glasses, watches, wristbands, or belts, are either fully or partially composed of planar and rigid materials, which require the use of obtrusive, hard supports or additional bendable strips to be mounted on the human body. Therefore, clinical devices that use the existing wearables cause discomfort and limit monitoring of human physiological data in the laboratory. This is the big limitation factor to overcome despite the ever-growing market for wearables in broader screenings outside of the clinic. By this account, it is necessary to replace the bulky and rigid plastics and metal components in the sensors and electronics with skin-like materials for enhanced wearability and functionality.The concept of WFHE poses a possible solution to address the aforementioned difficulties by providing user comfort, compliant mechanics, soft integration, multifunctionality, and smart diagnostics with embedded machine learning algorithms. Specifically, such electronics would provide stable and intimate contact to the soft human skin without adding any mechanical and thermal loadings or causing skin breakdown. Current development strategies and approaches for advanced WFHE focus on soft, flexible form factors, nonirritating and nontoxic characteristics, fully autonomous energy components, seamless wireless communications, Recent advances in soft materials and system integration technologies have provided a unique opportunity to design various types of wearable flexible hybrid electronics (WFHE) for advanced human healthcare and humanmachine interfaces. The hybrid integration of soft and biocompatible materials with miniaturized wireless wearable systems is undoubtedly an attractiveprospect in the sense that the successful device performance requires high degrees of mechanical flexibility, sensing capability, and user-friendly simplicity. Here, the most up-to-date materials, sensors, and system-packaging technologies to develop advanced WFHE are provided. Details of mechanical, electrical, physicochemical, and biocompatible properties are discussed with integrated sensor applications in healthcare, energy, and environment. In addition, limitations of the current materials are discussed, as well as key challenges and the future direction of WFHE. Collectively, an all-inclusive review of the newly developed WFHE along with a summary of imperative requirements of material properties, sensor capabilities, electronics performance, and skin integrations is provided. Wearable Flexible Hybrid ElectronicsThe ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Thermoelectric power generation represents one of the cleanest methods of energy conversion available today. It can be used in applications ranging from the harvesting of waste heat to conversion of solar energy into useful electricity. Remarkable advances have been achieved in recent years for various thermoelectric (TE) material systems. The introduction of various nanostructures is used to tune the transport of phonons, and band structure engineering allows for the tailoring of electron transport. In this overview, top-down approaches for phonon engineering such as atomic construction of new materials will be reviewed. Bottom-up approaches for electron engineering such as the formation of ordered nanostructures will also be discussed. The assembly of Thermo-Electric power Generating (TEG) devices is still particularly challenging, and consequently, thermal-to-electric conversion utilizing these devices has been realized only in niche applications. In this review paper, we will discuss some of the challenges that must be overcome to enable widespread use of TE devices. These include: thermal stability at the material level, and reliable contact at the device level.

The formula for maximum efficiency (η max ) of heat conversion into electricity by a thermoelectric device in terms of the dimensionless figure of merit (ZT) has been widely used to assess the desirability of thermoelectric materials for devices. Unfortunately, the η max values vary greatly depending on how the average ZT values are used, raising questions about the applicability of ZT in the case of a large temperature difference between the hot and cold sides due to the neglect of the temperature dependences of the material properties that affect ZT. To avoid the complex numerical simulation that gives accurate efficiency, we have defined an engineering dimensionless figure of merit (ZT) eng and an engineering power factor (PF) eng as functions of the temperature difference between the cold and hot sides to predict reliably and accurately the practical conversion efficiency and output power, respectively, overcoming the reporting of unrealistic efficiency using average ZT values.thermoelectrics | engineering figure of merit | engineering power factor | conversion efficiency | cumulative temperature dependence A thermoelectric (TE) generator produces electric power directly from a temperature gradient through TE material (1-4). The maximum efficiency of a TE generator was first derived based on a constant property model by Altenkirch (5) in 1909, and its optimized formula has been commonly used since Ioffe (6) reported the optimum condition for the maximum efficiency in 1957, which is (7)where T h and T c are the hot-and cold-side temperatures, respectively, and ΔT and T avg are their difference, T h − T c , and average (T h + T c )/2, respectively. The TE conversion efficiency by Eq. 1 is the product of the Carnot efficiency (ΔT/T h ) and a reduction factor as a function of the material's figure of merit Z = S 2 ρ −1 κ −1 , where S, ρ, and κ are the Seebeck coefficient, electrical resistivity, and thermal conductivity, respectively. Since the 1950s, the dimensionless figure of merit (ZT), such as the peak ZT (8-10) and the average ZT (2, 11, 12), has been used as the guide to achieve better materials for higher conversion efficiency.The maximum efficiency by Eq. 1 is inadequate when Z is temperature dependent. Due to the assumption of temperature independence, Eq. 1 only correctly predicts the maximum efficiency at a small temperature difference between the cold and hot sides, or in limited TE materials (13-15) that have Z almost constant over the whole temperature range. By ignoring the assumption and simply using Eq. 1, incorrect efficiency that is much higher than is practically achievable (16, 17) is often reported. In most cases for S, ρ, and κ that are temperature dependent, ZT values are not linearly temperature dependent (18)(19)(20)(21)(22) and they operate at a large temperature difference, so the prediction by Eq. 1 cannot be reliable. To overcome the inadequacy, complicated numerical simulations based on the finite difference method were carried out to calculate the efficiency while accounting for ...

Improvements in thermoelectric material performance over the past two decades have largely been based on decreasing the phonon thermal conductivity. Enhancing the power factor has been less successful in comparison. In this work, a peak power factor of ∼106 μW·cm −1 ·K −2 is achieved by increasing the hot pressing temperature up to 1,373 K in the p-type half-Heusler Nb 0.95 Ti 0.05 FeSb. The high power factor subsequently yields a record output power density of ∼22 W·cm −2 based on a single-leg device operating at between 293 K and 868 K. Such a high-output power density can be beneficial for large-scale power generation applications.half-Heusler | thermoelectric | power factor | carrier mobility | output power density T he majority of industrial energy input is lost as waste heat. Converting some of the waste heat into useful electrical power will lead to the reduction of fossil fuel consumption and CO 2 emission. Thermoelectric (TE) technologies are unique in converting heat into electricity due to their solid-state nature. The ideal device conversion efficiency of TE materials is usually characterized by (1)where ZT is the average thermoelectric figure of merit (ZT) between the hot side temperature (T H ) and the cold side temperature (T C ) of a TE material and is defined aswhere PF, T, κ tot , S, σ, κ L , κ e , and κ bip are the power factor, absolute temperature, total thermal conductivity, Seebeck coefficient, electrical conductivity, lattice thermal conductivity, electronic thermal conductivity, and bipolar thermal conductivity, respectively. Higher ZT corresponds to higher conversion efficiency. One effective approach to enhance ZT is through nanostructuring that can significantly enhance phonon scattering and consequently result in a much lower lattice thermal conductivity compared with that of the unmodified bulk counterpart (2). This approach works well for many inorganic TE materials, such as Bi 2 Te 3 (2), IV-VI semiconductor compounds (3, 4), lead-antimony-silver-tellurium (LAST) (5), skutterudites (6), clathrates (7), CuSe 2 (8), Zintl phases (9), half-Heuslers (10-12), MgAgSb (13, 14), Mg 2 (Si, Ge, Sn) (15, 16), and others.However, nanostructuring is effective only when the grain size is comparable to or smaller than the phonon mean free path (MFP). In compounds with a phonon MFP shorter than the nanosized grain diameters, nanostructuring might impair the electron transport more than the phonon transport, thus potentially decreasing the power factor and ZT. In contrast, improving ZT by boosting the power factor has not yet been widely studied (17)(18)(19)(20). To the best of our knowledge, there is no theoretical upper limit applied to the power factor. Additionally, the output power density ω of a device with hot side at T H and cold side at T C is directly related to the power factor by (21)where L is the leg length of the TE material and PF is the averaged power factor over the leg. As contact resistance limits the reduction of length L, higher power factor favors higher power density when h...

Coarse-grained reconfigurable architectures (CGRAs) present an appealing hardware platform by providing the potential for high computation throughput, scalability, low cost, and energy efficiency. CGRAs consist of an array of function units and register files often organized as a two dimensional grid. The most difficult challenge in deploying CGRAs is compiler scheduling technology that can efficiently map software implementations of compute intensive loops onto the array. Traditional schedulers focus on the placement of operations in time and space. With CGRAs, the challenge of placement is compounded by the need to explicitly route operands from producers to consumers. To systematically attack this problem, we take an edge-centric approach to modulo scheduling that focuses on the routing problem as its primary objective. With edge-centric modulo scheduling (EMS), placement is a by-product of the routing process, and the schedule is developed by routing each edge in the dataflow graph. Routing cost metrics provide the scheduler with a global perspective to guide selection. Experiments on a wide variety of compute-intensive loops from the multimedia domain show that EMS improves throughput by 25% over traditional iterative modulo scheduling, and achieves 98% of the throughput of simulated annealing techniques at a fraction of the compilation time.

Thermoelectric power generation is one of the most promising techniques to use the huge amount of waste heat and solar energy. Traditionally, high thermoelectric figure-of-merit, ZT, has been the only parameter pursued for high conversion efficiency. Here, we emphasize that a high power factor (PF) is equivalently important for high power generation, in addition to high efficiency. A new n-type Mg 2 Sn-based material, Mg 2 Sn 0.75 Ge 0.25 , is a good example to meet the dual requirements in efficiency and output power. It was found that Mg 2 Sn 0.75 Ge 0.25 has an average ZT of 0.9 and PF of 52 μW·cm ·K−2 over the temperature range of 25-450°C, a peak ZT of 1.4 at 450°C, and peak PF of 55 μW·cm·K −2 at 350°C. By using the energy balance of one-dimensional heat flow equation, leg efficiency and output power were calculated with T h = 400°C and T c = 50°C to be of 10.5% and 6.6 W·cm −2 under a temperature gradient of, respectively.thermoelectrics | magnesium | tin | power factor | output power T hermoelectric power generation from waste heat is attracting more and more attention. Potential fuel efficiency enhancement by recovering the waste heat is beneficial for automobiles and many other applications (1, 2). In addition, solar thermoelectric generator provides an alternative route to convert solar energy into electrical power besides the photovoltaic conversion (3). Thermoelectric generator (TEG) can be regarded as a heat engine using electrons/holes as the energy carrier. The conversion efficiency of a TEG is related to the Carnot efficiency and the material's average thermoelectric figure of merit ZT (4):where ZT = (S 2 σ/κ)T, and S, σ, κ, and T are Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. Pursuing high ZT has been the focus of the entire thermoelectric community by applying various phonon engineering via nanostructuring approaches to reduce the thermal conductivity (5-7), or by exploring new compounds with intrinsically low thermal conductivity, such as compounds having complex crystalline structure, local rattlers, liquid-like sublattice, and highly distorted lattice (8-11). However, for practical applications, efficiency is not the only concern, and high output power density is as important as efficiency when the capacity of the heat source is huge (such as solar heat), or the cost of the heat source is not a big factor (such as waste heat from automobiles, steel industry, etc.). The output power density ω is defined as the output power W divided by the cross-sectional area A of the leg, i.e., ω = W/A, which is related to power factor PF = S 2 σ by the following:Eq. 2 contains two main parts: square of the temperature difference divided by leg length, and material power factor PF = S 2 σ.Clearly, to achieve higher power density for a given heat source, we have to either increase the power factor PF or decrease the leg length. However, decreasing the leg length could cause severe consequences such as increase of large heat flux that will incr...

Mg 3Àx Na x Sb 2 has been prepared successfully by mechanical alloying plus hot pressing to investigate the effects of Na doping on the thermoelectric properties. Thermoelectric properties were characterized by the Seebeck coefficient, electrical resistivity, thermal conductivity, and thermoelectric figure of merit (ZT) from 298 to 773 K. Transport measurements reveal that an optimum doping of 1.25 at.% Na on Mg achieved ZT of 0.6 at 773 K. The enhancement in ZT is attributed to increased carrier concentration and power factor. The low cost, abundance, and nontoxicity makes this material a potentially promising thermoelectric material for power generation at a heat source below 773 K.

The figure of merit (ZT) has been used for more than half a century to guide the research of thermoelectric materials. In this perspective viewpoint, we emphasize that the power factor (PF) is more important than the thermal conductivity for thermoelectric power generation for a given ZT. We also propose the newly defined engineering parameters (ZT)eng & (PF)eng are the more practical indicators to show the temperature dependent efficiency & output power, as compared with conventional ZT & PF.