Resistance Temperature Detectors Fabricated via Dual Fused Deposition Modeling of Polylactic Acid and Polylactic Acid/Carbon Black Composites.

Planar-type resistance temperature detectors (P-RTDs) were fabricated via fused deposition modeling by dual nozzle extrusion. The temperature-sensing element of the fabricated sensor was printed with electrically conductive polylactic acid/carbon black (PLA/CB) composite, while the structural support was printed with a PLA insulator. The temperature-dependent resistivity change of PLA/CB was evaluated for different stacking sequences of PLA/CB layers printed with [0°/0°], [-45°/45°], and [0°/90°] plies. Compared to a PLA/CB filament used as 3D printing source material, the laminated structures exhibited a response over 3 times higher, showing a resistivity change from -10 to 40 Ω∙cm between -15 and 50 °C. Then, using the [0°/90°] plies stacking sequence, a P-RTD thermometer was fabricated in conjunction with a Wheatstone bridge circuit for temperature readouts. The P-RTD yielded a temperature coefficient of resistance of 6.62 %/°C with high stability over repeated cycles. Fabrication scalability was demonstrated by realizing a 3 × 3 array of P-RTDs, allowing the temperature profile detection of the surface in contact with heat sources.

Thermocells for Hybrid Photovoltaic/Thermal Systems.

The photovoltaic conversion efficiency of solar cells is highly temperature dependent and decreases with increasing temperature. Therefore, the thermal management of solar cells is crucial for the efficient utilization of solar energy. We fabricate a hybrid photovoltaic/thermocell (PV/T) module by integrating a thermocell directly into the back of a solar panel and explore the feasibility of the module for its practical implementation. The proposed PV/T hybrid not only performs the cooling of the solar cells but also produces an additional power output by converting the heat stored in the solar cell into useful electric energy through the thermocell. Under illumination with an air mass of 1.5 G, the conversion efficiency of the solar cell can improve from 13.2% to 15% by cooling the solar cell from 61 °C to 34 °C and simultaneously obtaining an additional power of 3.53 µW/cm2 from the thermocell. The advantages of the PV/T module presented in this work, such as the additional power generation from the thermocell as well as the simultaneous cooling of the solar cells and its convenient installation, can lead to the module's importance in practical and large-scale deployment.

High performance CNT point emitter with graphene interfacial layer.

Carbon nanotubes (CNTs) have great potential in the development of high-power electron beam sources. However, for such a high-performance electronic device, the electric and thermal contact problem between the metal and CNTs must be improved. Here, we report graphene as an interfacial layer between the metal and CNTs to improve the interfacial contact. The interfacial graphene layer results in a dramatic decrease of the electrical contact resistance by an order of 2 and an increase of the interfacial thermal conductivity by 16%. Such a high improvement in the electrical and thermal interface leads to superior field emission performance with a very low turn-on field of 1.49 V µm(-1) at 10 µA cm(-2) and a threshold field of 2.00 V µm(-1) at 10 mA cm(-2), as well as the maximum current of 16 mA (current density of 2300 A cm(-2)).

Height-renderable morphable tactile display enabled by programmable modulation of local stiffness in photothermally active polymer.

Reconfigurable tactile displays are being used to provide refreshable Braille information; however, the delivered information is currently limited to an alternative of Braille because of difficulties in controlling the deformation height. Herein, we present a photothermally activated polymer-bilayer-based morphable tactile display that can programmably generate tangible three-dimensional topologies with varying textures on a thin film surface. The morphable tactile display was composed of a heterogeneous polymer structure that integrated a stiffness-tunable polymer into a light-absorbing elastomer, near-infra-red light-emitting diode (NIR-LED) array, and small pneumatic chamber. Topological expression was enabled by producing localized out-of-plane deformation that was reversible, height-adjustable, and latchable in response to light-triggered stiffness modulation at each target area under switching of stationary pneumatic pressure. Notably, the tactile display could express a spatial softness map of the latched topology upon re-exposing the target areas to modulated light from the NIR-LED array. We expect the developed tactile display to open a pathway for generating high-dimensional tactile information on electronic devices and enable realistic interaction in augmented and virtual environments.

Twistable and bendable actuator: a CNT/polymer sandwich structure driven by thermal gradient.

We demonstrate a novel configuration of an electrothermal actuator (ETA), which is based on a polydimethylsiloxane (PDMS) slab sandwiched by upper and lower active layers of CNT-PDMS composite. When only one active layer of a single sandwich structure ETA is heated and the other is not, there exists a thermal gradient in the direction of the slab thickness, resulting in bending motion toward the unheated side. Moreover, a dual sandwich structure ETA, consisting of two parallel assembled sandwich structures on the same body, has the unique ability to act with a twisting motion as the two ETAs bend in opposite directions. We expect the advent of the bendable and twistable actuator to break new ground in ETAs.

Fibers of reduced graphene oxide nanoribbons.

Reduced graphene oxide nanoribbon fibers were fabricated by using an electrophoretic self-assembly method without the use of any polymer or surfactant. We report electrical and field emission properties of the fibers as a function of reduction degree. In particular, the thermally annealed fiber showed superior field emission performance with a low potential for field emission (0.7 V µm(-1)) and a giant field emission current density (400 A cm(-2)). Moreover, the fiber maintains a high current level of 300 A cm(-2) corresponding to 1 mA during long-term operation.

Effect of heat treatment with different heat transfer modes on the polymerization of tosylate-doped poly(3,4-ethylenedioxythiophene) films.

In this work, tosylate-doped poly(3,4-ethylenedioxythiophene) (PEDOT:Tos) films are prepared by thermally assisted oxidative polymerization either on a hot plate or in a convection oven. The main difference between these heat treatments is the way heat is transferred (conduction or convection) during polymerization. The surface morphology and structure, doped state, chemical composition, and the changes in the physical and chemical properties of the differently heat-treated films are analyzed using various instrumental methods. The hot plate-treated films exhibit a smooth and dense surface morphology with a low root-mean-square roughness of ~ 5 nm. The films have a quinoid-prevalent thiophene structure with a high electrical conductivity of 575 S/cm. By contrast, the oven-treated films show a rough and porous morphology with a surface roughness ranging from 30 to 80 nm depending on the scanning area, which yields high absorption capacity of more than 90% in the near-infrared range. The oven-treated films show a benzenoid-prevalent structure that provides relatively low electrical conductivity of 244 ± 45 S/cm. As a demonstration of these noticeable changes, PEDOT:Tos films are examined as a photothermal conversion layer to convert light energy to thermal energy, which is converted to electrical energy using a thermoelectric device by covering the films on the device.

An Electricity-Generating Window Made of a Transparent Energy Harvester of Thermocells.

Windows are primarily for admitting light or air and allowing people to see out. Presented here are windows that can generate electricity while retaining the primary functions. These windows are made of transparent thermocells that convert a temperature difference across the window to electricity. Interconnected p-type and n-type or p-n thermocells are introduced and utilized to scale up the output power of a thermocell window (T-window). The T-window consisting of 2 p-n thermocells provides an output voltage of 60 mV and a power density of 0.5 µW/cm2 for a small temperature difference of 10 °C with an optical transparency of ∼50% in the visible range. The T-window introduced here could pave the way to enhancing energy efficiency in residential environments by capturing naturally available low-grade heat, a new renewable energy source that is otherwise discarded to the surrounding environment.

A Light-Driven Vibrotactile Actuator with a Polymer Bimorph Film for Localized Haptic Rendering.

A vibrotactile actuator driven by light energy is developed to produce dynamic stimulations for haptic rendering on a thin-film structure. The actuator is constructed by adopting a thermal bimorph membrane structure of poly(3,4-ethylenedioxythiophene) doped with p-toluenesulfonate (PEDOT-Tos) coated onto a polyethylene terephthalate (PET) film. Upon irradiation of near-infrared (NIR) light, the light energy absorbed at the PEDOT-Tos layer is converted into thermoelastic bending deformation due to the mismatch in coefficient of thermal expansion between PEDOT-Tos and PET. Since the light-induced deformation is reversible, spatially localized, and rapidly controllable with designed light signals, the proposed actuator can produce vibrotactile stimulation over 10 dB at arbitrary areas in the human-sensitive frequency range from 125 to 300 Hz using a low input power of ∼2.6 mW mm-2, as compared with a complex electrical circuit and high input power needed to achieve such actuation performance. Together with its simple structure based on light-driven actuation, the advent of this actuator could open up new ways to achieve substantial advances in rendering textures at a flexible touch interface.

Manufacturing Process of Polymeric Microneedle Sensors for Mass Production.

In this work, we present a fabrication process for microneedle sensors made of polylactic acid (PLA), which can be utilized for the electrochemical detection of various biomarkers in interstitial fluid. Microneedles were fabricated by the thermal compression molding of PLA into a laser machined polytetrafluoroethylene (PTFE) mold. Sensor fabrication was completed by forming working, counter, and reference electrodes on each sensor surface by Au sputtering through a stencil mask, followed by laser dicing to separate individual sensors from the substrate. The devised series of processes was designed to be suitable for mass production, where multiple microneedle sensors can be produced at once on a 4-inch wafer. The operational stability of the fabricated sensors was confirmed by linear sweep voltammetry and cyclic voltammetry at the range of working potentials of various biochemical molecules in interstitial fluid.

Paper-Based Ionic Thermocouples for Inexpensive and High-Precision Measurement of Temperature.

Accurate and yet cost-effective temperature measurements are required in various sectors of academia and industry. Thermocouples (TCs) are most widely used for temperature measurements; however, their low temperature sensitivity and high thermal conductivity should be improved to ensure the reliable measurement of output voltage for small temperature differences. To address this, a paper-based ionic thermocouple (P-iTC) presented here utilizes a pair of paper strips soaked with the electrolytes of potassium ferri-/ferrocyanide and iron (II/III) chloride redox couples, which are used as p- and n-type elements, respectively. The fabricated P-iTC provides 70× higher temperature sensitivity (α, 2.8 mV/K) and 30× lower thermal conductivity (k, 0.8 W/m K) than those of commercial K-type TCs, thereby yielding a remarkably high α/k ratio of 3.5 mV m/W. Reliable sensing performance is measured during three weeks of operation, which indicates that the P-iTC should be stable in long-term operation. To demonstrate the practicality of the P-iTC, a 3 × 3 planar array of P-iTCs is fabricated to monitor the temperature profile of a surface in contact with heat sources. Using pencil-drawn graphite electrodes on paper, a highly cost-effective P-iTC with the material cost of ∼0.5 cents per device is also fabricated, which is successfully used to monitor cold chain temperatures while retaining its excellent temperature-sensing performance.

Crystal-like growth of a metal oxide/CNT composite fiber with electroplated "seed" from a CNT-dispersed nonaqueous electrolyte.

A fabrication technique is developed for the preparation of metal oxide/CNT composites. An essential feature of the technique lies in the use of nonaqueous electrolyte in place of the usual aqueous electrolyte, which ensures well-dispersed CNTs without surfactants. After a "seed" is formed by electroplating on the anode, the seed is simply pulled up at a certain speed to grow a 1D CNT composite structure. The technique leads to a uniform distribution of metal oxide and a high weight fraction of CNT in the composite structure. Moreover, the conductivity of the composite is much higher than that of the CNT fibers fabricated with polymer.

H(2) sensing characteristics of SnO(2) coated single wall carbon nanotube network sensors.

SnO(2) nanoparticle coated single wall nanotube (SWNT) network sensors were fabricated by forming a SWNT network on the Pt patterned SiO(2)/Si substrate using a dip coating method and subsequently depositing SnO(2) nanoparticles on the SWNT network by rf magnetron sputtering. Their H(2) gas sensing properties were investigated. The SnO(2)-SWNT network sensors stably and reversibly responded to H(2) gas even at room temperature and could detect H(2) gas down to 100 ppm. In addition to the low temperature detection, a remarkable finding was that the gas sensing behavior of SnO(2)-SWNT network sensors was changed from p-type to n-type with increasing SnO(2) deposition time (i.e. surface coverage of SnO(2) on SWNT). A schematic model was proposed to explain the switching of sensing behavior depending on the surface coverage of SnO(2) nanoparticles on the SWNTs.

Fabrication of multifunctional Guar gum-silver nanocomposite hydrogels for biomedical and environmental applications.

Poly(acrylamide-co-acrylamidoglycolic acid)/guar gum@ Ag-nanocomposite (AgNC@PAAG) hydrogels has been fabricated by a green protocol utilizing rhubarb stem-extract as bioreductant. The prepared nanocomposites (NCs) are formulated by varying guar gum (GG) polymer and cross-linker content, and used remarkably to study the release of an anticancer drug, 5-fluorouracil (FU). The AgNC@PAAG has demonstrated its potential in bacterial inactivation and p-nitrophenol (PNP) reduction. The AgNC@PAAG hydrogels showed extended FU release time, which was up to 23 h in pH 7.4. The higher zone of inhibition was documented for AgNC@PAAG against B. subtilis and E. coli. It was noticed that, the inhibition activity of AgNC@PAAG, was directly proportional to cross-linker content than the GG polymer. The efficiency of AgNC@PAAG as a nanocatalyst was evaluated for a model reduction reaction of p-nitrophenol (PNP) reduction by aqueous sodium borohydride (NaBH4), with an apparent rate constant of 121.8 × 10-3 min-1 at ambient temperature. The proposed nanocatalysts are reliable and recyclable, demonstrated its catalytic recycle efficacy of 85% after the third successive run. These NCs robust its biological and catalytic activity after embedding silver nanoparticles (AgNPs) by the bioreduction process; these optimized nanocatalysts can be remarkably used in biomedical healthcare sectors and industrial catalysis.

Macroscopic single-walled-carbon-nanotube fiber self-assembled by dip-coating method.

Pure macroscopic single-walled-carbon-nanotube (SWNT) fibers are fabricated by using a dip-coating method without any additive or additional electrical equipment or complex apparatus. The present method only utilizes microfluidics, which includes capillary condensation, capillary flow, and surface tension, and results in the self-assembly and self-alignment of SWNT colloids.

Diffusion and Current Generation in Porous Electrodes for Thermo-electrochemical Cells.

Carbon-based porous electrodes have led to remarkable improvements in the performance of thermochemical cells or thermocells that electrochemically harvest low-grade waste thermal energy. However, the output current from the thermocells is hampered by the diffusion effect, which leads to depleted ion concentration as the ions permeate through the porous electrode. Here, we advance a theoretical basis for a quantitative description of the diffusion effect on current generation in such porous electrodes. One single dimensionless parameter of Thiele modulus describes the effect according to the theory adopted from the well-established results in the literature. Experimental results for carbon fiber electrodes are illustrated and quantified by the theory. The theory presented here would provide a basis for the choice and design of porous electrodes for thermocells. The results should also provide a basis for devising electrochemical devices with highly porous electrodes.

Self-healable and dual-functional guar gum-grafted-polyacrylamidoglycolic acid-based hydrogels with nano-silver for wound dressings.

Dual-functional carbohydrate polymer-based silver nanocomposite (AgNC) hydrogels with self-healing, injectable, and bacterial inactivation properties have attracted particular attention in the wound dressing field. In this study, a rapid formation of AgNC hydrogels were prepared via in situ addition of guar gum-grafted-polyacrylamidoglycolic acid (GG-g-PAGA) polymer and silver nitrate (AgNO3) and sodium borohydride (NaBH4). The GG-g-PAGA polymer and its AgNC hydrogels were analyzed by FTIR, 1H and 13C NMR, UV-vis spectra, FE-SEM, EDX, and FE-TEM. The GG-g-PAGA@AgNC hydrogels exhibited self-healing ability, injectability, stretchability, flowability, high swelling, porosity, upright mechanical behavior, and biodegradability. Moreover, their bacterial inactivation and cytotoxicity were tested against wound pathogens and skin fibroblast cells, respectively. Therefore, incorporating GG-g-PAGA@AgNC hydrogels could be a versatile strategy to speed up wound healing processes, but a clinical trial is still required for its medical applications.

Iron (II/III) perchlorate electrolytes for electrochemically harvesting low-grade thermal energy.

Remarkable advances have recently been made in the thermocell array with series or parallel interconnection, however, the output power from the thermocell array is mainly limited by the electrolyte performance of an n-type element. In this work, we investigate iron (II/III) perchlorate electrolytes as a new n-type electrolyte and compared with the ferric/ferrous cyanide electrolyte at its introduction with platinum as the electrodes, which has been the benchmark for thermocells. In comparison, the perchlorate electrolyte (Fe2+/Fe3+) exhibits a high temperature coefficient of redox potential of +1.76 mV/K, which is complementary to the cyanide electrolyte (Fe(CN)63-/Fe(CN)64-) with the temperature coefficient of -1.42 mV/K. The power factor and figure of merit for the electrolyte are higher by 28% and 40%, respectively, than those for the cyanide electrolyte. In terms of device performance, the thermocell using the perchlorate electrolyte provides a power density of 687 mW/m2 that is 45% higher compared to the same device but with the cyanide electrolyte for a small temperature difference of 20 °C. The advent of this high performance n-type electrolyte could open up new ways to achieve substantial advances in p-n thermocells as in p-n thermoelectrics, which has steered the way to the possibility of practical use of thermoelectrics.

Single-walled carbon-nanotube networks on large-area glass substrate by the dip-coating method.

Highly uniform and large-area single-walled carbon-nanotube (SWNT) networks are realized by the dip-coating method, which is based on fundamental fluid-dynamic phenomena such as capillary condensation and surface tension. The changes in the polarity and hydration properties of the substrate affect the morphology of the SWNT networks and result in nonlinear growth of the networks in the repetitive dip-coating process. The density and the thickness of the SWNT networks are controlled by processing variables including number of dip coatings, concentration of SWNT colloidal solution, and withdrawal velocity. The networks have uniform sheet resistances and high optical transmittance in the visible wavelength range.

Cellulose long fibers fabricated from cellulose nanofibers and its strong and tough characteristics.

Cellulose nanofiber (CNF) with high crystallinity has great mechanical stiffness and strength. However, its length is too short to be used for fibers of environmentally friendly structural composites. This paper presents a fabrication process of cellulose long fiber from CNF suspension by spinning, stretching and drying. Isolation of CNF from the hardwood pulp is done by using (2, 2, 6, 6-tetramethylpiperidine-1-yl) oxidanyl (TEMPO) oxidation. The effect of spinning speed and stretching ratio on mechanical properties of the fabricated fibers are investigated. The modulus of the fabricated fibers increases with the spinning speed as well as the stretching ratio because of the orientation of CNFs. The fabricated long fiber exhibits the maximum tensile modulus of 23.9 GPa with the maximum tensile strength of 383.3 MPa. Moreover, the fabricated long fiber exhibits high strain at break, which indicates high toughness. The results indicate that strong and tough cellulose long fiber can be produced by using ionic crosslinking, controlling spinning speed, stretching and drying.