Journal of Materials Research

Critically sized large bone defects commonly result from trauma, radical tumor resections or infections. Currently, massive allografting remain as the clinical standard to treat these critical defects. Unfortunately, allograft healing is limited by the lack of osteogenesis and bio-integration of the graft to the host bone. Based on its widely studied anabolic effects on the bone, we have proposed that teriparatide [recombinant parathyroid hormone (PTH(1-34))] could be an effective adjuvant for massive allograft healing. In support of this theory, here we review studies that have demonstrated that intermittent PTH(1-34) treatment enhances and accelerates the skeletal repair process via a number of mechanisms including: effects on mesenchymal stem cells (MSC), angiogenesis, chondrogenesis, bone formation and remodeling. We also review the current literature on the effects of PTH(1-34) therapy on bone healing, and discuss this drug's long term potential as an adjuvant for endogenous tissue engineering.

While atomic layer deposition (ALD) has been used for many years as an industrial manufacturing method for microprocessors and displays, this versatile technique is finding increased use in the emerging fields of plasmonics and nanobiotechnology. In particular, ALD coatings can modify metallic surfaces to tune their optical and plasmonic properties, to protect them against unwanted oxidation and contamination, or to create biocompatible surfaces. Furthermore, ALD is unique among thin-film deposition techniques in its ability to meet the processing demands for engineering nanoplasmonic devices, offering conformal deposition of dense and ultra-thin films on high-aspect-ratio nanostructures at temperatures below 100 °C. In this review, we present key features of ALD and describe how it could benefit future applications in plasmonics, nanosciences, and biotechnology.

Nanoindentation methods are well suited for probing the mechanical properties of a heterogeneous surface, since the probe size and contact volumes are small and localized. However, the nanoindentation method may introduce errors in the computed mechanical properties when indenting near the interface between two materials having significantly different mechanical properties. Here we examine the case where a soft material is loaded in close proximity to an interface of higher modulus, such as the case when indenting bone near a metallic implant. Results are derived from both an approximate analytical quarter-space solution and a finite element model, and used to estimate the error in indentation-determined elastic modulus as a function of the distance from the apex of contact to the dissimilar interface, for both Berkovich and spherical indenter geometries. Sample data reveal the potential errors in mechanical property determination that can occur when indenting near an interface having higher stiffness, or when characterizing strongly heterogeneous materials. The results suggest that caution should be used when interpreting results in the near-interfacial region.

An antigenic mimic of the Ebola glycoprotein was synthesized and tested for its ability to be recognized by an anti-Ebola glycoprotein antibody. Epitope-mapping procedures yielded a suitable epitope that, when presented on the surface of a nanoparticle, forms a structure that is recognized by an antibody specific for the native protein. This mimic-antibody interaction has been quantitated through ELISA and QCM-based methods and yielded an affinity (K(d) = 12 × 10(-6) M) within two orders of magnitude of the reported affinity of the native Ebola glycoprotein for the same antibody. These results suggest that the rational design approach described herein is a suitable method for the further development of protein-based antigenic mimics with potential applications in vaccine development and sensor technology.

The ultrastructure of nanoscale apatite biomimetically formed on an organic template from a supersaturated mineralizing solution was studied to examine the morphological and crystalline arrangement of mineral apatites. Needle-shaped apatite crystal plates with a size distribution of ~100 to ~1000 nm and the long axis parallel to the c axis ([002]) were randomly distributed in the mineral films. Between these randomly distributed needle-shaped apatite crystals, amorphous phases and apatite crystals (~20-40 nm) with the normal of the grains quasi-perpendicular to the c axis were observed. These observations suggest that the apatite film is an interwoven structure of amorphous phases and apatite crystals with various orientations. The mechanisms underlying the shape of the crystalline apatite plate and aggregated apatite nodules are discussed from an energy-barrier point of view. The plate or needle-shaped apatite is favored in single-crystalline form, whereas the granular nodules are favored in the polycrystalline apatite aggregate. The similarity in shape in both single-crystalline needle-shaped apatite and polycrystalline granular apatite over a wide range of sizes is explained by the principle of similitude, in which the growth and shape are determined by the forces acting upon the surface area and the volume.

In the literature, a few biological cells have been used as templates to form microcapsules of a variety of shapes and sizes. In this study, we proved the concept that living cells like platelets can be encapsulated with polyelectrolytes using electrostatic layer-by-layer self-assembly (LBL), and, most importantly, the encapsulation process did not induce activation of the platelets. Glycol-chitosan and poly-L-glutamic acid were electrostatically deposited onto platelets, and the encapsulation was confirmed using confocal laser scanning microscopy and scanning electron microscopy. Transmission electron microscopy observation further confirmed that the encapsulation process was mild and the activation of platelets was negligible. The encapsulation of living biological cells like platelets can serve as a model system in a wide range of biomedical applications including local and sustained drug delivery, immune protection of artificial tissues, and versatile artificial blood.

Significant shifts of the frequency of the Raman spectra of the tangential mode of single walled carbon nanotubes (SWNTs) and fluorinated tubes (FSWNTs) in composites of polybutadiene (PB) were observed relative to the pristine SWNTs indicative of the interaction between the polymer and the SWNTs. Proton NMR line width measurements demonstrate partial suppression of polymer segmental motion for both types of nanotube composites and spin-lattice relaxation results indicate that short time-scale localized motions are also affected by SWNT inclusion, more so for FSWNTs. Hardness measurements as a function of wt% SWNTs and FSWNTs in the polymer show larger enhancements of hardness in the composite with the fluorinated tubes.

Ultrasmooth nanostructured diamond (USND) films were synthesized on Ti-6Al-4V medical grade substrates by adding helium in H(2)/CH(4)/N(2) plasma and changing the N(2)/CH(4) gas flow from 0 to 0.6. We were able to deposit diamond films as smooth as 6 nm (root-mean-square), as measured by an atomic force microscopy (AFM) scan area of 2 μm(2). Grain size was 4-5 nm at 71% He in (H(2) + He) and N(2)/CH(4) gas flow ratio of 0.4 without deteriorating the hardness (~50-60 GPa). The characterization of the films was performed with AFM, scanning electron microscopy, x-ray diffraction (XRD), Raman spectroscopy, and nanoindentation techniques. XRD and Raman results showed the nanocrystalline nature of the diamond films. The plasma species during deposition were monitored by optical emission spectroscopy. With increasing N(2)/CH(4) feedgas ratio (CH(4) was fixed) in He/H(2)/CH(4)/N(2) plasma, a substantial increase of CN radical (normalized by Balmer H(α) line) was observed along with a drop in surface roughness up to a critical N(2)/CH(4) ratio of 0.4. The CN radical concentration in the plasma was thus correlated to the formation of ultrasmooth nanostructured diamond films.

Hydrogels pose unique challenges to nanoindentation including sample preparation, control of experimental parameters, and limitations imposed by mechanical testing instruments and data analysis originally intended for harder materials. The artifacts that occur during nanoindentation of hydrated samples have been described, but the material properties obtained from hydrated nanoindentation have not yet been related to the material properties obtained from macroscale testing. To evaluate the best method for correlating results from microscale and macroscale tests of soft materials, nanoindentation and unconfined compression stress-relaxation tests were performed on poly-2-hydroxyethyl methacrylate (pHEMA) hydrogels with a range of cross-linker concentrations. The nanoindentation data were analyzed with the Oliver-Pharr elastic model and the Maxwell-Wiechert (j = 2) viscoelastic model. The unconfined compression data were analyzed with the Maxwell-Wiechert model. This viscoelastic model provided an excellent fit for the stress-relaxation curves from both tests. The time constants from nanoindentation and unconfined compression were significantly different, and we propose that these differences are due to differences in equilibration time between the microscale and macroscale experiments and in sample geometry. The Maxwell-Wiechert equilibrium modulus provided the best agreement between nanoindentation and unconfined compression. Also, both nanoindentation analyses showed an increase in modulus with each increasing cross-linker concentration, validating that nanoindentation can discriminate between similar, low-modulus, hydrated samples.

Although alterations in the gross mechanical properties of dynamic and compliant tissues have a major impact on human health and morbidity, there are no well-established techniques to characterize the micromechanical properties of tissues such as blood vessels and lungs. We have used nanoindentation to spatially map the micromechanical properties of 5-mum-thick sections of ferret aorta and vena cava and to relate these mechanical properties to the histological distribution of fluorescent elastic fibers. To decouple the effect of the glass substrate on our analysis of the nanoindentation data, we have used the extended Oliver and Pharr method. The elastic modulus of the aorta decreased progressively from 35 MPa in the adventitial (outermost) layer to 8 MPa at the intimal (innermost) layer. In contrast, the vena cava was relatively stiff, with an elastic modulus >30 MPa in both the extracellular matrix-rich adventitial and intimal regions of the vessel. The central, highly cellularized, medial layer of the vena cava, however, had an invariant elastic modulus of ~20 MPa. In extracellular matrix-rich regions of the tissue, the elastic modulus, as determined by nanoindentation, was inversely correlated with elastic fiber density. Thus, we show it is possible to distinguish and spatially resolve differences in the micromechanical properties of large arteries and veins, which are related to the tissue microstructure.

Surface-modification of the elastomer poly(dimethylsiloxane) by exposure to oxygen plasma for four minutes creates a thin, stiff film. In this study, the thickness and mechanical properties of this surface-modified layer were determined. Using the phase image capabilities of a tapping-mode atomic-force microscope, the surface-modified region was distinguished from the bulk PDMS; specifically, it suggested a graded surface layer to a depth of about 200 nm. Load-displacement data for elastic indentation using a compliant AFM cantilever was analyzed as a plate bending on an elastic foundation to determine the elastic modulus of the surface (37 MPa). An applied uniaxial strain generated a series of parallel nano-cracks with spacing on the order of a few microns. Numerical analyses of this cracking phenomenon showed that the depth of these cracks was in the range of 300-600 nm and that the surface layer was extremely brittle, with its toughness in the range of 0.1-0.3 J/m(2).

The corrosion of aluminized Type 310S stainless steel in the wet seal of molten carbonate fuel cells was investigated. Samples prepared with two aluminizing methods were used: thermal spray and slurry coating. At first, the diffusion of Fe and Cr into the aluminum is limited to the interfacial area. With time at 650°C, Fe and Cr aluminides precipitate in the Al layer. The slurry-coated layer contains a higher concentration of FeAl and Fe₃Al than does the thermal spray layer. Because of the high iron content, the corrosion protection of the slurry layer is lower than that of the thermal spray layer

The compounds Ge_xNbTe₂ (0.39⩽x⩽0.53) have been studied for their thermoelectric properties. By changing x, the carrier concentration can be adjusted so that the material changes from a p-type metal to a p-type semiconductor. The maximum germanium concentration at about Ge_0.5NbTe₂, is also the most semiconducting composition. High and low temperature electrical resistivity, Hall effect, Seebeck coefficient, and thermal conductivity were measured. The thermal conductivity is reasonably low and glasslike with room temperature values around 20-25 mW/cm K. However, the power factor is too low to compete with state of the art materials. The maximum ZT found in these compounds is about 0.12. The low ZT can be traced to the low carrier mobility of about 10 cm²/Vs. The related compounds Si_0.2NbTe₂ and Ge_0.5TaTe₂ were also studied

In situ gravimetric measurements and microscopic examinations were used to determine the mechanisms of oxygen removal from synthetic ilmenite disks between 823 and 1353 K. Under a hydrogen atmosphere, iron was observed to form a layer of low porosity on the surface of samples early in the reduction. This created diffusion limitations for hydrogen to the reaction front and for the escape of water vapor. A shrinking core reduction model, modified to include the growth of this iron film, was capable of predicting the conversion-time relationships of ilmenite samples. An activation energy of 43.2 +/- 2.6 kcal/gmole was determined to be representative of reaction control over the temperature range 823-1023 K.

Thin films of the 80 K-phase of BiCaSrCu-oxide superconductor having the composition of Bi2Ca1.05Sr2.1Cu2.19O(x) and high degree of crystalline perfection have been grown on c-axis oriented twin free single crystal substrates of NdGaO3. This has been achieved by carefully establishing the growth conditions of the LPE experiments. The temperature regime of 850 to 830 C and quenching of the specimens on the termination of the growth period are found to be pertinent for the growth of quasi-single crystalline superconducting BCSCO films on NdGaO3 substrates. The TEM analysis reveals a single crystalline nature of these films which exhibit 100 percent reflectivity in infrared regions at liquid nitrogen temperature.

A directionally solidified alloy based on the NiAl-(Cr, Mo) eutectic was examined by transmission and scanning electron microscopy to characterize the microstructure and room temperature deformation and fracture behavior. The microstructure consisted of a lamellar morphology with a group of zone axes (111) growth direction for both the NiAl and (Cr, Mo) phases. The interphase boundary between the eutectic phases was semicoherent and composed of a well-defined dislocation network. In addition, a fine array of coherent NiAl precipitates was dispersed throughout the (Cr, Mo) phase. The eutectic morphology was stable at 1300 K with only coarsening of the NiAl precipitates occurring after heat treatment for 1.8 ks (500 h). Fracture of the aligned eutectic is characterized primarily by a crack bridging/renucleation mechanism and is controlled by the strength of the semicoherent interface between the two phases. However, contributions to the toughness of the eutectic may arise from plastic deformation of the NiAl phase and the geometry associated with the fracture surface.

There is controversy in the literature regarding the existence of the metastable γ′ phase with an ordered L12 structure in rapidly solidified Fe - Ni - Al - C alloys. In this study, the quench rate - metastable structure dependence was examined. The effect of silicon on the kinetics of phase formation was studied by adding two weight percent silicon to a base alloy of Fe - 20Ni - 8Al - 2C. The nonequilibrium γ′ and γ phases were found in foils 65 to 100 μm thick. At higher quench rates, i.e., thinner samples, the matrix was disordered fcc γ with κ-carbide precipitates. Samples containing silicon had a matrix composed of γ′ and γ structures when the foils were thicker than 40 μm. At higher quench rates, the matrix was disordered fcc γ with κ-carbide precipitates. The nonequilibrium γ′ and γ structures are present in samples with or without silicon, but are observed at higher cooling rates with the addition of silicon. This sensitivity to cooling rate and composition may explain the differences reported in the literature.

Chemical reaction can occur at the fiber/matrix interface of intermetallic matrix composites, leading to a degradation of mechanical properties. Fe-40Al matrix composites were fabricated using SiC, B, W, Mo-base, and Al2O3 fibers. Composite samples were heat treated up to 1500 K to study the reaction kinetics, and reaction rates were determined from reaction zone thickness measurements. The Al2O3 and W fibers were found to be compatible with the Fe-40Al matrix, while the Mo-based fibers reacted moderately and the B and SiC fibers reacted severely. Experimental results are compared to theoretical thermodynamic predictions.

We employ the first-principles GW+Bethe Salpeter equation approach to study the electronic structure and optical absorption spectra of uniaxial strained graphene with many-electron effects included. Applied strain not only induces an anisotropic Fermi velocity but also tilts the axis of the Dirac cone. As a result, the optical response of strained graphene is dramatically changed; the optical absorption is anisotropic, strongly depending on the polarization direction of the incident light and the strain orientation; the characteristic single optical absorption peak from {\pi}-{\pi}* transitions of pristine graphene is split into two peaks and both display enhanced excitonic effects. Within the infrared regime, the optical absorbance of uniaxial strained graphene is no longer a constant because of the broken symmetry and associated anisotropic excitonic effects. Within the visible-light regime, we observe a prominent optical absorption peak due to a significant red shift by electron-hole interactions, enabling us to change the visible color and transparency of stretched graphene. Finally, we also reveal enhanced excitonic effects within the ultraviolet regime (8 to 15 eV), where a few nearly bound excitons are identified.

We present an overview of various theoretical methods with detailed emphasis on an intermediate Coulomb-U coupling model. This model is based on material-specific ab-initio band structure from which correlation effects are computed via self-consistent GW-based self-energy corrections arising from spin-fluctuations. We apply this approach to four isostructural intermetallic actinides PuCoIn5, PuCoGa5, PuRhGa5 belonging to the Pu-115 family, and UCoGa5, a member of the U-115 family. The 115 families share the property of spin-orbit split density of states enabling substantial spin fluctuations around 0.5 eV, whose feedback effect on the electronic structure create mass renormalization and electronic `hot spots`, i.e., regions of large spectral weight. A detailed comparison is provided for the angle-resolved and angle-integrated photoemission spectra and de Haas-van Alphen experimental data as available. The results suggest that this class of actinides is adequately described by the intermediate Coulomb interaction regime, where both itinerant and incoherent features coexist in the electronic structure.

Polyimide-metal oxide (Co3O4 or CuO) composite films have been prepared via in situ thermal decomposition of cobalt (II) chloride or bis(trifluoroacetylacetonato)copper(II). A soluble polyimide (XU-218) and its corresponding prepolymer (polyamide acid) were individually employed as the reaction matrix. The resulting composites exhibited a greater metal oxide concentration at the air interface with polyamide acid as the reaction matrix. The water of imidization that is released during the concurrent polyamide acid cure and additive decomposition is believed to promote metal migration and oxide formation. In contrast, XU-218 doped with either HAuCl4.3H2O or AgNO3 yields surface gold or silver when thermolyzed (300 C).

The permeability of oxygen through pure, large-grain Ag membrames has been found to be linear and repeatable over the 400-800 C range, but only at a magnitude that is a factor of 3.2 smaller than ascribed by prior research. AES data indicate the pertinence of grain-boundary considerations, due to the virtual undetectability of intragranular oxygen. Vacuum-desorption of oxygen-saturated Ag is found to occur at the critical temperature of 630 C; this is consistent with the increased mobility of oxygen atoms in the higher temperature regime.

Surface migration and volume diffusion in the Ag2Se-AgGaSe2 system were investigated using reactive diffusion couples which were analyzed by X-ray diffraction, optical microscopy, and electron probe microanalysis. The surface diffusivities of all mobile species are found to be much larger than volume diffusivities. The results of the study suggest that Se moves together with Ag and Ga to maintain binary (Ag2Se and Ga2Se3) stoichiometry and electroneutrality. The dominance of surface migration kinetics can account for the uniform annihilation of second-phase precipitates during heat treatments.

Amorphous iron and titanium-based alloys containing various amounts of chromium, phosphorus, and boron exhibit high corrosion resistance. We report some physical properties of Fe and Ti-based metallic alloy films deposited on a glass substrate by a dc-magnetron sputtering technique. The films were characterized using differential scanning calorimetry (DSC), stress analysis, scanning electron microscopy (SEM), x-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), electron microprobe, and potentiodynamic polarization technique.

This paper reconsiders the question of stability of a planar solid-liquid interface in an undercooled alloy melt without making the restrictive assumption of no heat flow in the solid (i.e., Gs = 0). The results of this analysis indicate that, provided the thermal gradient on the solid side of the interface, Gs, is positive, stability can be achieved in an undercooled alloy melt for growth rates R greater than Ra (where Ra is the absolute stability limit of Mullins and Sekerka, 1964). Thus, the absolute stability criterion for steady-state planar growth in an undercooled alloy melt is the same as derived earlier by Mullins and Sekerka for directional solidification. Relaxing the restrictive assumption of Gs = 0 also reveals that there is a regime of stability for low growth rates and low supercoolings.

A new \textit{D0$_{23}$} metastable phase of Cu$_3$Au is found to grow at the interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent of formation of this novel alloy phase depends upon an optimal range of interfacial width primarily governed by the deposition wattage of the dc-magnetron used. Such interfacially confined growth is utilized to grow a $\sim$ 300 nm thick Au/Cu multilayer with thickness of each layer nearly equal to the optimal interfacial width which was obtained from secondary ion mass spectrometry (SIMS) data. This growth technique is observed to enhance the formation of the novel alloy phase to a considerable extent. SIMS depth profile also indicates that the mass fragment corresponding to Cu$_3$Au occupies the whole film while x-ray diffraction (XRD) shows almost all the strong peaks belonging to the \textit{D0$_{23}$} structure. High resolution cross-sectional transmission electron microscopy (HR-XTEM) shows the near perfect growth of the individual layers and also the lattice image of the alloy phase in the interfacial region. Vacuum annealing of the alloy film and XRD studies indicate stabilization of the \textit{D0$_{23}$} phase at $\sim$ 150$^{\circ}$C. The role of interfacial confinement, the interplay between interfacial strain and free energy and the hyperthermal species generated during the sputtering process are discussed. Comment: Accepted in Journal of Materials Research

The effect of environment on the mechanical properties of an Nb-67Al-7Cr-0.25W-0.5Y alloy was investigated experimentally in the temperature range 800-1200 K. It is found that the severity of environmental attack in the alloy is determined by both matrix plasticity and oxidation kinetics. The former determines the ability of the matrix to accommodate the localized stresses generated during deformation and oxidation, while the latter governs the rate of formation of a protective oxide scale. The environmental degradation of the alloy can thus be reduced or eliminated by increasing atomic mobility.

Six near stoichiometric, NiAl single-crystal alloys, with 0.05-1.5 at.% of Hf and Zr additions plus Si impurities, were microstructurally analyzed in the as-cast, homogenized, and aged conditions. Hafnium-rich interdendritic regions, containing the Heusler phase (Ni2AlHf), were found in all the as-cast alloys containing Hf. Homogenization heat treatments partially reduced these interdendritic segregated regions. Transmission electron microscopy (TEM) observations of the as-cast and homogenized microstructures revealed the presence of a high density of fine Hf (or Zr) and Si-rich precipitates. These were identified as G-phase, Nil6X6Si7, or as an orthorhombic NiXSi phase, where X is Hf or Zr. Under these conditions the expected Heusler phase (beta') was almost completely absent. The Si responsible for the formation of the G and NiHfSi phases is the result of molten metal reacting with the Si-containing crucible used during the casting process. Varying the cooling rates after homogenization resulted in the refinement or complete suppression of the G and NiHfSi phases. In some of the alloys studied, long-term aging heat treatments resulted in the formation of Heusler precipitates, which were more stable at the aging temperature and coarsened at the expense of the G-phase. In other alloys, long-term aging resulted in the formation of the NiXSi phase. The stability of the Heusler or NiXSi phases can be traced to the reactive element (Hf or Zr) to silicon ratio. If the ratio is high, then the Heusler phase appears stable after long time aging. If the ratio is low, then the NiHfSi phase appears to be the stable phase.

The versatility of high energy grinding for instigating mechanochemical reactions in inorganic systems has been studied. High-energy grinding can be used to produce amorphous carbon from synthetic graphite and some forms of natural graphite. Elemental sulfur can be amorphized by prolonged energy grinding. The presence of iron resulting from wear of the grinding media strongly affects phase transformations of alphaFe2O3 and mechanochemical reactions of this phase with ZnO and NiO. Data obtained confirm that low-temperature mechanochemical method is a robust process route for production of a wide range of materials.

Reaction of Ni-Al alloys within the beta-NiAl phase with CrB2 was studied at 1473 K as a function of Al concentration in the alloy. Reaction of 49-50 at. pct Al alloys with CrB2 occurred by interdiffusion of Ni into CrB2 and Cr into the alloy without forming a new product phase. On the other hand, a new product phase, rich in Ni and B, formed by the reaction of alloys having Al concentrations 48 at. pct or lower with CrB2. The reaction product was observed both at the CrB2/alloy interface and along the alloy grain boundaries.

Studies of the reactive melt infiltration of silicon-niobium alloys in microporous carbon preforms prepared by the pyrolysis of a polymer precursor have been carried out using modeling, Differential Thermal Analysis (DTA), and melt infiltration. Mercury porosimetry results indicate a very narrow pore size distribution with virtually all the porosity within the carbon preforms open to infiltrants. The morphology and amount of the residual phases (niobium disilicide and silicon) in the infiltrated material can be tailored according to requirements by careful control of the properties (pore size and pore volume) of the porous carbon preforms and alloy composition. The average room temperature four-point flexural strength of a reaction-formed silicon carbide material (made by the infiltration of medium pore size carbon preform with Si - 5 at. % Nb alloy) is 290 +/- 40 MPa (42 +/- 6 ksi) and the fracture toughness is 3.7 +/- 0.3 MPa square root of m. The flexural strength decreases at high temperatures due to relaxation of residual thermal stresses and the presence of free silicon in the material.

The solidification of a series of Al-Cu-Fe alloys containing an icosahedral phase has been studied by scanning electron microscopy, energy-dispersive x-ray spectroscopy, powder x-ray diffraction, and differential thermal analysis. The constitution and morphology of alloys solidified at different rates are presented. The overall compositional range of the icosahedral phase was determined in as-cast, slowly solidified alloys and those annealed at 800 and 600 °C. The solidification of this phase follows closely the Al3Fe-AlCu compositional direction. The first icosahedral phase solidified close to Al65.5Cu21.5Fe13; its compositional region at 800 °C lies between about Al64.5Cu23Fe12.5 and Al62Cu26.5Fe11.5, while at 600 °C it lies between Al62Cu26.5Fe11.5 and Al60.5Cu29.5Fe10. The formation of the icosahedral and related structures observed in Al-Cu-Fe alloys is discussed.

The nucleation behavior of α(TiCrSiO), a 1/1 Fibonacci crystal approximant phase, was investigated in alloys made near the stoichiometric composition. Containerless solidification studies were made with electromagnetic radio-frequency levitation and the 105-m NASA Drop Tube. The solidification microstructures indicate that the α-Ti hexagonal solid solution was the primary crystallizing phase in these alloys, growing dendritically. The α(TiCrSiO) phase nucleated in the remaining liquid. The competition between these two phases resulted from the high oxygen concentration needed to form α(TiCrSiO), which also stabilized the hexagonal-close-packed α-Ti phase.

Metastable solidification of small droplets of Nd11.8Fe82.3B5.9 and Nd14Fe79B7 alloys was performed in an 8-m drop tube filled with helium. The results showed that the solidification path of the droplets depends on the alloy composition and on the droplet size. For Nd11.8Fe82.3B5.9 alloy, larger droplets are solidified by primary iron formation and subsequent Nd2Fe14B crystallization from the residual liquid phase, whereas smaller ones tend to be frozen by metastable primary Nd2Fe17Bx growth (x = 0–1). A similar transition of the solidification path from primary iron formation to primary Nd2Fe17Bx formation occurred in Nd14Fe79B7 alloy with reducing droplet size. However, metastable primary growth of Nd2Fe14B was also observed within a wide droplet size range prior to the appearance of the metastable Nd2Fe17Bx phase. Nucleation and growth of different phases were considered to produce an explanation of the observed phase selection phenomena in these two alloys.

Previous studies of a single lot of NiAl powder which had been ground under high intensity conditions in liquid nitrogen (cryomilling) indicated that this processing leads to a high strength, elevated temperature NiAl-AlN composite. Because this was the first known example of the use of the reaction milling process to produce a high temperature composite, the reproducibility of this technique was unknown. Two additional lots of NIAl powder and a lot of a Zr-doped NiAl powder have been cryomilled, and analyses indicate that AlN was formed within a NiAl matrix in all three cases. Compression testing between 1200 K and 1400 K has shown that the deformation resistance of these heats is similar to that of the first lot of NiAl-AlN; thus cryomilling can improve the creep resistance of NiAl by a factor of six. Based on this work, it is concluded that cryomilling of NiAl powder to form high temperature, high strength NiAl-AlN composites is a reproducible process.

Chemical compatibility of several reinforcement materials with three niobium aluminides, Nb3Al, Nb2Al, and NbAl3, were examined from thermodynamic considerations. The reinforcement materials considered in this study include carbides, borides, nitrides, oxides, silicides, and Engel-Brewer compounds. Thermodynamics of the Nb-Al system were reviewed and activities of Nb and Al were derived at desired calculation temperatures. Criteria for chemical compatibility between the reinforcement material and Nb-Al compounds have been defined and several chemically compatible reinforcement materials have been identified.

The distribution and chemistry of hydrides produced in single and dual phase alloys with a composition near TiAl have been investigated by using a combination of TEM and X-ray diffraction techniques. The alloys were exposed at 650 C to 13.8 MPa of gaseous H2 for 100 h. In the single-phase gamma alloy, large hydrides preferentially nucleated on the grain boundaries and matrix dislocations and a population of small hydrides was distributed throughout the matrix. X-ray and electron diffraction patterns from these hydrides indicated that they have an fcc structure with a lattice parameter of 0.45 nm. EDAX analysis of the hydrides showed that they were enriched in Ti. The hydrides were mostly removed by vacuum annealing at 800 C for 24 h. On dissolution of the hydrides, the chemistry of hydride-free regions of the grain boundary returned to the matrix composition, suggesting that Ti segregation accompanied the hydride formation rather than Ti enrichment causing the formation of the hydride.

Unidirectional SrO Al2O3 2SiO2 glass-ceramic matrix composites reinforced with uncoated Chemical Vapor Deposited (CVD) SiC (SCS-0) fibers have been fabricated by hot-pressing under appropriate conditions using the glass-ceramic approach. Almost fully dense composites having a fiber volume fraction of 0.24 have been obtained. Monoclinic celsian, SrAl2Si2O8, was the only crystalline phase observed in the matrix by x-ray diffraction. No chemical reaction was observed between the fiber and the matrix after high temperature processing. In three-point flexure, the composite exhibited a first matrix cracking stress of approx. 231 +/- 20 MPa and an ultimate strength of 265 +/- 17 MPa. Examination of fracture surfaces revealed limited short length fiber pull-out. From fiber push-out, the fiber/matrix interfacial debonding and frictional strengths were evaluated to be approx. 17.5 +/- 2.7 MPa and 11.3 +/- 1.6 MPa, respectively. Some fibers were strongly bonded to the matrix and could not be pushed out. The micromechanical models were not useful in predicting values of the first matrix cracking stress as well as the ultimate strength of the composites.

A novel stearic acid (SA)/3-aminopropyltrethoxysilane (APS) composite structure was fabricated using the combined method of the Langmuir-Blodgett technique and self-assembly monolayer (SAM) technique. Its frictional, adhesive properties and interface contact types between the atomic force microscope tip and the samples were evaluated based on Amonton's laws and the general Carpick's transition equation, respectively. The results showed that the tip-sample contacts corresponded to the Johnson-Kendall-Robert/Derjaguin-Muller-Toporov (DMT) transition model for SiO2, APS-SAMs, and the unheated SA-APS composite structure, and for the heated SA-APS bilayer to the DMT model. Frictional forces for the four samples were linearly dependent on external loads at higher loads, and at lower loads they were significantly affected by adhesive forces. Frictional and scratching tests showed that the heated SA-APS composite structure exhibited the best lubricating properties and adhesion resistance ability, and its wear resistance capacity was greatly improved due to the binding-mode conversion from hydrogen bonds to covalent bonds. Thus, this kind of composite bilayer might be promising for applications in the lubrication of nano/microelectromechanical systems. I.

Morphology and structure of ZnO films deposited on (0001) sapphire and glass substrates by off-axis sputtering are investigated at various temperatures and pressures. All films show highly textured structures on glass substrates and epitaxial growth on sapphire substrates. The full width at half maximum of theta rocking curves for epitaxial films is less than 0.5 degrees. In textured films, it rises to several degrees. The trend of surface textures in films grown at low pressures is similar to those grown at high temperatures. A morphology transition from large well-defined hexagonal grains to flat surface was observed at a pressure of 50 mtorr and temperature of 550 C. The experiment results are explained by the transport behavior of depositing species.

A laser sintering/creep technique has been established to determine the creep behavior of thermal barrier coatings under steady-state high heat flux conditions. For a plasma sprayed zirconia-8 wt. % yttria coating, a significant primary creep strain and a low apparent creep activation energy were observed. Possible creep mechanisms involved include stress induced mechanical sliding and temperature and stress enhanced cation diffusion through the splat and grain boundaries. The elastic modulus evolution, stress response, and total accumulated creep strain variation across the ceramic coating are simulated using a finite difference approach. The modeled creep response is consistent with experimental observations.

Two-dimensional crystals are an important class of materials for novel physics, chemistry, and engineering. Germanane (GeH), the germanium-based analogue of graphane (CH), is of particular interest due to its direct band gap and spin-orbit coupling. Here, we report the successful co-deposition growth of CaGe2 films on Ge(111) substrates by molecular beam epitaxy (MBE) and their subsequent conversion to germanane by immersion in hydrochloric acid. We find that the growth of CaGe2 occurs within an adsorption-limited growth regime, which ensures stoichiometry of the film. We utilize in situ reflection high energy electron diffraction (RHEED) to explore the growth temperature window and find the best RHEED patterns at 750 {\deg}C. Finally, the CaGe2 films are immersed in hydrochloric acid to convert the films to germanane. Auger electron spectroscopy of the resulting film indicates the removal of Ca and RHEED patterns indicate a single-crystal film with in-plane orientation dictated by the underlying Ge(111) substrate. X-ray diffraction and Raman spectroscopy indicate that the resulting films are indeed germanane. Ex situ atomic force microscopy (AFM) shows that the grain size of the germanane is on the order of a few micrometers, being primarily limited by terraces induced by the miscut of the Ge substrate. Thus, optimization of the substrate could lead to the long-term goal of large area germanane films.

Epitaxial thin films of Bi2FeCrO6 have been synthesized by pulsed laser deposition on SrRuO3 on (100)- and (111)-oriented SrTiO3 substrates. Detailed X-ray diffraction and cross-section transmission electron microscopy analysis revealed a double perovskite crystal structure of the Bi2FeCrO6 epitaxial films very similar to that of BiFeO3 along with a particularly noteworthy Fe3+/Cr3+ cation ordering along the [111] direction. The films contain no detectable magnetic iron oxide impurities and have the correct cationic average stoichiometry throughout their thickness. They however exhibit a slight modulation in the Fe and Cr compositions forming complementary stripe patterns, suggesting minor local excess or depletion of Fe and Cr. The epitaxial BFCO films exhibit good ferroelectric and piezoelectric properties, in addition to magnetic properties at room temperature, as well as an unexpected crystallographic orientation dependence of their room temperature magnetic properties. Our results qualitatively confirm the predictions made using the ab-initio calculations: the double-perovskite structure of Bi2FeCrO6 films exhibit a Fe3+/Cr3+ cation ordering and good multiferroic properties, along with the unpredicted existence of magnetic ordering at room temperature. Comment: Accepted for publication in Journal of Materials Research

Nacre from mollusk shell is a high-performance natural composite, composed of microscopic mineral tablets bonded by a tough biopolymer. Under tensile stress these tablets slide on another in a highly controlled fashion and over large volumes, and this unique mechanism leads to an overall toughness of nacre three orders of magnitude higher than the brittle mineral. This degree of toughness amplification is currently not matched by any synthetic composite. This article presents the first synthetic material based on the structure of nacre that successfully duplicates the mechanism of tablet sliding. This material was made of millimeter size poly-methylmethacrylate (PMMA) tablets arranged in columns and held by fasteners. Strain hardening was provided by tablet waviness, leading to strains at failure 3-5 times greater than bulk PMMA. Analytical and finite element models successfully captured the locking mechanisms, enabling a rigorous design and optimization of similar composites based on different materials or at different length scale. This work demonstrates how key features and mechanisms in natural nacre can be successfully harnessed in engineering material, and unveils two new mechanisms, the effect of free surfaces and unzipping. Both mechanisms may be relevant to natural materials such as nacre or bone. Keywordsnacre-biomimetics-tablet-analytical model-finite element model

Films of Bi$\rm _2$Sr$\rm _2$CaCu$\rm _2$O$\rm _8$ and Bi$\rm _2$Sr$\rm _2$CuO$\rm _6$ have been grown using Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely-related metastable compounds like Bi$\rm _2$Sr$\rm _2$Ca$\rm _7$Cu$\rm _8$O$\rm _{20}$ (2278) and rare-earth-doped compounds like Bi$\rm _2$Sr$\rm _2$Dy$\rm _x$Ca$\rm _{1-x}$Cu$\rm _2$O$\rm _8$ (Dy:2212) to form heterostructures with unique superconducting properties, including superconductor/insulator multilayers and tunnel junctions. Transmission electron microscopy (TEM) has been used to study the morphology and microstructure of these heterostructures. These TEM studies shed light on the physical properties of the films, and give insight into the growth mode of highly anisotropic solids like Bi$\rm _2$Sr$\rm _2$CaCu$\rm _2$O$\rm _8$. Comment: 17 pages, submitted to J. Materials Research. Email to chaiken@llnl.gov if you want to receive copies of the figures

Improvement of copper to graphite adhesion by thin interfacial films of titanium and chromium was investigated. Graphite fibers and highly oriented pyrolytic graphite flats were sputter-coated first with 10 nm of titanium or chromium and then with 50 nm of copper. After annealing to 970 C in argon/5%-hydrogen at atmospheric pressure for 5 min, copper without an interfacial bond layer agglomerated into nearly spherical particles, copper with the chromium bond layer agglomerated into particles with a contact angle less than 90 deg, indicating improvement in adhesion, and copper with the titanium bond layer exhibited a continuous metal film. In the latter case, most of the interfacial titanium was observed to have migrated into the copper and to the free surface, where the titanium reacted with contaminants in the annealing ambient.

We present a route for direct growth of boron nitride via a polyborazylene to h-BN conversion process. This two-step growth process ultimately leads to a >25x reduction in the RMS surface roughness of h-BN films when compared to a high temperature growth on Al2O3(0001) and Si(111) substrates. Additionally, the stoichiometry is shown to be highly dependent on the initial polyborazylene deposition temperature. Importantly, CVD graphene transferred to direct-grown boron nitride films on Al2O3 at 400{\deg}C results in a >1.5x and >2.5x improvement in mobility compared to CVD graphene transferred to Al2O3 and SiO2 substrates, respectively, which is attributed to the combined reduction of remote charged impurity scattering and surface roughness scattering. Simulation of mobility versus carrier concentration confirms the importance of limiting the introduction of charged impurities in the h-BN film and highlights the importance of these results in producing optimized h-BN substrates for high performance graphene and TMD devices.

Preliminary procedures have been developed for the ball milling of pristine and brominated P-100 graphite fibers. Because of the lubricative properties of graphite, large ball loads (50 percent by volume) are required. Use of 2-propanol as a milling medium enhances the efficiency of the process. The fibers, when allowed to settle from the milling medium, tend to be preferentially aligned with rather few fibers standing up. Milled, brominated P-100 fibers have resistivities that are indistinguishable from their pristine counterparts, apparently because of loss of bromine. This suggests that bromine would not be the intercalate of choice in applications where milled fibers of this type are required. It was found that brominated graphite fibers are stable in a wide variety of organic solvents.

The oxidation behavior of reaction-formed silicon carbide (RFSC) ceramics was investigated in the temperature range of 1100 to 1400 C. The oxidation weight change was recorded by TGA; the oxidized materials were examined by light and electron microscopy, and the oxidation product by x-ray diffraction analysis (XRD). The materials exhibited initial weight loss, followed by passive weight gain (with enhanced parabolic rates, k(sub p)), and ending with a negative (logarithmic) deviation from the parabolic law. The weight loss arose from the oxidation of residual carbon, and the enhanced k(sub p) values from internal oxidation and the oxidation of residual silicon, while the logarithmic kinetics is thought to have resulted from crystallization of the oxide. The presence of a small amount of MoSi, in the RFSC material caused a further increase in the oxidation rate. The only solid oxidation product for all temperatures studied was silica.