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Cryogenics 46 (2006) 237–242

The reactive liquid Mg infiltration process to producelarge superconducting bulk MgB2 manufacts

Giovanni Giunchi a,*, Giovanni Ripamonti a, Tommaso Cavallin b, Enrico Bassani b

a EDISON SpA, R&D Division, Foro Buonaparte 31, 20121 Milano, Italyb CNR-IENI, Sezione di Lecco, Corso Promessi Sposi 29, 23900 Lecco, Italy

Received 17 November 2005; accepted 17 November 2005

Abstract

An alternative and simple manufacturing process is presented to produce high density bulk MgB2 superconducting objects of largedimensions. The process avoids the use of high pressure apparatus and consists in the reactive infiltration of liquid Mg in B powderspreforms. With an appropriately designed stainless steel container for the reactants, several manufacts of different shape have beenobtained, including tubes, cylinders, rings or disks, of dimensions of the order of the tens of centimeters.

The MgB2 material, unlike high temperature oxide superconductors, allows an easy percolation of the supercurrents across the grainsboundaries, even if it is in the polycrystalline form. Due to this property, the large MgB2 manufacts obtained by the reactive liquid infil-tration present high superconducting characteristics, as demonstrated by transport and magnetic measurements up to 35 K. In particularthe magnetic levitation and the magnetic shielding capability appear as the most promising applicative fields in which there is a need forlarge and homogeneously superconducting bulk pieces.

For space applications, the use of MgB2 superconductors may enable a substantial improvement in the compactness and weight ofcryogenic systems, with respect to the actual systems based on the liquid He temperatures. Furthermore, with respect to other high tem-perature oxide superconductors, MgB2, besides the disadvantages of the need of a 20–30 K cryogenic system, presents the advantages ofa relative lower density (2.4 g/cm3), higher mechanical strength and easier processability, like that here described.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Processing (A); Liquid neon (B); Magnetic levitation (F); Magnetic shielding (F)

1. Introduction

The superconductivity of MgB2 up to 39 K, discoveredearly 2001 [1], added a new member to the family of thesuperconducting materials that are able to overcome theneed for liquid helium refrigeration. MgB2 is a hard mate-rial and for its practical use it is important, from themechanical point of view and for the superconductingcharacteristics, to reach densities at least 90% of the theo-retical density (dtheor = 2.63 g/cm3). To achieve these den-sity values, the material needs to be compacted by hot

0011-2275/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.cryogenics.2005.11.011

* Corresponding author. Tel.: +39 02 62223194; fax: +39 02 62223074.E-mail address: Giovanni.giunchi@edison.it (G. Giunchi).

pressing, otherwise its thermal instability at high tempera-tures and normal pressure causes its decomposition to Mgand MgB4 [2]. This is the case either if the MgB2 powdersare already available (‘‘ex situ’’ process) or if the material isprepared by the reaction of Mg and B (‘‘in situ’’ process).An alternative technique to hot pressing, to obtain highdensity MgB2, has been discovered by EDISON SpA(Italy) and involves the reaction of the liquid Mg withthe B powders in a closed metallic container [3]. Avoidingcumbersome hot pressing mechanical apparatus, whichmakes manufacturing of large pieces more complex, thereactive liquid Mg infiltration (RLI) can use conventionalovens which allows an easy preparation of large bulkmanufacts. These are of particular interest in the powerapplications of superconductivity and also in space

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applications where relative low density of material is a pre-mium. In the present work the various aspects of the infil-tration technology are presented, particularly as they applyto the manufacture of large bulk pieces, several tens of cen-timeter in size. In such cases, a big role is played by themechanical and thermal characteristics of the materialsinvolved in the process, i.e. the reacting elements and themetallic container. Several examples of prototype manu-facts of different shape are discussed below and the super-conducting characteristics of some of them are emphasized.A final remark is dedicated to the cryogenic systems to beused for this superconducting material, that should operatein the range of temperatures between 20 and 30 K and withhigh reliability. Furthermore, for space applications, thechoice of a cryogenic system is dictated by more stringentconstrains in term of weight, volume and efficiency.

2. The reactive liquid Mg infiltration technology

The reaction between liquid Mg and B powders toobtain MgB2 benefits from favourable phase diagram inwhich the MgB2 is the most stable product at temperaturesof a few hundred K above the Mg melting point and atmoderate pressures [4]. Furthermore, during the reaction,a high volume shrinkage occurs and drives the percolationof further liquid Mg inside the B powders.

A typical setup of the infiltration process includes theuse of a stainless steel container in which the reacting ele-ments are located. The disposition of the Mg and B reac-tants preferably is such that: (a) the boron powders arepacked at a bulk density of about 50% of the boron fulldensity and are placed in contact with a Mg bulk piece;(b) the B powders must be set as much as possible in directcontact with the container walls, as discussed below; (c) theMg is in the form of a solid block; (d) the lid of the con-tainer is inserted so that no free space remains inside thecontainer, after the charge of the reactants. Then the con-tainer is sealed with conventional welding techniques andsubjected to a thermal treatment, at temperature rangingfrom 750 �C to 950 �C and for a duration from half to sev-eral hours. After cooling, the container is removed bymachining and the resulting MgB2 manufact occupiesalmost all the space initially occupied by the B powders.The space initially occupied by Mg is either empty or par-tially filled with residual Mg, if a stoichiometric or higheramount of Mg has been used.

Among the variables which influence the process anddetermine the quality of the resulting manufact, the grainsize of the boron powder is one of the most important[5]. An easy infiltration occurs, several centimeters deep,when crystalline boron powders are used, ground typicallyto less than 100 lm. In contrast, when using amorphousboron, normally of micronic grain size, it is more difficultto realize the infiltration, probably due to the difficultiesto control the cleanness of their surface in such powders.On the other hand, when such amorphous powders arewetted by the liquid and successfully infiltrated, they pro-

duces a very homogeneous micron scale morphology ofthe MgB2 product, with outstanding mechanical and super-conducting properties. According to the needs of the appli-cations we select the most appropriate boron powder.

The application of the RLI technology works well inmanufacturing either bulk material or wires. In the lattercase, hollow wires have been obtained showing, for shortlengths, very high critical current densities [6]. The exten-sion of the RLI technology to long wires and coils requiresfurther improvements, related mainly to the control of theliquid Mg movement.

During the attempts to better understand the reactionsteps of the RLI process and to describe carefully the com-position of the resulting MgB2 product, a new Mg boridecompound has been identified, Mg2B25, that grows duringthe first part of the infiltration process at temperatureslower that 750 �C and remains as small islands (severalhundred nanometer in size) inside the large crystallineMgB2 grains [7–9]. To our knowledge, the presence of thisminority phase does not prevent the flowing of the super-currents in the MgB2 grains, even if it can cause a reductionof the optimal current density.

3. Manufacturing issues for large dimension objects

For manufacts on the order of tens of centimetres wideusing the RLI technology, several precautions must betaken in order to obtain a uniform object with no defectsinside. The main ones are:

(a) Homogeneous packing of the boron powders.(b) Appropriate design of the steel container in order to

counteract the increasing internal pressure and tominimize the relative expansion of the reacting boronpowders.

(c) Disposition of the reacting Mg and B species insidethe container, in such a way that the liquid infiltra-tion occurs with minimal turbulence.

(d) Careful control of the thermal stresses, induced bythe different thermal expansion/contraction coeffi-cients of the steel container and of the MgB2 product,during the thermal transients.

As a typical example of the difficulties that must beavoided, we consider manufacturing of a disk plate. In thiscase two different directions of Mg liquid infiltration can bechosen, as shown in Fig. 1. But Fig. 1a choice is preferabledue to the direct contact of the B powders with the con-tainer walls, which induce a pressure on the reacting speciesand guarantee a more dense material and a planar surfaceof the final MgB2 product.

A further aspect, that must be carefully taken undercontrol, is the expansion of the final product with respectto the initial volume occupied by the boron powders pre-form, as illustrated in the following example. A MgB2 cyl-inder of external diameter of 115 mm and thickness of10 mm was obtained by the RLI process according to the

Fig. 1. (a) Different liquid Mg infiltration directions in the Boron powderspreform. The (b) case must be avoided due to turbulent motion of theliquid, which does not guarantee the maintaining of a planar shape of thefinal product.

Fig. 2. (a) The steel container for the reactants; (b) longitudinal cross-sections of the rods to show the quality of the RLI process.

Fig. 3. MgB2 tubes made by the RLI process.

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manufacturing conditions reported in the following. Inorder to counteract the internally generated pressure dur-ing the reaction, the container with the reactants was fur-ther encased in other steel containers. After reaction,both the inner and the outer containers bulged where theboron powders were located and the diameter expansionof the MgB2 cylinder was of the order of 3 mm, i.e. about3% of the initial diameter.

4. Examples

The following examples illustrate the approaches to pro-duce large bulk MgB2 manufacts by the RLI technique.Each manufact requires a careful design of the steel con-tainer and the distribution of the reactants.

4.1. Rods

In a steel tube (OD = 12 mm, ID = 10 mm) with theends enlarged to an internal diameter of 15.5 mm, as shownin Fig. 2a, were fitted two Mg bulk pieces, for a totalweight of 12 g. Inside the tube, crystalline Boron powders(10.0 g) were compacted with a pressure of about 1 GPa.The ends of the tube were sealed by TIG welding and thetube was encased by other tubular steel containers up toa final external diameter of 35 mm. The containers werethermal treated vertically, at 900 �C for 3 h. After reaction,a residual small amount of Mg was found only in the bot-tom extremity and the rod about 100 mm long was homo-geneously infiltrated and transformed into MgB2, as shownin Fig. 2b.

4.2. Tubes

Several MgB2 tubes have been manufactured by the RLIprocess. The maximum length was 26 cm for a diameter(ext/int) = 9/7 mm (Fig. 3). The manufacturing procedureincludes the filling of a stainless steel tube with a Mg rod,coaxially with the tube, and with crystalline boron pow-ders, ground and sieved to <100 lm. The relative weightof the reactants is Mg/B = 16.5/13.1. The tubular steel con-tainer was sealed by TIG welding. To guarantee itsstraightness at high temperature it was surrounded by semi

cylindrical halves, inserted in an other stainless steel tube.All the system was kept vertical during the thermal treat-ment performed at 900 �C for 2 h. After cooling, the stain-less steel tube was machined out and a MgB2 tube with aregular cylindrical cavity was obtained. Then the samplewas externally polished by diamond tool grinding, reveal-ing a full density microstructure.

4.3. Cylinders

We found it convenient, in order to manufacture cylin-drical objects, to apply the infiltration process on boronpowders located between two concentric steel cylinders,with the Mg percolating concurrently, from both top andbottom side, into the powders.

A thick wall cylinder was produced, as in Fig. 4. Thefinal dimensions, after grinding and after cuttingfour 1 mm thick rings, were: diameters(ext/int) = 48.25/25.57 mm, h = 58.74 mm. An amount of crystalline boronpowders (130 g, size <100 lm) was packed inside a doublewall cylindrical steel container, with two Mg rings of a totalweight of 150 g, positioned at the top and at the bottom of

Fig. 4. MgB2 cylinder made by RLI process. Several sliced rings cut byelectro erosion are also shown, revealing the uniformity of the infiltrationand a full density microstructure.

Fig. 6. Large flat rings sliced and surface grinded out of a single MgB2

ring, produced by RLI process.

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the boron powders. The container was sealed with TIGwelding and was heat treated at 900 �C for 2.5 h.

Fig. 5. (a) The stainless steel containers used to densify a MgB2 cylinder of100 mm diameter. The shiny section is the not oxidized surface of the innercontainer due to its expansion against the outer one (shown cut). (b) Slicedsections of the resulting MgB2 cylinder, extracted from the innercontainer, cut by electro erosion.

The MgB2 cylinder was cut from the container by elec-troerosion and its surface finishing was done by diamondtools grinding. The finished product shows a density of2.35 g/cm3.

A large diameter MgB2 cylinder, 100 mm wide, 75 mmhigh and about 10 mm thick, was manufactured by insert-ing 290 g of crystalline boron (<100 lm), packed betweentwo Mg rings of total weight of 345 g, inside a multilayersteel container. Fig. 5a shows the inner container, afterthe reaction at 850�C for 3 h, and a section of one of theouter containers. The shiny area is the not oxidized surface,corresponding to the place where the boron was locatedand the MgB2 was emerging, exerting a strong pressureagain the external container. In Fig. 5b are shown severalcircular pieces cut by electro erosion from the MgB2 cylin-der. They can be useful as magnetic guides for a levitatingrotor.

4.4. Flat rings

Two large MgB2 rings (diameter ext/int = 15/10 cm;h = 9 mm) (Fig. 6) were sliced by electro erosion from asingle MgB2 ring, manufactured by RLI process. Theboron powders (<100 lm crystalline boron) were com-pressed inside an annular cavity machined out of a cylindri-cal stainless steel container. To achieve an uniforminfiltration, two Mg rings were placed on both sides ofthe boron preform and a lid was sealed directly in contactwith the reactants. The amount of Mg for each ring was78.1 g (internal) and 121.7 g (external) and the B powderamount was 156.7 g, for an Mg/B wt. ratio of 1.27. Thereaction was performed at 850 �C for 1 h.

5. Superconducting properties

The presented manufacts are under characterization fordifferent superconducting applications such as currentleads (the tubes), magnetic levitators (the rings) or mag-netic shields (the cylinders). The transport and magnetic

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properties of such bulk MgB2 materials have been mea-sured on selected similar small pieces and already reported[7,10]. Here we summarize the values of the critical currentdensity at a typical temperature and magnetic field. In thecase of RLI bulk materials with large crystalline grains,typical values of the critical current density range between10 and 20 A/mm2 at 20 K and applied magnetic inductionof 3 T. For micron size grain material, at the same experi-mental condition, the critical current density reaches 50 A/mm2.

The magnetic shielding can be considered as one of themost easy applications, especially in the space environ-ment. In this respect we have evaluated the shielding capa-bility of rings, subjected to external DC + AC magneticfields coaxial with the sample main axis. The best perfor-mance has been measured for a macro-porous bulk MgB2

ring (diam. ext/int = 25/21 mm, h = 15 mm) which shieldsa total field of 1.25 T, made by a DC component of0.35 T plus an AC component, at 6 Hz, of 0.9 T [11].

As far as the magnetic levitation is concerned we haveverified that our MgB2 bulk samples can operate in theinduced persistent current mode, even if polycrystalline.An example of the magnetic levitation behaviour of discsproduced by the RLI process has been recently described[12] and shows that the levitation characteristics of thematerial remain almost constant up to 30 K.

6. Cryogenics issues

The superconducting characteristics of the bulk MgB2

enable applications in the range of temperatures between15 K and 30 K, that are intermediate between the liquidN2 temperature, expected to be used for the high tempera-ture superconducting cuprates, and the liquid helium tem-perature, used for the NbTi material. This means thatMgB2 has to face a double trade off between the supercon-ducting characteristics and the cost of the cryogenics: in thecase of the higher temperatures as well as in the other caseof the lower temperatures. At higher temperatures, the useof the cuprates is very limited up to now, both for technicaland economic reasons, so the real competition is with NbTiat 4.2 K. In this respect, space applications represent thefirst and most favourable niches that should demonstratethe great benefit of the MgB2 which, working at highertemperatures, avoids a massive use of liquid helium.

In order to reach the intermediate temperature rangesuitable for the MgB2 applications, only two cryogenic flu-ids, H2 or Ne, are available, but other options are dispos-able, several of which are listed below.

6.1. Conduction cooling

The superconductor will be thermally connected withthe cold stage of a cryocooler and shielded by a multilayerinsulation (MLI) blanket, to prevent the radiation losses.Furthermore, a vacuum chamber which reduces gas con-duction and convention to the warm walls is necessary.

6.2. Liquid/vapour convection cooling

The superconductor will be in contact with a flowingcryogenic liquid (Ne or H2) and the thermal exchangewill cause the vaporisation of the cryogen fluid, that willbe re-liquefied in an appropriate place, outside the cryostatcontaining the superconductor, but connected in a closedcycle.

6.3. Pool liquid cooling

The superconductor is immersed in the liquid cryogen,with the possibility to regulate the temperature by adjust-ing the pressure on the liquid to a certain extent.

6.4. Gas convection cooling

At the previously mentioned temperatures, practicallyonly He gas can be used as heat exchanger, even if its cool-ing capacity is very limited.

6.5. Solid cryogenic cooling

Possible use of solid N2, Ne or H2, as cryogenic environ-ment, was also taken into consideration in special cases[13].

Among these possibilities we are in favour of the solu-tions that use neon as the cryogen which is much safer thanH2. For this purpose we have developed a neon liquefierthat allows the continuous cooling of objects in a cryostatof about 3 l, and can be used for laboratory superconduc-ting measurements [14]. The actual higher cost of the neon,compared to the helium, is mostly due to a lack of a widemarket, rather than technical issues.

7. Conclusions

The advent of the MgB2 superconductivity opens newpossibilities in the application of superconducting sys-tems in space devices. The manufacturing of highly densi-fied bulk MgB2 objects of large dimension have beendemonstrated, relying on the application of the reactiveliquid infiltration process. The superconducting charac-teristics of the material so processed are equivalent tothose obtained by more elaborated hot pressing proce-dures. Among the various superconducting bulk applica-tions, magnetic levitation and magnetic field shieldingappear as the most promising and easily affordableones. With respect to low temperature superconductingmaterials, MgB2 shows the advantages of a more friendlycooling system and of a lower density, which is an appre-ciable advantage for bulk products. Comparing MgB2 withhigh temperature superconducting materials, it offers,together with a lower density, the advantages of bettermechanical characteristics and of an easier manufacturingprocess.

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