Tellurium crystals ; their production and certain electrical properties

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1 University of Iowa Iowa Research Online Theses and Dissertations 1915 Tellurium crystals ; their production and certain electrical properties Wilbur Earle Tisdale State University of Iowa This work has been identified with a Creative Commons Public Domain Mark 1.0. Material in the public domain. No restrictions on use. This thesis is available at Iowa Research Online: Recommended Citation Tisdale, Wilbur Earle. "Tellurium crystals ; their production and certain electrical properties." MS (Master of Science) thesis, State University of Iowa, Follow this and additional works at:

2 Tellurium Crystals Their Production and Certain Electrical Properties By W. E. Tisdale A Thesis submitted to the faculty of the Graduate College of the State University of Iowa in partial fulfillment of the requirements for the degree of M a s t e r o f S c i e n c e July 30, 1915

3 These researches on Tellurium may properly he divided into four sets of experiments, and each set taken up and discussed separately. I. Production of Tellurium Crystals. II. III. Light Sensibility of Tellurium Crystals. The Resistivity and its Temperature Coefficients. IV. Thermo-electric Power of Tellurium.

4 I. Production of Tellurium Crystals The question of producing Tellurium crystals was first suggested by the striking changes produced in the element Seleniun?- when it is changed from an amorphous into a crystal state. Crystal masses give uncertain results because of the fact that the physical and electrical properties of crystals differ along the different axes, and the attempt was therefore made to produce single crystals in order to know with certainty that the results were not the summation of these different effects. The most pronounced effect in Selenium crystals is their change of electrical conductivity when acted upon by light of different wave lengths and because of the fact that in the periodic system of atomic weights, Tellurium and Selenium are in the same group, it was thought that it, too, might show the property. The crystals were produced by the process of sublimation, the variable factors being the atmosphere, pressure, and temperature in which they formed. The first crystals produced were made in air at a pressure of 1 cm. of mercury and about 400 C. They crystallized in the hexagonal system, but showed considerable interference in their formation because there were no absolutely perfect crystals. The diameter of the 1. Physical Review; N. S., Vol. IV., No. 2, August, 1914.

5 crystals was about the same as the length -- from one to two millimeters. They are opague to ordinary light, and show a very high reflecting power, although no quantitative measurements were taken. Crystals were then produced in an atmosphere of hydrogen, under a pressure of two and one-half atmospheres at 450 C. These crystallized in the same system as those in air -- the hexagonal but with the difference that the ratio of diameter to length of crystal was greatly changed. While in the vacuum the ratio was approximately unity, in the hydrogen the length was from 20 to 100 times the diameter. This difference might have been due to either of two factors -- the hydrogen atmosphere, or the increased pressure due to heating the hydrogen gas. To test this fact, dry hydrogen was run through the tube for a half hour and then evacuated, so that the pressure of formation was ^0 cm. of mercury at 4^0 C. The resulting crystals were a mixture of the kinds typical to those of the vacuum, and the high pressure hydrogen -- the difference being in that the long crystals were not as long as those in the high pressure hydrogen, and the short ones were shorter, but thicker than those of the low pressure. There seems, therefore, to be a relation between the ratio of the length to diameter of crystal and the pressure uhder which they form, and this relation is

6 that the higher the pressure the greater the ratio of length.to diameter of crystal. The question of the hydrogen playing a part in the formation is taken care of by the fact that Tellurium and hydrogen do not combine at this temperature and that the hydrogen atmosphere simply prevents the formation of one of the many Tellurium oxides which would prevent crystallization. Figure I. Figure I. is a photograph of crystals produced in hydrogen, and is actual size. It is highly important that the temperature of formation of the crystals should be as nearly constant as possible, because crystals formed at different temperatures, and consequently different pressures, show varying results, and if, therefore, a single crystal be the sum of parts formed at differing temperatures, experimental results would be of small value, because they could not be duplicated. To

7 maintain this constant temperature, a device represented in Figure II. was used, in which the parts and construction are shown, and Figure III. shows diagrammatically the connections. Figure II. Figure III. In the oven, which consisted of a porcelain tube 5 cm. in diameter, wound with platinum iridium wire and insulated by asbestos, there was a very steep temperature gradient, as shown in Figure IV.; and by Figure IV.

8 placing this regulator at A, in the lower part of the temperature curve, the maximum temperature, at B, could he maintained within 2 to 3 degrees.

9 II. Light Sensibility of Tellurium Crystals Tellurium cells hare been investigated for light sensitiveness by Adams and by Siemens1 with contrary conclusions, but apparently there is no record of experiments with crystals for the phenomena. Therefore, in these experiments, tests were made to establish the fact of the light sensitiveness of the Tellurium crystals. Some were mounted, by welding platinum wires to them, and others by the method of pressure contacts, and in all cases the results were the same, that the electrical resistance did not change by as much as 6x10 5 ohms, or.001$, when illuminated at points other than at the point of contact by a light which did not appreciably heat the crystal. The arrangement of the apparatus is shown in Figure IV a Figure IVa. 1. "Allatropic forms of Selenium," by A. P. Saunders, p

10 The light was not allowed to fall on the whole crystal and its contacts, because a very small difference in area of the two ends of the crystal caused a thermo current due to the different rates at which these ends heated up; and it was therefore necessary to screen the contacts as shown in the diagram. The thermo-electric power, hereinafter discussed, is so great that these crystals cannot be balanced in a bridge if exposed to the slightest currents of air or changes of temperature, because of the different heat capacities of the two ends, and they were therefore immersed in glycerine and parafine in the experiments. The method of procedure was to balance the bridge and note the deflection of the galvanometer when the crystal was in the dark, then illuminate the crystal at its center, and again note the deflection. The reverse, too, was tried, in that the crystal was illuminated and the bridge balanced, and then the light cut off. In all the experiments, there was no apparent change in the deflection of the galvanometer in changing the crystal from dark to light, or from light to dark, using the ordinary light rays. III. The Resistivity and its Temperature Coefficient In these first experiments on finding the resistance of a crystal of tellurium1-to which platinum had 1. No record of experiments on Tellurium crystals has been found.

11 been welded, the results were inconsistent and varied to a certain extent with the actual time involved in balancing the bridge. If the crystal was allowed to remain in the bridge circuit, the deflection of the galvanometer was not constant, but fluctuated continually. It was first thought that this peculiarity was due in some way to poor electrical contact, but when constant pressure contacts were employed, the results were identical in character, differing only in degree. It thus seemed that the effect of the passing of a current through the crystal was to change its resistance, and therefore a series of experiments was undertaken in which were measured the current through the crystal, and the voltage drop across it at the same instant, from which the resistance was calculated. Figure V.

12 Figure V. shows a crystal 7 mm* long. *4 nun. average diameter, placed in air. The I-E curve was plotted from the data taken, and the curve is the conductance curve calculated from the observed data. Figure VI. Figure VI. is a curve showing the same relations for a crystal 5 long,.4 mm. of diameter, placed in hydrogen. figure vii.

13 Figure VII. is for a crystal 4 mm. long,.3 mm. in diameter, placed in glycerin. The character of the I-E curves is alike in that: (a) they at first show a decided increase in E with a small increase in I; (b) they show a maximum E at which point the E decreases with increase of I; (c) they show a minimum E at which point E again increases with increase of I. The conductance curves will of course show a relative similarity, and it is to be noticed that they show at all times an increasing conductance, indicating a constantly decreasing resistance with increasing I. There seemed no apparent reason for the peculiar shape of these I-E curves, and it appeared certain, from the fact that the same type of curve was obtained in the different gases and liquids about the crystal, that there could not be a compound of Tellurium that was responsible; and it was therefore decided that the effect could be due to nothing other than increase of temperature, the current being the heating agent. With this point as an objective, experiments were undertaken to show the relation between resistance and temperature.

14 Figure VIII. Figure VIII. is such a curve, and. the crystal, to which platinum contacts had been welded, was taken in a parafine bath from the temperature of liquid air (-14K) C.) to *380 C. It will he noticed that the curve presents three distinct parts, which are approximately straight lines from -10 C. to -140 C.; from -4*10 C. to C.; and from-h70 C. to +380 C. which present three different resistance coefficients, two minus and one plus, considering the lower temperature of each range as R0, and cover practically the same range of temperature, 120. These coefficients, for use in the typical formula R^ = RQ (if at), are, using the mean of several different crystals:

15 R-L between -140 and -10 « R2 between +10 and +130 A2 = R^ between fl70 and 4380 A^ The above results are for individual crystals of Tellurium, and in order to determine whether the temperature effect was different in crystals and in either amorphous or crystal masses of Tellurium, small wires of Tellurium were made by splashing the molten material upon a glass plate. These wires were then mounted against platinum and the same experiments repeated, from which (Table IX.) it is seen that the curves are Tables VUIb. and IX. of the same character, but that the temperature of maximum resistance has been shifted from 0 to 100 C. Inasmuch as this shifting is apparently due to the state of the material, and the uncertain!ty of the

16 state of any sample other than crystals, no consistent results can be obtained from either supposedly amorphous or crystal aggregate samples. The question as to whether or not the fact of having platinum sealed in the Tellurium crystals might not be responsible for a part of this curve, due to their different coefficients of expansion, was tested experimentally by using pressure contacts, and the results are in Table VIII. (b). These experiments were not followed throughout the range of the other, but they indicate that the changes occur at the same temperature in both methods. The disadvantage of this method of contacts is that there is uncertainty as to pressure and cleanliness of the contacts, and it is difficult to repeat the curves. The mean specific resistance of Tellurium crystals made in hydrogen at A^O C,t was found to be.015 ohms at 25 C. along the crystal. It is probable that this is not a correct value for specific resistance across the crystal, although no exact determinations were made on this point. The specific resistance of hard drawn copper wire is given in Smithsonian tables as 16x10"?, which is 1 0 "4 times that of these crystals.

17 IV. Thermo-electric Power of Tellurium In the investigation of the thermo-electric power of tellurium, it was not convenient to use crystals because of their brittleness and small size, and it therefore became necessary in some manner to produce a long thin wire. The only way found that was at all successful was to fill a small capillary tube with the Tellurium and join this in circuit by freezing either copper or platinum in it at its ends. The tube was bent in a V-shapei for the greater convenience of placing it in the liquids used to vary the temperature. It was found that a Tellurium-copper junction gives a greater thermo-electromotive force than Tellurium- platinum over the same range, and the former was therefore used. The ordinary method of procedure was followed in that one end was kept in melting ice at 0 C. and the other run through the various ranges. The thermoelectric power of a thermo couple is the electromotive force produced by one degree C. difference in temperature between the junctions, and is expressed by the equation de : Q s A i Bt (l) dt where Q is the thermo-electro power, and A and B are constants, and t is the mean temperature of the junctions.

18 If this equation he integrated, it gives B a At * Bt2 f C (2 ) 2 Where E is total electro-motive force for a difference of temperature of the junctions of t degrees. When the difference of temperature of the junctions is zero, there is no electro-motive force in the circuit and C in (2 ) becomes zero, hence the equation may be written E a At + Bt2 (3 ) 2 and is seen to be a parabola. If (l), the first derivative of this equati on be placed equal to zero, that is, the thermo-electric power equal to zero, the value of t = -A, which is known as the neutral point of B the thermo-electric curve. Using the data of Table X. in equation (3), by the Table X. method of least squares, the values of A and B were found to be A = 227 B «.396

19 where A is in micro volts and B in micro volts per degree C. to he The value of -A is from these values found B -A = -573 B and is in degrees Centigrade. It will be noticed that this temperature can never be reached, and therefore that there is no actual neutral point in the thermoelectromotive power curve of Tellurium. The true significance of these values of A and B are shown to better advantage by comparing them with those of the elements more commonly used in making thermo-couples. A copper-platinum1 couple is used for comparison, and equation (l) for Tellurium, copper, and platinum is plotted in Figure XI. A t^dlrmo-couple made Figure XI. 1. "Smithsonian Tables',' 1914, p. 288.

20 of platinum and copper, where one junction is at 0 C. and the other at-fl00oc., would give a total electromotive force represented by the area ABEF, while one of Tellurium-copper would give an electromotive force represented by the area ACDF, which is about sixty-six times greater than ABEF. If a copper-tellurium-thermo couple has one end at 0 C. and the other at 50 C., the thermal electromotive force generated is enough that it can be read quite accurately on a voltmeter reading to.01 volts. These values of A and B are lower than those found by Haken^, but can be explained by his experiments in that the thermo-electric power is a function varying with the rapidity of cooling of the Tellurium in the capillary. 1. "Ann. der Phys.", Vol. 32, p. 291, 1910.

21 SUMMARY It has been shown that: 1. Tellurium crystallizes in but one I system -- the hexagonal. 2. Within limited ranges, the dimensions of the crystals can be varied by varying the pressure in which they form. 3* Tellurium crystals made in hydrogen at 4-50 C. and two and one-half atmospheres pressure do not change their resistance when acted upon by light. 4. The resistance of Tellurium crystals is a function of the temperature, and shows three definite temperature coefficients: from -140 to -10 * from +10 to +130 *^ from +170 to +380 * The specific resistance of Tellurium crystals made in hydrogen at 450 C. and two and onehalf atmospheres pressure is.015 ohms along the crystal. 6. The thermo-electric power of Tellurium is much greater than that of any of the other elements commonly used in making thermo couples.

22 CONCLUSION The large thermo-electric power of Tellurium, coupled with its relatively low resistance, offers a profitable field for experiment in thermo-pile construction, provided Tellurium can be made ductile enough to be drawn into thin wires. An experiment not recorded in this thesis was made in which a Coblenz thermo-pile and one Tellurium- platinum junction were compared. The one junction of the Tellurium-platinum couple gave as large a galvanometer deflection as did the standard thermo-pile at the same distance with the same illumination. Tellurium is classified as a metal, and the best determinations of its atomic weight are given as , although there seems as yet to be an opinion1 that Tellurium consists of two different elements, one of atomic weight about 1 2 7, and the other heavier. In Curve VIII. it will be observed that below 0 C. the curve is typically that of a metal, in that its resistance decreases with decreasing temperature. Above 0 C. the curve is not that of a metal, but suggests rather an alloy. No definite experiments were made on molten Tellurium, except to note that the resistance increases with temperature. Curve VIII. is 1. Roscoe and Schorlemmer, "Treatise on Chemistry " p

23 a cubic curve tending towards minus infinity at one end and it should therefore tend towards plus infinity at the other, which agrees with the experimental data, and is again the property of a metal. There is also the possibility that at 0 and 1^0 the breaks in Curve VIII. are due to a change in the crystal structure which changes the resistance. This is more reasonably to be applied at the 1500 break where the change is more gradual and far less in amount than at 0 when the curve undergoes a complete reversal. At 450, the melting point, the curve undergoes another complete reverse, due to the melting of the crystal, which is a. radical change in the structure of the conducting Tellurium, and it seems that at 4 ^0 and at 0, since the curve undergoes at each of these points a reversal, that the causes might be similar -- a marked change in the crystal -- but in the solid state it is impossible to detect changes in so small a crystal, and no conclusion other can be made.