9.1. Goal of the work
Determination of the dependence of the thermoelectromotive force of a thermocouple on the temperature difference between the junctions.
In a closed circuit (Fig. 9.1), consisting of dissimilar conductors (or semiconductors) A and B, an electromotive force (emf) E T arises and a current flows if contacts 1 and 2 of these conductors are maintained at different temperatures T 1 and T 2. This e.m.f. is called thermoelectromotive force (thermo-emf), and an electrical circuit of two dissimilar conductors is called a thermocouple. When the sign of the junction temperature difference changes, the direction of the thermocouple current changes. This
the phenomenon is called the Seebeck phenomenon.
There are three known reasons for the occurrence of thermo-EMF: the formation of a directed flow of charge carriers in a conductor in the presence of a temperature gradient, the entrainment of electrons by phonons, and a change in the position of the Fermi level depending on temperature. Let's look at these reasons in more detail.
In the presence of a temperature gradient dT / dl along the conductor, electrons at its hot end have greater kinetic energy, and therefore a greater speed of chaotic movement compared to electrons at the cold end. As a result, a preferential flow of electrons occurs from the hot end of the conductor to the cold one, a negative charge accumulates at the cold end, and an uncompensated positive charge remains at the hot end.
The accumulation continues until the resulting potential difference causes an equal flow of electrons. The algebraic sum of such potential differences in the circuit creates the volumetric component of the thermo-emf.
In addition, the existing temperature gradient in the conductor leads to the emergence of a preferential movement (drift) of phonons (quanta of vibrational energy of the crystal lattice of the conductor) from the hot end to the cold end. The existence of such a drift leads to the fact that electrons scattered by phonons themselves begin to make a directed movement from the hot end to the cold one. The accumulation of electrons at the cold end of the conductor and the depletion of electrons at the hot end leads to the appearance of a phonon component of the thermo-emf. Moreover, at low temperatures, the contribution of this component is the main one in the occurrence of thermal emf.
As a result of both processes, an electric field appears inside the conductor, directed towards the temperature gradient. The strength of this field can be represented as
E = -dφ / dl = (-dφ / dT)· (-dt / dl)=-β·(-dT / dl)
where β = dφ / dT.
Relation (9.1) relates the electric field strength E to the temperature gradient dT/dl. The resulting field and temperature gradient have opposite directions, so they have different signs.
The field defined by expression (9.1) is the field of external forces. Having integrated the strength of this field over the section of circuit AB (Figure 9.1) from junction 2 to junction 1 and assuming that T 2 > T 1, we obtain an expression for the thermal emf acting in this section:
(The sign changed when the integration limits changed.) Similarly, we determine the thermal emf acting in section B from junction 1 to junction 2.
The third reason for the occurrence of thermo-emf. depends on the temperature of the position of the Fermi level, which corresponds to the highest energy level occupied by electrons. The Fermi level corresponds to the Fermi energy E F that electrons can have at this level.
Fermi energy is the maximum energy that conduction electrons in a metal can have at 0 K. The higher the density of the electron gas, the higher the Fermi level will be. For example (Fig. 9.2), E FA is the Fermi energy for metal A, and E FB for metal B. The values of E PA and E PB are the highest potential energy of electrons in metals A and B, respectively. When two dissimilar metals A and B come into contact, the presence of a difference in Fermi levels (E FA > E FB) leads to the occurrence of a transition of electrons from metal A (with a higher level) to metal B (with a low Fermi level).
In this case, metal A becomes positively charged, and metal B negatively. The appearance of these charges causes a shift in the energy levels of metals, including Fermi levels. As soon as the Fermi levels are equalized, the reason causing the preferential transfer of electrons from metal A to metal B disappears, and a dynamic equilibrium is established between the metals. From Fig. 9.2 it is clear that the potential energy of an electron in metal A is less than in B by the amount E FA - E FB. Accordingly, the potential inside metal A is higher than inside B by the amount)
U AB = (E FA - E FB) / l
This expression gives the internal contact potential difference. The potential decreases by this amount during the transition from metal A to metal B. If both thermocouple junctions (see Fig. 9.1) are at the same temperature, then the contact potential differences are equal and directed in opposite directions.
In this case they compensate each other. It is known that the Fermi level, although weakly, depends on temperature. Therefore, if the temperatures of junctions 1 and 2 are different, then the difference U AB (T 1) - U AB (T 2) at the contacts makes its contact contribution to the thermo-emf. It can be comparable to volumetric thermal emf. and is equal to:
E contact = U AB (T 1) - U AB (T 2) = (1/l) · ( + )
The last expression can be represented as follows:
The resulting thermal emf. (ε T) consists of the emf acting in contacts 1 and 2 and the emf acting in sections A and B.
E T = E 2A1 + E 1B2 + E contact
Substituting expressions, (9.3) and (9.6) into (9.7) and carrying out transformations, we obtain
where α = β - ((1/l) (dE F / dT))
The quantity α is called the thermo-emf coefficient. Since both β and dE F / d T depend on temperature, the coefficient α is also a function of T.
Taking into account (9.9), the expression for thermo-emf can be presented as:
The quantity α AB is called differential or at effective thermo-EMF given pair of metals. It is measured in V/K and significantly depends on the nature of the contacting materials, as well as the temperature range, reaching about 10 -5 ÷10 -4 V/K. In a small temperature range (0-100°C), the specific thermal emf. weakly depends on temperature. Then formula (9.11) can be represented with a sufficient degree of accuracy in the form:
E T = α (T 2 - T 1)
In semiconductors, unlike metals, there is a strong dependence of the concentration of charge carriers and their mobility on temperature. Therefore, the effects discussed above, leading to the formation of thermal emf, are more pronounced in semiconductors, the specific thermal emf. much larger and reaches values of the order of 10 -3 V/K.
9.3. Description of the laboratory setup
To study the dependence of thermo-emf. on the temperature difference between the junctions (contacts), in this work we use a thermocouple made of two pieces of wire, one of which is a chromium-based alloy (chromel), and the other an aluminum-based alloy (alumel). One junction together with a thermometer is placed in a vessel with water, the temperature T 2 of which can be changed by heating on an electric stove. The temperature of the other junction T 1 is maintained constant (Fig. 9.3). The resulting thermal emf. measured with a digital voltmeter.
9.4. Experimental procedure and results processing
9.4.1. Experimental technique
The work uses direct measurements of the emf generated in the thermocouple. The temperature of the junctions is determined by the temperature of the water in the vessels using a thermometer (see Fig. 9.3)
9.4.2. Work order
- Plug in the voltmeter's power cord.
- Press the power button on the front panel of the digital voltmeter. Let the device warm up for 20 minutes.
- Loosen the clamp screw on the thermocouple stand, lift it up and secure it. Pour cold water into both glasses. Drop the thermocouple junctions into the glasses to approximately half the depth of the water.
- Write it down in the table. 9.1 the value of the initial temperature T 1 of the junctions (water) according to the thermometer (for the other junction it remains constant throughout the experiment).
- Turn on the electric stove.
- Record the emf values. and temperatures T 2 in table. 9.1 every ten degrees.
- When the water boils, turn off the electric stove and voltmeter.
9.4.3. Processing of measurement results
- Based on the measurement data, construct a graph of the emf. thermocouples 8T (ordinate axis) from the temperature difference between the junctions ΔT = T 2 - T 1 (abscissa axis).
- Using the resulting graph of the linear dependence of E T on ∆T, determine the specific thermal emf. according to the formula: α = ΔE T / Δ(ΔT)
9.5. Checklist
- What is the essence and what is the nature of the Seebeck phenomenon?
- What causes the appearance of the volumetric component of thermo-emf?
- What causes the appearance of the phonon component of the thermo-emf?
- What causes the occurrence of a contact potential difference?
- What devices are called thermocouples and where are they used?
- What is the essence and what is the nature of the Peltier and Thomson phenomena?
- Savelyev I.V. Course of general physics. T.3. - M.: Nauka, 1982. -304 p.
- Epifanov G.I. Solid State Physics. M.: Higher School, 1977. - 288 p.
- Sivukhin D.V. General course in physics. Electricity. T.3. - M.: Nauka, 1983. -688 p.
- Trofimova T.I. Physics course. M.: Higher School, 1985. - 432 p.
- Detlaf A. A., Yavorsky V. M. Physics course. M.: Higher School, 1989. - 608 p.
Devices for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM Sensor Lab are designed for measuring thermo-EMF coming from primary thermoelectric converters that measure the temperature of liquid metals (cast iron, steel, copper and others) and EMF generated by oxygen activity sensors.
Description
Operating principle
Thermo-EMF signals from the primary thermoelectric converter (thermocouple) and EMF from oxygen activity sensors (mV) supplied to the “measuring” input of the device for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM2 Sensor Lab are converted into digital form and, using the appropriate program, are converted into temperature and oxygen activity values. These signals are perceived by clocks with a frequency of up to 250 s-1. The device has 4 inputs: Ch0 and Ch2 - for measuring signals from thermocouples, and Ch1, Ch3 - for measuring EMF signals from oxygen activity sensors.
In the process of temperature measurements, the change in the incoming input signal is analyzed in order to determine its output to stable readings (characterized by the parameters of the so-called “temperature platform”, determined by the length (time) and height (temperature change). If during the time specified by the length of the platform, the actual If the temperature change does not exceed its specified height (i.e., the permissible temperature change), then the site is considered selected. Next, a device for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM Sensor Lab averages the clock temperature values measured along the length of the selected site and displays average value as the result of measurements on the screen.
In a similar way, areas are identified that correspond to the EMF reaching stable readings, the dimensions of which are also specified by length (time) and height (permissible change in the EMF value).
In addition to measuring the bath temperature, the device allows you to determine the liquidus temperature of liquid steel, which can be converted into carbon content using an empirical equation. Based on the results of measurements of the EMF generated by oxygen activity sensors, the activity of oxygen in liquid steel, cast iron and copper, the carbon content in steel, the content of sulfur and silicon in cast iron, the activity of FeO (FeO+MnO) in liquid metallurgical slag and some other parameters are determined by calculation. , related to the thermal state and chemical composition of liquid metals. The device also has the ability to determine the bath level (the position of the slag-metal boundary) by analyzing the rate of temperature changes when a thermocouple is immersed in the bath and determining the thickness of the slag layer with special probes.
Devices for measuring the temperature of liquid metals and EMF of iM2 Sensor Lab oxygen activity sensors have two modifications, which differ in the presence or absence of an LCD touch screen (Figure 1). In the absence of a screen, the device is controlled from an external computer or from an industrial tablet. In this case, special software is supplied to enable communication between them.
The touch screen is located on the front panel of the device and displays the progress of measurements, its results and other information related to measurements in digital and graphic forms. A menu in the form of text tabs is also displayed on the screen, with the help of which the device can be controlled, diagnosed, and viewed.
Sheet No. 2 Total sheets 4
previously measured measurements. In the “no screen” modification, all of the above information is displayed on the screen of a computer or industrial tablet.
The electronic boards of the device for measuring the temperature of liquid metals and EMF of the iM2 Sensor Lab oxygen activity sensors are installed in a dust-proof steel case, made according to the 19” standard for installation on a mounting rack or mounting in a panel.
Signals from primary converters can be transmitted to the device in two ways - via cable and via radio. In the latter case, the device is connected to the receiving unit (Reciver Box) via a serial interface, and a transmitting device (QUBE) is installed on the handle of the submersible wands, which converts the signals coming from the sensors into radio signals transmitted to the receiving unit. The latter receives them and transfers them to the device for processing.
The device is not sealed.
Software
Installation of software is carried out at the manufacturer. Access to a metrologically significant part of the software is impossible.
The design of the measuring instrument excludes the possibility of unauthorized influence on the software of the measuring instrument and measurement information.
Level of firmware protection against unintentional and intentional changes
High according to R 50.2.077-2014.
Specifications
Metrological and technical characteristics of devices for measuring the temperature of liquid metals and EMF of iM2 Sensor Lab oxygen activity sensors are given in Table 1. Table 1
* - without taking into account the error of the primary converter, extension cable and EMF sensor.
Type approval mark
The type approval mark is printed on the title page of the operational documentation by printing and on the front panel of the device using offset printing.
Completeness
The complete set of the measuring instrument is shown in Table 2. Table 2
Verification
carried out according to MP RT 2173-2014 “Instruments for measuring the temperature of liquid metals and EMF of oxygen activity sensors iM2 Sensor Lab. Verification methodology”, approved by the State Central Inspection Center of the Federal Budgetary Institution “Rostest-Moscow” on October 26, 2014.
The main means of verification are given in Table 3. Table 3
Information about measurement methods
Information about measurement methods is contained in the instruction manual.
Regulatory and technical documents establishing requirements for instruments for measuring the temperature of liquid metals and emf of oxygen activity sensors iM2 Sensor Lab
1 Technical documentation from the manufacturer Heraeus Electro-Nite GmbH & Co. KG.
2 GOST R 52931-2008 “Instruments for monitoring and regulating technological processes. General technical conditions".
3 GOST R 8.585-2001 “GSP. Thermocouples. Nominal static characteristics of transformation".
4 GOST 8.558-2009 “GSP. State verification scheme for temperature measuring instruments.”
when performing work to assess the conformity of products and other objects with mandatory requirements in accordance with the legislation of the Russian Federation on technical regulation.