1. Surface

2. High pressure phases

3. References

4. See also

Nickel hydride describes an alloy made by combining nickel and hydrogen. Hydrogen's content in nickel hydride is up to 0.002% by weight.

Hydrogen acts as a hardening agent, preventing dislocations in the nickel atom crystal lattice from sliding past one another. Varying the amount of alloying hydrogen and the form of its presence in the nickel hydride (precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting nickel hydride. Nickel hydride with increased hydrogen content can be made harder and stronger than nickel, but such nickel hydride is also less ductile than nickel. Loss of ductility occurs due to cracks maintaining sharp points due to suppression of elastic deformation by the hydrogen, and voids forming under tension due to decomposition of the hydride.^[1] Hydrogen embrittlement can be a problem in nickel in use in turbines at high temperatures.^[2]

In the narrow range of concentrations that make up nickel hydride, mixtures of hydrogen and nickel can only form a few different structures, with very different properties. Understanding such properties is essential to making quality nickel hydride. At room temperature, the most stable form of nickel is the face-centred cubic (FCC) structure α-nickel. It is a fairly soft metallic material that can dissolve only a very small concentration of hydrogen, no more than 0.002 wt% at {{convert|1455|C}}, and only 0.00005% at {{convert|25|C}}. The solid solution phase with dissolved hydrogen, that maintains the same crystal structure as the original nickel is termed the α-phase. At 25°C 6kbar of hydrogen pressure is needed to dissolve in b=nickel, but the hydrogen will come back out of solution if the pressure drops below 3.4 kbar.^[3]

Surface

Hydrogen atoms bond strongly with a nickel surface, with hydrogen molecules disassociating in order to do so.^[4]

Disassociation of dihydrogen requires enough energy to cross a barrier. On a Ni(111) crystal surface the barrier is 46 kJ/mol, whereas on Ni(100) the barrier is 52 kJ/mol. The Ni(110) crystal plane surface has the lowest activation energy to break the hydrogen molecule at 36 kJ/mol. The surface layer of hydrogen on nickel can be released by heating. Ni(111) lost hydrogen between 320 and 380 K. Ni(100) lost hydrogen between 220 and 360 K. Ni(110) crystal surfaces lost hydrogen between 230 and 430 K.^[3]

In order to dissolve inside the nickel, hydrogen must migrate from on the surface through the face of a nickel crystal. This does not take place in a vacuum, but can take place when the hydrogen coated nickel surface is impacted by other molecules. The molecules do not have to be hydrogen, but they appear to work like hammers punching the hydrogen atoms through the nickel surface to the subsurface. An activation energy of 100 kJ/mol is required to penetrate the surface.^[3]

High pressure phases

A true crystallographically distinct phase of nickel hydride can be produced with high pressure hydrogen gas at 600 MPa.^[3] Alternatively it can be produced electrolytically.^[8] The crystal form is face centred cubic or β-nickel hydride. Hydrogen to nickel atomic ratios are up to one, with hydrogen occupying an octahedral site.^[5] The density of the β-hydride is 7.74 g/cm³. It is coloured grey.^[5] At a current density of 1 Amp per square decimeter, in 0.5 mol/liter of sulfuric acid and thiourea a surface layer of nickel will be converted to nickel hydride. This surface is replete with cracks up to millimeters long. The direction of cracking is in the {001} plane of the original nickel crystals. The lattice constant of nickel hydride is 3.731 Å, which is 5.7% more than that of nickel.^[6]

The near-stoichiometric NiH is unstable and loses hydrogen at pressures below 340 MPa.^[3]

References

See also

• Solid solution
• Lattice energy

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