Study of Porosity in Corroded Refractories

In this work, computer image analysis was used in order to characterize the complex porosity found in ceramic refractory materials. The porosities of SiC and Al₂O₃ refractory bricks before and after slagging tests have been examined by both mercury porosimetry and image analysis techniques; the results obtained by these two methods were compared and their differences discussed. Mercury porosimetry underestimated the volume fraction of pores larger than 100 μm, which contributed significantly to the total pore volume of analyzed materials. In contrast, image analysis did not reveal finer porosity, the contribution of which was, however, less important, Image analysis appeared to have the advantage of quantification of local heterogeneities of porosity, and the capabilities of this technique to quantify the variations in the volume of pores adjacent to reaction zones are documented and supported by microscopic observations.     

Microstructural Characterization of Alumina and Silicon Carbide Slip-Cast Cakes

The effect of solids loading, particle-size distribution, and suspension viscosity on the resultant microstructure of slipcast monolithic ceramics prepared from aqueous suspensions of alumina and silicon carbide was studied. Unimodal alumina suspensions (average particle size = 0.6 μm) were prepared at 35, 37, and 42 vol%. Silicon carbide suspensions (average particle size = 0.7 μm) were produced with different quantities of dispersant at 37 vol%. Similarly, aqueous alumina suspensions of 42 and 50 vol% were produced with a bimodal particle-size distribution. The slip-cast microstructures were characterized by mercury porosimetry and small-angle neutron scattering, which provided pore size (distribution), pore fraction, and pore morphology. Essentially, the combination of these techniques deciphered packing differences obtained in the cake microstructures. For the alumina cakes produced from the 35,37, and 42 vol% suspensions, the individual characterization techniques, mercury intrusion, and the neutron scattering measurements showed that the cake microstructures were similar in pore size and quantity. However, comparison of the techniques and their assumptions showed differences in the pore shape. Mercury porosimetry and neutron scattering showed bimodal porosity for the cake produced from a mixture of 85% 6-μm particles and 15% 0.6-μm particles. Pore volume fraction and pore size increases were correlated with increased viscosity in the silicon carbide suspensions. In addition, the silicon carbide cake microstructures were measured, and homogeneity was evaluated as a function of position in the cast.     

Manufacture and characterization of ultra and microfiltration ceramic membranes by isostatic pressing

This paper describes the manufacture of tubular UF and MF porous and supported ceramic membranes to oil/water emulsions demulsification. For such a purpose, a rigorous control was realized over the distribution and size of pores. Suspensions at 30 vol.% of solids (zirconia or alumina powder and sucrose) and 70 vol.% of liquids (isopropyl alcohol and PVB) were prepared in a jar mill varying the milling time of the sucrose particles, according to the pores size expected. The membranes were prepared by isostatic pressing method and structurally characterized by SEM, porosimetry by mercury intrusion and measurements of weight by immersion. The morphological characterization of the membranes identified the formation of porous zirconia and alumina membranes and supported membranes. The results of porosimetry analysis by mercury intrusion presented an average pore size of 1.8 μm for the microfiltration porous membranes and for the ultrafiltration supported membranes, pores with average size of 0.01-0.03 μm in the top-layer and 1.8 μm in the support. By means of the manufacture method applied, it was possible to produce ultra and microfiltration membranes with high potential to be applied to the separation of oil/water emulsions.     

Characterisation of porous titania yttrium oxide compounds by mercury intrusion porosimetry and X-ray refractometry

Aiming at ceramic materials of low thermal radiation emittance titania yttrium oxide ceramics have been prepared by sintering in air at 1,100 and 1,400 °C, respectively, and characterised. Due to its high infrared refractive index, average pore sizes up to 1 μm for optimal Mie scattering cross sections and over 35% porosity the material is a promising candidate for elevated temperature insulations. The essentially in large pore growth is achieved by graphitic nano and micro particle additives. The microstructure parameters of porosity, specific surface area, median and average pore sizes are determined by mercury intrusion porosimetry. Beyond this classical technique supplementary measurements by X-ray absorption and X-ray refractometry reveal considerably larger average pore sizes due to closed pores not accessible for mercury. An additional advantage of the X-ray techniques is their non-destructive application, which allows other treatments afterwards. The combined application of both methods provides additional information and reliability in case of complex ceramic microstructures and may serve for improved developments of porous ceramics with optimised thermal scattering for energy savings in high temperature applications.     

Specific Permeability of Porous Compacts as Described by a Capillary Model

The specific liquid permeabilities of a wide variety of phosphatebonded, porous alumina compacts prepared from alumina powders of differing size distribution, and ranging in porosity from 32 to 50 vol%, were not described well by the Carmen-Kozeny model but were described by a capillary model based on mean entry pore radius. The specific permeability varied directly with the square of the mean entry pore radius determined by mercury intrusion. The compacts containing the lowest phosphate bond phase obeyed Poiseuille's law.     

Effect of various pore formers on the microstructural development of tape-cast porous ceramics

Various types of pore formers have been used for the fabrication of ceramics with controlled porosity. This study addresses a detailed and systematic comparison of different pore formers (e.g. graphite, polymethyl methacrylate, sucrose and polystyrene) with distinct features such as size, distribution and morphology of particles and decomposition/oxidation behavior. Investigations also involve their effect on the rheological properties of the slurries and the microstructural development of laminated porous ceramic tapes.Morphological features of the pore former particles were characterized using laser diffraction, B.E.T. surface area measurement and scanning electron microscopy (SEM) techniques as their thermal decomposition/oxidation behavior were determined by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) methods. Tape compositions were developed and optimized in order to incorporate identical volumetric loadings of the materials in the tape formulations with different pore formers for a reliable comparison of their pore forming characteristics. Porous yttria stabilized zirconia (YSZ) ceramics were fabricated without macroscopic defects (e.g. cracks, warpage and delamination) by developing heating profiles based on the identified thermal properties of the pore formers. Characterization of the sintered porous ceramics by SEM and mercury intrusion porosimetry techniques revealed novel relationships between the physical properties of the utilized pore formers, processing parameters and final pore structures.     

Use of X-ray tomography to study the porosity and morphology of granules

X-ray computed tomography (XRCT) is a technique that uses X-ray images to reconstruct the internal microstructure of objects. Known as a CAT scan in medicine, it has found wide application for whole-body and partial-body imaging of hard tissues (e.g., bone). A modern tabletop XRCT system with a resolution of about 4 μm was used to characterize some pharmaceutical granules. Total porosity, pore size distribution, and geometric structure of pores in granules produced using different conditions and materials were studied. The results were compared to data obtained from mercury porosimetry. It was found that while XRCT is less precise in the determination of total porosity in comparison to mercury porosimetry, it provides detailed morphological information such as pore shape, spatial distribution, and connectivity. The method is nondestructive and accurate down to the resolution of the instrument.Tomographic images show that the pore network of individual granules comprises relatively large cavities connected by narrow pore necks. The major structural difference between granules produced at different conditions of compaction and shear is a reduction in the pore neck diameter; the cavity size is relatively insensitive to these conditions. Comparison of pore size distributions determined from tomographic images and mercury porosimetry indicates that mercury intrusion measures the pore neck size distribution, while tomography measures the true size distribution of pores ca. 4 μm or larger (the instrument resolution).     

Direct Study of the Behavior of Flaw-Forming Defect in Sintering

The morphological change of natural pores with densification was examined in an alumina green body. The liquid-immersion technique was used for observing pores inside the body before and after sintering. In this technique the specimen is made transparent by an adequate immersion liquid and observed with a transmission optical microscope. Growth of large pores with densification was observed and is discussed along with past thermodynamic consideration of pore growth. Supplemental investigation of the results was conducted by SEM and mercury porosimetry.     

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10 µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5 µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9 MPa and 12.4 MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600 °C.     

Mercury porosimetry of ceramics

Mercury porosimetry has been used in ceramics for the characterization of products and for studies of processing. Specifications including PSD (pore size distribution by mercury porosimetry) are now applied to magnesia refractories used in basic oxygen furnaces and to building brick which may be exposed to frost. Other products cited as examples are the silica fiber tile used on the space shuttle, plasma sprayed coatings, carbon composites and filters.In ceramic processing research, PSD has proved valuable for evaluating the firing of basic refractories, brick and sanitary ware. Pore growth during the early stages of sintering several materials was first identified by PSD. The character of clay agglomerates and the presence of alumina aggregates in compacts have been measured, the latter showing bimodal PSD. The progressive change in PSD while compacting glass spheres outlines the stages of compaction. The most frequent pore diameter in plaster slip casting molds correlates directly with plaster consistency.In dense or vitrified ceramics, errors may occur due to closed pores or pores with narrow openings. However, in ceramic compacts and highly porous ceramics, pores have several openings so PSD is a realistic measure of structure.     

Highly Porous Geopolymers Through Templating and Surface Interactions

The natural porosity of geopolymers and the simplicity of geopolymer synthesis make them a potential candidate for the formation of highly porous ceramics. Here, the synthesis of highly porous (≈70 vol% or more) metakaolin geopolymer is demonstrated using a novel emulsion preparation with one‐pot curing and hydrophobization of the interior pores with alkylalkoxysilanes. Using mercury intrusion porosimetry, tailoring of the characteristic percolation pore size is demonstrated over a range of ≈200 nm to 10 μm. Using powder X‐ray diffraction, reactivity was shown to be decreased versus typical geopolymers, but substantial geopolymerization still occurred and the samples formed cohesive monoliths. Optional calcination under inert conditions allowed for formation of a glassy ceramic with a notable SiC phase, as well as further increasing the porosity by removing the hydrophobic pore coatings.     

Process control and optimized preparation of porous alumina ceramics by starch consolidation casting

This work concerns details of porosity and pore size control in starch consolidation casting of alumina ceramics using corn starch. In particular, the influence of the solids loading (68-78 wt.% alumina in suspensions with nominal starch contents of 20-50 vol.%) on the porosity, bulk density and shrinkage of alumina ceramics is studied. The results indicate a linear decrease of the linear shrinkage and the bulk density (and a corresponding increase in porosity) as the alumina concentration increases, with slopes that are independent of the starch content. The pore size is characterized via microscopic image analysis, the pore throat size via mercury porosimetry. Relations between the volumetric shrinkage, porosity and the volume fractions of starch and water in the suspensions are discussed, and a new concept, called “affine limit porosity” is proposed to explain the apparently paradoxical finding that the porosity increases with increasing alumina content in the suspension.     

Study of the Bimodal Pore Structure of Ceramic Powder Compacts by Mercury Porosimetry

Mercury porosimetry was used to study the pore-size distribution of both green and partially sintered alumina compacts. Experiments showed that using the mercury-penetration volume data to study the pore structure changes during sintering for a compact with a bimodal pore-size distribution could lead to wrong conclusions because the mercury-penetration volume was not linearly proportional to the porosity. Instead, the porosity distribution should be used. When the porosity of large pores was <10%, some large pores within the compact were observed to become isolated or channeled to the compact surface; this caused errors in the porosity distribution measured by mercury porosity.     

球形氧化铝制备过程中影响因素的研究

介绍了球形氧化铝的各种制备工艺及成形方法，并以油柱成形法制备球形氧化铝。利用低温液氮吸附仪、压汞仪以及智能强度测定仪对其物化性质进行了测定，重点考察了不同铝源合成的水合氧化铝、加料方式、扩孔剂和胶溶剂用量对球形氧化铝性质的影响。实验结果表明，制备水合氧化铝的铝源是影响球形氧化铝的重要因素；并流加料方式能制得大孔容、低表观密度、高强度的球形氧化铝；球形氧化铝的制备过程中，要制得适合于油柱成形的铝溶胶，胶溶剂加入量有最佳的用量范围，胶溶剂用量愈大，球性氧化铝的强度、比表面、孔容愈大，而孔径却随之变小；活性炭粉能有效地提高球形氧化铝的压汞孔容。     

Mercury porosimetry and its application to the analysis of coal pore structure

Mercury porosimetry, gas adsorption, helium and mercury densities and small angle X-ray scattering were the methods used for studying porosity characteristics of a number of Czechoslovak coals ranging from lignite to semi-anthracite. Total pore volumes were measured in the pore radii range 0.1 – 7500 nm and then divided into the following groups: micropores (smaller than 1.8 nm), mesopores (1.8 – 30 nm) and macropores (over 30 nm). Coal porosity values ranged from 1.3 up to 43%. With low and medium-rank coals (carbon content under 76%), porosity is mostly due to mesopores. Micropores and mesopores account for a substantial share of the porosity of bituminous coals (carbon content over 80.6%). The distribution of mesopores was determined for an identical pore radii interval on the basis of data obtained independently by means of mercury penetration, gas adsorption and small angle X-ray scattering. High-pressure mercury penetration is an effective method of obtaining the coal porosity spectrum within a broad interval of pore radii, provided that a correction is made for the coal compressibility when evaluating the measured data. The recommended value of the mercury—coal contact angle is 1352 for brown coal and 130° for bituminous coal.     

Pore size distribution in Separon using mercury contact angles in pores and hysteresis in porosimetry

The porous co-polymer Separon HEMA 1000 for HPLC column packing was examined by using a special method of mercury porosimetry evaluation. The advancing and receding contact angles of mercury in pores were determined and the method used is described in detail. The pore distribution curves obtained from mercury porosimetry and from the nitrogen desorption isotherms are in good agreement. The dependence of the hysteresis of the porosimetry curves and mercury retention in the pores on the intrusion volume was investiaged to characterize the porous structure of the sample. The influence of the pore potential on the hysteresis was evaluated.     

MnO_2和Fe_2O_3对氧化铝质压裂支撑剂微观结构和抗破碎能力的影响(英文)

采用无压烧结技术制备了软锰矿掺杂的高强度氧化铝质压裂支撑剂.通过X射线衍射、压汞式孔隙分析、扫描电子显微镜和筒压法分别研究了由软锰矿引入的MnO2和Fe2O3对支撑剂物相组成、孔结构、晶粒尺寸和抗破碎能力的影响.结果表明:当软锰矿掺杂量为0～5%(质量分数,下同)时,烧结样品中包括氧化铝、莫来石和钛酸铝相,软锰矿的掺入未明显改变晶体结构;当软锰矿掺量为5%时,Fe3+取代Al3+与组分中的TiO2反应并形成固溶体,MnO2固溶于Al2O3晶粒中,促进了Al2O3晶粒生长,过剩的Fe2O3和MnO2存在于陶瓷晶界处并在高温煅烧F形成液相促进致密化烧结;未掺杂样品中存在大量连通气孔,显气孔率为14.79%:掺入5%软锰矿后,显气孔率降低至5.29%,样品内部多为均匀分布的近球形闭气孔;在52MPa压力条件下,5%软锰矿掺杂样品的破碎率与未掺杂样品相比减少80.95%,抗破碎能力显著提高.     

Pore Sizes and Filtration Rates from Two Alumina Slips

The relationship between filtration rate and the resultant green body microstructure was examined for aqueous alumina slips cast at two different deflocculation states. The volume loading of both slips was 40%. Slip viscosities of 500 and 60 mPa·s were produced by different tetrasodium pyrophosphate additions. The filtration rate of these slips varied by a factor of 2; however, mercury porosimetry results showed the same average pore size for both samples. Single and multiple small-angle neutron scattering results showed the specimen cast with the higher-viscosity slip to possess a bimodal pore size distribution. The body cast with the low-viscosity slip showed unimodal porosity and, consequently, the filtration is attributed to the toroidal region between the packed particles. These results showed that mercury porosimetry does not provide a pore size that predicts filtration behavior of slips with different degrees of dispersion.     

Novel fabrication route for porous silicon carbide ceramics through the combination of in situ polymerization and reaction bonding techniques

For the first time, an in situ polymerization technique was applied to produce mullite‐bonded porous SiC ceramics via a reaction bonding technique. In this study, SiC microsized particles and alumina nanopowders were successfully coated by polyethylene (PE), which was synthesized from the particle surface in a slurry phase reactor with a Ziegler–Natta catalyst system. The thermal studies of the resulting samples were performed with differential scanning calorimetry and thermogravimetric analysis. The morphology analysis obtained by transmission electron microscopy and scanning electron microscopy (SEM) confirmed that PE was successfully grafted onto the particle surface. Furthermore, the obtained porous ceramics were characterized in terms of their morphologies, phase composition, open porosity, pore size distribution, and mechanical strength. SEM observations and mercury porosimtery analysis revealed that the quality of the dispersion of nanosized alumina powder into the microsized SiC particles was strongly enhanced when the particles were coated by polymers with in situ polymerization. This resulted in a higher strength and porosity of the formed ceramic porous materials with respect to the traditional process. In addition, the X‐ray diffraction results reveal that the amount of mullite as the binder increased significantly for the samples fabricated by this novel method. The effects of the sintering temperature, forming pressure, and polymer content on the physical and mechanical properties of the final porous ceramic were also evaluated in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40425.     

Effect of Curing Conditions on the Capillary Porosity of Hardened Portland Cement Pastes

The capillary pore structure of hardened portland cement pastes cured by high-pressure steam, chemical acceleration, high-pressure steam with reactive SiO₂, water immersion, water immersion and high-pressure steam, and hot-pressing was measured using mercury porosimetry to 50,000 psi. Differences of > 2 orders of magnitude exist in the average capillary pore diameters of the cement pastes studied. The largest pores (∼1 to 3 μm in diameter) are associated with high-pressure steam-cured pastes. The smallest average capillaries observed were 0.02 μm for pastes steam-cured with reactive SiO₂. Hot-pressed pastes had essentially no porosity accessible to mercury. The application of pore size control to problems of polymer-impregnated concrete is discussed.