Browse Articles

A mathematical model of the heat transport in nanofluids using the fractal theories (the scale relativity theory in the topological dimension DT = 2) is established. Through a scale covariance form of the Newton’s equation, a Navier-Stokes type equation with an imaginary viscosity coefficient and particularly a Schrödinger’s type equation are obtained. Some applications of the model are given like the heat transport through an effective thermal conductivity, by Brownian motion or by liquid layering at liquid-nanoparticle interface. It results that the heat transport in nanofluids is performed through an unique mechanism, the above standard sequences mentioned being imposed by the interaction scales. Moreover, the quantum thermal conductance of electrons in an one-dimensional wire is obtained. Keywords: nanofluid, heat transfer, fractal, thermal conductance

The paper proposes an analysis on the influence of the elastic rubber elements as component parts of the elastic tram wheels, as well of the railroad, on the stress state, on the wear and the loss in time of the mechanical properties under dynamic conditions. It is noticed the importance of the proping of the rubber elastic element (all degrees of freedom or vulcanized on a metallic support) going to a „brick” element shape proposal, which is absolutely „covered” for practical applications. This approximation leads to new correction coefficients, as well as to the validation of the simplified „brick” shape of specimens for a future experimental test standartization. Keywords: elastic wheel, stress, mechanical testing, finite element analysis

A degradation mathematical model based on the dependence of the carbonyl index by the degradation time to measure the polyolefins degradability was proposed. The mathematical model was verified with good results with own and literature degradation data. It was demonstrated that the three aging parameters of the mathematical model that can be calculated by fitted method, reflect the degradation behaviour of primary and secondary polyolefins and can be used for quantitative estimation of the polyolefins degradability. The possibility of using the mathematical model for estimation the opportunity of using the mechanical recycling as recycling method is the practical importance of the obtained results. There is also, a mathematical demonstration of the reason for which the polymers can not be infinitely recycled as other materials like metals, glass e.g. Keywords: mechanical recycling, ageing parameters, contamination

The frictional factor of new hip endoprosthesis was measured using one hip testing device designed in Biomechanics Laboratory of Medical Bioengineering Faculty. Hip simulator respect physiological condition (variable load, variable motion). The friction factor value for fluid film regime through the gait cycle was founded variable like a consequence of UHMWPE visco-elasto-plastic behaviour. Keywords: friction factor, gait cycle, visco-elasto-plastic behaviour

The railways mounting must ensure by strength and stability of the components, the guiding safety and running quality of the railways vehicles at the prescribes speeds and loads. The upper structure of the railroad is made from the ballasts, ties and rails. The steel rails mounted on ties with fastening systems, indication to maintain the railway gauge, rails inclination, stability and lasting of the railroad. Keywords: dynamic tests, rubber plates, fastening, Centipede, MGCplus

The present paper refers to the possibility of obtaining inorganic-organic nanohybrids, starting from nanoporous silica, in which the acrylonitrile template polymerization proceeds. In order to enhance the homogeneity between constitutive parts of the nanocomposite several methods were tested like increase of the imbibition period of the silica with the monomer, a better absorption capacity of the silica for the monomer and the use of the ultrasonication technique. The influence of the ultrasonic field on the template polymerization of acrylonitrile in inorganic structures was proved by FTIR and TG/DTG analyses. Ultrasonication was employed before the polymerization, in order to prevent the aggregation of particles, to well disperse the silica particles in the vinyl monomer and for a good imbibition. The results demonstrate that ultrasonication is effective in homogenizing the constitutive parts of the hybrid materials by yielding a fine microstructure of sintered nanomaterials. Keywords: inorganic-organic hybrid, porous silica, ultrasonication, template polymerization, nanocomposites

The most appealing aspect related to nano-composites use, consists of their biomechanical properties; these properties succeed in giving good mechanical strength, a minimal polymerization contraction and an exceptional aesthetics and polish due to the features of the nano-filler. Investigating the effect upon both the filler level and the size of the filler particles on some experimental composites’ mechanical and physico-chemical properties, some authors have come to the conclusion that by increasing the filler level can be obtain first of all an increasing of the compression resistance and secondly a significant decrease of water absorption. The vast majority of the investigated composites, based on borosilicate glass, are showing an important increase of their resistance to wear together with the increase of the filler level. In this particular study the resistance to traction and respectively to fracture and also the structure (SEM) for some experimental nanocomposites (produced in our laboratory) and for a commercial product (Premise - Kerr Corporation) has been determined. The samples of material were made according to the ISO 4049 specifications; they were tested with the use of the Instron Universal Lloyd apparatus and the Scanning Electronic Microscope (SEM - FEI). Keywords: nanocomposites, physical properties, flexural strength, diametral tensile strength, SEM

Two series of new self-coloured segmented polyurethane elastomers were successfully synthesized via the reaction of poly (tetramethylene ether) glycol (Terathane 1400) and hexamethylene diisocyanate / 4,5-dibromofluorescein with glycerine or castor oil as hard segments in a N,N-dimethylformamide solution. Polyurethane structure formation has been proven by infrared (IR) spectroscopy. Photosensitive-dibromofluorescein polyurethane elastomers exhibited high thermal stability and the addition of 4,5-dibromo-fluorescein in the preparation of the fluorescent polyurethane elastomers enhanced the tensile strength. This may be attributed to an increased cross-linking due to strong intermolecular interaction between the fluorescein polyurethane molecules. The fluorescence performance for the fluorescent polyurethane elastomers films appears at 560-580 nm, and increased concentration of 4,5-dibromofluorescein enhances the fluorescence performance of these polyurethane elastomers. Keywords: polyurethanes, fluorescence, chain extender, mechanical properties, thermal behavior

The paper presents the experimental results regarding the investigation of some physico-chemical and structural characteristics of starch modified by electron beam and microwave combined treatment. The combined electron beam (EB) and microwave (MW) treatment of granular starch was performed in four different regimes, successive and simultaneous. The experimental data showed that the acidity of starch aqueous solutions increased while the apparent viscosity reduced as a function of the treatment parameters and regime. No significant changes appeared in the morphological characteristics of the samples treated with combined EB-MW, but the spectral characteristics suffered minor changes suggesting that the stability of hydrogen inter- and intramolecular bonds is affected. At the same time, the degree of order increased after successive combined treatments indicating some molecular associations or rearrangements in amorphous component of starch due to the microwave field. The evolution of the molecular weights and molecular weight distribution indicated that the macromolecular degradation cannot be correlated with the specific absorbed energy as a consequence of the microwave field contribution. Keywords: starch, electron beam, microwave, viscosity, SEM, GPC

The thermal properties of a novel family of xerogels prepared from diepoxy-terminated poly(ethylene glycol)s of approximate molecular weights 600, 1000, 2000 and 4000 Da and various aliphatic polyamines (ethylenediamine, 1,4-diaminobutane, hexamethylenediamine, 1,8-octanediamine, 1,10-decanediamine, 1,12-dodecanediamine, diethylenetriamine, triethylenetetramine, tris(2-aminoethyl)amine) were studied by DSC and TGA measurements in order to establish the influence of several structural parameters that may affect the physical state of the network-forming polymer at a certain temperature. All xerogels synthesized displayed crystallinity which was affected by the MW of the PEG precursors, number of carbon atoms within the diamine chain, amine functionality and structure and amine/epoxy groups mole ratio. Both melting temperature and amount of crystalline phase were lower for networks with either shorter PEG chains, longer diamine chain, higher amine functionality and stoichiometric amine/epoxy groups ratio, while the thermal stability was higher for either longer PEGs, diamines with higher carbon atom numbers, higher amine functionality and stoichiometric amine/epoxy groups ratio. Keywords: networks; poly(ethylene glycol); aliphatic polyamines; melting temperature; glass transition temperature; thermal stability