This Small Business Innovation Research Phase I project will investigate the commercial potential of flash-sintering to enhance the manufacturability and performance of Solid Oxide Fuel Cells (SOFCs). The co-sintering of heterogeneous multilayer ceramics remains a technical barrier to the commercialization of SOFCs for two reasons: 1) the materials for the multilayer structure require different processing temperatures, which can hinder optimum material selection, and 2) batch style manufacturing techniques are expansive and have low throughput. Flash-sintering can address both of these technical issues by lowering the sintering temperatures and reducing the processing times. These advances will in turn permit: a) the use of higher-performing low-temperature cathode materials, b) single-step sintering, c) a change in the manufacturing protocol from batch to continuous, d) significant reduction in manufacturing energy consumption, and, e) the use of lower-cost tooling materials due to reduced processing temperatures.
The broader impact/commercial potential of this project will be to significantly enhance the commercial viability of Solid Oxide Fuel Cell (SOFC) technology. This will be done by improving manufacturing cost and performance of intermediate-temperature SOFCs. Improved cost will in turn enable greater proliferation of this environmentally-clean, high-efficiency power generation system, and when deployed, the concomitant reduction of greenhouse gases. The technique developed has broader impacts as a disruptive technology for the entire field of heterogeneous ceramic interfaces (for example: ceramic gas separation membranes, electrocatalysts, sensors, multilayer batteries, and metal-ceramic composites) by enabling new material combinations to be co-sintered. Finally, SOFCs are suitable for a wide range of fuels, including natural gas, allowing them flexibility in the constantly changing energy market. The size of SOFCs can range from a few kW to a few MW, suitable for all types of applications from powering homes and small businesses, to large-scale energy production.
The research generated during the completion of this phase I SBIR tested the viability of a new manufacturing technique for sintering multilayer ceramic structures for solid-oxide fuel cell (SOFC) application. This new method is called flash-sintering. The co-sintering of heterogeneous multilayer ceramics remains a technical barrier to the commercialization of SOFCs for two reasons: (i) The materials for the multilayer structure require different processing temperature, which hinders optimum material selection for the electrodes, and (ii) Batch style manufacturing techniques are expensive and have low throughput. Flash-sintering can address both of these technical issues by lowering the sintering temperatures and reducing the processing times. These advances will in-turn permit: (a) The use of higher performing low temperature cathode materials, (b) Single step sintering, (c) Changes in the manufacturing protocol from batch processing to continuous processing, (d) Significant reductions in energy cost for manufacturing, and (e) The use of lower cost tooling materials due to reduction in sintering temperature. Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs) present challenges to low cost manufacturing that flash-sintering is uniquely suited to address. The SOFC architecture investigated for this proposal was a Ni-YSZ anode supported design, with YSZ electrolyte and LSM cathode. Conventional approaches to sintering this architecture require multiple steps at 1300°C, at multiple hours each. If successful, flash-sintering will process the high performance IT-SOFC in a single step below 1000°C, in less than 1 minute. During this award FAST Ceramics executed a project plan to handle three project milestones 1) SOFC development 2) Field assisted Sintering Equipment Development 3) SOFC testing. The tasks were distributed according to expertise. Boston University handled the SOFC formulation and green-state production. FAST Ceramics worked on equipment development and testing of the new sintering process. The overall project goals of this SBIR were 1) Development of flash-sintering as a single step manufacturing process for SOFCs using temperatures below 1000C and sintering times below 1min. 2) Manufacturing the multilayer architecture free from crack and defects 3) Ensure that the process can produce the microstructure necessary for optimum SOFC performance. (Good Adhesion between layer and with requisite density and porosity) 4) Manufacture an SOFC that can generate 2Wcm-2 at 700C. Of the aforementioned goals the project was able to complete all but the last. Development of the Multi-Layer Architecture required understanding the co-sintering behavior of the 3 constituent materials: (1) Nickel-Yttria Stabilized Zirconia, (2) Yttria-Stabilized Zirconia, (3) Lanthanum Strontium Magnetite; which make up the Anode, electrolyte and cathode respectively. This was the successfully handled by the Boston University team (Professor Srikanth Gopalan and Professor Uday Pal) [1]. FAST Ceramics developed a flash sintering based on previous research carried out at the University of Colorado [2], but more specifically designed to handle the complexities of the SOFC geometry. Some of the notable improvements were to develop a way to deliver high frequency AC energy into the ceramic, this was to help prevent any metallization due to oxygen migration under electrical fields. Additionally, the new machine was able to better maintain the SOFC flatness during sintering. Figure 2 shows the flash sintering system. During the work for this SBIR FAST Ceramics successfully developed the AC flash-sintering system. The new design demonstrated good sintering of the heterogeneous laminate structure showing the required densities in each layer. However, the project failed to sinter enough square area for testability under SOFC conditions. While Flash Sintering continues to be a promising technology for rapid-efficient sintering of ceramics. Flash-Sintering remains an inherently unstable process, and in some cases the ceramics to be sintered can have trouble with metallization during the application of high DC electric fields. Because this process leads to increases in substrate conductivity it presents a complex positive feedback loop that must be overcome. Wither better alignment of the electrodes, and a faster control loop this process still remains a leading candidate for fast sintering of ceramics. Overall this project dramatically accelerated Flash Sintering towards manufacturing. While it is clear from the project report that FLASH sintering is not quite ready for manufacturing of SOFC’s. It has proven viable for densifying the constituent components. Furthermore, the technology seems to show immediate promise for manufacturing for other ceramic materials, with more homogenous microstructures (Cutting Tools and Thermoelectrics to name two). FAST ceramics continues to develop this nascent technology into a viable and useful manufacturing process, with full expectation that it will become an important tool for developing next generation technical ceramics. 1) Yoon, K. J., Ye, G., Gopalan, S., & Pal, U. B. (2010). Cost-Effective Single Step Cofiring Process for Manufacturing Solid Oxide Fuel Cells Using HSCTM Anode. Journal of Fuel Cell Science and Technology, 7(2), 021010. doi:10.1115/1.3177449 2) Francis, J. S. C., & Raj, R. (2011). Flash-Sinterforging of Nanograin Zirconia : Field Assisted Sintering and Superplasticity. Journal of the American Ceramic Society, 9, 1–9. doi:10.1111/j.1551-2916.2011.04855.x
