Isothermal phase diagram of CaO–SiO2–Nb2O5–La2O3 system at 1300 °C and 1200 °C

Thermodynamic properties of CaO–SiO₂–Nb₂O₅–La₂O₃ slag system are of great significance for the utilization and application of niobium and rare earth resources. In the present work, the phase equilibria in CaO–SiO₂–Nb₂O₅–La₂O₃ system at 1300 °C and 1200 °C have been experimentally investigated. A total of 12 equilibrium phase regions have been determined in the constructed isothermal spatial phase diagram, including five four-phase regions: Liquid + SiO₂+CaNb₂O₆+LaNbO₄, Liquid + CaNb₂O₆+Ca₂Nb₂O₇+LaNbO₄, Liquid + Ca₂Nb₂O₇+CaO·3SiO₂·2La₂O₃+LaNbO₄, Liquid +10CaO·6SiO₂·Nb₂O₅+Ca₂Nb₂O₇+LaNbO₄, Liquid + LaNbO₄+SiO₂+CaO·3SiO₂·2La₂O₃; six three-phase regions: Liquid + SiO₂+LaNbO₄, Liquid + CaNb₂O₆+LaNbO₄, Liquid + Ca₂Nb₂O₇+LaNbO₄, Liquid + CaO·3SiO₂·2La₂O₃+Ca₂Nb₂O₇, L + CaNb₂O₆+Ca₂Nb₂O₇, Liquid + SiO₂+CaNb₂O₆; four two-phase regions: Liquid + SiO₂, Liquid + CaNb₂O₆, Liquid + LaNbO₄, Liquid + Ca₂Nb₂O₇. Subsequently, the isothermal section of CaO–SiO₂–Nb₂O₅-(5%, 10%, 15%, 20%, 25%, 30%) La₂O₃ pseudo-ternary systems have been obtained.     

Phase equilibria and microwave dielectric properties in the ternary La2O3–TiO2–Nb2O5 system

High-temperature phase relations in the La₂O₃–TiO₂–Nb₂O₅ ternary system were investigated at 1290 °C using SEM, EDS and XRD, and a subsolidus phase equilibrium diagram was constructed. It was demonstrated that the addition of the La_1/3NbO₃ to the La₂O₃:3TiO₂ mixture completely stabilizes a perovskite-La_2/3TiO₃ compound, which is otherwise not stable at stoichiometric composition. On the other hand, a mixture La₂O₃:TiO₂ =1:3 dissolves in La_1/3NbO₃ up to 15 mol% TiO₂. Moreover, a newly defined composition of compound La₃₀Ti₁₆Nb₄O₈₇ formed in the La₂O₃-rich part of the system and a solid solution La₃Ti₅Nb₁₀O_39.5–La₂Ti₂Nb₈O₂₇ were proposed. In the binary subsystem LaTiNbO₆-La_0.462Ti_0.386Nb_0.614O₃, the components exhibit opposite temperature dependence of resonant frequency in the microwave frequency range. Thus, the dielectric properties of the prepared ceramics can be tailored by varying the component concentration and thermally stable dielectric properties can be achieved for a composition with a LaTiNbO₆/La_0.462Ti_0.386Nb_0.614O₃ molar ratio of 0.27.     

Chemical compatibility of proton conducting LaNbO4 electrolyte with potential oxide cathodes

The chemical and physical compatibility of the proton conducting electrolyte material LaNbO₄ with the potential partner materials LaMO₃ (MMn, Fe, Co) and La₂NiO₄ is investigated via the reaction of fine-grained powders and solid-state diffusion couples. Results show generally good chemical compatibility for LaNbO₄ with perovskite type compositions, particularly the LaFeO₃ and LaMnO₃ systems. In contrast, Ruddlesden–Popper type phases are predicted to be poor candidates for use with LaNbO₄. Investigation of the La₂O₃–NiO–Nb₂O₅ and La₂O₃–CoO–Nb₂O₅ phase diagrams finds two new perovskite-related materials of stoichiometry LaNb_1/3M_2/3O₃ (MNi, Co), and indicates coexistence of LaNbO₄ with the binary oxides NiO and CoO. Additionally, reduction of a LaNbO₄–NiO composite confirms coexistence of LaNbO₄ with Ni, and so it is concluded that doped-LaMO₃|LaNbO₄|Ni-LaNbO₄ type electrochemical cells may be manufactured via a direct co-firing route without the formation of secondary phases at inter-phase boundaries.     

Microwave dielectric characteristics of Nb2O5-added 0.9Al2O3–0.1TiO2 ceramics

The sintering characteristics, phase composition, and microwave dielectric properties of Nb₂O₅-added 0.9Al₂O₃–0.1TiO₂ ceramics sintered at 1300–1500 °C have been investigated. Results show that Nb⁵⁺ and Al³⁺ can co-substitute for Ti⁴⁺ and form Ti_0.8Al_0.1Nb_0.1O₂, which can lower effectively the sintering temperature, and improve the quality factor of 0.9Al₂O₃–0.1TiO₂ ceramics.     

Sintering capacity of refractory composites based on lanthanum chromite

Refractory lanthanum chromite-dielectric composites are synthesized. X-ray phase analysis of composites after high-temperature treatment indicate compatibility of lanthanum chromite with such oxides as Y₂O₃, LaYO₃, MgO, TiO₂, LaAlO₃, LaAl₁₁O₁₈, La₂TiO₅, La₂Ti₂O₇, La₄Ti₉O₂₄, La₂SiO₅, La₂Si₂O₇. Intensification of LaCrO₃ sintering in the presence of dielectric additions coexisting with it is demonstrated. The effect of the form of dielectric on the sintering capacity of lanthanum chromite alloyed with calcium and without it is demonstrated. Translated from Novye Ogneupory, No. 7, pp. 45–47, July 2008.     

Enhanced sinterability and conductivity of cobalt doped lanthanum niobate as electrolyte for proton-conducting solid oxide fuel cell

La_0.95Ca_0.05Nb_1-xCo_xO₄ (x?=?0, 0.01, 0.02, 0.03, 0.05) compounds have been synthesized by a conventional solid state reaction method at 1150?°C. Co and Ca have been simultaneously introduced into LaNbO₄ solid solution for the first time, taking the place of La and Nb sites, respectively. Dense ceramic pellets of La_0.95Ca_0.05Nb_1-xCo_xO₄ have been prepared by sintering at 1300?°C with the utilization of cobalt as the sintering aid. The conductivity measurement has been carried out for all the samples in wet air. The results demonstrate that conductivity of La_0.95Ca_0.05Nb_1-xCo_xO₄ compounds are higher than that of LaNbO₄ attributed to additional oxygen vacancies generated by Co and Ca co-doping. The strategy of doping cobalt as a sintering aid proposed in this work could be served as a valid way to enhance the sinterability and electrical conductivity of LaNbO₄ based proton conducting oxides.     

The transition from non-ohmic to ohmic characteristics in La2O3 doped ZnO–MgO–TiO2 linear resistance ceramics

La₂O₃ doped ZnO–MgO–TiO₂ based linear resistance ceramics were prepared by the solid phase sintering method. The doping content of La₂O₃ is from 0.0 wt% to 2.5 wt%. The solubility of La₂O₃ in ZnO is less than 0.06 mol% (0.5 wt%), La_0.66TiO_2.993 phases will be formed at grain boundary and change the distribution of spinel phase when La₂O₃ is excessive. For I–V test, undoped sample exhibits typical non-ohmic characteristics, but La-doped samples show excellent ohmic behaviors under low DC and high pulse current (PC). The complex impedance spectrum and the frequency dependent conductivity furtherly demonstrate that La-doped samples possess linear characteristics because there is no grain boundary effect which can affect the electron transmission at grain boundaries. Besides, the decrease of the grain boundary barrier from 0.2135eV for undoped sample to 0.0031eV for 0.5 wt% La doped samples can account for the elimination or reduction of grain boundary effect. In this work, the transition from non-ohmic to ohmic properties by doping La₂O₃ in ZnO–MgO–TiO₂ multiphase ceramics is realized.     

Grain growth kinetics and growth mechanism of columnar Al2O3 crystals in xNb2O5–7.5La2O3-Al2O3 ceramic composites

xNb₂O₅–7.5La₂O₃-Al₂O₃ ceramic composites with in-situ-grown columnar Al₂O₃ crystals were successfully prepared by microwave sintering at 1450–1525?°C using α-Al₂O₃, Nb₂O₅, and La₂O₃ powders as raw materials. X-ray diffraction results indicated that the main phases were Al₂O₃, LaNbO₄, and Nb₂O₅ in the prepared samples. A field emission scanning electron microscope (FESEM) showed that the Al₂O₃ crystals appeared as columnar in the structure. Moreover, the grain size of the columnar Al₂O₃ crystals increased with the Nb₂O₅ content. The ratio of the major axis to the minor axis of the crystals was largest when the Nb₂O₅ content was 15?vol%. Furthermore, the grain-growth kinetics index (n), growth activation energy (Q), and growth mechanism of the columnar Al₂O₃ crystals were studied. The results indicated that the Nb₂O₅ addition could promote formation and growth of columnar Al₂O₃ crystals, and the grain-growth activation energy indicated that the dissolution process controls the crystal growth. The growth mechanism of the columnar Al₂O₃ crystals was also studied. The present work demonstrated that Nb₂O₅ is a good additive for the preparation of Nb₂O₅–7.5La₂O₃-Al₂O₃ composite ceramics with columnar Al₂O₃ crystals.     

Sintering and Electrical Characterization of La and Nb Co‐doped SrTiO3 Electrode Materials for Solid Oxide Cell Applications

Single‐phase lanthanum and niobium co‐doped strontium titanate (Sr_1–3x/2La_xTi_0.9Nb_0.1O₃; x = 0–0.02) ceramics were prepared. Dilatometry in reducing atmosphere showed an increase in the sintering rate and sintered density with an increase in La amount. Microscopy of fractured surfaces of sintered samples showed that the average grain size increased drastically in reducing conditions with increasing La content (and associated A‐site vacancies). By incorporating 2 mol.% La, the electronic conductivity significantly improved from 80 to 135 S cm⁻¹ at 1,000 °C, and even larger improvements were observed at lower temperatures. These observations demonstrate the flexibility in tailoring the microstructure and electronic transport properties by doping small amounts of La into the Nb‐doped SrTiO₃ and show that Sr_1–3x/2La_xTi_0.9Nb_0.1O₃ is a potential electrode material for solid oxide cells.     

The Influence of La Doping and Heterogeneity on the Thermoelectric Properties of Sr3Ti2O7 Ceramics

La‐doping mechanisms and thermoelectric properties of Sr₃Ti₂O₇ Ruddlesden–Popper (RP) ceramics sintered under air and flowing 5% H₂ at 1773 K for 6 h have been investigated. Changes in lattice parameters and conductivity revealed a limited interstitial anion mechanism (~1 at.%) based on La³⁺ + ½O^2?→Sr²⁺, which resulted in insulating samples when processed in air. In contrast, electronic donor‐doping (La³⁺ + e^? → Sr²⁺) and oxygen loss [O^2? → ½ O₂ (g) + 2 e^?] are the dominant mechanism(s) in 5% H₂‐sintered ceramics with a solution limit of ~5 at.%. The increased solubility limit is attributed to the formation of Ti³⁺ during reduction, which compensates for the extra positive charge associated with La on the A‐site and also to the occurrence of oxygen loss due to the reducing conditions. For 5% H₂‐sintered samples, an insulating surface layer was formed associated with SrO volatilization and oxygen uptake (during cooling) from the sintering. Unless removed, the insulating layer masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive perovskite‐based (Sr,La)TiO_3?δ (ST). This highlights the superior power factor properties of reduced perovskite‐type ST phases compared to reduced RP‐type Sr₃Ti₂O₇ and serves as a precaution for the need to identify low levels of highly conducting perovskite phases when exploring rare‐earth doping mechanisms in RP‐type phases.     

Ni–Nb-Based Mixed Oxides Precursors for the Dry Reforming of Methane

Perovskite-related mixed-oxides based on La Ni Nb and La Sr Ni Nb were synthesized by the auto combustion method to use as precursors materials for the catalytic reforming of methane at 700 ºC, atmospheric pressure, CH₄:CO₂ = 1:1. LaNiO₃ and LaNbO₄ were used as reference. XRD analysis show that the synthesis method produce a new series of precursor family formed by a mixture of oxides where Ni crystallized as part of a perovskite and Ruddlesden–Popper structure while Nb formed lanthanum orthoniobate LaNbO₄, a scheelite-type structure alternating with oxide layers, with phase distribution depending on niobium content. For Nb (x ≤ 0.3) Ni crystallizes as LaNiO₃ perovskite-type oxide while for Nb (x ≥ 0.7) it forms mainly the orthoniobate phase LaNbO₄ a scheelite-type structure. At higher calcined temperatures (~1100 °C) La₂Ni_0.8Nb_0.2O₄ was formed with a Ruddlesden–Popper structure consisting of three perovskite type layers along the c-axis alternating with a layer of the rock salt type phase. TEM analysis showed the presence of cubic particles with sizes varying between 5 and 60 nm depending on the extent of substitution of Ni by Nb. Reduction of the perovskite-related precursor oxides produced a series of Ni⁰/La₂O₃–NbOx oxides with high metallic dispersion which favors the activity and stability of the catalysts. Introduction of doping quantities of Sr into LaNi_0.8Nb_0.2O_3±λ structure produced a mixture of oxides with Sr dissolved in the lanthanum orthoniobate LaNbO₄ scheelite-type structure due to the similarity of ionic radii of La and Sr. Under the reaction conditions conversions near the thermodynamic equilibrium were attained which remains for long periods of time assessing the stability of the synthesized catalysts.     

BaO–TiO2 microwave ceramics

In this paper, the structure and dielectric properties of BaO–TiO₂ system ceramics were studied. By adding ZnO and Nb₂O₅ as sintering agents to the raw materials, the BaO–TiO₂ system ceramics were sintered at a temperature of 1260 °C for 2 h and have superior dielectric properties at 1 GHz: quality factor Q=12,500, relative dielectric constant ε_r≈37, temperature coefficient of dielectric constant α_ε=0±30 ppm/°C. XRD pattern shows that the main crystal phase of the ceramics is Ba₂Ti₉O₂₀, accompanied by a small number of additional phases: BaTi₄O₉, Ba₄Ti₁₃Zn₇O₃₄, Ba₄Ti₁₃O₃₀ and Ti₂Nb₁₀O₂₉, etc. The initial Ba/Ti ratio has a great effect on the dielectric properties of the ceramics, which can be explained by the variance in the formation of phases due to different Ba/Ti ratios.     

Phase stability and equilibria in the La2O3–TiO2 system

XRD, electron probe wavelength and energy dispersive X-ray analyses were used to reexamine the phase relations in the La₂O₃–TiO₂ system. The diagram was redrawn to include the compound La₄Ti₃O₁₂ in addition to La₄Ti₉O₂₄, La₂Ti₂O₇ and La₂TiO₅. Above 1455°C a cation deficient perovskite La_2/3TiO₃ is stabilized by a small number of Ti³⁺ ions and remains stable on cooling in air. The proposed diagram represents a section through the system at the normal oxygen pressure in air at 1 atm, compositions being expressed in terms of the oxide components stable at room temperature.     

Effect of Li2CO3–Bi2O3 doping on the low-temperature sintering,structure, and dielectric and mechanical properties of MgO–TiO2(w) ceramics

Dense MgO–12% TiO₂(w) ceramics containing 12 wt% TiO₂, which were doped with Li₂CO₃–Bi₂O₃ composite sintering aids, were prepared at a low sintering temperature of 950 °C in this study. The effects of sintering additives on the sintering characteristics, phase composition, microstructure, and dielectric and mechanical properties of the ceramic samples were systematically investigated, and the influences of their phase composition and microstructure on the dielectric and mechanical properties were examined. The introduction of sintering aids produced a new Bi₄Ti₃O₁₂ phase in the sample structure, while the residual Bi₂O₃ mixed with the newly formed Mg₂TiO₄ and Bi₄Ti₃O₁₂ phases distributed at MgO grain boundaries formed a structure surrounding MgO grains. This structure filled the pores in the ceramic sample, which increased its density and enhanced the mechanical properties. At a Li₂CO₃–Bi₂O₃ content of 15 wt%, the density, flexural strength, and Vickers hardness of the ceramic samples reached their maximum values of 3.4 g/cm³, 218.9 MPa, and 778.7 HV, respectively. However, the further increase in the Li₂CO₃–Bi₂O₃ content deteriorated their dielectric properties although the dielectric constant and dielectric loss remained below 13.4 and 2.1 × 10⁻³, respectively. The findings of this work indicate that Li₂CO₃–Bi₂O₃ sintering aids can significantly lower the sintering temperature of MgO–12% TiO₂(w) ceramics and control their dielectric and mechanical properties through microstructural changes.     

Structural dependence of microwave dielectric properties in ilmenite-type Mg(Ti1-xNbx)O3 solid solutions by Rietveld refinement and Raman spectra

Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were prepared by the conventional solid-state reaction method. The phase composition, sintering characteristics, microstructure and dielectric properties of Ti⁴⁺ replacement by Nb⁵⁺ in the formed solid solution Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics were systematically studied. The structural variations and influence of Nb⁵⁺ doping in Mg(Ti_1-xNb_x)O₃ were also systematically investigated by X-ray diffraction and Raman spectroscopy, respectively. X-ray diffraction and its Rietveld refinement results confirmed that Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics crystallised into an ilmenite-type with R-3 (148) space group. The replacement of the low valence Ti⁴⁺ by the high valence Nb⁵⁺ can improve the dielectric properties of Mg(Ti_1-xNb_x)O₃ (x = 0–0.09). This paper also studied the different sintering temperatures for Mg(Ti_1-xNb_x)O₃ (x = 0–0.09) ceramics. The obtained results proved that 1350 °C is the best sintering temperature. The permittivity and Q × f initially increased and then decreased mainly due to the effects of porosity caused by the sintering temperature and the doping amount of Nb₂O₅, respectively. Furthermore, the increased Q × f is correlated to the increase in Ti–O bond strength as confirmed by Raman spectroscopy, and the electrons generated by the oxygen vacancies will be compensated by Nb⁵⁺ to a certain extent to suppress Ti⁴⁺ to Ti³⁺, which was confirmed by XPS. The increase in τ_f from ?47 ppm/°C to ?40.1 ppm/°C is due to the increment in cell polarisability. Another reason for the increased τ_f is the reduction in the distortion degree of the [TiO₆] octahedral, which was also confirmed by Raman spectroscopy. Mg(Ti_0.95Nb_0.05)O₃ ceramics sintered at 1350 °C for 2 h possessed excellent microwave dielectric properties of ε_r = 18.12, Q × f = 163618 GHz and τ_f = ?40.1 ppm/°C.     

Effect of ZnO on the microstructure and electrical properties of WO3–Bi4Ti3O12 ceramics

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT-based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi₂O₃ plays a critical role during sintering. Both Bi₂Ti₂O₇ and Zn₂TiO₄ secondary phases are stabilized when adding ZnO. Actually, when WO₃ and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi₂Ti₂O₇ phase and avoiding the incorporation of W⁶⁺ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO₃, suggesting that WO₃ does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi₂Ti₂O₇ phase.     

Dielectric behaviour of (Mg0·95Ca0·05)TiO3 for application in terahertz resonator

Abstract

The 0·15 THz resonator based on the (Mg_0·95Ca _0·05)TiO₃ (abbreviated as 95MCT hereafter) ceramic was designed, and the dielectric property of 95MCT for application has been studied. La₂O₃ and Nb₂O₅ were selected as liquid sintering aids to lower the sintering temperature. X-ray diffraction patterns indicated that MgTi₂O₅ secondary phase could be effectively suppressed by La₂O₃ and Nb₂O₅ additions. When the Nb₂O₅+La₂O₃ codoping content was 0·25 wt-%, the ceramic could be densified at 1320°C and also has good dielectric behaviours of Q_f?=?69720 GHz (6·8 GHz), ?_r?=?20·18 and τ_f?=??7·56 ppm °C^?1. The terahertz resonator designed at 0·15 THz exhibited that with the increasing height of inner cylinder, the two modes’ resonance frequencies decreased.     

Excellent energy-storage property and thermal stability of PLZT-based antiferroelectric ceramics

(Pb, La)(Zr, Ti)O₃ antiferroelectric (AFE) materials are promising materials due to their energy-storage density higher than 10 J cm⁻³, but their low energy-storage efficiency and poor temperature stability limit their application. In this paper, the (1 − x)(Pb_0.9175La_0.055)(Zr_0.975Ti_0.025)O₃–xPb(Yb_1/2Nb_1/2)O₃ (PLZTYN100x) AFE ceramics were prepared via two-step sintering method and investigated thoroughly. With the doping of Yb³⁺ and Nb⁵⁺, the phase structure transforms from the orthorhombic phase (AFE_O) to the coexistence of the orthorhombic-and-tetragonal phases. This structure reduces the free energy difference between the AFE and ferroelectric phases and reduces the fluctuation of energy with temperature, improving the energy storage efficiency and temperature stability. When the x = 0.05 (PLZTYN5), the AFE ceramic exhibits excellent temperature stability and ultrahigh energy storage performance, whose recoverable energy density (W_rec) is 6.8–8.2 J cm⁻³ at 30 kV mm⁻¹ in the temperature range from −55 to 75°C, and efficiency (ƞ) is 78%–86.7%. In addition, the change of W_rec is less than 15%, exceeding the performance of most AFE ceramics. The results demonstrate that the PLZTYN5 ceramic has great potential in pulse power capacitors.     

Flash synthesis of Magnéli phase (TinO2n-1) nanoparticles by thermal plasma treatment of H2TiO3

A flash synthesis method of titanium suboxides (TSO) Ti_nO_2n-1 nanoparticles with Magnéli phases by thermal plasma using rutile-phase metatitanic acid (H₂TiO₃) as the raw material was proposed and investigated, comparing with common sintering hydrogenation process of H₂TiO₃. TSO powders with diverse colors were prepared and characterized by means of XRD, SEM and TEM. The results showed Magnéli phase nanoparticles with perfect spherical shape and particle size of 20–100 nm were rapidly synthesized by thermal plasma treatment under Ar and Ar/H₂ atmospheres. Typical Magnéli phases of Ti₅O₉ and Ti₄O₇ with peacock blue color were formed under argon atmosphere. An increase of hydrogen content in the atmosphere made nano-H₂TiO₃ efficiently reduced to form a different TSO phases with diverse colors of dark blue to blue black. Notably, parts of nanoparticles were of carbon shell/Magnéli phase core structure. The UV–visible absorptivity and electric conductivity of samples were tested and the results indicate that thermal plasma is an efficient way to improve the optical and conductive properties of materials. But the electric resistivity of nano Magnéli phase powders still keep 10⁵–10⁶ Ω^.cm.     

Role of different rare earth oxides on the reaction sintering of magnesium aluminate spinel

Role of three rare earth oxides, viz., La₂O₃, CeO₂ and Yb₂O₃ on reaction sintering of magnesium aluminate spinel having molar ratio of MgO:Al₂O₃?=?1:2 from its solid oxide precursors was investigated in static and dynamic heating conditions. Effect of these additives (3?wt%) on densification behavior, phase assemblage and microstructure development were studied in the temperatures of 1500–1700?°C. Yb₂O₃ enhanced the sintering of spinel, while La₂O₃ and CeO₂ negatively impacted the sintering of magnesium aluminate spinel which can be discerned from the shrinkage curve of TMA as well as from static firing regime. This is ascribed to the formation of secondary phases in La₂O₃ and CeO₂ containing samples which have different crystalline structures to that of spinel. This anisotropy due to different crystallinity hindered the pore shrinkage and pore removal and thereby retarded the densification. Whereas, the cubic structure of the secondary phase formed in Yb₂O₃ containing sample which is isotropic with the crystalline orientation of the parental spinel phase assisted the densification.