Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30..539m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 539
Computer Science
2
Scientific paper
Chondrules may have formed by flash melting in the solar nebula [1-3]. It is thus important to understand the dynamics of heat transfer to mm-sized silicate spheres during the initial seconds/minutes of a brief duration heating event. Several studies have tried to reproduce the features of chondrules (and CAIs) by flash melting charges in the lab [4-8], but can "flash heating" be duplicated in conventional furnaces? Typically, a thermocouple is used to monitor the temperature of the experiment and it is usually assumed that the temperature of the charge is that registered by the thermocouple. In traditional long duration heating experiments of 1/2 hour or more the difference between the heating/cooling characteristics of a Pt100/Pt90Rh10 thermocouple and a pellet of silicate starting material probably does not matter. However, if a charge is heated for only seconds or minutes the actual temperature it experiences may not be accurately reflected in the thermocouple reading. A thermocouple constructed from new wires with a new weld (i.e. temperature sensor tip) gives a stable reading in about 1 minute (Fig. 1). The higher the temperature of the furnace, the faster the thermocouple registers the desired temperature. An old, heavily oxidized thermocouple (wires and weld) may take as much as 1 minute more than one with new wires/weld. A thermocouple with oxidized wires but a new weld stabilizes only marginally slower (~15 sec) than a completely new thermocouple. Such variation in the rate at which a thermocouple registers stable temperature suggests that heating is faster than indicated by the thermocouple. A group of experiments have been conducted in order to determine the point at which a charge melts. Experiments were performed in a Del-tech VT-31-OS furnace at 1450 degrees C and 1600 degrees C using 10 mg and 25 mg pellets of an oxide starting powder of CAI composition (liquidus~1595 degrees C). Time was recorded as soon as the samples entered the hot spot of the furnace (as opposed to waiting for the thermocouple to register stable temperature); samples were pulled and air quenched. At 1450 degrees C and less than 1 second in the furnace, both mass amounts were still in their original pellet shape and pressed powdered form. After 10 seconds of being in the furnace they were still in the pellet shape but were fused to the sample wire. At this point, the samples could no longer be nudged off the wire or changed from pressed to loose powder if handled, as was the case after less than 1 sec in the furnace. After a total time of 15 sec in the furnace, both mass amounts melted extensively (i.e. charges were spherical, ~1 mm and ~2 mm, and hung down on the sample wire). With the furnace at 1600 degrees C all samples that spent less than 1 sec in the furnace were still in their original pellet shape/powdered form. After 1 sec, the 10 mg samples were fused to the sample wire and after 2 sec, they melted; the 25 mg pellets, however, were still in powdered form. After 5 sec, the 25 mg samples were fused to the wire and after 7 sec, they melted. It is interesting to note the thermocouple reading during these brief melting events. In the 1600oC case, for example, at 2 sec when the 10 mg samples melted, the thermocouple read only ~900 degrees C (Fig. 1). The charge temperature must have been rather high, however, as only spinel and glass are present. A similar trend can be noted upon cooling. Fig. 2 shows the rate at which a thermocouple cools down. When the thermocouple registered 300 degrees C, charges could be touched and handled at ease with bare hands, illustrating that they could not have been 300 degrees C but were instead much cooler. The given values probably represent the minimum amount of time needed for a charge to melt, as small sample amounts and oxide starting materials, which would dissolve faster than silicates, were used. In order to obtain an approximation of the maximum amount of time a charge may need to melt, we placed 100 mg pellets in the furnace at 1400 degrees C. In 20 sec, they were fused to the wire, in 30 sec they melted. Thus, 30 sec may be used as an approximation of the maximum amount of time a charge may need before it is melted; greater amounts of starting material and lower temperatures than used here would require longer times for melting to occur. Inferred heating and cooling paths for charges (Figs. 1 and 2, respectively) constructed by using the above data show that a conventional high temperature furnace is suitable for studying flash heating. References: [1] Morfil G. et al. (1993) in Protostars and Planets III (E. H. Levy and J. Levin, eds.), pp. 939-978, Univ. of Arizona. [2] Boss A. P. and Graham J. A. (1993) Icarus, 106, 168-178. [3] Hood L. L. and Horanyi M. (1991) Icarus, 93, 259-269. [4] Wdowiak T. J. (1983) in Chondrules and Their Origins (E. A. King, ed.), pp. 279-283, LPI. [5] Connolly H. C. et al. (1993) Meteoritics, 28, 338-339. [6] Maharaj S. V. and Hewins R. H. (1994) GCA, 58, 1335-1342. [7] Yu Y. and Hewins R. H. (1994) LPS XXV, 1535-1536. [8] Maharaj S. V. and Hewins R. H. (1995) LPS XXVI, 883-884.
Hewins Roger H.
Maharaj Susan V.
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