R. Parodi

Growth of niobium nitrides by nitrogen-niobium reaction at high temperature

Abstract The reaction between nitrogen and bulk niobium under high purity conditions was studied in the range 1100–1900 °C at 150 kPa. Two different nitride layers are formed: an inner β phase (Nb 2 N) and an outer δ phase (NbN). The latter undergoes a transformation to γ-NbN and ϵ-NbN during cooling down. Nitrogen dissolves in the niobium core forming α solid solution. Both the weight gain and the thickness of the nitride layers follow the parabolic rate law, indicating that the reaction is mainly dominated by diffusional transport through the nitrides. Two parabolic kinetic stages are observed, the first before α phase saturation and the second after saturation. Data analysis based on a multiphase, moving boundary diffusion model allows the calculation of the effective nitrogen diffusion coefficients, yielding the expressions D β = 1.11[±0.5] exp(−3.23[±0.4] × 10 5 /RT) cm 2 s −1 and D δ = 3.27[±1.5] exp(−3.19[±0.25] × 10 5 / RT ) cm 2 s −1 .

Study of niobium nitrides for superconducting rf cavities

This paper deals with the first results of a study on Nb‐N system having as the main goal the performance of niobium nitride rf cavities. The nitrides were obtained by diffusion of nitrogen in bulk niobium at high temperature. Metallurgical and resistive measurements have been done to determine the phases and the critical temperature of the samples. Furthermore, in order to study the rf behavior of the material, a 4.5‐GHz niobium nitride cavity has been built and tested. The rf measurements showed, above 4 K, a surface resistance in good agreement with the computed values (1.15 μΩ at 4.2 K), i.e., about a factor of 10 better than the value obtained for niobium at the same temperature and frequency. At temperatures below 4 K, the residual resistance became prevalent, and its value was found to be 0.69 μΩ.

Self field effects in the critical current measurements of superconducting wires and cables

Abstract In the present work self field correction of the critical current measurements of superconducting cables and wires is discussed. It is shown that the so called ‘peak field correction’ is not correct due to the inhomogeneity of the magnetic field at the cable. To correctly handle the measured values of critical current, a general criterion, based on an effective field correction, is proposed. Some experimental results were analysed and the two criteria are compared.

Inductive method for critical current measurement of superconducting cables for high energy physics applications

Abstract In order to measure the critical current, the quench current and the n-value of superconducting cables for high field dipole magnets, an inductive method was developed taking advantage of an existing facility at INFN in Genova. The current is induced in a single turn of a superconducting cable by using the same superconducting magnet that generates the background field (6.5 T in 500 mm bore). The sample is soldered on a copper ring that holds the magnetic forces and thermally stabilizes the cable. The method was tested by measuring the critical current of a cable of HERA dipole magnets, previously measured by the standard method, i.e. using a dc power supply to feed the sample.

Nitridation of niobium-46 wt.% titanium alloy in nitrogen at 1300 °C

Abstract The nitridation of niobium-46 wt.% titanium alloy (βTi-Nb) was studied at 1300 °C in 15 kPa nitrogen under high purity conditions for times ranging from 0.5 to 52 h. Three different reaction layers were observed: (i) an outer, thin continuous layer of superconducting (Nb,Ti)N with f.c.c. structure; (ii) an intermediate, duplex layer formed by hexagonal Nb 2 N containing platelets of TiN; (iii) an inner duplex layer formed by the growth of TiN needles into the titanium-depleted βTi-Nb alloy. The observed microstructure is dominated by the different thermodynamic stability of the nitrides existing in the present conditions; in particular Ti is preferentially nitrided at the interior of the alloy. The TiN needles have the tendency to grow with linear kinetics and to form ordered patterns in the alloy; as a consequence TiN segregation can be tentatively described as a cooperative steady state process. The maximum value of T c , measured from the variation of a.c. susceptibility with temperature, was 17.9 K after 24 h reaction; at longer times a plateau is reached.

Niobium and niobium-titanium nitrides for RF applications

The application of niobium and niobium-titanium nitrides in RF devices is studied. Nitride samples, prepared by diffusion of nitrogen in bulk niobium and alloy, have been characterized by X-ray diffraction, SEM and AC magnetic measurements. Several phases of niobium nitride have been observed: the phase with higher nitrogen content, at the sample surface, has critical temperature about 15 K but inclusions of both lower T/sub c/ and nonsuperconducting phases enhance the RF surface resistance. The niobium-titanium nitride surface is composed by a phase having critical temperature up to 17 K and unwanted phases were not observed.<<ETX>>

rf surface resistance measurements of binary and ternary niobium compounds

The superconducting properties of binary and ternary niobium compounds for rf applications are investigated. The materials under study are niobium nitride (NbN), niobium‐titanium nitride (NbTiN), and niobium tin (Nb3Sn). Preparation techniques of the compounds are discussed. NbN and NbTiN are obtained by thermal reaction of bulk Nb in nitrogen atmosphere. Nb3Sn has been obtained using a technique originally developed for high‐field magnet fabrication and modified to be applied to rf cavity production. The experimental apparatus and measurement technique are described. In particular the raw experimental data have been carefully analyzed to obtain important informations on fundamental material parameters. The effect of field penetration in the superconductor and the influence on the experimental results of the normal metallic substrate on which the superconductor is grown is also considered. The measurements show that NbN and Nb3Sn are effectively potentially good materials for rf applications due to their ...

Preparation and characterization of YBa2Cu3O7-x superconducting films deposited by electrophoresis

Abstract The coatings of YBCO superconducting materials deposited on various shapes of silver substrates, by using an electrophoretic technique, have been studied and the effect of several parameters involved in the process has also been discussed. The experimental results show the importance of the milling treatment of the powders before the electrophoretic process. It is found that no deposition could be obtained without an adequate milling procedure to the powders. A series of deposition processes under various conditions with two different kinds of powders, prepared using the standard solid-state reaction method and the pyrolytic method, have shown that the successful deposition of the YBCO films is related to the electrical changing of the powder grains during the milling process. After deposition and thermal treatment the samples are characterized using X-ray diffraction, DC resistive measurements, AC susceptibility measurements. The performances of the films at RF fields are also investigated.

Multi-phase structure of thermally diffused niobium nitride

In order to study superconducting niobium nitrides for RF applications, Nb-N samples obtained by diffusion of nitrogen in bulk niobium were investigated. X-ray diffraction and AC susceptibility measurements were performed. From these measurements, the composition and the critical parameters (H/sub c1/ and J/sub c/) of the material were deduced. The results for several samples are compared. It is shown that the proposed method allows detection of the presence of different superconducting phases and granularity of materials. It was used to understand the RF behaviour of Nb-N (high residual surface resistance). Although the measured samples did not show granularity, the presence of several materials with different superconducting properties was observed. Low-T/sub c/ phases were always detected in the samples, sometimes also at the surface.

Critical current of prototype conductors for LHC dipole magnets

Electrical measurements on conductors developed for Large Hadron Collider (LHC) dipole magnets are reported. Two kinds of measurements were carried out: critical current on the prototype cables and AC magnetic measurements on the strands. To perform critical current measurements the samples were placed in the bore of the superconducting magnet MARISA, which allows measurements up to 6.5 T while temperature ranges from 1.8 to 5 K. The current is supplied using an inductive method: the background magnet is the primary winding of a transformer having the sample as a secondary winding. The current is measured by Hall probes placed near the samples; the critical current is measured by the resistive criterion rho =10/sup -14/ Omega m. Critical currents up to 60 kA were measured for four different samples. AC magnetic measurements, like AC susceptibility and penetrated and trapped magnetic flux, were carried out in order to study the connections between the critical current and the effective critical field. Information about the filament coupling at low field is also obtained.