3.1. Pt/PA Wrapped Yarns
Four types of covered yarns with 1000 twist/m were finally obtained.
Figure 4 shows the scanning electron microscope (SEM) images of all the samples from pure metal fibers to the different wrapped yarns. The free-standing Pt metal fibers (Young’s modulus: 160GPa, Poisson’s ratio: 0.38), with two different diameters of 20 μm (
Figure 4a) and 30 μm (
Figure 4d), respectively, have a smooth surface and circular-like cross-sections. The electrical resistance is 3.79 Ω·cm
−1 and 1.70 Ω·cm
−1; the tensile strength is 20.26 cN and 27.40 cN; the maximum strain is 1.04% and 0.90% for both two Pt fibers. Both were covered by different PA filaments, i.e., PA
F and textured PA (PA
D) yarns. From their SEM pictures, the Pt metal core is almost completely covered by two layers of PA filaments and the yarn twist is approximately uniform. The samples with PA
D filaments have an average diameter of 220 μm, which is a little larger than those with PA
F filaments, whose mean diameter is 190 μm. This can also be confirmed from their cross-sectional views, where the wrapped yarns with PA
D filaments have a looser structure while the core is more tightly packed in those with PA
F layers. In addition, observed from both axial and cross-sectional views, the core metal fiber has a spiral path in the whole structure, and a small segment is exposed on the surface of the yarn, in particular, with the PA
F filaments as the outer layer. This phenomenon is intentionally produced by adjusting the yarn tension in the fabrication process for the increment of the tensile strain of the metal fiber.
To examine whether the mechanical performance of the pure Pt fibers has been enhanced by being covered with PA filaments, samples of wrapped yarns underwent a tensile test as elaborated above. The observed load-strain-resistance relations of all the samples are plotted in
Figure 5, from which the critical force and strain at electrical failure are identified and summarized in
Table 1. The breaking force of the pure metal fiber with 20 μm diameter is 17.79 (±0.56) cN, dramatically increasing to 47.36 (±2.39) cN and 46.58 (±2.33) cN at electrical failure after being wrapped by PA
F and PA
D filaments, respectively. An increase of critical strain is also observed as raised from 1.16% to 2.12% and 2.84%. Similarly, the metal fiber with a diameter of 30 μm has a significant increment in its critical load from 31.31 (±0.75) cN to 88.06 (±2.24) cN and 78.57 (±1.69) cN and corresponding elongation from 1.13% to 2.28% and 2.84% at electrical failure via yarn-covering method by both PA
F and PA
D filaments. Hence, it can be concluded that the yarn-covering technology has had a positive effect on the mechanical performance of the pure metal fibers due to their spiral shape in the wrapped structure. During the whole process, the curved pure metal fiber was straightened first before it could undertake tension. Additionally, when PA
F filaments were used as covered layers, the critical load was 2.69 and 2.81 times of those of Pt fibers with different diameters; their corresponding elongation was 1.82 and 2.02 times of pure Pt fibers. By contrast, when PA
D filaments are taken as the outer layers, the load was 2.62 and 2.53 times; their strain was 2.45 and 2.52 times of both pure metal fibers. The slight discrepancy in mechanical enhancement of the metal fibers may be attributed to the tight and loose structures of the PA filaments in the wrapped yarns, where the later could bear a little larger strain with a smaller force due to their textured filaments.
To see whether the temperature-sensitive performance of the pure metal fiber is influenced by the wrapped structure, the relative resistance variation with temperature was investigated. Both a temperature-controlled oil bath and digital hotplate were used to control the temperature ranging from 30 °C to 50 °C, which is consistent with human body temperature range. The resistance-temperature curves of all the samples are plotted in
Figure 6. It can be observed that for each sample, the electrical resistance raised almost linearly while temperatures increased from 30 °C to 50 °C with either oil bath or hotplate. The coefficients of linearity r
2 were calculated through linear fitting and summarized in
Table 2. All the obtained coefficients of linearity surpassed 0.999, suggesting that the yarn-covering technology has no negative effect on the linearity between the resistance and temperature of the pure Pt fibers. The TCRα, i.e., the slope of resistance-temperature curves of all the samples was calculated and is summarized in
Table 2. All the TCR values generally maintained (with an averaged value of 0.00358 °C
−1), suggesting there is no significant difference in sensitivity between the wrapped yarns and the pure Pt fibers. Those experimental data demonstrate that the temperature-resistivity characteristics of the wrapped yarns remain at the same level as those of the free-standing metal fibers since the mechanism of the temperature sensor is only based on the changes of the metal resistance when subject to the temperature variation. In addition, the slopes of the resistance-temperature curve for both heating and cooling processes were exactly the same, suggesting that there was nearly no hysteresis during the loading-unloading cycle of temperature.
The response time, i.e., the time required for the electrical resistance of the sensor to achieve 63.2% of its final value when subjected to a step change in surrounding temperature, was used to characterize the sensor response [
42]. In this work, the step change of temperature was calculated by moving the samples from air at ∼20 °C into an oil bath or onto a hotplate at around 50 °C. The transient trend of resistance was observed as gradually rising exponentially with temperature, which was continuously collected by Agilent 34970A with a sampling rate of 50 ms
−1.
The response time was identified from the curve of real-time resistance response when the samples were subjected to a step change of temperature. As illustrated in
Figure 7, the electrical resistance of all the samples gradually rose and reached a plateau during this sudden change in temperature. It can be observed from
Table 3 that the typical response time of the free-standing Pt metal fibers is much less than 1 s either in oil-bath or hotplate conditions, while the response time of wrapped yarns became longer. This is because the PA filaments act like a thermal shield, preventing heat transfer to the pure metal fibers, yielding a slower response time with wrapped yarns.
To understand the above results, an ideal serial model of heat transfer (
Figure 8) was considered, where heat flow is vertical to the interface between the two components, i.e., along the direction of thickness.
As shown in above figure, the heat transfers through the PA66 fibers and air gaps to reach the Pt core (
Figure 8a) can be simplified as heat flow across the PA66 layer and equivalent air gap, when subjected to an oil bath and hotplate. Thermal conductivity coefficients and volumes of the corresponding material are denoted as λ and
v, as shown in
Figure 8. According to the equation of overall thermal conductivity coefficient of two pure materials in serial
it easy to see that the function of overall thermal conductivity coefficient λ shown above is an increasing function of λ
i as well as decreasing function of
vi when λ
i < 1. Hence, it can be concluded that the resultant thermal conductivity for wrapped yarns (PA66+Pt) is always lower than pure Pt fibers, which explains a longer response time of wrapped yarns. Moreover, as shown in
Figure 8b,c, the involved components for the wrapped yarns are Pt, PA66 layer, and air. As can be seen from
Table 4, the thermal conductivity coefficient of PA66 is much larger than that of air, and both far less than Pt core [
43]. In this case, the volume ratio of the two(
v2/
v1) in the wrapped structure is the determinant factor of the overall λ. Since the PA
D structure has a bigger bulk volume due to the fluffy curl shape, it keeps more air afterwards and leads to a smaller λ than that of the PA
F wrapped structure.
Table 3 also shows that the response time of the PA
F wrapped structure is slightly shorter than that of the PA
D wrapped structure. Moreover,
Figure 8c also explains that for wrapped yarns subjected to a hotplate, longer response time was observed mainly due to the partial contact between hotplate and PA66 layer.
Moreover, the response rate of the wrapped yarns is much slower in the hotplate condition than that in oil bath, suggesting that the operation conditions contribute a lot to the response time of the sensor. This can be explained by
Figure 9, where it can be seen that in the flowing liquid of heat convection, the yarn surface could make contact with the liquid media, which helps heat transfer easily to the core fiber, yielding a faster response time. While on a hotplate, however, due to the non-planar structure of the wrapped yarn, the wrapped yarn cannot sufficiently contact the flat mounting area, leading to a much slower response. Hence, the response time of the wrapped yarns depends on the working condition under which the sensing element is operating. Exact conditions of the test must first be specified together with the response time constant before applying the wrapped yarns into applications.
3.2. Fabric Temperature Sensors
Easy to see that the wrapped yarns have higher breaking tensile strength (> 40 cN), the same sensitivity, and acceptable response time compared with pure metal fibers, and thus are more suited and easier to incorporate into the woven structure for fabric temperature sensors. Hence, a wrapped yarn was woven into an organized floating pattern into a fabric composed of cotton yarns as both transverse and longitudinal elements using an automatic weaving machine.
Figure 10 shows microscopic images of the samples of the fabric sensors. It can be seen that all the electrodes are almost hidden in the woven fabric with free-standing metal fibers (
Figure 10a,d). A small segment of metal fibers may be exposed when using the wrapped yarn in the fabric, particularly when PA
D filaments were used as covering yarns. Due to the difference in bulk shape between covering yarn and core yarn, a reverse twist in the wrapped yarn would lead to a large discrepancy of PA
D wrapped structure, in which the core yarn is more likely to be uncovered. Since the diameter of the wrapped yarns is much larger than the pure metal fibers, the courses of the metal fiber in the sensitive area are 13 with a spacing of 800 μm and a total length of 170 mm, while the course numbers of the wrapped yarns are 11 with a spacing of 880 μm and a total length of 145 mm either covered by PA
F filaments or PA
D filaments.
As above, the response time of the fabric temperature sensors was also characterized by moving the samples with both sides from air at ∼20 °C into an oil bath around 50 °C. The resistance of the samples was continuously measured by Agilent 34970A at a sampling rate of 50 ms
−1. As illustrated in
Figure 11, the electrical resistances of all the samples gradually rose and reached a plateau during this sudden change of temperature from 20 °C to 50 °C. The typical response time of the fabric sensors with the free-standing Pt metal fibers as well as with wrapped yarns were summarized in
Table 5. It can be observed that for fabric sensors with free-standing Pt fibers, the response time was about 0.7 s in the oil-bath condition, slightly smaller than that of fabric sensors with wrapped yarns, suggesting that the wrapping method is acceptable as it does not significantly change the sensing property of the fabric sensors. This result can be explained by our previous analysis for the wrapped yarns. The silicone oil filled the holes and openings of the fabric, leading to a direct heat transfer between the silicone oil and temperature-sensitive material in the fabric. Hence, for fabrics woven with pure Pt filament, the heat-transfer process was more effective compared to that with wrapped structure, in which heat had to transfer through PA filaments, of which the thermal coefficient is far lower than the Pt fiber. Moreover, there is no obvious difference in the response time for front and back sides of the fabric sensors because the flowing liquid media could permeate and reach the Pt core in wrapped yarns from both sides.
Since the response time of the fabric temperature sensors is also affected by environment and operation condition of the sensors, samples of fabric temperature sensors underwent a sudden change of temperature by moving the samples from air at ∼20 °C onto a hotplate at around 50 °C. The resistance of the samples is continuously collected by Agilent 34970A at a scanning speed of 50 ms
−1. As illustrated in
Figure 12, the electrical resistance of all the samples gradually rose and reached a plateau during this sudden change of temperature. As summarized in
Table 6, the typical response time of all the fabric temperature sensors is much larger than that of those in oil-bath conditions. This is because in the convection liquid, the sensing material, i.e., Pt fiber, could sufficiently contact the liquid media to receive easy heat transfer, and thus a faster response time. On the hotplate, however, due to the non-planar nature of the fabric structure, the curved fabric sensor cannot fully conform to a flat mounting area, leading to a much slower heat transfer and longer response time. The response time of samples with wrapped yarns was generally larger than that with pure Pt fibers, due to the protection effect of the covering yarns. In addition, the response time is much larger when the fabric back contacts the hotplate than with front sides, due to the smaller segment of exposed contact areas of Pt/PA wrapped yarns. As a result, heating transfer efficiency from plate to fabric face is higher than that to fabric back. This explains that when the fabric back made contact with the plate, it took more time to respond than the fabric front. For instance, sample No.6 of the fabric temperature sensor with back side gives a value of 7.56 s as a maximum response time, while when its face contacts the plate it takes only 5.26 s, leading to the largest difference of front and back response time, i.e., 2.3 s.