Acrylamide-styrene copolymerization with the 4,4′-azo-bis-(4-cyanovaleryl)-bis-(m-chlorobenzoyl) diperoxide azoperoxy initiator

Summary The copolymerization of acrylamide with acrylic acid by use of 4,4-azo-bis-(4-cyanovaleryl)-bis-(m-chlorobenzoyl) diperoxide as initiator was studied. The thermolysis or photolysis of the mentioned initiator was studied. The thermolysis or photolysis of the mentioned initiator was used to prepare polystyrene with final peroxidic groups, which was further reacted with acrylamide to yield poly(styrene-acrylamide).     

Learning and the sources of corporate growth

This paper explores the link between learning and corporategrowth by developing different models of learning and showingthat they produce observably different models of corporate growth.Using data on the growth of a number of firms in the US automobileindustry during the 20th century, we compare these differentmodels of growth in an effort to identify the major sourcesof learning which these firms seem to have relied on. Althoughthere are interesting differences between growth processes beforeand after the Second World War, the basic conclusion that weare drawn to is that learning in this sector is largely unsystematicand opportunistic.     

Microstructure, toughness and flexural strength of self-reinforced silicon nitride ceramics doped with yttrium oxide and ytterbium oxide

Self-reinforced silicon nitride ceramics with additions of either yttrium oxide or ytterbium oxide have been investigated at room temperature after various processing heat treatments. Devitrification of the intergranular phase in these materials is very sensitive to the heat treatment used during processing and does not necessarily improve their strength and toughness. Hot-pressed ceramics without a subsequent devitrification heat treatment were the strongest. The ytterbium oxide-doped silicon nitride ceramics were consistently tougher, but less strong, than the yttrium oxide-doped silicon nitride ceramics. In all the ceramics examined, the fracture toughness showed evidence for R -curve behaviour. This was most significant in pressureless sintered ytterbium oxide-doped silicon nitride ceramics. A number of toughening mechanisms, including crack deflection, bridging, and fibre-like grain pull-out, were observed during microstructural analysis of the ceramics. In common with other silicon nitride-based ceramics, thin amorphous films were found at the grain boundaries in each of the ceramics examined. Arrays of dislocations left in the elongated silicon nitride grains after processing were found to belong to the {101¯0}<0001> primary slip system.     

Matrix polymerisation of poly(vinylacetate)

The matrix polymerisation of poly(vinylacetate) was investigated. Using scanning electron microscopy and gel permeation chromatography, the morphology and molecular weight distribution of poly(vinylacetate) were determined. The experimental data prove the matrix influence upon the molecular and morphological characteristics of poly(vinylacetate) thus obtained.     

Split-plot design optimization for trace determination of lead by anodic stripping voltammetry in a homogeneous ternary solvent system

A split-plot design has been used to simultaneously optimize reagent conditions and solvent medium for Pb²⁺ determination by anodic stripping voltammetry (ASV). Three mixture components, N,N-dimethylformamide (DMF), ethanol and water, and two process variable levels, ammonium acetate (supporting electrolyte) and hydrochloric acid concentrations, were varied. The calculations of main-plot, subplot and main-subplot interaction ANOVA sums of squares for regression and lack of fit are illustrated. These values are shown to be useful for model development. Six different models were evaluated. The bilinear-special cubic model has only a very slight lack of fit and is preferred. Standard error estimates were calculated using a method described in the literature by Cornell, and approximate critical values for t-tests on the model coefficients were investigated. Normal probability graphs seem appropriate for this analysis. The significant terms in this model are capable of describing how the mixture response surfaces change as process level conditions are varied. Optimized mixture proportions for each factorial design level are determined. Optimum conditions in the (− −) main-plot quadrant for the determination of lead by ASV in the homogeneous ternary solvent system were achieved with a solvent composition of 8.0 g of N,N-dimethylformamide (DMF), 7.0 g of ethanol and 5.0 g of water, corresponding to 40% m/m of DMF, 35% n/m of ethanol and 25% m/m of water. Water is essential since the sample and the buffer (0.1 mol kg⁻¹ ammonium acetate containing 0.00880 mol kg⁻¹ HCl) are added to the system as aqueous solutions. The other three main-plot quadrants presented optimum analytical signals for 95% aqueous solutions. The split-plot design seems to be especially appropriate for simultaneously optimizing process and mixture variables of this chemical system facilitating operational procedures by means of block randomization.     

Matrix polymerisation of vinyl acetate in the presence of molecular sieves

Polymerisation of vinyl acetate in the presence of some molecular sieves, Sephadex and CaX type Zeolite, led, in well defined experimental conditions, to polymers with a regular structure. Evidence of the role of molecular sieves as a matrix for stereoregular polymerisation was provided by n.m.r. spectra and scanning electron microscopy, while the pyrolysis- gas chromatography and elemental analysis have pointed out the separation of polymer from matrix. The experimental data obtained on polymerisation of vinyl acetate in the presence of molecular sieves allowed us to postulate the presence of a donor-acceptor type reaction mechanism.     

Spacial arrangement of poly(vinylacetate) synthesised by matrix polymerisation

The tactic structure of PVAc synthesised by matrix polymerisation was reported. At temperatures below 0°C, for a ratio, matrix to monomer, equal to 3, a higher packing degree of the macromolecular chains was obtained. The third peak of the X-ray diffraction spectra of matrix samples suggests the existence of isotactic polymer.     

Joining Ti-47Al-2Cr-2Nb with a Ti/(Cu,Ni)/Ti clad-laminated braze alloy

The joining of Ti-47Al-2Cr-2Nb using Ti-15Cu-15Ni (wt%) as braze alloy was investigated. Experiments were conducted at 980 and 1000°C for 10 min. The microstructure and the chemical composition of the interfaces were studied by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. For both processing conditions the reaction between the -TiAl alloy and the braze alloy produced layered interfaces, which are essentially composed of ₂-Ti₃Al and of Ti-Ni-Cu-Al and Ti-Ni-Cu intermetallic compounds. Microhardness tests showed that all reaction layers are harder than either the or the (₂ + ) lamellar grains of the intermetallic alloy.     

Accurate and computer efficient modelling of single event transients in CMOS circuits

A new analytical modelling approach to evaluate the impact of single event transients (SETs) on CMOS circuits has been developed. The model allows evaluation of transient pulse amplitude and width (duration) at the logic level, without the need to run circuit level (Spice-like) simulations. The SET mechanism in MOS circuits is normally investigated by Spice-like circuit simulation. The problem is that electrical simulation is time-consuming and must be performed for each different circuit topology, incident particle and track. The availability of a simple model at the logic gate level may greatly improve circuit sensitivity analysis. The electrical response of a circuit to an ionising particle hit depends on many parameters, such as circuit topology, circuit geometry and waveform shape of the charge injection mechanism. The proposed analytical model, which is accurate and computer efficient, captures these transistor-level effects of ionising particle hits and models them to the logic level of abstraction. The key idea is to exploit a model that allows the rapid determination of the sensitivity of any logic gate in a CMOS circuit, without the need to run circuit simulations. The model predicts whether or not a particle hit generates a SET, which may propagate to the next logic gate or memory element, making possible to analyse the sensitivity of each node in a complex circuit. Model derivation is strongly related to circuit electrical behaviour, being consistent with technology scaling. The model is suitable for integration into CAD tools, intending to make automated evaluation of circuit sensitivity to SET possible, as well as automated estimation of soft error rate