Abstract:

Zirconium diboride and boron carbide particles were used to improve the ablation resistance of carbon-carbon (C-C) composites at high temperature (1500°C). Our approach combines using a precursor to ZrB2 and processing them with B4C particles as filler material within the C-C composite. An oxyacetylene torch test facility was used to determine ablation rates for carbon black, B4C, and ZrB2-B4C filled C-C composites from 800 to 1500°C. Ablation rates decreased by 30% when C-C composites were filled with a combination of ZrB2-B4C particles over carbon black and B4C filled C-C composites. We also investigated using a sol-gel precursor method as an alternative processing route to incorporate ZrB2 particles within C-C composites. We successfully converted ZrB2 particles within C-C composites at relatively low temperatures (1200°C). Our ablation results suggest that a combination of ZrB2-B4C particles is effective in inhibiting the oxidation of C-C composites at temperatures greater than 1500°C. © 2010 Elsevier Ltd.

Using an organic solvent-based formulation, flexible and homogeneous ZrB2 tapes were cast for potential use as advanced aerospace exploration vehicles. Dispersant concentrations were optimized for attrition-milled ZrB2 powder using gravitational sedimentation and viscosity measurements. Tape cast slurry formulations with varying amounts of binder (6-8 vol%), plasticizer (7-11 vol%), and solids loading (20-23 vol%) were used to optimize the casting slurry. An optimal slurry formulation was determined based on cast tape uniform particle distribution and flexibility without cracking. Thermal gravimetric analysis of the optimized tape was used to create a binder burnout schedule that did not alter the tape structure or particle distribution. Our organic solvent-based tape casting approach results in minimum oxygen contamination after colloidal processing and robust cast tapes with a thickness of 280 mu m and green density of similar to 41% theoretic density after binder burnout.

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.

Abstract:

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.