Sen Mei

In situ preparation of weakly flocculated aqueous anatase suspensions by a hydrothermal technique.

Weakly flocculated aqueous anatase suspensions were prepared in situ by hydrothermally treating amorphous titania particles peptized with different amounts of tetraethylammonium hydroxide (TENOH). The simultaneous formation of hydrous TiO2 polyanions in the presence of OH- and tetraethylammonium cations are two essential conditions for the peptization process to occur. The absence of either of these conditions will cause reprecipitation. Transmission electron microscopy (TEM) revealed that the morphology of the particles formed at low TENOH concentrations consisted of well-dispersed anatase crystals, changing to asterisk-like structured particles with increasing concentrations of TENOH. Because of the extremely high absolute zeta potential (over -70 mV in all the samples) and ionic strength values, nontouching particle networks may be formed in situ in the mother solution in all samples, as predicted by DLVO theory. A trend to coagulation was observed in the suspensions with increasing concentrations of TENOH due to a more pronounced secondary minimum in the particle pair potential curves. Assuming the particles remained in the secondary minimum throughout the hydrothermal treatment may lead to the formation of the asterisk-like hard agglomerates. This may arise from the condensation of the -OH-rich TiO2 particles or from the deposition of material in the interparticle gap during the particle growth process. The green packing density of slip-cast bodies from a suspension containing 20 wt% solids was around 46%.

Hydrothermal processing of nanocrystalline anatase films from tetraethylammonium hydroxide peptized titania sols

Abstract Nanocrystalline anatase films were obtained by hydrothermally treating dip-coated TiO 2 gel films onto silicon substrates from tetraethylammonium hydroxide (TENOH) peptized titania aqueous sols diluted with different solvents (isoproponol, acetone and propanediol) at 240°C for 2 h. SEM, AFM and XRD analysis were used to characterize the microstructure and phase evolution of the films. The highest degree of homogeneity and good wettability towards the substrate could only be obtained with mixed solvents of acetone and isopropanol, yielding denser structure with homogeneously distributed fine grains among all the three solvents. A uniform distribution of fine grains of around 50 nm was obtained after hydrothermal treatment, while the film calcined at 500°C/2 h displayed larger grains of around 80 nm and were not so uniformly developed. All these results suggest that (a) TENOH peptized titania sol is a promising novel stock precursor for synthesizing TiO 2 films; (b) hydrothermal technique is quite efficient in preparing titania nanocrystalline films compared with calcination method.

Cordierite-based glass-ceramics processed by slip casting

Abstract In this research work, cordierite-based glass ceramics were prepared by slip casting from slurries with different solids loading. The amount of dispersant imposed different influence on the rheological behaviour of the cordierite–glass suspensions at different solids loading. A shear thinning behaviour was observed in the lower solids loaded suspensions (50 wt.%) with different amounts of dispersant within the whole range of shear rate. As the solids loading increased (60, 70 wt.%), the rheological behaviour of the suspensions gradually changed to shear thickening at higher shear rates. An optimal amount of dispersant for the cordierite–glass suspensions was found, which could lead to green bodies with homogeneous microstructures and to the smallest pore diameters. The sintered bodies prepared by slip casting exhibit higher density and microstructural homogeneity than those obtained by dry pressing.

Microstructural evolution in sol–gel derived P2O5-doped cordierite powders

Abstract The purpose of this paper is to analyse the effect of various amounts of P2O5 additive on the microstructural evolution of cordierite. In this research work, cordierite powders were prepared by sol–gel processing. The phase and microstructural evolution of the powders were characterised by DTA, XRD and SEM. The P2O5 additive was found to promote the μ-cordierite to α-cordierite transition at low concentrations, while a non-steady effect was shown for higher concentrations. The effect of P2O5 on the activation of cordierite can be attributed to the nucleation of P2O5 and the formation of mullite and xMgO-P2O5 based compounds (x=3,2,1).

Hydrothermal synthesis of well-dispersed TiO2 nano-crystals

Well-dispersed anatase and rutile nano-particles were prepared via hydrothermal treatment of tetrabutylammonium hydroxide-peptized and HNO 3 -peptized sols at 240 °C. A broad particle size distribution of anatase crystals was observed in the nonpeptized TiO 2 species hydrothermally treated at 240 °C. X-ray diffraction and transmission electron microscopy, as well as zeta potential measurement, were used to characterize the particles. The formation of the well-dispersed anatase and rutile particles from the peptized samples could be attributed to (i) homogeneous distribution of the component in the peptized sols, and (ii) the high long-range electrostatic forces between particles in the presence of both peptizers, which were not present in the nonpeptized samples. This work provided a new way to prepare nano-crystals of titania.

Optimisation of parameters for aqueous tape-casting of cordierite-based glass ceramics by Taguchi method

Abstract Aqueous suspensions of cordierite-based glass ceramics were prepared by using four types of dispersants and binders and different solids loading. The experiments were designed according to the Taguchi method, which shows great advantages in optimising more than two factors that need to be considered in an experimental design. Different parameters such as the type and concentration of the dispersants and the binders, and the solids loading were optimised to obtain homogeneous and crack-free green tapes. Dolapix CE 64 (1.0 wt.%) and Duramax B-1080 or Duramax B-1070 (10 wt.%) with 65 wt.% solids loading represent an optimal selection of the parameters to obtain low viscosity suspension, and crack-free green tapes with the highest green and sintered density. Microstructural differences between crack-free and cracked samples were observed by scanning electron microscopy (SEM). The crack-free green tapes show homogenous microstructures from top to bottom with organic additives uniformly surrounding the powders, whereas cracked samples exhibit heterogeneous microstructures and non-uniform distribution of the organics.

Sol-gel derived P2O5-doped cordierite powders: characterization and phase transformation

Abstract The application of cordierite glass-ceramics as electronic materials has been studied extensively because of their excellent dielectric properties. In this study, stoichiometric and P 2 O 5 -doped cordierite powders were synthesized by sol-gel method. The effect of P 2 O 5 additive on the crystallization and phase transformation was investigated using differential thermal analysis (DTA), infrared spectroscopy (IR) and X-ray diffraction (XRD). The activation energies for μ-cordierite to α-cordierite transition with various additive concentrations were obtained using Kissinger equation. It was found that P 2 O 5 promotes the μ-cordierite to α-cordierite transition at low additive concentrations, while it plays a non-steady inhibitory role at higher concentrations. IR spectra and XRD results revealed the presence of farringtonite at lower P 2 O 5 additive concentrations and magnesium pyrophosphate at the higher concentrations, which lead to the increment of activation energies.

The densification and morphology of cordierite-based glass-ceramics

Abstract In this study, five different proportion of a borosilicate glass (from 30 to 70 wt.%) were added to stoichiometric cordierite to improve the densification behaviour of cordierite-based glass-ceramica. The phase and microstructural evolution of the samples were characterised by DTA and SEM. The results show that glass content plays an important role in the sintering of cordierite-based glass-ceramics. The sintered density increases as the glass content increases at low temperature (950°C). At high temperatures, the sintered density passes through a maximum value when the glass content is about 60 wt.%, followed by a decrease as the glass content further increases. DTA results show that densification is a non-reactive liquid phase sintering process. The microstructural evolution of the samples is in good agreement with the measurements of sintered densities. At the lower glass content, the sample shows more pores and a non-uniform pore size distribution. With the glass content increasing, the pore size becomes more uniform and sintered density increases.

The fabrication and characterisation of low-k cordierite-based glass-ceramics by aqueous tape casting

Abstract In this work, cordierite-based glass ceramic tapes were prepared by tape casting from suspensions containing different solids loading and different average particle sizes (PS). Three different empirical models were used to fit the experimental data from relative viscosity measured at a fixed shear rate (500 s −1 ) versus solids loading. The green tapes were characterised by using scanning electron microscopy (SEM) and Hg porosimetry. The results showed that high values of solids volume fraction and PS ratio between glass and cordierite powders favour homogeneous and dense substrates. The dielectric properties of the sintered tapes were also measured, which met the requirements for the substrates in microelectronic and packaging industries.

Comparison of dispersants performance in slip casting of cordierite-based glass-ceramics

In this paper, four different dispersants (Tiron, Duramax D-3021, Targon 1128, and Dolapix CE 64) were selected as processing aids and their effects on electrophoretic properties of cordierite and glass powders, on the rheological behaviour of the suspensions and on the microstructures of the slip-cast green bodies were compared. Dolapix CE 64 rendered the particles the highest absolute values of zeta potential, the lowest relatively viscosities to the suspensions, and gave rise to the densest and more homogeneous green microstructures, compared with the other three dispersants. The differences are probably due to different thickness of the adsorbed layers and/or different conformations of the dispersants at the surface of the particles caused by differences in molecular weights as well as hydrophilic/hydrophobic ratios.