Formation and thermal properties of fluorite-pyrochlore composite structure in La2(ZrxCe1 − x)2O7 oxide system

Rare-earth oxides of La₂(Zr_xCe_1 ? x)₂O₇ for thermal barrier coatings (TBCs) are fabricated via a solid-state reaction at 1600 °C. As the phase formation, microstructure, and thermal properties of these oxides are examined, a fluorite–pyrochlore composite structure is found in the La₂(Zr_xCe_1 ? x)₂O₇ system. This composite structure is composed of coarse Ce-rich fluorite and fine Zr-rich pyrochlore grains. From XRD and microstructural analysis, the lattice parameter and volume fraction of each phase are evaluated in order to obtain the intrinsic thermal conductivity value of composite-structured oxide with porosity calibration. The thermal conductivity of the composite structure is similar to that of pyrochlore La₂Zr₂O₇, which is attributed to phonon scattering by phase boundaries.     

Low-temperature synthesis of Li7La3Zr2O12 with cubic garnet-type structure

In this paper, we report the direct synthesis of Li₇La₃Zr₂O₁₂ with the cubic garnet-type structure at low temperature with a lattice constant of 13.0035 Å. The synthesis condition is optimized to be at 750 °C for 8 h with 30 wt% excess lithium salt. No intermediate grinding was involved in this straightforward route. Without the adventitious of Al³⁺, the cubic Li₇La₃Zr₂O₁₂ is unstable above 800 °C and has an ionic conductivity of the order of 10^?6 S cm^?1.     

Investigation of La3 + Doped Yb2Sn2O7 as new thermal barrier materials

Low thermal conductivity is one of the key requirements for thermal barrier coating materials. From the consideration of crystal structure and ion radius, La^3 + Doped Yb₂Sn₂O₇ ceramics with pyrochlore crystal structures were synthesized by sol–gel method as candidates of thermal barrier materials in aero-engines. As La^3 + and Yb^3 + ions have the largest radius difference in lanthanoid group, La^3 + ions were expected to produce significant disorders by replacing Yb^3 + ions in cation layers of Yb₂Sn₂O₇. Both experimental and computational phase analyses were carried out, and good agreement had been obtained. The lattice constants of solid solution (La_xYb_1 − x)₂Sn₂O₇ (x = 0.3, 0.5, 0.7) increased linearly when the content of La^3 + was increased. The thermal properties (thermal conductivity and coefficients of thermal expansion) of the synthesized materials had been compared with traditional 8 wt.% yttria stabilized zirconia (8YSZ) and La₂Zr₂O₇ (LZ). It was found that La^3 + Doped Yb₂Sn₂O₇ exhibited lower thermal conductivities than un-doped stannates. Amongst all compositions studied, (La_0.5Yb_0.5)₂Sn₂O₇ exhibited the lowest thermal conductivity (0.851 W·m^− 1·K^− 1 at room temperature), which was much lower than that of 8YSZ (1.353 W·m^− 1·K^− 1), and possessed a high coefficient of thermal expansion (CTE), 13.530 × 10^− 6 K^− 1 at 950 °C.     

(TiZrHf)P2O7: An equimolar multicomponent or high entropy ceramic with good thermal stability and low thermal conductivity

ZrP₂O₇ is a promising material for high temperature insulating applications. However, decomposition above 1400 °C is the bottleneck that limiting its application at high temperatures. To improve the thermal stability, a novel multicomponent equimolar solid solution (TiZrHf)P₂O₇ was designed and successfully synthesized in this work inspired by high-entropy ceramic (HEC) concept. The as-synthesized (TiZrHf)P₂O₇ exhibits good thermal stability, which is not decomposed after heating at 1550 °C for 3 h. It also shows lower thermal conductivity (0.78 W m⁻¹ K⁻¹) compared to the constituting metal pyrophosphates TiP₂O₇, ZrP₂O₇ and HfP₂O₇. The combination of high thermal stability and low thermal conductivity renders (TiZrHf)P₂O₇ promising for high temperature thermal insulating applications.     

Preparation,structure and electrical conductivity of pyrochlore-type samarium–lanthanum zirconate ceramics

A series of zirconate compounds with the general formula Sm_2–xLa_xZr₂O₇ (0 ? x ? 1.0) were prepared by pressureless-sintering method at 1973 K for 10 h in air. The relative density, structure and electrical conductivity of Sm_2–xLa_xZr₂O₇ ceramics were investigated by the Archimedes method, X-ray diffraction and impedance spectroscopy measurements. Sm_2–xLa_xZr₂O₇ (0 ? x ? 1.0) ceramics exhibit a pyrochlore-type structure. The measured electrical conductivity of Sm_2–xLa_xZr₂O₇ ceramics obeys the Arrhenius relation and gradually increases with increasing temperature from 673 to 1173 K. Sm_2–xLa_xZr₂O₇ ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10^?4 to 1.0 atm at all test temperature levels. The electrical conductivity of Sm_2–xLa_xZr₂O₇ ceramics decreases with increasing lanthanum content at identical temperature levels.     

Electrical conductivity and stability of A-site deficient (La,Sc) co-doped SrTiO3 mixed ionic-electronic conductor

Mixed ionic–electronic conductors with high ionic conductivity play an important role in modern solid-state ionic devices. The ionic conductivity of SrTiO₃-based materials can be significantly improved by creating deficiency on the A-site and acceptor-doping on the B-site. We report in this paper a remarkable enhancement of ionic conductivity, sinterability and thermal stability of (La, Sc) co-doped SrTiO₃ by creating deficiency on the A-site. The ionic conductivity of (La_0.3Sr_0.7)_0.95Sc_0.10Ti_0.90O_3–δ varies from 0.005 S/cm at 500 °C to 0.01 S/cm at 700 °C and to 0.018 S/cm at 950 °C in 5%H₂/Ar. These values are nearly eight times higher than that of La_0.3Sr_0.7TiO_3–δ at T ≥ 700 °C. The A-site deficiency in (La, Sc) co-doped SrTiO₃ also improves the thermal and electrical stabilities in various atmospheres. A possible charge compensation mechanism among defects in the (La_0.3Sr_0.7)_0.95Sc_0.10Ti_0.90O_3–δ is also discussed.     

Development of mixed conducting dense nickel/Ca-doped lanthanum zirconate cermet for gas separation application

La_1.95Ca_0.05Zr₂O_7-δ (LCZ) and Ni–LCZ cermet have been prepared by combustion synthesis and conventional solid state mixing methods respectively. Both the materials are sintered in air and controlled atmosphere (5% H₂ in Ar). The density obtained for the material sintered at 1400 °C in controlled atmosphere is found to be more than 99.5%. This sintering temperature (1400 °C) is considered to be much lower compared to the conventional sintering temperature. The corresponding total conductivity for such Ni–LCZ cermet materials is ～400 S/cm measured at 750 °C having 40 vol% of Ni and 60 vol% LCZ.     

Initial phase hot corrosion mechanism of gas tunnel type plasma sprayed thermal barrier coatings

The hot corrosion resistance of the top layer in TBC is one of the main constructive factors which determines the lifetime of the coatings under critical operating environments. In the present study, 8 wt% yttria stabilized zirconia (8YSZ), lanthanum zirconate (La₂Zr₂O₇) and equal weight percentage of its composite (50%8YSZ + 50% La₂Zr₂O₇) coatings were prepared by using gas tunnel type plasma spray torch at optimum spraying conditions. The hot corrosion performances of the above thermal barrier coatings were examined against 40 wt%V₂O₅–60 wt%Na₂SO₄ corrosive ash at 1173 K for 5 h in open air atmosphere. After hot-corrosion testing, the coating surface was studied using a scanning electron microscope to observe the microstructure and X-ray diffraction techniques were used to identify the phase compositions. The results showed that LaVO₄ and YVO₄ are the main hot corrosion products along with the ZrO₂ phase transformation from tetragonal to monoclinic phases in La₂Zr₂O₇ and 8YSZ coatings respectively. The microstructure and phase formation mechanism of the hot corrosion products varied with each coating and among these, composition of 50%8YSZ + 50%La₂Zr₂O₇ coating exhibited least degradation against V₂O₅–Na₂SO₄ corrosive environment compared to the other coatings.     

Electrical properties and thermal expansion of cobalt doped apatite-type lanthanum silicates based electrolytes for IT-SOFC

The thermal expansion and conductivities have been investigated for Co³⁺ doped lanthanum silicates. The apatite-type lanthanum silicates with formula La₁₀Si_6?xCo_xO_27?x/2 (x = 0.2, 0.4, 0.6, 0.8, 1.0, 1.5) were synthesized by sol–gel process. The thermal expansion coefficient (TEC) of La₁₀Si_6?xCo_xO_27?x/2 was improved with increasing cobalt content because of the lower valence and larger radius of Co³⁺ ion compared to Si⁴⁺. Analysis of AC impedance spectroscopy showed that conductivity increased first and then decreased with increasing cobalt content. There is an optimum doping amount of cobalt and La₁₀Si_5.2Co_0.8O_26.6 exhibits the highest conductivity of 3.33 × 10^?2 S/cm at 800 °C. When x ≤ 0.8, the local distortion caused by doping with Co³⁺ can significantly affect the oxygen channels and assist the migration of the interstitial oxide ions, resulting in the improvement of ionic conductivity. However, excess Co³⁺ dopant (0.8 < x ≤ 1.5) reduced the number of interstitial oxide ions and decreased the conductivity.     

Investigation on the calcining condition and sintering temperature of perovskite-type La0.7Ca0.3CrO3 complex oxides by a combustion method

Perovskite-type La_0.7Ca_0.3CrO₃ composite oxides were synthesized by a combustion method. The calcining condition of the synthesized powders and sintering temperature of ceramic specimens were studied. It was found that a pure perovskite structure was direct acquired in the combustion ash and the fine morphology (～100 nm) was observed in the sample calcined at 600 °C. In view of the relative density, microstructure and electrical conducting properties of the sintering ceramics at different temperatures, the preferred sintering temperature was ascertained to be 1300 °C, at which the sample attains a high relative density of 96.8%, showing an electrical conductivity of 53.6 S cm^?1 at 800 °C and much lower activation energy of 0.122 ev. Compared with La_0.7Ca_0.3CrO₃ synthesized by the conventional solid state reaction method, that synthesized by the combustion method exhibits fine morphology and superior sintering activity, effectively lowering the sintering temperature and enhancing electrical conducting properties of the material.     

Room temperature synthesis of high temperature stable lanthanum phosphate–yttria nano composite

A facile aqueous sol–gel route involving precipitation–peptization mechanism followed by electrostatic stabilization is used for synthesizing nanocrystalline composite containing lanthanum phosphate and yttria. Lanthanum phosphate (80 wt%)–yttria (20 wt%) nano composite (LaPO₄–20%Y₂O₃), has an average particle size of ～70 nm after heat treatment of precursor at 600 °C. TG–DTA analysis reveals that stable phase of the composite is formed on heating the precursor at 600 °C. The TEM images of the composite show rod shape morphology of LaPO₄ in which yttria is acquiring near spherical shape. Phase identification of the composite as well as the phase stability up to 1300 °C was carried out using X-ray diffraction technique. With the phases being stable at higher temperatures, the composite synthesized should be a potential material for high temperature applications like thermal barrier coatings and metal melting applications.     

Synthesis of La0.85Sr0.15Ga0.85Mg0.15O2.85 materials for SOFC applications by acrylamide polymerization

The La_0.85Sr_0.15Ga_0.85Mg_0.15O_2.85 (LSGM) powders are synthesized using a novel method based on acrylamide polymerization technique. The phase evolution was determined by using XRD analysis. The sintering property was studied by using dilatability analysis. The electrical conductivity was also measured. XRD pattern indicates that the perovskite phase is formed at 1000 °C, and the impurity phase, LaSrGa₃O₇ still exists in the sintered sample. The shrinkage curve shows that the fast sintering temperature is 1432 °C. The sinterability was investigated as a function of sintering time and temperature. The results show that the densification rate of the sample was fast at first 5 h. The electrical conductivity was 0.093 S/cm at 800 °C. And a transitional temperature in the Arrhenius plot is 700 °C.     

Phase structure evolution and thermal expansion variation of Sc2O3 doped Nd2Zr2O7 ceramics

(Nd_1−xSc_x)₂Zr₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7) compounds were synthesized by solid state reaction at 1700 °C for 10 h, and characterized by XRD, Raman spectroscopy, SEM and high-temperature dilatometer. Nd₂Zr₂O₇ exhibited pyrochlore phase, and its lattice parameter increased after Sc₂O₃ doping, which could be attributed to the presence of Sc³⁺ interstitial ions in pyrochlore lattice. Fluorite phase formed in the doped Nd₂Zr₂O₇, and (Nd_0.3Sc_0.7)₂Zr₂O₇ exhibited pure fluorite phase. The thermal expansion coefficient (TEC) of Nd₂Zr₂O₇ was significantly enhanced by 10 mol% Sc₂O₃ doping, but higher Sc₂O₃ doping decreased the TEC. The reduced crystal energy due to the presence of Sc³⁺ interstitial ions could cause the initial increase in the TEC, and the formation of fluorite phase might contribute to the reduced TEC. Considering the alleviation of the thermal expansion mismatch stress for the high-temperature applications of Nd₂Zr₂O₇, Sc₂O₃ was an excellent dopant and there existed an optimal Sc₂O₃ content for the optimization design of compound compositions.     

Facile synthesis of cubic fluorite nano-Ce1−xZrxO2 via hydrothermal crystallization method

Nano-Ce_1?xZr_xO₂ (x = 0.15, 0.25, 0.5) were synthesized via co-precipitation using NH₄OH as precipitant and hydrothermal crystallization. The XRD results confirmed that the cubic fluorite nano-Ce_1?xZr_xO₂ can form in NH₄OH solution (pH > 10) at 150 °C for 12 h, and well crystallized 20–50 nm nano-Ce_1?xZr_xO₂ were obtained at 200 °C for 22 h. The crystal growth of Ce_1?xZr_xO₂ was suppressed under higher OH^? concentration and crystallite size decreased with increasing concentration of NH₄OH. Ce3d XP spectra showed that the main valence state of the cerium on Ce_1?xZr_xO₂ surface is +4, and substituting Ce⁴⁺ with Zr⁴⁺ has no obvious influence on Ce³⁺/Ce⁴⁺ ratio.     

Al2TiO5–Al2O3–TiO2 nanocomposite: Structure,mechanical property and bioactivity studies

Novel biomaterials are of prime importance in tissue engineering. Here, we developed novel nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite as a biomaterial for bone repair. Initially, nanocrystalline Al₂O₃–TiO₂ composite powder was synthesized by a sol–gel process. The powder was cold compacted and sintered at 1300–1500 °C to develop nanostructured Al₂TiO₅–Al₂O₃–TiO₂ composite. Nano features were retained in the sintered structures while the grains showed irregular morphology. The grain-growth and microcracking were prominent at higher sintering temperatures. X-ray diffraction peak intensity of β-Al₂TiO₅ increased with increasing temperature. β-Al₂TiO₅ content increased from 91.67% at 1300 °C to 98.83% at 1500 °C, according to Rietveld refinement. The density of β-Al₂TiO₅ sintered at 1300 °C, 1400 °C and 1500 °C were computed to be 3.668 g cm^?3, 3.685 g cm^?3 and 3.664 g cm^?3, respectively.Nanocrystalline grains enhanced the flexural strength. The highest flexural strength of 43.2 MPa was achieved. Bioactivity and biomechanical properties were assessed in simulated body fluid. Electron microscopy confirmed the formation of apatite crystals on the surface of the nanocomposite. Spectroscopic analysis established the presence of Ca and P ions in the crystals. Results throw light on biocompatibility and bioactivity of β-Al₂TiO₅ phase, which has not been reported previously.     

Structural and thermal studies of H2La2/3Ta2O7, a protonated layered perovskite

We have synthesised the new protonated layered perovskite H₂La_2/3Ta₂O₇ which is related to the Ruddlesden–Popper family. This compound is obtained by ionic exchange starting from Li₂La_2/3Ta₂O₇ maintained in dilute HNO₃ at 60 °C. Thermal X-ray diffraction and DTA/TGA revealed interesting dehydration properties with formation of a layered anhydrous phase leading at higher temperature (1550 °C) to La_1/3TaO₃. This latter compound exhibits the original lanthanum ordering expected similarly to that of the Li form, while at 900 °C a metastable form, presenting a disordered La distribution, is observed.     

Flux growth and thermal properties of LiBaB9O15 single crystal

A large LiBaB₉O₁₅ single crystal has been grown by the top-seeded solution growth (TSSG) method using a Li₂Mo₃O₁₀ flux system. The crystal obtained exhibits (1 1 0), (1 1 3) and (1 0 2) faces. For the first time, thermal properties of the as-grown crystal, including thermal expansion, specific heat and thermal conductivity, have been investigated as a function of temperature. The specific heat of the LiBaB₉O₁₅ crystal was measured to be 0.663–1.110 J g^?1 K^?1 over the temperature range of 20–400 °C. The crystal exhibits thermal expansion along the a- and b-axis, coupled with thermal contraction along the c-axis, over the measured temperature range of 25–500 °C. The average thermal expansion coefficients along the a- and c-axis of the LiBaB₉O₁₅ crystal from 25 to 500 °C are calculated to be α_a = 6.56 × 10^?6 K^?1 and α_c = ?4.82 × 10^?6 K^?1, respectively.     

High permittivity and low dielectric loss of Na0.5Bi0.5−xLaxCu3Ti4O12 ceramics

The phase structure, microstructure and dielectric properties of Na_0.5Bi_0.5?xLa_xCu₃Ti₄O₁₂ (NBLCTO) ceramics were investigated. La³⁺ substitution had a great influence on the phase structure and dielectric properties. The results showed that the pure phase could be more easily obtained when substituting La³⁺ for Bi³⁺. Under the same processing condition (970 °C for 7.5 h) and measuring condition (10 kHz around room temperature), NBLCTO ceramics with x = 0.10 possessed the highest permittivity (1.02 × 10⁴) and lowest dielectric loss (0.022). The obtained NBLCTO ceramics with x = 0.10 also had good frequency stability and good temperature stability (?1.87% to +3.27%) from ?60 °C to 120 °C at both 1 and 10 kHz. Complex impedance results revealed that the grain resistance R_g was 7.18 Ω cm and the grain boundary resistance R_gb was 1.19 × 10⁶ Ω cm.     

Electrical conductivity of LaxSr2−xFe1−yRuyO4±δ

Materials of the K₂NiF₄ structure type have been prepared and the electrical conductivity in air determined for a number of compositions in the La_xSr_2−xFe_1−yRu_yO_4±δ solid solution series including three with Ru substituted for Fe on the B site: La_0.2Sr_1.8Fe_0.6Ru_0.4O_4±δ, La_0.4Sr_1.6Fe_0.7Ru_0.3O_4±δ, and La_0.6Sr_1.4Fe_0.8Ru_0.2O_4±δ. Overall the total conductivity values measured were lower than expected for the Ru-doped materials, with a peak conductivity of ≈2 S cm⁻¹ at 700 °C. In the undoped La_xSr_2−xFeO_4±δ materials, a significant jump in conductivity was observed between the x = 0.7 and 0.8 compositions and was related to the bonding in the materials and the Fe³⁺/Fe⁴⁺ redox behaviour. In all materials, the conduction behaviour was found to follow a semi-conducting trend.     

Fabrication and electrical properties of Si-based La10−xBix(SiO4)6O3 apatite ionic conductor

In this paper we report an investigation about the relationship between Bi content and conductivity for the La_10?xBi_x(SiO₄)₆O₃ (x = 0.5–2) specimens. Increasing Bi content in apatite-type La_10?xBi_x(SiO₄)₆O₃ specimen leads to increasing the average grain size and sinterability because some Bi ion deposited at grain boundaries which promotes grain growth. The relative sintered density of the La₈Bi₂(SiO₄)₆O₃ specimen is 98%. There is a small shift in diffraction angle of XRD peaks, complicated lattice distortion was observed in the X-ray pattern with increasing content of Bi. Electrical conductivity of La_10?xBi_x(SiO₄)₆O₃ specimen increased with increasing Bi contents and the value of the La₈Bi₂(SiO₄)₆O₃ specimen is 2.4 × 10^?4 S cm^?1 at 700 °C.