B. Chevalier

Sonochemical Approach to the Synthesis of Fe3O4@SiO2 Core−Shell Nanoparticles with Tunable Properties

In this study, we report a rapid sonochemical synthesis of monodisperse nonaggregated Fe(3)O(4)@SiO(2) magnetic nanoparticles (NPs). We found that coprecipitation of Fe(II) and Fe(III) in aqueous solutions under the effect of power ultrasound yields smaller Fe(3)O(4) NPs with a narrow size distribution (4-8 nm) compared to the silent reaction. Moreover, the coating of Fe(3)O(4) NPs with silica using an alkaline hydrolysis of tetraethyl orthosilicate in ethanol-water mixture is accelerated many-fold in the presence of a 20 kHz ultrasonic field. The thickness of the silica shell can be easily controlled in the range of several nanometers during sonication. Mossbauer spectra revealed that nonsuperparamagnetic behavior of obtained core-shell NPs is mostly related to the dipole-dipole interactions of magnetic cores and not to the particle size effect. Core-shell Fe(3)O(4)@SiO(2) NPs prepared with sonochemistry exhibit a higher magnetization value than that for NPs obtained under silent conditions owing to better control of the deposited silica quantities as well as to the high speed of sonochemical coating, which prevents the magnetite from oxidizing.

Addition of nanosized Cr2O3 to magnesium for improvement of the hydrogen sorption properties

Abstract The combination of (i) the catalytic effects of Cr2O3, (ii) reactive mechanical grinding (RMG), and (iii) nanosize particles allow a huge improvement in the sorption properties of magnesium. For short milling duration, the absorption kinetics were already as good as that reported for nanocrystalline (Mg+Cr2O3) mixtures. The desorption process is also improved but not in a similar manner. It is assumed that the RMG of nanosized particles of Cr2O3 results in the formation of some Cr atoms in the mixture which greatly influence the sorption behavior.

Hydrogen absorption and desorption in Nd2Fe17 and Sm2Fe17

The uptake of hydrogen by Nd2 Fe17 and Sm2Fe17 has been monitored in a thermopiezic analyser as a function of temperature at an initial pressure of 1 bar (105 Pa). The first stage of hydrogen absorption around 250° C yields R2Fe17Hy (R = Nd, Sm) withy ≅ 2.2; this compound retains the Th2Zn17 structure of the starting alloy but the cell volume is increased by about 3%. The Curie temperature increases from 57 to 175° C for R = Nd and from 115 to 253° C for R = Sm. A second stage of hydrogen absorption at about 600° C corresponds to disproportionation of the alloy into α-Fe + RH2−ɛ.

Supercritical fluid processing: a new route for materials synthesis

Supercritical fluids exhibit a range of unusual properties that can be exploited for new reactions which are qualitatively different from those involving classical solid state chemistry. After giving a brief introduction to these fluids we describe their use in inorganic chemistry and related fields. We then present two examples concerning different areas of solid state chemistry: (i) the formation of novel inorganic nanoparticles; (ii) the preparation of new open-structure oxy(hydroxy)fluorides, thus showing the advantages of this supercritical fluid processing that can be seen as an alternative method to regular solution chemistry or solid-gas reactions.

Magnetic order in ternary compounds RRh2Si2 and RRu2Si2

Abstract Investigations by neutron diffraction and magnetic measurements are reported on the ternary suicides RRh2Si2 (R  Ce, Nd, Tb) and RRu2Si2 (R  Nd, Tb) with the tetragonal structure of the ThCr2Si2 type. CeRh2Si2 orders antiferromagnetically below 36 K with a wave vector k = [ 1 2 1 2 0] and magnetic moments parallel to the c-axis (m0) = 1.50 μ/Ce). The magnetic structure of TbRh2Si2 is antiferromagnetic (k = [001]) below TN = 94 K, the magnetic moments (8.5 μB/Tb at T = 15 K) are also parallel to the c-axis. NdRh2Si2 has the same magnetic structure below TN = 57 K, with m0 = 2.80 μB/Nd parallel to the c-axis. NdRu2Si2 exhibits a more complicated magnetic structure: below TN = 24 K it develops a sine-wave modulation (k = [0.130.130]) of the magnetic moments always parallel to the c-axis, with an amplitude Ak = 3.23 μB/Nd; a squaring of the magnetic structure occurs at about 15 K (m0 = 2.84 μB), and at T

Magnetic ordering in TbRh2Si2 and CeRh2Si2

Abstract Magnetic measurements and a neutron diffraction study have been carried out on the ternary compounds TbRH2Si2 and CeRh2Si2 with a tetragonal structure of ThCr2Si2-type. TbRh2Si2 is found to order antiferromagnetically below TN = 92K with moments along the c-axis. The structure of wave vector k=[001], consists of a stacking of ferromagnetic (001) planes with a +−+− sequence. CeRh2Si2 develops below TN = 36 K a structure described by a wave vector k=[ 1 2 1 2 0] in which the coupling within a (001) plane is antiferromagnetic. This change of coupling together with the large negative value of θp=−61K has been attributed to a change of the sign of the first nearest neighbour in-plane interaction which would correspond to an exchange mechanism different from the usual RKKY one in CeRh2Si2.

Reversible switching between p- and n-type conduction in the semiconductor Ag10Te4Br3

Semiconductors are key materials in modern electronics and are widely used to build, for instance, transistors in integrated circuits as well as thermoelectric materials for energy conversion, and there is a tremendous interest in the development and improvement of novel materials and technologies to increase the performance of electronic devices and thermoelectrics. Tetramorphic Ag(10)Te(4)Br(3) is a semiconductor capable of switching its electrical properties by a simple change of temperature. The combination of high silver mobility, a small non-stoichiometry range and an internal redox process in the tellurium substructure causes a thermopower drop of 1,400 microV K(-1), in addition to a thermal diffusivity in the range of organic polymers. The capability to reversibly switch semiconducting properties from ionic to electronic conduction in one single compound simply by virtue of temperature enables novel electronic devices such as semiconductor switches.

Hydrogen sorption of Mg-based mixtures elaborated by reactive mechanical grinding

The use of mechanical grinding (MG) under H2 of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process that allows suppressing the activation procedure generally requested for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface. A critical milling time exists below which the diffusion process is improved and above which no further improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 h. The addition of Co (catalyst), YNi (hydrogen pump) or oxides (abrasive element and nucleation centre) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of our mixtures are comparable with thus reported for MgH2–metal mixtures.

A new family of rare earth compounds, the ternary silicides RE2RhSi3 (RE = Y, La, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er) - crystal structure electrical and magnetic properties

Abstract New ternary silicides RE 2 RhSi 3 (RE = Y, La, Ce, Nd, Sm, Gd, Tb, Dy, Ho, Er) have been prepared. They crystallize with a hexagonal symmetry structure which is derived from the AlB 2 -type. Si and Rh atoms are inside RE 6 distorted trigonal prisms and are ordered in a two dimensional sublattice perpendicular to the c axis. The a and c parameters are approximately twice those of the corresponding disilicides RESi 2 . La 2 RhSi 3 and Y 2 RhSi 3 are not superconducting down to 1.6 K. Nd 2 RhSi 3 is ferromagnetic at 15 K, but the other silicides order antiferromagnetically with metamagnetic phase transitions for certain of them.

Structural, thermal, and electrical properties of CrSi2

Stoichiometric CrSi2 was prepared by arc melting and compacted by uniaxial hot pressing for property measurements. The crystal structure of CrSi2 was investigated using the powder x-ray diffraction method. From the Rietveld refinement, the lattice parameters were found to be a = 4.427 57 (7) and c = 6.368 04 (11) A, respectively. The thermal expansion measurement revealed an anisotropic expansion in the temperature range from room temperature 800 K with αa = 14.58×10−6/K, αc = 7.51×10−6/K, and αV = 12.05×10−6/K. The volumetric thermal expansion coefficient shows an anomalous decrease in the temperature range of 450–600 K. The measured electrical resistivity ρ and thermoelectric power S have similar trends with a maxima around 550 K. Thermal conductivity measurements show a monotonic decrease with increasing temperature from a room temperature value of 10 W m−1 K−1. The ZT values increase with temperature and have a maximum value of 0.18 in the temperature range studied. An analysis of the electronic band structure is provided.