Synthesis and sintering behaviour in CeO2-ZrO2 ceramics

The co-precipitation method has been employed to prepare CeO₂-ZrO₂ ceramics. The application of a wet chemical method is expected to yield highly sinterable material at lower sintering temperatures. The characteristics of the synthesized powders are evaluated with respect to the particle size distribution, calcination step, and the degree of agglomeration. The sintering behaviour of the prepared powder is studied at various temperatures to obtain different phase distributions and grain sizes. The amount of the monoclinic phase in the as-sintered specimen is decreased with increasing CeO₂ contents in CeO₂-ZrO₂. 13.7 mol% CeO₂ is sufficient to achieve a tetragonal phase in the CeO₂-ZrO₂ system. In addition, Y₂O₃ and MgO dopants in CeO₂-ZrO₃ reduce the grain size and result in a fully tetragonal phase for the 10 mol% CeO₂ matrix.     

Ageing effect on microstructural and mechanical properties of mullite-ZrO2-TiO2 composites

The microstructural and mechanical properties of mullite-zirconia composites with TiO₂ (0.25 and 1.0 mol) additions have been studied, after ageing the samples over a wide temperature range (1000 to 1500° C) for long periods of time (100 to 200 h). In the sample with 0.25 mol TiO₂ addition, changes in mullite composition and in the solid state compatibility at temperatures below 1450° C were detected. In the sample containing 1 mol TiO₂, decomposition of Al₂TiO₅ occurs atT1200° C. Both compositions exhibit no increment in zirconia average grain size during ageing and, concomitantly, there is no strength degradation until higher temperatures (>1400° C) are reached, which become more drastic when Al₂Ti₅ is present.     

Structural and electrical studies of CeO2 modified lead zirconate titanate ceramics

The present study is an investigation of the structural and electrical properties of solid solution processed lead zirconate titanate (PZT) ceramics modified by CeO₂. The PZT composition studied is Pb(Zr_0.535Ti_0.465)O₃ modified with 0.0–0.2 mol % CeO₂. Studies of the loss of lead oxide during sintering have been performed. The lead loss increases with increasing Ce in the sample up to 0.1 mol % Ce and becomes nearly constant on further addition of Ce. The lattice parameters and cell volume of the tetragonal phase of PZT also increase with addition of Ce up to 0.1 mol % Ce. The lead loss and X-ray data indicate that Ce is being reduced to Ce³⁺ and is going onto the A-site of the PZT perovskite system. The study concludes that controlled modification of the MPB composition of PZT using 0.1 mol % CeO₂ results in substantial enhancement of piezoelectric strain coefficients, dielectric constant and electromechanical coupling coefficients.     

Effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide

The effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide, at temperatures ranging from RT to 1000°C, has been studied. Electrical resistivity decreases with increase in temperature up to 1000°C in VC and MoSi₂, whereas minimum electrical resistivity is observed at ∼600°C in B₄C infiltrant.     

Low-temperature fabrication and electrical property of 10 mol% Sm2O3-doped CeO2 ceramics

Ten mol% Sm₂O₃-doped CeO₂ solid-solution (20SDC) powders have been synthesized via carbonate coprecipitation using ammonium hydrogen carbonate (AHC) and urea as the precipitants, respectively. Characterizations were achieved by elemental analysis, X-ray diffractometry, differential thermal analysis/thermogravimetry, and FESEM. An amorphous hydroxyl carbonate precursor (Ce,Sm)(OH)CO₃·2H₂O having nanosized (~10 nm) spherical particles was formed with AHC, while a mixture of crystalline (Ce,Sm)₂(CO₃)₂(OH)₂·H₂O and (Ce,Sm)₂O(CO₃)₂·H₂O phases exhibiting irregular particle morphologies was obtained with urea. Both the precursors convert to oxide solid solutions without any phase detected corresponding to Sm₂O₃ during calcination. The oxide powder processed via the AHC method can be sintered to >99% of the theoretical at a low temperature of 1200 ?C, due to the good dispersion and ultrafine size (~15 nm) of the particles, while that from the urea method can only reach ,67.2% dense at the same temperature. Electrical conductivity of the densified ceramic was measured in air in the range 400—700 ?C by the DC three-point method, and an activation energy of ~60.5 kJ/mol was derived from the experimental data.

© 2003 Elsevier Ltd. All rights reserved.     

Enhanced densification and microwave dielectric properties of Mg2TiO4 ceramics added with CeO2 nanoparticles

We report the study of the effects of processing parameters and additive concentration on the structure, microstructure and microwave dielectric properties of MTO–CeO₂ (x wt.%) ceramics with x = 0, 0.5, 1.0 and 1.5 prepared by solid-state reaction method by adding CeO₂ nanoparticles as a sintering aid. The pure Mg₂TiO₄ ceramics were not densifiable below 1450 °C. However, when CeO₂ nanoparticles were added to MTO, the densification achieved at 1300 °C along with the increase in average grain size with the uniform microstructure and improved microwave dielectric properties. This is mainly driven by the large surface energy of CeO₂ nanoparticles and their defect energy during the sintering process. While the addition of CeO₂ nanoparticles in MTO ceramics does not change the dielectric constant (?_r), the unloaded quality factor (Q_u) was altered significantly. MTO–CeO₂ (1.5 wt.%) ceramics sintered at 1300 °C exhibit superior microwave dielectric properties (?_r ～ 14.6, Q × f₀ ～ 167 THz), as compared to the pure Mg₂TiO₄ ceramics. The observed results are correlated to the enhancement in density and the development of uniform microstructure with the enhanced grain size.     

The effects of CeO2 addition on crystallization behavior and pore size in microporous calcium titanium phosphate glass ceramics

In this research the effect of the addition of CeO₂ to microporous Calcium Titanium Phosphate glass ceramics was studied. Different molar percentages of CeO₂ were added to three samples of a base glass whose composition was P₂O₅ 30, CaO 45, TiO₂ 25 (mol%). The first sample had 2 mol% CeO₂, the second sample had 4 mol% CeO₂, and the third sample had 6 mol% CeO₂. The fourth sample did not contain any CeO₂. The glass samples were melted and crystallized to bulk glass ceramics by a conventional method. Differential Thermal Analysis (DTA) was utilized to determine the appropriate nucleation and crystallization temperatures. Among the samples, the DTA curve of the sample which had 2 mol% CeO₂ had the sharpest crystallization peak. Therefore, this sample was chosen to prepare the glass ceramics. Using X-ray Diffraction (XRD) it was found that in all samples β-Ca₃(PO₄)₂ and CaTi₄(PO₄)₆ were the major phases. The β-Ca₃(PO₄)₂ phase was dissolved away by soaking the glass ceramics in HCl, leaving a porous skeleton of CaTi₄(PO₄)₆. CeO₂ addition increased the glass transition temperature and decreased the crystallization time and temperature. It was shown that CeO₂ addition resulted in an increase in the mean pore diameter while the specific surface area decreased. The median pore diameter and specific surface area were determined as 27 nm and 14 m²/g, respectively, for the sample containing 2 mol% CeO₂.     

掺钽对二氧化钛压敏电阻性能的影响

研究了钽对二氧化钛压敏电阻性能的影响。研究中发现掺入 0 .2 5mol %Ta2 O5的样品显示出最低的反转电压 (Eb= 6V/mm)、最高的非线性常数 (α =8.8)以及最高的相对介电常数 (εr=6 .2× 10 4 ) ,与样品电容和电阻的频谱特性相一致。样品的性能变化可用Ta5+ 对Ti4 + 的掺杂取代和该取代存在的饱和值来解释     

Ion storage properties of CeO2 and mixed CeO2/SnO2 coatings

Ion storage CeO₂ and CeO₂/SnO₂ coatings were prepared by the sol-gel dip-coating method using an aqueous-based process. The influence of added SnO₂ in the CeO₂ oxide coatings on the inserted/extracted charge was determined by chronocoulometric measurements. It was found that for 60 nm thick film, the inserted/extracted charge was twice (Q=10 mC cm^–2) as large for films containing 17 mol% SnO₂, if compared to pure CeO₂. The effect of the addition of SnO₂ to the mixed oxide coatings on their optical properties and structural characteristics was studied.     

晶态骨架介孔CeO2和CeO2-ZrO2的合成与结构表征

以乙酸铈、乙酸锆为无机源,嵌段共聚物P123为模版剂,采用溶胶-凝胶法合成了介孔CeO2及CeO2-ZrO2复合体,用XRD、氮气吸附脱附、TEM、HRTEM、SAED等对材料进行了表征.结果表明,所得介孔CeO2具有规则的晶态孔道结构,孔径约在10nm左右,特别是其孔壁晶粒高度定向排列且平行于孔道方向.原位高温TEM观测表明,该介孔CeO2在800℃仍具有很高的孔道完整性.掺杂ZrO2后发现,在ZrO2摩尔含量低于30%时仍具备高度定向排列结构,高于30%时骨架晶粒取向逐渐趋向无序化.     

CeO2掺杂TiO2光催化剂的性能研究

采用液相合成法制备CeO2掺杂TiO2光催化粉体,利用X射线衍射仪、紫外-可见分光光度计、FT-IR傅里叶变换红外光谱仪等对粉体的晶相组成、光谱吸收性能进行了表征,结果表明,掺杂后的TiO2经800℃热处理后仍呈锐钛矿型;CeO2/TiO2吸收光谱的阈值波长发生红移,但Ce掺杂浓度的增大对粉体的可见光吸收影响不大.光催化降解甲醛的结果表明,粉体在普通日光灯下对甲醛气体的降解率明显优于P25.