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Furnace for High-Temperature Metal (Carbide)-Carbon Eutectic
Fixed-Point
B. Khlevnoy * , M. Sakharov, S. Ogarev, V. Sapritsky
All Russian Research Institute for Optical and Physical Measurements (VNIIOFI), Moscow, Russia.
* A guest-scientist at the NMIJ at the time of investigation.
Y. Yamada
National Metrology Institute of Japan, AIST, Tsukuba, Japan.
K. Anhalt *
PTB, Berlin, Germany. * A guest-scientist at the NMIJ at the time of investigation.
Abstract. A new large-area furnace with maximum
temperature of 3500 K was designed at the VNIIOFI as a
furnace for high-temperature M(C)-C fixed points, and
then investigated at the NMIJ. Temperature uniformity was
investigated as to its dependence on various heater and cell
holder arrangements. One Re-C and one TiC-C cells were
made using BB3500YY and then compared with a Re-C
cell made in a NAGANO furnace at the NMIJ and with a
VNIIOFI-made TiC-C cell.
Introduction
High-temperature metal-carbon (M-C) and metal
carbide-carbon (MC-C) fixed points have been attracting
attention of many researchers and hold promise to take an
important role in radiometry and photometry in the nearest
future [1]. Three type of furnaces are conventionally used
for the highest temperature M(C)-C fixed-points:
Thermogage, Nagano and VNIIOFI-made BB3200pg/
BB3500 [2]. It was shown [3, 4] that the temperature
uniformity of furnaces is very critical for reproducibility
and plateau quality of the fixed points. To get the better
uniformity a new, BB3500YY, furnace was designed at the
VNIIOFI and then investigated at the NMIJ.
Furnace design
The BB3500YY furnace has a design similar to the
pyrographite (PG) blackbody BB3200pg [5]. The
cross-section of the BB3500YY is shown on Fig.1. It has a
cylindrical heater consisting of a set of PG rings,
compressed by a spring between front fixed copper and
rear movable graphite electrodes, and surrounded by
carbon cloth thermoshield.
Figure 1. Cross-section of the BB3500YY furnace
In comparison with previous PG furnaces the
BB3500YY has longer thermoshield and set of PG rings:
455 mm and 355 mm respectively instead 370 and 290 mm.
This makes the central part of the furnace more uniform.
The length of the rings set can be increased to 400 mm if
necessary. The inner diameter of the rings is 47 mm
instead 37mm, which also improves the uniformity
because of better conditions for radiation heat flux
exchange. The larger space between the cell, which is
placed in the center of the furnace tube, and the rings
allows placement of an additional screen for further
improvement of uniformity.
Advantages of the rings set design: numerous baffles
and a cell holder can be easily placed at any position
between the rings; different resistance ring order can be
chosen to change the temperature gradient.
BB3500YY has a rear radiation channel that can be
used for a monitor/control pyrometer or thermocouple.
Temperature uniformity at 1500 o C
Because of difficulties of temperature distribution
measurement at high temperatures the uniformity was
investigated first at 1500
o C using two type R
thermocouples. One of them was fixed in the rear channel
and used for furnace control and the second one was
moved through the front opening to make a scan along the
furnace axes.
At the first step the arrangement of the furnace was as
follows: a graphite cell holder of 80 mm length and 30 mm
in outer diameter designed to hold a cell of 45 mm length
and 24 mm in diameter. The holder was wrapped in 3-mm
graphite felt. Inside the holder there were eight thin-sheet
PG baffles: four in front of and four behind the cell. The
cell itself was not in place during the measurements. In
front of the holder and behind it there were sets of outer
baffles from the same thin-sheet PG material placed
between the heater rings. Fig.2 shows a family of
distributions measured. Curve 1 was measured when all
baffles and the felt were at their places as described above.
Then outer baffles were removed and curve 2 was
measured. Then outer baffles were put back, inner ones
removed and curve 3 was measured. Curve 4 was
measured with all baffles but without the felt. Curve 5
represents the distribution measured for just the same
heater except four rings around the rear end of the graphite
holder replaced by those with about two times higher
resistance according to resistance measured at room
temperature. One can see that neither baffles (inner or
outer) nor felt hardly changes the distribution. What
significantly changes it is resistance of the rings.
Proceedings NEWRAD, 17-19 October 2005, Davos, Switzerland 277