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Title: Improved Wetting of Mixed Ionic/Electronic Conductors Used in Electrochemical Devices with Ternary Reactive Air Braze Filler Metals

Abstract

This paper reports on the wetting behavior, reactivity, and long-term electrical conductance of a series of ternary filler metals being considered for brazing lanthanum strontium cobalt ferrite (LSCF) based oxygen separation membranes. Mixed ionic/electronic conducting perovskite oxides such as LSCF and various doped barium cerates are currently being considered for use in high-temperature electrochemical devices such as oxygen and hydrogen concentrators and solid oxide fuel cells. However to take full advantage of the unique properties of these materials, reliable joining techniques need to be developed. Furthermore, if the proposed joining technique were to yield a hermetic ceramic-to-metal junction that was also electrically conductive, it would additionally benefit the device by allowing current to be drawn from or carried to the electrochemically active mixed conducting oxide component without requiring an separate current collector. A newly developed brazing technique known as air brazing is one such method of joining. In its present form, air brazing uses a silver-copper oxide based filler metal that can be melted directly in air to form a compliant joint that is electrically conductive. Recently, it has been shown that the addition of titania can enhance the wetting behavior of Ag-CuO filler metals on alumina. Here the effectmore » of this wetting agent on the surface wettability, long-term electrical resistance at 750°C, and reactivity with La0.6Sr0.4Co0.2Fe0.8O3- (LSCF-6428 or LSCF) substrates is discussed.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
903243
Report Number(s):
PNNL-SA-51501
Journal ID: ISSN 0013-4651; JESOAN; AA1510100; TRN: US200719%%651
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the Electrochemical Society; Journal Volume: 154; Journal Issue: 3
Country of Publication:
United States
Language:
English
Subject:
08 HYDROGEN; 30 DIRECT ENERGY CONVERSION; BARIUM; BRAZING; COBALT; ELECTRIC CONDUCTIVITY; FERRITE; FILLER METALS; HYDROGEN; LANTHANUM; MEMBRANES; OXIDES; PEROVSKITE; SOLID OXIDE FUEL CELLS; STRONTIUM; SUBSTRATES; WETTABILITY; WETTING AGENTS; ternary air braze; silver; copper oxide; perovskite

Citation Formats

Hardy, John S, Kim, Jin Yong Y, Thomsen, Ed C, and Weil, K Scott. Improved Wetting of Mixed Ionic/Electronic Conductors Used in Electrochemical Devices with Ternary Reactive Air Braze Filler Metals. United States: N. p., 2007. Web. doi:10.1149/1.2430691.
Hardy, John S, Kim, Jin Yong Y, Thomsen, Ed C, & Weil, K Scott. Improved Wetting of Mixed Ionic/Electronic Conductors Used in Electrochemical Devices with Ternary Reactive Air Braze Filler Metals. United States. doi:10.1149/1.2430691.
Hardy, John S, Kim, Jin Yong Y, Thomsen, Ed C, and Weil, K Scott. Fri . "Improved Wetting of Mixed Ionic/Electronic Conductors Used in Electrochemical Devices with Ternary Reactive Air Braze Filler Metals". United States. doi:10.1149/1.2430691.
@article{osti_903243,
title = {Improved Wetting of Mixed Ionic/Electronic Conductors Used in Electrochemical Devices with Ternary Reactive Air Braze Filler Metals},
author = {Hardy, John S and Kim, Jin Yong Y and Thomsen, Ed C and Weil, K Scott},
abstractNote = {This paper reports on the wetting behavior, reactivity, and long-term electrical conductance of a series of ternary filler metals being considered for brazing lanthanum strontium cobalt ferrite (LSCF) based oxygen separation membranes. Mixed ionic/electronic conducting perovskite oxides such as LSCF and various doped barium cerates are currently being considered for use in high-temperature electrochemical devices such as oxygen and hydrogen concentrators and solid oxide fuel cells. However to take full advantage of the unique properties of these materials, reliable joining techniques need to be developed. Furthermore, if the proposed joining technique were to yield a hermetic ceramic-to-metal junction that was also electrically conductive, it would additionally benefit the device by allowing current to be drawn from or carried to the electrochemically active mixed conducting oxide component without requiring an separate current collector. A newly developed brazing technique known as air brazing is one such method of joining. In its present form, air brazing uses a silver-copper oxide based filler metal that can be melted directly in air to form a compliant joint that is electrically conductive. Recently, it has been shown that the addition of titania can enhance the wetting behavior of Ag-CuO filler metals on alumina. Here the effect of this wetting agent on the surface wettability, long-term electrical resistance at 750°C, and reactivity with La0.6Sr0.4Co0.2Fe0.8O3- (LSCF-6428 or LSCF) substrates is discussed.},
doi = {10.1149/1.2430691},
journal = {Journal of the Electrochemical Society},
number = 3,
volume = 154,
place = {United States},
year = {Fri Jan 19 00:00:00 EST 2007},
month = {Fri Jan 19 00:00:00 EST 2007}
}
  • In this paper we report on the results of a series of sessile drop experiments designed to examine the effect of TiO2 on the wetting behavior of Ag-CuO air braze filler metals. It was found that TiO2 concentrations as small as 0.5 mol% can significantly decrease the contact angle of Ag-CuO on alumina over a compositional range of 1 – 34mol% CuO. The effect appears to maximize at a copper oxide concentration of ~4 mol% CuO regardless of the titania content.
  • A silver-based joining technique referred to as reactive air brazing (RAB) has been recently developed for joining high temperature structural ceramic components of the type used in high-temperature electrochemical devices. In prior work, it was found that additions of CuO to silver have a significant effect on the wettability and joint strength characteristics of the resulting braze on polycrystalline alumina substrates. More recently, it has been found that by adding as little as 0.5 mol % titania to these Ag-CuO brazes, the wettability of the RAB on alumina surfaces is further enhanced. The results of wettabilty measurements of Ag-CuO-TiO2 RABmore » compositions on alumina will be presented along with the microstructural characterization of Ag-CuO-TiO2 braze joints in alumina.« less
  • Mixed ionic/electronic conducting perovskite oxides such as lanthanum strontium cobalt ferrite (LSCF) are strong candidates for potential use in a number of electrochemical devices, including gas separation membranes and solid oxide fuel cells (SOFC). Underlying the excitement over the these novel ceramics is the engineering challenge of effectively incorporating them into practical devices. Taking full advantage of the unique properties of advanced ceramics such as mixed conducting oxides depends in large part on being able to develop reliable joining techniques. Earlier studies have indicated that Ag-CuO reactive air braze (RAB) compositions are effective in joining to LSCF. Meanwhile, it hasmore » been found that small additions of as little as 0.5 mol% titanium oxide to Ag-CuO RAB compositions cause a dramatic increase in the wettability of RAB on many oxide ceramic surfaces. Therefore the wettabilty of Ag-CuO-TiO2 brazes on LSCF substrates will be examined and the flexural strength, microstructure, and conductivity of joints in LSCF made using Ag-CuO-TiO2 brazes will be discussed. Long-term aging effects on conductivity and microstructure will also be presented.« less
  • A series of silver-copper oxide ceramic brazing alloys was compositionally modified by doping with small amounts of titania. Subsequent contact angle measurements indicate that concentrations as low as 0.5 mol% TiO2 can significantly enhance wettability over a wide range of binary Ag-CuOx compositions.
  • The frequency dependent Electrochemical Strain Microscopy (ESM) response of mixed ionic-electronic conductors is analyzed within the framework of Fermi-Dirac statistics and the Vegard law, accounting for steric effects from mobile donors. The emergence of dynamic charge waves and nonlinear deformation of the surface in response to bias applied to the tip-surface junction is numerically explored. The 2D maps of the strain and concentration distributions across the mixed ionic-electronic conductor and bias-induced surface displacements are calculated. The obtained numerical results can be applied to quantify the ESM response of Li-based solid electrolytes, materials with resistive switching, and electroactive ferroelectric polymers, whichmore » are of potential interest for flexible and high-density non-volatile memory devices.« less