SEARCH · Engineering Papers
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
SiB6 is Calcium hexaboride structured and crystallizes in the cubic Pm-3m space group. The structure is three-dimensional and consists of one silicon molecule and one B framework. In the B framework, B is bonded in a 5-coordinate geometry to five equivalent B atoms. There is one shorter (1.65 Å) and four longer (1.75 Å) B–B bond length.
MgB12Si2 crystallizes in the orthorhombic Pnma space group. The structure is zero-dimensional and consists of sixteen boron molecules and four Mg(B4Si)2 clusters. In each Mg(B4Si)2 cluster, Mg2+ is bonded in a distorted bent 150 degrees geometry to two Si4- atoms. There are one shorter (2.45 Å) and one longer (2.64 Å) Mg–Si bond lengths. There are six inequivalent B+0.50+ sites. In the first B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.08 Å. In the second B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.07 Å. In the third B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.17 Å. In the fourth B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.00 Å. In the fifth B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.00 Å. In the sixth B+0.50+ site, B+0.50+ is bonded in a single-bond geometry to one Si4- atom. The B–Si bond length is 2.07 Å. There are two inequivalent Si4- sites. In the first Si4- site, Si4- is bonded in a 5-coordinate geometry to one Mg2+ and four B+0.50+ atoms. In the second Si4- site, Si4- is bonded in a 4-coordinate geometry to one Mg2+ and four B+0.50+ atoms.
High boron materials of high efficiency for thermoelectric power generation and capable of prolonged operation at temperatures over 1200 C are discussed. Background theoretical studies indicated that the low carrier mobility of materials with beta boron and related structures is probably associated with the high density of traps. Experimental work was mainly concerned with silicon borides in view of promising data from European laboratories. A systematic study using structure determination and lattice constant measurements failed to confirm the existence of an SiBn phase. Only SiB6 and a solid solution of silicon in beta boron with a maximum solid solubility of 5.5-6 at % at 1650 C were found.
A silicon carbide powder of a 5-micron grain size is mixed with 0.15 to 0.60 wt% mixture of a boron compound, i.e., boric acid, boron carbide (B4C), silicon boride (SiB4 or SiB6), aluminum boride, etc., and an aluminum compound, i.e., aluminum, aluminum oxide, aluminum hydroxide, aluminum carbide, etc., or aluminum boride (AlB2) alone, in such a proportion that the boron/aluminum atomic ratio in the sintered body becomes 0.05 to 0.25 wt% and 0.05 to 0.40 wt%, respectively, together with a carbonaceous additive to supply enough carbon to convert oxygen accompanying raw materials and additives into carbon monoxide.
Non-oxide ceramics such as silicon carbide (SiC), silicon nitride (Si3N4), and silicon borides (SiB4, SiB6) have thermal stability, oxidation resistance, hardness, and varied electrical properties. All these materials can be prepared in a fiber form from a suitable polymer precursor. The above mentioned fibers, when tested over a temperature range from 25 to 1400 C, experience degradation at elevated temperatures. Past work in ceramic materials has shown that the strength of ceramics containing both carbides and borides is sustained at elevated temperatures, with minimum oxidation. The work presented here describes the formation of ceramic fibers containing both elements, boron and silicon, prepared via the polymer precursor route previously reported by the authors, and discusses the fiber mechanical properties that are retained over the temperature range studied.
This report summarizes the evaluation and testing of high emissivity protective coatings applied to flexible insulations for the Reusable Launch Vehicle technology program. Ceramic coatings were evaluated for their thermal properties, durability, and potential for reuse. One of the major goals was to determine the mechanism by which these coated blanket surfaces become brittle and try to modify the coatings to reduce or eliminate embrittlement. Coatings were prepared from colloidal silica with a small percentage of either SiC or SiB6 as the emissivity agent. These coatings are referred to as gray C-9 and protective ceramic coating (PCC), respectively. The colloidal solutions were either brushed or sprayed onto advanced flexible reusable surface insulation blankets. The blankets were instrumented with thermocouples and exposed to reentry heating conditions in the Ames Aeroheating Arc Jet Facility. Post-test samples were then characterized through impact testing, emissivity measurements, chemical analysis, and observation of changes in surface morphology. The results show that both coatings performed well in arc jet tests with backface temperatures slightly lower for the PCC coating than with gray C-9. Impact testing showed that the least extensive surface destruction was experienced on blankets with lower areal density coatings.