Papers by Keyword: Figure of Merit (FOM)

Abstract: Thermoelectricity has gained special interest due to its potential applications, especially the advancements in the electronic devices with very low power consumption. Thermoelectric materials can be used to make energy conversion devices that generate power from thermal sources. Multiferroic oxides, in particular cobaltates, have been actively studied as a new type of thermoelectric material (1). The crystal structure of these cobaltates offers a possibility to manipulate Seebeck coefficient, electric conductivity, and thermal conductivity to optimize the figure of merit ZT. The theoretical explanation and experimental observations by some investigators proved the candidature of multiferroic materials for thermoelectric generation. Many semiconducting multiferroic oxides are showing spin dependent Seebeck coefficient (2-3). Moreover, most of these oxides are inherently stable at high temperatures in air, making them a suitable material for high temperature applications. In this work we have investigated the multiferroic and thermoelectric properties of thinfilms of doped cobalt oxide matrices. The observations confirmed that these materials are suitable for thermoelectric generation.

Abstract: We present, for the first time, simulations of thermoelectric properties of silicon carbide (SiC) nanowires as a function of the wire cross section at high temperature (500K), based on non-equilibrium classical molecular dynamics simulations for the lattice thermal transport and non-equilibrium green's function for the electrical transport. Our calculations show that figure of merit (ZT) was increasing with decreasing cross section area: ZT of SiC nanowire at 2x2 nm2 has maximum value in the range of 0.65 - 0.89 at 500K, which is 7 - 8 times larger than maximum ZT of SiC thin film value (0.125 at 973 K). These results show that SiC may be a promising material for thermoelectric applications operating at high temperature.

Abstract: Simple, physics-based, and accurate circuit models are reported for GaN power HEMTs and inductors; these models are then used to design high-performance chip-scale synchronous buck (SB) power converters to provide agile point-of-load (POL) low-voltage ( down to 1V) high-current (up to 10A) power to portable mobile devices from a battery. Excellent agreement between the measured and simulated results is demonstrated for load regulation for a 19V/1.2V, 800 kHz SB converter; for comparison, the same converter performance using the best commercially available state-of-the-art silicon power MOSFETs is also evaluated. It is shown that the conventional approach used for estimating power loss of a SB power converter is in error; a new application-specific Figure of Merit (FOM) for power switches is proposed that accounts for both input and output switching losses.

Abstract: An extensive study on the use of Si as a substrate for the growth of AlGaN/GaN layers for High-Electron-Mobility Transistor (HEMT) were studied and reported in this article. We have used thick buffers to grow high resistive i-GaN by MOCVD which offers a high breakdown voltage. While the leakage through buffer and substrate can be controlled by thick buffer, the leakage through gate is controlled using a thin 2-nm in-situ grown i-GaN cap layer. We have evidenced a high figure of merit (BV²/R_ON) of 2.6 x 10⁸ V²Ω^-1cm^-2 for AlGaN/GaN HEMTs grown on 4-inch Si substrate. The challenges before the MOCVD growth of GaN on Si is also discussed in detail.

Abstract: The magnetostatic interaction, the coercivity and the figure of merit of an irregular triangular nanowire array are studied with the help of Neel model. The results show that the broadening of the distribution of nanowire lengths decreases the thermal stability of the array and the minimum figure of merit is a good candidate for measuring the system’s thermal stability.

Abstract: In this study, a new electric power generation measurement system was developed for piezoelectric energy harvesting using unimorph-type piezoceramics. Relationships between electric power and material constants such as d₃₃, d₃₁, g₃₁, k₃₁, e33T/e0, s₁₁^E, Q_e and Q_m were investigated using various lead zirconium titanate (Pb(Zr,Ti)O₃, PZT) ceramics with different material constants. Using the equipment, pulse-type stress was applied to unimorph-type piezoceramics. Then, optimum measurement conditions were determined. Under these conditions, the electric power for piezoelectric energy harvesting was measured as a function of the material constants. Finally, it was clarified that for piezoelectric energy harvesting using a unimorph-type device, the figure of merit was combination of the 3 kinds of material constants such as large d₃₁, small e33T/e0, and large s₁₁^E.

Abstract: Thermoelectric (TE) semiconductor materials are widely used for miniaturized versatile cooling devices in a wide spectrum of equipments and energy generation in space vehicles. The bismuth telluride pseudobinary alloy family presents the best characteristics for room temperature TE cooling applications. Using appropriately oriented single crystals instead of the well known polycrystalline materials made by powder metallurgy methods, the efficiency of the TE device (Thermoelectric Cooler – TEC or Thermoelectric Generator – TEG) could be almost doubled. For having good quality TE material it is required to produce equally doped single crystals by the controlled crystallization process, namely with the Bridgman-Stockbarber method. Our experiments were made in the Universal Multizone Crystallizator Type UMC, developed by the ADMATIS Ltd., Miskolc, using a quartz tube under high vacuum conditions and automatically controlled thermal field parameters. The crystallographic analysis of the obtained samples was made by Scanning Electron Microscopy – (SEM), X Ray Diffraction – (XRD), and neutron diffraction (TOF spectrometry).

Abstract: Iron disilicide（-FeSi2）compounds were synthesized by field-activated pressure assisted synthesis (FAPAS) process, and their thermoelectric properties were measured. Fine microstructure with small pores was obtained in the fabricated samples. The average grain size was approximately 0.3 m. The thermal conductivity in the temperature range of 300-725 K was 3-4 Wm-1K-1, which was considerably lower than that of the same materials synthesized by other methods including SPS process. Lower thermal conductivity provided a higher figure of merit, ZT of 28.50×10-4 in the temperature range of 330-450 K.

Abstract: beta-FeSi2 films were prepared on Si(001) substrates by the molecular beam epitaxy method using an Fe source. The crystallographic orientation of beta-FeSi2 films on Si(001) substrates were characterized by using x-ray diffraction. Using scanning electron microscopy, surface morphology and film thickness of an oriented sample were observed and estimated. The mobility of beta-FeS2 films on Si(001) substrates was also characterized by Hall measurement at room temperature. The enhancement of figure of merit was evaluated as the functions of mobility and crystallographic orientation of samples.

Abstract: The calculation for 4H-SiC floating junction Schottky barrier diodes (Super-SBDs) was carried out by device simulation and the optimized device structure was fabricated. The best characteristics of the Super-SBDs were breakdown voltage of 2700V and the specific on-resistance of 2.57m*cm2. The world record of Bariga’s Figure of Merit (BFOM) for SiC-SBD expressed by 4Vbd 2/Ron was improved to 11,354MW/cm2.