Distribution of laser-induced dendrites on a steel surface

It is demonstrated that laser irradiation causes formation of complicated fractal structures (classical dendrites) on a steel surface. The distribution of dendrites over the irradiated region can be controlled using variations in the profile of the incident laser beam. Electron microscopy is used to evaluate the size distributions of dendrites and the surface distribution density.     

In situ synchrotron X-ray imaging on morphological evolution of dendrites in Sn–Bi hypoeutectic alloy under electric currents

The growth behavior and morphological evolution of dendrites in solidifying Sn–Bi alloy under electric currents [e.g., direct current (DC) and electric current pulse (ECP)] are in situ studied using synchrotron radiation X-ray imaging technique. The suppression of dendrite growth, floating and rotation of dendrites, refinement and remelting of dendrites are investigated by analyzing a series of animated images captured during the experiments. The modification mechanisms of dendrite morphology by electric fields are discussed based on the in situ and real-time observations. When DC is imposed on the samples, the growth of dendrites is significantly suppressed due to the effect of Joule heat, and a small dendrite freely flows up and rotates due to the common effect of natural convection. When ECP is imposed in the whole solidification process, the outset of solidification is delayed by Joule heat. And due to the accumulation of undercooling, dendrites suddenly nucleate, grow and finally become fine primary dendrite arm spacing. When ECP is imposed during the crystal growth stage only, the dendrites are remelted at first and then reappear along the original growing trajectories, showing the hereditary feature.     

Growth mechanism of Si-faceted dendrites

The growth mechanism of Si-faceted dendrite was studied using an in situ observational technique. We directly observed the growth processes of Si-faceted dendrites from Si melts. It is found that triangular corners with an angle of 60 degrees are formed at the dendrite tip. We present an original growth model for faceted dendrites based on the experimental evidence. The model fully explains the growth process of faceted dendrites.     

Diffusion-Limited Aggregates of Poly(Ethylene Oxide) on Alkali Halides

Poly(ethylene oxide) (PEO) was crystallized on alkali halides, such as NaCl, KCl, and KBr, from solution in isopentyl acetate. PEO dendrites first formed on alkali halide surfaces and grew to sheet the whole surface. Individual fibrils consisting of dendrites were arranged with the preferred orientation in the <110> direction on KBr and KCl, and mainly in the <100> direction on NaCl. Tetragonal lamellae grew not on the substrate directly but on the sheet platelet only, establishing a homoepitaxial relationship to the dendrites. Because the dendritic sheet that intervened between them controlled the orientation of the lamellae, tetragonal lamellae formed with a preferred orientation on alkali halide surfaces. The dendrites had morphological features of fractals, i.e., diffusion-limited aggregates (DLA). Their morphology was controlled by the diffusion process of polymer molecules on the substrate surface, as well as by epitaxial interaction. The fractal dimension (D) was obtained by analyzing the dendrites by taking the autocorrelation function of density: D ranges from 1.6 to 1.8, depending on the crystallization conditions, such as the kinds of substrate and crystallization temperature.     

Numerical study of growth competition between twin grains during directional solidification by using multi-phase field method

A multi-phase field model is established to simulate the growth competition and evolution behavior between seaweed and columnar dendrites during directional solidification.According to the effects of surface tension and interfacial energy,we quantitatively analyze the influences of factors such as inclination angles,pulling velocity,and anisotropic strength on twin growth.The results demonstrate that the pulling velocity and anisotropic strength have an important influence on the morphology and evolution of the seaweed and dendritic growth.The low pulling velocity and anisotropic strength are both key parameters for maintaining the stable morphology of seaweed during competitive growth in a bicrystal,showing that the lateral branching behavior is the root of the dendrites that can ultimately dominate the growth.And it is clarified that the lateral branching behavior and lateral blocking are the root causes of the final dominant growth of dendrites.With the increase of anisotropy strength,the seaweed is eliminated fastest in case 1,the seaweed is transformed into degenerate dendritic morphology,and eliminates the seaweed by promoting the generation and lateral growth of the lateral branches of the dendrites.The increase of pulling velocity is to increase the undercooling of favorable oriented grain and accelerate the growth rate of dendrites,thus producing more new primary dendrites for lateral expansion and accelerating the elimination rate of unfavorable oriented grain.     

Neuron participation in a synchrony-encoding assembly

Background

Caenorhabditis elegans actively crawls down thermal gradients until it reaches the temperature of its prior cultivation, exhibiting what is called cryophilic movement. Implicit in the worm's performance of cryophilic movement is the ability to detect thermal gradients, and implicit in regulating the performance of cryophilic movement is the ability to compare the current temperature of its surroundings with a stored memory of its cultivation temperature. Several lines of evidence link the AFD sensory neuron to thermotactic behavior, but its precise role is unclear. A current model contends that AFD is part of a thermophilic mechanism for biasing the worm's movement up gradients that counterbalances the cryophilic mechanism for biasing its movement down gradients.

Results

We used tightly-focused femtosecond laser pulses to dissect the AFD neuronal cell bodies and the AFD sensory dendrites in C. elegans to investigate their contribution to cryophilic movement. We establish that femtosecond laser ablation can exhibit submicrometer precision, severing individual sensory dendrites without causing collateral damage. We show that severing the dendrites of sensory neurons in young adult worms permanently abolishes their sensory contribution without functional regeneration. We show that the AFD neuron regulates a mechanism for generating cryophilic bias, but we find no evidence that AFD laser surgery reduces a putative ability to generate thermophilic bias. In addition, although disruption of the AIY interneuron causes worms to exhibit cryophilic bias at all temperatures, we find no evidence that laser killing the AIZ interneuron causes thermophilic bias at any temperature.

Conclusion

We conclude that laser surgical analysis of the neural circuit for thermotaxis does not support a model in which AFD opposes cryophilic bias by generating thermophilic bias. Our data supports a model in which the AFD neuron gates a mechanism for generating cryophilic bias.     

Surface‐enhanced Raman scattering on silver dendrite with different growth directions

A simple and cost‐effective strategy was developed to synthesize Ag dendrites via an aqueous chemical route based on spontaneous galvanic displacement between silver (I) ions and copper foil under hydrothermal condition. Ag dendrites with [100], [110], and [111] growth direction can be prepared by adjusting the content of polyvinyl pyrrolidone. The Ag dendrites exhibited fine and well reproducible surface‐enhanced Raman scattering effect using Rhodamine 6G as model molecule. There is a corresponding relation between the false color plot of Raman intensity and the dendrite morphology. Among the products, the Ag dendrites growing along [100] direction have the best surface enhancement ability, and a possible explanation was proposed. The results might open up new thinking on surface‐enhanced Raman scattering by proper crystal growth and design. Copyright © 2011 John Wiley & Sons, Ltd.     

基于深度学习激光熔覆层树枝晶的形貌识别

在增材制造技术中,树枝晶的表征对于分析激光熔覆层的机械性能至关重要,但目前树枝晶的标记主要由人工完成,耗时长且容易引入人为误差,而深度学习可提高目标识别准确度。基于U-net网络提出了适于识别分割树枝晶的BNC-Unet网络,将串行注意力机制和Batch Normalization层有效地部署在上采样和下采样区域,调整图像特征的权重信息。选取交并比作为分割结果的评价指标,并对比了原Unet以及不同的改进方法在该指标下的结果。在测试集中,BNCUnet网络分割树枝晶准确率指标为84.2%,比原U-net网络结果提升了8.97%。该指标表明网络能准确地从激光熔覆层金相图中识别出树枝晶形貌,且识别树枝晶的准确率显著提高,便于在激光熔覆试验后评估熔覆层性能。     

Numerical simulation on dendritic growth of Al-Cu alloy under convection based on the cellular automaton lattice Boltzmann method

A numerical model is developed by coupling the cellular automaton (CA) method and the lattice Boltzmann method (LBM) to simulate the dendritic growth of Al-Cu alloy in both two and three dimensions. An improved decentered square algorithm is proposed to overcome the artificial anisotropy induced by the CA cells and to realize simulation of dendritic growth with arbitrary orientations. Based on the established CA-LBM model, effects of forced convection and gravity-driven natural convection on dendritic growth are studied. The simulation results show that the blocking effect of dendrites on melt flow is advanced with a larger number of seeds. The competitive growth of the converging columnar dendrites is determined by the interaction between heat flow and forced convection. Gravity-driven natural convection leads to highly asymmetric growth of equiaxed dendrites. With sinking downwards of the heavy solute, chimney-like or mushroom-like solute plumes are formed in the melt in front of the columnar dendrites when they grow along the gravitational direction. More details on dendritic growth of Al-Cu alloy under convection are revealed by 3D simulations.     

Antennal morphology and sensilla ultrastructure of Dendroctonus valens LeConte (Coleoptera: Curculionidae,Seolytinae), an invasive forest pest in China

Dendroctonus valens LeConte, an invasive forest pest, is highly dependent on its olfactory system for activities such as host seeking, mating, oviposition and population aggregation. The antennae are the primary olfactory organs in bark beetles. We describe four morphological types of sensilla on the antenna of D. valens: (1) 3 subtypes of sensilla trichodea located at the apex and lateral surface of the club. Sensilla trichodea types 1 and 3 were innervated by 5 and 13 dendrites, respectively. Sensilla trichode type 2 was not found dendrites in a lumen, (2) sensilla chaetica found on the scape and funicle and innervated by 7 dendrites, (3) a pair of sensilla basiconica distributed primarily on the three sensory bands, each innervated by 2 dendrites at the base, and (4) one fluted cone type of grooved peg dispersed throughout the sensory bands. The possible functions of these sensilla are discussed in relation to their morphology and ultrastructure.     

Dendrite to symmetry-broken dendrite transition in directional solidification of non-axially oriented crystals

In this paper, the morphological transition from dendrite to symmetry-broken dendrite is investigated in the directional ;olidification of non-axially-oriented crystals using a quantitative phase-field model. The effects of pulling velocity and zrystal orientation on the morphological transition are investigated. The results indicate the orientation dependence of the ;ymmetry-broken double dendrites. A dendrite to symmetry-broken dendrite transition is found by varying the pulling telocity at different crystal orientations and the symmetry-broken multiple dendrites emerge as a transition state for the ;ymmetry-broken double dendrites. The state region during the transition can be well characterized through the variations ff the characteristic angle and the average primary dendritic spacing.     

树枝状银纳米结构的可控电化学合成及其SPR/SERS性质

采用电化学沉积的方法在导电玻璃衬底上合成出树枝状的银纳米结构. 通过调控电位、表面活性剂、硝酸银浓度等参数控制树枝状银纳米结构的尺寸、直径、结晶性和分枝密度. 三种典型的树枝状银纳米结构被用来做表面增强拉曼散射衬底,其中的一种可以清楚的探测浓度低于0.1 nmol/L的若丹明6G分子. 不同树枝状银纳米结构表面增强拉曼效果相差比较大,说明了分枝颗粒形状,特别是颗粒间距对表面增强拉曼散射有非常重要的影响.     

等温凝固多晶粒生长相场法模拟

采用Kim模型，利用耦合溶质场的相场模型对Al-2-mole-Cu合金等温凝固过程中多晶粒相互影响下枝晶的生长过程进行数值模拟，为了提高计算效率，采用差分去实现宏微观场之间的耦合.研究了不同过冷度对多晶粒枝晶形貌和溶质分布的影响，结果表明：成分过冷对枝晶生长速度和溶质分配有着重大的影响，溶质元素Cu在固液界面前沿重新分布，结果导致实际热过冷减小，进而影响枝晶的生长和溶质向外层的扩散，致使相互接触的枝晶产生萎缩而其余没有受到抑制的枝晶生长方向产生择优现象. 关键词： 相场法 模拟 择优生长 等温凝固 二元合金     

Influences of travelling magnetic field on the dendritic structures of Sn–1.8Cd peritectic alloy during directional solidification

The effects of melt flow driven by a travelling magnetic field (TMF) on solidification structures of Sn–1.8 wt.% Cd peritectic alloy have been investigated numerically and experimentally. Numerical results indicate that the flow velocity at the solid–liquid interface under a downward TMF is smaller than that under an upward TMF. The experimental results show that the growth directions of dendrites are chaotic, and several crotches among the dendrites are observed at the solid–liquid interface in the case of no field. It is concluded from TMF results that the ordered growth of dendrites at two different directions occurs, and only one crotch among the dendrites appears at the solid–liquid interface. The location of the crotch gradually approaches the interface center with increasing magnetic field intensity (B≤10.3 mT). Moreover, the growth of high-order branches occurs at the crotch under a downward TMF. A simple model is established for explanation and it well corresponds to the experimental results.     

Modeling of multi-burst mode pico-second laser ablation for?improved material removal rate

This paper deals with the unique phenomena occurring during the multi-burst mode picosecond (ps) laser ablation of metals through modeling and experimental studies. The two-temperature model (TTM) is used and expanded to calculate the ablation depth in the multi-burst mode. A nonlinear increment of ablation volume is found during the multi-burst laser ablation. The deactivation of ablated material and the application of temperature-dependent electron-phonon coupling are demonstrated to be important to provide reliable results. The simulation results based on this expanded laser ablation model are experimentally validated. A significant increase of ablation rate is found in the multi-burst mode, compared with the single-pulse mode under the same total fluence. This numerical model provides a physical perspective into the energy transport process during multi-burst laser ablation and can be used to study the pulse-to-pulse separation time effect on the ablation rate.     

Quantifying the quality of femtosecond laser ablation of graphene

The influence of beam intensity on laser ablation quality and ablation size is experimentally studied on graphene-coated silicon/silicon dioxide substrates. With an amplified femtosecond-pulsed laser system, by systematically decreasing the average power, periodic stripes with decreasing widths are ablated. Histogram analyses of the untouched and ablated regions of scanning electron microscope images of the fabricated structures make it possible to quantify the ablation quality. These analyses reveal that submicron ablation can be achieved while maintaining 75 % ablation accuracy by adjusting the beam intensity around the ablation threshold.     

Low-temperature STM/STS investigation of nanostructures created by an STM tip on Au(111) surfaces

The structural stability of rapidly solidified (about 10⁴ K/s) Sn–3.7Ag–0.9Zn eutectic solder was explored by high-temperature annealing. For the as-cast solders, the applied fast cooling rate had a significant influence on the microstructure of the solders. The faster the applied cooling rates, the smaller the β-Sn dendrites. After annealing at 473 K for 20 and 50 h, β-Sn dendrites congregated together into bulk ones for minimizing the interfacial energy, and Ag₃Sn intermetallic compounds (IMCs) as well as ternary Ag–Zn–Sn IMCs segregated on the grain boundary of the β-Sn dendrites. It seems that the coarsening of the β-Sn dendrites in the rapidly solidified specimen brought a significant softening during annealing of the explored Sn–Ag–Zn alloy. Finally, the β-Sn dendrites vanished gradually with increase of the annealing period, which leads to a kind of softening.     

Fractal Properties of CdTe Quantum Dots Dendrites

Optics and Spectroscopy - Here we present the analyses of fractal properties of CdTe dendrites. The spectral characteristics of dendrites obtained at different acids of the initial solution were...     

Growth of dendrites in a large bore HyBrID copper vapor laser

Rapid growth of copper dendrites on the surface of copper metal has been observed in the discharge region of a large-bore (80 mm diameter) HyBrID copper vapor laser. The dendrites have the shape of deposit morphology. And the maximum average growth rate is around 0.25 mm/h in height and width. Also, a dendrite of 25 mm in maximum height and width appears after an operating time of 150 h. As a result, the average laser output power decreases to 75% of the initial level of the output power owing to the dendrites obstructing the laser beam.     

基于双相延迟模型的飞秒激光烧蚀金属模型

为了分析飞秒激光烧蚀过程,在双相延迟模型的基础上建立了双曲型热传导模型.模型中考虑了靶材的加热、蒸发和相爆炸,还考虑了等离子体羽流的形成和膨胀及其与入射激光的相互作用,以及光学和热物性参数随温度的变化.研究结果表明:等离子体屏蔽对飞秒激光烧蚀过程有重要的影响,特别是在激光能量密度较高时;两个延迟时间的比值对飞秒激光烧蚀过程中靶材的温度特性和烧蚀深度有较大的影响;飞秒激光烧蚀机制主要以相爆炸为主.飞秒激光烧蚀的热影响区域较小,而且热影响区域的大小受激光能量密度的影响较小.计算结果与文献中实验结果的对比表明基于双相延迟模型的飞秒激光烧蚀模型能有效对飞秒激光烧蚀过程进行模拟.