SU1293606A1 - Method and apparatus for measuring thermal diffusitivity of materials - Google Patents

Abstract

The invention relates to the field of thermophysical measurements and can be used to measure the thermal diffusivity of low-temperature materials, including dielectric, that do not allow significant overheating of controlled samples. The aim of the invention is to improve the accuracy of measuring the thermal diffusivity of dielectric materials with low thermal conductivity. The method for measuring the thermal diffusivity of dielectric materials is based on the excitation of a plane temperature wave in a sample in the form of an unlimited plate. Measurements are made of the ratio of the amplitudes of the temperature wave on the sample surface. The frequency of the temperature wave is set equal to the value at which the length of the temperature wave becomes equal to the thickness of the monitored material sample. The desired thermal diffusivity is determined by the ratio a S F / 4 p, where is the sample thickness, F is the frequency of the temperature wave. The proposed method makes it more accurate and easier to measure the thermal diffusivity of dielectric materials. c (L N5 QD O5 O a „A.

Description

The invention relates to the field of thermophysical measurements and can be used to measure the thermal diffusivity of thermal conductivity materials that do not allow significant overheating of monitored samples.

The purpose of the invention is to improve the accuracy of measuring the thermal diffusivity of materials with low thermal conductivity.

The drawing shows a functional diagram of an apparatus for implementing a method for measuring a carrier.

thermal diffusivity of materials.

The device contains a frequency controlled first low-frequency generator 1, a voltage-modulated power supply source 2 of a flat low-inertia heater 3, which is mounted on the surface of the test sample 4.

(

The heater 3 is thermally insulated with a shell 5. The temperature of the non-heated surface of the sample is measured by a thermocouple 6 with a temperature-sensitive element. The AC output signal of the thermocouple converter 6 is amplified by a low-frequency amplifier 7, rectified by a phase-sensitive rectifier 8, whose reference input through a frequency doubler 9 and phase shifter 10 receives a double-frequency generator signal 1. The output of the phase-sensitive rectifier 8 is registered by indicator 11. A meter 12 of the frequency ratio, for example, an electron-counting frequency meter, is connected to the output of the frequency doubler 9 with the first input and the second with the output of the low-frequency generator 13; with digital display 14.

The essence of the method is as follows. In the absence of a variable component of heat exchange with the environment of a flat plate in which a temperature wave is excited, the temperature distribution Q along the coordinate axis X, is perpendicular

plate described by the expression

g with -J.X

 cos (tot S 0, e

 Y-)

2Q

where UL is the temperature at the heated

surface (); CJ - circular frequency of the temperature wave;

Q - coefficient), gyhuropr {1-

in one f: spines.

Under the condition of one-sided access to a controlled sample from the side of an unheated surface (for example, in the technological control of sheet and pynoiiHbix materials), a criterion for choosing the frequency o, which would allow to judge the temperature amplitude of the heated surface, is necessary. This is necessary in order to exclude the radiation component of the heat transfer coefficient, which is substantially large at temperatures above 400 K, and the conductive component of this coefficient, which contributes to heat transfer with a temperature gradient of the heated surface and surrounding more than a few tenths of a degree. This is especially important when inspecting materials with a low thermal diffusivity (for example, differences in dielectric materials based on polymers), since for tangible changes in temperature, an unheated surface is unacceptable.

overheat controlled material.

I

If the length L of the temperature wave

L 2W. some frequency W will be equal to the plate O, i.e. , the absorption of the temperature wave in thickness; the e plate is -.

e e 535 times

From this dependence follows the value of the frequency F of the temperature wave at which L 5

W- 2-7 2Q

from where

. .. "

Thus, the condition L S makes it possible to judge the variation of the temperature of the heated surface.

It is obvious from the last expression that it is possible to determine the thermal diffusivity coefficient Q if of izestna 7 and

Q "I F., 4TT

Thus, if it is assumed that the frequency of the temperature wave is

the largest, according to which the measured value Q is measured, the frequency at which the length of the temperature wave is equal to the thickness of the test sample is directly proportional to the coefficient of thermal conductivity

„Tf ,,. PjjyQ K-Q,

ATG

where K - is the measurement conversion factor. For example, the coefficient of thermal diffusivity of a sample of polyurethane foam with a thickness of 1 mm, determined by the proposed method, is equal to

S „

41G

5,097

 0, D056 10

The device works as follows.

The power of the heater 3 pulses at a frequency equal to twice the frequency of generator 1 and excites the temperature wave in the controlled sample 4. A thermal converter 6 with a temperature-sensitive element converts the temperature fluctuations of the unheated surface of sample 4 into a DC signal modulated by a variable frequency component equal to the temperature frequency the waves. The low-frequency amplifier 7 amplifies only the variable component of this signal, and the phase-sensitive rectifier 8, in the reference channel of which the frequency doubler 9 is connected and the variable phase shifter 10, which compensates for phase ramps in the measuring channel, produces a synchronous rectification of this component. Indicator 11 shows the amplitude of the variable component of temperature on the unheated surface of the sample.

If you set the frequency of the generator 13 low 1astot numerically equal

f - p - Hz

where S is the sample thickness in mm, then the frequency ratio meter will perform the division operation, the result of which is equal to

5J. 4th

„. |. -si, p ,,., o.: j

36064

Digital indicator 14 with the number n Q 10 m / s.

So, to measure the thermal diffusivity of the thermal diffusivity of a foam with a thickness of 1 mm at a temperature of 333 K, it is necessary to excite harmonic temperature fluctuations with an amplitude of about 5 K. Taking into account the value of the thermal conductivity for

The width of this material Q O, 4066 1 0 is the frequency of the alternating current, which creates a variable component of the temperature, is 2.5-3 Hz.

The proposed method for measuring the coefficient of thermal diffusivity of materials, mainly dielectric, and a device for its implementation improve the accuracy of measuring the thermal diffusivity of dielectric materials, which usually have low values of this coefficient and, therefore, require sensitive and accurate means of measuring very small fluctuations of the temperature of the unheated sample surface.

Claims (2)

Invention Formula 30 1. A method for measuring the thermal diffusivity of materials based on the excitation of a temperature wave in a sample simulated in a flat unrestricted plate, measuring the amplitudes of the temperature wave and calculating a definable quantity, characterized in that, in order to improve accuracy, when measuring the temperature wave re 40 they are gulled to establish its length, a multiple of the sample thickness, and the thermal diffusivity Q is determined by the ratio S 45   -47p -F where S is the sample thickness; F is the frequency of the temperature wave; p 1,2,3 - one of the numbers of natural p yes. I 2. A device for measuring the thermal diffusivity of materials, comprising a first low-frequency generator connected to a controlled power source, the output of which is connected to a flat heater mounted on one side of a test sample. Thermal converter mounted on the other side of the sample and connected to the ushshtiktel, indicator, characterized in that it is equipped with an additional phase-sensitive rectifier, frequency doubler, phase shifter, frequency ratio meter, second low-frequency generator, second digital indicator, while the first input of the phase-sensitive rectifier the miter is connected to the output of the amplifier, and the output is connected to the first indicator, the frequency doubler output is connected to the input of the phase converter 0, the output of which is connected to the second input of the phase-sensitive rectifier, the frequency doubler input is connected to the first generator, the first input of the frequency ratio meter, and the second input is connected to the second a low frequency generator; a second digital indicator is connected to the output of the frequency ratio meter. -Sh in Editor A.Revin Compiled by V. Guseva Tehred V.Kadar Proofreader M.Demchik Order 378/47 Circulation 777 Subscription VNIIPI USSR State Committee for inventions and discoveries 1130355 Moscow, Zh-35, Raushsk iab, D. 4/5 Production and printing company, Uzhgorod, Projecto st., 4 gz shshshlsh G