Previous efforts to directly write conductive metals have been narrowly focused on nanoparticle ink suspensions that require aggressive sintering (>200°C) and result in low-density, small-grained agglomerates with electrical conductivities <25% of bulk metal. Here, we demonstrate aerosol jet printing of a reactive ink solution and characterize high-density (93%) printed silver traces having near-bulk conductivity and grain sizes greater than the electron mean free path, while only requiring a lowtemperature (80°C) treatment. We have developed a predictive electronic transport model which correlates the microstructure to the measured conductivity and identifies a strategy to approach the practical conductivity limit for printed metals. Our analysis of how grain boundaries and tortuosity contribute to electrical resistivity provides insight into the basic materials science that governs how an ink formulator or process developer might approach improving the conductivity. Transmission line measurements validate that electrical properties are preserved up to 20 GHz, which demonstrates the utility of this technique for printed RF components. This work reveals a new method of producing robust printed electronics that retain the advantages of rapid prototyping and three-dimensional fabrication while achieving the performance necessary for success within the aerospace and communications industries.

Printing methods such as additive manufacturing (AM) and direct writing (DW) for radio frequency (RF) components including antennas, filters, transmission lines, and interconnects have recently garnered much attention due to the ease of use, efficiency, and low-cost benefits of the AM/DW tools readily available. The quality and performance of these printed components often do not align with their simulated counterparts due to losses associated with the base materials, surface roughness, and print resolution. These drawbacks preclude the community from realizing printed low loss RF components comparable to those fabricated with traditional subtractive manufacturing techniques. This review discusses the challenges facing low loss RF components, which has mostly been material limited by the robustness of the metal and the availability of AM-compatible dielectrics. We summarize the effective printing methods, review ink formulation, and the postprint processing steps necessary for targeted RF properties. We then detail the structure-property relationships critical to obtaining enhanced conductivities necessary for printed RF passive components. Finally, we give examples of demonstrations for various types of printed RF components and provide an outlook on future areas of research that will require multidisciplinary teams from chemists to RF system designers to fully realize the potential for printed RF components.

The coefficients of thermal expansion (CTEs) of single crystalline, monoclinic β-Ga2O3 were determined by employing high-resolution X-ray diffraction measurements. This work reports the CTE measurements on a single crystalline β-Ga2O3 substrate. The CTE values along the “a,” “b,” and “c” axes are 3.77 × 10−6 °C−1, 7.80 × 10−6 °C−1, and 6.34 × 10−6 °C−1, respectively, and the CTE of the angle β (the angle between the “a” and “c” axes) is determined to be 1.31 × 10−4 ° K−1. All CTE values reported here are linear under the temperature regime between room temperature and 1000 °C. All measurements were performed in a controlled nitrogen gas environment, and no surface degradation was observed after these measurements. Thermal strain calculations with different material combinations involving β-Ga2O3 are also presented relevant to both epitaxial and wafer bonding applications for Si, InP, 3C–SiC, 6H–SiC, GaN, and sapphire.

Printed radio frequency (RF) electronic components are often prohibitively lossy due to the materials challenges involved in additively manufacturing metals and dielectrics. We use aerosol jet printing of reactive silver inks to fabricate microstrip transmission lines onto commercial RF boards and subsequently extract the insertion loss of the printed silver through bisect de-embedding of the transmission lines. We directly compare the performance of our printed silver microstrips to conventional copper-clad microstrips to benchmark the efficacy of additive manufacturing against traditional processing methods. With an insertion loss nearing that of conventional copper, reactive silver ink printed traces offer dense continuous metals that can reliably act as conductors for RF applications. In addition to the morphological effects on loss from the printed metal itself, we also observe that the effect of substrate surface texture contributes to unexpected loss that may be mitigated by smoothing the surface or aligning the print direction to minimize these effects. Metallizing passive RF components using reactive inks offers a practical approach which will allow RF designers to take advantage of three-dimensional space. This is possible without sacrificing the necessary high conductivity and low loss needed to produce high performance devices for use within aerospace and communications.INDEX TERMS 3D printing, additive manufacturing, microstrip line, printed circuit board.