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Material Science & Engineering (MSE)
We are studying different functional materials that can be used for the fabrication of fuel cells. Solid Oxide Fuel Cells (SOFC) are electrochemical devices which directly convert chemical energy to electricity with very high efficiency (see Figure 1). However, the performance and stability of a SOFC are critically dependent on the activity and structural stability of various cell components such as the electrolyte, anode, cathode and interconnects. Lanthanum chromite-based perovskite oxides have been investigated as interconnects for high-temperature SOFCs and power stations due to the high stability against oxidizing and reducing environment and good electrical conductivity. In order to enhance the performance of LaCrO3 for such high temperature applications, optimization of the electrical conductivity of LaCrO3 is required.
Substitution on the A and B sites with alkali earth and transition metal elements modifies the electronic structure of the oxides and shows interesting catalytic properties. For example, doping a small amount of divalent cation Sr2+ (about 0.1%) in place of La3+ will significantly reduce the optical gap of LaCrO3, suggesting an increase of electrical conductivity as well. The main objective of our work is to optimize the electrical conductivity and to understand the mechanism of improvement in electrical conductivity of LaCrO3 by screening all Group IIa elements, M2+ (Be, Mg, Ca, Sr, Ba, Ra) as a substitution dopant to the La site in LaCrO3 (see Figure 2). We have applied molecular modeling to shorten the candidate list so that more focused experiments may be carried out efficiently.
Another area that we are interested in is multiferroics. They are materials that are both ferroelectric and magnetic. They are rare because in most ferroelectrics such as BaTiO3, the ferroelectricity is driven by a hybridisation of empty d orbitals with occupied p orbitals of the octahedrally coordinated oxygen ions. The mechanism requires empty d orbitals and thus cannot lead to multiferroic behaviour. This is the so-called “d0 ness” criterion.
However, “d0 ness” does not rule out other possible ways of achieving multiferroics in the magnetic perovskite oxides and related materials. For examples, the multiferroic property of the widely studied BiFeO3 is achieved by making use of the stereochemical activity of the lone pair on the large (A-site) cation to provide the ferroelectricity, while keeping the small (B-site) cation magnetic.
A second route to multiferroism is provided by ‘geometrically driven’ ferroelectricity, which is compatible with the coexistence of magnetism; the anti-ferromagnetic ferroelectrics YMnO3 and BaNiF4 fall into this class. The recent success of ferromagnetism in diluted magnetic semiconductors (DMS) or dilute magnetic oxide (DMO), also provides a another potential way for multiferroics by introducing dilute concentrations of magnetic species to ferroelectric oxide, thereby harnessing the benefits of a magnetic response without significantly affecting the ferroelectricity of the non-magnetic host.
The quest for multiferroic material by doping magnetic species in ferroelectric material, for example BaTiO3 crystals, requires first a thorough understanding of the nature (charge state, local symmetry, and electronic structure) of the defects/impurities present in these materials, and of ferromagnetic and ferroelectric coupling as well as their interaction with light. To this end, IHPC has initiated a series of first principles calculations in cooperation with NTU as the experimental partner. Most of work focused on the role of oxygen vacancy in mediating orbital and spin order.
Ong K. P., Wu P., Liu L. and Jiang S.P. “Optimization of electrical conductivity of LaCrO3 through doping: A combined study of molecular modeling and experiment” Appl. Phys. Lett. 90, 044109 (2007). DOI: 10.1063/1.2431780
Ong, K.P., P. Blaha, and P. Wu, “Origin of the light green color and electronic ground state of LaCrO3”. Phys. Rev. B, 77, 073102 (2008). DOI: 10.1103/PhysRevB.77.073102
Jiang S.P., Liu L., Ong K.P., Wu P., Li J. and Pu J. “Electrical conductivity and performance of doped LaCrO3 perovskite oxides for solid oxide fuel cells” Journal of Power Sources 176, 82 (2008). DOI: 10.1016/j.jpowsour.2007.10.053
Zhang Z., Wu P., Ong K.P., Lu L. and Shu C. “Electronic properties of A-site substituted lead zirconate titanate: Density functional calculations” Phys. Rev. B 76, 125102 (2007). DOI: 10.1103/PhysRevB.76.125102
Zhang, Z., P. Wu, L. Lu, and C. Shu, Defect and electronic structures of acceptor substituted lead titanate. Appl. Phys. Lett., 92, 112909 (2008) DOI: 10.1063/1.2898212
Ong K.P., Bai K.W., Blaha P. and Wu P. “Electronic structure and optical properties of AFeO(2) (A = Ag, Cu) within GGA calculations” Chem. Mater. 19, 634 (2007). DOI: 10.1021/cm062481c
Fig.1 Structure of one Solid Oxide Fuel Cell
Fig.2 Crystal structure of La1-x MxCrO3-x/2 (M=Ca, Sr, Ba)
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This page is last updated at: 26-MAY-2009