High temperature oxidation behaviour of Nb and HfO2 coatings on ZrB2

ZrB2 has a unique combination of properties such as high melting point (>3000°C) and low theoretical density, high strength and elastic modulus which makes itself a very attractive candidate for ultra-high temperature applications. However, its’ oxidation resistance is poor above 800°C which limits its application for aero-propulsion and hypersonic flight applications. Few studies have shown that the addition of transition metals into the ZrB2 material could improve the oxidation behaviour at high temperatures. In this study, two different materials were applied as oxidation protective coatings by means of magnetron sputtering technique on top of ZrB2 : the transition metal Nb and HfO2. The oxidation studies were performed at 1500 and 1600°C in a box furnace under synthetic air atmosphere for different times. Both coatings have shown promising results and the ZrO2 scale growth was drastically reduced. Formation of mixed oxides comprising of Nb and ZrO2 at the surface has reduced the oxygen transport at the ZrB2 reaction front. Similarly, HfO2 has acted as a barrier to the oxygen transport and a lower oxidation rate was achieved

High Temperature Interaction of Volcanic Ashes with 7YSZ TBC´s produced by APS: Infiltration behavior and phase stability

High-temperature infiltration behavior and phase stability of yttria-stabilized zirconia (7YSZ) thermal barrier coatings (TBC) produced by atmospheric plasma spray interacting with volcanic ashes (VAs) are presented here. Three VAs from the Colima, Popocatepetl, and Eyjafjallajökull volcanoes have been used in this work. Previous to infiltration experiments, physicochemical characterization of the VAs was carried out including thermal analyses by DSC, structural studies by XRD, and ICP chemical composition measurements. TBCs’ infiltration tests were carried out at 1250 °C for different times. Results showed that infiltration depth as a function of time behaves in a non-linear way. Mainly two important infiltration behaviors were identified corresponding to high- and slow-speed infiltration regimes. Higher infiltration kinetics was detected for VAs with lower SiO2 content. The extent of chemical degradation of the 7YSZ is directly related to the silica content. For greater SiO2 values, a higher content of monoclinic ZrO2 was observed leading to maximum values at intermediate annealing times between 2 and 5 h. This behavior can be correlated with the high-speed tetragonal to monoclinic ZrO2 transformation at short times (between 2 and 5 h) until a maximum monoclinic content is reached. After that, the reaction follows by the interplay of ZrSiO4 formation at the expense of previously formed m-ZrO2 together with the incorporation of Y and Zr to the glass melt

Reaction Products from High Temperature Treatments of (LaxGd1x)2Zr2O7 System and Volcanic Ash Powder Mixtures

Rare-earth zirconate (REZ) pyrochlores, specifically (La,Gd)2Zr2O7, are promising alternatives to substitute 7YSZ in thermal barrier coatings (TBC) due to their increased phase stability and resistance to calcium-magnesium-aluminosilicate (CMAS)-based damage at high temperatures. This work analyses the reaction products of isothermal treatments at 1250°C of (LaxGd1−x)2Zr2O7 powder mixtures in reaction with natural volcanic ashes (VAs) with different chemical compositions (Popocatepetl, Colima, and Eyjafjallajökull). The chemical composition in the (LaxGd1−x)2Zr2O7 system showed a strong influence on the formation of reprecipitation products (Zr,Gd,La oxides), reactive crystallization products (La-Gd apatites, lanthanum silicates, zirconolite, and garnets), and intrinsic crystallization products (hematite-like structures). The formation of garnets, which enhances the mitigation effect against molten VAs/Si deposit damage, was observed more promptly on (La0.5Gd0.5)2Zr2O7 promoted by high contents of Fe, Ca, Ti, and Mg on VAs. Thus, the (La0.5Gd0.5)2Zr2O7 system stands as a possible candidate to overcome today’s threats for the integrity of current TBCs

Evaluation of the reactivity of dense lanthanum‑gadolinium zirconate ceramics with Colima volcanic ashes

The effects of ingestion of airborne particles from pyroclastic events of active volcanoes by aircraft turbines and their subsequent reaction with thermal barrier coatings have attracted the attention of the scientific community in recent years. The reaction products of infiltration experiments of lanthanum‑gadolinium zirconate (LGZO) ceramics with molten ashes from the active Colima volcano at 1250 °C for 10 h are presented and discussed as a function of the Gd3+ content. Five ceramic compositions, varying the Gd3+ content in solid solution were synthesized by the chemical coprecipitation and calcination method of pressed powders. These compositions include pure lanthanum and gadolinium zirconates, LZO, and GZO, respectively. Penetration depth and identification, and in some cases quantification of the reaction products between the molten ash and LGZO ceramics were performed by scanning electron microscopy, chemical composition with energy dispersive X-ray spectroscopy, grazing incident X-ray diffraction as well as micro-Raman spectroscopy. The LZO ceramic exhibited the greatest infiltration resistance with an infiltration depth of approximately 23 μm from the surface. The phase characteristics of the reaction layers were dependent on the gadolinium content. LZO, LGZO25, and LGZO50 (x = 0, 0.25, and 0.5) showed the presence of apatite as well as monoclinic and tetragonal zirconia, while LGZO75 and GZO (x = 0.75 and 1), additionally showed the presence of cubic zirconia and anorthite. As the Gd3+ content increases in the LGZO solid solutions, the wavenumber value corresponding to the stretching vibrational mode of the silicon tetrahedra in apatite shifts from 862 to 877 cm−1, which is associated with a decrease in Sisingle bondO bond lengths. These findings indicate that the amount and kind of rare earth cations dissolved in the melt plays an important role in the precipitation of the reaction products

What's in a Name? A Cost-Effectiveness Analysis of the Noninvasive Follicular Thyroid Neoplasm with Papillary-Like Nuclear Features' Nomenclature Revision.

Background: The noninvasive subtype of encapsulated follicular variant of papillary thyroid carcinoma (eFVPTC) has been reclassified as noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) in 2016 to reflect the indolent behavior and favorable prognosis of this type of tumor. This terminology change has also de-escalated its management approach from cancer treatment to a more conservative treatment strategy befitting a benign thyroid neoplasm. Objective: To characterize the reduced health care costs and improved quality of life (QOL) from management of NIFTP as a nonmalignant tumor compared with the previous management as eFVPTC. Methods: A cost-effectiveness analysis was performed by creating Markov models to simulate two management strategies for NIFTP: (i) de-escalated management of the tumor as NIFTP involving lobectomy with reduced follow-up, (ii) management of the tumor as eFVPTC involving completion thyroidectomy/radioactive iodine ablation for some patients, and follow-up recommended for carcinoma. The model was simulated for 5 and 20 years following diagnosis of NIFTP. Aggregate costs and quality-life years were measured. One-way sensitivity analysis was performed for all variables. Results: Over a five-year simulation period, de-escalated management of NIFTP had a total cost of $12,380.99 per patient while the more aggressive management of the tumor as eFVPTC had a total cost of$16,264.03 per patient (saving $3883.05 over five years). Management of NIFTP provided 5.00 quality-adjusted life years, whereas management as eFVPTC provided 4.97 quality-adjusted life years. Sensitivity analyses showed that management of NIFTP always resulted in lower costs and greater quality-adjusted life years (QALYs) over the sensitivity ranges for individual variables. De-escalated management for NIFTP is expected to produce ∼$6-42 million in cost savings over a five-year period for these patients, and incremental 54-370 QALYs of increased utility in the United States. Conclusion: The degree of cost savings and improved patient utility of de-escalated NIFTP management compared with traditional management was estimated to be $3883.05 and 0.03 QALYs per patient. We demonstrate that these findings persisted in sensitivity analysis to account for variability in recurrence rate, surveillance approaches, and other model inputs. These findings allow for greater understanding of the economic and QOL impact of the NIFTP reclassification