Preparation And Properties Of Superfine Spherical Silver And UV Curable Polyurethane-based Conductive Silver Paste

As the important material in electronic industry, conductive silver paste has been widely used in printed circuit technology, semiconductor packaging, solar cell and other fields. However, due to the shortcomings of conductive silver paste including expensive cost, the migration of silver powder and pollution of the conductive paste, it has seriously restricted the application of conductive paste. Therefore, it’s necessary to improve the comprehensive performance and lower the cost of conductive paste. In this dissertation, superfine spherical silver was prepared by liquid phase reduction method. UV curable polyurethane-based conductive silver paste with high conductivity, fast curing, good adhesion and excellent flexibility was prepared by using the compound of superfine spherical silver and flake silver as conductive phase, bifunctional vinyl-terminated polyurethane acrylate prepolymer(PUA) and 1,6-hexanediol diacrylate(HDDA) as adhesive phase. The main research contents and results were as follows:Firstly, superfine spherical silver was prepared by liquid phase reduction method with ascorbic acid as reducing agent, silver nitrate(Ag NO3) as precursor, polyvinylpyrrolidone(PVP) and cetyltrimethyl ammonium bromide(CTAB) as dispersants and Ag seed as growth centers. The effects of the mole ratio of ascorbic acid to silver nitrate, PVP and CTAB content, the p H value, reaction temperature, Ag seed content, stirring rate and feedding time of Ag NO3 on the diameter of superfine spherical silver were investigated. It was found that the diameter of superfine spherical silver could been greatly decreased when the compound of PVP and CTAB were used as dispersants. Increasing the p H value would decrease the amount of byproduct. Moreover, adding suitable amount of Ag seed could also effectively decrease the diameter of superfine spherical silver because of the increase of growth centers of silver. When the concentration of silver nitrate was 1.0mol/L, the mole ratio of ascorbic acid to silver nitrate was 1.0, the mass ratio of PVP to silver nitrate was 1.0, the mass ratio of CTAB to PVP was 0.30, the p H value was 12.5, the reaction temperature was 30℃, the stirring rate was 300r/min, the feeding time of silver nitrate was 10 min and the volume fraction of Ag seed liquid was 16%, superfine spherical silver with the diameter of 130 nm was prepared.Secondly, sodium dodecyl sulfate(SDS), CTAB, PVP and sodium hexametaphoshpate were used to modify superfine spherical silver. Laser particle size analyser and ultraviolet spectrophotometer were employed to evaluate the stability of the dispersion. It was found that the stability of the dispersion could been improved after superfine spherical silver was modified by SDS. When the concentration of SDS was 0.19wt%. The maximum change of diameter of superfine spherical silver decreased from 35 nm to 17 nm during 22 h, and the maximum change of light transmittance of superfine spherical silver decreased from 14.1% to 4.6%. What’s more, the absolute value of Zeta potential was above 30 m V. It suggested that stable dispersion of superfine spherical silver was prepared in this condition.Thirdly, UV curable polyurethane-based conductive silver paste was prepared by using the compound of superfine spherical silver and flake silver as conductive phase, PUA and HDDA as adhesive phase. The effects of types of hard segments of PUA, molecular weight of soft segments, types and content of reactive diluents, superfine spherical silver and flake silver content on the properties of silver paste were investigated. The results showed that the conductivity and the bending resistance of the cured paste were improved by using the suitable compound of superfine spherical silver and flake silver. Improving the polarity of PUA could increase the conductivity of the cured paste, but it would worsen the bending resistance of the paste. When superfine spherical silver content was 6.6wt%, the flake silver content was 58.4wt%, the HDDA content was 17.0wt% and the PUA content was 17.0wt%, the conductivity of the paste was up to 1.33╳106S/m. Meanwhile, the cured paste had a high adhesion to polyethylene terephthalate and the level of cross-hatch test was 5B. Moreover, the cured paste exhibited an excellent bending resistance, which the conductivity was still 3.38╳105S/m after 10 times bending test.Fourthly, propanedioic acid, adipic acid, sebacic acid and oleic acid were used to remove the layer of organic adsorption coated on flake silver. The removal efficiency and mechanism of the layer of organic adsorption on flake silver were investigated by thermogravimetry, differential scanning calorimetry, laser raman spectroscopy and scanning electron microscopy. The results demonstrated that the conductivity of flake silver increased when propanedioic acid was used. The adsorption decreased from 1.12wt% to 0.23wt%. The conductivity of the paste reached 8.62╳105S/m when the flake silver with propanedioic acid treated was as the conductive phase.