Showing papers in "Journal of Materials Science in 2004"

Oxidation-based materials selection for 2000°C + hypersonic aerosurfaces: Theoretical considerations and historical experience

Abstract: Hypersonic flight involves extremely high velocities and gas temperatures with the attendant necessity for thermal protection systems (TPS). New high temperature materials are needed for these TPS systems. A systematic investigation of the thermodynamics of potential materials revealed that low oxidation rate materials, which form pure scales of SiO2, Al2O3, Cr2O3, or BeO, cannot be utilized at temperatures of 1800°C (and above) due to disruptively high vapor pressures which arise at the interface of the base material and the scale. Vapor pressure considerations provide significant insight into the relatively good oxidation resistance of ZrB2- and HfB2-based materials at 2000°C and above. These materials form multi-oxide scales composed of a refractory crystalline oxide (skeleton) and a glass component, and this compositional approach is recommended for further development. The oxidation resistance of ZrB2-SiC and other non-oxide materials is improved, to at least 1600°C, by compositional modifications which promote immiscibility in the glass component of the scale. Other candidate materials forming high temperature oxides, such as rare earth compounds, are largely unexplored for high temperature applications and may be attractive candidates for hypersonic TPS materials.

Microcapsule induced toughening in a self-healing polymer composite

Abstract: Microencapsulated dicyclopentadiene (DCPD) healing agent and Grubbs' Ru catalyst are incorporated into an epoxy matrix to produce a polymer composite capable of self-healing. The fracture toughness and healing efficiency of this composite are measured using a tapered double-cantilever beam (TDCB) specimen. Both the virgin and healed fracture toughness depend strongly on the size and concentration of microcapsules added to the epoxy. Fracture of the neat epoxy is brittle, exhibiting a mirror fracture surface. Addition of DCPD-filled urea-formaldehyde (UF) microcapsules yields up to 127% increase in fracture toughness and induces a change in the fracture plane morphology to hackle markings. The fracture toughness of epoxy with embedded microcapsules is much greater than epoxy samples with similar concentrations of silica microspheres or solid UF polymer particles. The increased toughening associated with fluid-filled microcapsules is attributed to increased hackle markings as well as subsurface microcracking not observed for solid particle fillers. Overall the embedded microcapsules provide two independent effects: the increase in virgin fracture toughness from general toughening and the ability to self-heal the virgin fracture event.

A review of anode materials development in solid oxide fuel cells

Abstract: High temperature solid oxide fuel cell (SOFC) has prospect and potential to generate electricity from fossil fuels with high efficiency and very low greenhouse gas emissions as compared to traditional thermal power plants. In the last 10 years, there has been significant progress in the materials development and stack technologies in SOFC. The objective of this paper is to review the development of anode materials in SOFC from the viewpoint of materials microstructure and performance associated with the fabrication and optimization processes. Latest development and achievement in the Ni/Y2O3-ZrO2 (Ni/YSZ) cermet anodes, alternative and conducting oxide anodes and anode-supported substrate materials are presented. Challenges and research trends based on the fundamental understanding of the materials science and engineering for the anode development for the commercially viable SOFC technologies are discussed.

Adhesively-bonded joints and repairs in metallic alloys, polymers and composite materials: Adhesives, adhesion theories and surface pretreatment

Abstract: In the present paper, the following topics are reviewed in detail: (a) the available adhesives, as well as their recent advances, (b) thermodynamic factors affecting the surface pretreatments including adhesion theories, wettability, surface energy, (c) bonding mechanisms in the adhesive joints, (d) surface pretreatment methods for the adhesively bonded joints, and as well as their recent advances, and (e) combined effects of surface pretreatments and environmental conditions on the joint durability and performance. Surface pretreatment is, perhaps, the most important process step governing the quality of an adhesively bonded joint. An adhesive is defined as a polymeric substance with viscoelastic behavior, capable of holding adherends together by surface attachment to produce a joint with a high shear strength. Adhesive bonding is the most suitable method of joining both for metallic and non-metallic structures where strength, stiffness and fatigue life must be maximized at a minimum weight. Polymeric adhesives may be used to join a large variety of materials combinations including metal-metal, metal-plastic, metal-composite, composite-composite, plastic-plastic, metal-ceramic systems. Wetting and adhesion are also studied in some detail in the present paper since the successful surface pretreatments of the adherends for the short- and long-term durability and performance of the adhesive joints mostly depend on these factors. Wetting of the adherends by the adhesive is critical to the formation of secondary bonds in the adsorption theory. It has been theoretically verified that for complete wetting (i.e., for a contact angle θ equal to zero), the surface energy of the adhesive must be lower than the surface energy of the adherend. Therefore, the primary objective of a surface pretreatment is to increase the surface energy of the adherend as much as possible. The influence of surface pretreatment and aging conditions on the short- and long-term strength of adhesive bonds should be taken into account for durability design. Some form of substrate pretreatment is always necessary to achieve a satisfactory level of long-term bond strength. In order to improve the performance of adhesive bonds, the adherends surfaces (i.e., metallic or non-metallic) are generally pretretead using the (a) physical, (b) mechanical, (c) chemical, (d) photochemical, (e) thermal, or (e) plasma method. Almost all pretreatment methods do bring some degree of change in surface roughness but mechanical surface pretreatment such as grit-blasting is usually considered as one of the most effective methods to control the desired level of surface roughness and joint strength. Moreover, the overall effect of mechanical surface treatment is not limited to the removal of contamination or to an increase in surface area. This also relates to changes in the surface chemistry of adherends and to inherent drawbacks of surface roughness, such as void formations and reduced wetting. Suitable surface pretreatment increases the bond strength by altering the substrate surface in a number of ways including (a) increasing surface tension by producing a surface free from contaminants (i.e., surface contamination may cause insufficient wetting by the adhesive in the liquid state for the creating of a durable bond) or removal of the weak cohesion layer or of the pollution present at the surface, (b) increasing surface roughness on changing surface chemistry and producing of a macro/microscopically rough surface, (c) production of a fresh stable oxide layer, and (d) introducing suitable chemical composition of the oxide, and (e) introduction of new or an increased number of chemical functions. All these parameters can contribute to an improvement of the wettability and/or of the adhesive properties of the surface.

Review of recent studies in magnesium matrix composites

Abstract: In this paper, recent progress in magnesium matrix composite technologies is reviewed. The conventional and new processes for the fabrication of magnesium matrix composites are summarized. The composite microstructure is subsequently discussed with respect to grain refinement, reinforcement distribution, and interfacial characteristics. The mechanical properties of the magnesium matrix composites are also reported.

Review on auxetic materials

Abstract: Although a negative Poisson's ratio (that is, a lateral extension in response to stretching) is not forbidden by thermodynamics, for almost all common materials the Poisson's ratio is positive. In 1987, Lakes first discovered negative Poisson's ratio effect in polyurethane (PU) foam with re-entrant structures, which was named anti-rubber, auxetic, and dilatational by later researchers. In this paper, the term 'auxetic' will be used. Since then, investigation on the auxetic materials has held major interest, focusing on finding more materials with negative Poisson's ratio, and on examining the mechanisms, properties and applications. Therefore, more materials were found to have the counter-intuitive effect of auxeticity due to different structural or microstructrual mechanisms. The present article reviews the latest advances in auxetic materials, their structural mechanisms, performance and applications.

Dehydration and rehydration processes of cement paste exposed to high temperature environments

Abstract: Microstructural changes of an OPC cement paste after being exposed at various elevated temperatures and further rehydration have been evaluated using 29Si MAS-NMR. Thermogravimetry and XRD are also employed to complement the information. NMR studies of cement paste exposed to high temperatures demonstrate a progressive transformation of C-S-H gel that leads at 450°C, to a modified C-S-H gel. For temperatures above 200°C to a progressive formation of a new nesosilicate. At 750°C, the transformation of C-S-H is complete into the nesosilicate form with a C2S stoichiometry close to larnite, but less crystalline. Also is observed an increase of portlandite that takes place up to temperatures of 200°C. A progressive increase of calcite formation up to 450°C is noticed. The ettringite disappearance below 100°C is confirmed and the portlandite and calcite are converted to lime at 750°C. The initial anhydrous phases as larnite and brownmillerite remain unaltered during heating. Rehydration of the heated samples (450 and 750°C) shows recrystallization of calcite, portlandite and ettringite, and the C-S-H reformation from the new nesosilicate. The larnite and brownmillerite remain unaltered during rehydration. The developing of damaged due to the formation of microcracking is detected and improved because of rehydration phenomena.

Processing, properties and arc jet oxidation of hafnium diboride/silicon carbide ultra high temperature ceramics

Abstract: The processing and properties of HfB2-20 vol%SiC ultra high temperature ceramics were examined. Dense billets were fabricated by hot-pressing raw powders in a graphite element furnace for 1 h at 2200°C. Specimens were then tested for hardness, mechanical strength, thermal properties and oxidation resistance in a simulated re-entry environment. Thermal conductivity of the current materials was found to be less than previous work had determined while the strength was greater. Oxidation testing of two flat-face models was conducted, at two conditions, for two 10-min durations each. It was concluded that passive oxidation of SiC plays a role in determining the steady-state surface temperatures below 1700°C. Above 1700°C, temperatures are controlled by the properties of a thick HfO2 layer and active oxidation of the SiC phase.

Electrical applications of carbon materials

Abstract: The electrical applications of carbons and their composites are reviewed, with emphasis on applications that are relevant to industrial needs. The applications include electrical conduction, electrical contacts, electrodes, electromagnetic interference shielding, resistance heating, thermoelectricity, sensing, electrical switching, electronic devices and thermal pastes. The carbons include graphite, coke, carbon fibers, carbon filaments, carbon black and flexible graphite.

The hypersonic environment: Required operating conditions and design challenges

Abstract: Hypersonic flight powered by airbreathing engines offers the potential for faster response time at long ranges, and reduced cost for access-to-space. In the present paper the operating environment of typical hypersonic vehicles are discussed, including results for the radiation equilibrium wall temperature of external vehicle surfaces and the flow properties through three sample engines spanning the range of hydrocarbon-fueled Mach 4-8 flight and hydrogen-fueled flight at speeds up to Mach 17. Flow conditions at several locations through the sample engines were calculated to provide indications of the required operating flow environment. Additional system consideration such a seals, joints, vehicle integration and in-service engineering are addressed.

Role of minor alloying additions in formation of bulk metallic glasses: A Review

Abstract: Minor alloying addition or microalloying technology has already shown dramatic effects on glass formation and thermal stability of bulk metallic glasses (BMGs). This paper intends to provide a comprehensive review of recent developments of this technology in the field of BMGs. The beneficial effects of minor alloying additions on the glass formation and the thermal stability of BMGs will be summarized and analyzed. In addition, principles and guidelines for future application of this technology will also be proposed.

Mechanochemical synthesis of nanoparticles

Abstract: The results of recent investigation of the mechanochemical synthesis of inorganic nanoparticles are reviewed. It was demonstrated that, by selecting suitable chemical reaction paths, stoichiometry of starting materials and milling conditions, mechanochemical processing can be used to synthesise a wide range of nanocrystalline particles dispersed within a soluble salt matrix. Selective removal of the matrix phase by washing the resulting powder with appropriate solvents can yield nanoparticles of the desired phase. This technique has been shown to have advantages over other methods of producing nanoparticles in terms of low cost, small particle sizes, low agglomeration, narrow size distributions and uniformity of crystal structure and morphology.

Oxidation of ZrB2- and HfB2-based ultra-high temperature ceramics: Effect of Ta additions

Abstract: Several compositions of ZrB2- and HfB2-based Ultra-High Temperature Ceramics (UHTC) were oxidized in stagnant air at 1627°C in ten minute cycles for times up to 100 min. These compositions include: ZrB2-20 vol% SiC, HfB2-20 vol% SiC, ZrB2-20 vol% SiC-20 vol% TaSi2, ZrB2-33 vol% SiC, HfB2-20 vol% SiC-20 vol% TaSi2, and ZrB2-20 vol% SiC-20 vol% TaC. The weight change due to oxidation was recorded. The ZrB2-20 vol% SiC-20 vol% TaSi2 composition was also oxidized in stagnant air at 1927°C and in an arc jet atmosphere. Samples were analyzed after oxidation by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy to determine the reaction products and to observe the microstructure. The ZrB2-20 vol% SiC-20 vol% TaSi2 showed the lowest oxidation rate at 1627°C, but performed poorly under the more extreme tests due to liquid phase formation. Effects of Ta-additions on the oxidation of the diboride-based UHTC are discussed.

Styrene butadiene styrene polymer modification of road bitumens

Abstract: This paper describes the polymer modification of road bitumens with SBS. Six polymer modified bitumens (PMBs) were produced by mixing bitumen from two crude oil sources with an SBS copolymer at three polymer contents. The rheological characteristics of the SBS PMBs were analysed by means of dynamic mechanical analysis using a dynamic shear rheometer (DSR). The results of the investigation indicate that the degree of SBS modification is a function of bitumen source, bitumen-polymer compatibility and polymer concentration. When the polymer concentration and bitumen-polymer compatibility allow a continuous polymer network to be established, modification is provided by a highly elastic network which increases the viscosity, stiffness and elastic response of the PMB, particularly at high service temperatures. However, ageing of the SBS PMBs tends to result in a reduction of the molecular size of the SBS copolymer with a decrease in the elastic response of the modified road bitumen.

Designing for ultrahigh-temperature applications: The mechanical and thermal properties of HfB2, HfcX, HfNX and αHf(N)

Abstract: The thermal conductivity, thermal expansion, Young's Modulus, flexural strength, and brittle-plastic deformation transition temperature were determined for HfB2, HfC0.98, HfC0.67, and HfN0.92 ceramics. The mechanical behavior of αHf(N) solid solutions was also studied. The thermal conductivity of modified HfB2 exceeded that of the other materials by a factor of 5 at room temperature and by a factor of 2.5 at 820°C. The transition temperature of HfC exhibited a strong stoichiometry dependence, decreasing from 2200°C for HfC0.98 to 1100°C for HfC0.67 ceramics. The transition temperature of HfB2 was 1100°C. Pure HfB2 was found to have a strength of 340 MPa in 4 point bending, that was constant from room temperature to 1600°C, while a HfB2 + 10% HfCx had a higher room temperature bend strength of 440 MPa, but that dropped to 200 MPa at 1600°C. The data generated by this effort was inputted into finite element models to predict material response in internally heated nozzle tests. The theoretical model required accurate material properties, realistic thermal boundary conditions, transient heat transfer analysis, and a good understanding of the displacement constraints. The results of the modeling suggest that HfB2 should survive the high thermal stresses generated during the nozzle test primarily because of its superior thermal conductivity. The comparison the theoretical failure calculations to the observed response in actual test conditions show quite good agreement implying that the behavior of the design is well understood.

Synthesis of nanotube from a layered H2Ti4O9 · H2O in a hydrothermal treatment using various titania sources

Abstract: In this paper, the synthesis of tubular titania was carried out through a soft chemical hydrothermal reaction of TiO2 powders in NaOH or KOH aqueous solution systems. It was found that nanotubular products prepared in our studies were identified as H2Ti4O9·H2O by X-ray diffraction analysis with their morphology and crystallinity being dependent on synthetic conditions, i.e. reaction time and temperature of the hydrothermal process. The photocatalytic activity of nanotubular H2Ti4O9·H2O was evaluated in a decomposition test of HCHO at 298 K in an aqueous system using radiation with a mercury lamp. The morphology and yield of these nanotubular products were found to be dependent on the hydrothermal synthetic conditions.

Polymer stabilized silver nanoparticles: a photochemical synthesis route

Abstract: This study describes a novel and convenient way for the preparation of polymer stabilized colloidal silver by an ultra-violet irradiation technique. Methoxypolyethylene glycol (MPEG) generates free radicals in presence of ultra-violet radiation and acts as the reducing agent towards the silver ion. MPEG also serves as a stabilizer of the silver particles formed.

Bio-composites produced from plant microfiber bundles with a nanometer unit web-like network

Abstract: Using plant microfiber bundles with a nanometer unit web-like network, a moulded product with a bending strength of 250 MPa was obtained without the use of binders. High interactive forces seem to be developed between pulp fibers owing to their nanometer unit web-like network. In other words, the area of possible contact points per fiber are increased, so that more hydrogen bonds might be formed or van der Waals forces increased. When 2% oxidized tapioca starch, by weight, was added, the yield strain doubled and the bending strength reached 310 MPa. The starch mixed moulded product had a similar stress strain curve to that for magnesium alloy, and three to four times higher Young's modulus and bending strength values than polycarbonate and GFRP (chopped). The mouldings have a combination of environmentally friendly and high strength properties.

Adhesion and sliding of wet snow on a super-hydrophobic surface with hydrophilic channels

Abstract: Adhesion and sliding of wet snow on a superhydrophobic surface with hydrophilic channel were investigated. Two different alignment (two dimensional and three dimensional) of the hydrophilic channels in a superhydrophobic surface were prepared and compared with merely a superhydrophobic surface and a hydrophilic surface. Both alignment samples exhibited intermediate level of wet snow adhesion between merely a superhydrophobic surface and a hydrophilic surface. Although the three dimensional sample also showed intermediate level for the wet snow sliding behavior, the two dimensional sample exhibits poorer snow sliding behavior than a superhydrophobic surface. Based on the experiments using a water-hollow glass beads composite, water movement to hydrophilic parts from wet snow occurs on both samples. It is deduced that the poor sliding behavior on the two dimensional sample was due to the increase of viscosity of wet snow on superhydrophobic parts as a result of the water movement to hydrophilic parts.

Carbothermal reduction synthesis of nanocrystalline zirconium carbide and hafnium carbide powders using solution-derived precursors

Abstract: Zirconium carbide (ZrC) and hafnium carbide (HfC) powders were produced by the carbothermal reduction reaction of carbon and the corresponding metal oxide (ZrO2 and HfO2, respectively). Solution-based processing was used to achieve a fine-scale (i.e., nanometer-level) mixing of the reactants. The reactions were substantially completed at relatively low temperatures (<1500°C) and the resulting products had small average crystallite sizes (∼50–130 nm). However, these products contained some dissolved oxygen in the metal carbide lattice and higher temperatures were required to complete the carbothermal reduction reactions. Dry-pressed compacts prepared using ZrC-based powders with ∼100 nm crystallite size could be pressurelessly sintered to ∼99% relative density at 1950°C.

The role of structure on the thermal properties of graphitic foams

Abstract: A high conductivity graphite foam developed at Oak Ridge National Laboratory (ORNL) owes its unique thermal properties to the highly aligned graphitic structure along the cell walls. The material exhibits a peak in thermal conductivity at temperatures similar to that of highly ordered natural graphite, indicating the foam has an extremely graphitic nature. This paper explores the graphitic structure of the foam and attempts to correlate the morphology of the ligaments with the bulk thermal properties, up to 182 W/m·K. First, the manufacturing process of the foam and the resulting material properties are reported. Then, several models for representing the bulk materials properties are reviewed. Examination by optical image analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was used to examine the structure of the graphite foam. In addition, crystallographic structure determined by X-ray diffraction is reported. A simple two parameter model of the morphology was developed and then used to predict the overall thermal properties of the foam based on the assumed highly ordered ligament structure. This new model correlated (within 5%) thermal conductivity to density of several foams, provided the average ligament conductivity could be accurately represented. From the new model and the material characterization data, it was determined that the average ligament thermal conductivity of the foam is >1650 W/m·K at room temperature, and increases to more than 2300 W/m·K at liquid nitrogen temperatures.

Processing and characterization of ZrB2-based ultra-high temperature monolithic and fibrous monolithic ceramics

Abstract: Zirconium diboride (ZrB2) based ultra-high temperature ceramics either unmodified or with SiC particulate additions of 10, 20, or 30 volume percent were prepared by conventional hot pressing. The ZrB2-SiC compositions had improved four-point bend strength compared to the ZrB2 prepared in our laboratory as well as other reported ZrB2 or ZrB2-SiC materials. Strength and toughness increased as the amount of SiC increased. Measured strengths ranged from ∼550 MPa for ZrB2 to over 1000 MPa for ZrB2-30% SiC. Likewise, toughness increased from 3.5 MPa to more than 5 MPa over the same composition range. The addition of SiC also improved oxidation resistance compared to pure ZrB2. Co-extrusion processing was used to produce ZrB2-based ultra-high temperature ceramics with a fibrous monolithic structure. Samples had dense ZrB2-30 vol% SiC cells approximately 100 μm in diameter surrounded by porous ZrB2 cell boundaries approximately 20 μm thick. ZrB2-based fibrous monoliths had four point bend strength of ∼450 MPa, about half of a conventional ZrB2-SiC ceramic with the cell composition. Preliminary analysis of fracture behavior found that ZrB2-based fibrous monoliths did not exhibit graceful failure because the difference in strength between the cell and cell boundary of the current materials was not sufficient.

Synthetic routes, properties and future applications of polymer-layered silicate nanocomposites

Abstract: This paper focuses on polymer nanocomposites and their syntheses, properties and future applications, several of these application will be successful in the near future. This new type of materials, based on smectite clays usually rendered hydrophobic through ionic exchange of the sodium interlayer cation with an onium cation, may be prepared via various synthetic routes comprising exfoliation adsorption, in-situ intercalative polymerization and melt intercalation. The whole range of polymer matrices covered, i.e., thermoplastics, thermosets and elastomers. Small addition—typically less than 6 wt%—of these nanoscale inorganic fillers promote concurrently several properties of the polymer materials, including tensile characteristics, heat distortion temperature, scratch resistance, gas permeability resistance, and flame retardancy.

Hot Isostatic Pressing (HIP) technology and its applications to metals and ceramics

Abstract: This review examines some of the components of this increasingly exploited technology as well as the application of which will surely increase as a result of constant development in equipment design and extensive research in the field of ceramic and metal materials in general for the production of fully dense and reliable parts. Newly developed high temperature HIP equipment can offer potential improvements to material properties relative to more conventional techniques as a possible solution to the manufacture of ceramic and metal components for airframe and structural components where critical and highly stressed applications are required. By the use the near net shape techniques, exotic materials can be used more cost effectively than machining from solid. Designers and manufacturers alike can make better products by introducing HIP to their production route.

Electrophoretic deposition—mechanisms, myths and materials

Abstract: Explanations of the deposition process during electrophoretic deposition (EPD) are presented and their boundary conditions discussed. It is suggested increasing resistance during EPD is due to the deposit and not dilution of current carrying species in the suspension. Dialysis membrane experiments demonstrate ions carry significant current. Side-effects of two suspension-conditioning agents are described, i.e., TMAH and PEI. The former can induce “aging” in suspension as its surface adsorption varies with time and reduces suspension pH. PEI appears to adsorb on all ceramic and metal powders, so may be a universal EPD agent for stoichiometric deposition of ceramic/ceramic and ceramic/metal powder-mixtures. Novel structures produced by EPD are presented.

Density analysis of direct metal laser re-melted 316L stainless steel cubic primitives

Abstract: This paper reports on the density of cubic primitives made by Direct Metal Laser Re-Melting, a process variant of selective laser sintering (SLS). Here, stainless steel 316L powder fractions are scanned and fused by a 90 W Nd:YAG laser in consecutive 100 μm layers in order to build a 3-Dimensional object. The effects of Q-Switch pulsing frequency, scanning speed and scan spacing on sample density are described. The samples are measured by two methods: a weight/volume analysis and a xylene impregnation technique. The results are supported by microscopy analysis for qualitative arguments. The results show the significant influence of pulsing the beam on the density of the fabricated material. Also reported is the relationship of material density with energy density (as a function of the process parameters; power, scan speed and scan spacing). Optical analysis of material cross sections shows a periodic occurrence of porosity across the whole range of samples. Causes for this are discussed.