Search Results (104)

Surface erosion of the coaxial thermocouple probe initiates continuous bridging of thermoelectric materials on the insulation layer surface, forming new temperature measurement junctions. This inherent ability to measure continuous self-erosion ensures the operational reliability of the coaxial thermocouples in high-temperature ablative environments. However, the fabrication of a high-temperature electrical insulation layer and a high-adhesion insulating layer in the coaxial thermocouples remains a challenge. Inspired by calcium carbonate/oxalate crystals in jujube leaves that strengthen the leaves, a bioinspired structural ceramic (BSC) mimicking these needle-like crystals is designed. This BSC demonstrates excellent high-temperature insulation (with insulation impedance of 2.55 kΩ at 1210 °C) and adhesion strength (35.3 Newtons). The BSC is successfully used as the insulating layer in a K-type coaxial thermocouple. The generation rules for surface junctions are systematically studied, revealing that stable and reliable measurement junctions can be created when the sandpaper grit does not exceed 600#. Static test results show that the K-type coaxial thermocouple ranges from 200 °C to 1200 °C with an accuracy of 1.1%, a drift rate better than 0.0137%/h, and hysteresis better than 0.81%. Dynamic test results show that the response time is 1.08 ms. The K-type coaxial thermocouple can withstand a high-temperature flame impact for 300 s at 1200 °C, as well as over forty cycles of high-power laser thermal shock, while maintaining good response characteristics. Therefore, the K-type coaxial thermocouple designed in this study provides an ideal solution for long-term temperature monitoring of the thermal components of aerospace engines under extremely high-temperature, high-speed, and strong thermal shock conditions. Full article

Incorporating a 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-based derivative (1,4-bis(diphenoxyphosphoryl)piperazine, DIDOPO) in combination with modified calcium sulfate whiskers (MCSWs) improved the flame retardancy, thermal stability, and rheological properties of a polyethylene oxide (PEO) composite. The synergistic flame-retardant effect of DIDOPO and MCSW on the PEO system was investigated. After introducing 5 wt.% MCSW and 10 wt.% DIDOPO into PEO, the UL-94 rating of the composite reached V-0, and the limiting oxygen index was increased to 26.5%. Additionally, the peak and average heat release rates and total heat release of the PEO/10% DIDOPO/5% MCSW composite decreased by 38.9%, 22%, and 20.07%, respectively. The results of a thermogravimetric analysis (TGA) revealed that PEO/10% DIDOPO/5% MCSW displayed an improved initial thermal stability and rate of char formation compared to those of the PEO matrix. The results of TGA/Fourier transform infrared analysis indicated that the composites exhibited phosphorus-containing groups during thermal degradation, based on the characteristic absorption peaks, and increased amounts of gas-phase volatiles. The morphologies and structures of the residues indicated that the PEO/10% DIDOPO/5% MCSW blend was less stable than PEO during combustion. The MCSW mixture formed a denser, more continuous carbon layer on the composite surface during combustion. The rheological behavior indicated that the high complex viscosity and moduli of PEO/10% DIDOPO/5% MCSW promoted the cross-linking network structure of the condensed phase during combustion. MCSW exhibited an excellent flame retardancy and improved thermal stability, which are potentially promising for use in fire safety applications. Full article

As a natural fiber, cashmere is favored for its softness, finesse, and warmth. However, its poor flame-retardant properties seriously affect the safety of cashmere. Current flame-retardant treatments for cashmere tend to lead to heavy metal pollution and significantly reduce wearer comfort. In this work, natural and environmentally friendly calcium alginate fibers were blended with cashmere to obtain blended fibers. The blended fibers exhibited good hygroscopicity and softness. The incorporation of calcium alginate fibers enhanced the flame retardancy of the blends, and the LOI of the blended fibers reached 40.2 without smoldering. It was due to a stable CaO protective layer formed by Ca²⁺ during combustion and the dense carbon layer with the decomposition intermediates of cashmere, which exerted a flame-retardant effect in the condensed phase. This study provided an eco-friendly approach to producing high-quality flame-retardant cashmere products. Full article

The inherent flammability and toxic smoke emission of epoxy resins (EPs) pose significant challenges to their advanced engineering applications. To address this limitation, we developed a novel flame-retardant additive through the organic modification of layered double hydroxides (LDHs) using a ternary MgAlLa hydrotalcite structure intercalated with phosphotungstic acid (PWA). This innovative design established a synergistic mechanism by combining the catalytic carbonization effect of lanthanum with the radical scavenging capability of PWA. The optimized MgAlLa-PWA/EP composite demonstrated remarkable flame retardancy and smoke suppression improvements, exhibiting 77.9% and 62.4% reductions in the peak heat release rate (pHRR) and total heat release (THR), respectively, compared to pure EP. Particularly noteworthy was the 72.6% decrease in total smoke release (TSR), accompanied by a significant elevation of the limiting oxygen index (LOI) value to 26.8% and achievement of UL-94 V-0 rating. Microstructural analysis revealed that the modified composite formed a continuous and uniform layer with increased density during combustion, effectively inhibiting oxygen exchange, smoke diffusion, and heat transfer. This study provides a novel strategy for designing multi-element synergistic LDHs additive for high-efficiency flame retardancy and smoke suppression of EP. Full article

As a zero-carbon fuel, ammonia can be directly employed in its liquid form. However, its unique physical and chemical properties pose challenges to its application in engines. The direct injection of liquid ammonia is considered a promising technique for internal combustion engines, yet its combustion behavior is still not well understood. In this work, the combustion characteristics of liquid ammonia direct injection under high-pressure conditions were investigated using direct numerical simulation (DNS) in a Eulerian–Lagrangian framework. The ammonia spray was injected via a circular nozzle and underwent combustion under high-temperature and high-pressure conditions, resulting in complex turbulent spray combustion. It was found that the peaks of mass fraction of important species, heat release rate, and gaseous temperature increase with increasing axial distance, and the peaks shifted to richer mixtures. The distribution of scalar dissipation rate at various locations is nearly log-normal. The budget analysis of species transport equations shows that the reaction term is much larger than the diffusion term, suggesting that auto-ignition plays a predominant role in turbulent ammonia spray flame stabilization. It can be observed that both non-premixed and premixed combustion modes co-exist in the ammonia spray combustion. Moreover, the contribution of premixed combustion becomes more significant as the axial distance increases. Full article

Polyvinyl alcohol (PVA) is used in various fields; however, its highly flammable property greatly limits its application. In order to improve the flame-retardant properties of PVA, one method is by adding flame retardants directly, while another method is through grafting, cross-linking and hydrogen bonding. A flame retardant, 9, 10-dihydro-9, 10-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-vinyltrimethoxysilane (VTES), was synthesized through the addition reaction of a P–H bond on the DOPO and unsaturated carbon–carbon double bonds on the VTES. Then, the DOPO-VTES and zirconium phosphate (α-ZrP) were blended with PVA to cast a film, in which DOPO-VTES was grafted onto the PVA by cross-linking the hydroxyl group in the molecular structure of DOPO-VTES with the hydroxyl group in PVA; α-ZrP was used as a cooperative agent of DOPO-VTES. The cone calorimetry test (CCT) showed a significant reduction in both the heat release rate (HRR) and total heat release rate (THR) for the flame-retardant PVA films compared to pure PVA. Additionally, thermogravimetric analysis (TGA) revealed a higher residual char content in the flame-retardant PVA films than in pure PVA. These findings suggested that the combination of DOPO-VTES and α-ZrP could improve the flame retardancy of PVA. The cooperative flame-retardant mode of action at play was possibly that DOPO in the DOPO-VTES acted as a mainly gas-phase flame retardant, which yielded a PO radical; VTES in the DOPO-VTES produced silicon dioxide (SiO₂), which acted as a thermal insulator; and α-ZrP catalyzed the carbonization of the PVA. By combining DOPO-VTES with α-ZrP, a continuous dense carbon layer was formed, which effectively inhibited oxygen and heat exchange, resulting in a flame-retardant effect. It is expected that flame-retardant films for PVA have a broad development prospect and potential in the fields of packaging materials, electronic appliances, and lithium-ion battery separators. Full article

Polyurethane foam (PF) is a versatile polymer widely used in various applications. By changing the composition of polyol and isocyanate, these foams can be classified into rigid polyurethane foams (PUFRs) and flexible polyurethane foams (PUFFs). The flexible polyurethane foam (PUFFs) is well known for its sound absorption capacities; nevertheless, its flammability poses significant safety hazards. The purpose of this study is to look into how flexible polyurethane foam reacts to fire, specifically its combustion properties, and the risks that come with them. The study aims to find out the rates of horizontal and vertical burning, the make-up of the reaction products, and the temperatures that build up inside the polyurethane foam mass when a support pole is placed in front of the stage and sound-absorbing material is added to stop stage sounds from reverberating. There were performed experiments to determine the fire behavior of the samples in contact with an ignition source in the form of a small flame and experiments to determine the ignition temperature of the sound-absorbing sponge, where it was found that vertical position accelerates combustion, and in practical applications, this aspect must be considered for fire prevention. To determine the combustion gases, several methods were used, namely spectrophotometric, ion chromatography, and gas-chromatographic methods. Analysis of the gases resulting from the combustion of the sound-absorbing sponge indicates the presence of dangerous toxic compounds (hydrogen cyanide, carbon monoxide, and hydrochloric acid), which can endanger human health in the event of a fire. Full article

This study investigates the formation of by-product species during flame spray synthesis (SFS) of superparamagnetic maghemite (γ-Fe₂O₃) nanoparticles. Four samples are synthesized by utilizing two standardized burner types (SpraySyn1 and SpraySyn2) and varying the iron (III) nonahydrate (INN) concentration (0.1 M and 0.2 M) in the precursor feed while using ethanol and 2-ethylhexanoic acid as solvent. Conducting complementary powder analysis revealed a predominant presence of carboxylates and carbonates as by-product species (~14–18 wt.%), while no strong indications for elemental carbon and precursor/solvent residues can be found. Carbonates/carboxylates are located on particle surfaces, and the particles’ surface loadings by these species are independent of the precursor concentration but depend on burner type, with SpraySyn2 exhibiting lower values, indicating a more complete combustion for this burner. Through time-resolved thermophoretic sampling, we further demonstrate that carbon forms temporally in the visible flame center when using SpraySyn1. Since carbon solely forms momentarily within large flame pulses and decomposes further downstream, its temporal formation is of minor relevance for the final particle purity. However, its local co-existence aside from γ-Fe₂O₃ in the flame has potential to bias in situ diagnostics. Full article

The alumina, in the form of α-Al₂O₃ tabular balls, considered in this study is a high-purity form of aluminum oxide that has been fired at high temperatures (well above 1900 °C), virtually removing porosity. However, the purity and inertness of the surface of the Al₂O₃ tabular balls minimize the catalytic activity, which is why lithium doping was tried. Thus, the target of this study was the effect of doping with lithium ions in some tabular balls of Al₂O₃ (the crystalline structure is corundum) on the improvement of the catalytic properties of alumina. This study examined the impact of a lithium catalyst on the combustion of various fuels within a porous inert medium (PIM) burner. This study specifically compared low calorific gaseous fuel (e.g., biogas) combustion in a PIM burner with and without the lithium catalyst. The experimental setup comprised a gas preparation unit for mixing CNG and CO₂ to simulate biogas and a PIM burner. The PIM burner comprised Al₂O₃ spheres (13 mm diameter, 45% porosity) in a random packing configuration. Three fuels, varying in composition and lower heating value (LHV ranging from 20.771 to 27.695 MJ/m³), were combusted at air ratios ranging from 1.67 to 1.79. The results indicated that the catalyst increased peak combustion temperatures by 23.2 °C to 51.4 °C, depending on the fuel type and air ratio. Significantly higher carbon monoxide (CO) concentrations were observed without the catalyst, particularly with fuel type F1, while nitrous oxide (NOx) levels remained consistently low. Upstream flame propagation was observed in the presence of the catalyst. These findings demonstrate the potential of lithium catalysts to enhance combustion stability and reduce emissions in porous media combustion burners. Following these studies, it can be stated that Li(I) has the role of promoter of the catalytic process. Full article

Mixtures of hydrogen with common hydrocarbon fuels are considered viable for reducing carbon footprint in modern industry, power production, and transportation. The addition of hydrogen alters the kinetics and thermophysical properties of the mixtures, as well as the composition and properties of combustion products, requiring detailed research into the features of flame propagation in hydrogen-enriched hydrocarbon–air mixtures. Of particular interest are also the safety aspects of such fuels. In this paper, experimental results are presented on the premixed laminar flame propagation in channels formed by two closely spaced plates (Hele-Shaw cell), with the internal straight walls forming a diverging (diffuser) channel with the opening angles between 5 and 25 degrees. Methane–hydrogen–air mixtures with the hydrogen relative contents of 0%, 25%, and 50% and global equivalence ratio of unity were ignited by a spark near the closed narrow end of the channel. Experiments were performed with the gap width of 3.5 mm; video recordings were processed in order to determine the quantitative features of the flame front propagation (leading and trailing point coordinate, coordinates of the cusps, cell sizes and shapes). The main features of flame propagation (fast initial expansion, development of cellular flame, self-induced longitudinal oscillations) are obtained and compared to clarify the effect of hydrogen contents in the fuel and channel geometry (gap width, opening angle). Full article

This study investigates the effects of inorganic flame retardants, zinc borate, and magnesium hydroxide, on the thermal, morphological, flame retardancy, and mechanical properties of polypropylene (PP)/calcium carbonate composites for potential construction industry applications. Polypropylene/calcium carbonate (50 wt.%) composites containing 5 and 10 wt.% flame retardants were prepared using a batch mixer, followed by compression moulding. The results demonstrated enhanced thermal stability, with the highest char residue reaching 47.2% for polypropylene/calcium carbonate/zinc borate (10 wt.%)/magnesium hydroxide (10 wt.%) composite, a notably strong outcome. Additionally, the composite exhibited an elevated limited oxygen index (LOI) of 29.4%, indicating a synergistic effect between zinc borate and magnesium hydroxide. The proposed flame retardancy mechanism suggests that the flammability performance is driven by the interaction between the flame retardants within the polypropylene/calcium carbonate matrix. Magnesium hydroxide contributes to smoke suppression by releasing water, while zinc borate forms a protective glassy foam that covers the burning surface, promoting char formation and acting as a physical barrier to heat transmission and fire spread. Scanning electron microscopy confirmed good dispersion of the additives alongside calcium carbonate within the polymer matrix. Despite the addition of up to 10 wt.% flame retardants, the composites maintained high-notched impact strength. Full article

In studying the effects of oxygen concentration and molecular structure on the morphologies of the soot particles produced by hydrocarbon fuels, ethylene and ethane were chosen as experimental fuels. With a Gülde laminar coaxial diffusion flame device, a soot particle device was used to sample soot particles at different oxygen concentrations (21%, 24%, 26%, 28%, and 31%) and at different heights above a burner (HABs = 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm). High-resolution transmission electron microscopy (HRTEM) was used to scrutinize and analyze the soot particles at varying oxygen concentrations. The findings suggest that at the same oxygen concentration, ethylene produces brighter and taller flames. With an increase in the oxygen concentration, ethylene flames and ethane flames gradually decrease in height and become brighter. With an increase in the HAB, the average primary soot particle diameter (D_p) increases initially and then decreases, the fractal dimension (D_f) increases, and the aggregates transition from strips and chains to clusters. At the same flame height (HAB = 30 mm), the D_p decreases, the D_f increases, the carbon layer torsion resistance (T_f) and the carbon layer spacing (D_s) increase, and the carbon layer changes from a parallel arrangement to a curved arrangement to form denser network aggregations. Full article

Incorporating outstanding flame retardancy and electromagnetic interference shielding effectiveness (EMI SE) into polymers is a pressing requirement for practical utilization. In this study, we first employed the principles of microencapsulation and electrostatic interaction-driven self-assembly to encapsulate polyethyleneimine (PEI) molecules and Ti₃C₂T_x nanosheets on the surface of ammonium polyphosphate (APP), forming a double-layer-encapsulated structure of ammonium polyphosphate (APP@PEI@Ti₃C₂T_x). Subsequently, flame-retardant thermoplastic polyurethane (TPU) composites were fabricated by melting the flame-retardant agent with TPU. Afterwards, by using air-assisted thermocompression technology, we combined a reduced graphene oxide (rGO) film with flame-retardant TPU composites to fabricate hierarchical TPU/APP@PEI@Ti₃C₂T_x/rGO composites. We systematically studied the combustion behavior, flame retardancy, and smoke-suppression performance of these composite materials, as well as the flame-retardant mechanism of the expansion system. The results indicated a significant improvement in the interface interaction between APP@PEI@Ti₃C₂T_x and the TPU matrix. Compared to pure TPU, the TPU/10APP@PEI@1TC composite exhibited reductions of 84.1%, 43.2%, 62.4%, and 85.2% in peak heat release rate, total heat release, total smoke release, and total carbon dioxide yield, respectively. The averaged EMI SE of hierarchical TPU/5APP@PEI@1TC/rGO also reached 15.53 dB in the X-band. Full article

Urea–formaldehyde (UF) resin is the most widely used adhesive resin. However, it is necessary to improve its flame-retardant performance to expand its applications. In this study, exploiting electrostatic interactions, anionic phytic acid and cationic chitosan were combined to form a bio-based intumescent flame-retardant, denoted phytic acid–chitosan polyelectrolyte (PCS). The molecular structure of the urea–formaldehyde resin was optimized by crosslinking with melamine and plasticizing with polyvinyl alcohol-124. Thus, by combining PCS with the urea–formaldehyde resin and with ammonium polyphosphate and ammonium chloride as composite curing agents, flame-retardant urea–formaldehyde resins (FRUFs) were prepared. Compared to traditional UF resin, FRUF showed excellent flame retardancy and not only reached the UL-94 V-0 level, but the limit of oxygen index was also as high as 36%. Compared to those of UF, the total heat release and peak heat release rate of FRUF decreased by 86.44% and 81.13%, respectively. The high flame retardancy of FRUF originates from the combination of oxygen and heat isolation by the dense carbon layer, quenching of phosphorus free radicals, and dilution of oxygen by a non-flammable gas. In addition, the mechanical properties of the FRUF remained good, even after modification. The findings of this study provide a reference for the flame-retardant application of FRUF for applications in multiple fields. Full article

Polyethylene terephthalate (PET) foams have the characteristics of being lightweight and high strength, as well as offering good heat resistance, minimal water absorption, etc., and they have been widely used in the wind power field. In addition, they are being promisingly applied in automotive, rail, marine, construction, and other related fields. Therefore, the flame retardancy(FR) of PET foams is an issue that requires investigation. The addition of flame retardants would affect the chain extension reaction, viscoelasticity, and foamability of PET. In this study, zinc diethyl hypophosphite (ZDP) and decabromodiphenylethane (DBDPE) were used to form a synergistic FR system, in which ZDP is an acid source and DBDPE is a gas source, and both of them synergistically produced an expanded carbon layer to improve the flame retardancy of PET foams. The ratio of ZDP and DBDPE is crucial for the carbon yield and the expansion and thermal stability of the char layers. At the ZDP/DBDPE ratios of 9/3 and 7/5, the thickness of the char layers is about 3–4 mm, the limiting oxygen index (LOI) values of FR modified PET are 32.7% and 33.6%, respectively, and the vertical combustion tests both reached the V-0 level. As for the extruded phosphorous/bromine synergism FR PET foams, ZDP/DBDPE ratios of 3:1 and 2:1 were applied. As a result, the vertical combustion grade of foamed specimens could still reach V-0 grade, and the LOI values are all over 27%, reaching the refractory grade. Full article