Silver Flakes Research Articles

Flexible and Printable Composite Ink for Thermal Management of Soft Electronics

AbstractSince heat generation in electronic devices causes thermal failure, heat dissipation is of critical importance. Furthermore, deformable devices are subjected to mechanical stress, therefore, mechanically stable thermal management material should be considered. Herein, a strategy for printable, thermally conductive, and mechanically stable composite ink for thermal management is introduced. Based on the galvanic replacement between eutectic gallium indium (EGaIn) nanoparticles and silver (Ag) flakes, decoration of the EGaIn nanoparticles on Ag flakes is resulted from the difference in standard reduction potential between Ag, Ga, and In. The resultant alloy formation(Ag–Ga or Ag–In) serves as the thermal transport junction between Ag flakes, leading to high thermal and electrical conductivity (≈140 W mK−1 and ≈106 S m−1, respectively). In addition, owing to the polymer binder, the printed ink is mechanically stable on a substrate exhibiting stable thermal conductivity and sheet resistance under the cyclic bending test. Notably, the heat dissipation of the light‐emitting diode (LED) showed better performance when applied with the developed composite ink compared to commercial Ag paste and thermal paste. The junction temperature of the LED is reduced effectively, resulting in a longer lifetime of the LED. The thermal management solution can be utilized in next‐generation soft electronics.

Validation of methodology to determine the contact resistivity of ECA-based bonds grounded on end–contact resistance measurements using redundant and modified TLM test structures

Accurate determination of the contact resistivity between electrically conductive adhesives (ECAs) and solar metallization paste is a key component of optimizing the electrical performance of bonds based on ECAs, such as shingled joints. This work proposed and validates a methodology that accounts for non–negligible inhomogeneities of test structures based on ECAs and silver screen-printed metallization, such as changes in width of the ECA, to improve the accuracy in the calculation of the contact resistivity. The proposed validation uses statistical theory based on the central limit theorem and the law of large numbers showing that the contact resistivity of bonds based on two different commercially available ECAs can be clearly differentiated with a minimum confidence level of 95% and a maximum error margin of 5%. Moreover, the necessary sample size to achieve such requirements is calculated and shown to be equal to 24 test structures. The validation was done using an acryl-based adhesive loaded with silver flakes (∼10−4 Ω cm), which was compared to an epoxide-based ECA filled with silver–coated copper particles (∼10−3 Ω cm). The validated methodology indicates that the contact resistivity when using the acrylic conductive adhesive is 0.1791 ± 0.0766 mΩ cm2 while when using the epoxide-based adhesive is 0.5025 ± 0.1218 mΩ cm2. Additionally, it is shown that the standard deviation of the contact resistivity is highly dependent on the composition of the conductive adhesive. Hence, the sample size varies depending on the ECA. It is expected that the presented methodology will be a powerful instrument for developers and researchers in the improvement and optimization of ECAs and ECA bonding processes.

Development and Performance Evaluation of Stretchable Silver Pastes for Screen Printing on Thermoplastic Polyurethane Films

Efficient, stretchable wiring electrodes are achieved when the resistance change during expansion and contraction is minimal. Herein, we prepared silver pastes specifically designed for screen printing on thermoplastic polyurethane films; they exhibit minimal resistance changes. The pastes were prepared using silver particles with sizes of 2 and 7 μm as well as a mixture of 2 and 7 μm silver particles (50:50 wt%). These pastes were analyzed using methods such as rheological measurements, thermogravimetric analysis, printability tests, tensile and torsion tests, and light-emitting diode (LED) tests. The most promising results were obtained when exclusively using 2 μm silver flake particles. The pastes demonstrated a viscosity of 24,880 cps, a thixotropic index value of 2.82, excellent printability, and consistent resistance measurements even after 100% stretch, thus indicating exceptional tensile properties. Moreover, the pastes exhibited substantial stability, with no change in brightness after the attachment of seven LEDs at 20% tension.

Silver-based conductive coatings as lightning strike protection for composite aircraft

Lightning strike protection of composite structures is an increasingly studied subject in the aerospace industry due to the need for finding novel protection solutions for low conductive composites. This work presents two silver-based lightning protective conductive coatings for potential replacement of expanded copper foil (ECF) as lightning strike protection (LSP). The first solution consists of an electroless method to produce silver-coated carbon fibres (SCCF). The SCCF are integrated as a sacrificial ply on a carbon fibre reinforced polymer (CFRP) panel with four different areal densities, using an epoxy-based primer or a PEDOT:PSS conductive ink as binder. The second solution is a silver-based commercial product consisting of a two-part epoxy with silver flakes, which acts as a sprayable thin coating beneath the paint layer.The protected CFRP panels were covered with aerospace-grade paint for realistic finishing and were subjected to high impulse current strikes. All four conductive coatings containing SCCF exhibited around 45%–50% retention of flexural strength, although largely inferior to painted expanded copper foil's resistance to emulated lightning strikes. The silver-based commercial product showed 83% retention of flexural strength and 97% retention of effective bending modulus, with a comparable performance to the painted expanded copper foil. The resulting retention of flexural strength, the absence of severe damage to the structure, its versatility for complex geometries and the easy manufacturability of the silver-based commercial coating makes this approach a good candidate for the replacement of ECF as a solution for LSP.

Recyclable Thin-Film Soft Electronics for Smart Packaging and E-Skins.

Despite advances in soft, sticker-like electronics, few efforts have dealt with the challenge of electronic waste. Here, this is addressed by introducing an eco-friendly conductive ink for thin-film circuitry composed of silver flakes and a water-based polyurethane dispersion. This ink uniquely combines high electrical conductivity (1.6×105 S m-1 ), high resolution digital printability, robust adhesion for microchip integration, mechanical resilience, and recyclability. Recycling is achieved with an ecologically-friendly processing method to decompose the circuits into constituent elements and recover the conductive ink with a decrease of only 2.4% in conductivity. Moreover, adding liquid metal enables stretchability of up to 200% strain, although this introduces the need for more complex recycling steps. Finally, on-skin electrophysiological monitoring biostickers along with a recyclable smart package with integrated sensors for monitoring safe storage of perishable foods are demonstrated.

Electrical Characterization of Carbon Nanotube Reinforced Silver and Copper Composites for Switching Contacts

Carbon nanotube (CNT)-reinforced silver and copper metal matrix composites—at three different reinforcement phase concentrations (0.5 wt.%, 0.75 wt.%, and 1 wt.%)—were produced via powder metallurgy and sintered via hot uniaxial pressing. Optical and electron microscopy techniques were used to characterize the powder mixtures and sintered composites. The latter were also electrically characterized via load-dependent electrical contact resistance (ECR) and surface fatigue tests. Particle size and morphology play a crucial role in CNT deposition onto the metallic powder. CNT were deposited exceptionally well onto the dendritic copper powder regardless of its larger size (compared with the silver flakes) due to the higher surface area caused by the grooves and edges of the dendritic structures. The addition of CNT to the metallic matrices improved their electrical performance, in general outperforming the reference material. Higher CNT concentrations produced consistently low ECR values. In addition, high CNT concentrations (i.e., 1 wt.%) show exceptional contact repeatability due to the elastic restitutive properties of the CNT. The reproducibility of the contact surface was further evaluated by the fatigue tests, where the composites also showed lower ECR than the reference material, rapidly reaching steady-state ECR within the 20 fatigue cycles analyzed.

A Micromechanical Analysis to the Viscoplastic Behavior of Sintered Silver Joints under Shear Loading.

Ag paste has been recognized as a promising substitute for Sn/Pb solder in SiC or GaN power electronic devices, owing to its ability to withstand high temperatures and facilitate low-temperature packing. The reliability of these high-power circuits is greatly influenced by the mechanical properties of sintered Ag paste. However, there exist substantial voids inside the sintered silver layer after sintering, and the conventional macroscopic constitutive models have certain limitation to describe the shear stress-strain relationship of sintered silver materials. To analyze the void evolution and microstructure of sintered silver, Ag composite pastes composed of micron flake silver and nano-silver particles were prepared. The mechanical behaviors were studied at different temperatures (0-125 °C) and strain rates (1 × 10-4-1 × 10-2) for Ag composite pastes. The crystal plastic finite element method (CPFEM) was developed to describe the microstructure evolution and shear behaviors of sintered silver at varied strain rates and ambient temperatures. The model parameters were obtained by fitting experimental shear test data to a representative volume element (RVE) model built on representative volume elements, also known as Voronoi tessellations. The numerical predictions were compared with the experimental data, which showed that the introduced crystal plasticity constitutive model can describe the shear constitutive behavior of a sintered silver specimen with reasonable accuracy.

Soft Electromagnetic Vibrotactile Actuators with Integrated Vibration Amplitude Sensing.

Soft vibrotactile devices have the potential to expand the functionality of emerging electronic skin technologies. However, those devices often lack the necessary overall performance, sensing-actuation feedback and control, and mechanical compliance for seamless integration on the skin. Here, we present soft haptic electromagnetic actuators that consist of intrinsically stretchable conductors, pressure-sensitive conductive foams, and soft magnetic composites. To minimize joule heating, high-performance stretchable composite conductors are developed based on in situ-grown silver nanoparticles formed within the silver flake framework. The conductors are laser-patterned to form soft and densely packed coils to further minimize heating. Soft pressure-sensitive conducting polymer-cellulose foams are developed and integrated to tune the resonance frequency and to provide internal resonator amplitude sensing in the resonators. The above components together with a soft magnet are assembled into soft vibrotactile devices providing high-performance actuation combined with amplitude sensing. We believe that soft haptic devices will be an essential component in future developments of multifunctional electronic skin for future human-computer and human-robotic interfaces.

Multimaterial Printing of Liquid Crystal Elastomers with Integrated Stretchable Electronics.

Liquid crystal elastomers (LCEs) have grown in popularity in recent years as a stimuli-responsive material for soft actuators and shape reconfigurable structures. To make these material systems electrically responsive, they must be integrated with soft conductive materials that match the compliance and deformability of the LCE. This study introduces a design and manufacturing methodology for combining direct ink write (DIW) 3D printing of soft, stretchable conductive inks with DIW-based "4D printing" of LCE to create fully integrated, electrically responsive, shape programmable matter. The conductive ink is composed of a soft thermoplastic elastomer, a liquid metal alloy (eutectic gallium indium, EGaIn), and silver flakes, exhibiting both high stretchability and conductivity (order of 105 S m-1). Empirical tuning of the LCE printing parameters gives rise to a smooth surface (<10 μm) for patterning the conductive ink with controlled trace dimensions. This multimaterial printing method is used to create shape reconfigurable LCE devices with on-demand circuit patterning that could otherwise not be easily fabricated through traditional means, such as an LCE bending actuator able to blink a Morse code signal and an LCE crawler with an on/off photoresistor controller. In contrast to existing fabrication methodologies, the inclusion of the conductive ink allows for stable power delivery to surface mount devices and Joule heating traces in a highly dynamic LCE system. This digital fabrication approach can be leveraged to push LCE actuators closer to becoming functional devices, such as shape programmable antennas and actuators with integrated sensing.

A new one-component electrically conductive adhesive with excellent electrical conductivity and mechanical properties using tetra-functional polyurethane acrylate oligomers and silver particle fillers

Electrically conductive adhesives (ECAs) are mainly used in integrated circuit (IC) packaging, light-emitting diode (LED) packaging and other electronic industries to replace traditional solders and lead-free solders. The electrical conductivity and mechanical properties are contradictory with an increase in the content of conductive fillers. Therefore, there is a continuous need to improve electrical conductivity without serious damage to the mechanical properties of ECAs. In this work, a tetra-functional polyurethane acrylate oligomer was used as the main composition of silver filled one-component ECAs to improve performance. By introducing the tetra-functional oligomer, low volume resistivity with good adhesion characteristics was achieved. The volume resistivity and lap shear strength of the silver flake filled ECAs were studied systematically. The ECA with 30.6 vol% content of silver flakes exhibits low volume resistivity of 1.02 × 10−7 Ω m, with a lap shear strength of 8.995 MPa.

Digitally Printable Magnetic Liquid Metal Composite for Recyclable Soft‐Matter Electronics

AbstractThe increasing interest in epidermal patches for wearable monitoring makes it urgent to move toward “greener” and recyclable materials in printed electronics. However, combining scalable fabrication, stretchable function, and recyclable material architecture is challenging to achieve. Herein, this article introduces a printable biphasic liquid metal (LM)‐based composite that combines excellent electrical conductivity, high stretchability, low‐electromechanical gauge factor, printability, sinter‐free functionality, and compatibility with heat‐sensitive substrates, thanks to the sinter‐free formulation. The novel eutectic gallium indium LM‐nickel‐styrene‐isoprene (SIS) thermoplastic elastomer composite takes advantage of a “greener” solvent composed of a mixture of methyl acetate, cyclohexane, and 4‐chlorobenzotrifluoride compared to the toluene used in block copolymers (BCPs). Furthermore, it replaces the costly micron‐sized silver flake filler particles with nickel, a lower‐cost alternative that enables facile recyclability by using a magnetic force. The use of BCPs with non‐permanent physical cross‐links enables the possibility of recycling. A green route is shown for recycling the composite, using only bio‐friendly acids and daily used products such as sugar and Epsom salt. The application of these composites is shown through the digital printing of stretchable circuits and magnetic switches.

Reconfigurable Electrical Networks within a Conductive Hydrogel Composite.

Soft materials that exhibit compliance, programmability, and reconfigurability can have a transformative impact as electronic skin for applications in wearable electronics/soft robotics. There has been significant progress in soft conductive materials; however, achieving electricallycontrolled and reversible changes in conductivity and circuit connectivity remains challenging. To overcome this limitation, asoft material architecture with reconfigurable conductive networks of silver flakes embedded within a hydrogel matrix is presented. The conductive networks can be reversibly created/disconnected through various stimuli, including current, humidity, or temperature. Such stimuli affect electrical connectivity of the hydrogel by controlling its water content, which can be modulated by evaporation under ambient conditions (passive dehydration), evaporation through electrical Joule heating (active dehydration), or absorption of additional water (rehydration). The resulting change in electrical conductivity is reversible and repeatable, endowing the composite with on-demand reconfigurable conductivity. To highlight this material's unique properties, it is shown that conductive traces can be reconfigured after severe damage and revert to lower conductivity after rehydration. Additionally, a quadruped robot is demonstrated that can respond to stimuli by changing direction following exposure to excess water, thereby achieving reprogrammable locomotion behaviors.

Screen-Printed Corrosion-Resistant and Long-Term Stable Stretchable Electronics Based on AgAu Microflake Conductors.

High-throughput production methods such as screen printing can bring stretchable electronics out of the lab into the market. Most stretchable conductor inks for screen printing are based on silver nanoparticles or flakes due to their favorable performance-to-cost ratio, but silver is prone to tarnishing and corrosion, thereby limiting the stability of such conductors. Here, we report on a cost-efficient and scalable approach to resolve this issue by developing screen printable inks based on silver flakes chemically coated by a thin layer of gold. The printed stretchable AgAu conductors reach a conductivity of 8500 S cm-1, remain conductive up to 250% strain, show excellent corrosion and tarnishing stability, and are used to demonstrate wearable LED and NFC circuits. The reported approach is attractive for smart clothing, as the long-term functionality of such devices is expected in a variety of environments.

Synthesis of Citrate-Capped Sn Nanoparticles with Excellent Oxidation Resistance for High-Performance Electrically Conductive Adhesives

Tin (Sn) metallic material is one of the best-known metals and has been extensively studied due to its ease of processing, low melting point, and low cost. However, it is still challenging to synthesize high-processing performance and antioxidant Sn nanoparticles with tunable sizes. This research reported a facile, easy-to-scale-up polyol-mediated synthesis of Sn nanoparticles assisted by sodium citrates as a capping agent. The presence of citrate capping facilitated uniform nucleation of Sn nanoparticles and enhanced their antioxidant capacity and dispersion properties. These nanoparticles can remain stable against oxidation for more than 270 days in an ambient atmosphere, even under continuous heating at 200 °C for over 12 h. The citrate capping also inhibits interparticle agglomeration and allows the preparation of high-quality suspensions in water and many conventional organics. Moreover, efficient size tuning over a wide range (60 nm–1 μm) can be achieved simply by changing the Sn2+ precursor concentrations. The above-mentioned antioxidant capacity and processability allow them to combine with silver flakes in thermosetting epoxy resin as complementary conductive fillers effectively, creating conductive pathways among the silver flakes to obtain high-performance electrically conductive adhesives (ECAs). By adding Sn nanoparticles as complementary conductive fillers, the resistivity of the ECAs can be reduced to 1/6000 of that of the ECAs filled with only the same mass ratio of silver flakes, exhibiting its great potential in future electronics.

Liquid Metal Enabled Elastic Conductive Fibers for Self‐Powered Wearable Sensors

AbstractRealizing stretchable conductive fibers with a trade‐off between stretchability and conductivity is important for wearables. Fibrous triboelectric nanogenerators (FTENGs) represent a promising device for wearable power sources and self‐powered sensors. However, the relationship between conductivity and triboelectric outputs of FTENG remains unfathomed. Herein, a simple strategy for fabricating stretchable conductive fibers with binary rigid‐soft conductive components and dynamic compensation conductive capability is reported. Wet‐spun thermoplastic polyurethane (TPU)/silver flakes (AgFKs) (TA) composite fiber is fabricated and coated by a water‐borne polyurethane (WPU) thin layer, bridging the subsequent liquid metal (LM) coating to obtain TPU/AgFKs/WPU/LM (TAWL) fibers. The TAWL fiber shows outstanding elongation (~600% strain), electrical conductivity of ~2 Ω cm−1 (~3125 S cm−1), and reversible resistance response within 70% tensile strain. Encapsulated by polydimethylsiloxane (PDMS), the TAWL fiber is demonstrated as single electrode FTENG with the maximum output voltage, current, and transferred charge of 7.5 V, 167 nA, and 3.2 nC, respectively. The FTENG shows 150% stretchability without output dropping, demonstrating the superiority of TAWL fibers to sustain large deformation and conductivity degradation but maintain stable triboelectric outputs. As self‐powered sensors, the FTENG can detect joint bending such as for fingers, elbows, and knees, as well as for pressure and location identification.

Deterministic and replaceable transfer of silver flakes for microcavities

Deterministic and replaceable transfer of silver flakes for microcavities

One-step synthesis of micro-sized flake silver particles as electrically conductive adhesive fillers in printed electronics

A novel one-step liquid phase reduction method was proposed to synthesize uniform micro-sized flake silver particles at room temperature. The La(OH)3 was used to replace other expensive agent (H2PtCl6 or H2PdCl4) as structure-directing agent. The role of the reagents and possible formation mechanism of micro-sized flake silver particles were investigated in detail. The effect of each synthesis parameter on morphology and particle size of micro-sized flake silver particles was also explored. It is found that La(OH)3 could control the reduction rate of Ag+ and its oriented growth by adsorbing on the (111) crystal plane of silver particles. The uniform micro-sized flake silver particles with a length of 4-55 μm and thickness of 0.2-5 μm could be synthesized by tuning the synthesis parameters. The synthesized micro-sized flake silver particles presented a smoother and straighter surface compared with commercial micro-sized flake silver particles prepared by ball milling. What is more important, the electrically conductive adhesive (ECA) filled by as-prepared micro-sized flake silver particles printed on polyethylene terephthalate film (PET) exhibited lower bulk resistivity (about 4.4×10-5 Ω·cm) than that filled by the commercial ball-milled micro-sized flake silver particles with the same Ag content of 80 wt. % (about 1.2×10-4 Ω·cm).

Parametric study on conductive patterns by low-temperature sintering of micron silver ink.

The fabrication of dense conductive patterns was achieved by low-temperature sintering of 1-3 μm micron silver flakes. A small amount of 20-50 nm nanosilver particles were added in the gaps of the micron silver flakes. The effects of sintering temperature, holding time and heating rate on the morphological evolution and formation mechanism of the sintered silver pattern were investigated in detail. Interestingly, rapid sintering (RS) can be achieved by removing the heating process from 70 °C up to the sintering temperature. The electrical resistivity of the sintered silver patterns was 10.8 × 10-6 Ω cm at 140 °C for 30 min under a pressure of 10 MPa. Moreover, the electrical resistivity of the sintered silver pattern for RS for 20 min does not change significantly after 6000 bending cycles. This work provides a new method to fabricate conductive patterns using micron silver flakes with the purpose of promoting the application of silver inks.

Ultrasensitive Strain Sensor Utilizing a AgF-AgNW Hybrid Nanocomposite for Breath Monitoring and Pulmonary Function Analysis.

Breath monitoring and pulmonary function analysis have been the prime focus of wearable smart sensors owing to the COVID-19 outbreak. Currently used lung function meters in hospitals are prone to spread the virus and can result in the transmission of the disease. Herein, we have reported the first-ever wearable patch-type strain sensor for enabling real-time lung function measurements (such as forced volume capacity (FVC) and forced expiratory volume (FEV) along with breath monitoring), which can avoid the spread of the virus. The noninvasive and highly sensitive strain sensor utilizes the synergistic effect of two-dimensional (2D) silver flakes (AgFs) and one-dimensional (1D) silver nanowires (AgNWs), where AgFs create multiple electron transmission paths and AgNWs generate percolation networks in the nanocomposite. The nanocomposite-based strain sensor possesses a high optimized conductivity of 7721 Sm-1 (and a maximum conductivity of 83,836 Sm-1), excellent stretchability (>1000%), and ultrasensitivity (GFs of 35 and 87 when stretched 0-20 and 20-50%, respectively), thus enabling reliable detection of small strains produced by the body during breathing and other motions. The sensor patching site was optimized to accurately discriminate between normal breathing, quick breathing, and deep breathing and analyze numerous pulmonary functions, including the respiratory rate, peak flow, FVC, and FEV. Finally, the observed measurements for different pulmonary functions were compared with a commercial peak flow meter and a spirometer, and a high correlation was observed, which highlights the practical feasibility of continuous respiratory monitoring and pulmonary function analysis.

MXene fillers and silver flakes filled epoxy resin for new hybrid conductive adhesives

The addition of V2CTx two-dimensional materials as auxiliary fillers in conductive adhesives can increase the contact area between conductive particles inside the matrix effectively reducing the resistivity of epoxy resin conductive adhesives. The V2CTx/Ag/rGO/MWCNTs fillers inside the epoxy resin will connect more Ag-clad Cu particles to form a conductive pathway, but its excessive content will be aggregated inside and thus increase the resistivity of the conductive adhesive. The volume resistivity of ECAs increases from 4.4 × 10−6 Ω m to 1.15 × 10−5 Ω m when the V2CTx/Ag/rGO/MWCNTs content of 0.1% increases to 0.34%. The Ag-clad Cu particles are interconnected inside the epoxy resin to form an electron transfer network. Inside the epoxy resin substrate Ag-clad Cu particles and V2CTx/Ag/rGO/MWCNTs interconnects to form a larger conductive network, so that the conductive adhesive shows good conductive properties.