Niobium Single Crystals Research Articles

Application of nuclear resonant reactions to the analysis of implanted Ne atoms in niobium crystal

Neon ions are implanted into niobium single crystal and their depth profile is determined employing 20Ne(p, γ) 21Na resonant nuclear reactions. The projected ranges obtained experimentally are close to the calculated mean depth of energy deposition. This implies the trapping of Ne atoms by radiation induced defects during implantation. The angular dependence for the backscattered protons and nhe nuclear reaction induced γ-rays is studied in Ne implanted niobium single crystal, and it is shown that a part of the implants occupies the octahedral interstice in niobium. The Ar + ion irradiation effect on the Ne atom distribution is recognized by reversing the sequence of the Ar + irradiation-Ne + irradiation. This effect is attributed to the radiation enhanced diffusion of Ne atoms under irradiation.

The deformation of niobium single crystals at temperatures between 77 and 4.2 K

Abstract The plastic deformation of niobium single crystals, purified by ultra-high-vacuum annealing, has been investigated in tension at temperatures between 77. and 4.2 K. The strongly orientation-dependent behaviour has been demonstrated by studying two tensile axis orientations, C (χ=l°, λ = 47°) and H (χ = 30°, λ = 45°). C specimens deformed readily even at 4.2 K. H specimens yielded at a high stress (two to three times that for C) with a large load drop. The deformation of C specimens was dominated by the anomalous slip, while the (101) [111] primary slip system was predominantly observed in H specimens. The load drop of H specimens was not due to twinning, but rather to the formation of shear bands. It has been demonstrated that the difference observed between C and H is caused by the difference in the operating slip systems, i.e. the anomalous slip in C specimens and the primary slip in H specimens, instead of an extrinsic cause. The effect of electron irradiation was examined at 30 K. Irradiation...

Electron irradiation hardening in niobium single crystals

Electron irradiation hardening was investigated in niobium single crystals with a tensile axis in the middle of the standard triangle. Specimens were irradiated at 15 K with 2 MeV electrons and deformed at 10 K in tension. The yield stress increased proportionally with the square-root of the dosage. Recovery behavior of the irradiation hardening suggested a formation of interstitial clusters at about 50 K. A simple model for the solid-solution hardening was proposed for a bcc metal which deformed by anomalous slip. The dependence of hardening on the interstitial concentration calculated using this model was compared with the experimental results. This calculation suggested that clustering of interstitials may also have taken place during irradiation at 15 K.

Cyclic deformation of Nb single crystals I. Influence of temperature and strain rate on cyclic hardening, shape changes and stress asymmetry

Abstract Niobium single crystals with single glide orientation have been cyclically deformed in tension-compression, at total axial strain amplitudes between 0.3 × 10−3 and 6.0 × 10−3. The cyclic hardening, the changes in shape of the crystals and the asymmetry of the hysteresis loop, have been studied at different temperatures (between 250 K and 354 K) and different strain rates (between 3.0 × 10−5 ' s−1 and 1.2 × 10−2 s−1). The cyclic stress-strain curves exhibit four different hardening regions, the main characteristic being the existence of a “plateau” region at low strain amplitudes where a limited amount of hardening or softening is observed. The length of this plateau increases as the temperature is decreased or the strain rate increased, and an interpretation is given in terms of the relative absence of dislocation-dislocation interactions. The shape of the cyclic stress-strain curves is found to depend on the cyclic strain history of the crystals, especially at the small strain amplitudes. The ch...

Cyclic deformation of Nb single crystals II. Influence of orientation on cyclic hardening, shape changes and stress asymmetry

Abstract Niobium single crystals with seven different orientations of the stress axis have been deformed cyclically in tension-compression at room temperature and at a constant strain rate of 6 × 10−4 s−1 over a range of total strain amplitudes between 0.3 × 10−3 and 6 × 10−3. The cyclic hardening behaviour has been found to be strongly dependent upon the crystal orientation. Those crystals whose c.s.s. curves exhibit four well-defined regions do not obey a cyclic equation of state, but for certain crystal orientations, the c.s.s. curves are flat and independent of the cyclic strain history. The magnitude of the crystal shape changes during cyclic deformation, varies with the crystal orientation, and for some orientations no changes in shape take place. These results are explained in terms of both the asymmetric slip and the observed active glide systems in tension and compression. The stress asymmetry was also found to vary in magnitude and sign with the orientation of the crystal axis in agreement with ...

Dislocation behavior in niobium single crystals with [110] and [100] tensile axis at low temperatures

Dislocation behavior in niobium single crystals with [110] and [100] tensile axis at low temperatures

Tunneling measurements on niobium single crystals with wax barriers

Using a wax-polishing-technique and evaporated lead counter-electrodes tunneljunctions on single crystal niobium are fabricated. Crystal surfaces normal to [100], [110] and [111] are investigated. No anisotropy is found in the tunneling curves within a precision of 1%. However, the evidence of these tunneling experiments is not sufficient, to exclude any gap anisotropy with the same certainty.

Étude diffractométrique de la dissolution de l'oxygène dans le niobium à pression moyenne (75 Torr) et entre 300 et 400 °C II: Traitement mathématique des profils des raies X

In this work we attempted to deduce the oxygen concentration profile in the thickness analysed from the form and the position of the niobium diffraction lines observed after oxidation and oxygen dissolution. This study was carried out, after oxidation of niobium single crystals at medium pressure (75 Torr) and in the temperature range 300–400 °C, in order to define the limiting concentration and the diffusion coefficient of oxygen in solid solution. Theoretical peaks were calculated using a computer. For the computations, curves of the solid solution parameter (compatible with oxygen diffusion) were assumed. The variation of these parameter curves allowed the adjustment of the theoretical and experimental peaks. The calculation was performed using the method of Gauss. The adjustment of the peaks necessitated the introduction into the calculations of a broadening factor for the elemental peak which depended on the gradient of the parameter variation. Models with one or two external zones (constant parameter) and a diffusion zone (variable parameter) were considered, to take into account solid solution transformation that appeared during cooling. Coincidence of the theoretical and experimental peaks then became satisfactory, confirming the proposed model based on the oxygen diffusion mechanism.

Étude diffractométrique de la dissolution de l'oxygène dans le niobium à pression moyenne (75 Torr) et entre 300 et 400 °C I: Analyse diffractométrique d'échantillons monocristallins et polycristallins

X-ray diffraction measurements were carried out on single-crystal niobium specimens oxidized in oxygen at medium pressure (75 Torr) and in the temperature range 300–400 °C. The following results were obtained. 1. (a) The deformations (shift and width) of the niobium diffraction lines were dependent on the temperature and duration of the oxidation and also on the crystallographic orientation of the surface of the specimen. 2. (b) Suboxides or constant concentration zones were identified in the solid solution. Continuous X-ray diffraction measurements on the niobium peaks were performed during the oxidation of polycrystalline niobium specimens. The deformation and the shift with time of the niobium diffraction peaks are shown. Further, although the NbO x suboxide precipitation was not observed during oxidation, it was observed during cooling with the appearance simultaneously of a constant concentration zone in the solid solution. Finally, the oxygen diffusion mechanism in the metal was verified from these continuous X-ray diffraction measurements.

Thermal conductivity of niobium single crystals in a magnetic field

The thermal conductivity in longitudinal magnetic fields up to 5 T and in the temperature range 3.5 to 15 K is measured in two high purity niobium single crystals having residual resistivity ratios of 22700 and 19200 and orientations of the rod axis [110] and [100]. The investigations show that by means of the longitudinal magnetic field the thermal conductivity may decrease only to a limiting value. In the crystal directions [110] and [100] for the ratio of the thermal conductivity in zero field and the thermal conductivity in the saturation field the temperature-independent factors 1.92 and 1.27, respectively, are determined. With the aid of these factors the thermal conductivity in the normal state is evaluated from the measured values of thermal conductivity below Tc in the magnetic field. The different conduction and scattering mechanisms are discussed. Die thermische Leitfahigkeit von zwei hochreinen Niobeinkristallen mit Restwiderstandsverhaltnissen 22700 und 19200 und Achsenrichtungen [110] und [100] wird im longitudinalen Magnetfeld bis zu 5 T zwischen 3,5 und 15 K untersucht. Die Messungen zeigen, das durch das longitudinale Magnetfeld die Warmeleitfahigkeit nur bis zu einem Grenzwert verringert werden kann. Fur das Verhaltnis zwischen der Warmeleitfahigkeit im Feld Null und der Warmeleitfahigkeit beim Sattigungsfeld wird fur die Orientierung [110] der temperaturunabhangige Faktor 1,92 und fur die Orientierung [100] der Faktor 1,27 bestimmt. Mit Hilfe dieser Faktoren wird die Warmeleitfahigkeit im normalleitenden Zustand aus den Meswerten der Warmeleitfahigkeit im Magnetfeld unterhalb Tc berechnet. Die verschiedenen Leitungs- und Streumechanismen werden diskutiert.

Low temperature internal friction in single crystal niobium

Low temperature internal friction in single crystal niobium

Fluxoid Pinning by Small Nitride Precipitates in Niobium

The magnetic hysteresis of superconducting niobium single crystals containing \ensuremath{\sim}80-\AA{}-diam nitride precipitates was investigated. Introducing precipitates into the samples produced no significant change in the critical-current density ${J}_{c}$ except near the critical fields ${H}_{c1}$ and ${H}_{c2}$. The samples trapped flux below ${H}_{c1}$ and exhibited dramatic peaks in ${J}_{c}$ within 3% of ${H}_{c2}$. These observations are interpreted as clear evidence of a threshold for fluxoid pinning, and reasonable agreement with theory is obtained.

Thermal and radiation effects on the electrical resistivity and internal friction parameters of oriented niobium single crystals

Thermal and radiation effects on the electrical resistivity and internal friction parameters of oriented niobium single crystals

Equilibrium properties of the fluxoid lattice in single-crystal niobium. I. Magnetization measurements

The field derivative of the superconducting magnetization ($\frac{\mathrm{dM}}{d{H}_{a}}$) of a single-crystal niobium sphere was measured with the applied magnetic field parallel to the three high-symmetry crystal axes. A field-sweep technique was used to make direct, continuous measurements of $\frac{\mathrm{dM}}{d{H}_{a}}$, and the data were numerically integrated to obtain the magnetization $M$. Excellent magnetic reversibility indicates the attainment of equilibrium conditions. The Meissner state, the intermediate mixed state, the mixed state, and the normal state show up as distinct field regions of the $\frac{\mathrm{dM}}{d{H}_{a}}$ vs ${H}_{a}$ plots separated by discontinuities in $\frac{\mathrm{dM}}{d{H}_{a}}$. The results are compared with microscopic theory and with related experimental studies of niobium. The magnetization measurements are compared directly with small-angle neutron-diffraction measurements of the fluxoid lattice in the same sample reported in the following paper.

Irradiation-induced lattice–site change of oxygen in niobium

Abstract The effect of 2·0 MeV 4He+ irradiation at 86 K and 293 K on the lattice site of oxygen (18O) dissolved in single-crystal niobium has been studied by use of the proton-channelling mothod. With an incident beam of 700 keV protons, angular scans of the yield from the nuclear reaction 18O(p, α)uN have been performed about the 〈100〉, 〈111〉 and {100} channels in Nb/0·18 at.% 180. The results indicate that oxygen atoms occupy octahedral interstitial sites in an unirradiated niobium crystal, but that the lattice site is altered when defects are introduced into the sample by irradiation at 86 K or 293 K with 2 MeV helium ions to a dose of 2–3 × 1018 ions cm−2. Using calculated channelled-flux distributions it is shown that the angular scans in irradiated samples are consistent with oxygen atoms being displaced by 0·5 ±0·1 A from the octahedral sites, the displacements occurring with equal probability in all 〈100〉 lattice directions. It was found that the angular scans in irradiated samples were not altere...

Dependence of blister-“deckeldicke” and of depth profiles of implanted He ions in Nb on angle of incidence

The dependence of blister deckeldicke and of depth profiles of 30 and 100 keV 3He ions incident on niobium single crystals at different angles of incidence has been measured by Rutherford backscattering (double alignment), scanning electron microscope, and nuclear reaction techniques. A strong dependence of deckeldicke and blister size on the angle of incidence is found. Deckeldicke is found to be larger than the most probable ion range. For small angles of incidence, the result agrees with the depth of the half maximum value of the 3He distribution.

Constant-volume thermodynamic functions for b.c.c. niobium-hydrogen solid solutions

Recent measurements of the composition and temperature variation of the elastic properties of niobium single crystals containing hydrogen in solid solution in the b.c.c. α-phase have been used to calculate the rigid-lattice thermodynamic functions of the solution from the measured constant-pressure thermodynamic functions. The results show large differences between the two sets of functions. The utility of rigid-lattice models is discussed. Nous avons utilisé des mesures récentes de la variation en fonction de la composition et de la température des propriétés élastiques de monocristaux de niobium contenant de l'hydrogène en solution solide dans la phase α c.c., pour calculer les fonctions thermodynamiques à réseau rigide de la solution à partir des fonctions thermodynamiques de la solution mesurées à pression constante. Les résultats mettent en évidence de grandes différences entre les deux ensembles de fonctions. Nous discutons alors l'utilité des modèles de réseau rigide. Aufbauend auf neueren Messungen von Zusammensetzung und Temperaturabhängigkeit der elastischen Eigenschaften von Niobeinkristallen, die Wasserstoff in fester Lösung in der krz. α-Phase enthalten, wurden die thermodynamischen Funktionen der Lösung in der Näherung des starren Gitters aus den gemessenen thermodynamischen Funktionen bei konstantem Druck berechnet. Die Ergebnisse weisen groβe Unterschiede zwischen den beiden Funktionssätzen auf. Der Nutzen von Modellen starrer Gitter wird diskutiert.

A channelling study of helium-ion irradiation damage in niobium I. Effect of dissolved oxygen

Abstract The method of ion channelling/backscattering has been used to compare the defect structures produced in degassed and oxygenated single-crystal niobium by irradiation at 86 K with 2·0 MeV 4He+ ions to doses up to 3 × 1016 ions/cm2 non-channelled, corresponding to approximately 1·3 × 10−2 displacements per atom, near the surface, for an assumed effective displacement energy of 62 eV. Channelling measurements were made, without warm-up, with 2·0 MeV 4He+ ions incident along 〈110〉 and 〈111〉 directions, and dechannelling rates and minimum backscattered yields at the surface were derived from the spectra of backscattered ions. Both the dechannelling rate and the minimum yield increased as a result of the irradiation, with saturation occurring at doses of about 1016 ions/cm2, and it was found that the increases were larger in samples containing 0·18 or 0·35 at.% of dissolved oxygen than in degassed samples. Heat treatments at 500 K produced only minor changes in the backscattered spectra. Most of the de...

Muon spin rotation in superconductors

By means of the muon spin rotation technique (μ+SR), the temperature dependence of the magnetic field inside the normal-conducting domains of high-purity tantalum crystals in the intermediate state has been measured in the temperature range 2.36K<T<3.5K. The results agree well with macroscopic measurements of the thermodynamical critical field. It is also shown that the distribution of magnetic fields in the mixed state of niobium single crystals may be determined by μ+SR. Possible applications of these findings to the study of long-range diffusion of positive muons at low temperatures are indicated.

Tunnel measurements on Nb single crystals

AbstractIn order to investigate the mechanism of superconductivity in transition metals, tunnel measurements on niobium single crystals annealed in ultrahigh vacuum are carried out. The residual resistivity ratio of the single crystals is r0 ≧ 20 000. The tunnel junctions yielding most information have the structure NbIAg. The insulator is produced by thermal oxidation of niobium. The value of the energy gap is Δ0 = (1.50 ± 0.05) meV. The interpretation of dU/dI and d2U/dI2 characteristics yields the electron‐phonon coupling constant Λ = 0.62 and the Coulomb pseudopotential μ* = 0.002. From tunnel curves values about the thickness and height of the tunnel barrier are estimated. The normal‐conducting properties of the tunnel junctions are discussed.