亮点
• 制备的MDCF-5碳泡沫在10%形变量下表现出优异的抗压性能（156 kPa）。
• MDCF-5样品在室温下热导率为0.0314 W·m⁻¹·K⁻¹，600℃下为0.2817 W·m⁻¹·K⁻¹。
• 该碳泡沫在密度、强度与热导率间实现了精细平衡。
• 工艺成熟可靠，可用于宏观大规模制造。

Highlights

•The prepared MDCF-5 carbon foam shows outstanding compression properties at 10% deformation (156 kPa).

•The MDCF-5 samples exhibit low thermal conductivities of 0.0314 W·m-1·K-1 at room temperature and 0.2817 W·m-1·K-1 at 600 ℃.

•The carbon foam achieves a delicate balance between density, strength and thermal conductivity.

•This process is mature and reliable, and can be used for large-scale macroscopic manufacturing.

摘要
制备低密度、高强度、低导热性的碳泡沫对航空航天热防护系统具有重要意义。传统聚合物热解制备碳泡沫的方法不可避免地面临严重体积收缩（通常约98%）和机械强度下降的问题。这些限制使得在降低热导率、最小化密度与增强抗压强度之间达成平衡成为关键挑战。本研究开创了一种热压技术，先制备高密度三聚氰胺泡沫前驱体，再通过热解制备兼具低导热性、高抗压强度与低密度的三聚氰胺衍生碳泡沫（MDCF）。优化的MDCF样品在微观层面表现出极小的骨架断裂缺陷，其机械强度显著优于现有报道的三聚氰胺基碳泡沫。具体而言，MDCF-5样品在10%形变量下的抗压强度达156.04 kPa。此外，MDCF-5保持了紧密交联的3D网络结构（平均孔径9.3 μm），同时具备优异的抗压回弹性与极端温度下的卓越隔热性能（室温、300℃、600℃下热导率分别为0.0314 W·m⁻¹·K⁻¹、0.0443 W·m⁻¹·K⁻¹、0.2817 W·m⁻¹·K⁻¹）。开发的MDCF材料在航天器热防护系统中展现出广阔应用前景，其核心优势体现在：制备工艺成熟可控、可实现宏观大规模制造、综合性能优异。

Abstract

The fabrication of carbon foams exhibiting low density, high strength, and low thermal conductivity holds significant importance for aerospace thermal protection systems. Conventional methods for producing carbon foams through polymer pyrolysis inevitably suffer from severe volume shrinkage (typically ∼98 %) and mechanical strength degradation. These constraints create a critical challenge in balancing thermal conductivity reduction, density minimization, and compressive strength enhancement. This study pioneers a hot-pressing technique to fabricate high-density melamine foam precursors, subsequently pyrolyzed to create melamine-derived carbon foam (MDCF) with integrate low thermal conductivity, high compressive strength, and low density. The optimized MDCF samples demonstrated minimal skeletal fracture defects at microscopic levels, achieving mechanical strength substantially superior to existing reported melamine-based carbon foams. Specifically, the compressive strength of MDCF-5 samples at 10 % deformation reached 156.04 kPa. Furthermore, MDCF-5 preserved a tightly cross-linked 3D network architecture with an average pore size of 9.3 μm, concurrently maintaining exceptional compressive resilience and superior thermal insulation across extreme temperatures (exhibiting thermal conductivities of 0.0314 W·m−1·K−1 at room temperature, 0.0443 W·m−1·K−1 at 300 °C and 0.2817 W·m−1·K−1 at 600 °C). The developed MDCF materials show promising applicability in spacecraft thermal protection systems, distinguished by three pivotal attributes: mature and controllable preparation process, macroscopic large-scale manufacturing, and excellent comprehensive performance.

引言
航天器（如空间站、卫星、行星探测器）在太空中受大气、太阳辐射及阴影效应等因素影响，长期处于冷热交替的严苛服役环境中。可靠的隔热防护系统对航天器的稳定运行至关重要[1]。多孔材料研究是实现航天器隔热材料低密度、低导热性与优异力学性能的关键路径[2,3]。目前，纤维基多孔隔热材料与泡沫基多孔隔热材料是航天隔热系统中的两大主要应用产品。其中，玻璃纤维毡、陶瓷纤维毡、金属纤维毡等纤维基多孔隔热材料已成功应用于不同温度范围的服务环境[4,5]。然而，纤维基隔热材料的密度通常高于150 kg/m³，这无疑降低了航天器被动温控系统的质量比，限制了其在未来航天领域的应用[6]。为提升航天器有效载荷，泡沫基多孔材料逐渐受到众多专家学者的关注[7]。其中，碳泡沫因低导热性、低密度、耐低温、优异的热疲劳性能与高温隔热性能[[8], [9], [10]]备受持续广泛关注。

Introduction

Spacecraft (such as space stations, satellites, and planet detectors) are affected by factors such as the atmosphere, solar radiation, and shadow effects in outer space, and are in a service environment with frequent alternations of hot and cold for a long time. A reliable heat insulation and protection system is of crucial importance for the stable operation of spacecraft [1]. The research on porous materials is a key approach to achieving low density, low thermal conductivity, and excellent mechanical properties of insulation materials for spacecraft [2,3]. Currently, fibrous mat porous insulation materials and foam porous insulation materials are two main application products in aerospace insulation systems. Among them, fibrous mat porous insulation materials such as glass fiber mats, ceramic fiber mats, and metal fiber mats have been successfully applied in service environments of different temperature ranges [4,5]. However, the density of fibrous mat insulation materials is generally higher than 150 kg/m3, which undoubtedly reduces the mass ratio of passive temperature control systems in the entire aerospace system and limits their application in the future aerospace field [6]. In order to increase the payload of spacecraft, foam porous materials have gradually attracted considerable attention from many experts and scholars [7]. Among them, carbon foam has been continuously and extensively noticed due to its characteristics like low thermal conductivity, low density, low-temperature resistance, good thermal fatigue performance, and excellent high-temperature insulation performance [[8], [9], [10]].

天然轻质多孔材料（如木材、珊瑚、骨骼）在外力作用下常表现出优异的承载能力[[11], [12], [13]]。这种高强度可归因于从宏观到原子尺度的多级特性。在分子层面，材料本身的原子键合强度决定了固体材料理论强度的上限[14,15]。除固有键合强度外，密度在很大程度上决定材料强度：高密度通常意味着高强度，而低密度往往对应低强度[[16], [17], [18]]。多孔碳泡沫的力学性能取决于三个特征密度：相对密度、节点密度与支柱密度[19]。通过巧妙调控这些特征密度，可制备高强度热解碳泡沫结构。例如，玻璃态碳蜂窝微晶格结构（600 kg/m³）通过牺牲尺寸稳定性（尺寸保留率<20%；体积保留率<1%）实现了惊人的高强度（≈1.2 GPa），其原理是提高节点密度并将支柱尺寸从1 μm减小至约200 nm（图1A）[9]。此外，通过最大化热解后的尺寸稳定性与体积保留率，可获得极低密度（<0.05 g/cm³）的热解碳泡沫结构。例如，通过将有机多孔泡沫（聚酰亚胺、聚氨酯泡沫等）浸渍低浓度热固性树脂溶液，泡沫可在热解后实现支撑密度最小化与支撑尺寸保留率最大化（图1B）[20,21]。然而，高体积保留率常伴随显著的质量损失，导致节点密度与支柱密度降低。低支撑壁厚、较大支撑尺寸与较低节点密度的结合意味着碳泡沫的力学强度较低。

Natural lightweight porous materials, such as wood, coral, and bones, often exhibit excellent load-bearing capacity under external forces [[11], [12], [13]]. Such a high intensity can be attributed to the multi-scale characteristics from the macro scale to the atomic level. At the molecular level, the strength of the atomic bonding of the material itself determines the theoretical upper limit of the strength of the solid material itself [14,15]. Apart from the inherent bonding strength, density largely determines the strength of the material: high density usually indicates strong strength, while low density often implies low strength [[16], [17], [18]]. The mechanical properties of porous carbon foams depend on three characteristic densities: relative density, nodal density, and strut density [19]. By skillfully adjusting these characteristic densities, high-strength pyrolytic carbon foam structures can be fabricated. For example, the glassy carbon honeycomb micro-lattice structure (600 kg/m3) can achieve astonishing high strength (≈1.2 GPa) by sacrificing dimensional stability (<20 % size retention; <1 % volume retention). The principle is to enhance the nodal density and reduce the strut size from 1 μm to approximately 200 nm (Fig. 1A) [9]. Additionally, by maximizing dimensional stability and retaining the volume after pyrolysis, extremely low-density (<0.05 g/cm3) pyrolytic carbon foam structures can be obtained. For instance, by impregnating organic porous foams (polyimide, polyurethane foams, etc.) with low-concentration thermosetting resin solutions, the foams achieve minimized support density and maximized support size retention after pyrolysis (Fig. 1B) [20,21]. However, high volume retention is often accompanied by a significant loss of mass, resulting in a reduction in nodal density and strut density. The combination of low supporting cell wall thickness, relatively large support size, and relatively low nodal density implies the low mechanical strength of carbon foams.

热能传递分为三种方式：热传导、热对流与热辐射[2]。一般而言，随着相对密度增加，碳泡沫的热导率也会相应升高[22]。低密度碳泡沫以高孔隙率为特征，热传导主要依赖气体对流与热辐射[23]。随着相对密度增加，材料孔隙率降低，节点密度与支柱密度增加，固相接触更紧密，固体热传导（碳骨架）成为主要传热机制[24,25]。在高密度碳泡沫中，固相热导率远高于气体对流与辐射，碳泡沫整体热导率也随之升高。例如，通过高温发泡热解获得的沥青基碳泡沫（RVC）虽具备强机械压缩性能，但其热导率与密度往往不尽如人意（图1C）[26]。由此推测，通过合成方法制备低密度、高强度、低导热性碳泡沫是材料科学领域的巨大挑战，因其特性相互矛盾、互斥。因此，亟需一种兼具低密度、高强度、低导热性的热解碳泡沫，其不仅需具备超越现有材料的力学强度，还需在大规模宏观制造的同时保持低导热性。

The transfer of thermal energy is divided into three ways: heat conduction, heat convection and heat radiation [2]. Generally speaking, with the increase of relative density, the thermal conductivity of carbon foams also increases accordingly [22]. Low-density carbon foams are characterized by high porosity, and heat conduction primarily depends on gas convection and heat radiation [23]. With the increase of relative density, the porosity of the material decreases, the node density and strut density increase, and the contact between solid phases becomes closer, making solid heat conduction (carbon skeleton) the main heat conduction mechanism [24,25]. In high-density carbon foams, the thermal conductivity of the solid phase is much higher than that of gas convection and radiation, and the thermal conductivity of the entire carbon foam also increases. For example, pitch-based carbon foams (RVC) obtained through high-temperature foaming pyrolysis have strong mechanical compression performance, but their thermal conductivity and density are often unsatisfactory (Fig. 1C) [26]. It can be speculated that preparing low-density, high-strength, and low-thermal-conductivity carbon foams through synthetic approaches is a huge challenge in materials science because these characteristics are mutually exclusive and contradictory. Therefore, there is an urgent need for a low-density, high-strength, and low-thermal-conductivity pyrolytic carbon foam. This pyrolytic carbon foam must not only possess mechanical strength surpassing existing counterparts, but also maintain low thermal conductivity while enabling large-scale macroscopic manufacturing.

本研究提出了一种极简单的高强度、低密度、低导热性热解三聚氰胺衍生碳泡沫制备工艺。该工艺优化了密度、强度与热导率的关系，突出三大核心特性：制备工艺成熟可控、可宏观大尺寸制造、综合性能优异。具体而言，首先制备低密度开孔三聚氰胺泡沫，并通过热压工艺赋予初始轻质三聚氰胺泡沫高密度特性；其次，结合碳化前的预热处理工艺，增强后续三聚氰胺碳泡沫的柔韧性；最终通过高温热解制备得到低密度、高抗压强度、低导热性的MDCF材料。

This study presents an extremely simple preparation process for high-strength, low-density, and low thermal conductivity pyrolytic melamine-derived carbon foam. This process optimizes the relationship among the density, strength, and thermal conductivity, highlighting three key characteristics: mature and controllable preparation process, macro-scale and large-size manufacturability, and excellent comprehensive performance. Firstly, low-density open-cell melamine foam was prepared and the initial lightweight melamine foam was endowed with high-density characteristics through hot-pressing process. Secondly, combined with the preheating treatment process before carbonization, the later melamine carbon foam was endowed with stronger flexibility. Finally, MDCF materials with low-density, high compressive strength and low thermal conductivity was prepared by high-temperature pyrolysis.

材料设计准则
通过热分解高孔隙率有机前驱体（如木材、海绵）可在惰性气氛中高温合成高孔隙率、低导热系数、低密度的热解碳结构[27,28]。本研究中使用的有机三聚氰胺泡沫是本质阻燃的开孔泡沫，以三聚氰胺甲醛树脂为原料，通过固化与微波发泡制备而成。除优异的阻燃性外……

Section snippets

Materials design criteria

Pyrolytic carbon structures with high porosity, low thermal conductivity coefficient, and reduced density can be synthesized through thermal decomposition of highly porous organic precursors (e.g., wood, sponges) under inert atmosphere at elevated temperatures [27,28]. The organic melamine foam used in this study is an intrinsically flame-retardant open-cell foam prepared by curing and microwave foaming with melamine formaldehyde resin as the raw material. Besides having excellent flame...

结论
航天器热防护系统领域亟需一种兼具低密度、高强度、低导热性的热解碳泡沫。此前，通过有机多孔泡沫初步热解后浸渍热固性树脂（如酚醛树脂）并再次热解的方法虽提升了碳泡沫的力学强度，但隔热性能不可避免……

Conclusions

A pyrolytic carbon foam featuring low density, high strength, and low thermal conductivity is urgently demanded in the field of aerospace thermal protection systems. Previously, the mechanical strength of carbon foam was enhanced through the initial pyrolysis of organic porous foam, followed by impregnation with thermosetting resin (such as phenolic) and subsequent pyrolysis. Although the mechanical strength of the prepared carbon foam was improved, the thermal insulation performance inevitably ...

实验部分
三聚氰胺树脂（MR）的合成分为两步。首先，将三聚氰胺与甲醛溶解于弱碱性介质（pH=8.5–9.0）。由于三聚氰胺与甲醛在酸性条件下会生成不溶性的亚甲基三聚氰胺沉淀，因此需在反应前将溶剂pH值调节至8.5–9.0，以确保反应过程中pH值维持在7.0–7.5。当外部温度达到……

Experimental section

The synthesis stage of melamine resin (MR) can be divided into two steps. Firstly, the melamine and formaldehyde are dissolved in a weakly alkaline medium (pH = 8.5–9.0). Because of melamine and formaldehyde will produce insoluble methylene melamine precipitation under acidic conditions, so it is necessary to adjust the pH value of the solvent between 8.5 and 9.0 before the reaction, so as to ensure that the pH value of the reaction process between 7.0 and 7.5. When the external temperature is...

致谢
本研究受江苏省产业前瞻与关键核心技术攻关项目（BE2022147）、海外教授项目（G2022181024L）、国家自然科学基金（52375188）、中国航空科学基金（2023Z057052003）、中国博士后科学基金（2024M754128）资助。作者感谢南京航空航天大学卢乐在SEM与XRD测试中的帮助，以及显微中心……

Acknowledgements

This work is supported by the Industry Foresight and Key Core Technology Competition Project of Jiangsu (BE2022147), the Overseas Professor Project (G2022181024L), the National Natural Science Foundation of China (52375188), the Aeronautical Science Foundation of China (2023Z057052003), the China Postdoctoral Science Foundation (2024M754128). The authors also appreciate Le Lu (Nanjing University of Aeronautics and Astronautics) for their help on SEM and XRD, as well as the Center for Microscopy ...

文章来源：《Chemical Engineering Journal》
https://www.sciencedirect.com/science/article/abs/pii/S1385894725054233