Mechanism Of Cavity Formation Research Articles

Study on the evolutionary mechanism of the cavity inside aluminum nanoparticles while reacting with oxygen

Aluminum nanoparticles are often considered attractive choices for fuels in energetic materials. In this study, molecular dynamics simulation with the reactive force field (ReaxFF) was used to study the oxidation process of three different particle sizes (4 nm, 5 nm, 6 nm) of ANPs (aluminum nano-particles). The cavity formation mechanism was obtained during the ANPs oxidation process. The cavity size, the oxide layer thickness, and the active aluminum content of ANPs with time were obtained. The results indicated that the formation of cavities is highly dependent on particle size. Large particle size prevents the formation of cavities. In 0–100 ps, the average growth rate of the oxide layer is 0.33, 0.25, and 0.20 nm/ps, respectively. Within 100–250 ps, the average growth rate of the oxide layer is −0.017, 0.038, and 0.117 nm/ps, respectively. Our results emphasize the effect of particle size on cavity formation, which provides insights into the atomic-scale oxidation mechanism of metal nanoparticles.

Influence of Mold Design on Shrinkage Porosity of Ti-6Al-4V Alloy Ingots

Mold design is one of the important ways to control shrinkage porosity. In this study, four mold forms with different tapers were first designed, the corresponding three-dimensional finite element models were built using the ProCAST software, and the influence of mold design on the filling and solidification processes of Ti-6Al-4V alloy was investigated. The results showed that the titanium alloy ingots exhibit typical characteristics of layer-by-layer solidification, and that the removal of the riser results in: (a) shortening the time it takes for molten metal to reach the bottom of the mold and the time needed to complete mold filling; (b) decreasing the maximum flow velocity and improving the filling stability; and (c) moving the shrinkage cavities up along the central axis of the ingot and decreasing the cavity volume. Meanwhile, it was also found that the shrinkage cavity volume decreases significantly with increasing mold taper, meaning a significant increase in ingot utilization rate. The shrinkage cavity formation mechanism was revealed through macrostructure analysis. During solidification, a grain frame is formed as a large number of equiaxed crystals intersect, thus creating an isolated liquid phase zone. When the liquid in this zone solidifies, the last zone to do so, its volume shrinkage cannot be compensated, thus leading to the formation of a shrinkage cavity.

Fundamental understanding and catalytic applications of hollow MFI-type zeolites

In this review, we summarize the fundamental understanding and recent progresses on hollow MFI-type zeolite synthesis, hollow cavity formation mechanism, and mass transport/catalytic performance intensification, especially from the viewpoints of industrial application. Obviously, introduction of hollow cavities into the micropores of MFI-type zeolites significantly facilitates both internal mass transport and catalytic performance in many Brönsted or Lewis acid catalyzed reactions, i.e. catalytic oxidation, methanol to hydrocarbons and catalytic cracking reaction. It is majorly assigned to the shortened diffusion path length within zeolite crystal, while there is no change of external surface, thus leading to lower mass transport energy barrier. In general, hollow zeolites are synthesized by two strategies, including selective desilication method and in-situ dissolution-recrystallization (also remarked as crystalline rearrangement) method, ascribing to the reassembly of dissolved species with low aggregation under the effect of organic templates in hydrothermal conditions in latter process. Notably, selective dissolution is the driving force of these two synthesis routes, thus it is key to tune the composition and dispersion of framework elements and intracrystalline defects for controlling the size and distribution of internal hollow cavities. Moreover, through rearrangement, previously impregnated metal nanoparticles with tunable composition and size can be encapsulated within zeolite crystal, thus showing relatively high thermal stability and catalytic activity in catalytic oxidation reactions, selective hydrogenation, cross-coupling, esterification, and C1 transformation.

Mechanism of molten KOH+NaOH etching of GaN revealed by the slopes of etch pits formed at threading dislocations

The etch pit method is used widely to reveal dislocations in GaN via a molten KOH+NaOH solution. To understand the pit formation mechanism better, we performed a statistical analysis of the three-dimensional pit geometry. Particular attention was focused on the slope of the pit facet that forms an angle θ with the {0001} plane. It was found that tanθ is linearly proportional to the apparent pit depth D for a given type of dislocation. Therefore, the slope of the linear fitting (SLF) of tanθ vs. D/t (t is the etch time) could be used to identify the dislocation type and was more reliable than using pit widths. Transmission electron microscopy was used to confirm the results. It is thought that the SLF reflects the configuration of atomic steps at the dislocation core. Our results imply that pit formation is governed by thermodynamic pit nucleation at the dislocation core, followed by kinematic facet development.

Revealing the role of micron-sized in situ TiC particles on tensile properties and fracture mechanism of martensitic wear-resistant steel at elevated temperature

In order to improve the performance of the TiC ceramic reinforced wear-resistant steel at medium and high temperatures, such as slag transportation and cement production, its high-temperature tensile properties need to be well understood. In this paper, an in situ micron-sized TiC ceramic particle reinforced wear-resistant steel with 1 vol% TiC particles was fabricated. The tensile behavior and fracture mechanism of the TiC ceramic reinforced steel at 25–600 °C were investigated. The role of micron-sized TiC particles on tensile properties and fracture mechanism at elevated temperature was studied. Results indicated that the high temperature strength of TiC ceramic reinforced wear-resistant steel was enhanced by improving the thermal stability of the matrix. In the low and medium temperature range (25–500 °C), micron-sized TiC ceramic particles played a role in improving the strength, but had little effect on the elongation to failure. However, the micron-sized TiC ceramic particles greatly reduced the elongation to failure of the experimental steel when the temperature exceeded 500 °C. In the range of 25–600 °C, the fracture mechanism matched the typical ductile fracture. However, the mechanism of cavity formation varied at 500 °C. At 25–500 °C, the micro-cavities mainly formed due to fracture of the micron-sized TiC particles with sizes in the range of 1–9 μm and defects of the matrix. Above 500 °C, the interfacial debonding between the micron-sized TiC and the matrix was the main mode of crack initiation.

Study of the Double Wedge Cut Technique in Medium-Depth Hole Blasting of Rock Roadways

A double wedge cut was used to improve the cut effects in the medium-depth hole blasting of rock roadways. First, the cavity formation mechanisms of single wedge cut and double wedge cut were analysed theoretically. The results show that the cavity formation resistance of the second-order wedge cut is reduced by the first-order wedge cut. Second, simulations of the two cut modes were conducted, which visualize the stress wave evolution and indicate that the stress wave of the first-order cut holes can cause pre-damage to the rock mass in the second-order cut cavity. A comparison of the stress field distribution shows that the first-order cut holes significantly increase the stress field intensity within 0 ~ 1.6 m to enhance the failure of the rock mass in the first-order cut cavity. Hence, the rock mass in the first-order cut cavity can easily be thrown out to create a new free face, which reduces the clamping effect of the second-order wedge cut and exacerbates the tensile failure of the rock mass in the second-order cut cavity. Thus, the difficulty of second-order wedge cut is reduced. Finally, blasting experiments were performed in a rock roadway. Using the double wedge cut, the average footage increased by 0.31 m, the average utilization rate of blastholes increased by 12.4%, and the average specific charge decreased by 0.20 kg·m−3, which proves the superiority of this cut mode.

Pit Formation Mechanism of Monolayer MoS2 By Thermal Oxidation

Generating pits and thereby active edge sites of molybdenum disulfide (MoS2) monolayers is desirable for many electrochemical catalytic reactions including hydrogen evolution reaction (HER). Thermal oxidation is one of the potentially scalable and facile methods to effectively create the pits on MoS2 monolayers. Therefore understanding the thermal oxidation mechanism is very important to precisely control the generation of active edge sites of MoS2-based electrocatalysts. To date, pits are assumed to be favorably formed on MoS2 at undercoordinated sites such as sulfur (S) vacancies at high temperatures. However, the thermal oxidation studies have not considered the existence of adventitious carbon (C) that exists almost everywhere and interact with oxygen at elevated temperatures. Herein, we investigated the influence of adventitious C on the thermal oxidation of MoS2 monolayers. We employed in situ environmental transmission electron microscopy (ETEM) to demonstrate the pit formation mechanism with the presence of adventitious C at the oxidation temperature of 300 °C. The in situ ETEM results show that the adventitious C is agglomerated at high temperatures. Then, the interfaces between monolayer MoS2 and C nanoparticle provide preferred sites for thermal oxidation of MoS2 and thus pit formation while the individual S vacancies exist intact. Density functional theory (DFT) calculations show the interfaces between MoS2 and C nanoparticle make the sequential adsorption of oxygen atoms thermodynamically favorable, unlike only S vacancy sites. We also tested the electrochemical performance of MoS2 with pits for HER and it reduced the overpotential by ~ 130 mV. These results demonstrate the combination between ETEM experiment and DFT calculation can be effectively used to study the chemical reaction mechanism between 2-dimensional materials such as MoS2 and gaseous species. Furthermore, we provide the potential to control the active edge site of MoS2-based electrocatalyst by understanding the fundamental pit formation mechanism by thermal oxidation.

Threshold conditions for thermocapillary transition to deep penetration mode in selective laser melting of metal powder bed

The keyhole mode of selective laser melting (SLM) of metal powder bed in additive technology is characterized by an intensive hydrodynamic process in a thin molten layer. Such a mode is widely used also in laser and electron beam welding indicating the similarity of hydrodynamic processes in these technologies despite a significant difference of operating parameters. The threshold conditions of thermocapillary keyhole mode transition for various metals (Cu, Fe, Ti) in a wide range of beam parameters used for selective laser melting of metal powder layer and laser welding are investigated. The condition of threshold for thermocapillary transition into keyhole mode by sticking of viscous layer to the solid boundary is formulated. The fulfillment of this condition is confirmed by the convergence of estimated values of the viscous layer thickness and the molten layer depth during the transition to a keyhole mode. Analytical estimates of keyhole threshold and comparisons with experimental values of beam power and spot size corresponding to the transition in keyhole mode for SLM processes and laser welding are presented. The correlation of these values confirms the thermocapillary mechanism of cavity formation and the similarity of hydrodynamic processes in laser welding and SLM processing in keyhole mode at wide range of operating parameters.

Role of Cavity Formation on Long-Term Stress Corrosion Cracking Initiation: A Review

Plant life extension from the initial licensed life to beyond 60 years is now being discussed for light water cooled nuclear powered reactors (LWRs). Reliable prediction for material degradation is extremely important to keep the reliability of LWRs during such long-term operation. One of the specific perspectives for this prediction is to take into account the changes in material properties during long-term operation, such as cavity formation, even at the LWR operating temperature. The mechanism of cavity formation and the associated phenomena are closely intertwined with interdisciplinary technological and scientific knowledge. Therefore, historical key knowledge from both phenomenological and fundamental research studies related to cavity formation was first reviewed to understand the overall picture. Subsequently, current research results related to long-term stress corrosion cracking initiation in the LWR environment were summarized to explain what is known, what is still unknown, and what are the critical remaining subjects.

Conformational Characterization of Linker Revealed the Mechanism of Cavity Formation by 227G in BVDV RDRP.

RNA-dependent RNA polymerase (RdRp) is a relevant antiviral drug target. We investigated a potent benzimidazole inhibitor (227G; IC50 = 0.002 μM) against bovine viral diarrhea virus (BVDV) RdRp; however, its inhibition action was completely impaired in the presence of a resistant mutation, I261M. The binding of 227G in mutant RdRp affected the binding site loop conformations (especially Linker) that increased the volume of the binding site. It was also observed that the innate Linker's flexibility was retained, which was otherwise completely frozen in the wild-type complex. The functional role of Linker was hypothesized that it is a multidocking site for RNA template, inhibitors, and the other proteins involved in replication complex formation. The binding phenomenon requires significant molecular flexibility and the large-amplitude conformational dynamics of Linker, which is currently unknown. We observed a bidirectional "hinge"-like motion of Linker from crystal position, indicating its pronounced flexible behavior. This study underscores the importance of Linker's flexibility in the functionality of BVDV RdRp and proposes the template entrance site for selective anti-BVDV drug discovery.

Analysis of the Cavity Formation Mechanism of Wedge Cut Blasting in Hard Rock

The basic process of cut blasting is to break rock, throw fragments, and form a cavity. Based on the characteristics of cut blasting and the combined effect of stress waves and detonation gas, the evolution process of wedge cut blasting is divided into two stages, and a theoretical model is proposed to investigate the cavity formation mechanism by theoretical analysis and field tests. In phase one, rock breaking is caused by stress waves. By considering the dynamic strength of the rock, a computational model is built for the rock failure zone derived from the coupled cylindrical charge explosion. In phase two, the driving force of the detonation gas overcomes the total resistance of the surrounding rock mass, accelerates fragments, and then throws fragments to form a cavity. The criterion of cavity formation is established on the basis of the quasi‐static loading of the detonation gas. The theoretical model provides an overall interpretation of the cavity formation mechanism, in which stress waves break rock and detonation gas throws fragments. A specific case indicates that the range of the failure zone is approximately 18 times the borehole radius in granite and that the hole‐bottom spacing of the wedge cut can be designed as 50 cm; in addition, detonation gas is sufficient to overcome the total resistance, accelerate rock fragments, throw fragments, and form a cavity. Field tests present favourable blasting results, with a high utilization rate of boreholes and uniform fragment sizes. Therefore, the model could provide theoretical support and technical guidance for wedge cut blasting in hard rock.

Primary epithelial-myoepithelial carcinoma of�the�lung with cavitary lesion: A case report

Epithelial-myoepithelial carcinoma (EMC) typically arises in the salivary glands, whereas EMC of the lung is an extremely rare histological form that originates from the bronchial glands. Although cavitation in primary lung cancer is not uncommon, to the best of our knowledge, a case of EMC with a cavitary lesion has not been reported to date. We herein describe a case of cavity-forming pulmonary EMC. A 72-year-old man was referred to our department due to a thickened cystic wall discovered in the upper lobe of the left lung and underwent thoracoscopic left upper lobectomy. Microscopically, the tumor was characterized by biphasic architecture, with glands surrounded by myoepithelial cells. The pathological diagnosis was EMC. The patient has remained in good health for 2 years postoperatively, without any evidence of recurrence. As regards the mechanism of cavity formation, it was hypothesized that the bronchial gland in the primary cystic lesion had been present 3 years prior to the development of the EMC, and grew to become a cavitary lesion. Therefore, although the mechanism of cavity formation remains to be elucidated, EMC of the lung may include a cavitary lesion.

Numerical investigation of cavity formation mechanism for micron-waterdrop impact on deep pool

As one of the most fundamental and iconic fluid motion, droplet impact exists widely in scientific technologies and natural environment, and the phenomenon has been studied both for fundamental mechanism and for industrial applications in aerospace engineering, inkjet printing, agricultural irrigation and hydraulic structure erosion. Therefore, it is of great significance to study such basic movements for understanding the interfacial deformation of gas and liquid flow and improving the applications of droplet impact movement in engineering. Droplet impacting on a deep liquid pool has been extensively investigated for droplets with millimeter diameter. In this article, focusing on the cavity formation mechanism during a Micron-sized waterdrop impact on a deep pool, we perform systematic numerical simulations with adaptive mesh refinement technique and volume of fluid method to study the impact of a 290 μm water droplet on a deep water pool at velocities in a range of 2.5-6.5 m/s. The free surface motion, geometric variation of the cavity, local pressure field and vorticity field at selected times are presented to identify the pool-drop water mixing, capillary wave propagation, cavity formation, vortex ring generation and bubble entrapment phenomenon, and the dynamic mechanism of cavity motion is further explored. It is found that under the premise of neglecting the surface tension effects on the cavity whose depth is in a range of h∈(D, hmax), where D is the radius of initial droplet and hmax is the maximum depth, the cavity growth time to reach its maximum depth still scales as t∝h5/2, where t is time, but in the end, the formation of the bottom of the cavity is driven by capillary waves. There are two types of the initial cavity shapes: one is U-shape and the other is hemispherical shape, the former one generally changes into V-shape, and in the latter case, the bottom of the cavity will gradually transform into cylindrical shape, resulting in a thin jet and possible bubble entrapment. Cavity collapse is closely related to capillary wave propagation. When the impact velocity is low (Fr=567.1, Re=1595, We=121.8), the low-pressure zone is initially generated at the junction between the cavity sidewall and the bottom, a large vortex ring is then generated near the free surface and the bottom of the cavity, respectively. Under high impact velocities (Fr=792.1, Re=1885, We=170.2), the thin jet is observed, the generation of the vortex ring is suppressed. The low-pressure zone is first generated at the junction between the wave bottom and the cavity sidewall, after the cavity becomes cylindrical, the cavity collapses before the capillary wave arrives at the bottom, causing a bubble entrapment.

Characterization and formation mechanism of pits on diamond {100} face etched by molten potassium nitrite

The morphology of diamond surface etched by molten potassium nitrate was investigated and the formantion mechanism of etched pits was disscussed. The inverted pyramid and square-shaped pits were formed on the {111} face and {100} face of diamond particles, respectively. The size and depth of etching pits on {100} face were apparently smaller than that of {111} face, which suggests {100} face has higher stability due to the role of oxygen functional groups absorb on the structure. At a low etching temperature (600°C), square-shaped pits were firstly formed on {100} face and then gradually changed into octagon shape with the increasing of etching time. The change of shape can be attributed to the configuration of atomic on {100} face of diamond and the change of oxygen partial in molten salt during the etching process. When etch temperature up to 700°C, the edges direction of the preferentially formed square shape pit became an angle of 45°with the edges of later formed square shape pit which indicated that partial oxygen pressure can effect stability of diamond surface and etching direction.

A Summary of Corrosion Properties of Al-Rich Solid Solution and Secondary Phase Particles in Al Alloys

The heterogeneous structure of Al alloys renders them susceptible to localized corrosion due to the different electrochemical properties existing in the Al-rich solid solution matrix and secondary phase particles. The galvanic interactions between these two phases can result in pit formation either through dissolution of the particles or corrosion of the matrix adjacent to the particles. This detrimentally localized corrosion behavior is closely related to the corrosion properties of the particles and the Al-rich matrix. The comprehensive characterization of this behavior under various and varying conditions is critical to understanding the mechanism of pit formation, selecting appropriate inhibitors, and developing protection strategies. The corrosion properties (corrosion potential, pitting potential and corrosion rate) of both secondary phase particles and Al-solid solutions in Al alloys are summarized in this review, aiming to provide a database for corrosion research applicable to the localized corrosion of Al alloys.

PERKEMBANGAN CACAT RONGGA PADA PROSES EKSTRUSI

Cavity defect occurs in an extruded product when the remaining billet reaches a certain thickness. This defect can create serious problem in the quality of products because it produces waste material and requires an extra operation for finishing. The requirement of wasting a certain portion of the product becomes undesirable especially for a relatively short, headed product. Since the occurrence of this defect is troublesome in industrial practice, it is important to study the conditions leading to the cavity formation. By using this prediction, it may be possible to stop the process before the cavity starts to develop. In this work, a new measuring technique to monitor the development of cavity defect in forward extrusion was studied. This technique uses electrical contact principle, which detects the contact between metal pins and the leading face of the billet. When the cavity develops, a void or gap between the punch and the billet interface develops, and breaks the electrical current of the circuit. The experiment of extrusion process was undertaken in parallel with a finite element analysis (FEA) using ABAQUS software. It was shown that at a certain distance the cavity defect was revealed. In this work, the cavity formation was investigated by a variation of die reduction. The influence of coefficient of friction, punch speed, and billet thickness was also observed. This study concentrated on improving the understanding of the behavior of metal flow and the development mechanism of cavity formation with the aim of providing general guidance for the extrusion industry in minimizing the cavity defect formation of the extruded product.

노후 매립관로로 인한 지하 공동발생 메카니즘 고찰

노후관로로 인한 지반내 공동형성과 지반이완 메카니즘을 분석하기 위해 현장시험을 수행하였으며 수치해석을 수행하여 현장시험과 비교하였다. 현장시험은 지반내 인위적인 공동을 만들기 위해 얼음을 이용하였으며, 시간경과에 따른 얼음의 융해로 인해 공동형성과 주변지반의 이완을 확인할 수 있었다. 다짐된 토사의 interlocking에 의한 이완 및 공동형성 거동을 고려하기 위하여개별요소법에 근거한 수치해석 프로그램인 PFC 2D를 이용하였다. PFC의 클럼프(clump) 요소를 도입하여 불규칙 형상의 입자를 모사하였으며, 이를 통해 얻은 공동형성과 지반이완 특성을 현장시험 결과와 비교함으로써 지반내 공동형성 및 거동특성을 파악하였다. In order to analyze ground relaxation and cavity formation mechanism due to deteriorated sewer pipe, field test was carried out and a numerical assessments were compared with the field test results. An artificial underground cavity was intended using the ice block overlaying the buried pipe and confirmed that the cavity and relaxation of the surrounding ground were gradually formed as the ice block starts to melt down. Such mechanism was highly suspected to be involved with soil particle interlocking as a soil compaction was a typical process for the buried pipes. In exploring such mechanism numerically, commercially available DEM (Discrete Element Method) code PFC2D was used and the interlocking induced cavern behaviors were successfully simulated and compared with field test results by utilizing the clump logic imbedded in PFC code.

Influence of Lateral Growth on the Surface Pit Formation of GaN Heteroepitaxial Film Grown by MOCVD

This study made an attempt to understand and control the heteroepitaxial growth of GaN from the view of the essential behaviors of crystal growth. Through a comparison of the nonpolar, polar and semipolar GaN epitaxial film, the influence of lateral growth on the surface pit formation mechanism has been investigated. For a-plane GaN, the lateral growth velocities of the less inclined {20-21} and {11-22} facets are approaching to the velocity of {10-11} facet under high temperature, so that the surface pit was transformed from a triangular shape to a pentagonal one. For c-plane GaN, the size of the surface pit produced by screw dislocation is determined by the lateral growth of the pit facets. A slow lateral growth rate of the inclined facets {10-11} compared with the vertical growth rate of (0001) facet under low V/III ratio would enlarge the pit size. For (11-22) semipolar GaN, surface pit is rarely observed, because the vertical growth velocity of {11-22} plane is slow compared with the lateral growth rate of the inclined facets, such as {11-20} and (0001).DOI: http://dx.doi.org/10.5755/j01.ms.22.2.12931

Understanding of surface pit formation mechanism of GaN grown in MOCVD based on local thermodynamic equilibrium assumption**Project supported by the National Natural Science Foundation of China (Grant Nos. 11204009 and 61204011) and the Beijing Municipal Natural Science Foundation, China (Grant No. 4142005).

Frank’s theory describes that a screw dislocation will produce a pit on the surface, and has been evidenced in many material systems including GaN. However, the size of the pit calculated from the theory deviates significantly from experimental result. Through a careful observation of the variations of surface pits and local surface morphology with growing temperature and V/III ratio for c-plane GaN, we believe that Frank’s model is valid only in a small local surface area where thermodynamic equilibrium state can be assumed to stay the same. If the kinetic process is too vigorous or too slow to reach a balance, the local equilibrium range will be too small for the center and edge of the screw dislocation spiral to be kept in the same equilibrium state. When the curvature at the center of the dislocation core reaches the critical value 1/r0, at the edge of the spiral, the accelerating rate of the curvature may not fall to zero, so the pit cannot reach a stationary shape and will keep enlarging under the control of minimization of surface energy to result in a large-sized surface pit.

Study of the pitting effects during the pre-ignition plasma–propellant interaction process

The propellant ignition mechanism has become a central issue in the electrothermal chemical (ETC) launch technology, and the pre-ignition plasma–propellant interactions are critical in determining the ignition characteristics. In this work, both an open-air ablation test and an interrupted burning test are conducted for three different propellants. A fused silica window, which is transparent in all relevant wavelengths, is utilized to investigate the role of the plasma radiation. Surface pitting of the propellants after interaction with the plasma is analyzed using a scanning electron microscope (SEM). The effect of pits on the plasma ignition is then studied and a possible formation mechanism of pits is proposed. The input heat flux and the surface temperature of the propellants are obtained by solving a pre-ignition plasma–propellant interaction model. The results shed light on the pre-ignition plasma ignition mechanisms and will assist in the development of propellants for an ETC launcher.