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Section 3. Technical sciences
https://doi.org/10.29013/AJT-20-1.2-16-20
Onarkulov Karimberdi Egamberdiyvich, Physics department Ferghana State University of Ferghana, Uzbekistan
Usmanov Yakub, Physics department Ferghana State University of Ferghana, Uzbekistan Gaynazarova Kizlarhon Isroilovna, Physics department Ferghana State University of Ferghana, Uzbekistan Azimov Toolanboy Ma'rifjonovich, Physics department Ferghana State University of Ferghana, Uzbekistan E-mail: tmazimov@mail.ru
SEMICONDUCTOR SENSOR FOR DETECTING VOLUME CHANGES AT LOW TEMPERATURES
Abstract. The article discusses the technological aspects of the production of stoichiometric alloys with the addition of Pb with high thermoelectric properties and their use in the film version as sensors for volume changes at low temperatures.
Keyword: adhesion, deformation, defect, alloy, thermoelectricity, strain sensitivity.
Introduction. To study the strength of struc- reference value for a plurality of homogeneous tures, it is necessary to measure local deforma- sensors of primary transducers. tions, forces, pressure, temperatures, displace- Material and Methods: Upon receipt of semi-ment, control the moments of occurrence of conductor materials, the thermoelectric properties defects and the speed of their development. At of alloys depend not only on the composition, but the same time, special requirements are imposed also on the purity of the starting components. Thereon measuring devices: ensuring high metrologi- fore, materials obtained from various batches of raw cal characteristics of each individual measure- materials often differ in their thermoelectric prop-ment, provided that the measurements are mas- erties. In practice, it is convenient to determine the sive. Mass measurements are understood to mean characteristics of the feedstock by the thermoelec-measurements during which a correspondence is tric properties of the base (undoped material) fused established between the measured value and the from this feedstock [1].
As is known, when Bi2Te3 and Bi2Se3 are alloyed the thermoelectric properties of the alloy change with a change in the batches of the feedstock, since while the properties of the base itself also change. Naturally, when the thermoelectric properties of the base change, the optimal concentration of the dopant, should also be changed. Usually, the optimum concentration of a dopant for a substrate with certain properties is found empirically by conducting a gray melt with a different concentration of dopant. For alloying Bi2Te3 and Bi2Se3, bases with the following thermoelectric properties are selected: electrical conductivity о = 200-600 Q-1-cm-1, thermoelectric coefficient a = 240-200 ^V/deg.
Discussion: The initial components chosen by us for the preparation of the alloy of the required composition were of the following purity: bismuth GOST 10928-64 brands В4-00, tellurium Г0СТ9514-60 brands ТА-1, antimony ГОСТ 1069-62 brands С-0 и lead С-00. Based on the works [2], the composition of the solid solution corresponding to 74 mol% was chosen as the basis for the study. Sb2 Te3 and 26 mol% Bi2 Te3.
To obtain a doped hole conductivity material in a quartz crucible with a gate, the following charge composition was taken: Bi - 16.179% the weight, Te - 56.993% the weight, Sb - 25.828% the weight, corresponding to the following thermoelectric properties of the base: a = 1000 П1 • cm-1, a = 200 ^V/deg. Lead was used as an alloying additive.
The dopant - lead, was located in the crucible between equal layers of tellurium.
To clarify the effect of additives on the main characteristics of the alloys, the following studies were carried out.
Effect of dopant concentration on thermoelectric and electro physical properties.
To determine the optimal concentration of the dopant, lead was introduced into the charge in an amount of from 0.05% weight to 0.25% weight.
Figure 1 shows the change in electrical conductivity (a), thermoelectric coefficient (a), thermoelectric power (a 2 a) and loss (M).
Figure 1. Investigation of the influence of the concentration of the Pb dopant on the properties of Bi2Te3 - Sb2Te3 material and the amount of losses
With increasing concentration of the dopant, the electrical conductivity and thermoelectric coefficient change almost linearly.
Optimum thermoelectric properties of the alloy are obtained with the introduction of 0.05% weight, dopant. In this case, specific conductivity a = 1500 Om-1-cm-1, thermoelectric coefficient a = 175 ^V/deg.
The percentage loss M decreases with increasing concentration of the dopant, which is associated with the formation on the melt surface of the thinnest layer of lead telluride, the vapor elasticity, which is much less than the vapor elasticity of antimony and bismuth telluride.
Temperature dependences of thermoelectric properties of Bi2Te3 - Sb2Te3 doped with lead.
To study the temperature dependence of the thermoelectric properties of the alloyed material, 0.05% lead weight was introduced into the base (ff = 1000 Om1 • cm1, a = 200 yV/deg.). Semi-elements were made from the ingot obtained, in which the changes in electrical conductivity (ff), thermoelectric coefficient (a), dielectric constant (%) and thermoelectric power (z = a 2 ff) were studied in the temperature range from 200 °C to 3000 °C (Fig. 2).
a
Ом'1-cm"1
mkB град
Z10 град
■-X10
1350 Kan
\ \ CMC град
120Ц ■210 \ ^Д. ^ -
1050 ■190
900 170 \ \ -
750 ■150 \ \
600 130 6 ■
X
450 110 \ Б ■
с
300 90 4 ■
150 -70 ...... 3 ■
3,2 2,9 2fi 23 2ß
1Л 1,1 0,8 ■0.5
0 50 100 150 200 250 300_^jt)c
Figure 2. Change in the thermoelectric properties of Bi2Te3 - Sb2Te3 doped with lead as a function of temperature
b) Investigation of the uniform distribution of thermoelectric properties along the length of the ingot.
When Bi2Te3 - Sb2Te3 was doped with lead, stirring was not performed. Therefore, the change in thermoelectric properties along the length of the ingot was studied. To determine the uniformity of distribution, 500 g of Bi2Te3-26 mol% and Sb2Te3-74 mol% alloys were fused, which corresponds to the following mixture composition: bismuth - 80.8940 g, tellurium - 284.9660 g, antimony - 134.1400 g and lead -
0.2500 g. A lead alloy was placed between two layers of tellurium. The mixture was melted at a temperature of7500 C for 30 minutes.
The obtained ingot was a cylindrical rod 9 cm long and 3.2 cm in diameter. The loss of material during smelting due to evaporation was 0.04% by weight. The ingot was cut into 10 equal parts, and semi-elements were made from each such part. The results of measurements of thermoelectric properties along the length of the ingot show good uniformity, which shows that alloy mixing during melting is not required (Fig. 3).
Figure 3. Investigation of the homogeneity of thermoelectric properties of a Bi2Te3 -- Sb2Te3, Pb ingot along its length
In order to study the tens metric properties of semiconductor film structures based on Bi2Te3-- Sb2Te3, a mechanical powder was made from the material of the alloy, we introduce 0.05% weight, and the dopant is lead. To obtain strain gauges operating in a wide temperature range, strain-sensitive sensors were obtained by thermal vacuum spraying under vacuum in the order of ~ 10-4 ^ 10-6 torr, which has the properties of a dry vacuum. Strain-sensitive semiconductor films were obtained from (BiSb) 2Te3: Pb on a polyamide film [2].
The substrate was heated to Tn = 90 °C. Spraying was performed at a temperature of ~ 700 °C from tantalum boats. The film thickness was d = 4-5.2 ^m. The resulting films have good adhesion to the substrate and possess the necessary electromechan-
circuit with a B7-23 instrument, ohmic contacts made of silver were applied in vacuum using special stencils.
Table 1 shows the values of the Hall coefficient Rx, mobility ^r, and the Hall concentration px mea-
ical properties for strain elements. The resistance of sured at room temperature for films obtained at the samples was measured by a two-probe electric Tn = 90 °C.
Table 1.
No. Thickness, pm R, -10-2 — KA m-2 CM p B ■ c P , 10 20cm-3 x '
16 4.9 1.2 49 5.2
38 4.4 0.9 37 7.3
76 5.2 1 42 6.25
109 5.1 0.95 40 6.4
9 S 10
Figure 4. Relative change in resistance from elongation of the film
The relative change in resistance under the influence of uniaxial deformation was measured at temperatures of 300.77, 4.2 ° K (Fig. 4).
The results of the studies showed that (BiSb)2Te3 films with an admixture of 0.05 atomic percent of Pb thermally sprayed on a polyamide substrate at room temperature have a high strain sensitivity of K = 100 - 1000, where K is the co-
efficient of strain sensitivity equal to the ratio of the relative change in resistance when films are deformed to elongation
£ AR iAL
E =-/ —
RL
We studied the temperature coefficient of film resistance in the temperature range 300-0.5 K. The films were cooled in a refrigerator with He 3 vapor evacuation to obtain a temperature of up to 0.5 K [3]. A change in the resistance of undeformed films during cooling was not detected (film dimensions 4 • 6 mm2, resistance 50 0 m).
The strain sensitivity of the films was measured at low temperatures up to 4.20 K using a cryogenic insert. The temperature of the strain film was changed by raising the cryogenic insert above the level of liquid He4 in the STG-25 transport vessel. Films (4 • 10 mm2 in size) were glued onto a bronze plate, which was bent using a special installation [4].
Elongation was determined by the formula
Al _ 6dAy
T " ~LT
where d is the film thickness, L is the distance between the stops, A is the mixing of the plate.
Table 2.
T, K 300 77 4,2
K 444 944 777
The results showed that the strain sensitivity of the films at helium and nitrogen temperatures is greater than the values at room temperature (Table 2).
The saturation of strain sensitivity at helium and
f Al \
nitrogen temperatures occurs faster, — 4 -10 4
f Al \ V l J than at room temperature — 7 • 10-4 .
V l J
To find out the cause of the spread of strain resistance, the noise voltage spectrum was measured at given currents flowing in the film. Experimental results showed that the source of voltage fluctuations is the noise resistance of the sample. To clarify the contribution of current contacts to the sample noise, we compared the noise voltage of the initial film with the contacts and the corresponding value of the noise signal obtained from the sample with a partially removed film and the same contacts. With partial removal of the film, an increase in noise was observed approximately in proportion to the increase in film resistance. Therefore, the film is responsible for the appearance of the noise signal, not the contacts.
At a temperature of 4.2 K, the character of the noise signal (dependence on frequency, current, and film resistance) did not change compared to values at room temperature.
In order to compare the relative value of the mean square fluctuation of the film resistance during tens
metric measurements and noise measurements, an estimate was made
R
—< if2P(f)df = —<lelnA = 1,5-10-
where R—, Rk is the average resistance of the sample, respectively, for noise and strain measurements, I is the current through the sample, p(f) is the spectral density of voltage fluctuations taken from the experiment, 1/f is the maximum characteristic time of the device B7-23, 1/f2 - the minimum characteristic time of the device B7-23.
Result: The fluctuations in the resistance of the
sample during tens metric measurements are greater, SR
—— ~ 0,1 than received during noise measurements
Rk SR
'h 10-5 and this is due to the mechanical backlash
R
of the screw. By eliminating the contribution of mechanical fluctuations in strain gauge films,
AZ l
SRh R
10 8 one can achieve a resolution of elon-
gation at room and helium temperatures.
Conclusion: This value can be compared, for example, with the relative elongation of metals during the superconducting transition, which indicates the possibility of recording volume changes during the normal metal - superconductor transition using the studied tens metric films — ~10-7 [5].
References:
1. Ioffe A. F. Semiconductor thermocouples. - M.-L. 1957.- 60 p.
2. Atakulov B. A., Abdullaev E. A., Ya A. Afuzov, Bilyalov E. I., Rakhimov A. U. Deformation effects in in-homogeneous semiconductors.- T. Fan. 1978.
3. Onarkulov K. E., Ubaidullaev M. I. Electrophysical properties of polycrystalline films (Bi2xSbx)2Te3 at temperatures of 0.5-300 K. Abstracts of the report of the regional scientific and practical conference. -Fergana. 1990.
4. Atakulov B. A., Zhurkin B. G., Ubaidullaev M. I., Tskhovrebov A. M. Investigation of the strain sensitivity of (BiSb) 2Te3: Pb films at temperatures of 0.5-300 K. Preprint - No. 89. - FIAN.- M. 1984.
5. Olsen J. L., Rohrer H. Helv. Phuzs. Akta. 1960.- 33 p. - 872 p.
