Niobium Discs Research Articles

Medium grain niobium SRF cavity production technology for science frontiers and accelerator applications

We present the cost-effective production of superconducting radio frequency (SRF) cavities made of medium grain (MG) niobium (Nb) discs directly sliced from forged and annealed billet. This production method provides clean surface conditions and reliable mechanical characteristics with sub-millimeter average grain size resulting in stable SRF cavity production. We propose to apply this material to particle accelerator in the science and industrial applications. The science applications require high field gradients(≥ 30 MV/m) particularly in pulsed mode. The industrial applications require high Q 0 values with moderate gradients (∼ 20 MV/m) in CW mode operation. This paper describes the MG Nb disc production recently demonstrated and discusses future prospects for application in advanced particle accelerators in the science and industrial applications.

Intraoperative loading of calcium phosphate-coated implants with gentamicin prevents experimental Staphylococcus aureus infection in vivo.

Orthopedic device-related infection (ODRI) is a potentially devastating complication arising from the colonization of the device with bacteria, such as Staphylococcus aureus. The aim of this study was to determine if intraoperative loading of a clinically approved calcium phosphate (CaP) coating with gentamicin can protect from ODRI in vivo. First, CaP-coated titanium aluminium niobium (TAN) discs were used to investigate the adsorption and release kinetics of gentamicin in vitro. Gentamicin loading and subsequent release from the coating were both rapid, with maximum loading occurring following one second of immersion, and >95% gentamicin released within 15 min in aqueous solution, respectively. Second, efficacy of the gentamicin-loaded CaP coating for preventing ODRI in vivo was investigated using a CaP-coated unicortical TAN screw implanted into the proximal tibia of skeletally mature female Wistar rats, following inoculation of the implant site with S. aureus. Gentamicin-loading prevented ODRI in 7/8 animals, whereas 9/9 of the non-gentamicin treated animals were infected after 7 days. In conclusion, gentamicin can be rapidly and simply loaded onto, and released from, CaP-based implant coatings, and this is an effective strategy for preventing peri-operative S. aureus-induced ODRI in vivo.

Critical issues related to instrumented indentation on non-uniform materials: Application to niobium subjected to high pressure torsion

Nanoindentation is a powerful tool for characterizing the mechanical properties of materials at small length scales. Since the loading rate can be accurately recorded and controlled during a nanoindentation test, the strain rate dependence of these properties can also be determined. However, there are still a few problems that need to be addressed when it is applied to some special materials. High pressure torsion (HPT) processed metals are examples of these materials with non-uniform microstructures and mechanical properties. In this work, commercially pure niobium disks with diameter of ~10.0mm and thickness of ~1.0mm were processed by HPT. Grain sizes from a few nanometers to a few micrometers were generated due to a strong radial strain gradient in the disk. Instrumented nanoindentation tests were conducted at different radial locations of the HPT niobium disk. During each test, the loading rate was controlled so that the indentation strain rate was kept constant. Some key issues associated with the nanoindentation experiment, such as contact stiffness, contact area and the effect of pile-up or sink-in were evaluated and discussed carefully. The work-based method was used in data processing and it was compared with other approaches. The effects of indentation location, i.e., grain size, as well as indentation strain rate were characterized.

Physical properties and structure of large grain/single crystal niobium for superconducting RF cavities

The R&D program on superconducting cavities fabricated from electron beam melted large grain/single crystal (LG/SC) niobium discs explores it's potential for production of approximately 1000 cavities for the European XFEL.Thermal, electrical, mechanical properties, crystal orientation and structure are investigated with the aim to make the fabrication procedure more efficient. In opposite to fine grain niobium the thermal conductivity of LG/SC has a pronounced maximum at 2K. Calculation found a correlation between thermal conductivity enhancement and phonon scattering at the grain boundaries. Detected enhancement is very susceptible to plastic deformation that can cause the complete elimination of the low temperature peak. The final annealing at 800°C of cavities made from large grain niobium is necessary for hydrogen outgassing, as well as for the thermal conductivity enhancement due to stress relaxation and recovery of crystal defects introduced at the cavity fabrication.The effects of annealing temperature up to 1200°C, heating rate, and holding time on the structure recovery after rolling are also established.Total elongation at the uniaxial tensile tests for LG is very high (50 - 110%) and depends significantly on the load direction, because only very few grains are in the gage length. The elongation after fracture by bi-axial testing (bulging test) for LG is lower (<15%) yet sufficient for deep drawing of half-cells.Metallographic investigation of and electron beam welding tests on, niobium single crystals show that an appropriate disc enlargement and annealing can be done without destruction of the single crystal. These tests showed that a cavity can be produced without grain boundaries even in the welding area. On base of the results a fabrication method of single crystal cavities is proposed.

Formation and size dependence of vortex shells in mesoscopic superconducting niobium disks

Recent experiments [I.V. Grigorieva et al., Phys. Rev. Lett. 96, 077005 (2006)] on visualization of vortices using the Bitter decoration technique revealed vortex shells in mesoscopic superconducting Nb disks containing up to L = 40 vortices. Some of the found configurations did not agree with those predicted theoretically. We show here that this discrepancy can be traced back to the larger disks with radii R � 1 to 2.5µm, i.e., R � 50 100�(0) used in the experiment, while in previous theoretical studies vortex states with vorticity L � 40 were analyzed for smaller disks with R � 5 20�(0). The present analysis is done for thin disks (mesoscopic regime) and for thick (macroscopic) disks where the London screening is taken into account. We found that the radius of the superconducting disk has a pronounced influence on the vortex configuration in contrast to, e.g., the case of parabolic confined charged particles. The missing vortex configurations and the region of their stability are found, which are in agreement with those observed in the experiment.

Paramagnetic Meissner Effect

The explanation for the paramagnetic Meissner effect (PME) in high-temperature superconductors (HTSC) and niobium disks, distinct from previous discussions, is proposed. It is based on the impurity mechanism of high-temperature superconductivity and the general localization approach to the superconductivity problem. Turns of the local magnetic moments of the two-level systems are considered to be the cause of this effect in HTSC. The estimations of the magnetic moment magnitude and the magnetic moment concentration are given. In niobium disks, this effect is also explained by turns of the local magnetic moments, but of another nature. The role of the layered structure is demonstrated. It is shown that the proposed models can explain all available experimental facts. The connection between this effect and the electric field effect in HTSC is discussed.

Heat flow at the niobium-superfluid helium interface : Kapitza resistance and superconducting cavities

By measuring the temperature jump in the presence of a heat flux across niobium disks surrounded by He II, we determined the Kapitza resistance, Rk , for different surface roughness and bulk purity. The experiments were conducted from 1.5 Kt oT* , under saturated vapour pressure. The influence of foreign atoms due to the presence of Ti near the interface was also investigated. The importance of Rk on the efficiency of superconducting cavities is discussed.

Electrochemical and XPS studies of the corrosion behavior of sputter-deposited amorphous Fe–Cr–Ni–Nb alloys in 6 M HCl

Amorphous Fe–Cr–Ni–Nb alloys containing 9–68 at.% niobium were successfully prepared by the D.C. magnetron sputtering method using targets consisting of a commercial Type 304 stainless steel disc and high purity niobium discs. The corrosion rate of the sputter-deposited Fe–Cr–Ni–Nb alloys containing more than 19 at.% niobium in 6 M HCl at 30°C was more than four orders of magnitude lower than that of bulk Type 304 stainless steel and almost the same as that of sputter-deposited niobium metal, although the corrosion rate of the alloys was higher than that of the binary Cr–Nb alloys. The corrosion potential of the alloys was higher than that of niobium and all the Fe–Cr–Ni–Nb alloys were spontaneously passivated in 6 M HCl at 30°C. XPS analysis revealed that the spontaneous passive films formed on the alloys after immersion in 6 M HCl for 48 h were rich in chromium and niobium cations and deficient in iron cation. The formation of the homogeneous double oxyhydroxide films containing chromium, niobium and iron cations is responsible for the high corrosion resistance of the alloys.

Detailed investigation of the superconducting transition of niobium disks exhibiting the paramagnetic Meissner effect

The superconducting transition region in a Nb disk showing the paramagnetic Meissner effect (PME) has been investigated in detail. From the field-cooled magnetization behavior, two well-defined temperatures can be associated with the appearance of the PME: T_1 (< T_c) indicates the characteristic temperature where the paramagnetic moment first appears and a lower temperature T_p (< T_1) defines the temperature where the positive moment no longer increases. During the subsequent warming, the paramagnetic moment begins to decrease at T_p and then vanishes at T_1 with the magnitude of the magnetization change between these two temperatures being nearly the same as that during cooling. This indicates that the nature of the PME is reversible and not associated with flux motion. Furthermore, the appearance of this paramagnetic moment is even observable in fields as large as 0.2 T even though the magnetization does not remain positive to the lowest temperatures. Magnetic hysteresis loops in the temperature range between T_1 and T_p also exhibit a distinct shape that is different from the archetypal shape of a bulk type-II superconductor. These behaviors are discussed in terms of the so-called 'giant vortex state'.

Observation of paramagnetic Meissner effect in niobium disks.

For several superconducting niobium disks, the dc field-cooled magnetization (FCM) shows a paramagnetic response below their superconducting transition temperature ${T}_{c}\ensuremath{\approx}9.2\mathrm{K}$ when the applied magnetic filed is normal to the disk. At 8.6 K, the FCM reaches a maximum in fields of 5 to 10 Oe and becomes diamagnetic for fields larger than 20 Oe. This FCM behavior is qualitatively similar to the paramagnetic Meissner effect (PME) recently observed in high- ${T}_{c}$ cuprate superconductors. Various surface treatments to the Nb disks result in changes in both the zero-field-cooled magnetization and the FCM, including the disappearance of the PME.

Hydrogen transport through non-porous membranes of palladium-coated niobium, tantalum and vanadium

Experiments are reported for hydrogen transport in palladium-coated membranes of niobium, tantalum, and vanadium. Two geometries are studied, coated discs 2 mm thick, and coated tubes of 0.25 mm wall thickness. The hydrogen is extracted at 100% purity. Fluxes are much higher than with current palladium alloy and polymeric membranes at these pressures, and are somewhat higher than with palladium-coated porous ceramics. Palladium-coated niobium discs show effective permeabilities about 0.32 μmol/m-sec-Pa 1 2 at 425°C and 2 atm pressure (including gas-phase resistance). The coated tubes have about 1 2 this permeability, and thus mass transport resistances about 1450 m 2-sec-Pa 1 2 /mol. Coated tantalum discs have effective permeabilities about 0.1 μmol/m-sec-Pa 1 2 , but are less susceptible to hydrogen embrittlement. Since these coated-metals are stronger and more durable than palladium, ceramics of polymers, considerable design flexibility is possible. Based on these measurements, the flux for advanced membranes would be 2000 scf/hr-m 2 for a driving force of 3 atm, suggesting attractive economics for petrochemical plant use.

Infrared spectra of the hydrogenated amorphous silicon layers on metallic substrates of solar absorbers

Infrared absorption spectra were investigated in hydrogenated amorphous silicon (a-Si(H)) films deposited both by d.c. reactive sputtering (RS) and by r.f. glow discharge (GD) of silane. The films were deposited onto polished niobium and tungsten solar test disks to form spectrally selective absorbers for comparative photothermal and photovoltaic studies. Spectra were observed from the normal spectral reflectance from 400 to 3900 cm -1 with a Fourier transform spectrometer. Hydrogen, alloyed typically at 10–35 at.% in these a-Si films, significantly reduces the gap state density and produces IR spectra characteristic of Si-H bonding complexes depending on the deposition method and the substrate temperature T D. Comparison between RS and GD films revealed significant differences in their IR spectra. RS films show a strong Si-O-Si bonding absorption from atmospheric oxygen. GD films have a marked Si-H wagging mode absorption resolved into three components. This band is very sensitive to the deposition conditions and decays with increasing T D. The SiH 3 rocking mode was identified at 500 cm -1 in the GD films. The hydrogen concentration from the IR stretching mode absorption was correlated with the H 2 evolution results of McMillan and Peterson on identical films. The transition with increasing T D could be traced from the room temperature state with polymeric complexes of hydrogen to the high temperature state with the single monohydride complex.

The interphase surface energy of superconducting niobium

The intermediate-mixed state laminar domain structure has been decorated on high purity dished niobium discs at 4.2°K. The Landau-Lifshitz-Sharvin theory is tested and used and the interphase surface energy is estimated at (3.7 ± 0.9) × 10 -3 ergs cm -2.