Phonon density of states of nanocrystalline Fe prepared by high-energy ball milling

Abstract

We performed inelastic neutron scattering measurements on nanophase Fe powders prepared by high-energy ball milling. Neutron energy loss spectra were measured for two states of the material: ͑1͒ as milled, when the material had a characteristic nanocrystallite size of 12 nm; and ͑2͒ annealed, when the material had a characteristic crystallite size of 28 nm. The longitudinal peak in the phonon density of states ͑DOS͒ of the nanophase Fe was broadened, compared to that of the annealed material. We attribute this broadening to short phonon lifetimes in nanocrystals. The nanophase material also showed an enhanced density of states at low energies below 15 meV, which may indicate the presence of intercrystallite vibrations. These differences in phonon DOS should have only a small effect on the difference in vibrational entropy of nanocrystalline and larger-grained Fe.

Vibrations of Micro-eV Energies in Nanocrystalline Microstructures

“…Similar measurements with incident neutrons of 45 meV energy were performed on samples of Ni 3 Fe with the LRMECS spectrometer. The results were essentially the same as those from prior measurements with a triple-axis instrument [7,10], although a better sampling of phonons at all Q is possible with time-of-flight instruments ( hQ is the momentum transfer).…”

“…Enhanced intensity above the high-energy cutoff of the bulk material has been attributed to phonon lifetime broadening caused by phonon interactions with grain boundaries [9][10][11] and recent measurements on iron have isolated a contribution from surface oxides [12]. The increased number of phonon modes at low energies [5][6][7][8][9][10][11] is less well understood. The phonon density of states (DOS) at low energies is up to a factor of 5 times larger than the DOS of control samples of material composed of larger crystals [10].…”

“…Earlier measurements of Debye-Waller factors and Lamb-Mössbauer factors showed large mean-squared atomic displacements in nanocrystalline materials [1][2][3][4]. More recently, neutron and x-ray inelastic scattering measurements revealed that nanocrystalline materials have an increased number of vibrational modes at the highest and lowest energies of their measured spectra, compared to materials with crystals of conventional sizes [5][6][7][8][9][10][11][12]. Enhanced intensity above the high-energy cutoff of the bulk material has been attributed to phonon lifetime broadening caused by phonon interactions with grain boundaries [9][10][11] and recent measurements on iron have isolated a contribution from surface oxides [12].…”

Phonons in Nanocrystalline57Fe

“…Very recently, neutron inelastic scattering measurements have shown some differences in the phonon density of states (DOS) of nanocrystalline and bulk materials [2-6]. One such effect was an enhancement of the phonon DOS at low energies [3][4][5][6][7]. A broadening of the peak from the longitudinal modes in the phonon DOS was also observed and attributed to the lifetime broadening of phonons in small crystals [5,6].…”

“…Unusual mechanical properties and soft magnetic properties were topics of numerous investigations on metallic nanocrystals [1]. Very recently, neutron inelastic scattering measurements have shown some differences in the phonon density of states (DOS) of nanocrystalline and bulk materials [2][3][4][5][6]. One such effect was an enhancement of the phonon DOS at low energies [3][4][5][6][7].…”