SU913198A1 - Method of complex determination of electroconductive material thermal physical properties - Google Patents

Description

The invention relates to teppofizicheskim measurements, in particular for complex measurements coefficients · heat conductivity, electrical conductivity, and may be used in materials science in studying the properties of electric conductivity ⁵ .nyh materials.

A stationary method is known for measuring the thermal conductivity of metals, because of the fact that a sample is heated by an electric current passing through it ¹¹ and the temperature distribution along the sample is measured at a certain heating power.

However, the stationary method of measuring thermal conductivity does not allow to simultaneously determine the coefficient of temperature of the sample conductivity.

The closest technical solution to the present invention is a method for measuring the thermophysical properties of electrically conductive materials ^ in which the sample is in the form of a thin rod, the ends of which have the same temperature,

2

• TPaUt on a medium with a constant temperature and heated by an electric current passed through it. After the steady state is established, the average temperature and two extreme sections of the rod equidistant from it are determined to be the current flowing through the sample and the voltage drop across its working section, and then the coefficients of heat and electrical conductivity are calculated according to the known relations [2].

However, this method does not allow for sufficiently high accuracy when expanding the number of measured characteristics, for example, the thermal diffusivity is figurative.

The aim of the invention is to improve the accuracy with increasing number of measured thermal characteristics.

This goal is achieved by the fact that in the method of determining the thermal physical properties of electrically conductive materials, in which a sample is in the form of a racing rod, the ends of which have the same

temperature, placed in a medium with a constant temperature, heated by electric current and in a steady state measure the temperature distribution along the length of the rod, the voltage drop on 5 It and the amperage flowing through the sample, after switching on the electric current, measure the temperature change over time in the middle section of the rod to establish a steady state c. <about this section and the data obtained determine the coefficient of thermal diffusivity.

The temperature field of a rod heated by an electric current in an environment of zero temperature, equal to the temperature of its ends and constant in time, is described by the dependence

913198 4

standing measure temperature in section) x 'at a time ΐ +

♦ 4? and +2 d'G, then from the expression (1) can be obtained ratio

which, taking into account the above notation is converted to the form

A 4 * 4

and?

-Y

where 4 = 4 (Χ, 'ί)

e ⁵ ·

+ ν

] *

(four)

4-4 * th * "'V l ¹ λ a

where <3 is the diameter of the rod,

K - coefficient of thermal conductivity of the rod,

If the conditions of heat exchange between the rod and the environment are chosen ^ in such a way that the complex is 6 ^ ~ ^;

2f plays the role of an amendment, from the expression (3.) · it is possible to obtain the calculation formula for the thermal diffusivity about 4 ** 4

4-4 e ¹

25 th = —— M

rod temperature in cross section at time)

- current coordinate, measured from the extreme section of the working section of the rod;

- time of heating the rod with electric current; *

B ι *** A l ~ notation of complexes

- Mr. -g— ι ⁴ - ''

Ϊ0

From the expression (1) for the stationary stage of sample heating, the calculated ratio for the coefficient of thermal conductivity can be derived

where and

C5> 5

3 - · sipa current passing through the rod ·;

έ - coefficient of electrical conductivity of the rod, ’

2 - the cross-sectional area of the rod, '

C - heat capacity of the material of the rod?

$ is the density of the rod

0 - coefficient of thermal diffusivity of the rod /

A - the effective heat transfer coefficient in the gap between the rod and the heater?

“Perimeter rod,

? - the length of the work area

rod.

If after switching on the electric current at the initial stage of heating the sample to establish a stationary co40

45

- • voltage drop at the working section of the rod ·;

DL - temperature difference between the middle and extreme sections of the working section of the rod after the establishment of a stationary state '

- amendment to the lateral heat exchange, close in magnitude to the amendment άς £ ·

The conductivity coefficient is read by the formula

l

(6)

FIG. Figure 1 shows the temperature distribution along the rod at the initial moment of time at K = 0 (straight line 1) at times ΐ-ι (curve 2), ^d C ^ = 'and ^ + hX (curve 3). and Ϊ3 + ΔΪ

(curve 4), as well as after the establishment of a steady state (curve 5).

From the character shown in FIG? 1 of the curves, it follows that the increase in the accuracy of measurement of the thermal diffusivity is provided by measuring the temperature, Ιχ and £ 3 in the middle section of the rod, where there is the largest .5 АГ А Ία λ <51

50

55

neck temperature change for a period of time Δ по 'compared with any: / · other section of the rod.

FIG. 2 schematically shows a device that implements the method.

. The device scheme consists of sample 6, made in the form of a rod with thickened ends 7, placed in the inner tube of the heater 8, heat * · protective sheath 9, thermocouples 10 'and 11 fixed in the extreme sections of the working section of the rod, and thermocouple 12 fixed in its average cross section.

After evacuating the installation by turning on the heater 8, the required temperature level is reached and maintained. An electric current is passed through the sample and at the initial stage of its heating to establish a steady state, temperatures 1 are measured.

and 1¾ in the middle section of the rod in

ten

20

times = Δΐ ¹ .

After establishing a steady state, the temperature difference 25 between the middle and extreme sections of the working section of the rod, as well as the voltage drop at its ends and the current passing through it are measured. The coefficients of temperature, heat and electrical conductivity are calculated by formulas (4), (5) and (6).

Additional measurement of three temperature values in the middle section of the rod, two equal consecutive periods of time at the initial stage of the sample — 35 heating the sample with an electric current until a steady state is established makes it possible to most accurately determine the coefficient of thermal diffusivity. Thus, the number of thermally determined ones characteristics without making any changes

913198 6

in a device that implements the stationary method of Kohlrausz.

When using the proposed method, there is no need to create a special installation for measuring the thermal diffusivity, the production cost of which is usually 10 thousand rubles, and the creation period is at least two years.

Claims (1)

Claim ♦ A method for complex determination of thermophysical properties of electrically conductive materials, in which a sample in the form of a thin rod, the ends of which have the same temperature, is placed in a medium with a constant temperature, heated by electric current, and in a steady state measure the temperature distribution along the rod, voltage drop; and the strength of the current going through the sample, differing in the fact that, in order to increase accuracy with an increase in the number of measured thermal characteristics, after switching on the electrical current measure the temperature change over time in the middle section of the rod to establish a stationary state in this section and from the data obtained determine the desired characteristic.