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 a nalysis (DTA), thermogravimetric analysis (TGA) and published results of fo urier transformer infrared spectroscopy (FTIR). Based on these analyses the optimum balance of binder degradation mechanism, heating rate, burnout tem perature and burnout atmosphere were determined. The maximum upper temperat ure at which pyrolysis can take place in an oxidising atmosphere, was impos ed at 450 degrees C, to avoid the risk of oxidising the thermoelectric mate rial above this temperature, which could degrade its thermoelectric propert ies. Thermoelectric cast green tapes made with PVB formulation were found t o leave char residue after pyrolysis at 450 degrees C, estimated to be almo st 20% of the total PVB content in the tape. Different pyrolysis atmosphere s 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 degrees C with a hold-out time of 5 h, which reduced the char residue t o only 1%. Even with this small percentage, the char residue was in the for m of a very fine black powder (soot) which covered the thermoelectric mater ial 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 thermoelec tric generator by the tape casting method. (C) 2000 Elsevier Science Ltd. A ll rights reserved.