GB2578365A

GB2578365A - A magnesium diboride construction and a method for forming the same

Info

Publication number: GB2578365A
Application number: GB1912593.9A
Authority: GB
Inventors: John Howard Wort Christopher; Stewart Eardley Edwin
Original assignee: Element Six UK Ltd
Current assignee: Element Six UK Ltd
Priority date: 2018-09-04
Filing date: 2019-09-02
Publication date: 2020-05-06
Also published as: WO2020048931A1; GB201814370D0; GB201912593D0

Abstract

A method for forming a magnesium diboride construction is disclosed which includes providing a mass of grains of magnesium diboride, placing the grains into a canister to form a pre-sinter assembly and sealing the canister. The pre-sinter assembly is then treated at an ultra-high pressure of 3GPa or greater and a temperature or between 800ºC to 1500ºC to sinter the magnesium diboride to form a polycrystalline construction. The construction has at least 95 weight% magnesium diboride and a superconducting transition temperature Tc of greater than 36K, and a critical flux density of greater than 6 x 108 A/m2 as measured at 27K. Also disclosed is a superconducting magnet, such as for an NMR imaging device, formed from the magnesium diboride construction.

Description

A MAGNESIUM DIBORIDE CONSTRUCTION AND A METHOD FOR FORMING THE

SAME

Field

This disclosure relates to a magnesium diboride construction formed of polycrystalline magnesium diboride material, a method for making the same and tools comprising the same, particularly but not exclusively for use as magnesium diboride based superconductors, for example those finding application in power transmission, high field magnets, nuclear magnetic resonance analysis devices, magnetic resonance imaging devices, and superconducting energy storage devices.

Background

Magnesium diboride has been found to be a superconductor with a critical temperature (Tc) of about 39.5 K, which is much higher than the best-known intermetallic superconductors. The material has many favourable properties compared to conventional cuprate superconductors and the classical intermetallic superconductors.

Superconductors formed of magnesium diboride (MgB2) in bulk form have the potential to produce permanent magnets with higher field strengths than conventional ferromagnetic materials. The low materials costs and significantly simpler processing routes compared to bulk superconductors having ReBCO (Rare earth-Barium-Copper Oxide) as the superconducting compound make MgB2 an attractive choice of material for use as superconductors, particularly for use in biomedical applications such as magnetic drug targeting and small-scale MRI. For these applications, superconducting materials with a high critical current density Jc which is dependent on the magnetic field B [Jc(B)] are required to maximise the trapped magnetic field.

To reduce the costs of, for example, NMR and MRI scanners, it is highly desirable tar supercoaducling magnets to operate at as high a temperature as possible. To date there are known superconductors (such as YBCO) that can operate at liquid nitrogen temperatures (77K), which allows the use of the cheapest cryogenuc systems; however, this cuss of superconductor is generally difficult to fabricate into large bulk magnets due to the intrinsic material properties. The next convenient cryogen boiling point is that of neon (27K) and magnesium diboride (vige2) superconducts at temperatures below 39.5K so can operate at liquid neon temperatures. In addition, MgB2 may be fabricated in bulk ceramic form using conventional pressure sintering techniques. However, the use of conventions: pressures (<1 GPa) result in ceramics with less than theoretical density and less than optimal superconductor and magnetic performance.

To fabricate a magnet horn a superconductor it is necessary to pin the magnetic flux efficiently and with as high a density as possible (giving stronger magnets). To date this has been achieved by adding dopants such as oxides, iron, carbon or excess magnesium. However, it has been appreciated that, in such doped materials, whilst the magnetic flux pinning is enhanced, the superconducting transition is supressed to the point that the magnet wilt bareiy work at liquid neon temperatures. This is an important point, as a superconducting transition at temperatures below 27K and a broadening of the transition may lead to the operating temperature of the superconductor dropping below 27K The consequence of this is that a gas having a lower cryogen boiling point such as helium would need to be used which is a far more expensive undertaking from a cryogenic oeneration system view point.

It has therefore been appreciated that the use of dopant species, such as carbon-containing elements, to pin magnetic flux centres in superconducting magnesium diboride conventionaiiy leads to a degradation in the critical transition temperature (To) from normal to superconducting states for the materiai. A reduction in Tc (39.5K for pure Mg82) is accompanied by a gradual transition and may result in poor material performance during transition and a lower operating temperature to achieve the desired properties.

It has further been appreciated that conventional methods for producing MgB2 superconducting materials and the conventional materials themselves have associated with them a number of problems and deficiencies. For example, due to the strong affinity of Mg towards 02, conventionally the reaction between Mg and B is required to be carried out in a concealed and/or an inert atmosphere. Furthermore, due to the wide difference in melting points (or vapour pressures) of Mg (650 C) and B (2080 C) it has been conventionally necessary for the reaction between Mg and B to be carried out at high pressures. Also, due to the porous and brittle nature of MgB2, conventional methods demand high pressure sintering or compaction in hot stage to obtain the MgB2 material in dense form. Due to the number of expensive process steps and the associated excessive consumption of energy involved in conventional production processes for superconducting MgB2 material, it has been appreciated that there is a need for a simple and inexpensive method of producing superconducting magnesium diboride material to make it attractive for commercial exploitation in the form of high quality powders, dense bulk bodies, and/or long composite conductors with desirable phase purity, microstructure and superconducting properties.

Thus there is also a need for production of a superconducting bulk material based on MgB, having good magnetic flux pinning at temperatures above 27K to enable feasible materials to be produced that would enable comparatively cheap cooling using neon instead of liquid helium in certain applications.

SUMMARY

Viewed from a first aspect there is provided a method for forming a magnesium diboride construction comprising: providing a mass of grains of magnesium diboride; placing said grains into a canister to form a pre-sinter assembly; sealing the canister; treating the pre-sinter assembly at an ultra-high pressure of around 3GPa or greater and a temperature of between around 800C to around 1500C to sinter the magnesium diboride to form a polycrystalline construction, the construction comprising at least around 95 weight% magnesium diboride and having a superconducting transition temperature Tc, of greater than around 36K, and a critical flux density of greater than around 6x10 Alm? as measured at 27K.

In some examples, the step of treating the pre-sinter assembly comprises treating the assembly at a pressure of between around 4GPa to around 8GPa. The step of treating the pre-sinter assembly may also comprise treating the assembly at a temperature of between around 900C to around 1300C.

In some examples, the step of treating the pre-sinter assembly comprises treating the assembly at the pressure and temperature for between around 3 minutes to around 30 minutes, or in some examples for between around 5 to around 15 minutes.

The step of treating the pre-sinter assembly may, in some examples, comprise treating the assembly to form a polycrystalline construction comprising between around 96 weight% magnesium diboride to around 100 weight % magnesium diboride.

The polycrystalline construction formed according to the above method may have a superconducting transition temperature T.. of between around 36K to around 39.5K, or between around 37K to around 39.5K, or between around 38K to around 39.5K.

In some examples, the method further comprises treating the grains of magnesium diboride in an inert atmosphere prior to the step of treating the pre-sinter assembly at the pressure and temperature to remove water vapour and/or oxygen species from the grains.

Viewed from a second aspect there is provided a polycrystalline magnesium diboride construction comprising at least around 95 weight% magnesium diboride; the construction having a superconducting transition temperature Tr: of greater than around 36K, and a critical flux density of greater than around 6x10r Aim as measured at 27K.

Viewed form a third aspect there is provided a permanent magnet comprising the above defined construction.

Viewed from further aspects there is provided a nuclear magnetic resonance analysis device comprising the above-defined permanent magnet and/or a magnetic resonance imaging device comprising said permanent magnet.

Viewed from another aspect there is provided a superconductor for use in any one or more of power transmission applications, nuclear magnetic resonance analysis devices, magnetic resonance imaging devices, and/or as a superconducting energy storage device and/or as a high field magnet comprising the above-defined construction.

Brief Description of Drawings

Non-limiting aspects will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a plot of the magnetic susceptibility against temperature data with a 5mT measurement field for four examples of a body of MgB2 material; and Figures 2a to 2d are plots of critical current density against magnetic field for four examples of bodies of MgB2 material produced under different sintering conditions.

Description

Non-limiting aspects will now be described by way of example. Various magnesium diboride constructions were sintered as follows. A pre-sinter assembly was prepared by placing an amount of magnesium diboride powder having an average grain size of between around 1 to around 10 microns into a canister in an inert atmosphere to inhibit oxidation. The canister was then sealed, removed from the inert atmosphere and placed in an ultra-high pressure high temperature press such as a cubic press. The canister was then subjected to a pressure of between around 4GPa to around 8GPa at a temperature of between around 900C to around 1500C for between around 5 to around 30 minutes to sinter the material.

Once sintered, the canister was removed from the press and the canister material was subsequently removed from the sintered bulk material to form a polycrystalline magnesium diboride construction. The construction was finished to the desired shape using any one or more of EDM cutting, or laser ablation techniques.

The various example constructions were tested for phase purity, microstructure and superconducting properties using conventional techniques.

In particular, four example constructions were made for testing. The first construction comprised magnesium diboride powder having an average grain size of between around 1 to around 10 microns which was prepared according to the above method and, once placed into the press, was subjected to a pressure of around SGPa and a temperature of around 11000 for a sintering time of 5 mnutes, The second example construction comprised magnesium diboride powder having an average grain size of between around 1 to around 10 microns which was prepared according to the above method and, once placed into the press, was subjected to a pressure of around 5GPa and a temperature of around 1100C for a sintering time of 15 minutes.

The third example construction comprised magnesium diboride powder having an average grain size of between around 1 to around 10 microns which was prepared according to the above method and was subjected to a sintering pressure of around 5GPa and a sintering temperature of around 1100C for a sintering time of 25 minutes.

The fourth example construction comprised magnesium diboride powder having an average grain size of between around 1 to around 10 microns which was prepared according to the above method and was subjected to a sintering pressure of around 5GPa and a sintering temperature of around 900C for a sintering time of 15 minutes.

In none of the example constructions and methods were dopants added to the pre-sinter grains of magnesium diboride.

The various example constructions were finished post sintering to form discs of MgB2 having a diameter of around 50mm.

The sintered MgB2 constructions were then subjected to a number of tests to determine purity, microstructure and superconducting properties. The results are shown in Figures 1 and 2a to 2d.

Figure 1 shows the variation in magnetic susceptibility with temperature using a 5mT measurement field for the above four examples of MgB2 material. The results for the first example are denoted on the graph by the line having reference numeral 10, those for the second example are denoted by the line having reference numeral 20, those for the third example are denoted by the line having reference numeral 30 and the line denoted by reference numeral 40 corresponds to the results for the fourth example. Figure 1 demonstrates the sharpness of the transition for all of the tested examples and shows that reduced sintering times may be preferable as they are believed to result in less grain growth during the sintering process.

Figures 2a to 2d show for examples one to four above the critical current densities of the four samples. The critical current densities were extracted using Bean's model from the width of the magnetisation loops for each of the example constructions. In Figures 2a to 2d, the lines denoted by reference numerals 100, 200, 300 and 400 represent the variation in critical current densities of examples 1 to 4 above with magnetic field at 4.2K. The lines denoted by reference numerals 102, 202, 302 and 402 represent the variation in critical current densities of examples 1 to 4 above with magnetic field at 20K. The lines denoted by reference numerals 104, 204, 304 and 404 represent the variation in critical current densities of examples 1 to 4 above with magnetic field at 27K. It will be seen that desirable critical current densities were obtained for the examples even at 27K.

The microstructure and composition of the four example constructions were also analysed. Whilst conventional XRD techniques may used to determine if there were any phases other than the desired MgB2 phase, it was found preferable to use mass spectrometry techniques including secondary ion mass spectrometry (SIMS) for this as they are more sensitive down to the ppm level and below. :1 was determined that the examples had a purity of rvigB; of greater than around 95 weight)/0 (the theoretical density of the MgB2 ceramic being greater than 98%), with a high transition temperature for each example of greater than around 35K) and high critical flux densities (greater than ey.10:' Air& as measured at 27K). Furthermore; an analysis of the microstructures using, for example conventional SEM imaging techniques, showed that for the fourth example above which was sintered using a temperature of 9000, a pressure of 5GPa and a sintering time of 15 minutes, very little grain growth was present and the resulting ceramic exhibited a Tc. 37K and a critical current density greater than 6x10a Alm2 which implies good flux pinning despite the:R.& of impurities at 27K (liquid neon temperature).

The UHPHT method described above, without the addition of dopants to the pre-sinter material, may result in a MgB2 construction that has high purity of greater than 95 weight %, in some cases greater than 96 weight%, or greater than 97 weight % or greater than 99 weight%. By selecting the pressures. temperature and sintering tunes it is passible to fabricate a material that not only has a Tc of around 39K but also a high trapped field. Whilst not wishing to be bound by a particular theory, it is believed that the high loads induced by the UHPHT sintering coriditiow.; do two things, firstly, suppress the grain growth whilst allowing full densification and secondly, induce defects into the MgB2 crystallites that can act as magnetic flux pinning centres_ Fuilheimore, the combination of finer grains (having an average grain size of 1-10 microns) and defective grains in pure, MgB2 after sintering may enable the formation of El construction having enhanced magnetic flux pinning without any need to introduce an impurity element whilst still allowing a high transition temperature Tc, and without broadening the transition. This a S5 SDS in enabling strong and stable magnetic flux pinning to be achieved at temperatures in excess of 27K, the boiling point of neon. As such. MgB2 bulk constructions of the present examples may be a feasible technology for:capped fieid magnets utilising hguld neon rather than helium cryostats for cooling. The constructions may therefore find application as the magnet in a mobile NMR system, negating the need for either liquid helium or a very high current power supply to energise a conventional wound superconducting electromagnet. instead. the trapped lield MgB2 may be pumped with a much lower magnetic flux from a small pulsed coil to trap sufficient flux in the MgB2 to generate the multiple tesla field needed for NMR, bearing in mind such a field must be generated turns above the MgB2 surface to allow for the thermal insulation above the superconductor.

In summary, MgB2 transitions between being superconducting and non-superconducting at 39.5. The material's superconductivity and magnetic pinning will improve as temperature is reduced so, the colder the operating temperature, the more efficiently it works. The temperature of 27K is the temperature achievable using liquid neon rather than helium in the cryostat in the end apparatus, however the superconductivity of MgB2 is worse at 27K than it is at, for example, 4K which would enable liquid helium to be used in the cryostat. The UHPHT methods of the examples may allow for a superconducting performance without out doping the material which has typically only been achievable by conventional doping of the material. The doping of conventional processes is considered to have a negative effect on transition temperature Tc whereas methods of the examples are believed to enable the Tc of the construction to be close to 39.5K which may be advantageous both from a cost perspective and working conditions perspective.

In some examples, it was considered additionally beneficial to purify and maintain the purity of the MgB2 powders both before and during sintering as far as possible. This is contrary to conventional beliefs that suggest that MgO and other impurities are required or at least assist in the magnetic flux pinning in MgB2 superconductor constructions. Typically MgO is an unintentional dopant (at grain boundaries) due to natural surface oxidation of the MgB2 starting material and impure powder handling techniques.

in some examples, the use of inert and or controlled atmospheres to remove water vapour and oxygen species from the MgB2 powders prior to sintering, and MgO removal techniques (including reduction in hydrogen) to maintain the purity or MgB2 powders pre and during sintering, may be included in the example methods.

It has been appreciated that the conventional sintering pressures for MgB2 of 50 MPa to 1GPa may have the problem that the resulting MgB2 ceramics are not sufficiently dense unless higher sintering temperatures are used, however it has been found that increasing the sintering temperature may be detrimental as the grains grow too much. The methods of the examples assist in ameliorating these problems, for example using sintering pressures of greater than around 3GPa and in some embodiments greater than 4GPa whilst keeping the sintering temperature relatively low to suppress grain growth, for example in the range of between around 800C to around 1500C. In some examples, the sintering temperatures are between around 900 C to around 1300C, and the sintering times may be for example between around 3 to around 30 minutes.

Claims

Claims 1. A method for forming a magnesium diboride construction comprising: providing a mass of grains of magnesium diboride; placing said grains into a canister to form a pre-sinter assembly; sealing the canister; treating the pre-sinter assembly at an ultra-high pressure of around 3GPa or greater and a temperature of between around 800C to around 1500C to sinter the magnesium diboride to form a polycrystalline construction, the construction comprising at least around 95 weight% magnesium diboride and having a superconducting transition temperature.1" of greater than around 36K, and a critical flux density of greater than around 6x103,4/m2 as measured at 27K.
2. The method of claim 1, wherein the step of treating the pre-sinter assembly comprises treating the assembly at a pressure of between around 4GPa to around 8GPa.
3. The method of any one of the preceding claims, wherein the step of treating the pre-sinter assembly comprises treating the assembly at a temperature of between around 900C to around 1300C.
4. The method of any one of the preceding claims, wherein the step of treating the pre-sinter assembly comprises treating the assembly at the pressure and temperature for between around 3 minutes to around 30 minutes.
The method of any one of claims 1 to 3, wherein the step of treating the pre-sinter assembly comprises treating the assembly at the pressure and temperature for between around 5 to around 15 minutes.
The method of any one of the preceding claims, wherein the step of treating the pre-sinter assembly comprises treating the assembly to form a polycrystalline construction comprising between around 96 weight% magnesium diboride to around 100 weight % magnesium diboride.
7. The method of any one of the preceding claims, wherein the step of treating the pre-sinter assembly comprises treating the assembly to form a polycrystalline construction having a superconducting transition temperature T, of between around 36K to around 39.5K.
8. The method of any one of claims 1 to 6, wherein the step of treating the pre-sinter assembly comprises treating the assembly to form a polycrystalline construction having a superconducting transition temperature TO of between around 37K to around 39.5K.
The method of any one of claims 1 to 6, wherein the step of treating the pre-sinter assembly comprises treating the assembly to form a polycrystalline construction having a superconducting transition temperature T0 of between around 38K to around 39.5K.
10. The method of any one of the preceding claims, further comprising treating the grains of magnesium diboride in an inert atmosphere prior to the step of treating the pre-sinter assembly at the pressure and temperature to remove water vapour and/or oxygen species from the grains,
11. A polycrystalline magnesium diboride construction comprising: at least around 95 weight% magnesium diboride; the construction having a superconducting transition temperature T" of greater than around 36K. and a critical flux density of greater than around 6)(108 Pirn2 as measured at 27K.
12. The construction of claim 11, comprising between around 96 weight% magnesium diboride to around 100 weight % magnesium diboride.
13. The construction of any one of claims 11 to 12, wherein the polycrystalline construction has a superconducting transition temperature To of between around 36K to around 39.5K.
14. The construction of any one of claims 11 to 12, wherein the polycrystalline construction has a superconducting transition temperature Tr; of between around 37K to around 39.5K.
15. The construction of any one of claims 11 to 12, wherein the polycrystalline construction has a superconducting transition temperature T" of between around 38K to around 39.5K.
16. A permanent magnet comprising the construction of any one of claims 11 to 15.
17. A nuclear magnetic resonance analysis device comprising the permanent magnet of claim 16.
18. A magnetic resonance imaging device comprising the permanent magnet of claim 16.
19. A superconductor for use in any one or more of power transmission applications, nuclear magnetic resonance analysis devices, magnetic resonance imaging devices, and/or as a superconducting energy storage device and/or as a high field magnet comprising the construction of any one or more of claims 11 to 15.