On the ion exchange defects in aluminosilicate lattices

Summary Samples of synthetic leucite and boron-substituted leucite are investigated by infrared spectroscopy and spectrally resolved thermoluminescence. Evidence is obtained in favour of the assumption that point defects in aluminosilicate lattices are originated by exchanges of Si⁺⁴ and Al⁺³ ions lying in different cells.     

Photonic crystals--a step towards integrated circuits for photonics.

The field of photonic crystals has, over the past few years, received dramatically increased attention. Photonic crystals are artificially engineered structures that exhibit a periodic variation in one, two, or three dimensions of the dielectric constant, with a period of the order of the pertinent light wavelength. Such structures in three dimensions should exhibit properties similar to solid-state electronic crystals, such as bandgaps, in other words wavelength regions where light cannot propagate in any direction. By introducing defects into the periodic arrangement, the photonic crystals exhibit properties analogous to those of solid-state crystals. The basic feature of a photonic bandgap was indeed experimentally demonstrated in the beginning of the 1990s, and sparked a large interest in, and in many ways revitalized, photonics research. There are several reasons for this attention. One is that photonic crystals, in their own right, offer a proliferation of challenging research tasks, involving a multitude of disciplines, such as electromagnetic theory, nanofabrication, semi-conductor technology, materials science, biotechnology, to name a few. Another reason is given by the somewhat more down-to-earth expectations that photonics crystals will create unique opportunities for novel devices and applications, and contribute to solving some of the issues that have plagued photonics such as large physical sizes, comparatively low functionality, and high costs. Herein, we will treat some basics of photonic crystal structures and discuss the state-of-the-art in fabrication as well give some examples of devices with unique properties, due to the use of photonic crystals. We will also point out some of the problems that still remain to be solved, and give a view on where photonic crystals currently stand.     

Indentation responses of piezoelectric films

Frictionless indentation responses of transversely isotropic piezoelectric film/rigid substrate systems under circular cylindrical indenter (i.e., punch), conical indenter (i.e., cone), and spherical indenter (i.e., sphere) are investigated. Both insulating and conducting indenters are considered. The technique of Hankel transformation is employed to derive the corresponding dual integral equations for the mixed boundary value indentation problems. For the two limiting cases of infinitely thick and infinitely thin piezoelectric films, closed-form solutions are obtained. For piezoelectric films of finite thickness, a numerical method is constructed to solve the dual integral equations and semi-empirical models having only two unknown parameters are proposed for the responses of indentation force, electric charge and electric potential, and contact radius. With the two parameters inferred from the numerical results, the semi-empirical formulae are found to provide good estimates of the indentation responses for the two limiting cases of infinitely thick and thin piezoelectric films, as well as those in between. The inferred parameters in the proposed semi-empirical formulae for normalized indentation force and electric charge are checked against four different piezoelectric materials and are found to be insensitive to the selection of piezoelectric materials. It is believed that the proposed semi-empirical indentation formulae are useful in developing experimental indentation techniques to extract the material properties of piezoelectric films.     

Planar nematic anchoring on SiO-coated substrates

Summary The nematic planar anchoring is usually explained by using simple elastic models: the surface easy axis corresponds to the surface direction that minimizes the excess of nematic elastic energy. When anisotropic rough substrates are used to align nematic liquid crystals, due to the complex surface morphology, usual elastic models are not directly applicable. This paper presents quantitative topographical data of rough substrates, obtained with oblique SiO evaporation under vacuum for nematic planar anchoring. Experimental data are obtained by means of Atomic Force Microscopy and they are used to demonstrate the self-affine nature of these substrates and to relate the nematic anchoring with the anisotropy of the local fractal properties of the substrate itself. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Topological order in the phase diagram for high-temperature superconductors with point defects

Applying a Lindemann like criterion obtained previously by Kierfeld et al. [J. Kierfeld, T. Nattermann, T. Hwa, Phys. Rev. B 55 (1997) 626], we estimate the magnetic field and temperature for a high-T_c superconductor, at which a topologically ordered vortex glass phase becomes unstable with respect to a disorder-induced formation of dislocations. The employed criterion is shown to be equivalent to a conventional phenomenological Lindemann criterion including the values for the numerical factors, i.e., for the Lindemann number. The positional correlation length of the topologically ordered vortex glass is calculated.     

聚乙烯单晶的退火效应

用混合晶红外光谱法和ＳＡＸＳ、ＤＳＣ和ＬＡＭ等方法研究了聚乙烯单晶在７５℃、９０℃和１０５℃退火前后的结构变化，发现在９０℃以下和１０５℃以上退火，遵循不同的退火机制     

核磁共振法测定高聚物的结构—核磁共振二维谱

介绍了核磁共振(NMR)波谱,尤其是核磁共振二维谱(2D NMR)在最近几年内的进展。内容包括怎样用NMR测定高聚物的序列分布、共聚物结构、聚合物的混溶性、聚合物液晶的取向度、分子量、短链度和长链支化度、凝聚态高聚物的多相结构等。     

The thermal decomposition of NaHCO3 powders and single crystals

The thermal decomposition of four commercial powders and of differently stored single crystals of sodium hydrogen carbonate is studied by power compensation DSC and by optical and FT-IR microscopy. Independently of manufacturer, specified purity and price, the thermal curves of all the commercial powders show a more or less pronounced low temperature peak preceding the one due to the main decomposition. Such small peak is not observed when samples of laboratory recrystallized material are used. However the thermal behaviour of the latter preparation differs remarkably depending on storage conditions: the material kept in closed glass containers decomposes at temperatures higher than those of the material stored in a dessiccator in the presence of concentrated H₂SO₄. The observation by optical microscopy of the behaviour of the surfaces of single crystals coming from different storage conditions when the temperature is raised in a Kofler heater helps the interpretation of the data collected. The mechanism of the decomposition is discussed and the relevant kinetic parameters reported.