GB/T 35807-2018Rubber, vulcanized—Determination of thermal diffusivity—Flash method (English PDF)

Foreword

This standard was drafted in accordance with the rules given in GB/T 1.1-2009.

Introduction

The test method for measuring the thermal diffusivity by the flash method has a wide measurement range, high temperature, high speed, and the measurement process can be in the oxidizing gas.

Features such as atmosphere, inert gas or vacuum environment, but also have the advantages of simple geometric structure, small size and fast test. widely used. Especially in the rubber field, the thermal diffusivity is measured by analyzing the thermodynamic properties of the tire, especially for the tire temperature field.

An important prerequisite for research. By monitoring the thermal diffusivity of the materials in various parts of the tire, it can effectively avoid or reduce the heat of certain parts of the tire during driving.

Heavy causes potential risk factors such as punctures. Studying and determining the thermal diffusivity of tire rubber materials can provide important protection for the safe use of tires.

Barriers, especially for the improvement of green tire performance, provide important technical basis and design reference.

Caution - Personnel using this standard should have practical experience in formal laboratory work. This standard does not indicate all possible security questions.

The user is responsible for taking appropriate safety and health measures and ensuring compliance with the relevant national regulations.

1 Scope

This standard specifies that a flash source is applied to the front side of the sample to calculate the material heat by detecting the temperature rise time of the back side of the sample. The method of diffusion coefficient.

This standard is applicable to the measurement of temperature in the range of 20 ° C ~ 250 ° C, the thermal diffusivity of the body above 0.01mm²/s evenly Isotropic vulcanized rubber.

Note 1. If the sample is accompanied by decomposition or morphological changes within the measured temperature range, the test temperature range is adjusted accordingly.

Note 2. Since the flash method is a non-contact measurement method, it may be different from the results measured by other physical quantity principles (eg, heat flow meter method, protective hot plate method). Wait).

2 Normative references

The following documents are indispensable for the application of this document. For dated references, only dated versions apply to this document. For undated references, the latest edition (including all amendments) applies to this document.

GB/T 2941 Rubber Physical Testing Methods General Procedures for Sample Preparation and Regulation

GB/T 14838 Determination of the standard precision of rubber and rubber products

3 Terms and definitions

The following terms and definitions apply to this document.

4 Symbols and footers

5 Method Overview

A small, thin disk-shaped sample is irradiated with a high-intensity energy pulse for a short period of time. The schematic diagram is shown in Fig. 2. Positive sample The surface absorbs the pulse energy and records the change in the back surface temperature (temperature self-recording curve) caused by it. Thermal diffusion coefficient through the thickness and back of the sample The time required for a certain ratio of temperature rise to the maximum value (see Figure 3) is calculated. When it is necessary to determine the heat of the sample in a temperature range When the diffusion coefficient is required, it needs to be tested separately at each required temperature.

Note: This test method is described in detail in a large number of published books and review articles, and the principles are outlined in Appendix A.

6 Equipment

6.1 The basic structure of the device is shown in Figure 4. It mainly includes flash source, sample cell, detector, signal processing device, etc.

6.2 Flash source. It can be a laser pulse, a flash lamp or other device capable of forming short-cycle high-energy pulses. The circumference of the energy pulse The period should be less than 2% of t1/2. The intensity of the pulse irradiated on the surface of the sample should be uniform. The pulse duration used should be 0.5% to 2% of t1/2 In order to further reduce the influence of the pulse width on the temperature rise curve, pulse width correction is required.

6.3 Environmental control equipment. For testing under non-room temperature conditions, it is necessary to configure environmental control equipment to achieve the required temperature of the sample. degree. The temperature control accuracy is 0.1 °C.

6.4 Detector. The detector that measures the temperature rise of the sample can be a thermocouple, an infrared detector, an optical pyrometer or other small temperature changes.

A device capable of providing a linear electrical signal output and should be capable of detecting a change of 0.05K above the initial temperature of the sample. Detector and its The response time of the matching amplifier should be less than 2% of t1/2. In order to be as close as possible to the sample, the temperature measuring element should be in close contact with or fixed to the sample holder.

On the sample holder. The temperature measuring element should not be in contact with the sample, and it is not allowed to be embedded in the sample.

6.5 Signal processing device. including electronic circuit, pulse peak filter, amplifier and analog-number reading the difference between sample temperature and ambient temperature Word converter.

6.6 Data Acquisition and Recording System. The data acquisition system should have a sufficiently fast speed, and the sampling frequency should be less than 1% of t1/2.

7 Sample

8 Calibration

8.1 Periodically verify equipment performance and evaluation errors by measuring one or several materials with known thermal diffusivity. Appendix B gives The verification procedure for the thermal diffusivity.

8.2 Temperature Calibration of the Flash Tester There are currently two main methods.

a) Curie point standard calibration method. find a series of standard samples whose Curie point transition temperature is within the measured temperature range of the instrument, and measure The deviation of the phase transition temperature from the theoretical value to correct the temperature inside the instrument;

b) External standard thermocouple method. The internal temperature of the instrument is corrected by a calibrated thermocouple.

The above two methods are user selectable. The user can perform a self-check or commission a qualified metrology agency to calibrate.

8.3 Use standard materials to verify the data of unknown materials, pay attention to the performance of the standard materials (including half-temperature rise time and thermal diffusivity) and Unknown similarity, and the same way of measuring the temperature rise curve is effective.

8.4 When making corrections, an important test for data validity is to change the thickness of the specimen. Because the half-temperature rise time varies with L2, that is, the sample thickness The degree is reduced by half and the half-heating time is correspondingly reduced to a quarter of its original value. Therefore, if the same material is representative of different thicknesses When the measured thermal diffusivity is the same (applying a suitable heat loss correction), the measured data can be considered to be true and effective.

9 Test steps

9.1 Determine and record the thickness of the sample.

9.2 Surface the sample (see 7.4) according to the sample and place it in the sample holder.

9.3 Determine the sample temperature.

9.4 Selection of energy pulses. Under the premise of ensuring measurable temperature rise, the lowest possible energy pulse should be used to ensure that the detector is in its line.

Working within the sex range (especially during low temperature testing), while preventing large pulse energy from causing a significant increase in the temperature of the absorber layer on the surface of the sample (In extreme cases, the surface of the sample may be decomposed).

9.5 After the pulse is emitted, the initial or processed temperature profile is monitored to determine the appropriate energy range.

9.6 Before or during the test, the stability of the temperature rise signal baseline should be verified manually or automatically to ensure that it is 4% of the maximum temperature rise. Within.

9.7 Determine the ambient temperature of the sample at baseline stability, test with the energy selected in 9.4, and collect baseline and instantaneous The temperature is raised and the cooling data is analyzed according to the provisions of Chapter 10. In the case of multiple sample tests, the samples are placed in sequence at the same temperature.

The rows are measured sequentially (saving time) and then warmed to the next temperature to be measured.

9.8 Change or set the ambient temperature of the sample according to the requirements, and repeat the data acquisition process, and test at different temperatures.

9.9 If necessary, cycle test the sample cooling or repeated heating process at each test temperature.

10 Calculation

11 Report

The test report should contain the following.

a) the name and number of this standard;

b) the thickness of the sample;

c) test temperature;

d) calculating the thermal diffusivity at x=50% at the test temperature;

e) repeat the results of the test at each temperature point;

f) a description of the correction process for heat loss and finite pulse time effects;

g) Manufacturer and model of the instrument used.

12 Precision

Appendix A (informative appendix) principle

A.1 Ideal situation - the physical model of the pulse method is based on the thermal properties of the adiabatic (isolated heat exchange) sheet material at a constant temperature Next, its front is subjected to an instantaneous energy pulse, and the model assumes the following.

a) one-dimensional heat flow;

b) there is no heat loss on the surface of the board;

c) uniformly absorbing the pulse on the front side;

d) the pulse duration is extremely short;

e) only a very thin layer of absorbed energy pulses;

f) the plate material is uniform and isotropic;

g) The properties of the material do not change with temperature under the test conditions.

Parker derives the number of thermal diffusivity calculations based on the temperature distribution equation in a solid thermal insulation material with a uniform thickness (thickness L).

For the expression, see the formula (A.1) given by Carslaw and Jaeger. In the formula. alpha---The thermal diffusivity of the material.

When an energy radiation pulse Q instantaneously illuminates the front side of the sample (x = 0) and is uniformly absorbed, the depth of the absorption layer is g, and the temperature is divided at this time. The cloth is. When 0 < x < g, see formula (A.2). When g < x < L, see formula (A.3). T(x,0)=0 (A.3)

Under this initial condition, equation (A.1) can be written as equation (A.4). In the formula. rho---material density; C---material specific heat.

For opaque materials, g is very small, so there is a formula (A.5).

On the back side, ie x=L, the temperature change over time can be expressed by equation (A.6).

Two dimensionless parameters, V and omega are defined by equations (A.7) and (A.8).

TM represents the highest temperature on the back, and the simultaneous (A.6)~ (A.8) formula (A.9). When V = 0.5, omega = 1.38, see equation (A.10). Or formula (A.11).

Where t1/2 is the time required for the backside temperature to reach half of its maximum temperature. The principle of the flash method can be illustrated in Figure 2.

A.2 Non-ideal situation---The actual test will violate the above assumption to some extent, so the above Parker method is introduced, it does not The foot is very obvious. Since then, researchers have used various theories to describe the actual process and to correct the boundary conditions that violate the hypothesis. ideal The correction should include all the factors, but so far there is no such correction. And someone uses a single or paired correction method to calculate the bias difference. The result is a revised series of data that is different from each other. From a historical point of view, this is understandable, each research Researchers focus on correcting one or two deviations in the ideal model, assuming that the other factors are ideal and constant. But this itself The principle is violated, because in reality the test will always be interfered to by certain test conditions to a certain extent, and all parameters are changing at the same time. some The situation may increase the influence of a certain condition, such as the pulse is too long, and some may cause other deviations, such as the front due to excessive pulse energy Excessive heat loss, etc. […]