Dispersibility of pretreated polyacrylic acid-modified SiC powder

A slurry with high dispersibility is crucial for slip casting technology. The weakly acidic SiC powder with high dispersibility was prepared with polyacrylic acid pretreated by the sodium hydroxide (adjusted pH = 5). The pretreated polyacrylic acid can be effectively adsorbed on the surface of SiC powder, and its utilization efficiency (37%) was better than the unpretreated one (21%). The modified SiC powder with pH = 6 was obtained by polyacrylic acid modification, and the maximum solid content was 61.2 Vol%. The optimum of the pretreated polyacrylic acid addition amount was 1 wt% based on Zeta potential, solid content, and viscosity analysis data. Compared with the unmodified SiC, the Zeta potential absolute value of the modified (−63 mV) was higher due to a large amount of anionic modifier adsorbed on particle surfaces. The modified SiC slurry was more stable because the modification increases the electrostatic repulsion among the SiC molecules.     

Surface modification of superfine SiC powders by ternary modifiers-KH560/sodium humate/SDS and its mechanism

SiC is a promising functional ceramic material with many great properties. High concentrated SiC slurry with excellent rheology and stability is required in some processes of ceramic forming. In this work, the dispersion of SiC powders was obviously improved by ternary modifiers: γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH560), sodium humate and sodium dodecyl sulfate (SDS). Modified SiC slurry showed the lowest viscosity of 0.168 Pa s at a solid content of 50 vol%. The maximum absolute value of zeta potential of SiC increased from 47.3 to 61.6 mV by modification. Sedimentation experiments showed that a highly stable suspension of modified SiC was obtained at pH 10. SiC green body with high density of 2.643 g/cm³ was prepared with modified powders by slip casting. X-ray photoelectron spectra (XPS) and thermogravimetry (TG) measurements indicated the adsorption of modifiers on SiC surface. Therefore, modified SiC powders could stably disperse in aqueous media due to the increase of electrosteric repulsion between particles. The novel strategy used in this study could further improve the dispersion of SiC powders.     

Effect of SiC powder on the properties of SiC slurry for stereolithography

Silicon carbide (SiC) is a kind of structural ceramics with excellent properties and it is widely used in industrial fields. Stereolithography (SL) is a potential additive manufacturing technique to fabricate fine complex SiC components, the resin-based SiC slurry with superior rheological and photo-polymerization properties is important for SL. In this paper, we investigated the influence of SiC powder on the properties of the SiC slurries for SL. The physical characteristics of SiC powder such as particle size, size distribution and appearance were tested and observed, and their influence on the dispersion, sedimentation and photo-polymerization property of the SiC slurry were investigated and discussed in detail based on their correlative theory, we finally prepared SiC slurry with superior rheological and photo-polymerization properties, and fabricated the fine complex SiC green body with low defects, high accuracy and high bending strength successfully. The SiC slurry with the solid content of 40 vol% was fabricated by the SiC powder with the median diameter D₅₀ ≈ 10.0 μm and a narrow particle size distribution, it is Bingham fluid with good fluidity and the viscosity of it is 464.40 mPa s under the shear rate of 51.08 s^?1, the cured SiC parts with Z – axis dimension change of 0.75% was finally fabricated, the three points bending strength of it is 50.18 MPa. Our research work provides some fundamental understanding of the SL technique for fabricating fine complex SiC components, explored a suitable way to fabricate high quality SiC green parts through SL, and offers some valuable references for preparing SiC slurry with superior rheological and photo-polymerization properties.     

Dispersion and densification of nano Si–(Al)–C powder with amorphous/nanocrystalline bimodal microstructure

The dispersion behavior and densification of nano Si–(Al)–C powder with amorphous/nanocrystalline bimodal microstructure were investigated. The Si–C powders synthesized by a mechanical alloying (MA) process had a near‐spherical shape with an average particle size of 170 nm. A solid loading of 62 vol% was achieved using polyethyleneimine (PEI) as a dispersant. The optimum dispersant amount was 1 wt% based on zeta potential, sedimentation, and viscosity analysis data. The high zeta potential value (73 mV) compared with that of the commercially available SiC (65 mV) was caused by modified surface properties and consequent promotion of the cationic dispersant adsorption. A Si–Al–C slurry containing 6.5 wt% of sintering additives with a solid loading of 60 vol% was also prepared. The relative density of the dried Si–Al–C slurry was 63.3% without additional compaction, which could be densified at 1650°C at a pressure of 20 MPa using a spark plasma sintering furnace.     

pH值对包覆改性SiC料浆分散特性和流变性的影响

通过zeta电位、沉降实验、流变特性、粘度等测试表征pH值对包覆改性SiC料浆分散特性和流变性的影响。研究表明:以偶联剂作为基础层,有机聚电解质作为分散功能层的有机包覆改性SiC粉体主要通过静电空间位阻效应实现稳定分散,调节pH值可以控制接枝聚合物水解产物的解离方式,从而改变了颗粒表面的电荷种类和电荷密度,包覆改性粉体的流动特性也发生变化。调整料浆pH值约11可制备出固相体积分数达56％、表观粘度为568 MPa·s的稳定料浆。     

Fabrication and electromagnetic interference shielding effectiveness of Ti3Si(Al)C2 modified Al2O3/SiC composites

A dense alumina fiber reinforced silicon carbide matrix composites (Al₂O₃/SiC) modified with Ti₃Si(Al)C₂ were prepared by a joint process of chemical vapor infiltration, slurry infiltration and reactive melt infiltration. The conductive Ti₃Si(Al)C₂ phase introduced into the matrix modified the microstructure of Al₂O₃/SiC. The refined microstructure was composed of conductive phase, semiconductive phase and insulating phase, which led to admirable electromagnetic shielding properties. Electromagnetic interference shielding effectiveness (EMI SE) of Al₂O₃/SiC and Ti₃Si(Al)C₂ modified Al₂O₃/SiC were investigated over the frequency range of 8.2–12.4 GHz. The EMI SE of Al₂O₃/SiC-Ti₃Si(Al)C₂ exhibited a significant increase from 27.6 to 42.1 dB compared with that of Al₂O₃/SiC. The reflection and absorption shielding effectiveness increased simultaneously with the increase of the electrical conductivity.     

Effects of sintering additives on dispersion properties of Al2O3 slurry containing polyacrylic acid dispersant

The effects of sintering aid adsorption on the dispersion properties of aluminum oxide slurries were investigated. We considered Al₂O₃ slurry without additives and Al₂O₃ slurry with a Mg additive with 0.1 mass% in oxide equivalent as a sintering aid. In this study, we evaluated the adsorption isotherm of polyacrylic acid (PAA) onto Al₂O₃ and the dispersion degree of Al₂O₃ slurries in sedimentation tests under gravity. The adsorption isotherm featured a characteristic adsorption isothermal line with a maximum value when Mg additive was present in Al₂O₃. In addition, the packing fractions did not correspond to the apparent viscosity. However, in slurry that was allowed to settle for several days, both of them agreed.Therefore, the disagreement between the packing fraction and the apparent viscosity immediately after preparation arose from changes of the dispersion state, such as the decrease of the distance between particles with time.     

Dispersion and properties of zirconia suspensions for stereolithography

Stereolithography is an attractive technique for the fabrication of complex-shaped ceramic components with high dimensional accuracy. One of the challenges in this technology is the development of high solid loading, low viscosity photosensitive ceramic suspension. In this study, the dispersion of zirconia in photocurable resin and the slurry properties were intensively investigated. Rheological measurements showed that DISPERBYK-103 proved to be an effective dispersant. 42 vol% ZrO₂ suspension was successfully prepared using 3.5 wt% DISPERBYK-103 as the dispersant, with a suitable viscosity (4.88 Pa·s) below the maximum allowable viscosity value (5 Pa·s) for stereolithography applications. The adsorption behavior of DISPERBYK-103 on the surface of zirconia powders was characterized by TG and FT-IR, confirming the dispersion effect of dispersant. Contact angle measurements were also conducted to show that the adsorption of DISPERBYK-103 could help to improve the wettability between powder and photocurable resin. Results showed that DISPERBYK-103 was effective for the preparation of suitable slurries for the development of ZrO₂ ceramics through stereolithography.     

聚乙烯亚胺表面改性SiC粉体的流动特性

采用聚乙烯亚胺（PEI）作为表面活性剂对工业用SiC粉体进行表面改性处理,研究了改性前后SiC粉体的流动特性以及PEI加入量和pH值的影响.实验结果表明：PEI加入量为3%（质量分数）、pH值为6.5时,改性后的SiC粉体取得较好的流动特性和较高的饱和吸附量,颗粒分散较好,颗粒尺寸和形状较为均匀.     

油基纳米氧化锌分散浆的制备及其流变特性

选用硬脂酸钠对ZnO进行表面湿法改性，以环十五硅氧烷硅油为溶剂，PEG-10聚二甲基硅氧烷为分散剂，通过机械球磨法制备了纳米ZnO分散浆。利用水接触角、热重、TEM和FTIR对纳米ZnO粉体进行表征。结果表明，硬脂酸钠改性后，粉体具有疏水性，且硬脂酸钠最佳包覆量为6%（以ZnO的质量计，下同）。硬脂酸钠包覆量为6%的疏水性纳米ZnO粉体，包覆层厚度约为2 nm，此时水接触角最大为145.4°。模拟防晒乳液的防晒性能测试中，纳米氧化锌分散浆的紫外屏蔽性能显著优于粉体。流变特性测试表明，分散浆为假塑性流体，流动曲线符合Ostwald-de Wale幂律方程，具有剪切稀化特性；分散浆的黏度低，触变性小，储存稳定性高；温度升高，黏度降低，配方生产中对温度的敏感程度较小 。     

Preparation of SiC porous ceramics by a novel gelcasting method assisted with surface modification

The SiC porous ceramics were successfully prepared by gelcasting process using Isobam as gelling agent. The pretreatment and surface modification of SiC was proposed to solve the problem that Isobam is difficult to crosslink and gelling with non-oxide ceramics. The SiC powder was firstly pre-oxidized, and then modified with a silane coupling agent to obtain a functional end group -NH₂ on the surface, so that it can be crosslinked with Isobam to obtain a green body with uniform structure. With low amount of organic matter, no additional debinding procedure was required before sintering. The suitable oxide thickness was investigated, which shows a linear relationship with the original SiC particle size. Moreover, the solid content of the slurry was adjustable between 40 vol% and 52 vol%, and the porosity of the products varied between 50% and 60% after sintering.     

Effect of Polyacrylic Acid Addition on Rheology of SiC‐Al2O3‐ZrO2(3Y) Mixed Suspensions

A suspension with good rheology and high stability is crucial for slip casting and gelcasting technology. However, a mixed suspension from two or more different powders usually has bad rheology because of the easy agglomeration of mixed powders caused by the attractive force between the powders with heterocharges. We studied the surface modification of the each single‐component powders (SiC, Al₂O₃, ZrO₂(3Y) powders) and the SiC‐Al₂O₃‐ZrO₂(3Y) mixed powders to increase the repulsive force by adjusting the pH value and adding polyacrylic acid (PAA) as dispersant. The PAA addition effects on the SiC‐Al₂O₃‐ZrO₂(3Y) mixture were investigated in terms of zeta potential, pH range for heterocharge region, dispersion of the mixed powders and rheology of the mixed slurry based on the study of each unary suspensions. The results show that before surface modification the SiC‐Al₂O₃‐ZrO₂(3Y) mixed powders were agglomerated severely because they were in the heterocharge region with a broad pH range from 3.5 to 8.25, while after surface modification (pH = 10.5, PAA = 0.8wt%) the heterocharge region was narrowed with a relatively narrower pH range from 2.6 to 3.7. The mixed powders with homocharges were dispersed well because of the great electrostatic repulsive force and steric hindrance offered by PAA and the mixed suspensions had favorable rheology.     

Preparation and characterization of ZrB2 and TaC containing Cf/SiC composites via Polymer-Infiltration-Pyrolysis process

To improve the thermo-chemical resistance of PIP–C_f/SiC composites, the SiC matrix is modified by adding ZrB₂ and Ta powder to the pre-ceramic slurry to form C_f/SiC–ZrB₂–TaC composites. Within this study the modified composites are investigated regarding their microstructure, chemical composition and physical properties (density = 2,39–2,72 g/cm³; porosity = 20,3–24,8 vol.-%; fiber volume content = 52–57 vol.-%). Mechanical properties are investigated in order to ensure that there is no negative influence by ZrB₂ and TaC matrix modification. The matrix modification is followed by an improvement in bending strength (up to 27% increase), Young’s modulus (up to 28% increase) and for interlaminar shear strength (up to 22% increase). Finally the thermo-chemical behavior of the C_f/SiC–ZrB₂–TaC composites is evaluated in a combustion chamber-like environment using the Airbus Group long-term material test facility (Environmental Relevant Burner Rig-Kerosene, ERBURIG^K). The results show that the thermo-chemical resistance of C_f/SiC–ZrB₂–TaC composites is improved and the oxygen permeability through the composite is decreased (from 5 to 1 layer).     

Highly concentrated aqueous SiC slurry containing fine SiC powder: A new approach for ultra-dense green body

SiC slurry with ultra-high concentration up to 70 vol% was prepared using oxidized fine and coarse SiC powder mixture, and dense SiC green body with a relative density of 76% was fabricated by drying the slurry at ambient condition. Three approaches were performed to prepare highly concentrated SiC slurry; preparation of SiC powder having good dispersion behavior, optimization of the oxidation condition, and optimization of bi-modal particle size distribution. An aqueous slurry with the solid loading up to 62 vol% could be prepared using fine (150 nm) SiC powder prepared by the mechanical alloying of Si and carbon. The surface property of the fine and coarse (10 μm) SiC powders was optimized using an oxidation treatment. The maximum solid loading of the fine SiC slurry prepared using oxidized powder was 66 vol%. By optimizing the mixing ratio of the oxidized fine and coarse SiC powder to 75:25, the solid loading of the SiC slurry could increase up to 70 vol%. The relative densities of the green bodies after drying 66, 68, and 70 vol% slurries were 69, 75.7, and 76.1%, respectively, which values were higher than those (58%) prepared by cold isostatic pressing under 200 MPa.     

An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder

A slurry based process was developed to achieve the uniform dispersion of a high volume fraction of carbon nanotubes (CNTs) in aluminum powder. Al powder was subjected to surface modification to introduce a hydrophilic polyvinyl alcohol (PVA) membrane on its surface, which has good wettability and strong hydrogen bonding interactivity with functionalized CNTs. It was found that, when mixed with a CNT aqueous suspension, the PVA-modified Al (Al@PVA) powder resulted in much better adsorption uniformity of CNTs than the untreated Al powder. The adsorption capacity of CNTs was greatly enhanced by using nanoflake Al powder, which has better geometric compatibility with the CNTs and a larger surface area than spherical powder. Consequently, a uniform dispersion of 20 vol.% CNTs was achieved in the nanoflake Al@PVA powders. The advantage of this approach is that it provides easy control over adsorption uniformity and capacity of CNTs in a metal matrix, through the combination of surface modification and thickness adjustment of the metal flake powders.     

碳化硅悬浮浆料粘度损失研究

用硅烷偶联剂对1.2μSiC粉接枝改性,和100μSiC粉调制浆料,研究了SiC粉表面Zeta电位和浆料固相体积含量、粘度增量之间的关系;在碱性条件下,SiC表面水解是导致Zeta电位变化和浆料粘度损失的原因。考察了润湿剂烷基聚氧乙烯醚(Alcohol Ethoxylate,AEO)对浆料Zeta电位和粘度损失的影响,...     

A nitride whisker template for growth of mullite in SiC reticulated porous ceramics

Silicon carbide reticulated porous ceramics (SiC RPCs) were fabricated by polymer replica technique. The effects of nitride whisker template on the growth of mullite, the strut structure and mechanical properties of SiC RPCs were investigated. Prepolyurethane (PU) open-cell sponge was first coated by SiC slurry consisting of SiC, reactive Al₂O₃, microsilica and Si powder, then it was nitridized at 1400 °C in a flowing N₂ atmosphere to prepare SiC preforms. Subsequently, these preforms were treated by vacuum infiltration of alumina slurry and fired at 1450 °C in air. The results showed that Si₂N₂O whiskers grew on the surface and in the matrix of SiC preforms after nitridation. The diameter of struts in SiC RPCs increased after vacuum infiltration process because alumina slurry was easily adhered by the surface nitride whiskers. In addition, such whiskers inside the strut of SiC preforms acted as the template to promote the growth of column-liked mullite in SiC RPCs. The mechanical properties and thermal shock resistance of SiC RPCs were greatly improved due to the special interfacial characteristics of multi-layered struts as well as better interlocked column-liked mullite in SiC skeleton.     

Effect of additives on slip casting rheology,microstructure and mechanical properties of Si3N4/SiC composites

The SiC/Si₃N₄ composites were fabricated with sintering process. To produce SiC/Si₃N₄ composite components, slurry mixtures containing Si/SiC powders were used by the slip casting method. In order to investigate the effect of dispersants and additives on the rheological properties and the body casted, slurries with concentration of 70% solid weight were prepared. It included a mixture of silicon and silicon carbide with weight ratios of 30 wt% and 70 wt%, respectively, and various weight percentages of Ball clay as lubricant and Tiron (sodium salt of benzene disulfonic acid) as dispersant at pH value of 7. After preparing the green bodies by slip casting method by using plaster mold, the samples were sintered at 1450 °C inside an atmospheric-controlled furnace under a pressure of 0.12 MPa of nitrogen gas for 2 h. By examining the rheological properties of the slurry and the sintering properties, it was concluded that the best slurry was obtained in terms of viscosity, density, porosity and strength using 5 wt% Ball clay and 0.5 wt% Tiron. Phase transformations, microstructure and morphology of the sintered specimens were accomplished by Field Emission Scanning Electron Microscopy (FESEM) examination and X-ray diffraction experimental analysis. XRD and FESEM results demonstrated that the composite fabricated by slurry containing 5 wt% Ball clay and 0.5 wt% Tiron had the least porosity without SiO₂ phase.     

Recrystallization sintering and characterization of composite powders composed of two types of SiC with dissimilar particle sizes

Pure silicon carbide (SiC) ceramics were prepared through recrystallization sintering by using two types of SiC powder, with different particle sizes, as the raw materials. The effects of the fine powder content on the bulk density, porosity, flexural strength, and grain morphology were investigated. In the synthesis process, silicon nitride (Si₃N₄) was used as the sintering additive that decomposed and transformed into SiC to promote the growth of SiC grains. The added fine powder was exploited in the evaporation and condensation process and grain amalgamation caused by the movement of the grain boundaries. Thus, a dissimilar fine powder content modulated the microstructure and mechanical strength of the SiC ceramics. The results indicate that the bulk density and flexural strength increase to a maximum of 2.12 g/cm³, 44.2 MPa, respectively, when the fine powder content is 40 wt.%. Three kinds of grain morphologies, that is, uniform equiaxed grains, round, equiaxed grains, and hexagonal platelet grains, and the maximum average pore size (3.62 μm) are obtained when the fine powder content is between 0 wt.% and 60 wt.%. In addition, the main crystal phase 6H-SiC is partially converted to 4H-SiC when the fine powder content is up to 60 wt.%.     

Surface modification of SiC powders by hydrolysed aluminium coating

SiC powders are surface modified to behave like alumina in aqueous suspensions by coating the powders with in situ generated hydrolyzed aluminium from dilute aqueous aluminum nitrate solutions in a pH range of 3–4.5 using hexa-methylene-tetramine as the base generator at ambient temperature. By examining the zeta potential and rheological properties of aqueous suspensions of the powder with different Al-coverage, it was observed that the coated powder begins to show alumina-like surface properties at an Al-coverage of 0.1 mg/m², in contrast to 0.5 mg/m² that was the minimum value reported earlier for observing the effect. This is explained by proposing a coating mechanism which proceeds through adsorption of a layer of cationic hydrolysed aluminum molecules, such as [Al(OH)(H₂O)₅]²⁺ and [Al₂(OH)₂(H₂O)₈]⁴⁺, during which the particles attain alumina-like surface charge properties. The modified powder retained alumina-like surface characteristics when stored under moist conditions and in acidic suspensions. The low value of Al-coverage and the resulting low increase (<10%) of powder surface area are advantageous for preparation of concentrated SiC and SiC–Al₂O₃ composite suspensions in acidic aqueous medium with rheological properties similar to that of their counterparts containing alumina only.