Profile-Based Load Scheduling in Wireless Energy Harvesting Sensors for Data Rate Maximization

In this paper, power management algorithms for energy harvesting sensors (EHS) that operate purely based on energy harvested from the environment are proposed. To maintain energy neutrality, EHS nodes schedule their utilization of the harvested power so as to save/draw energy into/from an inefficient battery during peak/low energy harvesting periods, respectively. Under this constraint, one of the key system design goals is to transmit as much data as possible given the energy harvesting profile. For implementational simplicity, it is assumed that the EHS transmits at a constant data rate with power control, when the channel is sufficiently good. By converting the data rate maximization problem into a convex optimization problem, the optimal load scheduling (power management) algorithm that maximizes the average data rate subject to energy neutrality is derived. Also, the energy storage requirements on the battery for implementing the proposed algorithm are calculated. Further, robust schemes that account for the insufficiency of battery storage capacity, or errors in the prediction of the harvested power are proposed. The superior performance of the proposed algorithms over conventional scheduling schemes are demonstrated through computations using numerical data from solar energy harvesting databases.

Autoignition in a non-premixed medium: DNS studies on the effects of three-dimensional turbulence

Direct numerical simulation (DNS) results of autoignition in anon-premixed medium under an isotropic, homogeneous, and decaying turbulence are presented. The initial mixture consists of segregated fuel parcels randomly distributed within warm air, and the entire medium is subjected to a three-dimensional turbulence. Chemical kinetics is modeled by a four-step reduced reaction mechanism for autoignition of n-heptane/air mixture. Thus, this work overcomes the principal limitations of a previous contribution of the authors on two-dimensional DNS of autoignition with a one-step reaction model. Specific attention is focused on the differences in the effects of two- and three-dimensional turbulence on autoignition characteristics. The three-dimensional results show that ignition spots are most likely to originate at locations jointly corresponding to the most reactive mixture fraction and low scalar dissipation rate. Further, these ignition spots are found to originate at locations corresponding to the core of local vortical structures, and after ignition, the burning gases move toward the vortex periphery Such a movement is explained as caused by the cyclostrophic imbalance developed when the local gas density is variable. These results lead to the conclusion that the local ignition-zone structure does not conform to the classical stretched flamelet description. Parametric studies show that the ignition delay time decreases with an increase in turbulence intensity. Hence, these three-dimensional simulation results resolve the discrepancy between trends in experimental data and predictions from DNSs of two-dimensional turbulence. This qualitative difference between DNS results from three- and two-dimensional simulations is discussed and attributed to the effect of vortex stretching that is present in the former, but not in the latter.

Continuous distribution kinetics for the thermal degradation of LDPE in solution

Polymer degradation in solution has several advantages over melt pyrolysis, The degradation of low-density polyethylene (LDPE) occurs at much lower temperatures in solution (280-360degreesC) than in conventional melt pyrolysis (400-450degreesC). The thermal degradation kinetics of LDPE in solution was investigated in this work. LDPE was dissolved in liquid paraffin and degraded for 3 h at various temperatures (280-360degreesC). Samples were taken at specific times and analyzed with high-pressure liquid chromatography/gel permeation chromatography for the molecular weight distribution (MWD), The time evolution of the MWD was modeled with continuous distribution kinetics. Data indicated that LDPE followed random-chain-scission degradation. The rapid initial drop in molecular weight, observed up to 45 min, was attributed to the presence of weak links in the polymer. The rate coefficients for the breakage of weak and strong links were determined, and the corresponding average activation energies were calculated to be 88 and 24 kJ/mol, respectively. (C) 2002 John Wiley Sons, Inc.

Electrochemically deposited crystalline thin film of polyaniline on nickel for redox reactions at positive potentials

Redox reactions which occur at positive potentials such as ferrous/ferric, hydroquinone/quinone, ferrocyanide/ferricyanide etc. in aqueous acidic electrolytes cannot be studied on non-platinum metals, for example, a Ni electrode. On the contrary, these reactions occur on polyaniline (PANI) modified Ni electrodes, as evidenced from cyclic voltammetry, amperometry and steady-state polarization experiments. Under identical experimental conditions of scan rate (v) and concentration (C), the peak current density (i(p)) values of Fe2+/Fe3+ redox reaction are greater on the PANI modified Ni than on Pt. Additionally, the peak potential separation (DeltaE(p)) of the voltammogram is lesser on the PANI modified Ni. With an increase in thickness of the PANI, DeltaE(p) increases suggesting that the redox reactions tend to depart from the reversibility. Scanning electron micrographs reveal the presence of a crystalline deposit of PANI on Ni when the thickness of PANI is about 0.08 mum. However, the PANI becomes amorphous and porous at higher thickness values. Raman spectroscopy and X-ray diffraction studies corroborate the observations made out of scanning electron microscopy. Higher catalytic activity of PANI is attributed to crystalline nature of PANI on Ni. Exchange current density and standard rate constant of Fe2+/Fe(3+)redox reaction are evaluated. (C) 2002 Published by Elsevier Science B.V.

Minimizing buffer requirements under rate-optimal schedule in regular dataflow networks

Large-grain synchronous dataflow graphs or multi-rate graphs have the distinct feature that the nodes of the dataflow graph fire at different rates. Such multi-rate large-grain dataflow graphs have been widely regarded as a powerful programming model for DSP applications. In this paper we propose a method to minimize buffer storage requirement in constructing rate-optimal compile-time (MBRO) schedules for multi-rate dataflow graphs. We demonstrate that the constraints to minimize buffer storage while executing at the optimal computation rate (i.e. the maximum possible computation rate without storage constraints) can be formulated as a unified linear programming problem in our framework. A novel feature of our method is that in constructing the rate-optimal schedule, it directly minimizes the memory requirement by choosing the schedule time of nodes appropriately. Lastly, a new circular-arc interval graph coloring algorithm has been proposed to further reduce the memory requirement by allowing buffer sharing among the arcs of the multi-rate dataflow graph. We have constructed an experimental testbed which implements our MBRO scheduling algorithm as well as (i) the widely used periodic admissible parallel schedules (also known as block schedules) proposed by Lee and Messerschmitt (IEEE Transactions on Computers, vol. 36, no. 1, 1987, pp. 24-35), (ii) the optimal scheduling buffer allocation (OSBA) algorithm of Ning and Gao (Conference Record of the Twentieth Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages, Charleston, SC, Jan. 10-13, 1993, pp. 29-42), and (iii) the multi-rate software pipelining (MRSP) algorithm (Govindarajan and Gao, in Proceedings of the 1993 International Conference on Application Specific Array Processors, Venice, Italy, Oct. 25-27, 1993, pp. 77-88). Schedules generated for a number of random dataflow graphs and for a set of DSP application programs using the different scheduling methods are compared. The experimental results have demonstrated a significant improvement (10-20%) in buffer requirements for the MBRO schedules compared to the schedules generated by the other three methods, without sacrificing the computation rate. The MBRO method also gives a 20% average improvement in computation rate compared to Lee's Block scheduling method.

A chemical approach to understand fragilities of glass-forming liquids

Fragility is viewed as a measure of the loss of rigidity of a glass structure above its glass transition temperature. It is attributed to the weakness of directional bonding and to the presence of a high density of low-energy configurational states. An a priori fragility function of electronegativities and bond distances is proposed which quite remarkably reproduces the entire range of reported fragilities and demonstrates that the fragility of a melt is indeed encrypted in the chemistry of the parent material. It has also been shown that the use of fragility-modified activation barriers in the Arrhenius function account for the whole gamut of viscosity behavior of liquids. It is shown that fragility can be a universal scaling parameter to collapse all viscosity curves on to a master plot.

Compressive behavior and fracture features of rubber bearing glass-epoxy composites exposed to aqueous media

Epoxy systems containing HTBN rubber material and reinforced with E-glass fibres, exposed to a fixed time duration in three separate media were subjected to compressive mode of deformation. The yield stress and fractographic features noted on the compression failed samples are reported in this work. The experiment reveals that the seawater exposed sample exhibits a drop in strength compared to dry (unexposed) sample. This kind of drop is maintained if the media is changed from seawater to distilled water. When HCl is included in seawater. the experiment shows a small rise in strength value. These changes have been attributed to various factors like medium ingress into samples assisting interface failure, the larger-sized Cl- influencing the extent of diffusion of medium into system and finally their participation in the deformation phenomena. The fractographic features reveal interface separations that show either scattered debris or a cleaner surface or display a whitish-coated matrix region depending on whether the tests are done on unexposed samples or on ones following the immersion in the media.

Comparison of CVD and MOCVD-grown Al2O3 coatings in the performance of cemented carbide cutting tool inserts

Low-pressure MOCVD, with tris(2,4 pentanedionato)aluminum(III) as the precursor, was used in the present investigation to coat alumina on to cemented carbide cutting tools. To evaluate the MOCVD process, the efficiency in cutting operations of MOCVD-coated tools was compared with that of tools coated using the industry-standard CVD process.Three multilayer cemented carbide cutting tool inserts, viz., TiN/TiC/WC, CVD-coated Al2O3 on TiN/TiC/WC, and MOCVD-coated Al2O3 on TiN/TiC/WC, were compared in the dry turning of mild steel. Turning tests were conducted for cutting speeds ranging from 14 to 47 m/min, for a depth of cut from 0.25 to 1 mm, at the constant feed rate of 0.2 mm/min. The axial, tangential, and radial forces were measured using a lathe tool dynamometer for different cutting parameters, and the machined work pieces were tested for surface roughness. The results indicate that, in most of the cases examined, the MOCVD-coated inserts produced a smoother surface finish, while requiring lower cutting forces, indicating that MOCVD produces the best-performing insert, followed by the CVD-coated one. The superior performance of MOCVD-alumina is attributed to the co-deposition of carbon with the oxide, due to the very nature of the precursor used, leading to enhanced mechanical properties for cutting applications in harsh environment.

Thermal cycling of Mg-MMCs

Creep resistant Mg alloy QE22 reinforced with maftec(R), saffil(R) or supertec(R) short fibres is cycled between room temperature and 308degreesC at different ramp rates in the longitudinal and transverse directions. From the careful analysis of the strain vs. temperature thermal cycling curves true material behaviour and artifacts from the dilatometer are deciphered. From this analysis true coefficient of thermal expansion and relaxation processes are deduced. Hysteresis at higher temperatures is attributed to the relaxation process, whereas hysteresis at low temperatures giving a tilt-ground shape to the thermal cycling curves is again an artifact due to the instrument. The change in ramp rate highlights this effect. Finally, the effect of thermal cycling on microstructure is examined.

V(2)O(5)-Anchored Carbon Nanotubes for Enhanced Electrochemical Energy Storage

Functionalized multiwalled carbon nanotubes (CNTs) are coated with a 4-5 nm thin layer of V(2)O(5) by controlled hydrolysis of vanadium alkoxide. The resulting V(2)O(5)/CNT composite has been investigated for electrochemical activity with lithium ion, and the capacity value shows both faradaic and capacitive (nonfaradaic) contributions. At high rate (1 C), the capacitive behavior dominates the intercalation as 2/3 of the overall capacity value out of 2700 C/g is capacitive, while the remaining is due to Li-ion intercalation. These numbers are in agreement with the Trasatti plots and are corroborated by X-ray photoelectron spectroscopy (XPS) studies on the V(2)O(5)/CNTs electrode, which show 85% of vanadium in the +4 oxidation state after the discharge at 1 C rate. The cumulative high-capacity value is attributed to the unique property of the nano V(2)O(5)/CNTs composite, which provides a short diffusion path for Lit-ions and an easy access to vanadium redox centers besides the high conductivity of CNTs. The composite architecture exhibits both high power density and high energy density, stressing the benefits of using carbon substrates to design high performance supercapacitor electrodes.

Enhanced Survival of Probiotic Lactobacillus acidophilus by Encapsulation with Nanostructured Polyelectrolyte Layers through Layer-by-Layer Approach

The encapsulation of probiotic Lactobacillus acidophilus through layer-by-layer self-assembly of polyelectrolytes (PE) chitosan (CHI) and carboxymethyl cellulose (CMC) has been investigated,to enhance its survival m adverse conditions encountered in the GI tract The survival of encapsulated cells in simulated gastric (SGF) and intestinal fluids (SIF) is significant when compared to nonencapsulated cells On sequential exposure to SGF and SIF fox 120 nun, almost complete death of free cells is observed However, for cells coated with three nanolayers of PEs (CHI/CMC/CHI) about 33 log % of the cells (6 log cfu/500 mg) survived under the same conditions The enhanced survival rate of encapsulated L acidophilus can be attributed to the impermeability of polyelectrolyte nanolayers to large enzyme molecules like pepsin, and pancreatin that cause proteolysis and to the stability of the polyelectrolyte nanolayers in gastric and intestinal pH The PE coating also serves to reduce viability losses during freezing and freeze- drying About 73 and 92 log % of uncoated and coated cells survived after freeze:drying, and the losses occurring between freezing and freeze-drying were found to be lower for coated cells

Enhanced endocytosis of nano-curcumin in nasopharyngeal cancer cells: An atomic force microscopy study

Recent studies in drug development have shown that curcumin can be a good competent due to its improved anticancer, antioxidant, anti-proliferative, and anti-inflammatory activities. A detailed real time characterization of drug (curcumin)-cell interaction is carried out in human nasopharyngeal cancer cells using atomic force microscopy. Nanocurcumin shows an enhanced uptake over micron sized drugs attributed to the receptor mediated route. Cell membrane stiffness plays a critical role in the drug endocytosis in nasopharyngeal cancer cells. (C) 2011 American Institute of Physics. [doi:10.1063/1.3653388]

Optimal maintenance policy of a machine subject to failure

Optimal maintenance policies for a machine with degradation in performance with age and subject to failure are derived using optimal control theory. The optimal policies are shown to be, normally, of bang-coast nature, except in the case when probability of machine failure is a function of maintenance. It is also shown, in the deterministic case that a higher depreciation rate tends to reverse this policy to coast-bang. When the probability of failure is a function of maintenance, considerable computational effort is needed to obtain an optimal policy and the resulting policy is not easily implementable. For this case also, an optimal policy in the class of bang-coast policies is derived, using a semi-Markov decision model. A simple procedure for modifying the probability of machine failure with maintenance is employed. The results obtained extend and unify the recent results for this problem along both theoretical and practical lines. Numerical examples are presented to illustrate the results obtained.

Time delay to firing of a triggered vacuum gap with barium titanate in trigger gap

The time delay to the firing of a triggered vacuum gap (t.v.g.) containing barium titanate in the trigger gap is investigated as a function of the main gap voltage, main gap length, trigger pulse duration, trigger current and trigger voltage. The time delay decreases steadily with increasing trigger current and trigger voltage until it reaches saturation. The effect of varying the main gap length and voltage on the time delay is not strong. Before `conditioning�¿ the t.v.g. two groups of time delays, long (>100�¿s) and short (<10�¿s), are simultaneously observed when a large number of trials are conducted. After conditioning, only the group of short time delays are present. This is attributed to the marked reduction of the resistance of the trigger gap across the surface of the solid dielectric resulting directly from the conditioning effect.

Electrical Treeing and the Associated PD Characteristics in LDPE Nanocomposites

Treeing in low density polyethylene (LDPE) filled with alumina nanocomposite as well as unfilled LDPE samples stressed with 50 Hz ac voltage has been studied. The tree inception voltage was monitored for various samples with different nano-filler loadings and it is seen that there is an increase in tree inception voltage with filler loading in LDPE. Treeing pattern and tree growth duration for unfilled and nano-filled LDPE samples have also been studied. Different tree growth patterns as well as a slower tree growth with increase in filler loading in LDPE nanocomposites were observed. The observed slow propagation of tree growth with filler loading is attributed to the changes in the polymer crystalline morphology induced by the presence of nano-particles and the greater ability of the nanoparticles to resist discharge growth. SEM studies carried out to determine the morphology of unfilled and nano-filled LDPE showed an increase in lamellae packing in LDPE nanocomposites and this increased lamellar density leads to a reduction in the tree propagation rate. Partial discharge activities were also monitored during the electrical tree growth in both the unfilled and the nano-filled LDPE samples and were found to be significantly different. PD magnitude and the number of PD pulses per cycle were found to be lower with electrical tree growth duration in LDPE nanocomposites as compared to unfilled LDPE. The same trend was seen with increased filler loading also.