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

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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

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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

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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

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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

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“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

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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

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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

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Gravimetry

No observable gain in weight

Oxide growth is visual, too small to measure

Oxide growth suspected to be dielectric SiO2 nanolayer

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oxidation oxide light refractionoxidation oxide light refraction

Temperature Based Oxidation

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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

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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

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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

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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

Acknowledgements

Beth Opila and the High Temperature Materials

Lab

… for support and guidance throughout

my research

Wade Jenson and Jerry Floro

… providing the material

National Science Foundation and Center for

Diversity in Engineering

… for research funding and programming

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