Method for manufacturing an Ag-based electrical contact material, an electrical contact material and an electrical contact obtained therewith

Abstract

A material and method for manufacturing an Ag-based electrical contact material includes synthesizing an intermetallic compound of Me.sub.xSn.sub.y type; ball milling the intermetallic compound; mixing the so obtained intermetallic compound powder with silver powder; packing the mixed powders into a green body; and forming a MeO-SnO.sub.2 cluster structure by internally oxidizing the intermetallic compound Me.sub.xSn.sub.y while sintering the green body.

Claims

1. A method for manufacturing an Ag-based electrical contact material characterized in that it comprises the steps of: a. synthesizing an intermetallic compound of Me.sub.xSn.sub.y type, wherein Me is an additive metal; b. ball milling the intermetallic compound; c. mixing the so obtained intermetallic compound powder with silver powder; d. packing the mixed powders into a green body; and e. forming a MeO-SnO.sub.2 cluster structure by internally oxidizing the intermetallic compound Me.sub.xSn.sub.y while sintering the green body.

2. The method of claim 1, further comprising the step of: f. densifying an obtained material by repressing and re-sintering to release extra strain.

3. The method of claim 1, wherein Me is selected among: copper, molybdenum, iron, manganese, nickel, indium, antimony.

4. The method of claim 3, wherein Me is copper.

5. The method according to claim 1, wherein synthesizing step (a) is performed by mixing Me powder with Sn powder; melting the mixed powders; and quenching and annealing the intermetallic compound.

6. The method according to claim 1, wherein step (b) of ball milling is performed so as to obtain particles of intermetallic compound with a diameter d comprised between 1 m and 20 m.

7. The method according to claim 6, wherein said diameter d is less than 5 m.

8. The method according to claim 1, wherein the powders packing step (d) is performed by pressing the powders at a pressure comprised between 50 MPa and 200 MPa.

9. The method according to claim 1, wherein after step (e) a further step (f) is performed which comprises: f. densifying an obtained material.

10. An Ag-based electrical contact material obtained by: a. synthesizing an intermetallic compound of Me.sub.xSn.sub.y type, wherein Me is an additive metal; b. ball milling the intermetallic compound; c. mixing the so obtained intermetallic compound powder with silver powder; d. packing the mixed powders into a green body; and e. forming a MeO-SnO.sub.2 cluster structure by internally oxidizing the intermetallic compound Me.sub.xSn.sub.y while sintering the green body.

11. An Ag-based electrical contact comprising at least one portion of a material obtained by the process of claim 10.

12. An Ag-based electrical contact material characterized in that it comprises a MeO-SnO.sub.2 cluster structure.

13. An Ag-based electrical contact material according to claim 12, wherein Me is selected among: copper, molybdenum, iron, manganese, nickel, indium, antimony.

14. An Ag-based electrical contact material according to claim 13, wherein Me is copper.

15. An Ag-based electrical contact comprising at least one portion of a material obtained by the process of claim 14.

16. An Ag-based electrical contact comprising at least one portion of a material obtained by the process of claim 13.

17. An Ag-based electrical contact comprising at least one portion of a material obtained by the process of claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the present invention will be more clear from the description of preferred but not exclusive embodiments of a method for manufacturing an Ag-based electrical contact material, according to the present invention, of an Ag-based electrical contact material and of an electrical contact, shown by way of examples in the description, examples and drawings (incorporated in the examples), wherein:

(2) FIG. 1 shows a time-temperature sintering diagram of the green body, during step e) of the method according to a preferred way to perform the present invention;

(3) FIG. 2 shows the energy adsorbed by three samples during charpy test;

(4) FIG. 3 shows the uni-axial tensile test results of the same three samples of FIG. 2;

(5) FIG. 4 illustrates mechanical lifetime test results of four samples;

(6) FIG. 5 illustrates electrical lifetime test results of the same four samples of FIG. 4.

(7) FIG. 6 is a SEM analysis illustrating the microstructure (right picture enlarged) of Ag/FeSn2 oxidized at 900 C. for 2 h;

(8) FIG. 7 is a SEM analysis illustrating the microstructure (right picture enlarged) of Ag/Ni3Sn4 oxidized at 900 C. for 2 h;

(9) FIG. 8 is a SEM analysis illustrating the microstructure of Ag/Cu3Sn with initial Cu3Sn particle size about 10 um (left) and 4 um (right) oxidized at 850 C. for 2 h; and

(10) FIG. 9 is a SEM analysis illustrating the microstructure of reference Ag/SnO2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) The method for manufacturing an Ag-based electrical contact material according to the present invention provides a first step a) which comprises synthesizing an intermetallic compound of Me.sub.xSn.sub.y type, wherein Me is a metal as defined above. In particular, stoichiometric Me and Sn powders are mixed and then melted at about 1000 C. for at least 30 min (please check). This step is preferably carried out under protective atmosphere. Afterwards, the intermetallic compound is subjected to quenching and annealing treatments under vacuum.

(12) As far as stoichiometry is concerned, x and y may vary over a wide range depending on the metal. However, it has been found that, for a given metal, preferred values of x and y in the Me.sub.xSn.sub.y intermetallic compound are those which give higher ratio of y/x within the availability of intermetallic phases since this enables larger proportion of SnO2 and thus higher arc erosion resistance. For example, when Me is iron, y/x=1 and 2 are both available, but FeSn2 is preferred. Other examples are Cu3Sn, Ni3Sn4.

(13) After step a), Me.sub.xSn.sub.y intermetallic compound is ball milled according to a second step b) of the present invention. This step is preferably carried out by use of WC (tungsten carbide) balls, in such a way to obtain the desired particle size. The particle size is modulated by varying milling time, milling balls type and the ball-material mass ratio. As better shown in the following examples, the Applicant found out that performing step b) in order to obtain particles of intermetallic compound with a diameter d comprised between 1 m and 20 m, and more preferably with grain size smaller than 5 m, the final Ag-based electrical contact material shows the higher fracture toughness.

(14) After step b), the so obtained intermetallic compound powder is mixed with silver powder, according to step c) of the method of the invention. This mixing is carried out with ZrO.sub.2 (zirconium dioxide) balls with a proper ball-material ratio.

(15) At this point, according to following step d), the mixed powders of silver and intermetallic compound, is packed into a green body. Preferably, it is a loosely packing step, which means that it is carried out by pressing the powders at a pressure comprised between 50 MPa and 200 MPa for a time lapse comprised between 1 s and 30 s.

(16) Later on, step e) is carried out. It is performed by thermally treating the green body, in order to cause the sintering thereof and the internal oxidation of the Me.sub.x-Sn.sub.y intermetallic compound. This internal oxidation causes the formation of MeOSnO.sub.2 cluster structures. They are complex cluster structures with a high SnO.sub.2 content core and a high metal content surface. This is due to the fact that the metal diffuses outward, compared to Sn. Therefore, the silver contacts mainly MeO and this in-situ formation of MeO in Ag enables a very good adhesion, overcoming the above toughness problems related to these kinds of materials. In other words, the combination of the steps of the present invention attains replacing the bad Ag/SnO.sub.2 interface with a good Ag/MeO interface. Moreover, the high content of SnO.sub.2 in the structure core ensures a good arc erosion resistance.

(17) According to preferred embodiments of the invention, step e) is carried out at a temperature of about 850 C. for about 2 hours under air, in the way shown as an example in FIG. 1.

(18) Advantageously, after step e), a further step f) of densifying the obtained material is carried out.

(19) This step aims to obtain a final material with desired microstructure and features. It preferably comprises pressing the material with a pressure comprised between 600 MPa and 900 MPa for a time lapse comprised between 1 s and 30 s and then sintering at a temperature comprised between 300 C. and 600 C. for a time lapse comprised between 1 h and 3 h.

(20) In preferred embodiments, the metal of the intermetallic compound is selected among: copper, molybdenum, iron, manganese, nickel, indium and antimony. However, the most preferred metal is copper, as it can be easily deducted from the examples below.

(21) According to a further aspect, the present invention also relates to an Ag-based electrical contact material comprising cluster structures of MeOSnO.sub.2.

(22) As mentioned before, the metal of the cluster structure may be chosen among metals or metalloids elements. However, molybdenum, iron, manganese, nickel, indium, antimony and, above all, copper, are the preferred to the aims of the present invention.

(23) The Ag-based electrical contact of the present invention comprises at least one portion of such a material comprising MeOSnO.sub.2 cluster structures.

(24) Preferably, the whole electrical contact is made of said material.

(25) Here follow examples of the present invention according to some preferred embodiments.

Example 1

(26) Intermetallic phase Cu.sub.3Sn is synthesized under protective atmosphere (step a).

(27) Stoichiometric Cu and Sn powders are mixed and melted at 1100 C. for 4 hours followed by quenching and further annealing at 650 C. under vacuum.

(28) The obtained Cu.sub.3Sn compound is ball milled with WC balls (ball-material mass ratio 100:1) (step b) to certain particle size. In particular, a first sample is ball milled up to 10 m diameter and a second sample is ball milled up to 4 m diameter in order to investigate the influence of the particle size of initial intermetallic phase Me.sub.xSn.sub.y on fracture toughness, as shown in FIGS. 2 and 3. As a matter of fact, these figures show the possibility of tuning microstructure and mechanical property through particle size control.

(29) Cu.sub.3Sn powder and Ag powder are mixed (step c) with ZrO.sub.2 balls (ball-material mass ratio 10:1).

(30) The mixed Ag/Cu.sub.3Sn powder is pressed with 100 MPa for 30 s (step d) and further sintered and oxidized (step e) at 850 C. for 2 h under air, as shown in the attached FIG. 1.

(31) The as sintered Ag/Cu.sub.3Sn samples are pressed with 750 MPa for 10 s and further sintered at 450 C. for 2 h under air, achieving at least 95% density (step f).

(32) As a comparative example also an Ag/SnO.sub.2 sample is manufactured with a prior art method. It is synthesized at CHCRC with composition 86 wt % Ag, 12 wt % SnO.sub.2 and 2 wt % Bi.sub.2O.sub.3.

(33) The three samples were tested showing the results reported in FIGS. 2 and 3.

(34) The attached FIGS. 2 and 3 show mechanical tests results on respectively: Ag/SnO.sub.2 (comparative) and Ag/(Me,Sn)O samples with different initial particle size, as indicated in the figures.

(35) In particular, FIG. 2 shows the energy absorbed during charpy tests and FIG. 3 shows the uni-axial tensile tests. As it is clearly visible from the figures, mechanical features of the materials manufactured by means of the method of the invention are largely enhanced with respect to the reference material obtained through the methods of the prior art.

Example 2

(36) The influence of different metals in the initial intermetallic phase Me.sub.xSn.sub.y on fracture toughness and electrical lifetime was investigated, as shown in FIGS. 4 and 5 respectively.

(37) In particular, four samples were prepared. As a comparative example, the first sample is an Ag/SnO.sub.2 sample that is manufactured according to a prior art method, with composition 86 wt % Ag, 12 wt % SnO2 and 2 wt % Bi2O3.

(38) While the remaining three were prepared using the method of the invention, starting from synthesizing three different intermetallic compounds with a particle diameter of 1-4 m: i. Intermetallic compound FeSn2; ii. Intermetallic compound Ni3Sn4; iii. Intermetallic compound Cu.sub.3Sn.

(39) The method used to manufacture Cu3Sn was the same used in Example 1.

(40) For FeSn2 and Ni3Sn4, a solid state reaction was adopted instead to minimize the synthesis time and cost. Under H2, after being heated up to 250 C. in 1 h, the sample was held at 250 C. for 2 h to allow liquid Sn to diffuse around, and then was heated up to 750 C. in 2 h, held at 750 C. for another 12 h, finally cooled down within 1 h. For Ni3 Sn4, we get trace amount of Sn besides the majority phase Ni45Sn55. For FeSn2, due to incomplete reaction, an additional annealing step at 475 C. was performed for 2 days. Afterwards the majority phase turns out to be FeSn2, with small quantities of FeSn and Sn.

(41) The obtained bar-shaped samples were characterized for charpy and tensile test to evaluate the fracture toughness. The attached FIGS. 4 and 5 show the results.

(42) Both tests results indicate a light enhancement of fracture toughness and electrical lifetime in the Ag/FeSn.sub.2 and Ag/Ni.sub.3Sn.sub.4 samples compared to Ag/SnO.sub.2 sample. At the same time, the two figures show a great enhancement of fracture toughness of Ag/Cu.sub.3Sn sample compared to Ag/SnO.sub.2 sample.

(43) Furthermore, SEM analysis of the fracture surface in oxidized Ag/Me.sub.xSn.sub.y samples (FIG. 6-8) have revealed much better improvement of interface adhesion compared to prior art sample (FIG. 9).

(44) It is clear from the above description and examples that the method according to the present disclosure, as well as the above illustrated Ag-based electrical contact material and the relevant electrical contact, fully achieve the intended aims and solved the above-highlighted problems of the existing Ag-based material manufacturing methods, Ag-based electrical contact materials and Ag-based electrical contacts.

(45) Indeed, they overcome the adhesion problem, improving the fracture toughness of the material of the present invention, while resulting inexpensive and safeguarding a high electrical conductivity, as previously pointed out.

(46) In addition to that, it has been found that the material of the invention are even more durable from an electrical point of view, as revealed by the above FIG. 5. For this reason it may be stated that the method of the present invention, improves both mechanical and electrical properties of the material obtained therewith.

(47) Several variations may be made to the method for manufacturing an Ag-based electrical contact materialas well as to the electrical contact material itself and to the relevant electrical contactsall falling within the scope of the attached claims.