Search

Microgravity experiments of polyethylene (PE) droplet combustion were conducted by a 3.6-s drop tower with the gravity level of 10−3~10−4 g to investigate the burning behaviors and fire hazards of molten thermoplastics in the spacecraft. Pre-ignited droplets with a diameter of about 3 mm were continually generated and detached from burning PE tubes. Once the drop capsule started free-fall, droplets entered the microgravity environment with an initial velocity of 10–35 cm/s (Stage I). A comet-shape flame with an intense bubbling and ejecting process of the moving droplet was observed, and the burning-rate constant (K) was found around 2.6 ± 0.3 mm2/s. After the droplet landed on the floor, it could rebound with a near-zero velocity, showing as a spherical flame (Stage II). The combustion of PE droplet followed the classical d -square law with K = 1.3 ± 0.1 mm2/s. The measured large burning-rate constant (or the volume shrinkage rate) of the moving droplet was caused by the robust bubbling process, which reduced the bulk density of molten PE and ejected unburnt fuel (about 25% of total mass loss). However, the actual mass burning rate of the PE droplet should be smaller than most hydrocarbon liquids because of a smaller mass-transfer number (B ≈ 2). The flame burning rate of PE droplet is 4 ± 1 g/m2-s per unit flame-sheet area that may be used to estimate the fuel mass-loss rate and fire heat release rate in microgravity. This novel microgravity combustion experiment on the thermoplastic droplet could expand the physical understanding of fire risk and hazard of plastic material in the spacecraft environment. [ABSTRACT FROM AUTHOR]

The enriched oxygen ambient may be applied to China's next generation space station. To understand the fire behaviors under oxygen-enriched microgravity environment, flame-spread experiments on extruded poly(methyl)methacrylate (PMMA) rods with 10-mm diameter were conducted in the SJ-10 Satellite. The opposed flame-spread behaviors were studied at the oxygen-enriched ambient (33.5% and 49.4%) under low flow velocities in the range of 0 to 12 cm/s. After the ignition in the middle of the sample, an opposed flame spread was achieved, rather than the forward flame spread. The flame-spread rate increases with the opposed flow velocity, due to the decreased flame width and the enhanced flame heat flux. Moreover, a blue flame sheet with a frequent burst of bubbles is found throughout the opposed-flow spread process, showing a near extinction behavior. For the oxygen concentration above 25%, normal-gravity experiments suggest that whether PMMA is cast or extruded should have a negligible effect on the opposed flame spread in microgravity. Compared to normal gravity, the microgravity flame spread rate in the oxygen-enriched atmosphere is slower which is the order of 0.1 mm/s, only one-tenth to one-fifth of that in normal gravity at the same nominal opposed flow velocity, and the acceleration of flame spread in microgravity by increasing oxygen concentration is also much smaller. This result suggests that (1) if the environmental gas flow is small, the fire hazard increased by raising oxygen level in microgravity space cabin can be much smaller than that on Earth; and (2) the fire risk of oxygen-enriched microgravity environment might be overestimated when a ground-based test method is employed to evaluate the burning characteristics of solid material. [ABSTRACT FROM AUTHOR]

Amorphous zinc-tin oxide ( a -ZTO) films are fabricated using a combustion solution process. Utilization of a -ZTO films as the novel active layers, ultraviolet (UV) and ethanol sensors are prepared for the first time. The sensor performances are strongly related to the film resistance, which is determinated by the surface O 2 − formed by O 2 + e = O 2 − during UV and ethanol detections. During UV exposure, the resistivity of a -ZTO films decreases from 6.50 × 10 5 Ω cm to 56.85 Ω cm, which are very sensitive towards UV (365 nm) light and the photodetectors own a sensitivity value of 650 from 0 to 30 V. While for ethanol gas, the detection regions can be in a wide range from 20 ppm to 500 ppm. The gas response can achieve a value of 31.18 at 500 ppm ethanol and good response/recovery speed (6 s and 3 s), which can be comparable with that of the common used nanomaterials. The novel a -ZTO film based UV and gas sensors are very promising researches for future UV and gas sensor applications. [ABSTRACT FROM AUTHOR]

A Horizontal Channel Apparatus (HCA) has been employed to investigate opposed flame spread over solid fuels in reduced buoyancy environments, which would be encountered at the diminished gravitational levels on extraterrestrial surfaces (e.g., the partial gravity on the Moon and Mars) as well. With a scaling analysis, the characteristic buoyant flow velocity in HCA is well correlated to the channel height and the Rayleigh (Ra) number. The experimental results show that, without forced flow, the flame spread rate and flammability in HCA can be correlated with those at different gravity levels by using Ra number, indicating HCA may have the ability to simulate the burning characters in partial gravity. Together with the Reynolds (Re) and Ra numbers , which signify the role of forced and buoyant flow, respectively, the flame spread rates and the map of flammability of the solid fuel obtained in the current HCA experiments are determined. Furthermore, four distinctively different flame spread patterns are found at various channel heights and opposed-flow velocities once the width of the cellulosic sample is extended. Likewise, the map of flame spread patterns is governed by the Re and Ra. It is further found through the examinations of the time average of the mass-loss rate that as long as Ra is below a critical value, the opposed flow in terms of Re mainly determines the oxygen transportation, while the reduced buoyancy in terms of Ra mainly contributes to the bifurcations of the potential hydro/thermal instabilities. [ABSTRACT FROM AUTHOR]

This work studies the flame spread over horizontal thick PMMA (polymethyl methacrylate) rods with three radii under different oxygen concentrations and upward cross flow velocities. The flame spread rate and the limit oxygen concentration are measured. Far away from the extinction limit, the flame spread rate increases with the flow velocity but decreases with radius. Near the extinction limit, the flame spread rate is insensitive to the flow velocity and radius. The flame spread rate can be correlated by the stretch rate, and it is found that the flame spread rates for different radii are close at the same stretch rate. A scaling analysis shows that the flame spread rates are approximately square-root dependent on the stretch rate. Prior to the extinction, the flame has entered the regressive burning regime where the flame leading edge will not spread forwardly but continuously retreat. The flame extinction is dependent on the local stretch rate and in-depth heat conduction. For a given radius, the limit oxygen concentration increases with the stretch rate. For a given stretch rate, the smaller cylinder can sustain at lower oxygen concentration due to the less solid-phase heat loss. [ABSTRACT FROM AUTHOR]

Amorphous silicon-zinc-tin-oxide (α-SZTO) thin films were prepared, and their properties were investigated physically and electrically, with an emphasis on the Si effects. An appropriate Si content in the matrix can not only achieve stable and dense films, but also suppress the formation of oxygen vacancies efficiently, due to its high oxygen bonding ability. Thin film transistors (TFTs) with α-SZTO active channel layers exhibited a field-effect mobility of around 1 cm2V-1s-1, an on/off current ratio of 107, and a subthreshold swing of 0.863 V/decade with a good long-term stability. The α-SZTO TFT is a potential candidate for electronic applications. [ABSTRACT FROM AUTHOR]

Texture surfaces of ZnO:Al (AZO) films are prepared by ammonium acetate, a neutral etching agent. The structural, optical, and electrical properties of AZO films are investigated in detail for improving the light trapping ability. Under the optimal etching conditions (e.g., solution concentration of 5.0 wt% and etching time of 20 min), AZO films have a typical crater-like structure with haze values in the 20–30% range, as well as a high visible transparence and electrical conductivity. A feasible mechanism model is proposed to explain the etching process. Both H + and OH − hydrolyzed in solution are involved in the etching reaction. The etching reaction and product diffusion co-dominate the process to form crater-like structures. In ammonium acetate, the etching process is much mild and controllable, making it easy to generate homogeneous texture morphologies in a large area. The high conversion efficiency of 10.75% is achieved for single-junction amorphous silicon thin-film solar cells using textured AZO films as the front contact layers. The ammonium acetate is an ideal etchant for preparing textured AZO films for commercial applications in solar cells. [ABSTRACT FROM AUTHOR]

We investigate the electrical properties of ZnO thin-film transistors (TFTs) under the ultraviolet (UV) light illumination to clarify the device reliability. The ON/OFF current ratio of ZnO TFTs shows an evident reduction due to the creation of UV photo-induced current. This behavior degradation becomes severe for devices exposed to a short-wavelength UV light. The persistent photoconductivity is observed in ZnO TFTs under the UV illumination with a much slower decay for the ON-state than the OFF-state, which is attributed to the reduced recombination under a positive gate bias. The UV light instabilities of ZnO TFTs can be explained on the basis of UV photoexcited oxygen vacancies. A feasible mechanism model is proposed in this paper. These observations and analytical model not only provide useful physical insight into the device behaviors, but also offer basic design guideline for oxide TFTs. [ABSTRACT FROM PUBLISHER]

The future generation of inhabited spacecraft will have a significantly different cabin environment from the present ones, characterized by low pressure and elevated oxygen concentration. This new atmosphere and the low-velocity gas flows in microgravity provide distinct conditions for the combustion of the solid materials used, and their influence on material flammability is of particular interest in the fire safety of spacecraft. Experiments have been conducted to investigate the effects of sub-atmospheric pressure and low flow velocity on the opposed flame spread and extinction behaviors over a thick PMMA. A flammability map was constructed that delineates the uniform regime, the flamelet regime, and extinction limits for thick PMMA under sub-atmospheric pressures. The limiting oxygen concentration increases with the reduced ambient pressure at a fixed opposed flow, while the flamelet regime becomes wider. Under low ambient pressure, the flame spread rate increases with the flow velocity, but the increasing rate slows down. At a constant oxygen concentration, the flame spread rate increases with the ambient pressure and gas-phase conduction dominates flame spread. At a constant oxygen partial pressure, the higher ignition temperature and less gas-phase conduction reduce the flame spread rate synchronously with the increased pressure. [ABSTRACT FROM AUTHOR]