Growth, structural, crystallisation, thermal decomposition and dielectric behaviour of melaminium bis(hydrogen oxalate) single crystal

摘要

Single crystals of melaminium bis (hydrogen oxalate) (MOX) single crystals have been grown from aqueous solution by slow solvent evaporation method at room temperature. X-ray powder diffraction analysis confirms that MOX crystallises in monoclinic system with space group C2/c. The calculated lattice parameters are a = 20.075 ± 0.123 ? b = 8.477 ± 0.045 ? , c = 6.983 ± 0.015 ? , a = 90o, β = 102.6 ± 0.33o, γ = 90o and V = 1,159.73 (A°)~3. Thermogravimetric analysis at three different heating rates 10, 15 and 20 ℃ min~(-1) has been done to study the thermal decomposition behaviour of the crystal. Non-isothermal studies on MOX reveal that the decomposition occurs in two stages. Kinetic parameters [effective activation energy (E_a), pre-exponential factor (ln A)] of each stage were calculated by model-free method: Kissinger, Kim-Park and Flynn-Wall method and the results are discussed. A significant variation in effective activation energy (E_a) with conversion progress (α) indicates that the process is kinetically complex. The linear relationship between the ln A and E_a was established (compensation effect). DTA analyses were conducted at different heating rates and the activation energy was determined graphically from Kissinger and Ozawa equation. The average effective activation energy is calculated as 276 kJ mol~(-1) for the crystallization peak. The Avrami exponent for the crystallization peak temperature determined by Augis and Bennett method is found to be 1.95. This result indicates that the surface crystallization dominates overall crystallization. Dielectric study has also been done, and it is found that both dielectric constant and dielectric loss decreases with increase in frequency and is almost a constant at high frequency region.