Nanocrystalline Barium Strontium Titanate Ceramics Synthesized via the “Organosol” Route and Spark Plasma Sintering

Dense nanocrystalline barium strontium titanate Ba_0.6Sr_0.4TiO₃ (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified “Organosol” process. X‐ray diffraction (XRD) and dielectric measurements in the temperature range 173–313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature T_C decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%–4% in the frequency range of 100 Hz–1 MHz and temperature interval 160–320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer‐sized ceramics.     

The influence of phase transition on electrocaloric effect in lead‐free (Ba0.9Ca0.1)(Ti1−xZrx)O3 ceramics

In this paper, the influence of phase evolution on polarization change and electrocaloric response in lead‐free (Ba_0.9Ca_0.1)(Ti_1?xZr_x)O₃ ceramics (BCTZ) was systematically investigated. With increasing Zr/Ti ratio, the phase structure and phase transition behavior were greatly changed, resulting in various temperature and electric field dependence of electrocaloric responses. For x=0.05, a peak electrocaloric temperature change 1.64 K (at 130°C) and corresponding entropy change 1.78 J·kg^?1·K^?1 were obtained for 0‐7 kV·mm^?1 electric field. Negative electrocaloric temperature change in ?0.1 K was obtained below Curie temperature (T_c), which may be induced by the orthorhombic‐tetragonal ferroelectric phase transition. With the increase in x, the peak value of the electrocaloric response decreased but much better temperature stability was observed. Simultaneously the negative electrocaloric response gradually disappeared with the disappearance of the low temperature ferroelectric‐ferroelectric phase transition. For x=0.2, electrocaloric response showed good temperature stability ranging from room temperature to 130°C, attributing to the relaxor ferroelectric feature.     

溶胶-凝胶法制备钛酸锶钡铁电薄膜的研究

钛酸锶钡(BST)薄膜是一类重要的铁电薄膜材料。采用溶胶-凝胶法制备了不同组分的具有钙钛矿结构的BST薄膜。利用X射线衍射技术(XRD),研究了不同退火条件下BST薄膜的结晶特性,结果表明制备的BST薄膜形成了单一的钙钛矿结构;利用扫描电子显微镜(SEM)和原子力显微镜(AFM)观察了薄膜的表面形貌,结果表明制备的BST薄膜光滑,平整,无明显的孔洞和裂纹,且生长良好。BST薄膜的晶粒细致,排列整齐,分布均匀,呈现球状。     

Recent Materials Characterizations of [2D] and [3D] Thin Film Ferroelectric Structures

A review is given of ceramic and single-crystal thin film ferroelectric oxides, emphasizing perovskite phases, together with some new developments on hafnia films. It is shown that single-crystal barium titanate films behave as bulk down to at least 77 nm, with no finite size effects, no phase transition temperature shifts, and no dielectric peak broadening or change from first- to second-order transitions, suggesting that the gradient defect model of Bratkovsky and Levanyuk correctly describes such effects as extrinsic in experimental studies of equally thin ceramic thin films. In ceramic barium–strontium titanate (BST) thin films, it is shown that there is also no intrinsic broadening or shifts in phase transitions, with sharp, unshifted, bulk-like transitions observed only as re-entrant upon warming from cryogenic temperatures; this shows that phase transitions in ceramic thin films are dominated by kinetics and not thermodynamics and are definitely not equilibrium measurements. At high fields (>1 GV/m), the films exhibit space charge-limited conduction; no variable-range hopping is observed, contrary to recent studies on SrTiO₃. Some novel, unconventional switching processes are discussed, comparing the "perimeter effect" (non-equilibrium, ballistic) with Molotskii's equilibrium model. Theory and experiment are described for [3D] nanotubes, nanorods, and nano-ribbons (or micro-ribbons). The layered-structure-perovskite–pyrochlore conversion in bismuth titanate is described together with the PbO+TiO₂ phase separation in lead zirconate titanate during electrical breakdown, as are novel HfO₂ precursors that demonstrate enhanced temperature crystallization from the amorphous state and hence commercial advantages for front-end processing.     

Preparation and characterization of Pb(Lu1/2Nb1/2)O3–Pb(In1/2Nb1/2)O3–PbTiO3 ternary ferroelectric ceramics with high phase transition temperatures

To explore new relaxor‐PbTiO₃ systems for high‐power and high‐temperature electromechanical applications, a ternary ferroelectric ceramic system of Pb(Lu_1/2Nb_1/2)O₃–Pb(In_1/2Nb_1/2)O₃–PbTiO₃ (PLN–PIN–PT) have been investigated. The phase structure, dielectric, piezoelectric, and ferroelectric properties of the as‐prepared PLN–PIN–PT ceramics near the morphotropic phase boundary (MPB) were characterized. A high rhombohedral‐tetragonal phase transition temperature T_R‐T of 165°C and a high Curie temperature T_C of 345°C, together with a good piezoelectric coefficient d₃₃ of 420 pC/N, were obtained in 0.38PLN–0.20PIN–0.42PT ceramics. Furthermore, for (0.8?x)PLN–0.2PIN–xPT ceramics, the temperature‐dependent piezoelectric coefficients, coercive fields and electric‐field‐induced strains were further studied. At 175°C, their coercive fields were found to be above 9.5 kV/cm, which is higher than that of PMN–PT and soft P5H ceramics at room temperature, indicating PLN–PIN–PT ceramics to be one of the promising candidates in piezoelectric applications under high‐driven fields. The results presented here could benefit the development of relaxor‐PbTiO₃ with enhanced phase transition temperatures and coercive fields.     

Enhancement of relaxor properties by Nb doping in Ba0.8Sr0.12Ca0.08TiO3 lead-free ferroelectric ceramics

Niobium-doped barium strontium calcium titanate (BSCTN) ceramics were prepared using a conventional solid-state reaction method. The effects of Nb contents on crystal structure, microstructure, dielectric properties and ferroelectric relaxor behavior of the BSCTN ceramics were investigated. The BSCTN ceramics showed dense microstructures with uniform crystal grains with Nb doping. It was demonstrated that Nb⁵⁺ entered the B-site of the perovskite BSCTN ceramic and substituted for Ti⁴⁺, which caused the expansion and distortion of crystal lattice of the tetragonal BSCTN ceramic. Doping of Nb resulted in more diffused phase transition and lower Curie temperature of the BSCTN ceramics. The diffuseness degree indicator γ increased until the addition of Nb dopant exceeded 1.5 mol% where a maximum γ of 1.98 was achieved. Among the compositions assayed in this work, the BSCTN ceramics with Nb contents of approximately 1.0–1.5 mol% yielded promising relaxor properties that made them alternative sources for development of environmental friendly lead-free relaxor ferroelectric materials.     

Dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates

The dielectric and microwave properties of barium strontium titanate (BST) thick films on alumina substrates have been investigated. The BST films were screen printed and sintered at temperatures below 1300 °C. At temperatures below the Curie point the BST films exhibit tunability in the range 15–35% under a DC bias field of 2 kV/mm. The dielectric loss is critically dependent on film thickness with lower losses (<10⁻²) for the thicker films (>100 μm). A relaxation process appears to take place for the BST films in the MHz to GHz frequency regime. The variation of permittivity with bias field exhibits hysteretic behaviour in both the ferroelectric and paraelectric regions. This is believed to arise due to the non-uniform composition and existence of micro/nano-polar phases in the films.     

Lead‐Free (Ba0.70Sr0.30)TiO3‐Modified Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics with Large Electric Field–Induced Strains

The dielectric, ferroelectric, and electric field–induced strain behavior of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT) ceramics modified with (Ba_0.70Sr_0.30)O₃ (BST) were investigated as a function of composition and temperature. The ceramic samples were synthesized by a solid‐state mixed oxide method and sintered at 1125°C for 2 h. The XRD and Raman spectra showed coexisting rhombohedral and tetragonal phases throughout the entire compositional range with the tetragonal phase becoming dominant at higher BST concentrations. For all compositions, the temperature dependence of the dielectric spectra revealed a frequency dependence that is characteristic of a relaxor mechanism. This suggests that these ceramics lacked long‐range order and it appears that the maximum disorder was observed for the composition with 5 mol% BST (BNKT–0.05BST sample). This was evidenced by the observation of pinched hysteresis loops, even at room temperature, and a significant decrease in the P_r and E_c values which resulted in large electric field–induced strains (S_max) of 0.40% and a normalized strain coefficient ( = S_max/E_max) of 732 pm/V. This significant strain enhancement at the composition of x = 0.05 may be attributed to both a composition‐induced structural phase transition and a field‐induced relaxor to ferroelectric phase transition.     

Microstructure and Dielectric Properties of Barium Strontium Titanate Thick Films and Ceramics With a Concrete-Like Structure

Barium strontium titanate (Ba_0.6Sr_0.4TiO₃, BST) thick films and ceramics with a concrete-like structure were fabricated by a novel sol–gel-based route. In this process, well-crystallized nano-sized or micrometer-sized BST powders or their combinations are dispersed into a sol–gel matrix. Crack-free thick films and dense BST ceramics have been fabricated at a low sintering temperature with a very inhomogeneous concrete-like structure. The dielectric properties of such thick films and ceramics have been investigated. The results show that the dielectric constant and loss tangent of the thick films are 1209 and 0.016, respectively, at 0°C, 10 kHz, and those of the bulk BST ceramics are 5510 and 0.02, respectively, at 0°C, 10 kHz.     

Tuning the electrocaloric effect in 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics by relaxor phase blending

The pseudo-first-order phase transition in 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ ceramics leads to a sharp increase in temperature change (ΔT) in the vicinity of the ferroelectric-to-relaxor transition temperature T_FR (~100 °C) [Appl. Phys. Lett. 110 (2017) 182904]. In this study, we add the 0.78Bi_0.5Na_0.5TiO₃-0.06BaTiO₃-0.16(Sr_0.7Bi_0.2)TiO₃ relaxor phase to the 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ ferroelectric matrix to tune its electrocaloric effect. The results show that the addition of the relaxor phase plays a vital role in phase and local-structure evolution. A transition occurs between the ferroelectric and ergodic relaxor phases when the mass fraction of the latter increases to 30% (x = 0.3), as verified by X-ray diffraction analysis, Raman spectroscopy, and polarization-electric field (P-E) hysteresis loops. Furthermore, addition of the relaxor phase reduces the T_FR from 76 °C at x = 0.1–55 °C at x = 0.2; however, this transition disappears at x = 0.3 and 0.4 composite. In-situ piezo-force microscopy (PFM) images illustrate that domains can be written into x = 0.1 and 0.2 ceramics with a valley in the piezoresponse curves. Increasing the temperature agitates the domain arrangement and decreases the contrast for PFM images; this indicates a gradual phase transition in the composite. The temperature corresponding to maximum ΔT exhibits a downward shift (0.58 K at 80 °C for x = 0.1 and 0.5 K at 65 °C for x = 0.2), while the temperature-ΔT curves are flat when x = 0.3 and 0.4. Moreover, the maximum ΔT shows a decrease with an increase in the relaxor phase content; this is believed to be related to a decrease in the latent heat due to a pseudo-first-order to second-order transition. Thus, we suggest that the incorporation of a relaxor phase into ferroelectric matrices is an effective technique to tune their electrocaloric effect and improve the thermal stability of ceramic composites.     

Effect of Yb Codoping on the Phase Transition,and Electrical and Photoluminescence Properties in KNLN:Er/xYb Ceramics

Ceramics 0.94(K_0.5Na_0.5)NbO₃?0.06LiNbO₃:Er/xYb with x = 0, 0.002, 0.004, 0.006, 0.008 were fabricated in this study, and phase structure, dielectric, piezoelectric, ferroelectric, and upconversion photoluminescence properties of the ceramics were systematically studied. Results show that all ceramics are in the polymorphic phase transition region near room temperature. However, a relaxor‐like phase transition was observed as Yb increasing to over x = 0.004. The optimized dielectric, piezoelectric, and ferroelectric properties, and maximized photoluminescence intensity ratio are obtained at x = 0.004. And the electrical and photoluminescence properties of the ceramics were discussed from the point view of the normal to relaxor‐like ferroelectric phase transition. The present study demonstrates that both the electrical and upconversion photoluminescence properties of the ceramics have an intimate correlation with the normal to relaxor‐like ferroelectric transition induced by Yb doping.     

Low Temperature Deposition of High Performance Lead Strontium Titanate Thin Films by in situ RF Magnetron Sputtering

Highly (100) oriented lead strontium titanate (Pb_0.4Sr_0.6TiO₃) thin films were deposited on LaNiO₃ ‐coated Si substrate via radio‐frequency magnetron sputtering method with substrate temperature ranging from 300 to 500°C. The PST thin films were crystallized at a temperature as low as 300°C, which may result from the well‐controlled stoichiometry and the in situ crystallization on seed layer. At an electric field of 400 kV/cm, high tunability of 43% and 57% can be achieved for PST films deposited at 300°C and 500°C, respectively. Moreover, the dielectric response shows weak frequency dependence and the loss factor stays relatively low. The results suggest that such films should be promising candidate for the microwave tunable devices compatible with the current Si technology.     

Mn和Zr共掺杂钛酸锶钡/氧化镁复合陶瓷材料的微结构和介电性能

通过固相反应法制备Mn、Zr共掺杂钛酸锶钡/氧化镁陶瓷粉体,经干压成型后在空气气氛中于1450℃烧结4h,通过扫描电子显微镜和X射线衍射研究了ZrO2和MnO2共掺杂的Ba0.6Sr0.4TiO3/MgO复合陶瓷材料的微结构和介电性能。结果表明:ZrO2可以显著降低材料的介电常数和介电损耗,有效提高了陶瓷材料的温度稳定性;随ZrO2添加量的增加,体系的晶胞参数略有增加,MgO在钛酸锶钡中以独立相的形式存在;制备出的BST铁电陶瓷材料的25℃相对介电常数较低(εr<110),介质损耗小于1.0×10–3(在频率为10kHz时),温度系数小于6.012×10–3,可调性大于20%(8.0kV/mm),适用于制作移相器。     

Microwave dielectric properties of the (BaxSr1−x)TiO3 thin films on alumina substrate

Barium strontium titanate, (Ba_xSr_1?x)TiO₃ (BST) thin films have been prepared on alumina substrate by sol–gel technique. The X-ray patterns analysis indicated that the thin films are perovskite and polycrystalline structure. The interdigital electrode with 140 nm thickness Au/Ti was fabricated on the film with the finger length of 80 μm, width of 10 μm and gaps of 5 μm. The temperature dependence of dielectric constant of the BST thin films in the range from ?50 °C to 50 °C was measured at 1 MHz. The dielectric properties of the BST thin films were measured by HP 8510C vector network analyzer from 50 MHz to 20 GHz.     

Effect of Nb doping on the relaxor behaviour of (Pb0.75Ba0.25)(Zr0.70Ti0.30)O3 ceramics

Strong influence of niobium admixture added to the lead–barium–zirconate–titanate ceramics of a chosen composition Ba/Zr/Ti 25/70/30 on grain structure, dielectric, and pyroelectric properties was confirmed. The Nb-modified ceramics exhibit classical relaxor ferroelectric behaviour similar to other complex lead perovskites such as lead–lanthanum–zirconate–titanate-type ceramics. Additional anomalies in ɛ′(T) curves in low frequency range were observed in the paraelectric phase for undoped ceramics. These anomalies and some disturbances in regularities typical for the relaxor ferroelectric behaviour in the vicinity of diffuse ferroelectric–paraelectric phase transition in undoped PBZT ceramics are eliminated by the Nb admixture. An attempt at a quantitative explanation is presented in the paper.     

Non-ergodic – ergodic transition and corresponding electrocaloric effect in lead-free bismuth sodium titanate-based relaxor ferroelectrics

The ferroelectric refrigeration technology based on electrocaloric effect (ECE) is a promising environmental way to replace the gas compression refrigeration. Lead-free bismuth sodium titanate (NBT)-based relaxor ferroelectric ceramics have advantages of a large ECE within a wide temperature range around the depolarization temperature (T_d), but the T_d of pure NBT is too high to meet the application requirements. Here, we systematically studied the electrocaloric effect in (0.95-x)(Na_0.5Bi_0.5)TiO₃-0.05SrTiO₃-x(K_0.5Na_0.5)NbO₃ ((0.95-x)NBT-0.05ST-xKNN) ceramics and its relation with the relaxor behavior. The addition of KNN enhances the relaxor character, and shifts the T_d from 120 °C to a low temperature even below 0 °C. The directly measured ECE shows a peak of high ?T_max = 0.88 K (@ 60 °C) for x = 0.05 under an electric field of 40 kV/cm and a wide temperature span of 79 °C (≥ 80% of ?T_max). The x = 0.07 sample exhibits the optimal room-temperature ECE performance with a high ?T_30 °C = 0.71 K because the non-ergodic – ergodic phase transition occurs near room temperature. The electric field–temperature phase diagram reveals the origin of the enhanced ECE as the electric-field-induced transition between polar nanoregions (PNRs) and long-range ferroelectric domain.     

Morphotropic phase boundary and electrical properties of lead-free (K0.5Bi0.5)TiO3–Bi(Ni0.5Ti0.5)O3 relaxor ferroelectric ceramics

Lead-free relaxor ferroelectric ceramics (1?x)(K_0.5Bi_0.5)TiO₃–xBi(Ni_0.5Ti_0.5)O₃ were prepared by a conventional solid-state route, the phase transition behavior and corresponding electrical properties were investigated. A typical morphotropic phase boundary (MPB) between rhombohedral and tetragonal ferroelectric phases was identified to be in the range of 0.05<x<0.07 where the optimum piezoelectric and electromechanical properties of d₃₃=126 pC/N and k_P=18% were achieved. Most importantly, a high Curie temperature ~320 °C, around which the material shows a typical relaxor ferroelectric behavior characterized by the presence of diffuse phase transition and frequency dispersion, was obtained in MPB compositions, significantly higher than those of some existing MPB lead-free titanate systems. These results demonstrate a tremendous potential of the studied system for device applications.     

Ergodicity of fine-grained canonical relaxor ferroelectric (Bi0.5Na0.5)1-xBaxTiO3 films

The drastic reduction in dimensions in thin films, together with the low crystallization temperatures used, normally results in a large reduction in the grain size. It has been reported that relaxor ferroelectric states are stabilized at room temperature for fine-grained ceramics and films that behave as normal ferroelectrics for large grains. In this work, the effects of the grain size reduction on the relaxor characteristics are analyzed for a composition that is already a canonical relaxor with a nonergodic state at room temperature: (Bi_0.5Na_0.5)_1-xBa_xTiO₃ (BNBT). The comparison of the local polar ordering within BNBT grains studied with piezoresponse force microscopy on large-grained ceramics and fine-grained thin films shows that the development of stable long-range ferroelectric order with the application of an electric field is hampered due to the small grain size of the grains. The ergodic character of the high-temperature phase is thus stabilized at room temperature, following a similar mechanism as the one discussed for other noncanonical relaxors.     

The effects of Barium Strontium Titanate (BST) on the soft magnetic properties and loss performance of MnZn ferrites

With the increasing power density of the switched mode power supply (SMPS) developed nowadays, higher efficiency is required from the magnetic core, where the MnZn ferrites are often adopted. However, the relatively high operating temperature of the SMPS often results in reduced resistivity of the MnZn, which increases the eddy current loss. To enhance the resistivity of MnZn ferrite at high temperature range (>100 °C), donor-doped barium strontium titanate (BST) with a positive temperature coefficient of resistivity (PTCR) is prepared and dopped in the MnZn ferrite. The influence of BST addition from 0.000 wt% to 0.020 wt% on the MnZn ferrite is investigated over a wide temperature range from 25 °C to 140 °C. The XRD result suggests ionic exchange between the spinel phase and perovskite phase. The SEM result shows a refined and more uniform microstructure of MnZn ferrite brought about by the BST addition. At the maximum of 0.020 wt%, the BST addition shows almost no influence on density and the saturation magnetic induction. However, the initial permeability is slightly reduced by the BST addition, due to the microstructural change. Moreover, the BST concentrating at the grain boundaries improves the DC-resistivity across the temperature range from 25 °C to 140 °C. Due to the addition of BST, the reduction in eddy current loss at 300kHz/100 mT is around 35% at 25 °C, and ∼20% reduction at 140 °C.     

High electromechanical strain properties by the existence of nonergodicity in LiNbO3–modified Bi1/2Na1/2TiO3–SrTiO3 relaxor ceramics

This study investigated the effect of LiNbO₃ modification on the dielectric, ferroelectric and electromechanical strain properties of Bi_1/2Na_1/2TiO₃–SrTiO₃ (BNT–ST) lead–free relaxor ceramics. The sintering temperature for lead–free BNT–ST relaxor ceramics was slightly decreased from 1175?°C to 1050?°C by modifying with LiNbO₃. We found that the sintering temperature affects the dielectric behavior of 0.76BNT–(0.24?x)ST–xLiNbO₃ (BNST–100xLN) ceramics at high temperature (near dielectric maximum temperature, T_m). The T_m for the low–temperature sintered sample was shifted to relatively higher temperature by comparison with the high–temperature sintered samples. Furthermore, the degradation of dielectric behavior near T_m in low–temperature sintered BNST–2LN ceramics was revealed after poling treatment and seem to be related to the existence of a high temperature stabilized nonergodic relaxor phase. Accordingly, we assume that the stabilized nonergodic relaxor phase is responsible for the relatively late transition from ferroelectrics to the relaxor. Therefore, we obtained the improved d₃₃^* of 616?pm/V as the highest value in low–temperature sintered BNST–2LN ceramics.