ABSTRACT
The mechanism of thermal decomposition and fire suppression, and the fire-extinguishing performance of HFO-1234yf, HCFO-1233xf and 2-BTP agents were investigated by using both experimental and theoretical methods. The different halogen atoms connected with the middle carbon atom result in the varied strength of C-X (X = F, Cl, Br) bonds, and thus different thermal stability of these agents, which could further affect the pyrolysis mechanism/products and the fire-extinguishing mechanism/performance of these agents. Owing to the generation of CF3Ë, ClË and BrË radicals, as well as some unsaturated small molecules produced by their pyrolysis, the HFO-1234yf, HCFO-1233xf and 2-BTP agents have minimum extinguishing concentrations (MECs) of 9.80 vol%, 7.28 vol% and 2.92 vol% (9.80 vol%, 7.28 vol% and 2.56 vol%) for suppressing propane-air (methane-air) flame, respectively, which are comparable to or even better than those of other hydrofluoroolefin (HFO) and hydrofluorocarbon (HFC) agents. Despite the contribution of directly produced BrË radicals, which have the lowest energy barrier and the highest efficiency in capturing free radicals, the BrË and CF3Ë radicals produced by the follow-up reactions with OHË/HË radicals may also contribute a lot to the best fire-suppressing performance of 2-BTP. Due to the high reactivity of these unsaturated halogenated olefins and their pyrolysis products, exothermic reactions could occur between the original agents (or their pyrolysis products) and the OHË/O: radicals, thus leading to the combustion-promotion effect of the HFO-1234yf, HCFO-1233xf and 2-BTP agents. The slightest combustion-promotion effect of the 2-BTP extinguishant may result from the easier generation and best performance of the BrË radicals, as well as the lowest energies released by the exothermic reactions.
ABSTRACT
In view of the appropriate physicochemical characteristics and environmental friendliness of the trans-1,1,1,4,4,4-hexafluoro-2-butene (HFO-1336mzz(E)) substance, the thermal-decomposition mechanism as well as the fire-extinguishing mechanism and performance of this agent were systematically studied by employing both experimental and theoretical methods in this work. We found that the HFO-1336mzz(E) agent not only has promising thermal stability at room temperature but also exhibits pronounced fire-extinguishing performance, which is comparable to that of HFC-236fa and even better than that of HFC-125 extinguishant. Additionally, the promising fire-extinguishing performance of HFO-1336mzz(E) may result from the physical and chemical extinguishing effect of its thermal-decomposition products including HFO-1336mzz(Z), HC≡CCF3, CF3C≡CCF3, and CF3H, which makes a significant contribution to capturing the free radicals in the flame, as well as cooling and diluting the combustible fuel-air mixture. Both experimental and theoretical results suggest that the HFO-1336mzz(E) agent is a highly recommendable candidate for Halon extinguishant, which is worthy of further investigation and evaluation of its practical applicability in fire-suppression utilization.
ABSTRACT
By employing particle-swarm optimization (PSO) and first-principles computations, we theoretically predicted five stable phases of graphene-like borocarbonitrides (g-BCN) with the stoichiometric ratio of 1:1:1 and uniformly distributed B, C, N atoms, which are the isoelectronic analogues of graphene. These g-BCN monolayers are effectively stabilized by their relatively high proportion of robust C-C or B-N bonds and strong partial ionic-covalent B-C and C-N bonds within them, leading to pronounced thermal and kinetic stability. The visible-light absorption and high carrier mobility of the investigated g-BCN monolayers indicate their possible applications in high-efficiency photochemical processes and electronic devices. Our computations could provide some guidance for designing the graphene-like materials with earth-abundant elements, as well as some clues for the experimental synthesis and practical applications of ternary BCN nanosheets.
