Modeling of compression curves of phase change graphite composites using Maxwell and Kelvin models

Abstract

Materials containing phase change and graphite composites are very attractive materials for energy storage and thermal management applications because of their high thermal conductivity and heat storage characteristics. Mostly, these composites are currently used in thermal management of lithium-ion batteries to regulate the battery temperature and protect the battery from undesirable thermal runaway and also it can be used in other thermal energy storage applications. Several samples with expanded graphite impregnated with phase change material composites such as paraffin wax were tested in quasi-static stress–strain compression tests. To provide these composites with flexibility and compressibility, a special silicon polymer added to the phase change material–expanded graphite composite resulted in a new phase change material with expanded graphite and polymer composite; for which several samples of this composite were tested as well. The compression tests were performed using an Instron 3300R floor model universal testing system at a constant platen speed of 52 mm/min. All tests were conducted at room temperature and they were compressed up to failure. All phase change material–expanded graphite composite samples were tested in-plane and through-plane relative to the expanded graphite compaction directions. Both phase change material–expanded graphite composite samples in in-plane and through-plane directions showed distinct and unique mechanical and thermal characteristic responses. Compression stress–strain tests for all samples were modeled using a combined constitutive viscoelastic polymeric foam model equation based on Kelvin and Maxwell models. In this research, the Maxwell–Kelvin viscoelastic model was used to calibrate the compression tests for expanded graphite, phase change material–expanded graphite, and phase change material with expanded graphite and polymer composites. It was found that ductility and viscous characteristics of the model are due to the presence of expanded graphite whereas brittleness characteristics are due to the presence of phase change material. The polymeric foam model equation is a powerful tool for designing new energy storage composites with targeted mechanical and thermal characteristics such as yield strength, Young's modulus, thermal conductivity, and latent heat. From curve fitting of the experimental tests of phase change material–expanded graphite composites with the viscoelastic model, several mechanical properties such as elastic and viscous coefficients were computed.