Fabrication of hierarchically porous spherical particles by assembling mesoporous silica nanoparticles via spray drying

A new type of hierarchically porous materials is fabricated by assembling mesoporous nanoparticles via spray drying. Well-dispersed mesostructured silica nanoparticles (MSN), whose particle size distribution was narrow in the range of 20 nm and 50 nm, were prepared by a thermal deposition method. By spray drying a MSN suspension, MSN were assembled into spherical secondary particles. After calcination, the spherical particles have two types of mesopores, mesopores of 3 nm in size inside of calcined MSN and larger inter-nanoparticle mesopores of about 15-20 nm. This hierarchical pore system should provide nanospaces for efficient mass transport of guest species with different sizes.     

Processing and Sintering of Agglomerate-free CaO-ZrO2 Powder

Coprecipitation supercritical fluid drying technology has been employed to synthesize calcia-stabilized zirconia ultrafine powder with low-cost inorganic salts as the starting materials. The sintering behaviors of these powders were also investigated. The results showed that supercritical fluid drying could effectively alleviate the hard agglomeration of grains during the gel drying process, and the morphology of the powder retained the network texture of the original gel. The resulting particles were characterized by small particle size (5-20 nm), better monodispersity and high surface area, which gave rise to high activity and sinterability. Consequently, these powders could readily be compacted into the desired shape and their densification could be carried out in shorter time and at lower temperatures. For instance, nanometer-sized powder calcined at 600癈 for 2 h could be cold-pressed into a green body and sintered at 1100?for 0.5 h to attain a dense body with bulk density of 5.9718 g/cm3 and speci     

利用冲击波技术制备TiO2纳米晶

以钛酸丁脂为原料,将溶胶-凝胶法与冲击波加载手段结合能制备出粒度均匀、晶粒小于50 nm的纳米TiO2.冲击波作用的瞬时性能有效地抑制晶粒的生长.冲击压力对粉体的晶粒度有一定影响,冲击压力增大将导致冲击温度升高,过高的冲击温度会造成晶粒长大.为了获得尺寸细小的纳米晶粒粉体,应在保证凝胶分解的前提下,降低冲击压力.     

Large- and small-nanopore silica prepared with a short-chain cationic fluorinated surfactant

A cationic partially fluorinated surfactant with four carbons in the chain 1-(3,3,4,4,4-pentafluorobutyl)pyridinium chloride is employed as a structure-directing agent to synthesize nanoporous silica. Samples are prepared in dilute ammonia solutions at room temperature with a range of surfactant:Si ratios. The sample with the largest surfactant:Si ratio forms particles with wormhole-like micropores with an average diameter of 1.6?nm, which corresponds to the anticipated small size of the surfactant aggregates. On the other hand, the sample with the smallest surfactant:Si ratio forms a gel that, upon drying, has uniform 11.1 nm pores. The formation and stabilization of the latter large-mesopore structure is unusual for a sample prepared and dried under ambient conditions, and may reflect favourable roles of the surfactant both in inducing gelation and in stabilizing the pore structure during drying.     

Effect of the drying conditions on the microstructure of silica based xerogels and aerogels

Nanostructured silica based xerogels and aerogels are prepared by sol-gel technology, using methyltrimethoxysilane as precursor. The influence of the drying method and conditions on the microstructure of the obtained materials is investigated, since the drying stage has a critical influence on their porosity. Two types of drying methods were used: atmospheric pressure drying (evaporative), to produce xerogels, and supercritical fluids drying, to obtain aerogels. Although the supercritical fluids drying technique is more expensive and hazardous than the atmospheric pressure drying, it is well known that aerogels are less dense than the xerogels due to less pore shrinkage. However, the ideal situation would be to use atmospheric pressure drying in conditions that minimize the pore collapse. Therefore, in this work, different temperature cycles for atmospheric pressure drying and two heating rates for the supercritical fluids drying are tested to study the gels' shrinkage by analyzing the density and porosity properties of the final materials. The best materials obtained are aerogels dried with the lower heating rate (approximately 80 degrees C/h), since they exhibit very low bulk density (approximately 50 kg/m3), high porosity (95%)-mainly micro and mesopores, high surface area (approximately 500 m2/g), moderate flexibility and a remarkable hydrophobic character (>140 degrees). It was proved that the temperature cycles of atmospheric pressure drying can be tuned to obtain xerogels with properties comparable to those of aerogels, having a bulk density only approximately15 kg/m3 higher. All the synthesized materials fulfill the requirements for application as insulators in Space environments.     

Synthesis of nanocrystalline Ni/Ce-YSZ powder via a polymerization route

Pechini process was used for preparation of three kinds of nanocrystalline powders of yttria-stabilized zirconia (YSZ): doped with 1.5 mol% nickel oxide, doped with 15 mol% ceria, and doped with 1.5 mol% nickel oxide plus 15 mol% ceria. Zirconium chloride, yttrium nitrate, cerium nitrate, nickel nitrate, citric acid and ethylene glycol were polymerized at 80 °C to produce a gel. XRD, SEM and TEM analyses were used to investigate the crystalline phases and microstructures of obtained compounds. The results of XRD revealed the formation of nanocrystalline powder at 900 °C. Morphology of the powder calcined at 900 °C, examined with a scanning electron microscope, showed that the presence of nickel and cerium inhibited the grain growth in the system. The average crystallite size of the material doped with nickel oxide (9.33 nm) was bigger than the one doped with cerium oxide (9.29 nm), while the YSZ doping with the two oxides simultaneously promoted the grain growth with crystallite size of 11.37 nm. Yttria-stabilized zirconia powder with a mean crystallite size of 9.997 nm was prepared successfully by this method.     

An X-ray diffraction and Mossbauer study of nano-crystalline Fe2O3–Cr2O3 solid solutions

A series of gels with nominal composition Fe_2−xCr_xO₃ (x=0–2) was prepared at room temperature by an inorganic sol–gel route and studied by X-ray diffraction and Mossbauer spectroscopy. The gels dried at 105°C were found to be X-ray amorphous, but Mossbauer studies revealed the gels to be nano-crystalline solid-solution particles of the composition above, with super-paramagnetic properties. Further heating to 600°C gave crystalline X-ray patterns which allowed lattice parameter and crystallite size calculations to be made. It was found that lattice parameters and crystallite sizes decreased with increasing chromia content, and that at the higher chromia/iron ratios, a partially collapsed Mossbauer pattern results, indicating reductions in crystallite size and hyperfine field with increasing chromia content. This revised version was published online in November 2006 with corrections to the Cover Date.     

Glycerol-assisted gel combustion synthesis of nano-crystalline LiNiVO4 powders for secondary lithium batteries

Glycerol assisted gel combustion route (GGCR) was used for the synthesis of nano-crystalline inverse spinel LiNiVO₄ powders, with metal nitrates, citric acid and glycerol for three different total metal ions to glycerol ratios (M/G) as 1:1, 1:2 and 1:3. In this study, the effect of total metal ion to glycerol ratio on the formation of the nano-crystalline LiNiVO₄ phase was investigated and discussed. The thermal behavior and structural coordination of the as prepared as well as the calcined polymeric intermediates at different temperatures were investigated, respectively, using the DSC and FTIR measurements. The microstructure of the polymeric intermediates was identified by taking the scanning electron microscopy (SEM) pictures. The formation of the inverse spinel LiNiVO₄ phase, obtained from the calcined intermediates, was confirmed by comparing the observed X-ray diffraction spectra with the respective JCPDS standard. The crystallite size was calculated using the Scherrer's formula and the smallest crystallite size of 39 nm was observed for the LiNiVO₄ powders obtained from the polymeric intermediate, calcined at 450 °C for 12 h, synthesized using the GGCR with M/G=1:1.     

超临界干燥制备PSNB气凝胶及其超疏水性能研究

以聚硼硅氮烷为原料,二乙烯苯为交联剂,通过硅氢加成反应结合超临界干燥工艺制备聚硼硅氮烷气凝胶。利用红外光谱、扫描电镜、比表面积与孔径分析仪对所制备气凝胶的形成及微观结构进行了分析,并通过接触角仪对样品的疏水性能进行了研究。结果表明:聚硼硅氮烷和二乙烯苯通过硅氢加成反应制得聚硼硅氮烷气凝胶;所制备的气凝胶的比表面积为307~458cm~2/g,孔体积为1.20~2.17cm~3/g,孔径分布为2.0~100nm,是一种具有三维网状结构的介孔材料,并具有超疏水性能,且当起始溶剂体积分数为85%时,气凝胶疏水性能最佳,接触角为151.5°。     

Effect of synthesis methods for mesoporous zirconia on its structural and textural properties

Zirconia was synthesized by sol–gel and post-hydrothermal treatment under autogenous pressure in order to study the effect of synthesis methods on its structural and textural properties. On the basis of thermal analysis, in situ X-ray diffraction and Raman spectroscopy techniques, the synthesis processes exhibit similar thermal behavior and zirconia phase transformation. The effect of in situ calcination temperature on the crystallization behavior, crystal phase transition, and crystallite size analysis was studied. The results obtained revealed that amorphous zirconia transformed into tetragonal phase above 400 °C and thermally stabilized up to 700 °C. A biphasic mixture of tetragonal and monoclinic zirconia was formed at 750 °C. Activation energy of sintering due to grain growth mechanism predicted that the zirconia phase transformation is due to the increase in the crystallite size of tetragonal phase above its critical size. The post-hydrothermal treatment resulted in the formation of high surface area mesoporous zirconia (213 m² g^?1). Upon increasing the calcination temperature, a pronounced decrease in the specific surface area of zirconia samples due to sintering process and phase transformation.     

Synthesis,structural characterization,and photocatalytic properties of iron-doped TiO<Subscript>2</Subscript> aerogels

Fe(III)-doped TiO₂ aerogels are prepared by acid catalyzed sol–gel method followed by supercritical drying, and then heat treatment. Raman spectra together with X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns of the iron-doped TiO₂ aerogel samples revealed the existence of both anatase and brookite crystalline phases. It was found that the brookite phase formation is favored by the increase of the iron content in the dried samples. XRD measurements show that the lattice constant c of anatase phase decreases with the dopant addition, while the value of a remains essentially unchanged. The microstructure of the investigated samples is relatively compact with small mesopores as revealed from transmission electron microscopy (TEM). The most enhanced photocatalytic activity was exhibited by the TiO₂ aerogel sample with 1.8 at.% Fe(III) whose apparent rate constant of the salicylic acid photodegradation was found to be of almost six times higher than that of Degussa P25.     

常压干燥法制备炭气凝胶

炭气凝胶是一种具有交联状结构的新型纳米多孔材料,它是用溶胶－凝胶法和超临界干燥工艺制成的有机气凝胶（RF）经炭化得到的。它具有比表面积大和电导率高等特性,可用作超级电容器和可充电电池理想的电极材料。通过优化传统制备炭气凝胶的工艺,成功地在常压条件下制备出了高RC比（间苯二酚为催化剂的摩尔比）的RF气凝胶和炭气凝胶。用扫描电子显微镜（SEM）、比表面积测试与孔径分布（BET）等方面对其微结构进行了研究。结果表明,常压干燥法制备的炭气凝胶具有纳米网络结构和大比表面积。用常压干燥代替了超临界干燥,大大降低了制备成本,扩展了其应用前景。     

Preparation of tin oxide gels with versatile pore structures

Hydrous tin oxide gels were subjected to consecutive solvent-evaporation (SE) and CO₂ supercritical (CS) drying steps, followed by re-hydration at low (30%) humidity, and the effects on pore structure of the SE drying duration and particle surface potential of the gel were investigated. The difference in surface potential has been found to impose significant variations in initial pore size distribution, while the SE drying step tends to narrow such differences and simultaneously reduce the mean pore size. It is demonstrated that varieties of pore structures have thus been obtained by simply varying these two parameters.     

Synthesis of titania-silica xerogels by co-solvent induced gelation at ambient temperature

A series of binary titania-silica mixed oxides was prepared by sol-gel synthesis at room temperature. The hydrolysis of titanium isopropoxide (Ti(ⁱOPr)₄) and tetraethylorthosilicate (TEOS) was facilitated by co-solvent induced gelation in acidic media. The resulting gels were dried, calcined, and then characterized by powder X-ray diffraction analysis, nitrogen sorption studies (at 77 K), diffuse reflectance spectroscopy, Raman microscopy, and transmission electron microscopy. The nitrogen sorption studies indicate that the porosities could be tuned when simple aromatic solvents such as toluene, p-xylene, or mesitylene were added as a co-solvent to the synthesis gel. The binary mixed metal oxide materials obtained in this study showed high activity towards the degradation of phenol, and possessed high surface areas, and large pore volumes with narrow pore size distribution without the need for additional hydrothermal synthesis or supercritical drying.     

疏水型SiO2气凝胶

以多聚硅为硅源,通过溶胶-凝胶、表面修饰及超临界干燥过程制备出了疏水型ＳｉＯ_２气凝胶．表面修饰后ＳｉＯ_２气凝胶孔洞由修饰前的２３．１ｎｍ减小到１８．２ｎｍ;比表面积由修饰前的 ４７７ｍ２ｇ^－１增加到 ５６３ｍ２ｇ^－１,骨架颗粒比修饰前略大,饱和水蒸汽吸附量由吸附前的０．０４（重量比）降低到０．００１２,且与水不浸润．     

Alumina-silica aerogel catalysts prepared by two supercritical drying methods for methane combustion

Palladium-supported alumina-silica (10 mol%) aerogels were prepared by two different supercritical drying methods. In one method, an alumina-silica wet gel was dried under supercritical conditions of ethanol in an autoclave at a temperature of 270C and pressure of 26.5 MPa. In the other, ethanol was supercritically extracted by carbon dioxide in an extractor at 80C and 15.7 MPa. Their catalytic properties for methane combustion were measured after being fired at 1200C for 5 h. The palladium-supported alumina-silica aerogels prepared in the autoclave showed higher activity (methane conversion) than the one prepared in the extractor. This was attributed to the autoclave-dried aerogel having a larger pore size and better palladium dispersion.     

Structural,optical and magnetic properties of nanocrystalline Zn0.9Co0.1O-based diluted magnetic semiconductors

With a view to understand the influence of nano size on various properties of cobalt-doped ZnO-based diluted magnetic semiconductors, a series of materials were prepared by the citrate gel route. The phase and morphology studies have been carried out by X-ray diffraction and transmission electron microscopy, respectively. All the samples of the present investigation are found to have hexagonal wurtzite structure and crystallite sizes are found to vary from 25 nm to 65 nm. From the optical absorption measurements it has been observed that upon doping with cobalt, the energy band gap is found to shift towards lower energy side (red shift) while it shifts towards higher energy side (blue shift) when the crystallite size is increased continuously. It has been observed from the XPS results that oxidation state of Cobalt is +2 and that the difference in binding energies of Co 2p_3/2 and Co 2p_1/2 is found to increase continuously with increasing crystallite size. Finally, all the samples are found to exhibit room temperature ferromagnetism and the specific magnetization decreases with increasing crystallite size.     

SiO2/M纳米复合材料的结构及催化性能

采用溶胶-凝胶法和超临界干燥法制备了SiO2/M(Cu,Co)纳米复合材料,对其结构、形貌进行了TEM和比表面分析,并研究了SiO2/M纳米复合材料的催化性能.结果表明,制得的SiO2/M纳米复合材料保留了SiO2气凝胶的纳米网络和高比表面积,金属组份Cu与Co均匀地分散在纳米级SiO2气凝胶骨架中,对CO的氧化反应以及CO-NO反应均表现出高的催化活性.     

纳米级微孔SiO_2玻璃材料制备技术研究Ⅱ.纳米级微孔SiO_2玻璃体材的研制

采用溶胶－凝胶法，以正硅酸乙酯为原料，在酸催化下，用草酸作化学添加助剂（ＤＣＣＡ），着重研究了草酸用量对产品性能的影响，在此基础上，成功地研制出本体尺寸较大的纳米级微孔玻璃体材，其孔径分布主要范围在１～２ｎｍ之间，比表面积可达２１５．３９ｍ２／ｇ，密度几乎为石英玻璃的１／２，在紫外－可见光区透光性好，是一种很好的功能性纳米级基质材料。     

Synthesis of sulfated zirconia nanopowders via polyacrylamide gel method

In the present study, polyacrylamide gel method was applied for synthesis nanosized sulfated zirconia powders, first time. An assessment of the influence of calcination temperature and sulfate ion loading on the properties of synthesized powders was performed and the samples were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscopy and thermal analysis. The results revealed that tetragonal phase was obtained on calcination at 500 °C and was stable towards higher temperatures (650 °C). The increase of crystallite size with increasing calcination temperature was observed. Calcination for long duration time led to evolution of some sulfur species, furthermore results in higher particle size (∼100–200 nm) as compared to calcination for short duration time resulting in lower particle size (<50 nm). The presence of sulfate had no significant effect on thermal stability of polyacrylamide gel network whereas structure and the nature of the sulfur species bound with the zirconia surface are affected by the sulfur content.