bluuebunny/crossref_metadata_2025_split huggingface dataset:
Transfer of Carbon Dioxide within Cultures of Microalgae: Plain Bubbling versus Hollow‐Fiber Modules
Authors: Carvalho Ana P. , Malcata F. Xavier 
Published: July 2002

AbstractIn attempts to improve the metabolic efficiency in closed photosynthetic reactors, availability of light and CO2 are often considered as limiting factors, as they are difficult to control in a culture. The carbon source is usually provided via bubbling of CO2‐enriched air into the culture medium; however, this procedure is not particularly effective in terms of mass transfer. Besides, it leads to considerable waste of that gas to the open atmosphere, which adds to operation costs. Increase in the interfacial area of contact available for gas exchange via use of membranes might be a useful alternative; microporous membranes, in hollow‐fiber form, were tested accordingly. Two hollow‐fiber modules, different in both hydrophilicity and outer surface area, were tested and duly compared, in terms of mass transfer, versus traditional plain bubbling. Overall volumetric coefficients (KLa) for CO2 transfer were 1.48 × 10−2 min−1 for the hydrophobic membrane, 1.33 × 10−2 min−1 for the hydrophilic membrane, and 7.0 × 10−3 min−1 for plain bubbling. A model microalga, viz. Nannochloropsis sp., was cultivated using the two aforementioned membrane systems and plain bubbling. The produced data showed slight (but hardly significant) increases in biomass productivity when the hollow‐fiber devices were used. However, hollow‐fiber modules allow recirculation of unused CO2, thus reducing feedstock costs. Furthermore, such indirect way of supplying CO2 offers the additional possibility for use of lower gas pressures, as no need to counterbalance hydrostatic heads exists.
Engineering Dental Pulp‐like Tissue in Vitro
Authors: Mooney David J. , Powell Courtney , Piana James , Rutherford Bruce 
Published: July 2002

AbstractInjury or infection of adult dental pulp often necessitates root canal therapy. This terminates dentin formation and subsequent tooth maturation. In addition, the synthetic materials currently utilized to replace lost tooth structure are not capable of completely replacing the function of the lost tissue, and often fail over time. This report describes a technique to engineer new pulp‐like tissues utilizing cultured cells and synthetic extracellular matrices. Fibroblasts were obtained from human adult dental pulps and multiplied in culture. These cells were subsequently seeded onto synthetic matrices fabricated from fibers (approximately 15 μm in diameter) of polyglycolic acid (PGA). The pulp‐derived fibroblasts adhered to the fibers, proliferated, and formed a new tissue over 60 days in culture with a cellularity similar to that of native pulp. These tissues may find application in the regeneration of oral tissues and may provide novel systems in which to study the biocompatibility of materials and chemicals used in dentistry.
Pattern of transmission of laser light through carious molar teeth
Authors: Chandler N. P. , Pitt Ford T. R. , Watson T. F. 
Published: July 2002

AbstractAimsThe aims of this study were to investigate the pattern of transmission of laser light through carious human molar teeth, and to examine the effect on light transmission of reactions to caries in the dentine.MethodologySectioned carious human molars were examined whilst illuminated from their external surfaces by a helium–neon laser probe of 0.5 mm diameter. The pattern of light transmission was observed and selected specimens were photographed. The passage of light through the surrounding dentine toward the pulp space was investigated by confocal microscopy. Further carious and restored teeth were illuminated from three buccal and three lingual sites to establish the probability of light being able to reach the coronal pulp space.ResultsFor all 31 teeth examined, it was posssible to illuminate the pulp space from a position on the buccal side. The presence of intracoronal restorations did not necessarily prevent light transmission to the pulp. Confocal microscopy demonstrated conduction of light within dystrophic deposits of irregular secondary dentine.ConclusionsThe presence of carious lesions or restorations in molar teeth does not necessarily interfere with transmission of laser light to the pulp space. Light is conducted within irregular secondary dentine. With appropriate probe placement, assessments of pulp health in diseased posterior teeth may be possible using laser Doppler flowmetry.
Modelling and exploring human sleep with event history analysis
Authors: YASSOURIDIS ALEXANDER , STEIGER AXEL , KLINGER ARTUR , FAHRMEIR LUDWIG 
Published: July 2002

SummaryIn this paper we propose the use of statistical models of event history analysis for investigating human sleep. These models provide appropriate tools for statistical evaluation when sleep data are recorded continuously over time or on a fine time grid, and are classified into sleep stages such as REM and nonREM as defined by Rechtschaffen and Kales (1968). In contrast to conventional statistical procedures, event history analysis makes full use of the information contained in sleep data , and can therefore provide new insights into non‐stationary properties of sleep. Probabilities of or intensities for transitions between sleep stages are the basic quantities for characterising sleep processes. The statistical methods of event history analysis aim at modelling and estimating these intensities as functions of time, taking into account individual sleep history and assessing the influence of factors of interest, such as hormonal secretion. In this study we suggest the use of non‐parametric approaches to reveal unknown functional forms of transition intensities and to explore time‐varying and non‐stationary effects. We then apply these techniques in a study of 30 healthy male volunteers to assess the mean population intensity and the effects of plasma cortisol concentration on the transition between selected sleep stages as well as the influence of elapsed time in a current REM period on the intensity for a transition to nonREM. The most interesting findings are that (a) the intensity of the nonREM‐to‐REM transitions after sleep onset in young men shows a periodicity which is similar to that of nonREM/REM cycles; (b) 30–45 min after sleep onset, young men reveal a great propensity to pass from light sleep (stages 1 or 2) into slow‐wave sleep (SWS) (stages 3 or 4); (c) high cortisol levels imposed additional impulses on the transition intensity of (i) wake to sleep around 2 h after sleep onset, (ii) nonREM to REM around 6 h later, (iii) stage 1 or stage 2 sleep to SWS around 2, 4 and 6 h later and (iv) SWS to stage 1 or stage 2 sleep about 2 h later. Moreover, high cortisol concentrations at the beginning of REM periods favoured the change to nonREM sleep, whereas later their influence on a nonREM change became weak and weaker. As sleep data are also available as event‐oriented data in many studies in sleep research, event history analysis applied additionally to conventional statistical procedures, such as regression analysis or analyis of variance, could help to acquire more information and knowledge about the mechanisms behind the sleep process.
COMPARATIVE EVALUATION OF THE INFLAMMATORY MEDIATORS IN PATIENTS UNDERGOING LAPAROSCOPIC VERSUS CONVENTIONAL CHOLECYSTECTOMY
Authors: Chaudhary D. , Verma G. R. , Gupta R. , Bose S. M. , Ganguly N. K. 
Published: July 2002

Background: Injury induces systemic inflammatory response. The degree of response is proportionate to the trauma. Methods: Patients with ultrasound‐proven gall stones posted for operation were randomized into two groups. Laparoscopic cholecystectomy was performed in one group while the other group underwent conventional cholecystectomy. Peripheral venous blood samples for inflammatory mediators, namely tumour necrosis factor, C‐reactive protein, oxygen release from monocytes and polymorphonuclear leucocytes were drawn 1 day before surgery and repeated on the first and third days after surgery. Partial pressure of arterial oxygen was also measured in both groups. Only patients who met selection criteria and had uncomplicated postoperative courses were included in the study.There was a total of 40 patients for the final analysis, 20 in each group. Results: There was no significant difference (P > 0.1) in age, sex or body mass index in both groups of patients. The level of all of the inflammatory mediators was elevated on the first postoperative day; however, the elevation was significantly less following laparoscopic cholecystectomy compared to conventional cholecystectomy (P < 0.01). Although the values came down on the third postoperative day, they remained above the respective controls. Postoperative hypoxaemia was significantly more in patients of conventional than laparoscopic cholecystectomy (P < 0.001). Conclusions: Trauma‐induced inflammatory response is significantly less following laparoscopic cholecystectomy compared to conventional cholecystectomy. It appears to be related to degree of trauma.
Electronic structure and deep impurity levels in [111] GaAs/AlxGa1−xAs semiconductor superlattices
Authors: Ren Shang Yuan , Dow John D. 
Published: July 2002

A theory of the electronic structures of periodic N1×N2 GaAs/AlxGa1−xAs superlattices grown along the [111] direction is presented. Deep levels associated with s- and p-bonded substitutional impurities in these superlattices are also predicted. It is found that: (i) in contrast with [001] superlattices, [111] superlattices are almost always direct band-gap superlattices. (ii) The [111] superlattices exhibit weaker quantum-well confinement than the corresponding [001] superlattices. (iii) As the thickness, t(GaAs), of each GaAs layer is reduced below a critical value (tc ≂13 Å or N1≂4 for x=0.7iii), common shallow donor impurities such as Si cease donating electrons to the conduction band and instead become deep traps. For [111] superlattices tc is smaller than the corresponding tc for [001] superlattices. The fundamental band gap and the band edges of the superlattice, and hence the ionization energies of deep levels, depend strongly on the layer thickness t(GaAs) but only weakly on t(AlxGa1−xAs). The T2- and A1-derived deep levels (of the bulk point group Td) are split and shifted, respectively, near a GaAs/AlxGa1−xAs interface: the p-like T2 level splits into an a1 ( pσ-like) level and a doubly degenerate e ( pπ -like) level of the point group for any general superlattice site (C3v), whereas the s-like A1 bulk level becomes an a1 (s-like) level of C3v. The order of magnitude of the shifts and splittings of deep levels at a GaAs/AlxGa1−xAs interface is less than 0.1 eV, depends on x, and becomes very small for impurities more than ≂3 atomic planes away from an interface. These predictions are based on a periodic
superslab calculation for unit superslabs with total thickness t(GaAs)+t(AlxGa1−xAs) as large as 65.3 Å or N1+N2=20 two-atom-thick layers. The Hamiltonian is a tight-binding model in a hybrid basis that is a generalization of the Vogl model and properly accounts for the nature of interfacial bonds. The deep levels are computed using
the Hjalmarson et al. theory [Phys. Rev. Lett. 44, 810 (1980)] and the special points method.
Growth mode and interface formation of Fe and Co films on ZnSe(001) (abstract)
Authors: Jonker B. T. , Prinz G. A. 
Published: July 2002

The incorporation of magnetic layers in semiconductor heterostructures is an increasingly active area of study. We are interested in the growth of ferromagnetic layers on diluted magnetic semiconductor (DMS) substrates as a means of magnetically polarizing the DMS medium, a necessary condition for these materials to exhibit the extraordinary magneto-optic and transport properties for which they are known. Since interfacial effects are expected to play an important role in thin-film heterostructures, we have examined the growth of Fe and Co films on ZnSe(001) to determine the mode of film growth, the formation of the interface, and the structure of the overlayer at the 1–10 monolayer level. The results obtained should apply to the growth of such overlayers on the ZnSe-based DMS compounds for modest magnetic ion concentrations as well, such as (Zn,Mn)Se, (Zn,Fe)Se, and (Zn,Co)Se. The ZnSe(001) samples were grown by molecular-beam epitaxy on GaAs(001) substrates, passivated with a Se coating in a manner analagous to the As passivation of GaAs, and transferred to a surface analysis system.
The Se coating was thermally desorbed in UHV at 150–200 °C just prior to metal deposition. The metal films were deposited from miniature electron-beam sources and characterized in situ with Auger electron diffraction, RHEED, and x-ray photoelectron spectroscopy. The coverages were determined by x-ray flourescence measurements. The coverage dependence of the forward scattering peaks in the Auger electron diffraction scans due to the occupation of second and third monolayer sites provides information on the initial mode of film growth. The angular position of these peaks provides a measure of the out-of-plane lattice spacing, and hence the structure of the overlayer. We find that the growth of Fe(001) on a ZnSe(001) epilayer is predominantly 2D in nature at a substrate temperature of 175 °C, while growth directly on the oxide-desorbed GaAs(001) bulk substrate shows more significant 3D clustering. While the growth of Co on GaAs(001) results in a nearly ideal bcc single-crystal structure, growth on ZnSe(001) epilayers yields a multicrystalline structure. Little interaction with the ZnSe is observed with XPS.
Effect of residence time on microwave plasma chemical vapor deposition of diamond
Authors: Celii F. G. , White D. , Purdes A. J. 
Published: July 2002

We have investigated the effects of reactant residence time on the properties of microwave-assisted chemical vapor deposited diamond films. Using a constant gas pressure of 40 Torr and gas composition of 1% CH4 in H2, the total gas flow rate was adjusted from 25 to 800 sccm which corresponds to apparent chamber residence times of 3–100 s for our reactor. The flow rate is one of the few parameters in the microwave plasma deposition system that is decoupled from all others, making this a relatively clean study on the effects of a single process variable. Two distinct types of diamond films were prepared in this work. One set was deposited directly on diamond-seeded silicon wafers. A second set was prepared separately, and under otherwise identical diamond film growth conditions, but deposited on a layer of microcrystalline diamond (MCD). The 5-h growths resulted in continuous polycrystalline diamond films of ∼2 μm thickness which were analyzed with scanning electron microscopy, Raman scattering and photoluminescence (PL) spectroscopies, infrared (IR) transmission, and x-ray diffraction. The Raman and PL spectra, and x-ray diffraction data exhibited flow-dependent behavior, while the electrical resistivity, IR transmission spectra, and deposition rate of the films, as well as the optical emission spectra of the deposition plasmas, were relatively insensitive to the flow rate. The x-ray diffraction results revealed that the diamond films were textured, although the preferred orientation was not particularly related to the surface morphology.
The surfae preparation strongly affected the texture, with films deposited on MCD exhibiting a [110] texture while those on Si had a [111]-preferred orientation. Furthermore, the extent of texture for the films on Si varied with the gas flow rate, unlike that of the films on MCD. The Raman and PL spectra varied with flow for both the films on Si and on MCD. Flow modeling showed the importance of convective flow in our reactor, and the insensitivity of deposition rate on gas flow rate can be rationalized by the flow pattern. The presence of vortices above the growth surface complicates the concept of residence time for this system. We argue that variation in the concentrations of gas-phase diamond growth species, arising from changes in gas residence time, are responsible for the flow-dependent variation in diamond film material properties.
Effect of isothermal annealing on the magnetic behavior of amorphous ferromagnets with composition near the percolation threshold. I. Outside the critical region
Authors: Kaul S. N. , Mohan Ch. V. 
Published: July 2002

Results of detailed ferromagnetic resonance measurements taken on amorphous (a-) Fe90+XZr10−X alloys with X=0 and 1 in the horizontal-parallel and vertical-parallel sample configurations at a fixed microwave frequency of ≂9.23 GHz in the temperature range 77 to 300 K before and after these alloys have undergone isothermal annealing treatment at 400 K for durations of time, tA, ranging from 10 to 240 min are presented and discussed. While only a single (primary) resonance is observed for T≲TC (the Curie temperature), an additional (secondary) resonance first appears at T≂TC and then gets fully resolved for T≳(TC+10 K). For the primary resonance (i) the ‘‘in-plane’’ uniaxial anisotropy field, Hk, and the Gilbert damping parameter, λ, both scale with the saturation magnetization, MS, in the temperature ranges 77 K≤T≲TC and 0.5TC≲T≲0.8TC, respectively, at all tA including tA=0; (ii) isothermal annealing has essentially no effect on MS(T), the ‘‘peak-to-peak’’ linewidth, ΔHpp(T), ΔH0 (the frequency- and temperature-independent contribution to ΔHpp) and λ(T); (iii) with increasing Fe concentration, the value corresponding to the peak in λ(T) decreases, λ(0.5TC) increases while λ(300 K) stays constant at 1×108 s−1, and (iv) isothermal annealing up to a time duration of 30 min results in an enhancement of about 1.5% in TC and a reduction of nearly 40% in the characteristic temperature T0 (which is a measure of the re-entrant transition temperature) whereas no further change occurs in both TC and T0 at higher annealing times.
By contrast, for the secondary resonance, (a) Hk≂0 at all temperatures T≳TC and values of tA covered in the present experiments, (b) isothermal annealing leaves the resonance field unaltered, and (c) a steep decline in the value of ΔHpp′ occurs for tA≤10 min but at higher values of tA, ΔHpp′ remains constant for a-Fe90Zr10 whereas it increases at first and then saturates in the case of a-Fe91Zr9. The effect of isothermal annealing on the quantities of interest, i.e., TC, T0, ΔHpp, etc., can be qualitatively understood in terms of a finite spin clusters plus an infinite ferromagnetic matrix model.
Some electrical properties of amorphous silicon/amorphous silicon nitride interfaces: Top nitride and bottom nitride configurations in MNS and TFT devices
Authors: Godet C. , Kanicki J. , Gelatos A. V. 
Published: July 2002

Top nitride (TN) and bottom nitride (BN) configurations of the a-SiN1.6:H/a-Si:H interface produced by plasma enhanced chemical vapor deposition (PECVD) have been characterized by using metal/a-SiN1.6:H/a-Si:H (MNS) devices and thin-film transistors (TFT). We observed that: (i) for MNS devices, the resonant frequency activation energy Eac from admittance, saturates at a higher value for the TN (≂0.3 eV) than for the BN (≂0.1 eV) MNS devices and the resonant frequency preexponential factors are found to be, respectively, in the 1012 s−1 and 108 s−1 range, (ii) in strong accumulation, the source-drain current activation energy is nearly similar for TN and BN TFTs (Edc≂0.1 eV), a slightly higher value being found in the latter configuration, (iii) the analysis of the transfer characteristics yields a very similar density of deep interface states (DOS) ≂4–5×1012 cm−2 eV−1 in both configurations, and (iv) the effective field-effect mobility is higher for the BN (μFE≳0.5 cm2/V s) than for the TN (μFE≲0.3 cm2/V s) TFTs.
The observed difference in mobilities for both TFTs structures is most likely associated with very high source/drain contact resistances in the TN TFT rather than with the quality of both interfaces. To explain some of these experimental results, the surface- and buried-channel models are proposed for TN and BN TFT configurations, respectively. The buried-channel concept is based on evidence of recycling/intermixing of nitrogen atoms into a-Si:H deposited on a-SiN1.6:H by PECVD. The nitrogen tail would produce a Si-rich a-SiNx:H alloy transition layer, followed by a nitrogen-doped n-type layer. The doped layer corresponding to the buried-channel formation is predicted to be located beyond 30 Å from the BN interface. In contrast, the surface-channel proposed for the TN interface is based upon this interface being considered as atomically sharp. The MNS results (i) are consistent with this model. In the case of TN MNS they are explained by thermionic emission of electrons from the traps to the conduction band edge in a-Si:H and in the case of BN MNS by hopping in the defective a-SiNx:H interlayer. The result (ii) on TFTs may be attributed to a broadening of the linear part of the conduction band tail of a-Si:H in the channel region, due to recycled nitrogen atoms.
Frequency control of semiconductor lasers
Authors: Ohtsu M. , Nakagawa K. , Kourogi M. , Wang W. 
Published: July 2002

This article reviews our recent works on frequency control of semiconductor lasers. The magnitudes of quantum noise limited frequency modulation (FM) noise, realized by the negative electrical feedback, are given for four methods of using an external Fabry–Perot cavity as a frequency demodulator. It is shown that the theoretical expression for the quantum noise-limited FM noise of the feedback laser contains a factor of 1/8 as compared with that of the free running laser, which is due to the different ways of injecting the vacuum fluctuations to the laser cavity and to the external Fabry–Perot cavity for negative electrical feedback. The FM sideband technique is shown to be an effective method to reject the contribution of laser power fluctuations to the FM noise detection for the negative electrical feedback system. As a candidate for a high reflectivity and frequency selective external reflector for the optical feedback, characteristics of the semiconductor laser as a phase conjugate mirror, i.e., the characteristics of the nearly degenerate four-wave mixing and the nondegenerate four-wave mixing in a semiconductor laser, are shown. Optical feedback by using a velocity selective optical pumping and polarization spectroscopy of an atomic vapor is proposed as an effective method to realize simultaneously the center frequency stabilization and linewidth reduction of the field spectrum of the laser, and also the fine detuning of the stabilized center frequency.
For the heterodyne frequency locking between two lasers, a spectroscopic method of using a Doppler-free spectrum of the three-level atomic vapor, obtained by using the phenomenon of coherent population trapping, is shown. In order to realize a highly efficient nonlinear optical frequency conversion for wideband frequency sweep of semiconductor lasers, a method of adding the output powers of several lasers, i.e., the coherent addition, is presented. After emphasizing that the wideband frequency sweep (covering from the near-infrared to the visible region) can be realized by using the techniques of nonlinear optical frequency conversions and the optical phase locking, relevant experimental results of nonlinear optical frequency conversions are presented which are the second harmonics generation, sum and difference frequency conversions. A highly accurate optical frequency measurement system is proposed using an optical frequency comb generator with a modulation sidebands up to several THz. Performances of the optical frequency comb generator used for this system are presented. As a candidate for an ultrafast and wavelength insensitive photodetector for optical frequency counting, nonbolometric optical response characteristics of a high transition temperature (Tc) oxide superconducting film are demonstrated.