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Properties of the Sr 3 MoO 6 electroceramic for RF/microwave devices
Abstract
The Sr3MoO6 (SMO) double perovskite has been studied due to its photoluminescence properties. Herein, the aim of this work is to study the dielectric properties of the SMO in radiofrequency (RF). From impedance spectra, we proposed an equivalent circuit using three associations of R-CPE in parallel. We also evaluated its performance as a microwave device (dielectric resonator antenna - DRA). The SMO-based DRA operated at 3.71 GHz (frequency of operation antenna), with reasonable gain (5.00 dBi) and directivity (5.70 dBi), where these values are close to the ones presented by commercial antennas. Additionally, a numerical simulation is performed to obtain the main parameters for proper device application. Based on our results, SMO electroceramic shows important properties, which can be used for devices operating in the S-band such as weather radar, surface ship radar and communications satellites.
... In this sense, frequency-dependent parameters are required, such as low dielectric constant (ε r < 15), low dielectric loss (tan δ < 1 × 10 -3 ), and near-zero temperature coefficient of resonant frequency (τ f ) [7,8]. A large number of dielectric ceramics have been studied for application in wireless communication, such as titanates, molybdates, etc. [9][10][11][12]. ...
... For example, the decrease in R values due to the temperature rise is associated with the increase of conductivity. These results confirmed that the profile found in the arcs reduced diameters with the temperature rise [9]. Furthermore, all samples presented a decrease in n values with temperature increasing, indicating a resistive behavior contribution for CPE elements. ...
A series of MCuSi4O10 (M = Ca²⁺, Sr²⁺ and Ba²⁺) electroceramics have been prepared by solid-state reaction and sintered at 1150 (CCSO (CaCuSi4O10)), 1100 (SCSO (SrCuSi4O10)), and 1150 °C (BCSO (BaCuSi4O10)). The structure and dielectric properties of the samples were studied by powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), impedance spectroscopy at radiofrequency (RF) and dielectric resonator antenna (DRA) at microwaves. The electroceramics presented values of thermo-activated process and activation energy (Ea) between 0.65 and 1.66 eV. In addition, fitting plots showed the equivalent circuit consisting of two parallel (R/CPE) connected in series, which is associated with the grain and grain boundary electrical contributions. The microwave dielectric properties showed dielectric constant (εr) of 3.47, 5.04, and 5.40 and quality factor (Q x f) of 11,350.00, 11,438.60, and 28,983.00 GHz for CCSO, SCSO, and BCSO, respectively. All the samples have a great potential for microwave applications, especially the CCSO- and BCSO-based DRA, due to the near-zero temperature coefficient of resonant frequency (τf) values (− 2.24 and − 7.41 ppm.°C⁻¹, respectively). The preliminary results indicate that MCuSi4O10 electroceramics are satisfactory for use as a commercial antenna for C- and X-band applications.
... In this study, each cylinder resonator was fed laterally by a 50 Ω SMA probe in which the dominant mode in this setup is HE 11δ mode. The numerical simulation of the DRA is considered adequate when reflection coefficient (S 11 ), real impedance (Z') and impedance imaginary (Z") obtained demonstrate good agreement with results experimental [38,39]. ...
... On the other hand, the addition resulted in the decreasing of operation frequency and bandwidth justified by the higher ε ′ r of composites [40,41]. All materials had bandwidth above 100 MHz with radiation efficiencies higher than 95%, values that are higher than others ceramics used for the manufacture of DRAs [38,42,43]. Results demonstrate that YNO -CYNTO system are interesting candidates in microwave operating devices, where YCT15% could be employed in S-band (2-4 GHz) applications, such as Wi-Fi devices and satellite communication transmissions, while YNO and others systems could be used in C-band (4-8 GHz) applications as weather radars and surface ship radars [44,45]. ...
The present work shows present the dielectric properties of YNO – CYNTO ceramic composites at Radio Frequency (RF) and Microwave (MW) regions. X-ray diffraction showed that the addition of CaTiO3 (CTO) on YNbO4 (YNO) caused the formation of the CaYTiNbO7 (CYNTO) phase. In the RF analysis, a typical universal dielectric response in the frequency of the temperature-dependent conductivity was found; the frequency-dependent AC conductivity at different temperatures indicates that the conduction process is thermally activated. Regarding the values of the temperature coefficient of capacitance (TCC) was observed a change of signal with the increment of CTO. Besides, the activation energies of composites were calculated and demonstrated an increase with addition of the CTO. In the MW range, it was observed that permittivity and dielectric loss enhance with the addition of CTO. Moreover, the addition caused the improvement of the thermal-stability of YNO where τf value for YCT5% was equal to +0.64 ppm.ºC⁻¹. The composites were analyzed as antennas presenting a reflection coefficient below - 10 dB at the resonant frequency, realized gain of 5.10–5.45 dBi, bandwidth of 129–355 MHz and radiation efficiency above 95%. The results obtained indicate that YNO – CYNTO system would be interesting candidates in microwave operating devices.
... Microwave magnetic devices are indispensable components in satellite and mobile communications systems [1][2][3][4][5][6]. The demand for microwave devices such as higher integration, higher operating frequency, and smaller size is an important reflection of the rapid development of the electronic communication industry [7][8][9]. There is a significant demand to improve the performance of magnetoelectronic units, which are the core components of microwave devices. ...
A simple and convenient method is demonstrated in this work by continuously applying uniaxial tensile strains to tune the high-frequency properties of flexible magnetic films. The magnetostriction effect causes the uniaxial magnetic anisotropy in the Ti/Fe/Ni81Fe19/Fe/Ti multilayer film when the flexible substrate transitions from the convex state to the planar state after preparation. In addition, the microstructure, magnetic domain morphology, and the high-frequency magnetic performance of the pre-strained Ti/Fe/Ni81Fe19/Fe/Ti multilayer films are investigated. The results show that the flexible Ti/Fe/Ni81Fe19/Fe/Ti multilayer films’ initial permeability can be monotonically varied over a hundred units, and the resonant frequency can be adjusted around 1.5 GHz. The flexible Ti/Fe/Ni81Fe19/Fe/Ti films, with their elastic-tunable magnetic performance, are promising candidate materials for flexible microwave devices.
... Using a lower dielectric constant (ε r ) material as a dielectric device can effectively reduce the latency [8]. The microwave dielectric ceramics used for 5G/6G components are required to have as following characteristics of low ε r , high Q׃ and near-zero temperature coefficient of resonance frequency (τ f ) [9][10][11][12][13]. Table 1 lists some of the dielectric resonator antennas that have been designed and fabricated by researchers using microwave dielectric ceramics [14][15][16][17][18][19][20][21][22]. As can be seen from Table 1, the dielectric constants of the dielectric resonator are concentrated around 15-20. ...
Garnet-typed ceramics of Y3Mg1-xMnxAl3SiO12 (0≤x ≤ 0.2) have been synthesized using the traditional solid-state reaction method. The optimal microwave dielectric properties (εr = 10.73, Q׃ = 62,824 GHz, τf = −34.8 ppm/°C) are obtained for Y3Mg0.9Mn0.1Al3SiO12 (x = 0.1) sintered at 1575 °C for 5h. A millimeter-wave dielectric resonator antenna is designed and fabricated using Y3Mg0.9Mn0.1Al3SiO12 as a dielectric unit due to its excellent characteristics of low dielectric constant and high Q׃. The designed DRA resonates at 24.94 GHz with a bandwidth ∼2.20 GHz (S11 < −10dB). The simulated gain and efficiency are 6.64 dBi and 91.08%, respectively. The results indicate that the Y3Mg0.9Mn0.1Al3SiO12 ceramic has a potential application as an antenna for the 5G/6G millimeter wave frequency band.
... Recently, the rapid development in the electronic communication industry has proposed higher requirements for microwave devices including higher operating frequency, lower loss, and higher integration level. As magnetoelectronic devices are the core components of microwave devices [4][5][6], there is an urgent demand to improve their magnetic properties at the high-frequency range. However, most microwave/radio-frequency magnetic components are discrete devices based on bulk materials, and directly reducing the size of high integration often leads to degradation in performance. ...
This work demonstrates the dependence between magnetic properties and the thickness of NiFe thin films. More importantly, a quantitative study of the surface composition of NiFe thin film exposed to atmospheric conditions has been carried out employing angle-resolved X-ray photoelectron spectroscopy (ARXPS). In this study, we fabricated Ni81Fe19 (NiFe) thin films on Si (100) substrate using electron beam evaporation and investigated their surface morphologies, magnetic properties, and the thickness of the surface oxide layer. The coexistence of metallic and oxidized species on the surface are suggested by the depth profile of ARXPS spectra. The thickness of the oxidized species, including NiO, Ni(OH)2, Fe2O3, and Fe3O4, are also estimated based on the ARXPS results. This work provides an effective approach to clarify the surface composition, as well as the thickness of the oxide layer of the thin films.
... Para obter informações mais detalhadas dos diagramas de Nyquist os resultados de impedância obtidos experimentalmente foram aproximados com a impedância de um circuito elétrico equivalente teórico composto por resistores (R), capacitores (C), indutores (I), elementos de fase constante (CPE) ou outros componentes.Dessa forma, é possível determinar as contribuições do grão, contorno de grão e/ou efeito de eletrodo na resposta elétrica do material[7,8]. Para a obtenção do circuito, foi empregado o software EISSA (Electrochemical Impedance Spectroscopy Spectrum Analyser) e os resultados obtidos para a temperatura de 440ºC são apresentados na Tabela 1.Durante o ajuste usando o software EISSA foi observado que existe a contribuição do grão e contorno de grão na resposta elétrica da argila analisada, sendo que o contorno de grão tem caráter mais isolante do que o grão (Rgb > Rb). ...
... As described previously in Z" spectra, the resistive contribution of grain, grain boundary and electrode effect decrease with temperature increasing. Grain and electrode are predominantly capacitive due to the fitted parameter n (deviation of ideal capacitance) for CPE element with increasing of the temperature [53]. ...
Molybdates from A 2 Mo 3 O 12 family have been widely investigated due to its low sintering temperature, low thermal expansion coefficient, and low dielectric loss. Fe 2 Mo 3 O 12 (FMO) is an oxide from this family and widely used in the catalytic field. The aim of this work is to evaluate the influence of the Bi 2 O 3 -B 2 O 3 as a sintering aid in the microstructure and dielectric properties of FMO. The diffraction results showed that the FMO with the monoclinic structure phase was obtained after the calcination process (650 °C). Mössbauer spectroscopy showed the formation of Fe 2 O 3 after the sintering process at 800 °C. The scanning electron-microscopic demonstrates an increase of the grain as a function of sintering aid concentration. The samples were analyzed by using the impedance spectroscopy at radiofrequency with temperature variation. The Nyquist diagram obtained in this temperature range was fitted from an equivalent circuit with three R-CPE associations, corresponding to the morphology of the electroceramics. For dielectric properties in the microwave, all the samples showed values of ε r lower than 10. Values of Q x f above 14132.35 GHz were achieved. The thermal stability was evaluated by the temperature coefficient of resonant frequency ( τ f ). The lowest τ f values of -6.55 ppm/°C and -4.35 ppm/°C (near-zero) were measured to FMO and FMO mixed with 7.5 wt% Bi 2 O 3 -B 2 O 3 , respectively. Based on these results, FMO can be used to low permittivity ceramic for low temperature co-fired ceramics ( LTCC) applications, antenna substrate, and millimeter-wave range.
... The value of M″ max is different for different temperature. As the value of M'' decreases, it suggests the occurrence of relaxation in the sample caused by hopping of charge carriers [38]. With increasing the temperature, the peaks shift in the direction of high-frequency area. ...
The polycrystalline sample of a double perovskite, Sm2NiMnO6 was synthesized by a solid-state reaction route. From the X-ray structural study, it is found that the structure of the material is monoclinic with lattice parameters: a = 4.1750(63) Å, b = 7.6113(63) Å, c = 5.9896(63) Å, and β = 112.70°. These parameters are very close to and consistent with those of such type of materials. The dielectric, impedance, AC conductivity, and electrical modulus properties of the sample were studied in the temperature range of 25-300 ℃ and the frequency range of 1 kHz-1 MHz. Typical relaxor behavior observed in the dielectric studies was confirmed by Vogel-Fulcher fitting. From the Nyquist plots, the temperature dependent contribution of grain and grain boundary effect was confirmed. The non-Debye type of relaxation was found using the complex impedance spectroscopy. The magnetic study revealed that the sample had paramagnetic behavior at room temperature. Magneto-electric (ME) coefficient was obtained by changing DC bias magnetic field. This type of lead-free relaxor ferroelectric compound may be useful for high-temperature applications.
... It is important that the reflection coefficient (S 11 ) of the DRA is below −10 dB in order to operate efficiently as an antenna, with less return loss [28]. As shown in Fig. 6a, all of the DRAs had good S 11 values, which were below −40 dB. ...
in the microwave range by measuring their di-electric properties comprising the temperature coefficient of the resonant frequency (τ f), relative dielectric permittivity (ε r), and dielectric loss (tanδ) in the C band (4-8 GHz). The Li 2 TiO 3 phase was obtained by solid-state reaction and the ceramic composites were obtained by blending phase 2 with 5, 10, 15 and 20 wt% of Al 2 O 3. We then analyzed these ceramic composites as dielectric resonator antenna. The results showed that the alumina addition of 5 wt% improved the τ f value for the composites (τ f = −0.58 ppm °C −1) but the other di-electric properties did not improve with the alumina concentration, where the permittivity decreased and the dielectric loss increased. According to the antenna measurements, all of the samples exhibited good performance in terms of the gain (3.91-5.67 dBi), bandwidth (338-500 MHz), and efficiency (70.58-96.84%) compared with previous findings.
... The aim of this study was to study the sludge generated in the textile industry treatment by electrocoagulation and to make feasible its use as a barrier layer capacitor and as a dielectric resonator antenna (DRA) [13][14][15][16][17][18]. ...
This work aims to develop an eco-friendly ceramic obtained from the sludge generated in the textile industry treatment by electrocoagulation to demonstrate a possibility of natural raw materials’ extraction decrease. The sludge generated by the electrocoagulation treatment was treated by some sintering processes. The presence of two crystalline phases was confirmed through the Rietveld refinement: Fe2O3 and NiCr2O4. In the microwave range it is possible to confirm the relative stability of dielectric permittivity over the wide frequency range. The Temperature Coefficient of Capacitance results demonstrate that the samples can be classified as a class 4 ceramic capacitor. The activation energy measurement profile is similar for all samples. The results showed that samples obtained from the sludge present low dielectric permittivity, two thermo-activated processes one to water loss and another to conduction with activation energy of approximately 0.6 eV for all analysed samples. The manufactured samples can be applied as a barrier layer capacitor and dielectric resonator antennas.
The development of multifunctional materials is an exceptional research area, which is aimed at enhancing the versatility of materials according to their wide fields of application. Special interest was devoted here to lithium (Li)-doped orthoniobate ANbO4 (A = Mn), in particular, the new material Li0.08Mn0.92NbO4. This compound was successfully synthesized by a solid-state method and characterized using various techniques, including X-ray diffraction (XRD), which confirmed the successful formation of an ABO4 oxide with an orthorhombic structure and the Pmmm space group. The morphology and elemental composition were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The vibrational study (Raman) at room temperature confirmed the existence of the NbO4 functional group. The effects of frequency and temperature on the electrical and dielectric properties were studied using impedance spectroscopy. In addition, the diminishing of the radius of semicircular arcs in the Nyquist plots (-Z'' vs. Z') showed the semiconductor behavior of the material. The electrical conductivity followed Jonscher's power law and the conduction mechanisms were identified. The electrical investigations showed the dominant transport mechanisms in the different frequency and temperature ranges, proposing the correlated barrier hopping (CBH) model in the ferroelectric phase and the paraelectric phase. The temperature dependence in the dielectric study revealed the relaxor ferroelectric nature of Li0.08Mn0.92NbO4, which correlated the frequency-dispersive dielectric spectra with the conduction mechanisms and their relaxation processes. The results demonstrate that Li-doped Li0.08Mn0.92NbO4 could be used both in dielectric and electrical applications.
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 εr′ = 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.
The metallic magnetic thin films are widely used in high-frequency magnetic devices, the reliability of which greatly depends on the saturation magnetization (Ms) and resonance frequency (fr) of the films. However, increasing the operating frequency of the magnetic thin film is extremely challenging. This study investigates the microstructure, magnetic domains and magnetic properties of obliquely deposited Ti/Fe/Ni81Fe19/Fe/Ti multilayer films. The coercivity distribution of the deposited films is investigated by first order reversal curve (FORC) measurements. The result of static magnetic properties revealed the in-plane uniaxial anisotropy with a high anisotropy field (Hk) up to 495 Oe. A magneto-optical Kerr microscope image displayed the multi-step magnetization reversal processes at different oblique angles. Complex permeability spectra indicated that the resonance frequency increases from 1.27 to 4.71 GHz as the deposition angle is increased from 0° to 25°. Thus, Ti/Fe/Ni81Fe19/Fe/Ti multilayer film with tunable magnetic performances is one of the good candidate materials for microwave devices.
Herein, we report a study on the suitability of the Sr3Mo1‐xWxO6 (0.1 ≤ × ≤ 0.9) solid‐solution system for radio frequency and microwave (MW) applications. The ceramic powders were synthetized via solid‐state reaction at 1423 K for 6 h. X‐ray diffraction and scanning electron microscopy were performed to confirm the double‐perovskite‐type structure and investigate the surface morphology, respectively. Electrical characterization of the system as a function of temperature was performed using impedance spectroscopy. The equivalent circuit with two parallel association resistor‐constant phase element (R‐CPE), connected in series, associated with grain and grain boundary effects was used to adjust the electric response of the solid solutions. Sr3Mo0.9W0.1O6 solid solution exhibited a resonant frequency temperature coefficient (τ f ) value close to zero (8.5 ppm K‐1), showing a great potential for MW applications. This article is protected by copyright. All rights reserved.
Molybdates from A2Mo3O12 family have been widely investigated due to its low sintering temperature, low thermal expansion coefficient, and low dielectric loss. Fe2Mo3O12 (FMO) is an oxide from this family and widely used in the catalytic field. The aim of this work is to evaluate the influence of the Bi2O3–B2O3 as a sintering aid in the microstructure and dielectric properties of FMO. The diffraction results showed that the FMO with the monoclinic structure phase was obtained after the calcination process (650 °C). Mössbauer spectroscopy showed the formation of Fe2O3 after the sintering process at 800 °C. The scanning electron-microscopic image demonstrates an increase of the grain as a function of sintering aid concentration. The samples were analyzed using the impedance spectroscopy at radiofrequency with temperature variation. The Nyquist diagram obtained in this temperature range was fitted from an equivalent circuit with three R-CPE associations, corresponding to the morphology of the electroceramics. For dielectric properties in the microwave, all the samples showed values of εr lower than 10. Values of Q x f above 14,132.35 GHz were achieved. The thermal stability was evaluated by the temperature coefficient of resonant frequency (τf). The lowest τf values of − 6.55 ppm/°C and − 4.35 ppm/°C (near-zero) were measured to FMO and FMO mixed with 7.5 wt % Bi2O3–B2O3, respectively. Based on these results, FMO can be used to low-permittivity ceramic for low-temperature co-fired ceramics (LTCC) applications, antenna substrate, and millimeter-wave range.
Ba5Li2W3O15 (BLWO) electroceramics with different amounts of lithium carbonate (10–15%) were synthesized using the solid-state route. Powder x-ray diffraction (PXRD) and Raman spectroscopy showed the presence of the BaWO4 (BWO) phase in the samples. The excess lithium in the BLWO matrices contributed to an increase in the BWO phase and improved the thermal stability. Images obtained by scanning electron microscopy (SEM) showed grains without a well-defined morphology and with a low porosity. The dielectric properties in the microwave (MW) range for the dielectric resonator based on BLWO were analysed by the Hakki–Coleman and Silva–Fernandes–Sombra methods. The near-zero temperature coefficient of resonant frequency (6.59 ppm°C−1) was obtained for the sample synthesized with 15% Li2CO3. Numerical studies based on the coefficient of reflection (S11) and impedance values of the BLWO-based dielectric resonator antennas (DRA) were also conducted. They agreed well with the experimental results, which indicates that this material can be applied as an antenna device in the MW range.
Eu³⁺ – Tb³⁺ co-doped Sr3MoO6 (SMO) phosphors have been synthesized by solid-state reaction. Powder X-ray diffraction (PXRD) and Scanning electron microscopy (SEM) were used for the characterization of the synthesized materials, and no significant structural changes have been observed. Photoluminescence (PL) was obtained under the 290 nm excitation where a strong energy transfer from host to active ions and weak energy transfer between the active ions with prominent emission from respective active ions. The fluorescence intensity ratio (FIR) was evaluated as a function of temperature in the range of 14 and 300 K using Eu³⁺/Tb³⁺ ratio to obtain absolute (SA) and relative (SR) sensitivities. SMO:Eu/Tb presented maximum SA of 0.0029 K⁻¹ at 300 K and SR of 0.24% K⁻¹ at 300 K, demonstrating that investigated system could be used in temperature sensing applications at low temperature.
Recently, increasing the application frequency of devices through spin wave resonance has received widespread attention. Exciting spin waves through the spin pinning effect of the Ni film on the surface of the Ni81Fe19 film is of great significance to increase the application frequency of the device. Herein, the Ni (t nm)/Ni81Fe19 (50 nm)/Ni (t nm) multilayer films, which were fabricated by magnetron sputtering. And the micromorphology, static magnetic properties, ferromagnetic resonance (FMR) linewidth (∆H) are discussed in detail. As the Ni film thicknesses increased, the saturation magnetization (4πMs) of the trilayers decreased from 7405 to 5805 Gs. When the Ni film thickness was below 10 nm, ∆H showed an upward trend, and the ∆H slightly decreased at 15 nm Ni film thickness. When Ni film thickness was less than 10 nm, the perpendicular standing spin wave (PSSW) mode appeared, which implied a strong pinning effect. With 10 nm and 15 nm Ni film, the PSSW mode disappeared, indicating that the large grain size was not conducive to the pinning effect of the film.
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.
Multilayer films with high saturation magnetization and low ferromagnetic resonance linewidth have great potential for various applications in the era of high frequency and miniaturization. In this study, Fe (t nm)/Ni81Fe19 (50 nm)/Fe (t nm) multilayer films were fabricated using electron beam evaporation, and the microstructure, saturation magnetization, coercivity, ferromagnetic resonance (FMR) linewidth and perpendicular standing spin-wave (PSSW) mode are discussed in detail. With increasing Fe film thickness, the saturation magnetization of the multilayer films increased from 10,547 to 14,494 Gs, becoming closer to the theoretical value, and the ferromagnetic resonance linewidth showed an overall increasing trend. It is worth noting that a strong pinning effect has been found via the appearance of the PSSW mode in the FMR spectrum, when the thickness of Fe reaches 4 nm. As the thickness of the Fe film increased from 4 to 15 nm, the perpendicular standing spin-wave field (Hp) increased from 133 to 379 Oe, and the perpendicular standing spin-wave linewidth (ΔHp) increased from 68 to 148 Oe. Remarkably, with 8 nm thick Fe film, the multilayer films achieved excellent comprehensive performances with high saturation magnetization (4πMs, 13,525 Gs), a low in-plane ferromagnetic resonance field (Hr, 725 Oe), and a low FMR linewidth (ΔH, 84.24 Oe).
Highly directional scatterers are designed using core-shell microspheres. Their structural parameters are optimized analytically by genetic algorithm to optimize directional scattering, and the results are verified by finite difference time domain calculations. Using such optimized directional scatterers, the front-to-back ratio and directivity of a dipole is enhanced to 9 and 4.4 respectively.
Highly directional scatterers are designed using core-shell microspheres. Their structural parameters are optimized analytically by genetic algorithm to optimize directional scattering, and the results are verified by finite difference time domain calculations. Using such optimized directional scatterers, the front-to-back ratio and directivity of a dipole is enhanced to 9 and 4.4 respectively.
The magnetic and electrical properties of complex oxide thin films are closely related to the phase stability and cation ordering, which demands that we understand the process-structure-property relationships microscopically in functional materials research. Here we study multiferroic thin films of double-perovskite La2NiMnO6 epitaxially grown on SrTiO3, KTaO3, LaAlO3 and DyScO3 substrates by pulsed laser deposition. The effect of epitaxial strains imposed by the substrate on the microstructural properties of La2NiMnO6 has been systematically investigated by means of advanced electron microscopy. It is found that La2NiMnO6 films under tensile strain exhibit a monoclinic structure, while under compressive strain the crystal structure of La2NiMnO6 films is rhombohedral. In addition, by optimizing the film deposition conditions a long-range ordering of B-site cations in La2NiMnO6 films has been obtained in both monoclinic and rhombohedral phases. Our results not only provide a strategy for tailoring phase stability by strain engineering, but also shed light on tuning B-site ordering by controlling film growth temperature in double-perovskite La2NiMnO6 films.
Electronic and optical properties of perovskites are related to the local structures of the compounds and define their functional applications. Herein we have prepared a double perovskite Cs2AgSbCl6, which crystallized in cubic structure with space group Fm-3m and the material is found to have a varied band gap associated with different body colors. The anti-site defect model was established to investigate transposition influence on optical and electronic properties of double-perovskite Cs2AgSbCl6, and the proposed model clearly explained the as-observed variable body color. Cs2AgSbCl6 perovskite possesses high decomposition temperature and is stable upon prolonged exposure to air and moisture, which emphasize its potential in the field of photovoltaic absorbers and optoelectronic applications.
A lead-free bismuth holmium cobalt titanate multiferroic(BiHoCoTiO6) was synthesized at high temperatures by a solid-state reaction (a mixed oxide) route. Structural analysis of the compound is performed using X-ray diffraction data and an orthorhombic crystal system is suggested for the material. Study of room temperature scanning electron microigrapgh exhibited better morphology for the material. The uniform distribution of the small rod-type of grains with dimension of 1–2 µm length and 0.2–0.3 µm diameters was visible. Analysis of temperature-frequency dielectric data exhibited two dielectric anomalies or phase transitions: first transition temperature(tc1) at 175 °C and the second one at 325 °C. Study of frequency and temperature dependence of resistive characteristics (performed using complex impedance spectroscopy) has shown significant contributions of grains and grain boundaries, which in turn, helps in understanding the electrical conduction mechanism and microstructure behaviour of the material in a better way. The impedance or Nyquist plots were modelled with an equivalent circuit containing capacitance, resistance and related parameters due to grain (bulk), grain boundaries and capacitance. The transport properties, AC conductivity and electrical modulus of the material were also investigated and reported here.
A novel non-rare-earth doped phosphor La2MgGeO6:Mn⁴⁺ (LMG:Mn⁴⁺) with near-infrared (NIR) long persistent luminescence (LPL) was successfully synthesized by solid-state reaction. The phosphors can be effectively excited using ultraviolet light, followed by a sharp deep-red emission peaking at 708 nm, which is originated from ²Eg→⁴A2g transition of Mn⁴⁺ ions. The luminescent performance were analyzed by photoluminescence (PL) and photoluminescence excitation (PLE) spectra. The crystal field parameters were calculated to describe the environment of Mn⁴⁺ in LMG host. The LPL behaviors as well as the mechanisms were systematically discussed. This study suggest the phosphors will broad new horizon in designing and fabricating novel NIR long phosphorescent materials.
Double perovskite Bi2FeMnO6 is a potential
candidate for the single-phase multiferroic system. In this work, we study the
magnetic, electronic, and optical properties in BFMO by performing the density functional theory
calculations and experimental measurements of magnetic moment. We also
demonstrate the strain dependence of magnetization. More importantly, our calculations of
electronic and optical properties reveal that the onsite local correlation on Mn and
Fe sites is critical to the gap opening in BFMO, which is a prerequisite condition
for the ferroelectric ordering. Finally, we calculate the x-ray magnetic circular
dichroism spectra of Fe and Mn ions (L2 and L3 edges) in
BFMO.
A new ultra-low fire glass-free microwave dielectric material Li3FeMo3O12 was investigated for the first time. Single phase ceramics were obtained by the conventional solid-state route after sintering at 540°C–600°C. The atomic packing fraction, FWHM of the Ag oxygen-octahedron stretching Raman mode and Qf values of samples sintered at different temperatures correlated well with each other. The sample with a Lower Raman shift showed a higher dielectric constant. Interestingly, the system also showed a distinct adjustable temperature coefficient of resonant frequency (from −84× 10−6/°C to 25 × 10−6/°C).
In this work, BBT has been synthesized by solid-state reaction. X-ray diffraction and Raman spectroscopy were used to structurally characterize the samples. The dielectric properties were analyzed by impedance spectroscopy for radiofrequency range. For microwave range, the dielectric properties were analyzed by the Hakki–Coleman method. The properties of a DRA made from BBT were obtained by numerical simulations and compared with experimental results. The results show that the synthesis of BBT was effective from 800 °C on, showing nanosize particles (35–45 nm) with tetragonal symmetry. Raman spectra exhibited all active modes characteristic of BBT. The dielectric properties of BBT presented high values of dielectric permittivity (ɛ′ = 52.40 for microwave range and ɛ′ > 100 for radiofrequency range). BBT ceramic based showed negative values of (−422.11 ppm °C−1) temperature coefficient of resonant frequency. The DRA BBT-based demonstrated the use as an antenna, exhibiting small gain of 1.1 dBi and efficiency of 32.5%.
We report multiferroic behavior in double perovskite Y2CoMnO6 with
ferroelectric transition temperature Tc = 80K. The origin of ferroelectricity
is associated with magnetic ordering of Co2+ and Mn4+ moments in a
up-up-down-down arrangement. The saturation polarization and magnetization are
estimated to be 65 uC/m2 and 6.2 Bohr magneton/f.u. respectively. The
magnetoelectric coupling parameter, on the other hand, is small as a 5 Tesla
field suppresses the electric polarization by only ~8%. This is corroborated
with observed hysteretic behaviour at 5K that remains unsaturated even upto 7
Tesla. A model based on exchange-striction is proposed to explain the observed
high temperature ferroelectricity.
In this paper is reported an experimental and numerical investigation of the microwave dielectric properties of BiFeO3 (BFO) ceramic matrix added with 3 and 10 wt% of Bi2O3 or PbO, obtained through a new procedure based on the solid-state method. The experimental and theoretical characteristics of the resonator like return loss, bandwidth, input impedance are in good agreement. The simulations of radiation patterns are presented. For BFO reference sample, frequency response bandwidth is of 6.40% (simulated) and 7.18% (experimental) for frequency operation around 2.94 GHz. The BFO reference sample showed a dielectric permittivity around 25, with loss around 10(-2). The adding of bismuth oxide (3 and 10 wt%) reduced the value of the dielectric permittivity as well as lead oxide 3 wt%, otherwise only the adding of 10 wt% increases its value. The temperature coefficient of resonant frequency (tau(f)) was also measured for all dielectric samples. The values obtained were in the range of -517 ppm degrees C-1 to -222 ppm degrees C-1. With the addition of lead oxide was observed a deterioration of the thermal stability of the ceramic, with respect to the central frequency of resonance. The results obtained confirm the necessity of producing composites of this material with others positive tau(f) materials for obtaining dielectric resonator antennas with tau(f) similar to 0.
Potentiodynamic electrochemical impedance spectroscopy (PDEIS) employs virtual instruments for probing the interface and gives, in real time and in a single potential scan, the multidimensional spectra that characterise complex impedance of the interface as a function of frequency and the electrode potential, along with the potentiodynamic voltammogram. The built-in equivalent electric circuit analyser provides semi-automatic extraction of the potential dependencies of the double layer capacitance, components of Faraday impedance (charge transfer resistance and Warburg impedance) and some additional impedance elements from the impedance spectra extended for the variable potential. PDEIS has been applied to probe the electrochemical response of metal monolayers and nanostructures on various supports.
In this work, barium zirconate (BaZrO3) ceramics synthesized by solid state reaction method and sintered at 1670 8C for 4 h were characterized
by X-ray diffraction (XRD), Rietveld refinement, and Fourier transform infrared (FT-IR) spectroscopy. XRD patterns, Rietveld refinement data and
FT-IR spectra which confirmed that BaZrO3 ceramics have a perovskite-type cubic structure. Optical properties were investigated by ultraviolet–
visible (UV–vis) absorption and photoluminescence (PL) measurements. UV–vis absorption spectra suggested an indirect allowed transition with
the existence of intermediary energy levels within the band gap. Intense visible green PL emission was observed in BaZrO3 ceramics upon
excitation with a 350 nm wavelength. This behavior is due to a majority of deep defects within the band gap caused by symmetry breaking in
octahedral [ZrO6] clusters in the lattice. The microwave dielectric constant and quality factor were measured using the method proposed by Hakki–
Coleman. The dielectric resonator antenna (DRA) was investigated experimentally and numerically using a monopole antenna through an infinite
ground plane and Ansoft’s high frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA
were investigated by adjusting the dimension of the same material.
Layered oxides, A2B'B"O6, with small ions at A-site mostly occupied by unusual-metals like transitional metals are of great importance due to their strong and unusual magnetic interactions and technical applications. Due to small tolerance factors it is very difficult to synthesize such materials in ambient pressure and temperature. Depending upon the variation in temperature and pressure, same composition of the above mentioned materials can be crystallized with different structural arrangements. The effect of pressure and synthetic condition of different materials has been described in this review. The literature data on the effects of the A-site occupation by small ions on the structure and properties are summarized. Finally structural changes and dissimilar properties of high temperature and pressure synthesized recent developed materials are compared with the materials with usual B-site ordered double perovskite.
We investigate the dependence of magnetic properties on the B-site cation ordering for a series of (Sr0.5Ba0.5)2FeSbO6 double-perovskite samples for which the Fe/Sb long-range ordering ranges from nearly nonexisting to essentially complete. Also the size of crystallographically ordered domains varies among the samples. All the samples are antiferromagnetic at low temperatures independent of the level of ordering, indicating multiple co-existing exchange and/or superexchange interactions. Nevertheless, a clear trend is seen for the Néel temperature and the effective magnetic moment both increasing with increasing degree of order and the size of the ordered domains.
Eu³⁺-activated BaLaMgNbO6 red-emitting phosphors were synthesized by a high-temperature solid-state reaction method. Phase analysis and luminescence were characterized by X-ray diffraction (XRD) and photoluminescence excitation and emission spectra. The XRD patterns showed that BaLaMgNbO6 had a monoclinic structure with space group P21/n. The excitation spectra consisted of a broad charge-transfer band and some sharp f-f absorption peaks characteristic of Eu³⁺. The intensity ratio of I615/I590 was used to detect the chemical environment of Eu³⁺. The chromaticity coordinates of BaLa0.7Eu0.3MgNbO6 were (0.67, 0.33), indicating that the BaLaMgNbO6:Eu³⁺ phosphors were excellent red-emitting phosphors. Under excitation by near-ultraviolet (UV) and blue light, the phosphor not only exhibited intense red emission but also showed high color quality. The Ozawa and Dexter energy-transfer theories were employed to calculate the theoretical quenching concentration and determine the concentration quenching mechanism. In addition, the activation energy of BaLa0.7Eu0.3MgNbO6 was calculated through the Arrhenius equation. A configurational coordinate diagram was used to explain the thermal quenching mechanism.
A BiCu2PO6 microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu2PO6 ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (ɛr) = ∼16, a quality factor (Q × f) = ∼39,110 GHz and a temperature coefficient of resonant frequency (τf) = ∼−59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO2 with a positive τf (∼+400 ppm/°C) was added to compensate the τf value. The composite material was found to have a near-zero τf (+2.7 ppm/°C) and desirable microwave properties (ɛr = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.
Novel members of the R2NiRuO6 family of double perovskites have been prepared via a nitrate-citrate route, followed by annealing treatments in air in the 1100-1300 °C temperature range. The crystal and magnetic structures were characterized from neutron powder diffraction (NPD) data at 300 K and 2 K. All the samples are monoclinic, P21/n space group, with a √2a0×√2a0×2a0 unit-cell, compared to the simple perovskite a0 edge. Ni²⁺ and Ru⁴⁺ ions occupy distinct octahedral positions, with a certain antisite disordering. The magnetic structures are defined by the propagation vector k = 0. The magnetic moments of the Ni²⁺ cations are antiparallel to the spins of the Ru⁴⁺ sublattice; the structure can be described as ferromagnetic [011] layers antiferromagnetically (AFM) coupled to each other. For R = Ho and Er perovskites, the rare-earth moments participate in the magnetic structure: Ho³⁺ cations are ordered along the direction (010) in alternate planes AFM coupled and the magnetic moments of Er³⁺ are ferromagnetically ordered along the c axis.
Environmental friendly, non-toxic double perovskite Sr 2 TiMoO 6 (STM) ceramics were synthesized using solid-state reaction route for high temperature thermoelectric applications. XRD and SEM studies confirmed the presence of single-phase solid solution with highly dense microstructure, which were achieved by optimizing sintering temperature and atmosphere. Effect of sintering atmosphere on the structure of STM samples was studied. Rietveld refinement of XRD data confirmed cubic structure of this double perovskite with space-group Pm3m. Temperature dependent thermoelectric measurement showed that STM ceramics converted from p-type into n-type behavior around 550 K. Conductivity mechanism of this double perovskite was further explained by small polaron hopping model.
The effect of calcium on the properties of SmBa1–xCaxCoCuO5+δ (x = 0.0–0.3) as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) is evaluated systematically. Samples exhibit a highly crystalline double perovskite phase, and their cell volumes decrease as x is changed from 0.0 to 0.3. The oxygen content and average thermal expansion coefficient (TEC) of SmBa1–xCaxCoCuO5+δ decreases as the calcium content increased. An average TEC of as low as 15.3 × 10⁻⁶ °C⁻¹ is obtained for SmBa0.7Ca0.3CoCuO5+δ. The area specific resistances at 700 °C decrease by approximately 50% when the calcium content is increased from x = 0.0 (0.173 Ω cm²) to x = 0.3 (0.086 Ω cm²). The maximum power densities of SmBa1−xCaxCoCuO5+δ-based single cells at 800 °C increase from 635 mW cm⁻² (x = 0.0) to 939 mW cm⁻² (x = 0.3).
Two microwave dielectric ceramics ReVO4 (Re = Nd, Sm) prepared through a conventional solid-state reaction method were investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis revealed that pure and dense NdVO4 and SmVO4 ceramics with tetragonal structure could be obtained when sintered in the temperature range 1100°C-1180°C for 4 h. The samples sintered at 1160°C exhibited the best densification with relative densities larger than 96%. NdVO4 ceramic exhibited promising microwave dielectric properties with a relative permittivity (εr) of 12.0, a quality factor (Q × f) of 36,440 GHz, and a negative temperature coefficient of resonance frequency (τf) of −44.3 ppm/°C. SmVO4 had a εr ∼ 11.7, a Q × f ∼ 38,620 GHz and a τf ∼ −48.4 ppm/°C.
Double perovskite R2NiMnO6 (R=Pr, Nd, Tb, Ho and Y) composites are prepared via solid state synthesis or nitrate route. Isothermal magnetic entropy change ( ) and relative cooling power (RCP) for all systems are calculated and compared. All of them possess relatively high values of and RCP as compared to many perovskite and double perovskite systems reported previously. Ho2NiMnO6 has the highest value for and RCP as 8.4 at 70 kOe and 59.18 JKg⁻¹ at 20 kOe respectively. This study highlights the potential of magnetocaloric refrigerant materials at low temperature.
LiLaMgWO6:Eu³⁺ phosphors have been synthesized by high-temperature solid-state reaction and their structure, ultraviolet-visible diffuse reflectance spectra, photoluminescence and decay properties have been investigated as a function of the Eu³⁺ concentration. The XRD results indicated that all the prepared phosphors can be assigned to a monoclinic double perovskite structure in space group C2/m. LiLaMgWO6:Eu³⁺ phosphor has exhibited excellent red emission and efficient excitation, consisting of a broad band ranging from 250 to 350 nm as well as several narrow excitation peaks between 450 and 580 nm. The presence of dominant red emission band centered at 615 nm suggested that Eu³⁺ enters in the lattice site without inversion symmetry. The quenching concentration of Eu³⁺ is up to 30 mol%. The theoretical quenching concentration and quenching mechanism were discussed herein. Moreover, the thermal quenching, CIE chromaticity coordinates, quantum yield, Judd-Ofelt analysis and energy transfer mechanism are discussed in detail.
There is a growing global interest for the development of green technologies that allow the use of products with less damage to environment, as well as for maximum and sustainable use of natural resources. The main aim of this study was to develop superparamagnetic nano-biocomposites for application as dielectric resonator antennas, from a combination of a cardanol-based thermoset plastic, chemically modified sponge gourd fibers (NaOH 10% and NaClO 1 wt%), and magnetite nanoparticles in different contents (1, 5, and 10 wt%). The magnetite particles exhibited nanometric size, high purity and crystallinity, and superparamagnetic character. All nano-biocomposites showed superparamagnetic behavior, excellent thermal stability, good biodegradation rates, and better mechanical strength for the material with magnetite 10 wt%. All dielectric resonator antennas showed satisfactory return loss and suitability for technological applications, especially for performance in broadband.
Double perovskite related oxides A2ZnTiO6 (A = Pr, Gd) have been successfully synthesized by solid state reaction and investigated as photocatalysts for the first time. The two layered titanates mainly demonstrate absorbances under UV irradiation, except for several sharp absorption bands above 400 nm for Pr2ZnTiO6. Therefore, a series of photocatalysts by doping A2ZnTiO6 (A = Pr, Gd) with Cr have been developed in the hope to improve their absorption in the visible light region. The successful incorporation of Cr was detected by XRD and XPS, and the prepared samples have also been characteriazed by SEM, UV–vis DRS and PL. The characterization results suggested that Cr was present mainly in the form of Cr3+, with only a small amount of Cr6+ species. It served as an efficient dopant for the extension of visible light absorbance and improved photocatalytic activities under solar light irradiation. For both Pr2ZnTiO6 and Gd2ZnTiO6, the valence band (VB) was composed of hybridized states of the Zn 3d, O 2p and the conduction band (CB) has major contribution from Zn 4s, Ti 3d orbitals. For Cr doped samples, the newly formed spin-polarized valence band in the middle of the band gap that primarily arise from Cr 3d orbitals was responsible for the improved optical and photocatalytic properties.
A new glass free low temperature sinterable CuMoO4 ceramic was prepared by solid state ceramic route. The structural, microstructural, electron dispersive spectrum and X-ray photoelectron spectroscopy analysis revealed the quality of the material synthesized. The CuMoO4 ceramic sintered at 650 oC exhibits densification of 96 % and low coefficient of thermal expansion (CTE) of 4.6 ppm/oC in the temperature range of 25-500 oC. It has relative permittivity (εr) of 7.9, quality factor (Qf) of 53000 GHz and temperature coefficient of resonant frequency (τf) of -36 ppm/oC (25-85 oC) at 12.7 GHz. The sintered ceramic also shows εr of 11 and low dielectric loss (tanδ) of 2.7 ×10-4 at a frequency of 1 MHz. The full width half maximum (FWHM) of A1g Raman mode of CuMoO4 ceramic at different sintering temperatures correlate well with the Qf values. The low sintering temperature, low relative permittivity, high-quality factor and matching coefficient of thermal expansion to that of Si make CuMoO4 a suitable candidate for ultra-low temperature cofired ceramic (ULTCC) applications.
Polycrystalline Pr2CuTiO6 (PCT) ceramics exhibits dielectric, impedance and modulus characteristics as a possible material for microelectronic devices. PCT was synthesized through the standard solid-state reaction method. The dielectric permittivity, impedance and electric modulus of PCT have been studied in a wide frequency (100 Hz–1 MHz) and temperature (303–593 K) range. Structural analysis of the compound revealed a monoclinic phase at room temperature. Complex impedance Cole–Cole plots are used to interpret the relaxation mechanism, and grain boundary contributions towards conductivity have been estimated. From electrical modulus formalism polarization and conductivity relaxation behavior in PCT have been discussed. Normalization of the imaginary part of impedance (Z″) and the normalized imaginary part of modulus (M″) indicates contributions from both long-range and localized relaxation effects. The grain boundary resistance along with their relaxation frequencies are plotted in the form of an Arrhenius plot with activation energy 0.45 eV and 0.46 eV, respectively. The ac conductivity mechanism has been discussed.
This work reports the dielectric properties of Sr3WO6 (SWO) double perovskite at radio frequencies as a function of the temperature and a study in microwaves range to evaluate the material as a novel dielectric resonator. Thermo-activated charge transfer process for SWO ceramic was observed and two resistor–capacitor associations were fitted for the Nyquist diagram. SWO presented negative values of temperature coefficient of capacitance (TCC) and the activation energies of dielectric relaxation processes were measured by conductivity (1.35 eV), imaginary modulus (1.38 eV) and imaginary impedance (1.36 eV). For the microwave range, SWO shows high dielectric permittivity (13.57) and dielectric loss (0.0281). The dielectric resonator antenna (DRA) fabricated from SWO ceramic presented an operation frequency in 4.1 GHz and return loss below −40 dB. It was possible to evaluate the performance of the antenna from numerical simulation. From these results, the SWO based DRA shows good properties to be used as a novel microwave dielectric resonator.
Despite the remarkable rise in efficiencies of solar cells containing the lead-halide perovskite absorbers RPbX3 (R = organic cation; X = Br(-) or I(-)), the toxicity of lead remains a concern for the large-scale implementation of this technology. This has spurred the search for lead-free materials with similar optoelectronic properties. Here, we use the double-perovskite structure to incorporate nontoxic Bi(3+) into the perovskite lattice in Cs2AgBiBr6 (1). The solid shows a long room-temperature fundamental photoluminescence (PL) lifetime of ca. 660 ns, which is very encouraging for photovoltaic applications. Comparison between single-crystal and powder PL decay curves of 1 suggests inherently high defect tolerance. The material has an indirect bandgap of 1.95 eV, suited for a tandem solar cell. Furthermore, 1 is significantly more heat and moisture stable compared to (MA)PbI3. The extremely promising optical and physical properties of 1 shown here motivate further exploration of both inorganic and hybrid halide double perovskites for photovoltaics and other optoelectronics.
Modern Antenna Handbook represents the most current and complete thinking in the field of antennas. The handbook is edited by one of the most recognizable, prominent, and prolific authors, educators, and researchers on antennas and electromagnetics. Each chapter is authored by one or more leading international experts and includes cover-age of current and future antenna-related technology. The information is of a practical nature and is intended to be useful for researchers as well as practicing engineers. From the fundamental parameters of antennas to antennas for mobile wireless communications and medical applications, Modern Antenna Handbook covers everything professional engineers, consultants, researchers, and students need to know about the recent developments and the future direction of this fast-paced field. In addition to antenna topics, the handbook also covers modern technologies such as metamaterials, microelectromechanical systems (MEMS), frequency selective surfaces (FSS), and radar cross sections (RCS) and their applications to antennas, while five chapters are devoted to advanced numerical/computational methods targeted primarily for the analysis and design of antennas.
We have studied the effect of addition of titania, as a sintering agent, to alumina on the dielectric characteristics of the ceramic composites obtained. The dielectric properties of the titania–alumina composites were studied by use of impedance spectroscopy and the Hakki–Coleman method in the radio-frequency and microwave regions, respectively. The temperature coefficient of the resonant frequency (τ
f
) was also studied. Dielectric permittivity (ε′) was increased and the dielectric loss (tan δ) was improved by addition of titania, as a result of better sintering; addition of 10 wt.% titania resulted in ε′ = 12.68 and tan δ = 8.23 × 10−4 in the microwave region. Increasing the concentration of TiO2 led to inversion of the τ
f
signal; values were positive when the concentration of TiO2 was >7.5 wt.%. The ceramic composites were evaluated as antennas; for all samples the return loss (S
11) was <−10 dB and the gain was approximately 3 dBi. Addition of 7.5 wt.% titania to the Al2O3 improved antenna performance. In conclusion, addition of the TiO2 to alumina improves its dielectric properties, resulting in the possibility of use of such composites as dielectric resonator antennas (DRA).
The polycrystalline sample of Ba2BiNbO6 (a member of double perovskite family) was synthesized by a high-temperature solid-state reaction method (calcination at 910 °C, and sintering at 950 °C). Preliminary analysis of room temperature X-ray data pattern of the sample confirms the formation of a single phase compound in orthorhombic crystal system. The room temperature scanning electron microscope image of the pellet sample clearly shows the uniform distribution of grains and formation of a high density sample. Detailed studies of dielectric and polarization characteristics with temperature confirmed the existence of ferroelectricity in the material. Studies of electrical properties exhibits a strong correlation with the micro-structure and resistive properties of the material. The electrical transport shows the existence of non-exponential-type of conductivity relaxation in the material. This lead-free ferroelectric has many interesting characteristics as compared to that of their lead-based counterpart, Pb2BiNbO6.
The B-site substituted perovskite oxides A2B′B″O6 have in the recent decades gained an increasing amount of interest due to their various interesting properties and possible applications. Here we survey the literature for ca. one thousand A2B′B″O6 perovskite compounds. Crystal structures and the various crystal chemistry features such as ordering and valence mixing of the B cations characteristic to these compounds are reviewed, together with their electronic and magnetic properties. Most importantly, the thorough examination of the research so far carried out allows us to make predictions for a number of new A2B′B″O6 compounds yet to be synthesized and reveal exciting but not yet fully explored puzzles related to this family of functional oxide materials.
Sr3−xEuxMoO6 and Sr2.7Eu0.3Mo1−yWyO6 red phosphors were successfully synthesized by solid-state reaction method at 1260 °C for 6 h. The photoluminescence (PL) properties of Eu3+ in this phosphor material were investigated by analyzing the excitation and emission spectra and the effect of Eu3+ concentration. X-ray diffraction (XRD) patterns indicate the formation of Sr3MoO6:Eu3+ pure phase. The excitation and emission spectra demonstrate that 30 mol% Eu3+:Sr3MoO6 shows a broad charge-transfer (CT) band at around 365 nm and an intense red emission (616 nm, 5D0→7F2 transition), with Commission Internationale de I׳Eclairage (CIE) chromaticity coordinates (0.619, 0.376). The substitution of Mo6+ with W6+ results in a considerable enhancement of luminescence intensity and improved red color purity. All the results imply that the studied phosphors may be a promising red component for the application in the white light emitting diodes.
In the present work, the microwave dielectric ceramic (Ag0.5Bi0.5)(Mo0.5W0.5)O4 was prepared by using the solid-state reaction method. (Ag0.5Bi0.5)(Mo0.5W0.5)O4 was found to crystallize in the scheelite structure, in which Ag(+) and Bi(3+) occupy the A site randomly with 8-coordination while Mo(6+) and W(6+) occupy the B site with 4-coordination, at a sintering temperature above 500 °C, with lattice parameters a = b = 5.29469(2) Å and c = 11.62114(0) Å, space group I41/a (No. 88), and acceptable Rp = 9.38, Rwp = 11.2, and Rexp = 5.86. High-performance microwave dielectric properties, with permittivity ∼26.3, Qf value ∼10000 GHz, and temperature coefficient ∼+20 ppm/°C, were obtained in the sample sintered at 580 °C. Its chemical compatibility with aluminum at its sintering temperature was revealed and confirmed by both X-ray and energy dispersive spectrometer analysis. This ceramic could be a good candidate for ultralow-temperature cofired ceramics.
A new microwave dielectric (AgBi)0.5WO4 ceramic with an ultralow firing temperature was prepared by the solid-state reaction method. (AgBi)0.5WO4 can be obtained as a dense single-phase bulk material with a calcination temperature of 500 °C and sintering temperatures of ca. 580 °C. The material has a microwave relative permittivity of ca. 35.9, a Qf value of ca. 13000 GHz, and a negative temperature coefficient of –69 ppm/°C at 7.5 GHz. The crystal structure of (AgBi)0.5WO4 was determined by a combination of X-ray and transmission electron microscopy (TEM) diffraction analysis. The compound crystallizes in the monoclinic C12/m1 (no. 12) space group with the lattice parameters a = 10.1330(8) Å, b = 11.0013(0) Å, c = 7.2756(4) Å, and β = 127.712(3)°. From an X-ray diffraction analysis, the (Ag0.5Bi0.5)WO4 ceramic reacts with silver after heat treatment at 560 °C to form new compounds, namely, Bi2WO6 and Ag2WO4. However, the (AgBi)0.5WO4 ceramic is chemically compatible with aluminum powder at 600 °C as an alternative cofired electrode material. All the results suggest that (AgBi)0.5WO4 ceramic is a promising new candidate material for ultralow-temperature cofired ceramic (ULTCC) technology.
A sintering-aid system using melting of B-Li glass for barium strontium titanate (BST)-based compositions to be used in low-temperature cofired ceramic (LTCC) layers is introduced. The effects of the sintering aid on the microstructure, dielectric properties, and application in LTCC were investigated. The composition Ba0.5Sr0.5TiO3-SrMoO4 with 3 wt.% B-Li glass sintered at 950°C exhibits optimized dielectric properties, including low dielectric constant (368), low dielectric loss (0.007), and moderate tunability (13%, 60 kV/cm) at 10 kHz. At 1.44 GHz, it possesses a dielectric constant of 218 and Q value of 230. LTCC multilayer ceramic capacitors fabricated by the tape-casting process have steady relative tunability of 12% at 300 V, suggesting that BST50-SrMoO4-B-Li glass composite ceramic is a promising candidate for electrically tunable LTCC microwave device applications.
A compositional dependence on the ferroelastic phase transition in scheelite solid-solution [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 ceramics was characterized by the microwave dielectric data over the temperature range 10–420 K. As x values increased from 0.0 to 0.125, a dielectric temperature-dependent anomaly consistent with a phase transition decreased linearly from 528 K for the pure BiVO4 end member to 264 K for the solid-solution composition at x = 0.125. With further increasing x, the transition temperature decreased sharply to approximately 45 K and became stable for x ⩾ 0.52. The phase transition point for pure tetragonal (Li0.5Bi0.5)MoO4 ceramics is approximately 43 K. As the x value increases, the Raman band of the [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 ceramics at room temperature broadens and overlaps. Ferroelastic domain structures were observed as a function of composition with diffraction contrast imaging and high-resolution imaging with transmission electron microscopy. Structure–property relations are inferred from the microscopy observations and the Raman spectra.
The influence of long term milling in an attritor of a mixture of BaCO3 and TiO2 powders on the reaction synthesis of BaTiO3 was studied. Thermal analysis (TG and DTA) of the unmilled and milled powders and X-ray diffraction of powders calcined at different temperatures were undertaken. Milling does not change the reaction sequence between BaCO3 and TiO2. BaTiO3 and Ba2TiO4 are produced in both kinds of powders. However, milling reduces the formation temperature and accelerates the formation rate of these two phases. The production of final BaTiO3 is incomplete in the milled powder, probably because of the formation of big crystals of Ba2TiO4. As a consequence, the milled powder contains large amounts of Ba2TiO4 after calcination at 1200°C. Milling produces allotropic transformations in TiO2 from anatase phase, to α-PbO2-like phase and finally to rutile structure.
Compound formation and compatibility relations have been studied for a portion of the system SrMoO at 1200°C. The ternary compounds were prepared: SrMoO4 with the tetragonal scheelite structure, SrMoO3 with the cubic perovskite structure and Sr3MoO6. This last compound, which had not been previously reported, gave X-ray powder data which could be indexed on a cubic unit cell with . It appeared to have a structure related to that of (NH4)3FeF6. X-ray powder data have been determined for SrMoO3 and Sr3MoO6. A previously reported fourth compound, Sr2MoO4, could not be prepared at 1000°C, 1200°C or 1400°C. The stable phase assemblage at its bulk composition was (Sr3MoO6 + Mo).
Temperature dependent impedance spectroscopic analysis of fine-grained magnetoelectric Pb(Zr0.53Ti0.47)O3–(Ni0.5Zn0.5)Fe2O4 (PZT–NZFO) composites was investigated. Debye-like impedance relaxation peaks were observed at intermediate frequency range. Maxwell–Wagner (MW) relaxation model was used to explain the space charge effect due to heterogeneous PZT and NZFO grain boundary in finer structure. The total resistivity was dominated by the grain boundary resistance due to the blocking effect arisen from the glass phase additive. The small value of conductivity measured in this system suggested the glass additive markedly modified the grain boundary properties. Electric modulus spectra reflected the contributions from two different effects: the large resolved semicircle arc was caused by the grain effect and the small poorly resolved semicircle arc was attributed to the grain boundary. The activation energy calculated from the impedance spectra was consistent with value estimated from the modulus spectra. Investigation on dielectric spectra revealed a polydispersive dielectric relaxation existing in the system, which was also demonstrated in the ac conductivity spectra. Small polaron relaxation and MW-type polarization mechanism were discussed through the analysis on the ac conductivity spectra.
The microwave dielectric properties of A5B4O15 (A=Ba, Sr, Mg, Ca, Zn; B=Nb, Ta) ceramics are investigated. The ceramics are prepared through the solid-state ceramic route. The dielectric properties are studied at microwave frequencies and structure and microstructure by XRD and scanning electron micrograph (SEM) methods. The ceramics show εr in the range 11–51, Q×f is in the range 2400–88,000 GHz and τf in the range −73–232 ppm/°C.
A series of (ABi)1/2MoO4 (A=Li, Na, K, Rb, Ag) compositions were studied in regard to the sintering behavior, phase composition, microwave dielectric properties and chemical compatibility with silver and/or aluminum for electrodes. All the (ABi)1/2MoO4 (A=Li, Na, K, Rb, Ag) ceramics could be sintered below 700°C with relative densities above 93%. Whereas the (KBi)1/2MoO4 ceramic can be sintered to a high density at around 630°C/2hrs with a relative permittivity ∼37, a Qf value of 4000GHz and a temperature coefficient of resonant frequency (TCF)∼+117ppm/°C. Furthermore, from the XRD analysis of co-fired ceramics, the (KBi)1/2MoO4 ceramic reacts with silver but not with aluminum at its densification temperature. The (ABi)1/2MoO4 (A=Li, Na, K, Rb, Ag) type ceramics can all be considered into the new field of ultra-low temperature co-firing dielectrics for multilayer applications.
Potentiodynamic electrochemical impedance spectroscopy (PDEIS) uses virtual instruments to acquire, by means of a common potentiostat, multidimensional dependencies that characterise variations of dc current and frequency response in the same potential scan. Unlike classical EIS, which finds the whole equivalent circuit in stationary states, PDEIS finds, in potentiodynamic systems, only those elements of equivalent circuits that are needed to decompose the ac response in a limited range of frequencies. The decomposition of ac response into components belonging to different elements is provided by a built-in spectrum analyser, which gives dependences of equivalent circuit parameters on variable potential. The new technique develops the idea, originally suggested by D.E. Smith, of versatile characterisation of the electrochemical response in a simple computerised experiment. PDEIS solves this problem with the use of multi-frequency potentiodynamic probing based on analysis of streams of wavelets. The use of the additional variable (electrode potential) helps to disambiguate the equivalent circuit analysis. The PDEIS performance is illustrated on systems of different kind: a reversible system (ferricyanide redox transformations on glassy carbon and platinum electrodes), a system that is locally reversible but shows different responses in forward and backward scans (Bi upd on Au) and strongly irreversible variable system (initial stages of aniline electropolymerisation on gold).
Colossal magnetoresistance - a huge decrease in resistance in response to a magnetic field has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half- metallic nature of these materials; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions and at crystal-twin or ceramic grain boundaries, although the operating temperature of such structures is still too low (≤150K) for most applications. Here we report a material-Sr2FeMoO6, an ordered double perovskite exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.
The anti-Stokes emissions or upconversion processes, for which emissions was found to exceed excitation energies by 10-100 kT, were presented. The dissolution of nanoparticles (6-8 nm) of Yb-Er- and Yb-Tm doped LuPO 4 as colloids in chloroform solutions was demonstrated. The Nd 3+-Yb3+ codoped YA nanocrystalline cemramics were also studied. It was observed that in thermoluminescence, where traps were emptied by excitation energies of the order of kT, constituted a field of anti-Stokes emission of its own.