Corine Bonningue

About the interesting properties of mixed-valence defect spinel ferrites for mass storage media

When the spinel ferrites present a large surface/volume ratio (i.e when they are in the form of thin films or fine powders) they can be oxidized into mixed-valence defect ferrites. These original spinels have magnetic, magneto-optical and optical properties making them interesting materials for high density recording media.

Cation-Deficient Spinel Ferrites : Application for High-Density Write-Once Optical Recording

Thin films of defect spinel ferrites can be used as write-once read-many media working with blue wavelengths. In fact, because these non-stoichiometric ferrites are metastable, they can be transformed into corundum phases at moderate temperatures by a laser spot. The transformed regions have different optical indices from the starting ferrite film, making the readout process possible. Recording density of about 5 bits/µm2 was demonstrated.

Integration of P-CuO Thin Sputtered Layers onto Microsensor Platforms for Gas Sensing

P-type semiconducting copper oxide (CuO) thin films deposited by radio-frequency (RF) sputtering were integrated onto microsensors using classical photolithography technologies. The integration of the 50-nm-thick layer could be successfully carried out using the lift-off process. The microsensors were tested with variable thermal sequences under carbon monoxide (CO), ammonia (NH3), acetaldehyde (C2H4O), and nitrogen dioxide (NO2) which are among the main pollutant gases measured by metal-oxide (MOS) gas sensors for air quality control systems in automotive cabins. Because the microheaters were designed on a membrane, it was then possible to generate very rapid temperature variations (from room temperature to 550 °C in only 50 ms) and a rapid temperature cycling mode could be applied. This measurement mode allowed a significant improvement of the sensor response under 2 and 5 ppm of acetaldehyde.

Highly Sensitive Sputtered ZnO:Ga Thin Films Integrated by a Simple Stencil Mask Process on Microsensor Platforms for Sub-ppm Acetaldehyde Detection

The integration of a 50-nm-thick layer of an innovative sensitive material on microsensors has been developed based on silicon micro-hotplates. In this study, integration of ZnO:Ga via radio-frequency (RF) sputtering has been successfully combined with a low cost and reliable stencil mask technique to obtain repeatable sensing layers on top of interdigitated electrodes. The variation of the resistance of this n-type Ga-doped ZnO has been measured under sub-ppm traces (500 ppb) of acetaldehyde (C2H4O). Thanks to the microheater designed into a thin membrane, the generation of very rapid temperature variations (from room temperature to 550 °C in 25 ms) is possible, and a rapid cycled pulsed-temperature operating mode can be applied to the sensor. This approach reveals a strong improvement of sensing performances with a huge sensitivity between 10 and 1000, depending on the working pulsed-temperature level.

Bi2(C2O4)3·7H2O and Bi(C2O4)OH Oxalates Thermal Decomposition Revisited. Formation of Nanoparticles with a Lower Melting Point than Bulk Bismuth

Two bismuth oxalates, namely, Bi2(C2O4)3·7H2O and Bi(C2O4)OH, were studied in terms of synthesis, structural characterization, particle morphology, and thermal behavior under several atmospheres. The oxalate powders were produced by chemical precipitation from bismuth nitrate and oxalic acid solutions under controlled pH, then characterized by X-ray diffraction (XRD), temperature-dependent XRD, IR spectroscopy, scanning electron microscopy, and thermogravimetric differential thermal analyses. New results on the thermal decomposition of bismuth oxalates under inert or reducing atmospheres are provided. On heating in nitrogen, both studied compounds decompose into small bismuth particles. Thermal properties of the metallic products were investigated. The Bi(C2O4)OH decomposition leads to a Bi-Bi2O3 metal-oxide composite product in which bismuth is confined in a nanometric size, due to surface oxidation. The melting point of such bismuth particles is strongly related to their crystallite size. The nanometric bismuth melting has thus been evidenced ∼40 °C lower than for bulk bismuth. These results should contribute to the development of the oxalate precursor route for low-temperature soldering applications.

FIB plan view lift-out sample preparation for TEM characterization of periodic nanostructures obtained by spinodal decomposition in Co1.7Fe1.3O4 thin films.

There is a miscibility gap in the CoFe2O4–Co3O4 phase diagram. In this miscibility gap, the oxides can be subjected to a spinodal transformation. It has already been observed in oxides consisting of crystals greater than or equal to 100 nm that spinodal decomposition leads to the formation of two alternating iron-rich and cobalt-rich spinel phases. The pseudo-periodic alternation occurs approximately every 5 nm. In the miscibility gap, thin films of pure iron cobaltites, consisting of crystallites of the order of 10 nm in diameter and around 300 nm in thickness, undergo transformation when they are treated at 600 °C for several hours. X-ray diffraction and Raman spectroscopy clearly reveal this transformation, which is accentuated as a function of the treatment time. An electron microscopy study of the cross-sections (view of the films along their thickness), confirms the progressive separation of the former spinel oxide in iron-rich and cobalt-rich spinel phases, without however revealing a pseudo-periodic organization of these phases, whatever the time of treatment. In an attempt to reveal this organization, a specific method of preparation has been implemented to extract the upper part of the films parallel to their basic plane and to observe the crystallites in plan view. The alternation of the iron- and cobalt-rich phases could, however, only be found in the largest crystallites. It seems that the nanometric size of the crystallites prevents the establishment of a pseudo-periodic organization of the phases during the periodic transformation. The observation of compositional anomalies in the grain boundaries seems to support the hypothesis related to a nanometric effect of the crystallization.