Showing papers by "Deyu Li published in 2019"

Pathways and challenges for efficient solar-thermal desalination

Abstract: Solar-thermal desalination (STD) is a potentially low-cost, sustainable approach for providing high-quality fresh water in the absence of water and energy infrastructures. Despite recent efforts to advance STD by improving heat-absorbing materials and system designs, the best strategies for maximizing STD performance remain uncertain. To address this problem, we identify three major steps in distillation-based STD: (i) light-to-heat energy conversion, (ii) thermal vapor generation, and (iii) conversion of vapor to water via condensation. Using specific water productivity as a quantitative metric for energy efficiency, we show that efficient recovery of the latent heat of condensation is critical for STD performance enhancement, because solar vapor generation has already been pushed toward its performance limit. We also demonstrate that STD cannot compete with photovoltaic reverse osmosis desalination in energy efficiency. We conclude by emphasizing the importance of factors other than energy efficiency, including cost, ease of maintenance, and applicability to hypersaline waters.

Thermoelectrics of Nanowires.

Abstract: The field of thermoelectric research has undergone a renaissance and boom in the past two and a half decades, largely fueled by the prospect of engineering electronic and phononic properties in nanostructures, among which semiconductor nanowires (NWs) have served both as an important platform to investigate fundamental thermoelectric transport phenomena and as a promising route for high thermoelectric performance for diverse applications. In this Review, we provide a comprehensive look at various aspects of thermoelectrics of NWs. We start with a brief introduction of basic thermoelectric phenomena, followed by synthetic methods for thermoelectric NWs and a summary of their thermoelectric figures of merit (ZT). We then focus our discussion on charge and heat transport, which dictate thermoelectric power factor and thermal conductivity, respectively. For charge transport, we cover the basic principles governing the power factor and then review several strategies using NWs to enhance it, including earlier t...

Drastically Reduced Ion Mobility in a Nanopore Due to Enhanced Pairing and Collisions between Dehydrated Ions.

Abstract: Ion transport through nanopores is a process of fundamental significance in nature and in engineering practice. Over the past decade, it has been found that the ion conductivity in nanopores could be drastically enhanced, and different mechanisms have been proposed to explain this observation. To date, most reported studies have been carried out with relatively dilute electrolytes, while ion transport in nanopores under high electrolyte concentrations (>1 M) has been rarely explored. Through systematic experimental and atomistic simulation studies with NaCl solutions, here we show that at high electrolyte concentrations, ion mobility in small nanopores could be significantly reduced from the corresponding bulk value. Subsequent molecular dynamics studies indicate that in addition to the low mobility of surface-bound ions in the Stern layer, enhanced pairing and collisions between partially dehydrated ions of opposite charges also make important contributions to the reduced ion mobility. Furthermore, we show that the extent of mobility reduction depends on the association constant between cations and anions in different electrolytes with a more drastic reduction for a larger association constant.

Distinct Signatures of Electron–Phonon Coupling Observed in the Lattice Thermal Conductivity of NbSe3 Nanowires

Abstract: The last two decades have seen tremendous progress in quantitative understanding of several major phonon scattering mechanisms (phonon-phonon, phonon-boundary, phonon-defects), as they are the determinant factors in lattice thermal transport, which is critical for the proper functioning of various electronic and energy conversion devices. However, the roles of another major scattering mechanism, electron-phonon (e-ph) interactions, remain elusive. This is largely due to the lack of solid experimental evidence for the effects of e-ph scattering in the lattice thermal conductivity for the material systems studied thus far. Here we show distinct signatures in the lattice thermal conductivity observed below the charge density wave transition temperatures in NbSe3 nanowires, which cannot be recaptured without considering e-ph scattering. Our findings can serve as the cornerstone for quantitative understanding of the e-ph scattering effects on lattice thermal transport in many technologically important materials.

The relationship between the Young’s modulus and dry etching rate of polydimethylsiloxane (PDMS)

Abstract: Polydimethylsiloxane (PDMS) has been the pivotal materials for microfluidic technologies with tremendous amount of lab-on-a-chip devices made of PDMS microchannels. While molding-based soft-lithography approach has been extremely successful in preparing various PDMS constructs, some complex features have to been achieved through more complicated microfabrication techniques that involve dry etching of PDMS. Several recipes have been reported for reactive ion etching (RIE) of PDMS; however, the etch rates present large variations, even for the same etching recipe, which poses challenges in adopting this process for device fabrication. Through systematic characterization of the Young's modulus of PDMS films and RIE etch rate, we show that the etch rate is closely related to the polymer cross-link density in the PDMS with a higher etch rate for a lower PDMS Young's modulus. Our results could provide guidance to the fabrication of microfluidic devices involving dry etching of PDMS.

Thermal transport through fishbone silicon nanoribbons: unraveling the role of Sharvin resistance.

Abstract: Heat conduction has been shown to be greatly suppressed in Si nanomeshes, which has attracted extensive attention for potential thermoelectric applications, yet the precise suppression mechanism remains to be fully understood. Attempting to further disclose the underlying mechanisms, we report on the thermal conductivity of the building block for nanomeshes, i.e., Si nanoribbons with fins attached to the two opposite sides. By expanding only the fin width while keeping both the period length and the backbone size constant, we observed an unexpected non-monotonic trend of the effective thermal conductivity normalized with the backbone cross-section. Further analysis showed that the corrected thermal conductivity extracted with appropriate consideration of the geometrical effect on diffusion followed a monotonically decreasing trend, reaching a maximum thermal conductivity reduction of 18% at 300 K for a ribbon with the maximum explored fin width of 430 nm, as compared to that of the straight ribbon of 66 nm backbone width. We attribute the thermal conductivity reduction to the thermal constriction resistance induced by the cross-section reduction between the fin and backbone sections. For ribbons with a larger fin width, the effective phonon mean free path is longer for phonons arriving at the constriction, which boosts the ballistic constriction resistance, i.e., Sharvin resistance, and leads to a lower thermal conductivity.

Kink as a new degree of freedom to tune the thermal conductivity of Si nanoribbons

Abstract: An attractive feature of nanomaterials is the possibility of tuning their properties through controlling their size and surface morphology, and understanding the effects of various parameters on thermal transport properties of nanostructures has been an active research topic in the past two decades. Through systematic studies of kinked silicon nanoribbons, we show how the kink morphology, a newly recognized degree of freedom for tuning thermal transport in nanostructures, modulates the thermal conductivity of these nanoribbons. For kinked Si nanoribbons that are 34 nm thick and 141 nm wide, the measured thermal conductivity first decreases as the period length reduces from 2 μm to 0.5 μm, reaching a 21% thermal conductivity reduction as compared to that of a straight counterpart at 300 K. However, as the period length drops to a level at which a straight heat transfer channel opens between the heat source and the sink, the thermal conductivity exhibits a steep increasing trend. Moreover, the comparison of thermal conductivity reduction for kinked ribbons along different crystalline directions indicates that phonon focusing could be exploited to further suppress thermal transport in kinked silicon nanoribbons. These results provide important guidelines on modulating heat transfer in nanostructures using kinks, which could be adopted to tune the thermal properties of nanostructures for different applications, such as thermoelectrics, microelectronic device thermal management, and functional thermal regulators.

Thermal transport in molecular beam epitaxy grown Si1 − xGex alloy films with a full spectrum of composition (x = 0–1)

Abstract: The thermal properties of Si1 – xGex alloys are important for two major reasons: one is their applications in high-temperature thermoelectrics and the other is the increasing heat dissipation demand for high power density devices. However, the large lattice mismatch between silicon and germanium leads to tremendous difficulties to obtain high-quality Si1 – xGex thin films, especially when x > 0.5. In this study, we obtained a series of high crystalline quality Si1 – xGex thin films with x covering all the way from 0 to 1 on Si substrates by molecular beam epitaxy. The out-of-plane thermal conductivities of these Si1 – xGex films were measured by the time-domain thermoreflectance approach. Results show that while the thermal conductivity can vary significantly with composition, it only changes marginally in the temperature range of 100 K–300 K for a specific Ge content x. A theoretical analysis indicates that alloy and boundary scatterings are the dominant mechanisms for the thermal transport in these Si1 – xGex (x = 0–1) alloy films.The thermal properties of Si1 – xGex alloys are important for two major reasons: one is their applications in high-temperature thermoelectrics and the other is the increasing heat dissipation demand for high power density devices. However, the large lattice mismatch between silicon and germanium leads to tremendous difficulties to obtain high-quality Si1 – xGex thin films, especially when x > 0.5. In this study, we obtained a series of high crystalline quality Si1 – xGex thin films with x covering all the way from 0 to 1 on Si substrates by molecular beam epitaxy. The out-of-plane thermal conductivities of these Si1 – xGex films were measured by the time-domain thermoreflectance approach. Results show that while the thermal conductivity can vary significantly with composition, it only changes marginally in the temperature range of 100 K–300 K for a specific Ge content x. A theoretical analysis indicates that alloy and boundary scatterings are the dominant mechanisms for the thermal transport in these Si1 ...