STRENGTH

CROSS-REFERENCE TO RELATED APPLICATIONS This invention claims priority to U.S. Provisional Patent Application No. 62/1 94,580 filed on July 20, 2015 and entitled, "Cryogenic Gas Mixtures with High Dielectric Strength", the contents of which are hereby incorporated by reference in its entirety.

GOVERNMENT SUPPORT STATEMENT

This invention was made with Government support under contract Nos. N00014-14-1 -0346 and N00014-14-1 -0377 awarded by the Office of Naval Research. The government has certain rights in the invention.

FIELD OF THE INVENTION

This invention relates to superconductors and cryogenic operating conditions. More specifically, the present invention provides low temperature gases and mixtures for use in high temperature superconductor and other systems that operate at cryogenic temperatures.

BACKGROUND OF THE INVENTION

Typical superconductors, or low temperature superconductors, must be cooled to 25 K (-243.15 "€) or below to achieve superconducting state . By comparison, high temperature superconductors achieve superconducting state above this value. Current materials have been observed at up to 138.15K (-135 "€), and search is on for new materials that show superconductivity at even higher temperatures. Specific compositions of material have been found to show high temperature superconducting properties, such as oxygen-doped copper compounds (cuprates) and some iron compounds. Similarly, some compositions show high temperature superconductivity when subjected to increased pressure, like hydrogen sulfide. Table 1 . List of superconductors and transition temperature for superconduction.

203 -70 H ₂ S (150 GPa)

133 -140 HgBa ₂ Ca ₂ Cu30x (HBCCO)

1 1 0 -163 Bi2Sr2Ca ₂ Cu ₃ Oio (BSCCO)

93 -180 YBa ₂ Cu ₃ 0 ₇ (YBCO)

55 -218 SmFeAs

41 -232 CeFeAs

39 -234 MgB ₂

26 -247 LaFeAs

18 -255 NbaSn

10 -263 NbTi

9.2 -263.8 Nb

4.2 -268.8 Hg

High temperature superconductor-based power cables have been used in power transmission (Maguire, et al., Progress and status of a 2G HTS power cable to be installed in the long island power authority (LIPA) grid. IEEE Trans. Appl. Supercond. 201 1 Jun; 21 (3): 961 -66; Demko, et al., Triaxial HTS Cable for the AEP Bixby Project. IEEE Trans. Appl. Supercond. 2007 Jun; 17(2): 2047-50) and are expected to enable electric power distribution of unprecedented power density as required on future all-electric vehicles, such as vessels. Low temperature, i.e. cryogenic, fluids are used to lower the temperature of the superconductor material to allow for superconduction. In many instances, liquid nitrogen is used, which was found to also provide suitable dielectric insulation for the power cable. However, shipboard applications prohibit the use of liquid nitrogen (LN2) since there exists a risk for asphyxiation hazards in case of a liquid nitrogen leak. The US Navy considers helium gas (gHe) as a viable option to cool HTS power cables without the risk for asphyxiation hazards since the gas inventory is smaller by several orders of magnitude and helium does not accumulate in the lower sections of the ship. Cryogenically cooled helium has been used in some high temperature superconductor applications, especially where the safety of liquid nitrogen is a concern. Cooling HTS cable with gHe has a number of additional advantages. Most importantly, it allows a wide temperature range including temperatures at which liquid nitrogen solidifies. The temperature range below the liquid nitrogen range increases the critical current in the individual HTS tapes and therefore the power density of the HTS cable. Another benefit is the tunability, allowing certain interconnects to operate temporarily at lower than nominal temperature during times of high demand. However, helium is a poor dielectric, and therefore has limited applications in power transmission.

Past efforts of cooling HTS power cables by gaseous cryogens focused exclusively on pure helium gas (Pamidi et al. , Installation and Testing of Helium Gas Cooled Superconducting DC Cable at FSU-CAPS. 1 1 th EPRI Superconductivity Conference (Houston, USA) 28-30 October 2013). An investigation was undertaken to determine the benefits of mixing small amounts of hydrogen or neon gas to helium to mitigate the limited dielectric strength of pure helium gas. The authors expected that this could to potentially improve dielectric characteristics while maintaining the thermal, non-flammable and non-corrosive properties of pure helium gas.

From the dielectric point of view, hydrogen gas is far superior to helium and neon. Hydrogen gas at room temperature has approximately 50% of the dielectric strength of nitrogen gas at room temperature (Vijh, Electric strength and molecular properties of gaseous dielectrics. IEEE Trans. Electr. Insul. 1977 Aug;12(4): 313-315). The noble gases helium and neon at room temperature exhibit a dielectric strength 15% and 25% respectively that of nitrogen gas at room temperature. It is known from other gas mixtures studies at room temperature that even a small amount of a superior gas can considerably improve the dielectric strength of the mixture more than the ratio would suggest. Typical examples for such investigations is the addition of small fractions of SF6 into N2 (Malik & Qureshi, A Review of Electrical Breakdown in Mixtures of SF ₆ and Other Gases. IEEE Trans. Electr. Insul. 1979 Feb; 14(1 ): 1 -13) or CO2 to a SF6-N2 mixture (Ohtsuka, et al., Effect of mixture of a small amount of CO2 in SF6/N2 mixed gas on the insulation performance under nonuniform field. IEEE Int'l Sympos Electr Insul. (Anaheim, USA 2-5 April 2000); p 288-291 ). Theoretical models to explain and predict the dielectric strength of gas mixtures are currently being developed (Zhao, et al., Prediction of the critical reduced electric field strength for carbon dioxide and its mixtures with 50% O2 and 50% H2 from Boltzmann analysis for gas temperatures up to 3500K at atmospheric pressure. J. Phys. D: Appl. Phys. 2014 Aug 13; 47(32):325203).

The challenges with pure helium's low breakdown voltage unfortunately limit its use. Accordingly, there is an unmet need in the field for compositions of gases that are both safe and possess high dielectric properties, enabling their use in high temperature superconductor applications. SUMMARY OF THE INVENTION

Past efforts of cooling superconducting devices by gaseous cryogens focused exclusively on helium. The limited dielectric strength of helium gas created a need for alternatives that could be used in the cryogenic temperature range (for example, but not limited to, 4-80 K). Gas mixtures containing helium with small amounts of hydrogen and/or neon gas to mitigate the limited dielectric strength of pure helium gas were examined, to determine if the gas mixtures could improve dielectric characteristics while maintaining the thermal properties of pure helium gas.

Thus a cryogenic gaseous mixture was prepared using a mixture of helium and hydrogen, wherein the gaseous mixture is up to 1 0 mol% hydrogen and up to 99 mol% helium. Exemplary gas mixtures include 1 mol% hydrogen/ 99 mol% helium, 2 mol% hydrogen/ 98 mol% helium, 3 mol% hydrogen/ 97 mol% helium, and 4 mol% hydrogen/ 96 mol% helium. Optionally, one or more other noble gases are added to the gaseous mixture at up to 4 mol%.

A limit of 4 mol% of hydrogen preferably ensures that the gas mixture is non-flammable in air, however improvement in the dielectric strength was found to be linear. Therefore, adding hydrogen at concentrations of 0.5 mol% to 1 0 mol% is effective in increasing dielectric properties of helium. Nonlimiting examples of concentrations of hydrogen include 0.5 mol%, 0.6 mol%, 0.7 mol%, 0.75 mol%, 0.8 mol%, 0.9 mol%, 1 .0 mol%, 1 .1 mol%, 1 .2 mol%, 1 .25 mol%, 1 .3 mol%, 1 .4 mol%, 1 .5 mol%, 1 .6 mol%, 1 .7 mol%, 1 .75 mol%, 1 .8 mol%, 1 .9 mol%, 2.0 mol%, 2.1 mol%, 2.2 mol%, 2.25 mol%, 2.3 mol%, 2.4 mol%, 2.5 mol%, 2.6 mol%, 2.7 mol%, 2.75 mol%, 2.8 mol%, 2.9 mol%, 3.0 mol%, 3.1 mol%, 3.2 mol%, 3.25 mol%, 3.3 mol%, 3.4 mol%, 3.5 mol%, 3.6 mol%, 3.7 mol%, 3.8 mol%, 3.9 mol%, 4.0 mol%, 5.0 mol%, 6.0 mol%, 7.0 mol%, 8.0 mol%, 9.0 mol%, 1 0.0 mol%, 1 1 mol%, 12 mol%, 13 mol%, 14 mol%, 15 mol%, 1 6 mol%, 1 7 mol%, 18 mol%, 1 9 mol%, and 20 mol%.

In some variations of the invention, one or more noble gases is added to the gaseous mixture. While the use of these noble gases does not improve dielectric properties, it was found that the gases do not significantly degrade dielectric strength. Further, the gases may be added to improve other properties of the gaseous mixture, such as heat transfer or gas detection, i.e. permitting detection of leaks in a HTS cable. Examples of useful gases include neon, argon, nitrogen, oxygen, methane and combinations thereof. These gases are added up to helium gas (balance) when employed. Examples include 0.1 mol%, 0.2 mol%, 0.3 mol%, 0.4 mol%, 0.5 mol%, 0.6 mol%, 0.7 mol%, 0.75 mol%, 0.8 mol%, 0.9 mol%, 1 .0 mol%, 1 .1 mol%, 1 .2 mol%, 1 .25 mol%, 1 .3 mol%, 1 .4 mol%, 1 .5 mol%, 1 .6 mol%, 1 .7 mol%, 1 .75 mol%, 1 .8 mol%, 1 .9 mol%, 2.0 mol%, 2.1 mol%, 2.2 mol%, 2.25 mol%, 2.3 mol%, 2.4 mol%, 2.5 mol%, 2.6 mol%, 2.7 mol%, 2.75 mol%, 2.8 mol%, 2.9 mol%, 3.0 mol%, 3.1 mol%, 3.2 mol%, 3.25 mol%, 3.3 mol%, 3.4 mol%, 3.5 mol%, 3.6 mol%, 3.7 mol%, 3.8 mol%, 3.9 mol%, and 4.0 mol%. For combinations of the gases, the combined mixture- excluding helium- is added at up to 4 mol%, as provided above.

The gaseous mixture is provided at a temperature of about 20 K to about 100 K. Useful temperatures include 20 K, 25 K, 30 K, 35 K, 37.5 K, 40 K, 42.5 K, 45 K, 47.5 K, 50 K, 52.5 K, 55 K, 57.5 K, 60 K, 62.5 K, 65 K, 67.5 K, 70 K, 72.5 K, 75 K, 773.5 K, 80 K, 85 K, 90 K, 95 K, 100 K, 1 05 K, 1 1 0 K, 1 15 K, and 120 K. In certain variations of the invention, the gaseous mixture is at a temperature of about 40 K to about 75 K.

Optionally, the gas is used at a predetermined pressure. In some variations of the invention, the gas is at between 0.5 MPa and 5.0 MPa. Nonlimiting examples include 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, 0.9 MPa, 1 .0 MPa, 1 .1 MPa, 1 .2 MPa, 1 .3 MPa, 1 .4 MPa, 1 .5 MPa, 1 .6 MPa, 1 .7 MPa, 1 .8 MPa, 1 .9 MPa, 2.0 MPa, 2.5 MPa, 3.0 MPa, 3.5 MPa, 4.0 MPa, 4.5 MPa, and 5.0 MPa.

The dielectric properties of the gaseous mixtures were studied at high temperature superconductor temperature ranges. In the range of 4K to 80 K, and at the predetermined pressure, the AC breakdown voltage of hydrogen-helium gas mixtures and neon-helium mixtures was determined, and compared to that of pure helium and pure neon. Breakdown experiments show that adding 4 mol% hydrogen by volume to helium increases the dielectric strength by approximately 80% while the mixture is still considered non-flammable at any ratio with air. Accordingly, a method is provided for insulating superconductors, such as high temperature superconductors. The system can also be used in cooling superconductors. Thus, the gaseous mixture allows for superconducting applications including, but not limited to, high temperature superconducting devices, high temperature superconducting power cables, and high power superconducting power generation systems in general. In addition, the present invention may be used in the cooling systems of non-power applications such as magnetic resonance imaging (MRI) and other medical applications. An enclosure, such as a cable coating, is provided which surrounds the high temperature superconductor. A gaseous mixture, as described above, is provided. Filling the enclosure with the gaseous mixture allows for cooling of the high temperature superconductor, as well as providing insulation to the high temperature superconductor via the dielectric properties of the gaseous mixture. The enclosure is optionally sealed to prevent the gaseous mixture from escaping from the enclosure. Alternatively, the enclosure is attached to one or more devices, sealing the gas inside the enclosure. The gaseous mixture is optionally cooled to high temperature superconductor temperatures, as provided above. Further, the gaseous mixture is optionally pressurizing, such as to about 0.5 MPa to about 5.0 MPa. Particularly useful pressures are provided above.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the invention, reference should be made to the following detailed description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic of the experimental setup to characterize cryogenic gas mixtures. The electrode system with uniform field in the gap between the electrodes.

FIG. 2 is a graph showing AC breakdown voltage as a function of pressure at a 2-mm gap distance in homogeneous field for helium (black circle), neon (gray square), 4 mol% hydrogen in helium mixture (dark gray diamond), and 4 mol% neon in helium mixture (light gray triangle). Error bars show the range of voltages recorded (maximum and minimum) at each pressure level for each gas mixture.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part thereof, and within which are shown by way of illustration specific embodiments by which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the invention.

As used herein, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a gas" includes a mixture of two or more gases and the like.

As used herein, "about" means approximately or nearly and in the context of a numerical value or range set forth means ±15% of the numerical.

As used herein, "gas" means a fluid material composed of one or more constituents which readily occupies a container and exhibits air-like properties at room temperature, i.e. are compressible. Gases include hydrogen, nitrogen, oxygen, methane, and some members of Group 17 (halogens) and 18 (noble gases) of the periodic table.

As used herein, "mixture" means a composition of two or more different substances that are mixed but are not combined chemically, and thus are capable of being separated. As used herein, "high temperature superconductor" means a material that exhibit zero electrical resistance and expulsion of magnetic flux fields, i.e. superconduct, at above 30K (-243.15 ^< €).

Example 1

Custom gas mixes comprising hydrogen, neon, and helium were obtained (Airgas, Inc., Radnor, PA). From the dielectric point of view, hydrogen gas is far superior to helium and neon. Hydrogen gas has approximately 50% of the dielectric strength of nitrogen gas at room temperature (Vijh, Electric strength and molecular properties of gaseous dielectrics. IEEE Trans. Electr. Insul. 1977 Aug; 12(4): 313-315). The noble gases helium and neon exhibit a dielectric strength 15% and 25%, respectively, of nitrogen gas at room temperature. Compositions were prepared for 99 mol% helium and 1 mol% hydrogen, 98 mol% helium and 2 mol% hydrogen, 97 mol% helium and 3 mol% hydrogen, and 96 mol% helium and 4 mol% hydrogen. In some instances, a container of a known volume of helium has a known volume of hydrogen added to form the composition discussed above. The improvement in the dielectric strength was found to follow a linear relationship, and thus improvements in dielectric properties are to be seen up to 10 mol% hydrogen. However, up to 4 mol% hydrogen is nonflammable in these compositions, making the gas composition safe.

The operating temperature of these gases is generally 20 K to 100 K, and typically between 40 and 75 K, for HTS applications.

Example 2 The dielectric properties of the gases prepared in Example 1 were examined by completing alternating current (AC) breakdown tests using an electrode system, as seen in FIG. 1 , at various pressure levels while at cryogenic temperatures, i.e. at 77 K.

Breakdown test apparatus 1 was formed of high pressure vessel 5 fitted with electrode system 10 in cryostat 2. Electrode system 10 is composed of electrodes 12 mounted onto G10 insulators 15. The electrodes have a profile similar to a Bruce profile, are highly polished, and consist of 31 6 stainless steel. The gap distance was adjustable, however was kept at 2 mm as this appeared to be a good trade-off between voltage resolution and upper voltage limitations of the setup. The lower electrode is grounded to a pose via ground 16, while the upper electrode is in electrical communication with a voltage source, such as an AC source operating at 60 Hz. The voltage source in connected through high voltage bushings 3. A gas injection and vacuum line is in communication with the interior of high pressure vessel 5. Liquid nitrogen 20 is filled into the cryostat to provide cryogenic temperatures. Test gas 20 is added to the interior of high pressure vessel 5 for testing, using the gas injection port.

The AC breakdown tests were performed by determining the voltage required to arc across a pair of electrodes installed inside a pressure vessel, filled with the various gas mixtures and immersed in a cryostat filled with liquid nitrogen. The electrodes were not changed throughout the duration of the experiment, i.e. maintained the 2 mm gap, to ensure that the results obtained from the gas mixtures were comparable. On completion of the experiment, the gap distance between the electrodes was re-measured and confirmed to be 2 mm. This shows that the electrodes did not become loose while performing the experiments at cryogenic temperatures. However, the distance between the electrodes is expected to be slightly different from 2 mm due to the thermal contraction of the materials at 77 K. The electrodes as well as all the other parts inside the pressure vessel were thoroughly cleaned by isopropyl alcohol before closing the vessel.

Before the beginning of the experiments, the pressure vessel was evacuated to 1 0-4 mbar or lower for at least 12 hours using a turbomolecular pump. The vessel was subsequently flushed twice with industrial grade helium gas to ensure all contaminants were removed before being filled with gas samples were injected into a pressure vessel of an electrode system, schematized in FIG. 1 .

Initial testing used research grade helium gas of 99.999% purity. The pressure vessel was immersed in a cryostat filled with liquid nitrogen, and the pressure inside the pressure vessel was adjusted to 2.0 MPa. The AC breakdown tests were not undertaken until equilibrium had been reached for the gas pressure, which ensured that the temperature of the helium gas and all components reached 77 K. Throughout the experiment liquid nitrogen was added to the cryostat to ensure that the whole pressure vessel including the bushing was immersed in liquid nitrogen. Initially, ten AC breakdown tests were performed to "season" the surfaces of the electrodes. It is a well-known effect that new electrodes tend to show lower breakdown voltages for the first few breakdown events. After seasoning the electrodes, fifteen AC breakdown tests were performed at 2.0 MPa, and the results recorded. The pressure within the vessel was reduced to 1 .5 MPa, then 1 .0 MPa and finally 0.5 MPa with fifteen AC breakdown tests being performed at each pressure level. Once the AC breakdown tests at 0.5 MPa had been completed, the pressure within the pressure vessel was released and a dry scroll vacuum pump was connected to remove the remaining helium gas from the vessel. The pressurized vessel was the flushed and the testing performed on a gaseous mixture of 96 mol% helium/4 mol% neon, pure neon and 96 mol% helium/4 mol% hydrogen. A lack of sufficient quantities of pure neon gas restricted the maximum operating pressure to 1 .8 MPa.

The result of the average AC breakdown voltage for the pure helium, pure neon, helium with 4 mol% neon and helium with 4 mol% hydrogen gas mixtures show neon to be the poorest dielectric material, as seen in FIG. 2. At the lowest pressure, neon exhibited an AC breakdown of about 2 kV, which increased with increasing pressure until 1 .5 MPa, at which point the breakdown voltage plateaued. A peak AC breakdown of about 5 kV was seen at 1 .8 MPa. Pure helium increased linearly from about 4 kV at 0.5 MPa to 15 kV a 2 MPa. The mixture of helium and neon (96 mol% He/ 4 mol% Ne) increased in a manner consistent with pure helium, and in fact overlapped the results seen with pure helium. A slight increase was seen at 1 .5 MPa, but was not statistically significant, as the results substantially overlapped. By contrast, the mixture of helium and hydrogen (96 mol% He/ 4 mol% H2) sharply increased from about 8 kV at 0.5 MPa to about 29 kV at 2 MPa, seen in FIG. 2.

On completion of the experiment, the electrodes were inspected by scanning electron microscopy (SEM) to check if the discharge activity created craters that might have locally enhanced the electric field, therefore resulting in a decrease of the breakdown voltage. While the craters were visible in the SEM images, they are of similar size as the machining marks (surface roughness) and thus expected not to play an essential role. The energy in the discharge pulses is very limited due to a high source impedance. Two distinct trends were observed when helium gas was mixed with neon and hydrogen. Firstly, mixing a small amount of a stronger dielectric gas with a weaker dielectric gas increases in AC breakdown strength. Adding 4 mol% hydrogen to helium, the AC breakdown voltage increased by approximately 80% for all pressure levels tested. Hydrogen is a diatomic gas with a dielectric strength far superior than those of noble gases. The second trend is the addition of a small amount of weaker dielectric gas being mixed with a stronger dielectric gas does not have any significant effect on AC breakdown voltage. The dielectric properties of neon are significantly lower than those of helium; however, the AC breakdown voltage for the pure helium and 4 mol% neon gas mixture is approximately identical with the pure helium gas for all pressure levels. From the viewpoint of dielectric properties, there does not appear to be a reason to add neon gas to the mixture. A small amount of (unintended) impurities does not have a negative effect on the dielectric strength. Thus, the addition of other noble gases can be added to improve other properties, such as improving heat transfer, or temperature insulation, or for increasing leak detection, as hydrogen is lighter than air thereby making it more difficult to detect leaks. In the preceding specification, all documents, acts, or information disclosed does not constitute an admission that the document, act, or information of any combination thereof was publicly available, known to the public, part of the general knowledge in the art, or was known to be relevant to solve any problem at the time of priority.

The disclosures of all publications cited above are expressly incorporated herein by reference, each in its entirety, to the same extent as if each were incorporated by reference individually.

While there has been described and illustrated specific embodiments of low temperature gases and mixtures for use in high temperature superconductor systems, it will be apparent to those skilled in the art that variations and modifications are possible without deviating from the broad spirit and principle of the present invention. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.