Non-uniform Deposition Research Articles

Comparative thermal analysis of Nd- and Yb-doped YAG and KGdW laser crystals under diode- and flashlamp-pumping

Temperature distribution in the Nd- and Yb- doped YAG and KGdW laser crystals under flashlamp- and diode-pumping was characterized by means of finite element analysis. For KGdW, two laser crystal orientations were considered for light propagation along the Np and Ng optical indicatrix axes, taking into account the anisotropy of thermal conductivity coefficient. The influence of the cooling conditions, pump spot size and dopant concentration on the temperature distribution was analyzed. For flashlamp-pumping conditions, the applicability of the quasi-steady-state model is discussed. The main concerns in the thermal management of KGdW laser host is the relatively low thermal conductivity that results in poor cooling and significant absorption coefficients under diode pumping that result in highly non-uniform volumetric heat deposition. “Athermal” Ng-cut KGdW crystal was found to produce higher temperature gradients that the “standard” Np-cut one, that should results in higher internal stresses and higher probability of thermally-induced cracks.

Polarization and angular effects of femtosecond laser-induced nanostructure-covered large scale waves on metals

In this paper, we find that nanostructure-covered large scale waves (NC-LSWs) can be produced on metals using both s- and p-polarized femtosecond laser pulses. We show that the period of NC-LSWs can be controlled by laser fluence, the number of irradiating pulses, and the incident beam angle. By modeling angular dependence of NC-LSW period induced by s-polarized light, we reveal that inhomogeneous energy deposition plays an important role in the formation of NC-LSWs. This allows us to establish a three-step model for NC-LSW formation, the formation of laser-induced surface unevenness, inhomogeneous energy deposition due to the interference of the incident light with the scattered light, and nonuniform energy deposition due to shielding by LSWs.

Effect of Powder Size and Volume Fraction of WC on the Microstructure of Laser Cladding WC-NiCrBSi Composite Coatings

WC-NiCrBSi composite coatings on 0.45 % carbon steel were fabricated by laser cladding. The volume fraction (33, 43 and 60 vol. %) of WC particles with the powder size of 2 μm, 5 μm and 50 μm were added respectively into NiCrBSi powders to deposit composite coatings. The microstructure of the powders and the coatings were examined by scanning electron microscope (SEM) and X-ray diffraction (XRD). Results show that the compactness and uniformity of the coatings are good when WC particles size was 2 μm and 5 μm. However nonuniform distribution and deposit of WC particles on the bottom of the coating will occur when the powder size of WC particles was 50 μm and the volume fraction of WC was 33 %. When the volume of WC addition was higher than 60 %, the volume fraction of the retained WC particles in the coatings is obviously decreased. The dissolving and decomposing of WC increase with decreasing the powder size and increasing the volume of WC addition and the laser power.

Colloidal and bacterial fouling during constant flux microfiltration: Comparison of classical blocking laws with a unified model combining pore blocking and EPS secretion

The instantaneous transmembrane pressure needs to be continuously increased to compensate for foulant accumulation during constant flux microfiltration. Herein, we compare predictions of a unified mathematical model and conventional blocking laws with laboratory data obtained during constant flux operation. Foulants employed included single species cultures of bacteria and coagulated natural colloids, both of which are known to form compressible cakes. The first principles model unifies fouling arising from pore blocking by individual cells, cake formation, as well as bacterial secretion of exopolymers. It also incorporates non-uniform spatial deposition patterns that have been observed during unstirred dead-end filtration. Mechanistically, these heterogeneities arise either from non-uniform membrane surface porosity or stochastic initial binding of foulants. Previous studies have shown that the initial patchy or uneven deposit morphology is magnified over longer time-scales during bacterial filtration by differential extracellular polymeric substances (EPS) secretion through quorum sensing. However, in this study, we are primarily interested in the averaged behavior (mainly flux and pressure). By spatially averaging the microscale variables we are able to compare with classical blocking law models. We show that blocking laws and the unified model both accurately model fouling under our experimental conditions. The unified model provides mechanistic insights into (bio)colloid deposition and associated fouling during constant flux microfiltration particularly since it is obtained excellent predictive agreement with experimental data using parameters taken exclusively from our recent study of constant pressure filtration.

Increasing the energy dynamic range of solid-state nuclear track detectors using multiple surfaces

Solid-state nuclear track detectors, such as CR-39, are widely used in physics and in many inertial confinement fusion (ICF) experiments. In the ICF experiments, the particles of interest, such as D(3)He-protons, have ranges of order of the detector thickness. In this case, the dynamic range of the detector can be extended by recording data on both the front and back sides of the detector. Higher energy particles which are undetectable on the front surface can then be measured on the back of the detector. Studies of track formation under the conditions on the front and back of the detector reveal significant differences. Distinct front and back energy calibrations of CR-39 are therefore necessary and are presented for protons. Utilizing multiple surfaces with additional calibrations can extend the range of detectable energies on a single piece of CR-39 by up to 7-8 MeV. The track formation process is explored with a Monte Carlo code, which shows that the track formation difference between front and back is due to the non-uniform ion energy deposition in matter.

Experimental Study of Asphaltene Deposition During Different Production Mechanisms

Sudden changes in key parameters such as pressure, temperature, and fluid composition may result in asphaltene precipitation and deposition, consequently reducing permeability and porosity as well as well injectivity and productivity. Sandstone cores of an Iranian reservoir were studied under high pressure and temperature. Asphaltene deposition was studied in recycled gas injection, CO2 injection, and natural depletion experiments. The authors observed that these processes could be ranked for the deposition severity viewpoint in the aforementioned order. Qualitatively investigation of cores indicated nonuniform deposition of precipitated asphaltene along a flooded core and reducing deposition from entering core terminal to the core outlet.

Effect of Solvent on the Morphology of Nickel Localized Electrochemical Deposition

Localized electrochemical deposition (LECD) was performed by microanode-guided electroplating (MAGE) in two different baths to investigate the effect of solvent on the deposit morphology. One of the baths contained an ordinary aqueous solution prepared from distilled water and ionic liquid by mixing choline chloride with ethylene glycol; the other bath contained the same concentration of nickel chloride. A nonuniform micro deposit was obtained in the ionic liquid and it appeared as if it had been directly accumulated with huge nodular nickel particles; a uniform micro pillar was formed in the aqueous solution and this micro pillar exhibited a smooth appearance. The distinct deposit morphology is governed by the nature of the bath, which is characterized by the electrical resistivity and viscosity. The activation energy that arose from the electrical resistivity (Eρ) and viscosity (Eη) in the different baths was estimated. The sum of activation energies (i.e., Esum = Eρ + Eη) is much higher in the ionic liquid than in the aqueous solution. The higher activation energy in the ionic liquid bath leads to the formation of nodular deposits. The lower activation energy in the aqueous solution bath leads to the formation of a micro pillar with fine uniform particles that have a smooth appearance. Therefore, loosely-packed coarse particles grow in the ionic liquid bath to form nickel nodules whereas tiny dense particles are formed in the aqueous solution bath to form a micro pillar with a smooth surface.

플라즈마 진단에 의한 PECVD SiO2증착의 불균일성 원인 연구

The cause of the thickness non-uniformity in the large area deposition of films by PECVD(Plasma Enhanced Chemical Vapor Deposition) was investigated by the plasma diagnostics. The spatial distribution of the plasma species in the chamber was obtained with DLP(Double Langmuir Probe) and the new-designed probe-type QMS(Quadrupole Mass Spectrometer). From the relationship between the spatial distribution of the plasma species and the depositing rate of the films, it was conformed that the non-uniform deposition of films was related with the spatial distribution of the oxygen radical density and electron temperature.

Effects of RF bias power on electron energy distribution function and plasma uniformity in inductively coupled argon plasma

Changes in the plasma non-uniformity and the electron energy distribution function (EEDF) by increasing RF bias power were observed in inductively coupled plasma using spatially resolved radial EEDF measurements. As the bias power was increased at a fixed ICP power at a low gas pressure, The EEDF was evolved from a bi-Maxwellian to a Maxwellian distribution. The plasma density was decreased in all radial positions and thus plasma non-uniformity was slightly changed. However, strongly improved plasma spatial non-uniformity was observed at a high gas pressure with a decrease in the center-plasma density and an increase in the radial edge-plasma density. This result could be understood by combined effects of the ion acceleration loss and the non-uniform power deposition due to the RF bias power.

Numerical simulation on turbulent flows in vertical chemical vapor deposition reactors

The issue of turbulence in chemical vapor deposition (CVD) reactors has received little attention, since the use of laminar flow conditions in the conventional operation of CVD reactors has been developed as a general preference, contrast to the common preference for turbulent flow conditions in other fields of chemical reactor operation. The turbulent flow can cause the instantaneous heat transfer to be highly nonuniform; however, on average the heat flux may be uniform in some certain part. It is not necessarily a problem in CVD for an instantaneously nonuniform deposition, and obtaining a uniform deposition is essential averaged over the required growth time. So it is worth to explore how to employ turbulent flow properties for efficient and uniform deposition in CVD. In this study, the effect of turbulence on the CVD process in a single wafer reactor is numerically studied with large eddy simulations (LES). The flow and temperature fields are obtained for the cases with different flow Reynolds number, Rayleigh number, geometric and other parameters. The time-averaged heat transfer (Nusselt number) is shown. Some optimum methods and designs to get much better uniformity characteristics across a large part of the wafer are discussed for the turbulent vertical CVD reactors.

On the Principles of Printing Sub-micrometer 3D Structures from Dielectric-Liquid-Based Colloids

Dielectrophoresis-assisted (DEP) on-demand printing of dielectric-liquid-based colloidal gold under room conditions is demonstrated and employed to print 2D and 3D structures with sub-micrometer feature sizes. The focus of the work is primarily on explaining the physics of the printing process, based on the formation of a controlled sequence of sub-micrometer drops. The physics of 3D structure formation on the substrate is explained through the visualization and analysis of various time-scales relevant to the printing process and the pinning of the contact line of the printed colloids. A parametric variation of the related variables, namely the applied voltage and the pulse length, is used to investigate the morphology and topography of a host of basic, printable 2D and 3D features. It is established that it is possible to obtain uniform particle deposits in 2D by filling up an initial coffee-ring-type non-uniform deposit with a series of subsequently formed drops, all obtained during a single electric pulse. Finally, on-demand production of multilayered, sub-micrometer gold tracks is demonstrated, where the annealed tracks exhibit exceptionally low electrical resistivity for their sizes, only two times higher than that of bulk gold.

Review of the formula of thermal focal length in side-pumped laser rods

The expressions of focal length in side-pumped rod lasers are studied. Good agreement is obtained between simulation and experiments reported previously, using Koechner's theory. The results reveal that underestimation of thermal lens accrues from overestimating the material's thermal conduction and without considering the nonuniform heat deposit. In the case of Nd:YAG rods, temperature-dependent variation of the refractive index constitutes the major contribution of thermal lensing, whereas the temperature- and stress-dependent variations of the refractive index make the major contributions of thermal lensing for Nd:Glass rod lasers.

Synthesis of hybrid nanographite particles using fluidized bed system

Nanographite coated with ferromagnetic substances such as iron or iron oxide is a potential material for microwave absorption because of its favorable structural, magnetic, and electrical characteristics. In this paper, deposition on surfaces of acid functionalized and microwave-exfoliated nanographite particles with the use of fluidized bed system is reported. Acid functionalization improves iron adhesion and exfoliation reduces the flake thickness. The parameters influencing the deposition process are considered. It is demonstrated that Faraday’s laws of electrolysis can be used for these systems if the charge transfer from solid cathode to bed particles is uniform. This requirement is satisfied only above some critical values of suspension density, electrolyte concentration, and stirring rate. The optimized values of current density are required for each specific system, as low current density leads to non-uniform deposition with local nucleation, when high a current density induces too rapid nucleation and promotes iron hydroxo complex formation. Deposition time also should be optimized for any specific system, as the expected amount of deposit cannot be formed longer because of side reactions.

Heterostructural coaxial nanotubes of CNT@Fe2O3 via atomic layer deposition: effects of surface functionalization and nitrogen-doping

This study attempted to synthesize one-dimensional (1D) coaxial nanotubes of Fe2O3 based on carbon nanotubes (CNT@Fe2O3) via atomic layer deposition (ALD) using ferrocene and oxygen as precursors. Results disclosed that undoped CNTs were suitable for the ALD of Fe2O3 (ALD-Fe2O3) only if they were chemically functionalized, due to their inert surface nature. It was further demonstrated that the effects of both covalent and non-covalent methodologies were limited in functionalizing undoped CNTs, leading to random and non-uniform deposition of Fe2O3. In sharp contrast, it was found that, as an alternative, nitrogen-doped CNTs (N-CNTs) contributed uniform and tunable ALD-Fe2O3, due to their active surface nature induced by incorporated N atoms. Consequently, various 1D heterostructural coaxial nanotubes were obtained with well-controlled growth of Fe2O3 on N-CNTs. For a better understanding, the underlying mechanisms were explored based on different N-doping configurations. In addition, high-resolution transmission electron microscopy and X-ray diffraction jointly demonstrated that as-deposited Fe2O3 is single-phase crystalline α-Fe2O3 (hematite). The as-synthesized heterostructural coaxial nanotubes of CNT@Fe2O3 may find great potential applications in photocatalysis, gas-sensing, and magnetic fields.

Effect of electroless nickel on the series resistance of high-efficiency inkjet printed passivated emitter rear contacted solar cells

Many existing and emerging solar cell technologies rely on plated metal to form the front surface contacts, and aluminium to form the rear contact. Interactions between the metal plating solutions and the aluminium rear can have a significant impact on cell performance. This paper describes non-uniform nickel deposition on the sintered aluminium rear surface of passivated emitter and rear contacted (PERC) cells patterned using an inkjet printing technique. Rather than being plated homogeneously over the entire rear surface as is observed on an alloyed aluminium rear, the nickel is plated only in the vicinity of the point openings in the rear surface silicon dioxide dielectric layer. Furthermore, this non-uniform nickel deposition was shown to increase the contact resistance of the rear point contacts by an order of magnitude, resulting in higher series resistance values for these fabricated PERC cells.

Asymmetric statistics of order books: The role of discreteness and evidence for strategic order placement

We show that the statistics of spreads in real order books is characterized by an intrinsic asymmetry due to discreteness effects for even or odd values of the spread. An analysis of data from the New York Stock Exchange (NYSE) order book points out that traders' strategies contribute to this asymmetry. We also investigate this phenomenon in the framework of a microscopic model and, by introducing a nonuniform deposition mechanism for limit orders, we are able to quantitatively reproduce the asymmetry found in the experimental data. Simulations of our model also show a realistic dynamics with a sort of intermittent behavior characterized by long periods in which the order book is compact and liquid interrupted by volatile configurations. The order placement strategies produce a nontrivial behavior of the spread relaxation dynamics which is similar to the one observed in real markets.

Kinetic model for ion pressure perturbations in inhomogeneous laser-matter interactions

A Boltzmann–Vlasov Fokker–Planck model is used to characterize the evolution of ion pressure perturbations from nonuniform laser deposition in a plasma slab. It is found that significant reduction and smoothing of ion pressure perturbations from nonuniform optically smoothed single beam laser deposition can be achieved on hydrodynamic time scales over a range of scale sizes. In addition, it is observed that the ion Fokker–Planck model predicts more smoothing of the ion pressure perturbations from the critical to ablation surface than the ion fluid model.

Multiscale Simulation and Optimization of an Atomic Layer Deposition Process in a Nanoporous Material

A multiscale simulator for alumina film growth inside a nanoporous material during atomic layer deposition (ALD) is developed. The model combines a continuum description at the macroscopic level of precursor gas transport inside a nanopore during exposure to each of the two precursor species (trimethyaluminum and water) with a kinetic Monte Carlo (kMC) simulation of the film growth on the microscopic scale. Simulation results are presented for both the kMC simulation and for the multiscale system, the latter illustrating how nonuniform deposition along the nanopore can occur when insufficient precursor exposure levels are used.

Optimisation of a thermophoretic personal sampler for nanoparticle exposure studies

A new Thermal Precipitator (TP) was developed as a personal sampler for nanoparticle exposure studies. Two parallel 20-mm-long plates with different but uniform temperatures were introduced into the TP with an appropriate gap distance, to achieve a uniform temperature gradient along the length of the plates. Particles are thermophoretically deposited on the colder plate in the TP which acts as the substrate. Analytical calculations were carried out to determine an optimal plate gap distance and temperature gradient in the TP. A simulation grid was created from the resulting geometry which was used for numerical modelling with a CFD Software. Results from the simulations showed a uniform deposition of particles up to the size range of about 300 nm for a temperature gradient of 15 K/mm and a 1-mm gap distance, independent of the orientation of the TP during sampling. In contrast to the old TP where up to 32 SEM images of its non-uniform particle deposition had to be evaluated to obtain an average particle size distribution, an evaluation of the uniform deposition with the new TP is much more simplified, remarkably reducing the time and cost of the evaluation, while providing more accurate results.

Fully Kinetic Fokker-Planck Model of Thermal Smoothing in Nonuniform Laser-Target Interactions

Using a fully kinetic 2D Fokker-Planck model, the generation and evolution of ion density perturbations from nonuniform laser deposition in a plasma slab have been studied. It is found that significant smoothing of the ion density perturbations from nonuniform optically smoothed single beam laser deposition can be achieved on hydrodynamic times scales over a range of scale sizes. In addition, it is observed that the Fokker-Planck model predicts more smoothing than the hydrodynamic Spitzer model.