Mesoscale Organization of Flower-Like La2O2CO3and La2O3 Microspheres

Well-dispersed spherical lanthanum oxycarbonate (La₂O₂CO₃) and La₂O₃ particles with a flower-like morphology were synthesized via a simultaneous polymerization–precipitate reaction, metamorphic reconstruction, and mineralization under a hydrothermal condition as well as subsequent calcination. The La₂O₂CO₃ and La₂O₃ microspheres obtained consist of about 20 or 40 nm thick nanosheets as petals, respectively. They have an open three-dimensional mesoporous and hollow structure and possess a high surface area, large pore volume, and marked thermal stability. These novel porous structural lanthanum-based materials could be used as catalyst supports for many reactions.     

Mechanical Properties of La1-xSrxCo0.2Fe0.8O3 Mixed-Conducting Perovskites Made by the Combustion Synthesis Technique

This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La_{1– x} Sr _x Co_0.2Fe_0.8O₃ ( x = 0.2–0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151–188 GPa and shear modulus of 57–75 GPa were measured. Biaxial flexure strength of ∼160 MPa was measured for lower strontium content batches. Strength greatly decreased to ∼40 MPa at higher strontium concentrations ( x = 0.6–0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (∼1.5 MPa·m^1/2) for low strontium ( x = 0.2) content and a lower value (∼1.1 MPa·m^1/2) for the other batches ( x = 0.4–0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.     

Preparation of the High-Quality Superconductor La1Ba2Cu3Oy

The relationship between the preparation procedure and superconducting properties of La₁Ba₂Cu₃O_y was studied. A series of samples was sintered in an N₂-gas atmosphere for various lengths of time ranging from 1 to 40 h, followed by a fixed annealing procedure in O₂. It was found that the shorter the sintering period, the higher was the oxygen content. The samples sintered for a period of less than 15 h contained excess oxygen compared with La₁Ba₂Cu₃O₇ and exhibited poor superconducting properties. The sample sintered for 40 h had an oxygen content y equal to 6.95, and had excellent superconducting properties. The mechanism for preparing high-quality La₁Ba₂Cu₃O_y is discussed.     

Densification of Si3N4 with LiYO2 Additive

This paper deals with the densification and phase transformation during pressureless sintering of Si₃N₄ with LiYO₂ as the sintering additive. The dilatometric shrinkage data show that the first Li₂O- rich liquid forms as low as 1250°C, resulting in a significant reduction of sintering temperature. On sintering at 1500°C the bulk density increases to more than 90% of the theoretical density with only minor phase transformation from α-Si₃N₄ to β-Si₃N₄ taking place. At 1600°C the secondary phase has been completely converted into a glassy phase and total conversion of α-Si₃N₄ to β-Si₃N₄ takes place. The grain growth is anisotropic, leading to a microstructure which has potential for enhanced fracture toughness. Li₂O evaporates during sintering. Thus, the liquid phase is transient and the final material might have promising mechanical properties as well as promising high-temperature properties despite the low sintering temperature. The results show that the Li₂O−Y₂O₃ system can provide very effective low-temperature sintering additives for silicon nitride.     

Microwave Dielectrics in the (La1/2Na1/2)TiO3–Ca(Fe1/2Nb1/2)O3 System

Dielectric properties of the system (1 − x)(La_1/2Na_1/2)TiO₃– _x Ca(Fe_1/2Nb_1/2)O₃, where 0.4 # x # 0.6, have been investigated at microwave frequencies. The temperature coefficient of resonant frequency (τ_f), nearly 0 ppm/°C, was realized at x = 0.58. These ceramics had perovskite structure and showed relatively low dielectric losses. A new dielectric material applicable to microwave devices having Q · f of 12000–14000 GHz and a dielectric constant (ε_r) of 59–60 has been obtained at 1300–1350°C for 5–15 h sintering.     

Refractory α-SiAlON Containing La2O3

Refractory Y-α-SiAlON with elongated grain morphology was obtained by utilizing La₂O₃ as a densification aid, which resulted in excellent room-temperature and high-temperature strength. Room-temperature strength of 1000 MPa was achieved when La₂O₃ was augmented by adding Y₂O₃ or removing AlN. With only La₂O₃, a temperature-independent strength of 800–950 MPa was maintained up to 1100°C, then gradually decreasing by 25% when reaching 1300°C. The R-curve measurements of fracture toughness showed relatively little dependence on microstructure, consistent with a strong interface that suppresses grain boundary decohesion. Compared with other densification aids such as SiO₂, Al₂O₃, Sc₂O₃, Y₂O₃, and Lu₂O₃, a finer microstructure was obtained by using La₂O₃. High nitrogen content in the residual La–Si–Al–O–N glass in equilibrium with the nitrogen-rich α-SiAlON is suggested to be the cause of these findings.     

Phase Transition and Ferroelectric Characteristics of Pb[(Mg1/3Nb2/3)1-xTix]O3 Ceramics Modified with La(Mg2/3Nb1/3)O3

The effects of 0–5 mol% addition of La(Mg_2/3Nb_1/3)O₃ (LMN) on the phase transition and ferroelectric behaviors of Pb[(Mg_1/3Nb_2/3)_1-xTi_x]O₃ (PMNT) ceramics with compositions near the morphotropic phase boundary (MPB) were studied. An evolution of structure from rhombohedral to tetragonal was found with increasing PbTiO₃ (PT) content across the MPB (at ∼32.5 mol% PT), and a coexistence of both rhombohedral and tetragonal phases was also found at the MPB. The dual-phase field extended toward the lower PT content side of the MPB, and, moreover, the rhombohedrality or tetragonality was reduced, especially for the compositions near the MPB, by the addition of La in PMNT. The ferroelectric transition was found to change from normal to diffuse as the La content increased and the compositions became more rhombohedral. In accordance with the structural evolution, the change of remanent polarization ( P _r) and coercive field ( E _c) also became gradually indistinct, and both P _r and E _c were reduced. For compositions near the MPB, both PMNT and La-modified PMNT had a similar electromechanical factor ( k _p) in a range around 0.55–0.60, but the mechanical quality factor ( Q _m) was significantly reduced for the La-modified PMNT. The piezoelectric coefficient ( d ₃₃), however, was largely improved with increasing La content in PMNT of compositions at MPB. A high value of d ₃₃∼ 815 pC/N was obtained for the 5-mol%-La-modified ceramics, but it was associated with a low value of Q _m.     

New Compound in the System La2O3-Al2O3

A new compound, 5La₂O₃-2Al₂O₃, is formed from an amorphous material prepared by the simultaneous hydrolysis of lanthanum and aluminurn alkoxides. It has an orthorhombic unit cell with a=0.9704 nm, b=0.5967 nm, and c=1.5473 nm. The structure contains tetrahedral AlO₄ groups and octahedral AlO₆ groups.     

La2Ti2O7 Ceramics

La₂Ti₂O₇ powders were prepared using three different techniques. Single-phase material was obtained at 1150°C by calcination of mixed oxides, at 1000°C by molten salt synthesis, and at 850°C by evaporative decomposition of solutions. Particle sizes and morphologies of the powders differed substantially, as did the sintered microstructures and dielectric properties. Very dense (99%), translucent, grain-oriented lanthanum titanate was fabricated by hot-forging at 1300°C under a 200-kg load. Anisotropy was demonstrated by X-ray diffraction, scanning electron microscopy, thermal expansion, and dielectric measurements.     

Thin-Layer Dielectrics in the Pb[(Mg1/3Nb2/3)1-xTix]O3 System

Thin layers of Pb[(Mg_1/3Nb_2/3)_{1– x} Ti _x ]O₃ (PMNT) were prepared by spin casting alkoxide-based solutions on platinized Si. The effects of additives, heat treatment, and composition ( x = 0 to 0.9) on perovskite phase development, ceramic microstructure, and dielectric properties are reported. Depending upon the processing conditions, ceramic thin layers could be formed in a nonferroelectric pyrochlore phase (A₂B₂O₆) or in a ferroelectric perovskite phase (ABO₃). The dimensions of the pyrochlore and perovskite units cells were related and increased with Mg and Nb contents. To minmize pyrochlore formation, the most effective processing method involved rapid heat treatment between successive solution depositions. Phase development and microstructure were also affected by solution additives. Additions of benzoic acid were found to affect the structure in solution and the later organic pyrolysis behavior from thin layers. The effect of composition on the dielectric and ferroelectric properties is also reported.     

Microwave Dielectric Properties of La5CrTi3O15 and La4MCrTi3O15 (M=Pr, Nd, and Sm) Ceramics

The La₅CrTi₃O₁₅ and La₄MCrTi₃O₁₅ (M=Pr, Nd, and Sm) microwave dielectric ceramics were prepared by the conventional solid-state ceramic route. The structure and microstructure of the ceramics were studied by X-ray diffraction and scanning electron microscopy methods. The dielectric properties of the ceramics were measured in the microwave frequency region using a network analyzer by the resonance method. The ceramics show a dielectric constant (ɛ_r) in the range of 37 to 39.5, a quality factor ( Q _u× f _o) 17,300 to 34,000 GHz, and a temperature coefficient of resonant frequency (τ_f) in the range from −22 to −38 ppm/°C.     

Neutron Powder Diffraction Study of Y-Sialon with La2O3 or Nd2O3 Addition

Neutron diffraction data of α-sialon and complementary phases were collected on the neutron powder diffraction (NPD) diffractometer installed in the NFL Studsvik, at a wavelength of 1.470 Å. Calculations were carried out by using the FullProf 2000 utilizing the crystal structure of the yttrium α-sialon phase. Selected profile and structure parameters were refined in the calculations. The calculated data showed that either La or Nd were also present in the α-sialon crystal structure in the presence of Y. The comparison of the present phases' weight contents, which were determined by the qualitative phase analysis between NPD and X-ray diffraction data, was carried out.     

Phases in the Binary Systems PbO—La2O3, Gd2O3, and Sm2O3

In the binary system PbO–La_zO₃ only one compound, 4PbO.La₂O₃, exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La₂O₃, corresponding to half the a parameter of the C form of La₂O₃. The solid solutions existing between the compositions La₂O₃–2Pb0 and pure La₂O₃ have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm₂O₃ and Gd₂O₈ are quite similar to that with La₂O₃. The compound Sm₂O₃.4Pb0 decomposes at 1000°C with evaporation of PbO; Sm₂O₃ remains in the B modification.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Structural Studies of Ordering in the (Pb1-xBax)(Mg1/3Nb2/3)O3 Crystalline Solution Series

The ordered structures of the (Pb_{1- x} Ba _x )(Mg_1/3Nb_2/3)O₃crystalline solution series were investigated by selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). At low Ba contents (e.g., x < 0.40), the ordered structure was found to be isostructural with Pb(Mg_1/3Nb_2/3)O₃, with a doubled unit cell characterized by 1/3{111} superlattice reflections. At higher Ba contents (e.g., x > 0.60), the ordered structure was characterized by 1/3{111} superlattice reflections. For intermediate Ba contents (e.g., x - 0.60), diffuse scattering along the {111} between diffuse 1/2{111} and 1/3{111} reflections was observed. The ordering is attributed to the distribution of the B-site cations between multiple sublattices. Strong fluctuations in the B-site cation ratio between ordered and disordered regions are believed not to exist; however, the possibility of weak fluctuations is consistent with the observed lattice images.     

Microstructure Characterization of the (1−x)La2/3TiO3·xLaAlO3 System

Microstructural characterizations on the (1− x )La_2/3TiO₃· x LaAlO₃ (LTLA) system were conducted using transmission electron microscopy. The presence of La₂Ti₂O₇ and La₄Ti₉O₂₄ phases in pure La_2/3TiO₃ is confirmed by the electron diffraction pattern. When x = 0.1, the ordering due to the A-site vacancies could be confirmed by the presence of antiphase boundaries (APBs) and return ½(100) superlattice reflection. As x increases, the ordering decreases and finally disappears when x = 0.6. The tilting of oxygen octahedra could be demonstrated by the presence of the ferroelastic domains in the matrix and return ½(111) and return ½(110) superlattice reflections in selected area electron diffraction patterns. In pure LaAlO₃, only the antiphase tilting of oxygen octahedra is present due to the presence of return ½(111) superlattice reflection. In the LTLA system of x = 0.1, both the antiphase and in-phase tiltings of the oxygen octahedra are involved; however, in the range of x from 0.3 to 0.9, the antiphase tilting of oxygen octahedra has appeared. The growth of the ferroelastic domains is influenced by the APBs in the matrix.     

Structure and Electrical Properties of A-site-Deficient Perovskite Compounds in the La2/3TiO3-La1/3NbO3 System

The structure and electrical properties of an A-site-deficient perovskite compound found in the La_2/3TiO₃-La_1/3NbO₃ system were investigated. The composition of the perovskite compound seemed to be very close to La_0.633(Ti_0.90Nb_0.10)O₃. X-ray diffraction analysis revealed a superstructure with a doubled c -axis parameter, resulting from an ordered arrangement of the A-site cation vacancies. Impedance measurements on the compound showed that La_0.633(Ti_0.90Nb_0.10)O₃ had high ionic conductivity at relatively low temperature (<770 K) and increased electronic conduction at high temperature (>770 K). The bulk ionic conductivity was comparable with that of La_0.683(Ti_0.95Al_0.05)O₃, which has the highest ionic conductivity among the La-(Ti,Al)-O perovskite compounds.     

Long-Time Relaxation from Relaxor to Normal Ferroelectric States in Pb0.91 La0.06 (Zr0.65 Ti0.35 O3

Pb_0.91 La_0.06 (Zr_0.65 Ti_0.35)O₃ (PLZT 6/65/35) is a relaxor ferroelectric near and above the temperature of the dielectric maximum (∼180°C). The relaxor state can persist to room temperature upon fast cooling. However, this relaxor state gradually changes to a normal ferroelectric over a long time period at 25°C, characterized by an elimination of relaxor-like dielectric dispersion and a significant rhombo-hedral broadening and subsequent splitting of the (220) X-ray diffraction peak. A transmission electron microscopy (TEM) bright-field image of a long-time relaxed sample revealed normal micrometersized ferroelectric domain contrast with relaxor-like "tweed" structure on the submi-crometer scale. The gradually structural evolution is discussed in terms of development of correlations between relaxor polar clusters.