Key Engineering Materials Vol. 492

Abstract: This paper gives an assessment on the electrical conductivity of different additive fillers (graphite, multi-walled carbon nanotubes) into carbon fiber-cement based composites (CFRC). Results show that cement matrix containing 0.4% carbon fiber (CF) and 0.5% multi-walled carbon nanotubes (MWCNTs) exhibits an excellent electrical conductivity of 33.65Ω·cm. When the content of CF is below the percolation threshold (0.4% CF), adding graphite is beneficial to the electrical conduction of CFRC, which has a tremendous drift from 3991.44Ω·cm to 524.33Ω·cm as the content of graphite varies from 0% to 30%. However, when the content of CF is above the percolation threshold, adding graphite makes no advantages in the electrical conductivity of CFRC because of leading to a porosity rising. MWCNTs are useful conductive constituents for CFRC and can increase electrical conductivity by two orders of magnitude. However, excessive adding MWCNTs into CFRC will have a rapid increase of electrical resistivity on the contrary.

Abstract: (K,Na)NbO₃-based piezoelectric ceramics are promising candidates for practical applications of lead-free piezoelectric materials due to their excellent piezoelectric properties. In this paper, lead-free piezoelectric ceramics (K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.10Sb_0.04)O₃ (KNL-NTS) were successfully fabricated by traditional ceramics processing. The effects of sintering temperature on the structure, density and electrical properties of KNL-NTS ceramics were investigated. Crystal phases of both calcined powders and KNL-NTS ceramics have orthorhombic structure similar to that of KNbO3 ceramics. The piezoelectric coefficient first increases and then decreases with sintering temperature in the 1100-1180 °C range. KNL-NTS ceramics sintered at 1160 °C shows the maximum piezoelectric coefficient of about 199 pC·N^-1 and the maximum remnant polarization of 18.75 μC·cm^-2, with the corresponding 10.95 kV·cm^-1 coercive field and 4.74 g/cm³ density.

Abstract: Lead-free (1-x-y)Na_1/2Bi_1/2TiO₃-xBaTiO₃-yBiFeO₃ ceramics were synthesized by ordinary sintering technique. The compositional dependence of phase structure and electrical properties of the ceramics was systematically investigated. All samples possessed pure perovskite structure. The dielectric and piezoelectric properties of ceramics were investigated with the amount of different BiFeO₃ substitutions. The addition of BiFeO₃ can not only decrease E_c and P_r but also lead to a significant degradation of the dielectric loss tanδ.

Abstract: In this paper, Na_0.54K_0.42Li_0.4NbO3 nano-powders were prepared by the sol-gel method, Na₂CO₃, K₂CO₃, Li₂CO₃ and Nb₂O₅ as raw materials, citric acid as coordination agent, ethylene glycol as esterifying agent, polyethylene glycol as dispersing agent. From the XRD, TG-DTA and TEM analysis, powders’ size, lattice constants and surface topography were studied. The results showed that under the condition of 600°C × 2h, nano-powders showed good dispersiveness and no reuniting, with pure orthorhombic perovskite structure and various grain sizes was 40.78 nm.

Abstract: BiFeO₃ thin films were spin-coated on conductive indium tin oxide (ITO)/glass substrates by a simple sol-gel possess annealed at 470-590°C. The crystal structure of as-prepared BiFeO₃ thin films annealed at different temperature was determined to be rhombohedral of R3m space and free of secondary phases was also confirmed. Cross section scanning electron microscope (SEM) pictures revealed that the thickness of BiFeO₃ thin film was about 320 nm. The double remanent polarization 2P_r of BiFeO₃ thin film annealed at 500°C is 2.5 μC/cm² without applied field at room temperature. Image of atomic force microscopy indicated that the root-mean-square surface roughness value of BiFeO₃ thin film was 6.13 nm.

Abstract: Dy₂O₃-doped bismuth titanate (Bi_4-xDy_xTi₃O₁₂: BDT) thin films with random orientation were fabricated on Pt/Ti/SiO₂/Si substrates by rf magnetron sputtering technique, and the structures and ferroelectric properties of the films were investigated. XRD studies indicated that all of BDT films consisted of single phase of a bismuth-layered structure with well-developed rod-like grains. The remanent polarization (P_r) and coercive field (E_c) of the BDT Film with x = 0.8 were 20 μC/cm² and 60 KV/cm, respectively. After 3 × 10¹⁰ switching cycles, 20% degradation of P_r is observed in the film.

Abstract: The electrical properties of Eu-doped bismuth titanate，Bi_4-xEu_xTi₃O₁₂ (BET) ceramics prepared by a conventional electroceramic technique were investigated. XRD analyses revealed Bi-layered perovskite structure in all samples. SEM micrographs showed randomly oriented and plate-like morphology. For the samples with x=0.4 and 1.0 the current-voltage characteristics exhibited negative differential resistance behaviors and their P-V hysteresis loops were characterized by large leakage current, whereas for the samples with x=0.6 and 0.8 the current-voltage characteristics showed simple ohmic behaviors and their P-E hysteresis loops were the saturated and undistorted hysteresis loops. The remanent polarization ( P_r ) and coercive field (E_c) of the BST ceramic with x=0.8 were above 16μC/cm² and 70KV/cm , respectively.

Abstract: Bi_3.25Gd_0.75Ti_2.97V_0.03O₁₂ (BGTV)ceramic were prepared by solid state reaction. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed plate-like grains with random orientation. BGTV caused a large shift of the Curie temperature ( T_C ) of Bi₄Ti₃O₁₂ (BIT) from 675°C to 398°C. The remanent polarization and the coercive field of the BGTV were 30μC/cm² and 52kV/cm at an electric field of 87kV/cm, respectively. Furthermore, the dielectric permittivity and dissipation factor were 300 and 0.003 at 1MHz, at 1V and at room temperature, respectively. These ferroelectric properties of BGTV are superior to V-doped Bi₄Ti₃O₁₂ (~20μC/cm² and 80kV/cm) and (Sr, Ta)-doped Bi₄Ti₃O₁₂ (~12μC/cm² and 71kV/cm) ceramics. In addition, the dense ceramics of BGTV could be obtained by sintering at temperatures 100─200°C lower than those of the SrBi₂Ta₂O₉ system.

Abstract: La3+ doped textured ceramics CaBi_4-xLa_xTi₄O₁₅ were prepared by oriented consolidation of anisometric particle method with tape casting. The orientation was improved by modifying sintering temperature. The doping of La³⁺ in orientated CaBi_4-xLa_xTi₄O₁₅ ceramics increased the dielectric constant. The higher dielectric constant was obtained in the direction which tropism parallels to the electric field. The relationship between dielectric constant and orientation was discussed.

Abstract: Tb-doped bismuth titanate (Bi_4-xCe_xTi₃O₁₂: BCT) and pure Bi₄Ti₃O₁₂ (BIT) thin films with random orientation were fabricated on Pt/Ti/SiO₂/Si substrates by rf magnetron sputtering technique. These samples had polycrystalline Bi-layered perovskite structure without preferred orientation, and consisted of well developed rod-like grains with random orientation. Ce-doping into BIT caused a large shift of the Curie temperature (T_C) from 675°C to lower temperature and a improvement in dielectric property. The experimental results indicated that Ce doping into BIT also result in a remarkable improvement in ferroelectric property. The P_r and the E_c values of the BCT film with x = 0.75 were 23 μC/cm² and 80 kV/cm, respectively.