In 1931, aerogels were first demonstrated as skeleton- or sponge-like porous materials [1]. These unique materials possess remarkable properties, including low bulk density (~ 0.003 g/cm3), high porosity (> 95%), large specific surface area (~ 103 m2/g), low thermal conductivity (~ 0.01 W m-1K-1), and low relative dielectric constant (~ 1.1). Due to these characteristics, aerogels find wide-ranging applications in lightweight construction, thermal and acoustic insulation, sensing, chemical/energy absorbers, catalyst supports, and energy technologies. The potential of aerogels to incorporate various functional nanomaterials within their structures has led to the development of nanocomposite aerogels, enabling the fabrication of novel composites. For instance, nanoparticle-infiltrated aerogels have proven to be valuable in catalysts, sensors, non-linear optics, plasmonic gain, electrodes for pseudo-supercapacitors, and electromagnetic interference shielding materials [2–5]. Notably, aerogels incorporating carbon nanotubes (CNTs) have demonstrated mechanoresponsive conductivity and pressure-sensing capabilities [6–8]. Despite the significant advancements in nanocomposite aerogels, their applications often suffer from challenges related to poor control and/or limited loading of nanoparticles or CNTs into the matrices, necessitating laborious synthetic processes [9]. To address this limitation and further enhance the potential of nanoparticle/CNT-aerogel composites, ongoing research aims to explore novel methods of nanoparticle/CNT integration, optimizing synthesis processes, and improving the overall performance and efficiency of these extraordinary materials.
Nitrocellulose (NC), also known as cellulose nitrate, is an important industrial polymer with a long history of diverse applications. Since the 1800s, NC has been widely used in coatings, inks, propellants, filtration, and bio-membranes [10]. The highly nitrated NC (wt% of nitrogen > 12%) is an energetic material and continues to play an indispensable role in the defense industries and micro propulsion [11, 12]. Energetic materials can be ignited by various methods, including flame, electrical spark, impact, friction, laser, radiofrequency, and chemical initiators. Compared to other ignition methods, laser ignition offers a range of benefits that make it a valuable and preferred ignition method for energetic materials in various industries. Its ability to provide precise control, improved safety, and enhanced performance makes it an essential technology for advancing the capabilities of energetic materials and their applications. However, NC's lack of an obvious light absorption band in the infrared radiation region makes it challenging to ignite using an infrared laser [13–16]. To overcome this limitation, researchers have explored the incorporation of various materials into NC to enhance its laser sensitivity. In addition to improving laser energy absorption, metal nanoparticles [9, 17–19] and carbonaceous nanomaterials [13, 20–32], can be incorporated into energetic materials to improve the controllability of laser ignition and combustion behaviors.
The utilization of plasmonic doped metal nanoparticles based on localized surface plasmon resonance has been a common approach to create "hotspots" for energetic materials [33–34]. For instance, Abboud et al. [17] demonstrated that aluminum (Al) nanoparticles can enhance and control local photothermal energy deposition through a photothermal effect. Fang et al. [18] experimentally confirmed the feasibility and effectiveness of gold nanoparticles (AuNPs) as optical sensitizers for micro-energetic cyclotrimethylenetrinitramine (RDX) crystals coated or doped with AuNPs. The AuNPs exhibit strong absorption at the laser wavelength and efficiently convert the absorbed energy into heat, leading to the formation of dense nano-hotspots. Subsequently, they reported that AuNPs are effective optical sensitizers for directly initiating RDX crystals using lower power pulse lasers [19]. Because of their plasmonic attributes, doped AuNPs offer three orders of magnitude greater sensitivity to pulsed laser irradiation than pure RDX.
As thermal conduits, carbonaceous nanostructures, such as graphene, graphene oxide (GO), and multi-walled CNTs (MWCNTs), play a crucial role in facilitating heat transfer from the reaction zone to the unburned portions of the propellant, resulting in the net thermal transport within the propellant along the direction of the reaction propagation wave. Several studies have demonstrated the significant impact of incorporating these nanostructures into energetic materials, particularly NC. For example, Jain et al. [20] found that doping graphene at a 3 wt% loading increased the burn rate of NC by 300%. Shen et al. [21] observed a 4.6% increase in burn rate when graphene was added to NC-triethylene glycol dinitrate-RDX propellant. Zhang et al. [13] reported the enhancement and controlled laser ignition and burn rates of NC microfilms when doped with 0.5% or more GO. By formulating NC/GO composite with controlled microstructure, Liu et al. [23] reported the enhancement in laser ignition and combustion properties of NC (e.g., a doped composite with 20% GO showed the maximum combustion efficiency and tremendous gas release). Very recently, Shi et al. [24] demonstrated improved thermal stability and thermal conductivity, reduced ignition delay time and weakened combustion flame when doping NC propellant with 0.25 ~ 2 wt% of GO. In addition to graphene and GO, MWCNTs have been explored for their potential to enhance thermal conductivity and augment burn rates of solid monopropellants. MWCNTs possess an exceptionally high thermal conductivity of 3,000 W m− 1K− 1 [35], far exceeding the thermal conductivity of most solid monopropellants, which typically falls within the range of 0.1 ~ 1 W m− 1K− 1 [36]. The incorporation of MWCNTs has shown promising results in enhancing thermal conductivity and augment the burn rates of solid monopropellants. For instance, Choi et al. [26] investigated the flame speed enhancement of a solid monopropellant (trinitramine) when coupled with MWCNTs, achieving an enhancement of up to 1,500 times. Molecular dynamics (MD) simulations by Jain et al. [30] demonstrated that doped MWCNTs can enhance the flame speed up to 3 times the bulk value.
Notably, Peterson et al. [12] reported an astoundingly simple preparative technique for robust, monolithic NC aerogels. The process involves mixing 4% NC in 1:1 ethanol and ethyl ether (referred to as "Collodion") with a co-solvent (e.g., ethanol, 2-propanol, or 1-butanol). Upon layering a non-solvent (e.g., hexanes) over the resulting NC solution, an opaque or translucent alcogel is formed. Through supercritical drying with carbon dioxide, a robust aerogel with a large surface area and a solvent-tunable, hierarchical, 3-dimensional nanoporous structure is synthesized. This gel matrix is highly suitable for back filling with high explosives (e.g., NC/RDX) and/or nanomaterials with unique optical, electric, thermal, and magnetic properties to create novel composite materials. The incorporation of nanomaterials could also enable pre-blast detection (e.g., through a taggant) or enhance detection post-blast (e.g., via a visual examination due to unique optical properties or by a simple readable magnetic signature for debris) [37].
Here we demonstrate a facile and efficient method for achieving uniform infiltration of optical sensitizers into NC aerogels directly from alcohol-based sol-gel mixtures, resulting in the formation of homogeneous NC-based nanocomposites. The optical sensitizers used in this process include AuNPs capped with self-assembled monolayers (SAMs) of hydroxyl and/or carboxylic terminal functional groups (denoted as OH- and/or COOH-AuNPs, respectively) as well as carboxylated MWCNTs (denoted as c-MWCNTs). The presence of these infiltrated optical sensitizers in the aerogels does not significantly alter the material’s structure; however, it does lead to enhanced laser initiation and combustion behavior.