Search

Highlights: [•] Dense and porous Hf and Zr-based UHTCs are sintered and structurally characterized. [•] They are spectrally selective, according to RT optical characterization. [•] Thermal emittance in the 1100–1800K temperature range shows promising properties. [•] Thus they appear promising for novel solar absorbers operating at high temperature. [ABSTRACT FROM AUTHOR]

Highlights: [•] ZrC/MoSi2 samples oxidized at various temperatures have been analyzed. [•] XPS and cross-section SEM/EDS revealed the compositions of the oxide layers. [•] Oxide phases involve both elements from the carbide and the additives. [•] We propose an oxidation mechanism based upon these observations and literature. [Copyright &y& Elsevier]

Abstract: Spark Plasma Sintering is used for the fabrication of highly dense HfB2 monolithic and HfB2–26vol.% SiCw composite. Reactive SPS from elemental reactants is preferred for the preparation of bulk HfB2 instead of classical sintering. The desired phase is rapidly formed through a solid–solid combustion synthesis mechanism, while full densification is achieved in 30min at 1350A when the applied pressure is switched from 20 to 50MPa after the synthesis reaction. A 99.4% dense whiskers-reinforced HfB2 ceramic matrix composite is also obtained in 30min by SPS (I =1350A, P =20MPa) using SHSed HfB2 powders and SiCw. Nevertheless, whiskers degradation into SiCp resulted under such conditions (temperature up to 1830°C). On the other hand, the presence of whiskers is clearly evidenced in 96% dense products obtained when the applied current was decreased down to 1200A (1700°C) while P was increased to 60MPa. [Copyright &y& Elsevier]

The evolution of the multi-scale microstructure of a (Zr,Ta)B 2 solid solution was studied as a function of time and temperature. The ceramics were produced by hot pressing a mixture of ZrB 2 with 15 vol% TaSi 2 followed by annealing at 2100 °C. Formation of a super-saturated solid solution led to the precipitation of TaC nano-needles within the micron-sized boride grain matrix. Phase stability diagrams were used to define the conditions of partial pressure within the sintering chamber that drove precipitation of nano-inclusions in the form of either metal or carbide. Through this approach, other systems containing various transition metals were explored to design other formulations for in-situ nano-composites with unprecedented strength at ultra-elevated temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

• Two novel test set-ups for high-temperature testing of materials for rocket nozzles. • ZrB 2 -based UHTCMCs with carbon fibers tested in high heat flux environment. • Temperature exceeded 2900 K for samples with higher erosion rates. • Excellent erosion resistance with respect to state-of-art materials. • Optimized manufacturing process favors thermal shock resistance. This paper presents the results of experimental tests for the characterization of Ultra-High-Temperature Ceramic Matrix Composite (UHTCMC) materials for near-zero erosion rocket nozzles. Two dedicated test set-ups were developed for preliminary screening of material candidates in a representative environment, characterized by relevant heat flux and temperature. The experimental set-up was based on a lab-scale 200N-class hybrid rocket engine, employing gaseous oxygen as the oxidizer and High-Density PolyEthylene as fuel; the configurations included free-jet test, in which small button-like samples were exposed to the supersonic exhaust jet of the rocket nozzle; and chamber inserts, in the shape and size of an annular element, placed inside the rocket combustion chamber. Computational Fluid Dynamic simulations, for modeling heat transfer and combustion chemical reactions, complemented the experimental observations and supported the characterization of test conditions. Samples with ZrB 2 -SiC matrix and continuous or chopped carbon fibers, sintered by either Hot Pressing or Spark Plasma Sintering were tested. Free-jet test samples demonstrated a substantially improved erosion resistance with respect to conventional graphite and in one case a negligible material recession. UHTCMC samples erosion was associated to the occurrence of a rapid rise in surface temperature, which achieved values over 2900 K. Chamber inserts, besides confirming the outstanding erosion resistance of UHTCMCs with respect to traditional materials (i.e. C/SiC), proved that long-fibers samples with sufficient porosity are more likely to withstand thermal shocks typical of the rocket combustion environment. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• ZrB 2 -based UHTCMC with carbon fibers tested in supersonic arc-jet wind tunnel. • Temperature jump on the front surfaces, at specific total enthalpies over 18 MJ/kg. • Excellent erosion resistance, despite formation of a porous oxide layer. • Jump related to increase in surface catalycity and reduction of thermal conductivity. An experimental campaign has been carried out to characterize a new class of Ultra-High-Temperature Ceramic Matrix Composites for near-zero ablation Thermal Protection Systems. Small-sized specimens, with ZrB 2 -based matrix and different carbon fiber architectures, were exposed to a simulated air supersonic flow generated by an arc-jet wind tunnel, achieving specific total enthalpies up to 20 MJ/kg and cold wall fully catalytic heat fluxes over 5 MW/m2, in an aero-thermo-chemical environment representative of atmospheric re-entry. Ablation rates were estimated by means of mass and thickness measurements before and after testing, demonstrating an excellent performance of the developed materials. Surface temperatures were monitored by means of infrared pyrometers and a thermo-camera, and during all the tests a spontaneous temperature jump was observed, with temperatures that reached values over 2800 K at the steady state. Post-test microstructural analyses revealed the formation of a porous oxide layer with a thickness of few hundred microns, mainly consisting of zirconia, with substantial removal of both SiC and carbon fibers. Below the oxide, the bulk material was unaffected. Computational Fluid Dynamics simulations allowed rebuilding the thermo-fluid-dynamic and chemical flow field. Moreover, it was possible to propose an innovative correlation of the temperature jump with an increased catalytic activity and a dramatic reduction of the thermal conductivity of the oxide layers forming on the exposed part of the sample, which anyway had a key role in preserving the unoxidized bulk materials at reasonable temperatures. Image, graphical abstract [ABSTRACT FROM AUTHOR]

High dissolution of anticancer drugs directly adsorbed onto porous carriers is indispensable for the development of drug delivery systems with high bioavailability. We report direct adsorption/loading of the anticancer drug letrozole (LTZ) onto the clinoptilolite (CLI) zeolite after the surface activation.In vitroLTZ dissolution from the CLI zeolites reached 95 % after 23 h in an acidic medium, being faster than the dissolution of the pure LTZ molecules. Fast dissolution occurs due to uniform exposure of the LTZ onto the external surface of the CLI zeolites, being accessible to the solvent for dissolution. On the other hand, the LTZ molecules were hidden in the bulk phase, giving a slow dissolution rate. Small positive value of the CLI/LTZ adsorption energy of 0.06 eV suggests that the release process is favourable in aqueous media. The main merit of the CLI/LTZ system is its quick onset of action and high bioavailability. This work demonstrates a possibility of enhancement of the dissolution of poorly soluble LTZ from the CLI zeolite, being promising for the further development of drug delivery systems., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Inc. All rights reserved.)