High-entropy spinel oxides: Self-propagating synthesis and densification by spark plasma sintering

The self-propagating room temperature method was utilized to synthesize high-entropy spinel oxides (HESOs): (Co,Cr,Fe,Mn,Ni)₃O_4-δ, (Mg,Cr,Fe,Mn,Ni)₃O_4-δ, (Mg,Co,Fe,Cr,Mn)₃O_4-δ, (Mn,Zn,Fe,Ni,Cr)₃O_4-δ, and (Co,Mn,Zn,Fe,Cr)₃O_4-δ. After thermal treatment at 1000 °C, XRD analysis confirmed their single-phased spinel$\left(\mathrm{Fd}\overline{3}m\right)$ structure. Densification by spark plasma sintering was successfully used for the first time on HESOs, resulting in relative densities from 94 % to 99 % while retaining the spinel structure. SEM/EDS mapping displayed a homogenous, dense microstructure with minimal porosity. (Mn,Zn,Fe,Ni,Cr)₃O_4-δ displayed the highest bending strength (171.5 MPa) and Young’s modulus (188 GPa). (Mg,Co,Fe,Cr_,Mn)₃O_4-δ demonstrated the highest hardness (8.8 GPa), while (Mg,Cr,Fe,Mn,Ni)₃O_4-δ exhibited the highest indentation fracture toughness (1.5 MPa m^–1/2). The lowest thermal diffusivity (0.67 – 0.51 mm² s^–1) was recorded for (Co,Mn,Zn,Fe,Cr)₃O_4-δ, while (Co,Cr,Fe,Mn,Ni)₃O_4-δ, had the highest thermal diffusivity (0.82 – 0.58 mm² s^–1). The study demonstrated a simple and efficacious method of synthesizing and densifying HESOs with structural, mechanical, and thermal properties favourable for different applications.