Showing papers by "Chalmers University of Technology published in 1988"

Band-gap tailoring of ZnO by means of heavy Al doping

Abstract: Films of ZnO:Al were produced by weakly reactive dual-target magnetron sputtering. Optical band gaps, evaluated from spectrophotometric data, were widened in proportion to the Al doping. The widening could be quantitatively reconciled with an effective-mass model for n-doped semiconductors, provided the polar character of ZnO was accounted for.

OJ 287 - Binary pair of supermassive black holes

Abstract: A historical light curve of the BL Lacertae object OJ 287 is constructed in the optical V band using observations between 1890 and the present The curve exhibits periodic outbursts at intervals of 116 yr or 9 yr The individual outbursts show an initial maximum with decaying submaxima at intervals of about 1 month It is proposed that these characteristics are caused by a binary in the nucleus of OJ 287, with the binary members having an accretion disk around them The outbursts at 9-yr intervals would be caused by the tidal action of the companion on the disk of the larger black hole A computer simulation is used to demonstrate that the inflow into the center of such a disk during the repeated periastron passages of a companion will produce an outburst similar to the ones observed Based on indirect evidence, probable masses of 5 billion solar and 20 million solar are determined for the two objects 38 references

Biomaterial and implant surfaces: on the role of cleanliness, contamination, and preparation procedures.

Abstract: Material specificity in implant-tissue interactions derives primarily from the surface properties (chemical composition, microstructure, etc.) of the implant. This article addresses several questions related to implant and biomaterial surfaces: What is the status of real implant surfaces (composition, cleanliness, contamination, microstructure, etc.), and how does it vary with preparation procedures? Can the surface status be varied and analyzed in a controlled manner? How significant are surface status variations for in vivo function? And so on. We discuss clean surfaces, how they are contaminated, and how the contamination may affect the properties. We also discuss different preparation procedures, such as conventional machining, plasma treatment, and sterilization. Three important conclusions are drawn: (i) The surface status of a particular implant material may vary widely depending on its preparation and handling history. (ii) The surface status of implants is expected to be important for in vivo function, and should thus be controlled and standardized. (iii) It is usually not possible to predict how a change in surface status will affect the long-term, in vivo function of an implant.

Photoluminescence of noble metals

Abstract: We have developed an explicit model to explain the radiative recombination in noble metals, arising from transitions between electrons in the spconduction band and holes in the d-band generated by optical excitation. We find that the observed photon distribution has its shape from two competing factors. The first is due to the optics and the final density of states for the exiting photons. This is off-set by a d-band density of states factor increasing as the number of available d-states increase. We give a satisfactory account of the observed spectrum, using constant matrix elements, and find that luminescence can be used as a complementary tool to the ordinary elastic light scattering, giving detailed information about occupied and unoccupied states, provided the elastic optical constants are measured on the same sample.

Monolithic Catalysts for Nonautomobile Applications

Abstract: The use of monoliths in automotive emission control systems has dominated the applications of monoliths. However, other applications have also become of interest due to unique features of monoliths. Several excellent reviews on emission control [1–3] and catalytic oxidations [4] have been published during the last few years. Deluca and Campbell [5] have reviewed monoliths used as catalyst supports. This paper reviews flow characterization, heat and mass transfer, mathematical modeling, properties of monolithic supports, and nonautomobile applications of monoliths.

Effect of dietary phytase on the digestion of phytate in the stomach and small intestine of humans.

Abstract: The purpose of this study was to investigate whether dietary phytase is involved in the hydrolysis of phytate in the stomach and small intestine of humans. The digestibility of phytase-deactivated wheat bran was studied on eight occasions and untreated wheat bran on two occasions in healthy ileostomates. Five subjects were studied for two 4-d periods and one subject on two occasions for two 4-d periods while fed a constant low fiber diet. The low fiber diet was supplemented with 16 g/d wheat bran in the second period. Three other subjects fed a low fiber diet were studied for 10 consecutive days, bran being added to the diet on d 5, 6 and 7. Inositol hexaphosphate and its degradation products were analyzed with a recently developed HPLC method. On average, 95% of the ingested phytate from phytase-deactivated wheat bran and 40% of the ingested phytate from untreated wheat bran were recovered in ileostomy contents. These results differ from those of a previous analysis using an iron precipitation method. Mucosal phytase and alakaline phosphatase, if present in the human small intestine, do not seem to play a significant role in phytate digestion in humans, whereas the dietary phytase may be an important factor for phytate hydrolysis. Iron precipitation methods are not adequate for determinations of phytate digestion.

Ferroelectric Liquid Crystals. Material Properties and Applications

Abstract: The last decade has seen a tremendous growth in the research and development activities in the area of ferroelectric liquid crystals. The field was opened up by the work of Meyer in 1974, and has now grown rapidly into one of the major research areas in liquid crystal science. The emphasis of the earlier work was on basic material characterization and phenomenology. Only very few liquid crystalline compounds exhibiting a ferroelectric phase were known at that time, and furthermore these Schiff-base organic compounds were chemically and thermally unstable and presented their ferroelectricity only at elevated temperatures. After the work by Clark and Lagerwall in 1980 on a fast electro-optic effect in a surface-stabilized ferroelectric liquid crystal (SSFLC) structure, both basic and applied research have accelerated, and today a great number of papers deal with development and characterization of ferroelectric compounds and mixtures and various aspects of device physics and manufacturing. It is of...

The soft-mode ferroelectric effect

Abstract: In this paper a presentation is given of some of the basic physics of the soft-mode ferroelectric effect, along with its experimental background. Further, possible applications in electrooptic devices are elucidated. A fast (sub-microsecond) electrooptic switching can be achieved in the A* phase, and in similar orthogonal smectic phases made up of chiral molecules. Instead of using the phase variable Φ, it uses the tilt angle θ, which in principle is a “hard” variable, but is expected to soften on approaching the transition to a lower-lying adjacent tilted smectic phase. However, the switching is efficient in the whole range of the orthogonal phase (in fact, less efficient near the tilting transition) and is observed whether an adjacent tilted phase is present or not. As compared to the surface-stabilized (SSFLC) electrooptic mode, this soft mode (SMFLC) is based on the electro clinic, effect of essentially ferroelectric nature very closely related to the presently more investigated ferroelectric...

Resonance frequencies of the human skull in vivo.

Abstract: Patients with skin penetrating titanium implants in the temporal bone, for attachment of bone‐anchored hearing aids, have made it possible to investigate the free‐damped natural frequencies (resonance frequencies) of the human skull in vivo. The resonance frequencies of the skull of six subjects were investigated. The resonance frequencies were extracted from two frequency response functions (acceleration/force) measured on each subject: One point measurement where the force and acceleration were both measured at the same point, and one transcranial measurement where the acceleration was measured contralaterally. Between 14 and 19 resonance frequencies were identified for each subject in the frequency range 500 Hz to 7.5 kHz. The two lowest resonance frequencies were found to be on the average 972 (range 828–1164) and 1230 (range 981–1417) Hz. The relative damping coefficients of all resonances were found to be between 2.6 and 8.9%. Due to the relatively high damping coefficients, it is assumed that the resonance frequencies do not significantly affect bone conducted sound. In the transcranial measurements, however, a few large antiresonances were found which may affect bone‐conducted sound. Intersubject variations were large, probably due to individual variations in skull geometry and in mechanical parameters. The results were shown to be consistent with previous results obtained on dry skulls. No obvious correlation between lowest resonance frequency and skull size was found.

Tunable superlattice p-i-n photodetectors: characteristics, theory, and application

Abstract: Extended measurements and theory on the recently developed monolithic wavelength demultiplexer consisting of voltage-tunable superlattice p-i-n photodetectors in a waveguide confirmation are discussed. It is shown that the device is able to demultiplex and detect two optical signals with a wavelength separation of 20 nm directly into different electrical channels at a data rate of 1 Gb/s and with a crosstalk attenuation varying between 20 and 28 dB, depending on the polarization. The minimum acceptable crosstalk attenuation at a data rate of 100 Mb/s is determined to be 10 dB. The feasibility of using the device as a polarization angle sensor for linearly polarized light is also demonstrated. A theory for the emission of photogenerated carriers out of the quantum wells is included, since this is potentially a speed limiting mechanism in these detectors. It is shown that a theory of thermally assisted tunneling by polar optical phonon interaction is able to predict emission times consistent with the observed temporal response. >

Dielectric properties near the smectic-C-smectic-A phase transition of some ferroelectric liquid-crystalline systems with a very large spontaneous polarization.

Abstract: The frequency and temperature dependence of the complex dielectric constant was measured near the smectic-${C}^{\mathrm{*}}$--smectic-A liquid-crystal phase (Sm-${C}^{\mathrm{*}}$--Sm-A) transition for two ferroelectric liquid crystals with a very large spontaneous polarization. The dielectric strengths as well as the corresponding relaxation frequencies of the dielectric modes were determined. Due to the large polarization we were able to resolve the contributions from the soft mode and the Goldstone mode in a comparatively large temperature interval in the Sm-${C}^{\mathrm{*}}$ phase close to the transition to the Sm-A phase. In the Sm-A phase, the relaxation frequency and the inverse dielectric strength of the soft mode decrease linearly when approaching ${T}_{c}$. Comparing the experimental data with a recent theoretical calculation of the complex dielectric constant, we conclude that we have a fairly good understanding of the behavior of the complex dielectric constant in the whole Sm-${C}^{\mathrm{*}}$--Sm-A transition range.

Asymptotic propagation properties of pulses in a soliton-based optical-fiber communication system

Abstract: An analytical investigation is made of the asymptotic propagation properties of pulses evolving from nonsoliton initial conditions in an optical-fiber communication system. Explicit analytical results are obtained for the characteristic parameters of the asymptotically emerging soliton as well as of the accompanying decaying nonsoliton part. The importance of the dispersively decaying part of the pulse for a soliton-based optical-fiber communication system is especially emphasized.

LINEAR DICHROISM(250–700 nm) OF CHLOROPHYLL a AND PHEOPHYTIN a ORIENTED IN A LAMELLAR PHASE OF GLYCERYLMONOOCTANOATE/H2O. CHARACTERIZATION OF ELECTRONIC TRANSITIONS

Abstract: We determined the polarized and isotropic visible/UV absorption spectra of chlorophyll a (Chi a) and pheophytin a (Pheo a) oriented in the lamellar phase of glycerylmonooctanoate/H2O. Resolution into mutually perpendicular X- and Y-polarized absorption spectra of the pigments was achieved assuming that the transition moments of Chi a and Pheo a are located in the plane of the chromophores. The polarized spectra were deconvoluted into harmonic progressions and the resultant assignment of band positions in the long-wavelength region was found to correlate well with independent spectral measurements, such as the polarisation of fluorescence. However, the correlation of the experimentally determined transitions with the results of theoretical calculations is not straightforward. The overall conclusions are of fundamental interest in the determination of orientations of the tetrapyrrole macrocycle in lipid lamellae and pigment-protein structures of photosynthetic membranes.

Orientation of DNA during gel electrophoresis studied with linear dichroism spectroscopy

Abstract: A method for in situ study of orientation of DNA during gel electrophoresis has been developed. Linear dichroism spectra measured by this phase-modulation technique can sensitively and selectively detect orientation of DNA during electrophoretic migration in gel. [Measurement of “electrophoretic orientation” was first reported in 1985 by B. Akerman, M. Jonsson, and B. Norden (1985) (J. Chem. Soc. Chem. Commun. 422–423)]. Restriction fragments of duplex DNA of lengths in the ranges of 300–2319 base pairs (bp) and 4361–23130 bp have been studied in 5% polyacrylamide and 1% agarose gels, respectively. The fragments become preferentially oriented with the DNA helix axis parallel to the migration direction. In agarose the orientation is found to increase sigmoidally, and in polyacrylamide, linearly, with the electric field strength, within the field ranges accessible to measurement (0–40 and 5–40 V/cm, respectively). In both types of gels a considerable increase in orientation with length of DNA was observed. Compared to dipole orientation in electric fields, the electrophoretic orientation is high: orientation factor S = 0.027 in agarose for 23130 bp at 10 V/cm and S = 0.004 in polyacrylamide for 2319 bp at 10 V/cm. In addition to orientation of DNA, the electrophoresis also leads to orientation effects in the gel structure owing to Joule heating. In agarose there is also an effect that is associated with the migrating DNA zones and that produces different orientations of the gel at the front and rear parts of a zone. Evidence is presented that this effect is due to a DNA-induced electroosmotic flow causing a contraction of the gel in the front of the zone and an expansion in the rear. The experimental results on DNA orientation are compared with the reptation theories for gel electrophoresis. The theory of Lumpkin et al. [O. J. Lumpkin, P. Dejardin, and B. H. Zimm (1985) Biopolymers24, 1573–1593] predicts no orientation length dependence, but it does predict a shape of the field dependence that resembles the shape observed in agarose. The theory of Slater and Noolandi [G. W. Slater and J. Noolandi (1986) Biopolymers25, 431–454] predicts an orientational length dependence that is an order of magnitude less than the experimental one, and a field dependence that agrees neither with the sigmoidal shape observed in agarose nor with the linear dependence in polyacrylamide.

Global L p -properties for the spatially homogeneous Boltzmann equation

Abstract: This paper studies the Lp-behavior for 1≦p ≦ ∞ of solutions of the nonlinear, spatially homogeneous Boltzmann equation for a class of collision kernels including inverse kth-power forces with k>5 and angular cut-off. The following topics are treated: differentiability in Lp together with global boundedness in time for Lp-moments that exist initially, translation continuity in Lp uniformly in time, and strong convergence to equilibrium.