Flat Aluminum Surface Research Articles

Persistent Sputtering Yield Reduction in Plasma-Infused Foams.

Aluminum microfoams are found to exhibit persistent sputtering yield reductions of 40%-80% compared to a flat aluminum surface under 100 to 300eV argon plasma bombardment. An analytical model reveals a strong dependency of the yield on the foam geometry and plasma sheath. For foam pore sizes near or larger than the sheath thickness, the plasma infuses the foam and transitions the plasma-surface interactions from superficial to volumetric phenomena. By defining a plasma infusion parameter, the sputtering behavior of foams is shown to be separated into the plasma-facing and plasma-infused regimes. While plasma infusion leads to a larger effective sputtering area, geometric recapture of ejected particles facilitates an overall reduction in yield. For a given level of plasma infusion, the reductions in normalized yield are more pronounced at lower ion energies since angular sputtering effects enable more effective geometric recapture of sputterants.

Evaporation of highly wetting fluids on aluminum microporous coating

Waste heat dissipation, using evaporation heat transfer, is highly desirable in applications involving components that generate a persistent heat flux, due to the temperature uniformity experienced during evaporation. This form of heat transfer requires continuous feeding of liquid to the entire heated surface and mitigating the formation of dry spots. With complete liquid coverage, evaporation heat transfer results in a temperature uniformity across the heated surface due to phase change at the liquid-vapor interface. Aluminum High-Temperature Conductive Microporous Coating (Al-HTCMC) is highly effective in spreading liquid over a surface. It is fabricated by brazing aluminum particles (dp = 11, 24, 66, or 114 µm) onto a flat aluminum surface. This coating is applied on a large vertical aluminum plate (50.8 mm wide × 152.4 mm tall × 3.3 mm thick). Rate of rise experiments with highly wetting fluids (ethanol, acetone, and methanol) are performed, at room temperature and atmospheric pressure, where the bottom of the coated plate is submerged in a pool of liquid to investigate the effect of the particle size on permeability and effective meniscus radius within the Al-HTCMC. Evaporation experiments are also conducted using ethanol at saturated conditions, where a 50.8 × 127 mm2 heater is attached to the aluminum plate. The effects of particle size, coating thickness, and fluid properties on the evaporation performance are evaluated. The temperature distribution across the entire surface is found to remain uniform with nearly zero superheat until the dryout heat flux occurs. A theoretical model is developed to predict the dryout heat flux based on pressure drop through the microporous layer, and the experimental results are in good agreement with the theoretical model.

Observation of a mixed regime for an impinging droplet on a sessile droplet

This article reports experimental results of the presence of a mixed regime where rebound or coalescence occurs when a liquid droplet with low impact velocity impinges on a static sessile droplet, deposited on a flat aluminum surface. In particular, the present study suggests possible reasons why this mixed occurs with nearly the same probabilities with the use of the sequential images captured by two high-speed cameras from the 767 experimental cases. It is also demonstrated that the mixed regime is substantially affected by static droplet volumes, Weber numbers, and offset distances between impinging and static droplets. In the mixed regime, rebound may occur because a relatively large air-film maintains to prevent film drainage for coalescence. The lubrication theory is utilized to estimate film thickness between two droplets and is compared to the critical film thickness. The upper limit of the Weber numbers in the mixed regime decreases with an increase in the offset distance, owing to a rotational droplet motion causing air-film elongation. The contact time during rebound slightly increases with the static sessile droplet volume. It is observed that the measured contact times increases with a decrease in impact velocity, showing cushioning effect by air-film. On the other hand, a coalescence is observed to intermittently in the mixed regime because the air-film layer existing at the interface has an irregular shape by oscillation of an impinging droplet.

Experimental investigations of thermal performance improvement of aluminum ceiling panel for heating and cooling by covering its surface with paint

Due to the low emissivity of raw metal surface (i.e. aluminum), the specific heating or cooling performance of non-painted surfaces is powered mainly on the natural convection and is relatively small. It can be sufficient for low energy buildings but not for the traditional ones. To increase the heat exchange through the radiation one can cover the raw surface with paint to increase its emissivity. To investigate the influence of paint cover on the heating and cooling performance of the ceiling-oriented flat aluminum surface the experimental investigations were done. The experiment was conducted on the aluminum ceiling panel with heat exchange surface of the dimensions 0.32 ⨯ 3.0 m. Heating performance of painted surface increased from 2.3 to 2.8 times and cooling performance increased from 1.5 to 1.7 times in the comparison to the non-painted one.

Frost formation and frost meltwater drainage characteristics on aluminum surfaces with grooved structures

Frost frequently forms on refrigeration and cryogenic engineering systems. Frost on finned surfaces increases the air-side thermal resistance and reduces the airflow area, which consequently reduce the heat transfer rate. Structured fin surfaces may be an effective solution to delay frost formation and promote meltwater drainage. This study experimentally studied condensation, frost crystal growth, frost melting and meltwater drainage characteristics on aluminum surfaces with parallel and crossed grooves with comparisons to these effects on a flat aluminum surface. The results show that condensed droplets freeze later with less frost accumulation on the grooved surfaces than on the flat surface and that the frost prefers to grow at the top edges and corners of the structures instead of inside the grooves on the grooved surfaces. During melting, the frost melts and the meltwater breaks up into small meltwater streams and droplets at the top of the structures or between the structures on the grooved surfaces. The parallel grooved surface has better drainage than the flat surface with a smaller meltwater retention ratio, while the surface with crossed grooves has worse drainage. In addition, the meltwater retention ratio is related to the frost thickness. Lower surface temperatures and longer frosting time results in more frost accumulation and larger meltwater films, which are more easily shed from the surface by gravity so that the meltwater retention ratio is smaller.

Facile Fabrication of Anodic Alumina Rod-Capped Nanopore Films with Condensate Microdrop Self-Propelling Function.

We report that aluminum surfaces can be endowed with condensate microdrop self-propelling (CMDSP) function by one-step voltage-rising mild anodization in hot phosphoric acid solution followed by fluorosilane modification. Via regulating reaction parameters, we can achieve anodic alumina self-standing rod-capped nanopore films and minimize their solid-liquid interface adhesion. Such low-adhesive nanostructured film owns remarkable CMDSP function, especially to condensate microdrops with sizes below 50 μm, differing from usual gravity-driven dropwise condensation on flat aluminum surfaces. Clearly, this work offers a facile, efficient, and industry-compatible approach to processing CMDSP aluminum materials, which is significant for developing innovative energy-saving air-conditioner heat exchangers.

Effect of Fruit Properties on Pomegranate Bruising

Pomegranate fruit quality is adversely affected by bruise damage. Bruises are formed by a variety of static loads and dynamic impacts on the fruit when it strikes any other object during handling, sorting, packaging, or transportation. In order to be able to reduce this damage, it is necessary to ascertain the influence of fruit properties on bruise susceptibility. An experimental study was performed with varying storage time, temperature, and impact region. In these experiments, pomegranates were dropped from three heights onto a flat aluminum surface. Significant effects on bruising in relation to storage time, temperature, impact region, and their interactions were found (p < 0.05). It was concluded that higher fruit temperature, firmness, and peel thickness reduced bruise damage to the pomegranate fruit. Moreover, storage time and increased radius of curvature increased the bruise volume and bruise area, respectively.

Atmospheric pressure plasma treatment of flat aluminum surface

The atmospheric pressure ambient air and oxygen plasma treatment of flat aluminum sheets using the so-called Diffuse Coplanar Surface Barrier Discharge (DCSBD) were investigated. The main objective of this study is to show the possibility of using DCSBD plasma source to activate and clean aluminum surface. Surface free energy measurements, X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM/EDX) and Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) were used for the characterization of the aluminum surface chemistry and changes induced by plasma treatment. Short plasma exposure times (several seconds) led to a significant increase in the surface free energy due to changes of its polar components. Various ageing effects, depending on the storage conditions were observed and discussed. Effects of air and oxygen plasmas on the removal of varying degrees of artificial hydrocarbon contamination of aluminum surfaces were investigated by the means of EDX, ATR-FTIR and XPS methods. A significant decrease in the carbon surface content after the plasma treatment indicates a strong plasma cleaning effect, which together with high energy efficiency of the DCSBD plasma source points to potential benefits of DCSBD application in processing of the flat aluminum surfaces.

The icephobic performance of alkyl-grafted aluminum surfaces.

This work analyzes the anti-icing performance of flat aluminum surfaces coated with widely used alkyl-group based layers of octadecyltrimethoxysilane, fluorinated alkylsilane and stearic acid as they are subjected to repeated icing/deicing cycles. The wetting properties of the samples upon long-term immersion in water are also evaluated. The results demonstrate that smooth aluminum surfaces grafted with alkyl groups are prone to gradual degradation of their hydrophobic and icephobic properties, which is caused by interactions and reactions with both ice and liquid water. This implies that alkyl-group based monolayers on aluminum surfaces are not likely to be durable icephobic coatings unless their durability in contact with ice and/or water is significantly improved.

Understanding improved osteoblast behavior on select nanoporous anodic alumina.

The aim of this study was to prepare different sized porous anodic alumina (PAA) and examine preosteoblast (MC3T3-E1) attachment and proliferation on such nanoporous surfaces. In this study, PAA with tunable pore sizes (25 nm, 50 nm, and 75 nm) were fabricated by a two-step anodizing procedure in oxalic acid. The surface morphology and elemental composition of PAA were characterized by field emission scanning electron microscopy and X-ray photoelectron spectroscopy analysis. The nanopore arrays on all of the PAA samples were highly regular. X-ray photoelectron spectroscopy analysis suggested that the chemistry of PAA and flat aluminum surfaces were similar. However, contact angles were significantly greater on all of the PAA compared to flat aluminum substrates, which consequently altered protein adsorption profiles. The attachment and proliferation of preosteoblasts were determined for up to 7 days in culture using field emission scanning electron microscopy and a Cell Counting Kit-8. Results showed that nanoporous surfaces did not enhance initial preosteoblast attachment, whereas preosteoblast proliferation dramatically increased when the PAA pore size was either 50 nm or 75 nm compared to all other samples (P<0.05). Thus, this study showed that one can alter surface energy of aluminum by modifying surface nano-roughness alone (and not changing chemistry) through an anodization process to improve osteoblast density, and, thus, should be further studied as a bioactive interface for orthopedic applications.

Characterization of Self-Assembled Monolayer on Anodized Aluminum by XPS, AFM and Low-Voltage SEM

Organic layers of tetradecylphosphonic acid (TDP) and octadecyltrietoxysilane (OES) are formed on flat anodized aluminum surfaces by a simple immersion process. The organic layers are characterized by dynamic contact angle measurements for water, AFM, low-voltage SEM and angle-resolved XPS. The results indicate that a self-assembled monolayer is formed using TDP, but not for OES. Plasma surface treatment prior to the coating was found to significantly influence the formation of organic layers.

Modeling of Repeated Rolling Contact on Rough Surface: Surface Topographical Change

An finite element analysis (FEA) of a repeated rolling contact over an elastic-plastic deforming rough surface is performed. The surface topographical change is calculated to determine the running-in phase to the steady-state rolling contact situation. A rigid hemisphere is repeatedly rolled over a rough flat aluminum surface and the effect of the contact load and the number of overrollings is studied. It is found that the change in surface topography due to the repeated rolling contact results in smoothening of the rough surface due to the flattening of the highest asperities. The result shows that the running-in of the repeated rolling contact takes place within the first few overrollings.

Morphological evolution of porous nanostructures grown from a single isolated anodicalumina nanochannel

Porous anodic aluminum oxide (AAO) membranes have been widely used as templates forgrowing nanomaterials because of their ordered nanochannel arrays with high aspect ratioand uniform pore diameter. However, the intrinsic growth behavior of an individualAAO nanochannel has never been carefully studied for the lack of a means tofabricate a single isolated anodic alumina nanochannel (SIAAN). In this study, wedevelop a lithographic method for fabricating a SIAAN, which grows into a poroushemispherical structure with its pores exhibiting fascinating morphological evolutionduring anodization. We also discover that the mechanical stress affects the growthrate and pore morphology of AAO porous structures. This study helps revealthe growth mechanism of arrayed AAO nanochannels grown on a flat aluminumsurface and provides insights to help pave the way to altering the geometry ofnanochannels on AAO templates for the fabrication of advanced nanocomposite materials.

Theory of below-threshold kinetic electron emission

We investigate the electron emission from clean monotonically flat crystalline aluminum surfaces due to the impact of rare gas atoms at the surface under grazing angles of incidence. Recent improvements of experimental methods allowed for the first time the measurement of electron yields as small as γ ≈ 0.01 electrons/projectile. Using these methods kinetic electron emission (KE) was found for projectile velocities well below the classical threshold vth for KE, the minimum velocity a projectile has to have in order to transfer in a binary collision enough momentum to an electron at the Fermi edge (with k = kF) to overcome the surface potential. Quantum mechanical effects smear out the Fermi edge creating electrons with momenta above kF. We present a calculation of γ in the below threshold region within the framework of a classical trajectory Monte Carlo simulation in which special emphasis is put on the description of the projectile trajectory and on an accurate determination of the momentum distribution, i.e. the Compton profile of the surface electronic structure. We employ different methods for calculating the electronic structure and study its influence on. We find that realistic momentum distributions can account for kinetic electron emission in the below threshold region.

Highly sensitive thermal detection of thrombin using aptamer-functionalized phase change nanoparticles

This paper describes a novel thermal biosensing technique for the highly sensitive and selective detection of thrombin using RNA aptamer-functionalized phase change nanoparticles as thermal probes. The presence of thrombin in solution leads to attachment of nanoparticles onto a substrate modified with the same aptamer by forming sandwiched complexes. The phase changes of nanoparticles from solid to liquid adsorb heat energy and generate sharp melting peaks during linear temperature scans, where the positions and areas of the melting peaks reflect the presence and the amount of thrombin, respectively. A detection sensitivity of 22 nM is achieved on flat aluminum surfaces, and the sensitivity can be enhanced by four times using silicon nanopillar substrates that have higher surface area. The thermal detection is immune to colored species in solution and has been used directly to detect thrombin in serum samples. By combining the high specificity of aptamers and the large surface area of silicon nanostructures, the thermal signals obtained during phase change of nanoparticles provide a highly sensitive, selective and low-cost method for thrombin detection.

Qualification of Bonding Process of Temperature Sensors to Extreme Temperature Deep Space Missions**

A process has been explored based on state of the art technology to bond a platinum resistance thermometer (PRT) on potential aerospace materials such as a flat aluminum surface and a flexible copper tube to simulate coaxial cable for flight applications. Primarily, PRTs were inserted into a metal plated copper braid to avoid stresses on the sensor while attaching the sensor with braid to the base material for long duration deep space missions. Appropriate pretreatment has been implemented in this study to enhance the adhesion of the PRTs to the base material. The NuSil product was chosen in this research to attach the PRT to the base materials. The resistance (∼1.1 kΩ) of PRTs was continuously electrically monitored during the qualification thermal cycling testing from −150°C to +120°C and −100°C to −35°C. The test hardware was thermal cycled three times the mission life per JPL design principles for the JUNO project. No PRT failures were observed during and after the PRT thermal cycling qualification test for extreme temperature environments. However, there were some failures associated with staking of the PRT pigtails as a result of the thermal cycling qualification test.

박막 알루미늄을 이용한 규칙적으로 정렬된 나노급 미세기공 어레이 제조기술 개발

An alumina membrane with nano-sized pore array by anodic oxidation using the thin film aluminum deposited on silicon wafer was fabricated. It Is important that the sample prepared by metal deposition method has a flat aluminum surface and a good adhesion between the silicon wafer and the thin film aluminum. The oxidation time was controlled by observation of current variation. While the oxalic acid with 0.2 M was used for low voltage anodization under 100 V, the chromic acid with 0.1 M was used for high voltage anodization over 100 V. The nano-sized pores with diameter of <TEX>$60\~120$</TEX> nm was obtained by low voltage anodization of <TEX>$40\~80$</TEX> V and those of <TEX>$200\~300$</TEX> nm was obtained by high voltage anodization of <TEX>$140\~200$</TEX> V. The pore widening process was employed for obtaining the one-channel with flat surface because the pores of the alumina membrane prepared by the fixed voltage method shows the structure of two-channel with rough surface. Finally, the sample was immersed to the phosphoric acid with 0.1 M concentration to etching the barrier layer.

Capacitance, induced charges, and bound states of biased carbon nanotube systems

Although it has long been known that the classical notions of capacitance need modification at the nanoscale, in order to account for important quantum effects, very few first-principles investigations of these properties exist for any real material systems. Here we present the results of a large-scale ab initio investigation of the capacitance properties of carbon nanotube systems. The simulations are based on a recently developed real-space nonequilibrium Green's-function approach, with special attention being paid to the treatment of the bound states present in the system. In addition, use has been made of a symmetry decomposition scheme for the charge density. This is needed both to speed up the calculations and in order to study the origins of the induced charges. Specific systems investigated include two and three nested nanotube shells, the insertion of a capped nanotube into another, a connected (12,0)/(6,6) nanotube junction, and the properties of a nanotube acting as a probe over a flat aluminum surface. First-principles estimates of the capacitance matrix coefficients for all these systems are provided, along with a discussion of the quantum corrections. For the case of the nanotube junction, the numerical value of the capacitance is sufficiently high, as to be useful for future device applications.

Quantum Calculations of Carbon Nanotube Charging and Capacitance

ABSTRACTAlthough it has long been known that the classical notions of capacitance are altered at the nanoscale, few first principles calculations of these properties exist for real material systems. With a recently developed ab initio formalism, which combines nonequilibrium Greens function techniques with real-space density functional calculations, we have investigated charging effects for carbon nanotube systems, which are described by the capacitance coefficients. Specifically, the capacitance matrix of two nested nanotube armchair nanotubes, the insertion of one nanotube into another, and the properties of a nanotube acting as a probe over a flat aluminum surface were considered.

Thermal Contact Conductance for Aluminum and Stainless-Steel Contacts

In the present study, existing analytical models developed to predict the thermal contact conductance for both aluminum/aluminum and aluminum/stainless steel surfaces in contact have been evaluated. The prediction of thermal contact conductance for these contacting pairs is difficult since a thin native oxide film generally occurs naturally on the aluminum materials. Therefore, one of the objectives of this study was to determine whether elastic or plastic thermal contact models could successfully predict both published and present experimental data for these commercially important alloys. A rigorous analytical analysis was conducted to check the validity of existing models (assuming either elastic or plastic deformation a priori) to predict the thermal contact conductance for nominally flat uncoated aluminum surfaces in contact under vacuum conditions. Both elastic and plastic deformation models were compared with new unpublished experimental data for nominally flat, roughened surfaces. NOMENCLATURE A Fraction of area in real contact Aa Apparent area a Contact spot radius (m) BJH Microscopic gap number b Flux tube radius (m) Cjn Microscopic constriction number E Young's Modulus (N/m2)