RESUMEN
A flexible, dense, defect-free, highly adhesive, and highly dissociation energy-rich protective coating is essential to enhance the atomic oxygen (AO) resistance of polymeric materials in a low Earth orbit (LEO). In this work, a dense, defect-free hybrid HMDSO/SiO2 thin film coating with compositional gradients on the surface of polyimide was synthesized using vacuum-ultraviolet (VUV) irradiation. The effects of VUV irradiation on the morphology, optical transmittance, and chemical components of plasma-polymerized HMDSO (pp-HMDSO) thin-film coatings deposited on the polyimide surface were investigated in depth. There were no defects such as cracks and holes in the surface morphology of pp-HMDSO films after VUV irradiation, but the surface roughness increased slightly, and the corresponding optical transmittance decreased slightly. The chemical components of pp-HMDSO films were changed in the depth direction starting from the top of the surface, forming hybrid HMDSO/SiO2 thin films with compositional gradients. The component gradient HMDSO/SiO2 composite coating further enhanced the atomic oxygen resistance of the polyimide due to the surface layer of the UV-modified coating enriched with high dissociation energy SiOx material. Therefore, this work provides a facile UV-induced synthesis method to prepare dense, defect-free, and highly dissociation energy-rich protective gradient coatings, which are promising not only for excellent AO protection in LEO but also for potential application in water-oxygen barrier films.
RESUMEN
Increasing the service temperature of TiAl intermetallics is the main challenge for the development of next-generation aircraft. Dispersion-strengthening, an effective means to further improve the high-temperature performance of metals, fails to implement in TiAl intermetallics due to difficulties in interface optimization. Here, we successively fabricate a TiAl naocomposite with fully lamellar microstructures and homogeneously dispersed Ti2AlC nanoprecipitates via spark plasma sintering. The composite consisted of semicoherent interfaces among γ-TiAl/Ti2AlC precipitates/α2-Ti3Al, in addition to continuous polysynthetic nanotwins. Strong pinning effects as well as strain-induced nanoscale TiCr2 precipitation uplift the operation temperature of TiAl nanocomposites by more than 50 °C. Furthermore, we experimentally proved that semicoherent interfaces among in situ Ti2AlC precipitates and its surrounding matrix serve as oxygen diffusion barrier during isothermal oxidization and significantly drop down the mass gain of TiAl nanocomposites during operation, making the present nanocomposite a highly potential candidate for use as light-weight structural materials in automotive and aerospace industries.
RESUMEN
Revealing the atomistic mechanisms for the high-temperature mechanical behavior of materials is important for optimizing their properties for service at high-temperatures and their thermomechanical processing. However, due to materials microstructure's dynamic recovery and the absence of available in situ techniques, the high-temperature deformation behavior and atomistic mechanisms of materials are difficult to evaluate. Here, we report the development of a microelectromechanical systems-based thermomechanical testing apparatus that enables mechanical testing at temperatures reaching 1556 K inside a transmission electron microscope for in situ investigation with atomic-resolution. With this unique technique, we first uncovered that tungsten fractures at 973 K in a ductile manner via a strain-induced multi-step body-centered cubic (BCC)-to-face-centered cubic (FCC) transformation and dislocation activities within the strain-induced FCC phase. Both events reduce the stress concentration at the crack tip and retard crack propagation. Our research provides an approach for timely and atomic-resolved high-temperature mechanical investigation of materials at high-temperatures.
RESUMEN
BACKGROUND: New generation antidepressant therapies, including serotonin-norepinephrine reuptake inhibitor (SNRIs) and selective serotonin reuptake inhibitors (SSRIs) were introduced in the late 1980s; however, few comprehensive studies compared the benefits and risks of various contemporary treatments for major depressive disorder (MDD) in pediatric patients. OBJECTIVE: Multiple-treatments meta-analysis (MTM) was conducted to assess efficacy, acceptability, and safety of contemporary interventions in children and adolescents with MDD. METHODS: Cochrane Library, AMED, CINAHL, EMBASE, LiLACS, MEDLINE, PSYCINFO, PSYNDEX, and Journal of Medicine and Pharmacy databases were searched for randomized controlled trials (RCTs) comparing medicinal interventions (citalopram, escitalopram, fluoxetine, mirtazapine, paroxetine, sertraline, venlafaxine), cognitive behavioral therapy (CBT), combined fluoxetine with CBT, and placebo treatment for acute MDD from January 1988 to March 2013. Treatment success, dropout rate, and suicidal ideation/attempt outcomes were measured. Bayesian methods were used to conduct a MTM including age and funding subgroups. RESULTS: A total of 21 RCTs (4969 participants) were identified. Combined fluoxetine/CBT exhibited the highest efficacy, with fluoxetine alone superior to CBT, paroxetine, sertraline, citalopram, escitalopram, and placebo treatment. Sertraline, paroxetine, escitalopram, and venlafaxine showed superior acceptability to fluoxetine and combined fluoxetine/CBT. Combined fluoxetine/CBT combination was less safe, though CBT was safer than fluoxetine alone. Combined fluoxetine/CBT, fluoxetine, and mirtazapine exhibited the highest efficacy; sertraline, escitalopram, venlafaxine, and paroxetine were the best tolerated; and mirtazapine and venlafaxine were the safest. CONCLUSIONS: Sertraline and mirtazapine exhibited optimally balanced efficacy, acceptability, and safety for first-line acute treatment of child and adolescent MDD.
