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Vapor Pressure and Heat of Sublimation of Tungsten

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Vapor Pressure and Heat of Sublimation of Tungsten JOURNAL OF RESEAR CH of th e Notiona l Burea u of Standards - A. Physics and Chemistry I Vo. 77A, No.2, March- April 1973 ~ Vapor Pressure and Heat of Sublimation of Tungsten E. R. Plante and A. 8. Sessoms Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234 (November 30, 1972) The va por pressure of tungs te n was measured by the Langmuir method in the te mpe rature range 2600 to 3100 K usin g a vac uum mi crobala nce. Four series of data gave concorda nt result s and three of the fo ur seri es gave second and third law heats of s ublimation in excell ent agreeme nt. A va por pressure equation representing the data is log P(atm)=-45385/T + 7.87 1, based on our mean third la w heat a nd ta bulated entropi es at 2800 K. The mean third law heat of s ublimati on al 298.15 K is 205.52 ± 1.1 kcal mol- I (859.90 ± 4.6 kJ mol- I) wh ere the uncertai nty is an overall estimated error. Rates of va pori­ zati on are about 1/2 those pre vio usly acce pted for tungsten. Key words: Heat of sublimation; Langmuir vaporizati on; rate of vapori zation; tungsten; vapor pressure. 1. Introduction right circular cylinders having nominal di ameters of 0.25 and 0.20 cm and length of 1.9 cm. A hole 0,1 This study was undertaken as part of an NBS con­ cm in diameter and 1.5 cm long, assumed to represent tribution to a program involving the measurement of blackbody conditions, was drilled along the cylinder vapor pressures of selected standard materials in axis and a suspension hole was drilled along a diameter various cooperating laboratories. The obj ect of these about 0.20 cm from the other end. measurements is to determine reliable standard The sample was suspended from on e arm of an vapor pressure data and to reveal, if possible, any equal-arm quartz 2 beam mi crobalance by a chain systematic differences in vapor pressures which mi ght of 0.025 cm di ameter sapphire or quartz rods con­ be attributable to different methods of meas urement. nected together by V- shaped hooks made by heating Currently, gold, and cadmium and silver [I]' have and bending the rods. The lower 10 cm of the sus­ been certified as standard reference materials for pension was 0.005 cm tungsten wire which passed vapor pressure measure ments. Certification of stand­ through the sus pension hole in the samples and over ards for platinum and tungsten are in process and are the hook on the lowest suspension rod. expected to be available in the near future [2]. The appendage of the vacuum chamber in whi ch Data leading to vapor pressures or heats of sublima­ the sample hung was a 20 mm O.D, Vycor 3 tube tion of tungsten have been reported by a number of made with a fu sed silica window at the bottom. The investigators [3- 9] using the Langmuir method. In window could be protected during s ublimation expe ri ­ addition, Golubtsov and Nesmeyanov [8] measured ments by a magnetically operated shutter; however, the vapor pressure by the Knudsen method. Thermo­ because the shutter must be kept open for a large dynamic data resulting from these and the current fraction of the time during short experiments when measurements are summarized in table 2 and dis­ the rate of vaporization is highest, the shutter was cussed in section 4 of this paper. left open during all the experiments reported here. To afford some additional protection to the window, 2. Experimental Method the side arm was extended so that the bottom of the sample was about 22.5 cm from the window. With > Data were obtained by Langmuir rate of sublimation this experimental arrangement changes in the window meas urements using a vacuum mi crobalance. Vacuum correction factor are not signifi cantly larger than in in the range 10- 7 to IO- B torr was maintained and those cases where an atte mpt was made to protect indicated by a commercial s putter-ion pump. Tech­ the window. niques and procedures were similar to those outlined Data were designated as belonging to a new ex­ previ ously [10]. perimental series when new window correction The tungsten SRM-749 stock material is in excess values were determined and when a sample was of 99.99 percent purity. Additional information will changed. The purpose of changing samples was to be included in the provisional NBS Certificate of Analysis. Samples were machined by arc erosion into 2 The terms qu artz and fused sili ca arc used interc hangeabl y in this pape r for fused silica . • 3 Re!erence to trade names is made onl y for completeness of description and does not I Figures in brackets indicate the literature references a l the end of Ihis paper. Im ply III any way the endorse ment of the prod uct by the National Bureau of Sta nda rd s. I 237 check on reproducibility of the measurements and procedure which eliminates static charge on the to see if changes in the rate of sublimation could be sample and its surroundings. detected for slightly different length to radius ratios Initial time for an experiment was taken when the of the samples because of temperature inho"inogeneity. brightness of the blackbody hole matched the bright­ Prior to experiments, a thin platinum coating which ness of the pyrometer filament previously set for a did not heat inductively was deposited on the interior temperature 50 K below the expected operating surface of the Vycor tube, and a grounding device, temperature for a particular power setting. Final time consisting of a split circular stainless steel ring with a was taken as the time the power was turned off. The magnetically actuated wire hinge, was positioned in the first temperature measurement was usually obtained Vycor tube so that it made contact with the platinum within the first minute of the experiment, at which coating. The Vycor tube was connected to the vacuum time the sample had attained its operating temperature. system by means of a standard taper joint using This method of determining the duration of the experi­ Apiezon W sealant. A wire connected the grounding ment was used because the rate of heating is slower device to an electrical ground. With the sample in than the rate of cooling. place the hinge of the grounding device could be Experimentally it is observed that the time required ( " magnetically deflected until it made contact with the to heat from the base temperature to the operating wire supporting the sample. This allowed for removal temperature is shorter, the greater is the temperature of any static charge generated during the high tempera­ difference between them. This probably is due to the ture heating. Heating was accomplished by induction fact that the power input can be changed almost of 450 kHz. The metal sample served as its own instantaneously, the power loss is proportional to susceptor. T4, and the energy absorbed by the sample during Temperatures were measured with an NBS-cali­ warm up becomes a smaller fraction of the energy brated optical pyrometer through a calibrated window input as the temperature difference gets larger. For and mirror. Calibration corrections for the window low temperature runs the length of time to go from the and mirror were determined in separate experiments base temperature to 50 K below the operating tempera­ using a band lamp. Corrections were determined in ture would be about 15 seconds while for high tempera­ terms of "A" values where A= (liT) - (l/Tw); Tis ture runs about 3 seconds would be required. This the brightness temperature of the source and T w is the method of timing tacitly assumes that the excess brightness temperature of the source with the window weight lost in heating from the base temperature to or mirror in the optical path. Window corrections were 50 K below the operating temperature exactly balances determined before and after each series of measure­ the deficiency in weight lost in going from 50 K below ments and the average value accepted. Mirror correc­ the operating temperature to the operating tempera­ tions were determined less frequently; an average of ture. We have assumed that systematic error resulting two independent sets of determinations was used. from this approximation is negligible. Corrections applied to the observed temperature because of the change in the window "A" value during an experimental series were less than 4 K at 3. Thermodynamic Treatment of Data 2800 K. Vapor pressures were calculated using the equation 4 In obtaining each datum point the following sequence of operations was followed: (1) the rest point of the p= -.!!!:..... (27TRT) 1/2 balance was determined, (2) the sample was heated (1) exat M to the base temperature, a temperature about 100 K below the lowest temperature where sublimation rate measurements were practicable, (3) an operating where m is the mass of material sublimed, t is the temperature of the sample was attained by adjusting duration of the experiment, a is the projected surface . j the heater power and holding it constant at a predeter­ area of the sample, T is the temperature on the mined setting, (4) the power ,was turned off, (5) the IPTS-68 scale [12], R is the gas constant, M is the sample and platinum coating on the Vycor sleeve atomic weight of the vaporizing species, monatomic were grounded, and (6) the rest point of the balance tungsten, and ex is the vaporization coefficient which was redetermined.
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