Experimental investigation on the self-ignition of pressurized hydrogen released by the failure of a rupture disk through tubes

Hydrogen is expected to be used as a clean energy carrier. However, when high-pressure hydrogen is suddenly released into the air through tubes, self-ignition can occur by a diffusion ignition mechanism. In this paper, the phenomena of self-ignition and flame propagation during the sudden release of high-pressure hydrogen were investigated experimentally. Experimental results show that self-ignition can occur when bursting pressure is sufficiently high in spite of the shortness of the tube. For example, self-ignition was observed at a bursting pressure as high as 23.5 MPa with 50 mm long tube. When self-ignition successfully occurs, a hydrogen jet flame is produced by the ignition. The flame is then stabilized at the tube outlet. From photodiode signals and flame images, the propagation of a flame inside the tube is confirmed and the flame is detected near the rupture disk as the bursting pressure increases. When the tube length is not long enough to produce self-ignition, a hydrogen flame is observed in the only boundary layer at the end of tube and it quenches after the flame exits the tube. Consequently, the formation of a complete flame across the tube is important to initiate self-ignition, which sustains a diffusion flame after jetting out of the tube into the air. Also, in order to establish a complete flame across the tube, it is necessary to have sufficient length such that the mixing region is generated by multi-dimensional shock–shock interactions.     

Theory of grain boundary motion during high-temperature DIGM

A theory of diffusion induced grain boundary migration (DIGM) is presented for high temperatures where volume diffusion of solute atoms out of the grain boundary is important. It is shown that due to the presence of a gradient term in the expression for the free energy of solid solution, even a relatively small discontinuity in the solute distribution across the gain boundary provides enough driving force for grain boundary migration. From the expression obtained for the grain boundary velocity the coefficient for the Ni diffusion across the grain boundaries in a Cu(Ni) polycrystal has been estimated.     

Classical trajectory study of the dissociation of hydrogen on copper single crystals: II. Cu(100) and Cu(110)

We report a second series of classical trajectory calculations for the activated dissociation of hydrogen on copper single crystals. We have modified the empirical interaction potential surface used previously by raising the energy requirements for dissociation, thereby lowering the dissociation probabilities and bringing their magnitudes in line with experiment. Dissociation probabilities on both a Cu(100) and Cu(110) surface for H₂ and D₂ are computed. The results, while in reasonable magnitude agreement with experiment, still do not fit the experimental translational energy dependence well and do not predict the correct isotope effect for Cu(110). A detailed comparison of the differences between the ground and first vibrationally excited states for the (100) and (110) surfaces suggest that within the approximations of this model calculation, the potential surfaces employed have underestimated the effects of surface roughness in determining the incident energy and angle dependence of the dissociation probabilities.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

The surface chemistry of H2O on clean and oxygen-covered Cu(110)

Adsorption of water at 100 K. on clean and oxygen-covered Cu(110) has been studied using UPS, TDS, Δφ and LEED measurements. The results indicate that two-dimensional hydrogenbonded islands are formed on the clean surface. The long-range order in these islands is in registry with the substrate lattice and gives rise to a c(2×2) LEED pattern. Upon the formation of multilayer ice, the ordering disappears. The presence of oxygen on the surface disrupts the hydrogen bonding, and composite oxygen-water layers are formed. A model of the arrangement of oxygen atoms and water molecules is presented, based upon the LEED observations for these layers and an estimate of the relative oxygen and water coverages. The intensity variation of a thermal desorption peak at 290 K, attributed to adsorbed OH species, with oxygen coverage is in accordance with this model. For low oxygen coverages, the TDS and Δφ results indicate that small oxygen-water clusters with an enhanced ratio of water molecules per adsorbed oxygen atom are present.     

Mechanism of Cu transport along clean Si surfaces

Cu diffusion along clean Si(111), (110) and (100) surfaces are investigated by Auger electron spectroscopy and low energy electron diffraction. The effective diffusion coefficients of copper are measured in the temperature range from 500 to 650°C. It is shown that the Cu transport along silicon surface occurs by the diffusion of Cu atoms through Si bulk and the segregation of Cu atoms to the surface during the diffusion process. It is found that the segregation coefficients of Cu to silicon surface during the diffusion process depend on surface orientation.     

利用Pr_(70)Cu_(30)晶界扩散改善烧结钕铁硼废料矫顽力的研究

钕铁硼磁体制备过程中出现的部分块体废料由于矫顽力较低,性能难以满足使用要求.本文主要通过晶界扩散技术来提高废料磁体的矫顽力.采用Pr_(70)Cu_(30)合金作为扩散介质,对烧结钕铁硼废料磁体进行了晶界扩散处理,研究了扩散温度、扩散时间和回火时间对扩散后的磁体性能的影响.结果显示,800℃下扩散3 h,磁体的矫顽力从原来的7.88 kOe(1 Oe=79.5775 A/m)提升至11.55 kOe,提升幅度为46.6%,同时剩磁没有明显降低.扩散后回火对矫顽力的提升有一定的作用.800℃下扩散4h后的磁体在500℃回火3h后,最高矫顽力可达11.97 kOe,比原磁体废料提高了51.9%,接近成品磁体的水平.显微组织分析证实了晶界扩散的作用.扩散处理后的磁体中,主相晶粒间形成了连续晶间相,起到有效的磁隔离作用,有利于矫顽力的提高.研究还发现,Pr_(70)Cu_(30)晶界扩散虽然可以使磁体腐蚀电位上升,但也会增加腐蚀电流密度,不利于磁体抗腐蚀性的改善.本文工作对于提高材料的成品率具有重要意义.     

A study of H2S adsorption kinetics on the clean and oxygen covered (110) surface of copper

The very low pressure adsorption kinetics of H₂S on the clean and oxygen covered Cu(110) face have been examined by Auger Electron Spectroscopy (AES) and Mirror Electron Microscopy (MEM, used for continuous surface potential variations of the copper surface). The AES experimental curves on the clean copper face have been interpreted using a model of island growth by surface diffusion. The presence of an adsorbed oxygen layer on the copper surface changes notably the induction times observed on both AES and MEM measurements.     

Theoretical studies on the adsorption and decomposition of H2O on Pd(1 1 1) surface

To provide information about the chemistry of water on Pd surfaces, we performed density functional slab model studies on water adsorption and decomposition at Pd(1 1 1) surface. We located transition states of a series of elementary steps and calculated activation energies and rate constants with and without quantum tunneling effect included. Water was found to weakly bind to the Pd surface. Co-adsorbed species OH and O that are derivable from H₂O stabilize the adsorbed water molecules via formation of hydrogen bonds. On the clean surface, the favorable sites are top and bridge for H₂O and OH, respectively. Calculated kinetic parameters indicate that dehydrogenation of water is unlikely on the clean regular Pd(1 1 1) surface. The barrier for the hydrogen abstraction of H₂O at the OH covered surface is approximately 0.2-0.3 eV higher than the value at the clean surface. Similar trend is computed for the hydroxyl group dissociation at H₂O or O covered surfaces. In contrast, the O-H bond breaking of water on oxygen covered Pd surfaces, H₂O_ad + O_ad → 2OH_ad, is predicted to be likely with a barrier of ∼0.3 eV. The reverse reaction, 2OH_ad → H₂O_ad + O_ad, is also found to be very feasible with a barrier of ∼0.1 eV. These results show that on oxygen-covered surfaces production of hydroxyl species is highly likely, supporting previous experimental findings.     

A spectroscopic study of the adsorption and reactions of methanol,formaldehyde and methyl formate on clean and oxygenated Cu(110) surfaces

The adsorption and reaction of methanoi (CH₃OH), methyl formate (CH₃OCHO) and formaldehyde (H₂CO) on clean and oxygen-covered Cu(110) surfaces has been studied with EELS, UPS and thermal desorption spectroscopy (TDS). The clean surface is relatively unreactive but adsorbed oxygen readily attacks the hydroxyl proton and formyl carbon atoms to generate the intermediate methoxy (CH₃O) and formate (HCOO). Methyl formate is split into two intermediates, methoxy and formate. By correlating the three techniques we analyse (a) the condensed multilayer at 90 K; (b) the weakly bound molecular monolayer states prior to dissociation or reaction and (c) the reactive intermediates at higher temperatures. Formaldehyde forms the surface polymer polyoxymethylene [(CH₂O)_n] in the monolayer on Cu(110) which subsequently reacts with oxygen to generate formate. No molecular formaldehyde was observed above 120 K. Correlation of the EELS and UPS results for polyoxymethylene shows that an earlier interpretation by Rubloff et al. [Phys. Rev. B14 (1976) 1450] of anomalous shifts in the formaldehyde UPS spectrum on surfaces is incorrect, and due simply to the new polymeric structure of surface formaldehyde. Methyl formate coordinates to copper via the carbonyl lone pair orbital and methanol via the oxygen lone pair orbital. No evidence was found for methyl formate synthesis by dimerization of formaldehyde (the Tischenko reaction) or dehydrogenation of methanol on the clean Cu(110) surface. These latter reactions are facile over copper catalysts at atmospheric pressure. The success of the oxidation experiments and the failure of the synthesis reactions in UHV is a consequence of the pressure dependence of the equilibrium constants for the different reactions. As found previously in Fischer-Tropsch studies, condensation reaction equilibria are pressure dependent and product formation is considerably suppressed at UHV pressures.     

Role of adsorption on surface composition of Pd–Cu nanoparticles

Monte-Carlo (MC) simulation is used to study the role of adsorption of hydrogen, oxygen and carbon monoxide (CO) on the surface composition and surface bond geometry of Pd–Cu nanoparticles. For clean particles the surface is found to be enriched in Cu. But in the presence of adsorbed hydrogen and CO there is a segregation reversal from Cu segregation at low coverage to Pd segregation at high coverage. In the presence of adsorbed oxygen, on the contrary, the extent of Cu segregation increases with coverage. For a 586-atom nanoparticle with 50% Pd in the bulk the corner sites are found to be occupied by Cu atoms up to one monolayer adsorption. But, while the occupancy of 7, 8 and 9-coordinated sites by Cu atoms decreases with increase of H and CO coverage, for oxygen adsorption this occupancy increases with coverage. The relevance of such results in catalysis studies is discussed.     

Grain boundary diffusion of Cu in TiN film by X-ray photoelectron spectroscopy

In this study, the grain boundary diffusion of Cu through a TiN layer with columnar structure was investigated by X-ray photoelectron spectroscopy (XPS). It was observed that Cu atoms diffuse from the Cu layer to the surface along the grain boundaries in the TiN layer at elevated temperature. In order to estimate the grain boundary diffusion constants, we used the surface accumulation method. The diffusivity of Cu through TiN layer with columnar structure from 400 °C to 650 °C is D_b≈6×10⁻¹¹exp(−0.29/(k_BT )) cm²/s. Received: 18 May 1999 / Accepted: 8 September 1999 / Published online: 23 February 2000     

第一性原理研究Zn偏析对CuΣ5晶界的影响

采用基于密度泛函理论的第一性原理方法研究了Zn偏析Cu晶界的原子构型和电子结构,分析了Zn偏析对Cu晶界力学性能的影响.结果表明,Zn以替换方式偏析到晶界处,Zn—Cu与Cu—Cu的成键方式类似,均为含有共价成分的金属键.Zn偏析导致少量电荷集聚于Zn与近邻Cu之间,有限地增强了晶界的结合.拉伸过程中Zn的d轨道定域性增强,Zn与近邻Cu间的电荷密度下降,削弱了Zn—Cu键,导致晶界断裂发生在Zn—Cu间.     

Studies on interdiffusion in Pd/Mg/Si films: Towards improved cyclic stability in hydrogen storage

The paper reports the diffusion coefficients of grain boundary diffusion and grain boundary assisted lattice diffusion of Pd in Mg in Pd/Mg/Si system, a useful material for hydrogen storage, at 473 K in vacuum. The grain boundary diffusivity is measured by Whipple model and grain boundary assisted lattice diffusivity by plateau rise method using Pd depth profiles constructed by Rutherford backscattering spectrometry. It is established that grain boundary diffusivities are about six orders of magnitude faster than lattice diffusivities. Fine grained microstructure of Pd film, high abundance of defects in Mg film and higher stability associated with Pd-Mg intermetallics are responsible for the diffusion of Pd into grain boundaries and subsequently in the interiors of Mg. Besides the indiffusion of Pd, annealing also brings about an outdiffusion of Mg into Pd film. Examination by nuclear reaction analysis involving ²⁴Mg(p,p′γ)²⁴Mg resonance reaction shows the occurrence of Mg outdiffusion. Minimization of surface energy is presumably the driving force of the process. In addition to Pd/Mg interface, diffusion occurs across Mg/Si (substrate) interface as well on increasing the annealing temperature above 473 K. These studies show that dehydrogenation of films accomplished by vacuum annealing should be limited to temperatures less than 473 K to minimize the loss of surface Pd, the catalyst of the hydrogen absorption-desorption process and Mg, the hydrogen storing element, by way of interfacial reactions.     

Grain Boundary Diffusion and Mechanisms of Creep of Nanostructured Metals

Comparative investigations of diffusion in coarse-grained (d = 20 μm), nanocrystalline (d = 0.04 μm) and nanostructured nickel (d = 0.3 μm) have been carried out in a temperature interval of 0.2–0.3 melting temperature. The reasons for difference of parameters of copper grain-boundary diffusion in the above materials are discussed. The effect exerted by grain boundary state and grain boundary diffusion fluxes of impurity on creep mechanisms of nanostructured nickel and copper in the temperature interval of 373–473 K have been studied. Significant change in the apparent creep activation energy under copper grain boundary diffusion fluxes is described as a consequence of different contribution of grain boundary sliding to overall deformation.     

Grain boundary interdiffusion in the case of concentration-dependent grain boundary diffusion coefficient

The grain boundary diffusion in a binary system which exhibits a grain boundary phase transition is considered in the framework of Fisher's model. The kinetic law of the growth of the grain boundary phase and the distribution of the diffusant near the grain boundary are calculated. The method of determining of the concentration dependence of the grain boundary diffusion coefficient from the experimentally measured penetration profiles of the diffusant along the grain boundaries is suggested. The experimental results on Zn diffusion in Fe(Si) bicrystals, Ni diffusion in Cu bicrystals and grain boundary grooving in Al in the presence of liquid In are discussed in light of the suggested model.     

Li_4SiO_4陶瓷小球水解吸行为实验研究

采用程序升温热解吸(TPD/TDS)方法对Li4SiO4陶瓷小球的水解吸行为进行了实验研究。结果表明:水解吸过程中主要存在四个解吸峰;其中100℃附近的峰可解释为物理吸附水;150,250,400℃附近的峰可分别解释为以氢键、Li-OH和Si-OH配位键形式存在的化学吸附水。氚的释放与水的解吸几乎同步进行,且氚的释放形式主要为氚水(HTO),据此推测,氚水可能存在三种释放机制:(1)-OT+H2O→-OH+HTO;(2)-OH+-OH→H2O,-OT+H2O→-OH+HTO;(3)-OT+-OH→HTO。     

Formation of copper islands on a native SiO2 surface at elevated temperatures

A combination of in situ X-ray photoelectron spectroscopy analysis and ex situ scanning electron- and atomic force microscopy has been used to study the formation of copper islands upon Cu deposition at elevated temperatures as a basis for the guided growth of copper islands. Two different temperature regions have been found: (I) up to 250 °C only close packed islands are formed due to low diffusion length of copper atoms on the surface. The SiO₂ film acts as a barrier protecting the silicon substrate from diffusion of Cu atoms from oxide surface. (II) The deposition at temperatures above 300 °C leads to the formation of separate islands which are (primarily at higher temperatures) crystalline. At these temperatures, copper atoms diffuse through the SiO₂ layer. However, they are not entirely dissolved in the bulk but a fraction of them forms a Cu rich layer in the vicinity of SiO₂/Si interface. The high copper concentration in this layer lowers the concentration gradient between the surface and the substrate and, consequently, inhibits the diffusion of Cu atoms into the substrate. Hence, the Cu islands remain on the surface even at temperatures as high as 450 °C.     

Creep and diffusion parameters in submicrocrystalline metals

We have studied the creep of nickel and copper in a submicrocrystalline (SMC) state in a vacuum and in the presence of a diffusion contact with an impurity (Cu and Al, respectively). It is shown that a reduction of the resistance in the presence of a diffusion contact with an impurity is observed in the SMC materials in the temperature range 398 to 473 K. This range is 200 to 400 K lower than the corresponding range for coarse-grained material. It is shown that in this temperature interval the coefficients of grain boundary diffusion for copper in SMC nickel are 5 to 6 orders of magnitude larger than in the coarse-grained material. We propose that the reduction in the temperature for the manifestation of a creep activation effect in the presence of a diffusion contact with an impurity in SMC materials is caused by the increase in the diffusion permeability of the submicrocrystalline grain boundaries. Institute of Materials and Strength Physics, Siberian Branch of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 77–82, March, 1998.     

Adsorption and decomposition of HNCO on Cu(111) surface studied by auger electron,electron energy loss and thermal desorption spectroscopy

No detectable adsorbed species were observed after exposure of HNCO to a clean Cu(111) surface at 300 K. The presence of adsorbed oxygen, however, exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociative adsorption of HNCO with concomitant release of water. The adsorption of HNCO at 300 K produced two new strong losses at 10.4 and 13.5 eV in electron energy loss spectra, which were not observed during the adsorption of either CO or atomic N. These loses can be attributed to surface NCO on Cu(111). The surface isocyanate was stable up to 400 K. The decomposition in the adsorbed phase began with the evolution of CO₂. The desorption of nitrogen started at 700 K. Above 800 K, the formation of C₂N₂ was observed. The characteristics of the CO₂ formation and the ratios of the products sensitively depended on the amount of preadsorbed oxygen. No HNCO was desorbed as such, and neither NCO nor (NCO)₂ were detected during the desorption. From the comparison of adsorption and desorption behaviours of HNCO, N, CO and CO₂ on copper surfaces it was concluded that NCO exists as such on a Cu(111) surface at 300 K. The interaction of HNCO with oxygen covered Cu(111) surface and the reactions of surface NCO with adsorbed oxygen are discussed in detail.