The Influence of Manufacturing Process, Metal Oxide Content, and

ICEC2006

The Influence of Manufacturing Process, Metal Oxide Content, and Additives on Switching Behavior of Ag/SnO2 in DC and AC Relays (2) Andreas Koffler,Peter Braumann, Bernd Kempf, Umicore AG & Co. KG, Hanau , Germany

Summary

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New findings and conclusions from a multi-year program are presented, which first part was published at the 22nd ICEC 2004.

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The current phase of work focused on investigating the influence of three separate parameters on the switching behavior of DC and AC relays: the grain size and dispersion of the components, the metal oxide content, and the In2O3 content. In automotive relays, materials with larger oxide particles achieve equivalent or better results. In the case of general purpose relays, the maximum life in regards to welding and erosion is achieved with a large metal oxide content (12%). Welding resistance and erosion behavior are counteracted less effectively by reducing the amount of In2O3 and using larger metal oxide particles. It has been determined that in general, a very high degree of oxide particle dispersion is only advantageous at low currents like those are switched in general purpose relays.

Key words: Silver tin oxide, relay applications, wet chemical precipitation

1. Introduction The historical development of the use of Ag/SnO2 in automotive relays and power relays has been previously reported in a comprehensive survey of the literature /1/. In switching experiments, the focus lay on comparing Ag/SnO2 materials manufactured using the classic mixed powder technology (PMT materials) with so-called NCF materials where the composite powder is produced using the wet chemical precipitation technology. The NCF method was presented for the first time in /2/ and explained in greater detail also in /1/. The major advantages of the NCF technology compared with the conventional powder blending technology can be summarized as follows according to /1/: - Extremely homogenous distribution of the components - High material deformability

Flexibility in the selection of the basic component SnO2 Flexibility regarding additives, including the use of In2O3

In /1/, numerous switching experiments were performed on automotive relays and power line relays. The switching performance of known material varieties was compared with new materials based on the NCF technology, and a few basic rules on the use of the materials within different load ranges were presented. Additional work concerning the overall topic addressed the following questions: - Under what conditions are extremely finely dispersed Ag/SnO2 materials advantageous? - What effect does varying the amount of SnO2 have on the switching performance? - What influence does the amount of In2O3 have? The results will be discussed in the following paragraphs.

2. Contact Materials Table 1 lists the Ag/SnO2 variants tested. Except for the comparative material Ag/SnO2 80/12 VS1011, all are based on wet chemical precipitation. Designation

Degree of dispersion AgSnO2 88/12 NCF1 extremely high AgSnO2 90/10 NCF1 extremely high AgSnO2 92/8 NCF1 extremely high AgSnO2 88/12 NCF2 extremely high AgSnO2 88/12 NCF3 high AgSnO2 88/12 VS1011 average

Table 1

In2O3 additive average content average content average content low content average content average content

Contact materials for switching tests

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processed into rivets. The chemical composition of the test material VS1011 corresponds to that of Ag/SnO2 88/12 NCF3.

Fig. 1

Ag/SnO2 88/12 NCF 1 (NCF with extremely fine oxide particles)

If we use Ag/SnO2 88/12 NCF1 as the reference, the metal oxide content was reduced from 12% to 8%. In the case of variant NCF2, the amount of In2O3 additive was reduced by 1/3 in comparison to NCF1. The variant NCF3 corresponds to NCF1 except for the somewhat coarser structure (lesser degree of dispersion).

Fig. 3

Ag/SnO2 88/12 VS1011 (conventional mixed powder technology with average metal oxide particles)

3. Automotive Relay Applications The experiments done on automotive applications centered on the influence of fineness of the metal oxide powder and the effect of the method for producing the composite powder on switching performance. For this reason, the variants Ag/SnO2 88/12 NCF1, Ag/SnO2 88/12 NCF3 and Ag/SnO2 88/12 VS1011 were included in the experiments. 3.1 Switching parameters

Fig. 2

Ag/SnO2 88/12 NCF 3 (NCF with fine metal oxide particles)

Fig. 1 shows the structure of Ag/SnO2 88/12 NCF1. This structure is characteristic of all NCF materials in which extremely fine metal oxide powder is used. By using less fine powder as in Ag/SnO2 88/12 NCF3 (Fig. 2), the structure is somewhat coarser in comparison to NCF1. The mechanical characteristics of NCF3 are more suitable for use in forming rivets as noted in /1/. In regard to component distribution, the material Ag/SnO2 88/12 VS1011 (Fig. 3) produced by powder blending has weaknesses in the form of oxide agglomerates. They only slightly affect the contact and switching performance; however, they can be starting points for cracks when

Model switch, NO, contact force 150 cN a) H4-lamp loads 1.5 s ON/ 3.5 s OFF, 50 000 ops. 4 H4 lamps: Ipeak = 170 A, Id = 22 A, We = 100 mWs, Wa =38 mWs 6 H4 lamps: Ipeak = 210 A, Id = 33 A, We = 130 mWs, Wa = 85 mWs b) Ohmic inductive loads 1 s ON/ 5 s OFF, 50 000 ops. 40 A, 0.42 mH: We = 12 mWs, Wa = 550 mWs 50 A, 0.42 mH: We = 24 mWs, Wa = 800 mWS The indicated values for the energy converted during the switch-on peak and in the switch-off arc We and Wa are typical averages calculated according to /3/. The failure criterion in all experiments was the first instance of failing to open within 2 s.

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3.2 Welding behavior Fig. 4 shows the number of operations at which welding first occurred. The results reveal generally large spreads that are also typical for relays not modified as model switches (series relays).

The picture is not uniform under ohmic-inductive loads of 40 A and 50 A. However, we can generally conclude that the extremely finely dispersed NCF1 does not offer any advantages.

Fig. 6

Fig. 4

Welding resistance under a lamp load and ohmic inductive load

Under a lamp load of 4 H4, model switch 1 tends to weld somewhat earlier. In this case, Ag/SnO2 88/12 NCF3 has a longer life than the extremely finely dispersed NCF1. The less homogenous comparative material VS1011 does much more poorly than the other two. In model switch 2 where both NCF1 and NCF3 reached the conclusion of the experiment of 50,000 switches, VS1011 failed after 20,000 operations.

Fig. 5

Relative erosion under ohmic-inductive loads

3.3 Erosion and material migration Under a lamp load (Fig. 5), the erosion was very slight in each case. The material migration was largely over a broad area and was not the result of failure. There was no significant difference between the tested materials. At a 40 A ohmic-inductive load (Fig. 6), we again saw only small differences, with a slight tendency toward greater erosion in the somewhat larger variants NCF3 and VS1011. We saw a clearer picture after increasing the current to 50 A: The extremely finely dispersed NCF1 revealed much less erosion and material migration.

Relative erosion under H4 lamp loads

As was expected, welding occurred after fewer operations under a 6 H4 lamp load. However, no clear tendencies can be concluded regarding the behavior of the different variants.

Fig. 7

Contact resistance (99.5% values)

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3.4 Contact resistance The contact resistance was measured from the decrease in voltage shortly before the contacts opened every time switching occurs. Fig. 7 presents the 99.5% values. As already known from /4/, the contact resistance under lamp load is clearly lower than under an ohmic-inductive load, with the three variants showing the same behavior. The weak inductive load produces resistances that are twice as high. There was no significant difference between the variants at 40 A and 50 A loads.

4. General Purpose Relay Applications Fig. 9 At least three relays were tested, and the results are shown in the Weibull diagram. The details of the switching conditions are given in the legends of the figures. The failure probability was plotted in reference to the target life. 4.1 Influence of the metal oxide content on the welding tendency and erosion Only NCF1 variants were investigated. The metal oxide content was 8%, 10% and 12%. The failures arose from contact welding at an inrush peak of 51 A (Fig. 8). The overtravel was sufficient in each case.

Weibull distribution of the failure rate by erosion General purpose relay, NO, 230 V, 5 A ohmic, 1 s ON/ 1 s OFF

Under a 5 A ohmic load, the failures were generated by insufficient contact from insufficient overtravel due to erosion. The results portrayed in Fig. 9 reflect the erosion behavior of the material variants investigated. There was no clear difference between NCF1 with 8% and 10% metal oxide. All initial failures were observed at approximately 90% of the target life. At 12% metal oxide, the lives were largely stable or lie above the target values.

When the metal oxide content was 8%, the relays clearly tended to weld before reaching their target life. Increasing the metal oxide content to 10% yielded a substantial improvement. At a metal oxide content of 12%, the life was consistently above the required target.

Fig. 10

Fig. 8

Weibull distribution of the failure rate from welding General purpose relay, NO, 230 V, Ipeak= 51 A, Id= 3 A, 5 s ON/ 5 s OFF

Weibull distribution of the failure rate from welding General purpose relay, NO, 230 V, Ipeak= 51 A, Id= 3 A, 5 s ON/ 5 s OFF

4.2 Influence of the amount of In2O3 and particle size on the welding tendency and erosion The results in Fig. 10 and 11 were determined in the same test series at a high inrush current of 51 A and at 5 A

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ohmic load. The results from Ag/SnO2 88/12 NCF1 from the prior section were used as a benchmark. These are contrasted with the results from AgSnO2 88/12 NCF2 (=1/3 of the In2O3 content) and Ag/SnO2 88/12 NCF3 (= somewhat coarser).

The results which are partially based on a very narrow statistical foundation need to be interpreted in light of the authors’ experience which confirms the conclusions. In particular, the following relationships can be derived from the results: 5.1 Automotive relays Ag/SnO2 88/12 NCF1, NCF3 and VS1011 were compared.

Fig. 11

Weibull distribution of the failure rate from erosion General purpose relay, NO, 230 V, 5 A ohmic, 1 s ON/ 1 s OFF

Under the given conditions, both the reduction of the In2O3 content and use of a less-fine metal oxide substantially increased the tendency for failures with welding before the target life was reached (Fig. 10). Under a 5 A load (Fig.11) as well, the use of the variants AgSnO2 88/12 NCF2 and NCF3 markedly shortened the life (due to greater erosion) in comparison with AgSnO2 88/12 NCF1.

5. Discussion of the results In comparing the different Ag/SnO2 variants, numerous switching conditions were used: DC, AC, high make current, ohmic and ohmic-inductive loads within different current ranges. In summary, we can say that none of the variants characterized by the production method of the composite powder, degree of component dispersion, or composition is significantly superior in comparison to the other in terms of switching. This conclusion agrees with the experiences of the authors that are far more wideranging than the results described in this article. All of the investigated variants, whether produced by the powder blending technology or the NCF method, are based on cutting-edge technology and the results of many years of material development. The performance level of all of the utilized Ag/SnO2 variants is hence very high.

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The welding resistance of finely dispersed materials produced using the NCF method (NCF1, NCF3) tends to be somewhat better for automotive relays under low lamp loads (4 H4 / Fig. 4). However, this difference becomes less significant as the switch-on current increases (6 H4 / Fig. 4).

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Under an ohmic-inductive load, all the three variants investigated have the same level of welding resistance (Fig. 4).

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There are no substantial differences in regard to erosion behavior and material migration under a lamp load (Fig. 5).

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Under an ohmic-inductive load of 40 A (Fig. 6), NCF1 has advantages in regard to erosion. At a higher current of 50 A, this tendency reverses and is much more significant. The tendency toward material migration is also larger at higher loads for NCF1 than for NCF3 or VS 1011 (Fig. 6).

In summary and again with reference to the authors’ wider experience, we can conclude that extremely finely dispersed Ag/SnO2 materials only have advantages over coarsely dispersed materials at low currents. For automobile relays, the variant Ag/SnO2 88/12 NCF3 hence represents an optimum compromise in regard to switching performance and processability. 5.2. General purpose relays -

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The use of Ag/SnO2 with a greater amount of metal oxides in general purpose relays can alleviate problems associated with high switch-on peaks. The results shown in Fig. 8 confirm this known relationship. When general purpose relays fail under an ohmic load due to the erosion of the contact, Ag/SnO2 containing 12% metal oxide is also advantageous (Fig. 9).

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In general purpose relays, optimum welding resistance is achieved at high inrush peaks with extremely finely dispersed materials such as Ag/SnO2 88/12 NCF1. A slightly coarser metal oxide as in NCF3 can have a negative effect. The same holds true when the In2O3 content is reduced to 1/3 as in Ag/SnO2 88/12 NCF2 (Fig. 10).

However, the results do not mean that variants NCF3 and NCF2 are unsuitable for general purpose relays. Their advantages (easier and hence more economical processability of NCF3, and less of the expensive In2O3 in NCF2) must be weighed against the switching advantages of NCF1. This is particularly true when considered in light of the fact that the somewhat higher welding resistance of NCF1 is only manifested at the threshold of welding. The advantages of improved processability can be critical, especially when an effective processing method such as direct riveting is desired. With regard to erosion behavior, the relationship of the three materials investigated, Ag/SnO2 88/12 NCF1, NCF2 and NCF3 is similar to welding: The somewhat coarser structure and lower In2O3 content reduces the life by approximately 20% in comparison to the results with NCF1 (Fig. 11). In regard to the use of the materials, the above arguments also apply. References We again refer to the comprehensive compilation of literature on the “Use of Ag/SnO2 in Automobile Relays and Power Relays“ in /1/. /1/

/2/

/3/

/4/

Braumann, P., Koffler, A.: The Influence of Manufacturing Process, Metal Oxide Content, and Additives on the Switching Behaviour of Ag/SnO2 in Relays. 50th Holm Conference on Electrical Contacts and 22nd International Conference on Electrical Contacts, Seattle, USA, 2004, p. 90-97 Heringhaus, F. et al.: On the Improvement of Dispersion in Ag-SnO2-Based Contact Materials. Proc. 20th ICEC, Stockholm, 2002, pp. 199 – 204 Braumann, P., Koffler, A.,: The Importance of Characterizing the Make and Break Operations to Allow Effective Contact Material Development. 19th International Conf. on Electric Contact Phenomena, Nürnberg, Sep. 1998, p. 325 - 333 Braumann, P.: Prüfung der elektrischen Lebensdauer von Kfz-Relais [Testing the electrical Life of Automobile Relays]. VDE Technical Report 55, Berlin-Offenbach: VDE-Verlag, 1999, (15th

Conference, Albert Keil Contact Seminar, Karlsruhe 1999) , p. 49 – 59 Andreas Koffler received the Dipl.-Ing. degree in 1992 in electrical engineering from the University of Applied Science GiessenFriedberg, Germany. In the same year he joined Umicore AG & Co. KG, Business Unit Technical Materials, Hanau, Germany. He is responsible for sales, applied technology, and the contact testing lab.