Showing papers by "Sofia University published in 1990"

Implications of shear-sense criteria for the tectonic evolution of the Central Rhodope massif, southern Bulgaria

Abstract: A systematic record of sense-of-shear criteria associated with peak conditions of amphibolite facies metamorphism from the Central Rhodope massif, Bulgaria, gives evidence for superposition of two distinct terranes. The lower terrane is characterized by south-vergent, flat-lying thrusting that involved eclogites. The upper terrane is characterized by east-vergent, flat-lying shearing possibly associated with crustal extension. We suggest that both regional metamorphism and deformation may be Cretaceous in age. There was erosion prior to emplacement of several crystalline sheets at shallow depths during the early Cenozoic. This was followed by Oligocene extension and rhyolitic volcanism. The Rhodope massif can no longer be regarded as a stable Paleozoic or older microcontinent.

Poly(ether/ester)s based on poly(butylene terephthalate) and poly(ethylene glycol), 1. Poly(ether/ester)s with various polyether: polyester ratios

Abstract: A series of poly(ether/ester)s 1 based on poly(butylene terephthalate)1 and poly(ethylene glycol) is synthesized. 1H and 13C nuclear magnetic resonance, infrared and differential scanning calorimetry (DSC) measurements prove a block structure of the prepared copolymers. The DSC curves suggest the existence of a three-phase morphology — two amorphous phases for both polyether and polyester segments, and a crystalline phase for the polyester segments. Small angle X-ray scattering (SAXS) measurements lead to the same conclusion. The degree of crystallinity assigned to the polyester fraction differs insignificantly and is similar to that of the homopolymer. SAXS data show approximately the same long spacing for all crystallizable copolymers. The tensile parameters of the studied polymers are similar to those of available commercial products based on poly(butylene terephthalate) and poly(tetrahydrofuran).

Electromagnetic waves in a plasma waveguide

Abstract: A detailed analysis of wave dispersion in a plasma-filled waveguide in a finite external magnetic field is presented. The mutual influence of modes on their dispersion curves is treated. A new phenomenon is demonstrated: the coupling of the waveguide EH and HE modes and the parallel appearance of a backward wave. The values of Ω and ωp (the cyclotron and plasma frequencies) and R (the waveguide radius) at which coupling between arbitrary EH and HE modes becomes possible are found. The dispersion relations and field distributions of two new mode families that arise owing to the presence of the anisotropic plasma are analysed.

Drainage of foam films in the presence of nonionic micelles

Abstract: The stratification (step-wise thinning) of horizontal foam films formed from micelar solutions of nonionic surfactants is studied experimentally. The effects of film area, temperature, oil solubilization, surfactant and electrolyte concentrations are investigated. The data support the colloid crystal model of stratification, according to which the step-wise thinning is due to a layer-by-layer destruction of an ordered structure of spherical nonionic micelles inside the film. The analysis of the data shows that the main factor governing the stratification is the volume fraction of the micelles. This fact leads to an unified interpretation of the observed effects, both with nonionic and with anionic surfactant solutions.

FIT, a computer program for decomposition of powder diffraction patterns and profile analysis of pair correlation functions

Abstract: FIT is an interactive computer program for fitting analytical models to powder diffraction patterns and to pair correlation functions. FIT has been written in Turbo C and runs on IBM XT/AT or compatible personal computers.

Poly(ether/ester)s based on poly(butylene terephthalate) and poly(ethylene glycol), 2. Effect of polyether segment length

Abstract: Structure-properties relationships of poly(ether/ester)s (PEEs) 1 based on poly(butylene terephthalate) (PBT) and poly(ethylene glycol) (PEG) are studied. By varying the length of the soft segments (PEG) of molecular weights 600, 1000 and 2000) two series of PEEs are obtained, (i) with constant mole ratio PBT:PEG (the PBT block length being approximately the same) and (ii) with constant weight ratio PBT:PEG (different PBT segment length). The block structure is proved by 1H nuclear magnetic resonance and differential scanning calorimetry studies. A three-phase morphology is established, i.e. two amorphous phases (polyether and polyester) and a crystalline phase (PBT), each one distinguished by its own transition temperature. At low temperatures (between −60 and 20°C) a fourth phase (crystalline PEG) appears. Small angle X-ray scattering intensity and long spacing increase abruptly for the compositions with PEG 2000. The mechanical characteristics of the studied PEEs depend on the weight ratio of the two types of blocks.

Micromechanical Description of Curved Interfaces, Thin Films, and Membranes I. Quasistatics

Abstract: Slow quasistatic deformations of a fluid interface or membrane are investigated theoretically from a micromechanical viewpoint. The surface rate-of-strain tensor is characterized by its invariants, a and/3, which account for the interfacial dilation and shear, respectively. The macroscopic interfacial stretching and shearing tensions and the bending and torsion moments are expressed through integrals over the components of the pressure tensor. The results are applied to derive the Gibbsian fundamental thermodynamic equations of an arbitrarily curved interface from the fundamental equations of the adjacent bulk phases. Then, by means of a variational principle, the conditions for interfacial mechanical equilibrium are derived. © 1990 Academic Press, Inc.

An enzyme-alkaline hydrolysis of feather keratin for obtaining aprotein concentrate for fodder

Abstract: The considered procedure for obtaining protein concentrate from feather can be applied for the utilization of waste feathers from the poultry industry.

MWC560 : a unique astrophysical object

Abstract: DESPITE its interesting behaviour1,2, the object MWC560 has been little studied since its discovery 50 years ago during the Mount Wilson Observatory Hα emission-line surveys3. To rectify this situation we have made systematic spectral and photometric observations of this star. Spectra of the hydrogen absorption Balmer lines show remarkable variability: they indicate that some components have radial velocities as large as −6,000 km s−1, and the profiles are complex, showing pronounced variability on times-cales of a few days. Photometric observations show that the V-band brightness of the star has risen by about three magnitudes over the past 20 years; during our own observations, this brightness was seen to change by a few tenths of a magnitude. We speculate that MWC560 may be a binary system containing a red giant and a compact companion—possibly a white dwarf—and exhibiting jet-like ejections along the line of sight4.

Study of some tungsten-containing chemical modifiers in graphite-furnace atomic-absorption spectrometry

Abstract: Several chemical modifiers based on tungsten have been evaluated: the individual modifiers W(VI) as WO3 in aq. 0.2 M NH3 and W(V) as W in H2O2 and the mixed modifiers W(VI)/NH3 + Pd(II), W(V)/H2O2 + Pd(II), and W(V)/H2O2 + PO4 3-. High efficiency of thermal stabilization for 18 analyte elements of high and moderate volatility has been demonstrated and possible mechanisms of stabilization are discussed.

Structure of segmented poly (ether ester)s as revealed by synchrotron radiation

Abstract: The structure and phase transitions of block copolymers of PBT and PEO having different composition were investigated by DSC, SAXS, and WAXS. Two different glass transitions and two melting points were observed. The increase of the integral SAXS intensity with temperature was much smaller than in the case of homopolymers, which is explained by a broad melting range. The long period strongly increases with temperature, which is attributed to growth of domains.

Coulomb blockade of resonant tunneling

Abstract: Ultrasmall double-barrier semiconductor structures are investigated in terms of the semiclassical sequential theory of resonant tunneling. The quantization of the charge buildup in the quantum well is taken into account. A peaked I-V characteristic is obtained, with each peak corresponding to an integer number of electrons in the well. A new explanation of the experiment of Reed et al. [Phys. Rev. Lett. 60, 535 (1988)] is proposed. The large value of the charging energy in their experiment ${\mathit{E}}_{\mathit{c}}$=${\mathit{e}}^{2}$/2C=43 meV makes semiconductor tunneling structures more convenient for observation of charging effects than the usual ones.

Pulsed laser photolysis of the PtCl62−—creatinine system in methanol

Abstract: The pulsed laser photolysis (excimer laser, XeCl; 308 nm) of the PtCl 6 2− —creatinine—methanol system was studied. The formation of an intermediate Pt III species (PtCl 5 2− cr, where cr = creatinine) was demonstrated and its decay kinetics were examined. Some kinetic and thermodynamic data of the photoinduced reaction were determined. The photolysis of the same system using stationary irradiation was also investigated allowing the end product of the reaction to be determined.

Ultrabroadband laser using prism-based “spatially-dispersive” resonator

Abstract: An ultrabroadband (up to 35 nm FWHM) generation by a pulsed dye laser is reported. The principle of laser operation is based on a recently proposed cavity configuration, which we have termed “spatially-dispersive” resonator. By means of a prism pair this spreads the various spectral components into different spatial areas in the active medium and thus reduces the competition between them. A generation of a spectrum containing a number of equally spaced lines is also demonstrated.

Inhibition of pancreatic lipase by ovalbumin and β-lactoglobulin A at the air/water interface

Abstract: The kinetics of ovalbumin and β-lactoglobulin A adsorption at the air/water interface were studied in the abence or presence of 1,2-didecanoylglycerol (dicaprin) monolayer. The lipolytic inhibition capacities of ovalbumin and β-lactoglobulin A are explained in terms of their adsorption kinetic properties. The adsorption of the proteins can be controlled by a diffusion and/or an adsorption energy barrier step. Assuming the adsorption to be diffusion-controlled, the apparent diffusion coefficient ( D app was calculated and compared with the diffusion coefficient ( D ) measured independently. When calculated, D app was found to be lower than D , which shows that an adsorption energy barrier exists. The energy barrier of β-lactoglobulin A was found to be lower than tat of ovalbumin. Assuming the adsorption energy barrier to be a rate-controlled process, the adsorption constant was calculated. β-Lactoglobulin A has a higher adsorption rate. The surface rheological dilatational properties during adsorption of the proteins and enzymes were also studied. In the presence of β-lactoglobulin A, the previously elastic properties of the lipid monolayer became viscoelastic. A mixed monolayer of ovalbumin/dicaprin or β-lactoglobulin A/dicaprin was spread and used to study the inhibition of horse pancreatic lipase activity at the air/water interface. A critical lipid/protein ratio was found to exist, above which enzymatic hydrolysis of dicaprin was observed. This molar ratio is 1 in the case of a mixed ovalbumin/dicaprin monolayer and 9 in the case of a mixed β-lactoglobulin A/dicaprin monolayer.

Completeness and Incompleteness in the Bimodal Base ℒ(R,−R)

Abstract: The paper deals with a modal language ℒ(R,−R), having an ordinary modality ⊞ (dual — S) with an usual Kripke-semantics x⊧⊞ϕ iff ∀y(Rxy ⇒ y⊧ϕ) and an additional modality ⊟ (dual — S), with the same semantics however over the complement −R of R: x⊧⊟ϕ iff ∀y(−Rxy ⇒ y⊧ϕ). Such a modality has been considered by some authors in different contexts — see e.g. [Hum] and [GPT], where the completeness theorems for the minimal normal ℒ(R,−R) — logic are independently proved. This language appears as a special case of the notion polymodal base, introduced by the author in [Gor]. This notion combines a polymodal language ℒ(□1,…,□n) with a set of formulae Φ, having a usual relational semantics over structures (frames) and a theory T in some language (for definiteness — first-order) for such structures. We shall denote such a base ℒT(R1,…,Rn). The models of the theory T will be called standard frames of this base. In particular, when the theory T determines some of the relations R1,…,Rn by means of the rest of them, the polymodal base becomes an enriched [poly]modal language. A typical example of it provides the modal language for tense logics — it is a bimodal base with a theory T−1 having a single axiom (−1) ∀xy(R1xy ↔ R2yx) and standard frames — it is an enriched modal language for . Another example is the language in question. ℒ(R,−R) being a bimodal base with theory T− with an axiom (−) ∀xy(R1xy ↔ −R2xy) and standard frames .

Spectrophotometric determination of amikacin kanamycin, neomycin and tobramycin

Abstract: A spectrophotometric method for determination of neomycin, kanamycin, tobramycin and amikacin is proposed, based on the Rimini test with disodium pentacyanonitrosylferrate(II) for primary and secondary aliphatic amines. The absorbance of the coloured addition product is measured. Beer's law is valid over a wide concentration range. The method is relatively fast and can be used in control of the manufacture of the antibiotics and their purity, instead of the much slower microbiological method. It is also applicable for determination of these antibiotics in formulations.