Browsing by Subject "phase transformation"

Now showing 1 - 2 of 2
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    First principles study of structural and thermodynamic properties of zirconia
    (2014) Zhang, Yi; Zhang, Jing
    Due to their high melting temperature and low thermal conductivity, zirconia (ZrO2) based ceramics have been widely used for thermal barrier coating materials. This study investigates zirconia's properties using the first principles calculations. Structural properties, including band structure, density of state, lattice parameter, as well as elastic constants for both monoclinic and tetragonal zirconia were computed. Pressure based phase transition of tetragonal zirconia (t-ZrO2) was also calculated, based on tetragonal distortion and band structure under compressive pressures. The results predicted a transition from monoclinic structure to a fluorite-type cubic structure at the pressure of 37 GPa. Moreover, monoclinic zirconia (m-ZrO2) thermodynamic property calculations were carried out. Temperature-dependent heat capacity, entropy, free energy, and the Debye temperature of monoclinic zirconia, from 0 to 1000 K, were computed and compared well with those reported in literature.
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    High-temperature stability and phase transformations of titanium carbide (Ti3C2Tx) MXene
    (IOP, 2021-06) Wyatt, Brian C.; Nemani, Srinivasa Kartik; Desai, Krishay; Kaur, Harpreet; Zhang, Bowen; Anasori, Babak; Mechanical and Energy Engineering, School of Engineering and Technology
    Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, are under increasing pressure to meet technological demands in high-temperature applications, as MXenes can be considered to be one of the few ultra-high temperature 2D materials. Although there are studies on the stability of their surface functionalities, there is currently a gap in the fundamental understanding of their phase stability and transformation of MXenes' metal carbide core at high temperatures (>700 °C) in an inert environment. In this study, we conduct systematic annealing of Ti3C2TxMXene films in which we present the 2D MXene flake phase transformation to ordered vacancy superstructure of a bulk three-dimensional (3D) Ti2C and TiCycrystals at 700 °C ⩽T⩽ 1000 °C with subsequent transformation to disordered carbon vacancy cubic TiCyat higher temperatures (T> 1000 °C). We annealed Ti3C2TxMXene films made from the delaminated MXene single-flakes as well as the multi-layer MXene clay in a controlled environment through the use ofin situhot stage x-ray diffraction (XRD) paired with a 2D detector (XRD2) up to 1000 °C andex situannealing in a tube furnace and spark plasma sintering up to 1500 °C. Our XRD2analysis paired with cross-sectional scanning electron microscope imaging indicated the resulting nano-sized lamellar and micron-sized cubic grain morphology of the 3D crystals depend on the starting Ti3C2Txform. While annealing the multi-layer clay Ti3C2TxMXene creates TiCygrains with cubic and irregular morphology, the grains of 3D Ti2C and TiCyformed by annealing Ti3C2TxMXene single-flake films keep MXenes' lamellar morphology. The ultrathin lamellar nature of the 3D grains formed at temperatures >1000 °C can pave way for applications of MXenes as a stable carbide material 2D additive for high-temperature applications.