Optimizing the performance of thermionic devices using energy filtering

Conventional thermionic power generators and refrigerators utilize a barrier in the direction of transport to selectively transmit high-energy electrons. Here we show that the energy spectrum of electrons transmitted in this way is not optimal, and we derive the ideal energy spectrum for operation in the maximum power regime. By using suitable energy filters, such as resonances in quantum dots, the power of thermionic devices can, in principle, be improved by an order of magnitude.Comment: 3 pages, 2 figure

Erfassung gasförmiger Emissionen vom Auslauf eines Jungviehstalles mittels aktiver Probenahmehaube

Open stable systems with outdoor yards are used in organic cattle farming. Until now there are no emission factors in Germany for outdoor yards of dairy barns or of cattle barns generally. The measuring of the emission potential of gaseous substances (NH3, CO2, N2O and CH4) is difficult because of the size of the outdoor yards and the direct access by climate. One possibility to measure those emissions is the use of an active sampling hood. The sampling hood provides defined laminar exhaust airflow. The product of flow rate and concentration results in the emission mass flow. In a preliminary study six cattle in calf with a total of 7.19 GV had access to an outdoor yard. This was divided virtually in a 9 x 11 à 1 m² grid. To each m² a dirt level (clean, slightly, medium and heavily soiled) has been assigned. The grid area was sampled randomly. The influence of a scraper was investigated, too. The levels of dirtiness were allocated to the NH3 emission mass flows. Based on the rating the results of the random samples were transferred on the partial area of the entire outdoor yard. The outcome of this is an emission factor for NH3 of 3.02 kg/GV*a for the outdoor yard before the scraper was used and 2.79 kg/GV*a afterwards. The preliminary results show that measuring NH3 emissions by an active sampling hood is practicable for the use at outdoor yards of cattle barns

Transverse rectification of disorder-induced fluctuations in a driven system

We study numerically the overdamped motion of particles driven in a two dimensional ratchet potential. In the proposed design, of the so-called geometrical-ratchet type, the mean velocity of a single particle in response to a constant force has a transverse component that can be induced by the presence of thermal or other unbiased fluctuations. We find that additional quenched disorder can strongly enhance the transverse drift at low temperatures, in spite of reducing the transverse mobility. We show that, under general conditions, the rectified transverse velocity of a driven particle fluid is equivalent to the response of a one dimensional flashing ratchet working at a drive-dependent effective temperature, defined through generalized Einstein relations.Comment: 4.5 pages, 3 fig

Optimizing Stimulation and Analysis Protocols for Neonatal fMRI

The development of brain function in young infants is poorly understood. The core challenge is that infants have a limited behavioral repertoire through which brain function can be expressed. Neuroimaging with fMRI has great potential as a way of characterizing typical development, and detecting abnormal development early. But, a number of methodological challenges must first be tackled to improve the robustness and sensitivity of neonatal fMRI. A critical one of these, addressed here, is that the hemodynamic response function (HRF) in pre-term and term neonates differs from that in adults, which has a number of implications for fMRI. We created a realistic model of noise in fMRI data, using resting-state fMRI data from infants and adults, and then conducted simulations to assess the effect of HRF of the power of different stimulation protocols and analysis assumptions (HRF modeling). We found that neonatal fMRI is most powerful if block-durations are kept at the lower range of those typically used in adults (full on/off cycle duration 25-30s). Furthermore, we show that it is important to use the age-appropriate HRF during analysis, as mismatches can lead to reduced power or even inverted signal. Where the appropriate HRF is not known (for example due to potential developmental delay), a flexible basis set performs well, and allows accurate post-hoc estimation of the HRF

Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts

The strength of the Zeeman splitting induced by an applied magnetic field is an important factor for the realization of spin-resolved transport in mesoscopic devices. We measure the Zeeman splitting for a quantum point contact etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to the transport direction. We observe an enhancement of the Lande g-factor from |g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first subband, six times larger than in GaAs. We report subband spacings in excess of 10 meV, which facilitates quantum transport at higher temperatures.Comment: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in Applied Physics Letter

Aircraft tire temperature at touchdown with wheel prerotation

Because of the skidding process that occurs when a heavy aircraft’s main landing gear tires touchdown, since the 1940s, a number of ideas have been patented to improve tire safety and decrease the substantial wear and smoke during every landing by spinning the gear wheels before touchdown. In this paper, a coupled structural–thermal transient analysis in ANSYS has been used to model a single wheel main landing gear as a mass-spring system. This model has been chosen to analyze the wheel’s dynamic behavior and tire tread temperature during the short period from static to a matching free-rolling velocity in which the wheel is forced to accelerate by the friction between the tire and ground. The tire contact surface temperature has been calculated for both the initially static and prespun wheels in order to compare the temperature levels for different initial rotations and to validate the use of the prespinning technique