Pyrolysis of polyvinyl butyral (PVB) binder in thermoelectric green tapes

The pyrolysis of polyvinyl butyral (PVB) binder and other organic additives in thermoelectric green tapes, are analysed through differential thermal analysis (DTA), thermogravimetric analysis (TGA) and published results of fourier transformer infrared spectroscopy (FTIR). Based on these analyses the optimum balance of binder degradation mechanism, heating rate, burnout temperature and burnout atmosphere were determined. The maximum upper temperature at which pyrolysis can take place in an oxidising atmosphere, was imposed at 450°C, to avoid the risk of oxidising the thermoelectric material above this temperature, which could degrade its thermoelectric properties. Thermoelectric cast green tapes made with PVB formulation were found to leave char residue after pyrolysis at 450°C, estimated to be almost 20% of the total PVB content in the tape. Different pyrolysis atmospheres of air, argon, CO2 and Ar/H2O were used to minimise the char content. The best pyrolysis for the PVB was obtained with the use of CO2 atmosphere at 450°C with a hold-out time of 5 h, which reduced the char residue to only 1%. Even with this small percentage, the char residue was in the form of a very fine black powder (soot) which covered the thermoelectric material powder in the tape, preventing its densification at the later stages of the sintering process. It was therefore concluded that the PVB system was not a suitable binder candidate to be used in the fabrication of thermoelectric generator by the tape casting method.