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.

 

Tools and methods

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.

 
oxides OR1 image 1

 
Repetition of this cycle lead to a film of the desired thickness. Structural, chemical and physical properties of the solid solution are then measured locally and statistically.
 
oxides OR1 image 2

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.

 

OXYDES OR1 high dielectric constant ceramic development, graph                                                                oxides OR1 image 3

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

OXYDES OR1 High dielectric constant ceramic development
Fig2 : Transmission and Scanning Electron Microscopy Images of CaCu3Ti4O12 covered with 10 nm of SiO2.