H. R. Shanks

Oriented Schwann cell growth on micropatterned biodegradable polymer substrates

This paper investigates the influence of substrate-mediated chemical and physical guidance on the growth and alignment of Schwann cells in vitro. Novel techniques were developed to fabricate microgrooves with adsorbed proteins on biodegradable polymer substrates made of poly(D,L-lactic acid). Compression molding and solvent-casting were used to transfer micropatterns from quartz and silicon substrates onto biodegradable polymer films. Laminin was selectively adsorbed onto the grooves and rat sciatic Schwann cells were seeded on the substrates. Laminin was found to improve adhesion of Schwann cells on the substrates. The microgrooves were found to cause the Schwann cells to align along the direction of the grooves. The groove width influenced Schwann cell alignment the most, while groove depth did not seem to play a significant role. The degradation of the grooves in the solvent cast films was much slower than those in the compression-molded films, making them the preferred substrates for Schwann cell culture.

Optical phonons of aluminum nitride

Oriented polycrystalline films of AlN on Si(111) substrates and amorphous AlN films on sapphire substrates have been prepared by sputter deposition of aluminum in a nitrogen plasma. Phonons at 303, 426, 514, 614, 663, and 831 cm−1 (±3%) have been observed in the Raman spectra of the crystalline films. The 303‐cm−1 phonon shifts to 288 cm−1 and the 663‐cm−1 phonon to 650 cm−1 in the amorphous sample. By means of infrared absorption measurements with the electric field perpendicular to the c axis we have tentatively identified E1(LO) and E1(TO) modes near 800 and 610 cm−1, respectively. A tentative assignment of the other modes is made from partial data from orientation dependent scattering and results for other wurtzite structure compounds. A comparison of the AlN phonon frequencies with those of other known wurtzite structure compounds shows that Martin’s scaling equation is satisfied.

Electrical Resistivity of Cubic Sodium Tungsten Bronze

The electrical resistivities of the nonstoichiometric compounds NaxWO3 have been measured as a function of sodium concentration from x=0.48 to x=0.88, and as a function of temperature from T=4°K to T=873°K. The minimum in electrical resistivity near x=0.75, which had been reported by earlier investigators, was absent in single crystals which were selected to be electrically homogeneous. Above room temperature, the electrical conductivity increased approximately linearly with sodium concentration; at liquid‐helium temperatures, the electrical conductivity increased approximately as the fourth power of sodium concentration.

Enhancement of the superconducting transition temperature near a phase instability in NaxWO3

Abstract The superconducting transition temperature of tetragonal I sodium tungsten bronze (NaxWO3) has been found to increase rapidly as the x-value is decreased to the metal-semiconductor phase transition. It is suggested that a soft mode instability is responsible for the increased electron-phonon interaction near the phase transition.

Growth of tungsten bronze crystals by fused salt electrolysis

Although tungsten bronzes and related compounds have been prepared by fused salt electrolysis for some time, very little detail about the actual growth methods and results have been reported. This work describes a successful electrolytic cell which has been developed for the growth of a variety of tungsten bronze crystals. This cell has been used to establish the parameters for the growth of NaxWO3 crystals with predictable composition and in addition the phases of bronze obtained as a function of melt composition and temperature. Growth rates were measured and found to vary considerably for the different phases. Large crystals of a new triclinic compound, Na4W14O43, were obtained during the study of different melt compositions. Crystals of the lithium isomorph of this compound were also prepared.

Control of dihydride bond density in reactive sputtered amorphous silicon

Data are presented which demonstrate how the dihydride bonding density in hydrogenated amorphous silicon produced by reactive rf sputtering may be controlled through variation of the rf sputtering power. It is suggested that as the power is increased, the temperature of the ions and neutral atoms above the substrate surface increases until particles incident on the surface have enough energy to prevent or destroy the dihydride bonding configuration. Conductivity data are also presented which show a decrease in dark conductivity and photoconductivity as dihydride bond density is increased. It is concluded that the dihydride bonding reduces carrier mobility.

A comparison of calcium sulfate dihydrate grown in clay gels and in sodium silicate gels

Abstract A comparison of crystals of calcium sulfate dihydrate grown in two different gel media, sodium silicate gel and bentonite clay gel, shows that the two media affect crystal habit and surface topography differently. Those crystals grown in silica gel at 40°C and pH 7.5 consist of elongated prisms, exhibit relatively smooth crystal faces, and are often twinned. Crystals grown in bentonite gel at the same temperature and similar pH tend to be tabular, exhibit a wide variety of kinks, steps, and pits on their surfaces, and are never twinned under our experimental conditions. A comparison of the bentonite-grown crystals with those commonly found in nature shows that bentonite resembles natural sediments in its effects on growth habit of gypsum.

Magnetic Susceptibility of the Cubic Sodium Tungsten Bronzes

The sodium tungsten bronzes (NaxWO3) in the cubic range 0.45<x<1.0 were found to have mass susceptibilities from 0.007×10—6 for x=0.49 to 0.053×10—6 for x=0.85. This feeble paramagnetism was found to be temperature independent from 70° to 300°K for three representative samples with x=0.49, 0.76, and 0.85. The susceptibility of WO3 was determined to be —0.060×10—6 emu/g and is also temperature independent. Satisfactory agreement between calculated and observed susceptibilities was obtained with a model which assumes that the bronzes consist of a dispersion of sodium ions in a WO3 lattice. The molar susceptibility, then, can be calculated from the equation χM=χ(WO3)+xχ(Na+)+χe. The term χe for the Pauli paramagnetism was obtained for two cases: (a) for nearly free electrons (m*=1.6m), and (b) for the density of states taken from the literature data on the low temperature heat capacity of the bronzes. Best quantitative agreement was obtained between the calculated and the observed susceptibilities for case b...

Thermal study of II–IV semiconductors: Heat capacity and thermodynamic functions of Mg2Pb from 5–300°K☆

Abstract The heat capacity of Mg2Pb has been measured at 5–300°K, and the lattice heat capacity, Debye temperature, and thermodynamic functions have been tabulated. The following values were found for the thermodynamic functions at 298.15°K: S°298.15 = 72.85 J (mole°K)−1 and −(G° 298.15 − H 0 °) T = 59.50 J (mole°K) −1 . The values of the experimentally determined thermodynamic functions are compared with values predicted utilizing a reduced temperature function, and are found to differ by more than experimental error.

The interaction of O2, NO, and N2O with Ge surfaces

Abstract The interaction of molecular oxygen (O 2 ), nitric oxide (NO), and nitrous oxide (N 2 O) with Ge(111), Ge(111) disordered by sputter-etching, and amorphous Ge (a-Ge) surfaces was studied at room temperature using high-resolution electron energy loss spectroscopy (HREELS), core level and valence band EELS, and Auger electron spectroscopy. The interaction of activated NO, N 2 O, and N 2 molecules with these surfaces was also examined. Results indicate that oxygen originating from O 2 bonds to surface and subsurface Ge atoms in a bridge configuration. Penetration is more pronounced at the disordered surfaces resulting in a dilute bridge bonding configuration (+ 1 oxidation state), whereas the + 1 to + 3 oxidation states are observed at the similarly treated Ge(111). The N 2 O molecules decompose to desorbing N 2 molecules and chemisorbed oxygen atoms. In contrast, nitrogen and oxygen are detected by Auger analysis at the NO-exposed surfaces. However, the germanium nitride vibrational modes are identified only after heating these surfaces to ~ 550 ° C. A precursor Ge nitride is observed at room temperature following exposure to nitrogen atoms and ions. Only the + 1 oxidation state is observed for the surfaces exposed to NO and N 2 O. This behavior is discussed and compared to O 2 chemisorption. Electronic EELS of Ge surfaces exposed to activated NO, N 2 O, and N 2 indicate that bonding occurs at the surface dangling bonds. Nitrogen related electronic losses are also identified.