Investigation of BiFeO3 modified PbTiO3-Bi(MgZr)O3-based complex perovskite ceramics

Bismuth containing crystalline solutions of (1 − x)Bi(MgZr)_0.5O₃-xPbTiO₃ (BMZ-PT) and [(BiFeO₃)_y − (BiMg_0.5Zr_0.5)_1−y]_x − [PbTiO₃]_1−x (BMZ-BF-PT) have been developed using conventional ceramic technology. X-ray diffraction analysis reveals that both the systems possess a perovskite structure, in which tetragonal to rhombohedral phase transformation appears for x = 0.55 in BMZ-PT and y = 0.20 in BMZ-BF-PT systems. SEM photographs reveal a uniform grain size distribution in the solid solution matrix with the presence of ferroelectric domains in few of the compositions. Ferroelectric hysteresis (polarization-electric field, P-E) loops reveal that increase in BiFeO₃ in BMZ-PT systems results in a decrease in residual polarization of the system with change and distortion in the shape of the (P-E) loops.     

Structural, dielectric, magnetic, magnetodielectric and impedance spectroscopic studies of multiferroic BiFeO3-BaTiO3 ceramics

Polycrystalline (1−x)BiFeO₃-xBaTiO₃ (x = 0.00, 0.10, 0.20 and 0.30) ceramics have been prepared via mixed oxide route. The effect of BaTiO₃ substitution on the dielectric, ferroelectric and magnetic properties of the BiFeO₃ multiferroic perovskite was studied. From XRD analysis it revealed that BaTiO₃ substitution does not affect the crystal structure of the (1−x)BiFeO₃-xBaTiO₃ system up to x = 0.30. Improved dielectric properties were observed in the prepared system. An anomaly in the dielectric constant (?) was observed in the vicinity of the antiferromagnetic transition temperature. Experimental results suggest that in the (1−x)BiFeO₃-xBaTiO₃ system, the increase of BaTiO₃ concentration leads to the effective suppression of the spiral spin structure of BiFeO₃, resulting in the appearance of net magnetization. The dependence of dielectric constant and loss tangent on the magnetic field is a evidence of magnetoelectric coupling in (1−x)BiFeO₃-xBaTiO₃ system. The impedance analysis suggests the presence of a temperature dependent electrical relaxation process in the material, which is almost similar for all the concentrations in the present studies. The electrical conductivity has been observed to increase with rise in temperature showing a typical negative temperature coefficient of the resistance (NTCR) behaviors analogous to a semiconductor and suggests a non-Debye type of electrical relaxation.     

X-ray characterisation of chemical solution deposited PbTiO3 films with high Ca doping

Single crystalline phase lead calcium titanate (Pb_1−xCa_xTiO₃) (PCT) thin films, with Ca²⁺ contents between 24 and 50 at.%, have been prepared onto Pt/(1 0 0)MgO by chemical solution deposition and rapid thermal processing. The sol-gel process used enables the synthesis of air-stable and precipitate-free solutions with high Ca²⁺ contents (up to 50 at.% of substitution of Pb²⁺ by Ca²⁺ in the PbTiO₃ perovskite has been tested). Structure and texture of the crystalline films have been determined by X-ray diffraction (XRD), using the Bragg-Brentano geometry. Strain of the films structure has been calculated by the variance method using the 1 1 1 perovskite peak of the grazing incidence XRD patterns. Data obtained by XRD and dielectric properties measured in the PCT films are compared with those reported for bulk ceramics.     

Preparation of the rod-like β-Si3N4 particles favoring the self-reinforcing Si3N4 ceramics

The β-Si₃N₄ particles were prepared by heating original α-Si₃N₄ powder with rare earth oxide Nd₂O₃ or Yb₂O₃ additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si₃N₄ was also investigated by XRD. The results showed that Yb₂O₃ could accelerate the transformation of Si₃N₄ more effectively than Nd₂O₃ and the powder heated at 1700 °C with over 4 wt.% Yb₂O₃ has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb₂O₃ had ideal β-Si₃N₄ rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si₃N₄ powder to prepare self-reinforced Si₃N₄ ceramic by hot-pressed sintering. The fracture toughness of the seeded Si₃N₄ ceramics increased to 9.1 MPa m^1/2 from 7.6 MPa m^1/2 of the unseeded Si₃N₄ ceramics, while the high value of strength was still kept at 1200 °C.     

Stoichiometric Pb(Mg1/3Nb2/3)O3 perovskite ceramics produced by reaction-sintering process

Stoichiometric lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), perovskite ceramics produced by reaction-sintering process were investigated. Without calcination, a mixture of PbO, Nb₂O₅, and Mg(NO₃)₂ was pressed and sintered directly. Stoichiometric PMN ceramics of 100% perovskite phase were obtained for 1, 2, and 4 h sintering at 1250 and 1270 °C. PMN ceramics with density 8.09 g/cm³ (99.5% of theoretical density 8.13 g/cm³) and K_max 19,900 under 1 kHz were obtained.     

Effect of Pr substitution on structural and electrical properties of BiFeO3 ceramics

Investigation on structural, vibrational, dielectric and ferroelectric properties of Bi_1−xPr_xFeO₃ (x = 0.0, 0.15, 0.25) ceramic samples has been carried out. Room temperature Rietveld-refined X-ray diffraction pattern shows the crystal structure of Bi_1−xPr_xFeO₃ is rhombohedral for x = 0 and triclinic for x = 0.15, 0.25. The changes in Raman normal modes with increasing doping concentration infer the structural transformation is due to Pr substitution at A-site in BiFeO₃. Raman spectra also reveal suppression of ferroelectric behavior due to Pr doping. The dielectric parameters, namely, dielectric permittivity (ε′) and loss tangent (tan (δ)) were evaluated as a function of frequency at room temperature. The ferroelectric polarization reduces in Pr doped bulk BFO samples due to structural change.     

Structural and impedance properties of KBa2V5O15 ceramics

The polycrystalline sample of KBa₂V₅O₁₅ ceramics was prepared by a mixed oxide method at low temperature (i.e., at 560 °C). The formation of the compound was confirmed using an X-ray diffraction technique at room temperature. Scanning electron micrograph of the material showed uniform grain distribution on the surface of the sample. Detailed studies of dielectric properties of the compound as a function of temperature at different frequencies suggest that the compound has a dielectric anomaly of ferroelectric to paraelectric type at 323 °C, and exhibits diffuse phase transition. Electrical properties of the material were analyzed using a complex impedance technique. The Nyquists plot showed the presence of both grain (>10³ Hz) and the grain boundary (<10³ Hz) effects in the material. Studies of electrical conductivity over a wide temperature range suggest that the compound exhibits the negative temperature coefficient of resistance behavior. The ac conductivity spectrum was found to obey Jonscher's universal power law.     

Crystallographic and dielectric properties of barium-substituted Pb(Mg1/3Ta2/3)O3 ceramics

Ceramic powders of (Ba,Pb)Pb(Mg_1/3Ta_2/3)O₃ were prepared via a B-site precursor route. Crystal symmetries and lattice parameters were determined. Monophasic perovskite was developed after the two-step reaction process, in which the lattice parameters showed linear changes in the entire composition range. Dielectric responses of the ceramics with compositional and frequency changes were investigated. The results were also compared with the (Ba,Pb)(Zn_1/3Ta_2/3)O₃ data.     

Structural,microstructural and magneto-electric properties of single-phase BiFeO3 nanoceramics prepared by auto-combustion method

Polycrystalline nano BiFeO₃ powders were synthesized by auto-combustion method using urea as fuel and metal nitrates (Fe(NO₃)₃·9H₂O, Bi(NO₃)₃·5H₂O) as oxidizers. In order to optimize the single-phase synthesis condition of BiFeO₃, different fuel to oxidizer ratios have been investigated. The preliminary structural investigation using X-ray diffraction shows the samples were of single phase and crystallize in rhombohedral structure (R3c). The ferroelectric and antiferromagnetic ordering temperatures of BiFeO₃ were found to be 832 °C and 364 °C respectively, from differential thermal analysis. The temperature dependent dielectric study shows an anomaly around 215 °C which corresponds to magneto-electric coupling in the material. Field-emission scanning electron micrographs show effect of fuel to oxidizer ratio on grain size evolution. The ferroelectric hysteresis loops for all the samples were measured at a frequency of 100 Hz confirming the ferroelectric nature. An evidence of magneto-electric coupling was also observed at room temperature from magneto-capacitance measurements.     

Structural and dielectric properties of ferroelectric Sr1−xBaxBi2(Nb0.5Ta0.5)2O9 and Sr0.5Ba0.5Bi2(Nb1−yTay)2O9 ceramics

Ferroelectric Sr_1−xBa_xBi₂(Nb_0.5Ta_0.5)₂O₉ and Sr_0.5Ba_0.5Bi₂(Nb_1−yTa_y)₂O₉ were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions (x=y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature (T_c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba²⁺ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba²⁺ substitution on Sr²⁺ site induces diffused phase transition and diffuseness increases with increasing Ba²⁺ concentration.     

CaMoO4:Dy3+,K+ near White Light Emitting Phosphor:Structural,Optical and Dielectric Properties

Undoped,Dy³⁺ doped and Dy³⁺,K⁺ codoped calcium molybdate phosphors have been synthesized by solid-state reaction method.X-ray diffraction studies reveal the tetragonal structure of the prepared phosphors having crystallite size 15-50 nm.Scanning electron microscopy（SEM） studies reveal the morphology and crystallite size of the prepared phosphors.Photoluminescence studies indicate that there are blue and yellow emissions at 489 and 576 nm,respectively corresponding to Dy³⁺ ion.The introduction of K⁺ ion significantly influences the blue and yellow emissions which causes the near white light emission from this codoped phosphor.The intense absorption peak of the codoped phosphor at 210 nm is attributed to the band gap and a shoulder at 240 nm appears due to charge transfer from oxygen ions to neighbouring molybdenum ions. The band gap of the codoped phosphor is calculated as 5.5 eV from the absorption studies.The dielectric properties such as permittivity and dielectric loss are studied as a function of frequency.     

Dielectric properties and substitution mechanism of samarium-doped Ba0.68Sr0.32TiO3 ceramics

Ba_0.68Sr_0.32TiO₃ ceramics of perovskite structure are prepared by solid state reaction method with addition of x mol% Sm₂O₃, and their dielectric properties are investigated. It is found that, integrating with the lattice parameters and tolerance factor t, there is an alternation of substitution preference of Sm³⁺ for the host cations in perovskite lattice. Owing to the replacement of Sm³⁺ ions for Ba²⁺ ions in the A site, T_c rises with the increase of Sm₂O₃ doping when the doping content is below 0.1 mol%; meanwhile, when the content is more than 0.1 mol%, Sm³⁺ ions tend to occupy the B-site, causing a drop of T_c. Owing to the modifications of Sm³⁺ doping, dielectric constant, dissipation factor and temperature stability of dissipation factor are influenced remarkably, making it a superior candidate for environment-friendly applications. Moreover, the creation of oxygen vacancies controls the dielectric constant when the addition is above 0.1 mol%, so the dielectric constant decreases with increasing of samarium.     

Influence of Bi2O3 and CuO addition on low-temperature sintering and dielectric properties of Ba0.6Sr0.4TiO3 ceramics

In this study, we tried to lower the sintering temperature of Ba_0.6Sr_0.4TiO₃ (BST) ceramics by several kinds of adding methods of Bi₂O₃, CuO and CuBi₂O₄ additives. The effects of different adding methods on the microstructures and the dielectric properties of BST ceramics have been studied. In the all additive systems, the single addition of CuBi₂O₄ was the most effective way for lowering the sintering temperature of BST. When CuBi₂O₄ of 0.6 mol% was mixed with starting BST powders and sintered at 1100 °C, the derived ceramics demonstrated dense microstructure with a low dielectric constant (? = 4240), low dielectric loss (tan δ = 0.0058), high tunability (Tun = 38.3%) and high Q value (Q = 251). It was noteworthy that the sintering temperature was significantly lowered by 350 °C compared with no-additive system, and the derived ceramics maintained the excellent microwave dielectric properties corresponding to pure BST.     

Preparation and electrical properties of bismuth layer-structured ceramic Bi3NbTiO9 solid solution

Bi₃NbTiO₉ (BNT) ceramic materials with the density ratios of 95-97% to the theoretical density were prepared by the conventional mixed-oxide method. The Curie temperature (T_c), 914 °C was found to be almost the highest one among those of all the ceramics known to date. The crystal structure of the ceramic was investigated by X-ray diffraction method. Anisometric plate-like crystalline grains were observed, which revealed the layered structure of the material. The temperature dependence of the electrical resistivity was measured. For the first time the dielectric properties of Bi₃NbTiO₉ in the vicinity of its transition temperature were measured. Finally, the hysteresis (P-E) loop measurement showed a good ferroelectricity in the Bi₃NbTiO₉ ceramics, and the piezoelectric constant (d₃₃) was found to be 7 pC/N.     

Effect of MgWO4 content on properties of Ba0.5Sr0.5TiO3 composite ceramics for tunable microwave applications

xMgWO₄-(1 − x) Ba_0.5Sr_0.5TiO₃ (x = 0.0, 5.0, 15.0, 25.0 and 35.0 wt%) composite ceramics were prepared via solid state reaction processing. Their structural and dielectric properties were systematically characterized. A significant increase in grain size was observed with increasing MgWO₄ content, which was accompanied by obvious variations in dielectric properties of the composite ceramics. It is found that the permittivity peaks of the samples gradually shifted to low temperatures with increasing MgWO₄ content. At the same time, tunabilities of the composite ceramics decreased, but their Q values increased. The sample with 35 wt% MgWO₄ possesses a high tunability of 16.8% (∼10 kHz), a low permittivity of 65 and an appropriate Q value of 309 (∼4.303 GHz), which meet the requirements of high power and impedance matching, thus making it a promising candidate for applications as electrically tunable microwave devices.     

Low temperature sintering and microwave dielectric properties of SmAlO3 ceramics

The effects of sintering aids on the microstructures and microwave dielectric properties of SmAlO₃ ceramics were investigated. CuO and ZnO were selected as sintering aids to lower the sintering temperature of SmAlO₃ ceramics. With the additions, the sintering temperature of SmAlO₃ can be effectively reduced from 1650 to 1430°C. The crystalline phase exhibited no phase differences at low addition level while Sm₄Al₂O₉ appeared as a second phase as the doping level was over 0.5 wt.%. In spite of the additions, the dielectric constants showed no significant change and ranged 19-21. However, the quality factor Q×f was strongly dependent upon the type and amount of additions. The Q×f values of 51,000 and 41,000 GHz could be obtained at 1430°C with 0.25 wt.% CuO and ZnO additions, respectively. The temperature coefficients depended on the additions and varied from −40 to −65 ppm/°C. Results of X-ray diffractions, EDS analysis and scanning electron microscopy were also presented.     

Alkoxide route to La0.5Sr0.5CoO3 epitaxial thin films on SrTiO3

An all alkoxide based sol–gel route was investigated for preparation of epitaxial La_0.5Sr_0.5CoO₃ (LSCO) films on 100 SrTiO₃ (STO) substrates. Films with 20–30 to 80–100 nm thickness were prepared by spin-coating 0.2–0.6 M (metal) solutions on the STO substrates and heat treatment to 800 °C at 2 °C min^− 1, 30 min, in air. The films were epitaxial with a cube-on-cube alignment and the LSCO cell was strained to match the STO substrate of 3.905 Å closely; a and b = 3.894 Å and 3.897 Å for the 20–30 and 80–100 nm films, respectively. The c-axis was compressed to 3.789 Å and 3.782 Å for the 20–30 and 80–100 nm films, respectively, which resulted in an almost unchanged cell volume as compared to polycrystalline film and nano-phase powders prepared in the same way. The SEM study showed mainly very smooth, featureless surfaces, but also some defects. A film prepared in the same way on an -Al₂O₃ substrate was dense and polycrystalline with crystallite sizes in the range 10–50 nm and gave cubic cell dimensions of a_c = 3.825 Å. The conductivity of the ca 30–40 nm thick polycrystalline film was 1.7 mΩcm, while the epitaxial 80–100 nm film had a conductivity of around 1.9 mΩcm.     

Microstructure and microwave dielectric characteristics of (1 − x)Ba(Zn1/3Ta2/3)O3-xBaTi4O9 ceramics

(1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ (0.1 ≤ x ≤ 0.85) composites are prepared by mixing 1150 °C-calcined BaTi₄O₉ with 1150 °C-calcined Ba(Zn_1/3Ta_2/3)O₃ powders. The crystal structure, microwave dielectric properties and sinterabilites of the (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics have been investigated. X-ray diffraction patterns reveal that BaTi₄O₉, ordered and disordered Ba(Zn_1/3Ta_2/3)O₃ phases exist independently over the whole compositional range. The sintering temperatures of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics are about 1240 - 1320 °C and obviously lower than those of Ba(Zn_1/3Ta_2/3)O₃ ceramics. The dielectric constants (?_r) and the temperature coefficient of resonant frequency (τ_f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of BaTi₄O₉ content. Nevertheless, the bulk densities and the quality values (Q × f) of (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramics increase with the increase of Ba(Zn_1/3Ta_2/3)O₃ content. The results are attributed to the higher density and quality value of Ba(Zn_1/3Ta_2/3)O₃ ceramics, the better grain growth, and the densification of sintered specimens added a small BaTi₄O₉ content. The (1 − x)Ba(Zn_1/3Ta_2/3)O₃-xBaTi₄O₉ ceramic with x = 0.1 sintered at 1320 °C exhibits a ?_r value of 31.5, a maximum Q × f value of 68500 GHz and a minimum τ_f value of 4.1 ppm/°C.     

Electrical properties of Pb(Mg1/3Nb2/3)O3-PbTiO3 ceramics modified with WO3

Ferroelectrics 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ (PMN-PT) + x mol% WO₃ (x=0.1, 0.5, 1, 2) were prepared by columbite precursor method. Electrical properties of WO₃-modified ferroelectrics were investigated. X-ray diffraction (XRD) was used to identify crystal structure, and pyrochlore phase were observed in 0.67Pb (Mg_1/3Nb_2/3)O₃-0.33PbTiO₃+2 mol% WO₃. Dielectric peak temperature decreased with WO₃ doping, indicating that W⁶⁺ incorporated into PMN-PT lattice. Lattice constant, pyrochlore phase and grain size contribute to the variation of K_max. Both piezoelectric constant (d₃₃) and electromechanical coupling factors (k_p) were enhanced by doping 0.1 mol% WO₃, which results from the introduction of “soft” characteristics into PMN-PT, while further WO₃ addition was detrimental. We consider that the two factors, introduction of “soft” characteristics and the formation of pyrochlore phase, appear to act together to cause the variation of piezoelectric properties of 0.67PMN-0.33PT ceramics doping with WO₃.     

Synthesis and thermodynamic stability of multiferroic BiFeO3

Ceramic BiFeO₃ samples were prepared by the sol gel combustion method using urea as fuel. The obtain powders were thermal treated at different temperatures (300-840  °C) and times (1-64  h) to investigate the best synthesis conditions of the material. The resulting materials were analysed by TGA, FTIR, SEM/EDS and XRD. Rietveld analysis was applied to the diffraction data. The temperature and time of the heat treatment are critical for a high BiFeO₃ phase content. Thermal treatment of 1  h at 600  °C yielded 99% molar of the BiFeO₃ phase with a mean particle size of 120  nm. Upper or lower calcinations temperatures yielded higher content of the secondary phases Bi₂Fe₄O₉ and Bi₂₅FeO₃₉. Further heat treatment in air or in argon, up to 64  h, induces a decomposition of the BiFeO₃ phase according to the reaction 49 BiFeO₃ BiFeO₃ → 12 Bi₂Fe₄O₉ + Bi₂₅FeO₃₉ pointing out that BiFeO₃ is not thermodynamically stable at 600  °C. The BiFeO₃ decomposition follows Avrami-Erofeev law with a slope of 1 indicating a one-dimensional kinetics.