Rheological behavior of cement pastes under Large Amplitude Oscillatory Shear

Cement pastes exhibit virtually all the rheological features of complex fluids. Thus, several rheological methods and setups have been used in the literature to characterize these materials. In the present investigation Large Amplitude Oscillatory Shear (LAOS) is for the first time exploited for cement pastes. LAOS can be used to characterize all the rheological properties within a single procedure. This technique is tested in the case of three different cement mixes: a Portland cement paste, nanoclay blended cement paste and a cement paste containing a hydro-soluble polymer. These mixes were selected in order to get rheological properties that are different both quantitatively and qualitatively. Indeed, addition of a low amount of nanoclay increased significantly the yield stress and the shear-thinning/thixotropic aspects of the cement paste, whereas addition of cellulose ether led to the decrease of yield stress and thixotropy. These non-linear rheological properties are discussed within the framework of LAOS.     

Carbon black pastes as coatings for improving thermal gap-filling materials

Carbon black pastes were found to be effective as coatings for improving the performance of thermal gap-filling materials, including flexible graphite, aluminum and copper. The thermal contact conductance across copper mating surfaces was increased by up to 180%. A fluidic form of carbon black paste (based on polyethylene glycol) was more effective than a thixotropic form (based on polyol esters). The carbon black pastes were much more effective as coatings than a commercial silver paste. With a carbon black paste coating, aluminum foil (7 μm thick) was a superior gap-filling material compared to similarly coated flexible graphite (130 μm thick). However, without a coating, flexible graphite was superior to aluminum. Commercial silicone-based gap-filling materials were inferior to flexible graphite or aluminum (whether coated or not).     

太阳能电池无铅导电电极的制备及电性能研究

以Bi2O3、B2O3和SiO2为主要原料制备无铅玻璃粘合剂,将其与导电银(Ag)粉、Al2O3、MnO2等无机添加剂和α-松油醇等有机载体进行混合制备无铅导电银浆,在800℃的温度下烧结20s形成Ag电极。采用四点探针法测量Ag电极电阻率ρ,通过SEM观察其断面形貌并用Keithley2400数字源表测定电池的相关性能参数,研究了Ag电极中导电Ag粉含量对电极性能的影响,确定了无铅导电Ag浆的质量配比为:导电银粉75%,玻璃粘合剂(Glass frit,GF)4%,无机添加剂1%,有机载体20%时,Ag电极的电性能趋于最佳。     

染料敏化太阳能电池丝网印刷浆料中有机造孔剂对电池性能的影响

本文采用丝网印刷工艺制备染料敏化太阳能电池的氧化钛多孔薄膜,并将其组装成电池器件.主要研究了丝网印刷浆料中造孔剂-乙基纤维素和松油醇的含量对氧化钛薄膜光电性能的影响.光电性能测试和扫描电镜结果显示,当乙基纤维素N50和N10的比例为1:0、松油醇的含量为75%时,薄膜的结构和光电转换性能结果最优,单层光电转换效率从0.84%提升到1.23%.     

Effect of properties and crystallization behavior of BaO–ZnO–B2O3–SiO2 glass on the electrical conductivity of Cu paste

The extensive application of multilayer ceramic capacitors provides an attractive development for terminal electrode pastes. However, the growing requirement for advanced glass materials used in terminal electrode pastes is substantiated. Therefore, to advance the development of electrode pastes, better development and deeper exploration of glass powder are required. Here, a series of BaO–ZnO–B₂O₃–SiO₂ (BZBS) glasses were prepared by melt-quenching technique, which are used to investigate the effect of the introduction of BaO on structure and properties of the ZnO–B₂O₃–SiO₂ (ZBS) glass. With the introduction of BaO, the relative amount of [BO₄] was much less, which made the glass network structure loosen, decreased the glass transition temperature (T_g) and increased the coefficient of thermal expansion of the glass. The decreasing contact angle was observed on the surface of a barium titanate (BaTiO₃) substrate. When the BaO content was around 3–7 mol%, the stability of ZBS glass frit could be strengthened by inhibiting the precipitation of Zn₂SiO₄ crystal. In addition, to further characterize the effect of glass on terminal electrode paste, BZBS glass powder was adopted to prepare copper electrode paste, which was printed on the BaTiO₃ substrate and subsequently fired at 800°C for 10 min. With the related copper paste containing ZBS glass doped with 7 mol% BaO, the optimum value of acid resistance and sheet resistance (1.99 mΩ) were exhibited, at which the coated copper paste formed a dense conducting layer.     

Effects of Strain on Solid-Solution Formation in an Ag-Pd Paste

A paste of Ag-Pd in orthosilicate glass, when fired, forms a useful conductor for the semiconductor industry. To understand the interaction of the Ag and Pd during firing, the paste and the fired conductor were studied by X-ray diffraction analysis. A complete structural analysis of 80% Ag-2% Pd pastes and the starting Ag and Pd revealed that the initial structure of the Pd controlled the final form of the solid solution in the fired conductor, Pastes made from Pd with internal strain and faulting gave inhomogeneous solid solutions in the temperature ranges studied, whereas pastes made from Pd with no internal strain or faulting gave homogeneous solid solutions. Kinetic studies were made in an effort to under- stand the mechanism of the Ag-Pd reaction for different structural types of Pd. A model mechanism is proposed, and possible effects on the physical characteristics of the pastes are noted.     

Plastic forming of alumina from coagulated suspensions

A processing system has been investigated in which alumina suspensions are initially dispersed at very high solids loadings and then subsequently coagulated by the introduction of salt to the system creating a plastic paste with rheological properties suitable for extrusion. Such coagulated pastes containing an alumina content of 81 wt% (50.7 vol%) have been extruded to produce sintered components possessing an average three point bend strength of 445 MPa. This corresponds to an improvement of 35–45% over samples prepared by a flocculated processing route at the solids loading required for extrusion. This increase in strength is attributed to the initial dispersion of samples at the high solids loading, which serves to maximise suspension homogeneity and leads to a smaller critical flaw size in the final sintered extrudate even though the microstructures and densities are very similar. At present the coagulated pastes produce relatively soft extrudates which can be subsequently moulded into more intricate shapes. Further work is required to formulate stiffer pastes resulting from higher alumina contents.     

Preparation and rheological behaviors of a nano-yttria anti-bonding paste

Anti-bonding pastes are used on areas that should not be welded during superplastic forming/diffusion bonding processes in multi-layer, complex metal structures. We prepared a nano-Y₂O₃ anti-bonding paste to prevent the coated anti-bonding paste from flowing to unwanted areas and to improve the thickness uniformity of the coated paste. Nano-spherical Y₂O₃ powders with a purity of more than 99.99 wt% and a narrow size distribution of 50 to 100 nm were prepared by liquid precipitation followed by sintering. The relationship between the rheological behaviors and dispersion structure of both pastes with and without 1.5 wt% rheology control additive was analyzed in detail. The results indicated that a strong three-dimensional network flocculation structure had formed in the pastes, especially in those with a rheology control additive. The resulting large yield stress, high thixotropy, and solid-like elastic behavior of the paste are favorable for maintaining the original coated shape and offer long-term stability, but are unfavorable for the screen printing (coating) process and thickness uniformity. To solve these problems, a high printing pressure and the use of a screen with high tension and large mesh count (325 mesh) could be adopted. The thermal analysis results show that the paste after SPF/DB processing was easy to clear with no negative influence on the process.     

Rheological analysis of ceramic pastes with ethyl cellulose for screen-printing

Pastes of ceramic particles are used in many industrial applications such as in printed electronic products and solid oxide fuel cells. Ethyl cellulose (EC), a polymer component in the paste, is very useful to control the rheological properties of the pastes for screen printing. In this study, to prepare ceramic La_0.6Sr_0.4Ti_0.3Fe_0.7O_3−δ (LSTF) pastes, EC was mixed with an organic solvent, ethylene glycol monobutyl ether acetate, and LSTF powder. The effects of EC and LSTF particle contents were investigated on the rheological properties of the pastes, using shear, stress, frequency-dependence, and a 3-step recovery tests. The recovery ratio of the pastes in the 3-step tests was found to depend strongly on their EC and LSTF particle contents, decreasing at higher contents due to the formation of a strong network. We also investigated the microstructure and surfaces of thick films fabricated through screen-printing, and confirmed that pastes with a low recovery ratio and high viscosity afford smooth surface profiles.     

How rheological properties affect fine-line screen printing of pastes: a combined rheological and high-speed video imaging study

Fine-line screen printing is still a great challenge due to the lack of overall understanding between the rheological properties and screen printing process. Here, we prepared ZnO pastes including ethyl cellulose or Thixatrol Max as an additive introducing different physical mechanisms of structure and flow control. Yield stress, viscosity, and its recovery after high shear were obtained using rotational rheometry. Filament breakup was determined in uniaxial elongational tests. Pastes were printed using a commercial screen designed for Si-solar cell front-side metallization, and the process was monitored with high temporal and spatial resolution using a transparent glass substrate and a high-speed imaging setup. Length of the pre-injection zone ahead of the squeegee scales inversely with yield stress and length of the cling zone behind the squeegee is proportional to filament rupture strain, irrespective of used additive. Paste spreading observable at the busbar and fine-line intersection takes place within 100 ms, irrespective of sample composition, demonstrating that fine-line electrode width is determined within the pre-injection zone where the paste is under pressure. A simple flow model is proposed relating electrode width to the reciprocal product of yield stress and high shear viscosity consistent with experimental data including both types of pastes.     

Factors influencing rheological characteristics of silver thick film paste and its correlation to multilayer ceramic processing

The rheological properties of silver thick film paste are influenced by the particle size, morphology of silver particles and by the nature of the filler. Rheological properties in-turn influence the multilayer processing parameters and properties. Silver particles, synthesised with gum Arabic (Ag/GA) as the dispersant resulted in agglomerated particles with irregular morphology. Ag/GA-based thick film paste exhibited thixotropic characteristics. Silver powder synthesised with polyvinyl alcohol (Ag/PVA) as the dispersant resulted in well-dispersed, spherical, free-flowing powder with narrow particle size distribution and exhibited pseudoplastic behaviour. The rheological characteristics of Ag/GA-based thick film paste are not influenced by the filler addition. Influence of filler addition is distinctly noted in the rheological characteristics of Ag/PVA-based thick film paste. The homogeneous mixing of Ag/PVA particles with filler and reproducible rheological characteristics of the corresponding thick film paste resulted in excellent printability with good edge-definition and repeatable properties.     

An improvement of color electrophoretic paint via ultrafine modified pigment paste

The ultrafine paste used in electrophoretic paint was prepared via ultrasonic method with pigment yellow 83 (P.Y.83), octadecyl dimethyl benzyl ammonium chloride, and deionized water. The average particle size of the ultrafine paste was only 196?nm and zeta potential was 30?mV with ultrasound for 30?min and 2?wt.% cationic dispersant. The conductivity of the paint was improved as the cationic dispersant increased. Centrifugal stability of the paint with ultrafine paste reached the maximum at 2?wt.% cationic dispersant. Compared with the performances of cathodic electrophoretic paint with the ultrafine yellow paste, the anodic film was smooth, uniform, and neat with 0.5?wt.% cationic dispersant in the ultrafine yellow paste. Deposited amount of the anodic film was 27?g/m², which was higher than the cathodic film. The curing property, adhesion, and chemical resistance of the cathodic film were better than the anodic film. Properties of five color cathodic paints (blue, yellow, red, black, and green) were discussed and were compared with the film of electrophoretic paint containing commercial pastes. Except the green film, the blue, yellow, red, and black films were smooth, fine, and uniform, and also provided good L-effect. The deposited amount of these films with excellent adhesive force was about 14–19?g/m². The electrophoretic paints containing modified pigment pastes and commercial pigment pastes present good adhesion and chemical resistance.     

Fabrication of dye-sensitized solar modules based on a prototyping pilot line and their integration into energy storage microsystems

We successfully developed a prototyping pilot line for dye-sensitized solar modules up to 23 × 23 cm². Dedicated ceramic materials were studied for large area application: we formulated a lead-free glass frit sealant with low softening point based on the ternary system Bi₂O₃-B₂O₃-ZnO and a TiO₂ anatase screen-printable paste for semi-transparent photoanodes. Alongside traditional screen-printing, specific machines among which a thermal press, a dye injection equipment and an automatic electrolyte filler were designed and realized. A silver-free layout was adopted and the solar modules reached an efficiency close to 2%. Photovoltaic glass panels composed by interconnected DSSC modules were interfaced with custom integrated circuits to realize energy storage microsystems for two self-powered prototypes: a USB solar table for recharging of portable devices and a novel LED glass parapet for active night lighting are here presented.     

Screen printing of sol–gel-derived electrolytes for solid oxide fuel cell (SOFC) application

The idea of using the sol–gel technique for producing low-cost components for solid oxide fuel cell (SOFC) application has attracted great interest. Besides its economic advantages, the sol–gel technique additionally offers the chance to reduce either the thickness of the electrolyte and therefore to reduce ohmic resistances or to lower the sintering temperature of single components like the electrolyte layer, due to the clearly reduced particle sizes of colloidal distributed particles in the sol.The work presented here deals with the development of sols and their application in combination with yttria fully stabilized zirconia mixed-oxide powders for the preparation of screen-printing pastes. Besides physical, chemical and thermal characterization of the sols, variations of the composition of the sol as well as of the pastes composed of sol and mixed-oxide powder were evaluated for preparing dense, gas-tight layers sintered at various temperatures, resulting in sufficient gas-tightness to ensure high power density SOFCs. Additionally, technological screen-printing parameters were studied.Single cell tests (50 mm × 50 mm) revealed current densities of approx. 1 A/cm². These values are comparable to current densities obtained by cells based on normal electrolyte layers, which were prepared in parallel.     

Rheological Analysis of Concentrated Zirconia Pastes with Ethyl Cellulose for Screen Printing SOFC Electrolyte Films

Concentrated zirconia pastes require optimization before they can be used in large-scale screen-printing processes in solid oxide fuel cells (SOFC) manufacture. The present article investigates a series of terpineol-based zirconia pastes stabilized with ethyl cellulose of varying concentrations by combined rheological methods. Small additions of ethyl cellulose (up to 0.50 vol%) resulted in a significant increase in the shear viscosity, shear stress, and network strength of the pastes. In comparison with the properties of the ink vehicles, the observed elastic response of the ink pastes on the initiation of flow was attributed to the interaction of ethyl cellulose with the zirconia particles. This interaction was confirmed to be bridging flocculation by rheological methods. Start-up shear flow experiments assessed the elastic deformation of the pastes in greater detail. Application of transient network theory (TNT) to describe the properties of the pastes was confirmed by the linear dependence of the equilibrium shear modulus ( G _∞) with increasing ethyl cellulose concentration. TNT provides a basis for explaining the rheological effects of bridging polymers in network suspensions and assessing their interaction with other organic additives in high solids volume suspensions. This is a useful analysis for optimizing suspensions for screen-printing SOFC components among other applications.     

Isolating the effects of thixotropy in geopolymer pastes

The rheological properties of potassium-based geopolymers were investigated through a series of experiments intended to isolate the influence of shear rate, recovery time, and shear ramping on thixotropy for a greater understanding of geopolymer thixotropic properties within the context of the geopolymer setting reaction. It is shown that for thixotropic disruption to occur a critical shear rate of 100 s⁻¹ must be reached or surpassed, full thixotropic restructuring occurs at around 90–100 min of total undisturbed rest time, and that reaching a state of full thixotropic disturbance heavily depends on subjected processing parameters. In addition, a consistent crossover between the storage and loss modulus within 1–3 min of oscillation during cyclical oscillatory measurements greatly indicates the repeatability and reversibility of thixotropy in geopolymers and the potential for tailorable viscosity. Overall, it is found that geopolymer pastes exhibit strong evidence of thixotropy, which is favorable for additive manufacturing, and that allotted rest time before shear and shear rate greatly influence the overall rheological properties.     

Low-temperature sintering and electrical properties of Ba0.68Sr0.32TiO3 thick films

Ba_0.68Sr_0.32TiO₃ (BST) thick films were prepared by screen printing on a flexible fluorophlogopite substrate. In order to realise the co-firing of the BST film with a silver electrode at a lower temperature, the BST precursor was used as a solvent for the screen-printing slurry and the cold sintering technique was used to pretreat the film. The sintering temperature of BST thick films prepared by conventional sintering process was higher than 1200 °C. When sintered at 950 °C, the thick films exhibited a high porosity. The density of the thick films was significantly improved after pretreatment with the cold sintering process (CSP). After the cold-sintered thick films were sintered at 950 °C for 30 min and then fired with a silver electrode, the samples exhibited a relative dielectric constant of 773 (at 25 °C and 10 kHz), a dielectric loss of 0.025, a remanent polarization of 5.3 μC/cm², and a coercive field strength of 38.1 kV/cm. Therefore, the low-temperature co-firing of BST thick films with a silver electrode was successfully realised.     

Rheology,injectability, and bioactivity of bioactive glass containing chitosan/gelatin,nano pastes

Bioactive pastes containing bioactive sol–gel derived glass (BG) and various amounts of chitosan (Cn) and gelatin (Gel) were prepared in this study. To be exact, three pastes were prepared by mixing 25 parts by weight of glass powder with (a) 100 parts by weight of a 3 wt% acetic acid-based chitosan solution, (b) 100 parts by weight of a 3 wt% water-based gelatin solution, and (c) 100 parts by weight of a solution containing equal amounts of the above-mentioned solutions. The bioactivity of the composite samples was evaluated by the immersion of the prepared pastes into the simulated body fluid (SBF) solution. The samples were also analyzed by X-ray diffractometry (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscope (SEM), Fourier transform infrared (FTIR), and atomic absorption spectroscopy (AAS). The results indicated better apatite formation capacity on the glass-/chitosan-/gelatin-injected paste after 14 days. Furthermore, unlike the chitosan containing paste, the gelatin-containing sample was injectable and displayed viscoelastic behavior as determined by conducting the rheology test in oscillation mode. In addition, while chitosan made the paste more viscous, it improved the washout resistance when compared to the gelatin-containing sample. The experimental results also indicate the formation of spherical calcites in the pastes prior to immersion into the SBF solution.     

Chemical Resistance of Low-Melting Enamels for Decorating Glass Products

The chemical resistance of lead-free low-melting opaque enamels intended for decorating the external surface of household glass is determined depending on the duration of treatment with hot (98°C) water and 2% sodium carbonate solution. The synthesized enamels can be attributed to the class of enamels resistant to hot water and weakly alkaline solutions.     

Low temperature preparation and machinability of porous ceramics from talc and foamed glass particles

Porous ceramics were prepared by firing mixtures of talc (Mg₃Si₄O₁₀(OH)₂) and foamed glass particles (ceramic balloons, CB) with and without LiCl as a sintering acid. The mixing ratios of the starting materials were talc:CB = 7:3, 8:2, 9:1 and 10:0, with additions of LiCl of 0, 2 and 5 mass%. The mixtures were formed into pellets and fired at 600–1000 °C. The pellets without LiCl showed very poor strength even when fired at 1000 °C but those containing LiCl were much stronger, even when fired at only 600 °C. The crystalline phases in these samples changed to enstatite (MgSiO₃) at ≥ 700 °C by decomposition of the talc under the fluxing action of the LiCl. The resulting samples were machinable and easily cut and drilled. The cutting rate decreased with increasing bending strength, for example, from 105 mm²/s and 6.3 MPa to 50 mm²/s and 16.3 MPa, respectively. The drilling rate of the present sample was found to be only slightly less than Teflon (polytetrafluoroethylene, PTFE) but much faster than graphite, glass ceramics, etc.