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High-resolution electron microscopy and cathodoluminescence scanning electron microscopy of SrTiO3-based ceramics elucidating their varistor property.

Shiojiri M., Isshiki T., Nishio K., Saijo H., Nomura T., Hitomi A., Sato S.

Inżynieria Materiałowa

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1998

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R. XIX, nr 4

779-784

EN

Semiconductive SrTiO3-based ceramics practically used as varistors and capacitors have been investigated using HRTEM, EDX and CLSEM. They were produced with and without an additonal reducing treatment at Po2=10 raising to a 12th power Pa in the sintering process at Po2=10 raising to a -8 power Pa, followed by reoxidizing process in the air. (Sr0.35Ba0.35Ca0.30)1.026TiO3 varistors produced without the additional heat-treatment are composed of well-developed crystalline grains with facet boundaries, having a low varistor coefficient (alpha<4). CL revealed that the grain, particularly near varistor surface, has high conductive inside containing oxygen vacancies and less-conductive boundary layer without the oxygen vacancies. The boundary layers may work as the double Shottky barriers. The additional reducing treatment forms a lot of oxygen vacancies in the grains of the varistors and damaged the crystallinity near the grain boundaries. The damaged regions easily introduce oxygen atoms into the grains during the reoxydizing process. The additional treatment, hence, produces thick boundary layers and consequently gives rise to high varistor coefficient (alpha>4), which is favourable for practical use. (Sr0.94Ba0.01Ca0.05)0.99TiO3 capacitors were also produced without the additional reducing treatment. Their grains have the facet boundary structure. The dielectric boundary layer and semiconductive inside of the grain are also observed. The boundary layer is as thin as a few ten nanometers, which cause the ceramic a high capacitance.

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Investigations of materials in random-system by quantitative high-resolution transmission electron microscopy and electron diffraction

Shiojiri M., Endoh H., Saijo H., Isshiki T., Nishio K.

Electron Technology

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1998

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Vol. 31, No. 2

189-192

EN

A detail of our project, "analysis of materials in random system by accurate measurement of high-resolution transmission electron microscopy (HRTEM) contrast and electron diffraction (ED) intensity" is reviewed. The main method is quantitative HRTEM and ED using an imaging plate (IP) recording and processing system, with the aid of the simulation and compresion. X-ray and neutron diffraction and cathodoluminescence scanning microscopy are also used as complementary methods. We have been making investigations on: 1) the local atomic configuration in cellular random system such as BaTi₂₋xSnFe₄O₁₁ and Fe-Al alloys, and the relation between their structure and magnetic properties; 2) the distribution of atoms at the heterointerfaces in multilayers such as GaAs/GaInP/Al-GaInP and GaAs/AlGaAs, and in polymorphic Zn(Mg)Se; 3) the site of doped Y atoms in BaTiO₃; 4) the structure and crystallization of elements and metal oxides in topological random system.

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High-resolution transmission electron microscopy and cathodoluminescence scanning electron microscopu observations of SrTiO₃-based varistors

Shiojiri M., Isshiki T., Saijo H., Sato S., Hitomi A.

Electron Technology

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1998

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Vol. 31, No. 2

170-175

EN

(Sr₀.₃₅Ba₀.₃₅Ca₀.₃₀)₁.₀₂₆TiO₃ ceramic varistors, which were produced by sintering in different reducing ambiences and by reoxidizing, have been investigated by high-resolution electron microscopy, electron probe microanalysis, and cathodoluminescence (CL) scanning electron microscopy. Varistors produced by ordinary sintering Po₂=10⁻⁸ Pa, 1400°C, 2 h) have low varistor coefficient are compos4ed of well-developed crystalline grains with facet boundaries. A severe reducting treatment (Po₂=10⁻¹² Pa, 1100°C, 2 h), added in the sintering process, damages the crystallinity in the grains by introducing many oxygen vacancies and changes the boundary zone to amorphous-like microcrystallites, which causes high varistor coefficient favourable for practical use. CL images vizualize the oxygen vacancies in the grains and glassy segregate phase (SiO₂-BaO-MgO) at boundary among three grains. It is found that the surface layer, which is composed of bright enclosed by dark boundary zones in CL images, is responsible for the varistor property.

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An application of cathodoluminescence microscopy on morphology of photosensitive dyes on AgBr crystals

Saijo H., Shiojiri M.

Electron Technology

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1998

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Vol. 31, No. 2

176-182

EN

Silver halides have been a unique material to record images in high qualty and with high sensitivity. However, the silver halides do not have enough sensitivity to exposing lighgt of green to red region. Photosensitive dyes adsorbed on silver halide microcrystals work to absorb light of the wavelength and inject photoelectrons to cause latent images in the substrates. This technique, spectral sensitization, is a key technology of modern photographic materials. Photosensitive dyes, generally cyanine dyes, assemble with their counter ions to form small crystallites, J-aggregates, on the surface of silver halide microcrystals. The incident electrons to the aggregates in an electron microscope produce photon emission, so-called cathodoluminescence (CL). The CL micrographs provided the information on shape, size, nucleation and growth, adsorptin position and perferential adsorption of J-aggregates on AgBr microcrystals. The results agreed well with those of Low-voltage High-resolution Scanning Electron Microscopy and Atomic Force Microscopy. The population of aggregates adsorbed on an AgBr particle varies from almost nine to full coverage. The thickness of J-aggregates crystallites is uniform and depends on a comgimation of dye species and counter ion, and is 2.1nm for thiacarbocyanine with sidium ion, 1.04 nm for thiacarbocyanine with tosyl ion and 0.5 nm for an oxacarbocyanine. However, the area of the crystallite increases with the number of dye molecules in it, and is about (20 ÷ 30)x(30 ÷ 50) nm. The longer edges of the crystallites align along [210] axes of the substrate surface. The crystallites connected linearly on the surface and stacked high to form multi-layered islands or arrays.