Macroporosity Formation Research Articles

Geopolymer composite foams reinforced with refractory filler: The effect of curing regime on microstructure, porosity and thermomechanical properties

Porous geopolymers with sustainable dimensional stability at high temperatures represent a noteworthy challenge for the development of thermomechanical, reliable and multifunctional composites. This study reports on the porosity formation of refractory-reinforced geopolymer paste via hydrogen peroxide decomposition. Reinforcing the slurry with ceramic chamotte and cordierite increases the apparent viscosity more than eight times in the interval to 100 s−1 retaining a pseudoplastic behavior. Cordierite particles increase the plastic viscosity at 6 Pa s and yield stress above 28 Pa. Surprisingly, the particulate ceramic filler acts as a porosity-modifier thereby reducing the mesoporosity region. Microstructural and porosity behavior was studied via digital microscopy, SEM, MIP and micro-CT. The wrinkled morphology of ceramics significantly affects the macroporosity formation in the region of 100 μm reaching a total porosity of ∼60 vol%. The presence of large pores in the macro region and the high distribution of interconnected porosity in ceramics-reinforced composite foams resulted in compressive strengths ranging from 2 to 10 MPa. The effect of curing rate on porosity was most evident in the cordierite reinforcement which led to the occurrence of large interconnected pores. Based on thermomechanical analysis, the ceramic-reinforced foams show high dimensional stability with a reduction in total shrinkage to −0.71 % at 850 °C. These results may foster further developments of innovative routes for the synthesis of geopolymer composite foams for widespread technological applications.

Adjustment of Micro- and Macroporosity of ß-TCP Scaffolds Using Solid-Stabilized Foams as Bone Replacement.

To enable rapid osteointegration in bioceramic implants and to give them osteoinductive properties, scaffolds with defined micro- and macroporosity are required. Pores or pore networks promote the integration of cells into the implant, facilitating the supply of nutrients and the removal of metabolic products. In this paper, scaffolds are created from ß-tricalciumphosphate (ß-TCP) and in a novel way, where both the micro- and macroporosity are adjusted simultaneously by the addition of pore-forming polymer particles. The particles used are 10-40 wt%, spherical polymer particles of polymethylmethacrylate (PMMA) (Ø = 5 µm) and alternatively polymethylsilsesquioxane (PMSQ) (Ø = 2 µm), added in the course of ß-TCP slurry preparation. The arrangement of hydrophobic polymer particles at the interface of air bubbles was incorporated during slurry preparation and foaming of the slurry. The foam structures remain after sintering and lead to the formation of macro-porosity in the scaffolds. Furthermore, decomposition of the polymer particles during thermal debindering results in the formation of an additional network of interconnecting micropores in the stabilizing structures. It is possible to adjust the porosity easily and quickly in a range of 1.2-140 μm with a relatively low organic fraction. The structures thus prepared showed no cytotoxicity nor negative effects on the biocompatibility.

Facile Preparation of Macro-Microporous Thorium Oxide via a Colloidal Sol-Gel Route toward Safe MOX Fuel Fabrication.

The identification of new colloidal sol-gel routes for the preparation of actinide oxides, which have a homogeneous and accessible porosity that can easily be impregnated by any concentrated actinide solution, opens new perspectives for the preparation of homogeneous nuclear fuel for minor actinide transmutation. This homogeneity allows us to avoid "hot spot" formation due to the local accumulation of more fissile elements. Here, we report the preparation of macro-microporous ThO2 materials by a colloidal sol-gel route. Using a thorium salt with 6-aminocaproic acid as a complexing agent at a controlled pH, we were able to pilot the condensation of thorium hydroxo species forming colloids of tuned nanometric size and thus the sol stability. After a freeze-drying process to concentrate colloids and a thermal treatment allowing complexing agent removal and macroporosity formation by a brutal gas release during combustion, a loose packing of ThO2 nanoparticles with an ordered distribution of interparticular porosity and a fraction of nanometric crystallites, whose size depends on the initial colloidal size, were obtained. The sols, pastes, and final materials were characterized by small- and wide-angle X-ray scattering to determine the colloidal size and the final structure of the materials, which was also confirmed by transmission electron microscopy. The most promising material was finally successfully impregnated by a simulating minor actinide solution and thermally treated to prepare a mixed actinide oxide material. This safe technology, relying on the colloidal sol-gel process and the formulation of complex fluids forming tunable precursors, opens new perspectives for the reuse of nuclear waste solutions as new fuel.

Macro Porosity Formation: A Study in High Pressure Die Casting

Porosity formation in high pressure die casting (HPDC) impacts mechanical properties and casting quality. Much is published regarding micro porosity and its impact on mechanical properties, but there is limited research on the actual formation of macro porosity. In production applications, macro porosity plays a critically important role in casting quality and acceptance by the customer. This paper argues that the most useful definition of macro porosity is the limits of visual detectability. With this definition, it will be shown macro porosity presents stochastically within a controlled HPDC process. This means macro porosity has a random probability distribution or pattern that should be analyzed statistically and cannot be predicted precisely. The general region where macro porosity forms is predictable with simulation, but its actual size and distribution of the voids are random. These results challenge the industry accepted practices for inspections and process controls. This also underscores the importance of up-front design for manufacturability to avoid macro porosity-related quality issues.

Daily strength testing of the Portland cement mortars

The study of daily tests (1-28 days) for compressive and flexural strength of cement mortars is presented in the article. The mortars were made from ordinary Portland cement, two different classes of cement were considered. Graphs with daily test strengths were obtained from the results of the experiment and the test results were fitted by exponential curves. To estimate the strength rates of mortars, numerical derivatives are calculated from experimental data and the exponential curves are fitted by the obtained values of numerical differentiation. As a result of experimental data processing, it was found that the flexural strength rate decreases to zero on the 12-14th day hardening of mortars, which can be explained by the end of the formation of macroporosity (voids) of the cement paste and the end of the chemical-physical processes of the formation of most contacts between the cement paste and the aggregate (sand grains).

A laboratory study to disentangle hydrological, mechanical and structural mechanisms of soil stabilization by plant mucilage between eroding and depositional zones of a slope

AbstractBiological exudates, such as plant mucilage, can greatly stabilize soils, but as the mechanical and hydrological drivers depend much on soil particle size composition, eroding and depositional areas of a slope may respond differently. Soils from an eroded midslope and a depositional footslope in an arable farm were amended with chia (Salvia hispanica) seed mucilage at concentrations of 0 g C kg−1, 0.46 g C kg−1 and 2.3 g C kg−1 mucilage, formed into cores, and then imparted with wetting and drying (WD) cycles. Mucilage increased the stability of these inherently stable soils from 80% to >98% water‐stable macroaggregates at 0 WD cycles regardless of slope position. Aggregate stability was maintained after 5 WD cycles by mucilage, whereas the stability of unamended soil dropped by 66.7% in the footslope and 30.1% in the midslope compared with 0 WD. The underlying physical stability properties were measured by tensile strength and penetration resistance for mechanical properties, water sorptivity and repellency for hydrological properties, and micro‐, meso‐, macro‐ and total porosity for structural properties. Almost every soil physical property measured changed less with WD cycles if mucilage was present. Compared to unamended soil, 2.3 g C kg−1 mucilage amendment decreased water sorptivity from 0.289 mm s−1/2 to 0.122 mm s−1/2 in the midslope and 0.230 mm s−1/2 to 0.182 mm s−1/2 in the footslope after 5 WD cycles. Aggregate stability, total porosity and water sorptivity were correlated. In the midslope, hydrology and penetration resistance were affected the most, which was likely to be driven by mucilage deposition in the macropores of this more coarsely textured soil. In the footslope, the greater impact of mucilage on tensile strength was likely to be driven by buffering of macroporosity formation by WD cycles in this finer‐textured soil.Highlights We explored how slope position interacts with plant mucilage to drive soil physical stability. Changes in soil physical stability by plant mucilage have rarely been considered with slope position. Interactions between mucilage and soil particles caused greater physical stability in the midslope than footslope. Mucilage stabilized soil by easing changes in pore structure, DOC redistribution and water repellency, with particle bonding less important.

Early-stage macroporosity enhancement in calcium phosphate cements by inclusion of poly(N-vinylpyrrolidone) particles as a porogen

The incorporation of poly(DL-lactic-co-glycolic acid) (PLGA) particles into calcium phosphate cements (CPCs) is an effective strategy to enhance CPC macroporosity and degradation. However, bone regeneration is hindered until hydrolytic PLGA degradation starts a few weeks after implantation. Additionally, CPC and CPC/PLGA injectability and cohesion are suboptimal. In the current study, poly(N-vinylpyrrolidone) (PVP), a water-soluble polymer, was incorporated as a porogen in CPC and CPC/PLGA composites to enhance handling properties and early-stage macroporosity formation. Further, the effect of PVP molecular weight (Mw) and particle size was studied. The results showed that PVP incorporation increased both injectability and cohesion of the CPC pastes, especially with addition of high Mw PVP. Moreover, the in vitro degradation studies revealed that incorporation of PVP induced an initial mass loss during the first week of incubation. In combination with PLGA, small PVP particles induced a higher mass loss at an early stage than large PVP particles, but this effect was no longer apparent after 4 weeks of incubation. In contrast, the incorporation of low Mw PVP had a stronger effect on in vitro degradation in the long term compared to high Mw. Finally, the presence of PLGA porogens appeared to be necessary for adequate CPC degradation.

LABORATORY DIAGNOSTIC SYSTEM FOR WATER-PHYSICAL SOIL PROPERTIES

Actuality of the problem. Irrigation has become a determining factor in the formation of bioproduction processes of new agricultural crop varieties and hybrids due to global climate change for all soil-climatic zones of Ukraine. Moreover, irrigation efficiency is determined to a significant degree by the reliability of the soil water-physical properties. The purpose of comprehensive hydrophysical studies was to determine the basic soil water-physical properties and constants necessary to create favorable soil regimes of reclaimed lands, and to do the mathematical modeling of the soil water regime. Complex laboratory hydrophysical tests of soil samples of undisturbed structure make it possible to determine hydrophysical functions for each soil sample: water holding capacity, water conductivity and water-physical constants of full and minimum moisture capacity, wilting moisture and maximum hygroscopic moisture, which can be determined on the same soil sample located on the same soil desorption curve from full moisture capacity to maximum hygroscopic humidity. The primary saturation of the soil sample under vacuum to full moisture capacity provides a single curve of water retention capacity taking into account structural macroporosity, which is the main feature of this technique. The resulting capillary hysteresis loop has the algorithm: fast nonequilibrium desorption from full moisture capacity and slow equilibrium sorption enables to build a differential curve of the distribution of pore volume over radii, characterizing the structure of the soil pore space. These structural characteristics are sensitive to soil processes, which determine the direction of epigenetic changes in the structure of the soil pore space and the direction of evolution of the soil matrix. The threshold of structural soil macroporosity formation is established from the loop of capillary hysteresis by the ratio of meniscus radii exceeding . Conclusions. The proposed system of soil laboratory diagnostics has advantages over the existing diagnostic methods and significantly increases the information content of complex hydrophysical tests, provides qualitatively new information on soils and provides mathematical modeling with the necessary parameters of mass transfer processes in moisture-saturated soils of the aeration zone.

Multimodal pore formation in calcium phosphate cements.

Calcium phosphate cements (CPCs) are commonly used as bone substitute materials. However, their slow degradation rate and lack of macroporosity hinders new bone formation. Poly(dl-lactic-co-glycolic acid) (PLGA) incorporation is of great interest as, upon degradation, produces acidic by-products that enhance CPC degradation. Yet, new bone formation is delayed until PLGA degradation occurs a few weeks after implantation. Therefore, the aim of this study was to accelerate the early stage pore formation within CPCs in vitro. With that purpose, we incorporated the water-soluble porogen sucrose at different weight percentages (10 or 20 wt %) to CPC and CPC/PLGA composites. The results revealed that incorporation of sucrose porogens increased mass loss within the first week of in vitro degradation in groups containing sucrose compared to control groups. After week 1, a further mass loss was observed related to PLGA and CPC degradation. Macroporosity analysis confirmed that macroporosity formation is influenced by the dissolution of sucrose at an early stage and by the degradation of PLGA and CPC at a later stage. We concluded that the combination of sucrose and PLGA porogens in CPC is a promising approach to promote early stage bone tissue ingrowth and complete replacement of CPC through multimodal pore formation. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 500-509, 2018.

Hydrothermal Synthesis and Characterization of Bio-Sourced Macroporous Zinc Phosphates Prepared with Casein Protein

The development of an original and simple procedure of hydrothermal porous biosourced zinc phosphates synthesis from casein protein is reported in this study. The synthesis procedure does not require additional phosphorus source and structure-directing agent for macroporosity formation. The formation of zinc phosphates has been investigated as a function of the pH of the starting mixture (4.5–14.0) and of the temperature of calcination (from 150 to 750 °C). A material composed of hopeite (Zn3(PO4)2·4H2O) and casein was obtained after synthesis at pH 4.5 and 100 °C from a mixture of casein and zinc nitrate solutions. Macroporous zinc phosphates composed of α-Zn3(PO4)2 and α-Zn2P2O7 with large porous size distribution (pore diameter between 350 to 1000 nm) were successfully obtained after the complete casein decomposition at 750 °C. Samples were characterized by X-ray powder diffraction, solid-state 31P NMR spectroscopy, thermal analysis, scanning electron microscopy, nitrogen adsorption, and by fluorescence spectroscopy. The macroporous zinc phosphates have a good stability in water for at least 24 h with no detectable change in their structure, porosity, and crystal morphology.

The Effects of Entrainment on Pore Shape in Keyhole Mode Welding

This study theoretically investigates the effects of the entrainment accompanying mass, momentum, and energy transport on pore size during high power density laser and electron beam welding processes. The physics of macroporosity formation is not well understood, even though macroporosity often occurs and limits the widespread industrial application of keyhole mode welding. This work is an extension of a previous work dealing with collapses of keyholes induced by high intensity beam drilling. In order to determine the pore shape, this study, however, introduces the equations of state at the times when the keyhole is about to be enclosed and when the temperature drops to melting temperature. The gas pressure required at the time when keyhole collapses is determined by calculating the compressible flow of the two-phase, vapor–liquid dispersion in a vertical keyhole with varying cross sections, paying particular attention to the transition between annular and slug flows. It is found that the pore size increases as entrainment fluxes decrease in the lower and upper regions of the keyhole containing a supersonic mixture. The pore size also increases with decreasing total energy of entrainment and an increasing axial velocity component ratio between entrainment and mixture through the core region. With a subsonic mixture in the keyhole, the final pore size increases with entrainment fluxes in the lower and upper regions. This work provides an exploratory and systematical investigation of pore size induced by entrainment accompanied by mass, momentum, and energy transport during keyhole mode welding.

Prediction of pore size in high power density beam welding

This study is to investigate parameters responsible for the final pore size during high power density laser and electron beam welding processes. Dimensionless parameters include the surface tension parameter, Mach number and liquid pressure at the keyhole base, friction factor, melting temperature, ratio of specific heats at constant pressure and volume, and loss coefficient. The friction factor can also be treated as a combination of viscous shear stress and thermocapillary force. Although the formation of macro-porosity is recognized as an important problem that limits the widespread industrial application of laser and electron beam welding, the physics of the macroporosity formation is not well understood. In order to solve this problem, both the states at the times when the keyhole is about to be enclosed and the pore is completely formed are considered to be governed by the equation of state. The pore shape thus can be determined by the Young–Laplace equation. The gas pressure is determined by calculating a compressible flow of the two-phase, vapor–liquid dispersion in a vertical keyhole of varying cross-section, paying particular attention to transition between the annular and slug flows. It shows that regardless of a supersonic or subsonic flow at the base, the final pore size decreases as the dimensionless surface tension parameter, liquid pressure at the keyhole base, and melting temperature decrease. The pore size also decreases with increasing friction factor subject to a supersonic flow at the base. The effects of the ratio of specific heats at constant pressure and volume, friction factor and loss coefficient on the final pore size are insignificant for a subsonic mixture. This work provides a systematical understanding of the factors affecting the final pore size during keyhole mode welding.

Dynamic tissue engineering scaffolds with stimuli-responsive macroporosity formation

Macropores in tissue engineering scaffolds provide space for vascularization, cell-proliferation and cellular interactions, and is crucial for successful tissue regeneration. Modulating the size and density of macropores may promote desirable cellular processes at different stages of tissue development. Most current techniques for fabricating macroporous scaffolds produce fixed macroporosity and do not allow the control of porosity during cell culture. Most macropore-forming techniques also involve non-physiological conditions, such that cells can only be seeded in a post-fabrication process, which often leads to low cell seeding efficiency and uneven cell distribution. Here we report a process to create dynamic hydrogels as tissue engineering scaffolds with tunable macroporosity using stimuli-responsive porogens of gelatin, alginate and hyaluronic acid, which degrade in response to specific stimuli including temperature, chelating and enzymatic digestion, respectively. SEM imaging confirmed sequential pore formation in response to sequential stimulations: 37 °C on day 0, EDTA on day 7, and hyaluronidase on day 14. Bovine chondrocytes were encapsulated in the Alg porogen, which served as cell-delivery vehicles, and changes in cell viability, proliferation and tissue formation during sequential stimuli treatments were evaluated. Our results showed effective cell release from Alg porogen with high cell viability and markedly increased cell proliferation and spreading throughout the 3D hydrogels. Dynamic pore formation also led to significantly enhanced type II and X collagen production by chondrocytes. This platform provides a valuable tool to create stimuli-responsive scaffolds with dynamic macroporosity for a broad range of tissue engineering applications, and may also be used for fundamental studies to examine cell responses to dynamic niche properties.

Determination of optimal location to monitor temperature in low pressure die casting process

Controlling and eliminating defects, such as macroporosity, in castings is a continuing challenge that manufacturers must continually address. Since the encapsulation of liquid regions by a solid shell and subsequent formation of macroporosity cannot be detected during casting, the die temperature, which is routinely measured, has been used as an indirect indicator of this defect. A finite element model has been developed to predict the evolution of temperature as well as the volume of encapsulated liquid in a casting with a high propensity to form macroporosity. The boundary conditions in the model were iteratively adjusted until the temperature predictions matched the experimental data for a variety of operational conditions. A model based methodology has been developed to analyse the correlation between the die temperature and the encapsulated liquid volume. This methodology is employed to assess the suitability of different in-cycle die temperatures for use as indicators of macroporosity formation, and to help determine the optimal location to monitor temperature for the purpose of minimising macroporosity.

Simulation of feeding flow and prediction of macro porosity of SGI casting based on microstructure formation

Based on the model of microstructure formation of spheroidal graphite iron (SGI) casting which includes continuous nucleation and growth of graphite nodules, carbides and austenite, a method was proposed to calculate the feeding flow during solidification processes by considering mass conservation and using Darcy's laws. The method takes into account liquid shrinkage as well as shrinkage and expansion caused by different phase formation, and can calculate the flow in liquid and mushy regions. According to calculation of the flow, macro porosity formation could be described through treating free surfaces and boundary of the regions. The proposed method has been applied to an automotive wheel hub SGI casting with two different processing cases. The feeding flow during the casting solidification with two cases, especially in isolated regions, was investigated and discussed. With the flow calculation, expansion or shrinkage of liquid or mushy regions might be judged, which could offer important information for evaluating formation of macro porosity. The results of macro porosity prediction were compared with results of practical castings. It is indicated that two results were in quite good agreement, and the method could predict macro porosity formation more accurately.

Effects of pulsed and continuous Nd–YAG laser beam waves on welding of Inconel alloy

In the present work a 2·5 kW high power Nd–YAG laser is used in the bead on plate (BOP) and butt welding of Inconel 690 plates of thickness 3 mm. Welding is performed using a rectangular laser pulse, for which the peak to base power ratio Wr is reduced from an initial value of 10 to a value of 1, maintaining an identical mean power of 1·7 kW. Therefore, the welding mode changes from a pulsed wave to a continuous wave. The BOP results indicate that the depth of the weld penetration increases at a lower travel speed and/or a higher value of Wr. In the butt welding process, as Wr is increased from 1 to 10, the cellular microstructure of the weld remains relatively unchanged, but the macroporosity formation ratio decreases from 7·1% to 0·6%. At low values of Wr, macroporosity is identified primarily in the root region. However, as Wr increases, the associated periodic high power increases the agitation of the molten pool and probably causes bubbles to float upwards. Consequently, at higher values of Wr, the regions of macroporosity are distributed randomly throughout the weld. Although microcracks are not apparent within any of the welds, each weld exhibits slight microporosity. This microporosity decreases as Wr increases. The present results confirm that a pulsed laser beam with an appropriate peak power can be used to achieve a compromise between the mechanical properties and surface roughness of the weld for Inconel 690 in Nd–YAG laser welding.

Investigation on Pore Formation during Hybrid Laser-Tungsten Inert Gas Arc Welding of Magnesium Alloy AZ31B

This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) Welding of Magnesium Alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macro-porosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.

Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B—mechanism and remedy

One of the major concerns during high speed welding of magnesium alloys is the presence of porosity in the weld metal that can deteriorate mechanical properties. This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) welding of magnesium alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macroporosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.

Macroporosity free aluminum alloy weldments through numerical simulation of keyhole mode laser welding

A transport phenomena-based numerical model is developed to predict the keyhole geometry and temperature profiles in the weldment during laser welding. The model can be used to prevent macroporosity formation during laser welding of aluminum alloys. The experimental results show that the weld metal contains large pores when the welding mode changes from conduction to keyhole mode or vice versa due to changes in welding variables. Based on this observation, the mathematical model predicts macroporosity formation when welding is conducted under conditions where small changes in welding parameters lead to a change in the welding mode. The model has been used to predict the geometry of the keyhole and the fusion zone, and the weldment temperature field for laser beam welding of aluminum alloys 5182 and 5754. The calculated weld pool depth, width, and shape for different welding speeds agreed well with the experimental results. The calculations showed that the keyhole profiles for high-speed welding were asymmetric. Negative beam defocusing resulted in a deeper keyhole than that obtained with positive beam defocusing. The transition from keyhole to conduction mode was more abrupt for negative beam defocusing. The model could predict the formation of macroporosity during laser welding of aluminum alloys 5182 and 5754. The results provide hope that transport phenomena-based models can be useful to prevent the formation of macroporosity during keyhole mode laser welding of aluminum alloys.

Continuous wave-Nd: yttrium–aluminum–garnet laser welding of AM60B magnesium alloy

This study shows that the formation of macroporosity and overfill in the weld pool were the most pronounced problems during continuous wave Nd:yttrium–aluminum–garnet laser welding of AM60B die cast magnesium alloy. The influences of various welding parameters on the formation of porosity and overfill were investigated with particular emphasis on the mechanism and prevention of porosity formation. It was found that the macro pores in the weld pool were mainly formed by the expansion and coalescence of the preexisting pores in the base metal. The amount of macro- porosity in the weld pool could be reduced to approximately that in the base metal by reducing heat input, i.e., by increasing welding speed and decreasing laser power. Increasing the beam defocusing did not reduce porosity in the weld metal until the beam was highly defocused and a shallow weld pool, characteristic of conduction mode welding was obtained. Overfill was observed for deep penetration autogenous welds and its formation could be attributed to porosity formation and the resulting displacement of the liquid metal over the top surface of the workpiece.