All-Printed Differential Temperature Sensor for the Compensation of

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All-printed differential temperature sensor for compensation of bending effects Shawkat Ali, Arshad Hassan, Jinho Bae, Chong Hyun Lee, and Juho Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02885 • Publication Date (Web): 08 Oct 2016 Downloaded from http://pubs.acs.org on October 10, 2016

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All-printed differential temperature sensor for compensation of bending effects Shawkat Ali, Arshad Hassan, Jinho Bae*, Chong Hyun Lee, and Juho Kim Department of Ocean System Engineering, Jeju National University, 102 Jejudaehakro, Jeju 63243, South Korea *

E-mail address: [email protected]

Abstract Since printed resistance temperature detectors (RTDs) is affected by tension and compression of metallic pattern on flexible or curved surfaces, a significant temperature sensing error is occurred in general. Hence, we propose differential temperature sensor (DTS) to compensate the bending effect of the printed RTDs, which is composed of two serially connected similar meander patterns fabricated back-to-back on the polyimide polyethyleneterephthalate (PET) substrate through inkjet printer Dimatix DMP-3000 by using silver nanoparticles. Under mechanical deformation, the resistance of the proposed DTS is not varied significantly on same temperature environment since its patterns vary differentially as one side experience tension while the opposite side experience compression. A single meander pattern of the proposed DTS has a total length of 75 mm and device dimensions of 7×7 mm2. Total resistance variation is observed to be 15.5 Ω against temperature variation from 0 to 100 °C, and the temperature coefficient of

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resistance (TCR) is 1.076×10-3 °C-1. The proposed DTS exhibits no significant resistance change on bendability test down to 2 mm diameter due to mechanical deformation. In addition, it is also used to detect the curvature of a body shape down to 2 mm diameter due to its resistance changes by ±8.22% using a single meander pattern of DTS. The proposed sensor can be applied on curved or flexible surface to measure temperature relative accurate than single meander pattern.

Keywords: Differential temperature sensor, resistance temperature detector, bending effect, curved surface, Ag nanoparticles.

Introduction Recently printed electronics technology has been widely researched due to its various advantages over conventional rigid electronic technology such as low cost, flexibility, rapid, easy, and one step fabrication are the key features of the printed electronics1-6. Among the electronic devices, various sensors for temperature sensing have been studied from many aspects including materials, geometry, fabrication techniques, and substrates7-11. There are two main types of temperature sensors, contact based temperature sensor and non-contact temperature sensors. Contact type of temperature sensors are required to be in physical contact with the object being sensed and use thermal conduction to monitor temperature changes, whereas the non-contact type of temperature sensors use convection and radiation. These sensors can measure the temperature of solids, liquids, or gases over a wide range of temperature. To measure the temperature of a human skin, integrated circuit chips based on contact type temperature sensors were demonstrated11. Even though these sensors give high resolution and accuracy, the


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architecture is complex and rigid. To fabricate this type of sensors, conventional electronics manufacturing systems based on lithography are well precisely controlled, which is capable of reproducing the exact parameters of the device. However, this fabrication process is quite complicated, time consuming and requires various steps control of temperature and pressure, the throughput limitations, lack of batch processing, rigid substrates and use of corrosive chemicals12-17. Apart from the complex fabrication of the electronic chip sensors, many researchers have studied metal-based printed resistance temperature detectors (RTDs) utilizing platinum, gold, and copper materials to achieve higher temperature coefficient of resistance (TCR)12,16-17. However, these materials cannot be processed on flexible substrate as their curing temperature is very high14-15. Comparatively, silver (Ag) material is cheap and commercially available in the form of nanoparticles ink, which can be used for low cost fabrication of flexible temperature sensors through printing techniques. The melting temperature of Ag is compatible with plastic and paper substrates. In flexible electronics, PET substrate is commonly used which have melting temperature around 160 °C. Temperature sensors based on Ag are reported on flexible substrates using printing technologies18-19. However, when a metallic thin film pattern is deposited on a flexible substrate at low temperatures the resistivity varies along the bending diameter because of the compression and tension of the film particles. Under tension conditions the particles move away from each other while in compression they get closer to each other. Due to this physical change in the sensing material, its resistance varies, hence it introduces a measurement error in temperature as the resistance of the sensor represents temperature of the measuring object. The physical deformation put limitations on the RTDs and cannot be used in wearable electronics20. In order to measure accurate temperature of a flexible or curved surface such as in wearable electronics21-22 the sensor should be stable against mechanical stresses. The


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geometrical design could be change in such way to compensate the mechanical deformation of the sensor and make it sensitive only for the temperature of a measuring object.

In this paper, we propose a novel DTS device fabricated through inkjet material printer at room temperature in a single step. The proposed sensor is consisted of two back-to-back printed meander patterns connected in series, which has capability to measure the temperature of flexible or curved surfaces with minimum error as compare to simple RTD. Moreover, the sensor can measure a bending curvature of a body. The proposed sensor has three terminals that enable the sensor to be used in two different modes, differential mode and discrete mode. Terminal 1&3 configures differential mode for temperature reading, whereas terminal 1&2 or 2&3 configures discrete mode for curvature reading. In the differential mode, if the sensor is bent on either sides, one meander pattern experience tension (increase resistance), while the opposite side experiences compression (decrease resistance). In result, the overall resistance remains almost same because the resistance of patterns vary differentially and the values are sum up in series. In the discrete mode, from the resistance value of single side pattern, bending and curvature of a body can be measured. If the resistance of side A decreases and the opposite side B increases, this shows that side A experiences compression and opposite side experiences tension which indicates the body is curved at side A. Two similar meander patterns are fabricated on a plastic PET substrate by utilizing silver nanoparticles ink with line width of 300 µm and spacing between lines is 400 µm. To achieve precise patterns, parameters of the printing facility such as drop spacing, drop velocity, and substrate temperature were optimized. The fabricated device was cured at appropriate temperature and characterized for the temperature and strain sensing in straight and curved positions. Morphological characterizations were carried out through scanning electronic


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microscopy (SEM) and optical microscope. In this paper, experimental procedure is given in section “Fabrication”, experimental results are discussed in section “Discussion and verification”, and conclusion is summarized in last section.

Fabrication Schematic diagram of the meander pattern for the proposed differential sensor was designed in ACE 3000 version 7. To analyze the bending and temperature sensing effect of single meander pattern, four different patterns were designed. As the sensor’s geometry shown in Figure 1, to begin with, we have fabricated a single sided meander patterns to analyze temperature and bending characteristics, and then we have fabricated the DTS to study the measuring error from mechanical deformation effects of the flexible surface. In all cases, the track width is 300 µm, spacing between lines is 400 µm, and length of the patterns are 75, 150, 225, and 300 mm. These designs were exported to ACE 3000 software in drawing exchange format that contains all geometrical dimensions for the design of the sensor. The file was converted to bitmap image file format by using ACE 3000 software and then we exported the bitmap file in the Dimatix Drop Manager (software to control Dimatix Printer) which converted the bitmap file into its compatible format. Silver nanoparticles ink 50 wt%, particle size 115 nm, surface 24 dyn/cm and viscosity 24 cp (purchased from Sigma Aldrich) was loaded in the cartridge containing 16 nozzles. Prior to begin printing, the substrate was pretreated with ethanol and distilled water for 5 min each. Silver nanoparticle ink was printed with a commercial print head with 10 pL drop size. To overcome the coffee ring effect, the printhead temperature was set to 25 °C and the stage was set to 30 °C. The coffee ring effect occurs when the substrate temperature is not appropriate 23. Afterward, the samples were cured at 130 °C for 30 min. 5 ACS Paragon Plus Environment

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Figure 1. Layout diagram of the single sided meander pattern temperature sensor with dimensions.


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The fabricated temperature sensors on the plastic substrate are shown in Figure 2a, and Figure 2b shows the zoomed image of the single sensor. Microscopic image of the silver line is shown in Figure 2c, and it can be seen that the silver ink is deposited uniformly on the PET substrate. For the high-resolution morphology, SEM analysis was carried out through Jeol JSM-7600F as a high resolution image is shown in Figure 2d. Small porous areas were found in the silver line, and these increase the overall resistance of the silver line and provide a high resistance variation under bending. In other words, the resolution of the strain sensor is increased because of a porous film. To configure two meander patterns in DTS form, two back-to-back meander patterns were precisely fabricated on a PET substrate. Individual patterns were cured at 130 °C at hot plate and then patterns were connected in series by using silver epoxy and connecting wires. Two ends were combined to make series connection of the patterns whereas the other ends of the patterns were connected to connecting wires for the external interface connections. Silver epoxy was used to connect wires to the meander pattern to avoid contact resistance problem as the meander pattern is the silver material, hence both epoxy and meander pattern has the same Fermi level. One connection was taken from the common point of the meander patterns and other two connections were taken from the other ends of the patterns. In this way, the DTS became three terminals device. After connecting wires to the terminals of the DTS, it was encapsulated in scotch tape to avoid electrical interference of measuring body with DTS.


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Figure 2. (a) Fabricated temperature sensors on the PET substrate through inkjet material printer. (b) Zoomed image of the single sensor device. (c) Zoomed image of the silver trace. (d) SEM image of the silver line.

Discussion and verification Resistance temperature detector (RTD) is the contact based temperature sensors that changes its resistance along the change in temperature24. The energy of the atoms increases with the rise of


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temperature in metallic body. Hence, the atoms vibrate and there are a collision of moving electrons in the conduction band. These collusions result in zigzag flow of electrons and increase the resistance. This variation in resistance caused by temperature change is used to detect the temperature. The advantages of RTD type temperature sensors are, small size, high accuracy, short response time and simple architecture16. The TCR can be calculated by the following equation25.

=

− 1 ∆

Here, ∆T = − is change in temperature of the sensor, Ta is initial temperature at 0 ºC of the sensor, Tb is current temperature of the sensor, Ra is initial resistance of the sensor at 0 ºC and Rb is current resistance at a particular temperature. The sensitivity of the temperature sensor is calculated by

=

∆ 2

where, Ssens is sensitivity and ∆ = − . The variation in resistance with respect to temperature was observed by using our proposed sensor for different sizes of patterns. For this test, 4 different size sensors were fabricated having length of 75 mm, 150 mm, 225 mm, and 300 mm for sensor 1, sensor 2, sensor 3, and sensor 4, respectively. To get the average performance and find the variability between them, 8 samples for each design were fabricated and measured. These sensors were analyzed for their electrical characteristics by using Agilent Semiconductor Analyzer B5100 coupled with Probe Station MST8000C. The probe station contains a hot chuck with temperature controller (Hanyoung NP100). These sensors were placed on variable temperature chuck one by one to measure the change in resistance with respect to temperature. 9 ACS Paragon Plus Environment

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For the measurement, the terminals of the sensor were attached to probes, and then temperature was increased with incremental step of 10 °C ± 1% accuracy. To obtain the change in the resistance of the sensor, the temperature is increasing from 0 to 130 °C. For the lower temperature values from 0 to 20 °C, refrigerator was used, whereas hot chuck from 20 to 130 °C was used inside the probe station. It was observed that the sensor resistance has a linear relationship with the temperature as the curve is following a linear path from 0 °C until 100 °C with CTR of 1.111 x 10-3 °C-1. After 100 °C, the TCR changed and the relationship was no more linear as shown in Figure 3a. Sensor 2 showed a liner behavior from 0 °C until 95 °C with TCR of 1.112 x 10-3 °C-1 as shown in Figure 3b, and sensor 3 showed a linear region from 0 °C until 90 °C with TCR of 1.113 x 10-3 °C-1, and sensor 4 showed linear region from 0 °C until 86 °C with TCR of 1.116 x 10-3 °C-1 as shown in Figure 3c-d, respectively. These results show a trend between linearity and length of the sensor, as the length increases the linear region decreases accordingly. The linear region is reduced as length of the meander pattern is increased, and one possible reason is that the TCR get changed after a certain temperature due to grain boundaries26. Another possible reason of the temperature saturation could be that, the proposed sensor is composed of porous Ag nanoparticles as contrary to bulk silver, when atoms vibrate due to temperature they raise the resistance of the pattern. Because of the porous and non-homogenous nature of the inkjet print pattern the sensor reaches it resistance limitation at lower temperature as compare to bulk silver. This temperature saturation is directly proportional to the length of the pattern as in long length pattern the resistance is high. At lower temperature, it can enter in the saturation region which can be seen in Figure 3. The TCR was observed 1.138 x 10-3 °C-1 by using equation 1. Resistance values of all four sensors were measured for 100 times and their average values were used for the resistivity calculation by using equation 1. The sensitivity


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curves are shown in Figure 4. Here, the resistance value was recorded at 10 °C step for the temperature span of 0 to 80 °C as this region is linear for all the sensors. In all measured datum, the standard deviations were varied in between minimum 0 Ω and maximum 1.375 Ω.

Figure 3. Resistance variation against temperature of four types of sensors having length, (a) 75 mm, (b) 150 mm, (c) 225 mm, and (d) 300 mm (The linear range is inversely proportional to the length of the sensor.).


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Figure 4. Sensitivity vs temperature curve of the sensor 1, sensor 2, sensor 3, and sensor 4 for the temperature span from 0 to 80 °C.

The bending characterization of the proposed sensor was carried out at 25 °C and 30% relative humidity (RH) by using a homemade bending machine as shown in Figure 5a, the upper left inset shows the image of sensor that is under test for the bending characterization. Sensor was bent from 8 mm to 2 mm diameter and the resistance value was recorded, and it was observed that the resistance changes along the bending diameter. When the sensor is bent outward, the gap between Ag particles increases, whereas the gap between Ag particles decrease when it is bent inward. Here, the gaps between Ag particles changes the resistivity of the film as shown in 12 ACS Paragon Plus Environment

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Figure 5b. This characterization was applied on four different sensors sensor 1, sensor 2, sensor 3, and sensor 4. The sensors showed the variations in resistance under bending diameter of 5 mm. For all the sensor, we have normalized the resistance by using formula (Rb-Ra)/(Rmax-Ra), where Rmax is resistance value of the sensor at 100 ºC. Sensor 1 showed actual resistance change from 73.5 to 77 Ω, and the normalized values are shown in Figure 5c. Sensor 2 showed change in resistance from 139 to 152 Ω, and the normalized values are shown in Figure 5d. Sensor 3 showed change in resistance from 243 to 290 Ω, and sensor 4 showed resistance variation from 348 to 457 Ω, and their normalized resistance values are shown in Figure 5e and Figure 5f, respectively. If the total length of the sensor is increased, the sensitivity of the sensor also is increased. By using a single meander pattern, it is difficult to measure the temperature accurately of a flexible or curved surface because of mechanical deformation interference in the resistance. From all measured datum for 100 times at each points, we obtained the standard deviations were varied in between minimum 0 and maximum 0.0325. From these results, the single meander pattern based sensor is suitable for rigid and straight surfaces temperature measurement not for flexible surfaces.


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Figure 5. Strain sensing characterization of the single meander pattern. (a) Homemade bending machine. (b) Compression and tension phenomenon of the silver film. (c) Strain sensor with total wire length 75 mm, (d) 150 mm, (e) 225 mm, and (f) 300 mm.


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To overcome this problem, we proposed the DTS as the layout diagram shown Figure 6a. It is a back-to-back printed sensor on a flexible PET substrate with the same pattern and size. As the connection diagram shown in Figure 6b, both side patterns are connected in series. For the temperature sensing, terminals 1 and 3 are used, and terminals 1 and 2 or 2 and 3 are used for the strain measurement. As the equivalent schematic diagram shown in Figure 6c, two resistors RA and RB are representing the sensors on side A and side B. If the DTS is bent on side A, the resistance of meander pattern A will decrease because of compression while on side B the resistance will increase because of tension. Bending the differential sensor on either sides, the total resistance of the sensor remains stable under mechanical deformation as shown in Figure 6d due to the resistances RA and RB vary differentially. The resistance remains unchanged as one resistor observes -∆R while the other side resistance observe ∆R, hence both sides cancel out their change in resistance when they are in serial configuration. The total resistance of the DTS is changed by the temperature without the effect of the mechanical deformation. This differential resistance variation suggests that the back-to-back printed sensor can be installed on a curved, circular or flexible surface to measure the temperature. To demonstrate the proposed DTS for flexible surface temperature measurement applications, we have carried out the experiment by using two similar digital multi-meters model LG Precision DM-313. The DTS bending axis and bending directions are shown in the diagram given in the inset of Figure 6d. First, we connected one ohmmeter across a meander pattern of DTS and record the value of resistance while the sensor was bent on 4 mm diameter rod. Then we connected the second ohmmeter across the other meander pattern of the DTS, and there were no change in the previous ohmmeter readings after connecting the second ohmmeter. This result shows that there is no interference between the ohmmeters. One reason of no interference could be the same ohmmeters, and their same


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impedance and loading effect for each other’s. To analyze the bendability of the DTS, one ohmmeter was connected with terminals 1 and 2 and another ohmmeter was connected with terminals 2 and 3 of the DTS. The DTS was bend from 8 mm down to 2 mm on a plastic rod. It was observed that the resistance was drastically increased from 73 to 78 Ω on second ohmmeter for (RA+∆R), and from 73 down to 65 Ω on first ohmmeter for (RA - ∆R) as shown in Figure 6d. The ∆R is the change in resistance due to the bending effect of the sensor over the surface. By bending the sensor on either side, it observes change in resistance for RA and RB differentially. By measuring the resistance of one side sensor pattern, the curvature of the implanted body was predicted, as the bending axis and direction diagram is shown in the inset of Figure 6d. During the experiment sensor was bend on side A by using 2 mm rod, the resistance of side B was increased with +∆R of 8.22%, while the resistance of side A was decreased by -∆R of 8.22%. From this result, it shows that the body is bending on side A, and it indicates that the body shape is concave. By using only side A, the resistance is decreased which indicates the body shape is concave, hence the curvature is measured by using one side of the sensor at a time. When the ohmmeter was connected across terminals 1 and 3 of the DTS, a nominal changed (below 1 Ω) was observed against bendability of 2 mm diameter as shown in Figure 6d. To observe the temperature sensing behavior under bending conditions, the DTS was bend on 2 mm rod and temperature was increased from 0 to 100 °C with 10 °C step as shown in Figure 6e. The resistance linearly increased from 144 to 158 Ω, total resistance increased by 15.5 Ω with ±1 Ω. Sensitivity was calculated from the average resistance values against temperature and plotted in Figure 6e.


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Figure 6. (a) Layout diagram of the DTS showing two meander patterns on PET substrate backto-back, the inset shows cross sectional view of the DTS. (b) Connection diagram of the DTS. (c) Electrical equivalent schematic diagram of the DTS consisted of RA and RB. (d) Bending analysis of the DTS, only sensor A, only sensor B and differential sensor, that drastically reduces the mechanical deformation effect. (e) Resistance vs temperature analysis of the DTS at 2 mm diameter. (f) Resistivity of the DTS. 17 ACS Paragon Plus Environment

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To ensure the bending reliability of the DTS for wearable electronics, it was bent over 5 mm diameter for more than 300 cycles and at each cycle the resistance was measured. The sensor did not show any change in resistance value as shown in Figure 7 with solid line. When the sensor was bent over 2 mm diameter for 300 cycles, a negligible change of 1 to 2 Ω (as shown by solid red circles in Figure 7) in sensor’s resistance was observed. However, the overall behavior of the DTS remained stable under deformation. These results suggest that the proposed DTS can be used on flexible and curved surfaces for the measurement of the temperature. To demonstrate the practical application, we have installed the DTS on electric motor as shown in Figure 8a, and resistance against temperature was recorded. The sensor was installed on a curved electric motor body and run for 30 min. As the resistance of the sensor and temperature curve shown in Figure 8b, it can be seen that the sensor behavior is linear as compare to the straight position measurements of the sensor. In this case, the proposed DTS can measure the temperature variations without affected by the body shape of the motor body. On the motor body, the resistance RA = 73.35 Ω and RB = 74.4 Ω were measured, the bending shape was estimated as concave because RA is decreased and RB is increased.


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Figure 7. Bending endurance test of 300 cycles, the sensor in differential mode was bent on 5 mm and 2 mm diameter by using homemade bending machine.

Figure 8. (a) Temperature differential sensor is installed on a curved electric motor body to measure the temperature. (b) The response of the DTS sensor over motor body and on straight surface for the temperature span of 55 °C, at RH 30%. 19 ACS Paragon Plus Environment

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Conclusion We have successfully demonstrated the differential temperature sensor, which could measure temperature as well as strain of a flat, flexible, and curved surface. The Dimatix material inkjet printer DMP-3000 was used to fabricate the silver nanoparticles based back-to-back meander patterns on the PET substrate at ambient conditions. The DTS has reduced the temperature error caused by the mechanical deformation of the silver pattern. The resistance of the DTS sensor varies with temperature regardless of its bending diameter. The TCR of the temperature sensor was observed to be 1.076 x 10-3 °C-1. The sensor showed strain-sensing behavior under bendability from 8 mm down to 2 mm by the single meander pattern, and the resistance changed by 8.22%. The DTS showed reliability for more than 300 endurance cycles at 2 mm and 5 mm diameters. These characterizations can be good basis to develop a sensor for the temperature and bending sensing in wearable electronics.

Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2016R1A2B4015627).

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Table of contents

Figure 9. Layout diagram of the single sided meander pattern temperature sensor with dimensions.


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Figure 10. (a) Fabricated temperature sensors on a PET substrate through inkjet material printer. (b) Zoomed image of the single sensor device. (c) Zoomed image of the silver trace. (d) SEM image of the silver line.


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Figure 11. Resistance variation against temperature of four types of sensors having length, (a) 75 mm, (b) 150 mm, (c) 225 mm, and (d) 300mm (The linear range is inversely proportional to the length of the sensor.).


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Figure 12. Sensitivity vs temperature curve of the sensor 1, sensor 2, sensor 3, and sensor 4 for the temperature span from 0 to 80 °C.


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Figure 13. Strain sensing characterization of the single meander pattern. (a) Homemade bending machine. (b) Compression and tension phenomenon of the silver film. (c) Strain sensor with total wire length 75 mm, (d) 150 mm, (e) 225 mm, and (f) 300 mm.


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Figure 14. (a) Layout diagram of the DTS showing two meander patterns on PET substrate backto-back, the inset shows cross sectional view of the DTS. (b) Connection diagram of the DTS. (c) Electrical equivalent schematic diagram of the DTS consisted of RA and RB. (d) Bending analysis of the DTS, only sensor A, only sensor B and differential sensor, that drastically reduces the mechanical deformation effect. (e) Resistance vs temperature analysis of the DTS at 2 mm diameter. (f) Resistivity of the DTS. 31 ACS Paragon Plus Environment

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Figure 15. Bending endurance test of 300 cycles, the sensor in differential mode was bent on 5 mm and 2 mm diameter by using homemade bending machine.


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Figure 16. (a) Temperature differential sensor is installed on a curved electric motor body to measure the temperature. (b) The response of the DTS sensor over motor body and on straight surface for the temperature span of 55 °C, at RH 30%.


All-Printed Differential Temperature Sensor for the Compensation of

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