Complex permittivity and permeability of Co-substituted NiCuZn ferrite at rf and microwave frequencies

NiCuZn ferrite has recently attracted a lot of attention for its application in high frequency (up to a few GHz) multilayer chip inductors (MLCIs) and for other microwave devices owing to their favorable electromagnetic properties and low densification temperature. In order to study the effect of substitution of cations by cobalt in small concentration on the dielectric and magnetic properties at low and high frequencies, bulk polycrystalline ferrite samples of starting composition (Ni_0.2Cu_0.2Zn_0.6)_{1 − x} Co _x Fe₂O₄, having x = 0, 0.01, 0.03 and 0.05, were prepared by citrate precursor method. Pure spinel (cubic) ferrite formation was confirmed by powder X-ray diffraction technique. Complex permittivity and permeability were measured at microwave frequencies (X-band) using the cavity perturbation method, which is a non-contact method. The values of real part of permittivity (ε ′) vary in the range 7–9.6 and of the imaginary part (ε ″) vary from 0.020–0.120, whereas real part of the permeability (μ′) lies in the range 2.6–14.0 and the imaginary part of permeability (μ″) varies from 0.5–6.0. It is observed that there is an increase in μ′ and decrease in the magnetic loss (tan δ _μ) on increasing the cobalt concentration from x = 0 to x = 0.05. The variation of these parameters, both with frequency in X-band and with the cobalt concentration, is discussed in this paper.     

Magnetic and microwave absorbing properties of Ti and Co substituted M-hexaferrites in Ka-band frequencies (26.5 ~ 40 GHz)

Magnetic and microwave absorbing properties of M-hexaferrites have been investigated in Ka-band frequencies (26.5 ~ 40 GHz) with substitution of Ti⁺⁴ and Co⁺² ions for Fe⁺³ sites. The powders of composition BaFe_12-2xTi_xCo_xO₁₉ were prepared by conventional ceramic processing, where the substitution ratio x was varied from 0.0 to 2.0. With the calcined powders, polycrystalline bulk and composite sheet samples were prepared for the evaluation of magnetic and microwave absorbing properties. VSM measurements indicate that the saturation magnetization and coercivity of the M-hexaferrites decrease with the substitution of Ti and Co, which is attributed to the reduction in crystal anisotropy field by changing of easy-axis of magnetization from c-axis to basal plane. Ferromagnetic resonance frequency with the peak magnetic loss is decreased sensitively with the substitution of Ti and Co, which is well consistent with ferromagnetic resonance theory. Theoretical estimation of matching frequency and matching thickness for zero wave reflection is made using the solution map of impedance matching. A wide bandwidth of microwave absorption is predicted in the composite samples containing the ferrite powders of x = 0.5 in Ka-band frequencies.     

Influence of substitution on dielectric diffuseness in Ba<Subscript>(1-x)</Subscript>Bi<Subscript>(2+2x/3)</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics prepared by chemical precursor decomposition method

Barium bismuth niobate, Ba_(1-x)Bi_(2+2x/3)Nb₂O₉ (BBN with x = 0.0, 0.1, 0.2, 0.3, 0.4) ceramic powders in the nanometer range were prepared by chemical precursor decomposition method (CPD). The single phase layered perovskite was prepared throughout the composition range studied. No intermediate phase was found during heat treatment at and above 600°C. The crystallite size and the particle size, obtained from XRD and TEM respectively, were in the range of 15–30 nm. The addition of Bi₂O₃ substantially improved the sinterability associated with high density (96%) which was otherwise difficult in the case of pure BaBi₂Nb₂O₉ (BBN x = 0.0). The sintering was done at 900°C for 4 h. The relative permittivity of BBN ceramics at both room temperature and in the vicinity of the temperature of maximum permittivity (T_m) has increased significantly with increase in bismuth content and loss is also decreased to a certain level of bismuth doping. T_m increased with increase in Bi₂O₃. The diffuseness (γ) in the phase transition was found to increase from 1.54 to 1.98 with the increase in Ba²⁺ substitution level from x = 0.0 to x = 0.3.     

Structural evolution and microwave dielectric properties of (1 − x)BaZn1/3Nb2/3O3 + xBaMg1/2W1/2O3 ceramics

Structural evolution and microwave dielectric properties of (1 − x)BaZn_1/3Nb_2/3O₃ + xBaMg_1/2W_1/2O₃ (0 ≤ x ≤ 1) system have been investigated in this work. All samples exhibit single perovskite phase except for the samples with x = 0 and x ≥ 0.8 in which barium niobate and BaWO₄ second phase existed, respectively. 1:1 cations ordering existed in the samples with x ≥ 0.1, and the ordering degree increases with the increase of x. Liquid phase sintering was observed in the sample with x ≥ 0.8. Dielectric constant decreases almost linearly from 40.8 to 17.4 with increasing x. Q × f value monotonically increases from 26,162 GHz to 64,705 GHz with increasing x. The τ_f value changes from +30 ppm/°C to −27.8ppm/°C. Near zero τ_f value of −1.4 ppm/°C could be obtained at x = 0.4 composition.     

Investigation of La-doped 0.25Pb(Zn1/3Nb2/3)O3-0.75Pb(ZrxTi1-x)O3 ceramics near morphotropic phase boundary

La-doped 0.25Pb(Zn_1/3Nb_2/3)O₃-0.75Pb(Zr_xTi_1-x)O₃ (x = 0.51–0.57) ceramics were prepared by a conventional mixed oxide method. The piezoelectric and dielectric properties of various compositions near the morphotropic phase boundary (MPB) were investigated. The MPB was determined by X-ray diffraction (XRD) patterns. The optimal properties were obtained in the specimen with Zr/Ti = 53/47: d₃₃ = 570 pC/N, k_p = 0.60, ε_r = 3900, tgδ = 1.9%, T_c = 204°C, P_r = 35.21 μC/cm², E_c = 11.62 kV/cm, which means it is a good candidate for applications.     

Preparation, characterization and dielectric study of new ferroelectric relaxors crystallizing with the tetragonal tungsten bronze structural type

The replacement of sodium and niobium by lanthanum and iron in Pb₂NaNb₅O₁₅, have allowed to prepare the new solid state solution Pb₂Na_1−x La _x Nb_5−x Fe _x 0₁₅ (0 ≤ x ≤ 1), crystallising with the tungsten bronze structure. The latter synthesized, in air, by solid state reaction, has been studied by means of X-ray and dielectric measurements. The composition dependencies of ferroelectric—paraelectric transition temperature and the lattice parameters are reported. The dielectric constants ɛ′_r and ɛ″_r have been also investigated as a function of temperature in the range 20°–700°C, and as a function of frequency in the range 1 k–13 MHz. In the composition range 0 ≤ x ≤ 0.35, the materials exhibit a typical ferroelectric–paraelectric transition while, in the composition range 0.35 < x ≤ 1, a relaxor effect has been evidenced.     

Sintering behavior, structures and microwave dielectric properties of a rutile solid solution system: (AxNb2x)Ti1–3xO2 (A=Cu, Ni)

The sintering behavior, structures and microwave dielectric properties in a rutile solid solution system—(A_xNb_2x)Ti_1–3xO₂ (A=Cu, Ni)—were investigated and the samples were prepared by conventional solid state reaction method. Single phase of tetragonal rutile structure has been obtained through the entire range of compositions (0.02 ≤ x ≤ 0.20). The sintering temperature was lowered to 900°C by (Cu _{x /3}Nb_2x/3)⁴⁺ substituting for Ti⁴⁺ in the solid solution. Comparing with that of rutile TiO₂ (465 ppm/°C), the temperature coefficient of resonant frequency (TCF) of the rutile solid solution is much lower (about 250 ppm/°C), and the dielectric constant and the quality factor (Qf value) of the solid solution are about 70~80 and 7,000G Hz. The substitution of (Cu _{x /3}Nb_2x/3)⁴⁺or (Ni _{x /3}Nb_2x/3)⁴⁺ for Ti⁴⁺ in the solid solution improved the microwave dielectric properties of the rutile TiO₂ ceramics.     

Ni–Mn–Fe–Cr–O negative temperature coefficient thermistor compositions: Correlation between processing conditions and electrical characteristics

A study has been carried out to correlate the effect of sintering temperature on the microstructural, electrical and reliability aspects of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) negative temperature coefficient thermistor compositions prepared by solid-state route. The calcined and sintered compositions were characterized by X-ray diffraction and Scanning Electron Microscopy. The existence of cubic spinel single-phase region was determined by sintering Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ samples in air at temperatures 1150 to 1250 °C. X-ray diffraction patterns of samples sintered above 1200 °C shows additional Bragg reflections of a rock salt structured NiO phase besides normal cubic spinel. A maximum B-value of 4044 K was obtained for Ni_0.75Mn_1.95Cr_0.25Fe_0.05O₄ composition at a sintering temperature 1250 °C/3 h. The reliability of the thermistor compositions were evaluated by performing accelerated ageing based on thermal cycling test. We found that chromium enhances the reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) based NTC thermistor compositions. A maximum reliability of +0.25% resistance drift was observed at sintering temperature 1200 °C for 0.25 mol% chromium content. Excellent reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ NTC thermistor compositions makes it ideal candidates for high-performance thermal sensor applications.     

Magnetic properties of Ni-Zn ferrites prepared by microwave sintering method

The effect of zinc ion substitution for nickel on structural and magnetic properties of NiZn ferrites is reported. The spinel ferrite system Ni_1-xZn_xFe₂O₄ with x = 0.2, 0.3, 0.4 and 0.5 was prepared by microwave sintering method. The uniaxially pressed samples were sintered at various temperatures such as 900°C, 1000°C and 1100°C for 30 min. X-ray diffraction patterns of the samples indicate the formation of single-phase cubic spinel structure. SEM micrographs show that grain size increases with increasing zinc content and sintering temperature. The elemental composition of these ferrites was analyzed by EDS. Lattice constant increases with increase in zinc content, obeying Vegard’s law. The effect of composition and sintering temperature on initial permeability as the function of frequency and temperature was studied. The initial permeability of NiZn ferrite increases greatly with increasing Zn content and sintering temperature. The dependence of initial permeability with respect to temperature shows the decrease in the Curie point with increase in zinc content, is the normal behavior of ferrites. The relative loss factor (tand

Structural and ferroelectric characterization of BMZ–BF–PT ceramics

Bismuth containing crystalline solid solutions of [(BiMgZrO₃)_1 − y –(BiFeO₃) _y ] _x –(PbTiO₃)_1 − x , (BMZ–BF–PT) have been synthesized by high temperature solid-state reaction technique. The crystalline symmetry varied with the composition, indicating good solid state solubility of BMZ and BF with PT. X-ray diffraction (XRD) reveals that BMZ–PT and BMZ–BF–PT have a single-phase perovskite structure. The MPB of BMZ–PT lies in the region x = 0.55 to x = 0.6, which is supported by the transformation from rhombohedral to tetragonal phase. The SEM photographs exhibit the uniform distribution of grains in the matrix. Polarization–electric field hysteresis studies were carried out at Room Temperature for all the compositions up to an applied electric field of 3.5 kV/mm. It was found that with the increase of BiFeO₃ content in the composition the remnant polarization decreases and the ferroelectric loops get constricted. Variation of dielectric constant with temperature shows that there is an increase in Transition temperature (T _C) with the increase in applied frequency indicating a relaxor characteristic of the compound. Our results show that BMZ–BF–PT is a good low-lead (Pb) high-temperature ferroelectric ceramic.     

Yttrium iron garnet ceramic prepared from microwave-induced combustion

Yttrium iron garnet (YIG) nano-powders were successfully synthesized by microwave-induced combustion process. The as-prepared YIG powders were annealed at different temperature and investigated by XRD, SEM, and VSM. The as-received YIG powder shows the formation of garnet structure with saturation magnetization (M _s) of 12 emu/g, whereas upon annealing at 825 °C for 2 h, the saturation magnetization increases to 27 emu/g. The as-received yttrium iron garnet powders annealed at several temperatures revealed that the particle size ranged from 65 to 90 nm. Moreover, the linewidth of magnetic resonance peak (ΔH), complex permeability, and loss tangent of sintered YIG ceramic are also examined in this study.     

Fabrication,property and application of novel pyroelectric single crystals—PMN–PT

The pyroelectric properties and temperature stability of Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃(PMN–xPT) single crystals (0.13 ≤ × ≤ 0.40) were investigated. The best choice for pyroelectric performance is [111]-oriented PMN–0.26PT single crystal whose figures of merit for voltage responsivity and detectivity are 0.11 m²/C and 15 × 10⁻⁵ Pa^−1/2, respectively. However, the [001]-oriented PMN–0.37PT single crystal has much better temperature stability, whose temperature coefficient of pyroelectric property is 0.5%/K in the range of 20 °C to 55 °C, and Curie temperature is high: 175 °C. We also found that PMN–xPT possessed low thermal diffusivity D ~ 4.4 × 10⁻⁷ m²/s, low volume specific heat C _v~ 2.5 × 10⁶ J/m³ K and tunable permittivity ε ~ (300–7000). The pyroelectric performances of PMN–xPT single crystals are superior to those of conventional pyroelectric materials and promising for IR device applications.     

Effect of Eu-doping on the magnetic and magnetoresistive properties of La0.65Ca0.35MnO3

The effects of Eu-doping in the A-site on the magnetic and magnetoresistive properties of La_0.65-xEu_xCa_0.35MnO₃ (x = 0, 0.05, 0.10, 0.15) bulk samples have been investigated. All samples have a single perovskite structure. The Curie temperature T _C decreases with increasing Eu content from 268 K for x = 0 to 141 K for x = 0.15. The magnetoresistivity increases with increasing Eu content. Upon an application of a magnetic field of 2.0 T the maximum magnetoresistivity of 97.3% for the La_0.50Eu_0.15Sr_0.35MnO₃ was obtained. It provides an effective way to obtain the high magnetoresistance in the polycrystalline manganites.     

Thermal expansion and electrochemical properties of La0.7AE0.3CuO3−δ (AE=Ca, Sr, Ba) cathode materials for IT-SOFCs

Alkaline earth metals (Ca, Sr, Ba) substituted lanthanum copper oxides were investigated to evaluate their potentials as cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). The crystal structure, thermal expansion and electrochemical performance of La_1−x AE _x CuO_3−δ (x = 0.3, AE=Ca, Sr, Ba) were studied by X-ray diffraction, thermal dilatometer and impedance spectra, respectively. By lowering the size of the A-site cation in ABO₃ perovskite, a lower thermal expansion and polarization resistance were obtained forCa-doped LaCuO_3−δ cathode which showed an area specific resistance of 0.19 Ω cm⁻² under open circuit potential conditions at 800°C, and a polarization overpotential of 52 mV at a current density of 0.1 A/cm² at 700°C, being a potential candidate of cathode material for IT-SOFCs.     

Synthesis of (Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>)(Ti<Subscript>0.5</Subscript>Zr<Subscript>0.5</Subscript>)O<Subscript>3</Subscript> ceramics and effect of Sr content on room temperature dielectric properties

The compositions in the system (Ba_1−x Sr _x )(Ti_0.5Zr_0.5)O₃ with different Sr (x) content, were synthesized through solid oxide reaction route. The phase formation behaviors in the system were investigated by XRD. The room temperature dielectric properties of the compositions were investigated in the frequency range 10 Hz to 13 MHz. The solid solution system Ba_1−x Sr _x Ti_0.5Zr_0.5O₃ remains as cubic perovskite up to x < 0.6 and transforms into the tetragonal structure above x > 0.6. Composition with x = 0.6 contains a mixture of cubic and tetragonal phases with broadened diffraction pattern. It is observed that the increasing of Sr substitution results in the decreasing of bulk density, average grain size and dielectric constants etc. in the composition system. The AC dielectric conductivity of the ceramics also decreases with the increase in Sr-substitution due to decrease in loss as well as grain size with that substitution.     

Sintering and compositional effects on the microwave dielectric characteristics of Mg(Ta1−x Nb x )2O6 ceramics with 0.25 ≦ x ≦ 0.35

1,500 °C−sintered MgTa₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 30.5, Q × f = 56,900 GHz, and τ _f = 28.3 ppm/°C, whereas 1,400 °C-sintered MgNb₂O₆ ceramic exhibits microwave dielectric characteristics of ɛ _r = 21.7, Q × f = 89,900 GHz, and τ _f = −68.5 ppm/°C. In order to find the dielectric resonators with τ _f value close to 0 ppm/°C, the effects of sintering condition and composition on the microwave dielectric characteristics of Mg(Ta_1−x Nb _x )₂O₆ ceramics (0.25 ≦ x ≦ 0.35) prepared under sintering temperature of 1,300–1,450 °C are investigated. The results show that as the sintering temperature increases from 1,300 to 1,450 °C, the ɛ _r, Q × f and τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics all increase and saturate at 1,450 °C. On the other hand, as the Nb₂O₅ content decreases, the τ _f values of Mg(Ta_1−x Nb _x )₂O₆ ceramics will shift to near 0 ppm/°C. The optimized sintering conditions and composition to obtain the Mg(Ta_1−x Nb _x )₂O₆ dielectrics with τ _f close to 0 ppm/°C are sintering temperature of 1,450 °C, sintering duration of 4 h, and composition of x = 0.25, which exhibits the microwave dielectric characteristics of ɛ _r = 27.9, Q × f = 33,100 GHz, and τ _f = −0.7 ppm/°C.     

The electronic conductivity of Ba0.5Sr0.5CoxFe1?xO3?δ (BSCF: x?=?0?～?1.0) under different oxygen partial pressures

The electronic conductivity of sintered BSCF ceramics (Ba_0.5Sr_0.5Co _x Fe_1−x O_3−δ, 0 ≤  x  ≤ 1) was measured as a function of temperature up to 1273 K in air. The conductivity of BSC is thermally activated over 298–1273 K with an activation energy of 0.21 eV. The conductivity of BSF and BSCF (0.2 ≤  x  ≤ 0.8) is thermally activated below ∼673 K with activation energies of 0.21 eV–0.40 eV. Above 673 K, the formation of oxygen vacancies results in a decrease in p-type carrier concentration and a decrease in electronic conductivity. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF5582) was also measured under 10⁻⁵ atm ≤ pO₂ ≤ 1 atm. Below ∼673 K, the electronic conductivity of BSCF 5582 shows no dependence on pO₂. Above 673 K, the conductivity of BSCF5582 increases with increasing pO₂ for pO₂ ≥ 0.01 (p-type conduction) and decreases slightly with increasing pO₂ for pO₂  ≤ 0.01 atm. The activation energy for conduction above ∼673 K and at pO₂ ≥ 0.1 is ∼0.07 eV. Above ∼823K and at pO₂ ≥ 0.01 atm, the activation energy for conduction is ∼0.2 eV.     

Improved magnetic properties of bismuth ferrite ceramics by La and Gd co-substitution

Increased magnetic properties of La and Gd substituted bismuth ferrite (Bi_0.9La_0.1Fe_1-xGd_xO₃) (BLFGO) ceramics are reported. Considering perovskite structure of bismuth ferrite (BiFeO₃), Bi and Fe sites were partially substituted by La and Gd, respectively. These materials were synthesized by conventional solid state reaction method. Crystal structure and phase purity were confirmed by X-ray diffraction and Raman scattering spectroscopy at room temperature. A considerable improved ferromagnetic properties with double remnant magnetization of 0.184 emu/g was observed by increasing Gd ratio up to 5%. With different ionic sizes and due to magnetic moment of Gd, an induced deformation of spin cycloid structure had thereby resulted in net magnetization. Also, we monitor some decrease in dielectric loss upon La and Gd substitutions. Additionally, these ceramics showed significant magnetoelectric coupling. Such improvements on magnetic, insulation, and magnetoelectric properties demonstrated the potential of BLFGO for possible multiferroic device applications.     

Effect of heating rate on the structure evolution of (K0.5Na0.5)NbO3–LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃–xLiNbO₃ (abbreviated an KNLN) have been synthesized by traditional ceramics process. Effects of heating rate on the phase structure, microstructure evolution and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃–xLiNbO₃ were investigated. Results show that the heating rate has no effects on the phase structures. However, the fracture surface of the 0.94(K_0.5Na_0.5)NbO₃ −0.06 LiNbO₃ ceramics transforms from intergranular fracture mode to a typical transgranular fracture mode with the increasing of the heating rate. This result is ascribed to the presence of agglomerates of grains which exhibit different sintering behavior at diverse heating rates. The 0.94(K_0.5Na_0.5)NbO₃–0.06LiNbO₃ ceramic sintered at 1080°C with heating rate of 5°C/min shows the optimum piezoelectric properties(d ₃₃ = 210 pC/N, k _p = 0.403 and k _t = 0.498).