Nanostructuration and combinatorial chemistry

Oxides show remarkable magnetic and electrical properties. Those properties can depend on the intrinsic natural interfaces of the materials (grain boundaries, stones, etc.) as well as the artificial (composite materials, multilayers). New materials intrinsic properties can be exploited to build more efficient and reliable RAM memories - tunable capacities for energy efficiency  and high K capacitors.

 

Combinatorial PLD synthesis

Physical properties of oxide materials often result from multiple interactions in competition, and thus vary rapidly according to their composition and structure. To identify the compounds with enhanced or new properties, it is necessary to finely scan through the various phase diagrams. Doing it with a finite number of bulk materials leads to a discrete scan. At GREMAN we have developed and refined a synthesis technique which allows to grow continuous spread composition films: Combinatorial Pulsed Laser Deposition or CPLD. The idea is to explore lines of compositions taken through a ternary phase diagram (triangle below) on a single substrate. The successive deposition from 3 targets is spread on the substrate using a moving shadow mask synchronized with laser pulses. The local composition varies continuously; the one line solid solution is synthesized unit cell by unit cell during one deposition cycle.

 

High dielectric constant ceramics development

The laboratory developed un-doped and doped CaCu3Ti4O12 (CCTO) materials for multilayer ceramic capacitor. The dielectric performances of this material were improved, observed in figure 1 : (i) high dielectric constant (>104), (ii) low loss tangent , (iii) while keeping an industrial mode of production and iv) environment-friendly. It was proposed in agreement with the literature, that the colossal permittivity could find its origin from extrinsic properties of the material. Indeed, by association of semiconducting grains and insulating grain-boundaries, there would be a phenomenon of internal barriers they named “Internal Barrier Layer Capacitance” effect (IBLC). These results are very promising, nevertheless, some technological bottle-necks remain. In particular, the obtained capacitors possess prejudicial and insufficient breakdown voltages. To answer this problem and taking account the mechanism of these properties, materials with a new "Core-Shell” microstructure were elaborated. Such a structure consists of a core structure made of a dielectric material and covered entirely with a shell structure made of another insulator oxide (SiO2, TiO2 ...). This microstructure observed by scanning and transmission electron microscopy is presented in the figure 2. These micro-organizations were obtained by sol-gel technologies, but also by cold-plasma coating, see figure 3. Plasma treatments could lead to a significant benefit in the core / shell interaction, either by functionalizing the core in order to improve the adhesion of the shell, or by a direct plasma shell coating. This system of “semiconducting” grains isolated from each other by an insulating shell is expected to decrease the loss tangent and to increase the insulation resistance.

 

                                                              

Fig1 : Epsilon and loss tangent frequency dependence for CCTO coated with SiO2.            Fig 3 : Cold plasma for material deposition.


Fig2 : Transmission and Scanning Electron Microscopy Images of CaCu3Ti4O12 covered with 10 nm of SiO2.

Innovative non-thermal plasma coating for “core–shell” CaCu3Ti4O12 material

Colossal permittivity of CaCu3Ti4O12 (CCTO) makes it a very interesting candidate for capacitor applications. To improve its properties, an innovative physical method based on a pulsed non-thermal micro-plasma treatment was set up to realize the coating of CCTO’s grains (core) with silicon oxide (shell). This method is adapted to control the thickness and homogeneity of the shell, which will allow a better control of grain–grain boundary interfaces and improve the properties of this material. Best result is obtained for the set of gas mixture: Ar/O2/hexamethyldisiloxane (HMDSO) = 2028 N cm3 min−1 /7.84 N cm3 min−1 /523 mg h−1 , respectively, in plasma with a shell thickness of 50 nm. This study offers a new opportunity to quickly synthetize core–shell materials with a dry technique and without almost no secondary product resulting from the chemical reaction because it is in the gaseous state. A complete analysis of the plasma by optical emission spectroscopy in the UV-visible range shows that HMDSO molecules are totally dissociated in atomic (Si, C, and O) or simple radical species (C2 and CH) in the plasma phase. In addition, several thermometer species (OH°, CH, CN, N2, and N2 + ) are used to estimate excitation temperatures of the plasma (Trot, Tvib, and Te = 300 K, 2400–3700 K, and 5.3 eV, respectively) that clearly shows the non-equilibrium character and the efficiency of this plasma.
Published under an exclusive license by AIP Publishing.
Journal of Applied Physics 130, 163305 (2021); https://doi.org/10.1063/5.0061180