SU754526A1 - Device for determining thermophysical characteristics of specimens - Google Patents

Description

(54) DEVICE FOR DETERMINATION OF THERMOPHYSICAL CHARACTERISTICS OF SAMPLES

The invention relates to devices for determining the iocTH heat conductor and the thermal diffusivity of conductive electrically conductive materials by non-stationary methods and is intended for use in research and factory laboratories.

A device is known for measuring the thermal conductivity, heat capacity, and thermal diffusivity of dielectrics on samples in the form of plates, between which a flat twisted nichrome heater: wire, and thermocouples are staked into the sample on the opposite source side l.

A device for determining the | and the coefficient of heat-conducting materials, is known. containing a heater and located on either side of it, the packages, assembled from the plates of the investigated material and metal plates with thermocouples 2.

: as well as the absence of electrical insulation of the heating element, which does not allow the use of electrically conductive samples.

Closest to the present invention is a device for measuring the thermal conductivity of materials, containing two cylindrical specimens, between which is a flatbed heater from a thin flat helix, burned into a mica package. Conducting wires are made of red copper. Thermocouples are embedded in the samples.

A disadvantage of the known device 5 for measuring thermal conductivity and thermal conductivity is in TQM, that the conductors and potential leads intended to measure the voltage drop on the heater are channels of unacceptable heat leakage from the heater, and the electrical insulation of the sample heating element is thermal resistance

5 causes the appearance of additional thermal resistances: heater-insulation, insulation sample. In addition, when a short heat pulse is applied, the temperature of the heating element rises quickly, reaching 20x10 or more, leading to the destruction of the heater and breaking the electrical circuit, which requires a repetitive installation of the installation, which represents known difficulty. The purpose of the invention is to improve the measurement accuracy by eliminating additional thermal outflows from the heating element through the supply line and the presence of thermal resistances and the life cycle of the heating element. This goal is achieved by using the test samples in the measuring device as current leads, while the thermoelectropes of the same name are connected to the voltage drop meter, and in the case of opposite and like thermoelectrodes, an interrupter is activated, synchronized with the test samples with a pulsed source. FIG. 1 is a schematic diagram of an apparatus for determining the thermophysical characteristics of solid electrically conductive materials; in fig. 2 shows the temporal dependence of the temperature at the place of thermocouple termination. The device contains a heating circuit consisting of a pulsed source 1, a meter 2 of the current strength of samples 3 contacting in a plane, 4, and a measuring circuit consisting of thermocouples 5, a device 6, a thermocouple recording thermopower, a voltage drop meter 7 and a chopper 8. The device works in the following way. Pulse source 1 generates an electrical impulse (Fig. 2) with a duration of -CHz. During its passage through the heating circuit, the chopper 8 breaks the circuit of opposite thermoelectrodes, leaving the circuit of the same name unbroken, and the meter 7 registers the voltage drop at the contact site 4 (the voltage drop across the sample section between the thermocouple hot springs and the heater plane is negligible according to the voltage drop at the point of contact). At the end of the electric pulse, the snubber goes to the position of replacing the circuit of opposite thermocouples, forming a series connection of thermocouples, and opens the circuit of similar thermocouples to prevent shunting action of the measuring bodies 7. The thermocouples connected in series measure the temperature change of the samples at the place of the thermocouple sealing caused by the action of an electric pulse, transfered by contacting surfaces into a thermal one. Since the location of samples and temperature sensors is symmetrical relative to the plane of contact, the spatial-temporal variation of temperature in the samples will be immetrical. Therefore, the combination of thermocouples by the differential scheme allows us to force the signal of temperaure change twice. The formulas for calculating the thermal diffusivity a and thermal flux L are as follows: (HiV41. OfQ-t iw Cr;) dC-L (t T ОН -: m distance from the heater plane to the place of thermocouple incorporation, m; sample section area. Time to reach maximum temperature T at the place of thermocouple fitting, s; electric pulse duration, s; W (C) - power released on the heater, W. In the absence of heat outflow, the power of the thermal pulse that propagates in the samples is equal to the electric power contact surfaces and Yelits as W (f) L (-c) / (- i :-) where 3 (t) is the current flowing in the sample circuit, V (t) voltage drop across the contact and part of the sample between thermocouples. Since Since the electrical resistance of the sample is much less than the electrical resistance of the contact, then the measured voltage falls completely on the contact planes (in the case of precision measurements, the specific electrical resistance of the sample is entered, corrected for the voltage drop across the sample. The presence in the known device of thermal contact resistances such as a heater-electric insulator, an electric insulator-sample and the presence of an electric insulator that adds additional thermal resistance leads to an increase in time and, therefore, negative errors in measuring thermal diffusivity, and thermal conductivity. . Another source of error is the fastening of current leads and potential leads directly to the heating element, which leads to an error in measuring the power of the thermal impulse, since additional heat leaks occur from the heating element.