Ag Flakes Research Articles

Mechanically Flexible Conductors for Stretchable and Wearable E-Skin and E-Textile Devices.

Considerable progress in materials development and device integration for mechanically bendable and stretchable optoelectronics will broaden the application of "Internet-of-Things" concepts to a myriad of new applications. When addressing the needs associated with the human body, such as the detection of mechanical functions, monitoring of health parameters, and integration with human tissues, optoelectronic devices, interconnects/circuits enabling their functions, and the core passive components from which the whole system is built must sustain different degrees of mechanical stresses. Herein, the basic characteristics and performance of several of these devices are reported, particularly focusing on the conducting element constituting them. Among these devices, strain sensors of different types, energy storage elements, and power/energy storage and generators are included. Specifically, the advances during the past 3 years are reported, wherein mechanically flexible conducting elements are fabricated from (0D, 1D, and 2D) conducting nanomaterials from metals (e.g., Au nanoparticles, Ag flakes, Cu nanowires), carbon nanotubes/nanofibers, 2D conductors (e.g., graphene, MoS2 ), metal oxides (e.g., Zn nanorods), and conducting polymers (e.g., poly(3,4-ethylenedioxythiophene):poly(4-styrene sulfonate), polyaniline) in combination with passive fibrotic and elastomeric materials enabling, after integration, the so-called electronic skins and electronic textiles.

UV‐Curable Silver Electrode for Screen‐Printed Thermoelectric Generator

AbstractFabricating thermoelectric generators (TEGs) using the screen‐printing process has advantages, including mass production, device scalability, and system applicability. However, the thick film formed through the process typically has low film density, and reduced performance, because of the presence of pores in the film created by the vaporization of the resin during high‐temperature annealing. During the soldering process used for thermoelectric module fabrication, the printed solder infiltrates into the screen‐printed electrodes through the micropores in the electrodes, causing cracks of the electrode film and an increase in resistivity. In this paper, an ultraviolet radiation (UV)‐curable process for screen‐printed electrodes is reported. The paste for the electrodes is synthesized by mixing Ag flakes that can be cured at low temperature with a UV resin. Scanning electron microscope images show that the UV‐curing process significantly reduces pores and thereby results in a smooth‐surfaced electrode layer. The film density after crystallization is also enhanced. TEGs composed of 72 couples with UV‐curable Ag electrodes generate a high power density of ≈6.69 mW cm−2 at a temperature difference of 25 °C; the device resistance is ≈0.75 Ω, and the figure of merit of the device is recorded to be 0.57, which is the highest among the printed TEGs.

Carbon Nanotubes with Carbon Blacks as Cofillers to Improve Conductivity and Stability.

In this study, a simple solution-mixing method is used to develop a kind of excellent flexible, electrically conductive adhesives (ECAs). Carbon nanotubes (CNTs) and carbon blacks (CBs) as cofillers were added into Ag-based pastes. The use of the two fillers is due to the consideration that these two materials may provide positive synergistic effects for improving the conductivity of ECAs. The conductivity, flexibility, cyclability, and oxidation resistance of ECAs with different contents of carbon fillers were studied. It was found that a small amount of CNTs or CBs can dramatically improve the ECAs’ conductivity. Solution-mixing method brings excellent carbon nanofiller dispersion in polymer matrix. Highly dispersed CNTs and CBs among the Ag flakes formed three-dimensional conducting networks to improve the conductivity of ECAs. The conductivity of ternary hybrid ECAs (with addition of 3 wt % CNTs and 2 wt % CBs) with a low content of 55 wt % Ag flakes is higher than that of the ECAs filled with only the Ag content over 65 wt %. Meanwhile, by selecting thermoplastic polyurethane resin as the matrix, the ECAs exhibited excellent mechanical compliance. The resistivity did not change when the ECAs were bended at a 60% flexural strain or pressed under 1200 kPa. Additionally, the adhesion strength of the new composited ECAs is better than that of a commercial ECA (Abletherm 3188). Further, no obvious conductivity change was observed when the sample was stored in ambient air condition at 80 °C and 60% relative humidity (60%) for 15 days.

The development of a highly stretchable, durable, and printable textile electrode

In textile and wearable electronics, the demand for a stretchable, durable, and easily manufacturable electrode is ever increasing. This paper describes the development of a highly stretchable and durable textile electrode fabricated by simple stencil and screen printing methods. It specifically investigated the effects of an interface layer as a planarization layer between the conductive electrode and the textile on the durability of the textile electrode. A stretchable conductive paste was synthesized by mixing Ag flake powder in polyester. The conductive electrode was encapsulated with Ecoflex material. The stretchability and durability of the textile electrodes were evaluated via stretching, bending, Massachusetts Institute of Technology (MIT) folding, twisting, and dynamic endurance tests. The stretching and MIT folding tests indicated that the interface layer significantly enhanced the durability of the textile electrode. A highly stretchable and flexible textile electrode exhibited a low sheet resistance of 0.05 Ω/square, excellent stretchability of 70%, and a critical bending radius of 1.5 mm. The textile electrodes also withstood dynamic stretching and bending endurance tests of 10,000 cycles. The illumination of a light-emitting diode with the conductive electrode was also stable under 70% tensile strain and in water. The potential application of the textile electrode as a strain sensor was demonstrated by applying it to a glove to detect finger motion. The strain sensors responded well to the finger motion, with considerable stability and repeatability.

Interface Modified Flexible Printed Conductive Films via Ag2O Nanoparticle Decorated Ag Flake Inks.

A new approach to stable, low resistance inexpensive printed flexible conductive inks is proposed. Silver inks have been extensively studied and commercialized for applications in printed electronics due to the inherent high conductivity and stability of silver, even in particulate-based percolation networks processed at temperatures compatible with low-cost polymer films such as poly(ethylene terephthalate) (PET). Recent interest in flexible and even stretchable circuits, however, has presented new challenges for particle-based inks as mechanical strains can result in the opening of critical particle-to-particle contacts. Here we report a facile, low-cost method for the single-step synthesis of stable, printable nanoscale Ag2O decorated Ag flake inks which can be converted to highly conductive Ag films at 150 °C curing temperature without the use of limited shelf life organometallics or low metal loading nanoparticles to modify the interface between silver flakes. Analysis indicates that decoration of Ag flakes with Ag2O nanoparticles (NPs) during ink synthesis improves the conductivity and flexibility of printed silver films by forming bridging interconnections between Ag flakes after low temperature reduction of the Ag2O NPs. In this work, printed nanodecorated silver conductors with starting oxide to metal weight ratios of 5:95 exhibited lateral resistivities lower than 1.5 × 10-5 Ω·cm, which was 35% less than films derived from undecorated Ag flake inks of the same total Ag loading and binder system. This resistivity difference increased to 45% after cyclic bend testing showing increased resilience to repeated flexing for the nanodecorated inks. Through detailed compositional and morphological characterizations, we demonstrate that such improved conductivity and flexibility are due to a more effective bridging afforded by the in situ synthesized Ag NPs on the surface of Ag flakes. These properties, combined with the simplified syntheses method of the nanoink, make the material a viable, advantageous alternative to the limited number of stretchable conductors currently available.

Microstructure and mechanical properties of sintered Ag particles with flake and spherical shape from nano to micro size

This work firstly evaluated the microscale mechanical properties of sintered Ag, consisting of various Ag particles of a flake shape and spherical shape from nano to micro size. Microscale pillar specimens were fabricated by a focused ion beam process and the mechanical properties of the sintered Ag, were measured by micro-compression testing using a nano-indentation system. The microstructure of sintered Ag is dependent upon the size and the shape of the utilized Ag particles, as this significantly influences its fracture behavior and mechanical properties. Mechanical properties not only depend on the porosity of the sintered Ag, but also on the degree of necking growth of the Ag particles. To explore this further, the shear strength of Ag sinter joint using different sizes and shapes of Ag particles was also evaluated. This revealed that sintered Ag flake particles possess a much greater bonding strength above 30 MPa than do sintered Ag spherical shape particles from nano to micro sizes. TEM observation revealed that many nanoparticles with a size less than 10 nm were formed in the sintered Ag flake structure due to its high grain boundary density and dislocation density, which increased its grain bonding quality.

Highly stretchable and conductive conductors based on Ag flakes and polyester composites

In stretchable electronics, the development of conductors that have high stretchability and low electrical resistance is a crucial technical issue. In this study, we developed a highly stretchable Ag conductor composed of Ag flakes and a polyester binder. We focused on the IR-curing technique to develop the stretchable Ag conductor, which was printed on the polyurethane elastomer substrate. The effects of the temperature and duration of curing on the electrical resistance and electromechanical properties of the Ag conductor were investigated. The electromechanical properties of the stretchable Ag conductor were evaluated in stretching, bending, and cyclic endurance tests. We achieved stretchable conductors with a low sheet resistance of 46 mΩ/sq., a superior stretchability of 150%, and a bending radius of 1 mm. The Ag conductor also exhibited outstanding mechanical durability in the cyclic stretching and bending endurance tests. In this study, the optimized IR-curing duration and temperature were 5 min and 130 °C, respectively. However, in future research, the curing duration could be further reduced by the optimization of the IR parameters. The results of the pencil hardness test and the peel-off adhesion test (i.e., the cross-cut test) indicated that the stretchable Ag conductor had excellent hardness and strength of adhesion between the conductor and the substrate.

Green synthesis of micron-sized silver flakes and their application in conductive ink

A facile, green method for fabrication of conductive ink composed of Ag flakes was developed for use in flexible printed electronics. The Ag flakes were prepared using AgNO3, nontoxic l-ascorbic acid (Vc) and polyvinylpyrrolidone (PVP) serving as a metal salt, reducing agent and capping agent, respectively. The prepared Ag flakes were characterized by XRD, SEM, and TGA. The combination of PVP and FeCl3 was found to be critical for the formation of the Ag flakes, and reaction activity was affected by temperature. Samples obtained at 140 °C were composed of Ag flakes with an average size of 0.94 ± 0.3 μm. A new fabrication method for producing conductive patterns was designed using a syringe with varying dispersion areas, allowing the ink (Ag flakes and organic solvent) to be applied directly onto a flexible photo paper. The conductivity and flexibility of the pattern were experimentally tested under varying reaction temperatures and bending cycles. Increasing contact area and packing density between Ag flakes resulted in an improved conductivity of the bending pattern. After more than 5000 bending cycles, the patterns were sintered at 160 °C and the resistivity increased from 4.6 ± 0.6 to 22.4 ± 0.8 μΩ cm, acceptable values for practical applications. Sample conductive lines drawn by the Ag flake ink exhibited excellent conductive performance and mechanical integrity, demonstrating a promising method for the formation of flexible microelectrodes or electronic devices.

RF characteristics of flexible circuits patterned with hybrid Ag paste

Printed electronics based on nanomaterials has been a potential alternative to conventional electronic technology and is favorable for flexible polymer substrates. However, a printed circuit has insufficient electrical performance and mechanical reliability under flexible stress. We designed hybrid Ag pastes containing Ag nanoparticles (NPs, 30–50 nm) and Ag flake (F, 2.5–3.5 μm in diameter) particles to overcome the obstacles of printed electronics. Six types of Ag paste (Ag NPs-x wt% F paste, x = 0, 10, 25, 50, 75, and 100) were screen-printed onto a polyimide substrate and sintered at 200 °C for fabricating a flexible circuit. We simulated a coplanar waveguide pattern before measuring the S-parameter using high frequency structural simulation. To investigate the radio frequency characteristics of various Ag circuits under sliding stress, a network analyzer and Cascade’s probe system in the frequency range from 1 to 10 GHz were employed before and after 100,000 sliding cycles. Hybrid Ag circuits showed stable electrical conductance and signal transmission, while pure Ag particle circuits showed a definite drop of electrical conductance (over 30% increase of electrical resistance) under cyclic sliding stress. The microstructural crack behavior demonstrated that the Ag F content within the Ag hybrid circuits effectively suppressed crack propagation, leading to improved flexibility of the sintered Ag circuit.

Thermal Stability of Silver Paste Sintering on Coated Copper and Aluminum Substrates

The thermal stability of silver (Ag) paste sintering on coated copper (Cu) and aluminum (Al) substrates has been investigated. Instead of conventional zincating or nickel plating, magnetron sputtering was used to achieve coating with titanium (Ti) and Ag. Silicon (Si) chips were bonded to coated Cu and Al substrates using a mixture of submicron Ag flakes and particles under 250°C and 0.4 MPa for 30 min. The joints were then subject to aging testing at 250°C for duration of 200 h, 500 h, and 1000 h. Two types of joints exhibited satisfactory initial shear strength above 45 MPa. However, the shear strength of the joints on Al substrate decreased to 28 MPa after 1000 h of aging, while no shear strength decline was detected for the joints on Cu substrate. Fracture surface analysis revealed that the vulnerable points of the two types of joints were (1) the Ag layer and (2) the interface between the Ti layer and Cu substrate. Based on the results of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and simulations, cracks in the Ag layer were identified as the cause of the shear strength degradation in the joints on Al substrate. The interface evolution of the joints on Cu substrate was ascribed to Cu migration and discontinuity points that initialized in the Ti layer. This study reveals that Al exhibited superior thermal stability with sintered Ag paste.

Printable elastic conductors by in situ formation of silver nanoparticles from silver flakes.

Printable elastic conductors promise large-area stretchable sensor/actuator networks for healthcare, wearables and robotics. Elastomers with metal nanoparticles are one of the best approaches to achieve high performance, but large-area utilization is limited by difficulties in their processability. Here we report a printable elastic conductor containing Ag nanoparticles that are formed in situ, solely by mixing micrometre-sized Ag flakes, fluorine rubbers, and surfactant. Our printable elastic composites exhibit conductivity higher than 4,000 S cm-1 (highest value: 6,168 S cm-1) at 0% strain, and 935 S cm-1 when stretched up to 400%. Ag nanoparticle formation is influenced by the surfactant, heating processes, and elastomer molecular weight, resulting in a drastic improvement of conductivity. Fully printed sensor networks for stretchable robots are demonstrated, sensing pressure and temperature accurately, even when stretched over 250%.

Laser patterning of highly conductive flexible circuits

There has been considerable attention paid to highly conductive flexible adhesive (CFA) materials as electrodes and interconnectors for future flexible electronic devices. However, the patterning technology still needs to be developed to construct micro-scale electrodes and circuits. Here we developed the selective laser sintering technology where the pattering and curing were accomplished simultaneously without making additional masks. The CFA was composed of micro-scale Ag flakes, multiwalled carbon nanotubes decorated with Ag nanoparticles, and a nitrile–butadiene–rubber matrix. The Teflon-coated polyethylene terephthalate film was used as a flexible substrate. The width of lines (50–500 μm) and circuit patterns were controlled by the programmable scanning of a focused laser beam (power = 50 mW, scanning speed = 1 mm s−1). The laser irradiation removed solvent and induced effective coalescence among fillers providing a conductivity as high as 25 012 S cm−1. The conductivity stability was excellent under the ambient air and humid environments. The normalized resistance change of the pattern was smaller than 1.2 at the bending radius of 5 mm. The cyclability and adhesion of the laser-sintered line pattern on the substrate was excellent. A flexible circuit was fabricated sequentially for operating light emitting diodes during the bending motion, demonstrating excellent feasibility for practical applications in flexible electronics.

DTT functionalization of Ag particles for conducting adhesives

ABSTRACTWe have studied the influence of the organic molecule DL-Dithiothreitol on the properties of isotropic conductive adhesives for application in microelectronics. The Ag fillers were prepared from commercial micro flakes or submicron particles from waste jewelry. The results show that coating of the Ag recycled fillers with thiol avoided the agglomeration of the particles improving the dispersion and the electrical and mechanical properties. By comparing the samples, the better properties were obtained from adhesives with recycled Ag particles relative to the ones formulated with commercial Ag flakes and improvement can be attributed to the absence of the lubricant layer and smaller size of the particles produced from recycled silver.

3D-printable, highly conductive hybrid composites employing chemically-reinforced, complex dimensional fillers and thermoplastic triblock copolymers.

The use of 3-dimensional (3D) printable conductive materials has gained significant attention for various applications because of their ability to form unconventional geometrical architectures that cannot be realized with traditional 2-dimensional printing techniques. To resolve the major requisites in printed electrodes for practical applications (including high conductivity, 3D printability, excellent adhesion, and low-temperature processability), we have designed a chemically-reinforced multi-dimensional filler system comprising amine-functionalized carbon nanotubes, carboxyl-terminated silver nanoparticles, and Ag flakes, with the incorporation of a thermoplastic polystyrene-polyisoprene-polystyrene (SIS) triblock copolymer. It is demonstrated that both high conductivity, 22 939 S cm-1, and low-temperature processability, below 80 °C, are achievable with the introduction of chemically anchored carbon-to-metal hybrids and suggested that the highly viscous composite fluids employing the characteristic thermoplastic polymer are readily available for the fabrication of various unconventional electrode structures by a simple dispensing technique. The practical applicability of the 3D-printable highly conductive composite paste is confirmed with the successful fabrication of wireless power transmission modules on substrates with extremely uneven surface morphologies.

Fabrication of the hybrid Ag paste combined by Ag nanoparticle and micro Ag flake and its flexibility

The mechanical properties of screen-printed silver circuit were focused by fabricating hybrid silver pastes composed of silver nanoparticles and micron-sized silver flakes with various contents of silver flakes (0, 25, 50, 75, 100 wt%). All printed hybrid silver circuits on the polyimide (PI) substrate were sintered at 250 °C for 30 min in air. Surface porosity and pore volume of hybrid silver were analyzed by SEM image analyzer and BET method, respectively. Pore volume was increased as contents of silver flake from 1.7 cm3/g to 3.0 cm3/g. The low cycle flexibility of screen-printed hybrid silver circuits was evaluated by Massachusetts Institute of Technology (MIT)-type folding endurance test to confirm the flexibility of screen-printed circuit. A roll-type 90° peel test was conducted to measure the peel strength of the hybrid silver circuits. The highest flexibility and peel strength of screen-printed silver circuit was 50 wt% of silver flake contents. Silver flake improved the peel strength and folding endurance property of the silver circuit screen-printed silver nanopastes.

Development of electrically conductive-superoleophobic micropillars for reducing surface adhesion of oil at low temperatures

Electrically conductive and superoleophobic micropillars have been developed through the construction of biomimetic micropillars using Ag-filled epoxy composites and the incorporation of FDTS on the micropillar surface. These micropillars are found to be superoleophobic with an oil contact angle of 140°, demonstrating excellent self-cleaning properties. The conductivity of micropillars allows for the Joule-heating effect to actively reduce the adhesion and even unfreeze the frozen oil droplets by passing electrical current. Electrical resistance of the composite micropillars was modulated by two orders of magnitudes by varying the contents of Ag flakes from 45wt% to 65wt%. The effectiveness of conductive micropillars for surface un-freezing was investigated by applying DC current to decrease the adhesion strength of frozen oil droplets on surfaces. The results showed a pronounced reduction of frozen oil adhesion force by 60% when the resistance increased from 7.5Ω to 877Ω after applying DC current for 2min. By continuously applying DC current for 3min, the frozen oil adhesion decreased to 0.05N, reaching zero when the surface was heated up to −10°C after applying DC current for 5min. In contrast, when the droplet was heated up to −5°C by hot air, there is still a substantial force of adhesion. The research findings demonstrate the use of constructing conductive-superoleophobic composite micropillars at surface for eliminating the frozen oil from surfaces at low temperatures.

Effect of Silver Flakes in Silver Paste on the Joining Process and Properties of Sandwich Power Modules (IGBTs Chip/Silver Paste/Bare Cu)

In this study, a silver paste has been introduced for attaching chips onto bare Cu substrates (without coating) without applying pressure. Small nano-thickness Ag flakes, measuring 1 μm–5 μm length, were embedded uniformly in Ag nanoparticles for improving the density of the material. The presence of silver flakes in the silver paste affected the joining process and its microstructure. Microstructure characterization revealed that densification of the silver layer was affected by the presence of silver flakes as the flakes coarsened and formed reactive in situ nanoparticles, which facilitated the sintering between the flakes and the incorporated nanoparticles. Coarsening of silver flakes depended on the sintering temperature, time, and the atmosphere, which affected the decomposition and burning out of organics presented on the surface of the flakes. A high-density silver layer was obtained due to the presence of compact silver flakes. With an increase in the microstructure density, a higher bonding strength and a lower thermal impedance of the sintered joints were achieved. On performing pressureless sintering at 270°C for 30 min under 99.99% N2 or 4% H2/N2, the bonding strength and thermal impedance for 11 × 11 mm2 chips were excellent, measuring approximately 21.9 MPa and 0.077°C/W, respectively.

Novel interface material used in high power electronic die-attaching on bare Cu substrates

Silver paste sintering on bare Cu substrates can be of significant interest in power semiconductor manufacturing. However, the bare Cu substrates easily oxidized and the die attachment using the silver paste commonly applied pressure. In this study, these problems are solved by using a prepared novel silver paste, which could be pressure-less low temperature sintered completely under inert gas (99.99 wt% N2). The silver paste consists of wide size distribution particles as Ag nanoparticles, Ag sub-micro-sized particles and micro-sized Ag flakes. High content of organic system mixture in the silver paste need more evaporating time to avoid cracks in the sintered Ag film. During sintering, the Ag flakes in situ formation of highly reactive silver nanoparticles is crucial for the sintering to occur between the silver flake and particles, unlike the sintering between the silver particles. A dense silver film contributed to the sintered silver flakes, which has the nature of good densification, well sintering with the silver nanoparticle. The possible joining mechanisms of metallic bond and hydrogen bonds between bare Cu and sintered Ag were proved. Accordingly, the sandwich joints (chip/silver paste/bare Cu) include high bonding strength and low thermal impedance.

Electrically conductive adhesives based on thermoplastic polyurethane filled with silver flakes and carbon nanotubes

Flexible conductive adhesive could be used in flexible device electronic packaging. In this study, carbon nanotubes (CNTs) were used as one-dimensional conductive scaffolds to construct effective electrical networks among the silver flakes in electrically conductive adhesives (ECAs) for saving noble metal fillers and reducing the cost because CNTs used as conductivity fillers could bridge the neighboring silver flakes to accelerate the electron transport. The electrical, mechanical, and thermal properties have been enhanced after adding CNTs. The electrical conductivity increased 85.6% after 4.5 wt% CNT added into the 50 wt% Ag TPU adhesives. In addition, after the Ag flakes modified by succinic acid (SA), the electrical conductivity has also been improved. The development of highly conductive, flexible, and low cost ECAs will allow them to be widely used in flexible electronic devices packaging in the future.

Characterizing the conductivity of ICA joints by the mean intercept length of Ag flakes

Purpose – The purpose of this paper is to present a novel and alternative method for the characterization of isotropic conductive adhesive (ICA) joints’ conductivity by the calculation of the mean intercept length of conductive flakes in the cured joint. ICAs are widely used in the field of hybrid electronics or special printed circuit board applications, such as temperature sensitive or flexible circuits. The main quality parameters of the ICA joints are the conductivity and the mechanical strength. Design/methodology/approach – For the experiments, one-component Ag-filled thermoset ICA paste was used on FR4 printed circuit test board to join zero-ohm resistors. Six different curing temperatures were applied: 120, 150, 175, 210, 230 and 250°C. The conductivity of the joints was measured in situ during the curing process. Micrographs were taken from the cross-sectioned joints, and the mean intercept length was calculated on them after image processing steps. Findings – Results of the measured conductivity and the mean intercept length were compared, and acceptable correlation was found for what can be used for characterizing the conductibility of ICA joints. Research limitations/implications – Investigating and characterizing the conductivity of ICA joints by an image processing method. Originality/value – The main advantage of this method compared to the electrical measurements is that the conductivity characterization is possible on any kind of component. Therefore, this method can be used in any appliances not only in test circuits.