Romannevshupa, Jose Ramón Ares, ,*Jose Francisco Fernández, Adolfo Del Campo, Elisa Roman

Tribochemical Decomposition of Light Ionic Hydrides at Room Temperature

RomanNevshupa,†Jose Ramón Ares,‡,*Jose Francisco Fernández,‡ Adolfo del Campo,§ Elisa Roman|

† Spanish National Research Council, Institute “Eduardo Torroja” (IETCC-CSIC), C/Serrano Galvache 4, Madrid 28033, Spain

‡ Department of Physics of Materials, Autónoma University of Madrid, Madrid 28049, Spain

§ Spanish National Research Council, Institute of Ceramic and Glass (ICV-CSIC), C/Kelsen 5, Madrid 28049, Spain

| Spanish National Research Council, Institute of Material Science of Madrid (ICMM-CSIC), C/Sor Juana Inés de la Cruz 3, Madrid 28049, Spain

Supplementary information

XRD of MgH2 pellets

FigureS1. X-ray diffraction pattern of MgH2 pellet. Hydride phase is indexed to tetragonal -MgH2 (space group P42/mnm). Small diffraction peaks related to Mg and Mg(OH)2 phases also are detected.

Model of stress and deformation in the contact zone1

For the sake of simplicity it is assumed that both the pin and the sample surfaces are smooth. Under applied normal load, P, i.e. without sliding of a pin along the sample, the contact zone is a circle.

For elastic contact the diameter of the contact zone

, (S1)

whereR is the radius of the pin, E* is the contact modulus:

,(S2)

whereνi is Poisson’s ratio for ith component, Ei is the Young’s modulus for ith component, i=1 corresponds to theplane sample and i=2 corresponds tothepin.

Maximum contact pressure is determined as follows:

, (S3)

Under applied tangential force shear occurs not at the interface, but at certain depth corresponding to the maximum shear stress that can be determine from the following equations.

Maximum tangential stress:

.(S4)

The depth from surface where the shear stress is maximal:

. (S5)

The initial parameters used for calculation are shown on Table S1, whereas the results are given on Table S2.

Table S1. Parameters used for calculation of the contact zone

Parameter / Value / Reference
E1, (GPa) / 88.6-95.3 / 2
E2, (GPa) / 398-410 / 3
ν1 / 0.42
0.2-0.23 / 4
2
ν2 / 0.22-0.27 / 3
Density of MgH2, g/cm3 / 1.45 / 5
Molar mass of MgH2, g/mol / 26.3 / 5

Table S2. Calculation results of the contact zone

Normal load, N / 0.21 / 0.42
de, μm / 29 / 36
dp, μm / 51.7 / 73
z, μm / 6.9 / 8.7
σe,max, MPa / 478 / 602
τe,max, MPa / 161.7 / 193
Volume of the active zone, 10-6 cm3 / 0.92 / 1.45
Mass of the active zone, μg / 3.62 / 6.44
Hydrogen content in the active zone, nmole H2 / 138 / 244

Maximum “flash” temperature at sliding contact

Approximate solution for flash temperature (circular contact on half-space):6

, (S6)

wherer is the size of the contact zone along the direction of sliding, µ is the friction coefficient, σ is the mean contact pressure, V is the sliding velocity, λ is the thermal conductivity, Pe1 is the Peclet number for the flat sample:

, (S7)

where1 is the material density and Cp1 is the heat capacity of the flat sample. Subscripts 1 and 2 refer to the flat sample and the pin, correspondingly.

Table S3. Initial parameters for calculation of flash temperature: Specific heat (Cp), thermal conductivity (λ) and density (ρ), sliding velocity (V)

Alumina ceramics / MgH2
Cp (J kg-1 K-1) / 7803 / 1369 7
λ (W m-1 K-1) / 30-403 / 0.04-1.6 8
 (kg m-3) / 3750-39903 / 1.45 5
V (m s-1) / 0.18
Friction coefficient / 0.4
Radius of the contact zone, um / 25

Depending on the normal load and variation of other parameters the resulting “flash” temperature increase is ranged between 8 and 23 ºC.

Figure S2. Thermal desorption spectra of MgH2 powder measured at constant heating rate 20 ºC/min.

References:

(1)Popov, V., Contact between Rough Surfaces. In Contact Mechanics and Friction, Springer: Berlin, Heidelberg, 2010; pp 81-103.

(2)Zarshenas, M.; Ahmed, R.; Kanoun, M. B.; Haq, B. u.; Isa, A. R. M.; Goumri-Said, S., Physica Scripta 2013,88, 065704.

(3)Auerkary, P., Mechanical and physical properties of engineering Alumina ceramics. Technical notes. JULKAISIJA – UTGIVARE, Technical research center of Finland: Espoo, 1996.

(4)Danaie, M.; Mitlin, D., Journal of Alloys and Compounds 2009,476, 590-598.

(5)Li, X., Green Energy: Basic Concepts and Fundamentals. Springer London: 2011.

(6)Tian, X.; Kennedy, J. F. E., Journal of Tribology 1994,116, 167-174.

(7)Kelkar, T.; Kanhere, D. G.; Pal, S., Computational Materials Science 2008,42, 510-516.

(8)Shim, J.-H.; Park, M.; Lee, Y. H.; Kim, S.; Im, Y. H.; Suh, J.-Y.; Cho, Y. W., International Journal of Hydrogen Energy 2014,39, 349-355.