Preparation of fine Ni particles by the spray-pyrolysis technique and their film forming properties in the thick film me
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Fine Ni particles were directly prepared from Ni(NO3)2 and NiCl2 aqueous solutions by the spray-pyrolysis technique under an H 2 -N 2 atmosphere. Hollow Ni particles with rough surfaces were obtained at a low reaction temperature (700-900 °C). With increasing reaction temperatures from 900 °C to 1500 °C, the particles varied gradually to solid ones with a smooth surface, and the particles obtained at 1600 °C were perfectly spherical ones whose average size was —0.6 jam. From the point of view that aggregation free powders are considered to be good for thick film applications, Ni(NO3)2 was preferred to NiCl2 as a starting material. The perfectly spherical particles derived from Ni(NO3)2 gave a Ni film about 3 jitm in thickness having sheet resistance of 37.7 mO-square"1.
I. INTRODUCTION
In the manufacture of electronic devices such as hybrid integrated circuits and multilayer ceramic capacitors, the thick film method is an important technology used in the formation of conductive thick films from metal powders. Although precious metals (Au, Ag, Pt, Pd) have been used to make the conductive films, replacement of the costly precious metals by cheaper metals has been tried in recent years. For example, Ni is beginning to be used for forming internal electrodes of multilayer ceramic capacitors, replacing palladium.1 Metal powders in thick film technology have been mainly obtained through chemical precipitation from aqueous or organic solution of metal salts. But it is difficult to prepare Ni particles directly from solution; the reduction of precipitates to Ni powder is necessary, making the process troublesome and forming aggregates in the powder. In our previous work, spherical fine metal particles of Ag, Pd, Cu, and Ag-Pd alloy were obtained directly from their nitrate aqueous solutions by the spraypyrolysis technique.2"4 These powders had good and unique properties for thick film materials.4"7 In this study, the effects of spray-pyrolysis conditions on the properties of Ni particles produced are investigated, and the formation process of Ni particles is discussed. Furthermore, the film-forming properties of the particles are briefly investigated by a thick film process. II. EXPERIMENTAL PROCEDURE A. Spray-pyrolysis experiments
Ni(NO 3 ) 2 -6H 2 O and NiCl 2 -6H 2 O (Wako Pure Chemical Industries, Ltd., Osaka, Japan) were used for 2828
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 12, Dec 1990
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the study. The Ni salt was dissolved in distilled water at Ni2+ concentration of 1 mol • dm"3. Figure 1 illustrates the spray-pyrolysis reactor. All preparations were carried out at a pressure of 0.1 MPa. The solutions were atomized with an ultrasonic atomizer (exciting sound frequency: 1.7 MHz) and introduced into a reaction tube at the feed rate of 0.8-1.7 cm3 • min"1 with N2 (conveying gas) and H 2 . The ratio H 2 /N 2 was 0.2 in the main runs and varied from 0.03 to 0.2 in hydrogen flow rate effects. The reaction tube was heated using two electric furnaces. The product powder was co
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