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.
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 oC Because of difficulties of temperature distribution measurement at high temperatures the uniformity was investigated first at 1500 oC 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. Comparing curves 5 and 6 shows that the graphite holder does not improve the uniformity. Another holder we used was a 3 mm thick CC tube of 150 mm length rapped with 4 layers of graphite cloth. Such a holder, in distinction from the graphite one, improves the uniformity, which is shown on Fig.3.
Temperature Difference, C
40 20 0 -20
1. All baffles+felt 2. No outer baffles
-40
3. No inner baffles
-60
4. No felt 5. Hot rings at rear end
-80
6. Hot rings+graphite holder
-100 -100 -80 -60 -40 -20
0
20
40
60
80 100 120 140
Distance from Holder centre, mm
Figure 2. Temperature distributions measured by thermocouple at T=1500 oC for BB3500YY with a graphite holder 20
Temperature Difference, C
0
Work with eutectics cell
-20 -40
1. Hot ring s. C C tube 2. Hot ring s. N o tube
-60
3. Hot ring s. C C tube+cloth 4. No tube 5. CC tube+cloth
-80
-100 -120 -100 -80
them, narrow-beam fiber radiation thermometer looked through the rear channel of the furnace to a blind baffle placed at the rear end of the CC-tube holder and used for furnace temperature control. The second thermometer looked through the front opening to a movable target inside the holder. The target was a 10 mm length graphite tube with soot-blackened bottom at one end and 5 mm aperture at another one. At the beginning the target was placed inside the CC tube near its front end, the furnace was brought to the desired temperature of 2500 °C, and the temperature of the target was measured by means of the second radiation thermometer. Then the furnace was brought to the temperature of 1500 oC, the target was pushed by an alumina rod to be moved by one step of 10 to 20 mm, the furnace was brought back to the temperature of 2500 °C and the second point of the temperature distribution measurement was taken. This was repeated to cover the length of the CC tube. As a test of the validity of this method, the temperature distribution was measured at 1500 oC and was compared with measurement with the thermocouple with the same furnace arrangement. Both methods agreed within one degree. Then the method was applied to measure the temperature distribution at 2500 oC. The measurements showed that the hottest point was just in the center of the CC tube and temperature gradually decreased towards its ends. The central 40 mm and 80 mm parts of the tube were uniform within 2 and 5 degrees respectively.
6. NEW heater. CC tube+cloth
-60 -40 -20 0 20 40 60 Dis tance fr om Holde r ce ntr e , m m
80
100
120
Figure 3. Temperature distributions measured by thermocouple at T=1500 oC for BB3500YY with a CC-tube holder
Distribution measured first for the arrangement with some high resistance rings at the rear end of the cell position but without any holder (curve 2), then for the same arrangement but with the CC-tube (curve 1), and then with the tube wrapped in the cloth (curve 3). Comparison of the curves shows that the cloth has more significant effect than the tube itself. Curves 4 and 5 represent distributions measured without and with the tube wrapped with cloth respectively but for arrangement with low resistance rings around the rear end of the cell position. Finally a new set of rings was arranged, which showed (in combination with CC holder) the uniformity (curve 6) within 5 degrees along the distance of 110 mm.
Temperature uniformity at 2500oC Two radiation thermometers were used for temperature distribution measurement at higher temperatures. One of
The NMIJ Re-C cell of 45 mm length, 24 mm outer diameter and 3 mm aperture cavity was heated in BB3500YY furnace to observe melting and freezing plateaus. The same cell was then used for observing the plateaus in VR10-A23 furnace [6], which has operation temperature limited to 2500 oC. The former showed better melting plateau shapes. BB3500YY was used for filling one Re-C and one TiC-C cells. The Re-C cell was compared with a similar one but filled in VR10-A10 furnace (vertical variant of VR10-A23). The TiC-C cell was compared with a cell manufactured by VNIIOFI. The results of these comparisons will be presented in the paper. References [1] Yamada Y., Advances in High-temperature Standards above 1000oC, MAPAN - Journal of Metrology Society of India, v.20, No 2, 2005, pp.183-191. [2] Sapritsky, V., Ogarev, S., Khlevnoy, B., Samoylov, M., and Khromchenko, V., Metrologia 40 (2003), S128-S131. 2003 [3] N. Sasajima, M. K. Sakharov, B. B. Khlevnoy, Y. Yamada, M. L. Samoylov, S. A. Ogarev, P. Bloembergen and V. I. Sapritsky. 'A comparison of Re-C and TiC-C eutectic fixed-point cells among VNIIOFI, NMIJ and BIPM'. in 9th International Symposium on Temperature and Thermal Measurements in Industry and Science (TEMPMEKO). 2004. Dubrovnik. [4] Woolliams E., Khlevnoy B., Sakharov M., Samoylov M., Sapritsky V., Investigation of TiC-C and ZrC-C eutectic fixed-point blackbodies., Submitted to Metrologia. [5] Sapritsky V. and others, Applied Optics, 36, 5403-5408, 1997. [6] Y. Yamada, N. Sasajima, H. Gomi, and T. Sugai, in Temperature: Its Measurement and Control in Science and Industry, 7 (Ripple et al ed.), AIP Conference Proceedings, Melville, New York (2003) 965-990.
