Implementation of Thermal Inkjet Printing of 2D Materials into 3D Printers

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1 Implementation of Thermal Inkjet Printing of 2D Materials into 3D Printers Henry E. Crosby III REU program, College of William and Mary Dr. Russell Buzz Wincheski NASA Langley, Nondestructive Evaluation Sciences Branch August 8, 2016 Abstract Inks of conductive materials are currently being developed for methods of direct-write additive manufacturing of thin circuits onto materials and structures. Beyond standard direct-write printing methods, one can instead implement the use of thermal inkjet printing of such circuits. In particular, the printing of strain gauge sensors can be utilized for non-destruction evaluating techniques. Printing both silver-nanoparticle inks and graphene oxide inks, these circuit strain gauges were printed and further investigated for quality of prints. The investigation of these printed sensors onto materials such as standard printer paper, polyethylene terephthalate (PET), polymide films, and additive manufacturing thermoplastics. Using the techniques of thermal inkjet printing on 3D printers opens up a variety of possibilities in printing such inks onto printed parts. Supported by NSF REU grant PHY Introduction 1.1 Nondestructive evaluating strain sensors Strain gauge sensors are of the most basic form of non-destructive evaluating devices that offer a simple way to monitor the strain that a material experiences. Manufacturing said sensors vary greatly and have seen many changes over time in design, material, and construction. One such method for creating such an electronic device is by using a 3D printed direct writing utilizing traces of metallic inks that can be annealed and form a circuit. Similarly one could print these metallic inks using an inkjet printer instead. Thermal inkjet printers are an very effective method because they can relatively cheap to replace, they can print traces of inks of up to micron-scale line width prints, and already exist as a commercial standard in inkjet printing so they re very familiar in their effectiveness. 1.2 Thermal Inkjet Printing The interest in implementing the printing of such conductive inks is twofold. On one hand there is the practicality of printing using thermal inkjet printers in that they re the industry standard in printing industry. In this regard they re very interesting because they are very familiar and can be easily much more easily accessible. On the other hand they are also cheaper and this makes them 1

2 much more disposable. This is important because inkjet printer, in general, do have a tendency to jam and with metallic inks it can become difficult to clean once jammed. So in this respect, it is more appealing to work with these inkjet printheads, first, to understand the printing process and the potential resolutions of inkjet printing. 1.3 Additive manufactoring Since the early 80s, additive manufacturing has continued to see improvements up to the modern 3D printer that is now a standard tool in the creation of structures for many industries. Fused deposition modeling (FDM) or fused filament fabrication (FFF). Many of the popular 3D printer used in industry are open-sourced and this makes it very easy to modify things like extruder mounts to implement the thermal inkjet to supplement in printing these conductive inks directly onto a 3D printed structure D materials In 2010, Andre Geim and Konstantin Novoselov received the Nobel Prize in Physics for the discovery graphene for their experiment in Graphene ever since has been fascinating to many for its many appealing properties. It is a mono-layer crystalline sheet of carbon atoms in hexagonal shape. Graphene has the best thermal and electrical conductivity known to date. Being able to implement these 2D materials with similar features of graphene would be of great interest. 2 Experimental 2.1 Optimizing ink for printing Figure 1: Image of the four Novacentrix silver nano particle inks and Graphenea graphene oxide ink. 1,2 Four Novacentrix silver nano-particle inks dispersed ethylene glycol solutions of varying viscosities and concentrations were investigated for the optimal printed silver traces. These inks were labeled JS-B25P, JS-A101, JS-B30G, and JS-B40G. Each was to be studied and investigated in 2

3 order to determine which ink gave the best print possible and which was easiest to clean. The second kind of ink was a graphene oxide ink. This was an ink developed by Graphenea and contained flakes of exfoliated graphene oxide dispersed in a water solution. 2.2 Optimizing inkjet printer The thermal inkjet print-head used was a prototype thermal inkjet head developed by ST-Electronics. It had twenty variable nozzles each with an approximate size of 20 microns in width and height. Each nozzles resistor was pulsed high at 5V with an Arduino Pro Mini, which was programmed to set the pulse width, repetition rate, and chose which nozzles were to jetted on or off. This Arduino would send the pulses when given a digital input to do so from a TinyG micro-controller that also controlled a Lintech screw-driven xy stage. The printhead would be set at a fixed height above the stage and printed on the stage which was controlled by the TinyG and programmed through g-code to move into the shape of whatever design was desired to be printed. To better understand the printing process would want to know the printing parameters which included the height of the inkjet above the stage, the pulse width, the repetition rate of the pulse, the substrate to be printed on, the number of nozzles printing, and the print speed of the stage. Figure 2: Image of Printer Setup 2.3 Anneal Conditions Once the prints were made they were to be annealed at variable temperatures to evaporate the solvents. For the case of the silver nano-particles it was also a matter of partially melting the silver so that the particles could bunch together much easier, creating a better connection to conduct electricity. In the case of the graphene oxide, annealing was important to evaporate the water, but also to cure the graphene oxide into graphene. This was very important in the process because graphene oxide in fact does not conduct electricity. So in order to cure it, the graphene annealed at a variety of temperatures and also cured with ascorbic acid. 3

4 2.4 Print characterization Once the prints were made and being that they are continuous one wants to know the conductivity of the prints. To measure the dimensions of a print one would measure the print height and line width using an optical profilometer. For every print, three arbitrary points were picked along the entire print and then measured three times giving a total of nine data points for both a measurement in height and width. The length of the print was measure using a ruler as this was of larger scale. Lastly, the resistance of the prints was measured using an ohmmeter where they were measured from end to end. 2.5 Mechanical characterization Load tests were conducted to determine the gauge factor (GF) of the printed strain gauges. The following equation is for the gauge factor of a strain sensor, it describes the sensitivity of the gauge factors by displaying the amount percent change of a resistance divided by the percent change in strain: GF = Where the amount of strain (ɛ) the substrate experiences is simply: R R 0 ɛ (1) ɛ = L L 0 (2) This was done by putting a silver epoxy on both pads and then connecting them to an AC current bridge that would measure the resistance of the sensor. The ends of the substrate the gauges were printed on were placed taught into a load cell where the length that the load cell would pull the substrate was measured along with the changes of resistance in the sensor and the amount of load that the substrate was experiencing. 2.6 Material characterization For the case of the graphene oxide prints. One was not only interested in the electrical functionality of the printed ink, but also how well one could actually cure the graphene oxide into pure graphene. As previously mentioned, the graphene was annealed to better cure the graphene oxide into graphene. This quality of this cure was measured using Raman spectroscopy. Comparing this shift in what was called the Raman shift and the intensity of the shift, one could compare them to the spectrum response of a sheet of pure graphene to get a feel for how pure of graphene the reduced graphene oxide was annealed to. 4

5 3 3.1 Results and Discussion Optimizing ink for printing The thermal inkjet print-head had a a reported target ink viscosity of around 2 cp at 20 C. The inks labeled as JS-B30G and JS-B40G were far too viscous for the recommended, with viscosities of 9.0 and 7.8 cp at 18 C. The first ink tested for printing was the JS-B25P ink which had a reported viscosity of 3.3 cp at 20 C. The JS-A100 silver nano-particle ink was also tested, but was not given a reported viscosity value, it appeared to be less viscous in comparison to the JS-B40P and Js-B30P so it was not even attempted. After printing the silver inks JS-B25P and JS-A100, it became quite apparent that they were presenting themselves as problematic for the inkjet print-heads as they had a tendency to clog them. Imaging the ink with a scanning electron microscope (SEM) it became clear that the silver nano-particles in the ink will clump together as seen in Figure 3. Further test prints indicated that the JS-B25P ink was the most optimal for the printing process as it was the lesser common to jam the ink just printer after shorter periods of time. To better remedy the issue of jamming, the ink was filtered with a 0.2 micron filter and diluted in a 1:1 ratio with methanol. The dilution and filter helped to lessen the viscosity, making it even more optimal to print. The graphene oxide ink did not experience quite the same Figure 3: Silver nano-particle ink amount of optimizing rigor. The ink would run for a substan- before (top) and after (bottom) betial amount of more prints in comparison to the silver nano- ing filtered. particle ink. For the sake of testing and trying to prevent future prints from jamming the inkjet print-heads, the graphene oxide ink was further diluted 1:1, with 2.5 ml of graphene oxide ink and 2.5 ml of water. 3.2 Optimizing inkjet printer To better understand the printing parameters, we varied certain parameters while keeping others constant. For instance varying the substrates while keeping the rest of the printing conditions the same to compare the water adhesion. There were four different substrates printed on. More importantly though were differences in parameters and their effect on the actual dimensions of the print qualities. Due to the high amount of jamming it was hard to really consider an optimal printing height above the substrate. 5

6 Figure 4: Image of inkjet nozzles jamming and the cleaning process 3.3 Anneal conditions The silver nano particle inks were to be annealed to both evaporate the solvent they were dispersed in and to partially melt and better connect the silver nano particles to try and increase the conductivity of the printed sensors. The anneal temperatures and durations varied for the silver prints, but most importantly it depended on the substrate to which they were printed on. The two thermoplastics, could withstand higher temperatures and only needed to be annealed for short durations of up to an hour at temperatures ranging between 200 C to 300 C. There was miniscule differences in the anneal temperatures and even more in anneal durations for these anneals because once the process of evaporating the solvent was complete, it did not change much in regard to the actual quality of the conductivities of the silver ink prints. Annealing the silve rink onto the PET plastic and printer paper were another story, due to the fact that they were more susceptible to burning or scarring at such temperatures. As a result an optimal 175 C anneal time over the course of approximately 24 hours seemed to work best. Too much higher temperature and scarring would become extreme and if the duration lasted much longer it wouldnt change the print much at all since the solvent at this point would have evaporated. The anneals were varied between a hot plate versus an oven. Where for the shorter anneals, they were conducted on a hot plate, while the longer anneals were done in the oven. Annealing the graphene oxide ink was a similar process. The only issue with annealing the graphene is that it is much more important in the curing process because one is annealing the graphene oxide ink in order to reduce the graphene oxide flakes into flakes of graphene to conduct electricity. In fact the same exact anneal conditions applied to the graphene oxide as to that of the silver, in particular, it mattered most what the graphene oxide was printed on. Instead of annealing at the higher C range for the thermoplastics, the graphene oxide was instead exclusively annealed in the oven at 175 C for at a minimum of 24 hours. Another added caviat to reducing the graphene oxide is that a pre-anneal step was made to further reduce the graphene oxide by soaking it in ascorbic acid at 90 for up to 30 minutes. Very noticeable change to the quality of the graphene sensors was made when annealing in the process from start to finish. Further understanding of the quality of the graphene prints and the anneal conditions were analyzed using Raman spectroscopy. 6

7 3.4 Print characterization The dimensions of the printed silver depended heavily upon the substrate and the number of nozzle being printed. On average however, the On average, the silver nano particle had a conductivity of 3.38x10 6 (Ωm) 1. Compared to the accepted value of bulk silver, 6.29 x 10 7 (Ωm) 1, this is 5 percent of the accepted value. This fairly reasonable because it is an incredibly smaller scale and there is going to be an expected, even significant, drop in conductivity because of the fact that it is a bunch of silver nano particles group together and not solid silver. For the reduced graphene oxide prints, the most difficult measurement to make was the actual height of the traces. Because graphene is so small and the flakes, may be relatively think with respect to a monolayer of graphene, it is still incredibly small. Somewhere in the nanometer scale, and at this scale it becomes relatively hard to make a definitely measurement. Even using an atomic force microscope (AFM) the height was indefinite and appeared to be within the actual plane of the substrate that it was printed on. The other dimensions were just the same however as the silver ink. The resistances of the reduced graphene oxide prints were still very high as well, averaging in the mega-ohm range. On average the conductivites of the reduced graphene prints were around 12.8 (Ωm) Mechanical characterization Figure 5: Image of Printer Setup Only the silver nano-particle ink printed sensors gave resistances that were low enough to make a quality load test with initial resistances in the tens of ohms. An average gauge factor of approximately 11.3, the standard deviation here is almost the size of the average. This makes the number much more inconclusive, but it is important to take note of the fact that some of the silver sensors were cracked or were missing parts, that would lower their sensitivity to the strain of the material. The large standard deviation is mainly do to the large differences in the gauge factor values with only five load tests successfully completed. To get a better understanding for a more precise estimate of a typical gauge factor of silver one would want to do more tests. 7

8 3.6 Material characterization Figure 6: Picture of Raman Spectroscopic analysis of various graphene and reduced graphene oxide structures Lastly, one would want to better characterize the graphene oxide anneal process. The graphene oxide flakes In the graph above, Figure 6, one can see that the reduced graphene oxide (the top two lines) is not very close to the shape of a pure sheet of graphene (the bottom line). One can also see that in using Raman spectroscopy the issues of how substrate being printed on can skew the actual spectra. Here it appears that the green line may have the graphene spike, but in fact it is not the same exact shift and is a product of the PET substrate it was printed on. There is actually some response for a typical graphene response in the printed reduced graphene oxide, but it isn t strong enough to fully confirm. One way of fixing this and getting better results would be to print over the same trace a few times to pick up a stronger response and cloud the background substrate response. 4 Conclusions and Future Work Thermal inkjet printing indeed has it its own set of pros and cons that come with it. On one hand it is a commercial standard, it can printed up to relatively high resolutions, and it is fairly inexpensive to replace. On the other hand it does have a tendency to jam up and problems are more likely to arise when printing a lot of sensors or a very large sized one. To remedy these problems with printing one simply needs to do more research on both the printing process and cleaning the ink-jet print heads. Another idea would be to try and either implement a thermal inkjet print heads with a lot more nozzles or even try a piezoelectric printer head that flushes inks our of the ink chamber utilizing the piezoelectric effect from certain crystals. Another step in the step of this research is to further the understanding of the quality of the prints that are being made when printing from the inkjet print head, as well as actually printing with the inkjet printer head mounted to a 3D printer. Additionally, one could look into prinitng not just strain gauges, but things like photovoltaic cells and see how well the graphene, in particular, might be susceptible to storing energy from light. Also, further work does need to go into, especially the Raman Spectra analysis, to see how one could develop better quality prints of graphene. Ultimately this project is not a niche concept. It has a vast spectrum of application in which 8

9 one may desire to incorporate conductive inks into 3D printed structures and in particular, implementing the printing of 2D materials like graphene into these structures. So, in the end, it is not something that is specific even for nondestructive testing, but would have many applications as one would now be able to even easier embed said 2 material sensors onto 3D printed objects. 9

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