Preparation and rheological behavior of lead-free graphite conducting paste

A lead-free graphite conducting paste was prepared by using graphite powder, lead-free barium boron low-melting glass and ethyl cellulose solution. By analyzing the sheet resistance, Vickers hardness as well as the adhesion strength of the fired film prepared by the pastes formulated with different composition, the composition of the paste with preferable properties was evaluated in weight percent: organic vehicle 60, graphite powder 15–20, and low-melting glass 20–25. The films, prepared by the pastes using the lead-free glass with a glass transition temperature of 476°C, were perfectly flat and compact after fired at a peak temperature within the range of 540 to 590°C. The sheet resistance of the fired film with glass content of 25 wt% was 135 Ω/mm² at the film thickness of 25 ± 3 μm, while the Vickers hardness was 12.3 N/mm² and the adhesion strength to soda lime silicate glass substrate was 45.6 N/mm². In addition, the rheological, thixotropic, and viscoelasticity behaviors of the typical paste were characterized and the effects on the screen-printing ability were analyzed in detail. It is found that a slight flocculated network structure is formed in the paste at rest, but the flocculated structure is very slight due to the steric effect of the ethyl cellulose macromolecules and good dispersing. The rheological, thixotropic, and viscoelasticity behaviors of the typical paste were similar to those of the screen-printing pastes with a high solid content of filler.