From participation to dropout: Quantitative participation patterns in online university courses

The academic e-learning practice has to deal with various participation patterns and types of online learners with different support needs. The online instructors are challenged to recognize these and react accordingly. Among the participation patterns, special attention is requested by dropouts, which can perturbate online collaboration. Therefore we are in search of a method of early identification of participation patterns and prediction of dropouts. To do this, we use a quantitative view of participation that takes into account only observable variables. On this background we identify in a field study the participation indicators that are relevant for the course completion, i.e. produce significant differences between the completion and dropout sub-groups. Further we identify through cluster analysis four participation patterns with different support needs. One of them is the dropout cluster that could be predicted with an accuracy of nearly 80%. As a practical consequence, this study recommends a simple, easy-to-implement prediction method for dropouts, which can improve online teaching. As a theoretical consequence, we underline the role of the course didactics for the definition of participation, and call for refining previous attrition models.     

Mechanical Behavior and Microstructural Development of 6063-T1 Aluminum Alloy Processed by Equal-Channel Angular Pressing (ECAP): Pass Number Influence

Equal-channel angular pressing (ECAP) is a very interesting method for modifying the microstructure in producing ultrafine-grained materials and nanomaterials. It consists mainly of pressing test samples through a die containing two channels that are equal in cross section and intersect at a certain angle. As a result of pressing, the sample theoretically deforms by simple shear and retains the same cross-sectional area to allow repeat pressing for several cycles. A 6063-T1 aluminum alloy was investigated in this study. The specimens were processed for up to nine passes (one, three, six, and nine ECAP passes, respectively) using a die channel angle of 90°. After ECAP, the samples were cut from each specimen and prepared for metallographic analysis and mechanical testing. The microstructures of the ECAP treated and as-received material were investigated using both optical microscopy and scanning electron microscopy. All samples (ECAP processed and as-received) were mechanically investigated in compression tests. Ultimate compressive strength, yield strength, and compression modulus were obtained. Also, all ECAP processed specimens were investigated for microhardness and compared with the as-received material.     

Photosensitive micelles based on polysiloxanes containing azobenzene moieties

Photosensitive micelles based on amphiphilic azo-polysiloxanes were obtained and characterized. The amphiphilic polymers were synthesized in a two-step reaction, starting from a polysiloxane containing chlorobenzyl groups in the side chain. In the first step, the polysiloxane was modified with azo-aromatic groups (35–45% substitution degree) and in the second step the unreacted chlorobenzyl groups were quaternized, using different tertiary amines. The structure of the polysiloxanes and their aggregation/disaggregation capacity were evaluated by 1H-NMR, DSC, fluorescence and UV–VIS spectroscopy, dynamic light scattering and transmission electron microscopy. All the amphiphilic polysiloxanes are capable of generating micelles, the critical concentration of the aggregation values being situated in the range 10⁻³–10⁻² g/L. As a function of the ternary amine structure used in the quarterisation reaction, the micellar aggregation process is different, individual micelles or micellar clusters being obtained. The clusters' dimension cannot be controlled, the polydispersity index having high values. The disaggregation processes of the micelles under UV irradiation reveal that the polymer chemical structure influences the aggregates stability. A total micelles disaggregation was obtained for the polysiloxane modified with azophenol and amine containing a long hydrocarbon segment. In the other cases, only a partially disaggregation process takes place.     

A comparative study of the electrochemical deposition of molybdenum oxides thin films on copper and platinum

Molybdenum oxides thin films electrochemical deposition was performed using solutions of peroxo-polymolybdate at pH 2.3 and ammonium molybdate at pH 5.5 as precursors and smooth copper and platinum as supports. The deposition has been carried out at constant potentials in the range of −600 to −800 mV vs. Ag/AgCl (sat. KCl). The thin films deposited on copper were then heated at 350 and 450 °C in argon. In the case of thin films deposited from ammonium molybdate and heated at 450 °C, the XRD spectra reveal, along with MoO₂, the presence of Cu₆Mo₅O₁₈ phase. For the thin films prepared from peroxo-polymolybdate and subjected to the same heat treatment, the only XRD phase present was MoO₂. Thermogravimetric (TG) analysis was performed on samples prepared by scraping away the thin films (molybdate precursors) from the copper support. Before heat treatment, the AFM images of the as-deposited thin film reveal a granular morphology, with diameters in the 20–80 nm range.     

Model of the interband nonlinear absorption in amorphous chalcogenides: Quasi-linear analytical solutions

It is proposed a quasi-linear algorithm for the solving the kinetic equations system, which describes the excitation and relaxation processes in the electronic and phononic subsystems of amorphous chalcogenides. The nonlinear interaction between an optical field and an amorphous chalcogenide medium is analyzed. The nonlinear interaction between an excitation incident optical field and the non-crystalline chalcogenide materials, evidences the specific nonlinear phenomena in amorphous semiconductors, which have no correspondent in the case of crystalline semiconductors. The model is limited to phenomena occurring in chalcogenide semiconductors, simultaneously with the optical field propagation, by neglecting the excitation phenomena.     

On the aluminium hard anodization and the critical value V of the barrier layer voltage

The relation between the critical value V^* of the barrier layer voltage and the mean coordination number of the oxygen in the first coordination sphere around the aluminium atom was investigated in order to illustrate the transition from conventional to hard anodization. The modification from open to closed structure is very sharp in the neighbourhood of V^*.     

Synthesis and characterization of nanocrystalline Zn ferrites substituted with Ni

Nanocrystalline powders of nickel substituted zinc ferrite with general formula Ni_xZn_1−xFe₂O₄ (x = 0, 0.2, 0.4, 0.6, 0.8, 1) have been synthesized via sol-gel auto-combustion method using tartaric acid as combustion-complexing agent. Samples were sintered at 773 K and 973 K in static air atmosphere. The absence of the organic phase and the spinel formation were monitored by using Fourier transform infrared spectroscopy. The structure and crystallite size were analyzed from X-ray diffraction data revealing spinel mono-phase formation in the range of nanometric crystallite size confirmed also through scanning electron microscopy. Mean size of crystallites lay in the range 20-40 nm. The influence of nickel content on the microstructure was investigated considering the crystallite size, distance between adjacent crystal planes, lattice parameter and porosity. The variation of magnetic properties of the samples was studied by using vibrating samples magnetometer and discussed considering the proposed cation distribution, relative bond angles and canting angles. The highest maximum value of the magnetization (63 emu/g) was found for Ni_0.8Zn_0.2Fe₂O₄.     

The Role of NiO Doping in Reducing the Impact of Humidity on the Performance of SnO2‐Based Gas Sensors: Synthesis Strategies,and Phenomenological and Spectroscopic Studies

The humidity dependence of the gas‐sensing characteristics in SnO₂‐based sensors, one of the greatest obstacles in gas‐sensor applications, is reduced to a negligible level by NiO doping. In a dry atmosphere, undoped hierarchical SnO₂ nanostructures prepared by the self‐assembly of crystalline nanosheets show a high CO response and a rapid response speed. However, the gas response, response/recovery speeds, and resistance in air are deteriorated or changed significantly in a humid atmosphere. When hierarchical SnO₂ nanostructures are doped with 0.64–1.27 wt% NiO, all of the gas‐sensing characteristics remain similar, even after changing the atmosphere from a dry to wet one. According to diffuse‐reflectance Fourier transform IR measurements, it is found that the most of the water‐driven species are predominantly absorbed not by the SnO₂ but by the NiO, and thus the electrochemical interaction between the humidity and the SnO₂ sensor surface is totally blocked. NiO‐doped hierarchical SnO₂ sensors exhibit an exceptionally fast response speed (1.6 s), a fast recovery speed (2.8 s) and a superior gas response (R_a/R_g = 2.8 at 50 ppm CO (R_a: resistance in air, R_g: resistance in gas)) even in a 25% r.h. atmosphere. The doping of hierarchical SnO₂ nanostructures with NiO is a very‐promising approach to reduce the dependence of the gas‐sensing characteristics on humidity without sacrificing the high gas response, the ultrafast response and the ultrafast recovery.     

An atomic force microscopy statistical analysis of laser‐induced azo‐polyimide periodic tridimensional nanogrooves

The surface morphology of azo‐polyimide films was investigated after 355nm Nd: YAG laser irradiation with two different incident fluencies. Atomic force microscopy (AFM) was employed to correlate the laser‐induced tridimensional nanogrooved surface relief with the incident fluence and the number of irradiation pulses. The height images revealed that the grooves depth increased even tens of times by increasing the incident fluence, using the same numbers of irradiation pulses. For low incident fluence, the films were uniformly patterned till 100 pulses of irradiation. Instead, when using higher fluence, after 15 pulses of irradiation the accuracy of the surface relief definition was reduced. This behavior could be explained by means of two different mechanisms, one that suppose the film photo‐fluidization due to the cis‐trans isomerization processes of the azo‐groups and the second one responsible for the directional mass displacement. The dominant surface direction and parameters like isotropy, periodicity, and period were evaluated from the polar representation for texture analysis, revealing the appearance of ordered and directionated nanostructures for most of the experimental conditions. Also, the graphical studies of the functional volume parameters have evidenced the improvement of the relief structuration during surface nanostructuration. The correlation of these statistical texture parameters with the irradiation characteristics is important in controlling the alignment of either the liquid crystals or the cells/tissues on patterned azo‐polyimide surfaces for optoelectronic devices and implantable biomaterials, respectively. Microsc. Res. Tech. 76:914–923, 2013. © 2013 Wiley Periodicals, Inc.     

Azo-polymers modified with nucleobases and their interactions with DNA molecules

The photo-fluidization process which is specific for azo-materials opens a new perspective for their use in the field of molecules nano manipulation at the surface of the azo polymer films. This is possible considering that in the case of the UV irradiation from a polarized laser source the azo material has an unidirectional flow. Here, we investigated the structuring phenomena occurring on the surface of the azo-polysiloxanes films modified with nucleobases, upon UV irradiation. Measurements of topography and adhesive forces between polymeric substrates and a hydrophilic probe have been done by atomic force microscopy (AFM). The response of the material upon irradiation has been investigated also by using UV−VIS spectroscopy. This method allowed us to draw the photo-isomerization and relaxation curves. Also, preliminary tests were conducted to determine the capacity of the film surface to immobilize DNA molecules.