A Petri net approach for simple assembly line balancing problems

A simple assembly line balancing problem of type-1 (SALBP-1) aims to minimize the number of workstations for a given cycle time. In the relevant literature, several heuristics based on a branch-and-bound procedure, tabu search, and genetic algorithms (GAs) were proposed to solve SALBP-1. In this paper, an algorithm based on the reachability analysis of Petri nets is developed for SALBP-1. The proposed algorithm searches enabled transitions (or assignable tasks) in the Petri net model of precedence relations between tasks, and then the task minimizing the idle time is assigned to the station under consideration. The algorithm is coded in MATLAB, and its efficiency is tested on Talbot’s and Hoffmann’s benchmark datasets according to some performance measures and classifications. A computational study validates its effectiveness on Tonge’s 70-task problem by comparison with optimal solutions of traditional heuristics and a GA.     

High-Quality Flat-Top Micromachining of Silica by a CW CO2 Laser

This letter demonstrates a simple and fast technique that dramatically improves the micromachined surface quality of silica substrates processed by a continuous-wave CO₂ laser. When sufficient laser power is directed at the surface of a sample, its temperature increases beyond the boiling point. Melted material is ejected and a groove is created. Some of the ablated material redeposits on each side of the groove, altering the surface quality of the substrate. To circumvent this problem, we deposit a polymer overlay as a sacrificial film which protects the surface during the ablation process, resulting in a flat surface on either side of the groove at very high writing speeds. The grooves can be used as an aid to passive alignment for an optical fiber, as diffractive optical elements or to create high-quality microfluidic chips     

Preconditioning of AISI 304 stainless steel surfaces in the presence of flavins—Part I: Effect on surface chemistry and corrosion behavior

Stainless steel AISI 304 surfaces were studied after a mild anodic polarization for oxide growth in the presence and absence of two derivatives of vitamin B2 (riboflavin and flavin mononucleotide) that can be secreted by metal-reducing bacteria and act as a chelating agent for iron species. The alterations in oxide chemistry were studied by means of surface-sensitive techniques such as X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analysis. The complementary electrochemical characterization revealed a preferential growth of an oxide/hydroxide iron-rich film that is responsible for an altered pit initiation and nucleation behavior. These findings suggest that as the corrosion behavior is determined by the interplay of the chemical and electronic properties, only a mild anodic polarization in the presence of redox-active molecules is able to alter the chemical and electronic structure of the passive film formed on stainless steel AISI 304. This helps to achieve a profound understanding of the mechanisms of microbially influenced corrosion (MIC) and especially the possible effects of the redox-active biomolecules, as they may play an important role in the corrosion susceptibility of stainless steel surfaces.     

Some Biochemical Properties of Lipase from Bay Laurel (<Emphasis Type="Italic">Laurus nobilis</Emphasis> L.) Seeds

Lipase was isolated from bay laurel (Laurus nobilis L.) seeds, some biochemical properties were determined. The bay laurel oil was used as the substrate in all experiments. The pH optimum was found to be 8.0 in the presence of this substrate. The temperature optimum was 50 °C. The specific activity of the lipase was found to be 296 U mg protein⁻¹ in optimal conditions. The enzyme activity is quite stable in the range of pH 7.0–10. The enzyme was stable for 1 h at its optimum temperature, and retained about 68% of activity at 60 °C during this time. K _m and V _max values were determined as 0.975 g and 1.298 U mg protein⁻¹, respectively. Also, storage stability and metal effect on lipolytic activity were investigated. Enzyme activity was maintained for 9, 12, and 42 days at room temperature, 4 and −20 °C, respectively. Ca²⁺, Co²⁺, Cu²⁺, Fe²⁺, and Mg²⁺ lightly enhanced bay laurel lipase activity.     

The influence of reactive milling on the structure and magnetic properties of nanocrystalline MnFe2O4

Nanocrystalline manganese ferrites (MnFe₂O₄) have been synthesized by direct milling of metallic manganese (Mn) and iron (Fe) powders in distilled water (H₂O). In order to overcome the limitation of wet milling, dry milling procedure has also been utilized to reduce crystallite size. The effects of milling time on the formation and crystallite size of wet milled MnFe₂O₄ nanoparticles have been investigated. It has been observed that single phase 18.4 nm nanocrystalline MnFe₂O₄ is obtained after 24 h milling at 400 rpm. Further milling caused deformation of the structure as well as increased crystallite size. With the aim of reducing the crystallite size of 18.4 nm, MnFe₂O₄ sample dry milling has been implemented for 2 and 4 h at 300 rpm. As a result, the crystallite size has been reduced to 12.4 and 8.7 nm, respectively. Effects of the crystalline sizes on magnetic properties were also investigated. Magnetization results clearly demonstrated that crystallite size has much more effect on the magnetic properties than average particle size.     

Permeation through a bed on a vibrating medium: theory and experimental results

A dead-end filtration set-up with a vertically vibrated medium is used to study cake permeation. A key feature of these experiments is that a sudden increase in permeability at a certain critical vibration amplitude takes place, when the static loading is light. A theory to explain this phenomenon is put forward in terms of a relation to local fluidization near the medium, thus returning the clogged septum resistance to virtually its unclogged value. The fluidization is due to a particle stress induced by the vibration of the particle fluid mixture near the medium. This stress can be large enough to counteract the compressive stress that is caused by gravity and drag due to the fluid flow in the set-up.Estimates for the particle stresses are obtained; these are proportional to the amplitude decay inverse length λ. The latter is derived from the analysis of a vibrated particle-fluid mixture that is in a state of fluidization. It is argued that only in this state will the value of λ be large enough to generate the required particle stress. A much smaller value for λ is obtained when the particles in the medium make enduring contacts.The theory predicts a frequency dependence for the turnover point in the permeability according to the root of the applied frequency. This theoretical result is confirmed by the experiments. The theory also predicts that when the decay is too steep, so that the vibration amplitude vanishes at a distance of less than a particle diameter, no fluidization will occur. This is found to be true for larger cake masses.     

Chemical stability of KNbO3, NaNbO3, and K0.5Na0.5NbO3 in aqueous medium

In recent years, potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) has become popular and promising among perovskite lead‐free piezoceramic systems. In this study, the chemical stability of KNN powders in aqueous medium was investigated as a function of pH, time, and powder surface area. To better understand the dissolution behavior of the complex KNN stoichiometry, subconstituents such as potassium niobate (KNbO₃, KN) and sodium niobate (NaNbO₃, NN) were investigated separately first. Results showed that all of the cations in the structure underwent dissolution in different values. Indicating that KNN undergoes incongruent dissolution in aqueous medium, the dissolution of A site cations was higher at lower pH while the dissolution of B site cations increased at higher initial pH. The order of released cation concentrations (C_A1 = K > C_{A2 = Na} > C_B = Nb) fits with inverse relationship of cation field strength (FS) order, B = Nb⁵⁺_FS > A₂ = Na⁺_FS>A₁ = K⁺_FS, at pH 4, 7 and 10 for NN, KN, and KNN. Calculated diffuse layer thickness from the ICP data confirmed to outer amorphous layer in TEM image. Also, the ratio of normalized cation concentration versus surface area of powders showed that incongruent dissolution kinetic was driven by the diffusion step.     

Electrical characterization of a-Si:H(n)/c-Si(p) structure

In this study, n-type hydrogenated amorphous silicon (a-Si:H) was fabricated on p-type crystalline silicon (c-Si) substrates to obtain heterojunction diodes. The amorphous films were obtained by the Plasma Enhanced Chemical Vapor Deposition (PECVD) technique. Temperature dependent current-voltage (I-V-T) measurements and investigation of the dc current injection mechanism of a-Si:H(n)/c-Si(p) device structure have been performed. The series resistance (4.6-8.2 Ω) values displayed nearly temperature independent behavior and the ideality factor varied between 2.7 and 1.6 in the temperature range 100-320 K. The forward bias I-V-T characteristics of c-Si/a-Si:H heterojunctions are found to behave like the Schottky junctions where carrier injection is especially influenced by the carrier generation-recombination in the junction interface formed on the amorphous side. The temperature dependent ideality factor behavior shows that tunneling enhanced recombination is valid rather than thermionic emission theory. In the frame of this model, characteristic tunneling energy and characteristic temperature are found to be 9 meV and 1900 K, respectively. It is concluded that fabricate n-type hydrogenated amorphous silicon is a preferable semiconductor material layer with low interface state density because the temperature dependent interface state density calculations give values of the order of 10¹⁴ eV⁻¹ cm⁻².     

Properties of thermo-chemically surface treated carbon fibers and of their epoxy and vinyl ester composites

Carbon fibers were surface treated by a continuous gas phase thermo-chemical treatment. The surface and the mechanical properties of the fibers were investigated before and after treatment and compared to the properties obtained with a conventional industrial electro-chemical surface treatment. The increase of the oxygen atomic content was much sharper, the surface chemistry was better controlled, and the tensile strength of the fibers increased slightly with the thermo-chemical surface treatment. The thermo-chemical surface treatment created a topography which amplitudes were under 10 nm, thus creating some mechanical interlocking with the matrix. The electro-chemical surface treatment did not generate such a topography. The improvement of interfacial adhesion with a vinyl ester matrix was limited, revealing that oxidation of the carbon fiber surface alone cannot tremendously improve the mechanical properties of carbon fiber–vinyl ester composites.     

Evolution of connected health: a network perspective

In this study, the evolution of the connected health concept is analysed and visualized to investigate the ever-tightening relationship between health and technology as well as emerging possibilities regarding delivery of healthcare services. A scientometric analysis was undertaken to investigate the trends and evolutionary relations between health and information systems through the queries in the Web of Science database using terms related to health and information systems. To understand the evolutionary relation between different concepts, scientometric analyses were conducted within five-year intervals using the VantagePoint, SciMAT, and CiteSpace II software. Consequently, the main stream of publications related to the connected health concept matching telemedicine cluster was determined. All other developments in health and technologies were discussed around this main stream across years. The trends obtained through the analysis provide insights about the future of healthcare and technology relationship particularly with rising importance of privacy, personalized care along with mobile networks and mobile infrastructure.