Advanced Materials Research Vols. 47-50

Abstract: Novel PE- and PP-based electromagnetic wave shielding and absorbing materials with low combustibility, enhanced thermal and mechanical properties, containing graphite, grinded wood and fire retardants, were developed and investigated. Their flame-resistance, thermal and mechanical properties are investigated. Electromagnetic wave reflection coefficients of the materials over the frequency range 20-40 GHz were measured; despite low concentration (10%) of functional filler, reflection coefficient can be as low as -15 dB. Criteria of incombustibility of polymeric composites are formulated: thermochemical, kinetic, thermal, physical and mass transfer. These criteria allowed to set up the principles of decreasing combustibility of polymeric composites. Materials requirements, allowing to choose the most effective shielding and fireproof systems for multicomponent composites, containing polymers, wood and other fillers, were formulated.

Abstract: Strategy of the synthesis of multifunctional materials is developed on the basis of physical properties of composites, composition of fillers, the type of polymer matrix and distribution of ingredients in composite. Each of these factors is displayed in material in different extent depending on technological parameters of processing, and also properties and interaction of fillers in particular conditions. In homogeneous and, in particular in heterogeneous systems, such as metals and alloys, ferro - and ferrimagnetics, ferroelectrics, ferroelectromagnetics, polymer and ceramic matrix composites, high-temperature superconducting ceramics, etc. It is always possible to outline various types of hierarchy. Investigation of the effect of hierarchical structures on physical properties and the nature of interaction of various internal fields in inhomogeneous materials is inseparably linked with the development of methods of synthesis of new smart and intelligent structures.

Abstract: Electromechanical impedance (EMI) technique using lead zirconate titanate (PZT) transducers has been increasingly applied to structural health monitoring (SHM) of aerospace, civil and mechanical structures. The PZT transducers are usually surface bonded to or embedded in a structure and subjected to actuation so as to interrogate the structure at the desired frequency range. The interrogation results in the electromechanical admittance (inverse of EMI) signatures which can be used to estimate the structural health or integrity according to the changes of the signatures. In the existing EMI method, the monitored structure is only excited by the PZT transducers for the interrogating of EMI signature, while the vibration of the structure caused by the external excitations other than the PZT actuation is not considered. However, in real situation many structures work under vibrations. To monitor such structures, issues related to the effects of vibration on the EMI signature need to be addressed because these effects may lead to misinterpretation of the structural health. This paper develops an EMI model for beam structures, which takes into account the effect of beam vibration caused by the external excitations. An experimental study is carried out to verify the theoretical model. A Lab sized specimen with external excitation is tested and the effect of excitation on EMI signature is discussed.

Abstract: There are exists positive thermal expansion property for almost all materials. However, in many cases, the material property with negative thermal expansion is requested for engineering applications. This work is to develop high performance fiber-reinforced composites with negative thermal expansion by using high strength polyethylene fiber Dyneema®, high strength PBO fiber ZYLON®, aramid fiber Technora ® and carbon fibers.

Abstract: Modified natural rubber latex can be used as paperboard barrier coating in order to replace unrecyclable wax coating material. Natural rubber latex (NRL) was prevucanized in different time interval and the efficiency of crosslinking were determined by swelling ratio test. Prevulcanized natural rubber latex (Pre-VNRL) was blended with modified lignin dispersion to decrease sticking tendency of the coatings. Particle size of lignin dispersion was reduced by Mannish reaction using octylamine which can increase hydrophobility in lignin. Compared with Pre-VNRL coating, a reduction of water vapour transition rate (WVTR) was observed in blendingcoated paper. Such moisture-barrier behaviour is attributable to a good compatibility of the nanosized modified lignin particles with rubber. The blocking (sticking) tendency decreased with the content increase of lignin due to stiffness of lignin.

Abstract: Morphing aircraft wings require flexible skins that can undergo large strains, have low in-plane stiffness. In this paper, the sandwiched structure is designed to maintain airfoil shape throughout transition and not to suffer from large out-of-plane deformation under aerodynamic pressure loads. It consists of honeycomb and flexible skin. Honeycomb which is high-strain capable in one direction without dimensional change in the perpendicular in-plane axis provides distributed support to the honeycomb. Flexible skin is used to create the smooth aerodynamic surface. The morphing wing structure is developed together with the sandwiched skin technology. It is capable of changing in chordwise and increasing chord by 20%. Elastomeric and SMP skins are selected for use. Embedded heating wire springs act as the activation system for the SMP. Experiment results show the morphing wing model with elastomeric or SMP skins can be driven successfully by DC motor.

Abstract: The study of the kinetics of martensitic phase decomposition in the Cu-10wt.%Al alloy with Ag additions showed that the presence of Ag retarded the eutectoid decomposition reaction and enhanced martensite stabilization. This stabilization effect was attributed to Ag atoms redistribution as structure defects, increase in the numbers of Cu-Al pairs due to Ag-Al interaction and the Al atoms redistribution around one Cu atom at the sub-lattice of the martensitic crystal.

Abstract: Increasingly stringent regulations for limiting pollutant emissions for both aircraft and industrial gas turbines enforce further reduction of NOx emissions while maintaining flame stability. Application of premixed flames offers the possibility to reduce these emissions, but nevertheless it is strongly connected with flame instability risks. A possible solution to ensure the stability of premixed flames is to provide enhanced heat recirculation employing porous inert material. Experimental determination of flame stability and emissions of a porous burner containing a reticulate ceramic sponge structure are reported and the influence of the structural properties of the porous matrix on stable operating range was investigated. It was found, that the flame stability limit was significantly higher compared with free flame burners and nitric oxide (NOx) emissions were below 10 ppm for all cases.

Abstract: Preparation of NbB2-Al2O3 in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) involving thermite reactions. Three different thermite mixtures, including Al-Nb2O5, Al-B2O3, and Al-Nb2O5-B2O3, were adopted and mixed with Nb and/or B powders to formulate the sample compacts. Experimental observations show that the thermite reaction enhances sustainability of the synthesis reaction. Among them, the powder compact containing the thermite mixture of Al and Nb2O5 exhibited the highest flame velocity and combustion temperature. The lowest reaction propagation rate and temperature were observed in the sample with Al and B2O3 as thermite reagents. Due to insufficient reaction time for the sample containing Al and Nb2O5, the end products were composed of large amounts of two intermediate borides Nb3B4 and NbB in addition to NbB2 and Al2O3. The SHS process involving the thermite reaction of Al with B2O3 was favorable to produce NbB2-Al2O3 composites, confirmed by the least Nb3B4 and NbB left in the end products.

Abstract: In this work we present the influence of different particle size (surface areas: 120,150, 200 and 250 m2/g) and size distribution of precipitated silica on the mechanical properties of Ethylene- Propylene-Diene (EPDM) rubber. The vulcanization system employed was efficient. Compounds were prepared using a Banbury internal mixer. Tensile and tear properties of vulcanized blends were determined according to ASTM D412 and ASTM D624 procedures, respectively. Results show an increasing tendency on tensile properties when particle size decreases, due to the better dispersion of the filler and to a greater interaction with the rubber. Increases of up to 500% on tensile strength and 400% on tear strength were observed. However, there was an incoherent behavior for the silica with surface area of 200 m2/g (Si-200), so size distribution of the aggregates was experimentally determined by a microphotography study. Secondary aggregates size distribution was very different for each type of silica. Aggregates for Si-200 presented a bimodal distribution where the greater frequencies correspond to aggregates with surface area higher than the corresponding values for the Si-120. This fact could explain why the mechanical properties of the EPDM filled with Si-200 are not in between the values of the compounds filled with Si-150 and Si-250.