Novel Magnetic-to-Thermal Conversion and Thermal Energy Management Composite Phase Change Material

Superparamagnetic materials have elicited increasing interest due to their high-efficiency magnetothermal conversion. However, it is difficult to effectively manage the magnetothermal energy due to the continuous magnetothermal effect at present. In this study, we designed and synthesized a novel Fe3O4/PEG/SiO2 composite phase change material (PCM) that can simultaneously realize magnetic-to-thermal conversion and thermal energy management because of outstanding thermal energy storage ability of PCM. The composite was fabricated by in situ doping of superparamagnetic Fe3O4 nanoclusters through a simple sol–gel method. The synthesized Fe3O4/PEG/SiO2 PCM exhibited good thermal stability, high phase change enthalpy, and excellent shape-stabilized property. This study provides an additional promising route for application of the magnetothermal effect.


Introduction
Under the alternating magnetic field, magnetic nanoparticles can produce a large amount of heat energy by the magnetothermal effect as a result of Néel relaxation or Brownian relaxation [1][2][3][4]. More importantly, magnetic nanoparticles exhibit superparamagnetism when their size is reduced to a certain extent [5,6], which has relatively high magnetic susceptibility and no remanence or coercivity after removal of the magnetic field [7,8]. These advantages, coupled with magnetothermal effect of magnetic nanoparticles, provide various useful applications that range from cancer treatment [9,10] to magnetically triggered drug delivery [11,12]. However, the thermal energy from magnetothermal conversion of superparamagnetic materials was difficult to manage effectively due to the continuous magnetothermal effect [13][14][15].
Based on the above idea, we reported a novel Fe 3 O 4 /PEG/SiO 2 composite PCM via the facile doping of superparamagnetic nano Fe 3 O 4 through in situ sol-gel method. Superparamagnetic nano Fe 3 O 4 possess high saturation magnetization (Ms) and negligible remanence, and they were successfully combined with a form-stable PCM system in this work. The thermal energy dissipated by successfully combined with a form-stable PCM system in this work. The thermal energy dissipated by superparamagnetic nano Fe3O4 was stored by PCM in the way of phase transition ( Figure 1). Thus, this strategy can effectively control the temperature of the magnetothermal conversion system by managing the thermal energy from magnetothermal conversion of superparamagnetic materials.

Synthesis of Fe3O4/PEG/SiO2 Composite
The preparation of Fe3O4/PEG/SiO2 composite was divided into two procedures. The first procedure was the synthesis of superparamagnetic Fe3O4 nanoclusters. In this procedure, Fe3O4 nanoclusters were prepared using a typical solvothermal method. Firstly, FeCl3 (10 mmol) and Na3Cit (1 g) were added to a mixed solvent of DEG and EG (DEG:EG = 1:1, total volume = 80 mL) under mechanical stirring at 120 °C and dissolved to form a clear solution. Secondly, NaAc (50 mmol) was put in the clear solution and stirred vigorously for the next 1h. Then, the obtained homogeneous solution was transferred into a 100 mL of Teflon-lined stainless steel autoclave and heated to 200 °C. After maintaining this condition for 10 h, the sealed autoclave was cooled to room temperature. Finally, the obtained precipitate was separated from the mixed solvent by a magnet and rinsed with deionized water for several times.
In the second procedure, Fe3O4/PEG/SiO2 composite PCM was synthesized via a sol-gel method. Figure 2 illustrates a schematic of the preparation method of Fe3O4/PEG/SiO2 composite. A specified amount of TEOS was mixed with deionized water at a mole ratio of 1:10. After stirring for several minutes, 0.5 M of hydrochloric acid was dropped into the solution, and the pH of the solution was adjusted to 1-2. The hydrolysis reaction proceeded until the solution became clear. Subsequently, 100 g·L −1 of sodium carbonate solution was added to adjust pH to 5-6, and the melted PEG and Fe3O4 homogeneous aqueous solution was put in silica sol under stirring. Silica gel containing PEG and Fe3O4 was obtained after 100 g·L −1 of Na2CO3 solution was added to adjust the pH of the solution. The

Synthesis of Fe 3 O 4 /PEG/SiO 2 Composite
The preparation of Fe 3 O 4 /PEG/SiO 2 composite was divided into two procedures. The first procedure was the synthesis of superparamagnetic Fe 3 O 4 nanoclusters. In this procedure, Fe 3 O 4 nanoclusters were prepared using a typical solvothermal method. Firstly, FeCl 3 (10 mmol) and Na 3 Cit (1 g) were added to a mixed solvent of DEG and EG (DEG:EG = 1:1, total volume = 80 mL) under mechanical stirring at 120 • C and dissolved to form a clear solution. Secondly, NaAc (50 mmol) was put in the clear solution and stirred vigorously for the next 1h. Then, the obtained homogeneous solution was transferred into a 100 mL of Teflon-lined stainless steel autoclave and heated to 200 • C. After maintaining this condition for 10 h, the sealed autoclave was cooled to room temperature. Finally, the obtained precipitate was separated from the mixed solvent by a magnet and rinsed with deionized water for several times.
In the second procedure, Fe 3 O 4 /PEG/SiO 2 composite PCM was synthesized via a sol-gel method. Figure 2 illustrates a schematic of the preparation method of Fe 3 O 4 /PEG/SiO 2 composite. A specified amount of TEOS was mixed with deionized water at a mole ratio of 1:10. After stirring for several minutes, 0.5 M of hydrochloric acid was dropped into the solution, and the pH of the solution was adjusted to 1-2. The hydrolysis reaction proceeded until the solution became clear. Subsequently, 100 g·L −1 of sodium carbonate solution was added to adjust pH to 5-6, and the melted PEG and Fe 3 O 4 homogeneous aqueous solution was put in silica sol under stirring. Silica gel containing PEG and Fe 3 O 4 was obtained after 100 g·L −1 of Na 2 CO 3 solution was added to adjust the pH of the solution. The PCM whose PEG content was 80 wt % was obtained when the gel was dried in a vacuum drying oven at 50 • C. PCM whose PEG content was 80 wt % was obtained when the gel was dried in a vacuum drying oven at 50 °C.

Characterization
Scanning electron microscopy (SEM) images were obtained on a FEI Nova NanoSEM 450 apparatus. High-resolution transmission electron microscopy (HRTEM) was recorded on a FEI Tecnai G2 F30 microscope operating at 300 kV. X-ray powder diffraction (XRD) patterns of the products were performed on a Japan Rigaku D/Max 2400 automatic X-ray powder diffractometer equipped with Cu-Kα radiation (λ = 1.54178 Å ) within 5-80°. FTIR spectra of the products were recorded using a Nicolet 460 FTIR spectrophotometer from 4000-600 cm −1 . Thermogravimetric analyses (TGA) and differential scanning calorimeter (DSC) curves were performed by using a TA Instruments Universal Analysis 2000. All DSC measurements were both carried out from 5 °C to 80 °C at the heating/cooling rate of 10 °C/min and a N2 flow rate of 50 mL/min. All TG measurements were performed in N2 atmosphere and the range of heating is from room temperature to 700 °C. The M-H and ZFC-FC curves were measured with a Quantum Design MPMSXL-7 system. Digital photos were taken with a Nikon digital camera.
Magnetic-to-thermal conversion and energy storage test of the Fe3O4/PEG/SiO2 composites: the 1 g of composites were put into small bottles (internal diameter, 1.2 cm); a certain alternating magnetic field (1.36 MHz and 550 A·m −1 ) was applied to the composites with an alternating current generator; changing temperature of product was measured using a fiber sensor plugged into it.

Characterization of Superparamagnetic Fe3O4
Fe3O4 superparamagnetic nanoclusters were fabricated via a modified solvothermal method. The morphology and microstructure of nano Fe3O4 were investigated through SEM and TEM. The obtained results are shown in Figures 3a-d. Figure 3a,b revealed that the diameter of nanoclusters was about 159 nm on the average and the morphology of nano Fe3O4 was spherical. Lattice fringes were recorded for an isolated nanocluster, and whose diameter was 152 nm, as shown in the highresolution TEM (HRTEM) image in Figure 3c. Obviously, a cluster consisted of many small primary particles according to this image. The measured distance between two adjacent planes in the same direction was 0.25 nm, which conformed to the lattice spacing of (311) planes of cubic magnetite. The

Characterization
Scanning electron microscopy (SEM) images were obtained on a FEI Nova NanoSEM 450 apparatus. High-resolution transmission electron microscopy (HRTEM) was recorded on a FEI Tecnai G2 F30 microscope operating at 300 kV. X-ray powder diffraction (XRD) patterns of the products were performed on a Japan Rigaku D/Max 2400 automatic X-ray powder diffractometer equipped with Cu-Kα radiation (λ = 1.54178 Å) within 5-80 • . FTIR spectra of the products were recorded using a Nicolet 460 FTIR spectrophotometer from 4000-600 cm −1 . Thermogravimetric analyses (TGA) and differential scanning calorimeter (DSC) curves were performed by using a TA Instruments Universal Analysis 2000. All DSC measurements were both carried out from 5 • C to 80 • C at the heating/cooling rate of 10 • C/min and a N 2 flow rate of 50 mL/min. All TG measurements were performed in N 2 atmosphere and the range of heating is from room temperature to 700 • C. The M-H and ZFC-FC curves were measured with a Quantum Design MPMSXL-7 system. Digital photos were taken with a Nikon digital camera.
Magnetic-to-thermal conversion and energy storage test of the Fe 3 O 4 /PEG/SiO 2 composites: the 1 g of composites were put into small bottles (internal diameter, 1.2 cm); a certain alternating magnetic field (1.36 MHz and 550 A·m −1 ) was applied to the composites with an alternating current generator; changing temperature of product was measured using a fiber sensor plugged into it.

Characterization of Superparamagnetic Fe 3 O 4
Fe 3 O 4 superparamagnetic nanoclusters were fabricated via a modified solvothermal method. The morphology and microstructure of nano Fe 3 O 4 were investigated through SEM and TEM. The obtained results are shown in Figure 3a-d. Figure 3a,b revealed that the diameter of nanoclusters was about 159 nm on the average and the morphology of nano Fe 3 O 4 was spherical. Lattice fringes were recorded for an isolated nanocluster, and whose diameter was 152 nm, as shown in the high-resolution TEM (HRTEM) image in Figure 3c. Obviously, a cluster consisted of many small primary particles according to this image. The measured distance between two adjacent planes in the same direction was 0.25 nm, which conformed to the lattice spacing of (311) planes of cubic magnetite. The selected-area electron diffraction (SAED) pattern recorded for the same particle was diffraction rings and revealed that nano Fe 3 O 4 is polycrystalline. The diffraction spots were widened into narrow arcs, which indicated slight misalignments among the primary nanocrystals.
Mass magnetization of the as-prepared products was measured at room temperature (300 K) in an applied magnetic field, in which the cycle range of H was from −20 kOe to 20 kOe. Figure 3e indicated that Fe 3 O 4 nanoclusters had a saturation magnetization (Ms) of 66.7 emu g −1 and negligible coercivity and remanence, suggesting that the obtained nanoclusters are superparamagnetic at room temperature. To further demonstrate the superparamagnetic behavior of nano Fe 3 O 4 at room temperature, standard zero field cooling (ZFC) and field cooling (FC) curves were measured under a magnetic field of 500 Oe (Figure 3f). The maximum of the ZFC curve appeared at about 190 K, which was the superparamagnetic blocking temperature (T b ) of the Fe 3 O 4 nanoclusters [36], and the temperature illustrated a transition from a magnetically blocked state (at low temperature) to a superparamagnetic state (at high temperature). After this temperature, a bifurcation point was observed at around 260 K in the ZFC−FC curve, which is the irreversibility temperature where the ZFC magnetization curve departs from the FC curve. selected-area electron diffraction (SAED) pattern recorded for the same particle was diffraction rings and revealed that nano Fe3O4 is polycrystalline. The diffraction spots were widened into narrow arcs, which indicated slight misalignments among the primary nanocrystals. Mass magnetization of the as-prepared products was measured at room temperature (300 K) in an applied magnetic field, in which the cycle range of H was from −20 kOe to 20 kOe. Figure 3e indicated that Fe3O4 nanoclusters had a saturation magnetization (Ms) of 66.7 emu g -1 and negligible coercivity and remanence, suggesting that the obtained nanoclusters are superparamagnetic at room temperature. To further demonstrate the superparamagnetic behavior of nano Fe3O4 at room temperature, standard zero field cooling (ZFC) and field cooling (FC) curves were measured under a magnetic field of 500 Oe (Figure 3f). The maximum of the ZFC curve appeared at about 190 K, which was the superparamagnetic blocking temperature (Tb) of the Fe3O4 nanoclusters [36], and the temperature illustrated a transition from a magnetically blocked state (at low temperature) to a superparamagnetic state (at high temperature). After this temperature, a bifurcation point was observed at around 260 K in the ZFC−FC curve, which is the irreversibility temperature where the ZFC magnetization curve departs from the FC curve.

Characterization of Fe3O4/PEG/SiO2 Composite
X-ray diffraction (XRD) is an efficient means to investigate the crystallization property of materials. Figure 4a shows the XRD patterns of PEG/SiO2, Fe3O4/PEG/SiO2, and PEG6000. As shown in the picture, sharp and intense diffraction peaks were observed at approximately 18.9° and 23.1° for PEG/SiO2 and Fe3O4/PEG/SiO2 composite, respectively, similar to pure PEG6000. Therefore, SiO2 and Fe3O4 exerted no effect on the crystal structure of PEG6000 [37]. In addition, the lower intensity of peaks observed in XRD pattern of PEG/SiO2 and Fe3O4/PEG/SiO2 compared to PEG 6000. Notwithstaning, PEG/SiO2 and Fe3O4/PEG/SiO2 composite still possessed good crystalline properties, which guaranteed the energy storage ability of the PCMs because thermal energy storage was closely related to the crystallization property of PCM [38]. Figure 4b shows the FTIR spectra of PEG6000, PEG/SiO2, and Fe3O4/PEG/SiO2 composites. The pure PEG6000 had the following characteristic absorption bands: (i) stretching vibration of O-H at 3430 cm −1 , (ii) stretching vibration of CH3 at 2917 cm −1 , (iii) stretching vibration of CH2 at 2889 cm −1 , and (iv) symmetric stretching vibration of C-O-C at 1106 cm −1 . Compared to that of pure PEG, the FTIR spectra of the synthesized Fe3O4(4%)/PEG/SiO2 and PEG/SiO2 included a symmetric stretching vibration of Si-OH in the range of 3032-3700 cm −1 , asymmetric stretching vibration of Si-O-Si at 1050 cm −1 , and bending vibration of SiO-H at 796 cm −1 , besides all representative bands for pure PEG6000.

Characterization of Fe 3 O 4 /PEG/SiO 2 Composite
X-ray diffraction (XRD) is an efficient means to investigate the crystallization property of materials. Figure 4a shows the XRD patterns of PEG/SiO 2 , Fe 3 O 4 /PEG/SiO 2 , and PEG6000. As shown in the picture, sharp and intense diffraction peaks were observed at approximately 18.9 • and 23.1 • for PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 composite, respectively, similar to pure PEG6000. Therefore, SiO 2 and Fe 3 O 4 exerted no effect on the crystal structure of PEG6000 [37]. In addition, the lower intensity of peaks observed in XRD pattern of PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 compared to PEG6000. Notwithstaning, PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 composite still possessed good crystalline properties, which guaranteed the energy storage ability of the PCMs because thermal energy storage was closely related to the crystallization property of PCM [38].

Thermal Properties
The thermal characteristic plays a critical role in the thermal management ability of PCMs. Figure 5 presents the DSC curves of PEG6000, PEG/SiO2, and Fe3O4/PEG/SiO2 with different Fe3O4 contents. The endothermic and exothermic enthalpy values and the phase transition temperatures obtained from the DSC curves are listed in Table 1. The endothermic and exothermic enthalpy values of the Fe3O4/PEG/SiO2 PCMs were 114-122 and 109-114 J/g, respectively. Enthalpies of fusion of composites are 162.4 J/g in theory because additive amount of PEG is about 80% of total mass of composites. Compared to the theoretical values, enthalpies of fusion of PEG/SiO2 and Fe3O4/PEG/SiO2 composites decrease by 25-30%, which is likely to be caused by the interference of SiO2 network with the crystallization of PEG, this phenomenon is consistent with the result reported for confinement effects of SiO2 framework on phase change of PEG [39]. They had a high phase-change enthalpy and heat storage capacity, although the PEG/SiO2 and Fe3O4/PEG/SiO2 composites showed a partial loss of latent heat compared with the pure PEG ( Figure 5 and Table 1). Enthalpy of Fe3O4 (3%)/PEG/SiO2 with the average diameter of 95 nm (130.5 J/g) is higher than that of Fe3O4 (3%)/PEG/SiO2 with the average diameter of 159 nm (114.7 J/g). This is probably because that smaller size of doped Fe3O4 is more favorable to microphase separation and causes a more continuous PEG domain, which benefits crystallization of PEG in Fe3O4/PEG/SiO2 composites ( Figure S1). In addition, the phase transition temperatures listed in Table 1 are close to that of pure PEG6000.

Thermal Properties
The thermal characteristic plays a critical role in the thermal management ability of PCMs. Figure 5 presents the DSC curves of PEG6000, PEG/SiO 2 , and Fe 3 O 4 /PEG/SiO 2 with different Fe 3 O 4 contents. The endothermic and exothermic enthalpy values and the phase transition temperatures obtained from the DSC curves are listed in Table 1. The endothermic and exothermic enthalpy values of the Fe 3 O 4 /PEG/SiO 2 PCMs were 114-122 and 109-114 J/g, respectively. Enthalpies of fusion of composites are 162.4 J/g in theory because additive amount of PEG is about 80% of total mass of composites. Compared to the theoretical values, enthalpies of fusion of PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 composites decrease by 25-30%, which is likely to be caused by the interference of SiO 2 network with the crystallization of PEG, this phenomenon is consistent with the result reported for confinement effects of SiO 2 framework on phase change of PEG [39]. They had a high phase-change enthalpy and heat storage capacity, although the PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 composites showed a partial loss of latent heat compared with the pure PEG ( Figure 5 and Table 1). Enthalpy of Fe 3 O 4 (3%)/PEG/SiO 2 with the average diameter of 95 nm (130.5 J/g) is higher than that of Fe 3 O 4 (3%)/PEG/SiO 2 with the average diameter of 159 nm (114.7 J/g). This is probably because that smaller size of doped Fe 3 O 4 is more favorable to microphase separation and causes a more continuous PEG domain, which benefits crystallization of PEG in Fe 3 O 4 /PEG/SiO 2 composites ( Figure S1). In addition, the phase transition temperatures listed in Table 1 are close to that of pure PEG6000.

Shape-Stabilized Properties
Shape-stabilized properties are crucial aspects in the application of PCMs. Figure 6 shows digital photos of PCMs at different temperatures. The samples were placed in an oven at constant temperature. The changes in the macroscopic morphology of samples were observed and recorded by a digital camera immediately after 20 min. Pure PEG6000 appeared a melted ring and melted completely when it was placed in the oven for 20 min at 65 °C and 95 °C, respectively. However, the PEG/SiO2 and Fe3O4/PEG/SiO2 composites were shape-stabilized and did not show any leakage behavior at either 65 °C or 95 °C. This is because that the PEG is a long-chain polymeric PCM that can completely or partially interpenetrate the network of SiO2 gel. The SiO2 gel network can limit the leakage of PEG by capillary force and surface tension when PEG changes from solid to liquid [27,40]. Therefore, PCMs doped with superparamagnetic Fe3O4 possess an excellent shape-stabilized property.

Shape-Stabilized Properties
Shape-stabilized properties are crucial aspects in the application of PCMs. Figure 6 shows digital photos of PCMs at different temperatures. The samples were placed in an oven at constant temperature. The changes in the macroscopic morphology of samples were observed and recorded by a digital camera immediately after 20 min. Pure PEG6000 appeared a melted ring and melted completely when it was placed in the oven for 20 min at 65 • C and 95 • C, respectively. However, the PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 composites were shape-stabilized and did not show any leakage behavior at either 65 • C or 95 • C. This is because that the PEG is a long-chain polymeric PCM that can completely or partially interpenetrate the network of SiO 2 gel. The SiO 2 gel network can limit the leakage of PEG by capillary force and surface tension when PEG changes from solid to liquid [27,40]. Therefore, PCMs doped with superparamagnetic Fe 3 O 4 possess an excellent shape-stabilized property.

Shape-Stabilized Properties
Shape-stabilized properties are crucial aspects in the application of PCMs. Figure 6 shows digital photos of PCMs at different temperatures. The samples were placed in an oven at constant temperature. The changes in the macroscopic morphology of samples were observed and recorded by a digital camera immediately after 20 min. Pure PEG6000 appeared a melted ring and melted completely when it was placed in the oven for 20 min at 65 °C and 95 °C, respectively. However, the PEG/SiO2 and Fe3O4/PEG/SiO2 composites were shape-stabilized and did not show any leakage behavior at either 65 °C or 95 °C. This is because that the PEG is a long-chain polymeric PCM that can completely or partially interpenetrate the network of SiO2 gel. The SiO2 gel network can limit the leakage of PEG by capillary force and surface tension when PEG changes from solid to liquid [27,40]. Therefore, PCMs doped with superparamagnetic Fe3O4 possess an excellent shape-stabilized property.

Magnetic-To-Thermal Conversion and Thermal Energy Management Performance
Synthesized Fe 3 O 4 /PEG/SiO 2 composite PCMs exhibited excellent magnetothermal conversion and thermal energy management behavior under an alternating magnetic field. Figure 7 shows the magnetic-to-thermal energy conversion and thermal energy management curves of the Fe 3 O 4 /PEG/SiO 2 composites. As shown in the magnetic heating curves, when the content of nano Fe 3 O 4 in the composites increased, temperature rise rate increased-i.e., heating efficiency increased-so Fe 3 O 4 /PEG/SiO 2 possess the best heating efficiency when additive amount of Fe 3 O 4 is 4 wt %. After heating for 420s under the alternating magnetic field, the temperatures of the Fe 3 O 4 /PEG/SiO 2 materials with Fe 3 O 4 contents of 1% to 4% reached 53.4 • C, 58.5 • C, 78.6 • C, and 90.7 • C, respectively. There is a growth platform appeared between 42 • C and 57 • C for Fe 3 O 4 /PEG/SiO 2 PCMs (2, 3, and 4%) when the alternating magnetic field was applied (inset of Figure 7). The times that Fe 3 O 4 /PEG/SiO 2 PCMs (2, 3, and 4%) are in the growth platform stage of temperature are 120, 165, and 277 s, respectively. In this stage, thermal energy from magnetothermal conversion is stored in the way of latent heat. Fe 3 O 4 (1%)/PEG/SiO 2 did not have this growth platform because it had less magnetic particles, such that phase transition did not occur completely in the same period of time. Likewise, the phenomenon appeared in the process of natural cooling. Moreover, the temperature of PEG/SiO 2 increased slowly, which is a reflection of the fact that PEG/SiO 2 composite did not have the component of nano Fe 3 O 4 . In addition, heating rate increases and time of melting platform shortens, when the intensity of alternative magnetic field increases ( Figure S2). Overall, combining with PCM can achieve effective management of thermal energy from magnetothermal conversion.

Magnetic-To-Thermal Conversion and Thermal Energy Management Performance
Synthesized Fe3O4/PEG/SiO2 composite PCMs exhibited excellent magnetothermal conversion and thermal energy management behavior under an alternating magnetic field. Figure 7 shows the magnetic-to-thermal energy conversion and thermal energy management curves of the Fe3O4/PEG/SiO2 composites. As shown in the magnetic heating curves, when the content of nano Fe3O4 in the composites increased, temperature rise rate increased-i.e., heating efficiency increased-so Fe3O4/PEG/SiO2 possess the best heating efficiency when additive amount of Fe3O4 is 4 wt %. After heating for 420s under the alternating magnetic field, the temperatures of the Fe3O4/PEG/SiO2 materials with Fe3O4 contents of 1% to 4% reached 53.4 °C, 58.5 °C, 78.6 °C, and 90.7 °C, respectively. There is a growth platform appeared between 42 °C and 57 °C for Fe3O4/PEG/SiO2 PCMs (2, 3, and 4%) when the alternating magnetic field was applied (inset of Figure  7). The times that Fe3O4/PEG/SiO2 PCMs (2, 3, and 4%) are in the growth platform stage of temperature are 120, 165, and 277 s, respectively. In this stage, thermal energy from magnetothermal conversion is stored in the way of latent heat. Fe3O4 (1%)/PEG/SiO2 did not have this growth platform because it had less magnetic particles, such that phase transition did not occur completely in the same period of time. Likewise, the phenomenon appeared in the process of natural cooling. Moreover, the temperature of PEG/SiO2 increased slowly, which is a reflection of the fact that PEG/SiO2 composite did not have the component of nano Fe3O4. In addition, heating rate increases and time of melting platform shortens, when the intensity of alternative magnetic field increases ( Figure S2). Overall, combining with PCM can achieve effective management of thermal energy from magnetothermal conversion.

Reversible Stability
It is important for composite phase change materials that have good reversibility of energy storage and releasing when managing magnetothermal energy. Therefore, the magnetic-to-thermal conversion cycling tests were performed in this work. Figure 8a shows the results of the first and 50th cycles under the same conditions. The curve of the 50th cycle was nearly coincident with that of the first cycle. The DSC curves before and after the cycling test were also almost identical (Figure 8b). The peaks after the cycling test presented a very small shift compared with the peaks before the cycling test. These findings confirm that the synthesized Fe3O4/PEG/SiO2 materials possess high reversibility.

Reversible Stability
It is important for composite phase change materials that have good reversibility of energy storage and releasing when managing magnetothermal energy. Therefore, the magnetic-to-thermal conversion cycling tests were performed in this work. Figure 8a shows the results of the first and 50th cycles under the same conditions. The curve of the 50th cycle was nearly coincident with that of the first cycle. The DSC curves before and after the cycling test were also almost identical (Figure 8b). The peaks after the cycling test presented a very small shift compared with the peaks before the cycling test. These findings confirm that the synthesized Fe 3 O 4 /PEG/SiO 2 materials possess high reversibility.

Thermal Stability
Besides reversible stability, thermal stability is also vital for Fe3O4/PEG/SiO2 composite PCM. The TG and DTG curves in Figure 9 explain the thermal degradation behaviors of the PCMs. Both PEG/SiO2 and Fe3O4/PEG/SiO2 composites had a steep thermal degradation process that occurred between 325 °C and 476 °C, which is consistent with that of pure PEG and far above phase transition temperature. Furthermore, the weight losses of approximately 76.2 wt % and 75.4 wt % in composites were observed in TG curves from room temperature to 700 °C, which is nearly close to the theoretical content of PEG in composites (80 wt %, in accordance with the synthesis experiment). These results show that obtained PCM composites possess high thermal stability at phase transition temperature.

Conclusions
In conclusion, we designed and synthesized a novel composite of Fe3O4/PEG/SiO2 that can achieve effective thermal energy management for magnetic-to-thermal conversion process. The composite was fabricated by in situ doping of superparamagnetic Fe3O4 nanoclusters through a simple sol-gel method. Nano Fe3O4 effectively converted magnetic work into internal energy that was released into the surrounding environment in the form of thermal energy when applying alternative magnetic field, and the converted thermal energy was stored in the PCM and can be exploited. In addition, the synthesized Fe3O4/PEG/SiO2 composite demonstrated several outstanding characteristics, such as high thermal stability and excellent shape-stabilized properties, which meets the requirements for practical application. Thus, our composites exhibit the potential for realizing the conversion from electromagnetic energy to thermal energy, and moreover, management and control

Thermal Stability
Besides reversible stability, thermal stability is also vital for Fe 3 O 4 /PEG/SiO 2 composite PCM. The TG and DTG curves in Figure 9 explain the thermal degradation behaviors of the PCMs. Both PEG/SiO 2 and Fe 3 O 4 /PEG/SiO 2 composites had a steep thermal degradation process that occurred between 325 • C and 476 • C, which is consistent with that of pure PEG and far above phase transition temperature. Furthermore, the weight losses of approximately 76.2 wt % and 75.4 wt % in composites were observed in TG curves from room temperature to 700 • C, which is nearly close to the theoretical content of PEG in composites (80 wt %, in accordance with the synthesis experiment). These results show that obtained PCM composites possess high thermal stability at phase transition temperature.

Thermal Stability
Besides reversible stability, thermal stability is also vital for Fe3O4/PEG/SiO2 composite PCM. The TG and DTG curves in Figure 9 explain the thermal degradation behaviors of the PCMs. Both PEG/SiO2 and Fe3O4/PEG/SiO2 composites had a steep thermal degradation process that occurred between 325 °C and 476 °C, which is consistent with that of pure PEG and far above phase transition temperature. Furthermore, the weight losses of approximately 76.2 wt % and 75.4 wt % in composites were observed in TG curves from room temperature to 700 °C, which is nearly close to the theoretical content of PEG in composites (80 wt %, in accordance with the synthesis experiment). These results show that obtained PCM composites possess high thermal stability at phase transition temperature.

Conclusions
In conclusion, we designed and synthesized a novel composite of Fe3O4/PEG/SiO2 that can achieve effective thermal energy management for magnetic-to-thermal conversion process. The composite was fabricated by in situ doping of superparamagnetic Fe3O4 nanoclusters through a simple sol-gel method. Nano Fe3O4 effectively converted magnetic work into internal energy that was released into the surrounding environment in the form of thermal energy when applying alternative magnetic field, and the converted thermal energy was stored in the PCM and can be exploited. In addition, the synthesized Fe3O4/PEG/SiO2 composite demonstrated several outstanding characteristics, such as high thermal stability and excellent shape-stabilized properties, which meets the requirements for practical application. Thus, our composites exhibit the potential for realizing the conversion from electromagnetic energy to thermal energy, and moreover, management and control

Conclusions
In conclusion, we designed and synthesized a novel composite of Fe 3 O 4 /PEG/SiO 2 that can achieve effective thermal energy management for magnetic-to-thermal conversion process. The composite was fabricated by in situ doping of superparamagnetic Fe 3 O 4 nanoclusters through a simple sol-gel method. Nano Fe 3 O 4 effectively converted magnetic work into internal energy that was released into the surrounding environment in the form of thermal energy when applying alternative magnetic field, and the converted thermal energy was stored in the PCM and can be exploited. In addition, the synthesized Fe 3 O 4 /PEG/SiO 2 composite demonstrated several outstanding characteristics, such as high thermal stability and excellent shape-stabilized properties, which meets the requirements for practical application. Thus, our composites exhibit the potential for realizing the conversion from electromagnetic energy to thermal energy, and moreover, management and control of the resulting thermal energy can also be realized by phase transition component in composites. This study will be a helpful strategy for management of magnetothermal energy, such as temperature control in magnetic hyperthermia.