Mobile Learning

Works in progress from selected papers presented at the 2006 IADIS International Conference on Mobile Learning provide an overview of the diverse research being conducted in this field.     

Interfacial Strain Gradients Control Nanoscale Domain Morphology in Epitaxial BiFeO3 Multiferroic Films

Domain switching pathways fundamentally control performance in ferroelectric thin film devices. In epitaxial bismuth ferrite (BiFeO₃) films, the domain morphology is known to influence the multiferroic orders. While both striped and mosaic domains have been observed, the origins of the latter have remained unclear. Here, it is shown that domain morphology is defined by the strain profile across the film–substrate interface. In samples with mosaic domains, X‐ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using scanning transmission electron microscopy finds that within 5 nm of the film–substrate interface, the out‐of‐plane strain shows an anomalous dip while the in‐plane strain is constant. Conversely, if uniform strain is maintained across the interface with zero strain gradient, striped domains are formed. Critically, an ex situ thermal treatment, which eliminates the interfacial strain gradient, converts the domains from mosaic to striped. The antiferromagnetic state of the BiFeO₃ is also influenced by the domain structure, whereby the mosaic domains disrupt the long‐range spin cycloid. This work demonstrates that atomic scale tuning of interfacial strain gradients is a powerful route to manipulate the global multiferroic orders in epitaxial films.     

Light‐Fueled,Spatiotemporal Modulation of Mechanical Properties and Rapid Self‐Healing of Graphene‐Doped Supramolecular Elastomers

Gaining spatially resolved control over the mechanical properties of materials in a remote, programmable, and fast‐responding way is a great challenge toward the design of adaptive structural and functional materials. Reversible, temperature‐sensitive systems, such as polymers equipped with supramolecular units, are a good model system to gain detailed information and target large‐scale property changes by exploiting reversible crosslinking scenarios. Here, it is demonstrated that coassembled elastomers based on polyglycidols functionalized with complementary cyanuric acid and diaminotriazine hydrogen bonding couples can be remotely modulated in their mechanical properties by spatially confined laser irradiation after hybridization with small amounts of thermally reduced graphene oxide (TRGO). The TRGO provides an excellent photothermal effect, leads to light‐adaptive steady‐state temperatures, and allows local breakage/de‐crosslinking of the hydrogen bonds. This enables fast self‐healing and spatiotemporal modulation of mechanical properties, as demonstrated by digital image correlation. This study opens pathways toward light‐fueled and light‐adaptive graphene‐based nanocomposites employing molecularly controlled thermal switches.     

Robustness of optimal channel reservation using handover prediction in multiservice wireless networks

The aim of our study is to obtain theoretical limits for the gain that can be expected when using handover prediction and to determine the sensitivity of the system performance against different parameters. We apply an average-reward reinforcement learning approach based on afterstates to the design of optimal admission control policies in mobile multimedia cellular networks where predictive information related to the occurrence of future handovers is available. We consider a type of predictor that labels active mobile terminals in the cell neighborhood a fixed amount of time before handovers are predicted to occur, which we call the anticipation time. The admission controller exploits this information to reserve resources efficiently. We show that there exists an optimum value for the anticipation time at which the highest performance gain is obtained. Although the optimum anticipation time depends on system parameters, we find that its value changes very little when the system parameters vary within a reasonable range. We also find that, in terms of system performance, deploying prediction is always advantageous when compared to a system without prediction, even when the system parameters are estimated with poor precision.     

Robust Single Molecule Magnet Monolayers on Graphene and Graphite with Magnetic Hysteresis up to 28 K

The chemical functionalization of fullerene single molecule magnet Tb₂@C₈₀(CH₂Ph) enables the facile preparation of robust monolayers on graphene and highly oriented pyrolytic graphite from solution without impairing their magnetic properties. Monolayers of endohedral fullerene functionalized with pyrene exhibit magnetic bistability up to a temperature of 28 K. The use of pyrene terminated linker molecules opens the way to devise integration of spin carrying units encapsulated by fullerene cages on graphitic substrates, be it single-molecule magnets or qubit candidates.     

Multiservice platforms for data services

The potential for broadband and data services is large and continues to expand for the foreseeable future. Consideration of the cost-effective delivery and support of both new, advanced services, as well as existing services, has led to the driver for multiservice platforms (MSPs). The cost effectiveness results from the MSP potential to share network bandwidth between services, offer a potential simplification of the operational support systems (OSS), and enable new service features and functions. A generic ATM-based MSP is described in this paper where issues of service support, service interworking, operations and maintenance, and network architecture are discussed. The paper is intended as a tutorial on the concepts.     

Surface-modified maghemite as the cathode material for lithium batteries

The inorganic–organic hybrid maghemite (γ-Fe₂O₃)/polypyrrole (PPy) was synthesized and evaluated as cathode-active material for room temperature lithium batteries. The nanometer-sized core–shell structure of the hybrid consisting of the maghemite core with surface modified by PPy was evidenced from the morphological examination. The cathode fabricated with the as-prepared hybrid material delivered an initial discharge capacity of 233 mAh g⁻¹ and a reversible capacity of ∼62 mAh g⁻¹ after 50 charge–discharge cycles. A much higher performance with an initial discharge capacity of 378 mAh g⁻¹ and a reversible capacity of ∼100 mAh g⁻¹ was achieved with the cathode based on the segregated active material, which was obtained by subjecting the as-prepared hybrid material to an additional ball-milling process. The study demonstrates the promising lithium insertion characteristics of the nanometer-sized core–shell maghemite/PPy particles prepared under optimized conditions for application in secondary batteries.     

Germany's solar cell promotion: Dark clouds on the horizon

This article demonstrates that the large feed-in tariffs currently guaranteed for solar electricity in Germany constitute a subsidization regime that threatens to reach a level comparable to that of German hard coal production, a notoriously outstanding example of misguided political intervention. Yet, as a consequence of the coexistence of the German Renewable Energy Sources Act (EEG) and the EU Emissions Trading Scheme (ETS), the increased use of renewable energy technologies does not imply any additional emission reductions beyond those already achieved by ETS alone. Similarly disappointing is the net employment balance, which is likely to be negative if one takes into account the opportunity cost of this form of solar photovoltaic (PV) support. Along the lines of the international energy agency [IEA, 2007. Energy policies of IEA countries: Germany, 2007 review. International Energy Agency, OECD, Paris, p. 77], we recommend the immediate and drastic reduction of the magnitude of the feed-in tariffs granted for solar-based electricity. Ultimately, producing electricity on this basis is among the most expensive greenhouse gas abatement options.     

Long-term performance calculations based on steady-state efficiency test results: Analysis of optical effects affecting beam,diffuse and reflected radiation

There are a growing number of commercially available solar thermal collector types: flat plates, evacuated tubes with and without back reflectors and different tubular spacing or low concentration collectors, using different types of concentrating optics.These different concepts and designs all compete to be more efficient or simply cheaper, easier to operate, etc. at ever higher temperatures, and to extend the use of solar thermal energy in other applications beyond the most common water heating purposes.In view of the proper dimensioning of solar thermal systems and proper comparison of different collector technologies, for a given application, there is a growing need for existing and future simulation tools to be as accurate as possible in the treatment of these different collector types.Collector heat losses are usually considered to be well determined, under variable operating conditions, through the use of the heat loss coefficients provided by efficiency curve parameters. Yet, the traditional approach to the optical efficiency fails to describe accurately the optical effects affecting the amount of radiation which actually reaches the absorber.This paper develops a systematic approach to the proper handling of incident solar radiation, folding that with the information available from tests for determination of collector efficiency curve (steady-state) and the way the optics of different collector types uses incident solar radiation and transforms it into useful heat.     

Permittivity and Conductivity Measurements of Building Materials at 5.8 GHz and 41.5 GHz

The results of various experiments performed to characterize electromagnetic properties of typical building materials at BRAN (Broadband Radio Access Networks) and MVDS (Multipoint Video Distribution System) bands are presented and compared. Transmission and reflection coefficients were measured as functions of the angle of incidence, using a wideband sounder based on the swept frequency technique. A high precision angular positioning system was built and used in co-ordination with the measurement equipment.The dielectric constants (_r) were estimated by comparing values predicted with a multiple successive internal reflection model and measured transmission and reflection coefficients. It has been found that the variation of the dielectric constant value with the frequency does not necessarily follow the same trend for different materials. For example, the measured _r for plasterboard presents an increment withfrequency (2.02 at 5.8 GHz, 2.5 at 41.5 GHz – measured by theauthors –, 2.58 at 59.5 GHz, and 2.81 at 60.2 GHz – given byliterature –), while the values for glass follow a wanderingpattern (6.06 at 5.8 GHz, 5–10 at 10 GHz, 3.41 at 41.5 GHz, 7.51at 57.6 GHz, and 5.29 at 60.2 GHz). Moreover, the measured_r for a brick wall at 5.8 GHz (3.58) shows differenceswith values reported at 2 GHz (4.44).