Fenga et al. The use of PVP during wet ball milling leads to a more uniform dispersion of the PVP alumina, which helps in increasing the bonding of the grains and limiting the growth of grains during the sintering process. Hence, this will lead to an increase in the toughness of porous alumina ceramics. Fukushima et al. The results revealed that the compressive strength of porous SiC is in the range of 5. Meanwhile, the compressive strength of porous SiC ceramic using gelation freezing was The microstructure of porous SiC fabricated using the gelation freezing method consisted of a unique honeycomb with a 3D interconnected pore network, which gives excellent compressive strength to porous SiC ceramics compared to other methods, for example, the partial sintering method see Fig.
Sengphet et al. The study showed that with increasing organic content KP in porous clay ceramics the density decreased from 1. Meanwhile, the tensile strength decreased from The decrease in the tensile strength of porous clay ceramics is attributed to the increase in porosity, which is generated by the decomposition of the organic materials from the KP waste during the sintering process of porous clay ceramics.
Mohanta et al. The study showed that through increasing the ratio and the size of the rice husk powder, the porosity increased while the mechanical properties decreased.
This can be understood because the mechanical properties and the porosity are normally inversely related. Novaisn et al. By using PMMA as a pore forming agent at a particle size of mm and ratios of 2. Also using PP as a pore forming agent resulted in marked decrease in the mechanical properties of porous tiles ceramics.
Mao et al. In addition, the content of pore forming agent not only affects the porosity, but also the size, shape, channels, and pore distribution. Boron nitride Powder metallurgy and C — Ball milling Sugar 1 and 4 h — Bioactive glass Powder metallurgy C — Grinding 8 Stearic acid — — mm Mullite-bonded porous Powder metallurgy —C 30 Ball milling Graphite 4h 1. Guanghai Li et al. The study showed that the nano-Al2O3 can increase the thermal shock resistance, fracture toughness, and strength of the porous Al2O3 ceramics.
The authors reported that the increases in the mechanical properties were attributed to the growth of the interparticle contacts by surface diffusion.
Lee et al. It is found that the strength of the porous SiC and NiO-ceramics improves with the control of the additives. They reported that this enhancement in the strength of porous ceramics could be attributed to the bonded force between the grain and the neighboring grain without affecting the porosity much see Fig. Circular points correspond to strength and rectangular points are for porosity [57].
Kord et al. They reported that this higher strength may be attributed to good sinterability, which could result in decreased porosity, and hence high strength porous SiC ceramics. Gaiye et al. They mentioned that the unique microstructure is useful for producing enhancements in the bending strength and fracture toughness of macro-porous alumina ceramics as shown in Fig.
This enhances the compact area among the alumina particles and blocks the crack path. Thirdly, the addition of YSZ particles can improve the surface diffusion, which leads to the formation of the bonding neck to obtain the best mechanical properties [62]. It is found that the sodium aluminosilicate additives have a strong effect on the mechanical properties. This excellent compressive strength of porous silicate with sodium aluminosilicate compared to other additives, may be attributed to the structuration mechanism, which leads to the formation of crystalline silica and the aluminosilicate binder.
Lil et al. Therefore the machinability improved. Wang et al. Li and Yin [67] studied the effect of phase composition on the microstructure and mechanical properties of Lu2O3-doped porous Si3N4 ceramics by adding SiO2 as a ceramic additive to improve the mechanical properties of porous Si3N4 ceramics.
The study indicates that with increasing b-Si3N4 content in porous Si3N4 ceramics, the mechanical properties increase. In addition, during the sintering process, there is a full transformation from a-Si3N4 to b-Si3N4 by the solution process with an hexagonal rod morphology see Fig.
Wei et al. This means that the mechanical properties of porous cordierite were enhanced without decreasing the porosity. However, usually the mechanical properties of porous ceramics decrease with increased porosity. During the sintering process of the porous ceramics, La2O3 additives produced liquids located between the grains and these solidify during cooling resulting in the strong bonding of these grains.
At the junction of the cordierite particles, it can be observed that some neck-like connectors have formed. These connectors lead to bonding the cordierite particles together and improving the mechanical properties after sintering.
This means that there is an improvement in the mechanical properties of porous ceramic by ceramic coating. Feng et al. The study showed that through increasing the ceramic binder ratio, the porosity of porous SiC decreased, and hence the mechanical properties improved as shown in Fig. Yao et al. Larger grains were also strongly attached to smaller particles and there was strong bonding between the large SiC grains see Fig.
The strong porous structure between SiC and mullite would result in excellent strength for ceramics. In addition, with an increase in the Al OH 3 ratio, the amount of alumina Al2O3 increases and with increase in Al OH 3 content, the amount of Al2O3 increases and accelerates the reaction of mullization that leads to higher strength. The study showed that with decreasing porosity, the compressive strength of porous alumina ceramics increased.
After sintering at C, the compressive strength and the porosity of porous mullite—alumina ceramics were B It is a fact that porosity decreases with increasing sintering temperature, which results in increasing compressive strength. Also, in this case, the compressive strength of porous alumina ceramics with anisotropic pore channels will be generally lower in the perpendicular direction, compared with being parallel to the direction of freezing see Fig. The study showed that the mechanical properties of porous Si3N4 greatly improved after the CVI process because of the increase in the ability of loading for b-Si3N4 particles and in the connection strength between b-Si3N4 particles with a decrease in porosity.
The dielectric properties decrease due to the decrease in porosity. Eoma et al. Yu et al. Xu et al. Hong et al. The study showed that the mechanical properties of freeze cast porous zirconia ceramics enhanced upon impregnation with the silica aerogel. The compressive strength has an exponential decrease with increasing porosity. The compressive strengths of high porous zirconia ceramics were from 9. Therefore with increasing TiO2 contents the average pore size, strength and liquid content at sintering temperature increased and the neck bonds developed better, while the porosity decreased with optimized porous spinal ceramics at 1.
In addition, the increase in strength may be attributed to three factors. The second is the porosity. With increasing TiO2, the particles become denser, the porosity decreases and the neck bonds among the particles become coarser, which leads to an increase in strength [79].
Table 4 shows examples of the effects of ceramic additives on the mechanical properties of some porous ceramic materials. Porous ceramic materials have low mechanical properties because of the porosity. Therefore, several attempts have been made to improve the mechanical properties of porous ceramic.
Falamak et al. Flexural, compressive in situ mm and strength increased from 3. Zirconia Nano porous Sol gel and C — Mixing 10 — 50 min 10—30 nm Alumina Silicon car- Polymer- —C — Ball milling — 5 h with 1.
Some improvement in machinability [65] Li et al. This can relate to 1 improved sintering due to the generation of very active secondary particles during the sintering step and oxidation; 2 the existence of more bonds in the initial green compact due to the higher content of ductile Al particles. Oh et al. The study showed improvement in the fracture toughness and strength. A fracture toughness of 4. But this ceramic is not a porous ceramic.
Clegg and Paterson [82] reported ductile particle toughening of HAP using ammonium hexachloroplatinate ACP as a source of platinum particles. It is found that through increasing the volume fraction of platinum particles, the fracture toughness of porous HAP ceramics doubles the value of the untouched HAP. This improvement in fracture toughness may be attributed to the crack bridging mechanism. The study showed that the fracture toughness and Fig.
Sintering conditions of composites is C and 30 MPa for 1 h [81]. This enhancement in the mechanical properties is attributed to the crack bridging mechanism see Fig. The bending strength and the fracture toughness are and 2. These values are 2. Manoj et al. This variation in strength is attributed to the sintering reactions and the mullitization process. Hence, for the Al2O3 source, mullite—silica grains were strongly adhered and connected with the large SiC grains compared with the AlN source.
The mullite—silica grains had poor adhesion and weak bonding, therefore the strength of the porous ceramics with the Al2O3 source is higher than for the porous ceramics with an AlN source. Fung et al. The study showed that with a 4. This is due to thermal expansions, and thus different rates of shrinkage between the NiAl2O4 phase and the alumina during the sintering and cooling processes building up stress in the porous alumina.
This closure slowed down the growth of cracks leading to an improvement in strength for porous alumina ceramics see Fig. Table 5 shows examples of metal additive effects on the mechanical properties of some porous ceramic materials. The study showed that incorporating 0. It was found that the particle size strongly affects the microstructure and strength. With decreasing particle size, the apparent porosity decreases gradually; there was good neck development among the particles and an acceptable pore distribution leading to an increase in the compressive strength see Fig.
Alumina Al 4, 8, 12, Powder —C 8 Ball milling — 2h 1. The results revealed that the particle size strongly affects the formation of cordierite and mullite, and changes the pore characterization and strength. The strength increases with decreasing particle size and the increase in sintering temperature because of the porosity decrease, well distributed pores and good neck development. Table 6 shows examples of particle size effects on the mechanical properties of some porous ceramic materials.
Trecant et al. It is found that the mechanical properties of MBCP ceramics increase with increasing implantation times due to the decrease in microporosity and the precipitation of needle-like crystals. This means that a physico- chemical process could improve the mechanical properties of MBCP ceramics see Fig. Zhu et al. They reported that the enhancement in the strength of porous alumina ceramics following the degassing process was attributed to the increase in the relative density from 0.
Kawai et al. It is found that the strength of the porous Si3N4 ceramics does not always increase with the decreasing porosity but depends on the grain length of b-Si3N4; a longer grain leads to a higher strength.
Komlev et al. The effect depends on the properties of the microstructure of the matrix, the polymer and the conditions of the impregnation experiment. Antsiferov and Porozova [44] studied the effects of the mechano-chemical activation of the cordierite charge on the strength of porous cordierite using polymeric-matrix duplication. Before mixing the raw material in to a charge, these raw materials were subjected to wet grinding and drying.
The charge activation and mixing were conducted in the drying mode with the introduction of titanium oxide or carbide in an aqueous medium with the addition of Trilon B a disodium salt of ethylenediaminetetra acetic acid EDTA known as an active complexon. This process is known as mechano-chemical activation. The study showed that charge activation by the addition of Trilon B in an aqueous medium is possible, leading to attainment a steady increase in strength with a satisfactory quality of the cordierite obtained.
They reported that this improvement in strength was attributed to a glassy phase in the materials as shown in Fig. Cordierite— The pore size distribution changes from bi-peak mode to mono-peak mode, the porosity and the median pore size decrease but the strength increases [90] Li et al. Spinel 8. Another factor which affects the mechanical properties of porous ceramics is a gelatin coating. Dresslern et al. The study showed that a gelatin coating increases the toughness of porous coated ceramics.
They reported that this increase in toughness is attributed to the bridging of the crack, which is facilitated by the applied gelatin coating. The ceramics coated with higher concentration gelatin sols 0. Scanning electron microscope SEM micrograph of gelatin fragments on the fracture surface of a transversal crack for hydroxyapatite HAP -ceramics which have been coated with highly concentrated gelatin sol of 0.
In addition, with increasing gelatin concentration, the porosity decreases slightly. This effect is due to the increased amount of remaining gelatin material within the transversal cracks after drying see Fig. Applying different gel strengths at different temperatures has a slight effect on the compressive strength of the coated ceramics [96].
Takaaki and Todo [53] studied the effect of collagen coating on the compressive strength of porous bioceramic bone substitute beta-tricalcium phosphate b-TCP.
Junkes et al. Coating with polysiloxane gives a mechanical strength reaching B MPa, while with the silica suspension it has a mechanical strength close to B MPa. Kim et al. The solutions aluminum nitrate hydrate Al NO3 3. Microstructure of alumina sintered at C. Note that abnormal grain growth occurred in the CuO-doped alumina [99]. Meanwhile, porous ceramics soaked in a potassium solution present a lower strength than ordinary porous ceramics. The increase in the bending strength can be attributed to the formation of the mullite 3Al2O3.
Some researchers also studied the oxidation degree for ceramic powders during sintering and its impact on the mechanical properties of porous ceramics. Ramesh et al. All CuO-doped Al2O3 could be sintered at a low temperature e.
The hardness of Al2O3 containing up to 0. In contrast, the hardness of the undoped Al2O3 increases with increasing sintering temperature, i. The lower hardness of the doped ceramics reported could be due to the increased grain size with increasing sintering tem- perature see Fig.
Zhang et al. Silicon nitride Solid content Freeze casting C — Ball milling — 1. This chloride, was due to the formation of the MgCl, alumi- mullite phase num nitrate hydrate, Al NO3 3.
Si3N4 Grain size Powder — — — — — 0. These changes were due to the bonding necks among the particles, the oxidation degree of the Si3N4, microcracks mostly caused by crestabolite and the porosity see Fig. In addition, the solid content ratio is another factor reported by researchers as affecting the mechanical properties of porous ceramics.
Ye et al. The porosity decreased from The formation of these round pores obstructed the formation of the a-Si3N4 phase, but it was found to be useful for improving the mechanical properties of porous Si3N4 ceramics because of its unique pore structure. Hou et al. In general, the mechanical properties of porous ceramics were affected by the formation of a dense ceramic wall and the porosity.
In contrast, the compression molding process leads to a more homogenous microstructure than the other processes, resulting in an excellent compressive strength of Injection molding also leads to the partial segregation of the expanded microspheres, hence resulting in a moderate compressive strength of 9. Heness et al. It was found that the strength of highly porous ceramics increases with the increasing volume of highly porous ceramics.
Table 7 shows examples of other factors affecting the mechanical properties of some porous ceramic materials with different work conditions. This review article has highlighted the factors affecting the porosity, the mechanical properties, and the strength of porous ceramics along with the microstructural factors. The sintering temperature has a strong effect on the mechanical properties and the strength of porous ceramics.
Increasing the sintering temperature also results in increased mechanical properties. A pore forming agent affects the mechanical properties with increasing ratio, the particle size of the pore forming agent, and the size, shape, channels, and distribution of the pores.
As the porosity percentage increase, the mechanical properties decrease. Metal particle additives also have important effects on the mechanical properties of porous ceramics. Increasing the metal particle content can improve the mechanical properties due to its effect on the sintering process, the crack bridging technique, the increased density, the decreased porosity, and the phase formation.
The mechanical properties of porous ceramics increase with decreasing particle size. This is related to the well-developed neck, the homogenous pore distribution and the decreased porosity. In addition, there are many different factors affecting the mechanical properties, such as the oxidation degree, coating, template size, specimen size, forming methods, solid contents, grain structure dense ceramic wall structure , grain size distribution, and implantation time.
The ceramic materials used as porous ceramics in the selected papers studied have a different percentage ratio of SiC Preparation of porous materials with controlled pore size and porosity. Journal of the European Ceramic Society 24 2 , — Effect of pure b-Si3N4 powder on microstructure and mechanical properties of porous Si3N4 ceramics.
R, Desa, J. Effects of pressure and temperature on pore structure of ceramic synthesized from rice husk: A small angle neutron scattering investigation. Journal of Alloys and Compounds , — Microstructure and electrical properties of porous PZT ceramics derived from different pore-forming agents. Acta Materialia 55 1 , — Processing and piezoelectric properties of porous PZT ceramics. Ceramics International 33 3 , — Microstructure, mechanical and dielectric properties of highly porous silicon nitride ceramics produced by a new water-based freeze casting.
Ceramics International 38 5 , — Macroporous ceramics: Novel route using partial sintering of alumina-powder agglomerates obtained by spray-drying.
Ceramics International 40 7 , — Mechanical characterization of micro- and nano-porous alumina. Ceramics International 41 9 , — Processing and properties of advanced porous ceramics: An application based review. Ceramics International 40, — Mechanical properties of ceramics—cement based porous material under impact loading.
Processing routes to macroporous ceramics: A review. Journal of the American Ceramic Society 89 6 , — Journal of Pharmaceutical Sciences 97 3 , — Macro-porous ceramics: Processing and properties. International Materials Reviews 57 2 , — Hierarchical porous materials made by drying complex suspensions. Langmuir 27 3 , — Micro structural investigations and mechanical properties of macro porous ceramic materials from capillary suspensions.
Journal of the American Ceramic Society 97 12 , — Journal of the American Ceramic Society 85 3 , — Sintering behavior of porous SiC ceramics. Ceramics International 30 6 , — Microstructure and properties of porous silicon carbide ceramics fabricated by carbothermal reduction and subsequent sintering process.
Materials Science and Engineering: A 1 , — International Journal of Applied Ceramic Technology 6 5 , — Effect of sintering temperature on compressive strength of porous yttria-stabilized zirconia ceramics. Ceramics International 36 5 , — Journal of Materials Science 45 1 , — Effects of sintering temperature on pore characterisation and strength of porous corundum—mullite ceramics.
Journal of Ceramic Processing Research 11 3 , — Fabrication and properties of porous mullite ceramics from calcined carbonaceous kaolin and a-Al2O3. Liquids go right through things that have porosity. Unconsolidated sediments tend to have higher porosity than consolidated ones because they have no cement, and most have not been strongly compressed.
Finer-grained materials e. A number of factors affect the permeability of soils , from particle size, impurities in the water, void ratio, the degree of saturation, and adsorbed water, to entrapped air and organic material.
The first equation uses the total volume and the volume of the void. The second equation uses the total volume and the volume of the solid. Absolute, effective, and relative permeability Reservoirs contain water and oil or gas in varying amounts. Each interferes with and impedes the flow of the others. Permeability is measured on cores in the laboratory by flowing a fluid of known viscosity through a core sample of known dimensions at a set rate, and measuring the pressure drop across the core, or by setting the fluid to flow at a set pressure difference, and measuring the flow rate produced.
The porosity and permeability of rocks is important in determining which rocks will make a good reservoir. A rock that is both porous and permeable would make a good reservoir rock as it allows oil and gas to move up through the pores in the rock closer to the surface where it can be extracted.
Low porosity usually results in low permeability , but high porosity does not necessarily imply high permeability. Thus, coarse-grained rocks are usually more permeable than fine-grained rocks, and sands are more permeable than clays.
Theoretically, grain size does not affect porosity for well sorted grains but in nature, sand grain size affects porosity probably because sand grain deformation from a spherical shape increases with decrease in particle size. Soil retains water in the interstitial spaces between the soil particles ; the amount of space in a soil sample is described as its porosity.
Generally speaking, larger sized particles don't pack together well, so they tend to have bigger spaces between them which can pass a lot of water through quickly. Fill one measuring cup to ml with sand, the second cup with ml of clay and the third with ml with small pebbles. Fill a graduated cylinder to ml with water. Slowly and carefully pour the water into the first cup until the water just reaches the top of the sand.
Porous rocks This is because the water can get into the gaps between the grains. Rocks that absorb water are described as being porous.
Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking grains. So porous rocks tend to be softer than non- porous rocks. Generally speaking, sand is more porous than clay because sand particles are larger and the pore spaces between the particles are also larger.
However, typically speaking, a sand is both more permeable and more porous than a clay - but only typically. But permeability is a different thing. It increases as particle size increases. That means capillarity increase as particle sizes decreases. At depths greater than about 8, feet, the porosity of the sand- size material is commonly greater than the porosity of the clay- size materials.
Bulk density typically increases as the ratio of solids in a soil increases, and conversely decreases as the ratio of solids decreases. Looked at from the porosity perspective, bulk density increases as pore space decreases. The relationship of bulk density and porosity is reciprocal. Porosity of rocks Porosity is the ratio of pore volume to its total volume.
Porosity is controlled by: rock type, pore distribution, cementation, diagenetic history and composition. Porosity is not controlled by grain size, as the volume of between-grain space is related only to the method of grain packing.
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