Niobium hydrogenation process: Effect of temperature and cooling rate from the hydrogenation temperature (original) (raw)
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Measurements of hydrogen content in bulk niobium by Thermal Desorption Spectroscopy
2002
The hydrogen content of bulk niobium has been studied by Thermal Desorption Spectroscopy. The work has been focussed initially on the influence of the vacuum firing and the surface chemical treatment. It is planned to extend the investigation to niobium samples of different quality and origin to ascertain the interest of using the Thermal Desorption Spectroscopy technique to qualify the raw niobium sheets to be used for cavity manufacturing
Kinetics of thermal decomposition of niobium hydride
International Journal of Refractory Metals and Hard Materials, 2012
High-purity niobium powders can be obtained from the well-known hydride-dehydride (HDH) process. The aim of this work was the investigation of the structural phase transition of the niobium hydride to niobium metal as function of temperature, heating rate and time. The niobium powder used in this work was obtained by hightemperature hydriding of niobium machining chips followed by conventional ball milling and sieving. X-ray diffraction measurements were carried out in vacuum using a high-temperature chamber coupled to an X-ray diffractometer. During the dehydriding process, it is possible to follow the phase transition from niobium hydride to niobium metal starting at about 380°C for a heating rate of 20°C/min. The heating rate was found to be an important parameter, since complete dehydriding was obtained at 490°C for a heating rate of 20°C/min. The higher dehydriding rate was found at 500°C. Results contribute to a better understanding of the kinetics of thermal decomposition of niobium hydride to niobium metal.
Superconductor Science and Technology, 2018
Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are not well known. Using first principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties.
Combustion of niobium in hydrogen and nitrogen. Synthesis of niobium hydrides and hydridonitrides
International Journal of Hydrogen Energy, 2001
This paper treats the combustion processes observed in the Nb-N-H system, and its application to the synthesis of niobium hydridonitrides. In particular the Self-Propagating high-temperature synthesis (SHS) process in Nb-H and Nb-N systems was studied. The combustion products were hydrides and nitrides, respectively. The determination of the main features of combustion process by obtaining binary compounds enabled to examine a ternary system including both N2 and H2 simultaneously. It was found that niobium combustion in the mixture of two reacting gases proceeded in three competetive ways, established earlier for IVA group metals. Various types of reaction occur depending on the partial pressure ratio.
Materials Today: Proceedings, 2016
Nanocrystalline powders of MgH 2 was synthesized under a high hydrogen gas pressure (50 bar), using reactive ball milling technique operated at ambient temperature under 50 bar of a hydrogen gas atmosphere. The MgH 2 powders obtained after 200 h of continuous RBM time composed of β and γ phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250 o C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics, three different types of nanocatalysts (metallic Ni, Nb 2 O 5 and (Ni) x /(Nb 2 O 5) y) were utilized with different weight percentages and independently ball milled with the as-synthesized MgH 2 powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results have shown that the as-prepared nanocomposite MgH 2 /5Ni/5Nb 2 O 5 powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for β-phase (68 kJ/mol) and γ-phase (74 kJ/mol). Moreover, this nanocomposite system shows excellent hydrogenation/dehydrogenization behavior indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt.% H 2 at 250 o C.
Hydrogen-induced defects in bulk niobium
Physical Review B, 2004
Our aim in the present work was to investigate changes of the defect structure of bulk niobium induced by hydrogen loading. The evolution of the microstructure with increasing hydrogen concentration was studied by x-ray diffraction and two complementary techniques of positron annihilation spectroscopy (PAS), namely positron lifetime spectroscopy and slow positron implantation spectroscopy with the measurement of Doppler broadening, in defect-free Nb ͑99.9%͒ and Nb containing a remarkable number of dislocations. These samples were electrochemically loaded with hydrogen up to x H = 0.06 ͓H/Nb͔, i.e., in the ␣-phase region, and it was found that the defect density increases with hydrogen concentration in both Nb samples. This means that hydrogen-induced defects are created in the Nb samples. A comparison of PAS results with theoretical calculations revealed that vacancy-hydrogen complexes are introduced into the samples due to hydrogen loading. Most probably these are vacancies surrounded by 4 hydrogen atoms.
The characterization of borided pure niobium
Surface and Coatings Technology, 2005
In this study, mechanical properties of borided pure niobium were investigated. Boronizing was carried out in a solid medium consisting of Ekabor powders at 940 8C for 2, 4 and 8 h. The formation of NbB 2 on the surface of pure niobium was confirmed by XRD analysis. Metallographic studies revealed an almost uniform and compact boride layer on the surface of the pure niobium. Experimental results showed that longer boronizing time resulted in thicker boride layers. The thickness of boride layer ranged from 8 to 22 Am with some scatters. The hardness of borided specimens decreased with the distance from the surface to the interior of the test material. The hardness of the boride on the substrate was 2500 HV while the hardness of the substrate was 110 HV.
Role of Nitrogen on Hydride Nucleation in Pure Niobium by First Principles Calculations
2018
It is well known that the formation and growth of niobium hydride degrades the superconducting radio frequency (SRF) properties of niobium cavities and the treatments that reduce hydrogen concentration improve the cavity quality factor. Recently it has also been shown that the addition of nitrogen through doping or infusion improves the quality factor of SRF niobium. Thus, in this work, the role of nitrogen solute addition in niobium on hydride precipitation is probed through first-principles calculations. In the presence of nitrogen the energetic preference for hydrogen to occupy interstitial sites in the vicinity is reduced. Furthermore, both interstitial octahedral and tetrahedral sites become equally favorable for hydrogen to occupy due to the presence of nitrogen. To examine the role of nitrogen on the nucleation of hydrides, we utilized valence charge transfer and density of states calculations. These quantum insights reveal a strong tendency for nitrogen to accumulate charge,...
Thermodynamic Evaluation of Hydrogen Absorption by Niobium During SRF Fabrication
2000
The properties and performance of the ultra high purity Nb used to fabricate superconducting radio frequency (SRF) particle accelerator cavities have been found to vary with processing conditions. One hypothesis for these variations is that hydrogen, absorbed during processing, is responsible for this behavior. The key assumption behind this hypothesis is that niobium can absorb hydrogen from one or more of the processing environments. This paper reviews work examining the validity of this assumption. It was determined that Nb will spontaneously react with water producing adsorbed atomic hydrogen that is readily absorbed into the metal. The passivating oxide film normally prevents this reaction, but this film is frequently removed during processing and it is attacked by the fluoride ion used in the polishing solutions for SRF cavities. However, during electropolishing that cathodic reduction of hydrogen is transferred to the auxiliary electrode and this should suppress hydrogen absorption.
On the Hydrogenation of a NaH/AlB2 Mixture
The Journal of Physical Chemistry C, 2015
A mixture of 3NaH/AlB 2 was prepared by ball-milling; its hydriding reaction was studied between 375−425°C and 25−50 bar hydrogen pressure by means of volumetric titration. The hydriding reaction was characterized by means of in situ synchrotron radiation powder X-ray diffraction and high-pressure differential scanning calorimetry. Hydriding reaction took place at the molten state, and its reaction products were NaBH 4 and Al. The scanning electron microscopy images of the material revealed that the material morphology changes after hydriding. A maximum hydrogen uptake of 4.7 wt % was registered for the hydriding experiment at 425°C and 50 bar hydrogen pressure. Dehydriding reaction was studied by means of volumetric titration and differential scanning calorimetry. The dehydriding reaction at 425°C and 1 bar argon pressure registered a release of 2.4 wt %. The low dehydriding level was attributed to the reduction of the available particle surface upon melting of the material during the hydriding reaction.