lowering the silver content in automotive conductive … · lowering the silver content in...

www.technology.matthey.com JOHNSON MATTHEY TECHNOLOGY REVIEW

http://dx.doi.org/10.1595/205651317X695271 Johnson Matthey Technol. Rev., 2017, 61, (2), 156–164

Lowering the Silver Content in Automotive Conductive Pastes Experiments in thrifting with selected base metals, base metal alloys and silver-coated base metals

By Edwin S. Raj* and Jonathan Booth Johnson Matthey Technology Centre, Blount’s Court, Sonning Common, Reading, RG4 9NH, UK

*Email: [email protected]

The high conductivity of silver and its oxidation resistance make it the metal of choice for automotive applications in defogging and other areas. There is scope to reduce the cost by reducing the content of silver, a costly metal. This article reports the results from testing formulations with reduced silver content. A range of silver-coated and base metal fillers were tested however none of these resulted in performance to match the commercially available silver automotive pastes.

Introduction

Automotive conductive pastes are formulations consisting of an active metal component, a glass frit and an organic medium; the latter two facilitating the adhesion and application on to the target substrate, which is usually glass. Applications for these pastes include print-based in-car defogging systems, antennae and alarm circuits. The term ‘frit’ refers to ceramic mixtures that have been melted to form a glass and then crushed into a fine powder. Modern glass frits used in the silver pastes are lead-free to meet environmental regulations and are often predominantly bismuth-rich

to effect good flow properties (1). Screen printing is the preferred mode of application for automotive pastes being cost-effective and scalable for industrial production.

Silver is the material of choice for automotive conductive paste due to its high electrical conductivity and oxidation resistance. In order to meet the industry defogger standards, the conductive tracks must operate with a 12 V line and the tracks need to have a resistivity of 6 mW sq–1 at a thickness of 10 μm after being fired (2), which can only be met by using a noble metal such as silver. Typically, the silver loading in the paste is in the range of 60–88 wt% depending on the intended function. Table I summarises the automotive silver pastes produced by Johnson Matthey (3).

Although silver has excellent conductivity and stability, it is expensive. It is estimated that as much as 40% of the cost of the automotive silver paste is that of the silver content and between 2–3.4 g of silver is used per car for defogging purposes (2).

Any potential replacement for silver in automotive pastes should meet requirements of conductivity, solderability, adhesion, chemical resistance, optimal stability of resistivity over temperature and overall compatibility with automotive glass technology. Thrifting of silver with fillers is not uncommon in silver pastes in applications such as photovoltaic current collectors where fillers such as metals, silver coated metals and alloys have been attempted (4–6). However, stringent requirements for automotive silver pastes such as needing to pass environmental tests makes the choice

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http://dx.doi.org/10.1595/205651317X695271 Johnson Matthey Technol. Rev., 2017, 61, (2)

Table I Silver Pastes for Automotive Glass Applications Available from Johnson Matthey

Product Ag content, % Resistance, mΩ sq–1 Firing range, ºC

AG7500-60 60 ±8.1 600–720

AG7500-80 80 ±2.8 600–720

AG7500-85 85 ±2.1 600–720

AG7500-88 88 ±1.8 600–720

of fillers rather limited. More than a decade ago, when the automotive industry considered the option of increasing the voltage in vehicles from 12 V to 24–42 V, some work was carried out to develop resistive silver pastes (to maintain the same power across the heating window) which involved thrifting silver pastes with base metals and their alloys (7). However, the industry abandoned the switch to high-voltage sources and no progress has been made in the area of thrifted automotive silver pastes since. Furthermore, in the past 10 years, the price of silver has decreased from its peak price of US$48.5 per troy ounce in April 2011 to US$17.9 per troy ounce in February 2017 (Figure 1) which has been a disincentive to the thrifting approach. However, there is still scope to further decrease the cost per kilogram of paste if a suitable replacement for some or all of the silver in automotive paste is found. This article provides an overview of the effects of reducing the silver content in 80% automotive silver

paste using fillers which includes selected base metals, silver-coated base metals and base metal alloys.

Experimental

Silver pastes and thrifted silver pastes were fabricated with 80% metal loading by triple roll milling appropriate quantities of metal component, glass frit, additives and organic medium to achieve a viscosity suitable for screen printing. For the sake of comparison, the same bismuth(III) oxide (Bi2O3)-based glass frit was used in the present study the loading of which was typically 2–5%. Printed glass tiles were fired in a pre-heated fast-fire roller kiln, set to the target temperature, in air. Electrical resistivity of the tracks was measured by the Van der Pauw four-point probe method (8) on meander circuits. Colour measurements were carried out in a Datacolor spectrophotometer after calibrating using standard black trap and white tile. Temperature-

US

Dol

lars

, $

45

40

35

30

25

20

15

10

5

0 2005 2010 2015

2005 2010 20152000

Fig. 1. Variation of silver price (US$ per troy ounce) between years 2000–2017 (© Kitco Metals Inc)

© 2017 Johnson Matthey

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158 © 2017 Johnson Matthey


programmed oxidation (TPO) studies were carried out in an Altamira AMI-200 instrument by heating ~200 mg of the powder to 600ºC in 10% O2/He gas.

Thrifting with Base Metals

A simple approach to thrifting silver is to replace with cheaper base metals. The closest conductivity match for silver (6.30 × 107 S m–1 at 20ºC) is copper (5.96 × 107 S m–1 at 20ºC) (9). However, copper suffers from poor oxidation resistance, with onset of oxidation around 140ºC, making it an unlikely candidate for firing applications which involve temperatures in excess of 600ºC. Other metals with a conductivity match close to silver are aluminium (3.50 × 107 S m–1 at 20ºC) and zinc (1.69 × 107 S m–1 at 20ºC). Zinc oxidises on firing to form zinc oxide (ZnO) whereas aluminium tends to either oxidise or alloy with silver (10), depending on the loading; all of which significantly increases the resistivity of the silver track. Although nickel is one of many carcinogenic metals known to be an environmental and occupational pollutant which requires extreme care during handling (11), it is a promising metal filler with conductivity of 1.43 × 107 S m–1 at 20ºC and resists oxidation on firing. In order to test the effect of adding nickel filler to the silver paste, screen printable pastes were prepared by triple-roll mixing a mixture of appropriate quantities of silver, nickel (average particle size 10 µm, 8.4–32.4 wt% loading), glass frit, additives and organic medium. The paste was screen printed

onto float glass substrates, dried around 100ºC and subsequently fired in a roller kiln.

Figure 2 shows the variation of resistivity of 80% nickel-thrifted silver tracks fired at temperatures between 620–680ºC in a fast-fire roller kiln. For comparison, resistivities of 80% Ag tracks fired at the same temperature are also plotted alongside. The resistivity of a pure silver track decreases with increasing firing temperature. For example, the resistivity of a silver track fired at 620ºC was 2.6 µΩ cm which dropped to 1.9 µΩ cm after firing at 680ºC, which is attributed to sintering of silver particles. As the nickel concentration in a silver track is increased, the resistivity of the track increases. The variation in resistivity (ΔR) with firing temperature between 620–680ºC is between 3.8–13.7%, in the studied composition range. ΔR increases initially with increasing nickel concentration and then drops to lower values. Apart from conductivity, addition of fillers will also

have an impact on properties such as acid resistance and reverse colour. The acid resistance test involves evaluating the chemical resistance of silver tracks to withstand acid attack. This was carried out in 0.1 N sulfuric acid at 80ºC followed by a tape test; the latter involving testing the adhesion of the tracks after the acid test using sticky tape. Acid resistances of silver pastes supplied by paste manufacturers vary significantly but most pastes pass a 2–4 h acid test. Acid resistance tests were carried out on silver tracks thrifted with 8 wt% nickel filler fired at 660ºC. The resistances of

1.2 × 10–5 8.4 wt% Ni 16.3 wt% Ni 24.2 wt% Ni 32.4 wt% Ni Pure Ag

Res

istiv

ity, o

hm c

m

8.0 × 10–6

4.0 × 10–6

0.0 620 640 660 680

Firing temperature, ºC

Fig. 2. Variation of resistivity of 80% silver paste and silver paste thrifted with nickel filler (various loadings) with firing temperature

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1

2

4


the thrifted silver tracks were measured before and after the acid test as well as after tape tests and are summarised in Table II.

Acid resistance of nickel-thrifted silver tracks was poor with all tracks falling apart after the tape test. This is expected in the light of the fact that base metals vigorously react with acid which opens up channels for the acid to further penetrate into the track and attack the glass frit which binds the metal particles to the glass substrate, ultimately leading to delamination of the track during the tape test. Due to poor acid resistance, base metals are unlikely candidates for fillers in automotive silver pastes.

Thrifting with Silver Coated Fillers

A straightforward strategy to reduce the silver loading in silver paste is to use silver coated fillers such as silver coated copper and silver coated nickel, which in principle should meet the fine balance between

conductivity and metal loading and may also offer better acid resistance due to the silver shell protecting the base metal core.

Investigations on the use of silver-coated copper powders as fillers for automotive silver pastes suggest that these Ag@Cu powders suffer the same fate as their pure copper counterparts (12). TPO studies carried out on two silver-coated copper powders with different silver loadings (10 wt% and 28 wt% silver) in 10% O2/He ambient pointed that both samples oxidise on heating, irrespective of the silver loading, with two distinct oxidation steps involved (Figure 3). Increased silver loading on copper just increases the oxidation onset temperature. The onset of oxidation occurs between 200–300ºC, which can be assigned to the formation of copper(I) oxide (Cu2O) which on further heating oxidises to copper(II) oxide (CuO) between 375–425ºC. The silver coating on copper delays the oxidation on heating by about 100ºC however does not prevent it. This can be explained on the basis of

Table II Resistance of Silver Tracks Containing 8 wt% Ni Filler Before and After Acid Test and also After Tape Test

Acid test duration, hour Resistance, ohm

Before acid test After acid test After tape test

0 6.8 × 10–2 – 6.8 × 10–2

6.3 × 10–2 6.2 × 10–2 Damaged

6.8 × 10–2 6.8 × 10–2 Damaged

6.3 × 10–2 6.1 × 10–2 Damaged

5

Ther

mal

con

duct

ivity

det

ecto

r sig

nal

0

–5

–10 Ag@Cu (10% Ag)

–15 Ag@Cu (28% Ag)

–20

–25 0 100 200 300 400 500 600 700

Temperature, ºC

Fig. 3. TPO profiles of 10Ag@Cu and 28Ag@Cu powders in 10% O2/He


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non-uniform coverage of silver on copper and also due to the presence of pin holes in the silver shell which allows the penetration of oxygen, eventually resulting in the oxidation of copper (13). Oxidation of silver-coated copper powders at temperatures above 200ºC makes them unsuitable for firing applications such as automotive pastes.

Silver-coated nickel (Ag@Ni) powders are also used extensively as filler material in inks and are claimed to have conductivity and chemical stability approaching pure silver (14). Hence, silver-coated nickel powder (40% Ag by weight) was tested as a filler for 80% silver paste. X-Ray diffraction (XRD) of 680ºC fired silver prints containing Ag@Ni filler showed no evidence of nickel oxidation. Figure 4 shows the variation of resistivity as a function of firing temperature and

Figure 5 shows the variation of resistivity as function of Ag@Ni loading in the silver paste. For comparison, the resistivity of 80% silver paste is also plotted alongside.

Resistivity of pure silver paste decreases with increasing firing temperature. Resistivity of silver tracks containing Ag@Ni also show similar behaviour, albeit linked to Ag@Ni loading (Figure 4). For example, at 8% Ag@Ni loading, the resistivity of a track fired at 620ºC was 2.9 µΩ cm which dropped to 2.2 µΩ cm after firing at 680ºC. However, at 32% Ag@Ni loading, the change of resistivity with increased firing temperature was almost negligible, remaining around 5 µΩ cm between 620 – 680ºC. The resistivity of the Ag@Ni containing silver paste increases with increasing Ag@Ni loading, which is more pronounced at higher firing temperatures. Although the resistivity

8 × 10–6 8 wt% Ag@Ni 16 wt% Ag@Ni 24 wt% Ag@Ni

32 wt% Ag@Ni Pure Ag

Res

istiv

ity, o

hm c

m

4 × 10–6

0 620 640 660 680

Firing temperature, ºC

Fig. 4. Variation of resistivity of 80% silver paste and silver paste thrifted with silver coated nickel, Ag@Ni, filler (various loadings) with firing temperature

8 × 10–6

620ºC 640ºC 660ºC 680ºC

Res

istiv

ity, o

hm c

m

4 × 10–6

0 8 wt% 16 wt% 24 wt% 32 wt% Pure Ag Ag@Ni Ag@Ni Ag@Ni Ag@Ni

Fig. 5. Variation of resistivity of thrifted 80% silver paste with Ag@Ni loading

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increases with increasing Ag@Ni loading, the latter promotes stable resistivity values over the typical firing range which is desired in automotive windscreen manufacturing.

Compatibility of thrifted silver pastes with automotive black enamel is critical as they are overprinted onto an enamel layer prior to firing and shaping of windscreen glass. Black enamels are made by pasting a mixture of black pigment and low melting glass (usually in the ratio of 1:4) and applied on to windscreens by either screen printing or inkjet printing techniques. Black pigments are usually derived from either copper chromite spinel, chrome iron nickel spinel or iron cobalt chromite spinel (15). The main purpose of the enamel layer is to protect the glue that holds the windscreen in place from degradation by ultraviolet (UV) light and also to hide the electrical connections. For enamel compatibility studies, thrifted silver pastes were printed onto the pre-printed and dried black enamel and fast-fired at 600ºC, 620ºC, 640ºC, 660ºC and 680ºC.

The difference between two colour samples is often expressed as ΔE, which is the difference between the L*, a* and b* values of the reference and sample. Therefore, ΔE displays the difference between two samples as a single value for both colour and lightness. A detailed explanation of coordinates from Commission Internationale de l’Eclairage (CIE) Lab Colour model L*, a* and b* are explained elsewhere (16). Reverse colour values L*, a* and b* were measured on black

enamel and black enamel under the silver print from which ΔE values were calculated using the formula (Equation (i)):

(L2 – L1)2 + (a2 – a1)2 + (b2 – b1)2 (i)ΔE = 3

Table III shows the results of compatibility testing of 8 wt% Ag@Ni-containing silver track on a hiding black enamel 1T3015 manufactured by Johnson Matthey. The reverse colour data suggest that the Ag@Ni thrifted silver paste is compatible with 1T3015 enamel. Both ΔL and ΔE values are lowest around 660ºC firing, which is similar to that observed for the pure silver paste. Although there is no general agreement on an ideal value; ΔE value of 2.3 has been proposed (17) as just noticeable difference (‘JND’), nevertheless, ΔE of 0.5 or less is preferred by windscreen manufacturers. Acid resistance tests were carried out on 660ºC fired,

8% Ag@Ni loaded silver tracks. Figure 6 shows the state of 8% Ag@Ni loaded silver tracks after 1 h, 2 h and 4 h acid test, respectively.

The resistance of the tracks was measured before and after acid tests as well as after tape tests (Table IV). Acid attack was evident in all tracks but 8 wt% Ag@Ni thrifted silver track passed the 1 h acid and tape test. This is an improvement compared to pure nickel fillers where delamination was observed in all samples exposed to acid.

Table III L*, a* and b* Coordinates of 8 wt% Ag@Ni Containing Silver Track Printed over 1T3015 Hiding Enamel

Firing temperature, ºC Sample L* a* b* ∆E

600 Black enamel 9.2 0.33 0.62 5.9

600 Black enamel under silver 19.4 0.09 0.12

620 Black enamel 6.4 0.49 0.58 5.0


640 Black enamel 5.4 0.61 0.7 1.8


660 Black enamel 5.5 0.63 0.59 0.2


680 Black enamel 6.3 0.58 0.29 0.5



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1

2

4


(a) (b) (c)

1 cm

Fig. 6. Silver tracks containing 8% Ag@Ni on glass after acid test: (a) 1 h; (b) 2 h and (c) 4 h

Thrifting with Base Metal Alloys

Suitable cost-effective base metal alloys can also be used as fillers for silver paste as long as they are stable at high temperatures and have conductivity in the target range. Alloy powders including brass and stainless steel were tested in silver pastes. The XRD pattern of silver track containing 32 wt% brass filler (70:30 Cu:Zn), fired at 680ºC is shown in Figure 7.

Major phases are silver and brass alloy along with minor amounts of CuO and ZnO, indicating that

a small amount of brass alloy had been oxidised during fast-firing at 680ºC. Typical resistivity of an 80% silver paste thrifted with 32 wt% brass filler and fired at 680ºC was 12 µW cm. However, the variation in resistivity ΔR with increasing firing temperature (620–680ºC) and increasing filler loading was >15% which could be related to the oxidation of brass filler, as evidenced from XRD. Base metal alloys that suffer from oxidation during firing will negatively impact the stability and compatibility of the silver paste with the other automotive glass components.

Even alloys that do not exhibit oxidation behaviour may still interact with either silver or glass components of the paste during firing, leading to undesired properties. XRD of silver tracks containing stainless steel (type 430-L) filler did not show evidence of oxidation on firing. Resistivity of 80% silver paste thrifted with 8–32 wt% stainless steel filler and fired at 680ºC varied between 4–16 µW cm. ΔR with increasing firing temperature (620–680ºC) was >15%. However, migration of silver into glass, commonly known as silver bleeding, was observed in fired silver tracks containing 430-L filler. Figure 8 shows the cross-sectional electron probe

Table IV Resistance of Silver Tracks Containing 8 wt% Ag@Ni Filler Before and After Acid Test and also After Tape Test

Resistance, ohm

Before acid test After acid test After tape testAcid test duration, hour

0 4.63 × 10–2 – 4.63 × 10–2

4.89 × 10–2 4.74 × 10–2 4.75 × 10–2

5.69 × 10–2 5.53 × 10–2 Damaged

4.51 × 10–2 4.56 × 10–2 Damaged

40 50 60 70 80 90 100 110 120 2θ, degree

25000

20000

15000

10000

5000

0

Cou

nts

PDF 04-001-2617 Ag PDF 04-004-8062 Cu0.76Zn0.24 PDF 01-089-7102 ZnO PDF 00-048-1548 CuO

Fig. 7. XRD pattern of silver track thrifted with 32 wt% brass filler, fired at 680ºC


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Cr level 1132

849

566

283

0 Average: 85

Ag level555

416

277

138

0 Average: 70

Si level

Average: 213 0

114 229 343 458 573 687 802 917

Cr 50 µm 50 µmAg Si 50 µm

Fe level 1414

1060

707

353

0 Average: 148

50 µmFe CrAgFe 50 µm

CrAgFe 1.18 AgFe 1.20

CrFe 10.51 CrAg 0.02

Fe 0.26 Ag 17.43

Cr 0.01

Element mix

SL 50 µm

2040 1785 1530 1275 1020 765 510 255

0 Average: 278

SL level

Fig. 8. EPMA cross-sectional element map of silver track thrifted with 24 wt% stainless steel filler, fired at 640ºC

microanalysis (EPMA) image of silver track containing 24 wt% stainless steel powder fired at 640ºC.

Both silver and stainless steel particles are homogeneously distributed in the track. The silver track is also less dense even after 640ºC firing which could be due to poor packing of silver and stainless steel particles; d90 of the latter being 25 µm. Glass frit distribution as seen from the silicon map is high at the interface between the glass substrate and the silver track which is expected due to glass melting and flowing under gravity. Overlap element maps point out that there is no evidence of detectable interaction between silver and stainless steel. On the other hand, silver bleeding into the glass substrate is evident. Silver bleeding occurs when silver ions are exchanged for sodium ions in glass (18), probably promoted by interaction with the filler. Stainless steel thrifted prints also showed poor acid resistance.

Summary

Attempts were made to reduce the silver content of the automotive silver paste by thrifting with selected base metals, base metal alloys and silver-coated base metals. Inevitably, the conductivity decreases with increasing filler loading which can be fine-tuned through the choice of fillers depending on the requirement. However, most fillers tested in this study show poor acid resistance which limits their use in automotive applications. The choice of the glass frit plays a significant part in improving the acid resistance which requires further research to fine tune the glass frit composition to the type of filler used, if the concept of thrifted silver pastes were to be a commercial success. Further considerations need to be made on the use of base metal fillers especially when the benefits of using them does not outweigh the cost.

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http://dx.doi.org/10.1595/205651317X695271

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2. K. Ghaffarzadeh, Y. Yamamoto and H. Zervos, “Conductive Ink Markets 2017–2027: Forecasts, Technologies, Players: Ten-Year Market Forecasts Segmented by Applications and Materials, Critical Application and Technology Appraisals, Company Profiles”, IDTechEx, Boston, USA, 2017, 456 pp

3. “Silver Pastes for Automotive Glass Applications”, Johnson Matthey Advanced Glass Technologies BV, Maastricht, The Netherlands, 1st March, 2016

4. D. Rudolph, S. Olibet, J. Hoornstra, A. Weeber, E. Cabrera, A. Carr, M. Koppes and R. Kopecek, Energy Procedia, 2013, 43, 44

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Johnson Matthey Technol. Rev., 2017, 61, (2)

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15. G. Sakoske, J. Ryan, V. N. Romanov, D. Klimas, G. Tünker and O. Heitman, ‘Pressure Forming of Automotive Glass and Challenges for Glass-Ceramic Enamels’, Glass Processing Days: The Sixth International Conference on Architectural and Automotive Glass, Today and in the 21st Century, Tampere, Finland, 13th–16th June, 1999

16. “Colorimetry: Understanding the CIE System”, ed. J. Schanda, John Wiley & Sons Inc, New Jersey, USA, 2007, 467 pp

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The Authors

Edwin S. Raj is a Principal Scientist in the Catalyst & Materials Group at Johnson Matthey Technology Centre, Sonning Common, UK. His current research activities are focused on conductive pastes for electronic, automotive and photovoltaic applications as well as investigating pigments for glass enamels.

Jonathan Booth is a Research Manager at Johnson Matthey Technology Centre. He has 29 years’ experience in applied materials research. He specialises in glass enamels and high-temperature conductive pastes for both automotive and photovoltaic applications. Jonathan has a particular interest in the relationship between glass structure and its macro properties.

http://dx.doi.org/10.1595/205651317X695271

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