共查询到20条相似文献,搜索用时 46 毫秒
1.
Uzma K. H. Bangi M. S. Kavale Seungsu Baek Hyung-Ho Park 《Journal of Sol-Gel Science and Technology》2012,62(2):201-207
The successful incorporation of multiwalled carbon nanotubes (MWCNTs) into silica aerogels prepared by sol–gel method is reported
herein. Pure silica aerogels prepared using sodium silicate precursor by ambient pressure drying are so fragile that they
cannot be used easily. MWCNTs were used as reinforcements to improve the mechanical properties of silica aerogels. Results
show that inserting small amounts of MWCNTs in the gels causes enhanced dimensional stability of silica aerogels. The silica
aerogels were prepared by doping MWCNTs in silica matrix before gelation. The influence of MWCNTs on some microstructural
aspects of silica matrix has been studied using nitrogen adsorption–desorption isotherms. From SEM study it is confirmed that
the silica particles get capped on the surface of MWCNTs suggesting an enhanced toughness. Further, FTIR, Raman, EDAX, thermal
conductivity and hydrophobicity studies of these doped aerogels were carried out. By addition of MWCNTs, silica aerogels were
formed with 706 m2/g BET and 1,200 m2/g Langmuir surface areas and 149o contact angle. Low density (0.052 g/cc) and low thermal conductivity (0.067 W/m K) MWCNTs doped silica aerogels were obtained
for the molar ratio of Na2SiO3::H2O::MWCNTs::citric acid::TMCS at 1::146.67::2.5 × 10−3::0.54::9.46 respectively with improved mechanical strength. 相似文献
2.
Ann M. Anderson Mary K. Carroll Emily C. Green Jason T. Melville Michael S. Bono 《Journal of Sol-Gel Science and Technology》2010,53(2):199-207
Hydrophobic silica aerogels have been prepared using the rapid supercritical extraction (RSCE) technique. The RSCE technique
is a one-step methanol supercritical extraction method for producing aerogel monoliths in 3 to 8 h. Standard aerogels were
prepared from a tetramethoxysilane (TMOS) recipe with a molar ratio of TMOS:MeOH:H2O:NH4OH of 1.0:12.0:4.0:7.4 × 10−3. Hydrophobic aerogels were prepared using the same recipe except the TMOS was replaced with a mixture of TMOS and one of
the following organosilane co-precursors: methytrimethoxysilane (MTMS), ethyltrimethoxysilane (ETMS), or propyltrimeth-oxysilane
(PTMS). Results show that, by increasing the amount of catalyst and increasing gelation time, monolithic aerogels can be prepared
out of volume mixtures including up to 75% MTMS, 50% ETMS or 50% PTMS in 7.5–15 h. As the amount of co-precursor is increased
the aerogels become more hydrophobic (sessile tests with water droplets yield contact angles up to 155°) and less transparent
(transmission through a 12.2-mm thick sample decreases from 83 to 50% at 800 nm). The skeletal and bulk density decrease and
the surface area increases (550–760 m2/g) when TMOS is substituted with increasing amounts of MTMS. The amount of co-precursor does not affect the thermal conductivity.
SEM imaging shows significant differences in the nanostructure for the most hydrophobic surfaces. 相似文献
3.
Digambar Y. Nadargi Sanjay S. Latthe A. Venkateswara Rao 《Journal of Sol-Gel Science and Technology》2009,49(1):53-59
The properties of silica aerogels are highly dependent on the post-treatment steps like gel washing, gel aging and gel drying.
The experimental results of the studies on one of the post-treatment steps i.e. gel aging effect on the physical and microstructural
properties of methyltrimethoxysilane (MTMS) based silica aerogels, are reported. These hybrid aerogels were prepared by two
step sol–gel process followed by supercritical drying. The molar ratio of MeOH/MTMS (M) was varied from 7 to 35 by keeping
the H2O/MTMS (W) molar ratio constant at 4. The as prepared alcogels of different molar ratios were aged from 0 to 5 days. It was
observed that 2 days of gel aging period is the optimum gel aging period for good quality aerogels in terms of low density,
less volume shrinkage and high porosity. The well tailored network matrix with low density (0.04 g/cm3), less volume shrinkage (4.5%), low thermal conductivity (0.05 W/mK) and high porosity (98.84 %) was obtained for 2 days
of gel aging period of M = 35. Further, the gelation time varied from 8 to 1 h depending on the M values. The gelation time was being more for lesser
M values. The aerogels were characterized by bulk density, porosity, volume shrinkage, thermal conductivity, Scanning Electron
Microscopy and the Fourier Transform Infrared spectroscopy. 相似文献
4.
A. Parvathy Rao A. Venkateswara Rao Uzma K. H. Bangi 《Journal of Sol-Gel Science and Technology》2008,47(1):85-94
The experimental results on the preparation of low thermal conductivity and transparent ambient pressure dried silica aerogels
with the sodium silicate solution, TMCS silylating agent with methanol, isopropyl alcohol, hexane and xylene solvents, are
reported. This study is focussed on the effect of preparation conditions such as varying the number of preparation steps,
pH of the hydrosol and hydrogel ageing temperature, for the production of the low thermal conductive silica aerogels and the
results are analysed. Density, thermal conductivity, % of optical transmission and contact angle of the aerogels were measured.
The Fourier Transform Infrared Spectroscopy (FTIR) studies revealed the presence of Si–C and C–H along with the Si–O–Si and
OH bonds and their intensities strongly depend on the processing steps, pH of the hydrosol and gel ageing temperature. The
UV–Visible spectra indicated the % of optical transmission of the aerogels decreased with increasing the number of processing
steps, increase in the pH of the hydrosol from 3 to 8 and decreased for ageing temperature up to 50 °C. Further increase in
temperature >50 °C, the % of optical transmission of the aerogels increased. The TGA-DTA data showed the thermal stability
of the aerogels with respect to hydrophobicity is 325 °C. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy
(SEM) analyses revealed the nanostructure of the aerogels. The porosity of the aerogels was studied using the pore size distribution.
Silica aerogels with low density (0.051 g/cc), low thermal conductivity (0.049 W/m K), optical transmission (65%), high hydrophobicity
(159°) and resistance to humid atmosphere >1 year was obtained in the present studies. 相似文献
5.
A. Venkateswara Rao Uzma K. H. Bangi Sunetra L. Dhere Hiroaki Imai Hiroshi Hirashima 《Journal of Sol-Gel Science and Technology》2011,57(1):95-100
The effect of iron acetylacetonate on the physico-chemical properties of waterglass based silica aerogels by ambient pressure
drying has been investigated. Doping the gels with iron acetylacetonat (FeAA) facilitates in the diminution of the density
of the aerogels. The well established silica network provides effective confinement of FeAA nanoparticles which resists the
collapse of silica network during ambient pressure drying. Therefore, in the present paper, the effects of FeAA on the physico-chemical
properties of the aerogels have been studied by varying the FeAA:Na2SiO3 molar ratio from 3 × 10−4 to 6 × 10−4. The aerogels were prepared via ambient pressure drying and characterized by the bulk density, thermal conductivity and water
contact angle. The aerogel’s surface morphology, elemental analysis and pore structure were characterized by means of EDAX
and FTIR, TEM and N2 adsorption- desorption analyzer. The high temperature hydrophobicity of these aerogels was checked by heating them in temperature
controlled furnace. Silica aerogels with low density ~0.050 g/cc have been obtained using the molar ratio of Na2SiO3:H2O:FeAA:Citric acid:TMCS at 1:146.67:3 × 10−4:0.54:9.46, respectively. EDAX and FTIR studies show that the iron species are entrapped in the mesoporous framework and not
took part in the bonding with silica. 相似文献
6.
Poonam M. Shewale A. Venkateswara Rao A. Parvathy Rao S. D. Bhagat 《Journal of Sol-Gel Science and Technology》2009,49(3):285-292
In the present paper, attempts have been made to produce transparent silica aerogels with low density and better hydrophobicity
by controlled sol–gel route and subsequent atmospheric pressure drying. The hydrogels were prepared by hydrolysis and polycondensation
of sodium silicate (Na2SiO3) in the presence of acetic acid catalyzed water followed by several washing steps with water, methanol and hexane, respectively.
The surface modification of the wet gel was carried out using a mixture of hexamethyldisilazane (HMDS) in hexane. Since, the
sol–gel chemistry provides a straightforward method to control the physical and optical properties of the aerogels, the influence
of various sol–gel parameters viz. gel washing time, molar ratios of CH3COOH/Na2SiO3 and HMDS/Na2SiO3 and silylation period on the physical and optical properties of the aerogels have been investigated. The aerogels have been
characterized by bulk density, Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric and Differential Thermal
Analysis (TG-DTA), Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM) studies and Contact angle measurements. 相似文献
7.
In the present paper the experimental results of the effect of sol-gel processing temperature on the physical properties of
the TEOS based silica aerogels are reported and discussed. The aerogels were produced by the two step sol-gel process at various
temperatures in the range of 26–70∘;C followed by supercritical drying using methanol solvent extraction. A remarkable reduction in the gelation time was observed
from three and a half days at room temperature to a mere 18 hours at 50∘;C. The best quality aerogels in terms of low density and high optical transmission were obtained for 6 hours hydrolysis time.
The aerogels were characterized by the measurements of bulk density, volume shrinkage, porosity, refractive index and optical
transmission. Monolithic aerogels with ultra low density (∼0.018 g/cm3), extremely high porosity (∼99%) and optimum optical transmission at 700 nm (∼75%) were obtained for the molar ratio of TEOS:MeOH:acidic
water:basic water at 1:99:10.42:14.58 respectively. 相似文献
8.
Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH4F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N2 adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m2/g were obtained by using NH4F (1 M), HCl (4 M), and a molar ratio of Na2SiO3::H2O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C. 相似文献
9.
Guangwu Liu Bin Zhou Xingyuan Ni Jun Shen Guangming Wu Ai Du Guoqing Zu 《Journal of Sol-Gel Science and Technology》2012,62(2):126-133
The experimental results of thermal process on the microstructural and physical properties of ambient pressure dried hydrophobic
silica aerogel monoliths are reported and discussed. With sodium silicate as precursor, ethanol/hexamethyldisiloxane/hydrochloric
acid as surface modification agent, the crack-free and high hydrophobic silica aerogel monoliths was obtained possessing the
properties as low density (0.096 g/cm3), high surface area (651 m2/g), high hydrophobicity (~147°) and low thermal conductivity (0.0217 Wm/K). Silica aerogels maintained hydrophobic behavior
up to 430 °C. After a thermal process changing from room temperature to 300 °C, the hydrophobicity remained unchanged (~128°),
of which the porosity was 95.69% and specific density about 0.094 g/cm3. After high temperature treatment (300–500 °C), the density of final product decreased from 0.094 to 0.089 g/cm3 and porosity increased to 96.33%. With surface area of 466 m2/g, porosity of 91.21% and density about 0.113 g/cm3, silica aerogels were at a good state at 800 °C. Thermal conductivities at desired temperatures were analyzed by the transient
plane heat source method. Thermal conductivity coefficients of silica aerogel monoliths changed from 0.0217 to 0.0981 Wm/K
as temperature increased to 800 °C, revealed an excellent heat insulation effect during thermal process. 相似文献
10.
The experimental results by using various exchanging solvents in the preparation of two step (acid and base) processed ambient
pressure dried hydrophobic silica aerogels, are reported. Silica alcogels were prepared by hydrolysis with oxalic acid and
condensation with NH4OH of ethanol diluted tetraethylorthosilicate (TEOS) precursor and hexamethyldisilazane(HMDZ) methylating agent. The exchanging
solvents used were: hexane, cyclohexane, heptane, benzene, toluene and xylene. The physical properties such as % of volume
shrinkage, density, pore volume, % of porosity, thermal conductivity, % of optical transmission, surface area, pore size distribution
and contact angle (θ) of the silica aerogels with water, were measured as a function of EtOH/TEOS molar ratios (R) for all the exchanging solvents. It was found that the physical and hydrophobic properties of the silica aerogels strongly
depend on the nature of the solvent and R. Heptane solvent resulted in highly transparent (≈90% optical transmission at 700 nm for 1 cm thick sample), low density
(≈0.060 g/cm3), low thermal conductive (≈0.070 W/m·K), high % of porosity (97%), high surface area (750 m2/g), uniform porosity and hydrophobic (θ ≈ 160°) aerogels compared to other solvents. On the otherhand, xylene resulted in aerogels with higher hydrophobicity (θ ≈ 172°) among other solvents. 相似文献
11.
Jyoti L. Gurav A. Venkateswara Rao Digambar Y. Nadargi 《Journal of Sol-Gel Science and Technology》2009,50(3):275-280
The experimental results on the study of thermal conductivity and effect of humidity on HMDZ modified TEOS based aerogels
dried at ambient pressure, are reported. Silica sol was prepared by keeping the MeOH/TEOS molar ratio, Acidic water (Oxalic
acid) and basic water (NH4OH) concentrations constant at 16.5, 0.001 and 1 M, respectively throughout the experiments and the HMDZ/TEOS molar ratio
(h) was varied from 0.34 to 2.1. Finally, the surface modified wet gels were dried at an ambient pressure. The thermal conductivity
of the aerogel samples was measured. Further, the humidity study was carried out in 80% humid surrounding at 30 °C temperature
over 80 days. The best quality aerogels in terms of low bulk density, thermal conductivity and durability (no moisture absorption)
with an only 2% of weight gain were obtained for TEOS: MeOH: Acidic H2O: Basic H2O: HMDZ molar ratio at 1:16.5:0.81:0.50:0.681, respectively. The thermal stability and hydrophobicity of the aerogel have
been confirmed with Thermo gravimetric and Differential Thermal (TG–DT) analyses and Fourier Transform Infrared Spectroscopy
(FTIR), respectively. Microstructural studies were carried out by Scanning Electron microscopy (SEM). 相似文献
12.
Evaggelos Economopoulos Theophilos Ioannides 《Journal of Sol-Gel Science and Technology》2009,49(3):347-354
Tetraalkylammonium fluoride salts have been employed as catalysts for the synthesis of silica aerogels by a two-step, sol–gel
method. Aerogel materials were characterized by N2 physisorption and SEM. The effect of the type of catalyst on the optical transparency of obtained aerogels has been examined.
It has been found that such compounds allow the synthesis of silica aerogels with the highest optical transmittance ever reported
for such materials. The optimal catalysts are tetrabutyl and tetraoctyl ammonium fluoride, with which aerogels with transparency
as high as 96% and extinction coefficient as low as 3.5 m−1 can be prepared. 相似文献
13.
Masoud Salavati-Niasari Fatemeh Davar Masoud Farhadi 《Journal of Sol-Gel Science and Technology》2009,51(1):48-52
Nanocrystalline Copper aluminate (CuAl2O4) was prepared by sol–gel technique using aluminum nitrate, copper nitrate, diethylene glycol monoethyl ether and citric acid
were used as precursor materials. This method starts from of the precursor complex, and involves formation of homogeneous
solid intermediates, reducing atomic diffusion processes during thermal treatment. The formation of pure crystallized CuAl2O4 nanocrystals occurred when the precursor was heat-treated at 600 °C in air for 2 h. The stages of the formation of CuAl2O4, as well as the characterization of the resulting compounds were done using thermo–gravimetric analysis, X-ray diffraction,
scanning electron microscopy and Fourier transform infrared spectroscopy. The products were analyzed by transmission electron
microscopy and ultraviolet–visible (UV–Vis) spectroscopy to be round, about 17–26 nm in size and E
g = 2.10 eV. 相似文献
14.
Oana Ştefănescu Marcela Stoia Mircea Ştefănescu Titus Vlase 《Journal of Thermal Analysis and Calorimetry》2009,97(1):251-256
Hybrid organic–inorganic materials, silica–diol, were synthesized by the sol–gel process from mixtures of tetraethylorthosilicate
(TEOS) and diols: ethylene glycol (HO–CH2–CH2–OH) and 1,3 propane diol (HO–CH2–CH2–CH2–OH), in acid catalysis. The gels have been synthesized for a molar ratio H2O:TEOS = 4:1 and different molar ratios diol/TEOS: 0.25; 0.5; 0.75; 1.0; 1.25 and 1.5. The resulting gels were studied by
thermal analysis and FT-IR spectroscopy, in order to evidence the interaction of diols with silica matrix. Thermal analysis
indicated that the condensation degree increases with the molar ratio diol/TEOS until a certain value. The thermal decomposition
of the organic chains bonded within the silica network in the temperature range 250–320 °C, leaded to a silica matrix with
modified morphology. The adsorption–desorption isotherms type is different for the samples with and without diol. Thus, the
specific surface areas have values <11 m2/g for the samples without diol and >200 m2/g for the samples with diols, depending on the annealing temperature. 相似文献
15.
P. R. Aravind P. Shajesh P. Mukundan K. G. K. Warrier 《Journal of Sol-Gel Science and Technology》2009,52(3):328-334
Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels
show densities in the range 0.34–0.38 g/cm3. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones.
The surface area for 5wt% titania aerogel has been found to be as high as 685 m2/g with a pore volume of 2.34 cm3/g and the 10wt% titania aerogel has a surface area of 620 m2/g with a pore volume of 2.36 cm3/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The
surface modified aerogels possess high surface areas in the range of 540–640 m2/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution
of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy
has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania
addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity
of the aerogel. 相似文献
16.
The present contribution reports on our results concerning the synthesis of different binary and ternary oxide systems by
using hybrid materials as “composite” precursors. In the last years, we have developed and explored a valuable strategy to
yield a very homogeneous dispersion of nanoparticles of early metal transition oxide, MO2 (M = Zr, Hf) inside a silica matrix. This route is based on the use of the sol–gel process to obtain organic–inorganic hybrid
silica-based materials embedding the oxide precursors (Zr and/or Hf oxoclusters), which are then calcined at high (T > 500 °C)
temperatures to give the desired oxides. The “precursor” hybrid materials are prepared by a modified sol–gel process, involving
the copolymerisation of the organically modified oxozirconium or oxohafnium clusters (M4O2(OMc)12 (M = Zr, Hf and OMc = methacrylate) with (methacryloxymethyl)triethoxysilane (MAMTES) or (methacryloxypropyl)trimethoxysilane
(MAPTMS). Free radical copolymerisation of the 12 methacrylate groups of the oxoclusters with the methacrylate-functionalised
siloxanes allows a stable anchoring of the oxoclusters to the silica network formed by the hydrolysis and condensation of
the alkoxy groups. The sol–gel reactions of the two methacrylate-modified silanes methacryloxymethyltriethoxysilane and methacryloxypropyltrimethoxysilane
were followed by using two independent time-resolved spectroscopic methods, viz., IR ATR and NMR with the aim to optimise
their pre-hydrolysis times and consequently their use as precursors for hybrid materials. As mentioned, thermal treatment
at high temperature of the hybrid yields a very homogeneous dispersion of ZrO2 and/or HfO2 nanoparticles in the silica matrix, since the molecular homogeneity of the starting hybrid is retained in the final mixed
oxide. This route was successfully applied both to the synthesis of bulk materials and thin films characterised by different
compositions (in term of M/Si molar ratios and M nature), heating route (conventional or microwave-assisted) and final temperature
of annealing (from RT to 1,100 °C). The first example of the ZrO2–HfO2–SiO2 ternary oxide system was also prepared by this approach. The prepared systems, both in the form of hybrid materials as well
as in the final form of binary or ternary oxides, were thoroughly characterised by a wide variety of analytical tools from
a compositional, structural, morphological point of view. Moreover, in the case of the binary ZrO2–SiO2 bulk materials, also the evolution under heating was followed by different methods. In particular, the composition of the
hybrid as well as of the final oxidic materials was determined by X-Ray Photoelectron Spectroscopy and elemental analysis,
whereas FT-IR and multinuclear solid-state NMR spectroscopies shed light on the changes occurring in the composition upon
thermal heating and the degree of condensation of the silica network. The morphology and the microstructure of the hybrids
and of the oxides were studied by nitrogen sorption and Scanning Electron Microscopy. X-Ray Diffraction, Transmission Electron
Microscopy and X-ray Absorption Fine Structure Spectroscopy X-ray Absorption Fine Structure Spectroscopy were used to follow
the conversion of the amorphous oxides to the final materials consisting of crystalline zirconia or hafnia dispersed in amorphous
silica. On selected systems, functional properties (surface reactivity, dielectric properties) were furthermore investigated.
The obtained binary oxides were also used as substrates for functionalisation experiments with (1) dialkycarbamates and (2)
long alkyl chains to produce functional materials for catalysis and HPLC applications, respectively. 相似文献
17.
SrAl2O4:Eu2+, Dy3+ powders were synthesized by sol–gel–combustion process using metal nitrates as the source of metal ions and citric acid as
a chelating agent of metal ions. The amounts of citric acid in mole were two times those of the metal ions. By tracing the
formation process of the sol–gel, it is found that decreasing the amount of NO3
− in the solution is necessary for the formation of transparent sol and gel, and the dropping of ethanol into the precursor
solution can decrease the amount of NO3
− in the solution. By combusting citrate sol at 600 °C, followed by heating the resultant combustion ash at 1,100–1,300 °C
in a weak reductive atmosphere containing active carbon, SrAl2O4:Eu2+, Dy3+ phosphors can prepared. X-ray diffraction, Thermogravimetry–differential thermal analysis, scanning electron microscopy and
fluorescence spectrophotometer were used to investigate the formation process and luminescent properties of the as-synthesized
SrAl2O4:Eu2+, Dy3+. The results reveal that the SrAl2O4 crystallizes completely when the combustion ash was sintered at 1,200–1,300 °C. The excitation and emission spectra indicate
that excitation broadband mainly lies in a visible range and the phosphors emit strong light at 510 nm under the excitation
of 348 nm. The afterglow of phosphors lasts for over 10 h when the excited source is cut off. 相似文献
18.
F. Attanasio G. Rialdi R. Swierzewski W. Zielenkiewicz 《Journal of Thermal Analysis and Calorimetry》2006,83(3):637-640
The physico-chemical properties
of poly(ethylene) glycol solutions in water have been studied with use of
pressure perturbation calorimetry. The three PEGs of average molecular mass
(Mr) 6000, 10000,
20000 were used. The concentration of polymers was changed in the range 0–30%
mass per volume (w/v%).
On the basic of VP-DSC measurements with use of PPC technique the dependencies
of thermal expansion coefficient (α) and excess specific heat capacity
(Cp,exc) on temperature
were determinated for PEG–water solutions. 相似文献
19.
Arnon Chaipanich Thanongsak Nochaiya 《Journal of Thermal Analysis and Calorimetry》2010,99(2):487-493
Thermal analysis (thermogravimetry and differential thermal analysis) was used with scanning electron microscopy technique
to investigate the hydration mechanisms and the microstructure of Portland cement-Fly ash-silica fume mixes. Calcium silicate
hydrate (C–S–H), ettringite, gehlenite hydrate (C2ASH8), calcium hydroxide (Ca(OH)2) and calcium carbonate (CaCO3) phases were detected in all mixes. In the mixes with the use of silica fume addition, there is a decrease in Ca(OH)2 with increasing silica fume content at 5 and 10% compared to that of the reference Portland-fly ash cement paste and a corresponding
increase in calcium silicate hydrate (C–S–H). 相似文献
20.
Je Kyun Lee George L. Gould Wendell Rhine 《Journal of Sol-Gel Science and Technology》2009,49(2):209-220
Less fragile lightweight nanostructured polyurea based organic aerogels were prepared via a simple sol–gel processing and
supercritical drying method. The uniform polyurea wet gels were first prepared at room temperature and atmospheric pressure
by reacting different isocyanates with polyamines using a tertiary amine (triethylamine) catalyst. Gelation kinetics, uniformity
of wet gel, and properties of aerogel products were significantly affected by both target density (i.e., solid content) and
equivalent weight (EW) ratio of the isocyanate resin and polyamine hardener. A supercritical carbon dioxide (CO2) drying method was used to extract solvent from wet polyurea gels to afford nanoporous aerogels. The thermal conductivity
values of polyurea based aerogel were measured at pressures from ambient to 0.075 torr and at temperatures from room temperature
to −120 °C under a pressure of 8 torr. The polyurea based aerogel samples demonstrated high porosities, low thermal conductivity
values, hydrophobicity properties, relatively high thermal decomposition temperature (~270 °C) and low degassing property
and were less dusty than silica aerogels. We found that the low thermal conductivities of polyurea based aerogels were associated
with their small pore sizes. These polyurea based aerogels are very promising candidates for cryogenic insulation applications
and as a thermal insulation component of spacesuits. 相似文献