Effect of SiC-mullite coatings on oxidation resistance of graphite

Abstract

Two layers oxidation protecting coatings of SiC and mullite were successfully created on the graphite substrate by pack cementation and plasma spray methods, respectively. Phase synthesis was studied by X-ray diffraction. Microstructure and morphology were investigated by optical and scanning electron microscopes (SEM). X-ray diffraction results of the first coating showed that the secondary SiC was completely synthesised by the heat treatment of Si, Al₂O₃, C and SiC at 1500°C. Alumina and kaolin reaction at 1400°C led to the formation of mullite as the second layer. The oxidation resistance of two layers coating was considerably improved in comparison with mono layer coating and raw graphite. Oxidation resistance was decreased at higher temperatures in all samples. Formation of SiO₂ glassy phase improved the graphite oxidation resistance which was confirmed by SEM.     

Development of mullite and spinel coatings on graphite for improved water-wettability and oxidation resistance

Mullite and spinel forming sols were prepared from hybrid precursors having both organic and inorganic origins. Refractory grade graphite flakes were coated by these sols and their performances were compared in terms of oxidation resistance and water-wettability. Particle size distribution, structural evolution and related characteristics of both mullite and spinel gels have been investigated by dynamic light scattering (DLS), differential thermal analysis (DTA), X-ray diffraction (XRD) and infrared spectra (IR) studies. Coated graphites have also been studied by IR and XRD tests along with scanning electron microscopy (SEM) with energy dispersive spectral (EDS) analysis. The better performance of spinel coated graphite was confirmed. The mechanism of spinel formation on graphite was suggested to take place via intermediate gamma alumina phase formation from boehmite sol. It was clarified by DLS, XRD and microstructural analysis of dried and calcined gels.     

Physicochemical aspects of graphite protection from oxidation in heat treatment of mixtures on ethyl silicate binder

Results of a study of the physicochemical processes that occur in heat treatment of graphite — ethyl silicate binder mixtures are presented. Diffusion “bridges” in plane 0001 of graphite between the latter and SiO₂ gel confirm diffusion matching of the ≡ Si — O — Si ≡ and ≡ C — C ≡ bonds in heat treatment of the mixtures at 1223 K with the formation of β-SiC, which diminishes the oxidation of graphite.     

Hydrogen-free deposition of diamond-like coatings from a graphite source

We successfully deposited diamond-like hydrogen-free carbon coatings by pulsed electron beam vapor deposition (by pseudospark discharges), which is a novel technique in this field. Transmission electron microscopy and electron diffraction demonstrate that these layers consist essentially of two components: (i) amorphous spherical particles with an average diameter of 50 nm, which form an amorphous particle network, and (ii) single crystalline diamond crystallites of up to 10 μm in diameter. The investigation of the amorphous regions by parallel electron energy loss spectroscopy permits to distinguish amorphous carbon and amorphous diamond.     

Protection of graphite and graphite-containing materials from oxidation

The results of a comprehensive study of newly synthesized single-layer and multiple-layer coatings based on modified corundum for protection of graphite from oxidation are reported. __________ Translated from Novye Ogneupory, No. 7, pp. 25–33, July, 2005.     

Lipid oxidation products of heated soybeans as a possible cause of protection from ruminal biohydrogenation

Heating oilseeds has been shown to improve the milk fatty acid profile when given to dairy cows, compared to raw oilseeds. However, results from published studies are conflicting. The conditions of heating and storage of the oilseeds could be responsible for these differences, probably partly through their effects on lipid oxidation, the products of which could act on ruminal biohydrogenation (BH). Thus, 15 different treatments were applied to ground soybeans: three levels of heating (no heating, 30 min at 110 or 150°C) × 5 ambient storage durations (0, 1, 2, 4, or 6 months). Soybeans were incubated in vitro with ruminal fluid for 6 h. Triacylglycerol (TAG) polymers, hydroperoxides and hydroxyacids (HOA), aldehydes, and fatty acids were assayed in soybeans and ruminal culture. No TAG polymer was detected in any treatment. Soybeans stored for a long time had a high content of HOA, whereas those heated at 150°C, whatever the storage duration, had high aldehyde contents. The percentage disappearance of cis‐9,cis‐12 18:2 and cis‐9,cis‐12,cis‐15 18:3 in incubates decreased significantly in cultures with heated soybeans, especially at 150°C, suggesting that this partial protection of polyunsaturated fatty acids (PUFA) from BH was at least in part linked to the aldehyde content of the heated soybeans. Practical applications: Oilseeds given to ruminants are often heated, and heat treatment is known to generate oxidation products. Knowing what oxidation products influence ruminal biohydrogenation of unsaturated fatty acids could result in technological processes allowing a better transfer of unsaturated fatty acids from oilseeds to ruminant products.     

Novel coatings from renewable resources for the protection of bronzes

End-capped poly(lactic acid)s with a benzotriazole moiety were synthesized by Ring Opening Polymerization of lactide, characterized by spectroscopic methods and tested as protective coatings on selected bronze surfaces. Performances of functionalized polymers were evaluated in terms of colour changes of the treated metal and stability of the coating. A comparison between end-capped polymers and a mixture of poly(lactic acid) and benzotriazole was also run. End-capped poly(lactic acid)s showed excellent stability to photochemical and thermo-hygrometric ageing and better performances than a blend of poly(lactic acid) and benzotriazole. These polymers show promising performances for metal's coating.     

Production of refractories from graphite metallurgical waste products

Conclusions Comparative investigations of natural graphite and that obtained from metallurgical waste showed the prospective use of the latter for making graphitic refractories for which it is necessary to speed up the organization of the industrial production of graphite for GSMO.Translated from Ogneupory, No. 1, pp. 38–40, January, 1987.     

BSL-based coatings for bamboo protection

Bhilawanut shell liquid is a natural product, and because of its phenolic nature it undergoes reactions similar to those of phenol. Surface coatings based on BSL were prepared from formaldehyde, styrene, hexamine, and epichlorohydrin for the protection of bamboo surfaces. BSL-based coatings were applied on bamboo and the performance of these coatings was evaluated by accelerated and chemical resistance tests. It was observed that these coatings can be used for the protection of bamboo from corrosive environments. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 487–493, 1998     

Anode materials for lithium ion batteries from mild oxidation of natural graphite

Mild oxidation of a natural graphite in an ammonium peroxydisulfate solution yields promising anode materials. X-ray photoelectron spectroscopy, FTIR spectroscopy, electron paramagnetic resonance, thermogravimmetry, differential thermal analysis, high resolution electron microscopy and surface area measurements provided results suggesting that oxidation eliminates some reactive structural defects in this graphite. In addition, the surface of natural graphite is recoated with a dense layer of oxides forming an effective passivating film to prevent the decomposition of electrolyte and the movement of graphene molecules along its a-axis. Consequently, its thermostability and the EPR signal increase. In addition, the numbers of nanosized pores and channels increases, which provide more inlets and outlets for lithium intercalation and deintercalation and more sites for lithium storage. As a result, the reversible lithium capacity and the coulombic efficiency in the first cycle increase significantly and the cycling behaviour improves markedly. The reproducibility of product properties can be well controlled, and this method is promising for industry.     

Graphene layers from thermal oxidation of exfoliated graphite plates

Graphite platelets of 1–5 μm in diameter consisting of a few graphenes were generated from commercially available exfoliated graphite by ultrasonic treatment in benzene (1 mg material in 20 ml solvent) for 3 h. Droplets from the suspension were dispersed on silicon wafer. The graphite platelets were characterized by atomic force microscopy (AFM). Successive oxidation of the sample was carried out at 450–550 °C in air. AFM measurements showed that the thermal oxidation removed 2–3 graphenes from the platelets and it left behind single graphene layers.     

Nanostructured coatings approach for corrosion protection

Nanostructured surface treatment coatings based on the Self-assembled Nanophase Particle (SNAP) approach were investigated as potential replacement for chromate-based surface treatments on aircraft aluminum alloys. In the traditional sol–gel method, hydrolysis-condensation processes are followed by condensation polymerization upon film application. This process sequence provides a low temperature route to the preparation if thin coatings which are readily applied to most metallic substrates. The recent discovery of a method of forming functionalized silica nanoparticles in situ in an aqueous sol–gel process, and then cross-linking the nanoparticles to form a thin film, is an excellent example of a nanoscience approach to coatings. This Self-assembled Nanophase Particle (SNAP) process can be used to form thin, dense protective organic surface treatment coatings on Al aerospace alloys. The ability to design coating components from the molecular level upward offers tremendous potential for creating multifunctional coatings.

The important components of Al alloy corrosion inhibition by chromate are storage and release of Cr^VI species, inhibition of cathodic reactions (primarily oxygen reduction), and inhibition of attack at active sites in the alloy. Unlike chromate-based treatments, current SNAP coatings provide barrier-type corrosion resistance but do not have the ability to leach corrosion inhibitors upon coating damage and minimize corrosion of the unprotected area. In this study, organic inhibitors were tested for corrosion protection of aluminum alloys in combination with the (SNAP). Scanning Vibrating Electrode Technique, anodic polarization, electrochemical impedance spectroscopy, and salt spray test were used to study this new approach for chromate replacement.     

Complex coatings for the protection of molybdenum

Conclusions The complex coating (MoSi₂+Al₂O₃) obtained by flame-spraying of aluminum oxide on the MoSi₂ layer on molybdenum has a higher oxidation and erosion resistance than a coating made up only of molybdenum disilicide. At 1200°C, mullite forms in the coating as a result of the interaction between amorphous silica and-Al₂O₃.Translated from Ogneupory, No. 6, pp. 53–54, June, 1968.     

防晒品配方设计

防晒产品的配方优化对防晒产品的设计起着至关重要的作用。介绍了降低风险、增强功能、光化学稳定性、溶解度问题、胶囊效能、防护增效剂和微粒化原料等。同时,阐述了供应商可持续提供健康的原材料,提高其产品效能,改善现有材料的透明度和含有微粒抗氧化物的化妆品为合格产品,并得到减少太阳紫外线对皮肤伤害的高效能产品。     

Adhesion of ceramic coatings sprayed onto graphite

In order to expand the fields of application and to improve the performance of graphite (Cg), it is necessary to reduce its permeability towards oxygen and to limit its reactivity and especially its oxidation. It is therefore essential to protect it from the environment through the use of ceramic coatings. Adhesion between ceramic coatings and graphite is controlled by the mechanical stresses in the coatings and the thermodynamic work of adhesion. Different metal-graphite systems were examined and it was shown that the adhesion depended particularly on the thermal expansion coefficient mismatch between the two materials and on the stability of the metal carbide. Thus, the role of the addition on the graphite surface of elements such as Cr, Mo, Al, Si, and O in the adhesion of metals or ceramics to graphite has been identified.     

Advanced coatings on the basis of Si(C)N precursors for protection of steel against oxidation

An effective and low-cost method for applying polysilazane-based barrier coatings on stainless steel is presented. Two different precursors in the system SiCN (ABSE) and SiN (PHPS) have been used for applications of polymeric and ceramic-like coating materials. With corresponding precursor solutions in toluene or ether, steel substrates were coated by means of a simple dip coating technique. The influence of different annealing atmospheres such as ambient air and nitrogen on the resulting coatings was determined. Therefore precursor-based powders and coatings were characterised by TGA, REM, TEM, GDOES and chemical composition measurements. Oxidation tests on coated samples were done in air at temperatures of 800–1000 °C. By measuring the weight gain and the oxidation depth, parabolic oxidation kinetics were determined. The precursor-based coatings protect stainless steel from oxidation up to 1000 °C.     

Hydrophobic benzoxazine-cured epoxy coatings for corrosion protection

A hydrophobic benzoxazine-cured epoxy coating (EPB) was prepared by a dip coating and thermal curing method using benzoxazine monomer (B-TMOS) as curing agent. Fourier transform infrared (FTIR) analyses confirmed the presence of thermal curing reactions and hydrogen-bonding interactions in the epoxy/polybenzoxazine system. The hydrophobicity of epoxy coatings induced by the incorporation of B-TMOS was enhanced significantly, and the water contact angles of resultant EPB coatings were higher than 98°. The corrosion protection ability of epoxy coatings was investigated by open-circuit potentials, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS) methods. The results showed that the charge transfer resistance (R_ct) of EPB coatings was increased by about three orders of magnitude compared with bare mild steel, and the protection efficiency values of all EPB samples were more than 98%. This increased corrosion protection property could be attributed to the high hydrophobic performance of EPB coatings.