Internal-strain-controlled tungsten bronze structural ceramics for 5G millimeter-wave metamaterials

Abstract

Miniaturization, power dissipation and temperature stability are the most critical factors restricting the microwave use of ceramic-based devices. Balancing them, especially for the ceramics with high dielectric constant, is a great challenge. Herein, a new strategy by trivalent ion (Al³⁺ and Ga³⁺) co-substitution is put forward in tungsten bronze structural Ba₄Nd_9.33Ti₁₈O₅₄ ceramics for millimeter-wave metamaterials, and a nearly 40% increase in Q_f values with stable temperature characteristics is obtained. The crystal symmetry, morphology and the vibration modes are characterized by XRD, EDS and Raman spectroscopy, suggesting the varied internal strain and oxygen octahedron tilt from the mismatch of different cations. The best performance of Ba₄Nd_9.33Ti_18−x(Al_0.4Ga_0.6)_xO_54−0.5x (BNT-AG_x) is observed when x is 2.4: ε_r = 67.71, Q_f = 13 675 GHz, τ_f = +4.4 ppm °C⁻¹. Considering the dispersion relationship of BNT-AG_x ceramics further, the structure of BNT-AG_x metamaterials is uniquely designed, and the sub-wavelength BNT-AG_x blocks are also fabricated to demonstrate the frequency selection filtering function with high temperature stability at the millimeter-wave band (>30 GHz), which is of great significance to achieve some eMBB (Enhanced Mobile Broadband) scenes of 5G (the 5th Generation) technology, from a metal loaded cavity to free space.