Cross-sectional Porosity Research Articles

Microscopic mechanism of ultrasonically welded joints: The role of terminal roughness and wire diameter

The ultrasonic welding technology is widely promoted as a new connection approach in the field of current energy vehicle wiring harness connection. In this paper, three kinds of 25mm2 copper wire harnesses with different wire diameters and T2 copper terminals with different surface roughness were welded by ultrasonic welding. The mechanical properties of the joints were investigated by tensile experiments and the microstructure of joints was characterised using SEM and EBSD techniques. Excessive roughness increases plastic deformation at the weld interface during ultrasonic welding. This increases the dislocation density at the weld interface and refines the grain size. However, at the same time it inhibits recrystallisation to a certain extent. The lower roughness facilitates recrystallisation, but the low density of HAGBs makes the interface susceptible to slip in extended crystallographic plane and direction. Appropriate roughness allows the weld interface to generate fine equiaxed grains and a high density of HAGBs. This facilitates the obstruction of dislocation movement and improves the strength of joint. In addition, the high porosity of a longitudinal cross-section of the conductor with its small diameter was investigated. This results in a large number of wires remaining on the terminals when force is applied. It was determined that the larger a diameter of wire, the higher a cross-sectional porosity. The copper wire breaks at a weak point in cross-section when the force is applied, resulting in the entire wire being left on terminal. At a wire diameter of 0.2 mm, the porosity of a cross-section reaches an equilibrium and the strength of joint is even higher than the strength of material itself, resulting in the joint pulling off. The maximum strength reaches 4703.77 N.

Investigation on Three-Dimensional Void Mesostructures and Geometries in Porous Asphalt Mixture Based on Computed Tomography (CT) Images and Avizo

To investigate the void mesostructure in porous asphalt mixtures (PA), computed tomography (CT) and Avizo were utilized to scan and reconstruct the three-dimensional (3D) void model of PA-16 specimens. The void mesostructure of the specimen was quantitatively characterized through the anisotropy evaluation index. The equivalent pore network model (PNM) was extracted using the medial axis method. Based on the PNM model, the topological structure of the specimen and the morphological characteristics of the connected pores were analyzed. The results showed that the void anisotropy evaluation method can reflect the microscopic morphology of voids in porous asphalt mixtures. The cross-sectional porosity of representative elementary volume (REV) is mainly distributed between 20% and 25%, and about 90% of the macropores have a diameter between 0.5 mm and 3 mm. The distribution of cross-sectional porosity is uneven along the REV height direction. As the smallest cross-section of the seepage path, the equivalent radius of the throat is mainly between 0.1 mm and 1.5 mm, which is much smaller than the equivalent radius of the pore. The topological structure of pores is quite different, and their coordination numbers are mainly concentrated within 18. The pores with coordination numbers 1 to 10 constitute the main body of the pores inside REV, accounting for over 98% of the total number of pores. In addition, the permeability calculation results show that there is a significant difference in the permeability of each axis of REV compared to the total permeability of the superpave gyratory compactor (SGC) specimen, which illustrates that the permeability distribution presents an obvious spatial anisotropy. This study effectively reveals the heterogeneity of the 3D void morphology of porous asphalt mixtures, and it provides a reference for a better understanding of the void flow rules in drainage pavements.

A hBN/B-Si-Al-O-glass composite coating growth on SiCp/Al composite using one-step PEO with nanoparticle addition for outstanding electrical insulation performance

A comprehensive study of the formation of electrical insulation coatings on SiCp/Al composite using plasma electrolytic oxidation (PEO) in an electrolytic system with hBN nanoparticles has been carried out. A complete coating can be formed under a high positive voltage (550–600 V) in a short period of time (3 min), and the coating is mainly composed of the hBN crystal phase as well as some amorphous phases. The high temperature of the local plasma in the PEO process impels these amorphous phases to form an amorphous B-Si-Al-O glass phase. As the treatment time goes on, the cross-sectional porosity of the coating first decreases rapidly and then increases slowly. This is attributed to the fact that the nanoparticles continuously deposit and sinter to repair the larger pores during the coating growth process, hence reducing porosity. Within 6 min, the coating thickness rapidly increases to 133.3 μm, showing excellent electrical insulation performance (with a breakdown voltage of 1460 ± 28 V, volume resistance of 5.52 ± 0.26 G·Ω, electrical resistivity of 4.18 × 1010 Ω∙cm) and high optical band gap value (above 5 eV), which is suitable for electronic packaging or dielectric materials.

Effects of negative voltage on microstructure, electrical insulating, anti-corrosion, and thermal physical performance of plasma electrolytic oxidation coating on SiCp/Al composite

PEO coating microstructure on SiCp/Al composites under the influence of negative pulse are comparatively studied. Results show that the cross-sectional porosity is significantly reduced to 5.1%. The electrical insulation performance of coatings initially improves and then declines as the negative voltage rises from 0 V to 120 V, which is attributed that either an excessively low or excessively high negative voltage is insufficient to improve the internal defects of the coating. Among all the coatings, the coating obtained at +500 V/−80 V (i.e., N80) possesses better electrical insulation (with breakdown voltage of 736 ± 15 V, dielectric strength of 16.3 ± 0.3 V/μm, electrical resistivity of 1.51 × 1011 Ω∙cm), better dielectric performance, higher optical band gap value, making it appropriate for usage as electronic packaging or dielectric materials. The impedance modulus at low frequency of N80 coating is two to three times higher than that of N0, N40, and N120 coating, providing a long-term anti-corrosion ability for the substrate, during the 72-hour immersion. Additionally, the coatings display a high infrared emissivity in the waveband of 8–20 μm (>0.9) to achieve good radiation and heat dissipation effect. Thus, a reasonable negative voltage used in coating preparation does effectively diminish internal structural defects, enhancing coating performance.

Microporous Polyethersulfone Membranes Grafted with Zwitterionic Polymer Brushes Showing Microfiltration Permeance and Ultrafiltration Bacteriophage Removal.

Virus removal from water using microfiltration (MF) membranes is of great interest but remains challenging owing to the membranes' mean pore sizes typically being significantly larger than most viruses. We present microporous membranes grafted with polyzwitterionic brushes (N-dimethylammonium betaine) that combine bacteriophage removal in the range of ultrafiltration (UF) membranes with the permeance of MF membranes. Brush structures were grafted in two steps: free-radical polymerization followed by atom transfer radical polymerization (ATRP). Attenuated total reflection Fourier transform infrared (ATR-FTIR) and X-ray photoelectron (XPS) verified that grafting occurred at both sides of the membranes and that the grafting increased with increasing the zwitterion monomer concentration. The log reduction values (LRVs) of the pristine membrane increased from less than 0.5 LRV for T4 (∼100 nm) and NT1 (∼50 nm) bacteriophages to up to 4.5 LRV for the T4 and 3.1 LRV for the NT1 for the brush-grafted membranes with a permeance of about 1000 LMH/bar. The high permeance was attributed to a high-water fraction in the ultra-hydrophilic brush structure. The high measured LRVs of the brush-grafted membranes were attributed to enhanced bacteriophages exclusion from the membrane surface and entrapment of the ones that penetrated the pores due to the membranes' smaller mean pore-size and cross-section porosity than those of the pristine membrane, as seen by scanning electron microscopy (SEM) and measured using liquid-liquid porometry. Micro X-ray fluorescence (μ-XRF) spectrometry and nanoscale secondary ion mass spectrometry showed that 100 nm Si-coated gold nanospheres accumulated on the surface of the pristine membrane but not on the brush-coated membrane and that the nanospheres that penetrated the membranes were entrapped in the brush-grafted membrane but passed the pristine one. These results corroborate the LRVs obtained during filtration experiments and support the inference that the increased removal was due to a combined exclusion mechanism and entrapment. Overall, these microporous brush-grafted membranes show potential for use in advanced water treatment.

PREDICTING THE ELECTRICAL CONDUCTIVITY OF DUAL-POROSITY MEDIA WITH FRACTAL THEORY

The microspatial structure of porous media affects the electrical properties of reservoir rocks significantly. In this work, a dual-porosity model is established to investigate the electrical properties of porous media, in which tree-like networks and capillary channels represent fractures and pores. By using fractal theory, we established an analytical equation for the conductivity of water-saturated dual-porosity media. The analytical equation, devoid of any empirical constants, expresses the electrical properties of the porous media as a function of some structural parameters ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]. We also examine the impact of various matrix structural parameters on conductivity. It is found that increasing the length of mother channel ([Formula: see text], length ratio ([Formula: see text], the number of branching layers ([Formula: see text], and tortuosity fractal dimension ([Formula: see text] leads to a decrease in conductivity, whereas increasing the diameter of mother channel ([Formula: see text], diameter ratio ([Formula: see text], the cross-sectional porosity ([Formula: see text], [Formula: see text], and the channel bifurcation number ([Formula: see text] enhances conductivity. Furthermore, we validated this analytical model by comparing it with the experimental data available, and the results demonstrate good agreement. This research has proposed an advanced conductivity model that enables us to better understand the underlying physical mechanisms of the electrical properties in porous media.

A statistical approach to determine data requirements for part porosity characterization in laser powder bed fusion additive manufacturing

A major factor in determining the fatigue life of fracture-critical parts is the effect of process-induced porosity. Prediction of critical pore size in different process regimes of a laser powder bed fusion (L-PBF) processed part could provide invaluable information for process development and qualification and certification efforts. However, the amount of data required to accurately populate the pore size distribution to predict the critical pore size is still unknown. To address this gap, the present study utilizes extreme value statistics to determine the data required to characterize porosity in the L-PBF additively manufactured parts fabricated using different processing conditions. 2D cross-sectional porosity data obtained via optical microscopy was used as an example to demonstrate the statistical modeling approach. The statistical modeling described here can also be applied to other manufacturing processes and other types of data such as 3D porosity measurements, grain size, and inclusions.

Power controlled microstructure and infrared properties of air plasma spraying based on YSZ coatings

Yttria stabilized zirconia (YSZ) exhibits low thermal conductivity as well as high electrical conductivity in a high temperature environment, which plays an important role in the infrared stealth field. This work studied YSZ coatings with different porosities by controlling the spraying power of air plasma spraying (APS), while analyzing the influence of spraying power on the microstructure and crystalline structure of the YSZ coating, and the mechanism of porosity on the infrared emissivity of the YSZ coating. The results showed that the cross-sectional porosity decreased from 19% to 9% with a spraying power increase from 55 kW to 95 kW. However, the infrared emissivity of the coatings at 3–5 μm decreased from 0.56 to 0.46 with porosity increases. Through the analysis of model simulation and theoretical calculation, it was found that YSZ coatings with higher porosity levels had more scatterers in the coating, which increased the backscattering of the coating and reduced the emissivity. The results showed that YSZ as a low emissivity coating has a dual effect and more potential in high temperature infrared stealth.

Preparation and characterization of nanoclay loaded polyacrylonitrile hollow fiber membranes

New Polyacrylonitrile (PAN) nanoclay hollow fiber membranes were successfully fabricated through the dry-jet wet spinning process. The effects of adding the nanoclay on the hollow fiber mechanical properties and permeation performance and dye removal from water were investigated. Direct red 80, Direct orange 26 and Direct brown 2 dyes were used for dye removal experiments. Field emission scanning electron microscopy (FESEM) was used to analyze the cross-sectional and surface morphology and porosity. X-ray diffraction (XRD) has been used to compare crystalline structure of the fabricated hollow fiber membranes. The results showed that permeation is increased by adding nanoclay to the spinning dope. In addition, hollow fibers spun with lower nanoclay content showed lower hydraulic permeability and higher dye removal efficiency. Tensile strength increased from 1.09 to 2.06 with 3 wt% nanoclay loading.

Interfacial Bonding Mechanism and Mechanical Performance of Continuous Fiber Reinforced Composites in Additive Manufacturing

The additive manufacturing of continuous fiber composites has the advantage of a high-precision and efficient forming process, which can realize the lightweight and integrated manufacturing of complex structures. However, many void defects exist between layers in the printing process of additive manufacturing; consequently, the bonding performance between layers is poor. The bonding neck is considered a key parameter for representing the quality of interfacial bonding. In this study, the formation mechanism of the bonding neck was comprehensively analyzed. First, the influence of the nozzle and basement temperatures on the printing performance and bonding neck size was measured. Second, CT scanning was used to realize the quantitative characterization of bonding neck parameters, and the reason behind the deviation of actual measurements from theoretical calculations was analyzed. When the nozzle temperature increased from 180 to 220 °C, CT measurement showed that the bonding neck diameter increased from 0.29 to 0.34 mm, and the cross-sectional porosity reduced from 5.48% to 3.22%. Finally, the fracture mechanism was studied, and the influence of the interfacial bonding quality on the destruction process of the materials was determined. In conclusion, this study can assist in optimizing the process parameters, which improves the precision of the printing parts and performance between the layers.

Effect of in-flight particle behavior on the morphology of flame sprayed Er2O3 splats

The morphology and microstructure of splats impact the comprehensive capability of a new coating methodology called chelate flame spraying (CFS). This study addresses the quantitative characterization of the spread morphologies of flame sprayed Er2O3 splats directly deposited under different spray conditions on aluminum alloy substrates with a mirror finish. The influence of the in-flight particle temperature and velocity, carrier gas type, and carrier gas ratio on the solidification mechanism of molten droplets was investigated. Image analysis methods were employed to identify single splats from the morphology observed with field-emission scanning electron microscopy (FE-SEM). In addition, Er2O3 films were synthesized on an Al–Mg alloy (A5052) substrate using N2 or O2 as the carrier gas. When O2 was used as the carrier gas, 109-μm-thick films were deposited on the A5052 substrate. The cross-sectional porosity of the films was 3.8%. In contrast, films with 101-μm thickness were synthesized on the A5052 substrate when N2 was used as the carrier gas. The cross-sectional porosity of these films was 13.8%. The results showed that the carrier gas type (N2) and carrier gas ratio had a significant effect on the flattening behavior of the molten droplets. A spraying method combined with multidimensional modes is proposed to control the morphology of the splats.

Magnetic solder to reduce porosity of solder joints formed in microgravity

Solder joints formed in microgravity are inferior to joints formed in Earth gravity. They are mechanically weaker and less electrically conductive due to voids forming in the joints when soldering in microgravity. Surface tension and lack of gravitational buoyancy combine to prevent flux and water vapors from escaping the molten solder in absence of gravity, resulting in more porous solder joints. Additionally, corrosive flux vapor products remain trapped in the voids causing further deterioration.Parabolic aircraft flight microgravity soldering data are presented, to compare the joint porosities of conventional nonmagnetic solders and new magnetic solders, to determine if the magnetic solder, when soldered in the presence of a magnetic field, would reduce porosity of solder joints formed in microgravity. Soldering irons were used to hand-solder resistor leads into PC board though-holes, for nonmagnetic and magnetic solders. Addition of 4 wt% and 6 wt% of iron microparticles to the solder pastes resulted in 24% and 28% reductions in the solder joint average cross-sectional porosity, respectively, relative to average nonmagnetic solder joint porosity. Joint porosity distributions were log-normal, so the Wilcoxon Rank Sum test for statistical significance was used to show these porosity reductions are statistically significant at α = 0.05. A fully-developed wire magnetic solder could enable in-space repair of electronics at the component level, giving space mission planners additional flexibility to eliminate some or all spare board-level electronics, and plan instead to perform electronics repairs at the component level. This could then reduce or eliminate required mass of these spare electronics boards.

Surface-porous and hollow poly(lactic acid) (SPH-PLA) and surface-porous and lotus-root-like PLA (SPL-PLA) nanofibres: preparation, quantitative analysis, and modelling

In this study, PLA nanofibres with unique microstructures were prepared by coaxial electrospinning. The cross-section of the prepared PLA nanofibres exhibited either a surface-porous hollow structure (SPH-PLA) or a surface-porous lotus-root-like structure (SPL-PLA). The morphology, hollow proportion, and porosity of the fibres under different electrospinning parameter conditions were analysed quantitatively; further, three-dimensional (3D) models of the PLA nanofibres were established under parameter conditions that provided the highest hollow proportion. The results of the analysis revealed that the diameter fluctuated only slightly with the process parameters. Additionally, variations in the different electrospinning parameters caused only a slight change in the surface porosity (i.e. only 31% ± 4%) of the PLA fibres. The total porosity of the section (termed the pore porosity), porosity of the hollow section (termed the hollow porosity), R value (R = hollow porosity/pore porosity), and number of small pores of the SPH-PLA nanofibres did not change significantly with the receiving distance; the change observed was approximately only by 38% ± 2%, 28% ± 2%, 75% ± 5%, and 11 ± 1, respectively. Further, the cross-sectional porosity of the SPL-PLA nanofibres also did not change significantly with the receiving distance, only by approximately 24% ± 2%. The hollow proportion was the highest (76.32% ± 8.34%) at a core/shell flow rate ratio of 0.3:0.9 (mL/h), spinning voltage of 19 kV, and receiving distance of 6 cm. Therefore, 3D models of the SPH-PLA and SPL-PLA nanofibres were established under these electrospinning parameters.

Analysis of deterioration characteristics in oil-immersed insulation pressboard with different durations of aging based on an image-processing method

Insulation pressboard samples were obtained by thermal aging (according to Montsinger’s formula, at 130 °C, the pressboard is heated for 0 to 32 days) and discharge experiments. SEM images of samples were analysed. Image segmentation was applied to calculate the fibre width, cross-sectional porosity, and carbon-trace area. Inter-layer fibre models were established to observe fibre morphology using 3-D reconstruction. The initial discharge voltage decreased with age, and the discharge amounts increased. After 16 days of aging, the fibre width had decreased to between 68.1% and 81.8% of unaged pressboard. As the aging increased, cellulose hydrogen bonds were broken, which affected the expansion of interlayer pores, increasing the porosity of the pressboard. After 32 days of aging, the porosity increased to 2.38 times that of a new pressboard. In addition, the longer the aging, the larger the area of carbon marks caused by the discharge breakdown. With the aggravation of thermal aging, the insulating property of pressboard decreased due to the decrease of fibre width and increase of porosity that further accelerated the damage to the fibre structure. It was concluded that the fibre width and porosity could be used as criteria to judge the degradation of pressboard.

Cross-sectional Porosity and Oil Sorption of PLA Nanofibers with Hollow and Lotus Root-like Structures

Cross-sectional Porosity and Oil Sorption of PLA Nanofibers with Hollow and Lotus Root-like Structures

Failure modes of insulating pressboard subjected to high electric fields and durations: Evidence of partial discharge and changes at the microscopic scale

The deterioration of insulation pressboard under needle-plate discharge was tested, and the degradation stage was divided according to observed experimental phenomena. Based on the improved Top-hat watershed image segmentation method, the white marks area of the pressboard were tested. Fibres with different discharge states were extracted from SEM images. The fibre width was calculated, and the porosity of the surface of the insulation pressboard at different degradation stages was calculated. Simultaneously, the 3D reconstruction technique was used to observe the 3D morphology of fibres at different discharge stages. The study found that with the deepening of the discharge process, the discharge of the pressboard increased, and the white marks of the pressboard continued to expand from point to surface. In addition, the diameter of the fibre of the insulating pressboard decreased obviously with the increase of the pressing time, and the fibre diameter was 89.6% after the breakdown. Moreover, the electrical stress had a great effect on the expansion of the interlayer pores of the pressboard; the cross-section porosity of the insulating pressboard gradually increased with the deepening of the discharge process, and after breakdown, the interlayer porosity reached 12.5%.

Influence of ultrasonic field during micro-arc oxidation on the structure and properties of calcium phosphate coatings

The study of the influence of applied external ultrasound (US) field with varied power of 35 or 100 W during the micro-arc oxidation (MAO) processing on the thickness growth, surface roughness, porosity, sizes of structural elements and structural-phase states of the calcium phosphate (CaP) coatings on the titanium surface was performed. It was shown that the US transmittance through the electrolyte for 10 min of MAO processing had led to the increase of the coating thickness from 50 to 60 µm, the surface roughness from 3 to 4.5 µm, and the cross-section porosity from 27 to 40%. However, under the action of external US there was a destruction of structural elements (spheres with pores) on the coating surface and a filling of pores’ spaces by fragments. That led to the decrease of the surface porosity from 30 to 12%. It was revealed by X-ray diffraction (XRD) that all types of the CaP coatings deposited under external US field and without it were in X-ray amorphous state. However, applied US field had led to the increase of the coating crystallinity with incorporation of CaHPO4 and β-Ca2P2O7 phases.

Effect of Bedding Planes on the Permeability and Diffusivity Anisotropies of Berea Sandstone

Evaluating the anisotropy of transport parameters in rocks is important for various applications, such as reservoir engineering and rock mechanics. Owing to their anisotropic pore structures, the tortuosity, constrictivity, and pore size distribution of rocks are often anisotropic in nature, which in turn affect the permeability and diffusivity. However, it has still not been determined whether the permeability and diffusivity are anisotropic in the same manner. This study used experiments and numerical modeling to examine the effect of the pore structure on the permeability and diffusivity anisotropies of rocks. The experimental results showed a clear difference in the anisotropy ratios of the permeability (k⊥/k‖) and diffusivity (De⊥/De‖) for Berea sandstone, which is the de facto standard porous sandstone. The analysis results from micro-focus X-ray computed tomography and simulation with the lattice Boltzmann method supported the experimental difference in anisotropy ratios. In the analysis and simulation, the relation between the minimum cross-sectional porosity area and characteristic pressure gradient was estimated. The analysis results suggest that the minimum cross-sectional porosity areas that influence the permeability anisotropy are too large to physically induce anisotropic NaCl diffusion, and thus, the diffusivity of Berea sandstone is nearly isotropic.

High voltage resistance ceramic coating fabricated on titanium alloy for insulation shielding application

To endow the insulation shielding of electromechanical devices, a TiO2 based ceramic coating with high voltage resistance was fabricated on titanium alloy by plasma electrolytic oxidation (PEO). The effects of PEO coatings with different phase composition and microstructure on insulation performance were determined. The results show that the ceramic coating formed in the silicate-based solution was mainly composed of rutile, anatase TiO2 and Al2TiO5. As the coating thickens from 10 to 36 µm, the increasing cross-sectional porosity enables the dielectric strength decrease from 32.94 ± 2.1 V/μm to 11.9 ± 0.71 V/μm and the electric resistivity drop from 1.17 × 108 Ω cm to 0.99 × 108 Ω cm respectively. With 2 g/L KOH added into the basic electrolyte, the thickness of coating significantly increased to 50.0 ± 2.1 µm, accompanied by the enhanced crystalline degree of rutile TiO2 phase and the decreasing of internal defects. In this way, the optimum coating with preferable resistivity of 1.08 × 1010 Ω cm, high breakdown voltage of 617.78 ± 26.31 V and dielectric strength of 12.36 ± 0.53 V/μm was achieved.

The Application of Globular Water-Atomized Iron Powders for Additive Manufacturing by a LENS Technique.

The water-atomized ATOMET 28, 1001, 4701, and 4801 powders, manufactured by Rio Tinto Metal Powders, were used for additive manufacturing by a laser engineered net shaping (LENS) technique. Their overall morphology was globular and rounded with a size distribution from about 20 to 200 µm. Only the ATOMET 28 powder was characterized by a strong inhomogeneity of particle size and irregular polyhedral shape of powder particles with sharp edges. The powders were pre-sieved to a size distribution from 40 to 150 µm before LENS processing. One particular sample—LENS-fabricated from the ATOMET 28 powder—was characterized by the largest cross-sectional (2D) porosity of 4.2% and bulk porosity of 3.9%, the latter determined by microtomography measurements. In contrast, the cross-sectional porosities of bulk, solid, nearly cubic LENS-fabricated samples from the other ATOMET powders exhibited very low porosities within the range 0.03–0.1%. Unexpectedly, the solid sample—LENS-fabricated from the reference, a purely spherical Fe 99.8 powder—exhibited a porosity of 1.1%, the second largest after that of the pre-sieved, nonspherical ATOMET 28 powder. Vibrations incorporated mechanically into the LENS powder feeding system substantially improved the flow rate vs. feeding rate dependence, making it completely linear with an excellent coefficient of fit, R2 = 0.99. In comparison, the reference powder Fe 99.8 always exhibited a linear dependence of the powder flow rate vs. feeding rate, regardless of vibrations.