JPS6138410B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for measuring thermal diffusivity using an unsteady method. The present invention relates to a thermal diffusivity measuring device using an unsteady method that measures thermal diffusivity by utilizing the fact that the modulation frequency of incident electromagnetic waves depends on the modulation frequency of incident electromagnetic waves.

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

A thermal diffusivity measuring device using an unsteady method calculates thermal diffusivity by forcibly creating a thermally non-equilibrium state within a sample and measuring the change in temperature distribution of the sample that occurs as the state relaxes. This method has the advantage that it generally takes less time to measure than a measuring device using a steady method, and only the temperature needs to be measured as a function of time.

Representative examples of this unsteady method include the Angstrom method and the Flash method. In a measuring device using the Angstrom method, one end of a rod-shaped sample whose cross-sectional area is sufficiently small compared to its length is brought into contact with a heat source that periodically heats and cools the rod.
A periodic temperature change is caused at one end of the sample, resulting in a temperature wave within the sample, and the state in which this temperature wave propagates within the sample is determined by the distance from the heating point in the wave propagation direction. Observation is made by measuring the temperature at two or more different measurement points, and the thermal diffusivity is calculated using the wave and phase of the temperature obtained at each measurement point.

On the other hand, in a measurement device using the flash method, a light absorption layer is provided on one surface of a flat plate sample, and this layer is irradiated with, for example, a xenon arc flash or laser pulse to instantaneously heat it by light absorption. The temperature rise in the absorption layer that occurs during flash irradiation 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, and the temperature-time curve obtained at this time is measured. Calculate the thermal diffusivity.

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 the measurement takes a relatively long time 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. Summer,
There are 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, the calculation of thermal diffusivity is performed on the assumption that heat loss does not need to be considered because the measurement is short.
This assumption is well satisfied for materials with high thermal conductivity such as metals, but the error increases as materials with low thermal conductivity such as polymer films are used.

In summary, conventional measuring devices have the disadvantages that contact resistance becomes a factor of error and that it is difficult to set adiabatic conditions.

SUMMARY OF THE INVENTION An object of the present invention is to provide a thermal diffusivity measuring device that can eliminate the above-mentioned drawbacks and accurately and easily measure thermal diffusivity regardless of the sample material.

The thermal diffusivity measurement device according to the present invention uses a double layer as a sample consisting of a layer to be measured and a thin absorbing layer that efficiently absorbs incident electromagnetic waves, and the sample is sealed in a gas (hereinafter referred to as background gas) sealed. The layer to be measured is in contact with the background gas in the container,
The amplitude of the pressure wave generated when the sample is irradiated with modulated electromagnetic waves, and the amplitude of the pressure wave similarly measured only in the absorption layer. The thermal diffusivity is determined from the slope of this value with respect to the square root of the modulation frequency of the electromagnetic wave.

Therefore, since the sample surface temperature is not directly measured,
Contact resistance, which causes errors, is removed 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 possible to actually fully satisfy the insulation conditions.

Fig. 1 shows the principle of the present invention, in which 1 is a sample, 2 is a sealed container, in which an electromagnetic wave entrance window 3 is formed on one wall, and a microphone 5 as a sound pressure detection element is placed on the other wall to ensure airtightness. It is attached to keep it in place. 6 is a light source (for example, a xenon lamp, a tungsten lamp, or various lasers); 7 is a chopper for modulating electromagnetic waves (the modulation frequency is 20 to 2000 Hz); 8 is an amplifier for amplifying the output of the microphone 5; 9 is the aforementioned This is a so-called lock-in amplifier that receives the reference signal from the chopper 7 and the output of the amplifier 8 and measures the amplitude and phase of the signal.A recorder (not shown) is connected to the amplifier 9. 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 has a layer to be measured 1a as shown in FIG.
and an absorbing layer 1b, and is pasted with adhesive tape, for example, so that the measured layer 1a is in contact with the background gas and constitutes a part of the wall surface of the container 2, as shown in FIG. 1 or 3. It can be attached to the inner surface of the entrance window 3 by means of, for example, or it can be attached to the wall surface 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 carbon black binder resin solution.

Next, the operation of the above measuring device will be explained. When the sample 1 is irradiated with the modulated electromagnetic wave beam 6a, the electromagnetic wave is absorbed by the absorption layer 1b, and the energy is converted into thermal energy within the absorption layer 1b, increasing the temperature of the absorption layer 1b.

Now, assuming that the sample 1 is composed only of the absorption layer 1b, the above 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. In this case, pressure waves are generated because the background gas has a constant volume. Assuming that the incident electromagnetic wave 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) = θ _p cos (ωt - ε) ...It is expressed by (1). However, if the absorption coefficient of the absorption layer 1b is sufficiently large, θo is a value determined by the thermal conductivity and specific heat of the absorption layer 1b itself, and ε is the initial phase angle. Furthermore, there is a relationship between the amplitude θo of the fluctuation component of the absorption layer surface temperature and the amplitude Q of the pressure fluctuation inside the sealed container 2 as follows: Q=γP ₀ θ ₀ /√2l _g a _g T ₀ ...(2) be. However, Po and To are sealed containers 2
, γ and a _g are the ratio of constant pressure specific heat to constant volume specific heat and thermal diffusivity of the background gas,
_g is the length of the sealed container 2 in the electromagnetic wave incident direction.
From these two equations, it can be seen that when the conditions inside the sealed container 2 are determined, the amplitudes of the pressure waves of the sample surface temperature are in a proportional relationship.

Now, the sample 1 consists of two layers: the layer to be measured 1a and the absorption layer 1b.
Consider the case of layers. Since the layer to be measured 1a and the absorption layer 1b are in contact with each other, the absorption layer 1 of the layer to be measured 1a
The surface temperature on the b side is also expressed by equation (1). This temperature wave propagates within the layer to be measured 1a, heats the background gas on the opposite surface, and generates a pressure wave in the sealed container 2 in the same way as when the sample 1 consists of only the absorbing layer 1b. At this time, the temperature θs(x,
t) is calculated under the following conditions, where x is the thickness of the layer 1a to be measured and a _s is the thermal diffusivity, that is, the heat flow occurs only in the direction perpendicular to the surface of the sample, and 1
It can be thought of as a dimensional problem, and when the surface where the measured layer 1a and the absorbing layer 1b are in contact is Z, and the condition that the temperature distribution is always symmetrical in the positive and negative directions is satisfied, θs(x, t ) = θ ₀ exp (−a _s x)
It is expressed in the form cos(ωt-a _s x-ε)...(3). Note that a _s is defined by the thermal diffusivity α _s as a _s =(ω/2α _s ) ^1/2 . From this, by comparing equations (1) and (3), θs(x,
The logarithm of the ratio of the amplitudes of t) and θ(t) R=θ _s (x, t)/θ(t) is lnR=A−a _s x=A−(ω/2α _s ) ^1/2 x … (4). Here A=ln{cos(ωt-a _s x-ε)/cos(ωt
−ε)}. Moreover, the phase difference between both waves is Δθ=−a _s x =−(ω/2α _s ) ^1/2 x (5).

Although this invention uses the relationship expressed by equation (4) or (5) in principle, there are problems in actual measurement as described below. That is, when the sample 1 is actually installed in the sealed container 2 and the absorbing layer 1b absorbs electromagnetic waves and generates heat, causing periodic temperature changes, the surface of the layer 1a to be measured that is in contact with the absorbing layer 1b is This is because the temperature distribution is not symmetrical in the positive and negative directions when Solutions to such one-dimensional heat conduction problems have generally been sought for a long time. The solution is very complicated 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 selected appropriately, the angular frequency range where the amplitude ratio R mentioned above is and sample layer thickness (4)
It is known that the formula is followed. The inventors, etc.
We found that the conditions for the linearity of and ω ^1/2 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 paint was stirred in a ball mill for more than 5 hours to form a coating with a film thickness of 6 μm, 10 μm, 19 μm, or 24 μm.
The carbon-butyral resin layer is applied to the absorbent layer 1b,
A double-layer sample with a polyvinyl chloride resin layer as the layer to be measured 1a was prepared, and this was installed as shown in Figure 4. White light was incident on the light source 6 using a xenon lamp, and the amplitude of the pressure wave was measured. We measured the dependence of the film on the film thickness using the frequency as a parameter. Note that the thickness of the absorption layer 1b is 5 to 10 μm.

FIG. 5 shows the dependence of the R value on the film thickness, which was obtained by dividing the value thus measured by the amplitude previously obtained with the reference sample of the absorbing layer 1b only. As can be seen from Figure 5, it is clear that the R value includes higher-order terms of x as shown in lnR=A−α _s0 x−α _s1 x2 (6), but the film thickness is small. In this range, lnR shows a linear relationship with x, so for substances with low thermal diffusivity such as vinyl chloride resin, by setting the film thickness in the range of 5 to 20 μm, the conditions to satisfy equation (4) are sufficiently established. It turned out that there was.

FIG. 6 shows the dependence of the R value on the square root of the angular frequency ω. The R value shows a linear relationship with √, and the slope of this is (1/2α _s ) ^1/2 x from equation (4). If we calculate the thermal diffusivity of polyvinyl chloride from the slope of this straight line, we get α _s = 1.07×10 ^-3 cm ²・sec ⁻¹ , which is higher than the values announced so far from 0.9 to 2.4×10 ⁻³ cm ²・
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 diameter was 0.15 mm.
Thermal diffusivity values were determined in the same manner as in the case of polyvinyl chloride resin using a copper plate with a thickness of . Its value is
1.38 cm ² · sec ^-1 , which is a fairly good agreement with the value of thermal diffusivity, 1.14 cm ² · sec ^-1 , which was calculated with reference to the thermal conductivity and specific heat listed in the chemical handbook, for example. 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 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 of the sample layer is adjusted, Sufficient reliability can also be guaranteed for the measured values of substances that have a low thermal diffusivity and are commonly called heat insulating materials.

FIG. 7 shows an embodiment of the present invention, in which formula (4) is used in principle as in the above one, but a reference sample consisting only of an electromagnetic wave absorbing layer and an absorbing layer are used. and the layer to be measured are separately attached to the sample mounts of two sealed containers of the same shape and size equipped with sound pressure detectors of the same performance, and the two sound pressures are measured when electromagnetic waves are irradiated to these containers. The logarithm of the ratio of the detector output signal intensities is measured.

In Figure 7, 1,101 is the sample, 2,10
2 is a sealed container, 3,103 is an electromagnetic wave entrance window, 4,
104 is an O-ring, 5,105 is a microphone,
6 is a light source, 7 is a chipper, 8,108 is an amplifier,
9 and 109 are lock-in amplifiers, 10 and 15 are arithmetic circuits, 11 is an X-Y recorder, 12 is a beam splitter, 13 and 113 are concave mirrors, and 14 is a scanner, which emits the electromagnetic wave beam 6a modulated by the chopper 7. is split by the beam splitter 12 into light beams 6b and 6c, which are then split by the concave mirrors 13 and 11.
3, the light is focused on the entrance window 3, 103 of the container 2, 102. The sample 1 includes an absorbing layer 1b and a layer to be measured 1.
The sample 101 consists of only the absorption layer 1b.

After the output signal of the microphone 5, 105 as a sound pressure detector is amplified by the amplifier 8, 108, only the signal having the same frequency component as the electromagnetic wave modulation frequency is detected by the lock-in amplifier 9, 109.
amplified. After this, it becomes an input to the arithmetic circuit 10,
The logarithm of the ratio of the output signal strengths of the two microphones 5 and 105 is obtained, and the calculation output 10a is
This becomes the Y-axis input of the recorder 11.

On the other hand, the modulation frequency of Chotsupa 7 is Scanner 14
The reference signal 7a is continuously swept as a function of time, and the reference signal 7a becomes an input to the lock-in amplifier 9,109. Further, the voltage output of the scanner 14 is converted into its square root value by the arithmetic circuit 15, and then the output signal 15a becomes the X-axis input of the X-Y recorder 11. The X-Y recorder 11 operates according to inputs to the X and Y axes, and records the relationship expressed by equation (4). By actually measuring the slope of this recorded straight line, the thermal diffusivity can be immediately determined.

Although the above embodiment is for measuring the absolute value of the thermal diffusivity, it can also be applied to determining the relative magnitude of the thermal diffusivity, such as evaluating the superiority or inferiority of heat insulating materials or evaluating thermal conductive materials. In other words, if the measurement conditions are the same, such as the thickness of the layer to be measured and the modulation frequency of the incident electromagnetic wave, the measured signal strength will be determined only by the degree of attenuation of the temperature wave within the layer to be measured. The magnitude of the signal intensity corresponds to the magnitude of the thermal diffusivity. Therefore, by selecting an appropriate film thickness, relative evaluation of thermal diffusivity can be performed quickly and easily simply by attaching the absorbing layer.

Further, the sealed container 2, 102 may be made of a pressure-electricity converting material so that it also serves as a sound pressure detector.

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 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, so the frequency is changed and the thickness of the layer to be measured is From this, the thermal diffusivity can be determined. Also,
Temperature can be measured by bringing the surface to be measured into contact with background gas in a sealed container, heating this gas, and measuring the resulting pressure fluctuations. Error factors caused by contact resistance of the sample can be eliminated.
Furthermore, by appropriately selecting the thickness of the layer to be measured, it is possible to
It is possible to accurately measure the value of thermal diffusivity over a wide range, even for materials with extremely low thermal diffusivity such as polymer films. In addition, a reference cell is formed by attaching the sample made of the absorbing layer to a second sealed container to obtain a reference signal, and it is possible to quickly detect deterioration of the absorber or medium gas. This also allows the measurement accuracy to be improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the measurement system showing the principle of this invention, Fig. 2 is a side view showing the structure of the sample, and Figs. 3 and 4 are cross-sectional views to explain how the sample is attached to the sealed container. , Fig. 5 is a characteristic diagram showing the relationship between the ratio of the signal intensity from the absorption layer to the measured layer and the signal intensity of the absorption layer alone and the film thickness of the measured layer, and Fig. 6 shows the frequency dependence of the signal intensity ratio. A characteristic diagram to explain
FIG. 7 is a block diagram of a measurement system showing an embodiment of the present invention. 1,101...sample, 1a...layer to be measured, 1b
...Absorbent layer, 2,102 ... Sealed container, 3,10
3... Electromagnetic wave incidence window, 4,104... O ring,
5,105...Microphone, 6...Light source, 7...
...Chiyotsupa, 8,108...Amplifier, 9,109
... Lock-in amplifier, 10, 15 ... Arithmetic circuit, 11 ... X-Y recorder, 12 ... Beam splitter, 13, 113 ... Concave mirror, 14 ... Scanner. Note that the same reference numerals in the figures indicate the same or corresponding parts.

Claims (1)

[Claims] 1 A first device having at least one electromagnetic wave incidence window or sample mounting window and having a required gas sealed inside.
and a second sealed container, an electromagnetic wave generator that generates electromagnetic waves to irradiate the sample, a modulator that modulates the electromagnetic waves, and a first sound pressure detector that detects pressure waves generated in the first sealed container. a second sound pressure detector that detects pressure waves generated in the second sealed container, and a logarithmic value of the ratio of the intensity of the output signals of the first and second sound pressure detectors. A first arithmetic circuit, a second arithmetic circuit that calculates the square root of the modulation frequency of the modulator, and a recorder that uses the outputs of the first and second arithmetic circuits as Y-axis and X-axis inputs. , attaching a sample consisting of a layer to be measured and an absorbing layer that absorbs electromagnetic waves to one of the first and second sealed containers, and a reference sample consisting of an absorbing layer to the other, and irradiating modulated electromagnetic waves; A thermal diffusivity measuring device characterized in that thermal diffusivity is determined from the slope of a straight line drawn on a recorder.