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Article Cite This: ACS Omega 2019, 4, 2964−2972
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Thermal Properties and Enhanced Thermal Conductivity of Capric Acid/Diatomite/Carbon Nanotube Composites as Form-Stable Phase Change Materials for Thermal Energy Storage Peng Liu,*,† Xiaobin Gu,*,†,‡ Liang Bian,†,‡ Xiangfeng Cheng,§ Lihua Peng,∥ and Huichao He‡ †
School of Gems and Materials Technology, Hebei GEO University, Shijiazhuang 050031, Hebei, China State Key Laboratory for Environment-Friendly Energy Materials, South West University of Science and Technology, Mianyang 621010, Sichuan, China § School of chemical engineering, The University of Queensland, Brisbane 4072, Australia ∥ School of Earth and Space Sciences, Peking University, Beijing 100871, China ACS Omega 2019.4:2964-2972. Downloaded from pubs.acs.org by 109.94.221.84 on 02/18/19. For personal use only.
‡
ABSTRACT: The capric acid (CA)/diatomite (DT)/carbon nanotube (CNT) ternary system was investigated to develop a shape-stabilized composite phase change material for thermal energy storage via the direct impregnation method. DT was used as the supporting material to absorb CA and prevent its leakage. It was found that good form stability could be obtained when the loading of capric acid in the CA/DT composite reached about 54%. Furthermore, CNTs were added into the CA/DT form-stable phase change material (FSPCM) to enhance the thermal conductivity of the binary system. Moreover, the X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy analyses were carried out to characterize the microstructure and chemical properties of the composite PCM. The thermal properties of the prepared form-stable phase change materials (FSPCMs) were determined using differential scanning calorimetry (DSC) and thermogravimetric analyses. The analysis results showed that the components of the FSPCMs were in good compatibility and CA is well-infiltrated into the structure of the DT/CNT matrix. DSC analysis indicated that the latent heat of fusion of the ternary system was 79.09 J g−1 with a peak melting temperature of 31.38 °C. The thermal conductivity of the CA/DT/CNTs increased from 0.15 to 0.48 W m−1 K−1, with only 7 wt % of CNTs. It is shown that the thermal conductivity of the ternary system was greatly enhanced by the addition of CNTs. The thermal conductivity increased by 1.56 times compared to that of the binary system. Moreover, the enhancing mechanisms of heat conduction transfer by CNTs were revealed by taking advantage of energy wave theory.
1. INTRODUCTION With the increasing pressure of energy shortage and environment pollution, energy storage technologies have received more and more attention all over the world in recent years. The use of phase change materials (PCMs) which store thermal energy mainly in the form of latent heat, was considered to be one of the most effective methods to store thermal energy since the PCMs could provide higher heat storage capacity and more isothermal behaviors during phase transition compared to other heat storages.1 PCMs that are used as storage media in the latent thermal storage system can be mainly divided into two categories according to the characteristics of medium, that is, organic and inorganic PCMs. Inorganic PCMs primarily include salt hydrates, salts, metals, and alloys, whereas organic PCMs are comprised of fatty acids/esters, paraffin, and polyalcohols.2 Among organic PCMs, fatty acids are of a particularly important class for thermal energy storage. However, low thermal conductivity restricts its feasibility in real application of thermal energy storage. Therefore, to resolve the above-mentioned problem, a © 2019 American Chemical Society
significant amount of research and development work on carbon nanotubes (CNTs),3−6 silver nanowire,7,8 graphene,9−12 carbon,12−14 and graphite is in progress in many countries by scholars.10,15,16 The previous work has confirmed that addition of highly thermal conductive materials does improve the thermal conductivity of organic PCMs. Among these thermal conductivity enhancers, the high thermal conductivity in the range of 2000−6000 W m−1 K−1 of CNTs makes it ideal for being used as one of the most suitable thermal conductivity enhancers. Due to the high thermal conductivity, good dispersion ability, large surface and high stability, CNTs are considered to be greatly promising materials to increase the thermal conductivity of PCMs.5,17 However, different PCM systems made-up of different PCMs, support, and thermal conductive materials could have different thermal properties. There is a great deal of Received: November 9, 2018 Accepted: January 14, 2019 Published: February 11, 2019 2964
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Figure 1. SEM images of samples (a) CA, (b) DT, (c) CA/DT (S2-3), (d) CTNs, and (e, f) CA/DT/CTNs (S3-4).
the disk-like structure surface can be clearly observed, which indicates high porosity of DT as expected. The SEM image in Figure 1b is almost the same as in some reports.18−22 Therefore, it can also confirm that the dark spots were the pores of DT. Because of this, CA does not leak from the surface of the FSPCM, when CA is in the melting state. As can be seen from Figure 1c, most surface of DT was covered by CA. Moreover, due to the melt-impregnation between CA and DT, many tiny pores on the disk-like structure were “missing”. The porous structure of DT could prevent the leakage of the melted CA. However, there are still some empty pores as shown in Figure 1c. Figure 1d displays the morphology of cotton-like structures of CNTs. As can be seen from Figure 1e,f, with the introduction of CNTs into the CA/DT FSPCM, it can be seen that CNTs together with CA were randomly dispersed into the surface of DT. As a result, the thermal conductivity of the CA/DT/CNT FSPCM could be improved in theory. In fact, the capillary and surface tension forces between its components are critical factors for retaining the structural stability of its composite PCM.23 Additionally, the pore structure and surface area of DT before and after the addition of CA were determined. The Brunauer−Emmett− Teller surface area changes of DT before and after the addition of CA were 5.5108 and 3.7928 m2 g−1, respectively. The pore volume changes of DT before and after the addition of CA were 0.0335 and 0.0052 cm3 g−1, respectively. Both the pore volume and specific surface area of DT decrease after the addition of CA, and the results are consistent with the SEM images. Energy-dispersive X-ray spectroscopy (EDS) analysis shows that the main elements of the cover material on the surface of the CA/DT composite are C and O. It proves that the loaded
arrangement and permutation among PCMs, support, and thermal conductive materials. Although many scholars have attempted to evaluate the improvement efficiency of the addition of thermal conductive materials, much more work is still required to make this process more efficient. Hence, in this study, diatomite (DT) characterized by high porosity, high oil and water absorption capacity, and low density was taken as a supporting matrix to absorb capric acid (CA) and prevent the acid leakage. Capric acid (CA) is one of the most promising solid−liquid organic phase change materials (PCMs) used in building energy conservation due to the suitable phase transition points and large latent heat for thermal energy storage. CNTs with high thermal conductivity were used as the thermal conductivity enhancer. The capric acid (CA)/ diatomite (DT)/carbon nanotube (CNT) ternary system was investigated to fabricate a shape-stabilized composite phase change material for thermal energy storage via the direct heating impregnation method. Moreover, the thermal properties of the form-stable phase change materials (FSPCMs), particularly, the enhancing mechanisms of heat conduction transfer by CNTs were revealed by taking advantage of energy wave theory.
2. RESULTS AND DISCUSSION 2.1. Morphology of the CA/DT/CNT FSPCM. To characterize the morphological changes, scanning electron microscopy (SEM) images of the CA, DT, and CNT composites were obtained and are shown in Figure 1. Figure 1a shows the morphology of CA with a paste-like surface structure. As illustrated in Figure 1b, the morphology of DT is mainly composed of a disk-like structure and lots of pores on 2965
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that CA is found in the pores of DT. Moreover, its crystal structures are unchanged. The findings further indicate that there is no appreciable interaction among the components of the CA/DT/CNTs. The physicochemical compatibility between CA, DT, and CNTs was investigated via Fourier transform infrared spectroscopy (FTIR) spectroscopy. Their FTIR spectra are illustrated in Figure 4. As seen from Figure 4, in the spectrum
material was CA. This also means that CA is absorbed and distributed in DT with good compatibility (Figure 2 and Table 1).
Figure 2. EDS image of CA/DT.
Table 1. Element Distribution in CA/DT atomic number
element symbol
element name
concentration percentage
certainty
8 14 6
O Si C
oxygen silicon carbon
57.8 38 4.2
0.97 0.96 0.95
Figure 4. FTIR spectra of the CA, DT, CNT, CA/DT FSPCM and CA/DT/CNT FSPCM.
2.2. Chemical Compatibility of the CA/DT FSPCM. Figure 3 depicts the X-ray diffraction (XRD) of the CA, DT,
of DT, it has large bands at the range of 471, 793, and 1090 cm−1. The band at 471 cm−1 indicates asymmetric stretching vibration of the Si−O functional group, whereas the peak of 793 cm−1 represents the vibration of the SiO−H functional group, and the strong peaks at 1091 cm−1 belongs to the stretching vibration of the Si−O−Si functional group.18,24,25 The pure CA possesses the absorption band at 939, 1410, 1710, 2850, and 2930 cm−1. The band at 939 cm−1 is associated with the stretching vibration of −OH. Additionally, the absorption peak at approximately 1710 and 1410 cm−1 may be due to CO stretching vibration and COO− stretching vibration, respectively. Moreover, the absorption peaks at 2850 and 2930 cm−1 are due to the symmetric stretching vibration of −CH3 and −CH2, respectively.26,27 In the FTIR spectrum of CNTs, the primary absorption bands were observed at 1600 and 2900 cm−1, which are attributed to the stretching vibration of −CC− and −OH groups, respectively.5,23 It can be found that the main peak in the FTIR spectra of CA, DT, and CNTs also appeared in the FSPCMs, respectively. No new absorption band occurred after impregnation. However, little changes in the wavenumbers of some bands may be attributed to the weak physical interactions described as the capillary and surface tension force. In a word, this means that there was no chemical interaction among the CA, DT, CNT, CA/DT FSPCM and CA/DT/CNT FSPCM. 2.3. Thermal Properties of the CA/DT FSPCM. The phase change behaviors and properties of CA and the selected FSPCM samples, such as phase change and latent heat, were obtained during heating and cooling periods by differential scanning calorimetry (DSC) analysis. The DSC thermogram curves of pure CA and FSPCMs are shown in Figure 5, and the corresponding data of thermal performances are given in Table 2. The DSC curve of CA exhibits a melting temperature at 30.92 °C and a freezing temperature at 27.69 °C. The latent
Figure 3. XRD of the CA, DT, CA/DT FSPCM and CA/DT/CNT FSPCM.
CA/DT (S2-3) FSPCM and CA/DT/CNT (S3-4) FSPCM. As shown in Figure 3, though the diffraction intensities were changed due to the varied content, all the diffraction peaks of CA can be observed on the XRD pattern of the composite, indicating that the raw materials among the prepared composite was a physical process and their structures remained. The main characteristic peaks of the CA, DT, and CA/DT FSPCM and CA/DT/CNT FSPCM are in good agreement. Moreover, there is no new diffraction peak observed on the XRD of the CA/DT FSPCM and CA/DT/ CNT FSPCM. In addition, a strong diffraction peak was exhibited at 2θ = 5.26 and 11.58° on the XRD pattern of CA, respectively. However, it may be due to the low loading of CA in DT, the corresponding peaks become quite weak on the XRD pattern of the FSPCM. Therefore, it can be speculated 2966
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is summarized in Table 2. It can be noted that the CA/DT/ CNT FSPCMs in this study has a higher latent heat value than some CA/mineral FSPCMs and significant potential for thermal storage in building applications. Compared with the values of the pure CA, the FSPCMs displayed slight changes in the phase change temperature. These results were due to the orientation of the crystal line region of fatty acid chains into the structure with/without CNTs.23 2.4. Thermal Stability of CA and CA/DT FSPCM. The thermogravimetric analyses (TGA) curves of pure CA, CA/DT (S2-3), and CA/DT/CNTs (S3-4) are determined by the TGA analysis technique and presented in Figure 6. As seen
Figure 5. DSC curves of the CA, CA/DT, and CA/DT/CNT FSPCM.
heat of melting and solidifying of CA are 166.37 and 167.95 J g−1, respectively. The CA/DT (S2-3) FSPCM melts at 30.99 °C with the latent heat of 87.58 J g−1 and solidifies at 28.32 °C with the latent heat of 87.96 J g−1, whereas the phase change latent heat values related with melting and solidifying were found to be 79.09 and 79.11 J g−1 for CA/DT/CNTs, respectively, which correspond to the melting temperature and solidifying temperature at 31.38 and 28.95 °C, respectively. However, in comparison with the pure CA, the latent heat values of melting and solidifying for the CA/DT sample decrease to a certain degree during phase changes processes. The solidifying point among the samples shows some minor difference, which is caused by the physical interactions implied in FTIR analysis.24 Compared with the CA/DT FSPCM, the latent values of CA/DT/CNTs were slightly lower than that of CA/DT. Even so, it is appropriate for thermal regulation and thermal storage application in buildings. The reduction in the latent heat of the FSPCM may be ascribed to the following reasons. The interference of porous matrices of DT, the restriction of crystal arrangement, and the orientation of CA molecular chains into the microporous of DT and CNTs caused by the drag and steric effects, which resulted in the decline of regularities of crystal line regions and the increase of lattice defects.5,18 Additionally, a comparison of thermal properties between CA/ DT/CNTs and some other FSPCMs reported in the literature
Figure 6. TGA curves of CA, CA/DT, and CA/DT/CNT FSPCM.
from Figure 6, the one-step degradation of pure CA started at 90 °C and terminated at approximately 230 °C, which corresponds to the evaporation of CA. The degradation temperature range of CA/DT (S2-3) and CA/DT/CNT (S34) FSPCMs exhibited a mass loss in the temperature range of 90−230 °C, whereas it is evaporated from FSPCMs at 90 and 210 °C range, which corresponds to the mass losses of composites as about 51−53%. If the heating process of the FSPCMs was continued to 400 °C, no degradation of FSPCMs appeared (Figure 7). However, by subtracting the residue of CA and other mass loss from the composite due to weighing error and impurities, it can be determined that the mass loss of CA is about 96.61% during degradation, whereas there is a FSPCM mass loss of about 52.55% (S2-3) and 51.44% (S3-4), respectively, even though the heating temperature is up to 320 °C. This is in agreement with the experimental value (54 wt
Table 2. Thermal Properties and Results Compared with Some Literature Studies
item
melting temperature (°C)
latent heat of melting (J g−1)
solidifying temperature (°C)
latent heat of solidifying (J g−1)
thermal conductivity (W m−1 K−1)
this work (CA) this work (CA54 wt % + DT 46 wt %) this work (CA54 wt % + DT 46 wt %/CNTs 7 wt %) CA (55 wt %) + expanded perlite + expanded graphite (10 wt %)26 CA (60 wt %) + halloysite + expanded graphite (5 wt %)27 50 wt % capric/myristic acid + 40 wt % expanded perlite + 10 wt % EG28 capric−palmitic acid (35 wt %) + atapulgite29 capric/myristic acid (20.0 wt %) + vermiculite30 capric−lauric acid (40 wt %) + expanded vermiculite31 capric−palmitic (40 wt %) + expanded vermiculite31 capric−stearic acids (40 wt %) + expanded vermiculite31
30.92 30.99 31.38 31.60 29.56 21.70 21.71 19.80 19.09 23.51 25.64
163.37 87.58 79.09 96.30 75.4 85.40 48.2 27.46 61.03 72.05 71.53
27.69 28.32 28.95 31.50 25.36 20.70
167.95 87.96 79.11 96.30 75.35 89.75
0.15 0.31 0.48 0.14 0.76 0.08
17.10 19.15 21.40 24.90
31.42 58.09 67.24 69.64
0.07 0.18 0.20 0.30
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thermal conductivity (0.15 W m−1 K−1) of pure CA, the thermal conductivity of the CA/DT (S2-3) FSPCM is higher than that of pure CA. This is due to the fact that DT also has an important influence on the thermal conductivity of CA. It can be remarkably noted that the effect of DT’s introduction on the thermal conductivity of CA is worth evaluating. At the same time, the thermal conductivity of the FSPCM samples including CA/DT/CNTs (S3-4) (1%), CA/DT/CNTs (S3-4) (3%), CA/DT/CNTs (S3-4) (5%), and CA/DT/CNTs (S34) (7%) was 0.39, 0.41, 0.46, and 0.48 W m−1 K−1, respectively. Compared with the thermal conductivity of pure CA, it is easy to find that the increase in the thermal conductivity of FSPCM samples were about 260.00, 273.33, 306.67, and 320.00%, respectively. The thermal conductivity increased by 1.56 times compared to that of the binary system. The thermal conductivity of the CA/DT/CNT FSPCM is much higher than that of some other CA/mineral FSPCM system.22,32 The dispersion of the CNT additive in the FSPCM does play a role in enhancing the heat transfer efficiency. Moreover, with the increase of the mass fraction of CNTs in the CA/DT/CNT FSPCMs, their thermal conductivity of CA/ DT/CNT FSPCMs also increases. The CNT additive into the FSPCM can enhance the thermal conductivity of fatty acid compared with the other enhancement additive, for example, expanded graphite, although the cost of CNTs is relatively high. However, it can provide a new choice of enhancement additive and the CNTs also have better perspective of application. The improvement mechanism of thermal conductivity of CA can be analyzed as the following aspects and the mechanism is displayed in Figure 8. First of all, as Figure 8 shows, the support material DT also plays a role in improving the thermal conductivity of CA and the mechanism is exhibited in Figure 8a,b. The reasons can be explained as follows. First of all, the thermal conductivity of DT is much higher than that of Pure CA. Moreover, the porous disk-like structure of DT provides a large number of heat transfer channels. Moreover, the thermal energy is in the form of an energy wave. As shown in Figure 8b,c, the heat transfer has certain directionality due to the structure of DT by means of the properties of the wave. When CA was dispersed in the porous structure channels of DT, it
Figure 7. Thermal conductivity of CA/DT FSPCMs with different mass fractions of the CNT additive.
%). The temperature limit (90 °C) for the thermal degradation of FSPCMs is drastically higher than that of their serving working temperatures, confirming that the FSPCMs had relatively high thermal stability.23 In a word, it can be obviously concluded that the fabricated FSPCMs have quite good thermal stability or high thermal resistance for them to be applied as FSPCMs.5,6 2.5. Thermal Conductivity of FSPCM and Enhancement Mechanism. It is well-known that the enhancement of thermal conductivity is efficient on improving the storage/ release rate with the introduction of CNTs.5 However, the enhancement mechanism of thermal conductivity of CA with the addition of CNTs is less discussed. On the basis of this fact, the increasing thermal conductivity of the prepared FSPCM is also focused on and investigated in this work. According to the previous literature, pure CA’s thermal conductivity was 0.15 W m−1 K−1, whereas it was about 0.35 W m−1 K−1 for DT. The thermal conductivity of CA/DT, CA/ DT/CNTs (1%), CA/DT/CNTs (3%), CA/DT/CNTs (5%), and CA/DT/CNTs (7%) was measured and is given in Figure 6. As shown in Figure 8, CA/DT (S2-3) FSPCM’s thermal conductivity was 0.31 W m−1 K−1. In comparison with the
Figure 8. Improvement in the mechanism of the thermal conductivity of CA. 2968
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can be transferred at different directions which could be decomposed into X, Y, and Z. Because of the difference in the heat source and low wave resistance, where CA dispersions are less, the heat transfer efficiency is higher during the working process. Furthermore, energy wave will undergo scattering and refraction during the heat transfer process, and it is subjected to the boundary conditions and contact conditions. So, when the heat transfer direction of CA in the porous structure is parallel to the direction of the porous structure, the heat transfer efficiency is lowered due to a larger wave resistance during the CA being heated, whereas when the heat transfer direction of CA in the porous structure is perpendicular to the direction of the porous structure, heat transfer efficiency is improved by the wall of the porous structure of DT with high thermal conductivity owing to lower wave resistance. This result is also in agreement with the literature.33 Moreover, it can be seen from the experimental results that the effect of CNTs on the thermal conductivity of FSPCMs with CNTs in this work was high enough to compare many reports. For example, the thermal conductivity of palmitic acid−stearic acid as the eutectic phase change material was enhanced by 20.2, 26.2, 26.2, and 29.7%, repectively for 5, 6, 7, and 8 wt % CNT addition.3,5 The thermal conductivity of palmitic acid as the phase change material was increased by 51.6% by introducing 1.0 wt % multiwall CNTs.5,34 It can be seen that the FSPCM with CNT composites in this study has a higher latent heat value than that of some clay/fatty acid composites and significant potential for thermal energy storage application in buildings. On the other hand, the enrichment of CA can be attributed to the high thermal conductivity of CNT additive that can provide a large heat transfer area and greatly reduce the heat resistance. As mentioned above, the addition of CNTs is dispersed and fully mixed with CA and DT. As seen from Figure 7c, on the basis of enhancing the CA’s thermal conductivity by DT support, CNT additive can further increase the heat transfer channels of CA and enrich the thermal conductivity of community by coupling CA and DT with its high thermal conductivity. Simultaneously, CNT additive can significantly reduce the wave resistance during the heat transfer process. Moreover, CNTs could also change the direction of heat transfer of CA in the porous structure of DT. Moreover, the bigger the CNT amount, the larger the heat transfer area. Thus, the thermal conductivity becomes higher and higher by increasing the CNT additive. These analyses are in good agreement with the test results of thermal conductivity and thermal storage/release performance of the selected samples. 2.6. Thermal Storage/Release Performance of the Prepared FSPCM. The melting and solidifying time and total heat storing and releasing time can also show and demonstrate the effect of enhanced thermal conductivity by CNT additive and the DT support material. Moreover, it can also confirm that the above-mentioned discussion is reasonable. Therefore, the thermal storage/release properties of CA, CA/DT (S2-3), and CA/DT/CNTs (S3-4) were estimated and are shown in Figure 9. The test method of the melting and cooling curves is as follows. All the test samples were kept at the same initial temperature by a refrigerator to accelerate the cooling rate, then immediately all the samples were placed in the water bath. The water bath was switched on and the maximum temperature was set as 60 °C. The temperature curve of the melting process was recorded by virtue of thermal sensors, which were connected with an intelligent paperless recorder to
Figure 9. Storage and release curves of the CA, CA/DT, and CA/ DT/CNT FSPCM.
record the temperature at all times. When the temperature of water bath increases to 60 °C, the temperature is kept at 60 °C for half an hour. After that, all the test samples were placed in the refrigerator again to solidify the samples. Simultaneously the temperature curve of the melting process was recorded. Thus, the melting and solidification curves were obtained. It is easy to judge whether or not the composite including CNT is better heat transfer than the other two composites, storage efficiency can usually be defined as temperature changes (thermal energy changes) of per time and the melting time (time from elapsed from initial time to the phase change point). As can be seen from Figure 9, the total heat storing and releasing times of CA, CA/DT (S2-3) and CA/DT/CNTs (S3-4) are about 13 and 10 min shorter than those of pure CA, respectively. In other words, as seen from the testing results, the heat storing and releasing time of FSPCMs were decreased with the increase of the amount of added CNTs. It is due to the enrichment of the thermal conductivity and enhancement of the heat transfer rate into the FSPCM composite as a result of CNT addition. This also demonstrates that improving the thermal conductivity of CA by CNT additive is effective and is in agreement with the discussion mentioned above. Moreover, it is evident that storage efficiency of CA/DT/CNTs (S3-4) is better than that of CA/DT (S2-3) and CA. On one hand, in the thermal melting process, it took about 12 min for pure CA to maintain the equilibrium temperature platform, whereas the CA/DT (S2-3) and CA/DT/CNTs (S3-4) took 6 and 4 min, respectively. Moreover, during the solidifying process, it took 13 min for the pure CA to maintain the equilibrium temperature platform compared with 7 and 4 min for CA/ DT (S2-3) and CA/DT/CNTs (S3-4), respectively. The test results of CA, CA/DT (S2-3), and CA/DT/CNTs (S3-4) indicate that the heat transfer rate and efficiency were likewise improved.
3. CONCLUSIONS A novel CA/DT/CNT FSPCM with enhanced thermal conductivity was prepared via the melting−impregnation method in this study. The morphology, chemical compatibility, thermal properties, thermal stability, thermal conductivity, and thermal storage/release performance were investigated in detail. Furthermore, the enhancement mechanism of thermal conductivity of CA by the introduction of CNTs and support 2969
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Figure 10. Preparation of composites.
Figure 11. Seepage behaviors of composites (a) S2-1, (b) S2-2, (c) S2-3 and (d) S2-4.
mixtures were stirred for 2 min by using a glass rod. After this, the beakers containing the mixtures were placed into a thermostatic water bath at 50 °C for 10 min and the liquid PCM was impregnated into the porous structure of DT. Simultaneously, the mixtures were constantly stirred manually using a glass rod to mix evenly, as shown in Figure 10. The whole operation process such as weighing samples and mixing samples should be as fast as possible. Fourth, after DT was mixed with CA, the mouth of the beaker was covered with a thin plastic wrap. After the fabricated composite was cooled for 1 h to solidify CA completely into the pores of DT, the CA/ DT composites were obtained. The optimum holding ratio of CA into DT was determined by applying leakage tests to each composite. For this process, the fabricated composite sample was heated on a heater platform at 50 °C for 60 s to check seepage behaviors of CA in the melted state. Before testing, we allow the samples to solidify completely in a refrigerator at 0 °C for 1 h. Then 3 g of composites were weighed in turn and the samples with the same weight were dispersed onto the center of medium speed qualitative filter paper with a diameter of 12.5 cm, respectively. Afterwards, all the samples were shaped into similar cylinders to the utmost extent, shown in Figure 11. Finally, the samples prepared were heated for 60 s at 60 °C on a magnetic stirrer apparatus with the constant temperature and constant humidity for leakage observation and investigation. The heating process is as follows. The sample is placed on the magnetic stirrer apparatus, and the temperature begins to rise. After the temperature rises to 50 °C for continuous 60 s, the sample continuously produces a strong CA odor at 50 °C, which is much higher than its working temperature. Thus, the state of the heated sample could be considered as completely melted. As shown in Figure 11, the same amount of testing sample was exploited to perform the experiment. During the experimental process both S2-3 and S2-4 produce a strong CA odor, and the stain area is basically the same on the filter paper. Moreover, after further
materials was revealed by taking advantage of energy wave theory. The melting and freezing temperatures of CA/DT/ CNTs are 31.38 and 28.95 °C, respectively. Correspondingly, its latent heat values are 79.09 and 79.11 J g−1, respectively. The support material DT can prevent the leakage of pure CA liquid and simultaneously enhance the thermal conductivity of CA. The addition of CNTs does play an important role in enhancing the heat transfer rate and performance of FSPCMs by improving the heat transfer channel and reducing the wave resistances. Compared with the thermal conductivity of pure CA, the increase in the thermal conductivity of FSPCM samples were about 260.00, 273.33, 306.67, and 320.00%, respectively. The thermal conductivity of CA/DT/CNTs (7%) increased by 1.56 times compared to that of the binary system. CA/DT/CNTs can be promising PCM candidates used in building due to their suitable phase change point, high latent heat, good chemical compatibility, good thermal stability, and so on.
4. EXPERIMENTAL SECTION 4.1. Materials and Methods. Analytically pure capric acid used herein was obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). The diatomite samples were supplied by China’s Damao Chemical Reagent Co., Ltd (Tianjin, China). The DT sample purchased in this work had been purified. Before the experiments, the diatomite samples were dried at 120 °C for 2 h to remove the humidity. Carbon nanotubes (CNTs) were obtained from Suzhou Tanfeng Tech. inc. Reagent Co., Ltd (Suzhou, China). 4.2. Preparation of FSPCM. On the basis of the previous exploratory research, the optimum absorption ratio can be further obtained by leakage tests in detail. The FSPCMs were synthesized by two steps using the direct melt-impregnation method. In the first step, CA and dry DT samples with different mass fractions (58:42, 56:44, 54:46, and 52:48) were placed in a 250 mL beaker in turn, and subsequently the 2970
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*E-mail:
[email protected] (X.G.).
continuous heating for some time, there is no leakage on S2-3 and S2-4. In other words, both S2-3 and S2-4 have no liquid stains on the filter paper. Thus, a burnt phenomenon occurs in S2-3 at the same temperature and it may be seriously evaporated. On the other hand, S2-3 can absorb relatively more CA and has relatively high latent heat. So, S2-3 was selected as the relatively preferred sample. It confirms that CA has been successfully loaded with DT support. It may also be concluded that when the mass fraction of CA is 54%, DT can prevent the leakage of CA and form the FSPCM. Therefore, the leakage test results obtained from the samples with different impregnation rations show that the optimum mass fraction of CA into the prepared FSPCM corresponds to 54 wt %. In the second step, the CA/DT/CNT FSPCMs were prepared. In this process, the specified amounts of CNTs were added to the previously prepared FSPCM. To guarantee the dispersion of CNTs uniformly into the CA/DT FSPCM samples, the suspension in the beaker was continually stirred with a magnetic stirrer. Four kinds of FSPCMs were prepared by arranging the amount of the added CNTs. All the prepared samples were dried at room temperature. All samples are marked according to Table 3 in this paper.
ORCID
Peng Liu: 0000-0002-8728-1209 Liang Bian: 0000-0002-2769-7018 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (41872039 and 41831285), Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (17kffk13), the Hebei Key Technology R&D Program of the Agency of Hebei province (17214016), Ph.D. Research Startup Foundation of Hebei GEO University (BQ2017020, BQ2017021) and Supported by the Opening Project of Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education (18zxgk03).
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Table 3. Samples Used in This Work sample name S1-1 S2-1 S2-2 S2-3 S2-4 S3-1 S3-2 S3-3 S3-4
the composition ratio of the CA/DT composite (wt %) pure CA 58% CA + 56% CA + 54% CA + 52% CA + 54% CA + 54% CA + 54% CA + 54% CA +
42% 44% 46% 48% 46% 46% 46% 46%
DT DT DT DT DT DT DT DT
+ + + +
1% 3% 5% 7%
CNTs CNTs CNTs CNTs
4.3. Characterization of FSPCM. X-ray diffraction (XRD, Rigaku D/max-rB, Japan) analysis of CA, DT, and the composite samples were carried out using a test current of 10 mA and a voltage of 20 kV. A scanning electron microscope (SEM, Phenom ProX, Netherlands) was used to observe the surface morphology of the CA, DT, CA/DT (S2-3) FSPCM and CA/DT/CNT (S3-4) FSPCM. The Fourier transform infrared spectroscopy (FTIR) spectra of samples were obtained via a Fourier transform infrared spectrometer (Thermo Scientific) using the wavenumber region from 400 to 4000 cm−1, with a resolution of 2 cm−1 using KBr pellets. A differential scanning calorimeter (DSC, TA, Q100) was employed to determine the thermal storage performance of samples at a heating rate of 5 °C min−1. The thermal stability of samples was investigated by a TGA Instrument (TGA, Q600) from 25 °C (the room temperature) to 600 °C at a heating rate of 10 °C min−1. The temperature fluctuation was 0.1 °C and the accuracy of DSC was 0.1%. To investigate the thermophysical properties of samples, the cooling curve of the samples were evaluated by an intelligent paperless recorder. Meanwhile, the thermal conductivity of the samples was measured via a hot disk thermal constants analyzer (TPS2500).
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REFERENCES
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DOI: 10.1021/acsomega.8b03130 ACS Omega 2019, 4, 2964−2972
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DOI: 10.1021/acsomega.8b03130 ACS Omega 2019, 4, 2964−2972
