
1 3
International Journal of Thermophysics (2020) 41:139
Page 3 of 17 139
2 Experimental Method
The method of ohmic pulse-heating was applied to resistively heat Ir and Re
sample wires with
in diameter and an approximate length of
from
room temperature (
) up to their boiling point. At this point, the sample
explodes due to the large discontinuous jump in volume and gives the method its
popular name exploding wire technique. Energy for the heating process is pro-
vided by a
capacitor bank that can be charged to a voltage between 3kV to
10kV. Once the experiment is started, a current that can peak in more than 10kA
runs through the wire sample and rapidly heats it with heating rates on the order
of
. These high heating rates lead to a very short experimental duration
of typically
to
, depending on the specimen and experimental parame-
ters used, and offer several advantages: First, the method can be considered quasi-
containerless, i.e., chemical reactions with the surrounding atmosphere may be
neglected. Second, the wire sample remains unaffected by gravity which allows
the experimenter to perform measurements in the liquid phase as the wire speci-
men does not collapse upon melting. Third, high heating rates additionally sup-
press an axial expansion of the wire specimen. As the experiment can be con-
sidered quasi-static, which implies an increased radial expansion to compensate
for the absent axial expansion, it is sufficient to monitor the radial expansion
throughout the experiment to derive the sample’s volume expansion as a func-
tion of time. For this purpose, a specialized CCD-camera system was used (PCO
imaging, controller unit by Theta System and Graz Univ. of Technol.) in com-
bination with a high-power photo flash (Multiblitz X10AC/DC,
) that
allows an image acquisition rate of
, i.e., approximately one shadow
image every
at an exposure time of
. Each shadow image has a reso-
lution of
. For further details on the expansion measurement setup,
the reader is referred to [5, 12].
In addition to the measurement of thermal radial expansion, the sample’s sur-
face radiance is monitored throughout the experiment by means of pyrometry to
derive the sample temperature as a function of time. The pyrometer used operates
at a central wavelength of
(FWHM =
, sampling rate:
).
Neutral density filters are applied to cover a broad temperature range.
To physically extend the accessible temperature range by raising the sample’s
boiling point, low-pressure pulse-heating experiments at
were com-
plemented by high-pressure pulse-heating experiments at
to
.
While the low-pressure experiments were conducted in nitrogen (Alphagaz 1
,
), distilled water was used in high-pressure experiments to build up
pressure. A detailed description of the setups may be found in [12, 16]. Note,
that the high static pressure has negligible impact on density measurement due
to the very small compressibility of these metals of
= 0.0082 GPa
and
= 0.0092 GPa
[17]. Therefore, density data obtained on different isobars
in the kbar-regime are typically indistinguishable for most metals, compare also
[13].