Feridoon Azough

DiC12: Magnesium titanate microwave dielectric ceramics

Abstract Magnesium titanate ceramics have been prepared by a chemical route (Pechini method) and by the conventional mixed oxide route. Dielectric properties of sintered samples were measured at 8 GHz by the Hakki and Coleman technique. Microstructures were examined by optical microscopy and scanning electron microscopy. The use of the chemical method enabled the sintering temperature to be lowered and good quality, single phase, dense ceramics were obtained at temperatures down to 1150°C. Dielectric constants were approximately 17.5 and Q values up to 21200 were obtained at 8 GHz. The effect of powder preparation processes, sintering temperature and cooling rate on the densification, microstructure and microwave dielectric properties are discussed.

The effect of Cr and La on MgTiO3 and MgTiO3–CaTiO3 microwave dielectric ceramics

Magnesium titanate and MgTiO 3 –CaTiO 3 ceramics were prepared by a chemical (Pechini) route and classical mixed oxide route. Selected specimens were doped with Cr or La. Specimens were sintered at 1350 °C and 1400 °C. Microstructures were examined by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Dielectric properties were determined at 8 GHz by the Hakki and Coleman method. The highest Q values were obtained for undoped, chemically prepared MgTiO 3 (20800); any dopants caused the Q value to be degraded. Additions of small amounts of Cr (≤1 mol %) to mixed oxide magnesium titanate increased the density to 97.1% theoretical, and increased the Q value (from 7000) to 13,000. Additions of La led to the formation of La 2 Ti 2 O 7 second phase and reduction in the Q value for both materials. Both Cr and La acted as effective sintering aids, increasing density (to a maximum of 99% theoretical for 1% mol La in chemically prepared samples) and relative permittivity (to 18.1 for the same specimens). The relative permittivity of MgTiO 3 –CaTiO 3 ceramics increased with calcium content, but the corresponding Q values decreased (∈ r = 19.9, Q = 8500 for Mg:Ca = 94: 6). Small additions of La to Mg–Ca titanates enhanced the dielectric Q values but decreased the relative permittivities.

Thermoelectric Power Generation from Lanthanum Strontium Titanium Oxide at Room Temperature through the Addition of Graphene.

The applications of strontium titanium oxide based thermoelectric materials are currently limited by their high operating temperatures of >700 °C. Herein, we show that the thermal operating window of lanthanum strontium titanium oxide (LSTO) can be reduced to room temperature by the addition of a small amount of graphene. This increase in operating performance will enable future applications such as generators in vehicles and other sectors. The LSTO composites incorporated one percent or less of graphene and were sintered under an argon/hydrogen atmosphere. The resultant materials were reduced and possessed a multiphase structure with nanosized grains. The thermal conductivity of the nanocomposites decreased upon the addition of graphene, whereas the electrical conductivity and power factor both increased significantly. These factors, together with a moderate Seebeck coefficient, meant that a high power factor of ∼2500 μWm(-1)K(-2) was reached at room temperature at a loading of 0.6 wt % graphene. The highest thermoelectric figure of merit (ZT) was achieved when 0.6 wt % graphene was added (ZT = 0.42 at room temperature and 0.36 at 750 °C), with >280% enhancement compared to that of pure LSTO. A preliminary 7-couple device was produced using bismuth strontium cobalt oxide/graphene-LSTO pucks. This device had a Seebeck coefficient of ∼1500 μV/K and an open voltage of 600 mV at a mean temperature of 219 °C.

Microstructural control of microwave dielectric properties in CaTiO3–La(Mg1∕2Ti1∕2)O3 ceramics

High-density xCaTiO3–(1−x)La(Mg1∕2Ti1∕2)O3 ceramics (x=0.0–1.0) were prepared by solid-state reaction. The products were characterized by scanning electron microscopy, x-ray diffraction, and transmission electron microscopy. CaTiO3–La(Mg1∕2Ti1∕2)O3 ceramics formed solid solutions throughout the compositional range with no evidence of a secondary phase. For most ceramics the symmetry was orthorhombic with Pbnm space group. The La(Mg1∕2Ti1∕2)O3 end member exhibited 1:1 ordering of Mg2+ and Ti4+ cations (in octahedral sites) superimposed on the perovskite distortions leading to monoclinic symmetry with P21∕n space group. Long-range 1:1 order of Mg2+ and Ti4+ cations was destroyed at x=0.3. The degree of distortion, unit-cell volume, and crystal lattice stress decreased with increasing CaTiO3 content. Twin and antiphase domains, generated by structural phase transitions on cooling, were observed in all the ceramics. Twin domain density increased with CaTiO3 content but the size of individual domains decreased...

Structural study of Ca0.7Nd0.3Ti0.7Al0.3O3 dielectric ceramics using synchrotron X-ray diffraction

Abstract The crystal structure of Ca 0.7 Nd 0.3 Ti 0.7 Al 0.3 O 3 has been determined using synchrotron X-ray diffraction. Data were collected from a polycrystalline sample for Rietveld analysis. At room temperature, the structure exhibits an octahedral tilted framework (perovskite-type) with an orthorhombic lattice ( a =5.3803 A, b =5.4003 A and c =7.6140 A). The positions of the atoms in the unit cell and their isotropic temperature factors have been determined, together with the occupations of sites by Ca/Nd and Al/Ti atoms. The structure was determined to feature an a − a − c + orthorhombic tilt system in association with displacement of the A-site cations.

Facile Surfactant‐Free Synthesis of p‐type SnSe Nanoplates with Exceptional Thermoelectric Power Factors

Abstract A surfactant‐free solution methodology, simply using water as a solvent, has been developed for the straightforward synthesis of single‐phase orthorhombic SnSe nanoplates in gram quantities. Individual nanoplates are composed of {100} surfaces with {011} edge facets. Hot‐pressed nanostructured compacts (E g≈0.85 eV) exhibit excellent electrical conductivity and thermoelectric power factors (S 2 σ) at 550 K. S 2 σ values are 8‐fold higher than equivalent materials prepared using citric acid as a structure‐directing agent, and electrical properties are comparable to the best‐performing, extrinsically doped p‐type polycrystalline tin selenides. The method offers an energy‐efficient, rapid route to p‐type SnSe nanostructures.

A transmission electron microscope study of commercial X7R-type multilayer ceramic capacitors

Abstract X7R-type multilayer ceramic capacitors produced by three commercial manufacturers (A, B and C) have been investigated by transmission electron microscopy. All the specimens contained grains with distinct core-shell structures; the cores were almost pure BaTiO3 and exhibited domain structures at room temperature. Specimens of type A had a grain size of 1–2μm, with ‘wide’ shells containing Pb, Bi, Nb, Ba and Ti. A bonding phase, rich in Si, Al, Ba and Ti was detected between the dielectric grains and Ag-Pd electrodes. Heating the samples to temperatures above 130 °C caused the ferroelectric domain structure to be lost and replaced by a dislocation network. Specimens of type B and C had a grain size of 0.5–1.0 μm; the ferroelectric cores were surrounded by ‘narrow’ shells rich in Nb, Co, Ba and Ti. The Ag-Pd internal electrodes were twinned. Type I and type II core-shell structures were found in all three sets of multilayer ceramic capacitors.

Intrinsic dielectric and spectroscopic behavior of perovskite Ba(Ni1∕3Nb2∕3)O3–Ba(Zn1∕3Nb2∕3)O3 microwave dielectric ceramics

Ceramics of 0.35Ba(Ni1∕3Nb2∕3)O3–0.65Ba(Zn1∕3Nb2∕3)O3 were prepared by the mixed oxide route. The effect of the cooling rate (2 °C–240 °C∕h) after sintering on the microwave dielectric properties of the ceramics was examined. While the extrinsic factors, such as porosity and secondary phases, markedly influence the dielectric properties in the low-frequency regime, they have minimal effect on these properties in the high-frequency regime. The mechanisms involved in modifying the high-frequency dielectric properties of the materials were investigated by Fourier transform infrared and Raman spectroscopy, in conjunction with the Rietveld analysis of x-ray diffraction (XRD) spectra. A reduction in the cooling rate after sintering results in an increase in the high-frequency Q×f (product of dielectric Q value and measurement frequency) from 42 to 58 THz in the high-frequency regime (∼1.5 THz). Such behavior correlates very well with the increase in the B-site occupancy by Nb (deduced from the Rietveld analyses...

The ikaite-to-vaterite transformation: new evidence from diffraction and imaging

Vaterite is one of three polymorphs of calcium carbonate (CaCO3) found in nature, the others being calcite and aragonite. Here the formation of vaterite from decomposition of ikaite (CaCO3·6H2O) was investigated using synchrotron powder diffraction and scanning electron microscopy. The crystallite sizes of vaterite (∼40 nm) were found to be much smaller than those of the precursor ikaite (∼0.5–1.0 µm) as a result of vaterite nucleating as ikaite dehydrates. The rate of decomposition to vaterite increases with temperature, indicating kinetic control of this transformation. It is postulated that the structural arrangement of the hydration sphere around Ca2+ in ikaite determines the orientation of Ca2+ and CO32− ions such that vaterite nucleates upon dehydration. This implies that the dehydration of a precursor hydrated phase such as ikaite is required for vaterite nucleation.

The effect of La2Ti3O9 second phase on the microstructure and dielectric properties of La2Ti2O7 ceramics

Abstract Lanthanum dititanate ceramics have been prepared by the mixed oxide route. Up to 6 mol% Nb 2 O 5 or 10 mol% excess TiO 2 was added to the starting mixes. Sintered products exhibited densities of ≥ 95% theoretical. La 2 Ti 3 O 9 was present as a second phase in all doped samples. With the addition of increasing amounts of Nb 2 O 5 or TiO 2 there was formation of La 2 Ti 3 O 9 within the La 2 Ti 3 O 9 matrix, and the average grain size decreased slightly. Dielectric properties were sensitive to the presence of the second phase: relative permittivity increased from 47 to ∼56 with additions; the dielectric loss tangent was independent of the amount of excess titania but increased dramatically when > 3 mol% Nb 2 O 5 was added; the temperature coefficient of capacitance (TCC) decreased monotonically from +50 ppm/ °C for undoped La 2 Ti 2 O 7 ceramics to −50 ppm/ °C for specimens containing ∼10 mol% additions. Material with zero TCC can be achieved with the addition of ∼5 mol%, Nb 2 O 5 or excess TiO 2 .