bluuebunny/crossref_metadata_2025 huggingface dataset:
Spectroscopic analysis of a dye–mineral composite–a Raman and FT‐IR study
Authors: Manciu F. S. , Ramirez A. , Durrer W. , Govani J. , Chianelli R. R.
Published: April 2008

In this investigation, we address the question of how organic thioindigo binds to inorganic palygorskite to form a pigment similar to Maya Blue. We also address how such binding, if it occurs, might be affected by varying the proportion of dye relative to that of the mineral, and by varying the length of heating time used in preparation of the pigment. In addition to samples of palygorskite and thioindigo both alone, four synthetic pigment samples were prepared; two samples of 8 wt.% dye, one heated at 170 °C for 3 h and one at 170 °C for 9 h, and two samples of 16 wt.% dye, one heated at 170 °C for 3 h and one at 170 °C for 9 h. All samples were examined using Fourier transform‐infrared (FT‐IR) and FT‐Raman spectroscopy. For the pigment samples, FT‐IR peaks at 1627 cm−1 are attributed to a downshifted CO stretching mode of thioindigo due to dye–clay interaction. This interpretation is corroborated by FT‐Raman CO peaks with 14 cm−1 shifts to lower wavenumber for the pigment relative to thioindigo alone. Additional Raman scattering between 550 cm−1 and 650 cm−1 also suggests dye–clay interaction through metal–oxygen bonding. We are thus led to the possibility of mostly hydrogen bonding between silanol and carbonyl at lower dye concentration, with a predominance of metal–oxygen bonding at higher dye concentration. Copyright © 2008 John Wiley & Sons, Ltd.
Heparin‐binding proteins of human seminal plasma: purification and characterization
Authors: Kumar Vijay , Hassan Md. Imtaiyaz , Kashav Tara , Singh Tej P. , Yadav Savita
Published: April 2008

Human seminal plasma (HuSP) contains several proteins that bind heparin and related glycosaminoglycans. Heparin binding proteins (HBPs) from seminal plasma have been shown to participate in modulation of capacitation or acrosome reaction and thus have been correlated with fertility in some species. However, these have not been studied in detail in human. The objective of this study was to purify major HBPs from HuSP in order to characterize these proteins. HBPs were isolated by affinity—chromatography on Heparin—Sepharose column, purified by reverse‐phase high‐performance liquid chromatography (RP‐HPLC) and Size‐exclusion chromatography and checked for purity on sodium‐dodecyl PAGE (SDS–PAGE). Identification of HBPs was done by matrix‐assisted laser desorption‐ionization‐time‐of‐flight‐mass spectrometry (MALDI‐TOF‐MS). Here we report the purification and identification of seven HBPs in seminal fluid. The major HBPs are lactoferrin and its fragments, semenogelin I fragments, semenogelin II, prostate specific antigen, homolog of bovine seminal plasma‐proteins (BSP), zinc finger protein (Znf 169) and fibronectin fragments. In this study we are reporting for the first time the purification and identification of BSP‐homolog and Znf 169 from HuSP and classified them as HBPs. Here we report the purification of seven clinically important proteins from human seminal fluid through heparin affinity chromatography and RP‐HPLC, in limited steps with higher yield. Mol. Reprod. Dev. 75: 1767–1774, 2008. © 2008 Wiley‐Liss, Inc.
In vitro and in vivo derived porcine embryos possess similar, but not identical, patterns of Oct4, Nanog, and Sox2 mRNA expression during cleavage development
Authors: Magnani Luca , Cabot Ryan A.
Published: April 2008

In vitro culture conditions stress the cleavage stage mammalian embryo and can contribute to reduced developmental potential of cultured embryos. One process that may be altered during embryo culture is the establishment and maintenance of pluripotency. Pluripotency is largely controlled by three genes: Oct4, Nanog, and Sox2. The objective of this study was to determine the expression pattern of Oct4, Nanog, and Sox2 in cleavage stage porcine embryos obtained in vivo or by in vitro fertilization and parthenogenetic activation. We used quantitative, real time PCR to assess the relative amount of each transcript in cleavage stage embryos. We found that Oct4 was transiently activated at the 2‐cell stage (P‐value <0.05) while Nanog and Sox2 were activated at the 4‐cell stage (P‐value <0.05) in in vitro embryos. Embryos derived in vivo showed a similar but not identical pattern of expression of Nanog mRNA been in highest abundance both at the 4 cell and the blastocyst stage. The activation observed at the 4‐cell stage for Nanog and Sox2 was shown to be RNA polymerase II dependent (P‐value <0.05). This study showed that Oct4, Nanog, and Sox2 possess similar, but not identical, patterns of expression between in vitro and in vivo derived porcine embryos. The difference between the amount of transcripts may reflect the reduced developmental potential observed in in vitro cultured embryos. Mol. Reprod. Dev. 75: 1726–1735, 2008. © 2008 Wiley‐Liss, Inc.
Methylation changes in porcine primordial germ cells
Authors: Petkov Stoyan G. , Reh Wade A. , Anderson Gary B.
Published: April 2008

Epigenetic re‐programming is an important event in the development of primordial germ cells (PGC) into functional gametes, characterized by genome‐wide erasure of DNA methylation and re‐establishment of epigenetic marks, a process essential for restoration of the potential for totipotency. In this study changes in the methylation status of centromeric repeats and two IGF2‐H19 differentially methylated domain (DMD) sequences were examined in porcine PGC between Days 24 and 31 of pregnancy. The methylation levels of centromeric repeats and IGF2‐H19 DMD sequences decreased rapidly from Days 24 to 28 in both male and female PGC. At Days 30 and 31 of pregnancy centromeric repeats and IGF2‐H19 DMD sequences acquired new methylation in male PGC, while in female PGC these sequences were completely demethylated by Day 30 and remained hypomethylated at Day 31. To characterize methylation changes that PGC undergo in culture, the methylation status of embryonic germ cells (EGCs) derived from PGC at Day 26 of pregnancy was examined. Centromeric repeats and IGF2‐H19 DMD sequences were similarly methylated in both male and female EGC and hypermethylated in female EGC compared with female PGC at the same embryonic age. Our results show that, similar to murine PGC, porcine PGC undergo genome‐wide DNA demethylation shortly after arrival in the genital ridges. When placed in culture porcine PGC terminate their demethylation program and may acquire new DNA methylation marks. To our knowledge, this is the first report regarding epigenetic re‐programming of genital ridge PGC in the pig. Mol. Reprod. Dev. 76: 22–30, 2009. © 2008 Wiley‐Liss, Inc.
A level set method for shape and topology optimization of large‐displacement compliant mechanisms
Authors: Luo Zhen , Tong Liyong
Published: April 2008

A parameterization level set method is presented for structural shape and topology optimization of compliant mechanisms involving large displacements. A level set model is established mathematically as the Hamilton–Jacobi equation to capture the motion of the free boundary of a continuum structure. The structural design boundary is thus described implicitly as the zero level set of a level set scalar function of higher dimension. The radial basis function with compact support is then applied to interpolate the level set function, leading to a relaxation and separation of the temporal and spatial discretizations related to the original partial differential equation. In doing so, the more difficult shape and topology optimization problem is now fully parameterized into a relatively easier size optimization of generalized expansion coefficients. As a result, the optimization is changed into a numerical process of implementing a series of motions of the implicit level set function via an existing efficient convex programming method. With the concept of the shape derivative, the geometrical non‐linearity is included in the rigorous design sensitivity analysis to appropriately capture the large displacements of compliant mechanisms. Several numerical benchmark examples illustrate the effectiveness of the present level set method, in particular, its capability of generating new holes inside the material domain. The proposed method not only retains the favorable features of the implicit free boundary representation but also overcomes several unfavorable numerical considerations relevant to the explicit scheme, the reinitialization procedure, and the velocity extension algorithm in the conventional level set method. Copyright © 2008 John Wiley & Sons, Ltd.
Analysis and reduction of quadrature errors in the material point method (MPM)
Authors: Steffen Michael , Kirby Robert M. , Berzins Martin
Published: April 2008

The material point method (MPM) has demonstrated itself as a computationally effective particle method for solving solid mechanics problems involving large deformations and/or fragmentation of structures, which are sometimes problematic for finite element methods (FEMs). However, similar to most methods that employ mixed Lagrangian (particle) and Eulerian strategies, analysis of the method is not straightforward. The lack of an analysis framework for MPM, as is found in FEMs, makes it challenging to explain anomalies found in its employment and makes it difficult to propose methodology improvements with predictable outcomes.

In this paper we present an analysis of the quadrature errors found in the computation of (material) internal force in MPM and use this analysis to direct proposed improvements. In particular, we demonstrate that lack of regularity in the grid functions used for representing the solution to the equations of motion can hamper spatial convergence of the method. We propose the use of a quadratic B‐spline basis for representing solutions on the grid, and we demonstrate computationally and explain theoretically why such a small change can have a significant impact on the reduction in the internal force quadrature error (and corresponding ‘grid crossing error’) often experienced when using MPM. Copyright © 2008 John Wiley & Sons, Ltd.
Preparation of microwave radiation induced graft copolymers and their applications as reinforcing material in phenolic composites
Authors: Kaith B.S. , Kalia Susheel
Published: April 2008

Flax fiber was modified through grafting of binary vinyl monomers mixtures such as methyl methacrylate (MMA)/vinyl acetate (VA), MMA/acrylamide (AAm), and MMA/styrene (Sty) under the influence of microwave radiations. 24.64% grafting was found at 210 W microwave power under optimum reaction conditions. Graft copolymers obtained were characterized with FTIR spectroscopy, scanning electron microscopy, and TGA/DTA techniques. Graft copolymers were found to be moisture retardant with better tensile strength. Phenolic composites using graft copolymers vis‐à‐vis flax as reinforcing material were subjected for the evaluation of different mechanical properties such as wear resistance, tensile strength, compressive strength, modulus of rupture (MOR), modulus of elasticity (MOE), and stress at the limit of proportionality (SP). Composites reinforced with graft copolymers showed better mechanical properties in comparison to composites reinforced with flax. Phenolic composites reinforced with Flax‐g‐poly(MMA/Sty) showed maximum wear resistance followed by reinforcement with flax, Flax‐g‐poly (MMA/AAm), and Flax‐g‐poly(MMA/VA). Composites reinforced with Flax‐g‐poly(MMA/Sty) and flax fibers have been found to show 150 N tensile strength with extension of 3.94 and 2.17 mm, respectively. It has also been found that composites reinforced with Flax‐g‐poly(MMA/Sty) showed maximum compressive strength (1,000 N) with compression of 3.71 mm in comparison to other graft copolymers and flax fibers reinforcement. Reinforcement of phenolic resin with Flax‐g‐poly(MMA/Sty) and flax fibers could improve the MOR and MOE. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers
On the application of liquid‐crystal thermography for the nondestructive detection of delamination in composite structures
Authors: Khatibi Akbar Afaghi , Maleki Hadi Rezghi
Published: April 2008

To prevent the catastrophic failure of composite structures, several nondestructive testing methods have been developed. In this study, a new technique to evaluate these structures by using thermochromic liquid crystal (TLC) thermography is presented. Temperature gradients produced by sensitive liquid‐crystals are used to detect delamination in composite specimens. The effects of constituent materials and delamination size/location are investigated. The results from TLC thermography are compared to those from infrared thermography. Finally, advantages/disadvantages of the new method are discussed. On the basis of the results from this study, it could be concluded that the TLC thermography can be used as an inexpensive nondestructive testing method in inspection of composite structures. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers
Mechanical properties and morphologies of polypropylene with different sizes of glass bead particles
Authors: Yang Kun , Yang Qi , Li Guangxian , Kuang Junjie , Jiang Ziqiang
Published: April 2008

A commercial grade of isotactic polypropylene (PP) was used to study the mechanical properties and morphologies of the PP composites filled with four sizes of glass bead particles. The glass bead particles used were with average particle sizes of 15 μm (GB15), 10 μm (GB10), 5 μm (GB5), and 2.5 μm (GB2.5), respectively. It was clear that the glass bead size was an important factor on the determination of mechanical properties of the composites. As a whole, in view of the scatter in the data, under the condition of same filler content, the yield strength and impact strength of the composites filled with smaller glass bead particles was higher than those of the composites filled with bigger ones. And the flexural strength and modulus of the composites filled with GB10, GB5, or GB2.5 particles could be regarded as the same. The flexural strength and modulus of the composites filled with GB15 particles were higher than those of the composites filled with other three sizes of particles. Among four sizes of glass bead particles, GB2.5 had the best toughening effect to improve the impact strength of PP matrix. And the major toughening mechanism of the PP/GB2.5 composites was the pinning effect introduced by GB2.5. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers
Phase transition studies of polymer–liquid crystal composite using dielectric and optical transmittance techniques
Authors: Agarwalla N.K. , Madhusudanan K.P. , Srivastava Abhishek Kumar , Manohar Rajiv , Shukla J.P.
Published: April 2008

Dielectric properties of polymer–liquid crystal composite comprised of the polymer poly(methyl methacrylate) and low‐molecular mass liquid crystal, cholesteryl palmitate are investigated in the frequency range from 1 kHz to 10 MHz and in the temperature range of 27–120°C. The dielectric permittivity and dielectric loss shows substantial changes with the addition of polymer. The significant feature is that the pure liquid crystal exhibits broad absorption, showing dielectric relaxation in the range of 10 – 100 kHz whereas the composite shows relaxation peaks at 10 MHz. The percentage optical transmittance of pure liquid crystal and composite have also been measured and compared to study its phase transition behavior. The smectic phase observed in the pure liquid crystal is suppressed on formation of polymer composites. The mass spectrometry measurement does not show any abduct formation between the pure liquid crystal and the composite. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers
Preparation and characterization of carboxymethyl polyvinyl alcohol–graphite nanosheet composites
Authors: Yu Chaosheng , Li Bin
Published: April 2008

Carboxymethyl polyvinyl alcohol (CMPVA) was prepared by introducing carboxymethyl groups to polyvinyl alcohol (PVA) with chloroacetic acid. Graphite nanosheets (NanoG) were obtained by treating expandable graphite at a high temperature in a muffle furnace, and then sonicating it in an aqueous ethyl alcohol solution. Nanocomposites of CMPVA with NanoG were prepared by dispersing NanoG in a CMPVA aqueous solution with the aid of sonication and then casted on plastic film to remove water. Carboxymethyl groups were proved to be linked to PVA by Fourier transform infrared (FTIR) spectra and the degree of carboxymethyl substitution on the PVA was 2.77% determined by conductometric titration. Experimental results show that carboxylate anions in introduced carboxymethyl groups facilitate the dispersal of NanoG with positive charges in the CMPVA solution. The CMPVA matrix was detected by X‐ray diffraction (XRD) and FTIR spectra to be less crystal, into which NanoG tend to disperse. Scanning electron microscopy was used to characterize the structure of the graphite and morphology of the composites to prove that NanoG was 40–70 nm thick. The percolation threshold of CMPVA/NanoG composites at room temperature was as low as 0.80 wt%. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers
Continuous subcutaneous insulin infusion (CSII insulin pump therapy) for type 1 diabetes: a Bournemouth perspective
Authors: Kerr D , Nicholls H , James J
Published: April 2008

The first patient in Bournemouth to use insulin pump therapy started in 1998. Subsequently, meta‐analyses have supported our clinical experience that, for carefully selected individuals with type 1 diabetes, insulin pump therapy is associated with: reductions in HbA1c of 0.5–0.9% compared to multiple injection therapy; fewer glycaemic excursions above and below target levels compared to traditional injection therapy with patients reporting marked improvements in well‐being; reduction in the risk of severe hypoglycaemic episodes, particularly at night; and improved quality of life.

Starting insulin pump therapy requires a specialist multidisciplinary team for selection of patients for continuous subcutaneous insulin infusion, and to provide the on‐going education. Before starting pump therapy, patients should have participated in a structured group education programme whilst on multiple daily injections of insulin.

Once established, the use of additional technologies including continuous glucose monitoring and the pump download facilities give significant added value to the clinical care of people with type 1 diabetes opting for this form of insulin delivery.

It is important that all patients with type 1 diabetes are made aware of these technologies in diabetes care and, if appropriate, have access to them. Copyright © 2008 John Wiley & Sons.
Autumn 2007 Meeting of the Association of British Clinical Diabetologists
Authors: Wroe J
Published: April 2008

Over 100 members attended the Autumn 2007 Meeting of ABCD to hear state of the art lectures on diabetes‐related clinical topics and to debate the pros and cons of the Quality and Outcomes Framework for Diabetes. As always, professional discussion and exchange of views and experiences outside the official programme of the Meeting were as important as the set lectures. These bi‐annual meetings afford an important insight into the state of affairs in the specialist diabetes service, not an altogether happy one at present. Copyright © 2008 John Wiley & Sons.
Preparation and characterization of multi‐walled carbon nanotubes/polymer gradient composite films
Authors: Hu Nantao , Zhou Hongwei , Dang Guodong , Chen Chunhai , Jing Jing , Zhang Wanjin
Published: April 2008

BACKGROUND: Recently, much work has focused on the efficient dispersion of carbon nanotubes (CNTs) throughout a polymer matrix for mechanical and/or electrical enhancement. However, there are still only few reports about gradient distribution of CNTs in polymer matrices. In the work reported here, CNTs embedded in a polymer film with a gradient distribution were successfully obtained and studied.

RESULTS: For composite films with gradient distributions of CNTs, the upper surface behaves as an intrinsic insulator, while the lower one behaves as a semiconductor, or even as a conductor. It is also found that with an increase of 1 wt% CNTs, the resistance of the bottom surface decreases by 2–3 orders of magnitude, as compared with pure polyarylene ether nitrile; furthermore, when the proportion of CNTs increases up to 5 wt%, the resistance of the bottom surface shows only very little change. As a result, sufficient matrix conductivity of the bottom surface could be achieved at a lower filler concentration with CNTs in a gradient distribution. Meanwhile, the thermal stability, glass transition temperature and tensile properties of the matrix are maintained.

CONCLUSION: There is considerable interest in such gradient composite films, which could be applied in the electrical engineering, electronics and aerospace fields, for their excellent mechanical properties, thermal stability and novel electrical properties. Copyright © 2008 Society of Chemical Industry
Bis(2‐(6‐methylpyridin‐2‐yl)‐benzimidazolyl)titanium dichloride and titanium bis(6‐benzimidazolylpyridine‐2‐carboxylimidate): Synthesis, characterization, and their catalytic behaviors for ethylene polymerization
Authors: Liu Shaofeng , Zuo Weiwei , Zhang Shu , Hao Peng , Wang Deligeer , Sun Wen‐Hua
Published: April 2008

A series of 6‐(benzimidazol‐2‐yl)‐N‐organylpyridine‐2‐carboxamide were synthesized and transformed into 6‐benzimidazolylpyridine‐2‐carboxylimidate as dianionic tridentate ligands. Bis(2‐(6‐methylpyridin‐2‐yl)‐benzimidazolyl)titanium dichloride (C1) and titanium bis(6‐benzimidazolylpyridine‐2‐carboxylimidate) (C2–C8) were synthesized in acceptable yields. These complexes were systematically characterized by elemental and NMR analyses. Crystallographic analysis revealed the distorted octahedral geometry around titanium in both complexes C1 and C4. Using MAO as cocatalyst, all complexes exhibited from good to high catalytic activities for ethylene polymerization. The neutral bis(6‐benzimidazolylpyridine‐2‐carboxylimidate)titanium (C2–C8) showed high catalytic activities and good stability for prolonged reaction time and elevated reaction temperature; however, C1 showed a short lifetime in catalysis as being observed at very low activity after 5 min. The elevated reaction temperature enhanced the productivity of polyethylenes with low molecular weights, whereas the reaction with higher ethylene pressure resulted in better catalytic activity and resultant polyethylenes with higher molecular weights. At higher ratio of MAO to titanium precursor, the catalytic system generated better activity with producing polyethylenes with lower molecular weights. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3411–3423, 2008
Effect of annealing on magnetic and magnetotransport properties of Fe84Zr3.5Nb3.5Cu1B8 ribbons
Authors: Celegato F. , Coïsson M. , Kane S. N. , Mazaleyrat F. , Modak S. S. , Tiberto P. , Varga L. K. , Vinai F.
Published: April 2008

Fe84Zr3.5Nb3.5Cu1B8 melt‐spun ribbons have been submitted to furnace annealing to relax quenched‐in stresses and, above the first crystallization temperature, to induce nanocrystallization. The volume fraction of the nanocrystalline phase and the crystallites average size have been monitored by means of X‐ray diffraction as a function of annealing temperature. Hysteresis loops have been measured on as‐quenched and annealed ribbons, revealing the expected softening of the magnetic properties (reduction of the coercive field) and increase of the saturation magnetization as long as the nanocrystalline phase grows to the expense of the amorphous matrix. Low frequency (100 kHz–30 MHz) giant magneto‐impedance is enhanced when the best soft magnetic properties of the alloy are exploited by suitable thermal treatments, but rapidly vanishes out on increasing the frequency of the exciting current. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Diffusion simulation of Cr–Fe bcc systems at atomic level using a random walk algorithm
Authors: Sebastian I. San , Aldazabal J. , Capdevila C. , Garcia‐Mateo C.
Published: April 2008

This paper proposes a model to simulate the diffusion of impurities in bcc atomic lattices. It works with three‐dimensional volume, divided in small cubic elements (voxels), containing more than one atomic cell each.

Once the domain is discretized, impurities jump from one voxel to another according to certain probability that takes into account the composition and geometry of the target voxel.

In the present work, a model was applied to a prismatic volume and in order to deduce the relationship between the atomic jumping frequency and the temperature two different cases were studied. One consists of a Fe matrix with Cr impurities, and the other is based on a Cr matrix with Fe impurities. Results obtained from these simulations were compared with profiles obtained by Dictra software.

Results for the atomic jumping frequencies were fitted to an Arrhenius type equation, as shown in following expressions:

magnified image

From these equations it is possible to obtain an activation energy for the atomic jumping phenomenon of ∼306 kJ/mol and ∼411 kJ/mol for the Fe‐matrix and Cr‐matrix systems, respectively. These energies match the empirical measured values for the diffusion of Cr and Fe impurities, 250 kJ/mol and 407 kJ/mol, respectively.

Results obtained in this work assure that the proposed model is suitable for simulating the three‐dimensional diffusion of substitutional impurities in Cr and Fe bcc systems. It could be easily expanded to other bcc matrix systems. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Narrow‐band photodetection based onM‐plane GaN films
Authors: Ghosh Sandip , Rivera C. , Pau J. L. , Muñoz E. , Brandt O. , Grahn H. T.
Published: April 2008

Rapid identification of a range of hazardous airborne biological and chemical agents requires simultaneous detection at several specific wavelengths, and consequently a set of photodetectors with very narrow‐band spectral responsivity. We demonstrate two ultraviolet photodetection configurations based on strainedM‐plane GaN films on LiAlO2(100) substrates grown by molecular‐beam epitaxy with a detection bandwidth below 8 nm. The optical band gap of the film depends on the orientation of the linear polarization of the incident light relative to thec‐axis of GaN, which lies in the film plane. The first configuration consists of a polarization‐sensitive planar Schottky photodetector and a filter. An orthogonal alignment of thec‐axis of the photodetector and the filter produces a detection system with a peak responsivity at 360 nm and a bandwidth of 6 nm. The second one consists of two planar Schottky photodetectors with theirc‐axes oriented perpendicular to each other. The difference signal between the two photodetectors produces a peak responsivity at 358 nm and a bandwidth of 7.3 nm. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth, fabrication, and characterization of InGaN solar cells
Authors: Chen X. , Matthews K. D. , Hao D. , Schaff W. J. , Eastman L. F.
Published: April 2008

The InGaN alloy system offers a unique opportunity to develop high efficiency multi‐junction solar cells. In this study, single junction solar cells made of Inx Ga1–x N are successfully developed, with x = 0, 0.2, and 0.3. The materials are grown on sapphire substrates by MBE, consisting of a Si‐doped InGaN layer, an intrinsic layer and an Mg‐doped InGaN layer on the top. The I –V curves indicate that the cell made of all‐GaN has low series resistance (0.12 Ω cm2) and insignificant parasitic leakage. Contact resistances of p and n contacts are 2.9 × 10–2 Ω cm2 and 2.0 × 10–3 Ω cm2, respectively. Upon illumination by a 200 mW/cm2, 325 nm laser, Voc is measured at 2.5 V with a fill factor of 61%. Clear photo‐responses are also observed in both InGaN cells with 0.2 and 0.3 Indium content when illuminated by outdoor sunlight. But it is difficult to determine the solar performance due to the large leakage current, which may be caused by the material defects. A thicker buffer layer or GaN template can be applied to the future growth process to reduce the defect density of InGaN films. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ultraviolet GaN‐based nanocolumn light‐emitting diodes grown on n‐(111) Si substrates by rf‐plasma‐assisted molecular beam epitaxy
Authors: Sekiguchi Hiroto , Kato Kei , Tanaka Jo , Kikuchi Akihiko , Kishino Katsumi
Published: April 2008

We report the fabrication of GaN/AlGaN nanocolumn LEDs on n‐(111) Si substrates by RF‐MBE for the first time. Clear diode characteristics with a turn‐on voltage of 4 V and an ultraviolet emission spectrum with a peak wavelength of 354 nm were observed at room temperature. When the Al composition of p‐Alx Ga1–x N was changed from 8.8% to 25.1%, the high Al content led a narrowing of the FWHM compared withthat for low Al contents due to the suppression of carrier overflow. We measured the electroluminescence (EL) under dc and pulsed operations. The integrated EL intensity under the pulsed operation was 3 times as strong as that under the dc operation at 100 mA due to the reduced generation of heat. The thermal resistance was estimated to be 40 °C/W from the EL peak wavelength difference between the dc and pulsed operations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Performance of high‐power III‐nitride light emitting diodes
Authors: Chen G. , Craven M. , Kim A. , Munkholm A. , Watanabe S. , Camras M. , Götz W. , Steranka F.
Published: April 2008

The performance of III‐nitride based high‐power light emitting diodes (LEDs) is reviewed. Direct color high‐power LEDs with 1 × 1 mm2 chip size in commercial LUXEON® Rebel packages are shown to exhibit external quantum efficiencies at a drive current of 350 mA ranging from ∼60% at a peak wavelength of ∼420 nm to ∼27% at ∼525 nm. The short wavelength blue LED emits ∼615 mW at 350 mA and >2 W at 1.5 A. The green LED emits ∼110 lm at 350 mA and ∼270 lm at 1.5 A. Phosphor‐conversion white LEDs (1 × 1 mm2 chip size) are demonstrated that emit ∼126 lm of white light when driven at 350 mA and 381 lm when driven at 1.5 A (Correlated Color Temperature, CCT ∼ 4700 K). In a similar LED that employs a double heterostructure (DH) insign instead of a multi‐quantum well (MQW) active region, the luminous flux increases to 435 lm (CCT ∼ 5000 K) at 1.5 A drive current. Also discussed are experimental techniques that enable the separation of internal quantum efficiency and extraction efficiency. One technique derives the internal quantum efficiency from temperature and excitation‐dependent photoluminescence measurements. A second technique relies on variable‐temperature electroluminescence measurements and enables the estimation of the extraction efficiency. Both techniques are shown to yield consistent results and indicate that the internal quantum efficiency of short wavelength blue (λ ∼ 420 nm) high‐power LEDs is as high as 71% even at a drive current of 350 mA. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Direct observation of indium compositional fluctuation in GaInN/GaN multi‐quantum wells using an X‐ray micro‐beam from the 8‐GeV storage ring
Authors: Miyajima Takao , Uemura Shigeaki , Kudo Yoshihiro , Terada Yasuko , Fuutagawa Noriyuki
Published: April 2008

We measured the micrometer‐scale fluctuation of the indium contents in a 50 μm x 30 μm region of annealed Ga0.8In0.2N/GaN multi quantum wells by mapping the counts of indium fluorescent X‐rays excited by a 1.3 μm x 3.8 μm X‐ray micro‐beam. The mapping indicates that two distinct regions – indium‐rich and indium‐poor regions – are formed by the annealing. The indium contents in the island‐shaped low‐indium regions are 20% less than in the surrounding high‐indium region. As the island‐shaped low‐indium regions clearly coincide with the low‐radiative regions as observed by Hg‐lamp‐excited fluorescent microscopy, we believe that the low‐radiative regions are a result not of indium segregation but of the generation of defects such as plane defects. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Determination of the Mg occupation site in MOCVD‐ and MBE‐grown Mg‐doped InN using X‐ray absorption fine‐structure measurements
Authors: Miyajima Takao , Uemura Shigeaki , Kudo Yoshihiro , Kitajima Yoshinori , Yamamoto Akio , Muto Daisuke , Nanishi Yasushi
Published: April 2008

We analyzed the atomic structure around Mg atoms in MOCVD‐ and MBE‐grown Mg‐doped InN using Mg K‐edge X‐ray absorption fine‐structure (XAFS) measurements. Our experimental data closely fit to the simulated data in which Mg atoms occupy the substitutional sites of In atoms. From this result, we conclude that Mg atoms essentially occupy not N atoms sites but In atoms sites, meaning that Mg atoms can act as acceptors in InN. We believe that observations of p‐type conductivity are prevented by problems such as carrier compensation and electron accumulation at the surface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
MOCVD growth of GaN‐based materials on ZnO substrates
Authors: Wang Shen‐Jie , Li Nola , Park Eun‐Hyun , Feng Zhe Chuan , Valencia Adriana , Nause Jeff , Kane Matthew , Summers Chris , Ferguson Ian
Published: April 2008

The metalorganic chemical vapor deposition (MOCVD) growth of GaN based materials on ZnO substrates has numerous technical issues that need to be investigated and resolved. These include the thermal stability of ZnO, out‐diffusion of Zn/O from the ZnO into the epilayers, and H2 back etching into the ZnO all of which can cause poor film quality. Cracks and pinholes were seen in the epilayers, leading to the epilayer peeling off. In this study, good quality InGaN films with a wide range of indium incorporation have been grown on (0001) ZnO substrates by MOCVD. No indium droplets and phase separation were observed even at high indium concentrations. The optical microscopy and field‐emission scanning electron microscopy revealed a mirror‐like InGaN surface with no evidence of indium droplets on the surface. Photoluminescence (PL) showed broad InGaN‐related emissions with peak energy lower than the calculated InGaN band gap, possibly due to Zn/O impurities diffused into InGaN from the ZnO substrate. More recently, Al2O3 coated ZnO substrates have been employed for growth to limit Zn diffusion as well as assist epilayer growth. HRXRD result shows that a single crystal InGaN film has been successfully grown on an annealed Al2O3 coated ZnO substrate. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Energy relaxation processes of photo‐generated carriers in Mg doped (0001)GaN and (1‐101)GaN
Authors: Saida J. , Kim E. H. , Hikosaka T. , Honda Y. , Yamaguchi M. , Sawaki N.
Published: April 2008

Energy relaxation processes of photo‐generated excess carriers in Mg doped GaN epitaxial layers were investigated at room temperature, with photoluminescence intensity correlation method using femto‐second pulse laser as the excitation source. The decay curve was well fitted by exponential decay with two time constants. The slow process of the order of 100 ps was attributed to the energy relaxation of electrons in the conduction band, while the fast process of the order of several pico‐seconds was attributed to the capture process in the impurity band near the valence band. Little difference has been found out between the results for (0001)GaN and those for (1‐101)GaN. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Marked improvements in electrical and optical properties for MOVPE InN annealed at a low temperature (300 °C) in O2 atmosphere
Authors: Sugita K. , Nagai Y. , Matsuoka D. , Hashimoto A. , Harima H. , Yamamoto A.
Published: April 2008

We have found that the material quality of MOVPE InN can be markedly improved after the annealing in the air at around 300 °C. By the annealing in the air, carrier concentration is reduced by about one order of magnitude. In accordance with the carrier reduction, PL intensity is increased and PL peak energy is shifted to the lower energy side by about 0.06 eV for the film annealed for 3 h. The reduction of carrier concentration is also conformed by the shift of LO phonon‐plasmon coupled mode in the Raman spectrum. The FWHM of the E2 (high) mode is decreased, indicating that the crystalline quality is slightly improved by the annealing. Since the FWHM of X‐ray rocking curve is not changed after the annealing, the improvement by the annealing is concluded not to be in macroscopic scale but microscopic scale. No improvements are found for the samples annealed in the N2 flow. No data that show the chemical oxidation of InN are also found. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonpolar GaN layers grown by sidewall epitaxial lateral overgrowth: optical evidences for a reduced stacking fault density
Authors: Paskov P. P. , Monemar B. , Iida D. , Kawashima T. , Iwaya M. , Kamiyama S. , Amano H. , Akasaki I.
Published: April 2008

Nonpolar a ‐plane and m ‐plane GaN layers grown by MOCVD employing sidewall epitaxial lateral overgrowth (SELO) are studied by photoluminescence (PL) and spatially resolved micro‐PL. The effects of the groove orientations and the groove/terrace width ratio on the emission spectra, particularly on the stacking fault (SF) related emission bands in the 3.29–3.42 eV spectral region, are examined. The PL spectra of both types of nonpolar layers reveal a significant reduction of the defect related emissions when the grooves are oriented perpendicular to the c‐axis of GaN and the groove/terrace width ratio is smaller than one. The suppression of SF formation in the areas where a lateral overgrowth along the [0001] GaN direction occurs is confirmed by micro‐PL measurements showing no SF related emissions over the terrace regions. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
New nitridation technique for mosaicity control in RF‐MBE InN growth
Authors: Iwao K. , Yamamoto A. , Hashimoto A.
Published: April 2008

Mosaicity control is necessary for improvement of crystal quality of III‐V nitride semiconductor materials, especially for InN. However, the mosaicity control for both of the tilt and the twist angle fluctuations has not been achieved still now in the direct growth of InN on the sapphire substrate by nitridation. Moreover, the role of nitridation process has not been so much clear from the view point of the mosaicity control. In this paper, we propose a new nitridation technique, two‐step nitridation, combined with the high‐ and the low‐ temperature nitridation to reduce the mosaicity and also to make clear the roles of the nitridation process in the InN growth. As the result of the new nitridation technique, the considerably improvement of the tilt and the twist angle fluctuations has been successfully achieved in the direct growth of InN without any buffer layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
MOVPE growth of high quality AlN layers and effects of Si doping
Authors: Thapa S. B. , Hertkorn J. , Scholz F. , Prinz G. M. , Feneberg M. , Schirra M. , Thonke K. , Sauer R. , Biskupek J. , Kaiser U.
Published: April 2008

High quality AlN layers were grown by MOVPE on c ‐plane sapphire substrates after optimization of various stages of the epitaxial process. AFM measurements revealed a rms surface roughness of 0.2 nm. The FWHM of HRXRD for the (002) and (114) reflections are 25 and 300 arcsec, respectively. This excellent quality is further confirmed by low temperature (10 K) CL spectra with a FWHM of 10 meV.

We observed degrading surface and crystal quality of Si doped samples along with increasing in‐plane tensile strain up to a Si concentration of approx. 2 × 1019 cm–3 and thereafter relaxation for higher concentrations as determined by HRXRD, and low temperature CL measurements. Furthermore, we obtained a weak n‐type conductivity, increasing with temperature with an activation energy of about 200 meV. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Raman scattering analysis of GaN with various dislocation densities
Authors: Kitamura T. , Nakashima S. , Nakamura N. , Furuta K. , Okumura H.
Published: April 2008

We characterized GaN crystals with various dislocation densities by micro‐Raman spectroscopy. Defects and Strain for the GaN layer were examined through measurements of the Raman shift and the width of the TO phonon bands. The broadening of Raman bands in GaN crystals occurred as the dislocation density increased. The up‐shift of the peak frequencies corresponding to compressive strain was observed for the heteroepitaxial samples grown by MOCVD and MBE. The in‐plane distribution of defects and strain in epilayers with various dislocation densities was also examined by Raman mapping. The Raman maps showed that the examined phonon frequency and band width in samples fluctuated spatially. The increase of dislocation density in GaN epilayers induced not only the broadening of Raman bands but also increase of fluctuation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Realization of AlGaN/GaN HEMTs on 3C‐SiC/Si(111) substrates
Authors: Cordier Y. , Portail M. , Chenot S. , Tottereau O. , Zielinski M. , Chassagne T.
Published: April 2008

Cubic SiC/Si (111) template is an interesting alternative for growing GaN on silicon. As compared with silicon, this substrate allows reducing the stress in GaN films due to both lower lattice and thermal expansion coefficient mismatch, and can provide better heat dissipation. In this work, we first developed the epitaxial growth of 3C‐SiC films on 50 mm Si(111) substrates using chemical vapor deposition. AlGaN/GaN high electron mobility transistors were grown by molecular beam epitaxy on these films. Both the structural quality and the behavior of transistors realized on these structures show the feasibility of this approach. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Fabrication and properties of semi‐polar (1‐101) and (11‐22) InGaN/GaN light emitting diodes on patterned Si substrates
Authors: Hikosaka T. , Tanikawa T. , Honda Y. , Yamaguchi M. , Sawaki N.
Published: April 2008

InGaN/GaN light emitting diodes (LEDs) were fabricated on semi‐polar (1‐101) and (11‐22) GaN templates which had been grown on patterned (001) and (113)Si substrates, respectively. The GaN templates were of 300 × 300 μm2 discs formed by the coalescence of GaN stripes grown by selective MOVPE method. The n‐type electrode was fabricated on the back surface of the Si substrate. The electroluminescence (EL) peaks were observed around 440 nm and 420 nm in (1‐101) and (11‐22) LEDs, respectively. The full width at half maximum (FWHM) of the peaks at 20 mA were 33 nm ((1‐101) LED) and 21 nm ((11‐22) LED). The emission peak was slightly blue‐shifted at high drive currents, which was smaller than that of conventional c‐plane LEDs. The turn‐on voltage of the diodes was of the order of 3‐4 V. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Controlled local filament growth and dissolution in Ag–Ge–Se
Authors: Schindler Christina , Szot Krzysztof , Karthäuser Silvia , Waser Rainer
Published: April 2008

Memory cells based on the cation migration and filament formation and rupture in a solid electrolyte have attracted much interest due to low switching voltages and a prospective high scalability. In this study we indirectly visualized the growth and dissolution of the conductive filament in Ag–Ge–Se samples with Ag bottom electrodes by surface analysis with Conductive Atomic Force Microscopy (CAFM). By application of a negative voltage to the inert CAFM tip, conductive filaments were grown on the scanned area and they were dissolved under reversed bias. The local conductivity changes directly corresponded to changes in the topography, i.e. to the filament protrusion and dissolution. Topography changes could be circumvented by limiting the maximum current. By placing the CAFM tip on a random spot on the sample, filaments with a diameter as low as 20 nm were grown by local current–voltage measurements. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Coriolis effects in homogeneous and inhomogeneous katabatic flows
Authors: Shapiro Alan , Fedorovich Evgeni
Published: April 2008

Katabatic flows along a planar slope in a viscous stably‐stratified fluid are investigated analytically and numerically, with an emphasis on flow features arising from the action of the Coriolis force. Two idealized flow types are considered: turbulent flow along a uniformly cooled slope, and two‐dimensional laminar flow induced by a cold strip of finite width running down the slope. In the case of turbulent flow along a uniformly cooled slope, the downslope velocity exhibits a boundary‐layer structure, but the cross‐slope velocity and buoyancy fields spread inexorably upward. This behaviour is qualitatively similar to that found in laminar flows along a uniformly cooled slope. In contrast, in the two‐dimensional strip flow, a steady state is reached, in which the cross‐slope wind and buoyancy fields vanish far above the slope, but the downslope and slope‐normal velocity fields do not vanish. These latter two flow components comprise two purely horizontal along‐isentrope counter‐flowing currents: an upslope current entering the top of the boundary layer on one side of the strip, and a downslope current flowing out of the boundary layer into the environment on the other side of the strip. The slope‐normal vorticity associated with these currents originates in the stretching of planetary vorticity in a broad zone of convergent flow over the cold strip. Copyright © 2008 Royal Meteorological Society
Improvements of parametric quantum methods with new elementary parametric functionals
Authors: Ruette Fernando , Marcantognini Stefania A. M. , V. Karasiev Valentin , Sánchez Morella
Published: April 2008

A series of elementary parametric functionals (EPFs) for resonance integral, electron‐electron repulsion, electron‐nucleus attraction, core‐core interaction, and bond correlation correction were included in the new version of CATIVIC method [Int J Quantum Chem 2004, 96, 321]. In the present work, a systematic way to improve the precision of parametric quantum methods (PQMs) by using several EPFs in the parameterization of a set of molecules is proposed. Based on the fact that a linear combination of elementary functionals from the exact Hamiltonian is also a functional, a linear combination of EPFs has been proved that can enhance the accuracy of PQMs by considering the convex condition. A general formulation of simulation techniques for molecular properties is presented and a formal extension of the minimax principle to PQMs is also considered. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008
A spectral technique for random satisfiable 3CNF formulas
Authors: Flaxman Abraham
Published: April 2008

Let I be a random 3CNF formula generated by choosing a truth assignment ϕ for variables x1, xn uniformly at random and including every clause with i literals set true by ϕ with probability pi, independently. We show that for any constants 0 ≤ η2,η3 ≤ 1 there is a constant dmin so that for all d ≥ dmin a spectral algorithm similar to the graph coloring algorithm of Alon and Kahale will find a satisfying assignment with high probability for p1 = d/n2, p2 = η2d/n2, and p3 = η3d/n2. Appropriately setting the ηi's yields natural distributions on satisfiable 3CNFs, not‐all‐equal‐sat 3CNFs, and exactly‐one‐sat 3CNFs. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2008
Hitting time of large subsets of the hypercube
Authors: Černý Jiří , Gayrard Véronique
Published: April 2008

We study the simple random walk on the n‐dimensional hypercube, in particular its hitting times of large (possibly random) sets. We give simple conditions on these sets ensuring that the properly rescaled hitting time is asymptotically exponentially distributed, uniformly in the starting position of the walk. These conditions are then verified for percolation clouds with densities that are much smaller than (n log n)‐1. A main motivation behind this article is the study of the so‐called aging phenomenon in the Random Energy Model, the simplest model of a mean‐field spin glass. Our results allow us to prove aging in the REM for all temperatures, thereby extending earlier results to their optimal temperature domain. © 2008 Wiley Periodicals, Inc. Random Struct. Alg., 2008