Development of Conductive Carbon Coated Copper Nanoparticle Inkjet Fluid

An aqueous inkjettable conductive fluid based on carbon coated copper nanoparticles has been developed. The fluid can be handled in atmospheric conditions and processed at low temperature (105 °C) with no thermal annealing. A layer conductivity exceeding 600 S/cm has been demonstrated. The particles were produced in a continuous flow reactor from copper chloride powder by hydrogen reduction at high temperature (950 °C). Results indicate that conductivity is enhanced through the formation of carbon nanotubes by addition of ethene and water to the reaction flow. The type and concentration of dispersing additive and co-solvents had a significant impact on dispersion stability and electrical conductivity of the deposited layer. Applicability of the fluid for direct patterning of coatings for e.g. antistatic purposes was demonstrated by inkjet printing of a conductor electrode pattern. Introduction There is currently a strong drive towards the development of low cost conductive metallic nanoparticle based fluids for printed electrically functional devices. Significant interest has lately been directed towards replacing silver with copper due to its relatively low price and high conductivity. A major challenge in the development of copper based fluids is the tendency of copper to spontaneously oxidize in ambient conditions. Several ways of protecting the copper nanoparticles from oxidation have been proposed [1]. For example, graphitic coatings have been used to stabilize metallic nanoparticles and to minimize their size in conductive and magnetic fluids [2, 3]. Conductivity levels in the order of 1 S/cm have been demonstrated in [2] for inkjet printed layers of a fluid based on graphene coated copper nanoparticles. Metallic nanoparticles having a relatively high specific gravity (compared to e.g. carbon black) are challenging to disperse in a fluid base, and typically require a polymeric stabilizing ligand which will hinder inter-particle contact and hence conductivity unless removed in a subsequent sintering step. This paper presents an alternative approach for synthesis of copper nanoparticles with a graphitic layer providing stability towards oxidation and carbon nanotubes that enhance the conductivity of the dried fluid layer. The effect of dispersing additive and co-solvent composition on dispersion stability and conductivity of deposited layers is investigated. Materials & methods Particle production Carbon coated copper nanoparticles were produced by a gas phase synthesis technique developed previously for production of metallic nanoparticles [4]. Copper chloride precursor powder (Sigma-Aldrich) was fed using a powder feeder on alumina (Al2O3) pellet bed within a quartz glass nanoparticle reactor. The bed was made of 8 g of porous aluminium oxide pellets with 3mm diameter (Sigma-Aldrich). From the pellet bed the precursor was evaporated at 800°C into a nitrogen flow. The gas flow carried copper chloride vapour into the reaction zone, where reaction with hydrogen (13.8 vol-%) at 950°C produced copper nanoparticles and hydrogen chloride. In order to prevent oxidation and sintering of particles during handling, transport and storage, copper particles were coated during their synthesis with a graphitic carbon layer. In these experiments ethene served as precursor for the coating. In the reaction zone 0.9 vol-% of C2H4 was mixed to the gas flow together with hydrogen. In order to promote the formation of carbon nanotubes (CNT) some water vapor was fed into the reaction zone as well. The actual formation of CNTs depended on the copper chloride to ethene ratio fed into the reactor. The flow coming out of the reactor was diluted and cooled with nitrogen to prevent further agglomeration and sintering of particles. Produced powder was then collected in a PTFE filter bag (GORE®). Hydrogen chloride was removed downstream the filter from the exhaust flow using two tanks filled with NaOHwater solution before the flow was directed to a fume hood. Fourier transform infrared spectroscopy (FTIR, Gasmet Dx4000) was applied to measure the concentrations of HCl in order to monitor the particle production rate. FTIR also measured the concentration of water vapor, gas impurities, CO, CO2 as well as the gaseous degradation products of ethene. The results from FTIR measurements agreed very well with the amount of powder collected from the filters. According to FTIR data approximately 90-95 % of copper produced in the reactor could be retrieved from the particle filters. Fluid preparation The dispersing capability of the nanoparticles was evaluated using three dispersing additives suitable for water-based carbon black pigment dispersions which were kindly provided by BYK Chemie Gmbh: DISPERBYK-190 (Solution of a high molecular weight block copolymer with pigment affinic groups), DISPERBYK-198 (Solution of a copolymer with basic pigment 458 ©2011 Society for Imaging Science and Technology affinic groups) and DISPERBYK -2012, (Solution of a structured acrylate copolymer with pigment affinic groups) hereafter denoted (1), (2) and (3), respectively. Ethylene glycol monomethyl ether (EGME) (Sigma-Aldrich), ethylene glycol monobutyl ether (EGBE) (Merck) and n-propanol (Honeywell Riedel-de Haen) were used as co-solvents for improving the fluid jetting and layer forming properties. The viscosity (at 20 °C), surface tension and boiling point of EGME, EGBE and npropanol are 2.0 mPas/33 mNm/125 °C, 6.4 mPas/27 mNm /171 °C and 2.2 mPas/ 24 mNm/97 °C, respectively. The nanoparticles were mixed with the dispersing additives, co-solvents and de-ionized water (DIW), and the suspension was sonicated for 10 minutes in an ice bath at an intensity of 30% (1.0 cycle) using a UP400S ultrasonic processor (Hielscher Ultrasonics). Thereafter, the suspension was left to settle for an hour followed by decanting. Prior to spin coating or printing the suspension was sonicated in an ultrasonic bath for 10 minutes. Layer deposition Inkjet printing of the conductive carbon coated copper fluid was carried out with the Dimatix DMP-2831 laboratory scale piezoelectric drop-on-demand printer (Fujifilm Dimatix). The printer utilizes printhead cartridges with 16 nozzles arranged linearly at a pitch of 254 μm. Cartridges generating a nominal drop volume of 10 pL were used. Printing was performed at a jetting frequency of 1 kHz. Printing of solid fill layers for electrical characterization as well as electrode patterns for demonstration was carried out at 20 μm inter-drop spacing (1270 dpi). Spin coating of the nanoparticle dispersions was carried out using an EC101D series spin coater (Headway Research) at 1000 rpm for 1 minute. Layers were deposited on microscopic glass slides (Thermo Scientific) and heat stabilized Teonex Q65FA polyethylene naphthalate (PEN) film (Teijin DuPont Films) with a thickness of 125 μm. Both substrates were cleaned by immersion in acetone under ultrasonication for 10 minutes followed by immersion in isopropanol under ultrasonication for 10 minutes and drying under nitrogen flow. A single layer for both inkjet printing and spin coating was applied. The spin coated and printed layers were immediately dried on a hot-plate for 60-90 seconds at 60 °C and then transferred to an oven for 1 hour at 105 °C. For electrical characterization, gold electrodes having a layer thickness of 100 nm, channel length and width of 800 μm and 10000 μm, respectively, were grown by thermal vacuum evaporation on the deposited copper-carbon layer. After oven drying, and prior to electrical characterization, samples were stored in a nitrogen glove box. Characterization methods Particle size distribution and structure of the dry particle powder was studied using transmission electron microscopy (TEM, Philips CM200 FEG/STEM). In addition, the specific surface area (SSA) of powder samples was measured using BET (ASAP2020, Micromeritics Instruments). The elemental composition of the particles was analyzed using x-ray fluorescence (XRF, Philips PW2404 X-ray spectrometer) using semi-quantitative SemiQ-program. The weight changes in materials were measured by thermo-gravimetric analysis (TGA, Mettler TGA 851e). Fluid viscosity was measured using an mVROC viscometer (RheoSense). Fluid surface tension was measured using an EZ-Pi tensiometer (Kibron). Dispersion stability was evaluated qualitatively by visual inspection at intervals of 1 day and 4-5 days. The topography of the dried spin coated and printed layers was characterized by optical microscopy (BX60, Olympus), stylus profilometry (Dektak 150, Bruker AXS) and SEM imaging (LEO DSM 982 FE-SEM). Electrical characterization of the dried copper-carbon layers was performed by 2-point probe resistance measurements from the thermal vacuum evaporated gold electrodes. Layer conductivity (σ) was calculated using measured values for resistance (R), electrode gap length (L) and width (W) and layer thickness (d).