Low Reflectivity Research Articles

Self-Assembly of Soft and Conformable Broadband Absorbing Nanocellulose-Gold Nanoparticle Composites.

Broadband light-absorbing materials are of large interest for numerous applications ranging from solar harvesting and photocatalysis to low reflection coatings. Fabrication of these materials is often complex and typically utilizes coating techniques optimized for flat and hard materials. Here, we show a self-assembly based strategy for generating robust but mechanically flexible broadband light-absorbing soft materials that can conform to curved surfaces and surface irregularities. The materials were fabricated by adsorbing large quantities of gold nanoparticles (AuNPs) on the nanofibrils of hydrated bacterial cellulose (BC) membranes by tailoring the interaction potential between the cellulose nanofibrils and the AuNPs. The highly efficient self-assembly process resulted in very dense multilayers of AuNPs on the nanofibrils, causing extensive broadening of the localized surface plasmon resonance band and a striking black appearance of the BC membranes. The nanocomposite materials showed an absorptance >96% in both the visible and the near-infrared wavelength range. The AuNP-functionalized BC membranes demonstrated excellent conformability to curved and structured surfaces and could adopt the shape of highly irregular surface structures without any obvious changes in their optical properties. The proposed self-assembly based strategy enables the fabrication of soft and conformable broadband light-absorbing nanocomposites with unique optical and mechanical properties using sustainable cellulose-based materials.

New semiconducting lead-free halide double perovskite Ag2LiGaF6 for optoelectronic and thermoelectric applications

A comprehensive study of lead-free halide double perovskite using density functional theory (DFT) including structural, electronic, dynamical, mechanical, optical, and thermoelectric properties has been carried out. The tolerance factor and the octahedral factor confirm that the compound belongs to face-centred-cubic structure with Fm-3m space group. The predicted band structure and DOS exhibit that the material is an indirect bandgap semiconductor. Our calculation reveals that material is dynamically stable because of no negative frequency found in phonon-dispersion curve. The material also possesses mechanical stability. The optical spectra of material show good absorbance and low reflectivity in ultraviolet region. The thermoelectric part discusses the Seebeck coefficient, thermal conductivity, electrical conductivity, power factor, and figure of merit (zT) at different chemical potential into temperature range 400–1200 K. The purpose of this investigation is to stimulate researchers to develop this kind of material and assess their potential for the advancement of contemporary thermoelectric and optoelectronic devices.

Examining flight time, cognitive reflection, workload, stress and metacognition on decision making performance for pilots during flight simulation

Despite technological advancements, human decision errors still contribute to civil aviation accidents. This study investigated whether flight time, cognitive reflection, task-load, metacognition, and perceived stress predicted decision-making (DM) performance during two in-flight training simulations with 104 commercial pilots at Bogota International Airport. Hierarchical regression analysis revealed that the predictors accounted for 56% of the variance. Cognitive reflection, flight time and performance task load emerged as significant positive predictors. Cognitive reflection significantly moderated the relationship between flight time and DM performance, with pilots scoring lower on cognitive reflection showing improved DM with increased flight time, while controlling for performance task load. The study did not find significant relationships between stress metacognition and DM performance. The study emphasises the significance of advanced training methods in improving pilots’ DM, especially for those with low cognitive reflection. Future research should expand to multiple airlines, address gender balance, and incorporate direct measures of metacognitive monitoring.

Compact and low-crosstalk multimode waveguide crossing utilizing subwavelength holey metamaterial waveguides

To further increase the transmission capacity of on-chip optical communication systems, hybrid division multiplexing technology has emerged as a crucial alternative solution, in which multimode waveguide crossings are highly desired. In this paper, we propose and experimentally demonstrate a compact multimode (i.e., three different modes) waveguide crossing that employs subwavelength holey metamaterial waveguides (SHMWs). The used SHMW, formed by inserting subwavelength periodic holes into a multimode interference (MMI) coupler, deservedly exhibits synergetic advantages of the two kinds of structures, enabling an attractive three-mode (e.g., TE0, TM0, and TM1) waveguide crossing with flexible design, small size, and good performance. Simulation results show that the realized device has a low insertion loss (< 0.74 dB), low reflection loss (<−13.1 dB), and low crosstalk (<−31.6 dB) at a central wavelength of 1550 nm for all the modes with a compact footprint of 27.4 µm × 27.4 µm. The experimental results prove that insertion losses are as low as 0.72 dB, 0.27 dB, and 0.90 dB for TE0, TM0, and TM1 mode, respectively, with the corresponding crosstalk below −38 dB at 1550 nm. The proposed device can be widely applied in photonic integrated circuits to construct photonic systems with the abilities of mode control and multiplexing.

Customizable gradient MXene/polyurethane modules for electromagnetic interference shielding with low reflection and thermal management

The conductive polymer composites (CPC) with elevated levels of conductivity typically exhibit enhanced electromagnetic interference shielding effectiveness (EMI SE), however, this enhancement is often paralleled by an augmented risk of secondary electromagnetic (EM) pollution, attributable to the increased EM reflection. Developing a facile method to construct an eco-friendly CPC for EMI shielding that simultaneously exhibits low reflection and high shielding performance remains a promising yet challenging pursuit. Moreover, excellent shielding materials should possess exceptional thermal management capabilities to protect individuals or equipment under extreme conditions. Herein, we have engineered the MXene/polyurethane (MXene/PU) sponge (MPS) modules characterized by the hierarchical porous structure. Through the strategic modular combination of impedance-matching and impedance-mismatching modules tailored for different application contexts, the customizable gradient MXene/PU sponge (GMPS) facilitates achieving a balance of exceptional average EMI SE (44.6 dB), a low reflection coefficient (R = 0.18), and superior photo/electrothermal conversion capability. Furthermore, both experimental results and finite element analyses are adopted to explore the effect of the arrangement order of GMPS on EMI shielding performance. Leveraging the green, health-conscious, and flexibly customizable approach, it manifests tremendous potential across various areas, including intelligent residences and multifunctional integrated systems.

Study of electronic, mechanical, optical, and thermoelectric properties of stable double perovskites Cs2K(Ga/In)I6 for solar cells renewable energy

Double perovskites are exceptional materials for thermoelectric and optoelectronic applications in pursuing green energy. In this study, a comprehensive analysis of Cs2K(Ga/In)I6 has been conducted, focusing on its mechanical, electronic, optical, and thermoelectric properties by using wien2k, BoltzTraP, ShengBTE, and Phonopy codes. The stability of these compounds has been ensured by considering the tolerance factor, formation energy, phonon spectra, and elastic constants. The elastic and thermophysical properties, including ductility, anisotropy, Debye temperature, and melting temperature, are reported to be noteworthy. The band gaps of Cs2K(Ga/In)I6, measured to be 1.92 and 2.08 eV, indicate the presence of absorption bands in both the visible and ultraviolet regions. Significant absorbance, polarization, low optical reflectivity, and energy loss have been discussed in relation to photovoltaic applications. In addition, the transport characteristics are examined by analyzing the Seebeck coefficient and thermal and electrical conductivities. Due to their high figure of merit and exceptionally low lattice thermal conductivity at room temperature, these materials are highly recommended for thermoelectric generators.

Carbon nanotube heat absorber for efficient direct-heating of liquid tin in a solar receiver

A new approach that use vertically-aligned carbon nanotubes (VACNT) as heat absorbers is proposed for efficient direct solar heating of liquid tin in a solar receiver integrated with beam-down optics. Plate-type CNT heat absorbers were prepared by adjusting the concentration of CNTs in the processing solvent m-cresol. The prepared CNT absorber exhibited an increase in bulk density and the formation of a densely packed nanotubes skeleton with a higher CNT concentration in m-cresol. Thermal conductivity, a key design factor for solar absorbers, showed a stronger correlation with skeletal density than with bulk density. Heat absorption characteristics of the solar liquid tin receiver (50 mm-i.d., 60 mm-high) with the CNT absorbers integrated with a Fresnel lens (0.98 m i.d.) were determined. The tin temperature in the receiver with the CNT absorber reached a maximum of 728 °C. Bare tin exhibited system efficiency of 27.7 % and receiver efficiency of 38.3 %, but tin heating through the CNT plates exhibited system efficiency of up to 48.4 % and receiver efficiency of 64.3 %, showing a system heat absorption rate 1.75 times higher. The thermal absorption efficiency was proportional to the skeletal density of the plates, and the thermal absorption of the system was dominated by the efficient transfer of absorbed heat in the energy absorption region. The energy flow and heat loss in the receiver system were analyzed. Although the bare tin receiver exhibited a high heat loss of 27.1 % owing to the low emissivity or high reflectivity of the tin surface, the heat loss in the CNT absorber plates with high absorptivity was only 0.9 %. The flat plate-shaped transmission window in the receiver caused significant reflective heat loss and convective heat loss owing to head-on wind. Based on the energy flow analysis, improvement approaches for enhancing the thermal efficiency of the proposed receiver system are suggested.

Active illumination mode with checkerboard pattern in focus variation microscopy: Analysis and application

Optical measurement methods for surface topography offer the advantages of high accuracy, rapid measurement, and non-destructiveness. Each method has its own suitable application scenarios. Among them, focus variation microscopy is extensively employed in precision manufacturing, aerospace, and medical industries due to its ability to measure rough and large slopes surfaces. However, since the measurement depends on local grayscale differences between focused and blurred images, it cannot measure surfaces with low reflectivity or insufficient texture information. In this work, we propose an active illumination mode for focus variation method that utilizes a digital micromirror device (DMD) to generate a checkerboard pattern. This method introduces additional texture information, resulting in a usable local gradient of image grayscale. Additionally, we analyze the selection criteria for the checkerboard pattern parameters, including the period and light-dark ratio. Furthermore, measurements of two standard steps with different heights demonstrate that the measurement repeatability of the proposed method can reach the nanometer level, rendering it suitable for high-precision measurements. More importantly, the measurement noise results indicate significantly superior performance of active illumination mode compared to the uniform illumination mode. Finally, we reconstruct the surface topography of the microchannels in a microfluidic chip through the encapsulation layer, demonstrating the feasibility of the proposed method.

DFT based computational investigations of the physical properties of chalcogenide perovskites CsXS3 (X=P, Ta) for optoelectronic and photovoltaic applications

In this study, we employed density functional theory (DFT) calculations to systematically characterize the structural, mechanical, electronic, thermal, and optical properties of CsXS₃ (X = P, Ta), one of the most promising members of the chalcogenide perovskite (CP) family. Our theoretical findings align strongly with reported syntheses of other CP's, highlighting CsXS₃ (X = P, Ta) as most stable compounds. These materials satisfy the Born stability criteria, indicating robust mechanical stability, exhibit anisotropy, and demonstrate acceptable resistance to deformation under external stress. CsPS₃ and CsTaS₃ feature direct bandgaps of 1.91 eV and 0.51 eV, with small photocarrier effective masses, high absorption coefficients (1.6 × 10⁶ and 2.1 × 10⁶ cm⁻1), low reflectivity (15 % and 20 %), reduced loss function (0.6 and 0.4) and low refractive index (2.5 and 3.1), respectively. These properties underscore the potential of CsXS₃ (X = P, Ta)-based CP's for developing more efficient and stable optoelectronic devices and indoor photovoltaics. Notably, CsPS₃ is a strong contender for thermoelectric applications due to its lower Debye temperature compared to other CP's. Overall, our results demonstrate the great promise of CsXS₃ (X = P, Ta) chalcogenide perovskites in advancing the field of renewable energy and optoelectronics.

Polarimetric radar observations of the Jersey tornadic supercell on 1–2 November 2023

AbstractA large tornado and severe hailstorm struck Jersey, Channel Islands, just before midnight on 1–2 November 2023. Radar data are analysed to explore the morphology of the parent supercell thunderstorm. A tornado debris cloud was evident in polarimetric fields, collocated with a reflectivity ‘debris ball’. The debris cloud comprised a small region of exceptionally low correlation coefficient (<0.8) collocated with differential reflectivity ≤0dB. In vertical section, the debris signature comprised a column of low correlation coefficient and high reflectivity that tilted north‐northeast with height and extended to at least 2km above ground level.

Dual-gradient MXene/AgNWs/Hollow-Fe3O4/CNF composite films for thermal management and electromagnetic shielding applications

With the increasing prevalence of electronic devices, there is a pressing need for composite films that offer both efficient thermal management and superior electromagnetic interference (EMI) shielding. To address this, we developed a novel composite film featuring a controllable conductive-magnetic dual-gradient structure, composed of cellulose nanofibers (CNF), MXene, silver nanowires (AgNWs), and hollow ferric oxide (Fe3O4). This film was fabricated using a straightforward, cost-effective layer-by-layer vacuum filtration method. Leveraging CNF as the matrix material endows the film with remarkable flexibility. The integration of 2D MXene, 1D AgNWs, and 0D hollow Fe3O4 significantly enhances the film's thermal conductivity through multidimensional particle interactions, achieving a maximum value of 2.92 W/mK. Additionally, the dual-gradient structure of the film-comprising a transition layer and a reflection layer-improves EMI shielding efficiency by balancing high EMI shielding effectiveness with low electromagnetic wave reflection. Specifically, for the dual-gradient configuration (MAF)-25-CNF, the absorption coefficient (A) of electromagnetic waves incident on the low conductivity side reaches 0.23, and the shielding effectiveness reaches 45.8 dB. These findings highlight the potential of MXene/AgNWs/Fe3O4/CNF composite films with a controllable conductive-magnetic dual-gradient structure for applications in electronics, electrical engineering, and wearable technologies.

Structural stability, optoelectronic, thermoelectric, and elastic characteristics of X2ScBiO6 (X= Mg, Ca, and Ba) double perovskites for energy harvesting: First-principles analysis

Double perovskites have emerged as a potential new material for optoelectronic and thermoelectric applications. We explored structural, electronic, optical, thermoelectric, and elastic properties of double oxides perovskite, X2ScBiO6 (X = Mg, Ca, and Ba) through density functional theory (DFT). The most stable structure, according to the optimized structural parameters, is a cubic Fm3m (225) symmetry with a nonmagnetic (NM) state. We have utilized TB-mBJ to estimate the band structure and density of states. The semiconductor nature is predicted with indirect bandgap values of 1.90 eV, 1.39 eV, and 1.11 eV for Mg2ScBiO6, Ca2ScBiO6, and Ba2ScBiO6, respectively. Notable optical responses such as higher absorption (>105 cm−1) and low reflectivity for X2ScBiO6 suggest appropriate candidates for optoelectronic device applications. The efficient thermoelectric order has been investigated under semiclassical Boltzmann theory and constant relaxation time approximation as implemented in the BoltzTraP (BT) code. Several parameters, such as the Seebeck coefficient, electrical conductivity, thermal conductivity, power factor, and figure of merit, have been calculated. The ductile character of the X2ScBiO6 (X = Mg, Ca, and Ba) compounds is further supported by the elastic properties. Consequently, the results show that the compounds investigated could be good alternatives for thermoelectric and photovoltaic applications.

Experimental study on the thermal performance of non-white near-infrared solar reflective coatings in a permafrost region

Solar reflective coating (SRC) offers a method to reduce surface temperature without the need for additional energy consumption, making it highly applicable in permafrost regions. However, many SRCs with high solar reflectance are white, which can cause glare and not meet the aesthetic pavement requirements of high-grade highways. Herein, we developed two non-white SRCs (black and gray) designed to achieve low reflectance in the visible spectrum while maintaining high reflectance in the near-infrared region. We then evaluated the thermal performance of a white SRC, two non-white SRCs and asphalt (as a control) by analyzing surface temperature, ground temperature, and the heat flux between the soil and pavement in a permafrost region of the Qinghai-Tibet Plateau. The results showed that three SRCs reduced the annual mean surface temperature and surface temperature fluctuation compared to the asphalt. The white SRC exhibited annual heat release, whereas the non-white SRCs (black and gray) reduced heat absorption by 34.53% and 69.44%, respectively, compared to the asphalt. By directly reducing the heat absorption of asphalt, the non-white SRCs can potentially slow permafrost degradation beneath asphalt pavements. This study provides a theoretical foundation and technical support for applying non-white near-infrared SRC on high-grade highways in permafrost regions.

Fabrication of MXene-encapsulated Co@C nanoparticles for efficient microwave absorption in the X-band

Two-dimensional MXene has structural advantages in electromagnetic wave scattering due to its layered structure, but MXene materials can lead to impedance mismatch problems due to their high dielectric constants, so it is still a challenge to design highly efficient wave-absorbing materials based on MXene with low reflection loss, thin thickness, and wide absorption frequency. In this study, composite wave-absorbing materials were fabricated from Co–Co PBA precursors and MXene using liquid nitrogen flash freezing and freeze-drying techniques. By treating MXene and the PBA precursor at a high temperature of 750 °C, a rich heterogeneous interface was formed between Co@C and MXene (CCM7), and the impedance matching was optimized to improve the reflection loss capability. The optimized sample has an effective absorption bandwidth of 4.1 GHz at 2.5 mm covering the entire X-band with a minimum reflection loss of −61.42 dB. It is also demonstrated that CCM7 is satisfactory for Radar Cross-Section of flat panels and unmanned aerial vehicles by CST calculations, and this work provides a fresh perspective on the use of effective MXene composites for microwave absorption.

First-principles study on the stability and optoelectronic properties of the novel C6O2 nanostructure

This study presents the prediction of a novel 2D nanostructure, C6O2, characterized as a direct bandgap semiconductor with a rectangular atomic arrangement. Employing computational codes based on density functional theory (DFT), we optimized the lattice parameters, yielding (a = 6.26 Å and b = 2.43 Å). Stability analysis, including cohesive energy (with a value of −7.85 eV/atom) and phonon dispersion within the first Brillouin zone, confirms the acceptable stability of the C6O2 structure. Electronic properties in the ground state were investigated using both HSE06 and GGA approaches. Our results indicate that the predicted structure exhibits a direct bandgap with energy values of 0.108 eV (PBE), 0.11 eV (mBJ), and 0.415 eV (HSE06) at the M point. Furthermore, we explored the optical properties of this nanostructure using the HSE06 approach. Notably, the ground state exhibits moderate absorption across the visible light spectrum (around 3–5 eV) and a low reflection rate. These findings suggest that C6O2 holds promise for future experimental endeavors in designing electro-optical applications.

Signal Components and Impedance Spectroscopy of Potential p‐Si/n‐CdS/ALD‐ZnO Solar Cells: EIS and SCAPS‐1D Treatments

AbstractA silicon heterojunction (SHJ) solar cell with the attractive and widely used atomic layer deposited (ALD)‐ZnO/n‐CdS/p‐Si configuration is examined in this work to learn more about its electrical properties. Using EIS and SCAPS‐1D, a comprehensive model of the device is created and then simulated. Theoretical aspects of the cell are examined through the use of similar electrical circuit models, focusing on the transmittance spectrum made possible by the ALD‐ZnO layer's low reflectance and high visible transmittance. In this study, the C–V tool is used to study the trap states in the silicon absorber layer under different lighting conditions and wavelengths. The doping concentration and built‐in potential are determined using the Mott–Schottky technique. In addition, the cell's properties are investigated by measuring its G–V, G–F, C–T, and C–F in different real‐world scenarios. As a means of visualizing the electrochemical impedance data, Nyquist plots—sometimes called Cole–Cole plots—are utilized. By utilizing absolute impedance and phase shifts, Bode plots are employed to examine the system's frequency response. Last, the results of the SHJ cell's spectral response measurements are given, which confirm the results of the Nyquist plots.

In-situ formed silicon oxycarbide nanowires into porous SiC(rGO) PDCs enable balanced enhancement of robustness and thermal management

With ongoing development of rocket combustion and hypersonic vehicles, urgent needs are created on thermal structures, protection and thermal insulating materials, particularly balanced enhancement of mechanical and antioxidant properties. Herein, we propose a design strategy to construct continuous in-situ formed SiOC nanowires (SiOCnws)-toughened self-healing SiOx coatings into porous SiC(rGO) PDCs. The key production technique is re-pyrolyzing coassembled flexible precursors/SiC(rGO)p/graphite blends, followed by decarbonization and silica sol impregnation-sintering (SIS) process. Well-distributed hierarchical pores are built to heighten thermal insulation properties by the integrated decarburization of graphite coupled with free carbon. High-yield SiOCnws, with high surface activity, are first cultivated via graphite-assisted vapor-solid (VS) mechanism without transition metal catalysts. They interconnect to form intricate 3D meshwork by graphite-assisted melt-spinning and have good compatibility with ceramic matrix and SiOx coatings by carbothermal reaction. Hierarchically porous SiOCnws/SiC(rGO)40% PDCs after once SIS exhibit favorable thermal conductivity of 0.27 W‧m−1‧K−1 and exceptional high robustness (compressive strength: 39.02 MPa, hardness: 10.27 GPa). Such composites display low reflection loss of −48.14 dB at 11.22 GHz and even good structural stability at about 1300 °C as burned by a butane blowtorch for 3600 s, shedding light on competitive components for uses in thermal protection fields.

Ultrablack surface with omnidirectional high absorption.

Black surface plays an important role in solar-thermal conversion systems and space optical systems. Despite the significant efforts in developing a black surface with strong broadband absorption, realizing scalable omnidirectional high absorption ultrablack surfaces remains challenging. Here, we report an ultrablack surface based on a structural black paint (SBP), realized by femtosecond (fs) laser fabrication of high aspect ratio microstructure mold followed by mold transferring on black paint coating. The SBP exhibits extremely low hemispheric reflectance (R < 1.2%) in the solar spectrum at a normal incident angle; even at an 80° incident angle, the SBP also has good anti-reflection performance (R < 9%). Based on such properties, we further show the enhanced solar-thermal performance over commercial black paints. Our approach holds promises for scalable and cost-effective manufacturing of ultrablack surface, with potential applications in solar-thermal conversion efficiency improvement and space optical system stray light suppression.

Synthesis and evaluation of thermal and optical properites of Fe2AlB2 phase alloy for thermal control in space applications

Ternary laminated Fe2AlB2 alloy holds its significance in displaying high strength, damage tolerance, oxidation and radiation cracking resistance. It is synthesized through a pressureless sintering route from elemental powders. Synthesis of Fe2AlB2 is studied from two different molar ratios (2/1/2 and 2/1.5/2) and three different sintering temperatures (1100°C, 1150 °C and 1200 °C). Phase analysis is carried out through an x-ray diffractometer and scanning electron microscopy (SEM). In addition, thermal decomposition, thermal conductivity and absorbance spectra are examined. Fe2AlB2 begins to decompose at 1169 °C into borides and aluminides. Thermal conductivity and diffusivity of Fe2AlB2 decreases with increase in temperature. Lower solar reflectivity and high thermal emittance of Fe2AlB2 implies the thermal control and optical behavior that can be expected for extreme space environments.

Radial Junctions on Laser-Cutting Tinplate Enable Semitransparent Thin-Film BIPVs with Low Reflection and Designable Transmission

Radial Junctions on Laser-Cutting Tinplate Enable Semitransparent Thin-Film BIPVs with Low Reflection and Designable Transmission