37,286 research outputs found
On ovothiol biosynthesis and biological roles: From life in the ocean to therapeutic potential
No full textCovering: up to 2018 Ovothiols are sulfur-containing natural products biosynthesized by marine invertebrates, microalgae, and bacteria. These compounds are characterized by unique chemical properties suggestive of numerous cellular functions. For example, ovothiols may be cytoprotectants against oxidative stress, serve as building blocks of more complex structures and may act as molecular messengers for inter- and intracellular signaling. Detailed understanding of ovothiol physiological role in marine organisms may unearth novel concepts in cellular redox biochemistry and highlight the therapeutic potential of this antioxidant. The recent discovery of ovothiol biosynthetic genes has paved the way for a systematic investigation of ovothiol-modulated cellular processes. In this highlight we review the early research on ovothiol and we discuss key questions that may now be addressed using genome-based approaches. This highlight article provides an overview of recent progress towards elucidating the biosynthesis, function and potential application of ovothiols
Thermoelectric effects in magnetic nanostructures
No full textElektronentransport is een belangrijk natuurkundig verschijnsel dat vaak wordt gebruikt in de hedendaagse technologie. Alle elektrische apparaten, variërend van stofzuigers tot computerchips, zijn in principe gebaseerd op dit type transport. Ondanks dat elektronen meerdere eigenschappen hebben, wordt in de praktijk echter vaak alleen de lading van het elektron gebruikt. De energie en het magnetisch moment van de elektronen zijn eigenschappen die in respectievelijk thermoelektriciteit en spin-elektronica (afgekort: spintronica) worden toegepast. Spintronica richt zich op de overdracht van het magnetisch moment om bijvoorbeeld informatie te transporteren, terwijl thermoelektrische verschijnselen, zoals het Peltier en Seebeck effect, toegepast kunnen worden voor elektrische verwarming/koeling en thermokoppels.
Dit proefschrift beschrijft de fundamentele interacties tussen de drie vormen van transport (lading, warmte en spin) in magnetische nanostructuren. Het experimentele werk is een onderdeel van een breder onderzoeksgebied dat spin-caloritronica wordt genoemd. Deze onderzoeksrichting binnen de spintronica bestudeert de rol van het magnetisch moment van elektronen in warmtetransport.
De meerwaarde van deze spin-caloritronische effecten ligt, ten opzichte van reguliere thermoelektrische verschijnselen, in het gemak van het controleren van de magnetische textuur op de nanoschaal. Dit biedt daarom een sterk gelokaliseerde en programmeerbare controle over warmtestromen en zou bruikbaar kunnen zijn voor het genereren van thermo-elektrische energie of voor koeling. De gemeten effecten zijn echter klein en zijn nog ver weg van directe toepassingen. Desalniettemin zouden nieuwe ontwikkelingen binnen dit onderzoeksveld op een dag kunnen leiden tot de implementatie van spin-caloritronica in onze dagelijkse elektronische apparatuur.
Electron transport is one of the most important physical phenomena used in todays technology. All modern electrical equipment, ranging from vacuum cleaners till high-end microprocessors is essentially founded on this type of transport. However, in most cases the electron's full potential is not used and most applications only exploit the negative elementary charge that it possesses. The energy and the magnetic moment of the electrons are properties which are used in thermoelectricity and spin-electronics (spintronics), respectively. Spintronics focuses on the transfer of magnetic moments for the information transport, while thermoelectric phenomena (e.g. the Peltier and Seebeck effect) have found their application in devices for electric heating/cooling or thermocouples.
This thesis describes the fundamental interactions between the three types of transport (charge, heat, spin) in magnetic nanostructures. The experimental work described here is part of a wider research direction, called spin-caloritronics. This research branch of spintronics studies the role of magnetic moments in heat transport.
The potential advantages of spin-caloritronic effects with respect to regular thermoelectricity can be found in the easy manipulation of magnetic textures at the nanoscale. This enables very localized and programmable control of heat flow which might prove useful for thermopower energy harvesting or refrigeration. However, the previously discussed effects are weak and far from direct applications. Nonetheless, a combination of new developments in this field and by exploring novel materials it could one day lead to the implementation of spin-caloritronics in our everyday electronic devices.
New strategies in Bioconjugation : chemical modification of nucleic acids and peptides
No full textOne of the open challenges in chemical biology is to identify reactions that proceed with large rate constants in water at neutral pH values. Once assembled, these conjugates may be used for a broad variety of applications (e.g., therapeutics, imaging probes or as a catalytic system). Herein we describe a novel approach for the chemical modification of nucleic acids using guided organometallic-catalysts. Customized dirhodium complexes were prepared using modular ligands bearing various functional groups and connected to peptide nucleic acids through stable oxime linkages. The final constructs have been optimized for aqueous catalysis and were tested in preliminary alkylation studies of single-stranded DNA via dirhodium-carbenoids generated from alpha-diazocarbonyl compounds. During the course of optimizing the rather slow kinetics of oxime formation, we have developed two highly efficient methods for rapid oxime-based bioconjugations. (1) Dialdehydes were found to react with O-alkylhydroxylamines at rates of 500 M-1 s-1 in neutral aqueous buffer in the absence of a catalyst. The key to these conjugations is an unusually stable cyclic intermediate, which ultimately undergoes dehydration to yield an oxime. The scope and limitations of this method are outlined, as well as its application in bioconjugation with a DNA 41-mer and a mechanistic interpretation that will facilitate further developments of reactions with O-alkylhydroxylamines at low substrate concentrations. (2) Oximes proximal to boronic acids form in neutral aqueous buffer with rate constants of more than 104 M−1 s−1, the largest to date for any oxime condensation. The reaction tolerates a variety of biological interfering additives and is suitable for the rapid modification of short peptide sequences. Once formed, the oxime products are stable for days and undergo slow interconversion through a hydrolysis-based mechanism. Boron's dynamic coordination chemistry confers an adaptability that seems to aid a number of elementary steps in the oxime condensation.
In conclusion both methods represent important improvements for oxime-based bioconjugations in water (pH 7) at low equimolar concentrations. The high reaction kinetics are achieved without the need for additional reagents, catalysts or variations of the reaction conditions. In addition, the possibility of using reacting pairs that are commercially available will greatly enhance the applicability of these methods for efficient conjugations of precious biomolecules in the future
Seebeck effect in polypyrrole/metal junctions
No full textConducting polypyrrole films were electrochemically prepared from aqueous solution containing sodium p-toluenesulphonate as supporting electrolyte at constant current density. The films produced have electrical conductivities of about 35.8 S/cm. Seebeck effect was observed in all three junctions formed by polypyrrole with aluminum, copper and stainless steel. The maximum Seebeck coefficient obtained is from polypyrole/A1 junctions, which is about 33.3uV/C
Large Seebeck effect by charge-mobility engineering
No full textThe Seebeck effect describes the generation of an electric potential in a conducting solid exposed to a temperature gradient. In most cases, it is dominated by an energy-dependent electronic density of states at the Fermi level, in line with the prevalent efforts towards superior thermoelectrics through the engineering of electronic structure. Here we demonstrate an alternative source for the Seebeck effect based on charge-carrier relaxation: a charge mobility that changes rapidly with temperature can result in a sizeable addition to the Seebeck coefficient. This new Seebeck source is demonstrated explicitly for Ni-doped CoSb3, where a marked mobility change occurs due to the crossover between two different charge-relaxation regimes. Our findings unveil the origin of pronounced features in the Seebeck coefficient of many other elusive materials characterized by a significant mobility mismatch. When utilized appropriately, this effect can also provide a novel route to the design of improved thermoelectric materials
Effect of sodium rich pretreatments and processing conditions on microstructure and property evolution of sodium cobalt oxide thermoelectric materials
No full textGlobal environmental and sustainability issues have led to a growth in interest in oxide based thermoelectric materials. Sodium cobalt oxide, which presents low toxicity, is one of the most promising p-type thermoelectric materials for high temperature power generation applications. However, reproducibility and ease of manufacture limits its common use.
NaCo2O4 bulk ceramic materials were prepared from powders synthesized using a solid state reaction (SSR) and sol gel (SG) method. The effect of time and temperature of treatment were investigated in order to determine their influence on microstructure and physical properties.
The effects of three different Na-enriching pretreatments were evaluated with respect to microstructural evolution and their impact on thermoelectric and electric behaviour of the materials. Such modifications were found to be a critical factor affecting the microstructure of the bulk ceramic materials. The Na-rich pretreatments were found to improve density by up to 15%, increase electrical conductivity and help to compensate for Na loss at high sintering temperatures. The thermoelectric figure of merit ZT was found to increase for Na-rich pretreatment samples due to increases in Seebeck coefficient and low thermal conductivity. The highest value of ZT was found to be for the infiltration pretreatment where the value of 0.025 was observed at 350K. Na rich pretreatments, when compared with unpretreatment samples, reduces thermal conductivity by up to 35%, electrical resistivity by up to 67%, increases Seebeck coefficient by up to 23% and as a consequence increases ZT for ball milling preatreatment by 28%, for mixing preatreatment by 71% and for infiltrating by 250%.
A range of films were also produced using a spin coating technique, with thicknesses ranging from 200 nm, for single sol gel layers, up to ~ 32μm for 4 (ink + 2 sol layers) structures. Several factors such as: process conditions, substrates, surfactant and base components used, were investigated in order to improve the quality of films. Process conditions were found to be a critical factor affecting the quality of films. The use of sol infiltration of each layer and a higher preheated temperature were found to reduce surface roughness by up to 23%. The films showed good electrical resistivity ranging from 260 to 500 μΩcm. The lowest value of electrical resistivity was found to be for films annealed at 700ºC
SEEBECK EFFECT OF NANO-SCALE P-TYPE SILICON FILMS ABOVE ROOM TEMPERATURE
No full text Nano-scale Al -doped silicon films are prepared by magnetron sputtering. The amount of Al doped in the films is controlled by regulating the Al sputtering power and duration. With appropriate amount of Al , the Seebeck coefficient of the Si films at room temperature is larger than 168 μV/K, which increases with temperature in agreement with the conventional theory for three-dimensional semiconductors. By reducing the Al sputtering time, however, step-like Seebeck coefficient versus temperature is observed. This anomalous behavior is explained by the step-like density of states for two-dimensional semiconductors. </jats:p
Logarithmic variance profiles and the corresponding f-1 spectra of temperature fluctuations in turbulent Rayleigh-Bénard convection
No full textWe report experimental results for the temperature variance 2(z) and the corresponding frequency spectra P(f) in turbulent Rayleigh-Bénard convection (RBC) in a cylindrical sample of aspect ratioT= D/L = 1:00 (D = 1:12 m is the diameter and L = 1:12 m the height). The measurements were conducted in the Rayleigh-number range 1011 < Ra < 1:35 1014 and Pr ' 0:8. For Ra = 1:35x1014, 2(z) could be described well by a logarithmic dependence on the vertical position z in a range of z 1 < z < z 2 with z 1 ' 70 and z 2 = 0:1L. Here L=(2Nu) is the thickness of a thin thermal sublayer adjacent to the horizontal plate where the heat flux (denoted by the Nusselt number Nu) is carried mostly by thermal diffusion. In the log layer, we found that the temperature spectra had a significant frequency range over which P(f) f with close to 1. As Ra decreased, increased so that the log layer became thinner. At Ra = 2:05 1011, z 2 < z 1 and therefore there was no range for a log layer. Correspondingly, the temperature spectrum near the horizontal plate did not have the f1 scaling form either
Interplay of Peltier and Seebeck Effects in Nanoscale Nonlocal Spin Valves
No full textWe have experimentally studied the role of thermoelectric effects in nanoscale nonlocal spin valve devices. A finite element thermoelectric model is developed to calculate the generated Seebeck voltages due to Peltier and Joule heating in the devices. By measuring the first, second, and third harmonic voltage response nonlocally, the model is experimentally examined. The results indicate that the combination of Peltier and Seebeck effects contributes significantly to the nonlocal baseline resistance. Moreover, we found that the second and third harmonic response signals can be attributed to Joule heating and temperature dependencies of both the Seebeck coefficient and resistivity.
Prime values of and , quadratic
No full textWe prove an asymptotic formula for primes of the shape with integers and of the shape with prime. Here is a binary quadratic form with integer coefficients, irreducible over and has no local obstructions. This refines the seminal work of Friedlander and Iwaniec on primes of the form and Heath-Brown and Li on primes of the form , as well as earlier work of the author with Lam and Schindler on primes of the form with a positive definite form.45 page
- …
