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Dielectric investigation of the Sr3WO6 double perovskite at RF/microwave frequencies
RSC Advances
Abstract
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.
... It is noted that the change of W for Mo provided different structures and opposite TCF values. In our previous work [33], we investigated Sr 3 WO 6 (SWO) double perovskite as a potential dielectric resonator antenna (DRA). The similarly between molybdenum (Mo 6þ ) and tungsten (W 6þ ) ionic ratio and oxidation state allows the formulation of new LTCCs based structures mixing Mo 6þ and W 6þ ions [25]. ...
... Both ε 0 and tand values decrease as a function of frequency, due to dielectric relaxation and the electrode effect, but they also tend to increase when the temperature of the system rises. ε 0 values for SMO are smaller than those previously found for SWO [33]. This occurs due to replacement of W 6þ with Mo 6þ , which promotes a minor polarization, likely due to the change of symmetry of the double perovskite unit cell: from monoclinic (SWO) to cubic (SMO) symmetry. ...
... From these plots, the E a for SMO was calculated in three different ways: M'' (1.10 eV), Z'' (1.27 eV) and s 0 ac (1.18 eV). These values are lower than for SWO electroceramics (~1.37 eV) [33], i.e., the conduction process occurs easier in SMO than in SWO. This temperature dependency is observed in other electric parameters, such as the imaginary parts of the dielectric permittivity (ε'') and the capacitance (C 00 ) (Figs. S6 and S7, respectively, SI); both rise with temperature as the conduction process is thermally activated. ...
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.
... Finally, it is worth mentioning that all samples operate with radiation efficiency above 75%. These values are near or greater than that presented by many found in the literature in which other ceramics were used in the manufacture of DRAs [34][35][36][37]. ...
This work presents the study of the dielectric properties in the microwave region (MW) of BaMoO4 ceramic and composites with CaTiO3 (CTO). The BaMoO4 matrix was synthesized by solid state reaction and characterized by X-ray diffraction. Ceramic composites were obtained by adding 20–80 mol% of CTO in the BaMoO4 matrix. The Silva-Fernandes-Sombra method was used to measure the resonant frequency temperature coefficient (τf), and the BaMoO4 composite with 20% CTO showed an adequate value of relative dielectric permittivity (εr = 8.90), low dielectric loss tangent (tan δ = 9.89x10^- 4) and τf close to zero (τf = 3.48 ppm/ºC). BMO and its composites were applied as dielectric resonator antennas, and their far-field parameters were obtained through numerical simulation.
... Fig. 3a b shows that from 1 Hz to 10 kHz the ε ′ r drops sharply, whereas from 10 kHz the dipoles can no longer follow the electric field taking the permittivity remains constant up to 1 MHz [26,27]. It is also verified that at low frequency, the magnitude of permittivity decreases, Fig. 3b, in relation to YNO, Fig. 3a, due to the increase in the CTO, this directly influencing the transition of long distance carriers, which occurs in the contour of grain, for carriers moving in the grain falling into power barriers [28]. ...
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.
... To know the highest probable information concerning the electrical characteristics of the compound, CIS method is adapted which contains four basic formalisms: impedance, admittance, permittivity, and modulus formalism. Every individual yields a broad extent of graphical investigation relating to these electrical characteristics [22,23]. To differentiate grain, grain boundary, and electrode response of the material, the impedance measurement has been done in the frequency range of 1 kHz-1 MHz and the temperature frequency of 25-300 ℃. ...
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.
... For this propose, Nyquist plots were fitted using Eis analyzer software [27]. The obtained fits were shown by the equivalent circuit to identify the electrical contributions into the ceramics. ...
Yttrium iron garnet and calcium titanate (Y3Fe5O12-CaTiO3, YIG-CTO) ceramic composite were obtained according to the well-established solid-state route. This system was studied by complex impedance spectroscopy as a function of temperature. All samples showed a thermo-activated relaxation with energy activation between 1.10 and 1.70 eV. The CaTi0.6Fe0.4O2.8 phase is formed during sintering process, which influences in the electrical behavior of the samples. Nyquist diagrams were adjusted by two equivalent circuits in series to represent electric response. The YIG-CTO system was also used to design a dielectric resonator antenna (DRA) for microwave applications. The results indicate that DRA YIG-CTO-based presented an operation frequency range around 1–7 GHz, and reflection coefficient below −38 dB. From obtained results, the YIG-CTO composite is a great candidate for usage in electronic devices, operating in the C-band, such as WiFi and ISM band applications.
... 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 resulting values are reported in Table 1. A TCε ′ value of −601 ppm°C −1 was obtained at 100 kHz, which is quite good compared with the TCε ′ values recorded for some of most common oxides at the same frequency, such as Sr 3 WO 6 (−1539) [23], STO (−2400), TiO 2 (−750), Ta 2 O 5 (+600), BaTi 4 O 9 (+110) [24]. However, for a component of the Ln 2 CuTiO 6 family a value of −64 was recorded, specifically for Ho 2 CuTiO 6 , which highlights the potentiality of such family com Fig. 2. Temperature dependences of (a) dielectric permittivity (ε ′ ) and (b) conductivity (σ) measured from room temperature to 150°C at several fixed frequencies of the AC-driving voltage. ...
The dielectric properties of YCTO bulk capacitors were investigated as a function of temperature from 25 °C to 150 °C and at microwave frequencies in comparison to a SiO2 bulk sample. The results confirm the high-k character of the YCTO ceramic, in addition to the low AC conductivity, namely ε′ = 40.1 and σ = 6 × 10⁻⁸ S cm⁻¹ at 1 MHz, and show a weak frequency and temperature (25 °C–150 °C) dependence. A temperature coefficient value of −601 ppm°C⁻¹ for the dielectric constant (TCε′) was estimated at 100 kHz. In the GHz regime, a comparison with bulk SiO2 confirms the higher YCTO dielectric permittivity. These results demonstrate high-k YCTO ceramic as a very promising material with high potentiality for electronic applications.
... Many of researcher focused their studies in a specific property of materials such as, solar cell efficiency [4], magnetic properties [5], optical properties [6], dielectric [7]. Much double perovskite materials synthesized for microwave application such as, Sr 3 WO 6 [8] , La 2 CoMnO 6 [9]. Mostly the perovskite materials were prepared by solid state reaction rote with high temperature (≤1200 o C for 24h) such as Ca 2 FeOsO 6 [10], Sr 2 TiMoO 6 [ 11]. ...
A cubic (Fm-3m) crystal structure Ba 2 Zn 1-x Ni x WO 6 double perovskite series were synthesized by solid state reaction method at high temperature (≤1200 o C for 24h). The impedance spectroscopy results show that the semicircle shape decreased with substitution increasing of the Ni + 2 ions with Zn + 2 ions that refer to the decreasing in the grain resistance with increasing substitution that means the semiconducting behavior increasing for the Ba 2 Zn 1-x Ni x WO 6 series, also the resistivity and permittivity of the sample that decreased with frequency increasing and vice versa that the electrical conductivity found to increase with frequency increasing. The efficiency of the solar panel was measured via the current-voltage (I-V) characteristic curve, and the results were showed. The efficiency of the Five samples was found to be η = 0.594, 0.435, 0.43, 0.478 and 0.435% for Ba 2 ZnWO 6 , Ba 2 Zn 0.75 Ni 0.25 WO 6 , Ba 2 Zn 0.50 Ni 0.50 WO 6 , Ba 2 Zn 0.25 Ni 0.75 WO 6 and Ba 2 NiWO 6 respectively.
... 15,16 The energy necessary to activate this process was calculated by using the conductivity and the relaxation frequency of Z spectrum in the Arrhenius relation, by which the values of the natural logarithm were plotted versus the absolute temperature inverse. [17][18][19] The plots of these linear Arrhenius relations are shown in Fig. 2c, which supports the thermally activated conduction process in the material. Fig. 2c shows the values of the activation energy for the conductivity of 0.854 eV (for σ measurements) and 0.850 eV (for Z measurements). ...
In this work, we present a dielectric study on the Sr3V2O8 (SVO) ceramic matrix in the radiofrequency spectrum by using Impedance Spectroscopy (IS). We calcined pure phase SVO through the conventional solid-state reaction at 950◦C and sintered the ceramic pellet at 1050◦C. In the analyzed by X-ray Diffraction, the SVO presented single phase. At the Scanning Electron Microscopic (SEM) study, the micrographs presented a dense SVO with discrete large grains. The IS study was performed varying the temperature and showed that SVO presents giant relative dielectric constant (εr) not only at room temperature but also in high temperatures.
... Many researchers are interested in double perovskite oxides that consist of transition metals [1]. These materials represent a large part of material science research because of the diversity in their physical and chemical characteristics, and their diverse applications [1][2][3][4][5][6], such as light harvesting (LaNiMnO 6 ) [2], ferroelectrics (Pb 2 Mn 0.6 Co 0.4 WO 6 ) [5], Multiferroic (Bi 2 NiMnO 6 , Bi 2 FeCrO 6 ) [6], superconductivity (Sr 2 YRu 0.95 Cu 0.05 O 6 ) [7], magneto resistance (Sr 2 FeMoO 6 ) [8], dielectric resonators (Ca 2 AlTaO 6 , SrAlTaO 6 ) [9], and photo-catalysis (Cs 2 BiAgCl 6 ) [10]. ...
The effect of Sr2+ substitution on the morphology, crystal structure, and optical properties of double perovskite oxide Ba2-xSr
x
ZnWO₆ (x = 0.00, 0.25, 0.50, 0.75, 1.00) were investigated. Scanning electronic microscopy demonstrated that all samples have similar microstructure morphology but differ in the range of grain sizes. X-ray diffraction measurements indicated that these materials crystallize in a (Fm-3m) cubic crystal structure, and also confirmed the tolerance factor. Rietveld analysis revealed that the lattice parameter decreased from 8.11834 to 8.039361 Å when the substitution of Ba2+ with Sr2+ cations increased from zero to 100%. Fourier transform infrared (FTIR) and Raman spectroscopies displayed a symmetric stretching vibration of WO₆ octahedra at 825 cm-1, and an anti-symmetric stretching mode of WO₆ was observed by FTIR at 620 cm-1. A strong peak at 420 cm-1 was also observed in the Raman spectra and is due to the W-O-W bending vibration modes. UV-Vis diffuse reflectance spectroscopy was carried out for the series, and the band gap energy decreased from 3.27 eV for Ba₂ZnWO₆ to 3.02 and 3.06 eV for Ba1.75Sr0.25ZnWO₆ and Ba1.5Sr0.5ZnWO₆, respectively. The excitation and emission photoluminescence properties were investigated at room temperature.
In this communication, a monoclinic BaCaZrMnO6 double perovskite was synthesized by a solid-state reaction technique. The Williamson-Hall method was used to calculate mean crystallite size of 53.1 nm and micro-lattice strains of 0.0256% respectively. The study of the microstructural properties of the sample was completed using a scanning electron microscope (SEM) where compositional purity was checked by using energy dispersive x-ray spectroscopy (EDAX). Raman spectrum shows all atomic modes of vibrations; which gives information about phase, polymorphic crystallinity & molecular interaction and confirms the presence of all constituent elements. The UV visible spectrum analysis suggests the bandgap energy of 1.68 eV, suitable for some optoelectronic devices. The presence of the Maxwell-Wanger type of dispersion was confirmed from the analysis of the dielectric study. The analysis of the impedance spectroscopy suggests that the sample has a negative temperature coefficient of resistance (NTCR) while the study of the modulus confirms a non-Debye type of relaxation process. The results of the ac conductivity study confirm a thermally activated relaxation process. The semiconductor character of the sample was confirmed from the depressed semi-circular arcs in both Nyquist and Cole-Cole plots; which may find some applications related to sensors and energy storage devices.
In this article, the influence of co-substitution of alkali and transition metals at the Ba/Mo site of host BaMoO4 (inorganic Scheelite) is reported. A new lead-free double perovskite BaLiFeMoO6 (BLFMO) is synthesized using a conventional solid-state reaction route, having tetragonal crystal symmetry with an average crystallite size of 93.2 nm and strain of 0.115%. Grains are grown by sintering and distributed uniformly through well-defined grain boundaries as obtained from scanning electron microscopy (SEM). The purity and compositional analysis through energy-dispersive analysis X-ray spectroscopy (EDX) technique confirms the presence of constituent elements Ba, Li, Fe, Mo, and O. Raman study supports the substitution of Li/Fe atoms at Ba and Mo sites in the host matrix. Direct bandgap energy of 2.99 eV is calibrated using the ultraviolet–visible (UV–Vis) spectrum; may be suitable for photovoltaic applications. Maxwell–Wagner dielectric response, non-Debye relaxation, activation energy, and negative temperature coefficient resistance (NTCR) behavior are the outcomes of the synthesized sample characterized using dielectric, impedance, modulus, and conductivity plots in a wider spectrum of frequency (1 kHz–1 MHz) and temperatures (25–500 °C). The contribution of both grains and grain boundaries in the conduction mechanism has been investigated using Nyquist's fitting data calibrated from ZSIMPWIN software. A decrease in the magnitude of grain boundary resistance with a rise in temperature in fitted Nyquist's plot suggests the presence of semiconducting nature in the material. Analysis of leakage current density suggests that the nature of the conduction mechanism follows to the space charge limited current process. The study of temperature-dependent resistance supports BLFMO as a good candidate for NTC thermistor-related device applications. The study of polarization–electric field hysteresis loop suggests the possibility of the ferroelectric nature in the studied sample.
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.
A series of novel Sr3WO6:xDy³⁺ (0.01 ≤ x ≤ 0.35) yellow emitting phosphors are synthesized using the conventional high-temperature solid-state method. The phase analysis and optical properties are discussed in detail. Through the X-ray diffraction (XRD) analysis, the obtained phosphors are identified to be pure phases. The Sr3WO6:Dy³⁺ phosphors emit bright yellow emission light at 578 nm under 352 nm excitation, which is ascribed to the ⁴F9/2-⁶H13/2 transitions. The optimum doping concentration is 0.02 mol. The activation energy Ea is calculated to be 0.154 eV. The Commission International de L'Eclairage (CIE) coordinates and correlated color temperature (CCT) of the Sr3WO6:xDy³⁺ (0.01 ≤ x ≤ 0.35) is located in the yellow region. All results indicate that the Sr3WO6:Dy³⁺ phosphors have the potential to be applied in the white light-emitting diode (w-LED) field.
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.
This work presents the dielectric properties of Ba2TiSi2O8 in the Radiofrequency (RF) and Microwave (MW) regions. X-ray diffraction analysis showed that the material was obtained as a single-phase without the presence of spurious phases. Complex impedance spectroscopy demonstrated that there was no significant change of permittivity with temperature, whereas the dielectric loss was less than 1. Nyquist diagrams were modelled through an equivalent circuit using two associations of R-CPE related to the grain and the grain boundary effects. The MW analysis showed ε′r = 11.01 and tan δ = 4.55 × 10–2, values that are close to the results obtained in the RF region. Moreover, the τf value for Ba2TiSi2O8 was equal to − 47 ppm/°C which is close to the values adequate for a microwave device application. The numerical simulation demonstrated the operation of the material as a Dielectric Resonator Antenna (DRA), where a reflection coefficient below − 10 dB, a realised gain of 6.739 dBi, a bandwidth of 452.96 MHz and a radiation efficiency around 100% were observed. The results indicate that Ba2TiSi2O8 would be an interesting candidate in microwave operating devices in the C-band, as well as in devices operating in RF.
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.
Bismuth vanadate (BiVO4: BVO) ceramics is synthesized by solid state reaction (SSR) technique sintered at 750 °C for 4 h. BVO exhibits monoclinic crystal structure of space group I2/b as obtained from the XRD analysis. Field emission scanning electron microscopy (FE-SEM) confirms the microstructural property, which reveals that, the grains have clear grain boundaries with least porosity and they are highly dense. For the investigation of dielectric and electrical properties of BVO at a temperature (230–390 °C) and frequency (300 Hz–1 MHz) range are chosen. The results show that, dielectric constant (ε′) is dependent on temperature variation. At 300 Hz, BVO exhibits very high dielectric constant ∼5651 at 390 °C. Impedance and modulus analysis confirm the temperature dependent relaxation and non-Debye behavior of the material, respectively. Activation energy associated with imaginary part of impedance is 0.49 eV.
This paper presents the application of the Li2MgTi3O8 (LMT) ceramic matrix added with small quantities (0 wt.%, 1 wt.%, 3 wt.%, and 5 wt.%) of Bi2O3 for building and studying dielectric resonator antennas (DRA) to operate in microwave frequency bands. A low-cost procedure of manufacturing was applied to produce a material with good dielectric characteristics such as near zero temperature coefficient of resonant frequency (τf) in − 8.37 ppm.°C⁻¹ for LMT added with 3 wt.% Bi2O3, relative permittivity (εr) varying approximately between 19 and 24 and loss tangent (tan δ) varying between 5.41 × 10⁻⁴ and 1.42 × 10⁻³. Antenna prototypes and computational models were built for each compound, showing excellent agreement between measurements and simulations. All prototypes and simulated models have bandwidths (BW) wider than 100 MHz with resonant frequencies near 3.5 GHz. The gains and radiation efficiencies of the simulated antennas remained above 6.3 dBi and 98%, respectively.
In this work, we have analysed the effects of adding CaTiO3 (CTO) and changing the temperature on the dielectric and electric properties of ceramic matrix BiVO4 (BVO) in the radiofrequency range. BVO was synthesized by a calcination process at 500 °C and ceramic composites were prepared by the addition of CTO (8, 16, 24, 28 and 32 wt%). These composites were moulded in ceramic pellets and sintered at 800 °C. The crystal structures of BVO and composites were analysed by X-ray diffraction and no spurious phase was detected in the synthesized BVO. Complex impedance spectroscopy observed the presence of a thermo-activated charge transfer process with activation energy increasing with CTO concentration in the samples. The dielectric constant (ε) measured in radio frequency for ceramic composites presented high values that ranged from 26 to 9 k for BVO to BV32, respectively, at room temperature and a frequency of 1 Hz. The electrical response obtained by composites were fitted through an equivalent circuit composed of three associations in parallel with the resistance of a constant phase element, which showed minor deviations between the fitted and experimental data. The temperature coefficient of capacitance displayed negative and positive values in CTO-based composites and pure BVO, respectively; these characteristics are favourable for the application of composites in the radio frequency band.
In this work, the dielectric and electric properties of the ceramic matrix BiVO4 (BVO) and the effects of the addition of TiO2 were analysed by impedance spectroscopy (IS). The BVO phase was calcined at 773 K and used to prepare the composite ceramic which the titanium oxide was added (15, 30 and 60 wt% TiO2), molded in pellet shape and sintered at 1073 K. These samples were characterized by X-ray diffraction (XRD). The thermo-activated charge transfer process for the ceramics BVO with the respective additions was observed and the electric results were compared with the electric response of equivalent circuit composed of three associations in parallel with R-CPE and represented by the Nyquist diagram. At room temperature and in the frequency range of 1 Hz, the samples presented high relative permittivity, εr = 26k to approximately 35k, and a dielectric loss at the order of 10⁻² at 1 MHz. The composites presented negative and positive values of the temperature coefficient of capacitance (TCC) along TiO2 composition. Through IS coupled with temperature variation, the activation energies were measured; the values showed decrease with increasing withTiO2 concentration. © 2018 Springer Science+Business Media, LLC, part of Springer Nature
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).
Charge compensated Sr3WO6:K+,Eu3+ was reported as an efficient red phosphor for white light emitting diodes (LEDs). It exhibited a strong red emission centered at 618 nm. The main excitations are from f–f transitions of Eu3+ ions (360–550 nm), which matched the near ultraviolet and blue LEDs. The effects of charge compensation (Li+, Na+, and K+) on the photoluminescence properties were investigated and K+ was the best one among Li+, Na+, and K+. The reason for the enhancement of the red emission was discussed in detail. K+ played a different role for different doping concentration. Comparing with Y2O2S:Eu3+ red phosphor, Sr3WO6:K+,Eu3+ is much more stable, efficient and suitable for phosphor conversion LEDs.
In the present work, the xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared via the solid state reaction method. All the ceramics can be densified at low sintering temperatures around 820 °C. At room temperature, the BiVO4 type scheelite monoclinic solid solution was formed in ceramic samples with a composition of x ≤ 0.10. When x lies between 0.1 and 0.7, a BiVO4 scheelite tetragonal phase is formed at room temperature. In the range 0.7 ≤ x < 0.9, the ceramic samples were found to be composites consisting of BiVO4 type tetragonal and Bi(Fe1/3Mo2/3)O4 type monoclinic scheelite phases, and when x ≥ 0.9, the Bi(Fe1/3Mo2/3)O4 type monoclinic scheelite solid solution was formed. In the BiVO4 type monoclinic solid solution region, the phase transition to tetragonal phase was studied by in situ Raman and Far-Infrared spectroscopies and by thermal expansion analysis. All of these methods indicated that the phase transition temperature almost linearly decreased from 255 °C for pure BiVO4 to about −9 °C for x = 0.1 sample. High performance microwave dielectric properties with a high permittivity of about 74.8, high Qf values above 11500 GHz, and a small temperature coefficient of resonant frequency within +20 ppm per °C in a wide temperature range of 20–140 °C can be obtained in the composite ceramic sample with 60 mol% x = 0.10 composition and 40 mol% x = 0.02 composition. The xBi(Fe1/3Mo2/3)O4–(1−x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics might provide useful candidate materials for microwave integrated capacitive devices, such as filters, antennas, etc.
Double-perovskite La2NiMnO6 (LNMO) nanoparticles were synthesized by co-precipitation process, and the adsorption of bovine serum albumin (BSA) protein on these nanoparticles was carried out. The powder samples were annealed at 750, 850, 950, and 1,050[degree sign]C, respectively. X-ray diffraction (XRD) results reveal that there are double perovskites and exhibit mixed orientations, without any impurity phases. Transmission electron microscopy results as well as the XRD estimate results show that the crystalline size is about 34 to 40 nm. The adsorption of BSA on the magnetic nanoparticles was analyzed using a UV spectrophotometer at room temperature. The results show that the as-prepared LNMO nanoparticles display a good adsorbing ability for BSA, and the nanoparticle sintered at 850[degree sign]C has the highest value of 219.6 mg/g, which is much higher than others.
In this paper an alternative method for the measurement of the temperature coefficient of resonant frequency (τf), is presented. The traditional method (based on the Courtney method) present some limitations of measuring the values of τf, for samples with high dielectric loss due to their inability to observe clearly the TE011 mode. The alternative experimental setup, to measure the τf value, is based on the variation of the temperature of the dominant mode of a dielectric resonator antenna. The method is quite compatible with the measurement of τf, based on the Courtney method. It presents the advantage that it is less sensitive to the sample loss. In the studied samples, with loss higher than 10−2, the τf were obtained. Samples of known τf were measured in both methods, using the configuration proposed by Courtney and the present study. The alumina (Al2O3) and calcium titanate (CaTiO3) were selected because they have well known values of τf and have low dielectric losses, the bismuth niobate and titanium (Bi3NbTiO9) was chosen because it is not possible to measure its τf by the traditional method due to its high dielectric loss. The obtained results, by measuring, the τf value of CaTiO3 and Al2O3, in this proposed method, present excellent agreement when compared to the traditional Courtney, transmission method. It was also very efficient for measurements of the τf value, of high dielectric loss materials (>10−2), as for the bismuth and titanium niobate (Bi3NbTiO9). The analysis of the temperature coefficient of resonant frequency (τf) in dielectric resonators is an important property for the development of electronic devices. This is because the τf is a fundamental parameter, for the production of new components like filters, oscillators and antennas, with high thermal stability.
In the present work, the preparation of a layered magnetic ceramic oxide Ba2Co2Fe12O22 (Co2Y) is described by the solid state reaction method. X-ray diffraction (XRD) technique was used to study the structural properties. The Rietveld refinement method has confirmed a hexagonal crystal structure with lattice parameters (a = b = 5.8560 Ǻ and c = 43.4977 Ǻ; α = β = 90° and γ = 120°). The dielectric and electrical modulus properties were studied over a range of frequency (1 Hz to 1 MHz) and temperature (313–493 K) using the complex impedance spectroscopy (CIS) technique. The impedance plot showed the first semicircle at high frequency which corresponds to grain effect and the second semicircle at lower frequency which corresponds to grain boundary (conduction phenomenon). A complex modulus spectrum was carried out in order to understand the mechanism of the electrical transport process, which indicates that a non-exponential type of conductivity relaxation is present in this material. The values of the activation energy of the compound (calculated both from dc conductivity and modulus spectrum) are very similar, and hence the relaxation process may be attributed to the same type of charge carriers. The study of the dielectric property, conductivity and loss of ferrite materials, as a function of temperature, is important for microwave absorption applications. The protection of sensitive circuits from the interference of external microwave radiation, is an important technological application of these materials.
A lithium ferrite with the general formula LiFe5O8 was prepared by a new variation of the ceramic method, in which high-energy mechanical milling and calcinations of Li2CO3 and Fe2O3 were used. In this paper, we present a study of the structural and electrical properties of this ceramic phase. We also investigate the effects of the use of galactomannan as a new binding option as compared with a traditional binding material like glycerol.
Export Date: 3 October 2010, Source: Scopus, CODEN: INOCA, doi: 10.1021/ic100598v, Language of Original Document: English, Correspondence Address: King, G.; Department of Chemistry, Ohio State University, 100 West 18th Avenue, Columbus, OH 43210-1185, United States; email: gking@chemistry.ohio-state.edu, References: Howard, C.J., Stokes, H.T., (1998) Acta Crystallogr., 54, pp. 782-789;
New multiferroic compounds with double-perovskite structures were synthesized. Bi2NiMnO6 was synthesized in bulk form by high-pressure synthesis and also in a thin-film form by epitaxial growth. The material showed both ferromagnetic and ferroelectric properties, i.e., the multiferroic property at low temperature. Bi2FeCrO6 was also fabricated in a (1 1 1) oriented BiFeO3/BiCrO3 artificial superlattice, with a 1/1 stacking period. The superlattice film showed ferromagnetic behavior and polarization switching at room temperature. In the compounds, Bi3+ ion, located at the A site in the perovskite structure, caused ferroelectric structural distortion, and the B-site ordering of the Ni2+ and Mn4+ ions (Fe3+ and Cr3+ ions) in a rock-salt configuration led to ferromagnetism according to the Kanamori-Goodenough rule.
A refratariedade e a resistência ao choque térmico dos materiais estão intrinsecamente ligadas às fases que os compõem, de modo que a identificação destas fases através da técnica de difração de raios X é atualmente imprescindível para a completa caracterização de matérias-primas de produtos cerâmicos. Em 1969, H. M. Rietveld desenvolveu um software de refinamento por mínimos quadrados que permite a completa caracterização cristalográfica de um material mono ou multifásico. Apesar de possuir mais de trinta anos, a técnica ainda não é utilizada de maneira regular na análise quantitativa de matérias-primas naturais. O presente estudo busca utilizar o método de Rietveld em conjunto com outras metodologias, na quantificação da fase vítrea de bauxitas refratárias, analisando as vantagens e limitações desta técnica, bem como os resultados obtidos. Foram escolhidas a partir da literatura quatro técnicas cuja proposta comum é a determinação da fase vítrea em refratários e/ou outras classes de cerâmicas utilizando-se o método de Rietveld como base teórica. Uma quinta técnica, denominada dissolução química, foi utilizada como meio de comparação. Os resultados ilustram que, apesar dos esforços, nenhuma das técnicas estudadas pode ser utilizada de forma rápida e principalmente confiável para a determinação da quantidade de fase vítrea em refratários.
The structural stabilities of double perovskite ceramics – Ba[(Mg(1−x)/2Yx/3□x/6)W1/2]O3(0.0≤x≤0.35) have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results demonstrate that small amount of Y substitution for Mg (x=0.05) increased the stability of the double perovskite. However, further increase in Y substitution would lead to the increase in BaWO4 impurity phase. Nonrandom distribution of Y3+, Mg2+ and vacancies on 4b-site within a short range scale was observed due to their large cation size and charge differences, although space group Fm-3m was adopted for all compositions. The substitution of Y3+ for Mg2+ increased the dielectric permittivity. The Q×f value was considerably improved by substitution of Y3+ (x=0.05). The temperature coefficient of resonant frequency changed from negative into positive value and near zero τf value could be obtained at x=0.3 composition. Excellent combined microwave dielectric properties with εr=21.9, Q×f= 133,000 GHz, and τf =−2.4 ppm/°C could be obtained at x=0.3 composition.
The structural, vibrational and microwave dielectric properties of double perovskite ceramics Ba2Zn1-xCaxWO6 (x = 0-0.4) were investigated. The samples were sintered at different temperatures in the range 1300-1400 degrees C for 4 h. The grown samples were characterized by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive Xray spectroscopy analysis. Microwave dielectric properties of the samples were measured using the TE01 delta resonance mode of the cylindrical pellets. The relative permittivity (epsilon(r)), calculated using Clausius-Mossotti equation is found to be comparable with the experimental results. Our analysis shows that the tolerance factor (t) as well as the temperature coefficient of resonant frequency (tau(f)) of these perovskites decreases with the increase in Ca content. The value of tau(f) is zero for the samples with x = 0.3 and 0.4.
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.
In this article is present the structural and dielectric microwave properties of Alumina – CaTiO3 ceramic composite with the addition of CaTiO3 and your performance as a dielectric resonator antenna (DRA). Alumina has a negative temperature variation of resonant frequency coefficient τf, while CaTiO3 has a high positive temperature coefficient. Also influence of calcium titanate on densification and porosity of the composite is presented. The CaTiO3 was added (2.5% to 10% wt. amount) in order to analyze changes in the dielectric properties. Samples were characterized by X-ray diffraction. Dielectric properties were investigated by Hakki-Coleman methods and the experimental characteristics parameters like return loss, bandwidth, and input impedance are presented and compared with numerical simulations. The results obtained shows the formation of secondary phases (Hibonite and TiO2), with the dielectric permittivity and dielectric loss increasing and decreasing with calcium titanate concentration, respectively. The application of DRA was verified and all samples tested presented return loss below −10 dB in the resonant frequency. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:963–969, 2015
The structural stability and microstructure of (1−x)Ba(Mg1/2W1/2)O3−xBa(RE2/3W1/3)O3 (RE = Sm, Dy, Y, Yb) ceramics have been investigated by X-ray diffraction (XRD), Raman spectrometry, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) in this work. Microwave dielectric properties were studied in the frequency range 7–13 GHz. Phase-pure solid solutions formed in space group Fm-3m within the compositional range of x = 0.01–0.05. The local ordering of the two cations (RE3+ and W6+or Mg2+) produced an internal lattice-induced strain and charge-imbalance clusters which greatly affected the structural stability and Q × f value.
Double-perovskite Sr-deficient Sr2−xMgMoO6−δ (x = 0–0.15) samples were prepared via a sol–gel route and evaluated as anode materials for solid-oxide fuel cells. The effects of Sr-site deficiency on lattice structure, electrical conductivity, thermal expansion coefficient (TEC) and electrochemical performance were systematically investigated. It is found that Sr-site deficiency within 0 < x < 0.10 increases the degree of B-site ordering but decreases the TEC value. The electrical conductivity increases with increasing x and reaches the maximum at x = 0.10. The Sr-site deficiency also increases the ratio of Mo6+, leading to enhanced oxygen vacancy concentration and reduced polarization resistance. With La0.8Sr0.2Ga0.83Mg0.17O3−δ as electrolyte and Ba0.5Sr0.5Co0.8Fe0.2O3−δ as cathode, the single cells with Sr2−xMgMoO6−δ (x = 0, 0.05, 0.10 and 0.15) anodes show the maximum power densities of 527, 551, 659 and 573 mW cm−2, respectively, in H2 at 800 °C. The x = 0.10 anode exhibits the highest power density due to its high conductivity, low overpotential and good catalytic activity. Single-cell tests indicate the potential application of the deficiency in Sr-site for Sr2−xMgMoO6−δ as SOFC anode material.
In the present work, the (Bi1-xCex)VO4 (x ≤ 0.6) ceramics were prepared via a solid-state reaction method and all the ceramic samples could be densified below 900 °C. From the X-ray diffraction analysis, it is found that a monoclinic scheelite solid solution can be formed in the range x ≤ 0.10. In the range 0.20 ≤ x ≤ 0.60, a composite region with both monoclinic scheelite and tetragonal zircon solid solutions was formed and the content of the zircon phase increased with the calcined or sintering temperature. The refined lattice parameters of (Bi0.9Ce0.1)VO4 are a = 5.1801(0) Å, b = 5.0992(1) Å, c = 11.6997(8) Å, and γ = 90.346(0)° with the space group I112/b(15). The VO4 tetrahedron contracts with the substitution of Ce for Bi at the A site, and this helps to keep the specific tetrahedron chain stable in the monoclinic structure. The microwave dielectric permittivity was found to decrease linearly from 68 to about 26.6; meanwhile, the quality factor (Qf) value increased from 8000 GHz to around 23900 GHz as the x value increased from 0 to 0.60. The best microwave dielectric properties were obtained in a (Bi0.75Ce0.25)VO4 ceramic with a permittivity of ∼47.9, a Qf value of ∼18000 GHz, and a near-zero temperature coefficient of ∼+15 ppm/°C at a resonant frequency of around 7.6 GHz at room temperature. Infrared spectral analysis supported that the dielectric contribution for this system at microwave region could be attributed to the absorptions of structural phonon oscillations. This work presents a novel method to modify the temperature coefficient of BiVO4-type materials. This system of microwave dielectric ceramic might be an interesting candidate for microwave dielectric resonator and low-temperature cofired ceramic technology applications.
The structure, sintering behavior and microwave dielectric properties of A1−3x/2Lax(Mg1/2W1/2)O3 (A = Ba, Sr, Ca; 0.0 ≤ x ≤ 0.05) ceramics have been investigated in this paper by XRD, Raman, SEM and network analyzer. XRD phase analysis shows that La-substituted samples maintain the same crystal symmetry as their corresponding un-substituted samples. BaWO4, SrWO4 and CaWO4 existed as impurity phase in BLMW, SLMW and CLMW series, respectively. Small amount of La substitution increases, whereas further increase of the substitution amount decreases the B-site ordering of tungsten-based double perovskite compounds. For a well densified sample, all compounds have dielectric constant in the range 19–20. The Q × f value varies greatly with bulk density, B-site ordering and second phase. The absolute value of τf decreases as the sequence of BLMW, SLMW and CLMW series. Excellent combination microwave dielectric properties were obtained for BLMW sample with x = 0.04 sintered at 1450 °C/2 h: ɛr ∼ 20, Q × f = 87,680 GHz and τf = −1.2 ppm/°C.
A rare earth double perovskite oxide strontium cerium tantalate, Sr2CeTaO6 (SCT) is synthesized by the solid state reaction technique for the first time. The determination of lattice parameters and the identification of phase are carried out by the Rietveld refinement method (RRM) using the Fullprof program in the space group P21/n (C52h). A structure of SCT is obtained from RRM. The bond angle and bond length are calculated and listed in Table 1 for SCT. A small amount of impurity of CeO2 is found in the refinement with space group Fm3m. The scanning electron micrograph shows the average grain size ∼2 μm. The ac electrical property is investigated in the temperature range from 303 to 703 K and in the frequency range from 0.1 kHz to 1 MHz using impedance spectroscopy. The relaxation mechanism of SCT is explained in detail by fitting experimental impedance and electric modulus data with the modified Debye (Cole–Cole) model. The frequency-dependent electrical data are analyzed in the framework of the conductivity and modulus formalisms. The σac data are fitted with Jonscher's universal power law. The dc conductivity (σdc) (calculated from σac) follows an Arrhenius law with the estimated conduction activation energy =0.78 eV. The scaling behavior of imaginary part of electrical impedance (Z″) shows that the relaxation describes the same mechanism at various temperatures.
In this study, the CaCu3Ti4O12 (CCTO) ceramic phase was synthesized by microwave heating in a much shorter time compared to the conventional heating methods. The results indicate that microwave processing is a promising method for preparing CCTO ceramics. CCTO was prepared using a domestic microwave oven operated at 2.45 GHz with 800 W. After a few minutes of microwave irradiation the formation of CCTO was confirmed by X-ray powder diffraction. The CCTO ceramic was studied in the medium-frequency (MF) range (100 Hz–1 MHz) and in the microwave range of frequencies. The experimental and theoretical characteristics of the dielectric resonator antenna are investigated.
A double-perovskite Sr2FeMoO6 (SFMO) has been synthesized with a combined citrate-EDTA complexing method. The material shows a double-perovskite structure after reduction in 5% H2/Ar at 1100 °C for 20 h. A single fuel cell using this material as anode is constructed with the configuration of SFMOǀLa0.8Sr0.2Ga0.83Mg0.17O3ǀBa0.5Sr0.5Co0.8Fe0.2O3. The cell exhibits a remarkable electrochemical activity in both H2 and dry CH4, respectively. With Oxygen as oxidant, the maximum power density is 863.7 mW cm−2 with H2 as the fuel and 604.8 mW cm−2 with dry CH4 as the fuel at 850 °C, respectively. SFMO has an almost linear thermal expansion coefficient from 30 to 900 °C and is very close to that of La0.8Sr0.2Ga0.83Mg0.17O3. A durability test of the single cell indicates that SFMO is stable in dry CH4 operation. Therefore SFMO can be recommended as a promising anode material for LaGaO3-based solid oxide fuel cells operating with both H2 and dry CH4.
The double perovskite Sr2CoMoO6−δ was investigated as a candidate anode for a solid oxide fuel cell (SOFC). Thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) showed that the cation array is retained to 800 °C in H2 atmosphere with the introduction of a limited concentration of oxide–ion vacancies. Stoichiometric Sr2CoMoO6 has an antiferromagnetic Néel temperature TN ≈ 37 K, but after reduction in H2 at 800 °C for 10 h, long-range magnetic order appears to set in above 300 K. In H2, the electronic conductivity increases sharply with temperature in the interval 400 °C < T < 500 °C due to the onset of a loss of oxygen to make Sr2CoMoO6−δ a good mixed oxide–ion/electronic conductor (MIEC). With a 300-μm-thick La0.8Sr0.12Ga0.83Mg0.17O2.815 (LSGM) as oxide–ion electrolyte and SrCo0.8Fe0.2O3−δ as the cathode, the Sr2CoMoO6−δ anode gave a maximum power density of 1017 mW cm−2 in H2 and 634 mW cm−2 in wet CH4. A degradation of power in CH4 was observed, which could be attributed to coke build up observed by energy dispersive spectroscopy (EDS).
The structure stability of double perovskite ceramics – Ba2Mg1−xCaxWO6 (0.0 ≤ x ≤ 0.15) has been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and Raman spectrometry in this paper. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results show that small amount of Ca substitution for Mg increases the Mg/CaO bond strength, and hence the stability of the double perovskite. But it cannot completely suppress the decomposition of Ba2Mg1−xCaxWO6 at high temperature. Although space group Fm−3m is adopted for all compositions, nonrandom distribution of Ca2+ and Mg2+ on 4b-site within the short range scale is observed due to their large cation size difference. Small level doping of Ca (x ≤ 0.1) increases the dielectric permittivity monotonically, but does not affect the Q × f value greatly. As expected, the substitution of Ca tuned the temperature coefficient of resonant frequency (τf value) from negative to positive value. Excellent combined microwave dielectric properties with ɛr = 20.8, Q × f = 120,729 GHz, and τf = 0 ppm/°C could be obtained for x = 0.1 composition. However the Q × f value degrades considerably when the sample was stored under ambient conditions for a long time.
Sr3−xWO6:xEu3+ (x varies from 0.01 to 0.06) phosphors have been prepared at high temperature by the solid-state method. The crystal structure of Sr2.95WO6:0.05Eu3+ phosphor has been determined as a triclinic P-1 space group with a=8.3608 (19) Å, b=8.2903 (24) Å, c=8.2145 (23) Å, α=89.79 (3)°, β=89.82 (3)°, and γ=89.753 (22)°. The excitation spectrum of Sr2.95WO6:0.05Eu3+ phosphor reveals five excitation bands: one is assigned to the charge-transfer (CTLM) band of Sr3WO6 host at 307 nm, and another is assigned to intra-4f transitions between 393 nm and 600 nm. The emission spectrum of Sr2.95WO6:0.05Eu3+ phosphor exhibits a series of emission bands, which are attributed to the 5D0→7Fj (j=0−4) transitions of Eu3+ ions. The luminescence studies revealed that the Eu3+ ions show high luminescent efficiency in emitting red light at 616 nm. The thermoluminescence glow curve shows one dominant glow peak observed at 56 °C which is related to the defects at shallow trap depth. The trap parameters mainly activation energy (Ea) and the order of the kinetics (b) were evaluated by using Rasheedy's three points method (TPM).
Two solid solutions have been obtained in the Sr3-xBaxWO6 system. One of them with 0 {slanted equal to or less-than} x {slanted equal to or less-than} 0, 75, shows non-linear behavior. Four transitions have been detected by X-ray, μ-D.T.A., and dielectric measurements. The HT phase has the structure of cubic (NH4)3FeF6, the other ones are related. Rising x decreases the transition temperatures. At room temperature the obtained phase is piezoelectric and ferroelastic, as discussed ferroelectric behavior may be expected.
Semiconducting ${\mathrm{Ni}}_{0.4}$${\mathrm{Zn}}_{0.6}$${\mathrm{Fe}}_{2}$${\mathrm{O}}_{4}$, prepared in different ways, has been investigated. It appeared that the ac resistivity and the apparent dielectric constant of the material show a dispersion which can be explained satisfactorily with the help of a simple model of the solid: there should be well-conducting grains separated by layers of lower conductivity. Dispersion formulas are given. There is good agreement between experiment and theory.
The program DBWS Tools is an application for DBWS users that compiles input control files for versions 9411 and 9807 and manages program execution using a Windows 95/98/NT interface.
In this work, we study for the first time the dielectric properties, at microwave frequencies, of the BiREWO6 (RE = Y, Gd, and Nd) dielectric resonator (DR) obtained by microwave assisted fast solid state synthesis. The DRs were excited by a wire antenna protruding above a ground plane and simulations were carried out using high-frequency structure simulator. The DRs presented permittivity values between 13 and 19 and dielectric loss of around 10−2. The agreement between experimental and simulated values for the resonant frequency and the bandwidth for all dielectric resonator antennas was considered good. The small size of the antenna and its compactness makes it especially attractive for use as an electric monopole in practical applications. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:18–23, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26486
In this work is reported an experimental and numerical investigation of magnetic resonator antennas (MRA) of magnetic the alloy BaxSr1−xFe12O19 obtained in a new procedure in the solid-state method. The experimental and theoretical characteristics of the antenna like return loss, bandwidth, input impedance, and radiation patterns are in good agreement. Numerical validation is done, taking into account the air gaps between the dielectric resonator and the metallic conductors. Frequency response bandwidth in the range of 9–10% was obtained for frequency operation around 4 GHz. The alloys BaxSr1−xFe12O19 showed a dielectric constant around 8, with loss in the range 10−2–10−3. The temperature coefficient of resonant frequency (τf) was also measured for magnetic alloys BaxSr1−xFe12O19. The values obtained were in the range of 44.90–87.83 ppm/°C. These results obtained confirm the potential use of those materials for small dielectric MRA. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 452–458, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24931
The double perovskite Sr2MgMoO6-δ has been recently reported as an efficient anode material for solid oxide fuel cells (SOFCs). In the present work, this material have been investigated by high temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC) and impedance spectroscopy to further characterise its properties as SOFC anode. DSC and XRD measurements indicate that Sr2MgMoO6-δ exhibits a reversible phase transition around 275 °C from triclinic (I over(1, -)) with an octahedral tilting distortion to cubic (Fm over(3, -) m) without octahedral distortion. This phase transition is continuous with increasing temperature without any sudden cell volume change during the phase transformation. The main effect of the phase transformation is observed in the electrical conductivity with a change in the activation energy at low temperature. La3+ and Fe-substituted Sr2MgMoO6-δ phases were also investigated, however these materials are unstable under oxidising conditions due to phase segregations above 600 °C. © 2009 Elsevier Inc. All rights reserved.
The crystal structures of the beta and gamma polymorphs of Sr(3)WO(6) and the gamma<-->beta phase transition have been investigated using electron diffraction, synchrotron X-ray powder diffraction, and neutron powder diffraction. The gamma-Sr(3)WO(6) polymorph is stable above T(c) approximately 470 K and adopts a monoclinically distorted double perovskite A(2)BB'O(6) = Sr(2)SrWO(6) structure (space group Cc, a = 10.2363(1)A, b = 17.9007(1)A, c = 11.9717(1)A, beta = 125.585(1)(o) at T = 1373 K, Z = 12, corresponding to a = a(p) + 1/2b(p) - 1/2c(p), b = 3/2b(p) + 3/2c(p), c = -b(p) + c(p), a(p),b(p), c(p), lattice vectors of the parent Fm3m double perovskite structure). Upon cooling it undergoes a continuous phase transition into the triclinically distorted beta-Sr(3)WO(6) phase (space group C1, a = 10.09497(3)A, b = 17.64748(5)A, c = 11.81400(3)A, alpha = 89.5470(2)(o), beta = 125.4529(2)(o), gamma = 90.2889(2)(o) at T = 300 K). Both crystal structures of Sr(3)WO(6) belong to a family of double perovskites with broken corner sharing connectivity of the octahedral framework. A remarkable feature of the gamma-Sr(3)WO(6) structure is a non-cooperative rotation of the WO(6) octahedra. One third of the WO(6) octahedra are rotated by approximately 45 degrees about either the b(p) or the c(p) axis of the parent double perovskite structure. As a result, the WO(6) octahedra do not share corners but instead share edges with the coordination polyhedra of the Sr cations at the B positions increasing their coordination number from 6 to 7 or 8. The crystal structure of the beta-phase is very close to the structure of the gamma-phase; decreasing symmetry upon the gamma-->beta transformation occurs because of unequal octahedral rotation angles about the b(p) and c(p) axes and increasing distortions of the WO(6) octahedra.
Theory and experimental results are presented to show the possibility of using a resonant post technique for characterizing dielectric and magnetic materials at microwave frequencies. Results of the temperature dependence of the relative dielectric constant of nonmagnetic materials with ε<sub>r</sub>, varying from 4 to 60 are presented and also loss tangent measurements at room temperature. The complex permittivity and permeability of a number of garnet materials has also been measured with 4πγM<sub>s</sub> / ω varying from 0.25 to 0.8. The measured real part of the permeability is in good agreement with the theoretical predictions of Schlomann and the imaginary part of the permeability agrees with measurements by Green et al. on similar materials.
A novel technique for the measurement of dielectric and magnetic properties of a homogeneous isotropic medium in the range of approximately 3 to 100 kmc is described. An accuracy of /l.chemc/ 1 per cent is possible in the determination of permittivity or permeability in those cases where the loss tangent is sulliciently small. The measuring structure is a resonator made up of a right circular cyndrical dielectric rod placed between two parallel conducting plates. For measurement of permittivity two or more resonant TE<sub>onl</sub> mode frequencies are determined whereas for the measurement of permeability two or more resonant TM<sub>onl</sub> mode frequencies are determined. The dielectric or magnetic properties are computed from the resonance frequencies, structure dimensions, and unloaded Q. Since the loss tangent is inversely proportional to the unloaded Q of the structure, the precision to which Q is measured determines the accuracy of the loss tangent.
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