Aerosol dynamics of palladium particle formation by spray pyrolysis

J. Aerosol Sci., Vol. 26. Suppl 1, pp. $833-$834, 1995 Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00

Pergamon

AEROSOL DYNAMICS OF PALLADIUM PARTICLE F O R M A T I O N BY SPRAY PYROLYSIS A. S. Gurav 1, T. T. Kodas 1, M. J. Hampden-Smith2, P. Ahonen3 and E. I. Kauppinen3 1Depai~ta-tents of Chemical Engineering and 2Chemistry, University of New Mexico, Albuquerque, NM 87131, U. S. A. 3Aerosol Technology Group, VTT Chemical Technology, P. O. Box 1401 FIN-02044 VTT, Finland

KEYWORDS spray pyrolysis, palladium metal, particle size distributions INTRODUCTION A wide variety of industries including electronic, medical, chemical and automotive have applications for palladium metal powders. Substantial demand comes from the electronic industry which uses palladium thick films as contacts and electrodes in multilayer capacitors and other components. For thick film applications, the ideal Pd particles should be phase-pure, submicron-sized (< 0.3 ~tm), monodisperse, dense, spherical, unagglomerated, and must have excellent oxidation resistance. Spray pyrolysis has been demonstrated as a useful technique, producing much better powders compared to the conventional liquidphase precipitation methods, for the synthesis of Pd powders (Pluym et al., 1993). Considering the stringent requirements on powder characteristics for applications in the electronics industry, there is a considerable need to study the aerosol dynamics during spray pyrolysis of materials such as palladium. METHODS Gas-phase particle size distributions were determined during spray pyrolysis of palladium particles over a temperature range of 900 to 1150 °C. The experimental set-up is shown schematically in Fig. 1. An aqueous nitrate solution of Pd(NO3)2 was atomized by an ultrasonic generator using nitrogen as a carrier gas. The droplets were carried through a hot-wall reactor in which solvent evaporation and precursor precipitation occurred followed by the decomposition of Pd(NO3)2 to PdO and finally to Pd. The aerosol was diluted with clean, dry air near the reactor exit to prevent condensation of H20 or HNO3. The aerosol dynamics was investigated using a Berner-type low-pressure

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A. S. GURAVet al.

impactor, electrical low-pressure impactor, and a scanning differential mobility analyzer with an ultrafine condensation particle counter. The data from these instruments were combined with information about the morphology, primary particle size and phase composition of the particles obtained by scanning and transmission electron microscopies, electron diffraction, energy dispersive spectroscopy and X-ray diffraction. The results indicate that at high reactor temperatures (1000 °C and above), small particles (modes at ~ 40 and 70 nm, respectively, at 1000 and 1150 °(2) are formed, probably due to particle explosions during drying in the reactor. Results of particle size distributions and characterization of the size classified samples provide useful strategies for better control over the powder characteristics. ACKNOWLEDGEMENT We thank NSF PYI No. CTS-9058538 for support. Ultrasonic Gas r------n ~

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Figure 1. Experimental set-up for studying the aerosol dynamics during spray pyrolysis of palladium metal

Pluym, T. C., S. W. Lyons, Q. H. PoweU, A. S. Gurav, T. T. Kodas, L. M. Wang and H. D. Glicksman (1993), M a t e r . R e s . B u l l . , Vol. 28, 369-376.