Quantitative determination of the volume fraction of intergranular amorphous phase in sintered silicon nitride

In this paper, a commercial liquid phase sintered silicon nitride with Y₂O₃ and Al₂O₃ additives is investigated. The material is characterised by a large, and stable, internal friction peak near 1150 °C. The peak is linked with the glass transition in intergranular amorphous volumes, whose presence is confirmed with transmission electron microscopy. An estimate of the volume fraction of the amorphous phase is calculated from the difference in stiffness below and above the glass transition temperature. The procedure relies on accurate Young’s modulus data, which were obtained with the impulse excitation technique (IET). The amount of amorphous pocket phase was estimated at 12.4 vol.%. For the first time, microstructural evidence supporting this estimate is obtained, using analysis of scanning electron microscopy (SEM) images. The rather large amount of amorphous matter explains the limited high temperature potential of the material, which was primarily and successfully developed for wear applications.     

Anions effect on the low temperature growth of ZnO nanostructures

Seed mediated aqueous chemical growth (ACG) route was used for the growth of ZnO nanostructures on Si substrate in four different growth mediums. The growth medium has shown to affect the morphology and the size of the different nanostructures. We observed that the medium containing zinc nitrate anions yields the nanorods, in a medium containing zinc acetate anions nano-candles are obtained. While in a medium containing zinc chloride anions ZnO nano-discs were obtained and in a medium containing zinc sulfate anions nano-flakes are achieved. Growth in these different mediums has also shown effect on the optical emission characteristics of the different ZnO nanostructures.     

Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.     

On the Path Independency of the Point-wise J Integral in Three-dimensions

The asymptotic solution in the vicinity of a crack front in a three-dimensional (3-D) elastic domain is provided explicitly following the general framework in M. Costabel, M. Dauge and Z. Yosibash, 2004, SIAM Journal of Mathematical Analysis, 35(5), 1177–1202. Using it, we show analytically for several fully 3-D displacement fields (which are neither plane strain nor plane stress) that the pointwise path-area -integral in 3-D is path-independent. We then demonstrate by numerical examples, employing p-finite element methods, that good numerical approximations of the path-area -integral may be achieved which indeed show path independency. We also show that computation of the path part of the on a plane perpendicular to the crack front is path dependent. However, one may still use this path integral computed at several radii, followed by the application of Richardson’s extrapolation technique (as R→ 0) to obtain a good estimate for -integral.     

Coherent array imaging using phased subarrays. Part II: simulations and experimental results

The basic principles and theory of phased subarray (PSA) imaging imaging provides the flexibility of reducing the number of front-end hardware channels between that of classical synthetic aperture (CSA) imaging--which uses only one element per firing event--and full-phased array (FPA) imaging-which uses all elements for each firing. The performance of PSA generally ranges between that obtained by CSA and FPA using the same array, and depends on the amount of hardware complexity reduction. For the work described in this paper, we performed FPA, CSA, and PSA imaging of a resolution phantom using both simulated and experimental data from a 3-MHz, 3.2-cm, 128-element capacitive micromachined ultrasound transducer (CMUT) array. The simulated system point responses in the spatial and frequency domains are presented as a means of studying the effects of signal bandwidth, reconstruction filter size, and subsampling rate on the PSA system performance. The PSA and FPA sector-scanned images were reconstructed using the wideband experimental data with 80% fractional bandwidth, with seven 32-element subarrays used for PSA imaging. The measurements on the experimental sector images indicate that, at the transmit focal zone, the PSA method provides a 10% improvement in the 6-dB lateral resolution, and the axial point resolution of PSA imaging is identical to that of FPA imaging. The signal-to-noise ratio (SNR) of PSA image was 58.3 dB, 4.9 dB below that of the FPA image, and the contrast-to-noise ratio (CNR) is reduced by 10%. The simulated and experimental test results presented in this paper validate theoretical expectations and illustrate the flexibility of PSA imaging as a way to exchange SNR and frame rate for simplified front-end hardware.     

Dynamic analysis of capacitive micromachined ultrasonic transducers

Electrostatic transducers are usually operated under a DC bias below their collapse voltage. The same scheme has been adopted for capacitive micromachined ultrasonic transducers (cMUTs). DC bias deflects the cMUT membranes toward the substrate, so that their centers are free to move during both receive and transmit operations. In this paper, we present time-domain, finite element calculations for cMUTs using LS-DYNA, a commercially available finite element package. In addition to this DC bias mode, other new cMUT operations (collapse and collapse-snapback) have recently been demonstrated. Because cMUT membranes make contact with the substrate in these new operations, modeling of these cMUTs should include contact analysis. Our model was a cMUT transducer consisting of many hexagonal membranes; because it was symmetrical, we modeled only one-sixth of a hexagonal cell loaded with a fluid medium. The finite element results for both conventional and collapse modes were compared to measurements made by an optical interferometer; a good match was observed. Thus, the model is useful for designing cMUTs that operate in regimes where membranes make contact with the substrate.     

不平衡和非线性负荷引起的电能质量和中性线电流问题

1　引言近年来 ,对电力质量的关心正在迅速增长 ,因为低质量的电力对所供负荷会引起许多问题 ,例如工作失常、寿命缩短等。当今 ,负荷设备对电力质量变化的敏感性比过去应用的设备更为敏感。许多新的负荷设备含有以微处理器为基础的控制器和电力电子设备 ,它们对许多形式的扰动     

Engineering solid oxide fuel cell electrode microstructure by a micro-modeling tool based on estimation of TPB length

In this study, a typical solid oxide fuel cell (SOFC) electrode microstructure is numerically optimized in terms of the volume fraction of the catalyst, electrolyte and pore phases via a novel tool based on Dream.3D for the synthetic microstructure reconstruction and COMSOL Multiphysics® Modeling for visualizing and computing three/triple phase boundaries (TPBs). First, the properties of the representative volume element are studied by a parameter independence analysis based on the average particle size. The results indicate that the size of the representative volume element should be at least 10 times greater than the largest average particle size in the microstructure, while the number of mesh elements should be selected such that the smallest average particle size in the system is divided into at least 5. The method is then validated with the available studies in the literature and seems to agree well. Therefore, numerical reconstruction of SOFC electrodes by the proposed method is found to be a very useful tool in the viewpoints of accuracy, flexibility and cost. Finally, SOFC electrode microstructures having the same particle size distribution of an average particle size of 0.5 μm for each phase but with various phase volume fractions are generated and the resultant TPBs are computed similarly. It is found out that the volume fraction of each phase should be close to each other as much as possible to maximize the active TPB density and among the cases considered, the highest active TPB density of 9.53 μm/μm³ is achieved for an SOFC electrode including 35 vol% catalyst, 35 vol% electrolyte and 30 vol% porosity. The active TPB density is also found to be around 93% of the total TPB density.     

The effects of starter cultures on chemical,biochemical and sensory properties of low‐fat Tulum cheeses during ripening

Three different commercial starter cultures, Choozit^? MA 11 (MA ), Choozit^? BT 01 (BT ) and Choozit^? Feta A (Feta), were used to remedy textural and aromatic defects and improve the overall quality of low‐fat Tulum cheeses. Chemical and sensory analyses as well as electrophoresis were performed. Supplemental yoghurt bacteria and Lactobacillus helveticus were found to be key contributors in proteolysis with varying protein breakdown capacities. The results suggest that using appropriate culture combinations could result in low‐fat Tulum cheeses with better sensory characteristics and proteolysis rates.     

Ultralow Pt loading on CVD graphene for acid electrolytes and PEM fuel cells

Continuous-phase, porous graphene was produced by CVD process and tested for suitability as catalyst and catalyst support for polymer electrolyte membrane (PEM) fuel cells. N-doping of CVD graphene was carried by NH₃ gas flown over graphene for a given time. Ultralow Pt was sputter deposited onto porous, continuous phase N-doped graphene. Oxygen reduction reaction (ORR) activity by Rotating Disc Electrode (RDE) in 0,1 M HClO₄ electrolyte and PEM fuel cell performance were measured. N-doping and thicker Pt sputtering increases reduction current in RDE measurements. Sputter deposition of 1 nm and 10 nm Pt on CVD graphene (2.1 and 21.45 μgPt/cm² loading) has shown orders of magnitude increase in current compare to only-graphene samples. In fuel cell testing, sputter deposited Pt layer of 10 nm has provided 2.5 A/mgPt at 0.5 V polarization while 1 nm samples shows 15 A/mgPt performance at the same voltage.