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Effects of CaTiO3 addition on the microwave dielectric properties and antenna properties of BiVO4 ceramics
Abstract
This paper investigated the microwave dielectric properties through the Hakki-Coleman method of BiVO4 ceramic matrix, obtained by the solid-state reaction method, using 8, 16, 24 and 32 wt% of CaTiO3. The X-ray diffraction (XRD) was used for the structural characterisation of the crystalline phases present in ceramic before the sinterisation process. The thermal-stability in microwave range was measured in all dielectric samples, and the temperature coefficient of resonant frequency (τf) was found varying from −244.03 ppm °C⁻¹ to −2.7 ppm °C⁻¹. The CaTiO3 addition reduced the value of the dielectric permittivity (ε’) close to 25% of the BiVO4 values, keeping the dielectric loss (tan δ) around 10⁻³. The experimental and numerical simulated values of the dielectric resonator antenna (DRA) results show the agreement of the input impedance, return loss, bandwidth and radiation patterns. Moreover, all antennae features realised gain above 5dBi. In antenna results, the CaTiO3 addition increased the bandwidth, compared to BiVO4 DRA, that presented bandwidth around 34 MHz.
... Recently, with the tendency to decrease the sintering temperature, new materials have attracted more attention. Besides molybdates, tungstates, and phosphates, some borates are good candidates for new materials with a low sintering temperature [13][14][15][16][17][18][19][20][21][22][23][24]. ...
... The average cell volume per atom va is also an important parameter that should be maintained in an optimal range. Qin et al. [41] stated that the ranges of the decisive parameters that favor creating materials with a low dielectric permittivity are ppm < 0. 15 leads to the conclusion that these parameters are close to the ranges indicated in [41] for low permittivity candidate materials. Figure 4a,b compare the frequency dependences of the dielectric permittivities and the dissipation factors of copper borate ceramics at 20 • C in the 0.12-2.5 THz range. ...
... The average cell volume per atom va is also an important parameter that should be maintained in an optimal range. Qin et al. [41] stated that the ranges of the decisive parameters that favor creating materials with a low dielectric permittivity are ppm < 0. 15 [26]. For Cu3B2O6, the average Cu-O bond length is 2.1 Å [28]. ...
New ceramic materials based on two copper borates, CuB2O4 and Cu3B2O6, were prepared via solid state synthesis and sintering, and characterized as promising candidates for low dielectric permittivity substrates for very high frequency circuits. The sintering behavior, composition, microstructure, and dielectric properties of the ceramics were investigated using a heating microscope, X-ray diffractometry, scanning electron microscopy, energy dispersive spectroscopy, and terahertz time domain spectroscopy. The studies revealed a low dielectric permittivity of 5.1–6.7 and low dielectric loss in the frequency range 0.14–0.7 THz. The copper borate-based materials, owing to a low sintering temperature of 900–960 °C, are suitable for LTCC (low temperature cofired ceramics) applications.
... In the microwave (MW) regime, the research for better performance parameters, such as operation frequency (f 0 ), return loss (S 11 ), bandwidth (BW), directivity (D), gain (G), efficiency (g), and s f has attracted interest in the BLSF class, operating as dielectric resonator antennas (DRAs) [15][16][17][18]. Aguiar et al. [15], reported that the behavior of s f for the BFTO phase is sensitive to different concentrations of V 2 O 5 , finding -304.3 to -192.6 ppm°C -1 (from 1 to 5 wt%). ...
... In an innovative character, these materials acting as ceramic capacitors are compared to previous works [12][13][14] (with EIA standard specification) and the CDRAs performance parameters are contrasted in detail with other published works [16][17][18] by experimental and simulated analysis. ...
... where c is the speed of light in a vacuum [16][17][18]. The antenna setup used can be described as a square copper ground plane connected to the coaxial probe (50 X) and Sub-Miniature version A (SMA) connector, perpendicularly positioned. ...
Bismuth layer structured ferroelectric Bi3R2Ti3FeO15 with R = Bi (BFTO), Gd (BGFTO), and Nd (BNFTO) composition were synthesized by solid-state route, and their morphology and dielectric properties were investigated. The BGFTO and BNFTO samples, show a decrease in dielectric characteristics compared to BFTO. This can be explained by structural distortion resulting from the replacement of lanthanides in the cations of A-site in the perovskite layer. Impedance spectroscopy was performed to establish a correlation between the electrical properties and the microstructure of the ceramics. A non-Debye relaxation induced by a thermally activated mechanism can be observed in all samples. The activation energy of the materials found in the range of 0.6–0.9 eV and indicates the association of doubly ionized oxygen vacancy. The three phases proved to be news candidates for X4D capacitors, with excellent temperature stability (29–150 °C, TCC ≤ ± 3.3%). In addition, the electroceramics were used to design a cylindrical dielectric resonator antenna (CDRA) for microwave applications. The Bi3R2Ti3FeO15-based CDRA showed great potential for microwave antenna application operating in the S-band. A change is observed to τf of BFTO phase (− 428.48 ppm °C−1) to BGFTO (+ 59.17 ppm °C−1) and BNFTO (+ 57.69 ppm °C−1). This result opens up a great opportunity for future work in CDRA with near-zero temperature coefficients (τf ∼ 0).
... Several studies presenting the dielectric properties of BVO in the microwave range have already been carried out, reporting a temperature coefficient of resonant frequency (τ f ) of approximately − 260 ppm°C −1 , a high dielectric constant (ε) of~68, a quality factor (Q f ) of~6500 GHz [16][17][18], and dielectric characteristics important for its application in devices as filters, antennas, and so on [19,20]. As BVO has a negative τ f (− 244 ppm°C −1 ) [21,22], the addition of ZnO was chosen because it has a positive value τ f (70.79 ppm°C −1 ) [23] and was used in the search for thermal stability in the mixture. ...
... The Smith charts shown in Fig. 7 indicate that the scattering parameters of the experimental and simulated models show excellent agreement in the impedance matching. Plotting the impedance on the reflection polar plane obtains the impedance Smith chart, which is a transformation relationship between the upper impedance plane half-cycle and the unit circle area in the reflection plane (lower halfcycle), wherein the real parts and the imaginary parts at the two sides should be equal [21,22,40]. ...
... The impedance value at the point where the two cycles come together can result in a positive reactance or inductive reactance if the points are located at the top (+) and a capacitive reactance if the points are located at the lower half (−) [21,22,36,[41][42][43]. ...
This study reports the dielectric and structural characterisation of composites formed by the mixture of a BiVO4 ceramic matrix with ZnO (25, 50, and 75 wt.%) obtained through a solid-state method procedure for applications in the microwave range. X-ray diffraction is used for structural characterisation. The dielectric properties and temperature coefficient of resonant frequency indicate a composition with ZnO close to zero. Dielectric constants varying with ZnO composition (7 to 67) and dielectric loss of ~ 10−2 are measured, and the results confirm that the ZnO can improve the thermal stability of the composites. The composites are analysed as dielectric resonator antenna, where the experimental and theoretical characteristics of the resonator, like return loss, bandwidth, and input impedance, are in good agreement. Experimentally, for the BiVO4 reference sample, bandwidths of 12.5% and 40.82% are found for the addition of 75 wt.% ZnO for a frequency operation of ~ 5.52 GHz.
... Two traditional techniques for dielectric ceramic modification with desired features are known: one is to produce a new dielectric ceramic material, and the other is to make a composite utilizing two or more dielectric materials with characteristic compensation. The most common strategy is to combine two or more chemicals with negative and positiveτ f values to achieve nearly equal zero τ f values [6,[9][10][11][12]. For example, 0.95MgTiO3 -0.05CaTiO3 ceramic shows (ε r ) ∼21, (Q × f) ∼56,000 GHz at 7 GHz and τ f ∼ 0 ppm/°C [11]. ...
... In recent work, Bai et al showed the applicability of CaTiO3 for microwave absorption in X and Ku bands [18]. The CaTiO3-based ceramics have been mostly used as one component of the composite materials to get good microwave dielectric properties due to their moderate dielectric value and positive τ f [6,11,12,[19][20][21][22][23][24]. The selection of materials for composite is essential to know its structural and microwave dielectric properties in detail. ...
This article presents studies on characteristics properties of CaTiO3, Ca0.8Sr0.2TiO3, and Ca0.6La0.8/3TiO3 ceramics. These ceramics were synthesized using the solid-state reaction process. Structural examination revealed that the grown ceramics have an orthorhombic structure with the Pbnm space group. The random distribution of particle size was shown through morphological investigation. Apparent density of developed ceramics was determined using the Archimedes technique and found to be ˂ 90%. The microwave dielectric properties of grown ceramics are compared on the basis of ionic polarizability. It is observed that partial replacement of Ca-ions by Sr-ions provides a high permittivity value (ε r = 168.93), higher quality factor Q × f = 9,330 GHz), and enhanced positive temperature coefficient of resonant frequency (τf = 908.17). However, the substitution of Ca-ions by La-ions offers a low permittivity value (113.35), higher quality factor (16,730 GHz), and decreased temperature coefficient of resonant frequency (229.49 ppm/°C). These materials can be used with the ceramics possessing a negative temperature coefficient of resonant frequency to balance its τf-value nearly to zero.
... Its radiation efficiency is estimated to be above 99.91%, while a large gain and bandwidth of 4.24 and 2.89 GHz are observed for it, respectively, making it particularly valuable for military, medical as well as telecommunication applications [39][40][41][42][43]. Table 6 enlists and compares the performance parameters of the investigated antenna with earlier reported work [43][44][45][46][47][48]. ...
The effect of doping of Co²⁺ and Sn⁴⁺ in Ba0.5Sr0.5Fe12-2xO19 hexaferrite with different concentrations (x = 0.2, 0.4, 0.6, 0.8, and 1.0) was studied by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), complex impedance spectroscopy, and analyzed as a dielectric resonating antenna (DRA) at room temperature. The samples were produced and sintered by the solid-state reaction method. Their structure appeared from the M-phase type while their increase in grain size evidenced a dependency on Co-Sn content. The dielectric constant and loss tangent, inferred from impedance spectroscopy, also decreased with the corresponding increase in doping. The real and imaginary impedance decreased with the frequency increment. An electrical equivalent circuit using the Resistance-Constant Phase Element (R-CPE) association was thus proposed, the best-simulated components fitting with the observed structural and microstructure properties. It enabled a better understanding of the microstructure through simulated values of grain/grain boundaries and its effect on tuning the electrical properties in the low-frequency regime. The ferrite characteristics were tested for dielectric resonator antenna applications owing to their good inherent behavior than microstrip patch antenna. Measurement of the radiation efficiency, gain and bandwidth parameters of all the produced ceramics showed that the non-doped one (x = 0.0) exhibits the optimum values: 99.91%, 4.24, and 2.89 GHz, respectively, making the most valuable for antenna applications.
Graphical abstract
... The spinels MM' 2 O 4 in which M and M' are 3d metals have found applications in many technological fields like opto-electronic [1,2], dielectrics [3,4], ceramics [5,6], sensors [7] and continue to attract increasing attention in both catalysis and photo-electrochemical conversion [8][9][10]. It is established now that the morphological, optical and transport properties of spinels are dependent on the preparative conditions [11]. ...
The solid solution NixCu1-xFe2O4 (x= 0, x=0. 2,….x= 1) crystallizing in the spinel structure were prepared by the Sol-Gel method at 800 °C. The novelty of the work is also devoted to the synthesis of the spinel ferrites NixCu1-xFe2O4 (0 < x < 1) using the sol gel technique and the measurement of their structural and magnetic properties by X-ray diffraction and magnetic measurements. The structural, magnetic, optical and electrical properties were investigated. The formation of the tetragonal phase with a good crystallization and stoichiometric contents were confirmed by X-ray diffraction (x = 0 and 0.2) while a transition to cubic symmetry is observed for the other compositions. NixCu1-xFe2O4 exhibits n-type conduction with an increased band gap (Eg) as Ni is introduced in the crystal lattice while it decreases with augmenting the Ni-content in the cubic phase. The transport properties are characteristic of n-type behavior where the electrical conductivity decreases up to x = 0.4. Then, it increases and the conduction is mainly governed by the thermal emission over the inter-crystallites. An average hole mobility of ~ 10⁻⁵ cm² V⁻¹ s⁻¹ was determined for both symmetries. Magnetic measurements revealed that the saturation magnetization (Ms) increases with the Ni-content whereas the coercivity decreases. NiFe2O4 has the largest saturation magnetization (38.16 emu/g). The optical properties namely the energy gap (Eg), reflection index (n), extinction coefficient (k), dielectric complex (ɛ), optical conductivity (σopt), dissipation factor (tan δ) and relaxation time (τ) were determined.
... 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.
... Surface of Bi_Urea_PVA coating looks smoother and composed of smaller particles, which are interconnected forming a kind of network. According to literature[45,46] the main function of PVA is to build up certain viscosity of solution and provide a polymeric network, which hinders mobility of cations and helps to maintain local stoichiometry. ...
BiVO4 is known as a promising material for the design of analytical systems dedicated for non-enzymatic photoelectrochemical (PEC) glucose determination. In this work BiVO4 coatings were deposited on fluoride-doped tin oxide substrate by simple sol-gel method. The influence of different stabilizers, such as polyethylene glycol, urea, citric acid and polyvinyl alcohol (PVA) on the morphology and photoelectrochemical activity of the BiVO4 coatings was investigated. X-ray diffraction, scanning electron microscopy, UV-vis optical absorbance and photoluminescence techniques were used to evaluate structural and optical properties of BiVO4 films. PEC performance was characterized by means of voltammetry, electrochemical impedance spectroscopy and chronoamperometry. It is shown that the combination of urea and polyvinyl alcohol leads to the formation of BiVO4 coatings with lower charge transfer resistance as well as higher affinity towards adsorption of glucose, resulting in higher sensitivity of these photoelectrodes in glucose sensing. Sensitivity of BiVO4 coatings synthesized with urea (Bi_Urea) or with urea and PVA (Bi_Urea_PVA) towards glucose was 0.0033 mA cm⁻² mM⁻¹ and 0.0056 mA cm⁻² mM⁻¹, respectively. The limit of detection was estimated to be 7.94μM for Bi_Urea and 6.87μM for Bi_Urea_PVA.
... In Figure 4a, sintered at 1200°C, the existence of a few larger grains can be seen, which might be attributed to calcium titanate attempting to minimize internal energy by reducing the total space of the grain boundary, resulting in the subsequent grain growth. 29 This means that substituting Sn 4+ for Ti 4+ in the perovskite lattice can demote the grain growth as shown in Figure 3a−e. This nature of the morphology has been previously described for CaTiO 3 ceramics. ...
The lead-free Ca(Sn x Ti1-x )O3, (0 ≤ x ≤ 0.8) sample has been successfully prepared through the ball milling process, sintered at 1200 °C for 3 h. The structural, morphological, vibrational, and microwave dielectric properties of synthesized samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and impedance analysis. All the samples have an orthorhombic phase structure with a space group of Pbnm formation, and the crystalline size and strain changes with respect to Sn4+ doping were observed in the XRD analysis. From a morphological point of view, on increasing the content "x", the grain size reduces from 3.29 to 1.37 μm. The existence of vibrations and the bridging stretching mode of Ti-O-Ti and Ti-O-Sn both are associated with the broadband in the region below 800 cm-1 verified by FT-IR. The variation in electrons hopping off the host compound with respect to Sn4+ ions was analyzed in AC conductivity. The changes of dielectric properties such as complex permittivity, modulus spectroscopy, and dielectric loss at room temperature with a different frequency range of 1.00-2.00 GHz are discussed.
... Results of far-field parameters are presented in Table 5 and demonstrate that systems had interesting values of gain, directivity, efficiency, whereas no relevant changes were observed in these properties with the enhancing of CTO in the composites. 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]. ...
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.
... As seen in SEM micrographs the close packing structure increase by increasing zirconium (x) and the number of pores decreases. The presence of a few bigger grains can be observed in the Fig. 3(e) sintered at 1300 C which may be due to calcium titanate trying to reduce the internal energy by reducing the total area of grain boundary, resulting in the subsequent grain growth [21]. ...
The new ceramic composite of Ca(ZrxTi1-x)O3 had been prepared using the conventional mixed oxide solid state reaction method. The effects of Zr⁴⁺ ions substation over Ti⁴⁺ ions on the microwave dielectric properties of the CaTiO3 ceramics were investigated. The phase analysis showed that the specimens Ca(ZrxTi1-x)O3 presented single phase with orthorhombic structure in the range of x = 0.0–0.7 when sintered at 1300 °C for 3 h. The addition of Zr⁴⁺ facilitated the formation of large round-like grains with porous microstructure. The solid solution with x = 0.7 sintered at 1300 °C for 3 h showed a good combination of dielectric properties: εr=64, Q*f=3937GHz and ρrel=81.08% (at 2.5 GHz).
... Recently, the need for miniaturization and reduction of the operating frequency has encouraged the use of materials with higher permittivity (ε' ≥ 60) and losses typically around (tanδ ≈ 10 −2 − 10 −3 ) in order to preserve an acceptable DRA gain and efficiency. This concerns ferroelectric materials such as Ba(Ti 1−x Fe x )O 3−x/2 [19], Sr(ZrxTi1−x)O 3 [20] or ferroelectric composite materials such as Y 3 Fe 5 O 12 /CaTiO 3 [21] and CaTiO 3 -BiVO 4 [22]. Perovskites or TTB (tetragonal tungsten bronze) dielectrics are also of interest, such as CaTiO 3 [23] and Ba 4 Nd 9.3 Ti 18 O 54 (BNT) [24] respectively. ...
In this paper, ferroelectric ceramics with (Sr2Ta2O7)100-x(La2Ti2O7)x (STLTO) compositions have been investigated and their dielectric properties have been characterized in wide frequency band (from few kHz to few GHz); their integration in Dielectric Resonator Antennas (DRA) was conducted. The dense STLTO ceramics have been obtained by high temperature sintering of powders synthetized by solid state chemistry route. STLTO crystalline cell parameters and volume vary linearly as a function of the chemical composition (x) thus demonstrating an ideal solid solution domain for 0 ≤ x ≤ 3. Dielectric characterizations highlight that the permittivity and the dielectric loss vary according to the composition (x) and that the lowest losses are obtained for x < 1.65 compositions. The latter corresponds to the transition between the ferroelectric and paraelectric compositions of the STLTO material at room temperature. A low profile DRA structure was realized using a cylindrical paraelectric STLTO resonator (with x = 0) with a permittivity of 83 and losses tanδ = 5 ×10⁻³ @ 3.3 GHz. The DRA prototype was simulated, produced and tested. It exhibits a hybrid HEM11δ mode, with a resonant frequency at 5.80 GHz, a 4.9% bandwidth and a gain of 6.4 dB. These features confirm the potential of the paraelectric STLTO compositions in compact antennas radiating at frequencies below 6 GHz.
... The requirements for such new materials include a low dielectric constant to minimize the signal propagation delay, a low dielectric loss to ensure frequency selectivity and to restrict power consumption, and a low sintering temperature to enable the use of multilayer LTCC/ULTCC (low/ultralow temperature cofired ceramics) technology. Along with the modification of materials with a low dielectric constant, such as silica, borosilicate glasses, cordierite, mullite, forsterite, diopside, willemite, and aluminates [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], which have been well-known for decades, less popular ceramics have been explored recently, such as borates, tungstates, molybdates, vanadates, and phosphates [25][26][27][28][29][30][31][32][33][34][35][36][37][38]. The use of ceramic-ceramic or glass-ceramic composites is an effective way to tailor microstructure, electric, and thermal properties of functional materials for microwave substrates. ...
New zinc metaborate Zn4B6O13–willemite Zn2SiO4 composites were investigated as promising materials for LTCC (low temperature cofired ceramics) substrates of microelectronic circuits for submillimeter wave applications. Composites were prepared as bulk ceramics and LTCC multilayer structures with cofired conductive thick films. The phase composition, crystal structure, microstructure, sintering behavior, and dielectric properties were studied as a function of willemite content (0, 10, 13, 15, 20, 40, 50, 60, 100 wt %). The dielectric properties characterization performed by THz time domain spectroscopy proved the applicability of the composites at very high frequencies. For the 87% Zn4B6O13–13% Zn2SiO4 composite, the best characteristics were obtained, which are suitable for LTCC submillimeter wave applications. These were a low sintering temperature of 930 °C, compatibility with Ag-based conductors, a low dielectric constant (5.8 at 0.15–1.1 THz), a low dissipation factor (0.006 at 1 THz), and weak frequency and temperature dependences of dielectric constant.
... The spinels MM' 2 O 4 in which M and M' are 3d metals have found applications in many technological fields like opto-electronic [1,2], dielectrics [3,4], ceramics [5,6], sensors [7] and continue to attract increasing attention in both catalysis and photo-electrochemical conversion [8][9][10]. It is established now that the morphological, optical and transport properties of spinels are dependent on the preparative conditions [11]. ...
The solid solution NiFe2-xMnxO4 (0 ≤ x ≤ 2) is synthesized by sol gel method and the physical properties are investigated for the first time. The thermal analysis shows that the phases are formed above 400 °C. The oxides crystallize in an inverse cubic spinel whose lattice constant (a: 0.83381–0.83985 nm) increases only slightly up to x = 1.6, according to the Vegard’s law; such result is supported by the FTIR spectra. The UV-Visible spectroscopy shows both direct (1.00–1.56 eV) and indirect optical transitions (0.39–1.65 eV) due to the crystal field splitting of 3d metal. Field-dependent magnetization of the solid solution was measured at 300 K in the region (±20 kOe) and the end member NiFe2O4 exhibits a high magnetism with a saturation magnetization (20 emu/g), comparable to that reported previously. The saturation magnetization varies between 0.5 and 20 emu/g while the coactivity fluctuates between 110 and 239 Oe. The thermal variation of the electrical conductivity indicates a conduction mechanism by low polaron hopping which follows an exponential law with variable activation energies (Ea: 0.12–033 eV). The thermo-power is positive and nearly constant (S300K: 144–130 µVK⁻¹), indicating p type conduction with a mobility more a less constant (1.5–8 × 10⁻⁶ V² cm⁻¹ s⁻¹). The hole densities (NA × 10¹⁶: 0.26–2.17) are determined electrochemically from the capacitances measurements in neutral medium (Na2SO4 0.5 M). The flat band potential (Efb: −0.04 to −0.29 V) does not change significantly indicating that the valence band derives mostly from 3d orbital.
... The spinel MM' 2 O 4 in which M and M' are 3d elements have found various applications in various technological fields like the optoelectronic [1,2], dielectrics [3,4], ceramics [5,6], sensors [7] and continue to attract a growing interest in catalysis and photo-electrochemical (PEC) conversion [8][9][10]. The morphological, optical, and electrical properties of spinels depend on the synthesis conditions [11]. ...
NiFe2−xMnxO4 (0 ≤ x ≤ 2) is prepared by sol–gel route and the physical properties are studied for the first time. The thermal analysis shows that the phases are formed at 600 °C. The oxides crystallize in an inverse spinel with a cubic symmetry whose lattice constant (a: 0.83381–0.83985 nm) augments slightly up to x = 1.6, according to the Vegard’s law; such result is corroborated by the FTIR spectroscopy. The diffuse reflectance shows direct (1.00–1.56 eV) and indirect (0.39–1.65 eV) optical transitions due to the lift of degeneracy of 3d orbital. Field-dependent magnetization of NiFe2−xMnxO4 was measured at 300 K in the range (± 20 kOe) and NiFe2O4 shows a strong magnetism with a saturation magnetization of 20 emu/g. The saturation magnetization varies from 0.5 to 20 emu/g, whereas the coercivity fluctuates between 110 and 239 Oe. The variation of the electrical conductivity with temperature shows a conduction mechanism by small polaron hopping which obeys an exponential law with activation energies (Ea: 0.12–033 eV). The thermo-power is positive and almost constant (S300K: 144–130 µV K⁻¹), indicating p type comportment with a mobility more a less constant (1.5–8 × 10–6 V² cm⁻¹ s⁻¹). The hole concentrations (NA × 10¹⁶: 0.26–2.17) are computed from the capacitances measurements in Na2SO4 solution (0.5 M). The flat band potentials (Efb: − 0.04 to − 0.29 V), determined electrochemically, does not vary significantly indicating that the valence band derives mainly from 3d character.
... Castro and coworkers investigated (Ba 2 CoNbO 6 ) 1-x -(CaTiO 3 ) x composite and reached near-zero τ f at x = 0.37[58]. Oliveira investigated the effect of CaTiO 3 addition in thermal stability of the BiVO 4 matrix[59] and Paiva investigated the Y 3 Fe 5 O 12 -CaTiO 3 composite and determined that near-zero τ f composition[60]. The FMO and FMO-7.5 bulks showed τ f values of -6.55 and -4.35 ppm.°C -1 , respectively. ...
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.
Due to the importance of ceramic materials, especially of the perovskite type, this study included the synthesis of calcium titanate using dolomite as a natural calcium source from the Iraqi environment. Firstly, the work included preparing of calcium sulfate dihydrate from dolomite using dilute sulfuric acid, and then studying the optimal conditions for the formation of calcium titanate (CaTiO3) through the reaction of titanium dioxide with calcium sulfate dihydrate. The optimum conditions included: the ratio of titanium to calcium which was 1:0.5, 1:1, 1:1.5 and 1:2, respectively, as well as, the calcination temperature (1000, 1200 and 1300 °C) and finally the calcination time (1 and 2 h). The formation of calcium titanate was tracked by X-ray diffraction, and the results demonstrated that the optimum conditions are 1:1 at a calcination temperature of 1300 °C for 2 h. Moreover, the work included converting calcium titanate into calcium titanate nanoparticles using an environmentally friendly method which was achieved by using ultrasound on a liquid composed of calcium titanate in deionized water. The formation of calcium titanate nanoparticles was proved by TEM, which showed the formation of regular and irregular square-like nanoparticles with an average particle size of 55 nm. From an industrial point of view, this method is considered to be of low cost for the production of high cost nano-calcium titanate.
Dielectric ceramics are promising in large-scale commercial millimeter-wave communication technology, such as 5G and the upcoming 6G, thanks to their excellent frequency selection characteristics and environmental stability. In this work, various contents of SbO bonds were introduced into the MgTa2O6 lattice using the solid-phase reaction method to investigate the effects on the lattice and microwave dielectric properties. XRD confirms that Sb ions successfully occupy Ta sites in the lattice and cause lattice shrinkage and crystallinity deterioration, which leads to a slight decrease in the quality factor. Furthermore, DFT calculations reveal that the doping leads to electron-biased aggregation toward O atoms, causing higher SbO ionicity, but also attenuates the degree of ionization of Ta and Mg ions, which makes the dielectric constant of the doped samples vary non-monotonically with gradient doping. Satisfactorily, Sb doping substantially enhanced the thermal stability of the ceramics, with TCF values reduced from 36 × 10−6 °C-1 to 15 × 10−6 °C-1.
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.
Low sintering temperature Bi[V1-x(Fe1/3W2/3)x]O4 (BVFWx) (0.02 ≤ x ≤ 0.08) ceramics have been prepared by solid-state reaction. Compositions with 0.02 ≤ x ≤ 0.08 were scheelite-structure, but distortion of [BO4] tetrahedra decreased with increase in x. Although the crystal class remained monoclinic, the phase transition temperature (TC) decreased from 255 ℃ (BV) to 51 ℃ (BVFW0.08), suggesting that higher concentrations of (Fe1/3W2/3) would result in the stabilization of tetragonal scheelite at room temperature. The decrease in the [BO4] distortion additionally resulted in a large reduction in the magnitude of the temperature coefficient of resonant frequency (TCF) with near-zero values obtained by sintering BVFW0.08:2BVFW0.06 at 780 oC to give TCF ≈ -5.9 ppm / °C, relative permittivity εr ≈ 77.4, and microwave quality factor, Q × f ≈ 8,100 GHz (@ ~ 4.3 GHz). These promising microwave dielectric properties coupled with the low sintering temperature suggest that BVFWx ceramics are of interest in low-temperature cofired ceramic (LTCC) technology, and for monolithic resonator and filter applications.
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.
A series of Mn²⁺-doped Mg1-xMnxTa2O6 (x = 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) ceramics were synthesized by solid-state reaction method. The influence of introducing Mn–O bonds as a partial replacement for Mg–O bonds on the lattice and microwave dielectric properties was systematically investigated. XRD and Rietveld refinement confirm that Mn²⁺ occupies the 2a Wyckoff position and forms a pure trirutile phase. Moreover, based on the chemical bond theory, the dielectric constant is mainly affected by the ionicity of the Ta–O bond. The lattice and dielectric properties remain relatively stable with Mn²⁺ doping below 0.1, but excessive Mn²⁺ doping leads to pronounced distortion of the lattice, which is not beneficial for lattice stability and microwave dielectric properties. Introducing an appropriate amount of Mn–O bonds with high bond dissociation energy facilitates MgO6 octahedron stability, which improves the thermal stability of the lattice. Accordingly, the microwave dielectric properties for 0.06 Mn²⁺-doped MgTa2O6 ceramics were determined: εr = 28, Q × f = 105,000 GHz (at 7.5 GHz), τf = 19.5 ppm/°C.
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.
In this work, a complex impedance spectroscopy study of bismuth vanadate (BiVO4) ceramics with different additions of ZnO (25, 50, and 75 wt %) was performed. BiVO4 (BVO) was synthesized by the reaction method in solid-state and calcined at 500 °C and BVO–ZnO composites were moulded in sintered ceramic pellets at 700 °C. X-ray diffraction (XRD) was used to analyse the crystal structure of BVO and the BVO–ZnO composites; none spurious phase was observed during the synthesis. Analysis by complex impedance spectroscopy (CIS) showed that increasing the concentration of ZnO reveals increased activation energy due to thermo-activated charge transfer for the sample with 25 wt % ZnO. At room temperature, the increase in the ZnO concentration in the BVO matrix maintained a high value for the dielectric constant (ε), in the order of 104 at a frequency of 1 Hz. Average normalized change (ANC) was used to identify the temperature at which the available density of trapped charge states vanishes in each sample. The temperature coefficient of capacitance was positive for BVO and negative for composites. The adjustment through the equivalent circuit presented excellent electrical response for the composites, and identified an association with three resistors, each in parallel a constant phase element, showing the influence of grain and grain boundary on the process of thermo-active conduction.
Ba1-xCaxMoO4 (0 ≤ x ≤ 0.20) ceramics were prepared from powders to form solid solutions by a solid-state reaction sintering process. The influence of the Ca²⁺ content on the microstructure, sintering, densification, microwave dielectric properties and chemical stability of BaMoO4-based ceramics with Ag metal was discussed in detail. The sintering temperatures of the Ba1-xCaxMoO4 ceramics were effectively reduced to less than 950 °C by the formation of the solid solutions. Structural analysis indicates that the Ba1-xCaxMoO4 ceramics belong to the class of tetragonal scheelites. The crystal grain size begins to decrease and become more regular as x increases from 0 to 0.12. However, as x continues to increase, a liquid phase begins to appear, and the grain boundaries are no longer clear. The εr value increases from approximately 8.6 to 9.8 as x increases from 0 to 0.2. The Ba0.92Ca0.08MoO4 sample possesses the best microwave dielectric performance, namely an εr = 9.3 and the maximum Q × f value of 33593 GHz. The addition of 15 wt% TiO2 or 10 wt% CaTiO3 can effectively change the τf values of the Ba1-xCaxMoO4 ceramics to approximately 0. The Ba1-xCaxMoO4 ceramic samples can coexist with silver during the LTCC cofiring process.
Due to the multiple dimensional and embeddable characterizations, the three-dimensional woven structure is of great potential as a platform for multifunctional composites. As an example of this concept, we proposed a light-weight and high-gain three-dimensional woven spacer microstrip antenna (3DWS-MA) for the first time by integrating microstrip antenna into 3D woven spacer composites. The single-element 3DWS-MA showed superb electromagnetic performance with the gain value of 7.1 dB, which is more than four orders of magnitude higher than traditional microstrip antenna (2.5 dB). Furthermore, the 3DWS-MA maintained proper resonant frequency and impedance matching after the impact of 18 J, exhibiting excellent structural integrity.
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
In this paper is reported an experimental and numerical investigation of the microwave dielectric properties of Na2Nb4O11 (NN), ceramic matrix added with (0, 2, 5, and 10 wt%) of Bi2O3, 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 NN reference sample, frequency response bandwidth is of 43MHz (simulated) and 35MHz (experimental) for frequency operation around 2.28 GHz. The NN reference sample showed a dielectric permittivity around 58.4, with loss around 7.3 × 10− 3. The adding of bismuth oxide (2 and 5 wt%) reduced the value of the dielectric permittivity, otherwise the adding of 10 wt% increases its value. The temperature coefficient of resonant frequency (τf) was also measured for all dielectric samples. The values obtained were in the range of −3378 to −250ppm/°C. The results obtained confirm the necessity of producing composites of this material with others positive τf materials for obtaining dielectric resonator antennas with τf = 0. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1211–1217, 2016
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%.
Bismuth vanadate (BiVO4) powder was successfully prepared by the sol–gel method. Bismuth nitrate and ammonium vanadate were used as the starting precursors with mole ratio of 1:1 in ethanol media at 70 ºC for 1 h. The yellow gel was calcined at 400–600 ºC for 2 h. The phase of BiVO4 powder was characterized by X–ray diffraction (XRD). The morphology and chemical composition of BiVO4 powder were investigated by scanning electron microscopy (SEM) and energy dispersive X–ray spectroscopy (EDXS). The functional groups of BiVO4 powder was identified by Fourier transform infrared spectroscopy (FTIR) and the surface area of BiVO4 powder was determined by Brunauer, Emmett and Teller technique (BET).
Thin films of bismuth vanadate (BiVO4) are deposited
through the solution combustion synthesis technique coupled
with the dip-coating process. Thermal gravimetric analysis
shows a total mass loss of 71 % besides the formation of the
monoclinic phase, about 300 °C, which is also revealed by
X-ray diffraction. UV–Vis optical absorption spectra show
direct bandgap transition about 2.5 eV for films, in good
agreement with semiconducting monoclinic phase. Scanning
electron microscopic images reveal that thermal annealing time
at 500 °C is a very important parameter to control the thickness
and shape of the particles and yields an average thickness of
about 800 nm for 10 dip-coated deposited layers, with roundshaped
nanometric-sized particles, homogeneously distributed
on the film surface. Photoelectrochemical degradation of methylene
blue by a bismuth vanadate film deposited on fluor-doped
tin oxide substrate shows up as a very efficient process. The
first-order rate constant for the photoinduced process is about
five times the rate constant for degradation in the dark, showing
the capacity of the BiVO4/fluorine-doped tin oxide film for
electrochemical degradation, mainly in the presence of light
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.
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.
This book teaches the skills and knowledge required by today's RF and microwave engineer in a concise, structured and systematic way. Reflecting modern developments in the field, this book focuses on active circuit design covering the latest devices and design techniques. From electromagnetic and transmission line theory and S-parameters through to amplifier and oscillator design, techniques for low noise and broadband design; This book focuses on analysis and design including up to date material on MMIC design techniques. With this book you will: Learn the basics of RF and microwave circuit analysis and design, with an emphasis on active circuits, and become familiar with the operating principles of the most common active system building blocks such as amplifiers, oscillators and mixers Be able to design transistor-based amplifiers, oscillators and mixers by means of basic design methodologies Be able to apply established graphical design tools, such as the Smith chart and feedback mappings, to the design RF and microwave active circuits Acquire a set of basic design skills and useful tools that can be employed without recourse to complex computer aided design Structured in the form of modular chapters, each covering a specific topic in a concise form suitable for delivery in a single lecture Emphasis on clear explanation and a step-by-step approach that aims to help students to easily grasp complex concepts Contains tutorial questions and problems allowing readers to test their knowledge An accompanying website containing supporting material in the form of slides and software (MATLAB) listings Unique material on negative resistance oscillator design, noise analysis and three-port design techniques Covers the latest developments in microwave active circuit design with new approaches that are not covered elsewhere.
The BiVO4 additive was found effective for low-temperature firing of ZnNb2O6 polycrystalline ceramics below 950degreesC in air without a serious degradation in their microwave dielectric properties. Dense BiVO4-doped ZnNb2O6 samples of a relative sintered density over 95% could be prepared even at 925degreesC. An optimally processed specimen exhibited excellent microwave dielectric properties of Q(.)f = 55000 GHz, epsilon(r) = 26, and tau(f) = -57 ppm/degreesC. With increasing BiVO4 addition up to 20 mol% relative to ZnNb2O6, while the quality factor Q(.)f was gradually decreased, the relative dielectric constant, epsilon(r), was linearly increased and the temperature coefficient of resonant frequency, tau(f) was slightly increased. The variations in Q(.)f and epsilon(r) are surely attributable to the residual BiVO4 in the ZnNb2O6 matrix. An unexpected slight increase in tau(f) is probably due to the formation of the Bi(V,Nb)O-4-type solid solution.
As solar energy is the most abundant energy source, it has been widely exploited for thermal and electrical power generation. However, owing to the greater convenience of chemical energy storage, such as H2, compared to electricity, solar fuels have been considered as one of the most promising technological concepts due to their potential higher efficiency and environmental suitability. In this context, the photocatalytic water splitting into O2 and solar fuels (e.g. H2) is a topic of current interest. Furthermore, the development of photocatalysts that can utilize the whole electromagnetic spectrum is preferable in order to enhance the overall water splitting efficiency. Direct photocatalytic water splitting is a challenging problem because the water oxidation (WO) reaction is thermodynamically uphill. Hence, several WO photocatalysts have been developed and assessed over the last few decades, and it has been reported that BiVO4 is one of the most active O2 evolution photocatalysts. In this review, a first introduction regarding the solar fuel production and the water oxidation reaction is reported. Subsequently, the crystal and electronic structures as well as the optical properties that are closely related to the photoelectrochemical properties of BiVO4 are described. Finally, the monoclinic BiVO4 synthesis methods and the optimization methods to improve the performances of BiVO4 are discussed. The information gained from this analysis contributes to the better understanding of the main parameters affecting the activity and will ultimately lead to the optimized synthesis of a more efficient BiVO4 photocatalytic material.
A series of dense ceramic samples in the (1 − x)BiVO4–xLi0.5Re0.5WO4 (Re = La, Nd; 0.05 ≤ x≤0.11) has been prepared by using the traditional solid state reaction method. A single solid solution phase with monoclinic structure is formed through the entire compositions and the unit cell volume increases linearly with x. When x = 0.09, the composition of 0.91BiVO4–0.09 Li0.5La0.5WO4 readily sintered at 750 °C shows good microwave properties with εr of 76.7, Q × f of 6789 GHz and τf
of 7.3 ppm/°C. As x = 0.08, the 0.92BiVO4–0.08Li0.5Nd0.5WO4 ceramic sintered at 750 °C possesses excellent microwave dielectric properties of εr = 71.8, Q × f = 7482 GHz and τf
= 0.8 ppm/°C. These ceramics have applications for microwave devices requiring high permittivity and ultra-low sintering temperature.
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
Dense ceramic samples in the (1−x) BiVO4−xLi0.5Sm0.5WO4 (0.05 ≤ x ≤ 0.1) system are prepared using the solid-state reaction process. A single solid–solution phase with monoclinic structure formed through the entire compositions and the unit cell volume increases linearly with x. When x = 0.07, the composition of 0.93BiVO4–0.07 Li0.5Sm0.5WO4 readily sintered at 750 °C has excellent microwave properties with a high permittivity of 73.9, and a Q × f value of 9054 GHz and a near-zero temperature coefficient of −1.61 ppm/°C. The ceramics have applications for microwave devices requiring ultra-low sintering temperatures.
x(Ag0.5Bi0.5)MoO4-(1 - x)BiVO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared by using the solid-state reaction technique. Ceramics with x < 0.10 had a monoclinic scheelite structure, while those with ≥0.10 were tetragonal scheelite solid solutions. This indicates that the phase transformation temperature of BiVO4 was lowered through the formation of a solid solution. The thermal expansion data of the x = 0.08 sample showed that the thermal expansion coefficient was increased suddenly from +8 to +15 ppm/°C at about 60.6 °C due to the phase transition. Similarly, a maximum value of microwave dielectric permittivity was revealed at about 65 °C for the x = 0.08 sample. All of the ceramics could be well sintered below 700 °C. Good microwave dielectric behaviors, with relative permittivity >75 and Qf > 9000 GHz, were obtained in ceramics with compositions near x = 0.10. Both the THz data and the infrared spectra were used to study the intrinsic dielectric behavior of the materials at microwave frequencies.
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.
Optical and dielectric properties of BiVO4 nanocrystals with
different particle sizes have been investigated. BiVO4
nanocrystals with different particle sizes were synthesized through a
solid-state reaction method followed by mechanical ball milling for
different time durations. The samples were characterized by X-ray
diffraction, UV-Vis spectroscopy, field emission scanning electron
microscopy, transmission electron microscopy and photoluminescence
spectroscopy. Direct and indirect band gap energies were found to vary
in the range 4.04-4.16 eV and 3.51-3.67 eV respectively, for
different particle sizes. The band gap energies are higher with respect
to their values in the bulk BiVO4 due to quantum confinement
effects. Dielectric properties of the BiVO4 nanocrystals were
investigated and it was found that the dielectric constant increased
from 32 to 41 for the reduction of particle size from 29 to 7 nm.
This study examines the microwave dielectric properties of La2Sn2O7 ceramics with a view to the use of these ceramics in mobile communication. La2Sn2O7 ceramics were prepared by the conventional solid-state method with various sintering temperatures and times. The highest density obtained, 6.61 g/cm3, was that of La2Sn2O7 ceramic that was sintered at 1,580 °C for 34 h. Dielectric constants (ε
r
) of 13.2–14.4 and quality factors (Q × f) of 34,300–40,500 GHz were obtained by sintering at temperatures in the range 1,520–1,610 °C for 34 h. Dielectric constants (ε
r
) of 10.3–14.4 and quality factors (Q × f) of 15,300–40,500 GHz were obtained using sintering times of 30–38 h at a sintering temperature of 1,580 °C. When La2Sn2O7 was sintered at 1,580 °C for 34 h, a dielectric constant (ε
r
) of 14.4, a quality factor (Q × f) of 40,500 GHz and a temperature coefficient of resonant frequency (τ
f
) of −54 ppm/ °C were obtained.
The dielectric properties and the sintering effect upon microstructure of (1−x) CaTiO3-x(Li1/2Nd1/2)-TiO3 Ceramics are investigated in this paper. Nd3+ and Mg2 + ions co-substitution for Ca2 + on A site improves the sintering characteristic of CaTiO3 ceramics with forming orthorhombic perovskite structure. The structure of (1 − x) CaTiO3-x(Li1/2Nd1/2)TiO3 changes from orthorhombic to tetragonal as (Li1/2Nd1/2)TiO3 addition increasing. Limited solubility of (Li1/2Nd1/2)TiO3 in CaTiO3 forming a part solid solution compound achieves the adjustment of τε for CaTiO3 at low sintering temperature. The proper dielectric properties with ε = 78, tan δ = 0.0006, τε = +7 ppm/∘C are obtained for 0.8Ca0.67(Nd,Mg)0.22TiO3-0.2(Li1/2Nd1/2)TiO3 ceramics.
The domain structure of the ferroelastic monoclinic phase of BiVO4 is examined by electron microscopy and electron diffraction methods. It consists of two orientation variants separated by two sets of quasi-mutually “perpendicular” walls. The orientation of the domain walls is not determined by symmetry but follows from the direction of the spontaneous strain.Die Domänenstruktur der ferroelastischen, monoklinen Phase von BiVO4 wird mittels Elektronenmikroskopie und Elektronenbeugungsmethoden untersucht. Sie besteht aus zwei Orientierungsvarianten, die durch zwei Sätze von quasi-gegenseitigen „Senkrecht”-Wänden getrennt sind. Die Orientierung der Domänenwände wird nicht durch Symmetrie bestimmt, sondern folgt aus der Richtung der spontanen Verzerrung.
The domain structure of ferroelastic BiVO4 crystals was investigated by X-ray diffraction, nuclear magnetic resonance, optical polarizing microscopy, transmission electron microscope, and electron diffraction techniques. From these results, it is found that the BiVO4 crystals have only prominent W walls and no non-prominent W' domain walls. A model of the twin structure is suggested, and all experimental results are explained in terms of this model. However, the prominent W wall obtained from our experimental results should not occur in the ferroelastic species 4/mF2/m previously reported. From the acoustic symmetry character of the elastic behaviour, it has been established that there exist two possible W and W' walls for permissible planar walls that are consistent with all experimental observations. Thus, it can be concluded that the BiVO4 crystals having only the W walls found by our group and W' walls obtained by other groups are consistent with the ferroelastic species 4/mmmF2/m rather than 4/mF2/m.
In this work, dense ceramic samples across the solid solution of the [(Li0.5Bi0.5)xBi1−x][MoxV1−x]O4 system are prepared using the solid-state reaction process. The monoclinic phase region could be obtained for 0≤x<0.098 and the scheelite tetragonal solid solution region could be obtained for 0.098<x≤1. When x=0.098, the boundary composition [(Li0.5Bi0.5)0.098Bi0.902][Mo0.098V0.902]O4 could be readily sintered at 650°C and also have excellent microwave properties with a high relative permittivity of 81, and a Q×f value of 8000 GHz and a low temperature coefficient of +9.7 ppm/°C. This ceramic is chemically compatible with both Al and Cu electrode materials at its sintering temperature. It can be an excellent candidate for ultra-low-temperature co-fired ceramic technology and is the highest permittivity microwave material found to date with sintering considerations as low as 650°C.
The BiVO4 additive was found effective for low-temperature firing of ZnNb2O6 polycrystalline ceramics below 950°C in air without a serious degradation in their microwave dielectric properties. Dense BiVO4-doped ZnNb2O6 samples of a relative sintered density over 95% could be prepared even at 925°C. An optimally processed specimen exhibited excellent microwave dielectric properties of Q·f= 55000 GHz, ɛr= 26, and τf=−57 ppm/°C. With increasing BiVO4 addition up to 20 mol% relative to ZnNb2O6, while the quality factor Q·f was gradually decreased, the relative dielectric constant, ɛr, was linearly increased and the temperature coefficient of resonant frequency, τf, was slightly increased. The variations in Q·f and ɛr are surely attributable to the residual BiVO4 in the ZnNb2O6 matrix. An unexpected slight increase in τf is probably due to the formation of the Bi(V,Nb)O4-type solid solution.
Bismuth vanadates (BiVO4) with various crystal structures (tetragonal scheelite, monoclinic scheelite, and tetragonal zircon) and morphologies (sphere-, nanosheet-, dendrite-, and flower-like) were controllably fabricated by using a mild additive-free hydrothermal treatment process under the different preparation conditions. The crystal structures, morphologies, and photophysical properties of the products were well-characterized. Subsequently, their UV- as well as visible-light photocatalytic performance was evaluated via dyes rhodamine B (RB) and methylene blue (MB) degradation. Special attention was paid to evaluate the correlation of the reactivity with crystal structure, morphology, and electronic structure of as-prepared BiVO4 samples.
The ongoing developments of low temperature co-fired ceramic (LTCC) technique are making it more and more attractive for applications in microwave and millimetre wave frequency band. The most active areas for high frequency applications include Bluetooth module, front end module (FEM) of mobile phones, wireless local access network (WLAN). Local multipoint distribution system (LMDS) and collision avoidance radar etc. The recent development of LTCC substrate material extend the applicable frequency of the technique up to 100GHz. Owing to the feature of low conductor loss, low substrate loss, and up to 50 laminated layers, LTCC provide a harmonic bed for embedded microwave and mm wave passive components and accessories including antennas. In addition, LTCC substrate material has a wide tuneable range of thermal expansion coefficient. It is this feature that makes the LTCC substrate very attractive for various integrated packaging. In the presentation, the present status of the technology will be reviewed, from its manufacturing process, material properties to RF and microwave performance of typical LTCC modules. The LTCC market projection and cost issue will be also discussed to have a globe view of the technology. A large number of practical examples for high frequency applications will be followed to display the current technology status. The examples include, but not limited to, (1) RF front end module for mobile phones, (2) antennas for WLAN and Bluetooth devices, (3) laminated waveguide and other new form of transmission lines for mm wave applications, (4) integrated mm wave antenna arrays, and (5) Integrated RF modules such as high performance VCO. balanced LNA, miniaturized RF filters and balun. Since the most challenge issue in LTCC module designs is the integrated passives, the emphasis of the examples will be placed on the embedded passives and system integration. Some practical LTCC modules developed in The Chinese University of Hong Kong are listed in the-
following. The details of these modules can be found in the listed references and will be further elaborated in the presentation.
Monoclinic BiVO4 is formed by the hydrolysis of bismuth and vanadyl double alkoxide, followed by washing and drying. Its average particle size is almost-equal-to 45 nm. The chemical structure is described by the formula Bi(VO4). Sintered pellets with nearly theoretical density are prepared by hot pressing for 2 h at 750-degrees-C and 29.4 MPa. The electrical conductivity of the pellets has been measured at 150-degrees to 550-degrees-C. Data are interpreted in terms of ionic conductivity due to oxygen ion vacancies.
The mixed (oxygen ionic-electronic) conductor bismuth vanadate (BiVO4) was studied with respect to its electrochemical properties. The ionic transference numbers, measured by the concentration cell method in the temperature range of 800 to 1000 K, vary from 0.7 to 0.3. The total conductivity of this ceramic material isone order of magnitude lower than found for cubic yttria-stabilized zirconia (YSZ). The activation enthalpy for the electronic conductivity is high (193 kJ/mol) compared to the ionic conductivity (71 kJ/mol). The PO2 dependence of the conductivity data in combination with the Seebeck measurements showed electrons to be the majority charge carriers, indicating that BiVO4 is an n-type mixed conductor.
The postresonator method proposed by Hakki and Coleman for the measurement of dielectric properties of solids in the microwave region is reexamined. Based on the experiments performed around 10 GHz, the limitations of the method in loss determination for materials with tan delta < 5 x 10(-4) are outlined. Alternative techniques for studying high-Q materials are described. Loss measurements performed on well-prepared ceramics have been used to draw comparisons among these techniques. Some of the experimental findings are new and are reported for the first time.
A highly efficient monoclinic BiVO(4) photocatalyst (C-BVO) was synthesized by an aqueous method with the assistance of cetyltrimethylammonium bromide (CTAB). The structure, morphology and photophysical properties of the C-BVO were characterized by XRD, FE-SEM and diffuse reflectance spectroscopy, respectively. The photocatalytic efficiencies were evaluated by the degradation of rhodamine B (RhB) under visible-light irradiation, revealing that the degradation rate over the C-BVO was much higher than that over the reference BiVO(4) prepared by aqueous method and over the one prepared by solid-state reaction. The efficiency of de-ethylation and that of the cleavage of conjugated chromophore structure were investigated, respectively. The chemical oxygen demand (COD) values of the RhB were measured after the photocatalytic degradation over the C-BVO and demonstrated a 53% decrease in COD. The effects of CTAB on the synthesis of C-BVO were investigated, which revealed that CTAB not only changed the reaction process via the formation of BiOBr as an intermediate, but also facilitated the transition from BiOBr to BiVO(4). Comparison experiments were carried out and showed that the existence of impurity level makes significant contribution to the high photocatalytic efficiency of the C-BVO.
A new class of hemispherical dielectric resonator antenna using mixed dielectric with beam agility is presented. The design of the antenna was optimized by extensive numerical simulations with the HFSS, Ansoft package. The antenna resonates at 5.6 GHz, with a diameter of 26 mm and Sn of -15 dB. Varying the position of a regional dielectric discontinuity allows the principle radiation direction to be moved continuously through 360 degrees in the azimuth plane, with a vertical beam-width of 60 degrees. The size of this antenna makes it ideally suitable for hand held wireless communicators.
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