Excess Bi Research Articles

The microstructure, ferroelectric and dielectric properties of Ni-doped Na0.5Bi0.5TiO3 nanocrystalline films: A-site nonstoichiometry study

Effects of A-site nonstoichiometry on 3at% Ni-doped Na0.5Bi0.5TiO3 thin film were investigated. The influence of Na deficiency and Bi excess on the microstructure and electrical properties for the films were determined in comparison to the stoichiometric sample. The polycrystalline perovskite structure can be maintained at all compositions without secondary phase. Compared with the stoichiometric film, either Na deficiency or Bi excess can result in a decreased grain size. The enhanced insulating characteristic, ferroelectric and dielectric properties can be obtained in the Na or Bi nonstoichiometric thin films. Especially, the Na0.5Bi0.5(1+0.08)Ti0.97Ni0.03O3 thin film exhibits larger remanent polarization (Pr) of 22.3 μC/cm2 and dielectric tunability of 24.4%. These results show that Na deficiency or Bi excess lead to improved electrical performances for Na0.5Bi0.5TiO3-based thin films to some extent, rendering it suitable for ferroelectric and dielectric applications.

Effect of excess Bi content on electrical properties of BiFe0.95Cr0.05O3 thin films

The Bi1 + xFe0.95Cr0.05O3 (BFCO) (x = 0, 5, 10, 15 and 20%) thin films are fabricated on FTO/glass substrate using a chemical solution deposition method and sequential-layer annealing process. The effects of the excess Bi content on crystalline structure, morphology, and electrical performance of BFCO thin films are investigated. All the BFCO thin films are crystallized into polycrystalline perovskite structure and belonging to the space group of R3c. The BFCO thin films with 5 and 10% excess Bi contents possess no impurity phase. Especially, a dense surface morphology and columnar crystal structure can be obtained for the film with 5% excess Bi content. Especially, the one possesses superior ferroelectricity with a relative high remnant polarization (P r) of 69.8 µC/cm2 and low coercive electric field (E c) of 291 kV/cm at 1 kHz due to the relatively low leakage current density of 3.04 × 10− 5 A/cm2 at 200 kV/cm.

Large piezoelectric responses of Bi(Fe,Mg,Ti)O3-BaTiO3 lead-free piezoceramics near the morphotropic phase boundary

The (1-x)Bi1+y(Fe0.96Mg0.02Ti0.02)O3-xBaTiO3 (abbreviated as B1+yFMT-xBT) system was designed to explore the morphotropic phase boundary (MPB) separating rhombohedral (R) and tetragonal (T) phases, and the mechanism of performance enhancement. The phase structure of samples was discreetly investigated by X-ray diffraction, and Rietveld refinement results of the XRD data revealed that the B1+yFMT-xBT ceramics consist of an R phase at x = 0.24, two R and T phases at 0.26 ≤ x ≤ 0.32, and a single T phase at x = 0.34. Because of the MPB and the improvement of resistivity with adding excess Bi compensation, excellent piezoelectric properties of d33 = 198 pC/N and kp = 30.9%, as well as high Curie temperature TC = 497 °C were achieved in the B1+yFMT-xBT system at x = 0.30 and y = 0.02. Here, the large d33 = 198 pC/N which is the maximum value for the BF-BT-Bi(Mg,Ti)O3 system so far, and high TC = 497 °C suggest that Bi(Fe,Mg,Ti)O3-BaTiO3 ceramics have good prospects in high-temperature piezoelectric devices.

Tuning the ferroelectric-relaxor transition temperature in NBT-based lead-free ceramics by Bi nonstoichiometry

In this study, the Bi-nonstoichiometric 0.99Bix(Na0.8K0.2)0.5TiO3-0.01SrTiO3 (BNKST) ceramics with x=0.5–0.535mol (Bi50-Bi53.5) were prepared by a conventional solid-state reaction method. The effects of Bi excess on structural transition and ferroelectric stability of BNKST ceramics were systematically investigated by the Raman spectra, dielectric analyses and electromechanical measurements. The introduction of excess Bi3+ could significantly break the long-range ferroelectric order and favor the presence of relaxor phase, then the ferroelectric-relaxor transition temperature (TFR) can be effectively tuned to around room temperature by Bi nonstoichiometry, giving rise to an enhanced room-temperature strain property. The positive strain Spos and dynamic piezoelectric constant d33* of Bi52.5 critical composition reach 0.33% and 440 pm/V, respectively at 6kV/mm. The high recoverable strain of Bi52.5 sample can be attributed to the electric-field-induced reversible relaxor-ferroelectric phase transition. The present work may be helpful for further understanding and designing high-performance NBT-based lead-free ceramics for piezoelectric actuator applications.

Bismuth sodium titanate lead‐free piezoelectric coatings by thermal spray process

AbstractBismuth sodium titanate (BNT) piezoelectric ceramic coatings with dense morphology and single perovskite phase were fabricated by thermal spray process. The melting and crystallization behaviors of BNT near congruent composition and the effect of excess Bi composition were investigated by analyzing the heating‐melting‐cooling cycle for the application in thermal spray process. Crystal structure and morphology of the thermal sprayed BNT coatings were examined, and their electrical and electromechanical properties were tested. An effective piezoelectric coefficient d33 of 50 pm/V was obtained under substrate clamping, measured with laser scanning method. The findings have shed light on the design of appropriate compositions for achieving high quality lead‐free piezoelectric ceramic coatings by the scalable thermal spray process.

Effect of Bismuth Excess on Piezoelectric and Dielectric Properties of BiFeO3-BaTiO3 Ceramics

The effects of an excess of Bi on the piezoelectric and dielectric properties of 0.60Bi1+xFeO3-0.40BaTiO3 (x = 0, 0.01, 0.03, 0.05, 0.07) were investigated. The ceramics were processed through a conventional solid state reaction method and then quenched after sintering at different temperatures in the range of 980~1070 °C. A single perovskite structure without any secondary phase was confirmed for all compositions and temperatures. It was found that excess Bi reduced the sintering temperatures, acted as a sintering aid and enhanced the properties in combination with quenching. Curie temperature (TC) was found to slightly increase due to the presence of excess Bi; electrical properties were also improved by quenching. At x = 0.03 and 1030 °C, remnant polarization (2Pr) was as high as 45.4 μC/cm2 and strain at 40 kV/cm was up to 0.176 %.

Excess Bi and Cooling Method on Phase Structure and Electrical Properties of BiFeO3-BaTiO3 Lead-free Ceramics

采用传统固相烧结法制备了0.7BiFeO3-0.3BaTiO3-xBi2O3(0≤x≤0.05)无铅压电陶瓷, 研究了Bi补偿量x和冷却方式对其相结构、微观形貌和综合电学性能的影响。结果表明：所有样品均为菱方相(R)和伪立方相(PC)两相共存, 0≤x≤0.01样品为纯的钙钛矿结构, 且x=0.01样品的两相比例CR/CPC接近1; x>0.01样品中出现富Bi杂相Bi25FeO40。与冷却方式相比, 优化Bi补偿量更有利于提升BFBT-xBi2O3陶瓷的压电性能。随着x增大, d33先增大后减小, 在x=0.01时获得最优值。由于较小的晶粒、较合适的CR/CPC以及较大的残余应变, 水冷BFBT-0.01Bi2O3陶瓷获得了最优的压电性能(d33水冷=141 pC/N、kp=27%)和高TC=507℃。研究结果表明, BFBT基陶瓷有希望成为兼具高压电性能和高TC的无铅压电材料体系之一。

Observation of hidden atomic order at the interface between Fe and topological insulator Bi2Te3.

To realize spintronic devices based on topological insulators (TIs), well-defined interfaces between magnetic metals and TIs are required. Here, we characterize atomically precisely the interface between the 3d transition metal Fe and the TI Bi2Te3 at different stages of its formation. Using photoelectron diffraction and holography, we show that after deposition of up to 3 monolayers Fe on Bi2Te3 at room temperature, the Fe atoms are ordered at the interface despite the surface disorder revealed by our scanning-tunneling microscopy images. We find that Fe occupies two different sites: a hollow adatom deeply relaxed into the Bi2Te3 quintuple layers and an interstitial atom between the third (Te) and fourth (Bi) atomic layers. For both sites, our core-level photoemission spectra and density-functional theory calculations demonstrate simultaneous chemical bonding of Fe to both Te and Bi atoms. We further show that upon deposition of Fe up to a thickness of 20 nm, the Fe atoms penetrate deeper into the bulk forming a 2-5 nm interface layer containing FeTe. In addition, excessive Bi is pushed down into the bulk of Bi2Te3 leading to the formation of septuple layers of Bi3Te4 within a distance of ∼25 nm from the interface. Controlling the magnetic properties of the complex interface structures revealed by our work will be of critical importance when optimizing the efficiency of spin injection in TI-based devices.

Tailoring transport properties through nonstoichiometry in BaTiO3–BiScO3 and SrTiO3–Bi(Zn1/2Ti1/2)O3 for capacitor applications

The ceramic perovskite solid solutions BaTiO3–BiScO3 (BT–BS) and SrTiO3–Bi(Zn1/2Ti1/2)O3 (ST–BZT) are promising candidates for high-temperature and high-energy density dielectric applications. A-site cation nonstoichiometry was introduced in these two ceramic systems to investigate their effects on the dielectric and transport properties using temperature- and oxygen partial pressure-dependent AC impedance spectroscopy. For p-type BT–BS ceramics, the addition of excess Bi led to effective donor doping along with a significant improvement in insulation properties. A similar effect was observed on introducing Ba vacancies onto the A-sublattice. However, Bi deficiency registered an opposite effect with effective acceptor doping and a deterioration in the bulk resistivity values. For n-type intrinsic ST–BZT ceramics, the addition of excess Sr onto the A-sublattice resulted in a decrease in resistivity values, as expected. Introduction of Sr vacancies or addition of excess Bi on A-site did not appear to affect the insulation properties in air. These results indicate that minor levels of nonstoichiometry can have an important impact on the material properties, and furthermore it demonstrates the difficulties encountered in trying to establish a general model for the defect chemistry of Bi-containing perovskite systems.

ChemInform Abstract: Synthesis, Crystal Structure, and Magnetism of A2Co12As7 (A: Ca, Y, Ce—Yb).

AbstractIntermetallic Ln2Co12As7 (Ln: Y, Ce—Yb) and Ca2Co12As7 are prepared by solid state reaction of the elements using excess Bi as a flux (evacuated silica tubes,1223 K, 8 d).

Effect of bismuth surfactant on InP-based highly strained InAs/InGaAs triangular quantum wells

We report the effect of Bi surfactant on the properties of highly strained InAs/InGaAs triangular quantum wells grown on InP substrates. Reduced surface roughness, improved heterostructure interfaces and enhanced photoluminescence intensity at 2.2 μm are observed by moderate Bi-mediated growth. The nonradiative processes are analysed based on temperature-dependent photoluminescence. It is confirmed that Bi incorporation is insignificant in the samples, whereas excessive Bi flux during the growth results in deteriorated performance. The surfactant effect of Bi is promising to improve InP-based highly strained structures while the excess of Bi flux needs to be avoided.

Effects of excess Bi on structure and electrical properties of BiFeO3 thin films deposited on indium tin oxide substrate using sol–gel method

As a kind of potential unleaded multiferroic material, BiFeO3 (BFO) is expected to be used instead of Pb(Zr,Ti)O3 in high density ferroelectric random access memories. Bi1+xFeO3 (x = 0, 5, 10, 15 and 30 %) thin films deposited on indium tin oxide/glass substrate were successfully fabricated using sol–gel method, spin coating and layer by layer technique. The XRD and SEM analysis indicated that the orientation of BFO thin films is sensitive to the content of excess bismuth. Both sense surface and suppression of impurity phases were realized in the BFOx=10% thin films. Adding appropriately excess Bi can cover the volatilization of Bi2O3 during the thermal treatment. Leakage current density and P–E hysteresis loop demonstrate that x = 10 % is the best excess Bi content. BFOx=10% thin films has a minimum current density of 7.8 × 10−9A/cm2 at applied electric field of 600 kV/cm, a maximum remnant polarization 71.18 μC/cm2 with a coercive field of 412.52 kV/cm at measured electric field value of 1213 kV/cm, and a largest dielectric constant 208 with a relatively lower dielectric loss of 0.042 at applied frequency of 105 Hz, of which the current conduction mechanisms are Ohmic conduction and grain boundary limited conduction.

Hardening in non-stoichiometric (1 − x)Bi0.5Na0.5TiO3–xBaTiO3 lead-free piezoelectric ceramics

The role of A-site non-stoichiometry was investigated in lead-free piezoelectric ceramics based on compositions in the 1 − x(Bi0.5Na0.5TiO3)–xBaTiO3 system near the morphotropic phase boundary, where x = 0.055, 0.06, and 0.07. Donor doping was introduced through the addition of excess Bi, however, there were no changes in the crystal structure. In contrast, acceptor doping was introduced through the addition of excess Na and was found to promote rhombohedral distortions. A significant improvement of dielectric properties was observed in donor-doped compositions and, in contrast, a degradation in properties was observed in acceptor-doped compositions. Compared to the stoichiometric composition, the acceptor-doped compositions displayed a significant increase in coercive field (E c) which is an indication of domain wall pinning as found in hard Pb(Zr x Ti1−x )O3. This result was further confirmed via remanent polarization hysteresis analyses. Moreover, all A-site acceptor-doped compositions also exhibited an increase in mechanical quality factor (Q m) as well as a decrease in piezoelectric coefficient (d 33), dielectric loss (tan δ), remanent polarization (P r), and dielectric permittivity, which are all the typical characteristics of the effects of “hardening.” The mechanism for the observed hardening in A-site acceptor-doped BNT-based systems is linked to changes in the long-range domain structure and defect chemistry.

Antiferromagnetic Kondo lattice in the layered compoundCePd1−xBi2and comparison to the superconductorLaPd1−xBi2

The layered compound ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$ with the tetragonal ${\mathrm{ZrCuSi}}_{2}$-type structure was obtained from excess Bi flux. Magnetic susceptibility data of ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$ show an antiferromagnetic ordering below 6 K and are anisotropic along the $c$ axis and the $ab$ plane. The anisotropy is attributed to crystal-electric-field (CEF) effects and a CEF model which is able to describe the susceptibility data is given. An enhanced Sommerfeld coefficient $\ensuremath{\gamma}$ of 0.191 J mol $\mathrm{Ce}{}^{\ensuremath{-}1} \mathrm{K}{}^{\ensuremath{-}2}$ obtained from specific-heat measurement suggests a moderate Kondo effect in ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$. Other than the antiferromagnetic peak at 6 K, the resistivity curve shows a shoulderlike behavior around 75 K which could be attributed to the interplay between Kondo and CEF effects. Magnetoresistance and Hall-effect measurements suggest that the interplay reconstructs the Fermi-surface topology of ${\mathrm{CePd}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{2}$ around 75 K. Electronic structure calculations reveal that the Pd vacancies are important to the magnetic structure and enhance the CEF effects which quench the orbital moment of Ce at low temperatures.

Polarization switching at room temperature of undoped BiFeO3 thin films crystallized at temperatures between 400 ≤ T ≤ 500 °C

Pure BiFeO3 perovskite thin films have been prepared on Pt-coated silicon substrates by chemical solution deposition at temperatures below 500 °C. Precursor solutions with and without Bi(III) excess have been used. Perovskite films without crystalline secondary phases, as detected by X-ray diffraction analysis, are obtained at the lowest temperature limit of 400 °C. However, the scanning electron micrographs of these films show surface microstructures formed by well defined grains surrounded by a fine grained phase, suggesting the appearance of a volume fraction of crystals in an early stage of crystallization. The films prepared with Bi(III) excess have better defined ferroelectric hysteresis loops than those without any excess, especially for the films annealed at 400 °C, which can be attributed to an improved connectivity of the ferroelectric phase. This together with the fact that leakage current densities in the films decrease with decreasing the processing temperature, make that the BiFeO3 films prepared with Bi(III) excess and annealed at 400 and 450 °C can be poled at room temperature, obtaining an effective switching of the ferroelectric polarization with the electric field. Remanent polarization values of PR ~ 10 and ~60 μC/cm2 with coercive fields of EC ~ 205 and 235 kV/cm were obtained for the films prepared at 400 and 450 °C, respectively. The demonstration of the functionality at room temperature of these low temperature processed undoped BiFeO3 thin films increases the interest in these materials for their integration in multiferroic devices.

BiFeO3 thin films via aqueous solution deposition: a study of phase formation and stabilization

This paper reports a thorough microstructural investigation of bismuth ferrite (BFO) thin films subjected to various processing conditions and discusses their influence on the stability of the BiFeO3 perovskite phase. The formation of secondary phases in BFO thin films is studied as a function of annealing temperature and time, film thickness, Bi excess, and Ti substitution. While films annealed at 600 °C consist of the desired BiFeO3 phase, higher temperatures induce the decomposition leading to a significant amount of secondary phases, particularly the iron-rich Bi2Fe4O9 phase. A longer annealing time at 700 °C further enhances the decomposition of BiFeO3. Qualitative microstructural analysis of the films is performed by electron backscattered diffraction which provides phase analysis of individual grains. The morphology of the single-crystalline Bi2Fe4O9 grains that are embedded in the BiFeO3 matrix drastically changes as a function of the film thickness. Nucleation of these Bi2Fe4O9 grains probably occurs at the film/substrate interface, after which grain growth continues toward the surface of the film through the depletion of the BFO phase. Addition of Bi excess or the substitution of Fe with Ti in the precursor solutions significantly reduces the formation of an iron-rich secondary phase. Influence of the secondary phases as well as Ti substitution on magnetic properties of BFO films was investigated.

Effect of Excess Bi<SUB>2</SUB>O<SUB>3</SUB> Content on the Structure and Dielectric, Piezoelectric Properties of Bi<SUB>0.5</SUB>(Na<SUB>0.8</SUB> K<SUB>0.2</SUB>)<SUB>0.5</SUB>TiO<SUB>3</SUB> Lead Free Ceramics

Effect of Excess Bi<SUB>2</SUB>O<SUB>3</SUB> Content on the Structure and Dielectric, Piezoelectric Properties of Bi<SUB>0.5</SUB>(Na<SUB>0.8</SUB> K<SUB>0.2</SUB>)<SUB>0.5</SUB>TiO<SUB>3</SUB> Lead Free Ceramics

Effects of Bismuth Oxide Buffer Layer on BiFeO3 Thin Film

Bismuth ferrite (BiFeO3) thin films with Bi2O3 buffer layers were prepared on Si/SiO2/TiO2/Pt substrates by sol–gel‐derived spin‐coating method. The structural and electrical properties of BiFeO3 was effectively improved by adding a Bi2O3 buffer layers either at Pt/BiFeO3 interface or on BiFeO3 surface, also strongly depending on the positions and the annealing conditions of buffer layers. A 500°C‐annealed Bi2O3 buffer layer could act as a Bi source for compensating Bi volatilization and a diffusion barrier for species from BiFeO3. A near stoichiometric BiFeO3 with less defects and substrate contamination was obtained by employing a 500°C‐annealed Bi2O3 buffer layer in between Pt substrate and BiFeO3. The structure change in BiFeO3 led by such a buffer layer should result from the interfacial constraint between buffer layer and BiFeO3. Furthermore, this crystalline BiFeO3 specimen exhibited a highly (100)‐textured, where this preferred orientation was attributed to the accumulation of Bi at Pt/BFO interface. Therefore, the Pt/500°C‐annealed Bi2O3/BiFeO3/Pt thin film exhibited the good ferroelectric and magnetic properties. As compared to the usual method for controlling BiFeO3 composition by adding excess Bi, this study indicates the more advantages using a Bi2O3 buffer layer.

Pulsed Laser Deposition of Epitaxial and Polycrystalline Bismuth Vanadate Thin Films

We report pulsed laser deposition (PLD) synthesis of epitaxial and polycrystalline monoclinic bismuth vanadate (BiVO4, BVO) thin films. X-ray diffraction (XRD), atomic force microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy were used to characterize the samples. Epitaxial, c-axis oriented growth was achieved using single crystal yttria-stabilized zirconia (100), a substrate temperature of 575–600 °C, and an oxygen pressure of 7.8 mTorr. The volatility of Bi necessitated a large excess (Bi:V = ∼6:1) of this element in the ceramic targets to obtain stoichiometric films. XRD confirmed a BVO (001)||YSZ (001) and BVO [100]||YSZ [100] epitaxial relationship. Film growth was 3-D, and the morphology was discontinuous, consisting of irregular, smooth grains. Additionally, dense, continuous polycrystalline films were deposited on fluorine-doped tin oxide (FTO) on glass substrates at room temperature with stoichiometric targets and postdeposition annealing in air. Evaluation of these samples as photoanodes yielded photocurrents of ∼0.15 and ∼0.05 mA cm–2 at 1.23 V vs RHE under backside AM1.5G illumination with and without a hole scavenger (Na2SO3), respectively. We argue that the photocurrents are due to the high oxygen content inherent in the PLD process and suggest that these continuous films may be well-suited to investigating oxygen-related defects in BVO.

Effect of excess Bi content on the electrical properties of Bi0.95La0.05FeO3 thick films

BiFeO3 (BFO)-based ferroelectric films thicker than 1 µm are promising in dielectric, piezoelectric, and pyroelectric applications. However, using the common sol–gel technology to prepare BFO-based thick films is difficult because of crack formation and poor crystallization. In this study, we demonstrate that it is possible to prepare well-crystallized Bi0.95La0.05FeO3 (BLFO) thick films with a thickness of 1.4 µm by modifying excess Bi content. The effect of excess Bi content on the electrical properties of the BLFO thick film was investigated. Most excess Bi particles were found to be concentrated in the grain boundaries of thick films instead of volatilizing. Adding appropriate excess Bi when preparing BLFO thick films was also found to promote crystal growth and improve electrical properties.