H. Dammak

Effects induced by high electronic excitations in pure metals: A detailed study in iron☆

Although high electronic excitations were neglected for a long time in radiation effect studies of metallic targets, it is now well established that they can play a dominant role in the damaging processes of some metals. Iron is especially interesting in so far as, according to the rate of energy deposition in electronic excitations (dE/dx)e, various behaviours are observed. Below (dE/dx)e ≈ 40 keV/nm, due to electronic excitations, the amount of damage introduced in the sample is smaller than that expected from the sole elastic collisions. On the contrary, at very high (dE/dx)e levels, a strong enhancement of the damage occurs. After describing the experimental results obtained during low temperature irradiations with a few 10 MeV/nucleon heavy (oxygen to uranium) ions, a phenomenological model which accounts for this intricate behaviour will be presented. Finally, a microscopic mechanism will be proposed to explain how the energy given to the electronic system can play a role in damage processes involving atomic motion. Molecular dynamics simulations validate this approach.

α→ω phase transformation induced in titanium during ion irradiations in the electronic slowing-down regime

Abstract This letter shows that the displacive phase transformation from the h.c.p. α-phase to the hexagonal co-phase can be triggered in titanium targets in an unusual way during swift heavy ion irradiations. This unexpected result is a consequence of the high level of energy deposition in electronic excitation. A model based on the Coulomb explosion mechanism can account for such a phase transformation.

Phase transformation induced by swift heavy ion irradiation of pure metals

Abstract It is now unambiguously established that high electronic energy deposition (HEED), obtained by swift heavy ion irradiation, plays an important role in the damage processes of pure metallic targets: (i) annealing of the defects created by elastic collisions in Fe, Nb, Ni and Pt, and (ii) creation of additional defects in Co, Fe, Ti and Zr. For Ti, we have recently evidenced by transmission electron microscopy observations that the damage creation by HEED is very important and leads to a phase transformation. Titanium evolves from the equilibrium hcp alpha-phase to the high pressure omega-phase. We studied the influence of three parameters on this phase transformation: ion fluence, electronic stopping power and irradiation temperature. The study of Ti and the results concerning other metals (Fe, Zr, etc.) and the semi-metal Bi allow us to propose criteria to predict in which metals HEED could induce damage: those which undergo a phase transformation under high pressure. As a matter of fact, beryllium is strongly damaged when submitted to HEED and seems to behave very similarly to titanium. The fact that such phase changes from a crystalline form to another form were only observed in those metals in which high pressure phases exist in the pressure-temperature diagram, strongly supports the Coulomb explosion model in which the generation of (i) a shock wave and (ii) collective atomic movements are invoked to account for the observed damage creation.

Translational and orientational order in lead zinc niobate: An optical and Raman study

Raman and optical experiments have been performed on a lead zinc niobate [Pb(Zn1/3Nb2/3)O3 or PZN] single crystal oriented along (111) under zero-field cooling, and zero-field heating after field cooling conditions. At high temperature, the Raman spectra originate from a phonon–polarization coupling whose selection rules depend upon the wave vector of the quasistatic polarization, and that activates modes in the entire Brillouin zone. At room temperature, ferroelectric macrodomains were induced and stabilized under field cooling conditions. Their optically uniaxial behavior along [111] reflects a rhombohedral orientational order. However, Raman analysis shows that a zone center line assignment on the basis of the C3v point group is not appropriate, because of translational symmetry breaking due to chemical and structural disorder. Raman spectroscopy was also performed on a single crystalline sample of PZN oriented along (001). Raman measurements evidence the similarity of the spectra for both crystals in ...

Existence of a direct phase transformation from haematite to maghemite

Abstract The observation of a direct phase transformation from haematite α-Fe2O3 (antiferromagnetic) to maghemite γ-Fe2O3 (ferrimagnetic) by the mechanical action of grinding is reported. A simple grinding process is used; it is characterized by the presence of a shearing component exerted on the α-Fe2O3 particles. All crystallographic and magnetic measurements show existence of a phase transformation a - γ. The interpretation is based on a mechanism using two crystallographic concepts: the orientation relationship between the two phases, and movement of the oxygen planes due to a periodic shearing sequence. The results are also consistent with the phenomena related to the inverse transformation γ → α.

Study of the irradiation-induced α → ω phase transformation in titanium: Kinetics and mechanism

Abstract An atomic-crystallographic mechanism is proposed for the α → ω transformation during irradiation. The mechanism is based on the propagation of a localized displacement wave of the [100]α close-packed rows. From the phonon dispersion curves along (010)α, the displacement wave is decomposed into two transverse optical phonons and one transverse acoustic phonon. The proposed mechanism accounts for the ω morphology and can explain the presence of (210)α planes of diffuse intensity in reciprocal space. The kinetic of the transformation was followed by in-situ length and electrical resistance measurements. An analytical model based on the proposed mechanism enabled us to fit the experimental curves. Finally the kinetic results were compared with those obtained previously for the transformation occurring under an applied static pressure.

Track formation in metals by electronic processes using atomic and cluster ions

Abstract It is shown here that electronic excitation and ionization arising from the slowing-down of swift heavy ions can lead to structural modifications in some metallic targets as it has been known for a long time in insulators. This damage is always created in the close vicinity of the projectile path and can exhibit various forms.

New pressure optical sensor for distributed sensing

This sensor is based on the optical coupling between two optical fibers, submitted simultaneously to the same constraint. Due to the geometrical deformations, photoelasticity, and evanescent field this coupling was studied theoretically and experimentally, and used to build a time multiplexed sensor network by coupling short pulses. This arrangement has high sensitivity and resolution, without some drawbacks of the classical OTDR.