Superposition approach for description of electrical conductivity in sheared MWNT/polycarbonate melts

The theoretical description of electrical properties of polymer melts, filled with attractively interacting conductive particles, represents a great challenge. Such filler particles tend to build a network-like structure which is very fragile and can be easily broken in a shear flow with shear rates of about 1 s–1. In this study, measured shear-induced changes in electrical conductivity of polymer composites are described using a superposition approach, in which the filler particles are separated into a highly conductive percolating and low conductive non-percolating phases. The latter is represented by separated well-dispersed filler particles. It is assumed that these phases determine the effective electrical properties of composites through a type of mixing rule involving the phase volume fractions. The conductivity of the percolating phase is described with the help of classical percolation theory, while the conductivity of non-percolating phase is given by the matrix conductivity enhanced by the presence of separate filler particles. The percolation theory is coupled with a kinetic equation for a scalar structural parameter which describes the current state of filler network under particular flow conditions. The superposition approach is applied to transient shear experiments carried out on polycarbonate composites filled with multi-wall carbon nanotubes

Materials and processes for 3D printed electronics

Dissertação de mestrado integrado em Engenharia de MateriaisThe traditional manufacturing of electronic components consists of complex and with high environmental impact methods. Those materials are potentially dangerous for both environmental and public health, during the manufacturing process and at the end of the product lifetime when not correctly handled. Thus, the goal consists of producing in a simpler/cheaper way and with lower environmental impact, materials to be used into electronic components. In this work inks based of a natural polymer (carrageenan) and ultrapure water (a “green” solvent) were used to produce more environmentally friendly printable electronic components. To achieve magnetic, conductive and dielectric properties CoFe2O4 (CFO), multiwalled carbon nanotubes (MWCNTs) and BaTiO3 (BTO) nanoparticles were added, respectively. To promote a better dispersion and, therefore, to improve the properties of the final product, Triton X-100 was used as a surfactant. Trition X-100 was selected among other surfactants, since it has shown better results on initial selection tests. For the printing process, the most suitable parameters were selected according to the ink viscosity to improve the process as well as to optimize the method to introduce the ink into the syringe. Morphological, thermal, and mechanical tests were performed in order to evaluate the effects of fillers addition and concentration. Dielectric tests were carried out to the samples with BTO. The higher dielectric constant has been obtained for the sample with 20 wt.% BTO content, reaching 1.3 x 104 at 10 kHz. The electrical conductivity evaluation in the samples with MWCNTs shows that a DC conductivity of 0.026 S.m-1 is achieved for the sample with 5 wt.% MWCTNs content. Vibrating sample magnetometer (VSM) test was performed to analyse the magnetic behaviour of the composite samples with CFO, a saturation magnetization of 11 emu.g-1 being obtained for the samples with 20 wt.% CFO content. The inks developed on this work highlights the relevance of the implementation of natural materials as a base for the development of functional and multifunctional materials. Adding to that, this work can also act as an incentive to the study of materials and manufacturing procedures with lower environmental risks with the capacity of still answering society’s needs.A manufatura tradicional de componentes eletrónicos consiste em métodos complexos, com elevado impacto ambiental, quer por gasto energético quer pelos materiais usados que são potencialmente nocivos para o ambiente e para a saúde pública, durante o processo de fabrico e no final de vida do produto, quando não corretamente processados. Visto isto, o objetivo deste projeto consiste em produzir de um modo simples, com baixo custo e com menor impacto ambiental materiais que possam ser usados em componentes eletrónicos. Assim, neste trabalho foram desenvolvidas tintas à base de um polímero natural (carragenina) e água ultrapura (usada como solvente “verde”) para produzir componentes eletrónicos impressos mais amigos do ambiente. De modo a fornecer propriedades magnéticas, condutivas e dielétricas foram adicionadas nanopartículas de CoFe2O4 (CFO), Multicamadas de Nanotubos de Carbono (MWCNTs) e BaTiO3 (BTO), respetivamente. Para promover uma melhor dispersão foi usado Triton X-100 como surfactante. No processo de impressão foram estudados os parâmetros mais adequados de acordo com a viscosidade da tinta para tornar o processo mais rentável assim como tentar encontrar o melhor método para introduzir a tinta dentro da seringa com a menor formação de bolhas possível. Os testes morfológicos, térmicos e mecânicos foram feitos para todas as amostras para comparar as propriedades fornecidas pela adição das partículas, avaliando a sua interferência com o aumento da concentração de filler. Os testes dielétricos foram realizados para as amostras de BTO. A constante dielétrica com valor mais elevado foi obtido para a amostra com concentração de 20 wt.% BTO, atingindo 1.3 x 104 a 10 kHz. A avaliação dos testes de condutividade elétrica nas amostras de MWCNTs, mostraram uma condutividade DC de 0.026 S.m-1 foi obtida para a mostra com concentração de 5 wt.% MWCTNs. O teste de mapeamento de fluxo de valor (VSM) foi realizado para analisar o comportamento magnético do compósito com partículas de CFO, a magnetização de saturação de 11 emu.g-1 foi obtida para a amostra com concentração de 20 wt.% CFO . As tintas desenvolvidas neste trabalho veem dar relevância à implementação de materiais naturais como base para o desenvolvimento de materiais funcionais e multifuncionais. Vem também promover o estudo de materiais e métodos de produção com menos impacto ambiental e que consigam manter a resposta às necessidades da sociedade

Developments in X-ray tomography characterization for electrochemical devices

Over the last century, X-ray imaging instruments and their accompanying tomographic reconstruction algorithms have developed considerably. With improved tomogram quality and resolution, voxel sizes down to tens of nanometers can now be achieved. Moreover, recent advancements in readily accessible lab-based X-ray computed tomography (X-ray CT) instruments have produced spatial resolutions comparable to specialist synchrotron facilities. Electrochemical energy conversion devices, such as fuel cells and batteries, have inherently complex electrode microstructures to achieve competitive power delivery for consideration as replacements for conventional sources. With resolution capabilities spanning tens of microns to tens of nanometers, X-ray CT has become widely employed in the three-dimensional (3D) characterization of electrochemical materials. The ability to perform multiscale imaging has enabled characterization from system-down to particle-level, with the ability to resolve critical features within device microstructures. X-ray characterization presents a favorable alternative to other 3D methods, such as focused ion beam scanning electron microscopy, due to its non-destructive nature, which allows four-dimensional (4D) studies, three spatial dimensions plus time, linking structural dynamics to device performance and lifetime. X-ray CT has accelerated research from fundamental understanding of the links between cell structure and performance, to the improvement in manufacturing and scale-up of full electrochemical cells. Furthermore, this has aided in the mitigation of degradation and cell-level failures, such as thermal runaway. This review presents recent developments in the use of X-ray CT as a characterization method and its role in the advancement of electrochemical materials engineering

Dynamic Materials and Devices for Controlling Solar Heat Gain in Buildings

Modern technology and materials science, coupled with an understanding of our energy consumption patterns, allows for the opportunity to realize large-scale energy savings through the implementation of more efficient, adaptive technologies. More efficient heating and cooling of both commercial and residential buildings is a strong candidate for such advancements. Indeed, a significant portion of the annual US energy budget is devoted to the heating, ventilation, and air conditioning (HVAC) of commercial and residential buildings in order to maintain safe, comfortable internal environments. Dynamic tint windows, both traditional electrochromic devices and reflective devices, are promising candidates to help in the effort to improved building energy efficiency. Use of these technologies, both as integrated devices and in retrofit applications, allows building designers and occupants to adjust the energy properties of the building in response to climate or meteorological conditions in such a way that optimizes the efficiency of the building. While these devices are focused on heat gained and lost through the fenestrations of a building, it is important to modernize the way we think about efficiency with regard to the opaque façade as well. To this end, thermochromic materials such as certain crystal phases of vanadium dioxide have been proposed for inclusion in opaque façade materials. In this paper we investigate the synthesis, integration, and performance of advanced materials for application in both dynamic insulating glass units (IGUs) and dynamic coatings for the opaque portion of the building façade. We examine multiple avenues for low cost processing of electrochromic devices, as well as the viability of various technologies for retrofit applications in a variety of window and building configurations

Study of parameters dominating electromechanical and sensing response in ionic electroactive polymer (IEAP) transducers

Ionic electroactive polymer (IEAP) transducers are a class of smart structures based on polymers that can be designed as soft actuators or sensors. IEAP actuators exhibit a high mechanical response to an external electrical stimulus. Conversely, they produce electrical signals when subjected to mechanical force. IEAP transducers are mainly composed of four different components: The ionomeric membrane (usually Nafion) is an ion permeable polymer that acts as the backbone of the transducer. Two conductive network composite (CNC) layer on both sides of the ionomeric membrane that enhance the surface conductivity and serve as an extra reservoir to the electrolytes. The electrolytes, (usually ionic liquids (IL)), which provides the mobile ions. And two outer electrodes on both sides of the transducer to either provide a distributed applied potential across the actuators (usually gold leaves) or to collect the generated signals from the sensors (usually copper electrodes). Any variation in any of these components or the operating conditions will directly affect the performance of the IEAP transduces. In this dissertation, we studied some of the parameters dominating the performance of the IEAP transducers by varying some of the transducers components or the transducers operating conditions in order to enhance their performance. The first study was conducted to understand the influence of ionic liquid concentration on the electromechanical performance of IEAP actuators. The IL weight percentage (wt%) was varied from 10% to 30% and both the electromechanical (induced strain) and the electrochemical (the current flow across the actuators) were studied. The results from this study showed an enhanced electrochemical performance (current flow is higher for higher IL wt%) and a maximum electromechanical strain of approximately 1.4% at 22 wt% IL content. A lower induced strain was noticed for IL wt% lower or higher than 22%. The second study was to investigate the effect of changing the morphology of the CNC on the sensing performance of IEAP stress sensors. In this study, small salt molecules were added to the CNC layers. Salt molecules directly affected the morphology of the CNC layers resulting in a thicker, more porous, and high conductive CNCs. As a result, the ionic conductivity increased through the CNC layers and sensing performance was enhanced significantly. In the third study, a non-linear angular deformation (limb-like motion) was achieved by varying the CNC layers of the IEAP actuators by adding some conjugated polymers (CP) patterns during the fabrication of the actuators. It was found that the segments with the CP layers will only expand and never contract during the actuation process. Depending on the direction of motion and the location of the CP layers, different actuation shapes such as square or triangular shapes were achieved rather than the typical circular bending. In the fourth study, the influence of temperature on the electromechanical properties of the IEAP actuators was examined. In this study, both electromechanical and electrochemical studies were conducted for actuators that were operated at temperatures ranging from 25 ðC to 90 ðC. The electromechanical results showed a lower cationic curvature with increasing temperature up to 70 ðC. On the other hand, a maximum anionic curvature was achieved at 50 ðC with a sudden decrease after 50 ðC. Actuators started to lose functionality and showed unpredictable performance at temperatures higher than 70 ðC. Electrochemically, an enhancement of the ionic conductivity was resulted from increasing temperature up to 80 ðC. A sudden increase in current flow was recorded at 90 ðC indicating a shorted circuit and actuator failure. Finally, in the fifth study, protons in Nafion membranes were exchanged with other counterions of different Van der Waals volumes. The ionic conductivity was measured for IEAP membranes with different counterions at different temperatures. The results showed higher ionic conductivities across membranes with larger Van der Waals volume counterions and higher temperatures. A different ionic conductivity behavior was also noticed for temperatures ranging from 30 úC to 55 úC than temperatures between 55 úC and 70 úC after fitting the data with the Arrhenius conductivity equation

Simultaneous Rheoelectric Measurements of Strongly Conductive Complex Fluids

We introduce an modular fixture designed for stress-controlled rheometers to perform simultaneous rheological and electrical measurements on strongly conductive complex fluids under shear. By means of a nontoxic liquid metal at room temperature, the electrical connection to the rotating shaft is completed with minimal additional mechanical friction, allowing for simultaneous stress measurements at values as low as 1 Pa. Motivated by applications such as flow batteries, we use the capabilities of this design to perform an extensive set of rheoelectric experiments on gels formulated from attractive carbon-black particles, at concentrations ranging from 4 to 15 wt %. First, experiments on gels at rest prepared with different shear histories show a robust power-law scaling between the elastic modulus G[superscript '][subscript 0] and the conductivity σ[subscript 0] of the gels—i.e., G[superscript '][subscript 0]∼σ[superscript α][subscript 0], with α=1.65±0.04, regardless of the gel concentration. Second, we report conductivity measurements performed simultaneously with creep experiments. Changes in conductivity in the early stage of the experiments, also known as the Andrade-creep regime, reveal for the first time that plastic events take place in the bulk, while the shear rate [dot over γ] decreases as a weak power law of time. The subsequent evolution of the conductivity and the shear rate allows us to propose a local yielding scenario that is in agreement with previous velocimetry measurements. Finally, to establish a set of benchmark data, we determine the constitutive rheological and electrical behavior of carbon-black gels. Corrections first introduced for mechanical measurements regarding shear inhomogeneity and wall slip are carefully extended to electrical measurements to accurately distinguish between bulk and surface contributions to the conductivity. As an illustrative example, we examine the constitutive rheoelectric properties of five different grades of carbon-black gels and we demonstrate the relevance of this rheoelectric apparatus as a versatile characterization tool for strongly conductive complex fluids and their applications.United States. Dept. of Energy. Office of Basic Energy Sciences. Joint Center for Energy Storage ResearchMIT-France Seed FundCentre National de la Recherche Scientifique (France) (PICS-USA Scheme 36939

Amplified, Synergistic (Photo) Catalytic Water-Splitting by Thin- Film Conducting Polymer Composites

There is currently great interest in harnessing sunlight to generate hydrogen from water. Hydrogen may serve as a future energy carrier that could one day supplant fossil fuels like gasoline or diesel. One of the major challenges with implementing this concept is that, present-day photoelectrochemical (PEC) water splitting systems are either inefficient in their capacity to catalytically split water and/or subject to photocorrosion. The problem typically lies at the interface at which the water-splitting catalytic reaction occurs. One potential solution is to develop a thin-film, catalytic, interfacial layer that may lie between the photo-activated species (e.g. the semiconductor) and the aqueous, liquid phase. Such an interfacial layer could be designed to catalyse water-splitting at a more accelerated rate than is possible in its absence, whilst simultaneously suppressing photocorrosion. Ideally, such a thin-film interface would provide the greatest possible catalytic effect, preferably by synergistic amplification of the catalysis beyond what may be achieved by the catalyst species themselves. This work aimed to study and develop thin-film composites, based on well-known conducting polymer supports, that may serve as such an interfacial layer and that display synergistically amplified water-splitting catalysis. Despite their potential for facilitating high activity, thin-film conducting polymer supports have, historically, expedited only relatively weak performances in, for example, catalytic water oxidation (with current densities in the μA/cm2 range)