Browsing by Author "Rost, Christina M."
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- High-entropy oxides: Harnessing crystalline disorder for emergent functionalityKotsonis, G. N.; Almishal, S. S. I.; Marques dos Santos Vieira, F.; Crespi, V. H.; Dabo, I.; Rost, Christina M.; Maria, J. P. (Wiley, 2023-06-24)High-entropy materials defy historical materials design paradigms by leveraging chemical disorder to kinetically stabilize novel crystalline solid solutions comprised of many end-members. Formulational diversity results in local crystal structures that are seldom found in conventional materials and can strongly influence macroscopic physical properties. Thermodynamically prescribed chemical flexibility provides a means to tune such properties. Additionally, kinetic metastability results in many possible atomic arrangements, including both solid-solution configurations and heterogeneous phase assemblies, depending on synthesis conditions. Local disorder induced by metastability, and extensive cation solubilities allowed by thermodynamics combine to give many high-entropy oxide systems utility as electrochemical, magnetic, thermal, dielectric, and optical materials. Though high-entropy materials research is maturing rapidly, much remains to be understood and many compositions still await discovery, exploration, and implementation.
- On the thermal and mechanical properties of Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O across the high-entropy to entropy-stabilized transitionRost, Christina M.; Schmuckler, Daniel L.; Bumgardner, Clifton; Bin Hoque, Md Shafkat; Diercks, David R.; Gaskins, John T.; Maria, Jon-Paul; Brennecka, Geoffrey L.; Li, Xiadong; Hopkins, Patrick E. (AIP Publishing, 2022-12-16)As various property studies continue to emerge on high entropy and entropy-stabilized ceramics, we seek a further understanding of the property changes across the phase boundary between "high-entropy"and "entropy-stabilized"phases. The thermal and mechanical properties of bulk ceramic entropy stabilized oxide composition Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O are investigated across this critical transition temperature via the transient plane-source method, temperature-dependent x-ray diffraction, and nano-indentation. The thermal conductivity remains constant within uncertainty across the multi-to-single phase transition at a value of ≈2.5 W/mK, while the linear coefficient of thermal expansion increases nearly 24% from 10.8 to 14.1 × 10-6 K-1. Mechanical softening is also observed across the transition.
- Searching for superconductivity in high entropy oxide Ruddlesden-Popper cuprate filmsMazza, Alessandro R.; Gao, Xingyao; Rossi, Daniel J.; Musico, Brianna L.; Valentine, Tyler W.; Kennedy, Zachary; Zhang, Jie; Lapano, Jason; Keppens, Veerle; Moore, Robert G.; Brahlek, Matthew; Rost, Christina M.; Ward, Thomas Z. (American Vacuum Society, 2021-11-29)In this work, the high entropy oxide A2CuO4 Ruddlesden-Popper (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)2CuO4 is explored by charge doping with Ce+4 and Sr+2 at concentrations known to induce superconductivity in the simple parent compounds, Nd2CuO4 and La2CuO4. Electron doped (La0.185Pr0.185Nd0.185Sm0.185Eu0.185Ce0.075)2CuO4 and hole doped (La0.18Pr0.18Nd0.18Sm0.18Eu0.18Sr0.1)2CuO4 are synthesized and shown to be single crystal, epitaxially strained, and highly uniform. Transport measurements demonstrate that all as-grown films are insulating regardless of doping. Annealing studies show that resistivity can be tuned by modifying oxygen stoichiometry and inducing metallicity but without superconductivity. These results, in turn, are connected to extended x-ray absorption fine structure results, indicating that the lack of superconductivity in the high entropy cuprates likely originates from a large distortion within the Cu-O plane (σ2 > 0.015 Å2) due to A-site cation size variance, which drives localization of charge carriers. These findings describe new opportunities for controlling charge- and orbital-mediated functional responses in Ruddlesden-Popper crystal structures, driven by balancing of cation size and charge variances that may be exploited for functionally important behaviors such as superconductivity, antiferromagnetism, and metal-insulator transitions while opening less understood phase spaces hosting doped Mott insulators, strange metals, quantum criticality, pseudogaps, and ordered charge density waves.
- Sensing performance of sub-100-nm vanadium oxide films for room temperature thermal detection applicationsScott, E. A.; Singh, M. K.; Barber, J. P.; Rost, Christina M.; Ivanov, S.; Watt, J.; Pete, D.; Sharma, P.; Lu, T. M.; Harris, C. T. (AIP Publishing, 2022-11-14)Vanadium oxide films are widely employed as thermal detectors in uncooled infrared detection systems due to their high temperature coefficient of resistance near room temperature. One strategy toward maximizing detectivity and reducing the thermal time constant in these systems is to minimize the system platform dimensions. This approach necessitates thinner film thicknesses (≪100 nm), for which there is little information regarding thermal sensing performance. Herein, we report on the sensitivity of reactively sputtered vanadium oxide thin film resistive thermometers nominally ranging from 100 to 25 nm and assess the influence of thermal annealing. We demonstrate that films in this minimum limit of thickness maintain a high temperature coefficient while additionally providing an enhancement in characteristics of the noise equivalent power.
- Size Effects on the Cross-Plane Thermal Conductivity of Transparent Conducting Indium Tin Oxide and Fluorine Tin Oxide Thin FilmsOlson, David H.; Rost, Christina M.; Gaskins, John T.; Szwejkowski, Chester J.; Braun, Jeffrey L.; Hopkins, Patrick E. (IEEE, 2018-08-06)Visibly transparent and electrically conductive oxides are attractive for a wide array of applications. Indium tin oxide (ITO) and fluorine tin oxide (FTO) are the subset of the larger transparent conducting oxide family and possess transmittance in the visible spectrum as well as high electrical conductivity. Even though their unique optical and electrical properties have been thoroughly examined, the thermal transport properties, namely, thermal conductivity in the cross-plane direction, have received much less attention. In this paper, using a series of ITO and FTO thin films comprising a range of thicknesses and grain sizes, we characterize the cross-plane thermal conductivity using time-domain thermoreflectance. We determine the heat capacity of the FTO films from simultaneous measurements of volumetric heat capacity and thermal conductivity on an 396-nm-thick FTO film. We show that the size effects have a considerable influence on the thermal conductivity from both the perspective of grain boundary and thin film scattering.
- Studies on the structure and the magnetic properties of high-entropy spinel oxide (MgMnFeCoNi)Al2O4Krysko, Evan; Min, Lujin; Wang, Yu; Zhang, Na; Barber, John P.; Niculescu, Gabriela E.; Wright, Joshua T.; Li, Fankang; Burrage, Kaleb; Matsuda, Masaaki; Robinson, Robert A.; Zhang, Qiang; Katzbaer, Rowan; Schaak, Raymond; Terrones, Mauricio; Rost, Christina M.; Mao, Zhiqiang (AIP Publishing, 2023-10-20)The study of high-entropy materials has attracted enormous interest since they could show new functional properties that are not observed in their related parent phases. Here, we report single crystal growth, structure, thermal transport, and magnetic property studies on a novel high-entropy oxide with the spinel structure (MgMnFeCoNi)Al2O4. We have successfully grown high-quality single crystals of this high-entropy oxide using the optical floating zone growth technique for the first time. The sample was confirmed to be a phase pure high-entropy oxide using x-ray diffraction and energy-dispersive spectroscopy. Through magnetization measurements, we found (MgMnFeCoNi)Al2O4 exhibits a cluster spin glass state, though the parent phases show either antiferromagnetic ordering or spin glass states. Furthermore, we also found that (MgMnFeCoNi)Al2O4 has much greater thermal expansion than its CoAl2O4 parent compound using high resolution neutron Larmor diffraction. We further investigated the structure of this high-entropy material via Raman spectroscopy and extended x-ray absorption fine structure spectroscopy (EXAFS) measurements. From Raman spectroscopy measurements, we observed (MgMnFeCoNi)Al2O4 to display a combination of the active Raman modes in its parent compounds with the modes shifted and significantly broadened. This result, together with the varying bond lengths probed by EXAFS, reveals severe local lattice distortions in this high-entropy phase. Additionally, we found a substantial decrease in thermal conductivity and suppression of the low temperature thermal conductivity peak in (MgMnFeCoNi)Al2O4, consistent with the increased lattice defects and strain. These findings advance the understanding of the dependence of thermal expansion and transport on the lattice distortions in high-entropy materials.
- A topological kagome magnet in high entropy formMin, L.; Sretenovic, M.; Heitmann, T. W.; Valentine, T. W.; Zu, R.; Gopalan, V.; Rost, Christina M.; Ke, X.; Mao, Z. (Springer, 2022-03-18)Topological kagome magnets RMn6Sn6 (R = rare earth element) attract numerous interests due to their non-trivial band topology and room-temperature magnetism. Here, we report a high entropy version of kagome magnet, (Gd0.38Tb0.27Dy0.20Ho0.15)Mn6Sn6. Such a high entropy material exhibits multiple spin reorientation transitions, which is not seen in all the related parent compounds and can be understood in terms of competing magnetic interactions enabled by high entropy. Furthermore, we also observed an intrinsic anomalous Hall effect, indicating that the high entropy phase preserves the non-trivial band topology. These results suggest that high entropy may provide a route to engineer the magnetic structure and expand the horizon of topological materials.
- What is in a name: Defining "high entropy" oxidesBrahlek, Matthew; Gazda, Maria; Keppens, Veerle; Mazza, Alessandro R.; McCormack, Scott J.; Mielewczyk-Gryń, Aleksandra; Musico, Brianna; Page, Katharine; Rost, Christina M.; Sinnott, Susan B.; Toher, Cormac; Ward, Thomas Z.; Yamamoto, Ayako (AIP Publishing, 2022-11-04)High entropy oxides are emerging as an exciting new avenue to design highly tailored functional behaviors that have no traditional counterparts. Study and application of these materials are bringing together scientists and engineers from physics, chemistry, and materials science. The diversity of each of these disciplines comes with perspectives and jargon that may be confusing to those outside of the individual fields, which can result in miscommunication of important aspects of research. In this Perspective, we provide examples of research and characterization taken from these different fields to provide a framework for classifying the differences between compositionally complex oxides, high entropy oxides, and entropy stabilized oxides, which is intended to bring a common language to this emerging area. We highlight the critical importance of understanding a material's crystallinity, composition, and mixing length scales in determining its true definition.