In this work, a broad overview in the field of strontium titanate (ST, SrTiO₃)-, barium titanate (BT, BaTiO₃)- and barium strontium titanate (BST, BaSrTiO₃)-based gas sensors is presented and discussed. The above-mentioned materials are characterized by a perovskite structure with long-term stability and therefore are very promising materials for commercial gas-sensing applications. Within the last 20 years, the number of papers where ST, BT and BST materials were tested as gas-sensitive materials has ten times increased and therefore an actual review about them in this field has been expected by readers, who are researchers involved in gas-sensing applications and novel materials investigations, as well as industry research and development center members, who are constantly searching for gas-sensing materials exhibiting high 3S parameters (sensitivity, selectivity and stability) that can be adapted for commercial realizations. Finally, the NO₂-sensing characteristics of the BST-based gas sensors deposited by the authors with the utilization of magnetron sputtering technology are presented.

The order-disorder transition (ODT) of A₂B₂O₇ compounds obtained enormous attention owing to the potential application for thermal barrier coating (TBC) design. In this work, the influence of ODT on the mechanical and thermophysical properties of dual-phase A₂B₂O₇ high-entropy ceramics was investigated by substituting Ce⁴⁺ and Hf⁴⁺ with different ionic radii on B-sites (Zr⁴⁺). The X-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) results show that $$r_{\rm{A}^{3+}}/r_{\rm{B}^{4+}}=1.47$$r A 3 + / r B 4 + = 1.47 is the critical value of ODT phase boundary with different doping B-site ion contents, and the energy dispersive spectroscopy (EDS) results further indicate the uniform distribution of elements. Interestingly, owing to the high intrinsic disorder derived from high-entropy effect, the A²B²O⁷ high-entropy ceramics exhibit unreduced modulus (E₀ ≈ 230 GPa) and enhanced mechanical properties (HV ≈ 10 GPa, K_IC ≈ 2.3 MPa·m^0.5). A₂B₂O₇ high-entropy ceramics exhibit excellent thermal stability with relatively high thermal expansion coefficients (TECs) (Hf0.25, 11.20×10⁻⁶ K⁻¹, 1000 °C). Moreover, the matching calculation implied that the ODT further enhances the phonon scattering coefficient, leading to a relatively lower thermal conductivity of (La_0.25Eu_0.25Gd_0.25Yb_0.25)₂(Zr_0.85Ce_0.15)₂O₇ (1.48–1.51 W/(m·K), 100–500 °C) compared with other components. This present work provides a novel composition design principle for high-entropy ceramics, as well as a material selection rule for high-temperature insulation applications.