CaZrTi2O7 zirconolite synthesis: From ceramic to glass-ceramic

The formation of CaZrTi₂O₇ zirconolite and its crystallization in sodium alumino-borosilicate glass has been investigated via sintering in air. The ceramic precursor is prepared using a soft chemistry route to ensure ultimate mixing of the reactants at the molecular level. A nearly phase pure CaZrTi₂O₇ zirconolite is formed after sintering the ceramic precursor at 1400°C for 12 hours. In order to form zirconolite glass-ceramics, various processing conditions are investigated including sintering temperatures, different glass composition precursors, and ceramic to glass weight ratios. Zirconolite crystallization on the surface and inside of glass-ceramic-pelletized samples is investigated. Higher ceramic to glass weight ratio leads to the formation of zirconolite as crystalline phase, which is quite dominant in glass.     

Bioaccumulation of lanthanum,uranium and thorium,and use of a model system to develop a method for the biologically-mediated removal of plutonium from solution

Removal of La³⁺, UO₂²⁺ and Th⁴⁺ from aqueous solution by a Citrobacter sp. was dependent on phosphatase-mediated phosphate release and the residence time in a plug-flow reactor (PFR) containing polyacrylamide gel-immobilized cells. In a stirred tank reactor (STR) lanthanum phosphate accumulated on the biomass rapidly, in preference to uranium or thorium phosphates. Thorium removal was not affected by the presence of uranium but was promoted in the presence of lanthanum. Analysis of the accumulated polycrystalline material by X-ray powder diffraction (XRD) analysis and proton induced X-ray emission (PIXE) suggested the formation of a mixed crystal of lanthanum and thorium phosphate. La³⁺, UO₂²⁺ and Th⁴⁺ are analogues of the corresponding species of Pu³⁺, PuO₂²⁺ and Pu⁴⁺. The La/U/Th model system was used to identify some potential problems in the bioremediation of wastes containing plutonium and to develop a method for the biologically-mediated removal of plutonium from solution, in a test solution of ²³⁹Pu ‘spiked’ with a ²⁴¹Pu tracer. © 1998 SCI.     

Synthesis and characterization of a pyrochlore solid solution in the Na2O-Bi2O3-TiO2 system

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 Na₂O-Bi₂O₃-TiO₂ 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 ‘Bi₂Ti₂O₇’ pyrochlore. In contrast to ‘Bi₂Ti₂O₇’, 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.     

IR transmission prediction,processing, and characterization of dense La2Ce2O7

High-throughput computation, based on density functional theory (HT-DFT), is used to predict the bounds for optical transparency, from the ultraviolet to the infrared, for materials in the pyrochlore family. The HT-DFT approach adopted here uses an initial screening from Materials-Project database, with millions of calculated properties. Band gaps and phonon spectra were calculated from selected pyrochlore crystal structures taken from the Materials Project database. Short and long wavelength bounds for optical transparency were calculated for chemistries with stable, cubic structures. The calculations predict that La₂Ce₂O₇ has one of the broadest range of transparency for the pyrochlore family. Based on these calculations, dense polycrystalline samples of La₂Ce₂O₇ were produced by sintering and hot-isostatic pressing. Transparency was characterized by methods that did not require large samples with high optical quality, obtaining 7.15 and 7.5 µm at 95% and 90% normalized transmittance, respectively. Bandgap calculations suggest a lower bound of UV transparency cut-off of 0.3 µm. The infrared wavelength cut-off is higher than that reported for other pyrochlores, and higher than for yttria, zirconia, or other common infrared transparent ceramics. We discuss our prediction and characterization methods as well as the suitability of pyrochlores for mid- and far-infrared optical applications.     

Synthesis and Unique Photoluminescence Properties of Eu2Ti2O7 and Eu2TiO5

The europium titania materials, pyrochlore Eu₂Ti₂O₇ and orthorhombic Eu₂TiO₅, were synthesized from a mixture of Eu₂O₃ and TiO₂ using the solid‐state reaction method. The structural and optical properties of these titania materials were investigated using X‐ray diffraction analysis, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and diffuse reflectance spectroscopy. Temperature dependence of the PL intensity was measured between T = 20 and 450 K and analyzed on the basis of various theoretical models. A remarkable increase in the PL intensity with increasing T was observed in these titania materials at higher temperatures, above ~140 K, and well explained by a trap/reservoir model. Interestingly, a dramatic decrease in the electric‐dipole emission component relative to the magnetic‐dipole one was observed in Eu₂Ti₂O₇ above T ~ 140 K. The schematic energy‐level diagram for Eu³⁺ in the Eu₂Ti₂O₇ host was proposed for the sake of a better understanding of the PL and PLE processes in this type of phosphorescent material.     

可见光响应催化剂Nd2InNbO7的制备、表征及光催化性能研究

以传统的高温固相法制备出烧绿石型半导体光催化剂Nd2InNbO7.采用XRD和uV—DRS手段对其进行了表征,测试结果表明,Nd2InNbO7属于立方晶系,晶格常数为1．0616nm,带隙宽度为2．68eV,具备可见光响应能力。以目标物亚甲基蓝的降解率为指标考察了催化剂用量、助催化剂的种类及负载量对Nd2InNbO7的光催化性能的影响。结果表明,同Ag和RuO2相比,Pt助催化剂更有利于光生电子和空穴的分离,进而使催化效率有较大提高。当催化剂的用量为0．2g／100mL,且负载0．2％Pt助催化剂后,催化活性最佳。在实验用可见光源照射下,反应4h后即可将亚甲基蓝完全降解。     

可见光响应催化剂Nd_2InNbO_7的制备、表征及光催化性能研究

以传统的高温固相法制备出烧绿石型半导体光催化剂Nd2InNbO7。采用XRD和UV-DRS手段对其进行了表征,测试结果表明,Nd2InNbO7属于立方晶系,晶格常数为1.061 6 nm,带隙宽度为2.68 eV,具备可见光响应能力。以目标物亚甲基蓝的降解率为指标考察了催化剂用量、助催化剂的种类及负载量对Nd2InNbO7的光催化性能的影响。结果表明,同Ag和RuO2相比,Pt助催化剂更有利于光生电子和空穴的分离,进而使催化效率有较大提高。当催化剂的用量为0.2 g/100 mL,且负载0.2%Pt助催化剂后,催化活性最佳。在实验用可见光源照射下,反应4 h后即可将亚甲基蓝完全降解。     

Bi2O3-CaO-ZnO-Nb2O5系陶瓷介电性能研究

借助XRD,SEM,HP4140B微电流计和HP4274A阻抗分析仪等测试手段,研究实验制备的BCZN[(Bi1.5Ca0.5)(Zn0.5Nb1.5)O7]陶瓷试样介电性能,并第一次提出了介电常数临界温度和介质损耗临界温度的概念。结果表明:BCZN陶瓷是高频稳定形陶瓷,具有面心立方焦绿石结构,且在最佳烧结温度和保温时间下,为均一的α相单相结构;在很宽的频率范围内,介电常数较高(εr≈81)且很稳定,介质损耗很小(tanδ≈-1.0×10-4);它的介电常数在温度低于介电常数临界温度时,与温度呈线性关系,在温度高于介电常数临界温度时,发生高温突变;介质损耗在温度低于介质损耗临界温度时几乎不变,在温度高于介质损耗临界温度时,发生高温突变;介电常数临界温度和介质损耗临界温度都随频率的升高而升高。     

He irradiation-induced lattice distortion and surface blistering of Gd2Zr2O7 defect-fluorite ceramics

Gd₂Zr₂O₇ ceramics with different grain sizes ranging from nanoscale to submicron scale (91, 204, and 634 nm) were irradiated at room temperature using 190 keV He ions with doses ranging from 5 × 10¹⁶ to 5 × 10¹⁷ ions/cm². We fully characterized the pre- and post-irradiation samples using grazing-incidence X-ray diffraction (GIXRD), scanning electron microscope (SEM), and atomic force microscope (AFM) as the grain size and degree of irradiation vary. The results suggested that all three Gd₂Zr₂O₇ samples demonstrate outstanding radiation tolerance to displacement damage by retaining their crystallinity after irradiation at 5 × 10¹⁷ ions/cm². which is equal to 16 displacement per atom (dpa) at peak positions. Although lattice expansion was observed at a He irradiation at 5 × 10¹⁶ ions/cm² and beyond, the lattice remained stable for the nanograin ceramic, while the degree of distortion for the sample with the largest grain size (634 nm) continuously increased. Moreover, a delayed He bubble evolution process was seen for the nanograin ceramic, which did not appear for the submicron-grained sample. Interestingly, the grain size-dependent surface blistering was also found to be a function of ion fluence. After He irradiation at 5 × 10¹⁷ ions/cm² the AFM root-mean-square(RMS) roughness variation for Gd₂Zr₂O₇ ceramics of 91, 204, and 634 nm were 4.8, 7.0, and 11.1 nm, respectively.     

Effects of Ni2+ substitution on the structure and dielectric properties of Bi1.5MgNb1.5O7 cubic pyrochlores

The dielectric properties of bismuth-based cubic pyrochlores strongly depend on the environment of the A-site ions, e.g. the Ni²⁺ ions doped into Bi_1.5MgNb_1.5O₇ (BMN) pyrochlores for tailoring dielectric properties. Both BMN and Bi_1.5NiNb_1.5O₇ (BNN) ceramics exhibit a cubic pyrochlore structure with preferential (222) planes. However, {442} reflections are observed in BNN pyrochlores, revealing an off-center displacement of A and O' ions. The dielectric constant of BNN pyrochlores is lower than that of BMN pyrochlores, besides BNN pyrochlores have a larger dielectric loss (0.002) than BMN pyrochlores (0.0007). Ni-doping results in a loose and flexible structure contributing positively to the dielectric tunability, besides creating a large amount of oxygen vacancies. The higher amount of oxygen vacancies increases the dielectric loss of BNN pyrochlores. However, BNN pyrochlores exhibit enhanced temperature stability, with a temperature coefficient of –57 ppm/^oC, which is significantly better than that of BMN pyrochlores (–362 ppm/^oC).     

2—羟基—3—羧基—5—磺基偶氮胂的合成及光度法测定钍的研究

研究了 2 -羟基 - 3-羧基 - 5 -磺基偶氮胂的合成及光度法测定钍的方法。在 1 .2 mol/L硝酸介质中 ,钍与 2 -羟基 -3-羧基 - 5 -磺基偶氮胂形成 1∶ 2的绿色配合物 ,最大吸收波长在 6 75 nm处 ,摩尔吸光系数为 8.5 4× 1 0 4 L· mol- 1·cm- 1 。钍含量在 0～ 30 μg/2 5 m L范围内符合比耳定律。应用于特种镁合金中钍的直接测定 ,结果非常满意。     

Structural evolution of Lu2-xCexTi2O7 pyrochlores under 400 keV Ne irradiation

Lu_2-xCe_xTi₂O₇ (LCTO) pyrochlores were irradiated by 400 keV Ne²⁺ with fluences (dose) of up to 5 × 10¹⁵ ions/cm² (1.875 dpa). The detailed damage process was investigated by combining grazing incident angle X-ray diffraction (GIXRD) and transmission electron microscopy (TEM). Subsequent to the 2% volume swelling at a fluence of 1 × 10¹⁴ ions/cm² (0.037 dpa), the initially swollen LCTO pyrochlore formed both a disordered fluorite phase and a nanocrystalline pyrochlore phase at a fluence of 5 × 10¹⁴ ions/cm² (0.185 dpa). At higher fluences, the fluorite phase diminished as amorphous domains increased in volume when the dose reached a fluence of 1 × 10¹⁵ ions/cm² (0.371 dpa), while the nanocrystalline pyrochlore phase persisted. At the highest fluence of 5 × 10¹⁵ ions/cm² (1.854 dpa), the amorphous fraction decreased, meanwhile the degree of crystallinity of nanocrystalline pyrochlore phase was enhanced, as evidenced by the increased intensity of superlattice diffraction maxima. The phase transformation and recrystallization can be explained by the release of strain in irradiation-induced swollen pyrochlore crystallites. The evolution of the damage process is mainly driven by the differences in the Gibb's free energies of fluorite phase as compared with the pyrochlore phase as a function of grain size. We have demonstrated that ion beam techniques can be used to manipulate the phase stability and crystallite size of pyrochlore. These results provide the basis for tailoring the mechanical strength and response of pyrochlores to extreme radiation environments.     

试剂偶氮氟胂-DBN的合成及其与稀土、钍、铋显色反应的研究

介绍了新试剂偶氮氟胂 DBN的合成路线及方法 ,并对其与稀土 (铈、钇 )、钍、铋显色反应的条件和应用进行了研究。与稀土在 62 8nm处 ,0～ 2 5 μg/2 5mL范围内符合比尔定律 ,表观摩尔吸光系数分别为εCe=1 2 9× 1 0 5L·mol-1·cm-1,εY=1 3 0× 1 0 5L·mol-1·cm-1。试剂与钍在 0～ 2 6μg/2 5mL范围内符合比尔定律 ,ε62 4=1 3 3× 1 0 5L·mol-1·cm-1,与铋在 0～3 5 μg/2 5mL范围内符合比尔定律 ,ε62 2 =1 2 3× 1 0 5L·mol-1·cm-1。应用于铸造生铁中微量铈、药品胃必治中微量铋的测定 ,结果满意。     

偶氮氟膦-DBF的合成及其与稀土、铋、钍显色反应的研究

合成了新显色剂偶氮氟膦-DBF,并对其与稀土(铈、钇)和铋、钍的显色反应条件及应用进行了研究。与稀土的反应酸度范围是pH0.49～2.09(铈)、pH0.61～2.09(钇),配合物的最大吸收在630nm,表观摩尔吸光率εCe=1.13×105L·mol-1·cm-1,εY=1.08×105L·mol-1·cm-1。与铋、钍的反应酸度范围分别为pH0.14～1.32、pH0.08～0.62,表观摩尔吸光率分别为εBi=1.02×105L·mol-1·cm-1,εTh=1.34×105L·mol-1·cm-1。试剂应用于胃必治中铋、海产品中微量钍的测定,结果满意。     

Phase transition of Eu2Ti2O7 under high pressure and a new ferroelectric phase with perovskite‐like layered structure

Ferroelectrics with perovskite‐like layered (PL) structure are well‐known for their high T_c and the application prospect of high‐temperature‐piezoelectric sensing. In this study, the PL‐structure Eu₂Ti₂O₇ was prepared by 1‐step high‐pressure sintering, which show the pyrochlore structure of Eu₂Ti₂O₇ would change into PL structure at 11 GPa, 1300°C. The PL‐structure Eu₂Ti₂O₇ is metastable, which will change back to pyrochlore structure at about 900°C in the air. The PL‐structure Eu₂Ti₂O₇ was confirmed as a high‐temperature ferroelectric material for the first time. The ferroelectric domain switching was directly observed using piezoelectric force microscope. The piezoelectric constant of the PL Eu₂Ti₂O₇ ceramic was measured as 0.7‐0.9 pC/N and its thermal depoling temperature (T_d) was determined as 800°C, which is associated with the PL‐pyrochlore transition.     

Disorders and oxygen vacancies in p-type Sn2B2O7 (B = Nb,Ta): Role of the B-site element

Sn₂Nb_2−xTa_xO₇ (x = 0.0–2.0) with pyrochlore structure is a promising material for p-type oxide semiconductors. A systematic study of its Nb/Ta ratio indicated that the hole–generation efficiency of the Nb end (Sn₂Nb₂O₇) was an order of magnitude lower than that of the Ta end (Sn₂Ta₂O₇). Although this occurs due to differences in oxygen-vacancy formation, the origins of the hole–generation efficiencies remain unclear due to limited information on local and global crystal-structure disorders in pyrochlore Sn₂Nb₂O₇ and Sn₂Ta₂O₇. In this study, the crystal structures of Sn₂B₂O₇ (B = Nb, Ta), composed of BO₆ octahedra and Sn₄O tetrahedra, were investigated using X-ray absorption spectroscopy and X-ray diffraction. A detailed investigation of the local and global crystal structures indicated a larger amount of disorder in the Sn₄O tetrahedra in Sn₂Nb₂O₇ compared to Sn₂Ta₂O₇; disorder in the BO₆ octahedra occurred only in Sn₂Ta₂O₇. This study indicates that an appropriate selection of the B-site element is vital for suppressing defect and disorder formation in Sn₄O tetrahedra and subsequently improving the hole–carrier–generation efficiency.     

The dependence of lattice thermal conductivity on phonon modes in pyrochlore-related Ln2Sn2O7 (Ln = La,Gd)

Rare-earth pyrochlore materials are promising thermal barrier coatings materials and fundamental understanding of their thermal transport is crucial for further improving its performance. In this work, using density functional theory (DFT) method, we calculated the intrinsic lattice thermal conductivities of Ln₂Sn₂O₇ (Ln = La, Gd) and conducted a comprehensive analysis on the mode thermal conductivity, relaxation time, Grüneisen parameters, group velocity, and specific heat, respectively. It is shown that in pyrochlore-type materials the number of the optical phonons is much larger than that of the acoustic phonon, and the thermal conductivity of acoustic phonons are suppressed, both of which increase the contribution ratio of optical phonons. Especially, through cumulative analysis, we found that the contribution of optical phonons is significant: the ratio of optical contribution is more than 50% and 64% in La₂Sn₂O₇ and Gd₂Sn₂O₇. This work provides a comprehensive picture illustrating the significant role of the optical phonons in the lattice thermal conduction in rare-earth pyrochlore materials, and points out an avenue to obtain low thermal conductivity in complex structural thermal insulation materials.     

Composition-dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La,Nd, Gd; B = Sn,Hf, Zr)

Point defects are closely correlated with various properties of pyrochlore oxides and therefore play a key role on their engineering applications. Here, the native point defect complexes in RE₂B₂O₇ (RE = La, Nd, Gd; B = Sn, Hf, Zr) under stoichiometric and nonstoichiometric compositions are studied by first-principles calculations. The O Frenkel defect complex is predicted to be the predominant defect structure in stoichiometric zirconates and hafnates, whereas the cation antisite defect complex is the predominant one in stannates. In the case of BO₂ excess, the formation of the B-RE antisite defect together with the RE vacancy and the oxygen interstitial is energetically favorable, whereas the RE-B antisite defect together with the oxygen vacancy and the RE interstitial is preferable under the RE₂O₃ excess environments. Additionally, the formation energies of the native defect complexes are greatly affected by the B-site and/or RE-site cations. The strategy on tailoring the intrinsic defect structures of these pyrochlore oxides is proposed. It is expected to guide the experiments on the defect-related property optimization through stoichiometric and nonstoichiometric compositions, so as to meet the specific engineering requirements and promote their commercial applications.     

Ta_2O_5掺杂Bi_2O_3–ZnO–Nb_2O_5陶瓷的晶体结构、结晶化学及介电性能

采用固相反应法制备Bi1.5ZnNb1.5–xTaxO7陶瓷,研究了不同掺杂量Ta2O5对Bi2O3–ZnO–Nb2O5陶瓷相结构、晶体化学特性和介电性能的影响。结果表明:当x≤0.1时,样品均保持单一的立方焦绿石结构(α–BZN)。通过对样品结晶化学计算发现,随着Ta2O5掺杂量的增加,晶格常数a逐渐减小,结晶化学参数键价和AV(O')[A4]增大,AV(O)[A2B2]减小,48f(O)坐标ξ增加。在组成样品晶体结构的多面体中,由6个48f(O)组成的八面体结构(BO6)逐渐变得扭曲,而6个48f(O)和2个8b(O')组成的六面体结构逐渐变得规则,向正立方体结构变化。室温下样品的介电常数和损耗随Ta2O5掺杂量的增加而减小,弛豫度逐渐减小。     

(Bi2-yCay)(Zn1/3 Ta2/3)2O7陶瓷的介电弛豫和介电性能

研究了(Bi2-yCay)(Zn1/3Ta2/3)2O7(0≤y≤1)材料的组成、结构与介电性能.当Ca含量增加时,材料的相结构由单斜焦绿石相转变为立方焦绿石相.样品在20～85℃,1 MHz时的介电常数温度系数由72×10-6/℃逐渐增加到470×10-6/℃,然后降为-100×10-6/℃,样品在微波频率下的品质因数Q值从1 250逐渐降低至40.在-60～160℃,观测到(Bi1.2Ca0.8)(Zn1/3Ta2/3)2O7样品出现介电弛豫现象.随着Ca含量的增加,介电损耗的弛豫峰向高温移动.比较了同为立方结构相的(Bi2-yCay)(Zn1/3Ta2/3)2O7(0.7＜y＜1)和(Bi1.5Zn0.5)(Zn0.5·Ta1.5)O7介电弛豫温区移动的差异并分析了其形成原因.