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- ItemGeometric models for isotropic random porous media: A review(London : Hindawi, 2014) Hermann, Helmut; Elsner, AntjeModels for random porous media are considered. The models are isotropic both from the local and the macroscopic point of view; that is, the pores have spherical shape or their surface shows piecewise spherical curvature, and there is no macroscopic gradient of any geometrical feature. Both closed-pore and open-pore systems are discussed. The Poisson grain model, the model of hard spheres packing, and the penetrable sphere model are used; variable size distribution of the pores is included. A parameter is introduced which controls the degree of open-porosity. Besides systems built up by a single solid phase, models for porous media with the internal surface coated by a second phase are treated. Volume fraction, surface area, and correlation functions are given explicitly where applicable; otherwise numerical methods for determination are described. Effective medium theory is applied to calculate physical properties for the models such as isotropic elastic moduli, thermal and electrical conductivity, and static dielectric constant. The methods presented are exemplified by applications: small-angle scattering of systems showing fractal-like behavior in limited ranges of linear dimension, optimization of nanoporous insulating materials, and improvement of properties of open-pore systems by atomic layer deposition of a second phase on the internal surface.
- ItemRoom temperature direct band gap emission from Ge p-i-n heterojunction photodiodes(London : Hindawi, 2012) Kasper, E.; Oehme, M.; Arguirov, T.; Werner, J.; Kittler, M.; Schulze, J.Room temperature direct band gap emission is observed for Si-substrate-based Ge p-i-n heterojunction photodiode structures operated under forward bias. Comparisons of electroluminescence with photoluminescence spectra allow separating emission from intrinsic Ge (0.8 eV) and highly doped Ge (0.73 eV). Electroluminescence stems fromcarrier injection into the intrinsic layer, whereas photoluminescence originates from the highly n-doped top layer because the exciting visible laser wavelength is strongly absorbed in Ge. High doping levels led to an apparent band gap narrowing from carrier-impurity interaction. The emission shifts to higher wavelengths with increasing current level which is explained by device heating. The heterostructure layer sequence and the light emitting device are similar to earlier presented photodetectors. This is an important aspect for monolithic integration of silicon microelectronics and silicon photonics.
- ItemPlasma rotation with circularly polarized laser pulse(London : Hindawi, 2015) Lécz, Z.; Andreev, A.; Seryi, A.The efficient transfer of angular orbital momentum from circularly polarized laser pulses into ions of solid density targets is investigated with different geometries using particle-in-cell simulations. The detailed electron and ion dynamics presented focus upon the energy and momentum conversion efficiency. It is found that the momentum transfer is more efficient for spiral targets and the maximum value is obtained when the spiral step is equal to twice the laser wavelength. This study reveals that the angular momentum distribution of ions strongly depends up on the initial target shape and density.
- ItemEffects of free surface and heterogeneous residual internal stress on stress-driven grain growth in nanocrystalline metals(London : Hindawi, 2013) Schneider, Andreas S.; Wang, F.; Zhao, J.; Huang, P.; Lu, T.J.; Xu, K.W.By reevaluating the experimental study of Zhang et al. (2005), here we demonstrate that the extent of grain growth, previously proposed to be solely driven by external stress, may have been significantly overestimated. A new physical mechanism, termed as free surface assisted stress-driven grain growth (or self-mechanical annealing), is proposed and discussed in detail. Representing the cooperative effect of free surface and heterogeneous residual internal stress, the proposed mechanism is considered more favorable than the traditional pure stress-driven mechanism for interpreting the abnormal grain growth widely observed in deforming nanocrystalline metals at room temperature.
- ItemSupport for a long lifetime and short end-to-end delays with TDMA protocols in sensor networks(London : Hindawi, 2012) Brzozowski, Marcin; Salomon, Hendrik; Langendoerfer, PeterThis work addresses a tough challenge of achieving two opposing goals: ensuring long lifetimes and supporting short end-to-end delays in sensor networks. Obviously, sensor nodes must wake up often to support short delays in multi-hop networks. As event occurs seldom in common applications, most wake-up are useless: nodes waste energy due to idle listening. We introduce a set of solutions, referred to as LETED (limiting end-to-end delays), which shorten the wake-up periods, reduce idle listening, and save energy. We exploit hardware features of available transceivers that allow early detection of idle wake-up periods. This feature is introduced on top of our approach to reduce idle listening stemming from clock drift owing to the estimation of run-time drift. To evaluate LETED and other MAC protocols that support short end-to-end delays we present an analytical model, which considers almost 30 hardware and software parameters. Our evaluation revealed that LETED reduces idle listening by 15x and more against similar solutions. Also, LETED outperforms other protocols and provides significant longer lifetimes. For example, nodes with LETED work 8x longer than those with a common TDMA and 2x-3x longer than with protocols based on preamble sampling, like B-MAC.
- ItemAdaptable security in wireless sensor networks by using reconfigurable ECC hardware coprocessors(London : Hindawi, 2010) Portilla, J.; Otero, A.; de la Torre, E.; Riesgo, T.; Stecklina, O.; Peter, S.; Langendörfer, P.Specific features of Wireless Sensor Networks (WSNs) like the open accessibility to nodes, or the easy observability of radio communications, lead to severe security challenges. The application of traditional security schemes on sensor nodes is limited due to the restricted computation capability, low-power availability, and the inherent low data rate. In order to avoid dependencies on a compromised level of security, a WSN node with a microcontroller and a Field Programmable Gate Array (FPGA) is used along this work to implement a state-of-the art solution based on ECC (Elliptic Curve Cryptography). In this paper it is described how the reconfiguration possibilities of the system can be used to adapt ECC parameters in order to increase or reduce the security level depending on the application scenario or the energy budget. Two setups have been created to compare the softwareand hardware-supported approaches. According to the results, the FPGA-based ECC implementation requires three orders of magnitude less energy, compared with a low power microcontroller implementation, even considering the power consumption overhead introduced by the hardware reconfiguration.
- ItemData link layer considerations for future 100 Gbps terahertz band transceivers(London : Hindawi, 2017) Lopacinski, Lukasz; Brzozowski, Marcin; Kraemer, RolfThis paper presents a hardware processor for 100Gbps wireless data link layer. A serial Reed-Solomon decoder requires a clock of 12.5GHz to fulfill timings constraints of the transmission. Receiving a single Ethernet frame on a 100 Gbps physical layer may be faster than accessing DDR3 memory. Processing so fast streams on a state-of-the-art FPGA (field programmable gate arrays) requires a dedicated approach. Thus, the paper presents lightweight RS FEC engine, frames fragmentation, aggregation, and a protocol with selective fragment retransmission. The implemented FPGA demonstrator achieves nearly 120 Gbps and accepts bit error rate (BER) up to 2e - 3. Moreover, redundancy added to the frames is adopted according to the channel BER by a dedicated link adaptation algorithm. At the end, ASIC synthesis results are presented including detailed statistics of consumed energy per bit.
- ItemEnergy conservation and harvesting in wireless sensor networks(London : Hindawi, 2019) Stojcev, Mile; Stamenkovic, Zoran; Dimitrijevic, Bojan[No abstract available]