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A1. There is no mistake. Does the colour of the paste matter? It may matter, of course, depending upon the intended application for the platinum paste.

For example, a platinum paste (made using high surface area platinum black) is typically of white appearance if sintered at high temperature during firing in low percentage oxygen atmospheres. This paste will be black if fired in excess oxygen at temperatures below its melting point.

Ultimately the colour of the fired film will also depend on the particle size and surface area of the platinum powder used in the paste. Finely divided high surface area platinum blacks may appear black in colour even after firing. Low surface area platinum powders of grey colour will form grey to silver coloured films on firing.

In a ZrO₂-type oxygen sensor (typical materials in an oxygen sensor) the materials have to be porous enough to allow oxygen to cross the zirconia/Pt interface. Platinum black powder of high surface area is not porous enough for use in a sensor.

To make a platinum powder that is porous, start with a coarse Pt black powder, for instance Rustenburg sponge. This will give a coarse powder suitable for use on an oxygen sensor. The sponge should be ground into a paste with resin and solvent, in a pestle and mortar for example, to get a good paste mixture. This paste can then be applied by dipping or painting if a tubular oxygen sensor is to be made.

A2. In this application the platinum paste should be dried at 100°C for 30 minutes and then fired at typically 1300°C for 10 minutes at peak temperature.

Note: if the platinum powder is too coarse ( > 90 microns in particle size) it will not be suitable for use in oxygen sensors as it will form a very rough paste which will not sinter properly.

Answer posted January 2005