Electrorheological response of swollen silicone gels containing barium titanate

We have investigated storage moduli of silicone gels containing barium titanate in the presence of dc electric fields. The gels containing barium titanate swollen by silicone oil showed a storage modulus reduction, i.e. negative electrorheological effect. Contrary, no negative electrorheological effect was observed in the unswollen gels and silicon gels without barium titanate. Swollen silicone gels and most of silicone/BaTiO₃ gels obeyed empirical quadratic dependence in electric field strength. Although an apparent phase separation was not observed in the swollen gel, microscopic phase separation may occur due to the difference in electric conductivity between particles (∼10⁻¹⁰ S/cm) and silicone oil (10⁻⁹ S/cm), as a result, the negative electrorheological effect appears. The effects of frequency, degree of swelling, and the field strength have been discussed.     

氧化物气凝胶催化剂的超临界CO2干燥法制备及其应用

超临界CO2干燥技术是一项绿色安全,制品结构可控的新的制备特殊材料的技术,综述了国内外探索其工艺和干燥机理的研究现状,阐述了利用其制备超细ZrO2气凝胶的工艺实验研究,并展望了氧化物气凝胶催化剂的应用前景.     

CO2‐driven synthesis of monodisperse barium titanate microspheres

Monodisperse BaTiO₃ microspheres were synthesized using Ba(OH)₂, a homemade amorphous TiO₂ precursor, and dissolved CO₂ as the raw materials. To dissolve sufficient CO₂ in the mixing solution, the reaction process was carried out in air. Then, a BaCO₃ layer, as a fixed barium source, was coated on the surface of the TiO₂ precursor template through a hydrothermal process at 90°C. Furthermore, the BaCO₃ reacted with the TiO₂ precursor to form pure BaTiO₃ through in situ conversion during subsequent heat treatment at 800°C. Compared to the commonly used acid soaking method for the removal of BaCO₃ after hydrothermal treatment, the method combining in‐air mixing with a low hydrothermal temperature and heat treatment yielded BaTiO₃ microspheres with a more monodisperse homogenous morphology, which have a greater potential in various applications.     

Continuous extraction rate measurements during supercritical CO2 drying of silica alcogel

We investigated the kinetics of supercritical CO₂ (SCCO₂)-based drying of silica aerogels, a common, but time consuming and energy intensive step in their manufacture. An apparatus was developed to continuously measure alcohol extraction rates from alcogels as a function of key process variables by two redundant techniques. Kinetics data are reported for the drying of 2.5 mm, 5 mm, and 7.5 mm thick annular alcogels by pumping SCCO₂ through a 10 mm-thick concentric annulus surrounding their exterior. The SCCO₂ was at a temperature of 323 K and a pressure of 12.4 MPa and its mass flow rate varied from 1 kg/h to 5 kg/h. Gel thickness and SCCO₂ flow rate were both shown to significantly effect drying rate and required drying time. The results of a conjugate mass transfer model assuming pure diffusion in the alcogel compared favorably with the data when the composition dependence of molecular diffusivity was captured utilizing available correlations.     

Synthesis of barium strontium titanate nanopowders by microwave hydrothermal method

Barium strontium titanate (BST) powders of high purity, good crystallinity and well dispersed with a diameter of 50–90?nm are prepared by microwave hydrothermal method using tetrabutyl titanate, barium nitrate and strontium nitrate as the main raw materials. The phase composition, element composition and microstructure of the powders are characterised by X-ray diffraction, Fourier transform infrared spectroscopy, energy dispersive spectroscopy and transmission electron microscopy. The results show that Ba_0.6Sr_0.4TiO₃ powders could be synthesised under the conditions that the reaction temperature is 70°C, the reaction time is 10?min, and the value of pH is 14, which indicates that the reaction temperature, time and the value of pH have a great effect on the crystallinity of nanopowders, and the dispersant (OP-10) plays a role in the dispersion of the nanopowders.     

Solvothermally synthesized tetragonal barium titanate powders using H2O/EtOH solvent

Multilayer ceramic capacitor (MLCC) miniaturization has increased the demand for superfine BaTiO₃ powder due to its thin dielectric layer. Hydrothermally synthesized BaTiO₃ powder has submicron size, high purity and good crystalline nature at low synthetic temperature. However, the powder has a pseudo-cubic phase resulting in poor dielectric properties due to size effect and hydroxyl ion inclusion in the BaTiO₃ lattice. We attempted a superfine (lower than 100 nm) highly tetragonal BaTiO₃ powder via a solvothermal method without precipitating agent. Solvent composition effects on the BaTiO₃ powder tetragonality were discussed.     

Drying of agar gels using supercritical carbon dioxide

The use of supercritical carbon dioxide (scCO₂) for the removal of water from agar gels has been investigated and compared to air and freeze drying. Experiments were conducted to evaluate how gel formulation (with and without sucrose) and drying conditions (with and without ethanol as a co-solvent, flow rate and depressurisation rate) affected the microstructure of the gels dried using scCO₂. X-ray micro-computed tomography (X-ray micro-CT) was used to determine the voidage (% open pore space) of the dried structures, which can be used to indicate the extent of drying-induced structural collapse (in general, the lower the voidage, the greater the collapse). For formulations containing sucrose, which displayed the best structural retention, voidage was found to increase in the order: air drying (4% voidage) < supercritical drying with pure CO₂ (48%) < supercritical drying with ethanol-modified CO₂ (68%) < freeze drying (76%). The relatively high voidage of samples dried in the presence of ethanol, was due in part to foaming of the gels, hypothesised to result from an interaction between the agar and ethanol, rather than an effect of the supercritical fluid. CO₂ flow rate (1 vs. 3 l/min) during supercritical drying and depressurisation rate (0.4 vs. 1.6 MPa/min) had no effect on the dried microstructure.     

Flash sintering of barium titanate

A novel technique of low temperature and fast sintering is expected to meet the productive requirements of reducing energy consumption and improving efficiency. In this work, a dense Nb-doped BaTiO₃ ceramic is obtained by a sintering method applying a direct current electrical field of 140?V?cm^?1 for 30?s at 1055?°C. In particular, the rapid densification mechanism of flash sintering Nb-doped BaTiO₃ ceramic is explained by a model about liquid film which is formed by wetting at particle contacts, due to the Joule heating runaway. It is believed that the capillary forces generated from liquid film play a dominant role during flash sintering, which ensure the compaction of local particles and achieve the densified specimen in a relatively short time.     

Facile synthesis of nanorods of tetragonal barium titanate using ethylene glycol

Tetragonal barium titanate (BaTiO₃) nanorods were synthesized from hydroxide precursor by a hydrothermal/solvothermal method with 10 vol% ethylene glycol as solvent. The hydroxide precursor slurry was prepared by the addition of 10 M NaOH to a mixed solution of BaCl₂ and TiCl₄. When the above aqueous slurry was heated with water only at 200 °C, cubic BaTiO₃ nanocrystals formed, whereas tetragonal BaTiO₃ nanorods were obtained when heated with 10 vol% ethylene glycol. The crystallization of cubic BaTiO₃ via dissolution–reprecipitation of precursor could be suppressed by the addition of ethylene glycol, resulting in the formation of tetragonal BaTiO₃ under hydrothermal treatment at 200 °C.     

Upcycling by grafting onto semi-crystalline polymers using supercritical CO2

The creation of graft copolymers by selectively grafting a second polymer to the amorphous fraction of a semi-crystalline polymer in supercritical CO₂ is demonstrated herein. The graft copolymer is synthesized by free radical polymerization of a vinyl monomer within the semi-crystalline polymer below its melt temperature. Such conditions afford selective grafting on the amorphous regions (block “B”) while leaving the crystalline domains (block “A”) unmodified. Accordingly, unique A-B, A-B-A, A-B-A-B-A, and so forth. block structures are formed. In this work, styrene is polymerized within polyamide 6, polyethylene terephthalate, and isotactic polypropylene. Purification of these material is performed to remove the un-grafted homopolymer, allowing for determination of the graft yield, the portion of polymer which covalently bonds to the semi-crystalline matrix. Grafting yields achieved in polyamide 6, polyethylene terephthalate, and isotactic polypropylene were 98%, 59%, and 15%, respectively. Property enhancements were observed upon further characterization of polystyrene-polyamide 6 copolymers, including high glass transition temperatures, the ability to be remelted, and tunable grafting molecular weight. Additionally, hydrophobicity is controlled by varying polystyrene composition. The remarkable range of accessed properties demonstrates this as a potential route to upcycling plastics.     

Effects on aqueous barium titanate tape properties of passivation of barium ion leaching by using dispersants

To passivate a barium titanate (BaTiO₃) ceramic powder surface from Ba²⁺ ion leaching in water, passivation agent layer (PAL) was formed by drying the slurry after adding a commercial polymeric dispersant. By following the several characteristic steps of actual MLCC production process, slip and green properties were compared for two different polymer-adding modes; one is the PAL and the other is the conventional dispersant mode. Compared to the conventional dispersant adding method, PAL mode was the only effective way in reducing the amount of Ba²⁺ leaching. However, slip viscosity and green body properties were not a function of adding mode but a function of dispersant itself, which means using PAL did not deteriorate any of slip and green properties of BaTiO₃.     

常压一步法合成纳米钛酸钡粉体

以廉价的四氯化钛、氯化钡和氢氧化钠为主要原料,采用常压一步法合成纳米钛酸钡粉体.研究了四氯化钛水解程度、pH、反应温度和添加表面活性剂等因素对制备的纳米钛酸钡粉体纯度和形貌的影响.结果表明:在最佳工艺条件下,通过该方法合成的纳米钛酸钡粉体为立方晶系,呈均匀球形,粒径分布在20 nm左右,纯度为99.85%,钡与钛物质的量比为1.000±0.005.与水热合成法相比,该法具有反应温度和压力低、无需焙烧等优点.     

Fractionation of menhaden oil ethyl esters using supercritical fluid CO2

Supercritical fluid CO₂ was used to fractionate menhaden oil fatty acid ethyl esters to obtain concentrates of the esters of allcis-5,8,11,14,17-eicosapentaenoic acid (EPA) and allcis-4,7,10,13,16,19-docosahexaenoic acid (DHA). Separation of the ethyl esters was found to occur primarily by carbon number, thus limiting the degree to which the ethyl esters of EPA and DHA could be concentrated. Urea fractionation of whole esters in order to remove saturates, monoenes and dienes prior to fractionation with supercritical fluid CO₂ resulted in concentrates of EPA and DHA in purities exceeding 90%. Several criteria are given for the selection of crude oils in order to maximize both purity and yield of concentrates.     

超临界CO2萃取与同轴喷嘴耦合制备花椒油微胶囊

采用正交实验研究了超临界CO2萃取花椒油的工艺,探讨了萃取压力、温度和时间等因素对花椒萃取率的影响。并初步研究了在一定条件下,利用同轴喷嘴将萃取出的花椒油直接喷雾干燥制备微胶囊过程中,壁材浓度、壁材流量、CO2流量、进风温度等对包埋率的影响。     

饱水竹木漆器的超临界CO2脱水干燥研究

用超临界CO2对战国时代的饱水竹木漆器(含水率大于200%)进行脱水干燥。结果表明,超临界CO2脱水干燥的最优工艺条件为:干燥温度50℃,压力25 MPa,CO2流量为20 kg/h,脱水干燥5h。在该条件下,脱水干燥的竹木漆器,其含水率下降到了15%以下,漆皮完好如初,色泽鲜亮,样品形状完好、无收缩开裂损坏等现象,符合文物脱水保存的要求。     

Synthesis of nanocrystalline barium titanate: Effect of microwave power on phase evolution

Nanocrystalline BaTiO₃ (BT) powder was synthesised using a polymer precursor route and the influence of conventional, microwave and combined hybrid heating methods on phase formation was investigated. A single-phase tetragonal BT (t-BT) nanocrystalline powder of about 20 nm primary particle size and decreased agglomeration were formed when high levels of microwave energy were used. This was accomplished at a lower processing condition of 700 °C for 30 min compared to conventional processing, which required 900 °C for 5 h, resulting in potential savings in time and energy. During the nano BT synthesis, the role of microwaves was determined by subjecting the samples to identical thermal histories, i.e. exactly the same time-temperature profiles, while using a range of different levels of microwave power. Significant reduction in the activation energy for the formation of the tetragonal phase was observed with increasing levels of microwave power and the results are explained in terms of a possible non-thermal mechanism. Furthermore, under otherwise identical thermodynamic conditions of temperature, time and (atmospheric) pressure, the co-occurrence of hexagonal crystal structure at < 200 W of additional microwave power along with formation of tetragonal crystal structure at ≥ 200 W and single phase tetragonal crystal structure at ∼1200 W was observed, demonstrating a new method of controlling the phase evolution during the synthesis of nanostructured barium titanate powder. The methodology could be applied to synthesise a variety of functional ceramic powders with tailored levels of crystallographic phases.     

超临界CO_2萃取干燥技术及其在饱水文物脱水中的研究进展

饱水文物的脱水干燥是文物保护领域中一个重要的环节,直接关系到饱水文物的存在与否及其品相的高低。超临界CO2萃取干燥技术利用CO2气体在临界温度以上,控制饱水文物内部液体在临界点之上,使气/液界面消失,在无液相表面张力情况下进行的干燥过程。与传统脱水技术相比,超临界CO2萃取干燥技术具有处理周期短、脱水效率高、干燥应力小、脱水过程同时对文物杀菌和清洗等优点。综述了超临界CO2萃取干燥技术及原理、萃取干燥工艺流程,对国内外应用该技术对饱水文物脱水研究进行了简要评述,并展望了其发展趋势。     

Synthesis,characterisation and dielectric properties of low-loss Zr-doped barium strontium titanate materials

ABSTRACT

A series of materials of the composition Ba_0.7Sr_0.3Zr_xTi_(1???x)O₃ (BSZT), where x?=?0.00, 0.01, 0.02 and 0.03, were synthesised through the sol–gel method whereupon the manifestations of partial substitution of Ti⁴⁺ by Zr⁴⁺ on the structural and dielectric behaviour of Ba_0.7Sr_0.3TiO₃ (BST) were studied. X-ray diffraction patterns of all samples showed the development of a single-phase crystalline perovskite structure. The dielectric behaviour of the ceramic samples has been studied in detail in the frequency range 10 Hz to 1 MHz at room temperature, and as a function of increase in temperature. The sample with 2% Zr recorded the highest dielectric constant. The dielectric loss values for all the BSZT compositions are remarkably low, tan?δ?=?0.021～0.026 at 1 MHz, in contrast to the literature reports. It is observed that small amount of Zr doping on BST, where 0?<?x?≤?0.03 results in the enhancement of dielectric constant while decreasing the bulk density and dielectric loss.     

Synthesis and electrorheological properties of polyaniline‐coated barium titanate composite particles

Polyaniline‐coated barium titanate composite particles were synthesized by surface grafting polymerization, in which five silane coupling agents with different terminal groups were used. The structure of composite particles was characterized by SEM, XRD, FTIR, and TGA, and the electrorheological properties were tested by rotational rheometer. The results show that polyaniline was coated on the surface of barium titanate particles to form the shell‐core structure. The yield stress of the electrorheological fluids based on polyaniline‐coated barium titanate composite particles is higher than that of polyaniline and barium titanate. Compared with PANI‐based electrorheological fluids (ERFs), the composite particles‐based ERFs have lower field‐off viscosity due to the molecular interaction. The yield stress of the ERFs, as well as density of the composite particles, was affected by the groups at terminal of silane coupling agents. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008