Thermoelectric energy conversion can be used to capture electric power from waste heat in a variety of applications. The materials that have been shown to have the best thermoelectric properties are compounds containing elements such as tellurium and antimony. These compounds can be oxidized if exposed to the high temperature air that may be present in heat recovery applications. Oxide materials have better stability in oxidizing environments, so their use enables the fabrication of more durable devices. Thus, although the thermoelectric properties of oxides are inferior to those of the compounds mentioned above, their superior stability may expand potential the high temperature application of thermoelectric energy conversion. In this paper, the thermoelectric properties of promising oxide materials are reviewed. The different types of oxides used for thermoelectric applications are compared and approaches for improving performance through doping are discussed.

Oxide materials for high temperature thermoelectric energy conversion

Energy harvesting with thermoelectric materials has been investigated with increasing attention over recent decades. However, the vast number of various material classes makes it difficult to maintain an overview of the best candidates. Thus, we revitalize Ioffe plots as a useful tool for making the thermoelectric properties of a material obvious and easily comparable. These plots enable us to consider not only the efficiency of the material by the figure of merit zT but also the power factor and entropy conductivity as separate parameters. This is especially important for high-temperature applications, where a critical look at the impact of the power factor and thermal conductivity is mandatory. Thus, this review focuses on material classes for high-temperature applications and emphasizes the best candidates within the material classes of oxides, oxyselenides, Zintl phases, half-Heusler compounds, and SiGe alloys. An overall comparison between these material classes with respect to either a high efficiency or a high power output is discussed.

High Power Factor vs. High zT—A Review of Thermoelectric Materials for High-Temperature Application

Circular dichroism (CD) in terahertz (THz) regions has been widely used in biomonitoring, analytical chemistry, communication sensing and other fields. Here, we present a simple design of a dual-band THz...

Dual-band terahertz chiral metasurface absorber with enhanced circular dichroism based on temperature tunable InSb for sensing application

NaxCoO2 (x = 0.55, 0.89) sodium cobaltites have been prepared by solid-state reactions; their structural parameters have been determined; their microstructure has been studied; and their thermal (thermal expansion, thermal diffusivity, and thermal conductivity), electrical (electrical conductivity and thermoelectric power), and functional (power factor, thermoelectric figure of merit, and self-compatibility factor) properties have been investigated in air at temperatures from 300 to 1100 K. The results demonstrate that, with increasing sodium content, the electrical conductivity and thermoelectric power of the materials increase and their thermal conductivity decreases. As a result, the power factor and thermoelectric figure of merit of the Na0.89CoO2 ceramic at a temperature of 1100 K reach 0.829 mW/(m K2) and 1.57, respectively. The electron and phonon (lattice) contributions to the thermal conductivity of the ceramics have been separately assessed, and their linear thermal expansion coefficients have been evaluated.

Synthesis and properties of NaxCoO2 (x = 0.55, 0.89) oxide thermoelectrics

Tunable metamaterials unfolded new scopes for active control of light absorption by engineering meta-atoms. In this study, for the first time, we propose a broadband electrically-tunable metamaterial absorber that can simultaneously modulate reflected light amplitude in both the key telecommunication wavelength bands, i.e., O-band and C-band. We leverage gap plasmon resonance (GPR) of a metal–insulator–metal (MIM) structure and epsilon-near-zero (ENZ) phenomenon of indium-tin-oxide (ITO) to enable real-time control of reflected light amplitude. The proposed metamaterial consists of an array of Au-nanograting on top of Al2O3–ITO–SiO2–Au stack. The coupling of GPR-mode with ENZ-mode is observed when an external bias-voltage is applied to accumulate free-carriers at Al2O3–ITO interface. This results in a broadening of resonance with more than 80 % absorption over a 370 nm spectral window ranging from 1.25 μm to 1.62 μm. Such dynamic control of absorption properties is then utilized for amplitude modulation of the reflected light. We achieved amplitude modulation with an extinction ratio as high as 20.5 dB and 16.7 dB at 1.55 μm and 1.31 μm of wavelength, respectively, while switching between 0 V and 5 V biasing conditions. Thus, the tunable metamaterial absorber could help realizing optical modulators operating at different wavelength bands for integrated nanophotonic systems. 

Dual-band electro-optic modulator based on tunable broadband metamaterial absorber

Circular dichroism (CD) is required in the applications of biological detection, analytical chemistry, etc. Here, we numerically demonstrated large-range switchable CD by controlling the phase change of Ge2Sb2Te5 (GST) in a zigzag array. At the amorphous state of GST (a-GST), the strong and dual-waveband CD effects are realized via the selective excitations of electric, magnetic, and toroidal resonances. With the transition from a-GST to crystalline state GST, CD strengths are tailored dynamically in large ranges. In detail, the CD magnitudes change by about 0.93 and the modulation depths exceed 94% at dual wavebands. The strong CD effects and large-range switch capability in the GST-based metasurfaces will boost the development of active chiroptical devices. 

Strong and switchable circular dichroism of GST based metasurfaces in dual near-infrared wavebands

High temperature electrical conductivity and thermoelectric power of NaxCoO2

In this paper, we demonstrate giant and tunable polarization selectivity of circularly polarized light (CPL) in transmission and reflection modes simultaneously by using photosensitive silicon-based zigzag metasurface. At 3.42 THz, the metasurface provides circular dichroism of transmission/reflection (CDT/CDR) of 0.89/−0.83 for forward-incident CPL and −0.86/0.69 for backward-incident CPL. Multipole expansion and Jones matrix deduction are performed to analyze the physical mechanism of polarization selectivity. To verify the actual control effect of polarization selectivity, a dynamic chiral imaging utilizing the proposed structure and its enantiomer is provided. By applying extra optical pumping to silicon, the switching between showing and hiding pre-designed pattern in transmission field is realized without refabricating the structures. The proposed metasurface with high-efficiency circular polarization selectivity has profound implications for photonic integrated devices. 

High-efficiency and tunable circular polarization selectivity in photosensitive silicon-based zigzag array metasurface

Most active chiral metasurfaces operate in a single band and have an unidirectionally tunable circular dichroism (CD). Here, we propose a zigzag metasurface composed of a Z-shaped metallic strip and a L-shaped graphene strip to realize the dual-band tunable and strong CD. Two strong CD values of -0.88 and 0.88 are found at f1 = 0.86 THz and f2 = 1.23 THz, respectively. The strengths and resonant frequencies of these two CD signals can be tuned by varying graphene's Fermi energy (EF). Strikingly, the CD value at 0.86 THz undergoes a continuous adjustment in a large range from 0.79 to -0.88 when EF increases from 0.32 eV to 1.00 eV, implying that the proposed metasurface supports the switching of CD signal between on-, off- and reverse-states. Based on the strong CD signals, the capability of the metasurface as a biosensor to detect Avian Influenza viruses is demonstrated. This work will advance the development of broadband tunable chiral-optical devices.

Dual-band tunable and strong circular dichroism in a metal-graphene hybrid zigzag metasurface.

Circular dichroism (CD) is originally obtained from three-dimensional spiral structures by simultaneously exciting electric and magnetic resonances. To simplify construction, multilayer stacked asymmetric structures and the symmetric structures relying on oblique incidence are proposed for enhancing CD. Herein, we achieved the enhancement of dual-waveband CD by adding a Ge2Sb2Te5 (GST) layer on the top of a Z-shape gold array in a normally incident system. Benefited from the polarization selective excitations of electric and magnetic dipole resonances, the CD in a simple planar structure is immensely enhanced from near zero to 0.73 at 1.58 µm. Furthermore, the CD strengths is dynamically tuned by controlling the phase of GST. With the GST phase transition from amorphous (a-GST) to crystalline state (c-GST), CD magnitudes are switched by about 0.73 and 0.27 at dual wavebands respectively. The enhancement of CD by adding a layer on a simple planar array offers a new method for designing planar metasurfaces with strong chirality. 

Enhancing circular dichroism by integrating a Ge2Sb2Te5 layer in an active metasurface

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