Long time stress relaxation of amorphous networks under uniaxial tension: The Dynamic Constrained Junction Model

The constrained junction model that represents the stress-strain relations of amorphous networks in equilibrium is modified to analyze stress relaxation. Deviation of stress from equilibrium when a network is stretched suddenly is represented by a time dependent constraint contribution that is of the same form as that of the equilibrium theory. The time dependent motions of the junctions are assumed to obey the Langevin equation. The only new term in the model is a time dependent κ parameter that vanishes at long times. Results of the model are compared with uniaxial stress relaxation experiments on polyisoprene networks with different degrees of cross-linking. Experiments show that the time dependent κ parameter obeys a stretched exponential form, with β = 0.4 and τ = 40 s, both of which are the same for all extensions and cross-link densities studied. The front factor κ₀ depends on cross-link density in the same way as in the equilibrium case. Comparison with stress relaxation experiments shows satisfactory agreement at a wide range of extensions and for different degrees of cross-linking. The relatively low value of the stretched exponent parameter, β = 0.4, is interpreted in terms of a molecular picture where entanglements contribute to relaxation at a wide spectrum of time scales.     

The effect of D‐glucose on milk clotting activity of Mucor miehei in a chemostat with biomass retention

In this study microbial production of rennin from a commercial strain of Mucor miehei (NRRL 3420) has been accomplished in a continuously fed fermenter. The effects of feed D ‐glucose concentration (2.5–30 g dm⁻³) on milk clotting activity and on other system variables were investigated at optimum mixing and dilution rates of 400 rpm and 0.0052 h⁻¹, respectively, without pH control. Maximum milk clotting activity (1.24 IU cm⁻³) occurred when the feed D ‐glucose concentration was 7.5 g dm⁻³. Enzyme production continued for 500 h, producing a total milk clotting activity of 10 230 IU. At the maximum milk clotting activity point, total protein concentration, milk clotting and proteolytic activities were analysed and compared with those of a commercial rennet. The ratio of milk clotting activity to proteolytic activity, and specific milk clotting activity were determined as 1.55 × 10⁻³ IU PU⁻¹ and 5.28 IU mg⁻¹ medium protein, respectively, denoting similar characteristics to a commercial rennet after concentration of the fermentation medium. © 1999 Society of Chemical Industry     

Preparation and physical/electrochemical characterization of Pt/poly(vinylferrocenium) electrocatalyst for methanol oxidation

Preparation and characterization of a platinum (Pt)-based catalyst using a redox polymer, poly(vinylferrocenium) (PVF⁺), as the support material was described. Pt was obtained from aqueous solution of K₂PtCl₄ in the complex form. Pt particles were reduced by chemical and electrochemical means. Chemical reduction was performed using aqueous hydrazine solution and electrochemical reduction was carried out in H₂SO₄ solution. The Pt/PVF⁺ catalyst system showed catalytic activity towards methanol oxidation. Cyclic voltammetry was used for the electrochemical characterization of the catalyst system. Scanning electron microscopy (SEM) images and energy dispersive X-ray spectrum (EDS) of the catalyst system were also recorded. The system was tested in a single fuel cell configuration at ambient temperature and atmospheric pressure. The open circuit voltage (OCV) was 680 mV for the system and the maximum power density was 0.31 mW cm⁻² at a current density of 0.63 mA cm⁻². Catalytic activity of Pt/PVF⁺ system towards methanol oxidation was comparable with the related catalysts in the literature.     

Using Nutritional Lipid and Sensorial Profiles for the Characterization of Turkish Olive (Memecik X Gemlik cv) Hybrids obtained from Controlled Crossbreeding

Olive breeding is an important method for improving of olive oil quality. In the early 1990’s, a controlled cross breeding project was started on Turkish domestic olive cultivars by Bornova Olive Research Institute, Izmir. This study was carried out with a total of 11 hybrids and 2 parents and these oil samples were obtained from “Memecik” [5 samples (MG) + 1 parent (MEM)] and “Gemlik” [6 samples (GM) + 1 parent (GEM)] domestic olive cultivars grown under the same agronomic and pedoclimatic conditions. In this study, agronomic criteria (ripening index), some nutritional values (total phenolic contents, major fatty acids [palmitic, oleic, linoleic acid, linolenic acid] and their parameters (linoleic/linolenic ratio, quality index), and sensory properties (panel test) of olive (Memecik and Gemlik) hybrid oils were analyzed during 2005–2008 four harvest years. All results showed that 11 advanced domestic olive hybrids had different oil characteristics relative to the parents of hybrids based on nutritional and sensory profiles. The Pearson correlation coefficients and principal components factor loadings observed among chemical/sensory characteristics and fatty acid profile (GM 19, GM 39, MG 11, MG 5 and MG 123 promising hybrids) could be of interest in future olive breeding programs. Considering the main objectives of olive breeding project, the hybrids of GM 19 and MG 5 exhibited superior agronomic (maturation–regular yield), technological–nutritional (high total polyphenol content, balanced omega3/omega6 ratio), and sensorial (high pungent and bitter qualifications) features. The official registrations of these hybrids were carried out by Ministry of Agriculture and Forestry of Turkey.     

Evaluating Enzymatic Productivity—The Missing Link to Enzyme Utility

Kinetic productivity analysis is critical to the characterization of enzyme catalytic performance and capacity. However, productivity analysis has been largely overlooked in the published literature. Less than 0.01% of studies which report on enzyme characterization present productivity analysis, despite the fact that this is the only measurement method that provides a reliable indicator of potential commercial utility. Here, we argue that reporting productivity data involving native, modified, and immobilized enzymes under different reaction conditions will be of immense value in optimizing enzymatic processes, with a view to accelerating biotechnological applications. With the use of examples from wide-ranging studies, we demonstrate that productivity is a measure of critical importance to the translational and commercial use of enzymes and processes that employ them. We conclude the review by suggesting steps to maximize the productivity of enzyme catalyzed reactions.     

The influence of computer-assisted instruction on students’ conceptual understanding of chemical bonding and attitude toward chemistry: A case for Turkey

In this study, the effect of computer-assisted instruction on conceptual understanding of chemical bonding and attitude toward chemistry was investigated. The study employed a quasi-experimental design involving 11 grade students; 25 in an experimental and 25 in a control group. The Chemical Bonding Achievement Test (CBAT) consisting of 15 two-tier questions and the Chemistry Attitude Scale (CAS) consisting of 25 item were the principal data collection tools used. The CBAT and CAS instruments were administered in the form of a pre-test and post-test. Analyses of scores of the two groups in the post-test were compared and a statistically significant difference was found between groups in favor of experimental group. It also seems students from the experimental group were more successful than the control group students in remediation of alternative conceptions. The results of this study suggest that teaching–learning of topics in chemistry related to chemical bonding can be improved by the use of computer-assisted teaching materials.     

SUPERCRITICAL EXTRACTION AND DESULPHURIZATION OF BEYPAZARI LIGNITE BY ETHYL ALCOHOL/NaOH TREATMENT 3. ELECTROCHEMICAL CHARACTERIZATION OF ORGANIC SULPHUR COMPOUNDS

ABSTRACT

The overall objective, of this study is to develop an electrochemical method, that will be complementary to the existing spectroscopic methods, for the characterization of sulphur compounds in coal extracts. Beypazan lignite was used in this study. Supercritical interaction between coal and ethyl alcohol/NaOH (or Ca(OH₂) ) was carried out using a microreactor of 15 ml capacity. Liquid products obtained were separated by liquid chromatography in a glass column have been determined and these provided a ground for the identification of various sulphur functional groups in coal liquids obtained from supercritical interaction between a lignite and ethyl alcohol/NaOH or ethyl alcohol/Ca(OH) ₂ or ethyl alcohol. Organic sulphur compounds determined by this method were dibenzydisulphide, dibenzothiophene and tiophenol     

Adaptive on-line parameter identification of a steer-by-wire system

Adaptive compliance control strategy can be a significant advantage for control of steer-by-wire systems. Initially the method is proposed for robotic applications where the main concern is the interaction forces between the robot and its environment. There are several studies about cooperative working of a robot and a human. As long as the steering system is a part of the vehicle where driver interaction is involved, it is reasonable to think that compliance control strategies can be adapted to steer-by-wire systems. Compliance control is a model reference control (MRC) strategy where the measured external force/torque is used as an input to a reference model to calculate its output and where the real system is controlled appropriately to track the reference system output. If a sensor is available to measure the external force/torque, system parameters need not to be estimated. A constant gain feedback controller can be used in such a case. However, if the parameter variations of the system are not within certain bounds, a model reference adaptive controller (MRAC) is needed. In addition to this, examining the change in the dynamics of the system due to the compliance of the driver arms is not possible by direct MRAC, because the driver effect is considered as a disturbance in this strategy. Therefore, in this study, instead of estimating controller parameters using direct MRAC where the main concern is the tracking performance, it is considered to use indirect MRAC in which the system parameters are estimated to observe their variations in the presence of parametric uncertainty and disturbances and to further examine the change in the dynamics of the system due to the compliance of the driver arms forming a closed kinematic structure by constraining the steering wheel. Hence, a steer-by-wire experimental setup including driver interaction and vehicle directional control units has been developed and three well-known adaptive on-line estimation methods, which are output-error method, equation-error method and modified recursive least squares method are evaluated on the driver interaction unit. These three methods are compared in terms of computational complexity, convergence, stability and applicability to real vehicles.     

A high performance Σ-Δ readout circuitry for μg resolution microaccelerometers

This paper reports a second order electromechanical sigma-delta readout for micro-g resolution accelerometers in addition to other high-sensitivity capacitive microsensors with large base capacitance. The chip implements a switched-capacitor readout front-end and an oversampled sigma-delta modulator, and hence can be used for both open-loop analog readout and closed-loop control and readout with direct digital output. The readout circuit has more than 115 dB dynamic range and can resolve less than 3 aF/√Hz. Also this IC includes start-up circuit and feedback mechanism for closed-loop control of the accelerometer with a single 5 V supply in a ±4 g range. Together with the accelerometer, bandwidth of the overall system is limited with the sensor resonance frequency (1.53 kHz) in the open-loop mode. However in closed loop mode, oversampling of the acceleration data increases the bandwidth of the system up to few hundred kilohertz which is limited with the cut-off frequency of the low-pass filter placed at the output of the system. The start-up circuit allows rebalancing of a thick silicon proof mass with the limited 5 V supply after system start from power down or in the case of over-range input acceleration. The readout chip has been combined with a Silicon-On-Glass lateral accelerometer, which has a high sensitivity of 1.88 pF/g with large proof mass and long finger structures. A digital filtration and decimation circuitry is also implemented to signal process the output bit stream of the readout circuit. The complete module consumes 16 mW from a ±2.5 V supply and has an adjustable sensitivity up to 8 V/g with a noise level of 4.8 μg/√Hz in open-loop.     

INVESTIGATION OF THE RELATION BETWEEN CHEMICAL COMPOSITION AND ASH FUSION TEMPERATURES FOR SOKE TURKISH LIGNITES

Abstract

The purpose of this paper was to investigate the relationship between ash composition and ash fusion characteristic temperatures for some Turkish lignites. The lignite samples used are from different areas in Turkey. Regression analysis was used to relate the ash composition to the ash fusion behaviour. The results suggest that the chemical composition has a significant effect on the ash fusion temperatures. To consider Fe₂O₃ as an acidic ash constituent makes the realtionships between the selected ash composition parameters and ash fusion temperatures clearer.