High Level Of Porosity Research Articles

Ultralight and high-strength bulk alumina/zirconia composite ceramic foams through direct foaming method

The mechanical strength of highly porous ceramic foams shows significant attenuation with the increase of porosity, especially for ceramic foams with very high porosity level above 95%. Ultra-light bulk composite ceramic foams were fabricated via introducing 20 wt% zirconia in alumina-based cell wall, aiming at improving mechanical strength of alumina based ceramic foams. The preparation and stability of alumina and zirconia particles synergistically stabilized foams were investigated. The obtained bulk ceramic foams manifested an exciting combination of both high porosity above 96% and excellent specific compressive strength, due to their closed pore structure, defect-free cell wall with uniform thickness, and uniformly distributed zirconia grains in alumina matrix. The alumina/zirconia composite ceramic foams with porosity of 96.7% exhibited compressive strength of 1.11 ± 0.07 MPa, which was much higher than that of alumina ceramic foams in the case of same porosity.

Renewable N-doped active carbons as efficient catalysts for direct synthesis of cyclic carbonates from epoxides and CO2

In the spirit of green chemistry and greenhouse gas mitigation, we explore herein the chemical utilization of CO2 upon synthesis of cyclic carbonates over N-doped activated carbons. The N-doped carbocatalysts were obtained from inexpensive N-rich bio-waste precursors and characterized by standard techniques (N2 physisorption, chemisorption, XPS, SEM, TEM, XRD, FT-IR and Micro-Raman spectroscopy). The materials exhibited excellent catalytic activity for direct carbonation of epoxides with CO2 to cyclic carbonates (yields upto 99%) under solvent free, moderate temperature (100–150 °C) and low CO2 pressure (5–50 bar) conditions. The observed catalytic activity of the N-doped carbocatalysts was attributed to the Lewis basic sites originating from pyridinic, pyridonic, and quaternary N-sites capable of activating the CO2 molecule. While control experiments with multiwalled carbon nanotubes (MWCNT) or commercial activated carbon, failed to produce cyclic carbonates due to lack of active (basic) sites. In terms of the catalytic performance, the N-doped carbocatalysts presenting a high porosity (634–1316 m2/g) and high levels of pyridinic (33%) and quaternary N-doping (30%), (i.e. CA500 and MA500), exhibited the highest activity and selectivity (TOF, conversion and cyclic carbonate yields upto 99% in 5–15 h). Most importantly, these materials demonstrated good operational stability and reusability.

A study on changes in the properties of silumin surface layers modified by yttrium oxide

In the present work, the analysis of the structure and properties of silumin, modified by deposition of yttrium oxide on the surface by electric-explosive alloying, was made, (alloying with a droplet-plasma flow produced by the electric explosion of a conductive foil with a sample of powder material). The structure and phase composition of silumin were studied by scanning and transmission diffraction electron microscopy. Formed coatings are characterized by a high level of roughness and porosity.

Preparation of a Highly Porous Carbon Material Based on Quinoa Husk and Its Application for Removal of Dyes by Adsorption.

A porous carbon material was prepared from quinoa husk (QH) by carbonization and chemical activation with KOH. A series of experiments, including SEM (Scanning electron microscopy), FT-IR (Fourier transform infrared), XRD (X-ray diffraction), Raman, X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption, were carried out on the porous carbon produced from quinoa husk (PC–QH). The results showed that PC–QH was mainly composed of activated carbon and graphite. Moreover, PC–QH exhibited a high level of porosity with a BET (the Brunauer–Emmett–Teller theory) surface area of 1713 m2 g−1. As a representative dye, malachite green (MG) was selected to evaluate the performance of PC–QH to absorb the contaminants in dyeing wastewater. In batch adsorption experiments, PC–QH exhibited a high adsorption rate toward malachite green (MG). An uptake capacity of 599.90 mg g−1 was achieved in the initial 5 min, and the MG adsorption capacity of PC–QH reached 1365.10 mg g−1, which was higher than many other adsorbents. The adsorption data were well fitted with the Freundlich isotherm model and the pseudo-second-order kinetic model. PC–QH also displayed a high absorption rate to rhodamine B (RhB), methyl violet (MV), methylene blue (MB), and methyl orange (MO). The results in this study suggest that PC–QH can be a promising adsorbent for quick treatment of dyeing wastewater.

Polymer film dewetting for fabrication of out-of-autoclave prepreg with high through-thickness permeability

Polymer film dewetting on a substrate (independent of fiber bed architecture) was explored, developed, and demonstrated as a method to produce out-of-autoclave, vacuum bag-only (OoA/VBO) prepregs with high transverse permeability and process robustness. The dimensions of the surface openings created by dewetting were measured,and thepercent surface area exposed was calculated. Prepregs were fabricated with continuous and dewetted (discontinuous) films to producetrial laminates. The laminates were cured under both standard and sub-optimal conditions, and were characterized before, during, and after cure.Laminates fabricated with dewetted resin consistentlyachieved near-zero porosity. In contrast,laminates with continuous filmdisplayed high levels of porosity,particularly during sub-optimal cure. The findings demonstrate that dewetting can be used effectively to produce OoA prepregs with high through-thickness permeability, whichcan yieldporosity-free laminates via VBO processing. Furthermore, these results elucidate aspects of resin dewetting that are critical in thecreation ofrobust OoA prepregs.

A Comparative Geophysical Study of the Sedimentary-Metamorphic Contacts in the Douala and Kribi-Campo Sub-Basins of Cameroon

The magnetotelluric method has been used in this work to compare the nature of the sedimentary-metamorphic contacts of the Douala and Kribi-Campo sub-basins in Cameroon. The results show that the sedimentary-metamorphic contact zones for the two sub-basins, marked by a subsidence of the low resistivity materials on the sedimentary formations and uplift of the high resistivity materials characterized by intrusive bodies of higher values of resistivity on the metamorphic formations, are structurally similar,. However, the very low values of resistivity of rocks in the Douala sub-basin is suggestive of high porosity, permeability and high level of saline ions dissociation leading to high conductivity. These rocks should be of unconsolidated sediments for the sedimentary formation and gneiss for the metamorphic formation. On the contrary, the very high values of resistivity for rocks in the Kribi-Campo sub-basin indicate the absence of free mobile electrons and ions and low porosity, permeability and non-conductivity. The sedimentary formation of this sub-basin should be composed of limestone and conglomerates rocks with some gneissic and unconsolidated granitic materials. The rocks in the metamorphic formation should be completely granitic in nature. The shallow depth of penetration of only 4 km of telluric current in the Douala sub-basin is enough evidence that the tectonic events responsible for the emplacement of this contact zone were limited to the earth crust. On the other hand, in the Kribi-Campo sub-basin the tectonic events should have originated from within the earth mantle as the depth of penetration of telluric current attains 150 km.

Preparation of High-Quality Porous Calcium Hexa-Aluminate Ceramics using Salt as Both Reaction Medium and Pore-Former

Porous calcium hexa-aluminate (CA6) ceramics were synthesized by a molten salt-mediated technique using CaCO3 and α-Al2O3 powders as the main raw materials. The effects of salt type and content, along with firing temperature on phase formation, microstructural evolution, pore generation and size distribution, and mechanical strength of fired samples were investigated and compared. Among the three tested salts (KCl, NaCl, and CaCl2), KCl showed the best whereas CaCl2 showed the worst accelerating effect on CA6 formation. In the case of the former, phase-pure CA6 was formed after 3 h at 1500 °C, whereas in the case of the latter, CA2 was formed instead as a main phase. The salt not only acted as a reaction medium, accelerating CA6 formation, but also acted as a pore former, creating high levels of porosity in the fired samples. The apparent porosity of the fired samples clearly increased with an increase in KCl content. But the majority of pores were still micron sized and the pore size distribution was relatively narrow. This, along with the possible toughening effect from the well-developed plate-like CA6, conferred relatively high mechanical strength on the final highly porous CA6 ceramics.

Understanding the effect of porosity on the polarisation-field response of ferroelectric materials

This paper combines experimental and modelling studies to provide a detailed examination of the influence of porosity volume fraction and morphology on the polarisation-electric field response of ferroelectric materials. The broadening of the electric field distribution and a decrease in the electric field experienced by the ferroelectric ceramic medium due to the presence of low-permittivity pores is examined and its implications on the shape of the hysteresis loop, remnant polarisation and coercive field is discussed. The variation of coercive field with porosity level is seen to be complex and is attributed to two competing mechanisms where at high porosity levels the influence of the broadening of the electric field distribution dominates, while at low porosity levels an increase in the compliance of the matrix is more important. This new approach to understanding these materials enables the seemingly conflicting observations in the existing literature to be clarified and provides an effective approach to interpret the influence of pore fraction and morphology on the polarisation behaviour of ferroelectrics. Such information provides new insights in the interpretation of the physical properties of porous ferroelectric materials to inform future effort in the design of ferroelectric materials for piezoelectric sensor, actuator, energy harvesting, and transducer applications.

3D-printing porosity: A new approach to creating elevated porosity materials and structures

We introduce a new process that enables the ability to 3D-print high porosity materials and structures by combining the newly introduced 3D-Painting process with traditional salt-leaching. The synthesis and resulting properties of three 3D-printable inks comprised of varying volume ratios (25:75, 50:50, 70:30) of CuSO4 salt and polylactide-co-glycolide (PLGA), as well as their as-printed and salt-leached counterparts, are discussed. The resulting materials are comprised entirely of PLGA (F-PLGA), but exhibit porosities proportional to the original CuSO4 content. The three distinct F-PLGA materials exhibit average porosities of 66.6–94.4%, elastic moduli of 112.6-2.7 MPa, and absorbency of 195.7–742.2%. Studies with adult human mesenchymal stem cells (hMSCs) demonstrated that elevated porosity substantially promotes cell adhesion, viability, and proliferation. F-PLGA can also act as carriers for weak, naturally or synthetically-derived hydrogels. Finally, we show that this process can be extended to other materials including graphene, metals, and ceramics. Statement of SignificancePorosity plays an essential role in the performance and function of biomaterials, tissue engineering, and clinical medicine. For the same material chemistry, the level of porosity can dictate if it is cell, tissue, or organ friendly; with low porosity materials being far less favorable than high porosity materials. Despite its importance, it has been difficult to create three-dimensionally printed structures that are comprised of materials that have extremely high levels of internal porosity yet are surgically friendly (able to handle and utilize during surgical operations). In this work, we extend a new materials-centric approach to 3D-printing, 3D-Painting, to 3D-printing structures made almost entirely out of water-soluble salt. The structures are then washed in a specific way that not only extracts the salt but causes the structures to increase in size. With the salt removed, the resulting medical polymer structures are almost entirely porous and contain very little solid material, but the maintain their 3D-printed form and are highly compatible with adult human stem cells, are mechanically robust enough to use in surgical manipulations, and can be filled with and act as carriers for biologically active liquids and gels. We can also extend this process to three-dimensionally printing other porous materials, such as graphene, metals, and even ceramics.

Fabrication of bioactive porous bredigite (Ca7MgSi4O16) scaffold via space holder method

AbstractBredigite has been recently evaluated as a biodegradable ceramic with high mechanical strength, biocompatibility, bioactivity and excellent cell behavior. The objective of this research was to fabricate a nanostructured bredigite scaffold for bone repair and restoration via the space holder technique. The crystallite size, phase composition, particle size, pore morphology and pore size distribution of the scaffold were characterized using X-ray diffraction, as well as transmission and scanning electron microscopy. The results showed the fabrication of a highly interconnected porous scaffold, with a total porosity of about 86%, a pore size of about 400–600 μm, and the compressive strength of about 1.1 MPa. The prepared scaffold with nanosized structure and high porosity level leading to a high specific surface area could be used as an ideal scaffold for bone regeneration in bone tissue engineering applications.

The Influence of Temperature on Stability of Aluminum Foam Cell Wall during Foaming Process

This study concerns about the influence of foaming temperature which is applied to foaming process of aluminum foam to improve the stability of aluminum foam cell wall. Powder metallurgical method with four major foaming temperatures of 750°C, 800°C, 850°C and 900°C have been selected. Furthermore, the porosity of the foam was determined by ImageJ Analysis Software. Microhardness testing on the cell wall of aluminium foam was conducted according to ASTM E 92 using microhardness tester LM24AT with 200 grams and 15 s for loading time. The universal testing machine was applied to characterize the effect of foaming temperature on compression strength. The aluminum foam was observed in macroscopic and microscopic level using optical microscope (OM). The result revealed that the foaming temperature of 800°C gave the lowest value of porosity, with the highest hardness and compressive strength of 55.29 HV and 1.41 MPa, respectively. In addition, the highest porosity level was acquired by foaming temperature which was set at 900 °C. The lowest hardness value of 38.50 HV was obtained by foaming temperature of 700°C and the minimum compressive strength value of 0.75 MPa was exhibited when the foaming temperature was set at 900°C.

Use of Ion Beam Modification and Gas-Thermal Oxidation in the Process of Formation of Structurally Heterogeneous Oxide Coatings for Titanium Medical Implants

A new promising method for preliminary ion beam processing of titanium implants has been suggested and tested. The method uses high-energy Не+ ion implantation to achieve a high level of open porosity needed to form thin-layered coatings by the method of gas-thermal oxidation in air. The mechanism of pore formation and the effect of the obtained structurally heterogeneous oxide coatings on the osteointegration of rod implants are described.

Ba0.5Sr0.5Co0.8Fe0.2O3-δ-based dual-gradient cathodes for solid oxide fuel cells

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)/Gd0.2Ce0.8O1.9 (GDC) dual-gradient cathodes for solid oxide fuel cells (SOFCs) were prepared via a spraying-sintering route, using corn starch as a pore-forming agent. The X-ray diffraction (XRD) analysis results revealed that the BSCF and GDC showed good chemical compatibility even at 1200°C. The scanning electron microscopy (SEM) results indicated that a graded pore structure, in addition to a gradient composition, can be obtained by regulating the dosage of the pore-forming agent. Consequently, the dual-gradient cathode possessed a low polarization resistance of 0.25Ωcm2 at 750°C, when the pore-forming agent contents were graded at 5wt%, 10wt%, and 15wt% from the inner sub-layer to the outer sub-layer, respectively. This comparative study indicated that high levels of porosity are beneficial for oxygen transport; however, the variation in the porosity of the adjoining sub-layers should be tailored to guarantee the provision of continuous conductive paths for charge carriers.

Heat Treatment of Cold-Sprayed C355 Al for Repair: Microstructure and Mechanical Properties

Cold gas dynamic spraying of commercially pure aluminum is widely used for dimensional repair in the aerospace sector as it is capable of producing oxide-free deposits of hundreds of micrometer thickness with strong bonding to the substrate, based on adhesive pull-off tests, and often with enhanced hardness compared to the powder prior to spraying. There is significant interest in extending this application to structural, load-bearing repairs. Particularly, in the case of high-strength aluminum alloys, cold spray deposits can exhibit high levels of porosity and microcracks, leading to mechanical properties that are inadequate for most load-bearing applications. Here, heat treatment was investigated as a potential means of improving the properties of cold-sprayed coatings from Al alloy C355. Coatings produced with process conditions of 500 °C and 60 bar were heat-treated at 175, 200, 225, 250 °C for 4 h in air, and the evolution of the microstructure and microhardness was analyzed. Heat treatment at 225 and 250 °C revealed a decreased porosity (~ 0.14% and 0.02%, respectively) with the former yielding slightly reduced hardness (105 versus 130 HV0.05 as-sprayed). Compressive residual stress levels were approximately halved at all depths into the coating after heat treatment, and tensile testing showed an improvement in ductility.

Influence of post-HIP temperature on microstructural and optical properties of pure MgAl2O4 spinel: From opaque to transparent ceramics

Transparent MgAl2O4 spinel ceramics were processed from sub-micrometric commercial powder by applying a two-step procedure: pressureless sintering under vacuum followed by hot isostatic pressing. To limit grain growth and to avoid secondary reactions or impurities, no additives or sintering aids were added to the powder. First, pressureless sintering at 1500°C during 2h under vacuum led to opaque samples due to a high level of porosity. To improve the optical quality of the MgAl2O4 ceramics and the in-line transmission in the visible range, a post-treatment by hot isostatic pressing was applied. Highly transparent ceramics were obtained after a post-treatment at 1800°C for 10h with an in-line transmission of 81% at 400nm and 86% from 950 to 3000nm for a thickness of 2mm (98.8% of the theoretical transmission).

Twinning induced plasticity in austenitic stainless steel 316L made by additive manufacturing

Additively manufactured (AM) 316L steel exhibits extraordinary high yield strength, and surprisingly good ductility despite the high level of porosity in the material. This detailed study sheds light on the origins of the observed high yield strength and good ductility. The extremely fine cells which are formed because of rapid cooling and dense dislocations are responsible for the macroscopically high yield strength of the AM 316L (almost double of that seen in annealed 316L steel). Most interestingly, twinning is dominant in deformed samples of the AM316. It is believed that twinning-induced plasticity (TWIP) behaviour to be responsible for the excellent ductility of the steel despite the high level of porosity. The dominant twinning activity is attributed to Nitrogen gas used in 3D printing. Nitrogen can lower the stacking fault energy of the steel, leading to the disassociation of dislocations, promoting the deformation twinning. Twinning induces large plasticity during deformation that can compensate the negative effect of porosity in AM steel. However, twinning does not induce significant hardening because (1) the porosity causes a negative effect on hardening and (2) twinning spacing is still larger than extremely fine solidification cells.

Long discontinuous carbon fibre/polypropylene composites for high volume structural applications

A processing route is presented to manufacture discontinuous carbon fibre-reinforced polypropylene composites, using much longer fibre lengths (25 mm) and higher volume fractions (up to 45%) than previously reported in the literature. Carbon fibre tows are coated with different ratios of polypropylene, blended with a maleic anhydride coupling agent, to investigate the influence of the interfacial shear strength at the microscale on the macroscale composite properties. Improvements in the tensile performance at the macroscale (70% increase) are not as high as those reported for the interfacial shear strength at the microscale (300%), following the addition of the coupling agent. Consequently, the tensile strength of the carbon fibre-reinforced polypropylene material is only 45% of values reported for carbon fibre/epoxy systems, however, the tensile stiffness is comparable. This demonstrates the potential for using carbon fibre-reinforced polypropylene for structural applications, following further process optimisation to overcome the current high levels of porosity (3.3% at 0.45 Vf) to improve the tensile strength.

An iterative micromechanical modeling to estimate the thermal and mechanical properties of polydisperse composites with platy particles: Application to anisotropic hemp and lime concretes

This work aims at providing a multi-scale model to estimate the effective properties of hemp and lime concretes. The microstructures of such materials are characterized by a relatively high filling rate, platy orthotropic particles distributed on a wide range of spatial directions, and a high level of porosity at meso and micro scales. An iterative micromechanical modeling is here enhanced with some numerical features allowing to deal with the shape and orientation of the particles and with the resulting effective anisotropy of concretes. The model thus constructed is first put into practice to identify the properties of hemp shives and air-slaked lime for various compaction degrees. The results obtained are then used as input data to estimate the thermal and mechanical properties of both a collection of moderately compacted concretes, and a set of concretes with several compaction degrees. Simulations results are eventually confronted with experimental data from the literature.

Investigation of remelting and preheating in SLM of 18Ni300 maraging steel as corrective and preventive measures for porosity reduction

One of the most critical defects in selective laser melting (SLM) is the porosity formation. Optimization of process parameters for reducing the porosity levels to lower than <1% is possible in most of the cases. Susceptibility to porosity formation can be higher for different alloys as function of chemical composition due to higher spark generation and molten pool instabilities. On the other hand, the probability of porosity formation increases in larger components due to an extended processing time. Powder recoater wear, increase in thermal load, and accumulation of particles in the processing chamber become more relevant as the processing time increases. Hence, the use of integrated monitoring and correction strategies becomes crucially important. In this work, three different correction strategies are discussed for the correction of porosity during the SLM of 18Ni300 maraging steel. The main aim is to develop a possible correction and prevention scheme to be used within a fully monitored SLM process. The 18Ni300 maraging steel is susceptible to high levels of porosity due to the empirically observed melt-pool instabilities as well as high spark and vapor generation. The correction methods consisted of remelting of the defected layer employing different scan strategies namely “double pass,” “soft melting,” and “polishing.” As a preventive strategy, preheating at 170 °C was also evaluated. At an initial stage, all the strategies were tested throughout the part built in order to assess their general capacity in improving the part density. Surface roughness, geometrical error, and material microhardness were also evaluated to assess the impact of the strategies on the other quality aspects. The results indicate the capacity of improving the part density and reduce the part roughness effectively.

Formation and electrical characterization of black lipid membranes in porous filter materials

Black lipid membranes (BLMs) are planar lipid bilayers that can be formed in porous supports such as filters. A BLM as a biomimetic membrane has important applications in, for instance, biosensing and drug-permeability assays. In the present study, BLMs were formed in the pores of three different filters to investigate their ability to support the formation of a high-quality lipid bilayer. Poly (vinylidene fluoride) (PVDF), poly(acrylonitrile) (PAN), and polypropylene/polyethylene (PP/PE) filters were utilized as filter supports, and the bilayer formation was monitored in situ by capacitance measurements. The maximum capacitance was obtained for PVDF (5.34 μF · cm−2) versus PP/PE and PAN (2.95 and 0.91 μF · cm−2, respectively). Additionally, BLM leakage current measurements by wave deviation provided consistent results. Thickness and compactness of the bilayers were evaluated using pore forming protein (ClyA) insertion. The results show that PVDF with the highest porosity level was the most suitable support for BLM formation. The optimal thickness for the BLM in PVDF was 3–5 nm, where the most efficient protein insertion took place, in comparison with other filters.