Bao Yang

Bio: Bao Yang is an academic researcher from University of Maryland, College Park. The author has contributed to research in topics: Thermoelectric cooling & Thermal conductivity. The author has an hindex of 44, co-authored 141 publications receiving 7219 citations. Previous affiliations of Bao Yang include Massachusetts Institute of Technology & University of California, Los Angeles.

Nanoscale imaging of domain dynamics and retention in ferroelectric thin films

Abstract: We report results on the direct observation of the microscopic origins of backswitching in ferroelectric thin films. The piezoelectric response generated in the film by a biased atomic force microscope tip was used to obtain static and dynamic piezoelectric images of individual grains in a polycrystalline material. We demonstrate that polarization reversal occurs under no external field (i.e., loss of remanent polarization) via a dispersive continuous-time random walk process, identified by a stretched exponential decay of the remanent polarization.

Partially coherent phonon heat conduction in superlattices

Abstract: In this paper, the phonon thermal conductivity of semiconductor superlattices is calculated with the use of a modified lattice dynamics model, in which an imaginary wave vector is added. The mean free path caused by diffuse interface scattering is included in the imaginary wave vector. This model combines the effects of phonon confinement and diffuse interface scattering on the thermal conductivity in superlattices, and is applicable to phonon transport in the partially coherent regime, where bulk and superlattice phonon modes mix up. The theoretical results for the GaAs/AlAs superlattices are compared with the experimental data. The period thickness dependence and temperature dependence of the thermal conductivity in the GaAs/AlAs superlattices can be well explained by this model.

Application of hybrid sphere/carbon nanotube particles in nanofluids

Abstract: Previous studies on nanofluids have focused on spherical or long-fibre particles. In this work, a new type of complex nanoparticle—a hybrid sphere/carbon nanotube(CNT) particle, consisting of numerous CNTs attached to an alumina/iron oxide sphere—is proposed for applications in nanofluids. In such hybrid nanoparticles, heat is expected to transport rapidly from one CNT to another through the centre sphere and thus leading to less thermal contact resistance between CNTs when compared to simple CNTs dispersed in fluids. CNTs have an extremely high thermal conductivity, but thermal resistance between the CNTs and the fluid has limited their performance in nanofluids. The proposed hybrid sphere/CNT particles are synthesized by spray pyrolysis followed by catalytic growth of CNTs. The spheres are about 70 nm in diameter on average, and the attached CNTs have a length up to 2 µm. These hybrid nanoparticles are dispersed to poly-alpha-olefin with sonication and a small amount of surfactants to form stable nanofluids. The thermal conductivity of the fluids has been measured by a 3ω-wire method over a temperature range 10–90 °C. The results indicate that the effective thermal conductivity of the fluids is increased by about 21% at room temperature for particle volume fractions of 0.2%.

Measurements of anisotropic thermoelectric properties in superlattices

Abstract: Thermoelectric properties, i.e., thermal conductivity, electrical conductivity, and the Seebeck coefficient, have been measured in the directions parallel (in-plane) and perpendicular to the interface of an n-type Si(80 A)/Ge(20 A) superlattice. A two-wire 3ω method is employed to measure the in-plane and cross-plane thermal conductivities. The cross-plane Seebeck coefficient is deduced by using a differential measurement between the superlattice and reference samples and the cross-plane electrical conductivity is determined through a modified transmission-line method. The in-plane thermal conductivity of the Si/Ge superlattice is 5–6 times higher than the cross-plane one, and the electrical conductivity shows a similar anisotropy. The anisotropy of the Seebeck coefficients is smaller in comparison to electrical and thermal conductivities in the temperature range from 150 to 300 K. However, the cross-plane Seebeck coefficient rises faster with increasing temperature than that of the in-plane direction.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Toward Safe Lithium Metal Anode in Rechargeable Batteries: A Review.

Abstract: The lithium metal battery is strongly considered to be one of the most promising candidates for high-energy-density energy storage devices in our modern and technology-based society. However, uncontrollable lithium dendrite growth induces poor cycling efficiency and severe safety concerns, dragging lithium metal batteries out of practical applications. This review presents a comprehensive overview of the lithium metal anode and its dendritic lithium growth. First, the working principles and technical challenges of a lithium metal anode are underscored. Specific attention is paid to the mechanistic understandings and quantitative models for solid electrolyte interphase (SEI) formation, lithium dendrite nucleation, and growth. On the basis of previous theoretical understanding and analysis, recently proposed strategies to suppress dendrite growth of lithium metal anode and some other metal anodes are reviewed. A section dedicated to the potential of full-cell lithium metal batteries for practical applicatio...

Physics of thin-film ferroelectric oxides

Abstract: This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

Bulk nanostructured thermoelectric materials: current research and future prospects

Abstract: Thermoelectrics have long been recognized as a potentially transformative energy conversion technology due to their ability to convert heat directly into electricity. Despite this potential, thermoelectric devices are not in common use because of their low efficiency, and today they are only used in niche markets where reliability and simplicity are more important than performance. However, the ability to create nanostructured thermoelectric materials has led to remarkable progress in enhancing thermoelectric properties, making it plausible that thermoelectrics could start being used in new settings in the near future. Of the various types of nanostructured materials, bulk nanostructured materials have shown the most promise for commercial use because, unlike many other nanostructured materials, they can be fabricated in large quantities and in a form that is compatible with existing thermoelectric device configurations. The first generation of these materials is currently being developed for commercialization, but creating the second generation will require a fundamental understanding of carrier transport in these complex materials which is presently lacking. In this review we introduce the principles and present status of bulk nanostructured materials, then describe some of the unanswered questions about carrier transport and how current research is addressing these questions. Finally, we discuss several research directions which could lead to the next generation of bulk nanostructured materials.

Engineered doping of organic semiconductors for enhanced thermoelectric efficiency

Abstract: The conversion efficiency of heat to electricity in thermoelectric materials depends on both their thermopower and electrical conductivity. It is now reported that, unlike their inorganic counterparts, organic thermoelectric materials show an improvement in both these parameters when the volume of dopant elements is minimized; furthermore, a high conversion efficiency is achieved in PEDOT:PSS blends.