N.M. Ferreira

On the adequacy of 802.11p MAC protocols to support safety services in ITS

The use of IEEE 802.11p for supporting intelligent transportation systems (ITS) enables enhancing the drive experience to provide vehicle users with useful information related to road efficiency and public safety. Safety services, such as collision or sudden hard braking warning, are used to improve passenger safety and reduce fatalities. Along with the delay-critical nature of those services, the highly dynamic communication environment of vehicular networks demand an efficient medium access control (MAC) protocol to assure minimal transmission collisions. This work-in-progress paper presents a preliminary study on the adequacy of existent 802.11p based MAC protocols regarding safety applications, and proposes ideas to implement such services in an effective way assuming an infrastructure based operation mode.

ZnO Nano/Microstructures Grown by Laser Assisted Flow Deposition

Nano/microstructures of zinc oxide (ZnO) were grown by the laser assisted flow deposition (LAFD) method. This new process has proved to be very efficient, allowing high yield ZnO deposits at high-rate applicable to large-scale substrates. Laser local heating promotes fast ZnO decomposition and recombination under a self-catalytic vapour–liquid-solid mechanism for the nucleation and growth. Three types of ZnO morphologies were obtained according to the temperature/oxygen availability inside the growth chamber. The morphology can also be controlled adding rare-earth elements to the initial composition. Particularly, tetrapod morphology was obtained by europium oxide addition to the precursors. The structural and microstructural characterizations confirm the good crystallinity of the wurtzite structure. The photoluminescence spectroscopy revealed high optical quality of the as-grown ZnO. Specifically, the free exciton recombination and a strong near band edge recombination due to donor bound exciton transitions can be clearly recognized, although deep level emission in the green spectral region is present.

Designing strontium titanate-based thermoelectrics: insight into defect chemistry mechanisms

Driven by a need to develop low-cost and thermally stable materials for thermoelectric applications, donor-substituted strontium titanate is considered as a promising alternative to traditional thermoelectrics. The complex defect chemistry of SrTiO3-based materials imposes various limitations on identifying the relevant effects exerted on the electronic band structure and heat transfer, being a subject of debate and intensive research. Based on combined XRD, SEM/EDS, HRTEM, XPS, and TGA studies and measurements of thermoelectric properties, this work uncovers the particular role of various structural defects in electrical and thermal transport in Sr1±yTi0.9Nb0.1O3±δ, selected as a model system. Introduction of A-site cation vacancies provides a synergistic effect of combining fast charge transport in the perovskite lattice and suppressing the thermal conductivity mostly due to simultaneous generation of oxygen vacancies. The presence of oxygen vacancies promotes more efficient phonon scattering compared to Ruddlesden–Popper-type layers. These findings provide a link between structural and thermoelectric properties, offering further prospects for seeking highly performing SrTiO3-based thermoelectrics by tailoring the defect chemistry mechanisms.

Optical and dielectric behaviour of EuNbO4 crystals

In this work, EuNbO4 samples were prepared, in the shape of fibres, by the laser floating zone (LFZ) technique. The fibres grown at pulling rates below 20 mm h−1 are transparent and monophasic, crystallizing in a monoclinic structure. At higher growth rates, non-stoichiometric europium orthoniobate phases were promoted. The physical properties analysed in this work were the electric, dielectric and photoluminescence (PL) properties. For the fibre grown at 2.5 mm h−1, the dc conductivity shows a value of 5.97 × 10−12 S m−1 and the ac conductivity, at 1 MHz and 300 K, is 3.08 × 10−4 S m−1. Under the same experimental conditions the dielectric constant is ∼39 and the loss tangent ∼0.14, also presenting a relaxation phenomenon centred at ∼120 kHz. An ionic polarization type behaviour was identified from the dielectric measurements, which justifies the temperature independent behaviour of the dielectric constant. This type of polarization was assigned to the NbO4 units. PL measurements with ultraviolet excitation reveal the intra-4f6 transitions of Eu3+ ions in the fibres, exhibiting narrow intraionic lines as expected for single crystal materials. From the PL spectra at low temperature, it was possible to identify multiple europium related centres, from different local site symmetry and/or environment. The luminescence of Eu3+ was found to be preferentially excited via a broad excitation band peaked at ∼280 nm.

Processing strategies for smart electroconductive carbon nanotube-based bioceramic bone grafts

Electroconductive bone grafts have been designed to control bone regeneration. Contrary to polymeric matrices, the translation of the carbon nanotube (CNT) electroconductivity into oxide ceramics is challenging due to the CNT oxidation during sintering. Sintering strategies involving reactive-bed pressureless sintering (RB + P) and hot-pressing (HP) were optimized towards prevention of CNT oxidation in glass/hydroxyapatite (HA) matrices. Both showed CNT retentions up to 80%, even at 1300 °C, yielding an increase of the electroconductivity in ten orders of magnitude relative to the matrix. The RB + P CNT compacts showed higher electroconductivity by ∼170% than the HP ones due to the lower damage to CNTs of the former route. Even so, highly reproducible conductivities with statistical variation below 5% and dense compacts up to 96% were only obtained by HP. The hot-pressed CNT compacts possessed no acute toxicity in a human osteoblastic cell line. A normal cellular adhesion and a marked orientation of the cell growth were observed over the CNT composites, with a proliferation/differentiation relationship favouring osteoblastic functional activity. These sintering strategies offer new insights into the sintering of electroconductive CNT containing bioactive ceramics with unlimited geometries for electrotherapy of the bone tissue.

Carbon nanotube-based bioceramic grafts for electrotherapy of bone

Bone complexity demands the engineering of new scaffolding solutions for its reconstructive surgery. Emerging bone grafts should offer not only mechanical support but also functional properties to explore innovative bone therapies. Following this, ceramic bone grafts of Glass/hydroxyapatite (HA) reinforced with conductive carbon nanotubes (CNTs) - CNT/Glass/HA - were prepared for bone electrotherapy purposes. Computer-aided 3D microstructural reconstructions and TEM analysis of CNT/Glass/HA composites provided details on the CNT 3D network and further correlation to their functional properties. CNTs are arranged as sub-micrometric sized ropes bridging homogenously distributed ellipsoid-shaped agglomerates. This arrangement yielded composites with a percolation threshold of pc=1.5vol.%. At 4.4vol.% of CNTs, thermal and electrical conductivities of 1.5W·m(-1)·K(-1) and 55S·m(-1), respectively, were obtained, matching relevant requisites in electrical stimulation protocols. While the former avoids bone damaging from Joules heat generation, the latter might allow the confinement of external electrical fields through the conductive material if used for in vivo electrical stimulation. Moreover, the electrically conductive bone grafts have better mechanical properties than those of the natural cortical bone. Overall, these highly conductive materials with controlled size CNT agglomerates might accelerate bone bonding and maximize the delivery of electrical stimulation during electrotherapy practices.

On the end-to-end delay analysis for an IEEE 802.11P/WAVE protocol

The use of IEEE 802.11p for supporting intelligent transportation systems (ITS) allows a wide spectrum of applications providing vehicle occupants useful information related to public safety and road efficiency. The Wireless Access for Vehicular Environment (WAVE) standard is specifically tailored for delivering safety and multimedia messages in a highly dynamic vehicular communication environment. Such dynamic characteristics along with the delay-critical nature of safety services turn the medium access control protocol (MAC) timings very important. Therefore, it becomes of great interest to analyze a major performance metric, the end-to-end delay.

Guidelines to design multicomponent ferrospinels for high-temperature applications

This work explores the possibilities to design magnetite-based spinels through multiple simultaneous co-substitutions with transition metal cations, with emphasis on redox behavior and electronic transport. For the first time this approach was assessed for high-temperature applications, which is of particular interest for the development of consumable anodes for pyroelectrolysis, an alternative carbon-lean steelmaking process. A Taguchi plan was used to assess the impact of the concentration of substituting chromium, titanium, manganese and nickel cations on the lattice parameter and electrical conductivity of the multicomponent ferrospinels. The results revealed a comparable decrease in the electrical conductivity, provided by Cr3+, Mn3+/2+ and Ni2+ cations. The impact of Ti4+ was found to be less negative, contributed by the formation of Fe2+ cations and increased hopping probability. The strongest structural impacts, exerted by manganese cations, are likely to affect the mobility of polarons, as revealed by the analysis of the correlation factors for combined effects. Ferrospinels, containing various transition metal cations, are more susceptible to oxidation and phase decomposition, which often result in a sudden conductivity drop and significant dimensional changes in the ceramics. The observed trends for redox behavior suggest that the potential applications of multicomponent ferrospinels in oxidizing conditions are limited to 1000–1400 K due to insufficient stability, while higher temperature applications, requiring significant electronic conductivity, are rather suitable.

An RSU coordination scheme for WAVE safety services support

The use of wireless communication technologies to increase road safety is rising within the automobile world. Vehicle to vehicle (V2V) communications is a very promising field but the slow vehicle renewal rate combined with the current world economic crisis turns V2V into a distant scenario. A more viable solution relies on Infrastructure to vehicle communications (I2V) and the use of the wireless access for vehicular environment (WAVE) standard, specifically tailored for delivering safety and multimedia messages in a highly dynamic communication environment. This work-in-progress paper addresses an open issue in a previous presented infrastructure based solution: the beacon coordination between adjacent road side units (RSUs) and also a safety message retransmission mechanism performed by such RSUs.

WAVE Based Architecture for Safety Services Deployment in Vehicular Networks

Abstract Vehicle manufacturers, highway concessionaries, governments and academic research are cooperating to find the best solution to add safety services relying on vehicle to vehicle communication systems (V2V) and among vehicles and the infrastructure (V2I) located on the roadside. Safety services, such as collision or sudden hard braking warning have delay-critical requirements. This paper proposes a WAVE (Wireless Access for Vehicular Environment) based model and a MAC protocol to disseminate time-critical messages for safety services in highways where the Road Side Units (RSUs) are not present in the instant a safety event is generated and, consequently, the message dissemination is done only through vehicles. The model allows to determine the retransmission slot number and the models feasibility is somewhat evaluated by analysing the maximum number of available slots for a typical scenario.