M.P. Seabra

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.

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.

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.

Evaluation of metal-ions containing sludges in the preparation of black inorganic pigments

Inorganic pigments were prepared from industrial wastes: galvanizing sludges resulting from Cr/Ni plating processes (S, G, and T) and a sludge generated from steel wiredraw process that is Fe-rich (F). These industrial wastes were characterized in order to determine the main compositional variations and discover their influence on the colour characteristics of pigments for glazes and ceramics. The toxic character was also investigated and established. Attempting to form the black spinel structure, several combinations of sludges were prepared and then calcined at 1000 °C. XRD and microscopy analysis confirmed the presence of nichromite for compositions with higher Ni amounts, while trevorite was detected in iron-rich formulations. The combination of S and F sludges (SF compositions) generates pigments with higher black colorimetric quality, which is similar to, and sometimes better than, a commercial black pigment. Pigments containing GF and TF sludges develop brown hues on glazes and on porcelain stoneware bodies, being this effect more evident upon industrial firing trials. The addition of minor amounts of cobalt or manganese enhances the black coloration, and might adjust some formulation deviations.

Nano-titania doped with europium and neodymium showing simultaneous photoluminescent and photocatalytic behaviour

Titanium dioxide (TiO2) nanomaterials are attracting increasing interest, mostly because of their superior photocatalytic and antibacterial properties. In this work, we report the synthesis, by a “green” aqueous sol–gel route, of TiO2 doped with Eu and Nd (1 and 5 mol%), in order to extend its photocatalytic activity to the visible range, and also take advantage of the luminescent features of the rare earth metals (REs). Gels were dried in an oven at 75 °C, and then thermally treated at 450 °C in a static air flow furnace. Semi-quantitative phase composition (QPA) and microstructure (crystalline domain shape and size distribution) of the synthetic powders were studied by means of advanced X-ray methods: Rietveld refinements, and the whole powder pattern modelling (WPPM) technique, respectively. From these X-ray techniques, it was seen that the unit cell volume of RE-modified titania underwent a huge expansion, thus suggesting the incorporation of RE into the TiO2 lattice, accompanied by a decrease in the average crystalline domain diameters of anatase, rutile and brookite. The optical properties were also investigated by diffuse reflectance spectroscopy (DRS). Photoluminescent behaviour was assessed in the visible and near infra-red (NIR) spectral ranges. Photocatalytic activity of the samples was assessed both in liquid–solid and in gas–solid phases. The influence of the lighting conditions (UV- and visible-light exposure) was also taken into account for evaluating the photocatalytic activity. Nd–TiO2 was shown to be a bifunctional material, having both photocatalytic activity and NIR photoluminescent emission induced by the same excitation source, an unprecedented result.

Evolution from Ferroelectric to Relaxor Behavior in the (1 − x)BaTiO3 − xLa(Mg1/2Ti1/2)O3 System

Abstract Ceramics of (1 − x)BaTiO3−xLa(Mg1/2Ti1/2)O3 series (x = 0, 0.025, 0.05, 0.075 and 0.1) were dielectrically investigated. Based on the dielectric and X-ray diffraction data, phase diagram of the system for the studied composition range has been determined. Apparent relaxor behavior was revealed to set in when the BaTiO3-type sequence of three phase transitions is reduced to only diffuse transition. No clear boundary was found between ferroelectric and relaxor compositions of the system. It was observed the continuous crossover from ferroelectric to relaxor behavior.

Waste glass from end-of-life fluorescent lamps as raw material in geopolymers

Nowadays the stunning volume of generated wastes, the exhaustion of raw materials, and the disturbing greenhouse gases emission levels show that a paradigm shift is mandatory. In this context, the possibility of using wastes instead of virgin raw materials can mitigate the environmental problems related to wastes, while reducing the consumption of the Earths natural resources. This innovative work reports the incorporation of unexplored waste glass coming from end-of-life fluorescent lamps into geopolymers. The influence of the waste glass incorporation level, NaOH molarity and curing conditions on the microstructure, physical and mechanical properties of the geopolymers was evaluated. Results demonstrate that curing conditions are the most influential factor on the geopolymer characteristics, while the NaOH molarity is less important. Geopolymers containing 37.5% (wt) waste glass were successfully produced, showing compressive strength of 14MPa (after 28days of curing), suggesting the possibility of their use in non-structural applications. Porous waste-based geopolymers for novel applications were also fabricated.

Sensing properties and photochromism of Ag–TiO2 nano-heterostructures

Achieving advanced multifunctional materials displaying several coexisting properties is currently one of the most exciting and innovative research topics. In this study, we report the engineering of a multifunctional material exhibiting, at the same time, tuneable photochromic behaviour and gas-sensing properties for acetone detection. The photochromic property of silver modified (1–10 mol% Ag) titanium dioxide (titania, TiO2) NPs was monitored under consecutive UVA-light exposure times, and the change in colour was thoroughly investigated with both spectroscopic and colourimetric analyses. All Ag modified samples exhibited a significant change in colour and visible spectra after only 15 seconds of exposure, and this increased with further exposure. It was shown that both the silver molar content in the Ag–TiO2 nano-heterostructure, as well as the UVA-light irradiation time, governed the tunability of the photochromic behaviour (the colour changed from pale yellow to dark blue in Ag-modified specimens, while it remained white in unmodified TiO2). The same nano-heterostructures were also tested as sensing materials for resistive metal oxide gas sensors (MOS). These Ag–TiO2 nano-heterostructures proved to be highly sensitive for the detection of acetone vapours at low concentrations (<1 ppm), superior to the best TiO2-based sensors so far reported. This is the first thorough study to qualitatively monitor, in real-time, the growth of Ag0 NPs on a TiO2 matrix, assessing both optical spectroscopy and colourimetric CIEL*a*b* analysis (e.g. what meets the eye), and to also demonstrate the superior acetone gas-sensing properties of such nano-heterostructures.