定向金属氮化纯铝熔体表面Mg蒸气传质行为

从热力学角度分析并模拟了Mg蒸气在定向金属氮化法制备AlN材料中的相转变过程,证实了在高纯氮气气氛下Mg蒸气层的存在。按照气体扩散传质动力学原理,求出了Mg蒸气层的厚度关系式。研究表明:熔液距坩埚上沿距离、反应前沿的氧分压和初始Mg含量均影响着Mg蒸气层厚度。氧分压降低,初始镁含量的增加,Mg蒸气层厚度增加,有利于铝熔液的渗透氮化。

DIMOX工艺制备复合材料熔体表面Mg蒸气扩散动力学研究

从热力学角度分析了 Al-Mg-Si合金直接氧化生长 Al2 O3/Al复合材料工艺过程中 ,熔体表面 Mg蒸气层的存在 ,按照气体扩散传质动力学原理 ,解析了 Mg蒸气层的厚度。结果表明 :熔体上方 Mg蒸气层的厚度δ与熔体表面距坩埚上沿的距离 L成正比 ,坩埚上沿处 O2的摩尔分数增加 ,Mg蒸气层厚度减小。 Mg蒸气层厚度与材料自氧化生长速度密切相关。

高温球体与水的沸腾传热特性研究

为研究高温流体与水的传热特性,采用理论分析与自由界面追踪数值技术对高温球体表面的沸腾传热过程进行研究,得到球体表面传热特性的变化规律.分析结果表明,当球体温度不变时,蒸气膜层厚度自前滞点往后逐渐增大,膜层内温度呈非线性分布降低,速率呈抛物型分布.当球体温度提高时蒸气膜层厚度变大,相应的蒸气速率峰值也变大,球面传热系数却变小.数值仿真结果显示了气-液界面上不稳定性波动发展和气泡成长过程,较为真实地反映出沸腾传热的动态过程.

三谈汽化(初二、初三)

汽化是物质由液态变成气态的过程．液体的汽化过程有很多特点，在日常生活、工农业生产等方面有着广泛的应用．

Influence of intermetallic Al-Mn particles on in-situ steam Mg-Al-LDH coating on AZ31 magnesium alloy

The influence of intermetallic Al-Mn particles on the corrosion behavior of in-situ formed Mg-Al layered double hydroxide(Mg-Al-CO32--LDH)steam coating on AZ31 Mg alloy was investigated.The alloy was pretreated with H3PO4,HCl,HNO3or citric acid(CA),followed by hydrothermal treatment,for the fabrication of Mg-Al-LDH coating.The microstructure,composition and corrosion resistance of the coated samples were investigated.The results showed that the surface area fraction of Al-Mn phase exposed on the surface of the alloy was significantly increased after CA pretreatment,which promotes the growth of the Mg-Al-LDH steam coating.Further,the LDH-coated alloy pretreated with CA possessed the most compact surface and the maximum coating thickness among all the coatings.The corrosion current density of the coated alloy was decreased by three orders of magnitude as compared to that of the bare alloy.

Heat Transfer Characteristics of Dropwise Condensation of Steam on Vertical Polymer Coated Plates

The plasma polymerization method and dynamic ion-beam mixed implantation method were employed to coat ultra-thin polymer films on copper plates. Experiments indicated that steady dropwise condensation of steam at atmospheric pressure occurred. The condensation heat transfer coefficients increased by approximately 3 and 5－7 times for the polytrimethylvinylsilane film and polytetrafluoroethylene film respectively, compared with the value for film condensation under the same experimental conditions. The temperatures on the condensing surface and inside the test block were found to be rapidly and randomly fluctuated. The properties of the coated films and advantages of the methods used in this investigation were discussed briefly.

The role of graphene coating on cordierite-supported Pd monolithic catalysts for low-temperature combustion of toluene

In the present work,a Pd/graphene/cordierite(Pd/Gr/Cor)composite was prepared as a monolithic catalyst for low-temperature combustion of toluene.We mainly focused on understanding the role of graphene coating through investigation of catalytic performance and adsorption behavior of the composite.Compared with the traditional Pd/Cor catalyst without graphene coating,Pd/Gr/Cor catalyst delivered much higher activity and stability for toluene catalytic combustion in both dry and moist conditions.Transmission electron microscopy(TEM)and hydrophobic characterizations indicated that graphene coating can considerably improve the dispersity of Pd nanoparticles and enhance the hydrophobicity of the cordierite support.The adsorption behavior of the above two catalysts,including adsorption isothermal,adsorption kinetics,and adsorption thermodynamics were carefully investigated.The simulation results indicated that a large amount of toluene was adsorbed on graphene surface through relatively weak interaction,whereas only a relatively small amount of toluene was adsorbed on Pd surface with strong affinity.The adsorption thermal calculation indicated that the adsorption of toluene on graphene was a process with reduced entropy,indicating highly-ordered assembly of toluene molecular on graphene.It is the significant concentration and affinity gap between graphene and Pd that ensures a simultaneously and rapid transfer of toluene during the reaction process.

Cyclic oxidation behavior of Fe-9Cr-1Mo steel in water vapor atmosphere

The cyclic oxidation behavior of Fe-9Cr-1Mo steel (9Cr-1Mo) in 10%H2O+90%Ar (volume fraction) atmosphere at 600, 650 and 700 ℃ for various time was studied. The oxidation mechanism of 9Cr-1Mo steel in 10%H2O+90%Ar atmosphere was discussed. The thermal stress was evaluated in two oxide layers to illustrate the spallation of the oxide layer. The experimental results indicate that there exists a duplex oxide scale with an outer layer of Fe2O3 and an inner layer of mixed (Fe, Cr)3O4 formed on 9Cr-1Mo steel during cyclic oxidation. Some cracks generated in both inner and outer oxide layers. Parts of oxide scales spalled from substrate during the cyclic oxidation. A higher tensile stress in the oxide layer is formed at the early oxidation stage than at the later oxidation stage during heating. This tensile stress results in the formation of cracks in the oxide layer.

A perspective of microplasma oxidation (MPO) and vapor deposition coatings in surface engineering of aluminum alloys

Over the past years, great achievements have been made in the development of coating technologies for surface improvement of aluminum alloys. Despite these achievements, the role in the market strongly depends on the ability of surface coating technology under technical and economic considerations to meet the increased demands for heavy tribological applications of aluminum alloys. Microplasma oxidation (MPO) technology has recently been studied as a novel and effective means to provide thick and hard ceramic coating with improved properties such as excellent load-bearing and wear resistance properties on aluminum alloys. The present work covers the evaluation of the performances of current single and duplex coatings combining MPO, physical vapor deposition (PVD), and plasma assisted chemical vapor deposition (PACVD) coatings on aluminum alloys. It suggests that the MPO coating is a promising candidate for design engineers to apply aluminum alloys to heavy load-bearing applications. The prospective future for the research on MPO coatings is introduced as well.

Effects of CeO_2 coating on oxidation behavior of T91 steel in water vapor

Oxidation behaviors of blank and CeO2 coated T91 steel were investigated at 600 ℃ in water vapor for up to 150 h. Gold marker was used to define the mass transport direction. The oxide scales were studied with X-ray diffractometry （XRD）, scanning electron microscopy （SEM） and electron probe microanalyzer （EPMA）. The oxidation resistance of the steel is improved by CeO2 coating, though the improvement is not remarkable. Ce-rich oxide band is located at the interface of the inner equiaxed layer and the outer columnar layer after oxidation, which is not consistent with the original surface. The results show that outward iron transport is blocked by the Ce-rich band. A new oxide nucleating and growing site （reaction front） is induced at the inner surface of the Ce rich band.

Meteorites Penetration into the Dense Layers of the Earth＇s Atmosphere： Physical Mechanism behind the Explosive Phenomena

This discussion paper is an attempt to consider new ideas about the nature of explosive phenomena of the meteorite bodies in the Earth＇s atmosphere. Authors attract approaches, based on the concepts of physics of combustion, explosion and detonation in order to explain the explosive collapse of the meteorite. Authors believe that the meteorite explosion may be the result of gas-detonation mechanism of overheated meteoric bodies explosive boiling-up （the ＂vapor explosion＂）, accompanied by the formation of a supersonic front of shock wave. The considered hypothesis regarding the role of the ＂vapor explosion＂ in geological disasters can be used to explain the mechanisms of explosive phenomena in Volcanology： （1） With respect to the description of nature of phreatic eruptions; （2） for interpretation of dynamic regularities of the volcano crater opening and the subsequent shock-wave emission of steam-gas ＂fountain＂ to a height of several kilometers with fragments of magma and rocks as a result of the ＂vapor explosion＂ of overheated magma mass under the dome of the volcano.