Latent Heat Method to Detect Melting and Freezing of Metals at Megabar Pressures

The high-pressure melting curves of metals provide simple and useful tests for theories of melting, as well as important constraints for the modeling of planetary interiors. Here, we present an experimental technique that reveals the latent heat of fusion of a metal sample compressed inside a diamond anvil cell. The technique combines microsecond-timescale pulsed electrical heating with an internally heated diamond anvil cell. Further, we use the technique to measure the melting curve of platinum to the highest pressure measured to date. Melting temperature increases from $\approx 3000$ K at 34 GPa to $\approx 4500$ K at 107 GPa, thermodynamic conditions that are between the steep and shallow experimental melting curves reported previously. The melting curve is a linear function of compression over the 0–20 % range of compression studied here, allowing a good fit to the Kraut-Kennedy empirical model with fit parameter $C = 6.0$ .

Zachary M. Geballe, Nicholas Holtgrewe, Amol Karandikar, Eran Greenberg, Vitali B. Prakapenka, and Alexander F. Goncharov, “Latent heat method to detect melting and freezing of metals at megabar pressures”, (2021) Phys. Rev. Materials 5, Iss. 3, 033803, DOI: https://doi.org/10.1103/PhysRevMaterials.5.033803 abstract

Schematic of electrical path (black), optical paths (red), and diamond anvils (blue) at the Carnegie Institution for Science. A regulated dc power supply charges a capacitor bank ( $C_{bank}$ : 470 $μ F$ , 70 V electrolytic). When triggered by the Delay generator (SRS DG645), the MOSFET (FQP30N06L) allows current to flow through a reference resistor ( $R_{ref} = 0.29 Ω$ ), and the platinum sample that is compressed between diamond anvils. The snubber capacitor ( $C_{snub}$ : 16 $μ F$ , 100 V electrolytic) limits current oscillations. The circuitry for measuring current and four-point-probe voltage are shown in thin black lines. The voltage dividers, Vdiv, reduce input voltage to within the 15 V range of the in-amp (AD842). Each divider is made of two resistors with typical values of 1 $k Ω$ and 10 $k Ω$ . The in-amp is operated with no gain, referenced to ground, and connected through output resistors ( $R_{out}$ : 105 $Ω$ ) to the oscilloscope (Tektronix DPO 3034). A simplified optical path is shown here; see McWilliams et al. [37] for elaboration. During each heating pulse, one flipper mirror (FM) diverts light from the left or right side of the diamond cell to a CCD camera (Point Grey Grasshopper3 Color) for two-dimensional imaging of thermal emissions. The other flipper mirror (FM) does not divert the light, allowing it to pass into a confocal filtering system, then into a spectrometer (Princeton Instruments Acton SP2300) and streak camera (Sydor ROSS 1000) for time-resolved measurements of thermal emissions. Solid red lines show the path of light in one configuration; dashed lines show the alternative configuration. Ovals represent lenses, line segments at $45^{\circ}$ represent mirrors, and broken line segments represent pinholes.

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