5:15 PM - 6:45 PM
[PPS08-P10] Experimental study of metalic iron whisker formation on airless bodies surfaces
Keywords:Metalic iron whisker , Regolith, thermal cycling
Introduction: Materials exposed to space gradually change their physical and chemical characteristics. This process is known as space weathering [e.g.1]. Studies of space weathering have long focused on silicate minerals. However, Metal iron whiskers have been reported on troilite surfaces in Itokawa, Ryugu, and lunar regolith particles [2,3,4]. Three possible formation processes for these whiskers have been suggested: ion irradiation, impact of micrometeorites, and thermal cycling. Ion irradiation and impact of micrometeorites have been simulated for iron sulfide [5,6]. In simulations of impact of micrometeorites, they succeeded in making the whiskers which is enriched Fe, Ni and depleted S, but the sample is completely melted, which is significantly different from the actual state. No simulation experiments have been conducted using thermal cycling. In the field of materials science, the driving force for whisker growth from solids is thought to be primarily rapid diffusion of elements due to stress [e.g.7]. Tin whiskers with similar characteristics reported on Itokawa etc. have formed only in thermal cycling experiments.
In this study, we attempt to reproduce metallic iron whiskers using pyrrhotite as a starting material by means of thermal cycling. Since there is no difference in the mechanism of whisker formation between vacuum and ambient pressure [8], the first we conduct whisker formation experiments under a nitrogen gas.
Methods: The samples were natural pyrrhotite grains from Chihuahua, Mexico. We placed the sample on a self-made platinum stage. The Limkam cooling-heating stage was performed under nitrogen gas for simulating the thermal cycling on the surface of the body at the Kyoto university. The stage was programmed to heat the sample to 600 °C and then cool it to -190 °C, and heating, cooling rate was 10 ~ 150 °C/m. The cycling time was programmed 15 m ~ 7.5h. The number of thermal cycles ranged from 1 ~ 100. G1(purity: >99.99995 vol%) and G3(purity: >99.9 vol%) of nitrogen gas were used. Gas flow was set up 0.4L/m ~ 10L/m. We recorded videos of the grains during the thermal experiments. We imaged the samples before and after cycling, and analyzed chemical composition maps by 5keV field emission-scanning electron microscope (FE-SEM, JEOL JSM-7001F) and energy dispersive X-ray spectrometer (EDS, Oxford instruments, X-MAXN 150) at the Kyoto university.
Results: Iron oxide whiskers formed at 0.4 L/min gas flow regardless of cooling, heating rate and cycling time, the number of cycles, gas grade. The compositions of the whiskers were Fe and O rich and S depleted. The samples surfaces were completely oxidized and the surface microstructure was rough in all of the experiments under conditions other than G3 grade, 10 L/min gas flow. Under G3 grade, 10 L/min gas flow, the sample surfaces were partially unoxidized and whiskers were not formed. In unoxidized part, the ratio of Fe to S changed little before and after experiments.
Discussion: Evaporation experiments on troilite have been conducted between 800°C and 970°C, and the release of S has been observed [9]. Although the release of S is also suggested to occur at temperatures below 800°C, the amount of S released and the time required for release are significantly higher [9]. There are two possible explanations for the results of pyrrhotite transforming iron oxide: 1. Oxidation of Fe exposed on surface after S departs from sample due to thermal cycle [Fe1-xS (s) ⇒ Fe (s) + S (g), Fe (s) + O2 (g) ⇒ Fe-oxide (s)], 2. Simultaneous departure of S (or S-O compounds) from sample and formation of iron oxide due to the reaction of pyrrhotite with residual oxygen [Fe1-xS (s) + O2 (g) ⇒ Fe-oxide (s) + S (or S-O compounds) (g) ].
References: [1] Hapke B.(2001) [2] Matsumoto et al. (2020) [3] Matsumoto et al. (2021) [4] Matsumoto et al. (2023) [5] Christoph et al. (2022) [6] Thompson et al. (2023) [7] J. D. Eshelby (1953) [8] Suganuma et al. (2011) [9] Tachibana et al. (1998)
In this study, we attempt to reproduce metallic iron whiskers using pyrrhotite as a starting material by means of thermal cycling. Since there is no difference in the mechanism of whisker formation between vacuum and ambient pressure [8], the first we conduct whisker formation experiments under a nitrogen gas.
Methods: The samples were natural pyrrhotite grains from Chihuahua, Mexico. We placed the sample on a self-made platinum stage. The Limkam cooling-heating stage was performed under nitrogen gas for simulating the thermal cycling on the surface of the body at the Kyoto university. The stage was programmed to heat the sample to 600 °C and then cool it to -190 °C, and heating, cooling rate was 10 ~ 150 °C/m. The cycling time was programmed 15 m ~ 7.5h. The number of thermal cycles ranged from 1 ~ 100. G1(purity: >99.99995 vol%) and G3(purity: >99.9 vol%) of nitrogen gas were used. Gas flow was set up 0.4L/m ~ 10L/m. We recorded videos of the grains during the thermal experiments. We imaged the samples before and after cycling, and analyzed chemical composition maps by 5keV field emission-scanning electron microscope (FE-SEM, JEOL JSM-7001F) and energy dispersive X-ray spectrometer (EDS, Oxford instruments, X-MAXN 150) at the Kyoto university.
Results: Iron oxide whiskers formed at 0.4 L/min gas flow regardless of cooling, heating rate and cycling time, the number of cycles, gas grade. The compositions of the whiskers were Fe and O rich and S depleted. The samples surfaces were completely oxidized and the surface microstructure was rough in all of the experiments under conditions other than G3 grade, 10 L/min gas flow. Under G3 grade, 10 L/min gas flow, the sample surfaces were partially unoxidized and whiskers were not formed. In unoxidized part, the ratio of Fe to S changed little before and after experiments.
Discussion: Evaporation experiments on troilite have been conducted between 800°C and 970°C, and the release of S has been observed [9]. Although the release of S is also suggested to occur at temperatures below 800°C, the amount of S released and the time required for release are significantly higher [9]. There are two possible explanations for the results of pyrrhotite transforming iron oxide: 1. Oxidation of Fe exposed on surface after S departs from sample due to thermal cycle [Fe1-xS (s) ⇒ Fe (s) + S (g), Fe (s) + O2 (g) ⇒ Fe-oxide (s)], 2. Simultaneous departure of S (or S-O compounds) from sample and formation of iron oxide due to the reaction of pyrrhotite with residual oxygen [Fe1-xS (s) + O2 (g) ⇒ Fe-oxide (s) + S (or S-O compounds) (g) ].
References: [1] Hapke B.(2001) [2] Matsumoto et al. (2020) [3] Matsumoto et al. (2021) [4] Matsumoto et al. (2023) [5] Christoph et al. (2022) [6] Thompson et al. (2023) [7] J. D. Eshelby (1953) [8] Suganuma et al. (2011) [9] Tachibana et al. (1998)