Haitong Yu

Ultrathin planar broadband absorber through effective medium design

Ultrathin planar absorbers hold promise in solar energy systems because they can reduce the material, fabrication, and system cost. Here, we present a general strategy of effective medium design to realize ultrathin planar broadband absorbers. The absorber consists of two ultrathin absorbing dielectrics to design an effective absorbing medium, a transparent layer, and metallic substrate. Compared with previous studies, this strategy provides another dimension of freedom to enhance optical absorption; therefore, destructive interference can be realized over a broad spectrum. To demonstrate the power and simplicity of this strategy, we both experimentally and theoretically characterized an absorber with 5-nm-thick Ge, 10-nm-thick Ti, and 50-nm-thick SiO2 films coated on an Ag substrate fabricated using simple deposition methods. Absorptivity higher than 80% was achieved in 15-nm-thick (1/50 of the center wavelength) Ge and Ti films from 400 nm to near 1 μm. As an application example, we experimentally demonstrated that the absorber exhibited a normal solar absorptivity of 0.8 with a normal emittance of 0.1 at 500 °C, thus demonstrating its potential in solar thermal systems. The effective medium design strategy is general and allows material versatility, suggesting possible applications in real-time optical manipulation using dynamic materials.

Calculation and experimental validation of spectral properties of microsize grains surrounded by nanoparticles.

Opacified aerogels are particulate thermal insulating materials in which micrometric opacifier mineral grains are surrounded by silica aerogel nanoparticles. A geometric model was developed to characterize the spectral properties of such microsize grains surrounded by much smaller particles. The model represents the materials microstructure with the spherical opacifiers spectral properties calculated using the multi-sphere T-matrix (MSTM) algorithm. The results are validated by comparing the measured reflectance of an opacified aerogel slab against the value predicted using the discrete ordinate method (DOM) based on calculated optical properties. The results suggest that the large particles embedded in the nanoparticle matrices show different scattering and absorption properties from the single scattering condition and that the MSTM and DOM algorithms are both useful for calculating the spectral and radiative properties of this particulate system.

New Insight into the Angle Insensitivity of Ultrathin Planar Optical Absorbers for Broadband Solar Energy Harvesting.

Two challenging problems still remain for optical absorbers consisting of an ultrathin planar semiconductor film on top of an opaque metallic substrate. One is the angle-insensitive mechanism and the other is the system design needed for broadband solar energy harvesting. Here, first we theoretically demonstrates that the high refractive index, instead of the ultrathin feature as reported in previous studies, is the physical origin of the angle insensitivity for ultrathin planar optical absorbers. They exhibit omnidirectional resonance for TE polarization due to the high complex refractive index difference between the semiconductor and the air, while for TM polarization the angle insensitivity persists up to an incident angle related to the semiconductor refractive index. These findings were validated by fabricating and characterizing an 18 nm Ge/Ag absorber sample (representative of small band gap semiconductors for photovoltaic applications) and a 22 nm hematite/Ag sample (representative of large band gap semiconductors for photoelectrochemical applications). Then, we took advantage of angle insensitivity and designed a spectrum splitting configuration for broadband solar energy harvesting. The cascaded solar cell and unassisted solar water splitting systems have photovoltaic and photoelectrochemical cells that are also spectrum splitters, so an external spectrum splitting element is not needed.

A New Route for Unburned Carbon Concentration Measurements Eliminating Mineral Content and Coal Rank Effects

500 million tons of coal fly ash are produced worldwide every year with only 16% of the total amount utilized. Therefore, potential applications using fly ash have both environmental and industrial interests. Unburned carbon concentration measurements are fundamental to effective fly ash applications. Current on-line measurement accuracies are strongly affected by the mineral content and coal rank. This paper describes a char/ash particle cluster spectral emittance method for unburned carbon concentration measurements. The char/ash particle cluster spectral emittance is predicted theoretically here for various unburned carbon concentrations to show that the measurements are sensitive to unburned carbon concentration but insensitive to the mineral content and coal rank at short wavelengths. The results show that the char/ash particle cluster spectral emittance method is a novel and promising route for unburned carbon concentration on-line measurements without being influenced by mineral content or coal rank effects.

Theoretical Predictions of Spectral Emissivity for Coal Ash Deposits

Coal ash inevitably forms deposits as combustion residue on the walls and heat transfer surfaces of coal-fired boilers. Ash deposits decrease the boiler efficiency, reduce the generating capacity, and cause unscheduled outages. The radiative heat transfer is the major heat transfer mechanism in utility boilers; thus, the ash deposit emissivity is critical to boiler efficiency and safety. This paper presents a radiative transfer model to predict the spectral emissivities of coal ash deposits. The model includes the effects of the microstructure, chemical composition, and temperature. Typical ash deposit microstructures are generated using diffusion-limited aggregation (DLA). The radiative properties are then calculated using the generalized multiparticle Mie-solution (GMM). The combined GMM and DLA model predicts spectral emissivity better than the original Mie theory and Tiens dependent scattering theory with the average relative difference between predicted results and experimental data decreasing from 17.8% to 9.1% for sample 1 and from 18.6% to 4.2% for sample 2. Maxwell-Garnett (MG) effective medium theory is used to calculate the ash deposit optical constants based on the chemical compositions to include the effect of chemical composition. Increasing temperatures increase the particle diameters and particle volume fractions and, thus, the spectral emissivities. The spectral emissivity ultimately remains constant and less than one. The homogeneous slab model gives the upper limit of the ash deposit spectral emissivity.

Simple Rectangular Gratings as a Near-Field “Anti-Reflection” Pattern for GaSb TPV Cells

We show theoretically that 2D rectangular gratings on the surface of GaSb can serve as an “anti-reflection” pattern for nano-gap thermophotovoltaic (TPV) devices, which significantly enhances near-field radiative flux from the emitter to a GaSb cell, thus improving output power and conversion efficiency. The system in this study is a 200-nm gap TPV power generation system with a planar infrared plasmonic emitter and GaSb cell. Rigorous coupled-wave analysis is used to calculate the spectral near-field radiative flux involving periodic structures. The simulation shows that when coupled with a near-infrared plasmonic bulk emitter, adding gratings on the GaSb cell surface results in strong spectral enhancement above the cell’s bandgap and suppression for low-energy photon transmission, an effect that cannot be fully predicted by the effective medium theory. The resultant peak spectral heat flux is 2.8 times as high as the case without surface structures and the radiative transfer efficiency increased to 24.8% from the original 14.5% with the emitter temperature at 1800 K. The influence of the grating’s geometry parameters on the enhancement and peak frequency is further discussed with rigorous calculation of the spatial distribution of thermal radiative transfer that provided insight into the physical mechanism.