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Names | |
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IUPAC name Boron nitride | |
Identifiers | |
3D model (JSmol) | |
ChEBI | |
ChemSpider | |
ECHA InfoCard | 100.030.111 |
EC Number |
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216 | |
MeSH | Elbor |
PubChem CID | |
RTECS number |
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UNII | |
CompTox Dashboard (EPA) | |
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Properties | |
BN | |
Molar mass | 24.82 g/mol |
Appearance | Colorless crystals |
Density | 2.1 g/cm3 (h-BN); 3.45 g/cm3 (c-BN) |
Melting point | 2,973 °C (5,383 °F; 3,246 K) sublimates (c-BN) |
Insoluble | |
Electron mobility | 200 cm2/(V·s) (c-BN) |
Refractive index (nD) | 1.8 (h-BN); 2.1 (c-BN) |
Structure | |
Hexagonal, sphalerite, wurtzite | |
Thermochemistry | |
Heat capacity (C) | 19.7 J/(K·mol) [1] [2] |
Std molar entropy (S⦵298) | 14.8 J/K mol [1] [2] |
Std enthalpy of formation (ΔfH⦵298) | −254.4 kJ/mol [1] [2] |
Gibbs free energy (ΔfG⦵) | −228.4 kJ/mol [1] [2] |
Hazards | |
GHS labelling: | |
Warning | |
H319, H335, H413 | |
P261, P264, P271, P273, P280, P304+P340, P305+P351+P338, P312, P337+P313, P403+P233, P405, P501 | |
NFPA 704 (fire diamond) | |
Related compounds | |
Related compounds | |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal form corresponding to graphite is the most stable and soft among BN polymorphs, and is therefore used as a lubricant and an additive to cosmetic products. The cubic (zincblende aka sphalerite structure) variety analogous to diamond is called c-BN; it is softer than diamond, but its thermal and chemical stability is superior. The rare wurtzite BN modification is similar to lonsdaleite but slightly softer than the cubic form. [3]
Because of excellent thermal and chemical stability, boron nitride ceramics are used in high-temperature equipment and metal casting. Boron nitride has potential use in nanotechnology.
Boron nitride was discovered by chemistry teacher of the Liverpool Institute William Henry Balmain in 1842 via reduction of boric acid with charcoal in the presence of potassium cyanide. [4]
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Boron nitride exists in multiple forms that differ in the arrangement of the boron and nitrogen atoms, giving rise to varying bulk properties of the material.
The amorphous form of boron nitride (a-BN) is non-crystalline, lacking any long-distance regularity in the arrangement of its atoms. It is analogous to amorphous carbon.
All other forms of boron nitride are crystalline.
The most stable crystalline form is the hexagonal one, also called h-BN, α-BN, g-BN, and graphitic boron nitride. Hexagonal boron nitride (point group = D3h; space group = P63/mmc) has a layered structure similar to graphite. Within each layer, boron and nitrogen atoms are bound by strong covalent bonds, whereas the layers are held together by weak van der Waals forces. The interlayer "registry" of these sheets differs, however, from the pattern seen for graphite, because the atoms are eclipsed, with boron atoms lying over and above nitrogen atoms. This registry reflects the local polarity of the B–N bonds, as well as interlayer N-donor/B-acceptor characteristics. Likewise, many metastable forms consisting of differently stacked polytypes exist. Therefore, h-BN and graphite are very close neighbors, and the material can accommodate carbon as a substituent element to form BNCs. BC6N hybrids have been synthesized, where carbon substitutes for some B and N atoms. [5] Hexagonal boron nitride monolayer is analogous to graphene, having a honeycomb lattice structure of nearly the same dimensions. Unlike graphene, which is black and an electrical conductor, h-BN monolayer is white and an insulator. It has been proposed for use as an atomic flat insulating substrate or a tunneling dielectric barrier in 2D electronics. . [6]
Cubic boron nitride has a crystal structure analogous to that of diamond. Consistent with diamond being less stable than graphite, the cubic form is less stable than the hexagonal form, but the conversion rate between the two is negligible at room temperature, as it is for diamond. The cubic form has the sphalerite crystal structure (space group = F43m), the same as that of diamond (with ordered B and N atoms), and is also called β-BN or c-BN.
The wurtzite form of boron nitride (w-BN; point group = C6v; space group = P63mc) has the same structure as lonsdaleite, a rare hexagonal polymorph of carbon. As in the cubic form, the boron and nitrogen atoms are grouped into tetrahedra. [7] In the wurtzite form, the boron and nitrogen atoms are grouped into 6-membered rings. In the cubic form all rings are in the chair configuration, whereas in w-BN the rings between 'layers' are in boat configuration. Earlier optimistic reports predicted that the wurtzite form was very strong, and was estimated by a simulation as potentially having a strength 18% stronger than that of diamond. Since only small amounts of the mineral exist in nature, this has not yet been experimentally verified. [8] Its hardness is 46 GPa, slightly harder than commercial borides but softer than the cubic form of boron nitride. [3]
Material | Boron nitride (BN) | Graphite [9] | Diamond [10] | |||
---|---|---|---|---|---|---|
a- [11] [12] [13] | h- | c- [14] [10] | w- | |||
Density (g/cm3) | 2.28 | ~2.1 | 3.45 | 3.49 | ~2.1 | 3.515 |
Knoop hardness (GPa) | 10 | 45 | 34 | 100 | ||
Bulk modulus (GPa) | 100 | 36.5 | 400 | 400 | 34 | 440 |
Thermal conductivity (W/m·K) | 3 | 600 ∥, 30 ⟂ | 740 | 200–2000 ∥, 2–800 ⟂ | 600–2000 | |
Thermal expansion (10−6/K) | −2.7 ∥, 38 ⟂ | 1.2 | 2.7 | −1.5 ∥, 25 ⟂ | 0.8 | |
Band gap (eV) | 5.05 | 5.9–6.4 [15] | 10.1-10.7 [16] | 4.5–5.5 | 0 | 5.5 |
Refractive index | 1.7 | 1.8 | 2.1 | 2.05 | 2.4 | |
Magnetic susceptibility (µemu/g) [17] | −0.48 ∥, −17.3 ⟂ | −0.2 – −2.7 ∥, −20 – −28 ⟂ | −1.6 |
The partly ionic structure of BN layers in h-BN reduces covalency and electrical conductivity, whereas the interlayer interaction increases resulting in higher hardness of h-BN relative to graphite. The reduced electron-delocalization in hexagonal-BN is also indicated by its absence of color and a large band gap. Very different bonding – strong covalent within the basal planes (planes where boron and nitrogen atoms are covalently bonded) and weak between them – causes high anisotropy of most properties of h-BN.
For example, the hardness, electrical and thermal conductivity are much higher within the planes than perpendicular to them. On the contrary, the properties of c-BN and w-BN are more homogeneous and isotropic.
Those materials are extremely hard, with the hardness of bulk c-BN being slightly smaller and w-BN even higher than that of diamond. [18] Polycrystalline c-BN with grain sizes on the order of 10 nm is also reported to have Vickers hardness comparable or higher than diamond. [19] Because of much better stability to heat and transition metals, c-BN surpasses diamond in mechanical applications, such as machining steel. [20] The thermal conductivity of BN is among the highest of all electric insulators (see table).
Boron nitride can be doped p-type with beryllium and n-type with boron, sulfur, silicon or if co-doped with carbon and nitrogen. [14] Both hexagonal and cubic BN are wide-gap semiconductors with a band-gap energy corresponding to the UV region. If voltage is applied to h-BN [21] [22] or c-BN, [23] then it emits UV light in the range 215–250 nm and therefore can potentially be used as light-emitting diodes (LEDs) or lasers.
Little is known on melting behavior of boron nitride. It degrades at 2973 °C, but melts at elevated pressure. [24] [25]
Hexagonal and cubic BN (and probably w-BN) show remarkable chemical and thermal stabilities. For example, h-BN is stable to decomposition at temperatures up to 1000 °C in air, 1400 °C in vacuum, and 2800 °C in an inert atmosphere. The reactivity of h-BN and c-BN is relatively similar, and the data for c-BN are summarized in the table below.
Solid | Ambient | Action | Threshold temperature (°C) |
---|---|---|---|
Mo | 10−2 Pa vacuum | Reaction | 1360 |
Ni | 10−2 Pa vacuum | Wetting [a] | 1360 |
Fe, Ni, Co | Argon | React | 1400–1500 |
Al | 10−2 Pa vacuum | Wetting and reaction | 1050 |
Si | 10−3 Pa vacuum | Wetting | 1500 |
Cu, Ag, Au, Ga, In, Ge, Sn | 10−3 Pa vacuum | No wetting | 1100 |
B | No wetting | 2200 | |
Al2O3 + B2O3 | 10−2 Pa vacuum | No reaction | 1360 |
Thermal stability of c-BN can be summarized as follows: [14]
Boron nitride is not attacked by the usual acids, but it is soluble in alkaline molten salts and nitrides, such as LiOH, KOH, NaOH-Na2CO3, NaNO3, Li3N, Mg3N2, Sr3N2, Ba3N2 or Li3BN2, which are therefore used to etch BN. [14]
The theoretical thermal conductivity of hexagonal boron nitride nanoribbons (BNNRs) can approach 1700–2000 W/(m⋅K), which has the same order of magnitude as the experimental measured value for graphene, and can be comparable to the theoretical calculations for graphene nanoribbons. [26] [27] Moreover, the thermal transport in the BNNRs is anisotropic. The thermal conductivity of zigzag-edged BNNRs is about 20% larger than that of armchair-edged nanoribbons at room temperature. [28]
BN nanosheets consist of hexagonal boron nitride (h-BN). They are stable up to 800°C in air. The structure of monolayer BN is similar to that of graphene, which has exceptional strength, [29] a high-temperature lubricant, and a substrate in electronic devices. [30]
The anisotropy of Young's modulus and Poisson's ratio depends on the system size. [31] h-BN also exhibits strongly anisotropic strength and toughness, [32] and maintains these over a range of vacancy defects, showing that the anisotropy is independent to the defect type.
In 2009, cubic form (c-BN) was reported in Tibet, and the name qingsongite proposed. The substance was found in dispersed micron-sized inclusions in chromium-rich rocks. In 2013, the International Mineralogical Association affirmed the mineral and the name. [33] [34] [35] [36]
Hexagonal boron nitride is obtained by the treating boron trioxide (B2O3) or boric acid (H3BO3) with ammonia (NH3) or urea (CO(NH2)2) in an inert atmosphere: [37]
The resulting disordered (amorphous) material contains 92–95% BN and 5–8% B2O3. The remaining B2O3 can be evaporated in a second step at temperatures > 1500 °C in order to achieve BN concentration >98%. Such annealing also crystallizes BN, the size of the crystallites increasing with the annealing temperature. [20] [38]
h-BN parts can be fabricated inexpensively by hot-pressing with subsequent machining. The parts are made from boron nitride powders adding boron oxide for better compressibility. Thin films of boron nitride can be obtained by chemical vapor deposition from boron trichloride and nitrogen precursors. [39] ZYP Coatings also has developed boron nitride coatings that may be painted on a surface. Combustion of boron powder in nitrogen plasma at 5500 °C yields ultrafine boron nitride used for lubricants and toners. [40]
Boron nitride reacts with iodine fluoride to give NI3 in low yield. [41] Boron nitride reacts with nitrides of lithium, alkaline earth metals and lanthanides to form nitridoborates. [42] For example:
Various species intercalate into hexagonal BN, such as NH3 intercalate [43] or alkali metals. [44]
c-BN is prepared analogously to the preparation of synthetic diamond from graphite. Direct conversion of hexagonal boron nitride to the cubic form has been observed at pressures between 5 and 18 GPa and temperatures between 1730 and 3230 °C, that is similar parameters as for direct graphite-diamond conversion. [45] The addition of a small amount of boron oxide can lower the required pressure to 4–7 GPa and temperature to 1500 °C. As in diamond synthesis, to further reduce the conversion pressures and temperatures, a catalyst is added, such as lithium, potassium, or magnesium, their nitrides, their fluoronitrides, water with ammonium compounds, or hydrazine. [46] [47] Other industrial synthesis methods, again borrowed from diamond growth, use crystal growth in a temperature gradient, or explosive shock wave. The shock wave method is used to produce material called heterodiamond, a superhard compound of boron, carbon, and nitrogen. [48]
Low-pressure deposition of thin films of cubic boron nitride is possible. As in diamond growth, the major problem is to suppress the growth of hexagonal phases (h-BN or graphite, respectively). Whereas in diamond growth this is achieved by adding hydrogen gas, boron trifluoride is used for c-BN. Ion beam deposition, plasma-enhanced chemical vapor deposition, pulsed laser deposition, reactive sputtering, and other physical vapor deposition methods are used as well. [39]
Wurtzite BN can be obtained via static high-pressure or dynamic shock methods. [49] The limits of its stability are not well defined. Both c-BN and w-BN are formed by compressing h-BN, but formation of w-BN occurs at much lower temperatures close to 1700 °C. [46]
Whereas the production and consumption figures for the raw materials used for BN synthesis, namely boric acid and boron trioxide, are well known (see boron), the corresponding numbers for the boron nitride are not listed in statistical reports. An estimate for the 1999 world production is 300 to 350 metric tons. The major producers and consumers of BN are located in the United States, Japan, China and Germany. In 2000, prices varied from about $75–120/kg for standard industrial-quality h-BN and were about up to $200–400/kg for high purity BN grades. [37]
Hexagonal BN (h-BN) is the most widely used polymorph. It is a good lubricant at both low and high temperatures (up to 900 °C, even in an oxidizing atmosphere). h-BN lubricant is particularly useful when the electrical conductivity or chemical reactivity of graphite (alternative lubricant) would be problematic. In internal combustion engines, where graphite could be oxidized and turn into carbon sludge, h-BN with its superior thermal stability can be added to engine lubricants. As with all nano-particle suspensions, Brownian-motion settlement is a problem. Settlement can clog engine oil filters, which limits solid lubricant applications in a combustion engine to automotive racing, where engine re-building is common. Since carbon has appreciable solubility in certain alloys (such as steels), which may lead to degradation of properties, BN is often superior for high temperature and/or high pressure applications. Another advantage of h-BN over graphite is that its lubricity does not require water or gas molecules trapped between the layers. Therefore, h-BN lubricants can be used in vacuum, such as space applications. The lubricating properties of fine-grained h-BN are used in cosmetics, paints, dental cements, and pencil leads. [50]
Hexagonal BN was first used in cosmetics around 1940 in Japan. Because of its high price, h-BN was abandoned for this application. Its use was revitalized in the late 1990s with the optimization h-BN production processes, and currently h-BN is used by nearly all leading producers of cosmetic products for foundations, make-up, eye shadows, blushers, kohl pencils, lipsticks and other skincare products. [20]
Because of its excellent thermal and chemical stability, boron nitride ceramics and coatings are used high-temperature equipment. h-BN can be included in ceramics, alloys, resins, plastics, rubbers, and other materials, giving them self-lubricating properties. Such materials are suitable for construction of e.g. bearings and in steelmaking. [20] Many quantum devices use multilayer h-BN as a substrate material. It can also be used as a dielectric in resistive random access memories. [51] [52]
Hexagonal BN is used in xerographic process and laser printers as a charge leakage barrier layer of the photo drum. [53] In the automotive industry, h-BN mixed with a binder (boron oxide) is used for sealing oxygen sensors, which provide feedback for adjusting fuel flow. The binder utilizes the unique temperature stability and insulating properties of h-BN. [20]
Parts can be made by hot pressing from four commercial grades of h-BN. Grade HBN contains a boron oxide binder; it is usable up to 550–850 °C in oxidizing atmosphere and up to 1600 °C in vacuum, but due to the boron oxide content is sensitive to water. Grade HBR uses a calcium borate binder and is usable at 1600 °C. Grades HBC and HBT contain no binder and can be used up to 3000 °C. [54]
Boron nitride nanosheets (h-BN) can be deposited by catalytic decomposition of borazine at a temperature ~1100 °C in a chemical vapor deposition setup, over areas up to about 10 cm2. Owing to their hexagonal atomic structure, small lattice mismatch with graphene (~2%), and high uniformity they are used as substrates for graphene-based devices. [55] BN nanosheets are also excellent proton conductors. Their high proton transport rate, combined with the high electrical resistance, may lead to applications in fuel cells and water electrolysis. [56]
h-BN has been used since the mid-2000s as a bullet and bore lubricant in precision target rifle applications as an alternative to molybdenum disulfide coating, commonly referred to as "moly". It is claimed to increase effective barrel life, increase intervals between bore cleaning and decrease the deviation in point of impact between clean bore first shots and subsequent shots. [57]
h-BN is used as a release agent in molten metal and glass applications. For example, ZYP Coatings developed and currently produces a line of paintable h-BN coatings that are used by manufacturers of molten aluminium, non-ferrous metal, and glass. [58] Because h-BN is nonwetting and lubricious to these molten materials, the coated surface (i.e. mold or crucible) does not stick to the material. [59] [60] [61] [62]
Cubic boron nitride (CBN or c-BN) is widely used as an abrasive. [63] Its usefulness arises from its insolubility in iron, nickel, and related alloys at high temperatures, whereas diamond is soluble in these metals. Polycrystalline c-BN (PCBN) abrasives are therefore used for machining steel, whereas diamond abrasives are preferred for aluminum alloys, ceramics, and stone. When in contact with oxygen at high temperatures, BN forms a passivation layer of boron oxide. Boron nitride binds well with metals due to formation of interlayers of metal borides or nitrides. Materials with cubic boron nitride crystals are often used in the tool bits of cutting tools. For grinding applications, softer binders such as resin, porous ceramics and soft metals are used. Ceramic binders can be used as well. Commercial products are known under names "Borazon" (by Hyperion Materials & Technologies [64] ), and "Elbor" or "Cubonite" (by Russian vendors). [50]
Contrary to diamond, large c-BN pellets can be produced in a simple process (called sintering) of annealing c-BN powders in nitrogen flow at temperatures slightly below the BN decomposition temperature. This ability of c-BN and h-BN powders to fuse allows cheap production of large BN parts. [50]
Similar to diamond, the combination in c-BN of highest thermal conductivity and electrical resistivity is ideal for heat spreaders.
As cubic boron nitride consists of light atoms and is very robust chemically and mechanically, it is one of the popular materials for X-ray membranes: low mass results in small X-ray absorption, and good mechanical properties allow usage of thin membranes, further reducing the absorption. [65]
Layers of amorphous boron nitride (a-BN) are used in some semiconductor devices, e.g. MOSFETs. They can be prepared by chemical decomposition of trichloro borazine with caesium, or by thermal chemical vapor deposition methods. Thermal CVD can be also used for deposition of h-BN layers, or at high temperatures, c-BN. [66]
Hexagonal boron nitride can be exfoliated to mono or few atomic layer sheets. Due to its analogous structure to that of graphene, atomically thin boron nitride is sometimes called white graphene. [67]
Atomically thin boron nitride is one of the strongest electrically insulating materials. Monolayer boron nitride has an average Young's modulus of 0.865TPa and fracture strength of 70.5GPa, and in contrast to graphene, whose strength decreases dramatically with increased thickness, few-layer boron nitride sheets have a strength similar to that of monolayer boron nitride. [68]
Atomically thin boron nitride has one of the highest thermal conductivity coefficients (751 W/mK at room temperature) among semiconductors and electrical insulators, and its thermal conductivity increases with reduced thickness due to less intra-layer coupling. [69]
The air stability of graphene shows a clear thickness dependence: monolayer graphene is reactive to oxygen at 250 °C, strongly doped at 300 °C, and etched at 450 °C; in contrast, bulk graphite is not oxidized until 800 °C. [70] Atomically thin boron nitride has much better oxidation resistance than graphene. Monolayer boron nitride is not oxidized till 700 °C and can sustain up to 850 °C in air; bilayer and trilayer boron nitride nanosheets have slightly higher oxidation starting temperatures. [71] The excellent thermal stability, high impermeability to gas and liquid, and electrical insulation make atomically thin boron nitride potential coating materials for preventing surface oxidation and corrosion of metals [72] [73] and other two-dimensional (2D) materials, such as black phosphorus. [74]
Atomically thin boron nitride has been found to have better surface adsorption capabilities than bulk hexagonal boron nitride. [75] According to theoretical and experimental studies, atomically thin boron nitride as an adsorbent experiences conformational changes upon surface adsorption of molecules, increasing adsorption energy and efficiency. The synergic effect of the atomic thickness, high flexibility, stronger surface adsorption capability, electrical insulation, impermeability, high thermal and chemical stability of BN nanosheets can increase the Raman sensitivity by up to two orders, and in the meantime attain long-term stability and reusability not readily achievable by other materials. [76] [77]
Atomically thin hexagonal boron nitride is an excellent dielectric substrate for graphene, molybdenum disulfide (MoS2), and many other 2D material-based electronic and photonic devices. As shown by electric force microscopy (EFM) studies, the electric field screening in atomically thin boron nitride shows a weak dependence on thickness, which is in line with the smooth decay of electric field inside few-layer boron nitride revealed by the first-principles calculations. [70]
Raman spectroscopy has been a useful tool to study a variety of 2D materials, and the Raman signature of high-quality atomically thin boron nitride was first reported by Gorbachev et al. in 2011. [78] and Li et al. [71] However, the two reported Raman results of monolayer boron nitride did not agree with each other. Cai et al., therefore, conducted systematic experimental and theoretical studies to reveal the intrinsic Raman spectrum of atomically thin boron nitride. [79] It reveals that atomically thin boron nitride without interaction with a substrate has a G band frequency similar to that of bulk hexagonal boron nitride, but strain induced by the substrate can cause Raman shifts. Nevertheless, the Raman intensity of G band of atomically thin boron nitride can be used to estimate layer thickness and sample quality.
Boron nitride nanomesh is a nanostructured two-dimensional material. It consists of a single BN layer, which forms by self-assembly a highly regular mesh after high-temperature exposure of a clean rhodium [81] or ruthenium [82] surface to borazine under ultra-high vacuum. The nanomesh looks like an assembly of hexagonal pores. The distance between two pore centers is 3.2 nm and the pore diameter is ~2 nm. Other terms for this material are boronitrene or white graphene. [83]
The boron nitride nanomesh is air-stable [84] and compatible with some liquids. [85] [86] up to temperatures of 800 °C. [81]
Boron nitride tubules were first made in 1989 by Shore and Dolan This work was patented in 1989 and published in 1989 thesis (Dolan) and then 1993 Science. The 1989 work was also the first preparation of amorphous BN by B-trichloroborazine and cesium metal.
Boron nitride nanotubes were predicted in 1994 [88] and experimentally discovered in 1995. [89] They can be imagined as a rolled up sheet of h-boron nitride. Structurally, it is a close analog of the carbon nanotube, namely a long cylinder with diameter of several to hundred nanometers and length of many micrometers, except carbon atoms are alternately substituted by nitrogen and boron atoms. However, the properties of BN nanotubes are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, a BN nanotube is an electrical insulator with a bandgap of ~5.5 eV, basically independent of tube chirality and morphology. [90] In addition, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure. [91] [92]
Boron nitride aerogel is an aerogel made of highly porous BN. It typically consists of a mixture of deformed BN nanotubes and nanosheets. It can have a density as low as 0.6 mg/cm3 and a specific surface area as high as 1050 m2/g, and therefore has potential applications as an absorbent, catalyst support and gas storage medium. BN aerogels are highly hydrophobic and can absorb up to 160 times their weight in oil. They are resistant to oxidation in air at temperatures up to 1200 °C, and hence can be reused after the absorbed oil is burned out by flame. BN aerogels can be prepared by template-assisted chemical vapor deposition using borazine as the feed gas. [80]
Addition of boron nitride to silicon nitride ceramics improves the thermal shock resistance of the resulting material. For the same purpose, BN is added also to silicon nitride-alumina and titanium nitride-alumina ceramics. Other materials being reinforced with BN include alumina and zirconia, borosilicate glasses, glass ceramics, enamels, and composite ceramics with titanium boride-boron nitride, titanium boride-aluminium nitride-boron nitride, and silicon carbide-boron nitride composition. [93]
Zirconia Stabilized Boron Nitride (ZSBN) is produced by adding zirconia to BN, enhancing its thermal shock resistance and mechanical strength through a sintering process. [94] It offers better performance characteristics including Superior corrosion and erosion resistance over a wide temperature range. [95] Its unique combination of thermal conductivity, lubricity, mechanical strength, and stability makes it suitable for various applications including cutting tools and wear-resistant coatings, thermal and electrical insulation, aerospace and defense, and high-temperature components. [96] [97]
Pyrolytic boron nitride (PBN), also known as Chemical vapour-deposited Boron Nitride(CVD-BN), [98] is a high-purity ceramic material characterized by exceptional chemical resistance and mechanical strength at high temperatures. [99] Pyrolytic boron nitride is typically prepared through the thermal decomposition of boron trichloride and ammonia vapors on graphite substrates at 1900°C. [100]
Pyrolytic boron nitride (PBN) generally has a hexagonal structure similar to hexagonal boron nitride (hBN), though it can exhibit stacking faults or deviations from the ideal lattice. [101] Pyrolytic boron nitride (PBN) shows some remarkable attributes, including exceptional chemical inertness, high dielectric strength, excellent thermal shock resistance, non-wettability, non-toxicity, oxidation resistance, and minimal outgassing. [102] [103]
Due to a highly ordered planar texture similar to pyrolytic graphite (PG), it exhibits anisotropic properties such as lower dielectric constant vertical to the crystal plane and higher bending strength along the crystal plane. [104] PBN material has been widely manufactured as crucibles of compound semiconductor crystals, output windows and dielectric rods of traveling-wave tubes, high-temperature jigs and insulator. [105]
Boron nitride (along with Si3N4, NbN, and BNC) is generally considered to be non-toxic and does not exhibit chemical activity in biological systems. [106] Due to its excellent safety profile and lubricious properties, boron nitride finds widespread use in various applications, including cosmetics and food processing equipment. [107] [108]
Boron is a chemical element. It has the 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 valence electrons for forming covalent bonds, resulting in many compounds such as boric acid, the mineral sodium borate, and the ultra-hard crystals of boron carbide and boron nitride.
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.
Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS2.
Carbon is capable of forming many allotropes due to its valency (tetravalent). Well-known forms of carbon include diamond and graphite. In recent decades, many more allotropes have been discovered and researched, including ball shapes such as buckminsterfullerene and sheets such as graphene. Larger-scale structures of carbon include nanotubes, nanobuds and nanoribbons. Other unusual forms of carbon exist at very high temperatures or extreme pressures. Around 500 hypothetical 3‑periodic allotropes of carbon are known at the present time, according to the Samara Carbon Allotrope Database (SACADA).
Graphene is a carbon allotrope consisting of a single layer of atoms arranged in a honeycomb planar nanostructure. The name "graphene" is derived from "graphite" and the suffix -ene, indicating the presence of double bonds within the carbon structure.
A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their unique properties, these materials are of great interest in many industrial areas including, but not limited to, abrasives, polishing and cutting tools, disc brakes, and wear-resistant and protective coatings.
Boron compounds are compounds containing the element boron. In the most familiar compounds, boron has the formal oxidation state +3. These include oxides, sulfides, nitrides, and halides.
The nanomesh is an inorganic nanostructured two-dimensional material, similar to graphene. It was discovered in 2003 at the University of Zurich, Switzerland.
Alex K. Zettl is an American experimental physicist, educator, and inventor.
Graphitic carbon nitride (g-C3N4) is a family of carbon nitride compounds with a general formula near to C3N4 (albeit typically with non-zero amounts of hydrogen) and two major substructures based on heptazine and poly(triazine imide) units which, depending on reaction conditions, exhibit different degrees of condensation, properties and reactivities.
Borophene is a crystalline atomic monolayer of boron, i.e., it is a two-dimensional allotrope of boron and also known as boron sheet. First predicted by theory in the mid-1990s, different borophene structures were experimentally confirmed in 2015.
Transition-metal dichalcogenide (TMD or TMDC) monolayers are atomically thin semiconductors of the type MX2, with M a transition-metal atom (Mo, W, etc.) and X a chalcogen atom (S, Se, or Te). One layer of M atoms is sandwiched between two layers of X atoms. They are part of the large family of so-called 2D materials, named so to emphasize their extraordinary thinness. For example, a MoS2 monolayer is only 6.5 Å thick. The key feature of these materials is the interaction of large atoms in the 2D structure as compared with first-row transition-metal dichalcogenides, e.g., WTe2 exhibits anomalous giant magnetoresistance and superconductivity.
In materials science, the term single-layer materials or 2D materials refers to crystalline solids consisting of a single layer of atoms. These materials are promising for some applications but remain the focus of research. Single-layer materials derived from single elements generally carry the -ene suffix in their names, e.g. graphene. Single-layer materials that are compounds of two or more elements have -ane or -ide suffixes. 2D materials can generally be categorized as either 2D allotropes of various elements or as compounds.
A two-dimensional semiconductor is a type of natural semiconductor with thicknesses on the atomic scale. Geim and Novoselov et al. initiated the field in 2004 when they reported a new semiconducting material graphene, a flat monolayer of carbon atoms arranged in a 2D honeycomb lattice. A 2D monolayer semiconductor is significant because it exhibits stronger piezoelectric coupling than traditionally employed bulk forms. This coupling could enable applications. One research focus is on designing nanoelectronic components by the use of graphene as electrical conductor, hexagonal boron nitride as electrical insulator, and a transition metal dichalcogenide as semiconductor.
A rapidly increasing list of graphene production techniques have been developed to enable graphene's use in commercial applications.
Boron nitride nanotubes (BNNTs) are a polymorph of boron nitride. They were predicted in 1994 and experimentally discovered in 1995. Structurally they are similar to carbon nanotubes, which are cylinders with sub-micrometer diameters and micrometer lengths, except that carbon atoms are alternately substituted by nitrogen and boron atoms. However, the properties of BN nanotubes are very different: whereas carbon nanotubes can be metallic or semiconducting depending on the rolling direction and radius, a BN nanotube is an electrical insulator with a bandgap of ~5.5 eV, basically independent of tube chirality and morphology. In addition, a layered BN structure is much more thermally and chemically stable than a graphitic carbon structure. BNNTs have unique physical and chemical properties, when compared to Carbon Nanotubes (CNTs) providing a very wide range of commercial and scientific applications. Although BNNTs and CNTs share similar tensile strength properties of circa 100 times stronger than steel and 50 times stronger than industrial-grade carbon fibre, BNNTs can withstand high temperatures of up to 900 °C. as opposed to CNTs which remain stable up to temperatures of 400 °C, and are also capable of absorbing radiation. BNNTS are packed with physicochemical features including high hydrophobicity and considerable hydrogen storage capacity and they are being investigated for possible medical and biomedical applications, including gene delivery, drug delivery, neutron capture therapy, and more generally as biomaterials BNNTs are also superior to CNTs in the way they bond to polymers giving rise to many new applications and composite materials.
Graphene-Boron Nitride nanohybrid materials are a class of compounds created from graphene and boron nitride nanosheets. Graphene and boron nitride both contain intrinsic thermally conductive and electrically insulative properties. The combination of these two compounds may be useful to advance the development and understanding of electronics.
Boron nitride nanosheet is a crystalline form of the hexagonal boron nitride (h-BN), which has a thickness of one atom. Similar in geometry as well as physical and thermal properties to its carbon analog graphene, but has very different chemical and electronic properties – contrary to the black and highly conducting graphene, BN nanosheets are electrical insulators with a band gap of ~5.9 eV, and therefore appear white in color.
A graphene morphology is any of the structures related to, and formed from, single sheets of graphene. 'Graphene' is typically used to refer to the crystalline monolayer of the naturally occurring material graphite. Due to quantum confinement of electrons within the material at these low dimensions, small differences in graphene morphology can greatly impact the physical and chemical properties of these materials. Commonly studied graphene morphologies include the monolayer sheets, bilayer sheets, graphene nanoribbons and other 3D structures formed from stacking of the monolayer sheets.
Two dimensional hexagonal boron nitride is a material of comparable structure to graphene with potential applications in e.g. photonics., fuel cells and as a substrate for two-dimensional heterostructures. 2D h-BN is isostructural to graphene, but where graphene is conductive, 2D h-BN is a wide-gap insulator.