Measurement of the relative enthalpy of pure a-Al2O3 (NBS heat capacity and enthalpy Standard Reference Material No. 720) from 273 to 1173 K

Abstract

Th e re lat ive e nth a lp y of NBS S ta nd ard Re fe re nce Matcr ia l No. 720 (99.98 perce nt pure. s in glec rys ta l a· AI, O". a ca lorim e tri c hea t-ca pac it y s tandard ) was meas ured ovpr th e range 273 to [173 K by th e drop me thod us ing a hi ghl y prec ise Bun se n ice ca lo rim e te r. Entha lpy d a ta ove r th e s ame te mp e ratun: int e rv a l we re o bt a in ed a lso o n th e Ca lorim etry Co nfe r'e nce Sa mpl e of thi s s ubs ta nce. T hese res ult s a re be lieve d to be more acc ura te th a n s imi lar NBS res u lts o n th e la tt e r sa mpl e pub lis hed in ] 956. a nd s how no s ig n ifi ca nt di sco ntinuit y with th e NBS data on th e sa me s ubs ta nce tha t cove red th e ra nges 13 to 380 K and ] · 173 to 2257 K. Th e ave rage deviation from th e mean for a ll e nt ha lpy meas ure me nt s o n th e S RM 720 s amp le was 0.0 17 pe rce nt , and the s mooth e nth a lpy valu es rl c rive d from th e data were es tim a te d to be acc ura te to 0.1 pe rce nt. Th e preca uti o ns obse rved in o rd e r to minimi ze meas urin g e rrors a re d esc ribed in detail. Th e dat a are co mp a red with many se ts o f th e mos t re li ab le pub lis he d data ava il ab le and new reco mm e nd ed va lues for the th er mody na mi c fun c ti o ns of a·A I, O" a re prese ntt'd for th e int e rv a l 0 to 1200 K. Key wo rd s: Alumina; a luminium ox ide; co r u ndum : drop ca lo rim e tr y: c nth a lp y; hea t ca pac it y s ta nd ard; s pe c ifi c hea t : s ta nd a rd re feren ce ma teria l; sy nt he ti c s apph ire; th ermodyna mi c function s.

Enthalpy and Heat-Capacity Standard Reference Material: Synthetic Sapphire (Alpha-Al2O3) From 10 to 2250 K

“…The molar enthalpy values in column 5 were obtained from net heat values (F-C differences from column 3) by division by the applicable specimen mass (column 4). The present, high-temperature enthalpy results (table 1) and those from [2] in the range 273.15 K to 1173.15 K were represented by a single smoothing function derived by the method of least squares (eq. (3) of [2]).…”

“…The present, high-temperature enthalpy results (table 1) and those from [2] in the range 273.15 K to 1173.15 K were represented by a single smoothing function derived by the method of least squares (eq. (3) of [2]). The last two columns of table 1 give the absolute and percent deviation of the present high-temperature data from this equation.…”

“…7, K. These corrections ranged from one to two percent of the measured heat. The enthalpy data necessary to calculate the corrections for carbon, tantalum, aluminum oxide, and molybdenum, were taken from references [7], [8], [9] and [2], respectively. The heat content of the aluminum oxide constituted about 55 percent of the total measured heat at all cc the corrections applied to yield the mnlar values (col. 5), which were used temperatures.…”

“…A detailed presentation of the original ice-calorimeter enthalpy data along with a description of the smoothing procedure used to obtain the enthalpy and heat-capacity values appearing in [1] has been given in [2]. Unfortunately, the original receiving-calorimeter enthalpy data were never published due to the death of one of the principal experimenters and subsequent personnel changes.…”

“…Standard Reference Material 720 (a-A1 2 0 3 ) has been offered by the NBS Office of Standard Reference Materials since 1970 [11 as a heat-capacity and enthalpy standard certified in the temperature range 273.15 K to 2250 K. The relative enthalpy data, whose smoothed representation appears in [1], were obtained in two different types of drop calorimeter: a Bunsen ice calorimeter was used from 273.15 K to 1173.15 K [2] and an adiabatic receiving calorimeter from 1173.15 K to 2250 K. The smoothed relative enthalpy values of [1] rely as well for the absolute ice-point enthalpy (H 273 ,3 KHo rJ upon a much earlier low-temperature heat-capacity study [3] on the "Calorimetry Conference Sample" of pure a-AI 2 0 3 .…”