High-Pressure, High-Temperature Equations of State Using Fabricated Controlled-Geometry Ni/SiO2 Double Hot-Plate Samples

To model and predict the structure, dynamics, and composition of Earth’s deep interior, accurate and precise measurements of thermal expansion and compressibility are required. The laser-heated diamond-anvil cell (LHDAC) coupled with synchrotron-based x-ray diffraction (XRD) is a powerful tool to determine pressure-volume-temperature (P-V-T) relationships. However, LHDAC experiments may be hampered by non-uniform heating caused by the mixing of transparent materials with opaque laser absorbers. Additionally, radial temperature gradients are exacerbated by small misalignments (1-3 µm) of the x-ray beam with respect to the center of the laser-heated hotspot. We have fabricated three-dimensional, controlled-geometry, double hot-plate samples. In this double hot-plate arrangement, a transparent oxide layer (SiO₂) is sandwiched between two laser absorbing layers (Ni) in a single, cohesive sample. These samples were mass manufactured (>10⁵ samples) using a combination of physical vapor deposition, photolithography, wet etching, and plasma etching. The double hot-plate arrangement coupled with the chemical and spatial homogeneity of the laser absorbing layers addresses problems caused by mixtures of transparent and opaque samples. The controlled-geometry samples have dimensions of 50 µm x 50 µm x 1.4 µm. The dimensions of the samples are much larger than the synchrotron x-ray beam. With a heating laser FWHM of ~50 µm, the radial temperature gradients within the volume probed by the x-ray are reduced. We conducted XRD experiments to P > 50 GPa and T > 2200 K at beamline 16-ID-B (HPCAT) of the Advanced Photon Source. Here we present relevant thermal modeling of the LHDAC environment along with Ni and SiO₂ P-V-T equations of state. Our photolithography method of sample fabrication can be extended to different materials including but not limited to Fe and MgO.