Anisotropic stick-slip friction of highly oriented thin films of poly(tetrafluoroethylene) at the molecular level

Lateral force microscopy (LFM) studies of poly(tetrafluoroethylene) (PTFE) films with molecular resolution are reported. Thin PTFE layers with a high degree of orientation were obtained by pressing and sliding a block of polymer on a clean, heated muscovite mica substrate. LFM nanographs obtained on these films by scanning at directions between ca. 40 and 90° with respect to the film orientation direction, i.e. with respect to the direction of the polymer chains, showed a stick-slip type frictional motion of the LFM probe tip at the molecular level. The friction force observed at constant load decreased with decreasing scan angles. Chain-chain packing distances obtained by LFM and contact-mode atomic force microscopy were the same to within the experimental error and had a value of 5.8 Å. Dual-mode contact AFM/LFM imaging was also performed by scanning in the chain direction. Here LFM nanographs showed no distinct stick-slip phenomenon. The contact mode AFM images, however, exhibited clear molecular resolution with the expected chain-chain periodicity. The disappearance of the stick component in LFM scans performed in the chain direction was attributed to the smooth surface of PTFE on the molecular scale.     

Control of the chromium-oxygen reaction in pure iron melts

Oxygen activities in Fe-O-Cr melts, equilibrated with chromium oxide, were determined as a function of chromium content by EMF measurements using ZrO₂(CaO) plug-type sensors with a Cr-Cr₂O₃ reference. Plug-type sensors with or without Cr₂O₃ saturator were used in the experiments. From the experimental results values for the activity coefficient f_O and the interaction parameters and were evaluated. Raoultian chromium activity features a slight positive deviation from ideal solution behaviour in the temperature range from 1550 to 1650°C. Taking into account the calculated interaction parameters, the equilibrium constant K_Cr-O was redetermined at 1550 to 1650°C. In comparison with previous studies a better agreement was obtained with existing thermochemical data.     

Mechanized proofs of opacity: a comparison of two techniques

Software transactional memory (STM) provides programmers with a high-level programming abstraction for synchronization of parallel processes, allowing blocks of codes that execute in an interleaved manner to be treated as atomic blocks. This atomicity property is captured by a correctness criterion called opacity, which relates the behaviour of an STM implementation to those of a sequential atomic specification. In this paper, we prove opacity of a recently proposed STM implementation: the Transactional Mutex Lock (TML) by Dalessandro et al. For this, we employ two different methods: the first method directly shows all histories of TML to be opaque (proof by induction), using a linearizability proof of TML as an assistance; the second method shows TML to be a refinement of an existing intermediate specification called TMS2 which is known to be opaque (proof by simulation). Both proofs are carried out within interactive provers, the first with KIV and the second with both Isabelle and KIV. This allows to compare not only the proof techniques in principle, but also their complexity in mechanization. It turns out that the second method, already leveraging an existing proof of opacity of TMS2, allows the proof to be decomposed into two independent proofs in the way that the linearizability proof does not.     

Isolierung und Identifizierung von Farbstoffen in Seifen

Isolation and Identification of Dyes in Soaps A method is described for isolation and identification of dyes in soaps. The principle depends on separation of the dyes with the help of various organic solvents or acids, adsorption on polyamide powder and identification through thin layer chromatography. The inorganic pigments are identified by classical analytical procedures.     

Butadiene rubbers: topological constraints and microscopic deformation by mechanical and small angle neutron scattering investigation

Summary Results of stress-strain measurements are reported for butadiene rubbers of varying crosslink density. The fluctuation of the effective tube diameter of the polymer networks was investigated under uniaxial elongation by mechanical measurements as well as by small angle neutron scattering (SANS) which probe the local orientation on a segmental scale. The effect of topological constraints on the microscopic deformation of the butadiene network chains is well described within a tube approach. For the first time, experiments at large deformations and for polydisperse sample are presented. Excellent agreement between the statistical mechanical model and the experimental results is obtained. Received: 20 November 2001 /Revised version: 5 March 2002/ Accepted: 5 March 2002     

Clearing the air on EU guidance projects for school buildings

ABSTRACT

Professionals engaged in the design of new or retrofit of existing school buildings currently struggle to find comprehensive guidelines for achieving healthy and energy-efficient school environments. Recently, two major and independent research projects aimed at improving indoor environmental quality and energy performance of school buildings in Europe have been completed: SINPHONIE and School of the Future. Their guidelines reflect the distinct priorities and limited scope of each project. This paper compares for the first time and, as far as possible, synthesizes these outputs to facilitate their use in practice and research. Overall, SINPHONIE is most useful in addressing indoor environmental quality of schools in terms of the physical, chemical and microbiological stressors that impact and potentially affect human health. It aims to influence the future development of policy and regulations. By contrast, School of the Future presents a broader design-oriented approach based on best practice in four European countries. For the most part they complement each other in their recommendations; however, this review highlights several areas that require further clarification and research including links between indoor environmental quality and cognitive performance, the consequent appropriate level for the designed ventilation rate, and the role of adaptive thermal comfort in schools.     

Material removal mechanism in ultrasonic-assisted grinding of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> by single-grain scratch test

Alumina is an advanced ceramic that is frequently used in high-performance applications. Grinding of alumina is usually associated with micro-cracks and deteriorated surface quality. Ultrasonic-assisted grinding has been introduced in several applications as a promising method to overcome these constraints. In order to get a deeper knowledge of the characteristics of material removal mechanisms in alumina during grinding with ultrasonic stimulation of the workpiece, single-grain scratch tests were performed and the theoretical and experimental kinematics of grain-workpiece engagement were investigated. It was shown that in the real contact conditions, interrupted contact conditions happen, which is analogous to the theoretical model. The measured workpiece resonance frequency and mode shape were very close to the design conditions. The investigations show that the superposition of ultrasonic vibration into the grinding process increases the material removal of each grain. This result fully correlates with the presented theoretical analysis. Additionally, it was found that the impact action of ultrasonic-assisted grinding induces chipping around the produced scratch.     

Isotopenhydrologische Methoden (2H, 18O) zur Bestimmung der Grundwasserneubildung in Trockengebieten: Potenzial und Grenzen

Grundwasser - Die Quantifizierung der Grundwasserneubildung in Trockengebieten stellt aufgrund klimatischer Gegebenheiten, mächtiger ungesättigter Zonen sowie hochangepasster Vegetation...