Number Of Active Nucleation Sites Research Articles

Transparent glaze containing high-alumina glass frit: Batch-to-melt conversion

The use of high alumina (HA) glass frits in the production of transparent glazes is of potential technological interest due to their excellent mechanical properties. However, their strong tendency to crystallize limits their practical application. A transparent glaze was obtained by adding 30% by weight of the HA frit to a low-melting glaze fired at 1453 K for 30 min with a hardness between 5.92 GPa and 6.46 GPa. The crystallization and melting behavior of an HA frit mixed with a low-melting glaze model batch is investigated by studying the interaction between a compacted raw low-melting glaze placed on the polished surface of bulk HA glass. With increasing temperature, the low-melting glaze batch melts while CaAl2Si2O8 and MgAl2O4 crystallize in the HA frit. The melt surrounding the HA frit decreases the number of active nucleation sites weakening surface crystallization. In addition, the alkali cations in the melt tend to diffuse into the crystallized HA frit, reducing the viscosity of the layer surrounding the crystals, which accelerates their dissolution. The results provide information on the batch-to-melt conversion mechanism of high alumina content transparent glazes.

Nucleation and Growth of Cu2O: Role of Potential, Electrolyte pH, and Substrate

The nucleation and growth mechanism of functional oxides has a direct bearing on the structural and electronic properties of the deposit. We study the effect of electrolyte pH and deposition potential on the nucleation and growth of Cu2O on polycrystalline metal oxide (FTO) & metal (Au) substrates. Modelling of the recorded current-time transients indicates that both instantaneous and progressive nucleation occur with growth limited by diffusion or lattice incorporation of electro-active species or both. The preferred orientation of Cu2O shows a strong dependence on electrolyte pH. The films are (100) oriented on both substrates at pH 9 except at high applied potential on FTO where the orientation changes to (111). Interestingly, irrelevant of electrolyte pH, the grain size of Cu2O decreases with potential on FTO whereas it increases on Au substrates. We attribute this to a difference in the number of active nucleation sites between the two substrates. The nucleation and growth at pH 12 is observed to be dependent both on diffusion and lattice incorporation of electro-active species. Additionally, the films are primarily (111) oriented on both substrates, which is correlated to the availability of OH− ions.

Pool-Boiling Performance on Thin Metal Foils with Graphene-Oxide-Nanoflake Deposit.

The pool-boiling performance of water on thin metal foils with graphene-oxide deposition was studied. The boiling performance was evaluated both on fully coated surfaces, achieved by spin-coating, and surfaces with a laser-textured nucleation site, into which graphene oxide was added via drop-casting. During the experiments, a high-speed IR camera was used to obtain the transient temperature and heat-flux distribution. At the same time, a high-speed video camera was used to acquire synchronized bubble-growth recordings. In addition, a surface-wettability analysis was conducted for all the samples. In the case of fully coated samples, graphene-oxide deposition resulted in an increased number of active nucleation sites and an increase in the nucleation temperature, leading to a lowered nucleation frequency. Meanwhile, samples with a single laser-textured nucleation site enabled the analysis of isolated vapor bubbles, confirming that graphene-oxide deposition leads to a higher nucleation temperature, consequently resulting in a larger bubble-departure diameter and longer growth time. Two explanations for the results are proposed: the wettability of graphene-oxide deposition and the filling of surface microcavities with graphene-oxide nanoflakes.

Flow boiling of R134a and HFE-7000 in a single silicon microchannel with microstructured sidewalls

Flow boiling heat transfer is investigated in a single silicon microchannel with microstructured sidewalls using R134a (at 0.19 reduced pressure) and HFE-7000 (at 0.035 reduced pressure) as working fluids. The channel is 51 mm long with a 400 μm x 400 μm square cross section. Sidewall microstructures in the form of triangular cavities for the entire height of the channel were introduced during the fabrication process. By including the cavity pattern in the photolithography mask design, high accuracy and reproducibility is ensured during the fabrication of the test sample. A smooth channel with the same geometry but without sidewall microstructures was also fabricated. Flow boiling experimental results are presented for both channel configurations. For R134a at mass velocity G = 800 kg m−2 s−1, the comparison of the boiling curves shows that: in the channel with the microstructures, the wall superheat at the onset of nucleate boiling (ONB) is reduced from 21 K to 6 K as compared to the smooth channel. An increase of the flow boiling heat transfer coefficient up to 40% is found in the microstructured channel, which must be due to the increased number of active nucleation sites. Heat transfer results, flow visualizations and comparison between R134a and HFE-7000 data provide a clearly evidence of the active role of bubble nucleation during flow boiling in microchannels. These results are of fundamental importance in the design of next generation thermal management systems for electronics.

Immobilization of Pt nanoparticles on hydrolyzed polyacrylonitrile-based nanofiber paper

The electrochemical activity of catalysts strongly depends on the uniform distribution of monodisperse Pt nanoparticles without aggregates. Here, we propose a new hydrolysis-assisted smearing method for Pt loading on a free-standing paper-type electrode. Polyacrylonitrile (PAN)-based nanofiber paper was used as the electrode, and it acted as a Pt support. Hydrolysis of the electrode tripled the number of active nucleation sites for Pt adsorption on the PAN nanofibers, thereby significantly enhancing the wettability of the nanofibers. This facilitated the uniform distribution of Pt nanoparticles without aggregate formation up to 40 wt% (about 0.8 mg/cm2) with a particle size of about 3 nm. The catalytic current of the hydrolyzed Pt electrode in CH3OH/H2SO4 solution exceeded 213 mA/cm2 Pt mg, which was considerably greater than the current was 148 mA/cm2 Pt mg for an unhydrolyzed electrode.

Gradient wick channels for enhanced flow boiling HTC and delayed CHF

Liquid supply to wick structures of flow boiling surfaces is fundamentally restricted by the capillary limit at which the pressure drop of the wicked liquid surpasses texture-amplified capillary forces. Gradient wick structures partially decouple permeability and capillary pressure, thereby delaying the capillary limit. In this study, gradient wick channels facilitating out-of-plane liquid delivery are introduced to postpone the capillary limit and thus enhance the two-phase flow boiling heat transfer coefficient (HTC) and delay critical heat flux (CHF). Here, the permeability of the gradient wick channels is augmented by large-pore-size meshes employed near the bulk fluid while capillary pressure is maximized by small-pore-size meshes utilized near the hot boiling surface. This combination of wick structures enables to preferentially guide the cooling liquid, deionized water, from the far-field cold liquid toward the bottom hot substrate. The spatial distribution of individual gradient wick channels promotes separate liquid-vapor pathways, thus facilitating the vapor escape process. Experiments conducted here reveal that the flow boiling performance metrics of the proposed heat sink leveraging the gradient wick channels outperform those of the homogenous wick channels and solid fin channels. The proposed heat sink demonstrates a strong liquid mass flux dependency due to a combination of convective boiling and amplified wickability effects. The enhanced convective boiling could be related to surface roughness, a high number of active nucleation sites, and a large surface area available through tortuous passages of wick channels. At higher mass flow rates, effective capillary pressure available for out-of-plane wicking action also increases, thereby further boosting the wickability effect and associated heat transfer processes. This could meaningfully delay the CHF. In fact, the CHF limit was not observed on the gradient wick channel surfaces in the mass flux and wall superheat range studied. The experimental results indicated a maximum heat flux of 870 W/cm2 with a gradient wick channel heat sink, a 60% improvement compared with a plain copper surface. Furthermore, a maximum HTC of 1000 kW/m2-K at a wall superheat of 3 °C was observed, a three-fold enhancement compared with that of the plain surface. The proposed gradient wick channel topology offers new pathways for designing innovative surface technologies with high heat removal capabilities, thereby potentially improving the energy economy in myriad modern energy applications.

Electrodeposition of Co and Pd Onto Carbon Fiber and Platinum Ultramicroelectrodes

The synthesis of Co and Pd structures with tuned magnetic properties have scarcely been investigated from the electrochemical point of view [1-7]. Probably, this is due that, these properties are highly dependent on their shape, size and composition. In this sense, the use of ultramicroelectrodes (UME) allow to form a limited number of nuclei onto its surface, which is useful to analyse the electrochemical variables that modify the shape, size and specific composition of these particles [8-11]. In order to analyze the influence of the electrochemical variables during the synthesis of Co and Pd materials employing ultramicroelectrodes; in this work we conducted voltamperometric and chronoamperometric studies from two aqueous solutions. The first one containing PdCl2 0.01 M + NH4Cl 0.1 M and the second one CoCl2 0.01 M + NH4Cl 0.1 M. The working electrodes were a platinum UME (Pt-UME) of 10 μm of diameter and a carbon fiber UME (CUME) of 11 μm of diameter. A graphite bar was used as a counterelectrode and all potentials are reported against an Ag/AgCl electrode. The electrochemical experiments were controlled by mean a bipotentiostat UNISCAN M370. Fig. 1a shows a set of transients obtained in the potential range [-0.8 – -0.95] V applying a potential pulse during 32 s from the system Pt-UME/CoCl2 0.01 M + NH4Cl 0.1 M, while in Fig. 1b are depicted the transients obtained from the system Pt-UME /PdCl2 0.01 M + NH4Cl 0.1 M in the potential range [-0.4 – -0.7] V. Figure 1c and 1d, shows the transients obtained onto CUME for the case of Co and Pd, respectively. All the transients reported in Fig. 1 were compared with the theoretical ones predicted by the mathematical model proposed by Correia et al [12], by means a nonlinear fitting employing the Levenberg-Marquardt algorithm. In all cases, the kinetic parameters such as the nucleation rate (A) and the number of active nucleation sites (N0) were potential dependent. Also, the N0 values in each potential allowed us to simulate the AFM image associated to each potential considering a random distribution on the UME surface. [1] E.O. Berlanga-Ramírez et al., Phys. Rev. B, 70 (2004) 014410. [2] T. Sondon and J. Guevara, J. Mag. Mag. Mat. 272-276 (2004) e1247. [3] S. R. Brankovic, X. Yang, T. J. Klemmer, M. Seigler, IEEE T. Magn., 42(2) (2006) 132. [4] S. Ouazi, S. Vlaic S. Rusponi, G. Moulas, P. Buluschek, K. Halleux, S. Bornemann, S. Mankovsky, J. Minar, J.B. Staunton, H. Ebert, H. Brune. Nature Communications, 3 (2012) 1313 [5] M. Rezaei, S. Hadi, D. Fatmehsari, Journal of Materials Chemistry A, 1 (2014) [6] C. Xu, Y. Liu, J. Wang, H. Geng, H. Qiu, ACS Appl. Mater. Interfaces, 12 (2011) 4626. [7] F. Nasirpouri, S. M. Peighambari, A. S. Samardak, A. V. Ognev, E. V. Sukovatitsina, E. B. Modin, L. A. Chebotkevich, S. V. Komogortsev, S. J. Bending. Journal of Applied Physics 117, (2015) 17E715 [8] M. Peña, R. Celdran, R. Duo, Journal of Electroanalytical Chemistry, 367, (1994) 85. [9] C. Barin, A. Correia, L. Avaca, S. Machado, Journal of the Brazilian Chemical Society, 11 (2000) 175. [10] A. Kandory, H. Cattey, L. Saviot, T. Gharbi, J. Vigneron, M. Fregnaux, A. Etcheberry, G. Herlem, The Journal of Physical Chemistry C, 121 (2017) 1129. [11] A. Correia, S. Machado y L. Avaca, Journal of the Brazilian Chemical Society, 5 (1994) 173. [12] A. N. Correia, S. A. Machado, J. C. Sampaio, L. A. Avaca, Journal of Electroanalytical Chemistry, 407, (1996), 37-43. [13] C. T. Rueden, J. Schindelin, M. C. Hiner, BMC Bioinformatics, 18: 529 (2017) Figure 1

Study of pool boiling heat transfer with FC-72 on open microchannel surfaces

The paper presents investigations into pool boiling heat transfer for open microchannel surfaces. The experiments were carried out with saturated FC-72 at atmospheric pressure. Parallel microchannels fabricated by machining were about 0.2 to 0.4 mm wide and 0.2 to 0.5 mm deep. Analyzed surfaces with microchannels allowed to obtain heat transfer coefficients within the range of 6.1 – 9.8 kW/m2K, which in relation to the flat surface gives a 3 – 5 - fold increase in HTC. One of the reasons for the increase in the heat transfer coefficient when increasing the heat flux was the growing number of active nucleation sites at the bottom of microchannels and its side surfaces.

Enhancement of loop heat pipe performance with the application of micro/nano hybrid structures

To further improve the flat-type loop heat pipe (LHP) performance, this study evaluates the practical potential of use of highly enhanced boiling structures. It is found that in our proposed new heat pipe (NHP) system, the working fluid from the evaporator outlet to the condenser inlet is in a liquid–vapor two phase flow, which is different from the classical LHP theory. A new P-T diagram is developed to better understand the thermal and hydraulic process during the NHP steady operation. In this study, by using the laser ablation technique two different types of micro- and nanoscale hybrid structures are synthesized on the boiling pool substrate. It is indicated that the formed valleys with a larger opening width play an important role in more effectively improving the bubble nucleation and bubble growth at the micrometer sites, which can subsequently lead to an increased number of active nucleation sites. The best loop performance is obtained with the micro-cone structured substrate at a heat load of 140 W, at which the maximum boiling pool heat transfer coefficient of 42.17 kW/m2·K is achieved. Compared with the polishing Cu substrate, it is enhanced by 110%. When maintaining the boiling pool temperature lower than 85 °C, the proposed new heat pipe system can tolerate a maximum heat flux of 35.12 W/cm2, which is larger than that of the most conventional LHPs with methanol as the working fluid.

Pool boiling heat transfer enhancement with segmented finned microchannels structured surface

An experimental investigation has been carried out to investigate pool boiling heat transfer characteristics of segmented finned (SF) microchannels structured surface and compare its performance with that of uniform cross-section (UCS) microchannels structured surface and plane surface. All three surfaces have been fabricated on individual copper block with a foot print area of 10 × 10 mm2. Pool boiling experiments have been performed with these surfaces in atmospheric pressure condition using deionized water. Experiments have been performed for applied effective heat flux range of 0–200 W/cm2. Both the structured surfaces show better heat transfer performance compared to plane surface. It has been observed that SF structured surface shows a heat transfer improvement up to a factor of 3 times the heat transfer coefficient in plane surface whereas uniform UCS structured surface shows the improvement by a factor of 2 times the heat transfer coefficient in plane surface. Thus segmented finned microchannels structured configuration shows better heat transfer performance compared to other two surfaces. The reason behind the heat transfer improvement in SF configuration might be due to more number of active nucleation sites, better rewetting phenomenon and favorable bubble growth and release mechanism.

Graphene oxide as stable electrocatalytic substrate for solid-state bifacial dye-sensitized solar cells

Bifacial dye sensitized solar cells (BDSSCs) are regarded as advanced photovoltaics for markedly increased light-harvesting efficiency and the ability of both-side incident illumination absorption. We present here the fabrication of graphene oxide electrocatalyst supported polyaniline (GO/PANI) composite film via an in-situ electrochemical polymerization method and used it as transparent counter electrode (CE) for solid-state BDSSCs. The GO provides a large number of active nucleation sites, resulting in the formation of continuous PANI films through strong covalent bonds, which significantly increase the specific surface area and catalytic activity of the CE and further accelerate the charge transfer in the corresponding BDSSC. Due to the improved electrochemical catalytic activity of the GO/PANI CE with GO mass ratio of 0.6 wt%, both-illuminated power conversion efficiency of 5.74% with short-circuit current density of 20.23 mA cm−2 and open circuit voltage of 0.63 V is obtained for the solid-state BDSSC, which is 155.6% of the conventional bare PANI based BDSSC under front-irradiation. Electrochemical and electron transport/recombination performances have also been demonstrated for this type of device.

Evolution of the Macrostructure of Gray Cast Iron from Eutectic to Hypereutectic Composition

This work studies the evolution of the macrostructure of gray cast iron as the carbon equivalent (CE) increases from near-eutectic to highly hypereutectic values. The macrostructure of spherical samples of flake graphite irons of CE ranging between 4.24% and 5.23% was revealed by using direct austempering after solidification technique (DAAS). The results were analysed by applying the concept of coupled zone for gray cast irons. The coupled zone is a region in the stable Fe-C-Si diagram in which solidification proceeds as a cooperative growth of both austenite and flake graphite. The coupled zone for gray cast iron is claimed to be asymmetrical as the eutectic reaction is extended towards the prolongation of the austenite liquidus line. All the samples investigated in the present study show the presence of relatively large austenite grains, demonstrating that the number of active nucleation sites of austenite per unit volume is small. The observation of the microstructure shows the presence of several units of coupled growth inside each austenite grain, what proves that many eutectic colonies have grown from the same austenite nucleus, even for highly hypereutectic compositions. Colour etching was employed to reveal the “hidden” dendrites present in the microstructure. The results suggest that proeutectic graphite is not a good nucleating agent for austenite. These results contribute to the understanding of the solidification of hypereutectic gray irons and verify the earlier understanding of the solidification of eutectic gray irons and its extension to hypereutectic compositions.

Characteristics of subcooled water boiling on structured surfaces

Characteristics of the process and heat transfer of subcooled water boiling on mesostructured surfaces obtained by microarc oxidation of titanium foil with formation of a TiO2 layer and deposition of Al2O3 particles from boiling nanofluid have been experimentally investigated. The experiments have been carried out in the forced flow of deaerated water in a vertical rectangular channel, 21 × 5 mm in size. The ranges of regime parameters are as follows: water mass velocity is up to 650 kg/(m2 s), subcooling is 30–75°C, pressure is ~105 Pa, and heat flux rate is 0.7–5.0 MW/m2. It is established that the number of active nucleation sites is (70–80) × 105 1/(m2 s) at the heat flux of 1.5–2.0 MW/m2. Significant subcooling of the liquid and good wettability of the structured surface provide intense deactivation and lead to random spatial distribution of the nucleation sites. The characteristic size of vapor bubbles is about 200–250 μm and the bubble lifetime is 200–500 μs. Application of the coating prepared by microarc oxidation enhances heat transfer by 20–30%. At high subcoolings of liquid, the characteristics of boiling on smooth surfaces and surfaces with the coating were fairly close.

ELECTRODEPOSITION OF COBALT NANOCLUSTERS FROM AMMONICAL CHLORIDE SOLUTIONS ONTO HOPG ELECTRODES. A KINETICAL AND MORPHOLOGICAL STUDY

A kinetical study about the Co electrodeposition onto Highly Oriented Pyrolytic Graphite electrodes from an aqueous solution containing 0.01 M CoCl2 + 1 M NH4CI (pH = 7) was conducted at overpotential conditions through potentiostatic studies. The entire chronoamperograms were adequately predicted, consider­ing the contribution to the total current of three different processes: a Langmuir-type adsorption process, a three-dimensional nucleation and growth and a proton reduction process. In all cases, the nucleation rate and the number of active nucleation sites are potential dependent. The Atomic Force Microscopy study revealed the presence of homogeneous cobalt clusters of less of 100 nm in height and 50 nm in diameter at different potential values.

Numerical Simulation of Evaporating Two-Phase Flow in a High-Aspect-Ratio Microchannel with Bends

Flow boiling was investigated on a hydrophobic surface by coating Teflon® onto a 1×1 cm2 copper surface, resulting in contact angle of 118°. The images depicted were taken using distilled water flowing at 299 kg/m2s with 3°C subcooling. In the first series, the number of active nucleation sites increased as heat flux increased. For lower values of heat flux (< 80 kW/m2), vapor bubbles remained almost stationary on the surface. The hydrophobic contact angle makes the horizontal component of surface tension force act radially outward, causing the bubble interface to grow. This leads to increased triple contact line and increased vertical component surface force. The buoyancy force due to the vapor bubble volume appears to be insufficient to overcome this vertical force for liftoff. This explains the stationary bubbles observed at the lower heat fluxes. The bubbles show an increase in size and number with heat flux. After this increasing trend, the bubble continues to grow larger when heat flux is higher than 80 kW/m2, eventually leading to the dryout at 117.5 kW/m2. The later bubble growth at high heat fluxes is caused primarily by the coalescences of neighboring bubbles. These larger bubbles are more affected by flow induced drag forces and move downstream. This can be seen in the lower sequential series at 100 kW/m2. The larger vapor masses slide across the surface, continue to absorb smaller bubbles as they move downstream, and are swept off the surface.

Nanosecond laser texturing of uniformly and non-uniformly wettable micro structured metal surfaces for enhanced boiling heat transfer

Microstructured uniformly and non-uniformly wettable surfaces were created on 25-μm-thin stainless steel foils by laser texturing using a marking nanosecond Nd:YAG laser (λ=1064nm) and utilizing various laser fluences and scan line separations. High-speed photography and high-speed IR thermography were used to investigate nucleate boiling heat transfer on the microstructured surfaces. The most pronounced results were obtained on a surface with non-uniform microstructure and non-uniform wettability. The obtained results show up to a 110% higher heat transfer coefficients and 20–40 times higher nucleation site densities compared to the untextured surface. We show that the number of active nucleation sites is significantly increased in the vicinity of microcavities that appeared in areas with the smallest (10μm) scan line separation. Furthermore, this confirms the predictions of nucleation criteria and proves that straightforward, cost-effective nanosecond laser texturing allows the production of cavities with diameters of up to a few micrometers and surfaces with non-uniform wettability. Additionally, this opens up important possibilities for a more deterministic control over the complex boiling process.

The study of boiling-up onset of highly superheated n-pentane in a glass capillary at different pressures. I. Visualization by high-speed video and nucleation sites

The lifetimes of strongly superheated n-pentane in a glass capillary were measured with simultaneous high-speed video of boiling-up patterns. The experiments were conducted in the temperature range of 84.7–162.8°C at a pressure on fluid of 0.10, 0.29, 0.78, and 1.28MPa. Video data make it possible to assert that viable bubbles arise on the inner wall of a capillary. The number of active nucleation sites is limited. Redistribution of activity between boiling-up centers takes place as the pressure increases.

Experimental investigation on the pool boiling heat transfer to aqueous multi-walled carbon nanotube nanofluids on the micro-finned surfaces

The pool boiling heat transfer characteristics of aqueous multi-walled carbon nanotube nano-fluids on modified surfaces are experimentally investigated up to the critical heat flux point. The surface has been modified with diamond-shaped micro-finned with different geometrical properties. Results demonstrated that the pool boiling heat transfer coefficient on the plain surface was deteriorated, while for micro-finned surfaces, it was enhanced up to 56% and 77% for wt.% = 0.1 and wt.% = 0.3 respectively. Likewise, results showed that modified surfaces decreased the rate of fouling in comparison with plain surface. It was also revealed that these surfaces can also form an irregular, non-uniform fouling layer, which improved the number of active nucleation sites and bubble formation. However, for a plain surface, a continual fouling layer was seen on the surface which created a huge thermal resistance and a significant decrease in heat transfer coefficient. In addition, deposition of carbon nanotubes on the surface caused some changes in surface characteristics such that the static contact angle value was found to be decreased and surface wettability to be increased as well. In this case, critical heat flux, CHF was enhanced up to 95% (for wt.% = 0.3). Besides, with increasing the mass concentration of nanofluids, static contact angle was found to be decreased too. In terms of bubble formation, experiments showed that the rate of bubble formation can be intensified, however the generated bubbles can discreetly depart the surface, which led the heat transfer coefficient to be enhanced due to the local agitation and available heat transfer area.

Pool boiling heat transfer to aqueous alumina nano-fluids on the plain and concentric circular micro-structured (CCM) surfaces

The present work mainly focuses on the pool boiling heat transfer of aqueous alumina nano-fluids up to critical heat flux point on a circular surface at two mass concentrations of 0.1%, 0.3% and nanoparticle size of 20nm and 50nm. Nano-fluids were prepared using two-step method and were stabilized by nonylphenolethoxylate (NPE), non-ionic surfactant. Surface of the heater was modified by CNC micro-machinery technique and concentric circular microstructures (CCM) were created on the surface with different geometrical specifications. Results demonstrated that the heat transfer coefficient (HTC), could be deteriorated and enhanced for the plain and micro-structured surface respectively. However, rate of enhancement depended in part, to the particle size and geometrical specifications of microstructures. Rate of enhancement was found to be intensified with increasing the heat flux and mass concentration, decreasing the nanoparticles size and the interspace between microstructures, however height of microstructures had no particular influence on HTC. In terms of critical heat flux point (CHF), nano-fluids expectedly enhanced the CHF value. The enhancement rate of CHF was intensified with increasing the mass concentration of nano-fluid, nano-particle size and interspace between microstructures. The main reason for enhancing the heat transfer coefficient over the micro-structured surface was due to the larger heat transfer area in comparison with plain surface. While for the plain surface, deterioration of heat transfer coefficient was due to the continual fouling formation of nanoparticles on the surface, which increased the static contact angle of nano-fluid on deposited layer and subsequently reduced the number of active nucleation sites. Capillary wicking action and constitution of inflow liquid inside the fouling layer were the main reasons for CHF enhancement since can diminish the surface dry-out regions and enhance the surface toleration against the CHF crisis. Accordingly, a modified mechanism was proposed for enhancing the CHF in nano-fluids, which was the combination of liquid absorption from the bulk of nano-fluids, by the deposition layer and inflow capillary wicking action inside the deposition layer, which leads to the rewetting phenomena in dry-out regions.

A Kinetic and AFM Study of the Electrodeposition of Palladium Nanoclusters onto Highly Oriented Pyrolytic Graphite (HOPG)

In the present work we have carried out a kinetic and morphological study of the palladium electrodeposition onto highly oriented pyrolitic graphite electrode from an aqueous solution [0.001 M PdCl2 + 1 M NH4Cl (pH 7)]. From the potentiostatic study were calculated the diffusion coefficient (3.2 × 10−6 cm2 s−1), the number of active nucleation sites (N 0), the nucleation rate (A) and the rate constant of the proton reduction process (k PR). A, N 0 and k PR values were potential dependent and they increased with an augment in the applied overpotential. At higher overpotential, it is possible to induce the formation of small Pd clusters with 5 nm in height and an average size of 20–30 nm in diameter.