Surface characterisation and modification of submicron and nanosized silicon carbide powders

The surface characteristics of two grades of silicon carbide powders were analyzed and modified with the aim to control and tailor their behaviour in colloidal suspensions. The as-received submicron and nanosized SiC particles were both found to be covered with a thin oxygen-containing layer, composed mostly of SiO₂ and SiC_xO_y; however, their electrokinetic behaviours in aqueous suspensions were observed to be different. We coated the powders with a thin layer of aluminium oxy-hydroxide, which was found to modify their electrokinetic behaviours in aqueous suspensions. The composition and structure of the aluminium-containing layer were characterized by X-ray photoelectron spectroscopy (XPS) and TEM analyses. This coating may subsequently serve as a homogeneously distributed source of a low amount of sintering additive in manufacturing the SiC-based material for fusion application.     

Densification of a SiC-matrix by electrophoretic deposition and polymer infiltration and pyrolysis process

With the aim to improve the properties of the SiC-matrix in a SiC_f/SiC composite for fusion applications, a new fabrication technique combining electrophoretic deposition (EPD) and polymer infiltration and pyrolysis (PIP) was introduced. By using EPD from a well-dispersed, aqueous suspension we were able to produce SiC green bodies with closely packed particles (>62%TD). This deposition was followed by vacuum infiltration of the green body with pre-ceramic polymer precursor and then pyrolysis and crystallization at 1600 °C. Due to the high initial packing density of the deposits, only a few polymer-infiltration and pyrolysis steps were needed to achieve a relatively high density of the material. The SiC samples fabricated by the combined EPD-PIP process reached the matrix density of ∼86.5% TD and average pore size of ∼90 nm after six consecutive PIP cycles, while high thermal conductivity values (>30 W/mK) were obtained already after one PIP cycle.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.     

Characteristics of SiCf/SiC hybrid composites fabricated by hot pressing and spark plasma sintering

Abstract

Abstract

SiC fibre reinforced SiC–matrix ceramic composites were fabricated by electrophoretic deposition (EPD) combined with ultrasonication. Fine β-SiC powder and Tyranno-SA fabrics were used as the matrix and fibre for reinforcement, respectively. Different amounts of fine Al₂O₃–Y₂O₃ were added for liquid phase assisted sintering. For EPD, highly dispersed slurry was prepared by adjusting the zeta potentials of the constituent particles to ?+40 mV for homogeneous deposition. The composite properties were compared after using two different consolidation methods: hot pressing for 2 h at 20 MPa and spark plasma sintering (SPS) for 3 min at 45 MPa at 1750°C to minimise the damage to the SiC fibre. The maximum flexural strength and density for the 45 vol.-% fibre content composites were 482 MPa and 98% after hot pressing, respectively, whereas those for SPS were 561 MPa and 99·5%, indicating the effectiveness of SPS.     

Effect of Solids Loading on Slip-Casting Performance of Silicon Carbide Slurries

The influence of solids loading and particle shape on the green microstructure of slip-cast bodies was investigated. Three commercial silicon carbide (SiC) powders (two coarse varieties with the same particle-size distribution (PSD) but different particle shapes and a finer powder) were used to prepare bimodal PSDs designed to maximize the packing density. Various surface-active agents (anionic, cationic, and non-ionic) were tested. Anionic dispersants were the most effective in dispersing aqueous SiC slurries. The effectiveness of dispersants was evaluated by sedimentation tests using very dilute slurries, by rheology, and by the packing density of slip-cast bodies prepared from suspensions loaded with 62.5 wt% solids, stabilized with a fixed amount of dispersant (0.25 wt%, relative to the solids). Then, the best dispersant was selected to study the effects of dispersant and solids concentrations on the degree of packing of bimodal suspensions that contained the sharper-edged coarse particles. It could be observed that the green density was dependent on both parameters, initially showing an increase to a maximum, followed by a decreasing trend. A high value of 74.5% of the theoretical density (TD) was obtained from suspensions that contained 70 wt% solids and 0.1 wt% dispersant. The substitution of the angular coarse particles by similarly sized but more spherical particles resulted in an additional increase in green density to >76 wt% TD. The results can be interpreted in terms of freedom of particles upon deposition on the cast layer, which enables particle rearrangement, and segregation phenomena.     

Characterization of polyetheretherketone particle suspensions for electrophoretic deposition

Suspensions of polyetheretherketone (PEEK) using mixture of ethanol and isopropanol as solvent were prepared to carry out PEEK electrophoretic deposition (EPD). The rheological behavior and suspension structure of PEEK particles dispersed in co‐solvents were investigated over a range of pH values (1–10) and shear rates (γ = 10¹?3 × 10² s^?1). These PEEK suspensions generally exhibited a pseudoplastic flow behavior, indicating the occurrence of particle aggregation in the liquid medium. The maximum solids fraction (?_m) showed an estimated value of  ?_m = 2.9 wt %. Using a suspension with 3 wt % PEEK concentration, PEEK coatings on stainless steel substrates were obtained by EPD at constant voltage condition. The influence of the electrolyte conductivity on PEEK EPD from ethanol–isopropanol suspensions was studied. Experimental results showed that high‐conductivity ethanol‐based suspensions yield non‐uniform deposits, while low‐conductivity suspensions resulted in uniform coatings. The difference in the deposition behavior is due to the different pH of the suspensions and the relationship of pH with suspension conductivity. pH = 8 was the optimal value for this system in terms of deposition results. The surfaces of EPD PEEK coatings were homogenous and a qualitatively good adhesion between the PEEK deposits and the substrate was confirmed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40953.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Fabrication of SiC/diamond composite coatings by electrophoretic deposition and chemical vapor deposition

In this research, SiC/diamond composite coatings were fabricated by a novel procedure that consisted of the electrophoretic deposition (EPD) of diamond particles onto graphite substrates followed by chemical vapor deposition (CVD) of SiC. Various concentrations of MgCl₂ were employed to increase the deposition rate and uniformity of the deposits during the EPD process by giving a positive charge to diamond particles. The CVD of SiC was found to have a tightly connected diamond‐graphite interface and spherical texture. With higher weight fraction of diamond particles deposits, the wear of steel ball increased, while the wear of SiC coating decreased.     

Bubble-free electrophoretic shaping from aqueous suspension with micro point-electrode

The electrophoretic deposition (EPD) is a well-suited process for shaping compacts of nanosized powders. Aqueous suspensions are favorable for industrial application due to the high polarity of water enabling high solid loadings and for environmental reasons. Moreover, for many applications a local deposition is of interest. In this case the electric field has to be focused on a small point. This is a problematic issue when the aqueous suspension has a high electrical conductivity.The local resolution of the deposition was improved by moving both electrodes closer to each other. The limiting factor was the formation of bubbles by water electrolysis, which disturb the electric field at the electrodes. Therefore the electric field distribution of several electrode-configurations was calculated and the most suitable setup was selected. The formation of bubbles was suppressed by using unbalanced pulses of alternating voltage. Point deposits with diameter smaller than 740 μm were obtained, which were only about 1.5 times larger than the point electrode diameter. Possible applications of this technique in the future are rapid prototyping or commercial manufacturing of individual structures like those required in the dental industry.     

Effects of particle size and dispersion methods of In2O3‐SnO2 mixed powders on the sintering properties of indium tin oxide ceramics

Three group samples were used to investigate the effects of particle size and dispersion methods of In₂O₃‐SnO₂ mixed powders on the sintering properties of ITO ceramics by BET, SEM, XRD, and EPMA, etc. High‐density (99.8% of TD) ITO ceramics, with dimensions of 350 × 250 × 8 mm³ for the industrial application, were obtained by the mixed powders of In₂O₃ calcined at 1000°C and SnO₂ with BET 6.0 ± 0.5 m²/g and collocation use of ball mill for 300 minutes, stirred mill for 60 minutes, and sand mill for 3 minutes. The results indicate that: (i) the larger the SnO₂/In₂O₃ particle size ratio, the higher the density of ITO ceramics, (ii) the dispersion of mechanical ball‐mill methods for nanosized In₂O₃ and SnO₂ powders is beneficial to the densification and structural homogeneity, and (iii) the smaller the relative grain size, the more uniform the distribution of grain size.     

Effect of heat treatment on the dispersion and sintering behaviour of tin doped indium oxide powders

Due to the importance of structural uniformity of ITO targets on the properties of ITO films, the untreated and heat treated tin doped indium oxide powders were used to study the effects of four different dispersants on the dispersion behaviour of nanosized ITO powders. The optimum dispersant is NH₄PAA and its optimum amount is 1.00?wt% when the pH value is 9.0. In addition, the effect of the treatment temperature of nanosized ITO powders on the dispersion and sintering behaviour was also studied by SEM, TEM and XRD. The solid loading of ITO slurries and the relative density of the sintered bodies prepared with ITO powders treated at 900?°C could reach 40?vol% (untreated, 25?vol%) and 98.53% (untreated, 95.04%), respectively. The results indicate that the heat treatment of powders at 900?°C allowed obtaining powders from which ITO aqueous suspensions with high solid loading could be prepared and dense bodies after sintering. In another word, the appropriate heat treatment process for tin doped indium oxide powders could reduce the sintering temperature by 50?℃ and refine the grain size.     

Deposition of Silicon Carbide/Titanium Carbide Laminar Ceramics by Electrophoresis and Densification by Spark Plasma Sintering

SiC/TiC laminar ceramic composites were fabricated using electrophoretic deposition (EPD) from acetone-based suspensions. The growth rate of the SiC was almost twice that of the TiC at the same deposition voltage and solids loading. Pressureless sintering and spark plasma sintering (SPS) of the composites were investigated. SiC in the composites without sintering additives could not be densified using pressureless sintering, even at 2000°C. SPS, however, could densify the SiC/TiC composites at 1800°C and 35 MPa. The relative density of the SPS sample was 98.9%.     

Processing and impact behavior of Al/SiCp composites fabricated by the pressureless melt infiltration method

In the current investigation, pressureless melt infiltration was applied to fabricate the Al/SiC composites based on the SiC porous preforms. The process was conducted by introducing the aluminum melt into the SiC preforms at 950 °C under the nitrogen atmosphere, without the aid of pressure. To explore development of melt infiltration, initial preforms were produced with variable SiC fractions (40, 50, and 60 vol.%) using three different SiC powders with the mean particle size of 20, 50, and 90 μm. While the infiltration of aluminum melt into the preforms with 40 vol.% initial SiC volume fraction (SiC particle size of 90 μm) resulted to the composites with final density of 0.94 theoretical density (TD), this value drops down to ～0.9 TD for the composites produced by preforms with the SiC (90 μm) volume fraction of 60 vol.%. On the other hand, composites fabricated by 50 μm SiC powder (SiC volume fraction of 40 vol.%) demonstrated the final density of ～0.91 TD. The impact resistance tests performed on the composites demonstrated an enhancement in the value of impact energy with an increase of SiC powder particle size. Results, additionally, revealed a significant superiority of impact energy for the composites fabricated by a combined melt infiltration and sintering (MIS) procedure compared to those produced by infiltration at 950 and 1350 °C.     

Influence of processing parameters on microstructure and ferroelectric properties of PZT-coated SiC fibers

SiC fibers have been coated with coprecipitated PZT powders by electrophoretic deposition. Zr and Ti hydroxides, respectively, and Pb carbonate are precipitated from homogenous nitrate solutions at pH values between 5 and 7. The platinum-coated SiC fibers were electrophoretically coated with these coprecipitated PZT powders after calcination and milling. The coated fibers were sintered at temperatures of about 1170 °C. With the low solid yield of the suspension and the low layer thickness compared to the sparking distance an almost constant growth rate of the layer is observed during electrophoretic deposition of the PZT powders at a coating voltage of 50 V and a coating time of up to 180 s. Remanence and coercive field strength characteristics of the fibers sintered at 1170 °C increase with increasing sintering time and density and range between 11 and 25 μC/cm², respectively, between 12 and 22 kV/cm in good correspondence with literature values for pure PZT fibers.     

Joining of silicon carbide ceramics using a silicon carbide tape

This paper reports the joining of liquid-phase sintered SiC ceramics using a thin SiC tape with the same composition as base SiC material. The base SiC ceramics were fabricated by hot pressing of submicron SiC powders with 4 wt% Al₂O₃–Y₂O₃–MgO additives. The base SiC ceramics were joined by hot-pressing at 1800-1900°C under a pressure of 10 or 20 MPa in an argon atmosphere. The effects of sintering temperature and pressure were examined carefully in terms of microstructure and strength of the joined samples. The flexural strength of the SiC ceramic which was joined at 1850°C under 20 MPa, was 343 ± 53 MPa, higher than the SiC material (289 ± 53 MPa). The joined SiC ceramics showed no residual stress built up near the joining layer, which was evidenced by indentation cracks with almost the same lengths in four directions.     

Si-SiC oxidation barrier coating on 3D printed Si-SiC strut-based architectures deposited by electrophoretic deposition

This work reports on the production and oxidation behavior at 1500 °C of SiC coated Si-SiC tetrakaidekahedron periodic structures. C-SiC lattices were first produced by 3D printing of polymers templates followed by the replica technique and pyrolysis. They were then coated by electrophoretic deposition (EPD) with submicron SiC particles. SiC particles were dispersed in water and deposited successfully on C–SiC structures by optimising voltage, frequency and duty cycle to minimize water electrolysis in the aqueous system. Different coating thicknesses were deposited on the substrates by varying the deposition time. Samples were infiltered with molten silicon, oxidized at 1500 °C and characterized.     

Preparation of manganese oxide - graphite electrodes by electrophoretic deposition

Manganese oxide is a promising active material for supercapacitors (SCs) with pseudocapacitance due to its high capacitance and its environmentally friendly character. This paper deals with the preparation of electrodes for supercapacitors consisting of manganese oxide supported onto graphite by electrophoretic deposition. Manganese oxide powders were characterized and dispersed in water by controlling the colloidal and rheological behavior in order to obtain stable suspensions. Optimized manganese oxide suspensions were deposited onto graphite electrodes by electrophoretic deposition. The deposited mass per unit area in the electrodes was optimized by controlling the applied current density and the deposition time. It has been demonstrated that the introduction of a binder helped to improve the adherence to graphite; otherwise the deposit thickness obtained by EPD is limited and no films can be obtained by simply dipping. These conditions allowed us to obtain more homogeneous deposits with higher specific energy than without binder.     

Highly-porous hierarchical SiC structures obtained by filament printing and partial sintering

Three-dimensional (3D) porous silicon carbide (SiC) structures with total porosity in the range of 64–85% are achieved from nanosized SiC powders by filament printing and partial sintering in a spark plasma sintering (SPS) furnace. The effects of the SPS temperature (1500 and 1700 °C) and the addition of oxide sintering aids (7 wt. % Y₂O₃+Al₂O₃) on the porosity of the scaffolds are quantitatively compared. More specifically, hierarchical porosity consisting of meso-pores (< 50 nm) in the struts and open macro-pores (in the range of 500–700 μm) defined by the patterned structure is verified for the 1500 °C SPS treatment. The strength of the structures as a function of the density as well as their cooling profiles, once subjected to rapid heating under a direct flame, are analyzed. By following this easy route, an adaptable family of robust light 3D SiC structures with hierarchical porosity is accomplished.     

Experimental and numerical analysis of electrophoretic deposition on charge selective surfaces in external electric fields

Abstract

In this paper, we present chronopotentiometric results of direct current (DC) membrane electrophoretic deposition (DC-M-EPD) experiments and analyse the influence of the parameters solid concentration, particle size and electrical conductivity of colloidal fumed silica model suspensions on voltage–current curves. The expected streamline patterns are numerically modelled based on coupled mass balances, Ohmic law, Navier–Stokes and Nernst-Planck equations. The results confirm that overlimiting current condition is an obligatory condition for the formation of EPD green deposits under suitable DC-M-EPD conditions. Furthermore, micro-EPD experiments in external alternating current (AC) fields (micro-AC-EPD) using video microscopy come to the conclusion that the observed formations of microvortices are in accordance to the numerically modelled streamline patterns. Finally, the formation of AC-EPD microdeposits exhibiting a torus-like microstructure is substantially explained based on microfluidic streamline pattern analysis.     

Mechanical properties and microstructure of oil-well cement stone enhanced with submicron SiC whiskers

In order to improve the intrinsically brittle nature of the oil-well cement stone, a new type of submicron-fibrous whisker, referred to as the silicon carbide whisker (SiC whisker) was added into cement paste. The cement-based composites were prepared with the various amounts of submicron SiC whiskers. The composites were analyzed via scanning electron microscopy (SEM), mechanical testing, X-ray diffraction (XRD), infrared spectrum analysis (FTIR), and thermal stability analysis in order to understand the enhanced effect that the submicron SiC whiskers had on the applied properties and mechanisms of oil-well cement stone. The compressive strength, the tensile strength, and the flexural strength of the sample with 1% of additional submicron SiC whiskers increased by 9%, 104%, and 26%. The ultimate strain was more than 5%, which increased 194%. The elasticity modulus was 4648.4MPa, which decreased 31.1% after 28 curing days. These improved the mechanical properties of the cement-based composites primarily because of the microscopic mechanism of reinforcement via the submicron SiC whisker, including the bridging effect, the crack deflection, and the pull-out effect.

• HIGHLIGHTS

• A submicron SiC whisker was used to enhance the oil-well cement

• The submicron SiC whisker could significantly increase the mechanical behaviors of the oil-well cement

• The reinforcing mechanism that the submicron SiC whisker had on the oil-well cement was discussed in detail.

• The influence that the submicron SiC whisker had on the integrity of the cement sheaths were evaluated