Multilayer composite material

Abstract

A multilayer laminar composite material (10) comprising a conductive substrate (1), an electrically conductive buffer structure (2) composed of one or more non-oxide films of metal nitrides and a superconducting film (3) composed of Fe(Se, Te), wherein Fe(Se, Te) is a compound comprising atoms of Fe and at least a chalcogen anion consisting of atoms of Se and atoms of Te in variable proportions; and wherein the conducting non-oxide film (2) is interposed between said conductive substrate (1) and said superconducting film (3).

Description

[0001] Multilayer composite material

[0002] The present invention relates to a superconducting multilayer laminar composite material and the related method for the preparation of said material.

[0003] In particular, the invention relates to a flexible laminar superconductor, formed by a conductive substrate, an iron-based superconducting (IBS) film of the sub-class of iron chalcogenides, and a film interposed between the two previous ones made of a conductive, but non-oxide, ceramic material.

[0004] In the present description as well as in the claims enclosed thereto, some terms and expressions are considered to assume, unless otherwise explicitly indicated, the meaning expressed in the following definitions .

[0005] Under "Fe(Se, Te) " an Iron chalcogenide is meant, and then a chemical compound comprising atoms of iron and at least a chalcogen anion consisting of a combination of Selenium and Tellurium atoms in variable proportions.

[0006] Under "nominal composition" of Fe(Se, Te) the stoichiometric ratio of the elements contained in the film precursor materials is meant, thereto the nominal composition is attributed, unless otherwise measured .

[0007] Under "laminar", with reference to the superconducting multilayer laminar composite material, a layered material is meant with a very reduced thickness with respect to at least one of its sizes on a plane, for example an overall thickness comprised between 50 and 300 m. Under "normal phase" a condition of the superconducting film above the critical temperature (Tc) is meant, wherein then it passes from the nature of superconductor to that of simple conductor.

[0008] Under " f our-terminal sensing technique" a technique for measuring the physical properties of the material involving the setting-up of four contacts on the sample is meant, with the subsequent measurement of the electric potential on a pair of the same, simultaneously to the transfer of an electric current between the other two.

[0009] Under "target" a sample of the material is meant, which is intended to be deposited in form of thin film on a substrate. In the processes of depositing thin films, the target is generally struck by an energy source which disintegrates it to allow the growing thereof on said substrate through a reassembly, spontaneous, or not.

[0010] The recent finding of the superconductivity in the iron-based compounds (IBS) [1] has aroused great interest in the scientific community: in few months many superconducting compounds belonging to this family have been prepared and characterized.

[0011] For the plainness of the crystalline structure, the simplest one among IBSs, and for the low level of toxicity of the elements composing it, the compound Fe(Se, Te) , of the sub-class of the iron chalcogenides, is one of the most studied [2, 3, 4] . When selenium is partially replaced with 50% tellurium, the critical temperature Tc, i.e. the temperature below of which the material becomes superconductor zeroing the electrical resistance thereof, reaches a maximum value of about 14-15 K. The development of epitaxial films of Fe(Se, Te) on monocrystalline substrates has revealed to be fundamental for the study of the intrinsic properties of this compound [5, 6, 7, 8] . In the films, Tc can be higher than the values obtained in the massive compound, up to about 21 K [9] .

[0012] The reason of this behaviour was detected in the compressing deformation of the crystal lattice due to the lattice adaptation between the substrate and the film

[0010] . Under the form of single crystals or epitaxial films, the material can transport a current of about 105Acnr2without dissipation, in presence of magnetic fields up to 30 Tesla [11, 12, 13] at the temperature of the liquid helium.

[0013] Considering these features, the technological exploitation of this material is possible for applications in high magnetic fields and low operating temperatures, for example melting, accelerators of particles having big sizes and nuclear magnetic resonance (NMR) .

[0014] Most of these systems require the manufacturing of superconducting materials under the form of wires and tapes for implementing cables. It will be appreciated that, even for implementing tapes it is required to implement a laminar material which can be further processed.

[0015] Compared to the currently most used superconducting cables, such as the NbsSn-based ones, the Fe(Se, Te)- based cables can be used in higher magnetic fields. Similar performances can be obtained with high critical temperature (HTS) superconductors (for example the YBCO compound of cuprate family) , however the costs for producing the HTS-based conductors due to the complex architecture and to the resulting complication of the production processes have limited the use thereof so far

[0014] .

[0016] Currently, the most used methods for producing superconducting wires and tapes are the metallurgic methods of the "powder in tube" (PIT) type and the processes based on the thin film technology. However, since the high reactivity of the elements limits the process conditions by jeopardizing the final performance

[0017] , the implementation of wires of Fe(Se,Te) with PIT techniques [15, 16] can be hardly achieved. On the contrary, the technologies for the deposition of the thin films are suited for the manufacturing of Fe(Se, Te) -based conductors [18, 19] .

[0017] The superconducting laminar materials, known in the state of art, are conductors implemented with a layered architecture comprising: a metal support called substrate, typically a foil of Ni or alloy of Ni, which must ensure the tape flexibility and eventually its cooperation with the superconducting film for the stabilization thereof, a superconducting film and one or more intermediate films called "buffer layer" (BL) having the main purpose of locking the diffusion of atoms from the substrate to the superconducting film. Among the films composing the BL structure, the one in direct contact with the superconducting film must ensure an optimum chemical and structural compatibility to promote the epitaxial growth of the superconducting film itself. The superconducting laminar materials , known in the state of art , are then finished by a protective film covering the superconducting film, a silver-made protective film in case of HTS , and by a coating, said stabili zing film, generally made of copper which guarantees the thermal and electrical stability of the superconducting film . The so- generated structure is better known at international level as "coated conductor" .

[0018] The two main production techniques of superconducting laminar materials di f fer in the features of the metal support used as substrate . I f the substrate has a cubic texture , that is a biaxial orientation of the grains composing it , the proces s is known as RABiTS [ 20 , 21 ] . After a thermomechanical treatment , the substrate , usually a Ni- and / or Cu-based alloy, develops a cubic texture and, by using techniques for growing the epitaxial film, the substrate texture is trans ferred to the superconducting fi lm through the BL .

[0019] With the IBAD ( Ion Beam Assisted Deposition) technique , the substrate is an alloy not characteri zed by texture , and the cubic texture is induced in one of the films contained in the BL structure through an ionic bombardment during deposition

[0022] .

[0020] One of the problems with the IBAD technique , is that the BL structure is very complex : films of oxides with amorphous structure to lock the di f fusion from the substrate and to provide a planar surface and layers above the film grown with the IBAD technique to guarantee the chemical and structural compatibility with the superconducting layer . The feasibility of Fe (Se, Te) -based superconducting laminar materials was demonstrated by using both IBAD and RABiTS techniques [13, 23, 24, 25, 26] . In particular, it was demonstrated that it is possible to obtain films of Fe(Se, Te) by using one single buffer layer [18, 27, 28] or, generally, less defined cubic textures with respect to those required for the HTS films

[0013] .

[0021] In the superconducting laminar materials, known to the state of art, the stabilizing film protects the superconducting film in case of accidental transition in the normal phase.

[0022] The normal phase is dissipative, and the increase in temperature by Joule effect can cause serious damages. To remove the current from the superconducting film in the normal state, it is possible to use a metal with low electrical resistance in contact with the superconducting film.

[0023] If the BL is formed by insulating materials, to protect the superconducting film one or more layers of metal material deposited on the superconducting film are required, forming the stabilizing film and the protective film.

[0024] By using conductive BLs, the thickness of the stabilizing film can be reduced or even the film can be removed.

[0025] Moreover, the protective film may be made of less expensive material. In principle, this would combine an improved performance with a finished product cost reduction .

[0026] By the IBAD technique, the BL structure includes always layers of insulating materials. On the contrary, the RABiTS technique allows the use of one or more conducting BLs . In the past, for developing HTS belts, BLs of conducting oxides were studied [29, 30, 31] . However, for the epitaxial growth of HTS films, a high temperature, higher than 800°C, and an atmosphere rich in oxygen were required .

[0027] Owing to the diffusion of oxygen through the BL, often unwanted not-conductive oxides arose at the substrate interface, lowering the electrical connection and the substrate effectiveness when it is used also as stabilizer.

[0028] In the past, TiN, a conductive ceramic material, was studied as BL for its capability in locking the diffusion of small metal ions from the substrate to the superconducting film, and to be able to exploit RABiTS substrates based on copper, Cu, or alloys of Cu [32, 33, 34, 35] . In SZWACHTA, G.; GAJEWSKA, M.; K C, S. Growth and Characterisation of Pulsed-Laser Deposited Tin Thin Films on Cube-Textured Copper at Different Temperatures. Archives of Metallurgy and Materials, 2016, 61.2B: 1031-1038, different methods for growing TiN on Cu are analysed, by finding a great sensitivity of TiN to the temperature conditions and to the pre-treatment procedures. In fact, TiN is highly reactive with oxygen above 400°C

[0036] , and it is necessary to protect it, by making the BL structures for HTS, containing a layer of TiN, complicated and unattractive.

[0029] One of the main problems for implementing superconducting laminar materials with conducting non-oxide BLs, especially when using TiN, is to avoid the formation of oxides at the interface between the substrate and the superconducting film. The formation of oxides between the substrate and the superconducting film reduces the electrical connection therebetween, and the effectiveness of the substrate as stabilizer too.

[0030] Among all the materials used as BL for growing films of Fe(Se,Te) , the cerium oxide (CeCh) represents the most promising selection, providing the growth of remarkable quality superconducting films [13,26,37] .

[0031] The epitaxial growth of CeCh films, or more generally of ceramic oxide films, requires the oxygen presence

[0038] . Disadvantageously, when the CeCh film is used as single buffer layer, particular strategies are required to avoid the substrate oxidation, which can complicate the production process: like growing a double layer of CeCh by depositing the first layer under vacuum or reducing atmosphere

[0039] .

[0032] US patent No. US 6,784,139 Bl relates to both conducting oxide and conducting non-oxide BLs containing nitrogen. The document mainly deals with the compatibility between a conductive substrate and the BL. According to some implementations, the invention comprises a superconducting film, in particular consisting of yttrium barium copper oxide (YBCO) grown above BL. The document stresses the importance of the lattice compatibility required to allow a good electrical conduction between the superconductor and the conducting substrate.

[0033] As it is already known, some oxides, in particular CeO2, meet the requirement of lattice compatibility, but in the phase of depositing the conducting oxides unwished non-conducting oxides could be created which reduce the ef fectiveness of the conducting substrate and of the electrical conductivity between the superconductor and the conductor . The nitrides are proposed as non-oxide BL in combination between them or with other oxides with the purpose of obtaining an architecture with a good compatibility with YBCO, unfortunately not generali zable to other types of superconductors .

[0034] Therefore , it is clear how important is the development of a superconducting multilayer laminar composite material with a simpli fied structure with good stabili zation performances and with reduced production costs .

[0035] A further problem relates the need of covering a superconducting film to prevent it be damaged by the temperature changes and by the magnetic fields acting thereupon . The coating must suit both to protect the superconductor and to leave the performances thereof , in terms of conductivity and magnetic field generation, substantially unchanged . In WEI , Shaoqing, et al . First performance test of FeSeO . 5TeO . 5-coated conductor coil under high magnetic fields . Superconductor Sci ence and Technol ogy, 2023 , 36 . 4 : 04LT01 , di f ferent comparative tests were carried out in order to identi fy a protective coating of a coated conductor of Fe ( Se , Te ) obtained with IBAD technique , by allowing to validate the applicability of this type of superconductors in applications with high- intensity magnetic fields .

[0036] The technical problem underlying the present invention is to provide a superconducting multilayer laminar composite material and a related method for producing said superconducting multilayer laminar composite material allowing to obviate the drawbacks mentioned with reference to the known art , which involve the formation of unwanted oxides at the interface between the substrate and the BL and between the BL and the superconducting film and / or the use of complex and expensive production methods .

[0037] In a first aspect , such a problem is solved by a superconducting multilayer laminar compos ite material as defined in the enclosed claim 1 .

[0038] The superconducting multilayer laminar composite material comprises a conductive substrate , an electrically conductive buf fer structure composed of one or more non-oxide films of metal nitrides , and a superconducting film composed of Fe ( Se , Te ) .

[0039] Preferably, the metal nitride is selected from a group consisting of : TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN and / or combinations thereof .

[0040] Still more preferably, the metal nitride is TiN .

[0041] In particular, the conducting non-oxide film is interposed between said conductive substrate and said superconducting f ilm .

[0042] Preferably, said superconducting multilayer laminar composite material is flexible .

[0043] The main advantage of the superconducting multilayer laminar composite material according to the present invention lies in the fact of providing a conducting laminar material pre ferably with a conductive buf fer structure consisting of only one non-oxide film, and a conductive substrate which can perform the double role of providing flexibility to the superconducting multilayer laminar composite material and to stabilize it in case of accidental transition in normal phase.

[0044] Another advantage consists in the non-oxide character of the film structure consisting of one or more metal nitrides, which plays the role of buffer layer. Indeed, conductive nitrides are not oxides, therefore it is allowed to grow them under a controlled atmosphere, deficient of O2, preventing the formation of non-conducting oxides at the substrate interface. In fact, the presence of nonconducting oxides at the interface reduces the electrical connection between the superconducting film and the conductive substrate and then the effectiveness of the substrate itself as stabilizer.

[0045] It will be appreciated that the superconducting film composed of Fe(Se, Te) is a non-oxide superconductor, therefore for its deposition the formation of non-conductive oxides at the interface between the superconducting film and the buffer structure and between the buffer structure and the conductive substrate is avoided.

[0046] Preferably, the superconducting film of Fe(Se,Te) comprises a plurality of epitaxial layers of Fe(Se,Te) grown on said buffer structure.

[0047] Each layer of said plurality of layers can be grown at a different temperature, preferably comprised between 200°C and 450°C.

[0048] Preferably, the superconducting film comprises a first epitaxial layer of Fe(Se,Te) , grown on said buffer structure, and a second epitaxial layer of Fe(Se,Te) grown on said first epitaxial layer. The presence of the first and second epitaxial layer improves crystallinity of the first layer and the second layer provides the superconducting properties between the buf fer structure and the superconducting layer, thereby preserving good superconducting properties and a low resistivity in normal phase .

[0049] Said first epitaxial layer may not be superconductive since it is arranged to improve the crystalline structure of the first layer, whereas the second epitaxial layer is arranged to guarantee superconductivity .

[0050] Preferably, said first epitaxial layer is grown at a temperature comprised between 300 ° C and 450 °C, more preferably 400 ° C, and said second epitaxial layer is grown at a temperature comprised between 200 ° C and 300 ° C, more preferably 250 ° C .

[0051] Advantageously, the higher temperature in growing the first epitaxial layer, and the lower one in growing the second epitaxial layer allows to check the crystalline structure and the stoichiometric composition of Fe ( Se , Te ) .

[0052] An additional advantage is given by the reduction of energy consumption for the deposition of the superconducting film of Fe ( Se , Te ) , by growing the first epitaxial layer with a higher degree of crystallinity at higher temperature , to decrease afterwards the temperature during the growth of the second epitaxial layer which has an intrinsic lattice compatibi lity, inasmuch as it is placed on the same material , then not requiring high temperatures to improve the lattice compatibility .

[0053] In a second aspect of the invention, such a problem is solved by a method for producing the superconducting multilayer laminar composite material as defined in the enclosed claim 13.

[0054] Such method includes setting up the conductive substrate on which the electrically conductive buffer structure, composed of one or more non-oxide films of metal nitrides, must be grown.

[0055] Once having completed the growing of the buffer structure, the superconducting film of Fe(Se, Te) is grown on the buffer structure.

[0056] As in case of the laminar composite material, the metal nitride is preferably selected from the group consisting of: TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN and / or combinations thereof.

[0057] Still more preferably, the metal nitride is TiN.

[0058] Advantageously, TiN keeps its conductive properties and the connection with the substrate in all method phases, allowing to achieve a superconducting multilayer laminar composite material with optimum superconductive properties and low resistivity in normal phase.

[0059] Preferably, in the method, the superconducting film is obtained by growing a first epitaxial layer of Fe(Se, Te) on the buffer structure and, subsequently, a second epitaxial layer of Fe(Se, Te) is grown on the first epitaxial layer.

[0060] The present invention will be described hereinafter according to a preferred embodiment thereof, provided by way of example and not with limiting purposes with reference to the enclosed drawings wherein : * figure 1A shows a schematic depiction, not in scale, of a superconducting multilayer laminar composite material according to the invention;

[0061] * figure IB shows a schematic depiction, not in scale, of a preferred embodiment of the superconducting multilayer laminar composite material according to the invention;

[0062] * figure 2A shows a XRD spectrum of a sample of Fe(Se, Te) deposited at 250°C on foil of TiN / NiW;

[0063] * figure 2B shows the (0-scans of the peaks (001) of Fe(Se, Te) , and of the peaks (002) of TiN and of NiW;

[0064] * figure 3 shows a diagram of the resistance in function of temperature R(T) of the sample of Fe(Se, Te) deposited on TiN / NiW at 250°C. In the inset, a magnification of the superconductive transition area is shown;

[0065] * figure 4 shows a diagram of the resistivity in function of temperature of the superconducting multilayer laminar composite material Fe(Se, Te) / TiN / NiW (stars) , of the conductive substrate (pellets) and of the conductive substrate coated by the conductive non-oxide film of TiN (rhombuses) . By comparison, the overall resistivity of a laminar material Fe(Se, TeJ / CeCh / NiW (black) , scale on the right, is shown.

[0066] With reference to the drawings, a superconducting multilayer laminar composite material, hereinafter more simply mentioned as "superconducting material", is designated with 10.

[0067] The superconducting multilayer laminar composite material 10 comprises a conductive substrate 1, an electrically conductive buffer structure 2 composed of one or more non-oxide films of metal nitrides and a superconducting film 3 composed of Fe(Se, Te) .

[0068] The buffer structure 2 is generally interposed between said conductive substrate 1 and said superconducting film 3.

[0069] The metal nitride is selected from the group consisting of: TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN and / or combinations thereof.

[0070] In the preferred embodiment example of the invention of figures 2A, 2B, 3, 4, the metal nitride is TiN.

[0071] It will be noted that, in the embodiments which are herein described, the TiN represents a preferred selection of metal nitride since, apart from being an oxide and then preventing the formation of nonconducting oxides at the interface with the conductive substrate 1, it may have a cubic crystalline structure which is particularly compatible with the conductive substrate 1, e.g. with a conductive substrate 1 composed of NiW and with a superconducting film 3 in Fe(Se, Te) , during all the phases for making the multilayer laminar composite material 10.

[0072] In this connection, with reference to the embodiment of figure 1A, the architecture of the superconducting material 10 is very simplified and it provides: an electrically conductive buffer structure 2 consisting of one single non-oxide film interposed between the substrate 1 and the superconducting film 3. The buffer structure 2 allows an effective current transfer from the superconducting film 3 to the conductive substrate 1 when the superconducting film 3 is in normal phase. An effective current transfer improves the electrical and, consequently, thermal stability of the superconducting material 10.

[0073] The normal phase is dissipative and the increase in temperature may cause serious damages to common superconducting materials, but, according to the present invention, the superconducting material 10, through the good thermal connection between the buffer structure 2 and the substrate 3, shows an overall low resistivity in normal phase, thus promoting the thermal and electrical stability.

[0074] The conductive substrate 1 is made of a metal provided with cubic texture which, in particular but not exclusively, is selected from a group consisting of: Ni, Cu, W, Al, Ag, Fe, V and / or their alloys.

[0075] In this embodiment, the conductive substrate 1 is composed of Ni and W, with an atomic percentage of W comprised between 4% and 10%, preferably equal to 5%, and the superconducting film has a nominal composition of Fe(Se<i-X), Tex) , preferably Fe(Seo.s, Teo.5) ; therefore, the iron-based superconducting film 3 comprises 50% in Se by atoms, and 50% in Te by atoms on the total Se and Te atom amount .

[0076] With reference to the embodiment of figure IB, the multilayer laminar composite material 10 comprises, in addition to the structure described with reference to figure 1A, a protective film 4 which is arranged adjacent to the superconducting film 3, and a stabilizing film 5 which is then placed onto the protective film 4. The protective f ilm 4 is composed of a material selected from the group consisting of : Ag, Al , Cu, Fe , nitrogen-containing compounds such as TiN and / or alloys or combinations thereof .

[0077] It will be appreciated that the superconducting material 10 , once it has been obtained under the form of foil , can be processed to make wires or tapes to possibly implement electrical cables .

[0078] Advantageously, this type of electrical cables can be used for applications in high magnetic fields and low operating temperatures , for example large si ze particle accelerators and nuclear magnetic resonance (NMR) .

[0079] In order to prepare the superconducting multilayer laminar composite material 10 it is requested the setting up of the conductive substrate 1 , growing the electrically conductive buf fer structure 2 consisting of one or more non-oxide films on the conductive substrate 1 , and at last growing the superconducting film 3 of Fe ( Se, Te ) on the buf fer structure 2 .

[0080] In the preferred embodiments shown in figures the buf fer structure consists of only one non-oxide film of metal nitride , preferably of TiN .

[0081] According to some embodiments of the invention, the superconducting f ilm 3 is obtained by growing a first epitaxial layer of the superconducting film 3 o f Fe ( Se , Te ) on the buf fer structure 2 and afterwards a second epitaxial layer of the superconducting film 3 of Fe ( Se , Te ) is grown on the first epitaxial layer .

[0082] According to the present invention, an example of a preferred method for producing the multilayer laminar composite material 10 includes the use of the pulsed laser deposition ( PLD) technique for the Fe ( Se , Te ) superconducting film 3 , which has conditions compatible with the presence of a film of TiN; therefore , it is preferable to use a film of this material as single film of metal nitride of the buf fer structure 2 between the superconducting film 3 and the conductive substrate 1 .

[0083] According to this embodiment , the conductive substrate 1 is a metal alloy provided with Ni- and / or Cu-based cubic texture . Under the typical conditions of depositing the Fe-based superconducting film 1 , the TiN acts as barrier against the di f fusion of metals coming from the substrate .

[0084] At last , the TiN film maintains its conductive properties and the connection with the substrate 1 after the production of the whole superconducting material 10 . In particular, a superconducting material 10 , Fe ( Se , Te ) / TiN / Ni-W allows to obtain samples with optimum superconducting properties and moreover a low resistivity in normal phase , up to 50 times lower than the architectures with Fe-based superconducting film currently among the most promising ones ( figure 4 ) .

[0085] The advantage of the superconducting material 10 is that the conductive substrate 1 can provide , partially or totally, the electrical and thermal stability required to the superconducting material 10 .

[0086] With reference to the above illustrated preferred embodiments of method and superconducting material 10, it will be appreciated that the conductive substrate 1 is a commercial metal foil based on nickel and tungsten, Ni-5 at . % W (NiW) , provided with cubic texture

[0021] . The films were deposited with the PLD technique by using preferably the fourth harmonic (266 nm) or wavelengths of 355 nm, 532 nm and 1064 nm with a solid-state laser at Nd:YAG Q- switched, the used repetition frequency is preferably of 3 Hz or between 10 and 20 Hz and the fluence on a target is comprised between 1 and 2 J / cm2. In other embodiments the films can be deposited with an excimer laser operating in the ultraviolet as KrF (248 nm) or XeCl (308 nm) .

[0087] According to alternative implementing procedures, the films can be deposited by using the sputtering technique .

[0088] Method implementation examples

[0089] The distance between the target and the conductive substrate 1 is arranged at about 40 mm. The layer of TiN has been grown in a deposition room. After having reached a base pressure of 0.2-2.2xl0~6mTorr, the chamber was filled-in with N2 gas until reaching a pressure comprised 20 and 0.1 mTorr, at a temperature between 600 and 400°C, starting from a first stoichiometric commercial target [32, 40] . The deposition of the superconducting film 3 of Fe (Se, Te) took place under vacuum with a pressure between 3xl0~7and 3xl0~6mbar. A second target, produced in laboratory had a nominal composition FeSeo.sTeo.s

[0041] . The growing speed of the superconducting film 3 of Fe(Se, Te) was about 0.06 nm / s . The structural properties were analysed by X-ray diffraction with 20 angular dispersion measurements. The quality of the crystallographic orientation outside the plane was estimated from the width to half height (Full Width at Half Maximum (FWHM) ) of scans in a> , )— scan. The dependence of resistance upon temperature R(T) was measured with the f our-terminal sensing technique .

[0090] The conductive substrate of NiW was coated with a buffer structure 2 consisting of a non-oxide film 2 of TiN grown at a temperature of 500°C

[0035] . The films were epitaxial, the FWHMs of ) —scans for the TiN(002) and NiW (002) peaks had values which on the average are 5.2° and 6.9°, respectively (figure 2B) .

[0091] The superconducting film 3 of Fe(Se,Te) was deposited by using a two-step approach

[0042] . A first superconducting epitaxial layer of Fe(Se,Te) was deposited on the buffer structure 2 of TiN and a second superconducting epitaxial layer was deposited on the first layer, by reducing the deposition temperature between the first and the second step.

[0092] Owing to the homo-epitaxy mechanisms, this approach allowed to control both the crystalline structure and the stoichiometric composition of the superconducting film 3 of Fe(Se,Te) , and it ensured excellent superconducting performances [42, 26, 37 43] .

[0093] Indeed, the lattice incompatibility between TiN and Fe(Se,Te) was of 27% (on the diagonal of the lattice parameter) or of 11% (cube on cube) .

[0094] Despite this high incompatibility, the superconducting multilayer laminar composite material 10 showed good superconducting properties and a low resistivity in normal phase, thanks to the two-step deposition which allows the formation of a good crystalline structure in the first epitaxial layer and provides the superconducting properties between the buffer structure 2 and the superconducting film 3 through the second epitaxial layer. Figure 2A shows the XRD spectrum of the superconducting composite material 10 by depositing on the conductive substrate of NiW the TiN at 500°C, the film of Fe(Se,Te) at 250°C on the surface of a film of the same material deposited at 400°C. The whole structure was oriented with the axis-c perpendicular to the conductive substrate, there are only the reflections (001) of the existing films.

[0095] Figure 2B shows the "Rocking Curves" (RC) , previously defined as o-scan, of the conductive substrate 1 of NiW, of the conducting non-oxide film of TiN, and of the superconducting film of Fe(Se, Te) , the value of FWHM of 3.2° shows an excellent epitaxial growth of the film of Fe(Se,Te) .

[0096] Figure 3 shows the dependence of resistance of sample Fe(Se, Te) / TiN / NiW in function of temperature, the same sample of figures 2A and 2B, and the inset shows a detail of the superconducting transition.

[0097] To evaluate the electrical connection between the superconducting film 3 and the conductive substrate 1 the overall resistivity of the sample was measured by using the Van der Pauw method, to determine the resistivity starting from measures of electrical resistance by knowing the thickness of the superconducting material 10

[0044] .

[0098] Figure 4 shows the resistivity curves in function of temperature for the conductive substrate 1 of NiW, the conductive substrate 1 of NiW with the conducting non-oxide film 2 of TiN and the superconducting film 3 Fe(Se, Te) / TiN / NiW.

[0099] By comparison, even the measurement on a sample grown on a same type of substrate is shown, thereon a film of Cerium-based oxide was deposited: Fe(Se, Te) / CeO2 / NiW.

[0100] The latter sample is characterized by a resistance curve in normal phase with a bell-like course. The bell-like course is characteristic of the material Fe(Se,Te)

[0045] to confirm the fact that the cerium oxide isolates the superconducting film from the substrate .

[0101] Considering instead the film of TiN, the resistivities of the sample with only TiN and of the sample with the superconducting film 3 are comparable, within the experimental uncertainty, and similar to that of the conductive substrate 1. This involves an effective current transfer from the superconducting film 3 to the substrate through the conducting non-oxide film 2.

[0102] The resistivity is 50 times lower than the Cerium oxide film, as evidence to the fact that the current transfer from the superconducting film 3 to the conductive substrate 1 is guaranteed by the film of TiN. These results show that the film of TiN provides an optimum electrical connection between the superconducting film 3 and the metal substrate 1.

[0103] To the above-described multilayer laminar composite material and to the related method for its preparation a person skilled in the art, with the purpose of satisfying additional and contingent need, could introduce several additional modi fications and variants , however all within the protective scope of the present invention, as defined by the enclosed claims .

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Claims

CLAIMS1. A superconducting multilayer laminar composite material (10) comprising- a conductive substrate (1) ;- an electrically conductive buffer structure (2) composed of one or more non-oxide films of metal nitrides;- a superconducting film (3) composed of Fe(Se, Te) , wherein Fe(Se, Te) is a compound comprising atoms of Fe and at least a chalcogen anion consisting of atoms of Se and atoms of Te in variable proportions; and wherein the buffer structure (2) is interposed between said conductive substrate (1) and said superconducting film (3) .

2. The multilayer laminar composite material (10) according to claim 1, wherein said superconducting film (3) comprises a first epitaxial layer of Fe(Se,Te) grown on said buffer structure (2) and, a second epitaxial layer of Fe(Se,Te) grown on said first epitaxial layer.

3. The multilayer laminar composite material (10) according to claim 1 or 2, wherein the metal nitride is selected from the group consisting of: TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN and / or combinations thereof .

4. The multilayer laminar composite material (10) according to claim 1 or 2, wherein the metal nitride consists of TiN.

5. The multilayer laminar composite material (10) according to any one of the preceding claims, whereinthe conductive substrate (1) is a metal selected from the group consisting of: Ni, Cu, W, Al, Ag, Fe, V and / or alloys of the same.

6. The multilayer laminar composite material (10) according to any one of the preceding claims, wherein the conductive substrate (1) is a metal foil with cubic texture.

7. The multilayer laminar composite material (10) according to any one of the preceding claims, wherein the conductive substrate (1) is composed of Ni and of W with W comprised between 4% and 10% in atomic percentage .

8. The multilayer laminar composite material (10) according to any one of the preceding claims, wherein the superconducting film (3) has a composition of FeSe(i-X)Texwith x comprised between 0 and 1.

9. The multilayer laminar composite material (10) according to any one of the preceding claims further comprising a protective film (4) , adjacent to the superconducting film (3) , and a stabilizing film (5) placed on the protective film (4) .

10. The multilayer laminar composite material (10) according to claim 9 wherein the protective film (4) is composed of a material selected from the group consisting of: Ag, Al, Cu, Fe, nitrogen-containing compounds such as TiN and / or alloys or combinations thereof .

11. The multilayer laminar composite material (10) according to claim 9 or 10 wherein the stabilizing film (5) is composed of a material selected from the group consisting of: Cu, Al and / or alloys or combinations thereof.

12. The multilayer laminar composite material (10) according to one of the preceding claims used in form of foil, tape, wire or cable.

13. A method for the preparation of a multilayer laminar composite material (10) , comprising the following steps:• setting up a conductive substrate (1) ;• growing an electrically conductive buffer structure (2) composed of one or more non-oxide films of metal nitrides on the conductive substrate ( 1 ) ;• growing a superconducting film (3) of Fe(Se, Te) on the buffer structure (2) .

14. The method according to claim 13, wherein the metal nitride is selected from the group consisting of: TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN and / or combinations thereof.

15. The method according to claim 13, wherein the metal nitride consists of TiN.

16. The method according to claim 13, wherein the conductive substrate (1) has a cubic texture and is composed of Ni-W, wherein the W is preferably comprised between 4% and 10% in atomic percentage.

17. The method according to one or more of claims 13 to 16, wherein the superconducting film (3) is obtained by growing a first epitaxial layer of Fe(Se, Te) , not necessarily superconducting, on the buffer structure (2) and, subsequently a second epitaxial layer of Fe(Se, Te) is grown on the first epitaxial layer .

18. The method according to claim 17, wherein saidfirst epitaxial layer of superconducting film (3) is grown at a temperature comprised between 300°C and 450°C and said second epitaxial layer of superconducting film (3) is grown at a temperature comprised between 200°C and 300°C.

19. The method according to one or more of claims 13 to 18, wherein, in order to grow the buffer structure (2) and the superconducting film (3) , the Pulsed Laser Deposition (PLD) technique is used.

20. The method according to one or more of claims 13 to 19, for the preparation of a multilayer laminar composite material (10) , comprising the following steps :• setting up a conductive substrate (1) on a heating element;• setting up a deposition room;• setting up a first target of TiN;• bringing the pressure in the deposition chamber between 3xl0~5Pa and 3xl0~4Pa;• bringing the substrate to a T comprised between 400°C and 600°C;• bringing the pressure of the deposition chamber between 0.01 Pa and 2.6 Pa by introducing N2;• growing a buffer structure (2) consisting of a non-oxide film of TiN on the conductive substrate (1) by using the PLD technique;• removing the first target;• setting up a second target of Fe(Se, Te) ;• bringing the pressure in the deposition chamber between 3xl0~5Pa and 3xl0~4Pa;• bringing the temperature between 300°C and 450°C;• growing a first epitaxial layer of thesuperconducting film (3) of Fe(Se, Te) on the buffer structure (2) by using the PLD technique ;• bringing the temperature between 200°C and 300 °C; and• growing a second epitaxial layer of the superconducting film (3) of Fe(Se, Te) on the first epitaxial layer of superconducting film (3) by using the PLD technique.

21. The method according to claim 20, wherein, for the PLD technique, a fourth harmonic (266 nm) of a Nd:YAG Q-switched solid-state laser with repetition frequency at 3 Hz and fluence on the second target comprised between 1-2 J / cm2is used.

22. The method according to claim 20, wherein the distance between the first target and the conductive substrate (1) is comprised between 30 mm and 50 mm.

23. The method according to claim 20, wherein the distance between the second target and the buffer structure (2) is comprised between 30 mm and 50 mm.

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