Pascal Dufour

Origin of colossal permittivity in BaTiO3 via broadband dielectric spectroscopy

Barium titanate (BT) ceramics with Ba/Ti ratios of 0.95 and 1.00 were synthesized using spark plasma sintering (SPS) technique. Dielectric spectroscopy (frequency range from 40 Hz to 1 MHz and temperature range from 300 K to 30 K) was performed on those ceramics (SPS BT). SPS BT showed extremely high permittivity up to ∼105, which can be referred to as colossal permittivity, with relatively low dielectric loss of ∼0.05. Data analyses following Debye relaxation and universal dielectric response models indicate that the origin of colossal permittivity in BT ceramics is the result of a hopping polaron within semiconducting grains in combination with interfacial polarization at the insulating grain boundary. Furthermore, the contributions of each polarization mechanism to the colossal permittivity in SPS BT, such as a hopping polarization, internal barrier layer capacitance effect, and electrode effect, were estimated.

Biomimetic nanocrystalline apatites: Emerging perspectives in cancer diagnosis and treatment

Nanocrystalline calcium phosphate apatites constitute the mineral part of hard tissues, and the synthesis of biomimetic analogs is now well-mastered at the lab-scale. Recent advances in the fine physico-chemical characterization of these phases enable one to envision original applications in the medical field along with a better understanding of the underlying chemistry and related pharmacological features. In this contribution, we specifically focused on applications of biomimetic apatites in the field of cancer diagnosis or treatment. We first report on the production and first biological evaluations (cytotoxicity, pro-inflammatory potential, internalization by ZR-75-1 breast cancer cells) of individualized luminescent nanoparticles based on Eu-doped apatites, eventually associated with folic acid, for medical imaging purposes. We then detail, in a first approach, the preparation of tridimensional constructs associating nanocrystalline apatite aqueous gels and drug-loaded pectin microspheres. Sustained releases of a fluorescein analog (erythrosin) used as model molecule were obtained over 7 days, in comparison with the ceramic or microsphere reference compounds. Such systems could constitute original bone-filling materials for in situ delivery of anticancer drugs.

Synthesis and Optical Properties of Cu2CoSnS4 Colloidal Quantum Dots

Monodisperse quaternary chalcopyrite Cu2CoSnS4 colloidal quantum dots have been synthesized by acid peptization of a tailored Cu2CoSnS4 precursor displaying loosely packed, ultrafine primary crystallites. Well-defined peaks shifted to higher energy compared to the Cu2CoSnS4 bulk band gap value were observed on the UV-Vis absorption curve consistent with a quantum confinement behavior. First investigations by room temperature time resolved photoluminescence (TRPL) spectroscopy suggest that the photoluminescence emission does not arise from a donor–acceptor recombination.

Novel contributions on luminescent apatite-based colloids intended for medical imaging

The setup of colloidal hybrid nanosystems based on biomimetic calcium phosphate apatites doped with europium ions has recently raised great interest in the pharmacological community, especially due to their bio-inspired character. This is especially relevant in relation with medical imaging for cancer diagnosis. Questions however remain in relation to a number of applicability aspects, some of which have been examined in this contribution. In a first part of this work, we explored further the luminescence properties of such colloidal nanoparticles. We pointed out, upon excitation of europium, the existence of some non-radiative de-excitation via the vibration of O-H oscillators located at the vicinity of the Eu3+ luminescent centers. The replacement of Eu3+ by Tb3+ ions, less prone to non-radiative de-excitation, was then tested in a preliminary way and can be seen as a promising alternative. In a second part of this work, we inspected the possibility to store these colloids in a dry state while retaining a re-suspension ability preserving the nanometer size of the initial nanoparticles, and we propose a functional protocol involving the addition of glucose prior to freeze-drying. We finally showed for the first time, based on titrations of intracellular Ca2+ and Eu3+ ions, that folic acid-functionalized biomimetic apatite nanoparticles were able to target cancer cells that overexpress folate receptors on their membrane, which we point out here in the case of T-47-D breast carcinoma cells, as opposed to ZR-75-1 cells that do not express folate receptors. This contribution thus opens new exciting perspectives in the field of targeted cancer diagnosis, thus confirming the promise of biomimetic apatites-based colloidal formulations.

Purification of biomimetic apatite-based hybrid colloids intended for biomedical applications: a dialysis study

The field of nanobiotechnology has lately attracted much attention both from therapeutic and diagnosis viewpoints. Of particular relevance is the development of colloidal formulations of biocompatible nanoparticles capable of interacting with selected cells or tissues. In this context, the purification of such nanoparticle suspensions appears as a critical step as residues of unreacted species may jeopardize biological and medical outcomes, and sample purity is thus increasingly taken into account by regulatory committees. In the present work, we have investigated from a physico-chemical point of view the purification by dialysis of recently developed hybrid colloids based on biomimetic nanocrystalline apatites intended for interacting with cells. Both Eu-doped (2mol.% relative to Ca) and Eu-free suspensions were studied. The follow-up of the dialysis process was carried out by way of FTIR, TEM, XRD, pH and conductivity measurements. Mathematical modelling of conductivity data was reported. The effects of a change in temperature (25 and 45°C), dialysis medium, and starting colloid composition were evaluated and discussed. We show that the dialysis method is a well-adapted and cheap technique to purify such mineral-organic hybrid suspensions in view of biomedical applications, and we point out some of the characterization techniques that may prove helpful for following the evolution of the purification process with time.

Complex diffusion behavior of oxygen in nanocrystalline BaTiO3 ceramics

(18)O/(16)O exchange annealing and subsequent Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis is used to investigate oxygen transport in dense, nanocrystalline (average grain size d ≈ 300 nm) ceramics of nominally un-doped BaTiO3. Isotope penetration profiles are obtained as a function of temperature, 973 < T/K < 1173, at an oxygen activity aO2 = 0.20 and as a function of oxygen activity, 0.002 < aO2 < 0.20, at T = 1073 K. All isotope profiles show the same unusual shape: a flattened profile over the first ∼10(2) nm, followed by a short, conventional diffusion profile. We demonstrate that the entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation based on an increase in the local oxygen diffusion coefficient close to the surface. This position-dependent increase is attributed to additional oxygen vacancies that are generated by diffusion of chlorine impurities out of the ceramics. The presence of chlorine derives from the chemical route necessary to produce nanometric powders: it thus indicates a new manner in which nanocrystalline ceramics may differ from their microcrystalline counterparts.

First-principles electronic structure calculations for the whole spinel oxide solid solution range MnxCo3−xO4 (0 ≤ x ≤ 3) and their comparison with experimental data

Transition metal spinel oxides have recently been suggested for the creation of efficient photovoltaic cells or photocatalysts. These compounds can be easily tuned by doping to adapt their electronic or magnetic properties. However, their cation distribution is very complex and band structures are still a subject of controversy. We propose a complete density functional theory investigation of MnxCo3-xO4 compounds, using different approximations in order to explain the variation of these properties as a function of composition (for 0 ≤ x ≤ 3) and determine the electronic structure over the whole solid solution range. A detailed study of their atomic structure, magnetic properties and electronic structure is given and compared with experimental data. The unit cell volume calculated for each composition is in agreement with the volume obtained experimentally in ceramics, while a cubic-to-tetragonal structural transition is predicted at x = 2.0. An antiferromagnetic to ferrimagnetic behavior is observed at the lowest ordering temperature depending on the composition. The band gap, deduced from our band structure calculations, strongly decreases upon doping of the end members Co3O4 and Mn3O4, but is partly restored by the tetragonal distortion. A direct band gap, close to 0.5-0.8 eV, is calculated for 0.25 ≤ x ≤ 2.25, justified by inter-metal transitions from Mn ions on octahedral sites.

Reliability of X7R Multilayer Ceramic Capacitors During High Accelerated Life Testing (HALT)

Multilayer ceramic capacitors (MLCC) are essential components for determining the reliability of electronic components in terms of time to failure. It is known that the reliability of MLCCs depends on their composition, processing, and operating conditions. In this present work, we analyzed the lifetime of three similar X7R type MLCCs based on BaTiO3 by conducting High Accelerated Life Tests (HALT) at temperatures up to 200 °C at 400 V and 600 V. The results were adjusted to an Arrhenius equation, which is a function of the activation energy (Ea) and a voltage stress exponent (n), in order to predict their time to failure. The values of Ea are in the range of 1–1.45 eV, which has been reported for the thermal failure and dielectric wear out of BaTiO3-based dielectric capacitors. The stress voltage exponent value was in the range of 4–5. Although the Ea can be associated with a failure mechanism, n only gives an indication of the effect of voltage in the tests. It was possible to associate those values with each type of tested MLCC so that their expected life could be estimated in the range of 400–600 V.

Recent progress in the shaping and sintering of barium titanate nanoparticles. Application to high permittivity capacitors

The miniaturisation of electronic components required for many devices involves an increase their volumic efficiency. This work focus on the size reduction of capacitors. They are two ways for increasing the capacity of such passive components. One is to find materials with high permittivity, the other is to reduce the thickness of the films used in multilayers ceramics. Barium titanate, BaTiO3 (BT) nanoparticles were sintered using Spark Plasma Sintering (SPS) because the material presents interesting properties when obtained by SPS [1-3]. In this case, very unusual interesting dielectric properties, a so called “colossal” permittivity value up to several thousands. Moreover, no temperature dependence is observed. However, the values of dielectric losses are too high to consider industrial applications. In order to reduce these dielectric losses, different approaches have been proposed to control the chemistry of grain boundaries. We present here the structural, microstructural and electrical properties of ceramics sintered by SPS from BaTiO3 powders elaborated by co-precipitation method. Different routes tested to improve the dielectric properties of the ceramics are presented and discussed.On the other side, thick films of BT have been prepared. The objective of this work was to develop a BaTiO3 material having an accurate particle size to reduce the thickness of sintered films down to 1 μm. The rheological properties of the slurry have been optimized and, thanks to a suitable tape casting process, homogeneous thin films of 1 μm thickness could be obtained. Some properties of the films are described.