A route of polyethylene glycol-based phase change heat storage wood with AlN as the thermal conductive filler

doi:10.21203/rs.3.rs-2354045/v2

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Research Article

A route of polyethylene glycol-based phase change heat storage wood with AlN as the thermal conductive filler

https://doi.org/10.21203/rs.3.rs-2354045/v2

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Journal Publication

published 06 May, 2023

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Wood is a great potential natural material with good energy storage and temperature adjustment for the energy crisis. In this study, wood was delignified, and the PEG based eutectic polymer (PGMA) was impregnated into the delignified wood, and styrene (SM) grafted AlN was selected as thermal conductive filler to synthesize a stable phase change heat storage wood. The result shows that the styrene (SM) polymerized on the surface of AlN to form polystyrene (PS), the dispersion of AlN was enhanced. The thermal stability and loss resistance of PCES-Wood are improved for the penetration of PS-AlN into the cavities, which is attributed to the PS-AlN playing a "limiting" role in the crystallization of PGMA. When the content of PS-AlN is 6%, the thermal conductivity of PCES-Wood reaches 0.5148 W/(m·K). 4% PS-AlN-PCES@Balsa has the lowest undercooling and heat loss rate, and the melting enthalpy and solidification enthalpy reach 125.60 J/g and 120.30 J/g, respectively. The phase change temperature range is between 19.59 ℃ and 33.87 ℃, which is in the comfortable temperature range for the human. Under the illumination of the simulated solar light source, the surface temperature of PS-AlN-PCES@Balsa rises rapidly and shows a lasting heat storage capacity at low temperatures. After 200 cold and hot cycle tests, the phase change enthalpy of PS-AlN-PCES@Balsa is still high, and there is a slight leakage phenomenon. The phase-change wood prepared in this study shows that it is a potential material to store and release solar energy in practical applications.

Wood

Phase change energy storage

Carbon black

Photothermic

Biocomposite

Energy is the basis of human survival. Since the energy crisis, to solve the depletion of mineral energy and the consequent environmental pollution, improving energy utilization efficiency and developing renewable resources have become the focus of research and development [1]. The energy storage technology is one of the crucial methods to improve energy utilization efficiency. Phase change heat storage technology can solve the contradiction of energy supply mismatch in time and space. It uses phase change materials (PCMs) to store or release energy [2–4]. PCMs can not only reduce the energy consumption of iSndustrial production and recover industrial waste heat, but also transfer the excess energy during the low heat use period to the peak heat use period. Therefore, PCMs have become a very active research branch in recent years [5–7].

According to the phase change type, PCMs can be divided into four categories: solid-solid, solid-liquid, solid-gas and liquid-gas PCMs [8, 9]. Compared with the other three classes, solid-liquid PCMs always shows the characteristics of low cost, non-toxic and non irritating, high latent heat and wide range of phase change temperature, etc [10]. Polyethylene glycol (PEG) is a solid-liquid phase change material with a wide temperature range, high latent heat, and good biocompatibility [11, 12]. However, there are problems of PEG such as liquid leakage and poor thermal conductivity [13], which seriously affect the energy storage performance. A meaningful way to overcome the leakage problem of PEG is shape stabilization technology, which mainly restricts PEG to support materials and prepares composite PCMs [14, 15]. Porous material can be used to encapsulate PEG to form networks. Here, wood is a natural material with the characteristics of light quality, high strength, thermal insulation and convenient installation. It is particularly suitable for building and indoor utilization. The capillary adsorption and surface tension of wood pores can shape and package the PCMs to prevent the leakage of PEG, and significantly improve the potential of wood for adjusting indoor temperature and energy transformation[16, 17]. For example, Li et al.[18] prepared transparent wood/PEG composite PCMs with delignified transparent wood as the matrix. The composite also has good thermal insulation performance and good optical transparency. Xu et al.[19] reported a phase change energy storage wood (PCES-Wood) by introducing silica-stabilized PEG into the porous structure of solid wood by temperature-assisted sol-gel method. The results showed that SiO₂ network was formed in modified wood and the modifiers distributing in both the wood cell wall and cell lumen. This composite material exhibited considerable heat storage capacity and superior dimensional stability. Thermal cycling test showed that the SiO₂/PEG modified wood has satisfactory thermal reliability. Li et al. [20] reported a kind of PCES-Wood by immersing polyethylene glycol-based energy storage polymer into the porous structure of delignified wood. The results showed that the PCES-Wood exhibited good dimensional stability at high-temperature when the content of PEG was 90% (90% PGMA-Wood). The melting enthalpy and solidification enthalpy of 90% PGMA-Wood were 25.12 J/g and 31.59 J/g, respectively, which meets the comfortable indoor temperature for humans. It is expected that the enthalpy of PCES-Wood may be reduced for the wood skeleton occupies a large amount of energy storage space. Therefore, it is significant to maximize the enthalpy of PCES-Wood without leakage.

According to the natural properties of wood, the main active group is the hydroxyl group. The groups that are easy to react with the hydroxyl group are mainly isocyanate, epoxy, cyclic anhydride and aldehyde group, etc [21]. Among them, the materials with more epoxy group are mostly less toxic. In this study, we chose glycidyl methacrylate (GMA) with both epoxy groups and double bonds as the crosslink agent to mix with PEG. Then the polymer was impregnated into the wood and cross-linked within the wood cell wall. In order to improve the thermal conductivity of the system, aluminum nitride (AlN) was selected as the thermal conductive filler in this study. AlN has excellent electrical insulation and high thermal conductivity, whose thermal conductivity is up to 320W/(m·K), and it is widely used in high-power integrated circuits and packaging materials. However, AlN is prone to hydrolysis when water exposure [22], which dramatically affects its thermal conductivity. In this study, styrene and γ-Aminopropyltriethoxysilane (KH550) were used to coat AlN to synthesize composite particles, thereby improving the chemical stability of AlN. Then the modified AlN was combined with wood as a thermal conductive filler to prepare phase change heat storage wood (PCES-Wood). The most appropriate sample is selected by comparing the thermal properties of PCES-Wood with different amounts of modified AlN. The schematic diagram of PCES-Wood is shown in Fig. 1. The PCES-Wood prepared in this research has great significance to the development of low-carbon household products, green building materials and building energy conservation as temperature regulators.

2.1. Materials and chemicals

Balsa wood was purchased from Guangyue Wood Company (Jiangsu, China), all samples were cut to the size of of 2cm × 2cm × 1cm. These samples were dried at 50 ℃ before use. Sodium sulphite (Na₂SO₃) and sodium hydroxide (NaOH) were provided by Sigma-Aldrich.

Polyethylene glycol with a molecular weight of 800g mol⁻¹and 1000g mol⁻¹, maleic anhydride (MAN), glycidyl methacrylate (GMA), ethylene glycol dimethacrylate (EDMA), benzoin dimethyl ether (DMPA), aluminum nitride (AlN); hydrochloric acid (HCl, concentration10mol/L), 3-Aminopropyltriethoxysilane (KH550), styrene, ethanol, acetone, (all chemicals were purchased from Aladdin-Holdings Group, Shanghai, China).

2.2 Preparation of MAN-Wood

(1)The delignification of balsa was produced by boiled using NaOH (2.5 mol/L) and Na₂SO₃ (0.4 mol/L) at 95 ℃ for 18 h. Then the wood sample was bleached in 10% H₂O₂ solution for 3 h to promote the remaining small amount of lignin was further removed. The delignified wood was stored in acetone solution for standby, which was named as PRE-Wood. (2) The MAN was put into acetone to prepare 10% concentration MAN, and then a few drops of pyridine was added as catalyst. PRE-Wood was placed in MAN solution for 2h, after that, the sample was wrapped with tin foil and placed at room temperature for 12h to obtain MAN-Wood.

2.4 Preparation of PS-AlN

(1) 1% mass ratio of KH550 was dispersed in 200 mL ethanol solution and adjusted the pH to 3~4 with 10 mol/L hydrochloric acid aqueous solution, then the solution was stirred at 40 ℃ for 1 h. 20 g AlN was dispersed into KH550 solution and stirred at 40 ℃ for 8 h. Finally, the mixture was washed with ethanol several times and vacuum dried at 120 ℃ for 12 h to obtain KH550-AlN. (2) 100 mL absolute ethanol and 15 g KH550-AlN were mixed and stirred at 70 ℃ for 3 h, then the styrene was added at a mass ratio of 5 % based on the total weight of solution to prepare PS-AlN. The content of styrene was changed and 10% PS-AlN and 15% PS-AlN were produced in the same way. The final samples were dried at 80 °C for 12 h and the best ratio was filtered out based on the experimental results.

2.5 Fabrication of PCES-Wood

(1) PS-AlN was dispersed into 15 mL of absolute ethanol and ultrasonically dispersed for 10 min. The content of PS-AlN is 1%, 2%, 4% and 6% of the phase change heat storage monomer, respectively. Then PS-AlN and MAN-Balsa were added to the autoclave and heated at 150 °C for 3 h to synthesize PS-AlN-based wood thermal conductive framework. (2) 6 mL PEG 800 and 14 mL PEG 1000 were blended, then they were melted into a clear liquid under heating in a 50 °C water bath, and the PEG blend was stirred at 800-1000 rpm for 15 min. The PEG blend was cooled to 35 °C and 1 mL GMA, 1 mL EDMA, and 0.01 g DMPA were added to synthesize the phase-change heat storage material, which was named as PGMA. After completion, the MAN-Wood thermal conductive framework and PGMA were added to the autoclave and heated at 100 °C for 2 h. Finally, the samples were taken out and left under the sunlight for 12 h, then the samples were wrapped in aluminum foil and heated in an oven at 130 °C for 2 h to obtain phase change energy storage wood, which was labeled as 70PEG1000-Balsa. (3) The PS-AlN-based wood thermal conductive framework and the PGMA were added to the autoclave and immersed at 100 °C for 2 h. Finally, the samples were taken out and left in the sunlight for 12 h, then they were wrapped in aluminum foil and heated in an oven at 130 °C for 2 h to obtain PS-AlN-based phase change energy storage wood, which was labeled as PS-AlN-PCES@Balsa.

2.6 Characterization

The morphologies of samples were measured by SEM (SU8010, Hitachi, Japan). The chemical characterization was observed by X-ray photoelectron spectroscopy (250XL, Thermo Fischer, USA) and FTIR spectrum in the range of 400-4000cm^- ¹ (NicoletIS10,ThermoFisher, USA). The X-ray diffraction (SmartLab, Rigaku, Japan) was used to measure the crystallization status in the 2θ range of 5^o–80^o. Thermal conductivity was measured with a Hot disk Thermal Constant Analyzer (Hot Disk TPS 2500S). The heat storage properties and heat stability were measured by DSC (DSC 4000, PerkinElmer, USA, heating and cooling rate: 5 °C/min, N₂) and TGA (STAR^e System, Mettler-Toledo, Switzerland, test range: 30–800 °C, test rate: 10 °C/min, N₂). The light absorption capacity of PS-AlN-PCES@Balsa was tested by ultraviolet spectrophotometer (UV5100, Metash, China). The phase transformation behavior of PS-AlN-PCES@Balsa was measured by comparing the mass change before and after heating at 80 ℃ for 4 h. The k-type thermocouple was chosen to mesure the temperature of PS-AlN-PCES@Balsa at different times to judge the heat storage and release efficiency.

3.1 Morphological and chemical analysis of PS-AlN

In the FTIR spectra in Fig. 3a. For AlN, the characteristic peak position between 500 cm^− 1-900 cm^− 1 is Al-N stretching vibration absorption peak [23]. The peak at 785 cm^− 1 of KH550 represents the out-of-plane bending vibration of N-H, 949 cm^− 1 represents the deformation vibration of = CH, 1070 cm^− 1 means the deformation vibration of C-O-C, 1390 cm^− 1 is the characteristic peak of the Si-O-CH₂CH₃, 2890 cm^− 1 and 2970cm^− 1 represent the stretching vibration and symmetry vibration of C-H [24]. In the spectrum of KH550-AlN, the characteristic peak at 1390 cm^− 1 of the Si-O-CH₂CH₃ group appear. In addition, the absorption peak of KH550-AlN between 3500 cm^− 1-4000 cm^− 1 which represents water becomes wider. There is no water added in the reaction conditions, and there is no characteristic peak of hydroxyl in the spectrum of AlN, indicating that the broad peak is not derived from the silanol generated by the hydrolysis of KH550, but may be the stretching vibration of N-H in the modifier. KH550 is physically coated on the surface of AlN. In the spectrum of PS, 692 cm^− 1-988 cm^− 1 is the out-of-plane bending vibration of C-H in substituted benzene, the peak around 1490 cm^− 1 is the skeleton vibration of C = C, and the peak around 3000 cm^− 1 is the stretching vibration of C-H [25]. In the spectrum of PS-AlN, there are no characteristic peaks of PS and new peak. The characteristic peaks of PS below 1000 cm^− 1 overlap with the characteristic peaks of AlN to form a strong and broad peak. The characteristic peak of KH550-AlN at 1390 cm^− 1 disappears, indicating that there may be electron transfer and sharing between PS and KH550 coating layers and PS is connected to AlN through KH550. In order to further verify whether PS is coated on the KH550-AlN surface, AlN, KH550-AlN and PS-AlN were soaked in 10 mL water at a content of 5mg/mL for one day, and then the pH value of the solution was tested. AlN is easily hydrolyzed into amorphous when exposed in water, and then Al(OH)₃ is formed at a certain temperature, thereby the pH value of the solution is increased. The hydrolysis mechanism of the AlN is as follows [26] :

AlN + 3H₂O = AlOOH(amor) + NH₄⁺+OH⁻

AlOOH + H₂O = Al(OH)₃

Figure 2 shows that the pH value of the AlN solution is higher than 11, and the pH value of the KH550-AlN solution is higher than 9, both of which show strong alkalinity. It indicates that the single KH550 modification cannot avoid the hydrolysis of AlN, so the KH550-AlN will hydrolyze to produce silanol groups when exposed to water. Water molecules can penetrate into the interior and react with AlN. The pH value of 5% PS-AlN is about 9, because the less content of PS cannot wholly cover AlN. When the content of PS is higher than 10%, the pH value of the PS-AlN solution is between 5–7, and the solution appears neutral, indicating that PS is successfully coated on the surface of KH550-AlN.

Figure 3b shows the XPS spectra of AlN, KH550-AlN and PS-AlN. These three samples all contain O, N, C, and Al elements. The C element in AlN mainly exists in the form of natural adsorption, and the O element primarily comes from the reaction of oxygen and AlN. The N in AlN is -3, which is in a low valence state and it also has reductive properties. Oxygen has oxidizing properties. The equation for the reaction between the two elements is: 4AlN + 5O₂ = 4NO + 2Al₂O₃. Comparing with AlN, the peak intensities of O and N in KH550-AlN and PS-AlN have been significantly reduced. In order to further verify whether there is a reaction between KH550 and AlN, as well as between PS and KH550-AlN, the O and N peaks were treated by peak separation (Fig. 3c, 3d). The results show that the chemical bond at 531.88 eV in AlN is a C = O bond, mainly from the adsorption of oxygen and carbon dioxide in the air and the NO produced by the reaction between AlN and oxygen. The chemical bonds at 530.88 eV, 530 eV and 529.58 eV mainly come from Al₂O₃ which was formed by the reaction between AlN and oxygen. In KH550-AlN (Fig, 3e, 3f), the decrease of C = O bond at 532.08 eV is due to the reduction of O in the adsorbed air and the AlN surface is covered by Si-O. The chemical bond at 531.08 eV is mainly from Al-O and the hydrolysis of KH550. In the N1s spectrum of AlN, the N element is mainly N-Al compound and a small amount of adsorbed N-C. The N1s spectrum of KH550-AlN is mainly N-Al compound. In the O1s spectrum of PS-AlN (Fig. 3g), both 531.88 eV and 530.88 eV belong to O-Al compounds, and the peak position and intensity are similar to those of KH550-AlN. In the N1s spectrum, 397.18 eV, 396.88 eV and 396.3 eV are all N-Al bonds. In the N1s of KH550-AlN and PS-AlN (Fig. 3h), there is a π-π vibrational transition, which attributing to the electron-donating groups (such as -OH, -NH₂) are introduced on the surface of AlN, and the final state of the atom is always maintained at the high-energy position. The above analysis shows that KH550 and PS are mainly adsorbed on the AlN surface in a physical combination.

It can be seen from the Raman spectrum (Fig. 4a) that all samples have peaks at 610 cm^− 1, 657 cm^− 1 and 905 cm^− 1, corresponding to the E2^low mode, E2^high mode and A₁ (LO) mode, respectively. Among them, the diffraction peak intensity at 657 cm^− 1 is the highest. The KH550 modification increased the diffraction peak intensity of AlN at 657 cm^− 1. After PS coating, the diffraction peak intensity of KH550-AlN at 657 cm^− 1 decreased, and the diffraction peak intensity of PS-AlN is positively correlated with the PS content. Since the vibration frequency of the high E2 phonon mode is linearly dependent on the strain or stress in the crystal, it indicates that KH550 grafting adds more chemical bonding, which improves the crystallinity of AlN and increases the strain capacity. PS is an amorphous random polymer, which will reduce the crystalline order of KH550-AlN and in turn leads to a decrease in the Raman peak. This result can also be verified from the XRD pattern.

It can be seen from the XRD pattern that the crystalline peak positions of all samples are the same, and the 2θ diffraction peaks near 33°, 36° and 37° are the characteristic peaks of AlN (Fig. 4b). It can be seen from the Table 1 that the (001) crystal plane, the interplanar spacing of (002) crystal plane and (101) crystal plane are basically the same, indicating that the modification does not change the crystal structure of AlN. The coating of PS reduces the diffraction peaks of KH550-AlN, indicating that PS will cause minor distortions in the unit cell of AlN as an amorphous polymer, but it does not affect the crystal integrity of AlN.

Ultraviolet-visible absorption spectroscopy can describe the ability of a substance to absorb light of different wavelengths. It can be seen from the Fig. 4c that all samples have a significant absorbance near 205 nm and the corresponding band gap is 6.02 eV. The maximum absorption wavelength of PS-AlN is also near 205 nm, but the absorption peak intensity increases and shifts to the long-wave direction. This is because the KH550 modification introduces the auxochrome group -NH₂, which increases the n→π transition. In addition, the benzene in PS has a cyclic conjugated system, and the maximum absorption wavelength of its E₂ band is 236 nm, so the absorbance of the E₂ band of PS-AlN between 200 nm and 250 nm is enhanced. In addition to the increase in absorbance in the ultraviolet region of PS-AlN, the absorbance in the visible area is also significantly enhanced. Among them, 10% PS-AlN has the largest absorbance in the visible region.

It can be seen from the thermal stability curve in Fig. 4d that all samples all have good thermal stability except for 5% PS-AlN. Among them, the thermal weight loss before 200°C is attributed to the evaporation of adsorbed water on the AlN surface and the melting of PS. There is a slightly steeper weight loss peak between 350°C and 400°C, which is attributed to the breakage of the PS molecular chain. The higher weight loss rate of 5% PS-AlN may be because the main chain of PS is a saturated carbon chain and the side groups are conjugated benzene rings, which makes the molecular structure irregular and increases the rigidity of the molecule. Due to the rigidity of the molecular chain, it is easy to cause stress cracking. When the PS content is low, the core-shell bonding strength is insufficient, and the thermal stability of the PS shell layer is poor, resulting in brittle fracture and peeling.

Table 1

Crystal planes and interplanar spacings corresponding to the XRD characteristic peaks of AlN, KH550-AlN and PS-AlN.
	(100)		(002)		(101)
	2θ/(°)	d/(Å)	2θ/(°)	d/(nm)	2θ/(°)	d/(Å)
AlN	33.23	2.69	36.05	2.48	37.95	2.36
KH550-AlN	33.24	2.69	36.07	2.48	37.96	2.36
5%PS-AlN	33.20	2.69	36.02	2.49	37.92	2.37
10%PS-AlN	33.20	2.69	36.02	2.49	37.92	2.37
15%PS-AlN	33.22	2.69	36.05	2.48	37.94	2.36

Figure 5 shows the SEM and TEM images of AlN and 10% PS-AlN. From the Fig. 5a and Fig. 5b, it can be seen that the AlN is in the form of flakes and the background has prominent shadows, indicating that AlN has agglomerated. But the surface of PS-AlN is covered with a layer of substance which is different from AlN. It can be seen that there is a coating layer of about 5nm on the surface of PS-AlN. Figure 5c and Fig. 5d shows the EDS mapping of AlN and PS-AlN. After AlN is modified, the atomic percentages of N, O and Al are significantly lower, while the atomic percentages of C and Si are increased. The above results all show that the surface of AlN is successfully coated with KH550 and PS.

AlN and PS-AlN were dispersed in water and PEG 400 at a concentration of 0.5 mg/mL to investigate their dispersion stability (Fig. 6). It can be seen that the dispersion effect of AlN and PS-AlN in PEG 400 is not obvious. However, AlN settled on the first day in water, while KH550-DA-BN still maintained good dispersion stability on the first day in water. PS-AlN began to settle after the second day. This result indicates that PS-AlN has better dispersion stability in water for long time. Combining with the absorbance analysis of the material, 10% PS-AlN not only has good ultraviolet-visible light absorption capacity, but also has high thermal stability and good dispersion ability, which can be used as a preferred thermally conductive filler.

3.2 Microstructure and chemical analysis of the PS-AlN-PCES@Balsa

The micro-morphology of the section of the PCES-Wood with the addition of PS-AlN of 1%, 2%, 4% and 6% respectively is shown in Fig. 7a. It can be seen that wood plays a supporting role as the skeleton of PCMs. The surface topography shows that with the increase of PS-AlN content, the smoothness of the PCES-Wood surface increases, which indicates that PS-AlN forms a thermal conductive coating on the wood surface. It can be seen from the longitudinal section that PS-AlN is permeated into the lumen of wood, but the PS-AlN in 1% PS-AlN-PCES@Balsa and 2% PS-AlN-PCES@Balsa appears to unite, and this irregular aggregation may lead to uneven heat conduction of wood. The distribution of PS-AlN is relatively uniform in 4% PS-AlN-PCES@Balsa and 6% PS-AlN-PCES@Balsa. It can be seen from the cross-section that the PCMs is penetrated into the cell wall and cell cavity of wood and there is no apparent separation between the tube wall and PCMs, which indicates that the compatibility between wood and PCMs is very good. However, when the content of PS-AlN is high, the cell wall contour of the PCES-Wood is also clear, because the higher content of PS-AlN reduces the tension and capillarity of the PCMs, which also effectively prevents the leakage of the PCMs.

Figure 7b shows the FTIR spectrum of 70PEG1000-Balda, 6% PS-AlN-PCES@Balsa and 6% PS-AlN-PCES@Balsa after 500 cold and heat cycles. As can be seen from the figure, The peak of 2870 cm^− 1 in 70PEG1000-balsa and 6% PS-AlN-PCES@Balsa is the stretching vibration peak of methyl and methylene groups in PGMA. The peak of 1100 cm^− 1 is the stretching vibration peak of (C-O) in PGMA. The addition of PS-AlN does not change the chemical structure of PGMA, which indicates that PS-AlN and 70PEG1000-Balsa are physically bound. After 500 cold and heat cycles, there is no change in the characteristic peak of 6% PS-AlN-PCES@Balsa, which indicates that the PCES-Wood has good use stability. The characteristic peak of hydroxyl group at 3470 cm^− 1 decreases, which may be attributed to the evaporation of water.

The less interaction between the nano particles and the matrix may lead to the agglomeration of particles, and the agglomeration of particles may form an independent crystallization zone, thus affecting the phase transformation ability of wood. Therefore, it is necessary to study the crystallization behavior of PCES-Wood. Figure 7c shows the XRD diffraction pattern of PCES-Wood with different content of PS-AlN. As can be seen from the figure, PS-AlN-PCES@Balsa show high intensity peaks at 2θ = 19.2 ° and 23.4 °, which are also characteristic peaks of PGMA. The positions of the (100), (002), (101), (102), (110), (103) and (112) crystal planes of PS-AlN and PS-AlN-PCES@Balsa are the same, the above results show that the addition of PS-AlN does not change the crystal structure of PGMA, but combines with PGMA in a physically dispersed form, which is also consistent with the detection results of FTIR. After calculation, the crystallinity of 1% PS-AlN-PCES@Balsa, 2% PS-AlN-PCES@Balsa, 4% PS-AlN-PCES@Balsa and 6% PS-AlN-PCES@Balsa is 53.01%, 31.36%, 25.91% and 30.61% respectively, which indicates that the lower content of PS-AlN is not enough to blend with PGMA, but it can cover the wood to prevent the leakage of PGMA, so the 1% PS-AlN-PCES@Balsa showing high crystallinity. However, when the content of PS-AlN exceeds 2%, PS-AlN can be impregnated into the wood cell wall and cell cavity and physically blended with PGMA, thereby limiting the movement of its adjacent PGMA molecular chain. Therefore, when the content of PS-AlN is high, the crystallinity of PS-AlN-PCES@Balsa decreases obviously.

3.3 Thermal properties of the PS-AlN-PCES@Balsa

3.3.1 Thermal conductivity

Table 2 shows thermal conductivity of Balsa, 70PEG1000-Balsa and PCES-Wood added with different content of PS-AlN. It can be seen that Balsa's thermal conductivity is 0.03472 W/(m K), and 70PEG1000-Balsa's thermal conductivity is 0.3207 W/(m K), which is 824% higher than Balsa's thermal conductivity. The reason is that the GMA and PEG polymerize in the wood to form a highly crystalline polymer, and the heat conduction is converted from the atomic vibration of the original wood to the lattice vibration, so the thermal conductivity is improved. For crystalline polymers with neat lattice arrangement, heat propagation in the lattice is similar to waves [27]. In addition, the micropores and part of the cell cavities of 70PEG1000-Balsa are filled with thermal storage monomers, and lattice vibration is used to replace air thermal convection, which reduces thermal resistance and dramatically improves the thermal conductivity of 70PEG1000-Balsa. As the content of PS-AlN filler increases, the thermal conductivity of the material also gradually increases. When the content of PS-AlN is 6wt%, the thermal conductivity of the composite system is 0.5148w/ (m·K), which is 1383% higher than that of Balsa. This is because PS-AlN particles contact each other to form a good heat conduction path in the wood.

Table 2

Thermal conductivity of Balsa, 70PEG1000-Balsa and PS-AlN-PCES@Balsa.
Sample	Balsa	70PEG1000 -Balsa	1%PS-AlN -PCES @Balsa	2%PS-AlN -PCES @Balsa	4%PS-AlN -PCES @Balsa	6%PS-AlN -PCES @Balsa
Thermal conductivity (W/m^− 1K^− 1)	0.03472	0.3207	0.3461	0.3962	0.4649	0.5148
TC enhancement (%)	/	824	897	1041	1239	1383

3.3.2 Heat storage capacity

The DSC curve of PS-AlN-PCES@Balsa is shown in Fig. 8a and 8b. Table 3 lists the phase change temperature (including melting point Tm, crystallization point Tf, peak temperature Tp, and termination temperature Te) and phase change enthalpy (including melting enthalpy ΔHm and solidification enthalpy ΔHf). The melting temperature range and crystallization temperature range of all samples are within the comfortable temperature range of the human body. All the samples have high enthalpy of melting and solidification. The above results indicate that the lower content of PS-AlN does not cause the decrease of phase change enthalpy of PCES-Wood. The degree of supercooling of PCES-Wood can be evaluated by the difference between melting point and crystallization temperature. The efficiency of storage and heat release can be evaluated by calculating the heat loss rate (1-ΔHf/ΔHm)×100%. Figure 8c and 8d show the comparison between undercooling and heat loss rate. It can be seen that the undercooling degree and heat loss rate of 4% PS-AlN-PCES@Balsa are minimum.

Table 3

Phase transition temperature and phase transition enthalpy of PS-AlN-PCES@Balsa.
Sample		Melting		ΔH_m (J/g)		Freezing		ΔH_f (J/g)
Sample	T_m (℃)	T_mp (℃)	T_me (℃)	ΔH_m (J/g)	Tf (℃)	T_fp (℃)	T_fe (℃)	ΔH_f (J/g)
1%PS-AlN-PCES@Balsa	9.52	35.42	39.90	127.30	26.70	20.98	15.38	120.60
2%PS-AlN-PCES@Balsa	10.20	35.87	40.48	114.50	25.06	21.21	13.22	109.30
4%PS-AlN-PCES@Balsa	10.55	33.87	39.29	125.60	25.08	19.59	12.63	120.30
6%PS-AlN-PCES@Balsa	10.55	34.17	39.19	125.80	24.57	18.53	11.16	121.10

3.3.3 Thermal stability

Figure 9a and 9b show the thermogravimetric curve (TGA) and differential thermogravimetric curve (DTG) of PS-AlN-PCES@Balsa. Table 4 shows the temperature points and final residual mass percentage of samples at different weight loss rates. It can be seen from the figure that there is only one peak in the DTG curve corresponding to all samples, and the maximum weight loss rate is at around 400 ℃. 70PEG1000-Balsa has an obvious weight loss step between 30 ℃ and 100 ℃. This is because the hygroscopicity of 70PEG1000-Balsa. When the temperature rises, the combined water in 70PEG1000-Balsa will evaporate, causing a decline in quality. There is no severe weightlessness step in the range of 30 ℃-100 ℃ of PS-AlN-PCES@Balsa, indicating that PS-AlN-PCES@Balsa has better moisture resistance. When the mass loss rate reaches 50%, the temperature points of all samples tend to be the same. The weight loss rate of all samples slowed down after 420 ℃, indicating that the thermal decomposition process of PGMA and wood has been completed. The residual mass ratio of 6% PS-AlN-PCES@Balsa at 800 ℃ is the largest, followed by 4% PS-AlN-PCES@Balsa, which is positively related to the content of PS-AlN.

In order to study the service stability of PCES-Wood, we chose 6% PS-AlN-PCES@Balsa as the test object and a total of 200 cold and hot cycles were conducted. As shown in Fig. 9c and 9d, after 200 cycles, the peak change of 6% PS-AlN-PCES@Balsa is not significant, the melting point increases by 1.86 ℃, and the change of temperature and phase change enthalpy is very small, which indicates that after 200 times of cold and hot cycle tests, the PCES-Wood still has good heat preservation performance and use stability.

Table 4

Thermal cycling data of PS-AlN-PCES@Balsa.
Sample		Melting		ΔH_m (J/g)		Freezing		ΔH_f (J/g)
Sample	T_m (℃)	T_mp (℃)	T_me (℃)	ΔH_m (J/g)	Tf (℃)	T_fp (℃)	T_fe (℃)	ΔH_f (J/g)
6%PS-AlN-PCES@Balsa	10.55	34.17	39.19	125.80	24.57	18.53	11.16	121.10
After 200 cycles	13.02	36.03	41.96	125.80	25.23	19.16	11.62	114.40

3.3.4 Optical properties

Figure 10a shows the ultraviolet absorption spectrum of PS-AlN-PCES@Balsa. It can be seen that all the samples have light absorption ability in the UV light segment, but the light absorption ability in the visible and infrared light segments is poor. 2% PS-AlN-PCES@Balsa shows the strongest light absorption in the UV segment, followed by 1% PS-AlN-PCES@Balsa. The UV absorption capacity of 6% PS-AlN-PCES@Balsa is the worst, which may be because more PS-AlN will form uneven agglomeration on the wood surface, and more PS-AlN will also cause PS to coagulate into blocks, thus affecting AlN’s absorption of UV light.

In order to study the photothermal conversion efficiency of PS-AlN-PCES@Balsa under sunlight, the sample was irradiated by a solar-simulated light source, and the time temperature curve of the sample was detected by a temperature patrol detector. Figure 10b shows the temperature-time curve of the sample irradiated by light simultaneously. It can be seen from the figure that at the initial stage of irradiation, the heating rate of PS-AlN-PCES@Balsa was faster than that of unmodified Balsa and PEG-Balsa. When the temperature reaches the melting temperature of the PCES-Wood, the heating rate of the sample decreases, and a relatively gentle heating platform appears. This is because the PCES-Wood starts to carry out the solid-liquid phase transition, converting heat energy into internal energy. At the end of the melting process, The surface temperature of 2% PS-AlN-PCES@Balsa reaches 26.8 ℃, the surface temperature of 4% PS-AlN-PCES@Balsa is 26.2 ℃, the surface temperature of 6% PS-AlN-PCES@Balsa is 25 ℃, the surface temperature of 1% PS-AlN-PCES@Balsa is 24.5 ℃, the surface temperature of 70PEG1000-Balsa is 23.7 ℃, while the surface temperature of PEG-Balsa and Balsa is lower as 12.4 ℃ and 11.4 ℃, respectively. In addition, when the surface temperature of PCES-Wood rises from 0 ℃ to 20 ℃, 1% PS-AlN-PCES@Balsa takes 744s, 2% PS-AlN-PCES@Balsa takes 619s, 4% PS-AlN-PCES@Balsa takes 590s, 6% PS-AlN-PCES@Balsa takes 619s, the heating rate of 4% PS-AlN-PCES@Balsa is the fastest. The above results show that PS-AlN improves the thermal conductivity of PCES-Wood, it can quickly gather heat to raise the surface temperature of wood. However, when the content of PS-AlN is too high, it may cause particle aggregation and uneven thermal conductivity. In the cooling stage, the cooling rate of Balsa is the fastest, falling from 11.4 ℃ to 0 ℃ in 223s, indicating that the thermal insulation performance of Balsa is poor. The cooling curve of PEG-Balsa is relatively gentle, the cooling rate of 70PEG1000-Balsa and PS-AlN-PCES@Balsa is faster than that of PEG-Balsa, because the lattice arrangement of 70PEG1000-Balsa is more orderly and which has the characteristic of solid-solid phase transition. In addition, the addition of PS-AlN also limits the flow of PCMs, thereby prolonging the time of phase transformation.

Figure 11 shows the thermal imaging of Balsa, PEG-Balsa, 70PEG1000-Balsa and PCES-Wood with different content of PS-AlN in the process of temperature rise and fall. The samples are exchanged at 0 ℃ and 70 ℃. It can be seen from the figure that the heating rate of Balsa is the fastest. The heating rate of 70PEG1000-Balsa is faster than that of PEG-Balsa, because the lattice ordering degree of PGMA is better than that of PEG, which improves the intrinsic thermal conductivity of wood. The heating rate of PS-AlN-PCES@Balsa is faster than 70PEG1000-Balsa, and the surface color is warmer after 600s, which indicates that PS-AlN improves the thermal conductivity of wood, and the thermal conductive coating formed by PS-AlN on the wood surface also has a positive effect on the thermal insulation of wood. In the cooling process, Balsa has the fastest cooling rate due to its lack of phase transformation ability. At 600s, PS-AlN-PCES@Balsa shows higher temperature, which further verifies the thermal insulation performance of PS-AlN on PCES-Wood.

3.3.5 shaping effect

The solid-liquid phase change behavior can be characterized by the leakage rate, which reflects whether there is leakage of PCMs at high temperatures. The leakage of PS-AlN-PCES@Balsa before and after temperature rise is shown in the Fig. 12a. The leakage situation is calculated by measuring the mass before and after temperature rise, as shown in Table 5. It can be seen from Fig. 12a that after heating at 80 ℃ for 4h, there is slight leakage of PS-AlN-PCES@Balsa due to evaporation of wood crystal water and melting of PGMA. However, the PCES-Wood with higher PS-AlN content shows lower leakage from the appearance. The leakage rate of all samples is less than 1%, because the PGMA and PS-AlN interweave each other in the wood to form a three-dimensional network structure, which limits the leakage of PGMA (Fig. 12b). In addition, PS-AlN forms a thermal conductive layer on the wood surface, which has high thermal stability and which can protect the packaging of PGMA.

Table 5

Leak rates for PS-AlN-PCES@Balsa.
Sample	Before heating m₁(mg)	After heating m₂(mg)	Leakage (g)	Leakage rate C(%)
1% PS-AlN-PCES@Balsa	2.98	2.95	0.03	1
2% PS-AlN-PCES@Balsa	3.18	3.16	0.02	0.6
4% PS-AlN-PCES@Balsa	3.20	3.18	0.02	0.6
6% PS-AlN-PCES@Balsa	3.31	3.28	0.02	0.6

In this study, the PCES-Wood was prepared by high temperature pressure method with delignified balsa wood as the heat storage skeleton, and AlN as the heat conduction reinforcement filler. The chemical modification of AlN and its ability to improve the thermal conductivity of PCES-Wood were investigated. The main conclusions are as follows:

(1) Unmodified AlN is easily to be hydrolyzed, resulting in lower thermal conductivity and agglomeration. In order to improve the hydrolysis resistance and dispersion of AlN, KH550 and SM were used to graft the surface of AlN. SM polymerized on the surface of AlN to form polystyrene (PS). The dispersion of PS-AlN in water was enhanced. PS-AlN not only has good thermal stability, but also its UV visible light absorption capacity is stronger than AlN.

(2) The penetration of PS-AlN into the cavities of PCES-Wood and the thermal stability and loss resistance of PCES-Wood are improved. This is because PS-AlN played a "limiting" role in the crystallization of PGMA. It can be seen from the cross- section that the PGMA penetrated into the cell wall and cell cavity of wood, and there is no apparent separation between the pipe wall and the PCMs, indicating that the compatibility between wood and the PGMA is very good. However, the addition of PS-AlN will reduce the crystallinity and hardness of PCES-Wood. When the content of PS-AlN is 6%, the thermal conductivity of PCES-Wood reaches 0.5148 W/(m·K), which is 1383% higher than that of the original wood. 4% PS-AlN-PCES@Balsa has the lowest undercooling and heat loss rate, and the melting enthalpy and solidification enthalpy reach 125.60 J/g and 120.30 J/g, respectively. The phase change temperature range is between 19.59 ℃ and 33.87 ℃, which is in the comfortable temperature range of human body. Under the illumination of simulated solar light source, the surface temperature of PS-AlN-PCES@Balsa rises rapidly and shows a lasting heat storage capacity at low temperatures. After 200 cold and hot cycle tests, the phase change enthalpy of PS-AlN-PCES@Balsa is still high and there is a slight leakage phenomenon, which indicates that the PS-AlN-PCES@Balsa has good durability.

Acknowledgements

This research was funded by the National Key R & D Program of China (2017YFD0601104), the Project for Cultivating Excellent Doctoral Dissertation of Forestry Engineering (LYGCYB202010, Northeast Forestry University)，the Fundamental Research Funds for the Central Universities (2018ZY12).

Author contributions

Conceptualization, Yanchen Li and Yi Liu; Methodology, Yanchen Li; Writing-original draft, Jingmeng Sun and Weiye Zhang; Formal analysis, Daihui Zhang; Methodology, Xiaoyang Fang and Beibei Wang; Writing-review and editing, Jingmeng Sun and Fangzhou Li; Project administration, Yi Liu; Funding acquisition, Hongwu Guo. All authors reviewed the manuscript.

Conflict of interest The authors declare that they have no conflict of interest.

Supplementary information Supporting data are available in the online version of the paper.

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No competing interests reported.

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A route of polyethylene glycol-based phase change heat storage wood with AlN as the thermal conductive filler

Status:

Journal Publication

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Abstract

Figures

1. Introduction

2. Material And Methods

2.1. Materials and chemicals

2.2 Preparation of MAN-Wood

2.4 Preparation of PS-AlN

2.5 Fabrication of PCES-Wood

2.6 Characterization

3. Results And Discussion

3.1 Morphological and chemical analysis of PS-AlN

3.2 Microstructure and chemical analysis of the PS-AlN-PCES@Balsa

3.3 Thermal properties of the PS-AlN-PCES@Balsa

3.3.1 Thermal conductivity

3.3.2 Heat storage capacity

3.3.3 Thermal stability

3.3.4 Optical properties

3.3.5 shaping effect

4. Conclusions

Declarations

References

Additional Declarations

Status:

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