Investigating the powder processing effects on a ZrB(2)25 similar to vol% SiC ceramic composite densified using spark plasma sintering (SPS) allows for identification of densification mechanisms and enables a reduction in sintering temperature to a minimum of 1650 degrees C. Attrition milling (AM) and ball milling (BM) were investigated as processing methods to produce a fine and coarse powder densified using SPS with or without a tube furnace preheat treatment. Ceramics formed from AM and BM powders contain 1.66 similar to wt% oxygen contamination, primarily ZrO2 and SiO2, and 0.35 similar to similar to wt% oxygen contamination as SiO2, respectively. Heat treatment slightly reduces oxygen contamination but has significant impacts on the densification mechanisms. Without heat treatment, powder coarsening dominates the initial sintering process in the SPS inhibiting densification until similar to 1350 degrees C. After heat treatment, sintering and densification is enabled at low temperature, 1000 degrees C1100 degrees C. The densification of ZrB2SiC composites can be broken into a two-step process with phase 1 as the sintering step based on powder surface area reduction and phase 2 as a forging step where high-temperature creep and pressure eliminate porosity after the primary grains have formed. A timetemperature-density plot illustrates the change in densification mechanism used to fully densify ZrB2SiC composites in SPS.
Sealant materials for solid oxide fuel cells (SOFCs) must meet a demanding set of performance criteria for operating lifetimes of up to 40,000 hr. The resulting seals must be gas tight at temperatures up to 1000°C, resist stresses from thermal gradients and expansion mismatch of different stack materials, and perform reliably over long times at high temperatures in both oxidizing and reducing atmospheres. Ceramic and metal filled glass composite sealants provide for greater design flexibility than other approaches. The seal properties can be tailored by varying the composition, amount, and microstructure of the particulate phase. Composite properties such as glass transition temperature, viscosity, and thermal expansion coefficient can be altered by rational control of the glass chemistry and composite microstructure. Several specific materials combinations have been engineered to meet the demanding set of criteria for sealing materials in SOFCs and characterized by means of viscosity measurements at the proposed operating temperature of 750°C The influence of the matrix/particle interactions has been separated from the mechanical effects of the added phase in studies that systematically vary the chemical composition of the particles. Models for composite suspension viscosity were also used to interpret the observed variation in viscosity with composition and volume fraction of the filler.
Sol-gel precursors to HfB2 and ZrB2 are processed by high-energy ultrasonication of Hf,Zr oxychloride hydrates, triethyl borate, and phenolic resin to form precipitate-free sols that turn into stable gels with no catalyst addition. Both precursor concentration and structure (a sol or a gel) are found to influence the synthesis of the diboride phase at high temperature. Decreasing sol concentration increases powder surface area from 3.6 to 6.8m(2)/g, whereas heat-treating a gel leads to residual oxides and carbides. Particles are either fine spherical particles, unique elongated rods, and/or platelets, indicating particle growth with directional coarsening. Investigation of the conversion process to ZrB2 indicates that a multistep reaction is likely taking place with: (1) ZrC formation, (2) ZrC reacts with B2O3 or ZrC reacts with B2O3 and C to form ZrB2. At low temperatures, ZrC formation is limiting, while at higher temperatures the reaction of ZrC to ZrB2 becomes rate limiting. ZrC is found to be a direct reducing agent for B2O3 at low temperature (similar to 1200 degrees C) to form ZrB2 and ZrO2, whereas at high temperatures (similar to 1500 degrees C) it reacts with B2O3 and C to form pure ZrB2.
Multiwalled carbon nanotubes were dispersed in a silicon carbide matrix to examine nanotube influence on mechanical properties of the resulting composite. The ceramic matrix was generated through high temperature conversion of poly(methylsilyne), a preceramic polymeric precursor. Nanotube alkylation was explored using two functionalization schemes: organic peroxide workup and alkyllithium displacement of fluorinated nanotubes, which promoted extensive mixing within precursor solutions, thereby ensuring nanotube dispersion within the polymer matrix while facilitating interfacial bonding. The former scheme was less effective at displacing inner nanotube shell bound fluorine and resulted in lower alkyl chain grafting density on the outer shell. Polymer nanocomposites were pyrolyzed and consolidated using an optimized spark plasma sintering scheme to generate fully densified ceramics. The pure polymer-derived ceramic displayed exceptional Young's modulus and Vickers microhardness of 126 +/- 12 and 9.6 +/- 0.5 GPa, respectively, while maintaining a fracture toughness of 2.8 +/- 0.3 MPa center dot m1/2. Increased sintering time further augmented the fracture toughness to 3.6 +/- 0.4MPa center dot m1/2, approaching the 4MPa center dot m1/2 that characterizes pure silicon carbide, while maintaining both Young's modulus and microhardness. Nanotube addition resulted in some loss of the intrinsic mechanical properties, but enhanced monolith damage tolerance behavior, raising the Vickers indent force needed to induce cracks to an excess of 98.1N in contrast to the pure polymer-derived sample, which began crack propagation below 49.0 N.
Si3N4 nanocomposites reinforced with 1-, 2-, and 6-vol% single-walled carbon nanotubes (SWNTs) were processed using spark plasma sintering (SPS) in order to control the thermal and electrical properties of the ceramic. Only 2-vol% SWNTs additions were used to decrease the room temperature thermal conductivity by 62% over the monolith and 6-vol% SWNTs was used to transform the insulating ceramic into a metallic electrical conductor (92Sm-1). We found that densification of the nanocomposites was inhibited with increasing SWNT concentration however, the phase transformation from α- to β-Si3N4 was not. After SPS, we found evidence of SWNT survival in addition to sintering induced defects detected by monitoring SWNT peak intensity ratios using Raman spectroscopy. Our results show that SWNTs can be used to effectively increase electrical conductivity and lower thermal conductivity of Si3N4 due to electrical transport enhancement and thermal scattering of phonons by SWNTs using SPS. © 2010 Elsevier Ltd.