Synthesis and electrical properties of uniform silver nanoparticles for electronic applications

Silver nanoparticles are considered to apply a silver paste for electrode because of their high conductivity. However, the dispersion of silver nanoparticles in electronically conductive adhesives (ECAs) restricts them used as conductive fillers. A simple method had enabled the synthesis of silver nanoparticles by reducing silver nitrate with ethanol in the presence of poly(N-vinylpyrrolidone) (PVP). Reaction conditions, such as silver nitrate concentration, PVP concentration, reaction time, and reaction temperature, had been studied. Fine dispersion and narrow size distribution of silver nanoparticles were obtained. They were added to ECAs by re-dispersing them in ethanol while it was used as the diluent to adjust the volatility of ECAs, preventing them from the aggregation and increasing the chance to fill the gaps between silver flakes. This proposed process offers the possibility to effectively use these synthesized silver nanoparticles for improving the conductivity of ECAs.     

Effect of silver nanowires on electrical conductance of system composed of silver particles

Silver nanowires were prepared by polyol process and doped to the system containing conventional micro-scaled sphere or flake silver particles. It is found that the conductance of doped system is much better than the undoped system with the same quantity silver. A plausible mechanism is described as that these nanowires are favor to act as bridges to establish perfect linkage among particles, and the chance of contact and contact area become more than the cases without wires. This development will be meaningful for the preparation of advanced electrically conductive adhesive (ECA) with high conductivity and good adhesive strength.     

Effect of different types of silver and epoxy systems on the properties of silver/epoxy conductive adhesives

In this study, the effect of amine adduct powder (AAP) on the electrical conductivity, thermal expansion, and flexural modulus of one-part system conductive adhesives are investigated. One-part system conductive adhesives are prepared using two types of silver (silver A and silver B) and different percentage of filler loadings, 10 vol.% ?40 vol.%. Silver A adhesive systems exhibit higher electrical conductivity with lower percolation thresholds, high flexural moduli, and low coefficients of thermal expansion (CTE) compared silver B adhesive systems. A comparison of the electrical conductivity and thermal expansion properties of the one part and two-part of silver A adhesives system is undertaken. The one-part silver A adhesives system shows high electrical conductivity and low CTE values compared to the two-part system. This is due to the higher cross linking density of the one-part system compared to that of the two-part system.     

PEDOT:PSS nano-gels for highly electrically conductive silver/epoxy composite adhesives

In this work, we report a methanol-facilitated approach to directly use aqueous Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) (PEDOT:PSS) in the silver/epoxy composites for preparation of highly electrically conductive adhesives (ECAs) and an investigation of the interaction between PEDOT:PSS nano-gels and silver microflakes. PEDOT:PSS nano-gel (18?<?d?<?30 nm) aqueous dispersion is immiscible with epoxy resin and difficult to incorporate into the conventional silver-filled ECAs. To overcome this challenge, we used methanol to facilitate the dispersion of PEDOT:PSS and silver microflake in epoxy resin. The synergetic interactions between PEDOT:PSS and silver and the effect of methanol were investigated using dynamic light scattering (DLS), atomic force microscopy, Kelvin probe force microscopy, and scanning electron microscope. When PEDOT:PSS was exposed to methanol, its morphology changed from coil to coil/linear structure; the contact potential difference between silver microflake and PEDOT:PSS increased from 9.47 to 22.56 mV, showing an increased conductivity between PEDOT:PSS and silver microflake. It was found that the introduction of a small amount of PEDOT:PSS (0.1 wt%) to the conventional ECA with 60 wt% silver microflake remarkably improved the electrical conductivity from 104 to 386 S/cm. A significantly high conductivity of 2526 S/cm was achieved by further increasing the PEDOT:PSS concentration to 1 wt%. The impact of PEDOT:PSS on the adhesive bonding strength towards copper substrate was also examined; the bonding strength slightly decreased when <?1 wt% PEDOT:PSS was used, but abruptly dropped when PEDOT:PSS content was further increased beyond 1 wt%. The incorporation of the optimal 1 wt% PEDOT:PSS into conventional ECAs with 60% silver microflake greatly increased the electrical conductivities by 25 times with limited impact on the shear strength. The results provide insights to the synergetic interplay of conductive polymer and metallic fillers, and might have profound technical implications on the development of advanced conductive composites.     

Influence of post-curing and coupling agents on polyurethane based copper filled electrically conductive adhesives

Effects of post curing and silane coupling agents with different functional groups such as epoxy, isocyanate and ureide on the electrical and mechanical properties of copper (Cu) filled electrically conductive adhesives (ECAs) were studied. Micron-sized Cu particles were used as conductive fillers and polyurethane resin was applied as the adhesive material. Significant differences could be observed on the as cured electrical resistivity and shear strength of the Cu filled ECAs joints prepared with different silane coupling agents. Silane coupling agents functionalized with epoxy groups yielded the lowest electrical resistivity and highest shear strength among the ECAs in this study. Besides, effect of post-curing at 170 °C for 1 h on the ECAs was also investigated. Results showed that ECAs after post-curing exhibited enhanced electrical conductivity and shear strength compared to the as cured ECAs.     

Conductivity improvement of silver flakes filled electrical conductive adhesives via introducing silver–graphene nanocomposites

In this study, silver–graphene nanocomposites (SGNs) were successfully prepared by spontaneous reduction of silver ions and graphene oxide. Silver nanoparticles (about 30 nm) with narrow size distribution were distributed randomly on the surface of graphene. Different amounts of SGNs were introduced into silver flakes filled electrical conductive adhesives (ECAs) to study the effect of SGNs on the properties of the ECAs. The results showed that the volume resistivity of the ECAs decreased first and then increased with the increase of weight ratios of SGNs to silver flakes. While the weight ratio of SGNs to silver flakes was 20:80 (%), the resistivity reached the lowest value of 2.37 × 10^?4 Ω cm. The lap shear strength decreased with the increase of the content ration of SGNs. And when the weight ratio of SGNs to silver flakes was 20:80 (%), the lap shear strength of ECA was about 10 MPa. According to the thermogravimetric analysis, the addition of SGNs can cause a slight decrease in the thermal stability of the ECA. In summary, SGNs are the promising candidates for the conductivity improvement of silver flakes filled electrical conductive adhesives.     

Effects of filler shape and size on the properties of silver filled epoxy composite for electronic applications

Epoxy composites filled with nano- and micro-sized silver (Ag) particulate fillers were prepared and characterized based on flexural properties, coefficient of thermal expansion, dynamic mechanical analysis, electrical conductivity, and morphological properties. The influences of these two types of Ag fillers, especially in terms of their sizes and shapes, were investigated. Silver nanoparticles were nano-sized and spherical, while silver flakes were micron-sized and flaky. It was found that the flexural strength of the epoxy composite filled with silver flakes decreased, while the flexural strength of the epoxy composite filled with silver nanoparticles showed an optimum value at 4 vol.% before it subsequently dropped. Both silver composites showed improvement in flexural modulus with increasing filler loads. CTE value indicated significant decrements in filled samples compared to neat epoxy. Results on the electrical conductivity of both systems showed a transition from insulation to conduction at 6 vol.%.     

Effect of different copper fillers on the electrical resistivity of conductive adhesives

The effects of different copper fillers with different morphology and particle size have been studied in terms of electrical resistivity and thermal stability on the electrically conductive adhesives. The copper fillers used in this study were prepared by wet chemical reduction, electrolytic and gas atomization method, respectively. The as cured ECAs filled with different type of Cu fillers showed significant difference in electrical resistivity. Cu filler with smaller particle size showed higher packing density and larger surface area, which would enhance formation of conductive channels and increased conductive network in the ECAs, leading to a lower electrical resistivity. In addition, thermal stability of the ECAs were investigated under high temperature exposure at 125 °C and high humidity aging at 85 °C/85% RH for 1,000 h. Results showed that ECAs with Cu fillers of relatively small particle size and rough particle surface have excellent thermal stability due to enhanced adhesion and contact area between Cu fillers and the polymer matrix. A very low resistivity at an order of magnitude of 10^?4 ?? cm could be maintained for these ECAs after 1,000 h at 125 and 85 °C/85% RH.     

Properties of polyacrylic acid-coated silver nanoparticle ink for inkjet printing conductive tracks on paper with high conductivity

Silver nanoparticles with a mean diameter of approximately 30 nm were synthesized by reduction of silver nitrate with triethanolamine in the presence of polyacrylic acid. Silver nanoparticle-based ink was prepared by dispersing silver nanoparticles into a mixture of water and ethylene glycol. The mechanism for the dispersion and aggregation of silver nanoparticles in ink is discussed. The strong electrostatic repulsions of the carboxylate anions of the adsorbed polyacrylic acid molecules disturbed the aggregation of metal particles in solutions with a high pH value (pH > 5). An inkjet printer was used to deposit this silver nanoparticle-based ink to form silver patterns on photo paper. The actual printing qualities of the silver tracks were then analyzed by variation of printing passes, sintering temperature and time. The results showed that sintering temperature and time are associated strongly with the conductivity of the inkjet-printed conductive patterns. The conductivity of printed patterns sintered at 150 °C increased to 2.1 × 10⁷ S m⁻¹, which was approximately one third that of bulk silver. In addition, silver tracks on paper substrate also showed better electrical performance after folding. This study demonstrated that the resulting ink-jet printed patterns can be used as conductive tracks in flexible electronic devices.     

银粉性质对太阳能电池浆料的影响

作为传统化石能源的有效替代品,太阳能电池引起了学者们的广泛关注。导电银浆作为太阳能电池的重要原料,影响着太阳能电池的光电转换率和度电成本。银粉作为银浆中的导电相,其性能对银浆的电性能、流动性、粘附性等性质起着关键作用。近年来针对银粉的形态、尺寸、分散度、粒度分布和振实密度对导电银浆电性能的影响有较多研究。研究表明银粉的形态和粒径是决定银浆导电性、烧结质量等特性的主要因素;银粉的振实密度也是影响烧结厚膜的致密性以及电池的光电转化率的重要因素。在银粉制备过程中,不同种类的分散剂会影响银粉的分散性,从而影响银浆的细度、粘附性和电阻率。     

Heterogeneously structured conductive carbon fiber composites by using multi-scale silver particles

This paper reports a new approach to enhance the through-thickness thermal conductivity of laminated carbon fabric reinforced composites by using nanoscale and microscale silver particles in combination to create heterogeneously structured continuous through-thickness thermal conducting paths. High conductivity of 6.62 W/(m K) with a 5.1 v% silver volume fraction can be achieved by incorporating these nanoscale and microscale silver particles in EWC-300X/Epon862 composite. Silver flakes were distributed within the inter-tow area, while nanoscale silver particles penetrated into the fiber tows. The combination of different sizes of silver fillers is able to effectively form continuous through-thickness conduction paths penetrating fiber tows and bridging the large inter-tow resin rich areas. Positive hybrid effects to thermal conductivity were found in IM7/EWC300X/sliver particle hybrid composites. In addition, microscale fillers in resin rich areas showed less impact on tensile performance than nanoscale particles applied directly on fiber surface.     

Effect of the different shapes of silver particles in conductive ink on electrical performance and microstructure of the conductive tracks

The electrical performance of the ink-jet printed conductive tracks composed of silver particles was investigated. Three different shapes silver particles were synthesized via chemical reduction method in the presence of poly vinyl pyrrolidone, and then they were used to study the shape influence on the electrical property and thermal stability of the conductive tracks. The resistivity variation and microstructure of the silver conductive tracks was monitored as a function of fillers content using a four-point probe and scanning electron microscopy as well as thermal analysis. In addition, we proposed the possible formation mechanism of conductive tracks with different fillers. It demonstrated that the conductive tracks filled with silver nanorods and nanoparticles could achieve the volume electrical resistivity of ~3.2?×?10^?5?Ω?cm after sintering at 160?°C for 15?min. Finally, we fabricated highly conductive silver patterns on a glass substrate by ink-jet printing.     

Preparation and properties of a novel electrically conductive adhesive using a composite of silver nanorods, silver nanoparticles, and modified epoxy resin

A novel preparation method for a high-performance electrically conductive adhesive (ECA) which consisted of silver nanorods, silver nanoparticles and modified epoxy resin was developed. Silver nanorods (100 nm in diameter and 5 μm in length) were synthesized by reduction of silver nitrate with ethylene glycol in the presence of Pd seeds and poly (vinyl pyrrolidone) (PVP). Silver nanoparticles (50~60 nm) were synthesized using N, N′-Dimethylformanide as the reducing agent and PVP as the stabilizer. The nanorods and nanoparticles were dispersed well and no agglomerate in the matrix. The volume electrical resistivity tests showed the volume electrical resistivity of the ECA was closely related with the various sintering temperatures and time, and the ECA could achieve the volume electrical resistivity of (3–4) × 10⁻⁵ Ω cm after sintering at 160 °C for 20 min. Moreover, the results showed the as-prepared ECA was able to achieve low-temperature sintering and possessed excellent electrical, thermal, and mechanical properties.     

Improving the through-thickness thermal and electrical conductivity of carbon fibre/epoxy laminates by exploiting synergy between graphene and silver nano-inclusions

Fibre-reinforced polymer composites typically feature low functional (e.g., electric and thermal conductivity) and structural (e.g. mechanical strength and fracture toughness) properties in the laminate’s thickness direction. In the event of lightning strikes, overheating, and impact by foreign objects, composite laminates may suffer wide spread structural damage. This research explores the synergistic physical interaction between two-dimensional nanostructured (graphene nano-platelets) and, zero- or one-dimensional conductive fillers (silver nanoparticles or silver nanowires, respectively) when both are dispersed in fibre–polymer laminates. The results reveal a synergistic improvement in the through-thickness thermal conductivity that is more than the additive improvements by each constituent. Specifically, the simultaneous inclusion of graphene nano-platelets and silver nanoparticles/nanowires at a combined loading of 1 vol% resulted in approximately 40% enhancement in the through-thickness thermal conductivity while the inclusion of graphene nano-platelets alone at the same loading resulted only in 9% improvement. Similarly, the through-thickness electrical conductivity of carbon fibre/epoxy laminates incorporating graphene nano-platelets together with silver nanoparticles/nanowires was notably higher (⩾70%) than can be achieved by graphene nano-platelets alone (∼55%). These results demonstrate that the presence of nano-reinforcements exhibiting varied phonon transport and electron transfer pathways, and geometric aspect ratios promote synergistic physical interactions. Small improvements were found in the mechanical properties, including tensile, flexural or compressive properties of the carbon fibre-reinforced laminates, due to the relatively low concentrations of the nano-fillers.     

高含量银基导电胶的制备与性能研究

以90%(质量分数)银含量,10%(质量分数)树脂为基体制备出高银含量,高性能的环氧导电胶。采用0.025 mol/L KI试剂对银粉进行简单的表面处理,原位取代银粉表面的部分润滑剂并且与银粉表面的氧化银发生反应。在日光照射条件下原位生成小尺寸的银纳米粒子,这些小尺寸的银纳米粒子在固化过程中,会发生明显的烧结现象,增加银粉中相邻银微米片和银微米球之间的相互接触,从而可以显著降低导电银胶的体电阻率。相比于未处理的银粉制成的导电银胶,其体电阻率降低了26%,线电阻降低了8%,将其应用在光伏电池板上,极大的提高了其导电性能。     

Effects of hybrid fillers based on micro- and nano-sized silver particles on the electrical performance of epoxy composites

In the present study, electrically conductive adhesives produced from hybrid fillers based on micro- and nano-sized silver (Ag) was developed. The influence of the hybrid filler composition on the electrical properties of the hybrid system was studied. The electrical conductivity of the epoxy composites filled with micro- and nano-silver was correlated with their morphologies. A positive effect was observed in the electrical conductivity result when the composition of micro- and nano-sized Ag particles reached a 50:50 weight ratio. The nano-sized Ag particles became interconnecting particles in the interstitial spaces between micro-sized particles. Micrograph scanning shows that the particles were well distributed and dispersed, the separation between lumps of Ag filler by the insulating matrix was significantly reduced, leading to the formation of continuous linkages. The increased electrical conductivity resulted in a charge around the particle distribution, which led to the high capacity. Hence, these particles increased the conductivity of the system.     

Effect of inorganic binders on the properties of silver thick films

The technology for realizing conductive thick films from metal powders is of considerable importance in the manufacture of various electronic devices, such as hybrid integrated circuits, multilayer ceramic capacitors, three-dimensional circuits for mobile communication, etc. Silver thick films have been extensively used to provide electrical contacts in silicon solar cells, hybrid circuits, and other devices, as they have the lowest resistivity as well as the lowest cost among the noble metals. Metal powders for thick-film pastes are normally obtained through chemical precipitation from an aqueous or organic solution of their corresponding salts. This method is able to provide the desired particle shape and size and also offers a low preparation cost. In this paper, we have reported the effect of inorganic binders on the properties of silver thick films. The thick-film pastes were formulated from specially treated silver powders and using the three different types of inorganic binders such as glass frit, metal oxides, and a mixture of both. The effect of these inorganic binders on the surface structure, adhesion with alumina, solderability, and electrical properties was determined by analyzing the physical and chemical behavior of the inorganic binders in the silver thick films during firing. The films with an addition of glass frit and mixtures of glass frit and metal oxide did not provide good adhesion and solderability simultaneously. However, the silver thick films containing 10 wt % bismuth oxides exhibited excellent characteristics, i.e. high adhesion strength, good solderability, and low sheet resistivity. As expected, the adhesion strength decreased with decrease in the binder content.     

低温快速固化环氧导电胶的制备与性能

采用E-51型环氧树脂为导电胶基体树脂, 低分子量聚酰胺树脂(PA)为固化剂, 填加经硅烷偶联剂(KH550)改性后的纳米级和微米级铜粉及助剂制备导电胶。首次采用了液态固化剂(低分子量的聚酰胺酯), 以解决导电胶制备过程中填料用量受限的难题。通过正交试验方法探讨了导电胶中导电填料的含量、导电填料配比、硅烷偶联剂用量和还原剂添加量对导电胶粘接性能和导电性能的影响, 对导电胶的制备工艺进行了优化, 获得了制备导电胶的最佳方案。测试结果表明该导电胶能够在60 ℃下4 h内快速固化。在填料质量分数为65%时, 导电胶具有最低的体积电阻率3.6×10-4 Ω·cm; 导电胶的抗剪切强度达到17.6 MPa。在温度为85 ℃、湿度(RH)为85%的环境下经过1000 h老化测试后导电胶电阻率的变化和剪切强度的变化均不超过10%。      

Electrical reliability of different alloying content on copper alloy fillers in electrically conductive adhesives

The thermal stability and electrical reliability of electrically conductive adhesives (ECAs) filled with Cu fillers alloying with different amount of Ag and Mg (0.2–1.5 at.%), respectively, were studied by comparing their electrical resistivity under high temperature exposure at 125 and 85 °C/85% RH for 1,000 h. Results showed that the Cu-Ag filled ECAs were superior to Cu-Mg filled ECAs in terms of thermal stability during aging under high temperature exposure and high humidity condition. A final resistivity on the order of 10⁻⁴ Ω.cm could be maintained for Cu-Ag filled ECAs after aging at 125 °C for 1,000 h. Cu-Mg filled ECAs showed relatively high electrical resistivity compared to Cu-Ag filled ECAs. Resistivity of Cu-Mg filled ECAs increased rapidly over time during high temperature exposure at 125 °C except for Cu-0.5 at.% Mg filled ECA which was thermally stable after 400 h of aging. The ECAs in this study could withstand high temperature exposure at 125 °C better than aging under a combination of elevated temperature and high humidity at 85 °C/85% RH.     

Silver-plated carbon nanotubes for silver/conducting polymer composites

Carbon nanotubes (CNTs) have advantages as conductive fillers due to their large aspect ratio and excellent conductivity. In this study, a novel silver/conducting polymer composite was developed by the incorporation of silver-plated CNTs. It is important to achieve a homogeneous dispersion of nanotubes and to improve the interfacial bonding to utilize the excellent properties of reinforcements in the matrix material. The homogeneous dispersion of nanotubes was achieved by an acid treatment process, and the interfacial contact was improved by electroless silver plating around nanotubes. The resistivity of the silver/conducting polymer composite was decreased by 83% by the addition of silver-plated single-walled carbon nanotubes. Conductive bumps were also screen-printed to demonstrate the capability of the composite as electrical interconnects for multi-layer printed circuit boards.