Victor M. Ferreira

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.

Dielectric spectroscopy of MgTiO3-based ceramics in the 109–1014Hz region

Magnesium titanate (MgTiO3) powder was prepared by a chemical route (Pechini method) and different dopants were added to prepare several compositions. These pure and doped compositions were sintered in air and dense ceramics were obtained. The pure MgTiO3 samples were also subjected to different heat treatments during sintering. Complex permittivity spectra of ceramic samples were determined by various techniques in the 109–1014 Hz range. These techniques included infrared spectroscopy in transmission and reflectivity modes and microwave dielectric measurements. Extrapolation to microwave frequencies from infrared data, according to the proportionality ɛ″(v) ∝ v, agrees quite well with the microwave data measured at 8 GHz and it is a useful procedure to estimate intrinsic microwave losses. Fast cooling from high temperatures of MgTiO3 samples increased dielectric loss, probably due to a structural disorder. Dopants have two types of effect depending on whether they form a distinct second phase or a solid solution with MgTiO3. In this last case intrinsic losses are strongly affected.

Structure and microwave dielectric properties of La(Mg0.5Ti0.5)O3-CaTiO3 system

(1-x)La(Mg 0.5 Ti 0.5 )O 3 (LMT)-xCaTiO 3 (CT) [0<x<1] ceramics were prepared from powder obtained by a nonconventional chemical route based on the Pechini method. The crystal structure of the microwave dielectric ceramics has been refined by Rietveld method using X-ray powder diffraction data. LMT and CT were found to form a solid solution over the whole compositional range. The 0.9LMT-0.1CT composition was refined using P2 1 /n space group, which allows taking into account B-site ordering. The compounds having x≥0.3 were found to be disordered and were refined using Pbnm space group. Microstructure evolution was also analysed. Dielectric characterization at microwave frequencies was performed on the LMT CT ceramics. The permittivity and the temperature coefficient of resonant frequency of the solid solutions showed a non-linear variation with composition. The quality factor demonstrates a considerable decrease with the increase of CT content.

Preparation and microwave dielectric properties of pure and doped magnesium titanate ceramics

Abstract Understanding the dielectric loss mechanisms in microwave ceramics is important since losses can hinder its applications. Cr, Fe and Co containing MgTiO 3 ceramics were prepared using a high purity powder obtained by a chemical route. Single-phase materials were obtained for all doped ceramics with the exception of the slowly cooled Fe-doped samples in which a surface layer of MgTi 2 O 5 was detected, containing a higher concentration of the dopant. The relationship between the obtained microstructures and the microwave dielectric properties is discussed. The main variations are observed in the dielectric loss which is affected by doping and processing parameters. Anharmonicity of lattice vibration modes caused by point defects and linear defects is suggested as responsible for the higher losses.

Structure evolution in La(Mg0.5Ti0.5)O3–SrTiO3 system

Crystal structure transformations of dielectric ceramics in the (1−x)La(Mg0.5Ti0.5)O3 (LMT)–xSrTiO3 (ST) [0 0.1), in-phase tilting (x>0.5), and transition from anti-phase two-tilting to one-tilting system (x>0.7), respectively.

Crystal Structure of Dielectric Ceramics in the La(Mg0.5Ti0.5)O3–BaTiO3 System

The crystal structure of microwave dielectric ceramics in the (1 - x)La(Mg 0 . 5 Ti 0 . 5 )O 3 (LMT)-xBaTiO 3 (BT) (0 ≤ x ≤ 0.9) system has been refined by Rietveld method using x-ray powder diffraction data. LMT and BT were found to form a solid solution in the whole compositional range. The increase of BaTiO 3 content results in the following sequence of structure transformations of those solid solutions: P2 1 /n (a - a - c + , B-site ordered) → Pbnm (a - a - c + ) → I4/mcm (a 0 a 0 c - ) → Pm3m (a 0 a 0 a 0 ). These structural changes are related to the disappearance of B-site cation ordering (x > 0.1), in-phase tilting (x > 0.3), and antiphase tilting (x > 0.5), respectively.

Stainless steel coatings sputter-deposited on tungsten carbide powder particles

Abstract The aim of this work was to study the feasibility of a sputtering technique to coat WC powder particles, regarding it as an alternative to the conventional mixture of powders. For such purpose, a stainless steel 304 (AISI) coating was sputter deposited on WC powder particles using a magnetron sputtering equipment specially developed to coat powder particles. The morphology of the coated powder was characterized by scanning electron microscopy observations, Brunauer–Emmett–Teller and laser diffraction measurements. The crystallographic structure was determined by X-ray diffraction. Inductively coupled plasma–atomic emission spectrometer and electron microprobe analysis were used to characterize the amount, chemical composition and distribution of the sputtered coating. The characterization results indicated that all WC particles were coated and that all the steel constituent elements were deposited in the same original proportion. The coating had a ferrite b.c.c. structure and presented a columnar growth with some porosity. The compaction behavior of the coated powders was characterized by unidirectional pressing using pressures between 60 and 250 MPa. The maximum of relative density was attained for P ≥190 MPa, with values of 57–58% of relative density, comparable to that of non-coated powders, and without the need of any pressing binder to obtain green compacts resistant to handling. High sintered densities, of approximately 95%, were obtained at a relatively low temperature of 1325 °C with only ∼6 wt.% of binder phase in the coated powders.

Synthesis of La(Mg0.5Ti0.5)O3 ceramics for microwave applications

Abstract La(Mg 0.5 Ti 0.5 )O 3 ceramics were prepared by a non-conventional chemical route, which was based on the Pechini method. For the synthesis of La(Mg 0.5 Ti 0.5 )O 3 powders, special attention was paid to calcination and milling conditions. Powder morphology and composition were evaluated. Fine La(Mg 0.5 Ti 0.5 )O 3 powders were obtained at lower temperatures than by conventional methods. Sintering under different conditions was also tested. Dense La(Mg 0.5 Ti 0.5 )O 3 ceramics were obtained at lower temperatures showing a single phase composition and a homogeneous microstructure. Preliminary dielectric characterization at microwave frequencies was also performed.

Ferroelectric-to-relaxor transition behaviour of BaTiO3 ceramics doped with La(Mg1/2Ti1/2)O3

Low-frequency dielectric studies were performed on the fine-grain ceramics of BaTiO3 doped with 2.5 mol% of non-ferroelectric perovskite La(Mg1/2Ti1/2)O3 (BT:2.5% LMT). In addition to the permittivity and P–E hysteresis loop measurements as a function of temperature, local electromechanical properties of BT:2.5% LMT were analysed by piezoresponse force microscopy. It was found that the temperature evolution of the dielectric properties detected by macroscopic methods (average over many grains) is similar to that observed on the nanoscale level (in a single grain). BT:2.5% LMT was revealed to exhibit the typical features of both the ferroelectric and the relaxor. The observed dielectric behaviour is discussed in terms of heterovalent substitution at both A-(Ba2+/La3+) and B-(Ti4+/Mg2+) perovskite sites.