Furnace windings

Twenty percent chromium-nickel and various chromium-aluminum-iron alloys are in common use as furnace windings. To achieve higher temperatures in air, a 10% Rh-Pt alloy can be used up to at least 1400°C (2550 °F). This alloy performs better than pure platinum because of higher strength and a lower rate of grain growth. A single crystal of the same dimensions as the resistance wire cross section tends to shear easily and cause failure. 

Molybdenum-wound furnaces are operated to at least 1500°C (2725 F). Because molybdenum oxidizes in air, such furnace windings are blanketed in a protective atmosphere of hydrogen. 

Coddet, High temperature corrosion and protection of materials 6, Proceedings, 6th International Symposium on High Temperatnre Corrosion and Protection of Materials, Les Embiez, France, May 2004, Part 1, Trans Tech Publications, Uetikon-Zuerich, Switzerland, 2004, p. 193. 

Brockway and A. Rowe, in Fundamentals of Gas-Surface Interactions, H. Saltsburg et al., editors. Academic Press, New York, 1967. p. 147. 

Mishra, in Metals Handbook, Desk Edition, 2nd edition, ASM International, Materials Park, OH, 1998, p. 718. 

---

The temperature control of the furnace is satisfactorily achieved via a thermocouple mounted very close to the furnace-winding. The maximum operational temperature may be obtained by using different thermocouples as indicated below ... 

The upper panel of the autotransformer has a scale graduated from 0 to 250 V, and a handle provided with a pointer. Turning of the handle changes the voltage of the current in the furnace winding and thus sets the required temperature. Connect a furnace to the mains via an autotransformer as follows place the pointer on the upper autotransformer scale opposite the zero graduation, next connect the furnace to it and connect the autotransformer to the mains in accordance with their voltage. 

If there is good communication between the heating elements and the sample or reference thermocouple junctions, then the control system can make its power adjustments based on one of those temperatures. If there is substantial insulation between these locations, which may be necessary for heat flow uniformity to both sample and reference or to permit the introduction of special gases, then a separate control thermocouple is used which is placed near the furnace windings. 

Random floating in the baseline is often observed, such as that illustrated in Figure 3.22. This occurs when the total heat capacities of the sample and reference are not matched, or if, for example, the reference container is positioned closer to the furnace windings than the sample container. This difference often... 

To address these concerns, the authors designed an all-silica reaction cell (Figure 25). The maximum operating temperature of this cell is approximately 1273 K, and it is limited only by the furnace windings and the silica window. This is the first published example of the use of silica as a window material. Although the thickness of the window was not stated, it likely limited the usable photon energy range. 

Use Alloy with platinum for high temperature thermocouples, furnace windings, laboratory crucibles, spinnerettes in rayon industry electrical contacts, jewelry, catalyst, optical instrument mirrors, elec-trodeposited coatings for metals, vacuum-deposited glass coatings, headlight reflectors. 

Several DTA instruments have been described by Barrall et al. (94, 95). A DTA calorimeter cell in which the AT-sensing thermocouples are attached to the sample container is shown in Figure 6.35. In this apparatus, copper-constantan thermocouples are soldered to a 4-mm-OD copper cup fitted with a copper lid. The thermocouples and sample cups are supported on ceramic insulator tubes which are attached to a metal base. All the cups were heated by thermal radiation received from the blackened copper radiation shield this prevented radiation hot spots due to furnace windings. The entire DTA cell was enclosed by a glass bell jar which provided a controlled atmosphere from reduced pressures to about 2 atm. 

The lem x rslure range of DTA depends [Pg.197]

Fig. 2-5 Cross-Section of Water-Cooled Fumance for STA 1 000 (STA 780) A Water cooled cold finger B Ceramic baffles C Ceramic tube D Micro-enviromental cup. . E Ceramic stem gas inlet F Furnace winding G STA hangdown assembly...

The first movement occurs while there is still attraction, so the sample position depends upon several influences. First consider that the sample is fairly reflective and that it is not in contact with any heat sink. Next consider that the magnet—gray after the first few heatings—is a good absorber of heat radiated from the furnace windings. The sample is warned principally from the heated air. 

---