Two-dimensional transparent Ag/Al metal temperature sensor
V HAJEESH KUMAR1and S SINDHU2,*
1Department of Physics, Amrita School of Arts and Sciences, Amrita Vishwa Vidyapeetham, Amritapuri 690546, India
2Department of Physics, Birla Institute of Technology and Science, Pilani 333013, India
*Author for correspondence (sindhunair@pilani.bits-pilani.ac.in) MS received 4 October 2020; accepted 12 January 2021
Abstract. Room-temperature thermocouple with good sensing properties is getting attracted increasingly because of their low power consumption and remarkable stability. Our study is based on the design and construction of a prototype transparent temperature sensor using thermocouple. 2D temperature thin film sensor was fabricated using magnetic masking. Thermocouples were made with thin-film, which showed a steady and reproducible temperature response. A potential difference was also shown for per degree temperature change. The transparency is obtained by special type of magnetic masking for thin-film process.
Keywords. Thermocouple; transparent temperature sensor; magnetic masking.
1. Introduction
Temperature-sensor devices [1–7] are designed to measure temperature of an object in its hot or cold state. In energy exchange process, temperature is a significant factor that should be taken into account. Thermometers are instruments specifically designed to measure temperature. Contact thermometer is a type of thermometer which should be placed in physical contact with the object or media that is being sensed. Thermocouple used here falls under contact temperature gauges [8]. Thermocouples have two wires made from different metals (either both of them are metal or one is metal and the other is a semiconductor). A junction is created by welding one end of the wire where the temper- ature is measured. A voltage is generated when temperature changes. Here, instead of wire, the sensor is fabricated with thin-films of different metals, such as aluminium (Al) and silver (Ag). The principle behind the thermocouple is See- beck effect which explains the accumulation of a potential differenceDVacross due to the spreading of charge carriers in the DT= Thot -Tcold, due to hot or cold condition. At equilibrium, the internal electric field will generate a gra- dient in the number of charge carriers when there is a movement from the hot to the cold state. The potential difference is regulated by the type of majority charge car- riers [9]. Transmittance is obtained by special type of masking and coating techniques. Many transparent con- ducting electrodes are fabricated for different purposes.
Many researchers [10–12] fabricated flat flexible transpar- ent conducting electrode using a crack template method in which a deposition was done using spray pyrolysis using a crackle precursor (CP). Metal deposition is done by vacuum
evaporation on specially patterned narrow crack to get transparent conducting thin film for thermocouple.
2. Experimental
The transparent thermocouples were fabricated using two types of metals, such as aluminium and silver, deposited on corning glass. The thin-film deposition is done with thermal vapour deposition method. The patterning of the glass plate used to prepare transparent electrode was done by magnetic mask-making technique [13] which is an elegant form of lithography. Figure 1a and b shows the arrangement of masking. A permanent bar magnet used is kept below the corning glass cleaned with a cleaning agent prepared by mixing 20 ml chromic acid and 300 ml of sulphuric acid which was subsequently cleaned with soap solution, isopropyl alcohol and distilled water. Plasma cleaning is also carried out to provide good adhesion for the film. The glass slide is masked with an aluminium plate having an L-shaped cavity and the iron filing is poured on this cavity. Figures 2 and3 show the design of the thermocouple. One leg of the thermocouple is alu- minium and another leg is silver and both are overlapped in the junction.
Here, the masking is done on the substrate (corning glass) by using a magnet, aluminium thin plate and iron filings.
The substrate is placed above the magnet and an aluminium plate having an L-shaped cavity is placed on the substrate by Kapton tape. The area of cross-section of junction is 9 mm921 mm and the dimension of legs are 9 mm955 mm [14]. Twenty-micron iron powder was sprinkled on the https://doi.org/10.1007/s12034-021-02442-1
0.005, 0.01, 0.015 and 0.02 g aluminium and silver with different heights of 10, 12, 14 and 16 cm at 2910-5mbar pressure. Aluminium is coated in the first leg of the ther- mocouple and subsequently silver, to get the thermocouple design. From this experiment, the optimized parameters for the thermocouple were found to be 0.01 g of aluminium and silver at a height of 14 cm, shown better results. The deposited sample was kept in vacuum chamber for 15 min for settling and condensing. The iron filing can be removed by taking the magnet away which will ensure continuous network like patterning.
3. Characterization of the metal film
The deposited network on film observed under microscope for understanding the continuous connectivity.
The continuous connection is very important for current to reach all over the sample. Figures4 and5 show micro- scopic images of Ag and Al films, respectively.
SEM image of the film is taken to study surface property.
Figures6and7show the SEM images of Al and Ag films, respectively, which show that there are no pores in the surface and possess good continuity. It shows that most of the area in the film is vacant which gives 100%
transmittance.
Figure 1. (a) Patterned substrate. (b) Sketch of magnet masking.
Figure 2. Masked structure.
Figure 3. Sketch of thermocouple. Figure 4. Microscopic image of Ag film.
The X-ray diffraction (XRD) result confirmed the film formation of aluminium and silver. Figure8shows the XRD pattern of silver.
The XRD results confirmed that the materials used here are aluminium and silver. XRD of silver film is coated in glass substrate. The four distinct diffraction peaks of the 2h values of 38.18, 44.37, 77.36, 81.62°can be assigned to the plane of (111), (200), (311), (222), respectively, indicates that the silver used is face centred cubic and is crystalline in nature [15]. The higher peak intensity of the plane Ag(111) shows that the Ag film has lowest surface energy in this plane [16]. The presence of amorphous substrate is domi- nated in the XRD because most of the area in the coated film is vacant. The XRD of aluminium in figure 9 shows that the polycrystalline nature of aluminium [16]. XRD patterns exhibited diffraction peaks at 38.52, 43.62 and 78.90°indicating pure Al phase.
Figure10shows the optical transmittance of the film. Ten milligrams of silver and aluminium are evaporated to get an Figure 5. Microscopic image of Al film.
Figure 6. SEM image of Al film.
Figure 7. SEM image of Ag film.
Figure 8. XRD pattern of silver film.
20 40 60 80
1000 1500 2000 2500 3000 3500 4000 4500 5000 5500
Intensity (arb.unit)
2Θ (deg)
Aluminium
Al(200)
Figure 9. XRD pattern of aluminium film.
optimum transparency and conductivity for the film. Alu- minium showed nearly 80% transmittance and silver showed 70% in visible range. The silver has maximum absorption at wavelength of 350 nm and this result is comparable to the results of Axelevitch et al [17]. The decay of the maxima is independent of thickness which always lies in the same wavelength [17–19]. Silver film possesses 64% above transmittance in visible range, whereas the aluminium reaches a maximum transmittance of 80% between 410 and 460 nm.
The conductivity of the film is determined by two-probe method. The resistance of silver film is 2.29 X, and for aluminium, it is 48.12 X. Figures 11 and 12 are the I–V characteristics of silver and aluminium films.
3.1 Calibration of sensor
The fabricated transparent temperature sensor is calibrated by using a specially designed device as shown in figure13a and b.
The calibration setup has cold and heat terminals. A resistive heater is used to heat one of the terminals and the other terminal is cooled by circulating cold water. The temperature in the hot and cold terminals is controlled by the temperature controller. Temperature of 20°C was kept in the cold terminal and varies the hot terminal from 30 to 70°C to take the output. The fabricated transparent ther- mocouple is placed between the terminals of the device.
The junction of the thermocouple comes in contact with a Figure 10. Optical transmittance of silver and aluminium films.
Figure 11. I–Vcharacteristics of silver.
Figure 12. I–Vcharacteristics of aluminium.
Figure 13. (a) Sketch of calibration setup. (b) Calibration device.
hot terminal, whereas the other free legs at the cold end.
The output is taken from the free end by connecting with a voltmeter [20]. Figure13b shows the setup for calibration.
The sensor will be placed in the device in such a way that the junction of the thermocouple comes at the hotter region.
The temperature at the hot end was increased from 40 to 70°
with the variac and corresponding voltage generated was measured for five different temperatures. A lower temper- ature range was selected to get good sensitivity. It is observed that the voltage varied from 1.5 to 8 mV for the temperature range of 40 to 70°. Figure 14shows voltage–
temperature characteristics of the thermocouple. Lines A, B, C and D are the voltage–temperature characteristics of five different thermocouples fabricated in the same optimum condition. They provide almost same voltage–temperature characteristic and linearly increased with the temperature.
Data was taken at an interval of 3°from initial temperature.
The aboveV–Tplot shows almost similar behaviour which shows the reproducibility of the transparent thermocouple.
The maximum output obtained is 7.5 mV at 70°C. The error with 5 sets of experiment is calculated is shown in the inset.
4. Conclusion
A good transparent 2-D temperature sensors were fabricated by thin film technology. We also introduced a new type of magnetic masking to achieve better transparency of above
70%. Surface, structural and optical qualities of the thin film have been studied and checked its functionality in temper- ature sensor. The film showed good transparency. The calibration of the sensors was done using a specially designed system. The sensor showed almost linear rela- tionship of voltage with temperature gradient.
References
[1] Chen B, Zhu Y Q, Yi Z, Qin M and Huang Q A 2015Sensors 1529871
[2] Zhu Y Q, Qin M, Huang J, Yi Z and Huang Q A 2016IEEE Sens. J.164300
[3] Green C F, Schaare P N and Bates C N 1983J. Exp. Bot.34 226
[4] Wang H and Mercier P P 2017Sci. Rep.74427 [5] Bentley J P 1984J. Phys. E: Sci. Instrum.17430
[6] Zhao R, Shao G, Li N, Xu C and An L 2016J. Sensors61 [7] Noriega J M, Ramı´rez R, Lo´pez R, Vaca M, Morales J,
Terres Het al2015J. Phys. Conf. Ser.582012029 [8] Mohammed A, Babani S, Sanka A I and Abdullahi N A 2014
Int. J. Mech. Prod. Eng. (IJMPE)32015
[9] Hofmann A I, Kroon R and Mu¨ller C 2019 Handbook of organic materials for electronic and photonic devices 2nd edn (Elsevier: Woodhead Publishing Series in Electronic and Optical Materials) p 429
[10] Gupta R, Rao K D M, Srivastava K, Kumar A, Kiruthika S and Kulkarni G U 2014ACS Appl. Mater. Interfaces613688 [11] Kim Minseok, Ha Dogyeong and Kim Taesung 2015 Nat.
Commun.66247
[12] Won Sejeong, Jung Hyun-June, Kim Dasom, Lee Sang-Hun, Lam DoVan, Kim Hyeon-Donet al2020Carbon158505 [13] Astaneh Sarah Hashemi, Sukotjo Cortino, Takoudis Christos
G and Feinerman Alan 2020 J. Vac. Sci. Technol. B 38 025001
[14] Sonker M K and Dewal M L 2015Def. Sci. J.65385 [15] Shameli K, Ahmad M, Shabanzadeh P, Zamanian A, Sang-
pour P, Abdollahi Yet al2012Int. J. Nanomedicine75603 [16] Hajakbari F and Ensandoust M 2016Acta Phys. Pol. A129
680
[17] Alex A, Gorenstein B and Gady G 2012Phys. Procedia321 [18] Lee C C, Wang D L, Chen C C, Chang J Y, Pong B J, Chi G C et al 2006 Sixth Int. Conf. Solid State Light. 6337 63370E
[19] Manikandan M, Gopal J and Chun S 2016RSC Adv.632405 [20] Parvis M, Grassini S and Barresi A 2012IEEE I2MTC - Int.
Instrum. Meas. Technol. Conf. Proc., p 1994 Figure 14. V–Tcharacteristic of the thermocouple.
