Effects of microreactor wall heat conduction on the reforming process of methane

In this paper, the effect of wall conduction of an autothermal tubular methane microreformer is investigated numerically. It is found that the axial wall conduction can strongly influence the performance of the microreactor and should not be neglected without a careful a priori investigation of its impact. By increasing the wall thermal conductivity, the maximum wall surface temperature is decreased. Due to the complex exothermic–endothermic nature of the chemistry of reforming, the axial variation of the wall temperature is not monotonic. Methane conversion and hydrogen yield are strongly dependent on the wall inner surface temperature, hence the heat conduction through the channel wall. The equivalence ratio and the wall thickness also significantly affect the reforming effectiveness and must be carefully considered in reactor optimization. Furthermore, it is found that exothermic oxidation reaction mechanisms, especially partial oxidation, are responsible for syngas (hydrogen and carbon monoxide) production near the inlet. Farther downstream, in the oxygen deficient region, endothermic steam reforming is the main hydrogen producing mechanism. By increasing the thermal conductivity, steam reforming becomes stronger and partial oxidation becomes weaker. For all investigated inlet conditions, the highest hydrogen yield is obtained for no or very low conductive walls.     

Stability analysis for multi-agent systems using the incidence matrix: Quantized communication and formation control

The spectral properties of the incidence matrix of the communication graph are exploited to provide solutions to two multi-agent control problems. In particular, we consider the problem of state agreement with quantized communication and the problem of distance-based formation control. In both cases, stabilizing control laws are provided when the communication graph is a tree. It is shown how the relation between tree graphs and the null space of the corresponding incidence matrix encode fundamental properties for these two multi-agent control problems.     

Decentralized Navigation Functions for Multiple Robotic Agents with Limited Sensing Capabilities

The decentralized navigation function methodology, established in our previous work for navigation of multiple holonomic agents with global sensing capabilities is extended to the case of local sensing capabilities. Each agent plans its actions without knowing the destinations of the others and the positions of those agents lying outside its sensing neighborhood. The stability properties of the closed loop system are checked via Lyapunov stability techniques for nonsmooth systems. The collision avoidance and global convergence properties are verified through simulations. This work was partially presented in [5].     

Fast and exergy efficient start-up of micro-solid oxide fuel cell systems by using the reformer or the post-combustor for start-up heating

The start-up process of a micro-solid oxide fuel cell system strongly influences its overall efficiency, especially for portable applications where a frequent switch-on and switch-off is required. We present herein a novel start-up process for such systems that exploits existing units, such as the post-combustor or the reformer, as a heat source to reach the operation temperature of the cell at 600 °C. Our experimental results show that the employment of platinum catalysts in the post-combustor or rhodium catalysts in the reformer for total oxidation of butane by air combined with an electrically heated wire led to a faster and more efficient start-up than conventional start-up methods using only electrical energy. By using the post-combustor as heat source, the start-up time could be reduced by 79% and the exergy cost by 86%. The latter includes the cost of the stand-alone fuel cell system to produce electrical energy for the joule heating of the wire (i.e. the system efficiency is accounted for). There are several advantages to use the reformer as heat source during start-up, such as prevention of coking of the fuel cell or improved heat transfer by internal heating of the other components. The start-up performance, however, was lower than that of the post-combustor: the start-up time could be reduced by 65% and the exergy cost by 68% compared to a conventional start-up.     

Magnetic properties and critical currents of epitaxial YBa2Cu3O7-x films

The authors report results of magnetic and transport measurements on thin epitaxial films of YBa₂Cu₃O_7-x which show critical current densities of 10⁷ A/cm² at 4.2 K. They exhibit well-formed symmetrical hysteresis loops and flux-trapping effects and linear susceptibilities at low fields. Magnetic and transport critical currents are in good agreement at low temperatures. The above properties are attributed to strong pinning from point defects which are suggested to be more numerous in films than in bulk single crystals. Diamagnetic shielding effects can be very large and are proportional to the critical current at zero field; however, there is a large penetration of H_a at all field values. Field-cooled magnetization is always very small, being only a few percent of the diamagnetic shielding. This small value is attributed to a balance between trapped flux and expelled flux in the cooling process. The strong pinning in attributed to a high density of defects in the film     

The bulk photovoltaic effect in ferroelectric Pb(Zr,Ti)O3 thin films

Abstract

The bulk photovoltaic effect (BPE) has been investigated in lead zirconate titanate (PZT) thin films. Measurements of the kinetics, spectral distribution and photocurrent hysteresis loops have been made. In the extrinsic spectral region, the steady-state photocurrent is primarily due to the BPE, where the photovoltaic tensor component has been determined to be G₃₁ = 10_?9 cm/V. However, in the intrinsic region, the BPE has not been determined due to the strong contribution from photoinjection currents. Finally, it is shown that the BPE may be the driving force for photoinduced hysteresis changes in PZT thin films, particularly in the extrinsic spectral region.     

Electronic and Ionic Trapping at Domain Walls in BaTiO3

We have coupled electron paramagnetic resonance (EPR) and polarization–voltage measurements to understand the effects of reducing ambients on the remanent polarization and density of paramagnetic centers in BaTiO₃, single crystals. Two types of reducing ambients were explored; one was done under vacuum (slightly reducing) and the second was performed in forming gas (very reducing). It is found that the vacuum anneal caused a reduction in the remanent polarization and a concomitant decrease in the isolated Fe³⁺ EPR resonance. The Fe³⁺–V_o complex EPR signal was relatively unaffected by this vacuum anneal. By injecting charge using an ultraviolet (UV) light and an applied bias combination, the polarization and the isolated Fe³⁺ signal intensity were restored, thereby suggesting that the suppression of the remanent polarization is due to trapping of electronic charge at the domain walls. For the forming gas anneal, we observe a much larger decrease in remanent polarization with an accompanying decrease in both the isolated Fe³⁺ and Fe³⁺–V_o complex EPR signals. For this anneal, charge injection by the UV light/bias combination did not restore the polarization nor the EPR densities. The remanent polarization, the isolated Fe³⁺, and the Fe³⁺–V_oV_o complex could be restored only by a reoxidizing anneal, suggesting that ionic defects {oxygen vacancies) are now responsible for pinning the domain walls. Collectively, these results suggest reducing anneals can suppress the amount of switchable polarization in BaTiO₃ by either electronic or ionic trapping mechanisms.     

Control of Leakage Resistance in Pb(Zr,Ti)O3 Thin Films by Donor Doping

Donor doping, with La and Nb, has been used successfully to improve the leakage resistance of Pb(Zr,Ti)O₃ (PZT) films. Donor doping of Pb(Zr_0.5Ti_0.5)O₃ films has led to an improvement in the leakage resistance of over 2 1/2 orders of magnitude at elevated temperatures (T 100°C). The effect on leakage resistance is the same for the A-site (La) and B-site (Nb) dopants. However, the improvement is only about 1 order of magnitude near room temperature. This temperature effect is due to an increase in the transition temperature from a low activation energy mechanism to a higher activation energy mechanism. Similar improvements in leakage resistance have also been obtained by increasing the Pb concentration in the starting solution, which implies that Pb vacancies are the dominant acceptor species in the undoped films. In addition, donor doping has been effective in improving the electrical breakdown strength at elevated temperatures. Consequently, donor-doped PZT films have been shown to be superior to undoped films for applications requiring high leakage resistance, such as decoupling capacitors.     

Hybrid model for the diffusion of simple and complex penetrants in polymers

A hybrid model, which combines the characteristic features of the Pace–Datyner molecular model with those of the Kulkarni–Stern free‐volume model, was developed to assess the effect of temperature, penetrant concentration, and polymer crystallinity on penetrant diffusivity. The predictive capabilities of the proposed model were tested by a direct comparison with experimental data. The diffusivity of ethylene and propylene vapors in semicrystalline polyethylene and isotactic polypropylene was experimentally measured using a magnetic suspension microbalance. Sorption kinetic measurements were carried out at temperatures up to 80°C and pressures up to 80 atm. The diffusivity was found to increase with temperature and penetrant concentration. Apparently, there was a very good agreement between the theoretical values predicted by the new hybrid diffusion model and the experimentally determined diffusivities. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Flow induced by ependymal cilia dominates near-wall cerebrospinal fluid dynamics in the lateral ventricles

While there is growing experimental evidence that cerebrospinal fluid (CSF) flow induced by the beating of ependymal cilia is an important factor for neuronal guidance, the respective contribution of vascular pulsation-driven macroscale oscillatory CSF flow remains unclear. This work uses computational fluid dynamics to elucidate the interplay between macroscale and cilia-induced CSF flows and their relative impact on near-wall dynamics. Physiological macroscale CSF dynamics are simulated in the ventricular space using subject-specific anatomy, wall motion and choroid plexus pulsations derived from magnetic resonance imaging. Near-wall flow is quantified in two subdomains selected from the right lateral ventricle, for which dynamic boundary conditions are extracted from the macroscale simulations. When cilia are neglected, CSF pulsation leads to periodic flow reversals along the ventricular surface, resulting in close to zero time-averaged force on the ventricle wall. The cilia promote more aligned wall shear stresses that are on average two orders of magnitude larger compared with those produced by macroscopic pulsatile flow. These findings indicate that CSF flow-mediated neuronal guidance is likely to be dominated by the action of the ependymal cilia in the lateral ventricles, whereas CSF dynamics in the centre regions of the ventricles is driven predominantly by wall motion and choroid plexus pulsation.