Screen printed resistive pressure sensors fabricated from polymer composites with carbon nanotubes

Screen printed resistive pressure sensors fabricated from polymer composites with carbon nanotubes Daniel Janczak1, Grzegorz Wróblewski1, Małgorzata J...
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Screen printed resistive pressure sensors fabricated from polymer composites with carbon nanotubes Daniel Janczak1, Grzegorz Wróblewski1, Małgorzata Jakubowska1,2, Marcin Słoma1,2, Anna Młoz˙niak2 1 ´w. A.Boboli 8, 02-525 Warszawa, (22)2348139, Department of Mechatronics, Warsaw University of Technology, ul. S e-mail: [email protected]

Institute of Electronic Materials Technology, ul. Wólczyn´ska 133, 01-919 Warszawa, (22)8353041 int. 457, e-mail: [email protected]

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The paper presents the results of the investigation into flexible layers based on carbon nanotubes used as measuring layer in force sensor. Results of mechanical fatigue tests show that carbon nanotubes layers are good for reinforcing or as a conductive additive in composite materials. Composition of carbon nanotubes in PMMA polymer resin was prepared by modified mixing process used in thick film material preparation. Sensor structure was fabricated by printing polymer-nanotube areas with polymer-silver paths as connection electrodes on polyester substrate foil. Second type of sensors was prepared with two comb electrodes and single carbon measuring layer. Composite materials were fabricated with different amount of nanotube content: 0,25 wt%, 0,5 wt%, 1 wt% and 2 wt% multiwall carbon naotubes (MWCNT). Different types of carbon-composites measuring layers were compared in the experiment. Results of mechanical fatigue tests conducted on carbon nanotubes layers showed that composition with polymer resin have good adhesion to polymer surface. Experiment shows CNT are good for reinforcing or as a conductive additive in different composite materials. Results of the observations show that dependence between sensor resistance and force tension is linear in logarithmic scale and similar for different samples. Resistance between sensor electrodes was measured for force tension changes in range 10N to 20kN. Better results were observed for sensors with comb electrodes and low content of carbon nanotubes. Keywords: resistive polymer nanocomposite, pressure sensors, carbon nanotubes, thick film technology, contact resistance.

Introduction Recently numerous investigations have been done on graphene and diverse application approaches of this material have been studied. Many scientific research and development institutions are focused on this carbon material because of its extraordinary properties like high electrical and thermal conductivity and extremely good mechanical properties [1]. Graphene layers are described in various applications like supercapacitors [2–4], FET transistors [5], photovoltaics [6], transparent electrodes [7], chemical and biochemical sensors [8–13]. Polymer composites containing different fillers like carbon nanotubes are often used in elastic sensors [14] including those made through printing techniques [15, 16]. Recently researches on screen printed force sensors fabricated from polymer composites containing carbon nanotubes were done in ITME [17, 18]. In this paper, screen printed force sensors containing

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graphene  nanoplatelets, which are a  novelty in field of graphene printed electronics, are presented. Due to high elasticity of developed graphene films, sensors have been printed on flexible substrates. Created flexible graphene force sensors can be used in numerous applications, for example mounted on elements with irregular shapes. ITME fabricated sensors are also interesting from the economical point of view, because they are produced using printing methods which are relatively cheap in field of electronic devices manufacturing, especially in  mass production. Screen printing is a  well known method in production of composites electrodes, used in mass production of one time usable sensors [19–20]. Using graphene nanoplatelets as composite filler allowed to develop force sensors with  higher resolution, than those based on carbon nanotubes. Moreover, the fact that layers are sensitive to the direction of stress was observed, what can be useful in the future, while

Screen printed resistive pressure sensors fabricated from polymer composites with carbon nanotubes

designing stress direction sensitive graphene force sensors. ITME based investigations allowed to elaborate layers with new properties and those layers were produced using low-cost and effective printing method.

Multiwall carbon nanotubes (MWCNT) were used in the present study as a reinforcing and conductive additive in composite with polymethylmethacrylate (PMMA) as a  carrier with butyl carbitol acetate solvent. Polymer-nanotube compositions were used to prepare measuring layers in screen printing technology. Characteristic dimensions carbon nanotubes (MWCNT) obtained by a gas phase nucleation process were observed on scanning electron microscope (SEM) AURIGA CrossBeam Workstation in Institute of Electronic Materials Technology (ITME). Carbon nanotubes (MWCNT) were produced and provided by Cheap Tubes Inc. Average diameter of the nanotubes in material used is 10–160 nm, and the length is 0,5–5 um. (Fig. 1.) Distribution, as a histogram, the average diameter and the average length of the nanotubes were showed in Fig. 1.c–d.

Fig. 1.c–d. Distribution, the average diameter and the average length of the nanotubes.

Fig. 1.a–b. SEM image of used Carbon Nanotubes (MWCNT) material.

Four types of pastes were prepared for the experiment. All samples based on 10 wt% PMMA with butyl carbitol acetate solvent. Composite materials were fabricated with different amount of nanotube content: 0,25 wt%, 0,5 wt%, 1 wt% and 2 wt% MWCNT. To achieve uniformity of the filler in the PMMA matrix mixtures were sonicated for two hours at room temperature. Then they were rolled twice on tree roll mills with silicon carbide (SiC) roller. The printing process of the final mixtures was done using screen printer AMI Presco type 242. After printing samples were cured at 120°C for 1 h. This compositions were used for fabrication measuring layers for resistive force sensors. Two types of such sensors have been designed and  constructed for the purpose of the experiment (Fig. 2.a–b). To print the first type sensors’ pressure-sensitive layers pasts with 0,5 wt%, 1 wt% and 2 wt% CNT concentrations were used. All types of prepared pasts

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Materials and Preparation

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Daniel Janczak, Grzegorz Wróblewski, Małgorzata Jakubowska, Marcin Słoma, Anna Młoz˙niak

were used to print measuring layers in sensors with combo electrodes. 125 um polyester foil was used as substrates. Substrate was warmed at 150 degrees for one hour to prevent changes in the dimensions of the substrate during the curing of the printed layers. Electrodes and conductive paths were screen printed on foil using silver paint L-121 produced in ITME and

cured at 130 degrees for half an hour. Barium titanate (BaTiO3) was used to screen printed nonconductive distance layer in sensor. Cured at 120 degrees for 1 hour. Sensor structure was fabricated by printing polymer-nanotube areas (Fig 2a. — 3) with polymer-silver paths (Fig. 2a. — 2) as connection electrodes on polyester substrate foil (Fig 2 — 1). Second type of sensors was prepared with two comb electrodes (Fig. 2b. — 2) and single carbon measuring layer(Fig. 2b. — 3). Dielectric distance was screen printed as last layer in structures (Fig. 2a,b. — 4).

Experimental Resistive pastes with multiwall carbon nanotubes were prepared. They were used to make samples to test electrical and mechanical properties of screen printed layers. Test shape had ten paths of different length. Resistance of all paths was measured and results were divided by the length of paths’ value to determined resistance per square of screen printed layers. Mechanical fatigue tests were conducted on prepared samples to prove samples adhesion to polymer substrate. Durability of printed layers was tested by bending repetitively, while resistance was being measured. Thickness of layers was measured by using contact profilometer Dektak 150f-my Vecco. Both types of sensors were used in experiment. Sensors were placed in hydraulic press, and pressure was applied to pressure-sensitive resistive area. Resistance changes measured at the electrodes were recorded with a frequency of 20 per second on a computer.

Fig. 3. Diagram of force sensor load. 1 — ground, 2 — sensor, 3 — source of pressure.

Results and Discussion Fig. 2.a–b. Two types of screen printed resistive pressure sensors: 1 — polymer substrate, 2 — silver electrode with path, 3 — carbon nanotube layers, 4 — distance.

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In Table 1. relation between carbon nanotubes wt % addition in pasta and resistance per square is shown. With increasing addition of nanotubes increase in conductivity was observed.

Screen printed resistive pressure sensors fabricated from polymer composites with carbon nanotubes

Table 1. Results of resistance per square measurement.

Resistance [kΩ/ ] 44,7 11,7 7,9 2,1

Fig. 4. Results of mechanical fatigue tests, sample made from a paste containing 0.5 wt% MWCNT.

The average thickness of the layers was 10 um, and it was constant for all samples. Results of mechanical fatigue tests show that carbon nanotubes layers are good for reinforcing or as a  conductive additive in composite materials. Figure 4 shows the effect of bending of the 0,5 wt% MWCNT samples changed little resistance (2–3%). Samples were bended repetitively, with bending radius of 8 mm. Figures 5 and 7 present resistance changes under different pressure for both types of investigated sensors. Resistance between sensor electrodes was measured for force tension changes in range 10 N to 20 kN. For all samples and both types of sensors dependence between sensor resistance and pressure was nearly linear on a logarithmic scale. During the first cycles of measurements high spread of resistance values was observed which was caused by low MWCNT content. Increasing pressure results in changes of polymer-nanotube composite resistance in the first part of the process with possible cause being increasing contact surface of measurement layers. The effect disappears after a  few cycles resulting in decreasing resistance after pressure is applied. A small hysteresis was observed in all types of sensors. Measurement errors were +/- 5%. The highest changes of resistance was observed for sensors with comb electrodes and the lowest multiwall carbon nanotubes decrease (shown in Fig. 8). Resistance of sensors (Fig. 2b) with 1 wt% and 2% wt% CNT concentrations on

Fig. 5. Resistance changes under pressure for the sensor 2 wt% MWCNT shown in Fig. 2a.

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Sample 0,25 wt% MWCNT 0,5 wt% MWCNT 1 wt% MWCNT 2 wt% MWCNT

measuring layer wasn’t changing more than 9 Ω within the measuring range. For the alternative sensors with two resistive layers (Fig.  2a) only sensors with 2 wt% CNT pasts on measuring areas worked, others being unmeasurable. Samples made of 0,25 wt% CNT and 0,5 wt% CNT  described [18] were measured for force tension changes in range from 400 N to 4 kN, resistance varied  from 1  MΩ to 1 kΩ. Similar characteristics were  obtained for 3,5% of Sakap-6 carbon black in polyesterimide resin [21]. Characteristics observed in  investigated were similar, different measuring range were obtained.

Fig. 6. Resistance changes under pressure on a logarithmic scale for the sensor 2 wt% MWCNT shown in Fig. 2a.

Conclusion Polymer composites containing carbon nanotubes were investigated. Samples with 0,25 to 2 wt% MWCNT

Table 2. Results of mechanical fatigue tests. Numerical results for a layer based on 0.5 wt% MWCNT.

Numbers of Cycles Resistance kOhm/sq

0 11,87

1200 11,86

4500 11,84

19500 11,87

34500 11,86

43500 11,89

61500 11,93

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Daniel Janczak, Grzegorz Wróblewski, Małgorzata Jakubowska, Marcin Słoma, Anna Młoz˙niak

References

Fig. 7. Resistance changes under pressure for the sensor 0,25 wt% MWCNT shown in Fig. 2b.

Fig. 8. Hysteresis for the sensor 0,25 wt% MWCNT shown in Fig. 2b.

content in composition were prepared. Two types of sensors with pressure-sensitive areas were produced. Investigation shows that screen printed layers with CNT can by used to detect pressure. Better results were observed for sensors with comb electrodes and low content carbon nanotubes. Sensors printed on flexible surfactants can be used not only on flat surfaces. Due to low thickness value they can be used to measure pressure in places hard to access by other means. Low cost technology allows disposable samples production. Sensors a  specially with combo electrodes can by connected to make a sensors matrix. Authors intent to use matrix to visualize 3d maps of pressure input of human foot. It can by helpful in medical research for example discovering and controlling possible growth disorders of children. Additionally sensors can by use in automobile industry to control correct work of breaks, and in security systems to detect and analyze pressures on the floor.

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