Search for: All records

This work systematically investigates the thermodynamic stability of Si_aO_b(M)_cC_dstructures derived from polymeric precursors incorporating metal fillers: Ta, Nb, and Hf, at 1200 and 1500°C. Structural characterization of the polymer derived ceramics (PDCs) employs X‐ray diffraction, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. Enthalpies of formation relative to crystalline components (metal oxide, silica, silicon carbide, and graphite) are obtained from thermodynamic measurements by high temperature oxide melt solution calorimetry. The enthalpies of formation (∆H°_{f, comp}) of Ta‐1200, Hf‐1200, Nb‐1200, Ta‐1500, Hf‐1500, and Nb‐1500 specimens are −137.82 ± 9.72, −256.31 ± 8.97, −82.80 ± 9.82, −182.80 ± 7.85, −292.54 ± 9.38, −224.98 ± 9.60 kJ/mol, respectively. Overall incorporation of Hf results in most thermodynamically stable structures at all synthesis temperatures. Si_aO_b(M)_cC_dspecimens employing Nb fillers undergo the most stable structural evolution in this temperature range. The results indicate strong thermodynamic drive for carbothermal reduction of metal oxide domains. Incorporation of Ta provides the greatest stabilization of SiO₃C mixed bonding environments. Ultimately, the choice of metal filler influences composition, structural evolution, and thermodynamic stability in PDCs.

This study presents new experimental data on the thermodynamic stability of SiC(O) and SCN(O) ceramics derived from the pyrolysis of polymeric precursors: SMP‐10 (polycarbosilane), PSZ‐20 (polysilazane), and Durazane‐1800 (polysilazane) at 1200°C. There are close similarities in the structure of the polysilazanes, but they differ in crosslinking temperature. High‐resolution X‐ray photoelectron spectroscopy shows notable differences in the microstructure of all polymer‐derived ceramics (PDCs). The enthalpies of formation (∆H°_{f, elem}) of SiC(O) (from SMP‐10), SCN(O) (from PSZ‐20), and SCN(O) (from Durazane‐1800) are −20 ± 4.63, −78.55 ± 2.32, and −85.09 ± 2.18 kJ/mol, respectively. The PDC derived from Durazane‐1800 displays greatest thermodynamic stability. The results point to increased thermodynamic stabilization with addition of nitrogen to the microstructure of PDCs. Thermodynamic analysis suggests increased thermodynamic drive for forming SiCN(O) microstructures with an increase in the relative amount of SiN_xC_4−xmixed bonds and a decrease in silica. Overall, enthalpies of formation suggest superior stabilizing effect of SiN_xC_4−xcompared to SiO_xC_4−xmixed bonds. The results indicate systematic stabilization of SiCN(O) structures with decrease in silicon and oxygen content. The destabilization of PDCs resulting from higher silicon content may reach a plateau at higher concentrations.

A liquid‐phase polymer‐to‐ceramic approach is reported for the synthesis of hafnium carbide (HfC)/hafnium oxide (HfO₂) composite particles from a commercial precursor. Typically, HfC ceramics have been obtained by sintering of fine powders, which usually results in large particle size and high porosity during densification. In this study a single‐source liquid precursor was first cured at low temperature and then pyrolyzed at varying conditions to achieve HfC ceramics. The chemical structure of the liquid and cured precursors, and the resulting HfC ceramics was studied using various analytical techniques. The nuclear magnetic resonance and Fourier transform infrared spectroscopy indicated the presence of partially hydrated hafnium oxychloride (Hf–O–Cl·nH₂O) in the precursor. Scanning electron microscopy of the resulting HfC crystals showed a size distribution in the range of approx. 600–700 nm. The X‐ray diffraction of the pyrolyzed samples confirmed the formation of crystalline HfC along with monoclinic‐HfO₂and free carbon phase. The formation of HfO₂in the ceramics was significantly reduced by controlling the low‐temperature curing temperature. Pyrolysis at various temperatures showed that HfC formation occurred even at 1000°C. These results show that the reported precursor could be promising for the direct synthesis of ultrahigh temperature HfC ceramics and for precursor infiltration pyrolysis of reinforced ceramic matrix composites.

Energy storage devices beyond lithium-ion batteries (LIBs), such as sodium-ion, potassium-ion, lithium-sulfur batteries, and supercapacitors are being considered as alternative systems to meet the fast-growing demand for grid-scale storage and large electric vehicles. This perspective highlights the opportunities that Si-based polymer-derived ceramics (PDCs) present for energy storage devices beyond LIBs, the complexities that exist in determining the structure-performance relationships, and the need forin situand operando characterizations, which can be employed to overcome the complexities, allowing successful integration of PDC-based electrodes in systems beyond LIBs.

The superior properties, such as large interlayer spacing and the ability to host large alkali-metal ions, of two-dimensional (2D) materials based on transition metal di-chalcogenides (TMDs) enable next-generation battery development beyond lithium-ion rechargeable batteries. In addition, compelling but rarely inspected TMD alloys provide additional opportunities to tailor bandgap and enhance thermodynamic stability. This study explores the sodium-ion (Na-ion) and potassium-ion (K-ion) storage behavior of cation-substituted molybdenum tungsten diselenide (MoWSe₂), a TMD alloy. This research also investigates upper potential suspension to overcome obstacles commonly associated with TMD materials, such as capacity fading at high current rates, prolonged cycling conditions, and voltage polarization during conversion reaction. The voltage cut-off was restricted to 1.5 V, 2.0 V, and 2.5 V to realize the material’s Na⁺and K⁺ion storage behavior. Three-dimensional (3D) surface plots of differential capacity analysis up to prolonged cycles revealed the convenience of voltage suspension as a viable method for structural preservation. Moreover, the cells with higher potential cut-off values conveyed improved cycling stability, higher and stable coulombic efficiency for Na⁺and K⁺ion half-cells, and increased capacity retention for Na⁺ion half-cells, respectively, with half-cells cycled at higher voltage ranges.

Three crystalline SiC fibers were studied: Tyranno, Hi‐Nicalon, and Sylramic. Thermodynamic stability of the SiC fibers was determined by high temperature oxide melt solution calorimetry. Results shed light on the thermodynamic penalty or benefit associated with microstructural modification of the ceramic fibers, and how energetics correlate to mechanical properties. Enthalpies of formation from components (SiC, SiO₂, Si₃N₄, and C, ∆H°_f,comp) for Tyranno, Hi‐Nicalon, and Sylramic are −12.05 ± 8.71, −58.75 ± 6.93, and −71.10 ± 8.71 kJ/mol Si, respectively. The microstructure in Sylramic offers the greatest stabilizing effect, thus resulting in its much more exothermic enthalpy of formation relative to elements and crystalline components. In contrast, the microstructure in Tyranno offers the least stabilization. The thermodynamic stability of the fibers increases with increasing mixed bonding (Si bonded to both C and O). From mechanical testing, Young's moduli of Tyranno, Hi‐Nicalon, and Sylramic are 112, 205, and 215 GPa, respectively. Greater thermodynamic stability is correlated with a higher Young's modulus.

Cross‐linking polymethylhydrosiloxane (PMHS) with divinylthiophene (DVT) via hydrosilylation in highly dilute conditions and subsequent supercritical drying in CO₂yield a polymeric aerogel containing aromatic sulfur integrally and uniformly distributed throughout the monolith. Fourier‐transform infrared (FT‐IR) spectroscopy indicates almost complete consumption of vinyl groups and SiH bonds in the product. Both FT‐IR and Raman spectroscopic analyses support loss of conjugation of vinyl groups with the retained double bonds of the thiophene ring. Scanning electron microscopy (SEM) indicates a condensed colloidal structure with characteristic particulate diameters of about 165 nm. SEM coupled with energy dispersive X‐ray spectroscopy elemental mapping shows that sulfur is distributed homogeneously in the polymeric aerogel. Porosimetry of the mesoporous aerogel indicates the effective average pore diameters are about 12 nm. Thermogravimetric analysis (TGA) establishes greater thermal stability of the PMHS‐DVT product than either of the pure unreacted components. TGA coupled with mass spectrometric (TG‐MS) identification of the volatiles released during pyrolysis shows that sulfur is driven from the cross‐linked polymer as thiophene and its derivatives. Recorded mass spectra support the hypothesis that cross‐linking DVT bridges between PMHS chains in the polymeric aerogel, and that this results in a more thermally stable monolith.

Successfully synthesized Si(Nb)OC composites through single source precursor route and as-pyrolyzed Si(Nb)OC demonstrate good rate capability due to uniformly distributed nanosized Nb₂O₅and graphitic carbon structure in the amorphous SiOC matrix.