Tantalum Single Crystals Research Articles

SiC and TaC as optical materials

The heteroepitaxial growth of β-SiC on silicon and TaC on single-crystal tantalum films for optical applications is reported. Both types of films were deposited by employing a horizontal chemical vapor deposition reactor at temperatures above 1200 °C. The β-SiC comprises single crystals with diameters greater than 2 in. The TaC was grown on single crystals of tantalum with (100) orientation and a crystal diameter greater than 0.5 in. Both materials exhibited reflectances greater than 40% at a wavelength of 100 nm and at angles of incidence less than approximately 70°. These measurements were made on the as-grown samples and are remarkable results given that the samples were not polished to a flatness of less than λ/80 r.m.s. in the visible region.

Low-Temperature Thermal Conductivity of Plastically Deformed Niobium and Tantalum Single Crystals of [001]Orientation

The thermal conductivity of [001] orientated niobium and tantalum single crystals plastically deformed in steps at room temperature is measured in the temperature range 0.3 to 20 K. From these measurements the thermal resistivity Wd induced by the plastic deformation is evaluated in the temperature range 0.3 to 5 K. After each deformation step a thermal resistivity Wd proportional to T−2 is found. The deformation behaviour and dislocation arrangement are studied by slip-line observation, Laue X-ray back-reflection, and transmission electron microscopy. Comparison between the experimental results and different models of the phonon-dislocation interaction reveals that the phonons are strongly scattered by static distortion fields rather than by dynamic phonon-dislocation processes. Die Warmeleitfahigkeit von bei Raumtemperatur schrittweise plastisch verformten[001]-orientierten Niob- und Tantaleinkristallen wird im Temperaturbereich zwischen 0,3 und 20 K gemessen. Aus diesen Messungen wird der durch die plastische Verformung erzeugte Warmewiderstand Wd zwischen 0,3 und 5 K bestimmt. Nach jedem Verformungsschritt wird ein zu T−2 proportionaler Warmewiderstand Wd gefunden. Das Verformungsverhalten und die Versetzungsanordnung werden mit Hilfe von Gleitlinien-beobachtungen, Laue Ruckstreu-Aufnahmen und urchstrahlungselektronenmikroskopie untersucht. Der Vergleich der experimentell gefundenen Ergebnisse mit verschiedenen Modellen der Phonon-Ver-setzungs-Wechselwirkung zeigt, das die Phononen eher eine starke Streuung an statischen Distorsions-feldern erfahren als durch dynamische Prozesse gestreut werden.

Carbide formation in Nb and Ta implanted by carbon ions at room temperature

Niobium and tantalum single crystals with 〈111〉 orientation were implanted with 200 keV C + ions at room temperature to fluences of 7.6 × 10 17 and 1.5 × 10 18 carbon ions per cm 2, respectively, which gave a calculated maximum carbon concentration of ∼ 46 and 52 at.% in each material. The depth profiles of carbon were analyzed with Auger spectroscopy, and the phase formations were investigated with cross-sectional transmission electron microscopy. Face-centered-cubic nonstoichiometric NbC and TaC were formed in the implanted Nb and Ta, respectively; no Nb 2C or Ta 2C was formed. Based on the observed crystallographic relations between the fcc carbides and the bcc host lattices, and on the morphologies of the carbides, it is suggested that the fcc carbides are formed by a diffusionless martensitic transformation.

Inverse Photoemission Spectroscopy of Transition Metals

Inverse photoemission spectroscopy using photons in the energy range 10-50 eV is now the preferred method for mapping the unoccupied bands of metals. With suitable apparatus it is possible to measure the energy, momentum and spin of bulk and surface states from the Fermi energy to above the vacuum zero. A wide range of clean transition metal single crystals and of metal overlayers on crystals have been studied in this way and the band dispersions compared with first principles band structure calculations. Tungsten and tantalum are typical of metals with a high density of unoccupied states that have been measured at several photon energies. Calculations have been made of the photon yield from single crystal tantalum using a relativistic version of the multiple scattering theory of Pendry. The agreement between the calculated and measured spectra is good up to over 5 eV above EF indicating that the one electron eigenstates derived in band structure calculations may be used with confidence for quasi-particles in transition metals.

Temperature and strain-rate dependence of the flow stress of tantalum single crystals in cyclic deformation

Temperature and strain-rate dependence of the flow stress of tantalum single crystals in cyclic deformation

Long-range internal stresses and low temperature phonon scattering in plastically deformed niobium and tantalum single crystals

The thermal conductivity of niobium and tantalum single crystals plastically deformed in steps at intermediate temperatures has been measured in the temperature range 0.3–20 K. From these measurements the thermal resistivity W d induced by the plastic deformation was evaluated in the temperature range 0.3–5 K. After each deformation step a thermal resistivity W d proportional to T −2 was found, indicating that phonons are scattered strongly by static strain fields rather than by dynamic phonon-dislocation processes. X-ray topography and transmission electron microscopy studies revealed that the dislocation arrangement is heterogeneous, consisting of kink walls and dislocation sheets. Since classical dislocation pile-ups have not been observed by the lattice thermal conductivity measurements nor by the dislocation arrangement studies, the long-range internal stresses causing phonon scattering have to be attributed to the heterogeneous dislocation arrangement as proposed according to Mughrabi's composite model.

Temperature and strain-rate dependence of the flow stress of ultrapure tantalum single crystals

Measurements of the temperature dependence of the cyclic flow stress of ultrapure tantalum single crystals (RRR ⪆ 14000) are extended to lower temperatures. After cyclic deformation well into saturation at 400 K, the temperature dependence of the flow stress is measured between 80 and 460 K at five different plastic resolved shear-strain rates, e, in the range 2 × 10−5 to 6 × 10−3 s−1. Below a critical temperature Tk the flow stress is dominantly controlled by the mobility of screw dislocations. A recent theory of Seeger describes the “thermal” component, σ*, of the flow stress (resolved shear stress) in the temperature and stress regime where the strain rate is determined by the formation and migration of kink pairs. The analytical expressions are valid in well-defined ranges of stress and temperature. The evaluation of the experimental data yields a value for the formation enthalpy of two isolated kinks 2Hk = 0.98 eV. From the low-stress (σ* < 10 MPa) data at higher temperatures the diffusion coefficient of kinks is evaluated as Dk = 2.0 × 10−6 m2 s−1. The product of the density of mobile screw dislocations and the distance between insurmountable obstacles is found to be 2 × 10−5 m−1. The stress dependence of the kink-pair formation enthalpy Hkp follows the theoretically predicted curve in the elastic-interaction stress regime. At the transition to the line-tension approximation (near σ* ≈ 80 MPa) the activation volume increases rather abruptly. Moreover, the quantitative analysis involves kinks other than those of minimum height. The most likely candidates are kinks on {211} planes.

Lattice thermal conductivity and phonon-dislocation interaction in niobium and tantalum single crystals plastically deformed at intermediate temperatures

Abstract The thermal conductivity of niobium and tantalum single crystals plastically deformed in steps at intermediate temperatures has been measured in the temperature range 0.3 to 20 K. From these measurements the thermal resistivity W d induced by the plastic deformation was calculated in the temperature range 0.3 K < T < 5 K. After each deformation step a thermal resistivity W d proportional to T −2 was found. The dislocation arrangement was studied by X-ray topography and transmission electron microscopy. The results are consistent with a model of phonon scattering by the static distortion field of dislocations.

Dislocation Internal Friction and Hydrogen Degassing Experiments in Niobium and Tantalum Single Crystals

Low temperature dislocation mobility in group-V transition metals is frequently determined by hydrogen-dislocation interactions as can be seen in internal friction experiments by the appearance of a hydrogen Snoek-Koester (S-K(H)) maximum. In order to study the intrinsic dislocation relaxation processes in niobium and tantalum single crystals we applied a hydrogen-degassing technique using a palladium surface layer as suggested by Rodrigues and Kirchheim. The effectiveness of the degassing treatments in our specimens could be controlled by measurements of the dislocation-damping spectrum. For the first time it was possible to achieve high-purity single crystals of niobium and tantalum in which the S-K(H) maximum was completely removed. The dislocation-damping spectrum of such crystals is presented.

Phonon‐Dislocation Scattering and Low‐Temperature Lattice Thermal Conductivity of Niobium and Tantalum Single Crystals Deformed at Low Temperatures

AbstractThe thermal conduction of niobium and tantalum single crystals plastically deformed in steps at low temperatures is measured between 0.3 and 20 K. The dislocation arrangement is studied by X‐ray topography and transmission electron microscopy. The thermal resistivity Wd due to the plastic deformation deduced in the temperature range 0.3 K &lt; T &lt; 5 K is found to be proportional to about T−3 after weak deformation. Strong deformation gives rise to a thermal resistivity proportional to T−2. The results are consistent with the model of scattering of phonons by the static distortion field of dislocations.

Anomalous slip in high-purity tantalum single crystals after tensile deformation at 77 K

Abstract High-purity tantalum single crystals having orientations in the central part of the stereographic triangle and a residual resistance ratio of 5760:1 have been deformed in tension at 77 K. In all crystals anomalous slip was observed. This result is remarkable in view of the fact that other authors have observed anomalous slip in crystals of comparable orientation only after deformation in compression. The. reason for this difference lies presumably in the higher purity of the crystals used in the present work, as indicated by etching experiments: etching in an acid solution subsequently suppresses anomalous slip owing to the influence of hydrogen interstitials.

Strong atomic displacements around nitrogen in tantalum determined by energy-dispersive x-ray diffraction

The nitrogen induced attenuation of Bragg reflections (static Debye-Waller Factor) of tantalum single crystals containing nitrogen has been measured by energy-dispersive x-ray diffraction. N has been dissolved in Ta up to a concentration ofc=3.3% N/Ta. From a detailed study of the concentration andK dependence (K=scattering vector) of the static Debye-Waller Factor exponent 2L we conclude:i) 2L increases linearly with the nitrogen content forc≦1.8% N/Ta,ii) The displacements of the atomic shells due to N on the octahedral sites areu 1=0.45 (5) A,u 2=(−)0.11 (2) A.u 3=0.25 (2) A,iii) The thermal vibrations of those atoms close to N are changed, due to the strong static displacements that they experience. The unusual strong and extended displacements around N in Ta are in contradiction to calculations where two-force models have been used. We propose arguments that displacements are responsible for the specific site chosen by hydrogen atoms when they are trapped by nitrogen in Ta.

Geometrical resonances in a high-injection-current nonequilibrium state of superconductor-normal-metal contacts.

A hysteretic transition to a high-injection-current nonequilibrium state in metallic contacts between silver and high-purity single-crystal tantalum was recently observed by the author and H. Lerchner. Oscillations of conductance with bias in this state are interpreted as geometrical resonances of the excess current caused by a bias-driven variation of the superconductor--normal-metal phase-boundary position. Quantitative numerical results are obtained from an extension of the one-dimensional Blonder-Tinkham-Klapwijk microconstriction model and are successfully compared with part of the experimental results. Lacking any self-consistency features, the model is unable to describe nonlinearities and feedback effects which apparently dominate much of the experimental behavior, but supplies some indication that Andreev reflection might also play an essential role in these effects.

Electron–Phonon Scattering and Low‐Temperature Thermal Conductivity of Niobium and Tantalum Single Crystals

AbstractThe thermal conduction of niobium and tantalum single crystals of different impurity contents is measured below 20 K. The investigation of the lattice thermal conductivity as limited by the interaction of phonons with electrons demonstrates that both, longitudinal and transverse phonon modes are scattered strongly.

Depolarisation studies of positive muons in copper, vanadium, niobium and tantalum single crystals

The behaviour of positive muons ( mu +) has been studied in Cu, V, Nb and Ta single crystals, using the muon spin rotation ( mu SR) technique. The measurements were performed in the temperature regime 5-365K, in transverse external magnetic fields of 50-5500 G. In FCC Cu the mu + diffusional motion can be described in terms of quantum mechanical jump processes of a small polaron among octahedral interstitial sites. An orientation average leads to an activation energy of E=79.2+or-3.1 meV, a pre-exponential factor of D(0)=(1.66+or-0.33)*10-8 cm2 s-1 and a tunnelling matrix element of J=(18.3+or-1.8)*10-6 eV. The mu + total volume expansion was found to be Delta V=3.91 AA3. Furthermore, the electric field gradient (EFG) at the mu + nearest-neighbour nuclei has been determined as q=0.30 AA-3. In BCC V, Nb and Ta the mu + diffusion is trap-limited. As a function of the temperature the mu + probe different classes of trapping sites associated with host metal impurities. At low temperature, before decaying, the mu + always end up in a trapped state which shows that the intrinsic mu + motion between traps is fast and seems to reflect properties of a band diffusion.

Chlorine adsorption on the Ta(110) surface

The interaction of chlorine with the (110) surface of tantalum single crystals is investigated using LEED and Auger spectroscopy. It appears that initial adsorption occurs via a mobile precursor state and with a high initial sticking coefficient. A variety of LEED superstructures is observed, depending on coverage and annealing procedures, corresponding to a sequence of commensurate phases.

Anomalous temperature behaviour of the shear elastic constant C44 in tantalum

The adiabatic elastic constants of a pure tantalum single crystal have been measured between 300 and 3000 K in a suitable geometry by means of an ultrasonic method. We find that the shear constant C 44 has an anomalous temperature behaviour similar to that found in niobium and vanadium.

タンタル単結晶の水素ぜい性

The effect of hydrogen on the ductility of tantalum single crystal was investigated in the temperature range of 77-396K using a variety of techniques including observation by the S.E.M., tensile testing, X-ray radiography, and electric resistivity methods. The solid solution of hydrogen in tantalum was shown to be highly ductile at temperatures above 300K. The ductility of the solid solutions decreased rapidly at temperatures below 300K, and was minimum at about 200K. The fracture mode was cleavage at the temperatures in the region of ductility minimum, while it was ductile at temperatures near 77K. The factors affecting the above embrittlement are considered to be inhomogeneous distribution of hydrogen in the specimen and formation of stress induced hydrides. The ductility near 77K may be caused by very slow diffusion of hydrogen at that temperature.

Anisotropy of the superconductive to normal transition in tantalum-nitrogen single crystals

Systematic experiments on the transition to the normal state in superconducting tantalum single crystals doped with interstitially dissolved nitrogen impurities are presented. On the low-κ side of the magnetic phase diagram, where at some low temperature 0 <T * <T ca transition from type I to type II superconductivity occurs, a new effect has been observed, as reported briefly elsewhere: because of the correlation ofH c 2 with crystal directions, at fixed temperatures nearT *, whether the sample is a type I or a type II superconductor depends on which crystal direction is oriented parallel to the external field. In the type II superconducting range all samples display anH c 2 anisotropy effect, which is commonly observed in cubic superconductors, even in the extreme dirty limit (α~12). However, the second anisotropy coefficient (l = 6 component) is found to be negligibly small in all tantalum samples.

Recovery and recrystallization in rolled tantalum single crystals

The recovery and recrystallization behavior of rolled tantalum single crystals having initial orientations of (001) [110], (112) [110] and (111) [110] has been studied. Each crystal developed a simple, single component texture upon rolling. The texture became more diffuse and the local lattice curvature more intense as the rolling plane deviated from (001) towards (111). The (001) [110] and (112) [110] crystals formed a uniform dislocation array and polygonization occurred on annealing. No recrystallization of the (001) [110] crystal took place at temperatures up to 1673 K. A few randomly-oriented recrystallized grains were nucleated in the (112) [110] crystal but these may have been artificially nucleated. Only the (111) [110] crystal reoriented during rolling; it also formed a cellular dislocation microstructure. This orientation was the only one to recrystallize easily, primarily because of the large lattice curvature. The recrystallized grains could be related to the deformation texture by a rotation of 23 to 24 deg about a common direction that was parallel to the normal direction of the sheet. The nature of the misorientation spread after rolling may have dictated this relation as a compromise to the one usually found in bcc materials,viz. 25 to 30 deg rotation about a common axis. Also, wide dislocation cell size distributions were noted in the (111) [110] crystal. The cell size distributions were log normal in nature. Subgrain growth was very much more rapid in this crystal. There was a large discrepancy between the observed subgrain growth and that predicted theoretically by the coalescence model.