Creating superconductivity in WB2 through pressure-induced metastable planar defects.

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tcof 17 K at 91 GPa. Upon further compression up to 187 GPa, the Tcgradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2 (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

HuBiogel incorporated fibro-porous hybrid nanomatrix graft for vascular tissue interfaces.

Native extracellular matrix (ECM) possesses the biochemical cues to promote cell survival. However, decellularized, the ECM loses its cell supporting mechanical integrity. We report, here, a novel biohybrid vascular graft of polycaprolactone (PCL), poliglecaprone (PGC) incorporated with human biomatrix as functional materials for vascular tissue interfacing by electrospinning, thus harnessing the biochemical cues from the ECM and the mechanical integrity of the polymer blends. The fabricated fibro-porous tubular small diameter graft (i.d. = 4 mm) from polymer blend was coated with a cocktail of collagenous matrix derived from human placenta called HuBiogel™. The compositional, morphological, and mechanical properties of graft were measured and compared with a non-coated tubular PCL/PGC graft using Fourier Transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). BCA assay was used to calculate the protein content and coating-uniformity throughout the hybrid graft. Mechanical properties such as tensile strength (1.6 MPa), Young's modulus (2.4 MPa), burst pressure (>1900 mmHg), and suture retention strength (2.3 N) of hybrid graft were found to be comparable to native blood vessels. Protein coating has improved the hydrophilicity and the biocompatibility (cell viability and cell-attachment) enhanced with human umbilical vein endothelial cells (HUVECs) seeded in vitro onto the lumen layer of the graft over two weeks. The overall results promise this new biohybrid graft to be a potential candidate for vascular tissue interface and regeneration.

Pressure-induced superconductivity in the giant Rashba system BiTeI.

At ambient pressure, BiTeI exhibits a giant Rashba splitting of the bulk electronic bands. At low pressures, BiTeI undergoes a transition from trivial insulator to topological insulator. At still higher pressures, two structural transitions are known to occur. We have carried out a series of electrical resistivity and AC magnetic susceptibility measurements on BiTeI at pressure up to ∼40 GPa in an effort to characterize the properties of the high-pressure phases. A previous calculation found that the high-pressure orthorhombic P4/nmm structure BiTeI is a metal. We find that this structure is superconducting with T c values as high as 6 K. AC magnetic susceptibility measurements support the bulk nature of the superconductivity. Using electronic structure and phonon calculations, we compute T c and find that our data is consistent with phonon-mediated superconductivity.

Persistent non-metallic behavior in Sr2IrO4 and Sr3Ir2O7 at high pressures.

Iridium-based 5d transition-metal oxides are attractive candidates for the study of correlated electronic states due to the interplay of enhanced crystal-field, Coulomb and spin-orbit interaction energies. At ambient pressure, these conditions promote a novel Jeff = 1/2 Mott-insulating state, characterized by a gap of the order of ~0.1 eV. We present high-pressure electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7. While no indications of a pressure-induced metallic state up to 55 GPa were found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in the proximity of a metal-insulator transition.

Pressure-induced unconventional superconducting phase in the topological insulator Bi2Se3.

Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.

Nanostructured diamond coatings for orthopaedic applications.

With increasing numbers of orthopaedic devices being implanted, greater emphasis is being placed on ceramic coating technology to reduce friction and wear in mating total joint replacement components, in order to improve implant function and increase device lifespan. In this chapter, we consider ultra-hard carbon coatings, with emphasis on nanostructured diamond, as alternative bearing surfaces for metallic components. Such coatings have great potential for use in biomedical implants as a result of their extreme hardness, wear resistance, low friction and biocompatibility. These ultra-hard carbon coatings can be deposited by several techniques resulting in a wide variety of structures and properties.

High pressure transport properties of the topological insulator Bi2Se3.

We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 toward increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance Δρ(xx) that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.

Nanomechanical properties of electrospun composite scaffolds based on polycaprolactone and hydroxyapatite.

Fibrous nanocomposite scaffolds were electrospun from dispersions of hydroxyapatite nanoparticles (nanoHA) in polycaprolactone (PCL) with varying nanoHA contents (from 0% to 50% by weight). Such scaffolds were produced to mimic the nano-features of the extracellular matrix (ECM) for natural bone tissue regeneration. NanoHA was found to be well dispersed in the PCL fibers up to the addition of 30 wt%, whereas beads and agglomeration of HA particles was observed above this nanoHA concentration. The structural and morphological characterizations were evaluated by scanning electron microscopy (SEM), Fourier-Transform Infrared Spectroscopy (FTIR), and X-ray Diffraction (XRD). The average fiber diameter decreased with increased nanoHA concentration. The nanomechanical properties of the as-spun fibrous scaffolds as well as pressure-consolidated (pelletized) composites were evaluated by nanoindentation. Elastic modulus increased with increasing HA content, but was especially pronounced for 40-50% HA content where the indenter tip is more likely to probe agglomerated HA particles.

An electrical microheater technique for high-pressure and high-temperature diamond anvil cell experiments.

Small electrical heating elements have been lithographically fabricated onto the culets of "designer" diamond anvils for the purpose of performing high-pressure and high-temperature experiments on metals. The thin-film geometry of the heating elements makes them very resistant to plastic deformation during high-pressure loading, and their small cross-sectional area enables them to be electrically heated to very high temperatures with relatively modest currents (approximately = 1 A). The technique also offers excellent control and temporal stability of the sample temperature. Test experiments on gold samples have been performed for pressures up to 21 GPa and temperatures of nearly 2000 K.

Synthesis and Mechanical Wear Studies of Ultra Smooth Nanostructured Diamond (USND) Coatings Deposited by Microwave Plasma Chemical Vapor Deposition with He/H(2)/CH(4)/N(2) Mixtures.

Ultra smooth nanostructured diamond (USND) coatings were deposited by microwave plasma chemical vapor deposition (MPCVD) technique using He/H(2)/CH(4)/N(2) gas mixture. The RMS surface roughness as low as 4 nm (2 micron square area) and grain size of 5-6 nm diamond coatings were achieved on medical grade titanium alloy. Previously it was demonstrated that the C(2) species in the plasma is responsible for the production of nanocrystalline diamond coatings in the Ar/H(2)/CH(4) gas mixture. In this work we have found that CN species is responsible for the production of USND coatings in He/H(2)/CH(4)/N(2) plasma. It was found that diamond coatings deposited with higher CN species concentration (normalized by Balmer H(α) line) in the plasma produced smoother and highly nanostructured diamond coatings. The correlation between CN/H(α) ratios with the coating roughness and grain size were also confirmed with different set of gas flows/plasma parameters. It is suggested that the presence of CN species could be responsible for producing nanocrystallinity in the growth of USND coatings using He/H(2)/CH(4)/N(2) gas mixture. The RMS roughness of 4 nm and grain size of 5-6 nm were calculated from the deposited diamond coatings using the gas mixture which produced the highest CN/H(α) species in the plasma. Wear tests were performed on the OrthoPOD(®), a six station pin-on-disk apparatus with ultra-high molecular weight polyethylene (UHMWPE) pins articulating on USND disks and CoCrMo alloy disk. Wear of the UHMWPE was found to be lower for the polyethylene on USND than that of polyethylene on CoCrMo alloy.

An electrospun triphasic nanofibrous scaffold for bone tissue engineering.

A nanofibrous triphasic scaffold was electrospun from a mixture of polycaprolactone (PCL), type-I collagen and hydroxyapatite nanoparticles (nano-HA) with a mixture dry weight ratio of 50/30/20, respectively. Scaffolds were characterized by evaluating fiber morphology and chemical composition, dispersion of HA particles and nanoindentation. Scanning electron microscopy revealed fibers with an average diameter of 180 +/- 50 nm, which coincides well with the collagen fiber bundle diameter characteristic of the native extracellular matrix of bone. The triphasic fibers, stained with calcein and imaged with confocal microscopy, show a uniform dispersion of apatite particles throughout their length with minor agglomeration. Scaffold fibers of triphasic (50/30/20), collagen/nano-HA (80/20), PCL/nano-HA (80/20), pure PCL and pure collagen were each pressure consolidated into non-porous pellets for evaluation by transmission electron microscopy and nanoindentation. While the majority of apatite particles are uniformly dispersed having an average size of 30 nm, agglomerated particles as large as a few microns are sparsely distributed. Nanoindentation of the pressure-consolidated scaffolds showed a range of Young's modulus (0.50-3.9 GPa), with increasing average modulus in the order of (PCL < PCL/nano-HA < collagen < triphasic < collagen/nano-HA). The modulus data emphasize the importance of collagen and its interaction with other components in affecting mechanical properties of osteoconductive scaffolds.

Control of phase composition in hydroxyapatite/tetracalcium phosphate biphasic thin coatings for biomedical applications.

Biphasic calcium phosphates comprising well-controlled mixtures of nonresorbable hydroxyapatite and other resorbable calcium phosphate phases often exhibit a combination of enhanced bioactivity and mechanical stability that is difficult to achieve in single-phase materials. This makes these biphasic bioceramics promising substrate materials for applications in bone tissue regeneration and repair. In this paper we report the synthesis of highly crystalline, biphasic coatings of hydroxyapatite/tetracalcium phosphate with control over the weight fraction of the constituent phases. The coatings were produced by pulsed laser deposition using ablation targets of pure crystalline hydroxyapatite. The fraction of tetracalcium phosphate phase in the coatings was controlled by varying the substrate temperature and the partial pressure of water vapor in the deposition chamber. A systematic study of phase composition in the hydroxyapatite/tetracalcium phosphate biphasic coatings was performed with X-ray diffraction. Tetracalcium phosphate in the coatings obtained at high substrate temperature is not formed by partial conversion of previously deposited hydroxyapatite. Instead, it is produced by nucleation and growth of tetracalcium phosphate itself from the ablation products of the hydroxyapatite target or by accretion of tetracalcium phosphate grains formed during ablation. This finding was confirmed by formation of calcium oxide, not tetracalcium phosphate, after annealing of pure hydroxyapatite coatings at high temperatures of 700-850 degrees C.

Novel gamma-phase of titanium metal at megabar pressures.

Group IV transition metals titanium, zirconium, and hafnium are expected to transform from an ambient hexagonal close packed (hcp, alpha-phase) to a body centered cubic (bcc, beta-phase) at high pressures. This transition path is usually facilitated by the occurrence of an intermediate hexagonal phase (distorted bcc, omega-phase). The existence of a bcc phase in zirconium and hafnium at high pressures has been known for the past ten years; however, its occurrence in titanium has been theoretically predicted but never observed. We report a novel unexpected transformation in titanium metal from an omega phase to an orthorhombic phase (distorted hcp, gamma-phase) at a pressure of 116+/-4 GPa.

Low-temperature growth of nanostructured diamond films.

Nanostructured diamond films are grown on a titanium alloy substrate using a two-step deposition process. The first step is performed at elevated temperature (820 degrees C) for 30 min using a H2/CH4/N2 gas mixture to grow a thin (approximately 600 nm) nanostructured diamond layer and to improve film adhesion. The remainder of the deposition involves growth at low temperature (< 600 degrees C) in a H2/CH4/O2 gas mixture. The continuation of the smooth nanostructured diamond film growth during low-temperature deposition is confirmed by in situ laser reflectance interferometry, atomic force microscopy, micro-Raman spectroscopy, and surface profilometry. Similar experiments performed without the initial nanostructured diamond layer resulted in poorly adhered films with a more crystalline appearance and a higher surface roughness. This low-temperature deposition of nanostructured diamond films on metals offers advantages in cases where high residual thermal stress leads to delamination at high temperatures.

Single-wall carbon nanotubes under high pressures to 62 GPa studied using designer diamond anvils.

Single-wall carbon nanotube samples were studied under high pressures to 62 GPa using designer diamond anvils with buried electrical microprobes that allowed for monitoring of the four-probe electrical resistance at elevated pressure. After initial densification, the electrical resistance shows a steady increase from 3 to 42 GPa, followed by a sharp rise above 42 GPa. This sharp rise in electrical resistance at high pressures is attributed to opening of an energy band gap with compression. Nanoindentation hardness measurements on the pressure-treated carbon nanotube samples gave a hardness value of 0.50 +/- 0.03 GPa. This hardness value is approximately 2 orders of magnitude lower than the amorphous carbon phase produced in fullerenes under similar conditions. Therefore, the pressure treatment of single-wall carbon nanotubes to 62 GPa did not produce a superhard carbon phase.

Electrical and mechanical properties of C70 fullerene and graphite under high pressures studied using designer diamond anvils.

We compare electrical and mechanical properties of C70 fullerene with high purity graphite to 48 GPa at room temperature using designer diamond anvils with embedded electrical microprobes. The electrical resistance of C70 shows a minimum at 20 GPa with transformation to an amorphous insulating phase complete above 35 GPa, while graphite remains conducting. Nanoindentation shows hardness values 220 times larger for the pressure quenched amorphous phase than for similarly treated graphite. Our studies establish that the amorphous carbon phase produced from C70 has unique properties not attainable from graphite.

Crystal structures at megabar pressures determined by use of the cornell synchrotron source.

X-ray diffraction studies have been carried out on alkali halide samples 10 micrometers in diameter (volume 10(-9) cubic centimeter) subjected to megabar pressures in the diamond anvil cell. Energy-dispersive techniques and a synchrotron source were used. These measurements can be used to detect crystallographic phase transitions. Cesium iodide was subjected to pressures of 95 gigapascals (fractional volume of 46 percent) and rubidium iodide to pressures of 89 gigapascals (fractional volume of 39 percent). Cesium iodide showed a transformation from the cubic B2 phase (cesium chloride structure) to a tetragonal phase and then to an orthorhombic phase, which was stable to 95 gigapascals. Rubidium iodide showed only a transition from the low-pressure cubic B1 phase (sodium chloride structure) to the B2 phase, which was stable up to 89 gigapascals.