Thin Film Substrates
Precious Metal are used extensively in such advanced material applications as electronic components, fuel cell electrodes, and high-performance catalysts. Fine precious metal powders are used in the electronics industry for the manufacture of fired conductors, multilayer ceramic capacitors, and conductive adhesives using thick-film technology. Multilayer ceramic capacitors are small electronic chips built up of alternating thin layers of nonconducting ceramic material sandwiched between an equal number of thin layers of the conducting electrode layer. The precious metal powders most commonly used are silver, gold, palladium, platinum, and combinations such as silver-palladium and gold-platinum.
Excellent conductivity, corrosion resistance, good solderability, and stability in air during firing of the metal ceramic films make precious metals the preferred electronic material for high-performance applications. The cost of using these more expensive materials is more than outweighed by their ease of firing, subsequent stability in air, and ease of attachment by soldering and welding. Precious metal pastes are used in a variety of sensors including systems for measuring oxygen, humidity, dew point, wind speed, flow rate, pressure, and temperature. These sensors make use of the catalytic characteristics of the metals as well as their moisture resistance, heat resistance, and conductivity. Gold, platinum, rhodium, and ruthenium are the principal precious metals used. The conductor pastes used in thick-film applications generally contain four components consisting of the metal powder, a glass powder included as a bonding agent for joining the metal to the substrate, an organic vehicle containing an organic resin that imparts the needed rheology to the paste for screen printing, and a solvent to control the viscosity and solids content.
The particle sizes for the metal powders range from 5 µm down to less than 0.1 µm. Uniform size distributions and spherical morphology are prerequisites for a good conductor paste. Precious metal particles with two-dimensional flake morphology are also used and are preferred for the manufacture of conductive adhesives and for polymer thick-film applications where the precious metal powders are not sintered. The majority of these fine precious metal powders are prepared by chemical precipitation from an appropriate aqueous salt solution. Homogeneous nucleation is the prerequisite for fine, smooth, and dispersed metal particles made by chemical precipitation. Any heterogeneous nucleation needs to be suppressed. In addition to controlling the nucleation mechanism, the prevention of aggregation or coagulation of the fine particles precipitating out of the solution is equally important. Van der Waals attraction forces are responsible for coagulation, while electrostatic repulsion forces arising from the electric double layers surrounding the particles act against coagulation. Without controlling the attraction forces, the particles will become flocculated, making it very difficult to control the size of the powders. Generally, changing the surface potential of the particles via adsorption of surfactants is the method used to prevent aggregation. By precipitating the metal powders directly from an organic phase, which can provide uniform and enhanced steric stabilization with no stabilization by electrostatic repulsion, finer and smoother particles are obtained.