Dielectric Properties of Bismuth Niobate Ceramics

The development of new dielectric materials that allow the reduction of size and weight of electronic components has been in the scope of the researchers. The bismuth-based dielectric ceramics are extensively studied for this purpose, namely, the bismuth niobate (BiNbO4). The first attempt to improve BiNbO4 occurred in 1992 when Kagata reported the microwave dielectric properties of bismuth niobate doped with the addition of oxides. This chapter will present a brief review of the several attempts that have been carried out to enhance the dielectric properties of BiNbO4 by modifying their structure through addition, doping, or atom(s) substitution. This manuscript focuses on a case study that involves bismuth substitution by europium ions. To investigate the inclusion of europium in BiNbO4 ceramics, (Bi1–xEux)NbO4 samples were prepared using the sol-gel method, in particular, the citrate route. The structure of the prepared samples was studied by X-ray diffraction (XRD) and Raman spectroscopy and the morphology by scanning electron microscopy (SEM). The dielectric properties were studied, in the microwave frequency range (MW), using the resonant cavity method, and in the radio frequency range (RF), with the impedance spectroscopy technique

Nanostructured LiFe5O8 by a biogenic method for applications from electronics to medicine

The authors are grateful for the FEDER funds through the COMPETE 2020 Program and National Funds through FCT-Portuguese Foundation for Science and Technology under the project UID/CTM/50025/2019, and Romanian Ministry of Education and Research, under Romanian National Nucleu Program LAPLAS VI.The physical properties of the cubic and ferrimagnetic spinel ferrite LiFe5O8 has made it an attractive material for electronic and medical applications. In this work, LiFe5O8 nanosized crystallites were synthesized by a novel and eco-friendly sol-gel process, by using powder coconut water as a mediated reaction medium. The dried powders were heat-treated (HT) at temperatures between 400 and 1000◦C, and their structure, morphology, electrical and magnetic characteristics, cytotoxicity, and magnetic hyperthermia assays were performed. The heat treatment of the LiFe5O8 powder tunes the crystallite sizes between 50 nm and 200 nm. When increasing the temperature of the HT, secondary phases start to form. The dielectric analysis revealed, at 300 K and 10 kHz, an increase of ε′ (≈10 up to ≈14) with a tan δ almost constant (≈0.3) with the increase of the HT temperature. The cytotoxicity results reveal, for concentrations below 2.5 mg/mL, that all samples have a non-cytotoxicity property. The sample heat-treated at 1000◦C, which revealed hysteresis and magnetic saturation of 73 emu g−1 at 300 K, showed a heating profile adequate for magnetic hyperthermia applications, showing the potential for biomedical applications.publishersversionpublishe

Propriedades eléctricas de vidros com alguns iões de terras raras

Doutoramento em FísicaEmbora a tecnologia do vidro tenha tido um significativo desenvolvimento nas últimas décadas, a compreensão da estrutura desordenada está ainda longe de ser clara. Alguns avanços têm sido conseguidos, mediante a utilização de técnicas de ressonância e de difracção, mas cada vez mais se revelam as potencialidades que proporcionam a medição de propriedades macroscópicas. Pretende-se com este trabalho estudar o efeito da introdução de diferentes iões de terras raras, como Gd, Nd, Eu e Dy, em vidros de borato e silicato de chumbo, a partir da medição de propriedades eléctricas, como a condutividade, a permitividade e a relaxação dieléctrica. A confirmação de estarmos na presença de uma estrutura amorfa foi efectuada com difracção de raios-X. No sentido de obter informações a partir da resposta dieléctrica, foram efectuadas medições de permitividade complexa, entre 20 mHz e 10 GHz, numa gama de temperaturas entre 12 K e 350 K, às quais se juntaram medições da resposta a um degrau de tensão e de condutividade em corrente contínua. Os dados foram analisados segundo vários modelos, consoante a zona de frequência de estudo. Assim, no domínio temporal, correspondente a baixas frequências, usou-se a função de Kohlraush-Williams-Watts, enquanto que no domínio da frequência se utilizaram as funções de Von Schweidler e de ColeDavidson. Foram então identificados dois mecanismos de relaxação separados por cerca de dez décadas de frequência. A relativa insensibilidade dos parâmetros de relaxação a baixa frequência à concentração e tipo de ião permite inferir da importância do vidro base neste processo de relaxação. Por outro lado, o mecanismo de relaxação a alta frequência é dependente da natureza do ião introduzido. Ambas as relaxações se identificam como sendo do tipo não-Debye. As medições de permitividade complexa em função da temperatura deram um contributo importante para identificar um fenómeno de agregação a baixas concentrações de óxido de terra rara incluído na matriz vítrea. O comportamento da permitividade com a concentração é semelhante para todos os sistemas estudados. A um inicial aumento de E' e EU até uma concentração crítica, segue-se um decréscimo para concentrações mais elevadas. No entanto aquele ponto é dependente do vidro base e ião de terra rara. Finalmente dão-se algumas pistas no sentido de se promover uma ligação entre os parâmetros de relaxação calculados e as características físicas dos sistemas.Despite the fact that the technology of glass has seen a great development in the last decades, the understanding of the disordered structure is far from clear. Some progress has been made, with techniques of ressonance and diffraction, but, more and more, the advantages of measurements of microscopic properties has been noted. With this work we intend to study the effect of introducing different rare earth ions, Gd, Nd, Eu and Dy, in lead borate and lead silicate glasses, with the measurement of electrical properties; conductivity, permitivity and dielectric relaxation. Confirmation ofthe amorphous structure was done by X-ray diffraction. In order to obtain information from dielectric response, we made measurements of the complex permitivity, between 20 mHz and 10 GHz, at temperatures between 12 K and 350 K, combined with pulsed field and d.c. conductivity measurements. Data was analysed with different models, dependent on the frequency of interest. In the time domain,· corresponding to low frequencies, we used the Kohlraush-Williams-Watts function, whereas in the frequency domain we used the Von Schweidler and Cole-Davidson functions. We identified two relaxation processes separated by ten decades of frequency. The relative insensivity of the low frequency relaxation parameters to the concentration and type of ion permits us to infer the role of base glass in this relaxation mechanism. On the other hand the high frequency relaxation process is dependent on the nature of ion. Both relaxation processes were classified as non-Debye type. Complex permitivity measurements as a function of temperature contributed to the identification of a phenomena of aggregation at low concentrations of rare earth oxide included in the matrix. The behaviour of permitivity with concentration is similar for all the systems studied. There is an inicial growth of E'and EU up to a criticaI concentration of rare earth ion, followed by a decrease for higher concentrations. This criticaI concentration depends upon the base glass and the rare earthion Finally, we give some ideas in order to find a connection between the relaxation parameters and the physical properties of the system

Temperature-dependent electrorheological effect and its description with respect to dielectric spectra

Electrorheological fluids consisting of large number and type of electrically polarizable particles have been presented in the literature. Nevertheless, there is a lack of temperature-dependent electrorheological effect analysis, which is their major feature from the application point of view. In this work, aniline oligomers were synthesized and carbonized in order to obtain suitable materials to be used in electrorheological fluids. The silicone oil suspensions were prepared and their temperature-dependent electrorheological performance was investigated in the temperature range between 25°C and 65°C. The electrorheological fluid based on particles with larger size exhibits higher sensitivity to the increase in temperature than the electrorheological fluid based on smaller particles. As an evaluative tool, dielectric spectra of the prepared electrorheological fluids were investigated. It has been shown that the activation energy of the relaxation process is higher for the electrorheological fluid based on larger size particles. The enhanced electrorheological effect at high temperature was ascribed to the shift of dielectric relaxation of the electrorheological fluids to higher frequencies. © The Author(s) 2015

Evaluating severe noise interference in IEEE 802.15.4 based location systems

This paper studies the impact of severe ISM noise interference in IEEE 802.15.4 based location systems, in particular the LOPES project. The architecture and operation of the location system are presented as well as the underneath physical mechanisms supporting the IEEE 802.15.4 noise immunity. A test-bed is proposed and the assessment scenario is described by focusing on the trial parameters. Results show that this technology is able to operate in extreme harsh environments. However, this has a significant impact in the location systempsilas performance

Nanocomposite Polymeric Materials Based on Eucalyptus Lignoboost® Kraft Lignin for Liquid Sensing Applications

This study reports the synthesis of polyurethane–lignin copolymer blended with carbon multilayer nanotubes to be used in all-solid-state potentiometric chemical sensors. Known applicability of lignin-based polyurethanes doped with carbon nanotubes for chemical sensing was extended to eucalyptus LignoBoost® kraft lignin containing increased amounts of polyphenolic groups from concomitant tannins that were expected to impart specificity and sensitivity to the sensing material. Synthesized polymers were characterized using FT-MIR spectroscopy, electrical impedance spectroscopy, scanning electron microscopy, thermogravimetric analysis, and differential scanning calorimetry and are used for manufacturing of all solid-state potentiometric sensors. Potentiometric sensor with LignoBoost® kraft lignin-based polyurethane membrane displayed theoretical response and high selectivity to Cu (II) ions, as well as long-term stability