The Influence Of Manufacturing Process, Metal Oxide Content, And

Transcript

The Influence of Manufacturing Process, Metal Oxide Content, and Additives on the Switching Behaviour of Ag/SnO2 in Relays Peter Braumann, Andreas Koffler Umicore AG & Co. KG, Technical Materials Division Hanau, Germany Abstract — As an introduction a brief historical summary of the use of Ag/SnO2 in general-purpose and automotive relays is given. The process for manufacturing contact materials by using wet-chemical precipitation technology (NCF technology) is explained and the potential of this technology for the production and the application of contact materials is demonstrated. The influence of the additives WO3, Bi2O3 and In2O3 is investigated with materials that are based on the powder blending technology and the NCF technology. I. INTRODUCTION A. Ag/SnO2 in general purpose relays In the area of general-purpose relays in regard to the use of Ag/CdO many alternative solutions have been described /1 - 3/, in which distinct advantages could be demonstrated for individual load types. However, to date no material has been found that is equal or superior to Ag/CdO on a broad scale. Accordingly, Ag/CdO has only been used in certain areas of general-purpose relays. It was only changes in legislation with the corresponding restrictions on the use of Ag/CdO that forced Ag/CdO to be replaced on a broader scale. /4/ Besides Ag/SnO2, another replacement for Ag/CdO in network relays is Ag/ZnO as a basic material /5/. AgNi could also be regarded as a possible alternative, although the limited welding resistance of this material group does set narrow limits. Both material groups should be viewed very critically, as in general-purpose relays there is a general trend towards higher start-up voltages with the corresponding demands on the materials for welding resistance. B. Ag/SnO2 in automotive relays In automobile relays Ag/SnO2 showed its great superiority compared to other contact materials from a very early stage. In /6, p. 684 et sqq./ the advantages of Ag/SnO2 compared to other materials such as AgNi or fine grain silver (AgNi 0.15) are explained; these are especially obvious in case of high inrush currents. The material Ag/CdO, which would otherwise certainly be rated as excellent in regard to its broad area of application, exhibited a much more distinct tendency to material migration compared to Ag/SnO2 in the automobile relay application /7/, /8, p. 87/ and has not been used in automobile relays for some time, regardless of the discussions about environmental impact and health. As the welding behaviour of contact materials under the conditions that prevail in the 12V automobile network are, to a great extent, determined by the anode, it is also quite possible to use asymmetrical material pairs /8/, in which case the anode must contain Ag/SnO2 and must offer fine grain silver for the cathode. While a solution such as this is very economical, it is not compatible with automobile manufacturers’ requirements for pole reversal. One statement in regard to the use of Ag/SnO2, which is relatively independent of the manufacturing process that is used and the individual composition of an Ag/SnO2 material, relates to the metallic oxide concentration. In addition to the better welding resistance, Ag/SnO2 materials tend to migrate less as the metallic oxide concentration is increased /9/. Because of this, a concentration of 12% is most common, although higher concentrations are also often seen in specific applications. In particular, when PdCu15 is used in the flasher relay, materials with a higher metallic oxide concentration have proved to be superior /10/. During the last few years the problem of switching in the planned 42 V automotive power system has often been discussed /11-15/, although in the meantime this discussion has become much less intensive. The subject will not be dealt with in detail in this study, but the following should be mentioned: extinguishing the arc within a sensible arc burning duration of around 5 ms in a 42 V automotive power system must be ensured through constructive measures on the relay, as is well known for other switch applications for higher voltages. Examples of this are the double break and the use of magnetic blow-out fields. This is also clearly pointed out in /16/. Because of the many positive experiences with other switch constructions it can be assumed that Ag/SnO2 will play an important role as a contact material within the scope of the 42V automotive power system (which is not foreseeable at present). II. CONTACT MATERIALS The contact materials used within the scope of these investigations were all based on powder technology. To manufacture the composite powder, as well as the traditional powder blending technology, dual-jet wet-chemical precipitation was also used. The technology of dual-jet wetchemical precipitation was presented in /17-19/ and is patented under /20/. We have named the contact materials that are manufactured with the help of wet-chemical precipitaction technology NCF materials. Extensive positive experiences have shown that NCF materials will become more significant in the future. Therefore it is logical to dedicate more attention to the NCF technology and its potential. Ag/SnO2 88/12 PMT1 Elongation: 24% 2 Tensile strength: 245 N/mm - A. Dual-Jet Wet-Chemical Precipitation To manufacture contact materials that are based on the wet chemical precipitation process, the first process step, blending the powder, is replaced by NCF technology. In the following known steps, compressing – sintering – extrusion, only the process parameters have to be adapted accordingly. Precipitating agent Ag/SnO2 88/12 NCF1 Elongation: 24% 2 Tensile strength: 310 N/mm - Ag +-solution Suspension of oxides Figure 1. Dual-jet wet-chemical precipitation of composite powders Fig. 1 shows the basic principle of a system for dual-jet wetchemical precipitation of composite powders: a silver salt solution and a precipitating agent are added to a suspension of metallic oxides (e.g. SnO2, In2O3) simultaneously; the precipitating agent can be either alkaline or acidic. In a chemical reaction silver or silver oxide precipitate. The suspended metallic oxide particles act as crystal nucleuses. When the NCF process is accordingly controlled, the individual components are distributed very homogeneously - a basic condition for a high-quality composite powder for the manufacture of contact materials with constant mechanical properties and constant switching characteristics. In addition to the stability of the material properties, the NCF process for manufacturing Ag/SnO2 materials has the following advantages: High formability of the materials: With a similar particle size distribution, NCF materials have much superior forming properties than materials that are produced by powder blending due to the improved Ag/SnO2 88/12 NCF3 Elongation: 30% 2 Tensile strength: 270 N/mm - Figure. 2 Influence of microstructures and manufacturing process on typical mechanical characteristics distribution of the metallic oxides in the silver matrix. This is an important prerequisite for manufacturing crackfree rivets. - Flexibility in regard to the choice of the basic component SnO2: On the other hand, the basically more favourable forming properties allow the use of extremely fine SnO2 without any serious negative effects in regard to ductility. - Flexibility in regard to active components: The use of active components to optimise the switching properties such as Bi2O3, WO3 or In2O3 generally has major negative effects on the mechanical properties, The materials Ag/SnO2 SPW and SPW4, which were manufactured by the powder blending process, have WO3 as an active component; variants Ag/SnO2 PMT1 on the other hand, contain Bi2O3. The variants Ag/SnO2 NCF1, NCF2 and VC1003 are based on the NCF technology and exhibit an extremely finely dispersed distribution of the metallic oxides. As two parameters, i.e. manufacturing process and structure were changed at the same time for the NCF materials compared to the powder blending-based materials, it was not possible to observe the influence of the structure and Ag/SnO2 NCF2 III. CPI101 VC1003 NCF NCF NCF In2O3, low Bi2O3 , med. In2O3, high GENERAL-PURPOSE RELAY APPLICATIONS A. The Influence of the Metal Oxide Content on the Erosion Rate If the erosion behaviour at resistance load is considered as a criterion when choosing the content of metallic oxide, a concentration of 8% is normally recommended for generalpurpose relays /1/. This rule corresponds to the authors’ experience for many applications, but it is not correct for all types of relays. Fig. 3 shows under this aspect the erosion rate in µg/Ws measured with 230 V, 5 A, resistance load, movable 2.5 mm, fixed contacts 2.7 mm diameter. The erosion rate was determined by measuring the weight lost and the energy on break. 0.5 230 V, 5 A, resistance load 0.4 Ag/SnO2 NCF1 0.3 0.1 fixed 0.2 movable B. Contact Materials for Switching Tests The Ag/SnO2 materials listed in Table 1 were used for the switching experiments. In addition to today‘s product names, for better orientation the R&D names used in /18/ are also listed. The NCF material VC1003 has only been available as an R&D variant to date. The metallic oxide concentration is stated for the individual experiments, although in automobile relays only variants with a concentration of 12% were used. SBC91 CPI31 Ag/SnO2 NCF1 Powder blending Powder blending Powder blending Active additive WO3 , low WO3 , med. Bi2O3 , med. fixed Ag/SnO2 88/12 PMT1 exhibits the rather coarser structure and the mechanical characteristics – 24% elongation with a tensile strength of 245 N/mm2 - that are characteristic for the powder blending process. Ag/SnO2 88/12 NCF1 is a variant which is characterised by the extreme fineness of the oxide particles. The tensile strength at 310 N/mm2 is much higher but the same elongation values of 24% are achieved. The Ag/SnO2 88/12 NCF3 variant, which is also based on the precipitation technology, has exactly the same composition as NCF1; however, a slightly coarser metallic oxide was used. With a tensile strength of 270 N/mm2 at expansion values of 30%, the mechanical characteristics of the material can be rated as very good. Ag/SnO2 SPW Ag/SnO2 SPW4 Ag/SnO2 PMT1 Man. process movable For "Formability" and "Flexibility" in regard to the choice of the basic component SnO2, Fig. 2 shows a comparison of Ag/SnO2 88/12 PMT1 as an example of a material manufactured by powder blending and Ag/SnO2 88/12 NCF1 and Ag/SnO2 88/12 NCF3 as examples of the NCF technology. (The different active components of the materials, see below, are not important for this comparison.) R&D fixed High flexibility in the use of In2O3: In2O3 is a very expensive component. When manufacturing powder via the NCF process, the In2O3 content can be chosen freely and is not required for the actual manufacturing process as is the case with internal oxidation. In2O3 can thus be regarded as a pure active component. The quantity is chosen in relation to the switching requirements. Name movable - manufacturing process changes separately within the scope of the experiments. TABLE 1. erosion rate [µg/Ws] which in turn limits their areas of application. A material based on the NCF process reacts more favourably in this respect and therefore offers much more freedom in the optimisation of switching properties when active components are being selected. 0 92/8 Figure 3. 90/10 88/12 Erosion rate against metal oxide content with resistance load (NO contacts) The diagram shows that in individual cases increasing the metallic oxide concentration from 8% to 12% can also reduce erosion. The decrease with a higher metal oxide content was caused by the 40% lower erosion of the movable contacts. An explanation for the behaviour of this relay can be the overheating of the movable contact. This is to an extent overcome by the 12% material. 9 relays were tested with all loads. The experiments only concerned the NO contact. A failure was deemed to be the first switching failure, defined by "no contact" or "no contact separation". 97 94 Cumulative frequency (%) B. The Influence of In2O3 and Bi2O3 on the lifetime with different loads For the following switching tests the NCF materials Ag/SnO2 88/12 NCF2 and Ag/SnO2 88/12 VC1003 were used. The metallic oxide concentration was chosen on the basis of preliminary experiments in which 12% had proved to be advantageous. 80 70 60 50 40 30 Ag/SnO2 88/12 VC1003 20 10 7 5 Ag/SnO2 88/12 NCF2 3 500 Switching conditions: Contacts 2.8 mm diameter Lamp load: 230 V, 2 kW incandescent bulbs, 1 s on / 3 s off Resistance load: 230 V, 10 A, 1 s on / 4 s off 230 V, 16 A, 1 s on / 4 s off Figures 4, 5 and 6 show the results in Weibull diagrams. Cumulative frequency (%) 97 94 80 70 60 50 40 30 20 10 Ag/SnO2 88/12 VC1003 3 500 1000 2000 5000 10000 20000 50000 100000 Operations Figure 4. Cumulative frequency of the first switching failures at 2 kW lamp load (NO, incandescent bulbs, 230 V, 1 s on/3 s off, peak current 120 A) Cumulative frequency (%) 97 94 80 70 60 50 40 30 Ag/SnO2 88/12 VC1003 20 10 7 5 Ag/SnO2 88/12 NCF2 3 500 1000 2000 5000 2000 5000 10000 20000 50000 100000 Operations Figure 8. Cumulative frequency (according to Weibull) of the first switching failures at a 16 A resistance load (NO, 230 V, 1 s on/4 s off) failures over the number of operations was also lower. In all cases, the failures were caused by contact welding. Under a 10 A resistance load (Fig. 5) the relationships reversed and Ag/SnO2 88/12 NCF2 showed the better results, characterised by a longer life with much less dispersion of the individual values. While the first failures with VC1003 occurred at 6,000 operations, the first failures with NCF2 did not occur until more than 20,000 operations. Again in this test series the failures were caused exclusively by welding of the contacts. Ag/SnO2 88/12 NCF2 7 5 1000 10000 20000 50000 100000 Operations Figure 5. Cumulative frequency (according to Weibull) of the first switching failures at 10 A resistance load (NO, 230 V, 1 s on/4 s off) In the case of lamp load (Fig. 4) Ag/SnO2 88/12 VC1003 led to longer lives than Ag/SnO2 88/12 NCF2. Dispersion of Under a 16 A resistance load (Fig. 6) there was a similar relationship between the two materials as with the 10 A load with advantages for material NCF2. However, it was noticeable that in spite of the higher load, higher numbers of operations were achieved with both materials This apparent contradiction can be explained by the different failure mechanisms of the two loads. While at 10 A contact welding determined the life, at 16 A it was erosion. Figures 9 and 10 show corresponding metallographic sections of Ag/SnO2 88/12 NCF2 as an example of the very similar behaviour of both materials in this respect. Under a load of 10 A (Fig. 7) there was a separation of silver and metallic oxides at the surface, in combination with the formation of silver-rich islands. This behaviour was illustrated in detail in /14/ and described as the (silver) bead effect. The occurrence of such Ag beads on the surface under a 10 A load can be regarded as the trigger for contact welding, which determines the end of life under a 10 A load. At a 16 A load (Fig. 8) a completely different structure formed as a result of the more intensive arcing effect on the contact surfaces: the melting zone was much thicker and, as can be seen by the dark grey areas, it contains considerable amounts of metallic oxide. Silver beads that could trigger welding were not observed. Ag/SnO 2 88/12 PMT1 Cumulative frequency 0.999 200 µm Figure 7. 0.99 0.90 Ag/SnO 2 88/12 SPW 0.50 0.25 0.10 0.01 Typical micro structure of Ag/SnO2 88/12 NCF2 after 10 A resistance load (see Fig. 5) 0 Figure 11. 10 20 30 40 50 Welding force /cN 60 70 Cumulative frequency of the welding forces (model switch, contacts 3 mm diameter/radius 5 mm on one side, 20 A, 6,600 operations, bounce time 1.5 ms, contact force 50 cN) The materials Ag/SnO2 88/12 SPW and Ag/SnO2 88/12 PMT1 were tested, which are based on the powder blending technology. That is, the comparison shows the direct influence of the additives WO3 and Bi2O3 on the welding forces. 200 µm Figure. 10 Typical micro structure of Ag/SnO2 88/12 NCF2 after 16 A resistance load (see Fig. 6) Under a 16 A load the end of life for the contacts is determined solely by the erosion resistance of the contact materials and does not occur prematurely through welding as with the 10 A load where there is still sufficient erosion reserve. Material Ag/SnO2 88/12 NCF2 seems to have the better properties compared to Ag/SnO2 88/12 VC1003, not just in regard to the beading effect but also in the area of erosion resistance. IV. AUTOMOTIVE RELAY APPLICATIONS A. Influence of WO3 and Bi2O3 Additives on Welding Forces The tendency of welding on make can best be presented from a quantitative aspect with the help of model switches. The results shown in Fig. 11 were determined by a switch that was developed within the scope of the "contact kinetics" project at Vienna Technical University /21/. The switch was adjusted in such a way that the welds were triggered solely by bouncing over 1.5 ms when the test current of 20 A was reached. Each of the two test series covered 6,600 individual measurements. The cumulative frequency of the forces that are required to open the contacts is shown. The results of the two test series for each material only show minor dispersion. If we take the 99% value of the forces as a comparison, we get values of 15 cN for Ag/SnO2 88/12 PMT1 and 33 cN for Ag/SnO2 88/12 SPW. In other words, the Bi2O3 additives more than halved the opening forces under the conditions that prevailed here. However, (unfortunately) this very positive result with the model switch can most certainly not be interpreted as saying that the opening force of relays can simply be halved by using Ag/SnO2 88/12 PMT1: if, for example, a relay tends towards bounces combined with strong surface melting, this could trigger welds, which cannot be overcome with any material. This is especially true – as our own investigations have shown – when the late bounces are short and the contacts re-close on a high current. /22/ Therefore targeted experiments within the scope of relay development in which the opening forces are varied and the bounce processes that occur are analysed are required in order to see which potentials are possible to reduce the opening forces with Ag/SnO2 88/12 PMT1 in practice. B. Relay Tests to Investigate the Influence of the In2O3 Content on Welding Resistance The NCF materials Ag/SnO2 88/12 NCF1 und NCF2 were investigated. Conditions: 300,000 operations, NC, contacts 3 mm diameter, 10 relays/material, lamp load (Ion = 100 A / Ioff = 20 A). The number of switching failures before the 300,000 switches were reached was defined as an assessment criterion. The switching failures that were observed were all due to slight welding of the contacts which opened again of their own accord. 10 10 9 Figure. 12 7 6 4 2 0.40 a 0 When the materials that were manufactured by powder blending are compared, it becomes clear that using Bi2O3 in PMT1 considerably reduces the rate of erosion. The use of NCF1, which was manufactured according to the NCF process, with a high In2O3 content produces a further considerable reduction in specific erosion. Lowering the In2O3 content to 1/3 in NCF2 increases the erosion rate to above the level of PMT1. b NCF1 c a b c NCF2 Proportion of relays that fulfilled the assessment criteria: a= none, b= 1, c< 10 switching failures (lamp load (Ion = 100 A / Ioff = 20 A), 300,000 ops., NO, 10 relays/material) Fig. 12 shows how many of the 10 switched relays reached the experiment duration of 300,000 operations without a switching failure (Column a), with maximum 1 switching failure (Column b) and with maximum 10 switching failures (Column c). In the case of Ag/SnO2 88/12 NCF1 only 2 of the 10 relays that were tested fulfilled criterion a. However, as the welds only occurred very sporadically, criterion b (maximum 1 switching failure) was fulfilled by 7 relays. None of the relays with this material exhibited more than 10 switching failures, which means that criterion c was fulfilled by all 10 relays. In the case of Ag/SnO2 88/12 NCF2 there were switching failures (= weldings) in only one of the 10 relays. However, with 13 switching failures this one relay did not fulfill either criterion b or c. Reducing the In2O3 content in Ag/SnO2 88/12 NCF2 compared to NCF1 produced an improvement in welding resistance under these conditions. C. Influence of Additives on Specific Erosion under Motor Load The materials Ag/SnO2 88/12 SPW4, Ag/SnO2 88/12 PMT1, Ag/SnO2 88/12 NCF1 and Ag/SnO2 88/12 NCF2 were investigated in the model switch. Conditions: 50,000 operations, NO, contacts 2.5 mm diameter, simulated motor load (Ion = 80 A / Ioff = 33 A). (for other test parameters see /19/) Wet-chemical precipitation Powder blending 0.35 Spec. erosion µg/Ws Fault criterion fulfilled 8 2 As the erosion of the contact pieces at this load is mainly caused by the break operation, the specific erosion was determined according to /23/ as mass loss in relation to the converted break energy. Fig. 13 shows a comparison of the specific erosion of the switched materials, each determined from two test series. 0.30 0.25 A 0.20 A 0.15 A A 0.10 0.05 0 Figure. 13 C C C C SPW4 PMT1 NCF1 NCF2 Specific erosion (A = anode, C = cathode, model switch, NO, 50,000 ops., simulated motor load (Ion= 80 A / Ioff= 33 A) This result shows that In2O3 has a direct influence on the switching characteristics and can thus be described as an active component. The quite considerable increase in the rate of erosion when the In2O3 content is reduced also suggests that the finely dispersed distribution of the metallic oxide components alone does not have any major positive effect on erosion under the conditions that prevailed here. D. Influence of Additives on the Life of Relays under Motor Load Ag/SnO2 88/12 SPW, Ag/SnO2 88/12 PMT1 were used in type A relays, while Ag/SnO2 88/12 NCF1 and Ag/SnO2 88/12 NCF2 were used in type B relays. Although relay types A and B were intended for the same areas of application, due to the different construction features the results of type A (materials manufactured by powder blending) are not comparable with those of type B (NCF materials). Conditions: 300,000 switches, break contact, 10 relays/material, motor replacement load (Ion = 40 A / Ioff = 20 A) As with the relay experiments with lamp load, the number of switching failures before the 300,000 switches were achieved was chosen as an assessment criterion. In this case, switching failures occurred in the form of interlocking and welding, caused by the surface formation resulting from strong erosion of the contacts. Fig. 14 shows how many of the 10 relays with the individual materials reached the test duration of 300,000 operations with no switching failures (column a), with maximum 1 switching failure (column b) and maximum 10 switching failures (column c). Relay type A Relay type B 10 10 9 8 8 Fault criterion fulfilled 7 6 4 7 6 4 2 0 Figure 14. a b c a b c a b c a b c SPW 4 PMT 1 NCF 1 NCF 2 Proportion of relays of types A and B which fulfilled the assessment criteria: a=none, b=1, c<10 switching failures (simulated motor load, Ion=40 A / Ioff=20 A, 300,000 switches, break contact, 10 relays/material) In the case of Ag/SnO2 88/12 SPW, only 4 of the 10 relays that were tested (Type A!!) fulfilled “criterion a” with 300,000 failure-free switches. However, 6 relays fulfilled “criterion c”. Under these extreme conditions Ag/SnO2 88/12 PMT 1 exhibited a much more stable behaviour with 7 from 10 completely failure-free switching relays. While in the case of Ag/SnO2 88/12 SPW the individual relays tended to show errors with an even frequency from 130,000 operations, with Ag/SnO2 88/12 PMT 1 the first switching failure occurred after 260,000 operations. Hence, PMT 1 operated completely without errors for twice the number of switches as SPW. The comparison shows that the additive Bi2O3 in Ag/SnO2 88/12 PMT 1 leads to an improvement of switching characteristics under motor load. In the case of Ag/SnO2 88/12 NCF1 only one single switching failure was registered for all 10 relays that were tested (type B!!), in other words, more than 3 million operations with no failure. Accordingly, 9 relays fulfilled “criterion a” and 10 relays “criteria b/c”. With Ag/SnO2 88/12 NCF2, 7 out of 10 relays worked completely failure free. One other relay only exhibited a few failures, and thus 8 relays fulfilled criteria b/c. Reducing the In2O3 content obviously has negative effects for the switching characteristics with motor load. The results achieved in model switches and relays concerning the influence of additives on the switching characteristic under engine loads correlate with each other to a great extent and can be summarised as follows: The use of Bi2O3 as an additive in place of WO3 considerably improves the switching characteristics under engine load. Even higher demands could be fulfilled with the NCF materials which have a high In2O3 content. Here we can also see the advantages of the NCF process in regard to flexibility in the use of additives, which were listed at the beginning: a material with a composition of Ag/SnO2 88/12 NCF1 is very difficult to manufacture by the powder blending process, even if powders with the coarser particle size are used. V. SUMMARY The potential of using the wet chemical precipitation technology (NCF technology) for manufacturing Ag/SnO2 was described: - High formability of the contact materials as an important prerequisite for the manufacture of crack-free rivets, - Flexibility in the choice of particle size for the silver metallic oxide that is used, - Flexibility in the choice of active components, - High degree of flexibility in the use of In2O3, the quantity can be freely chosen in regard to the switching-technical requirements. The further improved ductility of NCF materials was demonstrated using the example of the contact material Ag/SnO2 88/12 NCF3. In the general-purpose relay applications it was possible to demonstrate the positive effects of Bi2O3 on welding resistance under lamp load (Material: Ag/SnO2 88/12 VC1003). However, under a resistance load a material with a low In2O3 content (Material: Ag/SnO2 88/12 NCF2) achieved the better results. While at 10 A the life of the material was determined by welding caused by the so-called "beading effect", at 16 A arc erosion of the contacts was decisive for its life. Failures occurred earlier at a 10 A load. On the basis of switching experiments in the area of automobile relays the following correlations were derived: - - - The use of Bi2O3 (Material: Ag/SnO2 88/12 PMT1) in place of WO3 (Material: Ag/SnO2 88/12 SPW) considerably reduces the welding forces. Under lamp load the reduction of In2O3 content (Material: Ag/SnO2 88/12 NCF1) to 1/3 (in material: Ag/SnO2 88/12 NCF2) improved welding resistance. The additive Bi2O3 (Material: Ag/SnO2 88/12 PMT1) also reduced the specific erosion under motor load. A further optimisation of erosion behaviour under motor load can be achieved by using high concentrations of the additive In2O3 (Material: Ag/SnO2 88/12 NCF1). REFERENCES /1/ /2/ /3/ /4/ /5/ /6/ /7/ /8/ /9/ M. Huck, et al.: “Guidelines for the use of Ag/SnO2 contact materials in switching devices for low-power engineering”, Proc. 15th ICEC., Montreal, 1990, pp. 133 – 138. C. Veit: “Stand und Trends von Kontaktwerkstoffen in Relais”, VDE-Fachbericht 47, Berlin-Offenbach: VDEVerlag, 1995, (13. Kontaktseminar, Karlsruhe 1995), pp. 61 – 66. J. T. Schoepf: “Alternatives to silver cadmium oxide contact materials in relays”, Proc. 46th Relay Conf., Oak Brook Hills, Illinois, USA, 1998, pp. 7-1 – 7-7. V. Behrens, et al.: “Alternative contact materials for AC and DC power relays on the background of upcoming legal restrictions in regard to harmful substances”, Proc. 50th Relay Conf., Newport Beach, California, USA, 2002, pp. 11-1 – 11-6. T. J. Schoepf, et al.: “Development of silver zinc oxide for general-purpose relays”, IEEE Transactions on Components and Packaging Technologies, Vol. 25, No. 4, December 2002, pp. 656 – 662. Paul G. Slade: Electrical contacts: principles and applications, Marcel Decker Inc., New York - Basel, 1999. W. Balme, P. Braumann: “Welding tendency, contact resistance and erosion of contact materials in motor vehicle relays”, Proc. 15th ICEC, Montreal 1990, pp. 79– 84. C. Leung, A. Lee: “Silver tin oxide contact erosion in automotive relays”, Proc. 39th IEEE Holm Conf., Pittsburgh, 1993, pp. 61–67. L. Morin, N. Ben Jemaa, “Material transfer and welding in automotive power relays”, Proc. 48th Relay Conf., Lake Buena Vista, Florida, USA, 2000, pp. 2-1 – 2-7. /10/ T. J. Schoepf: “Substitute for palladium copper in DC flasher relays”, Proc. 48th Relay Conf., Lake Buena Vista, Florida, USA, 2000, pp. 5-1 – 5-6. /11/ T. J. Schoepf: “Auswirkungen von Bordnetzspannungen größer 12 V dc auf heutige Automobilrelais”, VDEFachbericht 55, Berlin-Offenbach: VDE-Verlag, 1999, (15. Fachtagung Albert-Keil-Kontaktseminar, Karlsruhe 1999), pp. 29 – 36. /12/ T. J. Schoepf: “Electrical contacts in the automotive 42 V DC power net”, Proc. 21st ICEC, Zuerich, 2002, pp. 43 – 55. /13/ O. Sakaguchi, et al.: “Performance investigation of contact materials in 42 V DC automotive relay”, Proc. 21st ICEC, Zuerich, 2002, pp. 56 – 61. /14/ L. Doublet, et al.: “Erosion and material transfer under 42 V DC”, Proc. 21st ICEC, Zuerich, 2002, pp. 62 – 68. /15/ V. Behrens, et al.: “Switching behaviour of silver based contact materials in 42 V DC applications”, Proc. 21st ICEC, Zürich, 2002, pp. 69 – 74. /16/ N. Ben Jemaa, et al.: “Arc duration and contact erosion in an automotive 42V DC network”, Proc. 50th Relay Conf., Newport Beach, California, USA, 2002, pp. 5-1 – 5-7. /17/ F. Heringhaus, et al.: “On the improvement of dispersion in Ag-SnO2-Based Contact Materials”, Proc. 20st ICEC, Stockholm, 2002, pp. 199 – 204. /18/ P. Braumann, et al.: “Neue Kontaktwerkstoffe auf der Basis von Ag/SnO2 für Kfz-Relais und Motorschalter”, VDE-Fachbericht 57, Berlin-Offenbach: VDE-Verlag, 2001, (16. Fachtagung Albert-Keil-Kontaktseminar, Karlsruhe 2001), pp. 157 – 162. /19/ F. Heringhaus, P. Braumann: “Anwendung der nasschemischen Fälltechnik zur Herstellung neuartiger pulvermetallurgischer Kontakt-werkstoffe”, Pulvermetallurgie-Fortschritte in Prozessen und Funktionalität, ISL Verlag, Hagen, published by Gemeinschaftsausschuß Pulvermetallurgie, Volume 18, Hagen 2002, pp. 31 – 48. /20/ DE 10017282 /21/ M. Hammerschmidt: Diagnostics of switching failure mechanisms in relays”, Proc. 21st ICEC, Zürich, 2002, pp. 101 – 104. /22/ P. Braumann: “Prüfung der elektrischen Lebensdauer von Kfz-Relais”, VDE-Fachbericht 55, Berlin-Offenbach: VDE-Verlag, 1999, (15. Fachtagung Albert-KeilKontaktseminar, Karlsruhe 1999), pp. 49 – 59. /23/ P. Braumann, A. Koffler: “The importance of characterizing the make and break operations to allow effective contact material development”, Proc. 19th ICEC, Nuremberg, Germany, 1998, pp. 325 – 333.