Rhenium Single Crystals Research Articles

The remarkable surface structure sensitivity of the ammonia synthesis over rhenium single crystals

The remarkable surface structure sensitivity of the ammonia synthesis over rhenium single crystals

Stationary creep and dislocation structure of Rhenium Single Crystals tested at moderate temperatures

AbstractCreep of rhenium single crystals is studied in tension along <11¯23> in the range of moderate temperatures (1233–1593 K). The kinetic equation is obtained to describe stationary creep rate as a function of temperature and stress, the parameters of this equation are determined. The activation energy is shown to be constant and independent of the stress applied. The X‐ray divergent beam technique (DB) is employed to show that glide occurs mainly in a basal plane, the preferential bending axis of its lattice lying along the tensile direction, and that the internal regions of the crystal comprise unstressed areas misoriented with respect to each other by an angle ≈ 10÷15'. The electron microscope studies of the dislocation structure suggest that hexagonal networks of dislocations located in basal glide planes are typical elements of the structural state.

The nuclear magnetic moment of190Ir and the directional distribution of gamma rays in190Os

Angular distributions of gamma transitions between levels in I9OOs fed in the decay of I9'Ir aligned in a rhenium single crystal have been measured at temperatures down to 2 mK. Multipole mixing ratios in both gamma and electron-capture transitions are deduced and compared with various nuclear models. The angular distribution coefficients are applied to interpret very small anisotropies observed when I9'Ir is polarised in an iron host matrix, cooled to 2.5 mK. The deduced magnetic dipole moment of the 4 + ground state of I9'Ir is Ip(I9'Ir; GS)~ = 0.04( l)pN. Possible single-particle descriptions compatible with this result are suggested. NUCLEAR MOMENTS: I9'Ir (from Re(a, n)): measured I,(@ from oriented nuclei in single crystal Re and polycrystalline Fe: deduced I9OOs S(E2/M1), Jp in electron capture, lpl(1901r ground state).

Electrical and superconducting properties of rhenium thin films

Single-crystal rhenium films were prepared by electron beam evaporation onto polished single-crystal sapphire at various substrate temperatures T s. Those with T s > 1100 K showed bulk behaviour in the superconducting transition temperature T c and in the resistivity ϱ at room temperature (1.7 K and 18.0 μΩ cm respectively). A residual resistivity ratio r of 15–21 was obtained for these films. Size effect measurements on films grown at 1300 K resulted in a value of 4.5 × 10 −11 Ω cm 2 for the product ϱ( T) l 0( T), where l 0 is the mean free path of the conduction electrons. Films grown at T s ⩽1000K had higher values of T c and of the residual resistivity ϱ 0. Annealing experiments indicated that these increased T c and ϱ 0 values are probably due to vacancies. Similar experiments were performed on films grown at rough-surfaced sapphire sunstrates. Interpretation of these results required the assumption of an inhomogeneous distribution of vacancies due to pinning to dislocations.

The low temperature electric quadrupole orientation of184Ir and185Ir in rhenium

The half-lives of184Ir,185Ir and186Ir have been determined to be 3.14(2) h, 14.4(1) h and 16.64(3) h, respectively. The quadrupole frequenciesv Q = e2qQs/h of184Ir and185Ir in rhenium single crystal have been determined to be Qs(184Ir) = + 2.0(3) b MHz and Qs(185Ir) = +2.0(3) b MHz. Adopting the value of −3.6(2)×1017 V·cm−2 for the electric field gradient eq at T=0 K, the values of the ground-state spectroscopic quadrupole moments of these isotopes are determined to be Qs(184Ir)=+2.0(3) b and Qs(185Ir)=−2.5(3) b. The185Ir moment is only consistent with the ground state Nilsson configuration $$I^\pi K = \tfrac{5}{2} - \tfrac{1}{2}$$ , the184Ir moment is consistent with K≥4 with a predominant K=4 component, in disagreement with K=0,1 found for186Ir. Our analysis differs in detail from the recent nuclear orientation study by Hagn et al.[1].

The influence of structural defects on the adsorption of simple molecules (H 2, C 2H 2, C 2H 4, C 2H 6, CO, NO) on rhenium single crystals

The influence of structural defects on the adsorption of simple molecules (H 2, C 2H 2, C 2H 4, C 2H 6, CO, NO) on rhenium single crystals

The influence of structural defects on the adsorption of simple molecules (H 2, C 2H 2, C 2H 4, C 2H 6, CO, NO) on rhenium single crystals

Using Thermal Programmed Desorption (TPD), Low Energy Electron Diffraction (LEED) and Auger Electron Spectroscopy (AES) we have studied the adsorption of hydrogen-containing molecules (H 2, C 2H 2, C 2H 4, C 2H 6) and oxygen-containing molecules (CO and NO) on two vicinal planes of the Re(0001) surface. The two surfaces are designated thus: ReS ¦14(0001)(101̄1)¦, ReS |6(0001)(167̄1) | . The structural defects have little effect on the adsorption of hydrogen and the hydrocarbons. They are more influential in the case of the oxygen-containing molecules. This is particularly true for CO; on the kink sites the CO molecules can completely dissociate whereas only a partial dissociation is possible on the steps. These results should be viewed in relation to the strong bond energy between carbon and oxygen in a CO molecule of 256 kcal/mole and the great affinity of oxygen for rhenium; E Re− O = 127 kcal/mole.

Influence of structural defects, steps and kinks, on oxygen adsorption on rhenium single crystals

Oxygen adsorption on Re (0001), Re (S) [14 (0001) × (1011)], and Re (S) [6 (0001) × (1676)] has been studied by Auger electron spectroscopy and by low energy electron diffraction. Presence of kinks on this last plane gives a more important amount of oxygen adsorbed than on the two other surfaces. On these three surfaces heating to 500 K is necessary to get ordered structures. The (2 × 2) pattern observed on Re (0001) corresponds to θ = 1/2 and results from the superposition of three domains with unit cell P (2 × 1) at 120° to each other. On the two other surfaces, the presence of steps favorises the development of one particular domain with the shortest side of its unit cell parallel to the step direction. Surface with kinks reconstructs when heated at 800 K after saturation with oxygen at room temperature, while the two other surfaces are stable under the same conditions. This reconstruction could be explained by the formation of a superficial layer of ReO3. On a étudié par spectroscopie des électrons Auger et diffraction des électrons lents l'adsorption de l'oxygène sur Re (0001) et sur deux faces vicinales de ce plan Re (S) [14 (0001) × (1011)] et Re (S) [6 (0001) × (1676)]. La présence de crans sur cette dernière face entraine à température ordinaire une adsorption plus importante de l'oxygène que sur les deux autres surfaces. Sur ces trois surfaces un chauffage à T = 500 K est nécessaire pour faire apparaǐtre une structure ordonnée. La structure 2 × 2 observée sur Re (0001) correspond à θ = 1/2 et est interprétée comme résultant de l'existence de trois domaines ayant une maille P (2 × 1) à 120° les unes des autres. Sur les deux autres plans la présence de marches favorise le développement du domaine dont la maille présente la petite dimension parallèle à la marche. La surface présentant des bords de marches crantés se reconstruit lors d'un chauffage à 800 K après saturation en oxygène à température ordinaire alors que les deux autres surfaces ne sont pas modifiées par le méme traitement. Cette reconstruction pourrait s'expliquer par la formation de l'oxyde superficiel ReO3.

Third-order elastic constants, pressure derivatives of the second-order elastic constants and anderson-grüneisen parameter δ of rhenium

Recently the second-order elastic (SEE) constants of single-crystal Rhenium and their pressure derivatives have been measured at 25 ~ by -~iA~aH~AN~ et al. (1). Rhenium is a h.c.p, crystal with c/a-ratio equal to 1.615 at 25 ~ which is slightly less than the ideal value. The object of the present paper is to evaluate the third-order elastic (TOE) constants of Rhenium which have nearly the ideal axial ratio value on the basis of a nearest-neighbour central interact ion model. These TOE constants are then used to evaluate the pressure derivatives of the SOE constants of Rhenimn which are compared with the experimental values (1). The Anderson-Griineisen parameter 6 (~,a) is also evaluated for Rhenium from its TOE data. SRINIVASAN and RA~JI RAo (~) worked out the thermal expansion of an ideal h.c.p. latt ice with a nearest-neighbour central interact ion potential of the type

Critical temperature of superconducting transition in plastically deformed rhenium single crystals

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Investigation of the Superconducting-Energy-Gap Behavior of Single-Crystal Rhenium by Tunneling

A 15.7% anisotropy in the superconducting energy gap has been measured by electron tunneling into single crystals of rhenium, using thin carbon films as the tunneling barrier to form Re-C-In sandwiches. The energy-gap behavior vs temperature is BCS-like. The anisotropy agrees with ultrasonic measurements of other hcp metals and suggests that phonon dispersion plays a dominating role in the gap behavior. However, gap values determined from multiparticle tunneling data exhibited reduced gap magnitudes and a dissimilar crystallographic behavior.

The effect of neutron irradiation on mechanical properties of rhenium

Abstract Polycrystalline and single crystal rhenium tensile specimens have been neutron irradiated at 50°C to fluences up to 2.6 × 1019 n/cm2 and subsequently tested in tension. The microstructure of the single crystals was examined by transmission electron microscopy. The yield strengths of the polycrystalline specimens increased from 19,000 psi in the unirradiated material to 143,000 psi after a fluence of 2.6 × 1019 n/cm2. Slip was observed on the basal and prismatic planes and (1121) twinning was found to be the main twinning mode in the single crystals. The single crystal yield strengths also increased markedly with fluence and were found to increase at a greater rate in those crystals oriented for prismatic slip. The radiation damage greatly retarded twin growth at the higher fluences but did not appear to affect twin nucleation. Channeling of dislocation through the small defect clusters occurred in the irradiated specimens. Deformation twinning did not result in channeling type behavior although a...

Tunneling into a superconducting rhenium single crystal

Tunneling measurements have been made of 2Δ(T)kBTc for a superconducting rhenium single crystal over the range 0.89 ≥ t ≥ 0.58. Within experimental error, the results follow a BCS curve, with 2Δ(0)kBTc = 3.38. Durch die Tunnel-effekt Technik haben wir die Messung der Temperaturabhängigkeit von der Energielücke 2Δ(t) für eine Kristallorrientierung des supraleitenden Rhenium-Einkristalles in dem Temperatur-intervall von t = 0.58 zu t = 0.89 gemacht. Die Folgerung aus der BCS-Theorie mit 2Δ(0)kBTc = 3.38 wurde für die Abhängigkeit gefunden.

LEED study of steps and facets on a Re surface

A rhenium single crystal with an initial (0001) orientation, recrystallized by twinning to yield {101̄ n} planes parallel to the surface. The LEED pattern displayed a one-dimensional superstructure which could be attributed to a system of regularly spaced steps. The 00 beam is diffracted from the (0001) plateaus of the steps, and not the high index plane. Heating the crystal in oxygen apparently modified the step system. Residual gas analysis showed that even after a “clean” LEED pattern has been obtained, carbon contamination is removed from the surface as CO by heating the sample in oxygen. Electropolishing of Re (0001) may result in basal plane etching. The faces of the rhombohedral pyramids thus formed were determined by analysis of LEED patterns and photomicrographs to be {112̄ n}, n = 2, 3, 4. The facet reflections are shifted in energy with respect to the normal reflections, due to a difference in inner potential. Also a dynamic interaction between the two sets of beams is suggested by observations. An intensity versus voltage plot for the normal 00 beam exhibits “forbidden” Laue maxima out to the 21st order. The absence of all but two secondary Bragg peaks may be explained with an Ewald sphere construction.

527. Field electron emission from lithium films on tungsten and rhenium single crystals. (USSR): A P Ovchinnikov and B M Tsarev, Fiz Tverd Tela, 9 (12), Dec 1967, 3512–3515 ( in Russian)

527. Field electron emission from lithium films on tungsten and rhenium single crystals. (USSR): A P Ovchinnikov and B M Tsarev, Fiz Tverd Tela, 9 (12), Dec 1967, 3512–3515 ( in Russian)

Unit cell and dimensional changes in irradiated single-crystal rhenium

Abstract Single-crystal rhenium rods exposed to 2·5 × 1018 and 5 × 1018 n/cm2 (E > 1 Mev) show fractional length and lattice parameter changes positive in the a-axis direction and negative in the c-axis direction. The magnitudes of the changes are such that the unit cell volume decreases after an initial expansion, while the crystal volume increases to an apparent saturation value. These results are interpreted as indicating a defect configuration of clustered interstitials and isolated vacancies.

Work Function of the (100) Surface of Tungsten-Rhenium Alloys

The work functions of the (100) surfaces of tungsten, tungsten with 2% bulk rhenium, and tungsten with 6% bulk rhenium single crystals were measured using the Shelton method. The values of 4.55 V, 4.56 V, and 4.59 V were obtained for the three samples, respectively. The A values for the three samples were 100, 110, 130 A/cm2 °K2, respectively. Electron-beam microprobe results indicated a surface concentration of 0.67% rhenium in the 2% bulk sample and 2% rhenium in the 6% bulk sample. The loss of rhenium from the surface has been attributed to the preferential sublimation during outgassing of the samples. The relative changes in work functions are consistent with the shift in the voltage knees on the ln i vs VR curves. The actual shape of the ln i vs VR curve is discussed.

Deformation twinning in rhenium single crystals

Abstract Experimental results are reported on the formation of deformation twins in rhenium single crystals, deformed in tension at room temperature. The twinning shear is large (∼0·6), in agreement with some earlier results for similar twins in zirconium. Low shear twins of the type, which are generally a feature of the deformation characteristics of hexagonal metals, were not observed. A possible reason for the choice of twinning system is proposed.

Elastic Constants of Rhenium Single Crystals in the Temperature Range 4.2°–298°K

The five independent elastic constants of single crystals of hexagonal close-packed rhenium have been determined by the ultrasonic pulse-echo technique over the temperature range 4.2°–298°K. The values at 298°K in units of 1011 dyn/cm2 are C11=61.25, C33=68.27, C44=16.25, C12=27.0, and C13=20.6. The bulk modulus derived from these constants shows a temperature variation somewhat analogous to that which has been reported for other 4d–5d transition metals. The Debye temperature calculated from the elastic constants at 4.2°K is 416.2°K.

The Preparation of High‐Purity Rhenium Single Crystals

Rhenium single crystals have been prepared by electron beam float zone melting with resistance ratios (R273°K/R4.2°K) exceeding 30,000. In one crystal a ratio in excess of 55,000 was obtained. It appears that purification occurs both by selective vaporization and zone refining.