Showing papers in "Journal of Materials Research in 1998"

Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants

Abstract: This paper reviews the past, present, and future of the hydroxyapatite (HAp)-based biomaterials from the point of view of preparation of hard tissue replacement implants. Properties of the hard tissues are also described. The mechanical reliability of the pure HAp ceramics is low, therefore it cannot be used as artificial teeth or bones. For these reasons, various HAp-based composites have been fabricated, but only the HAp-coated titanium alloys have found wide application. Among the others, the microstructurally controlled HAp ceramics such as fibers/whiskers-reinforced HAp, fibrous HAp-reinforced polymers, or biomimetically fabricated HAp/collagen composites seem to be the most suitable ceramic materials for the future hard tissue replacement implants.

Influences of pileup on the measurement of mechanical properties by load and depth sensing indentation techniques

Abstract: Finite element simulation of conical indentation of a wide variety of elastic-plastic materials has been used to investigate the influences of pileup on the accuracy with which hardness and elastic modulus can be measured by load and depth-sensing indentation techniques. The key parameter in the investigation is the contact area, which can be determined from the finite element results either by applying standard analysis procedures to the simulated indentation load-displacement data, as would be done in an experiment, or more directly, by examination of the contact profiles in the finite element mesh. Depending on the pileup behavior of the material, these two areas may be very different. When pileup is large, the areas deduced from analyses of the load-displacement curves underestimate the true contact areas by as much as 60%. This, in turn, leads to overestimations of the hardness and elastic modulus. The conditions under which the errors are significant are identified, and it is shown how parameters measured from the indentation load-displacement data can be used to identify when pileup is an important factor.

Radiation effects in crystalline ceramics for the immobilization of high-level nuclear waste and plutonium

Abstract: This review provides a comprehensive evaluation of the state-of-knowledge of radiation effects in crystalline ceramics that may be used for the immobilization of high-level nuclear waste and plutonium. The current understanding of radiation damage processes, defect generation, microstructure development, theoretical methods, and experimental methods are reviewed. Fundamental scientific and technological issues that offer opportunities for research are identified. The most important issue is the need for an understanding of the radiation-induced structural changes at the atomic, microscopic, and macroscopic levels, and the effect of these changes on the release rates of radionuclides during corrosion.

Determination of indenter tip geometry and indentation contact area for depth-sensing indentation experiments

Abstract: The phenomena of pile-up and sink-in associated with nanoindentation have been found to have large effects on the measurements of the indentation modulus and hardness of copper. Pile-up (or sink-in) leads to contact areas that are greater than (or less than) the cross-sectional area of the indenter at a given depth. These effects lead to errors in the absolute measurement of mechanical properties by nanoindentation. To account for these effects, a new method of indenter tip shape calibration has been developed; it is based on measurements of contact compliance as well as direct SEM observations and measurements of the areas of large indentations. Application of this calibration technique to strain-hardened (pile-up) and annealed (sink-in) copper leads to a unique tip shape calibration for the diamond indenter itself, as well as to a material parameter, a, which characterizes the extent of pile-up or sink-in. Thus the shape of the indenter tip and nature of the material response are separated in this calibration method. Using this approach, it is possible to make accurate absolute measurements of hardness and indentation modulus by nanoindentation.

Evaluation of Young's Modulus of Carbon Nanotubes by Micro-Raman Spectroscopy

Abstract: Micro-Raman spectroscopy is used to monitor the cooling-induced compressive deformation of carbon nanotubes embedded in an epoxy matrix. Young’s modulus of single- and multiwall nanotubes may then be derived from a concentric cylinder model for thermal stresses, using the D*-band shift for each tube type. The resulting values of the elastic moduli are in very good agreement with predicted theoretical values, and with the published experimental data set of Treacy et al., Nature (London) 381, 678 (1996).

Processing of Carbon Nanotube Reinforced Aluminum Composite

Abstract: Carbon nanotube reinforced aluminum (Al) composites were produced by hot-press and hot-extrusion methods. The interfacial structure between the carbon nanotube and Al was examined using a transmission electron microscope (TEM), and the mechanical properties were measured by a tensile test. TEM observations have shown that the nanotubes in the composites are not damaged during the composite preparation and that no reaction products at the nanotube/Al interface are visible after annealing for 24 h at 983 K. The strength of the composites is only slightly affected by the annealing time at 873 K, while that of the pure Al produced in a similar powder metallurgy process significantly decreases with time. These studies are considered to yield experimental information valuable for producing high performance composites.

Chemical Attachment of Organic Functional Groups to Single-walled Carbon Nanotube Material

Abstract: We have subjected single-walled carbon nanotube materials (SWNTM’s) to a variety of organic functionalization reactions. These reactions include radioactive photolabeling studies using diradical and nitrene sources, and treatment with dichlorocarbene and Birch reduction conditions. All of the reactions provide evidence for chemical attachment to the SWNTM’s, but because of the impure nature of the staring materials, we are unable to ascertain the site of reaction. In the case of dichlorocarbene we are able to show the presence of chlorine in the SWNT bundles, but as a result of the large amount of amorphous carbon that is attached to the tube walls, we cannot distinguish between attachment of dichlorocarbene to the walls of the SWNT’s and reaction with the amorphous carbon.

Quantitative analysis of strengthening mechanisms in thin Cu films: Effects of film thickness, grain size, and passivation

Abstract: Thermal stresses in thin Cu films on silicon substrates were examined as a function of film thickness and presence of a silicon nitride passivation layer. At room temperature, tensile stresses increased with decreasing film thickness in qualitative agreement with a dislocation constraint model. However, in order to predict the stress levels, grain-size strengthening, which is shown to follow a Hall–Petch relation, must be superimposed. An alternative explanation is strain-hardening due to the increase in dislocation density, which was measured by x-ray diffraction. At 600 °C, the passivation increases the stress by an order of magnitude; this leads to a substantially different shape of the stress-temperature curves, which now resemble those of aluminum with only a native oxide layer. The effect of passivation is shown to be very sensitive to the deposition and test conditions.

Nanocrystalline nickel and nickel-copper alloys: Synthesis, characterization, and thermal stability

Abstract: Pulsed electrodeposition is a simple, yet versatile method for the production of nanostructured metals. For n-nickel we determine the influence of the physical and chemical deposition parameters on the nanostructure of the deposits and demonstrate that the grain size can be tuned to values between 13 and 93 nm, with rather narrow grain size distribution. The thermal stability of our n-nickel as studied by x-ray diffraction and differential thermal analysis exhibits no detectable grain growth up to temperatures of about 380 K and an initial $$\sqrt t $$ behavior at 503 K followed by a regime of anomalous grain growth. For nanocrystalline Ni1-x Cux (Monel-metal™) we demonstrate that alloy formation occurs at room temperature and that both chemical composition and grain size can be controlled by the pulse parameters and by appropriate organic additives.

The role of microstructure and processing on the proton conducting properties of gadolinium-doped barium cerate

Abstract: The influence of grain boundary conductivity and microstructure on the electrical properties of BaCe0.85Gd0.15O3–δ have been examined. Grain sizes were varied by sintering at various temperatures. Impedance data were analyzed using the brick layer model, and some new consequences of this model are presented. The specific grain boundary conductivity exhibits an activation energy of ~0.7 eV, and for similar processing routes, is independent of grain size. An isotope effect was observed, indicating that protons (or deuterons) are the mobile species. TEM investigations showed the intergranular regions to be free of any glassy phase that could account for the differences in bulk and grain boundary properties. Single-crystal fibers, grown by a modified float zone process, were notably barium deficient, and exhibited a low conductivity, comparable to that of polycrystalline Ba0.96Ce0.85Gd0.15O3–δ.

On the design of 1-3 piezocomposites using topology optimization

Abstract: We use a topology optimization method to design 1–3 piezocomposites with optimal performance characteristics for hydrophone applications. The performance characteristics we focus on are the hydrostatic charge coefficient , the hydrophone figure of merit , and the electromechanical coupling factor . The piezocomposite consists of piezoelectric rods embedded in an optimal polymer matrix. We use the topology optimization method to design the optimal (porous) matrix microstructure. When we design for maximum and , the optimal transversally isotopic matrix material has negative Poisson's ratio in certain directions. When we design for maximum , the optimal matrix microstructure is layered and simple to build.

Bismuth quantum-wire arrays fabricated by a vacuum melting and pressure injection process

Abstract: Ultrafine bismuth nanowire arrays were synthesized by injecting its liquid melt into nanochannels of a porous anodic alumina template. A large area (1 cm × 1.5 cm) of parallel wires with diameters as small as 13 nm, lengths of 30–50 μm, and packing density as high as 7.1 × 1010 cm−2 has been fabricated. X-ray diffraction patterns revealed these nanowires, embedded in the insulating matrix, to be essentially single crystalline and highly oriented. The optical absorption spectra of the nanowire arrays indicate that these bismuth nanowires undergo a semimetal-to-semiconductor transition due to two-dimensional quantum confinement effects.

Melt infiltration casting of bulk metallic-glass matrix composites

Abstract: The authors describe a technique for melt infiltration casting of composites with a metallic-glass matrix. We made rods 5 cm in length and 7 mm in diameter. The samples were reinforced by continuous metal wires, tungsten powder, or silicon carbide particulate preforms. The most easily processed composites were those reinforced with tungsten and carbon steel continuous wire reinforcement. The Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 matrix was quenched to a glass after infiltrating the reinforcement. We analyzed the microstructure of the composites by x-ray diffraction and scanning electron microscopy. The measured porosity was less than 3% and the matrix was about 97% amorphous material.

Wetting and interface microstructure between Sn–Zn binary alloys and Cu

Abstract: Sn–Zn binary alloys have been examined as a lead-free solder. Zn distributes in a Sn matrix as platelets. The hypoeutectic alloys show two endothermic peaks in DTA, which correspond to the eutectic and the liquidus temperatures. Three reaction layers are formed at the Sn–Zn/Cu interface without containing Sn: the thick γ–Cu5Zn8 adjacent to the solder, the thin β′–CuZn in the middle, and the thinnest layer adjacent to Cu. Although many nonwetting regions and voids are formed at the interface because of poor wetting, soldering at 290 °C can form a rigid interface, and tensile strength reaches about 40 MPa.

Monodisperse magnetic nanoparticles: preparation and dispersion in water and oils

Abstract: Nanometric maghemite and cobalt ferrite particles are chemically synthesized. The process produces particles polydisperse in size. The positive charges of their surface allow one to disperse them in aqueous acidic solutions and to obtain dispersions stabilized through electrostatic repulsions. Increasing acid concentration (in the range 0.1 to 0.5 mol.L−1), interparticles repulsions are screened and phase transitions are induced. Using this phenomenon, we describe a two-step size sorting process, in order to get significant amounts of nanometric monosized particles (with diameters monitored between typically 6 and 13 nm). As the surface of the latter is not modified by the size sorting process, usual procedures are used to disperse them in several aqueous or oily media.

Importance of soft solution processing for advanced inorganic materials

Abstract: Based upon the analysis of materials cycling and processing on the earth, a thermodynamic concept for energetical and environmental problems has been proposed. It concludes that solution processing using aqueous solutions should be the most important processing even for advanced materials. According to this concept, energetical and environmental features of soft solution processing (SSP) are discussed in general, using also some particular examples, such as BaTiO3. Applications of the SSP are shown with special emphasis on hydrothermal and/or electrochemical synthesis of thin films and integration issues. Soft solution processing allows one to fabricate in aqueous solutions shaped/sized/oriented ceramics in only one step, without excess energies for firing/sintering or melting and without expensive equipment, providing an environmentally friendly route for the preparation of advanced ceramic materials.

Optical Properties of MS2 (M = Mo, W) Inorganic Fullerenelike and Nanotube Material Optical Absorption and Resonance Raman Measurements

Abstract: The optical properties of inorganic fullerene-like and nanotube MS2 (M = Mo, W) material are studied through absorption and resonance Raman, and compared to those of the corresponding bulk material The absorption measurements show that the semiconductivity is preserved Nevertheless, the positions of the excitons are altered in comparison to the bulk The Raman spectra of the nanoparticles show a close correspondence to that of the bulk However, the first-order peaks are broadened and, under resonance conditions, new peaks are observed The new peaks are assigned to disorder-induced zone edge phonons

Correlation Between Anatase-to-rutile Transformation and Grain Growth in Nanocrystalline Titania Powders

Abstract: During the heat treatment of an anatase nanocrystalline powder at a high temperature, the two processes of anatase-to-rutile (A → R) transformation and grain growth would occur simultaneously and affect each other. With decrease of the original anatase grain size, the A → R transformation temperature range became extended on both sides, which may be partially attributed to the prevention effect of grain growth on this transformation. On the other hand, the grain growth process could be significantly enhanced by the A → R transformation, which can be ascribed to the higher atomic mobility because of the bond breakage during the transformation.

Preparation of β-SiC nanorods with and without amorphous SiO 2 wrapping layers

Abstract: Preparation of β–SiC nanorods with and without amorphous SiO2 wrapping layers was achieved by carbothermal reduction of sol-gel derived silica xerogels containing carbon nanoparticles. The β–SiC nanorods with amorphous SiO2 wrapping layers were obtained by carboreduction at 1650 °C for 1.5 h, and at the end of 1.5 h the temperature was steeply raised to 1800 °C and held for 30 min; they are typically up to 20 µm in length. The diameters of the center thinner β–SiC nanorods within the amorphous SiO2 wrapping layers are in the range 10–30 nm, while the outer diameters of the corresponding amorphous SiO2 wrapping layers are between 20 and 70 nm. The β–SiC nanorods without amorphous SiO2 wrapping layers were produced by carbothermal reduction only at 1650 °C for 2.5 h, and their diameters are in agreement with those of the center thinner β–SiC nanorods wrapped in amorphous SiO2 layers. Large quantities of SiC rod nuclei and the nanometer-sized nucleus sites on carbon nanoparticles are both favorable to the formation of much thinner β–SiC nanorods. The formation of the outer amorphous SiO2 wrapping layer is from the combination reaction of decomposed SiO vapor and O2.

Observations of Grain Boundary Structure in Submicrometer-Grained Cu and Ni Using High-Resolution Electron Microscopy

Abstract: Submicrometer-grained (SMG) structures were produced in Cu and Ni using an intense plastic straining technique, and the grain boundaries and their vicinities were observed by high-resolution electron microscopy. The grain boundaries exhibited zigzag configurations with irregular arrangements of facets and steps, and thus they were found to be in a high-energy nonequilibrium state. A similar conclusion was reached earlier for SMG Al–Mg solid solution alloys which have much lower melting points than Cu and Ni, suggesting that nonequilibrium grain boundaries are a typical feature of metals processed by intense plastic straining.

Micro-Raman Spectroscopic Characterization of Nanosized TiO2 Powders Prepared by Vapor Hydrolysis

Abstract: Micro-Raman analysis was used to study the structure of TiO2 powders produced at low (260 °C) and high (600–900 °C) temperatures by vapor hydrolysis of titanium tetraisopropoxide (TTIP). Spatial inhomogeneity was discovered after the amorphous TiO2 powders produced at low temperature were calcined at 700, 800, and 900 °C for 3 h. The TiO2 powders produced at high temperatures (from 600 to 900 °C) were found to be spatially homogeneous and predominately anatase in structure. Small amounts of rutile and brookite are found for powders produced at 700, 800, and 900 °C after calcination at 600 °C for 3 h. The rutile and brookite impurities are believed to be concentrated on the surface of anatase based on a comparison of results of Raman and x-ray diffraction studies.

High-temperature Creep Resistance in Rare-earth-doped, Fine-grained Al2O3

Abstract: High-temperature creep in undoped Al2O3 and La2O3- or Y2O3- or Lu2O3-doped Al2O3 with a grain size of about 1 µm is examined in uniaxial compression testing at temperatures between 1150 and 1350 °C. The high-temperature creep resistance in Al2O3 is highly improved by the rare-earth oxide doping in the level of 0.045 mol %, and the creep rate is suppressed in the order La2O3

Porous NiTi alloy prepared from elemental powder sintering

Abstract: An elemental powder sintering (EPS) technique has been developed for the synthesis of porous NiTi alloy, in which Ni and Ti powders are used as the reactants and TiH2 powder is added as a pore-forming agent and active agent. Effects of various experimental parameters (sintering temperature, sintering time, and TiH2 content) on the porosity, pore size, and pore distribution as well as phase composition in experimental alloys are investigated. It is found that in order to avoid the formation of carcinogenic pure Ni phase, the porous NiTi alloy should be synthesized over a temperature of 1223 K. This gives NiTi as the main phase without any elemental phase. Substitution of Ti by TiH2 is more economic and more favorable to obtain homogeneous porous NiTi alloy. A proper selection of initial powders, ball-milling, pressing, and sintering process makes it possible to achieve the porous NiTi alloy with desired properties.

Structure and oxidation patterns of carbon nanotubes

Abstract: We discuss the oxidation of carbon nanotubes and how it is affected by structure and geometry. While graphite is known to oxidize primarily at defects to create etch pits, nanotubes have additional structural features such as high curvature, helicity, and contain five and seven membered rings which modify the initiation and propagation of oxidation. Oxidation does not necessarily start at the tip of the tubes, and there are pronounced differential oxidation rates between layers which depend on the helicity of the individual shells.

Auto ignition synthesis and consolidation of Al2O3–ZrO2 nano/nano composite powders

Abstract: An “Auto Ignition” technique was utilized in synthesizing Al2O3 –ZrO2 powders with nano/nano microstructure. The process used the corresponding nitrates as oxidizers and urea as the fuel. The as-synthesized powders were characterized by x-ray diffraction and transmission electron microscopy. It was observed that the microstructure consisted of crystallites of Al2O3 and ZrO2, both of which were nanocrystalline. As opposed to the other nanocomposite ceramics, this feature of the microstructure classifies the present powders as nano/nano type. This nanocrystallinity of the microstructure (crystallite size less than 100 nm) was maintained even after a soaking at 1200 °C for 2 h. Since the microstructure is stable at high temperatures, it was possible to densify the powders by hot isostatic pressing at 1200 °C. The product was 99% of the theoretical density and maintained nanocrystalline grain size. The average hardness and toughness values, as determined by an indentation technique, were 4.45 GPa and 8.38 MPa · m1/2, respectively. These values represent evidence of ductility in these composites since transformation toughening was ruled out in this case. The potential application of these results is expected to be in net shape deformation forming of ceramics.

Further analysis of indentation loading curves: Effects of tip rounding on mechanical property measurements

Abstract: The effects of indenter tip rounding on the shape of indentation loading curves have been analyzed using dimensional and finite element analysis for conical indentation in elastic-perfectly plastic solids. A method for obtaining mechanical properties from indentation loading curves is then proposed. The validity of this method is examined using finite element analysis. Finally, the method is used to determine the yield strength of several materials for which the indentation loading curves are available in the literature.

Cadmium-telluride—Material for thin film solar cells

Abstract: Due to its basic optical, electronic, and chemical properties, CdTe can become the base material for high-efficiency, low-cost thin film solar cells using robust, high-throughput manufacturing techniques. CdTe films suited for photovoltaic energy conversion have been produced by nine different processes. Using n-type CdS as a window-partner, solar cells of up to 16% efficiency have been made in the laboratory. Presently five industrial enterprises are striving to master low cost production processes and integrated modules have been delivered in sizes up to 60 × 120 cm2, showing efficiencies up to 9%. Stability, health, and environmental issues will not limit the commercial potential of the final product. The technology shows high promise for achieving cost levels of $0.5/Wp at 15% efficiency. In order to achieve this goal, scientists will have to develop a more detailed understanding of defect chemistry and device operation of cells, and engineers will have to develop methods for high-throughput manufacturing.

Resonant Raman Effect in Single-wall Carbon Nanotubes

Abstract: A resonant Raman study of single-wall carbon nanotubes (SWNT) using several laser lines between 0.94 and 3.05 eV is presented. A detailed lineshape analysis shows that the bands associated with the nanotube radial breathing mode are composed of a sum of individual peaks whose relative intensities depend strongly on the laser energy, in agreement with prior work. On the other hand, the shape of the Raman bands associated with the tangential C–C stretching motions in the 1500–1600 cm−1 range does not depend significantly on the laser energy for laser excitation energies in the ranges 0.94–1.59 eV and 2.41–3.05 eV. However, new C–C stretching modes are observed in the spectra collected using laser excitations with energies close to 1.9 eV. The new results are discussed in terms of the difference between the 1D electronic density of states for the semiconducting and metallic carbon nanotubes.

Continuous measurements of load-penetration curves with spherical microindenters and the estimation of mechanical properties

Abstract: Elastic and plastic properties of metals and Young's modulus of ceramics are determined in the microindentation regime by continuous measurements of load versus depth of penetration with spherical indenters. Calibration procedures, usually applied in nanoindentation experiments, are not needed in the microregime where spherical indenters (rather than sharp indenters with microscopical spherical tips) can be manufactured. As indenters of larger diameters are used, the elastic response of the specimen can be probed during the loading stage of the indentation tests (and not only during unloading, as is the case with nanoindenters). Hence, an accurate determination of Young's modulus can be achieved without a prior knowledge of possible “piling up” or “sinking in” which may occur at the perimeter of the contact area. The contact response of materials is shown to undergo four distinct regions: (i) pre-Hertzian regime, (ii) Hertzian regime, (iii) small-scale plasticity, and (iv) large-scale plasticity. A general methodology for estimation of yield strength and hardening exponent of metals is proposed in the last regime.

Stresses during thermoset cure

Abstract: Production problems attributed to excessive stresses generated during the cure of epoxies led us to develop a formalism to predict these stresses. In our first studies, we developed a fundamental understanding of the complex evolution of viscoelasticity as the cure progresses. We then incorporated these results into a proper tensorial constitutive equation that was integrated into our finite element codes and validated using more complicated geometries, thermal histories, and strain profiles. The formalism was then applied to the original production problem to determine cure schedules that would minimize stress generation during cure. During the pursuit of these activities, several interesting and puzzling phenomena were discovered that have stimulated further investigation.