Analysis of electrically conductive silver ink on stretchable substrates under tensile load

Microelectronics Reliability 50 (2010) 2001–2011

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Analysis of electrically conductive silver ink on stretchable substrates under tensile load Sari Merilampi a,*, Toni Björninen b, Veikko Haukka a, Pekka Ruuskanen a, Leena Ukkonen b, Lauri Sydänheimo b a b

Tampere University of Technology, Pori, Electronics, Pohjoisranta 11 A, 28100 Pori, Finland Tampere University of Technology, Department of Electronics, Rauma Research Unit, Kalliokatu 2, 26100 Rauma, Finland

a r t i c l e

i n f o

Article history: Received 26 January 2010 Received in revised form 16 June 2010

a b s t r a c t Electrical conductors were printed by the screen printing method on stretchable PVC substrates and on fabrics. Polymer thick film silver ink was used as the conductive medium. The electrical performance and the structure of the ink film were investigated in unloaded conditions and under strain. In addition, the ink film morphology was examined. The goal of this study was to provide information for developing a strain sensor for large strain levels using the materials under investigation. An additional aim was to assist the integration of electronics into other structures. The results showed that strain sensitive structures can be made using the materials selected for this study and these materials provide an opportunity to develop strain sensors. The structures also tolerated large strain levels and thus they can be integrated into other materials which are exposed to strain. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Printable electronics is a growing technology. Different printing techniques are used to print conducting patterns, passive components and passive microwave circuits. Screen printing, gravure printing, flexography, ink jet and lithography are examples of different printing technologies [1–8]. The performance of the applications and environmental testing is also performed on printed structures [9–11]. Established applications for thick film inks already exist. Screen printing is used to print thick film inks on ceramic substrates. The traditional cermet inks contain glass and after printing they are fired. Discrete SMD (surface mount device) resistors and capacitors are common examples of these applications. Material development has also made it possible to fabricate novel applications. Usually in these applications polymer thick film (PTF) inks are used. PTF inks do not contain glass and thus they do not need to be fired but are cured at lower temperatures (100– 200 °C). This makes it possible to use unconventional substrates such as paper and fabric. This creates an opportunity to develop novel light weight, flexible applications, such as large area embedded sensors. PTF inks can also be printed using printing methods other than screen printing. However, manufacture of the novel applications and the use of different printing techniques are not very common in the electronics industry. One goal of this study is to assist in the use of printable electronics based on polymer

* Corresponding author. E-mail address: sari.merilampi@tut.fi (S. Merilampi). 0026-2714/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2010.06.011

thick film inks, and to assist with the integration of electronics as part of other structures. The main goal of this paper is to provide information about the behavior of a conductive ink film on different stretchable substrates so that the strain behavior of the materials could be applied to flexible RFID tags to enable a wireless displacement measurement method. The reasons for the behavior of the materials are investigated. A wireless sensor based on the behavior of the materials will be developed in future studies. The PTF ink used in this study consists of an insulating polymer matrix, conductive silver flakes (fillers), solvents and additives. The relation of the physicochemical characteristics and the electrical conductivity of polymer matrix composites have been under investigation for example in articles [1,12–19]. The electrical conductivity of these composites in unloaded conditions and under tensile load is discussed in Section 2 (Background). The morphology of the ink film on different substrates is then evaluated in Sections 4.1 and 5.1 (Thickness and surface roughness of the ink film). After this, the electrical performance of the samples is investigated under tensile load in Sections 4.2 and 5.2 (Electrical performance of the samples under tensile load). Stretchable conductors are interesting because they can withstand large mechanical deformations during their use in electronics application, since the structure itself will deform without immediately breaking. In this way electronics could be integrated into clothes, for example, where mechanical deformations are applied. If the PTF ink is printed on a stretchable substrate, the structure is piezoresistive. The main reason for piezoresistivity is that the conductive fillers of the PTF ink film move apart from each

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other which increases the resistance when the conductor is under strain. The relationship between the mechanical strain and electrical conductivity has been investigated in, for example, articles [1,12,15–23] and it is further evaluated in this article. Besides investigating the relationship of strain and electrical conductivity the usability of the materials in strain sensing is discussed. Stretchable conductors have already been investigated in article by Merilampi et al. [1] and in article by Inoue et al. [24]. Merilampi et al. in [1] printed silver ink conductors with commercial silver inks and tested the printed structures. The conductors were strained up to 75% and the change in resistance was measured during stretching. The tensile tests were performed using three different silver inks on stretchable PVC substrate material. The study showed good stretchability and reasonably low resistance with one of the inks. Inoue et al. [24] dispersed silver particles in a silicone-based binder. This composite was used to fabricate superflexible conductors on silicone substrate by screen printing. The resistance was measured during the stretching of the conductors. Inoue et al. [24] reported electrical conduction in the siliconebased adhesive specimen with elongation of up to 100–180%. These studies encourage us to further investigate screen printed PTF structures in order to find suitable materials for embedded strain sensitive structures. The structure of this paper is as follows: after Section 1 (Introduction) and Section 2 (Background), the geometry, materials and fabrication of the sample structures are described in Section 3 (Fabrication of the samples). Section 4 (Measurements) presents the measurement setup of this study. The measurement results are introduced and discussed in Section 5 (Results and discussion). Conclusions and future work are presented in Section 6 (Conclusions).

2. Background 2.1. Electrical conductivity of polymer matrix composites The electrical conductivity of composites similar to the silver ink used in this study is discussed in, for example, articles [19–23]. The total conductivity is a function of the conductivity of the polymer matrix, the quantity and conductivity of conductive filler material (silver particles in our case), and the hopping conductivity [19–23]. The conductivity of the matrix (polymer) is negligible in our case. The behavior of piezoresistive composites has been analysed with different models. Taye et al. [16] described a history of models created for short fiber/elastomer matrix composites. It was found by Taye and Ueda 1987 that key factors affecting the conductivity of this kind of composite, in addition to the filler volume fraction, are filler aspect ratio and orientation distribution [16]. There are many complex models that also take the particle size and shape into account, but they require experimental parameters and are outside the scope of this study. The percolation theory can be applied to polymer matrix composite material such as silver inks to describe the relation between the physicochemical properties and the electrical properties [13]. The conductive particles of the composite form a conductive network at the percolation threshold and due to this the resistivity decreases rapidly and then levels off as the filler volume content is further increased. In articles [20–21] tunneling is suggested as being an important electrical transportation phenomenon at the percolation threshold. The effect of tunneling conduction decreases as the volume fraction of the filler particles increases. In our work, the filler material content is high and the particles are assumed to form a conductive network and the tunneling effect can be ignored when the samples are not under strain.

In the percolation model of a conductor–insulator composite the effective conductivity of a composite rc can be expressed according to Cohen et al. in 1978 as:

rc ¼ rf ðf  f  Þt

ð1Þ

where rf is the conductivity of the fiber, f is the conductor volume fraction, f* is the critical conductor volume fraction (f at the percolation threshold), and t is a conductivity exponent. Due to the power-law-type conductive behavior, small changes in f or f* can lead to large changes in the composite conductivity, especially close to the percolation threshold. The effective electrical conductivity of a composite is computed by predicting the threshold volume fraction of fiber in a given composite system [16]. 2.2. Electrical conductivity of polymer matrix composites under tensile load The effect of tensile strain on conductivity has been investigated by several scientists. When the polymer matrix composite is strained, several different phenomena are reported to occur. These include the breaking of the 3D network formed by the conductive filler particles, the loss of the contact between particles, the increase in the inter filler distance, the delamination and the reorientation of particles and the decrease of the volume fraction of the filler material as the material is extended [18–22]. Sevkat et al. [20] have also reported the decrease of resistance in the tensile test due to the Poisson effect. This means that more contacts in the lateral direction and fewer contacts in the longitudinal direction are formed. Sevkat et al. [20] reported that the resistance change and the tensile strain follow an exponential or power law. Hu et al. [21] mentioned that the change in the resistance is most sensitive to the strain level at the percolation threshold. At large strain levels, the conditions resemble the state, where the content of silver particles is close to the critical volume fraction. The aspect ratio affects the critical volume fraction due to a wider contact area between the particles with increasing aspect ratio, according to Lin and Chiu [25]. In this way the aspect ratio may also affect the electrical performance of an ink film under strain. If materials are used in high frequency applications, the frequency-dependent behavior should also be considered. In pure metals, the conductivity can be assumed constant until the frequency of the electromagnetic (EM) wave is of the same order of magnitude as the collision frequency of conduction electrons. In composites, the hopping conductivity may also be involved. If the hopping frequency is of the same order of magnitude as the frequency of the EM wave, conductivity may not be constant even at lower frequencies. The insertion loss of screen printed lines were studied by Björninen et al. [26] and it was found that the insertion loss (IL) of screen printed polymer thick film silver ink microstrip lines grows faster than the IL of the same microstrip line made of copper. In articles [27–29] the conductivity of the particle-reinforced composites was observed to depend on the frequency near the percolation threshold, but with higher particle loadings the conductivity was found to be practically independent of the frequency. If the particles move away from each other under large strain, the conditions are similar to the near percolation threshold and the hopping conductivity may be involved in the overall conductivity [21]. Variable models have been developed to describe the behavior of polymer matrix composites under tensile load. Carmona et al. proposed an extended percolation theory. The model involved the change in the filler volume fraction due to external loading. The sensitivity of the piezoresistivity of the composite depended on the elastic properties (i.e. Poisson’s ratio) of the matrix. McLachan et al. also pointed out that the reorientation and different movement of the two phases in the composite play an important

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role. Initially conductive composites become less conductive and suddenly become insulating. This was mainly due to the reorientation of fibers (fillers) during straining. Taya et al. found that the tunneling distance, which depended on the matrix material, affects the critical volume fraction of the filler. Taya et al. [16] found that the change in the volume fraction of fiber is negligibly small if the matrix is an elastomer (Poisson’s ratio for elastomers is 0.5). The reorientation of fiber along the direction of strain was suggested as being the main reason for the decrease and disappearance of the conductivity under strain. A fiber reorientation model was created to simulate the orientation of fibers under strain and through that the new critical volume fraction. The model was also discussed by Taya in article [15]. These models are also interesting in our case. The parameters of the commercial ink are still very difficult to specify, since the size distribution is large and the dimensions of the particles are impossible to determine precisely. In addition the tunneling distance both in our case and in general is very difficult to specify. Despite this the models give an idea of the phenomena occurring in the material during tensile load [15–16]. Hay et al. [19] used conductive silver ink and resistive graphite ink in the strain sensitive structures, which were experimentally examined. Strains from 0 to 0.002 were applied to samples. The structures showed an increase in resistance as strain was applied. Silver ink exhibited a nesting of large and small particulates while the graphite ink showed signs of leafing due to large agglomerations of the filler material. Silver ink film cracked when exposed to a strain of 0.002. Graphite-based ink film showed no such cracking when strained at this level; instead, a region of ‘‘uniform” particulate separation due to the elongation of the ink vehicle phase could be seen. The differing deformation mechanisms suggested that the silver ink possesses a larger modulus of elasticity than the graphite ink. Large differences in modulus values were attributed to the filler content of each composite. It was argued that the greater the filler content the more rigid the ink film becomes due to rigid fillers. This is also supported by Merilampi et al. [1]. According to Hay et al. [19] this effect is reinforced at higher solid content levels because the cross-linking between polymer chains during curing becomes more difficult due to obstruction by the high proportion of solid. This also results in reduced flexibility of the cured polymer matrix. The modulus values increase as particulate size decreases. In addition to this, particle size distribution has an effect on the modulus values of solid loaded polymer composites. Systems with larger particle size distribution exhibit the nesting of particles and possess higher modulus values because aggregates (the fusion of large and small particles) are able to carry a larger proportion of an applied load than large particles alone. Hay et al. [19] found that ink films formed from the graphite ink were more sensitive to microstrains than layers made from silver ink. The nesting of graphite particles resulted in greater particle density. Therefore, more junction failures occurred during small deformations. The spheroidal nature and low size distribution of the graphite particles were considered as proving that actual separation between particles occurs, resulting in a large increase in resistance. The flake nature and high particulate size distribution of the silver ink were considered to mean that two particulates may slide across each other during straining, while maintaining contact. This was assumed to increase the resistance. All strain sensitive structures in article [19] suffered from both hysteresis and drift [19].

investigated in this article since ink film on the PVC substrate had already shown interesting behavior. Two fabrics were also used as substrates to find a reference material, whose behavior is very different from samples on PVC substrate. The aim was to find different structures which could be used in strain sensors. The stretchability of the PVC substrate is based on the materials microstructure. The elastic behavior of Fabric 1 and Fabric 2 was based on the texture of the fabric. There are elastane ‘‘rubber bands” inside the woven fabric texture in the case of Fabric 1. When the fabric is stretched, the individual fibers do not stretch, but the woven structure does. No rubber bands were in Fabric 2. The stretchability of this fabric is based on the texture and the individual fibers also do not stretch in Fabric 2. Silver ink was selected on the basis of the results in article [1], which indicate that the ink film tolerates strains of up to 75% without breaking. The characteristics of the conductive silver ink are shown in Table 2. Rectangular conductors were made from materials described above. A schematic diagram of the conductors is given in Fig. 1. The width (8 mm) and the length (97 mm) of the conductors are the same as for the prototype strain sensitive tag antenna to be used in the future studies. The dimensions of the substrate are: length: 160 mm and width: 12 mm. The samples for thickness and microstructure measurement were made by setting the conductors in epoxy resin in a mold. After the epoxy had hardened the samples were ground so that cross sections could be examined. The mold was modified for 50% strained sample. Narrow cuts were made at the ends of the mold and the strained sample was tightened with a rack. The rack was designed to keep the sample under strain until the epoxy had hardened. 4. Measurements 4.1. Thickness and surface roughness of the ink film The morphology of the ink film was investigated on the different substrates to discover the effect of the substrate on the Table 1 Substrate materials. Substrate

Symbol

Thickness (lm)

Stretchable PVC Commercial elastic polyester/spandex fabric Commercial elastic fabric (no spandex)

PVC Fabric 1 Fabric 2

300 1000 500

Table 2 The characteristics of the conductive silver ink. Manufacturers description

Curing conditions (°C, min)

Viscosity (P)

Conductivity (MS/m)

One component silver ink consisting mainly of polyester resin and silver particles. Silver content is 60–65 wt.% and polyester resin content is 11– 14 wt.%. Particle sizes are mainly in the range from 3 to15 lm.

120, 20

200–300

1.25

3. Fabrication of the samples Screen printing technique was used to produce PTF conductors. The substrate materials are listed in Table 1. Stretchable polyvinylchloride (PVC) sheet proved to be a suitable substrate for printed conductors in article [1]. Samples mainly on PVC substrate were

Fig. 1. Schematic diagram of the stretchable conductors.

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behavior and performance of the conductive ink film on it. The effect of the thickness of the ink film on the ink film electrical performance was found to be essential, i.e. by Merilampi et al. [3]. This is why the thickness of the ink film was measured first. The measured results are also used in effective conductivity calculations later in the article (see Eq. (2)). The thickness of the ink film on PVC was measured when 0% and 50% strain was applied. Smaller strains were not measured since no significant changes in ink film thickness were expected with small strains. The ink films on fabrics were only visually examined with a microscope under 50% strain since the ink film does not stretch and the ink film thickness remained the same as without straining (as is found in Section 5.2). The thickness was measured using I-Solution lite software connected to an optical microscope. The thickness values are an average value from 20 different measurement points at the middle of the cross section, which is also at the middle of the conductor. Minimum, maximum and standard deviation of the thickness is also calculated by the software. The substrate affects the ink film thickness variation [1]. The fluctuation of the ink film was investigated to evaluate the effect of the substrate on the ink film. The surface roughness values (Ra) and (Rz) of the substrates and the ink film were measured using a profilometer when 0% or 50% strain was applied to the samples. The surface roughness value Ra is the arithmetical average of the deviations of average line and absolute measured value. The surface roughness value Rz is the average of the five highest points and the five lowest points of the surface. The surface roughness values were measured to evaluate the amplitude of the typical ‘‘waving” of the printed silver ink film and to see if the amplitude of the waving increases when strain is applied to conductors. Microscope examinations were also performed to investigate the morphology of the conductive ink film. Ra and the standard deviation of the ink film thickness give an idea of the average amplitude of the wavy surface. The minimum and the maximum thickness value and Rz give information about the extreme values. 4.2. Electrical performance of the samples under tensile load The resistance of the conductor was measured by 2-point measurement during straining to assist the future strain sensitive RFID tag development. Strain controlled measurements were performed on samples. The goal was to find differently behaving combinations of material so as to be able to develop different strain sensors. The 2-point measurement was considered to be a valid method since relative measurements were performed. In article [1] 4-point measurement results for the films of the same ink on flexible substrates in unloaded conditions are also presented. Mechanical contacts of the measurement heads were used in order to eliminate the effect of the adhesive material on the contact resistance under different strain levels. The alligator clips were pressed tightly on the ink film conductor ends. Note that the contact resistance of the mechanical connections may also change during the stretching of the conductors. However, as is later discussed, Figs. 14 and 16 indicate significant difference in the ink film under strain on different substrates and it is assumed that the main cause for the behavior of the samples is due to the ink film structure rather than the changed contact resistance. Still, the absolute measured values should be treated with care. The resistance of our samples was first measurement without load. The second measurement point was when 5% strain was applied and the third measurement at 10%. From 10% strain to 50% strain, the measuring was repeated after 10% steps. The strain rate was 50 mm/min. The width of the conductor was also measured at each measurement point to calculate the Poisons’ ratio and through that to evaluate the change in conductor thickness

during stretching. A slide gauge was used in width measurements and an accuracy multimeter was used in resistance measurements. First of all three stretchings were performed on samples on PVC (Samples 1–3), which is the main substrate material investigated, to see if the samples behave like samples in article [19], where strain sensitive structures behaved differently during the first cycle of straining compared to subsequent stretchings. An estimate of the effective conductivity of the ink film was calculated based on the measured conductor resistances and dimensions by:

R¼

ql A

¼

l

rA

!r¼

l ; RA

ð2Þ

where R is the measured DC resistance, l is the length of the conductor and A is the cross sectional area of the conductor. The change in conductor thickness during straining was ignored, because it is the same order of magnitude as the thickness variation and the error in thickness measurements. An ink film thickness of 27 lm (measured later) was used in calculations. The recovery of the conductor resistance was examined. The applied strains were the same as in the conductor resistance measurements. The resistance change in straining and in recovery was measured sequentially. In addition the recovery of the conductor width was examined by measuring the conductor width from the same measurement points as the resistance. A scanning electron microscope (SEM) was used to examine the microstructure of the conductive ink films on PVC in unloaded conditions and when strain of about 50% was applied. This is to observe the microstructure changes such as particle reorientation and failures in the ink film under loading (discussed in Section 2). The volume fraction of particles was analysed with Matlab software by using a code which calculates the area of particles compared with the total area of the ink films cross sectional area from SEM micrographs. These were taken from samples in unloaded conditions, during 50% strain and after straining. Two evaluation points from cross sections were randomly selected and the average was calculated from the results for each case. In addition to analysing the samples on PVC substrate, the resistance measurements were repeated with samples on fabric substrates to find out if the behavior of the ink film was significantly different on these substrates compared to when on PVC. One stretching was performed to samples on Fabrics. The recovery was examined and scanning electron microscope analysis was performed with samples on Fabric 1. 5. Results and discussion 5.1. Thickness and surface roughness of the ink film The measurement statistics of the sample conductors in unloaded conditions are presented in Table 3. The measurement statistics for the conductor on PVC under load are also presented in the table. The strain (%) in the Tables 3–5 and 7–9 as well as in Figs. 2–11 is the ratio of elongation and the original length of the conductor in percentage. The thickness variation in the ink film after screen printing is observed on all substrates. The porosity of fabrics increased the absorption of the inks inside the substrates which caused large variation in the thickness of the ink films. As shown in Fig. 15, on fabric substrate the ink partially penetrates between the fabric fibers. This makes quality of the ink film worse and the thickness values cannot be straightforwardly compared with the thickness values on non absorbing material like PVC. The thickness of the ink film on PVC substrate decreases slightly when the conductor is strained (See Table 3) due to the Poisson

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S. Merilampi et al. / Microelectronics Reliability 50 (2010) 2001–2011 Table 3 The thickness measurement statistics of the ink films on different substrates. Substrate

Strain (%)

Mean thickness (lm)

Minimum thickness (lm)

Maximum thickness (lm)

Standard deviation (lm)

PVC Fabric 1 Fabric 2 PVC

0 0 0 49

27 82 267 25

20 25 104 19

31 156 342 31

2 48 51 4

Fig. 2. Conductor width of Sample 1.

Table 4 The surface roughness values of the ink film and substrate. Material Ink film on PVC Ink film on Fabric 1 Ink film on Fabric 2 PVC Ink film on Fabric 1 Ink film on Fabric 2 Ink film on

PVC Fabric 1 Fabric 2

PVC Fabric 1 Fabric 2

Strain (%)

Screen (mesh/thickness of the thread in lm)

Ra (lm)

Rz (lm)

0 0 0 0 0 0 54 53 53 53 51 51

63/63 – 63/63 – 63/63 – – 63/63 – 63/63 – 63/63

2 1 21 21 34 37 1 4 67 28 26 18

8 6 96 80 151 111 5 29 142 117 90 69 Fig. 3. Conductor width of Sample 2.

effect. The Poisson’s ratio is the complement of the ratio of strain perpendicular to the applied load to the strain in the direction of the applied load. The Poisson’s ratio for the structure is calculated from the length and width change under tensile load. The measurement results of the change in the conductor dimension are presented in Section 5.2. Poisson’s ratio is: – (1 mm/8 mm)/ 0.5 = 0.25 which indicate that the change in thickness would be about 3 lm which is small in comparison with the fluctuation of the ink film surface (See Tables 3 and 4). The measured ink film average thickness under about 50% strain is 25 lm. According to calculations based on the Poison ratio it would be 24 lm. The stretching of the ink film on fabrics is based on the fabric stretching and the ink film itself does not stretch which can be observed from Fig. 16 and as is further discussed in Section 5.2. This is why the change in ink film thickness cannot be seen on fabric substrates under tensile load (see Fig. 16). The surface roughness of the substrates and the ink films are presented in Table 4. It can be observed from Table 4 that the surface roughness of the ink film is large on fabric substrates and it is also larger when the fabric is strained in the case of Fabric 1. Fabric 2 actually smoothens when it is strained and because the ink is partially inside the fabric the ink film is also smoother under straining. The large surface roughness of the ink film on fabrics is mostly due to the rough substrate surfaces, but as can be seen from

Fig. 4. Conductor width of Sample 3.

measurements from the PVC sample, the printing method also affects the ink film surface roughness. In the case of samples on PVC, the surface of the substrate is not completely smooth, but it still is smoother than the ink film surface. The wavy ink film surface is caused by the screen printing method. The ink film is thinner (the hollow of the wave) in the areas, where the threads of the screen are during printing even though the characteristics of the ink were designed for screen printing (‘‘high” viscosity pseudoplastic ink).

Table 5 Resistance and width of conductor on PVC during straining and recovery. Straining

Recovery

Strain (%)

DC resistance (O)

Estimated conductivity (kS/m)

Width (mm)

Strain (%)

DC resistance (O)

Estimated conductivity (kS/m)

Width (mm)

0 5 9 20 29 38 46

0.5 0.8 1.1 2.4 4.3 7.4 10.1

896 581 462 239 146 94 74

8.1 8.2 7.8 7.6 7.4 7.2 7.1

46 39 27 17 11 6 5

10.1 8.4 5.5 3.4 2.3 1.5 1.3

74 84 114 167 228 329 371

7.1 7.1 7.3 7.5 7.7 7.8 7.9

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Fig. 5. Resistance of Sample 1.

Fig. 9. Resistance of Sample 3.

Fig. 6. Calculated conductivity of Sample 1. Fig. 10. Calculated conductivity of Sample 3.

Fig. 7. Resistance of Sample 2.

Fig. 8. Calculated conductivity of Sample 2.

The straining does not have a significant effect on the PVC substrate surface roughness for strains P50%, but the effect on the ink film surface can be seen. Particularly for Rz values, which describe the extreme values, the difference between an unloaded sample and a sample under loading (Table 4) is significant. This supports the findings from microscope examination (thickness measurements and visual examination). Ra is taken from a larger area than

Fig. 11. The recovery of the conductor on PVC.

the standard deviation of ink film thickness. However, these two measurements still give a similar result. Both surface roughness values and standard deviations are larger when the sample on PVC substrate is strained. Presumably the thickness of the thin parts (wave hollows) decreases more than the thickness of the thicker parts under strain. Matrix cracking (See Fig. 12) is probably one of the reasons for a rougher ink film surface. The matrix cracking is further discussed in Section 5.2. Hay et al. [19] stated that the higher the vehicle content of the ink, the more uneven the ink film surface is when using the offset lithography method. The uneven ink film surface was also noted by Björninen et al. [2] with patterns printed with both gravure and screen printing methods, and by Siden et al. [30] with the flexography method. This indicates that uneven ink films are typical for patterns fabricated by different printing methods [2–3,19,30]. It was found by Björninen et al. [2] that if the conductive layer is uneven in high current density regions, the performance of an RFID tag antenna gets worse [2]. This is why in addition to the commonly examined physicochemical properties, it is important to examine the effect of the fluctuating ink film surface on the electrical performance of the printed applications. This is our future work.

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2007

Fig. 12. SEM micrograph from a crack (hole) in ink film on PVC under 50% load.

5.2. Electrical performance of the samples under tensile load 5.2.1. Samples on PVC substrate The width of the conductors on PVC during stretching was measured first as the results were later used in evaluating the effective conductivity of the conductors. The results are presented in Figs. 2–4. The error margin in the width measurement results is ±0.1 mm. The difference in width between the first, second and third stretching is not significant but a decrease in width of about 1 mm is found when 50% strain is applied. The measured changes in resistance and the effective conductivity of the conductors calculated on PVC are presented in Figs. 5–10. All three stretches are plotted in the same figures. The error margins are: length and strains: ±1 mm and resistance: ±0.1 O. The relatively large error margin in resistances is due to the drifting of the measurement value (decimals) of the resistance because of the settling of the ink film during measurement. The error margin of conductivity is ±0.2 MS/m. However, the relative change of conductivity during straining can still be estimated. The initial conductivity of all the samples on PVC was smaller than the value reported by the ink manufacturer. One reason for this might be the uneven conductor surface which has already been discussed in this article. The absolute thickness of the ink film cannot be precisely shown because of the thickness variation. An average value of 27 lm was used in the effective conductivity calculations although as discussed earlier, the thickness of the ink film changes when the conductor on PVC is strained (see Table 3). The effect of the thickness decrease during stretching was ignored, since the thickness change is small and the impact of it in effective conductivity calculations is insignificant. In the case of the conductivity of Sample 1, for example, during the second 50% stretch the difference in calculations (when using 27 lm and 24 lm) would be 8 kS/m which is less than the margin of error. The absolute values of calculated conductivity should be treated with care although the relative changes can be clearly seen. The original length of the samples was 97 mm before stretching. When the load was removed after the first 50% stretch the strain of Sample 1 was 6%. Strain 30 min later, before the second stretch, was 4%. When the load was removed after the second 50% stretch the strain was 7%. Strain 24 h later, before third stretch, was 2%. When the load was removed after the third 50% stretch the strain was 8%. Similar situation took place for samples 2 and 3. It can be seen from the resistance measurements (Samples 1–3) that the difference between the first stretching and the two following stretchings is larger than the difference between the second and the third stretchings. In article Hay et al. [19] found that a high degree of hysteresis was observed during the first cycle of straining, after which, the hysteresis reduced dramatically. This supports our findings of the difference between the first and the following stretchings. The resistance of our samples is lower and increases more after the first stretching. This is probably due the fact that the microstructural defects are created during the first stretching and they are already in the structure at the beginning of the two

subsequent stretchings. It is also observed that the recovery of the samples from very large strains (50%) is slow from 10% to 2–3% strains so the PVC substrate behaves anelastically. The anelastic behavior also explains why the starting resistance of stretching 2 might be larger than starting value of stretching 3. The recovery time is 24 h before stretching 3 and about 30 min before stretching 2. Also about 2–3% plastic deformation was found in the structure after 24 h. The permanent difference in the resistance (24 h after the stretching) in an unloaded condition is 0–0.5 O for Samples 1–3. If a lower strain rate was used, the ink film would have more time to settle, but since in many real applications the strain may be applied faster, the strain rate of 50 mm/min was selected. The behavior of Samples 1–3 indicates that after the first stretching the resistance increase steadies. If the materials are used in sensor applications, the first stretching should be performed before using the sensor. Dziedzic et al. [31] analysed carbon black/polymer composites during compression and decompression. The hysteresis of carbon black/polymer composite was observed to be much smaller when a lower maximum pressure was applied (2000 bar instead of 5000 bar). It would be interesting in future studies to find out, if in the case of smaller strains the hysteresis of the samples was smaller. The recovery and the hysteresis of printed conductors were further investigated to see if the structures behave like the samples in article [19], where a high degree of hysteresis was found during the first cycle of straining. The hysteresis can be seen in Fig. 11 and Table 5. The difference of the strained and recovered resistance is quite small for large strains. Notable differences occur under 10% strains since the structure does not have time to settle. Inoue et al. [24] reported that silicone-based specimens cured for 5 h at 100 °C almost recovered after removing the tensile stress. In article [1] there was delay in recovery due to the anelastic behavior of the PVC substrate which was also the substrate in our samples. Hay et al. [19] suggested that the hysteresis is ascribed to a viscoelastic effect occurring in the printed ink film, generating a slight delay between applied strain and actual particulate separation. The drift effect between repeated cyclical loadings was considered to be due to the mechanical settling of the ink film. Kure et al. [22] also found hysteresis in prototype sensors. The same kind of behavior can be seen with our samples. As had already been found in the straining test earlier in this study, the behavior of the PVC substrate is anelastic. This also affects the behavior of the whole structure so the resistance and conductor dimensions were studied during recovery as well. Table 6 illustrates the changes in resistance and conductor dimensions on PVC. The change in conductor width is within the margin of error since the width change in general is small. The effect of tensile strain on the conductivity of the ink film on PVC can be explained by microstructural changes in the ink film. The Poisson effect found in article by Sevkat et al. [20] cannot be observed with our samples because of the non-elastic polymer matrix cracking. Matrix microcracking is seen in Figs. 12 and 14.

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Larger cracks also occur (See Fig. 13). Although the ink film contains holes (Fig. 13), it remains conductive. (See Figs. 5–10). The cracking of the matrix leads to a partial breaking of the 3D network and a reduction of contact area between particles. The inter filler distance also increases and delamination of particles occurs. The decrease in the volume fraction of the particles in the ink film was evaluated from the cross section figures of the samples under loading. Table 7 shows about a 10% volume fraction reduction under approximately 50% strain. The volume fraction of particles recovers almost entirely after straining. Similar behavior was observed by Merilampi et al. [1]. Only particle delamination from the matrix could be seen after recovery. The volume fraction values are approximations from cross sectional areas of particles relative to the total ink film area. Since the volume fractions are calculated from two-dimensional cross section figures, the values are not absolute. However, comparisons between the samples can still be made. Although the absolute values of the parameters in Eq. (1) are not known in our case, it is observed that the volume fraction has an effect on the resistivity of the material. The effect of the volume fraction of conductive fillers was further examined in the case of carbon black/polymer matrix composites by Dziedzic et al. [31]. The behavior of carbon black/polyesterimide thick film resistors under high hydrostatic pressure (up to 5000 bar) was presented. When the pressure was increased, the much higher compressibility of the polymer matrix compared to carbon black affected the gradual increase of the carbon black volume fraction. The basic relationship of percolation theory q / ðf  f  Þt was used successfully for semi-quantitative analysis of the high pressure effect on the devices tested. In our case, the samples were exposed to strain instead of compression and the volume fraction is decreased. It should be also taken into consideration that, due to matrix cracking, the decrease in the particle volume fraction is not completely uniform. It would be useful to develop a model which takes this into account. Low levels of hysteresis were noted in article [31] when the resistance was measured at higher pressure and then at a lower pressure. Because the resistance after the whole compression/

decompression process returned to the initial value, the possibility of the destroying particle - particle contacts inside the film was excluded and the authors concluded that the observed hysteresis is the result of the partial plasticity of the polymer matrix. In our case a very small (0–0.5 O) permanent change in resistance was also found, which, it is assumed, was caused by the matrix cracking and by the delamination of particles which may also lead to small changes in particle–particle contacts. In our case, the polymer matrix is not an elastomer (Poisson’s ratio for polyesters is 0.39 [32]), but from Fig. 14 it would seem that the particles orientate in the direction of the load. In Fig. 14, the lower micrograph is taken from a sample under 0% strain. Although the particles mainly lie horizontally, because during curing the evaporation of solvents tends to orient the flakes in parallel to the substrate, more vertically placed particles are found in unloaded conditions than in the upper micrograph from a sample during 50% strain. These vertically positioned particles are circled in the lower micrograph. More research is needed to confirm these implications. The particle distribution of the ink of our study is large and strain levels are high. Thus matrix cracking and its consequences are believed to be the main cause for the increase in resistance. It should be noted that in addition to the microstructural changes the change in physical dimensions (width, length, thickness) affect the resistance. In addition, the thickness variation and surface roughness which were observed to increase when 50% strain was applied might reduce the conductivity.

Table 7 Evaluated average volume concentration of silver particles in ink film on PVC. Strain (%)

Particle volume fraction in ink film (%)

Notes

0 50 51 5

49 40 39 50

After 1st stretching After 3rd stretching After recovery

Table 6 Resistance and dimensions of conductors on PVC before and after the stretching. Sample

Stretching

Resistance (O) before stretching

Resistance (O) right after stretching

Change in width right after stretching

Change in length right after stretching

Change in resistance (O)

1 1 1 2 2 2 3 3 3 4

1 2 3 1 2 3 1 2 3 1

1.0 1.5 1.0 0.6 1.1 1.0 0.5 1.0 1.0 0.5

1.6 2.4 2.6 1.5 2.1 3.2 1.3 2.5 2.1 1.3

0.1 mm = 1.3% 0.2 mm = 2.5% 0.2 mm = 2.5% 0.3 mm = 3.7% 0.5 mm = 6.2% 0.4 mm = 4.9% 0.3 mm = 3.7% 0.2 mm = 2.5% 0.3 mm = 3.7% 0.2 mm = 2.5%

6 mm = 6.2% 7 mm = 7.2% 7 mm = 7.2% 7.3 mm = 7.5% 9.3 mm = 9.6% 7.8 mm = 8% 7 mm = 7.2% 7 mm = 7.2% 6 mm = 6.2% 5 mm = 5.1%

0.6 0.9 1.6 0.9 1.0 2.2 0.8 1.5 1.1 0.8

Fig. 13. SEM micrograph from a cross section of ink film on PVC under 51% strain after multiple stretchings.

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Fig. 14. Ink film on PVC under 50% and 0% strain.

As discussed earlier, the films of silver ink used in this study was shown to maintain electrical conductivity under 75% strain [1]. This means that this kind of silver ink formulation is more suitable for measuring large strain than very small deformations. If more sensitive strain measurements are needed, there are many possible solutions for measuring very small strains and deformations. They are discussed in articles [12,19,33] for example.

5.2.2. Samples on Fabric substrates The behavior of ink film on Fabrics was investigated to find a structure which behaves very differently from ink film on PVC under tensile load. The measurement results for the ink film on fabric substrates are presented in Tables 8 and 9. The resistance change during the recovery of the sample on Fabric 1 is also presented in Table 8. There was also a difficulty in measuring samples on Fabric 1 due to the web-like structure at higher strains (>30%, see Fig. 16). It was difficult to attach the measurement heads reliably and special attention had to be paid to this issue. The absolute values may not be precise, but the measured values still give the right order of magnitude. It can be concluded from the results that the behavior of Fabric 1 is considerably different compared to samples on PVC and it thus offers an interesting reference structure for PVC samples in strain sensing applications. The conductivity of the ink film on fabric substrates was not estimated by using Eq. (2) since the ink film is very uneven (see Table 3 and Fig. 15) and the ink is between the fibers, which causes gaps in the ink film. The performance of this kind of ink film is not the same as the performance of a smooth uniform ink film. The effective thickness of the ink film is very hard to estimate. It might be of the same order of magnitude as for samples on PVC since the printing process parameters were the same and since the DC resistances of samples on fabric were of the same order of magnitude in unloaded conditions as for samples on PVC. The recovery of resistance of samples on fabrics is slow under 30–50% strain, because the structure of the fabric and ink film settles. The recovery of the length of the conductor is very fast and almost no permanent deformation occurs. Also the width of the conductor does not change as much as it does on PVC.

Table 8 Sample on Fabric 1 under straining and recovery. Stretching/ recovery

Strain (%)

DC resistance (O)

Length (mm)

Width (mm)

Stretching Stretching Stretching Stretching Stretching Stretching Recovery Recovery Recovery Recovery Recovery Recovery

0 5 10 20 28 38 48 38 28 20 10 5 1

1 7 12 40 70 200 1,200,000 400 95 37 10 3 1

96.0 100.6 105.5 115.1 122.7 132.1 141.8 132.3 122.9 115.0 105.5 100.6 97.0

8.0 7.9 7.9 7.8 7.6 7.6 7.6 7.6 7.8 7.9 7.9 7.9 8.0

Table 9 The strains and the changes in the dimensions and resistances of ink film on Fabric 2. Strain (%)

DC resistance (O)

Length (mm)

Width (mm)

0 5 10 20 28 38 48

0.3 0.4 0.4 0.6 0.8 1.2 1.5

96.0 101.1 105.6 115,0 123.2 132.3 142.1

8.0 8.0 7.9 7.5 7.2 7.0 7.0

The recovery of Fabric 2 was not measured since there was about 19% plastic deformation after straining. Although the resistance was low even under large (50%) strain, this substrate is only suitable for structures, where no large strains are applied. This material is not a suitable substrate for strain sensitive tags. It is, however, a good substrate for the textile integrated conductors in places, where large stretching (50%) does not occur and thus the structure maintains its shape.

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Fig. 15. SEM micrograph of a cross section of ink film on Fabric 1 under 0% strain.

Fig. 16. SEM micrograph of a cross section of ink film on Fabric 1 under 50% strain.

The resistance of the sample on Fabric 1 changes more rapidly during straining compared to samples on PVC. It can be seen from Figs. 15 and 16 that the decrease of the conductivity of the fabric is mostly due to the structural change of the fabric rather than the microstructural changes of the ink film. The ink film does not stretch but when Fabric 1 is strained, the threads separate from each other as does the ink on the threads which can be seen in Figs. 15 and 16. Pictures on the lower left corners of Figs. 15 and 16 are taken from above the sample conductors and they illustrate the behavior of Fabric 1. The magnification in Fig. 16 shows that no microstructural changes in the ink film occur during straining. The different structure of Fabric 2 also affects the ink film behavior under loading. The investigation of the effect and the suitability of differently woven textiles are our future work. On the whole, stretchable PVC appears to be an interesting substrate for the future strain sensitive RFID tags, since the ink film on PVC forms a strain sensitive structure and the change in resistance and dimensions are measurable during straining and this should affect the functioning of the tag. Samples on PVC also show reasonable recovery from large strains. At least relatively large differences in strains can be detected by the structure if it is used in strain measurements. If more sensitive structures are needed, the ink can be modified by reducing the particle volume concentration near the percolation threshold although very large strains cannot then be measured, since the conductivity almost disappears when particle concentration is less than the critical particle volume fraction (percolation threshold). Dziedzic et al. [31] also noted that in the case of the smaller volume fraction of the filler, the change in

resistivity was more significant. This is interesting when considering sensor applications and it is interesting to further examine this in the future. More research is also needed to examine the time dependence of the samples including the hysteresis during cyclic stretching. Fabric 1 seems a suitable substrate for future strain sensitive RFID tags, as a reference structure to samples on PVC, due to the significantly larger increase in resistance during straining and due to the reasonable levels of recovery. 6. Conclusions Screen printed conductive PTF ink films on stretchable PVC and fabric substrates was investigated. The structures maintained their conductivity under large strain levels and they can thus be integrated into other materials such as clothes, where strains are applied. The resistance of the structures increased as a function of strain. The resistance change under load was found to be caused by different mechanisms on PVC and on fabrics. The behavior of the ink film was very different on PVC and on fabric. Both structures offer interesting opportunities in strain sensing. The substrate affected the resistance change and the morphology of the ink film. The ink composition has an important role in the behavior of the ink film under load. In addition to ink selection, it is possible to control the strain sensitivity as well as the recovery of the structure by selection of different substrate materials. The results of the study will be used in the future development of a wireless strain sensor based on RFID technology.

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Acknowledgements The financial support of the Finnish Cultural Foundation – Satakunta Regional Fund, the Ulla Tuominen Foundation and High Technology Foundation of Satakunta are gratefully acknowledged. The authors would also like to thank Adjunct Professor Antti Vuorimäki for guidance with electromagnetic theory.

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