A multilayer MoSi2-SiC-B coating to protect SiC-coated carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a multilayer MoSi₂-SiC-B coating was prepared on the SiC-coated C/C composites by a simple and low-cost slurry method. The phase, microstructure and element distribution of the as-received coating were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The as-received coating could effectively protect C/C composites against oxidation at 850 °C in air for 100 h without mass loss, which exhibits better oxidation protective ability than the multilayer MoSi₂-SiC coating prepared by the same method. At intermediate temperature (850 °C), the excellent oxidation protective ability of the coating is mainly attributed to the formation of the molten B₂O₃ for sealing the microcracks and preventing oxygen from attacking the C/C substrate.     

Synthesis of boron carbide powder from polyvinyl borate precursor

Polyvinyl borate (PVBO) was prepared by the condensation of poly(vinyl alcohol) (PVA) and boric acid, and used as a precursor for boron carbide. Boron carbide powder was synthesized by the pyrolysis of the PVBO precursor in air at 600 °C for 2 h, followed by heat treatment in Ar flow at 1300 °C for 5 h, which is a relatively low temperature compared with conventional carbothermal methods. Pyrolysis of the PVBO precursor resulted in submicron-size particles of B₂O₃ dispersed in a carbon matrix. In addition, the pyrolysis temperature governed the carbon content in the pyrolyzed product of the PVBO precursor, which led to the synthesis of crystalline boron carbide powder with little free carbon.     

One simple synthesis route to nanocrystalline tantalum carbide via the reaction of tantalum pentachloride and sodium carbonate with metallic magnesium

Nanocrystalline tantalum carbide (TaC) was synthesized via a simple route by the reaction of metallic magnesium with sodium carbonate and tantalum pentachloride in an autoclave at 600 °C. X-ray powder diffraction pattern indicated that the product was cubic TaC, and the cell constant was a = 4.451 Å. Field emission scanning electron microscopy image showed that it consisted of particles with an average size of 40 nm. The product was also studied by TGA in the flowing air. It had good thermal stability and oxidation resistance below 450 °C in air. The mechanism of the formation of TaC was also discussed in this paper.     

Low temperature synthesis of vanadium carbide (VC)

Nanocrystalline VC has been prepared via a convenient route by the reaction of metallic magnesium powder with vanadium pentoxide and basic magnesium carbonate in an autoclave at 650 °C. X-ray powder diffraction pattern indicated that the product was cubic vanadium carbide, and the cell constant was a = 4.155 Å. Scanning electron microscopy (SEM) image showed particles with an average size of about 60 nm. The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 350 °C in air.     

Atomic layer deposition of iron(III) oxide on zirconia nanoparticles in a fluidized bed reactor using ferrocene and oxygen

Conformal films of amorphous iron(III) oxide and α-Fe₂O₃ have been coated on zirconia nanoparticles (26 nm) in a fluidized bed reactor by atomic layer deposition. Ferrocene and oxygen were alternately dosed into the reactor at temperatures between 367 °C and 534 °C. Self-limiting chemistry was observed via in situ mass spectrometry, and by means of induced coupled plasma-atomic emission spectroscopy analysis. Film conformality and uniformity were verified by high resolution transmission electron microscopy, and the growth rate was determined to be 0.15 Å per cycle. Energy dispersive spectroscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy were utilized as a means to determine film composition at each deposition temperature. Over all of the deposition temperatures investigated, films were deposited as amorphous iron(III) oxide. However, after heat treatment at 850 °C in air and N₂ atmospheres, α-Fe₂O₃ was the predominant species.     

Nanocrystalline SiC films prepared by direct deposition of carbon and silicon ions

The method of direct deposition of carbon and silicon ions was used for preparation of nanocrystalline silicon carbide films. The deposition energy of carbon and silicon ions was 90 eV. The effect of substrate temperature in the range of 500-1150 °C on the structure of SiC films was studied by means of X-ray photoelectron spectroscopy (XPS) and X-ray diffractometry (XRD). According to XPS data, the films contained heterobonded Si-C atoms and homobonded Si-Si and C-C atoms, the relation between which varied as the function of substrate temperature. The data of XRD showed a noticeable growth of a nanocrystalline phase of cubic silicon carbide in the films at a temperature of about 700 °C. The content of 3C-SiC nanocrystalline phase reached 80 at.% at 950 °C. There was an established change from cubic polytype to rhombohedral polytype of silicon carbide α-SiC-21R at a substrate temperature higher than 1000 °C. The size of SiC crystal grains depended on the substrate temperature and changed from 4-5 up to 8-10 nm over the range of 700-950 °C. Besides, silicon unbonded with carbon also crystallized in nanocrystalline form with similar sizes of crystal grains. A possible model of the change of the polytypic composition of SiC film under the conditions of direct ion deposition was discussed.     

Large-scale synthesis of BN nanotubes using carbon nanotubes as template

Boron nitride nanotubes (BN-NTs) were synthesized in large scale by the reaction of NaBH₄ and NH₄Cl in the temperature range of 500-600 °C for 10-18 h, where carbon nanotubes (CNTs) were mixed together with the reactants to serve as template. Pure BN-NTs were obtained by oxidizing the product at about 800 °C in air atmosphere. The structure and morphology of the product with a surface area of 106.635 m²/g were characterized by X-ray diffraction, transmission electron microscopy, Fourier transformation infrared spectroscopy, and thermogravimetric analysis. Large scale preparation of BN-NTs could be realized by this simple and effective route.     

Composition and crystallinity of silicon nanoparticles synthesised by hot wire thermal catalytic pyrolysis at different pressures

The effect of pressure on the structure and composition of silicon nanoparticles synthesized by hot wire thermal catalytic pyrolysis (HW-TCP) of pure silane has been investigated. Light brown powders were produced at silane pressures of 10 and 50 mbar, at a flow rate of 50 sccm, using a tungsten filament at temperatures of 1900 °C and 1800 °C respectively. As determined by transmission electron microscopy and X-ray diffraction, the particles produced at lower pressure have sizes around 10 nm, whereas those produced at higher pressure are typically 50 nm. High resolution transmission electron microscopy (HR-TEM) shows a surface layer of between 2 and 5 nm thickness, which was confirmed by X-ray photoemission spectroscopy to be an oxide shell. Both X-ray diffraction and HR-TEM confirm a high degree of crystallinity in both sets of particles, with Raman spectroscopy indicating an increase in crystalline fraction with synthesis pressure.     

Oxidation and decarburization of an Fe-AI-C alloy

The oxidation behaviour of the Fe-5.5A1-0.55C (wt%) alloy was studied in air at 500 to 950° C. Specimens of two types of preparation were used: (i) homogenized for 4 h at 1100° C under an argon protective atmosphere to form carbide particles in the ferrite matrix (referred to Type 1 alloy); and (ii) homogenized for 96 h at 1100° C under an air atmosphere to form a ferrite matrix with full decarburization (Type 2 alloy). Type 1 alloy with or without a decarburized zone after oxidation had a higher oxidation rate than Type 2 alloy. The oxidation kinetics of Type 1 alloy are controlled by the stability of carbide particles. The interference of CO or CO₂ formation influences vitally the formation of protective aluminium oxide. During the oxidation of Type 2 alloy, two oxidation periods appear with slightly different parabolic constant values. The oxidation rate increased with temperature to a maximum at 600° C and then decreased to a minimum at 800° C.     

Effect of dopant on the thermal stability of core-shell ferrite nanoparticles

In this study, a series of core–shell ferrite nanoparticles were obtained by the thermal decomposition method of iron(III) acetylacetonate. Various amounts of Co³⁺, Mn³⁺, or Ni²⁺ ions as dopants were used. The prepared nanoparticles were treated at 500 °C for 24 h in the air atmosphere to determine the influence of doping on magnetite/maghemite crystalline structure and the following oxidation process. The nanoparticles before and after thermal treatment were evaluated by transmission electron microscopy, energy dispersive X-ray, X-ray diffraction and Mössbauer spectroscopy. The conducted studies show that doping primary structures by different 3d elements modulates the oxidation process of ferrite nanoparticles which, therefore, gives new possibilities for their application. However, the resistance to the oxidation of layered particles is different in comparison to the one-pot synthesis procedure. The susceptibility to oxidation varies not only due to the presence of different dopants in variable concentrations, but it also is dependent on layer positioning in the particle (core, shell, or both). The most protective element is Mn, and the least one – Co.     

Synthesis, thermal stability, and photocatalytic activity of nanocrystalline titanium carbide

Titanium carbide (TiC) was prepared via one simple route by the reaction of metallic magnesium powders with titanium dioxide (TiO₂) and potassium acetate (CH₃COOK) in an autoclave at 600 °C and 8 h. Phase structure and morphology were characterized by X-ray powder diffraction (XRD) and Scanning electron microscopy (SEM). The results indicated that the product was cubic TiC, which consisted of particles with an average size of about 100 nm in diameter. The product was also studied by the thermogravimetric analysis (TGA) and its photocatalysis. It had good thermal stability and oxidation resistance below 350 °C in air. In addition, we discovered that the cubic TiC powders exhibited photocatalytic activity in degradation of Rhodamine-B (RhB) under 500 W mercury lamp light irradiation.     

Effect of silicon content in steel and oxidation temperature on scale growth and morphology

The effect of high silicon content in steel, 1.6 wt.%Si and 3.2 wt.%Si, and high oxidation temperatures (850–1200 °C) on scale growth rate and morphology were investigated. The steels were oxidized in a 15% humid air with short isothermal oxidation times (15 min). The scale growth rate of the non-alloyed steel follows a parabolic law with time; it is an iron diffusion controlled oxidation. The presence of silicon delays scale growth by forming a silica SiO₂ barrier layer at the scale/metal interface, this effect is more important for the steel containing 3.2 wt.%Si and induces a discontinuous scale. Silicon oxides are concentrated at the scale/metal interface; their morphology depends on the oxidation temperature. For temperatures lower than 950 °C, silica is formed. Between 950 °C and 1150 °C, fayalite (Fe₂SiO₄) grains appear in the wüstite matrix close to the scale/metal interface. For temperatures higher than 1177 °C, a fayalite–wüstite eutectic is formed; this molten phase favours iron diffusion leading to high scale growth. After cooling, a continuous fayalite layer with small wüstite grains is obtained at the scale/steel interface.     

Cyclic oxidation of Haynes 230 alloy

The cyclic oxidation of Haynes 230 alloy (Ni-Cr-W-Mo alloy) was investigated in air at three different temperatures, 871, 982 and 1093 °C. Studies indicated that during cyclic oxidation, protective scales formed which were predominantly Cr₂O₃, with Kirkendall voids formed both at the scale/alloy interface and grain boundaries. Intergranular oxides were observed at temperatures above 982 °C while internal oxide particles were found above 1093 °C. Both intergranular and internal oxides were identified as aluminium oxide. A 50 m chromium-depleted zone developed after 70 h exposure at 1093 °C and was accompanied by disastrous scale spalling. The lowest chromium concentration within the depleted zone was 14 wt% which still provided a sufficient supply of chromium for development of a continuous Cr₂O₃ rich scale. Decarburization was observed at the higher temperature of 1093 °C, and a carbide-free zone developed. Also, it was found that Haynes 230 is subject to a sensitization process. At the lower exposure temperature of 871 °C, large amounts of chromium carbide formed preferentially at the grain boundaries. While at the surface region chromium carbide precipitation occurred at the twin boundaries.     

Synthesis of nano-TiC powder using titanium gel precursor and carbon particles

The paper presents a novel process for synthesis of nano-size titanium carbide by reaction between titanium bearing precursor gel and nano carbon particles derived from soot at different temperatures in the range of 1300-1580 °C for 2 h under argon cover. The HRTEM studies of TiC powder synthesized by heating at 1580 °C show the presence of cube shaped particles (~ 60-140 nm) and hollow rods (diameter ~ 30-185 nm). The average particle size of crystallites, calculated by Scherer equation is observed to be ~ 35 nm while the surface area-density measurements indicate it to be ~ 113 nm. The surface area decreases with increase in reaction temperature.     

Influence of the phase compositions on the transient-stage high-temperature oxidation behaviour of an NiCoCrAlY coating material

The oxidation behaviour of a commercial NiCoCrAlY coating material (H. C. Starck Inc.; Amperit 410) was investigated in the temperature range 850–1200°C. The measurements were made in air under isothermal and cyclic conditions. The oxidation process was quantified by thermogravimetry. The oxide compositions were characterized by energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectroscopy (SIMS). The corresponding oxide morphologies were studied by scanning electron microscopy (SEM). This study was focused on the oxide growth of a NiCoCrAlY alloy at temperatures between 850 and 1200°C up to oxidation periods of 25 h. Complementary information obtained from various surface-sensitive techniques are summarized in a simplified scheme of the oxide growth. The oxidation behaviour in the stated temperature range is explained by the different chromium and aluminium contents of the phases. Yttrium becomes effective at temperatures above 1000°C. The good protective properties of the NiCoCrAlY at temperatures above 1000°C and the fairly poor protection below 950°C are interpreted in terms of alumina formation and chromia formation as rate-determining factors.     

The effect of carbon containing atmosphere on the structure and mechanical properties of Fe-32Mn-9.4Al-1C-1.27Si alloy

The effect of a carbon containing atmosphere on the microstructure and mechanical properties of Fe-32Mn-9.4Al-1C-1.27Si alloy was investigated in this work. Surface oxide nodules and grain boundary oxides were found to form on this alloy when it was annealed in carbon-containing air at 1050 °C for 1 h. The oxidation reaction was thought to be the result of the green rot attack process. This alloy was embrittled severely by the carbon-containing air through the formation of surface oxide nodules and grain boundary oxide. The carbon-containing air enhanced the oxidation rate of this alloy at 1050 °C. The structure of the oxide nodules formed on this alloy in the carbon-containing air was similar to that observed on a FeMnAl alloy heated in 1000 °C air for 24 h.     

Oxidation behavior of green coke-based carbon–ceramic composites incorporating micro- and nano-silicon carbide

The oxidation resistance of the carbon–ceramic composites developed using green coke-based carbon and carbon black as carbon source, boron carbide, and micro- and nano-silicon carbide was carried out in the temperature range of 800 to 1,200 °C. Silicon carbide particulate as such and silicon carbide obtained by the reaction of green coke and silicon provided micro silicon carbide while silicon and carbon black and sol–gel silica and carbon black used as silicon carbide precursors led to the formation of nano-silicon carbide. The oxidation resistance of these composites at 800 to 1,200 °C for 10 h showed that the size of the silicon carbide influenced the oxidation resistance. The weight gain due to protective coating formed on oxidation was higher in composites containing nano-silicon carbide as compared to the composites containing micro silicon carbide.     

Properties of nanocrystalline SiC:Ge:H alloy deposited by hot-wire chemical vapor deposition using Organosilane and Organogermane

Nanocrystalline hydrogenated silicon carbide: germanium alloy (nc-SiC:Ge:H) films have been deposited by hot-wire chemical vapor deposition at a low substrate temperature of about 300 °C. Germanium incorporation into the films and film structure based on cubic silicon carbide were confirmed by X-ray photoelectron spectroscopy and X-ray diffraction. Optical absorption spectra of the films with a germanium mole fraction of about 2% shifted to lower energies by about 0.2 eV compared with that of nanocrystalline cubic silicon carbide films.     

Simultaneous carbothermic reduction of iron and titanium oxides to produce an iron-based composite by mechanically activated sintering method

In this research, for the first time, Fe–TiC nano-crystalline composite was produced via simultaneous reduction of iron and titanium oxides by petrocoke. Powder mixture of Fe₂O₃/TiO₂/petrocoke was mechanically activated in a high-energy ball mill at different times. X-ray diffraction method (XRD) and Scanning Electron Microscopy (SEM) were used to characterize the milled powders. The results showed that new phases were not formed during milling, even after 20 h of milling. However, crystallite size and lattice strain of hematite were remarkably decreased and increased, respectively. Thermogravimetry and Differential Thermal Analysis (TG–DTA) were done on 0, 10 and 20 h mechanically activated powders. These experiments showed a substantial decrease in reduction temperature of iron and titanium oxides as a result of mechanical activation. Then, the powders were cold compacted and sintered at 1200 °C in argon atmosphere for 1 h. XRD results of 20 h milled samples demonstrated that, in this condition, iron oxide was completely reduced to nano-crystalline iron and titanium dioxide was reduced to nano-crystalline titanium carbide and Fe–TiC nano-crystalline composite was successfully formed.     

Formation of WC powders using carbon coated precursors

This paper deals with the formation of tungsten carbide from carbon coated tungstic oxide precursors. This study makes use of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). DSC and TGA data for both coated and mixed 17.2 wt% carbon containing tungstic oxide demonstrate the superiority of the coated precursor in the formation of tungsten carbide, as conversion is initiated at lower temperature. XRD patterns of products from each 100°C isotherm from 900–1400°C, inclusive, illustrate the formation as it proceeds through lower oxides into tungsten then carburizes into tungsten carbide for both the mixed and coated samples.