Magnetomechanical damping by polycrystalline TbDy

Vibration damping in polycrystalline TbDy alloys was studied at cryogenic temperatures. Mechanical hysteretic losses were measured at various strains, frequencies, and loading configurations at 77 K. Some textured TbDy materials demonstrated 22.6% energy dissipation in mechanical measurements at low frequency (0.01 Hz) and a mean logarithmic decrement of 0.23 at a higher frequency (25 kHz). Ultrasonic velocities of longitudinal and shear elastic waves were measured on single and polycrystalline TbDy; little variation in ultrasonic velocities was found even for samples with large variation in crystallographic texture and magnetomechanical properties

Mössbauer Spectrometry Study of Thermally-Activated Electronic Processes in Li_xFePO_4

The solid solution phase of Li_xFePO_4 with different Li concentrations, x, was investigated by Mössbauer spectrometry at temperatures between 25 and 210 °C. The Mössbauer spectra show a temperature dependence of their isomer shifts (E_(IS)) and electric quadrupole splittings (E_Q), typical of thermally activated, electronic relaxation processes involving ^(57)Fe ions. The activation energies for the fluctuations of E_Q and E_(IS) for Fe^(3+) are nearly the same, 570 ± 9 meV, suggesting that both originate from charge hopping. For the Fe^(2+) components of the spectra, the fluctuations of E_Q occurred at lower temperatures than the fluctuations of E_(IS), with an activation energy of 512 ± 12 meV for E_Q and one of 551 ± 7 meV for E_(IS). The more facile fluctuations of E_Q for Fe^(2+) are evidence for local motions of neighboring Li^+ ions. It appears that the electron hopping frequency is lower than that of Li^+ ions. The activation energies of relaxation did not have a measurable dependence on the concentration of lithium, x

Phonons in nanocrystalline Ni3Fe

Inelastic neutron-scattering spectra were measured to obtain the phonon density of states (DOS) of nanocrystalline fcc Ni3Fe. The materials were prepared by mechanical alloying, and were also subjected to heat treatments to alter their crystallite sizes and internal strains. In comparison to material with large crystallites, the nanocrystalline material shows two distinct differences in its phonon DOS. The nanocrystalline DOS was more than twice as large at energies below 15 meV. This increase was approximately proportional to the density of grain boundaries in the material. Second, features in the nanocrystalline DOS are broadened substantially. This broadening did not depend in a simple way on the crystallite size of the sample, suggesting that it has a different physical origin than the enhancement in phonon DOS at energies below 15 meV. A damped harmonic oscillator model for the phonons provides a quality factor Qu, as low as 7 for phonons in the nanocrystalline material. The difference in vibrational entropy of the bulk and nanocrystalline Ni3Fe was small, owing to competing changes in the nanocrystalline phonon DOS at low and high energies

Phase Diagram of Li_xFePO_4

The phase diagram for LixFePO4 has been determined for different lithium concentrations and temperatures. The two low-temperature phases, heterosite and triphylite, have previously been shown to transform to a disordered solid solution at elevated temperatures. This disordered phase allows for a continuous transition between the heterosite and triphylite phases and is stable at relatively low temperatures. At intermediate temperatures the proposed phase diagram resembles a eutectoid system, with eutectoid point at around x=0.6 and 200°C. Kinetics of mixing and unmixing transformations are reported, including the hysteresis between heating and cooling. The enthalpy of this transition is at least 700 J/mol

Short-range ordering in face-centered-cubic Ni3Al

Films of fcc Ni3Al with suppressed short-range order (SRO) were prepared by physical vapor deposition of Ni3Al onto room-temperature substrates. Extended electron energy-loss fine-structure spectra were obtained from both Al K and Ni L23 edges. After the samples were annealed for various times at 150 °C, a moderate growth of SRO was observed in the first-nearest-neighbor environments of both the Al and Ni atoms. As prepared, these fcc Ni3Al materials, and presumably others having similar heat evolutions as measured by differential scanning calorimetry, have a high degree of chemical disorder

Magnetomechanical effects in textured polycrystalline Tb76Dy24

Uniaxial stress-strain measurements were performed on polycrystalline Tb76Dy24 alloys which exhibit "giant magnetostriction" at cryogenic temperatures. The Young's moduli were reduced by up to a factor of five at 77 K, in comparison to their values at 300 K. We attribute this reduction to a mechanical compliance from domain rotation. Large mechanical hysteresis is also found in nominally elastic stress-strain curves measured below the Curie temperature. Hysteretic curves from 0 to 25 MPa demonstrate up to 19% dissipation of the applied mechanical energy. The anisotropy of thermal expansion was also measured and used as a parameter for the degree of crystallographic texture. This anisotropy was correlated to bulk magnetostriction and to mechanical hysteresis

Intensities of backscatter Mössbauer spectra

The intensities of γ‐ray and x‐ray backscatter Mössbauer spectra of ^(57)Fe nuclei in different matrix materials were studied theoretically and experimentally. A previous analysis by J. J. Bara [Phys. Status Solidi A 58, 349 (1980] showed that negative peak intensities occur in backscatter γ‐ray spectra when the ^(57)Fe nuclei are in a matrix of light elements. We report a confirmation of this work and offer a simple explanation of the phenomenon. The present paper extends Bara’s analysis to the case of conversion x‐ray spectra; expressions for the intensity of conversion x‐ray spectra as a function of absorber thickness and absorber material parameters are presented. We show that negative peak intensities are expected in conversion x‐ray spectra when the ^(57)Fe nuclei are in a matrix of heavy elements

Phonon contributions to the entropies of hP24 and fcc Co3V

Inelastic neutron-scattering spectra and neutron-diffraction patterns were measured on the alloy Co3V at temperatures from 1073-1513 K, where the hP24 (ordered hexagonal) and fee structures are the equilibrium states of the alloy. Phonon density of states (DOS) curves were calculated from the inelastic-scattering spectra, allowing estimates of the vibrational entropy in the harmonic and quasiharmonic approximations. The vibrational entropy of the hP24-fcc phase transition at 1323 K was found to be 0.07k(B)/atom. The anharmonic contributions to the entropy over a temperature range of 100 K were comparable to the vibrational entropy of this phase transition. The anharmonic softening of the phonon DOS was only slightly larger for the hP24 than the fee phase, however, so the anharmonic effects contribute only slightly to the difference in entropy of the two phases. The simple Gruneisen approximation was inadequate for predicting the thermal softening of the phonon DOS

Vibrational entropy and microstructural effects on the thermodynamics of partially disordered and ordered Ni3V

Samples of Ni3V were prepared with two microstructures: (1) with equilibrium D022 order, and (2) with partial disorder (having a large D022 chemical order parameter, but without the tetragonality of the unit cell). For both materials, we measured the difference in their heat capacities from 60 to 325 K, inelastic neutron-scattering spectra at four values of Q at 11 and at 300 K, and Young's moduli and coefficients of thermal expansion. The difference in heat capacity at low temperatures was consistent with a harmonic model using the phonon density of states (DOS) curves determined from the inelastic neutron-scattering spectra. In contrast, at temperatures greater than 160 K the difference in heat capacity did not approach zero, as expected of harmonic behavior. The temperature dependence of the phonon DOS can be used to approximately account for the anharmonic contributions to the differential heat capacity. We also argue that some of the anharmonic behavior should originate with a microstructural contribution to the heat capacity involving anisotropic thermal contractions of the D022 structure. We estimate the difference in vibrational entropy between partially disordered and ordered Ni3V to be Spdis -Sord =(+0.038±0.015)kB /atom at 300 K

Highly Reversible Lithium Storage in Nanostructured Silicon

Anode materials of nanostructured silicon have been prepared by physical vapor deposition and characterized using electrochemical methods. The electrodes were prepared in thin-film form as nanocrystalline particles (12 nm mean diameter) and as continuous amorphous thin films (100 nm thick). The nanocrystalline silicon exhibited specific capacities of around 1100 mAh/g with a 50% capacity retention after 50 cycles. The amorphous thin-film electrodes exhibited initial capacities of 3500 mAh/g with a stable capacity of 2000 mAh/g over 50 cycles. We suggest that the nanoscale dimensions of the silicon circumvents conventional mechanisms of mechanical deterioration, permitting good cycle life