EP2761628A1

EP2761628A1 - Electric element including a layer of a polyuric material with electrical conductivity gradient

Info

Publication number: EP2761628A1
Application number: EP12775773.0A
Authority: EP
Inventors: Arnaud Allais; Jérôme Fournier; Lazhar KEBBABI; Lorrene Bayon; Anthony COMBESSIS; Lionel Flandin
Original assignee: Nexans SA
Current assignee: Nexans SA
Priority date: 2011-09-28
Filing date: 2012-09-27
Publication date: 2014-08-06
Also published as: AU2012314162A1; US20140246220A1; US9697925B2; AU2012314162B2; FR2980622A1; WO2013045845A1; FR2980622B1

Abstract

The present invention relates to an electrical element (100, 101, 102) including an electrically conductive element (3, 5, 10, 31, 32, 51), characterised in that the electrical element also includes a first layer (1) of a polymer material with electrical conductivity gradient obtained from a polymer composition including at least one polymer and conductive carbonaceous fillers.

Description

Electrical element comprising a layer of a polymer material with a gradient of electrical conductivity

The present invention relates to an electrical element comprising a layer of an electrically conductive gradient polymer material based on conducting carbon charges, intended to improve the breakdown resistance as well as the resistance to aging in a humid environment under electrical tension.

It applies typically, but not exclusively, to the fields of medium voltage (in particular 6 to 45-60 kV) or high voltage (especially greater than 60 kV) and up to 500 to 600 kV, or up to 800 kV), whether DC or AC.

It can be used as a polymeric layer surrounding the electrical conductor of this type of cable; or as a polymeric layer of an accessory used with this type of cable, such as for example a termination or a junction.

More particularly, the layer according to the invention is intended to be positioned at the interface between a conductive material (e.g., electrical conductor, semiconductor layer, etc.) and an electrically insulating material (e.g., electrically insulating layer).

The discontinuity of electrical properties between an electrically insulating material and a conductive (or semiconductor) material may cause a local reinforcement of the electric field by accumulation of space charges or charged species capable of initiating a tree under action. an electric field. In particular, the presence of moisture combined with the presence of an electric field with a polymer material promote the progressive degradation of the insulating properties of medium and high voltage power cables.

This degradation mechanism, well known under the terms "growth of electric trees due to water" (or "water treeing" in English), can thus lead to the breakdown of the electric cable concerned and therefore constitutes a considerable threat to the reliability of the transmission network with well-known economic consequences caused by short circuits.

The object of the present invention is to overcome the disadvantages of the prior art techniques by providing an electrical element, intended in particular for use in the field of medium voltage or high voltage energy cables, having a breakdown resistance electric and resistance to aging in a humid environment in the presence of an electric field, significantly improved.

The present invention relates to an electrical element comprising an electrically conductive element, characterized in that the electrical element further comprises a first layer of a polymer material with a gradient of electrical conductivity obtained from a polymeric composition comprising at least a polymer and conductive carbon charges.

Preferably, the electrical element may further comprise a second layer of an electrically insulating material, said first layer being positioned between the electrically conductive element and the second layer.

The Applicant has surprisingly discovered that the presence of a layer of a polymer material with a gradient of electrical conductivity, in particular when it is positioned between an electrically conductive element and an electrically insulating material, makes it possible to limit effectively, or even to avoid, the degradation related to electrical trees caused by space charges or charged species induced in particular by the presence of water in this type of electrical element.

In addition, in combination with an electrically insulating material, the layer according to the invention makes it possible to significantly reduce the reinforcements of the electric field on the surface of said electrically insulating material: the electric element thus has improved resistance to breakdown in AC or DC voltage. .

The term "electrically conductive element" means an element that may be a semiconductor material or an electrically conductive material.

In the present invention, the electrical conductivity of a semiconductor material can be at least 1.10 ^"9 S / m (siemens per meter), preferably at least ^{1.10" 3} S / m, and preferably can be less than 1.10 ³ S / m.

The electrical conductivity of an electrically conductive material may be at least 1.10 ³ S / m.

The electrical conductivity of an electrically insulating material may be at most 1.10 ^"9 S / m. In the present invention, the electrical conductivity of a material is conventionally determined according to ASTM D 991.

"Polymeric material with a gradient of electrical conductivity" is understood to mean a composite polymer material whose electrical conductivity varies gradually in the thickness of the layer constituted by said polymer material with an electrical conductivity gradient.

According to a first variant, the layer of the electrical conductivity gradient material may comprise several sublayers each having constant, and increasing and / or decreasing conductivities, so as to form said electrical conductivity gradient in a so-called discrete manner.

In a second variant, the layer of the electrical conductivity gradient material may be a single layer in which the electrical conductivity varies gradually (increasing and / or decreasing).

We will prefer the second variant which is more economical and easier to implement industrially.

The first layer may be defined as a layer having a thickness delimited by a first and a second surface, the first surface preferably being substantially parallel to the first surface.

The gradient of electrical conductivity in the thickness of the first layer can range from electrical conductivity:

from an electrically insulating material to that of a semiconductor material, from an electrically insulating material to that of an electrically conductive material, or

from a semiconductor material to an electrically conductive material.

These characteristics of electrical conductivity are in particular functions of the nature of the materials in physical contact with said first layer. Each of the faces of the first layer may have an electrical conductivity which is substantially of the same type as the electrical conductivity of the material with which each of its faces is respectively in contact.

More particularly, when the electrical element comprises the second layer, the electrical conductivity at the surface of the first layer closest to the electrically conductive element will preferably be greater than the electrical conductivity at the surface of the nearest first layer of the second layer.

In other words, the electrical conductivity gradient of the first layer is such that the electrical conductivity decreases progressively from the surface of the first layer closest to the electrically conductive element to the surface of the first layer closest to the second layer: the first layer therefore has a higher electrical conductivity on the electrically conductive element side second layer.

By way of example, the electrical conductivity gradient of the first layer may be between 1.10 ³ S / m and 1.10 ^"18 S / m (inclusive), and preferably between 1.10 ^{" 1} and 1.10 ^"11 S / m (terminals included). "Polymeric material" is understood to mean a material obtained from a composition based on one or more polymers, making it possible in particular to shape it by extrusion, injection molding or molding.

"Conductive carbon charge" is understood to mean any particle, or mixture of particles, mainly consisting of carbon atoms, functionalized or otherwise, grafted or not, and having electrically conductive properties.

By way of examples, the conductive carbonaceous fillers are chosen from carbon blacks, carbon fibers, graphites, graphenes, fullerenes, and carbon nanotubes, or a mixture thereof.

Carbon nanotubes are preferably used. The term "nanotubes" means nanoparticles of substantially elongate shape, and whose smallest dimension may be between 1 and 100 nm (inclusive) (dimension determined by microscopic analysis such as SEM (Scanning Electron Microscopy), TEM (Transmission Electron Microscopy) or by MFA (Atomic Force Microscopy) Nanotubes typically have a so-called "acicular" shape.

Carbon nanotubes have the advantage of having better compatibility with the polymer of the polymeric composition, compared with the other types of conductive carbon fillers mentioned in the present invention.

In addition, carbon nanotubes having a high form factor, especially at least 1000, they achieve the percolation with relatively low amounts of conductive carbonaceous charges compared to other carbonaceous charges. The form factor is typically the ratio between the smallest dimension of the conductive filler (ie, the diameter, for the carbon nanotubes) and the largest dimension of said conductive filler (ie, the length, for the carbon nanotubes).

Thus, thanks to the use of carbon nanotube type fillers, the mechanical and electrical properties as well as the adhesion properties of the first layer are optimized.

Carbon nanotubes can be of several types. They may be chosen from single-walled carbon nanotubes, double-walled carbon nanotubes, and multiwall carbon nanotubes, or a mixture thereof. Multi-walled carbon nanotubes, well known under the Anglicism "multi-walled nanotubes (MWNT)", will preferably be used.

The amount of conductive carbonaceous fillers in the polymer composition of the invention is in particular sufficient to constitute a percolating network.

By "percolating network" is meant an organization of conductive fillers creating one or more continuous electrical paths within the polymeric material of the first layer.

The polymeric composition of the first layer may comprise at most 30% by weight of conductive carbonaceous fillers, preferably at most 10% by weight of conductive carbonaceous fillers, and particularly preferably at most 5% by weight of conductive carbonaceous fillers. Preferably, it comprises at least 0.1% by weight of conductive carbonaceous fillers. The first layer is obtained from a polymer composition comprising at least one polymer in which said conductive carbon fillers are incorporated, to form a composite polymer material.

Preferably, the polymeric composition of the first layer is based on one or more polymers, so that it can be shaped easily, in particular by extrusion, injection or molding.

The polymeric composition may be a thermoplastic or elastomeric composition, crosslinkable or not.

The polymeric composition of the first layer may be a thermoplastic composition, that is to say that it mainly comprises one or more thermoplastic polymers with respect to the polymers constituting the polymeric composition.

The polymeric composition of the first layer may be an elastomeric composition, that is to say that it mainly comprises one or more elastomeric polymers with respect to the polymers constituting the polymeric composition.

When the polymeric composition is crosslinkable, it may further comprise one or more crosslinking agents.

The polymer of the polymeric composition of the invention may be chosen from an organic polymer and an inorganic polymer, or a mixture thereof

When the polymer of the polymer composition relating to the first layer is an organic polymer, said organic polymer may comprise at least one polyolefin and / or at least one polyepoxide. The term "polyolefin" as such generally means homopolymer or copolymer of olefin. Preferably, the olefin polymer is a homopolymer of ethylene, or a copolymer of ethylene (ie copolymer comprising at least ethylene).

By way of example of ethylene polymers, mention may be made of linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), low density polyethylene (LDPE) and medium density polyethylene (MDPE). ), high density polyethylene (HDPE), copolymers of ethylene and vinyl acetate (EVA), copolymers of ethylene and butyl acrylate (EBA), methyl acrylate (EMA), 2-hexylethyl acrylate (2HEA), copolymers of ethylene and alpha-olefins such as for example polyethylene-octene (PEO), polyethylene-butene (PEB), copolymers of ethylene and propylene (EPR) ) such as for example terpolymers of ethylene propylene diene (EPDM), poly (ethylene terephthalate) (PET), or a mixture thereof, and / or a derivative thereof.

It is preferable to use an EVA with a low level of vinyl acetate groups (less than 20% by weight) in order to limit the presence of polar functions, or more advantageously a polyethylene of the VLDPE, LDPE, LLDPE, MDPE or HDPE type.

The polymeric composition of the first layer may comprise more than 50.0 parts by weight of polyolefin per 100 parts by weight of polymer (s) (ie polymer matrix) in the composition, preferably at least 70 parts by weight of polyolefin per 100 parts by weight of polymer (s) in said composition, and particularly preferably minus 90 parts by weight of polyolefin per 100 parts by weight of polymer (s) in said composition.

In a particularly advantageous manner, the constituent polymer (s) of the polymeric composition of the first layer are only one or more polyolefins. In this case, it will be preferred to use a single type of polymer in the composition such as EVA with a low level of vinyl acetate moieties, or VLDPE, LDPE, LLDPE, MDPE or HDPE.

The term "polyepoxide" (or "epoxide polymer") as such generally means a multi-component polymer obtained by polymerization of epoxide monomers with a crosslinking agent, said crosslinking agent being of the acid anhydride type, phenol, or amine.

As an example of a polyepoxide, mention may be made of DiGlycidylether of Bisphenol A (DGEBA).

When the polymer of the polymeric composition relating to the first layer is an inorganic polymer, said inorganic polymer may comprise at least one polysiloxane. In the present invention, the inorganic polymers are therefore very different from the organic polymers.

In fact, the polysiloxanes, or silicones, are inorganic compounds formed of a silicon-oxygen chain (... -Si-O-Si-O-Si-O-...) on which groups can be attached to the silicon atoms.

By way of example, mention may be made of poly (dimethylsiloxane). The electrically conductive element of the invention may be, according to a first variant, an electrical conductor, preferably metallic, and in particular of elongate shape.

Considering the second layer and said metallic electrical conductor, the first layer is positioned between the second layer and the electrical conductor. The electrical conductivity gradient of the first layer is such that the electrical conductivity decreases progressively from the surface of the first layer closest to the electrical conductor to the other surface of the first layer (ie the surface closest to the second layer ): the first layer therefore has a greater electrical conductivity on the electrical conductor side than the second layer side.

The electrically conductive element of the invention may be, according to a second variant, a third layer of a semiconductor material.

By considering the second and third layers, the first layer is thus positioned between the second layer and the third layer. The electrical conductivity gradient of the first layer is such that the electrical conductivity decreases progressively from the surface of the first layer closest to the third layer to the other surface of the first layer (ie the surface closest to the second layer): the first layer therefore has a greater electrical conductivity in the third layer than the second layer side.

In the present invention, the semiconductor material may be a material obtained from a composition comprising at least one filler (Electrically) conductive (or semiconductor charge), in an amount sufficient to render said semiconductor composition.

The composition used to obtain a semiconductor material may comprise from 0.002 to 40% by weight of (electrically) conductive fillers, preferably at least 15% by weight of conductive fillers, and even more preferably at least 25% by weight of fillers. conductive.

The conductive filler may advantageously be chosen from all types of conductive fillers well known to those skilled in the art.

In a particular embodiment, the assembly formed by the first, second and third layers is a tri-layer insulation. In other words, the first layer is in direct physical contact with the second layer, and the second layer is in direct physical contact with the third layer.

The electrical element according to the invention may further comprise a fourth layer of a material identical to that of the first layer (ie polymer material with gradient of electrical conductivity), and a fifth layer of a semiconductor material, said fourth layer being positioned between the second layer and the fifth layer.

In a particular embodiment, the electrical conductivity gradient of the fourth layer, when positioned between the second layer and the fifth layer, is such that the electrical conductivity decreases progressively from the surface of the fourth layer closest to the fifth layer to the other surface of the fourth layer (ie the surface closest to the second layer): the fourth The layer therefore has a higher electrical conductivity in the fifth layer than in the second layer.

Preferably, the second layer, the third layer, and / or the fifth layer are layers of polymeric material.

In a particular embodiment, the first, second, third, fourth and / or fifth layers are extruded layers, in particular coextruded layers.

In another particular embodiment, the first, second, third, fourth and / or fifth layers may be thermoplastic layers, or thermosetting layers, crosslinked by methods well known to those skilled in the art.

According to a first embodiment of the invention, the electrical element may advantageously be an electric cable, in particular a medium or high voltage electrical cable, in which the electrically conductive element is the electrical conductor, this electrical conductor being surrounded by by the first layer. This electrical conductor may be an elongated metallic electrical conductor.

When the electric cable comprises the second layer, the latter surrounds the electrical conductor. The first layer is thus positioned between the second layer and the electrical conductor.

According to a second embodiment of the invention, the electrical element may advantageously be an electrical cable, in particular a medium or high voltage electrical cable, in which the electrically conductive element is the third layer of semi-electrical material. driver. The electric cable of this second embodiment comprises then the electrical conductor, this electrical conductor being surrounded by the first layer and the third layer. This electrical conductor may be an elongated metallic electrical conductor.

When the electric cable comprises the second layer, the first layer is positioned between the second layer and the third layer. Preferably, the second layer surrounds the third layer.

In this second embodiment, the electric cable may further comprise the fourth and fifth layers.

When the electric cable comprises the second layer, the fourth layer can then be positioned between the fifth layer and the second layer. Preferably, the fifth layer surrounds the second layer.

In a particularly preferred embodiment, when the electric cable comprises the second layer, the electric cable comprises said electrical conductor, and successively around this electrical conductor, are arranged the third layer, the first layer, the second layer, the fourth layer and the fifth layer.

More particularly, the assembly formed by the first, second, third, fourth and fifth layers constitutes a penta- layer insulation. In other words, the third layer is in direct physical contact with the electrical conductor, the first layer is in direct physical contact with the third layer, the second layer is in direct physical contact with the first layer, the fourth layer is directly in contact with the first layer. in physical contact with the second layer, and the fifth layer is in direct physical contact with the fourth layer.

The electrical cable of the invention may further comprise a metal screen surrounding the fifth layer.

This metal screen may be a so-called "wired" screen, consisting of a set of copper or aluminum conductors arranged around and along the fifth layer, a so-called "ribbon" screen composed of one or more conductive metallic ribbons placed (s) helically around the fifth layer, or a so-called "tight" screen type metal tube surrounding the fifth layer. This last type of screen makes it possible in particular to provide a moisture barrier that tends to penetrate the electrical cable radially.

All types of metal screens can play the role of grounding the electric cable and can thus carry fault currents, for example in the event of a short circuit in the network concerned.

In addition, the electric cable of the invention may also comprise an outer protective sheath surrounding the fifth layer, or more particularly surrounding said metal screen when it exists. This outer protective sheath can be made conventionally from suitable thermoplastic materials such as HDPE, MDPE or LLDPE; or materials retarding the propagation of the flame or resisting the spread of fire. In particular, if the latter materials do not contain halogen, it is called cladding type HFFR (for the Anglicism "Halogen Free Flame Retardant"). According to a third embodiment of the invention, the electrical element may be a junction for an electrical cable, in particular for medium or high voltage electrical cable, in which the electrically conductive element is a semiconductor material. This semiconductor material is defined in the present invention, and may refer to the third layer.

The junction for electric cable makes it possible to connect two electric cables, in particular two medium or high voltage electrical cables.

A junction is conventionally an elongated electrical element inside which are positioned two electrical cables whose electrical conductors are brought into contact. More particularly, one end of each of two cables is positioned within the junction, so that the junction surrounds said ends of the electrical cables.

Considering that these two electric cables are medium or high voltage energy cables comprising an elongated electrical conductor conventionally surrounded by a tri-layer insulation of the so-called internal semiconductor layer surrounded by an electrically insulating layer, the latter being surrounded by an so-called external semiconductor layer, a junction for an electric cable may typically comprise:

a third layer of a semiconductor material intended to be in contact with the electrical conductors of the two electric cables, a second layer of an electrically insulating polymeric material intended to be in contact with the electrically insulating layers of the two electric cables, and

a fifth layer of a semiconductor material intended to be in contact with the external semiconductor layers of the two electric cables.

The junction according to the invention may comprise said first layer positioned between the third layer and the second layer. Said junction may also comprise a fourth layer of a material identical to that of the first layer, this fourth layer being positioned between the second layer and the fifth layer.

More particularly, the various constituent layers of the junction may surround said ends of the electric cables.

A particular embodiment of this second variant may be a trifurcation, or in other words a "T" shaped junction for connecting three electric cables.

According to a fourth embodiment of the invention, the electrical element may be a termination for an electrical cable, in particular for medium or high voltage electrical cable, in which the electrically conductive element is a semiconductor material. This semiconductor material is defined in the present invention, and may refer to the third layer.

A termination, better known under the Anglicism "cone stress", is conventionally an elongated electrical element, cone-shaped, inside which is positioned an electric cable. In particular, one of the ends of the electrical cable is positioned inside the termination, so that the termination surrounds said end of the electrical cable.

Considering that said electric cable is a medium or high voltage power cable comprising an elongated electrical conductor conventionally surrounded by a tri-layer insulation of the so-called internal semiconductor layer type surrounded by an electrically insulating layer, the latter being surrounded by a so-called external semiconductor layer, an electrical cable termination may typically include:

a third layer of a semiconductor material intended to be in contact with the outer semiconductor layer of the electric cable, and

a second layer of an electrically insulating polymeric material, intended to be in contact with the electrically insulating layer of the electric cable.

The termination according to the invention may comprise said first layer positioned between the third layer and the second layer.

More particularly, the various constituent layers of the termination may surround said end of the electric cable.

Another subject of the invention relates to a method of manufacturing a layer of a polymer material with a gradient of electrical conductivity for an electric element as described in the present invention, characterized in that it comprises the step of heat-treating a layer of a polymeric material obtained from a polymeric composition as defined in the invention (ie a polymeric layer comprising at least one polymer and conductive carbonaceous fillers), said layer comprising a thickness delimited by a first and a second surface, these two surfaces being preferably substantially parallel, said processing step being carried out by applying a first temperature T1 to the first surface and a second temperature T2 to the second surface, so as to form a temperature gradient in the thickness of said layer and obtain a gradient layer of electrical conductivity (see first layer, and optionally fourth layer).

More particularly, at least one of the temperatures T1 or T2 is a temperature equal to or higher than the melting point T _f (eg semi-crystalline polymers) or glass transition T _g (eg amorphous polymers) of said polymer.

Of course, it is necessary that the temperature T1 is different from the temperature T2 to form said gradient of electrical conductivity.

The difference between the temperatures T1 and T2 may be at least 10 ° C, preferably at least 50 ° C, and particularly preferably at least 100 ° C.

This heat treatment (ie heating step) of the polymer composition (s) according to the invention (comprising conductive carbonaceous fillers) advantageously makes it possible to form a gradient of electrical conductivity in the thickness of the layer (s) obtained from said polymeric composition.

Indeed, thanks to the heat treatment, the relaxation of the polymer chains in the molten or viscous state, associated with flocculation forces, cause a modification of the microstructure of the composition polymer, thus promoting the formation or improvement of the percolating network.

The kinetics of this self-arrangement of the conductive carbonaceous charges is particularly dependent on the temperature. In other words, the high temperatures compared with the melting temperature T _f of the semi-crystalline or glass transition polymers T _g of the amorphous polymers result in high load network reinforcement kinetics and lead to high values of conductivities of polymeric materials. On the other hand, at temperatures close to T _f or T _g , the auto-setting rate remains low and the electrical properties of the polymeric material are only slightly affected. In other words, the completion of the network of conductive carbonaceous charges, associated with the conductivity levels presented by the material, is directly correlated with the temperature of the heat treatment called "curing treatment".

The characteristic temperatures of the physicochemical transitions of a polymer or of a polymer composition, of the crosslinked or non-crosslinked type, can be classically determined by differential scanning calorimetry (DSC) with a temperature ramp of 10 ° C./min under atmosphere nitrogen.

In a particularly advantageous embodiment, when the electric element is an electric cable according to the invention, the electric conductor of the electric cable is used as a heat source for a time necessary to form the electrical conductivity gradient in the thickness of the layer or layers comprising conductive carbonaceous fillers.

When the electric cable comprises only the first and second layers, and optionally the third layer, the heat treatment can be performed once these two or three layers shaped around the electrical conductor of the electric cable.

A current is applied through the electrical conductor of the electric cable to heat at a temperature T1, by Joule effect or by induction, the layer comprising the conductive carbon charges (see first layer), while the outside of the electric cable is maintained at a temperature T2 lower than the temperature T1.

In this case, the temperature T1 is a temperature greater than T _f (especially when the polymer is a semi-crystalline polymer), or T _g (especially when the polymer is an amorphous polymer). The temperature T2 is in turn a temperature lower than T _f (especially when the polymer is a semi-crystalline polymer), or T _g (especially when the polymer is an amorphous polymer).

When the electrical cable comprises all of the third, first, second, fourth and fifth layers, the heat treatment can be performed once these five layers shaped around the electrical conductor of the electric cable.

A current is passed through the electrical conductor of the electric cable to heat at a temperature T1, by Joule effect or by induction, the layer comprising the most carbonaceous conductive fillers. close to the electrical conductor (see first layer), the second layer remaining at a temperature T2 below the temperature T1.

The outside of the electric cable is heated to a temperature T1, in particular by convection, conduction or irradiation, the layer comprising the carbon-conducting charges closest to the outside of the electric cable (see fourth layer), the second layer remaining at a temperature T2 lower than the temperature T1.

The temperatures T1 and T2 being defined as those mentioned above (see for the electric cable comprising only the first and second layers, and optionally the third layer).

The heat treatment step is preferably performed after any implementation step of the one or more layers that make up the electrical element.

In addition, if the layer of a polymer material with a gradient of electrical conductivity is a crosslinked layer, the heat treatment step is preferably carried out prior to the crosslinking of said layer.

Preferably, the implementation of the layer or layers that make up the electrical element can be carried out by extrusion, injection or molding. In particular, coextrusion of said layers will be preferred.

Other features and advantages of the present invention will appear in light of the examples which follow with reference to the annotated figures, said examples and figures being given for illustrative and not limiting. Figure 1 shows a schematic cross-sectional view of an electric cable according to a preferred embodiment according to the invention.

FIG. 2 represents a schematic view in longitudinal section of a junction for an electric cable, according to the invention.

FIG. 3 represents a schematic view in longitudinal section of a termination for an electric cable, according to the invention.

FIG. 4 represents the evolution of the temperature for treating a polymeric composition according to the invention.

FIG. 5 shows the evolution of the electrical conductivity within the polymer composition treated according to FIG. 4.

For the sake of clarity, only the essential elements for understanding the invention have been shown schematically, and this without respect of the scale.

The medium or high voltage power cable 100, illustrated in FIG. 1, comprises an elongated central electrical conductor 10, in particular made of copper or aluminum, and, successively and coaxially around this central electrical conductor 10 is:

a layer 3 of a semiconductor polymeric material (i.e. "third layer"), said internal semiconductor layer,

a layer 1 of a polymer material with a gradient of electrical conductivity (i.e. "first layer"),

a layer 2 of an electrically insulating polymeric material (ie "second layer"), a layer 4 of a polymer material with a gradient of electrical conductivity (ie "fourth layer"), and a layer 5 of a semiconductor polymeric material (ie "fifth layer"), said outer semi-conducting layer.

The layers 1, 2, 3, 4 and 5 are extruded layers, and crosslinked or not.

The gradient of electrical conductivity in the thickness of the layer 1 is such that the surface of the layer 1 in contact with the layer 3 has an electrical conductivity greater than that of the surface of the layer 1 in contact with the layer 2.

A metal screen (not shown) of the cylindrical tube type, as well as an outer protective sheath (not shown), can also be positioned around the fifth layer.

The junction 101 for medium or high voltage electrical cable illustrated in Figure 2, is an elongated member of the tubular type for receiving at its center two electrical cables 100A and 100B to connect. The connection can be made by means of an electrically conductive part 11 directly in contact with the electrical conductors 10A and 10B of each of the two electrical cables 100A and 100B.

Considering that these two electrical cables 100A and 100B are medium or high voltage power cables each of the central electrical conductors 10A and 10B is surrounded by a tri-layer insulation of the so-called internal semiconductor layer type (layer not shown ) surrounded by an electrically insulating layer 2A, 2B, the latter being surrounded by a so-called external semiconductor layer 5A, 5B, the junction according to the invention comprises:

a layer 31 of a semiconductor material (i.e. "third layer"), in electrical contact with the electrical conductors 10A and 10B of the two electrical cables 100A and 100B, through the electrically conductive part 11,

a layer 1 of a polymer material with a gradient of electrical conductivity (i.e. "first layer"), covering the layer 3,

a layer 21 of an electrically insulating polymeric material (i.e. "second layer"), covering the layer 1 and being in contact with the electrically insulating layers 2A and 2B of the two electric cables,

a layer 4 of a polymer material with a gradient of electrical conductivity (i.e. "fourth layer"), covering the layer 21, and

a layer 51 of a semiconductor material (i.e. "fifth layer") covering the fourth layer 4 and being in contact with the outer semiconductor layers 5A and 5B of the two electric cables.

Thus, the first layer 1 is positioned between the third layer 31 and the second layer 21; and the fourth layer 4 is positioned between the second layer 21 and the fifth layer 51.

Terminator 102 for medium or high voltage electrical cable shown in FIG. 3 is an elongated cone-shaped element, within which the end of an electrical cable 100A is positioned. Considering that said electric cable 100A is a medium or high voltage power cable comprising an elongated central electrical conductor 10A surrounded by a tri-layer insulation of the so-called internal semiconductor layer type (layer not shown) surrounded by an electrically layer 2A insulator, the latter being surrounded by a so-called external semiconductor layer 5A, the termination according to the invention comprises:

a layer 32 of a semiconductor material (i.e. "third layer") intended to be in contact with the outer semi-conducting layer 5A of the electric cable,

a layer 22 of an electrically insulating polymeric material

(i.e. "second layer"), intended to be in contact with the electrically insulating layer 2A of the electric cable, and

a layer 1 of a polymer material with a gradient of electrical conductivity (i.e. "first layer") positioned between the third layer 32 and the second layer 22.

EXAMPLES A polymeric material having an electrical conductivity gradient according to the invention was made by applying a temperature gradient on a composite polymer sample by "melt" blending of a polymeric composition comprising:

97.5% by weight of EVA (with 12% by weight of vinyl acetate groups), marketed by ExxonMobil, under the reference Escorene UL0112, the melting temperature of the EVA being 96 ° C, and

2.5% by weight of multi-conductor carbon nanotubes sold by Arkema under the reference Graphistrength C100.

This mixture is produced according to the following two steps:

pre-mixing the conductive carbon nanotubes with the polymer matrix (i.e. EVA) melted in a Brabender brand internal mixer, for 15 minutes, at a temperature of 110 ° C, then

homogenizing the composite using a Scamia cylinder mixer for 20 minutes, the roll temperature being between 120 and 130 ° C.

Once the homogenization has been carried out, the polymeric composition is shaped by hot pressing to obtain plates 1 mm thick, according to the following process

3 minutes at 110 ° C without pressure,

3 minutes at 110 ° C, under a pressure of 3 t, and

quenched with cold water (15 ° C) for 2 minutes.

16 mm diameter pellets were then cut from the plates thus formed, and alternatively arranged with aluminum pellets of the same diameter and 12 μm thickness in a thermally insulating mold. The presence of the aluminum pellets only makes it easier to measure the electrical conductivity as a function of the distance to the heat source. The alternating stack of pellets of polymeric composition (ie composite polymer) and aluminum pellets form a test specimen, both ends of which consist of pellets of polymeric composition. The height of the specimen is 30 mm.

The specimen is then placed in a temperature gradient oven consisting of an upper tray and a lower tray. The temperature gradient was established by applying a set point of 300 ° C (upper plate) on the upper face of the test specimen, keeping the temperature of the underside of said specimen at room temperature (ie 25 ° C. lower plate). The corresponding temperature profile was measured at five points during the heat treatment period (see Figure 4).

The test was carried out for 90 minutes under a light stream of nitrogen gas. At the end of the treatment, the test piece was cooled for 15 minutes between the two trays regulated at 15 ° C.

After complete cooling of the system, the test piece was demolded and electrically tested element by element.

The DC electrical conductivity of the polymeric composition (i.e. pellets of polymeric composition) was measured by the four point method using a Keithley 2602 SMU.

FIG. 5 represents the evolution of the electrical conductivity of the polymeric composition as a function of the position relative to the heat source during the application of the temperature gradient according to FIG. 4. An electrical conductivity gradient is clearly highlighted. in the thickness of the polymeric composition according to the invention. Indeed, increasing the distance to the heat source - which is equivalent to a decrease in the heat treatment temperature - results in a decrease in microstructural rearrangement kinetics (ripening). This implies a variation of completion of the network of conductive carbon nanotubes which is translated on a macroscopic scale, by a reduced electrical conductivity.

In the present case, a variation of three orders of magnitude of the electrical conductivity was observed (measured minimum = 55.5 S. m- ¹ , and measured maximum = 4.2 × 10 ² S. m- ¹ ).

It is important to emphasize that the amplitude of the electrical conductivity gradient formed in the thickness of the material, as well as the maximum and minimum electrical conductivity values of said gradient can be modulated by controlling the profile of the applied temperatures, as well as the duration of the heat treatment.

Claims

Electrical element (100, 101, 102) comprising an electrically conductive element (3, 5, 10, 31, 32, 51), characterized in that the electrical element further comprises a first layer (1) of polymeric material gradient of electrical conductivity obtained from a polymer composition comprising at least one polymer and conductive carbon charges.

Electrical element according to claim 1, characterized in that the electrical element further comprises a second layer of an electrically insulating material (2, 21, 22), said first layer (1) being positioned between the electrically conductive element and the second layer.

Electrical element according to claim 2, characterized in that the electrical conductivity at the surface of the first layer closest to the electrically conductive element is greater than the electrical conductivity at the surface of the first layer closest to the second layer. .

Electrical element according to any one of the preceding claims, characterized in that the conductive carbonaceous fillers are chosen from carbon blacks, carbon fibers, graphites, graphenes, fullerenes, and carbon nanotubes, or one of their mixtures.

5. Electrical element according to any one of the preceding claims, characterized in that the carbon nanotubes are chosen from single-walled carbon nanotubes, double-walled carbon nanotubes, and multiwall carbon nanotubes, or one of their mixtures.

6. Electrical element according to any one of the preceding claims, characterized in that the electrical conductivity of the first layer (1) is between 1.10 ³ S / m and 1.10 ^"18 S / m (inclusive).

7. Electrical element according to any one of the preceding claims, characterized in that the polymeric composition of the first layer (1) comprises at most 30% by weight of conductive carbonaceous charges.

8. Electrical element according to any one of claims 1 to 7, characterized in that the electrically conductive element is an electrical conductor (10).

9. Electrical element according to any one of claims 1 to 7, characterized in that the electrically conductive element is a third layer (3, 5, 31, 32, 51) of a semiconductor material.

10. Electrical element according to any one of claims 2 to 9, characterized in that it further comprises a fourth layer (4) of a material identical to that of the first layer (1), and a fifth layer ( 5, 51) of a semiconductor material, said fourth layer (4) being positioned between the second layer (2, 21) and the fifth layer (5, 51).

Electrical element according to claim 8, characterized in that the electrical element is an electrical cable (100) in which the electrically conductive element is the electrical conductor (10).

12. Electrical element according to claims 2 and 11, characterized in that the second layer (2) surrounds the electrical conductor (10).

13. Electrical element according to claim 9, characterized in that the electrical element is an electrical cable (100) wherein the electrically conductive element is the third layer (3).

Electrical element according to claim 9, characterized in that the electrical element is a junction (101) for an electric cable in which the electrically conductive element is the third layer (31).

Electrical element according to claim 9, characterized in that the electrical element is an electrical cable termination (102) in which the electrically conductive element is the third layer (32).

16. A method of manufacturing a layer (1, 4) of a polymer material with an electrical conductivity gradient for an electrical element as defined in any one of the preceding claims, characterized in that it comprises the step method of thermally treating a layer of polymeric material obtained from a polymeric composition comprising at least one polymer and fillers conductive carbon layers, said layer comprising a thickness delimited by a first and a second surface, said treatment step being carried out by applying a first temperature T1 to the first surface and a second temperature T2 to the second surface, so as to form a gradient of temperature in the thickness of said layer and obtaining said layer (1, 4) of a material with gradient of electrical conductivity.

17. The method of claim 16, characterized in that at least one of the temperature T1 or T2 is a temperature equal to or greater than the melting temperature Tf or glass transition Tg of said polymer.

18. Manufacturing method according to claim 16 or 17, characterized in that, when the electrical element is an electric cable (100) according to claim 11 or 13, the electrical conductor (10) of the electric cable (100) is used as a source of heat for the application of the temperature gradient.