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Theses and Dissertations
1915
Tellurium crystals ; their production and certain
electrical properties
Wilbur Earle Tisdale
State University of Iowa
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Recommended Citation
Tisdale, Wilbur Earle. "Tellurium crystals ; their production and certain electrical properties." MS (Master of Science) thesis, State
University of Iowa, 1915.
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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
of
S c i e n c e
July 30, 1915
These researches on Tellurium may properly he
divided into four sets of experiments, and each set taken
up and discussed separately.
I.
II.
III.
Production of Tellurium Crystals.
Light Sensibility of Tellurium Crystals.
The Resistivity and its Temperature
Coefficients.
IV.
Thermo-electric Power of Tellurium.
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 amor­
phous 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 conduc­
tivity 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 sub­
limation, 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 crystal­
lized in the hexagonal system, but showed considerable
interference in their formation because there were no
absolutely perfect crystals.
1.
The diameter of the
Physical Review; N. S., Vol. IV., No. 2,
August, 1914.
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 pres­
sure.
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
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 atmos­
phere 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
maintain this constant temperature, a device represen­
ted in Figure II. was used, in which the parts and
construction are shown, and Figure III. shows diagrammatically the connections.
Figure III.
Figure II.
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.
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.
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.
There­
fore, in these experiments, tests were made to estab­
lish 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 6 x1 0 “5 ohms, or .0 0 1 $, 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. 494-5.
The light was not allowed to fall on the whole
crystal and its contacts, because a very small differ­
ence 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 there­
fore immersed in glycerine and parafine in the experi­
ments.
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 resist­
ance of a crystal of tellurium1-to which platinum had
1.
No record of experiments on Tellurium
crystals has been found.
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 con­
tinually.
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.
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 conduc­
tance 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.
f i g u r e vii.
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 resist­
ance 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.
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 4-130°C.; and from-H70°C. to +380°C. —
which present three different resistance coefficients,
two minus and one plus, considering the lower tempera­
ture of each range as R0 , and cover practically the
same range of temperature, 1 2 0 °.
These coefficients, for use in the typical
formula R^ = R Q ( i f at), are, using the mean of
several different crystals:
R-L between
-140 and -10
« + .00170
R 2 between
+10 and +130 A2
= - .00584
R^ between
fl70 and 4380 A^
• - .0 0 2 9 8
The above results are for individual crystals of
Tellurium, and in order to determine whether the tempera­
ture 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
state of any sample other than crystals, no consistent
results can be obtained from either supposedly amor­
phous 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 re­
sults 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 . 0 1 5 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.
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 Telluriumplatinum over the same range, and the former was there­
fore 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 thermo­
electric power of a thermo couple is the electromotive
force produced by one degree C. difference in tempera­
ture 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.
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
2
and is seen to be a parabola.
(3 )
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 « .3 9 6
where A is in micro volts and B in micro volts per
degree C.
The value of -A is from these values found
B
to he
-A = -5 7 3
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 thermo­
electromotive 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.
of platinum and copper, where one junction is at 0°C.
and the other at-fl00oC., would give a total electro­
motive 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 .0 1 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.
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:
5.
from -140 to -10
*+.0017
from +10 to +130
*^.00584
from +170 to +380
*-.0 0 2 9 8
The specific resistance of Tellurium
crystals made in hydrogen at 450°C. and two and onehalf atmospheres pressure is . 0 1 5 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.
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 Telluriumplatinum junction were compared.
The one junction of
the Tellurium-platinum couple gave as large a galvan­
ometer 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 1 2 7 .5 ,
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.
1.
Curve VIII. is
Roscoe and Schorlemmer, "Treatise on Chemistry "
p. 482-3.
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 1 5 0 0 break where
the change is more gradual and far less in amount than
at 0° when the curve undergoes a complete reversal.
At 45 0 , 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 con­
ducting 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.