An effect of antiphase boundaries on the kinetics of short-range ordering by a vacancy mechanism

In Monte Carlo simulations of chemical short-range ordering on a square lattice, the number of single-atom domains was found to depend on the presence of antiphase boundaries and on the mechanism by which ordering occurred. When antiphase boundaries were present and the ordering occurred by a vacancy mechanism, the number of single-atom domains was found to increase with decreasing temperature, in contrast to thermodynamic predictions. This is understood as a consequence of highly correlated vacancy motions in those regions of the lattice away from antiphase boundaries

Mössbauer Spectrometry

Mössbauer spectrometry gives electronic, magnetic, and structural information from within materials. A Mössbauer spectrum is an intensity of γ-ray absorption versus energy for a specific resonant nucleus such as ^(57)Fe or ^(119)Sn. For one nucleus to emit a γ-ray and a second nucleus to absorb it with efficiency, both nuclei must be embedded in solids, a phenomenon known as the “Mössbauer effect.” Mössbauer spectrometry looks at materials from the “inside out,” where “inside” refers to the resonant nucleus. Mössbauer spectra give quantitative information on “hyperfine interactions,” which are small energies from the interaction between the nucleus and its neighboring electrons. The three hyperfine interactions originate from the electron density at the nucleus (the isomer shift), the gradient of the electric field (the nuclear quadrupole splitting), and the unpaired electron density at the nucleus (the hyperfine magnetic field). Over the years, methods have been refined for using these three hyperfine interactions to determine valence and spin at the resonant atom. Even when the hyperfine interactions are not easily interpreted, they can often be used reliably as “fingerprints” to identify the different local chemical environments of the resonant atom, usually with a good estimate of their fractional abundances. Mössbauer spectrometry is useful for quantitative phase analyses or determinations of the concentrations of resonant element in different phases, even when the phases are nanostructured or amorphous. Most Mössbauer spectra are acquired with simple laboratory equipment and a radioisotope source, but the recent development of synchrotron instrumentation now allow for measurements on small 10 µm samples, which may be exposed to extreme environments of pressure and temperature. Other capabilities include measurements of the vibrational spectra of the resonant atoms, and coherent scattering and diffraction of nuclear radiation. This article is not a review of the field, but an instructional reference that explains principles and practices, and gives the working materials scientist a basis for evaluating whether or not Mössbauer spectrometry may be useful for a research problem. A few representative materials studies are presented

Charge Redistribution and Phonon Entropy of Vanadium Alloys

The effects of alloying on the lattice dynamics of vanadium were investigated using inelastic neutron scattering. Phonon densities of states were obtained for bcc solid solutions of V with 3d, 4d, and 5d transition metal solutes, from which vibrational entropies of alloying were obtained. A good correlation is found between the vibrational entropy of alloying and the electronegativity of transition metal solutes across the 3d row and down columns of the periodic table. First-principles calculations on supercells matching the experimental compositions predicted a systematic charge redistribution in the nearest-neighbor shell around the solute atoms, also following the Pauling and Watson electronegativity scales. The systematic stiffening of the phonons is interpreted in terms of the modified screening properties of the electron density around the solutes

First-principles study of phonon linewidths in noble metals

Phonon lifetimes in Cu, Ag, and Au at low and high temperatures were calculated along high symmetry directions using density functional theory combined with second-order perturbation theory. Both harmonic and third-order anharmonic force constants were computed using a supercell small displacement method, and the two-phonon densities of states were calculated for all three-phonon processes consistent with the kinematics of energy and momentum conservation. A nonrigorous Grüneisen model with no q-dependence of the anharmonic coupling constants offers a simple separation of the potential and the kinematics, and proved semiquantitative for Cu, Ag, and Au. A rule is reported for finding the most anharmonic phonon mode in fcc metals

Chemical Environment Selectivity in Mössbauer Diffraction from 57Fe3Al

Mössbauer diffraction was used to measure different autocorrelation functions for 57Fe atoms in different chemical environments. The sample was polycrystalline 57Fe3Al with the ordered DO3 structure. Diffraction peaks from a fcc structure with a doubled unit cell were detected when the incident radiation was tuned to the Mössbauer resonance of the Wyckoff 4(b) Fe site, but not for tuning to the 8(c) site, thereby distinguishing the spatial arrangements of these two Fe sites

Two-phase coexistence in Fe–Ni alloys synthesized by ball milling

We used mechanical alloying with a Spex 8000 mixer/mill to synthesize a series of Fe100–xNix alloys from x=0 to x=49. The Spex mill was modified so that it could also operate at a reduced milling intensity, and we compared the alloys synthesized after long times with the normal and reduced milling intensities. X-ray diffractometry and Mössbauer spectrometry were used to measure the volume fractions of the bcc and fcc phases in the alloys, and to determine the chemical compositions of the individual phases. We found that the composition ranges of the bcc and fcc single phase regions were extended well beyond their equilibrium ranges. At the higher milling intensity, we found that the bcc phase was destabilized with respect to the fcc phase, and the two-phase region shifted to lower Ni concentrations. For those alloys with coexisting bcc and fcc phases, we present evidence that the chemical compositions of the two phases are nearly the same. We explain the destabilization of the bcc with milling intensity as originating with a higher defect density in the bcc alloys than in the fcc alloys. We argue that this defect density is not homogeneous throughout the alloy, however, and the distribution of defect enthalpies can explain the two-phase coexistence in the as-milled alloys

Design of a digital compression technique for shuttle television

The determination of the performance and hardware complexity of data compression algorithms applicable to color television signals, were studied to assess the feasibility of digital compression techniques for shuttle communications applications. For return link communications, it is shown that a nonadaptive two dimensional DPCM technique compresses the bandwidth of field-sequential color TV to about 13 MBPS and requires less than 60 watts of secondary power. For forward link communications, a facsimile coding technique is recommended which provides high resolution slow scan television on a 144 KBPS channel. The onboard decoder requires about 19 watts of secondary power

Self-organized carbon nanostrips with a new LiC10 structure derived from carbon nanotubes

Single walled carbon nanotubes (SWNTs) were reacted with molten lithium at 220 °C for two weeks. This induced dramatic changes in their structure as shown by x-ray and electron diffractometry and Raman spectroscopy. A significant fraction of the initial SWNTs transformed into flat nanostrips having intercalated lithium in between them. Lithium forms a superlattice commensurate with that of the graphitelike nanostrips with [square root of]7×[square root of]3 in-plane distribution. This new structure corresponds to the LiC10 composition