Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

hydrogen absorption properties of thousands of dif-
ferent compositions at once, by measuring the opti-
cal transmission as a function of hydrogen pressure
of a single thin film wafer. It is even possible to determine
the enthalpy and entropy of hydride formation
optically. 9 By means of Hydrogenography it was
found that within the Mg-Ti-Ni ternary system, the
best alloy for hydrogen storage is Mg0.69Ni 0.26Ti 0.05 .
The hope is that Hydrogenography will help in discovering
new alloy systems with even better storage
performances. A comparison of existing energy
storage systems for cars is shown in figure 4.
b. Fastest hydrogen diffusion
High performance membranes are used to purify
hydrogen to the level needed in fuel-cell applications.
A key characteristic of a purification membrane
is its hydrogen permeability. It has long been known
that membranes made of Palladium and Copper
have interesting properties but only a small number
of Pd-Cu alloys has been thoroughly investigated
so far, as for each composition a new membrane
needs to be fabricated. With Hydrogenography it
is, however possible to measure simultaneously the
permeability of hydrogen through hundreds of
alloys. For this, one prepares a so-called gradient
sample in a multitarget sputtering system (see figure
5). The Cu and Pd targets are used to sputter a
Figuur 6
Reflectivity of a Mg0.7Ti 0.3 layer
deposited at the end of an optic
fiber during a hydrogen loadingunloading
program. At time t=0
the fiber end is brought in contact
with 0.4% hydrogen in argon.
At t=300 s, this atmosphere is
replaced by 20% oxygen in argon.
This cycle is repeated to check for
reproducibility.
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Schakelbare spiegels: een samenspel van licht en waterstof
gradient film on the 70 mm long glass substrate. The
Pd-concentration increases from 35 to 63%. A thin
layer of yttrium is subsequently sputtered on the
Pd-Cu as an optical indicator of hydrogen. Finally,
the bottom Pd target in the sputtering system is
used to deposit a pure Pd strip. This strip serves as
an entrance gate for hydrogen that is then forced to
diffuse laterally in the Pd-Cu gradient sample. After
a few hours at room temperature, the hydrogen
front exhibits a dumbbell shape. From its maximum
one can exactly determine the composition for optimal
permeability and the permeability.
c. Fiber Optic Hydrogen Sensors
Societal acceptance is a key ingredient for the implementation
of a future hydrogen-based economy. As
this acceptance depends essentially on safety of
operation there is a great need for sensitive, selective,
fast, reliable, stable and cheap hydrogen sensors.
Currently, commercial hydrogen detectors
are not useful for widespread use, particularly in
transportation, because they are rather bulky, not
stable over a long period of time and expensive.
Furthermore, most of the sensors available commercially
are based on electrical measurements at the
sensing point. This might be undesirable in potentially
explosive environments. These disadvantages
can be circumvented by using optical detectors
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