THzNDEAddMan
Previous studies on terahertz characterization of 3D printed structures [such as Squires and Lewis (2018)] print various filaments under so-called optimal printing conditions of printing temperature and printing speed. The terahertz refractive index and absorption coefficient are characterized for the optimally printed structures. For our work, there is a different emphasis: Do the printing parameters such as nozzle size, layer height, and orientation change the terahertz refractive index, absorption coefficient and birefringence of the optimally printed structures? The nozzle size, layer height, and orientation are not typically specified by filament or printer manufacturers. These printing parameters are typically chosen by the user depending on the desired results of the printed part. Typical real refractive indices and attenuation coefficients in the terahertz frequency range are shown in the figures below.
IIt is important to understand whether 3D printing parameters alter the terahertz refractive index and attenuation of printed components. If the terahertz refractive index, attenuation coefficient, or birefringence were to change with printing parameters, designers would need to take that into account in fabricating 3D printed terahertz devices . As an example, if the refractive index, attenuation coefficient, or birefringence of a printed terahertz waveguide were to vary due to printing parameters, then the device performance (e.g. propagation speed of guided terahertz, numerical aperture of the waveguide, loss per unit length of the waveguide) would all be dependent on the various printing parameters.
Here we present a few results on FDM printed plastics as well as nanoparticle jetted ceramics. The FDM printed polymer samples were designed and fabricated with the intent to observe any changes in the printed materials of real refractive index, attenuation coefficient, and birefringence. The largest changes in terahertz material properties was observed with horizontally versus vertically printed structures as shown in the figure below.
Horizontally printed samples have an average of a 1.9% increase in real refractive index over vertically printed samples, alongside lower attenuation coefficients for three of the four measurable filament materials, while only vertically printed samples had measurable birefringence. An overall average decrease of 125% in birefringence was measured with an increase in nozzle size from 0.4 mm to 0.8 mm. No correlation between attenuation coefficient and nozzle size was found. There was no direct correlation between layer height and change in refractive index or attenuation coefficient, but there was a clear trend that increasing layer height at the tested intervals caused an increase in the birefringence present within the samples. Based on these results, depending on the degree of precision in the refractive index, attenuation coefficient, and birefringence needed, performance variations in FDM 3D printed structures that rely on tailored optical properties such as various millimeter wave and terahertz-based 3D printed components including lenses, waveguides, and antennas should therefore take into consideration changing certain printing parameters during the fabrication process. Nozzle size and layer height are not correlated to changes in refractive index and attenuation coefficient, effectively reducing the parameters of concern for these tailored characteristics, unlike the birefringence of the structure, while notably print orientation has a measurable effect on all three optical properties.
The ceramic samples were fabricated using an XJet Carmel 1400 printer using nanoparticle jetting, a proprietary printing technology developed by XJet (https://xjet3d.com/), which falls under the category of binder jetting additive manufacturing. ZrO2 nanoparticles are suspended in a polymer binder and carrier agent to form an ink that is deposited layer by layer on top of one another. As the multiple print nozzles deposit these layers of the ZrO2 dispersion, as well as support structure material, the layers are dried using heat lamps and hot air dryers to evaporate the carrier agent. Once completed, the samples are submerged in water to dissolve the support material. After a period of drying in a desiccating chamber, the samples are considered green and ready for sintering. Sintering is required to join the individual ZrO2 particles into one dense structure at temperatures above 1200 °C. During firing, the binder decomposes leaving voids of varying size depending on the maximum temperature reached.The samples provided for terahertz characterization were all printed with identical printing parameters, but experienced different maximum sintering temperatures varying from 750 °C to 1450 °C in increments of 100 °C. The ceramic samples characterized in this research were provided by Dr. Paul Parsons from the University of Delaware. The measured relative permittivity is measured in the terahertz frequency range and also between 75-110 GHz (Couretsy of Dr. Paul Parsons,University of Delaware). The benefit of using two different systems to measure the relative permittivity, is to show that the data is both accurate and repeatable. This data confirms that as the sintering temperature is increased, the relative permittivity increases, while not varying over a relatively large frequency range. In the results, “Green State” refers to an un-sintered sample. At temperatures above 1200 °C, the relative permittivity reached a value where the increase between samples decreased dramatically, essentially becoming nil.
Terahertz transmission images of the ceramic plastes shows the variations in the dielectric constant of the material. Each sample shows clear isotropic volume reduction as the sintering temperature is increased, as to be expected as the particles begin to sinter, and voids begin to disappear with increased temperature. The samples decrease in volume from 1 % up to 18 % at the highest sintering temperature of 1450 °C. Also visible are the numerous horizontal striations seen in all the samples. These striations turned out to be an alignment issue with the XJet printer, verified by XJet staff. (This fabrication issue has since been resolved). That flaw ended up being misaligned print heads, effectively causing an inhomogeneity in the dispersion of the ZrO2 particles. In the 1450 °C sample, a clear nonlinear horizontal line about two thirds of the way up is visible (indicated by the white arrow). This line is a crack that runs throughout the entire length of the sample and suspected to be a product of a combination of material shrinkage and inhomogeneous particle density caused by the misaligned print heads. Notably, this crack is not fully visible under visual inspection, as it can only be seen on one side of the sample and only about one third of the way across that sample. The full extent of the crack was only observed post THz imaging.
