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Vanadium oxide thin films were deposited using pulsed direct current (dc) magnetron sputtering in an atmosphere containing argon and oxygen. The total pressure was varied from 2.5 to 15 mTorr, and the oxygen-to-argon ratio was varied from 2.5 to 30%. The resulting films were characterized using Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and glancing incidence x-ray diffraction (GIXRD). Electrical resistivity was calculated from I–V curves acquired from two-point-probe measurements and thicknesses measured from bright-field TEM images of cross-sectioned samples. TEM and GIXRD were used to characterize the crystallinity of each film. A transition from nanocrystalline to amorphous growth was observed with increasing partial pressure of oxygen. In all samples, the only crystalline phase observed was cubic vanadium oxide with the sodium chloride structure. Though the cubic VOx equilibrium phase field is limited to a maximum of x = 1.3, the cubic phase was observed with a value of x up to 2 in the present work. It was apparent from electron diffraction data that increased oxygen content correlated with an increase in the film disorder. The increase in oxygen content also corresponded with an increase in the film resistivity, which varied over 7 orders of magnitude from 1.18 × 10−3 to 2.98 × 104 Ω·cm. The temperature coefficient of resistance was found to increase with increasing oxygen content from −0.1 to −3.5%/°C. A direct correlation between film disorder and temperature coefficient of resistivity (TCR) was observed and could be exploited to engineer materials with the desired TCR.

The present study compares the feasibility of controlling the properties of a glass composite seal by adding nano‐ or micron‐scale yttria‐stabilized zirconia (YSZ) powders to a borate glass used for sealing electrolyte‐supported solid oxide fuel cells (SOFCs). The crystallization of the glass composites was found to be independent of the volume fraction of added YSZ, for both sizes of the additive. The variation of the flow properties of both composite seals was measured using a wettability test, and an increase of the contact angle was observed when the volume fraction of additives was increased. The major factor found to decrease spreading of the glass composite was the additive particle size, where shape retention was observed for the nanometer (nm)‐YSZ composites while spreading of the micrometer (μm)‐YSZ composites was observed under the same testing conditions. Examination of the microstructure showed that initially the Ba‐containing glass reacted with YSZ to form a BaZrO3 compound. Long‐time exposure at 800°C caused a large reduction in the coefficient of thermal expansion (CTE), which can be explained by increased formation of BaZrO3 and further change in glass composition. This change in CTE occurs rapidly for the nm‐YSZ composites, which is not observed for the μm‐YSZ composites. However, the adverse reactions occurring between the additives and the glass matrix were found to reduce the CTE of the glass composites to a value lower than the recommended limit for a system used for sealing SOFCs.

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Mixtures of Zr+B and Hf+B were shock compacted into bulk samples possessing relative densities above 95.5% and were subsequently converted to ZrB2 and HfB2 ceramic components by a heat treatment. The conversion temperature was varied between 1600° and 2000°C. The conversion temperature was found to have no effect on the final density of the ceramics. Theoretical densities of 72% and 62% were obtained for the converted ZrB2 and HfB2 ceramics, respectively. Increasing the heat‐treatment temperature promoted grain growth rather than densification for the ZrB2 samples. The grain size increased from 1.8±0.6 to 5.6±1.3 to 8.5±3.3 μm, for heat treatments at 1600°, 1800°, and 2000°C, respectively. No grain growth was observed for the HfB2 system, which exhibited a grain structure of 5.0±1.6, 3.3±1.5, and 4.4±2.2 μm for the same temperature range studied. Microhardness values for the ZrB2 decreased from 19.4±0.4 to 17.2±0.6 down to 13.7±0.6 GPa, while similar hardness results of 19.1±0.8, 17.1±1.0, and 17.8±0.5 GPa were observed for the HfB2 samples.

Rocksalt-structured vanadium oxide VO x (x = 0.8 -1.3) nanocrystal thin films are used in infrared imaging devices due to its high temperature coefficient of resistance (TCR). The stoichiometry of VO x is closely related with the defects in the crystalline lattice, which influence the TCR. However, it is very difficult to quantify the stoichiometry of VO x by energy-dispersive x-ray spectroscopy because of energy overlap and strong absorption of the low-energy V-L and O-K peaks. In general, electron energy loss spectroscopy (EELS) is useful for determining the stoichiometry and valence of transition metal oxides by using 1) the k-factor method, 2) the ratio of the L 3 /L 2 white line intensities or 3) the normalized white line intensity in reference to the continuum. But for VO x , the energy of the O-K edge is very close to that of the V-L edge. Consequently, EELS quantification techniques are still problematic due to difficulty of separating the O-K signal from the V-L signal. The energy loss near edge structure (ELNES) of the oxygen K-edge of vanadium oxides reflects the local density of states (DOS) at the oxygen site and can be taken as "finger print" of the oxidation states (Figs. 1a, 1b). The four standards have similar local symmetry in their crystal structures -VO 6 type octahedron structures around V atom [1], therefore have similar energy levels right above the Fermi level -two t 2g +e g branches in DOS due to the V 3d -O 2p hybridization [1,2]. The change in the ELNES of oxygen K-edge when going from VO to V 2 O 5 (Fig. 1a) is due to the increased contribution from t 2g peak to oxygen K-edge.Close comparison of EEL spectra of VO and V 2 O 3 standards (Fig. 1c) reveals also an increased energy "span" (∆E 2 in Fig. 1c) of the O pre-edge peak when changing from VO to V 2 O 3 . This is because with decreasing vanadium valence (going from V 2 O 3 to VO), the DOS above the Fermi level also decreases; thus it appears that the DOS of the t 2g energy level is "truncated" by the Fermi level [1]. As the result, the energy "span" of t 2g +e g branches decreases and energy width of O preedge peak decreases. Fig. 2a shows a comparison of EEL spectra from an ion-beam sputtered VO x nanocrystalline thin film with that of VO and V 2 O 3 standards and indicates that VO x thin films have both a ∆E 1 and a ∆E 2 very similar to those of the VO standard. This reveals that the VO x nanocrystal thin film has stoichiometry very close to VO (V 2+ ). Additional EELS data reveals that the same VO x thin film has spatial inhomogeneity in the stoichiometry. Fig. 2b shows a comparison of EEL spectra from different areas in the VO x thin film. The two spectra show differences both in chemical shift (∆E 1 ) and in the energy width of O pre-edge peak (∆E 2 ), corresponding to VO (area 1) and V 2 O 3 (area 2), respectively. Also, the V-L 3 , L 2 peaks exhibit broadening in the spectra from "area 2" suggesting