## 3 Results and Discussion
### Topography and Performance of Microheaters  
Figure 12 presents images of a typical microheater annealed at 750 \({}^{\circ}\)C, which were rendered based on data acquired with a non-contact optical 3D profiler. SEM images are presented in Figure 13. The average width of the narrowest part of the microheater could be estimated to be about 35 \(\upmu\)m. The cross-sectional areas of the narrowest part and the initial part of the arms adjacent to the narrowest part are in the ratio of about 1:2.5.  
Figure 11: SEM images of the nanoparticles contained in the ink. (**A**) Front surface of the film. (**B**) End surface of the film. The ink was printed on alumina substrate and then annealed in air atmosphere at 400 \({}^{\circ}\)C for 2 h to remove the binder.  
Figure 10: Size distributions of the colloidal particles obtained by dynamic light scattering. The green curve is characteristic of the as-prepared ink, and the red curve is characteristic of the top layer of the ink after 5 days of storage.  
The microheaters were wired to the copper laminated glass epoxy substrates with 80 \(\upmu\)m thick copper wires by attaching them to the terminal parts of the microheaters (see Figure 1) with PELCO colloidal silver (product No. 16031, Ted Pella, Inc., Redding, CA, USA). The wired microheaters were fed by a direct current (DC) from the power supply in constant voltage mode. To obtain the power dependence of the temperature of the hot part of the microheater, a series of images of its thermal radiation were acquired at various values of consumed power. Figure 14A demonstrates a typical thermal radiation image of the microheater, operated at a power of 200 mW. For comparison, Figure 14B presents an image of the same microheater operated at the same power taken on a Nikon Coolpix L820 digital camera. The temperature of the microheater at a given point could be retrieved from the signal of the corresponding image sensor by using the calibration coefficient. The calibration was performed on a small piece (2 \(\times\) 2 mm\({}^{2}\)) of sintered material exfoliated from the substrate, which was transferred to the large thick film heater with high temperature uniformity. The temperature of the transferred piece of sintered material during the calibration was controlled with a thermocouple. By this means, the emissivity of the microheater was not required in the calculations, since the calibration was performed on the material constituting a microheater.  
Figure 12: Images of a typical microheater annealed at 750 \({}^{\circ}\)C, which were rendered based on data acquired with a non-contact optical 3D profiler. (**A**,**B**) Perspective view in two color schemes. (**C**) Top view and profile of the narrowest part of the microheater. (**D**) Top view and profile of the initial part of the arms adjacent to the narrowest part. The presented profiles correspond to the positions of the light blue lines.  
Figure 15 presents the typical power dependence of the temperature of the hot part of the microheater. At a maximum temperature required for the operation of the MOS gas sensors of 500 \({}^{\circ}\)C, the power consumption was about 140 mW, being comparable with that of microheaters used in commercial MEMS-based MOS gas sensors [38]. The consumed power could be readily reduced by a factor of about 1.5 by printing each arm of the microheater with a wider angle (up to 30 degrees) between two sides. This would result in a sharper decrease in the temperature of the microheater arms in the direction of the boundary of the hole in the LTCC wafer, due to the sharper decrease in the current density.  
Figure 14: Thermal radiation image of the hot part of the microheater operating at a power of 200 mW, acquired with a FLIR SC655 thermal imaging camera (**A**), and an image of the same microheater operating at the same power, taken with a Nikon Coolpix L820 digital camera (**B**).  
Figure 13: SEM images of the microheater presented in Figure 12.