Novel Fumed Silica–Based Shape–Stabilized Phase Change Materials with Magnetically Boosted Charging Efficiency and Bi–Functional Thermotherapy Ability

Abstract

In this work, 1–octadecanol (OD) was combined with fumed silica (F–SiO₂) and Fe₃O₄ nanoparticles to form OD/F–SiO₂/Fe₃O₄ shape–stabilized phase change materials (SSPCMs) with low thermal conductivities (TCs) and magnetically boosted charging efficacy. The SSPCMs were studied with varying OD contents of 50–80% and Fe₃O₄ contents of 5.0–12.5%. F–SiO₂ possessed a suitable porosity to stabilize up to 70% OD without liquid leakage. The 70% OD/F–SiO₂/Fe₃O₄ SSPCM exhibited a high heat storage capacity of 147.6 J/g and excellent durability after 500 accelerated thermal cycles. In comparison to pure OD with a low TC of 0.284 W/(m·K), those of 50–70% OD SSPCMs were further reduced to 0.156–0.193 W/(m·K) owing to the addition of F–SiO₂ and Fe₃O₄ nanoparticles with low TCs. In addition, the SSPCMs possessed superparamagnetism inherited from Fe₃O₄ nanoparticles, enabling boosted magnetothermal conversion and storage ability. Indeed, under a low alternating magnetic field (intensity of 0.056 mT), the prepared 70% OD/F–SiO₂/Fe₃O₄ SSPCM was rapidly heated from ∼34 to 76 °C in 360 s, which was 6.4 and 3.8-fold superior to being heated by a convective oven at 100 and 150 °C, respectively. A thermal pad derived from the 70% OD/F–SiO₂/Fe₃O₄ SSPCM exhibited a bi–functional thermotherapy, simultaneously maintaining heat release within 50–55 °C for 30 min and 40–50 °C for 16 min, which was highly suited for high and mid–level thermotherapy, respectively. These merits endow the OD/F–SiO₂/Fe₃O₄ SSPCMs with a promising potential in practical thermotherapy applications.

1. Introduction

Thermal energy storage (TES) using shape-stabilized phase change materials (SSPCMs) is paramount for enhancing energy efficiency, sustainability, and reliability across various applications. By exploiting the high latent heat capacity during phase transitions, typically from solid to liquid, SSPCMs can store/release remarkable amounts of thermal energy at nearly fixed temperatures. , This ability to absorb or release heat without large temperature fluctuations makes PCMs ideal for applications such as building temperature regulation, renewable energy systems, electronic cooling, and thermal management of batteries. Moreover, TES using SSPCMs contributes to energy conservation by minimizing energy loss, improving load management, and enabling energy availability when direct sources are insufficient or intermittent. As the demand for clean and efficient energy systems grows, the role of PCMs in thermal energy storage continues to expand, offering promising solutions for achieving higher energy efficiency and reducing carbon footprints. ,

SSPCMs comprise phase change materials (PCMs) integrated into porous supports. While the PCMs are in charge of thermal energy storage, the porous supports play a critical role in enhancing the performance of SSPCMs by providing structural stability and preventing liquid leakage of PCMs during phase transition processes. In addition, the porous supports significantly affect the SSPCM’s thermal conductivities (TC), determining the heat transfer efficiency. Due to the inherently low TC of pure PCMs, a large number of reports have dealt with this issue by combining them with high–TC porous supports, including expanded graphite, , graphene, metal foams, , nanosilver, , and boron nitride. , With improved TCs, the obtained SSPCMs are promising in applications requiring rapid heat adsorption and release, including solar power plants and waste heat recovery systems. On the other hand, SSPCMs with low TCs are particularly useful in applications where controlled, gradual heat release or thermal insulation is desired. In building insulation systems, these materials are integrated into walls, roofs, or floors to enhance thermal comfort by slowly absorbing and releasing heat, thereby reducing energy consumption and improving efficiency. In packaging, low thermal conductivity SSPCMs are valuable for maintaining stable temperatures in insulated shipping containers for pharmaceuticals, food, and other temperature–sensitive goods over extended periods. Additionally, they are employed in thermal buffering applications where precise temperature regulation is necessary, such as in medical devices or biological sample storage. Their ability to provide sustained thermal energy release makes them ideal for personal heating systems, including thermotherapy pads and clothing designed for cold–weather environments. ,,

Fumed silica (F–SiO₂) is a known porous material for its extremely low TC (∼0.05 W/(m·K)), suitable porosity, high PCM adsorption (70–80%), low cost, non–toxicity, and high availability. , It was demonstrated that integrating PCMs with F–SiO₂ could decrease the PCM’s TCs by 30–50%. , While this property makes F–SiO₂ an ideal porous support for PCMs in applications requiring a prolonged heat release, the low TC significantly limits the heating rate during thermal charging processes, thus declining the overall performance of the obtained SSPCMs. Common SSPCM heating methods include convective, solar, and electric heating. Convective heating uses ovens or hot water as heat sources. However, the low thermal transfer of hot air in ovens and hot water’s high permeability and wetness make them unsuitable as effective heat sources. Solar and electric heating requires the addition of additives with high sunlight absorption or high electric conductivity, including carbon–based materials, − graphene aerogel, Mxene, , metal foams, , and nanometals. , Nevertheless, these additives usually possess high TCs, which results in TC improvement for the obtained SSPCMs. Boosting thermal charging performance while maintaining a low TC is, therefore, vital to fulfilling the perfection of low–TC SSPCMs.

Recently, magnetic heating has emerged as an effective heating method for SSPCMs. By integrating magnetic materials, e.g., Fe₃O₄ nanoparticles (NPs), into SSPCMs, the obtained SSPCMs can be rapidly heated under an alternating magnetic field because of the effects of the Néel and Brownian relaxation. , In addition, Fe₃O₄ NPs often possess low TCs, , which are expected to negligibly affect the low TC of the SSPCMs. With the discussion above, in this work, we combined 1–octadecanol (OD), F–SiO₂, and Fe₃O₄ NPs to fabricate OD/F–SiO₂/Fe₃O₄ SSPCMs with a magnetic charging ability. OD was selected as a PCM for its high heat storage capacity (∼240–350 J/g), , which is outstanding over other PCMs, including fatty acids (∼200 J/g), poly­(ethylene glycol) (∼160 J/g), and paraffin waxes (∼190 J/g). Moreover, OD exhibits low cost, non–toxicity, and a suitable phase change temperature (∼57 °C). Meanwhile, F–SiO₂ played a role as a porous support, and Fe₃O₄ NPs provided magnetism for magnetic heating. The OD/F–SiO₂/Fe₃O₄ SSPCMs were prepared with varying OD contents 50–80%, while the F–SiO₂/Fe₃O₄ ratio was fixed at 3:1. The obtained SSPCMs were first characterized for microstructure, morphology, and chemical compatibility. Then, thermal properties involving phase change behaviors, thermal stability, thermal conductivity, and others, including cycling durability, leakage resistance, and crystallinity of the SSPCMs, were comprehensively studied and discussed. Finally, the magnetic properties and practical magnetic heating ability were examined. Last but not least, the practical thermotherapy applications of the SSPCMs were also evaluated.

2. Materials and Methods

2.1. Materials

Fumed silica (Aerosil 200) was obtained from Evonik Operations (German). 1–Octadecanol (99%) was purchased from Alfa Aesar. Sodium acetate trihydrate (CH₃COONa·3H₂O), iron­(III) chloride (FeCl₃), ethylene glycol, and ethanol were all of AR type and obtained from Xylong Chemical (China).

2.2. Preparation of Fe₃O₄ NPs

The Fe₃O₄ nanoparticles (NPs) were synthesized using the method outlined in our previous report. Initially, 1.08 g of FeCl₃ and 4.6 g of CH₃COONa·3H₂O were added to 60 mL of ethylene glycol, and the mixture was ultrasonicated until a clear solution was obtained. The resulting solution was transferred into an autoclave and treated for 10 h at 200 °C. The precipitate was centrifuged, rinsed with water and ethanol, and finally dispersed in 60 mL of ethanol for future use. The Fe₃O₄ NPs were dried at 100 °C for 24 h instead of being dispersed in ethanol for characterization.

2.3. Preparation of OD/F–SiO₂/Fe₃O₄ SSPCMs

The OD/F–SiO₂/Fe₃O₄ SSPCMs were synthesized and investigated with varying OD contents of 50–80%. Previous reports showed that the Fe₃O₄ contents of approximately 5–15% were optimal for effective magnetic heating, while the remaining contents of porous supports were sufficient to stabilize PCMs. ,, Thus, the Fe₃O₄ contents in this work were varied within 5–12.5% by fixing the F–SiO₂/Fe₃O₄ ratio of 3:1. The composites were fabricated following a commonly employed solvent–assisted technique. Precisely measured amounts of OD, F–SiO₂, and Fe₃O₄ NPs were added to ethanol. After 1 h of stirring at the ambient temperature, the mixture was heated to ethanol’s boiling temperature to evaporate the solvent. Finally, the resulting materials were subjected to heat treatment at 70 °C for 24 h. Four SSPCMs were prepared by this procedure, and their specific compositions and abbreviated names are shown in Table .

1. Specific Compositions of Prepared SSPCMs.

SSPCM	abbreviated name	OD (%)	F–SiO2 (%)	Fe3O4 (%)
50% OD/F–SiO2/Fe3O4	50% OD	50	37.5	12.5
60% OD/F–SiO2/Fe3O4	60% OD	60	30	10
70% OD/F–SiO2/Fe3O4	70% OD	70	22.5	7.5
80% OD/F–SiO2/Fe3O4	80% OD	80	15	5

2.4. Characterization Methods

Scanning electron microscopy (SEM) was measured by a Hitachi S–4800 instrument (Hitachi, Japan). The N₂ adsorption–desorption isotherm was measured by a Belsorp–Max instrument (MicrotracBel, Japan). Fourier–transform infrared spectroscopy (FTIR) was measured by an FTIR 4600 model (JASCO, Japan). X–ray diffraction (XRD) was measured by an Empyrean Diffractometer (Malvern Panalytical, United Kingdom). Differential scanning calorimetry (DSC) was measured by a 214 Polyma instrument (NETZSCH, Germany). Thermogravimetric analysis (TGA) was measured by a Labsys Evo TG–DSC 1600 analyzer (Setaram Instrument) at a rate of 10 °C/min under a nitrogen atmosphere. Thermal conductivity was measured by TPS 2500S instrument (Hot Disk AB, Sweden). Vibrating sample magnetometry (VSM) was measured by a Microsens instrument (Microsens). Infrared thermal imaging was measured by an infrared camera (Uni–T 260 V, China).

3. Results and Discussion

3.1. Characterization

The porosity and microstructure of the pristine F–SiO₂, Fe₃O₄, and the prepared SSPCMs were characterized by SEM images (Figure ) and N₂ sorption isotherms (Figure ). Those of F–SiO₂ were described in our recent report. Briefly, F–SiO₂ possessed an interconnected porous structure formed by the aggregation of SiO₂ nanoparticles (Figure a). Its surface area was 195 m²/g, and its pore size ranged in micro and mesopores, as obtained from the N₂ sorption isotherms and the relevant pore size distributions (PSD) (Figure ). In addition, F–SiO₂ possessed macropores in the range of 50–150 nm with a large total pore volume of 17 cm³/g and a remarkable porosity of 88%, as obtained by mercury porosimetry. The interconnected micro–meso–macropore system and large porosity allow the penetration and transport pathways for OD. Meanwhile, the prepared Fe₃O₄ exhibited in the form of nanoparticles (NPs) of approximately 100–150 nm (Figure b), and each particles were formed by the aggregation of many tiny particles of ∼20 nm (Figure S1). The prepared Fe₃O₄ NPs, F–SiO₂, and OD were integrated to fabricate SSPCMs with varying OD contents of 50–80%. As seen in Figure c–d, the obtained SSPCMs showed a mixed combination of F–SiO₂, Fe₃O₄ NPs, and OD. With the ascended OD contents, the F–SiO₂ and Fe₃O₄ surfaces were increasingly covered with OD. The infiltration of OD into F–SiO₂ pores was confirmed by the gradually decreased N₂ adsorption and pore intensities with increasing OD contents (Figure ). Specifically, at 50% OD infiltrated, the pores below 20 nm almost disappeared (Figure b), indicating the smaller pores of F–SiO₂, i.e., micro and mesopores <20 nm, were filled with OD. The pores above 20 nm further declined with increasing OD content to 60% and completely disappeared at 70% OD. These results demonstrated that OD continued to fill the larger pores, i.e., mesopores >20 nm and macropores, after filling the smaller pores of F–SiO₂. The EDS images of a representative SSPCM, i.e., 70% OD (Figure g), showed an even dispersion of C, Si, O and Fe elements. These outcomes proved a successful preparation of OD/F–SiO₂/Fe₃O₄ SSPCMs, in which OD was restricted in the F–SiO₂’s porous network and on the Fe₃O₄ surfaces to some extent.

1.

SEM images of (a)­F–SiO₂, (b) Fe₃O₄, (c) 50% OD, (d) 60% OD, (e) 70% OD, (f) 80% OD, and (g) an EDS image of 70% OD.

2.

N₂ adsorption–desorption isotherms (a) and corresponding pore size distribution (b) of F–SiO₂ and the prepared 50–70% OD SSPCMs. The curves of F–SiO₂ were reused from our previous publication.

Figure a exhibits the FTIR patterns of two representative SSPCMs (50 and 70% OD) compared to pristine F–SiO₂, Fe₃O₄, and OD. For pristine OD, a broad and strong absorption peak located at 3323 cm^–1 was relevant to the vibrations of the O–H group; a distinct sharp peak at 1060 cm^–1 was relevant to the vibrations of the C–O group; and the C–H vibrations of the alkyl chain were evident as peaks at 2918, 2884, 1465, and 720 cm^–1. , The spectrum of F–SiO₂ shows a strong, broad absorption band located at 1060 cm^–1 and a weaker peak at 806 cm^–1 due to the vibration modes of Si–O–Si bonds, respectively, and a peak at 450 cm^–1 for O–Si–O bending; additionally, a broad O–H band between 3200–3700 cm^–1 indicated the presence of surface silanol groups (Si–O–H) and adsorbed water. , The prepared Fe₃O₄ also presented surface – O–H groups characterized by vibrations at 3424 cm^–1 and the vibrations at 570 cm^–1 characterized its Fe–O bonds. , Notably, the SSPCMs presented integrated properties of pristine OD and F–SiO₂ without new peaks. It is noted that the characteristic absorption of Fe₃O₄ was faintly displayed in the spectra of the SSPCMs, attributed to its low contents (7.5–12.5% Fe₃O₄ in the 50 and 70% OD SSPCMs).

3.

(a) FTIR spectra of F–SiO₂, Fe₃O₄, OD, 50% OD and 70% OD and (b) XRD patterns of the corresponding materials.

In addition to the FTIR, XRD patterns of the relevant substances were also measured, and the results are displayed in Figure b. The SSPCMs fully exhibited the two main characteristic diffraction patterns of OD at 2θ of 21.7 and 24.6°. Noting that the small peak on the left of the peak at 21.7° of pure OD also appeared in the 70% OD sample, but disappeared in the 50% OD sample. This indicated that a slight defect occurred in the crystalline structure of OD in the SSCPMs with low OD contents, which was due to interactions between OD and the porous supports (see Section for more discussion). Meanwhile, F–SiO₂ showed no diffraction signal in the SSPCMs due to the amorphous nature of the SiO₂ material. Notably, the prepared Fe₃O₄ showed weak peaks at 2θ of 30.3, 35.7, 43.5, 57.4, and 63.0°, respectively assigned to the characteristic crystalline planes (220), (331), (400), (511), and (440) of Fe₃O₄ (JCPDS Card No. 019–0629). , These diffractions faintly appeared in the two SSPCMs due to their inherently low intensities and the low Fe₃O₄ contents. Overall, the FTIR and XRD results demonstrated that OD, F–SiO₂, and Fe₃O₄ were physically combined, and the main crystalline characteristics of OD were preserved in the forms of SSPCMs.

3.2. Thermal Energy Storage Properites

The thermal energy storage properties of the prepared SSPCMs and pure OD were studied by DSC. The obtained DSC curves and specific phase change data are shown in Figure and Table , respectively. Pure OD was characterized to have three crystalline forms including rotator α, orthorhombic β, and monoclinic γ phase. The β–phase and γ–phase are stable at a lower temperature and transform to the α–phase at a higher temperature. , As shown in Figure , pure OD showed one endothermic peak at 56.9 °C during melting and two exothermic peaks at 55.9 and 51.7 °C during crystallization. The endothermic peak included a solid–solid (S–S) phase transition (β or γ–phase to α–phase) overlapped with a solid–liquid (S–L) transition due to their similar phase transition temperatures. In the crystallization process, the S–S transition (α–phase to β or γ phase) was supercooled and separated from the L–S transition, forming two distinct exothermic peaks. These results were in good agreement with previous reports. ,,

4.

Melting (a) and crystallization (b) DSC curves of OD and the prepared SSPCMs.

2. Phase Change Properties of OD and the Prepared SSPCMs.

	T M (°C)	T C,L‑S (°C)	T C,S–S (°C)	ΔH M (J/g)	ΔH C (J/g)	F (%)
50% OD	52.2	52.4	36.9	89.0	87.8	76.6
60% OD	52.6	53.9	39.7	118.5	116.4	85.0
70% OD	52.8	55.4	46.7	147.6	146.3	90.8
80% OD	53.1	55.3	47.6	174.2	173.2	93.7
OD	56.9	55.9	51.7	232.3	231	100.0
70% OD after 500 cycles	52.4	55.4	47.5	143.5	142.1	88.2

The phase transition behaviors of OD confined in the prepared SSPCMs were changed in both the melting and crystallization processes compared to pure OD. The melting DSC curves of the SSPCMs were separated into two peaks, and the corresponding melting temperatures were slightly decreased by 3.8–4.7 °C compared to pure OD. The crystallization temperatures of the SSPCMs were also reduced in both L–S transitions (by 2.0–3.5 °C) and S–S transitions (by 4.1–14.8 °C) compared to those of pure OD, and the phase transition temperature depression increased with lowering OD content in the SSPCMs. The phase transition temperature depression was a commonly observed phenomenon when a PCM was confined in nanopores. , According to the Gibbs–Thomson equation, the temperature depression degree increases with decreasing pore size that confines the PCM. , As described in Section , OD first filled the smaller pores of F–SiO₂ at low OD contents, causing the larger temperature depression. Subsequently, the larger pores were filled at higher OD contents, thus the temperature depression was decreased.

The phase change enthalpies of the SSPCMs are shown in Table . The SSPCMs exhibited an increase in phase change enthalpies with increasing OD content, reaching ∼174 J/g at 80% OD. This could be readily understood as OD was the only factor generating phase change enthalpies for the SSPCMs. Another possible reason was that interfacial interactions of OD with F–SiO₂ and Fe₃O₄ NPs could depress the crystallinity of confined OD, resulting in lowered phase change enthalpies. Previous reports demonstrated interfacial hydrogen bonds (H–bonds) between PCMs and surface functional groups of porous supports restricted the ordered arrangement of PCMs for crystallization, causing depressed crystallinities. , OD was known to H–bond with the hydroxyl (O–H) groups that were present on the surfaces of F–SiO₂ and Fe₃O₄. − The effects of these H–bonds on the crystallinity of OD could be examined by crystallization fractions (F (%)) calculated using eq ,

$$F=\frac{\mathrm{\Delta }{H}_{\mathrm{M},\mathrm{CPCM}}·100}{\mathrm{\Delta }{H}_{\mathrm{M},\mathrm{OD}}·x}$$ 1

where ΔH _M,SSPCM and ΔH _M,OD are melting enthalpies of the SSPCM and pure OD, respectively, and x denotes the corresponding mass fraction of OD in the SSPCM. The obtained F values (Table ) were all below 100%, suggesting incomplete crystallization of OD due to the H–bond interactions. In addition, the F values showed an increase from 76.6 to 93.7% with increasing OD content from 50 to 80%, respectively. As low OD contents, most of the OD was adsorbed on the surfaces of F–SiO₂ and Fe₃O₄ and thereby subjected to the H–bonds, leading to lower crystallization fractions. After OD completely covered the surfaces of F–SiO₂ and Fe₃O₄, further adsorbed OD would be excluded from direct contact with the surface – OH groups and, accordingly, free from the interfacial H–bonds. Therefore, the crystallization fractions increased with the ascending OD content in the SSPCMs.

3.3. Leakage-Proof Ability and Thermal Reliability

The leakage-proof ability of the SSPCMs compared to pure OD was examined by isothermally treating them at 80 °C (∼16 °C higher than their melting points) for 60 min. Afterward, the regions under the SSPCMs were thoroughly observed to determine the liquid leakage, and the digital photos during the test are shown in Figure . The SSPCMs with 50, 60, and 70% OD exhibited no observable leakage, while the one with 80% OD and pure OD experienced liquid leakage. These results indicated that F–SiO₂ effectively retained OD within its porous structure up to a certain loading capacity, preventing leakage by the effects of capillary, surface tension, and H–bond interactions. However, when the OD content reached 80%, the adsorption capacity of F–SiO₂ was surpassed, leading to liquid leakage. This suggests that the optimal confinement threshold lay between 70 and 80% OD, beyond which the structural integrity of the composite was compromised.

5.

Digital images during the leakage test of the prepared SSPCMs compared to pure OD.

The 70% OD SSPCM was considered the optimal material because of its excellent leakage–proof ability and high phase change enthalpy. Its thermal reliability was examined for 500 accelerated thermal cycles. Figure shows the thermal reliability of the 70% OD SSPCM characterized by DSC and XRD before and after the test, and the specific thermal data are shown in Table . The DSC curves (Figure a) showed small changes after 500 melting/crystallization cycles, causing slight changes in phase change temperatures, i.e., by 0.4 °C for melting temperature and 0.0 and 0.8 °C for L–S and S–S phase transition temperatures during the crystallization process, respectively. The melting and crystallization enthalpies of the cycling sample were achieved at 143.5 and 142.1 J/g, which were decreased by only 2.7 and 2.8% compared to those of the first cycle. In addition, the XRD patterns (Figure b) showed characteristic peaks of OD similar in position and intensity before and after the cycling test, indicating unchanged crystallization properties. These results demonstrated excellent thermal reliability for the 70% OD SSPCM, making it ready for long–term use.

6.

(a) DSC curves and (b) XRD patterns of the 70% OD SSPCM at the 1st and 500th phase change cycles.

3.4. Thermal Stability and Thermal Conductivity

The thermal stability of the prepared SSPCMs and pure OD was examined by TGA, and the resultant TGA curves are shown in Figure a. Pure OD showed a one–step and sharp thermal decomposition within 180–257 °C with a weight loss of almost 100%. Compared to pure OD, the prepared SSPCMs showed altered thermal decomposition behaviors with two–step weight loss, as marked in Figure a. The first weight loss sharply occurred within a temperature range close to that of pure OD, attributing the thermal decomposition of OD free from the surface H–bonds. In contrast, the second one gradually occurred within a long temperature range of approximately 257–600 °C, probably due to the thermal decomposition of OD H–bonded to F–SiO₂ and Fe₃O₄ surfaces. To further investigate this phenomenon, we additionally prepared composites of n–octadecane/F–SiO₂/Fe₃O₄ for comparison because n–octadacane has a similar structural formula as OD except for the −OH group at the chain end. Thus, n–octadecane was unable to form surface H–bonds with F–SiO₂ and Fe₃O₄ NPs, which is appropriate for comparing the effects of interfacial H–bonds on thermal decomposition with OD. As shown in Figure S2, the 50–80% n–octadecane/F–SiO₂/Fe₃O₄ composites showed sharp thermal decomposition in only one step, similar to that of pure n–octadecane. This result confirmed the second thermal decomposition step of OD/F–SiO₂/Fe₃O₄ SSPCMs related to surface H–bonded OD, which was more strongly retained in the SSPCMs compared to free OD, thus improving the thermal stability. In addition, the total weight loss of the 50, 60, 70, and 80% OD SSPCMs was calculated to be 50.6, 60.7, 69.8, and 79.3%, respectively, and is very close to the doped OD contents, indicating OD was well dispersed in the SSPCMs.

7.

TGA curves (a) and thermal conductivities (b) of OD and the prepared SSPCMs.

The thermal conductivities of the SSPCMs compared to pure OD are shown in Figure b. The TCs of the prepared SSPCMs were decreased to 0.156–0.217 W/(m·K) compared to a value of 0.284 W/(m·K) of pure OD. Of the components of SSPCMs, F–SiO₂ was characterized to have extremely low TC (∼0.05 W/(m·K)) because its pore size was close to the average free path of the air at ambient temperature, resulting in a lack of convection. Fe₃O₄ NPs also had poor TCs of 0.144–0.180 W/(m·K), as demonstrated in previous reports. , As a result, the combination of OD with F–SiO₂ and Fe₃O₄ NPs formed composites with lowered TCs compared to pure OD. Increasing OD contents accordingly resulted in a growth in TCs for the SSPCMs. Overall, the OD/F–SiO₂/Fe₃O₄ SSPCMs with low TCs were successfully prepared.

The thermal properties of the 70% OD SSPCM compared to other reported SSPCMs are shown in Table . Compared to other OD/SiO₂–based SSPCMs, including OD/mesoporous SiO₂ and OD/methylated SiO₂, the prepared 70% OD SSPCM exhibited a significantly higher heat storage capacity because of its higher crystalline fraction and OD content. Compared to other OD/non–SiO₂–based SSPCMs, the prepared 70% OD SSPCM showed a heat storage capacity comparable to OD/expanded perlite but lower than OD/Al₂O₃@expanded graphite, OD/expanded graphite/Fe₃O₄, and OD/SiC/expanded graphite. However, these SSPCMs were fabricated with high thermal conductivities (0.43–6.623 W/(m·K)), which is the reverse of the purpose of this work to fabricate low–thermal–conductivity SSPCMs. Therefore, they would be applicable in different applications.

3. Thermal Characteristics of the 70% OD SSPCM Compared to Other Reported OD–Based SSPCMs.

SSPCM	OD content (%)	ΔH (J/g)	F (%)	TC W/(m·K)	refs
OD/F–SiO2/Fe3O4	70	147.6	90.8	0.1936	this work
OD/mesoporous SiO2	70	47.03	28.6	0.51
OD/methylated SiO2	50	85.6	74.9	-
OD/expanded perlite	60	140.0	94.8	6.623
OD/Al2O3@expanded graphite	90	195.94	91.2	1.876
OD/expanded graphite/Fe3O4	80	185.2	95.1	4.598
OD/SiC/expanded graphite	91	192.3	92.4	1.674

3.5. Magnetothermal Conversion and Storage

Figure a presents VSM curves of the 50, 60, and 70% OD SSPCMs compared to pure Fe₃O₄ NPs. Inheriting from the high saturation magnetization of pure Fe₃O₄ (71.2 emu/g), the 50, 60, and 70% OD SSPCMs showed corresponding values of 7.7, 6.0, and 4.3 emu/g, which were directly proportional to their Fe₃O₄ contents of 12.5, 10.0, and 7.5%, respectively. In addition, the very small coercivity and remanence of pure Fe₃O₄ NPs were maintained in the SSPCMs. These results proved that the SSPCMs well preserved the superparamagnetic properties of Fe₃O₄ NPs.

8.

(a) VSM curves of 50–70% OD SSPCM compared to pristine Fe₃O₄, (b) apparatus for magnetic charging test, (c) temperature–time curves of 50–70% OD SSPCMs during magnetothermal conversion experiment, and (d) temperature–time curves of 70% OD SSPCMs charged by a magnetism and a convective oven at 100 and 150 °C.

Magnetic materials can be effectively heated when placed in an alternating magnetic field due to the Néel and Brownian relaxation effects. , This allows for rapid, localized heating without direct contact with external heat sources, improving thermal management applications. The integration of magnetism into SSPCMs offers a magnetothermal conversion and storage where the magnetically induced heat is stored in the SSPCMs owing to the high latent heat. To demonstrate the magnetothermal conversion and storage abilities of the prepared 50–70% OD SSPCMs, they were placed in an alternating magnetic field with an intensity as low as 0.056 mT produced from an inductor. The temperature variations during the test were monitored with an infrared camera, as shown in Figure b, and the resultant temperature–time curves are exhibited in Figure c. When the magnetic field was turned on, the 50–70% OD SSPCM’s temperatures rapidly increased from ambient temperatures to approximately 77 °C in 180, 230, and 360 s, indicative of effective magnetothermal conversion.

The temperature increments of the sample under magnetic power can be categorized into three steps. Initially, the temperatures rapidly increased from ambient temperatures to ∼55 °C because the magnetic energy was converted and reserved as sensible heat. Subsequently, the temperature heating rates were suddenly decreased, forming temperature plateaus within a temperature range of ∼55–58 °C. This step occurred because the magnetic energy was converted and reserved as the latent heat of melting of OD. Finally, the temperatures rapidly grew again because the magnetically induced heat was retained as sensible heat. As the magnetic power was stopped and the SSPCMs were placed at the ambient temperature, the SSPCMs’ temperatures first dropped to ∼55 °C. Then, the temperature–decreasing rates decreased, forming temperature plateaus, attributed to the heat release during the crystallization processes of OD. These results confirmed magnetothermal conversion and storage abilities for the prepared SSPCMs.

The magnetic heating efficacy was further compared to conventional convective heating. The prepared 70% OD SSPCM was placed in an oven at 100 and 150 °C for convective heating, and the temperature changes of the sample during the test were recorded. Figure d compares the temperature–time curves of the 70% OD SSPCM during the magnetic and convective heatings. The magnetic heating performance was superior to convective heating. While the magnetic heating needed only 360 s to increase the temperature of the SSPCM to 76 °C, the convective heating at 100 and 150 °C took up to 2300 and 1380 s, respectively, to do the same work. The magnetic heating performance was 6.4 and 3.8 times faster than the convective heating at 100 and 150 °C, respectively. The excellent magnetic heating performance would accelerate the thermal–charging rate of the prepared SSPCMs.

3.6. Thermotherapy Performance Evaluation

The unique ability to store and release large amounts of thermal energy at a nearly constant temperature for extended periods makes SSPCMs ideal for thermotherapy applications, where controlled and consistent heat is required. , Personal thermotherapy is categorized into high level (50–55 °C for 4–6 min), mid level (40–50 °C for 15–60 min), and low level (35–40 °C for 6–72 h). , In this work, the prepared 70% OD SSPCM exhibited two exothermic peaks at 54.6 °C (L–S phase transition) and 46.4 °C (S–S phase transition) during the crystallization process (see Table ), making it simultaneously suitable for high and midlevel thermotherapy. The practical thermotherapy efficiency of the prepared 70% OD SSPCM was examined by first compressing it at a pressure of 10 bar to form a thermal pad (110 × 55 × 5 mm³, Figure a), using 30 g of the material. The thermal pad was then heated in an oven at 70 °C to store the melting latent heat of OD. Afterward, it was applied to a mannequin, and the temperatures of the heat released to the mannequin were recorded using a thermocouple. Figure b shows digital and infrared images of the mannequin during the test, and the obtained temperature–time curve is shown in Figure c. As seen, when the temperature of the thermal pad went down to 55 °C, a temperature plateau appeared due to the heat release of the L–S phase transition. After the L–S phase transition was completed, the temperature continued to go down and subsequently formed another shorter temperature plateau at ∼47 °C due to the heat release of the S–S phase transition. The thermal pad released heat within 50–55 °C for 30 min and 40–50 °C for 16 min, which was highly qualified for the criteria of the high and midlevel thermotherapy. Thus, the thermal pad could be considered a bi–functional thermotherapy method. It is noted that although the thermal pad possessed an accelerated magnetic heating ability, it was not heated by magnetic power because the induction heater used in Section was of small scale and unable to fit with the thermal pad. However, the thermotherapy efficacy was not influenced by the different heating methods because it was monitored during the crystallization process.

9.

(a) The thermal pad produced from the prepared 70% OD SSPCM, (b) digital and infrared images during the thermotherapy test, and (c) the temperature–time curve obtained from the test.

4. Conclusions

In summary, OD/F–SiO₂/Fe₃O₄ SSPCMs with low TCs and accelerated magnetothermal conversion and storage were successfully demonstrated. In the SSPCMs, OD played the role of a heat storage material, F–SiO₂ was in charge of a porous support, and Fe₃O₄ NPs provided magnetism. They were physically combined in the SSPCMs, and the confined OD exhibited high crystallization fractions of 76.6–93.7% with varying OD contents of 50–80%. Up to 70% OD was stabilized in the SSPCMs with good leakage resistance owing to the suitable porosity of F–SiO₂. The 70% OD showed a high heat storage capacity of 147.6 J/g, which was negligibly reduced after 500 melting/crystallization cycles, indicative of good thermal reliability. Owing to the low TCs of F–SiO₂ and Fe₃O₄ nanoparticles, the TCs of the prepared 50–70% OD/F–SiO₂/Fe₃O₄ SSPCMs were reduced to only 0.156–0.193 W/(m·K), which were 45.1–32.0% lower than that of pure OD. In addition, the superparamagnetism of Fe₃O₄ nanoparticles was maintained in the SSPCMs, enabling the SSPCMs with accelerated magnetic heating ability. For example, under a low alternating magnetic field (intensity of 0.056 mT), the prepared 70% OD SSPCM could be rapidly heated from ∼34 to 76 °C in 360 s. For comparison, this heating rate was 6.4 and 3.8 times faster than convective heating at 100 and 150 °C, respectively. In the form of a thermal pad, the prepared 70% OD SSPCM simultaneously released heat within 50–55 °C for 30 min and 40–50 °C for 16 min, which was highly suited for high and mid–level thermotherapy, respectively. Thus, it could be regarded as a bi–functional thermotherapy method.

The data supporting this article have been included as part of the Supporting Information.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.5c06417.

• An SEM image of the prepared Fe₃O₄ and TGA curves of octadecane/F–SiO₂/Fe₃O₄ composites (PDF)

†.

N.B.T.N. and H.P. contributed equally to this work.

The authors declare no competing financial interest.