Thermoelectric and magnetic properties of spark plasma sintered REB66 (RE = Y,Sm, Ho,Tm, Yb)

A synthesis route was successfully established to reliably synthesize the high temperature thermoelectric material REB₆₆ (RE = Y, Sm, Ho, Tm, Yb) much more readily than the typical single crystal growth. Thermoelectric properties of HoB₆₆ and TmB₆₆ are also investigated for the first time. The REB₆₆ samples synthesized by spark plasma sintering showed thermoelectric properties equivalent to those measured on single crystals. A figure of merit ZT around 0.1 at 973 K was obtained with a sharply increasing trend toward higher temperatures. Strikingly, the thermoelectric properties of these compounds appear to be almost independent of the microstructure. The nature of the rare earth does not seem to strongly affect the thermoelectric properties, differences originating mainly from different compositions or the presence of secondary phases. Relatively large negative Curie-Weiss temperatures θ were observed, and an increasing coupling indicated from HoB₆₆ to YbB₆₆. This could be indicative of an unusual coupling mechanism.     

Microstructure evolution and hot corrosion mechanisms of Ba2REAlO5 (RE = Yb,Er, Dy) exposed to V2O5 + Na2SO4 molten salt

Ba₂REAlO₅ (RE?=?Dy, Er, Yb) powders were synthesized by a solid state reaction method, followed by cold pressing and sintering to produce pellets for hot corrosion tests. When exposed to V₂O₅?+?Na₂SO₄ molten salt at 900?°C and 1000?°C for 4?h and 20?h, REVO₄, Ba₃(VO₄) ₂ and BaAl₂O₄ formed as corrosion products due to chemical interactions between the ceramics and the molten salt, which were temperature and time independent. After the hot corrosion tests at 1000?°C, continuous, dense reaction layers with a thickness of ～80?μm formed on the sample surfaces, which had an effective function on suppressing further penetration of the molten salt. The hot corrosion mechanisms of Ba₂REAlO₅ are proposed based on Lewis acid-base rule, phase diagrams and thermodynamics. From a thermodynamics perspective, the molten salt directly reacting with Ba₂REAlO₅ is difficult compared with Yb₂O₃, Al₂O₃ and BaO.     

Upconversion Luminescence in Yb/Ln (Ln = Er,Tm) Doped Oxyhalide Glasses Containing CsPbBr3 Perovskite Nanocrystals

Yb/Ln (Ln=Er, Tm) doped TeO₂-based glasses containing CsPbBr₃ perovskite quantum dots were successfully prepared via in-situ glass crystallization. The nanocomposites yield typical green downshifting luminescence attributing to CsPbBr₃ exciton recombination under UV excitation, and produce Er³⁺ green, Er³⁺ red and Tm³⁺ blue upconversion emissions under 980 nm laser excitation. Impressively, specific Ln³⁺ emissions will be quenched with the precipitation of CsPbBr₃ in glass, enabling to finely tune upconversion emitting color. Spectroscopic characterizations evidence that the luminescence quenching is originated from non-radiative reabsorption effect induced by the precipitation of CsPbBr₃ rather than energy transfers from Ln³⁺ to CsPbBr₃. Finally, these nanocomposites are demonstrated to exhibit superior water resistance due to the effective protecting role of dense structural glass, particularly, about 95% downshifting luminescence of CsPbBr₃ and upconversion luminescence of Er³⁺ related to pristine ones are retained after immersing the products in water up to 30 days.     

Crystal structural and microwave dielectric properties of Ba4B'Nb3O12 (B′=Yb,Tm, Er,Y, Ho,Dy, Gd) ceramics

The influence of substitution of rare-earth ion (RE = Yb, Tm, Er, Y, Ho, Dy, Gd) for B′-site on phase composition, crystal structure, micromorphology, and microwave dielectric properties of Ba₄RENb₃O₁₂ ceramics are investigated. The results of XRD and Rietveld refinement indicate that the Ba₄RENb₃O₁₂ ceramic is composed of Ba(RE_1/12Nb_9/12)O₃ and Ba (RE_1/2Nb_1/2)O₃ phases. Porosity analysis shows that the relative density of Ba₄RENb₃O₁₂ ceramics can reach more than 95.25%. The variations of permittivity and temperature coefficient are associated with ionic polarization and cell polarization, respectively. The A_1g Raman‐active modes at 754 cm^?1 are oxygen-octahedron stretch vibration, reflecting the variation of the structure. Optimal microwave dielectric properties (ε_r = 38.75, Q × f = 41251 GHz, τ_f = 71.57 ppm/°C) of Ba₄RENb₃O₁₂ ceramics for RE = Yb are obtained at 1425 °C for 6 h.     

Hot corrosion behavior of Ba2REAlO5 (RE = Dy,Er, Yb) ceramics by vanadium pentoxide at 900–1000 °C

Hot corrosion behavior of Ba₂REAlO₅ (RE = Dy, Er, Yb) ceramics exposed to V₂O₅ molten salt at 900 °C and 1000 °C was investigated, providing a better understanding of their corrosion resistance as promising thermal barrier coatings. Obvious surface reactions occurred forming continuous, dense reaction layers on the top surfaces of the samples, the types of corrosion products being temperature and time independent. After heat treatment for 4 h and 20 h in V₂O₅ salt at the two temperatures, the corrosion products consisted of REVO₄, Ba₂REV₃O₁₁ and BaAl₂O₄ (RE = Dy, Er, Yb). Prolonged heat treatment and elevated temperature promoted the growth of Ba₂REV₃O₁₁ and REVO₄ grains. The reaction layer had a positive function on suppressing further penetration of the molten salt. The mechanism by which the corrosion reaction occurs is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

Upconversion thermal enhancement of 2H11/2→4I15/2 of Er3+ and blue emission of impurity Tm3+ in Sr3(PO4)2:Er3+/Yb3+

A series of Sr_2.99-x(PO₄)₂:.01Er³⁺/xYb³⁺ (x = .02, .04, .06, .08, .10) phosphors in the presence of impurity Tm³⁺ were synthesized by high temperature solid-state method, and X-ray diffraction results show that these samples are pure R-3 m(166) space group phase. The upconversion luminescence (UCL) of Er³⁺ and impurity Tm³⁺ under 980-nm laser excitation were investigated, and the results show that the intense blue UCL of impurity Tm³⁺ and thermal enhancement of ²H_11/2→⁴I_15/2 of Er³⁺ simultaneously exist. When Er³⁺ doping concentration is kept at .01, both the blue UCL intensity of impurity Tm³⁺ and green and red UCL intensity of Er³⁺ reach the maximum at Yb³⁺ doping concentration of .08. The thermal enhancement effect of ²H_11/2→⁴I_15/2 of Er³⁺ was observed as high as 3.27 times from 303 to 723 K, which is because of lattice distortion and phonon-assisted transition. In addition, the optical temperature performance of Sr_2.91(PO₄)₂:.01Er³⁺/.08Yb³⁺ sample was studied, and the maximum absolute temperature sensitivity was calculated as .00623 K⁻¹ at 538 K. This study suggests that Sr₃(PO₄)₂:Er³⁺/Yb³⁺ phosphors in the presence of impurity Tm³⁺ have a promising application prospect as optical temperature sensor at high temperature.     

The nature of current carriers and electric conductivity in the PbCl2-2PbO • SiO2 glasses

The nature of current carriers is revealed and the concentration dependence of the true transport numbers of chlorine ions η is studied by the Tubandt method in glasses of the PbCl₂-2PbO · SiO₂ system. It is demonstrated that, in glasses containing up to ∼20 mol % PbCl₂, the electricity transport occurs through protons and chlorine ions. At [PbCl₂] ≥ 20 mol %, the electric current is carried primarily by the chlorine ions. The known crystalline compounds in the PbO-PbCl₂ system are synthesized, and the temperature dependences of their electric conductivity are investigated for the first time. It is found that the introduction of PbCl₂ into PbO is accompanied, first, by an increase in the electric conductivity (to a PbCl₂ mole fraction of ∼0.5), and, then (when PbCl₂ single crystals are formed), by its decrease. The temperature-concentration dependence of the electric conductivity for glasses in the PbCl₂-2PbO · SiO₂ system is interpreted in terms of structural-chemical microinhomogeneity of the glass structure due to the selective interaction of components during the glass synthesis.     

Mechanical and thermal properties of δ-RE4Hf3O12 (RE = Yb,Lu)

Rare-earth (RE) hafnates are promising thermal and environmental barrier coating (TEBC) materials for SiC_f/SiC ceramic matrix composites. In this study, pure-phase and dense δ-RE₄Hf₃O₁₂ (RE = Yb, Lu) bulk ceramics have been fabricated via a hot-pressing method. The crystal structure, microstructure, mechanical, and thermal properties of δ-RE₄Hf₃O₁₂ were systematically investigated in order to probe their potential application as TEBCs. The high-temperature elastic moduli of δ-Yb₄Hf₃O₁₂ and δ-Lu₄Hf₃O₁₂ are measured to be 185 and 188 GPa at 1673 K, respectively, which are over 85% values of room temperature. The coefficients of thermal expansion are 7.64 × 10⁻⁶ and 7.46 × 10⁻⁶ K⁻¹ for δ-Yb₄Hf₃O₁₂ and δ-Lu₄Hf₃O₁₂, respectively. The relatively low coefficient of thermal expansion and thermal conductivity as well as their excellent high-temperature stability endow these hafnates as potential TEBC candidates.     

Microstructure and properties of porous SiC ceramics with AlN–RE2O3 (RE = Sc,Y, Lu) additives

The intrinsic microstructure and crystalline phases of porous SiC ceramics with 5 vol% AlN–RE₂O₃ (RE = Sc, Y, Lu) additives were characterized by high-resolution transmission microscopy with energy-dispersive spectroscopy and X-ray diffraction. The homophase (SiC/SiC) and heterophase (SiC/junction) boundaries were found to be clean; that is, amorphous films were not observed in the specimens. In addition, ScN, YN, and LuN were formed as secondary phases. The flexural strength and thermal conductivity of the ceramics were successfully tuned using different additive compositions. The flexural strength of the ceramics improved by a factor of ~3, from 11.7 MPa for the specimen containing Y₂O₃ to 34.2 MPa for that containing Sc₂O₃, owing to the formation of a wide necking area between SiC grains. For the same reason, the thermal conductivity improved by ~56%, from 9.2 W·m^?1·K^?1 for the specimen containing Lu₂O₃ to 14.4 W·m^?1·K^?1 for that containing Sc₂O₃.     

Facile synthesis of up-conversion Cit-NaYF4:Yb,Tm @phenol-formaldehyde resin@Ag composites for the sensitive detection of S2−

In this work, we synthesized the core-shell structure citric acid-modified up-conversion luminescent nanoparticles (Cit-NaYF₄:Yb,Tm) (denoted as UC)@ polymer of phenol-formaldehyde resin (PFR) particles. Ag nanoparticles were successfully loaded onto the PFR's shell by direct reduction of AgNO₃ in situ with rich hydroxyl groups in PFR and a new application of the UC@PFR@Ag composites for detection of S²⁻ was investigated. A good linear relationship between the fluorescence intensity of as-synthesis of UC@PFR@Ag composites and the concentration of S²⁻ could be found within the range of 2–100 nM (R² = 0.9929) with a limit of detection of 0.67 nM. The values of S²⁻ content in actual water samples obtained with the proposed method are much closer to unity as compared to the corresponding values obtained with the UV method.     

Oxidation behaviour of pressureless liquid-phase-sintered α-SiC with additions of 5Al2O3 + 3RE2O3 (RE = La,Nd, Y,Er, Tm,or Yb)

The oxidation behaviour of pressureless liquid-phase-sintered (PLPS) α-SiC was investigated as a function of the sintering additives of 5Al₂O₃ + 3RE₂O₃ (RE = La, Nd, Y, Er, Tm, or Yb) by thermogravimetry experiments in oxygen at 1075–1400 °C for up to 22 h. It was found that the oxidation is in all cases passive and protective, with kinetics governed by the arctan-rate law. This is because the PLPS SiC ceramics develop oxide scales having no cracks or open porosity and accordingly prevent the parent material from direct contact with oxygen. In addition, these oxide scales crystallize gradually during the exposure to the oxidizing atmosphere with the attendant reduction in the amorphous cross-section available for oxygen diffusion. It was also found that the rate-limiting mechanism of the oxidation is outward diffusion of RE³⁺ cations from the intergranular phase into the oxide scale, and that the activation energy of the oxidation increases with increasing size of the RE³⁺ cation. It was also observed that the oxidation of PLPS SiC increases with increasing size of the RE³⁺ cation, an effect that is especially marked for cation sizes above 0.9 Å because the oxidation rate becomes several orders of magnitude faster. This trend is attributable to the oxide scales being more crystalline, and containing crystals that are more refractory and amorphous residual phases that are more viscous as the size of the RE³⁺ cation decreases. Finally, implications for the design of PLPS SiC ceramics with superior oxidation resistance are discussed.     

Preparation of new heteronuclear (NH4)RE[FeII(CN)6]·nH2O complexes (RE = La,Ce, Pr,Nd, Sm,Gd, Dy,Y, Er,Lu) and their low-temperature decomposition for perovskite-type oxide

New heteronuclear (NH₄)RE^III[Fe^II(CN)₆]·nH₂O complexes (RE = La, Ce, Pr, Nd, Sm, Gd, Dy, Y, Er, Lu) were synthesized and their thermal decomposition products were investigated. The crystal structure of (NH₄)RE[Fe^II(CN)₆]·nH₂O would be a hexagonal unit cell (space group: P6₃/m), which was the same as that of La[Fe^III(CN)₆]·5H₂O. The hydration number n = 4 was estimated by TG results for all the RE complexes. The lattice constants depended on the ionic radius of the RE³⁺ ion for the heteronuclear complexes. The single phase of the perovskite type materials was directly obtained by decomposition of the heteronuclear complexes for RE = La, Pr, Nd, Sm, and Gd. A mixture of CeO₂ and Fe₂O₃ was formed for RE = Ce because of its oxidation to Ce⁴⁺. In the case of RE = Dy, Y, Er, and Lu complexes, the perovskite type materials formed at higher temperature via. mixed oxides such as RE₂O₃ and RE₄Fe₅O₁₃ due to the small RE³⁺ ionic radius.     

Sol–gel synthesis,characterization and application of lanthanide-doped cobalt chromites (CoCr2–xLnxO4; Ln = Tm3+ and Yb3+)

Cobalt chromite based pigments CoCr_2–xLn_xO₄ (Ln?=?Tm³⁺ and Yb³⁺) with different substitutional level of lanthanide (x?=?0–0.5) have been synthesized using aqueous sol–gel synthetic approach. The XRD analysis revealed that single phase spinel was obtained only with low content of lanthanide ions (x?=?0.01–0.04). The sol–gel derived powders with higher concentration of lanthanide (x?=?0.05–0.2) contained minor amount of orthochromite phase. At the highest substitutional level (x?≥?0.2) the perovskite phase became the main crystalline phase. The colour of obtained pigments and corresponding ceramic glazes were analogous. Depending on the dominant phase, the colour varied from bluish-green (prevailing spinel phase) to dark brownish green (the main perovskite phase). This study proved that the replacement of chromium ions by thulium and ytterbium was successful at low content of lanthanides influencing the shade of pigment and corresponding glazes.     

Simultaneous accommodation of Dy3+ at the lattice site of β-Ca3(PO4)2 and t-ZrO2 mixtures: Structural stability,mechanical, optical,and magnetic features

Structurally stable β-Ca₃(PO₄)₂/t-ZrO₂ composite mixtures with the aid of Dy³⁺ stabilizer were accomplished at 1500°C. The precursors comprising Ca²⁺, P⁵⁺, Zr⁴⁺, and Dy³⁺ have been varied to obtain five different combinations. The results revealed the fact that complete phase transformation of calcium-deficient apatite to β-Ca₃(PO₄)₂ occurred only at 1300°C, whereas the evidence of t-ZrO₂ crystallization is obvious at 900°C. The dual occupancy of Dy³⁺ at β-Ca₃(PO₄)₂ and t-ZrO₂ structures was evident; however, Dy³⁺ initially prefers to occupy β-Ca₃(PO₄)₂ lattice until its saturation limit and thereafter accommodates at the lattice site of ZrO₂. The typical absorption and emission behavior of Dy³⁺ were noticed in all the systems and, moreover, the surrounding symmetry of Dy³⁺ domains has been determined from the luminescence study. All the systems ensured paramagnetic response that is generally contributed by the presence of Dy³⁺. A gradual increment in the phase content of t-ZrO₂ in the composite mixtures ensured a significant improvement in the hardness and Young's modulus of the investigated compositions.     

Raman Spectra and the Structure of Melts in the Na2O–P2O5–SiO2System

Raman spectra of melts in the Na₂O–P₂O₅–SiO₂system are measured at high temperatures. The differences between the Raman spectra of melts and glasses with identical compositions are considered. It is demonstrated that the structural inhomogeneity of the system slightly increases with a decrease in temperature and vitrification of the melt.     

Electrical,structural and thermal properties of nanoceramic (Bi2O3)1−x−y(Ho2O3)x(Tm2O3)y ternary system

Ho₂O₃ and Tm₂O₃ doped Bi₂O₃ composite electrolyte type materials for solid oxide fuel cells (SOFCs) operating at intermediate-temperature were investigated. The bismuth-based ceramic powders were produced by using conventional solid-state synthesis techniques. The products were characterized by means of scanning electron microscopy (SEM), X-ray powder diffraction (XRD), differential thermal analysis/thermal gravimetry (DTA/TG), and the four-point probe technique (4PPT). XRD and DTA/TG measurements indicate that all of the samples have the stable fluorite type face centered cubic (fcc) δ-phase. 4PPT measurements were performed in the temperature range 150–1000 °C in air and these measurements showed that the electrical conductivity of the samples decrease with increasing amount of Tm₂O₃. This increase in the electrical conductivity of the samples could be attributed to the increase in the numbers of highly polarizable cations and oxide ion vacancies. The highest conductivity value was found as 5.31×10^?1 Ω cm^?1 for the (Bi₂O₃)_1?x?y(Ho₂O₃)_x(Tm₂O₃)_y ternary system (for x=20 and y=5 mol%) at 1000 °C. The activation energies of the samples were calculated from log σ graphics versus 1000/T. These calculated results showed that the translation motion of the charge carriers, oxygen vacancies, and space charge polarizations are responsible for the change in activation energy as a function of temperature.     

Thermal conductivity of single- and multi-phase compositions in the ZrO2–Y2O3–Ta2O5 system

Compositions in the ZrO₂–Y₂O₃–Ta₂O₅ system are of interest as low thermal conductivity, fracture resistant oxides for the next generation thermal barrier coatings (TBC). Multiple phases occur in the system offering the opportunity to compare the thermal properties of single, two-phase, and three-phase materials. Despite rather large variations in compositions almost all the solid solution compounds had rather similar thermal conductivities and, furthermore, exhibited only relatively small variations with temperature up to 1000 °C. These characteristics are attributed to the extensive mass disorder in all the compounds and, in turn, small interfacial Kapitza (thermal) resistances. More complicated behavior, associated with the transformation from the tetragonal to monoclinic phase, occurs on long-term annealing in air of some of the compositions. However, the phases in two of the compositional regions do not change with annealing in air and their thermal conductivities remain unchanged suggesting they may be suitable for further exploration as thermally stable TBC overcoats.     

Effect of ZnO on the structural,physio-mechanical properties and thermal shock resistance of Li2O–Al2O3–SiO2 glass-ceramics

The composition of lithium aluminosilicate (LAS) with different zinc oxide-magnesium oxide (ZnO–MgO) contents that ranged from 0 to 1.45 wt percent (wt%) was investigated to determine the thermal shock resistance properties of the glass-ceramics. The LAS glasses were melted in an alumina crucible at 1550 °C for 5 h, and the green compact samples were then heat-treated at 1100 °C for 3.5 h. The presence of zinc oxide (ZnO) in the compositions did not change the major crystal phase of β-spodumene. However, the addition of ZnO shifted the pronounced peak to a lower angle and increased the percentage of crystallinity from 55% to 59%. Additionally, the function of ZnO in LAS glass-ceramics as the network modifier was confirmed through Fourier Transform Infrared Spectroscopy (FTIR) analysis. The physio-mechanical properties were improved when 1.45 wt% ZnO was added to the LAS glass-ceramics. The results showed increased density (2.42 g/cm³), low porosity (0.85%), high flexural strength (125.23 MPa), and low coefficient of thermal expansion (25–800 °C) (CTE_{(25–800 °C)}) value of 1.73 × 10^?6 °C^?1. Meanwhile, the thermal shock resistance properties evaluation of the LAS glass-ceramics at different ZnO contents were conducted at different thermal shock temperatures of 200 °C, 500 °C, and 800 °C. The critical temperature of the LAS specimens with 1.45 wt% ZnO demonstrated the ability to withstand a thermal shock at 800 °C while preserving 87% of their initial strength of 108.40 MPa, exemplifying the best LAS glass-ceramics properties for rapid high-temperature change applications.     

Effect of Ta2O5 addition on the structure,crystallization mechanism,and properties of CaO–B2O3–SiO2 glasses for LTCC applications

CaO–B₂O₃–SiO₂–Ta₂O₅ (CBST) glass-ceramics, with different Ta₂O₅ content, (up to 6 mol%), have been prepared by using glass melt quenching followed by heat treatment between 800 and 880 °C. The Fourier Transform Infrared (FTIR) results showed that the stronger the attraction of Ta⁵⁺ to the oxygens in the BO₃^3? and SiO₃^2? structures, the more easily the B–O and Si–O bonds will be destroyed. The underlying reason is most probably the high field strength of Ta⁵⁺, which results in a weakening of the vibration intensities of the [BO₃] and [SiO₄] units. Moreover, the Differential Scanning Calorimetry (DSC) results showed that the softening point (T_g), crystallization starting temperature (T_c1), and exothermic crystallization peak temperature (T_p1), of the CaSiO₃ phase, shifted to higher values with the addition of Ta₂O₅. Also, the crystallization activation energy (E_a) and the glass stability factor (ΔT) of the CaSiO₃ phase increased, which indicated that the CaSiO₃ phase of the glass became inhibited by the addition of Ta₂O_5. It was, thus, obvious that there was a need of glass characterization. The results of the crystallization kinetics showed that the critical cooling rate decreased with the addition of Ta₂O₅, which indicated that the viscosity of the system had increased. The CBST glass-ceramics, containing 1 mol% Ta₂O_5, that were sintered at 875 °C for 15 min showed excellent dielectric properties: ε_r = 6.22 and tanδ = 1.19 × 10^?3 (1 MHz). To sum up, CaO–B₂O₃–SiO₂–Ta₂O₅ glass-ceramics are potential low temperature co-fired ceramic substrate materials.     

Failure mechanisms of (Gd0.9Yb0.1)2Zr2O7/Yb2SiO5/Si thermal/environmental barrier coatings during thermal exposure at 1300 °C/1400 °C

A novel tri-layer (Gd_0.9Yb_0.1)₂Zr₂O₇/Yb₂SiO₅/Si (GYbZ/YbMS/Si) thermal and environmental barrier coatings (TEBCs) was first proposed for protecting SiC-based ceramic matrix composites (CMCs). Wherein, the GYbZ layer by plasma spray physical vapor deposition (PS-PVD) was quasi-columnar structured while the YbMS and the Si layers by atmospheric plasma spray (APS) were lamellar structured. The oxidation behavior and the failure mechanisms of the GYbZ/YbMS/Si TEBCs at 1300 °C/1400 °C are revealed. At 1300 °C, the mud-cracks penetrated through the GYbZ/YbMS layer and transversely deflected in the Si layer are responsible for the oxidation at YbMS/Si interface. When the temperature increased to 1400 °C, the propagation of mud-cracks, cavities, and TGO channel cracks occurred due to the sintering of GYbZ and the fast growth of cristobalite. Eventually, these defects caused delaminating failure at interface. Moreover, another de-bonding failure of the coating was observed resulting from the significant thickening of oxide scale at the edge region.