Thermal modeling of laser-annealing-induced crystallization of amorphous NiTi thin films

Laser annealing of shape memory alloy thin films provides new opportunities in actuator design and fabrication for microelectromechanical systems applications. In this paper, we present a three-dimensional thermal model to simulate the crystallization process when a laser beam is swept across an amorphous NiTi thin film. Experimental crystallite nucleation and growth rates are included in the model to enable prediction of the size of the crystallized region as a function of laser annealing parameters. The model can also be used to study the crystallization of other material systems by means of laser annealing. PACS 42.62.-b; 81.20.-n; 81.30.Kf     

Mixing of iron with various metals by high-energy ball milling of elemental powder mixtures

The alloying of Fe with T=V, Cr and Mn by high-energy ball milling of elemental powder mixtures has been studied from the scale of a powder particle down to the atomic scale using X-ray and neutron diffraction, Mössbauer spectrometry and magnetic measurements for Fe_1?x T _x alloys with x=0.50, 0.65 for T=V, x=0.50, 0.70 for T=Cr and x=0.72 for T=Mn. Different alloying behaviours are observed according to T once powder particles have the final composition. The rather fast mechanical alloying of Fe with Mn reflects the statistical nature of the milling process in contrast to the slow mixing of Fe with V and of Fe with Cr. Hyperfine magnetic field distributions remain stationary in shape in the last milling stage at room temperature both for T=V and T=Cr. Magnetic measurements evidence the persistence with milling time of a large population of nanometer-sized Fe-Cr zones that are superparamagnetic at room temperature and at 400 K. By contrast, room-temperature Mössbauer spectra show only a single line for long milling times. The unmixed stationary state of milled p-Fe_0.7Cr_0.3 is discussed in the light of a recent model of systems driven by competing dynamics.     

Synthesis and characterisation of a novel europium-based graphite intercalation compound

In the lithium-europium-graphite system, a novel ternary compound was synthesised by direct immersion of a pyrolytic graphite platelet in a molten lithium-based alloy with a well chosen Li/Eu ratio at 400 °C. The ternary compound exhibits poly-layered intercalated sheets mainly constituted of two europium planes. Its chemical formula can be written Li_xEuC₄, since the amount of lithium is still not determined. The ¹⁵¹Eu Mössbauer spectra clearly indicate a +II valence for europium. The magnetic susceptibility and the magnetisation versus temperature reveal a complex behaviour which is qualitatively described thanks to structural hypothesis and analogies with the magnetic properties of the binary EuC₆ compound. A first ferromagnetic transition occurring at 225 K is attributed to interactions between both intercalated europium planes. The lower temperature susceptibility behaviour can be interpreted by antiferromagnetic interactions between in-plane neighbours and ferromagnetic interactions along the c-axis.     

Facile synthesis and characterization of naphthidines as a new class of highly nonplanar electron donors giving robust radical cations

Naphthidines 2 were prepared by nickel-catalyzed amination of 1-chloronaphthalene followed by oxidative homocoupling of 1-naphthalene amines 1 using titanium(IV) tetrachloride. The electronic and magnetic properties of materials 2 were investigated by cyclic voltammetry and other electrochemical techniques, EPR and UV-visible spectroscopies, and magnetic susceptibility. It was demonstrated that compounds 2 could be easily and reversibly oxidized via a two-electron-transfer reaction into their bis(radical cation) 2(2.2+), which displays a substantial stability at room temperature (the half-life of 2(2.2+) estimated by EPR at 25 degrees C was 10 days). B3LYP/6-31G optimized structures of N,N'-bis(4-methoxyphenyl)-(1,1'-binaphthyl)-4,4'-diamine 2g shows significant differences in the torsion angle between the naphthalene moieties depending on its oxidation state. Twisted structures are preferred for neutral compounds, whereas more planar are favored for the oxidized forms 2g*+ and 2g(2.2+) to realize spin and/or charge delocalizations over the whole pi-system. Such conformation changes concerted with the electron transfers contribute to explain the unusual two-electron process observed in the electrochemical behavior of 2g instead of the two single-electron transfers that would have been expected in the case of two successive oxidations. It is finally shown that the oxidation of 2g in CH2Cl2 with thianthrenium perchlorate (ThClO4) generates the dication 2g(2.2+) with singlet spin-multiplicity.