Pore Percolation Research Articles

Experimental Study on CO2 Flooding Characteristic in Low-Permeability Sandstone Considering the Influence of In-Situ Stress

Summary The pore network controlled by in-situ stress significantly influences the CO2 flooding in low-permeability reservoirs. In this study, the CO2/oil distribution and response of pore structure were monitored online using low-field nuclear magnetic resonance (LF-NMR), and the in-situ stress dependence of oil recovery was analyzed. The results show that the pore structure consists of adsorption pore (AP < 1 millisecond), percolation pore (1 millisecond < PP < 10 milliseconds), and migration pore (10 milliseconds < MP). Oil recovery was primarily influenced by AP and MP at lower in-situ stress, while PP and MP are the main contributors at higher in-situ stress. The matrix experienced compression deformation, microfracture generation, and shrinkage of pore, combined with an increase and followed by a decrease in oil recovery, responding to the increase of in-situ stress from 5 MPa to 15 MPa and from 15 MPa to 20 MPa. The reduction in gas channels promotes a piston-like advancement of oil displacement, resulting in an initial increase in oil recovery, while subsequent decline is linked to heightened pore heterogeneity caused by high in-situ stress. Increased heterogeneity reduces gas displacement stability, hampers CO2 sweep efficiency, and results in a granular distribution of residual oil. The findings provide insight on CO2-enhanced oil recovery (EOR) in low-permeability reservoirs.

Preliminary Validation of Persulfate Delivery Through Porous Pipe in the Vadose Zone: A Two-Dimensional Sandbox Experiment

ABSTRACT When remediating contamination in the vadose zone, which is a potential source of continuous groundwater and surface water pollution, technical challenges are often encountered. In this study, the feasibility of delivering sodium persulfate (SPS) oxidant to unsaturated soils was explored by integrating porous pipes, commonly used in agriculture, with in-situ chemical oxidation (ISCO). An evaluation of the operating characteristics of the pipe determined a recommended influent rate of < 0.1 L min−1 and an initial influent water pressure < 20 kPa to prevent percolation pore enlargement. Two-dimensional sandbox experiments revealed that pulse infiltration at an influent rate of 0.05 L min−1 was effective in creating matrix flow and lateral movement, minimizing preferential flow induced by continuous infiltration. Application of SPS (10% by wt.) under these conditions resulted in approximately 90% wetting of the sandbox area SPS flow at the experimental time that infiltrated from the pipe. After a 14-d post infiltration resting state, residual SPS concentrations ranged from 6,000 -12,000 mg/kg in soil, with soil pH levels of 3–4, ORP around 600 mV, and 40–60% decreases in soil organic carbon. This developed method helps retain the oxidant and promote chemical reactions, making ISCO a viable option for remediating unsaturated soil contamination.

Study on production performance characteristics of horizontal wells in low permeability and tight oil reservoirs

Low permeability and tight oil reservoirs are rich in microscale pores and contain a large amount of dissolved gas. With the progress of production, the volatile gas gradually forms a continuous gas phase and has an important impact on the productivity of oil wells. In this paper, the characteristics of oil and gas separation in the production process of low permeability and tight reservoirs are described, a productivity prediction vector model is established considering the characteristics of starting pressure in the percolation process, and the coupled influence of "oil film thickness - high pressure effect - multi-element gas effect" on dissolved gas flow is analyzed. Results show that: (a) The horizontal section length and production pressure difference of horizontal wells are the key factors affecting the productivity of low permeability or tight oil reservoirs, while the influence of wellbore radius and reservoir thickness is not obvious. (b) The increase in the length of the horizontal section effectively increases the well control range, increases the oil drainage volume, and thus improves the productivity. (c) With the development of low permeability or tight oil reservoirs, the reservoir pressure and oil saturation have decreased. This results in a decrease in oil film thickness and an increase in volatile gas content. With the change of volatile gas pressure and the change of percolation pore size, the Knudsen number changes in a large range, resulting in the change of the microscale transport mechanism of volatile gas.

Experimental study on dynamic pore-fracture evolution and multiphase flow in oil-saturated coal considering the influence of in-situ stress

The methane drainage in liquid-saturated coal is directly related to the multiphase flow in complex geological structures. In this study, the pore-fracture dynamic evolution induced by in-situ stress and the corresponding gas-liquid migration and morphology characteristics were investigated by a self-developed online LF-NMR triaxial stress system. The dynamic variation, obvious change, and slight response were occurred at the migration pore (MP＞3ms), percolation pore (0.3ms < PP < 3ms), and adsorption pore (AP<0.3ms) with the increased stress, and development of gas channel was contributed by MP (15%–20%), AP (5%–10%), and PP(0–5%) responding to the gas flooding. The pore-fracture successively experienced compression, dilation, expansion, and coalescence, and PP and MP dominated the fracture initialization and connection. The permeability is negatively correlated with the fractal dimension of the pore-fracture structure and is exponentially related to the porosity and MP porosity. The longitudinal shear fracture is beneficial for the gas-liquid migration, and the horizontal tension crack connects the scattered pore and branch fracture. The gas channel was influenced by the liquid capillary pressure and gas jiamin effect in poor pore-fracture and mainly influenced by the gas slippage effect, immiscible behavior, and fracture surface tension force in mature pore-fracture. The optimal gas channel occurred at the crack-propagation period, which was located at around 70% peak strength with 1.5md permeability, high liquid relative permeability and lower gas relative permeability. The findings provide significant insight into methane extraction.

Experimental study on stress-dependent multiphase flow in ultra-low permeability sandstone during CO2 flooding based on LF-NMR

Oil production performance in the unconventional reservoirs is significantly affected by the flooding agent and occurrence environment. In this study, the in-situ stress and displacement pressure effects are considered, the response of CO2/Oil and pore structure was monitored by online low field nuclear magnetic resonance (LF-NMR). The results show that the pore structure was composed of adsorption pore (AP＜3 m s), percolation pore (3 m s＜PP＜30 m s), and migration pore (30 m s＜MP). The stress sensitivity of adsorption pore is more obvious than that of percolation pore and gradually weakened responding to the increase of in-situ stress. The increased in-situ stress narrowed and closured pore throats and limited the migration of CO2/Oil, and the pore transformed from gas-channeling type into homogeneous type and the hydraulic conductivity of CO2 and oil decreased non-linearly responding to the pore shrinkage. The percolation pore and migration pore dominate the oil recovery, and the decreased MP resulted in the non-linearly and dynamic reduction of oil recovery. The higher relative permeability was presented and the end-point oil saturation non-linearly increased with the increasing in-situ stress. The increased displacement pressure effectively improved the oil recovery and dynamic gas channeling, and the incremental oil recovery was primarily contributed by the PP and MP due to the increased CO2 swept area. The positive effect of displacement pressure on oil recovery gradually reduced with the increase of in-situ stress. The findings provide a significant insight on the application of CO2-EOR in ultra-low permeability reservoirs.

Experimental study on fracture effect on the multiphase flow in ultra-low permeability sandstone based on LF-NMR

Multiphase flow affected by fractures is essential for enhanced oil recovery in ultralow-permeability reservoirs. In this study, CO2/water flooding of intact and fractured cores was performed under in-situ stress, and low-field nuclear magnetic resonance was employed to monitor the pore structure evolution and multiphase flow. The adsorption pore (AP), percolation pore (PP), and migration pore (MP) were determined in the ultralow permeability sandstone, and the PPs and MPs contributed to oil recovery during CO2 flooding. Three-scale pores contributed to oil production during water flooding. Compared with water flooding, CO2 flooding had a considerably higher average recovery efficiency, which was associated with pronounced gas channeling and lower ultimate oil recovery. In CO2 flooding, the oil production in PPs and MPs depended on the pressure difference, whereas that in APs was determined by oil swelling. But for water flooding, oil migration in the AP and MP was dominated by capillary imbibition and pressure difference, respectively. The fracture enhanced the oil recovery during CO2 flooding by increasing the CO2 sweep area in the PPs and MPs. It also improved oil recovery during water flooding based on the increment of the oil-water flow area in the APs and MPs. The mass transfer in CO2 flooding between the fracture and the matrix occurred through the PP and MP connected to the fracture, controlled by the pressure difference, number of PP and MP connected to the fracture, and oil swelling capacity. For water flooding, the oil-water exchange in the fracture-matrix system was dominated by number of pores connected to the fracture, hydrophilic ability of the rock, and pressure difference. These results can be used for enhancing oil recovery in fractured ultralow-permeability reservoirs.

Experiment and Model Investigation on the Permeability Evolution of Granite under Triaxial Compressive Damage

Deep granite is the ideal surrounding rock for nuclear waste geological disposal reservoir and its permeability is the key factor for nuclide migration. In order to study the permeability evolution characteristics of surrounding rocks in the near field of the disposal reservoir under different confining pressures during excavation, the mechanical and permeability properties of granite were explored through triaxial compression experiments associated with acoustic emission techniques. Experiment results suggest that the permeability exhibited typical pore percolation and fracture flow during the compressive damage process and it has an obvious relationship with the volumetric strain before and after dilation. According to such a relationship, a full-stage permeability model was proposed which is characterized by only 2 parameters. This permeability model was verified by the experiment data to be of good feasibility and effectiveness, exhibiting well fitness to the experiment under various confining pressures. According to the elastoplastic mechanical model that is available from other researchers, discussion was implemented and which convinced us that it is feasible to reproduce the real conditions of hydromechanical processes by combing the proposed permeability model and damage-based elastoplastic model in numerical modelling.

Experimental study on CO2/Water flooding mechanism and oil recovery in ultralow - Permeability sandstone with online LF-NMR

The investigation of the mechanism of CO2/water flooding in ultralow-permeability oil reservoir is critical to the enhanced oil recovery. In this study, a series of core flooding experiments in pore and pore-fracture cores subjected to different in situ stresses was conducted, and online LF-NMR technology was employed to dynamically monitor the multiphase flow and pore-fracture behavior. The results show that the percolation pore dominates the volumetric sweep efficiency, the capillary pressure caused by sandstone wettability controls the oil recovery in adsorption pore, and viscosity fingering and thief channeling mainly occurred in migration pore. The oil recovery is primarily contributed by the percolation pore and migration pore in CO2 flooding, and all pores averagely contribute to the oil recovery in water flooding. In addition, the oil recovery in the pore core decreased with the increase of the in situ stress (15–35 MPa) in CO2/water flooding. Compared with the increase of pore-fracture core oil recovery responding to the increased in situ stress in water flooding, the highest pore-fracture core oil recovery presented at 25 MPa followed by 35 MPa and 15 MPa in CO2 flooding. The oil saturated in fracture and fracture-connected pore was firstly displaced, and the evolved heterogeneous morphology of the residual oil was characterized by the increased fractal dimension from 1.5 to 1.7 in CO2 flooding and 1.78 to 1.82 in water flooding responding to the increased in situ stress. The pore core oil recovery rate in CO2 flooding is about 10 times than that in water flooding. The peak value of oil recovery presented at 25 MPa for CO2/water flooding, and the fracture influence is more obvious in water flooding. The findings provide significant guidance to the engineering practice of enhanced oil recovery in ultralow-permeability reservoir.

Visualization of mercury percolation in porous hardened cement paste by means of X-ray computed tomography

Mercury intrusion porosimetry (MIP) has been extensively used for the pore structure characterization of porous materials, but the percolation of mercury in porous cement-based materials during MIP has never been comprehensively investigated before. Here we visually probe the mercury percolation in a porous hardened cement paste (HCP) using X-ray computed tomography (X-CT). Novel double surface coatings on the HCP samples before and after MIP tests by an epoxy resin are specifically designed. The first coating enables the one-directional penetration of mercury in the HCP samples during MIP, while the second coating can partially seal the release of the entrapped mercury. MIP tests are operated under different maximum applied pressures (10,000 and 30,000 psi) and equilibrium time (5, 10, and 30 s). The high X-ray attenuation of mercury enables the visual trace of the intrusion mercury percolation pathways and the entrapped mercury clusters in the HCP samples by X-CT. The position-dependent entrapped mercury clusters in the pores account for the gray value changes and gradients in the HCP samples after MIP. The MIP characteristics, such as, total intrusion volume, volume size distribution and mercury entrapment, change with the maximum applied pressure and equilibrium time, but the percolation pore size shows the consistent values. The findings of this work deepen the understandings of MIP for the microstructure characterization of porous materials.

Study on Permeation Grouting Rules for Loess and Method for Predicting Migration Radius

Pressure grouting method can significantly enhance the strength of soil, and is widely used in foundation reinforcement. There already have some theoretical studies on pressure grouting, but the actual diffusion process of grouting can not be reflected exactly. Firstly, this paper conducts a field permeation grouting test on undisturbed loess of late pleistocene of quaternary, and acquires the pressure effected relationships of quantity vs. time. The test reveals that the grout quantity approximately increases linearly with the pressure until a certain grouting time point. The time point increases with grouting pressure and we try to define the pressure effected time point as the effective grouting time. After the effective grouting time, the grouting quantity remains constant due to the approaching of max migration radius and the stopping of grout migration. And then, considering the piston-like displacement effect of grout on porous mediums due to pressure, this study introduced the effective grouting time into the theoretical derivation process based on porous mediums’ fluid percolation pore pressure equation and Darcy’s law. The author combined the initial conditions and boundary conditions of pore pressure with the continuity conditions of permeation velocity, and performed the Boltzmann’s transformation and reduced-order processing to derive the equation for the migration velocity of the grout front and the migration radius of grout. Finally, by substituting the effective grouting time and the effective grouting quantity into the equation, we could obtain the grouting reinforcement radius. The equation can accurately predict the actual grouting reinforcement radius, and the applicability of the equation is verified by verification test.

Effect of optimized pore structure on sealing performance of drilling sealing materials in coal mine

To enhance the drilling sealing material's performance, SiO2, sodium silicate, and styrene-acrylic emulsion are selected to optimize the pore structure. Through the uniaxial compression experiment, NMR, SEM, and combined with fractal theory, each substance's influence on the pore structure and physical strength of the sealing material is analyzed qualitatively and quantitatively. The results show that the synergistic effect of styrene-acrylic emulsion and SiO2 on cement performance improvement is better than that of a single substance. The porosity of the optimized material decreases by more than 13.49%, and the peak stress of uniaxial compression is increased by more than 18.08%. Besides, the styrene-acrylic emulsion can improve the material's elastic modulus and enhance the material's ability to resist harmful external factors. Sodium silicate can hinder the optimization effect of SiO2 and styrene-acrylic emulsion. Compared with ordinary cement, the peak stress of materials containing these three substances is reduced by 3.05%, and the porosity is decreased only by 0.73%. Comparing the fractal dimension of each material, SiO2 and styrene-acrylic emulsion increases the total pore fractal dimension (Dw) and the percolation pore fractal dimension (Ds) of the material. The fractal dimension has a positive linear correlation with the porosity, which indicates that the pore structure of the material is optimized, and the pore connectivity is reduced.

Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads

With the characteristics of gradual instability in the supporting pressure area of roadway as the engineering background, this paper aims to explore the evolution law of pore and fracture in the coal sample under progressive loads. The low-field nuclear magnetic resonance (NMR) test was designed and conducted with the coal sample under different axial loads (0, 3, 5, 7, 9, and 11 MPa). The characteristic parameters such as the porosity, the pore size distribution, the transverse relaxation time (T2) distribution curve, and the magnetic resonance image (MRI) were obtained. As the test results show, significant difference in the NMR characteristics of the coal samples can be observed throughout the compaction stage and the elastic stage. In the compaction stage, the porosity of the coal samples decreases slightly; the T2 distribution curve moves to the smaller value as a whole, and the percolation pore (PP) displays a tendency to transform to the adsorption pore (AP). In the elastic stage, the porosity of the coal samples rises gradually as the load increases; the T2 distribution curve moves to the larger value as a whole, and the AP tends to transform to the PP. The MRI shows that some pores and fissures in the coal sample close up and disappear as the load increases gradually, while the main pores and fissures expand and perforate till the macro failure occurs. Compared with one-time loading, the progressive multiple loads can ensure the fracture of the coal sample to develop more fully and the damage degree higher. It indirectly reflects that the instability and failure of the coal under the progressive load has the stage characteristics, verifying that the coal in the supporting pressure area needs to be controlled in advance.

Study of a quantitative characterization parameter “oil and gas producing efficiency” of the producing degree of reserves in the developed area

Implementing the producing degree of reserves in the developed area is of great significance for timely understanding of oil and gas development trends in the developed area, and then taking targeted adjustment measures to improve oil and gas producing efficiency. On the basis of the analysis of different scale pore percolation capacity of reservoir and the study of four kinds of porosity (connected porosity, effective porosity, flowing porosity, filling porosity) and their corresponding calculation models of oil saturation, a quantitative characterization parameter - “oil and gas producing efficiency” of the producing degree of reserves in the developed area is proposed, and its theoretical discussion and case analysis are carried out. Research shows: 1) Oil and gas producing efficiency is the ratio of the reserves involved in percolation to geological reserves in the developed area of the oilfield, which is finally expressed as the ratio of the product of the flowing porosity and the original oil saturation of the flowing pores to the product of the connected porosity and the original oil saturation of the connected pores. 2) Reservoir flowing porosity and the original oil saturation of flowing pores are a function of displacement power, while the original oil saturation of connected pores is a function of reservoir forming power. Therefore, the oil and gas producing efficiency is essentially a coupling function of displacement power and reservoir forming power. 3) When the displacement power is greater than the reservoir forming power, the oil and gas will be fully developed, and the oil and gas producing efficiency will be 1; when the displacement power is less than the reservoir forming power, the oil and gas will be partially developed, and the oil and gas producing efficiency will be less than 1. 4) The calculation examples of X oilfield in Ordos Basin show that the connected porosity is 11.2%, the original oil saturation of the connected pores is 57.1%, the flowing porosity is 5.32%, the oil and gas producing efficiency is 0.832, and the oil and gas are partially developed. If the oil and gas are to be fully developed, the production pressure difference can be increased from 4.1MPa to 6.2MPa. 5) The study of oil and gas producing efficiency deepens the understanding of reserves producing status in the developed area of the oilfield, and has guiding significance for the targeted production adjustment and production pressure difference optimization of the oilfield.

Visualization of helium-water flow in tight coal by the low-field NMR imaging: An experimental observation

The low-field NMR imaging (NMRI) technology plays a key role of visualization of helium-water flow in tight coal. Combined with the traditional description by T2 spectrum, the NMRI description provides an auxiliary illustration on spatial distribution of water in various pores. To distinguish the different contribution of various pores in helium-water flow, a flow function-based classification of adsorption pore (AP), percolation pore (PP) and migration pore (MP) are suggested. It depends on the periodic appearance of peak and valley of the T2 spectrum. Although all pores contribute to porosity, the unchanged AP peak of T2 spectrum and the trapped water in 2D distribution shows AP contributes little to the helium-water flow. Further, the saturation evolution is clearly illustrated by CPMG sequence and NMRI visualization. Both descriptions show a strong dependence of saturation evolution on pore size distribution. Finally, the porosity is modified based on the squeezing effect of enhancing the peak of T2 spectrum. The experimental result shows the helium-water flow is properly demonstrated by the low-field NMRI method, but the drainage efficiency of helium displacing water is not high due to the trapped water in AP.

Multi-scale prediction of chemo-mechanical properties of concrete materials through asymptotic homogenization

In the present contribution, the effective mechanical, diffusive, and chemo-expansive properties of concrete are computed from a multi-scale and multi-physics approach. The distinctive features of the approach are that i) the mechanical and diffusive responses are modelled in a coupled fashion (instead of separately, as is usually done), and that ii) the multi-scale model considers three different scales of observation, which allows for including heterogeneous effects from both the micro- and meso-scales in the effective macro-scale properties of concrete. At the macro-scale, the concrete material is considered as homogeneous, whereas at the meso-scale it consists of particle aggregates embedded in a porous cement paste. At the micro-scale the porous cement paste is described as a two-phase material, composed of a solid cement phase and saturated capillary pores. Adopting a two-level asymptotic homogenization procedure, the effective meso-scale properties of the porous cement paste are computed first, using a unit cell that includes the cement paste and pore characteristics. Subsequently, the obtained meso-scale response of the porous cement paste, together with the aggregate characteristics, defines the material properties of a second unit cell, which is used for calculating the effective macro-scale response of concrete. The distributions of the pores and the aggregates within the unit cells are determined from a uniform, random distribution of points, and their radii are defined from a probability distribution function. The efficacy of the proposed framework is illustrated by studying the effective mesoscopic response of a porous cement paste, which demonstrates the influence of the micro-scale porosity and pore percolation. Next, the effective macroscopic response of concrete is analysed, by considering the influence of the aggregate volume fraction, the mismatches in elastic stiffnesses and diffusivity between the aggregate and the cement paste, and the porosity. The computed effective properties are compared with experimental data from the literature, showing a good agreement.

Connections Matter: On the Importance of Pore Percolation for Nanoporous Supercapacitors

Nanoporous supercapacitors play a key role in energy storage and thereby attract growing interest from the research community. Development of porous electrodes for supercapacitors is of the paramou...

Further insights into calcium sulfoaluminate cement expansion

There is still an ongoing debate on the mechanisms of expansion of calcium sulfoaluminate (CSA) cements, which can be either favourable, for example, in shrinkage compensating construction materials, or deleterious in cases of excessive expansion. In order to expand on previous studies, CSA cements with molar ratios (M) of calcium sulfate/ye'elimite of 0·1, 1, 1·5 and 2 were investigated from 30 min to 182 d. All systems showed expansion, and expansion was found to increase with increasing gypsum content. The sample with M = 2 showed a high expansion and severe cracking after 1 d of hydration. Based on the pore solution chemistry, crystallisation pressures were calculated, which revealed that not only ettringite, but also the other hydrate phases present (aluminium hydroxide (AH3), strätlingite, calcium aluminate decahydrate (CAH10), monosulfate) may contribute to the total crystallisation pressure. The percolation pore radii obtained by mercury intrusion porosimetry were used to calculate hydrostatic tensile stresses. It was found that in all investigated systems the hydrostatic tensile stresses exceeded the 1 d tensile strength, which is in line with the fact that all samples showed expansion.

Effective diffusivity of cement pastes from virtual microstructures: Role of gel porosity and capillary pore percolation

The role of capillary pores percolation and gel pores are investigated to explain the underlying differences between relative diffusivity obtained from different experimental techniques using microstructures generated from two different types of hydration model viz., CEMHYD3D (a voxel based approach) and HYMOSTRUC (a vector based approach). These models provide microstructures with different capillary pore connectivity for the same degree of hydration and the same porosity due to the underlying assumptions. In order to account for a C-S-H diffusivity at the micro-scale, a continuum micro-mechanics based model has been proposed. These simulations show that deperolation of capillary pores at around 20% of capillary porosity is essential in order to correctly predict diffusivity of cement paste with water-cement ratio by mass (w/c) in between 0.4 and 0.5. Furthermore from our analysis we present a viable postulate that the higher diffusivity measured by electric resistivity compared to other methods is due to differences in contribution from gel pores. For electrical resistivity measurement it is proposed that all gel pores are diffusive whereas for ion and tracer transport it is proposed that only nitrogen accessible gel pores are diffusive.

Relationship between the Size of the Samples and the Interpretation of the Mercury Intrusion Results of an Artificial Sandstone.

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.

Mesoporous titania films investigated by positron annihilation based on a slow positron beam

Tunable mesoporous titania (TiO2) thin films were synthesized via a sol-gel method using an amphiphilic triblock copolymer F38 as the structural template. The dependence of crystalization, pore morphology and interconnectivity of TiO2 films on the weight ratio of F38 was studied by wide-angle X-ray diffraction, field emission scanning electron microscopy and Doppler broadening of positron annihilation radiation spectroscopy based on a slow positron beam. By loading more F38, the crystallization of TiO2 films is enhanced, accompanied by a decrement in oxygen vacancies/grain boundaries. Smaller and isolated mesopores are formed in the films prepared with F38 less than 15 wt%. The pore percolation occurs when the weight ratio of F38 is up to 20 wt% and larger and interconnected worm-like pores are formed.