Characterization of Particle Orientations in Ceramics by Electron Paramagnetic Resonance

It is shown that electron paramagnetic resonance spectroscopy provides an accurate method for measuring the preferential orientation of axial crystallites in ceramics. Since the physical properties of ceramic materials are correlated with the degree of this orientation, EPR can be a convenient and useful tool for the characterization of ceramics.     

Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: I, Concepts and Theory

Electron paramagnetic resonance (EPR) spectroscopy is shown to be a powerful and versatile technique for the detection and characterization of preferred orientation effects in insulating ceramics. While the technique is applicable to both cubic and noncubic ceramics, in this work the conceptual and theoretical basis for the analysis of textured cubic materials is established. The practical utility of the EPR technique is illustrated for the specific case of a paramagnetic impurity (Mn²⁺) in the cubic ceramic MgO. For this example, EPR methods show that texturing occurs in a cold pressed, sintered body and that the preferred orientation corresponds to 〈111〉 axes lying nearly parallel to the direction of the cold-pressing force. The advantages of the EPR technique in characterizing preferred orientation effects in ceramic materials are discussed and compared with more traditional diffraction-based and optical characterization methods.     

Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: II, Formation and Properties of Textured MgO

A method for the formation of macroscopic quantities of faceted, nanophase MgO precursor powders has been developed that permits a degree of control over the particulate size distribution. By employing powders of this type it was possible to form textured MgO ceramics by means of cold-pressing and sintering. The theoretical basis for the application of EPR techniques to texture analysis as developed in Part I of this work was used to establish the nature of the preferred orientation in these MgO ceramics. Both EPR and X-ray analyses showed that preferred orientations of the 〈111〉 axes of the MgO microstructural grains occur along or near the direction of force used to compact the green body. EPR results also show that this texture develops during the sintering process and that no preferred orientation effects are present in the unsintered body formed by cold-pressing the faceted, nanophase MgO powder. Effects of parametric variations such as sintering temperature, compaction force, and faceted particle size on the degree of preferred orientation in these sintered MgO ceramics were investigated using EPR techniques. The results show that the amount of texturing increases with increasing compaction force or temperature up to a limiting value and that it also increases with decreasing faceted particle size.     

Three-Dimensional Electron Paramagnetic Resonance Imaging Technique for Mapping Porosity in Ceramics

A three-dimensional (3D) electron paramagnetic resonance imaging (EPRI) method was developed to probe the structure and size of pores in ceramic materials. The imaging device that was added to the EPR instrument consisted of a computer-controlled current source and magnetic field gradient. This add-on facility was tested using a well-defined diphenylpicrylhydrazyl phantom sample. Pumice was then used to demonstrate the potential of the technique. This stone was immersed in a 0.5 m M ¹⁵N-substituted perdeutereted tempone water solution to fill the pores with spin labels. Images were reconstructed using a filtered backprojection technique. A two-dimensional (2D) imaging plane was constructed by collecting 33 projection planes over 180°. A 3D image was derived from 22 planes each constructed by 22 projections. At present, the facility allows a resolution of 69 and 46μm for 2D and 3D imaging, respectively. Advancements of the imaging apparatus, software, and line width of the spin labels will be needed to enhance the resolution of this technique.     

Electron Paramagnetic Resonance Investigation of Stratum Corneum Lipid Structure

Electron paramagnetic resonance (EPR) in conjunction with a slow-tumbling simulation was utilized for defining stratum corneum (SC) lipid structure. We found that ordering calculated from the simulation is an appropriate index for evaluating SC lipids structure. The SC from two sites (mid-volar forearm and lower-leg) of human volunteers was stripped consecutively from one to three times using a glass plate coated with a cyanoacrylate resin. Aliphatic spin probes, 5-doxylstearic acid (5-DSA) and 3β-doxyl-5α-cholestane (CHL), were used to monitor SC ordering. EPR spectrum of 5-DSA incorporated in the SC demonstrated a characteristic peak for the first strip. However, EPR spectra of CHL in the SC did not show a clear difference for each strip, except for the peak intensity. The results imply that CHL is not incorporated into the lipid phase as easily as 5-DSA is. A slow-tumbling simulation of the EPR spectrum was performed to analyze the detailed lipid structure. The simulation results for 5-DSA show differences in values of the SC ordering as a function of depth. Thus, these results along with the simulation analysis provide a detailed SC layer structure.     

Tetrachloridocuprates(II)-Synthesis and Electron Paramagnetic Resonance (EPR) Spectroscopy

Ionic liquids (ILs) on the basis of metal containing anions and/or cations are of interest for a variety of technical applications e.g., synthesis of particles, magnetic or thermochromic materials. We present the synthesis and the results of electron paramagnetic resonance (EPR) spectroscopic analyses of a series of some new potential ionic liquids based on tetrachloridocuprates(II), [CuCl(4)](2-), with different sterically demanding cations: hexadecyltrimethylammonium 1, tetradecyltrimethylammonium 2, tetrabutylammonium 3 and benzyltriethylammonium 4. The cations in the new compounds were used to achieve a reasonable separation of the paramagnetic Cu(II) ions for EPR spectroscopy. The EPR hyperfine structure was not resolved. This is due to the exchange broadening, resulting from still incomplete separation of the paramagnetic Cu(II) centers. Nevertheless, the principal values of the electron Zeemann tensor (g(║) and g(┴)) of the complexes could be determined. Even though the solid substances show slightly different colors, the UV/Vis spectra are nearly identical, indicating structural changes of the tetrachloridocuprate moieties between solid state and solution. The complexes have a promising potential e.g., as high temperature ionic liquids, as precursors for the formation of copper chloride particles or as catalytic paramagnetic ionic liquids.     

电子顺磁共振技术

<正> 电子顺磁共振(EPR)又叫做电子自旋共振(ESR),是近二、三十年才迅速发展起来的一门新兴学科。它是利用具有未成对电子的物质在静磁场作用下对电磁波的共振吸收特性来进行分析的技术。这一研究手段具有深入物质内部,进行细致分析而不破坏样品;对化学反应过程无影响;灵敏度高和信息丰富等优点。目前已广泛地应用于物理、化学、生物学、医学以及各种材料科学领域。EPR 技术仅适用于含有未成对电子的物质,EPR 手段却能更多地研究那些以微量杂质、吸附中心、活性中心、缺陷、色心及自由基等微量顺磁中心的形式,广泛地散布在反磁性材料中的体系,即使在样品中不含有不成对电子,也可采用人工的方法在材料中形成不成对电子,然后用EPR 技术加以研究。例如吸     

Electron Paramagnetic Resonance, Optical, and Electronic Structure Studies of Photorefractive Barium Titanate

Barium titanate (BaTiO₃) is an important electrooptic ferroelectric oxide material which exhibits the unusual optical nonlinearity of photorefractivity. There is evidence that the photorefractive effect, which refers to the change in the index of refraction of the crystal by the action of light, is related to the presence of defects/impurities such as transition metal ions and/or cation or anion vacancies. This paper will review our electron paramagnetic resonance, optical, and theoretical eletronic structure studies aimed at probing the nature of the defects/impurities in BaTiO₃ and their role in the photorefractive effect.     

Ferroelectric-Domain Structure in Piezoelectric Ceramics

Twin-related (tetragonal (T), rhombohedral (R)) adjacent ferroelectric domains, which require a smaller energy for the rotation of the polarization vector, have been found to occur in lead zirconate titanate type piezoceramics at the morphotropicphase boundary, explaining the triplet splitting observed in electron diffraction patterns. The values of elastically stored energy density in the walls of the twinning domains are estimated, and a packing model of the ferroelectric domain structure as a T₁RT₂RT₁…succession is proposed to explain the coexistence of the two ferroelectric phases in the morphotropicphase boundary region. A crystallographic analysis of the transformation from the paraelectric to the ferroelectric phase is performed on the basis of martensite transformation theory. The calculated results are in good agreement with the observed transmission electron microscopy data. In particular, the crystallography and morphology of the "herringbone" domain structure, often observed in piezoelectrio ceramics, are well explained by this analysis.     

Electron Paramagnetic Resonance of Titanium Impurities in Reduced Yttria-Stabilized Zirconia

The electronic defects in single crystals of yttria-stabilized zirconia (YSZ) have been examined using electron paramagnetic resonance (EPR). The EPR spectra of hydrogen-reduced YSZ single crystals are almost identical to those of polycrystalline yttria (12 mol%)-stabilized zirconia-titania, indicating that the Ti³⁺ cation is the predominant electronic defect in both materials. The disparity between the discoloration of the polycrystals and single crystals (blue vs yellow) is attributed to the different crystalline environments of the Ti³⁺ cations in the grain boundaries and lattice. The concentration of titanium cations in the YSZ single crystals (ca. 600 ppm) is probably too dilute to contribute to the electronic conductivity.     

Electron Paramagnetic Resonance Spectroscopy Applied to Diffusion of Mn2+ in MgO

Electron paramagnetic resonance spectroscopy was used to measure the diffusion rates of Mn²⁺ in MgO single crystals at 1200 to 1520 K. It is shown that the spectral components of Mn²⁺ must be separated into one due to a high concentration and a set due to a low concentration of Mn²⁺ ions in the diffusion samples. It was found that the data fit a simple ion-diffusion model in which the activation energy E = 2.1±0.25 eV and D₀= (9.4±s2.8)× 10⁻⁶ cm² s ⁻¹.     

Charge-Compensating Defect in Ceria-Doped Strontium Barium Niobate by Electron Energy-Loss Spectroscopy and X-ray Photoelectron Spectroscopy

Undoped and CeO₂-doped Sr_0.5Ba_0.5Nb₂O₆ (SBN) powders were synthesized using the solid-state reaction method. The lattice parameters of undoped and CeO₂-doped SBN were evaluated using X-ray diffractometry. The valence state of the cerium ion in Ce-doped SBN was identified using X-ray photoelectron spectroscopy (XPS), and the valence of 3+ was confirmed. The charge-compensated defect of the cerium doping in SBN ceramics, i.e., excessive oxygen ions occupying the vacant O(4) or O(5) site, was further evidenced using electron energy-loss spectroscopy (EELS) and XPS. The relationships of charge-compensated defect, structure, binding energy, and temperature of the maximum of the relative permittivity ( T _m) were discussed.     

Electron Paramagnetic Resonance in Investigation of Polymer Fibres. A Review

The results obtained with EPR in studying polymer fibres suggest the following conclusions: EPR offers broad opportunities for studying the causes of the decrease in the strength of polymer fibres on exposure to temperature and light, and stabilization of the strength properties in irradiation (sterilization) of medical fibre materials allows tracing the causes of the decrease in or preservation of the therapeutic properties of native and immobilized enzymes; EPR with stable nitroxyl radicals (spin-probe method) allows investigating the kinetics of formation of new phases in polymer—solvent systems and the effect of the conditions of fabricating the fibres on the quantitative ratio between dense and loose packing of macromolecules in the amorphous structure of polymers; EPR allows identifying complexation of some metal ions with different functional groups in polymers.     

Characterization of Additive-Modified Beta Alumina Ceramics by Nuclear Magnetic Resonance

Sintered compacts of beta alumina have been made with additives of Li₂O, MgO, NiO, CaO, and Y₂O₃. The motional narrowing and quadrupole interactions of sodium were investigated as a function of temperature and additive concentration by wide-line NMR. The Li₂O, MgO, and NiO additives are incorporated in the beta alumina structure and enhance Na motion; CaO dissolves and inhibits Na motion; and Y₂O₃ forms a second phase and has no effect on the Na. The effects of the additives are explained by the size and charge of the additive cation, and intragranular and intergranular conductivity in doped beta alumina ceramics are predicted.     

Electron Paramagnetic Resonance Investigation of Acceptor Centers in Pb(Zr,Ti)O3 Ceramics

The nature of extrinsic point defects in lead zirconate titanate (PZT) ceramics of various compositions prepared by solution chemistry has been explored. Using electron paramagnetic resonance (EPR), several impurity sites have been identified in the as-received materials, which include Fe³⁺oxygen vacancy (V_O^‥) complex and isolated Cu²⁺ ions; both of these ions are incorporated into the lattice by replacing the Ti(Zr) ion. A Fe₃₊V_O^‥ complex serves as a sensitive probe of the local crystalline field environment of the ceramic; the symmetry of this defect is roughly correlated with its phase diagram as the composition is varied from PbTiO₃ to PbZrO₃. The Fe³⁺V_O^‥ complex experiences tetragonal, rhombic, or orthorhombic symmetry as the composition is varied from PbTiO₃ to PbZrO₃. As the composition of the Cu²⁺ ion is varied, it appears as though the addition of Zr, and not necessarily a change in phase, is largely responsible in determining the local environment of this acceptor impurity. Also, the Cu²⁺ resonance parameters weakly reflect the relative Ti–O(Zr–O) bond covalency in the perovskite lattice.     

Structural Characterization of High-Temperature Zirconia Ceramics by Perturbed Angular Correlation Spectroscopy

Perturbed angular correlation spectra of γ-rays emitted following the decay of dilute ¹⁸¹Hf in several zirconia ceramics are reported. Spectra for monoclinic and tetragonal zirconia, a tetragonal zirconia/yttria alloy, a cubic zirconia/yttria alloy, and two mixed tetragonal/cubic-phase zirconia/yttria alloys were measured as a function of temperature to 1470°C. The spectrum observed for each phase has a unique signature, and the spectrum of mixed-phase materials can be used to determine the relative amounts of the different phases. These data give a cubic/(cubic + tetragonal) phase boundary that is somewhat lower in temperature than indicated by current phase diagrams.     

CaZrO3改性KNN基无铅压电陶瓷结构与电学性能研究

通过固相反应法制备(1-x)(K0.48Na0.52)0.96Li0.04(Nb0.96Sb0.04) O3-xCaZrO3(KNN-xCaZrO3)无铅压电陶瓷,获得了正交-四方(O-T)多晶共存相.结果 表明:未经改性的KNN基陶瓷为正交相,CaZrO3引入后出现了四方相,体系的压电性能也得到了改善;随着CaZrO3含量的增加,四方相含量增加.在x=0.03 ～0.04范围内,O-T两相共存,并且在x=0.03时陶瓷具有优异的压电性能:d33=252 pC/N,kp=0.55.当x=0.06时,陶瓷为贋立方相结构,性能急剧恶化.CaZrO3的引入使KNN正交-四方相变温度(To-T)向室温偏移,同时也降低了居里温度(Tc),室温附近O-T相界共存,促进畴壁运动,是压电性能得以提高的根本原因.     

铋层状结构无铅压电材料的研究现状与发展趋势

铋层状结构无铅压电材料因其具有居里温度高、品质因数高、击穿强度及各向异性大等特性而受到人们的重视.本文介绍了铋层无铅压电材料的结构特点,综述了该体系压电陶瓷的微量元素与制备工艺对材料压电、铁电性能的影响规律,着重讨论了不同位离子取代对陶瓷性能和结构的影响,概括了制备高取向陶瓷的先进制备技术,并展望了铋层无铅压电材料未来的发展趋势.