Temperatures In Oxidizing Environments Research Articles

Low‐temperature curable adhesive for joining SiC/SiC components: Their application in wide range of temperatures

AbstractHerein we report the formulation of phenolic resin–based adhesive to join SiC substrates for a wide range of application temperatures. The formulation consists of phenolic resin as a binder and a mixture of B4C, Al2O3, Y2O3, Si, and SiO2 powders as active filler. The X‐ray diffraction, thermogravimetric analysis, porosity, and scanning electron microscopy analysis were carried out to understand the reactions occurring within this adhesive during its exposure to application temperatures. These results were also used to understand thermal stability and morphological changes that are involved in the formulated adhesive during its application. The shear strength values of lap joints of SiC substrates made with this phenolic adhesive after exposure to temperatures between 200 and 1500°C were studied to evaluate its adhesive capacity. Obtained SiC joints with this adhesive have shown an excellent lap shear strength value of 24 MPa after its cure. These joints were also found to be intact with good shear strength values of more than 5 MPa even after exposure to temperatures ranging from 200 to 1500°C. The results further confirm that the adhesive is potentially viable for joining SiC segments for their application at a wide range of temperatures in oxidative environments.

Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111).

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C1-C4) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111) using ultrahigh vacuum temperature programmed desorption (UHV-TPD). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The hydrogen-bonded (H-bonded) networks of small alcohols on Au(111), except for 1-butanol and isobutanol, have been previously imaged on the molecular level at low temperatures by scanning tunneling microscopy. Primary C1-C3 alcohols exhibit planar H-bonded long extended zigzag chain networks while 2-butanol arranges in tetramer clusters of H-bonded molecules due to steric hindrance inhibiting the proximity of molecules on Au(111). Herein, the desorption energy of small primary alcohols was shown to trend linearly with increasing C1-C4 carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. Butanol isomer studies allow the prediction of isobutanol long extended chains in contrast to tetramers. The distinction between the desorption of butanol isomers highlights the role of intermolecular interactions due to the difference in molecular packing structures on Au(111). Furthermore, by studying the energetics of terrace H-bonded networks in comparison with molecular adsorption at undercoordinated step edge and kink defect sites, it is shown that the contribution of stabilizing van der Waals forces to the overall adsorption energy is less for small alcohols adsorbed at kink sites (3.1 kJ mol-1 per CH2) and similar for those adsorbed at step edge (4.8 kJ mol-1 per CH2) and Au terrace sites (4.9 kJ mol-1 per CH2).

Modeling Strength Degradation of Fiber-Reinforced Ceramic-Matrix Composites Subjected to Cyclic Loading at Elevated Temperatures in Oxidative Environments

In this paper, the strength degradation of non-oxide and oxide/oxide fiber-reinforced ceramic-matrix composites (CMCs) subjected to cyclic loading at elevated temperatures in oxidative environments has been investigated. Considering damage mechanisms of matrix cracking, interface debonding, interface wear, interface oxidation and fibers fracture, the composite residual strength model has been established by combining the micro stress field of the damaged composites, the damage models, and the fracture criterion. The relationships between the composite residual strength, fatigue peak stress, interface debonding, fibers failure and cycle number have been established. The effects of peak stress level, initial and steady-state interface shear stress, fiber Weibull modulus and fiber strength, and testing temperature on the degradation of composite strength and fibers failure have been investigated. The evolution of residual strength versus cycle number curves of non-oxide and oxide/oxide CMCs under cyclic loading at elevated temperatures in oxidative environments have been predicted.

Modelled decomposition mechanism of flame retarded poly(vinyl acetate) by melamine isocyanurate

Model polymer poly(vinyl acetate) (PVAc) was combined with melamine isocyanurate (MIC) as intumescent flame retardant composite. This work emphasises on the study of the decomposition mechanism and compared with a single fire test in order to show its performance as flame retarded composite. As such, mass loss cone calorimetry tests were performed and compared with it inert and oxidative decompositions in lab tests. PVAc/MIC composites cannot be ignited with high loadings MIC. Difference curves in oxidative conditions revealed that the composites with MIC show only positive differences, indicating a high stabilisation, especially in the temperature region of a mild flame (500–700 °C). Experiments performed with TG coupled with mass spectroscopy and analyses on partially degraded residues with solid-state 13C-NMR enabled the construction of the complete decomposition mechanism. MIC has both a heat-sink FR activity as well as a crosslinking role in the stabilization of the polymer. The latter then creates a highly stabilised char at elevated temperatures in oxidative environments. The formed char starts to degrade only 100 °C higher than the pure polymer within a heating. The degradation of formed char of pure PVAc is autocatalytic; this behaviour disappears upon addition of sufficient amount of MIC. Therefore, the transport of fuel towards the gaseous phase is limited, creating an efficient flame retardant system.

High‐temperature oxidation of nickel and chromium studied with an in‐situ environmental scanning electron microscope

AbstractInvestigations of the morphology of metal oxide scales formed at high temperatures in oxidative environments are usually undertaken after exposure of the samples is completed. In this study, an environmental scanning electron microscope (ESEM) was used as a tool for the in‐situ observation of oxide scale formation. Pure nickel and chromium samples were oxidized at a temperature of 973 K in either pure oxygen or water vapour at a pressure of 667 Pa. The evolution of an oxide scale was followed in‐situ for up to 3 h. The morphology of the developing oxide scales was found to be a function of the metal substrate and the gaseous species. The growth mechanisms of the different metal oxide scales are reviewed and related to the analysed in‐situ images. Emphasis is placed on the relationship between oxidation mechanism and scale morphology. Nickel is seen to oxidise by outward diffusion of nickel probably on oxide grain boundaries when exposed to oxygen. Water vapour changes the scale morphology and a duplex‐type scale arises due to preferential overgrowth. The scale which develops due to chromium oxidation in oxygen is a fine‐grained, thin, and dense layer. In contrast, water vapour leads to whisker growth on chromium and an open, felt‐like structure forms. The applicability of the ESEM to the study of such systems is demonstrated, and its limitations are outlined. The results are encouraging examples of the possibilities which the in‐situ ESEM technique possesses.

Oxidative Constructing of Thin-Walled Ceramics (OCTC)

Modern engineering widely uses high temperatures in an oxygen atmosphere, i.e., in an oxidative environment. This calls for highly oxygen-resistant materials. Metals and metal alloys whose mechanical strength and plasticity are suitable for designing the most sophisticated shapes often do not meet the current demands on thermal stability, especially in the presence of silicates; borates; alkalis; sulfur-, vanadium-, and lead-containing compounds, etc. Oxygen-free ceramics, e.g., metal carbides and nitrides, are more heat resistant; however, even these materials become unstable at high temperatures in oxidative environments and fail after a time to eventually produce oxides. Therefore, using oxide ceramics in high-temperature processes has a crucial advantage. Modern methods for manufacturing materials from oxides eventually involve sintering powders either of these oxides or of their chemical precursors. Sintering imposes severe constraints on the shape of resulting products, especially in fabricating thin-walled profiles. We suggest a fundamentally novel method for creating thin-walled oxide ceramics, based on the controlled oxidation of metallic preforms with retention of the preform’s shape. Metal oxidation is a chemical process addressed in innumerable publications. However, primary emphasis heretofore has been placed on protecting metals from

The Stability of BN Interfacial Coatings in CFCC Systems During Oxidation and Exposure to Moisture

Abstract The mechanical reliability of ceramic matrix composites (CMCs) at elevated temperatures in oxidative environments is primarily dependent upon the chemical and structural stability of the fiber/matrix interface. Graphitic carbon coatings have traditionally been used to control the interfacial properties in CMCs, however, their use is limited in high temperature oxidative environments due to the loss of carbon and subsequent oxidation of the fiber and matrix. Thus, BN is being investigated as an alternative interfacial coating since it has comparable room temperature properties to carbon with improved oxidation resistance. The stability of BN interfaces in SiC/SiC composites is being investigated at elevated temperatures in either flowing oxygen or environments containing water vapor. The effect of several factors on BN stability, including crystallographic structure, extent of BN crystallization, and impurity content, are being evaluated. Nicalon™ fiber preforms were coated with ≈ 0.4 μm of BN by CVD using BCl3, NH3, and H2 at 1373 K. The coated preforms were densified using a forced-flow chemical vapor infiltration (FCVI) technique developed at ORNL.