REFINING SPECTRORADIOMETRIC TEMPERATURE DETERMINATION IN THE LASER-HEATED DIAMOND ANVIL CELL, WITH IMPACT ON THE EQUATION OF STATE OF IRON
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Date
2021-05
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The Ohio State University
Abstract
Quantification of the physical properties of iron under core conditions is necessary to understand the Earth's deep interior. Many experiments have been done to determine its high pressure, high temperature equation of state (EOS); however, many iron EsOS are mutually inconsistent. We explore sources of experimental temperature error with the potential impact that a lack of precision in experimental temperature may have on the EOS of iron in mind. Laser-heated diamond anvil cells (LHDACs) are widely used tools for researching materials at deep-Earth pressure and temperature conditions. Temperature in the LHDAC is determined by spectroradiometry, where the temperature (T) of a laser-heated sample is determined using the gray-body approximation of Planck's law of black-body radiation, which relates the measured intensity (I) of light emitted from the sample to the sample's temperature as a function of the wavelength of the emitted light (λ), assuming wavelength-independent emissivity (ε(T), "gray body assumption".
Sources of systematic temperature error in the LHDAC explored include effects of: (1) the gray-body assumption, (2) differences as a function of sample preparation, and (3) the effects of the optical system. To investigate these potential sources of error, we analyze apparent temperature as a function of wavelength as well as use synthetic data to quantify the effects of iron ε(λ,T) on temperature. We report the results of an analysis of 3,140 temperature measurements on iron collected at Argonne National Lab's Advanced Photon Source, at the beamlines HPCAT (16-ID-B) and GSECARS (13-ID-D), between 14-70 GPa and 1,000-2,900 K.
(1) We show with synthetic data that increasing emissivity as a function of wavelength results in a systematic underestimation of temperature, from 4% at 1000 K to over 10% at 2500 K. We find in our data that temperature generally decreases as a function of wavelength, consistent with an emissivity increase as a function of wavelength as is expected of iron. Further investigation is required to determine the source of the observed variation in temperature, which is likely caused by a combination of factors including but not limited to wavelength-dependent emissivity. (2) We find that foil iron samples have different trends in apparent temperatures as a function of wavelength than powder samples. We hypothesize that this could be due to the difference in surface properties or due to the emissivity response of other sample components besides iron. (3) We show with synthetic data that the difference between optical systems at HPCAT and GSECARS will cause a more severe underestimate of temperature by GSECARS. This prediction by synthetic data is reflected in our analysis of the apparent temperature as a function of wavelength of HPCAT and GSECARS data.
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Keywords
mineral physics, core, iron, diamond anvil cell