Fig. 3(d, e) show the dielectric pr

Fig. 3(d, e) show the dielectric properties of cold sintered samples over the temperature range between - 50 and 200 °C. Typical samples obtained at 150 °C for 15 h (Fig. 3(d)) and 225 °C for 1h (Fig. 3(e)) exhibit broad curves with a maximum permittivity at 125 °C, indicating ferroelectric to paraelectric transition. The sample obtained at 225 °C shows a slightly higher room temperature relative permittivity (ɛ’ = 1440 at 1MHz) than the one obtained at 150 °C (ɛ’ = 1330 at 1MHz). These values are lower than the value of the conventional BaTiO3 with coarse grains, but comparable with the values expected in nanocrystalline BaTiO3, which could be explained by size effect [36]. Indeed, it is well known that the dielectric properties of BaTiO3 are associated with its microstructure, especially grain sizes. Typically for BaTiO3, the peak of relative permittivity appears around grain sizes of 0.8 – 1 μm size range and decreases with decreasing grain sizes [25]. In addition, small grain sizes lead to a lower shift of the Curie temperature, especially below 100 nm [36]. The broadening of the curves (Fig. 3(d,e)) may be influenced by the broad grain size distribution in the cold sintered samples and the fact that several grains are within the size range highly influencing the shift in Curie temperature. In the paraelectric region, the relative permittivity of cold sintered samples follows the Curie-Weiss law (Fig. S3). The Curie constant (C) and the Curie-Weiss temperature (θc), estimated by the linear section of the inverse ɛ’ plots, are 1.9×105 and 74 °C for the sample sintered at 150 °C and 1.5×105 and 97 °C for the sample sintered at 225 °C, which are in good agreement with the trends reported by Frey et al. with nano hot pressed BaTiO3 [37]. The comparison of relative permittivity and Curie-Weiss temperature, depending on the size and sintering method, and these concepts are described elsewhere [25]. On the other hand, both samples exhibit slightly higher dielectric losses (tanδ ≈ 9 %) than the one reported for NaOH-KOH eutectic fluxes (tanδ ≈ 4 %) [25]. Such a difference in dielectric loss would be due to the previously discussed residual fluxes in the cold sintered BaTiO3.