Size effects on self-assembly and melting of silver-alkanethiolate on inert surfaces

Self-assembled materials produced in the reaction between alkanethiol and Ag are characterized and compared. It is revealed that the size of the Ag substrate has a significant role in the self-assembly process and determines the reaction products. Alkanethiol adsorbs on the surface of Ag continuous planar thin films and only forms self-assembled monolayers (SAMs), while the reaction between alkanethiol and Ag clusters on inert surfaces is more aggressive and generates a significantly larger amount of alkanethiolate. Two dissimilar products are yielded depending on the size of the clusters. Small Ag clusters are more likely to be converted into multilayer silver-alkanethiolate (AgSR, R = CnH2n+1) crystals, while larger Ag clusters form monolayer-protected clusters (MPCs). The AgSR crystals are initially small and can ripen into large lamellae during thermal annealing. The crystals have facets and flat terraces with extended area, and have a strong preferred orientation in parallel with the substrate surface. The MPCs move laterally upon annealing and reorganize into a single-layer network with their separation distance approximately equal to the length of an extended alkyl chain. AgSR lamellar crystals grown on inert surfaces provide an excellent platform to study the melting characteristics of crystalline lamellae of polymeric materials with the thickness in the nanometer scale. This system is also unique in that each crystal has integer number of layers – magic-number size (thickness). The size of the crystals is controlled by adjusting the amount of Ag and the annealing temperature. X-ray diffraction (XRD) and atomic force microscopy (AFM) are combined to accurately determine the size (number of layers) of the lamellar crystals. The melting characteristics are measured with nanocalorimetry and show discrete melting transitions which are attributed to the magic-number sizes of the lamellar crystals. The discrete melting temperatures are intrinsic properties of the crystals with particular sizes. Smaller lamellar crystals with less number of layers melt at lower temperatures. The melting point depression is inversely proportional to the total thickness of the lamellae – the product of the number of layers and the layer thickness.

Boundary condition effects in the simulation study of equilibrium properties of magnetic dipolar fluids

In this paper we investigate the equilibrium properties of magnetic dipolar (ferro-) fluids and discuss finite-size effects originating from the use of different boundary conditions in computer simulations. Both periodic boundary conditions and a finite spherical box are studied. We demonstrate that periodic boundary conditions and subsequent use of Ewald sum to account for the long-range dipolar interactions lead to a much faster convergence (in terms of the number of investigated dipolar particles) of the magnetization curve and the initial susceptibility to their thermodynamic limits. Another unwanted effect of the simulations in a finite spherical box geometry is a considerable sensitivity to the container size. We further investigate the influence of the surface term in the Ewald sum-that is, due to the surrounding continuum with magnetic permeability mu(BC)-on the convergence properties of our observables and on the final results. The two different ways of evaluating the initial susceptibility, i.e., (1) by the magnetization response of the system to an applied field and (2) by the zero-field fluctuation of the mean-square dipole moment of the system, are compared in terms of speed and accuracy.

Effects of temper level on the dependence of fatigue crack growth threshold and crack closure on the prior austenitic grain size

An attempt has been made to systematically investigate the effects of microstructural parameters, such as the prior austenite grain size (PAGS), in influencing the resistance to fatigue crack growth (FCG) in the near-threshold region under three different temper levels in a quenched and tempered high-strength steel. By austenitizing at various temperatures, the PAGS was varied from about 0.7 to 96 μm. The microstructures with these grain sizes were tempered at 200 °C, 400 °C, and 530 °C and tested for fatigue thresholds and crack closure. It has been found that, in general, three different trends in the dependence of both the total threshold stress intensity range, ΔK th , and the intrinsic threshold stress intensity range, ΔK eff, th , on the PAGS are observable. By considering in detail the factors such as cyclic stress-strain behavior, environmental effects on FCG, and embrittlement during tempering, the present observations could be rationalized. The strong dependence of ΔK th and ΔK eff, th on PAGS in microstructures tempered at 530 °C has been primarily attributed to cyclic softening and thereby the strong interaction of the crack tip deformation field with the grain boundary. On the other hand, a less strong dependence of ΔK th and ΔK eff, th on PAGS is suggested to be caused by the cyclic hardening behavior of lightly tempered microstructures occurring in 200 °C temper. In both microstructures, crack closure influenced near-threshold FCG (NTFCG) to a significant extent, and its magnitude was large at large grain sizes. Microstructures tempered at the intermediate temperatures failed to show a systematic variation of ΔKth and ΔKeff, th with PAGS. The mechanisms of intergranular fracture vary between grain sizes in this temper. A transition from “microstructure-sensitive” to “microstructure-insensitive” crack growth has been found to occur when the zone of cyclic deformation at the crack tip becomes more or less equal to PAGS. Detailed observations on fracture morphology and crack paths corroborate the grain size effects on fatigue thresholds and crack closure.

The Size-Dependent Elastic Properties of Nanofilms with Surface Effects

Size-dependent elastic constants are investigated theoretically with reference to a nanoscale single-crystal thin film. A three-dimensional _3D_ model is presented with the relaxation on the surface of the nanofilm taken into consideration. The constitutive relation of the 3D model is derived by using the energy approach, and analytical expressions for the four nonzero elastic constants of the nanofilm are obtained. The size effects of the four elastic constants are then discussed, and the dependence of these elastic constants on the surface relaxation and the ambiguity in the definition of the thickness of the nanofilm are also analyzed. In addition, the elastic moduli of the nanofilm in two kinds of plane problem are obtained and discussed in the case of a special boundary condition.

Size-Dependent Elastic Modulus And Fracture Toughness Of The Nanofilm With Surface Effects

The effective elastic modulus and fracture toughness of the nanofilm were derived with the surface relaxation and the surface energy taken into consideration by means of the interatomic potential of an ideal crystal. The size effects of the effective elastic modulus and fracture toughness were discussed when the thickness of the nanofilm was reduced. And the dependence of the size effects on the surface relaxation and surface energy was also analyzed.

Effects of diet and body size on phosphorus utilization of Liza haematocheila T. & S.

A 21-d laboratory experiment was conducted to study, the phosphorus (P) utilization of two different diets by redlip mullet Liza haematocheila T. & S. Sand-filtered water in salinity 30 and temperature 25 degrees C was used. Twenty-nine fish individuals were divided into three groups: 11 to group 1 (G1) fed on diet 1, 11 to group 2 (G2) fed on diet 2, and 7 to contrast group. Diet 1 was a commercial feed, more valuable in nutrition than diet 2 that similar to natural detritus. The results show the intake phosphorus (IP) of G1 was significantly higher than that of G2, and both increased linearly with body size at a certain amount of diet. The retention phosphorus (RP) in fish of G1 was lower than G2. The relationship between retention phosphorus and body size was positive and stronger in G2. Significant difference in faecal phosphorus (FP) was found between G1 and G2. Body size significantly impacted the excretion phosphorus (EP) in G1 but G2. The loss of intake phosphor-us in G1 was 10.83-20.27 mg per g fish weight gain, higher than that in G2 for 6.63-9.56. Of the phosphor-us, about 10% was allocated into growth, 50% in faeces, and the rest lost in excretion. The main part of phosphorus was lost in faeces but excretion. The phosphorus budget of the fish could be described as 100IP = 7.40RP + 47.39FP + 36.63EP (Diet 1) or 100IP = 11.93RP + 56.64FP + 21.76EP (Diet 2).

Studies on the Effect of High Energy Ball Milling On the Structural, Electrical and Magnetic Properties of Some Normal Spinets in the Ultrafine Regime

The present work is an attempt to understand the characteristics of high energy ball milling on the structural, electrical and magnetic properties of some normal spinets in the ultra fine regime, Magnetism and magnetic materials have been a fascinating subject for the mankind ever since the discovery of lodestone. Since then, man has been applying this principle of magnetism to build devices for various applications. Magnetism can be classified broadly into five categories. They are diamagnetic, paramagnetic, ferromagnetic antiferromagnetic and ferrimagnetic. Of these, ferro and ferri magnetic materials assume great commercial importance due to their unique properties like appropriate magnetic characteristics, high resistivity and low eddy current losses. The emergence of nanoscience and nanotechnology during the last decade had its impact in the field of magnetism and magnetic materials too. Now, it is common knowledge that materials synthesized in the nanoregime exhibit novel and superlative properties with respect to their coarser sized counterparts in the micron regime. These studies reveal that dielectric properties can be varied appreciably by high-energy ball milling in nanosized zinc ferrites produced by coprecipitation method. A semi conducting behaviour was observed in these materials with the Oxygen vacancies acting as the main charge carrier for conduction, which was produced at the time of coprecipitation and milling. Thus through this study, it was possible to successfully investigate the finite size effects on the structural, electrical and magnetic properties of normal spinels in the ultra fine regime

Effects of dietary calcium levels and limestone particicle size on the performance, tibia and blood of laying hens

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Effects of meal size and proximal-distal segmentation on gastric activity

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effects of Alloyed and Oxide Phases on Methanol Oxidation of Pt-Ru/C Nanocatalysts of the Same Particle Size

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Tuning the structural and magnetic properties of Sr 2 FeReO 6 by substituting Fe and Re with valence invariant metals

In the course of this work the effect of metal substitution on the structural and magnetic properties of the double perovskites Sr2MM’O6 (M = Fe, substituted by Cr, Zn and Ga; M’ = Re, substituted by Sb) was explored by means of X-ray diffraction, magnetic measurements, band structure calculations, Mößbauer spectroscopy and conductivity measurements. The focus of this study was the determination of (i) the kind and structural boundary conditions of the magnetic interaction between the M and M’ cations and (ii) the conditions for the principal application of double perovskites as spintronic materials by means of the band model approach. Strong correlations between the electronic, structural and magnetic properties have been found during the study of the double perovskites Sr2Fe1-xMxReO6 (0 < x < 1, M = Zn, Cr). The interplay between van Hove-singularity and Fermi level plays a crucial role for the magnetic properties. Substitution of Fe by Cr in Sr2FeReO6 leads to a non-monotonic behaviour of the saturation magnetization (MS) and an enhancement for substitution levels up to 10 %. The Curie temperatures (TC) monotonically increase from 401 to 616 K. In contrast, Zn substitution leads to a continuous decrease of MS and TC. The diamagnetic dilution of the Fe-sublattice by Zn leads to a transition from an itinerant ferrimagnetic to a localized ferromagnetic material. Thus, Zn substitution inhibits the long-range ferromagnetic interaction within the Fe-sublattice and preserves the long-range ferromagnetic interaction within the Re-sublattice. Superimposed on the electronic effects is the structural influence which can be explained by size effects modelled by the tolerance factor t. In the case of Cr substitution, a tetragonal – cubic transformation for x > 0.4 is observed. For Zn substituted samples the tetragonal distortion linearly increases with increasing Zn content. In order to elucidate the nature of the magnetic interaction between the M and M’ cations, Fe and Re were substituted by the valence invariant main group metals Ga and Sb, respectively. X-ray diffraction reveals Sr2FeRe1-xSbxO6 (0 < x < 0.9) to crystallize without antisite disorder in the tetragonal distorted perovskite structure (space group I4/mmm). The ferrimagnetic behaviour of the parent compound Sr2FeReO6 changes to antiferromagnetic upon Sb substitution as determined by magnetic susceptibility measurements. Samples up to a doping level of 0.3 are ferrimagnetic, while Sb contents higher than 0.6 result in an overall antiferromagnetic behaviour. 57Fe Mößbauer results show a coexistence of ferri- and antiferromagnetic clusters within the same perovskite-type crystal structure in the Sb substitution range 0.3 < x < 0.8, whereas Sr2FeReO6 and Sr2FeRe0.9Sb0.1O6 are “purely” ferrimagnetic and Sr2FeRe0.1Sb0.9O6 contains antiferromagnetically ordered Fe sites only. Consequently, a replacement of the Re atoms by a nonmagnetic main group element such as Sb blocks the double exchange pathways Fe–O–Re(Sb)–O–Fe along the crystallographic axis of the perovskite unit cell and destroys the itinerant magnetism of the parent compound. The structural and magnetic characterization of Sr2Fe1-xGaxReO6 (0 < x < 0.7) exhibit a Ga/Re antisite disorder which is unexpected because the parent compound Sr2FeReO6 shows no Fe/Re antisite disorder. This antisite disorder strongly depends on the Ga content of the sample. Although the X-ray data do not hint at a phase separation, sample inhomogeneities caused by a demixing are observed by a combination of magnetic characterization and Mößbauer spectroscopy. The 57Fe Mößbauer data suggest the formation of two types of clusters, ferrimagnetic Fe- and paramagnetic Ga-based ones. Below 20 % Ga content, Ga statistically dilutes the Fe–O–Re–O–Fe double exchange pathways. Cluster formation begins at x = 0.2, for 0.2 < x < 0.4 the paramagnetic Ga-based clusters do not contain any Fe. Fe containing Ga-based clusters which can be detected by Mößbauer spectroscopy firstly appear for x = 0.4.

Effects of stray magnetic fields and RF coupler asymmetry in the two-mile accelerator with sector focusing /

"AEC Contract AT(04-3)-400."

The effect of crack closure on the grain size dependence of fatigue crack growth threshold

Recent studies (I-7) clearly indicate a strong dependence of fatigue threshold parameter, A K on grain size in several alloy systems. Attempts to explain these observations on the basis of crat~tortuosity (1,8), fracture surface roughness (5,9) and crack closure (6) appear to present a fairly clear picture of the mechanisms that cause a reduction in crack growth rates at threshold. In general, it has been shown that coarse grained microstructures exhibit higher fatigue threshold in low carbon steels (1,5) aluminium alloys (7) and titanium alloys (6). In spite of these observations, there exists (10-1#) considerable uncertainity about the manner in which the AK~L depends on prior austenitic grain size in quenched and tempered steels. Studies in quenched and tempered steels demonstrating both an increase (3,12,14) as well as a decrease (11,12) in AKth with an increase in prior austenitic grain size can be sought to illustrate this point. Occasionally , the absence of any sensitivity of AKth to the variations in prior austenitJc grain size has also been reported (11,13). While a few investigators (5-7) comfortably rationalised the grain size effects on AK~L on the basis of crack closure by a comparison in terms of the closure-free component of the thresho~Ifc~, AK -f such an approach has yet to be extended to high strength steels, An attempt has been made in t~et ,pthrg sent study to explai. n the effect of pri, or austeniti.c grain size on &Kth on the basis of crack closure measurements in a high strength steel.

Effect of Structural Modification on the Quantum-Size Effect in II-VI Semiconducting Nanocrystals

The electronic structure of group II-VI semiconductors in the stable wurtzite form is analyzed using state-of-the-art ab initio approaches to extract a simple and chemically transparent tight-binding model. This model can be used to understand the variation in the bandgap with size, for nanoclusters of these compounds. Results complement similar information already available for same systems in the zinc blende structure. A comparison with all available experimental data on quantum size effects in group II-VI semiconductor nanoclusters establishes a remarkable agreement between theory and experiment in both structure types, thereby verifying the predictive ability of our approach. The significant dependence of the quantum size effect on the structure type suggests that the experimental bandgap change at a given size compared to the bulk bandgap, may be used to indicate the structural form of the nanoclusters, particularly in the small size limit, where broadening of diffraction features often make it difficult to unambiguously determine the structure.

Tuning the interfacial properties of supported metal nanoclusters

Nanoclusters are objects made up of several to thousands of atoms and form a transitional state of matter between single atoms and bulk materials. Due to their large surface-to-volume ratio, nanoclusters exhibit exciting and yet poorly studied size dependent properties. When deposited directly on bare metal surfaces, the interaction of the cluster with the substrate leads to alteration of the cluster properties, making it less or even non-functional. Surfaces modified with self-assembled monolayers (SAMs) were shown to form an interesting alternative platform, because of the possibility to control wettability by decreasing the surface reactivity and to add functionalities to pre-formed nanoclusters. In this thesis, the underlying size effects and the influence of the nanocluster environment are investigated. The emphasis is on the structural and magnetic properties of nanoclusters and their interaction with thiol SAMs. We report, for the first time, a ferromagnetic-like spin-glass behaviour of uncapped nanosized Au islands tens of nanometres in size. The flattening kinetics of the nanocluster deposition on thiol SAMs are shown to be mediated mainly by the thiol terminal group, as well as the deposition energy and the particle size distribution. On the other hand, a new mechanism for the penetration of the deposited nanoclusters through the monolayers is presented, which is fundamentally different from those reported for atom deposition on alkanethiols. The impinging cluster is shown to compress the thiol layer against the Au surface and subsequently intercalate at the thiol-Au interface. The compressed thiols try then to straighten and push the cluster away from the surface. Depending on the cluster size, this restoring force may or may not enable a covalent cluster-surface bond formation, giving rise to various cluster-surface binding patterns. Compression and straightening of the thiol molecules pinpoint the elastic nature of the SAMs, which has been investigated in this thesis using nanoindentation. The nanoindenation method has been applied to SAMs of varied tail groups, giving insight into the mechanical properties of thiol modified metal surfaces.