WO2007118337A1

WO2007118337A1 - Electrical contact assembly

Info

Publication number: WO2007118337A1
Application number: PCT/CH2006/000210
Authority: WO
Inventors: Kaveh Niayesh; Jan-Henning Fabian; Jens Tepper; Martin Seeger; Olof Hjortstam
Original assignee: Abb Research Ltd
Priority date: 2006-04-13
Filing date: 2006-04-13
Publication date: 2007-10-25

Abstract

A contact assembly (1, 2), in particular for a high voltage switchgear, has contacts (1; 2) that comprise a metal matrix (3) with embedded carbon nanotubes (4). The carbon nanotubes (4) improve the thermal and electrical conductivity of the electrodes (1, 2).

Description

Electrical contact assembly

DESCRIPTION.

Technical Field

The present invention relates to an electri- cal contact assembly as well as to a circuit breaker using such an assembly.

Background

Electrical contact assemblies, such as they are e.g. used in electrical switchgear or stationary contacts, usually have at least two metal contacts, which can be brought into contact with each other. Typical contact materials are copper or copper alloys. Lifetime and contact resistance are important parameters of electrical contact assemblies.

Summary of the invention

It is an object of the present invention to improve said parameters of an electrical contact assembly.

This object is achieved by the contact assem- bly and circuit breaker of the independent claims.

Accordingly, at least one of said contacts comprises a metal matrix with carbon nanotubes embedded therein. Embedding nanotubes in the metal matrix improves the electrical and thermal conductivity of the same, which, in turn, leads to lower electrical and thermal resistivity and improved lifetime. Embedding nanotubes in a metal matrix is especially advantageous for high voltage switchgear, such as circuit breakers, where arc discharges tend to erode the electrodes. The improved thermal conductivity de- creases the erosion rate, while the improved electrical conductivity allows larger load currents and reduces electrode heat-up.

The present invention takes advantage of the superior thermal conductivity of the carbon nanotubes to enhance the thermal properties of the composite material. As a result of the very high thermal conductivity of CNTs, it is possible to enhance the cooling capability of the contact system, which can result in lower erosion rates of the switching (arcing) contacts and lower work- ing temperatures (or higher current ratings) of stationary or nominal-current contacts.

Moreover, the ohmic losses of stationary contacts and power flow to the arcing contacts in the presence of an arc (or cathode fall) can be reduced because of the lower resistivity of the compound material and better charge carrier emitting behavior of the nanotubes.

Advantageously, the nanotubes are aligned along a preferential direction. Even if this alignment is not perfect, thermal and electrical conductivity will be largest along the preferential direction.

The invention also relates to electric switchgear comprising such a contact assembly. The invention can, in particular, be used in high voltage switch- gear. The term "high or medium voltage switchgear" desig- nates any switchgear for switching voltages of 1000 Volts or more. An especially advantageous application of the invention is its use in high voltage or medium voltage circuit breakers for switching voltages of 10 kV or more and, in particular, above 72 kV. The invention is also suited for stationary contacts, where it ' supports the operation of the contacts due to the CNT's increased electrical conductivity and helps to distribute the heat from local "hot spots" due to the CNT's improved thermal conductivity.

Short description of the figures

Further advantageous features of the present invention are disclosed in the dependent claims as well as in the following description and figures. The figures show:

Fig. 1 is a sectional view of a schematic contact arrangement in a high voltage or medium voltage circuit breaker,

Fig. 2 is a sectional view of part of an electrode,

Fig. 3 shows in (a) a simple model of the metal embedded carbon nanotubes and in (b) a simple building block of the compound material,

Fig. 4 shows the effective thermal conductiv- ity of the compound material (metal embedded carbon nanotubes) vs. the length of the intermediate region between two adjacent nanotubes for different volume ratios k,

Fig. 5 shows the power dissipation density due to heat conduction into the contacts vs . time for pure copper and carbon nanotubes embedded in copper (volume ratio of CNTs is k=10%) and

Fig. 6 shows a sectional view of an alternative embodiment of an electrode.

Ways for carrying out the invention

General :

The high voltage circuit breaker of Fig. 1 comprises a first, annular electrode 1 and a second, rod- shaped electrode 2. The two electrodes 1, 2 form the contact assembly of the switchgear. An actuator (not shown) is provided for axially moving one of the electrodes 1; 2 in respect to the other 2 ; 1 in order to open and close the switch. The basic design of this type of high voltage switchgear is known to the person skilled in the art. The present invention is described in the context of such a high voltage switchgear, but it can also be used for other applications.

One, or advantageously both, of the electrodes 1, 2 comprise a metal matrix with carbon nanotubes embedded therein. A macroscopic sectional view of a simple embodiment of such an electrode 1, 2 is shown in Fig. 2. The metal matrix 3 is advantageously^" a copper alloy or consists of copper. The carbon nanotubes 4 are embedded in the matrix 3 and are preferentially aligned along a preferential direction 5. Preferential direction 5 is substantially perpendicular to the surface 6 of the matrix 3, which also forms the contact surface of the electrode 1, 2.

Carbon nanotubes (CNTs) are cylindrical mole- cules with a minimum diameter of 1-2 nm and a length up to several tens of micrometers, or even longer. These molecules consist of carbon atoms, which are arranged in a hexagonal graphite layer that has been wrapped into cylindrical form. Two major types of CNTs can be distin- guished - Single-Wall Carbon nanotubes (SWCNTs) and

Multi-Wall Carbon nanotubes. MWCNTs are fullerene tubes consisting of several shells, which are arranged in coaxial fashion. The nanotubes can be characterized by the tube diameter and their twist of the rolled graphite lay- ers (chiral angel) .

Carbon nanotubes have a number of unique electrical and thermal properties, which make them suitable for a wide range of applications. For example, nanotubes are light and have a very high elastic modulus (> 1.5 TPa) . They are the strongest fibers discovered so far [1] • Metallic nanotubes of high quality can behave like ballistic conductors with a constant electrical resistance of about 6.5 kΩ. Note that this value is not dependent on the CNT-length) . This corresponds to a speci- 5 fie electrical conductance several times higher than copper for typical nanotubes of a few μm [5] . Moreover, the current density through a CNT could be as high as 10⁷ A/cm² (amps per square centimeter) .

CNTs are also excellent thermal conductors. 10 CNTs have measured thermal conductivities up to 3000 W/ (K m) and theoretical values in excess of 6600 W/ (K m) [3] .

Properties:

15 In the following, we provide estimates for the thermal conductivity and erosion performance of this type of a metal matrix 3 with embedded carbon nanotubes 4.

Thermal conductivity

20 As is described above, the high thermal conductivity of the carbon nanotubes 4 plays an important role in this invention. For this purpose, the thermal conductance of the composite 3, ^"4 consisting of the carbon nanotubes 4 and the embedding metal matrix 3 is esti-

25 mated using the simple model shown in Fig. 3. The effective thermal conductivity of the composite 3, 4 can be approximated by assuming that the total thermal resistance is a combination of three thermal resistances. So the effective thermal conductivity of the compound mate-

30 rial 3, 4 can be calculated as:

_λ _ (l + QC₁ )λ_meto/ (α₂ λ_metαl + λgyr* ) (l + Oc₂ ) λ_metαl + (X₁ (cc₂ λ_{melαl ,}+ λc^^* )

wherein 3 -j5_{q /}O_vC^ —= kjnt erface , „OC2 _= ^metal •

^LCNT ^ACNT AcNT is the surface area of one carbon nanotube, A_metai the surface area of the embedding metal part in one building block of the compound material 3, 4, L_CNT is the length of one carbon nanotube, I/i_nterface the length of the metallic part between two adjacent carbon nanotubes (see Fig. 3 part (b) ) . λ_metai is the thermal conductivity of the embedding metal 3 and λ_CNT the effective thermal conductivity of the nanotube 4. It must be noted that λ_CΛσ could be much lower than the thermal conductivity of nanotubes 4 depending on the value of thermal resistance between the nanotube 4 and the embedding metal matrix 3 :

¹I ^ΛCNT λ * ^■

1+ ^CNT^CNT ^•n thermal *^int erface

L cm

where λ_CNT is the thermal conductivity of nanotube 4 itself and R^'^_φce is the thermal resistance between one nanotube 4 and the embedding metal matrix 3.

It must be noted that for a given volume ratio k of the carbon nanotubes 4 in the compound material 3, 4, the above defined ratios OCi and (X₂ are related to each other as :

To give some quantitative measures, the carbon nanotubes 4 are assumed to have a length of 50 μm and a diameter of 2 nm. Furthermore, it is assumed that they are embedded in a copper matrix 3 (λ_metai = 390 W/ (m K)) and the effective thermal conductivity of one nanotube 4 in a metal matrix 3 is half of that of nanotubes 4 ( λ_CΛrr = 1500 W/ (m K)). Fig. 4 shows the thermal conductivity of the compound material 3, 4 as a function of the length of the interface region (Li_nterface) for different volume ratios k of the carbon nanotubes 4. As it can be seen, it is possible to achieve remarkably higher thermal conduc- tivities than conventional contact materials under reasonable conditions. So, for example, the thermal conductivity of the compound material 3, 4 would be more than twice that of copper in case of k=30%, Li_nterface=3 |im. Even for k=10%, i.e. for only 10 volume percent of CNTs 4 in the matrix 3 , a substantial increase of the thermal conductivity can be observed.

Erosion performance Using the calculated thermal conductivity of the compound material 3, 4, it is possible to evaluate the performance of this material as arcing contact 1, 2. The erosion of arcing contacts 1, 2 can be described as a mass loss due to contact material evaporation. This mass loss is proportional to an energy E^rosion ^{tnat can} be calculated as the difference of the net energy input to the arcing contact. E_jjet and energy losses at the contact like cooling due to radiation, convection and heat conduction EjjeatConduction into the contact. If the latter one dominates the energy losses, the contact erosion can be derived as

mass loss ~ E_Erosion = E_Net - E_HeatConducti_on

Fig. 5 shows the power dissipation density due to the heat conduction into the contact resulted by one-dimensional heat conduction simulations for the pure copper contacts and for the contacts made of the metal embedded carbon nanotubes 3, 4 with a thermal conductiv- ity of 870 W/ (m K) . As it can be seen, the time without any erosion (the plateau in the curves) has been increased from 2 ms to ca. 10 ms . Besides the better heat conduction in the compound 3, 4, application of carbon nanotubes 4 makes it possible to enhance the emitting of electrons at the contact surface 6, which can result in lower effective cathode fall voltage and thus in reduced power input P^et ^{to tne} contact surface 6. Manufacturing:

There are different established ways to fabricate metal matrix compounds 3, 4 with embedded CNTs 4. The detailed fabrication sequence plays no role for the invention.

One method of fabrication can e.g. comprise the following steps :

1) Fabrication of a CNT pre-form. This is an arrangement of carbon nanotubes e.g. on a substrate.

2) After optionally coating the pre-form with a wetting agent, liquid metal is cast around the pre-form in a casting process, such as injection casting. This step is carried out under a protective gas or under vacu- urn to prevent thermal oxidation of the CNTs .

To fabricate a pre-form to be used in step 1) , a suitable substrate, such as a silicon substrate, can be coated with a catalyst, such as Fe . The substrate is then coated with nanotubes using a CVD-type process. A suitable method is described by H. Kind et al . in "Patterned films of nanotubes Using Microcontact Printing of Catalysts" in Adv. Mater. 1999 (11) , No. 15, pp. 1285 - 1289. The details of CNT fabrication are described in the section "Experimental" of that publica- tion. In order to form preferentially aligned CNTs, the catalyst can e.g. be applied to the substrate as a Fe(NC^) ₃ solution of 0.0075 Mol/1, with the substrate being heated to at least 700 ⁰C during CVD. Details of this growth procedure can be found in - the Diploma thesis of Ph. Mauron, "Wachstum von CVD (Chemical Vapor Deposition) Nanotubeschichten" under the supervision of A. Zύttel of the group of solid state physics at the university of Fribourg, Switzerland, June 1999 as well as in - Philippe Mauron, "Growth Mechanism and

Structure of Carbon Nanotubes", Dissertation No. 1420, Universitat Fribourg (CH) (2003) , http: //www. ifres . ch/Homepage/DB/Diss_Mauron.pdf .

When using a pre-form of nanotubes on a substrate, the substrate can be removed after step 2. The resulting layer of a metal matrix 3 with embedded CNTs 4 can be used to form the surface 6 of the contact 1, 2. It may e.g. be mounted on a copper substrate 7, as shown in Fig. 6, or a plurality of such layers 3, 4 can be mounted on top of each other. Another way of producing metal matrix com- pounds 3, 4 with embedded CNT ¹S 4 is to sinter metal powders mixed with CNT' s. Prior to the sintering process the CNT and metal powder have to be well dispersed. The dispersion process can be performed either on dry powders or within a liquid medium. To further promote the formation of a stable dispersion, different types of chemical modifications of the powder surfaces can be used. In the dispersion process different techniques such as shaking, centrifugation, mixing, ball mixing and ultra sound can be used.

Instead of using a CNT pre-form, CNTs can also be loosely suspended in liquid metal. After solidifying the metal, the CNTs will generally not have a preferential direction. If a preferential direction is de- sired, the CNTs can e.g. be aligned at least partially by anisotropically deforming (e.g. by stretching or rolling) the metal matrix after embedding the CNTs, e.g. at elevated temperature where the metal is in a soft state. This procedure leads to a preferential alignment of the otherwise randomly distributed CNTs, as shown in the embodiment of Fig. 2.

The embodiments of Fig. 2 and 6 show a further advantageous aspect of the present invention. As can be seen, the metal matrix 3 comprises nanotubes 4 pro- jecting above the contact surface 6. This design can e.g. be achieved by removing part of the material of the matrix after embedding the nanotubes 4, for example by etching or grinding off some matrix material . The pro- jecting nanotubes 4 reduce the cathode fall (i.e. the voltage drop over the spark gap) , which leads to a decrease in power dissipation and erosion. ■ The contact system 1, 2 comprising at least one electrode 1 and/or 2 comprising a metal matrix 3 with carbon nanotubes 4 embedded in said metal matrix 3 can be part . of any type of electrical switch in any voltage range including low voltage and, in particular, can be part of a high or medium voltage switchgear. In particular, it can be used in a disconnector, earthing switch, circuit breaker or generator circuit breaker in the medium or high voltage range .

Additional cited references:

[1] Kreupl, F., Graham, A., and Hδnlein, W., "A status report on technology for carbon nanotube devices", Solid State Technology 4, S9-S16. 2002.

[2] Schδnberger, Ch., Foro, L., Physics World 6, 37-41. 2000.

[3] Berber, S., Kwon, Y. K., and Tomanek, D., "Unussually High Thermal Conductivity of Carbon Nanotubes", Physical Review Letters, vol. 84, no. 20, pp. 4613-4616, 2000. [4] Dai, H., "Controlling Nanotube Growth",

Physics World 6, 43-47. 2000.

[5] O. Hjortstam et al . , Appl . Phys . A 78, 1175 (2004) .

List of reference numbers

1, 2: Contacts

3 : metal matrix

4 : carbon nanotubes 5: preferential direction

6 : surface

7 : substrate

Claims

1. A contact assembly having contacts (1, 2), characterized in that at least one pf said contacts

(1, 2) comprises a metal matrix (3) with carbon nanotubes (4) embedded in said metal matrix (3) .

2. The contact assembly of claim 1 wherein said nanotubes (4) are arranged along a preferential di- rection (5) .

3. The contact assembly of claim 2 wherein said preferential direction (5) is substantially perpendicular to a surface (6) of said contact (1, 2) .

4. The contact assembly as claimed in any of the preceding claims, wherein said metal matrix (3) contains at least 10, in particular at least 30, volume percent (k) of nanotubes (4) .

5. The contact assembly as claimed in any of the preceding claims, comprising nanotubes (4) projecting above a surface (6) of said matrix (3) .

6. The contact assembly as claimed in any of the preceding claims, wherein said metal matrix (3) comprises copper.

7. The contact assembly as claimed in any of the preceding claims, wherein said metal matrix (3) is copper .

8. Switchgear, in particular high or medium voltage switchgear, comprising the contact assembly

(1, 2) as claimed in any of the preceding claims.