Showing papers in "Journal of Materials Science in 2005"

The effect of filler particle size on the antibacterial properties of compounded polymer/silver fibers

Abstract: Antibacterial activity has become a significant property of textiles used in applications such as medicine, clothing, and household products. In this study, we compounded polypropylene with either micro- or nano-sized silver powders. These polypropylene/silver compounds were prepared by direct melt-compounding using a conventional twin-screw mixer. We analyzed the characteristics of the compounds using wide-angle X-ray diffractometry (WAXS), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The DSC and WAXS results indicated that the crystallinity of the polypropylene component decreased slightly when compared with that of the pure polymer. The SEM micrographs indicated that the silver particles had good dispersibility in the matrix. We measured the mechanical properties of these materials using a universal tensile tester and evaluated the antibacterial activities of these compounds by performing quantitative antibacterial tests using the AATCC-100 test method. From these evaluations of antibacterial activity, we conclude that the compounds incorporating the silver nanoparticles exhibited superior antibacterial activity relative to the samples containing micron-sized particles.

The effect of silica nano particles and rubber particles on the toughness of multiphase thermosetting epoxy polymers

Abstract: The substantial increase in toughness achieved when nano-SiO2 particles were dispersed in a hot-cured single-part epoxy polymer was investigated. The synergistic effect of having a multiphase structure is based upon both nano-SiO2 and rubbery particles. The modulus, of the rubber-particulate epoxy polymer increases steadily as the wt.% of the silica nanophase was increased. The results suggested that the volume fraction of the rubbery-particulate phase which was formed was independent of the concentration of the nano-silicate phase present.

Review Nano and macro-structured component fabrication by electron beam-physical vapor deposition (EB-PVD)

Abstract: The objective of this paper is to demonstrate the versatility of electron beam-physical vapor deposition (EB-PVD) technology in engineering new materials with controlled microstructure and microchemistry in the form of coatings. EB-PVD technology is being explored in forming net-shaped components for many applications including space, turbine, optical, biomedical and auto industry. Coatings are often applied on components to extend their performance and life under severe environmental conditions including thermal, corrosion, wear, and oxidation. In addition, coatings have been used in designing and developing sensors. Performance and properties of the coatings depend upon its composition, microstructure and deposition condition. This paper presents recent results of various materials including ceramic, metallic, and functionally graded coatings produced by EB-PVD. Simultaneous co-evaporation of multiple ingots of different compositions in the high energy EB-PVD chamber has brought considerable interest in the architecture of functional graded coatings, nano-laminated coatings and designing of new structural materials that could not be produced economically by conventional methods. In addition, high evaporation and condensate rate allowed fabricating precision net-shaped components with nanograined microstructure for various applications. This paper will also present the results of various metallic and ceramic coatings including chromium, titanium carbide (TiC), hafnium carbide (HfC), tantalum carbide (TaC), hafnium nitride (HfN), titanium-boron-carbonitride (TiBCN), and partially yttria stabilized zirconia (YSZ), and HfO2-based TBC coatings deposited by EB-PVD for various applications.

Relationships between composition, structure and strength of inorganic polymers

Abstract: This article is the second in a two-part series and discusses inorganic polymers derived from fly ash. Part 1 [1] concerns inorganic polymers derived from a metakaolin precursor. For this study, 15 fly ash-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four component oxides (Na2O, SiO2, Al2O3 and H2O) and two alkali cations (Na and K) on the microstructure and compressive strengths were assessed. Similar to metakaolin-derived inorganic polymers, it was observed that high-strength fly ash inorganic polymers were related to low porosity and a dense, fine-grained microstructure. Such structures were characteristic of formulations with high silica mole fractions (SiO2/Al2O3 ∼ 3.9) and low water contents, as well as those with high alkali and low alumina contents. For the latter, not only was a characteristic slower strength development with increasing alkali content observed, but there was also a limit of alkali concentration (Na2O/Al2O3 ∼1) beyond which the strength deteriorated. Furthermore, SEM micrographs disclose that the fly ash precursor dissolves more readily in the sodium-based system compared to the potassium equivalent. The interrelation between microstructures of the respective formulations and their strength development are discussed. It is observed that the charge-balancing role of the alkali cations in the fly ash formulations may be dominant compared to initial alkali dissolution reaction of the aluminosilicate fly ash particles, which is partly responsible for initial strength development.

Composition and annealing effects in polythiophene/fullerene solar cells

Abstract: We have fabricated organic solar cells with blends of regioregular poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) as electron donor and electron acceptor, respectively. Blend composition and device annealing effects were investigated with optical absorption and photoluminescence spectroscopy, atomic force microscopy, photocurrent spectroscopy, and current-voltage characteristic measurements on devices under monochromatic or air mass (AM) 1.5 simulated solar light illumination. The highest efficiency was achieved for the 1:1 (P3HT:PCBM) weight ratio composition. The good performance is attributed to an optimized morphology that enables close intermolecular packing of P3HT chains. Inferior performance for the 1:2 composition is attributed to poorer intermolecular packing with increased PCBM content, while phase segregation on a sub-micron scale was observed for the 1:4 composition. The power conversion efficiency (AM 1.5) was doubled by the thermal annealing of devices at 140∘C to reach a value of 1.4%.

“Green” composites from recycled cellulose and poly(lactic acid): Physico-mechanical and morphological properties evaluation

Abstract: “Green”/biobased composites were prepared from poly(lactic acid) (PLA) and recycled cellulose fibers (from newsprint) by extrusion followed by injection molding processing. The physico-mechanical and morphological properties of the composites were investigated as a function of varying amounts of cellulose fibers. Compared to the neat resin, the tensile and flexural moduli of the composites were significantly higher. This is due to higher modulus of the reinforcement added to the PLA matrix. Dynamic mechanical analysis (DMA) results also confirmed that the storage modulus of PLA increased on reinforcements with cellulose fibers indicating the stress transfers from the matrix resin to cellulose fiber. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed that the presence of cellulose fibers did not significantly affect the crystallinity, or the thermal decomposition of PLA matrix up to 30 wt% cellulose fiber content. Overall it was concluded that recycled cellulose fibers from newsprint could be a potential reinforcement for the high performance biodegradable polymer composites.

Relationships between composition, structure and strength of inorganic polymers, part 2: fly ash-derived inorganic polymers

Abstract: This article is the second in a two-part series and discusses inorganic polymers derived from fly ash. Part 1 [1] concerns inorganic polymers derived from a metakaolin precursor. For this study, 15 fly ash-derived inorganic polymers were produced with various compositions. The effect of the concentration of each of the four component oxides (Na 2 O, SiO 2 , Al 2 O 3 and H 2 O) and two alkali cations (Na and K) on the microstructure and compressive strengths were assessed. Similar to metakaolin-derived inorganic polymers, it was observed that high-strength fly ash inorganic polymers were related to low porosity and a dense, fine-grained microstructure. Such structures were characteristic of formulations with high silica mole fractions (SiO 2 /Al 2 O 3 ∼3.9) and low water contents, as well as those with high alkali and low alumina contents. For the latter, not only was a characteristic slower strength development with increasing alkali content observed, but there was also a limit of alkali concentration (Na 2 O/Al 2 O 3 ∼1) beyond which the strength deteriorated. Furthermore, SEM micrographs disclose that the fly ash precursor dissolves more readily in the sodium-based system compared to the potassium equivalent. The interrelation between microstructures of the respective formulations and their strength development are discussed. It is observed that the charge-balancing role of the alkali cations in the fly ash formulations may be dominant compared to initial alkali dissolution reaction of the aluminosilicate fly ash particles, which is partly responsible for initial strength development.

Measurement of contact angle and work of adhesion at high temperature

Abstract: A critical review is given of the present state of knowledge and future perspectives in high-temperature contact angle measurement. Experimental results obtained by the different versions of the sessile drop method and by various procedures are given in order to illustrate the two main sources of scatter in wettability data, the first being related to the quality of the substrates and the second to control of the furnace atmosphere.

Bamboo—A functionally graded composite-correlation between microstructure and mechanical strength

Abstract: Bamboo is supposed to be one of the best functionally gradient composite materials available. In a piece of bamboo, not only the number of fibres (‘vascular bundles’) but also the fibre quality varies from outer to inner-most periphery. It has been observed that near the outer periphery, within 1 mm2 area, the number of fibres is approximately 8 whereas the same at the inner-most periphery is approximately 2. Again the cross-sectional shape of fibre at outer periphery is almost circular (diameter 0.14 mm) and compacted but at the inner-most periphery, a fibre (diameter of major axis 0.93 × diameter of minor axis 0.78 mm) has been sprayed, and contain matrix in it. This structural behaviour causes the variation of tensile strength, e.g., the strength of a fibre at the outer periphery is about 160 kg/mm2 and the same at the inner-most periphery is only 45 kg/mm2. It has also been observed that the matrix of bamboo can preferentially be removed from the fibre by alkali treatment. 10% NaOH can remove adhered matrix with little effect on fibres while 20% or stronger alkali reduces the strength of fibre.

Antibacterial properties of padded PP/PE nonwovens incorporating nano-sized silver colloids

Abstract: This paper deals with the effects that nano-sized silver colloids have on the antibacterial properties of PE/PP nonwovens against three kinds of bacteria: Staphylococcus aureus, Klebsiella pneumoniae, and Escherichia coli. These silver colloids comprise silver nanoparticles that are a non-toxic and non-tolerant disinfectant. PE/PP nonwovens are used as back sheets or coverstocks of baby diapers, adult diapers, sanitary napkins, and wipes. These materials are readily contaminated by bacteria present in moisture and dirt and can cause disease. We finished the nonwovens using a normal dipping–pad–dry method. From SEM images, we determined that the silver nanoparticles were generally dispersed well on the surface of the nonwoven fibers. We used the AATCC-100 test method to study the antibacterial properties of the treated fabrics. Bacteria were disinfected completely to below a count of 10 cells after 10 min when using the samples treated with 10 ppm of silver colloids. The ethanol-based silver/sulfur composite colloid (SNSE) has the best antibacterial efficacy when compared with the other nano-sized silver colloids. The silver particles having the smallest sizes gave the higher dispersibilities and the strongest antibacterial efficacies.

Nanostructured superhydrophobic surfaces

Abstract: A study on superhydrophobic silica surfaces based on roughness created by assembling different nanostructured materials was reported. Four different surface structures were created on a silica base material, which was subsequently covered with a self-assembled monolayer of tridecafluoro-1,1,2,2- tetrahydrooctyldimethylchlorosilane (TFCS). The surfaces consisting of a nanofiber mesh demonstrated the highest static contact angle, when covered with TFCS monolayers. However, the surfaces consisting of nanorod arrays were found to be possessing the best dynamic hydrophobicity due to their continuous surface structure.

Effect of organic acid treatment on the properties of rice husk silica

Abstract: Rice husk, an agro waste material, contains about 20% ash which can be retrieved as amorphous, chemically reactive silica. This silica finds wide applications as filler, catalyst/catalyst support, adsorbent and a source for synthesizing high performance silicon and its compounds. Various metal ions and unburned carbon influence the purity and color of the ash. Controlled burning of the husk after removing these ions can produce white silica of high purity. The present paper deals with the investigation carried out on two rice husk samples of different origin, one from the state of Andhra Pradesh (APRH) in the central part of India and the other from Kerala (KRH) the southern most part of the country. Leaching the husk with acetic and oxalic acids was attempted for the first time and the improvement in properties of the ash was studied. The husk samples were also treated with hydrochloric and nitric acids of different concentrations for comparison. The ashes produced by controlled burning of these samples before and after acid treatment, were characterized for the optical properties in addition to the chemical and physical nature. The APRH ash was found to be inferior to the KRH ash in all properties. Pretreatment of the husks with the organic acids improved the properties of ashes and the effect was comparable to that achieved by mineral acid leaching. Amorphous, reactive and high purity silica with high surface area and pore volume and good optical properties could be prepared from both the husks under specific conditions.

Surface tension of nickel, copper, iron and their binary alloys

Abstract: The surface tension of liquid binary alloys of the Cu-Ni-Fe system is measured by using the oscillating drop technique. The samples are processed contactlessly in an electromagnetic levitation chamber and hence, considerably large undercooling can be achieved. The alloys and the pure elements copper, nickel, and iron are investigated at various temperatures above and below their melting points. In addition, the surface tensions are also investigated as a function of the concentrations at constant temperature. The values agree well with predictions from theory and an analysis of the segregational behaviour is performed.

Numerical simulation of entangled materials mechanical properties

Abstract: A general approach to simulate the mechanical behaviour of entangled materials submitted to large deformations is described in this paper. The main part of this approach is the automatic creation of contact elements, with appropriate constitutive laws, to take into account the interactions between fibres. The construction of these elements at each increment, is based on the determination of intermediate geometries in each region where two parts of beams are sufficiently close to be likely to enter into contact. Numerical tests simulating a 90% compression of nine randomly generated samples of entangled materials are given. They allow the identification of power laws to represent the evolutions of the compressive load and of the number of contacts.

Absorption phenomena in organic thin films for solar cell applications investigated by photothermal deflection spectroscopy

Abstract: A high sensitive approach is presented to detect in particular the low level absorption features in pure and blended organic semiconductor films, revealing a.o. defect induced sub gap absorption and new interactions between the materials. Because sub bandgap absorption features are typically characterized by very low absorption coefficients, it is not possible to resolve them using common transmission and reflection measurements. Therefore the very sensitive and ground state spectroscopic technique of Photothermal Deflection Spectroscopy (PDS) has been developed, and introduced to characterize thin films of MDMO-PPV and PCBM, as well as films of MDMO-PPV containing an increasing amount of PCBM ranging from 5 to 90% weight fraction. The measured spectra of MDMO-PPV are interpreted in terms of defect induced absorption phenomena. The spectral position of the observed transitions in PCBM have been determined and verified. The PDS-study on MDMO-PPV/PCBM blended films revealed for the first time interaction between the two materials in the ground state. To get more insight in the interaction mechanism between the constituting materials a systematic Transmission Electron Microscopy (TEM) study has been carried out to reveal the morphology of the films. The obtained TEM-results on nanomorphology of the blended films show clear correlations with the PDS-results.

Mechanism and behavior of bitumen strength reinforcement using fibers

Abstract: This paper investigates the reinforcement mechanism of bitumen mixed with fibers. Fibers including cellulose, rock wool and polyester types were added to bitumen. The viscosity, toughness and tenacity, microscopy and rheological tests were conducted to characterize the engineering properties of bitumen-fiber mastics. Test results indicate that the reinforcing effect increases with increasing fibers up to a critical fraction. With higher mixing temperatures, there is a higher viscosity ratio of mastic to bitumen. The tensile strength of bitumen-fiber mastics also increases with increasing fiber concentrations because the fibers carry parts of tensile loads. With the increasing tensile strength, it is implied that there is a good adhesion between bitumen and fibers. Scanning electron micrographs show that fibers reinforce bitumen through a three dimensional structure. However, there is a critical fiber fraction when fibers start to interact with each other, resulting in lower toughness. The optimum fiber content is dependent on fiber type, length and diameter.

Finite element modeling concepts and linear analyses of 3D regular open cell structures

Abstract: Various Finite Element modeling concepts and linear analyses of 3D regular cellular solids (lattice structures) with relative densities ranging from 10% to 20% are presented. Continuum element based models and beam element based models are employed, the latter with and without an adaptation of stiffness in the vicinity of the vertices. Space filling unit cell models are used for a constitutive characterization of four different structures in terms of density and directional dependence of their Young's moduli. Finite structure models of different size are simulated for investigating the influence of free surfaces and being compared to results of uniaxial compression tests of samples fabricated by two different Rapid Prototyping techniques.

Correlation of capacitance and actuation in ionomeric polymer transducers

Abstract: Ionomeric polymer transducers are electromechanical actuators fabricated from ion-exchange membranes that have been surface plated with conductive metal. In this paper we discuss a series of experiments that characterize the electromechanical actuation response of three families of ionomers: Nafion (a product of DuPont), BPSH (sulfonated poly(arylene ether sulfone)) and PATS (poly(arylene thioether sulfone)). The first polymer is commercially-available while the second and third polymers are synthesized by the direct polymerization of sulfonated monomers in our lab. The mechanical properties and actuation response of Nafion-117, BPSH, and PATS of varying ionic content are studied in the Lithium cation form. The strain response of the materials varies from 50 μ strain/V to 750 μ strain/V at 1Hz. A linear correlation was found between the strain response and the capacitance of the material. This correlation was independent of the polymer composition and the plating parameters. All of the ionomers analyzed in this work exhibited a strain-to-charge response between 9 \(\frac{\rm \mu strain}{\frac{C}{\rm m^2}}\) and 15 \(\frac{\rm \mu strain}{\frac{C}{\rm m^2}}\). Due to the fact that the low-frequency capacitance of an ionomer is strongly related to charge accumulation at the blocking electrodes, this correlation suggests a strong relationship between surface charge accumulation and mechanical deformation in ionomeric actuators.

The role of back-pressure in equal channel angular extrusion

Abstract: Equal Channel Angular Extrusion (ECAE), developed and patented in Russia by Segal in 1977, has become in the last few years a very popular tool for studying the evolution of microstructure and properties under severe plastic deformation. It is believed that the strain-stress characteristics are uniform in a cross-section of the billet and this uniformity of the stress-strain distribution ensures the uniformity of microstructure and mechanical properties in ECAE processed billet. However, some experimental data such as the fracture of the extruded billet, which is initiated at the inner surface of the sample, has caused doubts about uniformity of stress-strain distribution. This non-uniformity has been proved recently by Finite Element Simulation. This paper reviewed our results from over six years of work using a unique machine for ECAE with computer controlled back-pressure and velocity of the backward punch. Theoretically back-pressure has been introduced in the earlier papers of Segal. However, practically back-pressure has not been widely used or often used in a primitive form of the consequent extruded sample and its role has not been understood.

A mechanical model for creep, recovery and stress relaxation in polymeric materials

Abstract: A mechanical model is presented, in which viscoelastic response is described by the action of time-dependent latch elements. The model represents viscoelastic changes occurring through incremental jumps as opposed to continuous motion. This is supported by the observation that polymeric creep, recovery and stress relaxation can be correlated with stretched exponential functions, i.e. Weibull and Kohlrausch-Williams-Watts, since (i) the former is also used in reliability engineering to represent the failure of discrete elements and (ii) there is evidence of the latter being an approximation to the Eyring potential energy barrier relationship, which describes motion in terms of molecular jumps.

Preparation of carbon aerogel electrodes for supercapacitor and their electrochemical characteristics

Abstract: A new process is presented for synthesizing the supercapacitor electrodes of carbon aerogel via pyrolyzing resorcinol-formaldehyde (RF) aerogel, which could be cost-effectively obtained by ambient drying of wet RF-gels instead of conventional supercritical drying. Defect free RF-aerogels instead of conventional supercritical drying. Defect free RF-aerogels with the linear shrinkage of less than 8% could be manufactured by ambient-drying of wet RF-gels. Carbon aerogels with high strength were prepared via pyrolyzing RF-aerogels in N2 atmosphere. The specific surface area (< 600 m2/g) and the electrical conductivity (∼ 50 S/cm) of carbon aerogels varied in sensitivity with the pyrolysis condition, while their densities (0.6 g/cm3) and porosities (70%) were found to be almost constant. Post heat-treatment of carbon aerogels around 300^∘C in air atmosphere was very effective for improving the electrochemical properties of electrodes. The carbon aerogel electrode pyrolyzed at 800∘C showed the specific capacitances of about 40 F/g in H2SO4 electrolyte solution and 35 F/g in KOH solution.

Wax morphology in bitumen

Abstract: Wax crystallisation and melting in bitumen is usually considered detrimental to bitumen quality and asphalt performance. The objectives of this paper are to study wax morphology in bitumen and to investigate effects of time, temperature, and thermal cycling on wax crystallisation. Various samples were selected, including eight waxy bitumens of different sources and three laboratory blends prepared by adding a slack wax and two isolated bitumen waxes to the non-waxy bitumen. Test methods used were differential scanning calorimetry (DSC), polarised light microscopy (PLM), confocal laser scanning microscopy (CLSM), and freeze etching (fracture) in combination with transmission electron microscopy (FF-TEM). The DSC results indicated that the selected bitumen samples differ widely in wax content and wax crystallisation starting and melting out temperatures. It was found that non-waxy bitumen displayed no structure or crystals neither in PLM, CLSM or FF-TEM, while waxy bitumens from different crude origins showed a large variation of structures. The morphology of wax crystals was highly dependent on crystallisation temperature as well as temperature history. The wax which has been isolated from waxy bitumen and mixed into non-waxy bitumen displayed similar morphology as the wax in the original bitumen. It was also found that bitumen wax usually melted at temperatures lower than 60°C although in one case a temperature of 80°C was needed until complete melting of the wax.

Structural, thermodynamical and optical properties of Cu2-II-IV-VI4 quaternary compounds

Abstract: The Cu2-II-IV-VI4 compounds (II = Zn, Cd; IV = Si, Ge, Sn; VI = S, Se, Te) with a tetrahedral coordinated structure have been investigated, as compared with the results of the products synthesized from respective elemental mixtures. These crystal structures and melting points are determined, using differential thermal analysis and powder X-ray diffraction, respectively. The optical band gaps of these balk crystals grown by the horizontal gradient freezing or Iodine transport method are also measured by optical absorption. These melting points, lattice constants and band gaps are found to vary linearly with increasing of mean atomic weight, and can be established from the empirical equations.

Texturization of multicrystalline silicon by wet chemical etching for silicon solar cells

Abstract: Two kinds of surface texturization of mc-Si obtained by wet chemical etching are investigated in view of implementation in the solar cell processing The first one was the acid texturization of saw damage on the surface of multicrystalline silicon (mc-Si) The second one was macro-porous texturization prepared by double-step chemical etching after KOH saw damage layer was previously removed Both methods of texturization are realized by chemical etching in HF-HNO3-H2O with different additives Macro-porous texturization allows to obtain effective reflectivity (Reff) in the range 9–20% from bare mc-Si This Reff value depends on the time of second step etching that causes porous structure modification The internal quantum efficiency (IQE) of cells with this kind of texturization has possibility to reach better conversion efficiency than the standard mc-Si solar cells However, low shunt resistance depends on morphology of porous layer and it is the main factor which can reduce open circuit voltage and conversion efficiency of cells The effective reflectivity is about 17% for acid texturized mc-Si wafer The investigation of surface morphology by scanning electron microscopy (SEM) revealed that the dislocations are appearing during chemical etching and they can reduce open circuit voltage The density of the dislocations can be reduced by controlling depth of etching and optimisation of acid solution

Turbostratic carbon nitride prepared by pyrolysis of melamine

Abstract: Since the theoretical calculations predicted that the hardness of C3N4 covalent compound might be comparable to or even higher than that of diamond [1, 2], many attempts have been made to synthesis this novel substance. Due to the great thermodynamic stability of N2, however, the ideal structural transition from precursor to crystalline carbon nitride is difficult to realize. In most cases, only amorphous products with low nitrogen content were obtained [3]. To solve this problem, it is good to prepare carbon nitride in graphitic or turbostratic form firstly, and then using it as precursor to synthesize other carbon nitride crystalline phases. Therefore, graphitic C3N4 have recently attracted more attention. Through different routes, graphitic C3N4 has been synthesized [4–6]. For previous work, the claimed graphitic-like C3N4 with unique (002) diffraction peak has the turbostratic structure actually [7–10], for the lake of other peaks in their XRD patterns. Meanwhile, the (002) spacing of those obtained turbostratic carbon nitrides are in the range of 0.32–0.33 nm. The graphitic C3N4 predicted by Teter and Hemley [11] can be described as a perfect de-ammonation polycondensate of melamine, therefore, melamine was often chosen as the carbon nitride precursor to synthesize graphitic C3N4 by electrodeposition [6] and solvothermal method [7]. Pyrolysis of melamine was investigated [12] in 1988, however, the authors focused their attention only on its thermal behavior, and they didn’t consider the possibility as potential candidate for carbon nitrides after complete polycondensate. Recently, pyrolysis of melamine under high pressure was studied at the temperatures up to 700 ◦C [13]. Obvious nitrogen loss made the attempt unsuccessful. In this letter, we report an improved pyrolysis route to prepare turbostratic carbon nitride from melamine. The pyrolysis was performed in two steps: first, the temperature was maintained at 300 ◦C in order to realize the primary condensation (melamine → melam) as complete as possible; and then it was risen to 650 ◦C to perform the advanced condensation. In the experimental, melamine was pyrolyzed in a quartz tube with a diameter of 35 mm, and an outer-thimble-shape heater was used. Melamine was placed at the middle of the quartz tube. The pyrolysis was conducted at 300 ◦C for 1 hr, then at 600◦ for 2 hr in atmosphere; after milling, the obtained powder was maintained at 300 ◦C for 0.5 hr, then at 650 ◦C for 1 hr in vacuum. Finally, a kind of brown carbon nitride powder was obtained. The chemical composition was analyzed by using elemental analyzer (LECO, CHN-1000) and EDX (EDAX INC., Phoenix). The product was characterized by X-ray diffraction (XRD) with Cu-Kα radiation (JEOL, ROTEX JRX-12), fourier transfer infrared (FTIR) spectroscopy (Bruker, EQUINOX55), scanning electron microscopy (SEM) (TOPCON, SM520), and transmission electron microscopy (TEM) coupled with selected area diffraction (SAED) (JEOL, JEM-2010) and thermogravimetry (TG) (NETZSCH, STA449C/6/G). The composition of the product characterized by elemental analysis and EDX is listed in Table I. SEM observation found that the particles dimension ranged from 5 μm to 20 μm. Most of the particles show the flake-like morphology. Fig. 1 shows a typical XRD pattern of the product. There is a single main peak at the position of 27.62 ◦, which suggests that the product was turbostratic. Its corresponding d-spacing is 0.321 nm similar to the (002) plane of the turbostratic/graphitelike carbon nitrides obtained in previous work [9, 14]. The FTIR spectrum of the prepared turbostratic carbon nitride and melamine are shown in Fig. 2. The IR spectrum of melamine presents three absorption bonds: 3000–3650 cm−1, 1100–1700 cm−1, and the last one centered at about 810 cm−1. The 3000– 3650 cm−1 band is assigned to N H stretching vibration modes, the 1100–1650 cm−1 band corresponds to the stretching vibrations related to C N, C N, and is generally associated with the skeletal stretching vibrations of these aromatic rings. The absorption at 810 cm−1 is characteristic of out-of-plane bending modes of these rings. The 460–850 cm−1 band is linked to the C NH2 group and the ring breadth or bending vibration modes [15]. Comparing the FTIR spectrum of the turbostratic carbon nitride with that of melamine, it reveals that after pyrolysis, the previous strong absorption peaks in the range of 3000–3650 cm−1 have disappeared, only a board absorption bond is left, which suggest that most of the N H bonds have been destroyed during the de-ammonation condensation. While the increased number of absorption peaks ranging from 1100 to 1700 cm−1 implies the condensation of 1,3,5-s-triazine rings making the related chemi-

Structural studies of geopolymers by 29Si and 27Al MAS-NMR

Abstract: A systematic study of geopolymers by 29Si and 27Al MAS NMR has been carried out in an attempt to understand polymer structural details. 27Al MAS NMR data shows that transient aluminium species are formed during the reaction of metakaolin with NaOH. Interaction of silicate anions with the aluminium sites of metakaolin was evident during the synthesis of geopolymers as observed from low field shift of 29Si MAS NMR resonance lines of silicate centres. As the reaction progresses, the coordination of aluminium (IV, V and VI) in metakaolin changes almost completely to IV. 29Si MAS NMR of selected compositions of the ternary system of sodium silicate, metakaolin and aqueous alkali reveals that geopolymerisation occurs in a distinct compositional region. At high alkalinity [> 30% (mol/mol) overall Na2O content], connectivity of silicate anions is reduced, consistent with poor polymerisation. At low alkalinity [<10% (mol/mol) overall Na2O content], a clear 29Si NMR resonance line due to unconverted metakaolin is observed. NMR spectra were recorded from a series of samples with a fixed Na2O content (20 mol%) and varied SiO2/Al2O3 ratio to observe aluminium substitution in the cross-linked silicon tetrahedra of polymer network. Aluminium insertion into the silicate network is confirmed from the observed 29Si NMR shift as a function of Si/Al ratio. The identification of the presence or absence of metakaolin in the cured geopolymer product is not possible even by 29Si NMR as the signal from metakaolin is indistinguishable from a broad 29Si NMR peak consisting of many resonance lines from the network of cross-linked silicon/aluminium tetrahedra. In an attempt to identify metakaolin signal, we prepared geopolymers with higher SiO2/Al2O3 molar ratios. Since aluminium substitutions in the silicate tetrahedral network are decreased, this results in better-resolved 29Si NMR lines. The 29Si NMR signal due to metakaolin is then distinguishable in the spectra of cured products in a series of samples with 3 to 11 mol% metakaolin. These results indicate that a geopolymer structure is a network of silicon/aluminium tetrahedra with some presence of unreacted metakaolin. The silicon/aluminium tetrahedra might have connectivity ranging from 1 to 4.

Novel production for highly formable Mg alloy plate

Abstract: The principle and advantages of multi-pass friction stir processing (FSP) for the production of a highly formable Mg alloy, and some convincing experimental results are reported in this paper. FSP is a solid state processing technique which involves plunging and traversing a cylindrical rotating FSP tool through the material. FSP achieved grain refinement and homogenization of the as-cast microstructure in Mg alloy AZ91D. Multi-pass FSP produced a fine homogeneous microstructure having a grain size of 2.7 μm throughout the plate. The plate containing this FSPed microstructure exhibited fracture limit major strains six times larger than the diecast plate in the fracture limit diagram (FLD). The present study shows that multi-pass FSP is an efficient production method for a large-scale plate of a highly formable Mg alloy.

Microstructure and mechanical properties of orientated thermoplastic starches

Abstract: Effect of orientation on microstructure and mechanical properties of thermoplastic starches with different amylose/amylopectin ratios was studied to understand the relationship between structure and properties in starch-based materials. Hydrogen bonds and the highly branched microstructure in amylopectin resist the orientation of the polymer chains. The unique microstructures of amylopectin form gel-balls and super-globes after gelatinization. A gel-ball contains mainly the chains from same sub-main chain in amylopectin, and remains in regular pattern and keeps a certain “memory”. The gel-balls and super-globes can be deformed under shear stress. However, the deformation does not alter the orientation ofthe polymer chains inside the gel-balls significantly. Orientation increases both modulus and yield stress but decreases the elongation, which is mainly contributed to by the orientation of amorphous phase. The oriented super-globe has large interior stress after retrogradation (crystallization) that results in micro-cracks and poor mechanical properties.

Tool penetration during friction stir spot welding of Al and Mg alloys

Abstract: The mechanism of tool penetration during friction stir spot welding of Al-alloy and Mg-alloy sheet materials is investigated and is explained as a progression of wear events, from mild wear to severe wear and then to melt wear in material beneath the base of the rotating pin. Melt wear can also occur under the rotating tool shoulder provided that sufficient penetration of the upper sheet occurs during the spot welding operation.