TITLE SUPERCONDUCTING MATERIALS AND METHODS OF SYNTHESIS

FIELD OF THE INVENTION The present invention relates to superconducting materials and methods of synthesis thereof. In particular, the present invention relates to doped superconducting materials comprising magnesium diboride (MgB ₂ ) and methods of synthesis thereof.

BACKGROUND TO THE INVENTION For a material to exhibit superconducting behaviour, the material must be cooled below its critical temperature (T _c ), the current passing through the cross-section of the material must be below the critical current density (J ₀ ) and the magnetic field to which the material is exposed must be below the critical magnetic field (H ₀ ).

The process of cooling low temperature superconductor (LTS) materials to their critical temperature is expensive and often difficult, thus limiting their use in commercial applications, whilst high temperature superconductor (HTS) materials tend to have limited applications because of their brittleness.

MgB ₂ has recently emerged as an alternative LTS material because of its low cost and ease of synthesis. By way of comparison, heat treatment of MgB ₂ wire requires 10 to 30 minutes at 700 ⁰ C whereas HTS wire requires 60 hours of heat treatment. MgB ₂ has been used in wires, tapes and powder-in-tube (PIT) processed strands and is a promising prospect in current carrying applications. In the principal PIT-based approaches, a tube or sheath is loaded with either pre-reacted MgB ₂ powder (the "ex-situ method") or a mixture of elemental Mg and B powder (the "in-situ method"), which readily lends itself to the

inclusion of property improving dopants. Furthermore, MgB ₂ has a critical temperature of 39K, which is almost twice that of the highest critical temperature of known LTS materials and more than four times that of the LTS workhorse, NbTi. MgB ₂ has a simple crystal structure, a large coherence length and the grain boundaries are transparent to current flow.

However, the J ₀ of pristine MgB ₂ drops rapidly with increasing magnetic field in response to its low upper critical field (H _c2 ) and MgB ₂ has poor flux pinning strength, both of which limit its application at higher magnetic fields. Nonetheless, irradiation, chemical doping and thermo-mechanical processing techniques have been used to enhance the critical current density and upper critical field of MgB ₂ . Chemical doping, for example, is an efficient way to introduce pinning centres into the superconductor structure because the fluxoids to be pinned are string-like and amenable to pinning by particles, precipitates and the like. For example, the use of SiC for doping MgB ₂ , as described in the Applicant's co-pending patent application no. PCT/AU03/00758, has been found the most useful particle to increase the J _c and many groups have tried to dope the MgB ₂ with carbon. For example, S. X. Dou, S. Soltanian, J. Horvat, X. L Wang, P. Munroe, S. H. Zhou, M. lonescu, H. K. Liu and M. Tomsic, Appl. Phys. Lett. 81 (2002) 3419, S. Soltanian, J. Horvat, X. L Wang, P. Munroe and S. X. Dou, Physica C 390 (2003) 185, H. T. Wilke, S. L Bud'ko, P. C. Canfield, D. K. Finnemore, R. J. Suplinskas, and S. T. Hannahs, Phys. Rev. Lett. 2004, 92, 17003, WK. Yeoh, J. H. Kim, J. Horvat, S. X. Dou, and P. Munroe, Superconductor Science and Technology, 19 (2006) L5-L8 have demonstrated that carbon in the form of silicon carbide, amorphous carbon and carbon nanotubes can enter the MgB ₂ structure by replacing the B to produce intraband scattering and hence significantly increase the upper critical magnetic field H _c2 .

Usually, nanometre-sized particles are necessary to ensure good pinning behaviour and the inventor has previously identified significant improvement in J ₀ (H), H _c2 and Hjrr in MgB ₂ through doping with nano-scale carbon, carbon nano-tubes and nano-scale SiC. However, it has been found that the doping effect is impeded by the agglomerate behaviour of the nano-particles. An attempt to address this problem includes a method in which ultrasound was used to mix the reaction powder as disclosed by W. K. Yeoh, J. H. Kim, J. Horvat, S. X. Dou and P. Munroe, Supercond. Sci. Technol. 19, L5-L8 (2006). Nonetheless, it is still difficult to disperse the nanometre-sized precursor powder into the reaction powders uniformly. Also, the nanometre-sized materials are very expensive and therefore producing superconductors in large quantities from such materials is uneconomical. It would therefore be desirable to find a cheap material to replace the nanometre-sized particles to produce a superconductor having improved superconducting properties.

Furthermore, the substitution of carbon for boron is limited at low temperature. A high level of carbon substitution can be achieved at higher temperatures, such as 1000°C, however, high temperature sintering causes a drastic grain growth, which is very undesirable for improving J _c at high temperatures. In order to explore the potential applications of MgB ₂ around 20 K or above, it is essential to search for new dopants that can maintain T ₀ and improve J ₀ . Yamada et al., Supercond. Sci. Technol. 19,(2006) 175-177, disclose using the aromatic hydrocarbons of benzene, naphthalene and thiophene as additives to improve the superconducting properties of in-situ processed MgB ₂ tapes. The observed improvement in J _c is almost comparable with MgB ₂ tapes prepared with 10%mol SiC powder addition. However, disadvantages of this approach include the high volatility of

aromatic hydrocarbons and the limited solubility of the carbon they release in the MgB ₂ lattice or matrix. The level of incorporation of carbon is limited to about 1%, thus limiting the enhancement of the upper critical field (H _c2 ).

In this specification, the terms "comprises", "comprising" or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.

OBJECT OF THE INVENTION It is the object of the present invention to produce a superconducting material that is cheaper to produce than the prior art superconducting materials produced with nanometre-sized materials.

It is a preferred object of the present invention to produce a superconducting material that has superconducting properties at least comparable with, if not superior to, those of the prior art superconducting materials produced with nanometre-sized materials.

SUMMARY OF THE INVENTION

In one form, although it need not be the only or indeed the broadest form, the invention resides in a method of synthesizing a MgB ₂ superconducting material including: a) mixing at least two starting materials capable of forming MgB ₂ with at least one organic dopant compound, said at least one organic dopant compound comprising carbon, hydrogen and oxygen; b) shaping the mixed materials into a desired shape; and c) heating the mixed materials to produce a carbon-doped MgB ₂ superconducting

material.

Suitably, the starting materials comprise Mg and B and preferably the starting materials are Mg powder of high purity and B powder of high purity.

Suitably, the starting materials include one or more powders of the following: Mg, B, MgB ₂ , MgH ₂ , MgB ₄

Preferably, the at least one organic dopant compound includes, but is not limited to, one or more of the following: a carbohydrate or derivative thereof, an organic acid, an ester, sugar (C ₆ Hi ₂ O ₆ ), malic acid (C ₄ H ₆ O ₅ ), tartaric acid (C ₄ H ₆ O ₆ ), citric acid (C ₆ H ₈ O ₇ ), dimethyl terephthalate (Ci ₀ HioO ₄ ), Nipasol (Ci ₀ Hi ₂ O ₃ ), pivalic acid (C ₅ HioO ₂ ), crotonic acid (C ₄ H ₆ O ₂ ), lactic acid (C ₃ H ₆ O ₃ ), isophthalic acid (C ₈ H ₆ O ₄ ), adipic acid (C ₆ H ₁₀ O ₄ ), succinic acid (C ₄ H ₆ O ₄ ), Ethylenediaminetetraacetic acid (EDTA) (C ₁₀ Hi ₆ N ₂ O ₈ ), 2-(3-benzoylphenyl)-propionic acid (Ci ₆ Hi ₄ O ₃ ), Methyl β-Naphthyl Ketone (Ci ₂ Hi ₀ O), starch (Ci ₂ Hi ₀ O ₅ ), sucrose (Ci ₂ H ₂₂ On), 1 ,8-naphthalic anhydride (Ci ₂ H ₆ O ₃ ).

Suitably, step b) includes compressing the mixed materials into pellets, suitably under a pressure of about 8MPa.

Suitably, step c) includes sintering the mixed materials at about 600°C-1000°C and preferably at about 650°C-900°C.

Suitably, step c) may be performed for about 10-240 minutes and preferably for about 30-120 minutes. Suitably, step c) may be performed under a high purity argon gas flow or under a gas flow of argon and hydrogen or in a sealed quartz tube.

Preferably, step a) further includes: i) dissolving the organic dopant compound with a solvent to form a solution; ii) mixing at least one of the starting materials capable of forming a

superconducting material with the at least one organic dopant compound in solution form to form a slurry; iii) drying the slurry in a vacuum chamber; and iv) mixing the dried slurry with at least one other of the starting materials capable of forming a superconducting material.

Alternatively, the method can include dry mixing of the starting materials capable of forming a superconducting material and the organic dopant compound without using a solvent.

Suitably, the solvent is a liquid and preferably an organic liquid. The solvent may be distilled water, but is preferably a non-aqueous solvent, such as benzene, toluene or hexane.

The method may further include the step of: d) furnace cooling the carbon-doped superconducting material to room temperature. In another form, although again not necessarily the broadest form, the invention resides in a MgB ₂ superconducting material comprising at least two starting materials capable of forming MgB ₂ and at least one organic dopant compound, said at least one organic dopant compound comprising carbon, hydrogen and oxygen, wherein the starting materials and the at least one organic dopant compound are mixed at a molecular level and heated to produce a carbon-doped MgB ₂ superconducting material.

Preferably, the at least one organic dopant compound includes, but is not limited to, one or more of the following: a carbohydrate or derivative thereof, an organic acid, an ester, sugar (CeHi ₂ O ₆ ), malic acid (C ₄ HeO ₅ ), tartaric acid (C ₄ H ₆ O ₆ ), citric acid (C ₆ H ₈ O ₇ ), dimethyl terephthalate (Ci ₀ HioO ₄ ), nipasol (Ci ₀ Hi ₂ O ₃ ), pivalic acid (C ₅ HI ₀ O ₂ ), crotonic

acid (C ₄ H ₆ O ₂ ), lactic acid (C ₃ H ₆ O ₃ ), isophthalic acid (C ₈ H ₆ O ₄ ), adipic acid (C ₆ HI ₀ O ₄ ), succinic acid (C ₄ H ₆ O ₄ ), ethylenediaminetetraacetic acid (EDTA) (C ₁₀ Hi ₆ N ₂ O ₈ ), 2-(3-benzoylphenyl)-propionic acid (Ci ₆ Hi ₄ O ₃ ), methyl β-Naphthyl ketone (Ci ₂ H ₁₀ O), starch (C ₁₂ Hi ₀ O ₅ ), sucrose (Ci2H ₂₂ On), 1,8-naphthalic anhydride (Ci ₂ H ₆ O ₃ ). Further features of the present invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example only, preferred embodiments of the invention will be described more fully hereinafter with reference to the accompanying drawings, wherein:

FIG 1 is a flowchart showing a method of synthesizing a superconducting material in accordance with embodiments of the present invention;

FIG 2 is a flowchart showing further steps of the method shown in FIG 1 in accordance with embodiments of the present invention; FIG 3 shows X-ray diffraction patterns of pure MgB ₂ and embodiments of the superconducting material comprising different levels of organic dopant compound;

FIG 4 is a graph of Full Width at Half Maximum (FWHM) values of the superconducting material as a function of a level of organic compound doping in accordance with embodiments of the present invention; FIG 5 is a graph of crystal parameters of embodiments of the superconducting material as a function of the level of organic compound doping;

FIG 6 is a graph of magnetic ac susceptibility of the superconducting material as a function of temperature for different levels of organic compound doping;

FIG 7 is a graph of critical current density of embodiments of the superconducting

material at temperatures of 5K and 2OK as a function of the level of organic compound doping;

FIG 8 shows a transmission electron microscope image and a selected area diffraction pattern of the superconducting material doped with 10% organic dopant compound in accordance with an embodiment of the present invention;

FIG 9 shows a transmission electron microscope image and a selected area diffraction pattern of an undoped sample of a prior art superconducting material;

FIG 10 is a graph of magnetization of embodiments of the superconducting material as a function of temperature for different levels of organic compound doping; FIG 11 is a graph of critical current density of embodiments of the superconducting material as a function of magnetic field for different levels of organic compound doping;

FIG 12 is a graph of upper critical field as a function of temperature ratio T/T _c of an embodiment of the superconducting material compared with prior art doped superconducting materials; FIG 13 is a graph of critical current density of the superconducting material according to an embodiment of the present invention as a function of magnetic field at temperatures of 5K and 2OK compared with a prior art doped superconducting material; and

FIG 14 is a graph of critical current density as a function of magnetic field for the superconducting material according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the flowchart in FIG 1, according to one aspect, the invention resides in a method 100 of synthesizing a magnesium diboride (MgB ₂ ) superconducting

material. The method 100 includes, at 110, mixing at least two starting materials capable of forming MgB ₂ with at least one organic dopant compound. According to the present invention, the organic dopant compound comprises carbon, hydrogen and oxygen.

According to embodiments of the present invention, the starting materials can comprise Mg and B and can include one or more of the following: Mg, MgH ₂ , B, MgB ₂ , MgB ₄ . In a preferred embodiment, the starting materials are in the form of high purity magnesium powder and amorphous boron powder of high purity. The purity level of the Mg and B starting powders is at least 99%. In alternative embodiments, the starting materials can be in an alternative solid form or in liquid form. The organic dopant compound can include, but is not limited to, for example, one or more of the following: a carbohydrate or derivative thereof, an organic acid, an ester, sugar (CβHiaOβ), malic acid (C ₄ HeO ₅ ), tartaric acid (C ₄ H ₆ O ₆ ), citric acid (C ₆ H ₈ Oy), dimethyl terephthalate (C10H ₁ 0O4), nipasol (C10H12O3), pivalic acid (C ₅ HI ₀ O ₂ ), crotonic acid (C ₄ H ₆ O ₂ ), lactic acid (0 ₃ H ₆ O ₃ ), isophthalic acid (CaH ₆ O ₄ ), adipic acid (C ₆ Hio0 ₄ ), succinic acid (0 ₄ H ₆ O ₄ ), ethylenediaminetetraacetic acid (EDTA) (Ci _O Hi ₆ N ₂ O ₈ ), 2-(3-benzoylphenyl)-propionic acid (Ci ₆ Hi ₄ θ ₃ ), methyl β-Naphthyl ketone (C ₁₂ H ₁₀ O), starch (Ci ₂ Hi ₀ O ₅ ), sucrose (Ci ₂ H ₂₂ On), 1 ,8-naphthalic anhydride (Ci ₂ H ₆ O ₃ ). Three examples will be described hereinafter in which the organic dopant compound is commercial sugar (C ₆ Hi ₂ O ₆ ), malic acid (C ₄ H ₆ O ₅ ) and tartaric acid (C ₄ H ₆ O ₆ ). With reference to FIG 1, the method 100 includes, at 120, shaping the mixed materials into a desired shape. In a preferred embodiment, this includes compressing the mixed starting materials in a pressure die having an inner diameter of 13mm and applying a pressure of 8MPa to press the mixed starting materials into precursor pellets. The precursor pellets are then sealed in iron tubes. According to one embodiment, the iron

tube has an outer diameter of 10 mm, a wall thickness of 1 mm, and is 10 cm long with one end of the tube sealed. The tube is filled with the mixed starting materials and the unsealed, open end is crimped, for example, by hand. In alternative embodiments, shaping can include, for example, one or more of mechanical pressing, rolling, extrusion, casting and/or swaging to produce the desired shape of, but not limited to, wires, tapes, coils or bulk material.

The method 100 of the present invention includes, at 130, heating the mixed materials to produce a carbon-doped MgB ₂ superconducting material. In a preferred embodiment, the method includes sintering the mixed powders at about 78O ⁰ C for about 1 hour under flowing argon gas. The argon gas is of high purity and the flow is maintained throughout the sintering process. In an alternative embodiment, a mixture of argon and hydrogen is employed. In a further alternative embodiment, sintering of the mixed materials is performed in a sealed quartz tube.

According to a preferred embodiment, the method 100 further includes at 140, furnace cooling the carbon-doped superconducting material to room temperature. In preparation for analyzing the carbon-doped MgB ₂ superconducting material, at 150, the precursor pellets were cut into bar-shaped samples of 1 *2*3 mm ³ and at 160, polished for magnetic measurements.

With reference to FIG 2, a preferred embodiment of the method 100 includes mixing the starting materials in the form of powders using a solution method. In this embodiment, mixing the starting materials at 110 further includes at 200 dissolving at least one organic dopant compound comprising carbon, hydrogen and oxygen in a solvent to form a solution. Mixing also includes at 210 mixing at least one of the starting powders capable of forming a MgB ₂ superconducting material with the organic dopant compound in

solution to form a slurry. The method further includes at 220, drying the slurry in a vacuum chamber and at 230, mixing the dried slurry with at least one other of the starting powders capable of forming a MgB ₂ superconducting material. In this embodiment, the solvent is distilled water. However, in alternative embodiments, the solvent is another organic liquid, such as benzene, toluene or hexane or other non-aqueous organic solvent. It should be appreciated that in alternative embodiments, the method includes dry mixing of the starting materials capable of forming a superconducting material and the at least one organic dopant compound without using a solvent.

In the preferred solution method of mixing the powders and the organic dopant compound is in the form of commercial sugar, the high purity powders of magnesium and amorphous boron and the commercial sugar were weighed out according to the weight ratio of MgB ₂ (CeHi ₂ Oe) x with x = 0, 0.02, 0.05, 0.1 and 0.15. The powder of boron and CeH- _I2 Oe were first mixed with the mortar and pestle by hand and the distilled water was used to help the mixing during the grinding process. Boron powder without sugar was also mixed with water and dried in the vacuum chamber in order to compare it with the doped samples. After the mixed powders were dried, they were mixed with Mg powder and ground with mortar and pestle by hand for 30 minutes.

The aforementioned method according to preferred embodiments of the present invention results in a polycrystalline superconducting material comprising MgB ₂ doped with different levels of organic dopant compound wherein the MgB ₂ and the organic dopant compound are mixed at a molecular level through an in-situ reaction process.

The phase and crystal structure of all the aforementioned 1 *2*3 mm ³ bar-shaped samples were analysed using X-ray diffraction (XRD) patterns using a Phillips diffractometer with Cu K _a radiation. The grain morphology and microstructure were also

examined using transmission electron microscopy (TEM).

The magnetization of the samples was measured over a temperature range of 5 to 30 K using a Physical Property Measurement System (PPMS, Quantum Design) in a time varying magnetic field with a sweep rate of 500Oe/s and amplitude 8.5 Tesla. The magnetic measurement was performed by applying the magnetic field parallel to the longest axis of the samples. The magnetic current density J ₀ was calculated from the height of the magnetization loop M using the following critical state model: J _c = 12Mb/d(3b-d), where b and d are the dimensions of the samples perpendicular to the direction of applied magnetic field and where d<b. J _c versus magnetic field was measured up to 8.5T. The low field J _c at 5K could not be measured due to flux jumping. The critical temperature T ₀ was determined by measuring the real part of the ac susceptibility at a frequency of 117 Hz and an external magnetic field of 0.1 Oe. T ₀ was defined as the onset of the diamagnetism.

Referring to FIG 3 and the XRD patterns of embodiments of the superconducting material of the present invention in the form of MgB ₂ doped with different levels of CβHiaOβ, the samples consist mainly of MgB ₂ , together with MgO as the main impurity phase. The C ₆ Hi ₂ Oe doped samples where the doping level is over 5% show an impurity peak which belongs to Mg ₂ C ₃ . The dashed lines show the positions of the (100) peaks and (002) peaks respectively of the samples. The (001) peak position shifts after doping with the C ₆ Hi ₂ Oe, and the peak position shifts to the larger 2θ angle as the doping level increases. This shift means that the a-axis is decreasing after the sugar doping. The decrease of the a-axis is an indication of carbon substitution for boron, which is consistent with the result of other researchers, such as S. X. Dou, W. K. Yeoh, J. Horvat, and M. lonescu, Appl. Phys. Lett 83, 4996 (2003). The position of the (002) peaks has no

obvious shift with the increase of sugar doping. The inventors conclude that the c-axis varies very little with the level of doping.

With additional reference to FIG 4, another feature of the XRD pattern is the peak broadening of the CeHi ₂ θ ₆ doped samples. FIG 4 shows the Full Width at Half Maximum (FWHM) for the (101) peaks. The FWHM increases as the doping level increases. The inventors speculate that this FWHM increase is an indication of a decrease in the crystal size and that the crystal size decrease can be attributed to two possible processes. A first process involves the sugar coating the outside of the boron powder, which prevents the magnesium from contacting the boron. This retards the reaction process and reduces the time available for growth of the MgB ₂ crystal. In a second process, the impurities from the sugar doping are distributed in the grain boundary, which pins the grain boundary. The grain boundary is thus prevented from moving and consequently the grain cannot grow any larger. The decrease in crystal size contributes to improving the Jc(H) behaviour as crystal grain size is a source for flux pinning. FIG 5 shows the crystal lattice parameters of the samples with different concentrations of sugar doping and the results confirm the phenomena of crystal parameter change. The results show that the a axis decreases with an increasing level of doping.

Reference is now made to FIG 6, which shows the transition temperature (T ₀ ) for embodiments of the doped superconductor of the present invention and undoped superconductor samples determined by ac susceptibility measurements. FIG 5 illustrates that the T ₀ drops as the level of organic dopant compound, in this embodiment sugar, increases. The T ₀ onset for the undoped samples is around 38K whereas the 15% doped sample has a T ₀ value of 32.5K. The T ₀ depression is a result of the carbon doping, which

is inconsistent with other literature, as disclosed by S. X. Dou, S. Soltanian, J. Horvat, X. L Wang, P. Munroe, S. H. Zhou, M. Ionescu, H. K. Liu and M. Tomsic, Appl. Phys. Lett. 81 (2002) 3419. It is important to maintain a high T _c in order to obtain a high J ₀ at higher temperature such as 2OK. So a compromised composition needs to be found to obtain good J ₀ (H) behaviour and a reasonable T ₀ .

With reference to FIG 7, the J _C (H) curve at 5K and 2OK show that the sugar doped samples exhibit an improvement in J ₀ compared with that of the pure MgB ₂ . At 5K, the J _c increases with the doping level until the doping level reaches 10%. Over 10% sugar doping produces a negative effect on the J ₀ value. The 15% doped samples shows a large drop in J ₀ compared with the 10% doped sample, but the J ₀ value still crosses over that of the pure MgB ₂ sample above 5T. The slope of J ₀ (B) dependences becomes less steep with an increasing level of sugar doping indicating a strong pinning effect caused by the sugar doping. At 6 Tesla and 5K, the highest J _c value achieved by the 10% doped sample is 3.6x10 ⁴ A/cm ² while the pure MgB ₂ sample only reached 2x10 ³ A/cm ² . At 8 Tesla, the highest J ₀ value reached is 1.5 * 10 ⁴ A/cm ² , which is close to that of the prior art SiC nanometre particle doped sample. Although the pure MgB ₂ sample shows relatively low J _c values due to un-optimised fabrication parameters, the enhancement of J ₀ for the doped samples is still impressive. At 2OK, the trend for J ₀ improvement is similar to those observed at 5K with some differences at low magnetic field values. At less than about 4 Tesla, the 2% doped sample has the highest J ₀ and the J ₀ values of the 5%, 10% and 15% doped samples decrease with increasing doping level. The inventors speculate that two factors possibly contribute to this decrease: one factor is likely to be the T ₀ drop caused by the carbon substitution for boron and the other factor is likely to be the introduction of impurity phases, such as Mg ₂ C ₃ , thus reducing the superconducting MgB ₂ volume.

Hence, a compromise needs to be sought to keep the T _c and J ₀ values at reasonable levels. At above 4 Tesla, the 10% doped sample surpasses the 2% doped sample and has the highest J ₀ . The rate of J ₀ drop of the sugar doped samples is much slower than that of the pure MgB ₂ , again indicating strong flux pinning induced by the sugar doping. With reference to FIGS 8 and 9, the microstructures of the 10% sugar doped sample and the undoped sample respectively were observed using a transmission electron microscope (TEM). Both samples contained some tiny particles, the size of which vary from 10nm to 50nm. Judging from the selected area diffraction (SAD) patterns shown as insets in FIGS 8 and 9, MgB ₂ and MgO rings are noticeable in both the samples and a weak Mg ₂ C ₃ ring is observable in the doped sample in FIG 8.

Energy Dispersive X-ray Analysis (EDX) was used to detect the element distribution and higher oxygen peaks were found in the doped sample compared with the undoped sample. In some locations, boron rich areas were found indicating a boron excess due to the reaction between magnesium, oxygen and carbon. Carbon was substantially uniformly distributed in the MgB ₂ matrix, which is an indication of good mixing of the sugar due to the solution mix method of a preferred embodiment of the method of the present invention.

According to another embodiment of the present invention, the at least one organic dopant compound comprising carbon, hydrogen and oxygen is in the form of malic acid (C ₄ H ₆ Os), which is used to produce carbon-doped MgB ₂ wires using a powder-in-tube (PIT) method through a in-situ reaction process. The method of synthesis is as depicted in FIGS 1 and 2 and details thereof for this embodiment follow.

Malic acid of 0wt% to 30wt% of MgB ₂ was dissolved in toluene solvent and the solution was mixed with amorphous boron powder having a purity of at least 99%. This

mixture was vacuum-dried and the dried mixture mixed with magnesium having a purity of at least 99% and thoroughly ground. A 10 cm long iron tube having an outside diameter (OD) of 10 mm, a wall thickness of 1 mm and one end sealed was filled with the mixed powder and the remaining end was blocked using an aluminium bar. The composite was drawn to a 1 mm to 1.4 mm diameter wire through a series of more than 30 dies with reduction rate about 10% every drawing. For fabrication of multifilament wires, a bundle of single core wires were inserted to an iron tube and the composite was drawn to a 1 mm to 1.4 mm diameter wire and several short samples about 2 cm in length were cut from the wire. These pieces were then sintered in a tube furnace at 800 ⁰ C and 83O ⁰ C for 30minutes with a heating rate of 3 ⁰ C per minute, and finally furnace-cooled to room temperature. A high purity argon gas flow was maintained throughout the sintering process. An un-doped sample was also made under the same conditions for use as a reference sample. The mixture was also pressed into pellets, which were sealed in an iron tube and sintered under the same conditions as for the wires. The phase and crystal structures of all the samples were obtained from X-ray diffraction (XRD) patterns using a Phillips (PW1730) diffractometer with Cu Ka radiation. Differential thermal analysis (DTA) was performed to study the heating rate effect on J ₀ . The grain morphology and microstructure were also examined by a scanning electron microscope (SEM) equipped with a focused ion beam (FIB) and a transmission electron microscope (TEM). The magnetization was measured at 5 and 20 K using a Physical Property Measurement System (PPMS, Quantum Design) in a time-varying magnetic field with sweep rate of 50 Oe/s and amplitude 8.5T. Samples of 1mmx2mmx3mm were used for all the magnetic measurements. Bar shaped samples with a diameter of 0.7 mm and length of 2.7 mm were cut from wire core for magnetic measurements. The magnetic

measurements were performed by applying the magnetic field perpendicular and parallel to the wire sample axis. Since there is a large sample size effect on the magnetic J ₀ for MgB2, all the samples for measurement were made to the same size for comparison. The magnetic J ₀ was derived from the width of the magnetization loop using Bean's model. J ₀ versus magnetic field has been measured up to 8.5 T.

Table 1 shows that doping with malic acid reduces the a-axis lattice parameter, indicating that carbon from the malic acid substitutes for boron in the MgB ₂ . The actual carbon content in the lattice is higher than those using other types of carbon-containing materials, suggesting that the freshly formed carbon due to decomposition of malic acid is highly reactive. The actual net % of carbon from 10wt%, 20wt% and 30wt% of malic acid doping is only 3wt%, 6wt% and 9wt% respectively. Thus, the level of carbon substitution for boron is much higher than any other dopants.

Table 1

20 wt% malic acid 3 .07464 3.52297 1.14581 0.0404

30 wt% malic acid 3 .07319 3.52147 1.14586 0.0460

Referring to FIG 10, it is evident that malic acid doping reduces the critical transition temperature, T _c by less than 2 K. T ₀ reduction is an indication of carbon substitution for boron which is essential for the improvement of H ₀₂ . However, the reduction for all three levels of dopants is the same and less than for other dopants.

FIG 11 shows the critical current density (J ₀ ) against magnetic field (H) for the malic acid doped samples. It is noted that there is no degradation in J ₀ in the low field range in

clear contrast to other dopants that cause the reduction of J ₀ in low fields. In higher fields, the J _c increases by more than an order of magnitude.

FIG 12 shows the comparison of upper critical field (H _c2 ) for malic acid doped MgB ₂ according to embodiments of the present invention with SiC doped MgB ₂ and MgB ₂ doped with single wall carbon nanotubes (SWCNT). Malic acid doping results in enhancement in

Hc2 compared to the undoped sample and compared to the SiC doped and single wall carbon nanotube doped samples.

FIG 13 shows the critical current density (J ₀ ) against magnetic field (H) for the

30wt% malic acid doped sample compared with 10wt% SiC doped MgB ₂ at 5 K and 20 K. Malic acid doping also results in enhancement in J _c compared to the SiC doped samples at both 5 K and 20 K. This is attributable to the high level of carbon substitution for boron in the malic acid doped MgB ₂ sample in comparison with that for the other dopants.

According to a further embodiment of the present invention, the at least one organic dopant compound comprising carbon, hydrogen and oxygen is in the form of tartaric acid (C ₄ H ₆ O ₆ ), which is used to produce doped MgB ₂ wires using the same fabrication process as for the malic acid doped sample described above. 10wt% tartaric acid was used in these samples. In the aforementioned method step 130, some samples were heated at 650 ⁰ C and some were heated at 900 ⁰ C. Undoped samples were heated at the same temperatures for comparison. FIG 14 shows the variation of J ₀ against H indicating a strong enhancement in J ₀ and flux pinning. It is interesting to note that J ₀ for the sample sintered at 650°C is better than that for 900 ⁰ C. This indicates that the free carbon from decomposition of the tartaric acid is highly reactive as in the case of SiC.

In summary, it was found that carbon from the aforementioned organic dopant

compounds comprising carbon, hydrogen and oxygen was doped into MgB ₂ . The substitution of carbon for boron enhances the flux pinning, but depresses T _c . Where the organic dopant is sugar, J ₀ was enhanced by two orders of magnitude at 6 T and 5K, or at 4 T and 2OK. The J ₀ was nearly 25,000A/cm ² at 2OK in a field of 4T. At 5K and 8 T, J _c reached 27,000A/cm ² .

Hence, the superconducting materials and methods of synthesis according to embodiments of the present invention address at least some of the aforementioned problems of the prior art superconducting materials and methods of synthesis. Significant advantages of the organic dopant compounds comprising carbon, hydrogen and oxygen described herein include the fact that they can dissolve in a solvent such that the solution can form a slurry with starting materials capable of forming a MgB ₂ superconducting material, such as boron in powder form. This method results in mixing at a molecular level, thus avoiding the agglomerate problem of the prior art nanoparticle dopants. After drying out the solvent, the organic dopants are coated onto the boron powder surface to form a highly uniform mixture. The resultant mixtures melt at lower temperatures and decompose at temperatures below the formation temperature of MgB ₂ , hence producing highly reactive and fresh carbon at the atomic scale as well as a reducing reagent, carbon monoxide, which can convert boron oxide to boron, thus reducing impurities in the boron powder. Because of the high reactivity of the freshly formed carbon, the carbon can substitute for boron at the same temperature (600 ⁰ C) as the formation temperature of MgB ₂ . The simultaneous dual reactions remotely incorporate the carbon into the lattice, which results in the enhancement of critical current density (J _c ), irreversibility field (Hj _rr ), and upper critical field (H _c2 ). Organic doping using the dopant compounds comprising carbon, hydrogen and oxygen described herein show a little depression in T ₀ , but

significantly reduce grain size, increase the carbon doping level and hence improve J _c , H _1n and H _c2 performance across all the measured temperatures and field ranges.

Organic dopant compounds comprising carbon, hydrogen and oxygen as specified herein, are both cheap and abundant and provide a cheap carbon source for doping with MgB2 superconducting starting materials such as Mg, B and MgB ₂ . Such dopants will decrease the fabrication cost of superconducting materials if they replace carbon nanotubes and/or nanometre-sized SiC particles, which are currently used widely as dopants. Furthermore, it has been demonstrated that the superconducting materials according to embodiments of the present invention have superconducting properties both comparable with, and superior to, prior art superconducting materials.

It is envisaged that the improved superconducting materials of the present invention are likely to have significant commercial implications for many applications in, for example, the medical, electronics, energy generation and transmission, and transportation sectors as well as other industry sectors. Throughout the specification the aim has been to describe the invention without limiting the invention to any one embodiment or specific collection of features. Persons skilled in the relevant art may realize variations from the specific embodiments that will nonetheless fall within the scope of the invention. For example, the ratio of organic dopant compound comprising carbon, hydrogen and oxygen to MgB ₂ may be varied from the specific amounts recited herein whilst achieving the advantages of the carbon doped MgB ₂ superconducting material of the present invention.