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Tel: 01223 334496
Email: thl34@cam.ac.uk
Biography
MPhil in Micro- and Nanotechnology Enterprise, University of Cambridge, UK, 2008
M.Sc. in Physics, National Taiwan University, Taiwan, 2005
B.S in Physics, National Taiwan University, Taiwan, 2003
Research ProjectUnder the enormous pressure of the global warming, fuel cells are considered as one of the most promising sources of next-generation energy. Compared with the heat engine which uses petroleum, fuel cells take advantage of their high efficiency (~70-90%), low noise (~40dB), and cleanness (no/less carbon dioxide would be produced). In addition, a number of products based on fuel cells have been designed or commercialized, such as vehicles (FCX Clarity, Honda 2008), mobile phones/laptops (the batteries can be sustained up to about 30 days), and portable or family-used power stations. Yttia stabilized zirconia(YSZ) has been widely used in solid oxide fuel cells because its high oxygen ion diffusivity, low electrical conductivity, fluorite, and low cost.
The fluorite structure consists of 14 cations (such as Zr4+) in fcc sites and 4 anions (such as O2-) inside the fcc cell arranged in the form of a cube, and pure ZrO2 condenses in the fluorite phase between 2600K and 3000K. However, fluorite ZrO2 can be stablized at room temperature by alloying with low valence oxides, such as CaO or Y2O3 (or both), to form solid solutions. The presence of Y3+ or Ca2+ causes the formation of oxygen vacancies to preserve the electricity neutrality, and these vacancies are free to move at sufficiently high temperature and therefore mediate the high oxygen ionic conductivity.
The video shows that when two Y3+ are located in nearest neighbour sites in ZrO2, an oxygen vacancy interacts strongly strong with the Y-Y cluster so it is free to move only at high temperature.
Research
The fact that the oxygen ion diffusivity of YSZ drops at room temperature limits its applications, and to reach the applicable voltage of each fuel cell battery, several YSZ plates have to be connected in parallel and consequently the price increases. There are several ways to improve the materials: 1) doping the zirconia with more than one impurity, 2) reducing the grain size, and 3) fabricating YSZ/insulator multilayer structures. In our previous research, YSZ and CSZ show superior oxygen ion diffusivity in a high temperature environment (~1500K), and this agrees well with the experimental results[ J. Gong et. al.(2000)]. Our simulation results also show that the YSZ or CSZ (calcia stabilized zirconia) with nano-scale grains decreases its oxygen ion diffusivity by a factor of 2-4, compared to that in the bulk materials[to be published]. Of particular interest is that experiments show that the YSZ-STO multilayer structure exhibits higher ion diffusivity along the interface by 8 orders of magnitude compared to pure YSZ bulk materials[ J.Garcia-Barriocanal (2008)]. It is general believed that the interface helps the motion of oxygen ions; however, there is little literature about the theoretical understanding of this. To understand the detail mechanism, we will construct a YSZ-STO multilayer model with both classical and quantum calculations, and the results will be compared to measurements mad in the materials chemistry group of Dr. Kumar in University of Cambridge.
(To the Top)Method
In this project, we use Molecular Dynamics(MD) to calculate the diffusivity of the oxygen vacancies. The program employed is DL_POLY, developed by Daresbury Laboratory. The calculations are performed using The Cambridge High Performance Computing Cluster Darwin, the 20th fast machine in the world in 2006[Top500 List] A simple MD algorithm is shown below
Following are some snapshots of the computational models:
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