Conductive Silicone Research Articles

Multiplexed electrospray emitters fabricated by rapid laser micromachining

This technical note documents the fabrication and performance of multiplexed electrospray emitters by rapid laser micromachining. A low cost 1064-nm fiber laser marker system is used to precisely, rapidly and selectively remove materials from a sheet of conductive silicone rubber, which is chemically inert. The laser pulse energy, scanning speed and machining path are judicially chosen to create clean and round micro posts that form external nozzle structures. This laser micromachining technique offers sufficient precision for ensuring uniform pressure drop across each fluid flow channel even at relatively low flow rate per nozzle. The uniformity of the operation from nozzle to nozzle is confirmed by measuring the diameter of each cone-jet. The results suggest that the laser micromachining is a rapid and convenient technique for fabricating multiplexed electrospray emitters.

Dry Textile Electrodes for Wearable Bio-Impedance Analyzers

In this paper the results of the study performed on dry textile electrodes for bio-impedance measurements are reported. The use of two different conductive fabrics is investigated: a silver-plated bi-elastic knitted fabric and a bi-layer structure comprising a layer of a silver-plated knitted fabric and a layer of conductive silicone. In this latter, the presence of the silicone layer guarantees a good contact with the skin, which is a fundamental condition for accurate bio-impedance measurements, and preserves the silver by oxidation due to sweat. The fabricated textile electrodes have been sewn on elastic cuffs and two different relative positions of the current and voltage electrodes have been investigated. The proposed cuffs are suitable to be embedded into a t-shirt for continuous and real-time bio-impedance measurements. The reliability of the developed prototypes is validated by comparison with data obtained from Ag/AgCl commercial electrodes. Promising results have been obtained for both the investigated conductive fabrics.

A bi-directional thermal-driven actuator with conduction‐to‐insulation transformation behavior and its applications in overheating protection and early warning of fire

To endow an actuator with multi functions in response to heat, a layer of electrically conductive silicone rubber (SR) filled with 80 wt% silver powder was coated on a double-layered actuator consisting of two layers of SR filled with different thermal expansion microspheres to obtain a triple-layered actuator. The layer of electrically conductive SR had a little effect on the actuation properties of the triple-layered actuator. The triple-layered actuator exhibited bi-directional deformation and conduction‐to‐insulation transformation behavior during actuation process. The relationships between electrical conductivity, bending curvature and heating time were established. Applications of the actuator in overheating protection and early warning of fire were demonstrated.

Conductive Electric Tapes Based on Silicone Pressure-Sensitive Adhesives

This paper assesses methods for the fabrication of single-sided electrically conductive tape. Silicone resin, dichlorobenzoyl peroxide, and various types of carbon fillers were used to prepare pressure-sensitive adhesives. Various carbon black and graphene filler concentrations in the range 1 to 25 wt.% were tested, whereas for carbon nanotubes the range was 0.25 to 10 wt.%. The adhesion, shear strength, tack, and shrinkage of the prepared tapes were determined. The most important property of the resulting tapes is their electrical conductivity. By using carbon black as a filler, it was possible to obtain a conductivity of 4.3 × 10−3 S/m, with carbon nanotubes 1.4 × 10−01 S/m and with graphene 5.9 × 10−3 S/m. Such electrically conductive silicone self-adhesive tapes are not yet commercially available on the market, but are expected to be useful in the electronic industry for the bonding of various materials. In the available literature provides no information about the self-adhesives tapes based on silicone pressure-sensitive adhesices which exhibit electrically conductive properties.

Stretchable conductor based on carbon nanotube/carbon black silicone rubber nanocomposites with highly mechanical, electrical properties and strain sensitivity

Stretchable conductive polymer composites are important in the field of flexible electronic devices, in which stretchable conductive silicone rubber composites has drawn great attention. However, the poor mechanical properties of the SR composites limit their practical application. How to compensate this drawback without compromising its electrical conductivity is an issue deserved to investigate. Herein, conductive carbon black grafted with a silicone coupling agent was prepared by free radical polymerization, and further reacted with carbon nanotube to obtain a new carbon black-carbon nanotube hybrid filler. This hybrid filler can not only improve the mechanical properties of silicone rubber composites, but also greatly enhance their electrical conductivity. Consequently, a silicone rubber composite with 5.76 vol% hybrid filler has high tensile strength (4.5 MPa) and elongation at break (211%). The minimum conductive percolation threshold and maximum conductivity of the composite are 0.24 vol% and 248.8 S/m, respectively. Besides, the composite has high strain sensitivity and superior deformation recovery performance, and can be applied to detect human motion behavior under a low strain. The preparation method of the hybrid filler provides a novel approach to the preparation and production of stretchable conductive composites with good mechanical properties and potential application for flexible electronic devices.

Vertically aligned silicon carbide nanowires/reduced graphene oxide networks for enhancing the thermal conductivity of silicone rubber composites

Ice-templated assembly strategy is an effective method to construct filler networks. The filler networks can be used to increase the thermal conductivity of polymer composites. Herein, silicon carbide nanowires, reduced graphene oxide and cellulose nanofiber were used as assembly units to construct vertically aligned filler networks by ice-templated assembly strategy. Polydimethylsiloxane grafted with poly(ethylene glycol) could enhance the interfacial interaction and improve the wettability between silicone rubber and the filler networks. Silicon carbide/reduced graphene oxide/silicone rubber composites were prepared by infiltrating the filler networks with silicone rubber. The thermal conductivity of the composites increased with increasing the contents of silicon carbide and reduced graphene oxide. The thermal conductivity of the composite with 1.84 vol% filler network was as high as 2.74 W/(m·K), exhibiting significant enhancement of thermal conductivity of 16 times compared with silicone rubber. This work provides an insight for preparing highly thermally conductive silicone rubber composites.

Practical and Durable Flexible Strain Sensors Based on Conductive Carbon Black and Silicone Blends for Large Scale Motion Monitoring Applications.

Presented is a flexible capacitive strain sensor, based on the low cost materials silicone (PDMS) and carbon black (CB), that was fabricated by casting and curing of successive silicone layers—a central PDMS dielectric layer bounded by PDMS/CB blend electrodes and packaged by exterior PDMS films. It was effectively characterized for large flexion-angle motion wearable applications, with strain sensing properties assessed over large strains (50%) and variations in temperature and humidity. Additionally, suitability for monitoring large tissue deformation was established by integration with an in vitro digestive model. The capacitive gauge factor was approximately constant at 0.86 over these conditions for the linear strain range (3 to 47%). Durability was established from consistent relative capacitance changes over 10,000 strain cycles, with varying strain frequency and elongation up to 50%. Wearability and high flexion angle human motion detection were demonstrated by integration with an elbow band, with clear detection of motion ranges up 90°. The device’s simple structure and fabrication method, low-cost materials and robust performance, offer promise for expanding the availability of wearable sensor systems.

Development of an sEMG sensor composed of two-layered conductive silicone with different carbon concentrations

To achieve robust sEMG measurements in an EMG prosthetic system, this study proposes a surface electromyogram (sEMG) sensor with a novel electrode structure composed of two-layered conductive silicone with different carbon concentrations. We hypothesized there is an optimal carbon concentration for achieving a large sEMG amplitude with robustness to external perturbation, and we empirically determined this optimal concentration. We produced fourteen sets of electrodes, with the weight ratio of carbon to silicone ranging from 1.7% to 4.0%. Using these electrodes, the user sEMG and electrical properties of the electrodes were measured. An external perturbation was applied on one side of the electrode to introduce a condition of unbalanced contact to the sEMG sensor. We defined an index of robustness for the sEMG sensor based on the signal-to-noise ratio in the balanced and unbalanced contact conditions. Based on the results of the robustness index, two optimal carbon concentrations, at weight ratios of 2.0%–2.1% and 2.6%–2.7%, were observed. Moreover, the double-peak property was correlated to the capacitance. Our results clearly demonstrate an optimal carbon concentration for robust sEMG measurements, and suggest that the robust measurement of sEMG is supported by the capacitance component of the sensor system.

Conductive silicone elastomers electrodes processable by screen printing

Conductive inks consisting of graphene and carbon black conductive fillers into a polydimethylsiloxane (PDMS) matrix, which can be processed into thin films by screen printing are developed. The influence of filler composition and content on mechanical and electrical properties of the conductive composites is investigated. The best composites were evaluated as electrode material for dielectric elastomer actuators and for piezoelectric sensors. With increasing filler content, the electrical properties of the resulting composites of graphite nanoplates (GNPs) or a binary mixture of GNPs and carbon black (CB) with PDMS (Mw = 139 kg/mol) are enhanced. Hence, PDMS composites filled with GNPs (42 wt.%) or a binary mixture of GNPs/CB (300/150 ratio, 30 wt.% of total filler loading) exhibited constant contact resistance values of 0.5 and 5 Ω determined in life-cycle test, respectively, thus rendering them suitable as electrode materials for piezosensors. On the other hand, dielectric elastomer actuators require more flexible electrode materials, which could be tuned by varying the polymer molecular weight and by reducing the filler content. Therefore, a composite consisting of PDMS (Mw = 692 kg/mol) and a binary filler mixture of GNPs/CB (150/75 ratio, 18 wt.% of total filler loading) was used for producing the electrodes of dielectric elastomer transducers (DETs). The produced DETs with different electrode thicknesses were characterized in terms of their performance. The negligible hysteresis of the electrode materials is favorable for sensor and actuator applications.

Improving Elasticity of Conductive Silicone Rubber by Hollow Carbon Black

Carbon black-based conductive rubber composites have important impacts on electromagnetic interference(EMI) shielding applications. However, an excessive amount of carbon black in the recipes of these conductive rubbers has caused their weak elasticity. Herein, hollow carbon black(HCB) particles were used to tune the elasticity of conductive rubber composites. Unique hollow morphology produced a better compression recovery of HCB than other solid carbon black, such as acetylene black. When the coupling agent was bonded to HCB, their conductive silicone rubber composites were featured by high stretching resilience, a fast compression recovery and excellent conductivity to satisfy the electromagnetic interference shielding requirements. Importantly, the rubber composites with coupling HCB had extremely low variations of mechanical property, conductivity and EMI shielding effectiveness after thermal accelerated aging tests. It is therefore revealed that the elasticity of HCB and its interfacial chemical coupling with rubber chains both play crucial roles in adjusting the elasticity of conductive rubber to sever long-term EMI protection.

Synthesis and characterization of electrically conductive silicone rubber — TiB2 composites

In this paper, electrically conductive composites comprised of silicone rubber and titanium diboride (TiB2) were synthesized by conventional mixing methods. Fine particles of TiB2 (in micron size) and 10 parts per hundred parts of rubber (phr) proportion of carbon black (XC-72) were used to make the composites with HTV silicone rubber. The composites were cured at appropriate temperature and pressure and the effect on the electrical properties was studied. The resistance of the silicone rubber is ~ 1015Ω which decreases to 1–2 kΩ in case of composites with negligible effect of heat ageing. The hardness increases by ~ 35% simultaneous to the decrease of ~ 47% in the tensile strength. Morphological characterization indicates the homogeneous dispersion of the fillers in the composite.

Penetrating Microindentation of Hyper-soft, Conductive Silicone Neural Interfaces in Vivo Reveals Significantly Lower Mechanical Stresses

ABSTRACTThere is growing evidence that minimizing the mechanical mismatch between neural implants and brain tissue mitigates inflammatory, biological responses at the interface under long-term implant conditions. The goal of this study is to develop a brain-like soft, conductive neural interface and use an improvised, penetrating microindentation technique reported by us earlier to quantitatively assess the (a) elastic modulus of the neural interface after implantation, (b) mechanical stresses during penetration of the probe, and (c) periodic stresses at steady-state due to tissue micromotion around the probe. We fabricated poly- dimethylsiloxane (PDMS) matrices with multi-walled carbon nanotubes (MWCNTs) using two distinct but carefully calibrated cross-linking ratios, resulting in hard (elastic modulus∼484 kPa) or soft, brain-like (elastic modulus∼5.7 kPa) matrices, the latter being at least 2 orders of magnitude softer than soft neural interfaces reported so far. Subsequent loading of the hard and soft silicone based matrices with (100% w/w) low-molecular weight PDMS siloxanes resulted in further decrease in the elastic modulus of both matrices. Carbon probes with soft PDMS coating show significantly less maximum axial forces (-587 ± 51.5 µN) imposed on the brain than hard PDMS coated probes (-1,253 ± 252 µN) during and after insertion. Steady-state, physiological micromotion related stresses were also significantly less for soft- PDMS coated probes (55.5 ± 17.4 Pa) compared to hard-PDMS coated probes (141.0 ± 21.7 Pa). The penetrating microindentation technique is valuable to quantitatively assess the mechanical properties of neural interfaces in both acute and chronic conditions.

Design of robot finger based on flexible tactile sensor

In this article, a flexible tactile sensor that made of conductive silicone rubber for dexterous robot hand is designed. The tactile sensor is made up of four microsensors. The maximum gripping force is simulated when the degree of a robot finger joint is 138. Meanwhile, a control system to analyze the creep and hysteresis characteristics and a processing system of the tactile sensor is designed. We also demonstrated an experiment for the application of robot grasp object, showing the finger’s flexibility and sensitivity. Then the feedback data is sent to control system to provide precise grasp action changes for the robot hand.

Back electrodes of dye-sensitized solar cells on textile fabrics

Dye-sensitized solar cells (DSSCs) are most often built on glass or similar rigid substrates. Transferring them onto textile fabrics would enable using DSSCs on large-scale textile architecture or for several textile-based stand-alone solutions, such as loading a mobile phone by a DSSC tent. For this approach, it is not only necessary to work with nontoxic and inexpensive materials, but also to create elastic and ideally even stretchable electrodes and other layers. Here we report on optimizing a conductive silicone rubber layer, filled with carbon black and graphite, on a viscose woven fabric to create a flexible counter electrode with inherent catalyst. Investigations of DSSCs with this coated textile, combined with a glass photo-electrode prepared with TiO2 and a natural anthocyanin dye, revealed effiencies up to the values gained with pure glass DSSCs.

3D Printing of Self‐Wiring Conductive Ink with High Stretchability and Stackability for Customized Wearable Devices

AbstractThe progress in 3D printing research has led to significant developments ranging from customized printing to rapid prototyping. However, the 3D printing of electrodes, especially stretchable electrodes for the fabrication of 3D printable electronic devices, is challenging due to the inherent weakness with respect to the printing material. A novel preparation method is reported for a 3D printable conductive ink with a self‐wiring effect during heat treatment, which pushes the silicone rubber outward and results in the accumulation of the conductors within the wire. This effect results in the formation of a polymer shell around the conductor, thus yielding conductors with larger stretchability and soft passivation characteristics. The conductive ink is prepared via the following steps: i) mixing of conductive filler, silicone rubbers, and solvent; followed by ii) soft heat treatment for soft curing and solvent evaporation. Furthermore, a capacitive sensor is fabricated using this dielectric polymer layer. As a demonstration, a mouse controller is fabricated using a capacitive sensor array prepared using the conductors developed in this study.

A Triboelectric Nanogenerator as a Self‐Powered Sensor for a Soft–Rigid Hybrid Actuator

AbstractTriboelectric nanogenerators are a promising technology for use in self‐powered sensors, which take advantage of multiple choices of materials and high response to low‐frequency triggering. Here, a conductive sponge/porous silicone‐based triboelectric nanogenerator (SS‐TENG) is demonstrated as a tactile sensor for a hybrid actuator. To fabricate the SS‐TENGs, liquid silicone and ethanol are mixed well and then infiltrated inside a conductive sponge. When heated, the ethanol evaporates from the silicone, leaving plenty of empty pores. Every pore supplies the contact and separation space for the conductive sponge and silicone to form a micro TENG, and the whole bulk makes up the SS‐TENG. When deformed, the SS‐TENG outputs electric signals. The open‐circuit voltage linearly increases from 3.9 to 17 V as the force increases from 2.9 to 9.9 N. In addition, when touched, the electric signals generated by SS‐TENGs can differentiate the corresponding objects. Finally, the SS‐TENGs are integrated with a soft–rigid hybrid actuator to form a bionic skeleton–muscle–skin hybrid gripper. The gripper can catch different objects and feedback electric signals, which show great potential for use in bionic robots.

Light-weight and flexible silicone rubber/MWCNTs/Fe3O4 nanocomposite foams for efficient electromagnetic interference shielding and microwave absorption

Light-weight and flexible methyl vinyl silicon rubber (VMQ)/multi-walled carbon nanotubes (MWCNTs)/ferriferrous oxide (Fe3O4) nanocomposite foams with efficient electromagnetic interference (EMI) shielding performance were firstly prepared by supercritical carbon dioxide (Sc-CO2) foaming process. The microwave-absorbing ability of the VMQ/MWCNTs/Fe3O4 foams has been greatly improved owing to the existence of cellular structure and magnetic Fe3O4 nanoparticles, thus significantly reducing the secondary EM wave pollution caused by reflection. These nanocomposite foams with density of ∼0.48 g−1 cm3 exhibited an EMI shielding effectiveness (SE) of 27.5 dB, and its average absorption ratio can reach up to 64% in the frequency range of 8.2–12.4 GHz. Such highly conductive (∼14.6 S/m) and specific EMI SE (∼72 dB·g−1cm3) can be obtained with filler loading of 1.78 vol%. Moreover, the flexible silicone rubber foams present excellent EMI shielding stability after repeated bending. The shielding mechanism of the composite foams was also discussed, and the results show that the main shielding mechanism of the foams was absorption with the addition of Fe3O4 nanoparticles. Furthermore, the foams exhibits excellent superparamagnetic and thermal properties, the maximum onset decomposition temperature can reach 467 °C. In view of the uncomplicated, low-cost and environmental protection preparation procedure, the light-weight, flexible, magnetic and highly conductive silicone rubber nanocomposite foams are expected to be used in lightweight and flexible electromagnetic shielding materials.

Enhancement of thermal conductivity and tensile strength of liquid silicone rubber by three-dimensional alumina network

ABSTRACTRapidly increasing demands for higher integration density and stability of electronic devices embrace higher requirements for thermally conductive silicone rubber, which is promisingly used in ultra-thin components. In this work, alumina whiskers (AWs) and alumina flakes (AFs) are used to modify liquid silicone rubber (LSR) by fabricating binary (AFs/LSR) or ternary (AWs/AFs/LSR) composites. The thermal conductivity and mechanical strength of the binary and ternary composites were investigated. Thermal conductivity of the binary AFs/LSR composite (25AFs/LSR) was 0.1990 W m−1 K−1, while the thermal conductivity of the ternary AFs/AWs/LSR composite (20AFs/5AWs/LSR) was 0.2655 W m−1 K−1. Furthermore, the tensile strength of the ternary AWs/AFs/LSR composites increased by 180.9% as compared with the binary system, increased to 7.81 MPa from 2.78 MPa due to the introduction of 1 wt% AWs. As a reason, a significant synergistic effect of AWs and AFs in the enhancement of both thermal and mechanical properties of the LSR was proved. Furthermore, the dielectric property measurements demonstrated that the ternary composites exhibited a lower dielectric constant and dielectric loss, indicating that the AWs/AFs/LSR composites were qualified to be applied in the field of electronic devices.

Thermoelectric cooling module with damping conductor on cold spays

Subject of Research.The paper presents results of study on a thermoelectric cooling module with damping heat and electrically conductive material for cold junctions. Telluride obtained by synthesis in a melting furnace is used as a thermoelectric element material. The size of a thermoelectric element is 2.0×2.0×1.5 mm. The modules were mounted on ceramic plates made of 96% Al2O3 with dimensions of 30×30×0.89 mm. Copper coated with a layer of nickel is used as the switching bus material. Modules were attached to the board by soldering. Tin and bismuth solder paste (melting point is equal to 139°С) was used as an assembly solder alloy. An electrically conductive EX-A302L silicone with specific resistance of 0.02 Ohm/cm, densityof 3.7 g/cm3 and thermal conductivity of 2.1 W/(m·K) was used for the damping compound. Methods. For efficiency increase of a thermal contact of a thermoelectric module, associated with large temperature gradient between hot and cold energy, especially for electrical n-elements, it is recommended to use an elastic electrically conductive adhesive. Elastic conductive adhesive based on silicone is proposed. Conductive silicone EX-A302L adhesive sealant consisted of single-component cold-vulcanized silicone with splashes of conductive microgranules. Working temperature from -50 to +120°C allows for the application of this adhesive both for conventional refrigeration modules, and for the cold junction of medium-temperature modules. A phased module assembly technology is developed. Main Results. Comparative tests are carried out for the experimental thermoelectric module and the standard industrial module of TB-31-2.0-1.5 thermoelectric module at measuring installations manufactured by Kryotherm company. The integrity of the modules was tested on Testo thermal imager. It is established that the difference between the working characteristics of experimental and industrial modules does not exceed 10% and lies within the permissible limits. It is found out that destruction dynamics of the modules after temperature cycling is virtually the same. Practical Relevance. The results obtained demonstrate the application possibility of electrically conductive adhesive as a contact layer for a thermoelement. The proposed technology will allow for replacing the materials of thermoelectric module branches with more efficient ones, but having different coefficients of temperature expansion, thereby increasing the efficiency of the thermoelectric device.

Rational Design of Soft, Thermally Conductive Composite Liquid‐Cooled Tubes for Enhanced Personal, Robotics, and Wearable Electronics Cooling

AbstractThermoregulatory garments composed of liquid‐cooled plastic tubes have users ranging from astronauts to multiple sclerosis patients and are emerging as a flexible cooling solution for wearable electronics and high‐power robotics. Despite the plethora of applications, the current cooling systems are cumbersome to use due to their excessive size. In this work this issue is resolved by developing soft, thermally conductive silicone–aluminum composite tubes. To achieve optimal device performance, the material must be designed to balance the decrease in bulk thermal resistance and the increase in interfacial tube‐substrate resistance due to composite stiffening. Thus, to enable the rational design of such tubes, a closed form thermomechanical model that predicts cooling performance as a function of tube geometry and filler fraction is developed and experimentally validated. Predictions via this model and experiments are used to reveal how the tube's geometrical and material design can be adjusted to minimize the required length of tubing and maximize the heat extracted from a metallic surface and skin. Lastly, through a holistic analysis, this work demonstrates that besides significantly increasing overall cooling capability, the use of low‐resistance tubing can provide a multifold reduction in the cooling system size and enable novel operating modes.