Glossary of Superconductor Terms

Back tension is the amount of force that is placed on the wire along its longitudinal axis during processing (such as wire transfer, reeling, winding, etc).  So an “x” amount of back tension is “x” amount of force applied along the longitudinal direction of the wire.

Ballooning

Ballooning is damage to the wire caused by the rapid expansion of cryogenic gases within the wire when it is warmed to room temperature from a cryogenic temperature.  When the wire is exposed to a pressurized liquid cryogenic environment, the liquid cryogen can diffuse into the wire unless the wire is hermetically sealed.  Upon warming to room temperature, this cryogenic liquid turns to gas within the wire causing it to balloon.  This balloon process will disrupt the material and thus the superconducting properties of the wire.  If you are considering using the wire in a pressurized liquid cryogenic environment such as liquid Nitrogen, please contact us about the advantages of using AMSC's Hermetic Wire (See Hermetic Seal).  Hermetic Wire has been designed specifically to withstand pressurized liquid cryogenic environments.

Bend Diameter

The bend diameter is the inner diameter of the wire when bent around a circular mandrel, spool, or reel.  The critical bend diameter is the smallest diameter to which the wire can be wound without damaging the wire.  For non-circular winding, there cannot be any section of the winding where the arc of the bend is tighter than the arc of the corresponding circle having the critical bend diameter.

BSCCO 2223

BSCCO 2223 is a commonly used name to represent the HTS material Bi_(2-x)Pb_xSr₂Ca₂Cu₃O₁₀.  This material is used in our multi-filamentary composite HTS wire and has a typical superconducting transition temperature around 110 Kelvin. 

Compressive Stress

Compressive stress arises from forces that attempt to condense or shrink the material or wire (squeezing as opposed to pulling).  A critical compressive stress for HTS wire is the amount of compressive stress that can be applied to the wire before the critical current is reduced below a specified level (such as 95% I_c retention).

Compressive Strain

Compressive strain is the percentage change in a material dimension that is under compression.  A critical compressive strain for HTS wire is the amount of compressive strain that occurs before the critical current is reduced below a specified critical level (such as 95% I_c retention).

Critical Current

The ideal physical definition of critical current is the current where a material has a phase transition from a superconducting phase to a non-superconducting phase.  For practical superconducting wire, the transition is not infinitely sharp but gradual.  In this case, the critical current is defined as the current where the voltage drop across the wire becomes greater than a specific electric field, usually 1 microvolt/cm.  Sometimes, for low-loss magnet applications, a lower electric field criterion is used.  The critical current is represented by the variable I_c. 

Critical Current Density

The critical current density, J_c is the critical current of a superconductor divided by the cross sectional area of the superconductor material.  The critical current density is useful when characterizing the quality of a superconductor material.  The critical current density, J_c should not be confused with the engineering critical current density, J_e (see Engineering Critical Current Density)

Critical Current Retention

The critical current retention is the relative ratio of the critical currents before and after a specific process or test.  For example, assume the initial critical current, I_co, of a wire is 100 amps.  After the wire is subjected to a certain test, the post-test critical current, I_c, is measured at 95 amps.   The critical current retention, I_c/I_co, is 95/100 = 0.95 or 95% for this wire under the test conditions.

Engineering Critical Current Density

The engineering critical current density, J_e, is the critical current of the wire divided by the cross sectional area of the entire wire, including both superconductor and normal metal materials.  This is different from the standard critical current density, J_c, which is the critical current divided by the cross sectional area of just the superconductor material of the wire.  The engineering critical current density is an important parameter used in the design of applications based on HTS wires.

Four Probe Resistance

A four probe resistance measurement is a technique for measuring the resistance of low resistance material.  Unlike two probe resistance measurements where the voltage drop is measured by the same probe that does the current excitation, the four probe technique separates the voltage drop measurement probes from the current excitation probes.  This separates the probe resistance from the actual sample resistance measurement.  For low resistance measurements, such as for highly conductive metals and especially for superconductors, it is necessary to perform the resistance measurement using a four probe technique.

Hermetic Seal

HTS wire can be hermetically sealed to prevent ballooning issues (see Ballooning).  To establish a hermetic seal, American Superconductor has created a special lamination technique.  This HTS Hermetic Wire prevents liquid nitrogen from entering the system even under a pressurized environment.

I-V Curve

The I-V curve or the “Applied Current vs the Voltage Drop” curve is the standard curve measured to observe a material’s superconductivity and critical current.  The voltage drop across the superconductor material is measured as a function of the applied current.  The I-V curve is used to determine physical parameters such as the superconducting critical current, the critical current density and the n-value of the material.

Inductive Winding

When wire is wound into a coil configuration, a current flowing in the coil produces a field around the coil, and the coil can be characterized by an inductance determined by the stored energy in that magnetic field.  Such a coil is termed an inductive winding, in contrast to special windings which cancel out the field and are thus “non-inductive.”    A flat inductive pancake coil is a convenient configuration for measuring n-values of long wire lengths at extremely low voltage levels.  Inductive winding is also the standard technique used to make superconductor magnets using HTS wires.

Kelvin

Kelvin is a unit of temperature starting at absolute zero and having the same scale as Celsius.  Thus zero degrees Celsius is 273.15 Kelvin.  Similarly -489.67 degrees Fahrenheit is absolute zero on the Kelvin scale.  Under ambient pressure, Helium liquefies at about 4.2 Kelvin, Nitrogen liquefies at about 77 Kelvin and water freezes at about 273.15 Kelvin.  The scientific symbol for the Kelvin unit is K.

Longitudinal

For our HTS wire the longitudinal direction is the direction along the length of the wire.  For example the longitudinal tensile stress is produced by a force stretching the wire along its length.  The longitudinal axis is also referred to as the axial, or rolling direction for HTS wire.

N-Value

The n-value describes the relationship of the voltage drop across the wire to the applied current.  For the transition from zero resistance (zero voltage drop) to a finite resistance (finite voltage drop), the I-V curve of HTS wires can almost always be fit with the power law

    E_(j) = E_c (j/j_c)ⁿ

Here E_(j) is the longitudinal voltage drop across the superconductor, E_c is the electric field criterion (see Critical Current), j is the applied current density,  j_c is the critical current density and n is the exponent.  For a sharper transition, the I-V curve has a higher n-value.  The n-value is often used to determine the quality of a bulk superconducting material (see inductive winding).

Self Field

The self field is the magnetic field that is induced when there is finite current flowing in a wire.   In the case of critical current measurements, the self field is the magnetic field that is induced in a straight piece of wire that is being measured.

Splice

A splice is a joint between two wire segments.  AMSC's High Strength HTS wires can be spliced together using a special soldering technique.  Typical splices have an electrical resistance less than 200 nOhm at 77 Kelvin. 

Superconducting Transition Temperature

The superconducting transition temperature T_c is the temperature at which a material undergoes a thermodynamic phase transition from a superconducting state to a non-superconducting or normal state.  High temperature superconductors are usually defined as those with transition temperature above 20 K - 40 K, when measured with small currents and no applied magnetic field.  It should be noted that T_c is suppressed by both applied current and magnetic field.

Tensile Stress

The tensile stress results from a force that would result in elongation of a material.  The critical tensile stress for HTS wire is the amount of tensile stress that can applied to the wire before the critical current is reduced below a specified level (such as 95% I_c retention).

Tensile Strain

The tensile strain is the percentage change in dimension that results from tensile stress on a material.  A critical tensile strain for HTS wire is the amount of tensile strain that occurs before the critical current is reduced below a specific critical value (such as 95% I_c retention).

Transverse

For HTS wire, the transverse direction is usually designated as being orthogonal to the length of the wire.  The two major transverse axes are along the width of the tape (also known as the ab-trans, lateral, or long transverse directions), and perpendicular to the face of the tape (also known as the c-axis, normal, or short transverse directions).