A comparison of microwave and convential crystallization characteristics in the albite-anorthite system

The melting and crystallization characteristics of three different compositions in the albite-anorthite system were studied in a conventional thermal field and a microwave field by using (1) a polarized light optical microscope test with refractive index oil immersion mounting, (2) x-ray diffraction and (3) electron microprobe analysis techniques.

The three different mixtures of albite (Ab) and anorthite (An) were melted and then held at a constant temperature (1200°C) for varying lengths of time in a 2.45 GHz electromagnetic field. The same mixtures of albite and anorthite were held at 1200°C for 64 hours and 128 hours in a conventional furnace. All heated charges were air quenched after a desired duration time at 1200°C.

All three conventionally heated compositions of albite and anorthite showed the presence of the crystalline phase. Only a trace of crystalline phase was found in 90 wt% Ab-10 wt% An at 1200°C for 64 hours in a conventional furnace. The amount of crystalline phase was found to increase as the anorthite content was increased. The mixture of 72 wt% Ab28 wt% An held at 1200°C for 128 hours in a conventional furnace had the largest amount of crystalline phase.

For the samples heated in a 2.45 GHz electromagnetic field, all compositions heated for 1 hour to 64 hours and the 90 wt% Ab-10 wt% An heated for 128 hours showed no trace of crystalline phase. The compositions of 81 wt% Ab-19 wt% An and 72 wt% Ab-28 wt% An showed some crystalline phase, but the amount of crystalline phase was much less than the samples processed in a conventional furnace.

The above results showed that a 2.45 GHz electromagnetic field affected the nucleation of plagioclase crystal in its two phase region. This effect results from the different dielectric constants of the crystalline phase and the liquidus phase. Electromagnetic field theory predicts that more power will be deposited into the lower dielectric constant region than into the higher dielectric constant region. This phenomenon results in the inhibition of the nucleation of the crystalline phase in the crystal-liquid two phase region in the albite-anorthite phase diagram.

The results of this work suggest that all ceramic phase diagrams may be different when heated using 2.45 GHz electromagnetic radiation.