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Glued laminated bamboo (glubam) is an engineered bamboo for the potential usage in structures as an alternative to timber-based glulam. This paper focuses on the impact toughness and static bending strength of glubam, with the goal to provide basic understanding and experimental data for impact design of engineered bamboo structures. Two types of glubam, i.e., thin-strip and thick-strip glubams were investigated through impact pendulum and quasi-static bending tests. For the thick-strip glubam, testing parameters also included three types of manufacturing conditions with different degrees of carbonization of composing bamboo strips. Both comparisons among different types of glubam, and between dynamic and static bending results can effectively identify the impact resistance and mechanical properties of glubam. The dynamic impact loading resulted in higher instability and had failure modes different from quasi-static tests for glubam specimens. Experimental values of toughness, modulus of elasticity (MOE) and modulus of rupture (MOR) at different loading speed conditions are obtained. [ABSTRACT FROM AUTHOR]

This paper concentrates on the deformation behavior of amorphous diamondlike carbon composite materials. Combined nanoindentation and ex situ cross-sectional transmission electron microscopy investigations are carried out on TiC/a-C nanocomposite films, with and without multilayered structures deposited by pulse dc magnetron sputtering. It is shown that by controlling the distribution of nanocrystallites forming nanoscale multilayers, the system can be used as a “microstructural ruler” that is able to distinguish various deformation patterns, which can be hardly detected otherwise in a homogeneous structure. It is shown that rearrangement of nanocrystallites and displacement of a-C matrix occur at length scales from tens of nanometer down to 1 nm. At submicrometer scale homogeneous nucleation of multiple shear bands has been observed within the nanocomposites. The multilayered structure in the TiC/a-C nanocomposite film contributes to an enhanced toughness. [ABSTRACT FROM AUTHOR]

Provides information on a study which carried out photothermal ionization spectroscopy measurements on a free-standing, high purity and high quality 4H-SiC epitaxial layer at various temperatures. Experimental details; Results and discussion.

IC functional failure is always a challenge for failure analysis engineers since it needs test pattern to access the defect location and electrically trigger it. Dynamic failure analysis is the only way to be used to do this kind of analysis. But it's time consuming and complex to employ dynamic analysis, so the success rate is lower and cycle time is longer. Static failure analysis is impossible to apply on these kinds of analysis since the test pattern and design information is needed. However, in this paper, the application of advanced FIB circuit edit (CE) was employed to isolate the suspected function block, and make it accessible to the DC bias. With static FA analysis, the defect location was successfully localized. Nanoprobing was employed on the further electrical analysis, and abnormal electrical performance was successfully observed. Combined with the device physics analysis, the suspected process was identified. Further PFA Wright etch was applied to visualize the defect which was a soft failure of Bipolar Junction Transistor (BJT) device. Failure mechanism was built up to explain the electrical and physical phenomena successfully. [ABSTRACT FROM AUTHOR]

As received 1100-O aluminum was cold rolled (CR) to 30%, 70%, and 80% reduction, respectively, to study the effects of microstructural evolution on the spall response using plate impact experiments. Previous results show a sharp increase in pullback velocity for 1100-O aluminum with increase in peak shock stress between 4.0 and 8.3 GPa due to hardening, followed by a decrease for peak shock stresses up to 12.0 GPa possibly due to softening. This maximum was not observed for the 30% CR, which showed only an increase in pullback velocity over the shock stress range of 4.0-12.0 GPa due to hardening (net increase in dislocation density). For the 70% CR aluminum, no change was observed in the pullback velocity over the range tested (4.0-11.0 GPa) probably due to saturation in dislocation density. Similar observations were made for the 80% CR, that is, no change was observed in the spall response between 4.0 GPa and 11.0 GPa. However, variations were observed in the spall response for the 80% CR, and these variations are attributed to material inhomogeneity possibly caused by increased cold rolling beyond saturation. The results also show a significant increase in Hugoniot Elastic Limit with increase in percent cold rolling. [ABSTRACT FROM AUTHOR]

We present a coupled theory on the piezoresponse force microscopy (PFM) responses by solving the electromechanical equations for transversally isotropic piezoelectric materials. The effective piezoelectric coefficients calculated from the coupled, the decoupled, and the indentation theories are then compared with the true piezoelectric coefficient d33. An approximately linear relationship between the effective piezoelectric coefficient from the coupled PFM analysis and d33 is obtained. Finally, the coupled analysis is extended to the piezoelectric film/substrate system. The obtained results are believed to be useful in quantitatively interpreting the PFM signals. [ABSTRACT FROM AUTHOR]

In this letter we report the controlled growth and microstructural evolution of self-assembled nanocomposite multilayers that are induced by surface ion-impingement. The nanoscale structures together with chemical composition, especially at the growing front, have been investigated with high-resolution transmission electron microscopy. Concurrent ion impingement of growing films produces an amorphous capping layer 3 nm in thickness where spatially modulated phase separation is initiated. It is shown that the modulation of multilayers as controlled by the self-organization of nanocrystallites below the capping layer, can be tuned through the entire film. [ABSTRACT FROM AUTHOR]

We described twenty polymorphic microsatellite loci derived from the expressed sequence tags of Puccinia striiformis f. sp. tritici, which causes yellow rust disease on wheat. The numbers of alleles range from two to six and eight microsatellite loci show significant similarities to known genes. Observed and expected heterozygosities ranged from 0.12 to 0.78 and from 0.24 to 0.87, respectively. [ABSTRACT FROM AUTHOR]

Nanoscale deformation in TiC/a-C nanocomposite coatings is revealed by combined nanoindentation and high resolution cross-sectional transmission electron microscopy. A process characterized by rearrangement of TiC nanocrystallites and the displacement of a-C matrix is found to dominate the deformation mechanism at length scales ranging from tens of nanometers down to 1 nm and contributes to toughening of the nanocomposite coatings by delocalizing local plastic deformations. [ABSTRACT FROM AUTHOR]

The fracture strain, strength, and flexibility of ZnO nanowires (NWs) with a large range of diameters (85–542 nm) are investigated at a quantitative level. Large strains up to 4%–7% have been obtained before the final elastic fracture, corresponding to fracture strengths close to the theoretical strength. The flexibility of a NW is discussed quantitatively in terms of the diameter and fracture strain. The fundamental mechanisms responsible for the observed exceptional properties are discussed. [ABSTRACT FROM AUTHOR]

Selenium is an important element for human health. Many studies have identified selenium deficiency in soil and water as an important factor in causing Keshan Disease (KD) in Northeast China. Previous studies have mainly focused on soil selenium content, staple food selenium content, and human selenium level, but there are few systematic studies on soil selenium's existing forms and their migration from soil to crops and the human body. This paper focused on inferring the barrier factors in the migration of selenium from soil to crop and the human body and transformation of its compounds. It provides a reference basis for the etiological analysis, prevention, control, and elimination of KD. The study used 121 183 samples of topsoil (0−20 cm), 30 295 soil parent samples of selenium and other geochemical indices in northeast China, and crop seeds and human hair samples from the KD endemic area. The surface soil selenium was dominantly selenium-sufficient in Northeast China. However, the soil selenium levels were generally low. The average topsoil selenium in Northeast China was 0.20 mg/kg, significantly lower than the world's average soil selenium content (0.4 mg/kg) and slightly lower than the Chinese average soil selenium content (0.24 mg/kg). Soil selenium mainly existed in strongly bounding by organic bound, with humic acid, and residue forms. The amount of selenium available to plants was sufficient in the selenium-sufficient and KD-endemic areas. However, the average selenium content of human hair was deficient, on average, with 0.16 mg/kg in KD endemic area. We assume that lower soil selenium content may be the basic factor influencing the biogeochemical deficiency of selenium in Northeast China. The sequestration of selenium by clay chemical constituents, such as iron and aluminum oxides and soil organic matter, especially in acidic soils, is another direct contributing factor to the low selenium content in biogeochemical food chain, which increases the risk of KD in the population. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) and optical gain (OG) spectra of a-plane GaN layers have been analyzed over a wide range of excitation intensities. The samples were fully coalesced layers grown by metalorganic chemical vapor deposition over r-plane sapphire substrates using epitaxial layer overgrowth (ELOG) and selective area lateral epitaxy (SALE) procedures. ELOG and SALE a-plane samples showed a strong stimulated emission line in backscattering-geometry PL spectra along with extremely high OG coefficient values (in SALE samples more than 2000 cm-1). Structures prepared with natural cleaved facet cavities based on these films were used to demonstrate optically pumped room-temperature lasing. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose and demonstrate an AlGaN/GaN/AlGaN double heterostructure (DH) with significantly improved two-dimensional (2D) confinement for high-power III-N heterostructure field-effect transistors (HFETs). The DH was grown directly on an AlN buffer over i-SiC substrate. It enables an excellent confinement of the 2D gas and also does not suffer from the parasitic channel formation as experienced in past designs grown over GaN buffer layers. Elimination of the GaN buffer modifies the strain distribution in the DH, enabling Al contents in the barrier region well over 30%. For the AlGaN/GaN/AlGaN DH design, the 2D electron gas mobility achieved was 1150 cm[SUP2]/V s at room temperature and 3400 cm[SUP2]/V s at 77 K, whereas the temperature independent sheet carrier density was N[SUBS] ≈ 1.1 × 10[SUP13]cm[SUP-2]. Compared to a regular AlGaN/GaN structure, the channel mobility-concentration profiling shows significant improvement in the carrier confinement. Sample DHFETs with 1-μm long gates demonstrate the threshold voltage of 3.5 V, with a peak saturation current of 0.6-0.8 A/mm. [ABSTRACT FROM AUTHOR]

We report on a study to compare the growth of thick A1GaN layers on GaN with different strain-relief interlayers. A set of ten period A1N/A1GaN superlattices was found to be the most efficient approach for the strain relief. The superlattice interlayer not only decreases the tensile strain but also improves the crystal structural quality. Thus, 2-μm-thick, high quality n[sup +]-A1[sub 0.2]Ga[sub 0.8]N layers can be grown on GaN epilayers without any cracks. [ABSTRACT FROM AUTHOR]

Polarization effects have been studied in GaN/AlGaN multiple quantum wells (MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence (PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [1&1macr;00]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs. Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. In the former case, the excitation-induced blueshift of the PL line is due to the screening of the built-in electric field by photoinjected carriers, which is consistent with the field strength of 1.23 MV/cm in the absence of excitation. [ABSTRACT FROM AUTHOR]

We report homoepitaxial GaN growth on freestanding (1&1macr;00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition. Scanning electron microscopy, atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (1&1macr;00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (1&1macr;00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence (PL) dynamics has been studied in AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by a pulsed metalorganic chemical vapor deposition (PMOCVD) procedure. The PL decay kinetics was found to be sensitive to the emission energy and temperature. The PL decay time increases with decreasing emission energy, which is a characteristic of localized carrier/exciton recombination due to alloy fluctuations. Its temperature dependence shows radiative recombination to be the dominant process at low temperatures, indicating a high quality of PMOCVD grown quaternary AlInGaN MQWs and establishing them as promising structures for the active region of deep ultraviolet light emitting diodes. [ABSTRACT FROM AUTHOR]

Low-temperature photoluminescence investigations have been carried out in the quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by pulsed metalorganic chemical-vapor deposition (PMOCVD). With increasing excitation power density, the emission peaks in both AlInGaN epilayers and MQWs show a strong blueshift and their linewidths increase. The luminescence of the samples grown by PMOCVD is attributed to recombination of carriers/excitons localized at band-tail states. We also demonstrate the luminescence properties of AlInGaN and A1GaN materials grown by a pulsed atomic-layer epitaxy and conventional MOCVD, respectively. [ABSTRACT FROM AUTHOR]

We investigate the built-in electrostatic polarization fields in GaN/AlGaN multiple quantum wells (MQWs) fabricated by the deposition of layers on sapphire substrates with different crystallographic surfaces. The analysis of the photoluminescence (PL) spectra of MQWs grown over (0001) C-plane, (1120) A-plane, and (1102) R-plane sapphire, as well as the theoretical calculations, revealed the formation of a strong (up to ∼1.7 MV/cm) built-in electrostatic field due to the spontaneous polarization and piezoelectric field in MQWs on C- and A-plane sapphire, whereas there was no indication of polarization in the identical structures grown on R-plane sapphire. The PL dynamics with the increase of excitation can be explained by quantum-confined Stark effect and screening of the built-in electrostatic field due to photoinjected carriers. The internal electrostatic-field-free quantum structures grown on R-plane sapphire may be promising for ultraviolet region optoelectronic applications due to higher emission intensity and stable spectral parameters. [ABSTRACT FROM AUTHOR]

Piezoresponse force microscopy (PFM) extends the conventional nano-indentation technique and has become one of the most widely used methods to determine the properties of small scale piezoelectric materials. Its accuracy depends largely on whether a reliable analytical model for the corresponding properties is available. Based on the coupled theory and the image charge model, a rigorous analysis of the film thickness effects on the electromechanical behaviors of PFM for piezoelectric films is presented. When the film is very thick, analytical solutions for the surface displacement, electric potential, image charge, image charge distance, and effective piezoelectric coefficient are obtained. For the infinitely thin (IT) film case, the corresponding closed-form solutions are derived. When the film is of finite thickness, a single parameter semi-empirical formula agreeing well with the numerical results is proposed for the effective piezoelectric coefficient. It is found that if the film thickness effect is not taken into account, PFM can significantly underestimate the effective piezoelectric coefficient compared to the half space result. The effects of the ambient dielectric property on PFM responses are also explored. Humidity reduces the surface displacement, broadens the radial distribution peak, and greatly enlarges the image charge, resulting in reduced effective piezoelectric coefficient. The proposed semi-empirical formula is also suitable to describe the thickness effects on the effective piezoelectric coefficient of thin films in humid environment. The obtained results can be used to quantitatively interpret the PFM signals and enable the determination of intrinsic piezoelectric coefficient through PFM measurement for thin films. [ABSTRACT FROM AUTHOR]

Abstract: Bacterial translocation (BT) has been suggested to be responsible for the high incidence of infections after intestinal transplantation (IT). The purpose of this study was to examine whether a probiotic supplement after orthotopic IT ameliorated ischemia-reperfusion injury and reduced BT at 4 or 6 days postoperative (sham or IT), as mesenteric lymph nodes (MLNs), liver, and splenic tissue samples from the six groups were assessed for BT by bacterial culture, measurement of tumor necrosis factor-α (TNF-α) in MLNs by competitive reverse transcription-polymerase chain reaction, and histological evaluation by Park''s classification. Oral administration of probiotics after IT did not improve short-term survival rates compared with the transplant-only groups (P > .05). However, the BT rates and levels of TNF-α in MLNs in groups with IT only were higher than the probiotic cohorts (P < .05). Histological injuries were significantly ameliorated in the group with six days of probiotic treatment compared with that in the nontreated hosts (P < .05). These data indicated that administration of probiotics after IT improved graft histology and reduced BT in rats. [Copyright &y& Elsevier]

The transmission of external jet noise through the double-leaf skin plate of an aircraft cabin fuselage in the presence of external mean flow is analytically studied. An aeroacoustic-elastic theoretical model is developed and applied to calculate the sound transmission loss versus frequency curves. Four different types of acoustic phenomenon (i.e., the mass-air-mass resonance, the standing-wave attenuation, the standing-wave resonance, and the coincidence resonance) for a flat double-leaf plate as well as the ring frequency resonance for a curved double-leaf plate are identified. Independently of the proposed theoretical model, simple closed-form formulas for the natural frequencies associated with the preceding acoustic phenomena are derived using physical principles. Excellent agreement between the model predictions and the closed-form formulas is achieved. Systematical parametric investigation with the model demonstrate that the presence of the mean flow as well as the sound incidence angles substantially affect the sound transmission behavior of the double-leaf structure. The influences of the panel curvature together with cabin internal pressure on jet noise transmission are also significant and should be taken into account when designing aircraft cabin fuselages. [ABSTRACT FROM AUTHOR]

A theoretical study on the vibroacoustic performance of a rectangular double-panel partition clamp mounted in an infinite acoustic rigid baffle is presented. With the clamped boundary condition taken into account by the method of modal function, a double Fourier series solution to the dynamic response of the structure is obtained by employing the weighted residual method (i.e., the Galerkin method). The double series solution can be considered as the exact solution of the problem, as the structural and acoustic-structural coupling effects are fully accounted for and the solution converges numerically. The accuracy of the theoretical predictions is checked against existing experimental data, with good agreement achieved. The influence of several key parameters on the sound isolation capability of the double-panel configuration is then systematically studied, including panel dimensions, thickness of air cavity, elevation angle, and azimuth angle of incidence sound. The present method is suitable for double-panel systems of finite or infinite extent and is applicable for both low- and high-frequency ranges. With these merits, the proposed method compares favorably with a number of other approaches, e.g., finite element method, boundary element method, and statistical energy analysis method. [ABSTRACT FROM AUTHOR]

Monitoring the pathogenic fungus of wheat stripe rust, Puccinia striiformis f. sp. tritici, plays a key role in effective control of the disease. In the present study, we developed a specific and sensitive polyiiierase chain reaction (PCR) assay for detecting the pathogen in wheat (Triticum aestivuin) leaves. A pair of primers (PSF and PSR) was designed based on the internal transcribed spacer (ITS) region sequence of P striiforinis f. sp. tritici. PCR products that were amplified with universal primers ITSI and ITS4 were cloned into pGEM-T Easy vectors and sequenced. The ITS sequence was compared with those of P. striiformis f. sp. tritici, P. triticina, P graminis f. sp. tritici, Blutneria graminis f. sp. tritici, Fusarium graminearurn, Rhizoctonia cerealis, and Bipolaris sorokiniana, which are associated with early symptoms of foliar diseases on wheat. Specificity of the primers was tested in the PCR assays using DNA extracted from all tested P. striiformis f..sp. tritici isolates, other fungal species, and healthy and infected wheat leaves sampled around stripe rust foci in wheat fields, different days after inoculation with P striiformis f. sp. tritici, as well as asymptomatic wheat leaves sampled around stripe rust foci in the fields. A PCR product of 169 bp was amplified from DNA of alt P. striiforrnis f. sp. tritici isolates. The primers did not amplify DNA from the other tested fungal species. The pathogen was detected from asymptomatic wheat leaves inoculated with P. striiformis f. sp. tritici under greenhouse conditions, as well as leaves sampled around stripe rust foci in wheat fields. Under optimum conditions, the PCR assay was highly sensitive and required only 0.1 pg of the target DNA for a detectable and reliable amplification with the PSF and PSR primers. [ABSTRACT FROM AUTHOR]

Focuses on the molecular and morphological characteristics of the causal agent of black knot in Prunus, Apiosporina morbosa. Similarities of the overall colony and conidial morphology in cultures of the A. morbosa isolates with an isolate of Cladosporium herbarum; Genetic features of A. morbosa.

Microcracks and their effect on the plastic deformation of γ-TiAl alloy with fully lamellar microstructure (FL) at relatively low strain rate (1 × 10-5/s) has been investigated. It is found that a large number of microcracks nucleate within the grains. The microcrack density increases with the increase of grain size. Most of the microcracks nucleate at the γ/α2 interfaces and gather at grains with soft-orientations. Based on the observation and analysis, a model of microcrack nucleation in FL γ-TiAl alloy is built up. The plastic elongation of γ-based TiAl alloys with FL microstructure changes non-monotonously with the increase of grain size, which results from cooperation of micro-deformations and microcracks. [ABSTRACT FROM AUTHOR]

The influences of polarization and structure parameters on the intersubband transition frequency within terahertz (THz) range and oscillator strength in GaN/AlGaN step quantum well have been investigated by solving Schrödinger and Poisson equations self-consistently. The results show that the Al mole compositions of step quantum well and space barrier have a significant effect on the THz intersubband transition frequency. A specific phenomenon is found that the minimum energy spacing between the ground state and first excited state can be achieved as the Al mole composition of space barrier is about twice of that of step well. In particular, an intersubband transition with energy of 19.8 meV (4.83 THz) can be obtained with specifically designed parameters. This specific phenomenon still exists in a wide range of step well width and a narrow range of well width with less than 3% fluctuation of the Al mole composition of barrier. In addition, oscillator strength and dipole matrix element versus the widths of well and step well, the influences of doping location and concentration on the absorption coefficient, are also investigated in detail in this study. The results should be of benefit to the design of devices operating in the THz frequency range. [ABSTRACT FROM AUTHOR]

The redshift (∼54 meV) of the photoluminescence (PL) peak energy of blue InGaN/GaN light-emitting diodes exposed to 60Co γ-rays was observed. Time-resolved PL indicates that the PL radiative time increases along with irradiation does. The temperature-dependent PL and photoreflectance techniques show that γ-ray irradiance leads to higher carrier localization energies and larger Stokes' shift, respectively. These facts suggest that the redshifts of the PL peak emission introduced by γ-ray irradiance mainly originated from the enhancement of indium fluctuation. The cathodoluminescent images of the samples before and after γ-ray irradiation are compared to verify the results. [ABSTRACT FROM AUTHOR]

Various acoustic and mechanical wave components have been developed. Most of them focused on the control of energy flow. However, these waves are not only energy flow but also wave signals with a frequency dependence. In this Letter, we propose a mechanical wave switch (MWS) capable of controlling energy flow and output frequency simultaneously. It consists of a difference frequency generation (DFG) unit and a wave filter. The DFG unit consists of two mass blocks and a tension/compression asymmetrical bilinear spring, and the wave filter is a one-dimensional mass-spring lattice. Systematical calculations were carried out to identify the critical condition for the design of MWSs. Both numerical and experimental results demonstrate the capability of the proposed MWS to tune the energy flow and the output frequency of acoustic waves via a small amplitude control wave signal. The scale-independent MWS can be implemented at micro- and nanoscales. [ABSTRACT FROM AUTHOR]

The properties of intersubband transition in AlGaN/GaN multi-quantum wells (MQWs) grown on different AlGaN templates by metalorganic chemical vapor deposition are investigated. The strain states of GaN wells are studied by Raman spectra and reciprocal space mappings, which shows that the GaN wells are compressively strained and the compressive strain is increased when the Al mole composition is varied from 0 to 0.3. The Fourier transform infrared spectrometer results show that the intersubband transition wavelength in the AlGaN/GaN MQWs can be tuned from 5.14 μm to 4.65 μm when the Al mole composition of the AlGaN template is increased. The results can be attributed to the quantum confined Stark effect. [ABSTRACT FROM AUTHOR]

The effect of long-range screening on the surface morphology of thin films grown with pulsed-dc (p-dc) magnetron sputtering is studied. The surface evolution is described by a stochastic diffusion equation that includes the nonlocal shadowing effects in three spatial dimensions. The diffusional relaxation and the angular distribution of the incident particle flux strongly influence the transition to the shadowing growth regime. In the magnetron sputtering deposition the shadowing effect is essential because of the configuration of the magnetron system (finite size of sputtered targets, rotating sample holder, etc.). A realistic angular distribution of depositing particles is constructed by taking into account the cylindrical magnetron geometry. Simulation results are compared with the experimental data of surface roughness evolution during 100 and 350 kHz p-dc deposition, respectively. [ABSTRACT FROM AUTHOR]

An analysis of dynamic roughening and smoothening mechanisms of thin films grown with pulsed-dc magnetron sputtering is presented. The roughness evolution has been described by a linear stochastic equation, which contains the second- and fourth-order gradient terms. Dynamic smoothening of the growing interface is explained by ballistic effects resulting from impingements of ions to the growing thin film. These ballistic effects are sensitive to the flux and energy of impinging ions. The predictions of the model are compared with experimental data, and it is concluded that the thin film roughness can be further controlled by adjusting waveform, frequency, and width of dc pulses. [ABSTRACT FROM AUTHOR]

In contrast to the commonly observed dynamic roughening in film growth we have observed dynamic smoothing in the growth of diamondlike-carbon nanocomposite (TiC/a-C) films up to 1.5 μm thickness. Analytical and numerical simulations, based on the Edwards–Wilkinson model and the Mullins model, visualize the effect of the diffusivity parameters and the noise strength on the interface evolution of dynamic smoothing. The prediction is in a good agreement with the measured roughness evolution. High-resolution transmission electron microscopy shows that the formation of an amorphous front layer 2 nm thick excludes possible influence of nanocrystallites on the dynamic growth behavior of the nanocomposite film. [ABSTRACT FROM AUTHOR]

Surface roughness and dynamic growth behavior of TiC/a-C nanocomposite films deposited by nonreactive pulsed-dc (p-dc) magnetron sputtering were studied using atomic force microscopy, cross-sectional scanning, and transmission electron microscopy. From detailed analyses of surface morphology and growth conditions, it is concluded that a transition in growth mechanisms occurs, i.e., a mechanism dominated by geometric shadowing at a p-dc frequency of 100 kHz evolving to a surface diffusion mechanism driven by impact-induced atomistic downhill flow process by Ar+ ions at a p-dc frequency of 350 kHz. It is shown that rapid smoothening of initially rough surfaces with rms roughness from ∼6 to <1 nm can be effectively achieved with p-dc sputtering at 350 kHz pulse frequency, leading to a transition from a strong columnar to a columnar-free microstructure. [ABSTRACT FROM AUTHOR]

We report on the strain reduction in AlGaN layers grown on porous GaN (P-GaN) by metalorganic chemical vapor deposition (MOCVD). The P-GaN was obtained by ultraviolet radiation-enhanced electroless wet chemical etching of thick hydride vapor phase epitaxial grown GaN (HVPE-GaN) templates over (001) sapphire substrates. By optimizing the growth conditions, lateral growth of AlGaN was enhanced resulting in air-bridge formation between the P-GaN and the AlGaN layers. X-ray diffraction studies showed significant strain relaxation in AlGaN layers primarily due to the strain sharing between P-GaN and the AlGaN layers. This allowed us to grow crack-free good optical quality layers with thickness exceeding the critical limits for AlGaN deposition on the conventional MOCVD GaN or HVPE-GaN. The obtained results demonstrate the potential of this approach for the development of efficient ultraviolet light emitters. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Originated from the art of paper cutting and folding, kirigami and origami have shown promising applications in a broad range of scientific and engineering fields. Developments of kirigami-inspired inverse design methods that map target three-dimensional (3D) geometries into two-dimensional (2D) patterns of cuts and creases are desired to serve as guidelines for practical applications. In this paper, using programed kirigami tessellations, we propose two design methods to approximate the geometries of developable surfaces and nonzero Gauss curvature surfaces with rotational symmetry. In the first method, a periodic array of kirigami pattern with spatially varying geometric parameters is obtained, allowing formation of developable surfaces of desired curvature distribution and thickness, through controlled shrinkage and bending deformations. In the second method, another type of kirigami tessellations, in combination with Miura origami, is proposed to approximate nondevelopable surfaces with rotational symmetry. Both methods are validated by experiments of folding patterned thin copper films into desired 3D structures. The mechanical behaviors of the kirigami designs are investigated using analytical modeling and finite element simulations. The proposed methods extend the design space of mechanical metamaterials and are expected to be useful for kirigami-inspired applications. [ABSTRACT FROM AUTHOR]

The capillary penetration of fluids in thin porous layers is of fundamental interest in nature and various industrial applications. When capillary flows occur in porous media, the extent of penetration is known to increase with the square root of time following the Lucas-Washburn law. In practice, volatile liquid evaporates at the surface of porous media, which restricts penetration to a limited region. In this work, on the basis of Darcy's law and mass conservation, a general theoretical model is developed for the evaporation-limited radial capillary penetration in porous media. The presented model predicts that evaporation decreases the rate of fluid penetration and limits it to a critical radius. Furthermore, we construct a unified phase diagram that describes the limited penetration in an annular porous medium, in which the boundaries of outward and inward liquid are predicted quantitatively. It is expected that the proposed theoretical model will advance the understanding of penetration dynamics in porous media and facilitate the design of engineered porous architectures. [ABSTRACT FROM AUTHOR]

Gas and liquid adsorption-induced deformation of ordered porous materials is an important physical phenomenon with a wide range of applications. In general, the deformation can be characterized by the pore-load modulus and, when the pore size reduces to nanoscale, it is affected by surface effects and shows prominent size-dependent features. In this Letter, the influence of surface effects on the elastic properties of ordered nanoporous materials with internal pressure is accounted for in a single pore model. A porosity and surface elastic constants dependent closed form solution for the size dependent pore-load modulus is obtained and verified by finite element simulations and available experimental results. In addition, it is found to depend on the geometrical arrangement of pores. This study provides an efficient tool to analyze the surface effects on the elastic response of ordered nanoporous materials. [ABSTRACT FROM AUTHOR]

Under quasi-static uniaxial compression, inserting aluminum foams into the interstices of a metallic sandwich panel with corrugated core increased significantly both its peak crushing strength and energy absorption per unit mass. This beneficial effect diminished however if the foam relative density was relatively low or the compression velocity became sufficiently high. To provide insight into the vaiying role of aluminum foam filler with increasing compression velocity, the crushing response and collapse modes of all metallic corrugate-cored sandwich panels filled with close-celled aluminum foams were studied using the method of finite elements (FEs). The constraint that sandwich panels with and without foam filling had the same total weight was enforced. The effects of plastic hardening and strain rate sensitivity of the strut material as well as foam!strut interfacial debonding were quantified. Three collapse modes (quasi-static, transition, and shock modes) were identified, corresponding to different ranges of compression velocity. Strengthening due to foam insertion and inertial stabilization both acted to provide support for the struts against buckling. At relatively low compression velocities, the struts were mainly strengthened by the surrounding foam; at high compression velocities, inertia stabilization played a more dominant role than foam filling. [ABSTRACT FROM AUTHOR]

Background: Although surgery is the gold standard treatment for anal fissure, the main concern remains its side effects and complications. Botulinum toxin injection and lateral internal sphincterotomy are technical options for patients suffering from chronic anal fissure. However, little is known about the efficacy of these two techniques. The aim of this meta-analysis was to compare the outcomes of botulinum toxin injection versus lateral internal sphincterotomy for chronic anal fissure. Methods: Original studies in English were searched from the MEDLINE database, PubMed, Web of Science, and the Cochrane Library database. Randomized control trials that compared botulinum toxin injection with lateral internal sphincterotomy were identified. Data were independently extracted for each study, and a meta-analysis was performed using fixed and random effects models. Results: Four hundred and eighty-nine patients from seven trials met the inclusion criteria. Patients undergoing lateral internal sphincterotomy had a higher-healing and incontinence rate. No statistically significant differences were noted in total complications between botulinum toxin injection and lateral internal sphincterotomy. Patients treated with lateral internal sphincterotomy had a significantly lower recurrence rate than the patients treated with botulinum toxin injection. Conclusions: Our meta-analysis shows that lateral internal sphincterotomy was superior to botulinum toxin injection in terms of healing rate and lower recurrence rate. Botox, however, is safe associated with a lower rate of incontinence and could be used in certain situations. Further studies with a long-term follow-up are required to confirm our observations. [ABSTRACT FROM AUTHOR]

Drying-induced cracking is widely encountered in nature and is of fundamental interest in industrial applications. During desiccation, the evolution of water content is nonlinear. Considering the inhomogeneous procedure of desiccation, it is worth considering whether water content will affect the crack pattern formation. To address this concern, in this paper, we report an experimental investigation on the effect of water content on the failure mode in drying colloidal films. A distinct failure transition from random cracking to curling is found when the initial water content increases gradually. When the water content is below a critical value for given film thickness, random desiccation cracking driven by shrinkage is observed. Beyond this critical water content, the film curls with the advent of several main cracks. It is also found that the critical water content corresponding to the transition point depends on the film thickness. In order to qualitatively interpret the experimental observation, a theoretical model is established by adopting the fracture mechanics based on the energy method. The model is found to agree well with the experimental results, elucidating the effects of initial water content on the crack patterns and the transition of failure modes. [ABSTRACT FROM AUTHOR]

In this letter, we have investigated the structural properties of thick InGaN layers grown on GaN by plasma-assisted molecular beam epitaxy, using two growth rates of 1.0 and 3.6 Å/s. A highly regular superlattice (SL) structure is found to be spontaneously formed in the film grown at 3.6 Å/s but not in the film grown at 1.0 Å/s. The faster grown film also exhibits superior structural quality, which could be due to the surface roughness suppression caused by kinetic limitation, and the inhibition of the Frank-Read dislocation generation mechanism within the spontaneously formed SL structure. [ABSTRACT FROM AUTHOR]

We have investigated the structural properties and relaxation phenomenon of InGaN multiple quantum wells (QWs) on [formula] semipolar GaN templates grown on patterned (001) silicon substrates by selective area growth technique. Our studies by transmission electron microscopy and x-ray diffraction reciprocal space mapping reveal that QWs emitting light at 540 nm experience significant strain relaxation along the in-plane [formula] direction by the generation of an array of basal stacking faults (BSF). The generation of BSFs in 540 nm QWs could be an important factor limiting its luminescence efficiency. [ABSTRACT FROM AUTHOR]

A strong enhancement of terahertz (THz) response in a GaAs/AlGaAs quantum well photodetector (QWP) is observed under perpendicular magnetic field. Photocurrent spectra show that besides partial contribution from an increase in the detector differential resistance, improvement of photoconductive gain induced by electron localization and Landau quantization of the in-plane electron motion in quantum wells is the predominant underlying mechanism. This study sheds light on the transport mechanism in THz QWPs and provides a possible means for enhancing THz QWP’s response. [ABSTRACT FROM AUTHOR]

A direct correlation has been established between stacking faults in a-plane GaN epilayers and luminescence peaks in the 3.29–3.41 eV range. The structural features of the stacking faults were determined by diffraction-contrast transmission electron microscopy, while the optical emission characteristics were observed by highly spatially resolved monochromatic cathodoluminescence. The studies were performed in the exact same regions of thinned foils. We find that stacking faults on the basal plane are responsible for the strong emission at ∼3.14 eV. Luminescence peaks at ∼3.33 and ∼3.29 eV are associated with the presence of stacking faults on prismatic a planes and partial dislocations at the stacking fault boundaries, respectively. [ABSTRACT FROM AUTHOR]

A photoreflectance (PR) and photoluminescence (PL) study has been performed on a Si-doped epitaxial GaN layer that contains impurity or defect related below band gap features in its PR spectrum. In the 300 K PR spectrum, these features appear at energies of 3.26 and 3.33 eV, respectively, but below 180 K they can no longer be seen. The 3.26 eV line evidently corresponds to a donor acceptor pair transition, also seen in PL. The origin of the 3.33 eV line is uncertain, but may correspond to a transition involving the nitrogen vacancy. [ABSTRACT FROM AUTHOR]

Carrier recombination dynamics in epitaxial a-plane GaN and fully coalesced epitaxial laterally overgrown (ELOG) a-plane GaN films has been studied by means of time-resolved photoluminescence under high photoexcitation. The results were compared with conventional c-plane GaN films grown under similar conditions. In a-plane GaN epilayers, the total efficiency of electron-hole plasma spontaneous luminescence decreases 20 times, whereas the luminescence decay time reduces from τLU=42 to τLU≤10 ps in comparison with c-plane GaN films. Meanwhile, an essential increase in total emission efficiency (by more than two orders of magnitude) and an increase of the decay time up to τLU=430 ps have been observed for an ELOG a-plane sample in comparison with a-plane GaN films. This confirms a significant reduction of the nonradiative recombination rate for nonequilibrium carriers. Assuming a saturation of the nonradiative deep-level transitions, the room-temperature free-carrier lifetime of τ=910 ps for ELOG a-plane GaN sample was obtained, which indicates on an excellent quality of the a-plane ELOG GaN films. [ABSTRACT FROM AUTHOR]

We report on the performance of AlGaN/GaN/A1N heterostructure field-effect transistors (HFETs) grown over slightly-off c-axis, single-crystal, bulk AlN substrates. Dc and rf characteristics of these devices were comparable to HFETs grown on semi-insulating SiC. The obtained results demonstrate that bulk AlN substrates are suitable for fabricating high-power microwave AlGaN/GaN transistors. [ABSTRACT FROM AUTHOR]

In this letter, we report the pulsed atomic-layer epitaxy of ultrahigh-quality AlN epilayers and AlN/Al[sub 0.85]Ga[sub 0.15]N multiple quantum wells (MQWs) on basal plane sapphire substrates. Symmetric and asymmetric x-ray diffraction (XRD) measurements and room-temperature (RT) photoluminescence (PL) were used to establish the ultrahigh structural and optical quality. Strong band-edge RT PL at 208 and 228 nm was obtained from the AlN epilayers and the AlN/Al[sub 0.85]Ga[sub 0.15]N MQWs. These data clearly establish their suitability for sub-250-nm deep UV emitters. [ABSTRACT FROM AUTHOR]