Vapor Layer Research Articles

Integration of Intense Pulse Light Annealing for Rapid Scalable Fabrication of Roll to Roll Slot Die Coated Perovskite Solar Cells

Perovskite Solar Cells (PSCs) is an exciting new photovoltaic technology and holds a great promise to reduce the cost of solar energy production. Flexible PSCs can offer an unique pathway towards manufacturing of terawatt-scale solar photovoltaic technology, it involves the fabrication of large area mass production of PSCs and modules by incredibly high throughput roll-to-roll (R2R) processing. Therefore, there is a great possibility of commercializing PSC technology through roll-to-roll printing method. Beyond high throughput R2R processing, flexible PSCs will also enable more diverse applications for solar energy harvesting including, their facile integration into buildings, consumer electronics, automobiles, and various other portable applications. The feasibility of upscaling the PSC technologies to a high-volume production using R2R slot-die coating is demonstrated in this study. Several attempts have been explored to optimize the slot-die coating technique to fabricate large-area PSCs on flexible ITO-coated PET (ITO/PET) substrates. PSCs are fabricated by R2R slot die coating on flexible ITO/PET substrates with the width of 6 inches and the web speed of 2 m/min. Initially, it involves R2R deposition of SnO2 electron transport layer (ETL), followed by a photoactive perovskite (MAPbI3) layer at ambient conditions compatible with industrial manufacturing. These coatings were developed by optimizing an appropriate meniscus length, flow rate, and drying conditions for attaining good quality films. Intense Pulse Light (IPL) annealing is integrated into the R2R allowing for rapid crystallization of SnO2 and Perovskite films within fraction of seconds by using high energy pulses at different pulse intervals. It eliminates the use of conventional ovens for thermal annealing and enables cost-effective large-scale production of Perovskite films and their corresponding solar cells rapidly. As obtained films were used for the device fabrication by spinning of Spiro-OMeTAD solution as a hole transporting layer and thermal evaporation of MoOx as a buffer layer and silver (Ag) as a top contact. The average stabilized power conversion efficiency (PCE) of devices made from different areas of the large area films is nearly 10% with an active are of 1cm2. The demonstrated achievement is very promising and paves an easy route towards future commercialization of perovskite-based solar cells technologies via R2R slot-die coating method. Figure 1

Production and Surface Modification of Cellulose Bioproducts.

Petroleum-based synthetic plastics play an important role in our life. As the detrimental health and environmental effects of synthetic plastics continue to increase, the renewable, degradable and recyclable properties of cellulose make subsequent products the “preferred environmentally friendly” alternatives, with a small carbon footprint. Despite the fact that the bioplastic industry is growing rapidly with many innovative discoveries, cellulose-based bioproducts in their natural state face challenges in replacing synthetic plastics. These challenges include scalability issues, high cost of production, and most importantly, limited functionality of cellulosic materials. However, in order for cellulosic materials to be able to compete with synthetic plastics, they must possess properties adequate for the end use and meet performance expectations. In this regard, surface modification of pre-made cellulosic materials preserves the chemical profile of cellulose, its mechanical properties, and biodegradability, while diversifying its possible applications. The review covers numerous techniques for surface functionalization of materials prepared from cellulose such as plasma treatment, surface grafting (including RDRP methods), and chemical vapor and atomic layer deposition techniques. The review also highlights purposeful development of new cellulosic architectures and their utilization, with a specific focus on cellulosic hydrogels, aerogels, beads, membranes, and nanomaterials. The judicious choice of material architecture combined with a specific surface functionalization method will allow us to take full advantage of the polymer’s biocompatibility and biodegradability and improve existing and target novel applications of cellulose, such as proteins and antibodies immobilization, enantiomers separation, and composites preparation.

An experimental investigation of the effect of coating on heat transfer during quenching

The work is aimed at studying the effect of galvanic nickel coating of a stainless steel cylinder on the quenching. In order to compare the results, the polished stainless steel cylinder was used as a sample. In addition, the influence of the formation of an oxide layer on the cooling process was studied. The experiments were carried out in water and ethanol with different subcoolings. The oxidized porous nickel coating led to increasing of the transition temperature from stable film boiling to intensive boiling regime. It was especially noticeable for cooling in water at high subcooling due to the higher cooling intensity caused by vapour layer thinning.

Thermodynamic Modeling of Hypergene Processes in Loparite Ore Concentration Tailings

Thermodynamic modeling was undertaken of hypergene processes in loparite ore concentration tailings at a temperature of 3 °C and 20 °C. We investigated water evaporation in the top layers of tailings in the summer months at low filtration coefficients. It was found that the main anions of pore solutions are CO32−, HCO3−, SO42−, HSiO3−, and cations—Na+ and K+. The main forms of REE migration have been revealed: lanthanum and cerium. Dominant newly formed phases in the system were shown to be gibbsite and goethite, minerals of the smectite group, muscovite, feldspars, silica, mica, apatite, secondary nepheline minerals, and strontianite.

PM2.5 Influence on Urban Heat Island (UHI) Effect in Beijing and the Possible Mechanisms

AbstractWhether the urban heat island (UHI) is affected by air pollution in urban areas has attracted much attention. By analyzing the observation data of automatic weather stations and environmental monitoring stations in Beijing from 2016 to 2018, we found a seasonally dependent interlink of the UHI intensity (UHII) and PM2.5 concentration in urban areas. PM2.5 pollution weakens the UHII in summer and winter night, but strengthens it during winter daytime. The correlation between the UHI and PM2.5 concentration has been regulated by the interaction of aerosol with radiation, evaporation and planetary boundary layer (PBL) height. The former two change the surface energy balance via sensible and latent heat fluxes, while the latter affects atmospheric stability and energy exchange. In summer daytime, aerosol‐radiation interaction plays an important role, and the energy balance in urban areas is more sensitive to PM2.5 concentration than in rural areas, thereby weakening UHII. In winter daytime, aerosol‐PBL interaction is dominant, because aerosols lower the PBL height and stabilize atmosphere, weaken the heat exchange with the surrounding, with more heat accumulated in the urban areas and the increased UHII. Changes in evaporation and radiation strengthen the relationship. At night, the change of UHII more depends on the energy stored in the urban canopy. Aerosols effectively reduce the incident energy during daytime, and the long‐wave radiation from the buildings of urban canopy at night becomes less, leading to a weakened UHII. Our analysis results can improve the understanding of climate‐aerosols interaction in megacities like Beijing.

High-resolution single-particle cryo-EM of samples vitrified in boiling nitro-gen.

Based on work by Dubochet and others in the 1980s and 1990s, samples for single-particle cryo-electron microscopy (cryo-EM) have been vitrified using ethane, propane or ethane/propane mixtures. These liquid cryogens have a large difference between their melting and boiling temperatures and so can absorb substantial heat without formation of an insulating vapor layer adjacent to a cooling sample. However, ethane and propane are flammable, they must be liquified in liquid nitro-gen immediately before cryo-EM sample preparation, and cryocooled samples must be transferred to liquid nitro-gen for storage, complicating workflows and increasing the chance of sample damage during handling. Experiments over the last 15 years have shown that cooling rates required to vitrify pure water are only ∼250 000 K s-1, at the low end of earlier estimates, and that the dominant factor that has limited cooling rates of small samples in liquid nitro-gen is sample precooling in cold gas present above the liquid cryogen surface, not the Leidenfrost effect. Using an automated cryocooling instrument developed for cryocrystallography that combines high plunge speeds with efficient removal of cold gas, we show that single-particle cryo-EM samples on commercial grids can be routinely vitrified using only boiling nitro-gen and obtain apoferritin datasets and refined structures with 2.65 Å resolution. The use of liquid nitro-gen as the primary coolant may allow manual and automated workflows to be simplified and may reduce sample stresses that contribute to beam-induced motion.

Minimum Leidenfrost Temperature on Smooth Surfaces

During the Leidenfrost effect, a thin insulating vapor layer separates an evaporating liquid from a hot solid. Here we demonstrate that Leidenfrost vapor layers can be sustained at much lower temperatures than those required for formation. Using a high-speed electrical technique to measure the thickness of water vapor layers over smooth, metallic surfaces, we find that the explosive failure point is nearly independent of material and fluid properties, suggesting a purely hydrodynamic mechanism determines this threshold. For water vapor layers of several millimeters in size, the minimum temperature for stability is ≈140 °C, corresponding to an average vapor layer thickness of 10-20 μm.

Explosion of metastable droplets in immiscible liquids

Evaporation of metastable droplets in an immiscible liquid can occur in a wide range of practical applications and in particular in cryogenic systems due to the low saturation temperature of the cryogenic liquid. In this article, we show experimentally that the incomplete explosion of a metastable droplet can lead to a ‘film evaporation’ configuration, where a remaining droplet is eventually separated from the host immiscible liquid by an established vapour layer. Based on a transient heat diffusion model, we identified that the necessary but not sufficient condition for the complete explosion of a metastable droplet is that the droplet must absorb, before explosion, an amount of heat larger than the latent heat required for a full evaporation, i.e. the global Jacob number Ja≥1. In addition, the measured superheat limits of pentane and isohexane are 420.65 K and 444.95 K respectively, which are very close to the theoretical Spinodal points (≤ 1.5 K) and other reported values in existing literature. Therefore, the experimental setup reported here can be used as an alternative way to quantify the superheat limit. The Droplet relaxation time was also found scaled to the square root of the diffusion characteristic time.

Role of extended surfaces on the enhancement of quenching performance

High heat transfer rate is essential in several industrial applications, including, nuclear reactors, storage and transport of cryogenic liquid, and metal forming, which deal with the rapid cooling of various materials. Rapid cooling of surfaces at temperatures significantly higher than the boiling point of the coolant is often restricted by formation of a vapor layer around the surface that retards the rate at which heat can be dissipated. In the present work we report on the effect of finned structures of millimetric length scale on the enhancement of heat transfer performance during quenching. The fins are observed to destabilize hydrodynamic vapor layer during film boiling. We show that the quench duration reduces by ~3.8 times and the maximum heat flux (MHF) increases by nearly 230% for the finned sample as compared to the smooth samples. The quench duration and heat transfer rate are shown to be dependent on spacing between fins and the optimum fin spacing is determined to yield the highest MHF and minimum quench time.

All-in-one photocatalysis device for one-step high concentration H2O2 photoproduction

Achieving multi-functions integrating reaction, separation and concentration into one-step catalysis process is a great challenge. Photocatalytic H2O2 generation is the cleanest and cheapest way for its production. Herein, an all-in-one photocatalytic device for H2O2 generation was designed by catalysis layer with ZIF-8/C3N4 composite and carbon dots-based evaporation layer (CDE). With this device, H2O2 solution with high concentration is obtained, and no need for extra separation and concentration processes. Under 2 sun illumination (3 h), H2O2 evolution reaches 17.13 μmol and H2O2 solution concentration gets to 10.15 mmol L−1. Mole fraction of H2O2 solution obtained by all-in-one device is 1.86 times more concentrated than that of not concentrated H2O2 solution. Besides, multiples devices can work concurrently to realize large-scale production and gain large amounts of H2O2 solution with high concentration. Moreover, all-in-one devices can be reused at least ten cycles. The relation between the structure and performance of the photocatalysis device is further studied by COMSOL Multiphysics software and mathematical analysis. All-in-one photocatalytic device promotes H2O2 photoproduction closer to the practical application due to realizing continuous production of amounts of aqueous solution of H2O2 with high concentration in one-step.

External group combustion of droplet clouds under two-stage autoignition conditions

The present study aims to extend the understanding of the external group combustion for droplet clouds under two-stage autoignition conditions. First, an exemplar case has been chosen to analyze the two-stage ignition behavior and flame structure. It was found that the fuel vapor transport of the droplet cloud is mainly supported by the thin vaporization layer near the edge of the cloud, which is the dominant factor of the ignition process. The case shows that a typical two-stage ignition includes the following detailed processes: first stage ignition, quasi-steady non-premixed cool flame, second stage ignition, and premixed hot flame. Three possible ignition modes for droplet clouds were identified through parametric studies, i.e. no ignition, single-stage ignition, and two-stage ignition. The transition between these modes is mainly controlled by the competition between chemical reaction and the vaporization of a thin layer of fuel droplets. Phase diagrams of different ignition modes at different ambient temperatures and sub parameters of the group number were presented. The previous group number G = 2πndRc2 demonstrated unsatisfactory prediction in the ignition modes of the droplet clouds. Basing on the vaporization layer feature of the droplet clouds, a new dimensionless group ignition number with much better predictability is first proposed in this work, i.e. Gig = 4πn2/3d2. The physical meaning of Gig is the dimensionless total surface area of droplets within a local layer.

Molecular dynamics simulations of nanoscale boiling on mesh-covered surfaces

• The boiling process above mesh-covered copper surfaces is studied by molecular dynamics simulations. • The heat transfer is significantly enhanced on mesh-covered surfaces. • Different mechanisms of boiling process are observed for mesh-covered surfaces. • Porous mesh-covered surface showed highest energy exchange rate, evaporation rate and heat flux. The surfaces with micro/nano porous mesh structures have shown great potentials to significantly enhance the boiling heat transfer performance. In this work, the nonequilibrium molecular dynamics simulations are carried out to investigate the boiling process on the mesh-covered copper surfaces with argon as a working fluid. Three different type of surfaces are incorporated: a porous mesh-covered surface (a surface with pores in between the mesh wires and base solid surface), a simple mesh-covered surface (a surface without pores under mesh wires) and a plain surface. The simulations results showed that both mesh-covered surfaces have significant effects on the density distribution of the system, argon temperature and evaporation rate. The heat transfer from the solid surface to argon is significantly enhanced for both mesh-covered surfaces due to larger solid–liquid contact area as compared to plain surface. Furthermore, different mechanisms of boiling process are observed for porous mesh-covered surface and other two surfaces. The argon atoms near the solid surface are quickly heated, forming a vapor layer which causes the separation of the liquid as the big clusters for both simple mesh-covered surface and plain surface. But liquid is present in the pores under the mesh wires which evaporates and delays the formation of vapor layer for the porous mesh-covered surface, and thus, when liquid cluster is separating from the solid surface the argon temperature, evaporation rate and heat flux are enhanced significantly as compared to simple mesh-covered surface and plain surface. Furthermore, some liquid remains in pores even after the separation of the liquid cluster from the solid surface, which evaporates with time while the liquid cluster rises up, further increasing the argon temperature and evaporation rate for the porous mesh-covered surface. It is found that simple and porous mesh-covered surfaces increased the evaporation rate by 19.16% and 23.89%, respectively. Moreover, the porous mesh-covered surface showed highest energy exchange rate and heat flux.

Effects of ignition position on the ignitability and flame behavior of stratified gasoline vapor

Stratified vapor–air mixtures are formed owing to evaporation after a leakage of gasoline in confined spaces. In this study, the effects of ignition position and ignition delay (tig), defined as the time interval between ignition and the moment of fuel spill, on the ignitability and flame behavior of the stratified vapor above a 0.8-mm-thick gasoline layer were experimentally investigated in a horizontal duct. Experimental results revealed that the ignitability of the stratified gasoline vapor depended on both ignition position and tig. The closer the ignition source was to the oil surface, the earlier the local stratified gasoline vapor became ignitable and thereafter lost flammability. A two-zone flame structure, including a blue premixed flame and a following yellow diffusion flame, was observed, and the blue flame propagated along only a certain layer of the gasoline vapor. The depth of the leading blue flame first increased and thereafter decreased with an increase in tig. The heat release owing to the yellow diffusion flame played an important role in the maximum explosion overpressure (Pmax), which closely related to both ignition position and tig. For a certain tig, Pmax was nearly independent of ignition position. The relationship between Pmax and tig varied with ignition position. The highest value of Pmax was obtained when ignition source was around the center of the duct, and the lowest value of 66 kPa appeared when ignition source was close to the duct roof.

Influence of the substrate permeability on Leidenfrost temperature

Leidenfrost state of droplets compromises heat dissipation in cooling applications because of the formation of a thin insulating vapor layer over which the coolant droplet levitates. Here we report on the dependence of the Leidenfrost temperature (LFT) of a deionized water droplet on the surface morphology and propose a pressure-based model to explain the associated droplet dynamics. We observe that the LFT increases with the height of micropillars and spacing between them. The LFT increases by ~ 270 °C compared to a smooth surface and reaches ~ 507 °C for a micropillar array with an interpillar spacing of 100 µm and height of 63 µm. The wall heat flux at the Leidenfrost state of the droplet varies between ~ 15 W cm−2 and ~ 52 W cm−2 for different microtextured surfaces. We show that irrespective of the height of the textures, the effect of surface roughness diminishes beyond a certain critical interpillar spacing. We develop a semi-analytical model to show that the excess vapor gap between the top of the pillars and the base of the droplet is dependent on the permeability of the substrate and influences the vapor pressure under the droplet. The excess vapor gap is shown to play a crucial role in determining the range of temperatures that sustain transition boiling and affects the rate of evaporation of the droplet in Leidenfrost state.

Photoacoustic laser streaming with non-plasmonic metal ion implantation in transparent substrates.

Photoacoustic laser streaming provides a versatile technique to manipulate liquids and their suspended objects with light. However, only gold was used in the initial demonstrations. In this work, we first demonstrate that laser streaming can be achieved with common non-plasmonic metals such as Fe and W by their ion implantations in transparent substrates. We then investigate the effects of ion dose, substrate material and thickness on the strength and duration of streaming. Finally, we vary laser pulse width, repetition rate and power to understand the observed threshold power for laser streaming. It is found that substrate thickness has a negligible effect on laser streaming down to 0.1 mm, glass and quartz produce much stronger streaming than sapphire because of their smaller thermal conductivity, while quartz exhibits the longest durability than glass and sapphire under the same laser intensity. Compared with Au, Fe and W with higher melting points show a longer lifetime although they require a higher laser intensity to achieve a similar speed of streaming. To generate a continuous laser streaming, the laser must have a minimum pulse repetition rate of 10 Hz and meet the minimum pulse width and energy to generate a transient vapor layer. This vapor layer enhances the generation of ultrasound waves, which are required for observable fluid jets. Principles of laser streaming and temperature simulation are used to explain these observations, and our study paves the way for further materials engineering and device design for strong and durable laser streaming.

PHOTOELECTRON SPECTROSCOPY STUDIES ON Al2O3 FILMS ON p-GaN(0001)

In order to determine its electronic and chemical properties, the Al2O3/p-GaN(0001) interface is studied in situ by the X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). Using physical vapor deposition (PVD) method, the Al2O3 film is deposited step by step under ultra-high vacuum (UHV) onto p-GaN(0001) surface covered with residual native Ga oxide. Prior to the first Al2O3 layer evaporation, binding energy of the Ga 3[Formula: see text] substrate line is equal to 20.5[Formula: see text]eV. The PVD method of deposition leads to an amorphous Al2O3 film formation. For the final 12.0[Formula: see text]nm thick Al2O3 film binding energy of the Al 2[Formula: see text] line is set at 76.0[Formula: see text]eV and for the O 1[Formula: see text] line at 532.9[Formula: see text]eV. The valence band offset (VBO) and the conduction band offset (CBO) of the Al2O3/p-GaN(0001) interface are determined to be equal to [Formula: see text]1.6[Formula: see text]eV and 1.8[Formula: see text]eV, respectively.

Self-excitation of Leidenfrost drops and consequences on their stability

Volatile liquids (water, alcohol, etc.) poured on hot solids levitate above a layer of vapor. Unexpectedly, these so-called Leidenfrost drops often suddenly start to oscillate with star shapes, a phenomenon first reported about 140 y ago. Similar shapes are known to be triggered when a liquid is subjected to an external periodic forcing, but the unforced Leidenfrost case remains unsolved. We show that the levitating drops are excited by an intrinsic periodic forcing arising from a vibration of the vapor cushion. We discuss the frequency of the vibrations and how they can excite surface standing waves possibly amplified under geometric conditions of resonance-an ensemble of observations that provide a plausible scenario for the origin, mode selection, and sporadic nature of the Leidenfrost stars.

Promoted stable combustion of alcohol-based fuel accompanied by inhibition of Leidenfrost effect in a wide temperature range

Combustion of bio-alcohols helps alleviate energy crisis and environmental pollution. However, the Leidenfrost effect (LE) of alcohols triggered by the adiabatic vapor layer greatly reduces the heat transfer efficiency, which makes alcohols exhibit a long ignition delay time (IDT) at low temperatures, hindering energy efficiency and clean combustion. Here, we first develop a scalable strategy to inhibit LE via self-propagating oxidation heat release and concomitant in-situ active radical generation. Reliable ignition and stable combustion of alcohol-based fuels are achieved in a wide temperature range by using a reactive organometallic compound methoxydiethylborane (MDEB). The theoretically released heat of MDEB self-oxidation is two orders of magnitude greater than the evaporation latent heat of ethanol or n-decane by thermodynamic calculation. Experiments show that the flammable temperature limits of MDEB/ethanol and MDEB/ethanol/n-decane are extended to 225 and 125 °C, which dropped by 370 and 465 °C, respectively. Moreover, the IDT of MDEB/ethanol/n-decane hybrid fuel significantly decreases to 27 ms. The 100% ignition achievement and disappearance of negative temperature behavior highlight the reliable start-up in the Liedenfrost region. This work thus provides a promising tool for promoting the ignition reliability and combustion stability of alcohols for ICEs at low temperatures, as well as pollutant control.

A comparative study of the annealing atmosphere effect on bismuth particles dedicated for silicon nanowires growth catalyst

In this work, we report on a facile fabrication method of bismuth nanoparticles consisting of evaporation of bismuth thin layers under a controlled environment. The investigated evaporation environments are vacuum, hydrogen gas, and plasma. The effects of the fabrication parameters on bismuth microstructural and morphological properties are studied using scanning electron microscopy (SEM) and x-ray diffraction (XRD) analysis. The parametric study demonstrates that, under the different conditions, the synthesized bismuth nanoparticles are quasi-spherical with a uniform density. Additionally, vapor–liquid-solid growth of silicon nanowires (SiNWs) catalyzed by bismuth is successfully achieved. The investigation of the effects of growth parameters shows that the density of the synthesized SiNWs was enhanced by increasing time. The SiNWs optoelectronics properties are also ameliorated in comparison to initial bismuth nanoparticles catalyst, namely the surface reflectivity and the minority carrier lifetime.

Moisture-Assisted Formation of High-Quality Silver Nanowire Transparent Conductive Films with Low Junction Resistance

Silver nanowire (AgNWs) transparent conductive film (TCF) is considered to be the most favorable material to replace indium tin oxide (ITO) as the next-generation transparent conductive film. However, the disadvantages of AgNWs, such as easy oxidation and high wire-wire junction resistance, dramatically limit its commercial application. In this paper, moisture treatment was adopted, and water was dripped on the surface of AgNWs film or breathed on the surface so that the surface was covered with a layer of water vapor. The morphology of silver nanowire mesh nodes is complex, and the curvature is large. According to the capillary condensation theory, water molecules preferentially condense near the geometric surface with significant curvature. The capillary force is generated, making the wire-wire junction of AgNWs mesh bond tightly, resulting in good ohmic contact. The experimental results show that AgNWs-TCF treated by moisture has better conductivity, with an average sheet resistance of 20 Ω/sq and more uniform electrical properties. The bending test and adhesion test showed that AgNWs-TCF treated by moisture still exhibited good mechanical bending resistance and environmental stability.