Chinese name: Lanthanum hexaboride
English name: lanthanum hexaboride
CAS number: 12008-21-8
Molecular formula: B6La
Molecular weight: 203.7715
Density (g/mL, 20 ℃): 4.76
Mohs scale hardness: 9.5
Room temperature resistance: 15-27 μ Ω
Vickers hardness: 27.7GPa
Work function: 2.66eV
Emission constant: 29A/cm2 · K2
Melting point (º C): 2715
CAS number: 12008-21-8
MDL number: MFCD00151350
EINECS number: 234-531-6
PubChem number: 24854496
Solubility: insoluble in water and hydrochloric acid
Chemical properties: The chemical properties are very stable, and do not react with water, oxygen, or even hydrochloric acid; At room temperature, it reacts only with nitric acid and aqua regia; In an aerobic atmosphere, oxidation occurs at 600 ~ 700 ° C. In a vacuum atmosphere, LaB6 materials are easy to react with other substances or gases to form substances with low melting points; At high temperature, the formed material will continue to volatilize, exposing the low escape work surface of lanthanum hexaboride crystal to the emission surface, and then lanthanum hexaboride has excellent anti-poisoning ability.
The crystal structure of borides determines a series of unique properties:
1) Due to the strong bonding force between boron atoms (lattice constant 4.145A), the refractory compound has a melting point of 2210℃.
2) The chemical properties are very stable, and do not react with water, oxygen, or even hydrochloric acid; At room temperature, it reacts only with nitric acid and aqua regia; Oxygen will oxidize at 600 ~ 700℃.
3) In a certain temperature range, the expansion coefficient is close to zero.
4) Good stability in the air, surface contamination during use can be restored by vacuum heat treatment.
5) Good ion bombardment resistance, can withstand high field strength.
6) Since there is no valence bond between the metal atom and the boron atom, the valence electrons of the metal atom are free. Therefore, borides have high electrical conductivity, and the resistance of lanthanum hexaboride is roughly the same as that of metallic lead. The temperature coefficient of its resistivity is positive.
7) If hexaboride is allowed to come into contact with refractory metal at high temperatures, the boron will diffuse into the lattice of the metal and form a intercalated boron alloy with the metal, at the same time the boron framework will collapse, allowing the metal atoms to evaporate.
8) When the boride is heated to a comparable temperature, the metal atoms on the crystal surface evaporate, but are immediately replaced by metal atoms diffused from the inside of the crystal lattice, while the boron framework remains unchanged, so that the loss of surface active substances is minimized.
Synthesis method:
Stoichiometric mixing of a small amount of lanthanum oxide, boron, and carbon black powder (about 1g in total) until uniform, the reactants are pressed into sheets [1in (1in=0.0254m) ×1/2in×1/16in]. The sheet is placed in a graphite crucible 1.5in high and 1.7in diameter. The latter is then put into a larger graphite crucible (4in high and diameter), through hydrogen, and heated at 1500℃ for 2h or 1800℃ for 1h.
Structural properties:
Since the B6 octahedral network requires two electrons to stabilize its structure, one of La's three valence electrons will be rich, and LaB6 acts as a metal conductor. The surface plasmon resonance absorption of metal conductor LaB6 nanomaterials is about 1000nm, which can make up for the poor blocking effect of infrared blocking materials such as ITO(indium tin oxide) in the near infrared region, making LaB6 an important infrared blocking material. Because of the strong covalent bond between the B atoms in the structure of LaB6, a tight spatial network is formed, so that it has the characteristics of low volatility, low escape work, high melting point, high strength and high stability.
Application: It has been successfully used in more than 20 military and high-tech fields such as radar aerospace, electronic industry, instrumentation, medical equipment, home appliances, metallurgy, environmental protection and so on. 1) Electron emission cathode. Due to the low electron escape work, cathode materials with the highest emission current at medium temperature can be obtained, especially high-quality single crystals, which are ideal materials for high-power electron emission cathodes. 2) High brightness point light source. For the preparation of core components of electron microscopy, such as selective optical filters, soft X-ray diffraction monochromators, and other electron-beam light sources. 3) High stability and long life system components. The excellent comprehensive performance makes it used in various electron beam systems, such as electron beam engraving, electron beam heat source, electron beam welding gun and accelerator engineering fields for making high performance components.
Packaging and storage: sealed in a dry, cool environment, should not be exposed to the air, prevent moisture oxidation agglomeration, affect the dispersion performance and use effect.
