Interconnects

1. INTERCONNECTS

2. CONTENTS
Introduction
Ceramic Interconnects (Lanthanum and Yttrium Chromites)
 Electrical Conductivity
 Thermal Expansion
 Thermal Conductivity
 Mechanical Strength
 Processing
Metallic Interconnects
 Chromium-based Alloys

3. Introduction
Roles Of The Interconnect In SOFCs
 Electrical Connection Between Cells
 Gas Separation Within The Cell Stack
Typical Requirements The Interconnect In SOFCs
 High Electronic Conductivity With Low Ionic Conductivity
 Chemical Stability In Both Fuel And Air
 Thermal Expansion Match To Other Cell Components
 High Mechanical Strength
 High Thermal Conductivity
 Chemical Stability With Regard To Other Cell Components
 Low Cost
 Ease Of Fabrication

4. Ceramic Interconnects (Lanthanum and Yttrium Chromites)
 Above 8OO°C, Doped Rare Earth Chromites Fit The Criteria.
 These Rare Earth Chromites Satisfy Most Of The Requirements, But Have Problems In
Fabrication And Have High Cost.
 Compositions From The System (La,Sr,Ca)(Cr,Mg)O3 Are The Leading Interconnect Materials.
 Compositions From The System (Y,Ca)CrO3 Also Have Acceptable Properties.
 Metallic Interconnects Are Easier To Fabricate And Potentially Less Costly Than Oxide Ceramics
But Their Lifetimes Under SOFC Operating Conditions Remain To Be Demonstrated.

5. Electrical Conductivity
 Electrical Conductivity must be >1 S/cm at 1000°C for adequate interconnect
working.
 Doping in YCrO3 or LaCrO3 is required in this regard.
 Upon exposure to reducing atmospheres, all oxides tend to lose oxygen resulting
in decrease of electrical conductivity.
 Chromites as interconnects remain single phase and do not dissociate at 1000°C
and 10-16 bar oxygen pressure.
 Chromites are only oxides available for use as interconnects.
 LaCrO3 doped with either Ca or Sr has sufficient conductivity in fuel atmospheres
to exceed 1 S/cm and therefore is preferred to Mg-doped LaCrO3.

6. Thermal Expansion
 Thermal expansion coefficients of electrolyte and interconnects in SOFCs
should match well.
 Thermal expansion coefficients of LaCrO3 and YCrO3 do not
match that of YSZ, but the addition of dopants makes the match
possible.
 Loss of oxygen in reducing atmosphere causes lattice expansion leading to
cracking problems.
 This expansion can be minimized but desirable properties
(electrical conductivity) can be lost.
 Usually we allow this expansion in the cell and stack design.

7. Thermal Conductivity
 Thermal conductivities of the SOFC components are in the order of 1.5-2 W/(mK).
 Low compared to 20 W/(mK) for stainless steel and 400 W/(mK) for copper.
 Heat dissipation (problem) must be considered in the SOFC design.
 True for monolithic and planar SOFCs, but is less problem in the tubular design.

8. Mechanical Strength
 Mechanical strength of many compositions of LaCrO3 is low compared to
YSZ and appears to be variable.
 related to structural flaws due to inhomogeneities in composition,
grain size and/or density.
 can alleviate this problem by improving processing.

9. Processing
 A number of methods are used to fabricate the interconnects depending upon the
SOFC design.
 For the Tubular design, electrochemical vapour deposition (EVD), plasma spraying,
laser ablation and slurry coating/sintering have been used.
 EVD and plasma spraying are favoured.
 Economics is an issue with EVD.
 Porosity and interfacial cracking are the difficulties with plasma spraying.

10.  For Monolithic design
 We co-sinters the electrolyte, cathode and anode, but fabricates the
interconnect separately.
 Have sealing problems of the planar cell design.
 Conventional planar cell designs build the gas distribution channels into the
interconnect.
 Made by using either glasses or cements which, when heated, give both
gas-tight seals and electrical contact.

11. Difficulties In Processing
 Cr-containing oxides are difficult to sinter
 vaporization of Cr-O species
 suppress densification
 Controlling the sintering atmosphere
 Extreme conditions are not compatible with the processing of the
other SOFC components
 Uneconomical
 Hot pressing for densifying LaCrO3
 LaCrO3 dissociates to Cr metal
 Cracking occurs due to Cr re-oxidation
 Unsatisfactory from economical point of view
 Liquid-phase sintering is a viable option.
 Suppressing the Cr-O volatility and enhancing mass transport
 Difficult process to control

12. Metallic Interconnects
 Metallic interconnect are feasible at down to (600- 850)°C.
 Advantages Over Ceramic Interconnects
 Lower material and fabrication cost
 Easier and more complex shaping possible
 Better electrical and thermal conductivity
 No deformation or failure due to different gas atmospheres across the
interconnection
 Fabrication Of Metallic Interconnects
 by machining, pressing or by near-net-shape sintering.
 Early attempts of using metallic materials as interconnect were not very successful.
 Because the materials contain a significant amount of Ni leading to large
thermal expansion mismatch between the metallic interconnect and the
ceramic SOFC components.
 The situation changed with the use of chromia-forming materials.

13. Chromium-based Alloys
 Chromium alloy containing 5 wt% iron and 1 wt% yttria (Cr 5Fe 1Y2O3) called DucrolIoy.
 Used with electrolyte-supported SOFCs
 Match its thermal expansion to that of the 8 mol% YSZ electrolyte
 Cr 5Fe 1Y2O3 is a chromia former and even after long-term exposure in oxygen or air,
the chromia scales are very thin.
 Thicker corrosion scales grow in carbon containing atmospheres (methane, coal gas)
due to formation of carbides.

14. Fabrication Of Interconnect Plates Of Cr 5Fe 1Y203
 Powder metallurgical methods
 High Energy Milling of Cr flakes with Fe and Y2O3
 Pressing and sintering in hydrogen atmosphere
 hot rolling in vacuum
 Electrochemical Machining for shaping
 DucrolIoy interconnects remain an expensive stack component.

15. Thank You