NATIONAL INSTITUTE OF NUCLEAR PHYSICSPADOVA UNIVERSITY
Camacho A., Rossi A., Palmieri V.
Sixth International Workshop on Thin Films and New Ideas for RF Superconductivity
Preparation of Nb3(Ga,Al)
superconductors by Electromagnetic
Induction Heating
Outline• Introduction of the common techniques used to prepare binary and
ternary superconducting A15 compounds.
• Electromagnetic Induction Heating Technique.
• Experimental procedure:
• Samples preparation.
• Heat Treatment Performed.
• Validation of the technique according superconducting properties and
quality of A15 phase on samples.
• Application of our technique on 6 GHz niobium cavities.
• Conclusions.
Introduction
• Common techniques used to prepare A15 superconducting phase:
Arc-melting process, chemical vapor deposition (CVD), sputtering,
etc.
• More specialized techniques such as:
• Melt-spin quenching technique: Nb3Ga 20.0K, V3Ga
15.0K, and Nb3Al 18.4K reported by Clemente [1]
[1] Clemente,“Superconducting properties of A15 compounds prepared by melt-spin quenching“
EM- Induction Heating Rapid heating
High temperatures during annealing process (~3000 °C)
Vacuumless
Self-heating of the sample
Short time of treatment
Clean quartz chamber
Economic system
Application on 6 GHz niobium Cavities
A15 compounds by EM-IH technique
6 GHz Nb Cavities
EM- Induction Heating System
Work head
15 KW
Powersupply
Pyrometer
Quartz tube
Flange
Flange
Argon or
Helium
Low overpressure
(250-3000)ºC
Exhaust gases
Coil
Input gas
Cavity or Sample
Experimental Procedure
Before annealing the samples
Chemical TreatmentBCP solution:
HF/HNO3/H3PO4 = 1:1:2
(20x10x5) mm
Experimental Procedure
How?
• Binary compounds Configuration I: Liquid Gallium (99.9% pure) Aluminum Foil (99% pure)
• Ternary compounds Configuration I and II: Paste: liq. Ga+ Al foil
Configuration I
Configuration II
Heat TreatmentChanging the voltage
and time
Rapid heating, quenching and transformation
Validation of the EM-IH TechniqueMaterials Number of samples Total
Nb-Ga 10
61Nb-Al 6
Nb-Al-Ga 45
Inductive
Measurement
Tc
Binary Compounds
Heat treatment for 10 minutes, changing the temperature from 1500 °C up to 1800 °C
Binary Compounds
• All the Tc are near to 12 K, an average of 3 K above of niobium transition
(9 K).
• The difficult to synthetize binary A15 compounds (Nb3Ga and Nb3Al) is
related to the competition from more stable phases, such as s phases
(Nb2Al and Nb5Ga3) and a-Nb phases (solid solution in bcc structure of
niobium).
• Samples annealed that not reach temperatures higher than 1500ºC, only
Nb superconducting transition was evidenced.
• The results suggest that the annealing process for 10 minutes at high
temperatures degrades the superconducting phase initially formed
Nb3Ga Nb3Al
Lattice parameter of 5.1809 Å, very close to the standard lattice parameter, 5.1800 Å
Lattice parameter of 5.2141 Å, much higher than the standard value, 5.1780 Å.
From our first attempt of A15 phase, we concluded...
1. Critical temperature results suggest higher diffusion of gallium atoms than aluminum atoms on niobium samples at the same heating conditions (corrosive property of gallium)
2. Wettability problems with liquid gallium which make the preparation of the samples before the heat treatment difficult
3. Very short time of heat treatment is necessary
TERNARY COMPOUNDS!!! Nb-Al-Ga
Ternary Compounds
Heat treatment for ~1 minute, changing the temperature from 1420 °C up to 2000 °C
Tc= (18±0.35)K
1
• Direct transformation of A15 phase from high temperatures of
niobium samples.
• Ternary compound seems to stabilize the A15 phase.
High Tc and sharp superconducting transition
Ternary Compounds
Heat treatment for ~1 minute, changing the temperature from 1420 °C up to 2000 °C
2
Broad superconducting transitions
• Gallium/ aluminum evaporate.
• Less control of the stoichiometry.
• Further studies are required in order toestablish a relation of the quantity of aluminum/gallium evaporated.
(210)
(200)
(310)
(320)(211)
X-ray diffraction in the planes: • (321) Nb3Ga• (110) Nb3Al
were not observed
Lattice parameter: 5.1904 Åa-Nb3Ga< a-Nb3(Al,Ga)< a-Nb3Al
Profile temperature vs time Cooling rate:
50 [˚C/s]
Heating rate: 90 [˚C/s]
Results Microstructure of Nb-Ga-Al_1 sample
Niobium Nb-Al-Gax
Niobium Nb-Al-Ga
Crack
x
Element wt% At.%Nb 82±1 66±1Ga 11,3±0,9 12,1±0,9O 1,9±0,1 9,0±0,6Al 4,7±0,2 12,9±0,4
73% at. Nb, 13.3% at. Ga and 14.2% at. Al. A15 and A2
Results
Nb-Al-GaNiobium
Microstructure of Nb-Ga-Al_1 sample
Nb-Al-Ga SamplesResults
Niobium Nb-Al-Gax
Microstructure of Nb-Ga-Al_1 sample
x
Niobium Nb-Al-Ga
Nb-Al-Ga Niobium
Results
Mapping that shows the interface between niobium and superconducting layer
Total Counts X-RaysElement Color Smin Smax
O K Red 11 99Ga L Green 17 405Al K Blue 19 296Nb L Yellow 151 2805Ga K Purple 21 366
Nb (L) Ga (K) Al (K)
Ga (L)-Nb (L) Al (K)-Nb (L)O (K)- Nb (L)
Ga (L)-Al (K) Ga (L)-Al (K)-Nb (L) Ga (L)-Al (K)-Nb (L)- O (K)
6 GHz niobium Cavities
Experimental Procedure
Before Coating: 1. Centrifugal Tumbling
Experimental Procedure
Before Coating: 2. Chemical Treatment, BCP solution
Before Coating: 3. High Pressure Water Rising
Experimental Procedure
Rotator
Liquid Gallium
Experimental Procedure
Yttria- stabilized Zirconium oxide
Inside Liq. gallium or paste with
Cavities Measurements
Sample Time [min.] Annealing Temperature, Max. [°C] Notes
1 1,3 2000 Melted
2 14,4 1731 Cavity with a small hole.
3 2,2 1770 Melted
4 3,0 1200 Normal conductor
510,0 1091 Normal conductor
66,1 2031 Normal conductor
71,4 1830 Normal conductor
Experimental Procedure
Direct transition of superconducting phase from high temperaturesusing E-M induction heating.
The temperature of the samples are very sensitive by changing voltageand time.
Nb+Al+Ga stabilized the A15 superconducting phase.
The best configuration is niobium+aluminum/gallium+niobium whichavoid the evaporation of gallium and aluminum at high temperatures more control of the stoichiometry SHARP HIGH SC TRANSITION
Conclusions
Thanks for your attention