Modulation of electrical transport in calcium cobaltite ceramics and thick films through microstructure control and doping

Ca₃Co₄O₉ is a promising p-type thermoelectric oxide material having intrinsically low thermal conductivity. With low cost and opportunities for automatic large scale production, thick film technologies offer considerable potential for a new generation of micro-sized thermoelectric coolers or generators. Here, based on the chemical composition optimized by traditional solid state reaction for bulk samples, we present a viable approach to modulating the electrical transport properties of screen-printed calcium cobaltite thick films through control of the microstructural evolution by optimized heat-treatment. XRD and TEM analysis confirmed the formation of high-quality calcium cobaltite grains. By creating 2.0 at% cobalt deficiency in Ca_2.7Bi_0.3Co₄O_9+δ, the pressureless sintered ceramics reached the highest power factor of 98.0 μWm^?1 K^-2 at 823 K, through enhancement of electrical conductivity by reduction of poorly conducting secondary phases. Subsequently, textured thick films of Ca_2.7Bi_0.3Co_3.92O_9+δ were efficiently tailored by controlling the sintering temperature and holding time. Optimized Ca_2.7Bi_0.3Co_3.92O_9+δ thick films sintered at 1203 K for 8 h exhibited the maximum power factor of 55.5 μWm^?1 K^-2 at 673 K through microstructure control.     

A Raman spectral characterization of ceramics in the system ZrO2-TiO2

Ceramics of zirconium titanate, ZrTiO₄, were prepared by the mixed oxide route, sintered at 1400 °C and cooled at various rates: water quench to 1 °C h^–1. Mixed oxide-prepared ceramics of Zr₅Ti₇O₂₄ were sintered at 1400 °C and cooled at 6 °C h^–1. TEM analysis of the ZrTiO₄ ceramics showed the development of an incommensurate superstructure in the more slowly cooled specimens. The Zr₅Ti₇O₂₄ ceramics exhibited a commensurate superstructure with a tripleda-axis. Raman spectra were obtained from polished ceramic specimens at room temperature. Major differences between spectra for ZrTiO₄ ceramics are believed to reflect differences in the degree of cation ordering. The Raman spectrum for Zr₅Ti₇O₂₄ is similar to that of slowly cooled ZrTiO₄ but there are significant differences.     

High-Temperature Structural Phase Transition in Ca0.7Ti0.7La0.3Al0.3O3: Investigation by Synchrotron X-Ray Diffraction

Mixed-oxide prepared Ca_0.7Ti_0.7La_0.3Al_0.3O₃ (CTLA) ceramics (≈96% dense), grain size 6–7 μm, with dielectric properties (at 4 GHz) of ɛ_r≈46, Q × f ≈38 000 GHz, and τ_f+13 ppm/°C, were studied at 25°–1300°C using synchrotron X-ray powder diffraction. At room temperature, CTLA exhibits a distorted orthorhombic structure, with two tilt systems: a =5.40383 (4) Å, b =5.41106 (6) Å, and c =7.64114 (7) Å with space group Pbnm . At 1050°±25°C, there is a transition from orthorhombic ( Pbnm ) to tetragonal ( I 4/ mcm ), with a simpler tilt arrangement. The lattice parameters at 1100°C were: a =5.44285 (4) Å and c =7.68913 (8) Å.     

Control of 0.2CaTiO3-0.8Li0.5Nd0.5TiO3 microwave dielectric ceramics by additions of Bi2Ti2O7

Ceramics of 0.2CaTiO₃-0.8Li_0.5Nd_0.5TiO₃) have been prepared by the mixed oxide route using additions of Bi₂O₃-2TiO₂ (up to 15 wt%). Powders were calcined 1100^∘C; cylindrical specimens were fired at temperatures in the range 1250–1325^∘C. Sintered products were typically 95% dense. The microstructures were dominated by angular grains 1–2 μm in size. With increasing levels of Bi₂O₃-2TiO₂ additions, needle and lath shaped second phases developed. For Bi₂Ti₂O₇ additions up to 5 wt%, the relative permittivity increased from 95 to 131, the product of dielectric Q value and measurement frequency increased from 2150 to 2450 GHz and the temperature coefficient of resonant frequency (τ _f ) increased from −28pp/^∘C to +22pp/^∘C. A product with temperature stable τ _f could be obtained at ∼2 wt% Bi₂Ti₂O₇ additions. For high levels of additives, there is minimal change in relative permittivity, the Qxf values degrade and τ _f becomes increasingly negative.     

High resolution TEM study of ceramic BaO·Pr2O3·4TiO2

Ceramic specimens of BaO·Pr₂O₃·4TiO₂ (Ba_4.5Pr₉Ti₁₈O₅₄) were prepared by the mixed oxide-route. The single phase products were examined by High Resolution Transmission Electron Microscopy (HRTEM). Orthorhombic symmetry, with cell parameters a22.2 Å, b12.2 Å and c7.6 Å, was confirmed. On the basis of space group pnam (No. 62) and the crystal structural data of Rawn et al., good agreement was obtained between the experimental HRTEM images and images simulated by the multislice method.     

Microwave dielectric ceramics in the system BaO–Li2O–Nd2O3–TiO2

Ceramics in the system BaO-Li₂O–Nd₂O₃–TiO₂ (BNT–LNT) were prepared by the mixed oxide route. Powders were mixed, milled, calcined and sintered at 1475°C for 4 h. Fired densities decreased steadily along the series from BNT to LNT. The microstructures of samples rich in BNT were dominated by small needle-like grains; the LNT samples comprised larger (6 μm) cubic grains. X-ray diffraction showed that there was a transition from orthorhombic BNT to cubic LNT; small amounts of LNT could be accommodated in BNT, but between 10–20% LNT there was the development of the second phase. Small additions of LNT led to a small increase in relative permittivity, but decreased the dielectric Q-value (from the maximum of 1819 at 4 GHz). As BNT and LNT exhibit negative and positive temperature dependencies of permittivity respectively, the addition of 10–20% LNT to BNT should yield samples with zero temperature dependence of _r Impedance spectroscopy showed that data could only be acquired at elevated temperatures for BNT rich samples (above 500°C), but at modest temperatures (less than 100°C) for the more conductive LNT.     

Structures and microwave dielectric properties of Ca(1−x)Nd2x/3TiO3 ceramics

Ca_(1?x)Nd_2x/3TiO₃ microwave dielectric ceramics were prepared by the mixed oxide route; powders were calcined at 1100 °C and sintered at 1450–1500 °C. High density, single phase products were obtained for all compositions. Grain sizes ranged from 1 μm to 100 μm. There was evidence of significant discontinuous grain growth in mid range compositions; all ceramics were characterised by complex domain structures. With increasing Nd content there was a evidence of a transition from an orthorhombic Pbnm structure to a monoclinic C2/m structure. This was accompanied by a decrease in relative permittivity (?_r) from 180 to 78, and decrease in the temperature coefficient of resonant frequency (τ_f) from +770 ppm K^?1 to +200 ppm K^?1. The product of dielectric Q value and resonant frequency (Q × f) varied in a grossly non-systematic way, exhibiting a peak at 13,000 GHz in Ca_0.7Nd_0.2TiO₃.     

Neodymium-Strontium Titanate: A New Ceramic for an Old Problem

High density ceramics based on neodymium-calcium titanate (RE_0.6II_0.1TiO₃) where RE = Nd, Pr, Sm and II = Ca, Sr, were prepared by the mixed oxide route. All products exhibited low thermal conductivity due to the presence of A-site vacancies assisting strong phonon scattering. The moderate electrical conductivity and high Seebeck coefficient resulted in the highest thermoelectric figure of merit (ZT) in neodymium-strontium titanate (NT-ST) ceramic with a value of 0.03 at 900 K. The temperature stable ZT behavior of NT-ST is promising for device applications.     

Microstructure and properties of Co-, Ni-, Zn-, Nb- and W-modified multiferroic BiFeO3 ceramics

BiFeO₃ polycrystalline ceramics were prepared by the mixed oxide route and a chemical route, using additions of Co, ZnO, NiO, Nb₂O₅ and WO₃. The powders were calcined at 700 °C and then pressed and sintered at 800–880 °C for 4 h. High density products up to 96% theoretical were obtained by the use of CoO, ZnO or NiO additions. X-ray diffraction, SEM and TEM confirmed the formation of the primary BiFeO₃ and a spinel secondary phase (CoFe₂O₄, ZnFe₂O₄ or NiFe₂O₄ depending on additive). Minor parasitic phases Bi₂Fe₄O₉ and Bi₂₅FeO₃₉ reduced in the presence of CoO, ZnO or NiO. Additions of Nb₂O₅ and WO₃ did not give rise to any grain boundary phases but dissolved in BiFeO₃ lattice. HRTEM revealed the presence of domain structures with stripe configurations having widths of typically 200 nm. In samples prepared with additives the activation energy for conduction was in the range 0.78–0.95 eV compared to 0.72 eV in the undoped specimens. In co-doped specimens (Co/Nb or Co/W) the room temperature relative permittivity was ～110 and the high frequency dielectric loss peaks were suppressed. Undoped ceramics were antiferromagnetic but samples prepared with Co or Ni additions were ferromagnetic; for 1% CoO addition the remanent magnetization (M_R) values were 1.08 and 0.35 emu/g at temperatures of 5 and 300 K, respectively.