Measurement of bistatic sea surface scattering with a parametric acoustic source.

This work presents the results from a series of bistatic sea surface scattering experiments conducted in shallow water using a parametric acoustic array as a source and a receiver comprising a horizontal linear array. The experiments measured scattering at three frequencies (4, 8, and 15 kHz) and at three incident grazing angles (13º, 20º, and 30º). The measurements were made over a 5 day period during which a variety of environmental conditions were encountered. This paper provides an outline of the experiments and presents some results for the forward scattering strength. The results show that the wave direction has a significant effect on the surface forward scattering. At each incident grazing angle, the fluctuations of scattering strength due to environmental conditions decreases as the frequency increases.

Weakly charged droplets fundamentally change impact dynamics on flat surfaces.

Electric charges are often found in naturally or artificially formed droplets, such as raindrops and those generated by Kelvin's water dropper. In contrast to the impact of neutral droplets on a flat solid surface upon which a thin convex lens shape layer of the gas film is typically formed, we show that the delicate gas thin film can be fundamentally altered for even weakly charged droplets. As the charge level is raised above a critical level of ∼1% of the Rayleigh limit for representative impact conditions, the Maxwell stress overcomes the gas pressure buildup to deform the droplet bottom surface. A conical liquid tip forms and pierces through the gas film, leading to a circular contact line moving outwards that does not trap any gas. The critical charge level only depends on the capillary number based on the gas viscosity. This finding applies to common liquids and molten alloy droplets, providing new insights into a range of applications such as mitigating pinhole defects in additive manufacturing.

Printing Functional 3D Microdevices by Laser-Induced Forward Transfer.

Slender, out-of-plane metal microdevices are made in a new spatial domain, by using laser-induced forward transfer (LIFT) of metals. Here, a thermocouple with a thickness of 10 µm and a height of 250 µm, consisting of platinum and gold pillars is demonstrated. Multimaterial LIFT enables manufacturing in the micrometer to millimeter range, i.e., between lithography and other 3D printing technologies.

Analysis techniques of lattice fringe images for quantified evaluation of pyrocarbon by chemical vapor infiltration.

Some image analysis techniques are developed for simplifying lattice fringe images of deposited pyrocarbon in carbon/carbon composites by chemical vapor infiltration. They are mainly the object counting method for detecting the optimum threshold, the self-adaptive morphological filtering, the node-separation technique for breaking the aggregate fringes, and some post processing algorithms for reconstructing the fringes. The simplified fringes are the foundation for defining and extracting quantitative nanostructure parameters of pyrocarbon. The frequency filter window of a Fourier transform is defined as the circular band that retains only those fringes with interlayer distance between 0.3 and 0.45 nm. Some judge criteria are set to define topological relation between fringes. For example, the aspect ratio and area of fringes are employed to detect aggregate fringes. Fringe coaxality and distance between endpoints are used to judge the disconnected fringes. The optimum values are determined by using the iterative correction techniques. The best cut-off value for the short fringes is chosen only when there is a reasonable match between the mean fringe length and the value measured by X-ray diffraction. The adopted techniques have been verified to be feasible and to have the potential to convert the complex lattice fringe image to a set of distinct fringe structures.

An Insight into the Mechanical Properties of Unidirectional C/C Composites Considering the Effect of Pore Microstructures via Numerical Calculation.

Pores are common defects generated during fabrication, which restrict the application of carbon/carbon (C/C) composites. To quantitatively understand the effects of pores on mechanical strength, this paper proposes a representative volume element model of unidirectional (UD) C/C composites based on the finite element method. The Hashin criterion and exponential degraded rule are used as the failure initiation and evolution of pyrolytic carbon matrices, respectively. Interfacial zones are characterized using the cohesive constitutive. At the same time, periodic boundary conditions are employed to study transverse tensile, compressive, and shear deformations of UD C/C composites. Predicted results are compared with the experimental results, which shows that the proposed model can effectively simulate the transverse mechanical behaviors of UD C/C composites. Based on this model, the effects of microstructural parameters including porosity, pore locations, the distance between two pores, pore clustering, and pore shapes on the mechanical strength are investigated. The results show that porosity markedly reduces the strength as porosity increases. When the porosity increases from 4.59% to 12.5%, the transverse tensile, compressive, and shear strengths decrease by 35.91%, 37.52%, and 30.76%, respectively. Pore locations, the distance between two pores, and pore clustering have little effect on the shear strength of UD C/C composites. For pore shapes, irregular pores more easily lead to stress concentration and matrix failure, which greatly depresses the bearing capacity of UD C/C composites.

Microchannels Formed Using Metal Microdroplets.

The metal microdroplet deposition manufacturing technique has gained extensive attention due to its potential applications in microstructure fabrication. In order to fabricate components such as microchannel heat sinks and microchannel reactors, this paper investigates the interactions and influences between microdroplets and substrates, as well as between microdroplets themselves. The transient phenomena during the fusion of metal microdroplets in contact with the substrate and the formation of inclined columns, as well as the solid-liquid coupling and morphology formation processes during the collision between microdroplets, are analyzed. The influence of microdroplet spacing on the morphology of microchannels during their formation is specifically studied. A three-dimensional finite element numerical model for the deposition of metal microdroplets forming inclined pillars is established based on the volume of fluid (VOF) method. The model treats the protective gas around the microdroplet as an empty zone and the microdroplet as a single-phase fluid. Simulation analysis is conducted to investigate the forming patterns of unsupported microdroplets at different spacing and their impact on the fusion morphology of microchannel components. Building upon this, a series of validation experiments are conducted using a piezoelectric microdroplet generator to produce uniform aluminum alloy microdroplets with a diameter of approximately 600 µm. A method for fabricating metal microchannel structures is obtained, which is expected to be applied in fields such as scattering structures for high-power electronic devices and microreactors in microchemical fields.

Direct Fabrication of Micron-Thickness PVA-CNT Patterned Films by Integrating Micro-Pen Writing of PVA Films and Drop-on-Demand Printing of CNT Micropatterns.

The direct fabrication of micron-thickness patterned electronics consisting of patterned PVA films and CNT micropatterns still faces considerable challenges. Here, we demonstrated the integrated fabrication of PVA films of micron-thickness and CNT-based patterns by utilising micro-pen writing and drop-on-demand printing in sequence. Patterned PVA films of 1-5 µm in thickness were written first using proper micro-pen writing parameters, including the writing gap, the substrate moving velocity, and the working pressure. Then, CNT droplets were printed on PVA films that were cured at 55-65 °C for 3-15 min, resulting in neat CNT patterns. In addition, an inertia-pseudopartial wetting spreading model was established to release the dynamics of the droplet spreading process over thin viscoelastic films. Uniform and dense CNT lines with a porosity of 2.2% were printed on PVA substrates that were preprocessed at 55 °C for 9 min using a staggered overwriting method with the proper number of layers. Finally, we demonstrated the feasibility of this hybrid printing method by printing a patterned PVA-CNT film and a micro-ribbon. This study provides a valid method for directly fabricating micron-thickness PVA-CNT electronics. The proposed method can also provide guidance on the direct writing of other high-molecular polymer materials and printing inks of other nanosuspensions.

Metal-organic frameworks/polydopamine synergistic interface enhancement of carbon fiber/phenolic composites for promoting mechanical and tribological performances.

Carbon fiber/phenolic composites have wide application prospects in the transmission of vehicles, where the combination of prominent mechanical and tribological properties is required. Multiscale metal-organic frameworks (MOFs) and polydopamine (PDA) as binary reinforcements were employed to construct a rigid-flexible hierarchical structure for improving the interfacial performances of friction materials. This unique rigid-flexible (MOFs/PDA) reinforcement could act as an effective interfacial linker, significantly facilitating the integration of fibers into the matrix and establishing a strong mechanical interlocking and chemical bonding onto the fiber/matrix interphase, thus boosting the mechanical and tribological properties of the composites. Benefiting from the MOF/PDA synergistic enhancement effects, the interlaminar shear strength of ZIF-8-composites (P1), MOF-5-composites (P2) and UiO-66-(COOH)2-composites (P3) was improved by 70.80%, 43.80% and 53.28%, respectively. In addition, the wear rate of P1 decreased from 3.55 × 10-8 cm3 J-1 to 2.45 × 10-8 cm3 J-1. This work provides a feasible approach for establishing rigid-flexible reinforced structures and opens up a double-component synergistic enhancement strategy to efficiently promote mechanical and tribological properties for fabricating high-performance carbon fiber/phenolic composites.

Experimental study and mechanism analysis on the effect of substrate wettability on graphene sheets distribution morphology within uniform printing droplets.

Uniform graphene films and micro-patterns are the cornerstones of graphene-based printed electronics. However, disk-like reduced graphene oxide (RGO) sheets trend to concentrate at the edge of the drop because of the famous coffee-ring effect, resulting in non-uniform patterns. To understand the physics of coffee-ring formation for RGO droplets on hydrophilic substrates, we propose a mechanical model to elucidate the influence and its mechanism of substrate wettability on the solute migration behavior and solute distribution morphology of RGO droplets. Stronger coffee-ring morphology and a slower velocity transition on the PMMA can be observed as compared to that on the glass slides. An explanation based on the mechanical model is provided as the large contact angle on the PMMA leads to a small hindrance force and finally results in more significant coffee-ring morphology. Remarkably, we have revealed one underlying mechanism by which the hydrophilic substrate with better wettability will form more uniform patterns. This study can provide fundamental insights into the relationship between substrate wettability and RGO sheets distribution morphology. It might contribute to morphology regulation of RGO droplets in the DOD printing of graphene films and micro-patterns.

Quantitative characterization of the carbon/carbon composites components based on video of polarized light microscope.

The components of carbon/carbon (C/C) composites have significant influence on the thermal and mechanical properties, so a quantitative characterization of component is necessary to study the microstructure of C/C composites, and further to improve the macroscopic properties of C/C composites. Considering the extinction crosses of the pyrocarbon matrix have significant moving features, the polarized light microscope (PLM) video is used to characterize C/C composites quantitatively because it contains sufficiently dynamic and structure information. Then the optical flow method is introduced to compute the optical flow field between the adjacent frames, and segment the components of C/C composites from PLM image by image processing. Meanwhile the matrix with different textures is re-segmented by the length difference of motion vectors, and then the component fraction of each component and extinction angle of pyrocarbon matrix are calculated directly. Finally, the C/C composites are successfully characterized from three aspects of carbon fiber, pyrocarbon, and pores by a series of image processing operators based on PLM video, and the errors of component fractions are less than 15%.

Interfacial microstructure and mechanical properties of Cf/AZ91D composites with TiO2 and PyC fiber coatings.

In spite of the effectiveness of the fiber coatings on interface modification of carbon fiber reinforced magnesium matrix composites, the cost and exclusive equipment for the coatings preparation are usually ignored during research work. In this paper, pyrolytic carbon (PyC) and TiO2 were coated on carbon fiber surface to study the effects of fiber coatings on interfacial microstructure and mechanical properties of carbon fiber reinforced AZ91D composites (Cf/AZ91D composites). It was indicated that both the two coatings could modify the interface and improve the mechanical properties of the composites. The ultimate tensile strength of the TiO2-Cf/AZ91D and the PyC-Cf/AZ91D composite were 333MPa and 400MPa, which were improved by 41.7% and 70.2% respectively, compared with the untreated-Cf/AZ91D composite. The microstructure observation revealed that the strengthening of the composites relied on fiber integrity and moderate interfacial bonding. MgO nano-particles were generated at the interface due to the reaction of TiO2 with Mg in the TiO2-Cf/AZ91D composite. The volume expansion resulting from the reaction let to disordered intergranular films and crystal defects at the interface. The fibers were protected and the interfacial reaction was restrained by PyC coating in the PyC-Cf/AZ91D composite. The principle to select the coating of fiber was proposed by comparing the effectiveness and cost of the coatings.

Quantitative characterization of carbon/carbon composites matrix texture based on image analysis using polarized light microscope.

A quantitative characteristic method was proposed for characterizing the matrix texture of carbon/carbon(C/C) composites, which determined the mechanical and physical properties of C/C composites. Based on the cloud theory that was commonly used for uncertain reasoning and the transformation between quantitative and qualitative characterization, so the relationship between the extinction angle and texture types was built by the cloud models for describing the texture of microstructure, moreover, linguistic controllers were established to analyze the matrix texture in accordance with the features of the polarized light microscope (PLM) image. On this basis, the extinction angle could be calculated from the PLM image of the C/C composites. In contrast to the results of measurement, the errors between calculative values and measured values were maintained 1-2° in basically. Meanwhile, the PLM image of C/C composites was segmented by the component, in particular, the matrix with mixed textures was further segmented by the difference of texture. It means that the quantitative characterization of C/C composites matrix based on single PLM image has been realized.

Research on precision-calibration techniques for selected area electron diffraction patterns of pyrocarbon.

The key techniques for determining orientation angle (OA) and interlayer space (d002) of pyrocarbon were investigated by analyzing selected area electron diffraction (SAED) patterns. A series of algorithms, which mainly include the five-point center-determined technique, the integral factor for the ellipse detection, the background subtraction operation and the Gaussian multipeak fitting algorithm, were designed for intensity sampling, data correction, and data fitting. The contribution ratio of the reflection intensity to the average d002 was considered. The algorithms were programmed and applied to evaluate SAED patterns of pyrocarbon in C/C composites by chemical vapor infiltration. Results showed that the proposed techniques can be effectively used to measure various SAED patterns, with a beam stop image or not, of pyrocarbon. The azimuthal intensities along the (002) arcs essentially obey the Gaussian distribution, although this is not obvious for the lower textural pyrocarbon. It is necessary for accurate OA to use the Gaussian multipeak fitting algorithm.