Kinetic and Thermodynamic Effects of Dopants (Mn/La) on the Microstructure of Yttria-stabilized Zirconia (YSZ) PDF Download

Author: Hui Li
Publisher:
ISBN: 9780438629882
Category :
Languages : en
Pages :

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Book Description
Nanocrystalline materials have been attracting wide attention because of their advanced technological application owing to their superior properties such as enhanced mechanical strength and ionic conductivity. The intrinsic presence of excess interfacial areas in nanomaterials causes their properties and functionalities to be strongly dependent on interfacial properties. From a thermodynamics perspective, this suggests both crystallographic stability and microstructural evolution are largely affected by interfacial energies, implying those energies can be used for microstructural control. However, there is still a dearth of understanding on the relationships between interfacial energies and processes such as sintering and coarsening of nanocrystalline ceramics, in particular in what concerns the usage of ionic dopants as sintering aids working on the modification of interfacial energies rather than from the classical kinetic approach (dopants affecting mechanisms of sintering). In this dissertation, nanocrystalline ceramic yttria stabilized zirconia (YSZ) was used as a model system to concomitantly evaluate the quantitative effect of dopants on both kinetics and thermodynamics of densification in order to provide a more comprehensive picture to allow refined control of sintering evolution. Doped and pure YSZ nanoparticles were synthesized using co-precipitation method and used as the starting powders for sintering. By using sophisticated calorimetric approaches, the interfacial energies of the nanoparticles were quantified, while the kinetic analysis was carried out using Kissinger method applied with sintering experiment in differential scanning calorimetry (DSC) with different heating rates. Manganese and lanthanum were the studied dopants, selected based on potential for segregation to interfaces, which would hypothetically cause a more pronounced interfacial energy change. Remarkable decrease in both surface and grain boundary energies, as well as an increase in the calculated dihedral angles, was observed with increasing Mn concentration in YSZ. Mn also caused a decrease in activation energy of densification of this system that could achieve high densities at moderate temperatures albeit with massive grain growth. La-doping, on the other hand, whilst decreasing interfacial energies and increasing dihedral angles in a similar manner as Mn, did not affect the activation energy of the process. Inhibition of both densification and grain growth was observed, implying that the design of favorable thermodynamic states is a necessary but not sufficient condition for densification to take place. Utilizing similar methodologies, studies proceeded on magnesium aluminate spinel with manganese and titanium as dopants to test if the concepts developed for YSZ could be extended to other systems. The results from analyzing Mn-doped spinel were perfectly aligned with the conclusions from the YSZ studies, such that a favorable thermodynamics and kinetics helped improve densification of the spinel. However, Ti-doped spinel showed significant deviations from the predicted behavior, as it had favorable thermodynamic and kinetic conditions but high densification could not be obtained during sintering. This anomalous behavior was attributed to abnormal grain growth that hindered pore elimination due to likely an energetic anisotropy in the grain boundaries.