High temperature oxidation of FeSiGe
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Transcript of High temperature oxidation of FeSiGe
High temperature
oxidation of FeSiGe
Jonathan E Valenzuela, Elizabeth J Opila, Ph.D
Wade Jenson, Jerrold Floro, Ph.D
High Temperature Materials Lab
Surface and Thin Films Research Symposium
NSF Grant #1157007
July 29th, 2015
High temperature
oxidation of FeSiGe
High temperature
oxidation of FeSiGe
Properties of ThermoelectricsThermoelectric materials convert a
thermal gradient to electric power
Mechanism: Seebeck Effect
● Figure of merit (ZT)
o Seebeck coefficient, S
o Electrical conductivity, σ
o Thermal conductivity, κ
Advantages of thermoelectrics
● Scavenges waste heat
o Increase efficiency
● Energy crisis sustainability effort
Thermal
Electrical
http://www.iams.sinica.edu.tw/project/chenkh/galleries/aml-gallery
2
current
FeSiGe alloys are candidates for
thermoelectric materials
Advantages of FeSiGe alloys
• Oxidation resistance from β-FeSi2
• Cost efficiency
• Further reduction in thermal
conductivity, κ, from Ge
3
Background on FeSiGeMaterials as-received
● Two different processing methods,
“Bulk” vs. “Ribbon”
o Bulk produced by arc melting
Coarse grain size
o Ribbon produced by melt
spinning
Fine grain size
4 μm
SEM Backscatter Image of Ribbon:
Grain MicrostructureSEM Backscatter Image of Bulk:
Grain Microstructure
20 µm
Processing of As-Received
MaterialsArc Melting: passing current through
bulk material, melt together
+ Control solidification rate of material
- Time consuming and expensive
4
Melt Spinning is performed by
passing melt on spinning Cu wheel
+ rapidly solidifies the material
SEM Backscatter Image of Bulk
Hirox Microscope Image
of Ribbon
Composition of FeSiGe• Multiphase alloy
β-FeSi2 & SiGe
• Main thermoelectric phase in the FeSiGe alloy
is β-FeSi2
• Ge bonds with Si phase, forms SiGe nanorods
55
β-
FeSi
2
SiGe
SEM
Backscatter
Image
EDS Map of β-
FeSi2
EDS Map of
SiGe
1 µm
SEM Backscatter Image
of SiGe nanorod
β-FeSi2 + Si
Fe Si
High Temperature Oxidation
6
“interaction between reactive gas environments
and... metals at high temperature”*
• “high temperature” = 500º-900º C
• “reactive gas environment” = standard air
composition
ºC
β-FeSi2 transforms to α-FeSi2 at 937ºC
Oxygen is the component of interest
in this reaction
Possible reactions and products of Fe, Si, and Ge with O
Behavior of oxidation dependent on
thermodynamics and kinetics
Multiphase oxidation
Difficult to predict oxidation, all dependent on thermodynamics and
kinetics of oxidation
3 general forms by which multiphase oxidation can occur
1. The two phases oxidize independently to form a non-uniform scale
2. Two phases oxidize cooperatively to form a uniform scale
3. The solute-rich second phase acts as a resevoir for the continued
growth of the solute scale
7Oxidation of multicomponent two-phase alloys, Gesmundo, F. Gleeson, B.G English
Thermodynamics of oxidation
8
Gibbs Free Energy (ΔG , kJ/mol)
• The “potential”; determines if reaction
releases energy or requires energy to
proceed
Negative Free Gibbs’ Energy (ΔG)
Within the high temperature
zone (500º–900º C), Si has the
lowest Gibbs energy value
• Should be the
thermodynamically favorable
oxidation reaction
release of energy
favorable reaction
Research question and
motivationHow does the oxidation of FeSiGe
behave at high temperatures?
● Interested in the application of
FeSiGe as a thermoelectric in high
temperatures (500º-900º C) in
standard air
● Objectives:
o Identify oxide produced by
FeSiGe
o Determine the oxidation behavior
undergone by FeSiGe as
outlined by Gesmundo
9
current
Oxidation of multicomponent two-phase alloys, Gesmundo, F. Gleeson, B.G English
Experiment Methodology
10
Sample Preparation
• Encapsulation
Sealing sample into silica
container with argon gas to
prevent oxidation during
annealing
• Annealing
Sample received as α-FeSi2,
place in furnace at 567º C for
56 hours to transform to β-
FeSi2
Oxidation Technique
• Temperature-based
Conducted 5 oxidation tests
(500º-900ºC) for 24 hours each
• Time-based
Conducted 2 oxidation tests
(600º C, 900º C) for 48, 72, 96,
120 hrs
Model of furnace set up
Insulating material
Alumina boat
Fused silica slide• Encapsulate to prevent oxidation during
transformation anneal
• Temperatures chosen based on best
thermoelectric use and extreme temp.
for thermoelectric phase
Analysis Methodology
Gravimetry: measurement of weight change
kinetics
• Weight measurements done before and after
each experiement to track weight change
Scanning electron microscopy (SEM)
• Provides topographical and compositional
information of material
Energy dispersive x-ray specstroscopy (EDS)
• Provides compositional information and X-ray
spectrum of elements in material
11
Gravimetry
No observable gain in weight
Oxide growth is visual, too small to measure
Oxide growth suspected to be dielectric SiO2 nanolayer
12
oxidation oxide light refractionoxidation oxide light refraction
Temperature Based Oxidation
13
SEM/EDS of Pre Oxidation FeSiGe SEM/EDS of 500ºC FeSiGe SEM/EDS of 600ºC FeSiGe
SEM/EDS of 700ºC FeSiGe SEM/EDS of 800ºC FeSiGe SEM/EDS of 900ºC FeSiGe
All ribbon material
Insignificant gain in oxygen maps
• Little to no oxidation occuring
14
Time Based Oxidation: 600º COxygen increase in EDS map α increase in oxidation
• Little to no oxidation until 96 hours
• %O not very significant in all maps
SEM/EDS of Pre Oxidation FeSiGe
SEM/EDS of 96 HR FeSiGe
SEM/EDS of 72 HR FeSiGe
SEM/EDS of 120 HR FeSiGe
15
Time Based Oxidation: 900º C
50 µm
50 µm
SEM/EDS of Pre Oxidation FeSiGe
SEM/EDS of 72 HR FeSiGe
SEM/EDS of 48 HR FeSiGe
SEM/EDS of 120 HRFeSiGe
Significant oxygen increase at 48 hours
Oxygen becomes dominating component in EDS map by 72 hours
• Remains dominant for 120 hours
Conclusion and Future Work
16
FeSiGe is a good candidate for high temperature oxidation!
• Insignificant oxide growth at thermoelectric use
temperatures and higher temperature
Further testing needed to confirm oxidation product and
behavior
• X-ray diffraction on nanoscale oxide layer to confirm
oxide composition and structure
• Larger samples to measure weight gain kinetics of oxide
growth