Wen-Pin Hsieh

Effect of mass disorder on the lattice thermal conductivity of MgO periclase under pressure

Thermal conductivity of mantle materials controlling the heat balance and thermal evolution of the Earth remains poorly constrained as the available experimental and theoretical techniques are limited in probing minerals under the relevant conditions. We report measurements of thermal conductivity of MgO at high pressure up to 60 GPa and 300 K via diamond anvil cells using the time-domain thermoreflectance technique. These measurements are complemented by model calculations which take into account the effect of temperature and mass disorder of materials within the Earth. Our model calculations agree with the experimental pressure dependencies at 300 and 2000 K for MgO. Furthermore, they predict substantially smaller pressure dependence for mass disordered materials as the mechanism of scattering changes. The calculated thermal conductivity at the core-mantle boundary is smaller than the majority of previous predictions resulting in an estimated total heat flux of 10.4 TW, which is consistent with modern geomodeling estimates.

Interpreting picosecond acoustics in the case of low interface stiffness

Analysis of data acquired in time-domain thermoreflectance (TDTR) experiments requires accurate measurements of the thickness of the metal film optical transducer that absorbs energy from the pump optical pulse and provides a temperature dependent reflectivity that is interrogated by the probe optical pulse. This thickness measurement is typically accomplished using picosecond acoustics. The presence of contaminants and native oxides at the interface between the sample and transducer often produce a picosecond acoustics signal that is difficult to interpret. We describe heuristics for addressing this common difficulty in interpreting picosecond acoustic data. The use of these heuristics can reduce the propagation of uncertainties and improve the accuracy of TDTR measurements of thermal transport properties.

Evidence for photo-induced monoclinic metallic VO2 under high pressure

We combine ultrafast pump-probe spectroscopy with a diamond-anvil cell to decouple the insulator-metal electronic transition from the lattice symmetry changing structural transition in the archetypal strongly correlated material vanadium dioxide. Coherent phonon spectroscopy enables tracking of the photo-excited phonon vibrational frequencies of the low temperature, monoclinic (M1)-insulating phase that transforms into the metallic, tetragonal rutile structured phase at high temperature or via non-thermal photo-excitations. We find that in contrast with ambient pressure experiments where strong photo-excitation promptly induces the electronic transition along with changes in the lattice symmetry, at high pressure, the coherent phonons of the monoclinic (M1) phase are still clearly observed upon the photo-driven phase transition to a metallic state. These results demonstrate the possibility of synthesizing and studying transient phases under extreme conditions.

Thermal conductivity of methanol-ethanol mixture and silicone oil at high pressures

4:1 methanol-ethanol (ME) mixture and silicone oil are common, important pressure transmitting media used in high pressure diamond anvil cell (DAC) experiments. Their thermal conductivities and elastic properties are critical for modeling heat conduction in the DAC experiments and for determining thermal conductivity of measurement samples under extreme conditions. We used time-domain thermoreflectance and picosecond interferometry combined with the DAC to study the thermal conductivities and elastic constants C11 of the ME mixture and silicone oil at room temperature and to pressures as high as ≈23 GPa. We found that pressure dependence of the thermal conductivity of ME and silicone oil are both well described by the prediction of the minimum thermal conductivity model, confirming the diffusion of thermal energy between nonpropagating molecular vibrational modes is the dominant heat transport mechanism in a liquid and amorphous polymer. Our results not only provide new insights into the physics of therma...

High-pressure Raman spectroscopy of phase change materials

We used high-pressure Raman spectroscopy to study the evolution of vibrational frequencies of the phase change materials (PCMs) Ge2Sb2Te5, GeSb2Te4, and SnSb2Te4. We found that the critical pressure for triggering amorphization in the PCMs decreases with increasing vacancy concentration, demonstrating that the presence of vacancies, rather than differences in the atomic covalent radii, is crucial for pressure-induced amorphization in PCMs. Compared to the as-deposited amorphous phase, the pressure-induced amorphous phase has a similar vibrational spectrum but requires much lower laser power to transform into the crystalline phase, suggesting different kinetics of crystallization, which may have implications for applications of PCMs in non-volatile data storage.

Emission properties of a dual ion/electron point emitter based on In–Bi alloy

A stable dual ion/electron point emitter based on In–Bi alloy has been fabricated. Its performance as a liquid metal ion source (LMIS) at 70–100 °C, which is much lower than the operating temperature of the Au–In emitter reported previously, is comparable to a typical Ga–LMIS. By terminating the ion emission using a specific solidification process under the presence of an extraction voltage, the solidified tip is transformed into a sharp field electron emitter with decent emission characteristics. The In–Bi alloy source is not only a LMIS for conventional focused ion beam systems but also a potential candidate for single-column dual focused ion/electron beam systems.

Ta and Au(Pd) alloy metal film transducers for time-domain thermoreflectance at high pressures

We studied the pressure dependence of the thermoreflectance and piezo-optical coefficients of metal film transducers—Al, Ta, and Au(Pd) alloy (≈5 at. % Pd)—at a laser wavelength, 785 nm, commonly used in time-domain thermoreflectance (TDTR) and picosecond acoustics experiments. Al has exceptionally high thermoreflectance at ambient pressure, dR/dT ≈ 1.3 × 10−4 K−1, but its applicability at high temperatures is limited by the low melting temperature. The thermoreflectance of Al also has an undesirable zero-crossing near 6 GPa. The thermoreflectance values of Ta and Au(Pd) are comparable to that of Al at ambient conditions but independent of pressure in the pressure range 0 < P < 10 GPa. Ta and Au(Pd) thin film transducers also show strong picosecond acoustic echoes at all pressures in this range. We conclude that Ta and Au(Pd) metal film transducers can replace Al in TDTR experiments and thereby facilitate the extension of TDTR methods to high pressures and temperatures.

Reduced lattice thermal conductivity of Fe-bearing bridgmanite in Earth's deep mantle

Complex seismic, thermal, and chemical features have been reported in Earths lowermost mantle. In particular, possible iron enrichments in the large low shear-wave velocity provinces (LLSVPs) could influence thermal transport properties of the constituting minerals in this region, altering the lower mantle dynamics and heat flux across core-mantle boundary (CMB). Thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of Earths lower mantle. Importantly, the pressure-induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity, but these effects remain largely unknown. Here we precisely measured the lattice thermal conductivity of Fe-bearing bridgmanite to 120 gigapascals using optical pump-probe spectroscopy. The conductivity of Fe-bearing bridgmanite increases monotonically with pressure, but drops significantly around 45 gigapascals due to pressure-induced lattice distortion on iron sites. Our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought. The decrease in the thermal conductivity of bridgmanite in mid-lower mantle and below would promote mantle flow against a potential viscosity barrier, facilitating slabs crossing over the 1000-km depth. Modeling of our results applied to LLSVPs shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease, which would enhance internal convective flow. Our CMB heat flux modeling indicates that, while heat flux variations are dominated by thermal effects, variations in thermal conductivity also play a significant role. The CMB heat flux map we obtained is substantially different from those assumed so far, which may influence our understanding of the geodynamo.

Hydration-reduced lattice thermal conductivity of olivine in Earth’s upper mantle

Significance Thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs. However, the effect of hydration on lattice thermal conductivity remains poorly understood. We studied lattice thermal conductivity of olivine (Mg0.9Fe0.1)2SiO4 (Fo90) to 15 GPa using ultrafast optics. The thermal conductivity of hydrous Fo90 with ∼7,000 wt ppm water is 2 times smaller than its anhydrous counterpart at transition zone pressures. Modeling thermal structure of a subducting slab shows that the hydration-reduced thermal conductivity in the oceanic crust further decreases the temperature within the subducting slab, which substantially lowers the olivine−wadsleyite transformation rate and extends the metastable olivine to greater depths. Such an effect could enhance water transportation to the transition zone. Earth’s water cycle enables the incorporation of water (hydration) in mantle minerals that can influence the physical properties of the mantle. Lattice thermal conductivity of mantle minerals is critical for controlling the temperature profile and dynamics of the mantle and subducting slabs. However, the effect of hydration on lattice thermal conductivity remains poorly understood and has often been assumed to be negligible. Here we have precisely measured the lattice thermal conductivity of hydrous San Carlos olivine (Mg0.9Fe0.1)2SiO4 (Fo90) up to 15 gigapascals using an ultrafast optical pump−probe technique. The thermal conductivity of hydrous Fo90 with ∼7,000 wt ppm water is significantly suppressed at pressures above ∼5 gigapascals, and is approximately 2 times smaller than the nominally anhydrous Fo90 at mantle transition zone pressures, demonstrating the critical influence of hydration on the lattice thermal conductivity of olivine in this region. Modeling the thermal structure of a subducting slab with our results shows that the hydration-reduced thermal conductivity in hydrated oceanic crust further decreases the temperature at the cold, dry center of the subducting slab. Therefore, the olivine−wadsleyite transformation rate in the slab with hydrated oceanic crust is much slower than that with dry oceanic crust after the slab sinks into the transition zone, extending the metastable olivine to a greater depth. The hydration-reduced thermal conductivity could enable hydrous minerals to survive in deeper mantle and enhance water transportation to the transition zone.

Effects of iron on the lattice thermal conductivity of Earth’s deep mantle and implications for mantle dynamics

Significance Presence of iron in Earth’s lower mantle may critically affect its physical properties and thermochemical structures. However, its effects on thermal conductivity and dynamics of deep mantle remain unknown. We studied lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using ultrafast optics. We observed an enhanced iron substitution effect in the low-spin iron-rich ferropericlase, with thermal conductivity that significantly drops across spin transition. Combined with bridgmanite data, we provided a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects and shows a twofold increase throughout the lower mantle. If ultralow velocity zones are hot and strongly enriched in iron, their exceptionally low thermal conductivity will delay their cooling, influencing lowermost mantle dynamics. Iron may critically influence the physical properties and thermochemical structures of Earth’s lower mantle. Its effects on thermal conductivity, with possible consequences on heat transfer and mantle dynamics, however, remain largely unknown. We measured the lattice thermal conductivity of lower-mantle ferropericlase to 120 GPa using the ultrafast optical pump-probe technique in a diamond anvil cell. The thermal conductivity of ferropericlase with 56% iron significantly drops by a factor of 1.8 across the spin transition around 53 GPa, while that with 8–10% iron increases monotonically with pressure, causing an enhanced iron substitution effect in the low-spin state. Combined with bridgmanite data, modeling of our results provides a self-consistent radial profile of lower-mantle thermal conductivity, which is dominated by pressure, temperature, and iron effects, and shows a twofold increase from top to bottom of the lower mantle. Such increase in thermal conductivity may delay the cooling of the core, while its decrease with iron content may enhance the dynamics of large low shear-wave velocity provinces. Our findings further show that, if hot and strongly enriched in iron, the seismic ultralow velocity zones have exceptionally low conductivity, thus delaying their cooling.