of 10
Supplementary Materials for: Interface pinning causes the
hysteresis of the hydride transformation in binary metal hydrides
Nicholas J. Weadock,
1
Peter W. Voorhees,
2, 3
and Brent Fultz
1
1
Department of Applied Physics and Materials Science
California Institute of Technology, Pasadena, CA, USA
2
Department of Materials Science and Engineering
Northwestern University, Evanston, IL, USA
3
Engineering Sciences and Applied Mathematics
Northwestern University, Evanston, IL, USA
(Dated: December 10, 2020)
I. ELECTRON MICROGRAPHS OF BULK PALLADIUM
FIG. 1. Scanning electron micrograph of a bulk Pd particle consisting of coalesced grains. The
inset shows the scale of individual particles; inset scale bar is 100
μ
m.
1
FIG. 2. Additional electron micrographs of the Pd nanopowder. a) Scanning electron micrograph of
the as-received nanopowder, showing significant agglomeration. Transmission electron micrographs
of the sonicated nanopowder in b) and c) show smaller porous agglomerates of nanocrystallites.
II. COMPARISON OF PRESSURE-COMPOSITION ISOTHERM MEASURE-
MENTS IN A SIEVERT’S APPARATUS AND
IN-SITU
X-RAY DIFFRACTION
SAMPLE CHAMBER
Pressure composition isotherms were measured during the
in-situ
x-ray diffraction studies
for bulk and nanocrystalline Pd samples. Figure 3 below compares the
in-situ
and Sievert’s
pressure composition isotherms measured for bulk (a) and nanocrystalline (b) Pd. For both
samples the plateau pressures for absorption and desorption are slightly greater in the
in-situ
setup than those measured for with the Sievert’s apparatus. Hysteresis values, however, the
same for both types of measurements.
We attribute the pressure discrepancy to the different heating stages in the equipment. On
the Sievert’s apparatus, a stainless steel reactor is completely enclosed by a large aluminum
block wrapped in a band heater. Several type K thermocouples are used for redundant
temperature measurement and PID control. The
in-situ
heating stage consists of a small
aluminum plate with a milled vertical sample channel. A 40 watt power resistor is affixed
to the back of the plate opposite the sample channel. Due to the restrictive geometry of
the hydrogen environment chamber feedthrough, only a single type K thermocouple is used
for both monitoring and PID control. This thermocouple is inserted into the aluminum
plate below both the sample channel and heater, which could result in an artificially high
temperature at the sample. We use the van’t Hoff equation and experimentally determined
2
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FIG. 3. Comparison of pressure-composition isotherms for a) bulk and b) nanocrystalline Pd-H at
333 K measured in a Sievert’s apparatus and in an
in-situ
hydrogen environment chamber.
enthalpies and entropies of formation for Pd-H to calculate that an increase in temperature
of only 4 K accounts for the observed pressure difference.[1, 2]
III.
IN-SITU
DIFFRACTION RESULTS
A complete set of diffraction patterns corresponding to the in-situ isotherms (second cy-
cle) of Fig. 3 are plotted in Figs. 4, 5 below. The color of each diffraction pattern corresponds
to the total hydrogen content at each step in the isotherm.
The diffraction patterns were refined to extract lattice parameters as a function of hydro-
3
gen concentration, as described in the main text. The variation of lattice parameter with
concentration was used to identify the phase boundaries in bulk PdH, as there is an abrupt
change between single- and two-phase regions. For nanocrystalline PdH, we also refined
the phase fraction, and report the fraction of the
α
-phase as a function of concentration
in Fig. 6. The
α
- and
β
-phase boundaries are identified as the hydrogen concentration at
which the
α
-phase fraction deviates from 1.0 (absorption) or 0.0 (desorption), respectively.
4
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FIG. 4.
In-situ
X-ray diffraction patterns acquired during absorption (left) and desorption (right)
by bulk Pd powder. The color of each trace corresponds to the total hydrogen content (in H/M)
as indicated by the central legend.
5