Points of melting ^ crystallisation and polymorphic transformations of sulfur in density ^ temperature coordinates

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1 High Temperatures ^ High Pressures, 2000, volume 32, pages 461 ^ ETP Proceedings pages 445 ^ 450 DOI: /htwu48 Points of melting ^ crystallisation and polymorphic transformations of sulfur in density ^ temperature coordinates Anatolii S Basin Department of Thermodynamics, Institute of Thermophysics, Siberian Branch of the Russian Academy of Sciences, Novosibirsk , Russia; fax: ; basin@itp.nsc.ru Boris G Nenashev Department of rystals, Institute of Mineralogy and Petrography, Siberian Branch of the Russian Academy of Sciences, Novosibirsk , Russia Presented at the 15th European onference on Thermophysical Properties, Wu«rzburg, Germany, 5 ^ 9 September 1999 Abstract. Experiments were carried out in situ and with automatic analogue signal registration during the continuous heating or cooling processes of sulfur samples in the temperature range 20 ^ with detailed investigation of the melting and crystallisation intervals. The transformation temperatures were measured, and the expansion coefficients, specific volumes, and density of sulfur with different purity grades were calculated. The investigation was carried out with the application of a narrow gamma-radiation beam passing through the sample. Hysteresis of heating and cooling processes as well as the dependence of temperature and volumetric characteristics of melting and solidification on the purity of sulfur samples were discovered. The peculiarities of the processes in connection with sulfur crystallisation and melting are discussed. 1 Introduction Sulfur and its compounds are substances of very broad practical application and importance. At the same time, pure sulfur is a very interesting substance with a number of unique individual properties, especially strikingly exhibited in liquid and gas states. Sulfur atoms have an exclusive ability to describe a set of molecular forms, from S 2 to S 20, including ring molecules, S 6,S 8,S 12, etc (Meyer 1965, 1976). A completely unique property of the molecular forms of sulfur is the ability to polymerise spontaneously in a liquid state (L-S) during transition through the l and m phases at temperature t l=m (Bellissent et al 1994; Alwarenga et al 1996). This self-polymerisation results in a sharp increase of liquid sulfur viscosity, a change of colour, and other macroproperties of sulfur. The special properties of S atoms and polymolecularity are also exhibited in the solid state of sulfur as an allotropy and a set of polymorphous transformations. Perhaps, the report about nineteen `melting points' follows from this (Meyer 1965). However, this large number of melting points remains disputable (Vezzoli and Walsh 1977). Only five crystallisation points of very pure sulfur in coordinates of density, r, and temperature, t, and approximately ten singular points were observed in our experiments between 20 and Some of them coincided with those observed earlier (Meyer 1965), but not all of them. The main purposes of our experiments consist in deriving precise data of sulfur density changes throughout the interval of melting ^ crystallisation temperatures. This area of physical transformations of substances is interesting no less than the l-phase points, but has not been investigated so far in detail. 2 Experimental The experiments were carried out by thermodilatometric analysis of the specific volume, v, as a function of temperature, t, and thermometric analysis of t as a function of time, t, by the gamma-ray attenuation technique. We used the same gamma-ray dilatometer as

2 462 A S Basin, B G Nenashev 15 ETP Proceedings page 446 for our iron research (Basin et al 1979), but with the improved measuring system (Basin and Alekseev 1991). As a whole, this technique is similar to that used in other works (Drotning 1981). Sulfur samples were prepared by a long vacuum distillation in glass silica ampoules. The mass of each sample was about 75 g; the volume of the ampoules was about 50 cm 3. The path of the 137 s gamma-radiation beam through the sample was approximately 37 mm. Two thermocouples were placed inside the sample. Gamma-ray intensity and thermocouple thermo-emf were measured by digital instruments and simultaneously registered with an x^y positional recorder. This double method for recording measured magnitudes allowed us to observe melting and crystallisation effects in a continuous process of heating and cooling of the sample and reliably to fix process features, which are not observed in experiments with a thermostatic sample. The experimental curves (figures 1 ^ 3) show separate fragments of a rather extensive complex of measurements. The experiments were conducted on five sulfur samples of high purity (99.999% for two samples and % for three samples) chemicals with an amount of bitumens less than and wt%, respectively. Such a purity was reached as the outcome of long vacuum degassing of the sulfur at 120 ^ and sublimation at 300 ^ The pressure inside the hermetic ampoules was no more than the saturation vapour pressure of liquid sulfur (5 225 mm Hg). v=cm 3 g S4.1cl L R id Figure 1. Experimental plot of gamma-radiation intensity versus thermo-emf, which has been transformed to a plot of specific volume, v, versus temperature, t. The present plot shows the result of the S4.1 sample test. The S4.1cl curve represents the process of liquid sulfur cooling (A!P!L!), solidification,!r!, and cooling of solid sulfur at t 5 t c. P A v=cm 3 g e L S4.2cl Figure 2. Experimental plot for test S4.2. The right curve shows the heating process (h) and melting, H! B! M. The left curve shows the liquid cooling process and solidification, L!! e! R!. R H M B S4.2h P S4.2h v=cm 3 g S1.2cl R1 R2 * R2 S1.1cl A Figure 3. Experimental plot for tests S1.1 and S1.2: processes of liquid cooling and solidification,! R1! R2!.

3 Melting ^ crystallisation and polymorphic transformations of sulfur ETP Proceedings page Results and discussion Figure 1 shows a typical experimental curve of sulfur-sample cooling, which begins in a liquid state for t ˆ t A, continues through the p! l transition (polymerisation point P) and the interval of solidification, L!! R!, finishing in a solid state at t The cooling rate of a sample for t 4 t L was set at approximately 2 8 min 1, but in the area R! it was spontaneously reduced to 41:3 8 min 1 as the outcome of sequential selection of heat. The process! R is the speed process of self-heating of sulfur because of initial spontaneous crystallisation. Spontaneous sulfur crystallisation finishes at t R, which corresponds to a site on the thermogram t(t). Therefore, temperature t R should be considered as a usual `point' of solidification. However, t R of sulfur is badly reproduced, as is seen in table 1. Besides, the closing stage of sulfur crystallisation, R!, considerably differs from an expected ideal, R! id at t R ˆ const, and is stretched across the interval Dt R It should be noted that the presence of the point was fixed for the first time, the temperature, t,was83 8, and this value did not appear among the singular points of sulfur before (Meyer 1965, 1976). Table 1. Measured temperatures of crystallisation of sulfur. Number of samples t A =8 t P =8 t 1 =8 t R1 =8 t 2 =8 t R2 =8 t D =8 t =8 and test S S S S S S S S a a Recalescence is not fixed in the given test. Similar nonideality of a solidification curve was observed for all investigated sulfur examples (figures 2, 3). However, the solidification curves were not reproduced even in sequential experiments with the same sample (figure 2). Figure 2 shows the features and compares the solidification curve and the melting curve belonging to the same sulfur sample, S4. A significant difference between process! R and the process observed in figure 1 is shown here. However, the main result is that the melting process H! B! M happens in a different temperature interval. The end of the melting point (point M in figure 2) in all experiments was closer to point P than to point L. The measured temperature, t R, was not below 130 8, and reached (that is close to a triple point, L=b=vapour, of sulfur) in one of the experiments. The position of marked melting start points (H) was close to the reference data of melting points a-s and g-s (Emsley 1991). Figure 3 shows the most impressive crystallisation features of a certain sulfur sample. Such features were observed in three experiments out of the eight. A feature of the crystallisation process here is the presence of two points, R1 and R2, and two temperatures, t R1 and t R2, which correspond to point R in figures 1 and 2. Point R1 in experiment S1.1 in figure 3 is poorly distinguishable in view of overwriting on sites L! and! R1, but the thermogram, t(t), of this experiment reliably recorded the start of undercooling of the melt (L! ) and its recalescence (! R1). Obviously, the second self-heating of a crystallised sample finishing at point R2 happened in an interval of two-phase conditions. The process of second self-heating was also observed for the first time, and the reasons for it are not vague. Some other features

4 464 A S Basin, B G Nenashev 15 ETP Proceedings page 448 of the melting ^ crystallisation processes of sulfur were also observed. Many of them are given in our detailed initial paper (Basin et al 1996), but their analysis requires additional experiments. Only the following is clear. Most of the features of melting ^ crystallisation processes of high purity sulfur are a consequence of polymorphism: the existence of independently identified a, b, g, and other crystalline states of sulfur and their slow transformations in an interval of 83 ^ In particular, the processes R! in figures 1 ^ 3 are stipulated by a parallel course of liquid l-s crystallisation under crystalline state b-s 8 and transformation of b-s 8 into a-s 8, which begins for t a=b 95:4 8 (Meyer 1965, 1976; Tackray 1970) and finishes at t The processes of undercooling, L!, recalescence,! R, and b! a transformations can also be influenced by nucleation centres g-s and crystalline phase g-s. On the whole, the results indicate a strong dependence of the melting and crystallisation characteristics of sulfur on the purity of certain samples. Moreover, the observable melting processes can be stimulated by a degree of completion of b! a transformation in a certain sulfur sample, stored for a long time at a room temperature of 20 ^ However, all this has a relatively small effect on the integrated characteristics of a specific volume (figures 1 ^ 3) and density of sulfur (figure 4). r=g cm a=l g=l b=l a=b R H 8 11 l=p M P Figure 4. Density ^ temperature plot of sulfur within intervals of melting ^ solidification and in their vicinity. Solid lines illustrate experimental results; dashed lines demonstrate results of extrapolation and generalisation: 1, a-s monocrystal; 2, a-s polycrystal; 3, a-s slow heating with a! b continuous transformation; 4, amorphous S; 5, b-s monocrystal; 6, b-s polycrystal; 7, amorphisation of undercooled l-s; 8, liquid S (l-s); 9, experimental crystallisation curve in test S3.1; 10, experimental cooling curve of solid sample S3.1; 11, generalised melting curve with respect to most of the tests on S4.2; a=b, g=l, a=l, b=l are temperature positions of sulfur basic transformations. Vertical lines show boundaries of experimental errors. 4 Generalised data In table 1 the experimental data on crystallisation temperatures and other transformations of sulfur are represented. Table 1 also includes the data on an upper bound of temperature of liquid sulfur in the given experiment (t A ). The data of t D in table 1 correspond to features of the experimental curve, characteristic for the start of formation of porosity in the hardening sulfur sample. Figure 4 shows our calculated scheme of ranges for the majority of observed states and processes for heating and cooling of sulfur, and singular points as well. Table 2 is similar to figure 4, but some differences for the data of densities for single and polycrystalline conditions are obvious as well as for liquid and amorphous sulfur. While compiling table 2 and figure 4 we also used the data from reviews by Meyer (1965, 1976), some modern papers (Kennedy and Wheeler 1983; Winter et al 1990), and others. In table 3 our experimental data on integrated magnitudes of volume modification for melting are represented. These data update numerical figures and the character of the processes, which were studied only by Kopp (1855), To«pler (1894), and Tamman (1903).

5 Melting ^ crystallisation and polymorphic transformations of sulfur ETP Proceedings page 449 Table 2. Sulfur density, r=g cm 3, in reference and singular points, recommended data. State 20 8 Temperatures of singular points= g-s 2.19 a-s sc a-s pc a-s b-s sc b-s pc l-s, l-s sc, monocrystalline solid state; pc, polycrystalline solid state; a, amorphous solid state; l, liquid state. Table 3. The volume and density change of sulfur at melting (L and refer to points L and in figures 1 ^ 3). Process t m =8 DV m V L V of melting cm 3 g 1 dr m r L r =r % this Kopp To«pler Tamman work (1855) (1894) (1903) a! l ± ± 9.8 7:5? b! l ± ± :5? g! l : Table 3 shows that data on the volume change, DV m, and density change, dr m,at melting, known earlier, corresponded to the b! l transition. The scattering of our data for DV m can be explained by the indeterminacy in a=b phase transformation of samples before the start of the next experiment with sulfur melting. This can be observed in other experiments, if the time of endurance in a solid state was not long. Quite a stable structure is the structure of liquid l-s only. The density of l-s can be represented by the following linear dependence: r l-s =g cm 3 ˆ 1:827 8: :4 Š, , with an error less than 0.6%. All other data, r(t), in table 2 have an error higher than that magnitude (except values for 20 8 ). However, to exclude distortion of the transformation pattern in figure 4, they are shown with an identical number of significant figures. References Alwarenga A D, Grimsdith M, Susman S, Rowland S, 1996 J. Phys. hem ^ Basin A S, Alekseev V A, 1991 J. Non-ryst. Solids 117/ ^ 239 Basin A S, Kolotov Ja L, Nenashev B G, Polovinkina R A, 1996 Russ. J. High Purity Substances (6) 51^69 Basin A S, Kolotov Ja L, Stankus S V, 1979 High Temp. ^ High Press ^470 Bellissent R, Descoª tes L, Pfeuty P, 1994 J. Phys., ondens. Matter 6 supplement 23A A211^A216 Drotning W D, 1981 High Temp. ^ High Press ^ 458 Emsley J, 1991 The Elements (London: larendon Press) Kennedy S J, Wheeler J, 1983 J. hem. Phys ^ 1527 Kopp H, 1855 Liebig Ann ^ 232 Meyer B (Ed.), 1965 Elemental Sulphur: hemistry and Physics (New York: Interscience) Meyer B, 1976 hem. Rev ^ 388

6 466 A S Basin, B G Nenashev 15 ETP Proceedings page 450 Tackray M, 1970 J. hem. Eng. Data ^ 497 Tamman G, 1903 Kristallizieren und Schmelzen (Berlin: Verlag von Johann Ambrosius Barth) To«pler M, 1894 Wied. Ann ^ 378 Vezzoli G, Walsh P J, 1977 High Temp. ^ High Press ^ 349 Wallis J, Sigalas I, Hart S, 1986 J. Appl. rystallogr ^ 274 Winter R, Egelstaff P A, Pilgrim W, Howells W, 1990 J. Phys., ondens. Matter 2 SA215 ^ SA218 ß 2000 a Pion publication printed in Great Britain

A comparison of a novel single-pan calorimeter with a conventional heat-flux differential scanning calorimeter

High Temperatures ^ High Pressures, 2, volume 32, pages 311 ^ 319 15 ECTP Proceedings pages 297 ^ 35 DOI:1.168/htwu26 A comparison of a novel single-pan calorimeter with a conventional heat-flux differential