In the Qpedia Issue 99 article, “Minimizing Thermocouple Errors in Electronics Thermal Characterization,” we examined how the use of thermocouples can alter the measurements that are made, whether it is because the thermocouple itself acts as a fin that dissipates heat, or any of a number of other reasons [1]. We saw that even use of a very fine 36-gauge thermocouple wire can introduce measurement error of more than 5%.
(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)
The influence of the sensor on the measurement can be even more challenging as the item under test becomes smaller and more sensitive to ambient influences. As an example, a thin film made of carbon nanotubes presents an extreme challenge, as the film is only 100 nm thick, or about 1/1000 of the diameter of an average human hair. At this scale, the material has practically no thermal mass, so the temperature of the film can easily be affected by most temperature transducers. In order to create a sensor small enough to measure temperatures on the thin film, Shrestha et al. chose to use a glass micropipette as a base [2]. A sensor was then fabricated by creating a junction of dissimilar metals, using the same thermoelectric principles as a thermocouple.
Thermocouple sensors had been made using micropipettes previously, but past examples used materials that were much more expensive, and the construction was very complex. The platinum and gold materials were chosen because of their use in a biological environment, but as a thermocouple junction, they had a low voltage output in response to temperature differences [3,4]. The sensors that Shrestha et al. created were meant to address these issues.
Glass micropipettes can be made with tips so small that they are commonly used to inject substances into individual living cells. The tip diameters that Shrestha et al. created ranged from 2 to 30 µm. The micropipettes were made using a micropipette puller which heated a 1.5mm glass tube and then pulled the tube apart, creating a finely tapered section (see Figure 1a). The micropipettes were then filled with a tin based soldering alloy (Figure 1b), and a micropipette beveler was used to sharpen and achieve the finished tip shape (Figure 1c).
Fig. 1 – Creating a Micropipette Thermocouple. [2]
The next step in the construction was to coat the outside of the micropipette with a nickel film using a sputtering process (Figure 1d). On the end of the tip where the tin was exposed, the deposition of nickel created the working thermocouple junction. The deposition conditions were varied to create different film thicknesses, which we will examine later. Finally, lead wires made of tin and nickel were attached to the respective materials in the sensor (Figure 1e).
The sensors were calibrated in a water bath at temperatures from 21 to 40°C, while the cold junction was maintained at a constant 24.5°C. The voltages generated by the sensors at each temperature were recorded using a Nano voltmeter, as the measurement scale was in the range of 100µV. The calibration curves for some of the sensors are shown in Figures 2 and 3 below, where a linear response is clearly demonstrated. It can also be seen that the thicker layer of nickel significantly increased the sensitivity of the sensor, more than doubling its voltage output for a given temperature.
Fig. 2 – Voltage response of a Micropipette Thermocouple with most-nickel deposition. [2]
Fig. 3 – Voltage response of a Micropipette Thermocouple with least-nickel deposition. [2]
Shrestha et al. [2] saw sensitivities as high as 8.9 µV/°C, compared to the average of 2.1 µV/°C for the platinum-gold thermocouples made by Kakuta et al. [4]. Fish et al. [3] achieved about 7 µV/°C from their Pt-Au thermocouples, but as a point of reference, a standard J-type thermocouple has a sensitivity of about 40 µV/°C [4]. With all of the thin film-based thermocouples, the output signal is low, and measurement noise can become a problem if the environment is not well controlled. Some of the researchers found it necessary to use preamplifiers to boost the output voltage of the sensors, which complicates setup and can introduce noise.
It should also be noted that because of the sizes of all of these sensors and the test subjects, the micropipette sensors need to be positioned with micromanipulators, a basic version of which is shown in Figure 4 below.
Fig. 4 – Sutter MM-33 Micromanipulator. [6]
Clearly these are not quite like the standard thermocouples that we use for standard thermal measurements, and the setup and fabrication of the sensors is more involved. The payoff is sensors that have an extremely fast response time to temperature changes, only a few microseconds in air [3], as well as very little thermal influence on the samples they are testing, which is critical at the small scales that Shrestha et al. examined. In addition, Shrestha et al. determined that the measurement accuracy of their sensors was 0.01°C, which compares very favorably to a standard J-type thermocouple. Even with the “special limits of error” grade wire, J-type accuracy is only about 1ºC. [5]
None of the three researchers quantified the possible heat transfer through the sensor, but they were confident that it was negligible even in the context of their micrometer-scale measurements. With sensor diameters from 2 to 30 µm, the cross-sectional area is so small that the thermal resistance via conduction is very high.
This approach may be excessive for most electronics thermal management purposes, but it does illustrate that even something as familiar as a thermocouple can be approached in many different ways. The sizes of these sensors allow them to make temperature measurements with pinpoint precision, and to identify very small heat sources and heat flow paths. One such application could be localizing hot spots on a CPU die, for example. Such small sensors could generate a temperature map by traversing the surface of a die.
They could also be used to measure any small component or heat sink where a standard thermocouple would draw too much heat away. As Shrestha et al. show with their temperature measurements on a 100 nm thick film [2], the applications can vary widely, and are limited only by our preconceptions of what thermocouples are capable of.
References
1. Advanced Thermal Solutions, Inc. (ATS), “Inaccuracies in Thermocouple Measurements,” Qpedia, Issue 99.
2. Shrestha, R., Choi, T. Y., Chang, W. S., “Measurement of Thermal Conductivity of Thin and Thick Films by Steady State Heat Conduction,” University of North Texas, Denton, TX, 2012.
3. Fish, G., Bouevitch, O., Kokotov, S., Lieberman, K., Palanker, D., Turovets, I., Lewis, A., “Ultrafast Response Micropipette-Based Submicrometer Thermocouple,” The Hebrew University of Jerusalem, Jerusalem, Israel, 1994.
4. Kakuta, N., Suzuki, T., Saito, T., Nishimura, H., Mabuchi, K., “Measurement of Microscale Bio-Thermal Responses by Means of a Micro-Thermocouple Probe,” University of Tokyo, Tokyo, Japan, 2001.
5. “Thermocouple – Thermocouple types – J,” http://www.thermometricscorp.com/thermocouple.html, [Dec. 16, 2015].
6. “Sutter Instrument Company – MICROMANIPULATION,” http://www.sutter.com/MICROMANIPULATION/mm33.html, [Dec. 22, 2015].
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.