Sophie Guillemet-Fritsch

Structure, thermal stability and electrical properties of zinc manganites

Zinc manganites ZnxMn3−xO4 were prepared by the thermal decomposition in air of oxalate precursors. The structure and the thermal stability of the oxides were determined and correlated with their electrical properties (resistivity, resistivity drift). The substitution of manganese by zinc in the spinel structure has a stabilizing effect against oxidation and phase transformation. The strong energetic Zn2+–O bonding slows down the cationic migration in the lattice. The zinc manganites are suited to high temperature NTC thermistors. The resistivity range is large and no ageing phenomenon is observed in the ceramics.

Origin of colossal permittivity in BaTiO3 via broadband dielectric spectroscopy

Barium titanate (BT) ceramics with Ba/Ti ratios of 0.95 and 1.00 were synthesized using spark plasma sintering (SPS) technique. Dielectric spectroscopy (frequency range from 40 Hz to 1 MHz and temperature range from 300 K to 30 K) was performed on those ceramics (SPS BT). SPS BT showed extremely high permittivity up to ∼105, which can be referred to as colossal permittivity, with relatively low dielectric loss of ∼0.05. Data analyses following Debye relaxation and universal dielectric response models indicate that the origin of colossal permittivity in BT ceramics is the result of a hopping polaron within semiconducting grains in combination with interfacial polarization at the insulating grain boundary. Furthermore, the contributions of each polarization mechanism to the colossal permittivity in SPS BT, such as a hopping polarization, internal barrier layer capacitance effect, and electrode effect, were estimated.

High-quality nickel manganese oxalate powders synthesized in a new segmented flow tubular reactor

Abstract High-quality nickel manganese oxalates have been prepared using an innovative approach for the production of homogeneous powders, the continuous Segmented Flow Tubular Reactor (SFTR). This new reactor is mainly composed of a mixer, a segmenter, a tubular section and a decanter. Mixed Ni–Mn oxalates are synthesized in the SFTR. The powders present controlled morphology, narrow particle size distribution, high purity and desired stoichiometry. Their characteristics are compared to those of powders obtained in a batch reactor. These oxalates are precursors for nickel manganites, used as negative temperature coefficient thermistor (NTC) ceramics.

X-ray and neutron diffraction studies on nickel zinc manganite Mn2.35−xNi0.65ZnxO4 powders

Abstract Nickel zinc manganite Mn 2.35− x Ni 0.65 Zn x O 4 powders were investigated by X-ray and neutron diffraction experiments. The variation of the cell parameters determined from X-ray data was used to get an estimate of the cationic distribution. Neutron diffraction experiments were performed to obtain room-temperature structural parameters, the tetrahedral and octahedral average scattering lengths and the oxygen–cation distances. Because of the opposite signs of the scattering lengths ( b Mn and b Zn ), the neutron data allow to determine, with high precision, the concentration of the cations on the different sites of the spinel structure. The cationic distribution determined from these data indicated that a small part of the Zn 2+ cations occupy the octahedral sites, for zinc-rich oxides ( x >1).

Magnetic properties of cobalt and manganese oxide spinel ceramics

Magnetic susceptibility measurements, magnetization and neutron diffraction results at low temperature for cobalt and manganese oxide spinel ceramics are presented. The Curie temperature varies similarly with the sample composition in ceramics and powders. The experimental molar Curie constant variation is explained by the presence of Co2+, CoIII, Mn3+ and Mn4+, and possibly Co3+ in the octahedral sites for the cobalt rich phases. The magnetic moments of the cations in tetrahedral and octahedral sites are not collinear and the global magnetization is oriented in a third direction.

Microstructure and electrical properties of Ni–Zn manganite ceramics☆

Abstract Nickel zinc manganite ceramics Mn 2.34− x Ni 0.66 Zn x O 4 (0≤ x ≤1.02) were investigated using structure, microstructure and electrical property interrelationships. The resistivity increased with zinc content whereas the activation energy remained constant. The drift of resistivity under thermal constraint, commonly observed in NTC thermistors was almost nil when the zinc content was sufficiently high ( x ≥0.7). Homogeneous microstructures and high densities were obtained when the ceramics were prepared from fine powders. The Ni/Zn/Mn/O system prepared by ‘chimie douce’ has interesting NTC properties for industrial applications.

Complex diffusion behavior of oxygen in nanocrystalline BaTiO3 ceramics

(18)O/(16)O exchange annealing and subsequent Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis is used to investigate oxygen transport in dense, nanocrystalline (average grain size d ≈ 300 nm) ceramics of nominally un-doped BaTiO3. Isotope penetration profiles are obtained as a function of temperature, 973 < T/K < 1173, at an oxygen activity aO2 = 0.20 and as a function of oxygen activity, 0.002 < aO2 < 0.20, at T = 1073 K. All isotope profiles show the same unusual shape: a flattened profile over the first ∼10(2) nm, followed by a short, conventional diffusion profile. We demonstrate that the entire isotope profile can be described quantitatively by a numerical solution to the diffusion equation based on an increase in the local oxygen diffusion coefficient close to the surface. This position-dependent increase is attributed to additional oxygen vacancies that are generated by diffusion of chlorine impurities out of the ceramics. The presence of chlorine derives from the chemical route necessary to produce nanometric powders: it thus indicates a new manner in which nanocrystalline ceramics may differ from their microcrystalline counterparts.

Contrasting conduction mechanisms of two internal barrier layer capacitors: (Mn, Nb)-doped SrTiO3 and CaCu3Ti4O12

The d.c. conduction is investigated in the two different types of internal barrier layer capacitors, namely, (Mn, Nb)-doped SrTiO3 (STO) and CaCu3Ti4O12 (CCTO). Scanning electron microscopy (SEM) and Capacitance - Voltage (C-V) analysis are performed to estimate the effective electric field at a grain boundary, EGB. Then, the d.c. conduction mechanism is discussed based on the J (Current density)-EGB characteristics. Three different conduction mechanisms are successively observed with the increase of EGB in both systems. In (Mn, Nb)-doped STO, non-linear J-EGB characteristics is temperature dependent at the intermediate EGB and becomes relatively insensitive to the temperature at the higher EGB. The J- EGB at each regime is explained by the Schottky emission (SE) followed by Fowler-Nordheim (F-N) tunneling. Based on the F-N tunneling, the breakdown voltage is then scaled by the function of the depletion layer thickness and Schottky barrier height at the average grain boundary. The proposed function shows a clear linear relationship with the breakdown. On the other hand, F-N tunneling was not observed in CCTO in our measurement. Ohmic, Poole-Frenkel (P-F), and SE are successively observed in CCTO. The transition point from P-F and SE depends on EGB and temperature. A charge-based deep level transient spectroscopy study reveals that 3 types of trap states exist in CCTO. The trap one with Et ∼ 0.65 eV below the conduction band is found to be responsible for the P-F conduction.

Thermal expansion of (Ba1 − xLax)Ti1 − x/4O3 solid solutions

Deformation and the thermal expansion coefficient of ceramic samples of (Ba1 − xLax)Ti1 − x/4O3 solid solutions (x = 0, 0.026, 0.036, 0.054) were studied in the temperature range 120–700 K. Based on an analysis of the data obtained, the temperature-composition phase diagram is refined, and the temperature dependence of the polarization is calculated. The results are discussed in combination with the dielectric measurement data.

Dielectric properties of colossal permittivity materials: An update

During the last ten years, a lot of interest has been devoted to the so-called colossal dielectric constant (CDC) material. The first materials exhibiting this behavior were the perovskyte-based ceramics based on the CaCu3Ti4O12 composition (CCTO). Relative dielectric permittivity can attain values up to (or even larger than) 105. Nevertheless, their losses are still high, the lower values ranging 10%, in a narrow frequency range, thus limiting their applications. Most work on this type of materials aims to reduce these losses and widen their useful frequency range. On the other hand, the underlying physical mechanisms responsible of the CDC are still under study. While the analysis of broadband impedance spectroscopy measurements leads most of the authors to propose an interfacial polarization mechanisms (at the electrodes or at internal barriers), there is a limited number of complementary electrical characterization techniques, for the moment, they comfort the proposed interfacial polarization mechanisms. In the present work, several characterization techniques like I-V, Sawyer-Tower (ST) measurements, and time-domain polarization are used to characterize these materials. One of our main results is the observation of a non-symmetrical response of these materials related to the direction of the polarization. These results are observed for both macroscopic level on bulk polycrystalline material and within individual grains of the same samples. These results do not fit current accepted models for polarization for CDC materials.