Direct write of a new class of composite materials containing copper and graphene in the powder phase is described. The composite was synthesized using batch electroless plating of copper for various times onto Nano Graphene Platelets (NGP) to control the amount of copper deposited within the loosely aggregated graphene powder. Copper deposition was confirmed by both Focused Ion Beam (FIB) and Auger electron spectroscopic analysis. A micro-cold spray technique was used to deposit traces that are ∼230 μm wide and ∼5 μm thick of the formulated copper/graphene powder onto a glass substrate. The deposited traces were found to have good adhesion to the substrate with ∼65x the copper bulk resistivity.

Advancements in electronic and sensor systems require high performance materials and fabrication methods that permit manufacturing of optimized designs. This requires further miniaturization and integration, while enhancing the functionality and lifetime of existing systems. New strategies in materials formulation and device fabrication are needed in order to eliminate the long lead times required for the fabrication of prototypes and evaluation of new materials and designs. Direct Write (DW) techniques, which do not need photolithographic work, support rapid prototyping, development and testing of new multifunctional materials. Direct write techniques are complementary to photolithography techniques in conformal patterning and rapid turnaround. Micro Cold Spray (MCS) is a variant of both bulk cold spray and aerosol DW which utilizes the cold spray process to deposit fine conductive features for microelectronic applications. MCS differs from cold spray in the types of targeted applications and feature sizes, and differs from aerosol DW in the deposition process. The MCS technique is capable of operating at room temperature in air while maintaining sub-mm resolution and without requiring post processing. Additionally, it is compatible with broad classes of materials desirable for electronic and sensor systems in the form of a single element or composite powders.

Ensuring energy efficiency in products requires materials with superior properties. Due to its beneficial properties, copper is often used in the construction of components with good electrical and thermal conductivities. Recent research advancements have resulted in the discovery of graphene material that has electrical (current density), thermal and mechanical properties exceeding those of bulk copper.1–3 Developing and implementing advanced scalable methods for manufacturing lightweight electrical conductors based on copper-graphene formulations will add to the efficiency of commercial and aerospace components.

The graphene powders used in this study are nano graphene platelets (NGP) from Angston Materials (Dayton, OH) with platelet typically 5 μm in length and approximately 2 nm in thicknesses. The surface of NGP needed to go through a surface activation prior to the copper electroless plating to enable uniform nucleation formation. The surface activation was conducted using a tin (II) chloride solution and palladium chloride solution.

The purpose of the activation process is to seed the surface of graphene platelets with palladium nuclei, which initiates the copper electroless plating reaction during the plating process.4–6 In this example, the surface activation was conducted by treating graphene platelets with the tin chloride solution (1 g/L Tin(II) chloride (SnCl2) and 1 ml/L Hydrochloric acid (HCl- 37%)) and palladium chloride solution (1g/L). A typical processing sequence included rinsing NGP with SnCl2 solution, followed by a deionized water rinse, a PdCl2 solution rinse and a final deionized water rinse. This sequence was repeated several times to achieve the desired level of graphene surface activation.

A few grams of graphene platelets were placed into a 3 inch OD 635 mesh sieve. A 100 ml of the SnCl2 solution was poured gradually into the sieve were the solution penetrated the graphene platelets in the sieve and drained out slowly. De-ionized water was poured into the sieve to rinse the sample until the SnCl2 solution was drained out completely. Then, 100 ml of the PdCl2 solution was added gradually into the sieve and allowed to drain out slowly, followed by rinsing with di-ionized water. The above process was repeated several times to achieve a desirable activation process for the graphene surface.

During the activation process, represented by the chemical reaction in Eq. (1), palladium ions on the surfaces of graphene platelets are reduced by tin (II) ions to form palladium nuclei, which catalyze the plating process.

Sn 2 + + Pd 2 + = Pd 0 + Sn 4 +
(1)

Copper electroless plating was used to deposit copper onto the activated graphene platelets using a plating solution (16.67g/L Copper (II) sulfate pentahydrate, 13.45g/L EDTA 2Na 2H2O, 1.28g/L Hydrazine) maintained at 40 °C using a water bath, with continuous mixing of the activated graphene platelets. Copper deposition was performed after solution pH was increased above 11.0 using controlled addition of sodium hydroxide.7 This pH restriction imposes the use of complexed copper ions in the electroless solution in order to prevent precipitation of Cu(II) hydroxide. Then ∼4% hydrazine solution was added slowly to make the hydrazine concentration equal to about 1.28 grams/L. Hydrazine serves to reduce the copper ions to metallic copper on the surface of the activated graphene platelets, as shown by the chemical reaction of Eq. (2). The plating can last from one to three hours. Once the plating process was completed, the copper coated graphene platelets were filtered then rinsed with deionized water and oven dried. A desired thickness of copper can be achieved by plating several times.

N 2 H 4 + 2 Cu 2 + + 4 HO N 2 + 4 H 2 O + 2 Cu
(2)

Figure 1 shows optical and SEM images of the coated powder. The sample in Figure 1(A) was collected after 1.5 hours of plating time. A new plating solution was added and allowed to continue an additional 1.5 hours; at that time sample B in Figure 1 was collected (3 hours total growth time). Sample C in Figure 1 was collected after a total plating time of 4.5 hours with a new plating solution added after sample B was collected.

FIG. 1.

Optical and SEM micrographs of copper-graphene powders after electroless plating of copper on graphene. Samples A, B and C were plated for 1.5, 3 and 4.5 hours respectively.

FIG. 1.

Optical and SEM micrographs of copper-graphene powders after electroless plating of copper on graphene. Samples A, B and C were plated for 1.5, 3 and 4.5 hours respectively.

Close modal

The growth mechanism of thin film coatings in electroless deposition follows three simultaneous crystal-building processes: nucleation, growth, and coalescence of three dimensional crystallites.8–10 The growth of copper on graphene platelets is expected to initiate at defect sites with carbon dangling bonds. The Image in Figure 1(A) shows copper particles grown into ∼100 nm size. The particles grew to larger sizes after 3 hours growth time (Figure 1(B)) with some of the neighboring particles coalescing to form larger particles after 4.5 hours (Figure 1(C)). Due to the large surface and the disagglomerated graphene platelets, the copper particles are found to grow to larger sizes before coalescing and forming a film.

Auger Electron Spectroscopy (AES) and Focused Ion Beam (FIB) analysis methods were performed on the powder sample to provide elemental information on extremely small regions of a powder surface and to image the copper coating within the graphene platelet agglomerates, respectively. With a sampling depth of only 2-10 atomic layers and a spot size on the order of 50 nm, AES spectra were taken on two labeled areas in Figure 2(A). The resulting chemical composition for area 1, which shows no copper particles in the SEM image, is 94.5% Carbon, 3% Oxygen and 2.5% Copper. The analysis indicates the presence of a very thin copper coating on the graphene surface. The composition at area 2 showing a copper particle is 75.3% Carbon, 12.4% Oxygen, 11.4% Copper, and 0.8% Tin. The FIB analysis in Figure 2(B) additionally illustrates that copper precursors penetrated throughout the graphene powder and coated the inside surfaces.

FIG. 2.

(A) SEM micrograph for Auger analysis of copper-graphene powder surface showing two selected points with and without copper particles. (B) SEM micrograph of copper-graphene powder, after cross sectioning with the FIB, illustrating the existence of copper particles and coatings within the agglomerated graphene.

FIG. 2.

(A) SEM micrograph for Auger analysis of copper-graphene powder surface showing two selected points with and without copper particles. (B) SEM micrograph of copper-graphene powder, after cross sectioning with the FIB, illustrating the existence of copper particles and coatings within the agglomerated graphene.

Close modal

Electroless deposition enables coating of loosely aggregated graphene platelets that are easily separated and dispersed. The process described here in enable effective low-cost transformation of graphene powder to custom developed dispersions and material composites.

Micro Cold Spray (MCS) is a variant of both cold spray and aerosol DW which utilizes the cold spray process to deposit fine conductive features for microelectronic applications. This differs from cold spray in application, and differs from aerosol DW in the deposition process. NDSU has been performing MCS research since 2008 and simultaneously investigating the associated focusing phenomenon.11 Conductive traces as small as 50 μm with near bulk conductivity have already been reported without optimized processing parameters and methods.12 Understanding cold spray and particle focusing has allowed for design of an improved MCS system which has the ability to significantly reduce feature size.

The MCS system is housed in a ventilated enclosure. The stages and gantry system are directly mounted to a two inch thick aluminum plate which is attached to the frame with rubber isolation mounts. An Aerotech ALS5000 stage controls translation in the x-direction and an Aerotech ALS25000 stage controls translation in the y-direction. Both stages are capable of traveling up to two m/s and holding 1 μm accuracy over 300 mm of travel. Each stage is powered by an A3200 amplifier. Attached to the stages is a precision machined polished flat (±1 μm) stainless steel plate on which substrates are mounted. The gantry is constructed from 95 mm construction rail and provides up to one meter of vertical adjustability for printing on tall substrates. The visualization components and deposition head are attached to the gantry. A CMOS camera is used to stream live video of the nozzle and substrate during the printing process. Illumination for this camera is provided by an LED spotlight. The second camera is attached to an adjustable microscope with a 10x objective allowing for measuring and viewing of finer features. This camera is illuminated by an LED ring light. The deposition head is attached to the gantry with a translational stage allowing for fine vertical adjustments. A heating coil with built in thermocouple is mounted around the deposition head to preheat gas to allow for higher flow rates and assist in deposition of material. A second thermocouple monitors deposition head temperature. Helium flow is managed by two MKS mass flow controllers (MFC) during printing – one MFC regulates accelerator flow over a range of 0-20 lpm and the other regulates carrier flow over a range of 0-1 lpm. The atomizer consists of an oscillating chamber with a gas agitation nozzle. The enclosure is ventilated by three fans pulling air and particulate through two-inch thick HEPA filters with an approximate cross section of 36 in2. The system is integrated into LabView via an NI X Series 6341Multifunction DAQ (National Instruments) providing control over the majority of the components so that the MCS system can be operated with minimal contact with deposition materials.

The deposition head, illustrated in Figure 3, is a custom design featuring a coaxial symmetric flow capable of reaching working pressures of 1 MPa. This design promotes steady operating conditions and deters clogging allowing for extensive runtime without the need for maintenance. The entire deposition head was designed in 3-D modeling software (Solidworks) which allows for future fluid dynamics analysis. The velocities of both the carrier and accelerator gases were designed to be symmetric throughout the deposition head allowing for less turbulence upon mixing. Temperature control is performed by a LabView integrated PID loop allowing for steady operation over a range of 0-300°C. The nozzle is also custom designed and consists of a linearly converging section 19.15 mm long with an inlet diameter of 800 μm and an exit diameter of 200 μm, and a second section approximately 17 mm long with a constant diameter of approximately 200 μm. This nozzle design allows for both focusing and acceleration of the aerosol particles. Due to the working gas (helium), subsonic particle velocities around 900 m/s can be reached.

FIG. 3.

(A) optical image of MCS deposition head. (B) CAD drawing of the deposition head with flow entrances and exits.

FIG. 3.

(A) optical image of MCS deposition head. (B) CAD drawing of the deposition head with flow entrances and exits.

Close modal

Copper coated graphene features were printed with the MCS system similar to how any other metals are printed. For these experiments the carrier gas flow was at 300 ccm, sheath at 3000 ccm; the deposition head was preheated to 240 °C, the nozzle was located 3 mm from the substrate and the substrate was translated at 10 mm/s. The substrate was a frosted microscope borosilicate glass slide, 1 mm thick. The printed feature was a line 1 cm long, approximately 230 μm wide and 1-5 μm thick with 1 mm x 1 mm square pads located at both ends of the line. This printed feature, shown in Figures 4(A) and 4(B) was specifically chosen so that a bulk resistivity measurement could be ascertained. The as-printed features appear dark due to loose powder on the surface, but if the surface is scratched by a probe it appears reflective.

After printing, no post-deposition processes were carried out. Resistance of the features was measured at 1 volt using an Agilent B1500A semiconductor analyzer, with both a voltage and resistance probe on each pad (four point probe) to remove any contact resistance errors from the measurements. Cross sectional area of the samples was measured via contact profilometry at 5 locations and averaged. Bulk resistivity of the samples was then calculated from the calculated area and a 1 cm length of the feature. A summary of four printed lines is shown in Table I. For reference, a minimum bulk resistivity of 65 was achieved if the material were to be assumed as pure copper.

TABLE I.

Summary of resistivity measurements of 1-cm printed traces.

Sample 1 2 3 4
Resistance (Ω)  33.1  28.8  51.4  39.2 
X-Sec Area (m2 3.28x10−10  5.65x10−10  4.88x10−10  4.90x10−10 
Resistivity (Ω-m)  1.09x10−6  1.63x10−6  2.51x10−6  1.92x10−6 
X-Bulk Cu  65  96.9  149  114 
Sample 1 2 3 4
Resistance (Ω)  33.1  28.8  51.4  39.2 
X-Sec Area (m2 3.28x10−10  5.65x10−10  4.88x10−10  4.90x10−10 
Resistivity (Ω-m)  1.09x10−6  1.63x10−6  2.51x10−6  1.92x10−6 
X-Bulk Cu  65  96.9  149  114 

The printed features were cross sectioned and milled with a focused ion beam (FIB), imaged and analyzed via SEM and EDS, respectively. The result of this analysis is shown in Figure 4(C). The bottom dark region in the SEM image is the glass substrate. The features near the substrate are less porous in nature while the features towards the surface are much more porous. When imaged with EDS (figure not included), it can be seen that the dense features are copper rich, while the porous features are carbon rich. In addition, the dense, lighter appearing feature on the top of the image is a sputtered platinum coating deposited to reduce charging during imaging. The observed coating porosity near the carbon rich area in the deposited samples is higher than conventional signal material deposition.8 Additionally, phase separation during the MCS process caused by density difference may lead to the observed stratification in Figure 4(B). The reason why increased porosities are observed in nanostructured powder deposition may be attributed to the irregular shapes of the powders, the significant difference in their densities and their hardness values. Optimization of deposition parameters is the key to achieve more homogeneous and less porous coating. In addition, deposition of nanostructured composite powders requires proper mixing of powders to prevent settling or stratifying in the feeder during deposition.

FIG. 4.

Images of MCS-printed copper-graphene features on frosted glass slide. The distance between pads is 1 cm (A) and line width is 230 μm (B). FIB cross section of the printed copper-graphene feature showing areas of rich graphene on the top and areas of rich copper near the glass substrate (C).

FIG. 4.

Images of MCS-printed copper-graphene features on frosted glass slide. The distance between pads is 1 cm (A) and line width is 230 μm (B). FIB cross section of the printed copper-graphene feature showing areas of rich graphene on the top and areas of rich copper near the glass substrate (C).

Close modal

Adhesion of the micro cold spray deposit is an essential part of development and quality assurance. There are several ways of quantifying the adhesion of a cold spray deposit into a substrate, including both qualitative and quantitative tests. The team performed a qualitative scotch tape adhesion test that attempts to separate the sprayed coating from the substrate. The adhesive tape was applied evenly and carefully to the printed trace and then pulled off in a direction normal to the substrate. The test was repeated three times and no signs of peeling or flaking of the sprayed coating were observed after inspection.

In conclusion, the deposition of thin copper-graphene traces in ambient has been demonstrated. While the resulted traces are not as conductive as bulk metals such as copper, the new materials may have applications in low power areas and as a materials with enhanced corrosion properties over copper. The low conductivity is attributed to the platelets thickness, purity and arrangements within the deposited trace. Additionally, porosity, phase separation and uniformity of copper distribution are likely contributors to the high conductivity. When you take a raw graphene powder and form a coating for instance, the small platelets are arranged in a disjointed manner. This increases the resistance of the macroscopic coating even though the conductivity across a single platelet may still be high. Graphene has been shown to have a higher current density than copper but this does not necessarily translate into a higher conductivity. Furthermore, most of the reported work on current density is performed on very pure, very small samples (nm range). Graphene however, does have many useful properties that can be imparted in a metal matrix composite such as enhancements of mechanical properties as well as light-weighting. With that in mind, this class of materials is worth additional investigations for enhanced properties through controlled deposition and percolation of the platelets within the coating to make MCS a leading consolidation process for nanostructured composite powders.

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