Authors: Konstantin Atroshchenko Antonio Alessandro Rossi

Supervisor: prof. Enzo Palmieri

"Presence of residual Tin drops in Thermally diffused Nb3Sn"

Advantages of the liquid Tin diffusion method

relatively cheap technique; uniformity of the film (stoichiometrically); can be used for covering surface of wide and complex shaped substrates (!). we don't need to manipulate dangerous substances as SnCl2 to create a nucleation centers, and the diffusion process is considerably faster

Liquid tin diffusion is a method of obtaining the superconductive A15 Nb3Sn coating over the 6 GHz cavities or other substrates. A bulk Nb 6 GHz cavity is introduced into molten Sn (dipping step) and after it follows the heat treatment (annealing step).

6 GHz cavities ready for treatments

Experimental stand

Inconel chamber: (chosen because of its stability at the high process temperature).

Alumina (Al2O3) crucible contains the Sn inside (99.99% nominal purity);

top chamber

upper furnace

lower furnace

linear feedthrough

cooling water

pumping

cavity

flanges

crucible with liquid Tin

cavity

droplets of Tin

Problem: • The residual droplets of

Tin on the banded parts (planes, which is horizontal) of the cavity

Problems of the methodProblem: • external furnace. Maximum temperature 1200OC,

but it’s hard to transmit it inside the chamber, so the temperature inside is not high enough to evaporate the residual Tin droplets

• Diffusion of contaminations through the wall of the chamber at high temperatures

external furnace

vacuum chamber

Residual Tin drops on the internal and external surface of the cavity

Nb3Sn

Sn

Sn

External surface Internal surface

The L - samples

For the experiment was designed the L-samples, which imitates the shape of the cavity. Samples is made of Niobium.

50

10

153

Basic surface treatment (for all samples)

washing in Rodaclean with ultrasonic (60 min) washing in deionized water with ultrasonic washing with deionized water drying with nitrogen BCP in the solution: HF/HNO3/H3PO4 = 1/1/2 washing with deionized water drying with nitrogen

All samples have been lapped using abrasive papers to reduce the residual roughness after machining.

1. Mechanical treatment

2. Basic chemical treatments

Preparing the surface. Glow Discharge

voltage connector

feedthrough with the sample inside

chamber

coil

baking time: 16 hours;baking temperature 120oC ;GD pressure: 10-3 mBar;

Cathode Parameters:current: 0,04 A

Coil Parameters:Current : 8 AMagnetic field 525 Gauss

Procedure:

Ultrasonic + Rodaclean 60 min

Clean with acetone

Clean with alcohol

Glow Discharge 1 min

Annealing. 4 hours. T = 1000OC

Ceramic tube

Sample

Nb wire

Flange

Glow Discharge. Results.

Horizontally fixed sample

Glow discharge. View from the bottom window

After glow discharge

After dipping

and annealing@ 1000oC

Preparing the surface. Anodization

Procedure:

BCP 10 min

Ultrasonic + Rodaclean 60 min

Clean with acetone

Clean with alcohol

Anodization inammonium citrate. V = 20V

Annealing. 4 hours. T = 1000OC

Ammonium citrate

sample

To the power supply

After anodization

After dipping

and annealing@ 1000oC

Ultrasonic + Rodaclean 60 min

Clean with acetone

Clean with alcohol

Chemical etchingHNO3 : HF = 1 : 1

Procedure:

Preparing the surface. Chemical etching

After annealing

After etching

Comparison of external resistive and internal heaters

Parameter Internal furnace External furnace

Maximum temperature < 2000OC 1200OC

Diffusion of the contaminations

through the wall of the chamber

No (chamber is cold) Yes (chamber is hot)

Chamber Could be used stainless steel chamber

Requires Inconel chamber

Time of heating to 1000OC < 5 min 3 hours

Cooling Doesn’t requires cooling Requires cooling the pumping zone

Linear feedthrough

Hot zone

Cold zoneVacuum chamber

heater

Pumping out

L - samples

Internal heater high temperature annealing

The cross-section of the system:

Vacuum Chamber

Vacuum chamber

Temperature control unit

baking control unit

Gate switch

Pumping control unit

Whole – metal valve

Comparison of annealing at 1000OC and 1300OC

annealing at 1300OCfor 5 minutes

annealing at 1000OCfor 4 hours

etching glow dischargeanodizationJust BSP

Outlook

Definition of optimal parameters of high temperature annealing: temperature and time of annealing, ets.

SEM measurements

Profilometric measurements

Covering of the Nb 6 GHz cavities;

RF – measurements

Thank you for attention!