G. Jyoti

alpha omicro Crystallography of the transition in shock-loaded zirconium

The orientation relations (ORs) between the alpha (hcp) and the omicro (three-atom hexagonal) structures have been determined for Zr, in which alpha transformed to omicro under shock loading. The ORs obtained are found to belong to the variant I OR given by Usikov and Zilbershtein in 1973 for statically compressed samples and also consist of an OR that is the same as that reported by Song and Gray in 1995 in recent dynamic experiments on Zr. The latter OR is shown to be a subset of the former. These observations show that the same type of mechanism of transformation is operative under both static pressure and shock compression. The crystallography of the transition is discussed in terms of the required strains.

Shock‐induced amorphization of q‐GeO2

Polycrystalline samples of the α‐quartz phase of GeO2 (q‐GeO2), recovered from peak shock compressions of 5, 6.8, and 10 GPa, have been examined by x‐ray diffraction, thermoluminescence, and Raman techniques. The measurements show that q‐GeO2 irreversibly amorphizes above 6.8 GPa. The estimates of the shear band temperature suggest that the mechanism of shock‐induced amorphization in q‐GeO2 is a solid‐solid one, in contrast to that in SiO2 quartz which has both solid‐solid and fusion‐quenched components.

Some aspects of pressure-induced ω → β transformation in group IVB elements

The ω (hexagonal) to β (bcc) transformation in Zr and Hf occurs at 30 and 71 GPa under static pressures. This transition has not been found in Ti up to 87 GPa. On the basis of full-potential linearized augmented plane wave calculations aided with thermal and entropy correction we predict an ω → β transition in Ti at around 102 GPa along the 300 K isotherm. In addition to this, we calculate the ω → β transitions in Zr and Hf at around 27 and 65 GPa respectively, which are in excellent agreement with the experimental values. The ω → β transition pressures, 102, 27 and 65 GPa for Ti, Zr and Hf respectively, do not follow the trend implied by the principle of corresponding states. We have analysed the causes for this anomalous trend. We observe that the ω → β transition depends on how the increased d-population due to s–d transfer under pressure is distributed in the different d-substates. For example, at ambient conditions, the bcc phase is unstable and the difference between the average charges in the bcc stabilizing d-t2g state and the destabilizing d-eg state are 0.008, 0.082 and 0.013 for Ti, Zr and Hf respectively. Compression increases this difference and stabilizes the bcc structure when it becomes about 0.1. The charge transfer needed for stabilizing the β structure is highest for Ti followed by Hf and then Zr, in line with the trend in transition pressures.

A new high-pressure phase transition in a zirconium-niobium alloy

It is reported for the first time that a g M y transformation can be induced in a g -stabilized zirconium alloy subjected to shock pressure. The y phase formed in the g matrix has been found to have a plate shape akin to martensitic plates. The lattice correspondence between the g and y structures has been found to be the same as that produced by thermal treatment. The formation of the plate-shaped y phase is explained in terms of a mechanism involving shear on <112> planes of the bcc lattice and the mechanical instability of the g phase.

On compressibility of osmium metal

On the basis of the high-pressure diamond anvil cell experiments on Os metal, Cynn et al. [Phys. Rev. Lett. 88, 135701-1 (2002)] have reported that this metal has lower compressibility than diamond. In the present work we have reanalysed the experimental data of Cynn et al. We find that the bulk moduli of Os and diamond are close to each other, implying that Os metal is as incompressible as diamond, but not more so. Our first principles total energy calculations using the full potential linearised augmented plane wave method on Os and diamond also suggest the same results.

Mass spectrometer calibration of high velocity impact ionization based cosmic dust analyzer

We are calibrating the time of flight mass spectrometer of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft. The CDA measures the flux of particles in the 10^(−15) to 10^(−9) g range at intersection velocities of up to 100 km/s. Of special interest are the chemical composition of the particles in orbit about Saturn and/or its satellites that are expected to be captured by CDA during ring plane crossings and upon close encounter with the satellites. Upon impacting a rhodium plate, particles are expected to partially ionize and their chemical composition is expected to be determined from mass analysis of the positive ions. In order to optimize impact ionization calibration experiments using a light gas-gun launched microspheric particles, we have done initial testing with a short duration pulsed laser (4 ns duration nitrogen laser (337 nm)). The beam is focused to deliver the 300μJ energy per laser pulse onto a 33 μm^2. The laser power density (≈10^10 W/cm^2) simulates the impact of particles with various combinations of density and velocities, e.g., 8 g/cm^3 (Fe) projectile at 23 km/s or 1 g/cm^3 projectile at 65 km/s. The CDA spectrometer will operate in the near vacuum of Saturnian zone environment is housed in a laboratory chamber at 10^(−6) mbar. The ions and electrons are separated by 680 V between target and grid. The laser ionization produces charge of 4.6pC (mostly Al^(+1)) in aluminum and 2.8pC (Fe^(+1)) in stainless steel. Estimating that each Al^(+1) and Fe^(+1) ion requires an energy of 5.98 and 7.90 eV/ion implies that ∼10−5 % of the laser pulse energy produces ions and the present system has a 10% detection efficiency. Using multi-channel plate detector to detect ions from aluminum alloy and kamacite yields well defined peaks at 24(Mg^(+1)), 27(Al^(+1)) and 64 (Cu^(+1)), and, 56(Fe^(+1)), 58(Ni^(+1)) and 60(Ni^(+1)) amu, respectively. Also contaminant ions at 23 (Na^(+1)) and 39(K^(+1)) amu are detected.

α→ω transition in shock compressed zirconium: a study on crystallographic aspects

In 1973, Usikov and Zilbershtein proposed that theα(hcp) →ω (a three atom hexagonal) transformation in Zr and Ti proceeds via theβ(bcc, a high temperature phase) intermediate. Based on this they derived two non-equivalent orientation relationships (OR) betweenα andω phases. Their transmission electron microscopy (TEM) study carried out on these elements, that wereα →ω-transformed under static high pressure, revealed only one of the two proposed ORs. Various TEM studies done thereafter on these elements and their alloys (ω transformed under static pressures) conform to either one of these ORs. In a recent TEM study by Song and Gray on Zr,ω-transformed under shock compression, a new OR has been observed which according to them is different than those given by UZ and they put forth the directα →ω transformation mechanism. In the present study, we have generated additional TEM data on shock compressed Zr samples and have reconciled the above conflicting results. We find all our ORs (which contain the OR of SG also) to be described by the OR reported by UZ. The latter OR (i.e. of SG) is shown to be a subset of the former. These observations show that the same type of mechanism of transformation is operative both, under static and shock compression. Mechanism of the transition is discussed in terms of the required strains.

Mass spectrometer calibration of Cosmic Dust Analyzer

The time-of-flight (TOF) mass spectrometer (MS) of the Cosmic Dust Analyzer (CDA) instrument aboard the Cassini spacecraft is expected to be placed in orbit about Saturn to sample submicrometer-diameter ring particles and impact ejecta from Saturns satellites. The CDA measures a mass spectrum of each particle that impacts the chemical analyzer sector of the instrument. Particles impact a Rh target plate at velocities of 1-100 km/s and produce some 10^(−8) to 10^(−5) times the particle mass of positive valence, single-charged ions. These are analyzed via a TOF MS. Initial tests employed a pulsed N2 laser acting on samples of kamacite, pyrrhotite, serpentine, olivine, and Murchison meteorite induced bursts of ions which were detected with a microchannel plate and a charge sensitive amplifier (CSA). Pulses from the N_2 laser (10^(11) W/cm^2) are assumed to simulate particle impact. Using aluminum alloy as a test sample, each pulse produces a charge of ∼4.6 pC (mostly Al^(+1)), whereas irradiation of a stainless steel target produces a ∼2.8 pC (Fe^(+1)) charge. Thus the present system yields ∼10^(−5)% of the laser energy in resulting ions. A CSA signal indicates that at the position of the microchannel plate, the ion detector geometry is such that some 5% of the laser-induced ions are collected in the CDA geometry. Employing a multichannel plate detector in this MS yields for Al-Mg-Cu alloy and kamacite targets well-defined peaks at 24 (Mg^(+1)), 27(Al^(+1)), and 64 (Cu^(+1)) and 56 (Fe^(+1)), 58 (Ni^(+1)), and 60 (Ni^(+1)) dalton, respectively.

Shock wave induced phase transitions from the trigonal phase to the coexisting amorphous and orthorhombic phases in α-FePO4

The shock induced response of berlinite form of α-FePO4, which has been studied recently under static pressure, has been investigated in order to examine the effect of shear and high temperature on the process of amorphization in this material. The samples were shock loaded up to 8.5 GPa in a gas gun and after recovery were analyzed using x-ray diffraction (XRD) technique. The sample retrieved from 5.2 GPa revealed an irreversible phase transformation of some of the material to an amorphous and a crystalline orthorhombic structure (space group Cmcm), which are co-existing. The XRD pattern of the 8.5 GPa sample on the other hand displayed the presence of only the orthorhombic phase along with the ambient structure. The absence of the the amorphous phase is attributed to the reverse transformation due to the high residual temperature in the 8.5 GPa sample. Since the Cmcm phase is equilibrium high pressure phase of such materials, the results could be interpreted on the three level free energy diagram. The c...

The orientation relations between the α and ω phases of shocked zirconium

From the recent shock wave measurements on Zr, Song and Gray (SG) reported a new orientation relationship (OR) between α and ω phases, which differed from those observed in earlier hydrostatic pressure experiments. Based on this, a different transformation mechanism was also proposed. To clarify the situation, we have performed selected area electron diffraction measurements on Zr samples shock loaded to 12 GPa. All the diffraction patterns are found to be consistent with the OR of Usikov and Zilbershtein (UZ). One diffraction pattern revealed even the OR of SG also. The superposition of the stereographic projections shows, however, that the ORs of UZ and SG are equivalent. Thus, the mechanism of phase transformation under static and shock loading is identical. Also, as both models (proposed by UZ and SG) lead to equivalent ORs, it is difficult to establish which one is operative.