Low-temperature crystallization of thin silicate layer on crystalline Fe dust
© The Society of Geomagnetism and Earth, Planetary and Space Sciences, The Seismological Society of Japan 2010
Received: 31 July 2008
Accepted: 6 October 2008
Published: 7 February 2015
The crystallization of an amorphous SiO layer covering Fe crystal grains has been clarified by high-resolution transmission electron microscopic (HRTEM) observation. Cristobalite crystals were produced preferentially on the (110) surface of Fe particles by the oxidation of silicon crystallites in the SiO layer, i.e. the oxidation energy of the silicon crystallites resulted in the epitaxial growth of the oxide layer on the Fe surface. The chemical reaction energy due to the oxidation of silicon crystallites in the SiO layer was concentrated at the interface of the crystal and the amorphous layer. Crystal growth took place from the Fe grain surface.
In a previous study (Kaito and Shimizu, 1984), a vacuumdeposited SiO film was shown to be composed of Si and SiO2 (gα-cristobalite) crystallites about 1 nm in diameter. By heating the film in air at various temperatures up to 500°C, the film changed from orange to being transparent while maintaining an amorphous structure with a halo diffraction pattern. Intensity analyses of the electron diffraction (ED) pattern and infrared sopectroscopy were carried out to evaluate the structure alteration (Morioka et al., 1998). The mixed film of Si and SiO2 altered from gα-cristobalite to ß-cristobalite (250°C) and ß-quartz (500°C) accompanied by the oxidation of Si crystallites in air. Infrared spectroscopy was carried out to evaluate the structure alteration (Morioka et al., 1998). In this paper, Fe crystal grains covered with SiO film were heated at 100°C in the stable region for gα-cristobalites in air. The alteration of the SiO film was detected by HRTEM and is discussed as a low-temperature crystallization effect due to the oxidation energy of silicon crystallites in the SiO film.
Fe wire (0.5-mmφ) was wound on the tungsten wire with a 0.5 mmφ filament, which had been preheated in a vacuum of 10-5 Torr. The chamber was a glass cylinder (17 cm diameter and 30 cm height) covered with stainless steel on top and connected to a high-vacuum exhaust through a valve at its bottom. After evacuation of the chamber, the iron particles were produced by evaporation in Ar gas at 80 Torr. The particles were collected on a glass plate 10 cm above the evaporation source. Immediately after the preparation of iron grains, the chamber was evacuated to 10-6 Torr, and SiO powder was evaporated from a W boat without exposure to air. The Fe particles covered with a SiO layer were removed from the vaccum system. The specimen was placed on the standard transmission electron microscope (TEM) grid and observed using an H-9000NAR TEM. The observation points were determined by placing the specimen on an electron microscopic holder, and the specimen was then heated in a furnace at 100°C in air. The alteration of the specimen was observed and analyzed at the same specimen position throughout the study.
Since the mean free path of phonons in the crystal is of micrometer order, and the mean free path in the amorphous SiO layer is a few nanometers, i.e., the crystallite size, the oxidation energy of silicon crystallites in the SiO layer diffuses to the iron crystal grain. Therefore, the thermal energy due to the oxidation accumulates at the iron grain interface. This energy causes the crystallization of the SiO layer to SiO2 due to the oxidation energy of the Si crystallites and diffusion reaction with air. The parallel relation at the interface between Fe and SiO2 occurred due to a minimum in interface energy. This result supports the theory of lowtemperature crystallization by Yamamoto et al. (2010). The oxidation energy of Si crystallites drives the crystallization of the SiO2 layer.
Simple silicates evaporate to form SiO, the Mg atom, the Fe atom, and O2 vapor and solid SiO, MgO, and FeO (Nuth et al., 1999). Iron exists largely as metallic species in the mixture film of Fe and SiO (Suzuki et al., 2000). If the iron grain is exposed in air, the grain surface is covered with magnetite phase and oxidation proceeds by the migration of iron atoms (Kaito et al., 1973). The reaction between Fe and SiO takes place above 1100 K (Hallenbeck et al., 1998). The crystalline SiO2 growth on the iron grain surface in O2 atmosphere results in low-temperature crystallization during the comet-formation process or commentary silicate crystallization as well as the previous experimental demonstration on the possibility of chemical-reaction-driven crystallization for the Mg-bearing silicate grain (Kaito et al., 2007). The migration of iron atoms to the SiO layer hardly occurred at low temperature.
The authors thank Joseph A. Nuth III of NASA/GSFC and the reviewers for valuable comments. This research was supported in part by the Ministry of Education, Science, Sports and Culture of Japan, Grant-in Aid for Scientific Research on Priority Areas, Development of Extra-Solar Planetary Science. This work was also supported in part by Ritsumeikan University Grant-in-Aid of Internationalization.
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