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.
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.
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]].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.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.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.
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...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 ...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...CRediT authorship contribution statement
Yapeng Wang: Writing – review & editing, Writing – original draft, Software, Investigation. Zhaofeng Chen: Supervision, Resources, Methodology. Lixia Yang: Supervision, Funding acquisition. Sufen Ai: Resources. Jianxun Zhang: Funding acquisition, Resources. Lihua He: Funding acquisition, Resources. Manna Li: Resources, Validation.Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ...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 ...From：《Chemical Engineering Journal》
https://www.sciencedirect.com/science/article/abs/pii/S1385894725054233