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1.
Lead magnesium niobate, Pb(Mg1/3Nb2/3)O3 (PMN) ceramics were prepared from the columbite method using calcined powders of various milling time (24–96 h). The effects on the grain size and dielectric properties of the ceramics were investigated. The results show that dielectric properties of ceramics are strongly influenced by the milling time of the starting precursors. Higher percentage of perovskite phase was found in the ceramics that was milled longer and thus the dielectric constant was found to increase when compared to the conventional 24 h milled results. Moreover, milling time also affected the particle size of the starting precursors and that of PMN powders. Therefore, milling time did not only affect the particle size of PMN powders but also the resultant grain size and the formation of perovskite phase, consequently affecting the dielectric constant of the ceramics.  相似文献   

2.
The structure, properties, and formation mechanisms of Y3Al5O12, Y2O3, and Lu2O3 laser ceramics are investigated. Their microhardness and fracture toughness are determined. It is shown that the change in mechanical properties is related both to the grain size and grain boundary structure. Processes of plastic deformation of crystals by mechanical twinning are considered. Mechanisms of formation and motion of twins in crystals with FCC structure are determined. It is shown that the realization of similar mechanisms in crystals with HCP structure results in the phase transformations. Models of the formation and motion of twin boundaries are proposed which result in pore healing when preparing monolithic samples of highly transparent ceramics.  相似文献   

3.
Nanocrystalline Tm3+(5%)-doped BaTiO3 (BT-Tm) has been synthesized by the sol–gel method. The morphology, structure, and optical properties of powders and ceramics were characterized. The average grain size of the gel precursor annealed at 700 and 900 °C was 20 nm and 30 nm, respectively. These powders were single phase and crystallized with a cubic structure while the BT-Tm sintered ceramics were crystallized with the tetragonal BaTiO3 structure. The photoluminescence spectra showed typical transitions of Tm3+ ions and a structure consistent with the Tm3+ ions incorporation in the BaTiO3 crystalline lattice. Thermoluminescence peaks recorded at 300 °C (for annealed samples) or at 230 °C for the ceramic sample were assigned to the recombination of the Tm2+-electron traps located mainly at the surface of the nano-crystals or inside the microcrystals, respectively.  相似文献   

4.
Nanocrystalline nickel oxide powders were calcined at 300, 600 and 900°C and pore structure evolution was followed by small angle neutron scattering (SANS). Pore size distributions at two widely separated size ranges have been revealed. Shrinkage of larger-sized pore with reduction in polydispersity has been observed with increasing heat treatment temperature. The pore structures at various heat treatment temperatures do not scale. This has been attributed to the grain boundary diffusion leading to an asymmetric shrinkage of the pores.   相似文献   

5.
The effect of the sintering temperature from 1070 to 1670 K of ceramic samples of lanthanum manganite La0.7Mn1.3O3 on their grain size, structure, magnetic and resistive properties has been studied. An increase in the sintering temperature to 1270 K is shown to lead to an insignificant increase in the grain size and an increase in the density, fraction of the ferromagnetic phase in a grain, and colossal magnetoresistance. The ceramics sintering at temperatures higher than 1470 K is found to sharply increase the grain size; simultaneously, the grain takes a layered structure. The grain growth at these temperatures is established to be accompanied by manganese precipitation at the grain boundaries and likely in the grain interior. The increase in the sintering temperature is accompanied by appearance of a magnetically phase heterogeneity and a decrease in the Curie temperature and magnitude of the colossal magnetoresistance effect.  相似文献   

6.
曾涛  董显林  毛朝梁  梁瑞虹  杨洪 《物理学报》2006,55(6):3073-3079
采用添加造孔剂的方法制备多孔锆钛酸铅(PZT)陶瓷,并研究了孔隙率和晶粒尺寸对多孔PZT陶瓷介电和压电性能的影响及机理.研究表明:孔隙率的增加降低了多孔PZT陶瓷的介电常数,提高了静水压优值,并证明在一定条件下孔隙率与介电常数关系可由Okazaki经验公式及Banno模型预测;晶粒尺寸增加,多孔PZT陶瓷的介电常数、压电系数和优值增加,并可用Okazaki空间电荷理论解释晶粒尺寸对试样介电和压电性能的影响.对于添加重量百分数为10%造孔剂的多孔PZT陶瓷,当烧结温度为1300℃时,孔隙率为34%,d关键词: 多孔PZT陶瓷 静水压优值 压电性能 介电性能  相似文献   

7.
Redox stability of cubic nanostructured zirconia ceramics, free of any secondary phases, has been investigated experimentally as a function of grain size. Pure 8 mol% Y2O3-doped ZrO2 powders were synthesized by a spray pyrolysis process and then compacted by uniaxial pressing, followed by cold isostatic pressing. Using appropriate thermal treatments, average grain sizes ranging from 25 to 242 nm and relative densities from 71% up to 98% were obtained. An electrochemical characterization was performed with comparison on ceramics of 3.2 and 7.6 μm and 98% of theoretical density starting from commercial YSZ powder.  相似文献   

8.
The results on surface modification of materials of different structures; morphology, grain sizes, density and porosity by exposure to nanosecond laser light are given. Laser induced changes in their surface characteristics are presented. Surface layers of Si3N4, SiC dense ceramics and BN graphite and turbostratic pressed powders are studied by scanning electron microscopy to reveal the new nanostructures (nanowires or nanotubes) and new morphologies. A pronounced evolution in structure and grain size of BN graphite powders was demonstrated in laser processing layers.  相似文献   

9.
《Solid State Ionics》2006,177(13-14):1227-1235
Both doped zirconia and ceria have been widely recognized as promising electrolytes in solid oxide fuel cells (SOFC). Total conductivity is an important parameter to evaluate solid electrolytes. It is well know that the contribution to the total conductivity by grain boundaries is especially pronounced for SiO2-contaminated electrolytes. In this study, we report on the different conduction behaviors of grain boundaries (GB) found in SiO2-containing (impure) 8YSZ (8 mol% Y2O3-doped ZrO2) and CGO20 (10 mol% Gd2O3-doped CeO2) ceramics. In the grain size range (∼ 0.5–10 μm) studied, the GB conductivity of impure CGO20 ceramics constantly decreases with increasing grain size, in contrast to that observed in impure 8YSZ electrolytes whose GB conductivity increases almost linearly with grain size. It is also found that the variation in GB conductivity versus grain size is different from case to case, depending on the sintering/annealing conditions used to fabricate the ceramics. Two mechanisms were proposed to explain the GB behaviors of the impure 8YSZ and CGO20 ceramics. For doped ceria, the GB phases are supposed to be inert, which do not react with or dissolve into the matrix. Increasing sintering temperature leads to not only grain growth but also change in viscosity and wetting nature of the GB phases. These two factors promote further propagation of the GB phases along the grain boundaries, leading to an increased GB coverage fraction. For doped zirconia, however, the major factor dominating the GB conduction is the further dissolution of SiO2 into zirconia lattice as a result of increase in sintering temperature or/and time. In addition, we will also evaluate and discuss the validities of the three models that are widely used to analyze the GB conduction in solid electrolytes.  相似文献   

10.
The effects of annealing temperature and manganese substitution on the formation, microstructure and magnetic properties of MnxZn1−xFe2O4 (with x varying from 0.3 to 0.9) through a solid-state method have been investigated. The correlation of the microstructure and the grain size with the magnetic properties of Mn–Zn ferrite powders was also reported. X-ray diffraction (XRD), a scanning electron microscope (SEM) and a vibrating sample magnetometer (VSM) were utilized in order to study the effect of variation of manganese substitution and its impact on crystal structure, crystalline size, microstructure and magnetic properties of the ferrite powders formed. The XRD analysis showed that pure single phases of Mn–Zn ferrites were obtained by increasing the annealing temperature to 1200–1300 °C. Increasing the annealing temperature to ?1300 °C led to abnormal grain growth with inter-granular pores and this led to a decrease in the saturation magnetization. Moreover, an increase in the Mn2+ ion substitution up to x=0.8 increased the lattice parameter of the formed powders due to the high ionic radii of the Mn2+ ion. Mn–Zn ferrites phases were formed and the positions of peaks were shifted by substituting manganese. The average crystalline size was increased by increasing the annealing temperature and decreased by increasing the substitution by manganese up to 0.8. The average crystalline size was in the range 95–137.3 nm. The saturation magnetization of the Mn–Zn-substituted ferrite powders increased continuously with an increase in the Mn concentration up to 0.8 at annealing temperatures of 1200–1300 °C. Further increase of Mn substitution up to 0.9 led to a decrease of saturation magnetization. The saturation magnetization increased from 17.3 emu/g for the Mn0.3Zn0.7Fe2O4 phase particles produced to 59.08 emu/g for Mn0.8Mn0.2Fe2O4 particles.  相似文献   

11.
The change of the lattice thermal conductivity of bulk nanostructured materials based on Bi2Te3-Sb2Te3 solid solutions with grain size distribution has been studied. These materials have a polycrystalline structure with grain sizes ranging from a few tens of a nanometer to a few micrometers. Large grains may contain inclusions or consist of several smaller parts which can be identified with coherent scattering regions seen in X-ray diffraction. The change of the lattice thermal conductivity mediated by additional scattering by inclusions and grain boundaries has been calculated. This calculation allows for the effect of nanoparticle size distribution. The calculated estimates are compared with the available experimental data.  相似文献   

12.
Nanocrystalline NiFe2O4 powders with different crystal sizes were hydrothermally synthesized with different alkali concentrations at 230 °C. Then the NiFe2O4 nanopowders were sintered at various temperatures to obtain NiFe2O4 ceramics with different grain sizes. The NiFe2O4 ceramic properties were strongly dependent on the grain size. With the increase in the grain size, saturation magnetization (Ms) and magnetic permeability (μ) increased, Curie temperature (Tc) decreased. Dielectric constant enhanced and dielectric loss decreased in nanoferrites. The possible mechanism of grain size dependences was discussed in detail.  相似文献   

13.
The Ba(Ce0.8Zr0.2)0.95Yb0.05O2.975 ceramics electrolyte was prepared via a Pechini method using metal nitrate salts as starting materials. An optimum annealing temperature of 1,400 °C was needed to obtain a pure perovskite-like phase with orthorhombic structure. Particle size distribution showed a bimodal distribution that corresponds to the loose powders and agglomerates size. Scanning electron micrograph revealed that the loose powders were in the nanosize range (70–200 nm). These ultrafine loose powders enhanced the densification of a pellet with relative density ∼95% obtained at 1,400 °C. The sample formed clear and compact grains with submicron sizes. Impedance results showed that the impedance semicircle of the grain was observed only at T ≤ 250 °C. The introduction of 20 mol% Zr improved the chemical stability of BaCe0.95Yb0.05O2.975 sample in atmosphere containing carbon dioxide at 600 °C. The sample also exhibited high proton conductivity in wet hydrogen.  相似文献   

14.
Single phase Sr substituted BaZr0.1Ti0.9O3 (also known as BZT) ceramics with a formula Ba1-xSrxZr0.1Ti0.9O3 (x=0.10, 0.20, 0.30, 0.40 and 0.50) are prepared using a solid state reaction of mixed oxides at 1250 °C for 15 h. Analysis of XRD patterns of the ceramic powders show that Sr substitutes into Ba sites and reduces the lattice parameter. The powders are sintered at 1600 °C for 6 h to investigate the microstructure and functional properties. It is found that strontium substitution significantly modifies the microstructure and greatly influences the dielectric properties. The increase in Sr content reduces the ceramic grain size and results in spherical grains. With an increase in Sr content, the Curie temperature is lowered, and the relative permittivity maximum is increased and broadened. In Ba1-xSrxZr0.1Ti0.9O3 with x=0.30, the relative permittivity reached a maximum of 26600 and with further increase of Sr content, the relative permittivity is lowered and the phase transition is found to be broad and diffused. The ferroelectric hysteresis characteristics are discussed in detail. PACS 77.84.-s; 77.80.Bh; 77.22.Ch; 77.84.Dy; 77.22.-d  相似文献   

15.
The effect of sintering aids of SiO2, ZrO2, B2O3, and MgO oxides on the optimum sintering temperature, ceramics grain growth, total volume of residual pores, and optical quality of obtained ceramics is studied. The best combinations of sintering aids are found; as a result, YAG:Nd (1 at%) samples of ceramics of high optical quality are obtained. An original method for characterizing laser properties of ceramics is developed. Comparative measurements of main laser characteristics of the obtained ceramics and ceramics of the Konoshima Chemical Corp. Ltd wellknown in the world practice, are performed.  相似文献   

16.
The double perovskite Sr2NiMoO6 powders and ceramics were prepared by two different (conventional and precursor) solid-state reaction methods. The phase structure was characterized by XRD and TEM techniques. It has been indicated that single-phase perovskite powders were obtained when calcined in air at 1300°C. However, nano-particles of the size 30–60 nm have been found in powders prepared with the precursor method, while those from the conventional route exhibit large irregular shaped particles with aggregation. The dielectric properties (ε r and tanδ) were also examined in the sintered ceramics. The results showed the transition point at 280°C for conventional route, while no clear phase change was observed in ceramics from the precursor route. These observations clearly indicate that the different starting processes affected the phase formation behavior and the electrical properties of Sr2NiMoO6 ceramics.  相似文献   

17.
Bulk specimens of Ce0.9Gd0.1O2-δ prepared with powders within a range of specific surface area were sintered in oxidizing, inert, and reducing atmospheres. The aim of this work is to investigate the effects of the sintering atmosphere on the microstructure and grain and grain boundary conductivities of the solid electrolyte. The lattice parameter determined by Rietveld refinement is 0.5420(1) nm, and the microstrain was found negligible in the powder materials. Specimens sintered in the Ar/4 % H2 mixture display larger average grain sizes independent on the particle size of the starting powders. The grain and grain boundary conductivities of specimens sintered under reducing atmosphere are remarkably lower than those sintered under oxidizing and inert atmospheres. The activation energy (~0.90 eV) for total electrical conductivity remains unchanged with both the initial particle size and the sintering atmosphere.  相似文献   

18.
MoO3/Al2O3催化剂中Mo分散的正电子研究   总被引:1,自引:0,他引:1       下载免费PDF全文
用浸渍法制备了一系列不同Mo含量的MoO3/Al2O3催化剂.测量了这些样品的正电子湮没寿命谱(PALS)与符合多普勒展宽(CDB)谱,以研究其孔洞结构以及Mo分散.正电子寿命测量结果表明,Al2O3载体中存在两种不同尺寸的孔洞.掺入MoO3之后,Mo原子主要进入Al2O3的大孔中,使孔洞体积减小.符合多普勒展宽谱结果表明,当MoO 关键词: 3/Al2O3催化剂')" href="#">MoO3/Al2O3催化剂 正电子湮没寿命谱 符合多普勒展宽 Mo 分散  相似文献   

19.
透明陶瓷透光性能的影响因素   总被引:1,自引:0,他引:1       下载免费PDF全文
 研究了影响激光透明陶瓷透光性能的主要因素,讨论了陶瓷内部气孔和杂质颗粒等散射粒子、晶界结构中晶界折射率与晶粒折射率的差异以及晶界表面粗糙度等因素对陶瓷透光性能的影响,并定量分析了激光陶瓷透过率随气孔尺寸、气孔率、晶粒相对晶界折射率以及晶界表面粗糙度的变化关系。结果表明: 陶瓷的透过率随着气孔率的减小而增大,但透过率随气孔尺寸的增大而呈现出周期性振荡,且当气孔尺寸与入射光波长可比拟时,陶瓷的透过率会明显降低;在晶界结构中,晶界的折射率与晶粒的折射率相差越小,陶瓷的透过率就越高;晶界表面粗糙度越大,透过率越低。然而,晶界折射率不同于晶粒折射率,这使得其陶瓷透过率降低的程度比对晶界表面粗糙度的影响明显得多。在陶瓷制备过程中,需要重点排除尺寸与入射波长可比拟的气孔, 以抑制晶界结构中第二相的产生。  相似文献   

20.
It has been shown for the first time that the structure of (La0.65Sr0.35)1 ? x Mn1 + x O3 ± Δ (x = 0, 0.1, 0.2) ceramics sintered at 1500°C substantially depends on the content of superstoichiometric Mn. It has been found that the average grain size increases from 10 to >100 μm with increasing x from 0 to 0.2. It has been revealed that grains of the ceramic samples sintered from powders with excess Mn have an internal nanoscale layered structure. A correlation has been revealed between the size and structure of grains and and the magnetoresistive properties.  相似文献   

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