Materials Science Forum Vols. 587-588

Abstract: A cost efficient powder coating technology developed in previous work [1-3] was modified to produce glass fibre/polyvinyl chloride (GF/PVC) towpregs that were subsequently processed by compression moulding and filament winding. The production of GF/PVC towpregs was optimized and a process window established. The composite structures obtained were submitted to mechanical testing and the results compared with the theoretical predictions made using the classical laminate theory (CLT). The obtained results showed that the GF/PVC composites properties are compatible with all major engineering applications.

Abstract: Thermoplastics are replacing traditional thermosetting resins as matrices in composite materials in many applications due to their enhanced properties. Thermoplastics exhibit better toughness, durability and damping properties and provide the options of continuous processing, reshaping, and reparability, as well as more favourable recycling and processing routes that do not involve chemical reactions [1]. However, their high melt viscosity makes it difficult to impregnate continuous fibres, which restricts commercial applications. Recently developed dry coating techniques allow the production of long fibre thermoplastic matrix towpregs without most of the previous impregnation problems [2, 3]. In this work, a new coating line was developed to produce long fibre dry coated thermoplastic matrix towpregs at rates compatible with industrial production. The polymer deposition rate was considerably increased to allow processing towpregs at greater speeds (approx. 10 m/min) than current equipment. A new coating chamber was used to allow improved control of the fibre and polymer contents and the towpreg impregnation quality. Other general improvements were also made in the equipment to allow better monitoring of the towpreg processing. Glass or carbon fibres with polypropylene towpregs (GF/PP and CF/PP, respectively) were produced on the new coating line which were then submitted to extensive tests to verify their polymer content and impregnation quality. This paper presents the results of the tests and discusses the optimization of the new dry coating line. The results show that the new deposition chamber allows production of economical thermoplastic matrix towpregs with improved efficiency that may be used in the industrial production of composites for commercial markets.

Abstract: The displacement devices using piezoceramic sensors play an important role in many fields. Soft type piezoelectric materials used for such sensors are more appealing because of their lower coercive field, relatively low modulus of elasticity, high electromechanical coupling factors and piezoelectric strain coefficients. We designed a PZT type material doped with niobium and nickel to increase the strain coefficient and still retaining a high coupling coefficient. The chemical formula was Pb0.97Ba0.08Nb0.03Ni0.06Zr0.46Ti0.40O3. The main piezoelectric parameters were: d33=680 pm/V, d31=-235 pm/V, kp=0.64 and k31=0.42. A unimorph transducer was made from a thin sheet of such a piezoceramic material, tightly bonded on a thin optically polished glass plate and mounted as a cantilever. The performance of this transducer was evaluated by means of a new optical method which makes use of the Moiré deflectometry and provides high precision. The method is rather simply and consists mainly in the following: the Moiré fringes rotate when the bimorph bends under the influence of the electric field and by measuring the rotating angle of the fringes one can easily determine the bimorph curvature and the free end displacement.

Abstract: An accurate technique for contact-less measurement of resistivity in non-magnetic conductive tubes has been developed. It is intended for application in non-destructive testing (NDT) in tube manufacturing by simple study of the induced currents. Inhomogeneities or minimal imperfections in the tube thickness are immediately detected. This measuring technique is also suitable for determining the thickness of very thin metal coatings on non-conductive tubes. The experimental setup corresponds to an electrical transformer, the tube being the core. A first coil is placed around the tube under test, whereas the detecting coil is placed inside it. The study of the magnetic screening caused by the currents induced in the tube, allows its conductivity to be determined. The use of low frequency guarantees the almost complete absence of the skin effect in such a way that the results are obtained in a very simple manner. The method is based on the determination of the phase angle of the complex mutual inductance between the two coils working under low frequency conditions.

Abstract: A new series of oligothienyl-imidazo-phenanthrolines 3 were synthesised in good to excellent yields by condensation of 5,6-phenantroline-dione 2 with formyl-thiophene derivatives 1 in the presence of ammonium acetate in glacial acetic acid. Furthermore, their solvatochromism and molecular optical nonlinearities were determined and comparatively studied. The experimental results indicate that phenanthrolines 3, due to their moderate solvatochromic properties and good optical nonlinearities, could be used as suitable probes for the determination of solvent polarity and as nonlinear optical materials.

Abstract: The synthesis of 5-arylazo- substituted bithiophenes and their UV-visible, solvatochromic and nonlinear optical properties (NLO) are described. In agreement with the solvatochromic data and also with the second-order molecular NLO characterization, the new donor-acceptor systems could find application as suitable solvatochromic probes and also as new NLO materials.

Abstract: The terahertz domain (500 GHz - 5 THz) has been object of unceasing research activities, due to the wide range of conceivable applications in these fields. This study focuses on the development of semiconducting YBa2Cu3O6+x (YBCO) thin films to be used as sensitive elements on future uncooled terahertz imagers working on a thermal principle. YBCO thin films have been hollow-cathode sputtered on MgO single-crystals under different conditions. Electrical and structural characterizations have then been carried out. The resistivity of the thin films and the temperature coefficient of resistance (TCR) have been determined. X-ray diffraction and atomic force microscopy analyses have then been performed. If compared with materials currently used as sensing element in commercial near-infrared imagers, electrical characterization shows values of the TCR comparable to amorphous silicon and almost two times better than VOx-compounds.

Abstract: The possibility of obtaining yttrium iron garnet (Y3Fe5O12, YIG) through thermal decomposition of two coordination compounds with malate and gluconate anions as ligands, (NH4)6[Y3Fe5(C4O5H4)6(C4O5H3)6]·12H2O and (NH4)6[Y3Fe5(C6O7H10)6(C6O7H9)6]·8H2O. A mixture of simple oxides, namely Y2O3 and α-Fe2O3, orthorhombic ferrite YFeO3 and small amounts Y3Fe5O12, is generated from the thermolysis of malate compound, while a clean Y3Fe5O12 is obtained from gluconate compound.

Abstract: This work presents the optical and structural characterization of p-type GaAs epilayers. The gallium precursor was the organometallic compound trimethylgallium (TMG). The influence of the doping in the optical and structural properties of the GaAs layers has been studied by photoluminescence (PL) and Raman dispersion measurements. The range of analyzed hole concentration was from 1017 to 1019 cm-3 as measured by the Hall-van der Pauw method. For carrying out doping p-type, it was necessary to modify the hydrogen activity in the growth atmosphere with the control of a H2+N2 mixture, which was used like transporting gas. The photoluminescence response and Raman dispersion of the layers are strongly dependence of the growth temperature, which were investigated based on the hole concentration. The PL response of the layers shows two radiative transitions, band-to-band and band-to-C-acceptor at low hole concentration and disappears at high concentrations. Raman scattering spectra show LO mode at 270 cm-1 for low doped samples and a LO-like mode at 290 cm-1 produced by the phonon-holeplasmon coupling for high doped samples.

Abstract: The electrochromic response of the Prussian blue (PB) system is produced by the electrochemical reaction of the colourless Everitt’s salt (ES), K2FeFe(CN)6, to form the blue oxidation product, KFeFe(CN)6. Reviews on the electrochemistry of PB and related compounds are available. Very little work, however, has been published on the quantitative aspects that affect film stability. The reported electrochemical deposition conditions for PB film formation vary considerably in terms of the electrochemical technique, pH, ferricyanide concentration, supporting electrolyte, as well as composition and concentration of the iron (III) salt. In this study, most PB films were prepared at ambient temperature from acidic K2SO4/H2SO4 (pH ≅ 2.0) solutions containing either Fe(ClO4)3 or FeCl3 and K3Fe(CN)6, using potentiostatic conditions. Dramatic increases in the film stability are obtained by simple acidifying; in fact, the switching speed increased from about 2 seconds at a pH of 7 to about 0.2 seconds at a pH of 2. Cycle lifetimes of at least 100,000 cycles were obtained in all of the acidified solutions. Factors such as the solution pH and heat treatment of the PB film also affect the position and shape of the observed cyclic voltammetric peaks. Summarizing, the PB films prepared by an electrodeposition method appear to be very promising for reaching stable, reversible and fast switching time (2 Hz) materials.