Synthesis and characterization of wood-based phase change material with high photothermal conversion efficiency

Phase change materials attract tremendous interest for building energy conservation due to their auto-temperature regulation and thermal energy storage capacity. However, its practical application is hindered due to the leakage problem and poor photothermal conversion efficiency. To address these issues, a scalable wood-based phase change material was prepared by impregnating polyethylene glycol (PEG) into wood particles doped with Fe3O4 and subsequent a hot press in this study. PEG was encapsulated by wood particles through its abundant pore structure and leakage rate of prepared wood-based phase change material (FWPCM) was only 2.9%, which solved the leakage problem effectively. FWPCM presented high latent heat of 73 J g−1 and slowed down the temperature change obviously. Addition of Fe3O4 powder endowed FWPCM a high photothermal conversion efficiency and thermal conductivity (0.3545 W/(m*K) was increased by 125% compared to PW. So FWPCM had potential to be used as building engineering material for energy collecting, storage and conversion benefited by its great thermal performance, superior durability, simple preparing process and acceptable mechanical property.


Introduction
Construction energy consumption, such as building heating and cooling system, take account almost one-third total energy consumption in the world, which brings about greenhouse effect and urban heat island effect. Therefore, phase change materials (PCMs), which can store or release energy as a form of latent heat within a certain temperature, has attracted tremendous attention [1][2][3]. As an emerged passive thermal storage material, PCMs can be used for achieving temperature-regulated textile, solar-cells and energy-saving building by collecting solar energy and environmental waster heat. The key to achieve an effective building PCMs is to have good shape-stability and high solar energy conversion efficiency [4][5][6][7].
In order to improve the shape-stability of PCM, a number of from-stable PCMs have been fabricated by impregnating PCM into porous substitute, such as expanded graphite [8][9][10][11], montmorillonite [12], calcium silicate [13], diatomite [14] and wood [15,16]. Wood with hierarchical and multi-porous structure, in recent years, has been used as a porous carrier for encapsulating PCM due to the low cost, free pollution and low carbon emission [17][18][19]. Yang et al prepared a novel wood-based PCM by infiltrating carbonized wood with dodecanoic acid via vacuum assisted impregnation method. Results showed that this composite has high enthalpy of 177.9 J g −1 and improved shape-stability [20]. Although these wood-based PCMs showed desired thermal performance and shape-stability, the volume of most wood-based PCMs were in the range of 8-60 cm 3 due to the limitation of impregnation method, which hindered their practical applications [21][22][23]. More importantly, the substitute of most wood-based PCMs are derived from the high-quality wood materials, which Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
is not the best solution to achieve low carbon emission and energy conservation [24,25]. It's greater impact in such developments could be achieved that if it's possible to create a sustainable wood-based PCM using waste wood materials, for instance, after failure, decomposition or fragmentation [26,27]. According to the US Environmental Protection Agency [28], in 2018, there were 3.1million tons recycled municipal waste wood in the United States, taking account only 17% of the total waste wood, which revealed a large number of waste wood were underutilized. Considering the better resource efficiency and environmental impact, it's evident to prepared sustainable wood-based PCM using waste wood is a priority. Therefore, a sustainable and form-stable wood-based PCM in a large scalable process is a huge challenge, but also essential [29].
Another drawback of wood-based PCMs is low photothermal conversion efficiency, which negatively effects the full use of sustainable energy sources, such as solar energy [30,31]. To improve the photothermal conversion efficiency, some materials have been doped into the composite PCM, including graphene, metal particles and carbon materials [32,33]. Graphene and carbon materials are often exhibited high solar energy conversion efficiency and thermal conductivity [34][35][36]. However, the high cost and complex process limit their wide applications. As a low-cost and common metal particle, Fe 3 O 4 showed not only high thermal conductivity but also outstanding solar-to-thermal conversion ability [37,38]. Therefore, the combination of wood-based PCMs and Fe 3 O 4 may be an effective method to develop a composite PCMs with high thermal conductivity and photothermal conversion efficiency.
In this work, a sustainable and form-stable wood-based PCM with high photothermal conversion efficiency was fabricated using waste poplar wood (shives, chips and residues) in a scalable process. This solves the current bottleneck of low wood utilization and small sample sizes caused by the use of solid wood. PEG and Fe 3 O 4 were impregnated into wood particles and followed by a facile hot press. The scalable and sustainable wood-based PCM was obtained and labeled as FWPCM. In addition, the pure wood (PW) and composites impregnated with only composite PEG (CWPCM) were used as the control groups. Leakage problem of PEG was solved by the encapsulation of poplar particle. The addition of Fe 3 O 4 endowed composite PCM with high thermal conductivity and excellent photothermal conversion efficiency. FWPCM has excellent heat storage capacity, outstanding thermal stability and acceptable mechanical strength, which can be used as wooden wallboard, ceiling and furniture material to regulate indoor temperature. So, this sustainable and scalable wood based composite PCM with low-cost, energy-saving and recyclable properties has great potential as building energy conservation material.

Materials
Following are the specific experimental materials of my research. Poplar wood particles was from Everbright Wood Industry Limited. Polyethylene glycol (molecular weight of 800,1000,1500) and Fe 3 O 4 nanoparticles were purchased from MacLean Biochemical Co., Ltd All reagents were analytical-grade reagents. Urea aldehyde resin glue with 65% solid content was provided by Shenglan Chemical Commercial Company.

Preparation of phase change energy storage particle board
The experimental process for preparing phase change energy storage particle board is as follows: Poplar particles were weighed and placed in an impregnation tank after over dried, 60% aqueous solution of PEG1500 and PEG800 was poured into the tank to carry out the impregnation. After 30 min vacuum treatment, the particles were dried at 103°C until constant weight, then mixed with 1 wt% Fe 3 O 4 nanoparticles and 15 wt% urea aldehyde resin glue. Finally, phase change energy storage particleboard FWPCM was obtained after hot pressing under the condition of 145°C and 1.5 Mpa. The prepared process is shown in figure 1. A total of six experimental groups were set up according to different ratios of PEG impregnating solution and the concentration of Fe 3 O 4 nanoparticles, and PW was the control group. The experimental group is shown in table 1.

Characterization
In this study, a variety of characterization methods were used to evaluate its performance. The specific characterizations are as follows: Scanning electron microscope (SEM, 6380LV, Nippon Electronics Corporation, Japan) was used to observe the morphology of PW, CWPCM, and FWPCM. The accelerating voltage is 3 kV for topography shooting, 15 kV for energy spectrum mapping, and the detector is SE2 secondary electron detector. All samples prior to imaging were sprayed with the Quorum SC7620 sputter coater to enhance imaging contrast.
The chemical structure of the composite materials was tested by flourier transformation infrared spectroscopy (FTIR, iS10, Nicolet, America) in the form of KBr pellet over the range of 400 ∼ 4000 nm.
The crystal structure of PW, PEG, Fe 3 O 4 , CWPCM and FWPCM was analyzed by an x-ray diffractometer (XRD, x-ray 6000, Shimadzu, Japan) from 5°to 90°with a scanning speed of 2°min −1 at room temperature.
Polarizing microscope (POM, THMS600, Japan) was used to observe the crystallization property of PW, PEG, and CWPCM at room temperature.
Differential scanning calorimetry (DSC, 204 Phoenix, Germany) was used to evaluate the thermal storage capacity. The temperature range was −10 ∼ 60°C with a heating or cooling rate at 5°C min −1 under nitrogen atmosphere.
The thermal stability was measured by thermogravimetric analysis (TG, Cloud Spectrum Instrument Corporation, America) under a nitrogen atmosphere from 30°C to 600°C at a heating rate of 5°C min −1 .
The thermal conductivities were conducted by a thermal constant analyzer (Hot Disk, TPS2500S, Sweden) at room temperature, the transient plane source method was adopted as the measurement method.
In order to further evaluate temperature regulation ability of composite PCMs, the samples of PW, CWPCM and FWPCM were heated at 70°C in the oven and cooled at −20°C in the refrigerator during heating and  cooling process, the temperature fluctuation curves were recorded and depicted by a temperature detector (JK-16C, Jinko, China).
To visually explore the photo-thermal conversion behavior, the temperature evolution of composite PCMs under simulated solar illumination conditions (tungsten lamp, wavelength 320-400 nm) were measured and depicted by a temperature recorder.
The absorbance was measured by UV/visible/near-infrared diffuse reflection test (Shimadzu UV-3600i Plus, Japan) in the range of 200∼800 nm.
Dimensional stability was carried out according to GB/T 17657. Shore hardness of the composites was tested with a LX-DS Digital Tester. The pressure pin is 10 mm far from the edge of the specimen (20 × 20 × 3 mm). The hardness is measured 5 times at different locations, and the average value is calculated.

Morphology of FWPCM
The morphology of PW, CWPCM, and FWPCM are shown in figure 2. Wood is a porous material with honeycomb-like structure, which is beneficial for encapsulating PCM (figures 2(a) and (b)). After impregnation and hot pressing, most wood lumens were filled with PEG, as confirmed by the SEM image of FWPCM in the radial (figure 2(d)) and longitudinal (figure 2(e)) sections. FWPCM showed a smoother wood cell wall after adding PEG in the enlarged image (figure 2(f)) which further demonstrated that PEG got into wood particles successfully and filled the lumens.
In order to improve the thermal conductivity and photothermal conversion efficiency, Fe 3 O 4 nanoparticles was introduced into the composite as thermo-promoters. As shown in figures 2(g) and (h), the mixture of PEG and Fe 3 O 4 nanoparticles was infiltrated into wood lumen. The high loading and uniform distribution of iron element in FWPCM were confirmed by EDS test (figure 2(i)). And the weight increase percentage (WIP) of composites was calculated by the following formula: here Wo is the average over drying weight of pure poplar particles before impregnation and Wt is the average over drying weight of pure poplar particles after impregnation with PEG.
A high weight increase percentage of CWPCM (WIP of 111%) and FWPCM (WIP of 109%) further revealed the effective impregnation of PEG and Fe 3 O 4 nanoparticles in composite PCM (figure 2(c)).

Chemical structure and crystallization of FWPCM
The chemical analysis of PW, PEG, CWPCM, FWPCM and Fe 3 O 4 was explored by FT-IR ( figure 3(a)). In the curve of PW, the peaks occurred at 2881 cm −1 , 1629 cm −1 and 1379 cm −1 were corresponding to -CH, -CH 2 and -OH groups, which could be attributed to the presence of cellulose in wood. In FWPCM, the absorption peak at 3411 cm −1 was due to the stretching of -OH group. The peaks at 2871 cm −1 , 1104 cm −1 , and 949 cm −1 characterized the tensile vibrations of PEG crystals, C-O and -CH 2 groups. The peak at 841 cm −1 was caused by internal -CH 2 . The absorption peak at 570 cm −1 was due to the vibrating of Fe-O group, and 1104 cm −1 was its frequency doubling peak. The curve of FWPCM was only a combination of PEG, PW and Fe 3 O 4 , and no additional peaks were observed, indicating that PEG, PW, and Fe 3 O 4 were physical combination. The crystallization of PW, PEG, CWPCM-3, Fe 3 O 4 and FWPCM were explored by XRD ( figure 3(b)). PW had two peaks at 18°and 22.5°, which could be assigned to the (101) and (002) planes of cellulose molecular chain. Characteristic diffraction peaks consistent with PEG appeared in the FWPCM curve after impregnation, further demonstrating the successful impregnation of PEG into wood particle. Three diffraction peaks of Fe 3 O 4 at 37.1°, 58.5°, and 63.1°also appeared in the curve of FWPCM. The refractivity of PW (figure 3(c)), PEG (figure 3(d)), and CWPCM (figure 3(e)) at room temperature was characterized using polarizing microscope. There was no obvious crystal region could be observed in wood, because most crystalline cellulose were coated by lignin. While PEG presented clear spherulitic crystal structure which indicated a great thermal energy storage capacity. CWPCM also showed apparent birefringence property, but the size of sphaerocrystal was much smaller than PEG, the change of spherulitic crystal structure finally lead to the decrease of enthalpy. The results of FTIR, XRD and POM confirmed that wood lumen was filled by PEG, and the system consisted by wood, PEG and Fe 3 O 4 exhibited outstanding chemical compatibility, which was beneficial for form-stable composite phase change materials.

Thermal energy storage performance of FWPCM
Thermal energy storage capacity and phase change temperature of FWPCM were determined by DSC (figure 4) and the corresponding values are shown in table 2. T f and T m are the freezing and melting temperature at the maximum heat flow during the phase transition. As seen in figures 4(a) and (b), enthalpy of CWPCM decreased apparently compared to pure PEG, because mixed PEG solution in wood channels was the only phase change material to storage or release thermal energy, while wood just as a carrier to solve the liquid leakage. After impregnation, the latent heat of CWPCM decreased with increasing PEG800 content in PEG mixed solution. CWPCM-3 had the largest freezing enthalpy of 78.74 J g −1 and melting enthalpy of 76.72 J g −1 . It's worth noting that a multi-peak phenomenon appeared in melting process due to the combination of PEG1500 and PEG800, while just one freezing peak observed resulted by the crystalline form transformation between them at high temperatures [39]. After the addition of Fe 3 O 4 , FWPCM showed a slight enthalpy decrease (figures 4(c) and (d)), but the phase transition temperature was similar to CWPCM, indicating that the addition of Fe 3 O 4 had little effect on the phase change behavior of FWPCM.

Thermal stability of FWPCM
The TG and DTG curves of PW, PEG, CWPCM, FWPCM are shown in figure 5. Wood showed one step degradation resulted by the decomposition of cellulose, hemicellulose and lignin. PEG also degraded in one step ranged from 300°C to 420°C owing to the break of C-C and C-O bond. The weight loss of CWPCM was carried out in two steps, which was attributed to the combined effect of PEG and wood. The first degradation ranged from 200°C to 330°C was resulted by the degradation of wood composition, and the second stage between 330°C-410°C was due to the degradation of PEG. The degradation rate of CWPCM was significantly lower than that of pure PEG, which was benefited by the thermal insulation protection of wood. FWPCM still presented two-step degradation similar to CWPCM because the superior stability of Fe 3 O 4 , suggesting this composite had great thermal stability when used for building temperature regulation.

Photo-thermal conversion performance and temperature management ability
The absorbance of PW, CWPCM-3, FWPCM-1, FWPCM-3 and FWPCM-5 are shown in figure 6(a). After impregnated by PEG, both of CWPCM and FWPCM presented excellent absorbance compared to PW except in the near-ultraviolet region. FWPCM had better absorbance than CWPCM after adding Fe 3 O 4 and its absorbance increased with the increase of Fe 3 O 4 content. This could be benefited to the black surface of FWPCM which effectively reduced the light reflection and improved the absorbance effectively. High absorbance led to a great photo-thermal conversion behavior, so the temperature evolution of composite PCMs under simulated solar illumination conditions (tungsten lamp, wavelength 320-400 nm) was measured by a temperature recorder ( figure 6(b)). Obviously, all composite PCMs had higher temperature than PW due to the superior absorbance. But it is needed to note that PW had faster temperature increase speed than CWPCM below about 73°C, this could be attributed to the thermal energy storage of PEG in CWPCM, which slowed down the temperature increase. The photo-thermal conversion performance of FWPCM also prior to CWPCM because of the Fe 3 O 4 nanoparticle. In addition, temperature of FWPCM-3 increased fastest due to the highest average absorbance value in the wavelength range of 320-400 nm.
Thermal conductivity is an important parameter to evaluate the thermal energy absorption and release efficiency of phase change materials. The comparison of thermal conductivity of PW, CWPCM and FWPCM are shown in figure 6(c). PW exhibited a low thermal conductivity of 0.147 W/(m * K) due to the unique porous structure of wood. While the thermal conductivity of CWPCM reached 0.2933 W/(m * K), which was improved by 99.5% because the formation of a continuous heat transfer path in wood after PEG addition. Furthermore, FWPCM-5 showed the highest thermal conductivity of 0.3545 W/(m * K) owing to the enhanced heat transfer network after adding Fe 3 O 4 nanoparticles, which was 2.25 times higher than that of PW. Therefore, it can be concluded that the introduction of Fe 3 O 4 nanoparticles improved the light collection ability and thermal conductivity of the composite.
Temperature evolution curves of wood, CWPCM and FWPCM during heating and cooling process were depicted using a temperature detector (figures 6(d) and (e)). During heating process, the temperature of all samples is gradually increased ( figure 6(d)). It is interesting that the curves of FWPCM and CWPCM appeared platform between 27∼40°C, which was related to the solid-liquid phase transition of PEG. In this temperature region, PEG in composite absorbed thermal energy and slowed down the temperature increase, which resulted to a plateau in the curve. In cooling process, CWPCM and FWPCM released the stored energy and prevent the temperature change ( figure 6(e)). An infrared thermal camera was used to help illustrating the temperature change of composite PCM under heating or cooling environment ( figure 6(g)). All samples were heated at 60°C   in an oven after cooled to 0°C in a refrigerator and kept a distance of 60 cm from the camera, surface temperature of the composite was recorded. The surface temperature of CWPCM was lower than that of PW in any time, indicating that CWPCM had better temperature regulation ability. It's worth noting that FWPCM showed a more sensitive temperature change compared to CWPCM due to the addition of Fe 3 O 4 . The melting enthalpy of FWPCM reaches 73 J g −1 , and there was a constant temperature interval about 4 min during heating and cooling process, which indicates that it has good heat storage and temperature regulation ability and has great potential for building energy saving [40].

Shape stability and physical properties of FWPCM
In order to explore the shape stability of composite PCM, we took images of PEG, CWPCM and FWPCM after heating at 60°C for 15 min ( figure 7(a)). PEG appeared liquid leakage after constant heating, while CWPCM and FWPCM remained stable without leakage because the porous structure and strong capillary adsorption effect of wood. FWPCM exhibited the same excellent shape stability as CWPCM, indicating that the addition of Fe 3 O 4 had no negative effect on the encapsulation of wood particles. The enthalpy of CWPCM-3 and FWPCM-5 were measured after 100 times heating-cooling cycles (figures 7(c) and (d)), the specific parameter was presented in table 3. After thermal cycles, the melting and freezing enthalpy of CWPCM-3 was 72.89 J g −1 and 66.47 J g −1 , respectively. And the melting and freezing enthalpy of FWPCM-5 was 69.47 J g −1 and 64.37 J g −1 , respectively. Both the enthalpy of CWPCM and FWPCM decreased slightly, indicating a great durability and recyclability. Figure 7(b) shows the Shore hardness of PW, CWPCM and FWPCM. CWPCM-4 showed the highest hardness of 54.3 HD, which was 39.2% higher than PW. After the addition of Fe 3 O 4 nanoparticles, FWPCM exhibited higher hardness than CWPCM, which was 17.6% higher than CWPCM-3, indicating that Fe 3 O 4 nanoparticles dispersed in FWPCM was beneficial for improving the hardness.  Dimensional stability of PW, CWPCM-3, FWPCM-1, FWPCM-3 and FWPCM-5 are shown in figure 7(g). It is apparent that the dimensional change rate of all samples in high humidity environment were higher than in dry heat condition due to the high-hydrophilicity of wood and PEG. Compared to the PW, both CWPCM and FWPCM showed a smaller size change rate (below 1% in dimensional change rate) because the swelling of PEG in wood.

Conclusion
In this study, a sustainable wood-based phase change material with high photothermal conversion efficiency was prepared successfully by combining wood, PEG and Fe 3 O 4 nanoparticle. DSC results showed that FWPCM had high latent heat of about 73 J g −1 , which significantly slowed down the temperature change rate. Thermal cycling test showed that leakage rate of PEG was only 2.9% which meant an excellent durability. Addition of Fe 3 O 4 nanoparticle obviously enhanced the photo-thermal thermal efficiency and thermal conductivity of FWPCM which was 2.25 times higher than that of PW. Physical properties including dimensional stability and hardness of FWPCM were also improved apparently after incorporating PEG and Fe 3 O 4 nanoparticle. The shore hardness of FWPCM increased by 17.6% than CWPCM-3 and dimensional change rate was below 1%. We suggest that FWPCM have the potential for application in building energy saving, due to their excellent thermal storage capacity, high photo-thermal efficiency, good thermal conductivity and great durability. Future studies should focus on improving compatibility between PEG and wood particle to enhance mechanical properties.