Showing papers by "Fritz Haber Institute of the Max Planck Society published in 1976"

On the Power-Characteristics of Electrochemical Solar Cells

Abstract: The output-power-characteristics of an electrochemical solar cell are analysed. Up to 9.5% efficiency with reference to the spectrum of a Xenon lamp has been reached with a CdS/[Fe(CN)6]3-/4--electrolyte cell which is equivalent to 5.5% efficiency for solar radiation. It is shown that the polarisation of the inert counter electrode and the semiconductor electrode has a critical influence on the efficiency and can lead to great losses. Die Leistungscharakteristik einer elektrochemischen Sonnenzelle wurde untersucht. Bezogen auf das Spektrum einer Xenon-Lampe konnte mit einer CdS/[Fe(CN)6]3-/4--Redox-Elektrolytzelle eine maximale Energiewandlung von 9,5% erreicht werden. Das entspricht einer Ausbeute von 5,5% fur das Sonnenspektrum. Es wird gezeigt, das die Polarisation der Gegenelektrode und der Halbleiterelektrode einen kritischen Einflus auf die Leistungsfahigkeit hat. Bei hoher Polarisierbarkeit der Elektroden entstehen an den Phasengrenzen grose Verluste.

Electroluminescence at Semiconductor Electrodes Caused by Hole Injection from Electrolytes

Abstract: Light emission from semiconductors has been studied at semiconductor-electrolyte interfaces, caused by minority carrier injection from the electrolyte at n-type specimen of SnO2, ZnO, CdS, GaP, and GaAs. Species of very high electron affinity are normally required in order to inject holes into the deep lying valence band of SnO2, ZnO, CdS, and GaP, but such species are not stable in aqueous solution. The short living species SO˙4 and OH˙ were found to have a long enough lifetime to inject holes into these materials when they are generated at the surface by an electrochemical reaction. The luminescence spectra caused by such hole injection have been studied. One can conclude from the spectral distribution of the emitted light that surface states play an important role in this radiative recombination. The quantum efficiency of light emission is in the order of 10−4 or below and depends on the voltage applied. The influence of the energy position of the electronic states of the radicals and of the band edges of the semiconductors on luminescence intensity is analyzed. An Halbleiterelektroden wurde die Emission von Lichtquanten untersucht, die durch die Injektion von Minoritatsladungstragern aus dem Elektrolyten in die n-type Halbleiter SnO2, ZnO, CdS, GaP und GaAs verursacht wurde. Substanzen mit sehr hoher Elektronen Affinitat sind normalerweise erforderlich um Defektelektronen in die energetisch tief liegenden Valenzbander von SnO2, ZnO, CdS und GaP zu injizieren. Derartige Substanzen sind in wasriger Losung nicht stabil. Die Untersuchungen ergaben, das die kurzlebigen Radikale SO˙4 und OH˙ eine genugend grose Lebensdauer besitzen, um Defektelektronen in diese Materialien zu injizieren. Die Radikale selbst wurden an der Oberflache der Halbleiter durch eine elektrochemische Reaktion erzeugt. Die Spektren der durch eine solche Defektelektroneninjektion erzeugten Lumineszenz wurden untersucht. Die spektrale Verteilung des emittierten Lichts last den Schlus zu, das Oberflachenzustande eine bedeutende Rolle bei der strahlenden Rekombination spielen. Die Quantenausbeute der Lichtemission liegt in der Grosenordnung von 10−4 oder niedriger und hangt vom angelegten Potential ab. Der Einflus der energetischen Lage des Elektronenniveaus der Radikale und der der Bandkanten der Halbleiter auf die Lumineszenz Intensitat wird analysiert.

The Chemistry of Enzyme Immobilization

Abstract: Publisher Summary Enzymes immobilized on or within a solid matrix by conjugation with synthetic water-insoluble polymeric supports can serve as reusable and removable highly specific reagents, which possess improved storage and operational stability. Continuous large-scale processes can be carried out in immobilized-enzyme reactors. Immobilized enzymes in conjunction with a detector have led to the development of highly specific electrode systems and similar analytical and monitoring devices. Immobilized enzymes are being explored for clinical application in the form of extracorporeal shunts or microcapsules. The clarification of some of the principles underlying the kinetic behavior of immobilized enzyme systems, that is, effects of the microenvironment imposed by the chemical nature of the support material and the effects of diffusional restrictions on the translocation of substrate and product, make possible the modulation of the properties of a bound enzyme by its conjugation to a support of predetermined chemical and physical characteristics.

The oxidative desintegration of sulfide crystals byThiobacillus ferrooxidans

Abstract: The autotrophic bacterium Thiobacillus ferrooxidans (Th.f) c a n acquire its energy through the oxidation of sulfur or iron(II). Twenty-five years ago Th.f was finally found to be responsible for the production of acid mine waters in sulfide-ore deposits [1]. Since then Th.f. has been grown systematically on many sulfide minerals. It has become the crucial agent for biological mining, a rapidly growing technique which is being successfully applied to extract metals from lowgrade ore and mineral tailings [2]. Recently Th.f has been grown successfully on chemically pure metal-sulfide compounds [3]. These experiments indicate that Th.f is able to desintegrate crystalline sulfides and to utilize their chemical energy while oxidizing them to the sulfates. We have cultivated Th.f. in shakerflask cultures adapting them to various synthetic sulfide compounds in 9K nutrient solution [4]. Naturally grown or synthetic crystals were then carefully selected and inoculated with Th.f The crystals were taken from the cultures after a period between several days and eight weeks. They were rinsed, dried and, after evaporation of a gold film, studied with a scanning electron microscope together with noninoculated reference crystals kept in the same solution. Clear marks of bacterial activity were found on natural leadsulfide crystals which split easily to yield very clean and even surfaces (PbS is known to be oxidized by Th.f [5]). In preliminary experiments other sulfide substrates (CuS, CdS) did not prove to be suitable, mainly because of rough surface structures, The traces of bacterial activity were evident as characteristic deepenings in otherwise even and undissolved surfaces. These deepenings do not have the appearance of typical chemical etching patterns and were completely absent on reference surfaces which were not exposed to bacterial activity. The bacteria which are approximately 1 Ixm in diameter can be seen sitting in the holes at various stages of their formation (Fig. 1). The cell surface appears to be rough. This is partially due to the fact that the bacteria were not chemically fixed and partially due to the loose fitting of the cell envelope, which is apparently characteristic for these gram-negative bacteria [6]. The following information about the bacterial oxidation of the sulfide surface was derived from scanning electron-microscope studies: (i) The deepenings start to develop at places of contact between the bacterial cell and the sulfide surface. (2) The depth and the dimensions of the depression increase with time of bacterial activity. The bacteria slowly sink deeper but a depth of approximately the dimension of the bacteria Fig. 1. Thiobacillusferrooxidans on PbS crystal surfaces. The dips they produce, show etching-type structures and uneven distribution. They are absent on surfaces treated with the same nutrient solution, but without bacteria (a)

Plastoelasticity of high polymers based on the analysis of polyethylene stretched till natural draw ratio

Abstract: A theory of the plastoelasticity of high polymers is developed, which takes into account the essential structural details explored with X-ray diffraction and electron micrographs. The structure consists of small microparacrystallites (mPC's), which are the nodules of a threedimensional network. This is deformed affine under stress. The theory takes into account the stretching phenomenon of affine transformation and retransformation down to colloidal dimensions and can solve therefore directly the differential equations of non-linear deformation behaviour. Within the microdomains of less than 0.1 mm diameter affine phenomena do not occur, because the microparacrystallites are disintegrated and plastically deformed. The theory is applied to the profile analysis of the shoulders in the neck of partically at room temperature drawn polyethylene. The retardation time of the microdomains reaches near the half width of the shoulder a minimum value of 2.5 min. Studying the stress-strain curve the rod firstly is deformed “homogeneously” till to an elongation of 8∼25% beyond the yield point. Till here those microdomains, which are plastically deformed, mostly are distributed randomly over the sample. In this way the deviation from the Hooke-law (linearity and immediate reversibility) can be explained, which becomes larger with increasing strain and increasing number of deformedmPC's. Above γ=25% cooperative deformations of adjacent microparacrystallites arise in planes orthogonal to the stretching direction as consequence of stress fields generated by the network knotted with plastically deformable knots. Now the macroscopically visible necking begins. At γ ∼ 2 both shoulders are separated and have gained their final profile, if the stretching velocity is constant and not too large and the temperature not too low. The Kelvin-elements, which represent the microdomains, are frozen in our example before they have reached the equilibrium elongation because outside of the shoulder the rod has become so cold that the plasticity of themPC's has decreased. This determines the so-called natural draw ratio. The deformation to natural draw ratio is absolutely reversible, if the time of passing of the microdomains the shoulder is small enough compared with the relaxation time of the Maxwell-elements. The effective Young-modulus in the phenomenological Hooke-part of the stress-strain curve has a maximal value of 5000 kp cm−2, whilst the entropy driven spring constant of the reversible deformable microdomains is much smaller with a value of 15 kp cm−2. The word “viscoelasticity” is not sufficiently descriptive.

Paracrystalline microdomains in monatomic liquids. I. Radial density functions of liquid lead at 350, 450, and 550°C

Abstract: An alignment procedure for $\ensuremath{\vartheta}\ensuremath{-}\ensuremath{\vartheta}$-goniometers was developed, using a NaCl single crystal floating on mercury. This method also allows the determination of the wavelength acceptance curve of the monochromator and thereby the elimination of the incoherent Compton scattering. The structure factors of liquid lead at 350, 450, and 550\ifmmode^\circ\else\textdegree\fi{}C were determined with a commerical x-ray diffractometer using $\mathrm{Mo} K\ensuremath{\alpha}$ radiation. The resultant radial density functions were compared to those of Kaplow et al. and North et al. and found to lie at most distances between their results.

Diamagnetic susceptibility of solid and liquid paraffins

Abstract: Experimental values for the diamagnetic susceptibility of a series of mono-disperse n-paraffins ranging between 6 and 50 carbon atoms in the solid (melt and solution crystallized) and in the liquid state are reported. From the dependence of the molecular susceptibility, ScHM, on the molecular weight, information about the intermolecular interactions between adjacent chain molecules and on the arrangement of the methyl end-groups is obtained. The ScHM values of melt- and solution-crystallized paraffins are, within experimental error, indistinguishable from each other. For C12H26 and C44H90 the specific susceptibility ScHM /M rises at the melting point within a few °C and reaches a plateau which characterizes the liquid state II. For the paraffin C24H50 an intermediate plateau between the melting point and the final true liquid state is observed and is called liquid state I. After cooling below the melting point TM, the ScHM value of the equilibrium state can be obtained after a short time only fro...

Hydrogen Peroxide Induced Periodical Catalysis on Copper‐Containing Sulfides

Abstract: The crystalline and stoichiometrical conditions for sulfides surfaces to produce oscillations during the reduction of hydrogen peroxide have systematically been investigated. Oscillations were observed with Cu5FeS4, CuFeS2, Cu2S, and CuS, however, did not appear with FeS, FeS2, PbS, MoS2, and CdS electrodes. The monovalent Cu+-ions have therefore to be considered the essential sites for autocatalysis. Oscillations start to occur as soon as through a cathodic polarization the Fermi-level in the sulfide is sufficiently raised to fill electrons into antibonding 4 s orbitals of Cu+-surface atoms. The positive charge of Cu+-ions is then accordingly reduced to make electron transfer from the perhydroxyle radical energetically impossible. It leaves an autocatalytic reaction mechanism involving two neighbouring reactants as the only remaining catalytical reaction possibility. The role of Cu(I)-oxide which is responsible for the pronounced light dependence of oscillation frequency and amplitude in the oscillation mechanism is further investigated. Investigations suggest that copper ions in sulfide surfaces play an analogous role as redox partners during the catalytic decomposition of hydrogen peroxide as copper ions in solution. The applied electrode potential permits the control of their catalytical reactivity and the energetical adjustment for the range of periodical catalytical behaviour. Die kristallinen und stochiometrischen Voraussetzungen fur das Auftreten von Oszillationen an Sulfidoberflachen wahrend der Reduktion von Wasserstoffperoxyd wurden systematisch untersucht. Oszillationen wurden bei Cu5FeS4, CuFeS2, Cu2S und CuS beobachtet, nicht jedoch bei FeS, FeS2, PbS, MoS2 und CdS. Einwertige Cu+-Ionen mussen infolgedessen als die wesentlichen, fur die Autokatalyse verantwortlichen Zentren betrachtet werden. Oszillationen beginnen aufzutreten, sobald durch eine kathodische Polarisation das Fermi-Niveau im Sulfid soweit angehoben wird, das antibindende 4s Orbitale von Cu+-Oberflachenatomen aufgefullt werden konnen. Die positive Ladung der Cu+-Ionen wird dadurch soweit reduziert, das der Elektronentransfer vom Perhydroxyl-Radikal energetisch unmoglich wird. Ein autokatalytischer Reaktionsmechanismus, der zwei benachbarte Reaktanten umfast, ist dann die einzige verbleibende katalytische Reaktionsmoglichkeit. Die Rolle von Cu(I)-Oxyd, welches fur die ausgepragte Lichtabhangigkeit von Oszillationsfrequenz und -amplitude verantwortlich ist, wird weiter studiert. Die Untersuchungen lassen schliesen, das Kupferionen in Sulfidoberflachen bei der katalytischen Zersetzung von Wasserstoffperoxyd eine analoge Rolle als Redox-Partner spielen wie Kupferionen in Losungen. Das Elektrodenpotential erlaubt es, ihre katalytische Reaktivitat zu kontrollieren und energetisch so anzupassen, das periodisches katalytisches Verhalten eintritt.

Exchange of holes between organic crystals and metal films evaporated in high vacuum

Abstract: Injection currents in organic crystals have been measured to determine the rate of hole exchange with metal contacts evaporated in high vacuum. At a gold contact the rate of hole injection increased with decreasing ionisation energy of the organic crystal but was depending on the evaporation procedure due to an influence of residual gas molecules. The excitation spectrum of the photocurrent agreed with hole generation through triplet excitons. Below the S0−S1 transition hole generation was due to surface states in the band gap. Der Austausch von Defektelektronen zwischen organischen Kristallen und im Hochvakuum aufgedampften Metallkontakten wurde als Injektionsstrom gemessen. Die Defektelektronen-Injektion aus einem Gold-Kontakt nahm mit abnehmender Ionisierungsenergie des organischen Kristalls zu, wobei jedoch eine Abhangigkeit von den Aufdampfbedingungen den Einflus von Restgasmolekulen erkennen lies. Das Anregungsspektrum des Photostroms entsprach einer Defektelektronen-Erzeugung durch Triplett-Exzitonen. Unterhalb des S0−S1 Ubergangs geschah die Defektelektronen-Erzeugung uber Oberflachenzustande.

Triplet Exciton Decay Processes in Crystalline Tetracene

Abstract: The decay of triplet excitons in crystalline tetracene is investigated. The monomolecular decay is characterised by a rate constant βt ≈ 5 × 103 s−1 and the bimolecular annihilation of triplet excitons is characterised by a rate constant γtt ≈ 2 × 10−11 cm3 s−1. The interaction between mobile triplet excitons and trapped holes is characterised by a rate constant γth ≈ 1 × 10−11 cm3 s−1. These values differ by two orders of magnitude from previous estimates. Some possible reasons for this are discussed. Der Zerfall von Triplett-Exzitonen in kristallinem Tetrazen wird untersucht. Der mono-molekulare Zerfall wird durch eine Geschwindigkeitskonstante βt ≈ 5 × 103 s−1 charak-terisiert, und die bimolekulare Annihilation zweier Triplett-Exzitonen wird durch eine Ge-schwindigkeitskonstante γtt ≈ 2 × 10−3 cm3 s−1 charakterisiert. Die Wechselwirkung zwischen einem mobilen Triplett-Exziton und einem an einer Haftstelle gefangenen Loch wird durch eine Geschwindigkeitskonstante γth ≈ 1 × 10−11 cm3 s−1 charakterisiert. Diese Werte unterscheiden sich um zwei Groβenordnungen von fruheren Schatzungen. Einige Moglichkeiten dafar werden diskutiert.

Elektrochemische Systeme zur Photolyse des Wassers mittels Sonnenenergie

Abstract: An Halbleiterelektroden konnen Redox-Reaktionen uber das Leitungsband oder uber das Valenzband ablaufen. Es werden die energetischen Bedingungen diskutiert, bei deren Erfullung die durch Lichtabsorption im Leitungsband erzeugten Elektronen Wasser reduzieren und die erzeugten Defektelektronen Wasser oxidieren konnen. Die Bedingungen fur eine optimale Ausbeute dieser Prozesse werden abgeschatzt. Redox reactions at semiconductors can proceed via the conduction- or the valence-band. The energetic conditions are discussed which are necessary to allow the reduction of water by electrons of the conduction band and the oxidation of water by holes of the valence band. Both electric charge carriers are generated by light absorption. Conditions for an optimal efficiency of this photolytic process are estimated.

Molecular Weight Distribution of γ-Irradiated Poly(methyl methacrylate)

Abstract: The molecular weight distribution (MWD) of poly(methyl methacrylate) (PMMA) was measured as a function of the dose absorbed during γ-irradiation MWD’s were obtained from gel-permeation chromatograms with the aid of an analog computer, applying essentially a method proposed by Tung If the initial MWD was rather broad, γ-irradiation caused it to become more narrow and, vice versa, if the initial MWD was rather narrow it became broader upon irradiation In all cases a limiting value of (Mw/Mn)=2 was approached This behavior meets with the expectations from the theory of random degradation and is, therefore, taken as evidence for radiation-induced random main-chain scissioning of PMMA

Chemical surface reactions in the presence of high electric fields

Abstract: Since the invention of the field ion microscope (FIM) by E. W. Muller1 and the first mass-spectrometric analysis of field ions by Inghram and Gomer,2 these tools for field ionization have been developed considerably, as described in several monographs and summarizing artilces.3–8 Field ion mass spectrometry (FIMS) can analyze solid surfaces with extreme sensitivity. The experimental facilities enable ultimately the identification of single surface particles, which can be correlated to individual surface sites within the atomic scale of a surface. The atom-probe, which was developed by E. W. Muller,9 can now analyze surface particles with high precision in mass resolution.

Paracrystalline Structure of Molten Lead

Abstract: It is well known that one can obtain informations of crystalline lattices from powder patterns of polycrystalline solids expanding a three-dimensional reciprocal lattice. The scattered intensity of molten metals can be analysed in a similar way by a threedimensional convolution polynom, which describes quantitatively the structure including the statistical relations within microdomains. In the case of Pb the micro-domains can be quantitatively described by a face centered arrangement of bimodal coordination statistics with the same neighbour distance as below the melting point T. The weight of a coordination statistic decreases from 12.0 to 11.7 at T3 indicating the generation of point defects. From the consideration of the shape of the coordination statistics one obtains some information concerning the diffusion of vacancies and an one- and two-dimensional collective motion of atoms. Bekanntlich erhalt man Informationen uber Kristallgitter aus den Pulverreflexen polykristalliner Festkorper, indem man ein dreidimensionales reziprokes Gitter aufspannt. Die von Metallschmelzen gestreute Intensitat kann in ahnlicher Weise durch ein dreidimensionales Faltungspolynom analysiert werden, das quantitativ die Struktur, insbesondere auch die statistischen Relationen innerhalb von Mikrodomanen beschreibt. Im Fall von Blei kann die Struktur der Mikrodomanen durch eine kubisch flachenzentrierte Anordnung bimodaler Koordinationsstatistiken beschrieben werden. Der mittlere Abstand nachster Nachbarn bleibt der gleiche wie unterhalb des Schmelzpunktes. Das Gewicht der Koordinationsstatistik sinkt von 12 auf 11,7 durch Einwandern von Leerstellen. Aus der Form der Koordinationsstatistik erhalt man Informationen uber Leerstellendiffusion und kollektive Atombewegung.

Grundlagen der Theorie des Parakristalls und ihre Anwendungsmöglichkeiten bei der Untersuchung der Realstruktur kristalliner Stoffe

Abstract: Es wird ein kurzer Uberblick uber die Grundvorstellungen der Theorie des Parakristalls und ihre Anwendung in der Rontgen-Feinstrukturanalyse gegeben. Weiterhin werden die Beziehungen erortert, die zwischen der Theorie des Parakristalls und den Flussigkeitstheorien von ORNSTEIN, ZERNIKE und ZERNIKE, PRINS sowie der von KRIVOGLAZ eingefuhrten Beschreibung gestorter Kristalle bestehen. A brief review is given concerning the basic ideas of the theory of paracrystals and its practical application in X-ray structure analysis. Moreover, the relationships are discussed which exist between the the ory of paracrystals and the theories of liquids as proposed by ORNSTEIN, ZERNIKE and ZERNIKE, PRINS, and between the theory of paracrystals and the treatment of imperfect crystals as introduced by KRIVOGLAZ.

Bedeutung des parakristallinen Zustandes für die Kolloidwissenschaft

Abstract: Wenn W 0. Ostwald, wie eingangs zitiert, schon vor mehr als 70 Jahren erkannt hatte, das kennzeichnend fur alle Kolloidstrukturen ausschlieslich die physikalischen Grenzflachen in Frage kommen, so ist dem hinzuzusetzen, das diese Grenzflachen die Folge der sie bildenden Partikel sind. Die Theorie des Parakristalls erkennt sie als Mikroparakristallite (mPC’s). Diese Partikel stellen sich als Mikrodomanen heraus mit speziellen, in der konventionellen Festkorperphysik noch wenig erkannten Eigenschaften. Diese sind namlich bedingt durch mehr oder weniger statistisch verteilte Punktdefekte, die in den hier behandelten Fallen so immobil sind, das die freie Energie innerhalb der Beobachtungsdauer ihren geringsten Wert nicht erreichen kann. Dann tritt innerhalb dieses Zeitraumes auch keine Alterung ein, auch keine Synarese, Aggregation, Ausflockung, Koagulation, Denaturierung, Gerinnung, Gelatinierung, oder welche Worte man sonst noch zu gebrauchen pflegt. Kurz und physikalisch interpretiert: Die mPC’s behalten ihren paraktristallinen g-Wert, sind mit W 0. Ostwald also keine Kristalloide, die kristallisieren konnen, sondern Mikroparakristalle (mPC’s), die sich in einem metastabilen thermodynamischen Gleichgewicht befinden.

Self‐Consistent Calculation of the Pair Correlation Function for Simple Fluids: Liquid Metals

Abstract: A self-consistent scheme for calculating the density response function of an interacting electron gas has recently been proposed by Singwi et al. Correlations between particles are introduced in this theory through equilibrium pair correlation function g(r). This method, when applied directly to a classical liquid, however, leads to convergence difficulties and does not seem to be applicable when the pair potential has a sharp highly repulsive part. A modification of this theory based on Pynn's suggestion takes into account the fact that g(r) is zero for small r. An effective field ψ(r) is parameterized in the hard-core region of the potential and a procedure to determine these parameters is given. This modified self-consistent iterative scheme is used to calculate the pair correlation function g(r) for liquid sodium and rubidium. It is concluded that the theory could be usefully applied to the study of the static and dynamical properties of classical liquids, provided the apparent sensitivity of the structure factor to initial values of parameters is removed.