Electrical element comprising a layer of a polymeric material having an electrical conductivity gradient

Abstract

The present invention related to an electrical element (100, 101, 102) including an electrically conductive element (3, 5, 10, 31, 32, 51), characterized 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.

Claims

1. An electrical element comprising: an electrically conductive element, wherein the electrical element has a first layerof a polymeric material having an electrical conductivity gradient, which material is obtained from a polymeric composition having at least one polymer and conductive carbon fillers and where said electrical conductivity of said polymeric material gradually varies in the thickness of the first layer.

2. The electrical element as claimed in claim 1, wherein the electrical element has a second layer of an electrically insulating material, said first layer being positioned between the electrically conductive element and the second layer.

3. The electrical element as claimed in claim 2, wherein the electrical conductivity at the surface of the first layer closest the electrically conductive element is higher than the electrical conductivity at the surface of the first layer closest the second layer.

4. The electrical element as claimed in claim 2, wherein said electrical element has a fourth layer of a material identical to that of the first layer, and a fifth layer of a semiconductive material, said fourth layer being positioned between the second layer and the fifth layer.

5. The electrical element as claimed in claim 2, the wherein second layer encircles the electrical conductor.

6. The electrical element as claimed in claim 1, wherein the conductive carbon fillers are selected from the group consisting of carbon blacks, carbon fibers, graphites, graphenes, fullerenes, carbon nanotubes or one of their mixtures.

7. The electrical element as claimed in claim 1, the carbon nanotubes are selected from the group consisting of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes, or one of their mixtures.

8. The electrical element as claimed in claim 1, wherein the electrical conductivity of the first layer is comprised between 110.sup.3 S/m and 110.sup.18 S/m (limits inclusive).

9. The electrical element as claimed in claim 1, wherein the polymeric composition of the first layer comprises at most 30% by weight conductive carbon fillers.

10. The electrical element as claimed in any one of claims 1 to 9, the electrically conductive element is an electrical conductor.

11. The electrical element as claimed in claim 10, wherein the electrical element is an electrical cable in which the electrically conductive element is the electrical conductor.

12. The electrical element as claimed in claim 1, wherein the electrically conductive element is a third layer of a semiconductive material.

13. The electrical element as claimed in claim 12, wherein the electrical element is an electrical cable in which the electrically conductive element is the third layer.

14. The electrical element as claimed in claim 12, wherein the electrical element is an electrical cable joint in which the electrically conductive element is the third layer.

15. The electrical element as claimed in claim 12, wherein the electrical element is an electrical cable terminal in which the electrically conductive element is the third layer.

16. A process for manufacturing a layer of a polymeric material having an electrical conductivity gradient, for an electrical element such as defined in claim 1, said method comprising the steps of: a heat treatment of a layer of a polymeric material obtained from a polymeric composition comprising at least one polymer and conductive carbon fillers, said layer having a thickness bounded 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 temperature gradient in the thickness of said layer and to obtain said layer of a material having an electrical conductivity gradient.

17. The process as claimed in claim 16, wherein at least one of the temperatures, T1 or T2, is a temperature equal to or above the melting point Tf or glass transition temperature Tg of said polymer.

18. The manufacturing process as claimed in claim 16, wherein when the electrical element is either one of an electrically conductive element of an electrical cable or a electrically conductive third layer, the electrical conductor of the electrical cable is used as a heat source to apply the temperature gradient.

19. The electrical element as claimed in claim 1, wherein said electrically conductive element is an element different from said first layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the present invention will become apparent in light of the following examples, which are described with reference to the annotated figures, said examples and figures being given by way of completely nonlimiting illustration.

(2) FIG. 1 shows a schematic cross-sectional view of an electrical cable according to one preferred embodiment according to the invention.

(3) FIG. 2 shows a schematic longitudinal-section view of an electrical cable joint according to the invention.

(4) FIG. 3 shows a schematic longitudinal-section view of an electrical cable terminal according to the invention.

(5) FIG. 4 shows the variation in temperature used to treat a polymeric composition according to the invention.

(6) FIG. 5 shows the variation in the electrical conductivity within the polymeric composition treated according to FIG. 4.

DETAILED DESCRIPTION

(7) For the sake of clarity, only elements that are essential to understanding the invention have been shown, schematically and not to scale.

(8) The medium- or high-voltage power cable 100 illustrated in FIG. 1 comprises an elongate central electrical conductor 10, especially made of copper or aluminum, and, in succession and coaxially about this central electrical conductor 10, there is: a layer 3 of a semiconductive polymeric material (i.e. the third layer), referred to as the internal semiconductive layer; a layer 1 of a polymeric material having an electrical conductivity gradient (i.e. the first layer); a layer 2 of an electrically insulating polymeric material (i.e. the second layer); a layer 4 of a polymeric material having an electrical conductivity gradient (i.e. the fourth layer); and a layer 5 of a semiconductive polymeric material (i.e. the fifth layer), referred to as the external semiconductive layer.

(9) The layers 1, 2, 3, 4 and 5 are optionally cross-linked, extruded layers.

(10) The electrical conductivity gradient in the thickness of the layer 1 is such that the surface of the layer 1 making contact with the layer 3 has a higher electrical conductivity than that of the surface of the layer 1 making contact with the layer 2.

(11) A cylindrical-tube type metal screen (not shown) and an external protective cladding (not shown) may also be positioned around the fifth layer.

(12) The medium- or high-voltage electrical cable joint 101 illustrated in FIG. 2 is a tubular-type elongate element that is designed to receive, in its center, two electrical cables 100A and 100B in order to connect them. This connection may be achieved by means of an electrically conductive part 11 making direct contact with the electrical conductors 10A and 10B of each of the two electrical cables 100A and 100B.

(13) 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 of which is encircled by a three-layer insulation system of the type made up of what is called an internal semiconductive layer (layer not shown) encircled by an electrically insulating layer 2A, 2B, the latter being encircled by what is called an external semiconductive layer 5A, 5B, the joint according to the invention comprises: a layer 31 of a semiconductive material (i.e. the third layer) making electrical contact with the electrical conductors 10A and 10B of the two electrical cables 100A and 100B, by way of the electrically conductive part 11; a layer 1 of a polymeric material having an electrical conductivity gradient (i.e. the first layer), covering the layer 3; a layer 21 of an electrically insulating polymeric material (i.e. the second layer), covering the layer 1 and making contact with the electrically insulating layers 2A and 2B of the two electrical cables; a layer 4 of a polymeric material having an electrical conductivity gradient (i.e. the fourth layer), covering the layer 21; and a layer 51 of a semiconductive material (i.e. the fifth layer), covering the fourth layer 4 and making contact with the external semiconductive layers 5A and 5B of the two electrical cables.

(14) 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.

(15) The medium- or high-voltage electrical cable terminal 102 illustrated in FIG. 3 is an elongate cone-shaped element inside of which the end of an electrical cable 100A is positioned.

(16) Considering that said electrical cable 100A is a medium- or high-voltage electrical cable comprising an elongate central electrical conductor 10A encircled by a three-layer insulation system of the type made up of what is called an internal semiconductive layer (layer not shown) encircled by an electrically insulating layer 2A, the latter being encircled by what is called an external semiconductive layer 5A, the terminal according to the invention comprises: a layer 32 of a semiconductive material (i.e. the third layer) intended to make contact with the external semiconductive layer 5A of the electrical cable; a layer 22 of an electrically insulating polymeric material (i.e. the second layer), intended to make contact with the electrically insulating layer 2A of the electrical cable; and a layer 1 of a polymeric material having an electrical conductivity gradient (i.e. the first layer), positioned between the third layer 32 and the second layer 22.

EXAMPLES

(17) A polymeric material having an electrical conductivity gradient according to the invention was produced by applying a temperature gradient to a composite polymer sample obtained by melt blending of a polymeric composition comprising: 97.5% by weight of the EVA (with 12% by weight vinyl acetate groups) sold by ExxonMobil under the reference Escorene UL0112, the melting point of the EVA being 96 C.; and 2.5% by weight of the conductive multiwalled carbon nanotubes sold by Arkema under the reference Graphistrength C100.

(18) This blend was produced in the following two steps: pre-mixing of the conductive carbon nanotubes with the molten polymer matrix (i.e. the EVA) in a Brabender internal mixer, for 15 minutes at a temperature of 110 C.; then homogenizing of the composite using a Scamia two roll mill for 20 minutes, the temperature of the rolls being comprised between 120 and 130 C. Once homogenized the polymeric composition was shaped by hot pressing in order to obtain 1 mm-thick sheets, the following process being used: 3 minutes at 110 C. without pressure; 3 minutes at 110 C. under a pressure of 3 t; and quenching in cold water (15 C.) for 2 minutes.

(19) 16 mm-diameter disks were then cut from the sheets thus formed and arranged in alternation with 12 m-thick aluminum disks of the same diameter in a thermally insulating mold. The aluminum disks were present merely to facilitate the measurement of electrical conductivity as a function of distance from the heat source.

(20) The alternated multilayer of disks of polymeric composition (i.e. composite polymer) and disks of aluminum formed a test sample the two ends of which consisted of disks of polymeric composition. The height of the sample was 30 mm.

(21) Next, the sample was placed in an oven having a temperature gradient, which oven consisted of an upper plate and a lower plate. The temperature gradient was established by applying a setpoint of 300 C. (upper plate) to the upper face of the test sample, while keeping the temperature of the lower face of said sample at room temperature (i.e. 25 C.; lower plate). The corresponding temperature profile was measured at five points throughout the heat treatment (see FIG. 4).

(22) The trial continued 90 minutes under a gentle flow of nitrogen gas. At the end of the treatment, the sample was cooled for 15 minutes between the two plates, set to 15 C.

(23) After the system had completely cooled, the sample was removed from the mold and tested electrically element by element.

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

(25) FIG. 5 shows the variation in the electrical conductivity of the polymeric composition, as a function of position relative to the heat source during the application of the temperature gradient in FIG. 4. An electrical conductivity gradient is clearly observed in the thickness of the polymeric composition according to the invention.

(26) This is because increasing distance to the heat sourcewhich is equivalent to decreasing the temperature of the heat treatmenthas the effect of slowing the dynamics of the microstructural rearrangement (maturing). This leads to a variation in the completeness of the network of conductive carbon nanotubes, which, on the macroscopic scale, results in a lower electrical conductivity.

(27) In the present case, a variation of three orders of magnitude was observed in the electrical conductivity (minimum measured=0.55 S.Math.m.sup.1, and maximum measured 4.210.sup.2 S.Math.m.sup.1).

(28) It is important to underline that the amplitude of the electrical conductivity gradient formed in the thickness of the material, just like the maximum and minimum electrical conductivity values of said gradient, may be modulated by controlling the applied temperature profile and the duration of the heat treatment.