Flexural creep of ZrB2/0–50 vol% SiC ceramics was characterized in oxidizing atmosphere as a function of temperature (1200°–1500°C), stress (30–180 MPa), and SiC particle size (2 and 10 μm). Creep behavior showed strong dependence on SiC content and particle size, temperature and stress. The rate of creep increased with increasing SiC content, temperature, and stress and with decreasing SiC particle size, especially, at temperatures above 1300°C. The activation energy of creep showed linear dependence on the SiC content increasing from about 130 to 511 kJ/mol for ceramics containing 0 and 50 vol% 2‐μm SiC, respectively. The stress exponent was about 2 for ZrB2 containing 50 vol% SiC regardless of SiC particle size, which is an indication that the leading mechanism of creep for this composition is sliding of grain boundaries. Compared with that, the stress exponent is about 1 for ZrB2 containing 0–25vol% SiC, which is an indication that diffusional creep has a significant contribution to the mechanism of creep for these compositions. Cracking and grain shifting were observed on the tensile side of the samples containing 25 and 50 vol% SiC. Cracks propagate through the SiC phase confirming the assumption that grain‐boundary sliding of the SiC grains is the controlling creep mechanism in the ceramics containing 50 vol% SiC. The presence of stress, both compressive and tensile, in the samples enhanced oxidation.
XRD and XAS were used to characterize the bulk structure, while XPS was used to characterize the surface structure, of commercially obtained nominally K4Fe(CN)6·3H2O, K3Fe(CN)6 and our synthesized Prussian Blue (PB) material. K4Fe(CN)6·3H2O was found to consist of a fully hydrated phase, which crystallizes in the monoclinic form and a less hydrated or anhydrous phase which crystallizes in the orthorhombic form. K3Fe(CN)6 was found to consist of the well-established orthorhombic form rather than the monoclinic form. The structure of our synthesized Prussian Blue (PB) was found to be consistent with that reported for (KOH)0.7Fe(III)1.33Fe(II)(CN)6·4.0H2O which crystallizes in the cubic form. XPS and XAS confirmed the presence of ferrous Fe(II) at the surface and bulk levels in K4Fe(CN)6·xH2O. However, XPS revealed the presence of Fe(II) (∼30%) and Fe(III) (∼70%) in the surface region of K3Fe(CN)6 while XAS confirmed the presence of mostly Fe(III) at the bulk level. Both XPS and XANES confirmed the presence of Fe(II) and Fe(III) in the surface and bulk regions of PB. This ex situ XAS study will be used to support the analysis of an in situ XAS data collected on a PB containing supercapacitor to understand the mechanistic origin of pseudocapacitance in these devices.
In situ X-ray absorption spectroscopy (XAS) along with electrochemical measurements were used to explore the redox activity of Prussian blue (PB) and its contribution to pseudocapacitance in an asymmetrical solid-state supercapacitor (SC). The SC, with a configuration of (activated carbon)/polymer electrolyte membrane/(activated carbon + PB), was fabricated with an embedded KCl saturated Ag/AgCl reference electrode. The potentials of the two electrodes were generally displaced proportionally with increase in the cell voltage. Upon negative polarization from the open circuit voltage (OCV) condition to −3 V, the charge capacity was greater than that upon positive polarization from the OCV condition to +3 V, in agreement with the higher fraction of Fe(III) than that of Fe(II) and the low concentration of K ions in PB. Detailed analysis of cyclic voltammetry data in the range between 0 and −3 V indicates that the battery-like contribution to the charge current was 96% that of the capacitor-like contribution at a scanning rate of 2.5 mV s−1 but only 15% at 100 mV s−1. The in situ XAS characterization results indicate that the majority of Fe was reversibly charged and discharged between Fe(III) and Fe(II) during positive and negative polarizations with minimal changes in local atomic structure.
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