Ultra-high temperature ceramic
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.
[5] UHTC research was largely abandoned after the pioneering mid-century Manlabs work due to the completion of the Space Shuttle missions and the elimination of the Air force spaceplane development.
In order to test real world performance of UHTC materials in reentry environments, NASA Ames conducted two flight experiments in 1997 and 2000.
Sharp B-1 had a HfB2/SiC nosecone with a tip radius of 3.5 mm which experienced temperatures well above 2,815 °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.
[8] Most research conducted in the last two decades has focused on improving the performance of the two most promising compounds developed by Manlabs, ZrB2 and HfB2, though significant work has continued in characterizing the nitrides, oxides, and carbides of the group four and five elements.
Nitrides such as ZrN and HfN have similarly strong covalent bonds but their refractory nature makes them especially difficult to synthesize and process.
The mechanism behind this enhancement in hardness maybe because of bonding behavior or some solid solution hardening effects arising from localized lattice strains.
[31] For applications based on combustion harsh environments and aerospace, Monolithic UHTCs are of concern because of their low fracture toughness and brittle behavior.
It has been found that the oxidative resistance of HfB2 and ZrB2 are greatly enhanced through the inclusion of 30% weight silicon carbide due to the formation of a protective glassy surface layer upon the application of temperatures in excess of 1,000 °C composed of SiO2.
Advantages of SHS include higher purity of ceramic products, increased sinterability, and shorter processing times.
Inexpensive precursor materials are used and reacted according to the reaction below: ZrO2 + B2O3 + 5Mg → ZrB2 + 5MgO Mg is used as a reactant in order to allow for acid leaching of unwanted oxide products.
[44] A second SHS reaction with Mg and H3BO3 as reactants along with the ZrB2/ZrO2 mixture yields increased conversion to the diboride, and particle sizes of 25–40 nm at 800 °C.
These coating materials exhibit uniform distribution of fine particles and porous microstructures, which increased hydrogen flow rates.
Nanocrystals of group IV and V metal diborides such as TiB2, ZrB2, HfB2, NbB2, TaB2 were successfully synthesized by Zoli's Reaction, reduction of TiO2, ZrO2, HfO2, Nb2BO5, Ta2O5 with NaBH4 using a molar ratio M:B of 1:4 at 700 °C for 30 minutes under argon flow.
After plasma of the reacting gases is created (by radio frequency or direct current discharge between two electrodes) the reaction takes place, followed by deposition.
The deposition takes place at lower temperatures compared to traditional CVD because only the plasma needs to be heated to provide sufficient energy for the reaction.
Thermal decomposition of Zr(BH)4 to ZrB2 can occur at temperatures in the range of 150–400 °C in order to prepare amorphous, conductive films.
The high melting points and strong covalent interactions present in UHTCs make it difficult to achieve uniform densification in these materials.
[54] Unfortunately, processing of UHTCs at these temperatures results in materials with larger grain sizes and poor mechanical properties including reduced toughness and hardness.
[56] In addition to improved mechanical properties, less SiC needs to be added when using this method, which limits the pathways for oxygen to diffuse into the material and react.
Hot pressing temperature, pressure, heating rate, reaction atmosphere, and holding times are all factors that affect the density and microstructure of UHTC pellets obtained from this method.
[59] Hot pressing may result in improved densities for UHTCs, but it is an expensive technique that relies on high temperatures and pressures to provide useful materials.
Spark plasma sintering often relies on slightly lower temperatures and significantly reduced processing times compared to hot pressing.
During spark plasma sintering, a pulsed direct current passes through graphite punch rods and dies with uniaxial pressure exerted on the sample material.
UHTCs, specifically Hf and Zr based diboride, are being developed to handle the forces and temperatures experienced by leading vehicle edges in atmospheric reentry and sustained hypersonic flight.
The material design challenges associated with developing such surfaces have so far limited the design of orbital re-entry bodies and hypersonic air-breathing vehicles such as scramjets and DARPA's HTV because the bow shock in front of a blunt body protects the underlying surface from the full thermal force of the onrushing plasma with a thick layer of relatively dense and cool plasma.
[65] Zirconium diboride is used in many boiling water reactor fuel assemblies due to its refractory nature, corrosion resistance, high-neutron-absorption cross-section of 759 barns, and stoichiometric boron content.
Due to the combination of refractory properties, high thermal conductivity, and the advantages of large stoichiometric boron content outlined in the above discussion of integral neutron absorbing fuel pellet cladding, refractory diborides have been used as control rod materials and have been studied for use in space nuclear power applications.
[68] While boron carbide is the most popular material for fast breeder reactors due to its lack of expense, extreme hardness comparable to diamond, and high cross-section, it completely disintegrates after a 5% burnup[69] and is reactive when in contact with refractory metals.
Hafnium diboride also suffers from high susceptibility to material degradation with boron transmutation,[70] but its high melting point of 3,380 °C and the large thermal neutron capture cross section of hafnium of 113 barns and low reactivity with refractory metals such as tungsten makes it an attractive control rod material when clad with a refractory metal.
