Polymorphic phase transition and thermal stability in squaric acid (H2C4O4)

Phase transformations in squaric acid (H2C4O4) have been investigated by thermogravimetry and differential scanning calorimetry with different heating rates beta. The mass loss in TG apparently begins at onset temperatures T-di=245 +/- 5 degrees C (beta=5 degrees C min(-1)), 262 +/- 5 degrees C (beta=10 degrees C min(-1)), and 275 +/- 5 degrees C (beta=20 degrees C min(-1)). A polymorphic phase transition was recognized as a weak endothermic peak in DSC around 101 degrees C (T-c(+)). Further heating with beta=10 degrees C min(-1) in DSC revealed deviation of the baseline around 310 degrees C (T-i), and a large unusual exothermic peak around 355 degrees C (T-p), which are interpreted as an onset and a peak temperature of thermal decomposition, respectively. The activation energy of the thermal decomposition was obtained by employing relevant models. Thermal decomposition was recognized as a carbonization process, resulting in amorphous carbon. (C) 2012 Elsevier Ltd. All rights reserved.