MTPCMs and their features. As a basic unit in the 3D structure, the thermal conductivity of NdFeB particles will directly impact the heat transfer of MTPCMs. Considering this, we adopted in situ chemical plating method to coat a silver shell on the surface of micro NdFeB particles modifying them to NdFeB@Ag particles (details see Supplementary Fig. 1 and the “Method” section)23. As shown in Fig. 1b, the color of magnetic particles changed from dark black to yellow which was similar to the silver microparticles, indicating NdFeB particles are completely covered by silver shells. This was also demonstrated by scanning electron microscopy (SEM) images and the corresponding energy-dispersive X-ray spectroscopy (EDX) element mappings in Supplementary Fig. 2. In contrast to bare NdFeB, the core particle was wrapped in a continuous silver layer, and there existed only the silver element on the surface of NdFeB@Ag. The homogeneous composite of NdFeB@Ag particles and paraffin can be obtained by simply adding and stirring NdFeB@Ag particles into melted paraffin, correspondingly the resulting composite presented a yellow appearance (Fig. 1c). Applying a magnetic field to the composite, NdFeB@Ag particles aggregated forming a porous structure that provided the strong capillary force to stabilize paraffin, consequently the magnetically tightened form-stable phase change material (MTPCM) was born. The mixing ratio of NdFeB@Ag particles is an important parameter for MTPCMs as they compromise their energy storage density for the leakage-proof and form-stable abilities. It was explored that the lowest volume mixing ratio for MTPCMs was 15.84 %, and as a comparison, we prepared three samples with different volume ratios in this paper. Unless specified, the composite refers to the unmagnetized mixture of NdFeB@Ag particles and paraffin in the following context to facilitate elucidation.
It is deduced that MTPCMs have leakage-proof and shape stability behaviors based on the theoretical analysis. To verify this point, a leakage test was performed on the heating platform at 50°C for MTPCM, unmagnetized composite, and pristine paraffin, which were all processed into 10⋅10⋅10 mm cubes. As shown in Fig. 1d, after being heated for a while, pristine paraffine melted completely into a fluid spreading out on the plate. As for the unmagnetized composite, phase separation occurred during the phase change process, paraffin flowed away from the composite leaving a stack of collapsed NdFeB@Ag particles. Unlike the above two materials, no leakage was observed on the MTPCM block, and it always retained its initial shape even after being compressed by a weight of 20 g, which is ascribed to the magnetic-induced strong capillary force inside the material. The test result confirms the outstanding leakage-proof and shape stability capacity of MTPCMs. This conclusion was presented more vividly by hung the three samples and heated them through a heat gun. During the phase change process, the paraffin and unmagnetized composite melted and dropped on the table while the MTPCM was always hung on the string with its original shape (see Supplementary Movie 1).
Hard magnetic particles can reassemble and then rebuild an interconnected framework when their original structure is destroyed by external stimuli, which enables MTPCMs a shape transformable function. Only by a simple heating operation, the MTPCM transformed from cylinder to cuboid while maintaining its leakage-proof ability intact (Supplementary Movie 2). This function makes it convenient to create or reconfigure MTPCMs to diverse geometries according to the specific application. Moreover, we fabricated MTPCMs into standard modules to make better use of their magnetism. The modules can attach spontaneously with good contact like “magnetic lego” due to the magnetic attraction. To further extend this concept, a series of intricate 3D architectures with different shapes and structures were realized via the modular assembly, and the generating object displayed a robust structure (Fig. 1e and Supplementary Movie 3). The above conformal shape and modular assembly features render MTPCMs favorable as they make instantly customize the shape of PCMs to meet complex practical requirements a reality.
The microstructure of the transverse section for MTPCMs was investigated using SEM and EDX. The cross-section images in Fig. 2a presented an oriented structure that is derived from the directional alignment of NdFeB@Ag particles and the accompanying passively arrayed paraffin. The anisotropic tendency in structure inevitably brings anisotropy performance to some extends. Besides, there were almost no voids inside the MTPCM, which is beneficial to its thermal and mechanical performance. The X-ray diffraction (XRD) pattern of the MTPCM in Fig. 2b only contained peaks of paraffin and pure Ag since the NdFeB particles are completely covered by silver shells. The XRD test states that NdFeB@Ag particles and paraffin is a physical combination and there is no new substance formed, revealing the force to stabilize paraffin is merely from magnetic-induced capillary action. To explore the magnetic properties of MTPCMs, we measured their magnetic hysteresis loops and surface magnetic flux density, as displayed in Fig. 2c-d. With the increase of volume ratio for MTPCM samples, their saturation magnetization and remanence rose from 50.22 to 58.72 emu g−1 and 32.81 to 38.15 emu g−1, illustrating the MTPCMs have a good magnetic response and strong magnetism. Furthermore, a typical sample with the 19.12 % volume ratio was tested at different temperatures at different time points. The magnetic hysteresis loops tested at 20, 30, and 40°C on day 1 and day 30 almost overlapped. Even though the magnetic loops of the sample had a smaller enclosed area at 50°C, the values of saturation magnetization and remanence were nearly the same as those at other temperatures, which were about 54 and 35 emu g−1. It is attributed to the high Curie temperature of NdFeB (315°C) that protect the magnetism of MTPCMs from high temperature. The results prove that MTPCMs own stable magnetism, which will not be affected by heating or long-time storage. The strong magnetism correspondingly generates high surface magnetic flux density in MTPCMs. For a 10⋅10⋅10 mm cubic MTPCM sample, the magnetic flux density on the surfaces which are parallel and perpendicular to the magnetic field reached up to 16.58 and 31.01 mT, respectively. Except for that, Fig. 2f shows the surface magnetic field profiles of MTPCMs with different shapes by magnetic observation cards. The samples all appeared a uniform magnetic field distribution and unambiguous boundary with their surroundings. The series of magnetic tests disclose MTPCMs have not only strong and stable magnetism but also homogeneous magnetic distribution without any defects.
Thermal and mechanical performance of MTPCMs. The phase change properties of MTPCMs were characterized by differential scanning calorimetry (DSC) (Fig. 3a). The curves of pristine paraffin and MTPCM samples with different volume ratios were very close, and the thermal parameters were extracted and summarized in Fig. 3b. The onset melting and solidifying point of paraffin were 38.65 and 36.23°C, respectively. The three MTPCM samples had nearly the same values with paraffin (38.95 and 37.33°C for 15.84 %, 39.5 and 37.13°C for 19.12 % and 40.05 and 37.07°C for 23.31% sample). The fusion enthalpy for neat paraffin was 163.75 J cm−3, as the addition of NdFeB@Ag particles, the value didn’t decrease but instead changed to 178.70, 172.59 and 141.63 J cm−3, which was ascribed to the magnetic particles increased the density of MTPCMs and thus improved their energy storage density. Moreover, unmagnetized composites were also measured by DSC and it was found that they had little difference with MTPCMs under the same volume ratio (Supplementary Fig. 3). These results indicate paraffin functions the dominant energy storage role in MTPCMs. The NdFeB@Ag particles only serve as the supporting material, and their magnetism does not affect the phase change characteristics of paraffin. Fig. 3c depicts the thermal gravimetric analysis (TGA) curves of paraffin and MTPCMs. Based on the data analysis, it can be achieved that the 5 % weight loss temperature of MTPCM was higher than that of paraffin, and this temperature boosted with the increase of volume ratio in MTPCMs. On the contrary, the weight loss rate declined with the addition of NdFeB@Ag particles. It is speculated that the capillary pores inside MTPCM protect paraffin from evaporation to some degree and finally delay its decomposition. Hence, MTPCMs have better heat resistance and thermal stability than pristine paraffin. The heat charging/discharging rate and temperature-hold time of PCMs are significant issues concerning their applications and they can be assessed via the heating and freezing process17, 24. The experiment system is shown in Supplementary Fig. 4, therein the MTPCM sample was produced into a 20⋅20⋅20 mm cubic block and the temperatures were set at 20 and 50°C with a heating/freezing rate of 1 K min−1. The heating-freezing process was cycled 10 times and the curves are shown in Fig. 3d. In a magnified period, it can be observed that the heat charging/discharging rate and temperature-hold time presented an inverse variation trend with the increase of volume ratio in MTPCMs. Specifically speaking, the sample with a larger volume ratio costs a shorter time to reach the equilibrium state in both heating and freezing processes, implying it has higher heat charging/discharging rate. And this results from the more NdFeB@Ag particles build better heat transfer pathways, accelerating the thermal storage and release efficiency of the sample. However, adding more NdFeB@Ag particles leads to the reduction of paraffin, and this will cut down the latent heat of the sample. Correspondingly, its phase change time, namely temperature-hold time, decreases. Hence, it needs to balance these two aspects of MTPCMs in practical applications. Notably, MTPCMs exhibited almost the same heat charging/discharging rate and phase change time in 10 cycles, signifying the outstanding thermal-cycle reliability and durability of MTPCMs. As an essential property of PCMs, the thermal conductivity of MTPCMs was studied and described in Fig. 3e. The interconnected 3D structure of MTPCM offers efficient thermal conduction routes inside the entity. As the elementary unit on this network, modified NdFeB@Ag particles possess a silver-like high thermal conductivity. Under the synergistic effect of the above two factors, the thermal conductivity of MTPCMs is greatly enhanced by one order of magnitude compared to organic paraffin (0.21 W m−1 K−1), reaching 2.97, 3.11, and 3.41 W m−1 K−1 for three different volume ratio samples. As stated above, the MTPCMs exhibited an anisotropic structure. Given the heat transfer mechanism in MTPCMs, we investigated the thermal conductivity of MTPCMs in the direction of parallel and perpendicular to the magnetic field (Fig. 3f). It was easy to find that MTPCMs had higher thermal conductivity along the magnetic field direction and the difference of thermal conductivity in these two directions slightly rose with the increase of volume ratio in the samples (0.61 W m−1 K−1 for 15.84%, 0.7 W m−1 K−1 for 19.12% and 0.76 W m−1 K−1 for 23.31% sample). The anisotropic thermal performance in MTPCMs is mainly caused by the anisotropy in the structure, and this effect is reinforced with the increase of volume ratio in MTPCMs.
Applying a magnetic field, both soft and hard magnetic particles will rotate and align eventually forming chains along with the magnetic flux profiles25, 26. The different point is soft magnetic particles collapse back into a pile of powders while hard magnetic particles can hold their configuration after removing the magnetic field. This characteristic arouses our inspiration to explore whether the alignment of NdFeB@Ag particles throughout the MTPCM can support the composite and strengthen its mechanical performance. On this background, the compression tests were conducted on the 10⋅10⋅10 mm cubic MTPCM modules (the measurement system is shown in Supplementary Fig. 5). We first performed the tests at different temperatures, and the results are depicted in Fig. 4a. At room temperature, MTPCMs displayed high compressive yield strength, and this capacity was reinforced with the increase of volume ratio in the sample. Yet at the temperature above the melting point of paraffin, the MTPCMs continuously deformed and were ultimately compressed into the flattened cuboid when subjected to the normal force, suggesting they have negligible compressive strengths at this moment. These can be observed from the state of typical compressed samples in the insets as well. The solid paraffin tightly around the NdFeB@Ag chains forming a protective layer that offers a lateral force when the sample is compressed by the normal force, which helps the magnetic chains keep their original direction and thus harvest a good compressive strength. Once the paraffin has melted, the magnetic chains lose this support and rapidly yield to the compression force. Therefore, the directional alignment of NdFeB@Ag particles together with the assistant of paraffin make MTPCMs gain excellent compressive strength. Remarkably, even though the paraffin melted at a high temperature, no leakage of the sample occurred throughout the whole compression process, proving the superior leakage-proof ability of MTPCMs again (Supplementary Movie 4). Additionally, the compressive strength in different directions was measured to survey the anisotropy in the mechanical performance of MTPCMs. As we expected, the sample had a better compressive strength in the direction parallel to the magnetic field, originating from the anisotropic structure of the MTPCM. Compared to conventional PCMs, MTPCMs possess good mechanical performance and this could broaden their application to the field of green building.
Energy conversion and storage of MTPCMs. Employing PCMs in the areas of solar energy utilization, heat recovery, and power supply and demand regulation is all due to their large latent heat endows them with great energy conversion and storage potentials27, 28, 29. Limited by the severe fluctuation of sunlight, the solar-thermal conversion system cannot support a persistent output. As a result, electric-thermal and thermo-electric conversion and storage systems have become promising alternatives1, 4, 30. On this occasion, we investigated the electricity-to-heat and heat-to-electricity conversion and storage performance of MTPCMs. As is known to all, organic paraffin is an insulator that is impossible to conduct electricity. However, as shown in Fig. 5a, the electrical conductivity of three MTPCM samples was 1.69⋅104, 2.25⋅104 and 2.83⋅104 S m−1, respectively. The values are among the conductivity of metal materials (σ>103 S m−1), certifying MTPCMs are good conductors of electricity. To figure out the underlying mechanism of this change (from insulator to a good conductor), we adopted the control variable mode, measuring the electrical conductivity of unmagnetized composites and the post-magnetized mixture of NdFeB and paraffin, respectively. It was found that the latter material was insulated and the unmagnetized composites had lower electrical conductivities than MTPCMs (Fig. 5b). This result elucidates that the modified NdFeB@Ag particles is a dispensable term to enable MPTCMs to become conductive, and on this basis, the interconnected 3D structure strengthens this effect further improving the electrical conductivity of MTPCMs. The electrical properties of MTPCMs are far beyond a high electrical conductivity. As discussed above, after being heated, NdFeB@Ag particles can reassemble building a new structure when the previous one is destroyed. With this feature, MTPCMs kept conductive regardless of encountering any deformations during the phase change process (Supplementary Movie 5). Owning such outstanding electrical performance is profitable to the electric-thermal conversion and storage of MTPCMs, and this is confirmed by the following experiments. A voltage of 3 V was applied on the MTPCM to trigger its electro-heat conversion, and the current stayed at 0.27 A in the whole process. Based on the temperature evolution of the sample, it can be calculated that the phase change time lasted for about 61 s and the electricity-to-heat harvest efficiency of MTPCM was 78.45 %. Generally, the phase transition time is relatively long under low voltage and the heat dissipation from the sample to surroundings increases accordingly, leading to reduced efficiency30. Therefore, such high efficiency at a low voltage demonstrates the prominent electricity-to-heat conversion and storage capacity of MTPCMs. Besides, a thermal infrared test was also implemented to express the electro-thermal conversion performance of MTPCMs qualitatively (Fig. 5c). A power of 8.88 W was input to the heart-shaped sample, and it turned to about 65°C in a short time of 30 s, suggesting the excellent performance of MTPCMs in the electricity-to-heat conversion and storage aspect again.
As the other energy harvesting system, the heat-to-electricity conversion and storage is executed via two MTPCM samples and an in-between thermoelectric generator. Herein, the paraffin in MTPCMs was replaced by EBiInSn (eutectic alloy of Bi, In, and Sn) whose melting point is 60°C to amplify the temperature gradient between the hot and cold side. The commercial thermoelectric device was a semiconductor that can convert the heat in the MPTCM into electrical energy according to the principle of the Seeback effect2, 27. As the heat source, the MTPCM was heated until the EBiInSn therein was melted. On account of the magnetic attraction feature of MTPCMs, the thermoelectric generator was tightly clamped having close contact with both two ends. We connected the system as a power supply to a self-programmed Bluetooth chip creating a hygrothermograph that can sensitively monitor the temperature and humidity of the local ambient in real-time (see Fig. 5d and Supplementary Movie 6). The evolution of temperature about the hot and cold side in the system and the corresponding voltage and current versus time are recorded in Fig. 5e. It can be seen that the MTPCMs made a high and long-lasting temperature difference, and accordingly, the power supply duration was over 4 min, and the maximum voltage and current reached 0.39 V and 0.13 A. To sum up, a 4.47 J net electrical energy was harvested from a small MTPCM block (size: 20⋅20⋅10 mm). These data suggesting the large latent heat and superior heat-to-electricity conversion and storage performance of MTPCMs.
Thermal management of MTPCMs. Being identified as an important application branch, thermal management demands PCMs master high thermal conductivity, large latent heat, and appropriate phase transition temperature at the same time, which are what exactly our MTPCMs highlight31, 32. On top of that, most PCMs need containers and binders to prevent leakage and help them stick to the heat source33, 34, 35. However, introducing extra several layers between the PCM and heat source increases the thermal resistance, combing the binders are usually organic materials with extremely low thermal conductivity, the temperature control effect of PCMs will be greatly weakened. But for MTPCMs, this problem can be easily solved via their leakage-proof capacity and magnetism, as shown in Fig. 6a, two MTPCM blocks can directly attract to each other clamping the heat source between them with compact contact, which not only excludes the containers or binders between objects but also minimizes the interface contact resistant through close contact. To illustrate the thermal management capacity of MTPCMs, polyimide electric heating films were used as the heat source and they were sandwiched between two MTPCM and two paraffin blocks, respectively (Fig. 6b). The two structures were hung in the air to eliminate the influence of heat transfer between PCMs and the table. Unlike MTPCM blocks, it needed to drop some molten paraffin in the gap between two paraffin blocks to glue the heating film. When the electric heating films began to work, their temperature kept rising until reached the melting point of paraffin, from now on, the temperature of these two films went in different tendencies. Pure paraffin absorbed heat and melted to liquid, eventually felling off the heating film. While during this process, the MTPCM blocks were always attached to the film to absorb heat and held their shape unchanged (see Supplementary Movie 7). It can be seen from the temperature variations versus time that the two films presented the same temperature in the initial stage. From the transition point, the temperature of one film rose sharply, while the other film kept at about 40°C for 5.8 min, and afterward its temperature also rose slowly.
The above is the case that the heat source is not magnetic, if itself is a magnetic substance or plating a magnetic layer on the surface of the heat source, the MTPCM can directly attach to it to carry out thermal management. For instance, as represented in Fig. 6c, polyimide electric heating films stuck to the iron plates acting as the heat source. In virtue of the magnetism from the iron plates, the MTPCM block could directly contact the film for temperature control, and as a comparison, the other film didn’t take any active heat dissipation steps. When the heating started, the temperature of the film without MTPCM rose rapidly reaching 65.33°C in 522 s under a power of 6 W, and once the power was cut off, its temperature dropped quickly to its beginning value (Fig. 6d). Overall, the temperature of the heat source fluctuated dramatically over a short period. But for the film using MTPCM block for thermal management, its temperature changed steadily and held at about 40°C for 43 s and 73 s in the heating and cooling section, respectively. This difference in temperature change for the two heating films can also be discovered intuitively by the sequential thermal infrared pictures. The aforesaid two experiments prove that MTPCMs have superb thermal management capacity and this arises from the combination of their good thermal properties and magnetism.
Batteries face a severe problem in practical application, that is, their performance is heavily dependent on the temperature36. Too high and too low temperature or even non-uniform temperature distribution will depress their energy and power capacity making batteries suffer from capacity fade and short lifespan37, 38. In this work, we engage MTPCMs for both heat dissipation at elevated temperature and low-temperature protection to regulate the working temperature of the flat-plate lithium-ion battery in a desirable range (Fig. 6e). The specific experimental details are recorded in the Methods section. When the battery discharged at a current of 17 A, its voltage dropped continuously until it reached the cut-off voltage at 168 s. While in this time the temperature increased slowly with the help of two MTPCM slices. It reached around 40°C after 96 s and stayed at this point for 16 s, then moved to the highest temperature of 41.75°C in the end. If the MTPCM slices were removed, for the same operation, the temperature in this process kept on rising and reached 60.85°C (Fig. 6f). It can be calculated that MTPCMs reduce the battery temperature rise by 31.39 % and this is attributed to the high thermal conductivity and large latent heat of MTPCMs. Towards the low-temperature situation, thanks to the excellent electrical properties, the MTPCM stripes can directly convert electricity to heat with high efficiency to warm up the battery from being frozen, forming an immediate protection scheme. In our experiment, the battery was cooled to -20°C by the incubator. Once the outer MTPCM stripes were powered on and the electric power of about 12.82 W was supplied to each stripe, the battery was heated up and its temperature rose to 46.07°C within 9.65 min. Even though the MTPCM stripes were powered off, the battery can still stay at the solidifying temperature of the MTPCM for 15 s and finally got a temperature of around 0°C through one heating operation (Fig. 6g). After the battery cooled down, we powered on the MTPCM stripes again and it can be seen that the battery presented almost the same temperature variation, suggesting the MTPCM slices own the cycling stability and can withstand the long and harsh working cycle conditions of the battery. The above heating and cooling experiments state that with prominent thermal performance and exclusive high electrical conductivity, MTPCMs have advantages in battery thermal management, they can control the battery temperature within normal values promptly, which is significant for high performance and long working time of the battery.