Phase-field simulation of polarization switching and domain evolution in ferroelectric polycrystals

A phase-field model is developed for predicting the polarization switching and domain structure evolution under an applied electric field in ferroelectric polycrystals. The model takes into account realistic grain structures as well as various energetic contributions, including elastic energy, electrostatic energy, and domain wall energy. A hysteresis loop – average polarization as a function of applied electric field – is computed, and the detailed domain evolution process during switching is analyzed. In particular, the role of grain boundaries in the nucleation and growth of new domains is studied. It is shown that switching takes place through the nucleation of 90° domains at grain boundaries and subsequent growth into the grain interiors instead of direct 180° domain switching. A correlation between the domain structures in neighboring grains was observed, and polarization switching in one grain was found to affect the switching in neighboring grains.     

Dielectric properties of PbTiO3/ZnO ceramic nanocomposites obtained by solid-state reaction method

In this paper, we report on the dielectric properties of PbTiO₃/ZnO ceramic nanocomposites prepared by a conventional solid-state reaction method with improvement in densification by the addition of ZnO nanowhiskers. Phase formation, densification, microstructure and dielectric properties of the composites were investigated as a function of the content of ZnO nanowhiskers. Densification behavior of the ceramic nanocomposites was significantly enhanced, as compared to pure PbTiO₃ ceramics. Moreover, the dielectric constant of the composites was higher than that of the pure PbTiO₃ ceramics.     

相场法研究Ni-Al间作用势对Ni75Al14Cr11合金L10预析出相的影响

采用微观相场法研究了Ni_(75)Al_(14)Cr_(11)合金第1个近邻到第4个近邻Ni-Al原子间相互作用势对L1_0预析出相沉淀过程的影响。结果表明,当第3近邻Ni-Al原子间相互作用势增大或第4种近邻Ni-Al原子间相互作用势减少时,L1_0预析出相和L1_2平衡相均提前沉淀,且L1_0预析出相体积分数减少而L1_2相的最终体积分数几乎不变;当第3近邻Ni-Al原子间相互作用势减少或第4近邻Ni-Al原子间相互作用势的增大时,L1_0预析出相和L1_2最终相都会推迟沉淀,且L1_0预析出相和L1_2相体积分数均增大;与上述2种情况相比,改变第1近邻Ni-Al原子间相互作用势对L1_0预析出相沉淀过程的影响较小,而改变第2近邻Ni-Al原子间相互作用势对沉淀过程几乎没有影响。进一步研究表明,Ni-Al原子间相互作用势改变不影响合金的沉淀机制,将影响L1_0预析出相、L1_2平衡相的析出时间、速度和2种相的体积分数,沉淀形貌等,从而影响Ni_(75)Al_(14)Cr_(11)高温合金的结构和性能,对合金优化设计有指导意义。     

Epitaxial 0.65PbMg1/3Nb2/3O3-0.35PbTiO3 (PMN-PT) thin films grown on LaNiO3/CeO2/YSZ buffered Si substrates

Ferroelectric PMN-PT thin films with a thickness of 600 nm were epitaxially grown on buffered Si (0 0 1) substrates at a substrate temperature that ranged from 550 to 700 °C using pulsed laser deposition (PLD). LaNiO₃ (LNO) electrode thin films with a resistivity of ∼1900 μΩ cm were epitaxially grown on CeO₂/YSZ buffered Si (0 0 1) substrates. The PMN-PT thin films grown at 600 °C on LNO/CeO₂/YSZ/Si substrates had a pure perovskite and epitaxial structure. The PMN-PT films exhibited a high dielectric constant of about 1818 and a low dissipation factor of 0.04 at a frequency of 10 kHz. Polarization-electric-field (P-E) hysteresis characteristics, with a remnant polarization of 11.1 μC/cm² and a coercive field of 43 kV/cm, were obtained in the epitaxial PMN-PT films.     

Improving the piezoelectric thermal stability by tailoring phase transition behavior in the new (1 − x)[0.65PbMg1/3Nb2/3O3-0.35PbTiO3]-xBiZn1/2Ti1/2O3 perovskite solid solutions

The phase transition behavior and its effect on thermal stability of the piezoelectric properties of the (1 − x)[0.65PbMg_1/3Nb_2/3O₃-0.35PbTiO₃]-xBiZn_1/2Ti_1/2O₃ ceramics with 0 ≤ x ≤ 0.06 were investigated. The phase transition from the monoclinic to tetragonal phase was determined by the dielectric constant and elastic constant measurements. The temperature independent piezoelectric response with −d₃₁ = 188 pC/N was obtained from 175 to 337 K for the composition with x = 0.02. The enhanced thermal stability of piezoelectric response was achieved by shifting the monoclinic-tetragonal phase transition to the lower temperature.     

不同退火气氛对0.7BiFeO3-0.3PbTiO3薄膜铁电及介电性能的影响

利用溶胶-凝胶法在LaNiO3/SiO2/Si衬底上制备了0.7BiFeO3-0.3PbTiO3（BFPT7030）薄膜,利用快速退火方式将薄膜分别在空气、氧气流、空气流、氮气流中进行后续退火处理。在空气、氧气流及空气流中退火的薄膜均完全结晶并呈现高度（100）择优取向。而在氮气流中退火的薄膜由于结晶很差,测试不出其电滞回线。空气中退火的BFPT7030薄膜表现出最大的剩余极化及最小的漏电流,Pr 为30μC cm-2, 而在空气流中退火的BFPT7030薄膜表现出最小的剩余极化(Pr: 13μC cm-2)及最大的漏电流。XPS测试结果表明,在空气、氧气流及空气流中退火的BFPT7030薄膜中Fe3 :Fe2 分别为2.09:1, 1.65:1 及 1.5:1。而在氧气流及空气流中退火的BFPT7030薄膜中Bi及Pb的相对含量低于在空气中退火的薄膜。铁离子的价态波动是产生氧空位的原因,增加氧气有助于抑制氧空位的产生。虽然Pb的挥发会导致较差的微观结构,但其挥发并不会导致氧空位的产生。     

Preparation and electrical properties of Bi0.5Na0.5TiO3-BaTiO3-KNbO3 lead-free piezoelectric ceramics

Lead-free (1 − x)Bi_0.47Na_0.47Ba_0.06TiO₃-xKNbO₃ (BNBT-xKN, x = 0-0.08) ceramics were prepared by ordinary ceramic sintering technique. The piezoelectric, dielectric and ferroelectric properties of the ceramics are investigated and discussed. The results of X-ray diffraction (XRD) indicate that KNbO₃ (KN) has diffused into Bi_0.47Na_0.47Ba_0.06TiO₃ (BNBT) lattices to form a solid solution with a pure perovskite structure. Moderate additive of KN (x ≤ 0.02) in BNBT-xKN ceramics enhance their piezoelectric and ferroelectric properties. Three dielectric anomaly peaks are observed in BNBT-0.00KN, BNBT-0.01KN and BNBT-0.02KN ceramics. With the increment of KN in BNBT-xKN ceramics, the dielectric anomaly peaks shift to lower temperature. BNBT-0.01KN ceramic exhibits excellent piezoelectric properties and strong ferroelectricity: piezoelectric coefficient, d₃₃ = 195 pC/N; electromechanical coupling factor, k_t = 58.9 and k_p = 29.3%; mechanical quality factor, Q_m = 113; remnant polarization, P_r = 41.8 μC/cm²; coercive field, E_c = 19.5 kV/cm.     

Highly (111)-oriented PbTiO3 films prepared by rf planar magnetron sputtering and their optical properties

Perovskite type PbTiO₃ films with highly [111] preferred orientation were synthesized successfully on the amorphous sapphire substrates by rf planar magnetron sputtering with an element target. Studies reveal that the film has a good alignment in the growth direction and a grain size approximately 180 nm. The mechanism of film growth was studied and the results manifested that an interfacial layer formed as self-buffer layer. This transition layer released the intrinsic stress, so that the film with excellent structure could be grown on amorphous substrates. The refractive index at room temperature was determined in the wavelength range 400–2500 nm from the transmittance at normal incidence. The dispersion was found to fit the individual dipole oscillator model very well, and the minimum refractive index of the film is deduced as 2.51.     

Magnetoelectric CoFe2O4/Pb(Zr0.53Ti0.47)O3 composite thin films of 2-2 type structure derived by a sol-gel process

CoFe₂O₄/Pb(Zr_0.53Ti_0.47)O₃ (CFO/PZT) magnetoelectric composite thin films of 2-2 type structure have been prepared onto Pt/Ti/SiO₂/Si substrate by a sol-gel process and spin coating technique. The optimal annealing process of composite thin films was determined by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It is found that the amount of the citric acid and concentration of CFO starting precursor solution have great impact on morphologies of composite thin films. Subsequent scanning electron microscopy (SEM) investigations show that the prepared thin films exhibit good morphologies and compact structure, and cross-sectional micrographs clearly display a multilayered nanostructure of multilayered thin films. The purpose of this work is to determine the optimal annealing processes of composite thin films and to prepare magnetoelectric composite thin films with good microstructure. It is shown that the films exhibit both good magnetic and ferroelectric properties, as well as a magnetoelectric effect.     

Effect of LiSbO3 on the phase structure, microstructure and electric properties of Sr0.53Ba0.47Nb2O6 ceramics

LiSbO₃ doped Sr_0.53Ba_0.47Nb₂O₆ ceramics were synthesized by conventional mixed-oxide method. The phase structure, microstructure, dielectric and ferroelectric properties of obtained ceramics were investigated. Pure tungsten bronze structure could be obtained in all ceramics and LiSbO₃ additive could promote densification and reduce the sintering temperature. The dielectric characteristics showed diffuse phase transition phenomena for all samples, which was proved by linear fitting of the modified Curie-Weiss law with γ value varying between 1.65 and 1.92. With increasing LiSbO₃ content, the transition temperature T_c decreased gradually to near room temperature. Normal ferroelectric hysteresis loops could be observed in all compositions, but the remnant polarization (P_r) and coercive field (E_c) all decreased gradually. Besides, the underlying mechanism for variations of the electrical properties caused by LiSbO₃ doping was explained in this work.     

X-ray diffraction study of Ba0.985Na0.015Ti0.985Nb0.015O3, Ba0.6Na0.4Ti0.6Nb0.4O3 and Ba0.3Na0.7Ti0.3Nb0.7O3 compositions

The Ba_0.985Na_0.015Ti_0.985Nb_0.015O₃, Ba_0.6Na_0.4Ti_0.6Nb_0.4O₃ and Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ compositions of the (1 − x) BaTiO₃–xNaNbO₃ (BTNNx) system have been studied by X-ray diffraction and by measurements of dielectric properties. The specimens with composition BTNN (x = 0.015, 0.40 and 0.70) have been refined by the JANA program from X-ray powder diffraction data. Ceramic samples with composition (1 − x) BaTiO₃ + xNaNbO₃ (where x = 0.015, 0.40 and 0.70) were prepared by calcinations from appropriate mixture of BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅. The calcined powder was sintered at temperature range 1200–1400 °C. As the composition x increased from 0.015 (and 0.70), the ferroelectric ceramics (x = 0.015, FE) with tetragonal phase changed to the ferroelectric relaxors (RFE, x = 0.40). RFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to 180 K) due to a frustration between RFE and FE state. These ceramics present the classical ferroelectric character when 0 ≤ x < 0.075 and 0.55 < x ≤ 1 and relaxor character when 0.075 ≤ x ≤ 0.55.     

Phase-field simulation of abnormal grain growth due to inverse pinning

The possibility of abnormal grain growth due to inverse pinning was verified using phase-field simulations. In bicrystalline systems with circular precipitates, the perfect wetting condition is required for the long-distance migration of the interface between the matrix grains. If the distance between precipitates that are perpendicular to the interface exceeds a critical value, the migration is not observed irrespective of the wetting condition. In polycrystalline systems, abnormal grain growth occurs with the aid of the driving force for grain growth even though l_lim exceeded the critical value, where l_lim is the minimum distance between precipitates. Furthermore, the perfect wetting condition is not required for the abnormal grain growth in the polycrystalline systems. These facts enlarge the possibility of inverse pinning in real alloy systems.     

Thermoelectric properties of rare earth doped SrTiO3

The thermoelectric properties of Sr_0.9R_0.1TiO₃ (R=Y, La, Sm, Gd, Dy) have been measured from room temperature to 1073 K. The electrical conductivities and Seebeck coefficients are independent of the kind of rare earth elements in the temperature range, so the figure of merits are influenced by the difference in the thermal conductivities. The thermal conductivities decrease with doping according to the rare earth atomic mass and ionic radius. Sr_0.9Dy_0.1TiO₃ shows the highest figure of merit of the investigated samples, reaching 3.84×10⁻⁴ K⁻¹ at 573 K.     

Phase-separation-induced self-assembly of controllable PbTiO3 nanodots on Si substrates

In this work, a phase-separation-induced self-assembly (PSIA) approach was developed to prepare PbTiO₃ nanodots on substrates. Lead titanate nanodots on Si wafer were obtained through Marangoni instability induced phase separation and following heat-treatment. It was found that acetylacetone added in the precursor played important roles not only in the incorporation of Pb into the nanodots, but also in the phase separation process. The size and density of PT nanodots could be controlled through the concentrations of precursor and phase separation adjuster, i.e., tetrabutyl-titanate and polyvinylpyrrolidone. The nanodots size and density were from 10 nm to ∼100 nm and from 0.5 × 10¹⁰ dots/cm⁻² to 4.6 × 10¹⁰ dots/cm⁻² respectively. Such preparation provides a simple way to prepare titanate nanodots on substrate.     

Structural and electrical properties of SrBi2(Ta0.5Nb0.5)2O9 thin films

SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBTN) thin films were obtained by polymeric precursor method on Pt/Ti/SiO₂/Si(1 0 0) substrates. The film is dense and crack-free after annealing at 700 °C for 2 h in static air. Crystallinity and morphological characteristic were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FEG-SEM) and atomic force microscopy (AFM). The films displayed rounded grains with a superficial roughness of 3.5 nm. The dielectric permittivity was 122 with loss tangent of 0.040. The remanent polarization (P_r) and coercive field (E_c) were 5.1 μC/cm² and 96 kV/cm, respectively.     

Film synthesis of SrTiO3 by the spray-ICP technique

SrTiO₃ films were synthesized by spray pyrolysis in combination with a high-temperature inductively-coupled plasma (ICP) as the heat source (spray-ICP technique). Sapphire and fused quartz were used as substrates. Ten minutes of film formation yielded films of 0.5–1.0 μm in thickness. The structure of the films consisted of crystallites about 0.1 μm in size. With respect to the homogeneity of the size and shape of the crystallites, sapphire is superior over fused quartz. X-ray diffraction measurements of the films revealed preferred orientation peaks of (2 0 0), (1 1 1), and (1 1 0) depending on the substrates and their locations from the ICP source.     

Structural and piezoelectric properties of LiNbO3-modified BiScO3–PbTiO3 ceramics

Piezoelectric perovskite materials based on the solid solution (1 − x)BiScO₃–xPbTiO₃ (BSPT) have been attracting attention for their high Curie temperature (T_c = 450 °C) and excellent piezoelectric properties. The LiNbO₃ (LN), which has a T_c as high as 1150 °C, has been recently reported forming a phase pure perovskite solid solution with some perovskite structure compounds. In the current work, the effects of LN substitution on the structural and electrical properties of BSPT ceramics were investigated in the 0.36BiScO₃–0.64{(1 − x)PbTiO₃–xLiNbO₃} (BSPTLNx) system. The results of LN addition in the BSPT ceramics show significant enhancement of the piezoelectric properties. The piezoelectric constant d₃₃, planar electromechanical coupling coefficient and remnant polarization P_r values reached 465 pC/N, 0.57 and 48 μC/cm², respectively, for x = 0.04. The T_c gradually decreases with increasing LN content in the BSPTLNx system, due to the structure transform from the tetragonal to the rhombohedral. A typical relaxor behavior is also produced with the LN substitution in the BSPTLNx system.     

Structural, ferroelectric and dielectric properties of In2O3:Sn (ITO) on PbZr0.53Ti0.47O3 (PZT)/Pt and annealing effect

Ferroelectric indium tin oxide (ITO) on PbZr_0.53Ti_0.47O₃ (PZT)/Pt structure, prepared by RF sputtering onto SiO₂/Si substrates, is studied in order to investigate the effect of ITO as a top electrode in these systems. X-ray diffraction, scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments were performed to study the structure and the surface morphology of the samples. From X-ray diffraction, we observe that the ITO thin film grows with the (1 1 1) texture and the peaks attributed to PZT are all from the perovskite phase. The average roughness (RMS) of the PZT surface is found to be 1.650 nm from AFM experiment. The ferroelectric and dielectric properties were inferred from polarization hysteresis loops, capacitance and dielectric constant measurements. These properties have been compared to those of the widely studied Pt/PZT/Pt system prepared under the same conditions. The effect of ITO/PZT/Pt annealing has been studied. Annealing at 400 °C leads to 13% increase in the dielectric constant ?_r.