Marian Stingaciu

An alternative method to modify the sensitivity of p-type NiFe2O4 gas sensor

Our objective was to determine the role of cooling rate on gas-sensing properties of annealed nano-grained nickel ferrite (NiFe2O4). The sol–gel auto combustion method was used for the preparation of NiFe2O4. To estimate structural and microstructural features, X-ray diffraction, and scanning electron microscopy techniques were used. For gas-sensing measurements different volatile organic compounds (VOCs) were used as testing gases. To identify the contribution of the different sensing layer elements to the conduction, ac impedance spectroscopy (IS) measurements were performed. It was found that the sensors cooled with lower rate exhibited better sensing performance, due to increase of resistance. Overall, this article covers an alternative method for modifying nickel ferrite gas sensor sensitivity.

The magnetodielectric effect in BaTiO3–SrFe12O19 nanocomposites

A nanoscale composite consisting of 30 vol% magnetic strontium hexaferrite SrFe12O19 embedded in a high dielectric permittivity matrix of BaTiO3 has been produced by the spark plasma sintering method. The morphology and phase composition were characterized by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) techniques. TEM micrographs indicate that no considerable grain growth occurs and different strained regions were formed by the sintering process. XRD investigations show no evidence of a chemical reaction between the constituents. The dielectric properties of such a nanocomposite were investigated versus temperature (50–300 K), frequency (100 Hz to 100 kHz) and magnetic field (0–5.6 Tesla). A magnetodielectric (MD) effect was detected in the whole studied temperature range. Above 150 K the occurrence of sharp magnetodielectric resonances around 1 kHz leads to a considerable enhancement of the effect. At temperatures below 150 K a positive MD effect was detected which is independent of the frequency.

Magnetoresistivity in CoFe2O4-BaTiO3 composites produced by spark plasma sintering

Ceramic composites of (CoFe2O4)x−(BaTiO3)(1−x) with x = 0.20, 0.25, and 0.30 have been synthesized by spark plasma sintering at 1000 °C under a uniaxial pressure of 75 MPa using commercially available nanopowders as starting materials. The composites reveal a negative magnetoresistivity effect of about − 1% at room temperature. An enhancement was observed at lower temperatures, and an effect of − 4.1% was found in the composite with x = 0.30 at 150 K by applying an external magnetic field of 25 kOe. The temperature dependence of the resistivity indicates variable range hopping involved in charge transport.

Introduction of Metal Oxides into Mg2Si Thermoelectric Materials by Spark Plasma Sintering

Oxide incorporation into thermoelectric Mg2Si-based materials was performed starting from commercial Mg2Si and commercial metal oxides by applying ball milling and spark plasma sintering (SPS) processing. The SPS conditions, such as sintering temperature, pressure, and holding time, were optimized with the aim of obtaining both full densification and oxide incorporation. Thermoelectric characterizations, such as Seebeck coefficient and electrical and thermal conductivity, were carried out and related to the pellet compositions. The morphology, composition, and crystallographic structure of the samples were characterized by field-emission scanning electron microscopy, energy-dispersive spectrometry, and x-ray diffraction analyses, respectively.