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 (∆
This content will become publicly available on November 1, 2024
This work systematically investigates the thermodynamic stability of SiaOb(M)cCdstructures 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. SiaOb(M)cCdspecimens 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.
more » « less- Award ID(s):
- 1743701
- NSF-PAR ID:
- 10469318
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- International Journal of Applied Ceramic Technology
- Volume:
- 20
- Issue:
- 6
- ISSN:
- 1546-542X
- Page Range / eLocation ID:
- 3395 to 3406
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract 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 SiNx C4−x mixed bonds and a decrease in silica. Overall, enthalpies of formation suggest superior stabilizing effect of SiNx C4−x compared to SiOx C4−x mixed 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. -
more » « less
Abstract Hafnium carbide (HfC) is an ultrahigh‐temperature ceramic with high melting point, chemical stability, hardness, and wear resistance. However, its low fracture toughness and poor thermal shock resistance limit its structural applications in extreme environments. In this study, co‐curing of liquid precursors was carried out prior to complete pyrolysis of individual polymeric precursors. First, HfC preceramic polymer precursor was cured, followed by silicon carbonitride (SiCN) precursor curing on a 2D carbon fiber (CF) cloth using the drop‐coating process. The infiltrated CFs were pyrolyzed at 800°C to achieve CF/HfC‐SiCN ceramic mini‐composites. The cross‐linked precursor‐to‐ceramic yield was observed to be as high as 65% when the procedure was carried out in an inert environment. Although stable up to 1200°C, CF/HfC‐SiCN samples demonstrated susceptibility to oxidation at 1500°C in ambient air. The oxidation of HfC in the presence of SiC leads to the formation of a hafnium‐containing silicate (Hf
x Siy Oz ) along with hafnia (HfO2). This compound of silicate and hafnia limits oxygen diffusion better than SiO2and HfO2individually. The incorporation of SiCN in HfC ceramic led to improved phase stability compared to a neat HfC system. The results of this study also show that the use of liquid‐phase precursors for HfC and SiCN in the polymer‐infiltrated pyrolysis method is a promising approach to fabricating high‐temperature structural ceramic matrix composites with good oxidation resistance. -
Among the series of stable closo -borate dianions, [B n H n ] 2− , the X-ray crystallographic structure of [B 7 H 7 ] 2− was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp 2 M) 2 B 9 H 11 ] (Cp = η 5 -C 5 H 5 ; M = Zr or Hf). The structures of [(Cp 2 M) 2 B 9 H 11 ] contain a pentagonal bipyramidal B 7 core, coordinated by two {Cp 2 M} and two {BH 2 } units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp 2 M) 2 B 9 H 11 ] complexes are substantially more stable than the parent dianion, in either [B 7 H 7 ] 2− or ( n Bu 4 N) 2 [B 7 H 7 ]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo -{Cp 2 M} units, as well as electrostatic interactions between the {Cp 2 M} units and the B 7 core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.more » « less
-
more » « less
Abstract (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high‐entropy ceramics (HEC) with a submicron grain size of 400 to 600 nm were fabricated by spark plasma sintering using a two‐step sintering process. Both X‐ray and neutron diffractions confirmed the formation of single‐phase with rock salt structure in the as‐fabricated (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C samples. The effect of submicron grain size on the thermal stability and mechanical properties of HEC was investigated. The grain growth kinetics in the fine‐grained HEC was small at 1300 and 1600°C, suggesting high thermal stability that was possibly related to the compositional complexity and sluggish diffusion in HEC. Compared to the coarse‐grain HEC with a grain size of 16.5 µm, the bending strength and fracture toughness of fine‐grained HEC were 25% and 20% higher respectively. The improvement of mechanical properties in fine‐grained HEC may be attributed to micromechanistic mechanisms such as crack deflection.
-
The present work details experimental phase stabilization studies for the disordered, multi-cation A6B2O17 (A = Zr, Hf; B = Nb, Ta) system. We leverage both high-temperature in situ and ex situ X-ray diffraction to assess phase equilibrium and metastability in A6B2O17 ceramics produced via reactive sintering of stoichiometric as-received powders. We observe that the A6B2O17 phase can be stabilized for any stoichiometric combination of Group 4B and 5B transition metal cations (Zr, Nb, Hf, Ta), including ternary and quinary systems. The observed minimum stabilization temperatures for these phases are generally in agreement with prior calculations for each disordered A6B2O17 ternary permutation, offering further support for the inferred cation-disordered structure and suggesting that chemical disorder in this system is thermodynamically preferable. We also note that the quinary (Zr3Hf3)(NbTa)O17 phase exhibits enhanced solubility of refractory cations which is characteristic of other high-entropy oxides. Furthermore, A6B2O17 phases experience kinetic metastability, with the orthorhombic structure remaining stable following anneals at intermediate temperatures.more » « less
