Structural and dielectric studies of lead-free ceramics: Na1/2Y1/2TiO3

The polycrystalline samples of Na_1/2Y_1/2TiO₃ were prepared by the mixed-oxide method. A preliminary X-ray structural analysis was shown to exhibit the formation of a single-phase compound with an orthorhombic structure. Microstructural analysis by scanning electron microscopy (SEM) exhibits well defined grains distributed uniformly through out the sample suggesting the compactness and homogeneity of the sample. Detailed studies of dielectric properties of Na_1/2Y_1/2TiO₃ in a wide frequency range (102–106 Hz) at different temperatures (31–500°C) show a dielectric anomaly at 105°C, which may be related to a ferroelectricparaelectric phase transition as suggested by hysteresis loop at room temperature. An ac conductivity (σ _ac) of the material is mainly governed by the polaron hopping mechanism, which is also influenced by both frequency and temperature. The activation energy was obtained from the plot of temperature with a.c. conductivity.      

Crystal structures and phonon modes of Ba(Ca1/2W1/2)O3, Ba(Ca1/2Mo1/2)O3 and Ba(Sr1/2W1/2)O3 complex perovskites investigated by Raman scattering

This work investigates the crystal structures and phonon modes of Ba(Ca_1/2W_1/2)O₃, Ba(Ca_1/2Mo_1/2)O₃ and Ba(Sr_1/2W_1/2)O₃ perovskites by Raman spectroscopy. The samples were produced by conventional solid‐state processing at 1200 °C. X‐ray diffraction showed that single‐phase homogeneous materials were produced, which are cubic or pseudo‐cubic in symmetry. The existing controversies in the literature for these complex perovskites were investigated by comparing experimental Raman data with group‐theory analysis. Ceramics with Ca and W or Mo were found to be cubic, space group Fm3 m. For these materials, four Raman‐active bands were observed and the fitting parameters showed that the Ba(Ca_1/2Mo_1/2)O₃ ceramic presents bands at lower wavenumbers if compared with the Ba(Ca_1/2W_1/2)O₃ sample. For the Ba(Sr_1/2W_1/2)O₃ material, two hypotheses were investigated for monoclinic or triclinic structures. The experimental results showed 12 Raman‐active modes for this ceramic, which is in perfect agreement with the theoretical predictions for a monoclinic (I2/m) structure. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigation on lithium ion conductivity and structural stability of yttrium-substituted Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript>

Advanced Li-air battery architecture demands a high Li⁺ conductive solid electrolyte membrane that is electrochemically stable against metallic lithium and aqueous electrolyte. In this work, an investigation has been carried out on the microstructure, Li⁺ conduction behaviour and structural stability of Li₇La_3-x Y _x Zr₂O₁₂ (x = 0.125, 0.25 and 0.50) prepared by conventional solid-state reaction technique. The phase analysis of Li₇La_3-x Y _x Zr₂O₁₂ (x = 0.125, 0.25 and 0.50) sintered at 1200 °C by powder X-ray diffraction (PXRD) and Raman confirms the formation of high Li⁺ conductive cubic phase (\( Ia\overline{3}d \)) lithium garnets. Among the investigated lithium garnets, Li₇La_2.75Y_0.25Zr₂O₁₂ sintered at 1200 °C exhibits a maximized room temperature total (bulk + grain boundary) Li⁺ conductivity of 3.21 × 10^?4 S cm^?1 along with improved relative density of 96 %. The preliminary investigation on the structural stability of Li₇La_2.75Y_0.25Zr₂O₁₂ in the solutions of 1 M LiCl, dist. H₂O and 1 M LiOH at 30 °C/50 °C indicates that the Li₇La_2.75Y_0.25Zr₂O₁₂ is relatively stable against 1 M LiCl and dist. H₂O. Further electrochemical investigation is essential for practical application of Li₇La_2.75Y_0.25Zr₂O₁₂ as protective solid electrolyte membrane in aqueous Li-air battery.     

Superconducting properties and structural phase transformation of Nb2N induced by C+ - and N+-implantation

Thin films of h.c.p. Nb₂N implanted uniformly wilh 25 at.% C⁺ or N⁺ ions were found to transform into the f.c.c. Bl structure; film surface temperatures did not exceed 200°C. The resistive superconducting transition sharpened markedly and T_c was found to increase after implantation, presumably due to the structural change. X-Ray diffraction measurements were used to determine crystal structure and lattice parameters. A possible mechanism for the transformation is discussed.     

Preparation and ferroelectric properties of pyrochlore-free Pb(Ni1/3Nb2/3)O3-based solid solutions

Polycrystalline ceramics of the perovskite solid solution 0.5Pb(Ni_1/3Nb_2/3)O₃-(0.5-x)-Pb(Zn_1/3Nb_2/3)O₃–xPb(Zr_1/2Ti_1/2)O₃; x=0.0–0.5 (PNN–PZN–PZT) were synthesized by a modified columbite method. Highly dense ceramics lacking parasitic pyrochlore phases were prepared at a calcination temperature of 950 °C by using a double-crucible configuration, excess PbO (2 mol %), and a fast heating/cooling rate (20 °C/min). The ceramics were characterized by a variety of techniques including X-ray diffraction, ferroelectric hysteresis loop measurements, field-induced longitudinal strain measurements, and electron microscopy. It was observed that the remanent polarization exhibited a significant increase with increasing x. In addition, the squareness of the hysteresis loop increased quasi-linearly as the molar fraction of PZT increased. The maximum spontaneous polarization and remanent polarization for the x=0.5 composition were 31.9 μC/cm² and 25.2 μC/cm², respectively. Moreover, the data were analyzed to show the evolution of the micro-domain state as a function of the molar fraction of PZT. PACS 77.22.-d; 77.80.Bh; 77.84.Dy; 61.10.Nz; 77.80.Dj     

Magnetoelectricity in the PbFe1/2Nb1/2O3 Ceramics

The magnetodielectric effect (the influence of a magnetic field H on the dielectric constant ?) and the magnetoelectric effect (the influence of an electric field E on the magnetoelectric constant ??) of the PbFe_1/2Nb_1/2O₃ ceramics have been investigated at temperatures T in the range from 50 to 200°C, including the Curie point T _C ? 98°C. It has been demonstrated that there is a correlation of these effects with the shift of the ferroelectric-paraelectric phase transition temperature in a magnetic field.     

Ionic displacements and structure of disordered PbFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript>

A sequence of structural models of unit cells of complex oxides with perovskite-type structure has been constructed to refine the average structure of PbFe_1/2Nb_1/2O₃ (PFN) at temperatures above the ferroelectric Curie point (T _C ～ 110°C). Owing to the analysis of the probability theory considerations, each model is characterized by only two positional fitting parameters. With the use of the intensities of 95 symmetrically independent X-ray reflections from a PFN single crystal at 160°C, a model with the R factor below 3% (K = 2.44%) was chosen. The ravine method has been used to verify the existence of a single minimum for the R factor with respect to the fitting parameter.     

A stable BaCeO3-based proton conductor for solid oxide fuel cells

In order to improve the chemical stability of BaCeO₃, Ti⁴⁺ was introduced into B site of BaCeO₃ to modify the chemical stability. XRD test demonstrates that \textBaC\texte_0.6\textT\texti_0.2\textY_0.2\textO_{3 - d} {\text{BaC}}{{\text{e}}_{0.6}}{\text{T}}{{\text{i}}_{0.2}}{{\text{Y}}_{0.2}}{{\text{O}}_{3 - \delta }} (BCTY) keeps its original pervoskite-type structure at a high doping level of 20%. After exposure in 94% N₂ + 3% CO₂ + 3% H₂O at 700 °C for 10 h, BCTY exhibited adequate chemical stability while decomposition was found in \textBaC\texte_0.8\textY_0.2\textO_{3 - d} {\text{BaC}}{{\text{e}}_{0.8}}{{\text{Y}}_{0.2}}{{\text{O}}_{3 - \delta }} (BCY). Accordingly, the conductivity of BCTY reaches 0.0072 S/cm at 700 °C in humidified hydrogen which is a little lower than BCY (0.0085). Besides, BCTY displayed better sintering characteristics than BCY at high temperatures and the relative density reaches 96.4% and 94.8%, respectively. The two samples also exhibited similar thermal expansion behavior from 30 to 1,000 °C. A fuel cell with BCTY as electrolyte exhibited 244 mW/cm² at 700 °C and the stable short-term performance further proved the stability of BCTY.     

Formation of conductive layers inside diamond by hydrogen ion implantation and subsequent thermal treatment at low or high pressures

(111) synthetic HPTP diamond plates are irradiated by H ₂ ⁺ 50 keV ions in the range of the fluences of 1?13 × 10¹⁶ sm^?2 and annealed in vacuum at 1 mPa (VPHT, 500?C1600°C) or at high HPHT parameters (4.0?C7.5 GPa, 1200?C1550°C). It is shown by measuring the layer conductivity and Raman light scattering that after VPHT annealing, a buried layer of glassy carbon 10?C100 nm thick with low resistance (??1 kOhm/??) is formed, followed by HPHT with high resistance (??1 MOhm/??) and hopping transport along defects.     

Phase evolution, Raman spectroscopy and microwave dielectric behavior of (Li1/4Nb3/4) doped ZrO2-TiO2 system

The phase evolution, Raman spectroscopy and microwave dielectric properties of (Li_1/4Nb_3/4) doped ZrO₂-TiO₂ system were investigated. The effects of the Zr/Ti ratio and the (Li_1/4Nb_3/4) substitution were addressed. X-ray diffraction and electron diffraction analysis showed that the crystalline phases of the (Li_1/4Nb_3/4) doped ZrO₂-TiO₂ ceramics depended greatly on the Zr/Ti ratio. The sample with Zr/Ti ratio of 7/9 crystallized as Zr₅Ti₇O₂₄ phase structure, a commensurate structure with a tripled a-axis superstructure and a Z^TTZ_TT sequence. Secondary phase of monoclinic ZrO₂ phase appeared when the Zr/Ti ratio was as high as 9/7. Raman analysis showed that the Raman peaks located at 651 and 624 cm⁻¹ were assigned to the vibration modes of Zr-O octahedron and Ti-O octahedron, respectively. The dielectric constant and quality factor (Qf value) of the (Li_1/4Nb_3/4) doped ZrO₂-TiO₂ ceramics decreased slightly as the Zr/Ti ratio changed from 6/10 to 9/7. The temperature coefficient of resonate frequency (TCF value) was sensitive to the Zr/Ti ratio and it showed a negative value when the Zr/Ti ratio was close to 5:7. Meanwhile, the TCF value of ZrO₂-TiO₂ ceramics could also be tailored by the (Li_1/4Nb_3/4) substitution.     

Successive phase transitions in the highly ordered complex perovskite PbLu1/2Nb1/2O3

The structural phase transitions and the electrical behaviour of the complex perovskite PbLu_1/2Nb_1/2O₃ have been investigated using X-ray powder diffraction, dielectric constant measurements, differential scanning calorimetry and measurement of the polarisation as a function of applied electric field. The high-temperature paraelectric phase is highly ordered. A first-order paraelectric-antiferroelectric phase transition occurs at 270°C and an antiferroelectric-ferroelectric phase transition, characterised by dispersion in the curves of dielectric constant as a function of temperature, occurs at ≈ 30°C. The antiferroelectric phase is isostructural with the orthorhombic form of PbYb1/₂Nb1/₂O₃. The low-temperature ferroelectric phase also has an orthorhombic crystal structure.     

A novel anode-supported stable BaCe0.7Ta0.1Y0.2O3− δ membrane prepared by all-solid-state process for solid oxide fuel cells

BaCe_0.7Ta_0.1Y_0.2O_{3− δ} (BCTY) and BaCe_0.8Y_0.2O_{3− δ} (BCY) were synthesized by solid-state reaction method at 1,300 °C for 20 h. After being exposed in 3% CO₂ + 3% H₂O + 94% N₂ at 700 °C for 20 h, the BCTY exhibited adequate chemical stability against carbonations while BCY decomposed into BaCO₃ and CeO₂. The BCTY showed the similar thermal expansion behavior to BCY from room temperature to 1,000 °C in air. The BCTY displayed a conductivity of 0.007 S/cm at 700 °C in humid hydrogen, lower than that of BCY (0.009 S/cm). A fuel cell with 10-μm thick BCTY membrane prepared through an all-solid-state process exhibited 1.004 V for OCV, 330 mW/cm² for maximum output at 700 °C, respectively. Short-term test shows that the fuel cell performance does not degrade after 20 h.     

ZrO2 films deposited by photo-CVD at low temperatures

Zirconium oxide layers have been successfully deposited by photo-CVD at low temperatures. ZrO₂ growth was observed at temperatures as low as 100 °C. When deposited at 250 °C and above, these films exhibited a polycrystalline structure with a mixture of different crystal phases. Deposition at 300 °C was found to form moisture-free ZrO₂ films with a high refractive index of 2.1, a very low effective density of trapped electrons of ∼8.8×10⁸ cm^-2 and an interface trap density of 6.6×10⁹ cm^-2 eV^-1 being readily obtained. Received: 17 December 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002     

Thermally activated crystallization of Nb2O5 grown on Pt electrode

The influence of the local crystallographic orientation of the polycrystalline bottom platinum electrode on the crystallization of niobium pentoxide thin films during their rapid thermal annealing was investigated by X-ray diffraction, X-ray reflectivity and transmission electron microscopy. The Nb₂O₅ thin films under study were reactively sputtered in a mixed O₂/Ar atmosphere and subsequently subjected to the annealing in argon atmosphere at temperatures ranging from 500 ^°C to 700 ^°C. The X-ray diffraction confirmed a transition from the amorphous niobium oxide to the crystalline orthorhombic Nb₂O₅ for temperatures between 500 ^°C and 600 ^°C. The X-ray reflectivity measurements showed that the crystallization process was accompanied by a continuous increase of the electron density in Nb₂O₅ and by a rapid increase of the surface roughness at 700 ^°C. It was further observed by transmission electron microscopy that Nb₂O₅ crystallizes selectively and that the crystalline domains of Nb₂O₅ possess a strong orientation relationship to the platinum from the bottom electrode. The orientation relationship $(\bar{1} 1 1)_{\mathrm{Pt}}\,{\parallel}\, (\bar{1} \bar{6}0)_{\mathrm{Nb}_{2}\mathrm{O}_{5}}$ was identified as the most beneficial one for crystallization of Nb₂O₅.     

A polarized micro-Raman study of a 0.65PbMg1/3Nb2/3O3 - 0.35PbTiO3 single crystal

Polarized micro-Raman spectra of a 0.65PbMg_1/3Nb_2/3O₃-0.35PbTiO₃ (0.65PMN-0.35PT) single crystal poled in the [001] direction are obtained in a wide frequency range (50-2000 cm^-1) at different temperatures. The best fit to the Raman spectrum at 77 K is achieved using 17 Lorenzians to convolute into it, and this is proved to be a reasonable fit. According to the group theory and selection rules of overtone and combinational modes, apart from the seven Raman modes that are from first-order Raman scattering, the remaining ones are attributed to being from second-order Raman scattering. A comparison between the experimental results and theoretical predictions shows that they are in satisfactory agreement with each other. Our results indicate that at 77 K the sample belongs to the rhombohedral symmetry with the C_3v⁵ (R3m) space group (Z=1). In our study, on heating, the 0.65PMN-0.35PT single crystal undergoes a rhombohedral to tetragonal to cubic phase transition sequence. The two phase transitions occur at 340 and 440 K, which correspond to the disappearance of the soft mode near 106 cm^-1 recorded in VV polarization and the vanishing of the band around 780 cm^-1 in VH polarization, respectively.     

Size-dependent luminescence in Y2Si2O7 nanoparticles doped with Ce3+ ions

Powders and thin films of nanocrystalline yttrium disilicate (Y₂Si₂O₇) doped with Ce³⁺ have been prepared by a sol–gel method. Structure and morphology of the synthesised samples have been determined and spectroscopic properties compared. The triclinic α-Y₂Si₂O₇ form (space group P 1-) for the powders annealed between 1000°C and 1200°C has been found. A total conversion into a thortveitite-type monoclinic β-Y₂Si₂O₇ polymorph after annealing of powder samples at 1400°C (space group C2/m) has been observed. In the case of films even at 1300°C the basically pure triclinic α-Y₂Si₂O₇ phase was observed with luminescent spectroscopy. The influence of grain size, controlled by thermal treatment, on the structure and luminescence properties of the fabricated materials are presented and discussed.     

Control of the degree of long-range compositional order in ceramics of the compound perovskites Pb(B′1/2B″1/2)O3 using recrystallization

It is established that an ordered state forms in Pb(Yb_1/2Nb_1/2)O₃ at fairly low temperatures (650–800°C). Subsequent high-temperature heat treatment of the ceramic without additives (sintering or additional annealing) does not produce any significant change in the degree of long-range compositional order s because of the low diffusion rate of the Yb and Nb ions. The addition of Li₂CO₃, which forms a liquid phase, creates conditions for the dissolution of grains with a high value of s, the nucleation of new crystallization centers, and the growth of grains with a new equilibrium value of s at the sintering (annealing) temperature. Zh. Tekh. Fiz. 69, 24–30 (March 1999)     

Effects of strontium on the characteristics of Pb(Zr1/2Ti1/2)O3--Pb(Zn1/3Nb2/3)O3 ceramics

The dielectric and piezoelectric properties of pyrochlore-free lead zirconate titanate-lead zinc niobate ceramics were investigated systematically as a function of Sr doping. The powders of Pb_{(1? x )}Sr _x [0.7(Zr_{1 / 2}Ti_{1 / 2})–0.3(Zn_{1 / 3}Nb_{2 / 3})]O₃, where x?=?0–0.06 were prepared using the columbite-(wolframite) precursor method. The ceramic materials were characterized using X-ray diffraction, dielectric spectra, hysteresis and electromechanical measurements. The phase-pure perovskite phase of Sr-doped PZN--PZT ceramics was obtained over a wide compositional range. The results showed that the optimized electrical properties were also achieved at composition x?=?0.0, which were K _P?=?0.69, d ₃₃?=?670?pC?N^?1, P _r?=?31.9?µC?cm^?2 and ε_rmax?=?18600. Maximum dielectric constant values of the systems decreased rapidly with increasing Sr concentration. Moreover, with increasing Sr concentration dielectric constant versus temperature curves become gradually broader. The diffuseness parameter increased significantly with Sr doping. Furthermore, Sr doping has been shown to produce a linear reduction in the transition temperature (T _m)?=?294.1–12.7x°C with concentration (x). Sr shifts the transition temperature of this system at a rate of 12.7°C?mol^?1%.     

Structural and electrochemical properties of LiNi1/3Co1/3Mn1/3O2 material prepared by a two-step synthesis via oxalate precursor

LiNi_1/3Co_1/3Mn_1/3O₂ (LNMCO) powders were formed by a two-step synthesis including preparation of an oxalate precursor by ??chimie douce?? followed by a solid-state reaction with lithium hydroxide. The product was characterized by TG-DTA, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), Raman spectroscopy, electron spin resonance (ESR), and SQUID magnetometry. XRD data revealed well-crystallized layered LNMCO with ??-NaFeO₂-type structure (R-3?m space group). Morphology studied by SEM and TEM shows submicronic particles of 400?C800?nm with a tendency to agglomerate. The local structure investigated by vibrational spectroscopy (FTIR, Raman), ESR, and SQUID measurements confirms the well-crystallized lattice with a cation disorder of 2.6% Ni²⁺ ions in Li(3b) sites. Electrochemical tests were carried out in the potential range 2.5?C4.5?V vs. lithium metal on samples heated at 900?°C for 12?h. Initial discharge capacity is 154 mAh/g at C/5, while a capacity of 82 mAh/g is still delivered at 10 C by the two-step synthesized LiNi_1/3Co_1/3Mn_1/3O₂ as cathode material.     

Acoustic emission and thermal expansion of Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> and Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-PbTiO<Subscript>3</Subscript> crystals

The temperature dependence of the elongation per unit length for Pb(Mg_1/3Nb_2/3)O₃ crystals unannealed after growth and mechanical treatment is investigated in the course of thermocycling. It is revealed that this dependence deviates from linear behavior at temperatures below 350°C. The observed deviation is characteristic of relaxors, is very small in the first cycle, increases with increasing number n of thermocycles, and reaches saturation at n≥3. In the first cycle, a narrow maximum of the acoustic emission activity is observed in the vicinity of 350°C. In the course of thermocycling, the intensity of this maximum decreases and becomes zero at n>3. For (1?x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ crystals, the dependence of the temperature of this acoustic emission maximum on x exhibits a minimum. It is assumed that the phenomena observed are associated with the phase strain hardening due to local phase transitions occurring in compositionally ordered and polar nanoregions.