Open AccessArticle

Fabrication of Flexible SWCNTs/Polyurethane Coatings for Efficient Electric and Thermal Management of Space Optical Remote Sensors

Huiqiao Yang

^1,*,

Yueting Wang

²,

Bo Yang

³,

Fulong Ji

¹,

Haitong Jiang

¹ and

Lei Li

State Grid Location Based Service Co., Ltd., Beijing 102200, China

Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100094, China

Tencent Technology (Beijing) Co., Ltd., Beijing 100080, China

Author to whom correspondence should be addressed.

Processes 2024, 12(12), 2650; https://doi.org/10.3390/pr12122650

Submission received: 18 September 2024 / Revised: 1 November 2024 / Accepted: 21 November 2024 / Published: 25 November 2024

(This article belongs to the Section Materials Processes)

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Figure 1
Schematic illustration for fabricating SWCNTs/WPU. "> Figure 2
SEM images of SWCNTs (a,b), SWCNTs/WPU-1 (c,d), SWCNTs/WPU-2 (e,f), SWCNTs/WPU-3 (g,h), SWCNTs/WPU-4 (i,j), and WPU (k,l). "> Figure 3
Raman spectra of SWCNTs before and after acid treatment (a). XRD patterns of SWCNTs/WPU (b). FTIR spectra of SWCNTs (c) and SWCNTs/WPU (d). TGA (e) and DTG (f) curves of SWCNTs/WPU. "> Figure 4
Effect of SWCNT content on the electrical conductivity (a) and thermal conductivity (b) of r-SWCNTs/WPU and p-SWCNTs/WPU. "> Figure 5
Stress–strain curves of p-SWCNTs/WPU (a). Tensile strength of r-SWCNTs/WPU and p-SWCNTs/WPU (b). Stability of electrical and thermal conductivities of p-SWCNTs/WPU-4 with bending experiment of 0–1000 cycles (c). "> Figure 6
Schematic illustration for heat dissipation solution. "> Figure 7
Heat transformation diagram (a), structural diagram (b), and cross-sectional photograph (c) of thermal radiator. "> Figure 8
Temperature variation of various components and positions of the radiator during the in-orbit flight. ">

Versions Notes

Abstract

Given the requirement of high-efficiency thermal dissipation for large-aperture space optical remote sensors, a radiator based on single-walled carbon nanotubes (SWCNTs) filled with waterborne polyurethane (SWCNTs/WPU) coatings was proposed in this work. In situ polymerized SWCNTs/WPU coatings allowed for the uniform distribution of acid-purified SWCNTs in WPU matrix. Modified oxygen-containing groups on purified SWCNTs enhanced the interfacial compatibility of SWCNTs/WPU and enabled an improved tensile strength 9 (26.3 MPa) compared to raw-SWCNTs/WPU. A high electrical conductivity of 5.16 W/mK and thermal conductivity of 10.9 S/cm were achieved by adding 49.1 wt.% of SWCNTs. Only 2.85% and 4.2% of declined ratios for electric and thermal conductivities were presented after 1000 bending cycles, demonstrating excellent durability and flexibility. The designed radiator was composed of a heat pipe, SWCNTs/WPU coatings and an aluminum honeycomb core, allowing for −1.6~0.3 °C of temperature difference for the in-orbit temperature and thermal balance experimental temperature of the collector pipe. Moreover, the close temperature difference for the in-orbit and ground temperatures of the radiator indicated that the designed radiator with high heat dissipation met the mechanical environment requirements of a rocket launch. SWCNTs/WPU would be promising electric/thermal interface materials in the application of space optical remote sensors.

Keywords:

SWCNTs/WPU coating; thermal conduction; mechanical property; structural design; optical remote sensor

1. Introduction

Space optical remote sensors can observe and analyze electromagnetic radiation data collected from different spectral regions ranging from ultraviolet to infrared. As the essential component of modern space missions, they play a significant role in resource surveys, environmental monitoring, terrain mapping, military surveillance, and astronomical observations [1,2,3]. Space optical remote sensors are operated in harsh and complex thermal environments in orbit, which are distinctly influenced by direct solar radiation, the infrared radiation and solar reflection of Earth, cold dark space, and internal heat sources [4,5]. Meanwhile, with the miniaturization, high integration, and high power of electronic systems for space optical remote sensors, thermal control issues such as limited space for heat dissipation, uneven heat flow distribution, and excessively high heat flow density are becoming increasingly prominent. For instance, the thermal power consumption of a specific solid-state memory board has increased from 7–8 W to 80–100 W with the heat generated by a single field programmable gate array from 2–3 W to 20–25 W [6,7,8]. Temperature fluctuations and uneven distribution would induce the thermal deformation of supporting structures and optical elements, thereby reducing imaging quality. The high dark current and thermal noise in charge-coupled devices caused by extreme temperature in focal plane assemblies would decrease and fail the reliability of electronic components, even leading to the failure of the entire space observation mission in severe cases [9]. Excessive temperatures not only affect the regular operation of the devices but also create significant thermal stress due to the large temperature difference between the internal components and the external environment, reducing the devices’ performance and reliability and thereby affecting their lifespan.

The main challenge of thermal management for high-power equipment is how to effectively minimize interface thermal resistance, enhance interface heat transfer, quickly transfer the heat generated by high-power components to the equipment casing, and eventually dissipate it into the cold space [10]. Besides the design of a complex heat conduction structure, numerous materials including carbon fiber, metals, conductive polymers, and phase change materials have been applied in the thermal management of spacecraft [11]. Among these, carbon fiber composites, which are commonly used to ensure the dimensional stability of highly stable structures, are restricted due to relatively low thermal conductivity [12,13]. Aluminum and copper foil or pipes, which are typically used for the isothermal design of large complex structures, are limited owing to their high density and thermal expansion coefficients [14,15]. Phase change materials, which can relieve the impact of large amounts of heat, are constrained by their relatively low thermal conductivities [16].

Polymer-based coating materials for space optical remote sensors have low density, excellent flexibility, easy processing, and good chemical stability and have been a promising substitute for metal-based coatings materials [17]. However, most polymer materials have poor electrical/thermal conductivity. It is necessary to introduce functional fillers (such as ceramics, metals, graphite, graphene, MXene, etc.) into the polymer aggregate to improve their electrical/thermal conductivity [18,19,20,21,22]. Recently, carbon-based materials with high thermal conductivity including graphene-based composites, carbon nanotubes (CNTs), diamond-based composites, and highly thermal conductive carbon foams have emerged as promising fillers for conductive polymers given their high thermal conductivity, radiation resistance, low thermal expansion coefficient, and light weight [23,24,25,26]. As a one-dimensional carbon allotrope, carbon nanotubes including single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have low density, a large aspect ratio, and extraordinary thermal and electrical conductivity, which makes them excellent functional fillers for coating materials on space optical remote sensors. For example, Hong et al. reported that the highest thermal conductivity of an MWCNT-filled polymer was 3.44 W/m/K with a filling amount of 4.0 wt.% [27]. Gou et al. prepared MWCNTs/silicone composites with a thermal conductivity of 4.27 W/m/K by introducing 2.0 wt.% MWCNTs into silicone materials [28]. Moreover, compared with highly thermally conductive materials with a single function, high-efficiency antistatic-reinforced polymers with electric conductivity are more suitable for coatings on space optical remote sensors. However, there is a scarcity of reports on the latest developments in applying SWCNT-based coating materials with high electrical and thermal conductivity in the thermal management of space optical remote sensors. Moreover, SWCNTs of extraordinary physical and mechanical properties make them ideal candidates for multifarious applications. SWCNTs exhibit remarkable chirality-dependent mechanical phenomena [29]. Fazelzadeh et al. developed an anisotropic elastic shell model to study the vibration characteristics of chiral SWCNTs [30]. SWCNTs exhibit remarkable chirality-induced anisotropic elastic properties including chirality-dependent Young and shear moduli, coupling of extension and twist, and chirality-dependent critical buckling strain. The application of SWCNTs with a specific chirality in both engineering applications and scientific studies to assign chirality (n, m) to a given SWCNT would be desirable for future work.

In this study, SWCNTs with high conductivity were combined with environmentally friendly waterborne polyurethane (WPU) matrix as coatings for space optical remote sensors. By adjusting the mass ratio of SWCNTs and WPU, the thermal and electrical conductivity and comprehensive properties of the SWCNTs/WPU coatings were systematically investigated. The flexibility and durability of SWCNTs/WPU were also investigated for 1000 cycles. By designing the radiator with SWCNTs/WPU coatings, thermal management for in-orbit and thermal balance experiments were performed to evaluate the potential of the SWCNTs/WPU-based radiator in the application of space optical remote sensors.

2. Materials and Methods

2.1. Materials

Raw single-walled carbon nanotubes (r-SWCNTs) with a length of 5–30 μm, an average diameter of <2 nm, a purity of >95 wt.%, and a specific surface area of >490 m²/g were obtained from Xianfeng Nano Co., Ltd. (Nanjing, China). Nitric acid, hydrochloric acid, dimethylol butanoic acid (DMBA), isophorone diisocyanate (IPDI), ethylenediamine (EDA), triethylamine (TEA), acetone, and polycaprolactone diol (PCL diol, average Mn = 10,000) were purchased from Sigma–Aldrich Co., Ltd. (Darmstadt, Germany).

2.2. Preparation of SWCNTs/WPU

To enhance the purity of r-SWCNTs, r-SWCNTs were treated in the mixture of nitric acid, hydrochloric acid, and distilled water with a mass ratio of 30 wt.%/30 wt.%/40 wt.% for 6 h and termed as purified SWCNTs (p-SWCNTs). The fabrication of SWCNT/WPU films is illustrated in Figure 1. Specifically, p-SWCNTs with specific weights (i.e., 4.0, 6.0, 8.0, and 10.0 g) were dispersed in acetone (300 mL) to achieve homogeneous dispersions with the assistance of sonication (1 h). After they were transferred into a three-neck flask, an SWCNTs/polyurethane prepolymer was formed by adding DMBA (1.2 g), IPDI (6.8 g), and PCL diol (15.4 g) at 80 °C for 2 h with continuous stirring under N₂ flowing. Then, carboxyl groups of the formed prepolymer were neutralized with the addition of TEA (0.6 g). Acetone was removed by adding deionized water to the SWCNTs/prepolymer suspension. Afterward, the SWCNTs/WPU suspension could be formed by ball-milling the SWCNTs/prepolymer aqueous solution with EDA (0.3 g) at a rotation speed of 500 rpm for 8 h. Then, the SWCNTs/WPU suspension was poured into a mold and dried at 50 °C for 4 h. According to the four different contents of p-SWCNTs in SWCNTs/WPU films (19.8 wt.% 28.7 wt.% 37.6 wt.% and 49.1 wt.%), as-prepared products were termed as SWCNTs/WPU-1, SWCNTs/WPU-2, SWCNTs/WPU-3, and SWCNTs/WPU-4, respectively. r-SWCNTs was also filled with the WPU matrix according to the above experimental description for comparison and termed as r-SWCNTs/WPU.

2.3. Characterization

The morphology of samples was observed using scanning electron microscopy (SEM, Nova™ Nano, Nagoya, Japan). The chemical bonds were determined by a Fourier transform infrared spectrometer (FTIR, Nicolet IS5, Waltham, MA, USA) at wavelengths ranging from 3500 to 500 cm⁻¹. The crystalline structure was recorded by an X-ray diffractometer (R-AXIS RAPID, Nagoya, Japan). The thermal behavior was studied using a thermal gravimetric analyzer (TGA) with derivative thermal gravimetric (DTG) (METTLER TGA 2, Waltham, MA, USA) under an N₂ atmosphere with a heating rate of 10 °C/min in the temperature range from 80 °C to 800 °C. The thermal conductivity was recorded on Hot Disk (TPS 2500 S, Göteborg, Sweden). The electrical conductivity was measured using a digital four-probe tester (Keithley2450, Xi’an, China). Raman spectra were recorded on a Raman spectroscope via an Argon-ion laser at the excitation wavelength of 532 nm. All samples were prepared using a dog-bone shape punch with dimensions of 30 mm length × 10 mm width. Uniaxial tensile tests were performed to study the mechanical properties of samples using an Instron 5965 tester (Instron, Norwood, MA, USA) at a fixed velocity of 1 mm/min.

3. Results and Discussion

3.1. Characterization of SWCNTs/WPU

The distribution of SWCNTs in a polymer matrix always suffers from poor dispersion. The high diffusion resistance of SWCNTs will induce their agglomeration on the granule surface of a polymer. Thus, an in situ polymerization approach was applied in this study to decrease the diffusion resistance given the low molecular weight of organics. The diffusion of SWCNTs in WPU granules could be achieved, benefiting the formation of an electrical-conductive channel. As the content of p-SWCNTs in SWCNTs/WPU increased, p-SWCNTs were uniformly distributed in the matrix of WPU without obvious agglomeration (Figure 2b–e). Three-dimensional networks derived from the interconnection of p-SWCNTs would be promising channels for the efficient conduction of electrons and heat.

The Raman technique is an effective approach to evaluate the contents of defects and graphitic structure in carbon-based materials. As shown in Figure 3a, the intensity ratios of the D band at 1350 cm⁻¹ and the G band at 1580 cm⁻¹ distinctly changed from 0.058 (r-SWCNTs) to 0.116 (p-SWCNTs), indicating that intensive detections on p-SWCNTs were introduced via the acid treatment. These defects would deteriorate the electrical and thermal conductivity of SWCNTs. XRD pattern (Figure 3b) of WPU displayed a broad diffraction peak from 15° to 30°, which was the characteristic peak in the soft segment area of WPU, indicating the amorphous structure of WPU. With the addition of SWCNTs in composite films, strong diffraction peaks near 2θ = 13.8° and 25.2° were observed in the patterns of SWCNTs/WPU, and the intensity of these peaks increased with the rising content of SWCNTs. Thus, SWCNTs were successfully combined in the matrix of WPU. A higher crystalline structure derived from added SWCNTs would induce more a regular structure of SWCNTs/WPU, which was conducive to the formation of thermal-/electrical-conductive networks of SWCNTs in WPU. For FTIR spectra shown in Figure 3c, broad peaks located at the wavelength of 3500–3000 cm⁻¹ are attributed to the stretching vibration of –OH. Multiple peaks corresponding to the stretching vibration of C=O, C=C, C-O-C, and C-O were observed at wavelengths of 1718, 1571, 1188, and 1108 cm⁻¹ [31,32]. The peaks ascribed to the oxygen-containing functional groups are clearly visible in the FTIR spectrum of p-SWCNTs and are very weak in the spectrum of r-SWCNTs. After the in situ polymerization of SWCNTs/WPU, obvious changes were observed, as exhibited in Figure 3d. For instance, the stretching vibration absorption peaks of –C–H in –CH₃ and –CH₂ were displayed at the wavelengths of 2940–2900 cm⁻¹ and 2900–2850 cm⁻¹, respectively. The absorption peak of the ester C=O group in the carbamate bond was located at 1700 cm⁻¹. The asymmetric stretching vibration peak of the carbamate bond was at 1236 cm⁻¹, and the peak of -C-O-C- stretching vibration was at 1090 cm⁻¹. SWCNTs/WPU containing SWCNTs with different mass ratios showed typical infrared characteristic peaks of WPU and SWCNTs.

The thermal stabilities of SWCNTs/WPU-1 to SWCNTs/WPU-4 were also evaluated. From TGA curves (Figure 3e) and DTG curves (Figure 3f) of SWCNTs/WPU, two stages of weight loss were clearly exhibited. Specifically, the first stage, from 270 °C to 400 °C, was attributed to the thermal decomposition of the WPU matrix. The second stage, from 450 °C to 680 °C, was ascribed to the decomposition of SWCNTs. Since the main matrix of WPU could be removed at the first stage, the weight residuals of SWCNTs/WPU at the temperature of 450 °C were 19.8 wt.% for SWCNTs/WPU-1, 28.7 wt.% for SWCNTs/WPU-2, 37.6 wt.% for SWCNTs/WPU-3, and 49.1 wt.% for SWCNTs/WPU-4. The thermal stability of samples could be evaluated by the heat-resistance index (T_HRI) [33], which was calculated as T_HRI =0.49 × [T₅ + 0.6 × (T₃₀ − T₅)], where T₅ and T₃₀ are the decomposition temperatures of SWCNTs/WPU at the weight loss of 5% and 30%, respectively, as listed in Table 1. The T_HRI values of WPU were significantly lower than those of SWCNTs/WPU, which presented an increasing trend from 152.7 °C (SWCNTs/WPU-1) to 156.1 °C (SWCNTs/WPU-3). In contrast, the T_HRI values of SWCNTs/WPU-4 were reduced to 151.3 °C, which should be due to the reduced interfacial compatibility between excessive SWCNTs and WPU.

3.2. Electric and Thermal Conductivity of SWCNTs/WPU

Figure 4 shows the measured electrical conductivity and thermal conductivity of r-SWCNTs/WPU and p-SWCNTs/WPU as a function of SWCNT filling content. The electric conductivity of p-SWCNTs/WPU exhibited a rising trend with SWCNT content (Figure 4a), i.e., 0.1 S/cm for p-SWCNTs/WPU-1 (19.8 wt.% of SWCNTs), 2.0 S/cm for p-SWCNTs/WPU-2 (28.7 wt.% of SWCNTs), 9.4 S/cm for p-SWCNTs/WPU-3 (37.6 wt.% of SWCNTs), and 10.9 S/cm for p-SWCNTs/WPU-4 (49.1 wt.% of SWCNTs). The electrical conductivities of p-SWCNTs/WPU were obviously higher than those of r-SWCNTs/WPU prepared with the same content of SWCNTs. The improved ratios were in the range of 17.5–51.1%. A similar increasing tendency of thermal conductivity with the rising content of p-SWCNTs was also presented as expected (Figure 4b), i.e., 1.83 W/mK for p-SWCNTs/WPU-1, 2.68 W/mK for p-SWCNTs/WPU-2, 3.85 W/mK for p-SWCNTs/WPU-3, and 5.16 W/mK for p-SWCNTs/WPU-4. Compared with r-SWCNTs/WPU, the thermal conductivity of p-SWCNTs/WPU enhanced with ratios from 3.9% to 20.5%.

On the one hand, the acid treatment of r-SWCNTs inevitably introduced more defects to the carbon backbone of nanotubes, which was negative for the electrical and thermal conductivity. On the other hand, hydroxyl and carboxyl groups were modified on the surface of SWCNTs by acid treatment, which would greatly enhance the interfacial compatibility of p-SWCNTs with a WPU matrix. The interfacial compatibility between the filler and polymer matrix was an essential issue for the functionalization of pristine polymer [34]. In this study, the enhancement of interfacial compatibility played a more obvious role in the enhancement of the thermal conductivity of p-SWCNTs/WPU.

3.3. Mechanical and Conductivity Stability of SWCNTs/WPU

Since polymer composites with enhanced electrical or thermal conductivity are commonly utilized in multiple real applications, it would be crucial to evaluate their mechanical stability as well as electrical/thermal stability during deformation. As illustrated in Figure 5a, the tensile strengths of p-SWCNTs/WPU with four contents of p-SWCNTs were higher than those of r-SWCNTs/WPU, for instance, 26.3 MPa for p-SWCNTs/WPU-1 and 20.1 MPa for r-SWCNTs/WPU-1. It was deduced that the interfacial interaction of p-SWCNTs and WPU would be enhanced because of the existence of hydroxyl and carboxyl groups on the surface of p-SWCNTs, which was significantly beneficial for the compatibility of p-SWCNTs with WPU matrix, and even more defects were introduced in the carbon backbone of p-SWCNTs by the acid treatment. Moreover, the tensile strengths of p-SWCNTs/WPU presented a reduced tendency from 26.3 MPa to 15.2 MPa as the contents of p-SWCNTs increased.

The flexibility of p-SWCNTs/WPU was studied via the bending experiment of 0–1000 cycles. Taking p-SWCNTs/WPU-4 as an example, the film was bent for 1000 cycles with an angle from 0° to 180° as shown in the inset of Figure 5b. The electric and thermal conductivities decreased with ratios of 2.85% and 4.20%, respectively, after the bending test of 1000 cycles. Cyclic p-SWCNTs/WPU-4 film kept well structural integrity and electric/thermal conductivity, exhibiting excellent durability and flexibility. Therefore, p-SWCNTs/WPU film could be a promising candidate as electric/thermal interface materials for space optical remote sensors.

3.4. SWCNTs/WPU in Designed Radiator

The schematic of the heat dissipation system is illustrated in Figure 6. The heat collected through the heat pipe was transferred to SWCNTs/WPU-3 coating via two groups of external sticking pipes in this study. The focal plane of a large-aperture camera in geostationary orbit was spliced by nine groups of focal plane circuits, and the heating power of each group of focal plane circuits was 26 J. The total heating power of these nine groups of focal plane circuits was 234 J. Because the satellite platform could follow the position of the sun, there was a long-term shadow surface on the surface of the optical remote sensor, and the area where the shadow surface could be used for heat dissipation was a part of a cylindrical surface, which was unfolded. There was only a weak external heat flow of earth reflection, infrared heat flow of Earth, and infrared radiation of other equipment on the satellite surface, which were all long-term stable external heat flows. It was required that the temperature of the heat source, that is, the heat collecting pipe, be within 0 ± 2 °C.

The coating is exhibited in Figure 7b, and the cross-sectional photograph of the radiator is presented in Figure 7c. The radiator consisted of a heat pipe, SWCNTs/WPU-3 coating, an aluminum honeycomb core, and SWCNTs/WPU coating from top to bottom. Two groups of heat pipes were installed on the outer surface of the coating. The joint length between the two groups of heat pipes and the radiator was 2 m, the width of the heat pipes was 0.03 mm, and the total contact area was 0.12 m². The thermal resistance of heat transfer from the heat pipe to the SWCNTs/WPU coating was mainly thermal resistance and contact thermal resistance. Fourier’s law states that the negative gradient of temperature and the time rate of heat transfer is proportional to the area at right angles to that gradient through which the heat flows. Fourier’s law is the other name of the law of heat conduction. The derivation of Fourier’s law was explained with the help of an experiment which demonstrated that the rate of heat transfer through a plane layer is proportional to the temperature gradient across the layer and heat transfer area. According to Fourier’s Law of heat conduction, heat transfer formula, and contact thermal resistance calculation formula, the contact thermal resistance was inversely proportional to the cross-sectional product of the heat transfer channel. The large contact area could ensure small total thermal resistance from the heat pipe to the SWCNTs/WPU-3 coating, so as to solve the problems of low thermal conductivity in the thickness direction of the SWCNTs/WPU-3 coating and large temperature difference during the high-power heat transfer.

The SWCNTs/WPU-3 coating was bonded with the honeycomb core structure by an adhesive to form a honeycomb plate, which provided good mechanical support for the SWCNTs/WPU coating to improve the rigidity of the whole radiator, ensuring that the radiator would not be greatly deformed in harsh mechanical environments including rocket launches. Coatings with smaller thickness would be more efficient for thermal conductivity. Considering the antistatic requirements of the high-speed rail cooling coating, SWCNTs/WPU-3 coating with a thickness of 0.2 mm was selected to guarantee the requirements of electrical conductivity and sufficient mechanical strength. The heat would eventually be transferred from the honeycomb core to the SWCNTs/WPU-3 coating with the lowest temperature, and, finally, the heat would be dissipated into outer space through the radiation heat exchange.

3.5. Thermal Management for in Orbit Flight

Considering that the visible focal plane circuit did not work for a long time, only eight heating loops on the heat collecting pipe were selected to analyze the temperature for the convenience of comparative analysis. The analysis was carried out as the whole thermal control system reached the thermal balance. At this time, the heating power value of the heat collecting pipe could be accurately obtained by calculating the heating time using the thermal controller.

The temperature change curves of the measuring points in the middle of the radiator, the measuring points of the external heat pipe, and the temperature control points of eight heating circuits on the heat collecting pipe during the in-orbit flight of the radiator are shown in Figure 8. The temperature of the heat collecting pipe met the thermal balance criterion [35], that the temperature fluctuation value did not exceed 0.5 °C for 4 consecutive hours, and the thermal control system reached a steady thermal balance meanwhile. The externally attached heat pipes were all covered with multiple layers along the way, and the heat insulation was fixed. The heat leakage of the heat pipes along the way could be ignored. The total electric heating power on the heat collecting pipes was basically equivalent to the heat dissipation power of the radiator.

The temperatures of the radiator at the thermal balance test stage and the in-orbit flight stage are listed in Table 2. When the electric heating power of the collector pipe was 234 ± 2 W, the in-orbit temperature and thermal balance experimental temperature of the collector pipe ranged from −1.6 °C to 0.3 °C, which was within the index requirements of 0 ± 2 °C. During the in-orbit flight, the total electric heating power of the collector pipe was 4.2 W higher than that in the ground thermal balance experiment. The average temperature of the heat sink in the ground thermal balance experiment was 98 K, and the temperature of the heat sink in outer space was 4 K. The temperature of the radiator was influenced by the constant temperature control of the collector pipe, and the in-orbit temperature level of the radiator was basically the same as that of the ground. The in-orbit background radiation heating power was small, and the in-orbit heat dissipation power of the radiator was large.

As shown in Table 3, the average temperature difference between the external heat pipe and the measuring point in the middle of the radiator at different stages was between 6.3 °C and 6.5 °C. It is indicated that the designed radiator in this study presented excellent heat dissipation and temperature equalization performance. Meanwhile, the heat dissipation performance of the designed radiator had little change after the ground test and rocket launch vibration, and the radiator had good mechanical properties, which could meet the mechanical environment requirements of a rocket launch.

SWCNTs have not been used in industry until recently owing to the absence of technology for their mass production and, as a consequence, their high price. Few companies have developed the technology to produce SWCNTs at a low price. For instance, the OCSiAl company today unveiled a breakthrough technology for the production of SWCNTs, which enables the large-scale commercial production of SWCNTs. Once the mass production of SWCNTs at a low price can be achieved, their broad application in multiple fields would be desirable.

4. Conclusions

In summary, p-SWCNTs/WPU films with excellent flexibility, electrical conductivity, and thermal conductivity were fabricated via an in situ polymerization approach. Specifically, hydroxyl and carboxyl groups on the surface of p-SWCNTs introduced by acid treatment allowed for the uniform distribution of SWCNTs in a WPU matrix, and SWCNTs/WPU had enhanced interfacial compatibility and improved tensile strength (26.3 MPa) compared to raw-SWCNTs/WPU. High electrical conductivity (5.16 W/mK) and thermal conductivity (10.9 S/cm) could be achieved for SWCNTs (49.1 wt.%)/WPU with only a 2.85% and 4.2% declined ratio after 1000 bending cycles, presenting excellent durability and flexibility. Moreover, the excellent consistency of the thermal balance test value and the in-orbit flight value of the designed radiator indicated the stable heat dissipation of the radiator after the ground test and rocket launch vibration, verifying the outstanding mechanical properties of the SWCNTs/WPU coating for rocket launches and remote sensors in orbit. This work provided a valuable reference for the application of carbon-based composited polymers as interfacial electrical-/thermal-conductive materials in the space field.

The use of SWCNTs-based high-thermal-conductivity materials faces the following issues. (1) SWCNTs exhibit significant anisotropic thermal conductivity. This means that heat conduction is more efficient along the plane, while it is more difficult in the direction perpendicular to the plane. The main challenge is to address the temperature gradient issues caused by the anisotropy in thermal conductivity at the joints of the flexible tape. (2) SWCNTs-based high thermal conductivity materials have a lower thermal expansion coefficient than other materials in space optical remote sensors (such as metals and ceramics). Particularly in the application of large-size titanium alloy, aluminum alloy, and carbon fiber-enhanced composite with high thermal conductivity, this mismatch in thermal expansion coefficients can lead to thermal stress and deformation at the interface, affecting the system’s performance and reliability. For the future prospects, it is necessary to deeply understand the multi-scale thermal conduction mechanisms, including phonon heat conduction, carrier conduction, phonon–electron coupling, and the transmission of electrons and phonons at interfaces, thereby providing a theoretical basis for the design of SWCNTs-based high-thermal-conductivity materials. In this process, molecular dynamics, density functional theory, and large-scale parallel computing technologies will increasingly be important in revealing thermal transport mechanisms at multiple scales.

Author Contributions

H.Y.: Conceptualization, Investigation, Data curation, Methodology, Characterization, Writing-original draft, Writing-review & editing. Y.W.: Validation, Supervision, Funding acquisition. B.Y.: Writing-review & editing. F.J.: Writing-review & editing. H.J.: Writing-review & editing. L.L.: Resource. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author.

Conflicts of Interest

Authors Huiqiao Yang, Fulong Ji, Haitong Jiang and Lei Li were employed by the company State Grid Location Based Service Co., Ltd. Author Bo Yang was employed by the company Tencent Technology (Beijing) Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The companies had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Schematic illustration for fabricating SWCNTs/WPU.

Figure 2. SEM images of SWCNTs (a,b), SWCNTs/WPU-1 (c,d), SWCNTs/WPU-2 (e,f), SWCNTs/WPU-3 (g,h), SWCNTs/WPU-4 (i,j), and WPU (k,l).

Figure 3. Raman spectra of SWCNTs before and after acid treatment (a). XRD patterns of SWCNTs/WPU (b). FTIR spectra of SWCNTs (c) and SWCNTs/WPU (d). TGA (e) and DTG (f) curves of SWCNTs/WPU.

Figure 4. Effect of SWCNT content on the electrical conductivity (a) and thermal conductivity (b) of r-SWCNTs/WPU and p-SWCNTs/WPU.

Figure 5. Stress–strain curves of p-SWCNTs/WPU (a). Tensile strength of r-SWCNTs/WPU and p-SWCNTs/WPU (b). Stability of electrical and thermal conductivities of p-SWCNTs/WPU-4 with bending experiment of 0–1000 cycles (c).

Figure 6. Schematic illustration for heat dissipation solution.

Figure 7. Heat transformation diagram (a), structural diagram (b), and cross-sectional photograph (c) of thermal radiator.

Figure 8. Temperature variation of various components and positions of the radiator during the in-orbit flight.

Table 1. Thermal parameters of SWCNTs/WPU.

Samples	Temperature of Weight Loss (°C)		T_HRI (°C)
Samples	T₅	T₃₀	T_HRI (°C)
WPU	116.3	306.1	112.8
SWCNTs/WPU-1	276.7	334.8	152.7
SWCNTs/WPU-2	284.1	337.4	154.9
SWCNTs/WPU-3	288.1	338.9	156.1
SWCNTs/WPU-4	272.8	332.7	151.3

Table 2. Tensile strength and modulus of SWCNTs/WPUs.

Samples	Tensile Strength (MPa)	Young’s Modulus (MPa)
p-SWCNTs/WPU-1	26.3 ± 1.2	2.51 ± 0.19
p-SWCNTs/WPU-2	23.2 ± 1.8	2.52 ± 0.17
p-SWCNTs/WPU-3	19.9 ± 1.1	2.34 ± 0.09
p-SWCNTs/WPU-4	16.4 ± 0.9	2.17 ± 0.13

Table 3. Thermal parameters of SWCNTs/WPU.

Sample	Heat Pipe Temperature (°C)	Point Temperature of External Heat Pipe (°C)	Center Point Temperature of Radiator (°C)	Temperature Difference of External Heat Pipe with Middle Point Radiator (°C)
Thermal balance test value	−1.5~0.3	−8.7~−7.7	−14.9~−14.1	6.3
In-orbit flight value	−1.6~0.2	−8.9~−7.9	−15.3~−14.5	6.5

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Yang, H.; Wang, Y.; Yang, B.; Ji, F.; Jiang, H.; Li, L. Fabrication of Flexible SWCNTs/Polyurethane Coatings for Efficient Electric and Thermal Management of Space Optical Remote Sensors. Processes 2024, 12, 2650. https://doi.org/10.3390/pr12122650

AMA Style

Yang H, Wang Y, Yang B, Ji F, Jiang H, Li L. Fabrication of Flexible SWCNTs/Polyurethane Coatings for Efficient Electric and Thermal Management of Space Optical Remote Sensors. Processes. 2024; 12(12):2650. https://doi.org/10.3390/pr12122650

Chicago/Turabian Style

Yang, Huiqiao, Yueting Wang, Bo Yang, Fulong Ji, Haitong Jiang, and Lei Li. 2024. "Fabrication of Flexible SWCNTs/Polyurethane Coatings for Efficient Electric and Thermal Management of Space Optical Remote Sensors" Processes 12, no. 12: 2650. https://doi.org/10.3390/pr12122650

APA Style

Yang, H., Wang, Y., Yang, B., Ji, F., Jiang, H., & Li, L. (2024). Fabrication of Flexible SWCNTs/Polyurethane Coatings for Efficient Electric and Thermal Management of Space Optical Remote Sensors. Processes, 12(12), 2650. https://doi.org/10.3390/pr12122650

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