SiC Suspensions Research Articles

Stabilisation, rheology and application of aqueous suspensions of oxidised vs. non-oxidised SiC powder

The study aimed at evaluating the stabilisation and rheological properties of aqueous suspensions (30 vol%) of pristine (non-oxidised) and oxidised fine SiC powders which were stabilised either with NaOH or TMAH (tetramethylammonium hydroxide). The extensive oxidation of the SiC powder surface was confirmed by the results of XPS and FT-IR analyses. The optimal amount of a given dispersant depended, according to the results of zeta potential measurements, on a pH range guaranteeing formation of a stable suspension. This effect allowed the authors to propose a mechanism of stabilisation of the SiC suspensions. Herein, the maximum amount of silanol groups is formed on the SiC particles surface in the pH range of 9.5–11 which then adsorb Na+ or TMA+ cations, thus creating a favourable system of electrostatic interactions. In turn, it increases the electrostatic repulsion between SiC particles and improves the stabilisation of the suspensions and their rheological characteristics. An increase in the surface oxidation of the SiC particles leads to reducing the repulsive forces between the powder particles. Apparent viscosity of the suspensions made of SiC UF10 powder, regardless of the type and amount of the fluidizer, is very low (5–10 mPa s) and shows a slight increase as the shear rate increases. The oxidation of the SiC powder changes the rheological characteristics of the suspensions, which may be caused by their rapid sedimentation. Stability of the prepared SiC suspensions was also assessed by casting them into a plaster mould and then by sintering and characterisation of the resulting samples.

Adsorption and conformation of polyethylene imine on the SiC/water interface and effects on the dispersion stability of slurry

The adsorption of polyethylene imine (PEI) on the SiC/water interface was studied in details by adsorption isotherms and kinetics. The adsorption results indicated that PEI formed saturated monolayer coverage on the SiC surface and the adsorption reached the equilibrium within 30 min. Langmuir and pseudo-second-order models could better describe the adsorption process. The adsorption was mainly dominated by electrostatic interaction between positively charged PEI and negatively charged SiC surface. With the pH increasing from 5 to 8, the conformation of PEI on the SiC surface transformed from flat trains to loops and tails, and then the PEI adsorption capacity increased gradually so that the contact angles of modified SiC decreased from 20.9° to 13.7°, the maximum values of zeta potential increased from 37.1 to 43.9 mV and the maximum stability of modified SiC suspensions at pH 5 also enhanced. Moreover, modified SiC powder with the best dispersion was obtained with 0.5 wt% PEI at pH 6.5 and modified slurry had the low viscosity of 664 mPa·s at high solid loading of 55 vol%.

Gas/liquid two-phase flow with perforated hollow filament spacer to enhance the filtration performance in spiral wound membrane module

In brackish water and seawater desalination process, flat sheet and spiral wound membrane modules are commonly used to remove particulate pollutants and/or inorganic ions. In these membrane modules, feed spacer is used to construct the flow channel, enhance the flow field and corresponding filtration performance. However, two negative effects also occur for the spacer. One is the high pressure drop due to flow around the filament. The other is the stagnant zone near the filament, which results in localized fouling. In this paper, a new perforated hollow filament spacer was proposed. Gas/liquid two-phase flow was used to mitigate the membrane fouling. Filtration experiments were performed by microfiltration of SiC suspension. Results showed that compared with conventional spacer, the hollow filament spacer can improve the steady permeate flux by 56 %, with lower pressure drop. Particle deposition on membrane surface was effectively retarded, without localized fouling observed. Gas/liquid Flow patterns in the membrane channel were studied with high-speed photography. With the increase of Gas/liquid ratio, the flow pattern changes from bubble flow, slug flow to churn flow. The best filtration improvement performance was obtained with slug flow pattern. This hollow filament spacer is expected to mitigate the membrane fouling and improve permeate flux in the membrane-based desalination process.

Gas production from hydrothermal and radiolytic reactions at silicon carbide-water interfaces

The present work explored the surface reactions in SiC suspensions subjected to high temperature and 60Co-gamma irradiation, respectively. We observed for the first time the pH-dependent SiC dissolution by water at 300 °C and 10 MPa produces a substantial CH4, while the formation of another product H2 is decreased by the reaction time. On the other hand, the continuous H2 production from water radiolysis is greatly enhanced by the presence of an interface possibly due to the interfacial charge transfer processes. Both of the reactions are accelerated by the increased active surface contacts that are associated with the decreased SiC particles size from 250 nm to 50 nm and rising weight percentage from 3% up to 7%. The XPS, FTIR, SEM, and XRD data found little microstructure changes by gamma irradiation, but noticeable corrosive surface during hydrothermal reactions. The study pointed out that gas release is an important factor that should be considered when SiC is applied for LWR fuel assemble.

Electrospark deposition of wear and corrosion resistant Ta(Zr)C-(Fe,Mo,Ni) coatings to protect stainless steel from tribocorrosion in seawater

The offshore and coastal infrastructure needs additional protection from wear, corrosion, and tribocorrosion. Herein, electrospark deposition (ESD) was employed to deposit composite TaC-(Fe,Mo,Ni) and (Ta,Zr)C-(Fe,Mo,Ni) coatings with a metallic matrix (similar in elemental composition to that of stainless steel) reinforced with carbide nanoparticles. The coatings were produced using TaC–Mo–Ni and TaC–ZrC–Mo–Ni electrodes under different energy regimes by varying frequency, voltage, and pulse duration to obtain different carbide contents. The obtained coatings have a bilayer composite structure: core-shell TaC–ZrC crystallites embedded in an Fe-based metal matrix with a (Ta,Zr)C network (zone 1) and approximately 5 nm Fe-based nanocrystallites surrounded by amorphous interlayers (zone 2). The tribological properties of TaC-(Fe,Mo,Ni) and (Ta,Zr)C-(Fe,Mo,Ni) coatings were superior to those of uncoated AISI 304 stainless steel, both in the 3.5% NaCl solution and in 3.5% NaCl + SiC suspension. The electrochemical characteristics of the best coatings were comparable to those of the stainless steel. Tribocorrosion tests indicated that when load is applied, the open-circuit potential values of steel reduce more significantly than those of the coatings.

Adsorption of epichlorohydrin-dimethylamine at the SiC-water interface: A study on wetting, electrokinetics, dispersion stability

In this work, the adsorption of epichlorohydrin-dimethylamine (EPI-DMA) at the SiC-water interface and its influence on the properties of SiC powders were investigated. The adsorption measurements showed that EPI-DMA had a high adsorption affinity to the SiC surface and the adsorption equilibrium was reached in 30 min. The Langmuir model and pseudo-first-order model were the best to fit the equilibrium and kinetic data, respectively. The wetting, electrokinetics, dispersion stability of SiC powders modified by different adsorption capacity of EPI-DMA (2.2 mg/g, 2.6 mg/g and 3.4 mg/g) were analyzed and compared. The results showed that with the increase of adsorption capacity of EPI-DMA, wetting properties and dispersion stability of SiC powders were improved. Due to the adsorption of EPI-DMA, the isoelectric point of SiC powders shifted toward alkaline pH and high electrostatic repulsion was obtained at about pH 4. The superior dispersion behavior of SiC powders modified by 3.4 mg/g EPI-DMA in water was attributed to electrosteric stabilization mechanism and small hydrodynamic diameter, leading to low viscosity (0.424 Pa s) of 54 vol% SiC suspension. Furthermore, the conformation of EPI-DMA at the SiC-water interface was proposed.

Dispersants for silicon carbide in water—A comparative study

AbstractThe present work describes a comparative study on a pool of 12 dispersants for the de‐agglomeration and stabilization of silicon carbide in aqueous suspensions with solids loading relevant for dip coating applications. As silicon carbide slurries may include sintering aids, different functional groups, molecular weight, and stabilization mechanisms were considered for the dispersants to be able to stabilize both slurry components. Additionally, pH effect, toxicity, additive compatibility, and foaming properties were considered, giving all the necessary information for developing new aqueous formulation of SiC suspensions, including advantages and disadvantages of the different candidates. Different de‐agglomeration procedures, powder surface area, and calcination temperature were also considered to study the effect of the SiC surface properties. The outcome is that slurry stabilization provided by an alkaline environment at pH larger than 8‒9 is significantly more effective than slurry stabilization by any of the tested dispersants. Alkaline environments facilitate a negative surface charge on SiC particles and provide a stable electrostatic stabilization mechanism not observed in neutral or acidic environments. One among the dispersant candidates (FA 4404) seems to broaden slightly the range of stability toward the acidic regime. Anionic surfactants or block co‐polymers tested exhibited no significant interaction with the SiC particles.

Effect of poly(diallyldimethylammonium chloride) adsorption on the dispersion features of SiC particles in aqueous media

The dispersion behavior of SiC suspensions was improved by PDADMAC adsorbed on the SiC surface effectively through electrostatic interaction.

Influence of TMAH and NaOH on the stability of SiC aqueous suspensions

The study aimed at evaluation of the rheological properties of aqueous suspensions of SiC stabilized with either TMAH (tetramethyloammonium hydroxide) or NaOH in the amount of a given dispersant in the range of 0–1.0 wt% as calculated on the dry mass of SiC powder. The optimal amount of the dispersants was finally evaluated at 0.4–0.6 wt %, based on the results of rheological studies (flow curves and viscosity curves) and the sedimentation rate, which corresponded to the pH of the suspension within the range of 9.5–10.5. The amount of the dispersant here did not depend on the solid volume fraction in the suspension, but rather, according to the zeta potential measurement results, the pH range guaranteed the formation of a stable suspension. This effect allowed to propose a dispersant mechanism for stabilizing the suspension. The maximum amount of silicate anions formed, accordingly, in the pH range of 9.5–10.5 on the surface of SiC particles, which in turn could be able to adsorb Na+ or TMA+ cations and thus increase the thickness of the Stern layer, increasing electrostatic repulsion between the SiC particles and improving the stabilization of suspensions and their rheological properties. However, the increase in suspension pH deteriorated their stabilization – most likely due to the occurrence of polymerization of amorphous silica and the presence of additional cations in water increasing its ionic strength. These suggestions were confirmed by the results of XPS analysis of the chemical state of SiC particle surfaces.

Electrophoretic deposition of bi-layered nano-sized silicon carbide/mullite coating from stabilized suspensions

Nano-sized silicon carbide/mullite bi-layered film was applied onto the surface of graphite using electrophoretic deposition (EPD) method from stabilized suspensions. The suspensions were prepared adding 5 g L−1 SiC and 10 g L−1 mullite in ethanol. The effects of adding various amounts of polyethylene imine (PEI) on the stability of suspensions were investigated by visual inspection (sedimentation height), measuring the zeta potential, and particle size distribution. As a result, the addition of 6.0 dwb% and 1.5 dwb% PEI to SiC and mullite suspensions were found to be an effective way in dispersing of nanoparticles. The zeta potential values of well-stabilized suspensions were determined 37.6 mV and 42.4 mV for SiC and mullite suspensions, respectively. Therefore, the coating processes were conducted with the optimal amounts of PEI. In the deposition process, the optimal distance between electrodes was determined 10 mm. In order to investigate the coating process parameters, the optical microscopy (OM) was used for single-layer coatings. The results reveal that applied voltages of 40 V and 10 V with deposition periods of 50 s and 60 s bring about uniform coatings. In addition, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were carried out to evaluate the microstructure. The thicknesses of green coatings were measured about 24.15 μm for SiC layer and 7.23 μm for the mullite layer.

Electrophoretic kinetics of nanomullite, nanoSiC and their composite suspensions

In detail characterisation of suspensions including nanomullite, nanosilicon carbide and various percentages of their composites has been studied in this Letter. Mechanism of particle ionisation, alternate pH and surface charge, particle size distribution, zeta potential, current density, electrophoresis movement and kinetics of time–weight curves has been studied, thoroughly. It was illustrated that triethanolamine cannot stabilise powders as well. However, poly-vinyl pyrrolidone could stabilise the particles as well but the particles could not move easily in the presence of applied electrical field due to the lack of enough surface charge. In this case, iodine (I) could be utilised as the preferred surfactant due to good stability and enough surface charge even though it could increase the concentration of free ions and conductivity of particles may be decreased. It was also found that in the case of utilising optimum surfactant, the amount of zeta potential for mullite suspension is more than composite powder and these two have a greater zeta potential than SiC suspension. Last but not least, it was found that the velocity of deposition reduces by increasing the time and its reduction depends on resistance and permittivity of suspension.

Properties of Porous SiC Ceramics Processed by Gelation and Consolidation of Boehmite Coated SiC Suspensions

ABSTRACTSiC powder was dispersed in various concentrations of boehmite sol. The boehmite coated SiC powder was characterized by zeta potential measurement, TEM and FT-IR analyses. Mullite bonded porous SiC ceramics were fabricated by sintering boehmite coated SiC powder compact via a pressureless sintering in air at 1300oC and were characterized by XPS, XRD, SEM analyses. The sintered ceramics have an interconnected open porous network with near uniform pore diameter. Rod shaped mullite crystals were observed in the neck region that bonded the SiC particles together. Effect of boehmite sol concentration on phase composition, microstructure, flexural strength, porosity and pore size distribution was studied. The ceramics synthesized from 30 wt% boehmite sol coated SiC powder contained 26 wt% mullite with ∼30 vol% porosity.

Dispersion of silicon carbide in poly alpha olefins-6 and trimethylopropane trioleate

In this paper, the dispersion behavior and interactions among SiC particles, dispersant and solvent were investigated. A hyperdispersant (Tech-5080) was adopted to study the stability mechanism in poly alpha olefins-6 (PAO6)/trimethylopropane trioleate (TMPTO) solvent system. Optical microscope and SEM images were used to assess the degree of dispersion. FT-IR analysis results showed that the dispersant were adsorbed onto the particle surface by hydrogen bonding. The stability measurements were studied by Turbiscan Labexpert and the size distribution was examined by Zetasizer Nano ZS to evaluate the effect of dispersant content on the stabilization of SiC suspension. A model was illustrated to explain the changing process of SiC particles in the solvent, which brings new insight into the dispersion phenomenon. Results showed that well dispersed SiC slurry could be developed with 50vol% PAO6 as solvent and using 0.5wt% Tech-5080 as the dispersant.

Fabrication of SiCf/SiC composites by hybrid techniques of electrophoretic deposition and polymer impregnation and pyrolysis

In the current study, hybrid technologies consisting of electrophoretic deposition (EPD) and polymer impregnation and pyrolysis (PIP) were adopted in order to prepare high-performance SiCf/SiC composites with high efficiency. Stable ethanol-based SiC suspensions were prepared for conducting EPD technique. PIP process by 7 cycles were performed for achieving a relative density as high as 91.9%. High-strength (289MPa) SiCf/SiC composites were fabricated, which displayed non-brittle fracture behavior. The relationship between microstructures and mechanical response was also investigated.

The application of saw-tooth promoter for flat sheet membrane filtration

A novel saw-tooth turbulence promoter was developed to improve the filtration performance of flat sheet membrane module (FSMM). The flux improvement mechanism was investigated through flow field test by particle image velocimetry (PIV). Results showed that the vortex produced by saw-tooth promoter intensified the mixing of fluid, which was favorable for the back transport of particles. Membrane shear rates in saw-tooth membrane modules (STMMs) were above 1.98 times higher than that in normal flat sheet membrane module. Particle deposition on membrane surface was effectively retarded. Flux improvement was verified by microfiltration of SiC suspension. Results showed that compared to the normal filtration method, saw-tooth promoter can improve the flux by 115.7%–258.0%, with a reduction of specific energy consumption above 33.8%. It was demonstrated that the saw-tooth promoter can improve filtration performance of flat sheet membrane module through flow field intensification.

Fabrication and Characterization of SiC-Coated Multi-Walled Carbon Nanotubes Reinforced Reaction Bonded SiC Composite

SiC-coated multi-walled carbon nanotubes (SiC-coated MWNTs) made by RF magnetron sputtering were introduced into the SiC suspensions to enhance the mechanical behaviors of reaction bonded silicon carbide ceramics. The composites were prepared by the epoxy resin curing and liquid silicon infiltration at 1550°Cfor 60min. The distribution, morphology, and reinforcing behaviors of MWNTs were investigated. X-ray diffraction (XRD) reveals that the MWNTs were reserved during the reaction sintering. The flexural strength and fracture toughness increase with MWNTs ranging from 0wt% to 0.5wt%, reaching the peaks value of 347MPa and 4.3MPam1/2 respectively at the MWNTs fraction of 0.5 wt%. This improvement derives from the MWNTs pullout. The consequent decline at MWNTs fraction of 1wt% is resulted from the corrosion of agglomerated MWNTs and the increase of residual silicon.

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB2–SiC (YZS) composite were conducted. YAG gel coated ZrB2–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB2 and SiC in the YZS composite were 3.8 and 1.3μm, respectively. The flexural strength, fracture toughness and microhardness were 375±37MPa, 4.13±0.45MPam1/2 and 14.1±0.5GPa, respectively.

Aqueous colloidal processing of SiC with Y3Al5O12 liquid-phase sintering additives

Abstract The aqueous colloidal processing of SiC with Y3Al5O12 liquid-phase sintering additives was investigated for two different additive systems, one the mixture of Y2O3 and Al2O3 in a 3:5 molar ratio and the other directly Y3Al5O12. The investigation involved the study of the colloidal stability of the different components, and the comparison of the rheological behaviour of concentrated suspensions of SiC, SiC + 3Y2O3:5Al2O3, and SiC + Y3Al5O12 as a function of the sonication condition, dispersant content, and solid loading. This allowed appropriate conditions for the preparation of well-dispersed, single-phase, and multi-component concentrated suspensions of SiC to be identified. It was found that the multi-component suspensions have better rheological behaviour than the single-phase ones, and that in terms of rheology and slip casting the Y3Al5O12 additives are more functional than the conventional 3Y2O3 + 5Al2O3 additives. It was also demonstrated that the Y3Al5O12 additive is as effective as the 3Y2O3 + 5Al2O3 additive in densifying SiC via liquid-phase sintering, with there existing no differences either in the microstructure or in room-temperature mechanical properties (hardness, toughness, and fracture mode). Implications of interest for the wet-shaping of complex SiC parts are discussed.

The Dispersion of SiC Nanopowders in Ethanol

The dispersion of SiC nanopowders in ethanol solution was studied by sedimentation test, particle size measurement and TEM analysis. The dispersion behavior of SiC nanopowders in ethanol solution was strongly dependent on the pH values, types and amounts of dispersant. PEI was found to be effective for the dispersion of SiC nanopowders in ethanol solution. With the addition of PEI, the isoelectric points of SiC nanopowders in ethanol solution were at pH 9.5, and shift to pH 12.3. The stability of SiC suspension increased with the dispersant content increasing until reached 2.5 wt% PEI. The suitable pH value for the dispersion of SiC nanopowders should lower than 10.

Aqueous gelcasting of ZrB2–SiC ultra high temperature ceramics

ZrB2–SiC ultra high temperature ceramics containing B4C and C as sintering additives were successfully prepared by aqueous gelcasting and pressureless sintering. Polyacrylic acid (PAA) was used as the dispersant throughout this research. The various effects of zeta potential, pH value, dispersant concentration, solid loading and ball-milling time on the rheology and fluidity behavior of ZrB2–SiC suspension were investigated in detail. A well-dispersed suspension with 50vol.% solid loading was prepared at pH 10 with 0.4wt.% PAA after ball milling at 240rpm for 24h. Then crack-free green ZrB2–SiC ceramics were obtained by gelcasting process and then pressureless sintered at 2100°C to about 98% relative density. The microstructure and mechanical properties were examined, and the flexural strength and fracture toughness were 405±27MPa and 4.3±0.3MPam1/2, respectively.