Valenzuela URN poster 0330

High Temperature Oxidation Studies of FeSiGeJonathan E. Valenzuela, Wade A. Jensen, Jerrold A. Floro, Elizabeth J. Opila

Department of Materials Science and EngineeringUniversity of Virginia, Charlottesville, VA 22904

ObjectiveCharacterize the oxidation behavior of FeSiGe at high temperatures.

Results: Oxidation Time DependenceOxidation 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.

SummaryFeSiGe 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

AcknowledgementsMany 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 TestsPreparation 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

IntroductionWhat is a thermoelectric material?Material that converts a thermal gradient to electrical energy1, 2.

Material SystemFeSiGe 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: CharacterizationCharacterization• 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


Ge (at%) O (at%)

120 20.2 53.2 8.4 18.2


Ge (at%) O (at%)

96 21.6 54.2 7.4 16.8


Ge (at%) O (at%)

48 21.2 51.2 7.4 20.2


Ge (at%) O (at%)

96 22.3 47.7 7.5 22.5


Ge (at%) O (at%)

120 21.3 43.5 7.9 27.3


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

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 microns Fig. 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

Valenzuela URN poster 0330

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