Titanium diboride (TiB2) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability and wear resistance. TiB2 is also a reasonable electrical conductor,[J. Schmidt et al. "Preparation of titanium diboride TiB2 by spark plasma sintering at slow heating rate" Sci. Technol. Adv. Mater. 8 (2007) 376 free download] so it can be used as a cathode material in aluminium smelting and can be shaped by electrical discharge machining.
Physical properties
TiB
2 shares some properties with
boron carbide and
titanium carbide, but many of its properties are superior to those of B
4C & TiC:
Exceptional hardness at extreme temperature
-
2nd hardest material at 3000°C (diamond)
-
3rd hardest material at 2800°C (cBN)
-
4th hardest material at 2100°C (Boron carbide)
-
5th hardest material at 1000°C (Boron suboxide)
Advantages over other borides
Other advantages
-
High thermal conductivity (60-120 W/(m K)),
-
High electrical conductivity (~105 S/cm)
Drawbacks
-
Difficult to mold due to high melting temperature
-
Difficult to sinter due to the high covalent bonding
-
Limited to pressing to small Monolithic pieces using of spark plasma sintering
Chemical properties
With respect to chemical stability, TiB
2 is more stable in contact with pure iron than
tungsten carbide or
silicon nitride.
TiB2 is resistant to oxidation in air at temperatures up to 1100 °C,
Production
TiB
2 does not occur naturally in the earth. Titanium diboride powder can be prepared by a variety of high-temperature methods, such as the direct reactions of
titanium or its oxides/hydrides, with elemental
boron over 1000 °C, carbothermal reduction by thermite reaction of
titanium oxide and
boron oxide, or hydrogen reduction of boron halides in the presence of the metal or its halides. Among various synthesis routes, electrochemical synthesis and solid state reactions have been developed to prepare finer titanium diboride in large quantity. An example of solid state reaction is the borothermic reduction, which can be illustrated by the following reactions:
(1) 2 TiO2 + B4C + 3C → 2 TiB2 + 4 CO
(2) TiO2 + 3NaBH4 → TiB2 + 2Na(g,l) + NaBO2 + 6H2(g)
The first synthesis route (1), however, cannot produce nanosized powders. Nanocrystalline (5–100 nm) TiB2 was synthesized using the reaction (2) or the following techniques:
-
Solution phase reaction of NaBH4 and TiCl4, followed by annealing the amorphous precursor obtained at 900–1100 °C.
[S. E. Bates et al. "Synthesis of titanium boride (TiB)2 nanocrystallites by solution-phase processing" J. Mater. Res. 10 (1995) 2599]
-
Mechanical alloying of a mixture of elemental Ti and B powders.
[A. Y. Hwang and J. K. Lee "Preparation of TiB2 powders by mechanical alloying " Mater. Lett. 54 (2002) 1]
-
Self-propagating high-temperature synthesis process involving addition of varying amounts of NaCl.
[A. K. Khanra et al. "Effect of NaCl on the synthesis of TiB2 powder by a self-propagating high-temperature synthesis technique" Mater. Lett. 58 (2004) 733]
-
Milling assisted self-propagating high-temperature synthesis (MA-SHS).
-
Solvothermal reaction in benzene of metallic sodium with amorphous boron powder and TiCl4 at 400 °C:
[Y. Gu et al. "A mild solvothermal route to nanocrystalline titanium diboride" J. Alloy. Compd. 352 (2003) 325]
- :TiCl4 + 2 B + 4 Na → TiB2 + 4 NaCl
Many TiB2 applications are inhibited by economic factors, particularly the costs of densifying a high melting point material - the melting point is about 2970 °C, and, thanks to a layer of titanium dioxide that forms on the surface of the particles of a powder, it is very resistant to sintering. Admixture of about 10% silicon nitride facilitates the sintering,
Thin films of TiB2 can be produced by several techniques. The electroplating of TiB2 layers possess two main advantages compared with physical vapor deposition or chemical vapor deposition: the growing rate of the layer is 200 times higher (up to 5 μm/s) and the inconveniences of covering complex shaped products are dramatically reduced.
Potential applications
Current use of TiB
2 appears to be limited to specialized applications in such areas as impact resistant
armor,
,
, neutron absorbers and wear resistant coatings.
TiB2 is extensively used for evaporation boats for vapour coating of aluminium.
of TiB2 can be used to provide wear and corrosion resistance to a cheap and/or tough substrate.
Compare
See also
