Pyrolysis of poly-methyl methacrylate (PMMA) binder in thermoelectric green tapes made by the tape casting method

doi:10.1016/S0955-2219(99)00169-7

Journal of the European Ceramic Society

Volume 20, Issue 3, March 2000, Pages 335-345

https://doi.org/10.1016/S0955-2219(99)00169-7 Get rights and content

Abstract

The pyrolysis of poly-methyl methacrylate (PMMA) binder in thermoelectric green tapes, are analysed through differential thermal analysis (DTA), thermogravimetric analysis (TGA). These analyses confirmed that the main mechanism of the PMMA binder decomposition is the depolymerization or the unzipping mechanism. The best pyrolysis procedure was found to be in air to a maximum temperature of 410°C with 1.30–2 h hold out time. Some small amount of binder residue was found to remain in tapes after the pyrolysis procedure. This was suspected to be due to an interaction between the thermoelectric powder and the PMMA binder. Despite this small residue, the thermoelectric tape was sintered successfully to almost 95% of its theoretical density, in vacuum at 1200°C after 4 h.

Introduction

The tape casting process has been investigated as a new low cost fabrication process for thermoelectric generator. The process involve the production of flexible thermoelectric tapes by the addition of polymer binder, plasticizer and dispersant to the thermoelectric powder in a solvent mixture.

The role of these organic additives are temporary with a limited function of imparting strength and flexibility in green tapes during the process of cutting, shaping and lamination of these tapes. Once the tapes are laminated, the organic additives must be completely removed from the tapes before densification can reach an advanced stage, otherwise, residues may be retained within the tapes and alter the desired properties of the thermoelectric materials. The removal of the organic additives is generally called the pyrolysis process or organic burnout, and consists of thermal decomposition and evaporation of the organic additives and the subsequent removal of the volatile compounds from the thermoelectric tape.

In a previous paper,¹ the pyrolysis of the polyvinyl butyral (PVB) binder system formulation for the thermoelectric tapes was investigated. It was concluded that the PVB system was not a suitable binder candidate to be used in the fabrication process, due to the inability to remove all the its residues from the tape. These residues were identified as a unsaturated hydrocarbone, and was in the form of a very fine black powder (soot), which covered the entire thermoelectric material, preventing its densification during sintering. To tackle the problem, an alternative polymer binder system a poly-methyl methacrylate (PMMA) was used due to its low burnout temperature and its low char content.

Section snippets

Experimental procedure

Several cast thermoelectric green tapes with Iron Disilicide powders were made with PMMA formulation in order to optimise the green tape quality, the details of which are published in Ref. 2. The Iron disilicide powder samples were prepared by the mechanical alloying technique.³ The average particle size of the powder was 3.15 μm, the specific surface area was 1.5633 m²/cm³ and of irregular shape. Table 1 shows the general slurry composition with PMMA binder.

The PMMA binders were of two types

Results and discussion

While the PVB binder is commonly used in the tape casting process, because of its excellent green tape characteristic,1, 2 the PMMA binder was used in this project for its low burnout temperature. The PMMA binders in general leave very small amounts of char after burnout.

The B-67 and the B-7MEK binders have been optimized for low char burnout by altering their chemical structures.⁴ These changes are a company proprietary, but it is thought that these changes involve the addition of oxidative

Conclusions

Acknowledgments

This work was performed as a part of a postdoctoral research contract with British Gas plc in 1994–1996. The authors acknowledge the help of Mr. David Lander from British Gas with the thermogravimetric analysis.

References (8)

Salam, L., Matthews, R.D. and Robertson, H. Pyrolysis of polyvinyl butyral (PVB) binder in thermoelectric green tapes...
Salam, L., Matthews, R.D. and Robertson, H. Optimisation of thermoelectric green tape characteristics. J. of Material...
Salam, L., Matthews, R.D. and Robertson, H. Formation of β-FeSi2 by planetary ball milling: — optimisation of milling...
Private communication with Dr. D. Whiteman, Rhom & Hass Company, USA,...

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