Phase evolution and electrical behaviour of samarium-substituted bismuth ferrite ceramics

Bi_1-xSm_xFeO₃ (x?=?0.15–0.18) ceramics with high density were produced using spark plasma sintering. The effects of composition, synthesis conditions and temperature on the phase evolution were studied, using XRD, TEM and dielectric spectroscopy. The coexistence of the ferroelectric R3c, antiferroelectric Pnam and paraelectric Pnma phases was revealed, with relative phase fractions affected by both calcination conditions and Sm concentration. Experiments on powdered samples calcined at different temperatures up to 950?°C suggest higher calcination temperatures promote Sm diffusion, allowing samples to reach compositional homogeneity. The structural transitions from the Pnam and R3c phases to the Pnma phase were comprehensively investigated, with phase transition temperatures clearly identified. The dielectric permittivity, electrical resistivity and breakdown strength were increased upon Sm-substitution, while ferroelectric switching was suppressed. The polarization-electric field loop became increasingly narrow with increasing Sm-content, but double hysteresis loops, which may reflect a reversible antiferroelectric to ferroelectric transformation, were not observed.     

Effects of Nd and Co co-doping on phase,microstructure and ferromagnetic properties of bismuth ferrite ceramics

In this work, the samples of Bi_0.95Nd_0.05Co_xFe_1?xO₃ (BNFCO) with x=0, 0.03, 0.05 and 0.07 mol fraction, were prepared by a solid state reaction. The effect of Nd and Co co-doping concentration on phase, densification, microstructure and ferromagnetic properties were examined. The BNFCO powders were prepared using a mixed-oxide method and calcined at 800 °C for 2 h before being pressed and sintered at various temperatures in the range of 825–900 °C for 2 h. An increase in Co co-doping content increased density of the ceramics. Phase analysis by X-ray diffraction indicated the existence of rhombohedral phase for all BNFCO powders and ceramics. Microstructural investigation using the scanning electron microscope showed a reduction of grain size with increasing Co content. Magnetic hysteresis loops showed that remanent magnetization and coercive magnetic field of the Co-doped samples were improved.     

Multi-doped bismuth ferrite thin films with enhanced multiferroic properties

Thin films of multi-doped bismuth ferrite, Bi_0.97−xLa_xSr_0.03Fe_0·94Mn_0·04Co_0·02O₃ (BL_xSFMC, x = 0.00–0.18), are synthesized on a fluorine-doped tin oxide (FTO)/glass substrate. The structure and multiferroic properties of the film samples are characterized and tested. The results indicate that on doping, the structure of the BL_xSFMC film changes has been changed. The concentrations of both oxygen vacancies and Fe²⁺ are decreased. The BL_0.18SFMC thin film exhibits Ohmic conduction, which reduces the influence of the built-in electric field E_bi of the space-charge region at the interface between an Au electrode and the BL_xSFMC during polarization. The BL_0.18SFMC thin film also exhibits enhanced ferroelectric properties than the undoped film, with a higher residual polarization of 188 μC/cm² and a higher squareness ratio of 1.21. Meanwhile, the reduced number of oxygen vacancies also reduces the Fe²⁺/Fe³⁺ ratio, thereby enhancing the Dzyaloshinskii–Moriya interaction of Fe–O–Fe bonds, and so the BL_0.18SFMC thin film exhibits enhanced ferromagnetism, with a saturation magnetization of M_s ≈ 3.94 emu/cm³. Thus, multi-ion doping can improve both the ferroelectric and ferromagnetic properties of BL_xSFMC thin films.     

Densification and dielectric properties of strontium bismuth tantalate ceramics

Abstract

The effects of B₂O₃ doping on the sintering processes and dielectric properties of SrBi₂Ta₂O₉ were investigated in this study. The sinterability of SrBi₂Ta₂O₉ was significantly enhanced by doping B₂O₃ due to the formation of liquid phase at elevated temperatures. The addition of B₂O₃ also led to inhibited thermal decomposition and enhanced grain growth of SrBi₂Ta₂O₉. The amount of doped B₂O₃ was found to substantially influence the dielectric constants of SrBi₂Ta₂O₉ ceramics. When a proper amount of B₂O₃ was added, the dielectric constants greatly increased. For facilitating the densification process and improving the dielectric properties of SrBi₂Ta₂O₉, it is important to control the doping amount of B₂O₃.     

Enhanced magnetic and dielectric properties of Y doped bismuth ferrite nanofiber

Yttrium doped Bismuth ferrite (BFO) nanofiber was fabricated via a sol-gel-based electrospinning process with the fiber diameter in the range of 60–220 nm. The crystal structure, magnetic and dielectric properties were investigated at room temperature. The Rietveld refinement results indicate the phase transition from space group R3c to Pbnm by the Y doping. Dramatic increase of magnetization has been achieved in Y doped BFO nanofiber. Compared with BFO nanoparticle, the Bi_0.95Y_0.05FeO₃ nanofiber exhibits nearly eighteen-fold improved magnetization, which is the strongest in the reported Y doped BFO at the same doping level. The largely improved magnetization mainly originates from the serious suppression of spiral spin structure by the small crystal size of nanofiber structure. Moreover, the Bi_0.95Y_0.05FeO₃ nanofiber holds the lower dielectric loss and obvious dependence of the capacitance on bias voltage, indicating the improved ferroelectricity due to the decreased leakage current. The simultaneous enhancement of ferroelectricity and magnetization in Y doped BFO nanofiber suggests that nanofiber structure plays an important role in improving multiferroic performance.     

Enhanced piezoelectric strain response in 〈100〉 grain-oriented lead-free bismuth ferrite–based piezoelectric ceramics

The 〈100〉 grain-oriented 0.11(Bi_0.5K_0.5)TiO₃–0.23BaTiO₃–0.02Bi(Mg_0.5Ti_0.5)O₃–0.64BiFeO₃ piezoelectric ceramics were prepared by a reactive templated grain growth method using a platelike H_1.08Ti_1.73O₄·nH₂O (HTO) template and Bi₂O₃–KHCO₃–MgO–Fe₂O₃–BaCO₃ matrix particles. The high degree of texturing (a Lotgering orientation factor of 80%) and high density (95%) were achieved by employing weight-pressing treatment during the binder-removal and sintering treatment along with optimizing the sintering temperature. The water-quenching treatment has a significant impact on the enhancement of dielectric, ferroelectric, and piezoelectric properties with the increase of dielectric constant, remanent polarization, and piezoelectric strain constant from 764, 13.6 μC/cm², and 384 pm/V for the as-sintered ceramics to 812, 29.9 μC/cm², and 526 pm/V after the water-quenching treatment at 850°C, respectively. The obtained piezoelectric strain constant with a 1.8 times enhancement compared to that of the ceramics with randomly oriented grains is significantly higher than those reported for other lead-free piezoelectric ceramics with Curie temperature >300°C. This study suggested the strong potentiality of this material system for high-temperature actuator application.     

Effect of BNBTKNN on the electrical properties of bismuth ferrite thin films

BiFeO₃/[0.93(Bi_0.50Na_0.50TiO₃)-0.05BaTiO₃-0.02K_0.50Na_0.50NbO₃] (BFO/BNBTKNN) bilayered thin films were fabricated on Pt/TiO₂/SiO₂/Si substrates without any buffer layers by a combined sol-gel and radio frequency sputtering route. Effect of BNBTKNN on electrical properties of BFO/BNBTKNN thin films was investigated. A higher phase purity and a denser microstructure are induced for the BFO/BNBTKNN bilayered thin film by using the bottom BNBTKNN layer, resulting in its lower leakage current density. Moreover, the enhancement in dielectric behavior is also demonstrated for such a bilayer, where a high dielectric constant and a low dielectric loss are obtained. The BFO/BNBTKNN bilayered thin film has an improved multiferroic behavior: 2P_r ∼ 76.8 μC/cm², 2E_c ∼ 378.1 kV/cm, 2M_s ∼ 52.6 emu/cm³, and 2H_c ∼ 453.6 Oe, together with a low fatigue rate up to ∼1 × 10⁹ switching cycles.     

Crystal structure and electrical properties of bismuth sodium titanate zirconate ceramics

Lead-free bismuth sodium titanate zirconate (Bi_0.5Na_0.5Ti_1-xZr_xO₃ where x = 0.20, 0.35, 0.40, 0.45, 0.60, and 0.80 mole fraction) [BNTZ] ceramics were successfully prepared using the conventional mixed-oxide method. The samples were sintered for 2 h at temperatures lower than 1,000°C. The density of the BNTZ samples was at least 95% of the theoretical values. The scanning electron microscopy micrographs showed that small grains were embedded between large grains, causing a relatively wide grain size distribution. The density and grain size increased with increasing Zr concentration. A peak shift in X-ray diffraction patterns as well as the disappearance of several hkl reflections indicated some significant crystal-structure changes in these materials. Preliminary crystal-structure analysis indicated the existence of phase transition from a rhombohedral to an orthorhombic structure. The dielectric and ferroelectric properties were also found to correlate well with the observed phase transition.     

铁酸铋光催化剂改性的研究进展

铁酸铋是一种钙钛矿型半导体光催化剂,因具有合适的光学带隙,良好的化学稳定性及可见光吸收性等特点而备受青睐。然而,铁酸铋的光生电子空穴对复合率高,载流子效率较低,导致其光催化活性较弱,限制了其实际应用。在概述铁酸铋结构和光催化机理的基础上,着重综述了贵金属沉积、半导体复合、金属离子掺杂等铁酸铋改性方法的作用机理与研究进展,探讨了铁酸铋光催化剂未来的研究方向。     

Effect of calcination temperature on structural and morphological properties of bismuth ferrite nanoparticles

Bismuth Ferrite (BiFeO3) is one such materials which has shown very promising multiferroic and excellent optical properties. In this paper, we report effect of annealing temperature on the structural, morphological and optical properties of BiFeO₃ nanoparticles synthesised through sol-gel auto-combustion method. Nanoparticles prepared were calcined at three different temperatures, 400 °C, 500 °C and 600 °C, and named as BFO1, BFO2 and BFO3, respectively. X-ray diffraction confirmed the rhombohedral structure with R3c space group as a primary phase. However, a secondary phase Bi₂Fe₄O₉ was also observed which decreases with increasing temperature. The crystallite sizes were found to increase with increasing temperature with BFO2 as anomaly. Field emission scanning electron microscopy (FESEM) shows clear grain formation for all the samples. TEM micrographs and SAED patterns show crystalline grains with rhombohedral structure. All the functional groups observed in the Fourier infrared spectroscopy (FTIR) measurement are indexed. The FTIR spectra shows presence of two prominent vibrational modes in the wave number range 447 and 560 cm^-1 corresponding to the stretching of Fe-O bonds. Raman analysis shows presence of a peak at ~527 cm^-1 for (BFO3) which was absent in other two samples. Also, the intensity of the A₁-1 mode was found stronger than that of A₁-2 mode in all the samples which confirmed the stability of the structure, except for BFO1.     

Modulating self-biased near-UV photodetection of Gd-doped bismuth ferrite ceramics by introducing zinc oxide as electron transport layer

ZnO semiconductor offers many advantages as an electron transport layer (ETL) in photovoltaic-based devices, including high charge carrier mobility and hole-blocking ability. In this work, ZnO thin film is introduced as an ETL between (Bi_0.93Gd_0.07)FeO₃ (BFO7Gd) ferroelectric and ITO thin film to form ITO/ZnO/BFO7Gd/Au heterostructure. The device with the ETL exhibited a superior photoresponsivity than the one without ETL, reaching ～32% enhancement. Furthermore, a subsequent E-field poling on the ITO/ZnO/BFO7Gd/Au heterostructure resulted in an additional ～25% increase in photoresponsivity. The enhancement is mainly attributed to two factors: (1) high electron mobility and lower recombination rate resulting from the introduction of ZnO ETL, and (2) efficient charge separation facilitated by the polarization-driven internal E field that superimposes with the interfacial built-in E fields. The introduction of ZnO ETL and the utilization of the ferroelectric polarization prove to be an alternative route to further modulate the photosensing performance of BiFeO₃-based near-UV photodetectors.     

Electrical and magnetic properties of ZrO2-doped lithium-titanium-zinc ferrite ceramics

This report documents the effect of 0–3 wt% ZrO₂ additive on the electrical and magnetic properties of LiTiZn ferrite. Ferrite powder of Li_0.65Fe_1.6Ti_0.5Zn_0.2Mn_0.05O₄ composition was synthesised at 900 °C for 4 h in air. Ferrite ceramics doped with ZrO₂ were sintered at 1010 °C for 2 h in air. A spreading resistance analysis showed that LiTiZn ferrites exhibited nonuniform distribution of depth DC resistivity, which varied in the range of (0.25–2.3) × 10⁹ Ω⋅cm depending on the amount of additive. Zirconia also affected the magnetic properties of ferrite so that the magnetisation increased and the initial permeability decreased as the ZrO₂ content increased. In addition, the Curie temperature varied. The permeability spectra measured in the frequency range from 10 MHz to 18 GHz changed depending on the zirconia content.     

铁酸铋光催化剂改性的研究进展

铁酸铋是一种钙钛矿型半导体光催化剂,因具有合适的光学带隙、良好的化学稳定性及可见光吸收性等特点而备受青睐.然而,铁酸铋的光生电子-空穴对复合率高,载流子效率较低,导致其光催化活性较弱,限制了其实际应用.在概述铁酸铋结构和光催化机理的基础上,该文着重综述了贵金属沉积、半导体复合、金属离子掺杂等铁酸铋改性方法的作用机理与研究进展,探讨了铁酸铋光催化剂未来的发展方向.     

Preparation,thermal and phase evolution and functional properties of non-stoichiometric strontium-doped lanthanum manganite perovskite ceramics

Powders of (La_0.85Sr_0.15)_0.98MnO_3-δ (LSM85) and (La_0.80Sr_0.20)_0.98MnO_3-δ (LSM80) perovskites have been synthesized and characterized in detail, and then sintered to evaluate their electrical and magnetic properties at low temperatures and at near room temperature. Microstructural observations/analyses after the dilatometric essays show that the perovskites have compositions that deviate from the nominal ones. Furthermore, magnetic characterization of both samples reveals a ferromagnetic behavior with a Curie temperature above 300 K for LSM80 and of 260 K for LSM85. In addition, LSM85 is insulator from 15 up to 300 K, whereas LSM80 is metallic up to 160 K and insulator up to 300 K. This significant discrepancy of behaviour is attributable to structural and compositional differences between the two perovskites.     

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 growth behavior in bismuth titanate-based ceramics

Bismuth titanate and lanthanum-doped bismuth titanate ceramics were prepared from freeze-dried powders employing conventional solid state reaction and sintering procedures. The sintering process was carried out at 1150 °C from 4 up to 48 h. X-ray diffraction analysis showed that preferred orientation was reduced in bismuth titanate ceramic as sintering time increased while lanthanum-doped sample showed much less degree of preferred orientation and was independent of sintering time. Grain growth studies also showed that initial anisotropic grain growth rate was the main factor controlling the grain morphology, rendering the plate-shaped grain in both pure and lanthanum-doped bismuth titanate ceramics. Based on established grain growth law, pore-controlled diffusion could be the major mechanism determining the observed microstructure in these layered compounds.     

Microstructure and magnetic properties of nickel-zinc ferrite ceramics fabricated by spark plasma sintering

Dense nickel-zinc (NiZn) ferrite ceramics were successfully fabricated within tens of seconds via spark plasma sintering. The phase composition and microstructure of the sintered samples were characterized by X-ray diffraction and scanning electron microscopy, respectively. The static magnetic properties at room temperature and Curie temperature of the samples were investigated by vibrating sample magnetometry. The results indicated that the main phase of the sintered samples was Ni_0.75Zn_0.25Fe₂O₄ with spinal structure, and the sintering temperature and heating rate observably affected the microstructure and density, then the magnetic properties of the sample. The Joule heat generated by NiZn ferrite during spark plasma sintering was very important for the rapid preparation of the sample with high density and small grain size. The low sintering temperature and heating rate would be helpful to obtain samples with small grain size, high density, and then good magnetic properties. The samples sintered at 900 °C with the heating rate of 5–10 °C/s were characterized of the relative density above 95%, 4πM_s value beyond 4000 Gs and coercivity below 27.7 Oe.     

PZT-nickel ferrite and PZT-cobalt ferrite comparative study: Structural,dielectric, ferroelectric and magnetic properties of composite ceramics

Comparative study of different PZT-based composite materials ((x)PbZr_0.52Ti_0.48O₃ + (1-x)CoFe₂O₄ and (x)PbZr_0.52Ti_0.48O₃ +(1-x)Ni_0.7Zn_0.3Fe₂O₄ (x = 0.8 and 0.9)) is presented in the frame of structural, dielectric, ferroelectric and magnetic properties. PZT and NZF/CF powders were synthesized by auto combustion technique. The composites were synthesized by mixing the appropriate amount of individual phases using conventional sintering. XRD data indicated the formation of well crystallized structure of PZT and NZF/CF, without the presence of undesirable phases. SEM micrographs revealed a uniform grain distribution of both, ferroelectric and ferromagnetic phases. Non-saturated hysteresis loops are evident in all samples due to the existence of non-ferroelectric ferrite phase. All the samples exhibit typical ferromagnetic hysteresis loop, indicating the presence of the order magnetic structure. Dielectric investigations revealed that ferrites are the main source of charge carriers, which must be of electronic origin. The activation energy of effective electrical resistivity is heavily influenced by the ferroelectric phase.     

铁酸铋的制备及其在光催化领域的研究进展

不同的制备方法,可以得到不同形貌的BiFeO_3晶体,从而使其具备不同的性能。该材料的制备方法主要有固相反应法、共沉淀法、水热法、溶胶-凝胶法等。旨在对目前已见报道的BiFeO_3的制备方法进行比较,并对该材料在水处理方面的光催化应用进行了综述。集光催化性及铁电磁性于一身的BiFeO_3,将在光催化领域有着广阔的应用前景。