Deformable temperature sensors are required for applications such as soft robotics, biometric sensing, cryopreservation of organs, and flexible electronics. In this paper, we demonstrate Cu–CuNi temperature sensors on flexible Kapton substrates by a novel method consisting of rapid aerosol jet printing of nanoparticles followed by laser sintering at low powers of 100 mW and 400 mW under a shroud of an inert gas to minimize oxidation. The sensors showed a highly linear response as a function of the temperature and the highest sensitivity among film-based sensors yet reported in literature (RajagopalM. C. Rajagopal, M. C. Sens. Actuators, A2018272253; YangF. Yang, F. Sci. Rep.201771721; MurakamiR. Murakami, R. J. Cryst. Growth20184877277). The sensor film microstructure was investigated using scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS), transmission electron microscopy (TEM), and selective area electron diffraction (SAED). The Cu and CuNi film morphology consisted of fused nanoparticles with varying degrees of coalescence and porosities ranging from 9% to 24% through the thickness of the films. No surface oxidation was observed for CuNi films but oxide phase was detected for the Cu films, which did not affect the sensor performance after repeated tests up to a temperature of 140 °C. The sensor performance was independent of the manufacturing conditions of the aerosol jet printing process. Flexibility tests showed a stable device performance (variation of Seebeck coefficient within 2.5%) after 200 bending cycles at three different radii and 200 twisting cycles. The superior performance of the sensor films to the bending and twisting tests was attributed to the porosity of the sintered nanoparticles that allows significant strain without a proportional build-up of the stresses in the film. These results demonstrate the suitability of nanoparticle-based bottom-up fabrication methods for a range of deformable high-performance electronic devices.
A hands-on symmetry project is proposed as an innovative way of teaching point groups to undergraduate chemistry students. Traditionally, courses teaching symmetry require students to identify the point group of a given object. This project asks the reverse: students are instructed to identify an object that matches each point group. Doing so requires students to think about symmetry in their everyday environment and aids in the development of a more intrinsic understanding of the assignment of symmetry classifications.
Direct-write methods, such as the Aerosol Jet® technology, have enabled fabrication of flexible multifunctional 3-D devices by printing electronic circuits on thermoplastic and thermoset polymer materials. Conductive traces printed by additive manufacturing typically start in the form of liquid metal nanoparticle inks. To produce functional circuits, the printed metal nanoparticle ink material must be postprocessed to form conductive metal by sintering at elevated temperature. Metal nanoparticles are widely used in conductive inks because they can be sintered at relatively low temperatures compared with the melting temperature of bulk metal. This is desirable for fabricating circuits on low-cost plastic substrates. To minimize thermal damage to the plastics, while effectively sintering the metal nanoparticle inks, we describe a laser sintering process that generates a localized heataffected zone (HAZ) when scanning over a printed feature. For sintering metal nanoparticles that are reactive to oxygen, an inert or reducing gas shroud is applied around the laser spot to shield the HAZ from ambient oxygen. With the shroud gas-shielded laser, oxygen-sensitive nanoparticles, such as those made of copper and nickel, can be successfully sintered in open air. With very short heating time and small HAZ, the localized peak sintering temperature can be substantially higher than that of damage threshold for the underlying substrate, for effective metallization of nanoparticle inks. Here, we demonstrate capabilities for producing conductive tracks of silver, copper, and copper-nickel alloys on flexible films as well as fabricating functional thermocouples and strain gauge sensors, with printed metal nanoparticle inks sintered by shroud-gas-shielded laser.
Carbon Aerogels (CAs) are potential candidates for electrochemical double layer electrode materials due to their unique properties such as high Specific Surface Area (SSA) up to ~1000 m2/g, controllable interconnected 3D-mesoporosity, good electrical conductivity and longer cycle stability. CAs are commonly prepared through a sol-gel polycondensation of resorcinol (R) with formaldehyde (F) and water by using Na2CO3 catalyst. The synthesis involves four steps: sol-gel formation, solvent exchange, supercritical drying, and pyrolysis. Supercritical drying is vital in creating mesoporosity while relative composition of R-F to water dictates pore-size distribution [1]. Recent studies [2] suggest that micropores of size comparable to that of electrolyte ions are crucial in obtaining high specific capacitance due to partial/complete de-solvation of ions into micropores. Therefore, our study is focused on synthesis of microporous CAs by R-F polymerization using (NH4)2CO3catalyst that allows drying aqua gel precursors at ambient conditions. Ambient drying has an additional advantage of producing high microporosity while eliminating two synthesis steps, e.g. solvent exchange and supercritical drying, which would significantly reduce the cost of the materials. Organic gels (OGs) were formed by polymerization of resorcinol and formaldehyde with varying amounts of distilled water (0.883g in Gel1, 2.894g in Gel2 and 4.904g in Gel3 respectively) using (NH4)2CO3 catalyst. The organic gels formed after polymerization were dried at ambient conditions for two days followed by pyrolysis at 900oC in inert (N2) atmosphere. Nitrogen adsorption-desorption measurements indicate that all three CA samples dried at ambient conditions have high microporosity and exhibit Langmuir type of monolayer adsorption isotherms. However, SSA of the materials decreases with amount of water content. BET analysis, SEM and TEM data will be presented for understanding the differences implied by the relative amount of R-F to water content. Supercapacitor electrodes were made from slurry prepared by mixing CA materials in 1wt.% PVDF in NMP and deposited onto etched copper foil (from MTI®) using doctor-blade followed by drying in vacuum and hot pressing at 2500 PSI. The electrochemical properties of the cells assembled in symmetric CR 2325 coin cell supercapacitor geometry using Celgard® polypropylene (PP) membrane separator will be discussed in terms of cyclic voltammetry, galvanostatic charging-discharging, electrochemical impedance and durability tests. The structure dependent electrochemical properties such as specific capacitance, energy density, power density, and cycle stability in aqueous (6M KOH) and organic electrolyte (NEt4BF4/1M Acetonitrile) will also be discussed in comparison to graphene platelets from STEM Chemicals as a baseline. References: [1] Horikawa T, Hayashi Ji, Muroyama K. Controllability of pore characteristics of resorcinol–formaldehyde carbon aerogel. Carbon. 2004;42:1625-33. [2] Simon P, Gogotsi Y. Materials for electrochemical capacitors. Nat Mater. 2008;7:845-54.
Three types of silica materials with different morphology, specifically SiO2 hollow microspheres, mesoporous silica, and silica aerogel were tested as potential precursors for synthesis of silicon nano- and meso-structures that resemble the original morphology of the precursors. In the optimized magnesium thermal reduction process, magnesium vapor was delivered to silica surface through a stainless steel mesh placed on top of a zirconia boat filled with silica precursor. This approach allowed for better control of silicon nanostructure formation by minimizing reaction by-products that can affect performance of lithium ion battery anode. Material morphological properties of the reduced silica precursors are discussed in terms of X-ray diffraction, BET, BJH pore size distribution, Raman spectroscopy, and TGA.
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