J.M. Summers, A. Vu, J. Maldonado, E. Bustamantes, C. Maldonado, H. Luyen, N.D. Shepherd
University of North Texas,
United States
Keywords: Additive manufacturing, Direct write, Boron nitride, Pressureless sintering, Microwave ceramics
Summary:
In this study, notoriously difficult to sinter boron nitride has been demonstrated to sinter without pressure in direct-write additively manufactured (AM) hexagonal boron nitride (h-BN) foams. This was achieved using an electrosteric slurry design containing mixtures of cubic boron nitride (c-BN) nanoparticles and h-BN flakes. At temperatures at or above 1700°C, the exothermic phase transition of the cubic to hexagonal phase decreased the unoccupied volume, promoted necking, and yielded coalescence. The sintered foams exhibited low dielectric constants, low dielectric loss, and high Q-factors at 15 GHz, demonstrating their suitability for microwave RF applications. Specifically, high Q×F values up to 134,300 GHz, dielectric constants (ε_r) up to 3.50, and loss tangents (tanδ) up to 4.63E-4 were achieved at ~15 GHz across the range of sintering temperatures between 1700 and 2100°C. Thermal conductivities up to 9.1 W/mK were achieved and correlated well with sintering temperature. By adjusting the ratio of h-BN to c-BN, the RF properties and thermal conductivity can be tuned for specific applications. Modelled lens antennas indicate bandwidths larger than 2 GHz and gains exceeding 10 dBi are achievable. These results are promising for the development of 3D printed boron nitride as a high temperature, low dielectric constant, low loss microwave ceramic material for customized next-generation RF components.