Temperature and temperature gradient measurement method in the laser-heated diamond anvil cell
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Date
2007-06
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The Ohio State University
Abstract
The determination of material properties under the conditions of the Earth's interior requires experiments performed at temperatures up to 6000 K, and pressures in excess of 100 million times atmospheric pressure. To achieve these conditions, samples of less than 100 ng are heated with an infrared laser focused to an area of about 7x10-6cm² inside a laser heated diamond anvil cell (LHDAC). The temperature measurement method is based on spectroradiometry of blackbody thermal emission. Most current methods suffer from chromatic aberrations in which blue light is focused more tightly than red light due to wavelength-dependent refraction through optical components. Existing spectroradiometry systems are subject to aberrations from the dispersion of the diamond and refractive optics. These systems also do not constrain temperature and wavelength-dependent emissivity. Although the peak temperature measurement is accurate, the temperature gradient can be underestimated by as much as 100% (Kavner and Panero, 2004). I present a calibration of a newly designed optical system to accurately measure the temperature and temperature gradient of a high pressure sample. The redesigned optical system splits the emitted, focused light from a small sample into two paths. One system, at the end of the first path is a spectrometer and CCD that collects intensity at the center of the sample where the light is selected by a 10 ?m slit. The other path leads to a high-dynamic range CCD camera that collects a 15x magnified image of the sample to measure total intensity as a function of position. With this data, the peak temperature, least affected by chromatic aberrations, provides a scaling factor for the spatial measurement to produce a two dimensional measure of the temperature across the sample. Data sets are collected simultaneously on both optical paths and then processed in a manner to relate temperature to intensity on an image of the sample. In this way, temperature can be measured at any point on the sample in situ. By use of a scaling factor in temperature measurements this setup lessens the effects of chromatic aberrations as well as creates opportunity to perform experiments which necessitate a higher degree of accuracy than allowed by current methods.