Compressed perovskite aqueous mixtures near their phase transitions show very high permittivities: New prospects for high‐field MRI dielectric shimming

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High permittivity perovskite composites have been largely studied in the past few years, mostly because of their innumerous technological applications 1. These electromagnetic materials are highly demanded nowadays 2. The perovskite barium titanium oxide (BaTiO3) is a dielectric microwave material greatly used in the electronic industry; applications in several fields can be stated, such as in (i) multilayer, high‐capacity capacitors and dielectric resonator antennas, resulting from high permittivities with few losses at a given frequency; (ii) in thermometers and infrared detectors, considering the thermic sensitivity; (iii) in motion, pressure and humidity sensors, owing to piezoelectricity; and (iv) in frequential filters, medical imaging, and antiradar surfacing 4. Research has been led to improve the production methods to achieve the most auspicious features in novel structures 9, in fine nanoparticles 11, and additionally in increasing the dielectric constant 14. Moreover, new composite materials based on titanate powders mixed with a polymer matrix are currently being developed for 3D‐printing devices such as antennas 15, despite the lowering of the relative permittivity from values higher than 1000 to 4 to 5, as a result of depolarizing effects in the composite medium. Although this class of composites offers great opportunities in terms of processing the material at a low cost, it is not suited for high permittivity applications.
Nowadays, one of the most prominent applications of perovskites is in the domain of high‐field MRI, in dielectric shimming processes—the purpose of which is to generate a more homogenous radiofrequency (RF) magnetic field. The concept of a dielectric resonator was first adapted to MRI by Alsop et al 16, and later on by Wen et al 17 and Yang et al 18, who explored this concept by using water pads placed in specific configurations around the body to manipulate the RF field; later, these pads were replaced by titanate‐water saturated mixtures 19, in which the feasibility and increased performance of the procedure in the presence of BaTiO3 and calcium titanium oxide (CaTiO3) were demonstrated: The high dielectric polarizability of titanate compounds enhances the permittivity of the water. The development of these pads is currently a subject of research, and the optimization of thickness, size, geometry and permittivity, needed for specific applications, is still required 22. Additionally, in certain occasions, the fluidity of the pad solution can cause it to deform and disturb the spatial distribution when placed under the patient 20.
The adjustment of the permittivity of perovskite mixtures to very specific values has been reported very little in the literature; usually, the suspensions are prepared to obtain a saturation point, which has been reported to correspond to the highest permittivity values of both CaTiO3 and BaTiO319. It remains unclear whether higher permittivity values can be found when exploring lower water concentrations, thus beyond the saturation point.
Recently, the use of BaTiO3 slurries of sintered beads was proven to be suitable for dielectric shimming pads, identically to slurries of BaTiO3 powder; in the case of beads, relative permittivity values were reported to reach up to 487, as opposed to 319 (the case of powder slurries) at 300 MHz 27. Nonetheless, the most currently used solutions for dielectric shimming are aqueous mixtures of CaTiO3 and BaTiO3 powders, which permittivities do not exceed 110 and 300, respectively 19. These values are a result of powder‐saturated solutions of approximately 45 to 50% v/v.
Therefore, this study aims to assess the dielectric influence of water ranging from 0 to 100% v/v in the perovskite powders BaTiO3 and CaTiO3, from 50 to 800 MHz.

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