Abstract
In this study, the preparation, characterization and electrical properties of silver nanowires (AgNws) were investigated. This study contains five parts including (1) the effects of the synthesis procedures on the continuously fabricating AgNws, (2) modification, preparation and properties of the AgNws conductive composites, (3) electrical, morphological, and electromagnetic interference shielding properties of silver nanowires and nanoparticles conductive composites, (4) enhanced thermal and mechanical properties of epoxy composites filled with silver nanowires and nanoparticles, and (5) preparation, morphological, and microwave absorbing properties of spongy iron powders/epoxy composites. The first part of this research aims to fabricate the AgNws by the design of continuously feeding. The optimum reaction temperature is around 150~160℃ to obtain higher yield of AgNws. The yield in the product tank is over 95%. The dynamics of process had been analysed. Products of AgNws were used to fabricate electrical and thermal conductive composites. In the second part of this research, silane modification was successfully applied to prepare the AgNws/epoxy-resin (hydrophobic) composites by in-situ and ex-situ methods. FT-IR results reveal that both interfaces between AgNws and epoxy of in-situ and ex-situ methods were successfully modified by 3-Aminopropyltriethoxysilane (APTES). From the electrical property tests which show that the AgNws/epoxy film prepared by ex-situ process (4.5 x 10-1 Ω/sq) possesses lower surface electrical resistance than that of the in-situ process (7.4 x 101 Ω/sq) for both contain 100phr of AgNws due to higher viscosity of epoxy resin in the curing process. Moreover, AgNws blended with hydrophilic resin, such as polyvinyl alcohol (PVA) and Arcrylic resins were prepared and compared with AgNws/epoxy-resin. Results show that AgNws/PVA-resin conductive film possesses the lowest surface electrical resistance (4.0 x 10-3 Ω/sq with 300 phr AgNws) due to good compatibility and adhesive force between AgNws and PVA. In the third part of this study, high electrical conductive films were prepared by blending AgNws with hydrophilic and hydrophobic resins. APTES silane was utilized to modify the surface of AgNws. The electrical properties and weathering resistance of AgNws based conductive films were compared with those of silver nanoparticles (AgNps) conductive films. The loading ratio of AgNws to AgNps blended with PVA was 50phr：400phr, when the volume electrical resistivities was around 1.2 x 10-2 Ω-cm. The nano-silver loading in the conductive film could be reduced significantly by adding AgNws due to the high aspect ratio of AgNws. Furthermore, morphological observation and weathering test comfirmed that the use of AgNws could increase the ratio of the binder, in comparison with the use of AgNps; therefore, the conductive film was more stable. With the increasing of the AgNws loading, the shielding effectiveness (SE) of electromagnetic interference (EMI) of the AgNws conductive film was increased. The minimum silver contents are 100phr for AgNws and 400phr for AgNps respectively, which aimed at achieving the SE under -20dB (>99%) for all ranges from 3~17 GHz frequency. In the fourth part of this research, high thermal conductivity films were prepared by blending AgNws with epoxy resins. APTES was used to modify the AgNws surface and to prepare a high aspect ratio and high thermal conductivity nano-material. A thermal interface material with a high thermal conductivity coefficient was used to form a thermal conductive channel in epoxy resins, which possesses superior thermal conductivity when the composite contained AgNws was higher than 50 phr (7.99 W/(m．k) for 50 phr, 29.17 W/(m．k) for 100 phr ). The loading ratio of AgNws to AgNps was 50phr:300phr when the thermal conductivity coefficient of the composite reached approximately 8 W/(m．k). The composite density ratio containing AgNws (50 phr) to AgNps (300 phr) was 1.512:3.650 (g/cm3), which suggests the density can be reduced by more than 50% in weight when AgNws, rather than AgNps, were used to prepare thermal conductive composites. Furthermore, the adhesion test indicates that the composite containing a lower loading (50 phr) of AgNws exhibits higher adhesive property than that of the composite containing a higher loading (300 phr) of AgNps with a similar thermal conductivity coefficient, resulting in enhanced adhesiveness between devices. In the fifth part of this study, the spongy iron powders (SIPs) which possess high specific area (1.57 m2/g) were successfully prepared and used for microwave absorbent. The specific area was about two times higher than that of commercial available carbonyl iron powders (CIPs) (0.73 m2/g). The SIPs were incorporated with epoxy resin as the microwave absorbing composites. The compositions and return loss (R.L.) of the composites made of the CIPs and SIPs were compared at S, C, X and Ku bands. Results showed that the SIPs/epoxy composites are lighter and thinner than those of the CIPs /epoxy composites. The comparing weight of per unit area at the four bands were 8.0, 5.0, 2.7, 2.0 kg/m2 (for SIPs/epoxy) to 15.3, 7.9, 5.0, 3.3 kg/m2 (for CIPs/epoxy), respectively. The thicknesses of SIPs/epoxy and CIPs/epoxy at the four bands were 4.4, 2.5, 1.5, 1.1 mm to 6.2, 3.0, 2.0, 1.2 mm, respectively. Compared with the microwave absorbing performance, the maximum return loss (R.L.) for the composites blended with the SIPs (14.5, 23.8, 15.9, 16.4 dB) were still larger than the composites blended with the CIPs (14.2, 11.9, 15.6, 15.1 dB), respectively. The microwave absorbing composites made of the SIPs were lighter, thinner and better microwave absorbing than those of composite with the CIPs.