WO2009080372A1

WO2009080372A1 - A bulk electric conductive member

Info

Publication number: WO2009080372A1
Application number: PCT/EP2008/053822
Authority: WO
Inventors: Erik Lewin; Ulf Jansson; Åke ÖBERG
Original assignee: Abb Research Ltd
Priority date: 2007-12-20
Filing date: 2008-03-31
Publication date: 2009-07-02

Abstract

A bulk electrically conductive member (4, 5) for an electric device, such as an electric contact or an electric conductor, said member (4, 5) comprising a nanocomposite material having a matrix (2) and crystallites (3) of nano-size, i.e. here defined as having a size in the range of 1 -200 nm in at least two dimensions, embedded therein, portions of said matrix (2) separating adjacent said crystallites (3) of said nanocomposite material have a thickness (T) providing said matrix (2) and hence said member (4, 5) with an electrical conductivity determined by a substantially two-dimensional character of said matrix (2), such as for so-called pseudo-2D materials like graphene.

Description

A bulk electric conductive member

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a bulk electrically conductive member for an electric device, such as an electric contact or an electric conductor, said member comprising a nanocomposite material having a matrix and crystallites of nano-size, i.e. here defined as having a size in the range of 1 -200 nm in at least two dimensions, embedded therein.

In this description and the subsequent claims, the expression "bulk" is referring to an object substantially consisting of the same material in the body as well as on the surfaces, i.e. the object has a homogenous composition. A bulk material is here defined as being in the size range of at least 0.5 mm.

In an object "having a size in the range of 1 -200 nm in at least two dimensions" at least one section can be made, wherein two points on the periphery of said section can never be more separated than 200 nm. Such an object can e.g. be a particle with a diameter between 1 -200 nm or a rod with a diameter between 1 - 200 nm.

Electric contacts, such as arcing contacts and sliding contacts, are used in a wide range of electrotechnical applications such as circuit breakers, generator breakers, contactors, power interrupters, disconnectors, relays, vacuum interrupters, fuses, current limiters, selector switches or for establishing and interrupting electric contacts in contact arrangements of plug-in type. If a contact material can be fabricated in bulk, it can also be used as an electric conductor.

In the present technology such electric contacts and conductors are, for example, made of metals such as Cu, or metal-matrix composite materials such as Ag or Cu, in combinations with a metal, such as W or Ni, or a ceramic with high melting point and/or hardening effect, for example SnO₂, WC, or graphitic carbon. Such materials are often expensive, and are not easy to optimize with regard to electric conductivity, thermal shock, arc erosion/melting, welding, wear and corrosion.

Therefore, there is a need for a bulk electrically conductive member for an electric device, such as an electric contact or an electric conductor, the member comprising an electrically conductive material which is easier to optimize with regard to, for example, electric conductivity, thermal shock, arc erosion/melting, welding, wear and corrosion, compared to conventional bulk electrically conductive members.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a bulk electrically conductive member being improved with respect to bulk electrically conductive member already known by at least par- tially addressing said need.

This object is according to the invention obtained by providing a bulk electrically conductive member of the type defined in the introduction, in which said member comprising a nanocomposite material having a matrix and crystallites of nano-size, i.e. here defined as having a size in the range of 1 -200 nm in at least two dimensions, embedded therein, portions of said matrix separating adjacent said crystallites of said nanocomposite material have a thickness providing said matrix and hence said member with an electrical conductivity determined by a substantially two- dimensional character of said matrix, such as for so-called pseudo-2D materials like graphene.

Pseudo-2D materials are a novel type of materials which have been thoroughly investigated over the last few years. The most widely investigated is graphene (see for instance: "The rise of graphene", A. K. Geim and K. S. Novoselov, Nature Materials, vol. 6 (2007) pp. 183-191 ), which is a material ideally consisting of a single layer of graphite. These pseudo-2D materials have superior electric conductivity, almost reaching superconductivity.

By providing for a nanocomposite comprising crystallites of nano-size embedded in a matrix, where said matrix separating adjacent said crystallites by only a few atomic layers of matrix, thus creating a network of pseudo-2D material with crystallites of nano-size embedded therein, it has surprisingly been seen that the contact resistance of said nanocomposite significantly decreases.

"Matrix" is in this disclosure to be interpreted to not only relate to a continuous majority phase in which said crystallites of nano- size are contained. The matrix may only consist of a few atomic layers around said crystallites of nano-size. The matrix is not only to be interpreted as a binding phase, but also as a phase significantly contributing to the electric conductivity and/or the contact resistance of the nanocomposite.

According to an embodiment of the invention the thickness of the matrix between the majorities of said adjacent crystallites does not exceed 10, 7, 5, 3, 2 or 1 atomic or molecular layer/layers. The significant decrease in contact resistance, associated with the pseudo-2D character of the matrix, is only achieved when the matrix between the adjacent crystallites is thin enough. When increasing the thickness the contact resistance is rapidly increasing.

According to another embodiment of the invention said matrix comprises a material which has a sheet-like structure on the molecular level, such as graphene sheets in graphite and corresponding sheets in hexagonal BN. It has been shown that the effect of high electrical conductivity in pseudo-2D materials is associated with the hybridization of the atoms included in the material. The high conductivity can be reached if the pseudo-2D materials have a high content of sp2 hybridized atoms, which is the case of sheet-like structures on the molecular level, e.g. almost one hundred percent of the C atoms in pure graphite are sp2 hybridized. The high conductivity discussed above may also be reached if the atoms in the matrix material are partly or fully sp3 hybridized, as long as the matrix separating adjacent crystallites is sufficiently thin.

According to another embodiment of the invention said matrix consists of carbon, boron, silicon, carbides, nitrides, borides or suicides, preferably carbon. In the field of "nanotube research", where the carbon nanotubes were the first to be explored, a wide range of new materials, showing similar properties, have been discovered. In analogy to the field of nanotubes, similar discoveries are to be expected for the field of pseudo-2D materials. Graphene was the first pseudo-2D material explored and in the backwater of that discovery a wide range of materials has been investigated, and there are doubtless more materials to come showing similar properties. The above mentioned materials can show the desired sp2 hybridization, mentioned above, if prepared correctly.

According to another embodiment of the invention the structure of said matrix comprises defects, such as point defects, extended defects or dislocation defects. Defects in a structure can enhance the electric conductivity and/or affect the mechanical properties positively, e.g. strengthen the material.

According to another embodiment of the invention said defects comprise at least one doping agent. Doping agents are known in the art to affect the electric conductivity of a material. By choosing one or several proper doping agents the electric conductivity of said matrix can be further increased. According to another embodiment of the invention said doping agent is a transition metal or a p-element, preferably Fe, Co, Ni, Ag, Ta, F, H or O, more preferred Ni. Said doping agents may provide a significant increase in the electric conductivity of said matrix.

According to another embodiment of the invention said crystallites consist of a metal, metal alloy, metal carbide, metal nitride, metal boride or metal suicide, preferably a metal carbide or a metal nitride. Properties additional to high electrical conductivity and/or contact resistance can be tailored by the embedded crystallites. If e.g. corrosion resistance is demanded by the nanocomposite film, crystallites showing good corrosion resistance is embedded in said matrix, and if e.g. a hard nanocompo- site film is required of a certain contact arrangement, hard crystallites, consisting of e.g. metal carbide or metal nitride, are chosen correspondingly.

According to another embodiment of the invention said crystal- lites consist of niobium carbide or titanium carbide. These two materials provide the nanocomposite film with high wear resistance. Niobium carbide and titanium carbide are also highly electrical conductive, which further increases the electrical conductivity of the nanocomposite film.

According to another embodiment of the invention said crystallites have a diameter-like dimension in at least two dimensions in the range of 1 - 200, 30 - 70, 50 - 200, 100 - 150 or 5 - 50 nm. If the crystallites are too large the composite will assume the bulk properties of the material of the crystallites. Furthermore, the thickness of the matrix between the adjacent said crystallites is dependent upon the sizes of the crystallites. If the crystallites are too large, it is impossible to achieve a thin matrix between said adjacent crystallites. According to another embodiment of the invention said crystallites are substantially elongated in one direction, i.e. being in the form of fibers, such as whiskers or nanorods. The increased electric conductivity achieved by providing a pseudo-2D material as discussed above can also be achieved by providing a bundle of fibers, wherein said fibers are separated by a thin layer of matrix, as mentioned above. By this, e.g. cables or other elongated conductors having superior electric conductivity can be manufactured. Also, members with anisotropic electric and mechanical properties can be formed by using the features of this embodiment of the invention.

According to another embodiment of the invention said crystallites substantially are in the form of particles, i.e. having substantially the same extension in all directions. By this an electric conductive member with isotropic electric and mechanical properties is obtained. Further processing of the member, by e.g. mechanical treatment, is more feasible when the member has these mechanical properties.

Another object of the present invention is to provide a contact element for making an electric contact to a second contact element for enabling an electric current to flow between said contact element and said second contact element, at least one of said contact element or said second contact element comprising an electric conductive member according to the invention. Said contact element has the advantages of the invention discussed above.

According to another embodiment of the invention the thickness of said contact element in a direction perpendicular to the contact surface is in the range of 0.0005-0.001 , 0.001 -0.01 , 0,005- 0.05 or 0.0005-0.1 m. A thickness in this interval makes it possible to use a bulk material for the production of the contact ele- ment, which gives a cost efficient production of said contact element. Another object of the present invention is to provide an electric contact arrangement comprising the contact element mentioned above. Said contact arrangement has the advantages of the invention discussed above.

According to another embodiment of the invention said contact arrangement is a sliding electric contact arrangement.

According to another embodiment of the invention said contact arrangement is an arcing contact arrangement.

Another object of the present invention in to provide a conductor element for enabling an electric current to flow through said con- ductor element, said conductor element comprises a member according to the invention.

According to another embodiment of the invention the method of manufacturing a member for an electric device, such as an elec- trie contact or an electric conductor, according to the invention comprises a sintering method. The member may be directly sintered into a single piece or by machining or forming a larger piece into the member. Using a sintering method makes it possible to use large quantities for the production of the member, which gives a cost efficient production of said member.

According to another embodiment of the invention the sintering method of manufacturing said member, according to the invention comprises manufacturing of a powder of said crystallites coated with said matrix and sintering of said manufactured crystallites to form said member. By using this manufacturing method the matrix of said member achieves the electric properties associated with the pseudo-2D character of the matrix discussed above; hence the member achieves superior electric conductivity. According to another embodiment of the invention the member for an electric device, such as an electric contact or an electric conductor according to the invention is used in any conducts, including cables, bundle conductors and/or in any of the follow- ing contacts: circuit breakers, generator breakers, contactors, power interrupters, disconnectors, relays, vacuum interrupters, fuses, current limiters, selector switches.

Other advantages and advantageous features of the invention will appear from the dependent claims and the subsequent description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.

In the drawing:

Fig 1 illustrates very schematically a nanocomposite according to the invention,

Fig 2 shows a graph where the contact resistance is plotted versus the matrix thickness of a nanocomposite according to the invention,

Fig 3 illustrates very schematically an electric contact element according to an embodiment of the invention,

Fig 4 is a sectioned view of an electric contact element of helical contact type according to another embodiment of the invention,

Fig 5 illustrates very schematically a contact arrangement according to the present invention in a disconnector, Fig 6 illustrates very schematically a sliding contact arrangement in a tap changer of a transformer according to an embodiment of the invention,

Fig 7 illustrates very schematically a contact arrangement according to the present invention in a relay, and

Fig 8 is a sectional view of a contact arrangement according to the present invention in a medium voltage vacuum interrupter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

A nanocomposite 1 according to the invention is very schematically shown in Fig 1 . Said nanocomposite 1 comprises a matrix 2 and crystallites 3 of nano-size, i.e. here defined as being in the dimension range of 1 - 200 nm embedded therein. Said crystallites 3 are separated by said matrix 2, having a thickness T between adjacent said crystallites 3 not exceeding 10, 7, 5, 3, 2 or 1 atomic or molecular layer/layers. The matrix 2 comprises a material which has a sheet-like structure on the molecular level, such as graphite and hexagonal BN, and said crystallites 3 consist of a metal, metal alloy, metal carbide, metal nitride, metal boride or metal suicide, preferably a metal carbide or a metal nitride.

Fig 2 shows a graph where the contact resistance is plotted ver- sus the matrix 2 thickness T of a nanocomposite film according to the invention, such as a film comprising the nanocomposite in

Fig 1 , in this exemplifying embodiment the matrix 2 consists of

C, i.e. graphite, and the crystallites 3 consist of nano-crystalline

TiC. It should be noted that the data on the thickness T of the matrix 2 should be interpreted as a rough average, the average thickness T of the matrix 2 between adjacent crystallites 3 was calculated from X-ray diffraction data and transmission electron microscopy images. As can be seen in the graph of Fig 2, the contact resistance significantly decreases when the matrix thickness T reaches a thickness corresponding to about two molecu- lar layers of graphite (-0.3 nm). The graph also indicates that the resistance starts to increase again when the thickness T of said matrix 2 between adjacent crystallites 3 nears zero. The low contact resistance shown in Fig 2, corresponding to a thickness T of the matrix 2 of about two molecular layers of C has never been achieved earlier in similar nanocomposites. Similar contact resistance is to be expected when changing the matrix 2 from being C, to being another material which has a sheet-like structure on the molecular level according to the above discussion. The crystallites 3 can also be changed to any other metal, metal alloy, metal carbide, metal nitride, metal boride or metal suicide, without departing from the basic ideas of the present invention. The above described measurement of contact resistance of the nanocomposite of the invention is measured on a thin film but is of course also valid for the nanocomposite in bulk state, thus similar contact resistance is expected for the bulk member of the present invention.

A contact element 4 forming an electric contact to a second contact element 5 for enabling an electric current to flow between said contact element 4 and said second contact element 5 is very schematically shown in Fig 3. At least one of the contact elements 4, 5 comprises the nanocomposite according to the invention. This gives the contact element 4, 5 the excellent properties of e.g. electric conductivity reported above. Depend- ing on the application of the contact element 4, 5 the properties of the total contact structure can be optimized by changing the material in said crystallites 3. If e.g. corrosion resistance is demanded by the nanocomposite 1 , crystallites 3 showing good corrosion resistance is embedded in said matrix 2, and if e.g. a hard nanocomposite 1 is required of a certain contact arrange- ment, hard crystallites 3, consisting of e.g. metal carbide or metal nitride, is chosen correspondingly.

A contact element 4, 5 having the following advantages may thus be obtained:

a) a low contact resistance over a broad range of contact loads (forces)

b) high resistance to wear

d) high corrosion resistance

e) good high-temperature properties,

f) a large potential to various properties by tuning as described above.

Fig 4 illustrates an example of a contact arrangement in which it is advantageous that at least one of the contact elements comprises the nanocomposite according to the invention for forming a contact with very high electric conductivity. This embodiment relates to a helical contact arrangement having a first contact element 6 in the form of a spring-loaded annular body, such as a ring of a helically wound wire, adapted to establish and maintain an electric contact to a second contact element 7, such as an inner sleeve or a pin, and a third contact element 8, such as an outer sleeve or tube. The first contact element 6 is in a contact state compressed so that at least a contact surface 9 thereof will bear spring-loadedly against the contact surface 10 of the second contact element 9 and at least another contact surface 1 1 of the first contact element 6 will bear spring-loadedly against at least a contact surface 12 of the third contact element 8. According to this embodiment of the invention at least one of the contact elements 8 - 10 comprises a nanocomposite according to the invention. Such a helical contact arrangement is used for example in an electrical breaker in a switch gear.

Fig 5 illustrates very schematically how an electric contact arrangement according to the invention may be arranged in a disconnector 13 with at least one contact element 14, 15 comprising a nanocomposite according to the invention, said contact elements 14, 15 are movable with respect to each other for establishing an electric contact therebetween and obtaining a visible disconnection of the contact elements.

Fig 6 illustrates schematically a sliding electric contact arrangement according to another embodiment of the invention, in which the first contact element 16 is a movable part of a tap changer 17 of a transformer adapted to slide in electric contact along second contact elements 18 to the secondary winding of the transformer, accordingly forming second contact elements, for tapping voltage of a level desired from said transformer. The first contact element 16 and/or the second contact elements 18 comprises a nanocomposite according to the invention. The first contact element 16 may in this way be easily moved along the winding while maintaining a low resistance contact thereto.

Fig 7 illustrates very schematically a contact arrangement according to another embodiment of the invention used in a relay 19, and one or both of the contact elements 20, 21 comprises a nanocomposite according to the invention, which will result in less wear of the contact element and make them corrosion resistant as a consequence of the character of said contact element material.

Finally, Fig 8 illustrates very schematically a contact arrangement according to another embodiment of the invention used in a medium voltage vacuum interrupter 22. The vacuum inter- rupter 22 comprises a vacuum insulated vessel 23 having metallic end plates 24, 25 and a cylindrical insulating wall 26 ar- ranged between the end plates. The cylindrical insulating wall and the end plates are enclosing a volume 27 that is hermetically sealed. The volume 27 comprises a first contact element 28. The first contact element 28 makes a breaking contact to a second contact element 29. The first contact element 28 and the second contact element 29 are arranged opposite each other and arranged at the end of a first and second conductor 30, 31 , respectively. Usually the contact elements 28, 29 are arranged at the conductors 30, 31 by soldering, however, the contact ele- ment 28, 29 can also be manufactured in one piece with its conductor 30, 31 , respectively if the contact elements and the conductors are made from the same material. The first conductor 30 is connected with the end plate 24 through bellows 32 enabling movement along the longitudinal axis of the first conductor 30 without breaking the vacuum in the vessel 23. An arc shield system 33 is arranged inside the insulating walls to prevent metallic contamination and thereby preventing flash- overs. According to this embodiment the first and second contact elements 28, 29 comprises the nanocomposite of the inven- tion. Also the conductors 30, 31 can optionally comprise the nanocomposite of the invention.

The contact elements 28, 29 according to the embodiment above are suitable for low as well as high voltage breakers.

A contact element and an electric contact arrangement according to the present invention may find many other preferred applications, and such applications would be apparent to a person with ordinary skill in the art without departing from the basic idea of the invention as defined in the appended claims.

It is also pointed out that other transition metals than those mentioned above may be suited to form said metallic/ceramic crystallites of nano-size for meeting different demands put on the contact layer in different applications.

Claims

1 . A bulk electrically conductive member (4, 5) for an electric device, such as an electric contact or an electric conduc- tor, said member (4, 5) comprising a nanocomposite material having a matrix (2) and crystallites (3) of nano-size, i.e. here defined as having a size in the range of 1 -200 nm in at least two dimensions, embedded therein, characterized in that portions of said matrix (2) separating adjacent said crystallites (3) of said nanocomposite material have a thickness (T) providing said matrix (2) and hence said member (4, 5) with an electrical conductivity determined by a substantially two-dimensional character of said matrix (2), such as for so-called pseudo-2D materials like gra- phene.

2. A member (4, 5) according to claim 1 , characterized in that said matrix (2) has a thickness between the majorities of adjacent said crystallites (3) not exceeding 10, 7, 5, 3, 2 or 1 atomic or molecular layer/layers.

3. A member (4, 5) according to claim 1 or 2, characterized in that said matrix (2) comprises a material which has a sheet-like structure on the molecular level, such as graph- ite and hexagonal BN.

4. A member (4, 5) according to claim 3, characterized in that said matrix (2) consists of carbon, boron, silicon, carbides, nitrides, borides or suicides, preferably carbon.

5. A member (4, 5) according to any of the preceding claims, characterized in that the structure of said matrix (2) comprises defects, such as point defects, extended defects or dislocation defects.

6. A member (4, 5) according to claim 5, characterized in that said defects comprise at least one doping agent.

7. A member (4, 5) according to claim 6, characterized in that said doping agent is a transition metal or a p-element, preferably Fe, Co, Ni, Ag, Ta, F, H or O, more preferred Ni.

8. A member (4, 5) according to any of the preceding claims, characterized in that said crystallites (3) consist of a metal, metal alloy, metal carbide, metal nitride, metal bor- ide or metal suicide, preferably a metal carbide or a metal nitride.

9. A member (4, 5) according to claim 8, characterized in that said crystallites (3) consist of niobium carbide or titanium carbide.

10. A member (4, 5) according to any of the preceding claims, characterized in that said crystallites (3) have a diameter- like size in at least two dimensions in the range of 1 - 200, 30 - 70, 50 - 200, 100 - 150 or 5 - 50 nm.

1 1 . A member (4, 5) according to claim 10, characterized in that said crystallites (3) are substantially elongated in one direction, i.e. being in the form of fibers, such as whiskers or nanorods.

12. A member (4, 5) according to claim 10, characterixed in that said crystallites (3) substantially are in the form of particles, i.e. having substantially the same extension in all directions.

13. A first contact element (6, 14, 16, 20, 28) for making an electric contact to a second contact element (7, 15, 18, 21 ,

29) for enabling an electric current to flow between said first contact element (6, 14, 16, 20, 28) and said second contact element(7, 15, 18, 21 , 29), characterized in that at least one of said first contact element (6, 14, 16, 20, 28) or said second contact element (7, 15, 18, 21 , 29) com- prises a member according to any of claims 1 -12.

14. A contact element (6, 7, 14, 15, 16, 18, 20, 21 , 28, 29) according to claim 13, characterized in that the thickness of said contact element (6, 7, 14, 15, 16, 18, 20, 21 , 28, 29) in a direction perpendicular to the contact surface is in the range of 0.0005-0.001 , 0.001 -0.01 , 0,005-0.05 or 0.0005- 0.1 m.

15. An electric contact arrangement (13, 17, 19, 22), charac- terized in that it comprises a contact element (6, 7, 14, 15,

16, 18, 20, 21 , 28, 29) in according to claim 13 or 14.

16. An electric contact arrangement (13, 17, 19) according to claim 15, characterized in that said contact arrangement (13, 17, 19) is a sliding electric contact arrangement.

17. An arrangement according (22) to claim 15, characterized in that said contact arrangement (22) is an arcing contact arrangement.

18. A conductor element for enabling an electric current to flow through said conductor element, characterized in that said conductor element comprises a member (4, 5) according to any of claims 1 -12.

19. Method of manufacturing a member (4, 5) for an electric device, such as an electric contact or an electric conductor, according to any of claims 1 -12, characterized in that said method comprises a sintering method.

20. Method of manufacturing a member (4, 5) according to claim 19, characterized in that said sintering method comprises manufacturing of a powder of said crystallites (3) coated with said matrix (2) and sintering of said manufac- tured crystallites (3) to form said member (4, 5).

21 . Use of a member (4, 5) for an electric device, such as an electric contact or an electric conductor according to any of claims 1 -12 in any conducts, including cables, bundle conductors and/or in any of the following contacts: circuit breakers, generator breakers, contactors, power interrupters, disconnectors, relays, vacuum interrupters, fuses, current limiters, selector switches.