Effects of Nd3+-substitution for Bi-site on the leakage current,ferroelectric and dielectric properties of Na0.5Bi0.5TiO3 thin films

Perovskite Na_0.5(Bi_1?xNd_x)_0.5TiO₃ (x = 0, 0.01, 0.03, 0.05; xNd: NBT) ferroelectric films were synthesized on indium tin oxide (ITO)/glass substrates via chemical solution deposition. Structural characterization shows the similar phase-pure perovskite structures in all the films and gradually decreased grain sizes with Nd³⁺ doping amount increasing. For all the films, the leakage behaviors are dominant by the Ohmic conduction in low electric field region and interface-limited Fowler-Nordheim tunneling mechanism in high electric field region. Additionally, the space-charge-limited conduction is involved in 0.03Nd: NBT sample. Compared with the sample of x = 0, the resistivity can be improved through Nd³⁺-substitution in NBT. Enhanced ferroelectricity can be obtained from the dynamic polarization-electric field test, and the reversible domains switching in film can be confirmed by static dielectric constant-electric field measurement. Especially, the 0.03Nd: NBT possesses optimal electrical performances with a large remanent polarization (P_r = 26.7 μC/cm²) and a high dielectric tunability (19.6% at 100 kHz).     

Significant reduction of dielectric loss of Ba0.51Sr0.34TiO3 film modified by Y/Mn alternate doping and preheating

To achieve significant reduction of dielectric loss and enhancement of dielectric tunability for actual applications, a Ba_0.51Sr_0.34TiO₃ film modified by Y/Mn alternate doping (Y/Mn-BST_0.85) is prepared by an improved sol-gel method, where the film is composed of 12 layers, and odd number layers are preheated. For comparison, BST_0.85, Y-BST_0.85 and Mn-BST_0.85 films are also prepared. XRD shows four films are ABO₃ cubic perovskite structures. The BST_0.85 film shows the smallest lattice parameter and donor doping because rich Ti⁴⁺ ions enter into A sites to replace some Sr²⁺ and Ba²⁺ ions. Besides the donor doping, three doped BST_0.85 films exhibit slightly larger lattice parameters and acceptor doping because Y³⁺ or Mn²⁺ ions replace some Ti⁴⁺ ions at A sites. The Y/Mn-BST_0.85 film shows the largest lattice parameter and the strongest crystallinity because Y-BST_0.85 layer and Mn-BST_0.85 layer match well in lattice structure. Compared to stoichiometric films, four nonstoichiometric films show markedly decreased dielectric losses. The Y/Mn-BST_0.85 film exhibits the lowest dielectric loss with 0.45 ~ 0.54% under the conditions of ??40 to 40V and 100?kHz, and 0.43 ~ 2.29% under the conditions of 10?Hz ~ 1?GHz and 0?V, thus can satisfy actual applications including tunable microwave application at high frequencies. The related mechanisms are studied by measuring leakage current density, observing AFM and analyzing XPS besides analyzing the XRD patterns.     

Tailoring structural and electrical properties of A-site nonstoichiometric Na0.5Bi0.5(Ti0.97Ni0.03)O3 ferroelectric films deposited on LaNiO3(100)/Si substrate

Highly (l00)-oriented Ni-doped Na_0.5Bi_0.5TiO₃ (NBTNi) thin films with different A-site cation nonstoichiometry were deposited on the LaNiO₃ (100)/Si substrates. We find that low levels of Na/Bi nonstoichiometry in the original composition of NBTNi films have obvious influence on the crystal structure and ferro-/dielectric properties. Na deficiency or Bi excess can lower the leakage current compared to the stoichiometric sample due to the decreased oxide-site vacancies. However, the mechanisms for the two types of films are different. That is, the mobile oxygen vacancies are tied by the Na vacancies in Na deficiency film whereas the formation of oxygen vacancies is suppressed for Bi-rich film. A good combination of ferroelectric property (P_r = 22.7?μC/cm²) and dielectric property (ε_r = 360 and tan?δ?=?0.11) can be achieved in Bi-rich NBTNi (Na_0.5Bi_0.54TNi) film. Besides, the effect of voltage and frequency on the capacitance and dielectric tunability for the Na_0.5Bi_0.54TNi film is investigated solely. These results show that NBT-based thin film is quite flexible in A-site nonstoichiometry, which provides a broad space for performance improvement.     

Effects of Mn doping on dielectric properties and energy-storage performance of Na0.5Bi0.5TiO3 thick films

Lead-free Na_0.5Bi_0.5Ti_1−xMn_xO₃ (NBTMn_x, x=0, 0.01, 0.03 and 0.05) ferroelectric thick films have been fabricated on LaNiO₃/Si(100) substrate by using a polyvinylpyrrolidone-modified sol-gel method and the effects of Mn content on their microstructure, dielectric properties and energy-storage performance were investigated. Compared with the pure Na_0.5Bi_0.5TiO₃ (NBT) thick films, NBTMn_x thick films exhibited a large enhancement in dielectric properties and energy-storage performance. Particularly, a giant recoverable energy-storage density (W) of 30.2 J/cm³ and the corresponding efficiency (η) of 47.7% were obtained in NBTMn_0.01 thick film at 2310 kV/cm. Moreover, the NBTMn_0.01 thick film displayed good energy-storage stability over a large temperature range at different frequency.     

Dielectric and optical properties of Ni doped Na0.5Bi0.5TiO3

Polycrystalline Ni doped Na_.5Bi_0.5TiO₃ samples (Na_0.5Bi_0.5)Ti_1-xNi_xO_3, (x?=?0.5, 0.10, 0.15) have been prepared by solid state reaction. The appearance of the additional peak in X-ray diffraction pattern indicates the ordering of Ti⁴⁺ and Ni²⁺ ions. Polygonal grains are converted into flakes with an increase of Ni concentration. Replacement of Ti⁴⁺ by Ni²⁺ strongly modifies the relative contribution of two peaks in the Raman bands within 200–400?cm^?1. Oxygen vacancy is observed in X-ray photoelectron spectrum to maintain charge neutrality due to aliovalent doping. Broad diffuse phase transition centered at the dielectric constant maximum indicates relaxor behaviour. Comparison between impedance and electric modulus spectrum suggests non-Debye relaxation. The ac conductivity follows the power law with the frequency exponent lies 0.52???0.72. The generation of holes by divalent Ni dopant at tetravalent Ti sites enhances optical band gap.     

Temperature Stable Dielectric Behavior of Sol–Gel Derived Compositionally Graded SrTiO3/Na0.5Bi0.5TiO3/SrTiO3 Thin Films

Polycrystalline sol–gel‐derived SrTiO₃/Na_0.5Bi_0.5TiO₃/SrTiO₃ (ST/NBT/ST) thin films were designed to achieve the electrical isolation of the NBT, and to mediate the temperature dependency of the dielectric properties. Proper thermal annealing of particulate phase enabled us to achieve compositionally graded elemental profiles between individual ST and NBT layers. The dielectric and ferroelectric properties were investigated with respect to the electrical behavior of the monophasic ST and NBT thin films. The dielectric characteristics of the multilayer thin film were marked by a temperature stable behavior (temperature coefficient of dielectric constant of 780 ppm/°C) in the measured ?50°C to 200°C range, frequency‐independent response at room temperature and improved dielectric loss characteristics compared with the NBT; however, on the expense of decreased permittivity and a reduced ferroelectric stability. Nevertheless, stable dielectric properties were achieved and properties of multilayer may well be exploited in functional devices that demand insensitive operation over wide temperature and frequency ranges.     

Effects of annealing temperature on the microstructure,ferroelectric and dielectric properties of W-doped Na0.5Bi0.5TiO3 thin films

The effects of annealing temperature on the structure, morphology, ferroelectric and dielectric properties of Na_0.5Bi_0.5Ti_0.99W_0.01O_3+δ (NBTW) thin films are reported in detail. The films are deposited on indium tin oxide/glass substrates by a sol-gel method and the annealing temperature adopted is in the range of 560–620 °C. All the films can be well crystallized into phase-pure perovskite structures and show smooth surfaces without any cracks. Particularly, the NBTW thin film annealed at 600 °C exhibits a relatively large remanent polarization (P_r) of 20 μC/cm² measured at 750 kV/cm. Additionally, it shows a high dielectric constant of 608 and a low dielectric loss of 0.094 as well as a large dielectric tunability of 62%, making NBTW thin film ideal in the room-temperature tunable device applications.     

Compensation for volatile elements to modify the microstructure and energy storage performance of (W,Ni)-codoped Na0.5Bi0.5TiO3 ceramic films

This work reports the characteristics of nonstoichiometric Na_0.5+xBi_0.5+yTi_0.96W_0.01Ni_0.03O₃ (x?=?0.0%, y?=?1.0%; x?=?0.5%, y?=?2.0%; x?=?1.0%, y?=?4.0%) ceramic films derived from chemical solution deposition and the role played by excess Na/Bi in modifying microstructure and electrical properties. Single perovskite phase structure can be maintained in all compositions. Decreased grain size can be obtained with the increasing compensation for volatile Na/Bi elements. Particularly, extra amounts of 0.5?mol% Na and 2.0?mol% Bi leads to reduced leakage and enhanced ferroelectric polarization. Meanwhile, due to the high breakdown electrical field strength and large difference between maximum and remanent polarization, an excellent energy storage performance can be achieved in Na_0.505Bi_0.52Ti_0.96W_0.01Ni_0.03O₃ sample, which is distinguished by a recoverable energy storage density of 40.5?J/cm³ and an energy storage efficiency of 43.6% at 2515?kV/cm as well as a good frequency stability. Hence, the regulation for the content of volatile elements is effective to modify the electrical response of Na_0.5Bi_0.5TiO₃-based materials.     

Investigation of Tb-doping on structural transition and multiferroic properties of BiFeO3 thin films

Pure BiFeO₃ (BFO) and Bi_1−xTb_xFeO₃ (BTFO) thin films were successfully prepared on FTO (fluorine doped tin oxide) substrates by the sol–gel spin-coating method. The effects of Tb-doping on the structural transition, leakage current, and dielectric and multiferroic properties of the BTFO thin films have been investigated systematically. XRD, Rietveld refinement and Raman spectroscopy results clearly reveal that a structural transition occurs from the rhombohedral (R3c:H) to the biphasic structure (R3c:H+R-3m:R) with Tb-doping. The leakage current density of BTFO_x=0.10 thin film is two orders lower than that of the pure BFO, i.e. 5.1×10⁻⁷ A/cm² at 100 kV/cm. Furthermore, the electrical conduction mechanism of the BTFO thin films is dominated by space-charge-limited conduction. The two-phase coexistence of BTFO_x=0.10 gives rise to the superior ferroelectric (2P_r=135.1 μC/cm²) and the enhanced ferromagnetic properties (M_s=6.3 emu/cm³). The optimal performance of the BTFO thin films is mainly attributed to the biphasic structure and the distorted deformation of FeO₆ octahedra.     

An effective strategy to realize superior high-temperature energy storage properties in Na0.5Bi0.5TiO3 based lead-free ceramics

To develop and fabricate environmentally friendly dielectric capacitors used in high-temperature environment, in this work, we prepare La³⁺ doped 0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ lead-free relaxor ferroelectric ceramics with high and wide phase transition temperature. With the introduction of La³⁺, due to the enhancement of the A- and B- site cation ion disorder, the dielectric relaxation characteristics of the ceramics are more obvious. Therefore, the polarization-electric field loops become slimmer and the remnant polarization (P_r) reduces. In addition, because La³⁺ as a donor dopant has lower mobility than A-site cation ions in the ceramic matrix, the grain sizes decrease with increasing La³⁺ content, which significantly leads to an increase in the breakdown strength (E_b). As a result, both a large recoverable energy density (W_rec) of 1.92 J/cm³ and a high energy efficiency (η) of 85.7% are obtained in the ceramic with 12 mol% La³⁺ content. More importantly, even at 200 °C and a low driving electric field of 155 kV/cm, the W_rec and η of this kind of ceramic are still as high as 1.2 J/cm³ and 89.4%, indicating good temperature stability. This work provides an effective and simple way to prepare environmentally friendly dielectric capacitors that are applicable in high-temperature environment.     

Bi(Mg0.5Ti0.5)O3 addition induced high recoverable energy-storage density and excellent electrical properties in lead-free Na0.5Bi0.5TiO3-based thick films

(1-x)Na_0.5Bi_0.5TiO₃-xBi(Mg_0.5Ti_0.5)O₃ (NBT-BMT) thick films were designed for achieving large recoverable energy-storage density (W_rec). A large W_rec of 40.4 J/cm³ was detected in the thick film for x = 0.4, which was more than 4 times larger than that of the pure NBT film. The addition of BMT induced slim polarization hysteresis (P-E) loops at room temperature. The slim P-E loops improved the difference between the maximum polarization (P_max) and the remnant polarization (P_r). Besides, a breakdown strength field (BDS) of 2440 kV/cm was also detected in the thick film for x = 0.4. The high BDS was caused by the reduced leakage current density. Furthermore, the thick film for x = 0.4 possessed superior energy-storage stability under different temperature, frequency and electric-field cycling. In addition, 90% of the pulsed discharge energy density could be released in less than 1100 ns by using a pulsed discharge measurement.     

Multiferroic and magnetoelectric properties of lead-free Ba0.8Sr0.2Ti0.9Zr0.1O3-Ni0.8Zn0.2Fe2O4 composite films with different deposition sequence

Ba_0.8Sr_0.2Ti_0.9Zr_0.1O₃/Ni_0.8Zn_0.2Fe₂O₄(BN) and Ni_0.8Zn_0.2Fe₂O₄/Ba_0.8Sr_0.2Ti_0.9Zr_0.1O₃ (NB) composite film were deposited on Pt/Ti/SiO₂/Si substrates by the sol-gel method and spin-coating method. The results show that the deposition sequences of the composite films have significant influence on the ferroelectric, ferromagnetic and magnetoelectric properties of the composite films. Two composite films possess not only good ferroelectric and ferromagnetic properties but good magnetoelectric properties as well. The NB composite film has clear interface between the ferroelectric film and ferromagnetic film and possesses greater magnetoelectric coupling effect than the BN composite film under the same H_bias. The maximum value of α_E is 70.14?mV?cm^?1 Oe^?1 was obtained in the NB composite film when H_bias is 638?Oe.     

Dielectric relaxation and resistive switching of Bi0.96Sr0.04Fe0.98Co0.02O3/CoFe2O4 thin films with different thicknesses of the Bi0.96Sr0.04Fe0.98Co0.02O3 layer

Bi_0.96Sr_0.04Fe_0.98Co_0.02O₃/CoFe₂O₄(BSFCO/CFO) bilayered thin films with different thicknesses of the BSFCO layer are synthesized on FTO/glass substrates by chemical solution deposition method (CSD). The influence of BSFCO thickness on the microstructure, dielectric relaxation, ferroelectric properties and resistive switching (RS) of the thin films are researched. Strain exists in the prepared thin films and gives rise to structural distortion, which has an effect on charged defects and ferroelectric polarization. Dielectric relaxation that is closely related to the interfacial polarization at the BSFCO/CFO interface is observed, and the dielectric loss peaks along with decreasing intensity shift to high frequency with decreasing strain. The Maxwell-Wagner two-layer model is adopted to investigate the mechanism of dielectric relaxation, and the relaxation time τ is calculated and it shown to be directly proportional to the strain. It is found that the dielectric properties, including low dielectric loss, can be improved by controlling the BSFCO layer thickness. The ferroelectric properties improve with the decreasing strain, the 12-BSFCO/CFO thin film possesses a large P_r ~ 102.9?μC/cm² at 660?kV/cm. The observed resistive switching (RS) behavior is attributed to the interfacial conduction mechanism, it is found that strain-dependent the ferroelectric polarization switching modulates the width of depletion layer and the height of potential barrier at the interface, resulting in the different resistance states.     

Improved ferroelectric photovoltaic effect in Mn-doped lead-free K0.5Na0.5NbO3 films

The pure and Mn-doped K_0.5Na_0.5NbO₃ (KNN) films were deposited using solution-gelation method. The crystal structure, ferroelectric properties, spectral response and J-V performance of photovoltaic effect were systematically investigated. Both the ferroelectric and leakage properties are obviously enhanced for Mn-doped KNN films. A fascinating phenomenon is observed that the ferroelectric photovoltaic effect is enhanced in Mn-doped KNN films, which is originated from the improved ferroelectric polarization and narrower band gap. The transition element Nb partially substituted by Mn results in the lattice distortion and further destroys the symmetry space structure, which enhances ferroelectric polarization. And the narrower band gap effectively decreases the internal potential barrier to separate the carriers. This work gives a clear relationship between the lattice distortion, ferroelectric and photovoltaic response. It is certain that lead-free transparent K_0.5Na_0.5NbO₃ films can be potentially applied in viable ferroelectric based solar cells.     

Effect of Donor, Acceptor, and Donor–Acceptor Codoping on the Electrical Properties of Ba0.6Sr0.4TiO3 Thin Films for Tunable Device Applications

We have investigated the effects of donor, acceptor, and donor–acceptor codoping on both the dielectric properties and the leakage current behavior of Ba_0.6Sr_0.4TiO₃ thin films prepared by the metalorganic solution deposition technique. La and Co were selected as donor and acceptor dopants, respectively. The electrical properties depend strongly on the type of dopants. Compared with others, codoped BST films have a much lower loss tangent, higher figure of merit, and lower leakage current. The electronic conduction mechanisms of the three types of dopants are reported.     

Dielectric,ferroelectric, and photoluminescent properties of Sm-doped Bi4Ti3O12 thin films synthesized by sol-gel method

We report on the structure, dielectric, ferroelectric, and photoluminescent properties of Sm³⁺-doped Bi₄Ti₃O₁₂ thin films which were prepared on fused silica and Pt/Ti/SiO₂/Si substrates by sol-gel method. The X-ray diffraction analysis confirmed that the Bi_4-xSm_xTi₃O₁₂ (BSmT) thin films were well crystallized in layered perovskite structure without any secondary phase. Raman spectra indicated that the structure of BSmT thin films was significantly distorted because of the Sm³⁺ doping. An appropriate doping amount of Sm³⁺ ions leads to obvious enhancement in ferroelectric and dielectric properties of BSmT thin films due to structure distortion and reduction in defects. In addition, the BSmT thin films also show orange-red color emission at 601?nm and long florescence lifetime (> 0.6?ms). This study indicated that lead-free BSmT thin films, which are featuring good electrical and photoluminescent properties, may have potential applications in integrated optoelectronic devices.     

Resistive switching behavior and improved multiferroic properties of Bi0.9Er0.1Fe0.98Co0.02O3/Co1-xMnxFe2O4 bilayered thin films

Bilayered Bi_0.9Er_0.1Fe_0.98Co_0.02O₃/Co_1-xMn_xFe₂O₄ (BEFCO/CM_xFO) thin films were deposited by the sol-gel method. Structural variations between the triclinic-P1 and trigonal-R3c:H (two-phase coexistence) phases in the BEFCO layer were observed owing to the trigonal-R-3m:H phase existing in the CM_xFO layer. The oxygen vacancy concentrations of the BEFCO/CM_xFO bilayered films are reduced by Mn-doping in the bottom CFO layer. The BEFCO/CFO films showed high oxygen vacancy concentrations with a high leakage current. This induced changes of the significant potential barrier at the interface between the BEFCO and CM_xFO layers in the processes of electron capture and release. Thus, the BEFCO/CFO film exhibited obvious resistive switching (RS) effect. The high leakage current also caused a fake polarization phenomenon with a blow up of the P-E loop in the BEFCO/CFO films. However, the real and outstanding ferroelectric properties, which resulted from the fewer oxygen vacancies and the 38% triclinic-P1 structure, were obtained in the BEFCO/CM_0.3FO films (P_r~156.3?μC?cm^?2). In addition, the typical capacitance-voltage curve further confirmed its superior ferroelectric performance. The RS effect almost disappeared in the BEFCO/CM_0.3FO bilayered films. Moreover, the enhanced ferromagnetic properties (M_s~100.36?emu?cm^?3, M_r~55.38?emu?cm^?3) were obtained for the BEFCO/CM_0.1FO films, which was attributed to the magnetic properties of BEFCO (a more triclinic-P1 phase and numerous Fe²⁺ ions), in addition to the CM_xFO layer. The introduction of the doped magnetic layer into the bilayered films thus represented a highly effective method for enhancing the multiferroic properties of BFO.     

Thickness dependent growth and ferroelectric/dielectric properties of phase-pure 0.65Pb(Mg1/3Nb2/3)O3-0.35PbTiO3 thin film derived from a modified sol-gel process

Annealing parameter and thickness are two significant factors affecting microstructure and electrical performance of sol-gel derived 0.65Pb(Mg_1/3Nb_2/3)O₃?0.35PbTiO₃ (0.65PMN-0.35PT) thin film. In this paper, various durations are firstly selected for the investigations on annealing parameter of 0.65PMN-0.35PT thin film. Enhanced insulating and ferroelectric properties can be obtained for the film annealed for 1 min due to its phase-pure and homogeneous perovskite structure. Based on this, a series of 0.65PMN-0.35PT thin films with various thicknesses by modifying deposition layer are synthesized annealed for 1 min and the effects of thickness on crystalline, insulating, ferroelectric and dielectric properties are characterized. It reveals that thickness-dependent behavior can be noticed for 0.65PMN-0.35PT thin film with the results that the 8-layered film possesses a relative large remanent polarization (P_r) of 23.34 μC/cm², and reduced leakage current density of 10^?9 A/cm² with low dissipation factor (tanδ) of 0.03 can be achieved for the 14-layered film.     

Improved dielectric energy storage performance of Na0.5Bi0.5TiO3-based lead-free relaxation ferroelectric ceramics achieved by domain structural regulation and enhanced densification

There is still a problem of low energy storage density in dielectric capacitors which is a core component of power systems. For the improvement of the energy storage density, the linear dielectric material CaTiO₃ (CT) was introduced in Na_0.5Bi_0.5TiO₃ (NBT) ceramics in this paper. By modifying the A site, a new relaxor ferroelectric ceramic was successfully synthesized and attained a recoverable density (W_rec) of 2.34 J/cm³ at x = 0.18. Moreover, the preparation process was optimized in this paper. Through the viscous polymer process (VPP) route, the energy density (W_A) of 82NBT-18CT_VPP ceramic further reaches 6.45 J/cm³ at 340 kV/cm, with efficiency (η) up to 75% and a W_rec of 4.82 J/cm³. At the same time, the change of W_rec is small at temperature (30–150 °C) and frequency (1 Hz–300 Hz), which demonstrates its excellent stability. The discharge power density reaches about 180 MW/cm³ and the discharge time is 0.117 μs, which indicates its excellent pulse discharge performance. The results show that 82NBT-18CT lead-free relaxation ferroelectric material is expected to become ideal for high-energy storage applications.     

Microstructural,ferroelectric, and photoluminescent properties of (100)‐oriented Sm3+‐doped Na0.5Bi0.5TiO3 thin films

(100)_C‐oriented Na_0.5Bi_0.5‐xSm_xTiO₃ (NBST) lead‐free ferroelectric thin films were prepared on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method, and their microstructural, dielectric, ferroelectric, and photoluminescent properties were studied. X‐ray diffraction and scanning electron microscopy analysis indicated that both the grain size and (100)_C orientation degree of NBST thin films were decreased by doping Sm³⁺ ions. Raman spectra showed that structural symmetry of NBST thin films decreased at low Sm³⁺ doping concentration and then increased at high doping concentration of Sm³⁺ ions. An appropriate amount of Sm³⁺ dopants was confirmed to enhance dielectric and ferroelectric properties of the NBST thin films. Among all the compositions, the Na_0.5Bi_0.492Sm_0.008TiO₃ thin film exhibited the largest remnant polarization (2P_r) of 27.3 μC/cm² and high dielectric constant of 1068, as well as a low dielectric loss of 0.04. Temperature‐ and frequency‐dependent dielectric characteristics illustrated the relaxor ferroelectric behavior of Na_0.5Bi_0.492Sm_0.008TiO₃ thin film. Meanwhile, the Na_0.5Bi_0.492Sm_0.008TiO₃ thin film also showed optimal orange‐red emission at 600 nm, which is originating from the ⁴G_5/2 → ⁴H_7/2 transition of Sm³⁺ ions.