Journal of Nanoparticle Research

, 11:2043

Uncapped silver nanoparticles synthesized by DC arc thermal plasma technique for conductor paste formulation

Authors

  • Manish Shinde
    • Centre for Materials for Electronics Technology (C-MET)
  • Amol Pawar
    • Centre for Materials for Electronics Technology (C-MET)
  • Soumen Karmakar
    • Department of PhysicsPune University
  • Tanay Seth
    • Centre for Materials for Electronics Technology (C-MET)
  • Varsha Raut
    • Centre for Materials for Electronics Technology (C-MET)
  • Sunit Rane
    • Centre for Materials for Electronics Technology (C-MET)
  • Sudha Bhoraskar
    • Department of PhysicsPune University
    • Centre for Materials for Electronics Technology (C-MET)
Research Paper

DOI: 10.1007/s11051-008-9569-7

Cite this article as:
Shinde, M., Pawar, A., Karmakar, S. et al. J Nanopart Res (2009) 11: 2043. doi:10.1007/s11051-008-9569-7

Abstract

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.

Keywords

SilverNanoparticlesPlasma synthesisTEMConductivitySEMThermal plasma reactorThin film

Introduction

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.

Experimental details

Uncapped silver nanopowder was synthesized using DC Arc Thermal Plasma reactor which consists of a tungsten cathode mounted inside a stainless steel reactor chamber, a DC power supply, a graphite crucible for housing metallic silver block, vacuum system, gas feed, and a closed loop water cooling system. Metal precursor, a commercial silver block (~25 × 25 × 5 mm and 99.99% pure), was placed in a graphite crucible, which acts as an anode (Fig. 1). Arc was generated using the mixture of argon and helium as plasma gases with 2:3 ratio maintained at 500 torr pressure and at 150 A arc current for obtaining the silver nanopowder. For structural determination, powder X-ray diffractograms were recorded with Rigaku Miniflex X-Ray Diffractometer using Ni filtered CuKα radiation. Fine-scale microstructure examination was accomplished by Transmission Electron Microscopy (TEM) using JEOL-2100X TEM instrument.
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Fig. 1

Schematic of DC Arc Thermal Plasma (DATP) reactor

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

X-Ray diffractogram of silver powder (Fig. 2) reveals formation of cubic (fcc) silver (JCPDS # 04-0783). Diffraction peaks corresponding to silver oxide are not detected. The crystallite size calculated by using Scherer formula is estimated to be ~25 nm.
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Fig. 2

X-ray diffractogram of silver powder synthesized by DATP technique

The TEM image (Fig. 3) indicates that as synthesized nanoparticles are non-agglomerated, mostly spherical in shape (showing faceted growth in some particles) and have wide polydispersity with size variation between 20 nm and 150 nm. This variation in particle size can be advantageous in thick films technology since smaller particles can serve as ‘neck joints’ for larger particles, which can lead to reduced cavity formation and, in turn, better electrical conductivity.
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Fig. 3

TEM image of silver powder synthesized by DATP technique

SEM photomicrograph depicting the microstructure corresponding to unfired film and its cross-section are presented in Fig. 4a, b, respectively. SEM photomicrographs of the thick films fired at 550, 600, 700, and 850 °C are shown in Fig. 5a–d and their corresponding cross-sections are shown in Fig. 6a–d, respectively. The salient aspects of SEM analysis for the above five types of thick films corresponding to (i) Fig. 4a, b; (ii) Fig. 5a–d; and (iii) Fig. 6a–d, respectively, are as follows:
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9569-7/MediaObjects/11051_2008_9569_Fig4_HTML.jpg
Fig. 4

SEM images of unfired thick films of silver on alumina substrate a top view, b cross-section

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Fig. 5

SEM images of thick films of silver on alumina substrate fired at a 550 °C, b 600 °C, c 700 °C, and d 850 °C

https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9569-7/MediaObjects/11051_2008_9569_Fig6_HTML.jpg
Fig. 6

SEM images of cross-section of thick films of silver on alumina substrate fired at a 550 °C, b 600 °C, c 700 °C, and d 850 °C

  1. (i)

    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.

     
  2. (ii)

    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).

     
  3. (iii)

    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).

     
Thus, our investigations reveal that use of the metal nanoparticles while formulating thick film conductor pastes leads to the reduction in peak firing temperature of the resultant thick films, which in turn can assist in circumventing the problem of silver penetration into alumina substrate. This reduction in processing temperature is probably ascribable to self-sintering mechanism in nanoparticles arising due to increased surface energy and internal strain. However, a more detailed experimental work and study is required to further reduce this processing temperature. It is felt that this can be achieved by controlling the powder preparation conditions. From the plot of sheet resistivity versus firing temperature (Fig. 7), the sheet resistance was found to be minimum at firing temperature of 600 °C. It increases gradually with increase in firing temperature. The sheet resistance is higher in case of film fired at 550 °C, which is expected due to insufficient sintering of silver particles. Higher value of sheet resistivity for the thick film fired at 550 °C as compared to film fired at 600 °C may also be due to presence of unburnt polymer moieties in the film, while the increase in resistivity for films fired above 600 °C is attributed to increase in cavity formation.
https://static-content.springer.com/image/art%3A10.1007%2Fs11051-008-9569-7/MediaObjects/11051_2008_9569_Fig7_HTML.gif
Fig. 7

Variation in sheet resistance with firing temperature of DATP-silver-based conductor thick films

Conclusions

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.

Acknowledgments

Generous funding by Department of Information Technology (DIT), India under the nanotechnology initiative is acknowledged.

Copyright information

© Springer Science+Business Media B.V. 2008