Concentration Of Microvoids Research Articles

Experimental Evaluation of Tensile Properties of Epoxy Composites with Added Cellulose Nanofiber Slurry

Cellulose nanofiber (CNF) is one of natural fibers, and its Young modulus and tensile strength have been estimated close to 140 GPa and at least 2–3 GPa, respectively. As the homogeneous dispersion method of CNF in polymer matrix, the chemical modification of the CNF surface or solvent exchange process are often used. However, the environmental load of these processes is large, and the chemically modified CNF is expensive. In this study, mechanically defibrillated CNF reinforced epoxy resin matrix (Epoxy-CNF) composites with various CNF volume fraction were fabricated. Their tensile modulus and ultimate strength of the epoxy composites were deteriorated by the CNF slurry addition, while the fracture elongation was increased. This can be attributed to the interaction of epoxy and water, concentration of microvoids, and CNF agglomeration. Thus, the reduced water content in Epoxy-CNF composites improves their tensile properties.

Experimental and Numerical Investigation of Second Harmonic Generation by Creep Induced Micro-voids

Background: Identification of damage at an early stage is crucial for critical structural components. Objective: Creep induced micro-voids in heat treated polycrystalline pure copper are experimentally and numerically characterized. Methods: This is accomplished by the use of non-linear ultrasonic waves. Numerically, the study is carried out on an elastic material with randomly located micro-voids. The finite difference in time domain method is used. Results: Experimentally it is found that material damage due to micro-voids that are developed before 40% creep life may effectively be detected with nonlinear ultrasonic waves in the low power regime. Increase in second order nonlinearity parameter with percentage creep life is observed along with the corresponding increase in micro-void concentration. Optical metallography and micro-hardness measurements were used to corroborate the experimental results. For the numerical studies, void dimensions selected for study are much lower than the probing wave length. Concentration of micro-voids is varied from 0.01% to 3%. The second order nonlinearity parameter showed a slight increase in the initial stages and an abrupt increase with higher micro-void concentration in simulations. This agrees with experimental trends. Conclusion: The feasibility of detecting micro-voids in the early stages of creep using nonlinear ultrasonic waves is experimentally demonstrated and a simple model with voids is numerically studied to show the sensitivity of the second order nonlinearity parameter to micro voids.

Magnetic and Electric Properties of FeNbB Melt Spun Ribbons

In the paper Fe82Nb2B16, Fe80Nb2B18 and Fe78Nb2B20 amorphous alloys, obtained by melt spinning, were examined. It was shown that the alloys studied in the as quenched state and in the relaxed amorphous state or in nanocrystalline state (after a suitable annealing) belong to very good soft magnetic materials with relatively high resistivity. The influence of annealing on brittleness as well as magnetic and electric properties measured at room temperature was examined. A correlation between sample microstructure (in the as quenched state and after annealing) and different physical properties is discussed. It was shown that the observed changes of relative magnetic permeability can be explained by changes of magnetoelastic energy, concentration of microvoids frozen during production process, effective anisotropy constant and magnetic polarisation.

Nanocrystallization-induced structural evolution of intergranular amorphous phase in Fe 78B 13Si 9 alloy

The nanocrystallization-induced structural evolution of the intergranular amorphous phase in a Fe 78B 13Si 9 alloy was investigated by X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM), and positron annihilation spectroscopy. Crystallization occurs at 773 K, where nanocrystallites of α-Fe with an average grain size of a few tens of nanometers are formed in an amorphous matrix. With increasing annealing temperature up to 973 K, the average grain size increases up to ∼80 nm. In the as-prepared sample corresponding to an amorphous precursor, more than 90% of the positrons are localized at vacancy-sized free volumes dominantly surrounded by Fe atoms and other positrons are trapped by microvoids. Along with the appearance of nanocrystallites and their growth due to annealing, the concentration of microvoids is increased in the intergranular amorphous phase.

Crystallisation and optimisation of soft magnetic properties in amorphous FeCuXSi13B9 (X = Mn, Co, Zr)‐type alloys

AbstractIt has been shown that annealing of FeCu1XSi13B9 (X = Mn, Co, Zr) alloys at suitable temperatures T op and times τ p leads to an increase of the magnetic permeability and a decrease of the coercive force (optimisation effect of soft magnetic properties) determined at room temperature. The optimisation effect of soft magnetic properties in these alloys has been investigated by different experimental techniques. This effect has been attributed to the formation of a nanocrystalline phase, a change of the magnetostriction coefficient, stress, and a change of the concentration of microvoids. The diffusion parameters of the optimisation effect of soft magnetic properties, the Curie temperature and the temperatures of first and second stages of the crystallisation process have also been determined. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress oscillation-induced modulated phase transformation and yielding in syndiotactic polypropylene

The polymorphic behavior and yielding mechanisms developed in syndiotactic polypropylene (sPP) subjected to step-wise strain has been spatially resolved by simultaneous small- and wide-angle X-ray microdiffraction using synchrotron radiation. The scan carried out along adjacent transparent/opaque bands generated by ‘stress oscillation’ shows a gradual and non reversible crystal–crystal transformation related to a permanent deformation, in contrast to the elastic behaviour showed by sPP after several cycles of stretching and relaxing at constant velocity. However there is no change in the degree of crystallinity along the scan (∼30%). The μSAXS results reveal a modulation in the process of void formation with a higher concentration of microvoids in the opaque band than in the transparent one.

On the reversibility of hydrogen effects on the properties of amorphous silicon carbide

We performed a theoretical investigation on the effects of hydrogenation and dehydrogenation in the structural properties of amorphous silicon carbide (a-SiC). The calculations were carried out combining interatomic potentials with free volume Monte Carlo simulations. Our prototype amorphous silicon carbide systems contained as much as 85 000 atoms, allowing for a good statistical sampling. We addressed the role of hydrogen on the local and mid-range order of the material. We found that carbon atoms segregate, forming small clusters, embedded in an extensive silicon network. Ring statistics showed that hydrogenation increases the concentration of micro-voids in the structure and reduces its mid-range order. On the other hand, dehydrogenation recovers most of the original structural order.

Spatial distribution of defects in ultra-fine grained copper prepared by high-pressure torsion

A study of ultra-fine grained copper produced by high-pressure torsion is presented. Positron annihilation spectroscopies (slow positron implantation, lifetime and Doppler broadening spectroscopies), transmission electron microscopy, X-ray diffraction spectroscopy, and microhardness measurements have been employed in order to identify defects and their spatial distribution, i.e. lateral distribution and depth profile, in the specimens. Two types of defects could be identified: dislocations in the distorted regions along grain boundaries and microvoids of size of 3–4 vacancies inside the grains. No change in grain size as a function of the distance from the center of the sample disk was observed. The mean coherent domain size close to the surface was found to be 80 ± 20 nm and it slightly increases with depth. The concentration of microvoids has been found to decrease with depth and it slightly varies with the distance from the center of the disk, being lowest in the center. No position dependence of the concentration of dislocations was observed.

Thermal stability of ultrafine grained copper

Thermal stability of ultrafine grained (mean grain size 150 nm) copper prepared by high pressure torsion was studied by means of positron-lifetime spectroscopy correlated with transmission electron microscopy. The microstructure of the material studied is strongly inhomogeneous. The grain interiors with low dislocation density are separated by distorted regions with high number of dislocations. We have found that positrons are trapped at dislocations inside the distorted regions and in the microvoids situated inside the grains. Calculations of the lifetime of a positron trapped at a microvoid as a function of its size were performed to obtain information about sizes of the microvoids. Abnormal grain growth, when isolated recrystallized grains grow inside the deformed matrix, takes place from $160\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ From 280 to $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ recrystallization occurs. Strongly inhomogeneous spatial distribution of defects does not allow application of the simple trapping model. Therefore a model of positron behavior in the ultrafine grained materials was developed in the present work. The model takes into account inhomogeneous spatial distribution of defects and allows for determination of dislocation density, concentration of microvoids, linear size of coherent domains, and volume fraction of the distorted regions. Moreover using this model it was possible to determine the activation energy of the recrystallization.

Positron states in hydrogenated amorphous silicon

Low-hydrogen-concentration a-Si:H grown by HW-CVD forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. Over a wide temperature range we observe a single positron state in the amorphous network with a temperature-independent lifetime of 322 ps. From the temperature dependence of the positron diffusion we show that this is a localized state and present direct observation of hopping diffusion of positrons. On annealing up to 400 °C the amorphous network is seen to relax and the first stages of crystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low-activation energy, around 0.1 eV, and is interpreted in terms of a reconfiguration of the fundamental defect identifed by positron annihilation.

Annealing and recrystallization of hydrogenated amorphous silicon

Using a combination of positron annihilation and x-ray-diffraction techniques, we have shown that low hydrogen concentration hot wire chemical vapor deposition grown $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ forms a continuous random network with no detectable free volume in the form of microvoids, and no evidence of a microcrystalline phase. On annealing up to 400 \ifmmode^\circ\else\textdegree\fi{}C, the amorphous network is seen to relax and the first stages of recrystallization occur. There is also evidence of vacancy clustering to form a low concentration of microvoids. The structural relaxation has a very low activation energy, around 0.1 eV, and is probably caused by a reconfiguration of hydrogen-terminated dangling, bond defects. The formation of microvoids and the recrystallization can both be interpreted by the migration of unterminated dangling-bond defects.

Thermoelastic behavior of carbon fiber/polycarbonate model composites

Model composites of polycarbonate (PC) containing single, multiple and chopped carbon fibers (CF) with and without and epoxy sizing were prepared by hot pressing. The thermoelastic behavior of model CF/PC composites was characterized by stretching calorimetry at room temperature. For small strains £ (i.e., e 0.01) the specific mechanical work, specific heat effects and specific internal energy changes AU were completely reversible in stretching/contraction cycles and quantitatively obeyed the standard relationships for elastic solids. Young's moduli E and ΔU were significantly higher, whereas the linear thermal expansivities α L were lower for model CF/PC composites compared to those for the neat PC. Smaller values of the above parameters for composites reinforced with sized CF suggested weaker CF/PC interfacial interactions. Current theoretical models of thermoelastic properties of composite materials suggest the existence of unusually stiff, highly oriented PC structures in fairly thick boundary layers around CF. The onset of inelastic deformation, as well as mechanical failure in CF/PC model composites at significantly smaller strains compared to the neat PC were tentatively explained by the yield and subsequent plastic flow of the matrix polymer initiated by heat effects of fiber fragmentation processes, and by higher concentration of microvoids generated in fiber fragmentation/debonding events, respectively.

The effect of substrate temperature on P-CVD deposited a-SiGe : H films

Undoped a-SiGe : H films were deposited by the RF plasma chemical vapor deposition method. Films deposited at different substrate temperatures ranging between 100°C and 300°C were studied for their optoelectronic and structural properties. Structural defects like vacancies and microvoids were studied by positron lifetime spectroscopy (PLTS) at room temperature. Optoelectronic properties of the films were correlated with the PLTS measurements. The observations show a decrease in the deposition rate with substrate temperature. Good optoelectronic properties and proper structural relaxation have been obtained with a decrease in microvoid concentration.

Numerical simulation of diffusion growth of a grain-boundary microvoid

We study the influence of a concentration of stresses in the vicinity of grain-boundary microvoids on their growth by the diffusion mechanism. For this purpose, we use the boundary-value diffusion problem formulated earlier on the basis of thermodynamical analysis of the “metal-microvoid” two-phase system. The distribution of stresses in the vicinity of microvoids, which is required to solve the problem, is determined by the method of finite elements. The results of calculations demonstrate that, at a high concentration of microvoids on the boundary of a grain, the influence of concentration of stresses is insignificant. Only for large values of the relative radius of microvoidsR/λ and a low concentration of microvoids (λ≥50μm) is the effect of concentration of stresses as large as 15–20%.

Relationship between morphology and solvent interactions of a polyester composite material

Abstract A special morphology is shown by injected parts of polyester bulk molding compounds (BMC): a skin with thickness 100 nm, is made of polyester–styrene polymer; the fillers and glass fibers are located in the core of the material; the matrix is composed of a nodular phase of polyester–styrene embedded in a continous phase of low profile additive poly(vinylacetate). The low profile mechanism of the matrix is induced by the formation of microcracks and/or microvoids at the interface of the two phases and in the thermoplastic phase. This work shows that the global chemical composition is independent of the compound flow length but the microvoid concentration is not. These microvoids strongly influence the interactions with organic solvents: diffusion and equilibrium concentration are higher in porous areas.

Microvoids in chalcogenide glasses studied by positron annihilation

Abstract Microvoids have been detected in glassy As2Se3 by positron lifetime spectroscopy. The positron lifetime in perfect and defected crystalline As2Se3 was calculated and the results correlated with the size of the (open volume) defects. This correlation suggests that the observed mean lifetime τ = 345(2) ps for the glassy material corresponds to microvoids of on average about three missing atoms (i.e., 2.5–3 A radius). The lower limit of the microvoid concentration per atom is estimated to lie between 0.1 and 100 ppm (corresponding to a volume fraction 10−5−10−2%), well below the detection threshold of small-angle X-ray scattering. The microvoids are stable on heating to 260°C, i.e., after annealing at temperatures 80°C above the glass transition temperature. The present results are in conflict with earlier suggestions that positrons are trapped only in the vicinity of point-like negatively charged defects in amorphous As2Se3. Frequent observation of long positron lifetimes leads to the conclusion that microvoids are a feature common to a wide range of chalcogenide glasses. The consequence of these results for the recent model of Elliott, in which the first sharp diffraction peak in the total structure factor of covalently bonded glasses is attributed to the chemical ordering of microscopic voids around cation-centred units, is discussed.

Hydrogen retrapping after thermal charging of hydrogen in iron single crystal

A theory to estimate and describe the behavior of supersaturated hydrogen in interstitial sites of a normal lattice and in trap sites is suggested, and the experimental proof is provided by thermal analysis. In this theory, variation with temperature changes of the equilibrium state between hydrogen in trapping sites and in normal lattice sites, which occurs during rapid cooling after hydrogen charging at high temperature, is considered. Two evolution rate peaks of hy-drogen corresponding to a reversible trap, a dislocation, and to an irreversible trap, a microvoid, are observed, respectively, at 388 and 538 K in a thermal analysis plot. The hydrogen amount released from the reversible trap is increased with decreasing microvoid concentration, even though the reversible trap density is maintained at the same level. According to the theoretical analysis, supersaturated hydrogen dissolved in a normal lattice site by a rapid cooling of hydrogen-charged iron from high temperature is predominantly retrapped into the vacant irreversible trap-ping sites. The remaining hydrogen exists in the normal lattice interstitial sites and will maintain local equilibrium with hydrogen in the reversible trap sites. The apparent hydrogen diffusivities at 293 K with each type of trap are estimated to be 1 × 10-6 cm2/s for reversible traps and 4 × 10-8 cm2/s for microvoid traps, based on changes in the hydrogen amount released from each type of trap with the room-temperature anneal time in vacuum.

Structure changes by annealing and doping of a-Si:H prepared at high deposition rate studied by means of positron annihilation

Doppler broadening measurements of γ-ray spectrum on the positron annihilation (DOBPA) have been applied to a-Si:H prepared at high deposition rate in order to explore vacancy-like microvoids. The influences of annealing and/or doping to the concentration of microvoids have been investigated. The lineshape parameters in DOBPA for the annealing show three regions for the annealing temperature. They are constant until 350 °C, slowly increasing from 350 °C to 500 °C, and then steeply decreasing until crystallization temperature. These correspond to the change in the concentration of microvoids. However, for boron-doped a-Si:H, the constant region disappears. The doping of boron itself increases the concentration of microvoids, but little dependence on the amount of boron is seen. Further, no definite evidence for positronium formation could be pointed out for these samples even in deconvoluted spectra, but the electron momentum distribution increases at about 2mc=2 × 10 −3 presumably resulting from the formation of Si-H 2 bonds near microvoids.

The effects of degree of mixing on the properties of filled elastomers

AbstractA new technique has been established for improving the degree of mixing of the filler in an elastomer matrix. This technique includes a unique heating and colling mixing procedure in conjunction with surface modification of the carbon black and proper choices of polymers. The heating cycle is for decreasing the microvoid concentration and for enhancing polymer–filler adhesion. The cooling cycle is for improving the degree of mixing of the filler. Experimental data obtained from several polychloroprene systems strongly substantiate the new mixing technique. The results clearly show that a good degree of mixing obtained using the foregoing mixing technique can indeed enhance the mechanical and permeability properties of filled elastomers.

Effect of cold working on the hydrogen trapping phenomena in pure iron

Hydrogen trapping in iron with various types and amounts of trapping sites was investigated through permeation experiments. The apparent hydrogen diffusivity decreases as the degree of cold working increases. This decrease is attributed to an increase in dislocation density and microvoid concentration. Analysis of the permeation data suggests that hydrogen binding energies of dislocation and microvoid are 25.6 kJ/mole and 29.1 kJ/mole, respectively. The data further show that the apparent hydrogen diffusivity increases as applied hydrogen pressure increases. Energy level diagrams around various trapping sites are proposed.