Boron-bearing species in ceramic matrix composites for long-term aerospace applications

Boron-bearing refractory species are introduced in non-oxide ceramic matrix fibrous composites (such as SiC/SiC composites) to improve their oxidation resistance under load at high temperatures with a view to applications in the aerospace field. B-doped pyrocarbon and hex-BN have been successfully used as interphase (instead of pure pyrocarbon) either as homogeneous or multilayered fiber coatings, to arrest and deflect matrix cracks formed under load (mechanical fuse function) and to give toughness to the materials. A self-healing multilayered matrix is designed and used in a model composite, which combines B-doped pyrocarbon mechanical fuse layers and B- and Si-bearing compound (namely B₄C and SiC) layers forming B₂O₃-based fluid healing phases when exposed to an oxidizing atmosphere. All the materials are deposited by chemical vapor infiltration. Lifetimes under tensile loading of several hundreds hours at high temperatures are reported.     

Redox reactions: inconsistencies in their description

The paper is aimed at defining reduction, oxidation, and redox reactions based both on the oxidation number and charge changes in reacting species. It is rationalized that the processes of oxidation and reduction, usually occurring simultaneously, can occur also as independent processes. It is explained that in balancing chemical equations of redox reactions the “gain” or “loss” of electrons should be understood as changes in oxidation number. A formal expressions “+n e^?” and “?n e^?” represent in reality a decrease and increase in oxidation number by n units, respectively.     

Sol–gel synthesis of porous inorganic materials using “core–shell” latex particles as templates

Here we report on the sol–gel synthesis of porous inorganic materials based on manganese, molybdenum, and tungsten compounds using the “core–shell” siloxane-acrylate latex as a template. The chemical composition and structural characteristics of the materials obtained have been investigated. It was shown that temperature conditions and gaseous media composition during the template destruction controlled the composition and structure of porous materials. To obtain porous inorganic materials for catalytic applications, the “core–shell” latex template was preliminarily functionalized by gold and palladium nanoparticles obtained by thermal reduction of noble metal ions-precursors in a polycarboxylic “shell”. Upon the template removal, noble metals nanoparticles of a size of dozens of nanometers were homogeneously distributed in the material porous structure. The evaluation of the catalytic activity of macroporous manganese, tungsten, and molybdenum oxides under the conditions of liquid phase catalytic oxidation of organic dyes has been performed. The prospects of employing macroporous oxide systems with immobilized nanoparticles of noble metals in the processes of hydrothermal oxidation of radionuclide organic complexes in radioactive waste decontamination have been demonstrated.     

Effect of natural clay components on sorption of Cs, Pu and Am by the clay

Sorption of Cs, Pu and Am on natural clay of complex composition was studied to better understand the sorption mechanisms. It was found that cesium sorption to natural clay was affected by its coatings and by the ionic strength of solution. The sorption of Pu and Am on the clay was compared with that on synthetic goethite, hematite and magnetite, representing components of the clay coatings. The sorption was quantitatively interpreted using models assuming ion exchange and/or complex formation on the “layer sites” and “edge sites” of the clay and its coatings. Constants characterizing properties of the sites and sorption equilibria were determined.     

Effect of synthesis conditions of pyrocarbon coatings on their corrosion resistance in H2SO4-HNO3 oxidizing mixture

Synthesis conditions of pyrocarbon coatings in a fluidized bed via pyrolysis of gaseous hydrocarbons from a CH₄-Ar mixture at temperatures of 1300–2000°C and from C₃H₆-Ar at 1200–1500°C were examined. The corrosion resistance of the pyrocarbon coatings in HNO₃-H₂SO₄ oxidizing mixture was studied.     

Modification of a structure of cermet coatings Co-Ni-Cr-B-Si-TiB2, (TiCr)B2 in their fusion

Peculiarities of variation in hardness and structure of the matrix alloy, Stellite, depending on a composition of the ceramic-metal system were found out in studying the effects of hardening of the base layer of cermet coatings. A new type of structure of matrix alloy, which was called “meshwork,” were discovered and examined as well as a new type of inclusions: plasma conglomerates, in which a fragmentary structure refinement was found. Under the conditions of the plasma stream new composite coatings of multilayer structure characterized by improved performance properties were produced.     

A new methodology for producing pyrocarbon composites containing zerovalent iron nanoparticles

A magnetic pyrocarbon composite containing nanoparticles with an overwhelming predominance of zerovalent iron has been synthesized. The nanocomposite has a core–shell–matrix structure in which Fe⁰ nanoparticles with an average size of 50 nm are located in the pyrocarbon matrix and coated with a ferrite shell preventing their aggregation and oxidation. The composite is distinguished for its high thermal stability, magnetic properties 59 G cm³/g, and electrical conductivity as high as that of graphite.     

Ultrathin polymer coatings by complexation of polyelectrolytes at interfaces: suitable materials,structure and properties

The article presents the state‐of‐the‐art of alternating physisorption of oppositely charged polyelectrolytes, the so‐called “layer‐by‐layer” method or “electrostatic self‐assembly” (ESA), for the preparation of thin polymer coatings. In comparison to other, more established self‐organization techniques, this recent method is distinguished by its simplicity, versatility, and speed. In particular, the tendency for self‐healing is unique. Emphasis is given to the role of the molecular structure of the polyelectrolytes, and to the nature of the support. Also, various parameters for the preparation of multilayer films are highlighted, which are very important due to the kinetic control of the build‐up process. The structure of the resulting coatings, their quality and stability, chemical reactions in the films, and potential applications are discussed.     

The anodic oxidation of alkanes in anhydrous hydrogen fluoride and related superacid media

The anodic oxidation of several alkanes in anhydrous hydrogen fluoride at various acidity levels, from “basic” media (KF) to “acid” media (SbF₅) has been studied to establish optimum conditions for the formation of carbenium ions. The oxidation potential of an alkane depends on structure and on the acidity level of the medium. Taking into account thermodynamical data, possible mechanisms for the anodic oxidation are proposed. Furthermore, it is shown that alkanes undergo spontaneous chemical oxidation in acid media (HF + SbF₅ which may be investigated in its kinetic and thermodynamic aspect by means of electroanalytical methods.     

Carrier gas UV laser ablation sensitizers for photopolymerized thin films

Designed carrier gas UV laser ablation sensitizers were synthesized and proved to greatly enhance the UV laser ablation of photopolymerized thin films. Polymers containing dense ester groups are reported to have better laser ablation performance because of the tendency of the ester groups to decompose into gaseous products (“carrier gases”) during the ablation process. In order to introduce this mechanism to cationic UV curable coatings for better laser ablation, a series of “carrier gas” sensitizers were synthesized by reacting hydroxyl containing reactive diluents such as oxetane and polyester polyols with monomethyl oxalyl chloride or dimethyl oxalate; the oxalyl group is considered a “carrier gas” generating moiety. Furthermore, a UV absorbing chromophore, naphthalene, is either chemically bound to the oxalyl containing molecules or blended with the synthesized oxalyl containing compounds to produce a synergistic effect. The “carrier gas” sensitizers were added into a typical cationic UV curable formulation to form sensitized coatings, which were then characterized by thermogravimetric analysis, real time FTIR and ablated by a 355 nm laser. The ablation vias were examined using optical profilometry and SEM. Compared to the control, the sensitized coatings were found to have similar thermal decomposition temperatures and higher functional group conversion during photopolymerization. All of the sensitized coatings containing the “carrier gas” sensitizers exhibited better UV laser ablation performance than the control. The combination of naphthalene derivatives and the oxalyl group gave a better ablation result, suggesting a synergistic effect. The chemical combination of the naphthalene and oxalyl group exhibited better ablation sensitization than their blends, suggesting a more efficient intramolecular laser energy utilization process.     

Magnetic characteristics of iron-modified oxide layers on titanium

Iron-modified TiO₂ coatings on titanium exhibiting ferromagnetic properties are formed by combining plasma electrolytic oxidation (PEO) and impregnation with subsequent annealing. It is found that iron is contained in the composition of dispersed particles with sizes of 1–10 μm distributed over the surface of the coatings. It is shown that the coercive force of the coating on titanium samples is 20–70 Oe. The composition, structure, and magnetic properties of the samples with coatings prepared via direct PEO and by combining PEO and impregnation are compared. It is concluded that impregnation and annealing procedures can be used to impart ferromagnetic properties to PEO coatings with different compositions, e.g., protective coatings and coatings applied in catalysis or medicine.     

微芯片上Ag2S电化学纳米开关的构筑及电学测量的初步研究

初步探索了利用电化学方法“自下而上”地构筑了Au/Ag/Ag2S-Au固体电化学纳米开关, 并确定了较适宜的开关工作条件. 小于1 nm的Ag2S-Au间隔是Ag2S开关器件的关键结构, 以保证电子的量子遂穿和间隔中Ag凸起的生长与收缩. 测量结果表明, Ag2S开关转换具有较好的可逆性和稳定性, 开关转换电流相差3个数量级以上.     

Carbon Nanotube‐Directed Polytetrafluoroethylene Crystal Growth via Initiated Chemical Vapor Deposition

Initiated chemical vapor deposition (iCVD) has been shown to be suitable for blanketing surfaces with thin polymer coatings of ≈1–2 nm and greater. In this work, iCVD coatings of polytetrafluoroethylene (PTFE) deposited on carbon nanotube (CNT)‐based surfaces show CNT‐templated PTFE single crystal growth. While the coating forms disoriented agglomerates when deposited on an amorphous carbon background, “shish‐kebab” structures are observed when grown on single‐walled carbon nanotubes (SWCNT) as well as CNT buckypaper. It is shown that the shish‐kebab structure is composed of PTFE lamellae arranged with the chain backbones running parallel to the SWCNT axis. This result allows one to control not only the surface chemistry using PTFE but also the coating surface topology.     

Some new applications of the biamperostat catalytic-kinetic determination of copper, peroxidase, glucose oxidase, thyroxine and 5-chloro-7-iodo-8-hydroxyquinoline.

Biamperometrically observable reactions can be evaluated by the “potentiostat” method with a current-to-voltage transducer. Some further examples of the application of this “biamperostat” are described: the copper(II)-catalyzed autodecomposition of hydrogen peroxide, the horseradish peroxidase-catalyzed oxidation of iodide with hydrogen peroxide, the glucose oxidase-catalyzed oxidation of glucose with molecular oxygen and the iodine (in organic compounds )-catalyzed oxidation of arsenic(III) with cerium(IV). 1,10-Phenanthroline is determined indirectly by its inhibitory action on copper(II).     

Thermal degradation and fire performance of intumescent silicone‐based coatings

This paper deals with the thermal degradation and fire performance of silicone‐based coatings for protecting steel. In this study, the fire performance of silicone coatings as virgin or formulated materials is evaluated using two homemade fire testing methodologies: one similar to the “torch test” fire testing method and the other using a heat radiator test. It was shown that the performance of the silicone‐based coating used as thermal barrier can be improved incorporating a modifier (a mixture of polydimethylsiloxane and silica coated by a silane). In this case, silicone‐based coating swells and exhibits same fire performance as commercial intumescent coating at the torch test. It is shown that the incorporation of modifier in the silicone makes it to swell upon heating resulting in the formation of expanded material exhibiting low heat conductivity. Thermal degradation of the coating is also investigated: it occurs in three main steps leading to the formation of a tridimensional network characterized by the formation of Q⁴ structure at high temperature. Copyright © 2012 John Wiley & Sons, Ltd.     

Transparent ultraviolet-barrier coatings

A number of phenyl polyesters have been synthesized to furnish molecules whose backbones rearrange under ultraviolet irradiation to an o-hydroxybenzophenone structure. This photochemical Fries rearrangement produces ultraviolet opacity in the irradiated film while maintaining visual transparency. Thin coatings of these polyesters completely protect substrates ordinarily sensitive to ultraviolet light. Spectroscopic analysis of various rearranged films and coatings clearly shows that the o-hydroxybenzophenone polymer formed is concentrated at the irradiated surface of the original polyester coating as a “skin”. Such a skin, formed in situ during the irradiation, functions to protect both the original polyester coating as well as the coated substrate from degradation by ultraviolet irradiation. Furthermore, a significant “healing” mechanism appears inherent in these coatings, for as the exposed skin ultimately degrades under extended ultraviolet irradiation, more of the underlying polyester layer apparently rearranges to compensate for the loss. Thus the clear coating functions both as a protective skin and a rearrangeable reservoir. Modified structures of the polyesters have been prepared which possess, in addition to their protective film properties, a useful solubility spectrum and a good solution shelf life.     

Improving the oxidation resistance of AlCrN coatings by tailoring chromium out-diffusion

In this work, we have studied the improvement on the oxidation resistance of AlCrN-based coatings by adding a subsurface titanium nitride barrier layer. Since oxidation is interrelated with the inward diffusion of oxygen into the surface of Al_xCr_1−xN (x = 0.70) coatings and the outward diffusion of Cr to the surface, the oxidation behaviour of the aluminium-rich AlCrN coatings can be tuned by designing the coating in an appropriate layered structure. The buried depth of the embedded layer and the oxidation time were varied, and the changes in the AlCrN/TiN depth composition profiles and surface oxidation stoichiometry were analysed by means of Glow Discharge Optical Emission Spectroscopy (GDOES) and Cross Sectional SEM (X-SEM) maps. It was observed that when a TiN diffusion barrier of 300 nm was deposited near the top surface (500 nm from the surface) the inhibition of the inward diffusion of oxygen and formation of beneficial alumina surface layers was promoted and consequently an increase of the oxidation resistance is achieved. This is explained in terms of a limited surplus of chromium from the coating to the surface. This was corroborated after performing experiments using CrN as embedded barrier layer which resulted in a continuous surplus of chromium to the surface and the formation of Cr-rich oxides. GDOES, in combination with X-SEM elemental maps, was proved to be a fast and accurate technique to monitor composition in-depth changes during oxidation, providing unique information regarding the oxide structure formation.     

Concepts of Trapping Topologically by Shell Molecules

Concepts of the synthesis of shell topological compounds, which consist of a guest molecule (or molecules) trapped by a host molecule with a spacial, egg shell-like structure are discussed. Generally, both constructing the shell molecule in the presence of a guest molecule and constructing the guest molecule in the presence of the shell (host) are ways to “shell” topological compounds. The preparation of shell molecules may consist of the completion of “preshell” molecules or of obtaining cascade branched oligomers and polymers. Cyclodextrins and substances like triquinance are considered to play a role in preshell molecules. Shell molecules may also be obtained by polyreaction of a monomer of the XRY_n type, which results in a cascade branched molecule of shell structure (spherical form). When the polyreaction is continued, the cascade branched molecule becomes a “cast” one. It is theoretically possible to enclose a guest molecule inside the shell during the cascade branching process if there is a good solvent (of high expansion coefficient value) in respect to the growing branches. A spacially developed molecule of both “empty” and “cast” structure may be obtained also by the known “step by step” cascade branching process which involves, for instance, a repeated cyanoethylation-reduction reaction.     

Deposition and Study of Composite Coatings with Oxygen-containing Manganese Compounds on the Surface of D16 Aluminum Alloy

Fundamental aspects and specific features were studied of how composite coatings with oxygen-containing compounds of manganese are formed on the surface of D16 alloy in the course of the microarc oxidation from a silicate-alkaline electrolyte in the anodic-cathodic mode with interrupted discharge. The influence exerted by the concentration of potassium permanganate in the electrolyte on the elemental composition, morphology, and structure of the coatings was examined. It was found that the coatings are promising as catalytically active and corrosion-protection systems.