Sylvain Marinel

Fast synthesis of nanocrystalline Mg2Si by microwave heating: a new route to nano-structured thermoelectric materials

The ultra fast synthesis of nanocrystalline Mg(2)Si was carried out using microwave radiation. The elemental precursors were first milled together under dry conditions to get fine particles. The resulting mixture of powders of Mg and Si was cold pressed before being heated by microwave irradiation. Precursors and products were analyzed by X-ray diffraction and scanning electron microscopy. The high energy ball milling parameters utilized to prepare the reactive powders have quite an influence on the behavior of the mixture under irradiation. Moreover, SEM imaging demonstrates that the power and time of irradiation are crucial for the grain growth of the Mg(2)Si and must be adequately controlled in order to avoid the decomposition of the phase. Our results show that we successfully managed to easily and quickly synthesize homogeneous nanocrystalline Mg(2)Si with particle size smaller than 100 nm using a microwave power of only 175 W for two minutes on powders ball milled for two hours.

Top-seeding melt texture growth of single-domain superconducting pellets

The top-seeding melt texture growth (TSMTG) process is investigated in a small-vertical-thermal-gradient furnace in order to better understand the conditions for growing large single-grain pellets. An crystal was used as seed in a Y123 - Y211 composite with additions. It was shown that the role of the seed is to initiate the sympathetic nucleation growth mechanism. Indeed, the additional driving force induced by the seed is clearly demonstrated by a decrease in the undercooling of the system. The seeding crystal introduces a heterogeneous nucleation centre, leading to a very well controlled nucleation rate. Furthermore, we discuss the relation between intrinsic anisotropic growth rate and the processing rate, showing the importance of the thermal conditions of the texturing process to stabilize a sympathetic growth. By careful control of the texturing parameters, we succeeded in producing single-grain pellets as confirmed by a pole figure experiment and neutron diffraction measurements. The levitation force for pellets of 2 cm in diameter reaches 16 N when an NdFeB magnet of the same diameter is used.

Microstructure and transport properties of YBCO zone melted samples processed in a microwave cavity and infra-red furnace

Abstract The unidirectional solidification method was used to prepare well textured YBa 2 Cu 3 O 7− δ (Y123) bars for current lead applications using two different processes. The first one consists in using the Melt Textured Growth method (MTG) in a classical furnace, applying an axial thermal gradient ( G ) of about 60°C/cm. In the second process, a microwave cavity working at 2.45 GHz is used, allowing to obtain larger G (about 260°C/cm). The different values of G corresponding to the two processes were considered to study the effect of G on the Y123 growth rate, microstructure, crystal orientation and transport properties. Thus, for both processes, the maximum pulling rate which permits a continuous growth of Y123 was determined. Microstructure of samples was studied, using polarised optical microscopy for each process. Moreover, the microstructure was correlated with the transport properties of textured bars, using pulse current with the four-point technique. Although the crystal orientation was not perfect ( a – b planes are not aligned with the bar axis), high values of self field J c (higher than 20 kA/cm 2 ) along several centimetres were measured with a high reproducibility for samples presenting single-grain features independently on the texturing process. However, when the pulling rate increases ( R >2 mm/h), the measured J c values were higher for samples obtained using the microwave melt process than for samples processed under lower thermal gradient. The difference in the term of J c is well correlated with the different microstructure.

Experimental evidence for an intermediate phase in the multiferroic YMnO3

We have studied YMnO3 by high-temperature synchrotron x-ray powder diffraction, and have carried out differential thermal analysis and dilatometry on a single crystal sample. These experiments show two phase transitions at about 1100 K and 1350 K, respectively. This demonstrates the existence of an intermediate phase between the room temperature ferroelectric and the high-temperature centrosymmetric phase. This study identifies for the first time the different high-temperature phase transitions in YMnO3.

Mg substitution in CuCrO2 delafossite compounds

Abstract A detailed investigation of the series CuCr1−xMgxO2 ( x = 0.0 – 0.05 ) has been performed by making high-temperature resistivity and thermopower measurements, and by performing a theoretical analysis of the latter. Microstructure characterization has been carried out as well. Upon Mg2+ for Cr3+ substitution, a concomitant decrease in the electrical resistivity and thermopower values is found, up to x ∼ 0.02 – 0.03 , indicating a low solubility limit of Mg in the structure. This result is corroborated by scanning electron microscopy observations, showing the presence of MgCr2O4 spinels as soon as x = 0.005 . The thermopower is discussed in the temperature-independent correlation function ratio approximation as based on the Kubo formalism, and the dependence of the effective charge carrier density on the nominal Mg substitution rate is addressed. This leads to a solubility limit of 1.1% Mg in the delafossite, confirmed by energy dispersive X-ray spectroscopy analysis.

Vibrational spectroscopy study of the lattice defects in CaZrO3 ceramics

Abstract CaZrO 3 materials were prepared by a solid state reaction. The samples were pre-calcined and sintered at 1673 K for 6 h. They were then analysed using Fourier Transform-Infrared spectroscopy. Microwave dielectric function ( e = e ′+i e ″) were determined by Kramers–Kronig analysis. The reflectance spectra were then adjusted using the classical dispersion equations taking into account the LO/TO (longitudinal/transverse optic) mode splitting. The most unfavourable mode in terms of dissipation was found to be the first one ( ω T =106.9 cm −1 ). A simple adjustment of this mode transverse optic damping constant was then carried out to fit the reflectance spectra of slightly non-stoichiometric CaZrO 3 materials. This procedure was conclusive except for excessive Zr non-stoichiometry (extrapolated to >2.78%).

Effect of nano-particles SnO2 and CeO2 addition on the Y123/Y211 sample grown in a microwave furnace

Abstract It is well known that the critical current density ( J c ) of Y123 bulk superconductor can be increased by the introduction of non superconducting flux pinning centers such as small secondary phases precipitates (1 μm 3 ). Thus, several studies have shown that the introduction of CeO 2 and SnO 2 powders allows the formation of micron size secondary phases homogeneously distributed in the Y123 matrix allowing an increasing of J c . As the precipitates size should be a crucial parameter that affects pinning properties of Y123, this work is devoted to compare, in terms of microstructure and superconducting properties, the effect of micrometric and nanometric SnO 2 particles addition in the ‘SnO 2 +CeO 2 ’ doped sample. It was shown that the addition of nanometric SnO 2 particles, compared with an addition of micronic SnO 2 particles, induces an increasing of the reactivity between the liquid phase and the dopants. Then, Y211 coarsening is not reduced and micronic secondary phases is formed conducting to obtain no improvement of the superconducting properties.

Controlled growth of single-grain YBa2Cu3O7 by top-seeding melt-texturing

A Top-seeding-melt-texturing (TSMT) process is investigated by quenching, in order to understand the growth mechanism better. A SmBa2Cu3O7 melt-processed crystal has been used as a seed crystal for growing a large single-grain YBa2Cu3O7 (Y-123) pellet in a Y-123/Y-211 composite with PtO2-addition. It was found that the growth is three-dimensional with a growth ratio R100R001 to 1. By comparing the seeded and un-seeded pellets, quenched at different temperatures during growth stage, their difference in nucleation temperatures were identified. This causes the appearance of a temperature window in which the growth of a large single-grain monolith can be realized. Finally, the growth front morphology was observed and the growth mechanism is discussed in terms of a highly supersaturated melt by the dissolution of Y-211 particles.

Sintering of CuO and ZnO in a Single Mode Microwave Cavity with Shrinkage Control

Abstract A specific TE10m microwave cavity has been designed to follow-up the shrinkage during the microwave sintering of ceramics powders using an optical based position sensing device. The basic principle consists in measuring the distance from a laser source to the sample surface by means of a triangulation method. The spatial resolution device is around a few micrometers that enables to accurately measure the shrinkage versus time of a microwave irradiated sample. The shrinkage curves have been recorded during the direct microwave sintering of CuO and ZnO. Sintering kinetics has been found extraordinarily fast as only a few seconds are needed to achieve the maximum shrinkage for both materials. This new method is undoubtedly powerful to increase our understanding of microwave sintering and very useful to control the microstructure of microwave sintered ceramics.

Effect of lithium salts addition on sintering temperature, cationic ordering and dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramic

The effect of different lithium salts (LiF, Li2CO3, LiNO3) addition on sintering temperature, cationic ordering and dielectric properties of Ba(Mg1/3Ta2/3)O3 ceramic has been investigated. This perovskite was synthesised by solid state reaction from BaCO3, MgO and Ta2O5. It was shown that, by addition of LiNO3, the sintering temperature was decreased to 1300°C (versus 1550°C–1600°C for the pure Ba(Mg1/3Ta2/3)O3 ceramic) without altering dielectric properties at 1 MHz. A cationic ordering was also observed. Sintering with co-fired nickel electrodes in Ar/H2 atmosphere was also successfully performed for Multi Layer Ceramic Capacitors applications.