Mishra, Durgamadhab

While studying thermo-acoustic properties of binary, ternary liquid mixtures or that of ferro-fluids, one not only need to measure the variation of acoustic parameters like acoustic impedance, hydration number, Rao's constant, Wada's constant, apparent molar volume, apparent molar isentropic compressibility at different temperature values but also need to see whether parameters like relative density, viscosity of the substance changes with change in temperature. This study is of utmost importance because while estimating the acoustic parameters sometimes relative density, sometimes viscosity and sometimes both of it appears in the formula. Electrical properties of the substances are also of similar importance. Hence, investigation of variation of viscosity, relative density of specific ferrofluids, with temperature is an important area of study. In this communication we have reported variation of viscosity, relative density of water based Zinc Spinel ferrite with temperature. The electrical properties of the water based Zinc Spinel ferrite are also reported. The study is carried out in the concentration range 0.001 M to 0.01 M. At concentration lower than 0.001 M, the behaviour of ferrofluids begins to deviate from the bulk fluid properties and as the concentration approaches 0.01 M, potential aggregation effects and saturation phenomena made the solution not to disperse as desired. The Zinc spinel ferrite is synthesized by sol-gel technique.

In this work, we report fabrication of flexible elastomeric sheets for Ku-band stealth applications by impregnation of cobalt substituted nickel spinel ferrites Ni1-xCoxFe2O4 (x = 0, 0.1, 0.3, 0.5, 0.7, and 1) based functional powders in Nitrile Butadiene Rubber (NBR) matrix, for the first time. The series of spinel ferrites have been synthesized using sol-gel auto combustion method followed by three step sequential heat treatments first at 650 °C, then subsequently at 950 °C, and finally at 1250 °C. Phase purity of the entire prepared Ni-Co ferrite series have been confirmed by X-ray diffraction analysis. M-H loop indicates that substitution of Co2+ at Ni2+ sites in spinel structure, enhances both the saturation magnetization (Ms) and coercivity (Hc). The reported synthesis process is scalable for the bulk production of ferrite materials, which is essential for making large area flexible sheets. Flexible rubber sheets are fabricated by loading the synthesized series of ferrite samples in NBR matrix with the help of Two Roll Mixing (TRM) Mill and High Temperature Rubber Compression and Moulding (HTRCM) Press. Electromagnetic (EM) studies suggest that out of all the spinel ferrite composition, Ni0.9Co0.1Fe2O4/NBR sheet has maximum magnetic loss, although Ms and Hc values are higher for other Co-substituted ferrites. Further, the best values of reflection loss (RL) over Ku-band (12.4–18 GHz) of microwave spectrum is obtained by varying weight percent (wt%) loading of Ni0.9Co0.1Fe2O4 ferrite composition in NBR. A maximum RL of −48 dB at ∼17 GHz, with Effective Absorption Bandwidth (EAB) of ∼3 GHz, has been observed for 80 wt% ferrite loaded NBR sheet (NC0.1R-80) at a thickness of 5.7 mm. This flexible microwave absorbing elastomeric sheet may be applied quickly on any shape and size of airborne/ground based military platform for its stealth treatment over the investigated frequency range of Ku-band. © 2025 Elsevier Ltd