A fuel cell is an electrochemical device consisting of an electrolyte, an anode, and a cathode which continuously converts the chemical energy of a fuel directly into electrical energy. Various types of fuel cells are available, including direct methanol fuel cells, alkaline fuel cells, proton exchange membrane fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells (SOFCs). The salient features of an SOFC are all-solid construction and high-temperature electrochemical-reaction-based operation, resulting in clean, efficient power generation from a variety of fuels. Solid oxide fuel cells are being considered as the premium power-generation devices for a broad range of applications, such as in portable electronic devices, automobiles, power generation, and aeronautics. SOFCs of two different designs, tubular and planar, are currently under development. Planar SOFCs offer simple manufacturing and a relatively short current path resulting in power densities and efficiencies higher than for the tubular design. However, planar SOFCs require hermetic seals to separate and contain the fuel and oxidant within the cell and to bond cell components together. The requirements for SOFC sealing materials are severe because the cells will operate at 600 to 1000 °C for thousands of hours, with sealing materials exposed to both oxidizing and reducing conditions. The seals must be chemically and mechanically compatible with different oxide and metallic cell components and should be electrically insulating. Also, they must survive cycling between room and SOFC operating temperatures.
Various types of seals--such as rigid seals, compressive seals, compliant seals, and hybrid seals--are being tried for SOFCs. For compliant and rigid seals, a number of glass and glass-ceramics based on borates, phosphates, and silicates are being investigated. Silicate glasses are expected to have performance superior to that of the borate and phosphate glasses. A barium calcium aluminosilicate (BCAS) glass of composition 35BaO-15CaO-5Al2O3-10B2O3-35SiO2 (mol%) was developed for use as sealing material for planar SOFCs, but during thermal cycling of the SOFC, the glass seal is prone to cracking. The strength and fracture toughness of this glass need to be improved to alleviate this problem. To achieve this goal, researchers reinforced the glass with boron nitride (BN) nanotubes that were produced at the NASA Glenn Research Center. Panels of glass containing 4 wt% of the nanotubes were hot pressed and machined into test bars. Mechanical and physical properties of the glass composites--including four-point flexure strength, fracture toughness, elastic modulus, microhardness, and density--were determined at room temperature. As shown in the graph, the addition of BN nanotubes improved the flexure strength of the glass by as much as 90 percent and the fracture toughness of the glass (measured by the single-edge v-notched beam method) by 35 percent. The addition of BN nanotubes decreased the elastic modulus and microhardness.
Flexure strength and fracture toughness of glass reinforced with 4 wt% BN nanotubes. Strength and fracture toughness data for glass are also included for comparison. Error bars indicate ±1.0 standard deviation.
Thus, it was demonstrated that the strength of the SOFC seal glass can be improved as much as 90 percent and the fracture toughness as much as 35 percent by reinforcing the glass with just 4 wt% of BN nanotubes. This is important because the addition of such a small amount of BN nanotubes will have little effect on the viscous behavior of the glass at the fuel-cell operating temperatures. Leak tests for the glass composite seals need to be carried out. The glass/BN nanotubes composites are expected to result in much improved seals for SOFCs. This is the first time that a glass matrix composite reinforced with BN nanotubes has been produced and that significant improvements in mechanical properties have been demonstrated.
Bansal, Narottam P.; Hurst, Janet B.; and Choi, Sung R.: Boron Nitride Nanotubes-Reinforced Glass Composites. J. Am. Ceram. Soc., vol. 89, no. 1, 2006, pp. 388-390.
Bansal, Narottam P.; and Gamble, Eleanor A.: Crystallization Kinetics of a Solid Oxide Fuel Cell Seal Glass by Differential Thermal Analysis. J. Power Sources, vol. 147, nos. 1-2, 2005, pp. 107-115.
Glenn contact: Dr. Narottam P. Bansal, 216-433-3855, Narottam.P.Bansal@nasa.govLast updated: October 11, 2006
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