Eric Beaugnon

Magnetic alignment of carbon nanofibers in polymer composites and anisotropy of mechanical properties

Engineering applications of carbon nanofibers and nanotubes require their alignment in specific directions. Single-walled carbon nanotubes can be aligned in a magnetic field due to the presence of small amounts of catalyst elements, such as Ni and Co. However, for carbon nanofibers, their extremely low magnetic susceptibility is not sufficient for magnetically induced alignment. We present a method of solution-coating of NiO and CoO onto the surface of the carbon nanofibers. Due to the NiO- and CoO-coating, these nanofibers can be well aligned in the polymer composites under moderate magnetic field (3 T). Both transmission electron microscopy and scanning electron microscopy results show the well-aligned nanofibers in a polymer matrix. Mechanical testing shows a pronounced anisotropy in tensile strength in directions normal (12.1 MPa) and parallel (22 MPa) to the applied field, resulting from the well-aligned nanofibers in the polymer matrix. The mechanism of magnetic alignment due to coating of NiO and C...

Processing of large Y1Ba2Cu3O7-x single domains for current-limiting applications

Bulk-textured YBa2Cu3O7-x single domains could be used for current-limiting applications by cutting and assembling long meanders, which would be submitted to network electric fields before using a breaker to interrupt a fault current. For that purpose, large YBaCuO single domains up to 93 mm in diameter can be isothermally grown by using a standard melt texturing growth (MTG) process with a SmBaCuO seed. The essential parameters that have to be controlled in order to reach this size are the temperature growth window, the substrate reactivity and the temperature homogeneity in the sample. Standard 45 mm diameter single domains show excellent superconducting properties, such as Jc above 105 A cm-2 and a homogeneous superconducting-to-normal transition at 91.8 K for 20 cm long conductors cut in these pellets. These measurements demonstrate the long -range homogeneity of single domains regarding Tc and Jc. Nevertheless the high Jc values lead to a too large a dissipation in the normal state at T = 77 K. Different methods to reduce the critical current density are described in order to fulfill the conditions for a safe recovery of the material after undergoing a magnetothermal transition.

Texturing by cooling a metallic melt in a magnetic field

Abstract Processing in a magnetic field leads to the texturing of materials along an easy-magnetization axis when a minimum anisotropy energy exists at the processing temperature; the magnetic field can be applied to a particle assembly embedded into a liquid, or to a solid at a high diffusion temperature close to the melting temperature or between the liquidus and the solidus temperatures in a region of partial melting. It has been shown in many experiments that texturing is easy to achieve in congruent and noncongruent compounds by applying the field above the melting temperature Tm or above the liquidus temperature of alloys. Texturing from a melt is successful when the overheating temperature is just a few degrees above Tm and fails when the processing time above Tm is too long or when the overheating temperature is too high; these observations indicate the presence of unmelted crystals above Tm with a size depending on these two variables that act as growth nuclei. A recent model that predicts the existence of unmelted crystals above the melting temperature is used to calculate their radius in a bismuth melt.

Isothermal growth of large YBa2Cu3O7-x single domains up to 93 mm

Abstract The growth of YBa 2 Cu 3 O 7−x single domains larger than 30–40 mm by a classical Top Seeding Melt Texturing (TSMT) method appears to be difficult. Because of the very narrow solidification range between the onset of the nucleation from the seed and the nucleation of grains in the liquid, reaction with the substrate as well as uncontrolled thermal gradients introduce severe limitations to the growth. After suppressing nucleation from the substrate and improving the temperature homogeneity around the sample, the growth rate measured in situ was used to carefully adjust the process. A YBa 2 Cu 3 O 7−x single domain has been grown up to 93 mm without applied thermal gradient nor large overheating. The texture is confirmed on a large volume by neutron scattering and an homogeneous 211 size distribution and content are found across the sample. This suggests that steady growth conditions have been maintained and that even larger samples could be grown.

Growth of YBaCuO single-domains up to 7 cm

Abstract The growth of large top seeded YBaCuO single-domain has been studied by means of in situ video recording during the process. The growth for samples (O ≤ 4 cm), textured under a thermal gradient, follows a model based on yttrium diffusion through the melt. The growth appears to be limited (3 × 3 cm 2 ) for an isothermal texturation whereas single-domains up to 7 cm are grown using whether a radial thermal gradient or a overheating process.

In-situ YBa2Cu3Ox growth rate analysis

Abstract The growth of large mono-domain YBa 2 Cu 3 O x (Y-123) pellets has been studied with levitation applications in mind. The Top-Seed Melt-Texturing method (TSMT) was applied using a SmBa 2 Cu 3 O x seed. During the process, a video recording was made of the evolution on the sample surface. Using this data, a model for the growth of mono-domains, inside a temperature window, is discussed and a strategy is defined to overcome the limitation in crystal size. 40 mm diameter mono-domains have already been obtained following this strategy.

Growth of YBaCuO mono-domains: growth limit and mono-domains with a diameter up to 7 cm

In order to grow very large mono-domains from a SmBaCuO seed, three growth conditions have been investigated. For a growth with a large overheating or with a thermal gradient, mono-domains with a diameter of 7 cm were easily grown. On the other hand, for a growth in an isothermal furnace, a growth limit has been observed. In this case, the EDS analysis of Y-211 content in the mono-domain shows a Y-211 concentration gradient and a growth limit when an amount of 48 mol% is reached. In order to limit the Y-211 increase, the time spent in the liquid phase has to be reduced. This technique enables larger mono-domains to be grown, but the growth is then stopped by parasitic Y-123 grain nucleation in the remaining liquid which blocks the growth front advancement.

Anomalous magnetism and normal field instability in supercooled liquid cobalt

Supercooled liquids may offer fascinating phenomena as compared with the normal state. In the case of supercooled paramagnetic liquids, completely different phenomena in high magnetic fields should be observed thanks to the high undercooling leading to higher magnetization and very strong magnetic coupling in the liquids. Here, we report the measurement of an undercooled pure liquid metal with a higher magnetization than the solid at the same temperature. We also observe the normal field instability pattern formation in the undercooled melt. Although the origin of the high magnetism of the liquid remains unclear, we discuss the possibility of a short range ordering effect.

Materials processing in a magnetic force opposed to the gravity

High magnetic fields can levitate diamagnetic substances. Conducting paramagnetic liquids can be levitated by combining electromagnetic and magnetic forces. Some recent examples of magnetic processing are given to enlighten some experimental difficulties related to solidification. Levitation appears as a particular magnetic processing. There are other concepts to damp convection in magnetic liquids, to modify and orient the solidified microstructure. An exciting perspective is the convection suppression in a magnetic liquid by using much smaller magnetic forces than the levitation force.

Dilatation measurements for the study of the α/γ transformation in pure iron in high magnetic fields

Magnetic field processing is a new promising tool for the structural and functional control of materials. A significant potential exists for tailoring microstructures and impacting kinetics of phase transformation in steels. A high magnetic field modifies the Gibbs free energy. As a result, the phase diagram is shifted upwards so that the Ac1 and Ac3temperatures increase as the magnetic field is increased. In this work, a new device for the heat treatment and in situ control of the transformation is described. For the first time, a dilatation measurement is used to study the shift of the ferrite/austenite equilibrium in high magnetic field up to 16 T and to quantify the ferrite concentration during the transformation. Experimental results for the transformations in pure iron are presented. Comparisons are made with the expected values based on the Weiss molecular field model near the Curie point.