JPH0718828B2 - Specific heat measurement method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for measuring the specific heat of solids and liquids, and in particular, it has a high specific heat capacity even in a high temperature range of 700 ° C. or higher, which has been difficult to measure in the past. It enables measurement.

[Conventional Technology and Problems] Conventional methods for measuring specific heat include an adiabatic method, an alternating current heating method, a dropping method, a levitation method, a differential scanning calorimetry method, and a direct pulse current heating method. (For example, "Magrich, Cesaryan, Perecky", "Introduction to Thermophysical Properties, Volume 1, Review of Measurement Techniques" (1984) Planham Press, New York; Maglic, Cezairliyan, Peletsky, "Compendium o
f Thermophysical Property Measurement Methods, Volu
me 1, Survey of Measurement Techniques ", (1984
), Plenum Press, New York) 7 of these methods
The methods applicable to the high temperature region of 00 ° C or higher are the dropping method, the levitation method, and the direct pulse current heating method. The dropping method and the levitation method are based on the increase in the water temperature when the sample held at a high temperature is dropped in the water bath. Is measured and the specific heat is calculated from the temperature differential of the enthalpy, so the measurement accuracy of the specific heat is not always sufficiently high, and the measurement requires a very long time (measurement at several points per day).

The levitation method and the direct pulse current heating method are limited to conductive materials, require elaborate and expensive measuring equipment, and sophisticated measuring technology, and are performed only at several research institutions in the world.

For the above reasons, there is a demand for a specific heat measuring method capable of measuring the specific heat up to 700 ° C. or higher in a short time not only for the conductive material but also for the non-conductive material. In order to meet these demands, the surface of a disk-shaped small sample (diameter 5 to 15 mm, thickness 0.5 to 3 mm) is instantaneously heated by a high-power pulse laser, and the laser flash is used to determine the specific heat from the temperature rise of the sample. Attempts have been made to develop a method for measuring specific heat capacity. This method can be applied to non-conductive materials, and in principle, it is possible to measure up to a high temperature at which the furnace can be heated.

The biggest problem in laser flash specific heat measurement is that it is not easy to accurately measure the amount of heat absorbed by the sample and the rise in sample temperature. In order to obtain the energy absorbed by the sample, the absolute value of the absorptance of the sample surface at the wavelength of the irradiation laser beam is necessary together with the energy density of the irradiation beam. The energy density of the irradiation beam is usually spatially non-uniform and varies from pulse to pulse (± 10%), and is high due to many error factors such as loss due to window materials, mirrors, lenses, etc., laser power calorimeter accuracy, etc. Accuracy assessment is extremely difficult.

The following method is mentioned as a first attempt to solve these problems. First, at the sample position, install a standard sample with a known specific heat (heat capacity) with a thin plate with a fixed absorption rate attached,
Calibrate the energy absorbed by the sample with a standard sample.
Next, the standard sample is removed, and the measurement sample to which the same thin plate for laser beam absorption is attached is placed at the same position. The absorbed energy of the sample is evaluated for each pulse by monitoring the energy fluctuation of each pulse, and the specific heat is measured from the value of the temperature rise of the measurement sample at that time.

(For example, Yoichi Takahashi, "Measurement of Thermophysical Properties by Laser Flash Method", Published by The Japan Society for Thermophysical Properties, Thermophysical Properties, Volume 1,
No. 1, 1987, p8-11) Due to factors such as adhesion between thin plate and sample and its reproducibility, monitoring accuracy of laser beam energy fluctuation, reproducibility of installation positions of standard sample and measurement sample, etc. There is a possibility of error, and high precision measurement is not easy. In addition, since a spatially non-uniform multi-mode pulse laser is usually used as a pulse heating source,
It takes several seconds for the entire sample to reach a uniform temperature, the sample temperature changes due to heat radiation, and a thermocouple is used for temperature measurement. I haven't been.

As a second attempt, a known laser beam with a spatially constant energy density was used, and the absorptance of the sample surface with respect to the laser beam and the emissivity at the effective wavelength of the radiation thermometer for measuring the backside temperature were measured to determine the specific heat. A method of directly obtaining the absolute value has been proposed. (Terao Arai, Tetsuya Baba, Akira Ono "Possibility of local heat capacity measurement by laser flash method", published by The Japan Society for Thermophysical Properties, Vol. 1, No. 2, 1987, p78
~ 80) This method does not require a thin plate for absorbing a laser beam and a standard sample, and does not involve the complexity of measurement like the former measurement. In addition, since the temperature rise on the back surface of the sample is measured using a radiation thermometer without using a thermocouple, and the spatially homogenized laser beam is used, thermal diffusion in the in-plane direction of the sample is accompanied, so that the temperature is within 50 ms. Since the measurement is completed in a time, it is less affected by the cooling due to the heat radiation from the sample surface, and the specific heat can be measured at a higher temperature than the former method.

However, even with this method, it is easy to obtain the absorbed energy of the sample with an accuracy of ± 10% or more due to factors such as the reflection and absorption of the laser beam by the window material, the measurement accuracy of the laser calorimeter, and the monitoring accuracy of the laser beam energy fluctuation. Not. Further, it is considered that there is an uncertainty of at least about ± 5% in the radiation temperature measurement on the back surface of the sample, so that highly accurate measurement is extremely difficult.

[Object of the Invention] An object of the present invention is to provide a specific heat measuring method capable of measuring the specific heat of conductive and non-conductive solid materials from room temperature to the maximum temperature at which the furnace can be heated with high accuracy and in a short time. To do.

[Means for Solving the Problems] According to the present invention, the object of the present invention is to obtain a spatially uniform energy for a surface of a standard sample / measurement sample which are installed in close proximity and whose front and back surfaces are blackened under the same conditions. Simultaneous irradiation with a pulsed radiant heating source having a distribution, the rise in the backside temperature of both is measured using either a radiation thermometer or a thermal imager, and the ratio of the temperature rise of the measurement sample to the standard sample and the value of the standard sample are measured. This is achieved by deriving the specific heat of the measurement sample from the assigned standard value of the specific heat.

[Operation] In the above means, since the specific heat of the measurement sample is obtained from the ratio of the temperature rise of the measurement sample to the standard sample with reference to the specific heat of the standard sample, both the absolute value of the absorbed energy of the sample and the absolute value of the temperature rise of the sample are It is not necessary and it is necessary to evaluate the measured value of the pulsed radiation heating laser beam energy density, which is the largest error factor in the laser flash specific heat measurement, and the evaluation of the sample absorbed energy based on the sample absorption rate, which is the next error factor. Sex can be removed.

Since it is a method of measurement by radiant heating and radiant temperature measurement, there is no upper limit of the measurement temperature in principle, and the upper limit of the actual measurement temperature depends on the upper limit of the operating temperature of the sample heating furnace and the heat resistant temperature of the sample and the blackened surface. Determined.

Embodiments of the Invention Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an example of a specific heat measuring device according to the present invention, in which a laser beam 2 emitted from a high-power pulse laser 1 has a spatially uniform energy distribution on a sample surface by a laser beam homogenizing optical system 3. Convert to obtain. The homogenized laser beam 4 is reflected by the mirror 5 and is uniformly applied to the standard sample 9 and the measurement sample 10 in the vacuum chamber 6. Standard sample 9
And the measurement sample 10 is installed close to the sample holder 8,
It is heated to the measurement temperature by the heater 7. The inside of the vacuum chamber is kept at a high vacuum better than 10 ^-5 ) torr in order to suppress heat loss from the sample due to convection and to prevent oxidation and contamination of the sample. The temperature rise on the back side of the sample is targeted at the center of the standard sample surface and the center of the back side of the measured sample.
(Or a thermal imager). This is two 1
It is composed of a set of radiation thermometers of the same structure, and it is necessary to adjust each radiation thermometer so that they have the same sensitivity. The two-channel output signal from the radiation thermometer is recorded in the transient memory 12 and transferred to the personal computer 13, where the specific heat of the measured sample is calculated from the temperature rise ratio, the standard value of the standard sample specific heat, and the mass of the standard sample / measured sample. It is calculated.

Hereinafter, the principle of calculating the specific heat will be described in detail with reference to FIG. The standard sample 9 and the measurement sample 10 placed close to each other receive the homogenized laser beam 4. At this time, the disc-shaped standard sample and the measurement sample have the same diameter, the surfaces are blackened in the same state, and the energy absorbed by both samples is the same.
The increase in thermal radiation after pulsed radiation heating depends on the emissivity of the back surface of the sample, but the ratio of the outputs of the two target radiation thermometers 11 depends on the emissivity because the surfaces of both samples are blackened to the same state. Exactly without.

Under these measurement conditions, the specific heat C _m of the measurement sample is calculated as follows from the standard specific heat value C _s of the standard sample. The absorptance of the blackened surface of the standard sample and the measurement sample for the pulsed radiation heating source is α, the emissivity for the target radiation thermometer is ε, the mass of the standard sample is M _s , and the mass of the measurement sample is M _m . The two samples are installed in a vacuum so that the contact area with the sample holder is minimized, and if the measured temperature is lower than 500 ℃ and the effect of heat radiation is small, consider that they are insulated from the external environment. You can In this state, from the definition of specific heat for each standard sample and measurement sample, Where ΔT _s is the temperature rise of the standard sample, ΔT _m is the temperature rise of the measurement sample,
A is the area of the standard sample and the measurement sample to be radiatively heated, and q is the energy density of radiant heating. The radiation thermometer measures not the change in the true temperature of the sample but the change in the spectral radiance L (λ, T) of the sample at the effective wavelength λ of the radiation thermometer. The spectral radiance of the sample is represented by the product of the spectral radiance L _b (λ, T) of the black body determined only by the wavelength and the temperature and the spectral emissivity ε of the back surface of the sample at the wavelength λ. Therefore, the change ΔV in the radiation thermometer output when the sample temperature rises from T to T + ΔT is expressed by the following equation.

ΔV = aL (λ, T + ΔT) -aL (λ, T) = aεL b (λ, T + ΔT) -aεL b (λ, T) = a, ε · ΔT · ЭL b (λ, T) / ЭT (3) Here, a is the sensitivity of the radiation thermometer.

Therefore, letting ΔV _{s be} the change in the radiation thermometer output that displays the brightness temperature for the standard sample, and ΔV _m be the change in the radiation thermometer output that displays the brightness temperature for the measurement sample.

ΔV _s = aε ・ ΔT _s・ ΦL _b (λ, T) / ΦT (4) ΔV _m = aε ・ ΔT _m・ ΦL _b (λ, T) / ΦT (5) (1) Formula (4), Substituting equation (5) into equation (2) and taking the ratio of the two, Therefore Becomes This equation does not include the energy density q of radiant heating, the absorptivity α of the sample surface, the emissivity ε, the sensitivity a of the radiation thermometer, and the specific heat of the measured sample is q, α, ε without measuring, It means that it is obtained from the value of the output ratio ΔV _s / ΔV _m of the radiation thermometer. In this way, the ratio, not the absolute value of the output, needs to be accurately determined. Therefore, it is not necessary for the two-target radiation thermometer to have an accurate temperature scale, and the two-target radiation whose sensitivity is adjusted to the same value. An equivalently accurate specific heat measurement can be performed using a meter.

The heat radiation from the sample surface increases in the measurement at high temperature of 500 ℃ or more, and even if the heat transfer due to convection and conduction from the sample is suppressed to the minimum, the adiabatic condition of the sample is not achieved. In such a case, the temperature rise of the back surface of the sample after pulsed radiation heating does not converge to a constant value, and approaches 0 exponentially as sampled in FIG. FIG. 3A shows a method for correcting the heat loss due to heat radiation for the standard sample and FIG. 3B for the measurement sample. The outputs ΔV _s ′ and ΔV _m ′, which correspond to the case where the adiabatic condition is satisfied, are obtained by extrapolating the exponential function up to the time of pulsed radiation heating. At that time, the specific heat of the measurement sample is given by the following equation.

In the present invention, the standard sample and the measurement sample are heated with the same energy density by the spatially uniform radiant heating source,
In reality, a completely uniform beam cannot be obtained. Also, the sensitivities of the two-target radiation thermometers do not match perfectly. The error due to such a cause may be obtained by exchanging the relative positions of the standard sample and the measurement sample with respect to the heating beam and the two-target radiation thermometer and using the average value of the measurement results at the original position and the exchange position. The energy density of the radiant heating applied to the standard sample and the measurement sample in situ is (1 + δ) q and (1-δ), respectively.
q, 2 The sensitivity of the target radiation thermometer is (1
+ Sigma) a, When a (1-σ) a for the measurement sample, the output [Delta] V _s1 to the standard sample in situ, the output [Delta] V _m1 to the measurement sample are expressed by the following equation.

ΔV _s1 = (1 + σ) aε ・ ΦL _b (λ, T) / ΦT ・ αA (1 + δ) q / (M _s C _s ) (8) ΔV _m1 = (1-σ) a ε ・ ΦL _b (λ, T) / ΦT ・ αA (1-δ) q / (M _m C _m ) (9) Let ΔV _{s2 be} the output for the standard sample at the conversion position and ΔV _m2 be the output for the measurement sample, and ΔV _s2 = (1-σ) aε ・ ΦL _b (λ, T) / ΦT ・ αA (1-δ) q / (M _s C _s ) (10) ΔV _m2 = (1 + σ) aε ・ ΦL _b (λ, T) / ΦT ・ αA (1 + δ) ) Q / (M _m C _m ) (11) Define the average value of the output as ▲ ▼ _s = (ΔV _s1 + ΔV _s2 ) / 2 ▲ ▼ _m = (ΔV _m1 + ΔV _m2 ) / 2 (8) (9) From the equations (10) and (11), the following equation is obtained, The non-uniform heating parameter δ and the non-uniformity parameter σ of the two target radiation thermometer sensitivities disappear, and errors due to these are eliminated.

Isotropic graphite (trade name: POCO AXM5Q
An example of measuring the specific heat of 1) at 400K will be described with reference to FIG. Sapphire (single crystal alumina) was used as a standard sample. Sample size is 6 mm in diameter and 2 in thickness
mm, and the front and back surfaces of both samples are blackened in the same state. Spatially uniform energy density (about 1.7 Jcm ^-2 )
The increase in the backside temperature of the sample after pulsed radiation heating at 2 is measured by a two-target radiation thermometer. For the standard sample sapphire, Fig. 4 (a) shows that isotropic graphite Is shown in FIG. 4 (b).
The maximum change of the radiation thermometer output is ΔV _s = 0.227V for the standard sample sapphire, and ΔVs is 0.227V for the measurement sample graphite.
V _m = 0.478V. The standard specific heat value of the sapphire standard sample is C _s = 0.942Jg ^-1 K _-1 , and the mass of the standard sample is M _s =
The specific heat of the measurement sample is 0.219g and the mass of the measurement sample is 0.0978g. Is required. For the POCO AXM5Q1 graphite, the recommended value of 0.995 Jg ^-1 K ^-1 was given by the National Institute of Standards and Technology (NIST) (JGHust, A fine-grained, isotropic gr
aphite for use as NBS thermophysical property RM's
from 5 to 2500K.Natl.Bur.Stand.Special Publ.260-8
9 (1984)), the difference from the measured value is within 1%.

[Effects of the Invention] As described above, according to this specific heat measuring method, the specific heat of all solid materials including non-conductive materials, which has been difficult to measure up to now, can be reduced to a high temperature of 2000 ° C or higher in a short time. It is possible to measure with high accuracy. INDUSTRIAL APPLICABILITY The present invention is applicable to new materials such as new ceramics developed for the purpose of advanced energy utilization, reactor materials and nuclear fuels in the field of peaceful use of nuclear power, composite materials and functionally graded materials in the field of aerospace, etc. On the other hand, it will be a highly accurate and most practical method for measuring the specific heat up to high temperature, and it will promote the development and utilization of new materials in these fields.

[Brief description of drawings]

FIG. 1 is a block diagram of a measuring device for sampling an embodiment of the present invention. FIG. 2 is a principle diagram of measurement showing a procedure for calculating specific heat. FIGS. 3 (a) and 3 (b) are diagrams showing a method of correcting heat loss due to heat radiation in high temperature measurement. Figure 4 (a)
And (b) are graphs of measurement examples demonstrating the action of the present invention. 1 ... Large output pulse laser 2 ... Laser beam 3 ... Laser beam homogenizing optical system 4 ... Uniform laser beam 5 ... Mirror 6 ... Vacuum chamber 7 ... Heater 8 ... Sample holder 9 ... … Standard sample 10 …… Measured sample 11 …… 2 Target radiation thermometer 12 …… Transient memory 13 …… Personal computer

Claims (1)

[Claims] 1. The front and back surfaces of both the standard sample and the measurement sample are blackened under the same conditions, and the absorptance, which is the proportional coefficient of temperature change, and the emissivity, which is the proportional coefficient of thermal radiation change, are measured with the standard sample, respectively. The surfaces of the standard sample and the measurement sample, which are placed in close proximity to each other, are simultaneously irradiated with the pulsed radiation heating sources that have the same energy distribution in the samples, and the relative values of the backside temperature change of both are heat-treated. A specific heat measuring method characterized by deriving the specific heat of a measurement sample based on a ratio of changes in thermal radiation from the two and a standard value of the specific heat valued to the standard sample, measured by a radiation detector.