Characterization,spectroscopic investigation of defects by positron annihilation,and possible application of synthesized PbO nanoparticles

Nanocrystalline samples of highly pure lead oxide were prepared by the sol-gel route of synthesis.X-ray diffraction and transmission electron microscopic techniques confirmed the nanocrystallinity of the samples,and the average sizes of the crystallites were found within 20 nm to 35 nm.The nanocrystallites exhibited specific anomalous properties,among which a prominent one is the increased lattice parameters and unit cell volumes.The optical band gaps also increased when the nanocrystallites became smaller in size.The latter aspect is attributable to the onset of quantum confinement effects,as seen in a few other metal oxide nanoparticles.Positron annihilation was employed to study the vacancy type defects,which were abundant in the samples and played crucial roles in modulating their properties.The defect concentrations were significantly larger in the samples of smaller crystallite sizes.The results suggested the feasibility of tailoring the properties of lead oxide nanocrystallites for technological applications,such as using lead oxide nanoparticles in batteries for better performance in discharge rate and resistance.It also provided the physical insight into the structural build-up process when crystallites were formed with a finite number of atoms,whose distributions were governed by the site stabilization energy.     

Vibrational Spectral Characterization of a Lead Lanthanum Zirconate Titanate During Various Stages of Sol-Gel Processing

Lead lanthanum Zirconate Titanate (PLZT 7/65/35) powders were prepared by a sol-gel process, using lead acetate, lanthanum acetate, zirconium n-propoxide and titanium n-propoxide as precursors along with 2-methoxyethanol as the solvent. Nitric acid was used to catalyze the sol-gel reaction. FTIR spectroscopy characterized the starting precursors, monitored the sol to gel transformations involving solutions with different molar ratios of water to alkoxide precursors, and tracked the crystallization behavior of the gels upon heat treatment. Raman spectroscopy was employed at room temperature to follow vibrational spectral changes for the samples that were cooled after various stages of heat treatment and crystallization of the gels. The resulting spectral changes were analyzed in terms of the structural changes that occurred during the sol-gel process.     

Effect of La3+ and Pr3+ co-doping on structural,thermal and electrical properties of ceria ceramics as solid electrolytes for IT-SOFC applications

In the present investigation, the effect of La³⁺ and Pr³⁺ co-doping on structural, thermal and electrical properties of ceria ceramics useful as solid electrolytes in intermediate temperature solid oxide fuel cells (IT-SOFCs) has been studied. The co-doped ceria Ce_0.8Pr_0.2–xLa_xO_2-δ samples have been prepared successfully via sol-gel auto-combustion synthesis. The high dense ceramic samples have been achieved by carry out an optimized conventional sintering at 1300 °C for 4 h. The powder X-ray diffraction analysis of all the co-doped ceria ceramics revealed the single phase with cubic-fluorite structure formation. Crystallographic information has been carried out from the powder X-ray diffraction and Rietveld refinement analysis. The scanning electron microscope and energy dispersive spectroscopy analysis revealed the smaller grain size with high density in microstructure and stoichiometric elemental confirmations. Raman spectra of prepared ceramics revealed the information of phase and oxygen vacancy formation in the entire compositions. The dilatometric studies of prepared co-doped ceria ceramics revealed the moderate coefficients of thermal expansion. The electrical parameters such as total conductivities and activation energies have been studied with the help of impedance spectroscopy. Among all these co-doped ceria ceramic samples, Ce_0.80Pr_0.10La_0.10O_2−δ found to exhibit the highest value of total ionic conductivity with minimum activation energy and this makes it could be a promising electrolyte material for IT-SOFC applications.     

Additive coloring of CaF<Subscript>2</Subscript> optical ceramic

The specificity of additive coloring of CaF₂ optical ceramic (formation of color centers in it and photothermochemical transformation of these centers in colored ceramic samples) has been considered. Under the same coloring conditions, this process occurs more slowly in ceramics rather than in crystals; at the same time, the limiting concentration of color centers that can be introduced into ceramics is much higher. The photothermochemical transformations of color centers in crystals and ceramics, which occur under illumination at different wavelengths and upon heating, have been studied. The specific features of introduction of color centers into ceramic and their transformation under illumination and heating are likely to be related to the mass twinning of ceramic grains.     

Nonlinear mechanical behavior of piezocomposites for ultrasonic transducers

A comparative study is carried out between the nonlinear behavior of a composite and the piezoceramic used to obtain it. This characterization is necessary for using the composite in power transducer applications. A study of the losses and the resonator stiffness variations has also been done. Both these effects, as well as the possibility of the frequency hysteresis, show different behavior in the composites, since the increases in the ceramics are different from those in the composites. In this study two measurement methods are used: principally the motional impedance increase with the motional current measurements. The results obtained are normalized in order to make them independent of the resonator size, and thus make the comparison between the composite and the ceramic easier. The figure of the mechanical loss tangent tan delta m versus the mean strain shows that the losses can be greater in the ceramic than in the composite for soft ceramics. The dependence behavior of the losses and stiffness variations versus the mean strain is studied for both resonators.     

A semi-empirical equation for oxygen nonstoichiometry of perovskite-type ceramics

Explicit equations correlating oxygen nonstoichiometry to oxygen partial pressure and temperature are important for applications of perovskite-type ceramics as membranes, adsorbents and catalysts in various chemical reaction and separation processes. A semi-empirical equation for oxygen nonstoichiometry on perovskite-type ceramics is reported in this paper. Though derived from the results of a point defect model on a perovskite-type ceramic material, La_0.1Sr_0.9Co_0.5Fe_0.5O_3−δ, this equation describes very well the experimentally measured oxygen nonstoichiometry data for two perovskite-type ceramics measured in this work and three perovskite-type ceramics reported in the literature. The major advantage of this semi-empirical equation lies in its simplicity, explicitness and accuracy. This equation is coupled with oxygen permeation equation to predict oxygen permeation current density through two perovskite-type ceramic membranes. The predicted data agree very well with the results reported in the literature using a complex defect reaction model.     

放电等离子烧结制备ZrB2-SiC超高温陶瓷的力学性能及氧化行为

在服役环境中,超高声速飞行器表面与空气剧烈摩擦导致温度极高。超高温陶瓷相较于一般陶瓷而言具有高熔点和良好的抗氧化烧蚀性能,是目前极具前景的热防护材料之一。采用放电等离子两步烧结工艺将ZrB2纳米粉末和SiC粉末在1700℃下制备超高温陶瓷材料ZrB2-20%SiC,通过纳米压痕微观实验、三点弯实验研究其力学性能及其在高温环境下的氧化行为,着重分析1000、1200、1400和1600℃4种不同氧化温度下ZrB2-20%SiC超高温陶瓷的氧化表面、氧化截面和氧化层厚度。结果表明:ZrB2-20%SiC超高温陶瓷的硬度为18 GPa,弹性模量为541 GPa,断裂韧性为5.7 MPa·m1/2;当氧化温度为1600℃时,超高温陶瓷内部的SiC由被动氧化转变为主动氧化,并且随着氧化温度升高,超高温陶瓷氧化层厚度与氧化温度呈正相关。     

Gelation and Swelling Behavior of Semi-Interpenetrating Polymer Network Hydrogels Based on Polyacrylamide and Poly(vinyl alcohol)

Novel semi-Interpenetrating Polymer Network (semi-IPN) hydrogels based on partially Hydrolyzed Polyacrylamide (HPAM) and Poly(Vinyl Alcohol) (PVA) were prepared by solution crosslinking using chromium triacetate. Effects of PVA content on the gelation process and swelling behavior in tap water and different electrolyte solutions were investigated. Study of the gelation behavior using dynamic rheometery showed that the limiting storage modulus of the semi-IPN gels decreased with increasing PVA content. It was also found that increasing the PVA content increases the loss factor, indicating that the viscous properties of this gelling system increase more strongly than the elastic properties. The swelling ratio of the semi-IPN gels in tap water decreased as the concentration of the PVA increased. However, the semi-IPN gels showed lower salt sensitivity factor in synthetic oil reservoir water as compared with HPAM gels. Therefore, they are potentially good candidates for enhanced oil recovery applications.     

The use of ceramics for retrospective dosimetry in the Chernobyl exclusion zone

The use of luminescence techniques with ceramic materials is playing an increasingly important role in retrospective dosimetry. Thermoluminescence measurements with ceramics at Hiroshima and Nagasaki and in areas downwind of the Nevada Test Site have shown that dose estimates may be obtained which are of value in comparisons with the results of modelling calculations. The Chernobyl accident has provided a recent example where retrospective dosimetry is urgently required in order to advance epidemiological studies of the population. This paper examines some of the approaches which are being used with luminescence techniques to provide dose estimates for samplesfrom Pripyat and how they can be used to contribute to the wider problem of dose reconstruction in the 30 km Exclusion Zone.     

Mechanically reliable ceramics: Needs and opportunities to understand and control fracture

The approaches used to obtain mechanically reliable ceramics are first reviewed, with the focus on their brittle failure because this is the major problem. After outlining needs to improve these reliability approaches, more basic issues of (1) crack formation, detection, characterization and propagation; and (2) development of more reliable materials by developing tougher or new materials are addressed. While the focus is on bulk ceramics, some specific discussion of ceramic coatings is also presented. Needs and opportunities noted for understanding of cracks range from many of the specifics of their occurrence and behavior on both a continuum and especially microstructural scale to very fundamental issues such as fracture on an atomic scale and particle and electromagnetic emission during fracture. Similarly needs and opportunities for understanding to obtain more reliable materials range from specifics of a variety of toughening mechanisms to the development of material property estimation techniques so that unique new materials, e.g. high temperature, analogs of jade, can be more effectively sought.     

Thermally induced birefringence in Nd:YAG ceramics

Analytical expressions for eigenpolarizations and phase delays in grains of thermally loaded Nd:YAG ceramic rods have been derived. It is shown that the depolarization of radiation in polycrystalline ceramics results in beam modulation with a characteristic size of the order of the ceramic grain size. It is reasonable to increase the ratio of rod length to grain length both to diminish this modulation depth and to compensate for birefringence by use of known techniques.     

Low-temperature sintering and electromagnetic properties of ferroelectric-ferromagnetic composites

Ferroelectric-ferromagnetic composites were prepared by the usual ceramic technology combined with the sol-gel method and sintered at 900 °C to adapt to the low-temperature co-fired ceramic (LTCC) technology. The ferroelectric ceramics in reasonable amounts can effectively decrease the RF loss, dielectric loss and increase the saturation magnetization. Variations of permeability, dielectric constant and loss tangent with the frequency in the range of 1 MHz-1.8 GHz have been discussed. The microstructures of the sintered ceramics were analyzed by scanning electron microscope (SEM). The influences of different composition on the electromagnetic properties of the composites have been investigated.     

感耦等离子体发射光谱/质谱分析新型陶瓷材料研究进展

新型陶瓷材料在信息、航空航天、生命科学等现代科学技术各个领域中发挥了极其重要的作用.微量、痕量杂质对材料的光、电等性能影响巨大.因此微量、痕量杂质含量的精确测定对材料制备和性能调控至关重要.文章综述了近10年来等离子体发射光谱/质谱(ICP-AES/MS)在新型陶瓷材料的微、痕量杂质分析方面的应用.结合作者的研究工作,着重介绍和评论了等离子体发射光谱/质谱各种进样方法的优缺点,并展望其发展趋势.     

Three-dimensional magnetic resonance microscopy of materials

Several aspects of magnetic resonance microscopy are examined employing three-dimensional (3D) back-projection reconstruction techniques in combination with either simple Bloch-decay methods or MREV-8 multiple-pulse line narrowing techniques in the presence of static field gradients. Applications to the areas of ceramic processing, catalyst porosity measurements and the characterization of polymeric materials are presented. The focus of the discussion centers on issues of sensitivity and resolution using this approach compared with other methods. Advantages and limitations of 3D microscopy over more commonly employed slice selection protocols are discussed, as well as potential remedies to some of the inherent limitations of the technique.     

Effect of akermanite morphology on precipitation of bone-like apatite

Bioactivity in vivo of ceramic materials has been related to their surface micro-topography and may be estimated by means of simulated body fluid method in vitro. In order to investigate the effect of surface topographies of akermanite ceramics on bioactivity in vitro, akermanite ceramics were synthesized by sol-gel method and different surface topographies of disc-shaped akermanite ceramics were prepared by polishing with different SiC sandpapers. Atomic force microscopy (AFM) was used to evaluate the surface morphology and roughness. The bioactivity in vitro of ceramics with different surface states was evaluated by soaking the ceramics in simulated body fluid (SBF). And the samples after being soaked were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometry (EDS). The results showed that the amounts of precipitated apatite on the ceramics with different surface roughness after being soaked in SBF were different and the bioactivity in vitro of ceramic with rough surface was significantly higher than that of ceramic with smooth surface. The study suggested that suitable surface roughness may improve the bioactivity in vitro of akermanite ceramics.     

溶胶-凝胶法制备的高压电常数（Bi0.5Na0.5）1－xBaxTiO3系无铅压电陶瓷

测量了使用溶胶-凝胶工艺制备的(Bi0.5Na0.5)1-xBaxTiO3(x=0.00，0.04，0.06，0.08,0.12)系陶瓷的介电、压电、铁电和热释电性能.由于使用了溶胶-凝胶工艺制备的粉料，因此所有样品的压电性能都得到了较大提高.其中(Bi0.5Na0.5)0.94Ba0.06TiO.3系陶瓷具有该系列最大的压电常数，d33=173×10-12C/N，与传统工艺相比，d33提高了近40%.同时，在一定范围内，随Ba含量的增加，材料的剩余极化Pr和矫顽场Ec逐渐减小，退极化温度逐渐降低.对于(Bi0.5Na0.5)0.94Ba0.06TiO.3系陶瓷，剩余极化和矫顽场分别为25μC/cm2和28kV/cm，退极化温度约为80℃. 关键词： 溶胶-凝胶 压电常数 剩余极化 矫顽场     

Ferroelectricity and ferromagnetism in fine-grained multiferroic BaTiO3/(Ni0.5Zn0.5)Fe2O4 composites prepared by a novel hybrid process

Dense, homogeneous, and fine-grained multiferroic BaTiO₃/(Ni_0.5Zn_0.5)Fe₂O₄ composite ceramics are synthesized by a novel powder-in-sol precursor hybrid processing route. This route includes the dispersion of nanosized BaTiO₃ ferroelectric powders prepared via conventional sold-state ceramic process into (Ni_0.5Zn_0.5)Fe₂O₄ ferromagnetic sol-gel precursor prepared via a sol-gel wet chemistry process. The composite ceramics show coexistence of obvious ferroelectric and ferromagnetic hysteresis loops at room temperature. Very low dielectric loss of about 0.02–0.0067 in the range of 10 kHz–10 MHz can be achieved, which is about an order of magnitude lower than the results of many reports using conventional processes at room temperature. The combination of high permeability and permittivity with low losses in the ceramics enables significant miniaturization of electronic devices based on the ceramics.     

Synthesis and characterization of single- and co-doped SnO2 thin films for optoelectronic applications

Doped SnO₂ thin films have been prepared by sputtering from two different targets: antimony doped tin oxide (ATO) and antimony and zinc doped tin oxide (AZTO). In the case of ATO ceramic, the antimony amount only reaches 0.012 mol per formula unit due to its evaporation at high temperature while the presence of Zn²⁺ in AZTO prevents the antimony evaporation, greatly enhances the ceramic density and allows the deposition of thin films with a high deposition rate. Both types of thin films have a dense morphology with a smooth surface and they are polycrystalline. For post-annealed ATO thin films, the Drude model was applied to deduce the carrier concentration, the optical mobility as well as the resistivity. The carrier concentration is around ten times higher for ATO thin films compared to AZTO. The two combined effects (higher carrier concentration and mobility) for ATO thin films doped with 1.2% of Sb lead to the best optoelectronic performances, confirming previous results obtained with ceramics. Nevertheless, we have a better opportunity to modulate the conductivity in the case of AZTO thin films.     

Molecular simulation of polymeric networks and gels: phase behavior and swelling

Polymer gels are commonly used in industrial, analytical, and domestic applications; their uses are likely to continue expanding as gels with novel chemical and structural characteristics are developed. These applications often rely on the precise control of the adsorption behavior of a gel. Development of useful gels, however, has been hampered by a lack of molecular-level understanding of the physics underlying phase transitions in such materials. In this report, we review recent molecular simulation work related to the study of fundamental aspects of network elasticity and of phase transitions in polymeric gels. In particular, simulations of simplified (coarse-grained) molecular models are described which provide insights into the general behavior of gels, as opposed to studies concerned with the properties of specific materials. Methodological aspects unique to the simulation of different properties of polymeric gels are emphasized. We also pay special attention to the role of entropic factors (such as network topology, backbone stiffness, chain length asymmetry), over that of energetic interactions (such as hydrofobic interactions or ionic forces) on the onset and characteristics of phase transitions in gels. In spite of the important advances made over the last years in methodology and computer hardware, many challenges remain if phase transitions for more realistic gel models are to be simulated.     

Infrared-optical characteristics of ceramics at elevated temperatures

In this work the infrared-optical properties of ceramic samples at ambient and elevated temperatures are presented. The aim of this work is the determination and interpretation of the spectral transmittance, reflectance and emittance of three oxide ceramic samples (MgO, Al₂O₃/ZrO₂, TiO₂/Y₂O₃) and one carbide ceramic sample (SiC) as a function of the temperature from 300 K up to 1100 K. The semi-transparency of the ceramics in the near infrared region has to be considered when performing the measurements and evaluating the data.The experimentally derived spectra are compared with the values derived from theoretical calculations. The complex dielectric function of a ceramic can be deduced from the Lorentz oscillator model. The complex refractive index is correlated with the complex dielectric function by the Maxwell relation. Scattering effects can be described by radiative transfer models. Finally the infrared-optical properties transmittance, reflectance and emittance depend on the complex refractive index and the morphology of the ceramic sample.Of special interest is the Christiansen wavelength as well as the location of the free running frequencies of the longitudinal and transversal optical oscillations. Usually the ceramic is highly reflecting between the free running frequencies of the longitudinal and transversal optical oscillations whereas at the Christiansen wavelength the reflectance vanishes which leads to an emittance of one for an optically thick sample at the Christiansen wavelength. Due to the known emittance it is possible to use the Christiansen wavelength for determining the temperature of the investigated sample.Finally the obtained infrared-optical spectra are presented and their temperature dependencies are discussed which are mainly due to phonon excitations. Additionally it has been shown that for example the location of the Christiansen wavelength can be influenced by varying the composition of the ceramic.