Influence of lanthanum and niobium doping on the relaxor behaviour of (Pb0.75Ba0.25)(Zr0.70Ti0.30)O3 ceramics

The lead–barium–zirconate–titanate (PBZT) ceramics of composition (Ba/Zr/Ti: 25/70/30) exhibit classical relaxor ferroelectric behaviour similar to other complex lead perovskites such as lead–lanthanum–zirconate–titanate or lead–magnesium–niobiate. Influence of Nb or La admixture on grain structure, dielectric and pyroelectric properties was showed earlier. The effect of simultaneously lanthanum and niobium modified PBZT 25/70/30 ceramics on properties have been studied and compared with earlier results. The influence of the additives on the values of characteristic parameters describing the relaxor behaviour of the studied ceramics (ε^′_max, T_m, T_f, T_B) was determined.     

Influence of external electric field on relaxor behaviour of (Pb0.75Ba0.25)(Zr0.70Ti0.30)O3 ceramics

The effects of the external d.c. and a.c. electric field on the dielectric response and relaxor behaviour of lead–barium–zirconate–titanate (PBZT) ceramics of the composition Ba/Zr/Ti 25/70/30 have been studied. The significant influence of the strength of the d.c. bias field and the amplitude of the a.c. field on the dielectric constant maximum and frequency dispersion was determined. The magnitude of the dielectric response strongly decreases under the bias field and increases with the increase of the a.c. field amplitude. All parameters describing the relaxor behaviour of the studied ceramics changed under the external d.c. and a.c. electric field. The experimental results can be explained in terms of existing models of relaxors.     

The influence of axial pressure on relaxor properties of BaBi2Nb2O9 ceramics

On the basis of our studies it results that dielectric properties of BaBi₂Nb₂O₉ ceramics are sensitive to axial pressure applied. The pressure causes an increase of dispersion in the real part of dielectric permittivity ?′(T,f) and a rise in the temperature T_m at which the maximum in ?′(T,f) dependence occurs. The applied pressure induces in the ?′(T) dependence an additional step-like anomaly, which appears at the temperature T_A < T_m. The applied pressure shifts both T_m and T_A at the same rate, i.e. dT_A/dX = dT_m/dX = +14 °C/kbar at high axial pressure range, above the threshold pressure X_thresh. The Vogel–Fulcher relationship is employed to determine the axial pressure influence on relaxor properties of BBN ceramics. The simulated order parameter q takes non-zero values below Burn‘s temperature T_B, where the polar clusters appear on cooling. For pressures higher than 0.8 kbar, the T_B changes at the rate dT_B/dX = −200 °C/kbar. The decrease in the difference between Burn's T_B and the freezing T_f temperatures induced by the applied axial pressure is observed. This could be ascribed to the narrowing of temperature range of relaxor behavior.     

Effect of MnO2 addition on relaxor behavior and electrical properties of PMNST ferroelectric ceramics

A novel ferroelectric system, 0.2Pb(Mg_1/3Nb_2/3)O₃–0.8Pb(Sn_0.46Ti_0.54)O₃ with MnO₂ addition (PMNST-Mn), was prepared. The dielectric, ferroelectric and piezoelectric properties were investigated. The results demonstrated that the addition of MnO₂ suppressed the dielectric relaxor behavior of PMNST. With the increase of MnO₂ addition, the diffuse phase transition (DPT) behavior weakened gradually. The addition of MnO₂ contributed to the decrease of the dielectric loss (tanδ) and the enhancement of ferroelectric polarization. The optimum ferroelectric and piezoelectric properties were obtained when the addition of MnO₂ is 0.75 mol%, and the remnant polarization (P_r) and mechanical quality factor (Q_m) were about 25% and 60% higher than those of PMNST, respectively. It was suggested that the formation of oxygen vacancies made the important contribution to suppressing relaxor behavior and improving the electrical properties of PMNST due to the substitution of Mn for B-site. This work provided a practicable strategy to tune electrical properties of ferroelectrics.     

Role of (La,Gd) co-doping on the enhanced dielectric and magnetic properties of BiFeO3 ceramics

The effect of the La³⁺ and Gd³⁺ co-doping on the structure, electric and magnetic properties of BiFeO₃ (BFO) ceramics are investigated. For the compositions (x=0 and 0≤y≤0.15) in the perovskite structured La_xGd_yBi_1−(x+y)FeO₃ system, a tiny residual phase of Bi₂Fe₄O₉ is noticed. Such a secondary phase is suppressed with the incorporation of ‘La’ content (x). The magnitude of dielectric constant (ε_r) increases progressively by increasing the ‘La’ content from x=0 to 0.15 with a remarkable decrease of dielectric loss. For x=0.15, the system La_xGd_yBi_1−(x+y)FeO₃ exhibits highest remanent magnetization (M_r) of 0.18 emu/g and coercive magnetic field (H_C) of ~1 T in the presence of external magnetic field of 9 T at 300 K. The origin of enhanced dielectric and magnetic properties of La_xGd_yBi_1−(x+y)FeO₃ and the role of doping elements, La³⁺, Gd³⁺ has been discussed.     

Study of relaxor behavior in a lead-free (Na0.5Bi0.5)TiO3-SrTiO3-BaTiO3 ternary solid solution system

In this work, single phase lead-free (0.8-x)(Na_0.5Bi_0.5)TiO₃−0.2SrTiO₃-xBaTiO₃ (NBT-ST-BT) ceramics were prepared by conventional solid state reaction method. The effect of BT on the structure and on the dielectric and ferroelectric properties of NBT-ST-BT were investigated. A structural transformation from pseudo-cubic to tetragonal along with possible phase coexistence was witnessed as the BT content was increased. A diffuse phase transition with considerable frequency dispersion in the dielectric response and slim P-E loops evidenced strong relaxor behavior for the ternary system at higher compositions of BT. An analysis of the frequency dependent T_m according to V-F and Power law indicated substantial interaction between the polar nano-regions and relatively broad distribution of freezing temperatures. The study of the dielectric constant at much higher and lower temperatures than T_m in the range of Burn's temperature (T_B) to freezing temperature (T_f) to provide useful information about the growth rate of polar nano-regions and their interactions for a better understanding of the relaxor behavior exhibited by the present ternary system.     

Temperature induced dielectric polarization evolution in polycrystalline Pb(Cd1/3Nb2/3)O3

The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), dilatometric and dielectric measurements performed for the same polycrystalline Pb(Cd_1/3Nb_2/3)O₃ (PCN) sample are presented. These results reveal the complex character of polycrystalline PCN dielectric properties; in particular, no structural phase transition (PT) was observed. The thermal expansion coefficient indicates glass-like behaviour of PCN. The frequency and temperature dependencies of complex dielectric permittivity were measured and analyzed in terms of diffused/relaxor transition of the ferroelectric polarization. Temperature induced evolution of the relative dielectric permittivity was found to involve two processes. Dielectric permittivity changes, originating from thermally induced evolution of thermal equilibrium of interactions between main structural lattice and two sub-lattices present in the sample were interpreted in terms of thermal evolution of polar cluster sizes in the region of relaxor/glass-like transition.     

Vanadium doping effects on microstructure and dielectric properties of barium titanate ceramics

V-doped barium titante ceramics were prepared by conventional solid state reaction method. XRD patterns show that V⁵⁺ ions have entered into the tetragonal perovskite structure of solid solution to substitute for Ti⁴⁺ ions on the B sites. Addition of vanadium accelerates grain growth of BTO ceramics and there is abnormal grain growth of barium titanate ceramics with higher vanadium concentration. Vanadium doping can increase the Curie temperature and decrease the dielectric loss of barium titanate ceramics. As vanadium concentration increases, the remnant polarization of V-doped BTO ceramics begins to increase and reaches the maximum and then decreases. The coercive electric field for V-doped barium titanate ceramics decreases with the increasing of vanadium concentration. As temperature rises, the remnant polarization and the coercive electric field of V-doped barium titanate ceramics decrease simultaneously.     

The effects of hot isostatic pressing temperature on the structure and properties in GdMnO3 ceramics

In this paper, the effects of hot isostatic pressing (HIP) temperature on the crystal structure, optical, dielectric and magnetic properties of GdMnO₃ ceramics were studied. All samples form a single-phase structure without structural transformation, while HIP temperature induces the changes in lattice parameters. HIP causes the change in Mn ions valence state, oxygen vacancy concentration, Raman vibration modes and microscopic morphology of GdMnO₃. Vacancy concentration of the samples prepared by HIP at 800 °C increases compared with that of the samples without HIP, then remains unchanged when the HIP temperature is from 800 to 900 °C, and finally decreases with the further increase of HIP temperature. Appropriate HIP temperature can increase the dielectric constant and decrease the dielectric loss. The transition temperature of paramagnetism to antiferromagnetism and magnetization can be significantly affected by HIP temperature. Magnetic transition temperature and magnetization are closely related to Mn²⁺ ions concentration and cation vacancy concentration, respectively.     

Processing of dense nanostructured HAP ceramics by sintering and hot pressing

A nanosized HAP powder was sintered and hot pressed, in order to obtain dense HAP ceramics. In a first series of experiments, the powder was isostatically pressed into uniform green compacts and sintered at temperatures ranging from 1000 °C to 1200 °C in air atmosphere for different times. In a second series, the isostatically pressed green compacts were hot pressed in argon atmosphere at 900 °C, 950 °C and 1000 °C. The SEM micrograph of the sample sintered at 1200 °C for 2 h showed a uniform 3 μm mean grain size dense microstructure. In the case of hot pressed HAP compacts, full dense, translucent nanostructures were obtained having mean grain size below 100 nm and improved mechanical properties. With the grain size decreasing from 3 μm to 50 nm, the fracture toughness of pure HAP ceramics increased from 0.28 MPa m^1/2 to 1.52 MPa m^1/2.     

Effects of Al2O3 addition on the microstructure and microwave dielectric properties of Ba4Nd9.33Ti18O54 ceramics

Ba₄Nd_9.33Ti₁₈O₅₄·x wt%Al₂O₃ (BNT-A) ceramics (x=0, 0.5, 1.0, 1.5, 2.0, 2.5) were prepared by the conventional solid state reaction. The effects of Al₂O₃ on the microstructure and microwave dielectric properties of Ba₄Nd_9.33Ti₁₈O₅₄ (BNT) ceramics were investigated. X-ray diffraction and backscatter electronic images showed that the Al₂O₃ additive gave rise to a second phase BaAl₂Ti₅O₁₄ (BAT). The formation mechanism and grain growth of the BAT phase were first discussed. Dielectric property test revealed that the Al₂O₃ additive had improved the dielectric properties of the BNT ceramics: increased the Q×f value from 8270 to 12,180 GHz and decreased the τ_f value from 53.4 to 11.2 ppm/°C. A BNT-A ceramic with excellent dielectric properties: ε_r=70.2, Q×f=12,180 GHz, τ_f=20 ppm/°C was obtained with 2.0 wt% Al₂O₃ added after sintering at 1320 °C for 4 h.     

Highly-reliable dielectric capacitors with excellent comprehensive energy-storage properties using Bi0.5Na0.5TiO3-based relaxor ferroelectric ceramics

The development of capacitors with high reliability and good comprehensive performances is of great significance for practical applications. In this work, lead-free relaxor ferroelectric (FE) ceramics of (1-x)(0.5(Bi_0.5Na_0.5)TiO₃-0.5SrTiO₃)-xBi(Mg_2/3Nb_1/3)O₃ ((1-x)(BNT-ST)-xBMN) were prepared by a conventional solid-state reaction method. The introduction of BMN was found to enhance local structure disorder, leading to the significantly reduced size of FE nanodomains, which is responsible for the slim polarization-electric field hysteresis loops. A giant energy-storage density of 6.62 J/cm³ and a high efficiency of 82 % can be achieved simultaneously under a moderate electric field of 34 kV/mm at x = 0.08. It also exhibits high discharge density ～ 2.74 J/cm³, large power density ～ 248 MW/cm³ and ultrafast discharge rate ～ 28 ns at 20 kV/mm in addition to excellent temperature (10–130 °C) and frequency (1–100 Hz) stabilities. These results demonstrate that the (1-x)(BNT-ST)-xBMN ceramic system is a promising lead-free candidate for advanced pulsed power capacitor applications.     

Characterization and properties of lanthanide based titanium tantalate and aluminate electronic ceramic mixtures

The 0.9LnTiTaO₆ + 0.1Ln′AlO₃ (Ln = Ce, Pr and Nd and Ln′ = Pr, Nd and Sm) ceramic mixtures are prepared through the solid state ceramic route. The materials are characterized using X-ray diffraction analysis, scanning electron microscopy and energy dispersive spectrometry. The dielectric properties in the radio as well as in the microwave frequencies are measured. The photoluminescence property of a representative sample is also analyzed. The materials have dielectric constant in the range 35.5-28.6, the temperature coefficient of resonant frequency in the range +35 ppm/°C to +14.1 ppm/°C and high quality factor. The measured values of ?_r and τ_f are compared with the corresponding predicted values calculated using mixture rules. Most of the compositions are useful in the field of optoelectronics and microwave communication.     

Effect of Ce and La substitution on dielectric properties of bismuth titanate ceramics

Effect of Ce and La substitution on the microstructure and dielectric properties of bismuth titanate (BT) ceramics was investigated. Bismuth titanate ceramics (Bi_4−xA_xTi₃O₁₂) (A = Ce or La; x = 0, 0.5, 1) were processed by sintering of pressed pellets, prepared from nanopowder synthesized by the modified sol-gel method. Pure and La modified bismuth titanate ceramics have single Bi₄Ti₃O₁₂ phase of Aurivillius type, whereas a small amount of Bi₂Ti₂O₇ pyrochlore phase appears in Ce modified bismuth titanate ceramics. In the same time addition of La and Ce improved sinterability of BT ceramics. The results of the measurement of dielectric constant and loss tangent at different frequencies (100 Hz-1 MHz) as a function of temperature reveal that Ce modified ceramics has a diffuse phase transition. Temperature T_m, corresponding to the maximum value of the dielectric constant, is shifted to higher temperature and the maximum value of the dielectric constant is decreased with increasing frequency, which indicate that relaxor behavior is caused by Ce substitution.     

Grain-size-insensitive dielectric properties of Sr0.6Ba0.4Nb2O6 relaxor ferroelectric ceramics with tetragonal tungsten bronze structure

The influence of grain size on the dielectric properties of Sr_0.6Ba_0.4Nb₂O₆ (SBN60) ceramics with a tetragonal tungsten bronze structure was examined. The conventional and two-step sintering procedures were used to fabricate dense and phase-pure SBN60 ceramics with grain sizes ranging from 2.2 μm to 17.3 μm. In the temperature dependence of the dielectric permittivity, all samples showed a relaxor-like broad dielectric peak with a maximum at around 60 °C. The maximum dielectric permittivity and dielectric maximum temperature of the SBN ceramics were found to be less dependent on the grain size. The reason for the grain-size-insensitive dielectric properties of the SBN60 ceramics is discussed based on the analysis of the dielectric properties and Raman spectra.     

Mechanical properties of zirconia composite ceramics

Composite materials based on 8 wt% yttria partially stabilized zirconia, with additions of gadolinium zirconate, lanthanum lithium hexaaluminate, yttrium aluminum garnet and strontium zirconate were characterized. Samples were fabricated by hot-press sintering at 1550 °C. The effect of the secondary phase content on the mechanical properties of the composites was evaluated. Hardness, elastic modulus and fracture toughness of the fabricated composites were determined by means of depth-sensitive indentation testing. The fracture toughness of the samples as determined by the indentation method was found to increase with increasing YSZ content, reaching 3 MPa·m^0.5 for samples with 80 wt% YSZ. The fracture toughness appeared to be affected by thermal expansion coefficient mismatch, crack bridging and crack deflection.     

Dielectric,pyroelectric, and ferroelectric properties of gadolinium doped Sr0.53Ba0.47Nb2O6 ceramic

Strontium barium niobate doped with gadolinium, with the stoichiometric formula Gd_ySr_{(0.53–3y/2)}Ba_0.47Nb₂O₆ (GSBN) was synthesized using the solid-state reaction method, with varied mol% compositions of Gd (y=0.00, 0.01, 0.03, 0.05 and 0.07). Gadolinium was chosen as a dopant with the goal of enhancing the ferroelectric and pyroelectric properties of SBN. The X-ray diffraction spectra showed that all compositions exhibit a single-phase tetragonal tungsten bronze structure. The influence of Gd as dopant on the microstructure was examined by using field emission scanning electron microscopy. The dielectric characteristics of the samples showed diffuse phase transitions. The Curie temperature of the samples shifted to lower temperature with increasing Gd concentration. The relaxor characteristic of the GSBN (above and below the Curie temperature) was described using the Curie-Weiss Law, a Gaussian distribution, and a quadratic equation. SBN doped with 3 mol% of Gd exhibits the highest remnant polarization, P_r=8.8 μC/cm², while 1 mol% Gd-doped SBN shows the highest pyroelectric coefficient of 285 µC/m² K. These qualities can be useful in security, healthcare, pollution monitoring, fire sensing, and smart energy system applications.     

Dielectric and ferroelectric properties of Ta-modified Bi3.25La0.75Ti3O12 ceramics

B-site modified Bi_3.25La_0.75Ti_3-xTa_xO₁₂ ceramics were prepared by the conventional solid-state reaction method. The influence of Ta₂O₅ on microstructure and electric properties of the ceramics was investigated. The results demonstrated that Ta⁵⁺ ions were dissolved into the perovskite lattice and homogeneously distributed in the matrix without forming any minority phase. The conduction mechanism and dielectric response behavior were transformed with Ta substation, which is triggered by varied structural distortion characteristics and defect diploes. The Curie temperature decreased gradually with increasing Ta content and a relaxor-like behavior was observed for x = 0.09 sample. The internal bias field is decreased with Ta doping, because the substitution of Ta⁵⁺ at B-site contributes to release the involved oxygen vacancies in defect diploes. Moreover, further increasing Ta content causes a reduction in the oxygen vacancies located at lattice misfits, resulting in a decrease of coercive fields. An improved ferroelectric properties were obtained for x = 0.09 sample with a relatively lower coercive field and a larger spontaneous polarization.     

Effect of consolidation parameters on structural,microstructural and electrical properties of magnesium titanate ceramics

The aim of this study is to explore the influence of the processing route on the structural and physical properties of bulk MgTiO₃ ceramics. Commercially available MgO and TiO₂ powders were mechanically activated in a planetary ball mill. Green bodies were formed by an isostatic pressure of 300 MPa. The sintering of these samples was done either by the Two-Step Sintering (TSS) approach or by conventional pressureless sintering followed by Hot Isostatic Pressing (post-HIPing). The first set of compacts was sintered by TSS in air at 1300 °C for 30 min and the next step was performed at 1200 °C for 20 h. The density of the obtained samples after the two-step sintering reached almost 90% of the theoretical density (%TD). The second set of compacts was sintered at 1400 °C for 30 min in air. The samples without open porosity were post-sintered by the HIP at 1200 °C for 2 h in an argon atmosphere at a pressure of 200 MPa. The density significantly increased up to 96%TD. The differences between samples prepared by these two techniques were also analyzed by XRD and SEM. The lattice vibration spectra were obtained using Raman spectroscopy and they indicate a high degree of lattice disorder, as well as high values of the oxygen vacancy concentration. Electrical characteristic were established in the frequency range from 10 kHz to 10 GHz. The choice of the processing route had considerable influence on structural and physical properties of MgTiO₃ ceramics.