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

• Published in: Thermochimica acta Vol. 526; no. 1; pp. 58 - 64
• Main Authors: Chen, Minzhi, Du, Muting, Jiang, Jing, Li, Dawei, Jiang, Wei, Zhuravlev, Evgeny, Zhou, Dongshan, Schick, Christoph, Xue, Gi
• Format: Journal Article
• Language: English; Dutch
• Published: Oxford Elsevier B.V 10.11.2011; Elsevier
• Subjects: Analytical chemistry; annealing; benzoic acid; Calibration; calorimeters; Chemical and thermal methods; Chemistry; Chips; cooling; Cooling rate; Esters; Exact sciences and technology; Fast scanning calorimetry; heat; Heating; Liquid crystal standard; Liquid crystals; phase transition; Scanning; Symmetry; temperature; Calibration; Fast scanning calorimetry; Symmetry; Liquid crystal standard; Scanning calorimetry; Calorimeter; Biphenyl derivatives; Thermal analysis; Liquid crystals; Ultrafast process
• ISSN: 0040-6031; 1872-762X
• DOI: 10.1016/j.tca.2011.08.020

► Phase transitions of 8OCB, HP53 and BCH52 were investigated by chip calorimeters. ► Phase transitions critically depend on the stability of the crystalline state. ► Peak temperature of the N–I transition of 8OCB is continuous and symmetric. ► Suggest 8OCB as a candidate for the symmetry verification of chip calorimeters. 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 für 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.