Nitin P. Padture

Ceramic materials for thermal barrier coatings

A method for identifying ceramics suitable for use as thermal barrier coatings is presented, based on parameters associated with thermal conductivity, oxygen diffusivity, thermal expansion coefficient, maximum temperature capability, hardness, elastic modulus, density, and chemical reactivity. A ceramic thermal barrier coating and method of manufacture is further presented, the ceramic comprising yttrium aluminum garnet (Y3 Al5 O12, or YAG)-based ceramics. Such ceramics are based on yttrium aluminum garnet or other ceramics with the garnet structure and alloys thereof. The ceramics in accordance with the present invention have low thermal conductivity, and are more potentially durable than prior art zirconia based ceramics.

Functionally Graded Materials

In the paper,the origin,exploitative flow,current status of material design and prospect of functionally graded materials (FGM) are introduced.The adopting method of FGM design,FGM synthesizing and thermal and mechanical evaluation are discussed in detail.Finally,the developing directions of FGM are analysed.

Thermal conductivity of dense and porous yttria-stabilized zirconia

The thermal conductivity of dense and porous yttria-stabilized zirconia (YSZ) ceramics has been measured as a function of temperature in the range 25 to 1000 °C. The dense specimens were either single crystal (8 mol% YSZ) or sintered polycrystalline (3 mol% and 8 mol% YSZ). The porous specimens (3 mol% YSZ) were prepared using the “fugitive” polymer method, where different amounts of polymer spheres (of two different average sizes) were included in the starting powders before sintering. This method yielded materials with uniformly distributed porosities with a tight pore-size distributions. A theory has been developed to describe the thermal conductivity of dense YSZ as a function of temperature. This theory considers the reduction in the intrinsic thermal conductivity due scattering of phonons by point defects (oxygen vacancies and solute) and by the “hopping” of oxygen vacancies. It also considers an increase in the effective thermal conductivity at high temperatures due to radiation. This theory captures the essential features of the observed thermal conductivity. The Maxwell theory has been used to analyze the thermal conductivity of the porous materials. An adequate agreement was found between the theory and experiment.

Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films.

A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelectric, almost nothing is known about their ferroelectric properties experimentally. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelectric domains in high-quality β-CH3NH3PbI3 perovskite thin films that have been synthesized using a new solution-processing method. The size of the ferroelectric domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelectric domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelectric behavior of hybrid perovskites, in particular in situ photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics.

Failure modes in plasma-sprayed thermal barrier coatings

Commercial plasma-sprayed thermal barrier coatings (TBCs) were investigated in an effort to elucidate the failure modes during thermal-cycling. Residual stresses in the thermally grown oxide (TGO) was measured using the Cr3+ photoluminescence piezo-spectroscopy (PLPS) method and the microstructures of the TBCs were characterized as a function of thermal cycles. The average residual stress in the TGO was found to be of the order of 1 GPa. The average thermal-cyclic life of the TBCs was found to be ∼350 cycles. Microstructural observations revealed that as the TGO thickened, cracking occurred at the bond-coat/TGO interface, and in some instances cracking also occurred at the TGO/top-coat interface, but primarily at crests of bond-coat undulations. The bond-coat-TGO separation resulted in ‘layering’ of the TGO at crests due to enhanced TGO thickening in those regions. In the troughs of bond-coat undulations, cracking occurred within the top-coat when the TGO was thick. Thus, the primary failure modes in these TBCs were: (i) cracking of the bond-coat/TGO interface; (ii) cracking within the top-coat; and (iii) linking of these microcracks by fracture of the TGO. A semi-quantitative failure model has been used to rationalize some of the observed cracking modes. Based on this analysis some suggestions are made for improving TBC durability.

TOWARDS DURABLE THERMAL BARRIER COATINGS WITH NOVEL MICROSTRUCTURES DEPOSITED BY SOLUTION- PRECURSOR PLASMA SPRAY

The feasibility of a new processing method—solution precursor plasma spray (SPPS)—for the deposition of ZrO2-based thermal barrier coatings (TBCs) with novel structures has been demonstrated. These desirable structures in the new TBCs appear to be responsible for their improved thermal cycling life relative to conventional plasma-sprayed TBCs. Preliminary results from experiments aimed at understanding the SPPS deposition mechanisms suggest that nanometer-scale particles form in the plasma flame, followed by their deposition by sintering onto the substrate in the intense heat of the plasma flame. The SPPS method, which offers some unique advantages over the conventional plasma-spray process, is generic in nature and can be potentially used to deposit a wide variety of ceramic coatings for diverse applications.

Room-temperature crystallization of hybrid-perovskite thin films via solvent–solvent extraction for high-performance solar cells

The room-temperature solvent–solvent extraction (SSE) concept is used for the deposition of hybrid-perovskite thin films over large areas. In this simple process, perovskite precursor solution is spin-coated onto a substrate, and instead of the conventional thermal annealing treatment, the coated substrate is immediately immersed in a bath of another solvent at room temperature. This results in efficient extraction of the precursor-solvent and induces rapid crystallization of uniform, ultra-smooth perovskite thin films. The mechanisms involved in the SSE process are studied further, and its versatility in depositing high quality thin films of controlled thicknesses (20 to 700 nm) and various compositions (CH3NH3PbI(3−x)Brx; x = 0, 1, 2, or 3) is demonstrated. Planar perovskite solar cells (PSCs) based on SSE-deposited CH3NH3PbI3 perovskite thin films deliver power conversion efficiency (PCE) up to 15.2%, and most notably an average PCE of 10.1% for PSCs with sub-100 nm semi-transparent perovskite thin films. The SSE method has generic appeal, and its key attributes—room-temperature process, rapid crystallization, large-area uniform deposition, film-thickness control, ultra-smoothness, and compositional versatility—make the SSE method potentially suitable for roll-to-roll scalable processing of hybrid-perovskite thin films for future multifunctional PSCs.

Methylamine-Gas-Induced Defect-Healing Behavior of CH3NH3PbI3 Thin Films for Perovskite Solar Cells

We report herein the discovery of methylamine (CH3NH2) induced defect-healing (MIDH) of CH3NH3PbI3 perovskite thin films based on their ultrafast (seconds), reversible chemical reaction with CH3NH2 gas at room temperature. The key to this healing behavior is the formation and spreading of an intermediate CH3NH3PbI3⋅xCH3NH2 liquid phase during this unusual perovskite-gas interaction. We demonstrate the versatility and scalability of the MIDH process, and show dramatic enhancement in the performance of perovskite solar cells (PSCs) with MIDH. This study represents a new direction in the formation of defect-free films of hybrid perovskites.

Square‐Centimeter Solution‐Processed Planar CH3NH3PbI3 Perovskite Solar Cells with Efficiency Exceeding 15%

The preparation of uniform, high-crystallinity planar perovskite films with high-aspect-ratio grains over a square-inch area is demonstrated. The best power conversion efficiency (PCE) of 16.3% (stabilized output of ≈15.6%) is obtained for a planar perovskite solar cell (PSC) with 1.2 cm2 active area, and the PCE jumps to 18.3% (stabilized output of ≈17.5%) for a PSC with a 0.12 cm2 active area.

Chronic Fine Particulate Matter Exposure Induces Systemic Vascular Dysfunction via NADPH Oxidase and TLR4 Pathways

Rationale: Chronic exposure to ambient air-borne particulate matter of <2.5 &mgr;m (PM2.5) increases cardiovascular risk. The mechanisms by which inhaled ambient particles are sensed and how these effects are systemically transduced remain elusive. Objective: To investigate the molecular mechanisms by which PM2.5 mediates inflammatory responses in a mouse model of chronic exposure. Methods and Results: Here, we show that chronic exposure to ambient PM2.5 promotes Ly6Chigh inflammatory monocyte egress from bone-marrow and mediates their entry into tissue niches where they generate reactive oxygen species via NADPH oxidase. Toll-like receptor (TLR)4 and Nox2 (gp91phox) deficiency prevented monocyte NADPH oxidase activation in response to PM2.5 and was associated with restoration of systemic vascular dysfunction. TLR4 activation appeared to be a prerequisite for NAPDH oxidase activation as evidenced by reduced p47phox phosphorylation in TLR4 deficient animals. PM2.5 exposure markedly increased oxidized phospholipid derivatives of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphorylcholine (oxPAPC) in bronchioalveolar lavage fluid. Correspondingly, exposure of bone marrow–derived macrophages to oxPAPC but not PAPC recapitulated effects of chronic PM2.5 exposure, whereas TLR4 deficiency attenuated this response. Conclusions: Taken together, our findings suggest that PM2.5 triggers an increase in oxidized phospholipids in lungs that then mediates a systemic cellular inflammatory response through TLR4/NADPH oxidase–dependent mechanisms.