Crack growth in transforming ceramics under cyclic tensile loads

The mechanisms of stable growth of short fatigue cracks (crack length up to 1 mm) at room temperature in magnesia-partially stabilized zirconia subjected to cyclic tensile loads were investigated. Single edge-notched specimens were fractured in the four-point bend configuration under cyclic and quasi-static tensile loads. At a load ratio of 0.1, the threshold stress intensity factor range, K, for fracture initiation in cyclic tension is as low as 3.4 M Pam^1/2, and catastrophic failure occurs at K=6.6 M Pam^1/2. For crack length less than 1 mm and for plane strain conditions, growth rates are highly discontinuous, and periodic crack arrest is observed after growth over distances of the order of tens of micrometres. Crack advance could only be resumed with an increase in the far-field stress intensity range. The mechanisms of short crack advance in cyclic tension are similar to those observed under quasi-static loads, and the tensile fatigue effect appears to be a manifestation of static failure modes. A model is presented to provide an overall framework for the tensile fatigue crack growth characteristics of partially stabilized zirconia. Experimental results are also described to demonstrate the possibility of stable room temperature crack growth under cyclic tension in fine-grained tetragonal zirconia polycrystals, partially stabilized with Y₂O₃. The growth of cracks in transformation-toughened ceramics is found to be strongly influenced by the crack size and shape, stress state and specimen geometry.     

Antibacterial LDPE Nanocomposites Based on Zinc Oxide Nanoparticles/Vermiculite Nanofiller

Polyethylene (PE) nanocomposites with the zinc oxide-nanoparticles/vermiculite nanofiller were prepared in two-steps. In first step, the ZnO-np/V nanofiller were prepared by the mechanochemical method followed by a heat treatment at 650?°C for 90 min. In second step, this nanofiller was used in concentration 3, 6, 10 and 15 wt% for PE nanocomposites preparation via melt compounding technique, the nanocomposites plates were pressed. The particles morphology of the ZnO-np/V nanofiller, arrangement of the ZnO-np/V nanofiller and surface of the PE plates were studied using scanning electron microscopy, optical light microscopy and atomic force microscopy. The surface roughness of the PE plates was evaluated from AFM measurements. Structural changes of the ZnO-np/V nanofiller in PE nanocomposites were monitored using X-ray diffraction analysis and Fourier transform infrared analysis. The gradual and long-term antibacterial effect of PE nanocomposites was tested on the Gram positive bacteria E. faecalis by counting the colony forming units number.     

Asymmetric Aza‐Morita–Baylis–Hillman‐Type Reactions: The Highly Enantioselective Reaction between Unmodified α,β‐ Unsaturated Aldehydes and N‐Acylimines by Organo‐co‐catalysis

The highly enantioselective organo‐co‐catalytic aza‐Morita–Baylis–Hillman (MBH)‐type reaction between N‐carbamate‐protected imines and α,β‐unsaturated aldehydes has been developed. The organic co‐catalytic system of proline and 1,4‐diazabicyclo[2.2.2]octane (DABCO) enables the asymmetric synthesis of the corresponding N‐Boc‐ and N‐Cbz‐protected β‐amino‐α‐alkylidene‐aldehydes in good to high yields and up to 99% ee. In the case of aza‐MBH‐type addition of enals to phenylprop‐2‐ene‐1‐imines, the co‐catalytic reaction exhibits excellent 1,2‐selectivity. The organo‐co‐catalytic aza‐MBH‐type reaction can also be performed by the direct highly enantioselective addition of α,β‐unsaturated aldehydes to bench‐stable N‐carbamate‐protected α‐amidosulfones to give the corresponding β‐amino‐α‐alkylidene‐aldehydes with up to 99% ee. The organo‐co‐catalytic aza‐MBH‐type reaction is also an expeditious entry to nearly enantiomerically pure β‐amino‐α‐alkylidene‐amino acids and β‐amino‐α‐alkylidene‐lactams (99% ee). The mechanism and stereochemistry of the chiral amine and DABCO co‐catalyzed aza‐MBH‐type reaction are also discussed.     

Light Quality and Intensity Modulate Cold Acclimation in Arabidopsis

Plant survival in temperate zones requires efficient cold acclimation, which is strongly affected by light and temperature signal crosstalk, which converge in modulation of hormonal responses. Cold under low light conditions affected Arabidopsis responses predominantly in apices, possibly because energy supplies were too limited for requirements of these meristematic tissues, despite a relatively high steady-state quantum yield. Comparing cold responses at optimal light intensity and low light, we found activation of similar defence mechanisms—apart from CBF1–3 and CRF3–4 pathways, also transient stimulation of cytokinin type-A response regulators, accompanied by fast transient increase of trans-zeatin in roots. Upregulated expression of components of strigolactone (and karrikin) signalling pathway indicated involvement of these phytohormones in cold responses. Impaired response of phyA, phyB, cry1 and cry2 mutants reflected participation of these photoreceptors in acquiring freezing tolerance (especially cryptochrome CRY1 at optimal light intensity and phytochrome PHYA at low light). Efficient cold acclimation at optimal light was associated with upregulation of trans-zeatin in leaves and roots, while at low light, cytokinin (except cis-zeatin) content remained diminished. Cold stresses induced elevation of jasmonic acid and salicylic acid (in roots). Low light at optimal conditions resulted in strong suppression of cytokinins, jasmonic and salicylic acid.     

Crosstalk between Light- and Temperature-Mediated Processes under Cold and Heat Stress Conditions in Plants

Extreme temperatures are among the most important stressors limiting plant growth and development. Results indicate that light substantially influences the acclimation processes to both low and high temperatures, and it may affect the level of stress injury. The interaction between light and temperature in the regulation of stress acclimation mechanisms is complex, and both light intensity and spectral composition play an important role. Higher light intensities may lead to overexcitation of the photosynthetic electron transport chain; while different wavelengths may act through different photoreceptors. These may induce various stress signalling processes, leading to regulation of stomatal movement, antioxidant and osmoregulation capacities, hormonal actions, and other stress-related pathways. In recent years, we have significantly expanded our knowledge in both light and temperature sensing and signalling. The present review provides a synthesis of results for understanding how light influences the acclimation of plants to extreme low or high temperatures, including the sensing mechanisms and molecular crosstalk processes.     

The environmental chemistry of copper (II) in aquatic systems

The speciation of copper(II) in fresh water has been examined by considering inorganic and organic complexation and adsorption and precipitation processes. It is concluded that in most aquatic systems these processes are capable of reducing free copper levels to very low values even in the presence of high levels of total copper.     

Moving-wire device for carbon isotopic analyses of nanogram quantities of nonvolatile organic carbon

We describe a moving-wire analyzer for measuring 13C in dissolved, involatile organic materials. Liquid samples are first deposited and dried on a continuously spooling nickel wire. The residual sample is then combusted as the wire moves through a furnace, and the evolved CO2 is analyzed by continuous-flow isotope ratio mass spectrometry. A typical analysis requires 1 microL of sample solution and produces a CO2 peak approximately 5 s wide. The system can measure "bulk" delta13C values of approximately 10 nmol of organic carbon with precision better than 0.2 per thousand. For samples containing approximately 1 nmol of C, precision is approximately 1 per thousand. Precision and sensitivity are limited mainly by background noise derived from carbon within the wire. Instrument conditions minimizing that background are discussed in detail. Accuracy is better than 0.5 per thousand for nearly all dissolved analytes tested, including lipids, proteins, nucleic acids, sugars, halocarbons, and hydrocarbons. The sensitivity demonstrated here for 13C measurements represents a approximately 1000-fold improvement relative to existing elemental analyzers and should allow the use of many new preparative techniques for collecting and purifying nonvolatile biochemicals for isotopic analysis.     

The garden city idea in the CIAM discourse on urbanism: a path to comprehensive planning

Over a period of three decades, from 1928 to 1959, the Congrès Internationaux d'Architecture Moderne (CIAM) formed an unconventional working team and a complex laboratory of progressive ideas for designing the city. During the founding years of the organization and the transatlantic emigration of many of its major figures during the second half of the 1930s, a profound engagement with the most important trends in the European and North American debates on planning had become the critical foundation for their common task. The organization's consequent basic convictions and demands summed up the most effective maxims of the discipline since the turn of the century. Officially, CIAM tried to distinguish itself from traditional urban planning and instead advocated modern urban planning as the antithesis to everything that existed previously. Seen from a historical perspective, however, numerous points of connection – in particular to the garden city movement – become very clear. The various members of CIAM were deeply influenced ideologically by Ebenezer Howard's visions for urbanism. Personal contact to representatives of the garden city movement, active involvement in its institutions, and broad implementation of Howard's ideas in the context of European and North American planning led CIAM from the analysis of the functions of the existing city to a comprehensive design of the modern city.