Nucleation Site Density Research Articles

Heat flux partitioning and macrolayer observation in pool boiling of water on a surface with artificial nucleation sites

In this study, the heat transfer mechanisms in pool saturated boiling of water on a sapphire heat transfer surface with a controlled nucleation site density (NSD) were observed using a high-speed infrared camera. Artificial nucleation sites were arranged on the wall surface by ink-jet printing of a superhydrophobic agent, and NSD was varied between 24 and 318 sites/cm2. The heat transfer characteristics of each fundamental heat transfer process and its contribution to the total wall heat transfer were evaluated through the heat flux partitioning analysis. There was an upper limit to the heat transfer enhancement by increasing the NSD. The boiling heat transfer coefficient (HTC) did not increase monotonically with increasing NSD but had a maximum value at an NSD between 89 and 130 sites/cm2. When NSD was increased excessively, the microlayer formation was inhibited by bubble–bubble interaction. The contribution of the microlayer evaporation to the wall heat transfer also decreased, resulting in a decrease in the boiling HTC. At all tested NSDs, liquid-phase heat transfer dominated the wall heat transfer, as in the case of the bare surface. The contribution of the microlayer evaporation that occupies a small area to the total wall heat transfer was less than 35%. In the high heat flux region, the macrolayer, a liquid layer with suppressed liquid motion, was successfully observed at the bottom of the coalesced bubble.

Highly ductile hypereutectic Al-Si alloys fabricated by selective laser melting

In the endeavor to maximize the refinement effect of primary Si and alleviate the inherent brittleness of hypereutectic Al-Si alloy, the approach of coating P as a modifier on powder was adopted. The ultimate aim was to create more heterogeneous fine AlP nucleus and enhance the nucleation efficiency of primary Si on AlP to refine the coarse primary Si to nano-scale during 3D printing. In the combination of large undercooling and high density of nucleation sites, the size of primary Si was successfully refined to 200–300 nm and the divorced eutectic was also induced to modify the microstructure of matrix. In the presence of nano-scale primary Si, the melting pool boundary (MPB) feature disappeared and the fracture mechanism also changed from load transfer to interfacial fracture. Compared with the pristine alloy, the ductility was increased four times without significantly changing the ultimate tensile strength (UTS) and wear resistance. The improvement of ductility is attributed to the refinement of primary Si, the disappearance of MPB features and the formation of divorced eutectic. The optimal tensile properties were: UTS-482 MPa, yield strength-320 MPa and ductility of 8.1% at 0.05 wt.% P. These are comparable to those for high-strength Al alloys.

Simultaneous measurement of heat flux and droplet population during dropwise condensation from humid air flowing on a vertical surface

A new experimental apparatus for the investigation of dropwise condensation from humid air flowing over a vertical aluminum surface at controlled velocity is presented. Differently from other works on this subject, here we measure simultaneously, during condensation of flowing moist air, the total heat flux (by a heat flux sensor), the latent heat flux (by weighing the mass of condensate), the small and the large droplet population. Experimental tests were performed on two aluminum specimens that display different wettability: a mirror-polished surface (56° advancing contact angle, 46° contact angle hysteresis) considered as a baseline, and a coated surface functionalized by using a sol–gel coating (87° advancing contact angle, 15° contact angle hysteresis). The effects of the wall subcooling, moisture content, relative humidity and air velocity on the heat and mass transfer during dropwise condensation (DWC) are investigated. Enhanced condensation is observed on the coated surface but, with the increase of relative humidity and wall subcooling, the advantage of using the coated surface diminishes. Time-lapse videos of the condensation process, featuring droplets detection down to three microns, are used to investigate droplet population (both large and small droplets) and nucleation sites density. Usually, DWC models assume tentative values of the nucleation site density and tentative trends of the droplet population. In this paper we measure and discuss such parameters. The determination of the nucleation site density is crucial because it affects the droplet interactions and the drop size distribution, determining the overall heat transfer. The measurement of the nucleation site density is currently rare in the literature, especially during condensation of flowing humid air. Here the nucleation site density is determined with a double approach and it is found to vary between 3.3 × 108 sites/m2 and 6.1 × 108 sites/m2 in the investigated range of experimental conditions. The experimental droplet population is also compared against the predicted one, finding some disagreement which should be properly addressed for the development of improved DWC models.

Atomic Layer Deposition of CoxOy Films: Oxidants versus Composition

AbstractCobalt oxides (CoxOy) have shown great potential for applications in catalysts, sensors, and energy storage fields, and their performance remarkably depends on their oxidation states. Herein, CoxOy films with precisely controlled composition are first introduced by atomic layer deposition (ALD) using bis(N,N0‐di‐iso‐propylacetamidinato)cobalt(II) (Co(iPr2‐Me‐AMD)2) as Co precursor and H2O/O3 as oxidants. The results show that the ALD processes using both oxidants exhibit typical self‐limiting characteristic, where cubic‐CoO films, with growth rate of 0.045 nm per cycle, are obtained at 150–200 °C using H2O oxidant, while cubic‐Co3O4 films, with growth rate of 0.05 nm per cycle, can be deposited at 200–225 °C using O3 oxidant. Both CoO and Co3O4 films show dense, smooth microstructure, and good crystallinity with typical columnar crystal feature, where the Co3O4 film shows smaller columnar size because the O3 promotes rapid nucleation with relatively higher density of nucleation sites. The thermodynamic growth mechanism of ALD process is established via density functional theory calculations, which demonstrates that the exothermic reaction path of O3 is more energetically than that of H2O. Both films possess relatively low resistivity of ≈10−1 Ω cm−1 with p‐type semiconductor behavior, which shows promising application potentials of these films.

Experiments to understand bubble base evaporation mechanisms and heat transfer on nano-coated surfaces of varying wettability under nucleate pool boiling regime

Absolute surface wettability effects on the characteristics of nucleate pool boiling heat transfer under saturated conditions are investigated with water as working fluid. Wettability of the base surface (ITO-coated sapphire) is modified by coating 60 nm thick SiO2 film using different thin film deposition techniques. In this way, two surfaces with modified wettability (30 o and 65 o static contact angles) have the same surface chemistry with negligible variation in surface roughness, while the base surface itself acts as the third surface with 90 o static contact angle. Experiments are conducted for various constant heat flux conditions. Bubble dynamics and temperature distributions of the substrate surface are mapped in-situ using high speed videography and IR thermography, respectively. Pool boiling curves and overall heat transfer coefficient curves, obtained from the recorded IR images, showed that the overall boiling performance increases with enhanced surface wettability. The observed enhancement has been explained on the basis of the modified bubble dynamics and the associated bubble base evaporation mechanism. The bubble dynamics parameters such as bubble departure diameter and departure frequency, and nucleation site density (NSD) were found to get altered so as to augment the total evaporative heat flux with increasing surface wettability. The heat transfer coefficients and heat transfer rates corresponding to bubble base evaporation for isolated vapor bubble revealed that higher wettability surfaces outperform the surfaces with lower wettability in heat transport through bubble base evaporation. The corresponding underlying mechanisms have been identified. In addition, it was found that the microlayer evaporation mechanism is more efficient as compared to that based on pure contact line evaporation in transporting the heat away from the surface.

Assessment on population balance model and wall boiling model for subcooled boiling flow of steam generator of nuclear power plant

Assessments on the combinations of the population balance model (PBM) and wall boiling model are meaningful for the calculation of the steam generator (SG) of the nuclear power plant (NPP). Firstly, the non-dimensional analysis between the report experiment and the SG of the NPP are carried out. Then, the influences of the models of the active nucleation site density Nn combined with the PBM kernels are analyzed. Lastly, the error analysis on the combinations is illustrated. The results show that the one model combination performs relatively better for the calculation of the Sauter mean diameter (SMD) dp (maximum relative error 6.09%), void fraction α (maximum relative error 20.06%) and superficial liquid velocity jl (maximum relative error 2.06%) of the SG of the NPP. This study demonstrates an integral method and process for the selection of the model combinations in the subcooled boiling flow conditions.

Quantification of Nucleation Site Density as a Function of Surface Wettability on Smooth Surfaces

AbstractDew water is recognized as a valuable source of clean water for human consumption. Given the developing concerns over global water scarcity, great focus has been turned toward increasing the efficiency of existing dew harvesting methodologies. Droplet nucleation is a critical first stage in the condensation process – and therefore key to dew water harvesting. In this paper, the droplet nucleation site density (Ns) as a function of surface wettability on smooth thin polymer films is quantified. A custom‐built environment chamber, operated in low supersaturation conditions relevant to atmospheric water harvesting, allows strict experimental control over temperature and humidity. Droplet growth through coalescence is quantified, and an exponential increase in the rate of coalescence is seen as the test surface wettability increased. Ns declines exponentially as wettability decreases according to the fitted equation Ns = 1.1 × 1011e−0.043θ , where theta is the contact angle of the smooth surface, which can be used to predict Ns from a known contact angle, currently not available. The average distance between droplet centers is found to increase at a linear rate. This nucleation behavior is in line with those of droplets in a Rayleigh distribution.

Experimental and Theoretical Investigation of Nucleation Site Density and Heat Transfer During Dropwise Condensation on Thin Hydrophobic Coatings

AbstractDropwise condensation (DWC) has the potential to enhance heat transfer compared to filmwise condensation (FWC). The heat transfer rates achieved by DWC depend on the drop size distribution, which is influenced by nucleation processes of newly formed drops. In DWC modeling, the nucleation site density Ns is used as an input parameter to obtain the drop size distribution of small drops. However, due to the small scale of the condensate nuclei, direct observation is difficult, and experimental data on the nucleation site density are scarce. In the literature, values in the range of 109 m−2 to 1015 m−2 can be found for Ns. In this paper, we report DWC experiments on SiO2 and 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PFDTES) thin hydrophobic coatings that show significantly different nucleation site densities. Nucleation site densities are estimated from high-speed imaging of small drops during initial condensation and from model calibration using established DWC theory. We have found the values for Ns to be in the range from 1.1×1010 m−2 to 5.1×1011 m−2 for the SiO2 coating and 1011 m−2 to 1013 m−2 for the PFDTES coating. Our results show that there can be large differences in the nucleation site density under similar conditions depending on the surface properties. This underlines the importance of investigating nucleation site density specifically for each surface and under consideration of the specific process conditions used for DWC.

Magnetization and microhardness of iron–chromium alloy films electrodeposited from an aqueous solution containing N, N-dimethylformamide

Iron-chromium (Fe–Cr) alloy films were electrochemically synthesized from an aqueous solution containing N,N-dimethylformamide (DMF). The chromium content of the alloy films increased up to ca. 32 at%, with an increase in the cathodic overpotential during the electrodeposition. Based on the XRD profiles and electron diffraction patterns, it was revealed that the electrodeposited Fe–Cr alloy films have an amorphous nanocrystalline structure. The crystal grain size of the alloys decreased due to an alloying effect of Cr atoms as well as an increase in the nucleation site density of the alloys that were electrodeposited, accompanying a significant overpotential from the stable complex ions. The saturation magnetization of the electrodeposited Fe–Cr alloy films decreased with an increase in the chromium content. On the contrary, the microhardness increased due to the synergistic contribution of solid solution strengthening and crystal grain refinement. The microhardness of the electrodeposited Fe–Cr alloy films reached up to 422.0 kgf/mm2 (HV0.05) and greatly exceeded that of the solidified Fe–Cr alloy ingots.

How Coalescing Bubbles Depart from a Wall.

Bubble evolution plays a fundamental role in boiling and gas-evolving electrochemical systems. One key stage is bubble departure, which is traditionally considered to be buoyancy-driven. However, conventional understanding cannot provide the full physical picture, especially for departure events with small bubble sizes commonly observed in water splitting and high heat flux boiling experiments. Here, we report a new regime of bubble departure owing to the coalescence of two bubbles, where the departure diameter can be much smaller than the conventional buoyancy limit. We show the significant reduction of the bubble base area due to the dynamics of the three-phase contact line during coalescence, which promotes bubble departure. More importantly, combining buoyancy-driven and coalescence-induced bubble departure modes, we demonstrate a unified relationship between the departure diameter and nucleation site density. By elucidating how coalescing bubbles depart from a wall, our work provides design guidelines for energy systems which can largely benefit from efficient bubble departure.

Silicon microchannels flow boiling enhanced via microporous decorated sidewalls

Silicon microchannels with microporous decorated sidewalls were applied to enhance water flow boiling heat transfer, especially the critical heat flux (CHF). In experiments, a CHF of 473 W/cm2 based on the heater areas (210 W/cm2 if based on the effective heating areas) was achieved in the proposed microchannels at the mass flux of 389 kg/cm2, accounting for a 205% CHF enhancement compared with that in plain-wall microchannels without compromising the pressure drop. In addition, an improvement of 228% in effective heat transfer coefficient was achieved in the early nucleate boiling regime at the mass flux of 113 kg/cm2 compared with the plain-wall microchannels owing to the increased nucleation site density on the porous walls. Moreover, a visualization study via a microscope-linked high-speed camera was conducted to investigate the mechanism of liquid distributions and wetting phenomena in porous wall microchannels. The sustainable liquid film induced by the microporous structure greatly delays the local dryout and promotes more efficient heat transfer mechanisms such as thin-film evaporation. The exit vapor quality of the proposed microchannels has been increased above 0.3 at the tested mass flux range 113–389 kg/m2s, two times higher than that in the plain-wall microchannels (∼0.1).

Enhanced pool boiling on micro-nano composited surfaces with nanostructures on micro-pin-fins

In the present study, the effect of the distribution of nanostructures on the pool boiling heat transfer performance of micro-nano composited surfaces in FC-72 was investigated. For this purpose, a micro-pin-fined surface (MPF), a micro-nano composited surface with nanoforests covering the top and bottom sides of micro-pin-fins (MPF-NF), a micro-pin-fined surface with nanostructures etched at the top corners of micro-pin-fins (MPF-TC), and a micro-pin-fined surface with nanostructures etched at the top edges of micro-pin-fins (MPF-TE) were fabricated. The pool boiling heat transfer performance was compared under the liquid subcooling (ΔTsub) of 0, 15, 25, and 35 K. It is found that MPF-NF has the lowest wall superheat at the onset of the nucleate boiling point due to its highest nucleation site density, while MPF-TE has the highest maximum heat transfer coefficient because its critical heat flux (CHF) is significantly higher than that of MPF-NF. The bubbles of MPF-TC and MPF-TE are mainly generated on the tops of micro-pin-fins, thereby alleviating the blockage of the capillary wicking channels by the bubbles in the high heat flux region. Furthermore, the condensation effect of the subcooled liquid on the bubbles generated on the tops micro-pin-fins, with a thinner thermal boundary layer, is stronger compared with that generated in the microchannels. On the contrary, the bubbles of MPF are mainly generated in the microchannels, leading to a blockage of the capillary wicking channels at high heat fluxes. Therefore, the CHF of MPF-TE at ΔTsub = 35 K is increased by 25% compared to MPF. However, the wake effect dominates the liquid supply and numerous lateral bubble coalescence on MPF-TE weakens the wake effect on the liquid supply, resulting in a lower CHF than that of MPF at ΔTsub = 0 K.

CFD-modelling of boiling in a heated pipe including flow pattern transition

Flow boiling occurs when a subcooled liquid enters a heating pipe and its temperature near the heating wall exceeds the boiling onset temperature. Bubbles are generated on the heating wall and the more downstream the larger the average bubble size due to progressing evaporation and coalescence. Further downstream, the two-phase flow morphologies may change from bubbly to slug, plug, and annular flow. Since these flow patterns have a great impact on the heat and mass transfer rates, an accurate prediction of them becomes critical. In this work, the recently developed GENeralized-TwO Phase concept (GENTOP) was used for flow patterns transition modelling and their effects on the wall heat transfer during the upward subcooled flow boiling inside a vertical heating pipe. Furthermore, a previously developed mechanistic bubble dynamics model was implemented in the GENTOP framework as a sub-model. This model is based on the force balance on a single growing bubble considering evaporation of the microlayer underneath the bubble, thermal diffusion and condensation around the bubble as well as the dynamic inclination and contact angles. It does not require a recalibration of parameters to predict the bubble dynamics. For implementing this model in the Euler-Euler (E-E) framework an extension of the current nucleation site density and heat partitioning model was required. Eventually, for a generic test case, flow boiling regimes of water in a vertical heating pipe were simulated using ANSYS CFX 18.2.

Bubble dynamics of R-123 and R-134a on pore/sub-tunnel surfaces

Structured enhanced surfaces are widely used to promote nucleate boiling heat transfer in refrigeration and process industries. Despite the frequent usage of the enhanced surfaces, there appears a significant lack of understanding on the boiling mechanism or predictive models. To enhance the understanding, the bubble dynamic data, which are the building blocks of the predictive models, are of necessity. This study continues that by Mehdi et al. [15], who reported R-134a bubble dynamic data on a pored surface. In this study, the tests were extended to R-123. The nine samples included those having 0.1–0.3 mm pore diameters and those having 0.75–3.0 mm pore pitches. The results showed that, R-123 yielded larger bubble departure diameters and more nucleation site densities than R-134a. The reason was attributed to the larger surface tension and liquid thermal conductivity of R-123 over R-134a. For both refrigerants, the trends of bubble dynamics according to the pore geometry were almost identical. The bubble departure diameter and the bubble generation frequency increased as the pore diameter increased. The nucleation site density increased as the pore pitch decreased. An improved model was developed extending that of Chien and Webb [13]. The model reasonably predicted the heat transfer coefficients as well as the bubble dynamic data of the present surfaces (95% of the heat transfer coefficients within ± 40%).

Assessment of wall heat flux partitioning model for two-phase CFD

The Eulerian-Eulerian two-phase flow (EETF) model coupled with the Wall Heat Flux Partitioning (WHFP) model is slowly becoming an effective tool for two-phase hydrothermal analysis in nuclear applications, with prediction accuracy highly dependent on sub-models for three bubble parameters, namely nucleation site density (N), bubble departure diameter (Dw) and bubble departure frequency (f). In this study, the preformance of different sub-models combinations is evaluated under a wide range of conditions, especially focusing on the prediction of void fraction. The four most commonly used or investigator-recommended combinations are identified by a literature review. A large database of flow boiling experiments covering 52 sets of experiments is developed for the assessment, rather than the popular case-by-case adjustment for a limited number of experiments. One of the combinations is distinguished to be the optimal combination, with an overall 44.2% mean absolute error (MAE). A nearly fixed relationship between void fraction and steam quality is predicted by this combination. It suggests that there is an ‘optimal’ range of operating conditions for which the combination can accurately predict the results. Based on the current assessment, for water, this range is between 17 and 97 K for subcooling and a mass flux larger than 770 kg/(m2s). However, the prediction of bubble diameter and gas velocity are not accurate, since the Kurul and Podowski model simply assumes the bubble diameter as the function of liquid temperature. Models considering bubble coalescence and breakup are desired to predict the bubble diameter and gas velocity more accurately. It is also shown that the performance of combinations, which are genreally considered to be applicable to a broader range, is not satisfactory. Experimental measurements of bubble departure diameter and nucleation site density under a wider range of conditions are desired to evaluate those sub-models.

Regulation of phase transition temperature and preparation for doping-VO2 smart thermal control films

Vanadium dioxide (VO2) is considered one of the most promising smart thermal control materials due to its insulator-metal temperature (IMT) reversible phase transition, accompanied by large changes in its optical properties. However, as the crystal defects on IMT change and the optical property of VO2 is still unclear, the preparation of doped VO2 films by magnetron sputtering is still a great challenge. In this work, the IMT of 41 kinds of doping-VO2 systems were studied by high throughput calculation based on density functional theory (DFT). It was found that the IMT increased with the decrease of the β angle in M phase and expansion of cell volume difference of M-phase and R-phase for IIA elements, VIIA elements, transition elements, and rare earth element doped VO2, and increased with the increase of the β angle in M phase and a decrease of cell volume difference of M-phase and R-phase for IA, IVA, VA, and VIA element doped VO2. According to the rule, the IMT, electronic structures, and optical properties of W doped VO2 were studied based on DFT. The results show that IMT and bandgap decrease with the increase of W6+ ion concentration, which is due to the increased cell volume difference of M-phase and R-phase in W doped VO2; each doped atom can reduce the IMT of 20.2 °C, and the IMT of V0.98W0.02O2 is close to room temperature (Tc ≈ 27 °C). The rate of infrared emissivity (∆ɛ) of V0.98W0.02O2 is about 0.2 at 8–14 μm (0.088–0.155 eV) and the average solar absorption (αs) of M phase and R phase is about 0.53 and 0.59 at 0.3–1.5 μm (0.496–4.13 eV), respectively. Finally, radio frequency magnetron sputtering was used to achieve precise doping, which solved the problem of oxygen partial pressure in reactive magnetron sputtering, and V1-xWxO2 films with IMT close to room temperature and narrow hysteresis width were prepared. This is due to the fact that higher W doping content will greatly increase the density of defect-induced nucleation sites and promote nucleation. At the same time, the experimental results of IMT were consistent with the calculated results, which proved the reliability of the calculation. This will provide a theoretical basis for the development of new thermal control materials and a new method for the preparation of doping-VO2 films in the future.

Separate effect of oxidation on the subcooled flow boiling performance of Zircaloy-4 at atmospheric pressure

In this paper, we present the results of an experimental investigation aimed at elucidating the separate effect of oxidation on the subcooled flow boiling performance (i.e., heat transfer coefficient and critical heat flux) of Zircaloy-4. We also present the results of exploratory tests which we conducted in preparation to this investigation, which demonstrate that even nominally identical unoxidized samples (i.e., with the same exact roughness and wettability) can have very different boiling dynamics and boiling performance under the same exact operating conditions. Such differences can be even larger than those between unoxidized and oxidized samples. Thus, to carefully separate the effect of oxidation, as part of this work, we developed a special testing and characterization protocol. We conducted boiling tests on the same exact Zircaloy-4 sample, from unoxidized conditions throughout its pre-transition, transition and post-transition oxidation stages. All the flow boiling tests were run at atmospheric pressure with degassed and deionized water at 1 bar, with a subcooling of 10 K and a flow rate of 1000 kg/m2/s. Together with boiling experiments, we characterized the surface morphology at the micro- and nano-scale in order to clarify how the oxidation-induced changes of surface properties affect the performance and the dynamics of the boiling process. Crucially, to eliminate random effects, we conducted this characterization on the same exact spot of the boiling surface at all stages of oxidation. Our results reveal that, differently from pool boiling studies in the literature, the oxidation of the surface has little or no impact on the dynamics (i.e., on nucleation site density, bubble growth time and departure frequency, and bubble size) and the performance of subcooled flow boiling, except for the post-transition oxide. This oxide has smaller bubble departure diameter and growth time, higher nucleation site density, and a higher critical heat flux limit. This behavior seems to be triggered by micron-scale cracks on the oxidized surface, which make the surface wick liquid.

A comparative study of pool boiling heat transfer in different porous artery structures

• A novel porous artery structure is proposed to enhance the CHF of pool boiling. • Effects of microporous layer thickness and the artery depth are investigated. • The maximum heat flux reaches 465 W/cm 2 with DI water as the working fluid. • The inherent physical mechanisms of porous artery structure are elucidated. A novel porous artery structure is proposed and experimentally validated to enhance the pool boiling heat transfer performance based on the concept of “phase separation and modulation”. In the experiment, multiple rectangular arteries are formed in the bottom of a porous structure, and a thin copper microporous layer is placed between the heating surface and the rectangular arteries. Compared with conventional porous structures, this novel porous artery structure can effectively improve the pool boiling heat transfer performance due to i) increased nucleation site density, ii) improved liquid replenishment by capillarity, and iii) effective liquid/vapor phase separation. Experimental results show that comparing with boiling heat transfer on a plain surface, a 200% higher in CHF, and a 144% higher in heat transfer coefficient (HTC), together with a 59% lower in superheat at the onset of nucleate boiling (ONB) are obtained for this new structure. In addition, the effects of the top and bottom microporous layer thickness and the artery depth on the pool boiling heat transfer performance are investigated, and the inherent physical mechanisms are analyzed.

The wall heat flux partitioning during the pool boiling of water on thin metallic foils

• We study nucleate boiling on substrates of small thermal capacity using IR camera. • Microlayer does not fully evaporate due to limited thermal capacity of the heater. • Microlayer evaporation and rewetting are less important compared to thick heaters. • Most energy is removed by convective effects created by bubble growth and departure. • We re-derived heat flux partitioning model to quantify each heat transfer mechanism. In this work, we studied the wall heat flux partitioning during the pool boiling of water on thin metallic surfaces. We conducted boiling experiments on surfaces where we engineered nucleation sites by nanosecond-fiber-laser texturing. These nucleation sites form triangular lattice patterns with different pitches. We measured the time-dependent temperature and heat flux distributions on the boiling surface using an infrared camera. We developed post-processing algorithms to measure, based on these distributions, all the fundamental boiling parameters used in heat flux partitioning models (e.g., nucleation site density, bubble wait and growth time, and bubble footprint radius) and the actual partitioning of the heat flux, i.e., how much heat is transferred by evaporation of the microlayer, rewetting of the surface, and convective effects. This work reveals that the mechanisms of heat transfer on substrates of small thermal capacity are very different compared to substrates of large thermal capacity. With water, the bubble microlayer typically does not dry out and the surface temperature at rewetting is practically the same as the rewetting fluid temperature. These effects limit the efficiency of microlayer evaporation and rewetting heat transfer. Instead, convective effects generated by the bubble growth process remove most of the energy from the heated surface. This behavior is captured by a heat flux partitioning model that we re-derived from first principles to describe the heat transfer mechanisms on substrate of small thermal capacity.

Influence of amorphous carbon interlayers on nucleation and early growth of lithium metal at the current collector-solid electrolyte interface

Nucleation and early growth of Li metal is critical to the performance of anode-free solid-state batteries. We report the use of amorphous carbon deposited by direct current magnetron sputtering as an intermediate layer between the Cu current collector and the Lipon solid electrolyte. The density, conductivity, and microstructure of the carbon interlayer are varied and their influence on the reversible formation and removal of the Li metal anode is investigated. It is shown that thin films of amorphous carbon act as seed layers, reducing the overpotential for Li plating and increasing the critical current density for Li plating and stripping from 2 up to 8 mA cm−2. It is further demonstrated that the ionic conductivity of the Li ions in the carbon interlayers determines their optimum thickness to be 100 nm or less, and that the initial Li loss due to interphase formation can be reduced to a few tens of nm by decreasing the density of the carbon films.