Liu Xuejian

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.

Effect of Density and Surface Roughness on Optical Properties of Silicon Carbide Optical Components

The effect of density and surface roughness on the optical properties of silicon carbide optical components is investigated. The density is the major factor of the total reflectance while the surface roughness is the major factor of the diffuse reflectance. The specular reflectance of silicon carbide optical components can be improved by increasing the density and decreasing the surface roughness, in the form of reducing bulk absorption and surface-related scattering, respectively. The contribution of the surface roughness to the specular reflectance is much greater than that of the density. When the rms surface roughness decreases to 2.228 nm, the specular reflectance decreases to less than 0.7% accordingly.

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.

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.

水基浆料涂覆结合原位反应制备C f /SiC复合材料表面光学涂层

A novel surface modification method for C-f/SiC composites is proposed with aqueous slurry painting together with reaction bonding. Stable slurries were obtained by dispersing SiC to carbon black in aqueous solution, under centain binder content and solid loading conditions. The results show that porous carbonaceous C/SiC tape with porosity of 49% could be obtained by aqueous slurry painting. Si/SiC coating with high density and strong bonding onto the substrate is realized by infiltrating liquid silicon into the as-prepared tape. A reaction layer of similar to 15 um in thickness is formed between the coating and the substrate. This Si/SiC coating exhibits excellent mechanical properties, with HV hardness of (14.19 +/- 0.46) GPa and fracture toughness of (3.02 +/- 0.30) MPa.m(1/2). The coating is achieved fine surface of 2.97 nm RMS in roughness after precision grinding and polishing.本研究提出一种C f /SiC复合材料表面改性新方法为水基浆料涂覆结合原位反应烧结工艺。系统研究了SiC和炭黑在水基浆料中的共分散、粘结剂的量和浆料固含量对浆料流变性能的影响、涂层的微观结构和性能等。研究结果表明: 采用水基浆料涂覆工艺可在基材表面制备一层气孔率达49%的多孔C/SiC预涂层; 通过液相渗硅原位反应工艺, 多孔预涂层转变为高致密、与基材强结合的光学涂层, 并且在涂层与基材间形成了~ 15 μm的化学反应过渡层; Si/SiC涂层的维氏硬度为(14.19 ± 0.46) GPa, 断裂韧性为(3.02 ± 0.30) MPa·m 1/2 ; 经过精细研磨抛光, 涂层的表面粗糙度可达2.97 nm RMS。

Optical Coating on C

A novel surface modification method for C-f/SiC composites is proposed with aqueous slurry painting together with reaction bonding. Stable slurries were obtained by dispersing SiC to carbon black in aqueous solution, under centain binder content and solid loading conditions. The results show that porous carbonaceous C/SiC tape with porosity of 49% could be obtained by aqueous slurry painting. Si/SiC coating with high density and strong bonding onto the substrate is realized by infiltrating liquid silicon into the as-prepared tape. A reaction layer of similar to 15 um in thickness is formed between the coating and the substrate. This Si/SiC coating exhibits excellent mechanical properties, with HV hardness of (14.19 +/- 0.46) GPa and fracture toughness of (3.02 +/- 0.30) MPa.m(1/2). The coating is achieved fine surface of 2.97 nm RMS in roughness after precision grinding and polishing.本研究提出一种C f /SiC复合材料表面改性新方法为水基浆料涂覆结合原位反应烧结工艺。系统研究了SiC和炭黑在水基浆料中的共分散、粘结剂的量和浆料固含量对浆料流变性能的影响、涂层的微观结构和性能等。研究结果表明: 采用水基浆料涂覆工艺可在基材表面制备一层气孔率达49%的多孔C/SiC预涂层; 通过液相渗硅原位反应工艺, 多孔预涂层转变为高致密、与基材强结合的光学涂层, 并且在涂层与基材间形成了~ 15 μm的化学反应过渡层; Si/SiC涂层的维氏硬度为(14.19 ± 0.46) GPa, 断裂韧性为(3.02 ± 0.30) MPa·m 1/2 ; 经过精细研磨抛光, 涂层的表面粗糙度可达2.97 nm RMS。

lt;inf

采用碳热还原氮化法合成了Eu 2+ /Tb 3+ 掺杂的Sr 2 Si 5 N 8 基荧光粉, 并重点研究了Tb 3+ -Eu 2+ 共掺时Sr 2 Si 5 N 8 基荧光粉的发光性能。研究结果表明: 由于Tb 3+ 的f → d间的跃迁是自旋允许的, Sr 2 Si 5 N 8 :Tb 3+ 在330 nm激发光下, 在490、543、585和623 nm四处各有一发射峰, 它们分别来源于Tb 3+ 的 5 D 4 → 7 F j ( j = 6、5、4、3)能级跃迁; 掺入Tb 3+ 对Sr 1.96 Si 5 N 8 :0.04Eu 2+ 的激发谱和发射谱的形状及峰位无明显影响, 当共掺离子Tb 3+ 浓度为 x = 0.01时, 样品发射强度比未共掺的Sr1.96Si5N8:0.04Eu2+提高了约20%, Tb 3+ 主要通过电多极能量传递的方式转向Eu 2+ 。

gt;f

采用碳热还原氮化法合成了Eu 2+ /Tb 3+ 掺杂的Sr 2 Si 5 N 8 基荧光粉, 并重点研究了Tb 3+ -Eu 2+ 共掺时Sr 2 Si 5 N 8 基荧光粉的发光性能。研究结果表明: 由于Tb 3+ 的f → d间的跃迁是自旋允许的, Sr 2 Si 5 N 8 :Tb 3+ 在330 nm激发光下, 在490、543、585和623 nm四处各有一发射峰, 它们分别来源于Tb 3+ 的 5 D 4 → 7 F j ( j = 6、5、4、3)能级跃迁; 掺入Tb 3+ 对Sr 1.96 Si 5 N 8 :0.04Eu 2+ 的激发谱和发射谱的形状及峰位无明显影响, 当共掺离子Tb 3+ 浓度为 x = 0.01时, 样品发射强度比未共掺的Sr1.96Si5N8:0.04Eu2+提高了约20%, Tb 3+ 主要通过电多极能量传递的方式转向Eu 2+ 。

lt;/inf

以Sr2CO3、Si3N4和Eu2O3为原料,以C为主要还原剂,采用碳热还原氮化工艺合成Sr2Si5N8：Eu2＋荧光粉,着重研究了C、Sr2CO3添加量及Eu2＋浓度对产物物相及发光性能的影响。研究结果表明：当C与Si3N4的摩尔比nc/3 4Si Nn=9/5时,合成出Sr2Si5N8：Eu2＋单相荧光粉,添加适当过量的Sr2CO3可提高合成产物的N含量,且荧光粉的发光强度与其N含量呈现正相关关系。在450 nm蓝光激发下,受Eu2＋的4f^6 5d^1→4f^7跃迁作用,Sr2Si5N8：Eu2＋荧光粉在550~700 nm波段范围产生非对称宽带发射。随着Eu2＋掺杂浓度由1.5mol%增加到20mol%,荧光粉的发光强度先增强后减弱,达到2mol%时发生浓度淬灭现象;发射主峰由608 nm逐步红移至641 nm;CIE色坐标从（0.606,0.393）位移至（0.656,0.343）,是一种可用于白光LED的优质红色荧光粉。以Sr 2 CO 3 、Si 3 N 4 和Eu 2 O 3 为原料, 以C为主要还原剂, 采用碳热还原氮化工艺合成Sr 2 Si 5 N 8 :Eu 2+ 荧光粉, 着重研究了C、Sr 2 CO 3 添加量及Eu 2+ 浓度对产物物相及发光性能的影响。研究结果表明: 当C与Si 3 N 4 的摩尔比 n c / n Si 3 N 4 =9/5时,合成出Sr 2 Si 5 N 8 :Eu 2+ 单相荧光粉, 添加适当过量的Sr2CO3可提高合成产物的N含量, 且荧光粉的发光强度与其N含量呈现正相关关系。在450 nm蓝光激发下, 受Eu 2+ 的4f 6 5d 1 → 4f 7 跃迁作用, Sr 2 Si 5 N 8 :Eu 2+ 荧光粉在550~700 nm波段范围产生非对称宽带发射。随着Eu 2+ 掺杂浓度由1.5mol%增加到20mol%, 荧光粉的发光强度先增强后减弱, 达到2mol%时发生浓度淬灭现象; 发射主峰由608 nm逐步红移至641 nm; CIE色坐标从(0.606, 0.393)位移至(0.656, 0.343), 是一种可用于白光LED的优质红色荧光粉。以Sr 2 CO 3 、Si 3 N 4 和Eu 2 O 3 为原料, 以C为主要还原剂, 采用碳热还原氮化工艺合成Sr 2 Si 5 N 8 :Eu 2+ 荧光粉, 着重研究了C、Sr 2 CO 3 添加量及Eu 2+ 浓度对产物物相及发光性能的影响。研究结果表明: 当C与Si 3 N 4 的摩尔比 n c / n Si 3 N 4 =9/5时,合成出Sr 2 Si 5 N 8 :Eu 2+ 单相荧光粉, 添加适当过量的Sr2CO3可提高合成产物的N含量, 且荧光粉的发光强度与其N含量呈现正相关关系。在450 nm蓝光激发下, 受Eu 2+ 的4f 6 5d 1 → 4f 7 跃迁作用, Sr 2 Si 5 N 8 :Eu 2+ 荧光粉在550~700 nm波段范围产生非对称宽带发射。随着Eu 2+ 掺杂浓度由1.5mol%增加到20mol%, 荧光粉的发光强度先增强后减弱, 达到2mol%时发生浓度淬灭现象; 发射主峰由608 nm逐步红移至641 nm; CIE色坐标从(0.606, 0.393)位移至(0.656, 0.343), 是一种可用于白光LED的优质红色荧光粉。

gt;/SiC Composites via Aqueous Slurry Painting and Reaction Bonding

以Sr2CO3、Si3N4和Eu2O3为原料,以C为主要还原剂,采用碳热还原氮化工艺合成Sr2Si5N8：Eu2＋荧光粉,着重研究了C、Sr2CO3添加量及Eu2＋浓度对产物物相及发光性能的影响。研究结果表明：当C与Si3N4的摩尔比nc/3 4Si Nn=9/5时,合成出Sr2Si5N8：Eu2＋单相荧光粉,添加适当过量的Sr2CO3可提高合成产物的N含量,且荧光粉的发光强度与其N含量呈现正相关关系。在450 nm蓝光激发下,受Eu2＋的4f^6 5d^1→4f^7跃迁作用,Sr2Si5N8：Eu2＋荧光粉在550~700 nm波段范围产生非对称宽带发射。随着Eu2＋掺杂浓度由1.5mol%增加到20mol%,荧光粉的发光强度先增强后减弱,达到2mol%时发生浓度淬灭现象;发射主峰由608 nm逐步红移至641 nm;CIE色坐标从（0.606,0.393）位移至（0.656,0.343）,是一种可用于白光LED的优质红色荧光粉。以Sr 2 CO 3 、Si 3 N 4 和Eu 2 O 3 为原料, 以C为主要还原剂, 采用碳热还原氮化工艺合成Sr 2 Si 5 N 8 :Eu 2+ 荧光粉, 着重研究了C、Sr 2 CO 3 添加量及Eu 2+ 浓度对产物物相及发光性能的影响。研究结果表明: 当C与Si 3 N 4 的摩尔比 n c / n Si 3 N 4 =9/5时,合成出Sr 2 Si 5 N 8 :Eu 2+ 单相荧光粉, 添加适当过量的Sr2CO3可提高合成产物的N含量, 且荧光粉的发光强度与其N含量呈现正相关关系。在450 nm蓝光激发下, 受Eu 2+ 的4f 6 5d 1 → 4f 7 跃迁作用, Sr 2 Si 5 N 8 :Eu 2+ 荧光粉在550~700 nm波段范围产生非对称宽带发射。随着Eu 2+ 掺杂浓度由1.5mol%增加到20mol%, 荧光粉的发光强度先增强后减弱, 达到2mol%时发生浓度淬灭现象; 发射主峰由608 nm逐步红移至641 nm; CIE色坐标从(0.606, 0.393)位移至(0.656, 0.343), 是一种可用于白光LED的优质红色荧光粉。以Sr 2 CO 3 、Si 3 N 4 和Eu 2 O 3 为原料, 以C为主要还原剂, 采用碳热还原氮化工艺合成Sr 2 Si 5 N 8 :Eu 2+ 荧光粉, 着重研究了C、Sr 2 CO 3 添加量及Eu 2+ 浓度对产物物相及发光性能的影响。研究结果表明: 当C与Si 3 N 4 的摩尔比 n c / n Si 3 N 4 =9/5时,合成出Sr 2 Si 5 N 8 :Eu 2+ 单相荧光粉, 添加适当过量的Sr2CO3可提高合成产物的N含量, 且荧光粉的发光强度与其N含量呈现正相关关系。在450 nm蓝光激发下, 受Eu 2+ 的4f 6 5d 1 → 4f 7 跃迁作用, Sr 2 Si 5 N 8 :Eu 2+ 荧光粉在550~700 nm波段范围产生非对称宽带发射。随着Eu 2+ 掺杂浓度由1.5mol%增加到20mol%, 荧光粉的发光强度先增强后减弱, 达到2mol%时发生浓度淬灭现象; 发射主峰由608 nm逐步红移至641 nm; CIE色坐标从(0.606, 0.393)位移至(0.656, 0.343), 是一种可用于白光LED的优质红色荧光粉。