Showing papers by "Bulgarian Academy of Sciences published in 1968"

On the borothermic preparation of titanium, zirconium and hafnium diborides

Abstract: The formation of titanium, zirconium and hafnium diborides by reaction between metal dioxides and elemental boron in vacuum in the temperature range 1000 °–1750 °C has been investigated. It has been established that titanium diboride with a composition close to that of stoichiometric TiB2 is obtained after a heattreatment of the reactants at 1700 °C for I h; HfB2 is obtained after heat treatment at 1750 °C for 2 h. Experimental results show that higher temperatures are necessary for the preparation of stoichiometric ZrB2 by the borothermic method. The structure of HfB2, and its stability at boiling point and at room temperature in the presence of acids, mixtures of acids, mixtures of acids with oxidizing agents, and alkaline solutions has been studied.

On the borothermic preparation of some vanadium, niobium and tantalum borides

Abstract: The formation of vanadium, niobium and tantalum diborides by the reactions of V2O3, Nb2O5 and Ta2O5 with elemental boron under vacuum in the temperature range 1000 °–1750 °C has been investigated It has been established that niobium and tantalum diborides with a composition close to that of stoichiometric NbB2 and TaB2 are obtained after a heat treatment of the reactants at 1650 °C for 1 h Borothermic reduction of V2O3 does not give a single-phase product but a mixture of VB2 and VB A method for the chemical analysis of NbB2− and TaB2 samples has been proposed

Non‐steady State nucleation in the formation of isotropic and anisotropic phases

Abstract: The role of non-steady state nucleation in the formation of crystalline phases is studied theoretically applying the basic concepts of the theory of crystal growth. It is shown that the impediments associated with the incorporation of molecules from the ambient phase in the crystaline nuclei lead to much longer time lags than in the case of isotropic nuclei. This predicts considerable differences in the kinetics of crystalline and liquid nucleus formation which modify in some degree the interpretation of Ostwald's rule of stages. The treatment is applied to the following special cases of phase transitions: crystallization from the vapour phase, from undercooled melt, from supersaturated solutions and electrolytic nucleation of metals on inert electrodes. Some experimental data are reviewed. Es wird theoretisch die nichtstationare Keimbildung kristalliner Phasen untersucht, wobei die grundsatzlichen Voraussetzungen der Theorie des Kristallwachstums in Betracht gezogen werden. Die Schwierigkeiten, mit denen die Molekule der Ausgangsphase in den Kristall eingebaut werden, fuhren zu viel groseren Induktionzeiten als im Falle der Bildung isotroper Keime. Dies bedingt bedeutende Verschiedenheiten in der Keimbildungskinetik kristalliner und isotroper Phasen, und wenn die Moglichkeit ihrer gleichzeitigen Bildung besteht, fuhrt dies zu Erscheinungen, welche der sog. Ostwaldschen Stufenregel folgen. Die Wachstumsschwierigkeiten kristalliner Keime werden mittels eines fruher eingefuhrten dimensionslosen Koeffizienten beschrieben. Nichtstationare Kristallisation aus der Gasphase, aus unterkuhlter Schmelze, ubersattigter Losung wie auch die elektrolytische Abscheidung von Metallen werden betrachtet.

Capacitance Studies of Passive Iron in Neutral Solutions by the Potentiostalic Pulse Method

Abstract: The pulse potentiostatic method for measuring of electrode capacitance was applied in the study of passivation and of properties of the passive film on iron in a neutral borate buffer solution, pH 8.41. Capacitance at the corrosion potential C = 38 μF/cm2 is considered to correspond to the double layer capacitance. At anodic polarization in the active dissolution range an adsorption isotherm, resembling to the Langmuir isotherm, was observed. The capacitance-potential relation under steady state conditions in the passive range and the thickness-capacitance plot derived from it were used to calculate the dielectric constant of the barrier layer of the passive film. The zero-field potential extrapolated from the capacitance-potential plot is in agreement with previous measurements. Additions of nitrate ions up to 0.50 mol/l do not have any effect both on the passivation characteristics and on the properties of the passive film. Sulphate ions, on the other hand, are responsible for the reactivation in the passive range, as well as for an increased conductivity of the film, attributed to incorporation of these ions in the film. The dynamic measurements of capacitance in the range of steady state film growth yielded a linear relation between capacitance and log of time, from the slope of which the field coefficient B was estimated. Die potentiostatische Impulsmethode zur Messung der Elektrodenkapazitat wurde bei einer Untersuchung der Passivierung und der Eigenschaften der Passivschicht auf Eisen in neutraler Boratpufferlosung (pH = 8,41) angewandt. Es wird angenommen, das die Kapazitat beim Korrosionspotential, C = 38 μF/cm2, der Doppelschichtkapazitat entspricht. Bei anodischer Polarisation im aktiven Auflosungsbereich wurde eine Adsorptionsisotherme beobachtet, die der Langmuir-Isotherme ahnlich ist. Die Beziehung zwischen Kapazitat und Potential im stationaren Zustand im Passivbereich und die daraus abgeleitete Abhangigkeit der Kapazitat von der Schichtdicke wurden zur Berechnung der Dielektrizitatskonstanten der Sperrschicht des Passivfilms verwendet. Das Potential in Abwesenheit eines elektrischen Feldes, das durch Extrapolation des Kapazitats-Potential-Diagramms gewonnen wurde, stimmt mit fruheren Messungen uberein. Ein Zusatz von Nitrationen bis zu 0,50 mol/l hat keinerlei Wirkung auf die Passivierungscharakteristik und die Eigenschaften der Passivschicht. Sulfationen andererseits verursachen Reaktivierung im Passivbereich und erhohte Leitfahigkeit der Schicht, die dem Einbau dieser Ionen in die Schicht zugeschrieben wird. Die dynamische Messung der Kapazitat im Bereich des stationaren Schichtwachstums ergab eine lineare Beziehung zwischen der Kapazitat und dem Logarithmus der Zeit, aus deren Steigung der Feldkoeffizient B ermittelt wurde.

Insulin-precipitating antibodies in insulin-treated and untreated diabetic patients.

Abstract: Although the existence of precipitating insulin antibodies (PIA) has been questioned by many authors, others have shown that such antibodies can really be found in insulin-treated animals. The authors have studied the problem in insulin-treated and untreated diabetic patients, using the agar-precipitation test ofOudin. It has been possible to demonstrate PIA in 20% of the 276 unselected diabetics studied. All control examinations were negative. Unexpectedly, the precipitation reaction was more often positive in the group of diabetics not treated with insulin. — The data of the authors disclose systematically for the first time the existence of PIA in man, and the prevalence of such antibodies in diabetic patients that have never been treated with insulin and in prediabetic subjects. These findings raise the question of an autoimmune pathogenesis of some types of diabetes.

Polymerization of methacrylates containing furan nuclei with different electron-donor capacity

Abstract: The polymerization of furfuryl methacrylate (FMA) and carbomethoxyfurfuryl methacrylate (CMFMA) has been studied. It has been established that under equal conditions the rate of CMFMA polymerization is greater than the rate of polymerization of FMA. The temperature dependence of the rate constants showed that the polymerization of FMA is associated with a greater energy of activation. These results are explained by π-complex formation between the furan nuclei on the one hand and the methacrylic double bonds or the growing methacrylic radicals on the other. Complex stability depends on the electron-donor capacity of the furan nuclei. Die Polymerisation von Furfurylmethacrylat (FMA) und Carbomethoxyfurfurylmethacrylat (CMFMA) wurde untersucht. Unter gleichen Bedingungen ist die Polymerisationsgeschwindigkeit von CMFMA groser als diejenige von FMA. Die Temperaturabhangigkeit der Gechwindigkeitskonstanten seigte auserdem, das die FMA-Polymerisation mit einer groseren Aktivierungsenergie verbunden ist. Diese Ergebnisse wurden mit einer π-Komplexbildung zwischen den Furanringen einerseits und den Methacrylatdoppelbindungen oder den wachsenden Methacrylatradikalen anderseits erklart. Die Stabilitat der Komplexe hangt von der Elaktronenabgabekapazitat der Furanringe ab.

Charge compensation of trapped holes in silver bromide

Abstract: The temperature dependence of the hole photoresponse in silver bromide single crystals is measured in the temperature range 200 to 300°K. The bulk lifetime of photoexcited holes is shown to pass through a minimum at temperatures where the drift mobility exhibits a maximum value. The analysis of the experimental results suggests that the lifetime of holes is limited by a neutralization process, presumably by silver ion vacancies, preceded by multiple trapping at a level 0.41 eV above the valence band. The proposed scheme is in accordance with other experimental results concerning the properties of holes in silver bromides. Es wird die Temperaturabhangigkeit des Photoleitfahigkeitsverlaufs der Locher in Silberbromideinkristallen zwischen 200 und 300°K untersucht. Es wird beobachtet, das die Volumenlebensdauer der lichtangeregten Locher bei den Temperaturen, bei denen die Driftbeweglichkeit einen Maximalwert erreicht, durch ein Minimum geht. Die Analyse der experimentellen Ergebnisse weist darauf hin, das die Lebensdauer der Locher durch einen Neutralisationsprozes (wahrscheinlich durch Silberionenleerstellen) begrenzt wird, der durch Mehrfachtrapping an einem Energieniveau 0.41 eV uber dem Valenzband geleitet wird. Das vorgeschlagene Modell befindet sich in ubereinstimmung mit anderen experimentellen Ergebnissen uber die Eigenschaften der Locher in Silberbromid.

The Role of Holes in the Photographic Process

Abstract: Investigations on the photoresponse of silver halides carried out on large crystals have provided new information on the properties of photoexcited holes and electrons. This information may now be used to describe the mechanism of the photographic process, paying due attention to the fate of holes which until now has been more or less obscure. The experimental evidence now available favours the assumption that in less than a microsecond both holes and electrons, released on the absorbtion of light quanta in a silver bromide grain, are independently and separately trapped. According to the transport mechanism of Gurney and Mott, interstitial silver ions neutralize the trapped electrons, forming in this way the silver atoms of the latent image specks. The temperature dependence of the lifetime and mobility of photoexcited holes indicate that, in the volume of the crystal, trapped boles are also neutralized by mobile point defects of the lattice. Silver ion vacancies are likely to participate in this...

Temperature Dependence of the Diffusion of Photoexcited Holes in AgBr

Abstract: A detection technique based on the bleaching of a thin silver deposit is used to investigate the diffusion of photoinduced holes between -35 and +35 °C. The apparent diffusion coefficient is found to follow an Arrhenius-type temperature dependence exhibiting an activation energy of 0.53 eV. Following previous suggestions, it is again concluded that the holes become engaged in multiple trapping while migrating within the crystal lattice. Some trapping models are discussed. Eine Nachweistechnik, die auf dem Ausbleichen einer dunnen Silberschicht beruht, wird zur Untersuchung der Diffusion von photoinduzierten Defektelektronen zwischen _ 35 und +35 °C benutzt. Der scheinbare Diffusionskoeffizient zeigt eine Arrhenius-Abhangigkeit von der absoluten Temperatur mit einer Aktivierungsenergie von 0,53eV. Fruheren Annahmen folgend, wird wieder angenommen, das die Defektelektronen an Mehrfachtrapping wahrend der Wanderung im Kristallgitter beteiligt sind. Einige Haftstellenmodelle werden diskutiert.

On the preparation and some physical properties of gadolinium sesquisulphide and gadolinium monosulphide

Abstract: The optimum conditions for the preparation of stoichiometric gadolinium sesquisulphide and gadolinium monosulphide according to the reactions Gd 2 O 3 +3 H 2 S = Gd 2 S 3 +3 H 2 O (1) and Gd 2 O 3 +2 Gd 2 S 3 + 3 C = 6 GdS +3 CO (2) have been investigated. It has been established that stoichiometric Gd 2 S 3 is obtained at temperatures above 750 °C. Gadolinium monosulphide was prepared by carrying out reaction (2) in vacuo in two stages : i.e. , the stoichiometric mixture of the three components was heat-treated at 1600 °C for 2–3 h, ground and pressed into pellets, then heat-treatment of the pellets was carried out at 1700 °C for 2 h. The magnetic susceptibilities of Gd 2 S 3 and GdS have been measured and the effective magnetic moments have been determined. The latter are close to the magnetic moment of the Gd 3+ ion which indicates the prevalence of ionic bonds in both sulphides. The dependence on temperature of the work function, ϑ, for cathodes of Gd 2 S 3 and GdS has been investigated. The conductivities of both sulphides have been determined theoretically; Gd 2 S 3 is deduced to possess a semiconducting, and GdS a metal, conductivity.