Huang Liping

Optimizing the rheological behavior of silicon nitride aqueous suspensions

Abstract This work focuses on the optimization of the rheological behaviors of the Si 3 N 4 aqueous suspensions by changing the kind and concentration of dispersant, pH value of dispersing medium and other process parameters. Zeta potentials, sedimentation and rheological behavior measurements have been carried out to prepare a well dispersed, uniform and concentrated Si 3 N 4 aqueous suspension. The isoelectric point of Si 3 N 4 powders is at the pH value of 4.2 and the value of Zeta potential is up to its maximum near pH 11. A good agreement between sedimentation and electrophoresis tests is found which identifies the optimum pH value for promising dispersion. Na 2 SiO 3 is an efficient dispersant which increases Zeta potentials in magnitude and improves the fluidity of Si 3 N 4 suspensions effectively. The optimum concentration of dispersant is in the range of 1.0 to 1.2 wt%, which is independent on the solids volume fraction of slips. A ball milling of 8 h helps to improve the fluidity of suspensions while longer milling is detrimental.

Rheological properties of aqueous silicon nitride suspensions

The effect of surface modification of Si3N4 particle on the colloidal behavior and the rheological properties of aqueous Si3N4 suspensions under steady and oscillatory conditions are investigated in detail. Due to the decrease of the oxidizing level, the isoelectric point (IEP) of the modified particle shifts to basic region gently. Attempts have been made to apply rheological models to the suspensions with various solid volume fraction (φ). For the as-received suspensions, the Sisco model provides the best fit in the range of φ ≤ 0.30 while the Casson model in 0.35 ≤ φ ≤ 0.45. The shear behavior of modified suspensions fits to Sisco model in the range of φ ≤ 0.40 and Casson model in 0.45 ≤ φ ≤ 0.54. The rheological behavior of modified suspensions is improved efficiently. The critical strain decreases and the linear viscoelastic regime narrows continuously with increasing solid concentration. For the modified suspensions, the linear viscoelastic regime broadens and the corresponding elastic modulus decreases sharply. With increasing solid concentration, the characteristic frequency shifts toward lower frequencies and the suspension transforms from more viscous to more elastic.

The combined effects of nanotopography and Sr ion for enhanced osteogenic activity of bone marrow mesenchymal stem cells (BMSCs)

Both surface topography and chemistry have a significant influence on the biological performance of orthopedic implant coatings. In our study, a surface modification strategy embodying bioactive trace element incorporation and nanotopography construction was employed to enhance the osteogenic activity of calcium silicate (Ca-Si) coatings. We developed strontium-loaded nanolayer on plasma sprayed Ca-Si (CS) coating via hydrothermal treatment which was denoted as Sr-NT-CS. The original CS coating and the CS coating modified with similar nanotopography (NT-CS) were studied in parallel. We investigated the cellular effects of surface topography and released Sr ion on the adhesion, proliferation, differentiation, and mineralization of BMSCs and the associated molecular mechanisms. The results indicated that the nanotopography activated integrin β1, promoted the spread of BMSCs into a polygonal osteoblastic shape, and induced higher levels of collagen secretion. The Sr incorporation stimulated osteogenic differentiation and mineralization as indicated by the increases in ALP activity and mineralized nodules formation. The examination of gene expressions revealed that Sr ion exerted the effects by interacting with extracellular calcium sensitive receptor (CaSR), and combined with the nanotopographical cue for the up-regulation of osteogenic master transcription factor Runx2. The promoted Runx2 subsequently affected osteoblast (OB) marker genes (BMP-2, BSP, OPN, and OCN), thus driving BMSCs to differentiate into OBs. Moreover, the Sr incorporation inhibited osteoclastogenesis, as indicated by the down-regulation of interleukin-6 (IL-6) and the inhibition of RANKL/RANK system. Those results suggested that our developed Sr-NT-CS coating have combined the effects of nanotopography and Sr ion for enhanced osteogenic activity of BMSCs.

Effects of dispersant on rheological behavior of silicon nitride aqueous suspension

Colloidal processing techniques such as slip casting, centrifugal slip casting or pressure slip casting are considered as a powerful route to fabricate silicon nitride ceramic components with complex shapes [1, 2]. For this purpose, Si3N4 powder should be dispersed efficiently in liquid medium thus forming a desirable suspension with the lowest viscosity and highest solid content. Colloidal behavior of Si3N4 suspension with either aqueous or organic medium has been studied widely [3–7]. The same Si3N4 powder behaves differently depending on the various processing parameters applied. In order to achieve promising powder suspensions, dispersants are frequently used [7–10]. After that the particle surface has been modified with dispersant by either changing the surface charge, i.e. by electrolytes, or by coating with a large amount of organic barrier, i.e. by polymers, or by a combined action of both factors, i.e. by polyelectrolytes, Si3N4 particles can be stabilized to a promising level in liquid media. The introduction of dispersant to suspension generally has important influence on the electrokinetics of suspended particles and the rheological behavior of suspension. In this letter, some experimental results about the effects of dispersant on the rheological behavior of Si3N4 aqueous suspension will be discussed. Silicon nitride powder (H.C. Starck, Germany) with an average particle size of 0.60 μm and a BET specific surface area of 21.0 m2 g−1 was used for the present study. The dispersant used was an inorganic compound which stabilized the suspension by changing the surface charge of Si3N4 particles with its negatively charged anion group in electrostatic mechanism. The zeta potential of Si3N4 particles was measured with diluted Si3N4 dispersion (volume fraction, φ < 0.005) at 25 ◦C on Zetasizer 4 (Malvern, UK). The rheological behavior of Si3N4 suspension was characterized in steady shear mode with a couette (cup radius: 34.0 mm, bob radius: 32.0 mm, bob length: 33.3 mm) on Rheometric Fluid Spectrometer II (RFS II). The zeta potential of Si3N4 particles as a function of pH of suspension is shown in Fig. 1. As can be seen the isoelectric point (IEP) of Si3N4 particles is at the pH value of 4.2 and the particles have a maximum zeta potential at about pH = 11. The introduction of dispersant increases efficiently the zeta potential of Si3N4 particles in magnitude and shifts the IEP to acid region. Meanwhile, the pH values corresponding to the maximum of zeta potential for Si3N4 particles with different dispersant concentration remain constant. Therefore, in this work, all suspensions investigated are kept at pH = 11 expecting a good fluidity obtained. It seems that the changes of zeta potential and IEP of Si3N4 particles result from the increasing of the charging density of the particles caused by the adsorption of negatively charged anion groups of dispersant on the surface of Si3N4 particles. Fig. 2 illustrates the flow curves of 23.9 vol % Si3N4 aqueous slurry containing various amounts of dispersant. As is shown, the behavior of slurry transfers from an anti-thixotropy via near Bingham fluid to thixotropy with increasing amount of dispersant. It is clear that the Si3N4 suspension studied is a non-Newtonian time-dependent fluid which exhibits a pronounced thixotropy. When the amount of dispersant is below 0.8 wt % (weight percent, on a powder basis) the suspension exhibits an anti-thixotropic behavior and the anti-thixotropy decreases with increasing the dispersant amount. The anti-thixotropic behavior of the suspension is considered as due to the irregular morphology of Si3N4 particles. At appropriate dispersant range from 0.8–1.2 wt %, the particles are well dispersed in media so that the anti-thixotropic behavior disappeared and a near Bingham behavior is exhibited. With further increasing of the dispersant, the excessive dispersant moleculars are bridged together to produce some particle networks in suspension which could be broken down in the high applied shear field, so the suspension exhibits a thixotropic behavior at higher dispersant amounts.

真空等离子体喷涂B 4 C-Mo复合涂层耐磨性能研究

采用真空等离子体喷涂技术制备了B 4 C-Mo复合涂层, 并对其耐磨性能进行了研究。与B 4 C纯涂层相比, 复合涂层结构更为致密, (B, Mo)C过渡相的存在改善了B 4 C相与Mo相之间的润湿性, 进而有效提高了涂层的抗摩擦磨损性能。此外, Mo在喷涂过程中形成了大量的纳米晶, 这也在一定程度上促进了复合涂层耐磨性能的提高。

Types of superplasticm-δ curves of Ti-6Al-4V alloy and their intertransformation

The superplasticm-δ curves of Ti-6Al-4 alloy have been determined at different temperatures and strain rates; all were of themL=mmax type, and were of either fundamental, descending or ascending types. The latter two types were transformed from the former through the processes ofmL→mO, δL→δO andmL→mF, δL→δF, respectively. The highest total elongation, δF, was 1150% obtained at 950‡C and 1.55×10−3 s−1. The Chin Liu equation has been applied to all types ofm-δ curves. The parameters characteristic of all types ofm-δ curves have been obtained and were found to vary with temperature and strain rate. The total elongation, δF, is determined by the resultant effects of all other parameters, not by a singlem value, theoretically considered to be constant and determined practically by a tensile strain of about 30%–50% (m30%–50%) as usual.