Browsing by Author "Stamper, C"

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    Experimental confirmation of the universal law for the vibrational density of states of liquids
    (American Chemical Society, 2022-04-02) Stamper, C; Cortie, DL; Yue, ZJ; Wang, XL; Yu, DH
    An analytical model describing the vibrational density of states (VDOS) of liquids has long been elusive, owing to the complexities of liquid dynamics. Nevertheless, Zaccone and Baggioli have recently developed such a model which was proposed to be the universal law for the vibrational density of states of liquids. Distinct from the Debye law, g(ω) ∝ ω2, for solids, the universal law for liquids reveals a linear relationship, g(ω) ∝ ω, in the low-energy region. We have confirmed this universal law with experimental VDOS measured by inelastic neutron scattering on real liquid systems including water, liquid metal, and polymer liquids, and have applied this model to extract the effective relaxation rate for the short time dynamics for each liquid. The model has also been further evaluated in the prediction of the specific heat with comparison to existing experimental data as well as with values obtained by different approaches. © 2022 American Chemical Society
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    On the temperature dependence of the density of states of liquids at low energies
    (Springer Nature, 2024-08-13) Jin, S; Fan, X; Stamper, C; Mole, RA; Yu, Y; Yu, DH; Baggioli, B; Hong, L
    We report neutron-scattering measurements of the density of states (DOS) of water and liquid Fomblin in a wide range of temperatures. In the liquid phase, we confirm the presence of a universal low-energy linear scaling of the experimental DOS as a function of the frequency, g(w) = a(T)w , which persists at all temperatures. The low-frequency scaling of the DOS exhibits a sharp jump at the melting point of water, below which the standard Debye’s law, g(w) ∝ w2 , is recovered. On the contrary, in Fomblin, we observe a continuous transition between the two exponents reflecting its glassy dynamics, which is confirmed by structure measurements. More importantly, in both systems, we find that the slope a(T) grows with temperature following an exponential Arrhenius-like form, a(T) ∝ exp(−/T) . We confirm this experimental trend using molecular dynamics simulations and show that the prediction of instantaneous normal mode (INM) theory for a(T) is in qualitative agreement with the experimental data. © The Authors - Open Access This article is licensed under a Creative Commons Attribution 4.0 International License.
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    Phonon engineering in thermal materials with nano-carbon dopants
    (AIP Publishing, 2024-06-01) Stamper, C; Cortie, DL; Nazrul-Islam, SMK; Rahman, R; Yu, DH; Yang, G; Al-Mamun, A; Wang, XL; Yue, ZJ
    The unique geometric and thermal properties of carbon nanoparticles (NPs)—including nanotubes, graphene, and nanodiamonds—have led to their use as additives in many composite material systems. In this review, we investigate the mechanisms behind the altered thermal conductivity (κ) of thermoelectric (TE) and other thermal materials that have been composited with carbon NPs. We provide a comprehensive overview and analysis of the relevant theoretical and applied literature, including a detailed review of the available thermal conductivity data across five common classes of TE materials (Bi2Te3 variants, skutterudites, metal–oxide, SnSe, Cu2Se) in combination with carbon additives, including graphene, nanotubes, carbon black, carbon fiber, and C60. We argue that the effectiveness of carbon NPs in reducing κ in TE composites generally arises due to a combination of the presence of the carbon NP interfaces and significant changes in the microstructure of the host material due to compositing, such as suppressed grain growth and the introduction of pores, dislocations, and strain. Carbon NPs themselves are effective phonon scatterers in TE composites due to a significant mismatch between their high-frequency phonon distribution and the lower-frequency phonon distribution of the host material. While carbon NP doping has proven itself as an effective way to increase the performance of TE materials, there is still a significant amount of work to do to precisely understand the fundamental thermal transport mechanisms at play. Rigorous material characterization of nanocomposites and spectroscopic studies of the precise lattice dynamics will greatly aid the development of a fully quantitative, self-consistent model for the thermal conductivity of carbon nanocomposites. © 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/)