Redistribution Of Material Research Articles

Approaches Towards Improving Zinc-Nickel Batteries Performance

The zinc/nickel electrochemical system has long been proposed as a good candidate of secondary alkaline batteries due to its excellent performance versus other aqueous batteries, such as high practical specific energy, excellent specific power, high open circuit voltage, low cost and low toxicity [1,2]. These advantages make it suitable for replacing lead-acid and nickel-cadmium batteries [3]. However, the high solubility of zinc in concentrated alkaline electrolytes is still a significant problem that induces two main failure mechanisms: a shape-change of the zinc electrode and a redistribution of the zinc active material due to its dissolution/redeposition during cycling. Dendritic growth can occur as a consequence of zinc dissolution/redeposition, and if severe, may lead to separators’ perforation and internal electrical short-circuits [4]. These drawbacks reduce the cell's capacity and lifetime, especially compared to traditional competing systems [5]. In addition, since the hydrogen evolution reaction (HER) is thermodynamically possible (especially during charging), the coulombic efficiency of the zinc electrode can be lowered by this parasite reaction [6]. The undesirable HER consumes water and some of the active material, yielding zinc hydroxide which in turn can generate a passivation layer that lowers the usability of the zinc anode materials [7]. There are different approaches to overcome these problems, such as the integration of additives in the active material formulation and/or in the electrolyte.In this contribution, we will show how regeneration of the active material can be obtained via appropriate steps of rest submitted to the active material and without the need for additional energy input. The so-called “self-healing” of the active material allows to recover a substantial part of the electrochemical performance. The concept was deeply studied and monitored by scanning electron microscopy coupled with elemental mapping by X-ray energy dispersive spectrometry, and operando tomography. An increase in the coulombic efficiency has been demonstrated making this discovery very promising for the future of zinc-based alkaline batteries. Keywords: Zinc-nickel batteries, additives, self-healing

Research on Nickel Material Trade Redistribution Strategy Based on the Maximum Entropy Principle.

In the double carbon background, riding the wind of new energy vehicles and the battery high nickelization, nickel resources rise along with the trend. In recent years, due to the influence of geopolitical conflicts and emergencies, as well as the speculation and control of international capital with its advantages and rules, the world may face price and security supply risks to a certain extent. Therefore, to obtain the most objective trade redistribution strategy, this paper first constructs the nickel material trade network, identifies the core trading countries and the main trade relations of nickel material trade, and finds that the flow of nickel material mainly occurred between a few countries. On this basis, a trade redistribution model is constructed based on the maximum entropy principle. Taking Indonesia, the largest exporter, and the largest trade relationship (Indonesia exports to China) as examples, the nickel material redistribution between countries when different supply risks occur are simulated. The results can provide an important reference for national resource recovery after the risk of the nickel trade.

Minimization of dies volume using uniform distribution of hollows in the conditions of multi-cycle fatigue dependence on temperature

The research is devoted to development of a model of dies of optimized shape used in the field of metal forming. Use of solid-cast metal dies at the present stage of production requires large material resources, which makes it actual to develop dies models with minimal volume of material due to effective distribution of hollows in correspondence with the required strength resource. The dies mainly experience compressive non-uniform loading in the conditions of drawing and forming, which can lead to the accumulation of damages and lowering of its strength resource in the conditions of long-term operation of a die at high temperatures; therefore, the optimized area is represented in this work by alternating rod elements and hollows. The paper considers the problem of optimizing the volume of a metal die, presents a model for minimizing the die volume with restrictions on multi-cycle fatigue at various temperature conditions. The dependence between the volume of the optimized area and the number of loading cycles is obtained, taking into account the non-uniform loading of the die for axisymmetric forming. The main approach to reducing the die volume due to distribution of hollows was concluded in increase the stress state of the rod elements experiencing compressive stress to the limit values in accordance with the restriction on multi-cycle fatigue. The number of elements providing the required die stress state is calculated for different temperature conditions and preset geometrical parameters of the rod elements. When calculating the configuration of the optimized die area for axisymmetric forming, experimental curves of dependence between stress state and number of cycles for 12Kh1MF1 steel were considered at the constant temperature. The variants of the material distribution in the internal die area at preset temperatures are presented. Reduction of the optimized die volume was compared with a solid cast die. The constructed mathematical model and the results of calculation of the optimized die volume will significantly reduce the material costs for production of metal forming dies and can be used for further development of the methods for topological optimization of stamping tools, taking into account the features and duration of power and thermal loading. The research was carried out under financial support of the Russian Foundation for Basic Research within the framework of the scientific project No. 19-38-60020\19 “Development of optimization model for dies of stamping accessories via the method of efficient material redistribution”.

Buckling shape control in metal plates via material distribution

The aim of this study is to explain from a fundamental perspective, how distributing the material thickness of a metal plate in different ways can be used to control the shape of the elastic buckling mode. The Evolutionary Structural Optimisation (ESO) method is modified to control the buckled shape of plates simply supported on three sides with the remaining longitudinal edge free. The bucking objectives are considered both from a fundamental perspective, and for the application of kinetic façade design, that is, façade panels that undergo shape changes to facilitate ventilation or shading control. A series of buckling experiments of optimised aluminium plates validates the optimised designs. It is shown that in order to achieve a maximum opening in a buckled plate, the buckled shape should be controlled in such a manner as to create a relatively flat zone of maximum transverse displacement over much of the displaced free edge, a shape that differs from the fundamental mode of a uniform thickness plate. Notably, such a shape may be achieved via optimisation with objectives that relate either to the shape itself, or to the objective of maximising the opening. The design methods and experiments demonstrate that redistribution of plate material can efficiently control the buckling characteristics of metal plates for desired functions, and provide significant performance enhancements over uniform thickness plates.

Increasing warning resistance of engine valves by gas nitrogenization method

The article presents the results of tribological research on the most promising way to restore and increase the wear resistance of engine valves by developing a method of gas nitriding. It is established that with increasing operating time the guide bushings of the outlet connections wear out with the displacement of the axis of the forming surfaces of the hole. Characteristic significant displacement of the axes of the inlet connections is not detected, ie. their wear on the diameter of the hole is 1.5 ... 3 times less than the wear of the exhaust bushings, the values of the displacement of the axes are within the error of the measuring instrument. The average value of ovality is greater in the exhaust seats - the maximum beating values of the intake seats are 0.34 mm, exhaust - 0.22 mm. It is proved that the non-uniformity of the wear of the sleeve hole is determined by the balance of acting forces, which, in turn, are determined by deviations from the optimal ratios μ and e. the side of the rocker arm axis. Distortions of the valve in the longitudinal axis of the engine contribute to an earlier reduction in the tightness of the valve pairs. Redistribution of the valve end material with the formation of a wavy concentric surface, the shape of the contact spot on the rocker arm and the corresponding direction of wear of the saddle chamfer was observed in 43% of the studied connections. Technological means and methods for improving the quality of repair, measuring instruments for accurate study of the parameters of parts and connections of the valve group are given. The results of laboratory and operational tests are presented. A method of gas nitriding with an installation for its implementation has been developed, which provides an environmentally friendly method of low-temperature and high-temperature hardening, obtaining deeper and well-developed layers of the diffusion near-surface zone and reduces training, technological time in the process of strengthening and reducing energy consumption

Charcoal Whirlwinds and Post-Fire Observations in Serengeti National Park Savannahs

Whirlwinds and visible dust devils occur over semi-arid ecosystems and entrain particles from the ground surface. Fires produce abundant charcoal across savannahs and the resulting blackened surfaces create a large albedo contrast. Whirlwinds have been observed associated with active fires; yet, there are few published observations on post-fire landscapes. Spatiotemporal patterns of whirlwinds have been documented for a limited number of regions and have not been made for the ecosystems of eastern Africa. From field-based sightings in the Serengeti National Park, Tanzania, we report on whirlwinds over burned savannah patches that entrained large quantities of charcoal to produce black coloured charcoal devils that lofted charcoal into the atmosphere. Two occurrences of charcoal devils were sighted and photographed, one each in the Western Corridor (Bunda District) and Lamai (Serengeti District), Mara Region. The observations were compared with regional scale meteorological data and remote sensing satellite imagery and albedo estimates of the land cover conditions. Although direct meteorological or particulate matter measurements were not made, the observations show that both charcoal devils differed in colour, funnel shape, height, and savannah land cover types (different woody to grass fuel canopies), and thus different charcoal morphologies. Charcoal laden whirlwinds require further study and characterization to analyse the contribution to local-scale redistribution of matter and regional-to-global fluxes of terrestrially derived atmospheric particulates. Future research focusing on the spatiotemporal patterns of whirlwinds over burned patches of savannah, the formation, duration and dissipation mechanisms, and characterisation of the entrained material would contribute to our understanding of the phenomena. The redistribution of organic and clastic material would contribute to understanding of detrital fluxes to depositional environments, such as lakes, wetlands, and snow.
 Keywords: Atmospheric boundary layer; Convection; Detritus; Dust devils; Fires; Particulate matter.

Laser-Driven One- and Two-Dimensional Subwavelength Periodic Patterning of Thin Films Made of a Metal–Organic MoS2 Precursor

Laser-based surface processing is an established way for the maskless generation of surface structures and functionalities on a large variety of materials. Laser-driven periodic surface texturing and structuring of thin films is reported for metallic-, semiconductive-, and polymeric films. Here, we introduce subwavelength surface patterning of metal-organic thin films of [Mo2S4(S2CNnBu2)2], a MoS2 precursor. Accurate control of one- and two-dimensional (1D and 2D) periodic patterns is achieved on silicon wafers with a pulsed 532 nm ns laser. With suitable combinations of laser polarization, laser pulse energy, the thickness of the SiO2 passivation layer, and the MoS2 precursor's thin film thickness, high-quality 1D and 2D self-organized periodic structures are obtained in virtually unlimited areas. The material redistribution related to the pattern formation is thermally driven at low laser energies. Increasing pulse energies beyond a threshold level, in our experiments a factor of 2, fully converts the precursor to MoS2.

Possible particle ejection contributions to the shape and spin stability of small near-Earth asteroids

Top Shaped Asteroids (TSAs) have proven to be common amongst the near-Earth rubble pile population, with multiple examples confirmed via groundbased radar and spaceborne optical sensors through the past 20 years. A substantial body of literature has developed, exploring the formation of these unique shapes either through rotation-induced landslides and creep, or collisional reaccumulation. Models of such mass movements can provide good explanations for mid and low latitude material redistribution, but Bennu also shows a significant increase in radius in the high polar regions, which is harder to explain with these processes. The discovery of repeated and probably ongoing particle ejections around the 500 m diameter asteroid Bennu by the OSIRIS-REx mission suggests that we need to consider an alternate or additional mechanism which, we show, can anticipate the detailed variety of TSA shapes. This paper explores asteroid shape evolution as the result of particle ejections, modeled as being simply correlated with latitude via diurnal heating (or meteorite impacts), and re-accumulation using simulations including gravity and solar radiation pressure. Asteroid outlines are evolved with time as a function of particle ejection velocities and asteroid rotation rates. Bennu’s shape can be anticipated by our model with RMS surface errors of less than 1.1% (2.7 m) although some southern latitudes have errors up to 10 m. Straightforward variation in conditions can produce shapes matching other TSAs. However, the observed particle fluxes on Bennu are approximately 3 orders of magnitude too low for this to be the only shaping mechanism on Bennu. The time necessary to form these shapes by our mechanism alone is far longer than the lifetimes of near-Earth asteroids, unless fluxes were once much greater, or there was an underlying oblate shape.

Mechanical Performance of Lightweight-Designed Honeycomb Structures Fabricated Using Multijet Fusion Additive Manufacturing Technology.

Cellular structures including three-dimensional lattices and two-dimensional honeycombs have significant benefits in achieving optimal mechanical performance with light weighting. Recently developed design techniques integrated with additive manufacturing (AM) technologies have enhanced the possibility of fabricating intricate geometries such as honeycomb structures. Generally, failure initiates from the sharp edges in honeycomb structures, which leads to a reduction in stiffness and energy absorption performance. By material quantity, these hinges account for a large amount of material in cells. Therefore, redesigning of honeycomb structures is needed, which can improve aforementioned characteristics. However, this increases the design complexity of honeycombs, such that novel manufacturing techniques such as AM has to be employed. This research attempts to investigate the optimal material distribution of three different topologies of honeycomb structures (hexagonal, triangular, and square) with nine different design configurations. To achieve this, higher amount of material was distributed at nodes in the form of fillets while keeping overall weight of the structure constant. Furthermore, these design configurations were analyzed in terms of stiffness, energy absorption, and the failure behavior by performing finite element analysis and experimental tests on the samples manufactured using Multijet fusion AM technology. It was found that adding material to the edges can improve the mechanical properties of honeycombs such as stiffness and energy absorption efficiency. Furthermore, the failure mechanism is changed due to redistribution of material in the structure. The design configurations without fillets suffer from brittle failure at the start of the plastic deformation, whereas the configurations with increased material proportion at the nodes have larger plastic deformation zones, which improves the energy absorption efficiency.

Spreading pressure bumps in gas-dust discs can stall planet migration via planet-vortex interactions

ABSTRACT We investigate the gravitational interaction between low- to intermediate-mass planets ($M_p \in [0.06-210]\, \mathrm{ M}_{\oplus }$) and two previously formed pressure bumps in a gas-dust protoplanetary disc. We explore how the disc structure changes due to planet-induced perturbations and also how the appearance of vortices affects planet migration. We use multifluid 2D hydrodynamical simulations and the dust is treated in the pressureless-fluid approximation, assuming a single grain size of $5\, \mu {\mathrm{m}}$. The initial surface density profiles containing two bumps are motivated by recent observations of the protoplanetary disc HD163296. When planets are allowed to migrate, either a single planet from the outer pressure maximum or two planets from each pressure maximum, the initial pressure bumps quickly spread and merge into a single bump which is radially wide and has a very low amplitude. The redistribution of the disc material is accompanied by the Rossby Wave Instability and an appearance of mini-vortices that merge in a short period of time to form a large vortex. The large vortex induces perturbations with a spiral wave pattern that propagate away from the vortex as density waves. We found that these vortex-induced spiral waves strongly interact with the spiral waves generated by the planet and we called this mechanism the ‘Faraway Interaction’. It facilitates much slower and/or stagnant migration of the planets and it excites their orbital eccentricities in some cases. Our study provides a new explanation for how rocky planets can come to have a slow migration in protoplanetary discs where vortex formation occurs.

Phase-field modeling of the morphological evolution of ringlike structures during growth: Thermodynamics, kinetics, and template effects

Understanding and controlling the morphology of crystals growing from prepatterned shapes, as in selective-area epitaxy, is fundamental for the development of novel device architectures. Here, an in-depth analysis of the faceted growth of ring-shaped crystal structures is presented. This is a prototypical case for assessing the role of thermodynamic and kinetic driving forces, as they induce different faceting between the outer convex region and the inner concave one. The well-established concepts of equilibrium crystal shape, set by surface energy minimization, and its kinetic counterpart, determined by orientation-dependent growth rates, provide a qualitative indication of the outcome of growth experiments. However, they are insufficient to capture the whole evolution pathway. A phase-field growth model including deposition, incorporation, and surface diffusion dynamics is considered, and extensive two- and three-dimensional simulations are performed. A continuum transition in the crystal morphology is recognized by varying the magnitude of the anisotropic surface energy vs the incorporation times. The in-depth analysis of a realistic case, mimicking experiments in the literature, highlights the prominent role of kinetics and of material redistribution from the outer to the inner perimeter of the ring. Finally, the templating effect of the initial pattern is investigated by considering polygonal profiles with different orientations, showing their effect on the facet formation and evolution.

Properties of the Zagreb 22 March 2020 earthquake sequence

Although strong and damaging earthquakes have hit Zagreb in the past, the 22 March 2020 earthquake (Mw 5.4) is the first one that was recorded by a modern digital local seismic network, and which could be analysed not only by macroseismic methods, but also by microseismic ones. Herewith we used the 3003 carefully analysed and located events from the first year of the aftershock sequence to learn more about the aftershock rate decay, their magnitude distribution, focal mechanisms and hypocentral locations. The aftershock activity rate was found to closely follow the modified Omori law, and fault-plane solutions for 10 events indicated prevailing pure-reverse faulting. Our analyses suggest that the reverse North Medvednica boundary fault (NMBF) was the causative fault, as it fits with the focal mechanisms and with the geometry of aftershock locations. The epicentral area was of a triangular shape with the mainshock in one vertex, and the opposite side of the triangle lying parallel to the surface trace of the NMBF. The hypocentres of aftershocks were predominantly located in the hanging wall of the fault. No surface break was observed, so the rupture is assumed to be buried. These facts were interpreted as a combination of the effect of conservation of mass (seismic flow) requiring some fault-parallel stress redistribution and transfer of material, and the fault loading and activation in the compressive environment controlled by the stress partition at the brittle-ductile transition zone within the crust. The later process involves compression within the hanging wall during the interseismic stage when the fault segment in the brittle crust is locked, followed by sudden dilatation during the rupture phase.

Finite Element Calculation of the Polymer Stamp Material Redistribution under Restrictions on Fatigue Life

The numerical method of stamp topological optimization taking into account fatigue strength is presented in the work. It is proposed to take into account the restrictions on the stress state in accordance with the curve of the dependence of the maximum stresses on the number of loading cycles in the ESO topological optimization method. An approach to the selection of the evolutionary coefficient with a step-by-step increase in the rejection coefficient is proposed when constructing an iterative scheme for the rejection of elements by the method of topological optimization. The calculation of the stamp optimal topology with a decrease in volume due to the removal and redistribution of material was carried out in the study. The new geometric model of the optimal topology stamp is based on the predicted distribution of elements with a minimum stress level. The verification calculation of the stress state of the stamp of optimal topology with an assessment of fatigue strength was carried out in the work. The numerical calculation was carried out using the finite element method in the Ansys software package. The minimized stamp volume decreased by 35% according to the calculation results. The results of the study can be further applied in the development of topological optimization methods and in the design of stamping tools of optimal topology.

A Review on Part Geometric Precision Improvement Strategies in Double-Sided Incremental Forming

Low geometric accuracy is one of the main limitations in double-sided incremental forming (DSIF) with a rough surface finish, long forming time, and excessive sheet thinning. The lost contact between the support tool and the sheet is considered the main reason for the geometric error. Researchers presented different solutions for geometric accuracy improvement, such as toolpath compensation, adaptation, material redistribution, and heat-assisted processes. Toolpath compensations strategies improve geometric precision without adding extra tooling to the setup. It relies on formulas, simulation, and algorithm-based studies to enhance the part accuracy. Toolpath adaptation improves the part accuracy by adding additional equipment such as pneumatically or spring-loaded support tools or changing the conventional toolpath sequence such as accumulative-DSIF (ADSIF) and its variants. It also includes forming multi-region parts with various arrangements. Toolpath adaptation mostly requires experimental trial-and-error experiments to adjust parameters to obtain the desired shape with precision. Material redistribution strategies are effective for high-wall-angle parts. It is the less studied area in the geometric precision context in the DSIF. The heat-assisted process mainly concentrates on hard-to-form material. It can align itself to any toolpath compensation or adaptation strategy. This work aims to provide DSIF variants and studies, which focus on improving geometric accuracy using various methodologies. It includes a brief survey of tool force requirements for different strategies, sheet thickness variation in DSIF, and support tool role on deformation and fracture mechanism. Finally, a brief discussion and future work are suggested based on the insights from several articles.

Evaluation of functional layers thinning of high temperature polymer electrolyte membrane fuel cells after long term operation.

The operation related degradation processes of high temperature polymer electrolyte membrane fuel cell operated with hydrogen-rich reformate gas are studied. CO impurities from the reformate gas are strongly adsorbed by the catalyst surface, leading to poisoning and thus, reduction of the overall performance of the cell. Most of the studies are performed in a laboratory set-up by applying accelerated stress tests. In the present work, a high temperature polymer electrolyte membrane fuel cell is operated in a realistic configuration for 12 000 h (500 days). The fuel cell contains as electrocatalyst Pt in the cathode and a Pt-Ru alloy in the anode. The study of the degradation occurring in the functional layers, i.e. in different regions of cathode, anode and membrane layer, is carried out by scanning electron microscopy, (scanning) transmission electron microscopy and energy dispersive X-ray spectroscopy. We observed a thinning of the functional layers and a redistribution of catalyst material. The thinning of the cathode side is larger compared to the anode side due to harsher operation conditions likely causing a degradation of the support material via C corrosion and/or due to a degradation of the catalyst via oxidation of Pt and Ru. A thinning of the membrane caused by oxidation agents is also detected. Moreover, during operation, catalyst material is dissolved at the cathode side and redistributed. Our results will help to design and develop fuel cells with higher performance.

Инжиниринговая цифровая модель пространства сельскохозяйственных земель для подготовки и сопровождения агроэкологической составляющей точного земледелия

An automated computational engineering model is being developed to support the agro-ecological component of the agricultural production process. The authors solve the tasks of creating a computational model of spatially differentiated assessment, monitoring and management of heterogeneous productivity of land. The structure of model data, following the logic of the natural process of formation of soil fertility, as operational units is based on the hierarchy of structural units of landscapes from the regional level to the elementary range of the agro-landscape—geotope. The structure of the attributive data of the operating units of the model is determined by the spatial differentiation of the calculated values of abiotic factors responsible for the formation of the soil cover. Elementary soil ranges—geotopes—are visualized in the information-mathematical 3D geometric model of the relief surface and are automatically classified by position in the landscape catena and by natural ecological, including calculated microclimatic, characteristics. Geotopes, homogeneous in terms of material and energy flow redistribution, provide agro-ecological grouping of lands at the local field level. Within the boundaries of geotopes, representative points of study of the natural spatial differentiation of the structure and parameters of the vertical profile of soils and agrochemical survey data are automatically distinguished. An information-mathematical field model is created to identify, by using computational methods, the natural boundaries of heterogeneities, which determine the spatial differentiation of the agrochemical state of soils and the microclimate—the habitat of crops. Mathematical model makes it possible to calculate spatial differentiation of quantitative characteristics of soils and biologically active substances in representative points of geotopes based on agrochemical survey data. The revealed patterns of redistribution of biologically active substances within the field are necessary for differentiated planning and implementation of the corresponding agrotechnical techniques of precision agriculture.

Gravity Observations and Apparent Density Changes before the 2017 Jiuzhaigou Ms7.0 Earthquake and Their Precursory Significance.

An Ms7.0 earthquake struck Jiuzhaigou (China) on 8 August 2017. The epicenter was in the eastern margin of the Tibetan Plateau, an area covered by a dense time-varying gravity observation network. Data from seven repeated high-precision hybrid gravity surveys (2014–2017) allowed the microGal-level time-varying gravity signal to be obtained at a resolution better than 75 km using the modified Bayesian gravity adjustment method. The “equivalent source” model inversion method in spherical coordinates was adopted to obtain the near-crust apparent density variations before the earthquake. A major gravity change occurred from the southwest to the northeast of the eastern Tibetan Plateau approximately 2 years before the earthquake, and a substantial gravity gradient zone was consistent with the tectonic trend that gradually appeared within the focal area of the Jiuzhaigou earthquake during 2015–2016. Factors that might cause such regional gravitational changes (e.g., vertical crustal deformation and variations in near-surface water distributions) were studied. The results suggest that gravity effects contributed by these known factors were insufficient to produce gravity changes as big as those observed, which might be related to the process of fluid material redistribution in the crust. Regional change of the gravity field has precursory significance for high-risk earthquake areas and it could be used as a candidate precursor for annual medium-term earthquake prediction.

Study of In/GaAs nanodroplet formation in conditions of non-stationary supersaturation during droplet epitaxy

In this paper, we present the results of kinetic Monte Carlo study of the In/GaAs growth by droplet epitaxy in conditions of non-stationary vapor supersaturation. These conditions allow achievement of the independent control of size and surface density of nanostructures. The material redistribution is realized on the surface when indium deposition is interrupted and leads to a decrease in the critical thickness of droplet formation. An average droplet size increases with increase in interruption time whereas the surface density decreases. However, additional nucleation within the wetting layer can also be observed during the growth interruptions, which makes it possible to increase the surface density of droplets.

Lack of fusion mitigation in directed energy deposition with laser (DED-L) additive manufacturing through laser remelting

The void level of a component produced by additive manufacturing can be used to predict the mechanical properties and in most cases it is important to develop processing methodologies able to minimize it. Conventional techniques include parameter optimization and the hot isostatic pressing (HIP). However, in the former there is a limit where improvement becomes minimal and the latter, although it is effective for internal voids, is not efficient in the case of voids located near the surface, which are considered critical. This paper describes a study in which laser remelting was used as an alternative to achieve higher densification after processing using directed energy deposition with laser (DED-L). This technique has been investigated for powder bed (PB) processes, but has not been explored for DED. Understanding the behavior of different materials considering the same processing parameters is also relevant and has not been previously addressed. An ytterbium (Yb) fiber laser with a maximum nominal power of 10 kW and wavelength of 1070 nm was used. Iron and Inconel 625 powders were deposited on an SAE 1010 steel substrate to form beads with intentional voids. The DED-L was performed with a power of 450 W, scanning speed of 600 mm/min and powder feed rate of 10 g/min. The laser radiation was applied over the produced surface at three energy levels: 31.5 kJ/m, 38.2 kJ/m and 45 kJ/m. An in-house MATLAB image processing code was developed in order to assess the void percentage. The results indicated that most of the voids originating from the DED-L process were due to lack of fusion between beads and between the deposit and the substrate. For iron, the void percentage after DED-L varied between 5.3% and 7.1%. After remelting, this value reduced to 1.7% applying 45 kJ/m. It was also noted that in all samples the voids that were initially connected to the surface coalesced because of material redistribution. However, a low repeatability was observed. For Inconel, the void percentage after DED-L varied between 4.4% and 7.1%, and reduced to 3.7% applying 31.5 kJ/m. This reduction is not as significant as that observed for iron and also the degree of dispersion was lower for iron. Hence, laser remelting is able to mitigate voids resultant from DED-L and, consequently, achieve better quality due to a lower probability of component failure. Simultaneously, productivity can be enhanced since the equipment required is the same as that used for the deposition. However, new strategies are needed to increase the process robustness and reduce the degree of dispersion.

Time-lapse submicrometer particle motion reveals residual strain evolution and damaging stress relaxation in clinical resin composites sealing human root canals

Polymer based composites are widely used for treatment, for example as biofilm resistant seals of root canal fillings. Such clinical use, however, fails more frequently than other dental composite restorations, due to stress-related misfits. The reason for this is that the biomaterials used are inserted as viscous masses that may bond to the substrate, yet shrinkage stresses arising during setting of the cross-linking polymer, work against durable adhesion. Here we combine phase contrast enhanced time-lapse radiography (radioscopy), digital image correlation (DIC) and submicrometer resolution phase-contrast enhanced microtomography (PCE-CT), to reveal the spatial and temporal dynamics of composite polymerization and strain evolution. Radioscopy of cavities located in the upper part of human root canals demonstrates how the composite post-gelation “densification” is dominated by viscous flow with quantifiable motion of both particles and entrapped voids. Thereafter, these composites enter a “stress-relaxation” stage and exhibit several structural adaptations, induced by residual shrinkage stresses. Consequently critical alterations to the final biomaterial geometry emerge: (i) entrapped bubbles expand; (ii) microscopic root filling pull-out occurs; (iii) the cavity walls deform inwards, and (iv) occasionally delamination ensues, propagating out from the root canal filling along buried restoration-substrate interfaces. Our findings shed new light on the interactions between confined spaces and biomedical composites that cross-link in situ, highlighting the crucial role of geometry in channeling residual stresses. They further provide new insights into the emergence of structural flaws, calling attention to the need to find new treatment options. Statement of significanceThis work quantifies recurring spatial and temporal material redistribution in composites used clinically to fill internal spaces in teeth. This knowledge is important for both promoting biomaterial resistance against potentially pathologic biofilms and for improving structural capacity to endure years of mechanical function. Our study demonstrates the significant role of geometry and the need for improved control over stress raisers to develop better treatment protocols and new space filling materials. The use of high-brilliance X-rays for time-lapse imaging at submicrometer resolution provides dynamic information about the damaging effects of stress relaxation due to polymerization shrinkage.