S. Rivoirard - Academia.edu (original) (raw)

Papers by S. Rivoirard

IOP Conference Series: Materials Science and Engineering, 2018

Measurement of the thermophysical properties of liquid metals is challenging because of their hig... more Measurement of the thermophysical properties of liquid metals is challenging because of their high chemical activity and high temperatures. The electromagnetic levitation allows one to hold the electrically conductive liquid sample containerless in an inert atmosphere in thermal equilibrium while measurements on the sample can be taken in a noncontact way followed by extraction of some thermophysical properties. Yet, the electromagnetic forces within the skin layer inside the sample cause convective flow of the liquid thus disabling the data extraction. A static magnetic field imposed over a sample is known to damp the convective flow. With these ideas, an experimental setup with a DC magnetic field directed perpendicular to the gravity vector was constructed and first experiments were performed with liquid Ni and some other materials. In most of the experiments the instability of the levitated sample during a slow variation of the DC magnetic field was observed which is reported in the present article.

Magnetohydrodynamics, 2017

10 th PAMIR International Conference Fundamental and Applied MHD, Jun 2016, Cagliari, ItalyIntern... more 10 th PAMIR International Conference Fundamental and Applied MHD, Jun 2016, Cagliari, ItalyInternational audienc

26th International Conference on Electrical Contacts (ICEC 2012), 2012

The behavior of the arc formed during electrical circuit opening is examined, in the presence of ... more The behavior of the arc formed during electrical circuit opening is examined, in the presence of a pulsed magnetic field. The field is applied in the inter-electrode gap where the arc develops. The opening sequence may be separated into two temporal phases. During the first phase, the arc remains stuck at the same position as under zero field. This is ascribed to the existence of a sticking force, which overcomes the Laplace force. The sticking force is related to the high pressure existing in the plasma. During the second phase, the Laplace force dominates. The arc is blown out of the contact region in a time duration of 1 to 1.5 ms. A model is developed which provides a semiquantitative account of the experimental behavior.

Journal of Magnetism and Magnetic Materials, 2001

A polycrystalline sample of Nd 2 Fe 14 B was prepared by induction melting and solidification in ... more A polycrystalline sample of Nd 2 Fe 14 B was prepared by induction melting and solidification in an intense applied magnetic field, to study the alignment due to residual magnetocrystalline anisotropy of this compound near the melting point. A well-defined direction of the thermal gradient perpendicular to the magnetic field was achieved by means of an appropriate experimental setup. The results of measurements indicate that the sample is substantially textured, but not in parallel to the elaboration field. A numerical method is utilized for the determination of anisotropy parameters in Nd 2 Fe 14 B compound, involving the analysis of measured magnetization curves. The minimization of the free-energy density, expanded in terms of these constants, is performed to determine the sub-lattice terms which are necessary to calculate the polarization curves. A fitting of the measured magnetization curves obtained at room temperature is realized to obtain the anisotropy constants, texture parameters and saturation magnetization.

$Research paper thumbnail of Neutron Diffraction Texture Study of NdFeB As-Cast Alloys and Magnets$

Materials Science Forum, 2002

Extrinsic magnetic anisotropy in NdFeB permanent magnets results from the hot forging process of ... more Extrinsic magnetic anisotropy in NdFeB permanent magnets results from the hot forging process of NdFeB bulk alloys. This process leads to microstructural changes and among them, to the development of a texture induced by hot deformation of the polycristalline alloy. This neutron diffraction texture study aims to demonstrate the (001) crystallographic fibre texture of the Nd 2 Fe 14 B phase developed along the deformation direction. The starting alloy already shows a certain texture in the as-cast state due to thermal gradients on cooling. This initial texture does not really affect the specific orientation developed on forging, as soon as the deformation rate is sufficient. The comparison between forging (very high strain rate) and pressing (low strain rate) shows a higher texturation level associated to higher magnetic anisotropy when the hot deformation is slow.

European Journal of Materials

Journal of Alloys and Compounds

AIP Conference Proceedings

ABSTRACT We report here on a theoretical study of the stability and the electronic structure of T... more ABSTRACT We report here on a theoretical study of the stability and the electronic structure of Ti-V-Cr ternary compounds and their related hydrides. The total energy and electronic structure were calculated using the spin-polarised Korringa-Kohn-Rostoker method combined with coherent-potential-approximation. First, we have studied the stability of different phases with composition TiCr2, corresponding to disordered bcc and fcc, to semi-ordered bcc and ordered AB2 alloys ad compounds. The calculations show that the bcc disordered structure is the most favourable one. Consideration to the effective non-stoichiometry of the binary TiCr1.8 compounds, the metal disorder at the B site favours the AB2 phase stability. The calculations fairly agree with a formula that is found stable experimentally. Substitution of vanadium atoms to chromium for only e.g. 3 %, disfavours the AB2 type of phase. Upon subsequent increases of the vanadium concentration, the difference between the total energy values of bcc and AB2 becomes more noticeable. For several compositions, namely TiV0.8Cr1.2 and Ti0.7V0.9Cr1.4 we have studied the impact of hydrogen insertion on the phase stability. The bcc and fcc type of structures were compared. The calculations are found to agree fairly with the existence of a martensitic phase transition as observed experimentally. From this theoretical investigation, and unexpectedly, onset on magnetism has been evidenced. This is more particularly pronounced with the Cr-rich compounds, but made effectively realistic once the corresponding hydrides are formed.

Applied Physics Letters, 2015

Advanced Magnetic Nanostructures, 2006

Page 1. Chapter 12 HARD-MAGNETIC NANOSTRUCTURES S. Rivoirard CRETA (CNRS) 25 avenue des Martyrs B... more Page 1. Chapter 12 HARD-MAGNETIC NANOSTRUCTURES S. Rivoirard CRETA (CNRS) 25 avenue des Martyrs BP 166, 38042 Grenoble Cedex 9, France D. Givord Laboratoire Louis Ne'el (CNRS) 25 avenue des Martyrs BP 166, 38042 Grenoble Cedex 9, France ...

2010 Proceedings of the 56th IEEE Holm Conference on Electrical Contacts, 2010

The phenomenon of magnetic blowing allows drastic reduction in arc duration. It is of specific in... more The phenomenon of magnetic blowing allows drastic reduction in arc duration. It is of specific interest in DC applications, more specifically in automotive applications. Blowing exploits the action of the magnetic force on the transitory arc formed during circuit opening. A new contact was described in a previous paper in which the magnetic field required for blowing is generated by

Philosophical Magazine A, 2000

This work represents one of the ® rst attempts to probe the bulk crystallographic alignment of ho... more This work represents one of the ® rst attempts to probe the bulk crystallographic alignment of hot-forged Nd± Fe± B± Cu magnets using neutron di raction techniques. Optimizing the microstructure induced by the hotforging process is necessary to develop good magnetic properties. For that purpose, understanding the deformation mechanisms in terms of thermal and mechanical behaviour is of great importance. Ingots of Nd± Fe± B± Cu alloy were cast into steel tubes and hot forged at 980 8 C. The magnetic properties and induced texture of the magnet were investigated. The results show a fair correlation between microstructural parameters induced by the deformation mode and magnetic properties. Using neutron di raction analysis, the pole ® gures revealed the existence of a texture that was more pronounced in the centre than in the periphery of the forged specimen. The good magnetic properties are the result of grain size reduction during forging combined with Nd 2 Fe 14 B crystallite alignment (c axis aligned along the forging direction). As the Nd± Fe± B alloy is in a semisolid state at the processing temperature, the fraction of intergranular liquid phase has to be su cient to allow alignment of the solid Nd 2 Fe 14 B particles but not too important to allow internal stresses between solid grains (resulting in cracks and size reduction). The compromise optimizing the magnetic properties is studied in this work.

Solid State Phenomena, 2005

A high speed hot forging process was applied to Nd-Fe-B and Nd-Fe-V as cast alloys in order to de... more A high speed hot forging process was applied to Nd-Fe-B and Nd-Fe-V as cast alloys in order to develop both the microstructure and the crystallographic texture appropriate for permanent anisotropic magnet properties. Neutron diffraction texture analyses are used to demonstrate the effect of the hot forging process on both kind of alloys. Microstructural changes are an important feature on forging in both cases. Coercivity is developed in the Nd-Fe-B alloy mainly from grain size reduction and disappearance of free iron. Stabilisation of the Nd(Fe,V)12 hard magnetic phase is achieved from the iron and Nd-rich microstructure of the starting Nd-Fe-V material. A comparison of the crystallographic textures of Nd2Fe14B and Nd(Fe,V)12 phases is done to account for the development of extrinsic magnetic anisotropy. In both cases, a nearly fibre texture is obtained in correlation with the symmetry of the deformation. However, the orientations are quite different in both alloys and the conseque...

Materials Science Forum, 2005

In this study, a hot forging process is applied to Nd-Fe-V as cast alloys in order to develop bot... more In this study, a hot forging process is applied to Nd-Fe-V as cast alloys in order to develop both the microstructure and the crystallographic texture appropriate for permanent magnet properties. A neutron diffraction texture analysis is used to account for the extrinsic magnetic anisotropy: the stabilisation of the Nd(Fe,V)12 hard magnetic phase has been achieved during forging but its extrinsic anisotropy level remains low. Attempts to understand this phenomenon are made through a discussion on the Nd-Fe-V alloy rheological and mechanical behaviour and a comparison with Nd-Fe-B permanent magnets.

Revue de Métallurgie, 2005

ABSTRACT

Review of Scientific Instruments, 2009

This article contains a description of a laser interferometer coupled with a furnace for in situ ... more This article contains a description of a laser interferometer coupled with a furnace for in situ dilatation measurements under high magnetic field. The apparatus fits an 18 T superconducting magnet with a 32 mm diameter room temperature bore. This optical method was chosen for dilatation measurements because it is not perturbated by the magnetic field. The measured sample can be heated up to 1500 K under a controlled atmosphere, and heating and cooling rates can be varied within a range of 10 K/s. The resolution is below 50 nm. As an example of using in situ dilatation measurements to follow phase transformations, the increase in the ferrite/austenite transformation temperature in pure iron in a magnetic field up to 16 T was experimentally evidenced.