Large low-frequency resistance noise in chemical vapor deposited graphene

We report a detailed investigation of resistance noise in single layer graphene films on Si/SiO2 substrates obtained by chemical vapor deposition (CVD) on copper foils. We find that noise in these systems to be rather large, and when expressed in the form of phenomenological Hooge equation, it corresponds to Hooge parameter as large as 0.1-0.5. We also find the variation in the noise magnitude with the gate voltage (or carrier density) and temperature to be surprisingly weak, which is also unlike the behavior of noise in other forms of graphene, in particular those from exfoliation. (C) 2010 American Institute of Physics. doi:10.1063/1.3493655]

Convergence feedback and unstable low frequency oscillations in a simple coupled ocean‐atmosphere model

The role of convergence feedback on the stability of a coupled ocean‐atmosphere system is studied using model III of Hirst (1986). It is shown that the unstable coupled mode found by Hirst is greatly modified by the convergence feedback. If the convergence feedback strength exceeds a critical value, several new unstable intraseasonal modes are also introduced. These modes have very weak dependence on the wave number. These results may explain the behaviour of some coupled models and to some extent provide a mechanism for the observed aperiodicity of the El‐Nino and Southern Oscillation (ENSO) events.

A Low-Complexity Algorithm for Intrusion Detection in a PIR-Based Wireless Sensor Network

We present a low-complexity algorithm for intrusion detection in the presence of clutter arising from wind-blown vegetation, using Passive Infra-Red (PIR) sensors in a Wireless Sensor Network (WSN). The algorithm is based on a combination of Haar Transform (HT) and Support-Vector-Machine (SVM) based training and was field tested in a network setting comprising of 15-20 sensing nodes. Also contained in this paper is a closed-form expression for the signal generated by an intruder moving at a constant velocity. It is shown how this expression can be exploited to determine the direction of motion information and the velocity of the intruder from the signals of three well-positioned sensors.

Lubricated Tribology of a Eutectic Aluminium-Silicon Alloy in the Ultra-Mild Wear and Mild Wear Regimes for Long Sliding Times

Aluminium-silicon alloy, an important material used for the construction of internal combustion engines, exhibit pressure induced distinct regimes of wear and friction; ultra-mild and mild. In this work the alloy is slid lubricated against a spherical steel pin at contact pressures characteristic of the two test regimes, at a very low sliding velocity. In both cases, the friction is controlled at the initial stages of sliding by the abrasion of the steel pin by the protruding silicon particles of the disc. The generation of nascent steel chips helps to breakdown the additive in the oil by a cationic exchange that yields chemical products of benefits to the tribology. The friction is initially controlled by abrasion, but the chemical products gain increasing importance in controlling friction with sliding time. After long times, depending on contact pressure, the chemical products determine sliding friction exclusively. In this paper, a host of mechanical and spectroscopic techniques are used to identify and characterize mechanical damage and chemical changes. Although the basic dissipation mechanisms are the same in the two regimes, the matrix remains practically unworn in the low-pressure ultra-mild wear regime. In the higher pressure regime at long sliding times a small but finite wear rate prevails. Incipient plasticity in the subsurface controls the mechanism of wear.

Modelling, Design and Analysis of Low Frequency Platform for Attenuating Micro-Vibration in Spacecraft

One of the most important factors that affect the pointing of precision payloads and devices in space platforms is the vibration generated due to static and dynamic unbalanced forces of rotary equipments placed in the neighborhood of payload. Generally, such disturbances are of low amplitude, less than 1 kHz, and are termed as ‘micro-vibrations’. Due to low damping in the space structure, these vibrations have long decay time and they degrade the performance of payload. This paper addresses the design, modeling and analysis of a low frequency space frame platform for passive and active attenuation of micro-vibrations. This flexible platform has been designed to act as a mount for devices like reaction wheels, and consists of four folded continuous beams arranged in three dimensions. Frequency and response analysis have been carried out by varying the number of folds, and thickness of vertical beam. Results show that lower frequencies can be achieved by increasing the number of folds and by decreasing the thickness of the blade. In addition, active vibration control is studied by incorporating piezoelectric actuators and sensors in the dynamic model. It is shown using simulation that a control strategy using optimal control is effective for vibration suppression under a wide variety of loading conditions.

Low-Temperature Fluorination Of Some Nonmetals And Nonmetal Compounds With Fluorine

Low temperature fluorination with elemental fluorine of elemental phosphorus, sulphur, silicon, amorphous carbon and phosphorus trichloride, phosphorus pentoxide, triphenylphosphine, hexafluorodisilane, hexachlorodisilane, hexabromodisilane, tetrasulphur tetranitride, sulphur dioxide, thionyl chloride and sulphuryl chloride has been carried out in freon-11 medium. The corresponding fluoro compounds have been isolated in near quantitative yields, purified by low temperature fractional condensation and characterised by IR spectroscopy and elemental analysis.

Low-temperature internal friction of glass ceramics

Low-temperature internal-friction measurements have been used to study the universal low-energy excitations in glasses before and after crystallization in two glass ceramics, one based on MgO-Al2O3-SiO2 (Corning Code 9606) and one based on Li2O-Al2O3-SiO2 (Corning Code 9623). In the Code 9606 sample, the number density of excitations is greatly reduced, while in the Code 9623 sample, their number density remains practically unaltered in the crystallized state. These measurements confirm the conclusions reached by Cahill et al. (preceding paper), which were based on thermal measurements up to room temperature. These measurements also demonstrate the usefulness of internal friction as a tool for the study of these low-energy excitations, since internal friction is less sensitive to defects common to glass ceramics, like magnetic impurities and grain boundaries, which tend to dominate low-temperature specific heat and thermal conductivity, respectively.

Complex Permittivity Characteristics of Epoxy Nanocomposites at Low Frequencies

The complex permittivity characteristics of epoxy nanocomposite systems were examined and an attempt has been made to understand the underlying physics governing some of the unique macroscopic dielectric behaviors. The experimental investigations were performed using two different nanocomposite systems with low filler concentrations over the frequency range of 10(-2)-400 Hz, but for some cases, the data has been reported upto 10(6) Hz for a better understanding of the behaviors. Results demonstrate that nanocomposites do possess unique permittivity behaviors as compared to those already known for unfilled polymer and microcomposite systems. The nanocomposite real permittivity and tan delta values are found to be lower than that of unfilled epoxy. In addition, results show that interfacial polarization and charge carrier mobilities are suppressed in epoxy nanocomposite systems. The complex permittivity spectra coupled with the ac conductivity characteristics with respect to frequency was found to be sufficient to identify several of the nanocomposite characteristics like the reduction in permittivity values, reduction in the interfacial polarization mechanisms and the electrical conduction behaviors. Analysis of the results are also performed using electric modulus formalisms and it has been seen that the nanocomposite dielectric behaviors at low frequencies can also be explained clearly using this formalism.

Micromechanics Of Fatigue Crack-Growth At Low Stress Intensities In A High-Strength Steel

An attempt to systematically investigate the effects of microstructural parameters in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels has been made for a high strength low alloy steel to observe in general, widely different trends in the dependence of both the total threshold stress intensity range, DELTA-K(th) and the intrinsic or effective threshold stress intensity range, DELTA-K(eff-th) on the prior austenitic grain size (PAGS). While a low strain hardening microstructure obtained by tempering at high temperatures exhibited strong dependence of DELTA-K(th) on the PAGS by virtue of strong interactions of crack tip slip with the grain boundary, a high strength, high strain hardening microstructure as a result of tempering at low temperature exhibited a weak dependence. The lack of a systematic variation of the near-threshold parameters with respect to grain size in temper embrittled structures appears to be related to the wide variations in the amount of intergranular fracture near threshold. Crack closure, to some extent provides a basis on which the increases in DELTA-K(th) at larger grain sizes can be rationalised. This study, in addition, provides a wide perspective on the relative roles of slip behaviour embrittlement and environment that result in the different trends observed in the grain size dependence of near-threshold fatigue parameters, based on which the inconsistency in the results reported in the literature can be clearly understood. Assessment of fracture modes through extensive fractography revealed that prior austenitic grain boundaries are effective barriers to cyclic crack growth compared to martensitic packet boundaries, especially at low stress intensities. Fracture morphologies comprising of low energy flat transgranular fracture can occur close to threshold depending on the combinations of strain hardening behaviour, yield strength and embrittlement effects. A detailed consideration is given to the discussion of cyclic stress strain behaviour, embrittlement and environmental effects and the implications of these phenomena on the crack growth behaviour near threshold.

The Single Langmuir Probe under Orbital-Limited Conditions in a Low-Density Collisional Plasma

The cylindrical Langmuir probe under orbital-limited conditions was used to determine the charge density in a low-density collisional plasma. The Langmuir's theory was applied to both electron and ion saturation currents in their respective accelerating regions. Present study indicates that the length of the probe significantly affects the probe characteristics. A probe of suitable length under orbital-limited conditions may be useful under the experimental conditions where the radius of the probe is much smaller than the Debye lengt.

The contribution of polarons, bipolarons and intersite tunneling to low temperature conductivity in doped polypyrrole

The conductivity of highly doped polypyrrole is less than that of intermediately doped samples, by two orders of magnitude, at 4.2 K. This may be due to more number of bipolarons in highly doped samples. Bipolarons require four times more activation energy than single polarons to hop by thermally induced virtual transitions to intermediate dissociated polaron states than by the nondissociated process. The conduction process in these polyconjugated systems involve ionization from deep trapped states, having a View the MathML source dependence, hopping from localised states, having View the MathML source dependence, and intersite tunnel percolation, having T−1 dependence. The interplay of these factors leads to a better fit by View the MathML source. The mechanism for this exponential behaviour need not be same as that of Motts variable range hopping. Conduction by percolation is possible, if an infinite cluster of chains can be connected by impurity centers created by dopant ions. The tendency for the saturation of conductivity at very low temperatures is due to the possibility of intersite tunnel percolation is disordered polaronic systems.

A measurement technique for low frequencies

A technique for the measurement of frequency within a cycle of a periodic input is described. This can be useful for quicker measurement of low frequencies.

Low reynolds number flow of a dilute suspension in slowly varying tubes

We have discussed here the flow of a dilute suspension of rigid particles in Newtonian fluid in slowly varying tubes characterized by a small parameter ε. Solutions are presented in the form of asymptotic expansions in powers of ε. The effect of the suspension on the fluid is described by two parameters β and γ which depend on the volume fraction of the particles which we assume to be small. It is found that the presence of the particles accelerate the process of eddy formation near the constriction and shifts the point of separation.

HDL subspecies : association with low HDL-C, obesity, and metabolic syndrome

AIMS An independent, powerful coronary heart disease (CHD) predictor is a low level of high-density lipoprotein cholesterol (HDL-C). Discoidal preβ-HDL particles and large HDL2 particles are the primary cholesterol acceptors in reverse cholesterol transport, a key anti-atherogenic HDL mechanism. The quality of HDL subspecies may provide better markers of HDL functionality than does HDL-C alone. We aimed I) to study whether alterations in the HDL subspecies profile exist in low-HDL-C subjects II) to explore the relationship of any changes in HDL subspecies profile in relation to atherosclerosis and metabolic syndrome; III) to elucidate the impact of genetics and acquired obesity on HDL subspecies distribution. SUBJECTS The study consisted of 3 cohorts: A) Finnish families with low HDL-C and premature CHD (Study I: 67 subjects with familial low HDL-C and 64 controls; Study II: 83 subjects with familial low HDL-C, 65 family members with normal HDL-C, and 133 controls); B) a cohort of 113 low- and 133 high-HDL-C subjects from the Health 2000 Health Examination Survey carried out in Finland (Study III); and C) a Finnish cohort of healthy young adult twins (52 monozygotic and 89 dizygotic pairs) (Study IV). RESULTS AND CONCLUSIONS The subjects with familial low HDL-C had a lower preβ-HDL concentration than did controls, and the low-HDL-C subjects displayed a dramatic reduction (50-70%) in the proportion of large HDL2b particles. The subjects with familial low HDL-C had increased carotid atherosclerosis measured as intima-media-thickness (IMT), and HDL2b particles correlated negatively with IMT. The reduction in both key cholesterol acceptors, preβ-HDL and HDL2 particles, supports the concept of impaired reverse cholesterol transport contributing to the higher CHD risk in low-HDL-C subjects. The family members with normal HDL-C and the young adult twins with acquired obesity showed a reduction in large HDL2 particles and an increase in small HDL3 particles, which may be the first changes leading to the lowering of HDL-C. The low-HDL-C subjects had a higher serum apolipoprotein E (apoE) concentration, which correlated positively with the metabolic syndrome components (waist circumference, TG, and glucose), highlighting the need for a better understanding of apoE metabolism in human atherosclerosis. In the twin study, the increase in small HDL3b particles was associated with obesity independent of genetic effects. The heritability estimate, of 73% for HDL-C and 46 to 63% for HDL subspecies, however, demonstrated a strong genetic influence. These results suggest that the relationship between obesity and lipoproteins depends on different elements in each subject. Finally, instead of merely elevating HDL-C, large HDL2 particles and discoidal preβ-HDL particles may provide beneficial targets for HDL-targeted therapy.