In this article, we report the development of a new method for 3D printing of dielectrics. An aerosol jet printer was used to deposit overlapping layers of photopolymer material under ultraviolet (UV) floodlight in the assembly of ramping microstructures in situ without the need for supporting structures. Printing is conducted using an in-house photodielectric ink, the development of which is presented with an emphasis on dielectric and mechanical bulk material characterization. Low dielectric loss at the X-band and structural strength are demonstrated, followed by print characterization wherein the driving mechanisms of the new method are explored, tied to print conditions, and related to specific material properties. Finally, a complex structure in the form of a 3D flower is printed to demonstrate the controlled and repeatable performance of the proposed technique.
Direct write printing is restricted by the lack of dielectric materials that can be printed with high resolution and offer dissipation factors at radio frequency (RF) within the range of commercial RF laminates. Herein, we outline the development of dielectric materials with dielectric loss below 0.006 in X and Ku frequency bands (8.2−18 GHz), the range required for radio frequency and microwave applications. The described materials were designed for printability and processability, specifically a prolonged viscosity below 1000 cps and a robust cure procedure, which requires minimal heat treatment. In the first stage of this work, nonpolar ring-opening metathesis polymerization (ROMP) is demonstrated at room temperature in an open-air environment with a low-viscosity monomer, 5-vinyl-2-norbornene, using the second-generation Grubbs catalyst (G-II). Differential scanning calorimetry (DSC) was used to study how the catalyst activity is increased with heating at various stages in the reaction, which is then used as a strategy to cure the material after printing. The resulting cured poly(5-vinyl-2-norbornene) material is then characterized for dielectric and mechanical performance before and after a secondary heat treatment, which mimics processing procedures to incorporate subsequent printed conductor layers for multilayer applications. After the secondary heat treatment, the material exhibits a 55.0% reduction in the coefficient of thermal expansion (CTE), an increase in glass-transition temperature (T g ) from 32.4 to 46.1 °C, and an increased 25 °C storage modulus from 428 to 1031 MPa while demonstrating a minimal change in dielectric loss. Lastly, samples of the developed dielectric material are printed with silver overtop to demonstrate how the material can be effectively incorporated into fully printed, multilayer RF applications.
Here, we report a previously un-reported printed electronics/additive manufacturing (AM) approach to fabricate conductive/resistive features on novel insulating silver–barium strontium titanate (Ag–BST) printed composite films. Ag–BST composite functional ink was formulated by blending a conductive Ag nanoparticle ink and an insulating BST nanoparticle ink. The blending ratio of Ag and BST inks was optimized to obtain the insulating phase after the initial curing and the conductive/resistive phase following selective laser sintering under ambient conditions. Selective laser sintered Ag–BST resistors showed an ohmic behavior and the resistivity could be adjusted by varying the laser sintering parameters, such as the wavelength, power and the rastering speed/pitch of the laser. This insulator to conductor/resistor transitioning Ag–BST ink paves a new path for direct write printed electronics/AM applications. Proofs of concept for potential applications utilizing this functional ink are demonstrated. Also, this Ag–BST ink can be used as a conventional resistive ink for dispensing printers. Thermally sintered Ag–BST resistors showed less than 8% variation in resistance between −50 °C and 150 °C.
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