Titanium diboride (TiB₂) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability and

wear resistance Wear is the damaging, gradual removal or deformation of material at solid surfaces. Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion). The study of wear and related processes is referred to as tribology. Wear in ...

. 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) 37
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/ref> so it can be used as a cathode material in

aluminium smelting Aluminium smelting is the process of extracting aluminium from its oxide, alumina, generally by the Hall-Héroult process. Alumina is extracted from the ore bauxite by means of the Bayer process at an alumina refinery. This is an electroly ...

and can be shaped by

electrical discharge machining Electrical discharge machining (EDM), also known as spark machining, spark eroding, die sinking, wire burning or wire erosion, is a metal fabrication process whereby a desired shape is obtained by using electrical discharges (sparks). Material is ...

Physical properties

TiB₂ shares some properties with

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 titanium carbide, but many of its properties are superior to those of B₄C & TiC:

Exceptional hardness at extreme temperature

*2nd hardest material at 3000°C (

diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Diamond is tasteless, odourless, strong, brittle solid, colourless in pure form, a poor conductor of e ...

) *3rd hardest material at 2800°C (

cBN CBN, or cbn, may refer to: Broadcasting organizations * Radio stations in St. John's, Newfoundland: ** CBN (AM), CBC Radio One ** CBN-FM, CBC Music * Chronicle Broadcasting Network, the predecessor of ABS-CBN * CBN (Australian TV station), a TV ...

) *4th hardest material at 2100°C ( B₄C) *5th hardest material at 1000°C ( B₆O)

Advantages over other borides

*Highest boride

elastic modulus An elastic modulus (also known as modulus of elasticity (MOE)) is a quantity that describes an object's or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. Definition The elastic modu ...

*Highest boride

fracture toughness In materials science, fracture toughness is the critical stress intensity factor of a sharp Fracture, crack where propagation of the crack suddenly becomes rapid and unlimited. It is a material property that quantifies its ability to resist crac ...

*Highest boride

compressive strength In mechanics, compressive strength (or compression strength) is the capacity of a material or Structural system, structure to withstand Structural load, loads tending to reduce size (Compression (physics), compression). It is opposed to ''tensil ...

*3rd highest boride

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...

(3230 °C) ( HfB₂)

Other advantages

*High

thermal conductivity The thermal conductivity of a material is a measure of its ability to heat conduction, conduct heat. It is commonly denoted by k, \lambda, or \kappa and is measured in W·m−1·K−1. Heat transfer occurs at a lower rate in materials of low ...

(60-120 W/(m K)), *High

electrical conductivity Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...

(~10⁵ S/cm)

Drawbacks

* Difficult to

mold A mold () or mould () is one of the structures that certain fungus, fungi can form. The dust-like, colored appearance of molds is due to the formation of Spore#Fungi, spores containing Secondary metabolite#Fungal secondary metabolites, fungal ...

due to high melting temperature * Difficult to sinter due to the high

covalent bonding 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 atoms ...

* Limited to pressing to small Monolithic pieces using of spark plasma sintering

Chemical properties

With respect to chemical stability, TiB₂ is more stable in contact with pure iron than

tungsten carbide Tungsten carbide (chemical formula: ) is a carbide containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into shapes through sintering for use in in ...

silicon nitride Silicon nitride is a chemical compound of the elements silicon and nitrogen. (''Trisilicon tetranitride'') is the most thermodynamically stable and commercially important of the silicon nitrides, and the term ″''Silicon nitride''″ commonly re ...

. TiB₂ is resistant to oxidation in air at temperatures up to 1100 °C, and to hydrochloric and hydrofluoric acids, but reacts with

alkali In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The a ...

nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...

and

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

Production

TiB₂ 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 Titanium is a chemical element; it has symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...

or its oxides/hydrides, with elemental

boron 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 ...

over 1000 °C,

carbothermal reduction Carbothermic reactions involve the reduction of substances, often metal oxides (O2-), using carbon (C) as the reducing agent. The reduction is usually conducted in the electric arc furnace or reverberatory furnace, depending on the metal ore. Thes ...

by thermite reaction of

titanium oxide Titanium oxide may refer to: * Titanium dioxide (titanium(IV) oxide), TiO2 * Titanium(II) oxide (titanium monoxide), TiO, a non-stoichiometric oxide * Titanium(III) oxide (dititanium trioxide), Ti2O3 * Ti3O * Ti2O * δ-TiOx (x= 0.68–0.75) * Ti ...

and

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 ...

, 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 + 6H_2(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. *Mechanical alloying of a mixture of elemental Ti and B powders. * Self-propagating high-temperature synthesis process involving addition of varying amounts of NaCl. *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: ::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 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 ...

. Admixture of about 10%

facilitates the sintering, though sintering without silicon nitride has been demonstrated as well. Thin films of TiB₂ can be produced by several techniques. The

electroplating Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the redox, reduction of cations of that metal by means of a direct current, direct electric cur ...

of TiB₂ layers possess two main advantages compared with

physical vapor deposition Physical vapor deposition (PVD), sometimes called physical vapor transport (PVT), describes a variety of vacuum deposition methods which can be used to produce thin films and coatings on substrates including metals, ceramics, glass, and polym ...

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 ...

: 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 TiB₂ appears to be limited to specialized applications in such areas as impact resistant

armor Armour (Commonwealth English) or armor (American English; see American and British English spelling differences#-our, -or, spelling differences) is a covering used to protect an object, individual, or vehicle from physical injury or damage, e ...

cutting tool Cutting is the separation or opening of a physical object, into two or more portions, through the application of an acutely directed force. Implements commonly used for cutting are the knife and saw, or in medicine and science the scalpel an ...

crucible A crucible is a container in which metals or other substances may be melted or subjected to very high temperatures. Although crucibles have historically tended to be made out of clay, they can be made from any material that withstands temperat ...

s, neutron absorbers and wear resistant coatings. TiB₂ is extensively used for evaporation boats for vapour coating of

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

. It is an attractive material for the aluminium industry as an inoculant to refine the

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 ...

when

casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or ...

aluminium alloy An aluminium alloy ( UK/IUPAC) or aluminum alloy ( NA; see spelling differences) is an alloy in which aluminium (Al) is the predominant metal. The typical alloying elements are copper, magnesium, manganese, silicon, tin, nickel and zinc. There ...

s, because of its wettability by and low solubility in molten aluminium and good electrical conductivity.

Thin film A thin film is a layer of materials ranging from fractions of a nanometer ( monolayer) to several micrometers in thickness. The controlled synthesis of materials as thin films (a process referred to as deposition) is a fundamental step in many ...

s of TiB₂ can be used to provide wear and

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

resistance to a cheap and/or tough substrate.

References

Compare

Magnesium diboride Magnesium diboride is the inorganic compound of magnesium and boron with the formula MgB2. It is a dark gray, water-insoluble solid. The compound becomes superconducting at 39 K (−234 °C), which has attracted attention. In terms of its ...

Physical properties

Exceptional hardness at extreme temperature

Advantages over other borides

Other advantages

Drawbacks

Chemical properties

Production

Potential applications

References

Compare

See also