On the stability and dynamic behavior of cascaded thermoelectric devices

Abstract

Fig. 1. Schematic representation of the course of a light beam. showing a penetration and displacement. for total internal rdection. Motion of the absorbing medium within the penetration depth INTERNAL REFLECTION changes the absorption, hence modulates the light. SURFACE \LIGHT RAY PATH INPUT INTERNAL GERMANIUM OUTPUT INPUT INTERNAL GERMANIUM OUTPUT FERRITE. . ' . F E~R I TE BLOCK RING Fig. 2. Schematic drawing and the modulator shoaing a cutaway side view. The input and output faces of the light pipe were cut at an angle of 16".Fig. 3. Typical wavelength dependence of modulation amplitude at 12 c/s. 50 1 I t 4 0 1 L I 1 modulator. A high number of reflections and a large depth of penetration are obtained near the critical angle, both of which can be responsible for large modulations. A larger depth of penetration is especially necessary if the working surfaces are not optically flat.The angle of incidence for the modulator shown in Fig. 2 was chosen as 1 6 ' . (The critical angle for the germanium-air interface is 14.S0.)This reflection element is supported at the ends by a cylindrical ferrite ring (Ferroxcube type 2P-644-45/7A2) which was so chosen in order to minimize the differential thermal expansion. Good results were obtained when the ring support was slightly higher than the center ferrite and the element was mechanically depressed towards the ferrite block by means of a micrometer drive (not shown). A dc bias current applied to the solenoid was employed for fine adjustment of the distance separating the reflection element and the ferrite block. An ac signal current applied to the solenoid served to expand and contract the ferrite block and thus move its surface in and out of the penetration depth of the light, thereby modulating the absorption of the light. Figure 3 shows typical but not optimized measured modulation amplitude vs. the wavelength of the light for six internal reflections. Although a modulation of only about 15 percent at 2.5 microns is shown, a modulation depth of 25 percent a t 2.5 microns was easily obtained and a maximum of 40 percent was obtained by carefully adjusting the separation between the reflection cell and the ferrite block. Modulation was observed over the frequency range investigated, which was 12 c/s to 15 kc/s. A general decrease in modulation amplitude was observed with increasing frequency of as much as an order of magnitude a t the higher frequency end. This decrease was attributed to expected inertial effects.We believe this work demonstrates the feasibility of using this type of a modulation system for low-frequency (chopper) modulation applications where mechanical choppers are not desirable. Larger modulation amplitudes (close to 100 percent) are probably obtainable with the use of surfaces of improved flatness.The dynamic response of single-stage thermoelectric devices and the static behavior of single-stage and thermally cascaded devices have been studied by many researchers. However, little work has been done on the dynamic response of cascaded devices. The purp...