophysical environments:
· At the accretion disk
stage of star formation:
Amorphous silicates (with the
composition and structure inherited from the parent molecular cloud) entering
an accretion disk slowly spiral inwards towards the protostar to form layers
of progressively higher temperatures.
At sufficiently high temperature,
diffusion processes start. This results in a conversion of the amorphous
structure into an ordered lattice structure at different material-dependent
temperature ranges.
· At the AGB stage of
stellar evolution:
In cooling outflows of late-type
red giants having an oxygen-rich elemental composition, silicates condense
in a thermodynamic condensation sequence: e.g. Mg2SiO4
--> MgSiO3 --> Fe2SiO4[Tielens
1998].
At the same time the dust
is subject to the process of annealing. After particle condensation (but
also at the same time), the silicates transform into the crystalline state
- depending on the cooling rate of the outflow compared with the typical
annealing time.
· Approach to understanding annealing processes:
1. Development of some degree
of local order:
Due to thermally enhanced diffusion,
the dust material gradually develops some order in the local arrangement
of the atoms.
Lattice structure changes to
a microcrystalline material.
Atom-size dependent diffusion
leads to chemical fractionation, i.e. impurities may assemble at the surface
or in inclusions.
2. Crystallization
If annealing lasts sufficiently
long even crystallization may start at critical nuclei. Laboratory study
reveils which kind of crystalline minerals forms from an amorphous silicate
produced by laser ablation or pyrolysis.
3. Annealing time t:
Time of first indications for
formation of an ordered structure. We found IR spectroscopy to be very
sensitive to detect the presence of microcrystals. Therefore IR spectroscopy
is used to determine activation energy and the constant w0
:
t -1 = w0 * e(-Ea/kT)
with w0
= 2 * 1013 s-1
... mean vibrational frequency of the SiO4
tetrahedra,
Ea ... activation energy of crystallization
4. Short-range order:
Time tanneal
to form a local ordered structure of size d.
d² = 6 D tanneal,
D = 1/3 * a²
* w0 * e(-Ea/kT)
diffusion constant
Experiments indicated that silicate
smoke annealed at T = 1000 K develops a local order of approx. 1 nm within
one day. This can be seen fom IR spectra.
5. Stellar Outflows
Typical timescale for dust particle
growth 107 s (approx. 115 days).
Typical timescale for changes
in the outflow temperature 108 s (approx.
3.1 years)
As model calculations of circumstellar
shells around M stars proved, condensation of silicates starts at temperatures
in the range of 1000 K; Mg-silicates subjected to this temperature would
develop a short/long-range order of approx. 30 nm!
It can be seen from the calculated
timescales that condensed silicates stay in a sufficient hot (for annealing)
environment and can develop a lokal order. The degree of crystallization
of the silicates annealed in this temperature range has to be determined
by laboratory study [model data from Gail & Sedlmayer
1998a].
Annealing processes are most likely to take place in stellar outflows!
6. Protoplanetary accretion disk
· Model: Solar-type star with accretion rate 10-7 Msun yr-1
In such a model conversion of
the initially amorphous silicates into crystalline silicates starts at
a distance of 1.5 AU at approx. 1000 K.
At 1 AU crystallization is completed.
With progressing crystallization
a drop in opacity of the dust material could take place. This expected
drop in opacity has to be proved experimentally.
[model data from Gail & Sedlmayer 1998b].
Modelling of protoplanetary disks indicates that annealing processes have to be considered in disk dynamics!
Laboratory study:
The annealing experiments are performed in
oxygen atmosphere at 1013 mbar in quarz glass box in a high temperature
oven.
We focuse on the question:
Is there any difference in annealing kinetics
between bulk samples and nanometer-sized samples?
1. Bulk: Thin glassy silicate splats (thickness
0.1 mm) produced by rapid cooling of a melt
2. Micrometre-sized grains: Ground splats
with size-distribution 5 - 10 mm
3. Nanometer-sized grains: Magnesium
silicate smoke produced by laser ablation
4. Amorphous SiO2:
produced by sol-gel technique
Analytical methods: