# Development and Characterization of a DC-Driven Thermal Oscillator Using Acrylate-Based Composites
## 4 Measurement Results and Discussion  
### Transient Behaviors  
Figure 6a illustrates the experimental setup for measuring the behaviors of the proposed thermal oscillator. In order to carefully control the boundary temperature for better characterization, the device was mounted on a thermoelectric cooler (TEC1-12706, Hebei I.T. Co.). The hot side of the thermoelectric cooler was water-cooled. The temperature of the cold side of the cooler could be adjusted by the applied voltage. Three K-type thermocouples were used to measure the temperatures at the regions close to the PTC composite sensor (\(T_{PTC}\)), the heater (\(T_{h}\)), and the cold side of the thermoelectric cooler (\(T_{c}\)), respectively. Figure 4: Fabrication process for the proposed thermal oscillator. Figure 5: Fabrication results. (**a**) The bottom layer. (**b**) Assembled DC-driven thermal oscillator. The outputs of the thermocouples were acquired by a temperature input module (NI-9212, National Instruments Co.). The voltage drop across the divider resistor (\(V_{div}\)) was measured by a data acquisition device (USB-6341, National Instruments Co.). By using Figure 1c, \(R_{PTC}\) can be easily evaluated because \(V_{s}\), \(R_{div}\), and \(V_{div}\) are given values. Figure 6b shows a photo of the measurement of the thermal oscillator with the thermocouples and cooler. The initial resistances of the PTC composite sensor (\(R_{PTC-ini}\)) and the microheaters (at 25 \({}^{\circ}\)C) were measured by using a multimeter. To confirm the fabrication reproducibility of the thermal oscillator, these measurements were repeated for two other PTC composite sensors and two microheaters from separate batches of fabrication. The results show that the initial resistances of the PTC composite sensors range from 310 \(\Omega\) to 450 \(\Omega\), and the resistances of microheaters range from 24 \(\Omega\) to 26 \(\Omega\). The resistance vs. temperature curves for the devices with PTC films of different OA concentrations are shown in Figure 7. The graphite concentration was fixed at 25 wt%. Obviously, the resistivity of the material remained almost constant in the low-temperature region, but suddenly began to increase sharply when the temperature exceeded its \(T_{m}\). In addition, as shown in Figure 7, the \(T_{m}\) of acrylate-based PTC materials is a strong function of OA concentration. A PTC material with a \(T_{m}\) lower than room temperature must be cooled to below room temperature in order to make a semicrystalline-to-amorphous transition, although heating is not required to make an amorphous-to-semicrystalline transition. On the other hand, a PTC material with a \(T_{m}\) higher than room temperature can reach semicrystalline-to-amorphous transition with the assistance of the environment's temperature. Figure 6: (**a**) Illustration of experimental setup. (**b**) Measurement of the thermal oscillator with thermocouples and cooler. Figure 7: Thermal response of acrylate PTC composites. It is quite simple to implement microheaters with other MEMS components monolithtically, while active cooling devices are usually either bulky or difficult to integrate. Therefore, in order to avoid using an external highly efficient cooler, a PTC material with 80 mol% OA was chosen for the studies in the subsequent experiments. Figure 8 shows the typical transient behavior of the proposed device with three consecutive cycles. \(V_{h}\) and \(V_{s}\) were 8.5 V and 7 V, respectively. \(T_{c}\) was 25 \({}^{\circ}\)C, and \(R_{div}\) was 400 \(\Omega\). As the device reached periodical steady state, \(T_{h}\) oscillated from 40.7 \({}^{\circ}\)C to 92.5 \({}^{\circ}\)C. The temperature behaviors of the PTC temperature sensor lagged behind that of the heater by about 1/5 of a period, and \(T_{PTC}\) stably oscillated between 26.8 \({}^{\circ}\)C to 27.6 \({}^{\circ}\)C. In addition, the corresponding \(R_{PTC}\), which was calculated by using the given values of \(V_{s}\), \(R_{div}\), and \(V_{div}\), is also shown in the figure.