The present invention relates to printed electronics and, in particular, to multi-component nanoinks for direct write applications.
Additive manufacturing processes are generally associated with 3D printing of polymers and metals for prototyping and limited production of complex hardware. However, a suite of maskless digital printing technologies commonly referred to as direct write (DW) enable the printing of electronics systems and provide many advantages over gravure and sheet based techniques. Historically, printed circuit metallization has been accomplished using pure metal inks of either silver, gold, or copper particles dispersed in a solvent.
The present invention is directed to a nanoink comprising a mixture of at least two of copper, silver, and gold nanoparticles dispersed in an organic solvent. The ratio of copper to silver nanoparticles can be less than 0.2:1 by weight. The mixture can comprise less than 40 wt % nanoparticles. The size of the nanoparticles can be less than 100 nm and, preferably, between 5 nm and 30 nm. The mixture can further comprise ceramic nanoparticles, including oxides of Group 4 elements such as titania, zirconia, or hafnia nanoparticles. For example, the organic solvent can be xylene. The organic solvent can further comprise a co-solvent, such as terpineol or white spirits, to improve printability. The organic solvent can further comprise a surfactant, such as a hyperdispersant, to disperse the nanoparticles.
The invention is further directed to a method to print a nanoink, comprising directly writing a nanoink on a substrate, the nanoink comprising a mixture of at least two of copper, silver, and gold nanoparticles dispersed in a solvent. For example, directly writing can include aerosol jet printing, syringe printing, or inkjet printing. The method can further comprise sintering the printed nanoink to an electrically conductive state. For example, the sintering can include laser, photonic, pulsed, or flash lamp sintering, or bulk thermal sintering. The sintering temperature can be less than 500° C.
As an example of the invention, silver-copper and silver-copper-hafnium oxide nanoinks were examined. High purity single metal and ceramic nanoparticles were synthesized and subsequently used to formulate stable multi-component ink systems. Following an aerosol-jet deposition printing process, selective laser sintering was used to post process multi-component depositions to an electrically conductive state. Mixed silver-copper ink systems which were laser sintered exhibited both homogeneous and heterogeneous microstructures and were experimentally observed to be influenced by the copper-to-silver content of the printing fluids. To this end, phase field modeling was employed and constitutive equations established which were used to computationally verify experimental observations. Fundamental material properties such as wetting angle, particle surface energies, and thermal diffusivities dominate microstructure evolution. The simulations indicate that the addition of up to 0.2 parts of copper to 1.0 part of silver can improve DC resistance properties in printed ink systems. Modeling suggests this may be a function of porosity minimization supporting the evolution of a highly conductive percolative path.
The detailed description will refer to the following drawings, wherein like elements are referred to by like numbers.
The present invention is directed to single-solution, multi-component nanoinks for direct write processes. The mixing of metal, metal oxides (conducting or semi-conducting), and dielectric or insulating components to yield tunable microstructures by DW processes enables previously unobserved architectures in printed electronic networks. For example, the behavior of multi-component inks and particle suspensions leading to structural development can be viewed as phase separation between immiscible fluids, leading to complex structures (i.e., micelles, vesicles, lamellae) during aerosol-jet (AJ) deposition. The segregation of these components into complex structures is driven by mutual diffusion of immiscible components under total free energy gradients. Such complex structures are thought to result from non-uniformity of solvent evaporation rates, local variation in composition, post processing techniques, and ink rheological properties. These driving forces enable the development of complex structures similar to the complexity of diatom frustules on surfaces.
The following describes NP synthesis, particle dispersion and ink formulation, DW printing, characterization, and modeling for the multi-component nanoinks of the present invention. Solution precipitation and solvothermal synthesis of metal alkyls results in phase pure and monodispersed NPs suitable for custom ink formulation. Following material synthesis, metallic inks can be formulated by combining surfactants, NPs, and organic solvents. As deposited, NP ink systems are generally not electrically conductive and require post processing to impart functionality. A variety of mechanisms can be employed to sinter discrete nanoparticles to form conductive pathways and most notably include bulk thermal (hot plate or oven) and laser sintering.
The multi-component inks of the present invention comprise two or more of the coinage metals (i.e., Cu, Ag, Au). High quality Cu, Ag, and Au nanoparticles can be synthesized according to literature methods. See S. Bunge et al., Nano Letters 3, 901 (2003). In particular, both Ag0 and Au0 NPs were synthesized following literature solution precipitation (SPPT) processes. For SPPT methods, a sample is heated or added to solvent at high temperature to initiate a decomposition of the metalorganic or organometallic precursors. The growth processes follow a La Mer growth mechanism, wherein a supersaturated solution is obtained at high temperatures and the strain released by forming a nucleation shower. Growth of the subsequent nuclei follows an Oswald ripening mechanism.
For the Ag0 NPs, silver acetate was heated in a mixture of toluene and oleylamine (ON) at 120° C. to generate high quality NPs of <20 nm in size. Similarly, Au0 NPs were synthesized using auric acid (HAuCl4) in hexane and ON at reflux temperatures. These particles were also found to be <20 nm in size. Powder X-ray diffraction (PXRD) analyses indicated that these syntheses produced phase pure metallic species.
Likewise, Cu0 NPs were also synthesized following previous literature methods. See S. Bunge et al., Nano Letters 3, 901 (2003). The copper mesityl (Cu(Mes)) precursor is not commercially available, and was synthesized according to Eq. (1). The halide metathesis of the copper salt (CuCl) with the Grignard reagent ((Mes)MgBr) produced the metal alkyl Cu(Mes) in high yield, as an off-yellow powder when fully worked up.
Once isolated, the Cu(Mes) precursor was dissolved in octylamine (OA) and then injected into hexadecylamine (HDA) at 300° C., according to Eq. (2). This decomposes the Cu(Mes) and reduces the Cu(l) to Cu(0). It was found that the Cu(Mes) needed to be cleaned (washed with hexanes and extracted with toluene) to obtain the highest quality precursor. If not washed and extracted, a substantial amount of ‘organic’ contaminant can be present that complicates subsequent ink formulation. With ‘clean’ precursors, high quality Cu0 NPs were synthesized in two sizes; small (red, 10-15 nm; 5 min. growth time) and large (copper, 50-80 nm; >40 min. growth time). The color noted is representative of the size of the particles.
The multi-component ink can further comprise ceramic NPs. Therefore, the ink can be a high dielectric constant (k) material ink. High k materials are those that are capable of ‘holding’ a significant amount of charge. Some of the best of these materials come from the Group 4 family of compounds, such as zirconia (ZrO2), titania (TiO2), and hafnia (HfO2). For example, hafnia can be prepared from several commercially available precursors, including hafnium chloride, carboxylate, ethoxide, and a neo-pentoxide derivative. See T. J. Boyle et al., lnorg. Chem. 51, 12075 (2012). These hafnia NPs can be synthesized using a SPPT process that employs 1-methyl imidazole and water in a 4:1 v/v ratio. See Y. Lee et al., Nanotechnology 19 (2008). A reaction mixture of Melm:H2O and an appropriate hafnia precursor can be heated and refluxed for 1 hour. The precipitate can be subsequently removed by centrifugation, washed with hexanes, and air dried. PXRD patterns of hafnia NPs synthesized with this process showed that the ‘as generated’ material was amorphous as synthesized and extracted. However, crystalline material can be isolated when heated at to 800° C. Transmission electron microscope (TEM) images revealed random shaped particles that were polydispersed.
Silver and copper inks showed concentration dependent behavior towards aggregation. Initially, the Ag ink begins as a diluted state in a fine dispersion, but begins to aggregate rapidly. The Cu ink was present as >100 nm clusters, but dilution in xylene lead to greater aggregation and sedimentation. Adding terpineol did not lead to stabilization of either system, but it may slow the aggregation process and create a broader distribution of aggregate size.
While all NP systems evaluated were sensitive to dilution, the silver system showed ultrasonic agitation can affect the floc size and create a bimodal particle size distribution. A potential reason for the observed instability is a destabilization of the surfactant structure of the nanoparticle systems, leading to particle fusion. When additional solvent is added, there may be an equilibrium of surfactant molecules that is achieved, creating a less stable nanoparticle system. All inks exhibit the same behavior in the initial formulation, indicating more or less stability.
To improve the long-term stability and aging qualities of the copper ink to be used in the mixed phase metallic particle dispersions, hyperdispersants can be added to prevent particle agglomeration and precipitation from solution. Solsperse™ dispersing agents, commercially available from Lubrizol Corporation, are compatible with the copper ink chemistry. However, to maintain compatibility with the printing and laser processing techniques used during deposition, the dispersing agent must be removed at temperature below 500° C. Thermal gravimetric analysis (TGA) showed that Solsperse 9000 was well suited for ink formulation, as 85% of the dispersant's organic content can be removed below 400° C. with full elimination occurring just above 500° C. Therefore, a 20 weight percent copper nanoparticle ink was formulated using a mixture of xylene, white spirits, and Solsperse 9000. The addition of Solsperse 9000 resulted in well dispersed copper ink solutions that resisted agglomeration and maintained a uniform distribution required for subsequent printing. Dynamic light scattering (DLS) also showed a strong ability of the dispersing agent to resist bimodal particle distribution evolution in copper inks which is thought to be beneficial during the AJ printing and processing phases.
Eleven mixed ink systems were prepared that varied in composition from 100 percent pure silver ink to 100 percent pure copper ink. Commercially sourced silver NPs, available from UT-Dots, Inc., were used in formulating mixed inks of silver and copper for evaluation of phase separation properties. The commercial silver ink was shown to have a mean particle size particle of 9-30 nm, which was very close to that of the SPPT-synthesized copper NPs. However, the solids concentration of the silver ink was measured to be 40 weight percent, twice that of the copper ink. In all instances, the total volume of each mixed ink was maintained at 1 mL, a convenient amount for AJ printing. For example, a mixed ink having 14.2 wt % copper and 85.8 wt % silver was formulated by taking 0.3 mL of the 20 wt % copper ink and 0.7 mL of the commercial 40 wt % silver ink. Formulation details and mixture ratios of the various copper-to-silver inks used are shown in Table 1.
An AJ deposition method was used to precisely print each of the 11 inks shown in Table 1. The process allows for the non-contact deposition of a nanoparticle ink by first atomizing a small quantity of the liquid ink and subsequently focusing the aerosolized product through a series of aerodynamic lenses resulting in deposition through a 50 to 300 micron orifices. A simple modified Van Der Pauw test pattern was printed onto glass slides and used to evaluate DC electrical properties of the printed inks. The pattern allowed for the calculation of resistance by applying a fixed current to the circuit and allowing the compliance voltage to float as needed. The arrangement minimized the effects of contact resistance and resulted in a more accurate measurement than simple two point techniques.
Copper NPs readily oxidize when heated under atmospheric conditions and electrical properties consequently suffer.
Laser sintering is an attractive alternative to bulk thermal processing since oxidation kinetics have been shown to be minimized in comparison to conventional heating processes. See I. Heodorakos et al., Applied Surface Science 336, 157 (2015); and T. Park and D. Kim, Thin Solid Films 578, 76 (2015). A continuous wave 835 nm infrared (IR) laser having an effective spot size of 20 microns was used to selectively sinter printed test structures. Power densities ranging up to 2387 W/mm2 were evaluated as increasing laser power has been shown in the literature to result in improved bulk DC resistivity. Substantially lower resistance values for equivalent circuits were obtained by laser processing as compared to bulk thermal processing. As shown in
Visual inspection and optical microscope analysis of printed and laser sintered test structures revealed that varying the ratio of copper-to-silver present dramatically changed not only the electrical properties of a conductor but appeared to promote a phase separation of metallic elements in the laser sintered depositions. The effect was not observed with any samples heated on a hot plate and is thought to be the result of a laser-sintering-dependent phenomenon governed in part by reaction kinetics, differential surface energies of silver and copper nanoparticles, and the ability of discrete metal components in the ink to preferentially absorbed energy delivered from the IR laser source.
Microscopy revealed the formation of homogeneous and heterogeneous annealed phases of mixed silver-copper ink systems. For example, as shown in
TiO2 particles were introduced into the ink system described above. A well dispersed ink was formulated and laser sintering was effective in annealing the film. As shown in
Under continuous operation, microstructures of binary metallic particles evolve over time in response to a multitude of cues, e.g., temperature and stress. In order to understand and predict the stability of these morphologies and their temporal evolution over extended time scales, a mesoscale approach is required, which incorporates both atomic scale information and evolving microstructures. To this end, a diffuse-interface, phase field model was used to evaluate the microstructural evolution of three-phase systems (two metals, which are denoted A and B, and a pore phase) that accounts for bulk thermodynamics and interfacial energies of the evolving phases. The starting point of the phase field treatment is the introduction of structural order parameters that describe each phase in the system. Here, (φA, φB, φC) was used to describe a three-phase system that is comprised of a binary mixture of two metals, {A, B}, with (φA, φB), respectively, in addition to the solvent/pore phase (φC). For the Ag-Cu system, material A (B) denotes Ag (Cu). Moreover, by enforcing φA+φB+φC=1, where the phase fields now are measures of the volume fraction of phases, one can write φC in terms of the other two phase fields. Next, a coarse-grained free energy functional of this three-phase system, Ftot is written as:
where f0 is the homogeneous bulk free energy density. εA and εB are the gradient energy terms, which set the interface energy of A-pore and B-pore surfaces. The following form for the free energy density of the grain microstructure was adopted:
f
0
=H
AφA2(1−φA)2+HBφB2(1−φB)2+W[φA2φB2+2(1−φB−φA)2(φA2+φB2+φAφB)],
where HA, HB, and W are parameters that set the energy scale of the system. The interface energy and width of each phase are uniquely defined via the model parameters εA, εB, HA, HB and W.
The phase fields used represent conserved quantities; therefore, with the aid of variational principles, the governing equations for the spatio-temporal evolution of φA and φB follow from
where the model parameters MA and MB are the atomic mobility of material A and B, respectively. See Cahn and Hilliard, J. Chem. Phys. 28, 258 (1958). The following assumptions are made in the modeling treatment:
To examine binary mixtures of materials A and B, the packing algorithm by Skoge et al., Phys. Rev. E 74, 041127 (2006) was used to generate realizations of Nsphere hard spheres (disks in two dimensions) with various radii and total packing density. Motivated by the experimental setup, the particle radii of materials A and B were set to be the same and only the particle volume fraction of A, VA=NA/Nsphere, and B, VB=NB/Nsphere, where NA and NB are the total number of particles of A and B, respectively, were varied. In this simulation, Nsphere=2000 and the number of particles of A was set to NA=500, 1000 and 1500, which yielded a particle volume fraction of VA=25%, 50% and 75%, respectively.
At a simulation time of 26.6×103,
Next, the microstructural evolution of a binary mixture with a particle volume fraction of material A and B of 40% and 60%, respectively, was examined in three dimensions. For this simulation, a total of Nsphere=500 was used. The snapshots shown in
The present invention has been described as multi-component nanoinks for direct write applications. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.
This application claims the benefit of U.S. Provisional Application No. 62/269,681, filed Dec. 18, 2015, which is incorporated herein by reference.
This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention.
Number | Date | Country | |
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62269681 | Dec 2015 | US |