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Impedance spectroscopy study of Na2Nb4O11
Abstract
The effects of different additions of Bi2O3 (0, 2, 5 and 10 wt.%) on Na2Nb4O11 ceramics have been studied
by complex impedance spectroscopy analysis. The structural and dielectric properties of Na2Nb4O11-
ceramics with Bi2O3 additions are also discussed as a possible material for microwave and radio frequency
applications. Solid state reaction method was used for producing the target compound. The
present work also reports on sample preparation, where polyvinyl alcohol was used as a binder to reduce
the brittleness and the organic binder was burnt out during sintering process. X-ray diffraction at room
temperature was used to identify phases formed as well as optimum calcinations conditions the production
of Na2Nb4O11 powder. Analysis was carried out to identify the pure-phase specimen by Rietveld
refinement method. Based on this method a structure with the lattice parameters (a = 10.840 Å,
b = 6.162 Å and c = 12.745 Å; a = 90�, b = 106.4� and c = 90�) was found. The dielectric and electrical properties
of Na2Nb4O11 for appropriate amounts of Bi2O3 additions has showed a considerable enhancement
and overriding applications for microwave and radio frequency communications. The electrical behavior
(complex impedance Z*, complex modulus M*) were studied over a frequency range (1 Hz to 1 MHz) and
temperature (553–713 K).
Supplementary resource (1)
In this study, the dielectric and electrical properties of RE3+ substituted (1-x)2/3Pb(Mg1/3Nb2/3)O3-1/3PbTiO3:xCoFe1.97RE0.03O4 (RE = La3+ and Eu3+) particulate composites (with x = 10, 15, 20, and 30 mol %) synthesized via a solid-state approach have been presented. The temperature-dependent dielectric constant ε′(T) displays a prominent peak around 180–190 0C, which drifts towards the high-temperature side with increasing frequency, consistent with the diffuse phase transition from ferroelectric to paraelectric. The high dielectric constant (ε′) value and additional peak at the higher temperature side are more prominent in CoFe1.97Eu0.03O4 (CFEO) than CoFe1.97La0.03O4 (CFLO) based composites because of the high conductivity of CFEO than CFLO. It is also seen that CFLO-based composite has much lower loss than CFEO-based composite owing to the lower conductivity of CFLO. The magnitude of dielectric loss (tan δ) increases with the temperature and concentration of ferrites, which might be due to the occurrence of thermally active carrier hopping conductions in low-resistive ferrites. The Jonscher’s double power law is used to analyse frequency-dependent AC conductivity. The temperature-dependent power exponents confirm that the conduction arises due to the transitional hopping of polarons. The complex impedance spectroscopy data modelled using equivalent electrical circuits show a single semicircle for both sets of composites, indicating that the conduction mechanism is only due to thermally active conducting grains.
In this work, the electrical properties of Bi4Ti3O12 (BiT) were investigated; those presenting interesting electrical and dielectric properties in the radiofrequency (RF) and microwave (MW) range were analysed. BiT has many applications in telecommunication systems and MW circuits. It was synthesized by the solid-state reaction method, and X-ray diffraction was used for structural characterization of this phase. The properties in the RF range were measured by impedance spectroscopy. An imaginary impedance relaxation peak and high values for real impedance were found. The complex impedance plane plots show the presence of a semicircle that was adjusted by an R-CPE circuit, presenting a thermoactive process for the load carriers. The dielectric properties in the MW range were measured using the Hakki–Coleman method, and the thermal stability was measured using the Silva–Fernandes–Sombra (SFS) technique. BiT was tested as an antenna and distant field parameters were measured and simulated.
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.
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
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.
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
Ca2Fe2O5 powder sample, were prepared to investigate the origin of the weak ferromagnetic component reported in literature for calcium ferrite single crystals. In this work, the calcination method was used to produce nanocrystalline powders of Ca2Fe2O5. XRD measurement has shown the presence of Fe3O4 magnetite and CaO as impurity phases. The ferrimagnetic phase deeply influences the magnetic behavior with features very similar to those reported in literature for Ca2Fe2O5, both powders and single crystals. Our results support the hypothesis that the weak ferromagnetic component observed in Ca2Fe2O5 can be also due to the presence of magnetite impurity traces in the samples. The powders were submitted to calcination processes at 500℃ for 2 hours and 950℃ for 16 hours. The sintered sample was submitted at 1050℃ for 6 hours and characterized by X-Ray Powder diffraction (XRD), dielectric measurements, Magnetization and Scanning Electron Microscopy (SEM) analysis.
A study of the dielectric, magnetic and structural properties of composites based on M-type barium hexaferrite BFO (BaFe 12 O 19) and SBN (SrBi 2 Nb 2 O 9) is presented. The magneto-dielectric matrix composite (SrBi 2 Nb 2 O 9) x (BaFe 12 O 19) 100–x , (x = 0, 25, 50, 75 and 100 wt%) were prepared by using a new procedure based in the solid state method. X-ray powder diffraction patterns, Raman and Infrared spectroscopy, Mössbauer spectroscopy and scanning electron microscopy (SEM) were carried out for better understanding of the microstructural, dielectric and magnetic properties. Radiofre-quency (RF) dielectric permittivity, dielectric loss measurements and magnetic and electric hysteresis loops properties are also discussed throughout this paper. The hysteresis loops showed that composite samples preserve the ferrimagnet-ism and ferroelectricity for hexaferrite when SBN is added to the composite (BFO25P), although they become less co-ercive. In addition, the effects of organic binders group (TEOS, PVA and glycerin) on structural properties were also investigated.
Polyvinyl alcohol-polypyrrole composite polymer fil ms were synthesized by chemical oxidative polymerization. Iron (III) chloride, FeCl 3 had been used at different concentrations as dopant and oxidant. Dielectric properties were m easured in the frequency range from 20 Hz to 1 MHz at room temperature using HP 4284A LCR meter. Results show that the dielectric properties vary with the compositions of iron (III) chloride dopant.
Frequency and temperature dependences of dielectric permittivity and electric modulus of pure and Ba-doped Bi2Ti4O11 were studied in the ranges of 10−1–106 Hz and −150–350 °C, respectively. We found that the antiferroelectric phase transition temperature of Bi2Ti4O11 decreases with Ba doping. In the permittivity studies, we also observed dielectric relaxation peaks shift to higher temperature with increasing frequency. Furthermore, in the electric modulus formalism, conducting peaks were uncovered above 150 °C in addition to the dielectric relaxation peak. We discussed the mechanisms for the dielectric relaxation and conduction processes based on TiO6 octahedra distortion and a space-charge model. © 2003 American Institute of Physics.
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.
This paper has considered the problem of electrical dispersion phenomena in vitreous ionic conductors. The authors conclude that in cases in which the dispersion is connected with the long range transport of charge, macroscopic electric field relaxation times and distribution functions derived from an impedance formalism using the electric modulus, seem to be more appropriate for the microscopic process that is believed to give rise to the dispersion than those derived from an admittance formalism using the complex permittivity.
The helium pycnometer allows us to measure the cell‐wall density of dry woods and the basic density of wood samples soaked with water and/or a consolidant solution if a non‐volatile solvent is used. These parameters were correlated to the porosity, which for degraded waterlogged wood is related to the maximum water content. Moreover, this has revealed the possibility of investigating, by means of accurate cell‐wall density determination, the efficacy of several consolidants in the treatment of waterlogged woods.
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 double perovskite oxide barium gadolinium tantalate, Ba2GdTaO6 (BGT) is synthesized by solid-state reaction technique. The Rietveld refinement of the X-ray diffraction pattern of the sample shows cubic phase at room temperature. Fourier transform infrared (FTIR) spectrum shows two primary phonon modes of the sample at around 587 cm−1 and 858 cm−1. Raman spectrum of the sample taken at 488 nm excitation wavelength, shows three primary strong peaks at 832, 384 and 106 cm−1. Group theoretical study is performed to assign the different vibrational modes of BGT. Dielectric spectroscopy is applied to investigate the ac electrical conductivity of BGT in a temperature range from 333 K to 673 K and in a frequency range of 42 Hz–1 MHz. The complex impedance plane plots show that the relaxation (conduction) mechanism in this material is purely a bulk effect arising from the semiconductive grains. The relaxation time corresponding to dielectric loss is found to obey Arrhenius law with an activation energy of 0.32 eV. The electric modulus formalism is adopted to interpret the relaxation mechanism of BGT.
CoFe1.95Ho0.05O4 spinel ferrite has been prepared by mechanical alloying and subsequent annealing at temperatures (TAN) in the range 800–1200° C. As an effect of annealing temperature, the average grain size of the material has increased from 40 to 90 nm and magnetically transformed from single domain/pseudo-single domain (40–64 nm) to multi-domain state (67–90 nm) for TAN>1050 °C. Dielectric properties of the nano-grained CoFe1.95Ho0.05O4 ferrite have suggested a correlation with magnetic domain transformation. The dielectric constant of the material is the lowest for the grains in pseudo-single domain state (59 nm). The material showed non-Debye type dielectric relaxation with activation energy 0.47–0.58 eV, along with lowest value of activation energy due to electrical conduction inside grains. The dielectric contributions of grains and grain boundaries have been precisely separated using complex modulus formalism and various scaling plots. Results of the dielectric parameters of the samples (dielectric constant: 20–100 and dielectric loss: 0.11–0.4) seems to be interesting for microelectronic applications of ferrite materials.
Barium orthovanadate (Ba3V2O8), a derivative of perovskite family has been prepared using a mixed-oxide technique. The room temperature X-ray diffraction analysis has confirmed the formation of a single phase compound in trigonal crystal structure. The study of microstructure by scanning electron microscopy shows that the compound has well defined grains, distributed uniformly throughout the surface. The studies of dielectric parameters (εr and tan δ) of the compound as a function of temperature at three different frequencies (100, 500, 1,000 kHz) exhibit that they are almost temperature independent at low and medium temperature ranges. Detailed studies of impedance and related parameters exhibit that the electrical properties of the material are strongly dependent on temperature, and bear a good correlation with its microstructures. The bulk resistance, evaluated from complex impedance spectra, is found to be decreasing with rise in temperature. It shows that the material has negative temperature co-efficient of resistance similar to that of semiconductors. The same behaviour has also been observed in the study of I–V characteristics of the material. The complex electric modulus analysis indicates the possibility of hopping conduction mechanism in the system with non-exponential type of conductivity relaxation. The nature of variation of dc conductivity with temperature confirms the Arrhenius behavior of the material. The ac conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law.
The microwave dielectric properties of (Mg(1−x)Nix)2SnO4 ceramics were examined with a view to their exploitation for mobile communication. The (Mg(1−x)Nix)2SnO4 ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction (XRD) patterns of the (Mg0.95Ni0.05)2SnO4 ceramics revealed no significant variation of phase with sintering temperatures. A maximum density of 4.77 g cm−3 was obtained for (Mg0.95Ni0.05)2SnO4 ceramic, sintered at 1550 °C for 4 h. Dielectric constant (ɛr) of 8.72, quality factor (Q × f) of 103,100 GHz, and temperature coefficient of resonant frequency (τf) of −62.8 ppm °C−1 were obtained for (Mg0.95Ni0.05)2SnO4 ceramics that were sintered at 1550 °C for 4 h.
The phase transitions between various structural modifications of the natrotantite-structured system xAg2Nb4O11–(1−x)Na2Nb4O11 have been investigated and a phase diagram constructed as a function of temperature and composition. This shows three separate phase transition types: (1) paraelectric–ferroelectric, (2) rhombohedral–monoclinic and (3) a phase transition within the ferroelectric rhombohedral zone between space groups R3c and R3. The parent structure for the entire series has space group R3¯c. Compositions with x>0.75 are rhombohedral at all temperatures whereas compositions with x<0.75 are all monoclinic at room temperature and below. At x=0.75, rhombohedral and monoclinic phases coexist with the phase boundary below room temperature being virtually temperature-independent. The ferroelectric phase boundary extends into the monoclinic phase field. No evidence was found for the R3–R3c phase boundary extending into the monoclinic phase field and it is concluded that a triple point is formed.
Structural characterisation of Na2Nb4O11 powder has been carried out by Rietveld refinement of neutron powder diffraction data in the temperature range 10–573 K and measurements of second harmonic generation in the range 75 to 450 K. The space group changes from Rc to C2/c on cooling through a phase transition at 380 K. The origin of the phase transition in Na2Nb4O11 is mainly related to distortion of some Nb(1)O7 pentagonal bipyramids within the equatorial plane whereas other Nb(2)O7 pentagonal bipyramids become almost regular. Although Na2Nb4O11 ceramics show a maximum permittivity of 150 at 380 K and frequency-independent permittivity below this temperature, the room temperature polymorph is not ferroelectric. Comparison of the high and room temperature structures shows clear evidence of antiparallel atomic displacements in the room temperature structure and, therefore, a tendency towards low temperature antiferroelectric behaviour. A comparison is also made with the structurally-related ferroelectric Ag2Nb4O11.
The phases, microstructure, composition analysis and microwave dielectric properties of (1−x)MgWO4–xCaTiO3 ceramics with Li2CO3–4H3BO3 additions prepared by solid-state reaction method have been investigated by using X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and advantest network analyzer. The τf of (1−x)MgWO4–xCaTiO3 were dependent on phase constitutions. The microwave dielectric properties of 0.91MgWO4–0.09CaTiO3 ceramics with Li2CO3–4H3BO3 were characterized, the results indicated that the ɛr and Q×f were associated with the sintering temperature and amount of Li2CO3–4H3BO3. The sintering temperature of ceramics was reduced to 950°C from 1150°C and τf was modified to 0ppm/°C with good Q×f. Addition of 5.0wt% Li2CO3–4H3BO3 in 0.91MgWO4–0.09 CaTiO3 ceramics sintered at 950°C showed excellent dielectric properties of ɛr=15.5, Q×f=20,780GHz (f=7.1GHz) and τf∼0ppm/°C. The material has a chemical compatibility with silver, making it a very promising candidate materials for LTCC applications.
Impedance spectroscopy is an effective method to investigate the dielectric properties of powders by dispersing particles in proper liquids. It allows reliable calculation of the dielectric constant by analysis of impedance spectra for a variety of particulate materials. The temperature dependence of the dielectric constant near the Curie temperature was investigated for hydrothermal barium titanate powders suspended in ethylene glycol or butoxyethanol. Room-temperature measurements revealed that the dielectric constant of hydrothermal BaTiO3 powders was ∼500. After annealing of powders at 1250°C for 1 h, the dielectric constant increased to ∼3000. The temperature dependence of the impedance spectra was determined by conducting measurements within the temperature range from 20° to 150°C. It was shown that hydrothermal BaTiO3 powders with a primarily cubic crystal structure had a suppressed temperature dependence of the dielectric constant near the Curie temperature. The dielectric constant of annealed powders with a tetragonal crystal structure was significantly higher at the Curie temperature similar to the values obtained for sintered bulk barium titanate.
Ceramic samples of Me2Nb4O11 (Me = Na and Ag) show ferroelectricity with a permittivity maximum of similar to 150 at T-C approximate to 110 degrees C and a possible second transition below room temperature. Hysteresis loops of Na2Nb4O11 at room temperature show remanent polarisation similar to 2 mu C cm(-2) at 30 kV cm(-1). The existence of spontaneous polarisation is not compatible with the centrosymmetric space group C2/c originally reported. The crystal structures were successfully refined in the non-centrosymmetric space group Cc.
A weighted form of the Durbin–Watson d statistic [Durbin & Watson (1971). Biometrika, 58, 1–19] has been used to quantify the serial correlation between adjacent least-squares residuals in Rietveld refinements of step-scan powder diffraction data. Analyses of X-ray and neutron data from a range of materials have shown that the d statistic: provides a sensitive measure of the progress of a refinement and remains discriminating when other agreement indices fail, for example, when comparing results at different step widths; provides quantitative information about the significance of serial correlation present in the residuals; provides a convenient means of assessing the reliability of the estimates of the parameter variances; and provides a basis for the selection of values of step width and intensity corresponding to optimum and/or minimum use of experimental beam time.
The ion transport properties of glasses and supercooled melts are studied using the scaling properties of complex conductivity data. It is shown that the frequency response of the conductivity reflecting the transport mechanism is very similar in glasses and in melts. In the crossover regime from dc to dispersive conductivity, the frequency response depends on composition, while a universal response is found at higher frequencies. The outcomes of an alternative analysis based on the complex electrical modulus formalism are examined in detail, and it is demonstrated that the shape of the M″ peak is not significant with respect to the ion transport mechanism.
Using Ca(NO3)2·4H2O, Mg(NO3)2·6H2O, Si(OC2H5)4, LiNO3 and Bi(NO3)3·5H2O as raw materials, CaO–MgO–SiO2 submicron powders were prepared at low temperature by sol–gel method. The crystallization temperature was decreased enormously by the introduction of Li–Bi liquid phase sintering aids into Ca–Mg–Si sol, and the powders with average particle sizes of 80–100nm and 200–400nm were obtained at the calcining temperature of 750°C and 800°C, respectively. The sintering characteristic and dielectric properties of powders calcined at 750°C with different content of powders calcined at 800°C were studied. When the content of powders calcined at 800°C was 10wt%, the dielectric ceramic sintered at 890°C had compact structure, and possessed excellent microwave dielectric properties: ɛr=7.16, Q×f=25630GHz, τf=−69.26ppm/°C.
In this paper we studied the effects of Bi2O3 and PbO addition on BiFeO3 (BFO) ceramic matrix. The structural, dielectric and magnetic properties of fifteen BFO samples were discussed in view of possible applications in RF and microwave devices. The present work also reports the preparation of the samples. Polyvinyl alcohol (PVA) and tetraethyl orthosilicate (TEOS) were also added as a binder in the fabrication procedure. The samples have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and magnetic hysteresis measurements. Further, a study based on impedance spectroscopy also has been done. Dielectric permittivity (ε′) and dielectric loss (tan δ) were measured at room temperature in the frequency range 100 Hz–10 MHz, as well as a.c. conductivity. The –Im[Z(f)] versus Re[Z(f)] plot has been obtained. The samples were investigated in view of possible applications like miniaturized filters, diplexers and dielectric resonator antennas (DRA). In the RF and MW frequency region, the application of magneto-dielectric and multiferroic perovskite composite materials is desirable for the miniaturization of components.
A study of the effect of correlated ion motion on the electrical conductivity relaxation in single-crystalline yttria-stabilized zirconia is presented. Complex admittance in the radio frequency range show power-law dependencies in the real part of the conductivity at high frequencies of the form ωn and asymmetric electric modulus plots as a result of correlations. An analysis of the frequency dependence of the electric modulus is conducted to obtain time decay functions of the form exp[-(t/τ)Β] from an analytical distribution of relaxation times. Correlation times, and parameters n and Β characterizing the relaxation in time and frequency domains are compared to show the equivalence of time and frequency representations. The common origin of ac and dc processes is discussed in view of the frequency dependence of the complex conductivity. From a macroscopic activation energy for ion motion E=1.16 eV and a Β value of 0.43, a single-ion microscopic activation energy Ea=0.5 eV is obtained as ΒE according to Ngai's coupling model. The microscopic activation energy is related to the association energy of oxygen vacancies.
a b s t r a c t In this paper the structural and microwave dielectric properties of the (1 À x)(Mg 0.95 Zn 0.05)TiO 3 –(x) (Ca 0.6 La 0.8/3)TiO 3 ceramic composites have been investigated with the variation of x as well as sintering temperature. The grown samples have been characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray (EDX) spectroscopy analysis. The Rietveld anal-ysis of the XRD data has been carried out for structure refinement of the phases. The relative permittivity ð r Þ, quality factor (Q) and temperature coefficient of resonant frequency s f of the grown samples have been thoroughly studied. Out of all samples of this series, the sample with x = 0.21 shows excellent dielectric properties with r $ 26:26, Q Â f $ 60,738 GHz (at 6.44 GHz) and a nearly zero s f $ À0.8 ppm/°C. Two types of dielectric resonator antennas with different feeding mechanisms have been fabricated using this sample to study their performance. The experimental results have been compared with the simu-lated results obtained using Ansoft High Frequency Structure Simulator software. Ó 2012 Elsevier B.V. All rights reserved.
We report a study of the structural and thermal characteristics of calcium titanate alloys Ca[(Li1/3Nb2/3)xTi1−x]O3−δ (abbreviated as CNLTO). CNLTO alloys were prepared using a new procedure in the solid-state reaction method. Then they were studied using x-ray diffraction, Raman scattering spectroscopy and microwave properties. The symmetry of the atomic structure of the samples belongs to the spatial group Pbnm, showing orthorhombic structure. The microwave dielectric properties of CNLTO for samples ball-milled with the ratios of 1 and 2.4 balls g−1, calcinated at 900 °C (with two different times of exposure: 3 and 5 h) and sintered at 1000 and 1100 °C (during 3 h) were investigated. For both calcination treatments (900 °C, during 3 and 5 h), the dielectric permittivity decreased with decreasing titanium concentration. It was also observed that a higher temperature of calcination contributes to increasing the εr values. Dielectric permittivity values in the range 10–50 were obtained. We noted a decrease in the temperature coefficient of resonant frequency (τf) with an increase of the x value (titanium substitution). CNLTO has excellent microwave properties with τf close to zero for concentrations between 0.925 and 0.950.
The electrical properties of the grain boundary region of electroceramic sensor temperature based on inverse spinel Zn <sub>7</sub> Sb <sub>2</sub> O <sub>12</sub> were investigated at high temperature. The zinc antimoniate was synthesized by a chemical route based on the modified Pechini method. The electric properties of Zn <sub>7</sub> Sb <sub>2</sub> O <sub>12</sub> were investigated by impedance spectroscopy in the frequency range from 5 Hz to 13 MHz and from 250 up to 600 °C. The grain boundary conductivity follows the Arrhenius law, with two linear branches of different slopes. These branches exhibit activation energies with very similar values; the low-temperature (≤350 °C) and high-temperature (≥400 °C) regions are equal to 1.15 and 1.16 eV, respectively. Dissimilar behavior is observed on the relaxation time (τ) curve as a function of temperature, where a single slope is identified. The negative temperature coefficient parameters and nature of the polarization phenomenon of the grain boundary are discussed. © 2003 American Institute of Physics.
The advantages of plotting a.c. data in terms of impedance, electric modulus and dissipation factor simultaneously are illustrated. Complex impedance is generally employed for ionic conductors because it can easily distinguish between bulk and grain boundary effects. However, comparison with the modulus and dissipation factor data allows easier interpretation of the microscopic processes responsible for the measured a.c. response. In particular, the difference between localized (i.e. dielectric relaxation) and non-localized conduction (i.e. long range conductivity) processes within the bulk of the material may be discerned by the presence or the absence of a peak in the imaginary modulus versus frequency plot. Similarly, the absence or presence of a peak in the imaginary impedance versus frequency plot can be correlated to space charge effects and non-localized conductivity. Long-range conductivity results in nearly complete impedance semicircles but no frequency dispersion in the permittivity while localized conductivity is reflected in a frequency dependent permittivity but no measurable conductance. The degree to which these assignments may be made is related to the dielectric relaxation ratio () and the differences between the time constants of the different relaxation processes present in the material being examined.
The significance of electrical conductivity and electrical modulus spectra with respect to ion transport mechanisms in melts, glasses, and crystals is discussed. It is shown that in the typical frequency range of an ac impedance experiment, the real part of the conductivity, σ′, is exclusively determined by the ion transport dynamics, while the imaginary part of the modulus, M″, is additionally influenced by vibrational and electronic polarisations. Accordingly, conclusions from the shape of M″ spectra about ion transport mechanisms cannot be drawn without properly taking into account the influence of these polarisations. Generally, it can be argued that analyses in terms of conductivity or dielectric spectra are preferable to analyses in terms of modulus or resistivity spectra if the electrical properties of a sample can be represented by resistances acting in parallel.
Experiments which access the quantity ε*(ω) are usually termed dielectric relaxation methods, although ε*(ω) and ε(t) actually refer to dielectric retardation. The true dielectric relaxation, the modulus M(t), corresponds to the decay of the electric field under the conditions of a constant dielectric displacement. We have measured the polarization in terms of a real dielectric relaxation technique by studying the decay of the electric field E(t) ∝ M(t) for times 10−3 s≤t≤106 s under constant charge conditions. This conceptually straightforward method bears the advantage of the ability to measure extremely large retardation times. Using the equivalent thermally stimulated technique we also investigate the equilibrium and non-equilibrium dynamic response of amorphous condensed matter well below its caloric glass-transition.
The paper considers a number of problems arising from the test of serial correlation based on the d statistic proposed earlier by the authors (Durbin & Watson, 1950, 1951). Methods of computing the exact distribution of d are investigated and the exact distribution is compared with six approximations to it for four sets of published data. It is found that approximations suggested by Theil and Nagar and by Hannan are too inaccurate for practical use but that the beta approximation proposed in the 1950 and 1951 papers and a new approximation, called by us the a + bdu approximation and based, like the beta approximation, on the exact first two moments of d, both perform well.
The power of the d test is compared with that of certain exact tests proposed by Theil, Durbin, Koerts and Abrahamse from the standpoint of invariance theory. It is shown that the d test is locally most powerful invariant but that the other tests are not.
There are three appendices. The first gives an account of the exact distribution of d. The second derives the mean and variance to a second order of approximation of a modified maximum likelihood statistic closely related to d. The third sets out details of the computations required for the a + hdu approximation.
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