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A low cost and scalable powder-based method for producing nanoporous Ni is demonstrated, and the only processing byproduct is water vapor. This is achieved by creating a metal-matrix composite of Ni and NiO that is then reduced under dilute hydrogen to create pure Ni with nanocrystalline structure and nanoscale porosity. The technique is demonstrated in loose powder and in thin and bulk compacts with centimeter dimensions. These compacts can withstand extensive deformation without disintegrating, performing similarly to wrought Ni in specific strength. The unique processing strategy and resulting material can be readily extended to other metals and alloys. Accepted version

Carbon nanomaterials are consistently providing new excitement over their properties and potential applications, but many of these material have yet to fully live up to their expectations commercially. The barrier to adoption often exists as a result of complex processing, fragility of the as-produced material, or difficulty scaling beyond laboratory quantities. This work provides a new approach for utilizing fibrous carbon nanomaterials to advance the technology toward new applications and industrial utility. This is accomplished by creating tailored device architectures through in-situ integration of activated carbon powder using carbon nanofiber deposition. The resulting hybrid materials and components can serve in diverse applications, with each instance able to be fine-tuned through a combination of processing parameters. The applications of such materials are anticipated to directly serve current carbon-based technology in filtration, energy storage and delivery, and thermal management, but the concepts are not limited to current carbon applications.

Mechanical alloying has been repeatedly demonstrated as an effective means to create unique structural materials, but it is comparatively rare that functional alloys are produced. Multi-phase functional alloy production is demonstrated with an oxide-dispersed Ni–Cu catalyst. These catalyst particles are designed to form porosity and catalyze carbon nanofiber deposition in situ, which results in a six-fold increase in deposition rate over traditional preparation methods. This technique serves as a template for many other systems possible with MA.

Surface oxidation of metal powders is a common challenge in powder metallurgy processing, and the oxides must eventually be broken up to produce high-quality, structural components. Here it is proposed that surface oxidation may be beneficial in the creation of porous metals through intraparticle expansion, which creates porosity by the reduction of oxides which are distributed within a metallic matrix. Previous work utilized the addition of separate oxides to the metal as a two-component (Cu powder + CuO powder) cryogenic ball milling process. The feasibility of controlling pore formation by directly oxidizing copper powder before milling is examined, and it is then extended to room temperature processing. Both pore distribution and morphology vary greatly with oxygen content, which can be controlled during processing. The pore volume and interconnectedness are maximized at intermediate oxygen concentration, indicating competing effects of the oxide loading. When room temperature high-energy ball milling is applied, the oxidized powder was found to exhibit even greater expansion upon annealing and larger pore size, though cold welding of the copper was encountered at short milling times. Using a single-component method at room temperature may prove valuable to reduce the cost and complexity of large scale production of expandable powder feedstock, and key considerations are discussed.

By leveraging decades of research on the catalytic synthesis of carbon nanofibers and combining it with unique processing methodology, low density monolithic carbon can be created. The process is simple, scalable and controllable. Analysis of deposition kinetics reveals that the final density of centimeter-scale carbon monoliths can be determined through basic process parameters, and the synthesis can be performed with a single step in as little as 1 h. In general, this material can be created over a large range of density (20–700 mg/cc), possesses high surface area (∼215 m 2 /g), is comprised of a mixture of straight and twisted carbon nanofibers (∼150–500 nm in diameter) with low crystallinity, which results in low electrical conductivity (1.32–2.83 S/m) and low thermal conductivity (∼0.03 W/mK), even at the highest densities. The materials are mechanically robust, with density-dependent elasticity values of 60–600 kPa. The methods and properties are discussed in context with other low-density carbon materials. The principal advantages of the process used in this work include the simplicity, direct control of density, and most notably, the ability to create this material with specific bulk geometry.

Predicting grain growth in nanocrystalline materials requires modeling approaches that incorporate grain boundary thermodynamics and kinetics. In this work, the thermokinetic model of Chen et al. (2012) for grain growth was applied to experimental X-ray diffraction measurements from a binary nanocrystalline alloy in an effort (1) to understand the influence of thermodynamic, kinetic, and material parameters in the model; and (2) to extend the thermokinetic model by incorporating temperature dependence. The model performs well for the grain boundary saturated case in the binary nanocrystalline alloy, where it is assumed that solute segregates to the grain boundaries and thermodynamically/kinetically reduces the driving force for grain growth. In this work, a sensitivity analysis of parameters (Monte Carlo global sensitivity analysis) identifies the important thermodynamic/kinetic parameters and their correlation with one another for the present model. This model was then extended to include the change in these independent thermodynamic/kinetic parameters as a function of temperature and to model the effect of initial grain size distribution. This research shows that the thermodynamic and kinetic contributions can describe grain growth in nanocrystalline materials and this extended model can be parameterized for grain size evolution and stabilization with temperature for nanocrystalline systems.

Microscale and nanoscale CuO was dispersed in Cu using room temperature high-energy ball milling over time intervals of 5, 30, 60, 120, and 240 min. These samples were then annealed under a reducing atmosphere for 1 h at temperatures of 400, 600, 800 and 1000 °C to create porosity by the reduction of the entrained oxides. Increases in porosity exceeding 40% were achieved using intermediate milling times and annealing temperatures. When considered cumulatively, the most effective processing conditions were a milling time of 30 min and expansion at 800 °C, but variations exist within each sample type. The complex relationship between milling time and annealing temperature is investigated in terms of particle size, morphology and microstructure. The findings indicate that room temperature milling is more efficient at producing porosity than comparable cryogenic methods, and this may enable industrial scaling of the process.

Porous metals can be created through a wide variety of processing techniques, and the pore morphology resulting from these processes is equally diverse. The structural and functional properties of metal foams are directly dependent on the size, shape, interconnectedness and volume fraction of pores, so accurately quantifying the pore characteristics is of great importance. Methods for analyzing porous materials are presented here and applied to a copper-based metallic foam generated through solid state foaming via oxide reduction and expansion. This process results in large voids (10s of microns) between sintered particles and small pores (10 microns to less than 50 nm) within particles. Optical and electron microscopy were used to image the porosity over this wide range, and the pore characteristics were quantified using image segmentation and statistical analysis. Two-dimensional pore analysis was performed using the Chan-Vese method, and two-point correlation and lineal path functions were used to assess three-dimensional reconstructions from FIB tomography. Two-dimensional analysis reveals distinct size and morphological differences in porosity between particles and within them. Three-dimensional analysis adds further information on the high level interconnectedness of the porosity and irregular shape it takes, forming tortuous pathways rather than spherical cells. Mechanical polishing and optical microscopy allow large areas to be created and analyzed quickly, but methods such as focused ion beam (FIB) sectioning can provide additional insight about microstructural features. In particular, after FIB milling is used to create a flat surface, that surface can be analyzed for structural and compositional information.

In binder jetting additive manufacturing (BJAM), the part geometry is generated via a binding agent during printing and structural integrity is imparted during sintering at a later stage. This separation between shape generation and thermal processing allows the sintering process to be uniquely controlled and the final microstructural characteristics to be tailored. The separation between the printing and consolidation steps offers a unique opportunity to print responsive materials that are later “activated” by temperature and/or environment. This may allow a new paradigm in multi-scale, multifunctional materials. This concept is preliminarily demonstrated using a foaming copper feedstock, such that the copper is printed, sintered and then foamed via intraparticle expansion in separate steps. The integration of foaming feedstock in BJAM could allow for creation of ultra-lightweight structures that offer hierarchical porosity, graded density, and/or tailored absorption properties. This work investigates processing protocol for copper foam structures to achieve the highest porosity. The copper feedstock was prepared by distributing copper oxides through the copper matrix via mechanical milling, and that powder was then printed into a green geometry through BJAM. The printed green parts were then heat treated using different thermal cycles to investigate the porosity evolution relative to various heating conditions. The heat treated parts were then examined for their resulting properties including porosity, microstructural evolution, and volumetric shrinkage. Parts that were initially sintered in air and then annealed in a hydrogen atmosphere led to higher porosity compared to those sintered in hydrogen alone. It was also found that the annealing of parts at 600 °C for 2 h resulted in the highest final porosity (59%) and the lowest volumetric shrinkage of 5%. Anisotropy in linear shrinkage in X, Y, and Z direction was also observed in the heat treated parts with the largest linear shrinkage occurring in the Z direction.

Mechanical alloying (MA) has been and continues to be thoroughly examined for creating structural materials, but the production of catalysts is relatively rare. This is especially true for catalysts used in the production of carbon nanofibers (CNFs), a versatile material for applications such as energy storage, catalyst support, advanced composites and others. The application of MA to create CNFs presents a valuable tool in reducing their cost and complexity, and thereby may increase their commercial potential. In this study, the effects of milling duration on CNF deposition are studied by the complementary methods of X-ray diffraction, compositional mapping, electron microscopy, particle size analysis and surface area analysis. These were used to determine microstructural and macroscale evolution of the catalyst powder and its effects on the kinetics and characteristics of carbon deposition using Ni and Ni 30 at % Cu. The results have important implications for low cost catalyst production and provide general guidance on the development of catalytic materials in miscible systems.

Pore expansion in solid metals is typically driven by an entrapped gas phase, such that temperature directly determines both the pressure within the pores and the ability of the matrix to plastically deform. This approach fundamentally limits the total porosity, as all pores are active simultaneously, which quickly results in coalescence and percolation. The method introduced here converts dispersed oxide particles to a gaseous reactant using reduction, such that independent stages of pore formation and expansion can be achieved by using more than one oxide chemistry. This is demonstrated using silver and copper oxides distributed within a silver matrix. As the temperature is raised, the silver oxide reduces first and creates porosity. As the temperature is raised further, the copper oxide reduces and creates additional porosity. This allows the pore morphology and grain size to be uniquely controlled while still maintaining the simplicity and scalability of the process. The microstructural development is studied through a combination of isothermal annealing, optical dilatometry, and focused ion beam cross-sectioning, and implications and strategies for other alloy systems are discussed. Keywords: Porous metals, Solid state foaming, Mechanical alloying, Dilatometry, Microstructure, Grain boundary pinning

In this study, we demonstrate the feasibility of forming a bulk consolidated, low-density high-entropy alloy, namely AlFeMgTiZn, which shows reasonable mechanical properties and high hardness. The fabrication of the high-entropy alloy from powdered precursors via high-energy mechanical alloying as a function of milling time is presented. In turn, the evolution of the alloy microstructure with postmilling anneal treatment is elucidated. Last, the severe plastic deformation processing methodology, i.e., equal-channel angular extrusion, chosen for consolidation, is described and shown to result in a bulk product with good results.

Entangled networks of carbon nanofibers are characterized both mechanically and electrically. Results for both tensile and compressive loadings of the entangled networks are presented for various densities. Mechanically, the nanofiber ensembles follow the micromechanical model originally proposed by van Wyk nearly 70 years ago. Interpretations are given on the mechanisms occurring during loading and unloading of the carbon nanofiber components.

Cryogenic high energy ball milling has been used to synthesize nanocrystalline Cu–14Al, Cu–12Al–2Zr and Cu–12Al–2Y alloys by mechanical alloying. The alloys were studied with the aim of comparing the effect of substituting Y and Zr in place of Al, in Cu–Al alloys, on the grain size stability at elevated temperatures. The as-milled alloys were subjected to annealing at various temperatures between 200 and 900 °C and the resulting grain morphology has been studied using X-ray diffraction and transmission electron microscopy. The addition of Y results in significantly reduced susceptibility to grain growth whereas in case of CuAl and CuAlZr alloys, the susceptibility to grain growth was much higher. The hardness is substantially increased due to Zr and Y addition in the as-milled CuAl powders. However, the hardness of Cu–12Al–2Zr gradually decreases and approaches that of Cu–14Al alloy after the annealing treatment whereas in case of Cu–12Al–2Y alloy, the relative drop in the hardness is much lower after annealing. Accordingly, the efficacy of grain size stabilization by Y addition at high homologous temperatures has been explained on the basis of a recent thermodynamic stabilization models.

Mitigating grain growth at high temperatures in binary nanocrystalline alloys is important for processing nanocrystalline alloy systems. The objective of this research is to develop a methodical design-based approach for selecting solutes in binary nanocrystalline alloys by revisiting grain boundary thermodynamics and the internal processes of grain growth and solute segregation in a closed system. In this work, the grain boundary energy is derived and systematically studied in terms of temperature, grain size, concentration, and solute segregation for binary systems of 44 solvents and 52 solutes, using readily-available elemental data, such as moduli and liquid enthalpy of mixing. It is shown that through solute segregation, the grain boundary energies of some binary systems can be reduced, resulting in thermodynamically stable grain structures and successful prediction of solutes that inhibit grain growth in some nanocrystalline alloys. Parametric studies reveal trends between equilibrium grain size, solute distribution and temperature for various binary systems culminating in the generation of nanocrystalline thermodynamic stability maps as a tool for solute selection in binary nanocrystalline alloys.

Nanocrystalline Cu86Al12B2 alloy with an as-milled average grain size of ∼11 nm was synthesized by high-energy ball milling at cryogenic temperature. The alloy was then annealed up to 900 °C (or 0.87 Tm of Cu). Microstructural changes with annealing were assessed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and microindentation. TEM investigation indicates that the newly-developed alloy retains its nanoscale grain size after annealing at 900 °C. The present investigation complements our previous findings of the thermal stability of cryomilled binary Cu88Al12 and Cu86Al14 alloys, where excellent thermal stability was observed and attributed to the in-situ formation of nanoscale Cu–Al precipitates and their Zener pinning effect. First principle calculation shows equilibrium in Al–Cu–B prefers segregation of B in the grain boundary of Cu–Al alloy. Superior thermal stability of the Cu86Al12B2 alloy was primarily ascribed to the synergistic grain boundary pinning of CuAl2 and AlB12 intermetallic phases. Moreover, the alloy maintains a higher hardness than both Cu88Al12 and Cu86Al14 alloy. The grain size dependent Hall–Petch strengthening was found to be the dominant mechanism with substantial contributions from solid solution strengthening and Orowan strengthening in annealed state.

porosity has typically been limited to � 40%. These relatively low porosity levels (compared to liquid state processes) constrain the applications of metal foams produced via solid state foaming. To extend the capabilities of solid state foaming, we have developed an additive means of creating porosity by intraparticle expansion. The current limitation of solid-state expansion by gas entrapment is dictated by voids formed between solid particles during consolidation. In this model, the initial gas pressure and foaming temperature determine the resulting porosity. However, if the expanding gas is not limited to just that which is trapped between particles, but is also located within particles, solid state foaming may assume a character more akin to expandable polymers which foam from the constituent pellets. This concept is a paradigm shift in terms of the solid state foaming process and the achievable levels of porosity. On the basis of this simple, powder feedstock expansion, there is universal application to powder metallurgy methods for foaming. In this work, we examine the microstructure and morphology of a Cu–Sb alloy that expands to porosities of close to 40% within individual particles, resulting in absolute porosity of 69% in sintered samples. The Cu–Sb alloy powder was formed by mechanically alloying Cu and Sb (Alfa Aesar, 99.9 and 99.5%, respectively) at cryogenic temperature(� 196°C) for 4h using a modified SPEX 8000M Mixer/Mill. The elemental powders were combined to achieve 5 at% Sb in Cu. The as-milled powders contained no appreciable porosity. Ball milling was used as a means to intimately mix the elements and refine and distribute preexisting oxides. Although oxygen exposure was controlled during milling and storage of powders, the manufacturersupplied precursors did contain appreciable oxygen content. The importance of this will be detailed later. The alloyed powder was annealed at 600°C for a period of 1h under 3% H2 (bal. Ar). During annealing, the powders underwent pore formation and expansion. When annealing was done in the absence of H2, it did not produce any expansion. Microscopic examination of the loose powders was carried out using an FEI Nova Nano Lab 600 dual beam microscope using scanning electron microscopy (SEM). Cross-sectional analysis was performed using a focused ion beam (FIB). The grain size and grain orientations were measured using focused ion beam

Nanocrystalline copper (Cu) was generated by cryogenic, high-energy ball milling. Antimony (Sb) was added to investigate its utility in stabilizing the grain structure during annealing up to a maximum temperature of 1073 K (800 °C). When alloyed with Sb in quantities up to 1 at. pct, thermal stability was maintained up to 673 K (400 °C). Cu and Sb have very different molar volumes which can drive segregation of the solute due to the elastic strain energy and hence stabilize the grain size by reducing grain boundary energy. The elastic mismatch of Sb in Cu is calculated to be quite large (113 kJ/mol) when molar volume is used, but when an equivalent equation using atomic radius is applied, the driving force is nearly an order of magnitude lower (~12 kJ/mol). The low elastic mismatch is corroborated by the large equilibrium solubility of Sb in Cu. The results for the Cu-Sb system are compared to the nanocrystalline Ni-W system and the large amount of equilibrium solubility of the solute in both cases is thought to hinder thermal stabilization since segregation is not strongly favored.

A unique type of nonwoven carbon material has been developed which is flexible, resilient, and produced at modest temperature and near ambient pressure using catalytic deposition. This material is comprised entirely of nanoscale carbon fibers, which are extensively interlaced to create a coherent, bulk material. The structure and basic mechanical and electrical properties of this material were investigated through cyclic compression and in situ resistance measurement. The material was highly elastic and capable of being repeatedly compressed without disintegration. The mechanical response varied with density, and the density was controlled by the amount of catalyst used. The material exhibited a high electrical resistivity, which varied nonlinearly with compression.

Cryogenic high energy ball milling was used to synthesize nanocrystalline Cu and Cu 99.5 Zr 0.5 alloys by mechanical alloying and consolidation by hot pressing at 550 °C temperature. The grain size stability of nanocrystalline Cu is improved by the Zr addition. Microstructural characterization using X-ray diffraction and transmission electron microscopy provided evidence for the formation of a Cu–Zr alloy solid solution with nanocrystalline size after hot pressing. The alloy exhibited a higher hardness (3.31 GPa), and shear strength (550 MPa) than nano-crystalline pure Cu however, the electrical resistivity is increased in the alloy.

Nickel-yttrium nanocrystalline alloys with an as-milled grain size of approximately 6.5 nm were synthesized using high-energy cryogenic mechanical alloying. The microstructural changes due to annealing were characterized using x-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy. Experiments demonstrated that increasing yttrium content led to stabilization of the nanocrystalline grain size at elevated homologous annealing temperatures. Additionally, it was found that inadvertent contamination with nitrogen during the milling process caused the formation of yttrium nitride (YN) precipitates, which, in turn, resulted in an additional nonlinear hardening effect beyond the expected hardening due to grain-size reduction. Results reveal that kinetic pinning by YN particles is effective in retaining a nanostructure to relatively high temperatures.

Alloys of copper (Cu) and zirconium (Zr) were generated by mechanical alloying via cryogenic, high-energy ball milling and then annealed to a maximum temperature of 1000 °C. The addition of only 1 at% Zr to Cu was found effective at stabilizing the grains in the nanocrystalline state to homologous temperatures in excess of 0.85. When Zr was added in concentrations of 2 and 5 at%, the alloys underwent substantial hardening during annealing, but grain size stability was not enhanced. The mechanism of grain size stabilization was investigated using thermodynamic and kinetic modeling. Zr is predicted to significantly reduce the grain boundary energy of Cu via segregation, but simplifications in the thermodynamic model do not capture high temperature behavior. Kinetically, good correlation between calculation and experimental observation was found by applying estimations for the limiting grain size and Orowan strengthening via second-phase pinning. Both thermodynamic and kinetic mechanisms may be active during annealing, but kinetic parameters appear to be sufficient in explaining the excellent stability of nanocrystalline Cu, even at low Zr concentration.

Widespread adoption of carbon nanomaterials has been hindered by inefficient production and utilization. A recently developed method has shown possibility to directly synthesize bulk nanostructured nonwoven materials from catalytically deposited carbon nanofibers (CNFs). The basic manufacturing scheme involves constraining carbon nanofiber growth to create three-dimensionally featured, macroscale products. Although previously demonstrated as a proof of concept, the possibilities and pitfalls of the method at a larger scale have not yet been explored. In this work, the basic foundation for using the constrained formation of fibrous nanostructures (CoFFiN) process is established by testing feasibility in larger volumes (as much as 2000% greater than initial experiments) and by noting the macroscale carbon growth characteristics. It has been found that a variety of factors contribute to determining the basic qualities of the macroscale fiber collection (nonwoven material), and there are tunable parameters at the catalytic and constraint levels. The results of this work have established that monolithic structures of nonwoven carbon nanofibers can be created with centimeter dimensions in a variety of cross-sectional shapes. The only limit to scale noted is the tendency for nanofibers to entangle with one another during growth and self-restrict outward expansion to the mold walls. This may be addressed by pregrowing carbon before placement or selective placement of the catalyst in the mold.

Copper (Cu) was cryogenically milled with tungsten (W) in a high-energy ball mill. The process created W particles dispersed in a nanocrystalline Cu matrix. These “alloys” were then annealed to a maximum temperature of 800 °C. The addition of W stabilized the Cu at∼40 nm during annealing to 400 °C for a 1 at% W composition and to 600 °C for 10 at% W. As evidenced through hardness measurement, the W provided a significant increase in strength over pure Cu, and the 10 at% W material maintained a 2.6 GPa hardness after annealing at 800 °C. The stabilization and strengthening mechanisms are compared against theoretical prediction and found to be in good agreement. Although the strength and stability are significantly improved over pure Cu, the maximum benefit was hindered by an extremely broad W particle size distribution (∼5–5000 nm). For the 10 at% W alloy, only half of the added W was reduced to nanoscale where kinetic pinning and strengthening become most effective.

The thermal stability of nanocrystalline cartridge brass (Cu–30 at.% Zn) and brass–Zr alloys were investigated. The alloys were produced by cryogenic ball milling and subsequently heat treated to a maximum temperature of 800 °C. The grain size of pure brass was found to be relatively stable in comparison to pure copper, and a high hardness was retained up to 600 °C. When 1 at.% zirconium was alloyed with the brass, the grain size was stabilized near 100 nm even at 800 °C. At the highest temperature, hardness was retained above 2.5 GPa for 1 and 5 at.% zirconium alloys, but the pure brass softened significantly. The stabilization is believed to be dominated by Zn–Zr interactions as a second phase of these two was observed in X-ray diffraction and transmission electron microscopy. Thermodynamic modeling indicates a zero grain boundary energy may be achieved depending on the mixing enthalpy value used (i.e., calculated vs. experimental) under ideal conditions, but microstructural features such as twinning and second phase particles are thought to be the dominant stabilization mechanism. Zr worked well in stabilizing the brass in the nanocrystalline state to nearly 90 % of its melting temperature.

Nanocrystalline Cu–30% Zn samples were produced by high energy ball milling at 77 K and room temperature. Cryomilled flakes were further processed by ultrahigh strain high pressure torsion (HPT) or room temperature milling to produce bulk artifact-free samples. Deformation-induced grain growth and a reduction in twin probability were observed in HPT consolidated samples. Investigations of the mechanical properties by hardness measurements and tensile tests revealed that at small grain sizes of less than ∼35 nm Cu–30% Zn deviates from the classical Hall–Petch relation and the strength of nanocrsytalline Cu–30% Zn is comparable with that of nanocrystalline pure copper. High resolution transmission electron microscopy studies show a high density of finely spaced deformation nanotwins, formed due to the low stacking fault energy of 14 mJ m –2 and low temperature severe plastic deformation. Possible softening mechanisms proposed in the literature for nanotwin copper are addressed and the twin-related softening behavior in nanotwinned Cu is extended to the Cu–30% Zn alloy based on detwinning mechanisms.

A series of Cu– x Nb ( x = 1–15 at.% 2 ) alloys have been investigated to study the metastable solid solubility extension of Nb in Cu by mechanical alloying. Analysis of X-ray diffraction and Gibbs free energy change confirmed that 7.5% of Nb was metastably dissolved in Cu after 8 h of milling at room temperature although Cu–Nb is a system with positive heat of mixing. The solid solubility could be extended up to 10% after enhancing milling duration to 16 h. Detailed thermodynamic analysis revealed that the additional energy stored during mechanical alloying could overcome the required energy barrier as per Miedema's model for the formation of disordered solid solution. The extended solid solubility has been explained along with the other possible mechanisms. Extensive annealing experiments and structural investigation revealed that the supersaturated solid solution is completely stable up to 400 °C. The matrix grains were stabilized and retained their size, ∼25 nm, even after annealing at 600 °C. Microhardness measurement and grain size analysis show that the dissolution of Nb in Cu has a larger strengthening effect than that of free Nb in the compositions.

The rate of catalytic carbon nanofiber formation from a mixture of ethylene and hydrogen at 550 °C was found to be dramatically faster over physical mixtures of palladium and cobalt micron scale particles than over either metal independently. The rate correlated with the metal fraction nearly identically for either Pd or Co rich mixtures. The highest rate increase over either pure metal was observed for a 1:1 mass ratio (∼150 times faster), although significant increases were found even at metal ratios of 11:1 (∼45 times faster). There was no direct evidence of extensive alloy formation from the mixed powders which suggests that a synergistic mechanism driven by proximity only may be responsible for the observed rate increases. It is thought a species (e.g. hydrogen atoms) formed at one metal (e.g. palladium) diffuses to the other where it accelerates carbon deposition by affecting the other catalyst material directly, or by generating favorable radical species. Kinetic synergism was also observed for Pd–Co alloys, although it was clearly less dramatic than that found for mixtures. Still, the fundamental similarity in behavior suggests that on the alloy surface two site types exist: one primarily Pd and one primarily Co.

Carbon nanofiber growth on palladium particles from ethylene–oxygen mixtures was investigated with respect to thermal history. Electron microscopy, combined with focused ion beam cross-sectioning show particles sinter quickly, but can be stabilized by the addition of a short carbon deposition step at a temperature below the general reaction temperature. This step generates a thin layer of carbon on the catalyst which reduces sintering once the temperature is raised to the optimal reaction temperature. For example, high temperature (e.g. 500 °C) catalyst pre-treatment leads to catalyst particle sintering, and subsequent fiber growth produces large diameter fibers. In contrast, small diameter fibers form on catalyst particles pretreated at low temperature (ca. 350 °C), even if the fibers are grown at a temperature at which deposition rates are faster (e.g. 550 °C). These results led to the development of unique multiple temperature fiber growth protocols that produce smaller diameter fibers while improving the deposition rate.

The nature of observed growth of solid carbon on palladium from ethylene−hydrogen mixtures is consistent with the supposition that the primary source of carbon for growth is homogeneously generated radicals. Evidence includes the lack of growth in the absence of a reacting mixture, sharp maxima as a function of temperature, and dramatic differences in temperature of growth as a function of mixture composition. The finding that the structure of the support strongly influenced the morphology of the solid carbon, and the temperature regime for deposition, is also consistent with this model. Carbon nanofibers were found to form on sputtered palladium films and palladium nanopowder (ca. 700 °C), whereas planar carbon structures deposited on palladium micrometer powder and foil (ca. 600 °C). A radical species growth mechanism is consistent not only with observations made herein but also with data presented in earlier studies.

The characteristics of carbonaceous materials deposited in fuel rich ethylene–oxygen mixtures on three types of palladium: foil, sputtered film, and nanopowder, are reported. It was found that the form of palladium has a dramatic influence on the morphology of the deposited carbon. In particular, on sputtered film and powder, tight ‘weaves’ of sub-micron filaments formed quickly. In contrast, on foils under identical conditions, the dominant morphology is carbon thin films with basal planes oriented parallel to the substrate surface. Temperature, gas flow rate, reactant flow ratio (C2H4:O2), and residence time (position) were found to influence both growth rate and type for all three forms of Pd. X-ray diffraction, high resolution transmission electron microscopy, temperature-programmed oxidation, and Raman spectroscopy were used to assess the crystallinity of the as-deposited carbon, and it was determined that transmission electron microscopy and X-ray diffraction were the most reliable methods for determining crystallinity. The dependence of growth on reactor position, and the fact that no growth was observed in the absence of oxygen support the postulate that the carbon deposition proceeds by combustion generated radical species.

Carbon nanofibers were generated over bimetallic catalysts in an atmospheric pressure chemical vapor deposition (APCVD) reactor. Catalyst compositions of Fe 30 at%, Cu and Ni 30 at% and Cu were mechanically alloyed using high-energy ball milling over durations of 4, 8, 12, 16, and 20 h. The catalyst powders were then used to produce carbon nanofibers in ethylene and hydrogen (4:1) at temperatures of 500, 550, and 600 °C. The microstructures of the catalysts were characterized as a function of milling time as well as at deposition temperature. The corresponding carbon deposition rates were assessed and are correlated to the microstructural features of each catalyst. The milling process directly determines the performance of each catalyst toward carbon deposition, and both catalysts performed comparably to those made by traditional co-precipitation methods. Considerations in miscible and immiscible nanostructured alloy systems are discussed.

Solid-state processes for creating metal foams are often complex and/or time consuming, and in methods such as gas entrapment, there are practical limits to the porosity achievable due to pore coalescence and percolation. A simple process has been developed to create copper-based metal foam with porosity greater than 65%. The process is unique as expansion occurs within particles, not between them. The method is applied to create a bulk part. The microstructure length scales of the foamed particles are examined using 1- and 2-point microstructure statistics.

In part 1, a visual library is reported for predicting stabilization in nanocrystalline binary alloys. A free energy model for grain boundary segregation and grain size stability has been applied for assessing the stability in 2288 distinct binary systems. The model incorporates accessible information that allows a more universal application. For each alloy pair, the values for elastic enthalpy, mixing enthalpy, minimum normalized grain-boundary energy, and the boundary concentration at the free energy minimum are reported. This data is presented in a periodic table format for each base solvent, where the pertinent parameters are reported numerically, and by color-coding to facilitate quick identification of stable alloys. The free energy of each system is dependent on the solvent, grain size, solute content, and temperature. In forthcoming work (part 2), the stabilization behavior of select alloys will be experimentally compared to the predictions reported here.

Extinction coefficients of gold nanoparticles with core size ranging from approximately 4 to 40 nm were determined by high resolution transmission electron microscopy analysis and UV-vis absorption spectroscopic measurement. Three different types of gold nanoparticles were prepared and studied: citrate-stabilized nanoparticles in five different sizes; oleylamide-protected gold nanoparticles with a core diameter of 8 nm, and a decanethiol-protected nanoparticle with a diameter of around 4 nm. A linear relationship between the logarithms of extinction coefficients and core diameters of gold particles was found independent of the capping ligands on the particle surface and the solvents used to dissolve the nanoparticles. This linear relation may be used as a calibration curve to determine the concentration or average size of an unknown nanoparticle or nanoparticle-biomolecule conjugate sample.

Carbon nanofibres were grown at low temperatures on a novel fibre catalyst, palladium particles, using a novel method, growth in fuel rich combustion conditions. The growth of carbon filaments by this process was found to be remarkably rapid. Preliminary analysis of how changes in operating parameters, for example gas flow rate and reactant ratio, showed that fibre diameter and other factors such as helicity are very sensitive to the precise conditions employed, suggesting simple means to impact overall properties of the fibres. The ability to control diameter, helicity, and surface roughness are discussed with possible importance to advanced composites.