Size Of Microvoids Research Articles

Microscopic fracture mechanism of sintered high purity chromium

The microscopic fracture mechanism was examined experimentally for high purity chromium. With the specimens prepared by sintering and swaging, a series of tensile tests was carried out in various test conditions. The high purity chromium became more ductile as the test temperature increased, but the ductile-to-brittle transition temperature (DBTT) was not found clearly in the temperature range of 300–623 K. In addition, it was very brittle when tested in water, probably due to hydrogen embrittlement. From a fractographical analysis, it was shown that there were many micro-voids on the fracture surface, when the test temperature was less than 423 K. These micro-voids on a local region in the grain boundary seemed to grow up during deformation and to act as the initiation sites of intergranular fracture, and subsequently followed by transgranular cleavage fracture. The fracture strain increased with decreasing size of micro-voids, irrespective of the test conditions. The growth of the micro-voids was supposed to be one of the predominant factors for controlling the fracture of high purity sintered chromium.

Statistical and Stochastic Description of Microvoid Evolution Observed in Dynamic Failure of Ductile Materials

A brief review is presented of statistical approaches on microdamage evolution. An experimental study of statistical microdamage evolution in two ductile materials under dynamic loading is carried out. The observation indicates that there are large differences in size and distribution of microvoids between these two materials. With this phenomenon in mind, kinetic equations governing the nucleation and growth of microvoids in nonlinear rate-dependent materials are combined with the balance law of void number to establish statistical differential equations that describe the evolution of microvoids' number density. The theoretical solution provides a reasonable explanation of the experimentally observed phenomenon. The effects of stochastic fluctuation which is influenced by the inhomogeneous microscopic structure of materials are subsequently examined (i.e. stochastic growth model). Based on the stochastic differential equation, a Fokker-Planck equation which governs the evolution of the transition probability is derived. The analytical solution for the transition probability is then obtained and the effects of stochastic fluctuation is discussed. The statistical and stochastic analyses may provide effective approaches to reveal the physics of damage evolution and dynamic failure process in ductile materials.

Characterization of microvoids in mulberry and tussah silk fibers using stannic acid treatment

To investigate the volume, size, and number of microvoids in mulberry and tussah silk fibers, stannic acid gel was used as a contrasting medium to the small-angle X-ray scattering (SAXS). The influence of the stannic acid treatment on the structure of silk fibers was first investigated by using the wide-angle X-ray diffraction prior to characterization of the microvoids. The changes in crystallite size and degree of orientation with increasing stannic acid gel fraction in fibers are investigated, and it was found that the stannic acid treatment does not cause serious changes in crystallite size and degree of orientation. The changes in crystallinity indices were observed when the volume fractions of stannic acid gel in the fibers exceeded about 10%. Thus, it was confirmed that the structure of silk fibers was retained in the region of the stannic acid gel fraction less than 10%. SAXS measurements revealed that the number and the fraction of the microvoids are larger, while the sizes of the microvoids are smaller, for the mulberry silk fibers compared with the tussah silk fibers. The fraction macrovoids, however, is considered to be larger for the tussah silk fibers. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 363–367, 1999

Microvoids in Bombyxmori silk. An electron microscope study

The size and distribution of microvoids in Bombyx mori silk were examined by transmission electron microscopy of silver sulphide ‘stained’ filaments. Silver sulphide deposited in voids and accessible regions of molecular structure appears as dense particles in thin transverse and longitudinal sections of silk filaments. Small particles (about 8 nm or less in diameter) occur around or adjacent to the periphery of the filaments. Larger particles (around 10–15 nm in diameter) occur in the form of dendritic arrays in the core region of the filaments. The leading edges of the dendritic arrays are oriented towards the fibre periphery. The particles (microvoids) appear to be either spherical or rod-like in shape and are aligned parallel to the long axis of the filament. A skin/core structure is proposed.

CO2 Sorption and Dilation of Poly(methyl methacrylate)

Sorption of CO2 in poly(methyl methacrylate) at 35−200 °C and concurrent dilation of the polymer at 35−85 °C over a pressure range up to 50 atm were studied. Dissolution and Flory−Huggins interaction parameters for the gas in the polymer, not only in the rubbery state but also in the glassy state, were estimated by analyzing the sorption data above the glass transition temperature (Tg0, 105 °C). Isothermal glass transition of the polymer/gas system was observed on isotherms of sorption and dilation below Tg0. Partial molar volumes of sorbed CO2 determined from the sorption and dilation isotherms increased with increasing concentration to the glass transition concentration. These isotherms were also analyzed on the basis of extended dual-mode models of sorption and dilation. From obtained parameters of the dual-mode models, nonequilibrium properties such as mean size and number of microvoids for the pure polymer and the CO2-sorbed polymer in the glassy state were evaluated. The mean size, dependent upon CO...

Accelerated life tests on a new water tree retardant insulation for power cables

This paper describes the results of an investigation in which 15 kV rated cables insulated with a new water tree retardant cross-linked polyethylene (TR XLPE) were subjected to accelerated aging tests under a controlled voltage stress and thermal load cycle conditions. Cables insulated with conventional XLPE and a commercially available TR XLPE were used as reference test populations to affirm the test methodology. Under the chosen conditions, cable life of the new TR XLPE as calculated using Weibull and log normal statistical distributions, was more than twice that for the reference TR XLPE. Extensive diagnostic measurements (water content, dissipation factor, water tree analysis) were performed on failed cable samples to bring out the differences between the three insulations. Electron micrographic investigations revealed the size and distribution of micro voids in the new TR XLPE to be smaller supporting its extended life under these tests. The experimental details of the accelerated life tests are also documented in a clear manner facilitating any archival of the data for future analysis and comparison.

The effect of small additional elements on the precipitation of reduced activation Fe9Cr2W steels

In order to study effects of small additional elements on precipitation of reduced activation Fe9Cr2W steels were irradiated up to 60 dpa at 693 K, 698 K and 733 K in FFTF. Micro-voids were observed in both materials of Fe9Cr2W with or without boron, the density of micro-voids in the steel with boron is larger than without boron, and the mean size of micro-voids is smaller than that without boron. However void swelling was less than 1%. Many precipitates were found to be M 23C 6 which consists of mainly Cr. Several precipitates which were Ti rich including Si and W were also observed at grain boundary at 733 K. Several Y 2O 3 particles was observed in an yttrium containing alloy. No precipitation including Al was observed in an Al containing alloy. Ti addition decreased precipitation of Ta-rich M 6C in 9Cr and 12Cr steels in this irradiation condition.

Precipitation Behavior of Irradiated Reduced-Activation Ferritic Steels.

In order to study the effects of small additional elements on precipitation behavior in reduced-activation ferritic steels under neutron irradiation, analytical transmission electron microscopy was used to examine the microstructure and the precipitation of the extraction replica of several reduced activation ferritic/martensitic steels which have different contents of small additional elements after 60 displacements per atom (dpa) irradiation at 693, 698 and 733K in the Fast Flux Test Facility (FFTF)/Materials Open Test Assembly (MOTA). All of reduced-activation ferritic/martensitic steels were found to have a good phase stability after irradiation. Micro-voids were observed in both materials of Fe-9Cr-2W with or without boron, the density of micro-voids in the steel with boron is larger than that without boron, and the mean size of micro-voids is smaller than that without boron. However void swelling was less than 1%. Most of the precipitates in the irradiated specimens were found to be M23C6 which consists of mainly Cr. The remainder of the precipitates were found to be Ta-rich M6C. Laves phase was observed only at 733K. Several precipitates which were Ti-rich including Si and W were also observed at grain boundary in Ti addition steels at 733K irradiation. Several Y2O3 particles were observed in an yttrium containing alloy. No precipitation including Al was observed in an Al containing alloy. Ti addition decreased precipitation of Ta-rich M6C in 9Cr and 12Cr steels in this irradiation condition.

Nondestructive Detection of Microvoids at the Interface of Direct Bonded Silicon Wafers by Scanning Infrared Microscopy

The density of microvoids with dimensions from several to a few tens of microns at the interface of bonded silicon wafers formed during annealing at different temperatures was investigated by using a submicron resolution scanning infrared microscope (SIRM). For low temperature heat‐treatment (400°C), the density and size (i.e., area) of microvoids have been found to be much larger in the case of bonding with hydrophilic wafers ( and up to 400 μm2, respectively) than for the hydrophobic one ( and up to 20 μm2). By increasing the annealing temperature to 1150°C, the density and size of microvoids decreased in both cases, but more significantly for hydrophobic wafer bonding (90% and <4 μm2 compared to 66% and <100 μm2). The cause of the different annealing behavior of microvoids between hydrophilic and hydrophobic samples is believed to be the native oxide forming only on the surface of the hydrophilic wafers during storage and surface treatment.

Random fractal percolation analysis of ductile fracture

The fractal percolation model of ductile fracture in plate specimens is established and the fracture probability P is calculated. If p (probability for forming microvoids) > 1−1/n, where n is the microvoid size, ductile fracture happens with a probability of 1. There is a critical probability p c with 0 < p c < 1−1/ n. If p > p c then there is a positive probability that ductile fracture happens and if p c > p > 0, ductile fracture does not happen. Fracture probability is sensitive to microvoid size, n, and probability p for forming microvoids. The critical probability p c decreases with increasing microvoid size or decreasing n. The fractal dimension can reflect the degree of damage of the specimen; the smaller the fractal dimension, the larger the degree of damage.

Probability of a ductile fracture

The present paper studies ductile fracture by a random fractal and calculates the fracture probability q of a ductile fracture. The fracture probability q is sensitive to the number n (microvoid size) and the probability p of forming microvoids. There are two critical probabilities p c1 and p c2. When p⩾ p c2 = 1-1/ n 2, the fracture probability q is 100%, the ductile fracture occurs with probability 1; when p c2 > p> p c1, there is a positive probability q (1> q>0), the fracture probability q increases with increasing probability p and/or decreasing number n; when p< p c1, the fracture probability q is zero (in fact less than 0.00001), the fracture does not occur. The critical probabilities p c1 and p c2 increase with increasing n. When 1-1/ n 2> p>1-1/ n, though a ductile fracture does not occur, there is at least a macrovoid which passes through the transverse plane of the specimen.

Study on the Formation of β-Crystalline from Isotactic Polypropylene Fiber

Abstract In this paper the conditions for the manufacture of high content β-form crystal isotactic polypropylene fiber in the presence of β nucleation and the β to α form transition during processing were studied. The results showed that the melt temperature had little effect on the relative content of β-form (K value) presented in as-spun fibers within a certain temperature range (210 to 230°C) while the cooling rate and cooling medium had a significant effect on it. In the case of a constant pump deliver (Q), the K value in the as-spun fiber decreased from 0.95 to 0 as the spinning speed increased from 3.25 m/min to 70 m/min. The smaller the thermal conductivity of cooling medium, the greater was the K value. After stretching the β-form PP fiber, there were many crevices at the fiber surface. It was confirmed by the mercury porosimetry that the drawn fiber possessed microvoid structure. The size and quantity of microvoids were closely related to the K value, stretching temperature and stretching ratio. Compared with PP fiber without nucleating agent, the drawn fiber had a lower apparent density and its moisture absorption was improved.

Argon sorption and partial molar volume in poly(ethyl methacrylate) above and below the glass transition temperature

AbstractSorption and dilation isotherms for argon in poly(ethyl methacrylate) (PEMA) are reported for pressures up to 50 atm over the temperature range 5–85°C. At temperatures below the glass transition (Tg=61°C), sorption isotherms are well described by the dual‐mode sorption model; and isotherms above Tg follow Henry's law. However, isotherms for dilation due to sorption are linear in pressure at all temperatures over the range investigated. Partial molar volumes of Ar in PEMA are obtained from these isotherms. The volumes are approximately constant above Tg (about 40 cm3/mol), whereas the volumes below Tg are smaller and dependent on both temperature and concentration (19–26 cm3/mol). By analyzing the experimental data according to the dual‐mode sorption and dilation model, the volume occupied by a dissolved Ar molecule and the mean size of microvoid in the glass are estimated to be 67 129 Å3, respectively. The cohesive energy density of the polymer is also estimated as 61 cal/cm3 from the temperature dependence of the dual‐mode parameters.

Sorption and diffusion of toluene in blends of low-density polyethylene with polyvinylchloride

Sorption and diffusion of toluene in LDPE-PVC blends in the form of powders are prepared by elastic-strain grinding were studied by the range method. LDPE is active, PVC inactive towards toluene vapour. It was found that the first cycle of toluene vapour sorption in films of polymer blends proceeds considerably more slowly than the second and subsequent sorptions; this is caused by slow relaxation of the non-equilibrium polymer structure formed during sample preparation. The limiting values of sorption increase with increasing content of PVC in the blend, evidently owing to the growing size of microvoids. The diffusion coefficient of toluene in LDPE-PVC blends decreases with increasing content of PVC. In polymer blends toluene is sorbed in the form of clusters.

Possible effects of collision cascades on bubble and void nucleation in metals

The localized and segregated production of point defects in collision cascades results in increased defect recombination, substantial defect clustering and strong temporal fluctuations in the local point defect concentrations. In the present paper, the role of these effects in cavity nucleation is discussed. Possible reaction paths starting from vacancy clustering in cascades and leading to the formation of bubbles and voids are characterized, and the relative significance of these paths in comparison with the corresponding effects expected for homogeneous displacement damage is assessed. The main processes considered are: (1) direct void nucleation from three-dimensional vacancy aggregates (“microvoids”) by stochastic fluctuations in the vacancy accumulation; (2) helium trapping by such microvoids which could reduce the effective helium diffusivity and enhance bubble nucleation; (3) collection of a surplus of vacancies in the cascade core by existing bubbles which could lead to a premature transformation from stable bubble growth to unlimited void growth. It is shown that, for realistic sizes, microvoids are unlikely to affect bubble and void nucleation significantly. The role of fluctuations in the frequency distribution of microvoid sizes is illustrated with the aid of “cascade significance plots” which indicate that tails of the microvoid size distribution may become more important than the commonly considered most probable or mean sizes. Future work is needed to clarify the role of microvoid size excursions. It is argued that, in the interesting temperature range of 1 3 to 1 2 of the melting temperature T m , the efficiency of cascade induced fluctuations in the local point defect concentrations is substantially reduced by correlations between the self-interstitial and vacancy contributions.

Characterization of microvoids in device-quality hydrogenated amorphous silicon by small-angle x-ray scattering and infrared measurements

The size, shape, and number density of microvoids in device-quality glow discharge deposited hydrogenated a-Si has been obtained by small-angle x-ray scattering (SAXS). By combining the SAXS results with infrared measurements, we deduce that the interior surfaces of these microvoids are largely unhydrogenated, containing at most 4--9 bonded H atoms. We suggest that these H atoms are the clustered H atoms previously detected by multiple-quantum NMR.

The observation of microvoids in device quality hydrogenated amorphous silicon

The size, shape, and number density of microvoids in device quality glow discharge deposited hydrogenated a-Si have been obtained by small angle x-ray scattering (SAXS). By combining the SAXS results with infrared measurements, we deduce that the interior surfaces of these microvoids contain ∼4–9 bonded H atoms. We suggest that these H atoms are the clustered H atoms previously detected by multiple quantum NMR.

Structure and superstructure of carbon fibres

AbstractThe structure of carbon fibres consists of stacks of carbon layers oriented parallel to the fibre axis. From X‐ray wide‐angle scattering studies (WAXS) one obtains various structural parameters characterizing the perfection of the stacking and the orientation of the layers. The latter is quantitatively related to the tensile modulus of the fibres. Small‐angle scattering studies (SAXS) are used to determine size, shape and orientation of microvoids. For carbon fibres with high heat treatment temperatures (HTT), a correlation between microvoid content and elongation at break is observed which indicates an optimum value for the microvoid content. Electrochemical intercalation is used to obtain information on the accessibility of the carbon layer structure.

Nucleation of Electrical Tress in Polyethylene

The inception voltage Vi for 1 pC partial discharges has been measured for vacuum molded needle-plane samples of low density polyethylene. The principal parameters studied are needle radius R (3≦R≦100 μm), ambient gas (air, N2, Ar, SF6), and frequency of the applied ac Voltage (60 Hz, 1 kHz). Experimental results during this "nucleation phase" indicate that electrons, field-emitted from the needle tip, transit through microvoids in the polymer before being stopped by collision with polymer chains. During transit they interact with molecules of the particular ambient gas which permeate the polymer's "free volume". In spite of the extremely small size of microvoids, Townsend avalanche-like behavior is inferred.

Positron annihilation studies of vacancy-type defects

The annihilation radiation of low energy positrons gives information on the electronic and defect structure of solids. There are three conventionally measurable quantities: the positron lifetime, the angular correlation of 2γ annihilation radiation and the Doppler-broadened annihilation line shape. In the presence of lattice defects the annihilation characteristics show considerable changes. This is due to positron trapping at defects like vacancies and their agglomerates, voids, dislocations and grain boundaries. The concentration of defects can be deduced from the ratio of trapped and free positrons.The annihilation characteristics are different for different defect configurations. Positrons reveal vacancy agglomeration and the lifetime of trapped positrons gives estimates on the size of microvoids in the range of 2–10 A. Various examples on the study of equilibrium and non-equilibrium defects, radiation damage and defect annealing are presented. Special emphasis is given to vacancy recovery and vacancy-impurity interactions in electron and neutron irradiated bcc transition metals like Fe, Mo, Nb, Ta.