No abstract

In the laboratory-type airlift tower reactor oxygen transfer from air in tap water and/or polyacrylamide solutions (Neuperm WF) was studied. In order to characterize the system, volumetric coefficient of oxygen transfer was determined by the gassing-out method. Two arrangements of the airlift tower reactor were compared, namely the reactor with and without motionless mixer. In addition, mean relative gas holdup and gas power output were determined for both arrangements.

We report on the preparation and properties of GaN and AlN layers grown by molecular-beam epitaxy on silver metal substrates. X-ray diffraction rocking curves show polycrystalline character of GaN with high preferential GaN͑11-22͒ orientation. An intermetallic phase of Ga 3 Ag is found at the GaN / Ag interface. On the other hand, AlN layers exhibit a monocrystalline structure with a growth direction of ͑0002͒. Schottky diodes prepared on GaN layers show good rectifying behavior and relatively low leakage current ͑ϳ10 −3 A/cm 2 ͒. These results indicate that the III-nitride growth on metallic substrates might be used for low-cost and large-area electronic and photonic devices. The growth and deposition of semiconductor thin films on metallic substrates has been well known for about three decades. Properties of silicon layers deposited onto tungsten and nickel substrates, 1 as well as on steel, 2 have been published. Gallium arsenide 3 grown by liquid phase epitaxy on a polycrystalline Mo metal substrate with a grain size sufficient for high efficiency solar cells has been reported in the past. Since the GaN layers were grown by plasma-assisted molecular beam epitaxy ͑MBE͒, 4 the photoluminescence properties of polycrystalline GaN grown on W, Mo, Ta, and Nb metal substrates by MBE were intensively studied in last decade. [5][6][7] The underlying reason for semiconductor growth on metal substrates might be good thermal conductivity to the environment or low-cost solutions for large-area circuits.The aim of our work is the growth of monocrystalline GaN and AlN on a silver metal substrate. We describe the growth process of the epitaxial layers that are characterized by x-ray diffraction ͑XRD͒ measurements. Schottky diodes on a GaN layer with AlN as a nucleation layer demonstrate the ability to fabricate GaN-based devices on a metal substrate.The III-N layers were grown by MBE using standard effusion cells and a radio-frequency ͑rf͒-plasma source operating at 13.56 MHz, rf power of 450 W, and 1 sccm nitrogen flux. The chamber pressure during the growth was 1 ϫ 10 −5 mbar. In a first series, GaN was grown directly on a silver substrate with a substrate temperature of 760°C. The Ga beam equivalent pressure ͑BEP͒ was varied from 2.5ϫ 10 −7 to 5 ϫ 10 −7 mbar for different runs. The thickness of the layers is around 500 nm. In a second series, a lowtemperature AlN nucleation layer with a substrate temperature of 600°C was grown on the silver substrate. The BEP of the Al cell was changed from 1.0ϫ 10 −7 to 1.4ϫ 10 −7 mbar for different runs. The thickness of this layer was approximately 100 nm. Subsequently, a GaN layer was grown on top of the AlN layer at a substrate temperature of 760°C. The thicknesses of the investigated layers were measured with a surface profilometer ͑Dektak͒, the surface morphology was characterized using atomic force microscopy ͑AFM͒, and the structural properties were determined with XRD. Figure 1 shows the XRD pattern of a GaN layer grown directly on the silver substrate. We observed a polycrysta...

SynopsisStatistical copolymers were prepared from N-carboxyanhydrides of L-valine and y-benzyl-L-glutamate in dioxan with triethylamine as an initiator. The copolymerization conversion was determined by ir spectroscopy, the copolymer composition by amino acid analysis, and the molecular weights by light scattering. The monomer reactivity ratios were found to be rvd = 0.14 and rG'olu(oBrl) = 6.4. High-molecular-weight copolymers are formed even at low conversions. The content of 8-structure in the copolymers was estimated from the ir spectra in copolymerization mixtures. The sequencelength distribution of L-valine and y-benzyl-L-glutamate copolymers was calculated and its dependence on copolymerization conversion is discussed. Relations between the sequence-length distribution and the content of 8-structure were studied. It was found that the content of /?-structure in samples with the same composition is different for low-and high-conversion copolymers. The formation of &structure in copolymers in the copolymerization mixture requires a certain minimal sequence length, which has been found to be about 6 valine units. INTRODUCTIONStatistical copolymers prepared by the copolymerization of two monomers to medium and high conversions are always chemically heterogeneous (i.e., the contents .of different monomer units in the individual macromolecules are different), and at the same time, are sequentially heterogeneous (i.e., the lengths of sequences of monomeric units differ in individual copolymer chains). In the case of binary statistical copolymers, according to classical copolymerization theory, the chemical composition of the arising copolymer and the ordering of monomeric units in its chains are determined, of all the kinetic parameters, only by reactivity ratios of both monomers; they also depend, of course, on the initial composition of the monomer mixture and on copolymerization conversion. Hence, the chemical heterogeneity and the sequence heterogeneity are interdependent, with their mutual relation determined just by the monomer reactivity ratio values.Further discussion is limited to the conversion heterogeneity, which reflects the change of the monomer mixture composition during copolymerization and, subsequently, results in the change in the composition of the forming c o p *To whom all correspondence should be a d d r d . This study extends the analysis of the heterogeneity of copolymers of Glu(OBz1) and Biopolymer~We also reexamine existing procedures used in the analysis of secondary structure by ir spectroscopy. Secondary structure of the copolymers must be investigated and quantitatively characterized directly in the copolymerization mixture. In the present study we try to correlate the sequence-length distribution of Val and Glu(OBz1) constitutional units calculated from the known initial composition of the mixture of monomers and conversion with the content of the secondary structure of various copolymers. THEORETICAL Sequence-Length Distribution in a Chemically Heterogeneous Statistical CopolymerL...

No abstract

No abstract

In this study, ultrafast GaAs metal-semiconductor-metal (MSM) photodetectors grown on aluminum mirrors/pseudo-substrates were fabricated and tested. Surface characterization measurements revealed the good quality of the surface morphology, while x-ray diffraction measurements showed several crystallographic orientations of the GaAs layer. The material exhibited a 50 fs carrier lifetime due to growth-induced defects. The response of the photodetectors showed a full width at half maximum of 300 fs. These results demonstrate that the growth of GaAs layers on lattice-mismatched metallic substrates with high thermal conductivity is a promising approach for low-cost and large-area fabrication of electronic and ultrafast photonic devices that require a highly effective thermal drain.

No abstract