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1.
Rare-earth zirconates with a pyrochlore structure have been developed for potential application in thermal barrier coating systems to further improve the performance and durability of gas turbines. The Sm 2Zr 2O 7 (abbreviated as SZ) powder was synthesized by solid state reaction and then deposited by air plasma spraying. The phase stability, microstructure and thermal conductivity of SZ and 8 wt% Y 2O 3 stabilized zirconia (8YSZ) coatings were investigated. The X-ray diffraction results indicated that the crystal structure of the as-sprayed SZ coatings was defect-fluorite, and after heat treating at 1200 °C for 50 h, it started to transform to pyrochlore, and the content of pyrochlore increased with increase in temperature of the heat treatment. The thermal conductivities of SZ coatings were significantly lower than those of 8YSZ coatings before and after heat treatments, which increased considerably after heat treatments compared to the as-sprayed states for both coatings due to sintering effects. 相似文献
2.
Pyrochlore glass-ceramics (GCs) have been investigated with samples fabricated via both sintering and hot isostatic pressing (HIPing) of a mixed oxide precursor. It has been demonstrated that sintering at 1200°C in air is necessary to obtain well-crystallized pyrochlore crystals in a sodium aluminoborosilicate glass through a one-step controlled cooling. The crystallization, structure, and microstructure of Eu 2Ti 2O 7 pyrochlore as the major phases in residual glass were confirmed with X-ray diffraction (XRD), scanning electron microscopy-energy dispersive spectroscopy, transmission electron microscopy, and Raman spectroscopy. The structures of major Eu 2Ti 2O 7 pyrochlore and minor [Eu 4.67O(SiO 4) 3] apatite in both sintered and HIPed samples were refined using synchrotron XRD data. While the processing atmosphere did not appear to affect the cell parameter of the main pyrochlore phase, very small volume expansion (~0.3%) was observed for the minor apatite phase in the HIPed sample. In addition, static leaching of the HIPed sample confirmed that pyrochlore GCs are chemically durable. Overall, pyrochlore GCs prepared via both sintering and HIPing with the Eu partitioning factor of ~23 between ceramics and the residual glass are suitable waste forms for minor actinides with processing chemicals. 相似文献
3.
Multicomponent pyrochlore solid solutions with and without uranium incorporation were fabricated and their thermal-mechanical properties were characterized. Multicomponent pyrochlore solid solutions without uranium exhibit comparable thermal conductivity and higher mechanical strength compared to baseline single component rare-earth titanate pyrochlore (A 2Ti 2O 7). Uranium incorporation reduces hardness as compared with single component compositions. High entropy pyrochlore with uranium displays the highest thermal conductivity within multicomponent pyrochlore solid solutions with significantly better mechanical properties than UO 2. The measured thermal conductivity correlates well with A-site cation mixing entropy and a modified size disorder parameter, and thus the size disorder and mixing entropy could be good indicators for predicting thermal conductivity of multicomponent pyrochlore solid solutions. This work opens up the possibility of designing multicomponent oxide solid solutions by controlling their chemical disorder/mixing entropy to achieve acceptable thermal-mechanical properties, desired radiation and corrosion performance for potential nuclear waste form and inert matrix fuel applications. 相似文献
4.
In this work, borosilicate based glass-ceramics with pyrochlore as crystalline phase for immobilization of high-level nuclear wastes (HLWs) were successfully synthesized by a one-step heat-treatment method. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) demonstrate that the obtained glass-ceramics show a regularly and uniformly distributed single pyrochlore (Ca,Na)(Nb,Ti) 2Nd 0.67O 6F crystalline phase. Moreover, the glass-ceramics prepared show LRNa, LRB, LRAl, LRSi, LRNd, LRTi and LRNb of about 6.8 × 10 −3, 3.7 × 10 −4, 1.5 × 10 −2, 2.2 × 10 −3, 3.0 × 10 −5, 5.1 × 10 −5 and 5.5 × 10 −6 g m −2 d −1 respectively after 28 leaching days, which are comparable to stable glass-ceramics for HLW immobilization. The results of this study are expected to provide an experimental reference for the engineering synthesis of glass-ceramics for the immobilization of certain HLWs. 相似文献
5.
AbstractIn magnetron sputtering of SrBi 2Ta 2O 9 (SBT) thin films, bismuth deficiency tends to result in formation of an unwanted pyrochlore phase. In this paper, the dependence of deposition parameters on phase evolution in Sr deficient SBT thin films is studied. Only when bismuth is over stoichiometric (Sr 0.74Bi 2.2Ta 2O 9+x) can one obtain pure SBT structure. Decreasing the Bi content encourages formation of the Bi deficient pyrochlore phase. At 1.04 of Bi, only the pyrochlore phase forms. The pyrochlore phase is very stable even at 900°C. 相似文献
6.
In this study, transparent LaErZr 2O 7 ceramic with high excess La and Er contents (nominally La 1.28Er 1.28Zr 2O 7.84) was successfully prepared by vacuum sintering at 1850?°C for 6?h using nanosized powder. The XRD, SEM, EDX and TEM results reveal that the single pyrochlore phase in the powder sample transforms to the coexistence of La-rich pyrochlore phase and Er-rich defect fluorite phase after high temperature sintering. The high excess amounts of La and Er favor the formation of pyrochlore structure. Despite the coexistence of two phases, the sample with 1?mm thickness shows excellent in-line transmittance in the visible to mid-infrared region (as high as 81% at 1100?nm). The upconversion and infrared emission under 980?nm exciting were measured and discussed as well. 相似文献
7.
Ln 2Ti 2O 7 (Ln=Y and Gd) pyrochlore glass‐ceramics have been fabricated successfully via internal crystallization. Subsequently, the phase evolution from Ln 2Ti 2O 7 pyrochlores to Ln 0.5U 0.5Ti 2O 6 brannerites in glass with uranium (U) substitutions on the Ln‐site of Ln 2Ti 2O 7 has been investigated using X‐ray diffraction, scanning electron microscope‐electron dispersive spectroscopy, transmission electron microscopy, Raman and diffuse reflectance spectroscopy. Combined characterization by XRD, SEM‐EDS and TEM SAED confirms the structures and phase evolution while Raman spectroscopy reveals characteristic vibration modes for both pyrochlore and brannerite. In addition, DRS of the U 5+ ion has been used to probe the phase evolution, with the corresponding f‐ f transition band of 2F7/2 energy level significantly shifting to longer wavenumbers due to the local coordination environment changing from eightfold coordination in pyrochlore to sixfold coordination in brannerite. 相似文献
8.
Ceramics of the perovskite multiferroics PbFe 0.5Nb 0.5O 3 (PFN) and PbFe 0.5Ta 0.5O 3 (PFT) were synthesized from new citrate polymeric precursors. X-ray tests pointed to trace amounts of the pyrochlore phase. SEM studies revealed the heterogeneous grain size distribution for PFN and the homogeneous one for PFT. Dielectric studies pointed to one diffuse T-C phase transition at 378 K for PFN and two diffuse M-T and T-C phase transitions, at 200 and 235 K, for PFT, respectively. X-ray photoelectron spectroscopy studies of PFN reveal that all ions exist in one valence state, however, with two chemical shifts for Pb 2+. Two valence states for the majority of ions of PFT seem to be connected with a higher volume fraction of the amorphous grain-boundary phase. The electronic energy band gap for both compounds is approximately 2.8 eV. Two magnetic transitions, ie, from the paramagnetic to the antiferromagnetic phase and then to the spin-glass phase, were observed at 156 and 10 K for PFN, and at 145 and 15 K for PFT, respectively. 相似文献
9.
Rare-earth (RE) doped glass-ceramics represent very interesting luminescent materials. The thermal annealing of the glass precursor causes the controlled precipitation of several crystalline phases, in which RE may be variously distributed, also with different oxidation states, e.g. Eu 2+ and Eu 3+. The present investigation demonstrates the feasibility of preparation of RE-doped alumino-boro-silicate glass-ceramics by direct firing in air (at 1000–1200 °C) of a preceramic polymer, filled with nano- and micro-sized particles, as an alternative to glass melting and annealing. In particular BaCO 3 or SrCO 3 micro-particles, mixed with nano-sized γ-Al 2O 3, were found to react with amorphous silica, available from the oxidative decomposition of a commercial silicone, yielding a strontium or a barium alumino-silicate phase. Boric acid micro-particles contributed both to the development of a liquid phase upon firing (promoting ionic interdiffusion) and to the formation of a La-borate phase, by interaction with La 2O 3 micro-particles. The blue and red luminescence of the obtained glass-ceramics is attributed to the incorporation of Eu 2+ and Eu 3+ ions, from nano-sized Eu 2O 3 filler, in alumino-silicate and borate phases, respectively. 相似文献
10.
The compositional limits of a previously reported (J. Am. Ceram. Soc., 61, 5-8. (1978)) but relatively unstudied sodium-bismuth titanate pyrochlore solid solution are revised and their electrical properties presented. The pyrochlore solid solution we report forms via a different mechanism to that originally reported and occurs in a different location within the Na 2O-Bi 2O 3-TiO 2 ternary system. In both cases, relatively large amounts of vacancies are required on the A-sites and on the oxygen sites, similar to that reported for undoped ‘Bi 2Ti 2O 7’ pyrochlore. In contrast to ‘Bi 2Ti 2O 7’, this ternary pyrochlore solid solution can be prepared and ceramics sintered using conventional solid-state methods; however, the processing requires several challenges to be overcome to obtain dense ceramics. This cubic pyrochlore series has low electrical conductivity (and does not exhibit any evidence of oxide-ion conduction) and exhibits relaxor ferroelectric behavior with a broad permittivity maximum of ~100 near room temperature. Variable temperature neutron diffraction data do not provide any conclusive evidence for a phase transition in the pyrochlore solid solution between ~4 and 873 K. 相似文献
11.
Because of the low content of TiO 2, the utilization of low and medium Ti-bearing blast furnace slag is a difficult problem. In this study, glass-ceramics with diopside as the main crystalline phase was made by using TiO 2 in the blast furnace slag as the nucleating agent and adding additional elements of TiO 2, Fe 2O 3 and Cr 2O 3. The effects of these three nucleating agents on the phase composition and structural properties of the prepared glass-ceramics were evaluated by DSC, XRD, and SEM to determine their optimal dosage. Research results show that the suitable mass percentages of the TiO 2, Fe 2O 3 and Cr 2O 3 crystal nucleation agents of the glass-ceramics are 7.69%, 0.96%, and 2.88%, respectively. The prepared glass-ceramics have excellent physical and chemical properties, e.g., a bending strength of 114.74 MPa, a bulk density of 2.77 g/cm 3, a water absorption rate of 0, and a mass-loss rate of only 0.085%. 相似文献
12.
The effects of different kinds of nucleating agents on crystallization, microstructure and performances of Magnesium Aluminosilicate (MgO-Al 2O 3-SiO 2, MAS) glass-ceramics which were fabricated by melting method in this study. Also, this paper systematically investigated the mechanism of glass stability, crystallization kinetics and element distribution of MAS glass-ceramics. Herein, we used three kinds of nucleating agents, which was TiO 2, ZrO 2 and composite nucleating agent (TiO 2/ZrO 2). The results showed after the doping of nucleating agent, the content of α-cordierite was increased, the stability and crystallization kinetics of glass was changed, the precipitated crystal phase was finer and more compact. Wherein, the sample with composite nucleating agents (TiO 2, ZrO 2) has the best performance due to the highest contents of α - cordierite, uniform distribution of elements without agglomeration in the crystal phase and the most compact structure, whose Vickers hardness and bending strength can reach 9.70 GPa and 312 MPa, respectively. 相似文献
13.
Generally, glass-ceramics have superior properties compared to their parent glasses. Here, we prepared a novel Nd 3+-doped MgO-Al 2O 3-SiO 2-based transparent glass-ceramics with excellent fluorescence properties. The effects of Nd 2O 3 content on the structure and properties of glass-ceramics were studied, aiming to provide a key guidance for preparing this transparent glass-ceramics. The results revealed that the glass stability increased originally and then decreased with increasing Nd 2O 3 content, so did the variation of wavenumbers in infrared spectra. And these glass-ceramics are mainly composed of cordierite with residual glassy phase. The three phenomenological intensity parameters (Ω 2,4,6) and radiative properties were estimated by Judd-Ofelt theory, and the values of Ω 2 first decreased and then increased with increasing Nd 2O 3 content. Three main emission peaks ascribed to the transitions from 4F 3/2 to 4I 9/2, 4I 11/2, 4I 13/2 at 898, 1057, 1330 nm were observed, respectively. The branching ratios for 4F 3/2→ 4I 11/2 transition increased as the Nd 2O 3 content raised, and the fluorescence lifetimes of the 4F 3/2 level were found to increase first and then decrease with Nd 2O 3 content (from 181 to 726 μs). The excellent fluorescence properties indicate that this novel glass-ceramics can be used as a potential solid-state optical functional material for 1.06 μm laser emission. 相似文献
14.
The objective of this study was to evaluate the effect of ZnO content on the physical, mechanical and chemical properties of CaO–Al 2O 3–SiO 2 (CAS) glass-ceramics produced from Colombian wastes, such as fly ash, granulated blast furnace slag and glass cullet. The CaO/SiO 2 molar ratio of the mixtures was held constant (0.36). ZnO was added to the mixtures in proportions of 4, 7 and 10 wt%. The glass-ceramics were produced by the controlled crystallization of a parent glass. The values of crystallization temperature (T p) show a fall up to 7 wt% and then shoots up with 10 wt% concentration of ZnO, but in general, ZnO addition lowers the temperature required for the formation of crystalline phases. In general, anorthite (CaAl 2Si 2O 8) is the main phase observed in all heat treated samples, in addition to albite (Na(AlSi 3O 8)) and labradorite (Na 0.45 Ca 0.55 Al 1.55 Si 2.45 O 8). The crystalline phases hardystonite (Ca 2ZnSi 2O 7) and willemite (Zn 2SiO 4) were also identified in the samples with 7 and 10 wt% ZnO. The densities of the glass-ceramics were between 2658 and 2848 kg/m 3, and it was found that ZnO helps to increase the density of glass-ceramics. The elastic modulus was in the 100–105 GPa range, the fracture toughness was between 0.45 and 0.64 MPa m 1/2, and the Vickers microhardness was between 632 and 653 MPa. With regards to the durability, the weight loss of the glass-ceramics immersed in alkaline solution (NaOH) did not exceed 1.5 wt% after immersion for 6 h at 80 °C. The results of this study confirm that the vitrification process is a favorable option to utilize these industrial wastes. 相似文献
15.
The selective partial oxidation of methane over the europium iridium pyrochlore, Eu 2Ir 2O 7, has been studied in situ by using powder X-ray diffraction and mass spectrometry. At a temperature of 873 K and 1 atmosphere pressure, with a CH 4O 2 ratio of 21 under argon dilution, high yields of synthesis gas are obtained. The pyrochlore structure of the iridate is modified during the initiation of the catalyst, giving an active solid that is shown to comprise particles of iridium metal, 30 Å in diameter, supported on europium oxide, Eu 2O 3. The modification of the pyrochlore structure was monitored by X-ray diffraction, and the product gases were continuously analysed by mass spectrometry. 相似文献
16.
The price of lithium-containing minerals and other chemical materials continues to increase, resulting in an increase in the production cost of Li 2O-Al 2O 3-SiO 2 (LAS) system glass-ceramics. In the LAS glass-ceramics component, the reduction in the amount of Li 2O used can reduce the cost of the product. It is worthwhile to study whether it is possible to prepare glass-ceramics with low expansion properties under low Li 2O content. The effect of Li 2O content on the glass-ceramics of LAS system was studied. In this paper, spodumene was used as the main raw material, and TiO 2 and ZrO 2 were added as crystal nucleating agents to prepare transparent glass-ceramics with low expansion coefficient. The effects of the change of Li 2O content on the crystal phase and microstructure of glass-ceramics were investigated by XRD, DSC, FTIR and SEM. The results show that the main crystalline phase of the low expansion transparent glass-ceramics is β-quartz solid solution. When Li 2O content is in the range of 2.99 wt% to 4.13 wt%, low expansion glass ceramics can be prepared by an appropriate method. With the increase of Li 2O content, the average coefficient of thermal expansion (CTE) in the temperature range of 30 °C–300 °C shows a decreasing trend. When Li 2O content is in the range of 3.51 wt% to 4.13 wt%, the thermal expansion coefficient of the glass ceramics is extremely small, and even a negative expansion coefficient occurs. 相似文献
17.
The effects of substituting Al 2O 3 with B 2O 3 on the structure, crystallization, mechanical properties, thermal properties and optical properties of translucent mica glass-ceramics were thoroughly investigated. The results demonstrated that the addition of 0.5 wt% B 2O 3 was optimum for glass precipitation, which increases the crystallinity of glass-ceramics and provides good translucency. When the content of B 2O 3 was greater than 0.5 wt%, both crystallinity and translucency decreased noticeably. The replacement of B 2O 3 for Al 2O 3 had no influence on the type of crystal phase and the precipitation of tetrasilicic fluoromica with non-stoichiometric ratio, while it did have an effect on the crystallinity and structure. The crystal sizes of glass-ceramics were in the nanoscale range and the transmittance test results indicated that they exhibit excellent translucency. 相似文献
18.
The Co2O3 doped MgO-Al2O3-SiO2-B2O3 (MASB) glass-ceramics were successfully fabricated by traditional melt quenching method and the following heat treatment. The effect of Co2O3 addition on crystallization behavior, glass networks, microstructure and physical properties was investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared absorption spectra (FTIR) and scanning electron microscopy (SEM) measurements. The results show that the metastable phase μ-cordierite and α-cordierite precipitated from the parent glass during the thermal treatment process. With the increase of Co2O3 content, the crystallization tendency of the parent glass increased, and the addition Co2O3 favored the transformation from μ-cordierite to α-cordierite. Meanwhile, the grain size of glass-ceramics increased obviously, Simultaneously, the mechanical strength of glass-ceramics increased first and then decreased, while the thermal expansion coefficient decreased. Glass-ceramic with excellent thermal and physical properties were prepared by adding 0.1 mol% Co2O3 content and then crystallizing it at 1020 ∘C for 3 h. 相似文献
19.
A series of ceramic samples were prepared to experimentally investigate sub-solidus phase relations in the La 2O 3-Sm 2O 3-ZrO 2 system at 1873 K and 1673 K. No ternary compounds have been observed, while the binary La 2Zr 2O 7 and Sm 2Zr 2O 7 pyrochlore phases form a continuous solid solution La 2?xSm xZr 2O 7 in the ternary system at the selected temperatures. X-ray diffraction and microstructure results demonstrated that the pyrochlore phase is stable in the ZrO 2-rich corner. The homogeneity range of the pyrochlore phase was carefully determined and the phase boundary of the cubic ZrO 2 (fluorite phase) which extends into the ternary system was also constructed via electron probe microanalysis. The as-obtained data were adopted to determine the mixing parameters for the pyrochlore and fluorite phases in the present thermodynamic modeling. A self-consistent database of the La 2O 3-Sm 2O 3-ZrO 2 system was accordingly established for the first time and the calculations agree well with the experimental data in the current work. 相似文献
20.
Pyrochlore-based glass-ceramics are attracting attention as a promising host material for immobilizing minor actinide in high-level wastes. However, the loading capacity of pyrochlore-based materials for actinide elements is to be improved, and only conventional synthetic approaches of powder sintering and glassy bulk crystallization were reported. Here, new (La,Y) 2(Zr,Ti) 2O 7 pyrochlore-based glass-ceramics were subtly designed and successfully prepared through a rapid melting and solidification process. The solidification behavior of samples with different cooling rates was investigated, the dense and crack-free (La,Y) 2(Zr,Ti) 2O 7 pyrochlore-based glass-ceramics were prepared at the cooling rate of 500 °C/s (within 1.5 s). The present amorphous La 2O 3-TiO 2-Al 2O 3 phase demonstrates the outstanding thermal stability compared with the currently used borosilicate glass for high-level waste immobilization due to the lack of conventional glass-forming components. The results show the great feasibility of this novel synthesis route for glass-ceramics by direct solidification and its good potential for immobilizing high-level wastes. 相似文献
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