The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials.
The TET (transient electro-thermal) technique is an effective approach developed to measure the thermal diffusivity of solid materials, including conductive, semi-conductive or nonconductive one-dimensional structures. This technique broadens the measurement scope of materials (conductive and nonconductive) and improves the accuracy and stability. If the sample (especially biomaterials, such as human head hair, spider silk, and silkworm silk) is not conductive, it will be coated with a gold layer to make it electronically conductive. The effect of parasitic conduction and radiative losses on the thermal diffusivity can be subtracted during data processing. Then the real thermal conductivity can be calculated with the given value of volume-based specific heat (ρcp), which can be obtained from calibration, noncontact photo-thermal technique or measuring the density and specific heat separately. In this work, human head hair samples are used to show how to set up the experiment, process the experimental data, and subtract the effect of parasitic conduction and radiative losses.
The TET technique1 is an effective approach developed to measure the thermal diffusivity of solid materials, including conductive, semi-conductive or nonconductive one-dimensional structures. In the past, the single wire 3ω method2-4 and the micro-fabricated device method5-9 have been developed to measure the thermal properties of one dimensional structures at the micro/nanoscale. In order to broaden the measurement scope of materials (conductive and nonconductive) and improve the accuracy and stability, the transient electro-thermal (TET) technique has been developed for characterization of thermophysical properties of micro/nanoscale wires. This technique has been used successfully for thermal characterization of free-standing micrometer-thick Poly (3-hexylthiophene) films10, thin films composed of anatase TiO2 nanofibers11, single-wall carbon nanotubes1, micro/submicroscale polyacrylonitrile wires12, and protein fibers. After eliminating the effect of parasitic conduction (if the sample is coated with a layer of gold to make it electronically conductive) and radiative losses, the real thermal diffusivity can be obtained. Then the real thermal conductivity can be calculated with a given value of volume-based specific heat (ρcp), which can be obtained from calibration, noncontact photo-thermal technique, or measuring the density and specific heat separately.
1. Experiment Procedure
2. Data Processing
Normalize the experimental temperature rise first, and conduct the theoretical fitting of that by using different trial values of the thermal diffusivity of the sample. This procedure is discussed in Guo's work1 in detail. Then subtract the effect of radiative losses and parasitic conduction on thermal diffusivity, and calculate the thermal conductivity. Details are given below.
Fitting of the experimental data for human head hair sample 1 (length 0.788 mm, coated with gold film only once) is shown in Figure 3. Its thermal diffusivity is determined at 1.67 x 10-7 m2/sec, which includes the effect of radiative losses and parasitic conduction. Figure 4 is a typical SEM image of human head hair. The short and long samples are coated with gold film twice and tested twice, respectively, based on Equation 12, the effect of parasitic conduction can be easily subtracted by curve fitting as shown in Figure 5. The point where the fitting curve intersects with the αeff-axis is the value of αeff when the resistance is infinite, which means the effect of parasitic conduction in Equation 12 is 0. Two human head hair samples with different lengths are measured to obtain two intersects. Details about the experimental conditions and measurement results are summarized in Table 1. By combining these two points, the relationship between αeff and L2/D can be revealed. From the measured pairs of ( α1, L12 / D1 ) and ( α2, L22 / D2 ), linear extrapolation (as shown in Figure 6) is done to the point of L=0 (meaning no effect of radiative losses), and thermal diffusivity at that point is 1.42 x 10-7 m2/sec [=α1 – (α1–α2 ) * L12 / D1 / (L12 / D1–L22 / D2) ]. This value reflects the thermal diffusivity of the sample without the effect of radiative losses and parasitic conduction.
For human head hair, the density is characterized by weighting several strands of hair and measuring their volume, and is measured at 1,100 kg/m3. The specific heat is measured by using DSC (Differential Scanning Calorimetry) and is measured at 1.602 kJ/kg K. So the real thermal conductivity is 0.25 W/m K. Details of experimental parameters and results for human head hair sample 1 and 2 are shown in Table 1.
Figure 1. A) schematic of the TET experiment setup and B) a typical V–t profile. Click here to view larger image.
Figure 2. The difference between T* and its approximation using Equation 9. Click here to view larger image.
Figure 3. Comparison between the experimental data and theoretical fitting result for the normalized temperature rise versus time (human head hair sample 1). Click here to view larger image.
Figure 4. A typical SEM image of human head hair. Click here to view larger image.
Figure 5. The fitting results for the thermal diffusivity change against 1/R for the human head hair sample 1 and 2. Click here to view larger image.
Figure 6. The fitting result for the real thermal diffusivity of human head hair samples. Click here to view larger image.
Human head hair samples | Sample 1 (short) |
Sample 2 (long) |
Length (mm) | 0.788 | 1.468 |
Diameter (mm) | 74.0 | 77.8 |
αreal+radiation (x 10-7 m2/sec) | 1.48 | 1.62 |
αreal (x 10-7 m2/sec) | 1.42 | |
ρcp (x 106 J/m3 K) | 1.76 | |
Real thermal conductivity (W/m K) | 0.25 |
Table 1: Details of experimental parameters and results for human head hair.
In the experiment procedure, three steps [step 2), 3) and 5)] are very critical for the success of characterizing thermal properties accurately. For step 2) and 3), much attention needs to be paid on applying silver paste only at the sample-electrode contact. It is very easy to contaminate the suspended sample with silver paste, and the thermal properties will increase if this happens. So in step 3), check the sample with microscope carefully, if any contamination-the silver paste is applied or extended to the suspended sample-is noticed, a new sample needs to be prepared for the experiment.
When Equation 10 is simplified to Equation 11, it is assumed that the experiment is conducted in a vacuum chamber at very low pressure (1-3 mTorr), so the gas conduction effect is negligible. After doing a series of test at different pressures, it is confirmed that, in Equation 10, the gas conduction coefficient h is proportional to the pressure p as h=γp. The coefficient γ is related to a parameter called thermal accommodation coefficient that reflects the energy coupling/exchange coefficient when the gas molecules strike the material surface. γ can be calculated as ξπ2DρcP / (4L2) where ξ is the slope of the thermal diffusivity against pressure. γ varies from sample to sample. This gas conduction factor can be strongly affected by the material surface structure and the spatial configuration in the chamber during TET characterization. For step 5), conducting the experiment at very low pressure (1-3 mTorr) will make sure that this complicated gas conduction effect is negligible.
Surface emissivity (ε) of the samples measured by this technique can also be calculated with the given value of volume-based specific heat (ρcp), which can be obtained from calibration, noncontact photo-thermal technique13-15 or measuring the density and specific heat separately. After subtracting the effect of parasitic conduction, the thermal diffusivity (αreal+rad) shown in Figure 6 only has the effect of radiative losses, . It is easy to know that:
(13)
Here T0 is the room temperature, L the diameter of tested samples, and D the diameter of the sample.
There are several limitations of the TET technique. First, the characteristic time ∆tc for the thermal transport in the sample, which equals to 0.2026L2/α1, should be much larger than the rise time (about 2 µsec) of the current source. Otherwise, the accuracy of the voltage evolution will be affected significantly. So it requires that the sample length L should not be too small or the thermal diffusivity α should not too big. Second, the temperature of the sample will rise by about 20-30° in the experiment. Within this range, the resistance of the sample should have a linear relationship to temperature. That is because in the part of theoretical background, it is known that the measured voltage change is inherently related to the temperature change of the sample. If the resistance of the sample does not have a linear relationship to temperature, the voltage evolution cannot stand for the temperature evolution. Third, the voltage of the sample should have a linear relationship to the DC current fed during the experiment. This means at a certain temperature, the resistance will not change when the DC current changes. It is well known that semiconductors do not have this property.
In conclusion, the TET technique is a very effective and robust approach to measuring the thermal properties of various types of materials. For the same material, just test two samples with different length each twice, all the important thermal properties of the materials, such as thermal diffusivity, thermal conductivity, and surface emissivity (if ρcp is given), can be characterized.
The authors have nothing to disclose.
Support of this work from Office of Naval Research (N000141210603) and the Army Research Office (W911NF1010381) is gratefully acknowledged. Partial support of this work from the National Science Foundation (CBET-0931290, CMMI-0926704, and CBET-0932573) is also acknowledged.
Digital Phosphor Oscilloscope | Tektronix | DPO 3052 | |
Sputter Coater | Denton Vacuum | DESK V | |
AC and DC Current Source | KEITHLEY | Model 6221 | |
Laboratory Microscope | Olympus | BX41 | |
Dual Stage Rotary Vane Vacuum Pump | Varian | DS102 | |
Vacuum Chamber | Huntington Mechanical Laboratories | Customized Product | The pressure in the chamber should be as low as 1-3 mTorr when working with the vacuum pump |
Colloidal Silver Liquid | Ted Pella | 16031 |