Lei Shao

Bio: Lei Shao is an academic researcher from Shanghai Jiao Tong University. The author has contributed to research in topics: Surface acoustic wave & Ion acoustic wave. The author has an hindex of 9, co-authored 16 publications receiving 1720 citations. Previous affiliations of Lei Shao include National Institute of Standards and Technology & University of Michigan.

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.

High thermal conductivity in amorphous polymer blends by engineered interchain interactions

Abstract: A high density of strong hydrogen bonds connecting two polymers that are homogeneously mixed in a thin film is shown to enhance the intrachain thermal conductance, boosting thermal transport in fully organic layers. Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost1. However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1–0.5 W m−1 K−1) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m−1 K−1 in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

Computational sprinting on a hardware/software testbed

Abstract: CMOS scaling trends have led to an inflection point where thermal constraints (especially in mobile devices that employ only passive cooling) preclude sustained operation of all transistors on a chip --- a phenomenon called "dark silicon." Recent research proposed computational sprinting --- exceeding sustainable thermal limits for short intervals --- to improve responsiveness in light of the bursty computation demands of many media-rich interactive mobile applications. Computational sprinting improves responsiveness by activating reserve cores (parallel sprinting) and/or boosting frequency/voltage (frequency sprinting) to power levels that far exceed the system's sustainable cooling capabilities, relying on thermal capacitance to buffer heat.Prior work analyzed the feasibility of sprinting through modeling and simulation. In this work, we investigate sprinting using a hardware/software testbed. First, we study unabridged sprints, wherein the computation completes before temperature becomes critical, demonstrating a 6.3x responsiveness gain, and a 6% energy efficiency improvement by racing to idle. We then analyze truncated sprints, wherein our software runtime system must intervene to prevent overheating by throttling parallelism and frequency before the computation is complete. To avoid oversubscription penalties (context switching inefficiencies after a truncated parallel sprint), we develop a sprint-aware task-based parallel runtime. We find that maximal-intensity sprinting is not always best, introduce the concept of sprint pacing, and evaluate an adaptive policy for selecting sprint intensity. We report initial results using a phase change heat sink to extend maximum sprint duration. Finally, we demonstrate that a sprint-and-rest operating regime can actually outperform thermally-limited sustained execution.

Utilizing Dark Silicon to Save Energy with Computational Sprinting

Abstract: Computational sprinting activates dark silicon to improve responsiveness by briefly but intensely exceeding a system's sustainable power limit. This article focuses on the energy implications of sprinting. The authors observe that sprinting can save energy even while improving responsiveness by enabling execution in chip configurations that, though thermally unsustainable, improve energy efficiency. Surprisingly, this energy savings can translate to throughput improvements even for long-running computations. Repeatedly alternating between sprint and idle modes while maintaining sustainable average power can outperform steady-state computation at the platform's thermal limit.

Advances in thermoelectric materials research: Looking back and moving forward

Abstract: BACKGROUND Heat and electricity are two forms of energy that are at opposite ends of a spectrum Heat is ubiquitous, but with low quality, whereas electricity is versatile, but its production is demanding Thermoelectrics offers a simple and environmentally friendly solution for direct heat-to-electricity conversion A thermoelectric (TE) device can directly convert heat emanating from the Sun, radioisotopes, automobiles, industrial sectors, or even the human body to electricity Electricity also can drive a TE device to work as a solid-state heat pump for distributed spot-size refrigeration TE devices are free of moving parts and feasible for miniaturization, run quietly, and do not emit greenhouse gasses The full potential of TE devices may be unleashed by working in tandem with other energy-conversion technologies Thermoelectrics found niche applications in the 20th century, especially where efficiency was of a lower priority than energy availability and reliability Broader (beyond niche) application of thermoelectrics in the 21st century requires developing higher-performance materials The figure of merit, ZT, is the primary measure of material performance Enhancing the ZT requires optimizing the adversely interdependent electrical resistivity, Seebeck coefficient, and thermal conductivity, as a group On the microscopic level, high material performance stems from a delicate concert among trade-offs between phase stability and instability, structural order and disorder, bond covalency and ionicity, band convergence and splitting, itinerant and localized electronic states, and carrier mobility and effective mass ADVANCES Innovative transport mechanisms are the fountain of youth of TE materials research In the past two decades, many potentially paradigm-changing mechanisms were identified, eg, resonant levels, modulation doping, band convergence, classical and quantum size effects, anharmonicity, the Rashba effect, the spin Seebeck effect, and topological states These mechanisms embody the current states of understanding and manipulating the interplay among the charge, lattice, orbital, and spin degrees of freedom in TE materials Many strategies were successfully implemented in a wide range of materials, eg, V2VI3 compounds, VVI compounds, filled skutterudites and clathrates, half-Heusler alloys, diamond-like structured compounds, Zintl phases, oxides and mixed-anion oxides, silicides, transition metal chalcogenides, and organic materials In addition, advanced material synthesis and processing techniques, for example, melt spinning, self-sustaining heating synthesis, and field-assisted sintering, helped reach a much broader phase space where traditional metallurgy and melt-growth recipes fell short Given the ubiquity of heat and the modular aspects of TE devices, these advances ensure that thermoelectrics plays an important role as part of a solutions package to address our global energy needs OUTLOOK The emerging roles of spin and orbital states, new breakthroughs in multiscale defect engineering, and controlled anharmonicity may hold the key to developing next generation TE materials To accelerate exploring the broad phase space of higher multinary compounds, we need a synergy of theory, machine learning, three-dimensional printing, and fast experimental characterizations We expect this synergy to help refine current materials selection and make TE materials research more data driven We also expect increasing efforts to develop high-performance materials out of nontoxic and earth-abundant elements The desire to move away from Freon and other refrigerant-based cooling should shift TE materials research from power generation to solid-state refrigeration International round-robin measurements to cross-check the high ZT values of emerging materials will help identify those that hold the most promise We hope the renewable energy landscape will be reshaped if the recent trend of progress continues into the foreseeable future

Organic thermoelectric materials for energy harvesting and temperature control

Abstract: Conjugated polymers and related processing techniques have been developed for organic electronic devices ranging from lightweight photovoltaics to flexible displays. These breakthroughs have recently been used to create organic thermoelectric materials, which have potential for wearable heating and cooling devices, and near-room-temperature energy generation. So far, the best thermoelectric materials have been inorganic compounds (such as Bi2Te3) that have relatively low Earth abundance and are fabricated through highly complex vacuum processing routes. Molecular materials and hybrid organic–inorganic materials now demonstrate figures of merit approaching those of these inorganic materials, while also exhibiting unique transport behaviours that are suggestive of optimization pathways and device geometries that were not previously possible. In this Review, we discuss recent breakthroughs for organic materials with high thermoelectric figures of merit and indicate how these materials may be incorporated into new module designs that take advantage of their mechanical and thermoelectric properties. Thermoelectrics can be used to harvest energy and control temperature. Organic semiconducting materials have thermoelectric performance comparable to many inorganic materials near room temperature. Better understanding of their performance will provide a pathway to new types of conformal thermoelectric modules.