Guan-Jun Yang

Development of Particle Interface Bonding in Thermal Spray Coatings: A Review

Thermal spray ceramic coatings deposited following the conventional routine exhibit a typical lamellar structure with a limited interface bonding ratio. The bonding between particles in the coating dominates coating properties and performance. In this review paper, the bonding formation at the interface between thin lamellae in the coating is examined. The effect of spray parameters on the bonding ratio is presented to reveal the main droplet parameters controlling bonding formation, which reveals that the temperature of the spray particle rather than its velocity dominates the bonding formation. The limitation to increase significantly the ceramic particle temperature inherent to the thermal spray process leads to the observation of a maximum bonding ratio of about 32%, while through controlling the surface temperature of the coating prior to molten droplet impact, the bonding at the lamellar interface can be significantly increased. Consequently, it is shown that with the proper selection of deposition conditions and control of the deposition temperature, the bonding ratio of ceramic deposits can be altered from a maximum of 32% for a conventional deposit to a maximum of 100%. Such wide adjustability of the lamellar bonding opens new possibilities for using thermal spray coatings in various applications requiring different microstructures and properties. The examination of recent studies shows that the bonding control makes it possible to fabricate porous deposits through surface-molten particles. Such an approach could be applied for the fabrication of porous materials, the deposition of high temperature abradable ceramic coatings, and for forming functional structured surfaces, such as a surface with super-hydrophobicity or a solid oxide fuel cell cathode interface with high specific surface area and high catalytic performance. Furthermore, complete interface bonding leads to crystalline structure control of individual splats through epitaxial grain growth.

Facile and Scalable Fabrication of Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells in Air Using Gas Pump Method

Control of the perovskite film formation process to produce high-quality organic-inorganic metal halide perovskite thin films with uniform morphology, high surface coverage, and minimum pinholes is of great importance to highly efficient solar cells. Herein, we report on large-area light-absorbing perovskite films fabrication with a new facile and scalable gas pump method. By decreasing the total pressure in the evaporation environment, the gas pump method can significantly enhance the solvent evaporation rate by 8 times faster and thereby produce an extremely dense, uniform, and full-coverage perovskite thin film. The resulting planar perovskite solar cells can achieve an impressive power conversion efficiency up to 19.00% with an average efficiency of 17.38 ± 0.70% for 32 devices with an area of 5 × 2 mm, 13.91% for devices with a large area up to 1.13 cm(2). The perovskite films can be easily fabricated in air conditions with a relative humidity of 45-55%, which definitely has a promising prospect in industrial application of large-area perovskite solar panels.

Formation of nanostructured TiO2 by flame spraying with liquid feedstock

Abstract A liquid feedstock flame spraying system was developed to deposit the nanostructured TiO2 coating using butyl titanate (Ti(OC4H9)4) and ethanol solution as precursor. The resultant deposit was characterized by infrared spectrum (IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that during spraying, butyl titanate had been completely decomposed and converted into nanostructured anatase TiO2, having a narrow size distribution from 10 to 20 nm.

Relationship Between Lamellar Structure and Elastic Modulus of Thermally Sprayed Thermal Barrier Coatings with Intra-splat Cracks

The elastic modulus of plasma-sprayed top coating plays an important role in thermal cyclic lifetime of thermally sprayed thermal barrier coatings (TBCs), since the thermal stress is determined by the substrate/coating thermal mismatch and the elastic modulus of top coating. Consequently, much attention had been paid to understanding the relationship between elastic modulus and lamellar structure of top coating. However, neglecting the intra-splat cracks connected with inter-splat pores often leads to poor prediction in in-plane modulus. In this study, a modified model taking account of intra-splat cracks and other main structural characteristics of plasma-sprayed yttria-stabilized zirconia coating was proposed. Based on establishing the relationship between elastic modulus and structural parameters of basic unit, effects of structural parameters on the elastic modulus of coatings were discussed. The predicted results are well consistent with experimental data on coating elastic modulus in both out-plane direction and in-plane direction. This study would benefit the further comprehensive understanding of failure mechanism of TBCs in thermal cyclic condition.

Phase formation of nano-TiO2 particles during flame spraying with liquid feedstock

The nanostructured TiO2 photocatalytic coatings were synthesized through flame spraying with liquid feedstock under different conditions. The nanostructured TiO2 deposit of substantial anatase phase was annealed at different temperatures. X-ray diffraction analysis showed that significant transformation from anatase to rutile occurred at a temperature above 600 °C. However, thermal analysis suggested that the phase transformation from anatase to rutile started at a temperature from 400 to 500°C. It was found that the grain size of rutile phase was larger than that of anatase. The deposits annealed at temperatures lower than 450°C were photocatalytically active. However, the deposit annealed at 500°C, which contained 95% anatase crystalline, became photocatalytically inactive. Based on the experimental findings, a model is proposed to explain the phase transformation of the nano-TiO2 particles and the phase formation in flame-spraying of nanostructured TiO2 deposit with liquid feedstock.

Material nucleation/growth competition tuning towards highly reproducible planar perovskite solar cells with efficiency exceeding 20%

Since there are still challenges in fabricating high-quality large-area perovskite films, perovskite solar cells have limitations for industrial application. Here, we develop a material nucleation/growth competition theory to guide us to tune the nucleation and grain growth process of perovskite. Subsequently, we introduce a gas-flow-induced gas pump approach for the large-area deposition of dense, uniform and full-coverage perovskite films, and this process is simple with high manufacturing efficiency and a wide process window. This enabled us to fabricate uniform perovskite films on substrates with the largest area of up to 144 cm2. Normal planar perovskite solar cells were fabricated at pressures of 100 Pa, 500 Pa and 1500 Pa, achieving average efficiencies of 19.25 ± 0.50%, 19.17 ± 0.46% and 18.98 ± 0.51% respectively for 0.1 cm2 devices (84 devices in total) with ultrahigh reproducibility. A high fill factor of up to 80% was obtained at different pressures. A champion cell with an efficiency of 20.44% was obtained which is one of the highest efficiencies for normal planar perovskite solar cells. Furthermore, we achieved an efficiency of 17.03%, the highest efficiency for normal perovskite solar cells with the device area exceeding 1 cm2 and an average efficiency of 15.63 ± 0.80% with an area of 1.1275 cm2 (for 30 devices).

Preparation of flexible perovskite solar cells by a gas pump drying method on a plastic substrate

A uniform and full coverage perovskite film is of significant importance for flexible perovskite solar cells. In this study, highly efficient flexible perovskite solar cells were assembled using a flexible conductive plastic substrate by a one-step gas pump drying method to prepare high performance perovskite films under air conditions. The SEM results show that the perovskite film deposited on the flexible conductive plastic substrate was uniform, compact, and pinhole-free. The AFM results show that the film presented an extremely smooth surface morphology with a root mean square roughness of 15.8 nm in a large and representative scan area of 18 × 18 μm2. The flexible planar perovskite solar cell was fabricated, and all devices were prepared at 100 °C or below under air conditions. The highest efficiency on flexible substrates had reached 11.34% with an average efficiency of 8.93% for 14 solar cell devices.

Atmospheric plasma-sprayed La0.8Sr0.2Ga0.8Mg0.2O3 electrolyte membranes for intermediate-temperature solid oxide fuel cells

La0.8Sr0.2Ga0.8Mg0.2O3 (LSGM) is considered a promising electrolyte material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) because of its high ionic conductivity. However, a main challenge in the application of LSGM is how to fabricate dense and thin LSGM membranes on electrode substrates at relatively low temperature (it is difficult to sinter LSGM to full density below 1500 °C). In this study, we report our findings on the preparation of thin LSGM electrolyte membranes using the low-cost atmospheric plasma spraying (APS) process. The phase composition, microstructure, and ionic conductivity of LSGM membranes deposited on an anode substrate depend sensitively on the particle size of LSGM powders because gallium (Ga) may evaporate during the APS process. When the particle size is 80% inter-lamellar bonding ratio), as referred from the thermal conductivity measurement of the LSGM deposit (>80% of the bulk thermal conductivity). Using LSGM powders with particle sizes >30 μm, we have fabricated LSGM membranes having ionic conductivity of ∼0.075 S cm−1 at 800 °C, ∼78% of the bulk value. Test cells based on plasma sprayed LSGM electrolyte membranes show excellent performance at 600–800 °C, suggesting that atmospheric plasma spraying is a promising approach for large-scale manufacturing of high-performance IT-SOFCs.

A Novel Plasma-Sprayed Durable Thermal Barrier Coating with a Well-Bonded YSZ Interlayer Between Porous YSZ and Bond Coat

Atmospheric plasma-sprayed YSZ (yttria-stabilized zirconia) thermal barrier coatings (TBCs) are widely used in industrial gas turbine engines to protect the superalloy blades from failure. The failure of TBCs in service occurs by the spalling of YSZ coating. Crack propagation leading to the failure of plasma-sprayed TBCs usually occurs within the YSZ coating near the YSZ/bond coat interface. In the present study, a novel durable TBC consisting of a YSZ interlayer of well-bonded lamellae between the bond coat and the conventional YSZ porous top coat was introduced. The YSZ interlayer was deposited at different coating surface temperatures, which resulted in the formation of YSZ with significantly improved interlamellar bonding. The result shows that the thermal cyclic lifetime of the novel TBCs with the 20-30-μm-thick YSZ interlayer increased by a factor of 4 compared with that of the conventional one. The improved thermal cyclic lifetime was attributed to the controlled transition of the cracking path from near the YSZ/bond coat interface to the YSZ top layer. The effect of the YSZ interlayer thickness on the lifetime of TBCs was also investigated.

Large-area high-efficiency perovskite solar cells based on perovskite films dried by the multi-flow air knife method in air

Perovskite solar cells are extremely promising high-efficiency low-cost photovoltaic devices. However, large-area and uniform perovskite film fabrication in air is still a big challenge for the mass production of highly efficient perovskite solar cells. Here, we first report the novel multi-flow air knife (MAK) method to control the formation of large-area perovskite films in ambient air. The proposed MAK method, based on the comprehensive understanding of the solution film evaporation process, entails the use of multiple gas flows that rapidly dry the solution and enable the production of large-area perovskite films in air. By adopting the MAK technology, high-quality films with full surface coverage and a low surface roughness of 4.98 nm over a 10 × 10 μm2 scan area were successfully fabricated. Highly reproducible power conversion efficiencies up to 11.70% were achieved with an active area of 1.0 cm2. The highest efficiency of 17.71%, obtained for a device with an area of 0.1 cm2, further demonstrated the promising potential of this technology. Thus, the MAK technology enables the low-cost fabrication of highly efficient perovskite solar cells, paving the way for the mass production of low-cost high-performance perovskite solar cells in ambient air.