High temperature broadband microwave absorbing materials

Abstract

Advancements in military and aerospace applications have increased the demand for high temperature microwave absorbing materials. While current state of the art composite material solutions contain strong absorption capabilities, they fail to perform in harsh thermal environments due to thermal limitations of the composite matrix. This work focused on fabricating a ferroelectric-ferrite ceramic composite capable of absorbing electromagnetic energy across a wide range of frequencies and temperatures. The dielectric properties of A-site non-stoichiometric sodium bismuth titanate (NBT) were investigated to determine if electrical losses could be tailored by altering the Na to Bi ratio. A bismuth deficient composition resulted in two electrical loss mechanisms at temperatures above 200°C associated with oxygen ion mobility and polaron hopping between Ti4+ and Ti3+. Nickel zinc ferrite (NZF) was selected as the second composite component for the flexibility it provided in both dielectric and magnetic properties based on composition and processing conditions. Highly conductive grains and resistive grain boundaries, attributed to electron hopping between Fe3+ and Fe2+ led to an electrical loss mechanism at temperatures up to 400°C. The magnetic and dielectric properties of composite samples xNZF + (1-x)NBT were evaluated at elevated temperatures and frequencies up to 1 MHz to determine the impact the two phase composite solution had on the individual properties of NBT and NZF. Three distinct thermally activated loss mechanisms were present in the composite samples leading to broadband absorption across the test frequencies of 1 kHz to 1 MHz and temperatures up to 600°C. Transmission and reflection data from X- and Ku-band rectangular waveguides (8-18 GHz) were used to evaluate the magnetic and dielectric properties of the composite samples at room temperature. xNZF + (1-x)NBT composites demonstrated significant attenuation across the microwave frequencies tested. The microwave absorbing capabilities of xNZF + (1-x)NBT composites at elevated temperatures were extrapolated by combining the thermally activated loss mechanisms studied at frequencies below 1 MHz, with room temperature electromagnetic properties collected at 8-18 GHz.