Preparation and Characterization of Modified Porous Wood Flour

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Preparation and Characterization of Modified Porous Wood Flour/LauricMyristic Acid Eutectic Mixture as a Form-Stable Phase Change Material liyun ma, Chuigen Guo, Rongxian Ou, lichao sun, Qingwen Wang, and liping li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03933 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Preparation and Characterization of Modified Porous Wood Flour/Lauric-Myristic Acid Eutectic Mixture as a Form-Stable Phase Change Material Liyun Ma, Chuigen Guo, Rongxian Ou, Lichao Sun, Qingwen Wang*, Liping Li* College of Materials and Energy, South China Agricultural University, Guangzhou, 510642, People’s Republic of China KEYWORDS: Porous wood flour; Lauric-myristic acid; Complex salt treatment method; Formstable phase change materials ABSTRACT: In this research, porous wood flour (WF) and a eutectic mixture of lauric acid (LA) and myristic acid (MA) were prepared as a form-stable phase change material (FSPCM) using a vacuum impregnation method. The effects of alkali (NaOH), cetyl trimethyl ammonium bromide (CTAB), and complex salts (CS) on the pore size of the WF were investigated. The results showed that the CS-treated WF (CS-WF) achieved the maximum pore size and impregnation ratio. The characterization by Fourier-transform infrared (FTIR) spectrometer, Xray photoelectron spectrometer (XPS), and X-ray diffractometer (XRD) illustrated that the combination of the LA-MA eutectic mixture and the CS-WF was a physical combination. The differential scanning calorimetry (DSC) results suggested that the optimum melting temperature and the latent heat of the CS-WF/LA-MA FSPCM were 33.1 °C and 98.2 kJ kg-1, respectively.

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The maximum impregnation ratio of the LA-MA eutectic mixture into the CS-WF was 60.3%. In addition, the thermogravimetric (TG) analysis indicated that the CS-WF/LA-MA FSPCM had better thermal durability than the pure LA-MA. Moreover, the CS-WF/LA-MA FSPCM had excellent thermal reliability after 500 thermal cycles. Thus, the CS-treatment of the WF was considered an excellent modification method. The prepared CS-WF/LA-MA FSPCM has the potential for latent heat thermal energy storage (LHTES) applications in terms of the proper phase-transition properties. 1. INTRODUCTION With the rapid development of the global industry, energy resource shortages, energy utilization ratios and environmental issues have become increasingly serious. Therefore, it is necessary to find a new method to recycle and reuse wasted energy, develop and utilize new types of energy, or store abundant energy. The phase change materials (PCMs) have high efficiency for storing excess energy; these materials have been used in thermal energy storage (TES); and have attracted a significant amount attention by many researchers and scholars.1 Furthermore, PCMs have been utilized in many areas, such as in the energy-efficient building industry2,3 and for solar TES systems4 due to the high-energy storage density and latent heat property, small temperature variation from storage to retrieval, and repeated utilization.5 In addition, there are many types of PCMs, such as inorganic PCMs (hydrous salt, metals, and alloys)6 and organic PCMs (paraffin, fatty acid, polyethylene glycol, and others). 7,8 PCMs, however, cannot be directly applied due to the leakage during the phase change although the leakage issue can be economically resolved by introducing a supporting material into the PCMs with a higher melting temperature than that of the PCMs.8


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Fatty acids are an organic PCM with typical solid-liquid phase-change characteristic. Lauric acid (LA) and myristic acid (MA) as fatty acids have been recommended as TES due to the high phase-transition enthalpy, the wide phase-transition temperature range, the lack of degradation after repeated hot and cold cycles, a long service life, low super-cooling, low-toxicity, noncorrosiveness, small volume variation, and the good compatibility with many supporting materials.9,10 However, some of the fatty acids have very high or very low phase-transition temperatures and cannot be directly applied in some fields. Therefore, according to the lowest eutectic point theory, a binary, ternary, or multivariate fatty acid eutectic mixture with a suitable phase-transition temperature is necessary for appropriate utilization. In fact, a fatty acid eutectic mixture also has a major shortcoming, which is the leakage problem during the phase-change process.11 The use of porous supporting materials can solve this problem. A variety of porous materials such as cellulose,12,13 inorganic porous materials (perlite, diatomite, vermiculite, and silicon dioxide), 14-16 porous carbon materials (graphite, activated carbon, and graphene),17-19 metal foam20,21 and porous polymers22 are frequently used to prevent the leakage of fatty acids during the phase-change process. Moreover, many studies14,23,24 indicated that the absorption of PCMs into porous materials was an effective way to limit the leakage of PCMs. Wen et al.14 reported that a capric acid (CA) and LA eutectic mixture was impregnated into porous expanded perlite (EP) and expanded vermiculite (EVM); the maximum impregnation ratios were 82.93% and 57.48% respectively without leakage. Zhang et al.23 discovered that porous cellulose acetate films retained cellulose acetate at a weight fraction of 86.9% without leakage. Wei et al.24 demonstrated that acid-treated EVM/carbon served as the capric-myristic-stearic acid (CA-MASA) supporting matrix. This not only prevented the leakage of the CA-MA-SA PCM, but also enhanced their thermal conductivity, reliability, and chemical stability.


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A series of porous materials have been investigated as the supporting substrate of form-stable PCM (FSPCM). However, some porous materials do not retain sufficient PCM due to a low specific surface area and porosity. Karaipekli et al.25 described that the only 20% of a CA-MA eutectic mixture was retained in the pores of vermiculite. Moreover, most of these porous materials are very expensive and the preparation technology of FSPCMs is difficult, which limits their use in many applications.26 Wood flour (WF) is a very common natural porous material that possesses many advantageous properties including an abundance of raw material, low-cost, and ease of fabrication for various applications. In fact, adding organic molecules to the WF results in unexpected hybrid properties; the porous structure not only increases the diffusivity of the PCMs but the leakage of the PCMs can also be prevented during the phase-change process. However, untreated WF has a very specific surface area and a blocky pore structure, making it difficult to use it as a PCMsupporting material. Therefore, the WF should be modified to improve the surface area, pore size and compatibility with the PCM-supporting material and it has been reported that effective methods have been developed. 24,27 An alkaline treatment (NaOH) can be tailored to remove lignin, hemicellulose and even cellulose from the lignocellulosic matrix, which can improve the porosity.28 As a cationic surfactant, cetyl trimethyl ammonium bromide (CTAB) can be easily infiltrated into the porous structure of supporting materials to improve the compatibility of the composites. 24, 29 A complex salt (CS) consists of the three ingredients NaCl, LiCl, and KNO3. Ortega-Liebana at al.30 reported that an approach for using a solution containing NaCl, LiCl and KNO3 (mass ratio 20:5:5) for impregnation and calcination to change the pore size of materials. In fact, organic PCMs such as LA and MA can be incorporated into the porous structure of


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modified WF by capillary force, surface tension, and hydrogen-bond interaction, thus preventing the leakage of the melted LA or MA from the FSPCM. In this study, a eutectic mixture of LA and MA (LA-MA) served as the PCM. The WF was modified by treatments with NaOH, CTAB, and CS and the effects of different treatment methods on the pore size of the WF were compared. The modified WF (MWF) was used as the supporting material. The MWF/LA-MA form-stable PCM (MWF/LA-MA FSPCM) such as NaOH-WF/LA-MA FSPCM, CTAB-WF/LA-MA FSPCM and CS-WF/LA-MA FSPCM.) were prepared by a vacuum impregnation method. 31 Subsequently, the structural and morphological properties of the prepared FSPCMs were analyzed with a scanning electron microscope (SEM) and an N2 adsorption instrument (ASAP 2460; Micromeritics, USA) techniques. The chemical compatibility and the thermal properties of the FSPCM were characterized by Fourier transform infrared spectrometer (FTIR), X-ray diffractometer (XRD), differential scanning calorimetry (DSC), and thermogravimetric (TG) analysis, respectively. The thermal reliability was also investigated by conducting an accelerated thermal cycling test using DSC characterization. 2. EXPERIMENTAL SECTION 2.1. Materials. LA (C12H24O2, CP) and MA (C14H28O2, CP) were purchased from the Ling Feng Chemical Reagent Co., Ltd., Shanghai, China. The poplar WF (80-100 mesh) was provided by the Pucheng Wood Industry Co., Ltd., Xuzhou, China. Sodium hydroxide (NaOH, 97%) and anhydrous ethanol were purchased from the Nanjing Chemical Reagent Co., Ltd., China. CTAB (AR) was bought from the Boao Biological Technology Co., Ltd., Shanghai China. Sodium chloride (NaCl, AR), potassium nitrate (KNO3, AR) and lithium chloride (LiCl, AR) were supplied by the Guangzhou Chemical Reagent Co., Ltd., China and were used to create CS solution.


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2.2. Determination and Preparation of LA-MA Eutectic Mixture. It is reported that the eutectic mass ratio of fatty acids can be calculated by the Schrader equation:14 =

−

∆

(1) where T is the melting temperature of the eutectic mixture; TA is the melting temperature of component A; ∆HA is the phase-change enthalpy of component A; XA is the molar fraction of ingredient A in the mixture; R is the gas constant.24 The thermal property parameters of the LA and MA obtained from the DSC test results are shown in Table 1. According to Equation (1), the theoretical mass ratio of LA-MA was calculated. Namely, when the mixture mass ratio of LA:MA=67.66:32.34, the LA-MA eutectic mixture has a lowest melting temperature, which is called the eutectic temperature. 32 The calculated minimum eutectic temperature T is 33.4 °C. The melting temperature of the mixture was measured by DSC. The results indicated that there was little difference between the measured temperature (34.6 °C) and the calculated temperature (33.4 °C) due to the influence of the external factor. Therefore, the LA-MA with a mass ratio of 67.66:32.34 was considered the eutectic composition. Table 1. Thermal Property Parameters of LA and MA Fatty acids

Melting temperatures (°C)

Latent heat of melting ( kJ kg-1)

Molar mass

LA

41.1

181.5

200.32

MA

53.4

221.9

228.37

The LA-MA eutectic mixture was prepared by the melt blending method. First, LA and MA were mixed in a beaker at the defined ratio (LA: 67.66%, MA: 32.34%). Then, the mixed LAMA was melted and stirred at 60 °C for 6 h on a magnetic stirring apparatus (Changzhou Aohua


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Co., Ltd., China). Finally, the LA-MA was slowly cooled down to the ambient temperature under atmospheric pressure. The LA-MA eutectic mixture was obtained. 2.3. Modification of the WF. In order to control the pore size of the WF improve its permeability and adsorption, and improve the compatibility between the WF and the PCM, The WF was modified by NaOH, CTAB and the CS treatment, respectively. The NaOH and CTAB treatments of the WF were conducted as follows: 30 g of WF and 100 mL of anhydrous ethanol were uniformly dispersed in each of two 500 mL three-necked flasks. Then, the NaOH (18 wt%) and CTAB (18 wt %) were added to the WF/anhydrous ethanol suspension. To guarantee the interaction of the WF with the NaOH solution and CTAB solution, the suspension was continually stirred at 60 °C for 2 h in a constant-temperature digital water bath (Guang Zhou Qian Hui Reagent Instrument Co., Ltd., China) with a mechanical stirrer. In order to remove the NaOH, CTAB, and anhydrous ethanol thoroughly, the obtained MWF was washed with distilled water and filtrated with a filtering device. The two resulting type of MWF were named NaOH-WF and CTAB-WF. The CS treatment of the WF was conducted as follows: 30 g of WF was put into a beaker and a certain amount of the self-matched CS solution was added slowly. This solution was made up of NaCl, LiCl, and KNO3 with a mass ratio of 4:1:1. The salt-soaked WF was calcined at 170 °C for 2 h to expand the pore size. Then, it was washed with distilled water and filtrated with a filtering device until no C1- was present (AgNO3 test); the product was named CS-WF. Finally, these samples were placed in an oven (Jiaxing Zhongxin Medical Instrument Co., Ltd., China) at 105 °C for 24 h. Then, the caked MWF was ground using a high-speed universal mill machine (FW-100, Tianjing Taisite Instrument Co., Ltd., China) to be suitable for impregnation;


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subsequently the MWF was sifted to 80-100 mesh and dried at 100 °C for 12 h to remove the residual water in a vacuum oven (Jiaxing Zhongxin Medical Instrument Co., Ltd., China). 2.4. Preparation of the FSPCM. Figure 1 schematically illustrates the procedure for preparing the FSPCM. The process is straightforward. First, the MWF was separately mixed with an excessive amount of the LA-MA eutectic mixture in a suction flask at room temperature. Then, the suction flask was connected to the vacuum pump and evacuated for 1h to complete the vacuum impregnation. In order to ensure the impregnation of the LA-MA eutectic mixture into the MWF, the LA-MA eutectic mixture was simultaneously heated at 60 °C during the impregnation process. Finally, the FSPCM was hot-filtered through filter paper to remove the liquid LA-MA eutectic mixture from the portion captured by the surface of the composites or not supported in the pores at 60 °C, until no leakage of the molten LA-MA eutectic mixture was observed or the weight loss of the FSPCM was less than 1%.24

Figure 1. Flow diagram of the preparation of FSPCM. To prove that the excessive LA-MA eutectic mixture was eliminated completely, a leakage test was carried out; specifically, the prepared MWF/LA-MA FSPCM and the pure LA-MA eutectic mixture were placed on a filter paper for comparison as shown in Figure 1a. Then, the samples


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were heated on a heating plate for 1 h at 60 °C. The test results are shown in Figure 1b. It can clearly be seen that the pure LA-MA has melted, but that the MWF/LA-MA FSPCM retains its overall shape and there are no traces of the melt exudation on the filter. This result indicates that the WF can be used as the supporting matrix and that the LA-MA eutectic mixture is well encapsulated. 2.5. Characterization of the MWF/LA-MA FSPCMs. To verify the effect of MWF on the physical, chemical, and thermal properties of the MWF/LA-MA FSPCMs, the prepared FSPCMs and the components were systematically characterized. The microstructure of the WF, MWF, WF/LA-MA, and the MWF/LA-MA FSPCMs was investigated by SEM (Quanta 200 model; Netherlands FEI). Prior to the measurements, the samples were coated with a gold/palladium alloy to prepare them for imaging. The pore size distribution and specific surface area of the MWF and the MWF/LA-MA FSPCMs were measured using the Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller (BET) methods respectively and using a N2 adsorption instrument (ASAP 2460; Micromeritics, USA). The physicochemical compatibility of the components of the FSPCMs was explored by using an FTIR (Perkin Elmer Spectrum 100; USA) in the wavenumber range of 400-4000 cm-1. The analysis was conducted at room temperature. Prior to the measurements, each sample was mixed with the KBr powder and then pressed into a small pellet. The crystalline phases of the WF, the LA-MA eutectic mixture, and the MWF/LA-MA FSPCMs were characterized by XRD (Ultima IV, Rigaku Corporation; Japan) in the angular range of 0-40° with 5°/min. X-ray photoelectron spectroscopy (XPS) (ESCALAB 250, USA) with Al Kɑ (1486.6 eV) as the X-ray source set at 150W and a pass energy of 30eV for high-resolution scans was used to characterize the WF, MWF, and FSPCM to illustrate the chemical affection.


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The melting and crystallization temperatures and latent heat values were determined for the pure LA-MA and MWF/LA-MA FSPCMs using the DSC (NETZSCH STA-449C model instrument, Germany). Samples of 3-8 mg were put into aluminum crucibles for the measurement. All the DSC measurements were carried out at a heating/cooling rate of 5 °C/min. All experiments were performed in a static nitrogen atmosphere of 20 mL/min and each sample was measured twice and the average values were used. The thermal stability of the LA-MA and MWF/LA-MA FSPCMs was investigated at a temperature range of 30-800 °C and using a TG method (Q500 V20.10 Build 36 instrument; USA), at a heating rate of 10 °C/min and under a nitrogen atmosphere; samples of 8~12 mg were put into aluminum crucibles for the measurement. To demonstrate the thermal storage efficiency, it is necessary to perform a thermal cycling test. Ten grams of the FSPCM were placed on a culture dish covered with filter paper. The containers were placed into the chamber with the thermostat set at 60 °C (which is 20-30 °C higher than phase transformation point of the FSPCM) to achieve complete melting. Subsequently, the FSPCM was cooled in a refrigerator set at -10°C (which is 20-30 °C lower than the crystallization temperature of the FSPCM) to achieve complete crystallization. The procedure was continuously repeated 500 times and the filter paper was changed after 10 cycles. The DSC was performed to determine the thermal properties of the FSPCM after the thermal cycles. 3. RESULTS AND DISCUSSION 3.1. Microstructure and Pore Structure Analysis Results. The pore diameter and specific surface area of the samples were obtained using the BJH and BET methods. Figure 2 shows the pore size distribution curves with partial enlargements of the details (Figure 2a-b) and the nitrogen adsorption-desorption isotherms (Figure 2c-d). As shown in Figure 2a-b, the pore size


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of all samples falls mostly in the mesopore category but nano-sized pores are more conducive to adsorption. As shown in Figure 2c-d, all samples exhibit type III adsorption.33 This means that the pore structure of all samples consists of open-ended tubular capillaries, which are beneficial to the adsorption. In the low-pressure zone, the nitrogen adsorption capacity is lower for the FSPCM than for the WF and MWF, which illustrates that the force between nitrogen and FSPCM is weaker than the force between the nitrogen and the WF or MWF. This shows that the pore structure of the WF and MWF has changed and that some pores have been blocked by the LA-MA. In the high-pressure zone, the adsorption curve increases. The nitrogen condensed in the pores and this led to some desorption hysteresis, because the pores were mostly filled with the LA-MA and the porosity decreased sharply. Table 2 shows the details of the changes in the pore structure. The average pore diameter of the natural WF is 8.73 nm and increased after modification to 13.34, 10.71, and 16.54 nm for NaOHWF, CTAB-WF, and CS-WF, respectively. CS-WF had the maximum pore size. The results also indicated that the average pore diameter after impregnation was decreased from the abovementioned values to 5.85, 8.29, 6.78, and 6.32 nm for the WF/LA-MA, NaOH-WF/LA-MA, CTAB-WF/LA-MA and CS-WF/LA-MA respectively. In other words, when the LA-MA eutectic mixture is incorporated into the porous structure of the MWF by capillary force, surface tension, and hydrogen bond interaction, the pore diameter decreases. Moreover, the maximum impregnation ratio occurred for the CS-WF/LA-MA FSPCM, because its average pore diameter decreased approximately 10 nm. In addition, the date in Table 2 also shows that the specific surface area of the WF/LA-MA, NaOH-WFLA-MA, CTAB-WF/LA-MA, and CS-WF/LA-MA decreased after impregnation. In conclusion, the CS-treated WF achieved the maximum pore size and impregnation ratio.


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Figure 2. Pore size distribution curves (a, b) and nitrogen adsorption-desorption isotherms (c, d) of WF, NaOH-WF, CTAB-WF, CS-WF (a-b);WF/LA-MA, NaOH-WF/LA-MA, CTABWF/LA-MA and CS-WF/LA-MA FSPCM (c). Table 2. Specific Surface Area and Pore Diameter of MWF and MWF/LA-MA FSPCMs WF

NaOH-WF

CTAB-WF

CS-WF

Specific surface area (m2·g-1)

7.69

5.12

6.53

4.45

Pore diameter (nm)

8.73

13.34

10.71

16.54

WF/LA-MA

NaOH-WF/LA-MA

CTAB-WF/LA-MA

CS-WF/LA-MA

Specific surface area (m2·g-1)

6.41

3.48

4.85

2.38

Pore diameter (nm)

5.85

8.29

6.78

6.32

The SEM images of the samples are shown in Figure 3. It can be seen from Figure 3a (WF) that the natural WF has a porous morphology. Figure 3b (NaOH-WF) and Figure 3d (CS-WF) show an increase in the pore diameter. However, the pore diameter of the WF changed little for


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the CTAB treatment (Figure 3c, CTAB-WF) and many pores remained. Figure 3e-h clearly shows that the microstructure of the porous WF has changed, indicating that the LA-MA eutectic mixture was impregnated into the pore structure of the WF. However, many pores are not completely filled in the WF/LA-MA FSPCM (Figure 3e) and WF/LA-MA FSPCM (Figure 3g), which may be due to the low interface energy, which led to poor compatibility. However, I the NaOH-WF/LA-MA FSPCMs (Figure 3f) and CS-WF/LA-MA FSPCM (Figure 3h), it can be seen that many fatty acids are attached to the surface of the WF, resulting in good compatibility. Interestingly, in the CS-WF/LA-MA FSPCM (Figure 3h), all the pores were completely filled with the LA-MA eutectic mixture and the mixture also covered the surface, resulting in hard to distinguish unclear interfaces after the impregnation treatment. This fact proved that the LA-MA was well impregnated into the porous CS-treated WF under the vacuum conditions, as expected. As a result, the CS-treated WF had the largest latent heat and the best adsorption performance due to the capillary force, surface tension, and hydrogen-bond interaction.

Figure 3. SEM images of the WF (a), NaOH-WF (b), CTAB-WF (c), CS-WF (d), WF/LA-MA FSPCM (e), NaOH-WF/LA-MA FSPCM (f), CTAB-WF/LA-MA FSPCM (g), and CS-WF/LAMA FSPCM (h).


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3.2. Thermal Properties of the FSPCM. The DSC curves for the LA-MA, and the MWF/LA-MA FSPCMs are shown in Figure 4. The melting temperature (Tm) and crystallization temperature (Tc) were obtained by drawing a line at the point of maximum slope of the DSC peak. The latent heat was calculated by numerical integration of the peak using software of DSC instrument.34 The results are listed in Table 3. Table 3. Thermal Properties of the LA-MA and FSPCM Heating process

Cooling process

Tm (°C)

△Hm (kJ kg-1)

Tc (°C)

△Hc (kJ kg-1)

LA-MA

34.6

163.0

33.1

157.5

WF/LA-MA

33.1

63.7

32.6

60.5

NaOH-WF/LA-MA

33.5

91.1

32.3

88.0

CTAB-WF/LA-MA

33.9

85.4

32.6

81.7

CS-WF/LA-MA

33.1

98.2

32.9

96.1

As shown in Figure 4 and Table 3, compared to the values of the pure LA-MA eutectic mixture, the melting temperature of the FSPCMs changed slightly and the latent heat values decreased due to the confinement of the LA-MA eutectic mixture’s molecules in the pores of the WF and MWF. The results show that the latent heats of the MWF/LA-MA FSPCMs were higher than those of WF/LA-MA FSPCMs and the ranking was WF/LA-MA < CTAB-WF/LA-MA < NaOH-WF/LA-MA < CS-WF/LA-MA. The reason is that the latent heats of the MWF/LA-MA FSPCMs largely depended on the impregnation ratio of the LA-MA eutectic mixture. The pore diameter of the WF was increased by the NaOH, CTAB, and CS treatments, and resulted in an increase the impregnation ratio of the LA-MA eutectic mixture. The LA-MA eutectic mixture impregnation ratio was calculated by Equation. (2):7


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∆

η= ∆ 100%

(2) where η is impregnation ratio; ∆HFSPCM and ∆HLA-MA are the latent heats of the FSPCM and the LA-MA eutectic mixture, respectively. Finally, according to Equation (2) and the values in Table 3, the adsorption rate was calculated. The eutectic mixture was retained in the WF/LA-MA FSPCM, NaOH-WF/LA-MA FSPCM, CTAB-WF/LA-MA FSPCM, and CS-WF/LA-MA FSPCM at 39.1 wt %, 55.9 wt %, 52.4 wt %, and 60.3 wt % LA-MA, respectively without leakage. The maximum impregnation ratio was 60.3% for the CS-WF/LA-MA FSPCM. Furthermore, the melting latent heat was 98.2 kJ kg-1 for the CS-WF/LA-MA FSPCM, which was 54.2%, 7.8% and 15.0% higher than the value of WF/LA-MA FSPCM, NaOH-WF/LA-MA FSPCM and CTAB-WF/LA-MA FSPCM, respectively. Similarly, the CS-WF/LA-MA FSPCM had the highest crystallization latent heat among all the FSPCMs. Therefore, the best thermal properties were observed for the CS-WF/LA-MA FSPCM. Based on the results of the impregnation ratio and the changes in the latent heat, it is concluded that the LA-MA eutectic mixture was introduced into the porous structure of the WF and MWF.

Figure 4. DSC curves of the LA-MA and the FSPCMs. Table 4 shows a comparison of the thermal properties of the CS-WF/LA-MA prepared in this study with those of FSPCMs reported in the literature. The melting temperature of the CS-


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WF/LA-MA differs from that reported in the literature. However, it has a relatively higher latent heats and impregnation ratio. Therefore, the prepared CS-WF/LA-MA as FSPCM has significant thermal energy storage ability for latent heat thermal energy system (LHTES) applications. Table 4. Comparison of the Thermal properties of the CS-WF/LA-MA Prepared in this Study and those of FSPCM reported in the Literature Tm

△Hm (kJ

Tc

△Hc (kJ

Impregnation

(°C)

kg-1)

(°C)

kg-1 )

ratio

22.8

80.45

22.0

-

57.48%

(14)

Lauric acid/expanded perlite

44.1

93.4

41.0

94.9

60%

(15)

Lauric acid/activated carbon

44.1

65.1

42.8

63.0

33.3%

(18)

22.9

86.4

21.0

80.4

54.4%

(24)

19.8

27

17.1

-

20%

(25)

98.2

32.9

96.1

60.3%

This work

FSPCMs Capric–lauric acid/expanded vermiculite

Capric-myristic-stearic/expanded vermiculite Capric-myristic/vermiculite Lauric-myristic/ CS-WF

33.1

References

3.3. Chemical Characterization Results. The FTIR spectra of the samples are shown in Figure 5. For the LA-MA eutectic mixture, the characteristic bands in the FTIR spectrum appear at 2922, 2852, 1712, 935, and 732 cm-1. The characteristic peaks at 2922 and 2852 cm-1 are the asymmetrical and symmetrical stretching vibration of the C-H group, which was attributed to the -CH2 and -CH3. There is an absorbance peak at 1712 cm-1, which belongs to the stretching vibration of the C=O. The peaks at 935 cm-1 and 732 cm-1 are attributed to the bending vibration and the swinging vibration of the O-H functional group.35 Wood consists of cellulose, hemicellulose, and lignin, which contain many functional groups, such as O-H, C-H, C-O groups of cellulose, hemicellulose, and lignin, and the C=O group of lignin. Therefore, for the WF (Figure 5a), the strong adsorption peak at 3297 cm-1 is associated with -OH stretching vibrations of WF. The peaks at 2920 and 2851 cm-1 are attributed to the asymmetrical and symmetrical stretching vibration of the C-H group, which was attributed to the


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Energy & Fuels

-CH2 and -CH3. There is an absorbance peak approximately at 1724 cm-1, which corresponds to the stretching vibration of the C=O group, the characteristic peak at 1032 cm-1 is attributed to the stretching vibration of the C-O functional group. However, for the MWF, the peak at 1724 cm-1 did not appear for the NaOH treatment (Figure 5b), which suggests that the hemicellulose, lignin, and extract in the WF were removed. For the CS-WF (Figure 5c-d), there are no new characteristic peaks of the functional groups, but the number of functional groups was lower and the peaks had moved, indicating that a large amount of hemicellulose, lignin, and extract was also removed. Thus, the pore size of the WF was enlarged. No additional functional groups were observed in all of spectra of the FSPCMs, indicating that the LA-MA eutectic mixture with the MWF consist of a physical combination. The FTIR results demonstrate that there is excellent physicochemical compatibility among the components of the FSPCMs.

Figure 5. FTIR spectra of the LA-MA, WF, NaOH-WF, CTAB-WF, CS-WF and the FSPCMs.


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Figure 6 shows the C1s spectra of the LA-MA, WF, CS-WF, and CS-WF/LA-MA FSPCMs. It was employed to illustrate the chemical affection between the LA-MA eutectic mixture and the CS-WF. There are two peaks corresponding to the C-C and O-C=O carbon bonds in LA-MA eutectic mixture. The raw C1s spectra of WF can be divided into 4 peaks that corresponding to C-C, C-O, C=O, O-C=O, respectively. Moreover, after the CS treatment, these bonds still occurred, suggesting that there is no chemical affection between the CS and WF. Similarly, for the CS-WF/LA-MA FSPCM, no additional functional groups were observed. Only the intensity of the functional groups changed, meaning that there is no chemical affection between the LAMA eutectic mixture and the CS-WF. As shown in Table 5, compared with the C1s spectra of the CS-WF, the intensity of the C-C and O-C=O peaks increases after the CS-WF was adsorbed into the LA-MA eutectic mixture, indicating that the LA-MA eutectic mixture was introduced into the porous structure of the CS-WF. Based on these results, there is no chemical affection between LA-MA eutectic mixture and CS-WF, and only physical combination occurred.

Figure 6. The C1s XPS spectra of the LA-MA, WF, CS-WF and CS-WF/LA-MA.


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Energy & Fuels

Table 5. Concentration of the Different C Groups Samples

C-C % At Conc.

C-O % At Conc.

C=O % At Conc.

O-C=O % At Conc.

LA-MA

93.46

0

0

6.54

WF

50.47

39.86

8.25

1.42

CS-WF

54.13

32.36

11.28

2.23

CS-WF/LA-MA

68.63

19.54

5.87

5.96

Figure 7 shows the XRD patterns of the CS-WF, the LA-MA eutectic mixture, and the CSWF/LA-MA FSPCMs. Characteristic peaks in the LA-MA eutectic mixture were observed at 2θ=9.1°, 21.5°, and 23.4°. Accordingly, the LA-MA eutectic mixture exhibited regular crystallization. In addition, there is only one strong diffraction peak located at 21.9° in the pattern of the CS-WF, which can be ascribed to the characteristic peak of the WF. After the LAMA eutectic mixture was imbibed into the CS-WF, the location of the strongest peak of the LAMA eutectic mixture remained unchanged although the intensity was reduced. This may be attributed to the confinement effect of the CS-WF in the LA-MA eutectic mixture. Thus, the crystallite size of the LA-MA eutectic mixture in the composites is smaller than that of the pure state. As can be seen from Figure 7, the CS-WF/LA-MA FSPCM exhibits the characteristic peaks of each component. No additional peaks from the other phase were discovered. This indicates that there is only a physical combination between the LA-MA eutectic mixture and the CS-WF, and no a chemical reaction.


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Figure 7. XRD patterns of the CS-WF, the LA-MA eutectic mixture, and the CS-WF/LA-MA FSPCM. 3.4. Thermal Stability of the FSPCM. Figure 8 shows the TG (Figure 8a) and derivative TG (DTG) curves of the CS-WF, the LA-MA eutectic mixture, and the CS-WF/LA-MA FSPCM. The results show that the degradation of the CS-WF occurred in three steps, which correspond to the cellulose, hemicellulose and lignin degradation in the different temperature ranges. The pure LA-MA eutectic mixture exhibited an one-step degradation in the temperature range of 130250 °C, which was due to the evaporation of the LA-MA eutectic mixture, whereas the CSWF/LA-MA FSPCM degraded at least two steps; during the stage, the weight loss of the CSWF/LA-MA FSPCM was about 60%. This ratio is in agreement with the encapsulation fraction (60.3 wt %) of the LA-MA into the CS-WF/LA-MA FSPCM. Furthermore, Figure 8a shows that the total weight loss values of the LA-MA and CS-WF/LA-MA FSPCM were 100% and 84.3%, respectively. The observed mass loss was ascribed to the degradation of the LA-MA eutectic mixture. The DTG curves in Figure 8b indicate a prolonged decomposition process of the CSWF/LA-MA FSPCM. It was concluded that the CS-WF/LA-MA FSPCM had good thermal stability.


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Energy & Fuels

Figure 8. TGA and DTG curves of the LA-MA and the FSPCM. 3.5. Thermal Reliability of the FSPCM. A good thermal reliability means that the material has high thermal storage efficiency. Figure 9 depicts the DSC curves of the CS-WF/LA-MA FSPCM before and after 500 thermal cycles. The thermal properties of the CS-WF/LA-MA FSPCM are shown in Table 6. The melting temperature (Tm) and crystallization temperature (Tc) of the CS-WF/LA-MA FSPCMs were 33.1 °C and 32.9 °C before thermal cycles, and 34.1 °C and 32.5 °C after 500 thermal cycles. The latent heats of heating and cooling the CS-WF/LAMA FSPCM changed from 98.2 kJ kg-1 and 96.1 kJ kg-1 to 91.4 kJ kg-1 and 90.7 kJ kg-1, respectively. However, the latent heat of the CS-WF/LA-MA FSPCM remained at a relatively high level. In addition, the melting and crystallization temperatures for heating and cooling the CS-WF/LA-MA FSPCM changed slightly. Therefore, the prepared CS-WF/LA-MA FSPCM has good thermal reliability after the thermal cycling. As a result, the thermal storage efficiency of the material is excellent.


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Page 22 of 25

Figure 9. DSC curves of the CS-WF/LA-MA FSPCM before and after 500 thermal cycles. Table 6. Thermal Properties of the FSPCM before and after Thermal Cycling Heating process

Cooling process

Tm(°C)

△Hm(kJ kg-1)

Tc(°C)

△Hc(kJ kg-1)

CS-WF/LA-MA FSPCM

33.1

98.2

32.9

96.1

CS-WF/LA-MA FSPCM (after cycling)

34.1

91.4

32.5

90.7

4. CONCLUSION In this study, the LA-MA eutectic mixture was first created with a mass ratio of LA:MA=67.66:32.34 based on the lowest eutectic point theory. Subsequently, the WF was modified by NaOH, CTAB, and CS treatments. Finally, the FSPCM were produced by the vacuum impregnation method. A comparison of the three different treatment methods and the analysis of the morphology, the chemical properties, and the TG analysis indicate that the CStreated WF exhibited the maximum pore size and impregnation ratio and the CS-WF/LA-MA FSPCM had superior properties compared with the NaOH- and CTAB-treated WF. The maximum impregnation ratio of the LA-MA eutectic mixture into the CS-WF/LA-MA FSPCM was 60.3 % compared to only 55.9% and 52.4% for the NaOH-WF/LA-MA FSPCM and CTABWF/LA-MA FSPCM, respectively. The latent heat and the thermal stability of the CS-WF/LAMA FSPCM were higher than for the other FSPCM. For the CS-WF and LA-MA, only physical incorporation was observed. Furthermore, the CS-WF/LA-MA FSPCM had good thermal reliability after the thermal cycling. As a result, the thermal storage efficiency was excellent. Based on these results, it is concluded that the CS-WF/LA-MA FSPCM is a promising candidate for use as energy-storing wallboard and packaging material for thermal energy storage applications due to the superior properties and simple technology. ■ AUTHOR INFORMATION


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Energy & Fuels

Corresponding Author *Tel.: +86-18344306921. E-mail: [email protected] E-mail: [email protected] ■ ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (31570572, 31670516 and 31600459). ■ REFERENCES (1)

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Preparation and Characterization of Modified Porous Wood Flour

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