Internal Resistive Heating Techniques for Studies of High Pressure-Temperature Material Properties in Solid and Liquid States

The laser-heated diamond anvil cell grants access to the conditions of deep planetary interiors in the laboratory. However, large thermal gradients and temporal heating instabilities can lead to uncertainties of several hundred Kelvin that limit the accuracy of measured data, can prohibit analyses requiring long acquisition times, and makes the production of stable melt volumes technically challenging. Resistive heating techniques provide improved temperature stability, precise control, reduced thermal gradients and significantly reduced uncertainties, but external heating techniques are limited to ~1500 K, beyond which the unpressurized parts of the anvils graphitize. Internal resistive heating (IRH), where heat is generated only within the pressure chamber, can extend the accessible temperature range significantly. We recently reported the development of a new IRH design that employs a novel 'split-gasket' approach to isolate the filament which allows stable, homogeneous heating of both metallic and non-metallic samples to temperatures over 3000 K at pressures up to ~65 GPa [1]. The IRH design is well suited to the rapid collection of high-resolution P-V-T datasets, the precise demarcation of phase boundaries, and experiments requiring long acquisition times. Here we report recent progress on further IRH development and its application to the study of melt phases. The design can sustain temperatures up to 4000 K and generate large, stable volumes of melt, either by wholly melting material contained in a distinct sample chamber or via melting of the metallic filament doubling as the sample. This provides a convenient experimental tool to determine high-pressure melt structure from x-ray diffuse scattering data. The filament melting temperature can also be inferred from changes in the electrical resistance of the heating circuit, providing a convenient lab-based melting criterion as a complement or alternative to the appearance of liquid diffuse scattering in an x-ray diffraction signal [2] or the appearance of a plateau in laser power vs temperature functions [3]. An application of IRH to the high-pressure melting curve of titanium is presented.

References: [1] Heinen et al. 2021 Rev. Sci. Instrum. 92, 063904 [2] Dewaele et al. 2007 Phys. Rev. B 76(14), 144106 [3] Lord et al. 2009 EPSL 284(1), 157-167.