Unusual thermoelectric behavior of packed crystalline granular metals

Loosely packed granular materials are intensively studied nowadays. Electrical and thermal transport properties should reflect the granular structure as well as intrinsic properties. We have compacted crystalline $CaAl$ based metallic grains and studied the electrical resistivity and the thermoelectric power as a function of temperature ($T$) from 15 to 300K. Both properties show three regimes as a function of temperature. It should be pointed out : (i) The electrical resistivity continuously decreases between 15 and 235 K (ii) with various dependences, e.g. $\simeq$ $T^{-3/4}$ at low $T$, while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near 60K, and (v) presents a rather unusual square root of temperature dependence at low temperature. It is argued that these three regimes indicate a competition between geometric and thermal processes, - for which a theory seems to be missing in the case of TEP. The microchemical analysis results are also reported indicating a complex microstructure inherent to the phase diagram peritectic intricacies of this binary alloy.Comment: to be published in J. Appl. Phys.22 pages, 8 figure

Formation and Structure of Nanocrystalline Al-Mm-Ni-Cu Alloys

This paper reports the results of a short investigation on the effect of Cu additions upon the nanocrystallization behaviour of an Al-Mm-Ni alloy. 2 at.%Cu added to a base alloy of Al$\text{}_{85}$Mm$\text{}_{10}$Ni$\text{}_{5}$ alloy by substitution for Mm (mischmetal). The control of cooling rate did not cause the formation of nanocrystals of fcc-Al phase. The nanocrystalline structure fcc-Al + amorphous phase in quarternary alloy was obtained by isothermal annealing and continuous heating method, but the last technique is more effective. The volume fraction, lattice parameter, and size of Al-phase were calculated

Structural Investigation of Rapidly Quenched FeCoPtB Alloys

Two sets of $Fe_{52-x}Co_{x}Pt_{28}B_{20}$ (x=0-26 at.%) and $Fe_{60-x}Co_{x}Pt_{25}B_{15}$ (x=0-40 at.%) alloys were prepared in the form of ribbons by the rapid quenching technique. Structure of the samples was characterized by Mössbauer spectroscopy and X-ray diffraction. In the as-quenched alloys the amorphous phase coexisted with the fcc-(Fe,Co)Pt disordered solid solution. Differential scanning calorimetry measurements performed in the range 50-720 °C revealed one or two exothermal peaks. The magnetically hard ordered $L1_{0}$ (Fe,Co)Pt and magnetically soft $(Fe,Co)_{2}B$ nanocrystalline phases were formed due to thermal treatment of the alloys. The influence of Co content on the structure of the as-quenched and heated alloys was studied

Mössbauer Study of Magnetic Properties of Fe80−xCoxZr7Si13Fe_{80-x}Co_{x}Zr_7Si_{13} (x = 0-30~at.%) Boron-Free Amorphous Alloys

Amorphous $Fe_{80-x}Co_{x}Zr_7Si_{13}$ (x = 0-30 at.%) alloys in which boron was completely replaced by silicon as a glass forming element have been prepared by melt quenching. Partial substitution of iron by cobalt caused the increase of the hyperfine field and saturation magnetization. The specialized rf-Mössbauer measurements revealed that all amorphous alloys studied are magnetically very soft. The rf-sidebands effect, related to magnetostriction, increases with the increase of Co content. In $Fe_{50}Co_{30}Zr_7Si_{13}$ sample the rf field exposure induced partial crystallization of amorphous phase that was attributed to mechanical deformations related to high frequency magnetostrictive vibrations induced by the rf field. The measurements of the hysteresis loop revealed that coercivity increases for higher Co content

Small-angle X-ray scattering from phase-separating amorphous meallic alloys undergoing nanocrystallization.

International audienceWe performed a combined small- and wide-angle X-ray scattering (SAXS/WAXS) study of nanocrystallization during annealing of binary Al92Sm8 and Al91Gd9 melt-spun glassy alloys. In course of the transformation an interference maximum develops in the SAXS region while WAXS spectra show formation of fcc-Al nanocrystals in the glassy phase. In order to determine the origin of the SAXS maximum the theoretical SAXS intensity and distance distribution function were calculated considering two structural models. The first model represented a single nanocrystal with a solute layer on its surface and the second one a phase-separated alloy with spatially correlated compositional fluctuations. The results of the calculations are compared with the experimental data. It is demonstrated that only the model representing correlated fluctuations reasonably reproduces the experimental results. We conclude that nano-scale glassy phase separation occurs in the investigated alloys and the nanocrystals form inside the Al-rich amorphous regions

Specific Heat Measurements on Amorphous and Nanocrystalline Al88Y5Ni5Co2

Specific heat at constant pressure, CP, was measured on amorphous and nanocrystalline Al88Y5Ni5Co2 alloys from differential scanning calorimetry experiments. Linear behavior of CP versus temperature from 323 to 423 K is explained by conduction electrons contribution and dilatation correction factor. Results indicate that the Fermi energy increases as nanocrystallization progresses, although the estimated values are clearly lower than those found for crystalline Al.Gobierno de España MAT2007-65227European Commission. Fondo Europeo de Desarrollo Regional MAT2007-65227Junta de Andalucía P06-FQM-0182