Investigation of decalibrated Nicrosil versus Nisil thermocouples: Quantitative sims analysis using indexed sensitivity factors and oxygen flooding

• Published in: Applications of surface science 1984-01, Vol.18 (3), p.287-298
• Main Authors: Christie, W.H., Eby, R.E., Anderson, R.L., Kollie, T.G.
• Format: Article
• Language: English
• Subjects: Analysing. Testing. Standards; Analytical chemistry; Applied sciences; Chemistry; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Materials testing; Metals, semimetals and alloys; Metals. Metallurgy; Physics; Specific materials; Spectrometric and optical methods
• ISSN: 0378-5963
• DOI: 10.1016/0378-5963(84)90316-7

Secondary Ion Mass Spectrometry was used to determine the failure mode of sheathed Nicrosil versus Nisil thermocouples exposed to temperatures above 1000°C. Four thermocouples, two sheathed in Inconel-600 and two sheathed in type 304 stainless steel, were studied. Relative sensitivity factors, indexed by a matrix ion species ratio, were used to quantify SIMS data for the alloys studied. Oxygen pressure ⩾ 2.7 × 10 -4 Pa in the sputtering region gave enhanced sensitivity and superior quantitative results compared to data obtained at instrument residual pressure. At sufficiently high oxygen surface coverage, the slope of the nonlinear sensitivity factor curves for the Nicrosil and Nisil alloys approached zero. Quantified SIMS data showed that these thermocouples decalibrated because significant alterations in the elemental composition of the Nicrosil and Nisil thermoelements occurred. The extent of the observed alteration was different for each thermocouple, for a given time and temperature of exposures, and was influenced by the particular sheath material used in the thermocouple construction.