Electrofluids
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The significant demand for energy storage systems has spurred innovative designs and extensive research on lithium-ion batteries (LIBs). To that end, an in-depth examination of utilized materials and relevant methods in conjunction with comparing electrochemical mechanisms is required. Lithium titanate (LTO) anode materials have received substantial interest in high-performance LIBs for numerous applications. Nevertheless, LTO is limited due to capacity fading at high rates, especially in the extended potential range of 0.01–3.00 V versus Li+/Li, while delivering the theoretical capacity of 293 mAh g−1. This study demonstrates how the performance of the LTO anode can be improved by modifying the manufacturing process. Altering the dry and wet mixing duration and speeds throughout the manufacturing process leads to differences in particle sizes and homogeneity of dispersion and structure. The optimized anode at 5 A g−1 (≈17C) and 10 A g−1 (≈34C) yielded 188 and 153 mAh g−1 and retained 73% and 68% of their initial capacity after 1000 cycles, respectively. The following findings offer valuable information regarding the empirical modifications required during electrode fabrication. Additionally, it sheds light on the potential to produce efficient anodes using commercial LTO powder.
Background: An electrofluid is, in essence, a liquid composite composed of a conductive filler material dispersed in a fluid matrix. Electrofluids are a promising area of research with applications in soft electronics as a substitute for conventional solid conductors. The versatility in choice of matrix as well as the inherent properties of a liquid system allow for tuning of mechanical properties such as stiffness as well as overcoming present challenges present in solid conductors such as fatigue or cold working, opening new possibilities for electromechanical applications. It is therefore very interesting to seek ways to control & characterize their rheoelectrical properties.
To this end, the functionalization of the surface of conductive filler particles becomes a promising research topic. By adsorbing or otherwise adding conductive polymers on the surface of filler particles their interaction with the matrix is changed and stability may be improved, most likely at the cost of reduced conductivity. The synthesis and characterization of silver particles functionalized with P3HT (poly(3-hexylthiophene-2,5-diyl)), as well as their comparison to particles functionalized with PTEBS (sodium poly[2-(3-thienyl)-ethoxy-4-butylsulfonate]) has been the main focus of this internship project.
Background: Advancements in soft robotics rely on chemistry and material science for stretchable wiring and functional materials. Carbon nanotubes (CNTs) hold potential in nanocomposites due to the large aspect ratio. This study aims to explore carboxyl-functionalized multi-walled carbon nanotubes (MWCNT-COOH) behavior in different solvents, an area yet to be extensively researched in this field.
Results: A range of MWCNT-COOH Electrofluids were fabricate with different concentrations to construct percolation curves. Thorough exploration of electrical and rheological parameters yielded essential properties of the nanocomposites.
Conclusion: Surface functionalization deteriorates electrical properties but establishes a robus non-covalent cross-linked network. A glycerol-water mixture provides an improved matrix for nanocomposites of these features and presents an attractive base for bio-sensing applications.
Soft-adaptive electronics require both sensor and conductor materials. The key parameter for these materials is their mechanoelectrical properties. Liquid metals and solid conductive composites have been exploited in this application field, but both are limited by either their chemical stability or limited flexibility, respectively. Electrofluids are a novel approach towards soft electronic components. They are concentrated colloidal suspensions of conductive particles, in which dynamic contacts retain electrical conductivity under deformation, filling the gap between liquid metals and solid composites. Here, we study the mechanical and electrical network interplay of electrofluids based on multi-walled carbon nanotubes (MWCNTs) in glycerol. These networks arise at different filler concentrations, showing a different response to external deformations. We found that electrical conductivity occurs without the presence of a rigid mechanical network, which allows MWCNT suspensions to be electrically conductive even under flow conditions. By performing rheoelectrical measurements, we observed how the mechanical and electrical networks evolved with the applied deformation. We demonstrated the applicability of electrofluids with tailored mechanoelectrical properties as soft electrical connectors.
Soft electrical components are highly demanded in human-machine interaction devices. ”Electrofluids” (EFs) as suspensions of electrically conductive filler particles in non-conductive solvents have been proposed as promising sensors and conductive materials since they can flow and retain electrical conductivity. As they remain liquid in working conditions, encapsulation and manufacturing of complex patterns remain as a challenge but would enable a wider variety of applications. We propose direct ink writing (DIW) as method to manufacture carbon-based EFs. We performed simple shear flow and Fourier-transform (FT) rheology to evaluate the printability of EFs containing different concentrations of Carbon Black and Graphene Powder by DIW. Electrofluids exhibited three important characteristics to be manufactured via DIW: yield stress behaviour (confirmed by flow curves), high brittleness, and a fast mechanical recovery within a range of 15 seconds. We created printability maps to distinguish printable and non-printable EFs. We used printable EFs to manufacture complex patterns. As a proof of the great potential of the EFs and DIW combination, we compared simple and multiline strain gauges enhancing the sensitivity of EF as strain sensor by 400%.
Silver nanowires (AgNW) find use in transparent conductive electrodes with applications in solar cells, touch screens, and wearables. Unprotected AgNW are prone to atmospheric corrosion and lose conductivity over time. Known passivation techniques either require submersion of pre-deposited AgNW in liquid compounds or the modification of AgNW inks prior to deposition, which alters viscosity and complicates deposition. Here, new possibilities for stabilization of pre-deposited AgNW networks without need for submersion are explored. It is demonstrated that AgNW networks can be stabilized either by argon or hydrogen plasma treatment or by solvent vapor annealing with ethanol, methanol, or ethyl acetate. These treatments yielded stable electrical resistance over at least nine weeks, whereas untreated or thermally annealed AgNW layers quickly lost conductivity. The potential of solvent vapor annealing is further explored by demonstrating a new processing technique for stable polymer matrix composites containing AgNW. Co-deposited layers of AgNW with polystyrene microbeads are annealed in ethyl acetate vapor to stabilize the AgNW while at the same time merging polymer beads into a closed film around the AgNW. The resulting composites maintained stable resistance and transmittance for at least seven weeks.
Flexible and stretchable electronics require both sensing elements and stretching-insensitive electrical connections. Conductive polymer composites and liquid metals are highly deformable but change their conductivity upon elongation and/or contain rare metals. Solid conductive composites are limited in mechanoelectrical properties and are often combined with macroscopic Kirigami structures, but their use is limited by geometrical restraints. Here, we introduce “Electrofluids”, concentrated conductive particle suspensions with transient particle contacts that flow under shear that bridge the gap between classic solid composites and liquid metals. We show how Carbon Black (CB) forms large agglomerates when using incompatible solvents that reduce the electrical percolation threshold by 1 order of magnitude compared to more compatible solvents, where CB is well-dispersed. We analyze the correlation between stiffness and electrical conductivity to create a figure of merit of first electrofluids. Sealed elastomeric tubes containing different types of electrofluids were characterized under uniaxial tensile strain, and their electrical resistance was monitored. We found a dependency of the piezoresistivity with the solvent compatibility. Electrofluids enable the rational design of sustainable soft electronics components by simple solvent choice and can be used both as sensor and electrode materials, as we demonstrate.
Silver-coated copper microparticles combine the oxidation resistance of silver with the low cost of copper. They are interesting components for printed conductive structures. We studied whether printed films of such particles can be printed and sintered at low temperatures in air to create highly conductive films and whether it is possible to recover the particles from them for recycling. Pastes containing 1.5 μm to 5 μm spheres and 3 μm flakes with L-ascorbic acid were prepared, screen-printed, and treated at temperatures of 110 °C to 300 °C in air. The bulk resistance of films treated below 160 °C were two orders of magnitude higher than that of bulk copper, ρCu, and limited by particle-particle contact resistances. They were reduced by treating the prints at 160 °C to 250 °C, leading to bulk film resistances down to 41ρCu. We demonstrate that the high mobility of silver enables the formation of necks that bridge the copper cores and reduce resistivity in this temperature window. The sintered prints retained their conductivity for at least 6 months. Treatments at higher temperatures in air were detrimental: resistances increased above 250 °C. These temperatures led to dewetting of the silver coating and fast copper oxidation, resulting in a continuously increasing resistance. In a final study, we demonstrated that films treated below 200 °C can be recycled by recovering the metal powder from the printed conductors and that the powder can be printed again.
We report on the unusual, advantageous ageing of flexible transparent electrodes (FTEs) that were self-assembled from oleylamine-capped gold nanospheres (AuNPs) by direct nanoimprinting of inks with different particle concentrations (cAu = 3 mg mL−1 to 30 mg mL−1). The resulting lines were less than 2.5 μm wide and consisted of disordered particle assemblies. Small-Angle X-ray Scattering confirmed that particle packing did not change with ink concentration. Plasma sintering converted the printed structures into lines with a thin, electrically conductive metal shell and a less conductive hybrid core. We studied the opto-electronic performance directly after plasma sintering and after fourteen days of storage at 22 °C and 55% rH in the dark. The mean optical transmittance [T with combining macron]400–800 in the range from 400 nm to 800 nm increased by up to ≈ 3%, while the sheet resistance Rsh strongly decreased by up to ≈ 82% at all concentrations. We correlated the changes with morphological changes visible in scanning and transmission electron microscopy and identified two sequential ageing stages: (I) post-plasma relaxation effects in and consolidation of the shell, and (II) particle re-organization, de-mixing, coarsening, and densification of the core with plating of Au from the core onto the shell, followed by solid-state de-wetting (ink concentrations cAu < 15 mg mL−1) or stability (cAu ≥ 15 mg mL−1). The plating of Au from the hybrid core improved the FTEs' Figure of Merit FOM = [T with combining macron]400–800·Rsh−1 by up to ≈ 5.8 times and explains the stable value of ≈ 3.3%·Ωsq−1 reached after 7 days of ageing at cAu = 30 mg mL−1.
Hybrid dielectrics were prepared from dispersions of nanoparticles with gold cores (diameters from 2.9 nm to 8.2 nm) and covalently bound thiol-terminated polystyrene shells (5000 Da and 11 000 Da) in toluene. Their microstructure was investigated with small angle X-ray scattering and transmission electron microscopy. The particles arranged in nanodielectric layers with either face-centered cubic or random packing, depending on the ligand length and core diameter. Thin film capacitors were prepared by spin-coating inks on silicon substrates, contacted with sputtered aluminum electrodes, and characterized with impedance spectroscopy between 1 Hz and 1 MHz. The dielectric constants were dominated by polarization at the gold–polystyrene interfaces that we could precisely tune via the core diameter. There was no difference in the dielectric constant between random and supercrystalline particle packings, but the dielectric losses depended on the layer structure. A model that combines Maxwell–Wagner–Sillars theory and percolation theory described the relationship of the specific interfacial area and the dielectric constant quantitatively. The electric breakdown of the nanodielectric layers sensitively depended on particle packing. A highest breakdown field strength of 158.7 MV m−1 was found for the sample with 8.2 nm cores and short ligands that had a face-centered cubic structure. Breakdown apparently is initiated at the microscopic maxima of the electric field that depends on particle packing. The relevance of the results for industrially produced devices was demonstrated on inkjet printed thin film capacitors with an area of 0.79 mm2 on aluminum coated PET foils that retained their capacity of 1.24 ± 0.01 nF@10 kHz during 3000 bending cycles.
