JPS5946337B2 - 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.
In more detail, the amplitude of the pressure wave in the sealed container induced by the standing temperature wave that occurs in the sample in the opto-eustatic effect is measured at two different modulation frequencies of the incident electromagnetic wave, and the thermal diffusivity is calculated from the ratio of the amplitudes. The present invention relates to a thermal diffusivity measurement method using an unsteady method for measuring .

The opto-eustatic effect is when a material placed in a sealed container filled with gas is irradiated with modulated electromagnetic waves that the material absorbs. A phenomenon that occurs.

The method of measuring thermal diffusivity using the unsteady method involves forcibly creating a thermally non-equilibrium state within the sample, and then determining the thermal diffusivity by measuring the change in the temperature distribution of the sample that occurs as the state relaxes. This method generally requires a shorter measurement time than a measuring device using a steady method, and has the advantage that only the temperature needs to be stagnated as a function of time.

Typical examples of this unsteady method include the Angstrom method and the Flash method.

In the Angstrom method, one end of a rod-shaped sample with a sufficiently small cross-sectional area compared to its length is brought into contact with a heat source that periodically heats and cools it, causing periodic temperature changes at one end of the sample. Temperature waves are generated within the sample, and the temperature is measured at two or more measurement points at different distances from the heating point in the direction of propagation of the waves. The thermal diffusivity is calculated using the amplitude and phase of the temperature wave obtained at each measurement point. On the other hand, in the measurement method using the flash method,
A light absorption layer is provided on one surface of a flat plate sample, and the light absorption layer is instantaneously heated by irradiating it with, for example, xenon arc flash or laser pulses, and the temperature rise in the absorption layer that occurs at this time. The temperature change that occurs on the surface of the sample on the opposite side of the irradiated surface as a result of propagation in the thickness direction of the sample is measured as a function of time after flash irradiation, and the thermal diffusivity is calculated from the temperature one-hour curve obtained at this time. .

Although thermal diffusivity can be measured using either of the above two methods, both have their own drawbacks.

That is, in the former case, it is necessary to mold the sample into a rod shape, which requires a large amount of sample material, or it is necessary to prepare an insulating system to minimize heat loss from the sample surface. becomes large-scale. In addition, since it takes a relatively long time to conduct the experiment, and the temperature detection element is brought into contact with the sample to measure the temperature, the contact resistance between the sample and the detection element and the contact resistance between the heating heat source and the sample are sources of error. There are some drawbacks, such as the fact that the measurement target is limited to substances with a relatively high thermal diffusivity. On the other hand, in the latter case as well, it is necessary to bring the temperature detection element into contact with the sample in order to measure the temperature, and the contact resistance at that time becomes an error factor.

In addition, thermal diffusivity is calculated based on the assumption that heat loss does not need to be taken into account because the measurement is short, and materials with high thermal conductivity such as metals satisfy this assumption well. The disadvantage is that the smaller the thermal conductivity, such as a polymer film, the greater the error. In summary,
These measurement methods have the disadvantages that contact resistance becomes a factor of error and that it is difficult to set adiabatic conditions.
Furthermore, a thermal diffusivity measurement method using an unsteady method that uses the opto-utilization effect has been known for a long time, but in this method, a double layer consisting of a layer to be measured and a thin absorbing layer that efficiently absorbs incident electromagnetic waves is used as a sample. This sample was placed in a sealed container filled with gas (hereinafter referred to as background gas) so that the layer to be measured was in contact with the background gas and formed part of the wall of the sealed container, and the sample was modulated. The ratio of the amplitude of the pressure wave generated at that time by irradiating the electromagnetic wave with the amplitude of the pressure wave similarly measured only in the absorbing layer is calculated, and the slope of the logarithm of this ratio with respect to the modulation frequency of the electromagnetic wave is calculated. Find the rate.

According to this method, since the amplitude of the temperature fluctuation on the sample surface is proportional to the amplitude of the pressure wave, there is no need to directly measure the temperature on the sample surface, thus eliminating the error factor caused by the above-mentioned contact resistance. Although this method makes it easier to set the heat insulation conditions, it has the following disadvantages. In other words, to determine the thermal diffusivity, the frequency dependence of pressure waves must be measured twice: once for the absorbing layer only, and once for the double layer consisting of the absorbing layer and the layer to be measured, which requires time and effort. . It is an object of the present invention to eliminate the above-mentioned drawbacks and to provide a method for measuring thermal diffusivity that can accurately and quickly determine the thermal diffusivity regardless of the sample material.

The thermal diffusivity measurement method according to the present invention uses a double layer as a sample consisting of a layer to be measured and a thin absorbing layer having an appropriate absorption coefficient and thickness for incident electromagnetic waves, and the sample is sealed in a hermetically sealed container containing a background gas. The sample is installed in a container so that the layer to be measured is in contact with the background gas and forms part of the wall of the sealed container, and modulated electromagnetic waves are irradiated onto this sample to modulate the amplitude of the pressure waves generated at that time. It is characterized by measuring at two modulation frequencies by changing the frequency, and determining the thermal diffusivity from the ratio of the two amplitudes.

Therefore, since the sample surface temperature is not directly measured, contact resistance, which can cause errors, is excluded from the measurement system. In addition, the completeness of the insulation condition can be confirmed by checking whether the logarithm of the ratio of the amplitudes of 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. By doing so, it is actually possible to fully satisfy the insulation conditions. Furthermore, thermal diffusivity can be determined by simply measuring the amplitude of pressure waves at two fluctuating frequencies for a sample consisting of an absorbing layer and a layer to be measured, which is extremely simple compared to conventional measurement methods. It is a measurement method. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is a sample;
Reference numeral 2 denotes a sealed container, on one wall of which an electromagnetic wave entrance window 3 is formed, and on the other wall a microphone 5 as a sound pressure detection element is adhered to maintain airtightness. 6 is a light source (e.g. xenon lamp, tungsten lamp, or various lasers); 7 is electromagnetic wave modulation (modulation frequency is 10 to 200
8 is an amplifier for amplifying the output of the microphone 5, and 9 is a so-called lock-in amplifier that receives the reference signal 71 from the chopper 7 and the output of the amplifier 8 and measures the amplitude and phase of the signal. So, this amplifier 9
Although not shown in the figure, a recorder is connected thereto.

Note that 4 is an O-ring for keeping the entrance window 3 part airtight. Further, the sealed container 2 is filled with a background gas. The sample 1 consists of a layer to be measured 1a and an absorption layer 1b as shown in FIG. 2, and the layer to be measured 1a is in contact with the background gas as shown in FIG. It is attached to the inner surface of the entrance window 3 by, for example, pasting with adhesive tape so as to constitute a part of the wall of the entrance window 3, or it is attached to the wall facing the entrance window 3 as shown in FIG. The absorption layer 1b absorbs the modulated electromagnetic wave beam, generates heat, 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 absorbent layer 1b is formed, for example, by applying a solution of carbon black binder and resin. Next, the operation of the above measuring device will be explained. When the sample 1 is irradiated with the modulated electromagnetic wave beam 61, the electromagnetic wave is absorbed by the absorption layer 1b, and the energy is modulated into thermal energy within the absorption layer 1b, causing periodic temperature changes in the absorption layer 1b.
Since the layer to be measured 1a and the absorption layer 1b are in contact with each other, the surface temperature of the layer to be measured 1a on the absorption layer 1b side also changes with the same period and phase. This temperature wave propagates within the layer to be measured 1a, heats the background gas on the opposite surface, and generates a pressure wave within the sealed container 2. The amplitude of this pressure wave (hereinafter referred to as sound pressure) is measured with a microphone 5. The sound pressure Q measured in this way can be thought of as a one-dimensional problem because the heat flow occurs only in the direction perpendicular to the surface of the sample, and the surface where the layer to be measured 1a and the absorbing layer 1b are in contact can be considered as a one-dimensional problem. Z
and satisfies the relationship of the following equation, which satisfies the condition that the temperature distribution is always symmetrical in the positive and negative directions. 1-
S However, Q.

is a constant specific to the Weiyodo device, ω is the modulation frequency, α8 is the thermal diffusivity of the layer 1a to be measured, λ is a constant determined by the magnitude of the optical absorption coefficient and thermal diffusivity of the absorption layer 1b, and is the thickness of the layer to be measured 1a. If the constant λ and the thickness X of the layer 1a to be measured are known, α8 can be found from the following relationship by measuring the sound pressures Ql and Q2 at two appropriate modulation angular frequencies ω1 and ω2. . Niru 2
] -1 We will explain how to find the constant λ to the 0th order. When the sample 1 consists of only the absorbing layer 1b, the sound pressure Q is given by the following equation. Q-
QO [λ (3) Therefore, we measure Q as a function of modulation angular frequency ω, and 10
The constant λ can be determined from the slope of the relationship between gQ and 10gω.

For the sake of convenience in determining α8 using equation (2), it is effective to obtain the absorption layer 1b in which λ is 1. Actually λ is 1
It will be shown below that an absorbing layer 1b having the following properties can be obtained. That is, 35% carbon black was dissolved in an appropriate amount of ethyl alcohol together with 65% polyvinyl butyral binder resin, and the paint was stirred in a ball mill for more than 5 hours and then applied to the electromagnetic wave entrance window 3, with a film thickness of 5 μM and 1 Op.
Absorption layers of m, l3 μm and 30 μm were obtained.

FIG. 5 shows the results of measuring the dependence of Q on electromagnetic wave modulation angular frequency ω for each of these absorption layers. As shown in FIG. 5, the relationship between 10gω and 10gQ is a linear relationship, and the constant λ can be calculated from the slope thereof. The value of the constant λ obtained in this way is 5 when the thickness of the absorption layer is 5.
1 for absorption layers of 5 μm or more, and all values greater than 1 were obtained for absorbing layers of 5 μm or more. Therefore, in the case of the paint used here, the constant λ for the absorption layer with a film thickness of 5 μm
was experimentally confirmed to be 1. Although this invention uses the relationship expressed by equation (2) in principle, there are problems in actual measurement as described below.

That is, the sample (1) is actually placed in the sealed container (2),
When the absorbing layer 1b absorbs electromagnetic waves and generates heat, causing periodic temperature changes, the temperature distribution becomes symmetrical in the positive and negative directions when the surface of the measured layer 1a in contact with the absorbing layer 1b is defined as Z. This is because it will not happen. Solutions to such one-dimensional heat conduction problems have generally been sought for a long time. The solution becomes very complex by introducing various parameters, but
If the heat loss parameter, which is a comparison of the magnitude of heat conduction in the positive and negative directions, is appropriately selected, the relationship between the logarithm of the sound pressure Q described above and the film thickness It is known to follow a linear relationship in the range. The inventors have discovered that the range of the thickness X of the layer to be measured 1a within which this linearity holds can be determined experimentally. That is, using the above-mentioned paint consisting of carbon and binder resin polyvinyl butyral, the film thickness was 6 μM, 10 μM9l9 μMF24 μM, 30 μM.
Pm, 34 μm polyvinyl chloride resin was coated, and a double layer sample was prepared with the carbon butyral resin layer as the absorbent layer 1b and the polyvinyl chloride resin layer as the absorbent layer 1a, which is shown in Figure 4. A xenon lamp was used as the light source 6, and white light was applied to the light source 6, and the dependence of the sound pressure on the membrane pressure was measured using the frequency as a parameter.

Note that the thickness of the absorption layer 1b is 5 μm. The relationship between the sound pressure and film thickness thus determined is shown in FIG. As can be seen from Figure 6, the relationship between the logarithm of the sound pressure Q and the film thickness The film thickness of materials with low thermal diffusivity, such as vinyl chloride resin, is 5 to
It has been found that by setting the thickness in the range of 20 μm, the conditions for satisfying the formula (1) are sufficiently satisfied. A polyvinyl chloride resin with a film thickness of 15 μm was used as the floating constant layer 1a, and a film with a film thickness of 5 μm formed from the above-mentioned paint consisting of carbon and polyvinyl butyral was used as the absorption layer 1b. When we calculated the thermal diffusivity of polyvinyl chloride, α
8=1.1X10-3CTL-Sec-1, and the values announced so far are 0.9 to 2.4x10-3cri1-
Since it is Sec-1, it can be seen that this is a sufficiently reasonable value.

Copper was selected as a sample with a large thermal diffusivity, and the thermal diffusivity value was determined in the same manner as in the case of polyvinyl chloride resin using a copper plate with a thickness of 0.15 mm. Its value is 1.38cTi1
- In Sec-1, the value of thermal diffusivity is 1.14, which is calculated by referring to the thermal conductivity and specific heat described in the chemical handbook, for example.
A fairly good match with cIL-Sec-1 is observed.
It is known that for substances with a large thermal diffusivity, the film thickness that can be used as a layer to be measured becomes large. Therefore, although there is a tendency that the higher the thermal diffusivity of a substance, the larger the thickness of the layer to be measured is required and the easier the measurement is, in this invention, as long as the thickness conditions of the sample layer are set, Sufficient reliability can also be guaranteed for the measured values of substances that have a low diffusivity and are commonly called heat insulating materials. As described above, according to the present invention, an absorption layer is provided on one surface of the layer to be measured, and the absorption layer is irradiated with modulated electromagnetic waves, thereby generating heat and causing temperature waves on one surface of the layer to be measured. The magnitude of the amplitude attenuation that occurs when this temperature wave is propagated to another surface depends on the thermal diffusivity, the distance of the wave propagation, and the frequency. Thermal diffusivity can be determined from the thickness.

Temperature stagnation can also be achieved by heating this gas by bringing the surface to be measured into contact with background gas in a sealed container and measuring the resulting pressure fluctuations. Error factors caused by contact resistance between the detection element and the sample can be eliminated. Furthermore, in the conventional method using the opto-astake effect, the frequency dependence of the pressure wave must be taken care of twice: when it consists of only an absorbing layer and when it consists of an absorbing layer and a layer to be measured. However, according to the present invention, there is an advantage that thermal diffusivity can be used by simply measuring sound pressure at two frequencies only in the case of a layer to be measured and an absorbing layer. In addition, by appropriately selecting the thickness of the layer to be measured, it is possible to adjust the temperature from a material with a high thermal diffusivity such as copper to a material with an extremely small thermal diffusivity such as a polymer film.
It has the advantage of being able to measure the value accurately over a wide range.

[Brief explanation of drawings]

Figures 1 to 5 show an embodiment of the present invention. Figure 1 is a block diagram of the measurement system, Figure 2 is a side view showing the structure of the sample, and Figures 3 and 4 are sealed. A cross-sectional view to explain how the sample is attached to the container, Figure 5 is a characteristic diagram showing the relationship between sound pressure and modulation frequency, and Figure 6 is a diagram showing the sound pressure from the absorption layer and the measured layer and the absorption layer only. FIG. 3 is a characteristic diagram showing the relationship between the ratio to sound pressure and the thickness of the layer to be measured. 1... Sample, 1a... Layer to be measured, 1b
...Absorbent layer, 2... Sealed container, 3...
...Electromagnetic wave incidence window, 4...O ring, 5.
...Microphone, 6...Light source, 7...
...Chiyotsupa, 8...Amplifier, 9...
...Lock-in amplifier.

Claims (1)

[Claims] 1. A sealed container having at least one electromagnetic wave entrance window or sample mounting window and in which a required gas is sealed, an electromagnetic wave generator that generates electromagnetic waves to irradiate the sample, and an electromagnetic wave generator that generates the electromagnetic waves. The modulated electromagnetic waves are irradiated onto a sample that is composed of a modulator that modulates, a layer to be measured, and an absorption layer that absorbs electromagnetic waves, and the layer to be measured forms a part of the inner surface of the sealed container. a sound pressure detector that measures the amplitude, phase, etc. of a pressure wave generated in the sealed container when The layer is irradiated with electromagnetic waves, and the thermal diffusivity is determined from the ratio of the output of the sound pressure detector at the first modulation frequency and the output of the sound pressure detector at the second modulation frequency. Thermal diffusivity measurement method. 2. Set the thickness of the layer to be measured on the sample to 5 to 20
The thermal diffusivity measuring method according to claim 1, characterized in that the value is μm. 3. The thermal diffusivity measuring method according to claim 1, wherein the thickness of the layer to be measured of the sample is 20 μm to 0.5 mm for a good thermal conductor such as metal. 4. The method for measuring thermal diffusivity according to claim 1, wherein the sealed container is made of a pressure-electricity converting material so as to double as a sound pressure detector.