Within the framework of a new optimization strategy based on self-compatible thermoelectric elements, the ability to reach maximum performance is discussed. For the efficiency of a thermogenerator and the coefficient of performance of a Peltier cooler, the constraint z T = ko = const. turned out to provide a suitable criterion for judging maximum performance. In this paper ko is calculated as an average of the temperature dependent figure of merit z T.

Gold nanoparticle–coated ZnO tetrapods have been utilized as a substrate for the detection of fluorescently labeled protein tetramethylrhodamine isothiocyanate bovine serum albumin and phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-(Lissamine rhodamine B sulfonyl) down to the concentrations of 15 pM and 79 nM, respectively. Our detection scheme is based on enhanced fluorescence excitation of the biomolecular analytes by the surface plasmon polaritons of gold nanoparticles coated on the ZnO tetrapod whiskers. This enhanced excitation is confirmed using COMSOL Multiphysics, where the optical near field is shown to be dependent on the coating density of the gold nanoparticles and branching of the ZnO nanostructures.

In the literature, a few biological cells have been used as templates to form microcapsules of a variety of shapes and sizes. In this study, we proved the concept that living cells like platelets can be encapsulated with polyelectrolytes using electrostatic layer-by-layer self-assembly (LBL), and, most importantly, the encapsulation process did not induce activation of the platelets. Glycol-chitosan and poly-L-glutamic acid were electrostatically deposited onto platelets, and the encapsulation was confirmed using confocal laser scanning microscopy and scanning electron microscopy. Transmission electron microscopy observation further confirmed that the encapsulation process was mild and the activation of platelets was negligible. The encapsulation of living biological cells like platelets can serve as a model system in a wide range of biomedical applications including local and sustained drug delivery, immune protection of artificial tissues, and versatile artificial blood.

In this article, we report the use of a microwave plasma in a microwave plasma–assisted spray (MPAS) technique to grow crystalline nanoparticles of the oxide thermoelectric material Ca3Co4O9. This unique growth process allows the formation of nanoparticle coatings on substrates from an aqueous precursor of Ca and Co salts. The particle size is controlled from few tens to few hundred nanometers by varying the concentration of the precursor. The resistivity, Seebeck coefficient, and the power factor (PF) measured in the temperature range of 300–700 K for films grown by MPAS process with varying concentrations of calcium and cobalt chlorides are presented. Films with larger nanoparticles showed a trend toward higher PFs than those with smaller nanoparticles. Films with PFs as high as 220 μW/mK2 were observed to contain larger nanoparticles.

One way to further optimize the thermoelectric properties toward a higher ZT is a temperature stable nanoengineering of materials, where the thermal conductivity is reduced by increasing the phonon scattering at the grain boundaries. To study this, Nb-substituted CaMnO3 perovskite-type material was synthesized by ultrasonic spray combustion (USC). The grain growth has been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Finally, the thermoelectric properties of compacted and sintered bulk samples from powder prepared by a continuous scalable USC process were measured up to 1050 K. The thermoelectric legs were prepared by an adapted sintering process. Here, a compromise between enhanced porosity to reduce the thermal conductivity and securing of mechanical stability and low resistivity should be obtained. Based on the grain growth mechanisms, an advanced sintering process for additional interconnection of the particles without particle growth is needed to further increase the thermoelectric performance.

Effect of the seed layer on Seebeck coefficient measurement of electroplated thermoelectric films was quantitatively analyzed. It is found that sheet, rather than bulk resistance of the seed layer, or more precisely its relative comparison to sheet resistance of the film, determines the effects. The analysis reveals that the seed layer’s effect can be neglected (within 1% error) only if the sheet resistance ratio [R□2(film)/R□1(seed)] is smaller than 0.01. This condition commonly requires film thicknesses of 100 μm or more, which is not practically relevant. Based on the analysis we proposed a new method that is applicable for 10 times thinner films. This approach for determining Seebeck coefficient is simple, and more importantly, electroplated films can be characterized in their statu nascendi. Finite element simulation and experiments verified the analysis.

Arrays of vertically aligned silicon wires of 250 nm–4 μm in diameter were fabricated in a top–down process using photolithography and deep reactive ion etching at cryogenic temperatures. Using the 3-omega method, thermal conductance of vertical silicon nanowires, i.e., nanopillars, was measured immediately on-chip without the need of breaking off single wires and mounting them into a special testing device. The Seebeck coefficient was measured with 2-mm2 arrays of pillars of 260 nm in diameter, which were pressure-joined with bulk chips for testing. Testing was performed in the temperature range between 50 and 470 °C at applied temperature gradients of up to 190 °C. We found a reduction of the thermal conductivity to less than 30% of the bulk silicon, confirming that arrayed vertical nanowires fabricated in an economical top–down process can strongly promote silicon as a thermoelectric material.

Graded and segmented thermoelectric elements are studied in order to improve the performance of thermogenerators that are exposed to a large temperature difference. The linear thermodynamics of irreversible processes is extended by assuming spatially dependent material parameters like the Seebeck coefficient, the electrical and thermal conductivities. For the particular case in which these transport coefficients exhibit a constant gradient, we present an analytical solution of the one-dimensional thermal energy balance in terms of Bessel functions. Given linear spatial material profiles, we discuss the optimization of performance parameters like the electrical power Pel and the efficiency η of a graded thermogenerator element of fixed length and fixed boundary temperatures. The results are compared with the constant properties model, i.e., physically and chemically homogeneous material, as a suitable reference for performance evaluation.

Four-leg thermoelectric oxide modules (TOMs) consisting of two p-type (La1.98Sr0.02CuO4) and two n-type (CaMn0.98Nb0.02O3) thermoelectric (TE) legs were produced with a manufacturing quality factor between 30 and 60%. The pressed sintered TE legs revealed 90% of the theoretical density to ensure a sufficient mechanical stability of the TE modules. The legs were connected electrically in series and sandwiched thermally in parallel between two Al2O3 plates serving as absorber and cooler, respectively. A solar cavity-receiver packed with an array of TOMs was subjected to concentrated thermal radiation with peak solar radiative flux intensities exceeding 600 kW/m2. Temperature distributions in the cavity, open-circuit voltage (VOC), and maximum output power (Pmax) were measured for different external loads and solar radiative fluxes (qin). Finally, the solar-to-electricity conversion efficiency (η) was calculated.

We have successfully developed a Seebeck coefficient Standard Reference Material (SRM™), Bi2Te3, that is essential for interlaboratory data comparison and for instrument calibration. Certification measurements were performed using a differential steady-state technique on 10 samples (15 measurements) randomly selected from a batch of 390 bars. The certified Seebeck coefficient values are provided from 10 to 390 K, and they are further supported by transient measurements. The availability of this SRM will validate measurement results, leading to a better understanding of the structure/property relationships and underlying physics of potential high-efficiency thermoelectric materials.

Variably spaced semiconductor superlattices (VSSLs) exhibited superior electron mobility and rectification because of electronic level alignment. We investigated the thermoelectric properties of VSSL structures using a self-consistent nonequilibrium Green’s function quantum model to capture the ballistic electron transport and anatomistic nonequilibrium Green’s function model to capture the phonon transport. A figure of merit was calculated as a function of temperature for two VSSL strain silicon–germanium materials and a non-VSSL material. Calculation of the figure of merit (ZT) versus temperature for a VSSL demonstrated a 17 times increase in power factor at the expense of a 4 times increase in thermal conductivity at room temperature compared to a comparable uniform superlattice. Calculation determined a ZT of 0.20 for a VSSL compared to a ZT of 0.04 for non-VSSL material at 400 K. VSSLs proved to be a candidate material to further increased ZT near room temperature for superlattice materials.

We report the results of an investigation on the structural evolution of a potential new thermoelectric material, Cu3SbSe3, as a function of temperature from 25 to 390 °C. From high-temperature x-ray diffraction data, the refined lattice parameters were seen to change nonlinearly, but continuously, with temperature, with an increased rate of thermal expansion in the a and b lattice parameters from around 125 °C to 175 °C and negative thermal expansion in the c axis from around 100 °C to 175 °C. Crystallographic charge flipping analysis indicated an increase in the disorder of the copper cations with temperature. This reversible order/disorder phase transition in Cu3SbSe3 affects the transport properties, as evidenced by thermal conductivity measurements, which change from negative to positive slope at the transition temperature. This structural change in Cu3SbSe3 has implications for its potential use in thermoelectric generators.