Experimental Methods

This section provides an account of the experimental methods followed. First, the nickel powder was ball milled according to the specified ball milling parameters. Next, scanning electron microscopy pictures were taken of the milled powder. Inks were made with the milled powder and printed via stencil casting by hand. The samples’ sheet resistances and thicknesses were then measured and used to determine the bulk conductivity of each ink.

3.3.1 Ball Milling

The ball milling jars and lids were custom machined out of stainless steel. The dimensions of the jar and the jar and lid are presented in Figure . The lids featured a rubber gasket and were able to be tightened to the jar with machine screws, creating a sealed enclosure inside.

Stainless steel grinding balls (Retsch) of 3mm, 5mm, 10mm, and 15mm diameter were used as the grinding media. The milling frequency was varied from 15Hz, 20Hz, and 30Hz, and the milling time was varied from 1 hour, 2 hours, and 8 hours.

Figure 3.5: Left: Drawing of ball milling jar. All dimensions are in mm; Right: picture of ball milling jar and lid with gasket (orange).

Prior to adding the powder, the jars, lids, gaskets, and balls were all thoroughly cleaned.

The jars, lids, and gaskets were first wiped down with water and a Texwipe to remove any residual Ni present. They were then wiped down with acetone and a Texwipe until there was no visible residue on the Texwipe, then placed into an ultrasonic water bath for one hour.

Once sonication was completed, the parts were placed in an oven at 50 C to dry, then wiped down one last time with acetone to remove any residue left behind by the water.

To clean the balls, as much residual Ni as possible was first removed with water and a Texwipe. The balls were then added to a plastic jar, which was filled partially with water and dish soap. The jar was closed and shaken vigorously, and the waste water was poured into a waste beaker through a funnel and sieve to avoid losing the grinding balls. This process was repeated with water and dish soap for a total of three times, then once more with acetone.

The balls were then also placed in an ultrasonic water bath for 1 hour, then removed and dried in an oven at 50 . Finally, they were rinsed once more with acetone and dried one more time.

The nickel powder and grinding balls were added to the jar in a mass ratio of 1:10 of powder to balls. Approximately 8g of powder was milled per jar per run. The powder was first added to the jars, and the balls were then carefully added on top, making sure not to cause the underlying powder to spill out. Isopropyl alcohol was then added to cover the powder and partially cover the balls in order to form a wet grinding environment inside the jars. The jars were then sealed and tightened with machine screws to form a closed environment. The sealed jars were placed in a planetary ball mill (Across International, PQ-N04). The direction of rotation was reversed every 15 minutes to ensure even milling of all the material inside.

Once ball milling was complete, the jars were removed from the ball mill and the lids removed. With the balls still inside the jars, both the jars and lids were placed inside an oven at 50 C until all of the isopropyl alchol was driven off. The powder and balls were then poured into a mesh sieve (30 mesh, or 0.5mm opening) in order to separate the balls from the powder. The powder was then poured into a vial for further use.

3.3.2 Scanning Electron Microscopy

A tabletop scanning electron microscope (Hitachi TM-1000) was used for all scanning elec-tron microscope images. First, aluminum SEM stages (Ted Pella) were covered with double-sided adhesive carbon tape to create a substrate upon which to secure the samples. For the nickel powder, a roughly 1cm² wide strip of carbon tape was firmly adhered to the SEM stage. A small amount of nickel powder was then spread onto the carbon tape and lightly pressed into the tape with the end of a wooden applicator (Puritan). The stage was then tapped a few times against the edge of a workbench to remove any excess unadhered nickel powder.

The chamber atmosphere was pumped down to vacuum prior to engaging the electron beam. Where possible, the software’s automatic focus and brightness/contrast settings were used when capturing images, but manual adjustment was necessary for some samples.

3.3.3 Ink Compositions

In order to investigate the effect of changing particle morphologies and sizes on printed sample conductivity, a slurry was made with each ball milled nickel powder. The composition was kept constant in order to isolate the effect of changing particle size on slurry conductivity.

The composition used was 80 wt% nickel powder (Alfa Aesar, 3-7Âțm median size) and 20 wt% poly(vinylidene fluoride)-hexafluoropropylene (PVDF-HFP, Kynar Flex 2801) with n-methyl-2-pyrrolidone (NMP) added as a solvent in a 4:1 mass ratio to the mass of PVDF-HFP. For each ink, 4g of nickel powder was used to yield 9g total of each ink. This composition is summarized in Table 3.3.

To make each ink, 1g of PVDF-HFP was first dissolved into 4g of NMP in a 20mL glass vial. Dissolution was aided by a combination of agitation on a vortex mixer and an ultrasonic water bath. After the polymer was fully dissolved, 4g of ball-milled nickel powder was added to the vial. The vial was then mixed for 5 minutes on the vortex mixer and then placed into the ball mill for 2 hours at 45Hz, changing directions every 15 minutes, in order to ensure more homogeneous particle dispersion. However, noticeable settling of the nickel powder occurred over time due to its density, so each ink was mixed for 10 minutes on the vortex mixer immediately prior to use for printing.

3.3.4 Stencil Casting

Nickel inks were printed by hand using a combination of stencil casting and doctor blade.

Inks were printed on a 75Âțm Kapton (polyimide) substrate (American Durafilm). A stencil was cut out of the same Kapton material to form a 2 ⇥ 4 array of 1cm² squares.

Printing was done on a 1/4 inch thick 6⇥6 inch MIC6 aluminum plate (McMaster-Carr) with a nominal flatness tolerance of 0.015 inches, which served as a heat-resistant and acceptably flat surface. The Kapton substrate was cut to size with scissors, and both the surface of the aluminum plate and both sides of the substrate were cleaned with a Kimwipe and acetone to remove any dust and debris that could be trapped between the subtrate and the plate. Prior to laying down the Kapton, a small amount of acetone was added to the aluminum plate upon which the Kapton was laid. As the Kapton was laid down, the capillary forces from the acetone pulled down the Kapton to the plate, which created smooth contact and eliminated the possibility for any trapped air bubbles as the acetone evaporated.

Excess acetone from under the sides of the substrate was wiped away with a Kimwipe, and the top surface of the Kapton was again wiped to remove dust and debris. Both the top and bottom ends of the substrate were then secured to the aluminum plate with masking tape.

The Kapton stencil was then aligned in the center of the substrate and cleaned and affixed using the same method as the substrate to the plate with acetone and masking tape. The acetone under the stencil was allowed to evaporate fully, especially along the edges of each 1cm² square, in order to allow for clean edges after printing.

A wooden applicator (Puritan) was used to place a small amount (0.1-0.2 mL) of ink ahead of each square. The doctor blade was then cleaned with a Kimwipe and acetone and drawn by hand towards the user from the top of the stencil to the bottom. The doctor blade was held at roughly a 60 angle from the plate towards the direction of casting to ensure only one edge of the blade was in contact with the stencil. Care was taken to ensure both sides of the doctor blade were drawn forward at the same speed and to apply enough pressure to maintain constant contact with the stencil but not so much that ink would be squeezed out between the stencil and the substrate. After printing, the stencil was removed and the plate and substrate placed in an oven at 80 C for at least 1 hour to drive off all solvent. The stencil and doctor blade were then cleaned for future use.

For all inks, 2 batches of 8 samples were printed for a total sample size of 16 per combi-nation of ball milling parameters.

3.3.5 Sheet Resistance Measurements

An inline four point probe was used for all conductivity characterization. The direct mea-surement being taken was sheet resistance, which was then converted to bulk resistivity and bulk conductivity by measuring the average thickness of each printed electrode.

Sample were printed on 75µm Kapton to as an insulating substrate. The printing method used introduced the possibility for some particle alignment along the printing direction, so resistance measurements were taken in two directions, one parallel and one perpendicular to the direction of casting. This is visualized in Figure 3.6.

Thickness measurements were taken with a handheld ratcheting screw micrometer (Mi-tutoyo). The micrometer was first zeroed to the thickness of the substrate. The sheet of samples was positioned such that the rotating side of the micrometer was in contact with the back of the substrate to avoid measurement errors that could be caused by shearing the sample, and the ratcheting function of the micrometer was utilized to minimize measurement error via compression of the sample.

Parallel

Perpendicular

Printing After Printing Measurement

Figure 3.6: Sample preparation for conductivity measurements. Left: Printing samples with doctor blade and Kapton stencil. The direction of the doctor blade introduces a possible axis along which particles in the slurry may be aligned. Middle: Electrodes cast on Kapton substrate. Right: Top, four point probe measurement perpendicular to the direction of casting; Bottom, four point probe measurement parallel to the direction of casting.