Open topics currently exist in Kaplan’s group for MSc and PhD students. Below is a brief description of each open topic. If you are interested in receiving more information, please contact Prof. Kaplan by e-mail or by telephone (04-829-4580).
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Graduate Research Topic: Experimental Measurement of Grain Boundary Structure in Metastable vs. Steady States
One research topic currently available for a graduate student involves the experimental measurement of the two-dimensional structure of a grain boundary in its metastable versus steady state. During sintering, grain boundaries initially form when adjacent particles join at a region commonly referred to as the “neck.” This region grows during the densification process, which defines sintering.
Typically, the grain boundary structure and chemistry we characterize are those present at the end of the sintering process. However, there is reason to believe that the structure and even the volume of the grain boundary at its initial formation differs significantly from that in its steady state. The goal of this project is to investigate these differences using advanced electron microscopy techniques.
For this purpose, ceramic powders will be dispersed on the surface of a single crystal substrate of the same material. Rapid heating will be employed to initiate the formation of grain boundaries between the particles and the single crystal. The specialized system used for rapid heating also allows for rapid cooling, enabling us to “freeze in” the as-formed (metastable) grain boundary structure.
Samples for transmission electron microscopy (TEM) will be prepared using the lift-out technique in our dual-beam focused ion beam (FIB) system. TEM will be used to characterize the structure and defects of the grain boundary. In addition, energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) will be used to analyze the chemical distribution and atomic bonding at the grain boundary, in comparison to the bulk.
A parallel set of samples will be annealed for longer times to allow the grain boundaries to reach their steady-state structure. The same characterization techniques will then be applied to study the differences between the metastable and steady-state grain boundaries.
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Graduate Research Topic: Solid-State Sintering of Zirconium Carbide (ZrC)
Zirconium carbide (ZrC) is an ultra high temperature ceramic with promising applications in the field of high-speed flight. However, the optimal processing route for ZrC remains elusive.
In our research group, we have recently developed a doping strategy, combined with controlled heating rates, that enables rapid densification of ZrC at relatively moderate temperatures. The goal of this research project is to study the influence of heating rates and sintering temperatures on densification behavior and grain growth in ZrC. In addition, the project will include measuring grain boundary mobility by analyzing grain growth in fully densified samples annealed at 2500 °C for various durations. Mechanical properties (hardness, wear resistance, and fracture strength in bending) will be measured.
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Graduate Research Topic: The Influence of Dopants on Surface Diffusion in Zirconium Carbide (ZrC)
Zirconium carbide (ZrC) is an ultra-high-temperature ceramic with promising applications in high-speed flight. Our recent studies have shown that specific dopants can significantly affect the densification and grain growth rates of ZrC.
This research project will focus on the mechanisms of densification, particularly the mobility of pores at grain boundaries. For a pore to remain attached to a grain boundary during grain boundary migration, surface diffusion must occur, and it is often the rate-limiting step in the process. Surface diffusion is strongly influenced by dopants and impurities that segregate to the surface, which is the central focus of this study.
In this project, dense ZrC samples will be sintered using various dopants previously identified by our group as important. Grooves will be etched into the polished surfaces of the ZrC samples, and their morphological evolution will be tracked as a function of annealing time at a selected temperature. Changes in groove geometry will be analyzed to extract quantitative measures of surface diffusion and compared for different dopants. The measured surface diffusion will then be correlated with the number of occluded pores in samples sintered with these specific dopants.
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The Influence of Ni on Grain Growth of Al2O3
An important microstructural feature of polycrystalline materials is grain size, since grain size and the respective area of grain boundaries affect mechanical and functional properties. As such, grain growth plays a significant role in microstructural evolution during sintering of polycrystalline materials. One of the most studied ceramic materials is α-alumina (α-Al2O3), which has served as a paradigm for the study of grain growth and is important for technological applications. Grain growth kinetics in alumina have been extensively studied, where major efforts have been made to understand the influence of key dopants and impurities on the average grain boundary mobility, and the effect of exaggerated grain growth.
While extensive measurements of grain growth have been conducted for alumina, it is only recently that the high temperature solubility limit of key dopants and impurities have been measured (by WDS). Mg, Ca, and Si have extremely low solubility limits at 1600°C (132 ppm, 51 ppm, and 188 ppm, respectively). Since these values are below the detection limits of most techniques used in the past, it is not clear if the grain boundary mobility measurements conducted on alumina were in the single phase region, or were influenced by particle drag (or a liquid phase) due to second phase precipitation.
Furthermore, recent studies of Ca doped alumina have now shown that even at dopant levels below the solubility limit, Ca causes a significant increase of grain boundary mobility. Thus, the accepted explanation that dopants such as Ca and Si cause liquid phase formation leading to rapid grain growth, are not complete. The actual mechanism by which dopants increase grain boundary mobility can only be speculated, and the goal of this research program is to address this important question.
The fundamental objective of this proposed research program is to quantitatively understand the influence of segregating Ni on the mobility of grain boundaries, at dopant levels below the solubility limit. The solubility limit at 1600C will be measured using techniques developed in our group (e.g. see). The outcome of this research will have fundamental contributions in the following manner:
- It will confirm existing theory regarding possible first-order interface transitions, within an experimental framework which provides fully defined relevant thermodynamic parameters. Dopant concentrations which show a discontinuous change in grain boundary mobility will serve to identify first order transitions in the state of grain boundaries.
- It will provide experimental evidence for the mechanism of grain boundary motion in alumina, and how dopant species affect the mechanism.
- It will elaborate the nature of dopant affected space charges at grain boundaries in alumina, and the possible influence of space charge on mobility.
- It will explore the influence of co-doping on grain boundary mobility, without the influence of secondary phases.
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Induction Sintering of Al2O3
Densification during sintering is always achieved by thermal activation. One of the challenges of homogenous heating, so as to minimize stresses during densification. These stresses can be significant, and limit the sintering rates of ceramics.
The concept to be explored in this project is thermal activation of non-conducting ceramics (alumina) by inductive heating of nanometer-sized particles of a secondary (conducting) phase distributed in the pre-sintered compact. Secondary phases to be tested include graphite, Ni, and W.
The density as a function of induction furnace conditions will be explored, and the general microstructure of the sintered bodies will be characterized using XRD, SEM, and TEM. The wear resistance of sintered samples will be measured.