Showing papers in "Journal of Materials Science in 2008"

Recent advances in shape memory polymers and composites : a review

Abstract: Shape memory polymers (SMPs) belong to a class of smart polymers, which have drawn considerable research interest in last few years because of their applications in microelectromechanical systems, actuators, for self healing and health monitoring purposes, and in biomedical devices. Like in other fields of applications, SMP materials have been proved to be suitable substitutes to metallic ones because of their flexibility, biocompatibility and wide scope of modifications. The shape memory properties of SMPs polymers might surpass those of shape memory metallic alloys (SMAs). In addition to block copolymers, polymers blends and interpenetrating network structured SMP systems have been developed. The present review mainly highlights the recent progress in synthesis, characterization, evaluation, and proposed applications of SMPs and related composites.

Development of lanthanum strontium manganite perovskite cathode materials of solid oxide fuel cells: a review

Abstract: The high-temperature solid oxide fuel cell (SOFC) is the most efficient and environmentally friendly energy conversion technology to generate electricity from fuels such as hydrogen and natural gas as compared to the traditional thermal power generation plants. In the last 20–30 years, there has been significant progress in the materials development and stack technologies in SOFC. Among the electrode materials, lanthanum strontium manganite (LSM) perovskites, till today, are the most investigated and probably the most important electrode materials in SOFCs. The objective of this article is to review and update the development, understanding, and achievements of the LSM-based materials in SOFC. The structure, nonstoichiometry, defect model, and in particular the relation between the microstructure, their properties (electrical, thermal, mechanical, chemical, and interfacial), and electrochemical performance and performance stability are critically reviewed. Finally, challenges and prospects of LSM-based materials as cathodes for intermediate and low-temperature SOFCs are discussed.

Hydrothermal processing of materials: past, present and future

Abstract: The hydrothermal technique provides an excellent possibility for processing of advanced materials whether it is bulk single crystals, or fine particles, or nanoparticles. The advantages of hydrothermal technology have been discussed in comparison with the conventional methods of materials processing. The current trends in hydrothermal materials processing has been described in relation to the concept of soft solution processing, as a single-step low energy consuming fabrication technique. Also some recent developments in multi-energy processing of materials such as microwave-hydrothermal, mechanochemical-hydrothermal, electrochemical-hydrothermal, sonar-hydrothermal, etc. have been discussed. An overview of the past, present and future perspective of hydrothermal technology as a tool to fabricate advanced materials has been given with appropriate examples.

Processing of nanographene platelets (NGPs) and NGP nanocomposites: a review

Abstract: The nanoscale graphene platelet (NGP) or graphene nanosheet is an emerging class of nanomaterials. An NGP is a nanoscale platelet composed of one or more layers of a graphene plane, with a platelet thickness from less than 0.34 to 100 nm. NGPs are predicted to have a range of unusual physical, chemical, and mechanical properties. Although practical electronic device applications for graphene are not envisioned to occur within the next 5–10 years, its application as a nanofiller in a composite material is imminent. The availability of processable graphene sheets in large quantities is essential to the success in exploiting composite and other applications. This review first describes the earlier processes for producing mostly multi-layer NGPs and their composites, which is followed by a discussion on the recent developments in the preparation of single-layer NGPs and their nanocomposites. Fundamental principles behind processing of nanographene materials are also briefly discussed.

Modification of eutectic silicon in Al-Si alloys

Abstract: The mechanical properties of Al–Si alloys are strongly related to the size, shape and distribution of eutectic silicon present in the microstructure In order to improve mechanical properties, these alloys are generally subjected to modification melt treatment, which transforms the acicular silicon morphology to fibrous one resulting in a noticeable improvement in elongation and strength. Improper melt treatment procedures, fading and poisoning of modifiers often result in the structure which is far from the desired one. Hence it is essential to assess the effectiveness of melt treatment before pouring. A much investigated reliable thermal analysis technique is generally used for this purpose. The deviation from the standard curve in thermal analysis helps in assessing the level of refinement of the Si structure. In the present review an attempt is made to discuss various aspects of modification, including mechanism, interaction of defects and non-destructive assessment by thermal analysis.

Emerging biodegradable materials: starch- and protein-based bio-nanocomposites

Abstract: This article provides a broad overview on the natural polymer-based bio-nanocomposite properties, processing and application. Bio-nanocomposites prepared with natural biopolymers, such as starch and protein, can be formed using a melt intercalation or a solvent intercalation method. Incorporation of layered silicates into the biopolymer matrices results in improved mechanical properties, water vapor barrier properties, and thermal stability of the resulting bio-nanocomposites without sacrificing biodegradability due to their nanometer size dispersion. Consequently, even though natural polymer-based bio-nanocomposite is in its infancy, it has a huge potential in the future.

Polymer-bioceramic composites for tissue engineering scaffolds

Abstract: Designing tissue engineering scaffolds with the required mechanical properties and favourable microstructure to promote cell attachment, growth and new tissue formation is one of the key challenges facing the tissue engineering field. An important class of scaffolds for bone tissue engineering is based on bioceramics and bioactive glasses, including: hydroxyapatite, bioactive glass (e.g. Bioglass®), alumina, TiO2 and calcium phosphates. The primary disadvantage of these materials is their low resistance to fracture under loads and their high brittleness. These drawbacks are exacerbated by the fact that optimal scaffolds must be highly porous (>90% porosity). Several approaches are being explored to enhance the structural integrity, fracture strength and toughness of bioceramic scaffolds. This paper reviews recent proposed approaches based on developing bioactive composites by introducing polymer coatings or by forming interpenetrating polymer-bioceramic microstructures which mimic the composite structure of bone. Several systems are analysed and scaffold fabrication processes, microstructure development and mechanical properties are discussed. The analysis of the literature suggests that the scaffolds reviewed here might represent the optimal solution and be the scaffolds of choice for bone regeneration strategies.

Calcium orthophosphate cements for biomedical application

Abstract: In early 1980s, researchers discovered self-setting calcium orthophosphate cements, which are a bioactive and biodegradable grafting material in the form of a powder and a liquid. Both phases after mixing form a viscous paste that after being implanted sets and hardens within the body as either a non-stoichiometric calcium deficient hydroxyapatite (CDHA) or brushite, sometimes blended with unreacted particles and other phases. As both CDHA and brushite are remarkably biocompartible and bioresorbable (therefore, in vivo they can be replaced with a newly forming bone), calcium orthophosphate cements represent a good correction technique of non-weight-bearing bone fractures or defects and appear to be very promising materials for bone grafting applications. Besides, these cements possess an excellent osteoconductivity, molding capabilities, and easy manipulation. Nearly perfect adaptation to the tissue surfaces in bone defects and a gradual bioresorption followed by new bone formation are additional distinctive advantages of calcium orthophosphate cements. Besides, reinforced formulations are available; those are described as calcium orthophosphate composites. The discovery of self-setting cements has opened up a new era in the medical application of calcium orthophosphates; several commercial formulations have already been introduced as a result. Many more compositions are in experimental stages. In this review, an insight into calcium orthophosphate cements, as excellent biomaterials suitable for both dental and bone grafting application, has been provided.

Silver, gold and bimetallic nanoparticles production using single-cell protein ( Spirulina platensis ) Geitler

Abstract: Interaction of single-cell protein of Spirulina platensis with aqueous AgNO3 and HAuCl4 was investigated for the synthesis of Ag, Au and Au core—Ag shell nanoparticles. Biological reduction and extracellular synthesis of nanoparticles were achieved in 120 h at 37 °C at pH 5.6. The nanometallic dispersions were characterized by surface plasmon absorbance measuring at 424 and 530 nm for Ag and Au nanoparticles, respectively. For bimetallic nanoparticles, absorption peak was observed at 509, 486 and 464 nm at 75:25, 50:50 and 25:75 (Au:Ag) mol concentrations, respectively. High-resolution transmission electron microscopy showed formation of nanoparticles in the range of 7–16 (silver), 6–10 (gold) and 17–25 nm (bimetallic 50:50 ratio). XRD analysis of the silver and gold nanoparticles confirmed the formation of metallic silver and gold. Fourier transform infrared spectroscopic measurements revealed the fact that the protein is the possible biomolecule responsible for the reduction and capping of the biosynthesized nanoparticles.

Stabilization of nanocrystalline grain sizes by solute additions

Abstract: This paper will review the grain growth in nanocrystalline materials with emphasis on the grain size stabilization that can result from solute additions. The grain growth in nominally pure nanocrystalline metals will be presented followed by descriptions of the stabilization of nanocrystalline grain sizes by kinetic approaches and thermodynamic strategies. The descriptions of nanocrystalline grain size by solute additions will be taken from the literature as well as from recent research in the authors’ laboratory. Examples of kinetic stabilization, which involves reduction of the grain boundary mobility, include second phase drag, solute drag, chemical ordering, and grain size stabilization. The thermodynamic stabilization, which is due to the lowering of the specific grain boundary energy by solute segregation to the grain boundaries, will be described for systems including Pd–Zr, Fe–Zr, Ni–W, Ni–P, and Co–P. Recrystallization during grain growth will be presented for the Ti–N system. Finally, a summary of alloys where nanocrystalline grain sizes can be maintained at annealing temperatures close to the melting point will be presented.

Estimation of lattice strain in nanocrystalline silver from X-ray diffraction line broadening

Abstract: The lattice strain contribution to the X-ray diffraction line broadening in nanocrystalline silver samples with an average crystallite size of about 50 nm is studied using Williamson-Hall analysis assuming uniform deformation, uniform deformation stress and uniform deformation energy density models. It is observed that the anisotropy of the crystallite should be taken into account, while separating the strain and particle size contributions to line broadening. Uniform deformation energy density model is found to model the lattice strain appropriately. The lattice strain estimated from the interplanar spacing data are compared with that estimated using uniform-energy density model. The lattice strain in nanocrystalline silver seems to have contributions from dislocations over and above the contribution from excess volume of grain boundaries associated with vacancies and vacancy clusters.

How to improve mechanical properties of polylactic acid with bamboo fibers

Abstract: Bamboo fibers (BF) were mixed in polylactic acid (PLA) to improve its mechanical properties: impact strength and heat resistance. Three different types of BF were extracted from raw bamboo by either sodium hydroxide (NaOH) treatment or steam explosion in conjunction with mechanical processing. They were designated as “short fiber bundle,” “alkali-treated filament” and “steam-exploded filament,” respectively. Composite samples were fabricated by injection molding using PLA/BF pellets prepared by a twin-screw extruding machine. Among them, the highest bending strength was obtained when steam-exploded filaments were put into PLA matrix. Impact strength of PLA was not greatly improved by addition of short fiber bundles as well as both filaments. In order to improve the impact strength of PLA/BF composites, PLA composite samples were alternatively fabricated by hot pressing using medium length bamboo fiber bundles (MFB) to avoid the decrease in fiber length at fabrication. Impact strength of PLA/MFB composite significantly increased, in which long fiber bundles were pulled out from the matrix. The addition of BF improves thermal properties and heat resistance of PLA/BF composites due to the constraint of deformation of PLA in conjunction with crystallinity promoted by anneal (at 110 °C for 5 h).

Metal Oxide Composites and Structures for Ultra-High Temperature Solar Thermochemical Cycles.

Abstract: Conceptually, thermochemical cycles are heat engines that drive endothermic chemical reactions, e.g., splitting water into hydrogen and oxygen. The two-step metal oxide cycles (typically ferrite-based) are particularly attractive since they are relatively simple, use non-corrosive materials, and involve gas–solid reactions requiring no difficult separations. Additionally, they are potentially the most efficient renewable-energy driven processes for hydrogen production. We are developing a novel concentrating solar power (CSP) driven metal-oxide-based heat engine, the CR5, at the heart of which are rings of a reactive solid that are thermally and chemically cycled to produce oxygen and hydrogen from water in separate and isolated steps. The monolithic ring structures must have high geometric surface area for gas–solid contact and for adsorption of incident solar radiation, and must maintain structural integrity and high reactivity after extensive thermal cycling to temperatures of at least 1,400 °C. We have demonstrated through laboratory and on-sun testing that cobalt ferrite/zirconia mixtures fabricated into monolithic structures suitable for the CR5 are mechanically robust and maintain productivity over tens of cycles. We have also demonstrated that carbon dioxide splitting (CDS) to carbon monoxide and oxygen is a thermodynamically favorable alternative to water splitting that can be conducted with both iron- and cerium-based materials.

Advances in the application of nanotechnology in enabling a ‘hydrogen economy’

Abstract: Hydrogen is gaining a great deal of attention as an energy carrier as well as an alternative fuel. However, in order to fully implement the so called ‘hydrogen economy’ significant technical challenges need to be overcome in the fields of production and storage of hydrogen and its point of use especially in fuel cells for the automotive industry. The purpose of this review is to present and discuss recent advances in the use of nanomaterials for solar hydrogen production and on-board solid state storage of hydrogen. The role of nanotechnology in enhancing the efficiency of fuel cells and reducing their cost is also discussed.

Surface modification of multi-walled carbon nanotubes using 3-aminopropyltriethoxysilane

Abstract: Multi-walled carbon nanotubes (MWCNTs) were functionalized via oxidation with a mixture of concentrated sulfuric acid and nitric acid. Thus functionalized nanotubes (f-MWCNTs) were silanized using a coupling agent, 3-Aminopropyltriethoxysilane (3-APTES). The f-MWCNTs and the reaction product of f-MWCNTs and 3-APTES (APTES–MWCNTs) were characterized by Fourier Transform Infrared Spectroscopy, Energy Dispersion Spectroscopy, Scanning Electron Microscopy, and Transmission Electron Microscopy. The results indicate the attachment of silane molecules on the surface of the functionalized MWCNTs. This silanization method allows for the improvement of the chemical compatibility of MWCNTs with specific polymers for application in nanotube-based polymer matrix composites.

Factors affecting the performance of metakaolin geopolymers exposed to elevated temperatures

Abstract: The effects of geopolymer binder systems exposed to elevated temperatures are examined. Geopolymers investigated were synthesized from metakaolin, activated by combinations of sodium/potassium silicate and sodium/potassium hydroxide. The specimens were then exposed to temperatures of 800 °C. The factors studied were: (1) calcining temperatures of kaolin; (2) Si/Al ratio of the geopolymer; (3) activator/metakaolin ratio; (4) curing temperature; and (5) alkali cation type. Altogether 30 geopolymer formulations were studied. The samples were subjected to compressive strength, thermogravimetry, and scanning electron microscopy tests. Results showed that Si/Al ratio has a significant influence on elevated temperature exposure deterioration. Lesser strength loss due to elevated temperature exposures were observed in geopolymer with high Si/Al ratios (>1.5). The geopolymer binders activated by potassium-based activators showed an enhanced post-elevated temperature exposure performance compared to sodium-based systems. The optimum calcining temperature of kaolin and curing temperatures for improved temperature performance are also reported.

Processing methods to control silk fibroin film biomaterial features

Abstract: Control of silk structural and morphological features is reported for fibroin protein films via all aqueous processing, with and without polyethylene oxide (PEO). Silk films with thicknesses from 500 nm to 50 μm were generated with controllable surface morphologies by employing soft-lithography surface patterning or by adjusting PEO concentrations. FTIR analysis indicated that water-annealing, used to cure or set the films, resulted in increased β-sheet and α-helix content within the films. Steam sterilization provided an additional control point by increasing β-sheet content, while reducing random coil and turn structures, yet retaining film transparency and material integrity. Increased PEO concentration used during processing resulted in decreased sizes of surface globule structures, while simultaneously increasing uniformity of these features. The above results indicate that both surface and bulk morphologies and structures can be controlled by adjusting PEO concentration. The combined tool set for controlling silk film geometry and structure provides a foundation for further study of novel silk film biomaterial systems. These silk film biomaterials have potential applicability for a variety of optical and regenerative medicine applications due to their optical clarity, impressive mechanical properties, slow degradability, and biocompatibility.

The nucleating effect of exfoliated graphite nanoplatelets and their influence on the crystal structure and electrical conductivity of polypropylene nanocomposites

Abstract: The focus of this research is to investigate how exfoliated graphite nanoplatelets, xGnP™, (graphene sheets ∼10 nm thickness, ∼1 μm diameter), a nanomaterial developed by the Drzal group, affects the crystallization of semicrystalline thermoplastics i.e., polypropylene (PP). In addition, this study explores how the presence of xGnP in combination with the processing conditions used to make the xGnP-PP nanocomposites alter the crystal structure and electrical conductivity of these systems. The nanocomposites are fabricated (i) by melt mixing followed by injection molding and (ii) by coating PP powder with xGnP with sonication in isopropyl alcohol followed by compression molding. PP was found to nucleate on the graphene surface of xGnP that is an effective nucleating agent for the β-form of PP crystals at low concentrations. The β-form of PP crystals has higher impact strength and toughness compared to the more common occurring α-form. It is found that the aspect ratio and concentration of xGnP combined with the crystallization conditions can be used to engineer the crystal structure such as the population and size distribution of PP spherulites and alter the electrical conductivity of xGnP-PP nanocomposites. The reason is that the presence of many small spherulites nucleated by the xGnP disrupts the percolated network formed by the conductive particles and thus increases the concentration required to reach conductivity and alters the conductivity value.

Review: degradation-induced embrittlement in semi-crystalline polymers having their amorphous phase in rubbery state

Abstract: The literature dealing with degradation-induced embrittlement mechanisms in semi-crystalline polymers having their amorphous phase in rubbery state is reviewed. It is first demonstrated that the decrease of molar mass resulting from a quasi-homogeneous chain scission process is responsible for embrittlement. The main specificity of the polymer family under study is that embrittlement occurs at a very low conversion of the degradation process, while the entanglement network in the amorphous phase is slightly damaged. In these polymers, chain scission induces chemicrystallization. The analyses of available data on this process show that it is characterized by a relatively high yield: about one half entanglement strands integrate the crystalline phase after one chain scission. A simple relationship expressing the chemicrystallization yield for a given polymer structure is proposed. Chain scission and chemicrystallization can lead to embrittlement through two possible causal chains: (1) chain scission → molar mass decrease → chemicrystallization → decrease of the interlamellar spacing → embrittlement. (2) Chain scission → molar mass decrease → chemicrystallization → decrease of the tie-macromolecule concentration → embrittlement. At this state of our knowledge, both causal chains are almost undistinguishable.

Application of lignin as natural adhesion promoter in cotton fibre-reinforced poly(lactic acid) (PLA) composites

Abstract: This study investigated how lignin—used as a natural adhesion promoter in biodegradable, thermoplastic cotton fibre-reinforced composites—influences the composites’ mechanical properties. Composites with fibre mass proportions of 40% were produced by compression moulding. Poly(lactic acid) (PLA), a biopolymer, served as matrix. Cotton/PLA composites with and without lignin content were manufactured. As reference samples of bast fibre-reinforced composites, kenaf/PLA composites were produced under the same conditions. The composites were tested for stiffness, tensile strength, elongation at break and impact strength. Fractured surfaces were analysed using scanning electron microscopy (SEM). The results of the composite investigations showed that the addition of lignin has an influence on the cotton/PLA composite characteristics. SEM investigations showed that the adhesion between fibre and matrix could be improved by the addition of lignin. Tensile characteristics like tensile strength and Young’s modulus could be improved clearly, while the impact properties were decreased.

Solvothermal reactions: an original route for the synthesis of novel materials

Abstract: Twenty years after the first development of solvothermal reactions, it appears important through the last research activities to trace the future trends taking into account their potentialities and the different economical constraints. During these last 20 years solvothermal reactions have been mainly used from preparing micro- or nanoparticles with different morphologies. Due to the importance to dispose of new materials for developing either basic research or industrial applications, such a presentation will be only focussed on the potentialities of solvothermal reactions in materials synthesis. Solvothermal reactions are mainly characterized by different chemical parameters (nature of the reagents and of the solvent) and thermodynamical parameters (in particular temperature, pressure). (a) The selection of the composition of the solvent opens new research areas for stabilizing materials belonging to different classes of materials (alloys, oxides, nitrides, sulphides…). (b) The mild temperature conditions generally used are able to improve chemical diffusion and reactivity in order to help the preparation of specific materials at the frontier between either different classes of inorganic materials (oxides-nitrides, nitrides-halides…) or inorganic/organic, inorganic/biologic frameworks. (c) The high pressure conditions, due to the small conveyed energy compared to temperature, allow also to stabilize metastable frontier materials (geo-inspired or bio-inspired materials). (d) In the future, taking into account, from one side: the economical and the environmental constraints, and from the other: the industrial demand of materials characterized by specific physical, chemical and biological properties, the potential developments of solvothermal processes will be analyzed.

Energy dissipation in carbon nanotube composites: a review

Abstract: In this article we discuss the energy dissipation that occurs when the interfacial slip of nanoscale fillers is activated in a host matrix material We consider both polymer (such as polycarbonate, PEO, PEG) and epoxy matrices The nanoscale fillers considered are carbon nanotubes (both singlewalled and multiwalled) as well as fullerenes The nano-composites are fabricated by using a solution mixing technique with tetra-hydro-furan as the solvent The interfacial friction damping is quantified by performing uniaxial dynamic load tests and measuring the material storage and loss modulus We study various effects such as impact of nanotube weight fraction, nanotube surface treatment (oxidation, epoxidation etc), test frequency, strain amplitude, operating temperature, as well as effect of pre-strain or biased strain The effect of geometry (ie, aspect ratio) is also considered by comparing the damping response of fullerene-composites with that of nanotube-composites

Deformation mechanism and texture and microstructure evolution during high-speed rolling of AZ31B Mg sheets

Abstract: High-speed rolling of AZ31B was carried out under various preheating temperatures from RT to 350 °C. The evolution of texture, grain sizes, and dislocation density distribution (Kernel average misorientation distributions) in the mid-thickness and surface layer were investigated. Computer simulations of deformation textures were also performed in order to understand deformation mechanisms. It is concluded that the temperature increase due to the plastic and frictional working during high-speed rolling makes the slip system more active and, hence, improves the ductility. The surface layer of the specimen has higher temperature and experiences severe shear stress; therefore the texture, microstructure, and dislocation density distribution are different from those of the mid-thickness of the specimen. Both mid-thickness and surface layer are dynamically recrystallized during the high-speed rolling.

A review of the influencing factors on anisotropic conductive adhesives joining technology in electrical applications

Abstract: New interconnect materials are always necessary as a result of evolving packaging technologies and increasing performance and environmental demands on electronic systems. Polymer-based conductive-adhesive materials have become widely used in many electronic packaging interconnect applications. Among all the conductive-adhesive materials, the anisotropic conductive adhesives (ACA) (or anisotropic conductive adhesive films, ACF) have gained popularity as a potential replacement for solder interconnects. The interest in using ACA instead of solder comes partly from the fact that the use of ACA for the direct interconnection of flipped silicon chips to printed circuits (flip chip packaging) offers numerous advantages such as reduced thickness, improved environmental compatibility, lowered assembly process temperature, increased metallization options, reduced cost, and decreased equipment needs. In this review, a summary of our understanding of the electrical, physical, thermal, chemical, environmental, and cost behaviors of ACA in conjunction with various packaging applications is elaborated. First, the formulation and curing kinetics of ACA materials, as well as the conduction mechanisms of ACA joints, are introduced; second, the influencing factors, including the boding process (boding temperature, boding pressure, curing conditions, reflow and misalignment processes, etc), the environmental factors (temperature, humidity, impact load, etc), and the properties of the components (the properties of the ACA, substrates, conductive particles, the bump height, etc), on the reliability of ACA joining technology are presented. Finally, future research areas and remaining issues are pointed out. The purpose is simply to pinpoint the most important papers that have played significant role for the advancement of the ACA bonding technology.

Experimental evaluation of interactions in supercritical CO2/water/rock minerals system under geologic CO2 sequestration conditions

Abstract: The hydrothermal autoclave experiments were conducted to simulate the interactions in the scCO2/water/rock minerals (quartz, biotite and granite) reaction systems using a Hastelloy C reaction cell at 100 °C. The dissolution characteristics of rock minerals and their surface texture alternation after hydrothermal treatment were examined by ICP-AES and SEM/EDX investigation, respectively. The results suggested that the hydrolysis of plagioclase phase should be mainly responsible for the elements dissolved from the Iidate granite samples. The dissolution was encouraged by the introduction of CO2 in the water/granite system, and generated an unknown aluminosilicate. No distinct chemical alternations occurred in the water-free scCO2/granite system, which indicated that rock minerals should be chemically stable in the water-free scCO2 fluids under the current mild experimental conditions. Both the highest concentration of Ca existing in the scCO2/vapor/granite system and the SEM observation results of calcite deposit, suggested that a meaningful CO2 minerals trapping process should be potential in the CO2-rich field during a short physicochemical interaction period.

Hydrothermal preparation of ZnO:CNT and TiO2:CNT composites and their photocatalytic applications

Abstract: ZnO:CNT and TiO2:CNT composites were fabricated under mild hydrothermal conditions (T = 150–240 °C) with an autogenous pressure. The as prepared composites were characterized using X-ray diffraction, Scanning electron microscopy and FTIR spectroscopy. Photocatalytic applications of the composites were investigated using indigo caramine dye. The effect of the catalyst content, pH of the medium, source and intensity of illumination on the photodegradation of the indigo caramine dye was studied and the efficiency of the composites were investigated based on different parameters like percent transmittance (%T), percent decomposition, and chemical oxygen demand of the dye solution to obtain optimum treatment conditions. The results obtained exhibit higher photocatalytic activity when compared to the reagent grade ZnO, TiO2 and hydrothermally prepared ZnO:AC and TiO2:AC composites.

Mechanical properties, fire performance and thermal stability of magnesium hydroxide sulfate hydrate whiskers flame retardant silicone rubber

Abstract: Halogen-free flame retardant silicone rubber (SR) composites, using magnesium hydroxide sulfate hydrate (MHSH) whiskers as flame retardant have been prepared by a two-roll mill. Moreover, microencapsulated red phosphorus (MRP) was used as a synergist. Mechanical tests were performed to determine the tensile strength, elongation at break, and shore hardness of the composites. The morphology of fracture surface was observed by environmental scanning electron microscopy (ESEM). The results showed MHSH slightly reduced the tensile strength of the composites, but had obvious influence on the elongation at break. Meanwhile, Shore A hardness presented uptrend with increasing MHSH content. The addition of vinyl silicone fluid (VSF) could improve the compatibility of the MHSH whiskers in SR matrix, and therefore improved the mechanical properties of composites. The flammability properties of composites were investigated by limited oxygen index (LOI), UL-94 tests, and cone calorimetry experiments. It is found that MHSH whiskers can effectively improve the flame retardancy of SR composites due to the endothermic degradation of MHSH whiskers accompanied with the release of water vapor, and the formation of fibrous magnesia acting as a barrier layer. The incorporation of MRP in SR/MHSH whiskers system had a synergic fire retardant effect in the condensed and gas phase. In addition, thermogravimetric analysis (TGA) indicated the presence of MRP enhanced thermal stability of the SR/MHSH composites at higher temperature range, and remarkably promoted char residue yield.

Geopolymerization reaction to consolidate incoherent pozzolanic soil

Abstract: Pozzolanic material-based geopolymer has been proposed as a solving methodology to the geohazards, due to pozzolanic collapsible soils widely present in the South Italy. The geopolymer was synthesized from pozzolana material under activation of NaOH 10 M or slurry of NaAlO2 in NaOH 10 M solution. The specimens were cured at 25 °C and 100% RH for different ageing times. The effect of the two activation methods on the properties of the geopolymer was investigated by means of X-ray diffraction, scanning electron microscopy (SEM), FTIR spectroscopy, nuclear magnetic resonance (27Al and 29Si NMR) and uniaxial compression tests. XRD, NMR and IR analysis indicate the geopolymer is generated by the dissolution of the silico-aluminate phases present in the pozzolana and the successive re-organization in amorphous and crystalline neo-formed phases. The spectroscopic evidences confirm that the 4-coordinated Al atoms present in the neat pozzolana and in the NaAlO2 change their coordination state splitting between 6- and 4-coordinated atoms, modifying the traditional chemistry of polysialate formation. SEM results show the synthesized geo-polymer maintained the granular morphology of the pozzolana and the geo-polymeric reactions occurred mainly at the surface of pozzolana particulates. Furthermore, uniaxial strength data increase gradually upon the curing time, until 40 MPa for the specimens activated with the slurry system.

Design of graded two-phase microstructures for tailored elasticity gradients

Abstract: Being one of new generation of composites, functionally graded materials (FGMs) possess gradually changed physical properties due to their compositional and/or microstructural gradients. In literature, exhaustive studies have been carried out in compositional modeling and design, while limited reports are available for microstructural optimization. This article presents an inverse homogenization method for the design of two-phase (solid/void) FGM microstructures, whose periodic base cells (PBCs) vary in a direction parallel to the property gradient but periodically repeat themselves in the perpendicular direction. The effective elasticity tensor at each PBC is estimated in terms of the homogenization theory. The overall difference between the effective tensor and their target is minimized by seeking for an optimal PBC material topology. To preserve the connectivity between adjacent PBCs, three methods, namely connective constraint, pseudo load, and unified formulation with nonlinear diffusion are proposed herein. A number of two-dimensional examples possessing graded volume fraction and Young’s modulus but constant positive or negative Poisson’s ratios are presented to demonstrate this computational design procedure.

Effect of sensitization heat treatment on properties of Al–Mg alloy AA5083-H116

Abstract: Al–Mg alloy AA5083 is a sheet and plate alloy used mainly for marine application as well as for structural components in transportation and military applications. The strength is derived from solid solution strengthening and strain hardening. The properties of as-received and sensitized samples of AA5083-H116 were investigated using microhardness measurements, tensile testing, optical microscopy, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Nitric Acid Mass Loss Test (NAMLT). The results show that both chemical and mechanical properties of the alloy decreased with increasing sensitization temperature and time. The deterioration in chemical property, which was measured in terms of the level of susceptibility to Intergranular Corrosion (IGC), is attributed to grain boundary precipitation of magnesium-rich particles. The loss in tensile and hardness properties is attributed to softening caused partly by decrease in Mg solute solid solution concentration with increasing sensitization time and temperature and partly by recrystallization at elevated temperatures.