Additive manufacturing of ultra low-loss microwave dielectrics for high-frequency applications
Creators: name: Ghosh, Avishek
Publisher: name: Loughborough University
Year: 2022
Subjects: classificationCode: 401699
subject: Materials engineering not elsewhere classified

description: The demand for miniaturising passive Radio Frequency (RF) components has witnessed exponential growth due to the rapid progress in wireless communication systems. The application of high-density, efficient, and multifunctional RF components with tailorable properties have drawn the attention of electronic industries towards Ultra-Low Temperature Co-Fired Ceramic (ULTCC) technology which offers substantial benefits over hermetic packaging technologies for high-speed communications and digital applications. Currently a range of ceramics materials (Alumina, Barium Titanate, Silica, Silicon Nitride Strontium Titanate, Quartz) available in the market that are considered for manufacturing components such as antennas, capacitors, couplers, filters, substrates for mobile communications, 3G, 4G and 5G applications. However, high sintering temperatures (&gt;1000 °C) place restrictions on these dielectric ceramics to be co-fired with the most common electrode materials such as gold, aluminium, silver, and copper. Moreover, depending upon the frequency of the RF signal, the waves propagating within dielectrics typically encounter loss during the operation. Communication systems are expected to require high performance and efficiency with tailorable properties for microwave-to-millimetre RF applications. Therefore, microwave dielectric ceramics of low sintering temperature (&lt;1000 °C), with high dielectric permittivity (εr), low dielectric loss (tanδ) and high-quality factor (Q ×f) are highly desirable to the communication industries. Additive Manufacturing (AM) or 3D printing (3DP) offers flexibility in production and independent operation while fabricating complex geometric structures of various shapes and sizes. Further requirements on functionalities and tailoring properties of the components can be demonstrated by utilizing the comprehensive range of design varieties presented by AM. Therefore, 3D-printed devices for microwave-to-millimetre RF applications could play a pivotal role in high-speed communications and digital applications.In this work, first-ever stable ink formulation of ultra-low loss microwave dielectric ceramics known as Bismuth Molybdate (Bi₂Mo₂O₉/BMO) and Silver Molybdate (Ag₂Mo₂O7/AMO) was developed with judiciary selected additives to achieve suitable rheological properties that are amenable for 3D printing. The formulated ink accommodates a high amount of solid content ~82 wt% and ~80 wt% respectively. The 3D printed BMO and AMO components with good structural integrity, and spatial resolution were sintered using the microwave and conventional heating methods, achieving ≥95% density. The 3D printed and sintered solid samples have exhibited excellent dielectric properties covering from GHz to THz range. The dielectric permittivity (εr) and loss tangent (tanδ) for the 3D printed BMO &amp; AMO solid components were found to be 38.20 ± 0.5, and 5x10-4, Q × f = 10,874 and 13.44 ± 0.3, 4x10-4, Q × f =24,071 respectively over to 8 GHz frequency range and the values were shown to be tailored using designed porosity. Further, the addition of metallic infills in the designed pores of the ceramic scaffolds has altered permittivity values. The 3D printed samples have demonstrated similar εr values in the 6G (70-100 GHz) frequency ranges.The work demonstrated the progress of extrusion additive manufacturing and the potential to tune the ceramic functionality for high-frequency applications via the design freedom offered by AM and this would enable further miniaturisation of microwave dielectric components for beyond 5G (above ~26 GHz) communication needs. The combination of novel materials and an efficient 3D printing technique can, therefore, deliver the components with desired properties. The present research leads a scope to demonstrate the ‘Powder to Product’ holistic approach to fabricate reliable and high-density components from novel microwave dielectric ceramics.