Micropore Characteristics Research Articles

Experimental Study on Thermal Expansion Properties and Micro-Pore Texture of High Strength Concrete in Early Age

The early age cracking of concrete is concerned with its thermal expansion properties, which is mainly reflected by the thermal expansion coefficient. Reasonably controlling the coefficient is an effective way of reducing cracks in the early age of concrete. While thermal expansion properties are related to the micro-pore texture characteristics of the concrete. Micro-pore textures of concretes of different mixing ratios and curing time were measured via mercury intrusion porosimetry (MIP), and the thermal expansion coefficient was determined by the comparator. The analysis of test results indicates the correlation between the parameters of micro-pore texture and thermal expansion properties, and also shows a highly positive correlation between the pore area and the thermal expansion coefficient. The results provide a solution for reducing the thermal expansion coefficient, thus controlling the early age cracking.

Coal lithotypes before and after saturation with CO2; insights from micro- and mesoporosity, fluidity, and functional group distribution

Abstract Four lithotypes, vitrain, bright clarain, clarain, and fusain, were hand-picked from the core of the Pennsylvanian Springfield Coal Member (Petersburg Formation) in Illinois. These lithotypes were analyzed petrographically and for meso- and micropore characteristics, functional group distribution using FTIR techniques, and fluidity. High-pressure CO 2 adsorption isotherm analyses of these lithotypes were performed and, subsequently, all samples were reanalyzed in order to investigate the effects of CO 2 . After the high-pressure adsorption isotherm analysis was conducted and the samples were reanalyzed, there was a decrease in BET surface area for vitrain from 31.5 m 2 /g in the original sample to 28.5 m 2 /g, as determined by low-pressure nitrogen adsorption. Bright clarain and clarain recorded a minimal decrease in BET surface area, whereas for fusain there was an increase from 6.6 m 2 /g to 7.9 m 2 /g. Using low-pressure CO 2 adsorption techniques, a small decrease in the quantity of the adsorbed CO 2 is recorded for vitrain and bright clarain, no difference is observed for clarain, and there is an increase in the quantity of the adsorbed CO 2 for fusain. Comparison of the FTIR spectra before and after CO 2 injection for all lithotypes showed no differences with respect to functional group distribution, testifying against chemical nature of CO 2 adsorption. Gieseler plastometry shows that: 1) softening temperature is higher for the post-CO 2 sample (389.5 °C vs. 386 °C); 2) solidification temperature is lower for the post-CO 2 sample (443.5 °C vs. 451 °C); and 3) the maximum fluidity is significantly lower for the post-CO 2 sample (4 ddpm vs. 14 ddpm).

Meso- and Micropore Characteristics of Coal Lithotypes: Implications for CO2 Adsorption

Lithotypes (vitrain, clarain, and fusain) of high volatile bituminous Pennsylvanian coals (Ro of 0.56−0.62%) from Indiana (the Illinois Basin) have been studied with regard to meso- and micropore characteristics using low-pressure nitrogen and carbon dioxide adsorption techniques, respectively. High-pressure CO2 adsorption isotherms were obtained from lithotypes of the Lower Block Coal Member (the Brazil Formation) and the Springfield Coal Member (the Petersburg Formation), and after evacuation of CO2, the lithotypes were re-analyzed for meso- and micropore characteristics to investigate changes related to high-pressure CO2 adsorption. Coal lithotypes have differing Brunauer−Emmett−Teller (BET) surface areas and mesopore volumes, with significantly lower values in fusains than in vitrains or clarains. Fusains have very limited pore volume in the pore size width of 4−10 nm, and the volume increases with an increase in pore size, in contrast to vitrain, for which a 4−10 nm range is the dominant pore width. ...

Variations in pore characteristics in high volatile bituminous coals: Implications for coal bed gas content

Abstract The Seelyville Coal Member of the Linton Formation (Pennsylvanian) in Indiana was studied to: 1) understand variations in pore characteristics within a coal seam at a single location and compare these variations with changes occurring between the same coal at different locations, 2) elaborate on the influence of mineral-matter and maceral composition on mesopore and micropore characteristics, and 3) discuss implications of these variations for coal bed gas content. The coal is high volatile bituminous rank with R 0 ranging from 0.57% to 0.60%. BET specific surface areas (determined by nitrogen adsorption) of the coals samples studied range from 1.8 to 22.9 m 2 /g, BJH adsorption mesopore volumes from 0.0041 to 0.0339 cm 3 /g, and micropore volumes (determined by carbon dioxide adsorption) from 0.0315 to 0.0540 cm 3 /g. The coals that had the largest specific surface areas and largest mesopore volumes occur at the shallowest depths, whereas the smallest values for these two parameters occur in the deepest coals. Micropore volumes, in contrast, are not depth-dependent. In the coal samples examined for this study, mineral-matter content influenced both specific surface area as well as mesopore and micropore volumes. It is especially clear in the case of micropores, where an increase in mineral-matter content parallels the decrease of micropore volume of the coal. No obvious relationships were observed between the total vitrinite content and pore characteristics but, after splitting vitrinite into individual macerals, we see that collotelinite influences both meso- and micropore volume positively, whereas collodetrinite contributes to the reduction of mesopore and micropore volumes. There are large variations in gas content within a single coal at a single location. Because of this variability, the entire thickness of the coal must be desorbed in order to determine gas content reliably and to accurately calculate the level of gas saturation.

Characterization of char from rapid pyrolysis of rice husk

In the present study, one process was selected for a fundamental study of structural evolution during rapid pyrolysis, as well as for the study of the influence of such evolution on char reactivity. Chars were prepared at different situations from rice husk. The reactivity of resultant chars was measured using non-isothermal thermogravimetric analysis. The structure of fresh and partly reacted chars was characterized using proximate and ultimate analyses, physical adsorption/desorption measurements of N 2 (− 196 °C), mercury intrusion porosimetry (414 MPa), FTIR, Helium pycnometer as well as samples visualization by scanning electronic microscopy (SEM). Appreciable differences in the physical characteristics, depending markedly on the pyrolysis stage, were observed. SEM observation showed that surface of pore in char particle became increasingly rough in the middle of pyrolysis. Micropore characteristics obtained from adsorption/desorption measurement were complex. Release of volatile material led to the development of pores with different changing trends. The surface area of char increased with pyrolysis process to a maximum value of 56.95 m 2/g at pyrolysis reaction ratio ( R p = 0.90). Macropores in char particles which were evaluated by mercury intrusion porosimetry indicated that the porosity increases continually. Combined the analysis result of density with porosity data, it was showed that particle shrinkage happened at the first stage of rapid pyrolysis. The H/C, O/C and N/C ratios of the char changed with different trends when the pyrolysis reaction ratio increased. Furthermore, FTIR studies indicated a gradual decrease in the intensities of OH, C–H and C–O stretches with pyrolysis process. At the end of reaction, most bands disappeared, resulting in a char that was mainly an aromatic polymer of carbon atoms.

Changes in optical properties, chemistry, and micropore and mesopore characteristics of bituminous coal at the contact with dikes in the Illinois Basin

Changes in high-volatile bituminous coal (Pennsylvanian) near contacts with two volcanic intrusions in Illinois were investigated with respect to optical properties, coal chemistry, and coal pore structure. Vitrinite reflectance (Ro) increases from 0.62% to 5.03% within a distance of 5.5 m from the larger dike, and from 0.63% to 3.71% within 3.3 m from the small dike. Elemental chemistry of the coal shows distinct reductions in hydrogen and nitrogen content close to the intrusions. No trend was observed for total sulfur content, but decreases in sulfate content towards the dikes indicate thermochemical sulfate reduction (TSR). Contact-metamorphism has a dramatic effect on coal porosity, and microporosity in particular. Around the large dike, the micropore volume, after a slight initial increase, progressively decreases from 0.0417 cm3/g in coal situated 4.7 m from the intrusive contact to 0.0126 cm3/g at the contact. Strongly decreasing mesopore and micropore volumes in the altered zone, together with frequent cleat and fracture filling by calcite, indicate deteriorating conditions for both coalbed gas sorption and gas transmissibility.

Specifics of C-C bond formation and rearrangement in transformations of hydrocarbons on modified zeolites

Using high-performance catalysts prepared from zeolites Y, M, and ZSM-5, the dependence on the acid and micropore characteristics of modified zeolites was studied for the yields of products of dehydrocycloaromatization of C2+ alkanes, the isomerization of n-pentane, and the disproportionation of toluene. On the basis of the obtained experimental results and published data, it was shown that the initial activation of reactant molecules occurs on acid sites regardless of their acid strength and that the formation and rearrangement of C-C bonds are controlled by the molecular-sieve parameters of a zeolite and can be varied via the modification of the virgin zeolites used to prepare the catalysts.

Nanoporosity Characteristics of Some Natural Clay Minerals and Soils

AbstractLimited information is available on microporosity in soils. A study was undertaken to investigate the micropore characteristics of four soil samples, with different particle-size distributions, and four natural silicate clay minerals. Specific surface area and the differential micropore-size distribution were taken into account to characterize the microstructure of the soils and clays. The micropore-size distributions showed a maximum contribution to the total microporosity by pores having an effective pore radius of ∼20 Å, thus indicating that that category of pore contributes more than others to the total microporosity of the system. For both soils and clays a good exponential correlation was found between the maximum contribution to the microporosity and their specific surface area. A linear relationship was also found between the microporosity of the soils and their clay content. It has been concluded that the micropore system formed by 20 Å pores is mainly located in the clay fraction of the soil, and contributes significantly to defining some of the most notable physicochemical properties of soils and clays.

Preparation of macroporous biodegradable poly(L-lactide-co-epsilon-caprolactone) foams and characterization by mercury intrusion porosimetry, image analysis, and impedance spectroscopy.

Two poly(L-lactide-co-epsilon-caprolactone) random copolymers containing 5 and 40 mol % of epsilon-CL, namely P(LA-co-CL(5)) and P(LA-co-CL(40)), respectively, have been made macroporous by freeze-drying solutions in dimethylcarbonate. Most of the freeze-dried foams, prepared by varying polymer concentration and cooling rate, exhibited two main pore populations: (1). longitudinally oriented tube-like macropores with diameters >or=100 microm, and (2). interconnected micropores (10-100 microm). Pore characteristics, including macropore density, mean diameter, and interdistance, as well as micropore density, area, and shape, were determined by image analysis of scanning electron micrographs in order to study the influence of processing and formulation parameters on foam structure and properties. The pore orientation and the 3-D texture also were studied by image analysis and impedance spectroscopy. In the case of the P(LA-co-CL(5)), the macropore diameter increased with the cooling rate while the micropore diameter decreased. The micropores also became more circular when the cooling rate was increased. The pore size and morphology of the P(LA-co-CL(40)) were quite unchanged by varying the cooling rate. All the other conditions being the same, the P(LA-co-CL(5)) foams were better organized than the P(LA-co-CL(40)) foams, and pore orientation was improved at the higher cooling rate. Pore size and morphology also can be controlled by changing the polymer concentration (Cp), as we showed by studying P(LA-co-CL(5)) foams prepared by freeze-drying solutions in the 1-10 w/v % Cp range. Macropore density, average diameter, and interdistance of P(LA-co-CL(5)) foams increased with Cp, but the micropore characteristics remained almost unchanged no matter the Cp. The reliability of the characterization methods has been discussed, with special attention to mercury intrusion porosimetry, which is used primarily for measurement of pore volume and pore size distribution. However, this technique is reported here as a destructive and unreliable method for the characterization of fragile P(LA-co-CL(40)) foams. This study shows that image analysis and impedance spectroscopy can give reliable information relative to the pore morphology and anisotropy of freeze-dried foams.

Chemical activation of high sulfur petroleum cokes by alkali metal compounds

Chemical activation of high sulfur (∼7%) petroleum cokes with alkali metal compounds was investigated in the temperature range of 400–600°C. Gasification, desulfurization, and disintegration during the activation process were studied, and the pore characteristics of activated carbons and differences from coal were investigated. It was found that the surface area did not increase considerably until the sulfur removal rates exceeded 98%, and that the product activated carbons have micropore characteristics and hydrophilicity. Changes in major functional groups were analysed by FTIR spectroscopy. Hydrogen of about 460–1460 cm 3 g −1 coke evolved due to the carbonization using added alkali metal with the alkali to coke ratio in the range of 1–4. Changes in thermal and morphological properties were discussed.

Modification of the micropore characteristics of the desilicated ZSM-5 zeolite by thermal treatment

Controlled desilication of ZSM-5 zeolite, using an aqueous solution of 0.8 m sodium carbonate and 0.01 m sodium hydroxide, results in a significant decrease of the Si/Al atomic ratio, with sodium orthosilicate and its dimer, sodium pyrosilicate, being selectively removed from the zeolite framework. Micropores of 0.49 nm, i.e., those significantly smaller than those of the parent zeolite, are formed during desilication at the expense of the original micropores. Such a desilicated zeolite shows an ion-exchange activity (with Ca 2+) much higher than that of the parent zeolite, owing to the increase of the density of Al tetrahedral sites. Activation of the desilicated zeolite at a temperature higher than 300°C results in the enlargement of the newly formed micropores. The final system is quite homogeneous with only one type of micropore, ca. 0.56 nm in diameter. This is probably at the origin of further improvements in the ion-exchange performance. There is no significant change of the ZSM-5 zeolite structure in this method of secondary synthesis: Thus, the function of the thermal treatment is to stabilize the framework and to homogenize the pore network of the desilicated zeolite.

Special features of the formation of the structure of highly porous cellular materials based on reaction-sintered silicon carbide

Special features of the structure of highly porous cellular materials containing reaction-sintered silicon carbide are investigated. The effect of the ratio of the initial components of the synthesis (silicon and carbon) on the quantitative and qualitative characteristics of micropores formed during the reaction sintering of silicon carbide is studied. Formulas for calculating the microporosity are presented. The distribution of micropores in the structure components of highly porous cellular materials is analyzed. The influence of the chemical composition and the macro- and microporosity on the strength parameters of highly porous materials is investigated. It is established that despite the maximum microporosity, the strength of highly porous silicon carbide is the highest as compared to the investigated materials containing free silicon or carbon.

Fundamentals of the theory of adsorption in micropores of carbon adsorbents: Characteristics of their adsorption properties and microporous structures

At the basis of the theory of vapor adsorption in the micropores of carbon adsorbents of the most probable slit-like, limited-size type lie the dispersion interactions between the adsorbate and adsorbent atoms as well as between the atoms of the adsorbed substance themselves. As a consequence of commensurability of the pore sizes (half-widths) with those of the adsorbed molecules there occurs the volume filling of the available micropore space, i.e., the field of adsorption forces. Early stages of activation of carbons produce practically homogeneous micropores, the size of which may suitably be estimated by small-angle x-ray and calorimetry techniques involving measurements of the heats of immersion of adsorbents into liquids with different molecular size. The two methods give results that are in agreement. Dubinin and Radushkevich proposed an equation for homogeneous micropore structures with two parameters, the micropore volume W 0 and the characteristic energy of adsorption o . With deviations not considerably in excess of 10%, the product of micropore half-width x 0 by E 0 is a constant. For nonhomogeneous micropores the reasonable approximation is to assume the normal micropore volume distribution. The Dubinin-Stoeckli equation obtained for this general situation involves three parameters W 0, E 0, and dispersion δ characterizing the distribution range. The parameters are effective since active carbons also contain larger mesopores, which exhibit weaker adsorption than micropores. The effective values of W 0 0, E 0 and δ, determined from the experimental adsorption isotherm of the standard benzene vapor at 293 K, allow the adsorption isotherms of other vapors within a temperature range of about 100 degrees to be estimated with reasonable accuracy. The real parameters for adsorption isotherms corrected for the adsorption in mesopores, i.e., the isotherms with the real parameters W 0 0, E 0, and δ, can be used for calculating the geometric surface area of micropores and their volume within a given size range. Thus, the effective micropore characteristics W 0 0, E 0, and δ are, together with the specific surface area of mesopores S me , a quantitiative characteristic of the adsorption properties and microporous structure of each active carbon specimen. Benzene at 293 K and nitrogen at 77K are compared as reference vapors and their specificity is discussed.

Pore diffusion in silver catalysts

AbstractDiffusion rates were measured at atmospheric pressure and room temperature for helium and nitrogen in pelleted silver catalysts. Pellets were prepared from three silver salts giving different micropore characteristics. The data showed that the diffusion was of the bulk type and that the micropores had no effect, indicating that mass transfer was predominantly in the macropore region. For each kind of material data were obtained for five pellet densities corresponding to macropore volume fractions from 0 up to 0.7.Effective diffusivities and labyrinth factors ψ were evaluated and found to increase with macropore porosity. The linear relationship between ψ and void fraction proposed by Buckingham for diffusion through soils fitted the data at high void fractions. A generalized form of the Maxwell equation containing three constants was derived. A single‐constant form of this expression agreed reasonably well with the measured results over the entire range of macropore volume fraction.