Neck Growth Rate Research Articles

In situ transmission electron microscopy and scanning transmission electron microscopy studies of sintering of Ag and Pt nanoparticles

Transmission electron microscopy and scanning transmission electron microscopy studies were conducted in situ on 2–5nm Pt and 10–40nm Ag nanoparticles to study mechanisms for sintering and to measure relevant sintering kinetics in nanoscale particles. Sintering between two separated particles was observed to initiate by either (1) diffusion of the particles on the sample support or (2) diffusion of atoms or small clusters of atoms to the neck region between the two particles. After particle contact, the rate of sintering was controlled by atomic surface diffusivity. The surface diffusivity was determined as a function of particle size and temperature from experimental measurements of the rate of neck growth of the particles. The surface diffusivities did not show a strong size effect for the range of particle sizes that were studied. The surface diffusivity for Pt nanoparticles exhibited the expected Arrhenius temperature dependence and did not appear to be sensitive to the presence of surface contaminants. In contrast, the surface diffusivity for Ag nanoparticles was affected by the presence of impurities such as carbon. The diffusivities for Ag nanoparticles were consistent with previous measurements of bulk surface diffusivities for Ag in the presence of C, but were significantly slower than those obtained from pristine Ag.

Seedling growth and water relations of three Cedrela species sourced from five provenances: response to simulated rainfall reductions

The effects of different simulated rainfall regimes on water relations, growth, and biomass production and allocation of five provenances of Cedrela fissilis, C. saltensis and C. balansae were assessed in a pot-grown seedling experiment in greenhouse under uncontrolled conditions. Four simulated rainfall regimes were applied: 600 mm year−1 (severe water deficit), 800 mm year−1 (mild water deficit), 1,000 mm year−1 (mild wet), and 1,200 mm year−1 (well-watered) used as control. Provenances were compared in terms of water relations (midday leaf water potential, Ψ md and relative water content, RWC), growth (root, shoot and leaf length, neck diameter), growth rate (shoot- and neck growth rate, SGR and NGR), biomass production (root, shoot and leaf dry weight), and biomass allocation (root:shoot ratio, RSR). Multivariate analyses revealed that water relations differed significantly under all simulated rainfall treatments by means of several variables throughout time. Effects of species × treatment, and provenance nested within species × treatment interactions significantly were important. SGR and RSR also were important to explain differences in Ψ md and RWC between treatments and provenances. Broadly C. balansae and C. saltensis provenances seem to display a better performance than C. fissilis provenances under severe water deficit only. However, despite differences between provenances, it was not possible to clearly separate them according to their origin, based on water relations and growth performance. Even though the results of this study performed on pot-grown seedlings are based on relatively few observations and require additional confirmation, they allow an opportunity for speculation on the factors that are related to intraspecific variations that are likely to occur during the growth of seedlings from different geographical sites in field condition.

Peculiarities of the neck growth process during initial stage of spark-plasma, microwave and conventional sintering of WC spheres

This work involves an investigation of the neck growth kinetics of free-packed spherical shaped binderless tungsten carbide particles during microwave and spark-plasma sintering. The application of a classical sphere to sphere approach showed the possibility of identifying the main diffusion mechanisms operating during the initial stage of microwave sintering of tungsten carbide powder. An anomalous neck growth rate in the initial period during microwave and spark-plasma sintering processes, up to 100 times faster in comparison to conventional sintering, was also revealed. Volume diffusion was enhanced by a small amount of a liquid phase, and surface diffusion was proposed as the primary mass transport mechanism for microwave sintering. The simulation operation of grain-boundary diffusion and power law creep was responsible for neck growth during spark-plasma sintering.Numerical simulation of neck growth revealed high values of the diffusion coefficient for microwave (3.41×10−8m2s−1 at 1200°C) and spark-plasma sintering (5.41×10−8m2s−1 at 1200°C). In the case of conventional sintering, the diffusion coefficients calculated are in good agreement with values for diffusion of W and C in a W–C system (8.6×10−16m2s−1 at 1200°C).Low values of the apparent activation energy (Ea) for microwave and spark-plasma sintering (62 and 52kJmol−1) have been obtained. For conventional sintering, all data collected indicate grain-boundary diffusion as the primary sintering mechanism (272kJmol−1).

Molecular dynamics simulation of neck growth in laser sintering of different-sized gold nanoparticles under different heating rates

The neck growth in the laser sintering of different-sized gold (100) nanoparticles under different heating rates is investigated using a molecular dynamics method. The numerical simulations are carried out for four pairs of two spherical nanoparticles under three different heating rates. For each pair, one nanoparticle has the same diameter of 4 nm and the other nanoparticle’s diameter is varied, ranging from 4 nm to 20 nm. The results show that the solid state sintering automatically takes place by local potential at room temperature. The stable neck width increases as the size of the other nanoparticle increases. Once the limit stable neck width is reached, it no longer is affected by the nanoparticle size. For the subsequent laser heating to the same final temperature, a lower heating rate results in a larger stable neck width due to the longer sintering process. The neck growth mechanisms and rate under various sintering conditions are discussed.

A Microscopic Model of the Asymmetric Neck Growth during Sintering Process upon the Asymmetric Particle Arrangement

A kinetic Potts Monte Carlo model was used to investigate the microstructural evolution of a three particles configuration during sintering. The a series of peculiar phenomena was observed and analyzed quantitatively, which indicated that even if the particle shape and the contact area are both completely symmetrical, the asymmetric neck growth will arise due to a special particle arrangement. Although the linear relationship between neck size logarithm and time logarithm was consistent with the traditional theory, a slower neck growth rate comparing with that of the two sphere model displayed a result of the asymmetric neck growth. The analysis of the particle rotation was made to confirm the occurrence of the asymmetric neck growth. It was firstly observed that the morphology of the grain boundaries became bevel, and the reason for this morphology was discussed. All the special phenomena have proved that the asymmetric particle arrangement about the contact area can trigger the unstable neck growth.

Genetic and Morphological Evidence that Phoma sclerotioides, Causal Agent of Brown Root Rot of Alfalfa, Is Composed of a Species Complex

Phoma sclerotioides, causal agent of brown root rot of alfalfa, causes severe root and crown lesions on alfalfa and other perennial forage legumes in regions with harsh winters. Isolates of P. sclerotioides exhibit diverse cultural morphologies on potato dextrose agar (PDA), suggesting that they may exhibit a high degree of genetic diversity. To investigate the genetic relatedness of P. sclerotioides isolates, 154 isolates from North America were sequenced at 10 loci. Maximum parsimony and maximum likelihood analyses of the complete 10-locus data set placed isolates into multiple strongly supported clades, and analyses of gene-jackknife and single-gene partitions of the data set indicated robust support for six major clades and three subclades. Genetic differences corresponded closely to differences in conidial size and septation, pycnidial neck length, mycelial pigmentation, and growth rate in axenic culture at 18 and 25°C. Isolates exhibited morphologies broadly consistent with the species description of P. sclerotioides, and new species were not designated. On the basis of genetic and morphological differences, we propose establishing seven infraspecific varieties within P. sclerotioides: P. sclerotioides var. sclerotioides, champlainii, viridis, obscurus, steubenii, macrospora, and saskatchewanii. All varieties of P. sclerotioides caused brown root rot of alfalfa and grew well at low temperatures.

Neck growth kinetics during microwave sintering of nickel powder

Model experiments on initial stage of microwave sintering of nickel powder showed anomalous neck-growth rate during isothermal soaking, which is not the case for conventional sintering. Neck growth was determined as a function of time. Values for the neck growth exponent in the neck growth equation, ( x/ a) n = Bt, of 5.2, 5.4, 5.8, and 5.9 were found for within the temperature range 700–950 °C, respectively. The evidences of formation of liquid phase during microwave sintering have been revealed, that may support enhancement of mass transfer during sintering process. The activation energy of 48 kJ mol −1 was found for microwave sintering of nickel, according to sphere-to-sphere model. Value revealed is significantly lower then values for conventional sintering (136 kJ mol −1), and is on same level with activation energy for diffusion of metals in liquid state. An explanation and analysis of this phenomenon has been attempted.

The Solid-State Neck Growth Mechanisms in Low Energy Laser Sintering of Gold Nanoparticles: A Molecular Dynamics Simulation Study

Molecular dynamics (MD) simulations were employed to investigate the mechanism and kinetics of the solid-state sintering of two crystalline gold nanoparticles (4.4–10.0nm) induced by low energy laser heating. At low temperature (300K), sintering can occur between two bare nanoparticles by elastic and plastic deformation driven by strong local potential gradients. This initial neck growth occurs very fast (<150ps), and is therefore essentially insensitive to laser irradiation. This paper focuses on the subsequent longer time scale intermediate neck growth process induced by laser heating. The classical diffusion based neck growth model is modified to predict the time resolved neck growth during continuous heating with the diffusion coefficients and surface tension extracted from MD simulation. The diffusion model underestimates the neck growth rate for smaller particles (5.4nm) while satisfactory agreement is obtained for larger particles (10nm). The deviation is due to the ultrafine size effect for particles below 10nm. Various possible mechanisms were identified and discussed.

Material aspects of dynamic neck retardation

Neck retardation in stretching of ductile materials is promoted by strain hardening, strain-rate hardening and inertia. Retardation is usually beneficial because necking is often the precursor to ductile failure. The interaction of material behavior and inertia in necking retardation is complicated, in part, because necking is highly nonlinear but also because the mathematical character of the response changes in a fundamental way from rate-independent necking to rate-dependent necking, whether due to material constitutive behavior or to inertia. For rate-dependent behavior, neck development requires the introduction of an imperfection, and the rate of neck growth in the early stages is closely tied to the imperfection amplitude. When inertia is important, multiple necks form. In contrast, for rate-independent materials deformed quasi-statically, single necks are preferred and they can emerge in an imperfection-free specimen as a bifurcation at a critical strain. In this paper, the interaction of material properties and inertia in determining neck retardation is unraveled using a variety of analysis methods for thin sheets and plates undergoing plane strain extension. Dimensionless parameters are identified, as are the regimes in which they play an important role.

Estudo alométrico dos cortes de cordeiros Santa Inês puros e cruzas

Cento e três cordeiros, machos e fêmeas, Santa Inês puros (SS) e cruzas com Texel (TS), Ile de France (FS) e Bergamácia (BS) foram utilizados no estudo do crescimento alométrico dos cortes. Os animais foram confinados em gaiolas individuais e abatidos aos 15, 25, 35 e 45 kg de PV. Após abate e resfriamento da carcaça, foram feitos os cortes: pescoço, costela/fralda, costeleta, lombo, paleta e perna, sendo esses dois últimos sem os braços, para avaliação do crescimento alométrico individual em relação ao corpo vazio (PCVZ). O crescimento alométrico do pescoço foi isogônico em todos os grupos, à exceção do grupo Santa Inês x Bergamácia (crescimento heterogônico positivo), comprovando que esse corte cresce de forma semelhante ao PCVZ. Os machos TS e FS apresentaram ritmo de crescimento do pescoço menor que o dos machos BS. A costela/fralda, em todos os grupos, teve crescimento lento entre os pesos avaliados, observando-se que, nos grupos TS e FS o ritmo de crescimento foi maior que no grupo SS. Nos machos, o desenvolvimento da paleta foi semelhante ao PCVZ, enquanto, nas fêmeas, foi lento em relação ao PCVZ. O desenvolvimento da costeleta e do lombo foi lento em todos os grupos, à exceção das fêmeas FS, nas quais acompanhou o PCVZ; os coeficientes alométricos de costeleta nesses animais foram menores que nos demais outros grupos. O desenvolvimento da perna foi similar ao PCVZ nos machos SS, TS e FS e nas fêmeas FS, porém, foi menos intenso nos machos BS e nas fêmeas SS, TS e BS.

On the coalescence of gold nanoparticles

The present work investigates the coalescence process of two gold nanoparticles for a host of initial temperatures and starting radii in vacuum with the help of molecular dynamics (MD) simulations. Diverse mechanisms of the first sintering stage, characterized by a growing neck region, were found. The results are compared with a phenomenological macroscopic model based on an energy balance and supplemented by a model for the surface variation of the nanosystem under consideration. The model is also modified to account for the curvature dependence of the melting temperature of the nanoparticles. Accounting for the findings of the MD simulations for the neck growth rate, the validity of the analytical model with the initial temperature and radius of the particle is shown. The calculations were extended to particles having radii between 9.5 and 25 Å.

Effective diffusion coefficients in solid-state sintering

A numerical analysis of the sintering neck growth rate between rigid spherical particles of the same size is carried out. The contributions of the surface, grain-boundary and volume diffusion transport into sintering kinetics during the first and the second stages of sintering are estimated. It is shown that the three-dimensional problem of the matter redistribution during sintering can be reduced to a two-dimensional problem if the effective diffusion coefficients are introduced. The effective diffusion coefficients are the grain-boundary diffusion coefficients that include the contribution of volume diffusion. The effective diffusion coefficients are sensitive to the stage of sintering: they have different form during the first and the second stages. During the second stage of sintering, the unified effective diffusion coefficient is determined. It is demonstrated that the effective diffusion coefficients can be used also for the description of the pressure-assisted sintering.

Original Monte Carlo Methodology Devoted to the Study of Sintering Processes

A Monte Carlo methodology different from others based on the Potts model has been developed to solve capillary‐driven mass transport involved in the sintering process. The addition of a cohesive energy term to the energetic model enables quantification of the stress gradients within grains, as well as the induced mass fluxes. The study of two‐particle sintering involving volume diffusion has been chosen as a first example. The morphological evolution of the two‐particle system to a single particle is followed. Real‐time‐dependent neck growth and shrinkage rates are defined. The rheological behavior of the particles induced by the Monte Carlo procedure is discussed.

Initial sintering of cylindrical particles of different sizes by surface diffusion

Sintering of two cylindrical particles of different sizes by surface diffusion at the early stage is theoretically analysed. Formulae for the neck growth rate are derived using a variational principle and the continuity equation of vacancies. The size difference between two particles is found to influence the growth rate of the neck only when the difference is large.

The fate of the infrarenal aortic neck after open aneurysm surgery

Purpose: The long-term success of the endovascular repair of abdominal aortic aneurysms is dependent on the secure fixation of the stent graft at the proximal and distal attachment sites. A progressive dilatation of the infrarenal neck may jeopardize this success. The data regarding this issue are scarce. However, the long-term fate of the infrarenal neck can be studied in patients who have undergone open aneurysm surgery. This was the purpose of the present investigation. Methods: Between January 1989 and December 1993, 64 patients underwent open repair of infrarenal abdominal aortic aneurysms. Of the 36 patients who were eligible for the study, 19 had preoperative computed tomography scans that were available. The 19 patients also underwent a new computed tomography scanning at a mean of 71 ± 12 months after surgery. Results: The mean preoperative aortic diameter was 25.4 ± 3.7 mm at the infrarenal neck, 24.8 ± 3.4 mm at the level of the renal arteries, and 26.7 ± 3.0 mm at the level of the superior mesenteric artery (SMA). The mean aortic diameter increased at all of the 3 levels: +2.8 ± 3.1 mm ( P = .0014) at the infrarenal neck, +2.8 ± 3.0 mm ( P = .0013) at the level of the renal arteries, and +1.3 ± 3.0 mm ( P = .080) at the level of the SMA. The annual growth rate was 0.48 mm/y ( P = .0023) at the infrarenal neck, 0.46 mm/y ( P = .0010) at the level of the renal arteries, and 0.21 mm/y ( P = .5811) at the level of the SMA. No correlation was found between the preoperative infrarenal neck diameter ( r = .295, P = .2194), the preoperative aortic diameter at the level of the renal arteries ( r = .302, P = .2088), and the preoperative aortic diameter at the level of the SMA ( r = .314, P = .2043) and the corresponding growth rates. The patients were stratified into 2 groups—one with a small annual growth rate at the infrarenal neck (n = 11; ≤0.3 mm/y) and one with a larger annual growth rate (n = 8; >0.3 mm/y)—and no differences in the preoperative infrarenal neck diameter or the clinical characteristics were found between the groups. Conclusion: This investigation shows an aortic dilatation of the infrarenal neck and of the aorta at the level of the renal arteries of approximately 0.5 mm annually after open aneurysm surgery. This dilatation raises concern regarding the long-term success after endovascular repair. The data also indicate that 2 populations might exist with regard to the annual growth rate of the infrarenal neck—one with low growth rate and one with higher growth rate. This might be of interest for the future selection of patients for endovascular repair. (J Vasc Surg 1998;28:889-94.)

The mechanism of defluidization of iron particles in a fluidized bed

To develop a qualitative model for the fluidization characteristics of cohesive iron particles, a systematic investigation was performed focusing attention on the mechanism of defluidization and the particle-to-particle neck growth. The neck growth rate determined by a scanning electron microscope (SEM) agreed fairly well with the prediction by the surface diffusion model. Below 1200 K, cohesion force was successfully explained by the neck growth model with the surface diffusion mechanism. The tensile strength of a neck measured by the diametral compression test was 20 MPa. This value agreed with the one obtained from the bed breaking velocity measurement. Defluidization criterion was discussed based on the balance between bubble buoyancy force and neck force. The prediction was validated experimentally.

Possible Explanations of Transient Neck Formation between Pairs of (100) Faceted Particles

Rankin and Boatner have observed that (100) faceted MgO particles in contact along either corners or edges developed, on heating, necks that initially grew, but then shrank and broke. General thermodynamic models are provided that predict this transient neck formation for any cubic particles that share only a small fraction of an edge and for particles that share an edge that is less than 0.6 times as long as the orthogonal edges. A third model, which assumes that reaction of MgO particles with their carbon substrate removes MgO from the MgO–carbon interface at a constant rate, explains the observation that the neck shrinkage rate greatly exceeds the neck growth rate. Application of the theory to explaining the seeming prevalence of rounded surfaces in sintering powders is described.

Effects of nickel on the sintering behavior of Fe-Ni compacts made from composite and elemental powders

Injection-molded Fe-Ni parts made from composite and elemental powders were prepared, and the effect of nickel on the sintering of iron compacts was investigated. Dilatometry analyses showed that the alpha-gamma phase transformation temperature of the Fe-Ni compact changed from a fixed 912 °C for pure iron to a temperature range between 700 °C and 912 °C where two phases coexisted. The microstructure indicated that nickel impeded surface diffusion and slowed down the neck growth rate of iron powder in the early sintering stage. The dual phase and the small neck size at low temperatures suppressed the exaggerated grain growth, which usually occurs on carbonyl iron powders at 912 °C. It was also observed that nickel impeded the grain growth of iron at high temperatures. Thus, by reducing the exaggerated grain growth during phase transformation, impeding the grain growth at high temperatures, and with high diffusion rates of iron in Ni-rich areas, enhanced densification was obtained for Fe-Ni systems, particularly for those systems made from composite powders. However, when coarse nickel powder was added, expansion was observed due to the presence of large pores around nickel powders. These pores were formed because of the particle rearrangement which was caused by the Kirkendall effect.

Viscoelastic effects in the coalescence of polymer particles

A study was made of the coalescence of linear acrylic resins over a much greater dynamic range than previously investigated. The rate of neck growth between a sphere and a flat slab of the same material was measured in situ by optical microscopy and found to exhibit a number of features typical of polymer viscoelastic response. Results are compared with independent measurements of the melt rheology (torsional creep compliance). An analytic approximation is proposed which allows the neck-growth kinetics to be predicted from the recoverable creep compliance and viscosity of the melt. It predicts qualitative trends consistent with the limited data on molecular weight, structure, and particle size variations. This analysis helps resolve some apparent contradictions among earlier studies and implies a qualitative difference in the coalescence mechanisms for large particles (5 μm) and colloids (<0.5 μm). In either case, the ultimate equilibration to homogeneous bulk mechanical properties requires times on the order of the terminal relaxation time, regardless of particle size. Evidence is presented that, in addition to the recognized effects of chain inter-diffusion, stress relaxation may play a role in the equilibration kinetics.

Sintering of two phase lamellar microstructures

In this communication, we have attempted to semiquantitatively describe the effect of an inert phase on neck growth rates during sintering of lamellar structures. Our treatment is a simplified one. It is analogous to the “real” situation in the same sense Ashby's initial treatment of sintering diagrams is to later, more refined one (2). A more detailed treatment necessitates consideration of densification, as well as neck growth, rates. Such an effort would also incorporate coupling effects between surface diffusion and boundary diffusion during Stage I sintering. In spite of the simplifications used, the present treatment is believed a useful first step in assessing the effect of an inert phase on sintering of lamellar microstructures.