The overall goal of the following video is to introduce the transient electro thermal technique. This technique is an effective way to accurately measure the thermal diffusivity of solid materials. To begin the measurement, the sample is suspended between two copper electrodes.
Silver paste is applied to the sample electrode contact area to reduce the thermal and electrical contact resistances to a negligible level. The whole sample is then housed in a vacuum chamber to reduce the heat transfer to the air. During the measurement, a step current is fed through the sample to induce JUUL heating.
The induced voltage time profile will be recorded by the oscilloscope. A higher thermo diffusivity of the sample will lead to a faster temperature evolution, meaning a shorter time to reach the steady state. Therefore, this transient voltage temperature change can be used to determine the thermo diffusivity.
Theoretical fitting of the normalized experimental temperature rise is conducted by using different trial values of the thermo diffusivity of the sample. The value giving, the best fit of the experimental data is taken as thermo property of the samples. Please check the manuscript for the detailed theoretical background.
The TD techniques has several advantages of what the techniques people have used in the past. First, it broadens the material scope we can measure. Now we can measure both conductive and non-conductive materials.
Second, it significantly improves the environmental accuracy and the stability. Our graduate student, Juan Lin, will show the procedure in this work. Human head hair samples collected from a 30-year-old healthy female are used to show how to set up the experiment and process the experimental data.
First, suspend the sample between two copper electrodes. Silver paste is then applied at the sample electrode contact area to reduce the thermo and electrical contact resistances to a negligible level. Next, the microscope is used to do the preliminary check of the sample.
We must make sure that the silver paste hasn't contaminated the suspended sample by seeping further down the hair's length. Once the hair is contaminated with silver paste, the thermal properties will be drastically changed. If any contaminations are noticed, a new sample needs to be prepared for the experiment.
Since human head hair samples are not electrically conductive, a very thin layer of gold film about 40 nanometers is coated outside the sample to make it electrically conductive. This gold effect on the thermo diffusivity will be subtracted from the final result. When processing the experimental data, now put the sample in the vacuum chamber and pump it to one to three milour, resulting in negligible gas conduction effects.
A step DC current is then fed through the sample to introduce electrical heating. The induced voltage time profile will be recorded by the oscilloscope. At last.
A scanning electron microscope will be used to characterize the length and diameter of the samples. Theoretical fitting of the normalized experimental temperature rise is conducted by using different trial values of the thermal diffusivity of the sample. The value giving, the best fit of the experimental data is taken as the thermal property of the samples for human head hair.
Two samples are coated with gold film twice in tested twice respectively. We have four effective thermo diffusivity and from equation 12 in the manuscript. We know coding the sample with gold film will only change the resistance.
The point where the fitting curve intersects with a thermo diffusivity access is the value of effective thermo diffusivity when the resistance is infinite, which means there is no gold effect. By combining these two points, the relationship between the real thermo diffusivity, including the radiation effect and L squared over D can be revealed. We do a linear extrapolation to the point of L equals zero, which means no radiation effect, and the thermo diffusivity at that point is 1.42 times 10 to the negative seven meters squared per second.
This value reflects the thermo diffusivity of the sample without the effect of radiation in the influence of the gold coating. For real thermal conductivity, it can be easily evaluated by this equation if density in specific heat are given. The TD technique is a very effective and a robust approach to measuring the sum physical properties or materials now for the same material using two different lengths.
Finally, we can measure the sum connectivity, some diffusivity. Also the surface effectivity if the density and the specific heat of the material is given.
