U-Chan Chung

Controlling internal barrier in low loss BaTiO3 supercapacitors

Supercapacitor behavior has been reported in a number of oxides including reduced BaTiO3 ferroelectric ceramics. These so-called giant properties are however not easily controlled. We show here that the continuous coating of individual BaTiO3 grains by a silica shell in combination with spark plasma sintering is a way to process bulk composites having supercapacitor features with low dielectric losses and temperature stability. The silica shell acts both as an oxidation barrier during the processing and as a dielectric barrier in the final composite.

Resonant magnetic response of TiO2 microspheres at terahertz frequencies

Spray-drying technique is used to fabricate spherical microparticles out of dissolved TiO2 nanoparticles. We show both experimentally and through numerical calculations that the microspheres support a Mie resonance, leading to an effective magnetic response. For this purpose, nearly single layers of microspheres were prepared and characterized by time-domain terahertz spectroscopy. We developed an experimental approach allowing simultaneous measurement of complex transmittance and reflectance of a thin layer, which in turn enables evaluation of its effective dielectric permittivity and effective magnetic permeability. Numerical finite-element-method calculations of the electromagnetic response show that the prepared microparticles are suitable for preparing a metamaterial with negative effective magnetic permeability.

Near-field probing of Mie resonances in single TiO2 microspheres at terahertz frequencies

We show experimentally that poly-crystalline TiO2 spheres, 20-30 μm in diameter, exhibit a magnetic dipole Mie resonance in the terahertz (THz) frequency band (1.0-1.6 THz) with a narrow line-width (<40 GHz). We detect and investigate the magnetic dipole and electric dipole resonances in single high-permittivity TiO2 microspheres, using a near-field probe with a sub-wavelength (~λ/50) size aperture and THz time-domain spectroscopy technique. The Mie resonance signatures are observed in the electric field amplitude and phase spectra, as well as in the electric field distribution near the microspheres. The narrow line-width and the sub-wavelength size (λ/10) make the TiO2 microspheres excellent candidates for realizing low-loss THz metamaterials.

Linking hopping conductivity to giant dielectric permittivity in oxides

With the promise of electronics breakthrough, giant dielectric permittivity materials are under deep investigations. In most of the oxides where such behavior was observed, charged defects at interfaces are quoted for such giant behavior to occur but the underlying conduction and localization mechanisms are not well known. Comparing macroscopic dielectric relaxation to microscopic dynamics of charged defects resulting from electron paramagnetic resonance investigations we identify the actual charged defects in the case of BaTiO3 ceramics and composites. This link between the thermal activation at these two complementary scales may be extended to the numerous oxides were giant dielectric behavior was found.

From core–shell BaTiO3@MgO to nanostructured low dielectric loss ceramics by spark plasma sintering

We report a quite general way to design materials with tailored properties by combining thermolysis and fast sintering approaches. Submicrometric-sized BaTiO3 particles have been directly coated in a continuous nanocrystalline MgO shell through a thermal decomposition process. The electron microscopy study has evidenced a shell composed of randomly oriented MgO nanocrystallites. The final nanostructured composite, made of sub-micrometric MgO and BaTiO3 grains uniformly distributed, is obtained in situ during the SPS process. Such a rearrangement can be explained by the initial core–shell architecture, the weak cohesion of the MgO nanocrystallites and their soft plastic behavior under SPS conditions. The composite effect leads to significant modifications in both the dielectric properties and Curie–Weiss parameters compared to uncoated BaTiO3, especially a decrease and thermal stabilization of both the permittivity and the dielectric losses. We ascribe such changes to the stress generated during SPS through the extended interfaces between the two components.

Towards left-handed metamaterials using single-size dielectric resonators: The case of TiO2-disks at millimeter wavelengths

We report a strong magnetic activity using an all-dielectric metamaterial based on Mie resonances, designed to work at millimeter wavelengths over the 30–70 GHz band. A good agreement was achieved between numerical simulations and experiment in the case of one meta-layer based on TiO2-disks, manufactured using a simple bottom-up approach. We also demonstrate through numerical simulations a negative refractive index within the same investigated metamaterial made of high dielectric permittivity single-size pellets. Choosing the suitable aspect-ratio of the metamaterial building blocks, a broadband magnetic response and a left-handed behavior are simultaneously obtained. This is a promising step towards innovative and complex electromagnetic functions, involving cheap and easy made metamaterials for millimeter wave applications.

Innovative architectures in ferroelectric multi-materials: Chemistry, interfaces and strain

Breakthroughs can be expected in multi-component ceramics by adjusting the phase assembly and the micro–nanostructure. Controlling the architecture of multi-materials at different scales is still challenging and provides a great opportunity to broaden the range of functionalities in the field of ferroelectric-based ceramics. We used the potentialities of Spark Plasma Sintering (SPS) to control a number of key parameters regarding the properties: anisotropy, interfaces, grain size and strain effects. The flexibility of the wet and supercritical chemistry routes associated with the versatility of SPS allowed designing newferroelectric composite ceramics at different scales. These approaches are illustrated through various examples based on our work on ferroelectric/dielectric composites.

Interface control in BaTiO3 based supercapacitors

Core shell BaTiO3 based particles sintered using advanced processes provide a high control of grain boundaries in bulk composites. As a result, supercapacitor behavior was evidenced which came from the balance between inner grain conductivity and grain boundary dielectric barrier. Thanks to the core-shell structure of the starting particles, improved control of the effective dielectric parameters can be achieved.

Tailoring the Composition of Eu3+-Doped Y3NbO7 Niobate: Structural Features and Luminescent Properties Induced by Spark Plasma Sintering

The defective fluorite-related Y3NbO7 host lattice was doped with Eu3+ ions to understand the influence of spark plasma sintering (SPS) process on this host lattice. The intrinsic disorder due to the occurrence of oxygen vacancies results in amorphous-type responses of the luminescent cations, and the spectral distribution varies as a function of the niobium content. Two spectral fingerprints of europium emissions were clearly enhanced. The correlation between luminescence, X-ray diffraction, and electron diffraction characterizations shows the existence of local inhomogeneity. Indeed, the particular nonequilibrium sintering conditions allowed pointing out a lack of miscibility within the Y3NbO7 solid solution domain. Thus, the SPS pellet is a composite of two extreme compositions. This phase demixing is mainly induced by the pressure coupled with a current effect that makes possible ionic migration in this Y3NbO7 ionic conductive matrix.

Magnetic dipole and electric dipole resonances in TiO2 microspheres at terahertz frequencies

In a non-magnetic dielectric sphere of high-permittivity ( <20), effective magnetic response occurs as a result of the 1st Mie mode, known as the magnetic dipole resonance. This resonance produces a similar effect as split ring resonators, making it possible to use dielectric spheres as metamaterial components. In the terahertz (THz) part of the spectrum, where dielectrics with  ~100 can be found, all-dielectric metamaterials can potentially reduce absorption and provide isotropic and polarization-independent properties. In this contribution, we discuss TiO2 micro-spheres, ~1/10 of the wavelength in diameter. Such spheres are expected to support the magnetic and electric dipole resonances. To detect these resonances in a single TiO2 microsphere we use THz near-field microscopy with the sub-wavelength size aperture probe. This method allows detection of Mie resonances in single sub-wavelength spheres. Fano-type line-shape is observed in the near-field amplitude and phase spectra. The narrow line-width of the magnetic resonance and the subwavelength size of the TiO2 microspheres make them excellent candidates for realizing low-loss THz metamaterials.