Sinterable ceramic powders from laser-heated gases

Abstract

Extremely high quality ceramic powders have been synthesized from SiH4, NH3 and CH4 reactant gasses that are heated by absorbing energy emitted from a CO2 laser. Resulting Si, Si3N4 and SiC powders have been characterized in terms of parameters which are important for densification processes. The powders are virtually ideal. The fully dispersible powders have mean diameters ranging from 0.1-0.3 pm with a standard deviation that is typically 25-45%. As-synthesized powders are extremely high purity because the synthesis equipment is hermetic and cold-walled. The synthesis process has been modeled on a macro scale with respect to heat- transfer, fluid-flow and stability criteria. These results have permitted the process to be scaled safely to production rates up to 8-40 tons/year/nozzle. The particle formation and growth processes have also been analyzed experimentally and analytically in terms of a collision-coalescence model. Application of these models permitted particle sizes to be increased to useful dimensions while retaining complete dispersibility. Compound particles form by a 2-step reaction sequence between molten silicon particles and a reactive atmosphere only after the Si particles have grown to desired dimensions. The process is extremely efficient; >95% of the SiH 4 is reacted in a single pass through the laser beam and approximately 2 kwhr of energy are required per kilo of powder. Manufacturing costs are projected to be $1.50-5.00/kg plus the cost of the reactants. Resulting powders have been dispersed and shaped into flaw-free, maximum density green parts; colloidal pressing and centrifugal sedimentation techniques have been used successfully. Reaction bonded silicon nitride (RBSN) forms from the Si powders in unusually rapid, low temperature (e.g. 1150C, 1 hr and 1250°C, 10 min) exposures. The SiC powders sinter to virtually full density in 1 hr at 2050 0C. The properties of both RBSN and sintered SiC (SSC) parts made from the lasersynthesized powders are excellent. RBSN strengths (up to 690 MPa) are 3-5 times values normally observed at the same densities and are in the range normally associated with fully dense alpha-Si3N4. The strengths of the SSC parts are also much higher than are normally observed (up to 714 MPa). The oxidation resistance of the RBSN is approximately 10 times better than conventional RBSN and 5-10 times better than commercial hot pressed Si3N4 (HPSN) for 1000 and 14000C air exposures. The superior properties and consolidation kinetics result directly from the high quality of the green parts and the purity levels maintained in the powders through the firing stage, As a separate topic, the surface tensions and densities of A1203 melts with MgO, TiO2 and ZrO2 additions were measured in air, He and He-H2 atmospheres using the pendant-drop technique. Melts on the bottom ends of sintered rods were formed by CO2 laser heating. A curve fitting technique was developed that improved the experimental accuracy of analyzing the short pendant drops that are characteristic of these materials.