Thermoelectric materials are of interest for applications as heat pumps and power generators. The performance of thermoelectric devices is quantified by a figure of merit, ZT, where Z is a measure of a material's thermoelectric properties and T is the absolute temperature. A material with a figure of merit of around unity was first reported over four decades ago, but since then-despite investigation of various approaches-there has been only modest progress in finding materials with enhanced ZT values at room temperature. Here we report thin-film thermoelectric materials that demonstrate a significant enhancement in ZT at 300 K, compared to state-of-the-art bulk Bi2Te3 alloys. This amounts to a maximum observed factor of approximately 2.4 for our p-type Bi2Te3/Sb2Te3 superlattice devices. The enhancement is achieved by controlling the transport of phonons and electrons in the superlattices. Preliminary devices exhibit significant cooling (32 K at around room temperature) and the potential to pump a heat flux of up to 700 W cm-2; the localized cooling and heating occurs some 23,000 times faster than in bulk devices. We anticipate that the combination of performance, power density and speed achieved in these materials will lead to diverse technological applications: for example, in thermochemistry-on-a-chip, DNA microarrays, fibre-optic switches and microelectrothermal systems.

Thin-film thermoelectric devices with high room-temperature figures of merit

Recent worldwide interest in building a decentralized, hydrogen-based energy economy has refocused attention on the solid oxide fuel cell (SOFC) as a potential source of efficient, environmentally friendly, fuel-versatile electric power. Due to its high operating temperature, the SOFC offers several potential advantages over polymer-based fuel cells, including reversible electrode reactions, low internal resistance, high tolerance to typical catalyst poisons, production of high-quality waste heat for (among other uses) reformation of hydrocarbon fuels, as well as the possibility of burning hydrocarbon fuels directly. Today, SOFCs are much closer to commercial reality than they were 20 years ago, due largely to technological advances in electrode material composition, microstructure control, thin-film ceramic fabrication, and stack and system design. These advances have led to dozens of active SOFC development programs in both stationary and mobile power and contributed to commercialization or development in a number of related technologies, including gas sensors,1 solid-state electrolysis devices,2 and iontransport membranes for gas separation and partial oxidation.3 Many reviews are available which summarize the technological advances made in SOFCs over the last 15-35 yearssreaders who are primarily interested in knowing the state-of-the art in materials, design, and fabrication (including the electrodes) are encouraged to consult these reviews.4-12 This review focuses on the factors governing SOFC cathode performancesadvances we have made over † Dedicated to Brian Steele, 1929-2003. Researcher, Entrepreneur, Consensus Seeker. 4791 Chem. Rev. 2004, 104, 4791−4843

Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes

Electrochemical impedance spectroscopy (EIS) has been performed to investigate electronic and ionic processes in dye-sensitized solar cells (DSC). A theoretical model has been elaborated, to interpret the frequency response of the device. The high-frequency feature is attributed to the charge transfer at the counter electrode while the response in the intermediate-frequency region is associated with the electron transport in the mesoscopic TiO2 film and the back reaction at the TiO2/electrolyte interface. The low-frequency region reflects the diffusion in the electrolyte. Using an appropriate equivalent circuit, the electron transport rate and electron lifetime in the mesoscopic film have been derived, which agree with the values derived from transient photocurrent and photovoltage measurements. The EIS measurements show that DSC performance variations under prolonged thermal aging result mainly from the decrease in the lifetime of the conduction band electron in the TiO2 film.

Electrochemical Impedance Spectroscopic Analysis of Dye-Sensitized Solar Cells

This thesis explores transport phenomena in nanochannels on a chip. Fundamental nanofluidic ionic studies form the basis for novel separation and preconcentration applications for proteomic purposes. The measurements were performed with 50-nm-high 1D nanochannels, which are easily accessible from both sides by two microchannels. Nanometer characteristic apertures were manufactured in the bonded structure of Pyrex-amorphous silicon – Pyrex, in which the thickness of the amorphous silicon layer serves as a spacer to define the height of the nanochannels. The geometry of the nanometer-sized apertures is well defined, which simplifies the modeling of the transport across them. Compared to biological pores, the present nanochannels in Pyrex offer increased stability. Fundamental characteristics of nanometer-sized apertures were obtained by impedance spectroscopy measurements of the nanochannel at different ionic strengths and pH values. A conductance plateau (on a log-log scale) was modeled and measured, establishing due to the dominance of the surface charge density in the nanochannels, which induces an excess of mobile counterions to maintain electroneutrality. The nanochannel conductance can be regulated at low ionic strengths by pH adjustment, and by an external voltage applied on the chip to change the zeta potential. This field-effect allows the regulation of ionic flow which can be exploited for the fabrication of nanofluidic devices. Fluorescence measurements confirm that 50-nm-high nanochannels show an exclusion of co-ions and an enrichment of counterions at low ionic strengths. This permselectivity is related to the increasing thickness of the electrical double layer (EDL) with decreasing salt concentrations, which results in an EDL overlap in an aperture if the height of the nanochannel and the thickness of the EDL are comparable in size. The diffusive transport of charged species and therefore the exclusion-enrichment effect was described with a simple model based on the Poisson-Boltzmann equation. The negatively charged Pyrex surface of the nanometer characteristic apertures can be inversed with chemical surface pretreatments, resulting in an exclusion of cations and an enrichment of anions. When a pressure gradient is applied across the nanochannels, charged molecules are electrostatically rejected at the entrance of the nanometer-sized apertures, which can be used for separation processes. Proteomic applications are presented such as the separation and preconcentration of proteins. The diffusion of Lectin proteins with different isoelectric points and very similar compositions were controlled by regulating the pH value of the buffer. When the proteins are neutral at their pI value, the diffusion coefficient is maximal because the biomolecules does not interact electrostatically with the charged surfaces of the nanochannel. This led to a fast separation of three Lectin proteins across the nanochannel. The pI values measured in this experiment are slightly shifted compared to the values obtained with isoelectric focusing because of reversible adsorption of proteins on the walls which affects the pH value in the nanochannel. An important application in the proteomic field is the preconcentration of biomolecules. By applying an electric field across the nanochannel, anionic and cationic analytes were preconcentrated on the cathodic side of the nanometer-sized aperture whereas on the anodic side depletion of ions was observed. This is due to concentration polarization, a complex of effects related to the formation of ionic concentration gradients in the electrolyte solution adjacent to an ion-selective interface. It was measured that the preconcentration factor increased with the net charge of the molecule, leading to a preconcentration factor of > 600 for rGFP proteins in 9 minutes. Such preconcentrations are important in micro total analysis systems to achieve increased detection signals of analytes contained in dilute solutions. Compared to cylindrical pores, our fabrication process allows the realization of nanochannels on a chip in which the exclusion-enrichment effect and a big flux across the nanometer-sized aperture can be achieved, showing the interest for possible micro total analysis system applications. The described exclusion-enrichment effect as well as concentration polarization play an important role in transport phenomena in nanofluidics. The appendix includes preliminary investigations in DNA molecule separation and fluorescence correlation spectroscopy measurements, which allows investigating the behavior of molecules in the nanochannel itself.

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Transport phenomena in nanofluidics

Determination of effective capacitance and film thickness from constant-phase-element parameters

Fundamentals of Impedance Spectroscopy Theory Measuring Techniques and Data Analysis Applications of Impedance

Guy Allan

Papers

Heterojunctions and Semiconductor Superlattices