A synthetic approach for enhanced thermoelectric properties of PEDOT:PSS bulk composites

The thermoelectric properties of PEDOT:PSS/Bi 0.5 Sb 1.5 Te 3 polymer/inorganic bulk composites with different Bi 0.5 Sb 1.5 Te 3 content were investigated. The composites were prepared at various concentrations of Bi 0.5 Sb 1.5 Te 3 by a solution-phase process before grinding to fine powders in liquid N 2 for hot pressing into bulk polymer composite materials. The measured transport properties are well described within a theoretical model for effective media involving a tunneling mechanism induced by thermal voltage fluctuations. Our results present a strategy for the preparation of bulk polymer composites and demonstrate an avenue for optimization of the thermoelectric properties of PEDOT:PSS/Bi 0.5 Sb 1.5 Te 3 bulk composites. V C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4933254] Recently, polymers have been considered for thermoelectric applications primarily because of their unique combination of properties that are atypical of inorganic material, namely, mechanical flexibility, low cost, low temperature and low cost processing, and general non-toxicity

Historical influences on the current provision of multiple ecosystem services: is there a legacy of past landcover?

Ecosystem service provision varies temporally in response to natural and human-induced factors, yet research in this field is dominated by analyses that ignore the time-lags and feedbacks that occur within socio-ecological systems. The implications of this have been unstudied, but are central to understanding how service delivery will alter due to future land-use/cover change. Urban areas are expanding faster than any other land-use, making cities ideal study systems for examining such legacy effects. We assess the extent to which present-day provision of a suite of eight ecosystem services, quantified using field-gathered data, is explained by current and historical (stretching back 150 years) landcover. Five services (above-ground carbon density, recreational use, bird species richness, bird density, and a metric of recreation experience quality (continuity with the past) were more strongly determined by past landcover. Time-lags ranged from 20 (bird species richness and density) to over 100 years (above-ground carbon density). Historical landcover, therefore, can have a strong influence on current service provision. By ignoring such time-lags, we risk drawing incorrect conclusions regarding how the distribution and quality of some ecosystem services may alter in response to land-use/cover change. Although such a finding adds to the complexity of predicting future scenarios, ecologists may find that they can link the biodiversity conservation agenda to the preservation of cultural heritage, and that certain courses of action provide win-win outcomes across multiple environmental and cultural goods

Novel Macroscopic and Microscopic Concepts in Thermoelectricity

Thermoelectric phenomena have been observed for the past two centuries while still being an interesting source of new research because of the countless unknowns governing thermoelectric transport in a host of materials. Novel materials and material structures continue to be discovered, providing enhanced thermoelectric effects that can be used in thermoelectric devices for power generation and refrigeration. With the increasing demand for reusable and renewable energy sources, the importance and usefulness of thermoelectric phenomena grows. It is therefore important to understand thermoelectric transport on both the larger macroscale of devices that serve to define material properties and the smaller microscale where phenomena manifest to give rise to these macroscale properties. In this work, the detailed concepts of macroscopic transformation optics are applied to thermoelectric transport. Using these results, a control of thermoelectric flows is shown that can guide the coupled electric and thermal currents in a predesigned way. Metamaterial designs are given for structures that can cloak, rotate, concentrate, and diffuse the coupled transport. These effects are shown through COMSOL MULTIPHYSICS finite element simulations. The results here show the success of applying transformation optics to control thermoelectric transport. Additionally, the microscopic semiclassical description of thermoelectric transport is explored using multiband wave packets. These multiband wave packets are required to describe transport in material systems with band dispersion degeneracies and are an important generalization of their single band counterparts used in Boltzmann theory. The equations of motion for the real space and reciprocal space positions of these wave packets are derived in the general scenario and are considered in the specific cases of single band transport, degenerate band transport, and a pair of linearly crossing bands. A full model for multiband transport is then developed through the use of a density operator that generalizes the usual Boltzmann transport equation. Expressions for the electric and thermal currents for the case of degenerate bands are then obtained alongside the thermoelectric material properties. This model is currently incomplete but continues to be developed

Thermal Fluctuations Tunneling in Doped Conjugated Polymers

The possibility of using conducting polymers as organic alternatives to widely used inorganic materials for thermoelectric (TE) applications has received much attention in the past few decades. Since conducting polymers are generally inefficient compared to inorganic TE materials, research into their underlying transport mechanisms is required to improve their efficiency. We use a model based on the effects of local thermal fluctuations to characterize the transport in conducting polymer composites. With this model, full linear responses for the current and electronic heat current are obtained. From these responses, the local temperature dependent conductivity, electronic contribution to the thermal conductivity, and Seebeck coefficient are extracted and related to those of the composite material through an effective medium theory. The resulting simple expressions for the TE transport properties are easy to use and can improve our understanding of transport in conducting polymers. An example of how to use the model is given for a parabolic tunneling barrier and comparisons to experimental data are also provided

Thermoelectric flow cloaking via metamaterials

A thermoelectric cloak including an inner region and an external medium. The inner region has a cloaking effect and is simultaneously invisible from both heat and electric charge fluxes; and heat, electric currents, and gradients in the external medium are unaltered by the cloaking effect of the inner region

Polaronic Transport in Ag-based Quaternary Chalcogenides

Low temperature resistivity measurements on dense polycrystalline quaternary chalcogenides Ag2+xZn1-xSnSe4, with x = 0, 0.1, and 0.3, indicate polaronic type transport which we analyze employing a two-component Holstein model based on itinerant and localized polaron contributions. Electronic structure property calculations via density functional theory simulations on Ag2ZnSnSe4 for both energetically similar kesterite and stannite structure types were also performed in order to compare our results to those of the compositionally similar but well known Cu2ZnSnSe4. This theoretical comparison is crucial in understanding the bonding that results in polaronic type transport for Ag2ZnSnSe4, as well as the structural and electronic properties of both crystal structure types. In addition to possessing this unique electronic transport, the thermal conductivity of Ag2ZnSnSe4 is low and decreases with increasing silver content. This work reveals unique structure-property relationships in materials that continue to be of interest for thermoelectric and photovoltaic applications