Bulk Niobium Research Articles

Growth of magnesium diboride films on 2 inch diameter copper discs by hybrid physical–chemical vapor deposition

Magnesium diboride (MgB2) coating is a potential candidate to replace bulk niobium (Nb) for superconducting radio frequency cavities due to the appealing superconducting properties of MgB2. MgB2 coating on copper may allow cavity operation near 20–25 K as a result of the high transition temperature (Tc) of MgB2 and excellent thermal conductivity of Cu. We have grown MgB2 films on 2 inch diameter Cu discs by hybrid physical–chemical vapor deposition for radio frequency characterization. Structural and elemental analyses showed a uniform MgB2 coating on top of a Mg–Cu alloy layer with occasional intrusion of Mg–Cu alloy regions. High Tc values of around 37 K and high critical current density (Jc) on the order of 107 A cm−2 at zero field were observed. Radio frequency measurements at 11.4 GHz confirmed a high Tc and showed a quality factor (Q0) much higher than for Cu and close to that of Nb.

Detection and location of SRF bulk niobium cavities quench using second sound sensitive sensors in superfluid helium

We developed sensors and instrumentation dedicated to detection and location of thermal quenches in SRF cavities via 2nd sound events in HeII. We studied 2 types of Quench Detectors (QD): 1) Oscillating Super leak Transducer (OST), 2) LOw REsponse Time resIstive Thermometers (LORETIT). The QD were characterized in He II bath (Temperature T0 = 1.6 K- Tλ. The SRF cavity quench is experimentally simulated using resistors of different sizes and geometries. High pulsed heat flux qP (qP < 2MW/m2) were applied to these heaters and the dynamic response of QD were investigated as function of several parameters (T0, qP, distance to heater). The OST were used for locating quench on different SRF cavities resonating at 2 frequencies f0 (f0 =88 MHz or f0 =352 MHz). The quench dynamics and critical size of normal resistive area leading to quench were investigated. Furthermore, a Second Sound Resonator (SSR) equipped with a pair of OST at each extremity (2nd sound generator (G) and detector (D)), a low thermal capacity heater (G) and a LORETIT (D), was successfully operated in the resonating and in the pulsed mode. The measured 1st sound and 2nd sound spectra were compared theoretical results and a good agreement is obtained.

Spiral 2 Cryogenic System for The Superconducting LINAC

SPIRAL 21 is a rare isotope accelerator dedicated to the production of high intensity beams (E = 40 MeV, I = 5 mA). The driver is a linear accelerator (LINAC) that uses bulk Niobium made quarter wave RF cavities. 19 cryomodules inclose one or two cavities respectively for the low and the high energy sections. To supply the 1300 W at 4.2 K required to cool down the LINAC, a cryogenic system has been set up. The heart of the latter is a 3 turbines geared HELIAL®LF (ALAT2) cold box that delivers both the liquid helium for the cavities and the 60 K Helium gaz for the thermal screens. 19 valve-boxes insure cryogenic fluid distribution and management. Key issues like cool down speed or cavity RF frequency stability are closely linked to the cryogenic system management. To overcome these issues, modelling and simulation efforts are being undertaken prior to the first cool down trials. In this paper, we present a status update of the Spiral 2 cryogenic system and the cool down strategy considered for its commissioning.

The role of niobium component in heterogeneous catalysts

This review article is devoted to the materials containing niobium and modified by the addition of different metals or thio- and amino-organosilanes towards the creation of new surface properties and enhancement of catalytic activity. The intention is to show and discuss the role of niobium supported on molecular sieves in the enhancement of the redox properties of transition metals (copper, silver, gold, platinum) and acidic or basic properties of the catalysts as well as to consider redox properties of pure and modified bulk niobium(V) oxide. This paper covers the contribution to the knowledge of this area brought over the last 15 years by our research team.

Change of surface critical current in the surface superconductivity and mixed states of superconducting niobium

A systematic study of irreversible magnetization was performed in bulk niobium after different surface treatments. Starting with smooth surfaces and abrading them, a strong increase of the critical current is observed up to an apparent limiting value. An impressive change of the critical current is also observed in the surface superconductivity (SSC) state, reaching values of the same order of magnitude as in the mixed state. We explain also the observation of strong SSC for magnetic fields perpendicular to large facets in terms of nucleation of superconductivity along bumps of a corrugated surface.

Magnesium diboride coated bulk niobium: a new approach to higher acceleration gradient.

Bulk niobium Superconducting Radio-Frequency cavities are a leading accelerator technology. Their performance is limited by the cavity loss and maximum acceleration gradient, which are negatively affected by vortex penetration into the superconductor when the peak magnetic field at the cavity wall surface exceeds the vortex penetration field (Hvp). It has been proposed that coating the inner wall of an SRF cavity with superconducting thin films increases Hvp. In this work, we utilized Nb ellipsoid to simulate an inverse SRF cavity and investigate the effect of coating it with magnesium diboride layer on the vortex penetration field. A significant enhancement of Hvp was observed. At 2.8 K, Hvp increased from 2100 Oe for an uncoated Nb ellipsoid to 2700 Oe for a Nb ellipsoid coated with ~200 nm thick MgB2 thin film. This finding creates a new route towards achieving higher acceleration gradient in SRF cavity accelerator beyond the theoretical limit of bulk Nb.

Performance analysis of superconducting rf cavities for the CERN rare isotope accelerator

The first cryomodule of the new HIE-ISOLDE rare isotope accelerator has recently been commissioned with beam at CERN, with the second cryomodule ready for installation. Each cryomodule contains five superconducting low-beta quarter wave cavities, produced with the technology of sputtering a thin niobium film onto the copper substrate ($\mathrm{Nb}/\mathrm{Cu}$). This technology has several benefits compared to the bulk niobium solution, but also drawbacks among which the most relevant is the increase of surface resistance with accelerating field. Recent work has established the possible connection of this phenomenon to local defects in the $\mathrm{Nb}/\mathrm{Cu}$ interface, which may lead to increased thermal impedance and thus local thermal runaway. We have analyzed the performance of the HIE-ISOLDE cavities series production, as well as of a few prototypes', in terms of this model, and found a strong correlation between the rf properties and one of the model characteristic quantities, namely the total surface having increased interface thermal impedance.

Large-scale synthesis of NbSe 2 nanosheets and their use as nanofillers for improving the tribological properties of epoxy coatings

Abstract The paper presents a facile and effective method to exfoliate bulk niobium diselenide (NbSe2) into nanosheets which were used as nanofillers to improve the tribological properties of epoxy coatings. X-ray diffraction (XRD) analysis indicates that the hexagonal crystal structure of NbSe2 nanosheet is preserved after ball milling. Transmission electron microscopy (TEM) and atomic force microscope (AFM) observations show that NbSe2 nanosheets were produced with sizes in hundreds of square nanometers and graphene-like structure. The tribological properties of NbSe2/epoxy composite coatings were systematically investigated and results show that they possess better friction-reducing and anti-wear performances due to the addition of NbSe2 nanosheets. It is demonstrated that the superior tribological properties is ascribed to a transfer film formed by NbSe2 nanosheets preventing a direct contact between steel ball and coating during the friction process. Furthermore, this work shows the potential applications of two-dimensional NbSe2 nanosheets as reinforcing nanofillers in polymer matrix.

Effect of sintering techniques on the microstructure and mechanical properties of niobium borides

This study reports on the consolidation of niobium boride powders by cold pressing/pressureless sintering (PS, with or without Co addition) and spark plasma sintering (SPS) techniques. Niobium boride powders (containing NbB, Nb3B4 and NbB2 phases) which were mechanochemically synthesized from Nb2O5, B2O3 and Mg blends and purified by HCl leaching were used as precursor material. The effects of different sintering techniques on the consolidation behaviour, microstructure, densification rate and mechanical properties (microhardness, wear characteristics, elastic modulus, indentation response and fracture toughness) of the bulk products were investigated. PS with Co addition resulted in superior properties than those yielded after PS without Co addition and SPS. Bulk niobium boride products having relative density of 93–98%, microhardness of 24–28GPa, wear volume loss of 2.8–4.5×10−4mm3, elastic modulus of 508–552GPa and fracture toughness of 4.05–4.74MPam1/2 were obtained by Co (2 or 5wt.%) activated PS at 1500°C.

Influence of annealing on mechanical and electrochemical properties of cold sprayed niobium coatings

Abstract In the present study, thick and dense niobium coatings were obtained using cold spray technique by using air as a process gas. Inter-splat boundaries are completely removed in the coatings heat treated at 1500 °C by the formation of equiaxed grains. Heat treatment reduces the porosity level to ~ 0.1%. Inter-splat boundary bonding state of the heat treated coatings was investigated using micro-tensile testing, scratch testing and nanoindentation and compared with the bulk niobium. The elastic modulus of the cold spray coatings heat treated at 1500 °C exhibits as high as 103 GPa whereas the same for bulk is 105 GPa. The increase in mechanical strength of inter-splat boundary from as -sprayed condition to 1500 °C was estimated to be 750%. Similarly corrosion performance of heat treated coatings was also evaluated in 1 M KOH solution through potentiodynamic polarization as well as impedance spectroscopy studies. The corrosion rate for the coatings heat treated at 1500 °C was estimated to be 0.443 MPY which is comparable for the bulk (0.498 MPY). Coatings annealed at 1250 °C and above, which is very close to the recrystallization temperature of niobium, were found to perform almost as bulk niobium indicating exciting implications for various applications. Assessment of structure–property correlations was done based on the microstructure, porosity and inter-splat bonding state, together with the mechanical and corrosion properties of the heat treated tantalum cold sprayed coatings.

Apparatus and method for plasma processing of SRF cavities

An apparatus and a method are described for plasma etching of the inner surface of superconducting radio frequency (SRF) cavities. Accelerator SRF cavities are formed into a variable-diameter cylindrical structure made of bulk niobium, for resonant generation of the particle accelerating field. The etch rate non-uniformity due to depletion of the radicals has been overcome by the simultaneous movement of the gas flow inlet and the inner electrode. An effective shape of the inner electrode to reduce the plasma asymmetry for the coaxial cylindrical rf plasma reactor is determined and implemented in the cavity processing method. The processing was accomplished by moving axially the inner electrode and the gas flow inlet in a step-wise way to establish segmented plasma columns. The test structure was a pillbox cavity made of steel of similar dimension to the standard SRF cavity. This was adopted to experimentally verify the plasma surface reaction on cylindrical structures with variable diameter using the segmented plasma generation approach. The pill box cavity is filled with niobium ring- and disk-type samples and the etch rate of these samples was measured.

Superconducting NbTiN thin films for superconducting radio frequency accelerator cavity applications

Current superconducting radio frequency technology, used in various particle accelerator facilities across the world, is reliant upon bulk niobium superconducting cavities. Due to technological advancements in the processing of bulk Nb cavities, the facilities have reached accelerating fields very close to a material-dependent limit, which is close to 50 MV/m for bulk Nb. One possible solution to improve upon this fundamental limitation was proposed a few years ago by Gurevich [Appl. Phys. Lett. 88, 012511 (2006)], consisting of the deposition of alternating thin layers of superconducting and insulating materials on the interior surface of the cavities. The use of type-II superconductors with Tc &amp;gt; TcNb and Hc &amp;gt; HcNb, (e.g., Nb3Sn, NbN, or NbTiN) could potentially greatly reduce the surface resistance (Rs) and enhance the accelerating field, if the onset of vortex penetration is increased above HcNb, thus enabling higher field gradients. Although Nb3Sn may prove superior, it is not clear that it can be grown as a suitable thin film for the proposed multilayer approach, since very high temperature is typically required for its growth, hindering achieving smooth interfaces and/or surfaces. On the other hand, since NbTiN has a smaller lower critical field (Hc1) and higher critical temperature (Tc) than Nb and increased conductivity compared to NbN, it is a promising candidate material for this new scheme. Here, the authors present experimental results correlating film microstructure with superconducting properties on NbTiN thin film coupon samples while also comparing films grown with targets of different stoichiometry. It is worth mentioning that the authors have achieved thin films with bulk-like lattice parameter and transition temperature while also achieving Hc1 values larger than bulk for films thinner than their London penetration depths.

Stoichiometry and thickness dependence of superconducting properties of niobium nitride thin films

The current technology used in linear particle accelerators is based on superconducting radio frequency (SRF) cavities fabricated from bulk niobium (Nb), which have smaller surface resistance and therefore dissipate less energy than traditional nonsuperconducting copper cavities. Using bulk Nb for the cavities has several advantages, which are discussed elsewhere; however, such SRF cavities have a material-dependent accelerating gradient limit. In order to overcome this fundamental limit, a multilayered coating has been proposed using layers of insulating and superconducting material applied to the interior surface of the cavity. The key to this multilayered model is to use superconducting thin films to exploit the potential field enhancement when these films are thinner than their London penetration depth. Such field enhancement has been demonstrated in MgB2 thin films; here, the authors consider films of another type-II superconductor, niobium nitride (NbN). The authors present their work correlating stoichiometry and superconducting properties in NbN thin films and discuss the thickness dependence of their superconducting properties, which is important for their potential use in the proposed multilayer structure. While there are some previous studies on the relationship between stoichiometry and critical temperature TC, the authors are the first to report on the correlation between stoichiometry and the lower critical field HC1.

Laser joining of NiTi to Ti6Al4V using a Niobium interlayer

Joining NiTi to Ti6Al4V is of great interest for applications in the biomedical and aerospace fields. Despite the importance, no joining techniques have been developed that avoid the formation of brittle intermetallics to produce high strength joints. In this work, Niobium was used as an interlayer to prevent the formation of these brittle phases when joining NiTi to Ti6Al4V. The presence of this interlayer ensured that crack free welds were obtained and no brittle intermetallic compounds were observed. The Niobium interlayer was of a much higher melting temperature than the base materials so the bulk Niobium did not melt during the joining process, acting as a diffusion barrier between the NiTi and Ti6Al4V. The laser was focused on the Ti6Al4V side of the joint, which joined the Ti6Al4V and Niobium by fusion welding. At this interface a (Ti, Nb) region was formed due to dilution of the Niobium and mixing with the Ti6Al4V. At the NiTi–Nb interface a eutectic reaction was responsible for joining. Mechanical testing of the joints revealed that the minimum tensile strength matched the ultimate tensile strength of the weakest material, Niobium. These results highlight new possibilities for the use of high melting point filler materials when joining NiTi to dissimilar materials, so that the formation of undesired phases can be avoided.

Mechanical properties of niobium radio-frequency cavities

Radio-frequency cavities made of bulk niobium are one of the components used in modern particle accelerators. The mechanical stability is an important aspect of cavity design, which typically relies on finite-element analysis simulations using material properties from tensile tests on sample. This contribution presents the results of strain and resonant frequency measurements as a function of a uniform pressure up to 620kPa, applied to single-cell niobium cavities with different crystallographic structure, purity and treatments. In addition, burst tests of high-purity multi-cell cavities with different crystallographic structures have been conducted up to the tensile strength of the material. Finite-element analysis of the single-cell cavity geometry is in good agreement with the observed behavior in the elastic regime assuming a Young’s modulus value of 88.5GPa and a Poisson’s ratio of 0.4, regardless of crystallographic structure, purity or treatment. However, the measured yield strength and tensile strength depend on crystallographic structure, material purity and treatment. In particular, the results from this study show that the mechanical properties of niobium cavities made from ingot material with large crystals are comparable to those of cavities made of fine-grain niobium.

Analysis of Nb3Sn surface layers for superconducting radio frequency cavity applications

We present an analysis of Nb3Sn surface layers grown on a bulk Niobium (Nb) coupon prepared at the same time and by the same vapor diffusion process used to make Nb3Sn coatings on 1.3 GHz Nb cavities. Tunneling spectroscopy reveals a well-developed, homogeneous superconducting density of states at the surface with a gap value distribution centered around 2.7 ± 0.4 meV and superconducting critical temperatures (Tc) up to 16.3 K. Scanning transmission electron microscopy performed on cross sections of the sample's surface region shows an ∼2 μm thick Nb3Sn surface layer. The elemental composition map exhibits a Nb:Sn ratio of 3:1 and reveals the presence of buried sub-stoichiometric regions that have a ratio of 5:1. Synchrotron x-ray diffraction experiments indicate a polycrystalline Nb3Sn film and confirm the presence of Nb rich regions that occupy about a third of the coating volume. These low Tc regions could play an important role in the dissipation mechanisms occurring during RF tests of Nb3Sn-coated Nb cavities and open the way for further improving a very promising alternative to pure Nb cavities for particle accelerators.

Metal forming technology for the fabrication of seamless Superconducting radiofrequency cavities for particle accelerators

The world of Particle accelerators is rather unique, since in a few high-energy Physics great laboratories, such at CERN for example, there have been built the largest technological installations ever conceived by humankind. The Radiofrequency resonant cavities are the pulsing heart of an accelerator. In case of superconducting accelerators, bulk niobium cavities, able to perform accelerating gradients up to 40 MeV/m, are just a jewel of modern technology. The standard fabrication technology foresees the cutting of circular blanks, their deep-drawing into half-cells, and its further joining by electron beam welding under ultra high vacuum environment that takes several hours. However, proposals such as the International Linear Collider, to which more than 900 scientists from all over the world participate, foresee the installation of 20.000 cavities. In numbers, it means the electron beam weld one by one under Ultra High Vacuum of 360,000 hemi-cells. At a cost of 500 €/Kg of high purity Niobium, this will mean a couple of hundreds of millions of Euros only for the bare material. In this panorama it is evident that a cost reducing approach must be considered. In alternative the author has proposed a seamless and low cost fabrication method based on spinning of fully resonators. Preliminary RF tests at low temperatures have proved that high accelerating gradients are achievable and that they are not worse than those obtainable with the standard technology. Nevertheless up to when the next accelerator will be decided to be built there is still room for improvement.

Final Commissioning of the Superconducting Heavy Ion Linear Accelerator at IUAC, Delhi

The superconducting linac as a booster of the 15UD Pelletron accelerator was partly commissioned with one linac module housing eight quarter wave bulk niobium cavities along with the superbuncher and rebuncher cryomodules. Subsequently two more linac cryomodules were added to have in total 24 cavities for acceleration. In addition, a new Linde helium refrigerator of capacity 750W @ 4.2K was installed in parallel to the earlier CCI refrigerator. The new refrigerator was integrated with the earlier cryogenics network system through a specially designed liquid helium distribution line without any valve box. The cooling philosophy with this new system is modified to have a faster cool down rate in the critical zone (150 - 70K) to avoid Q disease. The helium gas pressure fluctuation in the cavities is reduced significantly to have stable RF locking. The full linac is being operated and beams with higher energy are being delivered to the users. The present paper will highlight the performance of the new cryogenic system with respect to cool down rate, and helium pressure fluctuation.

Enhancement in Quality Factor of SRF Niobium Cavities by Material Diffusion

An increase in the quality factor of superconducting radiofrequency cavities is achieved by minimizing the surface resistance during processing steps. The surface resistance is the sum of temperature independent residual resistance and temperature/material dependent Bardeen-Cooper-Schrieffer (BCS) resistance. High temperature heat treatment usually reduces the impurities concentration from the bulk niobium, lowering the residual resistance. The BCS part can be reduced by selectively doping non-magnetic impurities. The increase in quality factor, termed as Q-rise, was observed in cavities when titanium or nitrogen thermally diffused in the inner cavity surface.

Separation of 97Ru from niobium target using PEG based aqueous biphasic systems

Separation of no-carrier-added (NCA) 97Ru from bulk niobium target has been carried out for the first time using green analytical technique, aqueous biphasic system. 50 % (w/v) polyethylene glycol (PEG)-4000, against 2 M solutions of various salts such as Na-citrate, Na-tartarate, Na-malonate, Na2CO3, NaHSO3, Na2SO4, Na2S2O3 K2HPO4, K3PO4, K2CO3 and 4 M KOH were employed at room temperature for the extraction of NCA 97Ru from bulk niobium. Influence of molecular weight of PEG rich phase as well as pH of some salt rich phase (e.g., Na-tartarate) on the extraction behaviour of NCA 97Ru into PEG rich phase was also observed. In the presence of sodium-tartarate salt solution, when volume of PEG-4000: Na-tartarate was 3:1, 91 % of NCA 97Ru was extracted into the PEG rich phase without any contamination of niobium target. Dialysis of PEG rich phase containing NCA 97Ru was carried out against deionised water to obtained pure NCA 97Ru.