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    Research items
    Ph.D. degree in Materials Science and Engineering. My expertise is synchrotron x-ray characterization of materials and melts using x-ray diffraction (XRD). My current research area includes melting of metals at ultrahigh pressure, materials crystal structure at high pressure and high temperature, and high pressure / high temperature physics and chemistry.
    Research Experience
    Jan 2017
    Research Scientist
    • Research Scientist
    Oct 2013 - Dec 2016
    PostDoc Position
    • PostDoc Position
    Current institution
    Carnegie Institution for Science
    Current position
    • Research Scientist
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    Research Items (18)
    High-pressure melting anchors the phase diagram of a material, revealing the effect of pressure on the breakdown of the ordering of atoms in the solid. An important case is molybdenum, which has long been speculated to undergo an exceptionally steep increase in melting temperature when compressed. On the other hand, previous experiments showed nearly constant melting temperature as a function of pressure, in large discrepancy with theoretical expectations. Here we report a high-slope melting curve in molybdenum by synchrotron X-ray diffraction analysis of crystalline microstructures, generated by heating and subsequently rapidly quenching samples in a laser-heated diamond anvil cell. Distinct microstructural changes, observed at pressures up to 130 gigapascals, appear exclusively after melting, thus offering a reliable melting criterion. In addition, our study reveals a previously unsuspected transition in molybdenum at high pressure and high temperature, which yields highly textured body-centred cubic nanograins above a transition temperature.
    Understanding the ultralow velocity zones (ULVZs) places constraints on the chemical composition and thermal structure of deep Earth and provides critical information on the dynamics of large-scale mantle convection, but their origin has remained enigmatic for decades. Recent studies suggest that metallic iron and carbon are produced in subducted slabs when they sink beyond a depth of 250 km. Here we show that the eutectic melting curve of the iron-carbon system crosses the current geotherm near Earth's core-mantle boundary, suggesting that dense metallic melt may form in the lowermost mantle. If concentrated into isolated patches, such melt could produce the seismically observed density and velocity features of ULVZs. Depending on the wetting behavior of the metallic melt, the resultant ULVZs may be short-lived domains that are replenished or regenerated through subduction, or long-lasting regions containing both metallic and silicate melts. Slab-derived metallic melt may produce another type of ULVZ that escapes core sequestration by reacting with the mantle to form iron-rich postbridgmanite or ferropericlase. The hypotheses connect peculiar features near Earth's core-mantle boundary to subduction of the oceanic lithosphere through the deep carbon cycle.
    We have designed and constructed a new system for micro-machining parts and sample assemblies used for diamond anvil cells and general user operations at the High Pressure Collaborative Access Team, sector 16 of the Advanced Photon Source. The new micro-machining system uses a pulsed laser of 400 ps pulse duration, ablating various materials without thermal melting, thus leaving a clean edge. With optics designed for a tight focus, the system can machine holes any size larger than 3 μm in diameter. Unlike a standard electrical discharge machining drill, the new laser system allows micro-machining of non-conductive materials such as: amorphous boron and silicon carbide gaskets, diamond, oxides, and other materials including organic materials such as polyimide films (i.e., Kapton). An important feature of the new system is the use of gas-tight or gas-flow environmental chambers which allow the laser micro-machining to be done in a controlled (e.g., inert gas) atmosphere to prevent oxidation and other chemical reactions in air sensitive materials. The gas-tight workpiece enclosure is also useful for machining materials with known health risks (e.g., beryllium). Specialized control software with a graphical interface enables micro-machining of custom 2D and 3D shapes. The laser-machining system was designed in a Class 1 laser enclosure, i.e., it includes laser safety interlocks and computer controls and allows for routine operation. Though initially designed mainly for machining of the diamond anvil cell gaskets, the laser-machining system has since found many other micro-machining applications, several of which are presented here.
    An overview of the in situ laser heating system at the High Pressure Collaborative Access Team, with emphasis on newly developed capabilities, is presented. Since its establishment at the beamline 16-ID-B a decade ago, laser-heated diamond anvil cell coupled with in situ synchrotron x-ray diffraction has been widely used for studying the structural properties of materials under simultaneous high pressure and high temperature conditions. Recent developments in both continuous-wave and modulated heating techniques have been focusing on resolving technical issues of the most challenging research areas. The new capabilities have demonstrated clear benefits and provide new opportunities in research areas including high-pressure melting, pressure-temperature-volume equations of state, chemical reaction, and time resolved studies.
    We have fabricated novel controlled-geometry samples for the laser-heated diamond-anvil cell (LHDAC) in which a transparent oxide layer (SiO2) is sandwiched between two laser-absorbing layers (Ni) in a single, cohesive sample. The samples were mass manufactured (>10^4 samples) using a combination of physical vapor deposition, photolithography, and wet and plasma etching. The double hot-plate arrangement of the samples, coupled with the chemical and spatial homogeneity of the laser-absorbing layers, addresses problems of spatial temperature heterogeneities encountered in previous studies where simple mechanical mixtures of transparent and opaque materials were used. Here we report thermal equations of state (EOS) for nickel to 100 GPa and 3000 K and stishovite to 50 GPa and 2400 K obtained using the LHDAC and in situ synchrotron X-ray microdiffraction. We discuss the inner core composition and the stagnation of subducted slabs in the mantle based on our refined thermal EOS.
    We report the discovery of a long-sought-after phase of titanium nitride with stoichiometry Ti3N4 using diamond anvil cell experiments combined with in situ high-resolution x-ray diffraction and Raman spectroscopy techniques, supported by ab initio calculations. Ti3N4 crystallizes in the cubic Th3P4 structure [space group I4¯3d (220)] from a mixture of TiN and N2 above ≈75 GPa and ≈2400 K. The density (≈5.22 g/cc) and bulk modulus (K0=290 GPa) of cubic-Ti3N4 (c−Ti3N4) at 1 atm, estimated from the pressure-volume equation of state, are comparable to rocksalt TiN. Ab initio calculations based on the GW approximation and using hybrid functionals indicate that c−Ti3N4 is a semiconductor with a direct band gap between 0.8 and 0.9 eV, which is larger than the previously predicted values. The c−Ti3N4 phase is not recoverable to ambient pressure due to dynamic instabilities, but recovery of Ti3N4 in the defect rocksalt (or related) structure may be feasible.
    The pressure-dependent phase behavior of semiconducting chalcopyrite ZnSiP2 was studied up to 30 GPa using in situ X-ray diffraction and Raman spectroscopy in a diamond-anvil cell. A structural phase transition to the rock salt type structure was observed between 27 and 30 GPa, which is accompanied by soft phonon mode behavior and simultaneous loss of Raman signal and optical transmission through the sample. The high-pressure rock salt type phase possesses cationic disorder as evident from broad features in the X-ray diffraction patterns. The behavior of the low-frequency Raman modes during compression establishes a two-stage, order-disorder phase transition mechanism. The phase transition is partially reversible, and the parent chalcopyrite structure coexists with an amorphous phase upon slow decompression to ambient conditions.
    Supplementary Figures, Supplementary Tables, Supplementary Discussion, Supplementary Methods and Supplementary References
    Laser-assisted processing and in-situ characterization of a Ni0.7-Al0.1235-Co0.15-Ti0.0265 alloy was carried out under a range of simultaneous hydrostatic high pressures of ∼ 30 GPa and high temperature conditions ∼2000 °C using a laser assisted heating in diamond anvil cell with synchrotron x-ray micro-diffraction. The characterization of the microstructure and X-ray diffraction analysis at ambient conditions confirmed the formation of the cuboids of ordered γ’ phase in the disordered γ matrix. The isothermal bulk modulus (B0) and its first order derivative (B0’) of the alloy were determined to be B0 = 123 ± 9 GPa and B0' = 5.7 ± 2.8. The in-situ characterization of the alloy at high temperatures under high pressures revealed that the γ’ phase transforms into the tetragonaly-distorted D022 type structure. This transformation is similar to the transformation that occurs in the ordered Ni3Al, responsible for the improved strength at high temperatures. High pressure was found to increase the onset temperature of the structural distortion. The pressure-temperature phase diagram of the Ni0.7-Al0.1235-Co0.15-Ti0.0265 up to ∼ 30 GPa and ∼2000 °C was determined and is reported here.
    We demonstrate that the condensation theory of planet formation yields solids of suitable compositions in the solar nebula that accrete to form the terrestrial planetary bodies. The mineral chemistry of the condensed objects provides definite criteria to establish the pressure and temperature of their formation. The solids condensing at a high nebular pressure of 0.01 to 0.001 bar and temperature of ∼1530 K∼1530 K had the best chemical composition and density to form Mercury (64 wt% iron and 36 wt% oxides, density ∼5.32gcm3). Solids that condensed around a pressure of 0.0001 bar or less and a temperature of ∼700 K∼700 K formed Earth and Venus (31 wt% iron, Ni and S and 69 wt% oxides, density ∼4 gcm3), and Mars (33.6 wt% Fe and S and 66.4 wt% oxides, density ∼3.7 gcm3). Iron sulfide provided S (3 wt%) for the core. Hydrous minerals forming in the lower temperature region provided water to the mantle. These results are highly significant because we have used only the chemical composition of the solar nebula, thermochemistry and astrophysical data on densities of the planets.
    There is increasing demand for hermetic metal/ceramic bonds for application in biomedical engineering, in particular for use in neurostimulating prosthetic devices such as, cochlear implants, muscular stimulators and retinal prosthesis. Platinum/Alumina bonds are particularly interesting because of the proven biocompatibility of the two materials and their strong bonding. Yet, the true nature of their bonding is not clear. Platinum/alumina interactions in different atmosphere (i.e. air and hydrogen) and different temperatures were studied by means of high temperature X-ray diffraction, SEM and EDS analyses, to better understand the interfacial reactions and bonding mechanism. It was observed that upon heating the platinum/alumina system in the reduced atmosphere tetragonal Pt3Al formed in low temperature and transformed to cubic structure at higher temperatures. In addition to that, at temperatures above 1500 °C alumina could migrate and encapsulate the platinum particles, with particle migration mechanism.
    The elastic properties of high-quality ZnO crystals and nanopowder of grain size of about 65 nm are studied for both wurtzite (low pressure) and rock-salt high pressure phases. The measured values of bulk moduli for wurtzite and rock-salt phases of bulk ZnO crystals are equal to 156±13 and 187±20 GPa, respectively, and considerably larger for ZnO nanocrystals. The phase transition begins at a pressure of about 9 GPa and it is completed at a pressure of about 13.8 GPa for bulk crystals, whereas the values of pressure at which the phase transition occurs are lower for nanocrystals. A carefull Rietveld analysis of the obtained data does not exhibit the presence of any intermediate phases between low pressure wurtzite and high pressure rock-salt phases of ZnO. The phase transition is accompanied by a strong decrease in the near-band-gap photoluminescence intensity. In addition, the pressure coefficient of the near-band-gap luminescence in ZnO nanocrystals exhibits strong deviation from the linearity observed in bulk crystals. An analysis of the results shows that defects present in the nanopowdered sample are responsible for the observed effects.
    We measured the volume of hafnium at several pressures up to 67 GPa and at temperatures between 300 to 780 K using a resistively heated diamond anvil cell with synchrotron x-ray diffraction at the Advanced Photon Source. The measured data allows us to determine the P-V-T equation of state of hafnium. The previously described [Xia et al., Phys. Rev. B 42, 6736-6738 (1990)] phase transition from hcp (α) to simple hexagonal (Ï) phase at 38 GPa at room temperature was not observed even up to 51 GPa. The Ï phase was only observed at elevated temperatures. Our measurements have also improved the experimental constraint on the high P-T phase boundary between the Ï phase and high pressure bcc (β) phase of hafnium. Isothermal room temperature bulk modulus and its pressure derivative for the α-phase of hafnium were measured to be Bâ = 112.9{+-}0.5 GPa and Bâ'=3.29{+-}0.05, respectively. P-V-T data for the α-phase of hafnium was used to obtain a fit to a thermodynamic P-V-T equation of state based on model by Brosh et al. [CALPHAD 31, 173-185 (2007)].
    We have used the CALPHAD-compatible equation of state (EOS) based on the explicit Gibbs free energy concept for the solid state of ten important elements: V, Nb, Ta, Mo, W (groups VB and VIB), Pd, Pt (group VIIIB) and Cu, Ag and Au (group IB). The new formulation uses SGTE data for ambient pressure and converges to the quasi-harmonic model at the limit of extreme pressure to calculate the Gibbs free energy as a function of pressure and temperature. The model is based on the available pressure–volume–temperature (PVT) data on the elements and can be usefully extrapolated to extreme pressures. When compared to shock wave data, the modeled EOS holds well, but the fit is not totally satisfactory in the ultrahigh-pressure range. A great advantage of this formulation is that it can be used to calculate thermodynamic properties such as the heat capacity and entropy at very high temperatures and pressures.
    We will present a description of a system for the measurement thermal conductivity materials under high pressures (currently up to 50 GPa) and at high temperatures (1000C-3000C) using a single side laser heated diamond anvil cell (LH-DAC). We extract information about the material’s high-temperature thermal conductivity by measuring and analyzing the surface temperature distribution in the area of the laser heated spot. Heat transfer model typical of an LH-DAC, technical system design and several comparative results for a few chosen materials will be presented.
    Principal component analysis (PCA) is used to investigate interrelationship among material properties of hydrides. Property data which consist of ∼200 compounds (binary and ternary metal hydrides) were analyzed. A comparative study was carried out among the metal properties with that of their hydrides. The observed decrease or increase of entropy, molar volume and specific heat of hydrides from that of the metals can be attributed to hydrogen bond formation, charge transfer and corresponding change in crystal structure.
    A materials database that includes more than 2,500 solids with thermo-chemical and physical properties has been constructed to study the effect of chemical composition, structure, and constituent element properties on the solids. The correlation between compound entropy and the sum of constituent element entropy is clearly established and even better linear relations are possible when the solids are separated according to their crystal structure or compositional types such as carbide or oxide. A number of hydrides have been examined with this approach and many more studies involving other classes of compounds are possible.
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