JPS5945100B2 - Thermal diffusivity measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermal diffusivity measurement device, and more specifically, it generates pressure waves in a gas sealed in a sealed container by an opto-acoustic effect, and measures the pressure waves to measure the temperature of a sample. The present invention relates to a thermal diffusivity measurement device using an unsteady method for measuring thermal diffusivity.

Now, the thermal diffusivity αs is a coefficient of 5 defined as αs-/cρ, where c is the thermal conductivity, c is the specific heat, and ρ is the density.

In addition, the opto-acoustic stick effect is a method in which an absorber that absorbs electromagnetic waves (hereinafter referred to as electromagnetic waves in a broad sense including light rays, etc.) is placed in a sealed container of gas, and the electromagnetic waves are
When this absorber is irradiated with amplitude modulation at a modulation frequency of about 0 to 2,000 Hz, the absorber generates heat by absorbing the electromagnetic waves, but the amount of heat generated changes according to the modulation frequency. This is a phenomenon in which pressure waves with a frequency equal to the modulation frequency are generated in the gas within the container. The amplitude of the pressure wave in the gas is different when the absorber conducts heat directly to the gas and when it conducts heat to the gas through a layer of thermal diffusivity that is the sample to be measured. The thermal diffusivity of the sample to be measured is calculated from the ratio. In addition to the method of measuring the thermal diffusivity of a sample using the opto-acoustic effect, there are various methods of suppressing the thermal diffusivity using unsteady methods. The method is to forcibly create a thermally non-equilibrium state and measure the change in temperature distribution of the sample that occurs as the state relaxes.Compared to the steady state method, the measurement time is generally shorter, and it is possible to measure only the temperature. It has the characteristic that it can be measured as a function of time.

Typical unsteady methods are the Angstrom method,
There is a flash method, etc.

In 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 it, causing periodic temperature changes at one end of the sample. A temperature wave is generated that propagates in the length direction of the sample, and the state in which this temperature wave propagates within the sample is measured by measuring the temperature at one measurement point of two or more points in the length direction of the rod-shaped sample. The thermal diffusivity is calculated using the amplitude and phase of the temperature wave obtained at each measurement point. In this measurement method, it is necessary to make a rod-shaped sample, and a large amount of sample material is required.In order to minimize heat loss from the sample surface, the measurement device must be equipped with a heat insulating device. Therefore, the equipment becomes large-scale. In addition, since the measurement takes a relatively long time and the temperature is measured by bringing the temperature detection element into contact with the sample, the contact resistance between the sample and the temperature detection element and the heating heat source are brought into contact with the sample to heat the sample. Therefore, the contact resistance between the heating heat source and the sample becomes an error factor. Another drawback is that the samples that can be measured by this measurement method are limited to those that have a relatively large thermal diffusivity. On the other hand, in the flash method, a light absorption layer is provided on one surface of a flat plate sample, and this surface is irradiated with light such as a xenon arc flash or laser pulse for a short time to instantaneously heat the surface by light absorption. 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, and the resulting temperature is measured. Calculate the thermal diffusivity from the time curve.

In this measurement method, the temperature is also measured by contact between the temperature detection element and the sample, and the contact resistance at this 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. Materials with high thermal conductivity such as metals often satisfy this assumption, but The disadvantage is that the smaller the thermal diffusivity, such as a molecular film, the greater the error.
The drawbacks of the conventional unsteady measurement method as described above can be eliminated by a measurement method using the opto-acoustic effect.

A thermal diffusivity measurement method using the opto-acoustic effect and its apparatus will be described below with reference to the drawings. Figure 1 is a cross-sectional view showing an example of a sample used in the method of measuring thermal diffusivity using the opto-acoustic effect. Referred to as a layer to be measured, 1b is an absorption layer that absorbs electromagnetic waves and converts them into heat. The absorption layer 1b is prepared, for example, by applying a solution consisting of carbon black and a binder resin. Z indicates a contact surface between the layer to be measured 1a and the absorption layer 1b, and heat propagates from the absorption layer 1b to the layer to be measured 1a via this contact surface. FIG. 2 is a cross-sectional view showing an example of a conventional sealed container used in a thermal diffusivity measurement method using the opto-acoustic effect, where numerals 1, 1a, and 1b are the same as those in FIG. is a sealed container, 3 is an electromagnetic wave incidence window,
4 is an O-ring, and 5 is a microphone for detecting sound pressure (generally speaking, a sound pressure detector).

The sample 1 is attached to the electromagnetic wave entrance window 3 with an adhesive tape or the like, and the space inside the sealed container is kept airtight by an O-ring 4 and a gasket (not shown) provided at the mounting portion of the microphone 5. FIG. 3 is a sectional view showing another example of a conventional sealed container. In FIG. 3, the same reference numerals as in FIG. When it is transparent, the sample 1 may be attached to the wall surface on the opposite side from the electromagnetic wave entrance window 3, as shown in FIG. FIG. 4 is a block diagram showing an example of a thermal diffusivity measuring device using the opto-acoustic effect. The same reference numerals as in FIG. laser light sources, etc.

A chopper 7 performs amplitude modulation on the intensity of light irradiated from the light source 6 to the electromagnetic wave entrance window 3 using a predetermined modulation frequency.

6a shows this modulated light.

8 is an amplifier; 9 is a so-called lock-in amplifier for inputting the reference signal 71 from the chopper 7 and measuring the amplitude and phase of a frequency component equal to the frequency of the reference signal 71 among the outputs of the amplifier 8; Although not shown in FIG. 4, the output of the lock-in amplifier 9 is connected to a recorder for recording this output.

Next, a measurement method using the apparatus shown in FIG. 4 will be explained.

In the first measurement, the sample 1 is composed only of the absorbing layer 1b and does not include the layer to be measured 1a. If the angular frequency of the modulated signal output from the chopper 7 is ω, the absorption layer 1b absorbs the modulated light 6a and converts it into heat, and this heat is absorbed by the gas in the sealed container 2 that comes into contact with the back surface of the absorption layer 1b. (hereinafter referred to as background gas), the background gas causes periodic temperature fluctuations corresponding to ω, and since the background gas is sealed within a constant volume, pressure waves occur. Occur.吸収層１ｂがバツクグラウンドガスと接する側の表面での温度の変動成分θ（ｔ）はθ（ｔ）＝θ０ｃ０ｓ（ωｔ−ε） ゜゜゜゜゜゜゜゜゜８゛゜゛゜゜゜゜゜゜゜゜（１）で表される。

Here θ. If the absorption coefficient of the absorption layer 1b is sufficiently large,
This value is determined by the thermal conductivity and specific heat of the absorption layer 1b itself. Further, ε is the initial phase angle. By measuring the amplitude Q of the pressure fluctuation inside the sealed container 2, θ(t) is indirectly measured.

Q=γPOOO/? 1, a, T0...
...There is the relationship (2).

Here, PO is the pressure inside the sealed container 2, TO is the temperature inside the sealed container 2, γ is the ratio of constant pressure specific heat to constant volume specific heat of the background gas, A8 is the thermal diffusivity of the background gas, 18
is the thickness of sample (1) (length in the direction perpendicular to the sample surface). From equation (2), it can be seen that when the conditions inside the sealed container (2) are determined, the amplitude θo of the fluctuation component of the sample surface temperature and the amplitude Q of the pressure fluctuation are proportional to each other.

Next, the same measurement is performed on a sample consisting of two layers, the layer to be measured 1a and the absorption layer 1b.

In this case, the surface Z where the absorption layer 1b is in contact with the layer to be measured 1a
The temperature of is shown by equation (1). This temperature wave θ(t)
propagates within the layer to be measured 1a and becomes θ8(
X, t). Here, X indicates the thickness of the layer to be measured 1a. Similarly to the relationship between θ(t) and Q, the relationship between the temperature θ8(X, t) of the surface of the measured layer 1a in contact with the background gas and the amplitude of the pressure fluctuation that occurs in the background gas is expressed by the equation ( 2). Sato θ8 (
Although the relationship between X, t) and θ(t) is generally complicated,
The heat flow occurs only in the direction perpendicular to the surface of the sample (1) (in other words, the heat flow is one-dimensional), and the origin is the boundary surface Z, and the heat flow is in the direction of the thickness of the layer 1a to be measured. is positive and the opposite side is negative, if the condition that the temperature distribution is always symmetrical in the positive and negative directions is satisfied, and -C, then θs(X, t
)=θ0exp(-A8x)COs(ωt-A8x-ε
)・・・・・・・・・・・・It can be expressed in the form of (3).

In formula (3), Za,=(Q/2α,)...
......The value defined by (31), and the formula (31
) is the thermal diffusivity of the layer to be measured 1a.

R=θ8(X,t)/θ(t) ・・・・・・・・・(
32) Then, find the logarithm 1nR of R and obtain.

Here it is.

Further, the phase difference Δθ between θ8(X, t) and θ(t) is as follows.

Now, in equation (32), R is the amplitude ratio of θ8(X, t) and θ(t), and in equation (5), Δθ is the amplitude ratio of θ8(X, t) and θ(t).
), but in actual measurements using the relationship of equation (2) above, the conditions in the sealed container (2) were kept the same, and the sample with only the absorbing layer 1b and the absorbing layer 1b and the layer to be measured were measured. The same measurement is carried out for the sample having 1a, and the amplitude ratio of the output of the lock-in amplifier 9 for the different sample is set as R, and the phase difference is set as Δθ.

That is, in the measurement method using the opto-acoustic effect, the temperature is measured indirectly without bringing the temperature detection element into contact with the sample (1). Among the assumptions for deriving equations (4) and (5), the assumption that the temperature distribution is symmetrical in both positive and negative directions with respect to the Z plane differs from the actual situation.

When the flow of heat is one-dimensional, the temperature distribution in the sample (1) with plane Z as the boundary has been analyzed for a long time, but the shape becomes complicated and it is difficult to put it into practical use. Measured layer 1a
If the parameters of heat loss between and absorbing layer 1b are appropriately selected, ω
It is known that equation (4) holds true within a predetermined range of and x. Moreover, the range of x for which equation (4) holds is 5 to 20 μm for the layer to be measured 1a made of a heat insulating material such as a polymer film or glass, and 20 μm to 0.5 for the layer to be measured made of a conductive material such as a gold layer. It has been experimentally confirmed that it is a song. Therefore, if the thickness of the layer 1a to be measured is adjusted to the above-mentioned thickness range, sufficiently reliable measured values can be obtained even for substances with a small thermal diffusivity, which are commonly called heat insulating materials. However, the larger the thermal diffusivity of the material, the larger the film thickness can be made to make the measurement easier. Now, since it is difficult to calculate the value of A in equation (4), in actual measurement, at least two 1's corresponding to at least two different values of ω are calculated.
The thermal diffusivity α is calculated by measuring the 1NR(7> value and eliminating A from the two equations corresponding to equation (4). The preferred method is to calculate the thermal diffusivity α for as many ω as possible. Measure 1nR, and in formula (4), 2nR!;
As shown in Figure 1 (explained in a later section), there is a linear relationship between ; and 1nR are plotted on rectangular coordinates, and α8 is calculated from the slope of a straight line connecting these plotted points using the method of least squares.

According to this method, the reliability of this measurement method can be judged based on whether the plotted points are on a straight line or not, and it can also be concluded that the assumptions used to derive equation (4) are actually valid assumptions. It has the advantage of being able to determine whether it is hot or not. Although the method for measuring thermal diffusivity using the opto-acoustic effect has been described above, the conventional measuring device used in this measuring method has drawbacks that should be improved.

The drawback is that in the region where the modulation frequency ω is high, the S/
This is because the N ratio decreases. That is, the amplitude Q (formula (2)) of the pressure wave inside the sealed container 2 is inversely proportional to the modulation frequency ω,
Q<Xω1 (6), and as ω increases, Q decreases, and the S/N ratio of the output of the microphone 5 decreases,
Measurement reliability decreases. If the time constant of the lock-in amplifier 9 is increased in order to compensate for this decrease, the measurement time is increased, which not only deteriorates the measurement efficiency but also increases the possibility of measurement errors due to other causes. The present invention was made to eliminate the above-mentioned drawbacks of conventional devices, and to achieve this purpose, resonance to acoustic vibrations is generated in a sealed container to increase the sensitivity to pressure waves even at high modulation frequencies. Embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 is a cross-sectional view showing one embodiment of the present invention, showing only the portion corresponding to the portion shown in FIG. 2 of the conventional device.

In FIG. 5, the same symbols as in FIG. 2 indicate the same parts, and 2
Reference numeral 1 designates an acoustic cavity resonance generating rod-shaped body for generating acoustic cavity resonance at a predetermined frequency within the sealed container 2 . By changing the shape and dimensions of this rod-shaped body 21, the acoustic resonance frequency of the sealed container 2 can be changed. Furthermore, it is well known that in order to generate acoustic cavity resonance in the sealed container 2, members of various shapes and sizes in addition to the rod-shaped body 21 can be provided in the sealed container 2 to generate various acoustic cavity resonances. Therefore, the rod-shaped body 21 is generally referred to as a resonance member. In this way, in the sealed container 2 having an acoustic resonance frequency, the output of the microphone 5 is S-Q・μ・F・
...can be expressed as r. In equation (7), Q
is Q in equation (2), μ is the sensitivity of microphone 5, and F
is an acoustic characteristic factor. F is a function of the frequency ω, and is always 1 in a sealed container that does not have an acoustic resonance frequency.
FIG. 6 is a characteristic curve diagram showing an example of the frequency characteristics of the output of the microphone 5, where the horizontal axis shows the modulation frequency (logarithmic scale), and the vertical axis shows the output of the microphone 5 (logarithmic scale in μv units).

The solid line in Figure 6 has an inner diameter of 25nφ and a length of 801! 1 stainless steel cylinder is used as a sealed container (2), and a stainless steel cylindrical rod 21 with a diameter of 24 nφ and a length of 65 m1 is attached to this. Sample 1 is made of 35% carbon blank and 65% polyvinyl butyral binder resin. These are the values actually measured for a coating film obtained from a coating material which was dissolved in an appropriate amount of ethyl alcohol and stirred in a ball mill for 5 hours or more, and the absorption layer 1b was made of a coating film. The broken line in FIG. 6 is the output voltage value of the microphone 5 obtained when the sealed container 2 having no acoustic resonance frequency is used. The measurement method when using the measuring device shown in Fig. 5 is the same as that explained earlier with respect to Fig. 4, so a general explanation will be omitted, but as shown in Fig. 6, The value of is between 1 and 1 at frequencies above 500 Hz, and is approximately 7 near 1,400 Hz, so the time required to measure the R value in equation (4) is shorter than with conventional measuring equipment. For example, at 1,000Hz, 1/
2. At 1,400Hz, the time is reduced to 1/7, and the accuracy is significantly improved. Furthermore, as described above, the resonance frequency can be set at an appropriate point as the structure allows the dimensions of the resonance member 21 to be changed as necessary. FIG. 7 is a graph showing an example of measurement results using the measuring device of the present invention, in which the horizontal axis represents V; the vertical axis represents 1nR, and the measurement points are arranged on a straight line. It shows the reliability of the measurement.

Figure 7 shows measurements using the measuring device and sample used for the measurements in Figure 6, and measurements using a sample in which a polyvinyl chloride resin with a film thickness of 10 μm was brought into contact with the absorption layer 1b of the sample as the layer 1a to be measured. This is the value of 1nR obtained. Calculating the thermal diffusivity of polyvinyl chloride from the slope of the straight line in Figure 7, α8-1
．． 07×10-3C! ! 1SeC-1, and the previously announced value is 0.9 to 2.4 x 10-3cTi1sec1
Therefore, it can be estimated that the measured value is accurate. In addition, copper was selected as a sample with a large thermal diffusivity of 0.1
Using a copper plate with a thickness of 5 mm and using the apparatus shown in Figure 5, α8-1.38? - Obtained the value of Sec1.

This value agrees fairly well with the value 1.14i·Sec-1 calculated from the thermal conductivity K1 specific heat C1 density rho of copper described in the Chemical Handbook. Moreover, the time required for these measurements is 1/2 to 1/2 compared to when using conventional measuring equipment.
1/3, and the measurement S/N is improved at each frequency of 500 Hz or higher, so the reliability of the measured value is improved. In addition, in the measuring device of the present invention, instead of configuring the sealed container 2 and the microphone 5 separately as shown in FIG. Needless to say, the microphone may be configured with a type, electromagnetic type, electrodynamic type, etc. microphone.

FIG. 8 is a block diagram showing another embodiment of the present invention, in which the same symbols as in FIGS. 4 and 5 indicate the same or corresponding parts, and 2a, 3a, 5a, 8a, 9a, and 21a are respectively 2, 3, 5, 8, 9, and 21, respectively, a second sealed container, a second electromagnetic wave incidence window, a second microphone,
2, 3, 5, 8, 9, and 21 are respectively referred to as a second amplifier, a second lock-in amplifier, and a second resonance member, a sealed container, a first electromagnetic wave incidence window, a first microphone, a first amplifier, a first lock-in amplifier,
It will be referred to as a first resonance member.

10 is an arithmetic circuit, 11 is an X-Y recorder, 12 is a beam splitter, 13 is a first concave mirror, 13a is a second concave mirror, 14 is a scanner, and 15 is an arithmetic circuit.

The light 6a modulated by the chopper 7 is split into light 6b and light 6c by the beam splitter 12, and the light is entered into the electromagnetic wave entrance oblique windows 3, 3a through concave mirrors 1j, 13a, respectively.

The sample in the first sealed container 2 has only the absorbent layer 1b, and the second
The sample in the sealed container 2a is composed of the same absorption layer 1b and the layer to be measured 1a as the sample in the first sealed container 2. Therefore, the outputs of the lock-in amplifiers 9 and 9a are input to the arithmetic circuit 10, the logarithm of the ratio is created, and 1nR shown in equation (4) is obtained and applied to the Y axis of the XY recorder 11. The scanner 14 outputs a signal that sweeps the frequency ω of the modulation signal to control the frequency of modulation by the chopper 7, and at the same time inputs a signal representing the frequency ω to the calculation circuit 15.
5 outputs a signal representing Ji and inputs it to the XY recorder 11.

Therefore, the straight line shown in FIG. T is automatically drawn on the X-Y recorder 11. What should be noted in this case is that the acoustic resonance frequency characteristics (characteristics shown in FIG. 6) of the sealed containers 2 and 2a must be exactly matched. For this reason, either of the resonance members 21, 21a must be provided with an adjustment section to align the frequency characteristics of the acoustic cavity resonance of both sealed containers. The device of the present invention has been described above in relation to a method of measuring the absolute value of thermal diffusivity, but the relative value of thermal diffusivity is a problem, for example when evaluating the superiority or inferiority of thermal insulation materials or the evaluation of thermal conductive materials. Needless to say, the apparatus of the present invention can be used in cases where the following conditions occur, and measurements can be made more quickly and easily with the apparatus of the present invention.

In other words, if the measurement conditions include the thickness of the layer to be measured, the modulation frequency of the incident electromagnetic wave, etc., the measured signal strength is determined only by the degree of attenuation of the temperature wave within the layer to be measured, and the signal strength Since the magnitude corresponds to the magnitude of the thermal diffusivity, if the film thickness of the sample is appropriately selected, a relative evaluation of the thermal diffusivity can be performed quickly and easily simply by attaching an absorbing layer. As explained above, according to the present invention, the characteristics of the measurement method using the opto-acoustic effect are exhibited, and the amplitude of the pressure fluctuation, which decreases at a relatively high frequency, is amplified by using the acoustic cavity resonance of the sealed container. By performing the measurement, the measurement time can be shortened and the reliability of the measured values can be improved.

In addition, by appropriately selecting the thickness of the layer to be measured, it is possible to accurately measure a wide range of materials, from materials with high thermal diffusivity such as copper to materials with extremely low thermal diffusivity such as polymer films. The advantage is that it can be done.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing an example of a sample used in the thermal diffusivity measurement method using the opto-acoustic effect, and FIG. 2 is a cross-sectional view showing an example of a conventional sealed container used in the thermal diffusivity measurement method. Fig. 3 is a sectional view showing another example of a conventional sealed container, Fig. 4 is a block diagram showing an example of a thermal diffusivity measuring device using the opto-acoustic effect, and Fig. 5 is an embodiment of the present invention. FIG. 6 is a characteristic curve diagram showing an example of the frequency characteristics of the output of the microphone in FIG. 5, FIG. The figure is a block diagram showing another embodiment of the invention. In these drawings, 1 is a sample, 1a is a layer to be measured, and 1b is a sample.
is an absorption layer, 2 is a sealed container, 3 is an electromagnetic wave entrance window, 4 is an O-ring, 5 is a sound pressure detector, 6 is a light source, 7 is a chipper, 8
is an amplifier, 9 is a lock-in amplifier, 21 is a resonance member,
2a is a second sealed container, 5a is a second high pressure detector, 9a
is the second lock-in amplifier, 10 is the arithmetic circuit, and 11 is X.
-Y recorder, 12 is a beam splitter, 13 is a first concave mirror, 13a is a second concave mirror, and 21a is a second resonance member.

Claims (1)

[Claims] 1. A generator that generates an amplitude modulated electromagnetic wave whose modulation frequency can be changed; a first sample configured with an absorption layer that absorbs the amplitude modulated electromagnetic wave and converts it into heat; A second sample formed by adhering a layer to be measured whose thermal diffusivity is to be measured to an absorbing layer similar to that of the first sample, an absorbing layer of the attached sample with the first sample or the second sample attached. a sealed container configured to allow the amplitude modulated electromagnetic waves to be incident on the container;
a sound pressure detector for detecting a pressure wave generated in the gas sealed in the sealed container due to the heat converted from the amplitude modulated electromagnetic wave due to absorption by the absorption layer of the first sample or the second sample; and measuring the thermal diffusivity of the layer to be measured from the output of the sound pressure detector when the first sample is mounted in the sealed container and when the second sample is mounted in the sealed container. In the measuring device, a resonance member is provided in the sealed container so as to generate acoustic cavity resonance in the gas sealed in the sealed container at a frequency near the amplitude modulation frequency of the amplitude modulated electromagnetic wave generated by the generator. A thermal diffusivity measurement device characterized by: 2. The thermal diffusivity measuring device according to claim 1, wherein the resonance member is provided with an adjustment section that can change the frequency of acoustic cavity resonance. 3. The thermal diffusivity measuring device according to claim 1, wherein the sound pressure detector is constituted by part or all of a sealed container. 4. A first sealed container with a first sample attached thereto; a resonance member, a first sound pressure detector for detecting pressure waves in the first sealed container, a second sealed container in which a second sample is attached, and a container sealed in the second sealed container. a second resonant member disposed within the second sealed container to generate acoustic cavity resonance in the gas having the same frequency characteristics as the acoustic cavity resonance generated in the first sealed container;
Claim 1, further comprising a second sound pressure detector having the same characteristics as the first sound pressure detector and detecting pressure waves within the second container. Thermal diffusivity measuring device.