Microwave dielectric properties of the 5.5Li<Subscript>2</Subscript>O–Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>–7TiO<Subscript>2</Subscript> ceramics

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 5.5:1:7 was prepared by solid state reaction route. The phase and structure of the ceramic were characterized by X-ray diffraction and scanning electron microscopy (SEM). The microwave dielectric properties of the ceramics were studied using a network analyzer. The microwave dielectric ceramic has low sintering temperature (∼1075°C) and good microwave dielectric properties of ε _r=42, Q×f=16900 GHz (5.75 GHz), and τ _f =63.7 ppm/°C. The addition of B₂O₃ can effectively lower the sintering temperature from 1075 to 875°C and does not induce degradation of the microwave dielectric properties. Obviously, the LNT ceramics can be applied to microwave low temperature-cofired ceramics (LTCC) devices.     

Microwave dielectric properties of (1 − x)(Mg0.95Co0.05)TiO3–x(Na0.5La0.5)TiO3 ceramic system

The microstructures and the microwave dielectric properties of the (1 − x)(Mg_0.95Co_0.05)TiO₃–x(Na_0.5La_0.5)TiO₃ ceramic system were investigated. Two-phase system was confirmed by the XRD patterns and the EDX analysis. A co-existed second phase (Mg_0.95Co_0.05)Ti₂O₅ was also detected. The microwave dielectric properties are strongly related to the density and the matrix of the specimen. A new microwave dielectric material 0.88(Mg_0.95Co_0.05)TiO₃–0.12(Na_0.5La_0.5)TiO₃, possessing an excellent combination of dielectric properties: ε_r  22.36, Q × f  110,000 GHz (at 9 GHz), τ_f  2.9 ppm/°C), is proposed as a candidate dielectric for GPS patch antennas.     

Microwave dielectric properties of high quality factor La(Mg0.5−xCaxSn0.5)O3 ceramics

The microwave dielectric properties of La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were examined with a view to their exploitation for wireless communications. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The La(Mg_0.5−xCa_xSn_0.5)O₃ ceramics contained Ca₂SnO_4, CaSnO₃, and La₂O₃. The amount of Ca₂SnO₄ increased with increasing sintering temperature. However, the relative amount of CaSnO₃ decreased with increasing sintering temperature. An apparent density of 6.52 g/cm³, a dielectric constant (ε_r) of 20.2, a quality factor (Q×f) of 80,500 GHz, and a temperature coefficient of resonant frequency (τ_f) of −79 ppm/°C were obtained for La(Mg_0.4Ca_0.1Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h.     

Dielectric properties of Mg0.95Ni0.05TiO3 ceramic modified by Nd0.5Na0.5TiO3 at microwave frequencies

The effect of Nd_0.5Na_0.5TiO₃ addition on the microwave dielectric properties of Mg_0.95Ni_0.05TiO₃ ceramics was investigated. The Nd_0.5Na_0.5TiO₃ content plays a significant role in improving the dielectric properties. Microwave dielectric measurements show an increase in the dielectric constant (ε_r) and temperature coefficient of the resonant frequency (τ_f) and a decrease in the quality factor (Q × f value) with increasing Nd_0.5Na_0.5TiO₃ content. By properly adjusting the Nd_0.5Na_0.5TiO₃ content, a zero τ_f value can also be obtained, which make Mg_0.95Ni_0.05TiO₃ a promising material in wireless systems. At 1300 °C, the 0.81Mg_0.95Ni_0.05TiO₃–0.19Nd_0.5Na_0.5TiO₃ ceramics possess excellent microwave dielectric properties: a dielectric constant (ε_r) of 25.61, a Q × f value of 69,100 GHz, and a τ_f value of −6 ppm/°C.     

Structure evolution and microwave dielectric response of (Ca0.5+xSr0.5−x)[(Al0.5Nb0.5)0.5Ti0.5]O3 solid solutions

The phase assemblage, crystal structure evolution and microwave dielectric response of (Ca_0.5+xSr_0.5−x)[(Al_0.5Nb_0.5)_0.5Ti_0.5]O₃ ceramics (abbreviated as CSANT hereafter) are investigated. Single perovskite solid solution is formed in the CSANT ceramics in Sr-rich composition range of x < −0.05, however, Ca₄Ti₃O₁₀-type layered perovskite phase begins to segregate after x = −0.05. The CSANT perovskites crystallized in Fm3m cubic symmetry in the composition range of x ≤ −0.2, however, as the Ca²⁺ content in A-site increased, the oxygen octahedral began to be anti-phase tilted at x = −0.1 and the crystal structure transited to P2₁/n pseudo-orthorhombic space group thereafter. The microwave dielectric response of the CSANT ceramics is elaborately discussed in terms of their crystallographic structure and chemical composition. When sintered at 1500 °C for 4 h, a dielectric constant ɛ_r of 52.5, a Qf product of 28000 GHz and a τ_f of +25.4 ppm/°C microwave dielectric ceramic can be obtained in the CSANT ceramics at x = 0.3.     

Low-temperature sintering and microwave dielectric properties of Ca2Zn4Ti15O36 ceramics

The sintering behavior, microstructures, and microwave dielectric properties of Ca₂Zn₄Ti₁₅O₃₆ ceramics with B₂O₃ addition were investigated. The crystalline phases and microstructures of Ca₂Zn₄Ti₁₅O₃₆ ceramics with 0-10 wt% B₂O₃ addition were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). The sintering temperature of Ca₂Zn₄Ti₁₅O₃₆ ceramic was lowered from 1170 to 930 °C by 10 wt% B₂O₃ addition. Ca₂Zn₄Ti₁₅O₃₆ ceramics with 8 wt% B₂O₃ addition sintered at 990 °C for 2 h exhibited good microwave dielectric properties, i.e., a quality factor (Qf) 11,400 GHz, a relative dielectric constant (ε_r) 41.5, and a temperature coefficient of resonant frequency (τ_f) 94.4 ppm/°C.     

Preparation and characterization of CaTiO 3 –(Li 1/2 Nd 1/4 Sm 1/4 )TiO 3 microwave ceramics by a sol–gel combustion process

A novel sol–gel combustion process was adopted to synthesize reactive ceramic powder with composition of 0.25 CaTiO₃–0.75 (Li_1/2Nd_1/4Sm_1/4)TiO₃, and microwave dielectric ceramics were prepared at different sintering temperatures using the synthesized powder. The combustion behavior of citrate gel and the sintering feature of the synthesized powder were evaluated by using differential thermal analysis–thermogravimetric analysis and thermo-mechanical analysis techniques, respectively. The citrate gel exhibits a self-propagating behavior after being ignited in air at room temperature. The as-burnt powder is so highly reactive that it can be transformed into single-phase perovskite at 900 °C and it can be sintered at 1100 °C. The effects of sintering temperature on the density, microstructure, and dielectric properties of the sintered ceramics were investigated. The maximum values of density, dielectric constant, and Q×f were achieved after sintering at 1200 °C. At 1200 °C, very dense ceramics with uniform grains and good dielectric properties with a dielectric constant of 123.8, a Q×f value of 5110 (at 3.7 GHz), and a τ_f value of +12.5 ppm/°C were achieved via the sol–gel combustion route. PACS 77.84.Dy; 81.20.FW; 77.22.-d     

Microstructures and microwave dielectric properties of La1-xBx(Mg0.5Sn0.5)O3 ceramics

The microwave dielectric properties of La_1-xB_x(Mg_0.5Sn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La_1-xB_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.995B_0.005(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. A maximum apparent density of 6.58 g/cm³, a dielectric constant (ε_r) of 19.8, a quality factor (Q × f) of 41,800 GHz, and a temperature coefficient of resonant frequency (τ_f) of −86 ppm/°C were obtained for La_0.995B_0.005(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h.     

Thermogalvanic power and fast ion conduction in δ-Bi2O3 and δ-(Bi2O3)1−x(R2O3)x with R = Y, Tb---Lu

The thermogalvanic power (Seebeck coefficient) of O^2- conducting δ-Bi₂O₃ and δ-(Bi₂O₃)_1−x(Y₂O₃)_x has been measured directly as a function of temperature and partial oxygen pressure in N₂---O₂ mixtures. The of δ-(Bi₂O₃)_0.75(R₂O₃)_0.25 with R = Tb---Lu was indirectly determined using an isothermal concentration cell technique. Except for pure δ-Bi₂O₃, the heat of transport is much smaller than the activation energy for O^2- conduction for all materials. The vibrational freedom of O²⁻ ions in all δ-stabilized materials is reflected in their IR spectra at room temperature. Two prototypes of a thermogalvanic P_O2 meter were tested.     

Remarkable influence on the dielectric and magnetic properties of lithium ferrite by Ti and Zn substitution

The effect of Zn and Ti substitution on the magnetic and electrical properties of Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄ ferrites (x=0.0 to 0.30 in steps of 0.05) +0.5wt% Bi₂O₃ prepared by a standard ceramic technique has been investigated. Electrical conductivity and dielectric measurements at different temperatures from 300 K to 700 K in the frequency range from 100 Hz to 2 MHz have been analysed. The variation of the real part of dielectric constant (ε^′) and loss tangent (tanδ) with frequency and temperature has been studied; it follows the Maxwell–Wagner model based on the interfacial polarization in consonance with the Koops phenomenological theory. It is found that the permittivity of zinc and titanium substituted lithium ferrite improves and shows a maximum value ( 1.5×10⁵) at 100 Hz for the x=0.25 sample. The dielectric transition temperature (T_d) depends on the concentration of Ti and Zn in Li_0.5Zn_xTi_xMn_0.05Fe_2.45−2xO₄. The saturation magnetization and Curie temperature both decrease with increase in the concentration of Ti and Zn in the ferrite.     

Effect of the concentration of precursors on the microwave absorbent properties of Zn/Fe oxide nanopowders

Zn/Fe oxide compound powders were obtained by the hydrothermal method using ferric nitrate Fe(NO₃)₃·9H₂O and zinc nitrate Zn(NO₃)₂·6H₂O at 200 °C and different precursor molar ratios x = Fe³⁺/Zn²⁺ equal to 2.8/0.2, 2.5/0.5, 1.8/1.2 and 1.5/1.5. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray analysis (SEM–EDAX). Room temperature measurements of the frequency dependence of the complex magnetic permeability and complex dielectric permittivity, over the frequency range from 0.1 to 6 GHz, were performed. For precursor molar ratios x = 2.8/0.2, x = 1.8/1.2 and x = 1.5/1.5 the obtained samples showed a ferromagnetic-like resonance behaviour. This behaviour was assigned to the prevalent compounds in the obtained samples, Fe₂O₃ (for x = 2.8/0.2) and ZnFe₂O₄ (for x = 1.8/1.2 and x = 1.5/1.5). Based on the magnetic and dielectric measurements, the microwave absorbent properties of the four samples were analysed, and the sample containing mostly of ZnFe₂O₄ (for x = 1.8/1.2) was found to be the best electromagnetic absorber in the frequency range 1.36–6 GHz.     

Enhanced cyclability of triple-metal-doped LiMn2O4 spinel as the cathode material for rechargeable lithium batteries

To improve the cycle performance of spinel LiMn₂O₄ as the cathode of 4-V-class lithium secondary batteries, spinel phases LiM _x Mn_2 − x O₄ (M=Li, Fe, Co; x = 0, 0.05, 0.1, 0.15) and LiFe_0.05M _y Mn_{1.95 − y} O₄ (M=Li, Al, Ni, Co; y = 0.05, 0.1) were successfully prepared using the sol–gel method. The spinel materials were characterized by powder X-ray diffraction (XRD), elemental analysis, and scanning electron microscopy. All the samples exhibited a pure cubic spinel structure without any impurities in the XRD patterns. Electrochemical studies were carried out using the Li|LiM _x Mn_2 − x O₄ (M=Li, Fe, Co; x = 0, 0.05, 0.1, 0.15) and LiFe_0.05M _y Mn_{1.95 − y} O₄ (M=Li, Al, Ni, Co; y = 0.05, 0.1) cells. These cathodes were more tolerant to repeated lithium extraction and insertion than a standard LiMn₂O₄ spinel electrode in spite of a small reduction in the initial capacity. The improvement in cycling performance is attributed to the stabilization in the spinel structure by the doped metal cations.     

Ceramic materials for use in microwave electronics

A study is reported of the structure and electrical properties of BaLn₂Ti₄O₁₂ compositions (BLT), where Ln=La, Nd, or Sm, with calcium, strontium, or lead substituted for barium, and bismuth for the lanthanide. The BLT compound is shown to be characterized by a broad isomorphicity region, at the edge of which form solid solutions with the highest dielectric permittivity reached thus far, up to about 170. A number of thermally stable ceramic materials with a high dielectric permittivity of 80 to 120, which permit construction of microwave dielectric resonators with Q factors of up to 600 for ɛ=120 and up to 3000 for ɛ=80 at f=4 GHz, have been developed based on BLT solid solutions. Fiz. Tverd. Tela (St. Petersburg) 41, 882–884 (May 1999)     

Phase transitions in solid solutions (1 ? x)BaTiO3-xBaMg1/3B2/3O3 (B = Nb, Ta)

The dielectric properties of ceramic samples of (1 − x)BaTiO₃-xBaMg_1/3Nb_2/3O₃ and (1 − x)BaTio₃-xBaMg_1/3Ta_2/3O₃ solid solutions (x = 0–0.25) are investigated in the frequency range from 10 Hz to 100 kHz at temperatures of 77–450 K. It is shown that the (x−T) phase diagrams of these solid solutions at x = 0.05 have a multiphase point at which the lines of all three phase transitions of BaTiO₃ converge. Original Russian Text ? V.G. Zalesskiĭ, V.V. Lemanov, E.P. Smirnova, A.V. Sotnikov, N. V. Zaĭtseva, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 1, pp. 108–112.     

Enhancement microwave dielectric properties of La(Mg0.5Sn0.5)O3 ceramics by substituting La3+ with Sm3+

The microwave dielectric properties of La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics were examined with a view to their exploitation for mobile communication. The La_1?xSm_x(Mg_0.5Sn_0.5)O₃ ceramics were prepared by the conventional solid-state method with various sintering temperatures. The X-ray diffraction patterns of the La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics revealed no significant variation of phase with sintering temperatures. Apparent density of 6.59 g/cm³, dielectric constant (ε_r) of 19.9, quality factor (Q×f) of 70,200 GHz, and temperature coefficient of resonant frequency (τ_f) of ?77 ppm/°C were obtained for La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics that were sintered at 1500 °C for 4 h. The dielectric constant, and τ_f of La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃ ceramics were almost independent with the sintering temperature as the sintering temperature varied from 1450 to 1600 °C.     

Studies on Structural, Magnetic and Thermal Properties of xFe2TiO4-(1−x)Fe3O4 (0≤x≤1) Pseudo-binary System

The xFe₂TiO₄-(1−x)Fe₃O₄ pseudo-binary systems (0≤x≤1) of ulvöspinel component were synthesized by solid-state reaction between ulvöspinel Fe₂TiO₄ precursors and commercial Fe₃O₄ powders in stochiometric proportions. Crystalline structures were determined by X-ray powder diffraction (XRD) and it was found that the as-obtained titanomagnetites maintain an inverse spinel structure. The lattice parameter a of synthesized titanomagnetite increases linearly with the increase in the ulvöspinel component. ⁵⁷Fe room temperature Mössbauer spectra were employed to evaluate the magnetic properties and cation distribution. The hyperfine magnetic field is observed to decrease with increasing Fe₂TiO₄ component. The fraction of Fe²⁺ in both tetrahedral and octahedral sites increases with the increase in Ti⁴⁺ content, due to the substitution and reduction of Fe³⁺ by Ti⁴⁺ that maintains the charge balance in the spinel structure. For x in the range of 0 ≤x≤0.4, the solid solution is ferrimagnetic at room temperature. However, it shows weak ferrimagnetic and paramagnetic behavior for x in the range of 0.4<x≤0.7. When x>0.70, it only shows paramagnetic behavior, with the appearance of quadrupole doublets in the Mössbauer spectra. Simultaneous differential scanning calorimetry and thermogravimetric analysis (DSC-TGA) studies showed that magnetite is not stable, and thermal decomposition of magnetite occurs with weight losses accompanying with exothermic processes under heat treatment in inert atmosphere.     

Complex permittivity, complex permeability and microwave absorption properties of ferrite-paraffin polymer composites

We have investigated the electromagnetic (EM) characteristics of Co_xMn_1−xFe₂O₄ spinel ferrite (where x=0.0, 0.5 and 1.0) nanoparticles (NPs)/paraffin nanocomposite material at 8-20 GHz. Co_xMn_1−xFe₂O₄ NPs have been synthesized by cetyltrimethylammonium assisted hydrothermal route using NaOH. A variation in complex dielectric permittivity and magnetic permeability at room temperature with frequency in the range 8-20 GHz has been studied. Particles showed phase purity and crystallinity in powder X-ray diffraction (XRD) analysis. At the same time, Co_xMn_1−xFe₂O₄ NPs demonstrated a spinel cubic structure from XRD results. A reflection loss of −46.60 dB was found at 10.5 GHz for an absorber thickness of 2 mm. Co_xMn_1−xFe₂O₄ may be attractive candidates for EM wave absorption materials.     

4f-electric quadrupolar anomalies in PrCu2Si2 and its pseudoternary solid solutions

Experimental observations for possible presence of 4f quadrupolar Kondo effect in PrCu₂Si₂ are critically discussed. A comparison of the low temperature properties of the alloys, Pr _x Y_1–x Cu₂Si₂ (x=0.05, 0.2 and 0.4), seems to signal the existence of 4f electric quadrupolar glass phenomenon around 4K forx=0.2.     

Study of dielectric and ac impedance properties of Ti doped Mn ferrites

We have reported dielectric and ac impedance properties of Ti doped Mn_1+xFe_2−2xO₄ (0x0.5) ferrites prepared by solid-state reaction method, using dielectric and impedance spectroscopy in the frequency range of 42 Hz–5 MHz, between the temperatures (300K–473K). The dielectric constant and dielectric loss (tan δ) decreases with increasing frequency but these parameters increase with increasing temperature. The dielectric loss tangent curves exhibit dielectric relaxation peaks at high frequencies (3.6 kHz–5 MHz), which are attributed to the coincidence of the frequency of charge hopping between the localized charge states and the external field. The dielectric properties have been explained on the basis of space charge polarization according to Maxwell–Wagner’s two-layer model and the hopping of charge between Fe²⁺ and Fe³⁺ as well as between Mn³⁺ and Mn²⁺ ions at B-sites. The complex impedance analysis has been used to separate grain and grain boundary in studied samples. Two semicircles corresponding to grain and grain boundary have been observed at low temperature, while only one semicircle has been seen at high temperatures. The resistance of grain and grain boundary both increase with Ti⁴⁺ doping.     

Synthesis and characterization of MnFe2O4-BiFeO3 multiferroic composites

The mixed spinel-perovskite composites of xMnFe₂O₄-(1-x)BiFeO₃ with x=0, 0.1, 0.2, 0.3 and 0.4 were prepared by solid state reaction method. The structure and grain size were examined by means of X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. The XRD results showed that the composites consisted of spinel MnFe₂O₄ and perovskite BiFeO₃ phases after being calcined at the temperature 950 °C for 2 h. The grain size ranged from 0.8 to 1 μm. Magnetization was found to increase with increasing concentration of ferrite content. The variation of dielectric constant and dielectric loss with frequency showed dispersion in the low frequency range. Magnetocapacitance was also observed in the prepared composites, which may be the sign of magnetoelectric coupling in the synthesized composites at room temperature.