EP3830309B1 - Copper-silver composite material - Google Patents

Description

The invention relates to a massive composite material comprising copper and a volume quantity of silver less than 5% by volume, relative to the total volume of said material, a method of manufacturing said material, and the uses of said material in various applications.

The invention applies typically, but not exclusively, to the fields of microelectronics, industrial magnetoforming, conductors for electrical and/or telecommunications cables, and conductors for pulsed magnets. More particularly, the invention relates to a composite material having both good mechanical properties, in particular in terms of resistance to breaking, and good electrical properties, in particular electrical conductivity.

Pure copper has excellent electrical conductivity (100% IACS or International Annealed Copper Standard ), but has a low breaking strength, notably around 200-400 MPa. Thus, mechanically reinforced copper conductors have been proposed comprising grains of pure copper in the form of nanocrystals or nanograins, or grains formed of a copper alloy. For example, Sakai et al. described [Acta Materialia, 1997, 45, 3, 1017-1023 ] a copper-silver alloy comprising 24% by mass of silver, having an optimized breaking strength of approximately 1.5 GPa. However, its electrical conductivity is approximately 65% IACS. This conductivity does not allow the alloy to be used in pulsed magnets which would then undergo a drastic increase in temperature, and/or in high voltage electrical cables. The alloy is obtained by a process comprising the melting of a mixture comprising copper and silver, the pouring of the mixture into the mold, then cold drawing stages alternated with heat treatment stages (in particular at 330 -430°C). The process is energy-intensive and/or expensive since it requires numerous heat treatment steps.

Other solutions have been proposed, such as the manufacture of a copper-silver composite material. Especially, CN 102723144 B describes a copper-silver composite material comprising 24% by mass of silver, and having an acceptable breaking strength of approximately 970 MPa. However, here again its electrical conductivity remains very moderate (approximately 72% IACS). The composite material is obtained by a process comprising a step of inserting a silver bar into a copper tube, an electron beam welding step under vacuum, a heat treatment step at 500-700°C , an extrusion step, then several wire drawing, annealing, and shaping steps to form a composite single strand. Several composite single strands (eg 630 single strands) are formed with the aforementioned process, then inserted into a copper tube to repeat the aforementioned process. The process is very long, energy-intensive and/or expensive since it requires numerous heat treatment and shaping steps.

Thus, the materials of the prior art have improved mechanical properties, to the detriment of electrical conductivity. Indeed, the methods of the prior art introduce internal defects such as grain boundaries, or stacking faults, which induce a reduction in the electrical conductivity of the material obtained. Furthermore, the processes are often long and/or expensive.

Thus, the aim of the present invention is to overcome all or part of the drawbacks of the prior art and in particular to provide a composite material based on copper and silver, having improved electrical properties, in particular in terms of electrical conductivity. , while guaranteeing good mechanical properties, in particular in terms of resistance to breaking, said material being able to present performances suitable for use in the field of cables, in particular as an electrically conductive element of an energy cable and/or telecommunications, in the field of pulsed magnets, in the field of intense magnetic field installations and/or in the field of industrial magnetoforming. Another aim of the invention is to provide a simple and economical process for preparing such a material.

The invention therefore has as its first object a material comprising copper and silver, as defined in claim 1.

The massive material of the invention is distinguished in particular from the non-massive materials described in particular in the documents US2017/11327 Or CN106493353 .

The material of the invention has improved electrical properties, in particular in terms of electrical conductivity, while guaranteeing good mechanical properties, in particular in terms of resistance to breakage. In particular, it can have a conductivity greater than or equal to approximately 75% lACS, while guaranteeing a breaking strength of at least approximately 900 MPa.

In the composite material of the invention, the copper and silver are preferably in the form of grains having at least one of their dimensions of sub-micron size (i.e. less than 1 µm).

According to one embodiment of the invention, the copper (respectively silver) is in the form of grains having at least one of their dimensions less than or equal to approximately 700 nm, preferably less than or equal to approximately 500 nm, of more preferably ranging from approximately 50 to 400 nm, and more preferably ranging from approximately 100 to 300 nm.

Such grain sizes make it possible to guarantee good electrical properties and good mechanical properties.

Considering several grains of copper (respectively of silver) according to the invention, the term "dimension" means the average dimension in number of all the grains of a given population, this dimension being conventionally determined by well-known methods of a person skilled in the art.

The size of the grain(s) according to the invention can for example be determined by microscopy, in particular by scanning electron microscope (SEM) or by transmission electron microscope (TEM).

The material of the invention is a composite material. In the invention the expression “composite material” means a material comprising at least one pure copper phase and at least one pure silver phase. In other words, said material is an assembly of at least copper grains and grains of silver, the grains of copper and the grains of silver not being mutually soluble. It should be noted that a copper-silver composite material differs from a copper-silver alloy in which copper is combined with silver, for example by fusion or mechanofusion. In particular, copper-silver alloys consist of a two-phase eutectic structure in the form of copper-silver solid solutions, one rich in copper, and the other rich in silver. The composite material of the invention does not include a zone of mutual solubility of copper and silver. The absence of a zone of mutual solubility of copper and silver in the composite material of the invention can in particular be demonstrated by energy dispersive analysis (EDX).

The material of the invention is massive. In other words, it is in the form of a solid mass, or it is different from a material in the form of a powder or powdery material.

The material of the invention preferably has a conductivity of at least approximately 80% IACS, more preferably at least approximately 85% IACS, and more preferably at least approximately 90% IACS, in particular at 20°C. .

The material of the invention preferably has an electrical resistivity of at most approximately 2.15 µΩ.cm, more preferably at most approximately 2.03 µΩ.cm, and more preferably at most 1.91 approximately µΩ.cm, particularly at 20°C.

The material of the invention preferably has an electrical resistivity of at most approximately 0.70 µΩ.cm, more preferably at most approximately 0.60 µΩ.cm, and more preferably at most 0.50 approximately µΩ.cm, particularly at -196°C.

The electrical resistivity is preferably determined using a device sold under the trade name Sourcemeter KEITHLEY2450, by the company TEKTRONIX.

The material of the invention preferably has a breaking strength of at least 900 MPa, preferably at least 1 GPa, preferably at least approximately 1.05 GPa, more preferably at least 1 .1 GPa approximately, and more preferably at least 1.2 GPa approximately, in particular at -196°C.

The breaking strength is preferably determined using a device sold under the trade name INSTRON 1195, by the company INSTRON.

The material of the invention preferably has an elongation at break of at least approximately 0.5%, particularly at room temperature (i.e. 18-25°C).

The elongation at break is preferably determined using a device sold under the trade name Epsilon 3442 extensometer, by the company DOERLER Mesures.

The material comprises silver in a volume proportion of less than 5%, relative to the total volume of said material. The low proportion of silver in said material makes it possible to guarantee a homogeneous material, in which the silver grains are uniformly dispersed within the copper grains. Indeed, at 5% by volume or beyond, the dispersion of silver in the material is heterogeneous (e.g. presence of aggregates), leading to a weakening of its mechanical properties.

According to a preferred embodiment of the invention, the material comprises at most approximately 2% by volume of silver, preferably at most approximately 1.5% by volume of silver, and even more preferably at most 1% by volume. Approximately % by volume of silver, relative to the total volume of said material.

The material of the invention comprises at least 0.1% by volume of silver, and preferably at least 0.5% by volume of silver, relative to the total volume of said material.

The material of the invention may comprise at least approximately 98% by volume of copper, and preferably at least 99% by volume of copper, relative to the total volume of said material.

The material of the invention may comprise at most approximately 99.9% by volume of copper, and preferably at most 99.5% by volume of copper, relative to the total volume of said material.

The material comprises at most approximately 0.1% by volume of unavoidable impurities, relative to the total volume of said material.

The unavoidable impurities can be chosen from the elements Al, C, Fe, Ni, Pb, Si, Sn, Zn, Se, and one of their mixtures.

In a particular embodiment, the material comprises at most approximately 0.5% by volume, and preferably at most approximately 0.1% by volume, of other impurities chosen from O, S, P, Se, and one of their mixtures.

According to one embodiment of the invention, the material comprises only copper, silver, and possibly unavoidable impurities and/or other impurities as defined in the invention.

According to the invention, the material essentially comprises copper and silver. In other words, copper and silver represent at least 99.9% by volume, and more preferably approximately 100% by volume, relative to the total volume of said material.

The copper and/or silver are in the form of grains having a filamentary shape.

The material of the invention is preferably anisotropic. In other words, it is composed of copper grains (respectively silver) elongated in a preferred direction, also called filamentary grains.

• une longueur (L_Cu), s'étendant selon une direction principale d'allongement,
• deux dimensions (D_Cu1) et (D_Cu2), dites dimensions orthogonales, s'étendant selon deux directions transversales orthogonales entre elles et orthogonales à ladite direction principale d'allongement, lesdites dimensions orthogonales (D_Cu1, D_Cu2) étant inférieures à ladite longueur (L_Cu) et inférieures ou égales à 700 nm, de préférence inférieures ou égales à 500 nm environ, de préférence encore allant de 50 à 400 nm environ, et de préférence encore de 100 à 300 nm environ, et
• deux rapports (F_Cu1) et (F_Cu2), dits facteurs de forme, entre ladite longueur (L_Cu) et chacune des deux dimensions orthogonales (D_Cu1) et (D_Cu2), lesdits facteurs de forme (F_Cu1, F_Cu2) étant supérieurs à 50, de préférence supérieurs ou égaux à 75 environ, de préférence encore allant de 100 à 400 environ, et de préférence encore de 100 à 300 environ.

Copper grains having a filamentary shape are grains for example having:

• a length (L _Cu ), extending in a main direction of elongation,
• two dimensions (D _Cu1 ) and (D _Cu2 ), called orthogonal dimensions, extending in two transverse directions orthogonal to each other and orthogonal to said main direction of elongation, said orthogonal dimensions (D _Cu1 , D _Cu2 ) being less than said length (L _Cu ) and less than or equal to 700 nm, preferably less than or equal to approximately 500 nm, more preferably ranging from approximately 50 to 400 nm, and more preferably from approximately 100 to 300 nm, and
• two ratios (F _Cu1 ) and (F _Cu2 ), called shape factors, between said length (L _Cu ) and each of the two orthogonal dimensions (D _Cu1 ) and (D _Cu2 ), said shape factors (F _Cu1 , F _Cu2 ) being greater than 50, preferably greater than or equal to approximately 75, more preferably ranging from approximately 100 to 400, and more preferably from approximately 100 to 300.

According to a particular embodiment, the two orthogonal dimensions (D _Cu1 , D _Cu2 ) of a grain having a filamentary shape are equivalent or close. We then speak of a “stick” or “thread”.

According to another particular embodiment, a grain having a filamentary shape can be a “ribbon” in which the two orthogonal dimensions (D _Cu1 , D _Cu2 ) of the grain according to the invention are its width (I _cu ) (first orthogonal dimension ) and its thickness (E _Cu ) (second orthogonal dimension), the width (I _cu ) being notably much greater than the thickness (E _Cu ).

The length (L _Cu ) of the copper grains (respectively silver) can be of micrometric size (ie less than 1 mm), preferably less than or equal to approximately 500 µm, preferably less than or equal to approximately 200 µm, of more preferably ranging from approximately 1 to 150 µm, and more preferably ranging from approximately 10 to 70 µm.

• une longueur (L_Ag), s'étendant selon une direction principale d'allongement,
• deux dimensions (D_Ag1) et (D_Ag2), dites dimensions orthogonales, s'étendant selon deux directions transversales orthogonales entre elles et orthogonales à ladite direction principale d'allongement, lesdites dimensions orthogonales (D_Ag1, D_Ag2) étant inférieures à ladite longueur (L_Ag) et inférieures ou égales à 700 nm, de préférence inférieures ou égales à 500 nm environ, de préférence encore allant de 50 à 400 nm environ, et de préférence encore de 100 à 300 nm environ, et
• deux rapports (F_Ag1) et (F_Ag2), dits facteurs de forme, entre ladite longueur (L_Ag) et chacune des deux dimensions orthogonales (D_Ag1) et (D_Ag2), lesdits facteurs de forme (F_Ag1, F_Ag2) étant supérieurs à 50, de préférence supérieurs ou égaux à 75 environ, de préférence encore allant de 100 à 400 environ, et de préférence encore de 100 à 300 environ.

Silver grains having a filamentary shape are grains for example having:

• a length (L _Ag ), extending in a main direction of elongation,
• two dimensions (D _Ag1 ) and (D _Ag2 ), called orthogonal dimensions, extending in two transverse directions orthogonal to each other and orthogonal to said main direction of elongation, said dimensions orthogonal (D _Ag1 , D _Ag2 ) being less than said length (L _Ag ) and less than or equal to 700 nm, preferably less than or equal to approximately 500 nm, more preferably ranging from approximately 50 to 400 nm, and more preferably from approximately 100 to 300 nm, and
• two ratios (F _Ag1 ) and (F _Ag2 ), called form factors, between said length (L _Ag ) and each of the two orthogonal dimensions (D _Ag1 ) and (D _Ag2 ), said form factors (F _Ag1 , F _Ag2 ) being greater than 50, preferably greater than or equal to approximately 75, more preferably ranging from approximately 100 to 400, and more preferably from approximately 100 to 300.

According to a particular embodiment, the two orthogonal dimensions (D _Ag1 , D _Ag2 ) of a grain having a filamentary shape are equivalent or close. We then speak of a “stick” or “thread”.

According to another particular embodiment, a grain having a filamentary shape can be a “ribbon” in which the two orthogonal dimensions (D _Ag1 , D _Ag2 ) of the grain according to the invention are its width (I _Ag ) (first orthogonal dimension ) and its thickness (E _Ag ) (second orthogonal dimension), the width (I _Ag ) being notably much greater than the thickness (E _Ag ).

The length (L _Ag ) of the silver grains can be of micrometric size (ie less than 1 mm), preferably less than or equal to approximately 500 µm, preferably less than or equal to approximately 200 µm, more preferably ranging from 1 at approximately 150 µm, and more preferably ranging from approximately 10 to 70 µm.

The material of the invention preferably has a relative density of at least approximately 99%, and preferably at least approximately 99.5%.

In the invention, the relative density is determined by the Archimedes method at 20°C, the reference body being pure water at 4°C.

The material of the invention can be in the form of a wire, in particular with a diameter ranging from approximately 0.1 to 4 mm, preferably from approximately 0.2 to 1 mm, and more preferably from 0.25 to 0. .8 mm approximately.

1. i) une étape de dispersion de particules micrométriques de cuivre et de particules micrométriques ou sub-micrométriques d'argent, dans un milieu non-solvant,
2. ii) une étape de séchage pour former une poudre composite comprenant lesdites particules de cuivre et d'argent, ladite poudre comprenant une quantité inférieure à 5% en volume environ de particules d'argent, par rapport au volume total de ladite poudre,
3. iii) une étape de frittage flash à une température d'au plus 600°C environ, afin d'obtenir une masse solide composite, et
4. iv) au moins une étape de tréfilage à froid, afin de mettre en forme la masse solide composite de l'étape iii).

The second object of the invention is a process for preparing a massive composite material conforming to the first object of the invention, characterized in that it comprises at least the following steps:

1. i) a step of dispersing micrometric copper particles and micrometric or sub-micrometric silver particles, in a non-solvent medium,
2. ii) a drying step to form a composite powder comprising said copper and silver particles, said powder comprising a quantity of less than approximately 5% by volume of silver particles, relative to the total volume of said powder,
3. iii) a flash sintering step at a temperature of at most approximately 600°C, in order to obtain a composite solid mass, and
4. iv) at least one cold drawing step, in order to shape the composite solid mass of step iii).

Thus the process of the invention is simple and it allows in a few steps to obtain a composite material conforming to the first object of the invention, having improved electrical properties, in particular in terms of electrical conductivity, while guaranteeing good mechanical properties, particularly in terms of breaking strength. Furthermore, it avoids repeated annealing and/or heat treatment steps such as those carried out in the processes of the prior art, while avoiding the phenomena of diffusion and/or melting of copper and silver. Finally, such a process can easily be transposed to an industrial scale.

Step i)

Step i) makes it possible to form a homogeneous mixture of copper and silver, while avoiding metal diffusion phenomena.

Step i) can be carried out by dispersing a powder of micrometric copper particles and a powder of micrometric or sub-micrometric silver particles in said non-solvent medium.

The non-solvent medium is a liquid which does not solubilize the copper and silver grains. It allows in particular to form a suspension.

The non-solvent medium can be chosen from alcohols, water, ketones such as acetone, and one of their mixtures.

Examples of alcohols include ethanol.

• i-a) éventuellement disperser une poudre de particules micrométriques de cuivre dans un milieu non-solvant S₁,
• i-b) disperser une poudre de particules micrométriques ou sub-micrométriques d'argent dans un milieu non-solvant S₂, et
• i-c) mélanger la poudre de particules micrométriques de cuivre ou la dispersion de poudre de particules micrométriques de cuivre de la sous-étape i-a), avec la dispersion de poudre de particules micrométriques ou sub-micrométriques d'argent de la sous-étape i-b), notamment sous agitation.

In particular, step i) can be carried out according to the following substeps:

• ia) optionally dispersing a powder of micrometric copper particles in a non-solvent medium S ₁ ,
• ib) dispersing a powder of micrometric or sub-micrometric particles of silver in a non-solvent medium S ₂ , and
• ic) mixing the powder of micrometric copper particles or the dispersion of powder of micrometric copper particles from sub-step ia), with the dispersion of powder of micrometric or sub-micrometric particles of silver from sub-step ib) , especially under agitation.

The non-solvent media S ₁ and S ₂ can have the same definition as that given above for the non-solvent medium S.

Preferably, the non-solvent media S ₁ and S ₂ are identical.

The non-solvent media S ₁ and S ₂ are preferably mutually soluble.

Substep i-a) can be carried out with mechanical or magnetic stirring or in the presence of ultrasound.

Substep i-b) can be carried out with mechanical or magnetic stirring, in particular in order to avoid the degradation of the micrometric or sub-micrometric silver particles.

Substep i-c) can be carried out with mechanical or magnetic stirring or in the presence of ultrasound.

The micrometric copper particles can have at least one of their dimensions ranging from approximately 0.5 to 20 µm, preferably from 0.5 to 10 µm approximately, preferably from approximately 0.5 to 4 µm, and more preferably from approximately 0.5 to 1.5 µm.

The micrometric copper particles are preferably spherical micrometric particles.

The silver particles can have at least one of their dimensions ranging from approximately 0.1 to 150 µm, and preferably from approximately 0.5 to 70 µm.

The micrometric or sub-micrometric particles of silver can be spherical or filiform.

The micrometric or sub-micrometric spherical silver particles may have a diameter ranging from approximately 0.5 to 20 µm, preferably from approximately 0.5 to 10 µm, preferably from approximately 0.5 to 4 µm, and preferably another approximately 0.5 to 1.5 µm.

According to one embodiment of the invention, the micrometric or sub-micrometric silver particles are filiform.

• une longueur (L'_Ag), s'étendant selon une direction principale d'allongement,
• deux dimensions (D'_Ag1) et (D'_Ag2), dites dimensions orthogonales, s'étendant selon deux directions transversales orthogonales entre elles et orthogonales à ladite direction principale d'allongement, lesdites dimensions orthogonales (D'_Ag1, D'_Ag2) étant inférieures à ladite longueur (L'_Ag) et inférieures ou égale à 700 nm, et de préférence inférieures ou égale à 500 nm, et
• deux rapports (F'_Ag1) et (F'_Ag2), dits facteurs de forme, entre ladite longueur (L'_Ag) et chacune des deux dimensions orthogonales (D'_Ag1) et (D'_Ag2), lesdits facteurs de forme (F'_Ag1, F'_Ag2) étant de préférence supérieurs à 50.

In particular, they present:

• a length (L' _Ag ), extending in a main direction of elongation,
• two dimensions (D' _Ag1 ) and (D' _Ag2 ), called orthogonal dimensions, extending in two transverse directions orthogonal to each other and orthogonal to said main direction of elongation, said orthogonal dimensions (D' _Ag1 , D' _Ag2 ) being less than said length (L' _Ag ) and less than or equal to 700 nm, and preferably less than or equal to 500 nm, and
• two ratios (F' _Ag1 ) and (F' _Ag2 ), called form factors, between said length (L' _Ag ) and each of the two orthogonal dimensions (D' _Ag1 ) and (D' _Ag2 ), said form factors ( F' _Ag1 , F' _Ag2 ) being preferably greater than 50.

According to a preferred embodiment, the two orthogonal dimensions (D' _Ag1 , D' _Ag2 ) of a filiform particle are equivalent or close and represent the diameter (D' _Ag ) of its cross section. We then speak of a “stick” or “thread”.

According to another preferred embodiment, a filiform particle is a “ribbon” in which the two orthogonal dimensions of the particle according to the invention are its width (I' _Ag ) (first orthogonal dimension) and its thickness (E' _Ag ) (second orthogonal dimension), the width (I' _Ag ) being notably much greater than the thickness (E' _Ag ).

• les deux dimensions orthogonales (D'_Ag1, D'_Ag2) des particules filiformes vont de 50 nm à 400 nm environ, et de préférence de 100 nm à 300 nm environ ;
• la longueur (L'_Ag) va de 1 µm à 150 µm environ, et de préférence de 10 µm à 70 µm environ ;
• les facteurs de forme (F'_Ag1, F'_Ag2) sont supérieurs ou égaux à 75 environ, de préférence vont de 100 à 400 environ, de préférence encore de 100 à 300 environ, et de préférence encore sont de l'ordre de 200.

Advantageously, the micrometric or sub-micrometric filiform particles of silver according to the invention are characterized by at least one of the following characteristics:

• the two orthogonal dimensions (D' _Ag1 , D' _Ag2 ) of the filiform particles range from approximately 50 nm to 400 nm, and preferably from approximately 100 nm to 300 nm;
• the length (L' _Ag ) ranges from approximately 1 µm to 150 µm, and preferably from approximately 10 µm to 70 µm;
• the form factors (F' _Ag1 , F' _Ag2 ) are greater than or equal to approximately 75, preferably range from approximately 100 to 400, more preferably from approximately 100 to 300, and more preferably are of the order of 200 .

Step ii)

Step ii) allows the non-solvent media to evaporate.

It can be carried out using a rotary evaporator, particularly under vacuum.

The drying temperature preferably ranges from approximately 70 to 100°C, and is more preferably of the order of 80°C.

According to a preferred embodiment of the invention, the composite powder comprises at most approximately 2% by volume of silver particles, preferably at most approximately 1.5% by volume of silver particles, and even more more preferentially at most approximately 1% by volume of silver particles, relative to the total volume of said powder.

Step ii')

The process may further comprise a step ii') of reduction of the dried composite powder from step ii), in the presence of dihydrogen. This step ii') can make it possible to eliminate the layer of copper oxide which can form on the surface of the copper particles.

Step ii') can be carried out at a temperature T ₁ of approximately 100 to 300°C, preferably of approximately 110 to 240°C, and more preferably of approximately 120 to 160°C.

Step ii') can be carried out by heating the powder from room temperature to temperature T ₁ as defined in the invention, at a speed ranging from 1°C/min to approximately 5°C/min, and preferably still ranging from approximately 2°C/min to 3°C/min.

Step iii)

In the present invention, the term "flash sintering" means uniaxial pressure sintering based on the use of an electric current. Flash sintering is also well known as “ Spark Plasma Sintering” or SPS.

Step iii) makes it possible to consolidate the powder obtained in the previous step ii) or ii'), while avoiding the phenomena of diffusion and/or melting of the copper and/or silver.

This step iii) is preferably carried out at a temperature T ₂ of at most approximately 550°C, preferably ranging from approximately 375 to 525°C, and even more preferably ranging from approximately 390 to 450°C. These temperatures make it possible to obtain a solid composite mass having sufficient residual porosity to be able to be cold drawn in the subsequent stages (eg without breaks and/or cracks and/or ruptures).

• de la température ambiante à 350°C à une vitesse allant de 20 °C/min à 30°C/min environ, et
• de 350°C à la température T₂ à une vitesse allant de 40 °C/min à 60°C/min environ.

According to a preferred embodiment of the invention, sintering is carried out by heating the powder:

• from room temperature to 350°C at a speed ranging from approximately 20°C/min to 30°C/min, and
• from 350°C to temperature T ₂ at a speed ranging from 40°C/min to approximately 60°C/min.

Sintering is preferably carried out under primary or secondary vacuum, or under an argon or nitrogen atmosphere.

The pressure exerted on the composite powder resulting from step ii) or ii') preferably ranges from 20 to 100 MPa, and even more preferably from 25 to 35 MPa.

The duration of sintering varies depending on the temperature. This duration generally ranges from approximately 20 to 30 minutes.

According to a particularly preferred embodiment of the invention, the sintering is carried out under secondary vacuum, at a pressure of approximately 25 to 50 MPa, at a maximum temperature of 400 to 500°C, maintained for a period of 3 to 10 minutes . The total duration of the heat treatment is, in this case, less than 1 hour 30 minutes.

The intensity of the pulsed current can range from approximately 10 to 250 A. The duration of each current pulse is of the order of a few milliseconds. This duration preferably ranges from approximately 2 to 4 ms.

In particular, the composite solid mass obtained at the end of step iii) has a relative density ranging from approximately 85 to 97%, preferably from approximately 90 to 95%, and more preferably from approximately 92 to 96%. Indeed, these density ranges are adapted to be able to implement the following wire drawing step, avoiding the formation of cracks and/or fractures.

At the end of step iii) the composite material can be in the form of a cylinder or a bar, in particular having a height or length greater than its diameter. This can thus make it possible to promote the implementation of step iv).

According to a particular embodiment, the cylinder or bar has a diameter ranging from approximately 5 to 80 mm, and preferably from approximately 5 to 40 mm.

Step iii) makes it possible to preserve the micrometric size of the copper particles and the micrometric or sub-micrometric size of the silver particles, and thus to avoid the growth of metallic grains.

The solid composite mass obtained in step iii) is preferably isotropic. In other words, it does not present a preferential orientation of the copper grains (respectively silver), compared to its own macroscopic geometric shape.

Step iv)

The cold drawing step(s) iv) are preferably carried out at a temperature of at most approximately 40°C, preferably at most approximately 35°C, particularly preferably ranging from -196°C to 30°C. C approximately, and more particularly preferably at room temperature.

The ambient temperature corresponds to a temperature ranging from approximately 18 to 25°C.

The process can comprise several steps iv), in particular from approximately 20 to 80 steps iv), and in particular around forty steps iv).

In a preferred embodiment, the drawing step(s) iv) make it possible to obtain a composite material in the form of a wire, in particular with a diameter ranging from approximately 0.1 to 4 mm, preferably from 0.2 to 4 mm. approximately 1 mm, and more preferably approximately 0.25 to 0.8 mm.

In a preferred embodiment, the drawing step(s) iv) make it possible to obtain a composite material in the form of a wire of length ranging from approximately 0.1 to 1000 m, and preferably from 0.2 to 50 m approximately.

During step iv), the phenomena of rupture and/or cracks and/or breaks are greatly reduced, or even avoided.

The method may further comprise between steps iii) and iv) a step of cooling the solid composite mass, in particular at a cooling rate ranging from approximately 4°C/min to 7°C/min.

The process conforming to the second object leads to a material conforming to the first object.

The invention also relates to a massive composite material as defined in the first subject of the invention, capable of being obtained according to a process as defined in the second subject of the invention.

The third object of the invention is the use of a massive composite material conforming to the first object of the invention or obtained according to a process conforming to the second object of the invention, as an electrical conductor, in particular for electrical cables and/or telecommunications, as a conductor for continuous or pulsed field magnets, in the field of intense field installations, or in the field of industrial magnetoforming.

Such a massive composite material presents a good compromise between electrical conduction and breaking strength to be able to be used in high voltage cables or overhead electricity transmission lines, in particular as an electrical conductor, or in motors, alternators, transformers, or connectors.

In addition, its good electrical and mechanical properties make it possible to reduce its diameter, and thus the mass of a conductive wire made of said massive composite material, by improving or maintaining its performance. This makes it possible to consider its use in the fields of Aeronautics, Space and Defense; particularly in drones, planes, missiles, launchers, satellites, probes, or spacecraft; or in land transport, particularly in railway catenaries.

The massive composite material conforming to the first object of the invention can also be used in installations with intense magnetic fields, in particular non-destructive pulsed magnetic fields greater than 100 Tesla. In particular, the low electrical resistivity of this material can induce, at constant power, an increase in the duration of the pulse of the pulsed magnetic field and a reduction in the electrical power necessary to power the continuous magnets.

Finally, it can increase the lifespan of magnetoforming tools such as pulsed magnets via the integration of wires in massive composite material conforming to the first object of the invention. Indeed, the conductive wires in this area are generally mechanically stressed well beyond their elastic limit.

Thus, wires made of solid composite material conforming to the first object of the invention can be integrated into prototypes of magnetoforming magnets.

Wires made of solid composite material conforming to the first object of the invention can enable the winding of industrial magnets for magnetoforming.

EXAMPLES

• poudre de cuivre, 0,5-1,5 µm, Alfa-Aesar,
• AgNO₃, Aldrich
• éthylène glycol, Aldrich,
• polyvinylpyrrolidinone PVP, 55000 g/mol, Aldrich.

The raw materials used in the examples are listed below:

• copper powder, 0.5-1.5 µm, Alfa-Aesar,
• AgNO ₃ , Aldrich
• ethylene glycol, Aldrich,
• polyvinylpyrrolidinone PVP, 55000 g/mol, Aldrich.

Unless otherwise noted, all of these raw materials were used as received from the manufacturers.

EXAMPLE 1 Preparation of a composite material according to the invention

Silver nanowires were prepared using a solution growth method from silver nitrate (AgNO ₃ ), PVP, and ethylene glycol, as described by Sun YG et al., “Crystalline silver nanowires by soft solution processing,”. Nano Letters, 2002. 2(2): p. 165-168 , with a PVP/AgNO ₃ ratio of 1.53. The silver nanowires obtained have a length ranging from approximately 30 to 60 μm, and a diameter ranging from approximately 200 to 300 nm.

A suspension comprising 0.178 g of silver nanowires and 9 ml of ethanol was prepared.

The suspension of silver nanowires was mixed with 15 g of copper powder, then the resulting mixture was homogenized under ultrasound, then evaporated using a rotary evaporator at 80°C. A PC ₁ composite powder comprising 1% by volume of silver, relative to the total volume of the powder, was thus obtained.

The composite powder was reduced in the presence of dihydrogen for 1 h at 160°C in order to reduce the copper oxide formed on the surface of the copper particles.

The resulting powder was then sintered by SPS using a device sold under the trade name Dr Sinter ^2080® , by the company Syntex Inc.

To do this, the composite powder was placed in a die/matrix made of tungsten carbide and cobalt alloy (WC/Co) with an internal diameter of 8 mm, the interior of which was protected by a graphite film. The die was then closed by symmetrical pistons and then introduced into the chamber of the SPS machine. Sintering was carried out under vacuum (chamber residual pressure < 10 Pa) using pulsed direct currents defined over 14 periods of 3.2 ms, including 12 pulse periods and 2 non-pulse periods. The temperature was controlled using a thermocouple inserted into a hole (5 mm deep) drilled on the external surface of the die. A temperature of 500°C was reached in 2 stages: a ramp of 25°C.min ^-1 for 13 minutes to go from ambient temperature to 350°C, then a ramp of 50°C.min ^-1 for 3 minutes to go from 350°C to 500°C. This temperature was then maintained for 5 minutes. These temperature ramps were obtained by applying pulsed direct currents defined over 14 periods of 3.2 ms, including 12 pulse periods and 2 non-pulse periods. A pressure of 25 MPa was reached within 1 minute and maintained for the remainder of the sintering. The die was then cooled within the SPS chamber. The MSC ₁ composite solid mass obtained is in the form of a cylinder of 8 mm in diameter and 33 mm in length.

The composite solid mass obtained was then drawn at room temperature using a tungsten carbide die. After 40 passes, a composite material in the form of an FC ₁ wire of 0.29 mm in diameter and 25 m in length was obtained. No breakage of the wires was observed.

The composite powders and composite wires were analyzed by scanning electron microscopy (SEM) using a field effect gun, sold under the trade name JEOL JSM 6700F by the company JEOL, and operating at 200 kV.

The density of the composite solid masses and the composite wires was determined by the Archimedes method.

The electrical resistivity of the composite wires was determined to be 77K (liquid nitrogen) using the four-point method, with a maximum current of 100 mA to avoid heating of the wires.

The breaking strength was measured using a device sold under the trade name INSTRON 1195 by the company INSTRON, at 77K (liquid nitrogen) and at 293K on composite wires 170 mm long. The specific tensions encountered were measured with a force sensor (1000 N or 250 N; 1.6 X 10 ^-5 ms ^-1 ).

For comparison, an identical process (identical operating conditions) to that described above was used, replacing the volume proportion of silver which was approximately 1% by volume, by a volume quantity of 10% by volume. approximately. A PC _A composite powder comprising 10% by volume of silver, relative to the total volume of the powder, was thus obtained at the end of step i). The PC _A composite powder is not part of the invention. A composite solid mass MSC _A and a composite wire FC _A , not part of the invention, were also obtained.

The density of the composite solid masses MSC ₁ and MSC _A is approximately 94% (± 2%).

There figure 1 is a SEM image of the PC ₁ composite powder according to the invention (cf. figure 1a : scale 10 µm, and figure 1b : scale 2 µm), and the PC _A composite not in accordance with the invention (cf. figure 1c : scale 10 µm, and figure 1d : scale 2 µm). There figure 1 shows the uniform dispersion of the silver nanowires within the copper powder, inducing a homogeneous powder. Conversely, the use of a volume quantity of silver of approximately 10% by volume does not make it possible to obtain a homogeneous powder.

There figure 2 shows the resistivity (in pQ.cm) at 77K of a composite material in the form of an FC ₁ wire in accordance with the invention (curve with solid triangles) and of a composite material in the form of a wire FC _A not in accordance with the invention (curve with solid circles), depending on their respective diameter (in mm).

There Figure 3 shows the breaking strength (in MPa) at 77K of a composite material in the form of an FC ₁ wire in accordance with the invention (curve with solid triangles) and of a composite material in the form of a FC _A wire not in accordance with the invention (curve with solid circles), depending on their respective diameter (in mm).

The breaking strength at 77K of a composite wire according to the invention is twice that of a pure copper wire with equivalent diameters, while guaranteeing low electrical resistivity (0.38-0.50 pQ .cm). These electrical resistivity values are in particular lower than those obtained for alloys or composites of the prior art having a similar breaking strength, but comprising 20 times more silver.

Claims (14)

1. A material comprising copper and silver, characterized in that it is a solid composite material containing grains of copper and grains of silver which are not mutually soluble, and in that it comprises a volume amount of silver of at least 0.1 % by volume and less than 5 % by volume relative to the total volume of said material, the copper and silver representing at least 99.9 % by volume relative to the total volume of said material, and wherein the silver and copper are in the form of grains of filament shape.
2. The material according to claim 1, characterized in that the copper and silver are in the form of grains at least one dimension thereof being less than or equal to 500 nm.
3. The material according to claim 1 or 2, characterized in that it has conductivity of at least 80 % IACS.
4. The material according to any of the preceding claims, characterized in that it has a tensile strength of at least 1 GPa.
5. The material according to any of the preceding claims, characterized in that it comprises at most 1.5 % by volume of silver relative to the total volume of said material.
6. The material according to any of the preceding claims, characterized in that the copper grains have:

   - a length (L_Cu) extending in a main direction of elongation,

   - two dimensions (D_Cu1) and (D_Cu2) called orthogonal dimensions extending in two transverse directions orthogonal to each other and orthogonal to said main direction of elongation, said orthogonal dimensions (D_Cu1, D_Cu2) being less than said length (L_Cu) and ranging from 50 to 400 nm, and

   - two ratios (F_Cu1) and (F_Cu2) called form factors between said length (L_Cu) and each of the two orthogonal dimensions (D_Cu1) and (D_Cu2),said form factors (F_Cu1, F_Cu2) being greater than or equal to 75, and

   the silver grains have:

   - a length (L_Ag) extending in a main direction of elongation,

   - two dimensions (D_Ag1) and (D_Ag2) called orthogonal dimensions extending in two transverse directions orthogonal to each other and orthogonal to said main direction of elongation, said orthogonal dimensions (D_Ag1, D_Ag2) being less than said length (L_Ag) and ranging from 50 to 400 nm, and

   - two ratios (F_Ag1) and (F_Ag2) called form factors between said length (L_Ag) and each of the two orthogonal dimensions (D_Ag1) and (D_Ag2), said form factors (F_Ag1, F_Ag2) being greater than or equal to 75.
7. A method for preparing a solid composite material such as defined in any of claims 1 to 6, characterized in that it comprises at least the following steps:

   i) a step to disperse micrometric particles of copper and micrometric or sub-micrometric particles of silver, in a non-solvent medium,

   ii) a drying step to form a composite powder comprising said particles of copper and silver, said powder comprising an amount of less than 5 % by volume of silver particles relative to the total volume of said powder,

   iii) a flash sintering step at a temperature of no more than 600 °C to obtain a composite solid mass, and

   iv) at least one cold drawing step to form the composite solid mass of step iii),
8. The method according to claim 7, characterized in that the non-solvent medium at step i) is chosen from among alcohols, water, ketones and one of the mixtures thereof.
9. The method according to claim 7 or 8, characterized in that the micrometric particles of copper have at least one of their dimensions ranging from 0.5 to 20 µm.
10. The method according to any of claims 7 to 9, characterized in that the micrometric or sub-micrometric particles of silver are filiform particles having:

    - a length (L'_Ag) extending in a main direction of elongation,

    - two dimensions (D'_Ag1) and (D'_Ag2) called orthogonal dimensions extending in two transverse directions orthogonal to each other and orthogonal to said main direction of elongation, said orthogonal dimensions (D'_Ag1), D'_Ag2) being less than said length (L'_Ag), and

    - two ratios (F'_Ag1) and (F'_Ag2) called form factors between said length (L'_Ag) and each of the two orthogonal dimensions (D'_Ag1) and (D'_Ag2), and

    being characterized by at least one of the following characteristics:

    - the two orthogonal dimensions (D'_Ag1, D'_Ag2) of the filiform particles range from 50 nm to 400 nm,

    - the length (L'_Ag) ranges from 1 µm to 150 µm,

    - the form factors (F'_Ag1, F'_Ag2) are greater than or equal to 75.
11. The method according to any of claims 7 to 10, characterized in that step iii) is performed at a temperature ranging from 375 to 525 °C.
12. The method according to any of claims 7 to 10, characterized in that the composite solid mass obtained after step iii) has relative density ranging from 85 to 97 %.
13. The method according to any of claims 8 to 13, characterized in that it further comprises a step ii') to reduce the composite powder dried at step ii), in the presence of dihydrogen.
14. Use of a solid composite material such as defined in any of claims 1 to 6 as electrical conductor, as conductor for continuous or pulsed field magnets, in the field of intense field installations, or in the field of industrial electromagnetic forming.

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