Verifying the symmetry of ultra-fast scanning calorimeters using liquid crystal secondary temperature standards

The rapid development of fast scanning calorimeters requires new approaches for temperature calibration and symmetry verification at scanning rates above 10 K/s. Liquid crystal materials like 4-cyano-4'-octoxybiphenyl (8OCB), 4'-ethyl-4-(4-propyl-cyclohexyl)-biphenyl (BCH-52), 4-(4-pentyl-cyclohexyl)-benzoic acid-4-propyl-phenyl ester (HP-53), have been recommended by the Gesellschaft fur Thermische Analyse e. V. (GEFTA) as secondary standards for symmetry verification of standard DSC instruments. However, fast cooling by chip calorimeter creates metastable states that show, during subsequent heating scans, multiple peaks due to structural reorganization. After structural reorganization by annealing at appropriate temperatures only one peak is observed. We checked the rate dependence of the liquid crystal phase transitions on heating and cooling up to 20,000 K/s. The rate dependence of the nematic to isotropic (NI) transition of 8OCB is continuous between heating and cooling and quite symmetric. So we suggest that 8OCB is qualified as a material for symmetry verification of fast scanning chip calorimeters. (C) 2011 Elsevier B.V. All rights reserved.