Mass and heat transport in direct methanol fuel cells

The direct methanol fuel cell (DMFC) is a better alternative to the conventional battery. The DMFC offers several advantages, namely, faster building of potential and longer-lasting fuel, however, there are still several issues that need to be addressed to design a better DMFC system. This article is a wide-ranging review of the most up-to-date studies on mass and heat transfer in the DMFC. The discussion will be focused on the critical problems limiting the performance of DMFCs. In addition, a technique for upgrading the DMFC with an integrated system will be presented, along with existing numerical models for modeling mass and heat transfer as well as cell performance.     

Introduction

In the planning of a new passenger railway in Perth over many years, state agencies have actively pursued opportunities for transit-oriented development along its route. There is a strong policy framework demonstrating clear intent on the type of development required. Designing a transport system to compete with the car in an established low-density suburban environment has, however, raised some significant challenges for land-use planning. This article reports the results of research aimed at assessing changes in non-residential land use and the behavioural responses of local businesses in three emerging transit-oriented development precincts. It examines the first wave surveys of land use and businesses in 2006, the year before the railway opened, questioning the extent to which business has anticipated the new railway. The findings indicate that, despite good policy intent, planning in anticipation of the railway has been weak and insufficient attention was paid to implementation of important strategic planning principles and policies. While some land uses are compatible with transit-oriented development, these businesses employ small numbers, at odds with policies that required high trip generating development. Businesses have not shown a particularly strong interest in the new railway, their location decisions appear to reflect standard business practice rather than being influenced by opportunities created by the railway. There is a glimmer of anticipation—incoming businesses are more positive about the opportunities the railway may bring than those established in the station precincts before the 2001 railway route was announced. In particular, a significantly higher proportion of businesses in one case study station precinct are positive about opportunities created by the railway.     

Simulation of Flow and Mass Transport in a Meander Microchannel Subject to Electroosmotic Pumping

We have investigated the flow and mass transport within an electroosmotically pumped incompressible liquid through a meander microchannel system. We employ two-dimensional, time-dependent finite element simulations in conjunction with a matched asymptotic treatment of the electrical double layers. The electroosmotic pumping is realized for two idealized and two realistic electrical fields, while a pressure-driven flow is used for comparison. We focus on the aspects of the electroosmotic transport.

We find for most of the electroosmotically driven cases rather complex flow fields, involving recirculation regions. These recirculation regions in all cases increase dispersion. (i) The least dispersion is associated with a plug-type velocity profile, which is obtained for an idealized purely wall-tangential orientation of the electrical field. (ii, iii) We find that both the idealized horizontal electrical field and the real electrical field between two vertical plates give considerably higher dispersion than the pressure-driven flow. Vertical plate electrodes, therefore, do not allow for a electrical field, which minimizes dispersion. (iv) The arrangement of two point electrodes at the in and out sections likewise proves to be no optimal means to reduce dispersion beyond the pressure-driven flow. Thus, meander geometries of channels, in general, cause severe problems if electroosmotic pumping needs to be achieved in combination with minimized dispersion.     

How small the contacts could be optimal for nanoscale organic transistors?

We report on a study seeking an optimized contact configuration for organic transistors that minimizes contact effects but maintains smallest contact size. We begin with the bulk access resistance in staggered transistors which results from the charge transport through the organic semiconductor film. Bulk access resistance is an intrinsic contributor to the contact resistance which has been little understood due to lack of a reliable study tool. In this work, we utilize the inner transported power inside the semiconductor film as a medium to investigate the contact resistance and the relevant contact effects. We examine the influences of the organic film thickness (t_SC), the channel length (L), the underlying charge transport and various organic semiconductor materials with variable carrier mobility. A roughly optimal contact length (L_C) of L_C0 ≈ 6t_SC is obtained. The results reveal that besides the device architecture the underlying charge transport should be also taken into account in designing organic transistors for practical application.     

Estimation of temporal changes in oxidation rates of sulphides in copper mine tailings at Laver, Northern Sweden

Tailings containing pyrrhotite were deposited in an impoundment at a copper mine at Laver, Northern Sweden, which operated between 1936 and 1946. Since then the oxidation of sulphides has acidified recipient water courses and contaminated them with metals. Measurements from surface water sampled in 1993, 2001 and 2004-05 from a brook into which the tailing impoundment drains indicate that the amounts of sulphide-associated elements such as Cu, S and Zn released into the brook have decreased over time, while pH has increased. The mass transport of S in the brook during 1993 and 2001 corresponded well with the amount of S estimated to be released from the tailings by oxidation. Secondary precipitates such as covellite and gypsum, which can trap sulphur, were shown in earlier studies to be present in only low amounts. The annual release of elements from the tailings was estimated from the volume of tailings assumed to oxidise each year, which depends on movement of the oxidation front with time. The results indicate that the oxidation rate in the tailings has decreased over time, which may be due to the increased distance over which oxygen needs to diffuse to reach unoxidised sulphide grains, or their cores, in the tailings.     

高真空多层绝热槽车制造工艺要点分析

从绝热材料的准备、内容器的缠绕、内外容器罐体的套合到夹层抽真空等工艺要点入手,对高真空多层绝热槽车的制造工艺进行了详细叙述。     

Limiting current density in bis(trifluoromethylsulfonyl)amide-based ionic liquid for lithium batteries

The physicochemical and electrochemical properties of the binary ionic liquid (IL), lithium bis(trifluoromethylsulfonyl)amide (LiTFSA) dissolved in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)amide (DEMETFSA), were investigated. The ionic conductivity of the binary IL decreased with an increase in LiTFSA concentration. The self-diffusion coefficients of Li⁺, DEME⁺, and TFSA⁻ dissolved in the IL were measured by using the pulsed-field-gradient spin-echo (PGSE) NMR method. The self-diffusion coefficient of each ionic species was also found to decrease with increasing concentration of LiTFSA. The limiting current density in the IL electrolyte was evaluated by chronoamperometry using symmetric Li|IL|Li cell. The results suggest that the diffusion process of Li(I) in the IL dominates the limiting current density in the cell. The highest limiting current density is achieved at a concentration of 0.64 mol dm⁻³ of LiTFSA.     

Understanding the structure of the first atomic contact in gold

We have studied experimentally jump-to-contact (JC) and jump-out-of-contact (JOC) phenomena in gold electrodes. JC can be observed at first contact when two metals approach each other, while JOC occurs in the last contact before breaking. When the indentation depth between the electrodes is limited to a certain value of conductance, a highly reproducible behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. Molecular dynamics simulations of this process show how the two metallic electrodes are shaped into tips of a well-defined crystallographic structure formed through a mechanical annealing mechanism. We report a detailed analysis of the atomic configurations obtained before contact and rupture of these stable structures and obtained their conductance using first-principles quantum transport calculations. These results help us understand the values of conductance obtained experimentally in the JC and JOC phenomena and improve our understanding of atomic-sized contacts and the evolution of their structural characteristics.     

Adjoint Monte Carlo neutron transport using cross-section probability table representation

The probability table representation of cross-sections is generally used to deal with neutron interactions in the unresolved energy range. In the frame of neutron transport methods, the capability of the probability table representation of cross-sections on the whole neutron energy range has been mentioned by Cullen (1974) and it has been already demonstrated for the Monte Carlo transport calculations by Zheng et al. (1998). Such an advantage is also illustrated here with a simple neutron propagation configuration dealt with the TRIPOLI-4 Monte Carlo transport code.     

An algorithm for fast calculation of ion range and damage distributions based on the boltzmann transport equation

Boltzmann transport equation (BTE) method using large energy intervals in ion scattering calculations can lead to significant errors in the calculation of ion and damage distributions. We pointed out that the inaccuracy is caused by the fact that the flux distribution in a transited energy state is not uniform. The flux is higher at its higher energy end because of a relatively higher cross section for a small energy transfer. Neglecting this difference and assuming an evenly distributed flux will lead to significant errors. The issue can be alleviated by using small energy intervals which inevitably increases computational times. We proposed to redistribute the flux among neighboring energy states around a transited energy state, with the redistribution weighted by their proximities to the mean value of transited energy. Therefore, ion range distributions can be calculated accurately with large energy intervals, thus significantly improving computational efficiency, i.e. by at least one order of magnitudes for 100 keV Si self ion irradiation.