Design choice and evaluation

Coils designs were chosen based on their theoretical optimization as magnetic field pro-ducers, granting higher probabilities (depending on a few parameters like material used, thickness, etc.) of generating high-intensity fields. However, as inferred in figure4.1, that option has a critical problem associated since spiral and squared-spiral patterns possess one of the contacts in the center of the coil, hindering the connections with it. Without a viable contact, most of the test coils could not be evaluated nor usable in further proto-types. Therefore, following the substrate choice, the next step was to test various ways of connecting the inner contact of the coil to a power source.

The first approach was to solder a copper wire to the middle pad, allowing an easy connection to an outside power source by a crocodile junction or by intertwining it with another wire. This option would be an outstanding solution, creating a simple manner to resolve the initial problem. However, it is only possible due to Kapton’s use as a sub-strate because if it were PET, the temperatures used to melt the solder would evaporate the substrate. Yet, during the attempt to solder the wire, an unexpected phenomenon was observed, as demonstrated in the figure4.3. The silver ink used could not undergo temperatures near 300/400^◦C, otherwise, it would simply melt and disappear from the substrate, destroying the line or contact. This unforeseen event made unviable the in-tended approach since lowering the heated iron temperature to a point where the ink was unaffected meant not melting the solder, impeding the establishment of the external contact.

Following this result, a second approach was taken, which consisted of printing both sides of a Kapton substrate with the same design, being the middle pads aligned. With

*The L1 mesh is presented in appendix A, figureA.1

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PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS

(A) Initial state (before applying the heated iron) (B) Final state (after applying the heated iron)

FIGURE 4.3: Silver ink lines destruction when high temperatures were applied(T = 246^◦C)

that configuration, it was possible to have two external connection pads positioned on opposite sides of the substrate, with the bonus of having double the lines to generate the magnetic field, leading to double the intensity. After producing the double-sided printed substrate, the only missing piece was a way to connect both middle pads to allow the current to flow from one side to the other.

The first attempt tested was cutting a small hole in the Kapton substrate in the region where the middle pads were going to be before the screen printing. Ideally, the ink would get through it during the fabrication process, printing the inside borders of the orifice and creating the connection between the coils. Additionally, another alternative was printing the patterns first, aligned by the middle pad, and then perforating a hole in the middle, allowing the current to flow between sides by a screw and a nut in contact with each side.

The result of the second approach is demonstrated in the figure4.4.

However, due to the extreme flexibility of Kapton, the creation of a hole in the mid-dle of the substrate proved more demanding than expected. Nevertheless, two different methods were tried, with both obtaining mediocre results. Either with a drill or a blade, it was possible to understand the inviability to reproduce the intended goal regularly. With the drill, some parts of the adjacent substrate to the perforated Kapton would lift, creating unwanted cracks, leading to breaks in the surrounding printed lines that would destroy the entire pad or parts of the coil. In the case of the blade, the cuts were not precise, a problem that, added to the difficulty of the cut itself, led to continuous damage in some lines of the coils. Thus, it was not possible to connect both sides of the substrate in a reproducible manner, leaving those designs without viable options for further testing or

4. MAGNETIC FIELD GENERATORS 31

implementation.

FIGURE4.4: Screw and nut connection for double layer planar coil (front and back)

Nonetheless, among all the patterns presented in figure4.1, mesh-type coils were the only type automatically ready to use after production since they did not possess an inner pad. Of the four designs attempted, only three were successfully printed as functional coils and capable of being tested. The evaluation consisted of applying different current values while registering the produced magnetic field to the specimens. However, the ob-tained results were far from expected since, for an applied voltage of 10V, which would mean a current within the coil of about 0.2A, the produced field was negligible and de-tected as noise of few microTesla by the gaussmeter. The behavior was analogous in the three analyzed mesh type coils, leading to the performance of attempts with higher cur-rents, up to 1A, with a minimal increment on the field observed. Due to this unexpected intensity absence, a conductivity test of the coils was assembled to analyze possible con-tact damage during the voltage application. However, the attained results proved equal resistivity to the presented values in table4.1, concluding that the coils were still conduc-tive but did not display high magnetic fields.

Additionally, it was not possible to obtain any form of comparison with literature since there are close to no publications on planar coils fabricated by screen printing or microm-eter thickness samples. Moreover, the result also put in question the other planar coils, even with their shape being much more probable of getting higher magnetic fields since the increment would probably not be enough to get the intended values. In addition, the screen printing mesh quality started to fall off after some washes and device fabrication, which led to multiple failed samples and eventually made it unusable for planar coil pro-duction. Herein, the option was to pursue other approaches to create a variable magnetic field with high intensity and, possibly, be embedded.

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PROGRAMMABLE MAGNETIC ATTRACTIVE MATERIALS: AN APPROACH FOR SHAPE-CHANGING CARS’INTERIORS