Preparation and Characterization of Compositions Based on PbO-MgO-Nb2O5 Using the Sol-Gel Method

Lead Magnesium Niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), is an important relaxor ferroelectric material. A significant problem exists, however, in the preparation of this material: it is very difficult to prepare pure phase, without the presence of a pyrochlore impurity phase which degrades the dielectric properties. Depending on the processing conditions, the amount of pyrochlore phase varies.Considering the ternary diagram PbO-MgO-Nb₂O₅, different compositions have been prepared by a simple sol-gel method at room temperature using Pb(CH₃COO)₂, Mg(CH₃COO)₂, Nb(OC₂H₅)₅ as precursors. After the heat treatment, the samples obtained were analyzed by XRD and EPMA with the purpose of studying the compositions formed.From the analysis of these results, it seems to be that the appearance of pyrochlore can be related to the reactivity of the MgO and/or the presence of other phases of the binary system PbO-Nb₂O₅. An excess of Pb and Mg is necessary for compositions to be formed near the PMN. The control of the amount of these two elements is very important because an excess of MgO would lead to rich compositions in Mg as a secondary phase.     

Synthesis of lead magnesium niobate and La-modified lead magnesium niobate using different Mg precursors

Summary In the present study, the effect of the partial replacement of Pb²⁺ by La³⁺ in the lead magnesium niobate Pb(Mg_1/3Nb_2/3)O₃ (PMN) perovskite structure was examined, taking into account the Mg-source. Pure lead magnesium niobate (PMN) and lanthanum-modified lead magnesium niobate (PLMN) having composition (Pb_1-xLa_x) (Mg_1+x/3Nb_2-x/3)O₃ with x=0.2 were elaborated. The phase formation was investigated by DTA/TG methods correlated with X-ray diffraction, performed on materials obtained in non-isothermal conditions. The diffraction data for the ceramics obtained by isothermal treatments emphasized the influence of the lanthanum on the crystal structure, inducing the doubling of the unit cell parameter. SEM investigations pointed out the lanthanum inhibitor effect on the grain growth process, leading to an uniform grain distribution.     

合成工艺对PMN-PNN-PZT 四元系压电陶瓷性能的影响

研究了前驱体合成(又称铌铁矿合成)与传统的氧化物混合合成,对PMN-PNN-PZT四元系压电陶瓷性能的影响。结果表明,前驱体合成工艺能够在合成过程中避免焦绿石相的生成。并且相比较于氧化物混合合成的工艺,它能使得压电陶瓷的压电常数d33由475 pC/N提高到530 pC/N,机械品质因数Qm从96提高到185,介质损耗tanδ也从0.020降低到0.009。综合实验分析结果得出前驱体合成方法是一种更加优异的合成方法。     

Photocatalytic Water Oxidation by a Pyrochlore Oxide upon Irradiation with Visible Light: Rhodium Substitution Into Yttrium Titanate

A stable visible‐light‐driven photocatalyst (λ≥450 nm) for water oxidation is reported. Rhodium substitution into the pyrochlore Y₂Ti₂O₇ is demonstrated by monitoring Vegard′s law evolution of the unit‐cell parameters with changing rhodium content, to a maximum content of 3 % dopant. Substitution renders the solid solutions visible‐light active. The overall rate of oxygen evolution is comparable to WO₃ but with superior light‐harvesting and surface‐area‐normalized turnover rates, making Y₂Ti_1.94Rh_0.06O₇ an excellent candidate for use in a Z‐scheme water‐splitting system.     

The influence of sulfur substitution on the atomic displacement in Bi2Ti2O7

To clarify the role of A₂O′ and B₂O₆ networks on cation displacement observed in Bi₂Ti₂O′O₆, we used density functional theory calculations to examine the effect of sulfur substitution on the O′ and O sites on lone pair formation and resulting atomic displacement observed in Bi₂Ti₂O′O₆. Cation displacement in bismuth titanate is suppressed only when S is substituted on the O′ site. Analysis of the electronic structure shows that S substitution on the O′ site suppresses the formation of the asymmetric p-type lone pair by modifying the Bi-anion hybridization. Lone pair formation is favored in Bi₂Ti₂O′S₆ and the atomic displacement is larger than that observed in Bi₂Ti₂O′O_6. This enhanced displacement is due to weaker Bi-S versus Bi-O interactions leading to significantly stronger hybridization between the Bi and O′ states in Bi₂Ti₂O′S₆. We also induced lone pair formation in a metallic bismuth pyrochlore oxide (Bi₂Ru₂O′O₆) by modifying the Bi-O interactions through S substitution on the B₂O₆ network, indicating atomic displacement on the A₂O′ network may be achieved by modifying the B₂O₆ network.     

PZT-PMN压电陶瓷的制备和压电性能的研究

先驱体法制备了具有高纯钙钛矿相的Pb(Zr1/2Ti1/2)1-x(Nb2/3Mg1/3)xO3(PZT-PMN)体系压电陶瓷。按配比在650~800℃预烧合成了B位(ZrTiO4)1-x-(MgNb2O6)x的固溶体,X-射线衍射(XRD)分析表明了该B位先驱体氧化物在750℃预合成后呈类似ZrTiO4的单相结构,用该B位先驱体与PbCO3煅烧后可生成具有高纯的钙钛矿相的陶瓷粉料。通过对体系压电性能与配比x的关系研究发现,在准同型相界附近四方相一侧的配比x=0.1处,该系统压电性能达最大。通过掺杂Nb5 含量为0.02可使机电耦合系数(kp)达到0.637。     

Nanocomposites of Tantalum‐Based Pyrochlore and Indium Hydroxide Showing High and Stable Photocatalytic Activities for Overall Water Splitting and Carbon Dioxide Reduction

Nanocomposites of tantalum‐based pyrochlore nanoparticles and indium hydroxide were prepared by a hydrothermal process for UV‐driven photocatalytic reactions including overall water splitting, hydrogen production from photoreforming of methanol, and CO₂ reduction with water to produce CO. The best catalyst was more than 20 times more active than sodium tantalate in overall water splitting and 3 times more active than Degussa P25 TiO₂ in CO₂ reduction. Moreover, the catalyst was very stable while generating stoichiometric products of H₂ (or CO) and O₂ throughout long‐term photocatalytic reactions. After the removal of In(OH)₃, the pyrochlore nanoparticles remained highly active for H₂ production from pure water and aqueous methanol solution. Both experimental studies and density functional theory calculations suggest that the pyrochlore nanoparticles catalyzed the water reduction to produce H₂, whereas In(OH)₃ was the major active component for water oxidation to produce O₂.     

PZT陶瓷靶材的制备及相成分分析

采用固相反应法制备了一系列PZT陶瓷靶材,并用XRD对其进行了相成分分析.结果表明,PbO的富足可以大大抑制PbZrO3的分解,还可以抑制焦绿石相的产生,起到稳定钙钛矿相的作用.然而,过量的PbO会使材料局部区域的相成分发生波动,故而破坏相成分的一致性.此外,对单块的PZT样品进行烧结还会出现陶瓷分层现象,将多块样品放在一起烧结或采用先进的烧结工艺有利于抑制分层.     

多离子束反应共溅射制备PbTiO_3铁电薄膜的微结构研究

本文利用多离子束反应共溅射装置，分别在Ｓｉ和ＭｇＯ衬底上原位制备了ＰｂＴｉ氧化物薄膜。研究表明，利用多离子束反应共溅射技术，可以显著降低薄膜后续热处理的温度；薄膜中焦绿石结构的消失温度与薄膜中Ｐｂ的含量有关；较之Ｓｉ衬底，在ＭｇＯ衬底上的薄膜较易获得好的晶体结构和优良的薄膜表面形貌。对所观察到的现象，从衬底与薄膜相互作用的角度进行了讨论