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The structural and dielectric properties of BaMoO4 - CaTiO3 advanced ceramics in the Radio Frequency (RF) region (< 300 MHz) were studied in the present work. X-ray diffraction and Rietveld refinement showed no evidence of secondary phases, indicating no reaction between the BaMoO4 (BMO) and CaTiO3 (CTO) phases. Complex Impedance Spectroscopy (CIS) analysis demonstrated that BMO presents greater permittivity than composites can be explained by the low densification of the composites. Nyquist diagrams were fitted employing an equivalent circuit with two associations of R-CPE (constant phase element) related to the grain and the grain boundary effects. The composites showed a decrease in the values of Temperature Capacitance Coefficient (TCC) with CTO (CaTiO3) content increase, whereas composites BMO20 and BMO40 showed TCC values equal to zero at 1 MHz. Moreover, as obtained by CIS analysis, the activation energy of thermo-activated processes decreases with CTO concentration increase. [ABSTRACT FROM AUTHOR]

Tri-doped phosphor YNbO4:Er3+/Tm3+/Yb3+ was prepared by solid-state reaction method. The XRD and SEM results reveal that the product is a monoclinic phase of YNbO4 with high crystallinity and the particles presenting grains with well-defined boundaries with average size is about 1–5 μm. The temperature-dependent luminescence is investigated with 980 nm laser excitation, whereas temperature sensing behaviour was studied in the range of 300–500 K based on Stark sublevels 1G4(a)/1G4(b) (Tm3+), thermal coupling levels 2H11/2/4S3/2 (Er3+) and non-thermal coupling levels 4F9/2 → 4I15/2 (Er3+) /3F2 → 3H6 (Tm3+), 2H11/2 → 4I15/2 (Er3+) / 4F9/2 → 4I15/2 (Tm3+) and 4S3/2 → 4I15/2 (Er3+) /3F2 → 3H6 (Tm3+), utilizing fluorescence intensity ratio (FIR) technique. It is worth highlighting that 1G4(a)/1G4(b) ratio as is not much employed in thermometry. Regarding, NTCL 3F2/1G4(b) pair exhibited the highest absolute sensitivity (SA) is about 110.2 × 10− 3 K− 1 at 500 K, whereas NTCL 3F2/4F9/2 pair presented the highest relative sensitivity (SR) is 2.1% K− 1 at 339 K. Meanwhile, the temperature-dependent emission colours are discussed in detail. High sensitivity indicates that YNbO4:Er3+/Tm3+/Yb3+ white-light phosphor can be used in non-contact temperature sensors applications. [ABSTRACT FROM AUTHOR]

The present article evaluates the effect of the addition of TiO2 on the dielectric properties in the radiofrequency region of the YNbO4 ceramic matrix. X-ray diffraction (XRD) showed that the YNT (Y(Nb0.5Ti0.5)2O6) phase formed from the reaction between YNbO4 (YNO) and TiO2 in the investigated composites. Complex impedance spectroscopy (CIS) analysis was utilized and it was observed that the activation energy (Ea) values of the materials varied between 1.13 and 2.31 eV. Nyquist diagrams were fitted for the materials using three associations of resistor-constant phase element (R-CPE), with an exception for the YNO40 composite that used only two associations. All compositions presented values of n in the range of 0.6 ≤ n ≤ 1 for grain resistance (Rg) and grain boundary resistance (Rgb), indicating a capacitive character. Temperature capacitance coefficient (TCC) values were also calculated. At many frequencies, it was observed that the composites presented values in the range of ± 1500 ppm °C−1, indicating that the evaluated systems could be employed as Class 1 capacitors according to EIA RS-198. [ABSTRACT FROM AUTHOR]

The present work shows the dielectric properties of (100-X)Ba2TiSi2O8–(X)TiO2 composites (X in wt%) at radio frequency (RF) region (< 300 MHz). X-ray diffraction and Rietveld's refinement showed the presence of Ba2(Ti7/9Si2/9)9O20 (BTSO) phase in the studied composites. To analyze the relationship between the electrical properties and the microstructure of the matrix and composites, Impedance spectroscopy (IS) was used. The applied model that best describes the behavior found in the samples is the Havriliak–Negami model. From the IS it was observed that the dielectric relative permittivity presented high values in BTS10 (X = 10%) and BTS20 (X = 20%) which allows classifying them as Colossal permittivity materials. This is an important result due to the potential for applications in new storage systems, capacitive devices and microelectronics. Nyquist diagrams were employed to evaluate the contribution of the grain and grain boundary in the electrical response of the samples and fitted employing an equivalent circuit with two associations of R-CPE (constant phase element). Temperature coefficient of capacitance (TCC) was also obtained and it was possible to observe that there was an increase in the TCC value for the compounds in all frequencies. In addition, IS demonstrated that the activation energy decreasing with TiO2 addition indicating a decrease in the resistive character of the materials. [ABSTRACT FROM AUTHOR]

This work studies the dielectric properties in the microwave region (MW) of the Ba2TiSi2O8 (BTS) ceramic with TiO2 additions and its applications as a dielectric resonator antenna (DRA). In this study, structural characterization through x-ray diffraction (XRD) is performed and the Rietveld refinement is used to confirm the phases formed. Analysis of the morphology of the materials is performed using scanning electron microscopy (SEM). The resonant frequency temperature coefficient (τf) reveals a variation from − 47.0 ppm°C−1 to + 16.5 ppm°C−1. The dielectric properties in the MW region reveal an increase in the dielectric permittivity (εr) and a decrease in the loss tangent (tanδ) of the samples. Numerical simulation shows good fits of the experimental data, with gain and directivity standing out, ranging from 4 dBi to 6 dBi and radiation efficiency below 80%. The results demonstrate that the samples can operate in C-band electronics, Wi-Fi devices, meteorological radar systems, etc. [ABSTRACT FROM AUTHOR]

The effect of doping of Co2+ and Sn4+ in Ba0.5Sr0.5Fe12-2xO19 hexaferrite with different concentrations (x = 0.2, 0.4, 0.6, 0.8, and 1.0) was studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), complex impedance spectroscopy, and analyzed as a dielectric resonating antenna (DRA) at room temperature. The samples were produced and sintered by the solid-state reaction method. Their structure appeared from the M-phase type while their increase in grain size evidenced a dependency on Co-Sn content. The dielectric constant and loss tangent, inferred from impedance spectroscopy, also decreased with the corresponding increase in doping. The real and imaginary impedance decreased with the frequency increment. An electrical equivalent circuit using the Resistance-Constant Phase Element (R-CPE) association was thus proposed, the best-simulated components fitting with the observed structural and microstructure properties. It enabled a better understanding of the microstructure through simulated values of grain/grain boundaries and its effect on tuning the electrical properties in the low-frequency regime. The ferrite characteristics were tested for dielectric resonator antenna applications owing to their good inherent behavior than microstrip patch antenna. Measurement of the radiation efficiency, gain and bandwidth parameters of all the produced ceramics showed that the non-doped one (x = 0.0) exhibits the optimum values: 99.91%, 4.24, and 2.89 GHz, respectively, making the most valuable for antenna applications. [ABSTRACT FROM AUTHOR]

This paper reviews the dielectric properties of Sr2TiSi2O8 (STS) ceramic in the radio frequency (RF) and microwave (MW) regions. X-ray diffraction (XRD) analysis is used to demonstrate that a small secondary phase of SrTiO3 is present in a synthesised STS ceramic. Complex impedance spectroscopy is performed, and the typical permittivity values of ceramic materials are observed. Nyquist diagrams are fitted based on an equivalent circuit using two associations of R-CPE related to the grain and the grain boundary effects. A dependence study of the changes in AC conductivity with frequency at different temperatures demonstrates that the conduction process is thermally activated, and a value of 1.0 eV is obtained for the activation energy. In the MW range, we observe values of εr = 12.7, tan δ = 1.3 × 10–2, and τf = − 6.1 ppm/°C, indicating interesting properties for applications in devices that operate in the MW region. A numerical simulation is employed to evaluate STS ceramic as a dielectric resonator antenna, and we observe a reflection coefficient of below − 10 dB, a realised gain of 4.6 dBi, a bandwidth of 475 MHz, and a radiation efficiency of around 75%. The properties of the STS matrix presented here indicate that this ceramic would be a suitable candidate for the operation of devices in the C-band, as well as in devices operating in the RF range. [ABSTRACT FROM AUTHOR]

This paper analysed the effects of doping with Pr and Yb on the dielectric and down-conversion luminescence properties in the ceramic matrix LaNbO4. X-ray diffraction analysis showed that no additional phases were present in the materials. Impedance spectroscopy analysis demonstrated increasing conductivity due to the generation of oxygen vacancy, and other charge carriers caused by the change in the oxidation states of Pr as well as by lattice distortion caused with the insertion of Pr and Yb ions. Down-conversion measurements in 470 nm presented well-defined bands in the red region that decreased in intensity with the enhancement of Yb3+. Results obtained demonstrate that Pr-Yb co-doped LaNbO4 (LNO) could be used both in electrical and optical applications. [ABSTRACT FROM AUTHOR]

The solid-state reaction method was used for obtaining LaNbO4 (LNO) samples doped with Pr3+ (1 mol%). Results of XRD and Rietveld refinement confirms that the LNO phase remains in the doped sample without the formation of secondary phases. Down-conversion studies were performed under 470 nm excitation and bands in the red region were observed. The intensity of the most intense band (650 nm) is increased by approximately 65% with the dopant addition. Even under very low excitation power the doped sample demonstrates a well-defined luminescence profile, indicating that the LNO:Pr1% sample can be a promising red-emitting phosphor candidate. [ABSTRACT FROM AUTHOR]

LiFe5O8 (LFO) spinel-like material has been studied for use in ferrite resonator antennas (FRAs). Antenna parameters such as gain and return loss were greatly affected when an external magnetic field was applied to the FRA. The temperature coefficient of the resonant frequency (τf) for the FRA presented a value of − 482.16 ppm/°C. The magnetic hysteresis results showed that the LFO was a soft ferrite, considering the values of the remanent magnetization (Mr = 5.95 emu g−1), coercive field (0.76 mT), and saturation magnetization (32.15 emu g−1). The magnetodielectric resonator presented a tuning effect in the resonant frequency as a function of the external magnetic field. The antenna bandwidth was also affected by the presence of the magnetic field. LFO is a soft ferrite with applications in microwave circuits, antennas, and devices for operation at microwave frequencies due to its magnetization and demagnetization properties. Impedance study revealed increasing conductivity from room to higher temperature with low activation energy (0.36 eV). [ABSTRACT FROM AUTHOR]