Zirconium diboride (ZrB
2) is a highly covalent refractory ceramic material with a hexagonal crystal structure. ZrB
2 is an
ultra-high temperature ceramic (UHTC) with a melting point of 3246 °C. This along with its relatively low density of ~6.09 g/cm
3 (measured density may be higher due to
hafnium
Hafnium is a chemical element; it has symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in many zirconium minerals. Its existence was predicted by Dm ...
impurities) and good high temperature strength makes it a candidate for high temperature aerospace applications such as hypersonic flight or rocket propulsion systems. It is an unusual ceramic, having relatively high thermal and electrical conductivities, properties it shares with
isostructural titanium diboride and
hafnium diboride.
ZrB
2 parts are usually
hot pressed (pressure applied to the heated powder) and then machined to shape. Sintering of ZrB
2 is hindered by the material's
covalent
A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atom ...
nature and presence of surface oxides which increase
grain coarsening before
densification during
sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, plas ...
.
Pressureless sintering of ZrB
2 is possible with sintering additives such as
boron carbide
Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders,
as well as numerous industrial applications. With a Vickers har ...
and
carbon
Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
which react with the surface oxides to increase the driving force for sintering but mechanical properties are degraded compared to hot pressed ZrB
2.
Additions of ~30 vol% SiC to ZrB
2 is often added to ZrB
2 to improve
oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
resistance through SiC creating a protective oxide layer - similar to aluminium's protective alumina layer.
ZrB
2 is used in
ultra-high temperature ceramic matrix composites (UHTCMCs).
Carbon fiber
Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers ( Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon comp ...
reinforced zirconium diboride composites show high toughness while
silicon carbide fiber reinforced zirconium diboride composites are brittle and show a
catastrophic failure
A catastrophic failure is a sudden and total failure from which recovery is impossible. Catastrophic failures often lead to cascading systems failure. The term is most commonly used for structural failures, but has often been extended to many ot ...
.
Preparation
ZrB
2 can be synthesized by stoichiometric reaction between constituent elements, in this case
Zr and
B. This reaction provides for precise stoichiometric control of the materials.
At 2000 K, the formation of ZrB
2 via stoichiometric reaction is thermodynamically favorable (ΔG=−279.6 kJ mol
−1) and therefore, this route can be used to produce ZrB
2 by
self-propagating high-temperature synthesis (SHS). This technique takes advantage of the high exothermic energy of the reaction to cause high temperature, fast combustion reactions. Advantages of SHS include higher purity of ceramic products, increased sinterability, and shorter processing times. However, the extremely rapid heating rates can result in incomplete reactions between Zr and B, the formation of stable oxides of Zr, and the retention of
porosity
Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
. Stoichiometric reactions have also been carried out by reaction of attrition milled (wearing materials by grinding) Zr and B powder (and then hot pressing at 600 °C for 6 h), and nanoscale particles have been obtained by reacting attrition milled Zr and B
precursor crystallite
A crystallite is a small or even microscopic crystal which forms, for example, during the cooling of many materials. Crystallites are also referred to as grains.
Bacillite is a type of crystallite. It is rodlike with parallel Wikt:longulite ...
s (10 nm in size).
Reduction of ZrO
2 and HfO
2 to their respective diborides can also be achieved via metallothermic reduction. Inexpensive precursor materials are used and reacted according to the reaction below:
: ZrO
2 + B
2O
3 + 5Mg → ZrB
2 + 5MgO
Mg is used as a reactant to allow for acid leaching of unwanted oxide products. Stoichiometric excesses of Mg and B
2O
3 are often required during metallothermic reductions to consume all available ZrO
2. These reactions are
exothermic
In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
and can be used to produce the diborides by SHS. Production of ZrB
2 from
ZrO2 via SHS often leads to incomplete conversion of reactants, and therefore double SHS (DSHS) has been employed by some researchers.
A second SHS reaction with Mg and
H3BO3 as reactants along with the ZrB
2/ZrO
2 mixture yields increased conversion to the diboride, and particle sizes of 25–40 nm at 800 °C. After metallothermic reduction and DSHS reactions, MgO can be separated from ZrB
2 by mild
acid leaching.
Synthesis of UHTCs by
boron carbide
Boron carbide (chemical formula approximately B4C) is an extremely hard boron–carbon ceramic, a covalent material used in tank armor, bulletproof vests, engine sabotage powders,
as well as numerous industrial applications. With a Vickers har ...
reduction is one of the most popular methods for UHTC synthesis. The precursor materials for this reaction (ZrO
2/TiO
2/HfO
2 and
B4C) are less expensive than those required by the
stoichiometric
Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions.
Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...
and borothermic reactions. ZrB
2 is prepared at greater than 1600 °C for at least 1 hour by the following reaction:
: 2ZrO
2 + B
4C + 3C → 2ZrB
2 + 4CO
This method requires a slight excess of boron, as some boron is oxidized during boron carbide reduction.
ZrC has also been observed as a product from the reaction, but if the reaction is carried out with 20–25% excess B
4C, the ZrC phase disappears, and only ZrB
2 remains. Lower synthesis temperatures (~1600 °C) produce UHTCs that exhibit finer
grain size
Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which ...
s and better sinterability. Boron carbide must be subjected to grinding prior to the boron carbide reduction to promote oxide reduction and diffusion processes.
Boron carbide reductions can also be carried out via reactive
plasma spraying if a UHTC coating is desired. Precursor or powder particles react with plasma at high temperatures (6000–15000 °C) which greatly reduces the reaction time.
ZrB
2 and ZrO
2 phases have been formed using a plasma voltage and current of 50 V and 500 A, respectively. These coating materials exhibit uniform distribution of fine particles and porous microstructures, which increased hydrogen
flow rates.
Another method for the synthesis of UHTCs is the borothermic reduction of ZrO
2, TiO
2, or HfO
2 with B.
At temperatures higher than 1600 °C, pure diborides can be obtained from this method. Due to the loss of some boron as boron oxide, excess boron is needed during borothermic reduction. Mechanical milling can lower the reaction temperature required during borothermic reduction. This is due to the increased particle mixing and
lattice defect
A crystallographic defect is an interruption of the regular patterns of arrangement of atoms or molecules in crystalline solids. The positions and orientations of particles, which are repeating at fixed distances determined by the unit cell par ...
s that result from decreased
particle size
Particle size is a notion introduced for comparing dimensions of solid particles ('' flecks''), liquid particles ('' droplets''), or gaseous particles ('' bubbles''). The notion of particle size applies to particles in colloids, in ecology, in ...
s of ZnO
2 and B after milling. This method is also not very useful for industrial applications due to the loss of expensive boron as
boron oxide Boron oxide may refer to one of several oxides of boron:
*Boron trioxide (B2O3, diboron trioxide), the most common form
*Boron monoxide (BO)
*Boron suboxide
Boron suboxide (chemical formula B6O) is a solid compound with a structure built of eight ...
during the reaction.
Nanocrystals of ZrB
2were successfully synthesized by Zoli's reaction, a reduction of ZrO
2 with NaBH
4 using a molar ratio M:B of 1:4 at 700 °C for 30 min under argon flow.
: ZrO
2 + 3NaBH
4 → ZrB
2 + 2Na(g,l) + NaBO
2 + 6H
2(g)
ZrB
2 can be prepared from solution-based synthesis methods as well, although few substantial studies have been conducted. Solution-based methods allow for low temperature synthesis of ultrafine UHTC powders. Yan et al. have synthesized ZrB
2 powders using the inorganic-organic precursors ZrOC
l2•8H
2O,
boric acid
Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen orthoborate, trihydroxidoboron or boracic acid. It is usually encountered as colorless crystals or a white ...
and
phenolic resin
Phenol formaldehyde resins (PF), also called phenolic resins or phenoplasts, are synthetic polymers obtained by the reaction of phenol or substituted phenol with formaldehyde. Used as the basis for Bakelite, PFs were the first commercial synthetic ...
at 1500 °C.
The synthesized powders exhibit 200 nm crystallite size and low oxygen content (~ 1.0 wt%). ZrB
2 preparation from polymeric precursors has also been recently investigated. ZrO
2 and HfO
2 can be dispersed in boron carbide polymeric precursors prior to reaction. Heating the reaction mixture to 1500 °C results in the in situ generation of boron carbide and carbon, and the reduction of ZrO
2 to ZrB
2 soon follows.
The polymer must be stable, processable, and contain boron and carbon to be useful for the reaction. Dinitrile polymers formed from the condensation of dinitrile with decaborane satisfy these criteria.
Chemical vapor deposition
Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films.
In typical CVD, the wafer (electro ...
can be used to prepare zirconium diboride.
Hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
gas is used to reduce vapors of
zirconium tetrachloride
Zirconium is a chemical element; it has symbol Zr and atomic number 40. First identified in 1789, isolated in impure form in 1824, and manufactured at scale by 1925, pure zirconium is a lustrous transition metal with a greyish-white color that c ...
and
boron trichloride
Boron trichloride is the inorganic compound with the formula BCl3. This colorless gas is a reagent in organic synthesis. It is highly reactive towards water.
Production and structure
Boron reacts with halogens to give the corresponding trihalides ...
at substrate temperatures greater than 800 °C.
Recently, high-quality thin films of ZrB
2 can also be prepared by physical vapor deposition.
Defects and secondary phases in zirconium diboride
Zirconium diboride gains its high-temperature mechanical stability from the high
atomic defect energies (i.e. the atoms do not deviate easily from their lattice sites).
This means that the concentration of defects will remain low, even at high temperatures, preventing
failure
Failure is the social concept of not meeting a desirable or intended objective, and is usually viewed as the opposite of success. The criteria for failure depends on context, and may be relative to a particular observer or belief system. On ...
of the material.
The layered bonding between each layer is also very strong but means that the ceramic is highly anisotropic, having different thermal expansions in the 'z' <001> direction. Although the material has excellent high temperature properties, the ceramic has to be produced extremely carefully as any excess of either zirconium or boron will not be accommodated in the ZrB
2 lattice (i.e. the material does not deviate from
stoichiometry
Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions.
Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...
). Instead it will form extra
lower melting point phases which may initiate failure under extreme conditions.
Diffusion and transmutation in zirconium diboride
Zirconium diboride is also investigated as a possible material for
nuclear reactor
A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
control rods due to the presence of boron.
:
10B + n
th →
11B">sup>11B→ α +
7Li + 2.31 MeV.
The layered structure provides a plane for helium
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
to occur. He is formed as a
transmutation product of
boron-10
Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three v ...
—it is the
alpha particle
Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay but may also be produce ...
in the above reaction—and will rapidly migrate through the lattice between the layers of zirconium and boron, however not in the 'z' direction. Of interest, the other transmutation product,
lithium
Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...
, is likely to be trapped in the boron vacancies that are produced by the boron-10 transmutation and not be released from the
lattice.
Application in solar energy sector
Direct absorption solar collector are being considered of great interest in combination with colloidal systems dispersed in suitable liquids. The obtained fluids are able to carry out, at the same time, the double function of volumetric solar absorbers and heat transfer media. Optical properties of ZrB
2 have been investigated for this purpose.
References
{{Borides
Zirconium(II) compounds
Borides
Ceramic materials
Refractory materials