JPS5925974B2 - Thermal diffusivity measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for measuring thermal diffusivity using an unsteady method, and more specifically to a method for measuring thermal diffusivity using an unsteady method, and more specifically, it relates to a method for measuring thermal diffusivity using an unsteady method, and more specifically, it relates to a method for measuring thermal diffusivity using an unsteady method. The present invention relates to a thermal diffusivity measurement method using an unsteady method that measures thermal diffusivity by utilizing the fact that the amplitude depends on the modulation frequency of incident electromagnetic waves.

The opto-acoustic effect mentioned above is when a substance placed in a sealed container filled with gas is irradiated with modulated electromagnetic waves that the substance absorbs, causing a pressure wave with the same frequency as the electromagnetic wave modulation frequency to appear inside the sealed container. A phenomenon that occurs.

The characteristic of measuring thermal diffusivity using the unsteady method is that it forces a thermally non-equilibrium state within the sample and measures the change in temperature distribution of the sample that occurs as the force relaxes. This method generally requires a shorter measurement time than the steady-state method, and is characterized in that only the temperature needs to be measured as a function of time.

Typical unsteady methods include the Angstrom method and the Flash method. The Angstrom method involves bringing one end of a rod-shaped sample, which has a sufficiently small cross-sectional area compared to its length, into contact with a heat source that periodically heats and cools it, thereby causing periodic temperature changes at one end of the sample. As a result, a temperature wave is generated within the sample, and the state in which this temperature wave propagates within the sample is determined by measuring the temperature at two or more measurement points at different distances from the heating point in the direction of wave propagation. Thermal diffusivity is calculated using the amplitude and phase of the temperature waves obtained at each measurement point.This method requires forming the sample into a rod shape. A large amount of material is required, and the equipment requires a large amount of equipment due to the provision of an insulation system to minimize heat loss from the sample surface.In addition, it takes a relatively long time to measure, and temperature measurement is difficult. Since the temperature detection element is brought into contact with the sample, the contact resistance between the sample and the sensor and the contact resistance between the heating source and the sample become error factors, and the measurement target has a relatively low thermal diffusivity. On the other hand, the flash method has the disadvantage that it is limited to large substances.On the other hand, the flash method provides a light absorption layer on one surface of a flat plate sample, and irradiates this with, for example, a Xe arc flash or a laser pulse to generate light absorption. Instant heating is performed, and the temperature rise in the absorption layer that occurs at this time is propagated in the thickness direction of the sample, and the temperature change that occurs on the sample surface opposite to the irradiated surface is measured as a function of time after flash irradiation. Thermal diffusivity is calculated from the temperature one-hour curve obtained at this time.In this method, the temperature is measured by contact between the temperature detection element and the sample, and the contact resistance at this time is the cause of error. In addition, the thermal diffusivity is calculated based on the assumption that heat loss does not need to be taken into account because the measurement time is short. However, the disadvantage is that the smaller the thermal diffusivity is, such as a polymer film, the larger the error becomes.In this way, in the conventional unsteady method of measuring thermal diffusivity, the error factors due to contact resistance and the thermal insulation Difficulty in setting conditions was a drawback.This invention was made to eliminate the above-mentioned drawbacks. This sample is placed in a sealed container filled with gas (hereinafter referred to as background gas) so that the layer to be measured is in contact with the background gas and forms part of the wall of the sealed container. The ratio of the amplitude of the pressure wave generated at that time by irradiation with electromagnetic waves and the amplitude of the pressure wave previously measured in the same way only on the absorption layer is determined, and the slope of this value with respect to the square root of the modulation frequency of the electromagnetic wave is used to determine thermal diffusion. This is to find the rate.

Therefore, since the present invention does not directly measure the sample surface temperature, the conventional error factor caused by contact resistance is completely eliminated. In addition, the completeness of the insulation condition can be confirmed by checking whether the logarithm of the ratio of the pressure waves has linearity with respect to the thickness of the layer to be measured, and the thickness of the layer to be measured can be appropriately selected. In particular, it is possible to carry out measurements in a state where the adiabatic conditions are actually sufficiently satisfied. An embodiment of the present invention will be described below with reference to the drawings. First, as shown in Figure 1, sample 1
consists of a layer to be measured 1a and an absorption layer 1b.

The absorption layer 1b later absorbs the modulated light beam and generates heat as shown in FIG. 4, and has the role of causing periodic temperature changes on the surface of the layer 1a to be measured that is in contact with the absorption layer 1b. This absorption layer 1b is formed, for example, by vapor deposition of carbon, adhesion of soot by combustion of aromatic hydrocarbons, or application of a carbon black binder-resin solution. Sample 1 is the second
As shown in the figure, it is attached to the light entrance window 3 of the sealed container 2 with double-sided tape or the like. The light radiation window 3 is a normal O-ring 4
etc., to maintain airtightness. If the sample 1 is transparent to the incident light, as shown in Figure 3, the sample 1
may be placed on the wall opposite to the light entrance window 3. Figure 2,
In FIG. 3, a microphone 5 as a sound pressure detection element is mounted on the sealed container 2 so as to maintain airtightness. As shown in FIG. 4, this measurement system includes a light source 6 such as a xenon lamp, tungsten lamp, or various lasers, a chopper 7 for modulating light, and a sample 1.
Input the reference signal Y from the sealed container 2 shown in FIGS. 2 and 3 with the microphone 5 attached, the amplifier 8 for amplifying the output of the microphone 5, and the chopper 7, and calculate the amplitude and phase of the signal. It consists of a lock-in amplifier 9 for measurement and a recorder (not shown) for recording the output of the lock-in amplifier 9. When the sample 1 is irradiated with the modulated light beam 6/, the absorption layer 1b absorbs the light, and the absorbed optical energy is converted into thermal energy in the absorption layer 1b, causing an increase in the temperature of the absorption layer 1b. Now, assuming that sample 1 is composed of only the absorption layer 1b, this thermal energy is transferred to the background gas in contact with the absorption layer 1b by thermal conduction, causing a periodic temperature rise of the gas, and the background gas increases. Because of the constant volume, it generates pressure waves. Assuming that the incident light is modulated at the angular frequency ω, the temperature fluctuation component θ(t) on the surface of the absorption layer 1b in contact with the background gas is θ(t)−θ0
c0s(ωt-ε) ・・・・・・・・・(1)
It is represented by

Here, θo is a value determined by the thermal conductivity and specific heat of the absorption layer 1b itself, if the absorption coefficient of the absorption layer 1b is sufficiently large. Further, ε is the initial phase angle. Also, the amplitude θ of the fluctuation component of the absorption layer surface temperature. and the amplitude Q of the pressure fluctuation inside the sealed container 2 is Q-γPOθ.

/AlgagTO (2) There is a relationship. T here. , PO are the temperature and pressure inside the sealed container 2, respectively, and γ and Ag are the ratio of constant pressure specific heat to constant volume specific heat and thermal diffusivity of the background gas, respectively.
g is the length of the sealed container 2 in the direction perpendicular to the sample surface. No. (
From equation 2), it can be seen that when the conditions inside the sealed container 2 are determined, there is a proportional relationship between the sample surface temperature and the amplitude of each pressure wave. Now, let us consider the case where the sample 1 consists of two layers, the layer to be measured 1a and the absorption layer 1b. Since the layer to be measured 1a and the absorbing layer 1b are in contact with each other, the surface temperature of the layer to be measured 1a on the absorbing layer 1b side is expressed by equation (1). This temperature wave is
The sample 1 propagates through the layer to be measured 1a and heats the background gas on the surface opposite to the absorbing layer 1b.
Pressure waves are generated within the sealed container 2 in the same way as when the chisel is used.
At this time, the temperature θ of the sample surface in contact with the background gas
8(X, t) is calculated under the following conditions, where the thickness of the layer 1a to be measured is x and the thermal diffusion coefficient is A8, that is, the heat flow occurs completely only in the direction perpendicular to the surface of the sample. The surface where the layer to be measured 1a and the absorbing layer 1b are in contact is Z, which can be considered as a one-dimensional problem, and the direction from left to right in the paper of FIG. 1 in the direction of the thickness of the layer to be measured 1a is positive. When the opposite direction to this positive direction is considered negative, when the condition that the temperature distribution is always symmetrical with respect to the positive and negative directions is satisfied, θ8
(X, t)=θ. Exp(-A8x)COs(ωt-A
sx−ε)゛゜゜゜゜゛゜゜゜(3).

Hexagram, As is A8-(ω/2α8) due to thermal diffusivity α8
Defined by +. From this, equation (1) and equation (3)
), the ratio of the amplitudes of θs(X, t) and θ(t), R=θ8(X, t)/θ(t), is expressed as follows: 1nR=A−A8x.

Here, It is.

Also, the phase difference between the two waves is:

In principle, this measurement method uses the relationship of equation (4) or (5), but in actual measurement there are problems as described below, so ) expression cannot be used. That is, when the sample 1 is actually placed in the sealed container 2 and the absorption layer 1b is heated by light absorption and causes periodic temperature changes, when the surface of the layer 1a to be measured that is in contact with the absorption layer 1b is Z, This is because the temperature distribution is not symmetrical in the positive and negative directions. Solutions to such one-dimensional heat conduction problems have generally been sought for a long time, but the solution requires introducing various parameters and becomes extremely complex. If the heat loss parameter, which is a size comparison, is selected appropriately, the above-mentioned amplitude ratio R
What is the angular frequency range and sample layer thickness? It is known that Equation (4) is followed within the range of . The inventors have discovered that the conditions for establishing the linearity of R and ω+ can be determined experimentally by changing the thickness of the sample layer. That is, 35% carbon black was dissolved in an appropriate amount of ethyl alcohol along with 65% polyvinyl butyral binder resin, and the mixture was stirred in a ball mill for more than 5 hours to form a coating with a film thickness of 6 μm.
A double layer sample 1 was prepared by coating polyvinyl chloride resin of 10 μm, 19 μm, 24 μm, 30 μm, and 34 μm, with the carbon-butyral resin layer serving as the absorbing layer 1b and the polyvinyl chloride resin layer serving as the layer to be measured 1a. The thickness of the absorption layer 1b is 5 to 10 μm. This sample 1 was installed as shown in FIG. 3, a xenon lamp was used as a light source, and white light was incident thereon, and the dependence of the amplitude of the pressure wave on the film thickness was measured using the frequency as a parameter. FIG. 5 shows the dependence of the R value on the film thickness, which was obtained by dividing this value by the amplitude previously obtained only for the absorption layer 1b. As can be seen from this figure, the R value is expressed as an experimental formula: 1nR-A-A, Ox-A8, x2...
It was found that it contains a higher-order term of X as shown in (6). However, in a range where the film thickness is small, 1nR11
By showing a linear relationship with respect to ix, it is clear that for substances with low thermal diffusivity such as divinyl chloride resin, the conditions for satisfying equation (4) are sufficiently established by setting the film thickness in the range of 5 to 20 μm. I understand. Next, FIG. 6 shows the dependence of the R value on the square root of ω.

The R value shows a very good linear relationship, and the slope of this is V)(1/2α)+x according to equation (4). Calculating the thermal diffusivity of polyvinyl chloride from the slope of this straight line, α becomes 1.07 x 10 (7i1 SeC), which is 0.9 to 2.4 x 10 -3 d. -Sec
Since it is -1, it can be seen that it is a sufficiently reasonable value.

Furthermore, the modulation frequency used is in the range of 20 to 2000 Hz. Copper was selected as a sample with a large thermal diffusivity, and 0,
The value of thermal diffusivity was determined in the same manner as in the case of polyvinyl chloride resin using a copper plate with a thickness of 15 mm.

The obtained value is 1.38 CTI1-Sec-1, which matches the thermal diffusivity value of 1.14-Sec-1 calculated with reference to the thermal conductivity and specific heat described in the chemical handbook, for example. Good agreement is seen. It is known that for substances with a large thermal diffusivity, the measurable thickness of the layer to be measured becomes large. Therefore, although there is a tendency that the larger the thermal diffusivity of a material, the larger the thickness of the layer to be measured is required and the easier measurement, in this invention, as long as the thickness conditions of the sample layer are set, Sufficient reliability can be guaranteed even for measured values of substances with a small diffusivity, which are commonly called heat insulating materials. However, as the layer to be measured 1a, a conductive material such as metal having a film thickness of 20 ttm to 0.5 mm can also be used.

Furthermore, the sealed container may be made of a pressure-electricity converting material so that it also serves as a sound pressure detector. The above is a method for measuring the absolute value of thermal diffusivity. However, when the relative magnitude of thermal diffusivity is to be investigated, for example, when evaluating the superiority or inferiority of heat insulating materials or thermally conductive materials, the method according to the present invention can be used. According to this method, a faster and simpler measurement method can be obtained.

In other words, if the measurement conditions are the same, such as the thickness of the layer to be measured and the incident energy modulation frequency, the measured signal strength is determined only by the degree of attenuation of the temperature wave within the layer to be measured. The magnitude of the signal strength corresponds to the magnitude of the thermal diffusivity. Therefore, as long as an appropriate film thickness is selected, relative evaluation of thermal diffusivity can be performed quickly and simply by simply attaching the absorbing layer. As explained above, according to the present invention, an absorbing layer is provided on one surface of the layer to be measured, and the absorbing layer is irradiated with modulated light, thereby causing heat to be generated on one surface of the layer to be measured. A temperature wave is generated on one surface, and the magnitude of the amplitude attenuation that occurs when this temperature wave is propagated to the other surface depends on the thermal diffusivity and the distance and frequency of wave propagation. , the thermal diffusivity can be determined from the thickness of the layer to be measured.

Temperature can also be measured by heating this gas by bringing the surface to be measured into contact with the background gas in a sealed container and measuring the resulting pressure fluctuations, which has always been a problem with conventional methods. A common drawback, that is, an error factor due to contact resistance between the sample and the temperature detection element can be eliminated. In addition, by appropriately selecting the thickness of the layer to be measured, it is possible to accurately measure heat in a wide range of areas, from materials with high thermal diffusivity such as copper to materials with extremely low thermal diffusivity such as polymer films. It has the advantage of being able to measure the diffusivity.

[Brief explanation of drawings]

Figures 1 to 6 show an embodiment of the present invention, with Figure 1 being a side view showing the configuration of a sample, and Figures 2 and 3 being a cross section showing how the sample is placed in a sealed container. Figure 4 is a block diagram showing the concept of the measurement system, and Figure 5 shows the ratio of the signal intensity from the absorbing layer and the layer to be measured to the signal intensity from the absorbing layer alone, with respect to the film thickness of the layer to be measured. The characteristic diagram, FIG. 6, is a characteristic diagram in which values similar to those in FIG. 5 are expressed with respect to the square root of the modulation angular frequency.

Claims (1)

[Scope of Claims] 1. A sealed container having at least one electromagnetic wave entrance window or sample attachment window and equipped with a sound pressure detector inside; a required gas contained in the sealed container; Using a measurement system in which the layer to be measured of the sample consisting of a thin absorption layer that absorbs electromagnetic waves necessarily forms a part of the wall surface of the sealed container, the sample is irradiated with electromagnetic waves modulated at a modulation frequency. A pressure wave is generated in a sealed container, the output including the amplitude and phase of this pressure wave is measured by the sound pressure detector, and the modulation frequency is varied at least two points within the modulation frequency range for the sample. let me go,
Thermal diffusion is determined from the slope of the logarithm of the ratio of the output of the sound pressure detector of the pressure wave generated in the sealed container to the signal intensity previously measured in the same manner with a sample consisting only of an absorbing layer, with respect to the square root of the modulation frequency. A thermal diffusivity measurement method characterized by measuring the thermal diffusivity. 2. The thermal diffusivity measuring method according to claim 1, wherein the layer to be measured is made of a heat insulating material such as a polymer film or glass having a thickness in the range of 5 to 20 μm.