An official website of the United States government
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.
more »
« less
Dielectric properties of disordered A 6 B 2 O 17 (A = Zr; B = Nb, Ta) phases
Abstract 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.
more »
« less
- PAR ID:
- 10645706
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of the American Ceramic Society
- Volume:
- 107
- Issue:
- 10
- ISSN:
- 0002-7820
- Format(s):
- Medium: X Size: p. 6868-6875
- Size(s):
- p. 6868-6875
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract 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
-
Wafer-scale synthesis of p-type TMD films is critical for its commercialization in next-generation electro/optoelectronics. In this work, wafer-scale intrinsic n-type WS2films and in situ Nb-doped p-type WS2films were synthesized through atomic layer deposition (ALD) on 8-inchα-Al2O3/Si wafers, 2-inch sapphire, and 1 cm2GaN substrate pieces. The Nb doping concentration was precisely controlled by altering cycle number of Nb precursor and activated by postannealing. WS2n-FETs and Nb-doped p-FETs with different Nb concentrations have been fabricated using CMOS-compatible processes. X-ray photoelectron spectroscopy, Raman spectroscopy, and Hall measurements confirmed the effective substitutional doping with Nb. The on/off ratio and electron mobility of WS2n-FET are as high as 105and 6.85 cm2 V-1 s-1, respectively. In WS2p-FET with 15-cycle Nb doping, the on/off ratio and hole mobility are 10 and 0.016 cm2 V-1 s-1, respectively. The p-n structure based on n- and p- type WS2films was proved with a 104rectifying ratio. The realization of controllablein situNb-doped WS2films paved a way for fabricating wafer-scale complementary WS2FETs.more » « less
-
Determination of anisotropic optical properties of MOCVD grown m-plane α-(Al x Ga 1−x ) 2 O 3 alloysAbstract The anisotropic dielectric functions (DF) of corundum structuredm-planeα-(AlxGa1−x)2O3thin films (up tox= 0.76) grown onm-plane sapphire substrate by metalorganic CVD have been investigated. IR and visible–UV spectroscopic ellipsometry yields the DFs, while X-ray diffraction revealed the lattice parameters (a,m,c), showing the samples are almost fully relaxed. Analysis of the IR DFs from 250 to 6000 cm−1by a complex Lorentz oscillator model yields the anisotropic IR active phononsEuandA2uand the shift towards higher wavenumbers with increasing Al content. Analyzing the UV DFs from 0.5 to 6.6 eV we find the change in the dielectric limitsε∞and the shift of the Γ-point transition energies with increasing Al content. This results in anisotropic bowing parameters forα-(AlxGa1−x)2O3ofb⊥= 2.1 eV andb∣∣= 1.7 eV.more » « less
-
Abstract We report on the tunable and enhanced dielectric properties of tungsten (W) incorporated gallium oxide (Ga2O3) polycrystalline electroceramics for energy and power electronic device applications. The W‐incorporated Ga2O3(Ga2−2xWxO3, 0.00 ≤ x ≤ 0.20; GWO) compounds were synthesized by the high‐temperature solid‐state chemical reaction method by varying the W‐content. The fundamental aspects of the dielectric properties in correlation with the crystal structure, phase, and microstructure of the GWO polycrystalline compounds has been investigated in detail. A detailed study performed ascertains the W‐induced changes in the dielectric constant, loss tangent (tanδ) and ac conductivity. It was found that the dielectric constant increases with addition of W in the system as a function of temperature (25°C‐500°C). Frequency dependence (102‐106 Hz) of the dielectric constant follows the modified Debye model with a relaxation time of ∼20 to 90 μs and a spreading factor of 0.39 to 0.65. The dielectric constant of GWO is temperature independent almost until ∼300°C, and then increases rapidly in the range of 300°C to 500°C. W‐induced enhancement in the dielectric constant of GWO is fully evident in the frequency and temperature dependent dielectric studies. The frequency and temperature dependent tanδreveals the typical behavior of relaxation loses in GWO. Small polaron hopping mechanism is evident in the frequency dependent electrical transport properties of GWO. The remarkable effect of W‐incorporation on the dielectric and electrical transport properties of Ga2O3is explained by a two‐layer heterogeneous model consisting of thick grains separated by very thin grain boundaries along with the formation of a Ga2O3‐WO3composite was able to account for the observed temperature and frequency dependent electrical properties in GWO. The results demonstrate that the structure, electrical and dielectric properties can be tailored by tuning W‐content in the GWO compounds.more » « less
