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Ultra-high-temperature ceramics (UHTCs) are a class of refractory ceramics that offer excellent stability at temperatures exceeding 2000 °C being investigated as possible thermal protection system (TPS) materials, coatings for materials subjected to high temperatures, and bulk materials for heating elements. Broadly speaking, UHTCs are borides, carbides, nitrides, and oxides of early transition metals. Current efforts have focused on heavy, early transition metal borides such as hafnium diboride (HfB2) and zirconium diboride (ZrB2); additional UHTCs under investigation for TPS applications include hafnium nitride (HfN), zirconium nitride (ZrN), titanium carbide (TiC), titanium nitride (TiN), thorium dioxide (ThO2), tantalum carbide (TaC) and their associated composites. == History == Beginning in the early 1960s, demand for high-temperature materials by the nascent aerospace industry prompted the Air Force Materials Laboratory to begin funding the development of a new class of materials that could withstand the environment of proposed hypersonic vehicles such as Dyna-soar and the Space Shuttle at Manlabs Incorporated. Through a systematic investigation of the refractory properties of binary ceramics, they discovered that the early transition metal borides, carbides, and nitrides had surprisingly high thermal conductivity, resistance to oxidation, and reasonable mechanical strength when small grain sizes were used. Of these, ZrB2 and HfB2 in composites containing approximately 20% volume SiC were found to be the best performing. UHTC research was largely abandoned after the pioneering mid-century Manlabs work due to the completion of the Space Shuttle missions and the elimination Air force spaceplane development. Three decades later, however, research interest was rekindled by a string of 1990s era NASA programs aimed at developing a fully reusable hypersonic spaceplane such as the National Aerospace Plane, Venturestar/X-33, Boeing X-37, and the Air Force's Blackstar program. New research in UHTCs was led by NASA Ames, with research at the center continuing to the present through funding from the NASA Fundamental Aeronautics Program. UHTCs also saw expanded use in varied environments, from nuclear engineering to aluminum production. In order to test real world performance of UHTC materials in reentry environments, NASA Ames conducted two flight experiments in 1997 and 2000. The slender Hypersonic Aero-thermodynamic Research Probes (SHARP B1 and B2) briefly exposed the UHTC materials to actual reentry environments by mounting them on modified nuclear ordnance Mk12A reentry vehicles and launching them on Minuteman III ICBMs. Sharp B-1 had a HfB2/SiC nosecone with a tip radius of 3.5 mm which experienced temperatures well above 2815 °C during reentry, ablating away at an airspeed of 6.9 km/s as predicted; however, it was not recovered and its axially-symmetric cone shape did not provide flexural strength data needed to evaluate the performance of UHTCs in linear leading edges. To improve the characterization of UHTC mechanical strength and better study their performance, SHARP-B2, was recovered and included four retractable, sharp wedge-like protrusions called "strakes" which each contained three different UHTC compositions which were extended into the reentry flow at different altitudes. The SHARP-B2 test that followed permitted recovery of four segmented strakes which had three sections, each consisting of a different HfB2 or ZrB2 composite as shown in Figure 1.〔 The vehicle was successfully recovered, despite the fact that it impacted the sea at three times the predicted velocity. The four rear strake segments (HfB2) fractured between 14 and 19 seconds into reentry, two mid segments (ZrB2/SiC) fractured, and no fore strake segments (ZrB2/SiC/C) failed.〔 The actual heat flux was 60% less than expected, actual temperatures were much lower than expected, and heat flux on the rear strakes was much higher than expected. The material failures were found to result from very large grain sizes in the composites and pure ceramics, with cracks following macroscopic crystal grain boundaries. Since this test, NASA Ames has continued refining production techniques for UHTC synthesis and performing basic research on UHTCs. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ultra-high-temperature ceramics」の詳細全文を読む スポンサード リンク
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