Effect of KNN doping on the dielectric properties of BaTiO3 lead-free ceramics

Journal of Materials Science: Materials in Electronics - In this study, the phase structure, microstructure, and dielectric properties of BT-xKNN ceramic prepared by traditional solid-phase method...     

Effect of Bi2O3 doping on the dielectric properties of medium-temperature sintering BaTiO3-based X8R ceramics

Medium-temperature sintering X8R ceramics were fabricated based on BaTiO₃-based ceramics with Bi₂O₃ additives. The effects of sintering aids Bi₂O₃ on crystalline structure and electrical properties of BaTiO₃-based ceramics were investigated. The sinterability of BaTiO₃ ceramics was significantly improved by adding Bi₂O₃, whose densification sintering temperature reduced from 1,260 to 1,130 °C. However, the dielectric constant (ε) of BaTiO₃-based ceramics doped with Bi₂O₃ was decreased dramatically. Both low ε phase Bi₄Ti₃O₁₂ and the decrease of the tetragonality (c/a ratio), which are demonstrated by XRD pattern, are resulted in the decrease of ε. The ε of samples doped with 5.5 wt% Bi₂O₃ was higher than the other doped samples. The substitution of Bi³⁺ for the Ba²⁺ in BaTiO₃ resulted in the increase of electrovalence (from +2 to +3) of A-site ion, so the attractive force between A and B (Ti⁴⁺) sites becomes stronger. Thus Ti⁴⁺’s polarization enhances, then ε was increased to some extent. The X8R BaTiO₃-based ceramics could be sintered at as low as 1,130 °C by doping 5.5 wt% Bi₂O₃ additives into the BaTiO₃-based ceramics, with a ε greater than 2,430 at 25 °C, dielectric loss lower than 1.3 % and temperature coefficient of capacitance <±15 % (?55–150 °C).     

锰掺杂对PMN-PZT陶瓷介电性能的影响

研究了锰掺杂对富锆PMN-PZT（铌镁酸铅一锆钛酸铅）陶瓷材料的相组成、微观结构、介电性能等方面的影响，并对实验结果作出物理机理的解释。实验结果表明：适量的锰掺杂有助于陶瓷晶粒的生长，并能有效地降低PMN-PZT陶瓷材料的介电常数和介电损耗，在锰掺杂量为3．0％（原子分数）时，εr：197、tanδ=0．15％，作为用于红外热释电探测器的陶瓷材料具有良好的介电性能。     

Microstructure development and dielectric properties of fast-fired BaTiO3 ceramics

The technique of zone sintering was applied to three BaTiO₃ source materials; studies were made in air, oxygen and vacuum. The optimum firing conditions suggested for the zone sintering of these materials were a temperature of 1450° C and a time in the hot zone of 5 min. The temperature dependence of the dielectric constant could be closely related to the firing conditions and material microstructure. Lower room-temperature dielectric constants and lower Curie temperatures were found as the holding time in the hot zone was increased, yielding larger grained structures.     

Fast-sintering of hydrothermally synthesized BaTiO3 powders and their dielectric properties

Hydrothermally synthesized barium titanate (BaTiO₃) powders with a submicrometre particle size have been fast-sintered with a heating rate of ∼ 200 °C min^-1 at various temperatures (1250–1350 °C) for short times (5 and 15 min). The microstructures and dielectric properties of the sintered samples are studied and compared with those sintered conventionally. The sample fast-sintered at 1250 °C for 5 min had the highest dielectric constant value of approximately 3700 with an average grain size of about 1 μm. Both the dielectric constant and the Curie–Weiss temperature are found to be dependent on the grain size of the sintered samples, particularly when the average grain size is less than 5 μm. This has been attributed to the presence of internal stress in the fine-grained BaTiO₃. This revised version was published online in November 2006 with corrections to the Cover Date.     

Pyroelectric and dielectric bolometer properties of Sr modified BaTiO3 ceramics

The pyroelectric and dielectric bolometer properties of Sr modified BaTiO₃ ceramics were investigated for thermal IR detector applications. The (Ba_1–x, Sr_x)TiO₃ solid solution ceramics have been prepared by adding 10–40 at % SrTiO₃ to BaTiO₃ and with different sintering conditions and under dc fields. The relationships of processing parameters, microstructures, dielectric/pyroelectric properties and dc fields are discussed. And the figures of merit are compared between pyroelectric-type and dielectric bolometer-type IR sensors based on the experimental results.     

Effect of Al doping on structural and dielectric properties of PLZT ceramics

Polycrystalline samples of Al modified lead lanthanum zirconate titanate [PLZT] with Zr = 55% and Ti = 45%, have been synthesized by a high-temperature solid-state reaction technique. X-ray diffraction analysis of the compounds suggests the formation of a single-phase compound with tetragonal structure. Dielectric studies of the compounds as a function of temperature (from 30 to 350°C) at frequency (1, 10 and 100 kHz) show that the compounds undergo a phase transition of diffuse type.The transition temperature shifts towards higher side with increase in frequency a typical characteristic of a relaxor material. Diffusivity (γ) study of phase transition of these compounds provided its value between 1 and 2, indicating the variation of degree of disordering in the system. Measurement of dc resistivity (ρ) as a function of temperature (Room temperature, RT to 350°C) at a constant biasing field suggests the compounds have negative temperature coefficient of resistance (NTCR).     

Effect of atmosphere on the PTCR properties of BaTiO3 ceramics

The influence of low-temperature annealing, at < 360 °C, in various reducing and oxidizing atmospheres for a series of BaTiO₃ ceramics with a positive temperature coefficient of resistance (PTCR) is discussed. Combined impedance and modulus spectroscopy is used to analyse a.c. impedance data and shows that the total resistance of the sample can be composed of up to three components, dependent on the cooling rate from the sintering temperature. For quickly cooled samples the PTCR response is dominated by an outer shell on individuals grains, whereas for slowly cooled samples the grain boundary resistance dominates. Annealing in reducing atmospheres destroys the grain boundary PTCR effect whereas the outer-shell grain PTCR effect is relatively insensitive to the reducing atmosphere. It is proposed that the acceptor states responsible for the outer-grain and grain-boundary PTCR effects are predominantly intrinsic metal vacancies, i.e. Ba and/or Ti, and adsorbed oxygen, respectively.     

Microstructure and dielectric properties of BaTiO3-based X7R ceramics

The relationship between the microstructures and dielectric properties of BaTiO₃-based X7R ceramics has been investigated at different calcination temperatures. The XRD and SEM results show that calcinations of BaTiO₃ raw powders increase the grain size and stabilize the tetragonality (c/a ratio) of the ceramics. The grain growth caused by the calcination prevents the doped ions from diffusing into the interior of the grains, and then increases the volume fraction of the tetragonal phased core. This process greatly increases the dielectric constant by improving the ferroelectricity. As a result, the relaxation mechanism of the domain reorientation generates high loss tangent. The BaTiO₃ ceramics with X7R specifications were prepared at the calcination temperature of 1200 °C and the sintering temperature of 1240 °C, whose dielectric properties were ɛ _r ≥ 4500, Δɛ _r/ɛ _r25 ≤ ± 10%(−55∼125 °C), tanδ ≤ 0.012(25 °C), respectively.     

Effect of sintering atmospheres on the microstructure and dielectric properties of Yb/Mg co-doped BaTiO3 ceramics

The effect of Yb/Mg co-doping on the microstructure and electrical properties of BaTiO₃ ceramics sintered under oxidizing, reducing and annealed conditions was investigated. XRD analyses indicated the suppression of the solubilities of the acceptor ions in BaTiO₃ under the reducing and annealed conditions_. DSC measurements indicated that addition of too much Yb would cause the collapse of the core-shell structure in the BaTiO₃ ceramics. The effect of sintering atmospheres on the microstructure and dielectric properties was attributed to the change of the solubilities of the acceptor ions in this core-shell-structured material.     

Effect of phosphor doping on the microstructure and dielectric properties of barium titanate ceramics

The effect of phosphor doping on the sintering behaviour, microstructure and dielectric properties of BaTiO₃ has been investigated. Diisopropyl phosphinate is added to high-purity BaTiO₃ powder prepared by the wet chemical method with a final amount of 0.14 wt.% P₂O₅ with respect to the BaTiO₃. Phosphor-doped BaTiO₃ ceramics with a high density and uniform grain size have been produced by using wet processing and pressureless sintering without any binder. A scanning electron microscope, thermometric analysis, X-ray diffraction and an impedance analyser have been used to determine the microstructure as well as the dielectric properties. The phosphor cations can form a liquid phase belonging to the ternary system BaO-TiO₂-P₂O₅, leading to the formation of BaTiO₃ ceramics with high density at low temperature. Phosphor-doped BaTiO₃ ceramics with a high density of 96% D_th are obtained by sintering at 1200°C with a soaking time of 2 h. The dielectric constants of samples sintered at 1150 and 1200°C are as high as 6100 and 5500, respectively; the Curie temperature of samples decreases with decreasing sintering temperature. Doping with a small amount of phosphor can improve the sintering and dielectric properties of BaTiO₃ ceramics.     

Effects of BiNbO4 on the microstructure and dielectric properties of BaTiO3-based ceramics

The microstructure and dielectric properties of ternary system BaTiO₃ (BT)–Bi_0.5Na_0.5TiO₃ (BNT)–BiNbO₄ were investigated. In the case of 1 mol% BNT addition, the Curie point (T _c) of BaTiO₃ was increased from 130 to 140 °C. The dielectric constant peak at T _c was markedly depressed with the addition of BiNbO₄, whereas an enhancement in the dielectric constant at lower temperature was observed. The addition of BiNbO₄ improved the temperature dependence of the dielectric constants of BaTiO₃-BNT system ceramics and the temperature coefficient of capacitance curves satisfied the EIA X8R specification with 3–4 mol% BiNbO₄. The dielectric constant peak at T _c was suppressed evidently due to over high-sintering temperature. As the BiNbO₄ content was increased, the Curie point progressively moved to lower temperatures. SEM indicated that fine and homogeneous grains were observed with 1 mol% BiNbO₄ addition. However, secondary phase grains appeared with increasing the BiNbO₄ content, which were identified as Ba₂TiO₄, NaBiTi₂O₆, and BaTiNb₄O₁₃ by XRD. Moreover, the proportion of the secondary phase grains increased as the BiNbO₄ content increased.     

Mechanical and dielectric failure of BaTiO3 ceramics

Both at room temperature and above the Curie temperature, Weibull plots for the mechanical and dielectric strengths of BaTiO₃ thick films sintered 1300° C to 1400° C showed good a correlation. The present results indicate that the microstructure plays a similar role in both mechanical and dielectric failures and firmly suggest that the fracture origins in both failures are analogous, possibly surface flaws associated with grain size. Specimens sintered at 1450° C showed a bend in the Weibull distribution for mechanical strength. This bimodal Weibull distribution was explained by the assumption that the geometric relation between the ratio of grain sizes to specimen thickness and stress gradient under a three-point bending condition affects the effective tensile stress of the tips of surface flaws.     

Phase relations and dielectric properties of BaTiO3 ceramics heavily substituted with neodymium

Investigations on the phase relations and dielectric properties of (1 -x)BaTiO₃ +xNd_2/3TiO₃ (BNT) ceramics sintered in air below 1650 K have been carried out. X-ray powder diffraction studies indicate apparent phase singularity for compositions withx < 0.3. Nd₂Ti₂O₇ is detected at higher neodymium concentrations. The unit cell parameter changes continuously with neodymium content, and BaTiO₃ is completely cubic at room temperature withx 0.0525, whereas electron diffraction studies indicate that the air-sintered BNT ceramics withx > 0.08 contain additional phases that are partly amorphous even to an electron beam. SEM observations reveal that BaTiO₃ grains are mostly covered by a molten intergranular phase, and show the presence of randomly distributed Nd₂Ti₂O₇ grains. Energy dispersive X-ray analysis shows the Ba-Nd-Ti ternary composition of the intergranular phase. Differential thermal analysis studies support the formation of a partial melt involving dissolution-precipitation of boundary layers of BaTiO₃ grains. These complex phase relations are accounted for in terms of the phase instability of BaTiO₃ with large cation-vacancy concentration as a result of heavy Nd³⁺ substitution. The absence of structural intergrowth in (1 -x)BaTiO₃ +xNd_2/3TiO₃ under oxidative conditions leads to a separation of phases wherein the new phases undergo melting and remain X-ray amorphous. BNT ceramics with 0.1 <x < 0.3 have _eff 10⁴ with tan < 0.1 and nearly flat temperature capacitance characteristics. The grain-size dependence of _eff, variations of _eff and tan with the measuring frequency, the non-ohmic resistivities, and the non-linear leakage currents at higher field-strengths which are accompanied by the decrease in _eff and rise in tan, are explained on the basis of an intergranular (internal boundary layer) dielectric characteristic of these ceramics.     

Effect of the dispersibility of BaTiO3 nanoparticles in BaTiO3/polyimide composites on the dielectric properties

We have investigated the effect of coupling agents with different organic moiety on the dielectric properties of polyimide/BaTiO₃ (70 nm) composite films. INAAT (isopropyl tris(N-amino-ethyl aminoethyl)titanate, KR 44) and APTS (3-amino-propyl-triethoxysilane) were used as coupling agents, respectively, for homogeneous dispersion of BaTiO₃ particles into a polyimide matrix. The composite films were prepared by pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA)-based polyimide. Enhanced dispersion of BaTiO₃ particles was obtained by the use of INAAT with more organic moiety compared to that afforded by APTS. The polyimide composite with BaTiO₃ particles (BaTiO₃ content at 50 vol.%) treated by INAAT showed an increased dielectric constant of 19.03 while retaining an appropriate dielectric loss of 0.0109, as compared to the dielectric constant (14.64) of polyimide/APTS-treated BaTiO₃ composite. The results of this work demonstrate the potential use of an INAAT coupling agent with more organo functional groups for obtaining enhanced dielectric properties in a polyimide/BaTiO₃ composite for application in an embedded capacitor.     

Effect of vanadium doping on sintering and dielectric properties of strontium barium niobate ceramics

Strontium barium niobate, Sr_0.5Ba_0.5Nb₂O₆ (SBN50) ceramics doping up to 3 wt% V₂O₅ were fabricated by solid state reaction route, starting from raw materials (oxides and carbonates) of analytical grade. The phase composition, microstructure and dielectric properties were investigated by X-ray diffraction, scanning electric microscope and impedance analyzer. The results show that the addition of V₂O₅ improves sintering densification of SBN ceramic samples. The relative density of the samples firstly increases and then slightly decreases with increasing amounts of V₂O₅ and sintering temperature. With the help of the additive of 1 wt% V₂O₅, the relative density of the sample sintered at 1,280 °C for 3 h can reached 97.2%. Only single tetragonal tungsten bronze phase SBN exists in all the doped samples. With increase in V₂O₅ content, the dielectric constant of SBN ceramics at both room temperature and in the vicinity of the phase transition temperature increases significantly and the Curie temperatures (Tc) obviously shifts to low temperature as well as the dielectric loss remains below 0.06. The diffuseness in the phase transition is found to increase with increase in vanadium doping level. The addition of V₂O₅ results in an increased grain size associated with rod-like grain growth.     

La2O3掺杂对BST/MgO复合陶瓷显微结构和性能的影响

采用固相反应法制备了未掺杂和La2O3掺杂(0.5%、1%、2%(摩尔分数))的Ba0.55Sr0.45TiO3/MgO复合陶瓷材料,并研究了它们的显微结构和各种介电性能.研究结果表明,La2O3除一部分会进入BST晶格獭代Ba或Sr的位置外,还会有一部分与MgO等形成无定形态物质滞留在晶界,起到抑制BST晶粒生长的作用.BST/MgO复合陶瓷的居里温度随La2O3掺杂量的增大而降低,居里温度的降低导致了介电常数的减小.适量的La2O3掺杂提高了复合陶瓷的调谐性,而且La2O3掺杂明显降低了复合陶瓷的微波介电损耗.0.5%(摩尔分数)La2O3掺杂的BST/MgO复合陶瓷具有最佳的综合介电性能,其在10kHz下的调谐性为6.9%(2kV/mm),3.99GHz时的介电常数和介电损耗分别为87.5和3.35×10-3,基本可以满足铁电移相器的使用要求.