Jean-Claude M'Peko

Electrical properties of zirconia–alumina composites

Abstract Sintered zirconia–alumina composites, prepared in a wide range of compositions, are studied in terms of their electrical response. Both grain conductivity and dielectric constant show the typical characteristics expected from the percolation theory, with v c =0.14±0.2 as the critical zirconia volume fraction for the onset of conduction. When the conducting zirconia phase is calcined prior to forming the composite, the whole system still shows a strongly reduced conduction response even for zirconia volume fractions ( v ) in the range of 0.4–0.5, after which it is considerably enhanced for v =0.7. These results are discussed in terms of (i) the influence of the materials microstructure and (ii) the effect of stress resulting from the alumina sintering on the calcined zirconia grains on the overall electrical response of the composite.

One-step approach for preparing ozone gas sensors based on hierarchical NiCo2O4 structures

Nanostructured semiconducting oxides have been used as resistive gas sensors of toxic and non-toxic gases, but little emphasis has been placed on ozone sensing. Here we present a new ozone gas sensor based on hierarchical NiCo2O4 cubic structures synthesized via a facile urea-assisted co-precipitation method and annealed at 450 °C, which showed a low detection level. Ozone detection was carried out through electrical measurements with an optimized performance at 200 °C, with fast response (∼32 s) and recovery (∼60 s) time with suitable concentration range (from 28 to 165 ppb) for technological applications. Furthermore, NiCO2O4 platelets are selective to ozone compared to other oxidizing and reducing gases. The low detection level can be attributed to the coexistence of 3D structures based on hexagonal platelet-like and porous flower-like shape, which were revealed by field emission scanning electron microscopy (FE-SEM). In summary, NiCo2O4 is promising for detection of sub-ppb levels of ozone gas.

Fluoride-modified electrical properties of lead borate glasses and electrochemically induced crystallization in the glassy state

Lead fluoroborate glasses were prepared by the melt-quenching technique and characterized in terms of (micro)structural and electrical properties. The study was conducted on as prepared as well as temperature- and/or electric field–treated glass samples. The results show that, in the as-prepared glassy-state materials, electrical conductivity improved with increasing the PbF2 glass content. This result involves both an increase of the fluoride charge carrier density and, especially, a decrease of the activation energy from a glass structure expansion improving charge carrier mobility. Moreover, for the electric field–treated glass samples, surface crystallization was observed even below the glass transition temperature. As previously proposed in literature, and shown here, the occurrence of this phenomenon arose from an electrochemically induced redox reaction at the electrodes, followed by crystallite nucleation. Once nucleated, growth of β-PbF2 crystallites, with the indication of incorporating reduced ...

Ozone sensing properties of nickel phthalocyanine:ZnO nanorod heterostructures

We report on the chemiresistive gas sensing characteristics of ZnO nanorods (NRs) modified by a thin layer of nickel phthalocyanine (NiPc). Ozone detection was carried out through electrical measurements with an optimized performance at 250°C, good reproducibility and suitable concentration range (from 80 to 890 ppb) for technological applications. The hybrid NiPc:ZnO films had superior performance to pure ZnO nanorods in terms of response time and sensitivity. The response times were 22 s and 26 s, respectively, whereas the ratio of resistances under ozone and air was 3.27 for NiPc:ZnO films and 2.56 for the pure ZnO NRs. The improvement in response time is attributed to the large surface area generated with the coating of the ZnO nanorods with the NiPc layer. Significantly, images taken with field-emission scanning electron microscopy (FE-SEM) indicated that the ZnO nanorods were fully covered with NiPc. X-ray diffraction measurements (XRD) revealed a preferential growth of the nanorod-like structures along the [100] direction. In summary, a successful approach has been developed to functionalize ZnO nanorods, which is promising for detection of ppb levels of ozone gas.

Electric field-assisted ultrafast synthesis of nanopowders: a novel and cost-efficient approach

A current trend in materials processing is the synthesis of high-reactivity powders with a particle size averaged at the nanoscopic scale. This is normally achieved by considering chemical routes that allow processing of the materials at temperatures markedly lower than those required in the conventional method. Here we introduce a simple but novel approach that enables ultrafast synthesis of materials by using electric fields. The case of CaCu3Ti4O12, where traditional chemical methods have usually revealed unable to synthesize a nanosized single-phase powder, is presented. In zero-field processing, the end-product powder prepared here via a modified polymeric precursor method exhibited, for instance, an average particle size of 300 nm. In the following, we show that thermal cycling of the precursor powder under an electric field input leads to a substantial drop in synthesis temperature, attributable to enhanced charge diffusion processes, ending with an average particle size sensibly reduced and, finally, rendering possible the production of nanopowders (<100 nm) by adjusting the maximum electric current allowed to flow across the material during processing.

Ferro-and Piezo-Electric Properties of Nb-Doped PZT 65/35 Ceramics

The ferroelectric and piezoelectric properties of conventionally prepared Nb-doped PZT 65/35 ceramics are studied in this work. The influence of Nb content and excess PbO is considered. The measured properties are found to be basically dependent on the degree of densification and structural phase development. In particular, the nature of the ferro-to para-electric phase transition is noted to better fit a generalized rather than Smolenskii-Isupov equation. In nice agreement with Bokov model, substitution of Nb 5+ for (Zr,Ti) 4+ is found to induce only poorly diffuse phase transition in these materials. The electrical and electromechanical properties reported in this work are in magnitude comparable to those exhibited by PZT-based materials.