Introduction

Thermal insulation materials play a critical role in managing heat for a variety of applications, including residential heating and cooling systems1,2, thermal management in electric vehicles3,4, shielding equipment from high-temperature heat5, and surviving harsh environmental conditions6,7. The primary goal in creating effective thermal insulation is to block heat transfer into specific areas. In the past, significant attention has been devoted to developing materials that possess low thermal conductivity, a measure of their ability to conduct heat at steady state under a temperature difference8,9,10. In materials with a porous structure, heat conduction through their solid parts is significantly inhibited11, and heat transfer mainly occurs through air molecules’ movement, which has an average mean free path of about 70 nm at ambient pressure12. Creating nano-porous structures with a pore size smaller than this can result in materials with extremely low thermal conductivity. Aerogels, with thermal conductivities as low as ~0.01 W m−1K−1 13,14, represent one of the best thermal insulation materials exploiting this strategy (Fig. 1a).

Fig. 1: Schematics of thermal insulation materials under thermal shock.
figure 1

a The thermal response of aerogels exemplifies the ability to isolate thermal shocks due to its low thermal conductivity, effectively impeding steady-state heat transfer and maintaining significant temperature differentials. b Temperature control behavior of phase change materials (PCMs) enables efficient heat absorption and maintains a stable temperature, effectively mitigating the impact of thermal shock. c Design of PCM aerogels combining the advantages of both aerogels and PCMs, retains the latent heat absorption capacity to strengthen the protection ability against thermal shock while achieving low thermal conductivity with the porous structure.

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Despite their unparalleled thermal conductivities, the widespread use of aerogels has been hindered by their costly and energy-intensive production processes. For instance, for silica aerogel, the precursor for synthesis has a high cost, and the commonly adopted supercritical drying method is also energy-intensive15,16, which often necessitates complex solvent exchange steps17,18,19. Such requirements stem from the need to engineer nano-porous structures which are key to silica aerogel’s thermal insulation performance. In contrast, more conventional insulation materials, such as fiberglass mats, polyurethane sponges, and polystyrene foams, have seen widespread applications due to their straightforward and cost-effective production processes. However, the thermal insulation performance of these materials is compromised due to their micron-scale pore structure, which is less effective at limiting air molecules’ movement. As a result, these materials typically exhibit thermal conductivities ranging from 0.03 to 0.1 W m−1K−1 20.

An alternative strategy to hinder the heat flow is to decrease the material’s thermal diffusivity \(\alpha\), which is related to its thermal conductivity \(k\) through \(\alpha=k/(\rho {c}_{p})\)21,22. Thermal diffusivity plays a crucial role in transient thermal processes. For instance, when a material makes contact with a high-temperature source, the spatial penetration of heat into the material (and the corresponding region with a significant temperature rise) follows with time as \(\sqrt{\alpha t}\). Consequently, materials with lower thermal diffusivity can delay heat propagation. For instance, phase change materials (PCMs), by absorbing heat through phase transition, effectively enhances the heat capacity, leading to a small thermal diffusivity and delaying the transient heat transfer process (Fig. 1b). This delay is especially advantageous in applications where protection against a brief, intense heat pulse is necessary, such as protecting equipment from thermal shocks or insulating devices during transient high-temperature events23,24,25. For thermal shock protection, the ideal material should have both a low thermal conductivity and low thermal diffusivity, as the former ensures that the heat flux is small while the latter helps to delay the heat propagation. The challenge, however, is that the conventional strategy to decrease thermal conductivity through creating nano-porous structures can only modestly change the thermal diffusivity because the density also decreases. Therefore, even for aerogel samples with extremely low thermal conductivity, their thermal diffusivities are not significantly lower compared to conventional insulation materials.

In this study, we introduce a thermal insulation material that boasts low thermal conductivity and thermal diffusivity simultaneously (Fig. 1c). We develop a micron-porous structure with embedded PCMs using a cost-effective, scalable aqueous in-situ confining method. The incorporation of PCMs enhances the material’s effective heat capacity, resulting in substantial reduction in thermal diffusivity. When subjected to transient heat sources, the PCMs can undergo a phase change and absorb a significant amount of energy, effectively impeding the propagation of heat (illustrated with one dimensional heat transfer model in Supplementary Note S1). In such transient scenarios, the ability of this PCM aerogel to limit heat flow is comparable to halving the thermal conductivity of conventional insulation materials. We further demonstrate that this material exhibits thermomechanical stability up to 300 °C, along with sufficient mechanical strength and self-extinguishing properties. The combined benefits of phase change and thermal insulation make this PCM aerogel a promising candidate for large-scale thermal insulation applications, particularly when protection against thermal shocks is needed.

Results

Material design and characterization

To synthesize PCM aerogel, we employed an innovative approach involving in-situ encapsulation within a water environment. Specifically, we encapsulated water-soluble PCMs within a thermosetting framework, resulting in a thermal-insulating porous material with exceptional latent heat absorption capacity. This in-situ confining strategy successfully encapsulates the PCMs and ensures the stability of the micron-porous structure above the melting point of the PCM, which has been a challenge to create low thermal conductivity materials that contain PCMs26,27,28. As shown in Figs. 2a and 2b, the melamine-formaldehyde (MF) resin pre-polymer underwent polymerization around the chitosan template in water, in-situ confining the polyethylene glycol (PEG) chain inside the gel skeleton. After removing water during freezing drying, the porous aerogels with composite PCMs as the framework were successfully obtained, and named as chitosan-MF-PEG (CMP) aerogels (labeled as CMP1-5, Supplementary Table S1). An aerogel without PCM components was also synthesized as a comparison and named as CMP0. Due to the minimal generation of waste and the absence of harsh reaction conditions, the synthesis was cost effective and easily scalable compared to the commonly adopted synthesis routes for silica aerogels (Supplementary Table S2). Monolithic white CMP aerogels with various shapes and sizes could be achieved via this approach. (Fig. 2c).

Fig. 2: Design and characterization of chitosan-MF-PEG (CMP) aerogels.
figure 2

a Schematic of one pot synthesis with freeze drying. b Illustration of the skeleton structure of phase change material (PCM) aerogel. c CMP samples in the form of hydrogel and aerogel. d SEM image of CMP aerogel fractured in liquid nitrogen. e TEM image of CMP2 aerogel, area-colored of (120) crystal planes of polyethylene glycol with a molecular weight of 6000 (PEG6k) with a spacing of 4.6 Å and SAED pattern inset. f FTIR spectrum of CMP samples and reactants. g XRD patterns of CMP aerogels.

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Scanning electron microscopy (SEM) image revealed that micron-sized pores were uniformly distributed within the PCM aerogel (Fig. 2d and Supplementary Fig. S1). The temperature field direction was not intentionally controlled during pre-freezing, resulting in irregular pore orientations of the CMP aerogel, contributing to its isotropic thermal transport property29,30. The structure of these pore walls was investigated by ultrathin sectioning and visual observation using High Resolution Transmission Electron Microscopy (HRTEM). The TEM images revealed that the crystalline regions within the CMP aerogel framework were confined by amorphous regions (Fig. 2e, Supplementary Fig. S2). The crystal plane spacing of the yellow area measured 4.6 Å, corresponding to (120) crystal planes of PEG6k crystals which suggested the successful confinement of PEG molecular chains within the cross-linked intervals of thermosetting resin supported by chitosan template31. This cross-linked structure was also verified by Fourier transform infrared (FTIR) spectroscopy (Fig. 2f). The double stretching vibration peak of the –NH2 group at 3355 cm−1 and 3289 cm−1 disappeared after reaction, leaving a single vibration peak belonging to –NH– group at 3350 cm−1. And the bending vibration peak of –NH2 group at 1591 cm−1 was replaced by the –NH– group at 1558 cm−1 32,33. Moreover, the X-ray photoelectron spectroscopy (XPS) spectrum for the surface of CMP aerogel indicated that only the peaks corresponding to –NH– and melamine ring with binding energy less than 400 eV were detected (Supplementary Fig. S3). These results indicate a crosslinking reaction in the synthesis as we designed (Supplementary Fig. S4).

The crystal structure of CMP aerogels was analyzed via X-ray diffraction (XRD) to assess the potential impact of crosslinking reactions on the crystallization behavior of PEG. The XRD patterns revealed that the CMP0 sample had only one bulging peak, attributed to the arrangement of chitosan chains (Fig. 2g). With the increase in PEG content, the characteristic peak signal of PEG6k in CMP1 to CMP5 exhibited increasing intensity, while the positions of the corresponding crystal planes (120) and (032) remained virtually unchanged31,34. Furthermore, the differential scanning calorimetry (DSC) curves of CMP aerogels with varying PEG content showed distinct melting and solidification peaks with different heights but similar temperatures (Supplementary Fig. S5), indicating that PEG6k retained its ability to store and release latent heat after in-situ confinement, which is consistent with the XRD results. By varying the molecular weight of embedded PEG, the phase transition temperature of the CMP aerogels can be further tuned in a wide range from 26.8 to 51.2 °C (Supplementary Fig. S6).

Characterization and modeling of thermal transport

To understand the ability of CMP aerogels in thermal shock protection, we first measured the thermal conductivity of CMP aerogels. Because the freeze-drying method leads to pore sizes in the range of microns, the CMP aerogels have thermal conductivities higher than that of air35, and increase with the density, from 0.041 W m−1K−1 in CMP0 to 0.082 W m−1K−1 in CMP5 (Fig. 3b, Supplementary Table S3). These values are comparable to those of conventional insulation materials. Nonetheless, for transient thermal shock protection, CMP aerogels present advantages over conventional insulation materials because its high latent heat leads to an effective low thermal diffusivity and helps to delay heat propagation36. To demonstrate this, we used a pulsed laser heating setup to measure transient heat propagation through CMP aerogels, with CMP0 and CMP2 as representative examples. CMP0 and CMP2 have comparable thermal conductivities (the former is about 0.041 W m−1K−1 and the latter is about 0.045 W m−1K−1) while only CMP2 contains phase transition components. In the transient measurement, a laser pulse heats up the front surface of the sample while the temperature at the backside is monitored by an infrared camera (Fig. 3a). At room temperature (20 °C), both CMP0 and CMP2 have similar temperature rise curves, indicating their similar thermal diffusivities and therefore similar capability in allowing heat propagation (Fig. 3c). At 50 °C, while CMP0 still exhibits nearly the same temperature rising curve, the temperature rise for CMP2 is significantly delayed, indicating a substantial reduction in effective thermal diffusivity due to the heat absorbing effect of PCMs. The presence of latent heat during phase transition results in a transient increase in specific heat capacity (\({c}_{p}\)) of the PCMs, leading to a decrease in the thermal diffusivity. For an ideal PCMs, the phase transition occurs at one temperature point where the equivalent \({c}_{p}\) tends towards infinity. However, in the case of real materials, the phase transition process exhibits a temperature range. Herein, the \({c}_{p}\) and latent heat (\({L}_{H}\)) measured by comparative method is used to calculate through Eq. (1). To account for temperature intervals during phase transition, half-peak width of the heat flow curve of PCMs is taken as an approximate \(\Delta T\) to obtain the effective thermal diffusivity (\({\alpha }_{{eff}}\)) during phase transition (Supplementary Fig. S7).

$${\alpha }_{{eff}}=\frac{k}{\rho \cdot \left({c}_{p}+\frac{{L}_{H}}{\Delta T}\right)}$$
(1)
Fig. 3: Enhanced thermal shock resistance characterization.
figure 3

a Schematic of laser flash method (disk samples with thickness of about 2 mm). b Thermal conductivity and density of chitosan-MF-PEG (CMP) aerogels. Error bars in b are standard deviation from 3 samples. c Temperature response under transient laser thermal shock. d Ranges of thermal conductivity and thermal diffusivity for commonly reported aerogel materials and phase change materials (PCMs) considering latent heat (Supplementary Table S4)5,17,37,38,39,40,41,42,43,44,45.

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If considering the entire heating process rather than just the phase transition period, the \(\Delta T\) refers to the region where the heat source exceeds the critical temperature for phase transition. Then the effective thermal diffusivity is dependent on the heat source temperature (Supplementary Note S2, Fig. S8). The thermal conductivity and effective thermal diffusivity are further compared for different materials, including aerogels, PCMs and our PCM aerogels (Fig. 3d). Aerogel materials exhibit significantly low thermal conductivity but their thermal diffusivity are modest, while PCMs generally have high thermal conductivity. CMP aerogels in this work instead offer the advantages of both low thermal conductivity and low effective thermal diffusivity.

Thermal and mechanical stability

The excellent thermal insulation ability of PCM aerogel was attributed to its porous structure and phase change composition. The crystalline-to-amorphous phase transition in the PCM aerogel offers the capability to absorb heat. To further characterize the phase transition behavior of CMP aerogels during melting and solidification, we employed in-situ XRD. With increasing temperature, CMP aerogels gradually undergo phase transformation as seen by the crystalline PEG peaks disappearing and broader diffraction signals appearing which are characteristics of amorphous regions (Fig. 4a). This phase transition accompanied with heat absorption is complete approximately below 65 °C. DSC measurements revealed the phase change behavior of different samples (Fig. 4b and Supplementary Fig. S5). Compared with vacuum-dried PEG6k, the melting points of the as-prepared CMP aerogels are lower, varying from 40 to 50.7 °C. The enthalpy value of CMP aerogels varied with the amount of PEG6k added, reaching a maximum of 152.1 J g−1 with 86.6 wt.% addition. Considering the chemical composition, variations in the phase transition behavior of CMP aerogels could result from the confinement effect and differences in the hydrogen bonding environment. The terminal –OH groups of PEG6k formed hydrogen bonds with the –OH or –NH functional groups in chitosan, thereby impeding intermolecular motion of PEG6k chains and leading to a reduction in crystallinity, enthalpy, and melting point46. The increasing proportion of restricted PEG6k may also contribute to the decrease in latent heat efficiency of CMP aerogels (Supplementary Table S3).

Fig. 4: Thermal stability of chitosan-MF-PEG (CMP) aerogels.
figure 4

a In-situ XRD patterns during melting and solidification. b Latent heat and temperature of phase transition for CMP aerogels. c DSC curves of CMP aerogel with 50 heating-cooling cycles. d–f Infrared images for phase change materials (PCMs) disks being heated at 40 °C, 80 °C, 300 °C. g, h Pore size distribution of CMP aerogels before and after thermal cycles. i Comparison of high temperature shape stability between CMP sample and literature works (Supplementary Table S5)34,47,48,49,50,51,52,53,54,55,56,57,58,59.

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In the past, volume change and possible leakage of liquid PCMs above the melting point are significant challenges for creating a micron-porous material containing PCMs, because the shape variation can cause structure changes and even block the pores. Due to the in-situ confinement effect of thermosetting resins, our CMP samples instead possess good thermal stability. After undergoing more than 50 temperature cycles between 0 and 90 °C (Supplementary Fig. S9), the endothermic and exothermic curves of CMP aerogel across the phase transition temperature are nearly identical, indicating good thermal stability (Fig. 4c). The shape stability was assessed using a high-temperature platform equipped with weight loads. A circular block-shaped CMP3 sample with 76.4 wt.% PEG content, measuring 1.27 cm in diameter, was positioned on a heating plate at 100 °C, and subjected to the force of a 1 kg stainless steel weight (the equivalent pressure is about 0.125 MPa, see Supplementary Fig. S10). After continuous heating for 30 min, the heated samples maintain a stable shape meanwhile mechanically supporting the weight without any deformation or liquid leakage. We further increased the temperature to 300 °C to characterize its high-temperature resistance and compared it with chemically cross-linked PU, a representative solid-solid PCM, and pure PEG (Fig. 4d–f). While all materials initially maintained a stable shape at 40 °C, upon exposure to 80 °C, the PEG disc completely melted into a liquid state. Subsequent heating to above 300 °C resulted in the destruction of the polyurethane bond (–NH–CO–) within the PU disc60, rendering its shape also unstable. In contrast, the thermosetting resin skeletons of CMP were stable and its shape remained nearly unchanged in IR images. The CMP samples exhibited exceptional high-temperature resistance, robust mechanical supporting capability, and shape stability beyond 300 °C. Normally the high-temperature resistance of PCMs was related to its packaging methods where the packaging efficiency generally had a trade-off effect with latent heat efficiency. When compared with previous studies, our CMPs showed significant advantages in terms of high-temperature resistance (Fig. 4i).

We have further characterized the thermal stability of PCM aerogel at microscopic level, by conducting pore analysis using the mercury intrusion method for samples undergoing temperature cycles. As the density increases, the pore volume expands while its size diminishes, transitioning from 1 to 5 microns in CMP0 to 100-200 nanometers in CMP5. (Fig. 4g and Supplementary Fig. S11). The pore size and distribution of these aerogels are consistent with the corresponding SEM images (Supplementary Fig. S1). The shape, height, and position of peaks in the CMP pore size distribution curves remained essentially unchanged after repeated phase change cycles (Fig. 4h and Supplementary Fig. S11). Moreover, the morphological changes of CMP samples during one thermal cycle were observed using an in-situ heating stage equipped SEM, we observed negligible deformation during the phase transition, indicating microscopic shape stability (Supplementary Fig. S12). After multi-thermal cycles, the thermal conductivity of CMP2 aerogel was measured nearly the same as before (Supplementary Fig. S13). Therefore, CMP aerogels are verified to be stable during thermal cycles considering phase change behavior, shape stability, and insulating performance.

Thermal shock protection application

Safeguarding batteries against thermal shock or thermal runaway is one of the pivotal applications of thermal insulation materials4,23,61. PCM aerogels present a promising solution for such case as they can effectively absorb heat and help to thermally separate the batteries from each other (Supplementary Fig. S14), providing thermal insulation to protect the battery from thermal runaway events, especially when the active heat dissipation system failed (Fig. 5a, Supplementary Fig. S15). The proposed synthesis method allowed us to create CMP aerogels in arbitrary shapes through casting in designed molds. Therefore, conformal thermal protection components can be created to suit batteries in different form factors, such as 18,650 cylindrical cells, coin cells, and pouch cells (Fig. 5b). Compared with typical nano-porous silica aerogels, the micron-porous structure of CMP makes its thermal conductivity significantly higher. But the lack of nano pores also allows CMP aerogels to be produced by a simpler and more cost-effective method instead of the complex supercritical drying method (Fig. 5c). With the introduction of PCMs, the thermal protection performance of CMP aerogels is comparable with silica aerogels when facing transient thermal shock.

Fig. 5: Thermal shock protection application of chitosan-MF-PEG (CMP) aerogels.
figure 5

a Schematic of thermal shock mitigation in lithium battery packs with phase change aerogel, which can provide enhanced protection, isolating high temperature region, safeguarding the other components within the battery pack and delaying the onset of complete thermal runaway. b CMP aerogel envelopes for 18,650 cylindrical batteries and inset for coin-cell batteries. c Comparison of properties and characteristics between CMP2 aerogel and nano-porous silica aerogel from supercritical drying. d Schematic of thermal shock characterization set up. e Temperature curves of the protected coin-cells on CMP0 and CMP2 aerogels. f Temperature curves of the protected coin-cells on commercial silica aerogel and CMP2 aerogel. g, h Infrared photographs of the cells during thermal shock events.

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To study the thermal protection efficacy of PCM aerogel for batteries subjected to high temperature thermal shock, we conducted thermal shock tests in a simulated configuration (Fig. 5d). CMP0 and CMP2 samples of identical dimensions were fabricated and placed in between a heated surface and the coin cells as comparisons (Supplementary Fig. S16). The surfaces of the coin cells were uniformly coated with graphite layer to ensure accurate reading of temperature by the infrared camera. The sample-carrying heat-spreading plate was placed on the 200 °C hot plate, rapidly increasing its temperature to over 160 °C. During the initial 10 min, the PCM component continues to absorb heat without getting hot, so the CMP2-aerogel-loaded coin-cell demonstrated significantly lower temperature compared to CMP0 aerogel (Fig. 5e, g). After completion of the phase transition, the coin cell protected by CMP2 aerogel gradually heats up.

We have further compared CMP2 aerogel with a commercial silica aerogel which has typical nano-porous structures and low thermal conductivity of about 0.026 W m−1K−1 (Supplementary Figs. S17, S18 and detailed comparison in Supplementary Table S6). Despite having nearly doubled thermal conductivity (0.045 W m−1K−1), during the initial 4 min, the temperature of coin cells protected by CMP2 are even lower than the cells protected by silica aerogels (Fig. 5f, h). This also contributes to the phase transition components of CMP2 aerogel which help to absorb heat and delay heat propagation. After the completion of the phase transition, the thermal insulation capability of CMP2 relies mostly on thermal conductivity, leading to a higher cell temperature compared to that of silica aerogel. Nonetheless, it was evident that the incorporation of PCMs results in a significant reduction in the effective thermal diffusivity of CMP2 aerogel for transient thermal events, enhancing its ability to withstand thermal shock within a certain period that is comparable to a silica aerogel with lower thermal conductivity.

Repeated thermal shock and continuous high temperature may destroy the in-situ confined structure of CMP aerogels, resulting in the failure of the thermal shock protection. Therefore, we repeated the 200 °C thermal shock experiment for more than 50 times to analyze the thermal shock protection reliability (Supplementary Fig. S19a). The bottom temperature of the 3 mm thick CMP2 and silica aerogel sheets quickly rose from below 20 °C to nearly 200 °C within 6 s (Supplementary Fig. S19b). After the top temperature of CMP2 rose above 50 °C, the aerogel sheets were simultaneously transferred to a liquid nitrogen-cooled stainless steel platform and were rapidly cooled to below 20 °C, completing a thermal shock cycle. The temperature curve of the 50th thermal shock cycle is basically the same as the uncycled one. Compared with silica aerogel, the rise and drop of top temperature of CMP2 aerogel in 1st and 50th cycles were both obviously delayed, indicating that CMP2 provided stable transient thermal shock protection during rapid temperature rise over and cooling cycle shocks. In addition to cyclic thermal shock, the impact of prolonged high-temperature exposure on the protection performance of CMP was also studied (Supplementary Fig. S20). The 1 cm thick CMP2 sample was heated at a 200 °C heating plate for 4 h and its top surface remained low temperature with acceptable fluctuation between 56.2 and 58.5 °C all the time (Supplementary Fig. S20c). The bottom side of CMP2 aerogel has partially carbonized and turned brown color. SEM pictures showed that the brown surface still maintained a porous structure while the small-sized pores were reduced, which could be due to the failure of the thin-walled structure during the oxidation and decomposition of the polymer (Supplementary Fig. S20d, and S20e). It should be emphasized that the discoloration occurred only at a limited depth, and the cross-section of the CMP2 sample showed that most of the sample remained intact (Supplementary Fig. S20f). DSC curves of samples in different regions showed that the completely discolored region at the bottom appeared no phase transition behavior (Supplementary Fig. S20g), while the enthalpy of less discolored decreased to 54.4 J g−1 (loss of 46.6% from 101.4 J g−1), and the enthalpy of the uncolored region remained nearly unchanged. Mover, during cyclic thermal shock, the sample was exposed to high temperature for less than 1 min each time, so the oxidation of the sample was mild compared to the case of long-term high temperature exposure (Supplementary Fig. S20h). Therefore, 50 thermal shock cycles had limited effect on the compressive properties of the materials, but the mechanical strength of the samples exposed for 4 h heating was greatly affected (Supplementary Fig. S20i). It can be seen that CMP aerogel can offer reliable protection under cyclic thermal shock. When exposed to continues high temperature, the region near the high-temperature source slowly fail due to oxidative decomposition. Because the porous structure still existed and the failure occurred only in a limited depth, the material could still partially perform the function of insulation and thermal shock resistance. When exposed to more extreme temperatures, the carbonization of CMP aerogel would be accelerated. The furtherly conducted 600 °C thermal shock protection test showed that CMP aerogel still exhibited obvious thermal shock delay and thermal insulation ability before complete carbonization (Supplementary Fig. S21).

To further understand the ability of PCM aerogels in protecting from transient thermal shock compared to conventional materials, we have performed a finite element (FE) simulation for a transient heat conduction process using experimentally measured thermal properties as inputs. Here we compare three different cases: a conventional aerogel (CMP0 and commercial silica aerogel), bulk PCM (with latent heat same as CMP2), and PCM aerogel (CMP2) (Fig. 6a and Supplementary Fig. S22; see modeling details and parameters in Supplementary Note S3). We focus on the time-averaged heat flux at the protected surface when the other surface is subjected to a sudden temperature increase. This time-integrated heat flux represents the ability of insulation materials to resist heat flow from the high-temperature source into a given area. Bulk PCM exhibited a significant heat flow owing to its high thermal conductivity (Fig. 6b). The heat flow when CMP0 aerogel and silica aerogel are used is significantly lower, while the lowest heat flow is achieved when CMP2 aerogel is used for thermal protection especially for the initial period. The heat flow in CMP2 gradually reaches the level of CMP0 and beyond silica aerogel, because most PCMs have completed their phase transitions, which is consistent with the thermal shock experiments (Fig. 5e, f). To further understand the relative importance of thermal conductivity and latent heat in governing the transient thermal protection using PCM aerogel, we extended the simulation to a range of different thermal conductivity and latent heat values (Fig. 6c). The color of each data point represented the time-integrated heat flow into the protected area, with blue indicating better thermal protection performance while red indicating the opposite. It can be seen that, in terms of resisting transient heat propagation, a material with a thermal conductivity of 0.04 W m−1K−1 and latent heat of 120 J g−1 would lead to a similar effect as a conventional material with a thermal conductivity of 0.027 W m−1K−1. The color map showing the increased temperature of cold surface as the evaluation criteria presented a similar result (Supplementary Fig. S23). The simulation results demonstrated that our cost-effective PCM aerogels with reasonably low thermal conductivity could exhibit superior thermal shock resistance due to their high latent heat values, potentially matching or even surpassing the performance of aerogels with lower thermal conductivities.

Fig. 6: Finite element (FE) analysis on the impact of latent heat on thermal shock resistance.
figure 6

a Schematics of FE simulation of thermal shock resistance. b Time-integrated heat flux at the protected surface with different insulation materials. Inset is the instantaneous heat flux. c Time-integrated heat flux map with respect to material thermal conductivity and latent heat.

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In addition, the PCM aerogels also exhibit remarkable mechanical strength and self-extinguishing capabilities due to the thermosetting resin skeleton. Prior to reaching the fracture limit of 20%, the compressive strength of CMP aerogels exceeded 1 MPa, which appeared to be an acceptable threshold for organic aerogels and could be applicable in various insulation scenarios (Supplementary Fig. S24)62. Meanwhile, at significant higher temperatures, the chitosan and MF resin component in CMP aerogels could undergo decomposition (Supplementary Fig. S25), releasing nitrogen-containing inert gases and leaving behind a protective carbon layer (Supplementary Fig. S3), thereby exhibiting inherent self-extinguishing properties (Supplementary Fig. S26)63. The exceptional mechanical strength, flame resistance, and high temperature tolerance of PCM aerogels rendered them highly practical in various applications.

Discussion

The quest for reaching lower thermal conductivity in thermal insulation materials through reduction in pore sizes has faced practical limitations owing to the high cost involved in the manufacturing process. In this work, a cost-effective approach was developed to create a micron-porous aerogel structure with in-situ encapsulated PCMs. With a thermal conductivity down to about 0.04 W m−1K−1 that is on par with commercial thermal insulation materials, our materials offer additional advantages in thermal shock protection due to the heat-absorbing ability of the PCM components. Our thermal shock characterization revealed that, in terms of protection from transient thermal shock events, our material with a significantly higher thermal conductivity can reach a similar thermal insulation performance set by silica aerogels, while offering significant advantages in scalability and cost-effectiveness in manufacturing compared to the commonly adopted synthesis routes for silica aerogels (Supplementary Table S2 and Fig. S6). Furthermore, the PCM aerogel exhibited good thermal and mechanical stability up to 300 °C and possessed self-extinguishing properties. We believe the phase change incorporation strategy into aerogels present significant opportunities to broaden the thermal insulation application of aerogel materials and facilitate thermal protection in various scenarios such as battery thermal management and electronic equipment protection.

Methods

Materials

Polyethylene glycol (PEG6k, molecular weight ~6000, provided by Macklin), chitosan (low viscosity:<200 MPa s, provided by Macklin), melamine (purity 99%, provided by Aladdin), formaldehyde (37~40% in water, provided by Adamas), glacial acetic acid (HAc, purity HPLC, provided by Concord).

Preparation of chitosan solution

The chitosan powder (4 g) and HAc (2 g) were added to 100 ml of deionized water and dissolved through stirring. Subsequently, the mixture was allowed to stand for one week until complete dissolution of the chitosan occurred. Altering the amount of chitosan enabled obtaining solutions with different concentrations.

Preparation of melamine formaldehyde (MF) pre-polymer

Melamine and formaldehyde are mixed in a molar ratio of 1:3, followed by short-time heating at 75 °C for 8–10 min. Subsequently, the milky white mixture undergoes a transformation into a clear solution, indicating successful preparation. The solution should be promptly rinsed with cold water to prevent further aggregation. It was crucial to utilize the obtained solution within one hour, otherwise it would undergo additional cross-linking and polymerization resulting in the formation of white blocks.

Preparation of chitosan-MF-PEG (CMP) aerogels

The one pot synthesis method is briefly outlined in Fig. 2a. Initially, a mixture of 0–4 g PEG6k and 250 μL MF pre-polymer is introduced into a 10 mL chitosan solution after stirring for 60 min to achieve homogeneous solutions. Then the solutions are introduced into designed molds, sealed to prevent water from overflowing and heated at 80 °C for 120 min to make it gelled. When cooling to room temperature, the self-stand and transparent hydrogel can be obtained, from which CMP aerogels can be gained through freezing drying. The CMP samples are designated as CMP0 ~ 5 according to their PEG6k content (Supplementary Table S1). The preparation process of chitosan solution and MF pre-polymer is relatively common and has been detailed in SI. The gel concentration and gelling process are determined through the sol-gel experiment, with a primary focus on optimizing the curing conditions and gelation time (Supplementary Fig. S27, Table S7). The mixed solutions of chitosan and MF pre-polymer are obtained in various proportions, resulting in distinct crosslinking phenomena upon heating. As depicted in Supplementary Fig. S27b, a stable gel can only be formed with a sufficiently high concentration of reactants, with the minimum combination being CMP0.

Material characterization

The porous morphology of CMP aerogels were observed with a Hitachi SU8220 field-emission scanning electron microscopy at an acceleration voltage of 15 kV High-resolution transmission electron microscopy (HR-TEM) was performed using a Transmission Electron Microscopy, Tecani FEI F20 S-Twin TEM operated at 200 kV voltage. Attenuated total reflection Fourier transform infrared (ATR-FTIR) absorption spectroscopy was conducted on a Spectrum Spotlight 200 FT-IR microscopy (Perkin Elmer Company, US) in the frequency range between 4000 and 650 cm−1 range with a resolution of 2 cm−1. X-ray powder diffraction (XRD) patterns were obtained by a Rigaku X-pert3 powder (PANalytical B.V Netherlands) with Cu Kα radiation (λ = 0.154056 nm) at U = 45 kV, I = 100 mA. The scans were taken in the 10–50° range at 5°/min scan speed. In situ XRD was performed using a heating stage with a heating rate of 1 K/min and a holding time of 10 min. The phase change properties were investigated by using a differential scanning calorimeter (DSC) instrument (DSC 214 Polyma, Netzsch, Germany). An average capacity of three cycles was taken to estimation. During the cycling heating and cooling test, about 20 mg of CMP2 aerogels was sealed in an Aluminum crucible with a hole on the top and was engaged in repeated heating from 0 to 90 °C and then cooled back, over 50 times. The thermal stability of CMP aerogels was investigated on an SDT- Q600 (TA Instrument, USA), carried out from room temperature to 700 °C at a heating rate of 10 °C/min under air and nitrogen flow. The cycle heating experiment was carried out with Fast Temperature Cycle Test Chamber (Dongguan Huitai Machine CO., LTD). Two CMP2 samples were heated from 20 to 100 °C, held for 5 min, then cooled to 20 °C. The heating and cooling time was programmable controlled at 5 min. The heating and cooling cycles were carried out for 500 times. The thermal conductivity and thermal diffusivity were measured on samples with the diameter measuring 25.4 mm and thickness measuring ~2 mm. The temperature-dependent measurement was carried out at intervals of 5 °C within the temperature range from 20 to 90 °C by a Laser Flash Analysis instrument (LFA 460, Netzsch, Netherland). For thermal conductivity and thermal diffusivity tests below phase change temperature, the furnace was kept near 20 °C by thermostatic water bath with a natural temperature fluctuation of less than 0.01 °C. By controlling the main gain intensity and pulse time, the temperature rises of the material surface caused by pulse heat was between 5~7 °C. The detector could record a signal that meets the fitting accuracy, and the peak temperature was much lower than the starting temperature of the phase transition (over 40 °C). Three parallel samples were measured and averaged. XPS measurement of CMP aerogels before and after self-extinguishing was performed with a KRATOS AXIS 165 X-Ray photoelectron spectrometer with an Al Kα radiation source. The pore structure of the CMP aerogels before and after thermal cycles, including the distribution of pore sizes and specific surface area, was quantitatively analyzed by Mercury intrusion porosimetry using an AutoporeV 9620 (Micromeritics, USA). The compressive strength and compressive strain curves were measured on Instron 5843, with two flat-surface compression stages and 1000 N load sensor. The apparent density was determined using the standard sample method. A 1-inch diameter ring cutter made of cast iron was employed to precisely shape the cylindrical sample, while a blade was utilized to trim its thickness to 1 cm. Subsequently, the remained powder was expelled with compressed air, weighed the aerogels accurately, and the density was calculated using the appropriate formula. Three parallel samples were measured and averaged. The temperature measurement in the coin-cell battery thermal shock experiment was carried out with thermostatic heating platform, whose surface temperature was held at 200 °C. Infrared imaging was conducted by the PS800 infrared camera (Guide sensmart, China).