John S. Hardy

Structure and Conductivity of Thermally Grown Scales on Ferritic Fe-Cr-Mn Steel for SOFC Interconnect Applications

With the development of solid oxide fuel cells (SOFCs) that operate in the intermediate temperature range of 650-800°C, ferritic stainless steels have become promising candidate materials for interconnects in SOFC stacks. The SOFC interconnect requires that the alloy possess not only excellent surface stability, but also high electrical conductivity through the oxide scale that forms at elevated temperatures and contributes to the alloys surface stability. It appears that ferritic Fe-Cr-Mn alloys may be potential candidates due to the formation of an electrically conductive scale containing (Mn, Cr) 3 O 4 spinel. To improve the understanding of scale growth on manganese-containing ferritic stainless steels and evaluate their suitability for use in SOFC interconnects, the oxidation behavior (i.e., growth kinetics, composition, and structure of the oxide scale) and the scale electrical conductivity of a commercially available Fe-Cr-Mn steel developed specifically for SOFC applications were investigated. The results are reported and compared with those of conventional ferritic stainless steel compositions.

Thermal stability and phase transformation of electrochemically charged/discharged LiMnPO4 cathode for Li-ion batteries

Electrochemically active LiMnPO4 nanoplates at lithiated/delithiated state were subjected to thermal stability and phase transformation evaluations for safety as a cathode material for Li-ion batteries. The phase transformation and oxygen evolution temperature of delithiated MnPO4 were characterized using in situ hot-stage X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric-differential scanning calorimetry-mass spectroscopy (TGA-DSC-MS), transmission electron microscopy and scanning electron microscopy (SEM)-energy dispersive X-ray analysis (EDAX).

Reactive Air Brazing: A Novel Method of Sealing SOFCs and Other Solid-State Electrochemical Devices

High-temperature electrochemical devices operate via an ion gradient that develops across a solid electrolyte. Consequently, hermeticity across this membrane is paramount. Not only must the electrolyte contain no interconnected porosity, but it must be connected to device chassis with a gas-tight seal. Here we report a new method of brazing developed specifically for solid-state electrochemical applications. We demonstrate that the seal is hermetic and resistant to thermal aging, can be thermally cycled under rapid heating rates with no measurable loss in seal strength, and has shown promise in sealing full-size planar solid oxide fuel cell ~pSOFC! components.

Rupture Testing as a Tool for Developing Planar Solid Oxide Fuel Cell Seals

One of the critical issues in designing and fabricating high-performance planar solid oxide fuel cell (pSOFC) stacks is the ability to hermetically seal adjacent metal and ceramic components. In our pSOFC development program, we have designed a testing technique that allows us to screen through the numerous variables involved in developing glass seals. Using this test for example, we have found that the composition of the metal component plays an important role in the strength of the seal. Microstructural analysis of as-sealed specimens revealed that an interfacial reaction zone forms during joining, and it appears that the thickness and composition of this layer are the dominant parameters that control joint strength. In this paper the details of the seal test are reported. The results have proven particularly significant in the development of the next-generation stack design. Supporting microstructural and chemical analyses collected on the test specimens are also presented and used to interpret the seal test results in an effort to identify the necessary steps toward improving glass pSOFC seals.

Sintering of lanthanum chromite using strontium vanadate

Abstract Small proportions (5 and 10 wt.%) of strontium vanadate (Sr 3 (VO 4 ) 2 ) were added to strontium-doped lanthanum chromite (La 0.85 Sr 0.15 CrO 3 ) to produce high density fuel cell interconnect materials in air at 1550°C without adversely affecting the desirable properties of the material. Compositions investigated were shown to have good electrical conductivity at SOFC operating temperatures in air and reducing environments, phase stability from room temperature to 1000°C, negligible thermal expansion mismatch with yttria-stabilized zirconia electrolytes and relatively low dilation at p O 2 10 −16 atm.

Joining Mixed Conducting Oxides Using an Air-Fired Electrically Conductive Braze

Pacific Northwest National Laboratory, Richland, Washington 99352, USAThe lanthanum strontium cobalt ferrites ~LSCF! are a well-known family of mixed oxygen-ionic/electronic conductors that are ofinterest for use in high-temperature electrochemical devices, such as solid oxide fuel cells and oxygen separation membranes. Oneof the challenges however in developing these types of devices is joining the constituent metallic and ceramic components,particularly with a technique that is compatible with the electrochemically active membrane materials. A new joining process hasbeen developed which employs a silver-copper oxide braze material. In the present study, we found that braze compositionscontaining between 1.4 and 16 mol % CuO offer the best combination of wettability, joint strength, and electrical conductivity. Ourresults indicate that the wettability of the Ag-CuO brazes on LSCF substrates increases with CuO content and that electricalconductivity through the joint remains high as long as the silver content is greater than 66 mol %. Results from long-term,high-temperature resistance experiments confirmed the latter finding, as minimal degradation in conductivity was observed in thevarious test joints over 500 h of testing at 750°C. Data obtained from three-point bend testing showed that joints formed with abraze composition of 1.4 and 16 mol % CuO displayed maximal average flexural strength.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1764782# All rights reserved.Manuscript submitted September 26, 2003; revised manuscript received January 22, 2004. Available electronically June 17, 2004.

Alternative planar SOFC sealing concepts

Abstract One of the challenges in manufacturing planar solid oxide fuel cells (pSOFCs) is in hermetically sealing the ceramic and metallic components such that the resulting joint remains rugged and stable over the lifetime of the stack. Traditionally, glass joining or compressive sealing has been used. While short-term success has been achieved with these techniques, it is apparent that to meet the long-term operational needs of stack designers, alternative sealing concepts will need to be conceived. At Pacific Northwest National Laboratory we have been developing two such alternatives, air brazing and bonded compliant sealing, the details of which are outlined here.

Sintering mechanisms in strontium doped lanthanum chromite

The sintering behavior of (La0.7Sr0.3)xCrO3 (0.95 ≤ x ≤ 1.05) is investigated to compare liquid phase sintering phenomena occuring in stoichiometric and non-stoichiometric compositions. Shrinkage analysis revealed marked contrast between the densification characteristics of the A-site enriched (x > 1.00) and A-site depleted (x < 1.00) materials. A-site depleted samples typically exhibited a single liquid phase sintering event at 1250 °C attributed to the melting of an exsoluted SrCrO4 phase. A-site enriched samples indicated two rapid shrinkage events due to the melting of SrCrO4, and a Sr2.67(CrO4)2 phase with a melting temperature of 1450 °C. Sr2.67(CrO4)2 was shown to evolve from a decomposition reaction between SrCrO4 and La2CrO6, detected together in A-site enriched samples from 800–1000 °C. Maximum densities (93% theoretical density) were achieved for (La0.7Sr0.3)xCrO3 x = 1.00 after sintering at 1700 °C for two hours.

Silver-copper oxide based reactive air braze for joining yttria-stabilized zirconia

We are investigating a new method of ceramic-to-metal joining, referred to as reactive air brazing (RAB), as a potential method of sealing ceramic components in high-temperature electrochemical devices. Sessile drop wetting experiments and joint strength testing were conducted using yttria stabilized zirconia (YSZ) substrates and CuO-Ag based air brazes. Results from our studies indicate that the wettability of the braze improves substantially with increasing CuO content, over a compositional range of 1 - 8 mol% CuO, which is accompanied by an increase in the bend strength of the corresponding brazed YSZ joint. The addition of a small amount of TiO2 (0.5 mol%) to the CuO-Ag braze further improves wettability due to the formation of a titanium zirconate reaction product along the braze/substrate interface. However, with one notable exception, the bend strength of these ternary braze joints remained nearly identical to those measured in comparable binary braze joints. SEM analysis conducted on the corresponding fracture surfaces indicated that in the binary braze joints the failure occurs primarily at the braze/YSZ interface. Similarly in the case of the the ternary, TiO2-doped brazes joint failure occurs predominantly along the interface between the braze filler metal and the underlying titanium zirconate reaction layer.

New sealing concept for planar solid oxide fuel cells

A key element in developing high-performance planar solid oxide fuel-cell stacks is the hermetic seal between the metal and ceramic components. Two methods of sealing are commonly used: (a) rigid joining and (b) compressive sealing. Each method has its own set of advantages and design constraints. An alternative approach is currently under development that appears to combine some of the advantages of the other two techniques, including hermeticity, mechanical integrity, and minimization of interfacial stresses in the joint substrate materials, particularly the ceramic cell. The new sealing concept relies on a plastically deformable metal seal; one that offers a quasi-dynamic mechanical response in that it is adherent to both sealing surfaces, i.e., non-sliding, but readily yields or deforms under thermally generated stresses. In this way, it mitigates the development of stresses in the adjacent ceramic and metal components even when a significant difference in thermal expansion exists between the two materials. The pre-experimental design of the seal, initial proof-of-principle results on small test specimens, and finite-element analyses aimed at scaling the seal to prototypical sizes and geometries are described herein.