Page 243 - Carbon Nanotube Fibres and Yarns
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Sensors based on CNT yarns   233


              shown in  Fig.  9.10 shows a negative thermoresistive coefficient, that is,
              R decreases with increasing temperature like earlier reports. However, the
              transition behavior was not seen in the R-T curve even up to 300 K as seen
              in Fig. 9.10C. This indicates that some molecular rearrangement and pos-
              sibly tunneling could be happening during the heating and cooling cycles
              that affect the thermoresistivity. Further studies are needed to fully explain
              this behavior.
                 To demonstrate the multifunctional sensing capability of CNT yarn
              in a composite, Kahng et al. studied the strain and temperature response
              of multiwalled carbon nanotube (MWCNT) yarns on a stainless-steel
              test beam [76]. By arranging 4-ply CNT yarns in a Wheatstone bridge
              configuration on a stainless-steel test beam, the variation of strain with
              temperature was simultaneously measured. Rather than resistance, the
              bridge current (represented as a summation of two currents on the paral-
              lel paths on the test beam with each path having two yarns in series) was
              outputted as shown in Fig. 9.11. With CNT fiber strain sensitivities of
              1.39–1.75 mV/V/1000 με at room temperature, and temperature sensi-
                                                   −1
              tivity of the CNT yarn bridge at 91 μA°C , the bridge current increased
              with temperature. This increasing trend of the bridge current agrees with
              the negative temperature coefficient resistance of CNT yarns reported
              by other authors [69–73].



                     0.000135

                     0.000130

                   Bridge current (A)  0.000120  Bridge 1
                     0.000125
                                              Bridge 2
                                              Plot 1 Regr

                     0.000115

                     0.000110

                     0.000105
                          −60     −40    −20      0      20      40      60
                                            Temperature (C)
              Fig. 9.11  The bridge current measured from the external temperature monitoring se-
              ries resistor to the bridge over the same temperature range. The electrical current is less
              than 100 μA for each yarn [76]. (Source: S.K. Kahng, T.S. Gates, G.D. Jefferson, Strain and
              temperature sensing properties of multiwalled carbon nanotube yarn composites, in: NASA
              Technical Report, 2008.)
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