Davies, Peter K
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Publication Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics. IV. Tuning τf and the Part Size Dependence of Q × f(2006-07-01) Wu, Hui; Davies, Peter KHigh Q ceramics of Ba3W2O9 (BW)-substituted Ba(Zn1/3Nb2/3) O3 (BZN) were prepared with a zero τf through the partial substitution of Zn by Ni and Co. The small concentrations of B-site vacancies introduced by the substitution of BW accelerated the kinetics and stability of the cation ordering and lowered the sintering temperature. Dense, zero τzf, ordered solid solutions such as 0.99Ba(Zn0.3Co0.7)1/3Nb2/3O3–0.01BW with εr=34.4 and Q×f=82 000 at ~8 GHz could be obtained after sintering at 1380°C for 5 h and annealing at 1300°C for 24 h. Partially ordered ceramics in the Zn/Co and Zn/Ni solid solutions show a large gradient in the ordering throughout the pellets, which produces a resonant frequency dependence of their Q×f value. The ordering gradient is associated with the increased constraints on the growth of the 1:2 ordered structure within the interior of larger and thicker pellets and can be minimized by extended annealing.Publication Enhanced Tetragonality in (x)PbTiO3-(1-x)Bi(B′B″)O3 systems: Bi(Zn3/4WN1/4)O3(2006-09-28) Stein, David M; Suchomel, Matthew R; Davies, Peter KSolid solutions in the (x)PbTiO3–(1−x)Bi(Zn3/4W1/4)O3 system have been examined by x-ray diffraction, dielectric measurements, and thermal analysis. Bi(Zn3/4W1/4)O3increases the tetragonality and Curie temperature of PbTiO3 which reach a value of 1.08 and 530 °C, respectively, at the limit of the single-phase perovskite forming region (x~0.8). The observation of a sustained increase in the tetragonality in this system is similar to the behavior of the (x)PbTiO3–(1−x)Bi(Zn1/2Ti1/2)O3 system and highlights the unique properties of Bi-based systems when the B sites contain high concentrations of highly polarizable cations.Publication Ba8ZnTa6O24: A New High Q Dielectric Perovskite(2005-08-01) Thirumal, M.; Davies, Peter KThe hexagonal perovskite, Ba8ZnTa6O24, was prepared in single-phase form and was found to be a stable secondary phase, formed as a result of the loss of ZnO from Ba(Zn1/3Ta2/3)O3 microwave dielectrics. The experimental and calculated X-ray patterns of Ba8ZnTa6O24 indicate it is isostructural with Ba8Ta6NiO24 with an 8H (cchc)2 close-packed BaO3 stacking sequence and the lattice parameters, a=10.0825(14), c=19.0587 (38)Å . High-density ceramics of Ba8ZnTa6O24 could be prepared at temperatures considerably lower (1400ºC) than those used to sinter pure Ba(Zn1/3Ta2/3)O3, and exhibit very good microwave dielectric properties with ε=30.5, Qf=62300, and τf=+36 ppm/ºC at 8.9 GHz.Publication Pb-free Ferroelectrics Investigated with Density Functional Theory: SnAl½Nb½O3 Perovskites(2011-04-26) Grinberg, Ilya; Davies, Peter K; Bennett, Joseph W; Rappe, Andrew MInterest in Pb-free ferroelectrics has intensified as the search for less toxic Pb replacements continues. Since Sn is isoelectronic with Pb, it has generated great interest. Most of this effort has focused on SnTiO3. Even though it shows impressive ferroelectricity in calculations, synthesis has proved elusive.We therefore use density-functional theory (DFT) to investigate B-site alternatives that involve smaller size to promote perovskite phase stability. In this paper, Sn(Al½Nb½)O3 (SAN) is investigated. We demonstrate that SAN is likely to be synthesizable, will be ferroelectric, and perhaps a good piezoelectric material as well. We discuss how cation displacements and their interactions affect the polarization of the solid solution.We also explore the electronic properties of the SAN solid solution and correlate them to the structural findings.Publication Influence of Non-Stoichiometry on the Structure and Properties of Ba(Zn1/3Nb2/3)O3 Microwave Dielectrics: II. Compositional Variations in Pure BZN(2006-07-01) Wu, Hui; Davies, Peter KThe formation of non-stoichiometric cubic perovskite solid solutions based on BaZn1/3Nb2/3O3 (BZN) was examined along 10 different directions in the BaO–ZnO–Nb2O5 ternary system. Limited ranges of non-stoichiometry were observed along several pseudo-binaries and the BZN structure can accommodate a variety of different types of defects. Although the deviations from stoichiometry are quite small, typically ~1 mole%, they induce large changes in the extent and stability of the 1:2 B-site ordering, the sintering and microstructure, and the dielectric loss properties. The highest Q×fs (~110 000 at 8 GHz) in the system, which coincide with the highest degree of order, were located in two regions along the BZN–Ba5Nb4O15 and BZN–BaNb2O6 lines. The results of this study provide an explanation for the large variations in crystal structure and Q×fs previously reported for BZN and other related systems (e.g., Ba(Zn1/3Ta2/3)O3), and demonstrate that non-stoichiometric starting compositions provide a route to the highest Q values.Publication Cation Ordering in Pb(Mg1/3Nb2/3)O3–Pb(Sc1/2Nb1/2)O3 (PMN–PSN) Solid Solutions(2002-09-01) Farber, Leon; Valant, Matjaz; Akbas, Mehmet Ali; Davies, Peter KExtended thermal annealing treatments were used to modify the B-site cation order in the (1 - x)PMN–xPSN perovskite system (where PMN is lead magnesium niobate, Pb(Mg1/3Nb2/3)O3, and PSN is lead scandium niobate, Pb(Sc1/2Nb1/2)O3). Extensive 1:1 ordering could be induced in compositions with x ≧ 0.1. The substitution of PSN into PMN produced a large increase in the thermal stability of the 1:1 ordered phase, with the maximum disordering temperature of ~1360oC being observed for the x = 0.5 composition. By monitoring the change in the degree of order with temperature, the order–disorder boundary for the PMN–PSN system was established and the transition temperature for pure PMN was estimated to be ~950oC. The changes in stability across the system were consistent with the "random-site" description of the cation order. The change in enthalpy associated with the ordering was affected by the size difference of the two ordered cation sites and by the size and charge mismatch of the metal cations that occupy the random-site position.Publication Pb-free Semiconductor Ferroelectrics: A Theoretical Study of Pd-substituted Ba(Ti1-xCex)O3 Solid Solutions(2010-11-10) Grinberg, Ilya; Davies, Peter K.; Bennett, Joseph W.; Rappe, Andrew M.We use first-principles density-functional-theory calculations to investigate the ground state structures of Ba(Ti1−xCex)O3 solid solutions containing Pd. Previous studies have shown that the properties of BaTiO3, a Pb-free ferroelectric ABO3 perovskite, can be tailored via B-site substitution. In the present study, we substitute Ce for Ti to increase the overall volume of the perovskite, to then accommodate an O-vacancy-stabilized Pd substitution. Using the LDA+U method, we predict that these proposed materials will display a decreased band gap compared to BaTiO3 while maintaining polarization. These features, combined with their environmentally friendly characteristics make these materials promising candidates for use as semiconducting ferroelectrics in solar-energy conversion devices.Publication Effect of V2O5 Doping on the Sintering and Dielectric Properties of M-Phase Li1+x-yNb1-x-3yTix+4yO3 Ceramics(2004-06-01) Borisevich, Albina Y.; Davies, Peter KThe effect of the addition of V2O5 on the structure, sintering and dielectric properties of M-phase (Li1+x-yNb1-x-3yTix+4y)O3 ceramics has been investigated. Homogeneous substitution of V5+ for Nb5+ was obtained in LiNb0.6(1-x) V0.6xTi0.5O3 for x ≤ 0.02. The addition of V2O5 led to a large reduction in the sintering temperature and samples with x = 0.02 could be fully densified at 900oC. The substitution of vanadia had a relatively minor adverse effect on the microwave dielectric properties of the M-phase system and the x = 0.02 ceramics had εr = 66, Q x f = 3800 at 5.6 GHz, and τf = 11 ppm/oC. Preliminary investigations suggest that silver metallization does not diffuse into the V2O5-doped M-phase ceramics at 900oC, making these materials potential candidates for low-temperature cofired ceramic (LTCC) applications.Publication Structure and Dielectric Properties of the Ba(Mg1/3Nb2/3)O3–La(Mg2/3Nb1/3)O3 System(1998-08-01) Akbas, Mehmet Ali; Davies, Peter KA complete range of perovskite solid solutions can be formed in the (1 - x)Ba(Mg1/3Nb2/3)O3–xLa(Mg2/3Nb1/3)O3 (BMN–LMN) pseudobinary system. While pure BMN adopts a 1:2 cation ordered structure, 1:1 ordered phases are stabilized for 0.05 ≤ x ≤ 1.0. Dark-field TEM images indicate that the La-doped solid solutions are comprised of large 1:1 ordered domains and no evidence was found for a phase-separated structure. This observation coupled with the systematic variations in the intensities of the supercell reflections supports a charge-balanced "random-site" model for the 1:1 ordering. The substitution of La also induces a transformation from a negative to positive temperature coefficient of capacitance in the region 0.25 ≤ x ≤ 0.5.Publication Potential and Impedance Imaging of Polycrystalline BiFeO3 Ceramics(2002-12-01) Kalinin, Sergei V.; Davies, Peter K; Suchomel, Matthew R.; Bonnell, Dawn AElectrostatic-force-sensitive scanning probe microscopy (SPM) is used to investigate grain boundary behavior in polycrystalline BiFeO3 ceramics. Scanning surface potential microscopy (SSPM) of a laterally biased sample exhibits potential drops due to resistive barriers at the grain boundaries. In this technique, the tips acts as a moving voltage probe detecting local variations of potential associated with the ohmic losses within the grains and at the grain boundaries. An approach for the quantification of grain boundary, grain interior, and contact resistivity from SSPM data is developed. Scanning impedance microscopy (SIM) is used to visualize capacitive barriers at the grain boundaries. In SIM, a dc-biased tip detects the variations of local potential induced by the lateral ac voltage applied to the sample. Unlike the traditional dc and ac transport measurement, both of these techniques are sensitive to the variation of local potential (SSPM) or local voltage oscillation amplitude and phase (SIM), rather than to current. Therefore, special attention is paid to the relationship between SSPM and SIM images and data obtained from traditional impedance spectroscopy and dc transport measurements. For BiFeO3 ceramics excellent agreement between the local SIM measurements and impedance spectroscopy data are demonstrated.