Iron Doped Zinc Oxide Thin Films Based Thermoelectric Temperature Sensors for Industrial Applications
Authors: Sharma Anjali , Kumar Sujit , Gupta Vinay , Tomar Monika 
Published: May 2020

Summary In the present work, thermoelectric properties of pulsed laser deposited (PLD) Iron doped Zinc Oxide (Fe:ZnO) thin films of varying thickness have been utilized for the fabrication of temperature sensor. An optimum thickness of 380 nm of Fe:ZnO thin film showed the maximum values of Seebeck coefficient, figure of merit and power factor of 94.6x10-3 mV/K, 2.7x10-3 and of 8.9x10-3 W/K2-m respectively at room temperature. These results indicate use of thermoelectric (Fe:ZnO) thin films as temperature sensor for the industrial applications.

Introduction

In many industrial applications continuous monitoring of temperature is needed. Typical temperature sensors are based on thermocouples or resistance elements made up of metals. However, these are not always suitable for many applications such as temperature sensors for liquids or gases in pipelines. Standard sensors inside such a fluid flow have an influence on the flow itself or hinder a cleaning of the pipeline system. Thus, novel thermoelectric temperature sensor based on metal oxide have been fabricated. Research efforts have been focused to determine best possible thermoelectric material to achieve highly efficient temperature sensor. The factors that contribute to find better thermoelectric materials include light weight, low noise or vibration and environment friendliness in addition to higher figure of merit. Thermoelectric materials can be in the form of bulk or thin film. Thin films are, however, expected to have lower thermal conductivity values as compared to the bulk materials due to the presence of strong phonon scattering at their interfaces. Thermoelectric thin films demonstrate improvement in thermoelectric efficiency and the possibility of their applications in flexible microelectronic devices.

Experimental

Fe doped ZnO (Fe:ZnO) thin films were deposited over corning glass substrates by Pulsed Laser Deposition (PLD) technique using ceramic Fe doped ZnO target. Fe:ZnO target was prepared by adding Fe2O3 (5%) to the Zinc oxide powder using solid state reaction method. A high power Excimer Laser (energy =100mJ), operating at a wavelength, λ=248 nm has been used for ablating the targets. Deposition of thin films was carried out in 100% O2 environment at 20 mTorr pressure. Thickness of Fe:ZnO thin films was varied from 200 nm to 480 nm. Thermoelectric measurements of the prepared thin films were carried out an indigenously developed set-up. The two ends of the sample were kept at different temperatures and the temperature was measured using thermocouples. After maintaining the temperature gradient at ends of the sample the voltage generated was measured using the precision digital multimeter. The measured voltage could be used for the calibration of temperature measurement.

Results and Discussion

Figure 1 shows the variation in thermoelectric voltage generated as a function of temperature gradient at the ends of the Fe:ZnO thin films. It can be observed from fig. 1 that with the increase in the temperature gradient from 5K to 30K, the induced thermoelectric voltage is found to be increasing from 0.1 mV to 8 mV in Fe:ZnO thin films. It is also observed that with increase in thickness from 200 nm to 380nm the thermoelectric voltage is found to be increasing upto 8 mV. However, with further increase in thickness to 480 nm the voltage generated is decreasing. . An optimum thickness of 380 nm of Fe:ZnO thin film showed the maximum values of Seebeck coefficient, figure of merit and power factor of 94.6x10-3 mV/K, 2.7x10-3 and of 8.9x10-3 W/K2-m respectively at room temperature. The obtained high value of Seebeck coefficient indicates the sensor could be calibrated to give measurements of temperature with great precision. These results indicate use of thermoelectric material (Fe:ZnO) thin films as temperature sensor for industrial applications.

Conclusion

The thermoelectric studies of Fe:ZnO thin films have been exploited for the development of temperature sensor. Thickness of Fe:ZnO thin films has been varied from 200 nm to 480 nm. An optimum thickness of 380 nm of Fe:ZnO thin film showed the maximum values of Seebeck coefficient, figure of merit and power factor of 94.6x10-3 mV/K, 2.7x10-3 and of 8.9x10-3 W/K2-m respectively at room temperature. Thus the thermoelectric voltage generated in the prepared Fe:ZnO thin film could be calibrated for the development of temperature sensor.

Figure 1