Zirconium diboride (ZrB₂)–silicon carbide (SiC) composites are promising candidates for ultra-high temperature applications, yet optimizing their densification and mechanical performance without sintering additives remains a challenge. This study systematically investigates the independent and combined effects of three critical spark plasma sintering (SPS) parameters, that is, temperature, applied pressure, and dwell time, on the densification behavior, microstructure, and mechanical properties of ZrB₂–20 vol% SiC composites. Building upon prior work on powder preparation effects (e.g., Tungsten Carbide (WC) vs. ZrO₂ milling), this research uniquely focuses on how precise control of sintering conditions alone can tailor final material characteristics. The results demonstrate that optimizing sintering parameters yields significant property enhancement, achieving a maximum relative density of 99.2% (at 2100°C, 65 MPa, 15 min) and peak flexural strength of 516 MPa (at 2000°C, 65 MPa, 60 min). Hardness and fracture toughness reached 17.08 GPa and 3.85 MPa m^1/2, respectively, under optimized conditions. Through detailed microstructural and performance analysis, this work explains the fundamental role of individual sintering parameters in governing densification kinetics and mechanical outcomes. The findings offer practical guidance for additive-free, energy-efficient processing of ZrB₂–SiC ceramics for advanced aerospace and thermal protection systems.

Hypergolic ignition systems have traditionally relied on toxic propellants such as MMH/NTO, prompting a global shift toward greener alternatives. High-Test Peroxide (HTP), with its high oxygen content and clean decomposition products, has emerged as a promising oxidizer when paired with kerosene and suitable catalysts. However, a mechanistic understanding of HTP–fuel ignition, especially with metal-organic catalysts under varying conditions, remains underdeveloped. Here, a comprehensive experimental and kinetic study of hypergolic ignition using Mn (II) acetylacetonate-doped kerosene with HTP is presented across a wide parametric space (HTP: 85–98 %; catalyst: 0.5–10 wt%; O/F: 4.5–7.5; T: 20–50 °C). The results reveal that ignition delay times (IDTs) reduce by over 30 % with preheating and optimal catalyst loading, and deconvoluted phase-wise IDTs show that Mn(II)AA primarily accelerates HTP decomposition and chemical ignition. Derived apparent activation energies (E_a ≈ 10.0 kJ/mol) are consistently low, while the Arrhenius pre-exponential factor (A) increases significantly with catalyst and oxidizer concentration, indicating catalytic efficiency and diminishing returns beyond 5 wt%. Peak flame temperatures exceeding 1200 °C confirm robust energy release, with high-speed imaging further revealing a transition to rapid, spatially distributed ignition under optimal conditions. These findings offer quantitative mechanistic insights into catalyst-enhanced HTP ignition and establish a framework for optimizing green bipropellant systems for aerospace propulsion.

Hypergolic propellants have long been central to spacecraft propulsion because of their storability, reliability and rapid ignition. Conventional systems such as hydrazine derivatives paired with oxidisers like nitrogen tetroxide deliver ignition delays in the order of a few milliseconds but pose serious risks due to extreme toxicity and handling hazards. The search for safer and environmentally friendlier alternatives has therefore become a priority in recent years. This review examines ignition delay times reported in the literature for both conventional and green propellants under ambient experimental conditions. Data were collected from published studies between 2000 and 2025 using major scientific databases, including Scopus, Web of Science, and Google Scholar, and are compared across three categories of propellants: traditional hydrazine-based systems, self-igniting ionic liquids and amines, and systems enhanced with catalytic or reactive promoters. The analysis shows that while conventional propellants remain benchmarks with ignition delays typically between 1 and 5 ms, some new formulations, particularly those containing reactive additives such as borohydrides or iodide salts, are achieving similar or improved performance in laboratory tests. The review also highlights that variability in reported ignition delays often stems from differences in test methods, droplet size, oxidiser concentration, and diagnostic approaches. Beyond performance considerations, attention is given to safety and environmental aspects since several green candidates reduce acute toxicity but introduce other challenges, such as instability or corrosive byproducts. By bringing together data in a comparative format and emphasising methodological limitations, this review aims to support the future design and evaluation of practical green hypergolic propellants.