Nonlinear Effective Dielectric Properties of Barium Strontium Titanate Composites from 300MHz to 4GHz

• Published in: Applied composite materials 2023-02, Vol.30 (1), p.93-109
• Main Authors: Zhu, Xiaojun, Fairbanks, Andrew J., Crawford, Travis D., Garner, Allen L.
• Format: Article
• Language: English
• Subjects: Barium strontium titanates; Bias; Characterization and Evaluation of Materials; Chemistry and Materials Science; Classical Mechanics; Composite materials; Dielectric breakdown; Dielectric properties; Dielectric strength; Effective medium theory; Electric power supplies; High power microwaves; Industrial Chemistry/Chemical Engineering; Materials Science; Microwave frequencies; Nonlinearity; Permittivity; Phase shifters; Polymer Sciences; Strontium; Transmission lines
• ISSN: 0929-189X; 1573-4897
• DOI: 10.1007/s10443-022-10065-w

The nonlinear effective dielectric properties of barium strontium titanate (BST) composites at microwave frequencies are important for designing microwave phase shifters, tunable capacitors, and nonlinear transmission lines (NLTLs) for high-power microwave applications. Previous studies have reported the dielectric properties of these nonlinear materials in the linear regime at microwave frequencies or in the nonlinear regime at sub-microwave frequencies; however, a detailed assessment of the nonlinear permittivity of BST composites at microwave frequencies is lacking. In this study, we used two bias tees and a DC power supply to apply a volume-averaged bias field from 0 to 1.43 V/m to composites located in a coaxial air line to measure the nonlinear effective permittivity of BST composites with volume fractions up to 30% BST (Ba 2/3 Sr 1/3 TiO 3 ) from 300MHz to 4GHz. The measured permittivity exhibits negligible nonlinearity and loss over these frequencies and volume fractions at the highest applied bias field. A nonlinear effective medium theory based on the Maxwell-Garnett law suggests that achieving strong nonlinearity for composites containing volume fractions from 20 to 50% of BST requires a bias field above V/m. Thus, while including BST in NLTL composites may be important for increasing dielectric breakdown strength, it may only enhance nonlinear permittivity for strong bias fields and high volume fractions.