Combustion Methods for Measuring Low Levels of Carbon in Nickel, Copper, Silver, and Gold
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I.
INTRODUCTION
ANALYSES for low-level impurities in metals are important in both fundamental and applied metallurgy. Throughout history, carbon was always the default impurity, as it is often present in furnaces, as crucibles, or as a reactive carbon monoxide gas. Carbon in metals affect various physical properties, including melting, conductivity, and mechanical properties. But carbon solubility in Cu, Ag, and Au were long held to be low, or insignificant. Thus, melting points of Cu, Ag, and Au are routinely measured in carbon crucibles; a set-up that in recent decades has become a standard for high-precision temperature calibration.[1] Our recent survey of literature and some preliminary physical tests have shown that carbon solubility may not be as low as thought, and carbon may affect melting points similarly to oxygen.[2,3] Moreover, Au, Ag, and Cu have all been shown to be much more active (kinetically) with carbon[4,5] than was previously believed. The similar wetting contact angle[6] for Au, Ag, Cu on graphite substrate (138, 136, and 140 deg, respectively) is indicative of similar behavior toward carbon. Despite the widespread use of carbon crucibles for melting of gold, a major manufacturer of differential thermal analysis (DTA) equipment recommends against the use of carbon for gold.[7] We have seen that melting temperatures may vary significantly with the metal’s
NATHAN S. JACOBSON, Research Physicist, is with the NASA Glenn Research Center, Cleveland, OH 44135. Contact e-mail: [email protected] KAYVON SAVADKOUEI and CHRISTOPHE MORIN, Applications Scientists, are with Horiba Scientific, Edison, NJ 08820. JO FENSTAD, Consultant, is with FlexiFrame AS, Falkenborgveien, 7044 Trondheim, Norway. EVAN H. COPLAND, Research Scientist, is with CSIRO, Melbourne, Australia. Manuscript submitted January 4, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
‘‘thermal history’’ with the carbon crucible and—visually striking—that solidification of gold in a carbon crucible deviates markedly from the solidification of gold in an alumina crucible treated with oxygen gas (Figure 1). The tentative phase diagram for the gold-carbon system[8] indicates some carbon solubility and a eutectic. These observations suggest the need for further study of carbon solubility in gold. There are four steps to determine carbon solubility 1. 2. 3. 4.
Provide metal and carbon of necessary purity. Equilibrate metal and carbon at high temperature. Quench samples. Quantitative analysis for carbon in each sample.
Each of these comes with potential errors, but we found the step 4 to be especially critical. For common metals like steel and nickel, the universally approved method is combustion, whereby where the metal is heated in an oxidizing environment and all the carbon is converted to CO(g) and/or CO2(g). The quantities of these gases are measured with infrared (IR) spectroscopy and converted back to a quantity of carbon. Often an ‘‘accelerator’’ is added to promote heat transfer, melting, and complete oxidation and detection of the carb
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