Solution Precursor Plasma Spray Research Articles

Cost-effective superhydrophobic ZnO films with adjustable wetting behaviors deposited via solution precursor plasma spray process

In this work, ZnO films featuring hierarchical micron−/nanometer-sized structures were deposited for adjusting wettability by a simple solution precursor plasma spray (SPPS) process using cost-effective zinc acetate solution. The effects of solution flow rate on the surface microstructures, surface roughness, crystalline orientation, and wetting behaviors of the films were investigated comprehensively. The results showed that high solution flow rate facilitated the formation of apparent hierarchical surface structures featuring more uniformly-distributed micron-sized protrusions and higher surface roughness. The increase of solution flow rate also promoted the crystalline growth along the non-polar (100) plane. With the solution flow rate increasing from 10 ml/min to 40 ml/min, the synergy of the surface structures and crystalline growth orientation resulted in a sharp decrease of the surface energy of the films from ~74.4 mJ/m2 to ~12.6 mJ/m2 and a dramatic elevation of water contact angles from ~130° to ~155°. The wetting states of different films were elucidated by correlating the wetting behaviors with the surface structures. The SPPS-deposited ZnO films showed high adhesion to water droplets, similar to the wetting behaviors of the petal effect, which could be used as a “mechanical hand” for non-loss microdroplet transfer. The work shows that SPPS represents a promising method to deposit cost-effective superhydrophobic films with adjustable wetting behaviors.

Tuning Mo Incorporation in Ni-Fe-Mo Solid Solutions: Towards an Improved Hydrogen Adsorption Energy

The production of hydrogen by water electrolysis has emerged as a promising pathway to achieve a 100% renewable and sustainable hydrogen economy. Up to now, alkaline water electrolysis has been highlighted as a truly alternative strategy for hydrogen production due to the usage of non-expensive electrocatalysts. Ni-based electrocatalysts already have a long century history because of high conductivity, stability, and earth abundancy. However, their strong hydrogen interaction energy slows the Hydrogen Evolution Reaction (HER) activity. Tuning the morphology of the catalyst by increasing the active surface area (e.g., production of nanosheets, nanowires, nanoparticles, etc.) and enhancing the intrinsic activity (e.g., doping, alloying, etc.) can improve the catalytic activity. Binary and ternary alloys based on nickel are a common route to boost their intrinsic activity and tune the morphology. Ternary alloys like CuAlNi, NiMoFe and NiMoW have been previously proposed, although the role of each element is still not fully investigated. Among the alloys, Ni-Fe mixtures have shown better catalytic activity towards HER, and the Mo incorporation has resulted in a reduced onset potential due to favorable hydrogen-metal interaction and development of active sites. The strategy for NiFeMo synthesis is mainly by hydrothermal method or electrodeposition which are limited by low material loading and production volume. Therefore, searching for a technique which is capable of producing trimetallic alloys with a scalable amount is of high interest for the development of hydrogen production. Here, we studied the preparation of FeNi3Mox via a fast and straightforward solution precursor plasma spraying (SPPS) method. The liquid precursors containing metal salts of Ni, Fe, and Mo are used to produce trimetallic solid solutions. The non-stoichiometric nanostructured electrocatalysts are deposited onto stainless-steel mesh (SS-mesh), Ni foam, and carbon paper for the HER in alkaline. We found that tuning Mo content (~9 At%) in the solution precursor conducts to an improved overpotential of 112 mV (iR-corrected) at -10 mA cm-2 and a Tafel slope of 109 mV dec-1 in agreement with a Volmer-Heyrovsky HER route. Energy dispersive X-ray spectroscopy (XPS) has confirmed the reduction in the superficial Ni and Fe oxides with Mo insertion which is directly affecting the electron transport. In addition, to investigate the effect of Mo in more detail, we evaluated the work function using ultraviolet photoelectron spectroscopy (UPS) which shows that Mo incorporation changes linearly the electrocatalyst’s work function and it is highly correlated to the hydrogen binding energy. Consequently, the change in the work function, resulting in a weaker hydrogen adsorption energy which leads to an improved hydrogen production. Theoretical catalytic activity maps from Density Functional Theory (DFT) studies have revealed the enhanced HER activity is the responsibility of the active sites near the superficial Mo atoms. Figure 1

Multilayer GZ/YSZ thermal barrier coating from suspension and solution precursor plasma spray

Gas turbines rely on thermal barrier coating (TBC) to thermally insulate the nickel-based superalloys underneath during operation; however, current TBCs, yttria stabilised zirconia (YSZ), limit the operating temperature and hence efficiency. At an operating temperature above 1200 °C, YSZ is susceptible to failure due to phase instability and CMAS (Calcia-Magnesia-Alumina-Silica) attack. Gadolinium zirconates (GZ) could overcome the drawback of YSZ, complementing each other with the multi-layer approach. This study introduces a novel approach utilising axial suspension plasma spray (ASPS) and axial solution precursor plasma spray (ASPPS) to produce a double-layer and a triple-layer TBCs with improved CMAS resistance. The former comprised suspension plasma sprayed GZ and YSZ layers while the latter had an additional dense layer deposited through a solution precursor to minimise the columnar gaps that pre-existed in the SPS GZ layer, thus resisting CMAS infiltration. Both coatings performed similarly in furnace cycling test (FCT) and burner rig testing (BRT). In the CMAS test, the triple-layer coating exhibited better CMAS reactivity, as evidenced by the limited CMAS infiltration observed on the surface.

Photoactive Ce-doped TiO2 and CeO2-TiO2 composite coatings deposited by suspension/solution plasma spray

In this work, Ce-doped TiO2 coatings were produced using a hybrid suspension (SPS) and solution precursor plasma spraying (SPPS) approach. CeO2-TiO2 composite coatings were also deposited by the SPS process. The photocatalytic activity of the coatings was evaluated by measuring the methylene blue degradation rate under visible light. Cerium doping and CeO2 composite coatings were characterized by X-ray diffractometer measurements and verified by energy dispersive spectroscopy mapping, Raman spectra, and X-ray photoelectron spectroscopy analysis. The hybrid SPS/SPPS technique presented a promising method to deposit doped ceramic coatings among thermal spray processes. According to the results, modifying TiO2 coatings decreased the bandgap of TiO2 from 3.25 to 2.3 eV using cerium dopant. However, a lower photocatalytic efficiency of TiO2 -CeO2 composite and Ce-doped TiO2 coatings compared to TiO2 could be related to the lower active surface area of TiO2 covered by submicron CeO2 particles and non-optimal dosage of cerium dopant, respectively.

Effect of feedstock solution concentration on the spectroscopic and electrical transport properties of thermal plasma synthesized ZnO nanoparticles

The structural, optical, spectroscopic, and electrical transport properties of zinc oxide (ZnO) nanoparticles prepared using the solution precursor plasma spray (SPPS) technique were investigated for different feedstock solution concentrations such as 1-mol and 4-mol. Rietveld refined powder X-ray diffraction and Raman spectra reveal that the prepared ZnO crystallizes in a hexagonal wurtzite structure with no secondary phase. The crystallite sizes for 1-mol and 4-mol samples are 27.5 nm and 31.3 nm, respectively. Analysis of UV–vis spectra shows that ZnO's band gap has increased from 3.19 eV to 3.23 eV as the mole concentration has increased. The spherical nature and nano size of the ZnO are confirmed by the pictures obtained using field emission scanning electron microscopy (FE-SEM) and transmission electron microscope (TEM). The presence of elements zinc and oxygen in the Zn2+ and O2− ionic states are revealed by X-ray photoelectron spectroscopy (XPS). Electrical study demonstrates that the initial mole concentration of ZnO powder affected both its electrical conductivity and dielectric constant. The activation energy value of electrical conductivity for 1-mol and 4-mol is 0.661 eV and 0.552 eV, respectively. The impedance analysis shows the electrode effect for the 1-mol sample whereas it is not observed for the 4-mol sample, which is further confirmed by modulus analysis. The presence of defect in the 1-mol sample results in a lower conductivity (3.68 × 10−12 S/cm at 1 Hz) than the 4-mol sample. The comprehensive analysis confirms that the structural, spectroscopic and electrical properties of the ZnO nanoparticle can be tunable to meet demand by changing the feedstock mole concentration in the SPPS process.

Effects of gas viscosity and liquid-to-gas density ratio on liquid jet atomization in crossflow

Atomization of liquid jets in gaseous crossflows has many natural and industrial applications, for example, in fuel atomization in gas turbine engines, rocket engines, film cooling, and, recently, suspension and solution precursor plasma spraying processes for the development of advanced coatings. Viscosity and density of the gaseous medium may significantly vary in applications such as plasma spraying, which can affect the instability waves on the liquid jet column, resulting in a major change in the mechanism of primary and secondary breakups. In this study, a numerical model is used to investigate the impact of gas viscosity on breakup mechanisms for a wide range of density ratios and Weber numbers. Due to many challenges, only a few comprehensive atomization measurements have been performed on this subject. However, novel computational models could provide the atomization process with a thorough picture in the last two decades. The incompressible variable-density Navier–Stokes equations are solved by using finite volume schemes, and a geometric volume-of-fluid technique is used to track the gas–liquid interface. In our parametric study, three sets of density ratios and Weber numbers are chosen. In each set, four cases with different orders of magnitude of gaseous Reynolds number are simulated. Different characteristics of jet atomization are analyzed, such as the jet trajectory, breakup location, and surface instabilities generated along the jet column. Ultimately, the effects of gaseous Reynolds number, density ratio, and Weber number on jet deformation and breakup mechanisms are discussed.

Solution Precursor Plasma Spraying of TiO2 Coatings Using a Catalyst-Free Precursor

The microstructural characteristics and phase composition of solution precursor plasma-sprayed (SPPS) titania-based coatings using a catalyst-free precursor are reported in this work. An ethanol-based solution containing titanium isopropoxide was used to deposit TiO2 coatings. The thermal behavior of the solution precursor changed as its phase transformation temperature increased when the molar concentration was increased from 0.3 M to 0.6 M. Scanning electron micrographs showed that the surface of the coatings was composed of nano- and submicron-sized spherical particles (<1 μm) with sintered and melted particles. The cross-sections showed a porous structure using lower concentrations and dense coating formation with micropores using higher concentrations, with thicknesses of about 5 µm-8 µm. Moreover, the coatings when the number of spray passes was increased were 16 µm-20 µm thick, giving an average layer thickness of 0.6 µm deposited per spray pass in all cases. Phase analysis revealed the presence of both the anatase and rutile phases of TiO2 in coatings sprayed with various concentrations at various stand-off distances. More detailed discussion is presented with respect to the effects of the solution concentration, stand-off distance, and number of spray passes on the coating's phase composition and microstructure.

Wetting, infiltration, and interaction behavior of calcium-magnesium-alumino-silicate towards Gd/Yb-modified SrZrO3 coatings deposited by solution precursor plasma spray

Wetting of thermal barrier coatings (TBCs) with calcium-magnesium- alumino-silicate (CMAS) leads to sintering and phase transition, which are major issues in the aerospace industry. We prepared Sr(Zr1−2xYbxGdx)O3−x (x = 0, 0.05, 0.1, and 0.15) coatings using solution precursor plasma spraying with inter-pass boundaries (IPBs) and vertical cracks, and analyzed the CMAS wettability at 1350 °C using the sessile-drop method. The wetting and diffusion dynamics of the CMAS melt on the surface of the coating were studied using a CCD camera, revealing that the Sr(Zr0.7Yb0.15Gd0.15)O2.85 coating had the lowest spreading speed (2.60 × 10−4 mm/s, spreading balance process). Furthermore, a greater extent of crack bending and smaller crack diameter can prevent the coatings from penetration of the CMAS melt.

Hydrocarbon-induced reversible wetting behaviors of hierarchically-structured yttrium oxide coatings

Yttrium oxide is considerably used in protective coatings and functional films in the industry, while the wettability of yttrium oxide surfaces is seldom investigated. In this work, yttrium oxide coatings were deposited via solution precursor plasma spray process using yttrium nitrate solution as feedstock. The phase composition, microstructures, surface topographies, wetting behaviors, and surface compositions of coatings after different post-deposition treatments were characterized comprehensively. The as-sprayed coatings showed dual-scale hierarchical surface structures which were composed of nanometer-sized particles and splats superimposed on micron-sized columnar structures. The as-sprayed coatings were superhydrophilic, whereas they became superhydrophobic after vacuum treatment, exhibiting water contact angles larger than 160°, roll-off angles smaller than 5°, and complete water droplet rebound behaviors. The superhydrophobic coatings also showed high durability in harsh environments which were corroborated by the self-cleaning test, water flushing, and immersion test. The superhydrophobic coatings became superhydrophilic rapidly after high temperature treatment or Ar-plasma treatment, while the coatings regained superhydrophobicity after another vacuum treatment, showing reversible wetting behaviors. The investigations on the surface compositions of coatings with different wettability were conducted, indicating hydrocarbon adsorption on the surface in vacuum treatment and desorption during high temperature treatment or Ar-plasma treatment induced the reversible wetting behaviors.

Effect of Solution Precursor on Microstructure and High-Temperature Properties of the Thermal Barrier Coating Made by Solution Precursor Plasma Spray (SPPS) Process

Effect of Solution Precursor on Microstructure and High-Temperature Properties of the Thermal Barrier Coating Made by Solution Precursor Plasma Spray (SPPS) Process

Preparation and CMAS Wettability Investigation of CMAS Corrosion Resistant Protective Layer with Micro-Nano Double Scale Structure

Solution precursor plasma spray (SPPS) can prepare thermal barrier coatings (TBCs) with nanostructures, which can modify the adhesion and wettability of molten silicate environmental deposits (CMAS) on the surface of TBCs, thereby improving the resistance of TBCs to CMAS corrosion. In this study, SPPS layers with micro-nano double scale structures were prepared on the surface of conventional atmospheric plasma spraying (APS) coatings. The effect of process parameters on the micro-nano double scale structures and the wetting and infiltration behavior of molten CMAS on the surface of coatings were investigated. The results show that micron structure is more sensitive to process parameters. Lower precursor viscosity, closer spraying distance, and smoother APS layer are favorable to form more typical and dense micron structures. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% and the steady-state contact angle increased up to three times. The reason is that the micro-nano double scale structures can effectively trap air and form an air layer between the coating surface and the molten CMAS. In addition, nano-particles play a more important role in the formation of the air layer, which in turn determines the steady-state wettability properties. While micron structures can influence the time needed to reach the steady state. However, the SPPS layers composed of nano-particles have a very loose structure and weak cohesion, and they degrade and fail rapidly after the infiltration of molten CMAS. Therefore, maintaining the excellent CMAS wetting resistance of the SPPS layers while taking into account their lifetime and reliability has become the focus of further research.

YSZ/LSM Composite Cathode Deposited by Solution Precursor Plasma Spraying

In this paper, solution precursor plasma spraying (SPPS) was employed to prepare a porous YSZ/LSM composite cathode for solid oxide fuel cell (SOFC). The surface morphology and microstructure of the composite cathode deposits were characterized using SEM. The effect of annealing treatment on SPPS YSZ/LSM microstructure was examined. The results showed that the as-sprayed YSZ/LSM deposits presented a porous aggregate with a size range of 10–60 μm when the alcohol was used as the solvent and the spraying distance was 60 mm. The porous aggregate was found to be composed mainly of small particles ranging from 0.2–2 μm, the YSZ/LSM composite cathode showed a finely porous microstructure with grain sizes from micrometers to sub-micrometers. A further annealing treatment at 1050 °C for 2 h in air resulted in a continuous microstructure porous coating with a perovskite phase. The polarization test results demonstrated that the minimum polarizations were 1.26 and 0.083 Ω·cm2 for the composite cathode at 800 and 1000 °C, respectively.

Hot corrosion behavior of hybrid double-layered LZ/LZC + YSZ thermal barrier coatings made using powder and solution precursor feedstock

Lanthanum zirconate (La2Zr2O7; LZ) and cerium doped lanthanum zirconate (La2(Zr0.7Ce0.3)2O7; LZC) has gained immense attention as thermal barrier coating (TBC) materials because of their superior thermophysical properties, high-temperature phase stability and chemical inertness than yttria-stabilized zirconia (YSZ). In the present study, LZ and LZC double-layered TBCs are prepared by solution precursor plasma spray process (SPPS) as a top layer over the atmospheric plasma sprayed YSZ coatings, and their hot corrosion behavior is studied. The as-deposited SPPS coatings exhibited single phases, and the microstructural morphologies reveal a hierarchically structured surface with vertically cracked microstructure. Exposure to the high temperature corrosion process results in damage of the morphology and microstructure within the SPPS layer by forming lanthanum vanadate (LaVO4) and lanthanum cerium vanadate ((LaCe)VO4) in LZ and LZC coatings, respectively, while leaching out zirconia (ZrO2). In the case of conventional YSZ, the corrosive salts infiltrate entirely through the coating and result in complete densification and associated phase changes. While combining the above aspects, the hybrid double-layered architecture of LZ/YSZ and LZC/YSZ coatings can be beneficial to protect the underlying YSZ layer against the detrimental infiltration of molten corrosive salt.

Effect of CMAS viscosity on the infiltration depth in thermal barrier coatings of different microstructures

Calcium magnesium aluminum silicate (CMAS) is one of the leading concerns for the gas turbine industry. The effects of CMAS viscosity and the coating microstructure on CMAS infiltration depth were explored by conducting a time dependent interaction study. Three CMAS compositions were used from literature, and their viscosities predicted through FactSage viscosity module were drastically different. The interaction was carried out on three different TBCs synthesized using the solution precursor plasma spray process (SPPS): two of the TBCs were made of yttrium aluminum garnet (YAG) having different microstructures that promote different modes of CMAS infiltration, and one TBC was made of gadolinium zirconate (GZO). All samples had stress relieving vertical cracks and different intensities of horizontally banded porosity, (inter pass boundaries IPBs). A concentration of 100 mg/cm2 of CMAS was applied on the TBCs which were then subjected to a 5-minute interaction at 1300 °C. Samples were analyzed using scanning electron microscopy (SEM), electron dispersive X-ray spectroscopy (EDXS), and transmission electron microscopy (TEM). Low viscosity CMAS readily penetrated the TBCs while more viscous CMAS showed less penetration. The depth of CMAS infiltration depended on the coating microstructure. In the YAG with IPBs, the CMAS spread horizontally in the IPBs before infiltrating deeper, resulting in reduced infiltration depth compared to other samples in spite of having wider vertical cracks. TEM and EDXS analysis were performed to investigate the phases present in the CMAS-TBC interaction region in YAG. Two regions were chosen, the top TBC surface in direct contact with the sea of CMAS, and the region at the CMAS penetration ended within the coating. The results showed that no secondary phases like apatite were observed in YAG, thus it can be concluded that the arrest of CMAS happened solely because of CMAS viscosity and the short infiltration time.

Thermal Cycling Failure of Yb-Gd Co-Doped Srzro3 Coating Prepared by Solution Precursor Plasma Spray

Thermal Cycling Failure of Yb-Gd Co-Doped Srzro3 Coating Prepared by Solution Precursor Plasma Spray

Correction to: Deposition of Columnar-Morphology Lanthanum Zirconate Thermal Barrier Coatings by Solution Precursor Plasma Spraying

Correction to: Deposition of Columnar-Morphology Lanthanum Zirconate Thermal Barrier Coatings by Solution Precursor Plasma Spraying

Phase Composition and Thermal Properties of Yb-Gd Co-Doped SrZrO3 Coating Prepared by the Solution Precursor Plasma Spray

Phase Composition and Thermal Properties of Yb-Gd Co-Doped SrZrO3 Coating Prepared by the Solution Precursor Plasma Spray

A Study on YSZ Abradable Seal Coatings Prepared by Atmospheric Plasma Spray and Mixed Solution Precursor Plasma Spray

The yttria-stabilized zirconia (YSZ) ceramic has been proved to be a promising material for abradable seal coatings. In this research, abradable seal coatings named C-YSZ and M-YSZ were prepared by applying the conventional plasma spraying method with improved nanostructured powders and by the mixed solution precursor plasma spraying technique, respectively. A study on the microstructure, mechanical properties, oxidation resistance and hot corrosion resistance was conducted on both C-YSZ and M-YSZ coatings by using scanning electron microscopy, transmission electron microscopy, muffle furnace and x-ray diffractometer. The results show that the C-YSZ coating presents larger-size pores with denser YSZ structures and grains showing size larger than 100 nm distributed around the pores compared with M-YSZ, in which the pores are much smaller and the grain size is 20-30 nm. As a result, the M-YSZ coating displays lower hardness and bond strength compared with C-YSZ. The oxidation tests show that the M-YSZ coating possesses better oxidation resistance due to the nanostructure topcoat compared with C-YSZ. However, the hot corrosion tests indicate that both the C-YSZ coating and the M-YSZ coating fail to present ideal hot corrosion resistance.

Deposition of Zinc Oxide Coatings on Wood Surfaces Using the Solution Precursor Plasma Spraying Process

In the present work, the solution precursor plasma spray (SPPS) process was used to deposit zinc oxide (ZnO) coatings on wood surfaces using zinc nitrate solution as precursor to improve the hydrophobicity and the color stability of European beech wood under exposure to ultraviolet (UV) light. The surface morphology and topography of the wood samples and the coatings were characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The formation of ZnO was detected with the help of X-ray photoelectron spectroscopy (XPS) and by Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra of the coated samples showed the typical Zn–O band at 445 cm−1. According to the XPS analysis, the coatings consist of two different Zn-containing species: ZnO and Zn(OH)2. Variation of the deposition parameters showed that the most significant parameters affecting the microstructure of the coating were the solution concentration, the deposition scan speed, and carrier gas flow rate. The wettability behaviors of the coated wood were evaluated by measuring the water contact angle (WCA). The coatings that completely covered the wood substrates showed hydrophobic behaviors. UV-protection of wood surfaces after an artificial UV light irradiation was evaluated by color measurements and FTIR spectroscopy. The ZnO-coated wood surfaces were more resistant to color change during UV radiation exposure. The total color change decreased up to 60%. Additionally, the FTIR spectra showed that the wood surfaces coated with ZnO had more stability. The carbonyl groups formation and C=C-bonds consumption were significantly lower.

Photocatalytic behavior of titania coatings fabricated by suspension and solution precursor plasma spray processes

ABSTRACT Titania (TiO2) powder is the most extensively employed photocatalyst for the purification of water. Nonetheless, the usage of conventional powder catalysts in water treatment has the limitation of settling of the catalyst during the reaction and the additional step of separation of the powders from the liquid. To overcome this limitation and extend the application of TiO2 for practical applications, coatings are promising. In the present study, a comparison of the photocatalytic property of titania coatings deposited by suspension plasma spray (SPS) and solution precursor plasma spray (SPPS) methods using similar plasma spray parameters is presented. The coatings are characterized by field emission scanning electron microscopy, profilometry, X-ray diffractometry, and Raman spectroscopy. The photocatalytic performance and efficiency of photodecomposition of the developed coatings are studied using methylene blue as the organic pollutant. The study demonstrates a higher photocatalytic activity and photodecomposition efficiency for the SPPS titania coating compared to SPS titania coatings. The improved performance is attributed to the presence of oxygen vacancies and is corroborated with X-ray photoelectron spectroscopy. The SPPS titania coatings hold promise for industrial-scale applications as it is economical, simple and does not involve multiple steps.