JPS5925975B2 - Heat capacity measurement method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Thermal conductivity, heat capacity, and thermal diffusivity can be measured by instantaneously irradiating a sample with a hot ray and observing the temperature rise of the sample.

This method was proposed by Parker et al. and is known as the three-constant measurement method of heat. That is, if the relationship between the time from the time when the heat ray was irradiated and the temperature rise T on the back side of the sample is recorded, a curve as shown in FIG. 1A can be obtained in principle. The thermal diffusivity is calculated from the time tx when this curve reaches 1 of the maximum value Tm and the thickness of the sample, and the above maximum value Tm is compared with a similar value for a standard sample with a known heat capacity. Therefore, the heat capacity is required. Furthermore, the thermal conductivity can be calculated from the heat capacity and the density of the sample. This method has the advantage of being able to use an extremely small sample compared to other methods of measuring heat capacity and the like, as well as being able to perform measurements over a wide temperature range from low to high temperatures. However, in the past, the temperature change of the sample was directly detected, so it was not possible to accurately detect the temperature rise of the sample due to the influence of temperature changes in the outside world, and due to the heat loss of the sample, the curve as shown in Figure 1B was observed, and since the shape of this curve is affected by the properties of the sample, the atmosphere, and the temperature difference, it was not possible to accurately determine the maximum value Tm of temperature rise. For this reason, it had the disadvantage of being less accurate than other measurement methods such as the adiabatic method. The present invention eliminates the above-mentioned drawbacks by using a hot bath and determining the theoretical value of the maximum temperature increase by extrapolation, and will be described in detail below. FIG. 2 is a diagram showing a longitudinal section and a configuration of a part of the apparatus for carrying out the method of the present invention. Although not shown, the shielding tube 1 is placed in a heat-resistant glass container, the inside of which is evacuated, and The container is suspended in an electric furnace or in a refrigerant such as liquid nitrogen.

A cylindrical heat bath 2 made of a material with good thermal conductivity and large heat capacity, such as copper, is installed inside this shield tube 1, and a heater 3 is wound around the outside of the bath 2 and connected to a power source 4. be. Furthermore, the output of the thermocouple 5 attached to the heat bath 2 is added to the controller 6, and the power supply 4 is controlled so that the output becomes equal to a desired reference voltage. Bath 2
maintains the desired constant temperature. For example, a plate-shaped sample 9 is supported by a holder 8 with a quartz pick or the like inside this hot bath 2.
is installed, and the sample is surrounded by the heat bath so that as little heat conduction as possible occurs between the sample 9 and the heat bath 2. In addition, a thin plate 10 of carbon or the like is coated with vacuum grease or the like on the top surface of the sample 9.
is attached to absorb as much of the irradiated heat rays as possible, and the amount of absorption does not change depending on the surface condition of the sample. Furthermore, an auxiliary tube 11 is fitted inside the heat bath 2 to improve the stability of the heat applied to the sample, and contacts 12 and 13 of the differential thermocouple are attached to the bottom surface of the sample 9 and the heat bath 2. Both ends thereof are connected to a digital voltmeter 14. The output of this voltmeter 14 is connected to a computer 15, an XY plotter 16 and a printer 17 are connected to the computer, and a thermometer 19 is connected to another thermocouple 18 attached to the heat bath 2. Further, a reflective plate 20 and an aperture 21 that are translucent to the heat rays are provided so as to face the upper opening of the shielding tube 1, and an energy detector 22 is provided on the side of the reflective plate and its output is added to an integrator 23. There is. Although not shown in the drawings, each thermocouple wire is wound several times around an anchor 24 formed at the bottom of the heat bath 2 to stabilize the contact temperature. In addition to the above-mentioned device, aperture 2
As shown by the dotted arrow from above 1, parallel light emitted by, for example, a laser light source is irradiated for a short period of several milliseconds.

A part of this light ray is reflected by the reflection plate 20 and detected by the energy detector 21, and the integrated value of the heat ray energy is measured by the integrator 23. Also, a part of the above light beam is reflected by the reflection plate 2.
The heat rays contained in the rays transmitted through 0 are absorbed by the sample 9. Therefore, a temperature rise occurs in the sample 9, and the temperature reaches a maximum value with a slight time delay after irradiation with the heat ray, and thereafter the temperature gradually decreases (mainly due to the dissipation of radiant heat). In addition, since the heat bath 2 with a large heat capacity maintains a stable constant temperature during that time, the digital voltmeter 14 to which the output of the differential thermocouple made up of the contacts 12 and 13 is applied is as shown by the curve X in FIG. The output is sent and added to the computer 15. In this curve, the horizontal axis is time and the vertical axis is the temperature difference between the sample and the heat bath, and the part where the temperature decreases is due to heat radiation from the sample to the heat bath. Since the maple and hot bath maintain a constant temperature as mentioned above, the curve clearly forms an exponential curve. Therefore, if we extend this curve to find the value at the time of irradiation of the heat ray, that value will be calculated as the difference in temperature between the sample and the heat bath, assuming that there is no time delay in temperature rise. The maximum value is
As shown by curve A in FIG. 1, this corresponds to the maximum value Tm when there is no heat dissipation. In other words, the amount of heat lost per unit time from a sample whose temperature has increased due to irradiation with heat rays DQ/Dt
is proportional to the temperature difference T between the sample and the heat bath 2, so if the proportionality constant is h, then the following holds true.

This also holds true because the ratio Q/T between the amount of heat Q and the temperature difference T is defined as the total heat capacity C, which is the product of specific heat and mass.

Therefore, from the above two equations, the following can be obtained.

As is well known, the solution to this equation is L, where P is the constant of integration, and this equation (1) shows that the temperature difference T between the sample and the hot bath forms an exponential function curve. There is.

And the temperature difference T that occurs during heat ray irradiation is T as described above.
If m, then by setting time t as O, it can be written as Tm=P.

This formula is the virtual temperature difference Tm at the time of heat ray irradiation at t=o
is the maximum temperature difference between the sample and the heat bath. Furthermore, if we take the natural logarithm of both sides of the above equation (2), we get:

In this way, since the logarithm of the temperature difference T between the sample and the heat bath at each time draws a straight line, the computer 15 samples the output of the digital voltmeter 14 at appropriate time intervals, The natural logarithm is taken and stored, and it is added to the XY plotter 16 for recording. Find the linear range of the recorded curve, give a command to the computer 15, use the least squares method to calculate a more accurate straight line, and use extrapolation to calculate the virtual line when the phantom time t is O. The temperature difference Tm is determined and recorded on the printer 17. Furthermore, since the integrator 23 indicates a value proportional to the amount of heat rays irradiated to the sample 9, the amount of heat rays Qm can be calculated using a proportionality constant determined in advance using a known sample. Since there is a relationship between the heat capacity C of the sample, the temperature difference Tm, the heat ray amount Qm}, and the mass m of the sample, the heat capacity C can be easily calculated from the temperature difference Tm recorded on the printer. be able to.

As described above in the embodiments, the present invention utilizes the fact that when a sample is irradiated with heat rays, the temperature difference between the sample and the heat bath reaches a maximum value and then falls, forming an exponential function curve. This method uses extrapolation to find the theoretical maximum value of the temperature rise, which can prevent errors caused by heat loss in the sample.

In addition, since a heat bath is provided and the temperature difference between this heat bath and the sample is observed, the temperature around the sample is kept accurately constant, and the above-mentioned downward curve forms an exact exponential function curve. Therefore, the heat capacity of the sample can be determined accurately. Note that when observing the curve X in Figure 1 using the device described above and determining the time Tx for the temperature of the sample to reach Tm/2, when the thickness of the sample is e, the thermal diffusivity α is ! ! is given by ~.

Therefore, from this thermal diffusivity α, the heat capacity C obtained as described above, and the density d of the sample, the thermal conductivity K is K=α・C−
It is determined by d.

[Brief explanation of drawings]

FIG. 1 shows a curve for explaining the principle of the invention, and FIG. 2 shows an example of an apparatus for carrying out the method of the invention.

Claims (1)

[Claims] 1. Surround the sample whose heat capacity is to be measured in a heat bath with good thermal conductivity and large heat capacity so that no heat conduction occurs between them, and instantaneously apply a hot wire of known energy to the sample. At the same time as the irradiation, the temperature difference between the sample and the heat bath was continuously observed, and the hypothetical value at the heat ray irradiation time was determined by extrapolation from the relationship between the time and temperature difference in the process of decreasing the temperature difference. 1. A method for measuring heat capacity, characterized in that the heat capacity of a sample is calculated from the temperature difference, the mass of the sample, and the known energy.