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Liquid Corrosion and High-Temperature Oxidation Effects on Silicon Carbide/Titanium Diboride Composite
Journal of the American Ceramic Society
Abstract
A silicon carbide composite containing titanium diboride particulate reinforcement was subjected to room-temperature acidic corrosion in aqua regia solutions for 200 h, and 50% NaOH+50% H2O and 10% HF+57% HNO3+33% H2O solutions for up to 500 h. A small increase in room-temperature strength was observed after the aqua regia exposure. Separately, this ceramic was oxidized at 1400°C for 24 h and then tested for strength retention at room temperature. Contrary to the observations reported in the literature on similar material, there was no significant room-temperature strength decrease in this particular composite.
Two-dimensional materials, such as MXenes, are attractive candidates for energy storage and electrochemical actuators due to their high volume changes upon ion intercalation. Of special interest for boosting energy storage is the intercalation of multivalent ions such as Mg(2+), which suffers from sluggish intercalation and transport kinetics due to its ion size. By combining traditional electrochemical characterization techniques with electrochemical dilatometry and contact resonance atomic force microscopy, the synergetic effects of the pre-intercalation of K(+) ions are demonstrated to improve the charge storage of multivalent ions, as well as tune the mechanical and actuation properties of the Ti3C2 MXene. Our results have important implications for quantitatively understanding the charge storage processes in intercalation compounds and provide a new path for studying the mechanical evolution of energy storage materials.
SiC/TiB2 composites have been exposed to aqua regia solution for 200 h, to 50% aqueous NaOH, and to 10% HF/57% HNO3/H2O solution for up to 500 h at room temperature.
Formulating room-temperature ionic liquid (RTIL) mixed electrolytes was recently proposed as an effective and convenient strategy to increase the capacitive performance of electrochemical capacitors. Here we investigate the electrical double-layer (EDL) structure and the capacitance of two RTILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), and their mixtures with onion-like carbon electrodes using experiment and classical density functional theory. The principal difference between these ionic liquids is the smaller diameter of the BF4– anion relative to the TFSI– anion and the EMI+ cation. A volcano-shaped trend is identified for the capacitance versus the composition of the RTIL mixture. The mixture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance. These theoretical predictions are in good agreement with our experimental observations and offer guidance for designing RTIL mixtures for EDL supercapacitors.
Ceramics used for different applications are affected not only by gas atmospheres, they can also be exposed to liquids, viz. solutions and melts. Corrosion of metals in liquid media is usually subdivided into chemical (in nonelectrolytes and molten metals) and electrochemical (in electrolytic solutions and melts). Since the corrosion mechanisms of ceramic materials in liquid media have not yet been studied thoroughly enough, we use basic chemical laws to describe the process. Under real conditions the processes of corrosive damage of structural ceramics, especially conducting SiC- and B4C-based ones, can also be described from the point of view of electrochemistry using, e.g., the theory of microgalvanic cells by Akimov.
The knowledge of the corrosion behaviour of ceramic materials is rather important for determining their fields of application (refractories, high-temperature insulation and others), especially when oxidation-induced variations of mechanical properties (including local damages) have no effect on the performance of a ceramic design. However, to specify the load carrying capacity of components of gas turbines and Diesel engines, this information is insufficient, since corrosion-induced variations of composition and structure of a ceramic surface influence the strength of the material (Table 5.1), lead to the failure of components and the engine itself during operation. Oxidation also exerts a considerable influence on the physical properties, e.g. thermal and electrical conductivity. It involves the growth of SiO2 contents in ceramics and an increase of their thermal linear expansion coefficients as a result [5.1]. But conclusions of possible oxidation-induced changes of ceramic properties can be made only from the investigations of corrosion mechanism and kinetics. Therefore, the impact of corrosion on the physico-mechanical properties of ceramics requires further examination.
Polarization curves and Auger spectroscopy are used to study the kinetics, formation mechanism, and layer-by-layer phase composition
of oxide films resulting from the anodic oxidation of SiC–TiB2–B4C ceramics containing 10 wt.% and 40 wt.% TiB2 in 3% NaCl solution. It is shown that a two-layer oxide film forms in the former case (β-Ti2O3 is the lower layer and TiO2 the upper one) and a three-layer film forms in the latter case (nonstoichiometric β-TiO is the upper and lower layers and
β-Ti2O3 the intermediate one). All ceramic samples, especially those with 10 wt.% TiB2, have high corrosion resistance.
The thermodynamic stability of SiC/SiC composite structures proposed for fusion applications is presented in this paper. Minimization of the free energy for reacting species in the temperature range 773–1273 K is achieved by utilizing the NASA-Lewis Chemical Equilibrium Thermodynamics Code (CET). The chemical stability of the matrix (SiC), as well as several fiber coatings (BN and graphite) are studied. Helium coolant is assumed to contain O2 and water moisture impurities in the range 100–1000 ppm. The work is applied to recent Magnetic and Inertial Confinement Conceptual designs. The present study indicates that the upper useful temperature limit for SiC/SiC composites, from the standpoint of high-temperature corrosion, will be in the neighborhood of 1273 K. Up to this temperature, corrosion of SiC is shown to be negligible. The main mechanism of weight loss will be by evaporation to the plasma side. The presence of a protective SiO2 condensed phase is discussed, and is shown to result in further reduction of high-temperature corrosion. The thermodynamic stability of C and BN is shown to be very poor under typical fusion reactor conditions. Further development of chemically stable interface materials is required.
Fatigue-crack growth behavior under cyclic tension-tension loading has been examined in a 30-vol%-TiB2-reinforced β-SiC matrix composite, using compact tension specimens tested at temperatures from 750° to 850°C in a furnace air environment. Cyclic fatigue-crack growth rate was found to depend strongly on testing temperature and was faster at a lower temperature. Such an anomalous dependence of crack growth rate on temperature is shown to result from the reduction in crack growth driving force, ΔK, by the wedging action of the oxide film formed behind the crack tip. Subtraction of the contribution from oxide films leads to a “normal” Arrhenius relation between cyclic crack growth rate and temperature, with the activation energy equal to that for the viscous flow of grain boundary phase.
The oxidation behavior of MoSi2-based composites reinforced with 30 vol.% TiB2, ZrB2, HfB2 or SiC were tested under isothermal and cyclic conditions at temperatures from 500 to 1500°C. The effects of thermal cycling on subsequent pesting behavior were studied. A second series of cyclic oxidation and pesting tests was then performed on MoSi2 composites containing from 15 to 45 vol.% TiB2 or SiC, in order to determine the effects of reinforcement volume fraction and porosity on oxidation behavior. Oxidation kinetics of the composites under static pesting (500°C) conditions and cyclic (1200°C peak temperature) conditions were studied. Scanning electron microscopy analyses were performed to determine the structures and compositions of the oxide layers formed under various test conditions.
The effects of TiB//2 addition to alpha -SiC on mechanical, thermal, and electrical behavior were determined. Data on mechanical, thermal, and electrical properties of SiC-TiB//2 particulate composite material have demonstrated improvements especially in the mechanical strength and fracture toughness over a-SiC. The TiB//2 particles are found to control the matrix grain size by the mechanism of pinning. The ability of the TiB//2 particles to cause crack-tip deflection results in increases to inherent mechanical properties of the matrix material. Thus, SiC-TiB//2 has improved mechanical properties as a direct result of the TiB//2 particulate.
The effects of TiB/sub 2/ addition to ..cap alpha..-SiC on mechanical, thermal, and electrical behavior were determined. The reliability of ceramics for structural applications is, in general, limited by variability in mechanical strength and low toughness. Property variability is normally attributed to processing- or microstructure-related flaws. Examples of microstructural flaws are exaggerated grain growth, voids and inclusions, while processing-related flaws can include impurities and nonuniform sintered material densification. These flaws can exert significant influence on nominal mechanical properties and their associated time-dependent behavior.
The strength, fracture toughness, and oxidation behavior of a hot-pressed SiC-TiB/sub 2/ composite (with B and C additions) were investigated. Adding 15 vol% TiB/sub 2/ to an SiC matrix increased the mean strength and mean fracture toughness of the composite by 28% and 45%, respectively. Oxidation of the SiC-15-vol%-TiB/sub 2/ composite occurred slowly at 1000C and 1200C but was catastrophic at 1400C.
“Corrosion Resistance of Silicon Carbides to Acids and Bases”
- M. Srinivasan
- T. Young
- N. Burlingame
A Microanalytical Study of a SiC-Based TiB2Particulate Composite”
- A Zangvil
- J E Garnier
- . H Mcmurtry
Silicon Carbide for Mechanical Shaft Seals”
- R W Lashway
Burlingame “Corrosion Resistance of Silicon Carbides to Acids and Bases”; presented at the Fall Meeting of the Basic Science Division of the
- M Srinivasan
- T Young