Search for: All records

This study delves into the polarization properties of various hair colors using several techniques, including polarization ray tracing, full Stokes, and Mueller matrix imaging. Our analysis involved studying hair in both indoor and outdoor settings under varying lighting conditions. Our results demonstrate a strong correlation between hair color and the degree of linear polarization. Specifically, light-colored hair, such as white and blond, exhibits high albedo and low DoLP. In contrast, dark hair, like black and brown hair, has low albedo and high DoLP. Our research also revealed that a single hair strand displays high diattenuation near specular reflections but high depolarization in areas with diffuse reflections. Additionally, we investigated the wavelength dependency of the polarization properties by comparing the Mueller matrix under illumination at 450 nm and 589 nm. Our investigation demonstrates the impact of hair shade and color on polarization properties and the Umov effect.

 

Zintl phase Mg 3 Sb 2 , which has ultra-low thermal conductivity, is a promising anisotropic thermoelectric material. It is worth noting that the prediction and experiment value of lattice thermal conductivity ( κ ) maintain a remarkable difference, troubling the development and application. Thus, we firstly included the four-phonon scattering processes effect and performed the Peierls–Boltzmann transport equation (PBTE) combined with the first-principles lattice dynamics to study the lattice thermal transport in Mg 3 Sb 2 . The results showed that our theoretically predicted κ is consistent with the experimentally measured, breaking through the limitations of the traditional calculation methods. The prominent four-phonon scatterings decreased phonon lifetime, leading to the κ of Mg 3 Sb 2 at 300 K from 2.45 (2.58) W m −1 K −1 to 1.94 (2.19) W m −1 K −1 along the in (cross)-plane directions, respectively, and calculation accuracy increased by 20%. This study successfully explains the lattice thermal transport behind mechanism in Mg 3 Sb 2 and implies guidance to advance the prediction accuracy of thermoelectric materials. 

The Mg 3 Sb 2− x Bi x family has emerged as the potential candidates for thermoelectric applications due to their ultra-low lattice thermal conductivity ( κ L ) at room temperature (RT) and structural complexity. Here, using ab initio calculations of the electron-phonon averaged (EPA) approximation coupled with Boltzmann transport equation (BTE), we have studied electronic, phonon and thermoelectric properties of Mg 3 Sb 2− x Bi x (x = 0, 1, and 2) monolayers. In violation of common mass-trend expectations, increasing Bi element content with heavier Zintl phase compounds yields an abnormal change in κ L in two-dimensional Mg 3 Sb 2− x Bi x crystals at RT (∼0.51, 1.86, and 0.25 W/mK for Mg 3 Sb 2 , Mg 3 SbBi, and Mg 3 Bi 2 ). The κ L trend was detailedly analyzed via the phonon heat capacity, group velocity and lifetime parameters. Based on quantitative electronic band structures, the electronic bonding through the crystal orbital Hamilton population (COHP) and electron local function analysis we reveal the underlying mechanism for the semiconductor-semimetallic transition of Mg 3 Sb 2-− x Bi x compounds, and these electronic transport properties (Seebeck coefficient, electrical conductivity, and electronic thermal conductivity) were calculated. We demonstrate that the highest dimensionless figure of merit ZT of Mg 3 Sb 2− x Bi x compounds with increasing Bi content can reach ∼1.6, 0.2, and 0.6 at 700 K, respectively. Our results can indicate that replacing heavier anion element in Zintl phase Mg 3 Sb 2− x Bi x materials go beyond common expectations (a heavier atom always lead to a lower κ L from Slack’s theory), which provide a novel insight for regulating thermoelectric performance without restricting conventional heavy atomic mass approach. 

Thermal transport in amorphous lithium‐sulfur (a‐Li_xS) is systematically investigated using molecular dynamics and the contributions from different types of heat carriers are quantitatively evaluated. In general, the thermal conductivity (TC) ofa‐Li_xS changes largely by varying the concentration (x) of Li ions ina‐Li_xS. Interestingly, the TC ofa‐Li_xS shows three distinct regimes of dependence on Li concentration. For low Li concentration (x = 0.4–1.2), the TC grows slowly, followed by a rapid increase in TC for medium Li concentration (x = 1.2–1.6), where the growth rate is three times that of the first regime, and finally, the TC is independent of Li concentration (x = 1.6–2.0). The TC enhancement in the first and second regimes is mainly attributed to propagating and non‐propagating vibrational modes ina‐Li_xS, respectively. In contrast, the stable thermal transport regime is governed by the competition between propagating and non‐propagating phonons. These investigations provide quantitative TC data of various polysulfides for shuttling analysis, and a fundamental understanding of the thermal transport mechanism of complexa‐Li_xS structures, which is beneficial for the rational design of thermal management of Li‐S batteries.

 

The notion of metasurface has inspired the innovation of various functional devices in the terahertz band, but the intrisinc dispersion restricts their application in broadband scenarios. Here, two terahertz achromatic linear‐phase‐gradient metasurface devices are demonstrated, which are a beam deflector and a beam splitter, respectively. The phase and dispersion of the metasurfaces are simultaneously engineered by changing the geometric parameters of the unit cells made of silicon gratings and pillars. The simulated and experimental results demonstrate the achromatic feasibility of the beam deflector from 0.6 to 1.2 THz and of the beam splitter from 0.6 to 1.1 THz. The transmittances and the splitting ratios of the achromatic beam splitter are also analyzed. The metasurface based achromatic beam deflector and splitter presented here not only enrich the terahertz functional devices, but the methods and structures may also promote the research of broadband terahertz metasurfaces.