Research Areas ______________________________________________
- Constrained Sintering of Solid Oxide Fuel Cell Materials (Dustin Frame)
- Processing and Optimization of Thin CsHSO4 Electrolyte (Dustin Frame and Surya Kencana)
- Constitutive Laws and the Densification Behavior of YTZP Matrix Composites (Sam Salamone)
- Mechanical Property Evaluation of Ceramic Coatings on Metal
- Processing of Ceramics with Hierarchical Pore Structures
I: Constrained Sintering of Solid Oxide Fuel Cell Materials
In the manufacturing of Solid Oxide Fuel Cells, it is necessary to sinter multiple layers of ceramics together. Ceramics sinter at different rates, so it is difficult to predict how the layers will react when sintered together. If you try to sinter two thin pieces of ceramic together, they will most likely warp due to the stresses associated with the constraint of the neighbor ceramic. Currently, to overcome this problem, one of the layers is made much thicker so the stress from the second ceramic is inconsequential. The thick Anode creates a lot of resistance that robs energy from our fuel cell. Our work focuses on preventing warping of multiple thin ceramic layers through constrained sintering. We are using the current fuel cell materials YSZ and LaMnO3. By being able to control the sintering of these thin layers, we will be able to increase the performance of SOFC's.
II: Processing and Optimization of Thin CsHSO4 Electrolyte
The most important part of a fuel cell is the electrolyte. It determines the operating temperature of the fuel cell, the material choice for other parts of the fuel cell, the type of fuel consumed, and the applications of the fuel cell.
Types of fuel cells commercially used are:
1. Phosphoric acid (PAFC)
2. Molten carbonate (MCFC)
3. Proton exchange membrane (PEM)
4. Solid oxide (SOFC)
The electrolytes of PAFC, MCFC, PEM, and SOFC are liquid phosphoric acid soaked in matrix, molten carbonate, solid polymers, and ceramics respectively. Aqueous and molten electrolytes are corrosive to other fuel cell's parts and hard to manage; therefore, non-solid electrolytes are unattractive. Polymer electrolytes operate at temperatures below the boiling point of water, are a very thin membrane, and use platinum as a catalyst. PEMs have a limited choice of fuel and are very expensive since the process to produce very thin membranes and the cost of platinum catalysts are very expensive. Ceramic electrolytes operate at temperatures in the range of 700-1000°C. SOFCs have a wide choice of fuel; however, they are very expensive since to withstand such high operating temperature, other parts of fuel cell must be made of expensive materials. All fuel cells commercially used have flaws. Hence, much research has been done to find an electrolyte that operates at a temperature range of 100-600°C. Our research is focusing on a solid inorganic acid electrolyte, CsHSO4. It is our goal to produce and optimize the processing of a CsHSO4 electrolyte that will be as effective as other commercially used electrolytes.III: Constitutive Laws and the Densification Behavior of YTZP Matrix Composites
The observed densification behavior of 2.5 mol% YTZP matrix composites (with rigid, non-densifying inclusions) has been rationalized using both the constitutive parameters and microstructure development. In this study, we report on the densification behavior of a polycrystalline matrix composite during both constant heating rate and isothermal heating schedules. It is shown that the effect of rigid inclusions increases as the volume fraction of inclusions increases and as the sintering temperature decreases. In all cases the effect was significantly less severe than what has been reported in the literature for polycrystalline matrix composites. Noticeable differences between the pure matrix and the composite, under all heating schedules, were observed only for the composite with 10 and 20 vol% inclusions. In all cases, the constant heating rate experiments enabled the compacts to eventually reach a much higher final density. This being due to the more rapid densification kinetics of the constant heating rate experiments.
The characterization of the microstructure of a sintering crystalline compact requires at least two variables, density and grain size. However, the effect of grain growth on densification of composites has not been well studied. In this case, the matrix undergoes very little grain growth during sintering and the final grain size of the matrix with or without the inclusions are very similar.
The constitutive response of the powder compact can be used to predict constrained densification (e.g. sintering of composites). This study employed low stress sinter-forging techniques to obtain the uniaxial viscosity and Poisson's ratio of a porous body, Ep & np respectively. These constitutive parameters, in particular the creep and densification viscosities of the matrix and the composites, will be determined as a function of microstructural evolution and temperature.
IV:
I: Polymer Derived Ceramic Composites as Environmental Barrier Coatings on Steel
Polymer Derived Ceramics offer significant advantages including ease of processing, low temperature pyrolysis, and control over composition. One of the significant problems in converting pre-ceramic polymers to ceramics is the significant volume change (50-75%). One promising strategy to overcome this problem is the use of expansion agents. This research focuses on the use of poly(hydridomethylsiloxane) with titanium disilicide as a reactive filler for use as an environmental barrier coating on stainless steel, specifically to prevent oxidation and corrosion in steam-methane environments. Work has shown that it is possible to attain good adhesion and relatively dense coatings of ~25 microns thickness with these components and pyrolysis at 800ºC. Investigations continue in order to thoroughly characterize the physical, mechanical, and protective properties of these coating. (in details...)
II: Nanoscale Reinforced, Polymer Derived Ceramic Matrix Coatings for Coal-fired Environments
Good high temperature corrosion protection ceramic coatings for metallic structures must have a set of properties that are difficult to achieve using established processing techniques. The required properties include ease of coating complex shapes, low processing temperatures, thermal expansion match with metallic structures and good mechanical and chemical properties. Nanoscale reinforced composite coatings in which the matrix is derived from preceramic polymers have the potential to meet these requirements. In our research, we have developed coatings reinforced with micron scale reinforced particles and have preliminary evidence that the quality and performance of coatings will be significantly enhanced by using nanoscale reinforcements. The coatings that we develop will be based on low viscosity pre-ceramic polymers. Thus they can be easily applied to any shape using a variety of techniques including dip-coating, spray-coating and painting. The polymers are loaded with a range of nanoparticles. The nanoparticles have two primary roles: control of the final composition and phases (and hence the properties); and control of the shrinkage during thermal decomposition of the polymer. Thus the selection of the nanoparticles is the most critical aspect of this project. We have developed a systematic protocol to select the particles and will use it in this research. Based on the processing requirements, and sintered microstructure, a few systems will be selected for detailed characterization including their performance under simulated coal fired environment and under temperature cycling. The proposed research will be a detailed and systematic investigation of preceramic polymers and nanoparticles as a novel class of precursors for ceramic coatings,leading to the identification of optimal materials systems and processing strategies.
III: Carbon Nanotube Reinforced Polymer Derived Ceramic Composites
Preceramic polymers (polycarbosilanes, polysiloxanes) were doped with transition metal compounds (Fe, Co, Ni and mixtures thereof), crosslinked and pyrolyzed in argon atmosphere between 800 °C and 1200 °C. The pyrolysis products were investigated with respect to the formation of CNTs and their morphology. Characterization was carried out with thermal analysis, XRD, SEM and TEM analysis. The ceramic residue was in the range 70 to 80%. During pyrolysis the formation of transition metal nanoparticles as the catalyst for CNT growth was observed in the polymer derived ceramic matrix having a particle size of several tens of nanometers. All samples doped with transition metals showed the formation of CNTs. The carbon sources for CNT formation are the hydrocarbons released from decomposition reactions of the preceramic polymers. The morphology of the CNTs was found to be influenced by the type and concentration of transition metal, and the pyrolysis parameters. The in situ formation of CNTs from decomposition gases of preceramic polymers offer potential for the in situ formation of carbon nanotubes in a (polymer derived) ceramic matrix.
IV: Non-linear Finite Element Modeling of Concrete Columns Confined with Fiber Reinforced Polymers
Passive confinement of concrete structures has long been a technique used to either retrofit deteriorating structures or to bring under-designed structures up to seismic code. Within the last ten years, fiber reinforced polymers have become of interest for the passive confinement of concrete due to their light weight and ease of application when compared to the industry standard of steel. By wrapping the fiber reinforced polymer (FRP) around a concrete column, the composite jacket will effectively hold the concrete together when the concrete column begins fail in compression or flexure. This can have the effect of actually increasing the columns load and strain bearing capacity preventing catastrophic failure of the structure. The current predictive models for the behavior of these passively confined concrete column do not predict the full stress strain relationship very well. The goal of this research was to develop a non-linear finite element (FE) model that takes into account the linear orthotropic behavior of FRPs and the plastic behavior of the yielding concrete. A Drucker-Prager plasticity model was implemented to model the behavior of the concrete. The FE model developed in this research using ANSYS 6.0 was able to predict the stress-strain behavior very well. The model was also extended to a square column geometry giving good insight into the stress state and possible failure mechanisms in the FRP jacket.
I: Processing and Characterization of Porous Titanium with Gradient in Porosity for Biomedical Applications
The goal of my research is to create a more mechanically compatible orthopaedic hip stem implant, greatly reducing stress shielding effects and promoting post-implantation bone in-growth and attachment. Introducing porosity lowers the elastic modulus, possibly matching the stiffness of porous titanium to that of cortical bone. Pores of approporiate size range have also been shown to facilitate bone in-growth. The intended method is CPTi-powder slurry infiltration of sacrificial polyurethane foam scaffold, followed by heat treatments to pyrolize polymer foam and binder and sinter Ti particles. This, I believe, will result in structurally sound porous titanium. Functional gradients in porosity will be investigated and is the aspect of this project with unique scientific merit. It will be achieved through compression molding of polymer foam scaffold. It is through this gradient of porosity that the optimal design of a foam metal for biomedical applications will be achieved. The porosity can be tailored to match that of bone near the implant exterior while strength can be preserved toward the interior of the implant. An osteoconductive hydroxylapatite coating will also be applied, via sol-gel methods, to promote post-implantation bone attachment.
II: Novel Nanoparticle Composite Systems for Three-Dimensional Printing (3DP) Dental Ceramic Applications
This project involves the use of a ProMetal RX-D Three Dimensional Printer to print ceramic dental copings. The ProMetal RX-D is designed to print into a powder bed of stainless steel, but the powder bed can be filled with a variety of ceramic powders instead. The binder development involves the use of preceramic polymers as both a binder and as a way to increase the ceramic yield of firing.
