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Article

A Simpler Fabrication for Thermal Energy Storage Wood

by
Weihua Zou
*,
Cong Li
,
Delin Sun
and
Naike Zou
School of Materials Science and Engineering, Central South University of Forestry and Technology, Shaoshan South Road 498, Changsha 410004, China
*
Author to whom correspondence should be addressed.
Forests 2023, 14(6), 1190; https://doi.org/10.3390/f14061190
Submission received: 27 April 2023 / Revised: 5 June 2023 / Accepted: 7 June 2023 / Published: 8 June 2023
(This article belongs to the Section Wood Science and Forest Products)
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Versions Notes

Abstract

:
Using thermal energy storage wood with phase change materials (PCM) as a building material can save thermal energy during heat-induced phase transition, and can reduce the energy consumption of indoor heating. In our work, three thermal energy storage poplars (TESPs: TESP-1, TESP-2 and TESP-3) were prepared by directly infiltrating three PCMs (fatty alcohol/acid materials: lauryl alcohol, decanoic acid and myristic acid myristyl ester), respectively, into the longitudinal-cutting plantation poplar woods and by directly encapsulating the PCMs in the poplar-based materials with SiO2 films. The phase-changing temperature ranges of TESP-1, TESP-2 and TESP-3 were at 19–30 °C, 26–39 °C and 33–54 °C, respectively. The phase-changing temperature peaks were at ~24 °C, ~31 °C and ~42 °C, respectively. After the same heat treatment on TESPs and original poplar (OP), the average temperature of TESPs was higher than that of OP after 35 min, thus proving that TESPs can save more thermal energy than OP. The radial bending strengths of TESP-1, TESP-2 and TESP-3 had increased by 50.85%, 70.16% and 70.31%, respectively, as compared to with that of OP. Additionally, the radial bending elastic modules of TESP-1, TESP-2 and TESP-3 had increased by 47.14%, 67.38% and 74.57%, respectively, as compared to OP. The tangential section hardness of the TESPs also had also increased by 67.09%, 71.80% and 80.77%, respectively. These improved mechanical properties of TESPs are almost close to that of ash wood (ash wood is a common building material), therefore, this proves that our TESPs can be used as thermal energy-saving building materials.

1. Introduction

The energy consumption of building had exceeded 30% in global energy consumption due to population growth and the development of economy, part of which was caused by building heating. Therefore, using thermal energy storage technology to prepare building materials, such as temperature-induced phase transition, has garnered worldwide interest in recent years [1,2,3,4]. Phase change material (PCM) is a thermal energy storage material (TESM), that can release or absorb thermal energy during the phase-changing process of PCM [1,5,6]. Wood is a natural and renewable porous material, and it can also be used as the matrix material of TESM, therefore, some thermal energy storage woods had been prepared using PCM as the TESM and woods as the matrix material [1,2,3,4,5,6].
Using the thermal energy storage wood (TESW) as a building material can help to store thermal energy as the PCM of TESW absorbs heat, and can thus reduce the energy consumption of indoor heating. Over the years, many scholars have conducted several studies on TESW. Liu et al. prepared a delignified balsa-based TESW by incorporating a solid-solid PCM into the delignified balsa [2]. Montanari et al. prepared a delignified silver birch wood-based transparent wood for thermal energy storage and reversible optical transmittance [3]. Wang et al. also prepared a delignified balsa-based TESW using microencapsulated PCM [4]. Ma et al. prepared a delignified cedar-based TESW using a capric acid-palmitic acid mixture stable-form PCM [7]. Yang et al. prepared a delignified poplar-based thermochromic TESW using crystal violet lactone/bisphenol A/tetradecanol PCM [8], a delignified poplar-based self-luminous wood composite for both thermal and light energy storage [9], and a delignified balsa-based TESW using 1-tetradecanol and Fe3O4 nanoparticles for magnetic-thermal and solar-thermal energy conversion and storage [10]. Li et al. prepared a delignified balsa-based TESW using a polyethylene glycol-based PCM [11]. Liu et al. also prepared a delignified balsa-based fluorescent thermochromic TESW by encapsulating polyethylene glycol and aggregation-induced emission carbon dots into delignified wood [12]. Sun et al. prepared a delignified platane-based TESW using fatty acid PCM [13]. Many researchers used delignified wood as the matrix material of TESW due to their loose structure which increases their potential for infiltrating PCM into wood [2,3,4,7,8,9,10,11,12,13]. For preventing the instability and leakage of PCM, some researchers prepared microencapsulated PCM and filled it into a wood-based material [1,4,14,15,16,17,18,19]. Lin et al. prepared a TESW using graphene aerogel-encapsulated polyethylene glycol as the PCM and using the full poplar as the matrix material [1]. Wang et al. used two N-alkyl acryl-amide monomers to form the wall material of the microencapsulated PCM for preparing TESW [4]. Jamekhorshid et al. prepared a composite of wood–plastic microencapsulated PCM [14]. Geng et al. researched the influences of PVA modification on performance of microencapsulated thermochromic PCM [15]. Jeong et al. prepared a microencapsulated PCM using RT31 as the core material and using melamine as the shell material for preparing TESW [16]. Lin et al. prepared a TESW with high-performance anisotropic thermal conductivity by impregnating balsa with the polymer-encapsulated PCM and via the subsequent in situ chemical deposition of copper inside the wood cell lumen [17]. Mathis et al. used a resin-encapsulated bio-based PCM for preparing TESW [18]. Guo et al. prepared a microencapsulated PCM by containing dodecanol via in situ polymerization [19]. In comparison with directly filling the original wood with PCM to prepare the TESW, filling delignified wood with microencapsulated PCM would reduce the energy consumption of the fabrication process of TESW due to the removal of the lignin of the wood and the preparation of the microencapsulated PCM. The strength of wood was reduced after delignification, which limits the application range of delignified wood-based TESW in building materials. Therefore, research must be conducted to find a simpler fabrication process for preparing a TESW with better strength in directly filling the PCM into the full original wood.
In this work, three kinds of thermal energy storage poplars (TESPs) were prepared using a simpler fabrication process. After being subjected to the same heat treatment (60 °C, 30 min), it was found that the average temperatures of TESPs were higher than that of the original poplar (OP) after 35 min, thus proving that the TESPs can save more thermal energy than OP. Although the base material of TESPs was the poplar, the radial bending strengths, radial bending elastic modules and tangential section hardness of TESPs (see the mechanical analysis) were close to those of ash wood (ash wood is a common building material, with 114.20 MPa radial bending strength, 10.40 GPa radial bending elastic modulus and 4.60 kN tangential section hardness [20]), thus, the TESPs can be used as a thermal energy-saving building material.

2. Materials and Methods

The TESPs were prepared by directly infiltrating three kinds of PCMs (fatty alcohol/acid materials) into the full original poplar (OP) samples, and by directly encapsulating the PCMs in the poplar-based materials with the silicon dioxide films (Figure 1). In the fabrication of TESPs, the processes of delignification and the preparation of microencapsulated PCM were omitted, so this fabrication of TESPs has a simpler process and is more energy-saving.

2.1. Materials and Chemicals

Plantation poplar is widely planted in many countries; itis a kind of farmed green material [21,22,23]. We used the longitudinal-cutting plantation poplar wood as the matrix material in this work as it is a good substitute for the natural forest woods. Thirty full original poplar samples (Populus spp., 300 × 20 × 20 mm) were purchased from Shandong Menghui (Linyi, China).
Fatty alcohol/acid material is a common PCM [1,2,3,4,5,6,7,8]. For conforming to body temperature, we decided to select lauryl alcohol (C12H26O, >99%, melting temperature: 24 °C), decanoic acid (C10H20O2, >99%, melting temperature: 31.5 °C), and myristic acid myristyl ester (C28H56O2, >99%, melting temperature: 41 °C) as PCM-1, PCM-2, and PCM-3, respectively. These PCMs were purchased from Xilong Science Co., Ltd. (Shantou, China).
According to the composite mechanisms of SiO2 and -OH [24,25], and the mechanism of encapsulating organic PCM with SiO2 [6], we decided to select neutral silica sol as the encapsulation material for deep infiltration of the SiO2 into the lumen of wood. The neutral silica sol (mSiO2·nH2O, SiO2: 30%) was purchased from Longda (Zhengzhou, China).

2.2. Preparation of TESPs

The full original poplar (OP) samples were dried at 100 °C for 80 h via the 202-00T constant temperature drying oven (Lichen, Shanghai, China); After melting each PCM at 60 °C, the OP samples and the PCM liquid were put into the RV−620−2 vacuum reactor (YBIF, Shanghai, China), and the OP samples were immersed in the PCM liquid. Then, the PCM was infiltrated into the OP samples by vacuumizing at 60 °C for 3 h, −0.09 MPa. After the infiltration of PCMs, the samples were immersed in neutral silica sol at ~25 °C for 24 h. Then, the samples were dried at 100 °C for 24 h via the constant temperature drying oven.

2.3. Test Section

Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), Nexus-470 FT-IR (Nicolet, Madison, WI, USA) and PHI-5000 Versaprobe III XPS (ULVAC-PHI, Kanagawa, Japan), were used to detect functional groups. The FTIR spectra were obtained between 4000 and 400 cm−1. The binding energy of XPS survey were from 0 to 1350 eV.
Scanning electron microscope (SEM) and energy disperse spectroscopy (EDS), MIRA-4 SEM (TESCAN, Brno, Czech Republic), were used to observe microstructure at 10 nm, and the central parts of the sample (300 × 20 × 20 mm) were selected as the testing samples. The MIRA-4 SEM, equipped with EDS, was used to test the contents of C, O and Si in the sample.
Differential scanning calorimetry (DSC), WKTS-RC500 DSC (VicoMeter, Taizhou, China), was used to detect phase-changing temperature characteristics, and the samples were heated from 0 to 100 °C with a heating rate of 10 °C min−1.
The surface temperatures of heat-treated OP and TESPs were compared in order to test their heat storage [26]. The UT305A infrared thermometer (UNI-T. Ningbo, China) was used to test their surface temperatures after OP and TESPs were heat-treated at 60 °C for 30 min, and each temperature was measured at 5 min intervals. The weights of TESPs before and after heat treatment at 103 °C for 150 h were compared to test the heat resistances of TESP with or without the SiO2 film [23,24].
Tangential section hardness, radial bending strength, radial bending elastic modulus and longitudinal compressive strength were tested via GB/T 1941–2009, GB/T 1936.1–2009, GB/T 1936.2–2009 and GB/T 1935–2009, respectively.
Getting weight gain rate (WPG) was calculated based on Equation (1) [24]:
WPG = (M1 − M0)/M0 × 100%
  • M1—Mass of TESP when absolute dry (g)
  • M0—Mass of OP when absolute dry (g)

3. Results and Discussion

3.1. FT-IR Analysis

Figure 2 shows the FT-IR spectrum of OP, PCMs (PCM-1, PCM-2 and PCM-3) and TESPs (TESP-1, TESP-2 and TESP-3).
The FT-IR spectra of OP: The stretching vibrations at 1026 cm−1, 1235 cm−1, 1505 cm−1 and 1735 cm−1 represent C-O of cellulose, C-O of guaiacki lignin, C=C of lignin aromatic ring, and C=O of hemicellulose, respectively. The peaks at 2914 cm−1 and 3320 cm−1 are the stretching vibrations of C-H and -OH groups in OP, respectively [27,28,29,30,31].
The FT-IR spectra of PCMs: The strong signals at 1450 cm−1, 2850 cm−1 and 2900 cm−1 all correspond C-H groups of dimethyl and trimethyl in PCMs (PCM-1, lauryl alcohol, C12H26O; PCM-2, decanoic acid, C10H20O2; and PCM-3, myristic acid myristyl ester, C28H56O2), respectively [22,23,24,25].
The FT-IR spectra of TESPs: The stretching vibrations of 1026 cm−1 (C-O of cellulose), 1235 cm−1 (C-O of guaiacki lignin), 1505 cm−1 (C=C of lignin aromatic ring) and 1735 cm−1 (C=O of hemicellulose) all appeared clearly in the FT-IR spectra of TESP-1, TESP-2 and TESP-3, which proved that TESPs retained the functional group characteristic of OP. The stretching vibrations of 1450 cm−1, 2850 cm−1 and 2900 cm−1 also appeared in the FT-IR spectra of TESP-1, TESP-2 and TESP-3, respectively, which all were caused by the dimethyl and trimethyl of PCMs. The stretching vibrations of 800 cm−1 and 1067 cm−1 represent O-Si-O of SiO2 [32,33], and these stretching vibrations appeared in the FT-IR spectra of TESP-1, TESP-2 and TESP-3, which proved that SiO2 was filled into all the TESPs. TESP-1, TESP-2 and TESP-3 have lower -OH stretching vibrations at 3320 cm−1 than OP, which may be caused by the chemical combination between the Si of SiO2 and the -OH of wood [24], and due to the replacement of -OH was by -Si-O- in the TESPs. Upon the comparison of TESPs with the TESW that used delignified wood as the matrix material [2,3,4,7,8,9,10,11,12,13], it was found that the strengths of these TESPs had been improved due to retention of the lignin and filling of the SiO2 (see mechanical analysis).

3.2. SEM and EDS Analysis

Figure 3 shows the SEM images of longitudinal-cutting TESPs. As can be seen, the structure of poplar-based material remained basically the same, however, there are certain obvious attachments (see green arrows) and films (see yellow arrows) on the surface of poplar-based material, and the microstructure of OP is covered by the layer of film. According to the formation mechanism of SiO2 film [24], the layer of film should be the SiO2 film. In Figure 3a, there are some obvious particles under the SiO2 film; the particle should be PCM-1 (lauryl alcohol), pointed out by green arrows. In Figure 3b, there are some irregular blocks under the SiO2 film; the irregular block should be PCM-2 (decanoic acid),indicated by green arrows. In Figure 3c, there are some streamline lumps under the SiO2 film; the streamline lump should be PCM-3 (myristic acid myristyl ester), as shown by the green arrows. The SiO2 film covered the PCM and the poplar-based material; the film can be evidently observed when the covered matrix was in a flat state, as shown by the yellow arrows in Figure 3. We also found contents of C, O and Si in the EDS data of TESPs (Table 1). The data showed that radial-cutting sections of TESPs has higher weight percentage of Si than longitudinal-cutting sections of TESPs, which proved that the lumen has more Si than the duct wall surface, and it also proved that SiO2 films in lumen had encapsulated the PCMs in the lumen. According to SEM and EDS analyses, the PCM had been directly encapsulated in the lumen of the poplar-based materials by the SiO2 film and the duct wall of the poplar-based material. In comparison to the TESW that was filled with the microencapsulated PCM [1,4,14,15,16,17,18,19], these TESPs immersed the samples in neutral silica sol which is simpler and greener.

3.3. DSC Analysis

The phase-changing temperature characteristics of PCMs and TESPs were measured using the differential scanning calorimeter (DSC). Figure 4 shows the DSC curves of TESPs and PCMs.
The phase-changing temperature peaks were as follows: TESP-1 at 24.17 °C and PCM-1 at 25.45 °C (Figure 4a), TESP-2 at 31.22 °C and PCM-2 at 32.46 °C (Figure 4b), and TESP-3 at 41.50 °C and PCM-3 at 42.75 °C (Figure 4c), all of which show that the phase-changing temperature peak values of TESP and its corresponding PCM was basically similar. The phase-changing temperature ranges were as follows: TESP-1 at 19–30 °C and PCM-1 at 17–33 °C (Figure 4a), TESP-2 at 26–39 °C and PCM-2 at 28–40 °C (Figure 4b), and TESP-3 at 33–54 °C and PCM-3 at 31–60 °C (Figure 4c), which again show that similar to the phase-changing temperature peak, the phase-changing temperature range of TESP and its corresponding PCM was also basically similar. Since the phase-changing temperature peak value and range of TESP was close to that of its PCM, this proved that the PCM of a TESP had not chemically affected the poplar-based material and the SiO2 film of TESP [8,24], so each TESP retained its PCM’s phase-changing characteristics. The TESP had a narrower phase-changing amplitude than its PCM (Figure 4), which shows that the poplar-based material and the SiO2 film TESW had altered the base material and the encapsulated material, respectively [8,24], thus they had restricted the phase-changing amplitude and heat-saving capacity of TESP. The wood-based material of TESW restricted the phase-changing amplitude and heat-saving capacity of TESW, which is an omnipresent problem in many studies conducted on TESW [7,8,9,10,11,12,13,14,15,16,17,18,19]. Therefore, mechanisms to improve the positive effect of the poplar-based material and the SiO2 film on the thermal storage capacity of TESP has become the primary focus of our future work.

3.4. The Composite Mechanism of TESPs

Figure 5a–c show the Si 2p energy spectrum diagrams from the XPS tests of TESPs, wherein we can find that each Si 2p energy spectrum diagram was fitted by two peaks. A peak corresponded to Si-O at 102.90 eV in Figure 5a, at 102.75 eV in Figure 5b and at 102.34 eV in Figure 5c, and another peak corresponded to SiO2 at 103.49 eV in Figure 5a, at 103.71 eV in Figure 5b and at 103.43 eV in Figure 5c [32,33,34,35]. The Si-O and SiO2 were the main chemical compositions of the SiO2 film. The chemical reaction between the Si of SiO2 and the -OH of wood in the FT-IR analysis, the Si-O and SiO2 peaks in the Si 2p energy spectrum diagrams of the XPS tests, and the SEM and EDS analyses, all proved that the SiO2 film and the wood duct wall of poplar-based material entirely encapsulated PCM. The PCM had no chemical reaction with the poplar-based material and the SiO2 film (see DSC analysis). The relation of PCM, SiO2 film and poplar-based material is shown in Figure 5d: the SiO2 film and the duct wall of the poplar-based material formed by -Si-O- entirely encapsulated the PCM in the lumen of the poplar-based material, but the PCM had no chemical reaction with the SiO2 film and the poplar-based material in TESP.

3.5. Thermal Energy Storage and Heat Resistance

The surface temperature of samples were tested to analyze the thermal energy storage of TESP. Table 2 shows the tested results of surface temperature after heat treatment at 60 °C for 30 min; each temperature was measured at 5 min intervals [26]. The surrounding temperature was 17 °C when the surface temperature was tested. As Table 2 shows, the average temperature of TESP-3 was 10 °C higher than that of OP in 25 min, the temperature of TESP-2 was 5 °C higher than that of OP in 35 min, and the temperature of TESP-1 was 3 °C higher than that of OP in 35 min, and all of these TESPs had higher temperature than OP after 35 min. Therefore, TESP can save more thermal energy as compare to OP in 35 min, however, the TESP cannot save more thermal energy than its PCM due to the effects of the wood-based material [8,9,10].
The PCMs can absorb heat and release heat in the phase-changing process, and the TESPs are mostly related to thermal temperature, so we analyzed the heat resistance of TESPs. The heat resistances of the TESPs with or without SiO2 film were tested by comparing the weights of TESPs before and after heat treatment (103 °C, 150 h) [23,24]. Without the SiO2 film, the weights of TESP-1, TESP-2 and TESP-3 reduced by about 38%, 36% and 22%, respectively, after the heat treatment. With the SiO2 film, the weights of TESP-1, TESP-2 and TESP-3 reduced by about 7%, 5% and 4%, respectively, after the heat treatment. Therefore, this proves that the TESPs with the SiO2 film had better heat resistance than the TESPs without the SiO2 film, and the SiO2 film of TESPs improved the heat resistance ability of the TESPs as the SiO2 film prevents the PCM from flowing away from the TESP at the solid–liquid phase-changing temperature.

3.6. Mechanical Analysis

Table 3 highlights the comparison of weights between OP and TESPs. As can be seen, the average weight of TESPs was 1.37 times of OP matrix. According to Equation (1), the WPG of TESP-1, TESP-2 and TESP-3 increased by 34.09%, 34.93%, and 42.24%, respectively. This increase was induced by the filled SiO2 film and the PCM. Moreover, the mechanical properties of TESPs were also improved due to the improved WPG as compared to that of OP.
Table 4 shows the average values of mechanical properties in OP and TESPs. The radial bending strengths of TESP-1, TESP-2 and TESP-3 increased by 50.85%, 70.16% and 70.31%, respectively. The radial bending elastic modulus of TESP-1, TESP-2 and TESP-3 increased by 47.14%, 67.38% and 74.57%, respectively. The tangential section hardness of TESP-1, TESP-2 and TESP-3 increased by 67.09%, 71.80% and 80.77%, respectively. The longitudinal compressive strength of the TESPs were all close to OP. Although the structural pattern of SiO2 film and PCM was mainly along the lumen of wood and the longitudinal direction of wood (see Figure 5d), the PCM had no chemical reaction with the SiO2 film and the poplar-based material in TESPs, so the structural pattern had not improved the longitudinal compressive strength of TESPs but the radial bending strengths and elastic modulus improved. The Si of SiO2 had higher hardness than OP, so the SiO2 film improved the tangential section hardness of TESPs. Therefore, when the SiO2 film and the PCM improved the weight and WPG of the TESPs, the radial bending strengths, radial bending elastic modulus and tangential section hardness of the TESPs were also improved.
Not only the TESPs have better mechanical property than the OP, but also the TESPs with lignin have better mechanical property than many delignified TESWs. Ash wood is a common building material, with 114.20 MPa radial bending strength, 10.40 GPa radial bending elastic modulus and 4.60 kN tangential section hardness [20]. The radial bending strengths, radial bending elastic modulus and tangential section hardness of the TESPs were basically close to these mechanical properties of ash wood, therefore, the TESPs can also be used as the building material, and thus can meet the heat-saving capacity of building materials.

4. Conclusions

TESPs were prepared by directly infiltrating three kinds of fatty alcohol/acid materials (lauryl alcohol, decanoic acid and myristic acid myristyl ester), respectively, into the longitudinal-cutting plantation poplar woods, and by directly encapsulating the PCMs in poplar-based materials with the SiO2 film.
The TESPs mostly retained the functional group characteristics of OP, PCMs and SiO2. The SiO2 film and the duct wall of the poplar-based material had encapsulated the PCM in the lumen of poplar-based material. The phase-changing temperature ranges of TESP-1, TESP-2 and TESP-3 were at 19–30 °C, 26–39 °C and 33–54 °C, respectively, and their phase-changing temperature peak values were at ~24 °C, ~31 °C and ~42 °C, respectively. After the same heat treatment, TESP-3 was 10 °C higher than OP in 25 min, TESP-2 was 5 °C higher than OP in 35 min, and TESP-1 was 3 °C higher than OP in 35 min, which proves that TESPs can save more thermal energy than OP. The SiO2 film improved the heat resistance ability of TESPs as the SiO2 film prevented the PCM from flowing away from the TESPs at the solid–liquid phase-changing temperature. Both PCM and SiO2 increased the weight and WPG of the TESPs, and also improved some of their mechanical properties. The radial bending strengths, radial bending elastic modulus and tangential section hardness of the TESPs were almost close to the mechanical properties of ash wood (ash wood is a common building material), so our TESPs can be used as a thermal energy-saving building material. In the TESPs, the poplar-based material and the SiO2 film had neither any chemical reaction with the PCM and nor had any temperature-induced phase change but as the matrix material and the encapsulated material they had restricted the phase-changing amplitude and heat-saving capacities of the TESPs. Therefore, mechanisms of improving the positive effect of the poplar-based material and the SiO2 film on the thermal storage capacity of TESP that has now become the focus of our future work.

Author Contributions

Conceptualization, W.Z. and D.S.; methodology, W.Z. and D.S.; software, W.Z. and C.L.; validation, W.Z., C.L. and N.Z.; formal analysis, W.Z.; investigation, N.Z.; resources, W.Z.; data curation, W.Z. and C.L.; writing—original draft preparation, W.Z.; writing—review and editing, W.Z.; visualization, N.Z.; supervision, N.Z.; project administration, W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Scientific Research Project of the Education Department of Hunan Province, China (No. 22A0185); Hunan Provincial Natural Science Foundation of China (No. 2023JJ31002).

Data Availability Statement

Data is contained within the article.

Acknowledgments

The authors would like to thank Shiyanjia Lab (www.shiyanjia.com) for supporting the SEM, EDS and XPS tests.

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 14 01190 g001 550
Figure 1. Fabrication process of TESPs.
Figure 1. Fabrication process of TESPs.
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Figure 2. FTIR spectra of OP, PCMs (PCM-1, PCM-2 and PCM-3) and TESPs (TESP-1, TESP-2 and TESP-3).
Figure 2. FTIR spectra of OP, PCMs (PCM-1, PCM-2 and PCM-3) and TESPs (TESP-1, TESP-2 and TESP-3).
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Figure 3. SEM images of longitudinal-cutting TESPs: (a) SEM image of longitudinal-cutting TESP-1, (b) SEM image of longitudinal-cutting TESP-2, and (c) SEM image of longitudinal-cutting TESP-3.
Figure 3. SEM images of longitudinal-cutting TESPs: (a) SEM image of longitudinal-cutting TESP-1, (b) SEM image of longitudinal-cutting TESP-2, and (c) SEM image of longitudinal-cutting TESP-3.
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Figure 4. DSC curves of TESPs and PCMs: (a) DSC curves of TESP-1 and PCM-1, (b) DSC curves of TESP-2 and PCM-2, and (c) DSC curves of TESP-3 and PCM-3.
Figure 4. DSC curves of TESPs and PCMs: (a) DSC curves of TESP-1 and PCM-1, (b) DSC curves of TESP-2 and PCM-2, and (c) DSC curves of TESP-3 and PCM-3.
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Figure 5. The Si 2p energy spectrum of TESPs and the composite mechanism of TESPs: (a) Si 2p energy spectrum of TESP-1, (b) Si 2p energy spectrum of TESP-2, (c) Si 2p energy spectrum of TESP-3, and (d) The composite mechanism of TESPs.
Figure 5. The Si 2p energy spectrum of TESPs and the composite mechanism of TESPs: (a) Si 2p energy spectrum of TESP-1, (b) Si 2p energy spectrum of TESP-2, (c) Si 2p energy spectrum of TESP-3, and (d) The composite mechanism of TESPs.
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Table
Table 1. EDS data of TESPs (longitudinal-cutting and radial-cutting sections).
Table 1. EDS data of TESPs (longitudinal-cutting and radial-cutting sections).
C (wt%)O (wt%)Si (wt%)
TESP-1 (Longitudinal-cutting section)75.0224.200.78
TESP-1 (Radial-cutting section)63.0127.139.86
TESP-2 (Longitudinal-cutting section)75.9022.181.92
TESP-2 (Radial-cutting section)67.8026.595.61
TESP-3 (Longitudinal-cutting section)85.9613.220.83
TESP-3 (Radial-cutting section)81.7715.322.91
Table
Table 2. Temperature test on OP and TESPs after heat treatment.
Table 2. Temperature test on OP and TESPs after heat treatment.
Time (min)OP (°C)TESP-1 (°C)TESP-2 (°C)TESP-3 (°C)
0~48~49~49~49
5~32~36~37~39
10~26~29~31~37
15~24~27~29~34
20~22~24~27~32
25~19~22~24~26
30~18~21~23~22
35~17~20~21~20
40~17~18~19~17
45~17~17~18~17
50~17~17~17~17
Table
Table 3. The weight comparison between OP and TESPs (20 × 20 × 20 mm).
Table 3. The weight comparison between OP and TESPs (20 × 20 × 20 mm).
OPTESP-1TESP-2TESP-3
Average weight (g)4.38 ± 0.065.87 ± 0.055.91 ± 0.056.23 ± 0.06
Table
Table 4. Average values of mechanical properties of OP and TESPs.
Table 4. Average values of mechanical properties of OP and TESPs.
Type of Mechanical TestOPTESP-1TESP-2TESP-3
Longitudinal compressive strength (MPa)32.14 ± 2.1132.33 ± 2.3033.19 ± 1.9736.93 ± 2.02
Radial bending strength (MPa)66.98 ± 3.12101.04 ± 3.31113.97 ± 4.02114.07 ± 3.61
Radial bending elastic modulus (GPa)7.51 ± 0.2511.05 ± 0.1812.57 ± 0.1413.11 ± 0.18
Tangential section hardness (kN)2.34 ± 0.043.91 ± 0.034.02 ± 0.024.23 ± 0.04
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Zou, W.; Li, C.; Sun, D.; Zou, N. A Simpler Fabrication for Thermal Energy Storage Wood. Forests 2023, 14, 1190. https://doi.org/10.3390/f14061190

AMA Style

Zou W, Li C, Sun D, Zou N. A Simpler Fabrication for Thermal Energy Storage Wood. Forests. 2023; 14(6):1190. https://doi.org/10.3390/f14061190

Chicago/Turabian Style

Zou, Weihua, Cong Li, Delin Sun, and Naike Zou. 2023. "A Simpler Fabrication for Thermal Energy Storage Wood" Forests 14, no. 6: 1190. https://doi.org/10.3390/f14061190

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