Pore Surface Fractal Dimension Research Articles

NEUROSTIMULATION FOR PSYCHOTIC SYMPTOMS

The effects of pyrolysis temperature and heating rate on the porous structure characteristics of rice straw chars were investigated. The pyrolysis was done at atmospheric pressure and at temperatures ranging from 600 to 1000 °C under low heating rate (LHR) and high heating rates (HHR) conditions. The chars were characterized by ultimate analysis, field emission scanning electron microscope (FESEM), helium density measurement and N2 physisorption method. The results showed that temperature had obvious influence on the char porous characteristics. The char yield decreased by approximately 16% with increasing temperature from 600 to 1000 °C. The carbon structure shrinkage and pore narrowing occurred above 600 °C. The shrinkage of carbon skeleton increased by more than 22% with temperatures rising from 600 to 1000 °C. At HHR condition, progressive increases in porosity development with increasing pyrolysis temperature occurred, whereas a maximum porosity development appeared at 900 °C. The total surface area (Stotal) and micropore surface area (Smicro) reached maximum values of 30.94 and 21.81 m2/g at 900 °C and decreased slightly at higher temperatures. The influence of heating rate on Stotal and Smicro was less significant than that of pyrolysis temperature. The pore surface fractal dimension and average pore diameter showed a good linear relationship.

Evaluation of the porous structure development of chars from pyrolysis of rice straw: Effects of pyrolysis temperature and heating rate

The effects of pyrolysis temperature and heating rate on the porous structure characteristics of rice straw chars were investigated. The pyrolysis was done at atmospheric pressure and at temperatures ranging from 600 to 1000°C under low heating rate (LHR) and high heating rates (HHR) conditions. The chars were characterized by ultimate analysis, field emission scanning electron microscope (FESEM), helium density measurement and N2 physisorption method. The results showed that temperature had obvious influence on the char porous characteristics. The char yield decreased by approximately 16% with increasing temperature from 600 to 1000°C. The carbon structure shrinkage and pore narrowing occurred above 600°C. The shrinkage of carbon skeleton increased by more than 22% with temperatures rising from 600 to 1000°C. At HHR condition, progressive increases in porosity development with increasing pyrolysis temperature occurred, whereas a maximum porosity development appeared at 900°C. The total surface area (Stotal) and micropore surface area (Smicro) reached maximum values of 30.94 and 21.81m2/g at 900°C and decreased slightly at higher temperatures. The influence of heating rate on Stotal and Smicro was less significant than that of pyrolysis temperature. The pore surface fractal dimension and average pore diameter showed a good linear relationship.

Size‐dependent aggregation behavior in monodisperse silica: a nitrogen adsorption study

AbstractThe present study focused on the aggregation behavior that resulted from surface interaction between monodisperse silicas synthesized with Stöber method using nitrogen adsorption method at cryogenic temperature (77 K). Specific surface area, pore size distribution and surface fractal dimension from adsorption isotherm were evaluated. For relatively large particle, (190 nm in diameter), it was observed that nitrogen adsorption isotherm could be classified to type I. However, others with small size (below 100 nm) have shown the characteristic of type IV with the evident character of hysteresis loop in the isotherm. The increment of specific surface area (SBET) changed with the decrease of particle size, which could be fitted with an exponential decay equation. Pore size distribution (PSD) indicated that there exist an amount of mesopore (between 2 and ∼50 nm) and shift toward the smaller with decreasing the size of silica. Surface fractal dimension (DSF) from thermodynamic method demonstrated that the DSF increased with the decreasing the size of silica. PSD and DSF clearly embodied the size‐dependent aggregation of monodisperse silica nanoparticle, which could be used to explain the inferior performance of polymer nanocomposite filled with smaller monodisperse silica. Copyright © 2011 John Wiley & Sons, Ltd.

Research on the Pore Structure of Alkali-Slag Cement Stone Activated with Water Glass

In view of the phenomena that alkali-slag cement has high mechanical strength and good impermeability and they are closely related with the pore structure of cement stone,water glass was selected as the alkali activater, nitrogen adsorption method was adopted to study the pore structure of alkali-slag cement stone, while the pore surface fractal dimension was determined by fractal theory. The results show that the pore structure of alkali-slag cement stone posesses obvious fractal characters and that fits the Frenkel-Halsey-Hill(FHH) model quite well.So the pore surface fractal dimension can be used to synthetically evaluate the superior or inferior of the pore structure of cement stone.

Effect of polyferric chloride (PFC) doses and pH on the fractal characteristics of PFC–HA flocs

Changes in the fractal characteristics of PFC–HA flocs with polyferric chloride (PFC) doses and coagulation pHs were investigated in this study. A type of PFC coagulant with a hydrolysis degree ( B*) of 0.6027 and a Fe(III) concentration of 0.202 mol/L was employed to coagulate HA solutions at five constant coagulation pHs. Zeta potential tests were employed to reveal the coagulation mechanisms. The physical properties of flocs formed in the coagulation process were analyzed by their CCD image morphology, size distribution, and fractal dimensions. Next, microscopic observations and nitrogen-adsorption–desorption methods were conducted to characterize the pore structure and surface fractal dimensions of powders prepared from these flocs through a cryofixation–vacuum-freeze-drying procedure. The PFC doses and coagulation pHs had an obvious effect on the physical characteristics of PFC–HA flocs and their corresponding powders. Generally, in the low PFC dose range, PFC–HA flocs formed under acidic coagulation pHs and charge neutralization mechanisms were larger, had less irregular boundaries or surfaces (lower D 1 value), a more compact structure (lower D 2 value) and more irregular pore surface or pore size distribution in their dried powder than those formed under neutral and alkaline coagulation pHs and physical enmeshment and/or fresh precipitate adsorption mechanisms involved in the coagulation process. After the PFC doses exceeded this range, a gradual reversal phenomenon occurred. Furthermore, there were different specific PFC dosages that were above the critical reversal point for each property of these flocs or powders, such as the median diameter, one-dimensional fractal dimension, two-dimensional fractal dimension, and pore surface fractal dimension. In addition to these characteristics, the most compact PFC–HA flocs were formed at coagulation pH = 6.00 ± 0.05 and a PFC dosage of 4.04 × 10 −4 mol/L (Fe 3+), and the loosest flocs were produced at coagulation pH = 7.00 ± 0.05 and a PFC dose of 1.01 × 10 −4 mol/L (Fe 3+).

Simultaneous Removal of SO2 and NOx by Calcium Hydroxide at Low Temperature: Evolution of the Absorbent Surface Structure

The surface structure characteristics of the absorbent subjected to SO2 and NO2 for different reaction times were measured by using the N2 adsorption method. The pore surface and structure fractal dimensions (D1 and D2) calculated from the N2 adsorption isotherms data were applied to analyze the fractal characteristics of absorbent particles during reactions. On the basis of the measurement results and the reaction mechanism of SO2 and NO2 removal, the evolution mechanism of absorbent surface structure during reactions was discussed. The presence of reaction products on the surface of the absorbent can reshape the pore structure. With the comparison of the evolution of the surface structure in SO2 and NO2 single removal, the interaction of SO2 and NO2 on the absorbent surface has a great effect on the change of surface structure with simultaneous removal of SO2 and NO2. The decrement of the surface structure parameters values is lower than that in both SO2 and NO2 single removal. The change trend of the D...

Morphological Characterization of Super Fine Pulverized Coal Particle. Part 4. Nitrogen adsorption and Small Angle X-ray Scattering Study

Super-fine pulverized coal combustion is a new pulverized coal combustion technology that has better combustion stability, higher combustion efficiency, lower NOx and SO2 emissions, and higher comprehensive efficiency, than using conventional particle sizes. In this paper, we combined the pore surface fractal dimensions and pore structure fractal dimensions based on different methods to analyze the fractal characteristics of super fine pulverized coal particles. And the gray relational analysis was used for further study on the influencing factors of the fractal dimensions. Besides, the discrepancy between the two fractal dimensions influencing the phases during the coal combustion process was also discussed. It is found that all the fractal dimensions of the super fine pulverized coal particles range from 2.36 to 2.91, which belong to the interval 2< D < 3. It is interesting to observe that the surface fractal dimension increases with the increase of the pulverized coal particle size while the structure ...

Fractal analysis of polyferric chloride-humic acid (PFC-HA) flocs in different topological spaces

The fractal dimensions in different topological spaces of polyferric chloride-humic acid (PFC-HA) flocs, formed in flocculating different kinds of humic acids (HA) water at different initial pH (9.0, 7.0, 5.0) and PFC dosages, were calculated by effective density-maximum diameter, image analysis, and N2 absorption-desorption methods, respectively. The mass fractal dimensions (Df) of PFC-HA flocs were calculated by bi-logarithm relation of effective density with maximum diameter and Logan empirical equation. The Df value was more than 2.0 at initial pH of 7.0, which was 11% and 13% higher than those at pH 9.0 and 5.0, respectively, indicating the most compact flocs formed in flocculated HA water at initial pH of 7.0. The image analysis for those flocs indicates that after flocculating the HA water at initial pH greater than 7.0 with PFC flocculant, the fractal dimensions of D2 (logA vs. logdL) and D3 (logVsphere VS. logdL) of PFC-HA flocs decreased with the increase of PFC dosages, and PFC-HA flocs showed a gradually looser structure. At the optimum dosage of PFC, the D2 (logA vs. logdL) values of the flocs show 14%-43% difference with their corresponding Df, and they even had different tendency with the change of initial pH values. However, the D2 values of the flocs formed at three different initial pH in HA solution had a same tendency with the corresponding Dr. Based on fractal Frenkel-Halsey-Hill (FHH) adsorption and desorption equations, the pore surface fractal dimensions (Ds) for dried powders of PFC-HA flocs formed in HA water with initial pH 9.0 and 7.0 were all close to 2.9421, and the Ds values of flocs formed at initial pH 5.0 were less than 2.3746. It indicated that the pore surface fractal dimensions of PFC-HA flocs dried powder mainly show the irregularity from the mesopore-size distribution and marcopore-size distribution.

Changes in textural properties of selected minerals under acid and alkali treatment from mercury intrusion porosimetry

Bentonite, biotite, illite, kaolin, vermiculite and zeolite were acidified or alkalized with hydrochloric acid or sodium hydroxide of concentrations 0.0, 0.1, 1.0 and 5.0 mol dm − 3 at a room temperature during two weeks. The treated minerals were transferred to calcium homoionic forms from which aggregates were formed by hand (if possible) and subjected to a few cycles of wetting–drying to stabilize the structure. Aggregates of bentonite, illite, kaolin and zeolite, and loose beds of biotite and vermiculite (no aggregation) were studied by mercury porosimetry. From mercury intrusion curves pore volumes, size distributions, average radii and surface fractal dimensions were calculated for pores ranged from c.a. 0.01 to 10 μm in radius. In most cases with an increase of the concentration of acid and alkali the pore volumes increased indicating a loosening of the minerals texture. Depending on the mineral and the treatment both broadening of pore dimensions and formation of new fine pores was observed, therefore the average pore radii changed in different directions. No well defined trends in the pore surface fractal dimension were noted under both treatments, however in most cases they were lower for treated than for nontreated minerals.

Capillary pressure in a porous medium with distinct pore surface and pore volume fractal dimensions

The relationship between capillary pressure and saturation in a porous medium often exhibits a power-law dependence. The physical basis for this relation has been substantiated by assuming that capillary pressure is directly related to the pore radius. When the pore space of a medium exhibits fractal structure this approach results in a power-law relation with an exponent of 3-D(v), where D(v) is the pore volume fractal dimension. However, larger values of the exponent than are realistically allowed by this result have long been known to occur. Using a thermodynamic formulation for equilibrium capillary pressure we show that the standard result is a special case of the more general exponent (3-D(v))(3-D(s)) where D(s) is the surface fractal dimension of the pores. The analysis reduces to the standard result when D(s)=2, indicating a Euclidean relationship between a pore's surface area and the volume it encloses, and allows for a larger value for the exponent than the standard result when D(s)>2 .

Surface analysis of cryofixation-vacuum-freeze-dried polyaluminum chloride–humic acid (PACl–HA) flocs

The powder of polyaluminum chloride–humic acid (PACl–HA) flocs was prepared by cryofixation-vacuum-freeze-drying method. The FTIR spectra show that some characteristic functional groups in polyaluminum chloride (PACl), humic acid (HA), and kaolin still existed in the dried flocs. X-ray diffractometry (XRD) patterns indicate that these flocs are amorphous. Nitrogen adsorption–desorption isotherms were obtained for different samples of the dried PACl–HA flocs. The BET specific surface area, BJH cumulative absorbed volume and BJH desorption average pore diameter of them were determined. The peak values of 8.4–11.2 nm (pore diameter) for pore size distribution (PSD) curves indicate that the pores of the dried flocs are mostly mesopores. The surface fractal dimensions D s and the corresponding fractal scales determined from both SEM images and nitrogen adsorption–desorption data sets reveal the multi-scale surface fractal properties of the dried PACl–HA flocs, which exhibited two distinct fractal regimes: a regime of low fractal dimensions (2.07–2.26) at higher scales (23–387 nm), mainly belonging to exterior surface scales, and a higher fractal dimensions (2.24–2.37) at lower scales (0.80–7.81 nm), falling in pore surface scales. Both HA addition and kaolin reduction in dried floc can decrease the irregularity and roughness of external surface. However, for the irregularity and roughness of pore surface, the addition of HA or kaolin in dried floc can increase them. Furthermore, some difference was found between the pore surface fractal dimensions D s calculated from nitrogen adsorption and desorption data. The pore surface D s values calculated through thermodynamic model were much greater than three.

Study on the pore surface fractal dimension and surface acid–base properties of natural particles around Guanting reservoir

Nitrogen absorption–desorption isotherms were obtained on different natural particles around Guanting reservoir, and their data points were used to analyze the pore structure and fractal properties of the obtained solids. Other microstructure parameters of these particles are the following: 9.5669–4.5605 m 2 g −1 of BET specific surface area, 0.01784–0.06656 cm 3 g −1 of BJH cumulative absorbed volume and 7.20–7.74 nm of BJH desorption average pore diameter. The peak values of pore size distribution (PSD) curves were found at near 3.74 nm for samples 2, 3 and 4, and near 3.25 nm for sample 1, and near 5.17 nm for sample 5. The average pore diameter was larger than that corresponding to the peak value of pore size distribution (PSD). The micropore areas are very small in these particles, but their micropore volume of them can occupy over 15% of total pore volume. The self-similar and rough surface was observed on these particles, and the pore surface fractal dimensions D s of them were between 2.69 and 2.80, calculated by fractal FHH equation with the data of the adsorption branch of the isotherm, are a little difference from ones with the isotherm data of desorption branch, and also their corresponding fractal scales are different. It may be attributed to the different p/ p 0 range for D s values calculation and pore structure property. Otherwise, the computing D s values by thermodynamic model do not give meaningful indication for particle surface irregularity. There are about 1–2 specific surface sites ( D OH) in each square nanometers on these particles surface, and at most pH range, the average number of protons reacted per surface site ( Z) have negative values, and only when pH values are lower than about 4, Z can have positive values. Potentiometric titration experiment and two surface complexation model simulation results indicate that CCM simulation results can better interpret the experiment data points than DLM at 0.005 mol L −1 ion strength. Based on the CCM, the protonation and deprotonation reactions model can successfully depicted the acid–base behavior of natural particles surface, and the surface intrinsic acidic constants and the total concentration of surface sites for these natural particles, could be obtained.

Pore structure and mechanical properties of cement–lime mortars

Studies focusing on materials used in Cultural Heritage conservation projects are becoming increasingly important. In this paper, the pore structure and mechanical properties of lime–cement mortars are evaluated in order to analyze their potential use, because this kind of mortar could reduce the disadvantages presented by both lime-based mortars and cement-based mortars. The microstructure of these blended mortars is studied taking into account porosity, pore size distribution and surface fractal dimension. Compressive and flexural strengths are discussed as a function of several parameters: curing time, binder composition and B/Ag (Binder/aggregate) ratio. The mechanical strength versus the deformation of the material is also evaluated, by analysis of Young's modulus, as well as the elastic and plastic zones. Unlike cement-based mortars, blended mortars with a high percentage of lime present a large plastic zone, which could be useful in the service-life of these mortars as a result of their ability to absorb strains caused by wall movements.

Blended pastes of cement and lime: Pore structure and capillary porosity

Microstructure of blended pastes of lime and cement were studied in this paper. An increment of complexity of the microstructure was found when pastes increase their percentage in cement. Microstructural characteristics as porosity, morphology of the pores, pore size distribution and surface fractal dimension were evaluated in the different pastes studying the modification with the variation of composition. The capillary water absorption is also evaluated obtaining higher capillary coefficients values for the pastes with higher amounts of lime. The increase of complexity of the microstructure, due to the cement in the pastes, leads to slight deviations of the parallel tube model.

The fractal dimension of stone pore surface as weathering descriptor

Pore surface fractal dimension of building stones were measured using mercury porosimetry in a pore diameter range from 100 to 0.012 μm. Surface fractal dimension can be used as descriptor of the stone pore surface, allowing an easier differentiation between weathered and unweathered stone than the pore size distribution. For the studied materials, weathering processes cause an increase in the surface fractal dimension.

Pore structure and its effect on strength of high-volume fly ash paste

Pastes made with different fly ash contents and water/binder ratios were tested at different ages. The total porosity and pore size distribution were measured by mercury intrusion. The fractal dimension of pore-solid interface was first measured by small angle X-ray scattering (SAXS). The relationship between porosity and strength for high-volume fly ash (HVFA) paste was determined. It is obtained that the pore size distribution affected both compressive strength and flexural strength, but the effects on compressive strength and on flexural strength are different. Preliminary experimental results indicated that strength was closely related to the pore surface fractal dimension.