Carbon nanotubes prepared in situ in a cellular ceramic by the gelcasting-foam method

The synthesis of carbon nanotubes from an oxide solid solution foam is reported for the first time. A foam of Mg0.9Co0.1Al2O4 solid solution is prepared by the gelcasting-foam method using notably mono- and di-functional acrylate monomers. Using a surfactant in the alkylpolyglucoside family allows to prevent structure changes in the foam before the onset of polymerisation. The wet ceramic foam is dried in air, producing a foam with an open porosity. Total porosity is equal to about 98% and the diameter of the pores is in the range 25–300 mm, about 90% being smaller than 200 mm. The Mg0.9Co0.1Al2O4 foam is reduced in H2–CH4 atmosphere, giving rise to a CNTs–Co MgAl2O4 composite foam. Using the foam instead of the corresponding powder allows a fourfold increase in the production of carbon nanotubes, more than 95% of which have only 1 (70%) or 2 walls

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

Mixed manganese spinel oxides: optical properties in the infrared range

Spinel oxides in manganite family are studied in terms of optical properties in the infrared range (3–12 lm). The reflectivity is measured on sintered pellets. The complex refractive index is estimated by fitting hemispherical directional reflectance in both polarizations, perpendicular and parallel. The influence of different metallic cations (Ni, Co, Fe, Cu) is compared. In particular, in the case of manganese nickel copper oxides, the impact of variations in copper and nickel contents is evaluated. Cationic distribution is determined and correlated to the optical characteristics. These materials, usually used for NTC thermistor applications, are investigated for IR charges in coating

Structure and electrical properties of single-phase cobalt manganese oxide spinels Mn3xCoxO4 sintered classically and by spark plasma sintering(SPS)

Cobalt manganese oxide spinels Mn3�xCoxO4 (with 0.98pxp3) were prepared by the thermal decomposition in air of oxalate precursors. The influence of the thermal treatments on the structure of these materials is emphasized. Single-phase ceramics were obtained after optimization of the sintering parameters. A precise phase diagram for the Co–Mn–O system is proposed according to thermal stability and structure of oxide powders. The electrical measurements on single-phase ceramics show that low values of resistivity can be achieved. The conduction could take place through jumps of polarons between Mn3+ and Mn4+ on octahedral sites. These compounds present interesting electrical characteristics for negative temperature coefficient (NTC) thermistor application

Synthesis and characterization of non-stoichiometric nickel-copper manganites

Non-stoichiometric nickel–copper manganites Ni Cu Mn h O were synthesized by thermal decomposition of x y 32x2y 3d / 4 41d mixed Ni Cu Mn C O , nH O oxalates in air at low temperature (623–673 K). X-ray diffraction showed that, x / 3 y / 3 (32x2y) / 3 2 4 2 for a nickel content x $0.1, the oxalates precipitated presented a mixed crystal structure up to a limit value of copper Ni extent, whereas the oxalates obtained with x ,0.1 were not mixed. This could be explained by the intermediate structure Ni of nickel oxalate (b orthorhombic form) between those of copper and manganese (a monoclinic form) oxalates. The structure (a or b) of the mixed oxalates obtained was also investigated and their lattice parameters are given. The Ni Cu Mn h O oxides crystallize in the spinel structure in a wide range of composition and a stabilizing effect x y 32x2y 3d / 4 41d 2 21 of copper was evidenced. They are highly divided (Sw.100 m g ) however Sw tends to decrease with increasing y . Cu The non-stoichiometry d of such nickel–copper manganites was for the first time determined by selective titration (gas chromatography) of the oxygen released during TPR experiments in argon. The technique is presented and the results, along with those obtained with manganese oxide Mn O and nickel manganites synthesized in the same conditions, showed that d 5 8 depended both on the decomposition temperature of the oxalate and on the chemical composition of the oxide. Such results should provide interesting data concerning the cationic distributions of these non-stoichiometric nickel–copper manganites

Influence of the composition of a H2-CH4 gas mixture on the catalytic synthesis of carbon nanotubes-Fe/Fe3C-Al2O3 nanocomposite powders

CNTs–Fe/Fe3C–Al2O3 nanocomposite powders have been prepared by selective reduction of an a-Al1.9Fe0.1O3 solid solution in H2–CH4 gas mixtures (0, 1.5, 3, 4.5, 6, 9, 12, 14, 16, 18, 24, 30 and 45 mol% CH4). The powders have been studied using macroscopic and microscopic techniques. The CNTs are arranged in very long bundles homogeneously dispersed in the composite powder. Most CNTs have less than four walls and are free of pyrolytic or amorphous carbon deposits. The inner diameter is in the range 1–6 nm, which could indicate that the catalyst particles active for CNTs formation are in this size range. The reduction of the Fe3+ ions to metallic Fe is highly favoured by the presence of CH4 in the reduction atmosphere. There are more Fe3C than a-Fe particles located at the surface of the matrix grains for CH4 contents higher than 4.5 mol%, however, the exact nature of the catalytically active particles remains an open question. Compositions in the range 9–18 mol% CH4 give the best

Synthesis of carbon nanotube–Fe-Al2O3 nanocomposite powders by selective reduction of different Al1.8Fe0.2O3 solid solutions

Al1.8Fe0.2O3 solid solutions have been prepared as amorphous, η (cubic) and α (corundum) phases. The oxides have been reduced in a H2–CH4 gas mixture at 900 or 1000 °C, giving rise to composite powders containing alumina, α- and γ-Fe, Fe3C and different forms of carbon including nanotubes, thick tubes and spheroidal particles. The powders have been investigated using a combination of chemical analysis, X-ray diffraction, Mössbauer spectroscopy, scanning and transmission electron microscopy, thermogravimetric analysis and specific surface area measurements. Using the stable form (corundum) of Al1.8Fe0.2O3 as starting material favours the formation of carbon nanotubes compared to the other forms of carbon. This could partly result from the fact that the metal nanoparticles formed upon reduction of the α solid solution, which act as a catalyst for CH4 decomposition and possibly nanotube nucleation, are smaller than when using amorphous or η solid solutions. Moreover, the crystallization of these latter compounds during the reduction in some way provokes the entrapment of carbon within the oxide grains. The nanotubes, most of which are less than 10 nm in diameter, are arranged in bundles several tens of micrometers long

IR spectroscopic study of NO and CO adsorptions on nonstoichiometric nickel-copper manganites

The adsorptions and co-adsorption of nitric oxide and carbon monoxide on nonstoichiometric nickel and nickel–copper manganites have been investigated in situ by transmission infrared spectroscopy. Time-dependent and temperature-dependent data have been acquired to investigate the nature of the molecule–surface interactions. The surface chemistry of NO was found to be particularly rich, involving numerous surface species (mononitrosyls, dinitrosyls, adsorbed N2O, nitrites/nitrates), whereas that of CO was somewhat simpler. The competitive adsorption of NO and CO was evidenced. However, with time, NO tended in all cases to colonize the surface of the samples, and Cu+ cations were shown to be the only access points of CO to the surface of such oxides in the presence of NO. Finally, a time sequence of reactions was shown in the case of NO adsorption

Zirconia–spinel composites. Part II: mechanical properties

The mechanical properties (fracture strength, fracture toughness, and Vickers microhardness) of MgAl2O4 and x wt% ZrO2–MgAl2O4 (1 # x # 30) hot-pressed materials were measured at room temperature. Two kinds of materials, which were prepared from either ball-milled or attrited powders, were investigated. Compared with an unreinforced spinel, a twofold (or greater) increase in both strengthening and toughening was measured for materials with the highest zirconia content (30 wt%). It was found that dispersing only 1 wt% of zirconia in the spinel produced a marked increase in the Vickers microhardness. The total zirconia content appears to be a key parameter

Zirconia-spinel composites. Part I: synthesis of powders and dense materials

MgAl2O4 and x wt% ZrO2–MgAl2O4 (1 # x # 30) composite powders were prepared by the urea combustion route. The powders were further ground by either ball milling or attrition to refine the grain size. Zirconia was found solely in the tetragonal form. Dense materials were prepared by hot pressing. The spinel matrix grains were submicronic in size. The ZrO2 particles were homogeneously dispersed at the grain junctions of the matrix, and their average size increased from less than 0.20 mm to ca. 0.45 mm with increasing content of ZrO2. The formation of monoclinic ZrO2 particles proceeded accordingly, being easier in materials with a finer matrix grain size (ex-attrition)