Untangling the Efficient Boron-Initialized Hydroxyl-Terminated Polybutadiene Combustion for High Energetic Solid Propulsion Systems

Highly energetic boron (B) particles embedded in hydroxyl-terminated polybutadiene (HTPB) thermosetting polymers represent stable solid-state fuel. Laser-heating of levitated B/HTPB and pure HTPB particles in a controlled atmosphere revealed spontaneous ignition of B/HTPB in air, allowing for examination of the exclusive roles of boron. These ignition events are probed in situ via simultaneous spectroscopic diagnostics: Raman and infrared spectroscopy, temporally resolved high-speed optical and infrared cameras, and ultraviolet–visible (UV–vis) spectroscopy. The emission spectra unravel two stages of the B/HTPB ignitionthe exoergic ignition of boron followed by HTPB combustion. It was found that HTPB readily absorbs the energy from the irradiating carbon dioxide (CO2) laser but efficiently transfers that thermal energy to the densely arranged boron particles due to the lower heat capacity of the latter. This transferred energy causes a surge in temperature for the boron particles, leading to ignition (in an oxygen environment) in B/HTPB, unlike the case with HTPB alone. The accumulated energy from the second stage of boron ignition triggers the decomposition of HTPB in conjunction with hydrogen abstraction to produce radical precursors via boron oxides (BO and BO2)the key emitting intermediates detected. Along with conventional combustion products such as carbon dioxide (CO2) and water (H2O), the formation of partially oxidized gaseous products such as methanol (CH3OH) and methyl vinyl ether have also been detected as a tracer of diverse oxidation events, suggesting a complex oxidation chemistry within HTPB and overall depict crucial insights for its use as a solid rocket fuel.嵌入端羟基聚丁二烯（hydroxyl-terminated polybutadiene, HTPB）热固性聚合物中的高活性硼（B）颗粒，可构成稳定的固态燃料。在可控气氛中对悬浮态B/HTPB复合颗粒与纯HTPB颗粒开展激光加热实验，结果显示B/HTPB复合颗粒在空气中可自发点火，为探究硼的专属作用提供了可行途径。本研究通过多光谱同步诊断技术对上述点火过程进行原位探测，所用手段包括拉曼光谱、红外光谱、时间分辨高速光学与红外相机，以及紫外-可见（ultraviolet–visible, UV–vis）光谱。发射光谱揭示了B/HTPB复合体系点火的两个阶段：首先为硼的放热点火，随后是HTPB的燃烧过程。研究发现，HTPB可高效吸收辐照用二氧化碳（carbon dioxide, CO2）激光的能量，且由于硼颗粒的热容更低，其可将热能高效传递至密集排布的硼颗粒。该传递的热能使硼颗粒温度骤升，最终引发B/HTPB复合体系在含氧环境中的点火，而纯HTPB体系则未出现此类点火现象。硼点火第二阶段积累的能量，会触发HTPB的分解反应，同时伴随氢提取过程，通过硼氧化物（BO与BO2）生成自由基前驱体——这是本研究检测到的关键发光中间体。除二氧化碳（carbon dioxide, CO2）、水（H2O）等常规燃烧产物外，本研究还检测到甲醇（CH3OH）、甲基乙烯基醚等部分氧化气态产物，这些产物可作为多种氧化过程的示踪剂，表明HTPB内部存在复杂的氧化化学反应机制，相关研究结果为其作为固体火箭燃料的应用提供了关键理论认知。