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Monitoring of the production process and the force sensing quality of a Reduced Graphene Oxide (rGO) sensor
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1  Swinburne University of Technology
Academic Editor: Alberto Vallan


The advantage of Graphene oxide (GO) is its hydrophilicity in contrast to Graphene’s hydrophobicity, and aqueous dispersions of GO are stable due to the absorption of water. This property makes GO ideal for producing strain and force sensors. To monitor the production of a graphene sensor, we painted 2 silver electrodes on a wooden substrate, connected these electrodes to a voltage divider with a reference resistor, and continuously measured the drop voltage across the reference resistor. The signal of the open circuit was recorded at 2 Hz before painting the GO solution across the electrodes, thereby closing the circuit. As GO is an insulator, ionic impurities of the aqueous solution and possibly the negatively charged Graphene oxide sheets provided the initial conductivity. Closing the circuit increased the conductance from 0 to 5-20 microS. Heating the fluid GO dispersion with an air gun increased the conductance by a further 2-5 microS until the water started to evaporate, and the conductivity dropped to 0 when a dry layer of GO was left. This process of painting, heating and drying was repeated twice, and after painting the third time, the GO was left to air-dry for approximately 2 h. Subsequently, the temperature of the heat gun was pre-set to 350°C , and GO was reduced to graphene (rGO). During the reduction process, the conductivity spiked to 0.8-1 S within 0.5 s, but dropped immediately through a power decay with an exponent of –0.24. After 15 minutes, the signal stabilised at 90 microS, and we subjected the wooden substrate to a three-point bending test with the sensor on the compression side. A force with a triangular wave profile was applied with a frequency of 0.03 Hz, and the loading, unloading, and dwell (zero force) segments of 11, 10, and 12 s, respectively, with maximum force of 200 N. The measured voltage sampling frequency across the reference resistor was increased to 20 Hz. The calculated conductance of the sensor and the force were aligned and plotted against each other. A 4th order polynomial was fitted to the data to obtain a calibration curve (R2 = 0.9974). The force was recalculated from the calibration curve, resulting in a good match of the original force and the recalculated one, resulting in an RMSE of 3.4 N. The RMSE% was < 10% at forces > 50 N, and < 5% at forces > 75 N. Alternatively, instead of the force accuracy, the time accuracy can be calculated. Although the hysteresis was negligible, the calculated force lagged the original by 0.25 s at small forces, decreasing linearly to 0 s at the peak force (0.25 s corresponds to 1.2% of the loading-unloading cycle, and 0.76% of the total cycle including the dwell segment). The force and time accuracy data proved that the sensor is sufficiently accurate. The calibration of the sensor served for evaluation of the quality of the electrical signal, in terms of the signal (drift, noise) and the calibration curve (range of conductance, hysteresis, degree of non-linearity, ease of curve fitting), thereby providing vital information on the sensitivity and measurement range (accuracy, saturation point). From these data, the purpose and the application of the sensor will be selected.

Keywords: graphene; graphene oxide; force sensor; sensor production; sensor quality