Bond Defects Research Articles

Evaluating damage of rubberized cement-based composites under aggressive environments

Rubberized cement-based composites are suitable for large surface applications that require higher strain capacity and lower propensity for cracking owing to length change rather than prioritize strengths. However, bond defects between rubber aggregates and cement matrix affect durability of the composite, especially the sustainability of such materials under aggressive environments. An enhanced rubber-cement matrix bond using a rubber coating solution contributed to a significant improvement in transfer properties. Hence, this study investigated effect of this bond enhancement on resistance of rubberized cement-based applications to acid and sulfate attacks. Especially, damage variable was defined as changes of load-resisting areas to evaluate the durability of rubberized mortar specimens during chemical attack process. Results demonstrated that the bond enhancement at rubber-cement matrix interface lowered damage variable of mortar incorporating coated rubber aggregates compared to that of the untreated one. Microstructural analysis still revealed a better interface in coated rubberized mortar exposed to acetic acid and to external sodium sulfate environments.

Defect detection of composite adhesive joints using electrical resistance method

Mechanical joint and adhesive bonding are two typical joining methods for composite materials. The bolt fastening method is reliable but increases the weight of the structure and cuts the composite fiber, which may cause stress concentration and strength degradation. The adhesive bonding method has a higher bonding strength than the mechanical fastening, but the bonding strength can deteriorate depending on the various parameters that influence the environmental conditions and manufacturing process. In addition, bonding defects due to the presence of foreign substances on the bonding surface or immature surface treatment is called a Kissing Bond, and can significantly reduce the bonding strength. The electrical resistance method is a very promising technique for detecting kissing bond defects by measuring the electrical characteristics after the dispersion of CNTs in the adhesive.In this study, composite-to-composite single-lap joint specimens with CNTs were fabricated and the defect detection capability and the static strength of composite joints were evaluated using the electrical resistance method. A carbon/epoxy composite was used as an adherend and the electrical resistance of the adhesive joint was measured by varying the content of CNTs to 0.5 wt%, 1.0 wt%, and 2.0 wt%. The AC and DC resistance values of composite adhesive joints with artificial defects were measured using an LCR meter and a high-resistance meter, and their lap shear strengths were evaluated.

Dangling bond defects in silicon-passivated strained-Si1−xGex channel layers

Dangling bond defects (DBs) in silicon-passivated (1- or 3-nm thick Si cap) strained-(100)Si1−xGex (x = 0.25–0.55) layers at interfaces with 1.8-nm thick HfO2 gate dielectric are studied by means of Electron Spin Resonance (ESR) spectroscopy. The results suggest a dominant contribution of Si DBs (Pb0 centers), a considerable fraction of which is located at the interface between the Si substrate crystal and the pseudomorphic Si1−xGex film. The density of Si DBs in the Ge containing samples is significantly lower than at the reference (100)Si/ HfO2 interface and decreases below the ESR detection limit (≈ 0.8 × 1011cm−2) with increasing thickness and Ge concentration in the Si1−xGex layer. However, the beneficial effect of Ge becomes less pronounced when the thickness of the Si cap is reduced to 1 nm or in the case of direct deposition of HfO2 on top of uncapped Si1−xGex. From these observations we conclude that DBs are eliminated due to in-diffusion of Ge from the Si1−xGex channel into interfacial Si layers, bringing the concentration of Ge to the range in which generation of Si DBs becomes energetically unfavourable, in agreement with previous observations on condensation-grown Si1−xGex layers.

Effect of an enhanced rubber-cement matrix interface on freeze-thaw resistance of the cement-based composite

Abstract Bond defects at rubber-cement matrix interface are detrimental to durability of the cement composite. Therefore, coating rubber aggregates with copolymer has been suggested to overcome this defect. This paper aims to investigate the effect of an improved rubber-cement matrix bond on frost resistance. Freeze-thaw temperature cycles were controlled by a thermal sensor embedded inside the core of a mortar specimen. Measurements of relevant quantities, such as mass loss, length change, mechanical properties (relative dynamic modulus of elasticity, compressive and flexural strengths), and durability factor, demonstrated that rubberized cement-based materials were more resistant under freeze-thaw environments than the control one. Especially, regardless of slight length gain of mortar incorporating coated rubber aggregates, copolymer coating still made the composite durable in frost conditions owing to its improved strain capacity and higher residual post-peak tensile strength.

Microstructure and Mechanical Properties of Thick Plate Friction Stir Welds for 6082-T6 Aluminum Alloy

Abstract 6082-T6 aluminum alloy plate with thickness of 42mm was butt welded by friction stir welding (FSW) from two sides. The microstructures of the joints exhibited different grain sizes because of unequal frictional heating and plastic flow during FSW process. The transition from the heat affected zone (HAZ) to the nugget zone (NZ) in thermos-mechanical affected zone of advancing side (AS-TMAZ) was more sudden than thermos-mechanical affected zone of retreating side (RS-TMAZ). Kissing bond (KB) defect throughout the entire FSW joint was displayed both at the grain boundary and in the interior of the grain with semi-continuous bands. KB had no direct effect on tensile properties. Vickers hardness of the FSW joint was lower than the BM because its high heat input, dissolved and coarsened precipitates and little to the grain size after FSW. Hardness distribution of double-sided welding joint showed X-shaped area softening characteristics, that is to say the lowest hardness was the junction of two welding joint of NZ and the junction of TMAZ and HAZ. The tensile fracture position occurred in the lowest hardness region of the FSW joint, and it did not occur in the KB defect position.

The effect of structural defects on the electron transport of MoS2 nanoribbons based on density functional theory

Using non-equilibrium Green’s function method and density functional theory, we study the effect of line structural defects on the electron transport of zigzag molybdenum disulfide (MoS2) nanoribbons. Here, the various types of non-stoichiometric line defects greatly affect the electron conductance of MoS2 nanoribbons. Although such defects would be lead to the electron scattering, they can increase the transmission of charge carriers by creating new channels. In addition, the presence of S bridge defect in the zigzag MoS2 nanoribbon leads to more the transmission of charge carriers in comparison with the Mo–Mo bond defect. Also, we find that the different atomic orbitals and their bonding structure at the edge affect the electron conductance of MoS2 nanoribbons. Moreover, we calculate the spin-dependent transport of MoS2 nanoribbons and showed that the spin polarization increases at the non-zigzag edges and remains even in the presence of the defect. This study presents a deep understanding of created properties in MoS2 nanoribbons due to the presence of structural defects.

Intrinsic thin film properties study of hydrogenated silicon using the method of RF-PECVD

Effect of hydrogen dilution on deposition rate, energy band gap, localized state defect - through observation of energy absorption Urbach, and surface roughness of intrinsic thin film was investigated using RF-PECVD. Films were grown on ITO glass substrate. Analysis of energy band gap was conducted to determine changes in the structure of a thin film of a-Si:H. Energy band gap is important to determine the portion of the spectrum of sunlight that is absorbed solar cells. The sun’s rays with energy greater than the energy band gap will be absorbed by the solar cell, but the rest will be the thermal energy which results in low efficiency. Characterization using UV-Vis spectrometer and Tauc’s plot methods showed that wide energy band gap was greater for larger hydrogen dilution. Moreover, the increase of the hydrogen dilution will decrease the rate of deposition and the Urbach energy. It is estimated that with an increased dilution of hydrogen will be obtained μc-Si:H film structure. This structure is more conductive due to the reduction of residual bandtail defect or dangling bond defects.

INVESTIGATION OF EFFECTS OF HEAT RELEASE IN ELECTROCHEMICAL SYSTEMS AND THEIR USE IN TECHNOLOGIES FOR PRODUCTION OF ENERGY-INTENSIVE SOURCES FOR AIRCRAFT

The search for new, more energy-intensive types of fuel for the operation of the power plants of aircraft is the most important task in aviation. The unique fuel that has no analogues is hydrogen. The paper attempts to substantiate the technology of metal hydride hydrogen storage in electrochemical systems based on aluminum and its alloys as the most affordable materials from fossil metals, since the traditional methods based on the use of cylinders and cryostats are not effective in transport systems. It is shown that the volumetric storage of hydrogen in the porous structure of metals with the formation of hydrides on atomic bond defects is maximally suitable for the implementation of the system, eliminating the excessive pressure and the low temperatures. The porous structure of the material provides both a high degree of availability of the electrolyte solution to the electrode for the accumulation of hydrides in the entire volume of the metal, and not only on its surface, but also the conditions for the realization of the reduction effect that excludes the explosive nature of hydrogen extraction. The problem of increasing the temperature in the reaction zone, which sometimes causes a slowdown in the rate of certain stages of the electrochemical process, is considered. Using the example of galvanic chrome plating, it has been established that an increase in the temperature inhibits the process of the reducing of the metallic chromium. Therefore, the detailed account of the thermal effects in the electrochemical system allows us to determine the mechanism of the processes. The work revealed that the thermal effects arising at the cathode determine the kinetics of the hydrogen reduction processes during the formation of a hydride. And the thermal effects at the anode determine the kinetics of the formation of a porous structure in the metal. The authors proposed to use the principle of action associated with the transition to the technologies of the volumetric storage of hydrogen in a solid-phase system based on a metal hydride compound for the formation of a new class of aircraft - diaplan.

Detection and Characterization Method for Interface Bonding Defects of New Composite Materials

The defects such as voids and debonding within the bonding interface between the composite and the substrate are the key factors affecting the safe use of materials. In this study, Industrial Computed Tomography (ICT) was used to perform non-destructive tests of bonding defects. High-precision characterization methods are required to quantify the defects. This paper proposes an improved image segmentation processing method for accurate extraction of defect edges and quantitative characterization. With the proposed method combining mathematical morphology and Fuzzy C-Means (FCM) threshold segmentation, the irregular defects of voids and debonding can be visualized and established with corresponding quantitative indicators. The experimental results show that the defect area error of the tested bonding layer is within ±6%, which satisfies the requirements of accurate detection and characterization of interface bonding layer defects of composite components. This work provides a strong technical basis for the reliability assessment of new ceramic matrix composites (CMCs) bonded structural members.

Ultrasonic Testing Combined with Pattern Recognition for the Detection of Kissing Bonds

Kissing bonds are defects in the adhesive bonds with intimate contact of touching surface but considerably lowered shear strength. Their detection specifically in the aerospace area is so not satisfactory. Usually, kissing bonds are inconspicuous in ultrasonic C-scans. However, the determination of attributes in the time domain and the frequency domain of an ultrasound signal provides the opportunity to derive a pattern for bonded area. Deviations from the pattern found in inconspicuous bonding areas indicate kissing bonds. The survey described here deals with the manufacturing of adhesively joint samples that purposefully include kissing bonds, as well as potential solutions for detecting them through ultrasonic testing combined with pattern recognition. The properties of the epoxy-based adhesive were varied by changing the mixing ratios between resin and hardener. Samples with a mixing ratio far apart from the manufacturer’s recommendation with an inconspicuous appearance in a C-scan, but low shear strength values were taken for further evaluation. After a definition and learning phase, a 100 percent hit rate to separate good bondings from kissing bonds could be derived in a blind test. The discriminating feature found is due to the frequency shift between good and kissing bonds as well as the relative amplitude of the second peak.

Low temperature dependence of protein-water interactions on barstar surface: A nano-scale modelling

The dynamics of the hydration shell of the inhibitor barstar is analysed at low temperature (300–243 K), through all-atom molecular dynamics simulations, and compared with that of bulk water. The relaxation of residence times of solvent molecules in the protein hydration shell follows a stretched exponential function exp[−(t/τ)β] with β = 0.48 ± 0.01, independent of temperature, showing that the decay process is mainly dominated by long-range molecular relaxation channels (short-range for bulk water). The percentage of water molecules exhibiting 4 hydrogen bonds, xHB4, is found to be a parameter essential for understanding some room and low temperature dependent properties of the protein hydration shell, suggesting an explanation for the unfreezing of protein hydration water as temperature decrease below 273 K. Moreover the dynamical transition that proteins and their hydration water exhibit at ~225 K can be explained by the decrease of ‘hydrogen bond defects’ in the protein hydration shell as temperature goes down. If most of those water molecules would present a tetrahedral arrangement (nearly no ‘hydrogen bond defects’), the bioactivity of proteins would be negligible. Comparison with experimental results is provided all along the work. Experimental data are quantitatively reproduced.

Effect of quality control parameter variations on the fatigue performance of aluminum friction stir welded joints

This paper describes fatigue tests performed on 6061-T651 and 5083-H321 aluminum friction stir welded joints with dimensions and loading conditions typical for structural applications. Butt and lap joint details with various defects intentionally introduced were tested under tension-only constant and variable amplitude loading conditions. In this paper, the fatigue test results are presented along with supporting metallurgical and nonlinear fracture mechanics analyses. Based on this work, it is concluded that kissing bond defects on the order of 0.3–1.0 mm can result in a significant fatigue life reduction and a shift in the failure mode to the weld root. The investigated toe-flash defect had less of an effect on fatigue performance. The lap joint did not perform as well as the butt joint detail.

Experimental studies on friction-stir welding of AA6061 using Inconel 601 tool

Nickel-based superalloy Inconel 601 has been used as tool material during friction-stir butt welding of AA6061-T6 plates. The unique properties of Inconel 601 viz. excellent high-temperature strength, strong work-hardening tendency, and high resistance to wear have been reported beneficial to utilize the material for potential use of FSW tool. Welding has been carried out in different combinations of tool rotation and traverse speed; joint performance in terms of ultimate tensile strength and micro-hardness has been studied. Optical microscopy has revealed existence of fine equiaxed gains at the nugget zone promoting relatively more refined grain structure as compared to the parent material. In few specimens, welding induced defects such as weld flash, kissing bond, zigzag line and tunnel defects have been identified. Maximum tensile strength ~ 133 MPa has been obtained at tool rotational speed ~ 1120 RPM and traverse speed ~ 40 mm/min. Fractographic analysis has revealed two distinct types of fracture mode in the specimens which have failed before the joint and at the joint, respectively. The specimens, failed before the weld zone, have corresponded to ductile fracture mode indicating dimples and ripples. On the contrary, beach marks (wavy patterns) have been noticed in the fractographic analysis of specimens which failed at the weld zone. After completing all welding experiments, the tool has been found not affected by wear, profile distortion by plastic deformation. Hence, application of the said tool can be advised provided the cost of machining the tool with desired pin profile is compromised.

Electron-Selective Epitaxial/Amorphous Germanium Stack Contact for Organic-Crystalline Silicon Hybrid Solar Cells

ABSTRACT: Carrier transport properties can be improved through suppressing the charge carrier recombination and decreasing the contact resistance using the proper carrier-selective contact. In this work, an epitaxial/amorphous germanium (epi/a-Ge) stack thin film is introduced into the rear surface of organic-inorganic hybrid (OIH) solar cells as an efficient electron-selective contact. This novel electron-selective stack contact simultaneously contributes low recombinative and resistive losses through a three-material system composed of n-type silicon (n-Si), epitaxial silicon germanium (epi-SiGe), and amorphous germanium (a-Ge), which promotes the tailoring of band structures, the passivation of surface dangle bond defects and the formation of Ohmic contact. The results result in obvious improvement in the open-circuit voltage (Voc) (from 549.5 mV to 643.0 mV) and fill factor (FF) (from 70.5% to 75.4%), corresponding to the increase in the power conversion efficiency (PCE) of OIH solar cells (from 10.3%...

Rubber aggregate-cement matrix bond enhancement: Microstructural analysis, effect on transfer properties and on mechanical behaviours of the composite

Limited strain capacity and low tensile strength of cement-based materials make them brittle and sensitive to cracking, behavior that limits durability of cement-based applications. Rubber aggregates (RA) incorporation appeared to be a suitable solution to improve the strain capacity and to limit the propensity of such materials for cracking. However, bond defect between RA and cementitious matrix is well-known and detrimental to mechanical and transfer properties of rubberized cement-based composites. This paper is dedicated to the enhancement of rubber-cement matrix interface and then investigates effect of this bond on transfer properties and mechanical behaviors of rubberized mortars. To achieve this goal, RA were coated with styrene-butadiene copolymer and after densification of this copolymer resin on their surface, they were mixed with the premixed-cementitious mixture. Another approach implemented was the use of an air-detraining admixture to produce rubberized mixture with the similar air content as the control mortar. Microstructural studies firstly clarified that cement paste bonded firmly on copolymer-coated RA. Air-permeability of the rubberized mortar incorporating copolymer-coated RA was lower than that of the control mortar. Moreover, rubber coating approach was beneficial to reduce water capillary absorption and to limit reduction in direct tensile strength. It also results in a better residual post-peak behavior and higher fracture energy, contributing to improve the resistance of the composite to shrinkage cracking even under high restrained condition.

Effects of slow highly charged ion irradiation on metal oxide semiconductor capacitors

Measurements were performed to characterize and better understand the effects of slow highly charged ion (HCI) irradiation, a relatively unexplored form of radiation, on metal oxide semiconductor (MOS) devices. Si samples with 50 nm SiO2 layers were irradiated with ion beams of ArQ+ (Q = 4, 8, and 11) at normal incidence. The effects of the irradiation were encapsulated with an array of Al contacts forming the MOS structure. High frequency capacitance–voltage (CV) measurements reveal that the HCI irradiation results in stretchout and shifting of the CV curve. These changes in the CV curve are attributed to dangling Si bond defects at the Si/SiO2 interface and trapped positive charge in the oxide, respectively. Charge state dependencies have been observed for these effects with the CV curve stretchout having a dependence of Q∼1.7 and the CV curve shifting with a dependence of Q∼1.8. These dependencies are similar to the results of previous studies focused on the Q-dependence of the stopping power of HCIs.

Defects modified in the exfoliation of g-C3N4 nanosheets via a self-assembly process for improved hydrogen evolution performance

In this work, self-assembly g-C3N4 samples were prepared without templates by a facile method involving thermal oxidation etching of bulk g-C3N4 and subsequent reflux in the different organic solvent at a low temperature. The internal hydrogen bond, cyano and amino defects of g-C3N4 nanosheets can be partly repaired via refluxing in the different polarity of the organic solvent, which is attributed to the effect of hydrogen bond polarity, hydroxyl and carbon chain structure of organic solvent. Especially, refluxing in the solvent of isopropanol, g-C3N4 nanosheets have transformed into quasi-sphere structure via a self-assembly process utilizing the effect of Oswald, which evidently reduced defect density within a molecular of g-C3N4, because of strong interaction between the generation of C–O bond of g-C3N4 nanosheets during refluxing in the isopropanol solvent and molecular group of isopropanol (hydroxyl group and the carbon chain structure). The self-assembly quasi-sphere g-C3N4 samples exhibit much faster photocatalytic hydrogen evolution rate than other g-C3N4 samples under visible light irradiation and the hydrogen evolution rate is 30.67 μmol h−1, which is about 10 times higher than that of bulk g-C3N4. The enhanced hydrogen evolution performance for quasi-sphere g-C3N4 samples is ascribed to large surface areas and accelerated separation of charges.

Effect of tool geometry and welding speed on mechanical properties of dissimilar AA2198–AA2024 FSWed joint

Different tool geometries were used to investigate the joining of aluminum alloys (AA2198 to AA2024) by friction stir welding (FSW). Three shoulder profiles (flat, raised spiral, and raised fan) and five different pin profiles (cone, half threaded cylindrical, straight cylindrical, tapered cylindrical and square) were selected. Preliminary investigations were conducted by moving the tool into a seamless sheet made of the AA2024-T3 in order to select the tools that produce defect-free joints. Preliminary investigations showed the raised fan shoulder profile helps the material flow from the edge of shoulder to the pin creating a smooth surface finish with no flash in comparison with flat and raised spiral shoulder profiles. Pins with a minimum diameter equal to half the plate thickness produced lack of penetration (LOP) defects, while increasing minimum pin diameter to the plate thickness eliminates the LOP defects. Half threaded cylindrical pin produced tunneling defect, whereas defect free joint made by straight cylindrical, tapered cylindrical and cubic pin profiles. So they were selected for joining AA2024 to AA2198. Fracture locations of different joint variants were observed the vicinity of the thermomechanical affected zone (TMAZ) of AA2198-T3 alloy, and in the nugget on the AA2198-T3 side which have the minimum hardness and highest strain localization as confirmed by hardness maps and digital image coronation (DIC). Higher measured temperature than dissolution temperature of AA2198 main strengthening precipitates could be the reason of low hardness and fracture in TMAZ and center of nugget. Furthermore a raised fan shoulder with a tapered cylindrical pin produced highest elongation and yield strength and it was selected as the best candidate for optimization of the welding parameters. It was found that higher rotational and traverse speeds enhance the formation of tunneling and kissing bond defects, suggesting that longer pins have to be used for higher traverse speeds. Welding speed 750 rpm with 450 mm min−1 could create joint with highest yield strength.

Self-Diffusion in Amorphous Silicon by Local Bond Rearrangements.

Experiments on self-diffusion in amorphous silicon (Si) were performed at temperatures between 460 to 600° C. The amorphous structure was prepared by Si ion implantation of single crystalline Si isotope multilayers epitaxially grown on a silicon-on-insulator wafer. The Si isotope profiles before and after annealing were determined by means of secondary ion mass spectrometry. Isothermal diffusion experiments reveal that structural relaxation does not cause any significant intermixing of the isotope interfaces whereas self-diffusion is significant before the structure recrystallizes. The temperature dependence of self-diffusion is described by an Arrhenius law with an activation enthalpy Q=(2.70±0.11) eV and preexponential factor D_{0}=(5.5_{-3.7}^{+11.1})×10^{-2} cm^{2} s^{-1}. Remarkably, Q equals the activation enthalpy of hydrogen diffusion in amorphous Si, the migration of bond defects determining boron diffusion, and the activation enthalpy of solid phase epitaxial recrystallization reported in the literature. This close agreement provides strong evidence that self-diffusion is mediated by local bond rearrangements rather than by the migration of extended defects as suggested by Strauß etal. (Phys. Rev. Lett. 116, 025901 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.025901).

Analytical layerwise stress and deformation analysis of laminated composite plates with arbitrary shapes of interfacial imperfections and discontinuous lateral deflections

In the present article, a double power series analytical layerwise model is proposed for analysis of multilayer orthotropic plates with arbitrary shapes of interfacial bond defects. In contrast to the previous models, the discontinuity in the lateral deflections of the successive layers is also considered. The adhesive bonding layers are modeled by elastic elements with normal and shear stiffness coefficients; so that, the continuity conditions of the transverse shear and normal stresses are met at the layer interfaces. It is the first time that an analytical solution rather than the data-dependent finite element method is proposed for stress and displacement analysis of multilayer rectangular plates with arbitrary shapes of interfacial bond damages, including the discontinuity in the transverse displacements of the successive layers. Results of the transverse displacement and stresses are then refined according to the 3D theory of elasticity. Results reveal that even small violations in the bonding between layers, may increase the resulting stresses, lateral deflections, and the differences between lateral deflections of the successive layers to more than twice, thrice, and five times, respectively. Moreover, in contrast to the growth of the in-plane stresses, redistribution of the transverse shear stress is governed by the equilibrium rather than the constitutive-law-based conditions.