Uncapped silver nanoparticles synthesized by DC arc thermal plasma technique for conductor paste formulation
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- Shinde, M., Pawar, A., Karmakar, S. et al. J Nanopart Res (2009) 11: 2043. doi:10.1007/s11051-008-9569-7
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Uncapped silver nanoparticles were synthesized by DC arc thermal plasma technique. The synthesized nanoparticles were structurally cubic and showed wide particle size variation (between 20–150 nm). Thick film paste formulated from such uncapped silver nanoparticles was screen-printed on alumina substrates and the resultant ‘green’ films were fired at different firing temperatures. The films fired at 600 °C revealed better microstructure properties and also yielded the lowest value of sheet resistance in comparison to those corresponding to conventional peak firing temperature of 850 °C. Our findings directly support the role of silver nanoparticles in substantially depressing the operative peak firing temperature involved in traditional conductor thick films technology.
KeywordsSilverNanoparticlesPlasma synthesisTEMConductivitySEMThermal plasma reactorThin film
Silver is a versatile material, which has been proving its applicability in diversified fields like medical and healthcare (Li et al. 2006) due to its antimicrobial properties and in electronics (Wentworth et al. 1997) owing to its excellent conducting properties. Metallic silver has been used as functional material in classical (Savage 1976) as well as photoimageable (Umarji et al. 2005) thick film pastes, which, in turn, are used in various hybrid electronic circuits and devices. The thick film conductive paste mainly consist of (a) metal powder which provides the conductive phase, (b) glass frit which promotes the sintering of the metal powders during firing and enables to adhere the metal film to the substrate, and (c) the organic phase, which disperses the metal and binder in order to impart the desired rheological properties to the paste (Savage 1976). Being equally versatile and challenging, nanoparticles of silver, are being currently explored for applications in various fields, e.g., labels for chip-based DNA detection (Fritzsche and Taton 2003), in ‘embedded passive technology’ (Gonon and Aoudefel 2006), etc. There are reports on the effects of surfactants, and processing parameters on the properties of silver-based thick film (Rane et al. 2003a, b), but there is paucity of information about the effects of firing temperatures on the properties of such films (Rane et al. 2000). The effect of firing temperature could be advantageously prominent in case of silver nanoparticles as compared to bulk silver. This feat can be accomplished because of the reduction in firing/sintering temperature of nanoparticles of silver. In this direction, we ventured into the possibility of formulating the thick film paste using uncapped nano-silver powder synthesized by DC arc thermal plasma technique (DATP) and investigated the effect of different firing temperatures on the properties of resultant thick films. DC arc thermal plasma technique with distinguishing features, such as very high plasma temperature, spontaneous evaporation of the precursor material, and rapid quenching, was deliberately chosen owing to its capability of synthesizing large quantity of uncapped silver nanoparticles in a very short time span. The metallic precursor material, nanoparticles of which are to be synthesized itself acts as an anode and the plasma arc is directly impinged upon it, making it energy efficient process. Moreover, in electronics technology applications, where the purity is of profound importance, usage of uncapped silver has definite edge over the conventional means (i.e., using surfactants/capping agents/stabilizers) of controlling the agglomeration of nanoscale powder. We indeed could observe the significant downward trend in sheet resistance—firing temperature behavior for the thick films formulated with uncapped nano-silver powder synthesized by DC arc thermal plasma technique. The most preliminary account of this kind of hitherto unattempted work is furnished in this letter.
Thick film paste of synthesized silver powder was subsequently formulated by homogeneous blending of silver powder and lead borosilicate glass frit in the ratio 9:1 in an appropriate vehicle containing ethyl cellulose (temporary binder) and organic solvents. The paste thus formulated was screen-printed on five alumina substrates using a nylon mesh (size 250). The thick films were allowed to settle for 10 min; dried for 15 min and then fired in a conventional firing furnace (BTU TFF51-4-36N26GT) at different peak firing temperatures i.e., 550, 600, 700, and 850 °C for 10 min in a typical 60 min firing profile. The resultant thick films were investigated for morphology and thickness using SEM technique (Philips XL 30), while resistivity study was carried out using four probe technique.
Results and discussion
Unfired film: As expected, the silver particles appeared to be uniformly dispersed in the glassy matrix (Fig. 4a). The back-scattered SEM image of the cross-section of the unfired film (Fig. 4b) shows large number of pinholes.
Complete evolution of thick film paste can be observed from SEM images depicted in Fig. 5a–d with respect to grain growth. The increase in grain size with respect to firing temperature is noted. The fired films show almost uniform surface morphology with lateral grain growth of silver particles. However, formation of cavities in case of higher firing temperature is observed (Fig. 5c and d).
The comparative examination of cross-sectional SEM micrographs discloses the reduction in pores, when the films are fired at 550 °C (Fig. 6a). The compact microstructure with few cavities for the film fired at 600 °C (Fig. 6b) suggesting depression in sintering temperature of the silver particles is observed. The size of the cavities and penetration of silver into substrate tend to enhance with increase in firing temperature (Fig. 6c, d). The noticeable feature of the above images is that the penetration of silver into substrate begins to take place in case of film fired at 600 °C (Fig. 6b); however, the penetration of silver appears to be increased for thick film fired at 700 °C (Fig. 6c) and even more prominently enhanced for thick film fired at 850 °C (Fig. 6d).
Thick film conductor paste was formulated using silver nanopowder (size 20–150 nm) synthesized by hitherto unattempted DC arc thermal plasma technique. It was found that the resultant ‘green’ thick films fired at 600 °C showed better properties in terms of surface microstructure and sheet resistance than those of the films prepared at other firing temperatures viz., 550 °C, 700 °C, as also the conventional thick film peak firing temperature of 850 °C. It can be concluded that the nano-size silver particles are responsible for lowering the thick film processing temperature. It is opined that nanoparticles of silver can extend thick film paste technology to MEMS regime by aptly complementing with the downsizing trend in electronics devices.
Generous funding by Department of Information Technology (DIT), India under the nanotechnology initiative is acknowledged.