Search

Amorphous Fe76-xNixCr4(SiB)20 (x = 12 and 25) ribbons were prepared using melt spinning technique. Their structural, magnetic and magnetocaloric properties were investigated using X-ray diffraction, vibrating sample magnetometer, and Mossbauer spectrometry. The samples were found to be amorphous. Room Mossbauer measurements ensured the amorphous state of Fe76-xNixCr4(SiB)20 ribbons and get further information about the effect of Ni content in the hyperfine parameters. The magnetic transition was analyzed in terms of Landau theory around the transition temperature. The amorphous alloys exhibit a second order transition from ferromagnetic to paramagnetic phase. With increasing Ni concentration, the Curie temperature decreases as well as the maximum entropy change ( - Δ S m max ) and relative cooling power (RCP). In addition, a phenomenological model was used to predict reasonably the magnetocaloric parameters.

This work focuses on the study of structural and magnetic properties of Mg2+-substituted cobalt nanoferrite (Co1-xMgxFe2O4, 0.0 ≤ x ≤ 1.0). The samples were synthesized by a coprecipitation route and then annealed at 850 °C for 2 h under atmospheric air. The X-ray diffraction patterns show that the samples crystallize in a cubic spinel structure with space group F d 3 ‾ m , and 3% secondary phase was identified as hematite ( R 3 ‾ c ) for x = 1.0. The average crystallite size (D) and the lattice parameter were greatly influenced by Mg content (D decreased from 38 to 20 nm). Transmission electron microscopy investigations revealed that the grains have an almost spherical shape and furthermore that the samples are chemically homogeneous. Magnetic measurements at 5 K revealed a wasp-waist magnetic hysteresis behavior of Co–Mg ferrite. The coercivity of Co0.4Mg0.6Fe2O4 with D = 25 nm was enhanced, with a maximum value of 4000 Oe. The room temperature Mossbauer spectra were fitted with two Zeeman sextets, which reflects the incorporation of Fe3+ in the A and B sublattices of the spinel structure. The hyperfine field of these sites decreases similarly to the saturation magnetization with increasing Mg content. The spectrum corresponding to x = 1.0 was calculated with the help of the hyperfine field distribution. Moreover, Mossbauer spectra, fore x = 0.0 and 1.0 were recorded at 6 and 80 K and were adjusted by the superposition of three sextets: one sextet for the A site and two sextets for the B sites (B1 and B2). The hyperfine parameters such as the hyperfine field and the isomer shift were improved at 6 and 80 K.

The aim of this work is to study the cobalt-iron oxide nanoparticles. This material has been synthesized using flash autocombustion method. The as-prepared sample was calcined at different temperatures such as 450, 650 and 850°C for 2 hours. To investigate the structural, morphological and magnetic properties, the X-ray diffraction technique, scanning electron microscopy and Mussbauer spectrometry were employed. Indeed, X-ray diffraction results reveal that the annealing samples exhibit a cubic spinel structure with space group Fd-Sm- In addition, they do not exhibit any secondary peaks which confirms the formation of single phase, which is cobalt ferrite spinel (i.e. CoFe204). The sample annealed at 850°C shows fine and intense peaks reflecting a good crystallinity of the sample. Otherwise, the scanning electron microscope observations indicate an irregularity of grains shape while the X-ray microanalysis spectra confirm the presence of only Co, Fe and O ions signature of the purity and homogeneity of our samples. On other hand, the 57Fe Mossbauer spectra measured at room temperature present a Zeeman patterns, which are fitted with different components reflecting the ferrimagnetic behavior of the samples. Mossbauer hyperfine parameters were also calculated for each annealed temperature. 850°C is the optimal annealing temperature to obtain spinel ferrite with good crystallinity and hyperfine field of 474.4 kOe.

In the present work, the magnetocaloric effect of La1-xNaxMn1-yFeyO3 (x = 0.1, 0.2 and y = 0, 0.1) was investigated using magnetic characterization data. The X-ray diffraction confirms that all samples synthesized by flash combustion and calcined at 1000 °C have been crystallized in a rhombohedral structure with an R 3 ¯ c space group. According to the magnetization versus temperature results, LaMnO3 is a ferromagnetic material with a Curie temperature TC = 95 K. The substitution of the rare earth La by alkali magnetic metal Na leads to an increase of TC to 265 and 330 K accompanied with large magnetic entropy change / - Δ S M max / of 2.88 and 4.88 J.Kg-1K−1 for x = 0.1, 0.2, respectively. The addition of 10% of Fe in La1-xNaxMn1-yFeyO3 decreases the Curie temperature and the magnetic entropy change, while, the relative cooling efficiency increases. The Mossbauer spectrum recorded at room temperature represents a paramagnetic character of the manganites and proves the distribution of Fe3+ in the samples.

La0.62Nd0.05Ba0.33MnO3 nanoparticles were prepared using co-precipitation process. Single phase compound formation was confirmed using X-ray diffraction analysis. The elaborated sample annealed at 1000 °C in air during 10 h, crystallizes in the rhombohedral perovskite structure with R-3c space group. Magnetic measurements reveal the presence of a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) phase in the vicinity of Curie temperature (270 K). Furthermore, the Arrott plots reveal a second order magnetic transition. Interestingly, La0.62Nd0.05Ba0.33MO3 sample presents a large relative cooling power (RCP) it is 315 J.kg−1 at 5 T). The critical exponents were studied by both approaches: Modified Arrott plots and Kouvel Fisher.

Due to their relatively large magnetocaloric effect, Ba and K doped LaMnO3 can be used in magnetic cooling systems. In this study, La1-xBaxMnO3 (x = 0.1; 0.2) and La0.9K0.1MnO3 powders were synthesized by the flash combustion method using glycine as a fuel and chelating agent. A subsequent calcination at 1000 °C was performed. Different characterization tools were used to carry out a detailed study of the structural, morphological, thermal, electrical, and magnetic behaviors of the doped materials. The structure of un-doped and doped samples was examined by X-ray Diffraction. According to magnetic data, La0.9K0.1MnO3 exhibits a ferromagnetic-paramagnetic phase transition at 271 K and a magnetic entropy variation under a magnetic induction of 4 T /ΔSM/ = 5.33 J kg−1K−1. On the other hand, the Curie temperature of La1-xBaxMnO3 increases with the Ba content from 157 K for x = 0.1 to 200 K for x = 0.2, whereas /ΔSM/ decreases from 2.56 J kg−1K−1 to 2.07 J kg−1K−1, respectively. Accordingly, it has been proven that the highest cooling performance corresponds to the K doped compound. The evolution from ferromagnetic to paramagnetic state was also determined with electron paramagnetic resonance spectroscopy. The temperature dependence of the resistivity shows a semiconductor character in La0.9Ba0.1MnO3 and 2 maxima of metal-insulator (M-I) transitions in the K doped sample. Enhanced magnetoresistance effects have been observed in La0.9K0.1MnO3 close to the ferromagnetic-paramagnetic transition temperature.

The synthesis of electrode materials with controlled microstructural characteristics and high conductivity over 1000 S cm −1 is a key factor in improving the performance of fuel cells. In this work, we propose an alternative route to the partial sintering of a powder mixture to control the porosity of a ceramic–metal composite currently used as an anode material in Protonic Ceramic Fuel Cells working at 400–600 °C. This new method is based on the use of nanopowdered cermet materials and starch in gelling form for the elaboration of Ni–BaCe 0.9 Y 0.1 O 3 (Ni–BCY) cermets. The microstructure and the electrical properties have been investigated with respect to the initial starch content. A porous microstructure consisting of homogenously distributed Ni and BCY phases is observed. It is shown that the degree of open porosity and the electrical conductivity of the cermets are sensitive to the starch content. Anodes elaborated from initial compositions comprising 10 and 20 wt.% of starch offer sufficient mechanical strength and an open porosity >30 vol.%. The measured conductivities vary with the composition, from 1000 to 3000 S cm −1 at 600 °C. The use of starch in gelled form as porogen allows the engineering of the porosity and the control of pore shape and distribution.

In this paper, we present the synthesis and characterizations of NaSICON-type ionic conducting ceramics of the general formula Na 1+ x M 1.775 Si x −0.9 P 3.9− x O 12 with 1.8 ≤ x ≤ 2.2 and M = Zr or Hf. The effect of the total substitution of zirconium by hafnium on electric properties has been studied. The various compositions were prepared by using the sol–gel method and the synthesized precursors were characterized by coupled DTA–TG. The oxides obtained after pyrolysis of the precursors were identified by X-ray diffraction. A sintering study by thermodilatometry permits to select the best thermal cycle adapted to our ceramics. Furthermore, the electric conductivity of the sintered ceramic samples was characterized by complex impedance spectroscopy. These results show that ceramics containing Zr synthesized by soft method, present a higher total conductivity than those obtained in literature (to be around 10 −4 S cm −1 ). The total substitution of Zr by Hf still improves this conductivity for some compositions.

The lithium-substituted NASICONS materials, with general formula Li2.8Zr2−y Si1.8−4y P1.2+4y O12 where 0 ≤ y ≤ 0.45, were synthesized by the sol–gel method which consisted in calcinig the preliminary precursors at temperatures higher than their crystallization temperatures. The calcined powders have been characterized by physico-chemical techniques: coupled DTA-TGA, XRD and CIS. The coupled DTA-TGA was used to determine the crystallization temperatures and leading to the estimation of the calcined temperatures. The XRD analyses show the obtaining of only the single phase without any detection of other phases as: ZrO2, SiO2, Li2O, ZrSiO4, Li3PO4, Li2SiO3 or Li2ZrO3, according to their JCPDS file cards and suggest that the decrease in the zirconium concentration does not modify the stability of the lithium-substituted NASICONS, whereas the complex impedance spectroscopy results have highlighted the best total electric conductivity for y value equal to 0.225.

In this work, we present the synthesis and the characterization of ionic conducting ceramics of NaSICON-type (Natrium super ionic conductor). The properties of this ceramic make it suitable for use in electrochemical devices. These solid electrolytes can be used as sensors for application in the manufacturing of potentiometric gas sensors, for the detection of pollutant emissions and for environment control. The family of NaSICON that we studied has as a general formula Na{sub 2.8}Zr{sub 2-y}Si{sub 1.8-4y}P{sub 1.2+4y}O{sub 12} with 0=

This review reports on the recent advances in the synthesis and physico-chemical properties of palladium-containing perovskites. Initially, the perovskite structure is briefly reviewed, then palladium-containing perovskites synthesis and physico-chemical properties are detailed. The applications of palladium-containing perovskites in catalysis; namely, NO reduction, methane combustion, methanol as well as ethanol oxidation, are briefly highlighted. The involvement and the important contribution of palladium-containing perovskites in cross-coupling reactions, especially Suzuki-Miyaura, Sonogashira, Ulmann and Grignard, are discussed.

This review reports on the recent advances in the synthesis and physico-chemical properties of palladium-containing perovskites. Initially, the perovskite structure is briefly reviewed, then palladium-containing perovskites synthesis and physico-chemical properties are detailed. The applications of palladium-containing perovskites in catalysis; namely, NO reduction, methane combustion, methanol as well as ethanol oxidation, are briefly highlighted. The involvement and the important contribution of palladium-containing perovskites in cross-coupling reactions, especially Suzuki-Miyaura, Sonogashira, Ulmann and Grignard, are discussed.

Nanomaterials based on spinel ferrites have already numerous applications, mainly based on their magnetic properties. Recently, catalytic properties of nickel, copper and cobalt ferrites in the conversion of CO and CH4 were evidenced, opening a new field of applications for these materials [1]. Magnetic properties of nanoparticles, as well as catalytic properties, will depend on their size, but also on their shape and size distribution. These parameters are linked to the elaboration method. The properties of transition metal spinel oxides depend also on the nature of the transition metal and on his site occupation in the structure. The location of the cations in the spinel structure is related to their octahedral or tetrahedral sites preference, but also to the synthesis method. For magnetite Fe3O4, which adopts the inverse spinel structure, the tetrahedral sites are fully occupied by Fe3+ ions, whereas octahedral sites are occupied by Fe3+ and Fe2+. For Co3O4, the octahedral and tetrahedral sites are respectively occupied by Co3+and Co2+ (normal spinel structure). Intermediate situations occur for CoxFe3−xO4,. Thus, three different compositions were prepared CoFe2O4, Co0.6 Fe2.4O4, Co1.4 Fe1.6O4.

Langasite La3Ga5SiO14 (noted LGS) is a piezoelectric material presenting a strong electromechanical deformation at high temperature (up to 1,000–1,200°C). Classical solid state reaction at high temperature (1,450°C) is generally needed to synthesize the LGS phase. In this study, we present a new synthesis way for polycrystalline langasite (LGS), involving a series of specific thermal cycles at moderate temperatures (

Cermet electrodes of Ni–BaCe O.9 Y 0.1 O 2.95 and assemblies of Ni-cermet/thin film electrolyte/Ni-cermet have been elaborated from a ceramic nanopowder synthesized by flash combustion. Anode/electrolyte/anode assemblies with a fully dense electrolyte of thickness ca. 20 μm were fabricated by a co-pressing and co-sintering process followed by reduction of NiO to Ni metal. In the reduced state, the anode microstructures comprise a uniform and homogeneous distribution of Ni metal and ceramic phase with relatively high porosity and high conductivity. Impedance spectroscopy showed that there were two major rate-limiting processes for the electrode reaction and that cell performance is determined by the electrolyte resistance. A total specific resistance of 0.4 Ω cm 2 at 600 °C was obtained for optimized assemblies.

Controlled titanium alkoxide mineralization in the presence of phosphonated, dendrimeric nano-building blocks provides a new family of hierarchically porous dendrimer-bridged titanium dioxide materials.

Advanced cermet electrodes of Ni-BaCeO.9Y0.1O2.95, suitable for Intermediate Temperature PCFC, have been elaborated using gelling starch as pore forming agent. SEM and mercury porosimetry measurements show a strong dependence of the microstructure upon the starch content. As the starch content increase, a gradual creation of percolating starch network is clearly shown. An half cell showing a gradient of porosity and a thin electrolyte dense layer was successfully obtained by a simple co-pressing and co-sintering process. The polarization ASR (0.13 Ohm.cm2) is obtained for the cell architecture investigated in this work