Sol-gel derived CaCu3Ti4O12 ceramics: Synthesis, characterization and electrical properties

The giant dielectric constant material CaCu₃Ti₄O₁₂ (CCTO) has been synthesized by sol-gel method, for the first time, using nitrate and alkoxide precursor. The electrical properties of CCTO ceramics, showing an enormously large dielectric constant ? ∼ 60,000 (100 Hz at RT), were investigated in the temperature range from 298 to 358 K at 0, 5, 10, 20, and 40 V dc. The phases, microstructures, and impedance properties of final samples were characterized by X-ray diffraction, scanning electron microscopy, and precision impedance analyzer. The dielectric permittivity of CCTO synthesized by sol-gel method is at least three times of magnitude larger than that synthesized by other low-temperature method and solid-state reaction method. Furthermore, the results support the internal barrier layer capacitor (IBLC) model of Schottky barriers at grain boundaries between semiconducting grains.     

Processing, dielectric properties and impedance characteristics of Na0.5Bi0.5Cu3Ti4O12 ceramics

Na_0.5Bi_0.5Cu₃Ti₄O₁₂ (NBCTO) ceramics were prepared by conventional solid-state reaction method. The phase structure, microstructure and dielectric properties of NBCTO ceramics sintered at various temperatures with different soaking time were investigated. Pure NBCTO phase could be obtained with increasing the temperature and prolonging the soaking time. High dielectric permittivity (13,495) and low dielectric loss (0.031) could be obtained when the ceramics were sintered at 1000 °C for 7.5 h. The ceramics sintered at 1000 °C for 7.5 h also showed good temperature stability (−4.00 to −0.69%) over a large temperature range from −50 to 150 °C. Complex impedances results revealed that the grain was semiconducting and the grain boundaries was insulating. The grain resistance (R_g) was 12.10 Ω cm and the grain boundary resistance (R_gb) was 2.009 × 10⁵ Ω cm when the ceramics were sintered at 1000 °C for 7.5 h.     

Dielectric constant and mixing model of BaTiO3 composite thick films

BaTiO₃ (BT) composite thick films of X7R BT particles with different BT gel fractions were prepared by using an aqueous BT sol. The dielectric constant versus different BT gel volume fractions showed a sigmoidal behavior as the BT gel phase filled the interstitials of the X7R BT particle compact. To explain and predict the effect of the BT gel as a second phase based on the experimental results, various models such as series, parallel, cubic, Lichtenecker’s model, and Hashin-Shtrikman bounds were considered. None of the existing theoretical models fit the experimental results. An empirical sigmoidal fitting function was proposed to fit the experimental data.     

Relaxor ferroelectric like giant permittivity in PrCrO3 semiconductor ceramics

The electrical behavior of PrCrO₃ ceramics prepared by citric acid route and sintered at 1200 °C has been characterized by a combination of permittivity measurements, and impedance spectroscopy (IS). The effective permittivity obtained in frequency range 100 Hz to 1 MHz and temperature range 80–300 K, exhibits giant permittivity value of 3 × 10⁴ near room temperature. The response is similar to that observed for relaxor ferroelectrics. IS data analysis revealed the ceramics to be electrically heterogeneous semiconductor with room temperature resistivity <10² Ω m consisting of semiconducting grains with permittivity ?′ ∼ 100 and more resistive grain boundaries with effective permittivity ?′ ∼ 10⁴. We conclude, therefore that grain boundary effect is the primary source for the high effective permittivity in PrCrO₃ ceramics.     

Synthesis and microwave dielectric properties of CaO-MgO-SiO2 submicron powders doped with Li2O-Bi2O3 by sol-gel method

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 80-100 nm and 200-400 nm 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 10 wt%, the dielectric ceramic sintered at 890 °C had compact structure, and possessed excellent microwave dielectric properties: ?_r = 7.16, Q × f = 25630 GHz, τ_f = −69.26 ppm/°C.     

Synthesis, characterization, and dielectric properties of Ba(Ti1−xSnx)O3 nanopowders and ceramics

We prepared Ba(Ti_1−xSn_x)O₃ powders and ceramics by means of the sol-gel process, with dibutyltin dilaurate as the Sn precursor. The samples were characterized by means of Fourier-transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy, and also determined the dielectric properties of the ceramics. The powders synthesized by means of the sol-gel process had a grain size on the nanometer scale, with the grains mainly composed of a cubic BaTiO₃ phase. Sn can disperse into BaTiO₃ more uniformly in the sol-gel technique using dibutyltin dilaurate as the Sn precursor. With increasing Sn concentration, the grain size of the Ba(Ti_1-xSn_x)O₃ ceramics increased and the maximum dielectric constant (?_max) first increased and then decreased. At a Sn concentration of 5 mol%, ?_max reached its maximum value (19,235).     

Synthesis and characterization of Zn-B-Si-O nano-composites and their doped BaTiO3 ceramics

We synthesized Zn-B-Si-O (ZBSO) nano-composites via sol-gel process, and then used them to dope BaTiO₃ ceramics. The ZBSO nano-composites and their ceramics were characterized by means of thermogravimetric, Fourier-transform infrared, and X-ray diffraction methods, and using scanning and transmission electron microscopy. We also characterized the dielectric properties of the ceramics. The results indicated that the ZBSO nano-composites were nanometer-scale powders with an amorphous structure. The particle size of the powders increased with increasing pH value, but initially decreased and then increased with increasing calcining temperature. At pH about 2 and with calcining at 400 °C, the nano-composites attained minimum particle size (about 30 nm). The sintering temperature of the BaTiO₃ ceramics could be reduced to 1100 °C by adding 5 wt% of the ZBSO nano-composites. Uniform, fine-grained BaTiO₃ ceramics with a high permittivity (?_r = 2946 and ?_max = 5072) were obtained by adding nano-composites; these properties were superior to the ZBSO glass doped BaTiO₃ ceramics.     

Structural and dielectric properties of A(Fe1/2Ta1/2)O3 [A = Ba, Sr, Ca]

The complex perovskite oxide barium iron tantalate (BFT), BaFe_1/2Ta_1/2O₃, strontium iron tantalate (SFT), SrFe_1/2Ta_1/2O₃ and calcium iron tantalate (CFT), CaFe_1/2Ta_1/2O₃ are synthesized by a solid-state reaction technique. Rietveld refinement of the X-ray diffraction data of the samples shows that BFT and SFT crystallize in cubic structure, with lattice parameter a = 4.06 Å for BFT and 3.959 Å for SFT, whereas CFT crystallizes in orthorhombic structure having lattice parameters a = 5.443 Å, b = 5.542 Å and c = 7.757 Å. Fourier transform infrared spectra show two primary phonon modes of the samples at around 450 cm⁻¹ and 620 cm⁻¹. The compounds show significant frequency dispersion in its dielectric properties. The complex impedance plane plots of the samples show that the relaxation (conduction) mechanism in these materials is purely a bulk effect arising from the semiconductive grains. The relaxation mechanism of the samples is modelled by Cole-Cole equation. The frequency dependent conductivity spectra are found to follow the power law.     

Fine-grained multiferroic BaTiO3/(Ni0.5Zn0.5)Fe2O4 composite ceramics synthesized by novel powder-in-sol precursor hybrid processing route

Dense, homogeneous, and fine-grained multiferroic BaTiO₃/(Ni_0.5Zn_0.5)Fe₂O₄ composite ceramics are synthesized by a novel powder-in-sol precursor hybrid processing route. This route includes the dispersion of nanosized BaTiO₃ ferroelectric powders prepared via conventional sold-state ceramic process into (Ni_0.5Zn_0.5)Fe₂O₄ ferromagnetic sol-gel precursor prepared via sol-gel wet chemistry process. Uniformly distributed slurry is obtained after ball milling and used in the fabrication of the ceramics with low sintering temperatures. The ceramics show coexistence of ferromagnetic and ferroelectric phases with obvious ferromagnetic and ferroelectric hysteresis loops at room temperature, besides exhibiting excellent magnetic and dielectric properties in a wide range of frequency. The combination of high permeability and permittivity with low losses in the ceramics enables significant miniaturization of electronic devices based on the ceramics.     

Dielectric relaxation in complex perovskite oxide In(Ni1/2Zr1/2)O3

The dielectric study of indium nickel zirconate, In(Ni_1/2Zr_1/2)O₃ (INZ) synthesized by solid state reaction technique is performed in a frequency range from 500 Hz to 1 MHz and in a temperature range from 303 to 493 K. The X-ray diffraction analysis shows that the compound is monoclinic. A relaxation is observed in the entire temperature range as a gradual decrease in ?′(ω) and as a broad peak in ?″(ω) in the frequency dependent real and imaginary parts of dielectric constant, respectively. The frequency dependent electrical data are analyzed in the framework of conductivity and electric modulus formalisms. The frequencies corresponding to the maxima of the imaginary electric modulus at various temperatures are found to obey an Arrhenius law with activation energy of 0.66 eV. The Cole-Cole model is used to study the dielectric relaxation of INZ. The scaling behaviour of imaginary part of electric modulus suggests that the relaxation describes the same mechanism at various temperatures. The frequency dependent conductivity spectra follow the universal power law.     

Low temperature sintering and microwave dielectric properties of SmAlO3 ceramics

The effects of sintering aids on the microstructures and microwave dielectric properties of SmAlO₃ ceramics were investigated. CuO and ZnO were selected as sintering aids to lower the sintering temperature of SmAlO₃ ceramics. With the additions, the sintering temperature of SmAlO₃ can be effectively reduced from 1650 to 1430°C. The crystalline phase exhibited no phase differences at low addition level while Sm₄Al₂O₉ appeared as a second phase as the doping level was over 0.5 wt.%. In spite of the additions, the dielectric constants showed no significant change and ranged 19-21. However, the quality factor Q×f was strongly dependent upon the type and amount of additions. The Q×f values of 51,000 and 41,000 GHz could be obtained at 1430°C with 0.25 wt.% CuO and ZnO additions, respectively. The temperature coefficients depended on the additions and varied from −40 to −65 ppm/°C. Results of X-ray diffractions, EDS analysis and scanning electron microscopy were also presented.     

Crystallographic and dielectric properties of barium-substituted Pb(Mg1/3Ta2/3)O3 ceramics

Ceramic powders of (Ba,Pb)Pb(Mg_1/3Ta_2/3)O₃ were prepared via a B-site precursor route. Crystal symmetries and lattice parameters were determined. Monophasic perovskite was developed after the two-step reaction process, in which the lattice parameters showed linear changes in the entire composition range. Dielectric responses of the ceramics with compositional and frequency changes were investigated. The results were also compared with the (Ba,Pb)(Zn_1/3Ta_2/3)O₃ data.     

Improved high-Q microwave dielectric resonator using CuO-doped MgNb2O6 ceramics

The microwave dielectric properties and the microstructures of MgNb₂O₆ ceramics with CuO additions (1-4 wt.%) prepared with conventional solid-state route have been investigated. The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. It is found that MgNb₂O₆ ceramics can be sintered at 1140 °C due to the liquid phase effect of CuO addition. At 1170 °C, MgNb₂O₆ ceramics with 2 wt.% CuO addition possesses a dielectric constant (ε_r) of 19.9, a Q×f value of 110,000 (at 10 GHz) and a temperature coefficient of resonant frequency (τ_f) of −44 ppm/°C. The CuO-doped MgNb₂O₆ ceramics can find applications in microwave devices requiring low sintering temperature.     

Solid state and solution synthesis of Ba(Mg1/3Ta2/3)O3: A comparative study

Ba(Mg_1/3Ta_2/3)O₃ [BMT] dielectric ceramics are prepared by solid state (one step, two step and molten salt synthesis) and wet chemical methods (precipitation, citrate gel and sol-gel). The formation mechanism of BMT in each synthesis technique is discussed. The formation temperature and particle size of the formed BMT were found to be much lesser (in nanometer range) for solution synthesized powders. It is found that synthesis by sol-gel method resulted in the formation of ultra pure nanopowders of BMT at about 600 °C with average crystallite size of about 18 nm where as in solid state synthesis the formation of BMT was formed at about 1100 °C with average crystallite size of 220 nm. On sintering these powders, densification and grain growth of the chemically derived powders were found to be lower than that of solid state synthesized BMT powder. This has resulted in a slight decrease in density and microwave dielectric properties of the solution synthesized BMT samples. It is found that the microwave dielectric properties improved with increase in the average grain diameter of the sintered BMT ceramics.     

Effect of Pb(Fe1/2Nb1/2)O3 modification on dielectric and piezoelectric properties of Pb(Mg1/3Nb2/3)O3-PbZr0.52Ti0.48O3 ceramics

10 mol% Pb(Fe_1/2Nb_1/2)O₃ (PFN) modified Pb(Mg_1/3Nb_2/3)O₃-PbZr_0.52Ti_0.48O₃ (PMN-PZT) relaxor ferroelectric ceramics with compositions of (0.9 − x)PMN-0.1PFN-xPZT (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) were prepared. X-ray diffraction investigations indicated that as-prepared ceramics were of pure perovskite phase and the sample with composition of x = 0.8 was close to morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase. Dielectric properties of the as-prepared ceramics were measured, and the Curie temperature (T_c) increased sharply with increasing PZT content and could be higher than 300 °C around morphotropic phase boundary (MPB) area. At 1 kHz, the sample with composition of x = 0.1 had the largest room temperature dielectric constant ?_r = 3519 and maximum dielectric constant ?_m = 20,475 at T_m, while the sample with composition of x = 0.3 possessed the maximum dielectric relaxor factor of γ = 1.94. The largest d₃₃ = 318 pC/N could be obtained from as-prepared ceramics at x = 0.9. The maximum remnant polarization (P_r = 28.3 μC/cm²) was obtained from as-prepared ceramics at x = 0.4.     

Microwave dielectric properties and microstructures of the 11Li2O-3Nb2O5-12TiO2 ceramics with B2O3 addition

The effects of B₂O₃ addition, as a sintering agent, on the sintering behavior, microstructure and microwave dielectric properties of the 11Li₂O-3Nb₂O₅-12TiO₂ (LNT) ceramics have been investigated. With the low-level doping of B₂O₃ (≤2 wt.%), the sintering temperature of the LNT ceramic could be effectively reduced to 900 °C. The B₂O₃-doped LNT ceramics are also composed of Li₂TiO₃ss and “M-phase” phases. No other phase could be observed in the 0.5-2 wt.% B₂O₃-doped ceramics sintered at 840-920 °C. The addition of B₂O₃ induced no obvious degradation in the microwave dielectric properties but increased the τ_f values. Typically, the 0.5 wt.% B₂O₃-doped ceramics sintered at 900 °C have better microwave dielectric properties of ?_r = 49.2, Q × f = 8839 GHz, τ_f = 57.6 ppm/°C, which suggest that the ceramics could be applied in multilayer microwave devices requiring low sintering temperatures.     

Microwave dielectric properties and microstructures of CuO- and ZnO-doped LaAlO3 ceramics

The microwave dielectric properties and the microstructures of 0.25 wt.% CuO-doped LaAlO₃ ceramics with ZnO additions have been investigated. The sintered LaAlO₃ ceramics are characterized by X-ray diffraction spectra and scanning electron microscopy (SEM). Tremendous reduction in sintering temperature can be achieved with the addition of sintering aids CuO and ZnO. The ceramic samples show that dielectric constants (ε_r) of 22−24 and Q×f values of 33,000−57,000 (at 9.7 GHz) can be obtained at low sintering temperatures 1340−1460°C. The temperature coefficient of resonant frequency varies from −24 to −48 ppm/°C. At the level of 0.25 wt.% CuO and 1 wt.% ZnO additions, LaAlO₃ ceramics possesses a dielectric constant (ε_r) of 23.4, a Q×f value of 57,000 (at 9.7 GHz) and a τ_f value of −38 ppm/°C at 1400°C for 2 h.     

Modified tunable dielectric properties by addition of MgO on BaZr0.25Ti0.75O3 ceramics

Phase composition, microstructure and tunable dielectric properties of (1 − x)BaZr_0.25Ti_0.75O₃-xMgO (BZTM) composite ceramics fabricated by solid-state reaction were investigated. It was found Mg not only existed in the matrix as MgO, there was also trace amount of Mg²⁺ ions dissolved in the BZT grains, which led to Curie temperature of the BZTM composites ceramics shifting to below −100 °C. Dielectric permittivity of the BZTM composite ceramics was reduced from thousands to hundreds by manipulating the content of MgO. Johnson's phenomenological equation based on Devonshire's theory was used to describe the nonlinear dielectric permittivity of the ceramics with increasing applied DC field. With increasing content of MgO, anharmonic constant α_(T) increased monotonously. Dielectric permittivity was 672, while dielectric tunability was as high as 30.0% at 30 kV/cm and dielectric loss was around 0.0016 for the 0.6BaZr_0.25Ti_0.75O₃-0.4MgO sample at 10 kHz and room temperature.     

Effects of Ca and Zr substitution upon structure and properties of Ba5LaTi3Ta7O30 low-loss dielectric ceramics

Effects of Ca and Zr substitution upon the dielectric properties of Ba₅LaTi₃Ta₇O₃₀ ceramics were investigated together with the structural characterization. All the samples of Ba₅La(Zr_xTi_1−x)₃Ta₇O₃₀ formed a filled tungsten-bronze structures, whereas the solid solution limit was determined as x=0.25 in (Ca_xBa_1−x)₅LaTi₃Ta₇O₃₀. Beyond this limit secondary phase of CaTa₂O₆ was detected and it would become the major phase for the Ca-rich compositions. The temperature coefficient of dielectric constant was improved with increasing Zr content while the dielectric constant decreased and the low dielectric loss varied little (in the order of 10⁻⁴). In the case of (Ca_xBa_1−x)₅LaTi₃Ta₇O₃₀, small temperature coefficient of dielectric constant could be obtained with increasing Ca content while the dielectric constant decreased significantly, and a small amount substitution of Ca for Ba induced decrease in dielectric loss.     

Comparative investigation of structure and dielectric properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 (65/35) and 10% PbZrO3-doped Pb(Mg1/3Nb2/3)O3-PbTiO3 (65/35) ceramics prepared by a modified precursor method

Relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) and 10% PbZrO₃-doped Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramics were both prepared by a modified precursor method, which was based on the high-temperature synthesis of an oxide precursor that contained all the B-site cations for the consideration of B-site homogeneity. The dielectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramic was more of normal ferroelectric behavior, but the high dielectric constant (?_m = 34,200 at 1 kHz) and piezoelectric constant (d₃₃ = 709 pC/N) were observed for this composition close to the morphotropic phase boundary. Comparatively, introduction of 10% PbZrO₃ into Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (65/35) ceramics enhanced the diffuse phase transition as well as the rhombohedral to tetragonal phase transition temperature, while it also kept the high dielectric constant (?_m = 29,600 at 1 kHz) and piezoelectric constant (d₃₃ = 511 pC/N).