review on stress sensitivity part i

Stress review - CERN, 4.11.2008

Review on stress sensitivity Part I

R. FlükigerB. Seeber

Group of Applied Physics (GAP)

University of Geneva

1


Outline

General problematics of stresses in superconductors

ITER and NED requirements

Uniaxial tensile stresses: Jc vs.

Models for description

Transverse compressive stresses: Jc vs. t

What do we actually know?

2


Requirement for ITER ≠ Requirement for NED

Requirements for Jc and stresses:

ITER: Jc(non Cu) > 1’100 A/cm2 at 12 T

low a.c. losses filament diameter < 20 mm

No impregnation, no particular mechanical protection

No cracks up to 30 MPa: twist pitch/bending

Direct contact between strands: transverse stresses !!!

No relevant degradation of Jc after > 20 years (neutrons)

NED: Jc(non Cu) ≥ 1’500 A/cm2 at 15 T

No cracks up to 120 MPa

Impregnation, reduces problems of transverse stresses

No relevant degradation of Jc after 10 years (neutrons)

3


The ITER TF Model Coil

ø 40 mm, 1.5 mm thick steel

Conduit rated current:

70 kA/11.8 T/4,6 K

1028 strands

Nb3Sn + 1/3 Cu

Nb3Sn Conductor

4


Internal Sn Diffusion Technique

Example: Oxford Instruments, for ITER Type I)

* 0.81 mm (NbTi)3Sn strand * 19 subelements *) * Single Ta barrier * Cu:non-Cu ratio 1 * Jc ~1200 A/mm2 (Type I) ~1100 A/mm2 (Type II) * Non-Cu hysteresis losses: 900 kJ/m3 (Type I) 700 kJ/m3 (Type II) * Unit lengths: up to 8 km

*) Agglomeration of original filaments during reaction: Characteristics of Internal Sn wires

Courtesy A. Vostner, ITER5


Problem: All high field superconductors are brittle. At fracture ≤ 0.05 %: Formation of cracks

Only exception: NbTi, with Tc = 10K, Bc2(0) = 14 T

Question: How can one built large magnets based on superconducting wires with

irr ≥ 0.6% ?

Answer: Microfilamentization Reason: Relationship between contact surface and volume (or: Ratio between Interface and total filament surface)

6


Microfilaments

Bronze Route wire 100 nm

2 m 7


wire name wire N. of filaments

Fil. eff. diameter

A15 layer thickness

Cu A15 volume fraction

GAP#14-#24 Ti, Ta 1.25 mm 14641 4-5 m 2-3 m 20% 23%2%

OST #7069 Ti 0.8 mm 18 70 m 10-30 m 59% 23.5%1%

OST #8056 Ti, Ta 0.9 mm 60 100 m 20-25 m 30% 42%2%

OST #8298 Ta 0.9 mm 60 100 m 20-25 m 30% 38%2%

SMI Ta 1 mm 192 50 m 10 m 45% 25%0.5%

BronzeInternal Sn

PIT

4-5 m

70 m

70 m

50 m

Internal Sn wire PIT wire

Filament size D: D(bronze) << D(Internal Sn, PIT)irr(bronze) > irr(Internal Sn, PIT) > 0.8 % ≤ 0.4 %

8


10 m

24 h/800°C

1 m

24h/850 °C

Main Limitation: above 560°C, the submicron size filaments are interrupted, due to the formation of Nb3Sn : “Spherodization”

10 m 1 m

The unfulfilled dream of Nb3Sn wires: « in situ » wires, with filament sizes < 100 nm

9


Strengthening of « In Situ » wires (effect submicron filaments)

Increase from 0.3 to 0.7 %

Dendrite sizes after casting

Final wire: Sizes < 100 nm

10

« in situ » techniquegiven upJc too low


Tensile stresses

11

650°C 4.2 K

Cu

Cu/Sn

Nb

Nb3Sn

cool down

m

•Cu and Cu/Sn in extension

•Nb and Nb3Sn in compression

Nb3Sn technical wires


Origin of precompression in superconducting wires

12

-1.5 -1.0 -0.5 0.0 0.5105

106

107

108

109

Eng

inee

ring

Crit

ical

Cur

rent

Den

sity

(A

m-2)

0.1

1

10

100

1000

Temperature: 4.2 K

Crit

ical

Cur

ren

t (A

)

23 T

Magnetic Field: 8 T

Applied Strain (%)

Nb3Sn Wire

Why is the effect of tensile strain important?

Stress review - CERN, 4.11.2008 13


Change of physical properties when applying a tensile stress

In Nb3Sn, the application of tensile stress has been recognized to change primarily the phonon spectrum rather than the electronic density of states(Markiewicz 2005, Hampshire et al., 2006)

14


Tc/Tcm

Asymmetric behavior of Bc2()

Bc2()Bc2m

Effect of tensile stress much stronger on Bc2 than on Tc

Bc2()/Bc2m = 1 – a|o|

15

10% reduction of Tc/Tcm: m: - 0.89%10% reduction of Bc2()/Bc2m: m: - 0.45%


Elastic tetragonal distortion under the effect of uniaxial tensile stress


GAP (Nb,Ti)3Nb - #14GAP (Nb,Ta,Ti)3Sn- #14

Uniaxial strain behavior of Nb3Sn wires

Internal Sn wires (Type I) are more strain sensitive than Bonze Route Wires. Asymmetry of Jc() observed for all wire types. Explanation by asymmetric distortion at both sides of m.

17

q

c

p

cc B

B

B

BBJ

2

1

2 1),(

SBB cc 022

7.11)( maS

0

20

2

1

1)(

a

ma

C

CS

Ekin model ten Haken model

Kramer’s law

Field and strain scaling laws for Nb3Sn

strain dependent critical field


-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.20.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

21T, 4.2KB

c2=25T

intrinsic strain (%)

Ic/Ic

m

-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.20.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0=

0.25

0.15

0.08

0.02

Ca=

303540

50


Ic/Ic

m-0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

a=900110013001500

21T, 4.2KB

c2=25T

B = 21 T


Ic/Ic

m

Ekin’s model

ten Haken’s model



Devices for Tensile Stress Measurements

Principle: Gradual release of the precompression

20


a: The Pacman strain device (University of Twente)

21


b) The Walters Spiral (Univ. Geneva)

•Max current 1’000 A

•Wire length up to 1 meter

•Max voltage tap distance 50

cm

•Jc criterion 0.01V/cm

Measurements: up to 21 T

Strain e: applied by an axial rotation

see: B. Seeber (next speaker)

22


Steel reinforced Nb3Sn wires

Stainless Steel 316LN leads to a higher precompression

* higher non-hydrostatic tetragonal deformation

* higher hydrostatic compression

Depending on the Steel:Nb3Sn ratio, em increases from 0.25 to 0.87%

For 40% steel, a decreases of Bc2 by 3 T is observed

Lower Jc values are measured: for em = 0.87%, Jc/Jco = 0.12.

23


Steel reinforced Nb3Sn wires

J.Ekin, W.Specking, R.Flükiger, J.Appl. Physics, 54(1983)2869

Fe/Nb3Sn

Jc/Jco

Stainless steel

Cu

24


Transverse compressive stresses


Fixed part

Moving part

Specifications:

- F = 5KN- I = 1000 A- Field 21 T

26

0 100 200 300 400 500

t


Conclusions

•Jc(B,) have been measured at very high fields (21 T) for Nb3Sn Bronze Route, Internal Sn and PIT wires.

•The results show a dominant effect of the axial components 1 D approximation: Jc(B,) curves usually analysed with the Ekin and ten Haken models

•3D distribution revealed by crystallography. Calculations still needed for larger filament sizes (subelements)

• Transverse compressive stresses: still no theoretical understanding. The very low reversibility of Jc suggests that nano- and microcracks are the major responsible for the much observed effects, which are much stronger than for uniaxial tensile stresses.


review on stress sensitivity part i

Documents

low stress review cern

stress sensitivity

iter type

jcnon cu

iter requirement

formation of nb3sn

kjm3 type

amm2 type