Showing papers by "University of Patras published in 1990"

Dependence of catalytic rates on catalyst work function

Abstract: THE catalytic activity and selectivity of metals can be altered dramatically and reversibly by supplying or removing oxide anions, O2–, at the metal catalyst surface by interfacing the catalyst with an O2– conductor such as zirconia1–6. This effect, termed non-faradaic electrochemical modification of catalytic activity (NEMCA), leads to a steady-state catalytic reaction rate increase up to 3 × 105 times higher than the rate of supply or removal of O2– at the catalyst surface1,3,4. Here we report that β"-Al2O3, which is a Na+ conductor, can also induce the NEMCA effect. Furthermore we show that the origin of the NEMCA effect lies in the controlled variation of catalyst work function on polarization of the metal-solid-electrolyte interface, and demonstrate that the rates of metal-catalysed reactions depend exponentially on the average catalyst work function. Thus the influence of catalyst electronic structure, rather than surface geometry, is here the key factor in catalytic activity.

Vibration isolation using open or filled trenches Part 2: 3-D homogeneous soil

Abstract: The isolation of structures from ground transmitted waves by open and infilled trenches in a three-dimensional context is numerically studied. The soil medium is assumed to be elastic or viscoelastic, homogeneous and isotropic. Waves generated by the harmonic motion of a surface rigid machine foundation are considered in this work. The formulation and solution of the problem is accomplished by the boundary element method in the frequency domain. The infinite space fundamental solution is used requiring discretization of the trench surface, the soil-foundation interface and some portion of the free soil surface. The proposed methodology is first tested for accuracy by solving three characteristic wave propagation problems with known solutions and then applied to several vibration isolation problems involving open and concrete infilled trenches. Three-dimensional graphic displays of the surface displacement pattern around the trenches are also presented.

Listric faulting, sedimentation, and morphological evolution of the Quaternary eastern Corinth rift, Greece: First stages of continental rifting

Abstract: The 30-km-wide Corinth rift is one of a series of west-northwest-trending Quaternary grabens in western Greece. In the north, a major listric fault marks the southern edge of a deep asymmetric graben filled by the sea.The footwall of this fault, which forms the southern part of the rift, has experienced epeirogenic uplift and 18% Quaternary extension along smaller listric faults. Rates of motion are determined by offsets in Quaternary rift-filling sediments. The uplift and listric extension of the southern part of the rift took place in the first two million years of rifting.This tectonic style occurs in other Greek Quaternary rifts, but comparable records have been destroyed by erosion in older rifts. Quaternary sediments exposed in the uplifted southern part of the rift comprise a northward-prograding deltaic sequence, the deposition of which continues today in the Gulf of Corinth. These Quaternary sediments are principally alluvial fan and coastal lacustrine conglomerates in the south, lacustrine Gilbert-type fan-delta conglomerates and marls in the center, and predominantly lacustrine marls overlain by thin marine or fluvial "caprock" in the north. The oldest marine "caprock" is mid-Pleistocene; this facies is generally transgressive over the lacustrine deltaic deposits. Mesoscopic structural features are unusually well developed in these sediments. Large listric faults are accommodated by minor synthetic or antithetic faults, rollover structures with counter faults, and low-dipping shear zones in the place where mesoscopic listric faults meet the decollement horizon. Some extension joints have been reactivated to form shear faults. The landscape on the uplifted southern part of the rift has flights of terraces previously interpreted as providing a detailed record of sea-level changes in the past 0.5 m.y. Most terraces result from fault disruption of the same delta-top surface, in places with marine caprock, that was also influenced by synsedimentary faulting. The marine caprock and associated alluvial facies are thin, and there is no evidence for river dissection due to lowered base level prior to the middle Pleistocene isotopic stage 6 (130-180 Ka); this indicates that marine highstands were only a little higher than normal lake level. In the late Pleistocene, lake levels dropped substantially during marine lowstands, leading to the dissection of the landscape and the deposition of thick alluvium during marine highstands. Caprock sediments preserve only an incomplete record of marine highstands.

A linear programming approach to reasoning about probabilities

Abstract: We continue our study, initiated in [9], of the following computational problem proposed by Nilsson: Several clauses (Boolean functions of several variables) are given, and for each clause the probability that the clause is true is specified. We are asked whether these probabilities are consistent. They are if there is a probability distribution on the truth assignments such that the probability of each clause is the measure of its satisfying set of assignments. Since this is a generalization of the satisfiability problem of predicate calculus, it is immediately NP-hard. In [9] we showed certain restricted cases of the problem to be NP-complete, and used the Ellipsoid Algorithm to show that a certain special case is in P. In this paper we use the Simplex method, column generation techniques, and variable-depth local search to derive an effective heuristic for the general problem. Experiments show that our heuristic performs successfully on instances with many dozens of variables and clauses. We also prove several interesting complexity results that answer open questions in [9] and motivate our approach.

Efficacy of prophylactic antimicrobial regimens in preventing infectious complications after transrectal biopsy of the prostate.

Abstract: In a prospective study on 81 patients undergoing transrectal needle biopsy of the prostate, the efficacy of prophylaxis in preventing postbiopsy infectious complications was determined. The patients were divided randomly into four groups, and a comparison of the rate of postbiopsy complications in each group was made. In 11 and 17% of the patients in Group A (n=18) who received povidone-iodine enema alone, bacteriuria and bacteraemia, respectively, occurred. When parenteral piperacillin alone in Group B (n=22) was administered, the rates of the same complications were 9 and 14%, respectively, while both rates were as low as 4% in Group C (n=25) when piperacillin in combination with povidone-iodine enema was given. On the other hand, in 31 and 37.5% of the patients in Group D (n=16), who served as controls, bacteriuria and bacteraemia developed. The study has thus shown that parenteral piperacillin in combination with povidone-iodine enema significantly reduces the incidence of infectious complications associated with transrectal prostatic biopsy.

The folded t distribution

Abstract: Measurements are frequently recorded without their algebraicsign. As a consequence the underlying distribution of measurements is replaced by a distribution of absolute measurements. When the underlying distribution is t the resulting distribution is called the “folded-t distribution”. Here we study this distribution, we find the relationship between the folded-t distribution and a special case of the folded normal distribution and we derive relationships of the folded-t distribution to other distributions pertaining to computer generation. Also tables are presented which give areas of the folded-t distribution.

Tectonic stages along a traverse cross cutting the Rhodopian zone (Greece)

Abstract: In a cross section along the Rhodopian zone from the Greece/Bulgarian border through the postalpine basins in the Xanthi area to Thasos island detailed structural mapping and collection of field data enable us to determine a sequence of tectonic stages that affected this area. This allows us to propose a model of continental collision. During the early Eocene, NE-SW directed movements produced a southwest facing pile of crystalline nappes with internal deformation comprising NW and NE trending isoclinal folds and a NE-SW trending stretching lineation. After a short time interval, nappe movements started again under brittle deformation conditions and produced composite thrust sheets, WNW- and NNE trending chevron folds and thick cataclasite. Late erogenic uplift and extension caused the formation of NW- and NE trending normal faults. These faults bounded the Oligocene flysch- and molasse basins and provided pathways for voluminous magmatic suites. This tectonic process was interrupted at the Oligocene/Miocene boundary by a late orogenic compression reflected in NW open folds and dissolution cleavages. Subsequently, differential uplift and subsidence associated with NW- and NE trending normal faults has continued up to the present time. From the early Pliocene new fragmentation and thinning of the crust reflected in WNW listric faults and NNE transfer faults has affected the southern parts of the Rhodopian zone. Nappe forming movements are the result of a progressive collisional process involving a Kimmerian microcontinent lying between the Eurasia and Apulia continents. In the final stages of this collision, uderplating processes led to regional uplift and extension. Additional extensional movements were related to the formation of the Aegean back arc basin caused by the Late Cenozoic Hellenic subduction zone.

Modern fan deltas in the western Gulf of Corinth, Greece

Abstract: Rivers in the western Gulf of Corinth have built gravelly fan deltas into waters 100–300 m deep. Seismic profiles (3.5 kHz) and gravity cores show that sand-filled channels extend seaward of modern distributary mouths and lead to sandy depositional lobes in deeper water. Much of the fan delta slope is underlain by mud. Closely spaced gullies cut the interchannel ridges on either side of the main channels. Incised slope valleys occur on east-facing slopes, where the wave fetch is greatest, and may be formed by rip-current-induced turbidity currents.

Vertical distribution and nocturnal migration of zooplankton in relation to the development of the seasonal thermocline in Patraikos Gulf

Abstract: Vertical distribution and nocturnal migration of zooplankton species in relation to the development of the seasonal thermocline in the shallow waters (90 m) of Patraikos Gulf (Ionian Sea, Greece) were investigated using a WP-2 closing net. Juvenile and adult copepods accounted for a mean of 91% of the total collected in three sampling periods, i.e. May, July and September 1985.Ctenocalananus vanus, Paracalanus parvus andOithona plumifera were the dominant copepods. The majority of the zooplankton tend to aggregate at the thermocline layer. Among copepods the two congeneric speciesClausocalanus pergens andC. furcatus exhibited different migratory responses to the development of the thermocline.C. pergens occurred in the lower part of the thermocline andC. furcatus in the upper region or above. The diel vertical migration of all species could be divided into four types: (1) no vertical migration; (2) upward migration at night; (3) occasional migration; and (4) reverse migration (down at night). In July when the strongest thermocline developed, most zooplankters rose close to the surface at night. For most species, temperature discontinuity did not limit their diel migration.

Some possible effects of a catheter on the arterial wall.

Abstract: When a catheter is inserted through an artery, the wall shear stress increases and the secondary flow exhibits an increased number of instantaneous vortices. These effects become more evident when the annular gap between the catheter and the artery becomes more narrow. Accordingly it is suggested that these factors may affect the preferential distribution of the atheromatous plaques at the sites of the wall with an increased permeability.

Are-examination of NaCI effects on phosphoenolpyruvate carboxylase at high (physiological) enzyme concentrations

Abstract: Inhibition of phosphoenolpyruvate carboxylase (EC 4.1.1 31) from the C4-halophyte Salsola soda L. by NaCI is compeutive to phosphoenolpyruvate (PEP). Physiological (betaine, glycerol) and synthetic (polyethylene glycol) osmotica and the allosteric activator glucose-6-phosphate (G6P) increase the apparent affinity of the enzyme for PEP and also alleviate the inhibition by NaCl. Physiological osmotica that either increase the Km(PEP) (proline) or are neutral (sorbitol), do not protect the enzyme against NaCI attack. In the absence of cosolutes and, G6P, the enzyme is self-protected when its concentration in the assay medium is increased to more physiological values. In addition, the amount of betaine needed for complete protection is inversely related to native protein concentration in the assay. Exogenous protein (bovine serum albumin or bovine skin gelatin) have no effect on either Km(PEP), or extent of NaCl inhibition. These results can be better explained with the exclusion volume theory and the inferred assumption that both cosolutes and high protein concentration strengthen intrinsic aggregation properties of enzymes. It is suggested that the extremely high phosphoenolpyruvate carboxylase concentration in the cytoplasm and the accumulation of compatible solutes in response to water stress fully protect the enzyme in vivo against the chaotropic effects of NaCI.

Prevalence of Toxoplasma gondii antibodies in gravidas and recently aborted women and study of risk factors.

Abstract: Two hundred seventeen parturients and eighty six recent aborters were screened for IgM and IgG toxoplasma antibodies. Age, profession, educational level, residence (urban/rural), presence of cat and other domestic animals were recorded for each subject. None of the subjects was IgM-positive. Prevalence of IgG positivity was 52.3% in the parturients and 50.2% in the recently aborted women. None of the personal or social characteristics investigated could be related to IgG positivity. However, the frequency of toxoplasma antibodies was found to be higher in recent aborters from rural areas where contact with soil is common regardless of whether cats are kept as pets or not. This study confirms other investigators' conclusion on the importance of soil contact as a risk factor for infection.

Chondroitin proteoglycans from squid skin. Isolation, characterization and immunological studies.

Abstract: Two populations of proteochondroitins were isolated from 4 M guanidine hydrochloride extracts of squid skin by a combination of ion exchange, gel chromatography and density gradient centrifugation. The proteoglycans, Mr 4.8 x 10(5) and 2.8 x 10(5), contained four and two chondroitin chains respectively and unusual oligosaccharides with uronic acid and sulphate groups, and had different amino acid and neutral sugar composition. The chondroitin chains isolated after alkaline borohydride treatment contained varying amounts of glucose, galactose, mannose, fucose and xylose, most likely as branches. Both proteoglycans were antigenic to the rabbit and showed considerable cross-reactivity as assessed by competition experiments using the ELISA technique. The proteoglycans reacted neither with exogenous hyaluronic acid nor with each other to form aggregates.

Using geometrical models in a process of reflective thinking in learning and teaching mathematics

Abstract: In this paper we investigate how geometrical models can be used in learning and teaching mathematics, in connection with the development of a process of reflective thinking, which we study first in general. Some more specific questions — arising from the use of geometrical models in the classroom — have led us to an experimental study, the results of which are presented and discussed in the paper.

Ureaplasma urealyticum colonization of full term infants: perinatal acquisition and persistence during early infancy.

Abstract: In a prospective study 225 (35%) of 640 pregnant women who delivered at term had vaginal colonization with Ureaplasma urealyticum at the time of delivery. One hundred ninety-three full term infants born to U. urealyticum-colonized mother were cultured from the throat, eyes and vagina within the first 3 days of life. One hundred seven infants (55%) had at least one culture site positive for U. urealyticum (throat 41%, eyes 20%, vagina 40%). Rupture of membranes for greater than or equal to 12 hours and the mode of delivery did not affect vertical transmission of U. urealyticum. We were able to follow 108 infants during the first 3 months of life. Sixty-eight, 33 and 37% of the infants who were initially colonized with U. urealyticum in the throat, eyes and vagina, respectively, were still colonized when the follow-up cultures were obtained 3 months later. Fourteen of the 108 infants whom we followed developed a lower respiratory tract illness. In the pharyngeally colonized infants there was no increased risk for lower respiratory tract illness during early infancy compared with the pharyngeally noncolonized infants.

Phonological and lexical encoding processes in beginning readers: Effects of age and word characteristics

Abstract: The aim of this study was two-fold. First to find out whether the prelexical phonological route develops prior to the visual route or whether the direct visual route develops first in the process of learning to read Greek. Second, to see whether the subjects' chronological age deviation affects decisively the overall reading accuracy and the processes employed in word reading. For this an investigation was conducted involving two age groups of Primary-one class children regarded as normal readers and differing between them by 10 months. The subjects were tested in three word (and nonword) reading tasks in which psycholinguistic properties of words (such as grammatical class, imagery level, semantic content, regularity and frequency) had been manipulated. The main findings were: first that reading accuracy was not significantly affected by the main psycholinguistic properties of words. Second, that the chronological age factor is unlikely to influence decisively and consistently reading acquisition in the long term. Third, that Primary-one children although tending to use mainly phonological information in reading, nevertheless seem to have started making the transition towards the use of the direct visual route as well.

Angiogenesis is associated with collagenous protein synthesis and degradation in the chick chorioallantoic membrane

Abstract: Extracts of the chick embryo chorioallantoic membrane (CAM) obtained from 7–20 day old embryos, contained enzyme activity that could degrade type IV collagen. Peak enzyme activity was observed on days 8–10 of embryogenesis, which coincides with the stage of maximum angiogenesis. This activity decreased to lowest values at days 13–15 and increased thereafter up to day 20. Maximum rate of collagen biosynthesis in CAM was observed between days 7 and 10, with a drastic decrease at day 12, when vascular density has reached a plateau. The type IV collagen-degrading activity of CAM was of the metalloprotease type, since it was inhibited by 1,10-phenanthroline and EDTA but was also partially inhibited by serine and thiol protease inhibitors.