Conductive ink based on metallic nanoparticles has been investigated in various studies as the demand for flexible electronics continue to increase. The scientific potential of this versatile topic includes devices such as: flexible OLED displays as wearable electronics; physiological sensors; and photovoltaic cells for mass production and optimized sunlight capture.
Gold and silver are the common conductive metals; their electrical properties make them very attractive materials in circuitry. However, as precious metals, use of their pure forms are impractical as they are prohibitively expensive for mass production and flexible electronics. The use of copper nanoparticles as conductive inks is promising since copper has comparable bulk electrical conductivity (1.68 μΩ . cm) to silver (1.59 μΩ . cm) or gold (2.44 μΩ . cm) while costing significantly less. Copper-based conductive inks could make flexible electronics more affordable and widely used in various industries.
The use of copper-based conductive ink, however, is mainly limited by its chemical reactivity. Copper in nanoparticle form is prone to oxidation [1] , and the process can be even further accelerated when heat is applied (such as during sintering). The authors have developed a hybrid copper ink and its fabrication method to achieve oxidation stability; the developed composition and method showed that it could resist oxidation even in high temperature without applying a coating or protective layer after deposition [2] . However, the additives used in the hybrid copper ink also increased the required sintering energy compared to the pure copper ink. The increased sintering energy poses a challenge in the sintering process due to the possibility of thermally damaging the substrates [3] .
Tin (Sn) and zinc (Zn) nanoparticles are potential candidates for additives to lower the required sintering energy. These metals are commonly used as solder materials due to
Conductive ink based on metallic nanoparticles has been investigated in various studies as the demand for flexible electronics continue to increase. The scientific potential of this versatile topic includes devices such as: flexible OLED displays as wearable electronics; physiological sensors; and photovoltaic cells for mass production and optimized sunlight capture.
Gold and silver are the common conductive metals; their electrical properties make them very attractive materials in circuitry. However, as precious metals, use of their pure forms are impractical as they are prohibitively expensive for mass production and flexible electronics. The use of copper nanoparticles as conductive inks is promising since copper has comparable bulk electrical conductivity (1.68 μΩ . cm) to silver (1.59 μΩ . cm) or gold (2.44 μΩ . cm) while costing significantly less. Copper-based conductive inks could make flexible electronics more affordable and widely used in various industries.
The use of copper-based conductive ink, however, is mainly limited by its chemical reactivity. Copper in nanoparticle form is prone to oxidation [1] , and the process can be even further accelerated when heat is applied (such as during sintering). The authors have developed a hybrid copper ink and its fabrication method to achieve oxidation stability; the developed composition and method showed that it could resist oxidation even in high temperature without applying a coating or protective layer after deposition [2] . However, the additives used in the hybrid copper ink also increased the required sintering energy compared to the pure copper ink. The increased sintering energy poses a challenge in the sintering process due to the possibility of thermally damaging the substrates [3] .
Tin (Sn) and zinc (Zn) nanoparticles are potential candidates for additives to lower the required sintering energy. These metals are commonly used as solder materials due to
Conductive ink based on metallic nanoparticles has been investigated in various studies as the demand for flexible electronics continue to increase. The scientific potential of this versatile topic includes devices such as: flexible OLED displays as wearable electronics; physiological sensors; and photovoltaic cells for mass production and optimized sunlight capture.
Gold and silver are the common conductive metals; their electrical properties make them very attractive materials in circuitry. However, as precious metals, use of their pure forms are impractical as they are prohibitively expensive for mass production and flexible electronics. The use of copper nanoparticles as conductive inks is promising since copper has comparable bulk electrical conductivity (1.68 μΩ . cm) to silver (1.59 μΩ . cm) or gold (2.44 μΩ . cm) while costing significantly less. Copper-based conductive inks could make flexible electronics more affordable and widely used in various industries.
The use of copper-based conductive ink, however, is mainly limited by its chemical reactivity. Copper in nanoparticle form is prone to oxidation [1] , and the process can be even further accelerated when heat is applied (such as during sintering). The authors have developed a hybrid copper ink and its fabrication method to achieve oxidation stability; the developed composition and method showed that it could resist oxidation even in high temperature without applying a coating or protective layer after deposition [2] . However, the additives used in the hybrid copper ink also increased the required sintering energy compared to the pure copper ink. The increased sintering energy poses a challenge in the sintering process due to the possibility of thermally damaging the substrates [3] .
Tin (Sn) and zinc (Zn) nanoparticles are potential candidates for additives to lower the required sintering energy. These metals are commonly used as solder materials due to
Conductive ink based on metallic nanoparticles has been investigated in various studies as the demand for flexible electronics continue to increase. The scientific potential of this versatile topic includes devices such as: flexible OLED displays as wearable electronics; physiological sensors; and photovoltaic cells for mass production and optimized sunlight capture.
Gold and silver are the common conductive metals; their electrical properties make them very attractive materials in circuitry. However, as precious metals, use of their pure forms are impractical as they are prohibitively expensive for mass production and flexible electronics. The use of copper nanoparticles as conductive inks is promising since copper has comparable bulk electrical conductivity (1.68 μΩ . cm) to silver (1.59 μΩ . cm) or gold (2.44 μΩ . cm) while costing significantly less. Copper-based conductive inks could make flexible electronics more affordable and widely used in various industries.
The use of copper-based conductive ink, however, is mainly limited by its chemical reactivity. Copper in nanoparticle form is prone to oxidation [1] , and the process can be even further accelerated when heat is applied (such as during sintering). The authors have developed a hybrid copper ink and its fabrication method to achieve oxidation stability; the developed composition and method showed that it could resist oxidation even in high temperature without applying a coating or protective layer after deposition [2] . However, the additives used in the hybrid copper ink also increased the required sintering energy compared to the pure copper ink. The increased sintering energy poses a challenge in the sintering process due to the possibility of thermally damaging the substrates [3] .
Tin (Sn) and zinc (Zn) nanoparticles are potential candidates for additives to lower the required sintering energy. These metals are commonly used as solder materials due to 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to their relatively low melting temperatures (Zink 419.5 o C and Tin 232 o C) compared to Copper (1084 o C). The copper-tin alloy powder has already been studied as a material for low energy laser sintering in additive manufacturing research [4] . However, the addition of these materials could also reduce the overall conductivity of the sintered traces as they have lower electrical conductivities than copper. It is necessary to determine the optimal compositional ratios of the conductive ink to lower the required energy of sintering without decreasing conductivity of the sintered path.
For the sintering method, the direct laser sintering technique is used in this study. Among different sintering techniques, the photo-sintering techniques such as the intensive pulsed light (IPL) and direct laser sintering techniques are widely studied. These methods could provide high power energy in a relatively short time, minimizing the heat affected zone. The IPL and the direct laser sintering techniques are similar to each other and applicable in different situations; however, direct laser sintering has the advantage over IPL sintering in durability of the sintered conductive path as the laser sintering gives more uniform microstructures [5, 6] .
Some researchers have found the sintering temperature of copper nanoparticles experimentally, despite the challenges. Park et al. [7] used a K-type thermocouple with 1 ms response time and embedded the thermocouple in the substrate, measuring the temperature from the bottom of the sintered conductive layer. They reported that copper nanoparticle sintering starts somewhere between 134 to 274℃ by indirectly calculating the surface temperature with an analytical transient heat transfer model [7] . Mittal and Jung [8] also found that copper nanoparticles with a diameter of 20 nm rigorously sinter at the temperature ranging from 217 to 234 ℃ using high-resolution TEM and calorimeter; however, this study did not use a laser sintering process.
The objective of this study is to identify the optimal hybrid copper ink composition and laser sintering parameters to create a conductive path. Three different hybrid ink compositions with varying ratios of additives (pure copper, copper and tin, copper and zinc) were experimentally investigated at varying laser intensities. Finite element analysis (FEA) was also used to estimate the localized temperature at the point irradiated with laser for each setup and verified by experimental results. The resulting electrical conductivity and thermal behavior was compared to provide optimal ink composition and sintering parameters.
Experimental investigations first started by preparing the samples with different compositions. The samples were then sintered using direct laser irradiation followed by characterization. Figure 1 illustrates the schematic view of the direct writing steps of hybrid copper inks; samples were prepared mainly in three different ink compositions as illustrated in Table 1 . The first is copper nanoparticle (Cu, dia. < 100 nm) as the main metallic component. The other two compositions introduced tin (Sn, dia. < 150 nm) and zinc (Zn, dia. < 60 nm) nanoparticles ranging from 5 to 20 wt.% at increments of 5 wt.%. Table 1 describes the metallic nanoparticle compositions of each sample. Other chemicals utilized for ink preparation are: poly(N-vinylpyrrolidone) (PVP, Mw: 40000 g mol−1), diethylene glycol (DEG), and formic acid (HCOOH). The detailed sample ink preparation procedures are described in a previous study [2] . All inks were vortexed for 1 min then sonicated for 20 mins before usage. The doctor-blade method was used to coat the hybrid inks onto glass slides covered with patterned masks. The thickness of the film was approximately 70 μm. Following this, the inks were annealed on a hot plate at 80°C for 10 min to evaporate the solvents. Figure 2 illustrates the scheme of doctor blade method used in this study. For the direct laser irradiation, a multimode fiber-coupled diode laser (BWTEK-BWF1) with a wavelength of approximately 808 nm was used. With the given spot diameter of 0.25 mm, the maximum power density capable of the system is 1.47 W/cm 2 . In this study, the irradiation power density was adjusted from 0.8 to 1.4 W/cm 2 with 0.2 W/cm 2 increments while scanning velocity was maintained constant at 7 mm/s. The direct laser sintering experiments were performed at ambient atmosphere conditions. Figure 3 illustrates the schematic view of laser sintering setup and irradiating path, which has 15% overlap on the previous path. Thus, three categories of conductive ink were produced: pure Cu, Cu-Sn mixtures, and Cu-Zn mixtures. The effect of these metallic additions to copper ink will be compared based on electrical conductivity and temperature profile.
In order to reduce the variability of experimental results and examine the reliability of fabrication process, each sample was replicated at least 3 times.
Sheet resistance is a far more accurate way of measuring the films conductivity and is most commonly used for comparing thin films of uniform thickness; thus, a four-probe station connected with a multimeter (Keysight 34460A) was utilized to measure the electrical resistivity of conductive films.
Identifying the maximum temperature at the irradiated samples is crucial as the high maximum temperature during the sintering process is not suitable for applications with thermally weak substrates.
In order to analyze the thermal behavior of selective laser sintering process and determine the process parameters, finite element numerical method was conducted. For this aim, a three-dimensional thermal FE model is developed on ABAQUS Explicit TM using DFLUX subroutine, written in FORTRAN, to calculate the heat flux as a function of time and location within the surface of elements. The surface heat flux distribution is computed based on the Gaussian function as follows [10] : (1) where η is the heat absorption coefficient of the irradiated surface, P is the heat source power (W), R is the radius of heat source irradiated to the surface of inks (0.125 mm), and x and z are the distances of a point away from the center of the heat source. Most previous works utilized surface heat flux rather than volumetric heat flux due to the very small layer thickness (<100 μm) of powders [11] [12] [13] [14] .
Boundary heat transfer is modelled by natural heat convection and radiation. Simulated convection follows Newton's law, the heat loss rate per unit area in W/m 2 due to convection is [15] : (2) where hc is the coefficient of convection heat transfer (20 W/m 2 ), Ts is the temperature of irradiated surface, and Ta is the ambient temperature (25 o C). The heat loss rate per unit area in W/m 2 due to radiation is [16] :
where ε is the surface emissivity, whose value depends on the surface conditions and the temperature of the metal plate. A constant surface emissivity of ε=0.6 is used for estimation of heat loss due to radiation [17] .
In this FE model, the absorption coefficient (η) of copper nanoparticle was considered 63.2% [18] , and the energy was similar to the experimental conditions. The material properties of copper nanoparticles and substrate such as thermal conductivity, specific heat, thermal expansion, etc. were varied as functions of temperature [19] . The initial temperature of nanoparticles was considered as room temperature and they were meshed with a 20-node quadratic brick element, known as DC3D20, in the ABAQUS TM element library [20] . As shown in Figure 4 , higher density of meshes have been placed in the laser path to obtain more accurate results while coarser meshes are placed around and outside the laser path to reduce the runtime. The dimensions of fine meshed regions are 1×0.2×10 -2 cm, in which there are 10,000 elements with 100µm length, 75µm width, and 25µm height. Figures 5 and 6 illustrate the temperature distribution of copper ink during the laser sintering process on the top surface and across the thickness of sample, respectively. As shown in these figures, the surrounding areas of laser spot are affected by its temperature and can play a preheating role for the next irradiation paths. According to the FEM simulation, there is a gradient of temperature around the irradiation path. In order to compensate for this gradient and keep the sintering temperature constant, different values of overlap (5%, 10%, 15%, and 20%) were examined based on FEM simulations and experiments; 15% overlap showed better integrity compared to other values. It is notable that the time delay between each two paths is very short (negligible), while the process time largely depends on the irradiation speed.
The proposed model and simulation of the thermal profile of the sintering path is compared with experimental results in this section. The method in which experimental temperature of the sintered surface was determined will be elaborated (involving embedded thermocouples within the conductive inks themselves). Additionally, the metric in which electrical performance is measured and compared is presented. Experimental data will be gathered based on this metric and compared between inks as well as the bulk properties of relevant metals.
The temperature of laser sintering process is difficult to determine experimentally. The small window of irradiation time makes it difficult to measure temperature changes. Also, the direct irradiation of the laser beam affects the temperature measuring devices, especially the infrared laser thermocouples.
In this study, the in-situ temperature monitoring of the laser sintering was performed utilizing a thermo-couple-based circuit (50 μm junction, Lab supplies, UK), single channel thermocouple amplifier (TCA-MS-K-1) with sensitive factor of 4 mV ℃ ⁄ , and a high-rate data acquisition system (DAQ, NI 6001) at 80 kHz. Fig. 7 illustrates the obtained results of temperature distribution from experimental test and FE model of pure copper ink (CSG). The sample was also simulated under following conditions: 0.25 mm spot diameter, 7 mm/sec irradiation speed, 1.4 W/cm 2 power density and 15% overlap.
The maximum temperature of sintering process obtained from the experiment was approximately 179 ℃ , while the finite element model showed the value of 211℃ which has 18.58% deviation compared to the experimental results. This deviation is attributed to some possible factors such as limitations and assumptions of the developed FE model and the differences in experimental conditions such as: particle diameter variance, copper oxidation affecting thermal conductivity, and the exact composition of the ink.
The electrical resistance of direct laser sintered samples was measured at varying laser power densities. Each sample's conductivity measurements were done at three different points. Figure 8 illustrates the obtained resistance values for the three samples: CSG, CSG-Sn and CSG-Zn. Zinc and tin content greater than 5 wt.% significantly increased the electrical resistance even at the maximum power density rendering the sample unusable. From the experimental analysis, the optimal sample was CSG-Zn sample with 5 wt.% of zinc nanoparticles. This sample was irradiated at 1.4 W/cm 2 power density and 7 mm/s scan velocity resulting in an electrical resistivity of 0.90 Ω.cm compared to samples CSG-Sn and CSG at 6.33 Ω.cm and 9.89 Ω.cm, respectively.
In regard to the bulk metal conductivity, copper has the lowest electrical resistance with a value of 1.68 μΩ . cm; comparatively, zinc has a resistance value of 5.9 μΩ . cm and tin has a resistance value of 10.9 μΩ . cm. The superior performance of the Cu-Zn alloy relative to the Cu-Sn alloy can be attributed to the much lower resistance of bulk zinc. However, alloying elements in copper should strictly reduce conductivity of a material. This is due to the interference of copper's naturally high conductivity as well as disrupting the crystal lattice (all of which will impede electron motion). However, as shown by the thermal analysis, the sintering temperature reached approximately 179 o C. Bulk copper has a melting temperature of 1084 o C, while the Copper nanoparticle's melting temperature is around 200 o C. Zinc and tin bulk have melting temperatures of 419.5 o C and 232 o C; thus, zinc and tin sintered to a larger degree increasing overall conductivity by bridging more copper particles. By introducing physical connectivity, superior conduction performance was achieved at low energy requirements relative to pure copper sintering.
There are several future works in order to improve and characterize the hybrid inks. The mechanical performance was not explored in this study. Bronze and brass are colloquial terms for Cu-Sn and Cu-Zn alloys, respectively. Copper alloying of this nature is historically done to improve mechanical toughness and workability. Thus, the hybrid inks can be expected to have new mechanical properties much like the bulk-alloys. The various mechanical properties relevant to flexible electronics to be studied include toughness, wear resistance, flexibility, and conductivity performance after mechanical wear. Moreover, the long-term oxidation analysis needs to be performed to examine formation of oxides in the inks.
In this study, three different compositions of hybrid copper inks were investigated. Optimal sintering parameters and compositions that consumes the least amount of energy for the highest electrical conductivity was determined. It was found out that the addition of zinc nanoparticles to the copper ink significantly reduced required energy for sintering; this resulted in the highest electrical conductivity at all power density levels. For power level, 1.4 W/cm 2 could sinter deeper into the samples resulting in higher conductivity. The superior performance of the added zinc was attributed to zinc's lower melting temperature. This allowed zinc to effectively bridge the copper nanoparticles improving the electron path. Tin also had a similar effect in increasing conductivity relative to the pure copper ink; however, the worse bulk conductivity of tin gave lower conductivity than the hybrid ink containing zinc and copper. 
CSG CSG-Sn CSG-Zn