Polycrystalline Nb Research Articles

On the correlation of blister diameter and blister skin thickness in helium-ion-irradiated Nb

A systematic study of the correlation between blister diameter and blister skin thickness has been performed for helium-ion irradiation of monocrystalline and polycrystalline Nb for ion energies ranging from 20 to 500 keV. The results indicate that a relationship Dmp∝t1.50 between the most probable blister diameter, Dmp, and blister skin thickness, t, which has been suggested by other authors, does not exist for the various types of Nb targets studied. For example, for room-temperature irradiation of annealed polycrystalline Nb the experimentally determined relationship is Dmp=10.3t1.22. Furthermore, the D-t relationship was found to depend on the irradiation temperature in contrast to theoretical predictions by the lateral stress model of blister formation. These results do not appear to support the lateral stress model which predicts the relationship D∝t1.5. However, the experimentally determined relationships can be explained in part by the gas pressure model of blister formation.

Characterization of Nb<inf>3</inf>Sn diffusion layer (A-15) material

Microstructural characterization of Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn diffusion layers grown by Sn-vapor reaction with single crystal Nb substrates has been carried out using Auger and X-ray spectroscopy, X-ray and reflection electron diffractometry, and scanning electron microscopy. The layers exhibit preferred crystallographic orientation traceable to the underlying Nb-substrate orientation. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\langle 111 rangle</tex> -Nb-substrate layers differ substantially from all other-Nb-direction layers. In particular, they exhibit surface-like composition profiles over several hundred angstroms into the bulk, a very rough micro-structure including numerous microcracks, and a surface superconducting transition temperature of < 1 K. Non- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\langle 111 rangle</tex> -Nb-substrate layers with 1 μm columnar grain structure and surface T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> 's ∼ 18 K appear to have breaking stresses far in excess of bulk polycrystalline Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn.

Anisotropy of the generalized Ginzburg-Landau parameters x 1 and x 2 in clean, cubic, type II superconductors

From the nonlocal GLAG theory expressions are derived for chi/sub 1/ and chi/sub 2/ of a clean, cubic crystal with a general Fermi surface. In the region of weak nonlocality near T/sub c/ the correction terms in H/sub c/2, chi/sub 1/, and chi/sub 2/ due to the nonspherical Fermi surface are expressed by two dimensionless Fermi surface averages. For highly purified Nb single crystals and polycrystalline Nb samples, chi/sub 1/ and chi/sub 2/ were determined from magnetization measurements in the temperature range (0.3--0.95) T/sub c/. The experimental data show that the orientation dependence in chi/sub 2/ is twice as large as in chi/sub 1/, in agreement with the theoretical result near T/sub c/. The two Fermi surface averages that characterize the anisotropy of H/sub c/2, chi/sub 1/, and chi/sub 2/ near T/sub c/ are estimated from experimental data for Nb and V.

Computer studies of the scattering of low energy hydrogen ions from polycrystalline solids

Reflection of 50 eV to 10 keV H atoms from polycrystalline Cu, Nb and Au targets has been calculated using the binary collision cascade program MARLOWE. The fractions of particles and energy reflected (backscattered) increase with increasing atomic number of the target and decrease with increasing incident energy. The results indicate that the effects of polycrystallinity are modest, reducing the amorphous reflection coefficients by about 25%. The calculations agree quite well with the experimental data for Cu and Au, but are about a factor of two larger than is observed for Nb.

Some 14 MeV neutron sputtering characteristics of a polycrystalline Nb sample

A sample of cold-rolled, polycrystallinc niobium was exposed to a fluence of 1.5 X 10 14 MeV neutrons/cm. Material sputtered from this sample was collected on a single crystal silicon disk placed in close juxtaposition to the Nb target. The collector was examined by three techniques, IMMA, SEM and Rutherford back-scatter of He ions. The sputtered material was found to be in two forms. The entire collector was covered with a background of “atomically” sputtered material. Superimposed on this were a number of irregular areas of high Nb concentration, patches or clusters of sputtered Nb atoms. When the total sputter yield is calculated from both of these forms a value of the order of 2–5 × 10 −5 is obtained.

Some 14 MeV neutron sputtering characteristics of a polycrystalline Nb sample☆

A sample of cold-rolled, polycrystallinc niobium was exposed to a fluence of 1.5 X 10 14 MeV neutrons/cm. Material sputtered from this sample was collected on a single crystal silicon disk placed in close juxtaposition to the Nb target. The collector was examined by three techniques, IMMA, SEM and Rutherford back-scatter of He ions. The sputtered material was found to be in two forms. The entire collector was covered with a background of “atomically” sputtered material. Superimposed on this were a number of irregular areas of high Nb concentration, patches or clusters of sputtered Nb atoms. When the total sputter yield is calculated from both of these forms a value of the order of 2–5 × 10 −5 is obtained. Un échantillon de niobium polycristallin laminé à froid a été exposé à une fluence de 1,5 × 10 16 neutrons / cm 2 de 14 MeV. Le matériau pulvérisé de cet échantillon a été recueilli sur un disque de silicium monocristallin placé très près de la cible de Nb. Le collecteur a été examiné par trois techniques, IMMA, SEM et la rétrodiffusion Rutherford d'ions He + . La matière pulvérisée se présentait sous deux formes. La surface entière du collecteur était recouverte d'un fond de matière pulvérisée à l'état atomique. Superposées à ce fond se trouvait un nombre de surfaces irrégulières de zones ou d'amas d'atomes de Nb pulvérisés à haute concentration en Nb. Quand le rendement total de pulvérisation est calculé à partir de ces deux formes de matière pulvérisée, on trouve une valeur de l'ordre de 2 a ̀ 5 × 10 −5 . Eine kalt gewalzte polykristalline Nb-Probe wurde einer 14 MeV-Neutronendosis von 1,5 · 10 16 cm −2 ausgesetzt. Das von dieser Probe zerstäubte Material wurde auf einer einkristallinen Si-Scheibc gesammelt, die in nächster Nachbarschaft zum Nb-Target lag. Der Kollektor wurde nach drei Methoden untersucht, durch Ioncnstrahl-Mikroanalyse, Sekundärelektronenmikroskopie und Rutherford-Rückstrcuung von He + -Ionen. Das zerstäubte Material lag in zwei Formen vor. Der gesamte Kollcktor war mit einem Untergrund von atomar zerstäubtem Material bedeckt. Diesem waren mehrere uneinheitliche Bereiche hoher Nb-Konzentration überlagert, die aus Flecken oder Haufen von zerstäubten Nb-Atomen bestanden. Wenn man die gesamte Zerstäubungsausbeute aus beiden Gebilden berechnet, ergibt sich ein Wert von etwa 2 bis 5 · 10 −5 .

High dose He + bombardment of niobium at 800° tO 1400°C

In proposed fusion reactors, the first wall will be bombarded by low energy D +, T + and He + ions to very high doses. Room temperature irradiations with 9 keV He + (Roth et al. 1975) showed an initial phase of blistering between 0.05 and 0.25 C/ cm 2, and a final state of considerable surface roughness at 8 to 112 C/ cm 2(7 × 10 20 He +/ cm 2). The present irradiation experiments with 6 keV He + are carried out with polycrystalline Nb foils at temperatures between 800 and 1400°C in order to study the influence of He mobility and of Nb surface diffusion during irradiation. The applied doses range from 2.5 × 10 17 He +/ cm 2 (blistering) to 5 × 10 20 He +/ cm 2 which corresponds to at least several weeks of reactor operation. The resulting changes of the niobium surface structures are observed by scanning electron microscopy and are pictorially presented in this paper. Mainly, sponge-like open structures are seen to develop at high doses, with increasing physical dimensions at higher temperatures.

Irreversible effect of hydrostatic pressurization on yielding in niobium

The irreversible effects of hydrostatic pressurization on the mechanical properties of polycrystalline Nb were studied. It was found that for prior pressures up to and including 3.45 MPa, there is no detectable change in the yield stress. Prior pressurization at 6.9 MPa eliminates the yield drop and sharp yield point at room temperature and reduces the yield stress at all temperatures. Pressurization at higher pressures causes a correspondingly greater decrease in yield stress. The fractional change in yield stress for a given prior pressure is greater at lower temperatures. The mechanical behavior of pre-pressurized niobium differs from that of other bcc metals such as iron and chromium mainly in that the critical pressure to induce an irreversible effect on mechanical properties is about two orders of magnitude lower than those previously observed. (auth)

Superconductivity in ZrxNb1−2xMox alloys: Possible dominance of a soft phonon mode

Normal-state specific heat and superconducting transition temperature measurements were performed on pure polycrystalline Nb and isoelectronic bcc ZrxNb1−2xMox alloys (x = 0.02, 0.05, and 0.10). Measured T c, γ and θ D all decline nearly linearly from the pure Nb values with increasing x. The changes are 15, 10, and 6%, respectively, for x = 0.1. T cH calculated by Hopfield's theory, using data and linearly interpolated values of η and θ D /〈θ2〉linear 1/2, increases with x by 28% for x = 0.1. By relaxing the linear interpolation of θD/〈θ2〉1/2, Hopfield's theory can be used to evaluate an average phonon frequency 〈θ2〉emp 1/2 from data. It is found to be nearly constant at about 229 K, in contrast to 〈θ2〉linear 1/2, which declines by 8% at x = 0.1. We suggest that one possible explanation for this behavior is the dominance of incipient soft-phonon longitudinal modes which occur in Nb and presumably in the alloys. Miedema's method has been used to calculate the variation of both γ and T c with alloying. The calculated γ is nearly constant, while the calculated T c declines more quickly than the measured variation. The band-structure electron density of states calculated with the aid of McMillan's equation for these isoelectronic alloys is found to decline by 8% at x = 0.1, in contrast to the prediction of the rigid band model. Normal-state low-temperature specific heat measurements on a pure annealed polycrystalline sample of Nb yield γ = 7.80 ± 0.02 mJ mole−1 K−2 and θD = 276 ± 2 K, in excellent agreement with previous measurements. The break in slope of C p/T vs. T 2 at 3.2 K is discussed.

Total backscattering yields of 3–15-keV protons from polycrystalline Nb

Polycrystalline Nb samples were bombarded by 3–15-keV proton beams at normal incidence. The energy and angular distributions of backscattered positively and negatively charged particles were measured quantitatively. By means of the charged fractions measured previously, the energy and angular distributions of the total number of backscattered particles including the neutrals were obtained. Integration of the distributions over all energies and angles yield the total reflection coefficients. These are considerably lower than existing theoretical values.

Microplasticity in b.c.c. metals

Stress relaxation was studied in polycrystalline Nb, Mo and W of 99.95% purity at 77 – 298 K. The relaxation following pre-straining to stresses not exceeding 70 – 80% of the macro-yield level was logarithmic, with the slope s = dσ/d ln ( t + t o) proportional to the tensile stress σ. At higher stresses the relation between s and σ was again linear, but d s/d σ assumed a new, constant, value. These observations, certains anomalies in the relaxation behaviour apparent particularly with high stresses and pre-staining rates, and the observed temperature dependence of s, are consistent with an interpretation in which mainly edge dislocations, impeded in their movement by structural defects and by impurities, control the relaxation in the low-stress region; at higher stresses dynamic recovery involving migration of jogged screw dislocations appears to be rate determining.

Mass spectroscopy of sputtered neutrals and its application for surface analysis

Mass spectroscopic studies of the neutral particles sputtered by Ar +ions at energies around 1 keV from polycrystalline targets of 11 different metals have been performed, using a hf plasma maintained in pure argon at some 10 −4 Torr by electron cyclotron wave resonance as an ionizing medium as well as an effective ion source. Besides sputtered atoms Me 1 and molecules Me 2 with intensity ratios R 21 between Me 2 and Me 1 in the order of 10 −2 to 10 −1, neutral triatomic complexes Me 3 with corresponding ratios R 31 of some 10 −4 are found. Possible applications of the experimental method for surface analysis are discussed for previously air exposed polycrystalline Ta and Nb surfaces.

Mass Spectroscopy of Sputtered Neutrals and Its Application for Surface Analysis

Using a hf plasma maintained in pure argon at some 10-4 torr by electron-cyclotron-wave resonance as an ionizing medium as well as an effective ion source, neutral sputtered particles are mass analyzed by a large aperture quadrupole mass spectrometer. Mass spectra of the neutral species sputtered from clean polycrystalline surfaces by normally incident Ar+–ions of 1 keV have been determined for 11 different pure metals. Besides sputtered atoms Me and diatomic molecules Me2 with intensity ratios R2 1 between Me2 and Me1 in the order of 10-2 to 10-1, neutral triatomic complexes Me3 with corresponding ratios R31 of some 10-4 are found. The results for noble metals are discussed in terms of the influence of σ bonding electrons. Possible applications of the experimental method for surface analysis by sputtered neutrals are demonstrated for air contaminated polycrystalline Ta and Nb samples. From the time dependent behaviour of the corresponding mass spectra of postionized sputtered neutrals, containing oxides and nitrides only as diatomic species MeO and MeN with an initial fraction of some 10-1, the depth profiles of the surface impurity layers have been determined.

Fatigue hardening in niobium single crystals

Nb single crystals of various orientations were cyclically deformed in tension-compression under strain control. At low strain amplitudes all crystals oriented for single slip and some oriented for multiple slip showed a two stage hardening. When present, the first stage was characterized with almost no cyclic work hardening. The rate of hardening in the second stage increased with strain amplitude and the amount of secondary slip; however in all cases it was considerably less than θ II in unidirectional deformation. In crystals oriented for single slip kink bands developed on their side faces during rapid hardening stage which resulted in considerable amount of asterism in Laue spots. A cyclic stress-strain curve independent of prior history was found to exist which was also independent of crystal orientation. Furthermore, this curve differed only slightly from that of polycrystalline Nb obtained from data in literature.

Spatial distributions of H 2 desorbed from Fe, Pt, Cu, Nb, and stainless steel surfaces

The spatial distributions of H 2 molecules desorbed from polycrystalline Fe, Pt, Cu, Nb and stainless steel (type 304) surfaces were measured with a rotatable ionization gauge. Prior to cleaning the surfaces by ion bombardment or by oxidation, the AES (Auger electron spectroscopy) spectra indicated that all of the sample surfaces were highly contaminated. (S, C, and P were the predominant impurities.) The spatial distributions measured under these conditions were considerably narrower than the commonly-assumed diffuse distribution; i.e., the concentration of desorbed molecules was higher in the vicinity of the surface normal than for the case of diffuse emission represented by cos θ, where θ is the angle of inspection measured from the surface normal. These measured distributions may be described approximately by the form cos d θ, where d ranges from ∼ 2.5 for Nb to ∼ 7 for Fe. However, as the surfaces were cleaned, the spatial distributions for Fe, Pt, and Nb tended to become diffuse (i.e., d → 1), which is consistent with the trend we observed previously for polycrystalline and single-crystal Ni. Copper proved to be an exception to this trend, since its distribution was far from being diffuse (i.e., d≅ 4) when the surface appeared to be clean according to our AES measurements. We suggest that this unusual characteristic of Cu is a consequence of an activation energy barrier associated with the dissociative adsorption of hydrogen on Cu.

Blistering of polycrystalline and monocrystalline niobium

Abstract Radiation blistering of niobium surfaces has been investigated for normally incident He+ and D+ projectiles. For the He+, the bombarding energies were 0.5 and 1.5 MeV, the targets were both monocrystalline and polycrystalline niobium at room temperature and at 900°C, and the total dose was from 0.01 to 1.0 C/cm2. For cold-worked samples at room temperature, 0.5-MeV He+ ions produced large blisters (up to 500 μm in diameter), many of which were ruptured; 1.5-MeV He+ ion implantation produced one big blister covering most of the irradiated area. For the D+ ion implantation, the bombarding energies were 0.250 and 0.5 MeV and the targets were all polycrystalline. Implanting 0.250-MeV D+ ions in annealed polycrystalline Nb at 700°C produced small blisters for doses ranging from 1.6 to 3.0 C/cm2. Preliminary results on the skin thicknesses of ruptured blisters indicate a close relationship between the thickness and both the projected range of the ions and the peak in the defect distributions.

Backscattering of Hydrogen and Rare Gas Ions from Metal Surfaces

Energy and angular distributions of H1+, Ne+, and Ar+ backscattered from polycrystalline Nb and W targets were measured. The bombarding energy used was 15 keV. The pressure in the target region during bombardment was kept below 2×10−9 Torr. As an energy spectrometer we used an electrostatic spherical condenser. Surface impurities were detected by observing the binary scattering peaks of Ne+. The shape of the energy spectra of backscattered protons depends strongly on the surface cleanness and on the crystalline structure of the target.

Vacuum Thermionic Work Functions of Polycrystalline Nb, Mo, Ta, W, Re, Os, and Ir

Experimentally measured effective vacuum thermionic work functions are reported for well-outgassed polycrystalline (but thermally preferentially oriented in many cases) surfaces of 70% (100)+20% (211)-oriented niobium (4.19 eV), 70% (100)+20% (211)-oriented molybdenum (4.0–4.3 eV), 70% (211)-oriented tantalum (4.25 eV), 96% (100)+4% (111)-oriented tungsten (4.52 eV), fairly randomly oriented rhenium (4.96 eV), randomly oriented osmium (4.83 eV), and 80% (111)+15% (100)-oriented iridium (5.27 eV) in a clean, 10−9 Torr background vacuum. Special techniques were employed to ensure accurate current, area, and temperature values. Purities, heat treatment histories, and x-ray crystallography measurements on the surfaces are included.

The internal friction of tantalum and columbium foils at ultrasonic frequencies

The internal friction and effective elastic moduli of polycrystalline Ta and Nb foils were measured at vibrational frequencies of about 37 kilocycles and (for Nb) 111 kilocycles. Relaxation effects readily identified with O and N impurities were observed, and correlated with the low frequency data of other investigators. Activation energies for O and N diffusion in both Ta and Nb were observed to decrease with increasing temperature, while the internal friction peaks were shifted upward on the temperature scale when the impurity content was increased. Measurements extended over the temperature interval from about −160°C to about 1000°C. Low temperature anomalies in the internal friction of Ta are believed to be caused by hydrogen. Room temperature plastic deformation of both Ta and Nb produced large but poorly defined decrement increases between 20°C and 200°C, a range which showed no anomalies for annealed specimens.