R. Legros

Ionic configuration and cation distribution in cubic nickel manganite spinels NixMn3−xO4 (0.57<x<1) in relation with thermal histories

Cation distribution has been determined in cubic manganite spinels NixMn3−xO4 (0.57<x<1) for samples slowly cooled (5 K/min) by combination of powder neutron diffraction and thermogravimetric oxidation results. In the composition range 0.80<x<1 a not negligible (⋍0.10) fraction of Mn3+ ions is found at A sites of the spinel lattice and an estimation of the Mn3+- O distance is carried out. Therefore, a fine analysis of overall data indicates that the Mn3+ concentration at A site cancels for x<0.65. The changes in electrical conductivity and in infrared spectrum are also accounted for by a cationic migration dependent on the thermal history of the specimens.

Recent progress in the fabrication of ceramic negative temperature coefficient thermistors

Abstract Negative temperature coefficient (NTC) thermistors are ceramics of industrial interest because of the very high dependence of their electrical resistance on temperature. Generally, these materials consist of transition metal (Cu, Co, Ni, etc.) spinel manganites. This work shows the simultaneous influence of the purity and the morphology of initial powders on the densification. Moreover, correlations between electrical properties, thermal stability and microstructure have been established. Results of a comparative study on ceramics prepared either by conventional or chemical methods are given. Increasing the performance yields thermistors which have great precision and stability.

Structural and electrical properties of iron manganite spinels in relation with cationic distribution

Abstract Single-phase iron manganite spinels, Mn 3−x Fe x O 4 with O ≤ x ≤ 1.05, were prepared by thermal processing of iron-manganese coprecipitated formate precursors. Powder X-ray diffraction (XRD) analysis shows a cubictetragonal transition for x = 1.05. The Mossbauer spectroscopy indicates the presence of only trivalent iron ions in both sites of the spinel structure. The electrical measurements show that these iron manganites have the characteristics of negative temperature coefficient thermistors. Moreover, in these solid solutions, the resistivity ρ decreases with increasing of iron content x . Correlation of the results obtained by XRD, Mossbauer spectroscopy and electrical measurements permits one to infer the cationic distribution, given by Mn 1 − y 2+ Fe y 3+ (Fe z 3+ Mn 2 − x 3+ Mn y 2+ O 4 2− with x = y + z .

Influence of chemical composition on sintering of bismuth-titanium-doped zinc oxide

Liquid-phase sintering (LPS) of ceramics issued from powders of bismuth-titanium-doped zinc oxide, made of spherical grains constituted of small crystallites, were studied by dilatometry. A correlation can be established between the shape of the shrinkage curves and the microstructure of the ceramics. During sintering the spherical grains are destroyed as soon as the liquid phase appears and the rearrangement proceeds directly between crystallites. Systematic studies were carried out on bismuth-titanium-doped ZnO. For small Bi contents, the beginning of a solid-phase sintering process occurs. For 1 at % Bi in the binary system Bi-doped ZnO, rapid shrinkage leads to a well-densified ceramic with a grain size of 70 μm on average. The influence of the amount of titanium iny% Ti-1% Bi-doped ZnO is discussed. Whatever the amount of Ti, LPS is observed. The shrinkage curves depend greatly on the amount of Ti: only a Ti content ofy=0.6 % leads to an increase in grain size (100 μm on average).

Structural properties of nickel manganite Ni x Mn3?x O4 with 0.5 ?x ? 1

AbstractSingle-phase nickel manganite spinels, NixMn3−xO4, with 0.5 ⩽x ⩽ 1, were prepared by a careful thermal processing of nickel-manganese coprecipitated oxalate precursors. Powder X-ray diffraction analysis of the spinel revealed the presence of cubic single spinel phase with parametera which decreases with nickel content. The lattice parameter variation can be explained in terms of the distribution of Ni2+ ions on the octahedral sites. Therefore, a fine analysis of data shows that some Ni2+ ions (forx>0.56) are located in tetrahedral sites. The percentage of nickel in A-sites increases with nickel content (x) following the relation % Ni2+ in A sites =P = − 82.1x2+192.4x−81.5 and thus the general formula for cation distribution is

TEM characterization of nickel and nickel-cobalt manganite ceramics

{\text{Mn}}_{{\text{1 - }}y}^{{\text{2 + }}} {\text{Ni}}_y^{{\text{2 + }}} [{\text{Ni}}_{x{\text{ - }}y}^{{\text{2 + }}} {\text{Mn}}_{{\text{2 - 2(}}x - y)}^{{\text{3 + }}} {\text{Mn}}_{x{\text{ - }}y}^{{\text{4 + }}} ]{\text{O}}_{\text{4}}^{{\text{2 - }}}

Cation distribution and oxidation states in nickel manganites NiMn2O4 and Ni0.8Mn2.2O4 from powder neutron diffraction

withy=xP/100This relationship explains the electrical properties of semiconducting nickel manganite ceramics.

The preparation, characterization and electrical properties of electroceramics made of copper—cobalt manganite spinel: Mn2.6−xCo0.4CuxO4, 0 ≤ x ≤ 1

Abstract The electrical conductivity of manganite spinels such as Cu x Mn 3− x O 4 , Co 0.4 Cu x Mn 2.6− x O 4 and Ni x Mn 3− x O 4 has been investigated versus composition x . For these three spinels the conductivity is described by a “hopping mechanism” between Mn 3+ and Mn 4+ ions on octahedral sites of the spinel lattice and reaches a maximum when the number of Mn 3+ ions is equal to the number of Mn 4+ ions.