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: III. Effect of the Muffling Environment(2006-07-01) Wu, Hui; Davies, Peter KThe effect of different muffling environments on the structure and dielectric losses of Ba(Zn1/3Nb2/3)O3 (BZN) microwave ceramics was investigated. The microwave dielectric losses of stoichiometric BZN pellets heated in ZnO-rich environments were severely degraded (e.g. Q×f~15 000 in ZnO powder) compared with samples muffled in their own powder (Q×f~80 000). Structure analyses and gravimetric measurements confirmed that the ceramics muffled in ZnO powder or vapor absorb excess ZnO to form non-stoichiometric solid solutions with reduced cation order and Q. By using starting compositions in the (1−x)BZN−(x)BaNb4/5O3 binary (x=0.04), the stoichiometry can be tailored to ensure that after the uptake of ZnO, the ceramics remain well ordered and are located in a high Q region of the system. For example, ZnO-vapor-protected (0.96)BZN−(0.04)BaNb4/5O3 reached a very high Q×f(~1 05 000) after sintering at 1400°C for 5 h.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 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 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 Crystalline Structure and Dielectric Properties of Li1+x-yNb1-x-3yTix+4yO3 M-Phase Solid Solutions(2002-03-01) Borisevich, Albina Y; Davies, Peter KThe structure and dielectric properties of the so-called M-phase solid solutions in the Li2O-Nb2O5-TiO2 system have been investigated. Detailed studies of the lattice parameters of these phases agree well with structure models based on intergrowths of LiNbO3 slabs with a titanium-rich corundum-type layer. The relative permittivity ranges from ~80 to ~55; microwave quality factors increase with the titanium content, reaching values of Q x f = 9000 at 6 GHz. The temperature coefficient of the resonant frequency changes sign within the solid solution region, thus permitting tunability to a zero value. All the compositions can be sintered to high density at temperatures ≤ 1100∘C.Publication Analysis of phase distributions in the Li2O–Nb2O5–TiO2 system by piezoresponse imaging(2001-02-01) Borisevich, Albina Y; Bonnell, Dawn A; Kalinin, Sergei V; Davies, Peter KThe M-phase solid solutions Li1+x-yNb1-x-3yTix+4yO3) (0.1 ≤ x ≤ 0.3, 0 ≤ y ≤ 0.175) in the Li2O–Nb2O5–TiO2 system have promising microwave dielectric properties. However, these compounds can contain small quantities of ferroelectric impurities that affect the polarization response of the material. Due to their low concentration and their chemical similarity to the host material, the impurities cannot be detected by x-ray diffraction or local elemental analysis. Scanning surface potential microscopy and piezoresponse imaging were used to analyze phase compositions in this system. Piezoresponse imaging demonstrated the presence of thin (<200–300 nm) ferroelectric layers on the grain boundaries oriented along the c-axis of the M-phase. Differences between the surface potential and the piezoresponse of ferroelectric multicomponent systems are discussed.Publication Structure and Polarization in the High Tc Ferroelectric Bi(Zn,Ti)O3-PbTiO3 Solid Solutions(2007-03-05) Grinberg, Ilya; Suchomel, Matthew R; Dmowski, Wotjek; Mason, Sara E; Wu, Hui; Davies, Peter K; Rappe, Andrew MTheoretical ab initio and experimental methods are used to investigate the [Bi(Zn1/2Ti1/2)O3]x [PbTiO3]1-x solid solution. We find that hybridization between Zn 4s and 4p and O 2p orbitals allows the formation of short, covalent Zn-O bonds, enabling favorable coupling between A-site and B-site displacements. This leads to unusually large polarization, strong tetragonality, and an elevated ferroelectric to paraelectric phase transition temperature.Publication Synthesis and Dielectric Properties of Li1-x+yTa1-x-3yTix+4yO3 M-Phase Solid Solutions(2002-10-01) Borisevich, Albina Y; Davies, Peter KThe synthesis, structure, and dielectric properties of the so-called M-phase solid solutions in the Li2O-Ta2O5-TiO2 system were investigated. Although the range of stability of the tantalate phases is more limited compared with their niobate counterparts, they have identical structures based on intergrowths of LiTaO3-type blocks separated by corundum type layers. The dielectric constants of the tantalate M-phases range from 68 to 52 and they all exhibit a negative temperature coefficient of capacitance. The temperature coefficient of the resonant frequency measured in the microwave region can be tuned to zero, and the system shows quite good quality factors with the highest value reaching a Q x f = 10500 at 6.7 GHz.Publication A-Site and B-Site Order in (Na1/2La1/2)(Mg1/3Nb2/3)O3 Perovskite(2004-05-01) Tarvin, Rachel; Davies, Peter K(Na1/2La1/2)(Mg1/3Nb2/3)O3 undergoes a series of phase transitions that involve cation order on the A- and B-sites of the parent perovskite structure. At high temperatures both sites contain a random distribution of cations; below 1275°C a 〈111〉 layering of Mg and Nb leads to the formation of a 1:2 ordered structure with a monoclinic supercell. A second transition was observed at 925°C, where the Na and La cations order onto alternate A-site positions along the 〈001〉 direction of the parent subcell. By quenching samples from above 1275°C to preserve the disorder on the B-site, a fourth variant of this compound was obtained by inducing A-site order through a subsequent anneal at 900°C. Although the changes in structure do not produce significant alterations in the relative permittivity (εr ~ 35), they do have a significant effect on the value of the temperature coefficient of the capacitance.