The document discusses different types of interconnect materials used in solid oxide fuel cells (SOFCs). It describes the requirements for interconnects including high electrical conductivity, thermal expansion matching other cell components, and chemical stability. It then summarizes ceramic interconnect materials like lanthanum and yttrium chromites that meet many requirements but have high costs and processing difficulties. The document also discusses metallic interconnects like chromium-based alloys that have easier fabrication but need to demonstrate long-term stability in SOFC operating conditions.

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