Value Of Τf Research Articles

Crystal structure and microwave dielectric properties of LiNb0.6Ti0.5O3 ceramics with Zn and Nb co-doped

The phase composition, microstructure, and microwave dielectric properties of the (Zn1/3Nb2/3)4+ substitution of Ti4+ in LiNb0.6Ti0.5O3 ceramics were systematically studied. The XRD pattern analysis results confirmed that Li1.075Nb0.625Ti0.45O3 solid solution single phase was formed in all components. Raman spectroscopy analysis showed that the substitution of (Zn1/3Nb2/3)4+ caused the expansion of the titanium oxide octahedral framework, and the increase in dielectric constant (εr) was attributed to the increase in bulk density and cation polarizability. The increase of oxygen vacancies after high-temperature sintering of doped ions has a negative impact on the quality factor (Q × f) value. As the doping content increases, the value of τf can continuously move to close to zero. LiNb0.6Ti(0.5-x)(Zn1/3Nb2/3)xO3 ceramic (x = 0.06) has good microwave dielectric properties when sintered at 1025 °C for 2 h: εr = 72.1, Q × f = 5370 GHz (at 4 GHz), τf = 3.52 ppm /°C.

Effect of substituting Al3+ for Ti4+on the microwave dielectric performance of Mg2Ti1-xAl4/3xO4 (0.01 ≤ x ≤ 0.09) ceramics

In this paper, Mg2Ti1-xAl4/3xO4 ceramics (0.01 ≤ x ≤ 0.09) were synthesized through conventional solid-state ceramic route. The cubic spinel structure, microstructure and microwave properties of Mg2Ti1-xAl4/3xO4 (x = 0.01, 0.03, 0.05, 0.07, 0.09) ceramics were investigated by X-ray diffraction, Raman spectra, infrared spectra. Rietveld refinements confirm that a spinel structure phase with space group Fd-3m is formed. The variation of the permittivity was concerned with the ionic polarizability, and the value of τf was influenced by the bond valence. Both Q × f values and relative density showed an identical trend. Intrinsic properties of Mg2Ti1-xAl4/3xO4 ceramics were analyzed by infrared spectra and Raman spectra. In addition, the Mg2Ti1-xAl4/3xO4 ceramic sintered at 1420 °C for 4 h possessed optimal dielectric properties (εr = 14.65, Q × f = 182347 GHz, τf = −57.7 ppm/°C) when x = 0.09.

Structural evolution, sintering behavior and microwave dielectric properties of Al(1-x)(Si0.5Zn0.5)xPO4 ceramics

Al(1-x)(Si0.5Zn0.5)xPO4 (0 ≤x≤0.6) ceramics were prepared via solid state reaction process. Their structural evolution, sintering behavior and microwave dielectric properties were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and with a network analyzer. Phase pure AlPO4 could not be obtained for the undoped composition. However, microscopically homogeneous single phase solid solution formed within the compositional range of 0 < x ≤ 0.05. Immiscibility with two-phase structure within apparently single phase solid solution could be observed for other doped compositions (0.1 ≤ x ≤ 0.6). Small level of doping with (Si0.5Zn0.5)3+ (x≤0.4) stabilized the orthorhombic cristobalite-like AlPO4 (C-AlPO4) phase; while further doping led to the transformation from orthorhombic α-C-AlPO4 into cubic β-C-AlPO4 phase. The doping with (Si0.5Zn0.5)3+ considerably improved the sinterability and reduced the sintering temperature to ˜900 °C when x = 0.6. The dielectric permittivity slightly increases and the Q × f value decreases with the increasing doping concentration. All composition demonstrate low permittivity (Ɛr ˜4) and negative value of τf. Good combined microwave dielectric properties with Ɛr ˜3.9, Q × f˜25,000 GHz and τf ˜ -25 ppm/oC could be obtained for the x = 0.2 composition after sintering at 1200 °C/2h.

Structure and synergy performance of (1-x)Sr0.25Ce0.5TiO3 -xLa(Mg0.5Ti0.5)O3 based microwave dielectric ceramics for 5G architecture

The crystal structure and microwave (MW) dielectric characteristics of the compound in the perovskite type series (1-x) (Sr0.25Ce0.5TiO3)-xLa(Mg0.5Ti0.5)O3, x = 0.0 to 0.6 have been studied. The structural phase evolutions from orthorhombic (for x = 0.0) to cubic (for x ≥ 0.15) as a function of La(Mg0.5Ti0.5)O3 have been investigated and are related to variation in the dielectric characteristics. For x = 0.0, the electron diffraction (TEM) revealed that Sr0.25Ce0.5TiO3 compound were tilted in both anti-phase and anti-parallel displacement of A-site cation. The nonlinear trend of the temperature coefficient of resonant frequency (τf) vs dielectric constant resulted from the tilted (for x = 0.0) to nontilted (for x ≥ 0.15) structure transition. The value of τf decreases markedly from +194 ppm/°C (x = 0.0) to + 0.8 ppm/°C (x = 0.6), but the Q × f value (Q-factor) increases from 19,238 GHz to 37,508 GHz. Low loss microwave (MW) dielectric properties: εr = 47.0, Q × f = 37508 GHz and τf = +15 ppm/°C and εr = 39, Q × f = 34098 GHz and τf = +0.8 ppm/°C were achieved for compositions with x = 0.45 and 0.6.

Cucurbit[7]uril-based fluorene polyrotaxanes

Poly[2,7-(9,9-dioctylfluorene)-alt-(2,7-fluorene/cucurbit[7]uril)] polyrotaxane was synthesized according to Suzuki coupling protocol in DMSO, by reacting 2,7-dibromofluorene encapsulated into the cucurbit[7]uril (CB7) cavity with 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester. The Ka value (9.5×103M−1) indicates that 2,7-dibromofluorene provides a preferential binding to CB7, which allows the synthesis of a polyrotaxane with a CB7/structural unit ratio of about 1/3 and higher molecular weights. The chemical structure was proven by FTIR and 1H NMR spectroscopy. The thermal, optical, electrochemical, wetting, and surface morphological properties of the polyrotaxane have been investigated and compared to those of the neat copolymer. The polyrotaxane exhibits an enhancement in the glass transition temperature, blue-shifted absorption by about 11nm, and a subtle effect on the LUMO energy levels. The fluorescence lifetimes follow a mono-exponential decay with a value of τF=0.7ns. The electrochemical band gap of the polyrotaxane (3.39eV) is smaller than that of the uncomplexed compound. Wetting properties reveal a change in the distribution of the dispersive and polar components of the surface free energy through the CB7 encapsulation. Atomic force microscopy indicated further that the polyrotaxane film is uniformly distributed over the substrate area with a regular consistency and lower roughness parameters.

Structural, dielectric and electrical analysis of Ba2−xLa4 + 2x/3Ti8O24ceramics system with frequency and temperature

In the present investigation, the study of structural, dielectric, and electrical properties of lanthanum doped barium titanate ceramics with structural formula Ba2−xLa4 + 2x/3Ti8O24 (x = 0.0, 0.2, 0.4, 0.6) was reported. The target samples were obtained by mixed oxide route technique. Preliminary X-ray diffraction studies revealed the single phase formation in the orthorhombic crystal structure. The surface morphology of the samples showed that there were well-defined grains of different sizes and shapes that were homogenously distributed on the surface of samples. The approximate grain size was also calculated. The dielectric properties like dielectric constant (er), loss tangent (tan δ), and AC conductivity (σac) and temperature coefficient of resonant frequency were studied in the temperature range of 298—623 K and frequency range of 10 KHz–1 MHz. Dielectric response showed low temperature dispersion in dielectric properties at low frequency. This showed the space charge phenomenon arising due to defects. DC resistivity measured at room temperature for different compositions decreased with increase in substitution. All four samples gave very small value of temperature coefficient of resonant frequency. Sample with x = 0.6 showed minimum value of τf. Thus, the prepared samples could be the dielectric materials for the practical applications like resonators, filters, antennas, etc. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:1679–1686, 2016

Microwave dielectric properties of double perovskite ceramics [formula omitted

The structural, vibrational and microwave dielectric properties of double perovskite ceramics Ba2Zn1−xCaxWO6(x=0−0.4) were investigated. The samples were sintered at different temperatures in the range 1300–1400°C for 4h. The grown samples were characterized by means of X–ray diffraction, Raman spectroscopy, scanning electron microscopy and energy dispersive X–ray spectroscopy analysis. Microwave dielectric properties of the samples were measured using the TE01δ resonance mode of the cylindrical pellets. The relative permittivity (εr), calculated using Clausius–Mossotti equation is found to be comparable with the experimental results. Our analysis shows that the tolerance factor (t) as well as the temperature coefficient of resonant frequency (τf) of these perovskites decreases with the increase in Ca content. The value of τf is zero for the samples with x = 0.3 and 0.4.

Structural, optical band gap, microwave dielectric properties and dielectric resonant antenna studies of Ba(1−x)La(2x/3)ZrO3 (0 ⩽ x ⩽ 0.1) ceramics

The structural properties of the Ba(1−x)La(2x/3)ZrO3 system were investigated by means of XRD, FT-Raman and FTIR. The XRD analysis demonstrated that lattice parameter gradually decreased with increase of La content. Raman analysis intended the formation of higher degree of symmetry to increase of La3+ content. It was also confirmed from FTIR analysis. The optical band gap was found to be decreased from 3.55eV to 3.01eV with increase of La3+ content. The surface morphology was studied by the SEM. The microwave dielectric constant and quality factor were investigated by the method as proposed by Hakki–Coleman. The microwave dielectric constant decreased from 38.40 to 30.47 and the value of τf changed from 384.71ppm/°C to 165.41ppm/°C. The Q×f values increased from 5731 to 9031 with increase of La3+ content.The dielectric resonator antenna (DRA) was investigated experimentally and numerically using a monopole antenna through an infinite ground plane and Ansoft’s high frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA were investigated in the composition between 0⩽x⩽0.1.

Microwave dielectric properties and microstructure of Sm(Mg0.5Ti0.5−xSnx)O3 ceramics

Sm(Mg0.5Ti0.5−xSnx)O3 (x=0.1–0.5) ceramics were prepared by a conventional solid-state ceramic route. The microstructure and microwave dielectric properties were investigated as functions of Sn4+ substitution and the sintering temperature. A pure perovskite structure was obtained for samples with x=0.1–0.4, whereas small amount of second phase Sm2Sn2O7 appears in the sample with x=0.5, which deteriorates the dielectric properties of the ceramics. Sn4+ substitution shows little influence on the sintering densification of the ceramics. With the increase of Sn4+ substitution, the dielectric constant and absolute value of τf for Sm(Mg0.5Ti0.5−xSnx)O3 ceramics decrease and increase, respectively. The Q×f of the ceramics can be improved by appropriate Sn4+ substitution. The Sm(Mg0.5Ti0.2Sn0.3)O3 ceramic sintered at 1625oC for 3h shows excellent microwave dielectric properties (εr=22.0, Q×f=72300GHz, τf =−41.8ppm/°C), indicating that it could be used as a candidate compensate for the material with positive temperature coefficient of resonant frequency.

Structure Stability and Microwave Dielectric Properties of (1 − x ) Ba(Mg1/2 W1/2 ) O3 + x Ba(Y2/3 W1/3 )O3 Ceramics

The structure stabilities of double perovskite ceramics- (1 − x) Ba(Mg1/2W1/2)O3 + xBa(Y2/3W1/3)O3 (0.01 ≤ x ≤ 0.4) have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and Raman spectrometry in this study. The microwave dielectric properties of the ceramics were studied with a network analyzer at the frequency of about 8–11 GHz. The results showed that all the compounds exhibited face-centered cubic perovskite structure. Part of Y3+ and W6+ cations occupied 4a-site and the remaining Y3+ and Mg2+ distributed over 4b-site, respectively, and kept the B-site ratio 1:1 ordered. Local ordering of Y3+/Mg2+ on 4b-site and Y3+/W6+ cations on 4a-site within the short-range scale could be observed with increasing Y-doping content. The decomposition of the double perovskite compound at high temperature was successfully suppressed by doping with Y on B-site. However, Ba2Y0.667WO6 impurity phase appeared when x > 0.1. The optimized dielectric permittivity increased with the increase in Y doping. The optimized Q × f value was remarkably improved with small amount of Y doping (x ≤ 0.02) and reached a maximum value of about 160 000 GHz at x = 0.02 composition. Further increasing in Y doping led to the decrease in Q × f value. All compositions exhibited negative τf values. The absolute value of τf decreased with increasing Y-doping content. Excellent combined microwave dielectric properties with εr = 20, Q × f = 160 000 GHz, and τf = −21 ppm/°C could be obtained for x = 0.02 composition.

Dielectric-Resonant Antenna Studies of Dysprosium Doped Barium Zirconate Ceramic

The structural properties of the Ba(1−x)Dy(2x/3)ZrO3 system were investigated by means of XRD, FT-Raman, and FTIR spectroscopy. XRD analysis exhibited that lattice parameter gradually decreased with increase of Dy content. FT-Raman and FTIR analyses showed that with increase of Dy content more ordered structure was formed in the composition. The surface morphology of sintered pellets was studied by the SEM. The microwave dielectric constant and quality factor were investigated by the method as proposed by Hakki–Coleman. The microwave dielectric constants of sintered pellets were found to be decreased from 38.40 to 21.13, and the value of τf changed from 384.71 to 62.71 ppm/°C. The Q × f values changed from 5731 to 5173 in the composition between x = 0.0 and 0.1. The dielectric resonator antenna (DRA) was investigated experimentally and numerically, using a monopole antenna through an infiniteground plane and Ansoft’s high-frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA were investigated in the composition between 0 ≤ x ≤ 0.1.

Structural, optical and microwave dielectric properties of Ba1−xSrxWO4 ceramics prepared by solid state reaction route

In this paper, Barium Strontium Tungstate (Ba1−xSrx)WO4 crystals with (x = 0; 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0) were prepared by standard wet milling ceramic preparation method. These crystals were structurally characterized by X-ray diffraction (XRD), Fourier transform Raman (FT-Raman) and Fourier transform infrared (FT-IR) spectroscopic techniques. The shape, growth and average crystal size distribution of these crystals were investigated by a scanning electron microscope (SEM). Their optical properties were investigated by ultraviolet visible (UV–vis) absorption and photoluminescence (PL) measurements. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all the crystals present a scheelite-type tetragonal structure without deleterious phases. FT-Raman spectra exhibited 6 Raman active modes in range from 100 to 1000 cm−1, while the FT-IR spectra presented 2 infrared active modes in range from 500 to 1000 cm−1. SEM micrographs showed well sintered BaWO4 grains, while the substitution of Sr induced modifications in the shape and reduction in the grain size. UV–vis absorption measurements evidenced an increase in the values of the optical band gap (from 4.36 to 4.53 eV) with the increase of Sr into BaWO4 lattice. Dielectric constant, temperature coefficient of resonant frequency (τf), quality factors were measured with Hakki–Coleman technique. The value of τf found −43.68 ppm/°C for BaWO4 which increased to −21.40 ppm/°C for the SrWO4.

Resonant frequency temperature stability of CaTiO3 based microwave dielectric ceramics

The determinants of resonant frequency temperature coefficient (τf) have been analyzed by the approximate treatment of Clausius-Mossotti equation. It is suggested that the value of τf can be adjusted by changing the contribution proportion of ionic polarization or electronic polarization for the dielectric constant. Results of electronic structure calculation and tolerance factor analysis show that B-site substitution of CaTiO3 with (Zn1/3Nb2/3)4+ could turn the value of τf from positive to negative, by enhancing the covalency in the BO6 octahedron and improving the contribution proportion of electronic polarization. Ca[(Zn1/3Nb2/3)xTi(1-x)]O3 dielectric ceramics were prepared by solid-state reaction method with niobate as precursor. Results of structure analysis and property measurement conform to the theoretical analysis. The Ca[(Zn1/3Nb2/3)0.7Ti0.3]O3 dielectric ceramic with near-zero τf was obtained.

Structural, microwave dielectric properties and dielectric resonator antenna studies of Sr(ZrxTi1−x)O3 ceramics

Compositionally induced phase transitions in the system Sr(ZrxTi1−x)O3 were analyzed using a combination of X-ray diffraction, FT-Raman and FTIR spectroscopy. Sr(ZrxTi1−x)O3 system showed at least two tilting of phase transitions, pm3m–I4mcm and I4/mcm–pnma. The structural transition occurred due to tilting of BO6 octahedra. Dielectric constant measured with Hakki–Coleman technique decreased from 253 to 25 with increase of Zr content. The value of τf found 1771ppm/°C for SrTiO3 which decreased to −82ppm/°C for the SrZrO3. The dielectric resonator antenna (DRA) was investigated experimentally and numerically using a monopole antenna through an infinite ground plane and Ansoft's high frequency structure simulator software, respectively. The required resonance frequency and bandwidth of DRA were investigated in the composition between 0≤x≤1.0.

Temperature compensated Sr2Al2SiO7 ceramic for microwave applications

The Sr–Gehlenite (Sr2Al2SiO7) ceramic has been prepared by the conventional solid-state ceramic route. Phase pure Sr2Al2SiO7 (SAS) ceramic sintered at 1525°C for 4h has ɛr=7.2 and Qu×f=33,000GHz. The SAS showed large negative τf of −37.0ppm/°C. A low value of τf was achieved by preparing SAS–CaTiO3 composite. The composite with 0.04 volume fractions (Vf) CaTiO3 sintered at 1500°C for 4h showed good microwave dielectric properties: ɛr=8.6, Qu×f=20,400GHz and τf=+8.5ppm/°C.

Tape Casting and Dielectric Properties of Sr2ZnSi2O7-Based Ceramic-Glass Composite for Low-Temperature Co-fired Ceramics Applications

The Sr2ZnSi2O7 ceramics sintered at 1475°C has ɛr=8.5, Qu×f=105,000 GHz (at 12.63 GHz), and τf=−51.5 ppm/°C. The sintering temperature of Sr2ZnSi2O7 has been lowered to 875°C by the addition of 20Li2O–20MgO–20ZnO–20B2O3–20SiO2 (LMZBS) glass. Addition of a small amount of SrTiO3 lowered the value of τf. Tapes were cast using the composition Sr2ZnSi2O7+15 wt% LMZBS. The stacked and sintered tapes exhibited ɛr∼7 and tan δ∼10−3 at 290 K. The chemical compatibility of Ag with the ceramic–glass composites was also investigated.

Effects of octahedral thickness variance on the temperature coefficient of resonant frequency of the B-site deficient hexagonal perovskites

The temperature coefficient of resonant frequency (τf) and the structure of B-site deficient hexagonal perovskite materials have been assembled with the intention of understanding the structure-property relationship. Based on the refined structure data, the authors proposed a structure-related parameter v to characterize the octahedral thickness variance. Therefore, a correlation between the value of τf and the octahedral thickness variance has been established. This approach illustrates that the octahedral thickness variance plays an important role in the value and sign of τf. It also gives a satisfying explanation for the variation of τf in the Ba5Nb4O15–Ba5Ta4O15 system.

THE EFFECT OF DELAY TIMES ON THE OPTIMAL VELOCITY TRAFFIC FLOW BEHAVIOR

We have numerically investigated the effect of the delay times τf and τs of a mixture of fast and slow vehicles on the fundamental diagram (the current-density relation) of the optimal velocity model. The optimal velocity function of the fast cars depends not only on the headway of each car but also on the headway of the immediately preceding one. It is found that the small delay times have almost no effects, while, for sufficiently large delay time τs, the current profile displays qualitatively five different forms, depending on τf, τs and the fractions df and ds of the fast and slow cars, respectively. The velocity (current) exhibits first-order transitions at low and/or high densities, from freely moving phase to the congested state, and from congested state to a jamming one, respectively. The minimal current appears in intermediate values of τs. Furthermore, there exist a critical value of τf above which the metastability and hysteresis appear. The spatial-temporal traffic patterns near the congested-jamming first-order transition is also presented.

Microwave Dielectric Properties of RE1−xRE′xTiNbO6 [RE = Pr, Nd, Sm; RE′= Gd, Dy, Y] Ceramics

The microwave dielectric properties of rare‐earth titanium niobates can be tailored by preparing solid‐solution phases between RETiNbO6 [RE = Pr, Nd, Sm] and RE′TiNbO6 [RE′= Gd, Dy, Y] microwave dielectric ceramics. Dielectric ceramics based on solid‐solution phases RE1−xRE′xTiNbO6 are prepared by the conventional solid‐state ceramic route for different values of x. The ceramic samples are characterized by XRD, SEM, and microwave methods. Ceramics based on RETiNbO6 belonging to aeschynite group show a positive value of τf, and those based on RE′TiNbO6 belonging to the euxenite group have a negative value of τf. The solid‐solution phases between aeschynites and euxenites show intermediate dielectric constant and τf values. The results indicate the possibility of tailoring the microwave dielectric properties by varying the composition of solid‐solution phases. The range of solid solubility of euxenites in aeschynites and aeschynites in euxenites are different for different rare‐earth ions. The ceramics have low sinterability and a high dielectric loss factor near the vicinity of aeschynite to euxenite transition. The value of dielectric constant changes abruptly close to the transition point. The abrupt variation in microwave dielectric properties indicate a first‐order phase transition at x= 0.86 in Pr1−x Gd xTiNbO6, x= 0.48 in Nd1−x Dy xTiNbO6 and x= 0.27 in Sm1−xY x TiNbO6 ceramics. The dielectric constant and the sign of τf of the solid‐solution phases are found to depend on the average ionic radius of the rare‐earth ions in RE1−xRE′xTiNbO6. The euxenite to aeschynite phase transition occurs at an average ionic radius of 0.945 Å in all three systems of solid‐solution phases.

Viscoelastic Behavior of a Supramolecular Polymeric System Consisting of Tri-3,7-dimethyloctyl cis-1,3,5-cyclohexanetricarboxamide and n-Decane

Viscoelastic behavior of a supramolecular polymeric system, so-called an organogel, consisting of tri-3,7-dimethyloctyl cis-1,3,5-cyclohexanetricarboxamide (DO3CH) and n-decane (C10) was examined varying the concentration of DO3CH (c) and temperature ranged from 20 to 50°C. The obtained storage and loss moduli for the system were well described with the sum of two Maxwell models possessing two sets of a relaxation time and strength: τf and Gf, and τs (>τf) and Gs, whereas the viscoelastic behavior of organogels consisting of N, N', N"-tris(3,7-dimethyloctyl)benzene-1,3,5-tricarboxamide (DO3B) and n-alkanes is well described with only one Maxwell model. The value of Gf is proportional to c2 as observed in entangled flexible polymer systems and the organogels of DO3B. On the contrary, the value of Gs is approximately proportional to c1.3 similarly to the behavior predicted for rigid rodlike polymer solutions. The value of τf is essentially independent of c, while that of τs is kept at a constant and is followed by increasing above c = 10 gL−1. The activation energies of both the relaxation times are less than that for the viscosity of C10. Supramolecular polymeric structure bearing 2-type, rigid rodlike and flexible, portions are generated in the system due to intermolecular hydrogen bonding. The fast relaxation mode is attributed to the entanglement release between the flexible portions as observed in the organogels of DO3B, and the other is attributed to rotational relaxation of the rigid rodlike portion.