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Title: Thermal Expansion of Iron-Rich Alloys and Implications for the Earth's Core

Abstract

Understanding the thermal-chemical state of the Earth's core requires knowledge of the thermal expansion of iron-rich alloys at megabar pressures and high temperatures. Our survey of literature revealed a significant lack of such data. We have determined the unit-cell parameters of the iron-sulfur compound Fe{sub 3}S by using synchrotron x-ray diffraction techniques and externally heated diamond-anvil cells at pressures up to 42.5 GPa and temperatures up to 900 K. The zero-pressure thermal expansivity of Fe{sub 3}S is determined in the form {alpha} = a{sub 1} + a{sub 2} T, where a{sub 1} = 3.0 {+-} 1.3 x 10{sup -5} K{sup -1} and {alpha}{sub 2} = 2.8 {+-} 1.5 x 10{sup -8} K{sup -2}. The temperature dependence of isothermal bulk modulus ({partial_derivative}K{sub T,0}/{partial_derivative}T){sub P} is estimated at -3.75 {+-} 1.80 x 10{sup -2} GPa K{sup -1}. Our data at 42.5 GPa and 900 K suggest that {approx}2.1 at. % (1.2 wt. %) sulfur produces 1% density deficit in iron. We have also carried out energy-dispersive x-ray diffraction measurements on pure iron and Fe{sub 0.864}Si{sub 0.136} alloy samples that were placed symmetrically in the same multianvil cell assemblies, using the SPring-8 synchrotron facility in Japan. Based on direct comparison of unit cellmore » volumes under presumably identical pressures and temperatures, our data suggest that at most 3.2 at. % (1.6 wt. %) silicon is needed to produce 1% density deficit with respect to pure iron.« less

Authors:
; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
930441
Report Number(s):
BNL-81186-2008-JA
Journal ID: ISSN 0027-8424; PNASA6; TRN: US0901390
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Proceedings of the National Academy of Sciences of the USA; Journal Volume: 104; Journal Issue: 22
Country of Publication:
United States
Language:
English
Subject:
43 PARTICLE ACCELERATORS; ALLOYS; DIAMONDS; EARTH CORE; IRON; JAPAN; PRESSURE RANGE GIGA PA; SILICON; SULFUR; SYNCHROTRON RADIATION; SYNCHROTRON RADIATION SOURCES; TEMPERATURE DEPENDENCE; TEMPERATURE RANGE 0400-1000 K; THERMAL EXPANSION; X-RAY DIFFRACTION; national synchrotron light source

Citation Formats

Chen,B., Gao, L., Funakoshi, K., and Li, J. Thermal Expansion of Iron-Rich Alloys and Implications for the Earth's Core. United States: N. p., 2007. Web. doi:10.1073/pnas.0610474104.
Chen,B., Gao, L., Funakoshi, K., & Li, J. Thermal Expansion of Iron-Rich Alloys and Implications for the Earth's Core. United States. doi:10.1073/pnas.0610474104.
Chen,B., Gao, L., Funakoshi, K., and Li, J. Mon . "Thermal Expansion of Iron-Rich Alloys and Implications for the Earth's Core". United States. doi:10.1073/pnas.0610474104.
@article{osti_930441,
title = {Thermal Expansion of Iron-Rich Alloys and Implications for the Earth's Core},
author = {Chen,B. and Gao, L. and Funakoshi, K. and Li, J.},
abstractNote = {Understanding the thermal-chemical state of the Earth's core requires knowledge of the thermal expansion of iron-rich alloys at megabar pressures and high temperatures. Our survey of literature revealed a significant lack of such data. We have determined the unit-cell parameters of the iron-sulfur compound Fe{sub 3}S by using synchrotron x-ray diffraction techniques and externally heated diamond-anvil cells at pressures up to 42.5 GPa and temperatures up to 900 K. The zero-pressure thermal expansivity of Fe{sub 3}S is determined in the form {alpha} = a{sub 1} + a{sub 2} T, where a{sub 1} = 3.0 {+-} 1.3 x 10{sup -5} K{sup -1} and {alpha}{sub 2} = 2.8 {+-} 1.5 x 10{sup -8} K{sup -2}. The temperature dependence of isothermal bulk modulus ({partial_derivative}K{sub T,0}/{partial_derivative}T){sub P} is estimated at -3.75 {+-} 1.80 x 10{sup -2} GPa K{sup -1}. Our data at 42.5 GPa and 900 K suggest that {approx}2.1 at. % (1.2 wt. %) sulfur produces 1% density deficit in iron. We have also carried out energy-dispersive x-ray diffraction measurements on pure iron and Fe{sub 0.864}Si{sub 0.136} alloy samples that were placed symmetrically in the same multianvil cell assemblies, using the SPring-8 synchrotron facility in Japan. Based on direct comparison of unit cell volumes under presumably identical pressures and temperatures, our data suggest that at most 3.2 at. % (1.6 wt. %) silicon is needed to produce 1% density deficit with respect to pure iron.},
doi = {10.1073/pnas.0610474104},
journal = {Proceedings of the National Academy of Sciences of the USA},
number = 22,
volume = 104,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • We conducted high-pressure experiments on hexagonal close packed iron (hcp-Fe) in MgO, NaCl, and Ne pressure-transmitting media and found general agreement among the experimental data at 300 K that yield the best fitted values of the bulk modulus K 0 = 172.7(±1.4) GPa and its pressure derivative K 0'= 4.79(±0.05) for hcp-Fe, using the third-order Birch-Murnaghan equation of state. Using the derived thermal pressures for hcp-Fe up to 100 GPa and 1800 K and previous shockwave Hugoniot data, we developed a thermal equation of state of hcp-Fe. The thermal equation of state of hcp-Fe is further used to calculate themore » densities of iron along adiabatic geotherms to define the density deficit of the inner core, which serves as the basis for developing quantitative composition models of the Earth's inner core. We determine the density deficit at the inner core boundary to be 3.6%, assuming an inner core boundary temperature of 6000 K.« less
  • The sound velocities of (Mg{sub .16}Fe{sub .84})O have been measured to 121 GPa at ambient temperature using nuclear resonant inelastic x-ray scattering. The effect of electronic environment of the iron sites on the sound velocities were tracked in situ using synchrotron Moessbauer spectroscopy. We found the sound velocities of (Mg{sub .16}Fe{sub .84})O to be much lower than those in other presumed mantle phases at similar conditions, most notably at very high pressures. Conservative estimates of the effect of temperature and dilution on aggregate sound velocities show that only a small amount of iron-rich (Mg,Fe)O can greatly reduce the average soundmore » velocity of an assemblage. We propose that iron-rich (Mg,Fe)O be a source of ultra-low velocity zones. Other properties of this phase, such as enhanced density and dynamic stability, strongly support the presence of iron-rich (Mg,Fe)O in localized patches above the core-mantle boundary.« less
  • Iron and nickel aluminides are ordered intermetallic alloys that are being developed for high-temperature structural applications because they have good oxidation resistance at > 1,000 C due to the formation of adherent alumina firms. More complex aluminide alloys have recently been developed to improve their mechanical properties behavior and to improve oxidation/corrosion resistance. The Fe[sub 3]Al alloy composition has a B2-ordered phase structure at about 550 to 900 C but transforms to the D0[sub 3] ordered phase at lower temperatures. While a considerable amount of mechanical property, structural microstructural, and corrosion data has been generated in the development of themore » iron- and nickel-aluminide alloys, there have been correspondingly little physical properties data. Physical properties like thermal expansion are basic material parameters used by engineers to select materials and by designers to perform stress analyses. The purpose of this paper is to present the preliminary thermal expansion data measured on leading or representative new alloys being developed from each of these three iron- or nickel-aluminide alloy types.« less