THIN GLASS-FILM BETWEEN ULTRAFINE CONDUCTOR PARTICLES IN THICK-FILM RESISTORS

Thick-film resistors are electrical composites containing ultrafine pa rticles of ruthenate conductor (Pb2RU2O7 in the present materials) dis tributed in a highly modified silicate glass. We show that conductor p articles remain flocced in the absence of any applied or capillary pre ssures, but are separated at equilibrium by a nanometer-thick film of glass. Microstructures show evidence for liquid-phase sintering, i.e., contact flattening of particles, under van der Waals attraction alone . Titania addition, which in dilute concentrations markedly increases the resistivity, decreases the temperature coefficient of resistance, and improves voltage stability and noise, is found to increase the equ ilibrium film thickness between particles by a few angstroms. STEM ana lyses show that the added titania preferentially concentrates in the s ilicate-rich grain boundary film, as well as at particle-glass interfa ces. The roles of interparticle forces and adsorption on the glass fil m thickness with and without titania are discussed. The large increase in resistivity caused by titania additions is attributed to the incre ase in film thickness as well as to local chemical changes of two poss ible types. Titania enrichment within the glass film itself is expecte d to decrease the local ruthenium ion solubility, and this along with the possible formation of a more insulating titania-substituted surfac e layer on ruthenate grains will decrease the tunneling conductivity b etween conductor grains.