Pore structure modification of silica matrix infiltrated with paraffin as phase change material

Temperature-controlling solid–liquid phase change material (PCM) infiltrated in a porous matrix is becoming attractive for aerospace applications. The properties of the pore structure are crucial factors in the selection of porous materials for PCM infiltration. The main purpose of this study is to adjust the pore structure of porous silica matrices with different molar ratios of ethanol (EtOH) and tetraethoxysilane (TEOS) for PCM infiltration. Five compositional ratios of EtOH/TEOS were introduced to prepare the silica with different pore size through sol–gel processing, and the pore structures were analyzed by N₂ adsorption–desorption measurements and scanning electron microscopy. Results indicate that the pore size increases with a larger value of the EtOH/TEOS molar ratio. Open cell pore structure and pore size of silica were observed and calculated. For EtOH/TEOS ratios of 10 and 20, the synthesized silica matrices had average pore sizes of 53.1 nm and 56.0 nm, respectively, exhibiting better infiltration. Moreover, the maximum mass fraction of paraffin as PCM in the silica matrices reached up to 75 wt%.     

Pore size control of porous silica by sol-gel process

Porous silica glass was prepared by sol-gel process from tetraethoxysilane (TEOS). The effects of solvents (water, ethanol), drying condition, heat treatment temperature on specific surface area and pore size distribution of porous silica were investigated. Gelation process accelerates with an increase of H₂O content, while retards with the increase of ethanol amount. Structure changes during heat treatment were studied by means of DTA, XRD. TEM micrographs show that the gel particles shrink after heated at 500°C, thus the average pore size decreases.     

Synthesis of mesoporous silica particles with controlled pore structure

Silica particles, with controllable porosity, were synthesized using two different precursors, tetraethylortosilicate (TEOS) and sodium silicate, but without the addition of template. Characteristics of silica particles (aggregates) prepared by these two methods were compared. The pore structure was tuned only by changing the processing parameters, such as precursor concentration, base concentration, temperature and reaction time. The pore structure of prepared silica particles (aggregates) is strongly influenced by processing conditions and easy controllable in broad range of the specific surface area, pore size, size distribution and pore volume. However, the silica particles synthesized from TEOS have very low total pore volume (ranging from 0.06 to 0.2 cm³/g) and a large portion of pores smaller than 4 nm. On the other side, the silica particles prepared from sodium silicate can be defined as a mesoporous silica with the average pore size up to 20 nm and much higher total pore volume (ranging from 0.8 to 1.5 cm³/g), which are important advantages for their application in encapsulation of enzymes.     

Silicon–titanium oxycarbide glasses as bimodal porous inorganic membranes

Porous oxycarbide Si–Ti–O–C glasses have been studied as potential materials for inorganic membranes. Such materials were prepared by pyrolysis of hybrid materials in nitrogen atmosphere. These hybrids were synthesized by the sol–gel process using tetraethylortosilicate (TEOS), polydimethylsiloxane (PDMS) and titanium orthotitanate (TBOT) as raw material. The influence of the TEOS/TBOT molar ratio on the pore size distribution has been studied in the range of pyrolysis temperatures between 400 and 1100 °C. The oxycarbide materials were characterized by FT-IR and NMR spectroscopies, XRD, mercury porosimetry, nitrogen adsorption and SEM. Bimodal pore size distributions showed one mode close to 0.02 μm and the other one in the range between 1 and 100 μm of pore diameter. Such pore sizes increase with the amount of TBOT. Reduced effective diffusivities were calculated by a theoretical model taking into account the mentioned pore size distributions. Diffusivities appeared in the range from 0.46 to 0.77 and increase with the titanium concentration in the oxycarbide.     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

Mesostructured silica for the reinforcement and toughening of rubbery and glassy epoxy polymers

Mesoporous forms of silica with wormhole framework structures prepared from tetraethylorthosilicate (denoted MSU-J-TEOS) or from sodium silicate (denoted MSU-J-SS) and an amine surfactant as the structure-directing porogen are highly effective reinforcing and toughening agents for rubbery and glassy epoxy polymers. The improvements in tensile strength and modulus provided by MSU-J silicas with a large average framework pore size (e.g., 5.9 and 21.3 nm) are superior to those provided by the corresponding silicas made from the same TEOS or SS precursors but with smaller framework pore sizes (e.g., 4.2 and 5.2 nm). The improved performance of the larger pore structures is realized even though the surface areas (∼670 m²/g) are substantially lower than the surface areas of the smaller pore analogs (812-1025 m²/g), most likely, because of more efficient polymer impregnation of the particle mesopores. In comparison to the MSU-J-TEOS silica assembled from TEOS, MSU-J-SS silica made from SS exhibits a more uniform pore distribution and smaller particles. These latter textural features lead to improved tensile strength and modulus without compromising the strain-at-break for both rubbery and glassy epoxy polymers. The exceptional strength and toughness provided by MSU-J-SS silica in comparison to MSU-J-TEOS silica are correlated with the high degree of dispersion of the mesophase particles in the epoxy matrix for the more effective distribution of stress and the deflection of microcracks.     

Effect of HF and NaOH etching on the composition and structure of SiOC ceramics

Porous SiOC ceramics were prepared with tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTES) as sol?gel precursors, and followed by etching with HF and NaOH solution. The microstructure evolution and chemical etching as a function of pyrolysis temperature were investigated. The amorphous carbon increases as rising the temperature from 800 ^oC to 1200 ^oC, and the graphitic carbon increases with further etching by HF and NaOH. However, the effect of pyrolysis temperature on the structure of C is more significant. The hydroxylation reaction and phase separation of SiOC ceramics results in the increase of SiO₄ unit, which reacts with HF and NaOH to form micro- and mesopores. The existence of mesopore after HF etching provides more specific surface area and pore volume. However, NaOH etching produces more micropores, and the contribution of micropores to specific surface area and pore volume is higher than that of mesopores. Although HF and NaOH etching increase the specific surface area of SiOC ceramics, the etching effect of NaOH is superior to that of HF etching, and the carbon-enriched SiOC ceramics are obtained after NaOH etching.     

Porous Silicon Oxycarbide Glasses

High-surface-area silicon oxycarbide gels and glasses were synthesized from mixtures of methyldimethoxysilane (MDMS) and tetraethoxysilane (TEOS) through acidic hydrolysis and condensation. A surface area of ∼275 m²/g and an average pore size of ∼30 Å was obtained for a 50% MDMS-50% TEOS glass at 800°C under a flowing argon atmosphere. The average pore size was increased by aging the precursor gels in ammonium hydroxide. The increased average pore size and the higher strength of the mesoporous gel network enhanced the surface-area stability of the glasses; in this case, surface areas >200 m²/g were retained at 1200°C under an argon atmosphere. ²⁹Si MAS NMR spectra revealed that an oxycarbide structure was established in the mesoporous glasses obtained after pyrolysis of the aged gels. The role of carbon was demonstrated by comparing the surface-area stability of the oxycarbide glasses with that of pure silica and that of oxycarbide glasses where all the carbon groups were removed through low-temperature plasma-oxidation treatments. In the absence of carbon, the thermal stability of the surface area decreased dramatically.     

Simultaneous control of rod length and pore diameter of SBA-15 for PPL loading

Mesoporous silica materials are attractive materials for immobilizing enzymes because of their well-ordered structures, large surface area are pore volume. Diffusion of large enzyme molecules such as porcine pancreatic lipase (PPL) through the lengthy channels of MPS takes place too slowly. Therefore, the squat of the enzyme at the pore mouth entrance, actually makes the rest of the channel useless. In this study, to overcome this problem, synthesis parameters of SBA-15 were changed, since along with pore diameter increasing, the mesochannel length becomes shorter. The main point to obtain a well-ordered 2D hexagonal pore structure was the pre-hydrolysis of tetraethyl orthosilicate (TEOS) before the addition of 1,3,5-trimethyl benzene as a micelle swelling agent. Due to the strong effect of zirconium in changing the morphology of SBA-15 particles, we modified SBA-15 in the presence of a small amount of ZrOCl₂ in the synthesis solution under acidic conditions. As a result, mesochannel length of SBA-15-Zr was shortened from 600 to <200 nm. The morphology of mesoporous silica was also changed from rod-like to platelet, because of the accelerating effect of Zr(IV) on the self-assembly rate of P123 and TEOS condensation. Characteristic results conducted by low angle XRD, high resolution transmission electron microscopy and nitrogen adsorption, confirmed tuning effect of Zr(IV) in SBA-15. Furthermore, it was shown that the number of pore entrances increases with decreasing the length of SBA-15 mesochannels, leading to obvious improvement of enzyme uptake. PPL has been successfully immobilized in the mesoporous channels of SBA-15-Zr. The total amount of lipase adsorbed on the mesoporous SBA-15-Zr was measured by thermal gravimetric analysis. The largest PPL adsorption capacity was 784 mg/g belonging to the SBA-15-Zr with the length of 150 nm and the mean pore size diameter of 9.22 nm.     

单分散大粒径介孔二氧化硅微球的制备及表征

以正硅酸四乙酯(TEOS)为硅源、N,N-二甲基甲酰胺(DMF)为乳化致孔剂,采用酸催化法得到部分缩合的聚硅酸乙酯(PES),再经碱进一步催化得到介孔二氧化硅微球。分析了酸催化剂用量、旋蒸温度对PES粘度及聚合程度的影响以及PES粘度对二氧化硅微球粒径的影响,并研究了氨水用量、乳化致孔剂种类、乳化致孔剂用量以及搅拌速度和后处理方式对二氧化硅微球性质的影响。利用扫描电子显微镜、旋转粘度计、氮吹吸附仪、傅里叶变换红外光谱仪等对所得微球进行表征。结果表明,所制备的介孔二氧化硅微球球形完整,粒径分布均匀,纯度高,微球平均孔径为10.164 4 nm,孔体积为1.023 023 cm³/g,比表面积为396.528 1 m²/g。     

Surface free energies and steam stability of methyl-modified silica membranes

Methyl-modified silica membranes have been prepared by acid-catalyzed co-hydrolysis and condensation reactions of tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES). The surface wettability, micro-structure and gas permeance of the methyl-modified silica membranes were investigated. The values of dispersion force γ_S^d, dipole force γ_S^P and hydrogen bonding force γ_S^h to the surface tensions for the silica membranes were evaluated by the extended Fowkes equation. The surface free energy and surface wettability of the silica membranes decrease greatly with the increasing of MTES/TEOS ratio mainly because of the contribution of hydrogen bonding force. FTIR analysis shows that the reason is the increase of Si–CH₃ group amount and the decrease of O–H group amount on the surfaces of silica membranes. After aging in steam circumstances, the increase of surface free energies results from the increase of O–H amount present on the silica membrane surfaces. The methyl ligands can make the mean pore size and total pore volume of silica membrane larger. Compared with the unmodified silica membrane, the gas permselectivities of the MTES-modified silica membranes have no obvious decrease in despite of the greatly increase of gas permeation rates. As the silica membranes are aged in steam circumstances, the decrease of gas permeation rates in the silica membrane with MTES/TEOS = 0 is far more than that in the silica membrane with MTES/TEOS = 0.8 while their H₂/CO₂ selectivities have no notable change.     

纳米孔超级绝热材料气凝胶的制备与热学特性

以正硅酸四乙酯(TEOS)为硅源，通过溶胶–凝胶及超临界干燥过程制备了SiO2气凝胶. 同时，采用相对廉价的多聚硅(E–40)为硅源，以三甲基氯硅烷(TMCS)为表面修饰剂，硅油为干燥介质, 在常压条件下制备了同样具有纳米多孔结构的SiO2气凝胶. 用透射电镜、扫描电镜及孔径分布仪对其结构进行了表征，并用动态热线法对其热学特性进行了测试. 结果表明: 两种方法制备的气凝胶均是典型的纳米孔超级绝热材料，后者热导率略高但成本低许多，所以更具应用推广潜力.     

Synthesis and characterization of porous media produced by a sol-gel method

Mesoporous silica materials were synthesized by sol-gel method using tetraethoxysilane (TEOS) as precursors and surfactants i.e., cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyoxyethylene cetyl ether (Brij 56) as templates. Surfactant templates were completely removed by calcination to form mesoporous structure. The effects of type and amount of surfactants on the characteristics of samples were studied. The textural characteristics such as surface area, pore volume, pore size, and pore size distribution were determined by nitrogen sorption isotherms. Fourier transform infrared (FTIR) spectroscopy was employed to qualitatively identify the chemical functionality and to confirm the removal of surfactant template. Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to directly observe surface morphology and mesoporous structure, respectively. The adsorption capacity of the synthesized adsorbent for toluene vapor was examined. We found that the pore volume and pore size of mesoporous materials affected the adsorption capacity. The sample prepared with high content of CTAB under basic condition (pH ∼7) yielded large pore volumes and pore sizes and subsequently possessed the high adsorption capacity for toluene vapor.     

Cylindrical pore arrays in silicon with intermediate nano-sizes: A template for nanofabrication and multilayer applications

An electrochemical technique for the fabrication of intermediate nano-sized pores in silicon (50-200) nm in aqueous HF solution is reported. Attempts to achieve similar nano-sized pores in the presence of organic solvents are also presented. Mesopore formation beginning with pore sizes <50 nm followed by pore widening in alkaline etch solution is also attempted. The desired pore size is obtained with a KMnO₄ oxidizing agent in the presence of surfactant. Our findings reveal that the applied current density during the fabrication process is a decisive factor. The concentration of HF and KMnO₄ should be controlled within appropriate ranges. A multilayer with two different pore morphologies is fabricated on the same substrate using the same electrolyte simply by changing the applied anodic current. Application of such cylindrical pore arrays as templates for polymer nanofabrication is demonstrated. By selective dissolution of the silicon membranes, we successfully obtained nanowires of polypyrrole (PPy) with ∼100 nm diameter size. The fabrication process, the electrochemical measurements, the formation mechanism and the different pore morphologies are investigated in detail.     

Physicochemical Characterization and Drug Release Properties of Methyl-Substituted Silica Xerogels Made Using Sol–Gel Process

In this work, a multi-analytical approach involving nitrogen porosimetry, small angle neutron and X-ray scattering, Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopies, X-ray diffraction, thermal analysis and electron microscopy was applied to organically modified silica-based xerogels obtained through the sol–gel process. Starting from a tetraethoxysilane (TEOS) precursor, methyltriethoxysilane (MTES) was added to the reaction mixture at two different pH values (2.0 and 4.5) producing hybrid xerogels with different TEOS/MTES molar ratios. Significant differences in the structure were revealed in terms of the chemical composition of the silica network, hydrophilic/hydrophobic profile, particle dimension, pore shape/size and surface characteristics. The combined use of structural characterization methods allowed us to reveal a relation between the cavity dimensions, the synthesis pH value and the grade of methyl substitution. The effect of the structural properties on the controlled Captopril release efficiency has also been tested. This knowledge facilitates tailoring the pore network for specific usage in biological/medical applications. Knowledge on structural aspects, as reported in this work, represents a key starting point for the production of high-performance silica-based hybrid materials showing enhanced efficacy compared to bare silica prepared using only TEOS.     

Influence of Catalyst (Citric Acid) Concentration on the Physical Properties of TEOS Silica Aerogels

In the present paper, the experimental results on the influence of catalyst (citric acid) concentration on the physical properties of TEOS silica aerogels, are reported. The aerogels have been prepared by hydrolysis and polycondensation of tetraethoxysilane (TEOS) using citric acid (CTA) as a new catalyst followed by supercritical drying in an autoclave. In order to obtain the best quality silica aerogels in terms of monolithy, high transparency, low density, large surface area and high porosity with uniform pore size distribution, the catalyst concentrations were varied from 0.0005 M to 0.1 M by keeping the molar ratio of TEOS : EtOH : H₂O constant at 1 : 5 : 7, respectively. It has been found that the lower (<0.001) CTA concentration resulted in low density, smaller surface area but opaque aerogels whereas higher (>0.005 M) CTA concentration resulted in high density, large surface area, highly transparent but cracked aerogels. On the other hand, medium (between 0.001 and 0.005 M) CTA concentration resulted in monolithic, low density, large surface area and highly transparent silica aerogels. The pore size distribution (PSD) for higher (0.1 M) and lower (0.0005) CTA concentrations shifted towards smaller and larger pore radii respectively, whereas for medium (0.001 M) CTA concentration, the PSD is narrow and uniform, which reduces the differential pressure during supercritical solvent extraction leading to monolithic silica aerogels. These results have been supported and discussed by considering the particle and pore sizes observed by Scanning Electron Microscopy (SEM). The surface area was measured by BET analysis.     

Microstructural Characterization of Porous Silicon Carbide Membrane Support With and Without Alumina Additive

Porous silicon carbide (SiC) membrane supports sintered at 1500°–1800°C were prepared by cold isostatic pressing (CIP) under different pressures and using different amounts of alumina additive (0%–4%). The relationship between processing factors and pore size and microstructure was examined. Varying the sintering temperature, the CIP pressure and the amount of additive used were found to be effective for controlling pore size and microstructure. The pore size and particle size of the membrane support prepared without alumina were found to increase with increasing sintering temperature. This was attributed to surface diffusion. Densification of the undoped support did not occur, however, because of concurrent pore development. In the SiC membrane support containing 4% alumina, small particles and a pore size of around 100 nm were retained. This was because of the formation of a limited amount of SiO₂–Al₂O₃ liquid phase during sintering.     

Effects of organic templates on the structural properties of porous clay heterostructures: a non-micellar template model for porous structure

In this study, a series of surfactants with different chain lengths were used to investigate the effect of organic templates on the structural properties of porous clay heterostructures (PCH). The variation tendencies of structural properties and chemical component were characterized by X-ray diffraction, CHN elemental analysis, major element analysis, and nitrogen adsorption/desorption at ?196 °C. Non-local density functional theory was employed to characterize pore size distributions in both micropore and mesopore regions owing to the bi-model pore structures of PCH. The major textural parameters of PCH samples increased with increasing the chain length and the intercalated amount of template agent, while the most probable pore size remained unchanged around 1.3–1.4 nm. Based on the structural and chemical component results of PCH, a non-micellar template model was presented to explain the formation mechanism of porous structure. By the extrusion and occupation of polymerizate of TEOS within interlayer, a fixed number of hydrocarbon chains congregate together to form surfactant aggregates (template). The aggregates with different chain lengths cause the increase of pore volume in depth dimension rather than in diameter. The obtained results also shed some light on the different roles of inorganic base clay and organic template agents in the formation of porous structure. Montmorillonite as the inorganic host supplies the space for pillaring reaction and controls the arrangement of template agent within interlayer. As the template agent and modifying agent, the cationic surfactant is the dominant factor in forming pores relative to amine. The neutral amine mainly serves as catalyst for the polymerization of TEOS besides its co-template effect.     

The Influence of the Amount of Fluoride Catalyst on the Morphological Properties of the Anilinepropylsilica Xerogel Prepared in Basic Medium

Anilinepropylsilica hybrid powder was synthesized by a sol-gel route in basic medium using HF as catalyst. The effect of the amount of HF catalyst on the morphologies and on organic content was studied. The xerogels were characterized using FTIR, SEM and N₂ adsorption-desorption isotherms. The increase in the HF quantity results in an slight increase in organic content and changes in the particle size, surface area and pore structure.     

Ionomer-silica hybrids via sol-gel reaction

Ionomer-silica hybrid materials were made from polyethylene-co-acrylic acid neutralized by a zinc salt (PI) and tetraethoxy silane (TEOS) via the sol-gel reaction. The effects of various experimental parameters such as solvents, H₂O/Si ratio and the amount of TEOS in the ionomer solution on the hybrid structure and properties were examined. The spectroscopic results show that solvents do not affect the structure of the hybrids, but influence the thermal properties. The hybrids made using highly polar solvent exhibit better thermal stability and dynamic mechanical properties at high TEOS contents. The amount of water used for hydrolysis and subsequent condensation play a significant role in the network formation. The varying amount of TEOS in solutions gives rise to different silica content of the hybrid. Above 50 wt%, the sample becomes opaque due to silica aggregation. The high ratio of H₂O/Si leads to phase separation during the reaction. Transparent hybrid materials can only be obtained when the ratio of H₂O/Si is below 5.