Phase evolution and chemical stability of Nd-doped Y3Fe5O12 waste forms synthesized in molten salt at a low temperature

Molten salt provides a fast mass transfer and nucleation process, so the ceramic solid solution for immobilization of high-level nuclear waste (HLW) can be synthesized at lower temperatures. The chemical stability in the process of interaction with groundwater determines the ability of the matrix phase to prevent radionuclides from entering the biosphere. Nd-doped Y₃Fe₅O₁₂ ceramics were prepared by the molten salt method at different sintering temperatures (1000℃, 1100℃, 1200℃) and different m (eutectic salt): m (oxide) ratios (2:1, 3:1, 4:1). The sintered ceramic is Y_3-xNd_xFe₅O₁₂, where x is the solubility of Nd in YIG. The results show that the optimum mass ratio of molten salt to oxide is 3:1. The solubility of Nd in garnet is 33.3 mol% (x = 1.1) and 56.7 mol% (x = 1.6) at 1100℃ and 1200℃, respectively. In the neutral medium, Nd does not transfer to the liquid phase. Acid leaching promotes the strong dissolution of the garnet matrix. In this case, the leaching rate of Nd from ceramics to the liquid phase is two to three orders of magnitude higher than that in a neutral medium. The experimental results suggest that garnet matrices can reliably immobilize actinides in subsurface repositories.     

Phase equilibrium in binary La2O3-Dy2O3 and ternary CeO2-La2O3-Dy2O3 systems

Cerium oxide doped with oxides of rare earth elements is a multifunctional material, a wide range of uses which is associated with its unique physicochemical properties. Phase diagrams of multicomponent systems are the physicochemical basis for the creation of new materials with improved characteristics.In this work, phase equilibria in ternary CeO₂–La₂O₃–Dy₂O₃ and binary La₂O₃–Dy₂O₃ systems in the whole concentration range were studied. No new phases have been identified in these systems. An isothermal section of the phase diagram of the CeO₂–La₂O₃–Dy₂O₃ system at a temperature of 1500 °С is constructed. No new phases have been detected in the system. It was found that in the studied ternary system solid solutions are formed on the basis of (F) modification of CeO₂ with structure of fluorite type, monoclinic (B), cubic (C) and hexagonal (A) modifications of Ln₂O₃.In the La₂O₃–Dy₂O₃ binary system (1500–1100 °С) three types of solid solutions are formed: based on hexagonal modification A-La₂O₃, monoclinic modification B-Dy₂O₃ and cubic modification C-Dy₂O₃ separated by two-phase fields (A+B) and (B+C), respectively. The boundaries of the regions of homogeneity of solid solutions based on A-La₂O₃ are determined by compositions containing 35–40, 20–25, 15–20 mol% Dy₂O₃ at 1500, 1250, 1100 °C, respectively. From the obtained data it follows that the solubility of Dy₂O₃ in the hexagonal modification of lanthanum oxide is 39 mol% at 1500 °C, 23 mol. % at 1250 °C and 16 mol% at 1100 °C. The limits of existence of solid solutions based on monoclinic B-modification are determined by compositions containing 30–35, 65–60 (1250 °С), 35–40, 55–60 (1100 °С) 40–45, 70–75 (1500 °C) mol% Dy₂O₃.In the studied system, with a decrease in temperature from 1500° to 1100°C, there is a decrease in the solubility of La₂O₃ in the crystal lattice of cubic solid solutions of C-type from 16 to 10 mol%.     

Experimental Phase Diagram Determination and Thermodynamic Assessment of the CeO2‐Gd2O3‐CoO System

New phase diagram data and a thermodynamic assessment of the CeO‐Gd₂O₃‐CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950°C–1050°C) of CeO₂‐based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100°C and 1300°C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300°C the Co solubility in the ternary compounds Ce_1?x?yGd_xCo_yO_2?x/2?y (fluorite) is 2.7 mol% and is less than 1 mol% in the Gd_2?xCe_xO_3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO_3?δ was found to be 1 mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2 mol% Co and 41.1 mol% Gd melting at an onset temperature of 1303 ± 5°C, which is close to the binary eutectic in the Gd₂O₃‐CoO system at 60 ± 2 mol% Co and 1348 ± 1°C.     

High‐Pressure Investigation in the System SiO2–GeO2: Mutual Solubility of Si and Ge in Quartz,Coesite and Rutile Phases

Mutual solubilities in crystalline phases in the system SiO₂–GeO₂ have been investigated up to 10 GPa pressure and 1500°C temperature, using a bulk composition of 50 mol% GeO₂. Solid solution of up to 40 mol% GeO₂ into the mineral quartz has been confirmed as well as solubility of Si into GeO₂ rutile (argutite) and Ge into SiO₂ rutile (stishovite) phases and limited Ge into coesite. Solubility of Ge in quartz is very high, and decreases with pressure, with the univariant quartz‐out reaction occurring near 3.4 GPa at 1200°C. The solubility of GeO₂ in coesite is highest at 3.4 GPa (about 8 mol%) and decreases with increasing pressure. Significantly more extensive solubility than previously reported for the rutile phases has been found and measured in detail as a function of pressure and temperature. Extensive solubility of SiO₂ in GeO₂ is found in argutite at 1200°C, increasing strongly with pressure and reaching a maximum of 25.2 mol% SiO₂ in GeO₂ at 9 GPa. At this point coesite (ss) plus argutite (ss) react to form a stishovite phase with 18 mol% GeO₂, and the mutual solubility in both phases decreases above this pressure. At 1500°C, similar solubilities are observed but the maximum SiO₂ solubility in argutite of just over 25 mol% occurs near 10 GPa. All these solid solutions can be recovered to ambient temperature and pressure. Phase diagrams and unit cell information of the phases are presented here. Based on these results, a useful and industrially relevant, application for accurately measuring high pressure is suggested.     

Effect of In, Ce and Bi dopings on sintering and dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

In, Ce and Bi doped Ba(Zn_1/3Nb_2/3)O₃ (BZN) ceramics were prepared by conventional mixed oxide technique. In doping between 0.2 and 4.0 mol% increased the density of BZN at 1300 °C, Ce doping caused a decrease in density at 1250 °C. Levels of Bi₂O₃ up to 1.0 mol% had negative effect on densification, while high level doping could significantly improve the densification of the specimens. XRD of the samples indicated that In, Ce and Bi doping resulted in single phase formation at all concentrations, except 0.5 mol% Bi. SEM of Bi doped BZN indicated only single phase structure and Ce doping even at 0.2 mol% gave some secondary phases. In and Ce doping increased the dielectric constant from 41 to around 66 at 1 MHz. Bi doping decreased the dielectric constant to about 37 at 0.2 mol%, and then higher doping led to dielectric constant to increase to about 63.     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.     

The microstructure,formation mechanism and sintering characteristics of Al2O3/ZrO2 supersaturated solid solution powders

In this study, the Al₂O₃/ZrO₂ supersaturated solid solution powders with different ZrO₂ contents were successfully synthesized by a novel combustion synthesis combined with water cooling (CS-WC) method. The solid solubility and formation mechanism of solid solution under the extremely non-equilibrium solidification condition were discussed in details. The ultra-high cooling rate greatly improves the solubility limit of Al₂O₃ in ZrO₂. When ZrO₂ content is 30 mol%, the Al₂O₃ has been almost dissolved into the ZrO₂ lattice. The formation mechanism of solid solution can be attributed to solute interception caused by the huge degree of supercooling. During the sintering process, the solid solution powders precipitate ZrO₂ particles and the Al₂O₃ matrix, which forms a fine and uniform nanostructure. Due to the synergistic effect of t-m phase transformation toughening and ZrO₂ nanoparticles toughening, the Al₂O₃/ZrO₂ nanoceramics exhibit excellent mechanical properties when ZrO₂ contents are at the range of 25–37 mol%.     

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.     

Synthesis of CeO2-ZrO2 solid solution by glycothermal method and its oxygen release capacity

The glycothermal (GT) reaction of Ce acetate and Zr alkoxide directly yielded CeO₂-ZrO₂ solid solutions in a region of low Ce content ≤40 mol%. Of the CeO₂-ZrO₂ solid solutions obtained by the GT method and subsequent calcination at 500 or 800 °C, the sample with 20 mol% Ce content had the largest BET surface area. This sample exhibited the highest Ce-based oxygen release capacity in the whole Ce/Zr composition range. The oxygen release capacities of CeO₂-ZrO₂ solid solutions synthesized by the GT method were much larger than those of the samples prepared by a coprecipitation (CP) method. The Reitveld analysis and the repetitive reduction-oxidation experiment indicated that the CeO₂-ZrO₂ solid solution synthesized by the GT method has a homogeneous structure as compared with that prepared by the CP method.     

Particle refinement of ZrB2 by the combination of borothermal reduction and solid solution

Flexible synthesis of ultra‐fine ZrB₂ powders was achieved by borothermal reduction in a mixture of ZrO₂, boron, and TiO₂. Without TiO₂ additive, coarse ZrB₂ powders with particle size of 0.81 μm were obtained, presumably due to good wettability and solubility of ZrB₂ in the byproduct B₂O₃. It was found that the particle growth of ZrB₂ was effectively inhibited by the solid solution of TiB₂ (≥1 mol%). The refinement mechanism was that the solid solution of in situ formed TiB₂ presumably lowered the wettability and solubility of ZrB₂ in the B₂O₃ liquid and significantly inhibited the coarsening of ZrB₂. The average particle size of resulting powders decreased to 0.37 μm with the addition of 10 mol% TiO₂.     

Phase formation and lattice distortion of Lu2O3-doped ZrO2 in comparison with Y2O3-doped ZrO2

Lu₂O₃ and Y₂O₃ doping of 8, 11, and 18 mol% in ZrO₂ were prepared by solid solution reaction, aiming to study the phase stabilization of Lu₂O₃-doped ZrO₂ and Y₂O₃-doped ZrO₂ in terms of phase formation and lattice distortion. The Rietveld refinement results indicated that Lu₂O₃-doped ZrO₂ and Y₂O₃-doped ZrO₂ followed the same trend in terms of cubic phase fraction, increasing from 25%–30% (8 mol%) to 95%–100% (11 and 18 mol%). This phase formation was confirmed by observing the same diffraction ring pattern observed for the Lu₂O₃-doped ZrO₂ and Y₂O₃-doped ZrO₂. The Vickers hardness of the Lu₂O₃-doped ZrO₂ was 4.3% higher than that of Y₂O₃-doped ZrO₂ at 8 mol%, but 9.7% and 14.8% lower at 11 and 18 mol%, respectively. This was likely caused by the lattice distortion effect of Y₂O₃ doping overpowering the field strength difference between Lu³⁺ and Y³⁺.     

Phase stability and thermophysical properties of CeO2-Re2O3 (ReEu,Gd, Dy,Y, Er,Yb) co-stabilised zirconia

A series of 16 mol% CeO₂-2 mol% Re₂O₃ co-stabilised zirconia (ZrO₂) (16Ce4ReSZ, ReEu, Gd, Dy, Y, Er, Yb) ceramic materials were synthesised using a chemical coprecipitation– high-temperature roasting method. Their phase structure, high-temperature phase stability, mechanical properties, thermal conductivity and coefficient of thermal expansion (CTE) were investigated. The results show that the ZrO₂ tetragonal phase co-stabilised by CeO₂ and Re³⁺ with a smaller radius has better stability. The 16Ce4ReSZ (ReDy, Y, Er, Yb) materials have high fracture toughnesses, low thermal conductivities, and high CTE values. As the radius of the Re³⁺ ions decreases, the lattice energy increased, while the lattice distortion decreases, the CTE decreases slightly and the thermal conductivity of the material increases slightly. Owing to the high phase stability of 16Ce4YbSZ, its mechanical properties are best after 100 h of sintering at 1400 °C.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.     

Erbium activating effect on the photoluminescence,morphology, and structure studies of nano-composite Phospho-silicate SiO2–P2O5

Smart nano-composites Phospho-silicate (SiO₂–P₂O₅) prepared in monolith form containing 16 mol% P₂O₅, and doped with different mol% of Er³⁺ ions between 0.5 and 5 mol%; the composite has been synthesized by Sol-Gel technique, and subsequently annealed at 850 °C.X-ray diffraction patterns show the structural properties of the mentioned prepared samples, giving rise to the crystallite sizes to increase in a range between 18, and 20.8 nm as the molar percent of the Er³⁺ ion increase from 0% to 5%. The morphology, and surface morphology of the prepared samples were characterized using TEM, and FESEM, respectively. The Raman analyses show that the active Raman bands are corresponding to silicate, and phosphate. These bands were enveloped by the strong asymmetric vibrations of Er₂O₃ at 420, and 840 cm⁻¹, their Bose-Einstein corrected intensities increased gradually by increasing the Er³⁺ ions concentration in the regions from 3400 up to 3450 cm⁻¹, and 3550 up to 3700 cm⁻¹. The optical studies show that the refractive index increased by increasing Er³⁺ ions concentration, from 1.7 up to 1.8 for as the concentration of Er³⁺ ions increase. The photoluminescence are exhibiting an emission with splitting at 1545, and 1555 nm, which is related to the intra 4F transition. It has been found that the optimal doping content of Er is 1 mol% then, after quenching caused by OH groups in Er³⁺ ions doped Phospho-silicate at higher concentrations. It is obviously that 1 mol% of Erbium ions is a suitable candidate for photonic applications such as Laser waveguide, and optical amplifier.     

Phase stability and mechanical properties of TZP with a low mixed Nd2O3/Y2O3 stabiliser content

The phase assembly of 1.0–5.0 mol% Nd₂O₃-doped ZrO₂ sintered at 1400 °C revealed that the tetragonal ZrO₂ phase could not be completely stabilised. Co-stabilising of 0.5–2.5 mol% Nd₂O₃ with 0.5–1.0 mol% Y₂O₃, however, allowed the preparation of fully dense (Nd,Y)-TZP ceramics by pressureless sintering in air at 1450 °C. The mixed stabiliser monoclinic zirconia nanopowder starting material was synthesized from a suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder in an alcohol/water mixture. A HV₃₀ hardness of 10 GPa combined with an excellent indentation toughness of 13 MPa m^1/2 could be achieved for the (1.0Nd,1.0Y)- and (1.5Nd,1.0Y)-TZP ceramics. The influence of the mixed stabiliser content on the phase stability and mechanical properties are investigated and discussed.     

Crystal structure and electrical properties of (Li,Ce, Nd)-multidoped CaBi2Nb2O9 high temperature ceramics

(Li, Ce, and Nd)-multidoped CaBi₂Nb₂O₉ (CBN) Aurivillius phase ceramics were prepared via a conventional solid-state sintering route. The crystal structure including bond lengths and bond angles, microstructure, dielectric constant, DC resistivity, and piezoelectric properties were systematically investigated. Rietveld-refinements of X-ray results indicated that small quantity of (Li, Ce, Nd) doping (< 2.5 mol%) increases orthorhombic distortion, because of the smaller ionic radii of doping ions. However, orthorhombic distortion obviously decreased with increasing (Li, Ce, Nd) doping concentration from 5 to 25 mol%. The replacement of asymmetric A-site Bi³⁺ with 6s2 lone pair electrons by symmetric Li⁺, Ce³⁺ and Nd³⁺ ions decreased the orthorhombic distortion. The morphologies and electrical properties of sintered ceramics were tailored by the introducing (Li, Ce, Nd) multi-dopants. The improvement of piezoelectric properties of modified-CBN ceramics were attributed to decreasing grain sizes and morphotropic phase boundary (MPB). Ca_0.85(Li_0.5Ce_0.25Nd_0.25)_0.15Bi₂Nb₂O₉ (CBNLCN-15) ceramics had optimum properties, and d₃₃ and T_c values were found to be ~ 13.1 pC/N and ~ 900 °C, respectively.     

CeO2 nano-precipitation in borosilicate glasses: A redox study using EELS

Precipitation of nanoscale phases in a series of five related alkali-borosilicate glasses is studied by scanning transmission electron microscopy and electron energy loss spectroscopy (EELS). All glasses studied are doped with 4 mol% of CeO₂, and contain varying concentrations of other oxides to participate in possible redox interactions, including Fe₂O₃, Nd₂O₃, AgO, and Cr₂O₃. The particular precipitates found vary from droplet shaped amorphous to dendritic shaped single crystals. The oxidation state of Ce and the existence and morphology of Ce precipitates is found to depend on both cooling sequence and the presence of other oxides. One aim of the study is to explore and showcase the applicability of high spatial resolution fine structure EELS for the study of the interrelated phenomena of redox interaction, phase separation, solubility, and nanoscale crystallization.     

Influence of the dual charge compensator on solid solution of the air-sintered Ca1-xCexZrTi2-2xFexCrxO7 zirconolite

In this study, a dual charge-compensator formulation was developed for zirconolite with the nominal composition Ca_1-xCe_xZrTi_2-2xFe_xCr_xO₇ (x = 0–0.30). The design strategy here was such that trivalent Fe and Cr were both targeted to substitute across the Ti site(s) to charge balance an inventory of CeO₂ included as a structural analogue for Pu. The targeted solid solution was prepared by sintering constituent oxides at 1400 °C for 10 h under an air atmosphere. By means of powder XRD refinement and selected area electron diffraction analysis, the dominant zirconolite polytype was confirmed to be 2M across the solid solution. The obtained product density was significantly increased when compared to the previously discussed Ca_1-xCe_xZrTi_2-2xCr_2xO₇ system, suggesting that the partial inclusion of Fe in a 1:1 molar ratio with Cr may improve sintering behaviour. The limit of solid solution was reached at approximately x = 0.30, for which the segregation of CeO₂ and Cr₂O₃ phases was clearly evidenced. An evaluation of obtained chemical compositions and bulk oxidation states was performed to inform the solid solution mechanism of Ce, Cr and Fe within zirconolite.     

Significant Enhancement of the Dielectric Constant through the Doping of CeO2 into HfO2 by Atomic Layer Deposition

Films of CeO₂ were deposited by atomic layer deposition (ALD) using a Ce(mmp)₄ [mmp = 1‐methoxy‐2‐methyl‐2‐propanolate] precursor and H₂O reactant. The growth characteristics and film properties of ALD CeO₂ were investigated. The ALD CeO₂ process produced highly pure, stoichiometric films with polycrystalline cubic phases. Using the ALD CeO₂ process, the effects of Ce doping into an HfO₂ gate dielectric were systematically investigated. Regardless of Ce/(Ce + Hf) composition, all ALD Ce_xHf_1?xO₂ films exhibited constant growth rates of approximately 1.3 Å/cycle, which is essentially identical to the ALD HfO₂ growth rates. After high‐temperature vacuum annealing at 900°C, it was verified, based on X‐ray diffraction and high‐resolution cross‐sectional transmission electron microscopy results, that all samples with various Ce/(Ce + Hf) compositions were transformed from nanocrystalline to stabilized cubic or tetragonal HfO₂ phases. In addition, the dielectric constant of the Ce_xHf_1?xO₂ films significantly increased, depending on the Ce doping content. The maximum dielectric constant value was found to be nearly 39 for the Ce/(Ce + Hf) concentration of ~11%.     

Effect of Ce dopant on upconversion and temperature sensing performances in homogeneous ultrasmall Y2O3:Yb3+/Ho3+ nanoparticles through flame aerosol synthesis

Flame aerosol synthesis (FAS) is an excellent strategy for continuous, fast, and mass production of small-size upconversion nanoparticles (UCNPs), which have high potential applications in fields like biological imaging, colour display and optical temperature sensing. However, flame-made UCNPs have received less attention, and relevant studies are limited. Herein, for the first time, we successfully fabricated cerium (Ce)-doped homogeneous ultrasmall Y₂O₃:Yb³⁺/Ho³⁺ UCNPs using a liquid-fed FAS method. Ce was doped to improve the upconversion luminescence (UCL) of the Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. The overall UCL intensity was enhanced ～77.9-fold for an optimal concentration of 20 mol% Ce-doped UCNPs, compared with the UCNPs without Ce doping with a relatively homogeneous ultrasmall size of 8–10 nm. Further studies confirmed that both trivalent (Ce³⁺) and tetravalent (Ce⁴⁺) simultaneously exist in the Y₂O₃ hosts and are critical in enhancing the UCL properties. In addition, the fluorescence intensity ratio (FIR) method was used to evaluate the thermal properties of the fabricated UCNPs. Ce doping significantly improved the thermal sensitivity of Y₂O₃:Yb³⁺/Ho³⁺ UCNPs. An excellent relative sensitivity (S_R) of 0.622% K^?1 at 598 K was obtained for flame-made UCNPs doped with 20 mol% Ce.