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Carbothermal reduction of oxidized tin and recovery of deep supercool

Joung, W; Pearce, J; Park, J (2018) Carbothermal reduction of oxidized tin and recovery of deep supercool. Metrologia, 55 (6). pp. 819-827. ISSN 0026-1394 (In Press)

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Abstract

Deep supercooling of tin is a characteristic sign of a high-purity sample having fewer nucleation sites. However, due to the risk of oxidation during the cell fabrication (especially during the initial sample filling process), oxidized tin can be present in the sample, which manifests itself as a nucleation agent and significantly affects the realization of the freezing temperature of tin. In this work, the effect of the oxidation of high-purity tin (nominally 99.9999 % pure) and an effective means of the reducing the oxidized sample were investigated. We fabricated an open-type tin freezing point cell using 1.15 kg of high purity tin shot, which was poured into the crucible in air, giving rise to a high risk of oxidation. The oxidized tin sample showed abnormally small supercools of about 0.1 °C which were caused by additional nucleation sites introduced by the oxidation. This is a problem because if the supercool is too shallow it can be difficult to establish a good quality freezing plateau. The oxidized sample also exhibited a melting temperature inversion over the entire range of the melting with a temperature depression of about 1.9 mK. The oxidized sample was carbothermally reduced by melting the sample in its graphite crucible and holding at a temperature of 750 °C for a total duration of 365 hours. The carbothermal reduction of the oxidized sample caused the satisfactory recovery of the deep supercool, and the recovered supercool was about 10.6 °C after about 100 hours of reduction, and 10.7 °C after 365 hours of reduction. At the end of the reduction process, the melting curve inversion was significantly alleviated, having 0.1 mK temperature depression over less than 10 % of the melting range. Importantly, the carbothermal reduction at 750 °C was found to be an effective means of removing the oxide nucleation sites and of recovering the characteristic deep supercool of tin.

Item Type: Article
Keywords: Tin; Fixed-point cell; Oxidation; Carbothermal reduction; Supercool; Melting temperature inversion
Subjects: Engineering Measurements > Thermal
Divisions: Engineering, Materials & Electrical Science
Identification number/DOI: 10.1088/1681-7575/aae494
Last Modified: 15 Nov 2018 15:01
URI: http://eprintspublications.npl.co.uk/id/eprint/8151

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