Conductive Silicone Rubber Research Articles

Highly sensitive and extremely durable wearable e-textiles of graphene/carbon nanotube hybrid for cardiorespiratory monitoring

Electroconductive textile yarns are of particular interest for their use as flexible and wearable sensors without compromising the properties and comfort of usual textiles. However, the detection of fine actions of the human body is quite challenging since it requires both the relatively higher sensitivity and stability of the sensor. Herein, highly sensitive, ultra-stable, and extremely durable piezoresistive wearable sensors were prepared by loading N-doped rGO and polydopamine-coated carbon nanotubes into silicon rubber tube. The wearable sensors thus produced show an excellent ability to sense subtle movement or stimuli with good sensitivity and repeatability. Furthermore, by bending the straight conductive silicon rubber tube (CSRT) into three different patterns, its sensitivity was then dramatically increased. Finally, the CSRT was found capable of sensing cardiorespiratory signals, indicating that the sensor would be an important step toward realizing bio-signal sensing for next-generation personalized health care applications.

A flexible strain sensor of porous conductive silicone rubber composites prepared from high internal phase emulsion

AbstractIn this work, a stretchable and compressible strain sensor was fabricated from conductive porous carbon nanotubes/polydimethylsiloxane (CNT/PDMS‐P) high internal phase emulsion (HIPE), which was obtained by simple mechanical mixing of CNTs, PDMS, and water. Micropores were uniformly distributed in the PDMS matrix, with diameters mainly in the range of 3–9 μm and averaging 5.3 μm. Due to the confined dispersion of CNTs in the porous PDMS framework, the CNT/PDMS‐P achieved a lower percolation threshold of 0.29 wt%. Compared to the non‐porous composite (CNT/PDMS), its percolation threshold was reduced by 25%. Besides, the CNT/PDMS‐P showed excellent sensitivity and cyclic stability. The sensitivity of CNT/PDMS‐P was 50% higher than that of CNT/PDMS in the tensile strain range of 0%–10%. The stability tests for 1000 loading/unloading cycles showed that the maximum offset rate of CNT/PDMS‐P was 58% lower than that of CNT/PDMS. The sensitivity of CNT/PDMS‐P compression sensor was S = 0.00259 kPa−1, which could detect the stress up to 600 kPa and maintain the standard waveform even after 1000 compression cycles. Finally, the potential application scenarios of the sensor, such as perception of human body limb movement, facial movement, gesture transformation, and human‐computer interaction scenarios, were explored.

Preparation and analysis of conductive and superhydrophobic silicone rubber

Conductive and superhydrophobic rubber products are of great significance to the safety and long-term use of outdoor equipment. However, the preparation of low-cost conductive, superhydrophobic rubber products with good extensibility, durability, stability, and environmental adaptability remains a challenge. In this paper, conductive and superhydrophobic silicone rubber composites with uniform concave-convex structure were prepared on the surface of silicone rubber by filling carbon nanotubes (CNTs) in the silicone rubber matrix and using 2000 mesh screen template replication method and spraying technology. Using the self-made conductive circuit, the good electrical conductivity of the silicone rubber composite was verified at 200% elongation and after 500 times of torsion. In addition, the silicone rubber composite had good droplet bounce, anti-icing and self-cleaning properties.

Improvement the flame retardancy, thermal conductivity and mechanical properties of silicone rubber via silane coupling agent modified CNTs‐β‐FeOOH

AbstractSilicone rubber with its odorless and non‐toxic, high and low temperature resistance and other advantages is widely used in many fields, but its shortcomings of flammability and low thermal conductivity limit its application and development. Therefore, the research on flame retardant and thermal conductive silicone rubber is practically valueable. In this study, iron oxyhydroxide (β‐FeOOH) was synthesized on carbon nanotubes (CNTs) by in situ growth and added to silicone rubber after surface modification with γ‐aminopropyltriethoxysilane (KH550). The results showed that the limiting oxygen index (LOI) of methyl vinyl silicone rubber (VMQ) with the addition of one part of KH550@CNTs‐β‐FeOOH increased to 29.8%, its peak heat release rate (pHRR) and total heat release (THR) decreased by 40.1% and 30.2%, respectively, and its thermal conductivity increased to 0.3680 W/m·k. The flame‐retardant performance and thermal conductivity of the VMQ composite were significantly improved. The improvement of its flame retardancy was mainly due to the catalytic carbonization of CNTs and β‐FeOOH, which promoted the formation of dense carbon layers. The carbon layer insulates the exchange of oxygen and combustible gases. In addition, the thermal conductive network constructed by KH550@CNTs‐β‐FeOOH in the composite effectively improved the thermal conductivity of VMQ.

A study on development of silicone rubber with conductive carbon, polyaniline, MWCNT composite for EMI shielding

The multiwall carbon nanotubes (MWCNTs) were grown by CVD method and using Ni impregnated zeolite as a substrate. The prepared MWCNT diameter varied from 10 to 60 nm and length in few microns. The silicone rubber (SR) was mixed well with conductive carbon, Polyaniline (PANI) and MWCNTs in two roll mill. The prepared silicone rubber materials were fabricated in the form of sheets with dimensions of 200 mm × 200 mm × 2 mm by using compression molding technique. The prepared sheets were subjected for EMI shielding efficiency measurements at low frequency (< 1.5 GHz) and high frequency range from 1 GHz to 18 GHz. At high frequency the shielding effectiveness of the Conducting Silicone Rubber and Conductive silicone rubber with MWCNT was found to be 24 dB and 48 dB. The volume resistivity measurements were also carried for all the prepared silicone rubber sheets, the results reveals that SR + MWCNT, CSR + MWCNT composites shows volume resistivity 4032 and 20.7 Ω.cm respectively. This confirms the conductivity of CSR + MWCNT is enough to exhibit good Shielding Effectiveness.

Transposable, highly thermal conductive silicone rubber/boron nitride composites with enhanced orientation of BN by multilayer stacked hot pressing according to rheological control

Composites with high out-plane thermal conductivity (TC) and great insulation are essential for thermal management in electronic devices. Herein, a transposable, highly thermal conductive silicone rubber (SR)/boron nitride (BN) composite was fabricated by multi-layer stacked hot pressing, in which the orientation of BN was enhanced based on rheological characterizations. Then the pre-pressed films were stacked under 180 °C to get a full-cured and thick composite. The enhanced orientation of BN and the excellent heat transfer capability of the composites were well verified by SEM, XRD, 2D WAXS, finite element analysis (FEA) and infrared thermography, which resulted in a wonderful thermal pathway and low interfacial thermal resistance. The TC of the composite reached 22.08 W/(m·K), and the high in-plane TC could be converted to high out-plane TC after simple vertically cut due to the flexibility and large thickness of composite. Meanwhile, for the first time, the excellent heat transfer capability of the composite was quantitatively characterized by the characteristic temperature rise time constant (τg). The rheological control was an efficient and potential strategy to get highly thermal conductive composites.

An embedded printed flexible strain resistance sensor via micro-structure design on graphene-filled conductive silicon rubber

Flexible strain sensors have been improved in sensing performance with the assistance of materials design, novel manufacturing, and microstructure fabrication. In this study, graphene was efficiently dispersed in ethanol and then re-dispersed into silicon rubber (SR) matrix, functioning as a flexible strain resistance sensor (FSRS) with functional macrostructure and modified microstructure to further improve the sensitivity. A stable dispersion of graphene was obtained in an ultrasound-aided ball milling process, where absolute ethanol was selected as the solvent and sodium dodecyl sulfonate as the surfactant. Graphene-filled conductive SR was embedded in the polydimethylsiloxane matrix as a conductive sensing layer, and the high sensing performance (GF = 25 ± 2) was achieved using a spiral printed. Micropores with an optimized interspacing of 10 mm were further introduced into the spiral CSM, and the results presented a significant improved sensitivity (GF = 51 ± 4) of the fabricated FSRS under a working strain (20%–30%) and cyclic test (>104 cycles). The FRSR was sensitive enough to monitor various movements of single and multi-joints of human body and identify the rhythm of music sound, which exhibited its potential application as a wearable flexible sensor.

Fabrication of Pressure Conductive Silicone Rubber Socket Device by Shape-Controlled Nickel Powders Produced by High-Energy Ball Milling.

The pressure conductive silicone rubber socket (PCR) is one of the promising test socket devices in high-speed testing environments. In this study, we report highly dense PCR device channels comprised of high aspect-ratio flake-shaped Ni powders. The shape-controlled Ni powders are prepared by the high-energy milling process. The scanning electron microscopy (SEM) and particle size analyzer (PSA) results of the synthesized powder samples showed well-defined flake type Ni powder morphology, and the powder sizes are distributed in the range of ~24–49 μm. The cross-sectional SEM study of the fabricated PCR revealed that the channels consisting of flake Ni powder are uniformly, densely distributed, and connected as face-to-face contact. The resistance of the PCR channels comprised of flake-shaped Ni powders showed ~23% lower resistance values than the spherical-shaped Ni powders-based channels, which could be due to the face-to-face contact of the powders in the channels. The magnetic properties study for the flake-type Ni powder showed a high remanence (~2.2 emu/g) and coercivity (~5.24 mT), owing to the shape anisotropy factor. Finally, the fabricated highly dense and conductive channels of the silicone rubber socket device by shape-controlled Ni powder could be a potential test socket device.

Bioinspired untethered soft robot with pumpless phase change soft actuators by bidirectional thermoelectrics

Soft pneumatic actuators play an important role in the technological advancement of soft robotics, but they face intrinsic limitations due to the presence of complex and bulky air compressors and tubes, that make it difficult to design completely soft robotic systems. In this study, we develop a first demonstration of the soft thermo-pneumatic actuating module that operates based on the thermally controlled gas-liquid phase transition with the soft thermoelectric device. The bi-directional thermal managing ability (heating and cooling) of the thermoelectric device is capable of controlling the inflation and deflation of the pneumatic chamber with the aid of the thermally conductive silicone rubber that enhances the heat transfer. For the first time to the authors’ best knowledge, we demonstrate the active cooling of the thermoelectric device that accelerates the deflation rate by the active cooling functionality that significantly reduces the whole operating time. Additionally, to validate its practical usage in soft robotics, the soft actuating module shows different motions, such as bending and rectilinear stretching, based on the assembly design that is employed to develop the soft gripper and the entirely untethered soft earthworm robot without a complicated and bulky compressor system.

Preparation method and properties of conductive silicone rubber

Carbon nano-conductive silicone rubber is a type of composite conductive polymer material. It has good electrical conductivity, thermal conductivity and high magnetic flux. It has a good application prospect to replace most traditional conductive materials. However, at present, its mechanical strength is poor, the strain is small, and its tensile strength is weak, which limits the application of carbon nano-conductive silicone rubber. For this reason, this paper uses carbon fiber as filler to prepare carbon-based nano-conductive silicone rubber by solution blending method, the preparation method analysis and performance research were carried out and the research results can provide a reference for the preparation of high-sensitivity, large-range flexible sensors.

Bioinspired structural silicone rubber composites with high thermal conductivity based on vertically aligned hybrid filler film

To improve the capability of thermal management of thermal conductive silicone rubber (SR) composites, a novel strategy to prepare SR composites with vertically aligned graphene/silicon carbide crystal whisker (SiC) hybrid film was proposed inspired by the typical architecture of the annual rings of trees. Frist of all, a SR/graphene-SiC bilayer film was prepared through drying SR solution and aqueous filler dispersion in turn. Then the bilayer film was rolled and subsequently cured to form SR composite. Poly(ethylene glycol) grafted poly(methylhydrosiloxane-co-dimethylsiloxane) was newly synthesized and used to enhance the interfacial interaction between SR and filler film. The bioinspired structural graphene/SiC/SR composite exhibited a very high thermal conductivity of 6.72 W/(m·K). The composite was determined to be an efficient thermal interface material to reduce the working temperature of light emitting diode, clearly showing the excellent thermal management capability of the obtained SR composites.

Preparation of thermally conductive flexible liquid silicone rubber composites by magnetic field‐induced self‐assembly of Fe3O4@C

AbstractEfficient heat dissipation is very crucial for the performance, service life, and reliability of modern electronic devices. It is very challenging to improve heat dissipation efficiency through innovative material structure design. Therefore, in the work, dopamine (PDA) is used to coat the surface of graphite (C) with its excellent surface coating modification ability, so that the C surface has oxygen‐containing groups to change its electronegativity, and Fe2+ and Fe3+ in the solution are attracted to the surface of C, and then in situ redox reaction occurs on the surface of C to generate magnetic graphite (Fe3O4@C). Fe3O4@C is uniformly dispersed into liquid silicone rubber (LSR) by solution blending, and Fe3O4@C performs integral directional filling in the LSR under the action of magnetic field (B = 140 mT), thus preparing Fe3O4@C/LSR composites. Through magnetic field orientation, the thermal conductivity of Fe3O4@C/LSR composites reach 1.25 W/m·K when the content of Fe3O4@C is 100 phr, which is 410% higher than that of pure LSR (0.2447 W/m·K). Infrared thermal imaging test shows that Fe3O4@C/LSR composites has good heat dissipation performance, Fe3O4@C/LSR composites have shown great potential in terms of flexibility and high thermal conductivity.

Conductive Silicone Rubber Coated by Reduced Graphene Oxide Pseudo-Photonic Crystal Film

Graphene aqueous dispersion is difficult to be prepared as conductive film by simple solvent evaporation deposition, not only because of the agglomeration in its aqueous solution, but also caused by the “coffee ring” phenomenon during solvent evaporation process. Herein, as a derivative of graphene, graphene oxide (GO) would be used as film forming material due to its good dispersion in aqueous solution and its liquid crystallinity. If GO can self-assemble as a compact pseudo-photonic crystal film by solvent evaporation deposition, it will be converted into a conducive graphene film after thermal reduction. In order to prepare a flexible and conductive elastomer material covered with graphene film, compact GO pseudo-photonic crystal film can be transferred onto the surface of a 10% stretched silicone rubber plate. After releasing the stretched force, GO pseudo-photonic crystal film can form a lot of folds, which provide allowance of shrinkage for this GO pseudo-photonic crystal film to avoid cracking during the high temperature reduction. Benefiting from transferring the GO film onto a stretched silicone rubber, a flexible, colored and conductive graphene/SR can be obtained.

Thermally conductive silicone rubber composites with vertically oriented carbon fibers: A new perspective on the heat conduction mechanism

The continual increase in power density and consumption of modern electronic devices calls for high-performance thermal interface materials (TIMs). In this work, silicone rubber composites with vertically oriented magnetic carbon fibers (o-MCF/SR) were successfully prepared via the cast molding and filler orientation upon uniform magnetic field in a vacuum environment. The through-plane thermal conductivity of o-MCF/SR composites showed an unusual power-law growth with increasing MCF loading. At a 9 vol% MCF loading, the o-MCF/SR composite exhibited a maximum through-plane thermal conductivity of 4.72 W/(m·K) with 2045.5 % TCE, about 3.55 W/(m·K) higher than that of SR composites with randomly dispersed MCF (λ = 1.17 W/(m·K), TCE = 431.8 %). To reveal the underlying through-plane heat conduction mechanism of o-MCF/SR composites, finite element simulation combined with classical effective medium theory model and machine learning method was used. A homogeneous unit fiber-filled composite (HUFC) was modelled, and a new parameter influencing the through-plane thermal conductivity, i.e., shape factor (b/a), was introduced here for the first time. Theoretically, this shape factor was dependent on the filler content and indirectly reflected the assembly of matrix and 1D filler in HUFC fit to a parallel-series hybrid model. Furthermore, a decrease of b/a was practically found with increasing MCF loading, and the dominant series-type assembly gradually transformed to parallel-type assembly of the o-MCF/SR composites. This work not only developed a method to fabricate desirable TIMs that hold great promise for advanced thermal management, but also introduced a new parameter to reveal the underlying mechanism for heat conduction capacity of polymer composites.

A scalable highly thermal conductive silicone rubber composite with orientated graphite by pre-vulcanizing and multilayer stacking method

Silicone rubber (SR)-based thermal conductive composites is a good candidate for heat-resisted thermal interface materials (TIM). It’s a challenge to increase thermal conductivity (TC) and get thick sample as the difficulty of vulcanizing SR in core layer. Herein, a highly thermal conductive and 10 mm SR/graphite (Gr) with oriented Gr (O-SR/Gr) was prepared by pre-vulcanization and multilayer-stacked hot pressing. A well cured thick composite with dense structure and excellent orientation of Gr was obtained. The in-plane TC of O-SR/Gr reached 13.93 W/(m·K) with 66.67 wt% Gr, while the samples without pre-vulcanization presented voids among sample. The dense composite and excellent orientation of Gr reduced interfacial thermal resistance (ITR) as fitted by Foygel model. They also presented rapid warming rate during actual warming process and remarkable heat resistance till 200 °C. The finding might provide inspiration for efficient design of high thermal conductivity composites on large scale.

Preparation and Properties of Conductive Foamed Silicone Rubber

The conductive foamed silicone rubber was prepared by Polymethylvinylsiloxane (PMVS) mixed with acetylene black, fumed silica, and AC foaming agent (azodicarbonamide). Influences of fillers on the microstructure, electrical conductivity and mechanical property were discussed. The results showed that the pore structure was affected by acetylene black. Acetylene black delayed the vulcanization speed of silicone rubbers, leading to bigger pores. Higher content of acetylene black decreased the mechanical properties and the electrical resistivity of rubbers. Fumed silica increased the mechanical properties and limited the aggregation of gas. Higher content of fumed silica led to small and obturator pores generated. Higher content of fumed silica increase the mechanical properties and the electrical resistivity of rubbers. AC foaming agents generated more gas, leading to bigger and irregular pores which affected the mechanical properties negatively, meanwhile AC foaming agents increased the conductivity of rubbers.

Vertically aligned carbon nanotubes/graphene/cellulose nanofiber networks for enhancing electrical conductivity and piezoresistivity of silicone rubber composites

The directional arrangement of a conductive filler is an effective method to improve electrical conductivity of composites. In this paper, aqueous dispersions of carbon nanotubes and graphene were prepared by high-pressure microfluidization technology. After mixing cellulose nanofiber into these dispersions, vertically aligned carbon nanotubes/graphene network foams were prepared using an ice-template assembly method, to which liquid silicone rubber was filled to obtain carbon nanotubes/graphene/silicone rubber composites. Polymethylhydrosiloxane was grafted with 3-(trimethoxysilyl) propyl methacrylate and the resultant copolymer was used as a coupling agent to improve interface interaction between silicone rubber and filler. With increasing carbon nanotubes content, the electrical conductivities of the foams and composites increased. The composite prepared with 4 g CNT and 1 g graphene aqueous dispersions had the conductive percolation threshold of 0.08% and exhibited high conductivity at low filler content. The addition of the copolymer increased the compressive strength of the composites. The composite prepared with 1 g CNT and 4 g graphene aqueous dispersions was encapsulated in an insole as a piezoresistor to effectively detect the number of pedestrian steps, and had a stable output of electrical signal. This work provides a method for preparing conductive silicone rubber composites with low conductive percolation threshold and excellent piezoresistivity.

Structural design of flexible interdigital capacitor based upon 3D printing and spraying process

Traditional substrates of metallic interdigital electrodes (IEs) are rigid and undeformable, flexible interdigital capacitors are therefore appealing as strain sensors. In this study, interdigital capacitors were parametrically designed by 3D printing and encapsulated by spraying process. The interdigital circuits of the structure were printed with conductive silicone rubber filled with silver-coated glass fiber and carbon fiber, and the circuits were encapsulated with polydimethylsiloxane. Herein, the interdigital-flexible structures were parametrically designed and firstly served as capacitive sensor, namely flexible interdigital capacitive sensors (FICSs). The spaces between IEs, are extremely sensitive to strain, therefore provide the capacitors with excellent electromechanical behaviors. The optimized FICS benefited for a wide working range of strain (0%∼45%), high sensitivity (gauge factor = 2.7) to a tiny strain of 0.3%, stable working duration at different stretching speeds (18 mm min−1, 36 mm min−1 and 72 mm min−1), prolonged service life (>800 cycles), as well as excellent capability to detect human movement (bulging, grasping and bending). Response mechanism of the FICS was modeled based upon its microstructure evolution, including the distances between IEs and the fillers migration. The printed FICSs with optimized structure provide a comprehensive thought in the design of electronics, further would inspire the branch of 3D printed electronics.

Structural design of flexible interdigital capacitor based upon 3D printing and spraying process

Abstract Traditional metallic interdigital electrodes are rigid and undeformable, flexible interdigital capacitors are therefore appealing as strain sensors. In this study, interdigital capacitors were parametrically designed by 3D printing and encapsulated by spraying process. The interdigital circuits of the structure were printed with conductive silicone rubber (CSR) filled with silver-coated glass fiber and carbon fiber (CF), and the circuits were encapsulated with polydimethylsiloxane (PDMS). Herein, the interdigital-flexible structures were parametrically designed and firstly served as capacitive sensor, namely flexible interdigital capacitive sensors (FICS). The spaces between interdigital electrodes (IEs), are extremely sensitive to strain, therefore provide the capacitors with excellent electromechanical behaviors. The optimized FICS benefited for a wide working range of strain (0~45%), high sensitivity (gauge factor, GF=2.7) to a tiny strain of 0.3 %, stable working duration at different stretching speeds (18 mm/min, 36 mm/min and 72 mm/min), prolonged service life (>800 cycles), as well as excellent capability to detect human movement (bulging, grasping and bending). Response mechanism of the FICS was modelled based upon its microstructure evolution, including the distances between IEs and the fillers migration. The printed FICS with optimized structure provide a comprehensive thought in the design of electronics, further would inspire the branch of 3d printed electronics.

Hybrid sol-gel-derived method for the synthesis of silicon rubber composites with hBN for characteristic applications in elastomeric thermal pads

The thermal conductivity of silicone rubber is very low and does not meet the required thermal conductive applications, so inorganic fillers are added to increase heat transfer which ultimately improves the thermally conductive path. In this study, silicone rubber/hexagonal boron nitride composite was synthesized by the hydrolysis-polycondensation method to enhance the thermal conductivity of the material while retaining acceptable flexibility properties. The introduction of MQ resin reduced crosslink density, thermal stability, tensile strength, and hardness of the silicone resin composite and improved elongation. The addition of vinyl-based MQT and aluminum-based MQT resins improved the properties of the silicone rubber, while the addition of vinyl-based MQT resin reduced the crosslink density, tensile strength and hardness, and improved its elongation and thermal stability properties. While aluminum-based MQT resin did not have a significant effect on crosslink density, tensile strength, or hardness, it also improved elongation and reduced thermal stability. The high filler concentration of hexagonal boron nitride in the composite enhanced thermal conductivity up to 3.253 Wm−1 K−1, while it reduced tensile strength to 1.248 MPa and elongation to 22% but increased hardness up to 75 shore A. The addition of silicone resin improved the thermal conductivity of all MQ, vinyl-based MQT3 and aluminum-based MQT3 resin composites up to 3.661, 3.962 and 4.817 Wm−1 K−1, respectively. For the same three resins, tensile strength was increased up to 1.274, 1.290, and 1.312 MPa, elongation at break was raised to 125%, 188%, and 150%, and hardness was reduced to 69, 71, and 72 shore A, respectively. The addition of silicone resin also showed an effect on density, volatile content, flame resistance, and volume resistivity.