to 3.2. Thermometry Construction and Application of Thermometer Elements ce temperature measurement is often the chief e of error in the drop method at high temperaconsiderable care was taken in this investigaensure the best possible knowledge of the le-capsule temperature. In this effort, the control e furnace temperature, the construction of the nocouple thermometers and the placement of ermometer elements were considered. e central silver core of the furnace which surds the sample capsule during temperature ibration was maintained as nearly as possible uniform temperature by the use of three indeent heaters (see [5]): besides the main heater, h surrounded the central core, an additional er surrounded each of the two silver guard ents, one above and one below. The temperadifference between each guard segment and the est end of the central silver core was kept less 0.1 °C as indicated by single-junction chromelel differential thermocouples installed between guard segments and the core. The drift from any value of the furnace temperature as indicated by thermometers in the central core was usually than 0.01 °C. ne temperature of the silver-core resistance furnace measured at and below 500 °C with a different, -stem, encapsulated platinum resistance therneter than was used in the earlier measurements the Calorimetry Conference Sample [2]. Above °C, the temperature was measured with each of new Pt-Pt10Rh thermocouples. In order to verify thermocouple calibration "in place", both of se thermocouples were also read at and below °C, the range in which the resistance thermomconsidered the primary thermometric was nent. The two thermocouples were constructed of 0.015 o.d. wires of thermocouple-grade Pt and Pt10Rh ■y. A large assortment of these wires was annealed I tested outside the furnace for homogeneity by a perature-gradient method. method. This consisted of jecting each annealed wire at uniformly spaced tions along its length to a much larger temperature dient than would normally be encountered under erating conditions in the furnace. Wires were osen from the assortment which yielded, under the Ove conditions of testing, parasitic emfs no greater an 0.1 μv. Two pairs of these wires were assembled the two thermocouples. In the furnace, each thermouple was contained in a length of new Degussa AL 23 mina tubing and had its junction protected with mina cement. Both thermocouples and the restance thermometer were calibrated on IPTS-48, amended in 1960, by the Temperature Section of e NBS. (All measured temperatures were later nverted to IPTS-68.) The resistance thermometer as calibrated at the ice, steam and sulfur points, and as checked at the zinc point with no sensible dis crepancy. Its ice point was frequently checked throughout the enthalpy measurements and did not vary from its calibration value. The resistance thermometer and both thermocouples are introduced at the furnace top. They extend into holes drilled in the silver core parallel to the furnace axis, terminating at midheight of the core (the same height at which the sample capsule is held). Each thermometer element is located at a different azimuth around the silver core, and its immersion in it is sufficient, according to calculation, to allow the element to attain the temperature of the core. b. Tests of Thermometer and Furnace Performance "Immersion" tests of all thermometer elements were conducted with the furnace controlled at 400 °C. These tests comprised measurement of the apparent temperature differences between one of the three thermometer elements positioned in its hole at furnace midheight and the other two elements, positioned in their holes, as the latter elements were withdrawn stagewise. This was repeated three times using each time a different one of the elements as the stationary one, and indicated that any temperature difference which may have existed over the upper half of the central silver core was probably less than 0.1 K. The same type of test was conducted with the thermocouples alone at 850 °C, and indicated an apparent temperature difference no greater than 0.2 K over the upper half of the central core. Intercomparison of the thermocouples and resistance thermometer in place in the furnace at and below 500 °C showed that each of the two thermocouples consistently registered a temperature 0.1 K above that of the resistance thermometer. The resistance thermometer was considered the more reliable of the two types of thermometer in this temperature range and as a result, each thermocouple-determined temperature above 500 °C was corrected by subtracting 0.1 K in processing the raw data. Though a helium-rich atmosphere is maintained at all times in both the furnace and calorimeter in order to promote temperature equilibration of the capsule, it was felt that a measurement of any temperature difference which might exist (laterally) between a typical sample capsule and the furnace core would be of value in estimating accuracy. Towards this end, one of the two calibrated thermocouples was paired through a common welded junction with a third similarly constructed Pt-Pt10Rh thermocouple and emf readings of each of these couples were taken over the range 0 to 900 °C. This pair of thermocouples was contained in the furnace in the same porcelain tube during comparison. The third couple was then detached, removed from the furnace and its junction attached inside a dummy capsule similar to those used in the measurements on a-Al2O3. The capsule was then suspended in exactly the same position in the furnace it normally occupies, and the emf's of both couples were again observed as the furnace assumed constant temperatures in the of the present results over those of previous similar NBS measurements on this substance, new light has been shed on the validity of certain corrections to the older NBS data which have been proposed in the literature [10]. Based partly on the present results, a new table of thermodynamic functions for the range 0 to 1200 K has been generated which the authors believe is the most accurate available today. In evaluating the data, use has been made of similar measurements completed recently by other investigators at NBS [22] using a high-temperature (1173 to 2300 K) drop-calorimetric apparatus of entirely different design [3]. 2. Samples Details describing the preparauion and analysis of the Calorimetry Conference a-AlO have already been given [2]. The new a-Al2O3 sample (SRM 720) was produced by the Linde Air Products Company, as was the Calorimetry Conference Sample. Singlecrystal rods of pure a-Al2O3 were grown using a modification of the Verneuil method [4]. The rods emerged from this process free of any obvious surface contamination and hence in no need of special chemical treatment as was required for the Calorimetry Conference Sample. The individual rods, not all of uniform diameter, were centerless-ground with diamond-impregnated wheels to establish a maximum diameter for the lot (approximately 2. mm). The rods were then bundled and each bundle cut with a diamond-impregnated saw into segments 4. to 6. mm long. No other cleaning process other than removal of grinding residue was carried out. The entire lot, comprising approximately 18 kg of these segments, was then subjected to a thorough visual examination and doubtful pieces (such as those showing discoloration or other possible contamination) removed. were Specimens for chemical analysis and enthalpy measurement were chosen from the remainder of the lot.3 Portions of four of these were encapsulated directly for enthalpy measurement. One portion of each of these four SRM 720 specimens was submitted to the Analytical Chemistry Division of the NBS for a qualitative spectrochemical analysis for metallic constituents. A specimen of the Calorimetry Conference Sample was concurrently analyzed by this method. These analyses indicated the purity (by weight) of all specimens to be the same: probably 99.98 percent, with the major impurities being magnesium, calcium, chromium, iron and silicon. An independent analysis was carried out in the same Division of the NBS by atomic absorption. spectrometry for magnesium on the surface and throughout the bulk of the SRM 720 specimens. This analysis indicated the surface contamination by magnesium to be 0.0001 percent by weight or less and the bulk of the material to contain 0.001 percent by weight or less. Tests also indicated that adsorbed matter (pre See section 3.4.a. for details of the sampling procedure followeu. sumably moisture on the ground surfaces of the SRM 720 sample) did not exceed 0.003 percent by weight. It light of these results, the effect of impurities on the specimen heat capacity in the present measurements i not likely to have exceeded 0.02 percent. This is less than the precision of measurement by at least a facto of two and about an order of magnitude less than the estimated accuracy of measurement. No account these impurities was taken in processing the data. 3. Calorimetric Procedure 3.1. Calorimeter Proper In the "drop" method, described elsewhere in great detail [5]. a specimen is held in an isothermal zone of a controlled-temperature furnace for a time sufficient to allow it to attain thermal equilibrium. In the series of measurements reported below, it is then dropped into Bunsen ice calorimeter, which measures the heat liberated by the specimen as it cools to 0 °C. In accurate work the specimen is usually encapsulated together with an inert gas; this procedure prevents any reaction of the specimen with the furnace atmosphere. Then, a second heat measurement at the same initial furnace temperature is made on the empty capsule (or one nearly identical to it), in order to obtain the desired relative enthalpy of the specimen alone (it is assumed that the capsule loses the same amount of heat both times). The furnace, ice calorimeter and thermometry of this investigation are very similar to those used previously in this laboratory for enthalpy measurements on the Calorimetry Conference Sample [2]. However, the calorimeter has been slightly modified by incorporating glass-tube segments between the calorimeter and the tempering coil ("T" in fig. 6 of [2]) and between the tempering coil and the mercury-accounting system ("B" and "C" in fig. 6 of [2]). Since both these seg ments are in the form of an inverted “U”, they form traps for gas bubbles or water thus assisting in a rapid diagnosis of leaks and improving one's ability to knowl edgeably manipulate the calorimetric fluids during as sembly and operation. The portion of the mercury transit line within the innermost calorimeter chamber has also been replaced by a glass tube, allowing one to completely clear the transit line for repairs without danger of contaminating the water inside the calorimeter. One point of technique worth mentioning involves the procedure used to fill the calorimeter. This is now done by using "R" (fig. 6 of [2]) as the evacuation and purified-water port and afterwards introducing mercury from "B" through valve "V" under atmospheric pres sure. Great care must be exercised to rid “V” of air before introducing mercury into the calorimeter and to ensure that the mercury does not splash onto the inner calorimeter parts. In this way, as large an amount of mercury as may be desired can be introduced into the calorimeter. Since temperature measurement is often the chief irce of error in the drop method at high temperaes, considerable care was taken in this investigan to ensure the best possible knowledge of the nple-capsule temperature. In this effort, the control the furnace temperature, the construction of the ermocouple thermometers and the placement of thermometer elements were considered. The central silver core of the furnace which surunds the sample capsule during temperature uilibration was maintained as nearly as possible a uniform temperature by the use of three indeendent heaters (see [5]): besides the main heater, nich surrounded the central core, an additional ater surrounded each of the two silver guard gments, one above and one below. The temperare difference between each guard segment and the arest end of the central silver core was kept less an 0.1 °C as indicated by single-junction chromelumel differential thermocouples installed between e guard segments and the core. The drift from any et value of the furnace temperature as indicated by ie thermometers in the central core was usually ss than 0.01 °C. The temperature of the silver-core resistance furnace as measured at and below 500 °C with a different, ong-stem, encapsulated platinum resistance ther1ometer than was used in the earlier measurements In the Calorimetry Conference Sample [2]. Above 00 °C, the temperature was measured with each of wo new Pt-Pt10Rh thermocouples. In order to verify he thermocouple calibration "in place", both of hese thermocouples were also read at and below 500 °C, the range in which the resistance thermomwas considered the primary thermometric lement. ter The two thermocouples were constructed of 0.015 no.d. wires of thermocouple-grade Pt and Pt10Rh alloy. A large assortment of these wires was annealed and tested outside the furnace for homogeneity by a temperature-gradient method. This consisted of subjecting each annealed wire at uniformly spaced stations along its length to a much larger temperature gradient than would normally be encountered under operating conditions in the furnace. Wires were chosen from the assortment which yielded, under the above conditions of testing, parasitic emfs no greater than 0.1 μv. Two pairs of these wires were assembled as the two thermocouples. In the furnace, each thermocouple was contained in a length of new Degussa AL 23 alumina tubing and had its junction protected with alumina cement. Both thermocouples and the resistance thermometer were calibrated on IPTS-48, as amended in 1960, by the Temperature Section of the NBS. (All measured temperatures were later converted to IPTS-68.) The resistance thermometer was calibrated at the ice, steam and sulfur points, and was checked at the zinc point with no sensible dis crepancy. Its ice point was frequently checked throughout the enthalpy measurements and did not vary from its calibration value. The resistance therinometer and both thermocouples are introduced at the furnace top. They extend into holes drilled in the silver core parallel to the furnace axis, terminating at midheight of the core (the same height at which the sample capsule is held). Each thermometer element is located at a different azimuth around the silver core, and its immersion in it is sufficient, according to calculation, to allow the element to attain the temperature of the core. b. Tests of Thermometer and Furnace Performance "Immersion" tests of all thermometer elements were conducted with the furnace controlled at 400 °C. These tests comprised measurement of the apparent temperature differences between one of the three thermometer elements positioned in its hole at furnace midheight and the other two elements, positioned in their holes, as the latter elements were withdrawn stagewise. This was repeated three times using each time a different one of the elements as the stationary one, and indicated that any temperature difference which may have existed over the upper half of the central silver core was probably less than 0.1 K. The same type of test was conducted with the thermocouples alone at 850 °C, and indicated an apparent temperature difference no greater than 0.2 K over the upper half of the central core. Intercomparison of the thermocouples and resistance thermometer in place in the furnace at and below 500 °C showed that each of the two thermocouples consistently registered a temperature 0.1 K above that of the resistance thermometer. The resistance thermometer was considered the more reliable of the two types of thermometer in this temperature range and as a result, each thermocouple-determined temperature above 500 °C was corrected by subtracting 0.1 K in processing the raw data. Though a helium-rich atmosphere is maintained at all times in both the furnace and calorimeter in order to promote temperature equilibration of the capsule, it was felt that a measurement of any temperature difference which might exist (laterally) between a typical sample capsule and the furnace core would be of value in estimating accuracy. Towards this end, one of the two calibrated thermocouples was paired through a common welded junction with a third similarly constructed Pt-Pt10Rh thermocouple and emf readings of each of these couples were taken over the range 0 to 900 °C. This pair of thermocouples was contained in the furnace in the same porcelain tube during comparison. The third couple was then detached, removed from the furnace and its junction attached inside a dummy capsule similar to those used in the measurements on a-Al2O3. The capsule was then suspended in exactly the same position in the furnace it normally occupies, and the emf's of both couples were again observed as the furnace assumed constant temperatures in the range 0 to 900 °C. The results indicated that when the temperature of the furnace core was not changing, any temperature difference between the capsule and core at equilibrium was probably always less than 0.1 K and much smaller at the lower furnace temperatures. How closely a given capsule, initially at room temperature before being lifted into the furnace, reaches temperature equilibrium with the furnace in the time allowed depends upon its composition, contents and the time it has resided in the furnace. (Any appreciable drift of the furnace temperature would, of course, produce additional error, but in practice this drift was negligible.) The time required to reach equilibrium at any temperature can be readily estimated [5, 6] by making at that temperature two enthalpy measurements, one with a grossly inadequate equilibration time. As a result of tests similar to this, up to an hour of equilibration time was allowed in the measurements on a-Al2O3 to ensure that the error due to this cause would be safely less than 0.01 percent. 3.3. Sample Containers The NBS high-temperature enthalpy measurements on the Calorimetry Conference Sample which were reported in 1956 [2] were made with the specimen contained in a capsule composed of the alloy 80 Ni-20 Cr. However, other enthalpy measurements upon this alloy itself in this laboratory [7] later disclosed that it undergoes a solid-solid phase transition of somewhat undetermined character in the vicinity of 600 °C. In order to avoid possible errors in the present a-Al2O3 enthalpy data arising from the use of such a capsule material, the present authors decided to adopt a material free of complicating transitions. The alloy Pt10Rh was chosen. Besides being inert with respect to the sample and the furnace atmosphere (helium), it has no solid-solid transitions of the type thought to introduce errors in enthalpy measurements [8, 14, 15], and maintains structural properties adequate for a capsule material at least up to 1500 °C. Each capsule was constructed from a segment of Pt10Rh tube (1/2 in o.d., 0.008 in wall thickness) with end caps of the same alloy (0.008 in thick) drawn to a cup shape and edge-welded by a heliarc process to the tube segment. The top of each capsule had welded in its center a 1.5 mm o.d. Pt10Rh alloy tube for the purpose of evacuation and introduction of helium gas. Final sealing was accomplished by pinching off and flame-cutting this small-diameter tube, while the absolute pressure of gas in the capsule was held at 1/4 atm. Implicit in the sample-container design was the consideration that a given container could not be conveniently opened, emptied and resealed. Therefore, all sample and empty capsules were fabricated as nearly as possible to identical dimensions, and each class of capsule component (wall, end caps and evacuation tube) was chosen from contiguous sections of 4 See section 4.1. for a description and results of this test. common pieces of stock. Insofar as the stock was homogeneous, each capsule should then contain equal proportions of Pt and Rh. As a further precaution against unsuspected inhomogeneities in the capsule material, two capsules of the seven fabricated were chosen at random to serve as empty capsules (hereafter also referred to as "blanks"). In order to test whether or not the capsules contained significantly different proportions of Pt and Rh, enthalpy measurements on each of the empty capsules were made before the main series of measurements was started. If there were no difference between the enthalpy data for the two empty capsules, it was felt unlikely that there would exist a significant difference between the empty capsule and sample capsule enthalpies. 3.4. Experimental Program a. Sampling It was desired that the enthalpy measurements be representative of those one would obtain for any specimen chosen at random from the lot of material known as SRM 720 (18 kg of rod segments in all). Towards this end, the measurements were made on four specimens chosen in the following manner (see fig. 1): The entire lot of rod segments was apportioned into 24 units (designated numerically 1 to 24) of approximately equal mass. Each of these was subdivided into pairs, each pair member (“portion") receiving the same numerical designation as the parent unit. Four groups of six units each were then formed by choosing at random from these 24 numbered units. Each of the four groups thus corresponded to twelve portions of rod segments labeled pairwise and referred to altogether as a "sublot." Each of the four sublots was then halved by eliminating one portion chosen at random from each numbered pair. Five grams of rods was then extracted from each FIGURE 2. Schedule of measurements. Each "X" indicates a single enthalpy measurement. Temperatures are spaced about 50 K apart from 0 to 900 °C. All measurements at any one temperature were completed before proceeding to another temperature. of the six remaining portions of each sublot and mixed together to yield four 30-g specimens, each characteristic of a different sublot. Hereafter, a reference to "sublot X" will imply "the specimen characteristic of sublot X." Each of four sample containers was then filled with rods from a different one of the 30-g specimens, the remainder of the specimens being retained for chemical analysis. The correspondence between numbers used in the sampling procedure and individual portions of SRM 720 was then dropped and all material save the specimens for measuring and analysis was mixed together. In addition, one sample container was filled with a specimen of the Calorimetry Conference Sample. b. Schedule of Measurements It was desirable to complete the enthalpy measurements on the seven capsules (four containing specimens of SRM 720, one containing a specimen of the Calorimetry Conference Sample, and two being blanks) with minimum effort and yet obtain sufficient data to allow analyzing the enthalpy data for any one capsule over the entire temperature range, 0 to 900 °C. Therefore the schedule of measurements described in figure 2 was followed in the main. The enthalpy measurements, indicated individually by "X." were made at temperatures spaced at about 50 K intervals. All measurements at any one temperature required by this program were completed before proceeding to the next temperature (randomly selected from those previ ously chosen for measurements). At least one duplicate measurement (on the Calorimetry Conference Sample or a blank) was included in each day's work as a daily monitor of precision. 4. Results 4.1. Measurements Before starting the main series of measurements, a few trial enthalpy measurements were made in an effort to determine whether the blanks and sample containers were sufficiently close in their alloy composition to justify the substitution of enthalpy data on the fabricated blanks for the desired data on the (empty) sample containers. If there were a significant difference between the two types of containers, this would not only manifest itself as systematic differences between the enthalpy data for the individual sublots of SRM 720, but might also show up as a difference between the enthalpy data for the blanks themselves. (For example, a variation of 0.1 percent in the rhodium content of the blanks would introduce approximately a 0.1 percent discrepancy among their enthalpy values, which should be easily detectable at 900 °C.) Triplicate enthalpy measurements on both blanks were made at 900 °C, and indicated that within the precision of thermal measurement (see fig. 3), the two blanks could be considered to have identical compositions. Triplicate enthalpy measurements at 900 °C on each of three of the four SRM 720 sublots were also made, using the enthalpy value for the blank determined above, and these also agreed with each other within the precision of measurement (0.01 percent in this case). With this foundation, the main series of enthalpy measurements was begun. The enthalpy data for the blanks are given by table 1 and represented in figure 3. Since no irregularities were anticipated in the enthalpy-temperature function of the blanks, it was decided to substitute smoothed blank enthalpy values for the observed blank data in all calculations, thereby reducing the effect of random errors in the blank data. The base line of figure 3 represents the following equation, which was chosen to smooth the data in columns 2, 3, and 4 of table 1: |