Stick‐slip with the slip phase as the starting point

In the literature, the stick‐slip phenomenon is always explained and modelled as starting from an initial stick phase. However, stick‐slip can also occur after a decrease in the imposed velocity, commencing above the critical (stick‐slip‐free) velocity. In this paper, it will be shown how stick‐slip can originate from a situation of pure sliding (without stick phases), if the slope of the friction force‐relative sliding velocity relation (F‐v_rel gradient) is negative. The F‐v_rel gradient used to study stick‐slip must, because of the interaction between the friction force and the dynamic behaviour of the mechanical system, be derived from velocity changes with the same time‐constant as the mechanical system of which the friction interface is part. It will also be shown that steady‐state friction data, and dynamic (non‐stationary) friction results obtained for relatively large (but fast) velocity variations, can lead to completely false conclusions related to stick‐slip.     

A sensorless PMSM drive using modified high-frequency test pulse sequences for the purpose of a discrete-time current controller with fixed sampling frequency

For electrical machines, accurate position estimations at low speed can be obtained from current responses on high-frequency voltage test pulses. Nevertheless, to apply these voltage test pulses the current controller is often interrupted. In this paper, a modified test pulse sequence is discussed for which the current can be controlled without interruption. To generate the modified test pulse sequence, an asymmetric pulse-width modulator is required. The improvement in the dynamic behaviour of the sensorless drive is discussed by simulations including a salient-pole permanent-magnet synchronous machine, the current control loop, the pulse-width modulator and the position estimator.     

Directional polymerisation and electronic properties of a cyanosubstituted dialkylpolyphenylenevinylene

Summary The synthesis of a monomer allowing for directional polymerisation based on the Knoevenagel condensation reaction is presented. The free running polymerisation gave a molecular weight (M _w = 24650 g mol^-1) polymer product. The polymerisation reaction could be followed by size exclusion chromatography (SEC) and the molecular weight of the polymer product could be controlled by termination of the polymerisation reaction at a suitable time. When terminated before completion a lower molecular weight product was obtained that could be separated into oligomers using preparative SEC. The polymer product and the oligomers were found to have very similar physical properties in terms of the optical band gaps, electronic energy levels and charge carrier mobilities as studied by UV-vis and pulse radiolysis time resolved microwave conductivity (PR-TRMC). Finally the positions of the filled energy levels were determined using ultraviolet photoelectron spectroscopy (UPS).This revised version was published online in September 2004. Due to technical problems, the previous version contained an incomplete PDF.     

The role of prenucleation clusters in surface-induced calcium phosphate crystallization

Unravelling the processes of calcium phosphate formation is important in our understanding of both bone and tooth formation, and also of pathological mineralization, for example in cardiovascular disease. Serum is a metastable solution from which calcium phosphate precipitates in the presence of calcifiable templates such as collagen, elastin and cell debris. A pathological deficiency of inhibitors leads to the uncontrolled deposition of calcium phosphate. In bone and teeth the formation of apatite crystals is preceded by an amorphous calcium phosphate (ACP) precursor phase. ACP formation is thought to proceed through prenucleation clusters--stable clusters that are present in solution already before nucleation--as was recently demonstrated for CaCO(3) (refs 15,16). However, the role of such nanometre-sized clusters as building blocks for ACP has been debated for many years. Here we demonstrate that the surface-induced formation of apatite from simulated body fluid starts with the aggregation of prenucleation clusters leading to the nucleation of ACP before the development of oriented apatite crystals.     

Intravascular contrast agent <Emphasis Type="Italic">T1</Emphasis> shortening: fast <Emphasis Type="Italic">T1</Emphasis> relaxometry in a carotid volunteer study

OBJECTIVE: To measure the T1 times in blood after the administration of the intravascular contrast agent gadofosveset trisodium in humans. MATERIALS AND METHODS: In a pilot study for parameter optimization, the T1-shortening induced by the injection of a single dose (0.03 mmol/kg body weight) of the MR contrast agent Vasovist (Bayer Schering Pharma AG) was measured at B (0) = 1.5 T as a function of time. In four sessions, T1 measurements were performed in the carotid vein of 9 volunteers up to 30 min after injection. T1 times were measured using a segmented saturation recovery turboFLASH (SSRTFL) pulse sequence with 7 different saturation recovery delay times in a total acquisition time of 20 s. RESULTS: The SSRTFL measurements showed T1 times of about 100 ms immediately after injection, which gradually increased to 175 ms at 30 min. The time curve of the R1 = 1/T1 averaged over all volunteers could be described with an exponential decay with a time constant T = 330 +/- 65 s and an amplitude DeltaR1 = 4.1 +/- 0.3 s(-1), and a constant offset of R1(0) = 5.7 +/- 0.2 s(-1). Mean relaxation values are in excellent agreement with theoretical predictions. CONCLUSION: An analytical expression for the initial T1-shortening of Vasovist was derived which can now be used for optimization of the pulse sequence parameters in clinical studies.     

Low band gap polymers for organic photovoltaics

Low band gap polymer materials and their application in organic photovoltaics (OPV) are reviewed. We detail the synthetic approaches to low band gap polymer materials starting from the early methodologies employing quinoid homopolymer structures to the current state of the art that relies on alternating copolymers of donor and acceptor groups where strategies for band gap design are possible. Current challenges for OPV such as chemical stability and energy level alignment are discussed. We finally provide a compilation of the most studied classes of low band gap materials and the results obtained in photovoltaic applications and give a tabular overview of rarely applied materials.