Liquid Flame Spray Research Articles

Fabrication of ultrathin multilayered superomniphobic nanocoatings by liquid flame spray, atomic layer deposition, and silanization

Superomniphobic, i.e. liquid-repellent, surfaces have been an interesting area of research during recent years due to their various potential applications. However, producing such surfaces, especially on hard and resilient substrates like stainless steel, still remains challenging. We present a stepwise fabrication process of a multilayered nanocoating on a stainless steel substrate, consisting of a nanoparticle layer, a nanofilm, and a layer of silane molecules. Liquid flame spray was used to deposit a TiO2 nanoparticle layer as the bottom layer for producing a suitable surface structure. The interstitial Al2O3 nanofilm, fabricated by atomic layer deposition (ALD), stabilized the nanoparticle layer, and the topmost fluorosilane layer lowered the surface energy of the coating for enhanced omniphobicity. The coating was characterized with field emission scanning electron microscopy, focused ion beam scanning electron microscopy, x-ray photoelectron spectroscopy, contact angle (CA) and sliding angle (SA) measurements, and microscratch testing. The widely recognized requirements for superrepellency, i.e. CA > 150° and SA < 10°, were achieved for deioinized water, diiodomethane, and ethylene glycol. The mechanical stability of the coating could be varied by tuning the thickness of the ALD layer at the expense of repellency. To our knowledge, this is the thinnest superomniphobic coating reported so far, with the average thickness of about 70 nm.

Ultrafast Processing of Hierarchical Nanotexture for a Transparent Superamphiphobic Coating with Extremely Low Roll-Off Angle and High Impalement Pressure.

Low roll-off angle, high impalement pressure, and mechanical robustness are key requirements for super-liquid-repellent surfaces to realize their potential in applications ranging from gas exchange membranes to protective and self-cleaning materials. Achieving these properties is still a challenge with superamphiphobic surfaces, which can repel both water and low-surface-tension liquids. In addition, fabrication procedures of superamphiphobic surfaces are typically slow and expensive. Here, by making use of liquid flame spray, a silicon dioxide-titanium dioxide nanostructured coating is fabricated at a high velocity up to 0.8 m s-1 . After fluorosilanization, the coating is superamphiphobic with excellent transparency and an extremely low roll-off angle; 10 µL drops of n-hexadecane roll off the surface at inclination angles even below 1°. Falling drops bounce off when impacting from a height of 50 cm, demonstrating the high impalement pressure of the coating. The extraordinary properties are due to a pronounced hierarchical nanotexture of the coating.

Antimicrobial characterization of silver nanoparticle-coated surfaces by "touch test" method.

Bacterial infections, especially by antimicrobial resistant (AMR) bacteria, are an increasing problem worldwide. AMR is especially a problem with health care-associated infections due to bacteria in hospital environments being easily transferred from patient to patient and from patient to environment, and thus, solutions to prevent bacterial transmission are needed. Hand washing is an effective tool for preventing bacterial infections, but other approaches such as nanoparticle-coated surfaces are also needed. In the current study, direct and indirect liquid flame spray (LFS) method was used to produce silver nanoparticle-coated surfaces. The antimicrobial properties of these nanoparticle surfaces were evaluated with the “touch test” method against Escherichia coli and Staphylococcus aureus. It was shown in this study that in glass samples one silver nanoparticle-coating cycle can inhibit E. coli growth, whereas at least two coating cycles were needed to inhibit S. aureus growth. Silver nanoparticle-coated polyethylene (PE) and PE terephthalate samples did not inhibit bacterial growth as effectively as glass samples: three nanoparticle-coating cycles were needed to inhibit E. coli growth, and more than 30 coating cycles were needed until S. aureus growth was inhibited. To conclude, with the LFS method, it is possible to produce nanostructured large-area antibacterial surfaces which show antibacterial effect against clinically relevant pathogens. Results indicate that the use of silver nanoparticle surfaces in hospital environments could prevent health care-associated infections in vivo.

Controlled time release and leaching of silver nanoparticles using a thin immobilizing layer of aluminum oxide

Silver nanoparticles are widely used as antibacterial agents in consumer products. There have been concerns about the environmental exposure and their toxic effect to organisms such as fish. Studies have quantified the release of silver from various products including textiles and plastics that use silver as an antibacterial agent, yet there is no unified standard for the measurement of silver release. Additionally, there is limited information about the release of silver from glass surfaces coated with silver nanoparticles. While immobilizing silver to substrates will ultimately reduce environmental exposure, deliberately controlling silver release will also reduce the amount of silver released into the environment. In this study, silver nanoparticles were synthesized and deposited onto glass using an aerosol pyrolysis process, i.e. the Liquid Flame Spray. The deposited silver nanoparticles were further coated with a thin layer of aluminum oxide that was fabricated by atomic layer deposition. The leaching of silver from the coated glass was measured in water over a period of six days. The results show that a 15nm thin layer of aluminum oxide is able to inhibit the release of silver up to 48h, thereby providing a way to control the release of silver in time.

Characterization of bidisperse magnetorheological fluids utilizing maghemite (γ-Fe2O3) nanoparticles synthetized by flame spray pyrolysis

In this study we have used liquid flame spray (LFS) process to synthetize γ-Fe2O3 nanoparticles of two different average sizes. Different sized nanoparticles were generated with two different liquid precursor feed rates in the spray process, higher feed rate resulting in larger nanoparticles with higher saturation magnetization. The nanoparticles were used in bidisperse magnetorheological fluids to substitute 5% of the micron sized carbonyl iron particles. To our knowledge this is the first time particles synthetized by the LFS method have been used in magnetorheological fluids. The bidisperse fluids showed significantly improved sedimentation stability compared to a monodisperse suspension with the same solid concentration. The tradeoff was an increased viscosity without magnetic field. The effect of the nanoparticles on the rheological properties under external magnetic field was modest. Finally, the dynamic oscillatory testing was used to evaluate the structural changes in the fluids under magnetic field. The addition of nanoparticles decreased the elastic portion of the deformation and increased the viscous portion.

Liquid flame spray fabrication of WO3-reduced graphene oxide nanocomposites for enhanced O3-sensing performances

WO3 is one of the inspiring sensing materials that show high response to O3; an efficient fabrication of WO3 film with incorporation of complementary additives is essential for enhanced sensitivity. Here we report film deposition by liquid flame spraying, characterization of nanostructured WO3-reduced graphene oxide (rGO) composites and their gas-sensing activities to O3. The starting feedstock was prepared from WCl6 and rGO for pyrolysis synthesis by flame spraying. Nano-porous WO3-rGO films were successfully fabricated and characterized by transmission electron microscopy, field emission scanning electron microscopy, Raman spectrometry, thermal analyses and X-ray diffraction. Nanosized WO3 grains exhibited oriented nucleation on rGO flakes whereas rGO retained intact its nano-structural features after spraying. Constrained grain growth of WO3 of 60–70 nm in size was realized in the rGO-containing films with as compared to ~220 nm in the pure WO3 film. The WO3-rGO film sensors showed quicker response to O3 and faster recovery than rGO-free WO3 film sensors. Addition of rGO in 1.0 wt% or 3.0 wt% in the films caused a significantly reduced effective working temperature of the film sensors from ~ 250 °C to ~ 150 °C.

Construction of WO3 coatings with micro-nano hybrid structures by liquid precursor flame spray for enhanced sensing performances to sub-ppm ozone

WO3 coatings with tunable micro-nano hybrid structures were fabricated by liquid precursor flame spraying. The coatings consisting of the sole component of monoclinic WO3 show porous topographical morphology with grains of ∼20nm and pores of ∼4–8μm. Assessment of gas sensing performances of the coatings to ozone reveals their optimum operating temperature of 150°C, and sensor response of 3.9 and 46.7 to ozone concentration of 0.5ppm and 2.0ppm respectively. The results shed light on construction of ceramic coatings with tunable hybrid structures by the precursor flame spray technical route for functional applications.

Developing polyimide-copper antifouling coatings with capsule structures for sustainable release of copper

Polyimide-copper layers consisting of individual capsule-like splats were one-step fabricated by solution precursor flame spray through controlling the reaction between dianhydride and diamine dissolved in copper nanoparticles-containing dimethylformamide solvent. The polyimide splat exhibited hollow structure with an inner pore of 10–15μm and a tiny hole of 1–5μm on its top surface. Transversal cut by focused ion beam milling of the individual splats and scanning electron microscopy characterization further revealed unique dispersion of the copper nanoparticles inside the polyimide shell. After 1000h exposure to the testing synthetic seawater, continuous release of copper from the coatings containing up to 30wt%Cu kept remarkable. Antifouling performances of the constructed layers were assessed by examining colonization behaviors of typical bacteria Bacillus sp. and marine algae Phaeodactylum tricornutum and Chlorella on their surfaces. Distribution of the inorganic nanoparticles endows the polyimide coatings with special capsule structure and exciting hydrophobicity and antifouling performances. The liquid flame spray route and the encapsulated structure of the polyimide-Cu coatings would open a new window for designing and constructing environment-friendly marine antifouling layers for long-term applications.

One-step flame synthesis of silver nanoparticles for roll-to-roll production of antibacterial paper

Nanoparticles are used in several applications due to the unique properties they possess compared to bulk materials. Production techniques have continuously evolved over the years. Recently, there has been emphasis on environmentally friendly manufacturing processes. Substrate properties often limit the possible production techniques and, for example; until recently, it has been difficult to incorporate nanoparticles into paper. Chemical reduction of a precursor in the presence of paper changes the bulk properties of paper, which may limit intended end-use. In this study, we present a novel technique for incorporating silver nanoparticles into paper surface using a flame pyrolysis procedure known as Liquid Flame Spray. Papers precoated with mineral pigments and plastic are used as substrates. Silver nanoparticles were analyzed using SEM and XPS measurements. Results show a homogeneous monolayer of silver nanoparticles on the surface of paper, which demonstrated antibacterial properties against E. coli. Paper precoated with plastic showed more nanoparticles on the surface compared to pigment coated paper samples except for polyethylene-precoated paper. The results demonstrate a dry synthesis approach for depositing silver nanoparticles directly onto paper surface in a process which produces no effluents. The production technique used herein is up scalable for industrial production of antibacterial paper.

Comparison of different coating techniques on the properties of FucoPol films

Plasma deposition, liquid flame spray (LFS) and atomic layer deposition (ALD) were used to form inorganic coatings in new exopolysaccharide (FucoPol) biodegradable films. Coated films were characterised in terms of surface, optical and barrier properties in order to evaluate their potential use in food packaging.FucoPol films presented dense and homogeneous surface with instant water contact angle of 95̊. Plasma deposition of perfluorohexane (PFH) on FucoPol surface has not shown significant improvement in the hydrophobic behaviour over the time. The FucoPol coating of SiO2 nanoparticles deposited by LFS and plasma deposition of PFH have shown higher instant water contact angle (135°) caused by coating surface roughness, but this hydrophobic behaviour was not stable over time. FucoPol films coated only with TiO2 deposited by ALD and combination of that with plasma deposition of PFH have shown stable water contact angle during time (90̊ and 115̊, respectively), transparency in the same order of magnitude and significantly lower permeability to water vapour (3.45×10−11mol/m s Pa and 3.45×10−11mol/m s Pa when compared to uncoated films with 5.32×10−11mol/m s Pa). Moreover, films coated with TiO2-PFH have also shown a permeability to oxygen of 1.70×10−16molm/m2sPa which is 67% lower than uncoated films.

Developing titania-hydroxyapatite-reduced graphene oxide nanocomposite coatings by liquid flame spray deposition for photocatalytic applications

Nanostructured titania has been extensively investigated for photocatalytic applications. Persistent challenge yet is how to effectively promote adhesion of microorganisms on the material surface for consequent enhanced photocatalytic disinfection. Here we report fabrication and characterization of titania-based nanocomposite coatings with addition of hydroxyapatite-reduced graphene oxide (HA-rGO). The nano features of TiO2, HA, and rGO were well retained during liquid flame spray deposition. Photocatalytic activities of the coatings were examined by degradation of methylene blue and sterilization testing of Escherichia coli bacteria. Addition of HA-rGO effectively increased the specific surface area of the coatings and markedly enhanced adherence of the bacteria for subsequent extinguishment. The TiO2–10wt.% (HA-rGO) coating showed the best photocatalytic performances and further overloading of HA-rGO resulted in enwrapping of TiO2 particles, resulting in deteriorated degradation activity. The results give clear insight into fabrication of novel photocatalytic nanocomposites by suspension thermal spray route for enhanced performances.

Liquid flame spray fabrication of polyimide-copper coatings for antifouling applications

Liquid flame sprayed polyimide-copper coatings were developed for marine antifouling applications. Mixture of polyimide precursor and copper particles was prepared as the starting feedstock for making the composite coatings. Synthesis of polyimide was achieved during the spraying and polyimide acts as binder to fix copper particles. Copper particles were enwrapped by polyimide matrix and distribute evenly in the coatings. Further electrochemical testing revealed excellent corrosion resistance of the coatings in artificial seawater. The coatings resist effectively colonization of E. coli and Bacillus sp. bacteria on their surfaces, suggesting their remarkable antifouling performances. The results shed light on construction of polymer-based antifouling layers for widespread marine applications.

Liquid Flame Spray—A Hydrogen-Oxygen Flame Based Method for Nanoparticle Synthesis and Functional Nanocoatings

Liquid Flame Spray—A Hydrogen-Oxygen Flame Based Method for Nanoparticle Synthesis and Functional Nanocoatings

Superamphiphobic overhang structured coating on a biobased material

A superamphiphobic coating on a biobased material shows extreme liquid repellency with static contact angles (CA) greater than 150° and roll-off angles less than 10° against water, ethylene glycol, diiodomethane and olive oil, and a CA for hexadecane greater than 130°. The coating consisting of titania nanoparticles deposited by liquid flame spray (LFS) and hydrophobized using plasma-polymerized perfluorohexane was applied to a birch hardwood. Scanning electron microscopy (SEM) imaging after sample preparation by UV laser ablation of coated areas revealed that capped structures were formed and this, together with the geometrically homogeneous wood structure, fulfilled the criteria for overhang structures to occur. The coating showed high hydrophobic durability by still being non-wetted after 500 000 water drop impacts, and this is discussed in relation to geometrical factors and wetting forces. The coating was semi-transparent with no significant coloration. A self-cleaning effect was demonstrated with both water and oil droplets. A self-cleanable, durable and highly transparent superamphiphobic coating based on a capped overhang structure has a great potential for commercial feasibility in a variety of applications, here exemplified for a biobased material.

Fast Pulse Combustion Process for Producing Lithium-Ion Cathode Materials

With the use of a fast pulse combustion process in a reactor we are able to produce LiFePO4, albeit with the use of an additional annealing step[1]. The role of latter is to introduce appropriate ordering into the structure. Disorder in the crystal structure of LiFePO4 has been identified by many groups[2–4], especially for materials synthesized via reactions without a high-temperature sintering step in an inert or reducing atmosphere, or using fast reactions on the order of minutes[5,6]. The most favorable intrinsic defect is the Li-Fe ‘anti-site’ pair in which the Li ion (on the M1 site) and the Fe ion (on the M2 site) are interchanged[2]. In this report we show that such a disordered structure can be obtained even with synthesis times shorter than 2 seconds; at such short times one usually obtains amorphous materials[7]. The disorder will be observed through Mossbauer analysis and Rietveld refinement of XRD data. The galvanostatic cycling behavior of such disordered LiFePO4 is shown together with the annealed version on the attached graph. We were able to cycle the former up to a current density of 340 mA/g (corresponds to 2C). Another group of interesting cathode materials is the lithium nickel manganese cobalt oxides. Using different ratios between the three transition metals different compositions can be obtained, some showing a very high capacity of 220 mAh/g when charged to 4.7 V[8,9]. With our method we are able to produce a wide range of compositions, also lithium rich ones. This is possible because of the very fast reaction from solutions of metal salts, where the mixing proceeds on the molecular level. The fast reaction is the reason that no phase separation or grouping of an element can occur, leading to a uniform composition across the whole material. Simultaneously the method also allows decoration of the particles with a carbon coating, which could help improve the capacity retention[9]. The synthesis of present cathode materials was carried out in a pulse combustion reactor[1], which uses the pulsating flow of flue gases from the combustion chamber to supply the necessary energy for the reaction. The pulsating flow is a result of pulsating fuel and air ignition. It has been shown previously that this kind of burning has a better heat transfer rate than similar turbulent flows[10]. The flue gases enter the reactor pipe that has a two fluid nozzle at the point of flue gas entry. The nozzle is used to produce droplets of precursor in the hot zone of the reactor. The solvent from the droplets evaporates at a very high rate causing formation of agglomerated nano powders, which are then fully or partly crystallized after passing through the heated reactor pipe. The desired temperature in the reactor depends on the product. The dried precursor undergoes a reaction, similar to that in the solution combustion synthesis[11] (SCS), because the precursor can be composed of nitrates and a carbohydrate, acting as a fuel. Another possible precursor type are flammable precursors, such as those used in liquid feed flame spray pyrolysis[7,12](LF-FSP), however, they are usually not used in production of cathode materials with this reactor.

Wetting hysteresis induced by temperature changes: Supercooled water on hydrophobic surfaces

The state and stability of supercooled water on (super)hydrophobic surfaces is crucial for low temperature applications and it will affect anti-icing and de-icing properties. Surface characteristics such as topography and chemistry are expected to affect wetting hysteresis during temperature cycling experiments, and also the freezing delay of supercooled water. We utilized stochastically rough wood surfaces that were further modified to render them hydrophobic or superhydrophobic. Liquid flame spraying (LFS) was utilized to create a multi-scale roughness by depositing titanium dioxide nanoparticles. The coating was subsequently made non-polar by applying a thin plasma polymer layer. As flat reference samples modified silica surfaces with similar chemistries were utilized. With these substrates we test the hypothesis that superhydrophobic surfaces also should retard ice formation. Wetting hysteresis was evaluated using contact angle measurements during a freeze–thaw cycle from room temperature to freezing occurrence at −7°C, and then back to room temperature. Further, the delay in freezing of supercooled water droplets was studied at temperatures of −4°C and −7°C. The hysteresis in contact angle observed during a cooling–heating cycle is found to be small on flat hydrophobic surfaces. However, significant changes in contact angles during a cooling–heating cycle are observed on the rough surfaces, with a higher contact angle observed on cooling compared to during the subsequent heating. Condensation and subsequent frost formation at sub-zero temperatures induce the hysteresis. The freezing delay data show that the flat surface is more efficient in enhancing the freezing delay than the rougher surfaces, which can be rationalized considering heterogeneous nucleation theory. Thus, our data suggests that molecular flat surfaces, rather than rough superhydrophobic surfaces, are beneficial for retarding ice formation under conditions that allow condensation and frost formation to occur.

Hydrophobisation of wood surfaces by combining liquid flame spray (LFS) and plasma treatment: dynamic wetting properties

Abstract The hydrophilic nature of wood surfaces is a major cause for water uptake and subsequent biological degradation and dimensional changes. In the present paper, a thin transparent superhydrophobic layer on pine veneer surfaces has been created for controlling surface wettability and water repellency. This effect was achieved by means of the liquid flame spray (LFS) technique, in the course of which the nanoparticulate titanium dioxide (TiO2) was brought to the surface, followed by plasma polymerisation. Plasma polymerised perfluorohexane (PFH) or hexamethyldisiloxane (HMDSO) were then deposited onto the LFS-treated wood surfaces. The same treatment systems were applied to silicon wafers so as to have well-defined reference surfaces. The dynamic wettability was studied by the multicycle Wilhelmy plate (mWP) method, resulting in advancing and receding contact angles as well as sorption behavior of the samples during repeated wetting cycles in water. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were employed to characterise the topography and surface chemical compositions and to elucidate the question how the morphology of the nanoparticles and plasma affect the wetting behavior. A multi-scale roughness (micro-nano roughness) was found and this enhanced the forced wetting durability via a superhydrophobic effect on the surface, which was stable even after repeated wetting cycles. The hydrophobic effect of this approach was higher compared to that of plasma modified surfaces with their micro-scale modification.

Preparation and antimicrobial characterization of silver-containing packaging materials for meat

Abstract In food technology, antimicrobial packaging materials could inhibit or limit the growth of spoilage bacteria and thus improve the shelf life of packaged products. The present study provides new insights into the preparation and antimicrobial characterization of silver-containing packaging materials and their efficacy against typical meat spoilage bacteria. Antimicrobial efficacy of packaging films produced by coextrusion or liquid flame spray process was determined by bioluminescence imaging and conventional antimicrobial assay. Fresh pork sirloin was packaged in selected films and composition of meat microbiota was analyzed by 16S rRNA amplicon sequencing. Shelf life of meat was not affected by any of the silver-containing packaging films, even though meat microbiota mostly consisted of bacteria that were inhibited or retarded in vitro by nanoscale silver coating. This may be due to different release dynamics of silver ions on meat surfaces compared to the circumstances in the antimicrobial assay or interactions between silver and amino acids.

Workplace performance of a loose-fitting powered air purifying respirator during nanoparticle synthesis

Nanoparticle (particles with diameter ≤100 nm) exposure is recognized as a potentially harmful size fraction for pulmonary particle exposure. During nanoparticle synthesis, the number concentrations in the process room may exceed 10 × 106 cm−3. During such conditions, it is essential that the occupants in the room wear highly reliable high-performance respirators to prevent inhalation exposure. Here we have studied the in-use program protection factor (PPF) of loose-fitting powered air purifying respirators, while workers were coating components with TiO 2 or Cu x O y nanoparticles under a hood using a liquid flame spray process. The PPF was measured using condensation particle counters, an electrical low pressure impactor, and diffusion chargers. The room particle concentrations varied from 4 × 106 to 40 × 106 cm−3, and the count median aerodynamic diameter ranged from 32 to 180 nm. Concentrations inside the respirator varied from 0.7 to 7.2 cm−3. However, on average, tidal breathing was assumed to increase the respirator concentration by 2.3 cm−3. The derived PPF exceeded 1.1 × 106, which is more than 40 × 103 times the respirator assigned protection factor. We were unable to measure clear differences in the PPF of respirators with old and new filters, among two male and one female user, or assess most penetrating particle size. This study shows that the loose-fitting powered air purifying respirator provides very efficient protection against nanoparticle inhalation exposure if used properly. © 2015, Springer Science+Business Media Dordrecht.

Roll-to-Roll Coating by Liquid Flame Spray Nanoparticle Deposition

ABSTRACTNanostructured coatings have been prepared on a flexible, moving paperboard using deposition of ca. 10-50-nm-sized titanium dioxide and silicon dioxide nanoparticles generated by a liquid flame spray process, directly above the paperboard, to achieve improved functional properties for the material. With moderately high production rate (∼ g/min), the method is applicable for thin aerosol coating of large area surfaces. LFS-made nanocoating can be synthesized e.g. on paper, board or polymer film in roll-to-roll process. The degree of particle agglomeration is governed by both physicochemical properties of the particle material and residence time in aerosol phase prior to deposition. By adjusting the speed of the substrate, even heat sensitive materials can be coated. In this study, nanoparticles were deposited directly on a moving paperboard with line speeds 50-300 m/min. Functional properties of the nanocoating can be varied by changing nanoparticle material; e.g. TiO2and SiO2are used for changing the surface wetting properties. If the liquid precursors are dissolved in one solution, synthesis of multi component nanoparticle coatings is possible in a one phase process. Here, we present analysis of the properties of LFS-fabricated nanocoatings on paperboard. The thermophoretic flux of nanoparticles is estimated to be very high from the hot flame onto the cold substrate. A highly hydrophobic coating was obtained by a mass loading in the order of 50–100 mg/m2of titanium dioxide on the paperboard.