A.F.L. Almeida

Dielectric permittivity and loss of CaCu3Ti4O12 (CCTO) substrates for microwave devices and antennas

The solid state procedure was used to produce bulk ceramics of CCTO (CaCu3Ti4O12). The samples of the CCTO ceramic were studied by X-ray powder diffraction, infrared and Raman scattering spectroscopy. The infrared and Raman scattering spectroscopy confirm the formation of the CCTO phase as seen by X-ray diffraction analysis. For one experimental procedure one uses an organic binder in the process of shaping the samples. In the second procedure the samples were prepared without the presence of the organic phase. For the second situation, we had higher dielectric constant (ɛr=7370) with high loss (D=0.2) at 1 KHz. For the first procedure one has the lower dielectric constant (ɛr=1530) and lower loss (D=0.11) at 1 KHz. Simple rectangular antenna prototypes were also designed on substrate samples (C1, C2, P1 and P2). For the antennas (with P2, C1 and C2 as substrates), the bandwidth (BW) is 90 MHz (around 3%). The antenna with P1 substrate presented a surprisingly high BW of 270 MHz that corresponds to a 10% bandwidth. In the microwave range one observes that higher values of the ɛr in the range of 3 GHz antennas is also presented by the P1 sample (ɛr=41.6), and the lowest one was presented by P2 (ɛr=33.7). Sample P1 also presents the highest value for the loss (Q−1) which is around 0.1 compared to the other samples which is around 0.03. Therefore, these measurements confirm the potential use for small high dielectric planar antennas. These materials are also attractive for capacitor applications and certainly for microelectronics, and microwave devices (e.g., cell mobile phones), where the miniaturization of the devices is crucial.

Structural studies of lithium triborate (LBO–LiB3O5) in borophosphate glass-ceramics

Abstract In this paper lithium borophosphate glasses and glass-ceramics in the system 0.75| x B 2 O 3 .(100− x )P 2 O 5 |.25Li 2 O. y Fe 2 O 3 with 0≤ x ≤100 mol.% and y =4 mol.%, were studied by X-ray powder diffraction, Mossbauer and infrared spectroscopy. All the samples in the system present a glass or glass-ceramic behavior which was confirmed by X-ray diffraction. From our Mossbauer analysis, high spin Fe 2+ and Fe 3+ in a distorted octahedral coordination are present in all samples. One can conclude that both iron ions are at sites of distorted octahedral coordination, acting as network modifiers (NWM) in a very broad site distribution, which is characteristic of an amorphous structural neighborhood. Such glasses and glass-ceramics containing nonlinear optical materials formed in a controlled crystallization process would be interesting candidates for applications in new nonlinear optical devices.

Study of structural, electrical, and dielectric properties of phosphate-borate glasses and glass-ceramics

In this work, phosphate-borate based glasses with molar composition 20.7P2O5–17.2Nb2O5–13.8WO3–34.5A2O–13.8B2O3, where A = Li, Na, and K, were prepared by the melt quenching technique. The as-prepared glasses were heat-treated in air at 800 °C for 4 h, which led to the formation of glass-ceramics. These high chemical and thermal stability glasses are good candidates for several applications such as fast ionic conductors, semiconductors, photonic materials, electrolytes, hermetic seals, rare-earth ion host solid lasers, and biomedical materials. The present work endorses the analysis of the electrical conductivity of the as-grown samples, and also the electrical, dielectric, and structural changes established by the heat-treatment process. The structure of the samples was analyzed using X-Ray powder Diffraction (XRD), Raman spectroscopy, and density measurements. Both XRD and Raman analysis confirmed crystals formation through the heat-treatment process. The electrical ac and dc conductivities, σac and σdc...