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Research Papers

Lifetime of Imidazolium Salts at Elevated Temperatures

[+] Author and Article Information
Daniel M. Blake

 NREL, 1617 Cole Boulevard, Golden, CO 80401Dan_blake@nrel.gov

Luc Moens, Daniel Rudnicki, Heidi Pilath

 NREL, 1617 Cole Boulevard, Golden, CO 80401

J. Sol. Energy Eng 128(1), 54-57 (Mar 08, 2005) (4 pages) doi:10.1115/1.2148976 History: Received June 18, 2004; Revised March 08, 2005

This report summarizes progress to date on the thermal stability of imidazolium salts being considered for application as heat transfer and thermal storage fluids in solar parabolic trough power systems. Imidazolium salts are a subset of the general class of molten salts. They are termed ionic liquids because many have freezing points at or below room temperature. This class of salts was selected for initial study because there were many examples that were reported to be stable at high temperatures. These reports were usually based on the results of standard thermal gravimetric analysis (TGA) methods. Work by our subcontractor at the University of Alabama and at NREL showed that slow heating rates or when the temperature is held constant for long times resulted in decomposition temperatures that are much lower than those found with the usual TGA methods. We have used a TGA technique that allows calculation of the rates of thermal decomposition as a function of temperature. The results lead us to the conclusion that the imidazolium salts known to be the most thermally stable would not have useful lifetimes above about 200°C. At present this determination is based on the rough approximation that the fluid in a solar trough system experiences a constant, high temperature. Better estimates of the useful lifetime will require a system model that takes into account the time at temperature distribution of a fluid moving through the different components in a solar plant.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Influence of anion on the thermal stability of imidazolium salts (DMIM=1,3-dimethylimidazolium; BMIM=1-butyl-3-methylimidazolium)

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Figure 2

Influence of cation structure on the thermal stability of imidazolium salts of the hexafluorophosphate anion (PMIM=1,2,3,4,5-pentamethylimidazolium; TMIM=1,3,4,5-tetramethylimidazolium)

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Figure 3

Influence of trifluoromethylsulfonate (Otf) versus methylsulfonate (Oms) as the anion on thermal stability of the ethylmethylimidazolium salt

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Figure 4

Percent of original weight remaining after 120min holding time at 200, 300, and 375°C compared to the standard TGA curve (Heating rate 20°C∕min) for [BMIM][NTf2]

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Figure 5

Dependence of the fraction of decomposition of BMIM PF6 on the log of the heating rate

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Figure 6

Half-life of BMIM PF6 as a function of temperature

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