Polarization and piezoelectric properties of grain-oriented ferroelectric Bi<Subscript>5</Subscript>FeTi<Subscript>3</Subscript>O<Subscript>15</Subscript> ceramics prepared by magnetic-field-assisted electrophoretic deposition method

Magnetic-field-assisted electrophoretic deposition method has been employed for synthesizing a(b)-axis-oriented Bi₅FeTi₃O₁₅ ceramics, and the effects of grain orientation and microstructure on the polarization and piezoelectric properties have been investigated. Grain-oriented Bi₅FeTi₃O₁₅ ceramics with a high relative sintered density of 98% is shown to exhibit enhanced polarization and piezoelectric properties with a remanent polarization (P _r) of 19 μC/cm² and a piezoelectric strain constant (d ₃₃) of 23 pm/V, which are much superior to those of randomly-oriented ceramics (P _r of 7 μC/cm² and d ₃₃ of 5 pm/V).     

Studies of structural,dielectric and impedance properties of Bi<Subscript>9</Subscript>Fe<Subscript>5</Subscript>Ti<Subscript>3</Subscript>O<Subscript>27</Subscript> ceramics

Bi₉Fe₅Ti₃O₂₇ is an eight-layered material belonging to the family of bismuth layered structured ferroelectromagnets. The polycrystalline sample of this compound was prepared by a standard solid-state reaction technique. The formation of the compound in an orthorhombic crystal structure was confirmed by an X-ray diffraction (XRD) technique (lattice parameters: a?=?5.5045[27] Å, b?=?5.6104[27] Å, c?=?76.3727[27] Å). Detailed studies of surface morphology of the compound using scanning electron microscopy (SEM) exhibit that the compound has domains of plate shaped grains. Studies of dielectric and electric properties in a wide temperature range (30–500 °C) at different frequencies (100 Hz–1 MHz) exhibit an anomaly at 291?±?2 °C, which is related to ferroelectric to paraelectric phase transition as suggested by hysteresis loop at room temperature. The values and nature of temperature variation of dc conductivity exhibit the NTCR behavior of the compound.     

Electrical and structural properties of Nb-doped SrTiO<Subscript>3</Subscript> ceramics

Niobium-doped strontium titanate synthesized via conventional solid-state reaction has been studied. Influence of niobium content on the lattice parameters and electrical conductivity has been reported. Various reduction conditions have been investigated. For samples reduced in hydrogen at 1400°C, a transition from thermally activated to metallic behavior has been observed. Maximum electrical conductivity (ca. 55 Scm⁻¹ at 650°C) has been observed for the SrTi_0.98Nb_0.02O_3-δ sample. The relation of electrical conductivity with the porosity of the samples has been shown.     

Normal-to-relaxor ferroelectric phase transition and electrical properties in Nb-modified 0.72BiFeO<Subscript>3</Subscript>-0.28BaTiO<Subscript>3</Subscript> ceramics

Preparation and electrical properties of sintered bodies consisting of monophase cubic spinel oxides, Cr_XMn_1.5Co_(1.0-X)Ni_0.5O₄ (0 ≦ X ≦ 0.42), were investigated. Specimens with compositions within X = 0.42 were prepared as starting materials. The element of Cr was used to exchange Co³⁺ in octahedral sites (B sites) with Cr³⁺ so that the hopping mechanism can be discussed. The sintered bodies with mono cubic spinel structure were confirmed to be prepared by heat-treatment for 48 h in air at 1000 °C to convert them into a cubic spinel structure after sintering at 1400 °C. The semiconductive characteristics of the sintered bodies were determined as p-type because the Seebeck coefficients were all positive. The electrical conduction of the sintered bodies was concluded to be controlled by the small polaron hopping mechanism. In the region 0.1 ≦ X ≦ 0.42, the lattice constant increases and electrical conduction (σ) decreases linearly with increasing Cr concentration. The decrease in σ and the increase in the lattice constant corresponded to the increase in Cr concentration by which the jumping distance of electrons between Mn³⁺ and Mn⁴⁺ is lengthened.     

Multiferroic,magnetoelectric and magneto-impedance properties of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/(Pb,Sr) TiO<Subscript>3</Subscript> bilayer films

We prepared NiFe₂O₄/(Pb, Sr)TiO₃ (NFO/PST) bilayer films by the chemical solution method and investigated their multiferroic, magnetoelectric and magneto-impedance properties. Multiferroic properties have been observed at room temperature. The bilayer films exhibit saturation polarization P _s ? 26.6 μC/cm² and saturation magnetization M _s ? 134 emu/cm³. With increasing Sr content, M _s. and P _s values of the NFO/PST bilayer films decrease. The variation may be ascribed to the influence of interfacial strain and decrease in tetragonality. High magnetoelectric coupling effect has been observed in the NFO/PST bilayer films with maximum value of α _E  = 6.35 Vcm^?1 Oe^?1 measured at H _DC  ~ 1 kOe and f ~ 10 kHz for Sr ~ 10% (NFO/PST10). The magneto-impedance measurements establish a strong dependence on magnetic field, further confirming magnetoelectric response in NFO/PST bilayer films. An alternative approach for impedance analysis of NFO/PST10 bilayer film provides direct evidence of strain mediated magnetoelectric coupling at room temperature. The results indicate that NFO/PST bilayer films can be considered as a potential multiferroic magnetoelectric material.     

Phase transition,microstructure and properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>1-x</Subscript>Ba<Subscript>x</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

In this article, (Na_0.5Bi_0.5)_1-xBa_xTiO₃ lead-free piezoelectric ceramics were prepared by solid-state reaction. The influence of Ba contents on phase structures, compositional distribution and electrical properties of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were systematically investigated to further understand the nature of phase transition. It was found that the phase structure of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ transforms from rhombohedral to tetragonal symmetry at x = 0.06 ~ 0.07 and Ba²⁺ segregation forms the coexistence of Ba-rich tetragonal and Ba-deficient rhombohedral phases close to MPB. The electrical properties of prepared samples regularly changed with Ba content, which is closely related to the distribution of rhombohedral and tetragonal phases. The prepared sample near MPB exhibited the largest dielectric constant and the excellent piezoelectric properties (the maximal measuring field reached 78 kV/cm and the piezoelectric constant d ₃₃ = 151pC/N).     

Influence of the calcination procedure on the thermoelectric properties of calcium cobaltite Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript>

Calcium cobaltite is one of the most promising oxide p-type thermoelectric materials. The solid-state reaction (or calcination, respectively), which is well known for large-scale powder synthesis of functional materials, can also be used for the synthesis of thermoelectric oxides. There are various calcination routines in literature for Ca₃Co₄O₉ powder synthesis, but no systematic study has been done on the influence of calcination procedure on thermoelectric properties. Therefore, the influence of calcination conditions on the Seebeck coefficient and the electrical conductivity was studied by modifying calcination temperature, dwell time, particle size of raw materials and number of calcination cycles. This study shows that elevated temperatures, longer dwell times, or repeated calcinations during powder synthesis do not improve but deteriorate the thermoelectric properties of calcium cobaltite. Diffusion during calcination leads to idiomorphic grain growth, which lowers the driving force for sintering of the calcined powder. A lower driving force for sintering reduces the densification. The electrical conductivity increases linearly with densification. The calcination procedure barely influences the Seebeck coefficient. The calcination procedure has no influence on the phase formation of the sintered specimens.     

Influence of the processing rates and sintering temperatures on the dielectric properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> ceramics

The stoichiometric CaCu₃Ti₄O₁₂ pellets were prepared by the solid state synthesis. X-ray diffraction data revealed the tenorite CuO and cuprite Cu₂O secondary phases on the unpolished CaCu₃Ti₄O₁₂ samples regardless of the heating rates. Also, the dielectric constant marked the highest for the CaCu₃Ti₄O₁₂ sample sintered at the lowest heating rate (1°C/min), which was explained by the increased grain conductivity due to the cation reactions. On the other hand, Cu₂O phase was found only on the unpolished CaCu₃Ti₄O₁₂ sample sintered over 1100°C and those are considered as the remains reduced from the CuO phase. The higher sintering temperature showed the increased dielectric constant and the loss tangent of the CaCu₃Ti₄O₁₂ samples, and this result could be interpreted by the impedance measurement data. The relationship between the processing condition and the dielectric properties was discussed in terms of the cation non-stoichiometry and the defect chemistry in CaCu₃Ti₄O₁₂.     

Structure and piezoelectric properties of K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>NbO<Subscript>3</Subscript> based lead-free piezoceramics with slight deviation from A-site K or Na stoichiometry

The dependence of structure and piezoelectric response d₃₃ on individual excess A-site K or Na in K_0.5Na_0.5NbO₃ based lead-free piezoceramics is reported. The coexistence of orthorhombic, tetragonal, and monoclinic phases at room temperature is observed in the investigated 0.94K_0.5+xNa_0.5NbO₃–0.06LiNbO₃ and 0.94K_0.5Na_0.5+yNbO₃–0.06LiNbO₃ ceramics (x, y = 0–0.025). The weight ratio of three phases, orthorhombic-tetragonal phase transition temperature, and d₃₃ value are highly sensitive to A-site K or Na nonstoichiometry. A-site Na nonstiochiometry plays a dominant role in enhancing the electrical properties of KNN by significantly reducing the weight ratio percentage of monoclinic phase and shifting the orthorhombic-tetragonal phase transition temperature to lower temperatures.     

Influence of fillers on the re-crystallization and dielectric properties of 60ZnO-30B<Subscript>2</Subscript>O<Subscript>3</Subscript>-10SiO<Subscript>2</Subscript> glass

This paper reports re-crystallization behaviour of 60ZnO-30B₂O₃-10SiO₂ pure glass. Influence of different fillers on the re-crystallization is also highlighted. Irrespective of the nature of the filler, glass transition temperature (T_g) remains the same as that of the pure glass at 570 °C. The onset crystallization temperature is enhanced by 15 to 25 °C owing to filler addition. The peak crystallization temperature corresponding to Zn₃B₂O₆, ZnB₂O₄ and willemite occurs at 735, 810 and 851 °C for the pure glass. In the presence of alumina, gahnite emerges first at 700 °C followed by ZnB₂O₄ phase. With mullite as filler, initially ZnB₂O₄ and willemite formed at 700 °C and gahnite phase emerged only at 750 °C and peaks at 787 °C. In the presence of fused silica only ZnB₂O₄ and willemite phases were observed. The dielectric properties of the pure glass and that of the glass + fillers are also reported. Re-crystallization of various low K phases ensures good dielectric porpertis with reduced glass content after sintenring.     

Effects of MnO<Subscript>2</Subscript> addition on microstructure and electrical properties of (Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>) <Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> ceramics

In this letter, MnO₂-doped (Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃ (BNBT-6) lead-free piezoelectric ceramics were synthesized by solid state reaction, and the microstructure and electrical properties of the ceramics were investigated. X-ray diffraction (XRD) reveals that all specimens take on single perovskite type structure, and the diffraction peaks shift to a large angle as the MnO₂ addition increases. Scanning electron microscopy shows that the grain sizes increases, and then decreases with increasing the MnO₂ content. The experiment results indicate that the electrical properties of ceramics are significantly influenced by the MnO₂ content, and the ceramics with homogeneous microstructure and excellent electrical properties are obtained with addition of 0.3 wt% MnO₂ and sintered at 1160°C. The piezoelectric constant (d₃₃), the electromechanical coupling factor (k _p ), the dissipation factor (tan δ) and the dielectric constant (ɛ _r ) reach 160 pC/N, 0.29, 0.026 and 879, respectively. These excellent properties indicate that the MnO₂-doped BNBT-6 ceramics can be used for actuators.     

Crystallization,microstructures and properties of low temperature co-fired CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> glass-ceramic

CaO-Al₂O₃-SiO₂ glass-ceramic were prepared by melt quenching technique. The crystallization behavior and properties were studied by means of a non-isothermal, thermal analysis technique, X-ray diffraction and scanning electron microscopy. The influence of sintering temperature on phase formation, microstructure, bending strength, dielectric and thermal properties were determined. The activation energy of crystallization and the Avrami parameter were also discussed. The X-ray diffraction results show that SiO₂ phase could be found in all samples and CaSiO₃ and anorthite phases could only be observed in the samples sintered at above 875°C. The densification of glass-ceramic starts at 730°C after the liquid glass is formed and stops at 803°C. Complete densification was achieved at 875°C and the highest mechanical strength was obtained at 850°C, but density significantly decreased at higher temperatures. The coefficient of thermal expansion and the dielectric constant increase with the increasing sintering temperature. The value of the Avrami parameter (n) is ~1.6 and the apparent activation energy (E) is 298 kJ/mol.     

Dielectric and Piezoelectric Properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-K<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-NaNbO<Subscript>3</Subscript>Lead-Free Ceramics

The ternary lead-free piezoelectric ceramics system of (1 – x) [0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃] – xNaNbO₃(x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized by conventional solid state reaction method. The crystal structure, dielectric, piezoelectric properties and P-E hysteresis loops were investigated. The crystalline structure of all compositions is mono-perovskite phase ascertained by XRD, and the lattice constant was calculated from the XRD data. Temperature dependence of dielectric constant _r and dissipation factor tan measurement revealed that all compositions experienced two phase transitions: from ferroelectric to anti-ferroelectric and from anti-ferroelectric to paraelectric, and these two phase transitions have relaxor characteristics. Both transition temperatures T_d and T_m are lowered due to introduction of NaNbO₃. P-E hysteresis loops show that 0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃ ceramics has the maximum P_r and E_c corresponding to the maximum values of electromechanical coupling factor K_p and piezoelectric constant d_33. The piezoelectric constant d₃₃ and electromechanical coupling factor K_p decrease a little, while the dielectric constant ₃₃^T/₀ improves much more when the concentration of NaNbO₃ is 8 mol%.     

Structure and properties of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Bi(Mg<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript>-0.38PbTiO<Subscript>3</Subscript> ceramics with MPB composition

0.62Bi(Mg_1/2Ti_1/2)O₃-0.38PbTiO₃-xwt%Bi₂O₃ (BMT-0.38PT-xBi₂O₃) ceramics were prepared by conventional powder-processing method. It indicated that the morphotropic phase boundary (MPB) region located in 0.0?≤?x?≤?0.3. For x?=?0.3, it exhibited good piezoelectric properties, d₃₃ ~245pC/N and k_p ~40 %. With the increase of Bi₂O₃ content, the Curie temperature (T_c) was found to increase, and the dielectric loss was found to decrease above 200 °C compared with BMT-0.38PT sample. Finally, it can be found that depolarization temperature was around 350 °C by thermal depoling method.     

Large area deposition of Pb(Zr,Ti)O<Subscript>3</Subscript> thin films for piezoelectric MEMS devices

Pb(Zr,Ti)O₃ (PZT) thin films deposited on insulating ZrO₂ buffered silicon wafer are intended to be employed for in-plane polarized piezoelectric MEMS devices. Multi-target reactive sputtering system for large area deposition of in-situ crystallized PZT thin films and the ZrO₂ buffer layer has been employed. The interface analysis of multilayer structures by high resolution transmission microscope, X-ray diffraction, optical refraction, and absorption spectra studies has been presented. At a substrate temperature of 520°C and excess lead deposition condition, the formation of a PZT superstructure has been revealed. The substrate temperature of 580°C leads to the crystallization of PZT directly into a single phase perovskite crystal structure. A pronounced Urbach behavior in our PZT thin films has been observed by optical absorption studies. The surface roughness of PZT films deposited on a ZrO₂ buffer layer is much higher than that on conducting platinized silicon wafer.     

Lead-free (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)<Subscript>0.95</Subscript>(LiSb)<Subscript>0.05</Subscript>Nb<Subscript>0.95</Subscript>O<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> piezoceramics

Lead-free (1-x)(K_0.5Na_0.5)_0.95(LiSb)_0.05Nb_0.95O₃-xBaTiO₃ (abbreviated as (1-x)KNNLS-xBT) piezoceramics were synthesized by conventional solid state sintering and the effect of BaTiO₃ on the microstructure, dielectric and piezoelectric properties was investigated. It was found that both orthorhombic-tetragonal (T _O-T) and tetragonal-cubic (T _C) phase transition temperatures decreased obviously with increasing BaTiO₃ content. Although proper amount of BaTiO₃ facilitated the sintering of (1-x)KNNLS-xBT ceramics, the addition of BaTiO₃ affected the relaxor behavior slightly and it was not beneficial to improve piezoelectric strain coefficient d ₃₃, remnant polarization P _r and piezoelectric coupling constant k _p.     

Raman and infrared spectroscopy of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript> - BaTiO<Subscript>3</Subscript> ceramics

Lead-free Na_0.5Bi_0.5TiO₃ -BaTiO₃ ceramics have been prepared in the whole range of concentrations and studied at room-temperature by means of X-ray, Raman scattering and infrared techniques. X-ray measurements revealed rhombohedral, rhombohedral-tetragonal boundaries and tetragonal modifacations depending on the contents of BaTiO₃. The distinct changes of the Raman and infrared spectra with increasing of BaTiO₃ content, which were correlated with X-ray results, were observed. The broad phonon spectra indicated the disorder in the A site of Na_0.5Bi_0.5TiO₃ -BaTiO₃ system.     

Microwave Dielectric Characteristics of MgTiO<Subscript>3</Subscript>/CaTiO<Subscript>3</Subscript> Layered Ceramics

MgTiO₃/CaTiO₃ layered ceramics with differently stacking were fabricated and the microwave dielectric properties were evaluated with TE₀₁₁ mode. With increasing CaTiO₃ thickness fraction, the resonant frequency decreased and the dielectric constant increased with a near-linear relation for the bi-layer ceramics, while the values of the tri-layer MgTiO₃/CaTiO₃/MgTiO₃ ceramics with thickness ratio of 1:1:1 derived much from the curves of the bi-layer ceramics. The finite element method was used to give an explanation for the differences between the bi-layer and tri-layer ceramics.     

Effect of Ca substitution by Fe on the thermoelectric properties of Ca<Subscript>3</Subscript>Co<Subscript>4</Subscript>O<Subscript>9</Subscript> ceramics

(Ca_1-xFe_x)₃Co₄O₉ polycrystalline samples (x?=?0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1 and 0.2) have been prepared by solid state reaction and sintered by spark plasma sintering. Their thermoelectric properties have been studied between 323 and 1000 K. The substitution limit is low (< 3%). For higher iron contents, formation of Co₃O₄, Ca₂FeCoO₅ and a calcium ferrite compound occurs. Concerning electrical conductivity and power factor, an optimum value exists for x?=?0.03 substitution. This optimum power factor is circa 10% higher than the one of the unsubstituted sample. The optimum thermal diffusivity is measured for the sample with x?=?0.02 and is circa 9% lower than the one of the unsubstituted sample. Therefore, the dimensionless figure of merit ZT is finally increased by 14% for the (Ca_0.98Fe_0.02)₃Co₄O₉ and (Ca_0.97Fe_0.03)₃Co₄O₉ compositions.     

Structural,dielectric and magnetic properties of particulate composites of relaxor (BaTi<Subscript>0.85</Subscript>Sn<Subscript>0.15</Subscript>O<Subscript>3</Subscript>) and ferrite (NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>) synthesized by gel-combustion method

Particulate composites of (1-x) BaTi_0.85Sn_0.15O₃ – x NiFe₂O₄ (with x?=?5, 10,15 and 20 wt%) were synthesized using the solid-state reaction method by sintering at 1350 °C for 4 h. Formation of the diphase composites was confirmed by X-ray diffraction (XRD) and Fourier Transform Infra-red (FTIR) techniques. Temperature (RT-200 °C) and frequency (20 Hz- 1 MHz) dependent of AC conductivity, dielectric constant and dissipation have been studied. The dielectric constant exhibits strong frequency dispersion in the range 20 Hz-1 kHz which is attributed to Maxwell-Wagner interfacial polarization occurring at grain-grain-boundaries interface/interface of grains of BTS-NF. The M-H curve of all the composites exhibited a hysteresis loops typical charcateistic of a ferromagnetic material. The ferromagnetic ordering in the composites on account of NiFe₂O₄ as a constituent is explained using bound magnetic polarons (BMPs) model. The experimental magnetic data have been fitted to BMP model. Value of M_s is smaller, whereas of H_c and M_r are higher of the composites compared to value for NiFe₂O₄. The temperature at which divergence in the M vs. T plot in ZFC and FC starts is higher for the composites than for NiFe₂O₄.