We report on the structure and dielectric properties of ternary A6B2O17(A = Zr; B = Nb, Ta) thin films and ceramics. Thin films are produced via sputter deposition from dense, phase‐homogenous bulk ceramic targets, which are synthesized through a reactive sintering process at 1500°C. Crystal structure, microstructure, chemistry, and dielectric properties are characterized by X‐ray diffraction and reflectivity, atomic force microscopy, X‐ray photoelectron spectroscopy, and capacitance analysis, respectively. We observe relative permittivities approaching 60 and loss tangents <1 × 10−2across the 103–105 Hz frequency range in the Zr6Nb2O17and Zr6Ta2O17phases. These observations create an opportunity space for this novel class of disordered oxide electroceramics.
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
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Abstract 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 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. -
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The electrical properties of the entropy stabilized oxides: Zr6Nb2O17, Zr6Ta2O17, Hf6Nb2O17and Hf6Ta2O17were characterized. The results and the electrical properties of the products (i.e. ZrO2, HfO2, Nb2O5and Ta2O5) led us to hypothesize the A6B2O17family is a series of mixed ionic-electronic conductors. Conductivity measurements in varying oxygen partial pressure were performed on A6Nb2O17and A6Ta2O17.The results indicate that electrons are involved in conduction in A6Nb2O17while holes play a role in conduction of A6Ta2O17. Between 900 °C–950 °C, the charge transport in the A6B2O17system increases in Ar atmosphere. A combination of DTA/DSC and in situ high temperature X-ray diffraction was performed to identify a potential mechanism for this increase. In-situ high temperature X-ray diffraction in Ar does not show any phase transformation. Based on this, it is hypothesized that a change in the oxygen sub-lattice is the cause for the shift in high temperature conduction above 900 °C–950 °C. This could be:
(i) Nb(Ta)4+- oxygen vacancy associate formation/dissociation,(ii) formation of oxygen/oxygen vacancy complexes(iii) ordering/disordering of oxygen vacancies and/or(iv) oxygen-based superstructure commensurate or incommensurate transitions. In-situ high temperature neutron diffraction up to 1050 °C is required to help elucidate the origins of this large increase in conductivity. -
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Abstract The direct selective laser sintering (SLS) process was successfully demonstrated for additive manufacturing of high-entropy carbide ceramics (HECC), in which a Yb fiber laser was employed for ultrafast (in seconds) reactive sintering of HECC specimens from a powder mixture of constitute monocarbides. A single-phase non-equiatomic HECC was successfully formed in the 4-HECC specimen with a uniform distribution of Zr, Nb, Hf, Ta, and C. In contrast, a three-layer microstructure was formed in the 5-HECC specimen with five metal elements (Zr, Nb, Hf, Ta and Ti), consisting of a TiC-rich top layer, a Zr–Hf–C enriched intermediate layer, and a non-equiatomic Zr–Ta–Nb–Hf–C HECC layer. Vickers hardness of 4- and 5-HECC specimens were 22.2 and 21.8 GPa, respectively, on the surface. These findings have important implications on the fundamental mechanisms governing interactions between laser and monocarbide powders to form a solid solution of HECCs during SLS.
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The use of thin Ta3N5films in tandem Si‐Ta3N5photoelectrochemical (PEC) devices motivates understanding of the surface Ta3N5properties, as they may have a strong effect on the device performance. The bulk and surface properties can change as a function of nitridation temperature; thus its effect is studied, ranging from 700 to 1000 °C, on the PEC performance, morphology, and composition of thin (10 nm) Ta3N5films deposited on planar and nanostructured Si substrates. Scanning electron microscopy (SEM), scanning Auger electron spectroscopy (AES), X‐ray photoelectron spectroscopy (XPS), and X‐ray diffraction (XRD) are employed to gain fundamental understanding in the differences of the Ta3N5films. By controlling Ta3N5morphology and composition with nitridation temperature, it is determined that Ta3N5with high crystallinity and surface N/Ta ratio, synthesized at 800 °C, yields the highest PEC performance with the earliest photocurrent onset and highest photocurrent. Samples nitrided at 700 °C have lower crystallinity and that likely leads to lower performance. For samples nitrided at temperatures above 800 °C, the N/Ta ratio decreases forming chemically reduced tantalum nitride phases, as well as N‐deficient and correspondingly O‐rich morphological domains that can adversely affect the PEC performance as hole‐blocking layers or O trap‐mediated recombination centers at the surface of Ta3N5.
