Phase evolution,microstructure and chemical stability of Ca1-xZr1-xGd2xTi2O7 (0.0 ≤ x ≤ 1.0) system for immobilizing nuclear waste

In order to ascertain the structural relationship of zirconolite and pyrochlore for their potential application in HLW immobilization, the Gd-doped zirconolite-pyrochlore composite ceramics (Ca_1-xZr_1-xGd_2xTi₂O₇) were systematically synthesized with x?=?0.0–1.0 by traditional solid-phase reaction method. The phase evolution and microstructure of the as-prepared samples have been elucidated by XRD and Rietveld refinement, Raman spectroscopy, BSE-EDS and HRTEM analysis. The results showed that zirconolite-2M, zirconolite-4M, perovskite and pyrochlore, four phases were identified in Ca_1-xZr_1-xGd_2xTi₂O₇ system and could be coexisted at x?=?0.4 composition. With the increase of Gd³⁺ substitution, the phase evolution was followed by zirconolite-2M→zirconolite-4M→pyrochlore. It is illustrated that the phase transformation from zirconolite-2M to zirconolite-4M was promoted by the preferential substitution of Gd³⁺ for Ca²⁺. And the solubility of Gd³⁺ in zirconolite-2M, zirconolite-4M and pyrochlore increased in sequence. The chemical stability test was also measured by the PCT leaching method. The normalized elemental release rates of Ca, Zr, Ti and Gd in Ca_1-xZr_1-xGd_2xTi₂O₇ system were fairly low and in the range of 10^?6?10^?8 g?m^?2 d^?1, which indicated a potential ceramics composite ensemble of CaZrTi₂O₇-Gd₂Ti₂O₇ system for nuclear HLW immobilization.     

High-temperature thermoelectric properties of polycrystalline CaMn1-xNbxO3-δ

The structural and thermoelectric (TE) properties of polycrystalline CaMn_1-xNb_xO_3-δ (0.025?≤?x?≤?0.25) were studied with X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and electrical transport measurements, with an emphasis placed on the Nb⁵⁺ content. The CaMn_1-xNb_xO_3-δ crystallized in an orthorhombic perovskite structure of the Pnma space group. The density and grain size of the CaMn_1-xNb_xO_3-δ samples gradually decreased when Nb⁵⁺ ions substituted Mn⁴⁺ ions. The CaMn_0.95Nb_0.05O_3-δ sample contained charge-ordered domains, stacking faults, and micro-twins. The substitution of Nb⁵⁺ for Mn⁴⁺ up to x?=?0.15 led to an increase in electrical conductivity, mainly due to an increased electron concentration. The CaMn_1-xNb_xO_3-δ samples with low Nb⁵⁺ contents (0.025?≤?x?≤?0.15) showed metallic behavior, whereas those with high Nb⁵⁺ contents (0.2?≤?x?≤?0.25) showed semiconducting behavior. The Nb⁵⁺ substitution lowered the absolute value of the Seebeck coefficient for the CaMn_1-xNb_xO_3-δ samples due to an increased electron concentration. The largest power factor (1.19?×?10^?4 W?m^?1 K^?2) was obtained for CaMn_0.95Nb_0.05O_3-δ at 800?°C. The partial substitution of Nb⁵⁺ for Mn⁴⁺ in CaMnO_3-δ proved to be highly effective for improving high-temperature TE properties.     

Magnetoelectric properties of 0.5BiFeO3–0.5Pb(Fe0.5Nb0.5)O3 solid solution

Magnetoelectrics are materials that join magnetic and electric orderings in the same phase. They exhibit magnetoelectric coupling which is important from the fundamental and practical point of view. The subject of the paper is a presentation of magnetic, electric and magnetoelectric properties of 0.5BiFeO₃–0.5Pb(Fe_0.5Nb_0.5)O₃ solid solution. The obtained material belongs to oxide perovskite magnetoelectrics of relatively high magnetic and electric ordering temperatures. Both temperatures are considerably above room what suggests potential application possibilities of the material. The magnetic properties were investigated using Mössbauer spectroscopy and magnetization measurements. The solid solution is an antiferromagnet with incomplete compensated magnetic moments. The electrical properties were determined using impedance spectroscopy analysis. There is an observed change of the electrical properties at the magnetic ordering temperature what indicates magnetoelectric coupling in the system. The electrical conductivity mechanism is also proposed. Magnetoelectric voltage coefficient was determined and possible explanation of its changes was proposed.     

Microstructure and electrical properties of (Na0.5K0.5)1−2xMgxNbO3–Bi0.5Na0.5TiO3 lead-free piezoelectric ceramics

0.975[(Na_0.5K_0.5)_1−2xMg_xNbO₃]–0.025(Bi_0.5Na_0.5TiO₃) (KNMN–BNT, x=0, 0.01, 0.02, 0.03, 0.04 and 0.05) lead-free piezoelectric ceramics were fabricated by the conventional solid-state sintering method. The dependence of Mg content on the microstructure and electrical properties of the ceramics is investigated. The X-ray diffraction (XRD) analysis revealed that an appropriate amount of Mg diffused into the KNN–BNT lattice to form a stable solid solution, the ceramics possessed a pure perovskite structure, and a morphotropic phase boundary (MPB) between the orthorhombic and tetragonal phases was observed with the composition of 0.02≤x≤0.05. The orthorhombic–tetragonal transition temperature (T_O–T) is less than 95 °C and the Curie temperature (T_c) is almost unchanged (~360 °C) with the increase of MgO content. The ceramics with x=0.02 showed enhanced piezoelectric and ferroelectric properties because of close proximity to the MPB, i.e., d₃₃~210 pC/N, k_p~0.41, 2E_c~22.4 kV/cm and 2P_r~39.2 μC/cm². Moreover, the dielectric properties exhibited optimal effects with x=0.02, that is ε_r~637 and tan δ~0.09. These results indicate that the introduction of MgO is an effective method to improve the density as well as the electrical properties and the temperature stability of the KNN–BNT ceramics. As a result, the KNMN–BNT ceramic is a promising candidate for lead-free piezoelectric materials.     

Structure and electrical properties of Nd2O3-doped 0.82Bi0.5Na0.5TiO3–0.18Bi0.5K0.5TiO3 ceramics

Nd₂O₃ doped 0.82Bi_0.5Na_0.5TiO₃–0.18Bi_0.5K_0.5TiO₃ (abbreviated to BNKT) binary lead-free piezoelectric ceramics were synthesized by the conventional mixed-oxide method. The results show that the BNKT ceramics with 0–0.15 wt.% Nd₂O₃ doping possesses a single perovskite phase with rhombohedral structure. The grain size of BNKT decreased with the addition of Nd₂O₃ dopant. The temperature dependence of the dielectric constant ?_r revealed that there were two-phase transitions from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric. A diffuse character was proved by linear fitting of the modified Curie–Weiss law. At room temperature, the specimens containing 0.0125 wt.% Nd₂O₃ with homogeneous microstructure presented excellent electrical properties: the piezoelectric constant d₃₃ = 134 pC/N, the electromechanical coupling factor K_p = 0.27, and the dielectric constant ?_r = 925 (1 kHz).     

Suppression of 3D mobility of carrier and superconductivity by Y substitution in Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ samples

Bulk Cu_0.5Tl_0.5Ba₂(Ca_2−xY_x)Cu₃O_10−δ superconductor ceramic samples were synthesized by the conventional solid-state method and characterized by X-ray diffraction, dc-resistivity, ac-susceptibility and Fourier Transform Infrared spectroscopy. The main purpose of this study was to investigate the role of charge carriers and the effect of Y substitution at Ca sites in between the CuO₂ planes on superconductivity. The superconducting properties are suppressed by Y substitution at Ca sites in between the CuO₂ planes of Cu_0.5Tl_0.5Ba₂(Ca_2−xY_x)Cu₃O_10−δ samples. It is most likely that Y³⁺ may create correlated domains in between the CuO₂ planes and localizes the carriers, which lowers the diamagnetic screening and suppresses the superconductivity. Therefore, cationic substitution reduces the three dimensional (3D) mobility of carriers, resulting in the reduction of the Fermi vector and velocity of the carriers, which in turn suppresses the superconducting properties of the material.     

Magnetic properties of hard (CoFe2O4)–soft (Fe3O4) composite ceramics

Composite ceramics of CoFe₂O₄/Fe₃O₄ with different weight ratios were synthesized by Spark Plasma Sintering (SPS) at a sintering temperature of 500 °C. The X-ray diffraction patterns demonstrate that all samples are composed of CoFe₂O₄ and Fe₃O₄ phases. The magnetization curves for all the composite ceramic are single-step loops indicating the existence of exchange spring effect. Due to the competition between the exchange interaction and the dipolar interaction, magnetic properties like coercivity (H_c) and remanence (M_r) are sensitive to the weight ratio of the soft phase.     

Crystal structure,Raman spectra and microwave dielectric properties of Li2Mg3Ti1-x(Mg1/3Nb2/3)xO6 (0 ≤x ≤ 0.25) ceramics

Li₂Mg₃Ti_1-X(Mg_1/3Nb_2/3)_XO₆ (0?≤x?≤?0.25) ceramics were prepared by a conventional solid-state reaction process. Their crystal structures, sintering characteristics, Raman spectra and microwave dielectric properties were then investigated. XRD patterns of the sintered samples indicated that all compositions showed a single phase and the rock-salt structure. As the (Mg_1/3Nb_2/3)⁴⁺ contents increase, the variations of ε_r values showed a downward trend, which could be explained by the changes of polarizabilities and the shift of Raman vibration modes. Q·f values initially increased to a maximum value and then decreased with increasing of x values. In addition, τ_f values decreased almost linearly with the x values, which significantly correlated with the thermal expansion coefficient. Excellent combined microwave dielectric properties with ε_r =?14.79, Q·f?=?204,900?GHz and τ_f =??18.43?ppm/°C were obtained for Li₂Mg₃Ti_.95(Mg_1/3Nb_2/3)_.05O₆ ceramic sintered at 1550?°C.     

Effect of SrZrO3 on phase structure and electrical properties of 0.974(K0.5Na0.5)NbO3–0.026Bi0.5K0.5TiO3 lead-free ceramics

(0.974−x)(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–xSrZrO₃ lead-free piezoelectric ceramics have been prepared by the conventional solid state sintering method. Systematic investigation on the microstructure, crystalline structures as well as electrical properties of the ceramics was carried out. With the addition of SrZrO₃, the rhombohedral–orthorhombic phase transition temperature of the ceramics increases. Both the rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions of the ceramics were modified to be around room temperature when x~0.05, and as a result remarkably strong piezoelectricity has been obtained in 0.924(K_0.5Na_0.5)NbO₃–0.026Bi_0.5K_0.5TiO₃–0.05SrZrO₃ ternary system, whose piezoelectric parameters were d₃₃=324 pC/N and k_p=41%.     

Effect of the Nb2O5 content on electrical properties of lead-free BaTiO3–Bi0.5Na0.5TiO3 ceramics

(1−x)BaTiO₃–xBi_0.5Na_0.5TiO₃ (BT–BNT) ceramics were prepared by the solid-state reaction method. With an increase of BNT content, both the Curie temperature and the room temperature resistivity increased. At 1 mol% BNT addition, the sample was not semiconducting, due to Bi₂O₃ volatilization resulting from the decomposition of pre-calcined BNT during sintering. Appropriate extra Nb₂O₅ doping in the raw materials could offset Bi₂O₃ volatilization and neutralize the redundant acceptor Na⁺ ions. When the extra Nb₂O₅ content was 0.6 mg, the sample room-temperature resistivity was 6.3×10³ Ω cm, with the Curie point about 135 °C and a high PTC effect of ∼3 orders of magnitude.     

Phase formation of REBa2Cu3O7−δ (RE: Y0.5Gd0.5, Y0.5Nd0.5, Nd0.5Gd0.5) superconductors from nanopowders synthesised via co-precipitation

Phase formation of REBa₂Cu₃O_7−δ (RE: Y_0.5Gd_0.5, Y_0.5Nd_0.5, Nd_0.5Gd_0.5) superconductors synthesised via co-precipitation (COP) method were investigated by thermogravimetric analysis (TGA), differential thermal analysis (DTA) and X-ray diffraction (XRD) analysis. All samples showed identical thermal decomposition behaviour from the thermogram in which 5 major weight losses were observed. However, XRD of the samples at different heat treatment temperatures showed different diffraction patterns indicating different thermolytic processes. Meanwhile, transmission electron microscopy and surface area analysis revealed that the powders obtained from COP have particle sizes ranging from 7 to 12 nm with relatively large surface area. Molar ratios of prepared samples obtained were near to the theoretical values as confirmed by elemental analyses using X-ray fluorescence (XRF). The T_C(R=0) for sintered YGd, YNd and NdGd were 87 K, 86 K and 90 K, respectively. Surface morphological study via scanning electron microscope showed the structures of samples were dense and non porous.     

Stress-dependent ferroelectric properties of Pb(Zr1/2Ti1/2)O3–Pb(Zn1/3Nb2/3)O3 ceramic systems

Effects of compressive stress on the ferroelectric properties of ceramics in PZT–PZN systems were investigated. (1 − x)Pb(Zr_1/2Ti_1/2)O₃(xPb(Zn_1/3Nb_2/3)O₃ or (1 − x)PZT–(x)PZN (x = 0.1–0.5) ceramics were prepared by a conventional mixed-oxide method. The ferroelectric properties under compressive stress of the PZT–PZN ceramics were observed at stress levels up to 170 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. It was found that with increasing compressive stress the area of the ferroelectric hysteresis (P–E) loops, the saturation polarization (P_sat), the remanent polarization (P_r), and the coercive field (E_c) decreased. These results were interpreted through the non-180° ferroelectric domain switching processes.     

Multiferroic ceramics in BaO–Y2O3–Fe2O3–Nb2O5 system

Multiferroic ceramics in BaO–Y₂O₃–Fe₂O₃–Nb₂O₅ system were synthesized and their dielectric, ferroelectric and magnetic properties were evaluated. XRD results showed that the ceramic composite consists of a major phase of tetragonal tungsten bronze structured Ba₂YFeNb₄O₁₅, and minor phases of monoclinic YNbO₄ and hexagonal Ba₃Fe₂Nb₆O₂₁. Three dielectric relaxations were observed in the temperature range from 125 to 575 K. The relaxor dielectric behavior in the temperature range from 125 to 350 K was attributed to the random occupation of Fe³⁺ and Nb⁵⁺ ions at B site of the tungsten bronze structure. The electrode polarization and the inhomogeneous structure contributed to the high-temperature and middle-temperature dielectric relaxations, respectively. Both the ferroelectric hysteresis loop and the magnetic hysteresis loop were measured, which suggested that the synthesized ceramic composite was a promising candidate of multiferroics.     

Structural,dielectric, piezoelectric,ferroelectric and electro-caloric properties of Ba1−xCaxTi0.975(Nb0.5Yb0.5)0.025O3 lead-free ceramics

Polycrystalline samples of Ba_1?xCa_xTi_0.975(Nb_0.5Yb_0.5)_0.025O₃ (where x = 0.15, 0.2 and 0.3, abbreviated as BCTYN) were prepared by the conventional solid state reaction method. The effect of calcium (Ca) substitution in BaTi_0.975(Nb_0.5Yb_0.5)_0.025O₃ (abbreviated as BTYN25) on the structural, dielectric, piezoelectric and ferroelectric properties and electro-caloric effects (ECE) was investigated. X-ray diffraction (XRD) results at room temperature showed that the BCTYN samples in the composition x < 0.3 exhibited a pure tetragonal perovskite structure. Dielectric measurements showed a classical ferroelectric behavior for all samples. With the increase of the Ca content, the Curie temperature (T_C) was still maintained with a small shift towards low temperature. The evolution of the Raman spectra was studied as a function of compositions and temperatures. The Raman bands confirmed the structure and the phase transition of the BCTYN ceramics. By adding Ca, the piezoelectric properties and the remanent polarization (P_r) are relatively maintained for the compositions x = 0.15 and x = 0.2. A piezoelectric coefficient of d₃₃ = 130 pC/N and a planar electromechanical coupling factor of k_p = 28% were obtained for these compositions. Two different methods were used to calculate the electro-caloric coefficients of the BCTYN ceramics. The incorporation of Ca was found to enhance the electro-caloric strength (ξ = ΔT/ΔE) within a broad temperature range with a best value of ξ = 0.2?Kmm/kV for x = 0.2.     

Bi(Zn1/2Ti1/2)O3 modified BiFeO3–BaTiO3 lead-free piezoelectric ceramics with high temperature stability

Lead-free high-temperature ceramics with compositions of 0.71BiFe_1−x(Zn_1/2Ti_1/2)_xO₃–0.29BaTiO₃ (BFZTx–BT, x=0–0.05 mol fraction) were fabricated by a conventional solid state reaction method. The effect of Bi(Zn_1/2Ti_1/2)O₃ (BZT) addition on the microstructure, electrical properties, relaxor behavior, and temperature stability has been studied. XRD patterns revealed that all compositions formed a single perovskite phase of pseudo-cubic crystal structure. The grain size was slightly affected by BZT addition. The diffuse phase transition and strong frequency dispersion of dielectric permittivity are observed for BZT modified ceramics. The addition of BZT into BFZTx–BT was also found to affect the piezoelectric properties and temperature stability of the ceramics with maximum values observed for x=0.5% and 1% BFZTx–BT compositions, respectively. The optimum piezoelectric properties with d₃₃=163 pC/N, together with high-temperature stability with a depolarization temperature T_d∼380 °C, reveal the BFZTx–BT ceramics to be promising high-temperature Pb-free piezoelectric materials.     

Excess conductivity in Cu0.5Tl0.5Ba2(Ca2−yMgy)(Cu0.5Zn2.5)O10−δ superconductor

Synthesis and characterization of Cu_0.5Tl_0.5Ba₂(Ca_2−yMg_y)(Cu_0.5Zn_2.5)O_10−δ (y=0, 1.0, and 1.5) superconductor with 0%, 50%, and 75% Mg-doping at the Ca sites are reported. The samples were synthesized by solid-state reaction and characterized by X-ray diffraction (XRD), dc-resistivity (ρ) and fluctuation induced conductivity (FIC) analysis. The zero resistivity critical temperature {T_c(R=0)} was decreased with the increase of Mg-doping at Ca sites. The microscopic parameters such as the cross-over temperature (T_o), zero temperature coherence length {ξ_c(0)} and interlayer coupling (J) were deduced from FIC analysis. According to Aslamazov Larkin equations, a distinct cross-over temperature (T_o1) from three-dimensional (3D) to two-dimensional (2D) fluctuation induced conductivity regions was observed in all samples. Another cross-over temperature (T_o2) from 2D to zero-dimensional (0D) fluctuations was also witnessed. FIC analysis revealed the deterioration of superconductivity with increased Mg-content.     

Crystallographic characterization of silicon nitride ceramics sintered with Y2O3–Al2O3 or E2O3–Al2O3 additions

Silicon nitride ceramics were sintered using Y₂O₃–Al₂O₃ or E₂O₃–Al₂O₃ (E₂O₃ denotes a mixed oxide of Y₂O₃ and rare-earth oxides) as sintering additives. The intergranular phases formed after sintering was investigated using high-resolution X-ray diffraction (HRXRD). The use of synchrotron radiation enabled high angular resolution and a high signal to background ratio. Besides the appearance of β-Si₃N₄ phase the intergranular phases Y₃Al₅O₁₂ (YAG) and Y₂SiO₅ were identified in both samples. The refinement of the structural parameters by the Rietveld method indicated similar crystalline structure of β-Si₃N₄ for both systems used as sintering additive. On the other hand, the intergranular phases Y₃Al₅O₁₂ and Y₂SiO₅ shown a decrease of the lattice parameters, when E₂O₃ was used as additive, indicating the formation of solid solutions of E₃Al₅O₁₂ and E₂SiO₅, respectively.     

Piezoelectric and dielectric properties of (Bi0.5Na0.5)0.94Ba0.06TiO3–Ba(Zr0.04Ti0.96)O3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics of (1 − x)(Bi_0.5Na_0.5)_0.94Ba_0.06TiO₃–xBa(Zr_0.04Ti_0.96)O₃ (abbreviated as BNBT–BZT100x, wherein x from 0 to 10 mol%) were fabricated. We have studied effects of amount of BZT content on the electrical properties and microstructures. X-ray diffraction analysis indicates that a solid solution is formed when BZT diffuses into the BNBT lattice, and further the crystal structure of sintered hybrid changes from rhombohedral to tetragonal symmetry along with increasing BZT content. Piezoelectric property measurements reveal that the BNBT–BZT4 ceramics has the highest piezoelectric performance, for example, the piezoelectric constant d₃₃ reaches to 167 pC/N and planar electromechanical coupling factor k_p is up to 0.27. In addition, the effect of Bi₂O₃ on the electrical properties and microstructure of the BNBT–BZT4 ceramics have also been studied, and found that the doping of Bi enhances the piezoelectric properties of ceramics.     

Dielectric characters of 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 ceramics fabricated at ultra-low temperature by the spark-plasma-sintering method

Solid solution (1 − x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ shows high dielectric constant near room temperature and is an ideal capacitor material. The composition 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃, which is located near the morphotropic phase boundary, were densified by the spark-plasma-sintering method at an ultra-low temperature (700 °C). Dielectric constant measurement shows that the thus prepared sample shows higher dielectric constant at room temperature and good temperature stability in a wide temperature range. The behavior is much different from that of samples sintered by conventional method and could be ascribed to size effect.