Micron-size Voids Research Articles

Size effect on void coalescence under intense shear

Void interaction, leading to coalescence, is the mechanism leading to ductile failure under intense shearing. Published unit cell model studies have demonstrated that micron-size voids collapse to form micro-cracks while continuous elongation and rotation of the voids thin the intervoid ligaments. At a final stage, the deformation leads to plastic flow localization in the ligament, and the material loses the load-carrying capacity. The micro-mechanism of void collapse, elongation, and rotation has been studied using 2D and 3D unit cell simulations but only within a conventional strain hardening material and, thereby, not accounting for micron scale size effects. However, the severe plastic deformation near the voids implies the development of significant plastic strain gradients, which must be accommodated by geometrically necessary dislocations (GNDs) that strengthens the matrix locally and elevates the stress level. The present research accounts for such gradient strengthening within the matrix in order to investigate the material size effect in ductile shear failure. The work presented leans on the unit cell model approach by Tvergaard (2009), but enables a constitutive length parameter to enter the analysis by substituting the matrix with a Fleck–Willis gradient enhanced material. The results reflect the combined effect of applied load, strain hardening, initial void volume fraction, and microstructure size. The general conclusion is that matrix strengthening, governed by size, delays the loss of load-carrying capacity and leads to less concentrated localization around small void. The results also show that the void collapse, elongation, and rotation mechanism is more sensitive to changes to the applied load, hardening, and initial void volume fraction at small scales.

Polyvinylidene fluoride aerogel with high thermal stability and low thermal conductivity

A polyvinylidene fluoride (PVDF) aerogel was fabricated by a phase inversion induced sol-gel process along with supercritical drying. The PVDF aerogel exhibited a hierarchical structure consisting of agglomerates and micron-size voids. The agglomerates consisted of nanoparticles and nanopores. The PVDF aerogel showed a water contact angle of 126° and was thermally stable up to 400 °C in aerial environment. The thermal conductivity of the PVDF aerogel was as low as 0.03602 W/(m·K). The PVDF aerogel is promisingly applied in thermal insulation under humid conditions and water treatment.

Fine-fibrous cellulose II aerogels of high specific surface from pulp solutions in TBAF·H2O/DMSO

Abstract Lightweight cellulose II aerogels featuring densities of about 40–70 mg cm−3 were prepared from 1 to 3% solutions of different pulps in hot (60°C) tetra-n-butylammonium fluoride (TBAF)·H2O/dimethyl sulfoxide (DMSO) by (i) the coagulation of cellulose with EtOH to afford self-standing, transparent and homogeneous gels, (ii) gel ripening and washing, (iii) solvent exchange and (iv) supercritical carbon dioxide (scCO2) drying. Size exclusion chromatography (SEC) analyses confirmed that the chemical integrity of cellulose is largely preserved at short dissolution times. Dissolution of more than 2% of cellulose at a sufficiently low viscosity for solution, casting was achieved after the water content of TBAF was reduced to a value equaling that of the monohydrate. Intriguingly, the obtained aerogels featured higher specific surfaces (≤470 m2 g−1) than comparable materials prepared from other cellulose solvents. This is due to the particular morphology of TBAF aerogels, which is supposedly formed by spinodal decomposition of the cellulose/solvent mixture upon exposure to the cellulose antisolvent. As a result, largely homogeneous three-dimensional (3D) networks of agglomerated cellulose spheres were formed, which simultaneously acted as supporting scaffolds for interconnected micron-size voids. As cellulose spheres are composed of very small interwoven nanofibers, TBAF-derived aerogels contain a high portion of micropores and small amounts of mesopores, too.

Void Formation and Intermetallic Growth in Pulse Electrodeposited Cu-Sn Layers for MEMS Packaging

Electrodeposited copper (Cu)-tin (Sn) based solid–liquid interdiffusion (SLID) bonding is becoming popular in wafer-scale packaging of inertial Micro-Electro-Mechanical-Systems (MEMS) sensors due to its inherent advantages of lower cost, low processing temperatures and less stringent surface uniformity requirements. However, eliminating micron-size voids within intermetallic compounds (IMCs) and bond interfaces has remained a challenging task. The present study focuses upon IMC growth and void formation at varying temperatures and times. Stacks of varying thickness of Cu and Sn were fabricated by electrodeposition, and the samples were annealed at temperature ranging up to 300°C. Scalloped shaped Cu6Sn5 (η-phase) and comparatively uniform Cu3Sn (e-phase) intermetallics were observed. Experimental results show that the growth of metastable Cu6Sn5 dominates IMC formation at lower temperatures but as temperature increases, Cu3Sn dominates over the Cu6Sn5 growth. This IMC growth transition from Cu6Sn5 dominant growth to Cu3Sn dominant growth depends on the annealing temperature and has a critical time duration. The IMC thicknesses are compared with those obtained by numerical simulation models. For given annealing temperatures, intermittent voids formed in the IMC layers show increasing size and decreasing void fraction trends with increasing annealing times. The results suggest that Cu-Sn SLID bonding performed at 275°C yields reliable bonding since the void growth is minimal. Based on these results, a test vehicle containing a kelvin structure and daisy chains (having large number of Cu-Sn bonded structure), was fabricated, resulting in electrical resistances lower than 30 m-ohms and 6 ohms, respectively.

Effect of pre-strain and work hardening rate on void growth and coalescence in AA5052

The effect of work hardening on void growth and coalescence was studied by pulling samples of aluminum alloy 5052 in tension to various amounts of pre-strains. To mimic voids forming during fracture, artificial micron-size voids were created in the pre-strained samples with an ultrafast laser. Void growth and coalescence was then followed in-situ under an optical microscope until failure. The effect of pre-strain on void growth rate and coalescence strains was extracted from the experiments. Comparison was made with the McClintock model for void growth which is only able to predict void growth in the samples without pre-strains. A finite element model without any adjustable parameters was able to capture the effect of pre-strain on both void growth and fracture provided that the proper material properties were used. We finally propose a simple model based on the local state of hardening between voids which is able to capture the change in failure strains due to pre-strain as well as void growth and coalescence.

Spatially resolved photoconductivity of thin films formed by colloidal octapod-shaped CdSe/CdS nanocrystals

We studied the optical absorption and photoconductive properties of thin films consisting of core-shell octapod-shaped nanocrystals, which consisted of CdS pods that branch out from a CdSe core. The current-voltage characteristics were measured at room and cryogenic temperatures and agreed well with a phenomenological exponential fitting model, from which we could extract the sheet resistance and the average voltage barrier for the charge tunneling between the octapods. The temperature dependence of the photocurrent showed temperature activated behavior above 220 K and a non-Arrhenius exponential (T/T(0))(n) dispersion below 220 K. Furthermore, we mapped the photocurrent generation within the octapod film via scanning photocurrent microscopy, which revealed photocurrent enhancement near micron-size voids and spatial shifts of the photocurrent maxima with bias voltage.

Emission of dislocations from nanovoids under combined loading

Among all directions available for dislocation emission from the surface of a cylindrical circular void, the direction of the most likely emission is determined. It is shown that this direction is different from the direction of the maximum shear stress at the surface of the void due to the applied loading. The critical stress and the direction of the dislocation emission are determined for circular nanovoids under remote uniaxial, pure shear, and arbitrary biaxial loading. The analysis includes effects of the loading orientation relative to the discrete slip plane orientation. It is shown that dislocations are emitted more readily from larger nanovoids and that wider dislocations are emitted under lower applied stress than narrow dislocations. Different mechanisms, under much lower stress, operate for growth of the micron-size voids.

Deuterium retention in rhenium-doped tungsten

Deuterium retention in rhenium doped tungsten (W–Re) was investigated by means of the thermodesorption technique. W–Re alloys of different structures and concentrations of Re were investigated. All tungsten rhenium alloys had a common structural feature – micron size voids in the bulk. The samples were irradiated by 200eV deuterium ions in the fluence range 1022–1024D/m2. The thermodesorption spectra of polycrystalline W measured with a linear ramp of 5K/s has two peaks at ∼450 and 650K. In TDS of W–Re alloys an additional high-temperature peak was found at 850K. The observed retention in W–Re alloys depends strongly on materials structure. In the range of 1–10%Re admixture the Re concentration in W–Re alloy does not influence noticeably the deuterium retention. Structure changes (filling of voids in material) was found to take place both at annealing and low-energy deuterium irradiation.

Simulation of Void Collapse in an Energetic Material, Part 2: Reactive Case

In Part 1 a numerical methodology was used to study collapse of voids in energetic materials by ignoring the contributions of the gas phase and chemical reaction. In this paper, the thermomechanical response of solid phase is combined with the vapor-phase compression and chemical reactions to study their effects on the energy deposition mechanisms. The formulation and numerical treatment of the coupled solid-gas interactions, including heat release caused by chemical reactions, are discussed. The effects of loading intensity and void size are studied. The results show that for the micron-size voids under consideration significant gas-phase chemical reactions occur. However, their influence on the void collapse itself is minimal. Results indicate that when voids completely collapse a significant amount of reaction products are created, but the collapse time is too short to set up thermal runaway at the hot spot before void collapse.