Crack-Growth Behavior in Thermal Barrier Coatings with Cyclic Thermal Exposure

Crack-growth behavior in yttria-stabilized zirconia-based thermal barrier coatings (TBCs) is investigated through a cyclic thermal fatigue (CTF) test to understand TBCs’ failure mechanisms. Initial cracks were introduced on the coatings’ top surface and cross section using the micro-indentation technique. The results show that crack length in the surface-cracked TBCs grew parabolically with the number of cycles in the CTF test. Failure in the surface-cracked TBC was dependent on the initial crack length formed with different loading levels, suggesting the existence of a threshold surface crack length. For the cross section, the horizontal crack length increased in a similar manner as observed in the surface. By contrast, in the vertical direction, the crack did not grow very much with CTF testing. An analytical model is proposed to explain the experimentally-observed crack-growth behavior

Crack-Resistance Behavior of an Encapsulated, Healing Agent Embedded Buffer Layer on Self-Healing Thermal Barrier Coatings

In this work, a novel thermal barrier coating (TBC) system is proposed that embeds silicon particles in coating as a crack-healing agent. The healing agent is encapsulated to avoid unintended reactions and premature oxidation. Thermal durability of the developed TBCs is evaluated through cyclic thermal fatigue and jet engine thermal shock tests. Moreover, artificial cracks are introduced into the buffer layer’s cross section using a microhardness indentation method. Then, the indented TBC specimens are subject to heat treatment to investigate their crack-resisting behavior in detail. The TBC specimens with the embedded healing agents exhibit a relatively better thermal fatigue resistance than the conventional TBCs. The encapsulated healing agent protects rapid large crack openings under thermal shock conditions. Different crack-resisting behaviors and mechanisms are proposed depending on the embedding healing agents

Microstructure design for blended feedstock and its thermal durability in lanthanum zirconate based thermal barrier coatings

The effects of microstructure design on the lifetime performance of lanthanum zirconate (La2Zr2O7; LZO)-based thermal barrier coatings (TBCs) were investigated through various thermal exposure tests, such as furnace cyclic thermal fatigue, thermal shock, and jet engine thermal shock. To improve the thermal durability of LZO-based TBCs, composite top coats using two feedstock powders of LZO and 8 wt.% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing in different volume ratios (50:50 and 25:75, respectively). In addition, buffer layers were introduced in layered LZO-based TBCs deposited using an air-plasma spray method. The TBC with the double buffer layer showed the best thermal cycle performance among all samples in all tests. For applications with relatively slow cooling rates, the thermal durability in single-layer TBCs is more effectively enhanced by controlling a composition ratio in the blended powder, better than introducing a single buffer layer. For applications with relatively fast cooling rates, the thermal durability can be effectively improved by introducing a buffer layer than controlling a composition in the top coat, since the buffer layer provides fast localized stress relief due to its high strain compliance. These research findings allow us to control the TBC structure, and the buffer layer is efficient in improving thermal durability in cyclic thermal environments

Thermal durability and fracture behavior of layered Yb-Gd-Y-based thermal barrier coatings in thermal cyclic exposure

The effects of structural design on the thermal durability and fracture behavior of Yb-Gd-Y-based thermal barrier coatings (TBCs) were investigated through thermal cyclic exposure tests, such as furnace cyclic thermal fatigue (FCTF) and jet engine thermal shock (JETS) tests. The effects of composition in the bond coat and feedstock purity for the buffer layer on its lifetime performance were also examined. To overcome the drawbacks of Yb-Gd-Y-based material with inferior thermal durability due to poor mechanical properties and low coefficient of thermal expansion, a buffer layer was introduced in the Yb-Gd-Y-based TBC systems. In FCTF tests, the TBCs with the buffer layer showed a longer lifetime performance than those without the buffer layer, showing the longest thermal durability in the TBC with the Co-Ni-based bond coat and the buffer layer of regular purity. In JETS tests, the TBC with the Ni-based bond coat and the buffer layer of high purity showed a sound condition after 2000 cycles, showing better thermal durability for TBC with the Co-Ni-based bond coat rather than that with the Ni-based bond coat in the single layer coating without the buffer layer. The buffer layer effectively enhanced the thermal durability in slow temperature change (in the FCTF test), while the bond-coat composition and the feedstock purity for the buffer layer were found to be important factor to improve the thermal durability of the TBC in fast temperature change (in the JEET test). Finally, these research findings allow us to control the structure, composition, and feedstock purity in TBC system for improving the thermal durability in cyclic thermal environments

Numerical analysis on the dynamic response of a plate-and-frame membrane humidifier for PEMFC vehicles under various operating conditions

PEMFC needs to be maintained at an appropriate temperature and humidity in a rapidly changing environment for automobile applications. In this study, a pseudo-multi-dimensional dynamic model for predicting the heat and mass transfer performance of a plate-and-frame membrane humidifier for PEMFC vehicles is developed. Based on the developed model, the variations in the temperature and relative humidity at the dry air outlet are investigated according to the air flow acceleration. Moreover, the dynamic response is analyzed as a function of the amplitude and period of the sinusoidal air flow rate at actual operating conditions. The effects of heat transfer on the dynamic response are more dominant than those of mass transfer. The settling time of the temperature and relative humidity at the dry air outlet decrease with the increase in air flow acceleration. In addition, the variations in the temperature and relative humidity at the dry air outlet increase with the increases in the amplitude and period of the sinusoidal air flow rate

Low Leakage in High‐k Perovskite Gate Oxide SrHfO3

Abstract Reducing the leakage current through the gate oxide is becoming increasingly important for power consumption reduction as well as reliability in integrated circuits as the semiconducting devices continue to scale down. Here, this work reports on the high‐k dielectric SrHfO3 (SHO) based devices with ultralow leakage current density via pulsed laser deposition (PLD). The ultralow current density is achieved by optimizing the growth conditions and the associated structural properties. In the optimized conditions, the dielectric properties of the 50‐nm‐thick SHO capacitors are measured: high dielectric constant (κ = 32), low leakage current density ( 4 MV cm−1). The surprisingly low leakage current density of SHO is ascribed to the large bandgap (≈6 eV), the large conduction band offset (CB offset > 3 eV) with respect to the semiconductor, and the low density of defect states inside the bandgap. The optimized SHO dielectric with high dielectric constant and ultralow leakage current density is proposed for future low‐power consumption devices based on Si as well as perovskite oxide semiconductors

Synthesis of a hybrid nanostructure of ZnO-decorated MoS₂ by atomic layer deposition

We introduce the synthesis of hybrid nanostructures comprised of ZnOnanocrystals (NCs) decorating nanosheets and nanowires (NWs) of MoS2prepared byatomic layer deposition (ALD). The concentration, size, and surface-to-volume ratio ofthe ZnO NCs can be systematically engineered by controlling both the number of ZnOALD cycles and the properties of the MoS2substrates, which are prepared bysulfurizing ALD MoO3. Analysis of the chemical composition combined with electronmicroscopy and synchrotron X-ray techniques as a function of the number of ZnO ALDcycles, together with the results of quantum chemical calculations, help elucidate theZnO growth mechanism and its dependence on the properties of the MoS2substrate.The defect density and grain size of MoS2nanosheets are controlled by thesulfurization temperature of ALD MoO3, and the ZnO NCs in turn nucleate selectivelyat defect sites on MoS2surface and enlarge with increasing ALD cycle numbers. Athigher ALD cycle numbers, the coalescence of ZnO NCs contributes to an increase inareal coverage and NC size. Additionally, the geometry of the hybrid structures can betuned by changing the dimensionality of the MoS2, by employing vertical NWs of MoS2as the substrate for ALD ZnO NCs,which leads to improvement of the relevant surface-to-volume ratio. Such materials are expected tofind use in newly expandedapplications, especially those such as sensors or photodevices based on a p−n heterojunction which relies on couplingtransition-metal dichalcogenides with NCs.Accepted Author ManuscriptChemE/Materials for Energy Conversion & Storag

Synthesis of a Hybrid Nanostructure of ZnO-Decorated MoS₂ by Atomic Layer Deposition

We introduce the synthesis of hybrid nanostructures comprised of ZnOnanocrystals (NCs) decorating nanosheets and nanowires (NWs) of MoS2prepared byatomic layer deposition (ALD). The concentration, size, and surface-to-volume ratio ofthe ZnO NCs can be systematically engineered by controlling both the number of ZnOALD cycles and the properties of the MoS2substrates, which are prepared bysulfurizing ALD MoO3. Analysis of the chemical composition combined with electronmicroscopy and synchrotron X-ray techniques as a function of the number of ZnO ALDcycles, together with the results of quantum chemical calculations, help elucidate theZnO growth mechanism and its dependence on the properties of the MoS2substrate.The defect density and grain size of MoS2nanosheets are controlled by thesulfurization temperature of ALD MoO3, and the ZnO NCs in turn nucleate selectivelyat defect sites on MoS2surface and enlarge with increasing ALD cycle numbers. Athigher ALD cycle numbers, the coalescence of ZnO NCs contributes to an increase inareal coverage and NC size. Additionally, the geometry of the hybrid structures can betuned by changing the dimensionality of the MoS2, by employing vertical NWs of MoS2as the substrate for ALD ZnO NCs,which leads to improvement of the relevant surface-to-volume ratio. Such materials are expected tofind use in newly expandedapplications, especially those such as sensors or photodevices based on a p−n heterojunction which relies on couplingtransition-metal dichalcogenides with NCs

PVDF-stimulated surface engineering in ZnO for highly sensitive and water-stable hydrazine sensors

Sensors based on multifunctional n-type metal oxide semiconductors are attracting significant interest in environmental monitoring owing to their distinct characteristics including low production cost, high detection response to different noxious analytes, nontoxic nature, and acceptable biocompatibility. Herein, we present an innovative approach that utilizes surface functionalization on ZnO thin-film transistor (TFT)-type sensors with a fluompolymer, poly (vinylidene fluoride-co-hexafluoropmpylene) (PVDF-HFP) to realize highly sensitive and water-stable liquid-phase sensors. ZnO sensors laminated with PVDF-HFP thin films demonstrate exceptional repeatable stability to DI water and liquid-phase hydrazine, indicating excellent sensitivity in addition to low hydrazine-detection limits approaching 0.01 nM (sub-ppt level) under ambient conditions. This detection limit is five orders of magnitude less than that of the legal limit for an 8 h exposure time-weighted average for hydrazine. Moreover, relatively acceptable repeatability and reproducibility of the sensors were guaranteed over 96% of their initial base current with hydrazine for a month. This outstanding sensing performance is attributed to the enhanced surface interaction between PVDF-HFP with a strong dipole moment and hydrazine, which is completely discriminated from the universal detection mechanism associated with oxygen ion species in ZnO

Potential of NiO_x/Nickel Silicide/n⁺ Poly-Si Contact for Perovskite/TOPCon Tandem Solar Cells

In this work, nickel silicide was applied to tandem solar cells as an interlayer. By the process of thermal evaporation, a layer of NiOx, hole transport layer (HTL) was deposited on n+ poly-Si layer directly. Nickel silicide was simultaneously formed by nickel diffusion from NiOx to n+ poly-Si layer during the deposition and annealing process. The I–V characteristics of NiOx/n+ poly-Si contact with nickel silicide showed ohmic contact and low contact resistivity. This structure is expected to be more advantageous for electrical connection between perovskite top cell and TOPCon bottom cell compared to the NiOx/TCO/n+ poly-Si structure showing Schottky contact. Furthermore, nickel silicide and Ni-deficient NiOx thin film formed by diffusion of nickel can improve the fill factor of the two sub cells. These results imply the potential of a NiOx/nickel silicide/n+ poly-Si structure as a perovskite/silicon tandem solar cell interlayer