1. High Temperature Oxidation Studies of FeSiGe
Jonathan E. Valenzuela, Wade A. Jensen, Jerrold A. Floro, Elizabeth J. Opila
Department of Materials Science and Engineering
University of Virginia, Charlottesville, VA 22904
Objective
Characterize the oxidation behavior of FeSiGe at high temperatures.
Results: Oxidation Time Dependence
Oxidation studies were carried out in a box furnace for 48-120 hours at 600°C
(temperature at which thermoelectric phase β-FeSi2 has maximum thermoelectric
efficiency) for bulk and ribbon FeSiGe and 900°C (maximum temperature before β-
FeSi2 transforms to α-FeSi2), for bulk FeSiGe. No significant weight change detected. Results: Oxidation Temperature Dependence
5 oxidation tests were carried out by box furnace from 500- 900°C for 24 hours each
with the ribbon material. SEM, EDS, and weight change was monitored.
Summary
FeSiGe is a good candidate for high temperature thermoelectric use!
• Insignificant oxide growth and microstructural coarsening effects at
optimum and maximum thermoelectric use temperatures, as
demonstrated by EDS.
• We hypothesize that a SiO2 is the oxide being grown, as
demonstrated by EDS and interference colors produced from the
oxide.
• Ge in FeSiGe is preferentially oxidized, as demonstrated by EDS
mapping.
References: (1.) Snyder, G. Jeffrey, and Eric S. Toberer. “Complex Thermoelectric Materials.” (2.) Ware, R.M.; McNeill, D.J., "Iron disilicide as a
thermoelectric generator material" (3.) Birks, Neil, Gerald H. Meier, and Frederick S. Pettit. Introduction to the High Temperature Oxidation of
Metals.(4.) Deal, B. E.; A. S. Grove (December 1965). "General Relationship for the Thermal Oxidation of Silicon". (5.) Gesmundo, F., and B.
Gleeson. “Oxidation of Multicomponent Two-Phase Alloys.” (6.) Pujilaksono, Bagas et al. “Oxidation of Iron at 400–600 °C in Dry and Wet O2.”
(7.) Nanko, Makoto et al. “Isothermal Oxidation of Sintered Β-FeSi2 in Air.” (8.) F.K. LeGoues, R. Rosenburg, T. Nguyen, F. Himpsel, and
B.S.Meyerson. “Oxidation Studies of SiGe”. (9.) J. Henrie, S. Kellis, S. Schultz, A. Hawkins. “Electric color charts for dielectric films on silicon”.
Future Work
Further testing needed to confirm
oxidation behavior.
• X-ray photoelectron spectroscopy on
nanoscale oxide layer to confirm
oxide composition
• Additional sample cross-section SEM
characterization
• Thermoelectric property
measurements after oxidation
Acknowledgements
Many thanks to Jerry Floro and
Wade Jensen for providing the
material for testing and the National
Science Foundation (NSF Grant
#1157007) for the funding for this
research.
Experimental: Processing
and Oxidation Tests
Preparation of Material
• Bulk: Arc melting, produces coarse
microstructure (Fig. 4)
• Ribbon : Arc melting and melt spinning for
rapid solidification, produces fine
microstructure (Fig. 5)
• All samples were encapsulated in Ar gas
and annealed at 567°C for 56 hours to
produce the thermoelectric β-FeSi2 phase7
Oxidation Time Dependence
• Oxidized for 48, 72, 96, and 120 hours at
600°C and 900°C in a box furnace (Fig. 6) in
ambient air
• 600°C for bulk and ribbon FeSiGe & 900°C
for bulk FeSiGe
Oxidation Temperature Dependence
• 5 samples of ribbon FeSiGe oxidized at
500, 600, 700, 800, 900°C for 24 hours in a
box furnace (Fig. 6) in ambient air
• 31 day oxidation for ribbon and bulk
FeSiGe at 700°C
Introduction
What is a thermoelectric material?
Material that converts a thermal gradient to electrical energy1, 2.
Material System
FeSiGe is a multiphase material composed of β-FeSi2 and SiGe formed by a
eutectoid decomposition of α-FeSi2 as shown in the binary phase diagram (Fig. 1). This
alloy is attractive for thermoelectric use because the raw materials used to create this
material are earth abundant, non-toxic and, presumably, oxidation resistant for waste
heat recovery.
Why are we doing this?
It is not known how this material will oxidize at higher temperatures. This is
especially important to know on the hot side of the thermoelectric device, in which
oxidation is most likely to happen. High temperature multiphase oxidation can
complicate the oxidation behavior of a material3. Three forms of oxide products4 (Fig. 2)
can form during the oxidation of multiphase materials like FeSiGe.
1. The two phases can oxidize independently to form non-uniform scale.
2. Two phases oxidize cooperatively to form uniform scale.
3. Solute rich second phase acts as reservoir for continued growth of solute scale.
(Oxidation of one phase dominates)
The particular oxidation product formed on a multiphase material is dominated
by the thermodynamics and kinetics of the possible oxidation reactions. Possible
oxidation reactions are listed in Fig. 35,6. Thermodynamic predictions on the oxide to be
formed indicate that SiO2 will be the oxide formed in these conditions.
High temperature thermal gradients could have various effects on the material.
Oxidation could consume the thermoelectric substrate at a higher rate, or the higher
temperature could transform the microstructure via coarsening, reducing the
effectiveness of the thermoelectric device. Therefore, it is important to learn the
behavior of this material to confirm that it is appropriate for high temperature use.
Fig. 8 Oxygen EDS maps to indicate oxidation on material. For oxidation at 600°C, little to no
oxidation was observed until 96 hours, where definitive oxygen gain on the surface of FeSiGe is seen.
Fig. 9 SEM images and oxygen EDS maps shown for oxidation exposures at 600°C. Some oxidations
occurs, increasing with time.
Fig. 10 SEM images and oxygen EDS maps of bulk FeSiGe after 900°C oxidation exposures. Oxygen
increases significantly compared to 600°C exposures. SiO2 growth hypothesized.
Fig. 12 Images of FeSiGe melt spun specimen before and after
oxidation at 900 C for 24 hours in air. Note the interference color on
the post-exposure due to formation of a thin oxide layer9.
Experimental: Characterization
Characterization
• Gravimetry: weight change measured for
oxidation kinetics
• Scanning Electron Microscopy (SEM): topography
and compositional data, secondary and
backscattered electrons measured
• Energy Dispersive Spectroscopy (EDS): elemental
and compositional data, x-rays analyzed Fig. 7 Electron interaction
volume. Electron/waves at
specific energies give different
info. on material.
oxidation oxide interference colors
600°C Ribbon: Insignificant oxidation
600°C Bulk: Minimal oxidation
900°C Bulk: Significant Oxidation
Preferential Oxidation of Germanium
Results: Oxidation Time Dependence (cont’d)
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
72 22.9 54.5 8.8 13.8
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
120 20.2 53.2 8.4 18.2
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
96 21.6 54.2 7.4 16.8
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
48 21.2 51.2 7.4 20.2
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
96 22.3 47.7 7.5 22.5
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
120 21.3 43.5 7.9 27.3
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
48 16.1 40.6 6.2 37.1
Ox. Time
(hours) Fe (at%) Si (at%)
Ge
(at%) O (at%)
72 12.6 40.4 5.6 41.4
Ox. Time
(hours) Fe (at%) Si (at%) Ge (at%)O (at%)
120 12.7 39.3 4.8 43.2
Fig. 11 Electron image and EDS maps of ribbon FeSiGe after 600°C for 96 hours. Definitive oxygen gain
detected on the surface of FeSiGe favoring Ge-rich sites consistent with prior literature for SiGe8.
Hypothesized SiO2 Formation
β-
FeSi2 SiGe
Fig. 4 Pre Ox.
Bulk FeSiGe
SEM Image
SiGe
β-
FeSi2
Fig. 6 Model of furnace set up
Insulating material
Alumina boat
Fused silica slide
Fig. 5 Pre Ox.
Ribbon FeSiGe
SEM Image
Fig. 1 Phase diagram of Fe-Si. α-FeSi2
transforms to β-FeSi2 at 937ºC.
Oxygen Germanium
Silicon
Fe Si Ge
𝑭𝒆 𝒔 +
𝟏
𝟐
𝑶 𝟐(𝒈)
→ 𝑭𝒆𝑶(𝒔)
𝑺𝒊 𝒔 + 𝑶 𝟐(𝒈)
→ 𝑺𝒊𝑶 𝟐(𝒔)
𝐆𝒆 𝒔 + 𝑶 𝟐
→ 𝑮𝒆𝑶 𝟐
𝟒𝑭𝒆 𝒔 + 𝟑𝑶 𝟐(𝒈)
→ 𝟐𝑭𝒆 𝟐 𝑶 𝟑
𝟑𝑭𝒆 𝒔 + 𝟐𝑶 𝟐(𝒈)
→ 𝑭𝒆 𝟑 𝑶 𝟒(𝒔)
Iron
Minimal Microstructural Coarsening Effects
Fig. 13 Spacing between FeSi2 remains consistently around 7-10 microns, and 2-3 microns, for bulk and
ribbon respectively.
7 microns
7 microns
10 microns
8 microns
2 microns 2 microns 3 microns 2 micronsFig. 2 Multiphase oxidation modes.
Fig. 3 Possible oxidation reactions in FeSiGe.
Bulk
Ribbon
600 C, 72 hours 650 C, 72 hours 700 C, 72 hours 700 C, 31 days
20 μm
4 µm