Absolute zero | EPFL Graph Search

Absolute zero is the lowest limit of the thermodynamic temperature scale; a state at which the enthalpy and entropy of a cooled ideal gas reach their minimum value, taken as zero kelvin. The fundamental particles of nature have minimum vibrational motion, retaining only quantum mechanical, zero-point energy-induced particle motion. The theoretical temperature is determined by extrapolating the ideal gas law; by international agreement, absolute zero is taken as −273.15 degrees on the Celsius scale (International System of Units), which equals −459.67 degrees on the Fahrenheit scale (United States customary units or imperial units). The corresponding Kelvin and Rankine temperature scales set their zero points at absolute zero by definition. It is commonly thought of as the lowest temperature possible, but it is not the lowest enthalpy state possible, because all real substances begin to depart from the ideal gas when cooled as they approach the change of state to liquid, and then to s

Access to long-chain polyketides by double-elongation reactions

Christopher Flowers

Due to the spongistatins' fascinating and complex molecular architectures, and their important biological activities, these marine macrolides have been the subject of intensive efforts directed towards their total synthesis. As an attractive alternative to isolation from marine microorganisms, which usually provides only minuscule quantities of this family of macrolides, total synthesis aims to deliver sizable amounts of the natural product, along with enabling access to novel structural analogues, for further biological evaluation. These factors, combined with the impressive molecular architectures themselves, present compelling and formidable challenges to contemporary organic synthesis with regard both to strategy and methodology for their total synthesis in the laboratory. To this end, efforts targeting such complex macrolides provide an arena in which the synthetic chemist can develop new and more efficient methodologies and display such advances to the ultimate task of complex target synthesis. Through this process, new levels of stereocontrol and novel strategies to streamline the entire synthetic route help evolve the field as a whole. The synthetic efforts directed at the AB and CD spiroacetals of the northern fragment are herein disclosed. In conjunction with the synthesis of the southern fragment within our group, this renders itself as an original and efficient formal total synthesis of spongistatin 1. The synthesis of these key fragments has been optimized to reduce protections, set up chirality in an efficient and absolute manner and avoid fluctuations in oxidation states. Achievements in these aspects, that are often used to define the art of modern day total synthesis, have resulted in a rapid synthesis of these key fragments. The synthesis of the AB and CD spiroacetals was completed in 7 and 11 steps, respectively, rendering the process competitive with the most rapid previous synthesis to date. The sequences are based around the use of 1,3-bis(trimethylsilyl)-2-methylidenepropane in double elongation Sakurai reactions with both aldehydes and ketones to form advanced functionalized polyketides.

EPFL2010

Efficiency enhancement and frequency-tunable capability in linear CMOS RF power amplifiers for wireless applications

Paulo Augusto Dal Fabbro

This research work deals with the design of linear CMOS RF power amplifiers. Two important aspects are treated: efficiency enhancement and frequency-tunable capability. For this purpose, two different integrated circuits were realized in a 0.11 µm technology, each one addressing a different aspect. With respect to efficiency enhancement, a dynamic supply RF power amplifier was realized. The circuit was designed for an operating frequency of 5.2 GHz, but measurements were also performed at 2.4 GHz. The integrated circuit includes a class A amplifier and a high-efficiency, switched-mode modulator. It occupies an area of 1.35 mm2. Two-tone measurement results at 2.4 and 5.2 GHz showed that the system can deliver over 16 dBm (40 mW) linear output power with efficiencies of 22.7% and 12.6%, respectively. Compared to a constant 2.5 V supply operation, the power amplifier operating with the dynamic supply delivers higher linear output power. Moreover, a relative efficiency improvement of a factor of 2.3 at 2.4 GHz and of a factor of 1.6 at 5.2 GHz at low output power levels is achieved. OFDM measurements at 2.4 GHz demonstrated that for an EVM lower than 3% and for an equal output power of 11 dBm (12.6 mW), the absolute improvement in efficiency is over 3.2%. In this work we also propose a tunable impedance matching network for use in multiband RF power amplifiers. This novel network is based on coupled inductors. A frequency-tunable RF power amplifier using this network was designed for operation at 3.7 and 5.2 GHz. Simulation results for the complete system integrated in a CMOS technology showed good results. However, the frequency-tunable behavior could not be observed in the characterization of the integrated circuit because of the low coupling factor of the integrated coupled inductors. A hybrid implementation using the integrated CMOS power amplifier, a discrete commercial RF transformer and a discrete bipolar transistor to control the current in the secondary winding of the transformer was carried out. The circuit was designed for operation at 200 and 300 MHz. The measurements have shown that at 200 MHz a relative improvement in efficiency of a factor of 1.4 was achieved. Moreover, less distortion was generated with the proposed tunable output matching network. The hybrid implementation allowed us to demonstrate the feasibility of the frequency-tunable RF power amplifier based on coupled inductors.

EPFL2009

Spectroscopy and dynamics at liquid water interfaces

Bruno Credidio

This thesis is a detailed description of three experimental investigations on aqueous interfaces. All projects made use of the microjet technology or the more recently developed flat-jet technique which enables the implementation of liquid water in vacuum chambers. In the first study presented here we show that a flat-jet created from the impingement of two liquid microjets generates a laminar turbulence-free water-water interface. By colliding a Luminol solution microjet with another microjet of hydrogen peroxide solution, the chemiluminescence of Luminol reveals where the liquids mix in the flat-jet. The liquids readily mix in the rims of the flat-jet, while in the middle part the different liquids only meet via diffusion. Flat-jets are stationary systems such that the longitudinal flow direction translates into a time dimension, providing access to the interface in different total interaction times ranging from 10 to 200 µs. This timescale is difficult to access by current techniques. The water-vacuum interface is the next topic, in which we have studied aqueous solutions using microjets and photoelectron spectroscopy (MJ-PES). For the first time, we have experimentally demonstrated the implementation of an absolute photoelectron energy reference in MJ-PES. We could show that there is a concentration-dependent shift of valence bands which were previously (without this absolute energy reference) undetectable. Even though this study was done using a tabletop He plasma light source, it creates the foundation of absolute energy referencing for MJ-PES works done in, e.g., synchrotron laboratories. Lastly, we have built an experimental apparatus to investigate the water-gas interface. Such interfaces are ubiquitous in nature but lack direct experimental works on the molecular scale, namely scattering studies. This is largely due to the technical difficulty imposed by the high vapour pressure of water. The dense layer of vapour shields the liquid surface in such a way that conventional molecular beams cannot travel to and from the liquid surface without interacting with the vapour phase, destroying the initial and final states required to understand the surface scattering event at the quantum level (transmission probability of ~e^(-100)). We propose an alternative method to this: by approaching the molecular beam source and the water flat-jet to a distance of 200 µm or less, the probability of scattering solely at the liquid surface becomes realistic. We have performed preliminary experiments using the developed apparatus.

EPFL2022