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Magnesium has received significant attention as a potential hydro-reactive metal particle fuel due to its superior ignition and combustion characteristics. In this work, a laser ignition testing system was designed for visualised combustion monitoring of moving magnesium particles with different average diameters. Combustion images were recorded and the effect of particle diameter on the combustion phenomena and combustion time were analysed. The statistical results show that the average combustion times are 4.5 ± 2.5, 15.2 ± 7.9, and 31.7 ± 17.9 ms for the magnesium particle fractions of 100, 150, and 200 µm, respectively. Moreover, a quasi-stable state, heterogeneous, and diffusion limited mathematical model was developed to describe the combustion process. In the model, a flame surface exists and divides the gas phase field between the particle surface and infinity into an internal zone and an external zone. Effects of particle relative movement on the heat transfer and external zone components were also considered.

Metal fuels have a high potential for applications in space propulsion owing to their superior energy density and distinct combustion characteristics. Therefore, research into the combustion properties of metal fuels in microgravity has become necessary. The principles, applications, benefits, and limitations of experimental methods for microgravity combustion of metal fuels were compared in this review. Drop towers, aircrafts, sounding rockets or microgravity balloons, acoustic or ultrasonic levitation, electrodynamic levitation, space stations, and numerical simulations are common methods used to provide microgravity for combustion research. Among them, the devices providing microgravity environment on the ground has become a good choice for simulating microgravity environment owing to its convenience, reliability and relatively low cost. Further, the physical and chemical properties of common metal fuels, such as aluminum, boron magnesium, and so on, were introduced, and the existing research achievements on the combustion of metal fuels in microgravity are succinctly summarized in this paper. The melting process, ignition temperature, combustion temperature, flame structure, and extinction of metal fuel combustion will be affected by microgravity. Moreover, the necessity and insufficiency of the research are also illustrated. Finally, the existing and potential applications of metal fuels in space propulsion and space weapons are introduced. Metal fuels’ superior energy density and combustion performance can benefit deep-space exploration, particularly Mars exploration, and bring new insights into the design of space weapons. This paper may throw some new light on the study of metal fuel combustion in microgravity.

The synthesis of renewable chemical fuels from CO2and H2O via photoelectrochemical (PEC) route reprensents a promising room-temperature approach for transforming greenhouse gas into value-added chemicals (e.g., syngas), but to date it has been hampered by the lack of efficient photocathode for CO2reduction. Herein, we report efficient PEC CO2reduction into syngas by photocathode engineering. The photocathode is consisting of a planar p-n Si junction for strong light harvesting, GaN nanowires for efficient electron extraction and transfer, and Au/TiO2for rapid electrocatalytic syngas production. The photocathode yields a record-high solar energy conversion efficiency of 2.3%. Furthermore, desirable syngas compositions with CO/H2ratios such as 1:2 and 1:1 can be produced by simply varying the size of Au nanoparticle. Theoretical calculations reveal that the active sites for CO and H2generation are the facet and undercoordinated sites of Au particles, respectively.

Action mechanism of nano titanium carbide (nTiC), nano zirconium carbide (nZrC), and nano silicon carbide (nSiC) was studied using a concentrated ignition experimental system, an X-ray diffractometer and a scanning electron microscope. Each nano-carbide was mixed with B in mass ratio of 2/8 and then pressed into tablets, respectively. Furthermore, a sample of amorphous B was also prepared as the control group. All the three nano-carbides showed positive influence on the ignition of B. During the first stage of combustion, only nTiC could increase the maximum emission spectral intensity of the sample, while both nTiC and nZrC helped to increase that during the second stage of combustion. Although the addition of nano-carbides intensified the energy release rate of B (except nSiC), the expansion and diffusion of generated carbon dioxide took large amounts of heat away and reduced the surface temperature of the samples The temperature change in solid-phase exhibited a relationship with the spectral intensity change in gas-phase. The temperature increase was slow during the ignition delay stage, while the first combustion stage proceeded with rapid increase in surface temperature. The high-temperature areas regressed during the second combustion stage, accompanied by the gradual stopping of gas-phase reactions. The components and microstructure of the condensed combustion products (CCPs) were analyzed after the ignition experiment. Results showed that both nTiC and nZrC could react with B to generate corresponding metal borides, capable of promoting the ignition of B. Moreover, the catalysis of produced TiO2and ZrO2explain the increase of emission spectral intensity during the second combustion stage. A lot of micropores formed in the CCPs after adding nTiC, which showed a destructive effect of generated gaseous CO2on the surface oxide layer. In general, nTiC is the top accelerant for B oxidation in this study, while nSiC is at the bottom.

Particle size and oxygen content are two of the key factors that affect the ignition and combustion properties of aluminum particles. In this study, a laser ignition experimental system and flame test system were built to analyze the ignition and combustion characteristics and the flame morphology of aluminum particles. A thermobalance system was used to analyze the thermal oxidation characteristics. In addition, the microstructure of aluminum was analyzed by scanning electron microscopy. It was found that the oxidized products were some of the gas phase products agglomerated. Smaller particle size samples showed better combustion characteristics. The combustion intensity, self-sustaining combustion time and the burn-off rate showed a rising trend with the decrease in the particle size. Increasing the oxygen content in the atmosphere could improve the ignition and combustion characteristics of the samples. Four distinct stages were observed in the process of ignition and combustion. Small particle size samples had a larger flame height and luminance, and the self-sustaining combustion time was much longer. Three distinct stages were observed during the thermal oxidation process. The degree of oxidation for small-sized samples was significantly higher than that for the larger particle size samples. Moreover, it was observed that the higher the oxygen content, the higher the degree of oxidation was.