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In this article, the dielectric properties of a Li4Ti5O12 (LTO) ceramic at the radio frequency (RF) and microwave (MW) regions were evaluated. X-ray diffraction showed that LTO was obtained without the presence of spurious and/or secondary phases. Complex impedance spectroscopy (CIS) analysis was conducted, whereas an activation energy (Ea) of 0.88 eV was observed. The temperature capacitance coefficient (TCC) was also calculated and demonstrated that LTO could be employed as a Class 1 ceramic capacitor. In the MW region, LTO presented ε'r = 25.4, tan δ = 5.7 × 10–4, and τf = -14.5 ppmºC−1, values that are interesting for devices that operate in the MW region. Numerical simulation demonstrated values of a realized gain of 4.78 dBi, a bandwidth of 227 MHz, and a radiation efficiency of 98%. Moreover, LTO was evaluated as a temperature sensor operating in the MW region and demonstrated a sensitivity of -0.06 MHz ºC−1. The values presented demonstrate that LTO could be employed in devices that operate in RF and MW regions. [ABSTRACT FROM AUTHOR]

This work presents experimental and numerical investigations of the microwave dielectric properties of the ceramic matrix CaMoO4 (CMO) with the addition of 8, 12, and 20 wt% TiO2, obtained through the solid-state reaction method. X-ray diffraction and Rietveld's refinement revealed no evidence of secondary phases, indicating no reaction between the CMO and TiO2 phases. The dielectric properties presented an improvement with the addition of TiO2, with the CMO8 sample presenting ε r ′ = 12.8, tan δ = 7.8 × 10–4, and τf = − 6 ppm°C−1, demonstrating that this material has thermal stability (τf < 0). The ceramic was tested as a dielectric resonator antenna (DRA) and numerical simulation results showed that the materials have a realized gain of 4.40–4.92 dBi, a bandwidth of 741‒1079 MHz, and a radiation efficiency above 86%. The results indicate that CMO‒TiO2 systems could be employed in devices operating in the S-band. [ABSTRACT FROM AUTHOR]

FeSbO4 powder was prepared using the solid-state reaction method in this work. Afterward, the dense and porous ceramics were obtained by sintering the pressed powder calcined at temperatures of 900 and 1000 °C for 4 h. Rietveld profile analysis of the X-ray powder diffraction data showed that FeSbO4 adopts the trirutile-type structure (space group P42/mnm, with a ≅ 4.63 Å and c ≅ 9.23 Å). SEM images showed that the powder calcined at 900 °C after being sintered at 1200 °C resulted in ceramics of higher crystallinity, larger grains, and consequently, low porosity. The dielectric properties were measured in the frequency range of 10−1 Hz–1 MHz as a function of temperature (25–250 °C). The real (σ′) and imaginary (σ″) parts of the complex conductivity increase with rising annealing temperature for both samples. The real conductivity in the AC region for = 100 kHz was 1.59 × 10 − 6   S · cm − 1 and 7.04 × 10 − 7   S · cm − 1 for the ceramic samples obtained from the powder calcined at 900 (C-900) and 1000 °C (C-1000), respectively. Furthermore, the dielectric constants (k′) measured at room temperature and f = 100   kHz   were 13.77 (C-900) and 6.27 (C-1000), while the activation energies of the grain region were Ea = 0.53 eV and Ea = 0.49 eV, respectively. Similar activation energy (Ea = 0.52 eV and 0.49 eV) was also obtained by the brick-layer model and confirmed by the adjustment of activation energy by DC measurements which indicated an absence of the porosity influence on the parameter. Additionally, loss factor values were obtained to be equal to 3.8 (C-900) and 5.99 (C-1000) for measurements performed at 100 Hz, suggesting a contribution of the conductivity originated from the combination or accommodation of the pores in the grain boundary region. Our results prove that the microstructural factors that play a critical role in the electrical and dielectric properties are the average grain size and the porosity interspersed with the grain boundary region. [ABSTRACT FROM AUTHOR]

Lead-free piezoelectric ceramics with perovskite structure can demonstrate excellent properties when their compositions are projected with sequences of specific phase transitions and/or coexistence of phases. The main problem, however, is that the use of isolate techniques to characterize the phase transitions of perovskite ceramics can be inconclusive or require intricate and expensive procedures. This study aimed to explore the use of the combination of different techniques to investigate the temperature-induced phase transitions and phase coexistence in ceramics of 0.66Ba(Zr0.2Ti0.8)O3–0.44(Ba0.7Ca0.3)TiO3 (0.44BCZT). The phase transitions of ceramics were characterized in temperature ranges from 0 to 160 °C using the techniques of dilatometry, differential scanning calorimetry, dielectric impedance, pyroelectric coefficient and X-ray diffraction. This study has shown the existence of three-phase transitions for the 0.44BCZT composition in the range of temperature and identified physical anomalies that confirm the phase coexistence. [ABSTRACT FROM AUTHOR]

In this paper, the optical absorption and radioluminescence ( RL) of polycrystalline ceramic and powder Ca WO4 are studied. The samples were synthesized via a solid-state method. Differential thermal analysis and thermogravimetric ( DTA/ TG) measurements indicate a minimum temperature of 700°C for calcination of precursors. The crystalline phase of the samples of calcium tungstate was investigated by X-ray diffraction ( XRD). The RL of the crystal powder and ceramics are in agreement with that present in the literature for bulk single crystals, presenting a single emission band centered at 420 nm when irradiated with β-rays. UV-vis absorption spectroscopy shows that the maximum absorption band of the Ca WO4 sample is at around 400 nm, indicating the presence of mixed valence states of W5+ and W6+, associated with a possible breaking of structural symmetry caused by WO4 tetrahedral distortion. [ABSTRACT FROM AUTHOR]

The electric field-assisted flash sintering method recently introduced in the literature was here considered for producing high-quality CaCu 3 Ti 4 O 12 electroceramics, from a powder originally synthesized by a Pechini-derived method and calcined at 800 °C for 2 h. The study included analyzing the dynamics of material shrinkage and densification. With increasing the electric field ( E ), three distinct regimes were observed: a conventional-like sintering behavior for E < 15 V/cm, followed by a region of accelerated (fast-dominated) sintering for 15 ≲ E < 30 V/cm, and then the flash-dominated regime, for E ≳ 30 V/cm, where sintering is not only accelerated but occurs suddenly. In consequence, sintering of the material under field input takes place at furnace temperatures sensibly lower, reaching a value as low as 750 °C for E = 60 V/cm versus 1050 °C in conventional processing. The probable physical mechanism behind each regime, including the role of the rise in sample temperature induced by the applied field (Joule heating), is also discussed. Finally, a correlation was found between the microstructure characteristics achieved during sintering and the dielectric response measured. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the production of bismuth germanate ceramic scintillator (Bi4Ge3O12) by combustion synthesis (SHS) method, focusing on the influence of the synthesis parameters on the crystalline phases and agglomeration of the nanoparticles. The synthesis and sintering conditions were investigated through thermal analysis, X-ray diffraction as function of temperature, dilatometry and scanning electron microscopy. Well-dispersed Bi4Ge3O12 powder was accomplished by the combustion of the initial solution at pH 9, followed by low temperature calcination and milling. Sintered ceramics presented relative density of 98% and single crystalline Bi4Ge3O12 phase. The luminescent properties of the ceramics were investigated by photo- and radio-luminescence measurements and reproduced the typical Bi4Ge3O12 single-crystal spectra when excited with UV, β and X-rays. The sintered ceramics presented light output of 4.4×103 photons/MeV. [Copyright &y& Elsevier]

We have investigated the potential use of Bi4Ge3O12 (BGO) laser-sintered ceramics in scintillator devices for radiation detection. Relevant results from the characterization made during the ceramic processing are presented, such as particle size analysis, X-ray diffraction studies, and thermal analysis. Furthermore, we have compared the microstructure, relative density, thermoluminescent (TL) emission, and transparency degree of ceramic bodies sintered by CO2 laser heating with a reference set of ceramics sintered in an electrical furnace, and also with single crystals grown by the Czochralski technique. It was observed that the laser-sintered BGO presented a relative density of 98% (±2%), better pore shrinkage, larger grains, and light transmission intensity 1.5 times higher than the BGO ceramics sintered in a furnace. The TL curves of all samples irradiated with UV and β—rays presented peaks at 75°, 102°, and 143°C, and provided strong indication that the intergrain defects have the same nature of the bulk defects and do not contribute with new traps in the temperature range studied. The relative areas of the total TL emission after IS irradiation were 6,0:4,4:1 for the reference ceramic, laser sintered ceramic, and single crystal, respectively. For UV irradiated samples, this relation was 7,1:4,7:1. From these results, we have concluded that laser-sintered ceramics have an amount of charge traps lower than the conventionally sintered BGO samples, thus having a higher radioluminescence yield. [ABSTRACT FROM AUTHOR]

This work reports on the influence of co-doping on the luminescence and persistent luminescence properties of BaAl 2 O 4 (BaO·Al 2 O 3) laser-sintered ceramics. The samples were doped with Eu and doubly co-doped with different rare earths (Dy, Er, Nd, Pr, and Tm) followed by laser-sintering without an atmospheric control, at a power density of 1.6 W/mm2. All sintered ceramics contained the hexagonal BaAl 2 O 4 phase and had a homogeneous microstructure. A photoluminescence emission band centered at 495 nm was observed, which was due to the 4f65 d1 → 4f7 Eu2+ transition in the BaAl 2 O 4 matrix. The persistent luminescence emission was monitored for 15 min after charging at 345 nm for 3 min. The combination of Nd and Dy co-doping led to substantial improvements in decay time and persistent luminescence intensity compared to individual co-doping. These results suggested that the two co-dopants interacted by modifying the trap configurations in the BaAl 2 O 4 matrix. • Laser sintering has proven effective in sintering BaAl 2 O 4 :Eu,RE ceramics, exhibiting persistent luminescence properties. • To achieve long-lasting persistent luminescence, it is crucial to choose the right co-dopants or trap depth. • The combination of Nd and Dy exhibits superior persistent luminescence results compared to samples co-doped individually. [ABSTRACT FROM AUTHOR]