Thermoelectrics, which can generate electricity from a temperature difference, or vice versa, is a key technology for solid‐state cooling and energy harvesting; however, its applications are ...constrained owing to low efficiency. Since the conversion efficiency of thermoelectric devices is directly obtained via a figure of merit of materials, zT, which is related to the electronic and thermal transport characteristics, the aim here is to elucidate physical parameters that should be considered to understand transport phenomena in semiconducting materials. It is found that the weighted mobility ratio of the majority and minority carrier bands is an important parameter that determines zT. For nanograined Bi–Sb–Te alloy, the unremarked role of this parameter on temperature‐dependent electronic transport properties is demonstrated. This analysis shows that the control of the weighted mobility ratio is a promising way to enhance zT of narrow bandgap thermoelectric materials.
Suppression of the bipolar conduction in narrow‐bandgap thermoelectric materials is crucial for improving their device efficiency. This work correlates the weighted mobility ratio in thermoelectric figure of merit and bipolar conduction in bismuth‐telluride‐based alloys. The results suggest that increasing the weighted mobility ratio suppresses the bipolar conduction most effectively among other parameters like carrier concentration and bandgap.
Band engineering is an effective strategy to improve the electronic transport properties of semiconductors. In thermoelectric materials research, density‐of‐states effective mass is an undoubted key ...factor in verifying the band engineering effect and establishing a strategy for enhancing thermoelectric performance. However, estimation of the effective mass is demanding or inaccurate depending on the methods taken. A simple equation is proposed, valid for all degeneracy: Log10 (md*T/300) = (2/3) Log10 (n) − (2/3) 20.3 − (0.00508 × |S|) + (1.58 × 0.967|S|) that utilizes experimentally determined Seebeck coefficient (S) and carrier concentration (n) to determine the effective mass (md*) at a temperature (T). This straightforward equation, which gives an accurate analysis of the band modulation in terms of md*, is indispensable in designing thermoelectric materials of maximized performance.
The density‐of‐states (DOS) effective mass of thermoelectric materials can be estimated accurately with the single parabolic band model. However due to complex Fermi integral calculations involved, an equation that is valid in degenerate materials is instead utilized erroneously. This work proposes another simple, universal, and accurate equation that can be used for any degeneracy to determine the DOS effective mass.
Filled skutterudites R(x)Co4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with ...the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics.
In analyzing zT improvements due to lattice thermal conductivity (κL) reduction, electrical conductivity (σ) and total thermal conductivity (κTotal) are often used to estimate the electronic ...component of the thermal conductivity (κE) and in turn κL from κL = ∼ κTotal − LσT. The Wiedemann-Franz law, κE = LσT, where L is Lorenz number, is widely used to estimate κE from σ measurements. It is a common practice to treat L as a universal factor with 2.44 × 10−8 WΩK−2 (degenerate limit). However, significant deviations from the degenerate limit (approximately 40% or more for Kane bands) are known to occur for non-degenerate semiconductors where L converges to 1.5 × 10−8 WΩK−2 for acoustic phonon scattering. The decrease in L is correlated with an increase in thermopower (absolute value of Seebeck coefficient (S)). Thus, a first order correction to the degenerate limit of L can be based on the measured thermopower, |S|, independent of temperature or doping. We propose the equation: L=1.5+exp−|S|116 (where L is in 10−8 WΩK−2 and S in μV/K) as a satisfactory approximation for L. This equation is accurate within 5% for single parabolic band/acoustic phonon scattering assumption and within 20% for PbSe, PbS, PbTe, Si0.8Ge0.2 where more complexity is introduced, such as non-parabolic Kane bands, multiple bands, and/or alternate scattering mechanisms. The use of this equation for L rather than a constant value (when detailed band structure and scattering mechanism is not known) will significantly improve the estimation of lattice thermal conductivity.
Bi2Te3 has been recognized as an important cooling material for thermoelectric applications. Yet its thermoelectric performance could still be improved. Here we propose a band engineering strategy by ...optimizing the converging valence bands of Bi2Te3 and Sb2Te3 in the (Bi1−xSbx)2Te3 system when x=0.75. Band convergence successfully explains the sharp increase in density-of-states effective mass yet relatively constant mobility and optical band gap measurement. This band convergence picture guides the carrier concentration tuning for optimum thermoelectric performance. To synthesize homogeneous textured and optimally doped (Bi0.25Sb0.75)2Te3, excess Te was chosen as the dopant. Uniform control of the optimized thermoelectric composition was achieved by zone-melting which utilizes separate solidus and liquidus compositions to obtain zT=1.05 (at 300K) without nanostructuring.
In this article, I propose a three-stage estimation model to examine the effect of parental divorce on the development of children's cognitive skills and noncognitive traits. Using a framework that ...includes pre-, in-, and post-divorce time periods, I disentangle the complex factors affecting children of divorce. I use the Early Childhood Longitudinal Study-Kindergarten Class 1998 to 1999 (ECLS-K), a multiwave longitudinal dataset, to assess the three-stage model. To evaluate the parameters of interest more rigorously, I employ a stage-specific ordinary least squares (OLS) model, a counterfactual matching estimator, and a piece-wise growth curve model. Within some combinations of developmental domains and stages, in particular from the in-divorce stage onward, I find negative effects of divorce even after accounting for selection factors that influence children's skills and traits at or before the beginning of the dissolution process. These negative outcomes do not appear to intensify or abate in the ensuing study period.
Reducing lattice thermal conductivity (κl) of a thermoelectric material is one of the most popular strategies to improve its thermoelectric performance. Particularly, many efforts have been focused ...on decreasing grain size to effectively scatter low‐frequency phonons by boundary scattering. In addition to the boundary scattering, we have recently demonstrated that dense arrays of dislocations formed in grain boundaries can further reduce the κl by dislocation scattering at room temperature and above. In order to closely examine the effect of the dislocation scattering, the κl of polycrystalline Bi0.5Sb1.5Te3 samples with and without dislocations were measured at low temperature (T < 200 K). Because other phonon scattering mechanisms like Umklapp and point‐defect scatterings are not dominant at low temperature, we clearly show the presence of the dislocation scattering in the sample with the dislocations by successfully describing its low temperature experimental κl with a theoretical model.
Flat metasurfaces with subwavelength meta‐atoms can be designed to manipulate the electromagnetic parameters of incident light and enable unusual light–matter interactions. Although hydrogel‐based ...metasurfaces have the potential to control optical properties dynamically in response to environmental conditions, the pattern resolution of these surfaces has been limited to microscale features or larger, limiting capabilities at the nanoscale, and precluding effective use in metamaterials. This paper reports a general approach to developing tunable plasmonic metasurfaces with hydrogel meta‐atoms at the subwavelength scale. Periodic arrays of hydrogel nanodots with continuously tunable diameters are fabricated on silver substrates, resulting in humidity‐responsive surface plasmon polaritons (SPPs) at the nanostructure–metal interfaces. The peaks of the SPPs are controlled reversibly by absorbing or releasing water within the hydrogel matrix, the matrix‐generated plasmonic color rendering in the visible spectrum. This work demonstrates that metasurfaces designed with these spatially patterned nanodots of varying sizes benefit applications in anti‐counterfeiting and generate multicolored displays with single‐nanodot resolution. Furthermore, this work shows system versatility exhibited by broadband beam‐steering on a phase modulator consisting of hydrogel supercell units in which the size variations of constituent hydrogel nanostructures engineer the wavefront of reflected light from the metasurface.
This paper reports an approach for plasmonic metasurfaces with hydrogel meta‐atoms at the subwavelength scale. Periodic arrays of hydrogel nanodots with tunable diameters are fabricated on silver substrates, resulting in humidity‐responsive surface plasmon polaritons. This work demonstrates that metasurfaces designed with spatially patterned hydrogel nanodots of varying size distributions benefit applications in anti‐counterfeiting, multicolored displays, and a phase modulator.
This article proposes a bandgap reference (BGR) recursive low-dropout (LDO) regulator chip that achieves a high power supply rejection (PSR) in the low- to mid-frequency range. The presented LDO ...design enables the total PSR of LDO to be free from the finite ripple-rejection of the BGR circuit, resulting in low design complexity and low power consumption. To improve the PSR further, the gate buffer is modified to provide an additional ripple feedforward cancellation. The modified gate buffer also offers fast transient response and stable operation. Moreover, a light-load stabilizer loop is also suggested to provide high stability over all load conditions. A prototype chip able to supply up to 300 mA output current was implemented by 0.5- μ m 5-V CMOS devices. The PSR was measured to be -102 to -80 dB at frequencies from 100 Hz to 0.1 MHz, which is higher than that of prior LDOs with C OUT ≥ 1 μ F. The proposed LDO consumes only 50 μ A at a load current of 300 mA, and a peak current efficiency of 99.98% was achieved. The line and load regulations were measured as 0.003%/V and 0.28%/A, respectively. This chip shows a figure-of-merit of 11 ps in the transient response.
Taming electronic and thermal transport properties is the ultimate goal in the quest to achieve unprecedentedly high performance in thermoelectric (TE) materials. Most state‐of‐the‐art TE materials ...are inherently narrow bandgap semiconductors, which have an inevitable contribution from minority carriers, concurrently decreasing Seebeck coefficient and increasing thermal conductivity. Nevertheless, the restraint control of minority carrier transport is seldom considered as a key element to enhance the TE figure of merit (zT). Herein, it is verified that the localized dislocation arrays at grain boundaries enable the suppression of minority carrier contribution to electronic transport properties, resulting in an increase of the Seebeck coefficient and the carrier mobility in bismuth antimony tellurides. It is also suggested that the suppression of minority carriers via the generation of dislocation arrays at grain boundaries is an effective and noninvasive strategy to optimize overall electronic transport properties without sacrificing predominant characteristics of majority carriers in TE materials.
The localized dislocation arrays at grain boundaries enable the selective suppression of minority carrier contribution to electronic transport properties without sacrificing characteristics of majority carriers, resulting in an increase of overall Seebeck coefficient and carrier mobility in bismuth antimony tellurides. This finding provides an effective and noninvasive paradigm to optimize electronic transport properties of conventional thermoelectric materials.
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