Synthesis, microstructure and performance evaluation of gadolinium incorporated cobalt ferrite ceramics

Spinel structured ferrites exhibit remarkable properties and phenomena, which are attractive for application in sensors and actuators, magneto-electronics, and electrochemical energy storage devices. Among spinel ferrites, cobalt ferrite CoFe2O4 (CFO) has attracted remarkable attention and widely studied because of their electro-magnetic properties, chemical stability and mechanical hardness. The properties and phenomena of CFO ceramics are dependent on microstructure and chemistry, which in turn depend on the synthesis processes and conditions employed for fabrication. Generally exact chemical composition, processing temperature, reactive or processing atmosphere (if any) and the ions that substitute Fe3+/Fe2+ ions dictate the structure and properties of a Co ferrite. Our recent approach demonstrated that doping with different rare-earth (RE) ions is a straightforward and versatile way to tune the structure and physical properties. However, technological challenges that limit the commercialization of these ceramics are: (1) fabrication with density values equal or approaching to theoretical density, (2) quantitative understanding of the RE ion induced effects, and (3) identifying dopants to enhance the resistivity and temperature stability while retaining the magnetrostrictive ferrite phase, the most important condition to design stress sensors and actuators for automotive and power generation systems. In this work, Gadolinium (Gd) incorporated cobalt ferrites (CoFe 2-xGdxO4, referred to CFGO) with variable Gd content (x=0.0-0.4) were synthesized by solid state ceramic method. Al/CFGO/Al capacitors were fabricated and tested under a wide range of frequency (f=20 Hz – 1 MHz) and temperature (T=30-900 °C). The CFGO ceramics exhibit lattice expansion due to larger Gd-ions. Frequency dependent dielectric measurements at room temperature obey the modified Debye model. The frequency and temperature dependent dielectric constant analyses indicate that pure CFO exhibits two dielectric relaxations in the frequency range of 1-10 kHz while Gd substituted CFO compositions exhibit only single relaxation at 1 kHz. Dielectric constant of CFGO ceramics is enhanced compared to pure CoFe2O4 due to the lattice distortion upon Gd incorporation. The tan δ (loss tangent) - T data reveals the typical behavior of relaxation loses. Activation energy of the dielectric relaxation calculated employing Arrhenius equation varies from 0.564 to 0.668 (±0.003) eV with increasing x values from 0.0 to 0.4. Thermally activated small polaron hopping mechanism is evident in temperature dependent electrical properties of CFGO. A two-layer heterogeneous model consisting of moderately conducting grain interior (ferrite-phase) regions separated by insulating grain boundaries (resistive-phase) accurately account for the observed temperature and frequency dependent electrical properties in CFGO ceramics. The results demonstrate that Gd incorporation promotes temperature tolerance, resistivity and performance of CFO to the high end as desired for several industrial applications