Matériaux diélectriques

Les matériaux diélectriques sont principalement mis en oeuvre pour la réalisation de condensateurs utilisés dans tous les domaines de l’électro-technique, de la microélectronique et des télécommunications. Certainsmatériaux diélectriques possèdent en plus des propriétés (ferroélectriques, piézoélectriques, pyroélectriques, optiques) ayant des applications dans le domaine des capteurs, des actionneurs et de l’optique non linéaire..

Sr2Ir1-xCuxO4 (x = 0, 0.05 and 0.1): Dilute magnetism in a mixed 3d–5d transition-metal oxide

Herein we have investigated the effect of Cu(3d − transition metal) substitution for the Ir – site on the structure and physical properties of Sr2IrO4. Interestingly, the critical octahedral rotation is found to decrease (increase in Ir – O – Ir angle) with Cu substitution. The resistivity measurements illustrate a significant decrease with Cu substitution, however, keep a semiconducting dependence. Magnetic measurements reveal a decrease in the magnetic transition temperatures. Observed magnetic moment per metal ion shows a decreasing trend as Cu content is increased in the structure. A study of the composition – structure – properties correlation for this interesting and mixed 3d − 5d transition metal system is presented and discussed here

A structural and magnetic study of Nd5Fe2B6 and Nd5Fe2B6D4.1

We investigated the structural and magnetic properties of the boride Nd5Fe2B6 and of the deuteride Nd5Fe2B6D4.1 by means of magnetization and specific heat measurements, Mössbauer spectroscopy and neutron powder diffraction. In Nd5Fe2B6D4.1, deuterium atoms are located in two inequivalent interstitial sites: one octahedral D1 [Nd6] and one tetrahedral D2 [Nd3Fe], both avoiding B as near neighbor. This insertion induces a pronounced anisotropic unit cell expansion along the c-axis. While the Nd5Fe2B6 compound exhibits a magnetic ordering below about 68 K, the deuteride exhibits a lower ordering temperature of about 21 K as a consequence of D insertion in the Nd environment. For both compounds powder neutron diffraction investigations have evidenced a ferromagnetic structure with magnetic moments along the c-axis. However, in the deuteride, the Nd magnetic moments values are reduced and that of Fe vanishes. 57Fe Mössbauer spectroscopy confirms the existence of a small ordered magnetic moment on iron in Nd5Fe2B6 and the presence of two environments for Fe in the deuteride

Indium oxide co-doped with tin and zinc: A simple route to highly conducting high density targets for TCO thin-film fabrication

Indiumoxideco-doped with tin and zinc (ITZO) ceramics have been successfully prepared by direct sintering of the powders mixture at 1300 °C. This allowed us to easily fabricate large highly dense target suitable for sputtering transparent conductingoxide (TCO) films, without using any cold or hot pressing techniques. Hence, the optimized ITZO ceramic reaches ahigh relative bulk density (∼ 92% of In2O3 theoretical density) and higher than the well-known indiumoxidedoped with tin (ITO) prepared under similar conditions. All X-ray diagrams obtained for ITZO ceramics confirms a bixbyte structure typical for In2O3 only. This indicates ahigher solubility limit of Sn and Zn when they are co-doped into In2O3 forming a solid-solution. A very low value of electrical resistivity is obtained for [In2O3:Sn0.10]:Zn0.10 (1.7 × 10−3 Ω cm, lower than ITO counterpart) which could be fabricated to high dense ceramic target suing pressure-less sintering

Presence of Peierls pairing and absence of insulator-to-metal transition in VO2 (A): a structure-property relationship study.

Layered vanadium oxides have been extensively explored due to their interesting metal-insulator transitions and energy conversion/storage applications. In the present study, we have successfully synthesized VO2 (A) polymorph powder samples by a single-step hydrothermal synthesis process and consolidated them using spark plasma sintering. The structural and electronic properties of VO2 (A) are measured over a large temperature range from liquid helium, across the structural transition (400-440 K) and up to 500 K. The structural analysis around this transition reveals an antiferrodistorsive to partially ferrodistorsive ordering upon cooling. It is followed by a progressive antiferromagnetic spin pairing which fully settles at about 150 K. The transport measurements show that, in contrast to the rutile archetype VO2 (R/M1), the structural transition comes with a transition from semiconductor to band-type insulator. Under these circumstances, we propose a scenario with a high temperature antiferrodistorsive paramagnetic semiconducting phase, followed by an intermediate regime with a partially ferrodistorsive paramagnetic semiconducting phase, and finally a low temperature partially ferrodistorsive antiferromagnetic band insulator phase with a possible V-V Peierls-type pairing

Effects of partial anion substitution on the thermoelectric properties of silver(I) chalcogenide halides in the system Ag₅<i>Q</i>₂<i>X</i> with <i>Q</i>=Te, Se and S and <i>X</i>=Br and Cl

A selection of mixed conducting silver chalcogenide halides of the general formula Ag5Q2X with Q=sulfur, selenium and tellurium and X=chlorine and bromine has been investigated due to their thermoelectric properties. Recently, the ternary counterpart Ag5Te2Cl showed a defined d10-d10 interaction in the disordered cation substructure at elevated temperatures where Ag5Te2Cl is present in its high temperature α-phase. A significant drop of the thermal diffusivity has been observed during the β−α phase transition reducing the values from 0.12 close to 0.08 mm2 s−1. At the same transition the thermopower reacts on the increasing silver mobility and jumps towards less negative values. Thermal conductivities, thermopower and thermal diffusivity of selected compounds with various grades of anion substitution in Ag5Q2X were determined around the silver -order/disorder β−α phase transition. A formation of attractive interactions could be observed for selenium substituted phases while no effect was detected for bromide and sulfide samples. Depending on the grade and type of substitution the thermopower changes significantly at and after the β−α phase transition. Thermal conductivities are low reaching values around 0.2-0.3 W m−1 K−1 at 299 K. Partial anion exchange can substantially tune the thermoelectric properties in Ag5Q2X phases

Flux growth at 1230 °C of cubic Tb2O3 single crystals and characterization of their optical and magnetic properties

In this work, we present the first crystal growth of cubic Tb2O3 single crystals by a controlled atmosphere flux method which uses a heavy metal free solvent working at less than half the melting temperature of this sesquioxide. Cubic millimeter-sized crystals extracted from as-grown boules are phase (powder XRD) and chemically (GDMS) pure and exhibit a Verdet constant in the visible and near-infrared spectral ranges, which is at least three times higher than that of a commercial Tb3Ga5O12 (TGG) crystal. The 1.36 mm thick crystals display a transmission coefficient higher than 77% over the 525 nm–1.38 μm spectral range. The absorption spectrum, magnetic susceptibility and specific heat measurements on the single crystals confirm the absence of detectable Tb4+ cations and other impurities

Unusual oxidation states give reversible room temperature magnetocaloric effect on perovskite-related oxides SrFe_0.5Co_0.5O₃

The magnetic properties and the magnetocaloric effect are presented for the perovskite-related oxide SrFe0.5Co0.5O3 prepared using electrochemical oxidation. SrFe0.5Co0.5O3 exhibits a second order paramagnetic-ferromagnetic transition close to room temperature (TC=330 K). The maximal magnetic entropy change ΔSMMax , the maximal adiabatic temperature change ΔTad and the refrigerant capacity are found to be equal to respectively 4.0 J/kgK, 1.8 K and 258 J/kg while raising the B-field change from 0 to 5 T

Synthesis, sintering and electrical properties of P-doped Bi₄V₂O₁₁ ceramics

Partial substitution of vanadium by phosphorus in Bi4V2O11, compound belonging to the Aurivillius family, leads to the formation of Bi4V2−xPxO11 solid solution with an orthorhombic symmetry and does not stabilize the γ-high temperature form down to room temperature. X-ray diffraction, density measurements, thermal analysis and IR spectroscopy have been used to confirm the formation of the solid solution. The influence of sintering temperature on the microstructure and the grains size of the samples with different compositions was investigated by the scanning electron microscopy (SEM). The evolution of the electrical conductivity with phosphorus ratio has been investigated by impedance spectroscopy and correlated to the structural transformations

Critical behavior and magnetic entropy change in Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ perovskite manganite

The aim of our study was to investigate the critical exponents of Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ material around its paramagnetic to ferromagnetic phase transition. Reliable critical exponents i.e. β=0.355, γ=1.329, and δ=4.867 at the critical temperature TC=117 K were refined using modified Arrott plot and Kouvel–Fisher methods. The critical exponent δ=4.760±0.129 was further separately determined from the isothermal magnetization data. Finally these critical exponents fulfill the Widom scaling relation δ=1+γ/β. Based on these analysis, the magnetization–field–temperature (M–H–T) data around TC collapses into two independent curves obeying the single scaling equation M(H,ε)=|ε|βf±(H/|ε|β+γ), with ε=(T−TC)/TC is the reduced temperature. Once again, the critical exponents are confirmed by the field dependence of the magnetic entropy change relation ∆SM|T=TC ∞Hn with n=1+(β−1)/(β+1). The obtained critical exponents for Nd0.7Sr0.1Ba0.1Ca0.1MnO3−δ manganite are in accordance with the prediction of the 3D-Heisenberg model with short-range magnetic interactions. The temperature dependence of specific heat and magnetic entropy change ΔSM further supports second order PM–FM phase transition and shed light the A-site-disordered perovskite effect on the magnetocaloric properties