Broadly defined synthesis and properties of phase change materials

Phase change materials are currently used in numerous applications such as sensor, memory, detector, etc. due to change in properties when external stimuli are applied. Over the past few decades, nanostructured phase change materials have shown enhanced properties and characteristics compared to the conventional bulk phase change materials. Transition metal chalcogenides are reported to undergo thermally triggered phase transitions, yet there is still significant room for improvements for nanostructured FeTe2. Also, two-dimensional MXenes are reported to have multiple phases due to change in the surface chemistry. This thesis proposes solution phase synthesis and properties of FeTe2 nanostructures with different Te vacancy concentrations and synthesis of nanostructured Ti3C2Tx MXenes. Te nanowires were first synthesized in the reactor with ethylene glycol solvent and Fe precursor was injected to form FeTe2. The morphology changed from flakes to necklace structure as the concentration of iron precursor changed from the stoichiometric ratio to the iron-rich, respectively. These materials were washed and sintered into a nanocomposite disk using spark plasma sintering. To study the properties of the FeTe2 nanocomposite disk, Seebeck coefficient measurement was applied on the nanocomposite disk within certain temperature range. Phase transitions from p-type to n-type conduction were observed at phase transition temperatures. Phase transition temperature changed with sintering time and initial molar ratio between Fe and Te. The longer sintering time and excess Fe injection during solution phase synthesis resulted in higher Te vacancy in FeTe2 and decrease in phase transition temperature. Two disks with different phase transition temperatures were integrated into one disk using spark plasma sintering. I-V characteristic measurement was applied to the integrated disk while heating. As temperature increased, p-n junction was formed as one side of the disk with the lower phase transition temperature changed to n-type and the other side of the disk with the higher phase transition temperature still remained p-type. I-V characteristic measurements were conducted while heating and cooling and reversible switching behavior was observed. MAX bulk disk was synthesized from TiH2, Al, and TiC powders by spark plasma sintering. The disk was ground into fine powders and a 325-mesh sieve was used to ensure the particle sizes to be small for effective etching. To produce MXene, MAX powders were etched with concentrated hydrofluoric acid to remove Al between Ti3C2 layers