Thermoresistive Characterization of Carbon Nanotube Yarn Monofilament Composites for Temperature Sensing

Abstract

Carbon nanotube yarn (CNTY) monofilament composites were investigated for integrated temperature sensing by embedding a single CNTY in a vinyl ester resin (VER) and measuring the electrical resistance change by tapping into the thermoresistive response of the CNTY. The effect of curing condition on the thermoresistive response was evaluated using dwell tests and repeated heating–cooling cycles, comparing specimens cured at room temperature (RT) with those post-cured at 140 °C for 1 h. RT-cured CNTY/VER monofilament composites exhibited electrical resistance drift, with the resistance failing to return to its initial value after each thermal cycle, resulting in a residual resistance change of ~8.85%. In contrast, post-cured (PC) specimens showed a much smaller residual change (−0.08%) after cycle completion. Thermal cycling from RT (~25 °C) to 100 °C produced a nearly linear negative thermoresistive response. The average heating and cooling TCR values were −7.98 × 10⁻⁴ °C⁻¹ and −8.32 × 10⁻⁴ °C⁻¹ for CNTY/VER, and −7.93 × 10⁻⁴ °C⁻¹ and −7.13 × 10⁻⁴ °C⁻¹ for CNTY/VER-PC, respectively. The hysteresis decreased from 21.65% for RT-cured specimens to 12.49% after post-curing, accompanied by improved linearity. The influence of heating rate on TCR was also examined for both freestanding CNTYs and CNTY/VER monofilament composites. The observed response is attributed to coupled matrix–yarn effects (wetting, resin infiltration, and shrinkage) together with temperature-dependent electron transport across CNT junctions. Finally, CNTY/VER monofilament composites demonstrated the ability to estimate internal temperatures under various thermal programs.

1. Introduction

Carbon nanotube yarns (CNTYs) exhibit excellent thermal, electrical, mechanical, and piezoresistive properties [1,2,3]. The performance of CNTYs is strongly influenced by the structural characteristics of the nanotubes and the yarn fabrication process. In particular, nanotube diameter, wall number, defect density, alignment, purity, and catalyst residues can affect the electrical and thermal transport behavior within the yarn. In addition, yarn-specific factors such as densification, twist, packing density, porosity, and hierarchical fiber architecture strongly influence the electrical conduction pathways and junction resistance between nanotubes and bundles [1,2,3]. Therefore, variations in CNTY synthesis and yarn processing can lead to significant differences in thermoresistive response and sensing performance in CNTY-based composites [1,2,3]. When integrated into thermosetting polymer composites, CNTYs may serve as effective in situ sensing elements for structural health and damage monitoring [3,4,5,6,7,8,9,10,11]. Although their use in flexible electronics [12], artificial muscles [13], flexible displays [14], energy storage [15], and strain sensing [9,13] is well documented, their potential application as temperature sensors, based on their inherent thermoresistivity, remains comparatively underexplored [16,17,18,19]. Thermoresistivity, the temperature-dependent variation in electrical resistance that underlies the operation of commercial thermistors and many other resistive sensing and electronic devices [20,21], is typically quantified through the temperature coefficient of resistance (TCR). In systems where this relationship is linear, sensitivity is quantified via the TCR [22]. Previous studies have highlighted the complex thermal behavior of CNTYs; for instance, Lekawa-Raus et al. [18] observed non-monotonic, nonlinear variations in resistance over a range of cryogenic and sub-zero temperatures (68 K to 273 K). Furthermore, Zhang et al. [2] reported a negative TCR for CNTYs within the range of 210 °C to room temperature (RT = 25 °C). Such negative TCR values are consistent with observations in freestanding CNTYs [23], individual carbon nanotubes [24], and traditional carbon fibers [25]. The negative thermoresistive response of carbon nanotube yarns (CNTYs) is commonly interpreted using thermally activated transport models such as fluctuation-induced tunneling (FIT) and variable-range hopping (VRH) [16,26,27]. As temperature increases, thermally assisted charge transport across tube–tube junctions and disordered CNT networks can produce a quasi-linear decrease in electrical resistance, consistent with the negative TCR behavior reported for many CNT-based assemblies [24]. Balam et al. [22] investigated the cyclic thermoresistive response of freestanding CNTYs and CNTY monofilament composites embedded in a vinyl ester resin under repeated heating–cooling cycles over two temperature ranges (−30 °C to RT and RT to 100 °C). The TCR remained negative for both the freestanding and embedded configurations. However, the magnitude of the TCR for the embedded CNTYs was approximately 30% lower than that of the neat yarns, indicating that the polymer matrix reduces thermoresistive sensitivity, likely by constraining yarn microstructure and junction-level transport within the monofilament composite.

Although the thermoresistive behavior of carbon nanotube yarns (CNTYs) embedded in thermosetting polymers has been reported, the role of the matrix cure state in sensing stability and reliability remains insufficiently understood. This study investigates the effect of post-curing on the thermoresistive response of CNTY/vinyl ester composites under controlled heating–cooling cycles with temperature dwells. By comparing room-temperature-cured and post-cured composites, this work demonstrates that post-curing improves resistance–temperature linearity and reduces thermal-history-dependent drift, identifying the matrix cure state as a key parameter for reliable in situ temperature sensing in CNTY-based composites.

2. Materials and Methods

2.1. Materials

The CNTY used in this study was fabricated by dry spinning from a vertically aligned CNT array at Nanoworld Laboratories (Cincinnati, OH, USA). The diameter, density, angle of twist, and average electrical resistivity of the densified CNTYs were approximately 30 μm, 0.65 g/cm³, 30°, and 1.7 × 10⁻³ Ω·cm, respectively [2,3,11,28]. Figure 1 shows Scanning Electron Microscopy (SEM) images of a continuous CNTY consisting of thousands of twisted multiwalled carbon nanotubes (MWCNTs) and CNT bundles in its cross-section with a diameter of ~30 µm at magnifications of 400× and 2300×, respectively. Derakane Momentum 470-300 (Ashland Global Specialty Chemicals, Covington, KY, USA), containing 33 wt% styrene, was used as the vinyl ester resin (VER). Methyl ethyl ketone peroxide (MEKP, FibreGlast Developments Corporation, Brookville, OH, USA) with a nominal concentration of 0.9 wt.% and cobalt naphthenate (CoNap, Sigma-Aldrich, Saint Louis, MO, USA) with a concentration of 0.6 wt.% were used as the crosslinking initiator and promoter, respectively [29].

2.2. Raman Spectroscopy

Raman spectroscopy was used to probe the vibrational and structural features of the CNT yarns (CNTYs), with particular attention to the D and G bands as metrics of defect density, graphitic ordering, and CNT alignment. Before analysis, CNTY specimens were cut into short segments and mounted on clean glass microscope slides using a minimal amount of adhesive at the ends to secure the yarn while keeping the measurement region unobstructed. Raman measurements were conducted on a Horiba Raman microscope using a 532 nm (2.33 eV) excitation laser, a 50× objective, and an incident laser power of ~5 mW at the sample. Spectra were collected over the range up to 2000 cm⁻¹ with a 10 s integration time and 3 accumulations, and the system was calibrated using the Si band at 520.7 cm⁻¹. To reduce the likelihood of laser-induced heating while maintaining a sufficient signal-to-noise ratio, the laser power was kept at 5 mW throughout. Raman spectra were acquired from selected surface regions of the CNTYs, including mapping over a typical area of 30 µm × 30 µm. This approach enabled consistent assessment of CNTY structural quality and vibrational response relevant to their electrical and thermoelectric performance.

2.3. Fabrication of CNTY Monofilament Composites

Four parallel AWG 426-DFV (~0.2 mm diameter) copper wire electrodes (Vishay Micro Measurements, Wendell, NC, USA) were stitched at the mid-center of a silicon mold (52 mm long, 12.7 mm wide, and 2.5 mm deep) using a needle. Then, a single CNTY was placed longitudinally over the copper wire electrodes at the center of the silicon mold (Figure 2). The CNTYs in all samples were prestressed using a constant hanging mass of 116 mg and attached to the mold using installation tape (Micro Measurements, Wendell, NC, USA). The CNTY end was adhered to the mold with tape (Scotch-Weld LO1000 adhesive, 3M, Saint Paul, MN, USA). Ohmic contact between the CNTY and electrodes was achieved using electric paint (Bare Conductive, London, UK). Then, 29.6 g of VER was mixed with 0.18 g of promoter (CoNap) and 0.27 g of initiator (MEKP) for 3 min. The VER formulation contains 33 wt.% styrene monomers. Next, 1.65 mL of the premixed VER was poured into the mold and cured at room temperature (RT ~25 °C) for 4 h [29]. For each experiment, three identical samples were fabricated, and representative experimental results are described next.

2.4. Experimental Setup for Thermoresistive Characterization

Thermoresistive characterization of CNTY monofilament composites was conducted using three different temperature programs in a digital tube oven (BR-12NT, Zhengzhou Brother Furnace Co., Ltd., Zhengzhou, China). The four-point probe electrical measurement technique was used to determine the electrical resistance (R) of the embedded CNTY by applying a constant current (I) between the outer terminals and measuring the voltage drop (V) from the other two internal wires as shown in Figure 2. The electrical resistance was measured by connecting the copper wire electrodes to an NI (Austin, TX, USA) PXI-4072 impedance–capacitance–resistance (LCR) digital multimeter card, mounted on a PXI-1033 chassis. The temperature ($T$) was measured during the experiment using a K-type thermocouple positioned close to the embedded CNTY . The thermocouple was connected to a NI-9211 thermocouple input module mounted in an NI cDAQ-9178 chassis. Data acquisition was performed at 1 Hz using NI SignalExpress software 2015. The electrical resistance and temperature were simultaneously recorded at 1 Hz using NI SignalExpress software. Figure 3 shows the schematic of the experimental setup.

2.5. Temperature Programs for Thermoresistive Characterization

The thermoresistive response of the CNTY/VER monofilament composites, cured at room temperature, and of the post-cured composites (CNTY/VER-PC) was investigated using different temperature programs to characterize their sensing capability. An incremental-dwell temperature program was performed to investigate the effect of dwell temperatures on the thermoresistive response of the CNTY monofilament composites, at which the temperature of the oven increased from T₀ ~ RT to T₁ = 45 °C using a heating rate of 0.9 °C/min (H₁) and maintained at T₁ for 180 min (D₁). Then, the temperature was ramped to T₂ = 55 °C at 0.9 °C/min (H₂) and maintained for 180 min (D₃). Finally, the temperature was ramped to T₃ = 80 °C at 0.9 °C/min (H₃) and maintained at T₃ for 180 min. The program was ended by cooling down the temperature to RT at a cooling rate of 0.2 °C/min. Figure 4 shows the schematic of the incremental-dwell temperature program.

The thermoresistive response of the CNTY/VER and CNTY/VER.PC monofilament composites were also determined using a cyclic temperature program. The temperature program was conducted using four continuous heating–cooling cycles by heating the samples above room temperature (RT ~ 25 °C) to 100 °C at a rate of 0.9 °C/min and cooling down to RT at a rate of 0.2 °C/min. A heating–cooling program was performed to investigate the monofilament composites’ response during the process of absorption and dissipation of thermal energy. To evaluate the signal noise level, the signal-to-noise ratio (SNR) of the fractional change in electrical resistance was calculated at dwell zones (D1, D2, and D3) according to:

SNR = 10 log10((ΔR/R₀)^Mean/(ΔR/R₀)^SD)

(1)

where (ΔR/R₀)^Mean is the mean value of fractional change in resistance and (ΔR/R₀)^SD is the corresponding standard deviation. The temperature coefficient of resistance (TCR) of the CNTY monofilament composites was quantified by the slope of the fractional change in electrical resistance per unit change in temperature at heating (β_H) and cooling (β_C) zones. β_H and β_C were calculated at heating and cooling zones according to [30,31]:

$${\beta }_{\mathrm{H}}={\displaystyle \frac{{(\Delta R/R_0)}_{heating}}{\Delta T}}$$

(2)

where (ΔR/R₀)_heating corresponds to the fractional change in electrical resistance in ramping zones, and ΔT is the corresponding temperature change.

$${\beta }_{\mathrm{C}}={\displaystyle \frac{{(\Delta R/R_0)}_{cooling}}{\Delta T}}$$

(3)

Three experimental parameters were used to quantify the thermoresistive hysteresis during each cycle: the maximum fractional change in electrical resistance (ΔR/R₀)_max, the maximum temperature change, ΔT_max, and hysteresis (H), which corresponds to the area under the hysteresis loop. (ΔR/R₀)_cooling corresponds to the fractional change in electrical resistance during cooling zones. The residual change in electrical resistance (ΔR/R₀)_res was quantified according to:

(ΔR/R₀)_res = (ΔR/R₀)_f − (ΔR/R₀)₀

(4)

where (ΔR/R₀)₀ corresponds to the fractional change in electrical resistance at the beginning of the experiment and (ΔR/R₀)_f corresponds to the fractional change in electrical resistance at the end of the experiment. The normalized hysteresis (H_N) was quantified according to:

$$H_{\mathrm{N}}={\displaystyle \frac{H}{{\Delta T}_{\max }(\Delta R/R_0{)}_{\max }}}$$

(5)

where H_N is the normalized hysteresis, H is the area under the hysteresis loop, (∆T)_max represents the maximum change in temperature achieved in each cycle, and (∆R/R₀)_max is the maximum fractional change in electrical resistance associated with the temperature change. Figure 5 shows the schematic of the experimental parameters used for characterization of the cyclic thermoresistive response of CNTY monofilament composites.

2.6. Evaluation of CNTY Monofilament Composites as Potential Temperature Sensors

To evaluate the implementation of the CNTY/VER-PC monofilament composites as potential temperature sensors, additional experiments were conducted to compare the response of the CNTY monofilament composites to that obtained using a J-type Iron-Constantan thermocouple (Omega Engineering, Inc., Stamford, CT, USA). The electrical resistance readings of the CNTY monofilament composites throughout a temperature program were converted to temperature values using an average TCR value calculated during heating and cooling cycles. Heating–cooling cycles with different heating and cooling rates were used in this section. The CNTY monofilament composites were subjected to two heating–cooling cycles with random temperature settings. The electrical resistance of the CNTY monofilament composite and temperature were simultaneously measured throughout the experiment. Then, the electrical resistance readings of the CNTY/VER-PC specimens were converted to temperature using a linear function such that:

T_cal = T₀ + (ΔR/R₀)/β_ave

(6)

where $T_{cal}$ is the calculated temperature, $\left(\Delta R/R_0\right)$ is the instantaneous fractional change in the electrical resistance of the embedded CNTY, $T_0$ is the reference room temperature (25 °C), and $R_0$ is the corresponding baseline resistance measured at $T_0$ at the start of the thermal program (or at the beginning of the selected stabilized cycle). At each time point, the instantaneous resistance $R\left(t\right)$ is recorded and the fractional resistance change is computed as $\Delta R/R_0=\left[R\right(t)-R_0]/R_0$. The average temperature coefficient of resistance, ${\beta }_{ave}$ (°C⁻¹), is obtained from the slope of the approximately linear ${\displaystyle \frac{\Delta R}{R_0}}$ temperature relationship over the chosen calibration range (e.g., RT–100 °C) by fitting the heating and/or cooling segments according to Equations (2) and (3) and then averaging the fitted slopes over multiple cycles to reduce noise and cycle-to-cycle variability. Once $R_0$ and ${\beta }_{ave}$ are defined, the instantaneous temperature can be estimated directly from the measured $\Delta R/R_0$ ; because ${\beta }_{ave}$ is negative for CNTY/VER composites, increasing temperature produces a negative $\Delta R/R_0$, and the ratio $(\Delta R/R_0)/{\beta }_{ave}$ yields a positive temperature rise above $T_0$.

2.7. Swelling Characterization

The network structure of the CNTY monofilament composites was evaluated through swelling experiments. The cured CNTY/VER and CNTY/VER-PC specimens were immersed in dichloromethane (ChemPure Chemicals, 99.3%, Baltimore, MD, USA) at approximately 25 °C. For this test, the samples were cut into cuboid pieces of approximately 1 cm. Prior to immersion, the specimens were dried at 80 °C, and their initial dry weight was recorded as W₀. The dried specimens were then immersed in 20 mL of dichloromethane for two weeks at 25 °C. After immersion, the swollen specimens were removed from the solvent and weighed to obtain W_sw. The degree of swelling was calculated for each specimen according to Equation (7) [32].

$$d_{\mathrm{SW}}={\displaystyle \frac{W_{\mathrm{SW}}-W_0}{W_{\mathrm{SW}}}}\times 100\%$$

(7)

where d_sw is the degree of swelling (%), W₀ is the initial dry weight of the sample, and W_sw is the weight of the swollen sample after immersion in dichloromethane.

2.8. Scanning Electron Microscopy

Scanning electron microscopy (SEM) was conducted using a JEOL IT-100 scanning electron microscope manufactured by JEOL Ltd. (Tokyo, Japan) to examine the surface morphology and microstructural features of carbon nanotube yarns (CNTYs). Specimens were cut into ~4 cm segments and mounted on alumina substrates using a carbon-based conductive adhesive. To enhance surface conductivity and mitigate charging, the mounted samples were sputter-coated with a thin gold layer. Imaging was conducted under high-vacuum conditions at an accelerating voltage of 20 kV, using secondary electron (SE) and backscattered electron (BSE) detectors to obtain surface and compositional contrast while minimizing imaging artifacts. Micrographs were acquired at multiple magnifications and at several locations along each yarn to evaluate diameter uniformity, CNT bundle organization, surface texture, and structural heterogeneity. This approach provided representative morphological information relevant to the electrical and thermoresistive behavior of the CNTYs.

3. Results

3.1. Raman Spectroscopy Characterization

Charge transport in CNT yarn networks is governed by sp² graphitic ordering and defect/junction scattering; therefore, Raman spectroscopy was employed to quantify the degree of graphitization and the defect population, which control the thermoresistive and electrical responses of the CNTYs.

The Raman spectrum shown in Figure 6 exhibits distinct D and G bands centered at approximately 1350.2 cm⁻¹ and 1580.2 cm⁻¹, respectively, confirming that the CNTY structure is dominated by sp² graphitic carbon [33]. The extracted linewidths indicate moderate band broadening (FWHM(D) ≈ 58.4 cm⁻¹ and FWHM(G) ≈ 65.5 cm⁻¹), consistent with defect- and disorder-related phonon scattering in the CNT framework. The defect level estimated from peak heights is I_D/I_G = 0.656, while the corresponding integrated area ratio is A_D/A_G = 0.747, indicating a moderate defect contribution when peak broadening is considered.

3.2. Effect of Temperature Dwells on Thermoresistive Response of CNTY Monofilament Composites

Figure 7 shows representative thermoresistive response curves of CNTY/VER monofilament composites subjected to the incremental-dwell temperature program described in Figure 4 (Section 2.5). To assess the effect of post-curing on the thermoresistive response, the electrical response of specimens cured at room temperature (CNTY/VER) was compared with that of specimens post-cured at 140 °C (CNTY/VER-PC). Representative responses for CNTY/VER and CNTY/VER-PC are presented in Figure 7a and Figure 7b, respectively. The fractional change in electrical resistance (ΔR/R₀) increased during the dwell segments for CNTY/VER (1.39% in D1, 5.10% in D2, and 6.60% in D3), whereas ΔR/R₀ decreased during the dwell segments for CNTY/VER-PC (−1.66% in D1, −3.1% in D2, and −4.5% in D3).

To evaluate the level of noise in the electrical response of CNTY monofilament composites during dwell sections (D₁, D₂ and D₃), the signal-to-noise ratio (SNR) of the resistance was calculated at dwell zones according to Equation (1). The average SNR values for both specimens were 30.68 ± 3.8, 32.62 ± 0.6 and 32.48 ± 1.2 for D₁, D₂ and D₃, respectively, which agree with previous results [30,31]. It is observed that the electrical resistance of CNTY/VER monofilament composites increases after the incremental-dwell heating cycles and does not return to its initial value when the specimen is cooled back to RT. The residual fractional change in electrical resistance, $(\Delta R/R_0{)}_{\mathrm{res}}$, was calculated using Equation (4) and yielded a positive value of $(\Delta R/R_0{)}_{\mathrm{res}}=8.85\%$. This resistance increase during the dwell zones may be attributed to changes in the CNT network structure within the CNTY/VER composite induced by the isothermal holds [34,35]. In addition, curing the matrix at RT may produce an incompletely crosslinked network containing unreacted styrene within the VER resin structure [36,37,38,39]. The isothermal dwell temperatures can therefore act as an effective post-curing step that alters the three-dimensional (3D) network structure of the VER resin [36,38]. Similar behavior has been reported for CNTYs embedded in epoxy resin [31]; Rodríguez-Uicab et al. reported a positive residual resistance change of 4.1% for an epoxy resin cured at RT [31]. To investigate the effect of post-curing on the thermoresistive response of CNTY/VER monofilament composites, the same incremental-dwell temperature program was repeated for post-cured specimens (CNTY/VER-PC). RT-cured specimens were post-cured at 140 °C for 1 h to promote full crosslinking of the VER resin [36,38]. Figure 7b shows the thermoresistive response of a post-cured specimen, for which $(\Delta R/R_0{)}_{\mathrm{res}}$ after cooling was −0.08%. Thus, post-curing reduced the residual resistance change from 8.85% to −0.08%, which may be attributed to an increased degree of crosslinking and a more stable final network structure [35,36,38]. The electrical response of the CNTY/VER monofilament composites can be separated into two contributions: irreversible resistance drift and reversible thermoresistive response. The irreversible drift is quantified by the residual fractional resistance change after cooling to room temperature and arises primarily from matrix-related processes, including additional curing or post-curing, relaxation of the vinyl ester network, removal of residual monomer/styrene, and stabilization or rearrangement of CNT bundles within the polymer matrix. In contrast, the reversible thermoresistive response corresponds to the nearly linear decrease in resistance during heating and the increase in resistance during cooling. This reversible response is attributed to the intrinsic negative thermoresistivity of the CNTY and temperature-dependent charge transport across CNT junctions, including tunneling and hopping mechanisms. Consequently, the residual drift reflects permanent or semi-permanent matrix-induced changes, whereas the cyclic resistance variation with temperature represents the reversible sensing response of the embedded CNTY [36,37,38,39].

3.3. Swelling Behavior of Vinyl Ester Resin Matrix

Changes in the degree of crosslinking before and after post-curing can be assessed via equilibrium swelling of the VER resin in methylene chloride (dichloromethane) [37,40]. Accordingly, swelling experiments were conducted for CNTY/VER and CNTY/VER-PC specimens. Dichloromethane was selected because its solubility parameter is close to that of VER [39]. The average degree of swelling, calculated, is 18.1 ± 4.13% for CNTY/VER specimens and decreased to 4.5 ± 1.21% after post-curing, indicating reduced solvent uptake consistent with a highly crosslinked 3D network [39,40,41]. Furthermore, increasing the post-curing temperature to 140 °C can facilitate the removal of residual monomer and styrene from the final network, yielding a more compact structure compared to the RT-cured vinyl ester [38].The calculated degree of swelling for each specimen is presented in

Table 1. Degree of swelling of CNTY/VER-PC and CNTY/VER monofilament composites..

Solvent	CNTY/VER-PC dsw (%)	CNTY/VER dsw (%)
Dichloromethane	4.50 ± 1.20	18.11 ± 4.10

.

Based on the degree of swelling calculated using Equation (7), both composite systems exhibited relatively low solvent uptake, indicating a high degree of crosslinking. However, the CNTY/VER specimens showed a higher degree of swelling than the CNTY/VER-PC specimens, suggesting that the room-temperature-cured resin had a lower crosslinking density. This result is consistent with previously reported swelling behavior for VER systems [34]. A higher crosslinking density restricts polymer chain mobility and reduces the free volume between crosslinks, thereby limiting solvent diffusion into the polymer network [31,42,43]. Therefore, post-curing the VER at an elevated temperature, as in the CNTY/VER-PC specimens, promotes the formation of a denser three-dimensional network structure with reduced free volume, resulting in lower solvent uptake compared with CNTY/VER [20,34,43,44].

3.4. Cyclic Thermoresistive Characterization of CNTY Monofilament Composites

The cyclic thermoresistive responses of CNTY/vinyl ester resin (VER) monofilament composites before and after post-curing are presented in Figure 8. Each specimen was subjected to four continuous heating–cooling cycles between room temperature (RT) and 100 °C. The VER matrix is amorphous and has a glass transition temperature of approximately 165 °C, which is well above the maximum temperature used in this characterization. To minimize the influence of prior thermal history, the first cycle was excluded from the data analysis. Figure 8a shows the thermoresistive behavior of CNTY/VER composites over three consecutive cycles. The temperature coefficient of resistance (TCR) was determined for both the heating and cooling segments according to Equations (2) and (3). The results indicate that the fractional change in electrical resistance decreases with increasing temperature above RT and increases upon cooling back to RT. This negative thermoresistive behavior is consistent with trends previously reported for individual CNTs [42,45,46] and CNT yarns [2,30,32].

The negative thermoresistivity of CNTYs could be explained by quantum mechanics mechanisms, such as fluctuation-induced tunneling (FIT) and variable-range hopping (VRH). It is believed that electron transport occurs via two mechanisms: electron tunneling and electron hopping. Mott and Davis [47] modeled electron hopping using a 3D VRH model, stating that at low temperatures, electrons hop beyond their nearest neighbors to find a state with an energy similar to their own, and that the electrical conductivity between the localized states obeys a T^−1/4 power law. Sheng et al. [14] suggested that fluctuation-electron-tunneling (FIT) occurs in disordered materials. Both mechanisms could be used to explain the thermoresistive behavior of CNTYs by accounting for the increase in the density of states and the number of electron charge carriers as a function of temperature during heating [14,47,48,49,50].

There are different contributing phenomena in the electrical response of the embedded CNTY during cooling. Upon cooling, it is expected that thermal shrinkage reduces the CNT-to-CNT distance, thereby decreasing electrical resistance [51]. Still, since the cooling rate is very low (0.2 °C/min), it may cause the rearrangement of CNT bundles within the yarn during cooling. Another contributing factor could be the intrinsic negative thermoresistivity of the CNTY, which increases tunneling resistance during cooling by separating electric charge carriers [47]. The increasing resistance observed during cooling sections indicates that the intrinsic thermoresistivity of CNTY is the dominant factor.

The average heating and cooling TCR values for CNTY/VER were (−7.98 ± 0.07) × 10⁻⁴ °C⁻¹ and (−8.32 ± 0.41) × 10⁻⁴ °C⁻¹, respectively. The maximum change in electrical resistance (∆R/R₀)_max was −5.94%. The residual change in resistance (∆R/R₀)_res yielded a small value of −0.13%. The normalized hysteresis was calculated according to Equation 5 and reported a value of H_N = 21.65% for CNTY/VER. This value agrees with the normalized hysteresis values reported previously [22,30]. Figure 8b represents the cyclic thermoresistive response of the CNTY monofilament composites after post-curing of the specimens at 140°C (CNTY/VER-PC). It is observed that the linearity in both the heating and cooling sections increases when the specimens are post-cured (average coefficient of determination (r²) of the linear fit was above 0.98 in the heating and cooling zones). Post-curing did not substantially change the heating TCR or the maximum resistance change; however, the cooling TCR and normalized hysteresis were affected, as shown in

Table 2. Thermoresistive parameters calculated from the analyzed heating–cooling cycles for CNTY/VER and CNTY/VER-PC monofilament composites. Values are reported as mean ± standard deviation.

Material	βH × 10−4 [°C−1]	βC × 10−4 [°C−1]	(ΔR/R0)max [%]	(ΔR/R0)res [%]	HN [%]
CNTY/VER	−7.98 ± 0.07	−8.32 ± 0.41	−5.94 ± 0.14	−0.13 ± 0.08	21.65 ± 2.5
CNTY/VER-PC	−7.93 ± 0.21	−7.13 ± 0.06	−5.91 ± 0.07	−0.20 ± 0.09	12.49 ± 1.4

. Therefore, the effect of post-curing was more pronounced in the cooling response and hysteresis behavior than in the heating TCR. However, the normalized hysteresis is reduced to H_N = 12.49% after the post-curing treatment. This value is closer to the hysteresis values reported previously in heating–cooling cycles of freestanding CNTYs from above RT to 100 °C [22]. Table 2 represents the thermoresistive parameters calculated for heating–cooling cycles of both specimens.

Figure 9 shows the cross-sectional SEM image of the CNTY/VER composite at 6660× and 13,300× magnification, respectively. The images show the potential infiltration of the polymer within the outer surface of CNT bundles, influencing the electrical resistance of the embedded CNTY by restricting the rearrangement of CNTs during heating–cooling cycles. Figure 9b shows the CNTY/VER interface, indicating that the CNT bundles closer to the interface of the yarn and resin are denser in comparison with the bundles at the center of the yarn cross-section. The difference between the coefficient of thermal expansion of the polymer and the CNTY during the heating section and viscoelastic relaxation after each cycle could change the contact and tunneling resistance between the CNT bundles by rearranging the CNT bundles within the yarn [22,28,29,30].

3.5. Effect of Heating Rate on Thermoresistive Response of CNTY Monofilament Composites

To investigate the effect of heating rate on the thermoresistive response of CNTY, TCR at heating (β_H) was investigated at different heating rates (Figure 10).

The specimens were heated from RT to 100 °C at 1.6 °C/min, 3.33 °C/min and 10 °C/min heating rates and the corresponding TCR values were calculated according to Equation (2). To investigate the effect of the constraint of the polymeric material on the thermoresistive sensitivity of the CNTY, the TCR of CNTY/VER monofilament composites was compared to that of the individual CNTYs considering the same heating rate. The thermoresistive sensitivity of the CNTY and CNTY/VER-PC monofilament composite is a function of heating rate, and the TCR decreases with increasing heating rate. This behavior has been reported previously for the CNTY embedded in silicone rubbers [30]. The TCR of CNTY/VER decreased by 28% by increasing the heating rate from 1.6 °C/min to 3.33 °C/min and decreased by 36% by increasing the heating rate to 10 °C/min. This could be due to the infiltration of VER within the CNTY bundles, which was evidenced by the SEM image [22]. The infiltrated resin reduced the mobility of the embedded yarn, therefore causing a decrease in thermoresistive sensitivity of the yarn [46]. In addition, the residual stresses due to the differential coefficients of thermal expansion between the yarn and VER affect the thermoresistive sensitivity of the embedded CNTY [43,51]. TCR decreased from −11.7 × 10⁻⁴ °C⁻¹ to −10.71 × 10⁻⁴ °C⁻¹, constituting a decrease of about 10%. It decreased to −9.75 × 10⁻⁴ °C⁻¹ by increasing the heating rate to 10 °C/min which was about a 9% decrease. It is observed that the effect of the heating rate is less evident in the freestanding CNTY in comparison to the embedded CNTY. By increasing the heating rate, the electrons in the conduction band cannot get enough energy to overcome the conduction band [45]. Therefore, the temperature coefficient of resistance decreases by increasing the heating rate.

3.6. Integrated Temperature Sensing

By comparing the temperature of the reference thermocouple (T_ref) with that calculated from the CNTY monofilament composite, it is concluded that the CNTY/VER-PC monofilament composite can accurately reproduce the temperature readings measured by the thermocouple. Practical implementation of the CNTY as an embedded temperature sensor in a post-cured polymeric matrix (CNTY/VER-PC) was evaluated using the incremental-cyclic temperature program described in Section 2.5. Three replicate CNTY/VER-PC monofilament composite specimens were tested. Each specimen was subjected to two successive temperature cycles with non-uniform (variable) heating and cooling rates. The specimens were first heated from RT (~25 °C) to 36 °C at 0.25 °C/min and then cooled to 27 °C at 0.04 °C/min. Next, the specimens were heated to 46 °C at 0.40 °C/min; the oven was then turned off, and the specimens were cooled to RT at 0.20 °C/min. Temperature was measured using a reference thermocouple (T_ref) placed close to the embedded yarn, while the electrical resistance of the CNTY monofilament composites was recorded simultaneously throughout the cycles.

Figure 11a shows the fractional change in electrical resistance (ΔR/R₀) together with the temperature measured by T_ref. During the first cycle, ΔR/R₀ decreased to −0.82% as the temperature increased from RT (~25 °C) to 46 °C and returned to −0.22% upon cooling to 27 °C. During the second cycle, ΔR/R₀ decreased to −1.62% upon heating to 46 °C and reached −0.10% after cooling back to RT. The average residual fractional resistance change, (ΔR/R₀)_res, was −0.11% after cooling back to RT. Figure 11b shows the temperature estimated from the resistance signal using Equation (6) and the average TCR obtained in Section 3.3 (β_ave = −7.53 × 10⁻⁴ °C⁻¹). The instantaneous ΔR/R₀ was converted to temperature and compared with the reference thermocouple (T_ref), showing that the calculated temperature follows the same trend as the thermocouple during both heating and cooling when using the average TCR. The temperature-sensing accuracy was quantified using the mean absolute error (MAE), which represents the average absolute difference between the CNTY-calculated temperature and the thermocouple reference temperature. The CNTY/VER-PC monofilament composite showed an MAE of approximately 0.58 °C, indicating good agreement between the CNTY-calculated temperature and the thermocouple reference temperature under the tested heating and cooling conditions. Therefore, the CNTY/VER-PC monofilament composite can accurately reproduce the thermocouple temperature history and demonstrates reliable performance as an embedded temperature sensor.

4. Conclusions

The dependence of the electrical resistance of carbon nanotube yarn (CNTY) on temperature (thermoresistivity) was investigated by embedding a CNTY in a vinyl ester resin (VER). The thermoresistive characterization of room-temperature-cured (CNTY/VER) monofilament composites and post-cured CNTY monofilament composites (CNTY/ER-PC) was studied, including the effect of temperature dwells on the residual change in fractional electrical resistance. It is observed that the residual change in electrical resistance increases to (∆R/R₀)_res = 8.85% after returning to the RT in CNTY/VER monofilament composites. This value decreased to −0.08% after post-curing the CNTY monofilament composites at 140 °C for 1 h. Polymer infiltration, elimination of residual monomers and styrene during thermal post-curing and an increase in the crosslinking network after post-curing could be attributed to the electrical response of CNTY/VER-PC composites. The thermoresistive characterization of the CNTY/VER monofilament composites was performed under heating–cooling cycles (25 °C to 100 °C). The thermoresistive response of the embedded CNTY was nearly linear for both CNTY/VER and CNTY/VER-PC monofilament composites. The average heating and cooling TCR values were −7.98 × 10⁻⁴ °C⁻¹ and −8.32 × 10⁻⁴ °C⁻¹ for CNTY/VER, and −7.93 × 10⁻⁴ °C⁻¹ and −7.13 × 10⁻⁴ °C⁻¹ for CNTY/VER-PC, respectively. This negative thermoresistive sensitivity could be attributed to the intrinsic thermoresistive behavior of the CNTYs. The hysteresis loops upon heating–cooling cycles were 21.65% in CNTY/VER composites. This value decreased to 12.49% after post-curing the specimens (CNTY/VER-PC) at 140 °C for 1 h. The evaporation of residual monomers and styrene, and the increase in the crosslinking density of the polymeric matrix upon post-curing the specimens, could be attributed to the lower hysteresis and higher linearity observed in the thermoresistive response of CNTY/VER-PC composites. Both specimens exhibited a negligible decrease in the fractional change in electrical resistance after each cycle. Different contributing factors, such as resin infiltration, as suggested by SEM, variation in contact resistance between the carbon nanotubes, and changes in the density and mobility of electric charge carriers, could account for these results. The thermoresistive sensitivity of the CNTY/VER-PC composite was investigated to evaluate the implementation of the CNTY monofilament composite as a temperature sensor. The electrical resistance of the CNTY monofilament composite was converted to temperature readings using the average TCR calculated at the heating–cooling sections. The calculated temperature was compared to the temperature readings of a reference thermocouple. It was shown that the CNTY/VER-PC monofilament provides a good reproduction of the thermocouple’s temperature readings. The experimental study on the thermoresistive response of CNTY monofilament composites suggests that CNTYs have the potential to enable temperature measurement within materials.