Conductive composite produced from coated powders

Abstract

Some embodiments are directed to the manufacture of functional composites (electrical conductors, thermal conductors, etc.) produced from coated powders.

Claims

1. A sintered dense conductive composite material only constituting of conductive particles, the conductive particles including a core of organic material clad with at least one layer of an electrical and/or thermal conductor material, wherein the entirety of the particles being sintered in order to be within an internal structure of the conductive composite material and in order to form an a continuous three-dimensional network of conductive material, the core of organic material possesses a particle size of between 50 μm and 300 μm, the core of organic material is a thermoplastics selected from polyethylenes (PE), polypropylenes (PP), polyetheretherketones (PEEK), polyetherketoneketone (PEKK), polyvinyl chlorides (PVC), polyvinylidene fluorides (PVDF), polytetrafluoroethylenes (PTFE), and silicones, the layer of conductive material is made of metallic or ceramic material, and wherein a proportion by mass of the layer of an electrical and/or thermal conductor material of the conductive composite material represents between 5% and 20% by weight of the total weight of the conductive composite material.

2. The conductive composite material as claimed in claim 1, wherein the conductive composite material is in the form of a film or a three-dimensional object.

3. The conductive composite material as claimed in claim 1, exhibiting an electrical resistivity of between 16.10.sup.−9 Ω.m and 100 Ω.m.

4. The conductive composite material as claimed in claim 1, exhibiting a thermal conductivity of between 2 W.m.sup.−1.K.sup.−1 and 50 W.m.sup.−1.K.sup.−1.

5. The conductive composite material as claimed in claim 4, exhibiting a thermal conductivity of between 5 W.m.sup.−1.K.sup.−1 and 10 W.m.sup.−1.K.sup.−1.

6. The conductive composite material as claimed in claim 1, wherein the core of organic material includes thermally and/or electrically conductive fillers.

7. The conductive composite material as claimed in claim 6, wherein the conductive fillers are coated with an electrical and/or thermal conductor material of graphite, graphene, carbon nanotube, vegetable fiber or conductive polymer type.

8. A method for manufacturing the conductive composite material as claimed in claim 1, the method comprising: a) supplying and/or producing charged or uncharged organic particles, the organic particles having a particle size of between 50 μm and 300 μm and being a thermoplastics particles selected from polyethylenes (PE), polypropylenes (PP), polyetheretherketones (PEEK), polyetherketoneketone (PEKK), polyvinyl chlorides (PVC), polyvinylidene fluorides (PVDF), polytetrafluoroethylenes (PTFE), and silicones; b) cladding the organic particles with one or more layers of at least one electrical and/or thermal conductor material, to form conductive particles, said one or more layers is made of metallic or ceramic material, c) shaping the conductive particles to form a dense conductive film or a dense component whose shape will have been defined beforehand, the step of shaping the conductive particles is carried out by techniques selected from sintering, wherein step b) of cladding the organic particles is carried out: either using a dry surface treatment technology, the particles being placed in suspension in a two-phase fluidized bed, or by mechanical means of rotation or vibration; or by using a wet surface treatment technology involving oxidation-reduction reactions of precipitation or of polymerization at the surface of the particles, the particles being placed in suspension in a three-phase fluidized bed, or by mechanical or magnetic agitation means.

9. The method for manufacturing as claimed in claim 8, wherein the sintering of step c) of shaping the conductive particles is followed by rolling, prototyping, thermoforming, or thermal spraying.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) Further characteristics and advantages of some embodiments will emerge more clearly from the reading of the description hereinafter, which is given as an illustrative and non-limitating example, and which refers to the attached figures, in which:

(2) FIG. 1 shows a diagrammatic view of the conductive particles according to some embodiments;

(3) FIG. 2 shows a diagrammatic view of the structure obtained after shaping of the conductive particles;

(4) FIG. 3A shows a microscopic view of the organic polyethylene cores before cladding;

(5) FIG. 3B shows a microscopic view of the polyethylene particles after cladding with silver by chemical coating;

(6) FIG. 4A shows a microscopic sectional view of the organic polyethylene cores coated with 20% by mass of silver;

(7) FIG. 4B show microscopic sectional view of the organic polyethylene cores coated with 20% by mass of silver;

(8) FIGS. 5A and 5B show microscopic sectional views of the organic PTFE cores coated with 40% by mass of silver;

(9) FIGS. 6A and 6B show microscopic sectional views of the organic PEKK cores coated with 30% by mass of tin oxide;

(10) FIG. 7 illustrates a component obtained after sintering of the silver-coated polyethylene (PE) particles;

(11) FIG. 8 illustrates the microstructure of the resulting component after sintering of the silver-coated PE particles;

(12) FIG. 9 illustrates the microstructure of a conductive material obtained from a mixture of polyethylene powder and silver powder.

(13) In these examples, unless otherwise indicated, all or most of the percentages and parts are expressed as percentages by mass.

EXAMPLES

Example 1, Inventive

(14) Silver cladding tests were carried out on a low-density polyethylene powder with a particle size of between 50 and 500 μm and an irregular morphology. Silver coating is conducted in an autocatalytic chemical bath (three-phase fluidized bed).

(15) Proportions by mass of silver of 10% (example 1B) and 20% (example 1A) relative to the total weight of the mixture of polyethylene+silver are applied in the form of a uniform coating to the surface of the polyethylene (PE) particles, as demonstrated by the images presented in FIGS. 3A, 3B, 4A and 4B.

(16) After cross-section analysis of the particles coated with 20% by mass of silver, the presence is found of a dense and continuous silver coating of approximately 1 μm on the surface of the polyethylene particles (FIGS. 4A and 4B).

(17) These coated powders can be used as any component according to the categories conventional in plastics technology. The shaping of these powders produces semi-finished or finished products by techniques such as extrusion, injection molding, sintering, prototyping, etc. It should be noted that the shaping technologies which give rise to high shear stresses on the material are not the most suitable for obtaining optimum conductivity performance.

(18) The polyethylene particles coated as indicated above are subsequently shaped by sintering (molding) under load, to give a disk with a diameter of 30 mm and a thickness of 5 mm. The shaping is carried out at a temperature of 160° C. for polyethylene. The objective of these preliminary tests is to characterize the structure of the materials, on the one hand, and their electrical resistivities, on the other (and therefore their electrical conductivities). The component obtained is shown in FIG. 7.

(19) The microstructure of the material is analyzed by optical microscopy after polishing of its surface. The images are shown in FIG. 8. Polishing the polyethylene-based material is made difficult because of its elasticity, which gives rise to plastic flow phenomena during the operation. A clear microstructure is therefore not easy to demonstrate. Nevertheless, the presence of the silver on the periphery of the particles can be made out, and there as well it forms a three-dimensional, interconnected network.

Example 2, Comparative

(20) For comparison, a conductive composite material was produced from a conventional mixture of polyethylene powder and a silver powder. The proportion by mass of silver powder was set at 70% relative to the total weight of the mixture. A mixture of this kind produces a conductive composite material having conductivity properties equivalent to the composite material produced according to some embodiments, namely including silver-coated organic particles, but with a very large proportion of silver powder. The microstructure of such a material is shown in FIG. 9. Clearly apparent is the presence of the silver in pulverulent form in substantial proportion. A volume proportion of silver of this kind allows the formation here of a sufficiently continuous network of the silver particles to produce a low resistivity within the material.

(21) Comparison of the Properties of the Conductive Composite Materials of Inventive Example 1 and of the Comparative Example

(22) The electrical resistance was measured by a micro ohmmeter, with an inter-electrode distance of 2 cm and without contact pressure. The results obtained are recorded in table 1 below:

(23) TABLE-US-00001 TABLE 1 Material Example Resistance (ohms) PE/Ag20% composite Example 1A 0.04 PE/Ag10% composite Example 1B 0.15 PE/Ag70% mixture Example 2 comparative 0.05

(24) Table 2 below lists the electrical resistivity and thermal conductivity values of some materials by way of example:

(25) TABLE-US-00002 TABLE 2 Electrical resistivity Thermal conductivity Materials (μohm .Math. cm) (W .Math. m.sup.−1 .Math. K.sup.−1) Ag 1.59 429 Cu 1.67 394 Al 2.65 234 Fe 9.71 80.4 C/diamond . 25-470

(26) Table 1 demonstrates the results of resistance measurements on various conductive materials (inventive or otherwise).

(27) The very low resistance (or resistivity) of the materials tested is noted. It is observed that for the composites produced from coated powders, a very low proportion of silver is sufficient to ensure maximum electrical conductivity. By way of comparison, 3.5 times more silver may be required in a conventional material (example 2) produced from a powder mixture than in a composite material according to some embodiments (example 1), to obtain a resistivity of the same order. Moreover, it may also be noted that a very substantial gain in terms of the density of these composite materials is obtained, this being the direct consequence of a lower proportion of silver. For a given resistivity, the density changes from 3.1 g/cm.sup.3 for the composite to 6.3 g/cm.sup.3 for the mixture of powders.

(28) Lastly, the mechanical characteristic of flexibility in polyethylene is only slightly affected for the composite material, whereas the material obtained by mixing tends to become rather stiff.

(29) It should be noted that different support powders may also be envisaged for coating, so as to render the composite materials more or less elastic and/or more or less hard (thermoplastics, thermosets and elastomers with variable molecular masses and variable densities, as for example PEs, PPs, PEEK, PEKK, PVC, PVDF, PTFE, silicone, epoxies, polyesters, polyurethanes, etc.).

(30) Various coatings on the particles are possible other than Ag: Cu, Nb, SnO.sub.2, AlN, Ti, etc.

(31) Some of these composites produced accordingly are very amenable to machining.

Example 3, Inventive

(32) Silver cladding tests were carried out on a PTFE powder having a particle size of between 10 μm and 100 μm and an irregular morphology. The application of silver is carried out in an autocatalytic chemical bath (three-phase fluidized bed).

(33) A proportion by mass of silver of 40% relative to the total weight of the mixture of PTFE+silver is applied in the form of a coating, with a thickness of approximately 1 μm, which is dense and continuous on the surface of the PTFE particles, as demonstrated by the cross-section analyses shown in FIGS. 5A and 5B.

(34) The shaping of these coated particles by techniques such as sintering, as described above, allows the material to be endowed not only with the electrical conductivity associated with the silver, but also with the self-lubricating and non-stick character inherent to PTFE.

Example 4, Inventive

(35) Tin oxide cladding tests were carried out on a PEKK (polyetherketoneketone) powder having a particle size of between 50 μm and 300 μm and a spongy morphology. The application of tin oxide is obtained by wet precipitation (three-phase fluidized bed).

(36) A proportion by mass of tin oxide of 30% relative to the total weight of the mixture of PEKK+tin oxide is applied in the form of a coating, with a thickness of between 1 and 2 μm, which is uniform on the surface of the PEKK particles, as demonstrated by the cross-section analyses shown in FIGS. 6A and 6B.

(37) The shaping of these coated particles by techniques such as sintering, as described above, allows the material to be endowed with an antistatic character, associated with the presence of a tin oxide, and allows a very high maximum permissible service temperature (˜250° C. continuously) to be achieved, which is one of the inherent characteristics of PEKK.