Growth and characterization of Ag–Al2O3 composites thin films for thermoelectric power generation applications

Thin films of Ag–Al₂O₃ composites were successfully grown on Si substrate by thermal evaporation method and their thermoelectric performance was modulated using post growth annealing technique. Pellet of Ag and Al mixture having 1:4 ratio was evaporated on Si substrate using the vacuum tube furnace. As grown sample was cut into pieces and post-growth annealing was performed at different temperatures using muffle furnace. XRD results suggested that as-deposited sample has amorphous nature, but crystallinity of the samples increase as an annealing temperature increase from 600 to 900^oC. This structural behavior of annealed samples was further verified by Raman spectroscopy measurements. We have reported an optimal annealing temperature (800 ⁰C) for the best thermoelectric performance of investigated composites. At this specific annealing temperature, charge carriers are highly mobile which resulted in the enhancement of thermoelectric power generation performance of Ag–Al₂O₃ composite. The value of power factor (1.38x10^?2 W/m-K^?2) reported in the current study is the highest value for Ag–Al₂O₃ composites so for reported in the literature according to the best of our knowledge.     

Stabilization of natural gas foams using different surfactants at high pressure and high temperature conditions

Natural gas foam can be used for mobility control and channel blocking during natural gas injection for enhanced oil recovery, in which stable foams need to be used at high reservoir temperature, high pressure and high water salinity conditions in field applications. In this study, the performance of methane (CH₄) foams stabilized by different types of surfactants was tested using a high pressure and high temperature foam meter for surfactant screening and selection, including anionic surfactant (sodium dodecyl sulfate), non-anionic surfactant (alkyl polyglycoside), zwitterionic surfactant (dodecyl dimethyl betaine) and cationic surfactant (dodecyl trimethyl ammonium chloride), and the results show that CH₄-SDS foam has much better performance than that of the other three surfactants. The influences of gas types (CH₄, N₂, and CO₂), surfactant concentration, temperature (up to 110°C), pressure (up to 12.0 MPa), and the presence of polymers as foam stabilizer on foam performance was also evaluated using SDS surfactant. The experimental results show that the stability of CH₄ foam is better than that of CO₂ foam, while N₂ foam is the most stable, and CO₂ foam has the largest foam volume, which can be attributed to the strong interactions between CO₂ molecules with H₂O. The foaming ability and foam stability increase with the increase of the SDS concentration up to 1.0 wt% (0.035 mol/L), but a further increase of the surfactant concentration has a negative effect. The high temperature can greatly reduce the stability of CH₄-SDS foam, while the foaming ability and foam stability can be significantly enhanced at high pressure. The addition of a small amount of polyacrylamide as a foam stabilizer can significantly increase the viscosity of the bulk solution and improve the foam stability, and the higher the molecular weight of the polymer, the higher viscosity of the foam liquid film, the better foam performance.     

煤炭地下气化过程中温度场及其传热特征研究进展

煤炭地下气化(UCG)技术作为一种环境友好的采煤方法,可被用来开发深部煤层与矿井遗留的煤炭资源。当地下气化时,随着气化时间的推移,煤炭在煤层内部"燃烧"逐渐形成气化炉,炉内温度最高可达1 200℃。从UCG过程中的放热反应出发,综述了热量来源、温度场的变化及其传热特征等,总结了研究温度场的重要手段。分析认为,UCG是一个复杂的温热动态变化过程,在此过程中氧化还原反应、吸热和放热的可逆反应同时进行,导致了气化炉的温度场及其传热机制的复杂性。研究气化过程传热机制的方法主要有物理模拟、试验监测、理论计算和数值模拟4种,各自具有不同的适应性。其中,物理模拟的可操控性强,但是难以解决地层接触关系导致的接触热阻误差;试验监测能真实反映温度场等的变化情况,但是实际操作的经济性差;理论计算通过对气化过程的拆分,从理论角度定性定量了UCG的温度场特征,但理论计算难以考虑到温度渗流等对温度的影响;数值模拟借助计算机进行温度场的多场耦合,综合考虑了渗流、变形、温度等对传热的影响,但多场的物理耦合计算难度较大,且煤层及岩石的热物理性质随温度变化会产生较大的差异,导致数值建模的计算难度增加,制约了数值模拟方法的...     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

Research and development of liquid hydrogen (LH2) temperature monitoring system for marine applications

Monitoring the temperature in liquid hydrogen (LH₂) storage tanks on ships is important for the safety of maritime navigation. In addition, accurate temperature measurement is also required for commercial transactions. Temperature and pressure define the density of liquid hydrogen, which is directly linked to trading interests. In this study, we developed and tested a liquid hydrogen temperature monitoring system that uses platinum resistance sensors with a nominal electrical resistance of approximately 1000 Ω at room temperature, PT-1000, for marine applications. The temperature measurements were carried out using a newly developed temperature monitoring system under different pressure conditions. The measured values are compared with a calibrated reference PT-1000 resistance thermometer. We confirm a measurement accuracy of ±50 mK in a pressure range of 0.1 MPa–0.5 MPa.     

Impact of air contamination by NOx on the performance of high temperature PEM fuel cells

High temperature PEM fuel cells show enhanced tolerances regarding fuel impurities like CO for use in various applications. However, the impact of air impurities like NO_x on the cell behavior is not completely understood yet. This study provides systematic investigation during 500 h of operation in presence of cathode air containing 10 ppm NO or NO₂. Nitrogen oxides provoke a strongly and linearly decreasing voltage of 245.3 ± 18.5 μV h⁻¹ and highly comparable damage that verifies similar HT-PEMFC degradation via both oxides. Cyclic voltammetry and electron microscopy reveal the loss of electrochemical catalyst surface by selectively poisoned surface and enforced catalyst particle growth. Impedance spectroscopy reveals besides increased electrode charge transfer resistances an affected proton conductivity. In contrast, SO₂/NO₂ impurity mixture in real occurring ratio causes less voltage decay due to a positive SO₂ impact through H₂SO₄ formation causing further shown and discussed effects like nitrate formation and discharge.     

Novel composite membrane based on zirconium phosphate-ionic liquids for high temperature PEM fuel cells

Composite membranes composed of zirconium phosphate (ZrP) and imidazolium-based ionic liquids (IL), supported on polytetrafluoroethylene (PTFE) were prepared and evaluated for their application in proton exchange membrane fuel cells (PEM) operating at 200 °C. The experimental results reported here demonstrate that the synthesized membrane has a high proton conductivity of 0.07 S cm^?1, i.e, 70% of that reported for Nafion. Furthermore, the composite membranes possess a very high proton conductivity of 0.06 S cm^?1 when processed at 200 °C under completely anhydrous conditions. Scanning electron microscopy (SEM) images indicate the formation of very small particles, with diameters in the range of 100–300 nm, within the confined pores of PTFE. Thermogravimetric analysis (TGA) reveals a maximum of 20% weight loss up to 500 °C for the synthesized membrane. The increase in proton conductivity is attributed to the creation of multiple proton conducting paths within the membrane matrix. The IL component is acting as a proton bridge. Therefore, these membranes have potential for use in PEM fuel cells operating at temperatures around 200 °C.     

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming in a tubular fixed-bed reactor

The effect of non-uniform temperature on the sorption-enhanced steam methane reforming (SE-SMR) in a tubular fixed-bed reactor with a constant wall temperature of 600 °C is investigated numerically by an experimentally verified unsteady two-dimensional model. The reactor uses Ni/Al₂O₃ as the reforming catalyst and CaO as the sorbent. The reaction of SMR is enhanced by removing the CO₂ through the reaction of CaO + CO₂ → CaCO₃ based on the Le Chatelier's principle. A non-uniform temperature distribution instead of a uniform temperature in the reactor appears due to the rapid endothermic reaction of SMR followed by an exothermic reaction of CO₂ sorption. For a small weight hourly space velocity (WHSV) of 0.67 h^?1 before the CO₂ breakthrough, both a low and a high temperature regions exist simultaneously in the catalyst/sorbent bed, and their sizes are enlarged and the temperature distribution is more non-uniform for a larger tube diameter (D). Both the CH₄ conversion and the H₂ molar fraction are slightly increased with the increase of D. Based on the parameters adopted in this work, the CH₄ conversion, the H₂ and CO molar fractions at D = 60 mm are 84.6%, 94.4%, and 0.63%, respectively. After CO₂ breakthrough, the reaction of SMR dominates, and the reactor performance is remarkably reduced due to low reactor temperature.For a higher value of WHSV (4.03 h^?1) before CO₂ breakthrough, both the reaction times for SMR and CO₂ sorption become much shorter. The size of low temperature region becomes larger, and the high temperature region inside the catalyst/sorbent bed doesn't exist for D ≥ 30 mm. The maximum temperature difference inside the catalyst/sorbent bed is greater than 67 °C. Both the CH₄ conversion and H₂ molar fraction are slightly decreased with the increase of D. However, this phenomenon is qualitatively opposite to that for small WHSV of 0.67 h^?1. The CH₄ conversion and H₂ molar fraction at D = 60 mm are 52.6% and 78.7%, respectively, which are much lower than those for WHSV = 0.67 h^?1.     

Validating the efficiency of γ-Al2O3/La0.6Sr0.4Co0.2Fe0.8O3-δ double-layer electrolyte for low temperature solid oxide fuel cell

Perovskite La_0.6Sr_0.4Co_0.2Fe_0.8O_3+δ (LSCF) as a promising cathode material possessed overwhelming electronic conduction along with certain ionic conductivity. Its strong electron conduction capability hinder the application of pure-phase LSCF as electrolyte in semiconductor membrane fuel cell (SMFC). In order to constrain the electron transport and take advantage of the decent ion conduction of LSCF, a thin layer of γ-Al₂O₃ with insulating property was added as an electron barrier layer and combine with LSCF to form a two-layer structure electrolyte. Through adjusting the weight ratio of LSCF/γ-Al₂O₃ to optimize the thickness of double layers, an open circuit voltage of 0.98 V and a maximum power density of 690 mW/cm² was received at 550 °C. At the same time, SEM, EIS and other characterization technology had proven that the LSCF/γ-Al₂O₃ bi-layer electrolyte can work efficiently at low temperature. The advantage of this work is the application of double-layer (γ-Al₂O₃/LSCF) structure electrolyte to instead of mixed material electrolyte in low-temperature solid oxide fuel cells. Structural innovation and the using of insulating materials provided clues for the further development of SMFC.     

Barium promoted manganese oxide catalysts in low-temperature methane catalytic combustion

In this study, a series of BaO-MnO_x mixed oxide catalysts were synthesized by the mechanochemical method and employed in lean methane catalytic combustion (MCC) at low temperatures. The synthesized catalysts were characterized by XRD, BET, TGA, FT-IR, H₂-TPR, O₂-TPD, and FESEM analyses. The results indicated that the 10 wt% BaO-MnO_x catalyst with a BET surface area of 25 m² g^?1 possessed the best catalytic performance. The higher activity of the 10 wt% BaO-MnO_x catalyst was due to the higher ability to supply oxygen through the components during the MCC process. The light-off temperature corresponding to 50% of the methane conversion was about 330 °C, which was about 50 °C lower than the pure MnO_x. Moreover, for the BaO(10)-MnO_x catalyst, the 10 and 90% of methane conversion temperatures were about 305 and 427 °C, respectively. Also, the 10 wt% BaO-MnO_x catalyst exhibited high catalytic stability under dry feed condition at 450 °C for 50 h. Furthermore, the influence of various parameters such as calcination temperature, feed ratio, GHSV, pretreatment condition, and presence of water vapor in the feedstock was studied on the catalytic performance.