Low resistivity contact to iron-pnictide superconductors

Abstract

Method of making a low resistivity electrical connection between an electrical conductor and an iron pnictide superconductor involves connecting the electrical conductor and superconductor using a tin or tin-based material therebetween, such as using a tin or tin-based solder. The superconductor can be based on doped AFe2As2, where A can be Ca, Sr, Ba, Eu or combinations thereof for purposes of illustration only.

Description

FIELD OF THE INVENTION

The present invention relates to a low resistivity electrical contact to a superconductor and, more particularly, to a low resistivity electrical contact to an iron-pnictide superconductor and to a contact making method.

BACKGROUND OF THE INVENTION

Superconductors are materials which carry electrical current without dissipation. However, feeding electrical current into a superconductor generates heat dissipation in the contacts and degrades maximum attainable current value. This degradation can be minimized by creating electrical contacts with low resistance.

The superconducting state is destroyed if the parameters of the experiment exceed some critical values. These are critical temperature T_c and critical magnetic fields, the higher of which called the upper critical field, H_c2 destroys superconducting state completely. The maximum density of current which the superconductor can carry without dissipation is called critical current density J_c. This value, in addition to temperature and magnetic field, depends on both intrinsic properties of materials used and the form of their preparation.

There is an ongoing effort to find superconductors with high intrinsic critical parameters. Discovery of superconductors with high transition temperatures, well exceeding the boiling point of liquid hydrogen, 20 K, opens the way for using high temperature superconductors for a variety of application. Just a few superconductors are known with transition temperatures on the order of 40 K, offering prospective for technological applications at 20 K. These include numerous compounds based on copper oxides and generally referred to as superconducting cuprates (T_c up to 135 K at ambient pressure) and MgB₂ (T_c=39K). A new family of materials based on iron-arsenide compounds known as iron pnictides, which are generally represented by AFe₂As₂ where A can be Ca, Sr, Ba or combinations thereof, offer a number of superior material parameters as compared to the cuprates. These include low anisotropy of the upper critical fields, high values of the upper critical field and small anisotropy of the superconducting critical currents.

Superconductivity can be induced in the iron pnictides by partial chemical (elemental) substitution (doping) of either of the three elemental constituents (A, Fe, As) and/or by application of pressure. Substituted iron pnictides can be represented by (A_1-xD1_x)(Fe_1-yD2_y)₂(As_1-zD3_z)₂, where A stands for alkali earth elements Ca, Sr, Ba, Eu or their combinations, and various elements can be added as dopants D1, D2 and D3.

Superconducting materials based on K-, P-, Co-, or Ni-doped BaFe₂As₂, with high transition temperatures to the superconducting state (above 20 K) were discovered in mid-2008 and their electrical properties were tested using commonly accepted in-laboratory electrical contact making techniques that involved attaching electrical conductor wires (e.g. silver wires) to the superconductor using silver epoxies or silver paint. However, these electrical connections were characterized by high contact resistance on the order of 10⁻³ ohm-cm², and are not suitable for high current applications.

Feeding of electrical current into the superconductor generates heat dissipation in the contacts and degrades maximum attainable current value. This degradation can be minimized by creating contacts with low resistance. However, the selection of contact materials heavily depends on the surface reactivity of the materials in contact. For unknown material, determination of right combination is impossible without heavy experimenting. For example, Indalloy Corporation provides a research soldering kit, targeting soldering non-standard metals. The number of alloys in the kit is above 280, which go in combination with 5 different fluxes. This gives more than 1400 combinations to test. Even when a mechanically strong soldered joint is made, there is absolutely no guarantee that it would give suitable contact resistance. The same is true with different approaches, used for contact making. These include, but are not limited to, different variants of vacuum evaporation (thermal, plasma, reactive magnetron sputtering, laser ablation). During contact making, new binary and ternary compounds are easily formed at the interface, which adds to the poor predictability of the contact electrical performance.

SUMMARY OF THE INVENTION

The present invention provides in an embodiment a method for making one or more low resistivity contacts to iron pnictide superconductors, such as for purposes of illustration and not limitation, those based on doped AFe₂As₂, where A can be Ca, Sr, Ba, Eu, or combinations thereof, as well as the electrical contact produced. The electrical contact is characterized by low surface contact resistivity suitable for use in both low and high current applications.

In an illustrative embodiment of the invention, an electrical conductor is connected to a surface of an iron pnictide superconductor using tin (Sn) or a tin-based material, such as a tin solder or tin-based solder, to provide a low resistivity electrical contact. The tin-based material can comprise a tin-silver alloy, a tin-copper alloy, a tin-lead alloy, or a ternary tin-silver-copper alloy. For purposes of illustration and not limitation, a low contact surface resistivity on the order of 10⁻⁹ ohm-cm² can be provided by practice of the invention.

Other advantages of the present invention will become apparent from the following drawings taken with the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of temperature dependence of resistance measured using two different contact configurations. In a usual four-probe scheme current was flowing between contacts 1 and 4, and potential drop was measured between contacts 2 and 3, in a 2-probe scheme current was flowing between contacts 2a and 3a, and voltage was measured between contacts 2 and 3.

FIG. 2 is a graph of voltage measured between contacts 2 and 3 (see inset in FIG. 1) as a function of current passing between contacts 2a and 3a. Above T, the curves are linear, reflecting Ohm's law. Below T_c the curves reveal broad plateau extending up to the critical current I_c.

FIG. 3 is a zoom (enlarged) view of the V-I curve measured in the 2-probe configuration, of the data of FIG. 2. Due to the contact resistance, voltage measured in the 2-probe scheme, is finite and obeys Ohm's laws with contact resistance R=8 μΩ, while the 4-probe measurements detect zero within experimental scatter value.

FIGS. 4 a and 4b are photographs showing experimental set-up used in contact resistance measurements where current was fed through either the current contacts at the ends of the superconductor sample (FIG. 4a) or potential contacts (FIG. 4b). The areas of the two contacts are A₁ and A₂: Since contacts are series-connected, measured R_c=R₁+R₂. With R₁=ρ□/A₁ and R2=ρ□/A₂, it is not difficult to show that ρ□=A₁A₂/(A₁+A₂) where ρ□ is contact surface area resistivity.

DETAILED DESCRIPTION OF THE INVENTION

An illustrative embodiment of the invention provides a method for making one or more low resistivity electrical contacts to iron pnictide superconductors which are represented by general formula (A_1-xD1_x)(Fe_1-yD2_y)₂(As_1-zD3_z)₂ wherein A comprises an alkali earth element selected from the group consisting of Ca, Sr, Ba, Eu, and combinations thereof and x is 0 to 1, y is 0 to 0.4 where y can be 0 to 0.2 in some particular embodiments, and z is 0 to 0.6. The element (e.g. dopant) D1 can be selected from the group consisting of Na, K, Rb, Cs and combinations thereof and can be present singly or collectively (if more than one is present) in an amount up to 100 atomic % on the alkali earth site of the superconductor material. Dopant D2 can be selected from the group consisting of Co, Ni, Pd, Rh, Ru, Pt and combinations thereof and can be present singly or collectively (if more than one is present) in an amount up to 20 atomic % on the iron site of the superconductor material. The element (dopant) D3 can be selected from the group consisting of P, Te, S, Se, Sb, Bi and combinations thereof and can be present singly or collectively (if more than one is present) in an amount up to 60 atomic % on the pnictide site of the superconductor material. The contact can be made from a tin-based alloy having a majority (greater than 50% by weight) of tin and that can include a minor amount of an element selected from the group consisting of Bi, Al, In, Pb, Ag, Au, Cu, Cd, Sb, and Zn or their combinations to improve the mechanical or soldering properties of the electrical contact and to improve a joint to the metallic conductor.

In a particular illustrative embodiment of the invention, the superconductor is represented by doped AFe₂As₂ with the general formula (A_1-xD1_x)(Fe_1-yD2_y)₂(As_1-zD3_z)₂, where A can be selected from the group consisting of Ca, Sr, Ba and Eu and combinations thereof, as well as the electrical contact produced, wherein an electrical contact is produced having low surface contact resistivity suitable for use in both low and high current applications.

The electrical conductor comprises a metallic electrical conductor which can include, but is not limited to, a metallic wire, metallic strip, metallic strap or other shape of an electrical conductor. The electrical conductor can include, but is not limited to, metallic conductors comprising Ag, Ag alloys, Cu, Cu alloys, Au, and Au alloys, and Al and Al alloys.

Pursuant to practice of the invention, the electrical conductor and the superconductor are electrically connected using tin (Sn) or a tin-based material, such as tin solder or tin-based solder wherein a tin-based solder material comprises a majority (i.e. 50% or more by weight) of tin in its composition. Ultrahigh purity tin solder comprising 99.999% by weight tin can be used in practice of the invention to provide a low resistivity electrical contact. Also, tin-based materials comprising an alloy of tin and lead, an alloy of tin and silver, and an alloy of tin and copper, and ternary alloys can be used in practice of the invention to provide a low resistivity electrical contact. For purposes of illustration and not limitation, a Sn—Ag solder having 3.5 weight % Ag and balance Sn can be used in practice of the invention. Also, a Sn—Pb solder having 37 weight % Pb and balance Sn can be used in practice of the invention. An illustrative ternary Sn—Ag—Cu solder alloy is described below.

In practice of the invention wherein the electrical conductor and a surface of the superconductor are connected by a tin or tin-based solder, a suitable soldering flux can be used which will permit a low resistivity electrical contact to be made. Suitable solder fluxes include, but are not limited to, commercially available Castolin 157 flux, Indalloy 2 flux and HCl.

The following examples are offered to illustrate, but not limit, embodiments of the present invention. In particular, electrical contacts to doped AFe₂As₂ were made using soldering techniques. A number of different solder alloys and fluxes summarized in Table 1 were tested and the measured surface resistivity ρ□ of the contacts (given as the contact resistance R divided by the contact area) are set forth in Table 1.

FIG. 1 shows the temperature dependence of electrical resistance measured on the sample of Ba(Fe_0.926Co_0.074)₂As₂, in which superconductivity with T_c=23K was induced by partial substitution of Fe by Co. The contacts were made by soldering silver wires to the clean surface of the Ba(Fe_0.926Co_0.074)₂As₂ single crystal with the help of high purity (99.99%) tin solder. Contact resistance was measured in two ways. First, a 4-probe configuration (FIG. 1) was used, with current flowing between contacts 1-4 and potential drop measured between contacts 2-3. Second, current was flowing through the contacts 2a and 3a, while potential was measured between contacts 2 and 3. The difference between these two measurements is caused by the fact that in the 2-probe measurement, potential difference includes voltage drop in the contact due to finite contact resistance. As can be seen, the two measurements give almost identical results, beyond the resolution of this experiment, set by a slight difference of current distribution in two sets of measurements. The contact resistance is shown to be definitely below 100 μΩ. However, because of its very small value it was difficult to measure it precisely in this geometry.

To obtain a better measurement of contact resistance, V-I (voltage vs. current) characteristics at temperatures below Tc of the compound were measured. Because of the zero resistance of the superconductor sample, finite voltage is generated when making measurements in two-probe technique, due to the resistance of the two contacts. In FIG. 2 a set of V-I curves taken in 2-probe configuration in isothermal conditions is shown. At temperatures above T_c, the curves are perfectly linear, as expected from Ohms law. Below T_c the curves reveal nonlinearity due to approach of the critical current density J_c. Since critical current density increases on cooling, at 20.1 K (2K below Tc), the measurement setup is not able to supply high enough current to reach J_c. Zoom of the curve at 20.1 K, FIG. 3, shows that actually the measured 2-probe resistance, though small, causes linear V-I curve with finite slope (corresponding to the contact resistance R_c=8 μΩ), as opposed to zero resistance as detected in four-probe measurements. This is due to the resistance of two contacts at the sample ends.

In FIG. 4a, 4b, photographs of two superconductor samples measured in two-probe measurements between either outer contact pair (FIG. 4a) or central contact pair (FIG. 4b). Since two contacts are series connected, measured resistance R_c is the sum of two contact resistances, R₁ and R₂. Taking contact areas of contacts 1 and 2 as A₁ and A₂, we come to the following relation: ρ□=R_cA₁A₂/(A₁+A₂). The experimentally determined values for different contact preparation techniques are summarized in Table 1 where “wire” is the electrical conductor and “alloy” is the solder alloy.

As can be seen in Table 1, use of pure tin solder and of tin alloy solders with Ag, Cu, Ag—Cu and Pb give contact resistivities in the nanoΩ-cm² range (10⁻⁹ Ω-cm²), comparable to the best examples known for other superconductors, as discussed below.

Sample Preparation for Table 1 Examples

Single crystals of Ba(Fe_1-yCo_y)₂As₂ were grown out of self flux using conventional high temperature solution growth techniques described in N. Ni et al. Phys. Rev. B 78, 214515 (2008), which is incorporated herein by reference. Small Ba chunks, FeAs powder and CoAs powder were mixed together according to the ratio Ba:FeAs:CoAs=1:4y(1−y):4y. Crystal dimensions can go up to 12*8*1 mm³.

Sample Characterization:

Elemental analysis of the samples was performed using wavelength dispersive x-ray spectroscopy (WDS) in the electron probe microanalyzer of a JEOL JXA-8200 Superprobe. According to elemental analysis these samples have y=0.074, while load during sample growth was y=0.10. They are characterized by T_c=22.5K.

Critical current measurements and resistivity measurements were performed using a Quantum, Design (QD) Physical Property Measurement System (PPMS).

Samples were cleaved into rectangular prisms, as shown in FIG. 4a, 4b. Four contacts were arranged in a usual four-probe contact configuration. Four contacts to the samples were made using silver wires attached to the sample using different combinations of soldering alloys and fluxes. In all cases to achieve reproducible results, the contacts were put on a fresh cleaved surface of single crystals of Ba(Fe_1-yCo_y)₂As₂. Contacts with reasonably low surface resistivity could be made with several soldering alloys based on Sn or ultrahigh purity Sn.

TABLE 1
Characteristics of the electrical contacts to doped AFe2As2 as a function of
preparation conditions.
No	Dopant				Rc, 20
Compound	type,				K	A1	A2	ρ□
	A	x or y	Wire	Alloy	Flux	Ω	cm2	cm2	Ω-cm2	Reference
I	Ba	Co,	Ag	Sn	Castolin	1.76*	1.73*	1.9*	1.59*	s9151
		0.074		99.999%	157	10−6	10−3	10−3	10−9
2	Ba	Co,	Ag	Silver	No	0.404	2.76*	2.49*	5.24*	s9148
		0.074		epoxy			10−3	10−3	10−4
3		Co,	Ag	Au	Ag	0.1	3*	3.23*	5.24*	s005
		0.074		Evaporation	paint		10−3	10−3	10−4
4	Ba	Co,	Ag	Indalloy 3	Castoline	7.27*	9.9*	9.8*	4.58*	s9142
		0.074			157	10−6	10−4	10−4	10−9
				In-10%
				Ag
5	Ba	Co,	Ag	Indalloy 3	Indalloy 1	2.41*	1.23*	4.92*	2.37*	s9143
		0.074				10−s4	10−3	10−4	10−8
				In-10%
				Ag
6	Ba	Co,	Ag	Indalloy 3	HCl	4.76*	2.47*	3.95*	7.27*	s9143b
		0.074			Concentrated	10−4	10−4	10−4	10−8
				In-10%
				Ag
7	Ba	Co,	Ag	Indalloy 3	Indalloy 2	Bad				s9144
		0.074
				In-10%
				Ag
8	Ba	Co,	Ag	Indalloy 3	Indalloy 3	Bad
		0.074
				In-10%
				Ag
9	Ba	Co,	Ag	Indalloy 3	Indalloy 4	Bad				s9143d
		0.074
				In-10%
				Ag
10	Ba	Co,	Ag	Indalloy 3	Indalloy	Bad				s9143c
		0.074			5A
				In-10%
				Ag
11	Ba	Co,	Ag	Indalloy	Indalloy 2	7.05*	1.03*	1.19*	3.89*	s9145
		0.074		121		10−6	10−3	10−3	10−9
				Sn-Ag
12	Ba	Co,	Ag	Indalloy	Castolin	7.73*	8.7*	1.30*	4.03*	s9149
		0.074		121	157	10−6	10−4	10−3	10−9
				Sn-Ag
13	Ba	Co,	Ag	Sn 63%	Castolin	8.1*	2.6*	3.41*	1.19*	s9146
		0.074		Pb 37%	157	10−6	10−3	10−3	10−8s
14	Ba	Co,	Ag	Sn 63%	Indalloy 2	9.98*	1.7*	1.76*	8.69*	s9147
		0.074		Pb 37%		10−6	10−3	10−3	10−9
15	Ba	K,	Ag	Indalloy	Castolin	9.93*	5.1*	6.4*	2.81*	s9150
		0.30		121	157	10−5	10−4	10−4	10−8
16	Ba	Ni,	Ag	Indalloy	Castolin	4.23*	2.19*	4.03*	6*10−7	s9153
		0.05		121	157	10−4	10−3	10−3
17	Ba	Co,	Ag	In	Castolin	1.85*	1.19*	1.19*	1.1*	s9152
		0.074		99.99%	157	10−4	10−3	10−3	10−7
18	Ba	Co,	Ag	Sn 97.5%	Castolin				5.7*	Average
		0.074		Al 2.5%	157				10−9	3 samples
19	Ba	Co,	Ag	Sn 57%	Castolin				7.2*	Average
		0.074		Bi 43%	157				10−8	3 samples
20	Ba	Co,	Ag	Sn 99%	Castolin				3.1*	Average 3
		0.074		Cu 1%	157				10−9	samples
21	Ba	Co,	Ag	Sn 99%	Castolin				1.58*	S10571
		0.074		Dy 1%	157				10−8
22	Ba	Co,	Ag	Sn 99%	Castolin				5.8*	Average 2
		0.074		Ti 1%	157				10−9	samples
23	Ba	Co,	Ag	Sn 85%	Castoline				10−7	Slightly
		0.074		Zn 15%	157					rectifying
										contacts
24	Ba	Co,	Ag	Ag 4.7%					1.4*	Average 3
		0.074		Cu 1.7%					10−9	samples
				Sn-
				93.6%
				ternary
				alloy

In Table 1, “reference” is sample number and % is weight % for the alloys under Alloy.

The contact resistance thus obtained was in a range of several μΩ and surface resistivity of 1 to 10*10⁻⁹ Ω-cm².

Example 1 of Table 1

Pure 99.999% tin was used as solder pursuant to the invention and Castoline 157 eutectic flux was used as a flux. Sample surface was covered with acidic flux and soldering was performed by heating sample to a melting point of the Sn. The melt showed very good wetting of the surface and the contact resistivity of the contacts was 1.59*10⁻⁹ Ω-cm².

Example 11 of Table 1

A commercially available Sn—Ag solder alloy (Indalloy # 121) was used pursuant to the invention in combination with Indalloy #2 flux. The alloy was heated to a temperature between liquidus and solidus lines of the composition. The contact resistivity was 3.89*10⁻⁹ Ω-cm².

Example 12 of Table 1

A commercially available Sn—Ag solder alloy (Indalloy # 121) was used pursuant to the invention in combination with Castolin 157 flux. The alloy was heated to a temperature between liquidus and solidus lines of the composition. The contact resistivity was 4.03*10⁻⁹.

Example 20 of Table 1

A eutectic Sn—Cu alloy was used pursuant to the invention in combination with Castolin 157 flux. The alloy was heated to a temperature between liquidus and solidus lines of the composition. The contact resistivity was 3.1*10⁻⁹ Ω-cm².

Example 24 of Table 1

A patented eutectic Sn—Cu—Ag alloy (Ag-4.7 wt %-Cu-1.7 wt %-Sn-93.6 wt % Sn) was used pursuant to the invention in combination with Castolin 157 flux. The ternary alloy was heated to a temperature between liquidus and solidus lines of the composition. The contact resistivity was 1.4*10⁻⁹ Ω-cm².

Other commercially available tin-based solder alloys known as Sn63Pb37 (37 weight % Pb and balance Sn) were used pursuant to the invention and produced results which were not too different from using pure tin solder (see Examples 14 and 15 of Table 1).

Example 15 of Table 1

To test if the soldering technique works with compounds in which superconductivity is induced by different dopants, contact resistance of the contacts made following procedures of Example 12 on samples of (Ba_1-xK_x)Fe₂As₂ were tested pursuant to the invention. The results are shown in Table 1 as Example 15. Contact resistivity was 2.81*10⁻⁹ Ω-cm². In part this value can be determined by the difference in the initial status of the surface, which was not cleaved but rather degraded due to exposure to air.

In another doping example, samples of Ba(Fe_1-yNi_y)₂As₂ were soldered pursuant to the invention following procedure of Example 12. Contact resistivity was determined as 6*10⁻⁹ Ω-cm².

The Examples shown above illustrate that tin and tin-based alloys provide very low surface resistivity contacts to the superconductors of the (A_1-xD1_x)(Fe_1-yD2_y)₂(As_1-zD3_z)₂ type described above.

Although the present invention has been described in connection with certain illustrative embodiments, those skilled in the art will appreciate that changes and modifications can be made therein within the scope of the invention as set forth in the appended claims.

Claims

1. A method of making an electrical connection between a metallic electrical conductor and an iron pnictide superconductor, comprising connecting the electrical conductor and superconductor using a tin or tin-based material disposed therebetween.

2. The method of claim 1 wherein the tin or tin-based material is a solder that is solidified between the electrical conductor and the superconductor.

3. The method of claim 1 wherein the superconductor is represented by (A1-xD1x)(Fe1-yD2y)2(As1-zD3z)2 wherein A comprises an alkali earth element selected from the group consisting of Ca, Sr, Ba, Eu, and combinations thereof and x is 0 to 1, y is 0 to 0.4, and z is 0 to 0.6 and wherein D1 is selected from the group consisting of Na, K, Rb, Cs and combinations thereof, D2 is selected from the group consisting of Co, Ni, Pd, Rh, Ru, Pt and combinations thereof, and D3 is selected from the group consisting of P, Te, S, Se, Sb, Bi and combinations thereof.

4. The method of claim 3 wherein y is 0 to 0.2.

5. The method of claim 1 wherein the tin or tin-based material comprises ultrahigh purity tin that is 99.999% by weight tin.

6. The method of claim 1 wherein the tin or tin-based material comprises an alloy of tin and lead, an alloy of tin and copper, an alloy of tin and silver, or a ternary alloy of tin-silver-copper.

7. An electrical connection between a metallic electrical conductor and an iron pnictide superconductor wherein the connection comprises a tin or tin-based material disposed between the electrical conductor and the superconductor.

8. The connection of claim 7 wherein the material comprises a solder that is solidified between the electrical conductor and the superconductor.

9. The connection of claim 7 wherein the superconductor is represented by (A1-xD1x)(Fe1-yD2y)2(As1-zD3z)2 wherein A comprises an alkali earth element selected from the group consisting of Ca, Sr, Ba, Eu, and combinations thereof and x is 0 to 1, y is 0 to 0.4, and z is 0 to 0.6 and wherein D1 is selected from the group consisting of Na, K, Rb, Cs and combinations thereof, D2 is selected from the group consisting of Co, Ni, Pd, Rh, Ru, Pt and combinations thereof, and D3 is selected from the group consisting of P, Te, S, Se, Sb, Bi and combinations thereof.

10. The connection of claim 9 wherein y is 0 to 0.2.

11. The connection of claim 7 wherein the material comprises ultrahigh purity tin that is 99.999% by weight tin.

12. The connection of claim 7 wherein the material comprises an alloy of tin and lead.

13. The connection of claim 7 wherein the material comprises an alloy of tin and silver.

14. The connection of claim 7 wherein the material comprises an alloy of tin and copper.

15. The connection of claim 7 wherein the material comprises an alloy of tin, copper and silver.

16. The connection of claim 7 wherein the material comprises a tin-based alloy that includes a minor amount of an element selected from the group consisting of Bi, Al, In, Pb, Ag, Au, Cu, Cd, Sb, and Zn.