Theoretical study on the effective thermal conductivity of silica aerogels based on a cross-aligned and cubic pore model

Aerogel nanoporous materials possess high porosity, high specific surface area, and extremely low density due to their unique nanoscale network structure. Moreover, their effective thermal conductivity is very low, making them a new type of lightweight and highly efficient nanoscale super-insulating material. However, prediction of their effective thermal conductivity is challenging due to their uneven pore size distribution. To investigate the internal heat transfer mechanism of aerogel nanoporous materials, this study constructed a cross-aligned and cubic pore model(CACPM) based on the actual pore arrangement of SiO_(2) aerogel. Based on the established CACPM, the effective thermal conductivity expression for the aerogel was derived by simultaneously considering gas-phase heat conduction, solid-phase heat conduction, and radiative heat transfer. The derived expression was then compared with available experimental data and the Wei structure model. The results indicate that, according to the model established in this study for the derived thermal conductivity formula of silica aerogel, for powdery silica aerogel under the conditions of T = 298 K, a_(2)= 0.85, D_(1)= 90 μm, ρ = 128 kg/m^(3), within the pressure range of 0–10^(5)Pa, the average deviation between the calculated values and experimental values is 10.51%. In the pressure range of 10^(3)–10^(4)Pa, the deviation between calculated values and experimental values is within 4%. Under these conditions, the model has certain reference value in engineering verification. This study also makes a certain contribution to the research of aerogel thermal conductivity heat transfer models and calculation formulae.

Nitrogen-doped hierarchical porous carbon from polyaniline/silica self-aggregates for supercapacitor

In this paper, nitrogen-doped hierarchical porous carbon(N-HPC) was prepared from polyaniline(PANI)/silica self-aggregates. H-bonding between N\\H groups in aniline/PANI and \\OH groups in nano silica template led to a self-assembly type, which enabled the formation of uniform N-HPC nanoparticles. Silica self-aggregates provided macroporous channels resulted in a decreased diffusion distance. After removing the hard template,the N-HPC had a high surface area(899 m^2·g^(-1)). Owing to two co-existed synergetic energy-storage mechanisms and the hierarchical porous structure, the obtained N-HPC exhibited a high specific capacitance of 218.75 F·g^(-1) at 0.5 A·g^(-1), compared with the nonporous nitrogen-doped carbon(N-C) derived from pure PANI. Moreover, the N-HPC electrode demonstrated excellent cycle life, retaining 99% of its initial specific capacitance after 1000 cycles.

Porous Properties of Nano-fibriform Silica from Natural Chrysotile

With the TEM and physical gas adsorption techniques, porous properties of nano-ribriform silica （MLD： 92.73%） from natural chrysotile are studied in this paper. The results indicate that porous nano-fibriform silica results from brucite octahedral sheets of nature chrysotile dissolved completely and Si-O tetrahedral sheets collapsed by acid leaching. Its length is at a micron or nanometer scale. There are two types of pores： pores among neighboring fibers and pores in nanoriber. These pores （less than 6.5 nm in diameter, mostly 2.1 nm and 3.8 nm） all belong to mesopores. The pores in fibers consist of those among SiO2 particles, those among aggregates, remnant nanotubes and capillary tubes. Nanoribriform silica proves better than the traditional silica as a carrier of catalyzer and a filler for reinforce rubber and plastics.

Poly(NIPAM-co-MPS)-grafted multimodal porous silica nanoparticles as reverse thermoresponsive drug delivery system

Hybrid drug delivery systems(DDS) have been prepared by grafting poly(NIPAM-co-MPS) chains on multimodal porous silica nanoparticles having an inner mesoporous structure and an outer thin layer of micropores. The hybrid thermoresponsive DDS were fully characterized and loaded with a model drug. The in vitro drug release tests are carried out at below and above the lower critical solution temperature(LCST) of the copolymer. The results have revealed that due to the presence of small diameter(~1.3 nm) micropores at the periphery of the particles, the collapsed globules of the thermoresponsive copolymer above its LCST hinders the complete release of the drug which resulted in a reverse thermoresponsive drug release profile by the hybrid DDS.

Production of porous silica by the combustion of rice husk ash for tundish lining

In order to study the production of porous silica compacts by the combustion of rice husk ash （RHA） for tundish lining, the experimental design technique was used to evaluate the effect of firing temperature, soaking time and compaction pressure on controlling both the porosity degree and compressive strength of rice husk ash compacts. The results revealed that while the porosity degree of the compacts decreased with the increase in the entire studied parameters, the compressive strength exhibited another trend especially at a lower soaking time. At a lower soaking time, the increase in firing temperature led to a slight decrease in compressive strength and then increasing thereafter. The porous silica compacts having 30% porosity and 〉 2.5 MPa compressive strength suitable for tundish lining could be obtained from the combustion of rice husk ash compacts.

Preparation of Scratch-Resistant Nano-Porous Silica Films Derived by Sol-Gel Process and Their Anti-reflective Properties

Structural strengthening of the nano porous silica films has been reported. The films were prepared with a base/acid two-step catalyzed TEOS-based sol-gel processing and dip-coating, and then baked in the mixed gas of ammonia and water vapor. The silica films were characterized with TEM, AFM, FTIR, spectrophotometer, ellipsometer, and abrasion test, respectively. The experimental results have shown that the films have a nanostructure with a low refractive index and can form an excellent scratch-resistant broadband anti-reflectance. The two-step catalysis noticeably strengthens the films, and the mixed gas treatment further improves mechanical strength of the silica network. Finally the strengthening mechanism has been discussed.

Exploring the potential of porous silicas as a carrier system for dissolution rate enhancement of artemether

Malaria is a parasitic and vector determined blood-conceived infectious disease transmitted through infected mosquitoes. Anti-malarial drug resistance is a major health problem, which hinders the control of malaria. A Results of a survey of drug-resistant malaria demonstrated safe proclivity to nearby all anti-malarial regimes accessible except from artemisinin and its derivatives. Artemether is a BCS class IV drug effective against acute and severe falciparum malaria;hence there is a strong need to improve its solubility. Silica is one of the most widely studied excipients. Silica can be used in solubility enhancement by preparing its solid solution/dispersion with the drug. The objective of this research was to improve dissolution rate of Artemether using non-precipitated porous silica(Aeroperl 300 Pharma) and precipitated silica like EXP. 9555, EXP. 9560, and EXP. 9565. Specific surface area calculated from BET method of porous silicas viz. APL 300(A), Exp. 9555(B), Exp. 9560(C), Exp. 9565(D) was found to be 294.13 m^2/g(A), 256.02 m^2/g(B), 213.62 m^2/g(C) and 207.22 m^2/g(D) respectively.The drug release from the developed formulation was found to be significantly higher as compared to neat ARM. This improved solubility and release kinetics of ARM may be attributed to high surface area, improved wettability and decreased crystallinity. Solid-state characterization of the developed optimized formulation F3 was carried out with respect to FTIR chemical imaging, XRD,SEM, and DSC. All the porous silicas which we have explored in the present context showed a significant capability as a carrier for solubility enhancement of ARM.

Structrue and Characteristics of Mesoporous Silica Synthesized in Acid Medium and Its Reaction Mechanism

Structrue and pore characteristics of the mesoporous silica synthesized in acid medium were studied by means of XRD, HRTEM, BET, FT-IR, DSC-TGA, and the reaction mechanism was also investigated deeply. The results show that mesopores in the sample possess hexagonal arrays obviously, whereas the structure of silica matrix is amorphous. The results also show that the acting mode of silica and CTMA+ inside the mesopores was chemical bonding force. The structure of mesoporous silica was mainly dependent on the aggregational condition of micelle of CTMA+ as well as their liquid-crystallized status. In addition, condensation and dehydration of silicate radicals were accompanied in the process of calcination, which resulted in the mesoporous structure ordered in local range and the pore sizes largening.

Preparation of porous silica from natural chlorite via selective acid leaching and its application in methylene blue adsorption

In this study,porous silica with high surface area was prepared through selective leaching of thermally activated chlorite in HCl solution.In the process,chlorite was activated by pre-calcining treatment,then activated components(MgO,Al_(2)O_(3),and Fe_(2)O_(3))were selectively leached by acid solution,resulting in the formation of nanopores in situ.The morphology,structure,surface area and pore-size distribution of the material were characterized by XRD,TG/DSC,^(27)Al MAS NMR,SEM,TEM and N2 adsorption−desorption isotherms.The highest specific surface area(SBET=333 m^(2)/g)was obtained by selectively leaching the 600℃ calcined chlorite from 3 mol/L HCl at 90℃ for 2 h.The pore sizes and specific surface areas can be controlled by calcination and leaching conditions.The ^(27)Al MAS NMR spectra of the samples revealed the relationship between structural transformation and the selective acid leaching properties of thermal-activated chlorite,demonstrating that AlVI transfers into AlV when chlorite changes into activated chlorite during thermal activation,and the coordinations of Al has a significant effect on acid solubility of chlorite.The as-prepared porous silica showed favorable adsorption abilities with capacity of 148.79 mg/g for methylene blue at pH of about 7 and temperature of 25℃,indicating its promising potential in adsorption application.

A comparative study of drying techniques on the morphology of porous silica gels synthesized via sol-gel process

The effect of drying techniques on the microstructure,morphology and pore structure of porous silica gels was studied in the paper.The gels were prepared by using sol-gel process and different drying routes:freeze-drying (FD),low pressure drying (LPD),high temperature drying (HTD) and chemical modification & ambient drying (CMD) techniques.Observation under pore distribution and structural properties showed that CMD technique leads to homogenous mesoporous silica material with specific surface area of 745 m2/g,and the average pore size around 20 nm,while LPD and HTD result in loosely packed particles with non-isotropic aggregation pattern.The specific surface areas of LPD and HTD samples are 419 and 513 m2/g respectively,and the pore size distribution of the samples are observed distributing widely in range of 10-100 nm.Freeze drying method is a new but prospective way to prepare mesoporous silica.The specific area of FD sample is around 500 m2/g.By the comparison for the properties of the gels,this paper wants to induce a further interest in finding a proper method to synthesize the porous silica gels for low price use.

Fabrication and Characterization of Porous Silica Monolith by Sintering Silica Nanoparticles

This paper aims to fabricate over centimeter size of porous silica monolith having meso-pores with large surface area. A precursor of porous silica monolith was obtained by sintering a monolithic SiO2-poly (vinyl alcohol) (PVA) nanocomposite at 600oC-1100oC. The sintering behavior was examined by means of Raman spectroscopy and a porosimetry. The PVA of the SiO2-PVA nanocomposite was combusted below 600oC, subsequently the silica nanoparticles of the nanocomposite were sintered above 900oC. The Raman spectroscopy suggested that amorphous structure of the porous silica monolith obtained above 1000oC was similar to that of a silica glass. The BET surface area and pore radius of the porous silica monolith decreased with increasing sintering temperature. These values were tailored in the range of ca. 0-291 m2.g-1 and 5-25 nm, respectively, by controlling the sintering temperature and time. The fabricated porous-silica monolith was translucent or opaque porous-silica depending on the pore size.

A novel fabrication of meso-porous silica film by sol-gol of TEOS

A homogeneous crack-free nano- or meso-porous silica films on silicon was fabricated by colloidal silica sol derived by hydrolyzing tetraethyl orthosilicate (TEOS) catalyzing with (C4H9)4N+OH- in water medium. The solution with ratio of H2O/TEOS15, R4N+ and glycerol as templates, combining with the hydrolyzed intermediate, controlled the silica aggregating; the templated silica film with heterostructure was developed into homogeneous nano-porous then meso-porous silica films after being annealed from 750 C to 850 C; the formation mechanism of the porous silica films was discussed; morphologies of the silica films were characterized. The refractive indexes of the porous silica films were 1.2561.458, the thermal conductivity < 0.7 W/m/K. The fabricating procedure and the sequence had not been reported before.

Kinetic Resolution of DL-Phenylalanine Methyl Ester by α -Chymotrypsin ImmobiIized on Porous Silica Beads

Kinetic resolution of DL-phenylalanine methyl ester was carried out using drimobilized α -chymotrypsin (IC) as catalyst. The effects of temperatUre, pH, concentration of substrate and reactionvessels on the resolution were investigated. High quality L-phenylalanine was obtained in good yieldby an IC column.

Sol-Gel-Derived Porous Silica: Economic Synthesis and Characterization

Porous silica was synthesized via the sol-gel process using clay obtained locally from Ijero-Ekiti in Ekiti State, Nigeria and compared with silica synthesized under similar conditions from sodium metasilicate (Na2SiO3) obtained comer- cially. The clay was initially refluxed with sodium hydroxide (NaOH) for 2 hours to extract SiO2 to form Na2SiO3, which was subsequently hydrolyzed to form a gel. The gel obtained was washed with deionized water to get rid of im-purities, dried and calcined at 800°C for 3 hours. The obtained silica powders were characterized using atomic absorp-tion spectrophotometer, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that the vibrational modes and diffraction patterns of the silica derived from com-mercial Na2SiO3 and that prepared from clay were similar containing pure amorphous SiO2. The morphology of the commercially obtained silica showed better arrangement of particles and exhibited slightly lesser porosity (62.4%) compared to that derived from clay which had a porosity of 65.5%. The result indicates that clay has a potential for use as an environmentally safe and economic starting material for preparing porous silica instead of high quality precursors.

Micromechanical interlocking-inspired dendritic porous silicabased multimodal resin composites for the tooth restoration

Porous silica particles have shown great potential application as reinforcing fillers in the field of dentistry due to their ability to construct the micromechanical interlocking effect at filler-matrix interface.However,how to accurately regulate the pore structure,especially the pore size,to increase the degree of the micromechanical interlocking and the performance of materials remains a challenge.Herein,we have proposed a facile self-assembly process to synthesize dendritic porous silica with tunable pore sizes(DPS-x)by adjusting the chain-length of the alcohols in the microemulsion.The mechanism of nucleation-growth is further put forward.The results indicate that the pore size of DPS-x indeed affects the mechanical property of composites,where the DPSpen particles with intermediate pore size are chosen as the optimal reinforcing fillers.The bimodal and multimodal filler formulations are further established to address the loading limitation of unimodal DPS-pen(46 wt.%).In virtue of the closepacked structure of identical spheres,the particle sizes of secondary silica embedded into the maximally loaded bimodal D3S7 composite(DPS-pen:Si430=30:70,w/w)are theoretically calculated without trials.Among all formulations,the developed multimodal D3S7+Si178+Si90 filler exhibits superior mechanical properties,the lowest shrinkage,and high polymerization conversion for dental composites,along with satisfied waster sorption and solubility,and good biocompatibility in vitro and in vivo,which are comparable to commercial composite Z350 XT(3M,USA).These DPS-x particles and their multimodal fillers can also be applied to other polymer-based biomaterials.

Provitamin D Doped Silica and Polymeric Films: New Materials for UV Biosensor

The original technologies of growing silica films, impregnated with 7-dehydrocholesterol (provitamin D3) on quartz substrates and free transparent films on the basis of polyvinyl alcohol and polyvinyl butyral have been developed. Provitamin D photoisomerization in the films under UVB irradiation was investigated by UV absorption spectroscopy. Remarkable changes in the absorption spectrum of 7-DHC were observed in silica and polyvinyl alcohol films as com-pared with ethanol solution, only in polyvinyl butyral film the spectrum was very nearly, while the spectral kinetics of 7-DHC photoisomerization in all the films was different from ethanol. We suggest that several films have potential as UV dosimeters to measure accumulated ‘antirachitic’ UV dose in the same manner as erythemic UV dose is measured by commonly used polysulphone film.

Single-step thermal carburization synthesis of supported molybdenum carbides from molybdenum-containing methyl-silica

A novel synthesis route to obtain highly dispersed molybdenum carbides in porous silica is described. The synthesis was carried out by a single-step heat treatment of molybdenum-containing and methyl-modified silica （Mo-M-SiO2） in argon atmosphere at 973 K. Mo-M-SiO2 precursor was facilely obtained via a one-pot synthesis route, using （NH4）6Mo7O24 4H2O （AHM） as molybdenum sources and polymethylhydrosiloxane （PMHS） as silica sources at the initial synthetic step. The optimal C/Mo molar ratio in reaction system for complete carburization of molybdenum species was 7. The carburization process of molybdenum species followed a nontopotactic route involving a MoO2 intermediate phase, which was evidenced by XRD, N2 adsorption-desorption and in situ XPS. Formation mechanism of Mo-M-SiO2 precursor was also proposed by observation of the reaction between AHM and PMHS with TEM. Furthermore, by adding TEOS into silica sources and adjusting TEOS/PMHS mass ratio, crystal phase of molybdenum carbides transferred from β-Mo2C to α-MoC1-x, and SiO2 structure changed from microporous to micro/mesoporous. Catalytic performances of samples were tested using CO hydrogenation as a probe reaction. The supported molybdenum carbides exhibited high selectivity for higher alcohol synthesis compared with bulk β-Mo2C and α-MoC1-x.

Preparation of Magnesia Insulation Materials by Walnut Shell Powder Impregnated with Silica Sol

In order to reduce the thermal energy loss of high temperature kilns and furnaces and lower the surface temperature of the kiln body,magnesia insulation materials were prepared using self-made magnesia porous aggregates(using high purity magnesia powder as starting material and potassium oleate as the foaming agent),middle grade magnesia powder,calcium aluminate cement,and SiO_(2) micropowder as starting materials,introducing walnut shell powder impregnated with silica sol(short for Sws)as a pore-forming agent.The effects of the Sws addition(0,10%,15%,and 20%,by mass)and the sintering temperature(1300,1350,1400,and 1480℃)on the properties of magnesia insulation materials were studied.The results show that(1)for the specimens fired at 1480℃,when the Sws addition is 10%,the cold compressive strength is 22 MPa;when the Sws addition is 20%,the thermal conductivity is 0.368 W·m^(-1)·K^(-1)(350℃);(2)nano-silica in the silica sol reacts with MgO in the matrix to form forsterite,which encapsulates the pores volatilized from the walnut shell powder and forms closed pores.

STUDY ON THE STRUCTURE OF ARTIFICIAL SILICA FUME AND THE ACTIVITY OF VOLCANIC ASH

The present experimental technique is used to study the image, crystal phase, pore structure and microstructure of artificial silica fume, and the experimental results show that artificial silica fume is a non-crystalline and porous matter with a high specific surface area and the high activity of volcanic ash. Therefore, it is a type of ideal material which can replace silica fume. (Author abstract) 5 Refs.

sol-gel法制备多孔纳米SiO_2薄膜

以NH3·H2O为催化剂,用正硅酸乙酯(TEOS)溶胶–凝胶工艺制备纳米多孔二氧化硅薄膜.强碱催化使二氧化硅胶粒溶解度增大并增大了体系的离子强度;丙三醇的加入,与水解中间体结合,有效抑制了二氧化硅溶胶胶粒的长大;PVA(聚乙烯醇)对二氧化硅胶粒有强的吸附作用,使胶粒聚联成大的网络结构,增加了成膜性能.通过改变反应物的剂量,调节添加剂的用量,可以制得折射率1.18～1.42,对应孔率60.50%～7.75%并且孔径小于100 nm的纳米多孔二氧化硅薄膜,.单层薄膜厚度在3 μm以内精确可控.