Development of Thermal Diffusivity Measurement Infrared Microscope for Astromaterials
Dec. 11, 2023 | Aerospace Project Research Associate, ISAS people
ISHIZAKI Takuya / Astromaterials Science Research Group, ISAS
Research Summary
Astromaterials Science Research Group operates the Extraterrestrial Curation Center, which is a planetary materials receiving facility to store and manage samples brought back from asteroids by the Hayabusa and Hayabusa2 sample return missions, and is commonly known as the Curation (https://curation.isas.jaxa.jp/en/). By using a high vacuum or nitrogen atmosphere clean chamber (Figure 1) to prevent contamination of valuable samples, the center obtains basic information such as appearance and weight for each individually identified sample and registers it in a database. We also distribute samples to researchers in Japan and abroad. The samples from NASA’s OSIRIS-REx, which is scheduled to return from the asteroid Bennu in October 2023, as well as samples from future planned sample return missions such as MMX, will also be curated for storage and management.
In addition to returned samples, we also store meteorite samples, and we are also working on research on the formation process of the solar system using the returned samples and meteorite samples, and working on methods for analyzing samples. Among them I am researching on a method to measure thermal diffusivity of the samples.
Thermal conductivity is one of the essential physical properties when considering the formation process of Solar System and is used to simulate the evolution of celestial bodies. If we know the thermal conductivity, we can calculate the balance between internal heat generation and heat dissipation to space and that enables to estimate the internal temperature of celestial bodies. In the Hayabusa2 mission, the analysis of physical properties of the returned asteroid Ryugu, including thermal conductivity, revealed that Ryugu's parent body*1 has a diameter of about 100 km and a temperature at which liquid water can exist in abundance inside the body. This is an important result that supports the hypothesis that phyllosilicate minerals, the main component of the Ryugu samples, were formed in water.
However, the returned samples, including the Ryugu samples, are very small (less than several millimeters) and have various shapes (Figure 2), and there has been no method to measure the thermal conductivity of such samples. Because they are so valuable, it is not possible to accumulate them for measurement, nor is it possible to shape them to an ideal shape.
I am, therefore, studying on a new method of measuring thermal diffusivity using a laser and an infrared microscope. By heating a sample with a laser beam and observing the resulting temperature change with an infrared microscope, the thermal diffusivity can be measured in a non-contact way, protecting the sample from contamination or damage. Thermal diffusivity is measured instead of thermal conductivity because it is impossible, in principle, to measure the thermal conductivity of natural shaped samples unlike shape-processed rectangles or cylinders, whereas the thermal diffusivity can be. By measuring thermal diffusivity, it can be converted to thermal conductivity together with specific heat and density. To increase the accuracy of the converted thermal conductivity, the thermal diffusivity must be measured with high precision. In this study, the cyclic heating method and a technique called lock-in are used to achieve highly accurate measurements (Figure 3).
Because the returned samples are small, simply applying a laser beam to the sample when heating will not produce a temperature difference on the surface and will not provide the necessary information. The laser beam is, therefore, turned on and off repeatedly at a frequency of tens of several hertz to generate a temperature wave on the sample surface, and the thermal diffusivity is analyzed from the relationship between the phase lag of the wave and the distance at which the wave diffuses. This is called the periodic heating method. Lock-in is a technique that reduces noise and amplifies the periodic component of temperature change by integrating the temperature response signal multiplied by reference sinusoidal signal. The most distinctive feature of this method is that it simultaneously realizes the non-contact measurement in addition to high-precision measurement by expanding the periodic heating method by combining the infrared microscopy with lock-in technique.
I am currently developing a clean-room version of the measurement device, and I think being a member of the curation group has the great advantage of having the return samples and meteorite samples that we are measuring close at hand, so I am able to carry out efficient development by trial and error in small verification experiments and by sharing information with the people involved.
Terminologies
- *1 parent body : The original celestial body from which the celestial body was formed. Ryugu is thought to have been formed by the reaccumulation of debris from the collisional destruction of its parent body.