Boundary Voids Research Articles

Se Inter-Diffusion Limits Absorber Layer Grain Growth in CdSe - CdTe Photovoltaics

Diffusion of Se from the CdSe window layer into the CdTe absorber improves the short circuit current density by narrowing the band gap and increasing the carrier lifetime. Thicker CdSe layers, however, show a dramatic loss in photocurrent collection due to Se over-alloying. Electron microscopy investigations show that this decrease in performance is due to the formation of small grains (∼783 nm average diameter), which exhibit grain boundary porosity in the Se inter-diffusion region. The larger grain boundary area and void free surfaces give rise to higher levels of nonradiative recombination, and therefore, a lower photocurrent. It is proposed that the small grain size is due to a drag force exerted by segregated Se solute atoms on a moving grain boundary, while faster Se diffusion along the grain boundaries results in vacancy build up and porosity due to the Kirkendall effect. The results indicate that the device processing conditions must be carefully controlled such that the negative effects of Se alloying (i.e., smaller grains, Kirkendall voids) do not undermine its benefits. Published by the American Physical Society 2024

Adsorptive performance and mechanism exploration of l-lysine functionalized celluloses for enhanced removal of Pb(II) from aqueous medium

In this study, two novel biosorbents of l-lysine grafted cellulose (L-PCM, L-TCF) were prepared for Pb(II) removal from aqueous solutions. Various adsorption parameters were surveyed, such as adsorbent dosages, initial concentration of Pb(II), temperature and pH, using adsorption techniques. At normal temperature, less adsorbent can achieve better adsorption capacity (89.71 ± 0.27 mg g−1 with 0.5 g L−1 of L-PCM, 16.84 ± 0.02 mg g−1 with 3.0 g L−1 of L-TCF). The pH range of application for L-PCM was 4–12 and that of L-TCF was 4–13. The adsorption of Pb(II) by biosorbents went through the boundary layer diffusion stage and void diffusion stage. The adsorption mechanism was chemisorption based on multilayer heterogeneous adsorption. The pseudo-second-order model fitted the adsorption kinetics perfectly. The Freundlich isotherm model adequately described Multimolecular equilibrium relationship between Pb(II) and biosorbents; the predicted maximum adsorption capacities of the two adsorbents were 904.12 and 46.74 mg g−1, respectively. The results showed that the adsorption mechanism was the electrostatic attraction between Pb(II) and -COOH and the complexation between Pb(II) and -NH2. This work demonstrated that l-lysine modified cellulose-based biosorbents have great potential in the field of Pb(II) removal from aqueous solutions.

Weldability of irradiated Stainless Steel 304 materials harvested from the National Research Universal (NRU) reactor

The weldability of the neutron-irradiated Stainless Steel 304 (SS 304) materials containing estimated helium levels of 12 to 44 appm has been studied through a Collaborative Research and Development Agreement between the Canadian Nuclear Laboratories and Oak Ridge National Laboratory. The work utilizes a SS 304 component harvested from the National Research Universal reactor. Laser beam welding was used to explore suitable welding conditions and associated parameters, to determine helium concentration limits for successful welding of irradiated stainless-steels. The experimental results show that maximum fusion zone void sizes, fusion zone void quantity and total heat affected zone helium-induced crack length generally increased with increasing He levels and effective weld heat input. A proper combination of welding parameters with low effective heat input, such as, laser power, weld travel speed and wire feed speed, were shown to improve the welding quality, and potentially reduce the length and probability of the formation of grain boundary helium-induced cracking and voids of irradiated SS 304 even with He concentrations as high as about 44 appm.

A Review on Failure Analysis of Turbine Blades of Aero Gas Turbine Engine

Almost all commercial electrical power on earth is generated with a turbine, driven either by wind, water, steam or burning gas. Turbine blades are subjected to very strenuous environments inside a gas turbine. They face high temperatures, high stresses, and a potentially high vibration environment. All these factors can lead to blade failure, resulting in catastrophic failure of turbine. In the event of turbine blade failure, turbine does not work and this leads to shutdown of power plant from one to four weeks or longer, depending on the extent of the damage and the procedure used to make the machine operational again, which results in economic loss and service to mankind also stops. For each day of a forced outage, a utility could lose hundreds of thousands of rupees in electrical power. The external and internal surface damages include corrosion, oxidation, crack formation, erosion, foreign object damage and fretting. The internal damage of microstructure include γ’ phase, CoNi3 [(Al, Ti)] phase aging (rafting), grain growth, brittle phases formation, carbides precipitation, creep and grain boundary void formation. These damages produce dimensional change which results in increase in operational stress that leads to deterioration in turbine efficiency. The deterioration of blade material is related to the high gas temperature, high steady state load levels (centrifugal load) and high thermal transient load (trips, start-ups, start downs). In this research, a review of common failures due to metallurgical defects found in gas turbine discussed is presented.

Short-Term Creep-Rupture Behaviour of AISI 310S Austenitic Stainless Steel Sheets Manufactured in Salem Steel Plant, a Special Steels Unit of Steel Authority of India Limited, Ministry of Steel, Government of India

The creep behavior of AISI 310S stainless steel taken from SAIL’s Salem stainless steel plant has been investigated by constant load tensile creep test at the temperatures of 973, 1023, and 1073 K and loads of 66.6, 74.8, 86.6, and 94.8 MPa. It exhibits steadystate creep behavior in most test conditions. The double logarithm plot of rupture life and applied stress yielded straight lines at all the three test temperatures indicating that power-law creep due to dislocation climb is the operating mechanism of creep deformation. Linear relationship was obtained for plots of logarithm of rupture life against inverse temperature obeying Arrhenius type of temperature dependence with activation energy of 340 kJ/mol. The stress-rupture data yielded a master curve of Larson-Miller parameter. The plot of Monkman-Grant relationship is typical indicating that rupture is controlled by growth of grain boundary cavities. The metallographic examination of crept samples revealed formation of grain boundary voids and cracks leading to intergranular creep fracture. Deformation twins and carbide precipitates were also observed. Creep-rupture properties are compared with that of AISI 600 ironbased superalloy to analyze quantitatively its behavior

Effects of Twin Boundary on Tensile Properties and Deformation Mechanism of Ni–Co Alloy Nanopillars

The influence mechanism of twin boundary in Ni–Co alloys is responsible for their deformation and technological applications. Herein, the uniaxial tensile loading of the Ni–Co single crystal nanopillars with twin boundaries is carried out by simulation. When the number of twin boundaries exceeds two layers, the effect of twinning boundary on dislocation is obvious, and the migration and reaction of twin boundaries lead to an increase in the number of dislocations accommodated inside, which is conducive to increasing the strength of the nanopillars. At low temperature, the dislocations are easy to penetrate and expand the twin grain boundary, while at high temperature, due to the aggravation of thermal motion, the resulting dislocations decrease. Meantime, the reaction between dislocations and twin boundaries leads to the disappearance of some original twin grain boundaries. As the strain rate increases, the main deformation mode of the nanopillar changes from the reaction of dislocation and twin boundary to twin boundaries migration. When the volume ratio of grain boundary void is less than 2.3%, the plastic deformation fluctuation is relatively stable. Nevertheless, when the volume ratio is greater than or equal to 2.3%, the fluctuation of the plastic deformation stage decreases rapidly.

Push-over bending and shear behavior assessments of hidden boundary void precast concrete slab with bi-tensional prestress

This paper presents a first proposed real-sized Hidden boundary Void precast concrete Slab, a so-called HVS. In order to assess and investigate push-over behaviors of the novel structural system, five types of real size specimens manufactured in factory are tested by corresponding static loading protocol experiments. Bending tests along with shear tests in both cases of presence and absence of topping concrete slabs are performed. Besides, composite slabs, which are two half-unit products connected by steel plates and U-bolts in the long edges, are surveyed to assess uniform characteristics of slab systems. Simultaneously, estimating calculations are proposed for the structural bearing capacity of partially prestressed concrete flagged sections. Proposed equations are developed according to provisions of several current global and local design standards. Moreover, this study provides another predicting approach using finite element analysis of MIDAS FEA for analytical performances of specimens. Thanks to these experimental and analytical results, the general characteristic of HVS may be observed, and then studied for realization in the field of prestressed precast concrete industries for construction.

Influence of Interphase Forces on Onset of Fully Developed Subcooled Flow Boiling at High Pressure

Accurate prediction of the onset point of fully developed boiling (FDB) in subcooled flow boiling is crucial for the design of industrial equipment. In this work, Eulerian−Eulerian two fluid model incorporated with the Rensselaer Polytechnic Institute (RPI) wall boiling model was used to investigate high-pressure subcooled boiling flow, with particular attention to the model’s capability of objectively predicting the onset of FDB. After using the experimental data to verify the model, the present work covered a working range of P=6.81−15.5MPa, G=966−2123 kg/m2s, q=750−1500 kW/m2. Results show that the onset of FDB coincided with the inflection point on the axial distribution of boundary void fraction. Lift force had no obvious effect on axial distribution of mean void fraction but had a significant influence on radially local void fraction and determination of the onset of FDB. The turbulent dispersion force, wall lubrication force had a great influence on both axial and radial distribution of void fraction, directly affecting the onset of FDB. Performance of interphase force models in boiling flow was found to be different from that in adiabatic flow, and the underlying mechanism was further revealed. At last, the most feasible interphase force models for predicting onset of FDB were recommended.

Correction Method for Hydraulic Conductivity Measurements Made Using a Fixed Wall Permeameter

Hydraulic conductivity measurement through a fixed wall permeameter is a common practice to obtain the fluid transmissibility characteristics of soil matrix; however, sidewall leakage due to rigid wall effect may significantly influence the observed values for coarse-grained soils. In this study, the boundary flow error was identified through characterizing the geometrical properties of voids adjacent to the sidewall, and a parameter known as the boundary void ratio (eb) was proposed to account for this effect. The findings suggest that a fixed wall cell containing coarse soils would unavoidably generate extra voids at the interface between soil grains and inner rigid wall, contributing to a larger eb at the wall than void ratio within the soil bed; the measured hydraulic conductivity is increased primarily due to the apparatus-induced error. A two-dimensional geometric model was then established to estimate the eb value for uniformly sized coarse soils confined by a rigid permeameter wall, based on which a method was obtained for eliminating the boundary flow error from a fixed wall cell. The mathematical method was finally validated against experimental data from existing literature. It can be concluded that the boundary condition at sidewall featuring unwanted gaps lead to overestimation of the coefficient of permeability; however, the proposed correction method could adequately eliminate the boundary flow error for uniformly sized coarse-grained soils tested within a rigid wall cell.

Structural and Mechanical Properties of AlCoCrNi High Entropy Nitride Films: Influence of Process Pressure

In the present study, novel AlCoCrNi high entropy nitride (HEN) films were deposited on Si substrate by a reactive direct current magnetron sputtering system. In order to investigate the influence of sputtering parameters on the microstructure and mechanical properties of the film, nitrogen flow ratio (RN: 25–100%) and process pressure (1.33 × 10−1–1.33 Pa) were controlled, respectively. All the films were identified as an amorphous phase with composition of near equiatomic ratios, regardless of the conditions of nitrogen flow ratios and process pressures. However, the limited mechanical properties were found for the films deposited under different nitrogen flow ratios with retaining the process pressure of 1.33 Pa. To enhance the mechanical properties of the AlCoCrNi HEN film, process pressure was adjusted. From the transmission electron microscopy (TEM) observation, the structure of the film deposited at the process pressure of 1.33 Pa is identified as a porous and open structure with a number of density-deficient boundary and nano-scale voids. On the other hand, densified morphology of the film was observed at pressure of 1.33 × 10−1 Pa. As a result, the hardness, elastic modulus, and H/E were improved up to 16.8, 243 GPa, and 0.0692, respectively.

Hot ductility behavior of AD730™ nickel-base superalloy

Hot ductility of the newly developed AD730™ nickel-base superalloy was investigated in the temperature interval 1050–1240 °C. The nil strength and nil ductility temperatures were determined by hot tensile testing using the Gleeble™ 3800 weld thermal simulation method. The influence of heating rate, representing the weld thermal cycle, on hot ductility behavior of the alloy was also investigated. The microstructure and the fracture mode of samples were examined by optical and scanning electron microscopy. The influence of heating rate on the extent of grain boundary liquation and void formation was determined and it is shown that the significant ductility loss near the NDT point could be related to the reduction of surface tension at the grain boundary-matrix interface. In addition, the contribution of hard precipitates, such as grain boundary MC carbides, voids, and cavities as other damage mechanisms responsible for ductility loss at high temperature, are discussed.

Life Testing of Ni-YSZ Fuel Electrodes under Electrolysis, Fuel Cell, and Current Switching Operation

An electrode supported Ni-YSZ/YSZ/Ni-YSZ symmetric cell was studied in life tests with constant current and switching current operation. At a current density of 0.2 A/cm2, there was no evidence of degradation. The cell electrochemical performance degraded slightly during life tests at current densities ≥ 0.8 A/cm2, with the ohmic resistance increasing and polarization resistance decreasing. The electrode/electrolyte interface structure was found to be seriously degraded after the life tests, with the formation of interfacial voids. In addition, grain boundary voids were observed within the YSZ electrolyte, especially after constant current operation. These observations presumably explain the observed electrolyte resistance increase. Both Ni and YSZ nanoparticles were observed in the electrode functional layers after life tests, possibly explaining the decrease of polarization resistance.

Finite element analysis of thermal and mechanical stresses due to the grain anisotropy of polycrystalline β-Sn

In order to investigate the stress and strain distributions caused by the grain anisotropy of polycrystalline β-Sn, a finite-element (FE) analysis was conducted using a polycrystalline model that considers the effect of grain anisotropy on the elastic and plastic properties. Even in the case of thermal free expansion with a temperature change of 75°C, plastic strains can be generated locally in polycrystalline β-Sn. This might be due to the high anisotropy of the thermal expansion coefficient and the low yield strength of β-Sn. It was found that both in-phase and out-of-phase thermal stresses can be present in the polycrystalline β-Sn simultaneously. The highest stress and strain appeared in the grains whose orientation is relatively different from those of neighboring grains, near the grain boundaries. This might cause the formation of a grain boundary void in the Pb-free solder. Comparing the strain distribution due to the thermal expansion coefficient anisotropy and the Young's modulus anisotropy, it was determined that the maximum value of the plastic strain due to the thermal expansion coefficient anisotropy is approximately 10% higher than that due to the anisotropy of Young's modulus, and the high strain areas are also different in the two cases. The thermo-mechanical fatigue life of β-Sn might be shorter than that estimated using the isothermal low cycle fatigue life at the same level as the macroscopic strain and could not be estimated by the previous method.

Deformation, diffusion and ductility during creep – continuous void nucleation and creep-fatigue damage

It is shown that the assumption of unit (negative) slope in the well known Monkman–Grant plot of time to failure against minimum creep rate is too restrictive. By acknowledging observed slopes in the range 0.8–1, a ductility–strain-rate relation is deduced where ductility decreases with reducing strain rate. This in turn has implications for the ductility exhaustion method as applied during stress relaxation in the dwell period of low cycle fatigue tests of austenitic steels at elevated temperature. The simple method is used to calculate the cyclic creep damage in typical tests on austenitic steels in the region 550–650 °C and is compared to other calculations as employed in the R5 high temperature assessment procedure. The assumption of a uniform nucleation rate of grain boundary voids with creep strain goes some way to predicting the slope of the ductility–strain-rate relation. Both the ‘unconstrained’ and ‘constrained’ (lower shelf) regions of void growth are discussed.

Growth behavior of voids randomly distributed at grain boundary on thin metal film with bamboo microstructures under thermal residual stress field and electric field

Based on the rigid grain hypothesis, an analytic expression is developed to investigate the grain boundary void growth on a thin metal film with bamboo microstructures under thermal residual stress field and electric field. The effect of the microstructure of the thin metal film, thermal residual stress, applied electric field, and diffusivity ratio between the grain boundary and the interface on the growth behavior of the voids and the stress evolution is evaluated. The result shows that the thin metal film with fine grains near the anode end may inhibit void nucleation and growth, which is beneficial for the optimization design of the thin metal film with bamboo microstructures.

Comparative molecular dynamics study of fcc-Al hydrogen embrittlement

Hydrogen embrittlement studies using reactive molecular dynamics have been conducted for Al metal nanoslabs with oxidized surfaces. These have provided evidence for possible grain boundary decohesion and void formation related range of hydrogen embrittlement mechanisms at different loading conditions with different initial defect configurations. Particularly, the effect of initial vacancy concentration on hydrogen diffusivity and localization, as well as on the amount of plasticity and grain formation is estimated. The results indicate significant reduction in diffusivity due to vacancy concentration, as well as hydrogen embrittlement effect through increase in dislocation emission and increase in dislocation slip propagation rate.

Exploring volume, compressibility and hydration changes of folded proteins upon compression.

Understanding the physical basis of the structure, stability and function of proteins in solution, including extreme environmental conditions, requires knowledge of their temperature and pressure dependent volumetric properties. One physical-chemical property of proteins that is still little understood is their partial molar volume and its dependence on temperature and pressure. We used molecular dynamics simulations of aqueous solutions of a typical monomeric folded protein, staphylococcal nuclease (SNase), to study and analyze the pressure dependence of the apparent volume, Vapp, and its components by the Voronoi-Delaunay method. We show that the strong decrease of Vapp with pressure (βT = 0.95 × 10(-5) bar(-1), in very good agreement with the experimental value) is essentially due to the compression of the molecular volume, VM, ultimately, of its internal voids, V. Changes of the intrinsic volume (defined as the Voronoi volume of the molecule), the contribution of the solvent to the apparent volume, and of the contribution of the boundary voids between the protein and the solvent have also been studied and quantified in detail. The pressure dependences of the volumetric characteristics obtained are compared with the temperature dependent behavior of these quantities and with corresponding results for a natively unfolded polypeptide.

A high-accuracy method for filling voids on remotely sensed XCO2 surfaces and its verification

The method for high accuracy surface modeling (HASM), inverse distance weighting (IDW) and ordinary Kriging (OK) are used to fill voids on XCO2 surfaces of GOSAT and SCIAMACHY. Inner voids and boundary voids are artificially made by cleaning out the downloaded data, where there are no voids, in different latitude belts of the northern hemisphere and the southern hemisphere at random. The cleaned-out data in the artificial voids are selected as the ‘true’ values for verification. The results demonstrated that HASM always has the highest accuracy compared with the classical methods of IDW and OK, whether voids are inner ones or boundary ones, data sets are from GOSAT or from SCIAMACHY, and the void areas are larger or smaller. HASM is an alternative approach to filling voids on XCO2 surfaces from the satellites.

Calculation of the volumetric characteristics of biomacromolecules in solution by the Voronoi–Delaunay technique

Recently a simple formalism was proposed for a quantitative analysis of interatomic voids inside a solute molecule and in the surrounding solvent. It is based on the Voronoi–Delaunay tessellation of structures, obtained in molecular simulations: successive Voronoi shells are constructed, starting from the interface between the solute molecule and the solvent, and continuing to the outside (into the solvent) as well as into the interior of the molecule. Similarly, successive Delaunay shells, consisting of Delaunay simplexes, can also be constructed. This technique can be applied to interpret volumetric data, obtained, for example, in studies of proteins in aqueous solution. In particular, it allows replacing qualitatively and descriptively introduced properties by strictly defined quantities, such as the thermal volume, by the boundary voids. The extension and the temperature behavior of the boundary region, its structure and composition are discussed in detail, using the example of a molecular dynamics model of an aqueous solution of the human amyloid polypeptide, hIAPP. We show that the impact of the solute on the local density of the solvent is short ranged, limited to the first Delaunay and the first Voronoi shell around the solute. The extra void volume, created in the boundary region between solute and solvent, determines the magnitude and the temperature dependence of the apparent volume of the solute molecule.

On High-Temperature Materials: A Case on Creep and Oxidation of a Fully Austenitic Heat-Resistant Superalloy Stainless Steel Sheet

The creep behavior of AISI 310S stainless steel taken from SAIL’s Salem stainless steel plant has been investigated by constant load tensile creep test at the temperatures of 973, 1023, and 1073 K and loads of 66.6, 74.8, 86.6, and 94.8 MPa. It exhibits steady-state creep behavior in most test conditions. The double logarithm plot of rupture life and applied stress yielded straight lines at all the three test temperatures indicating that power-law creep due to dislocation climb is the operating mechanism of creep deformation. Linear relationship was obtained for plots of logarithm of rupture life against inverse temperature obeying Arrhenius type of temperature dependence with activation energy of 340 kJ/mol. The stress-rupture data yielded a master curve of Larson-Miller parameter. The plot of Monkman-Grant relationship is typical indicating that rupture is controlled by growth of grain boundary cavities. The metallographic examination of crept samples revealed formation of grain boundary voids and cracks leading to intergranular creep fracture. Deformation twins and carbide precipitates were also observed. Oxidation tests were also carried out isothermally at 973 K, 1023 K, and 1073 K in dry air. The plots of mass gain versus square root time were linear at all the three test temperatures obeying parabolic kinetics of oxidation. It was found that scales are well adherent to the substrate. The plot of parabolic rate constant and inverse temperature was linear giving an activation energy value of 210 kJ/mol. The metallographic examination of an oxidized sample reveals duplex types of scales. Finally, rupture properties are compared with that of AISI 600 iron-based superalloy and oxidation weight gain analysis with surface nanocrystalline AISI 310S stainless steel to analyze quantitatively its behavior.