WEB Links between electron diffraction behaviour of phase change materials embedded in carbon nanotubes and in situ and ex situ thermal behaviourTuesday (22.09.2020) 15:55 - 16:10 C: Characterization 1 Part of:
Phase change materials exhibit the ability to undergo a reversible change between a crystalline and amorphous phase. Extreme confinement of such materials is achieved by enclosing them in single walled carbon nanotubes (SWCNTs) to produce ‘nano-confined’ PCMs or nC-PCMs. These structures have been identified as excellent vessels for the investigations of reduced dimensionality behaviour as their atomically thin structures generate internal Van der Waals surfaces, resulting in a precisely regulated growth of the encapsulated materials. Previous studies  have detailed the in situ beam amorphization of Sb2Te3 within SWCNTs, resulting in an observed in situ glass transition without any significant effects on the structural integrity of the SWCNT by using low accelerating voltage (i.e.< 86 kV) and electron beam dosages ranging from 0.8 to 1.5 pA/cm2. For this work high purity NanoIntegris (NI) SWCNTs were used with a diameter range of 1.3-1.7nm. These NanoIntegris nanotubes are produced in a ‘bucky paper’ state and therefore require predispersion for filling due to local adhesion. Gentle oxidation opens the nanotubes for filling via removal of the pentagonal lattice structures on the ends of the tubes. [2, 3] The Sb2Te3 filled NI sample was prepared via capillary melt filling which involved intimately grinding equal volume Sb2Te3 with a quantity of pre-treated SWCNTs and heating the mixture up to 675℃. The filled sample was characterised using thermoanalytical techniques which identified significant differences in the phase change behaviour of the confined material compared to its bulk form. DSC and TGA data obtained from the filled NI sample was used to inform the temperature regimen for in situ heating of the sample and the temperature used for ex situ thermal annealing of the sample to remove extraneous material which forms on the outer surface of the tubes. Electron diffraction (ED) studies of the sample illustrate the reversible transition of the filling material from a crystalline to an amorphous phase and then the recrystallisation in the cooling cycle. The innermost reflections correspond to the phase of the filling material which transitions from clearly defined reflections at room temperature to a diffuse ring-like pattern at 400℃, which suggests that the material has become glassy as a result of heating. The formation of the original reflections once the material is cooled back to room temperature indicate that this is a reversible process.