Wilhelm Bier

Gas to Gas Heat Transfer in Micro Heat Exchangers

Abstract By the method of mechanical manufacture of microstructures jointly developed by the Karlsruhe Nuclear Research Center (KfK) and Messerschmitt-Bolkow-Blohm (MBB) very compact cross-flow micro heat exchangers have been fabricated whose active volumes are 1 × 1 × 1 cm3 and whose typical channel cross-sections are 75 × 90 μm2. Measurements with nitrogen, argon and helium as heat transfer fluids were performed on two copper and stainless steel finned plate heat exchangers. With the stainless steel heat exchanger overall heat transfer coefficients up to 1700 W m−2 K−1 have been attained in the range investigated using helium as the fluid. For low flow rates greatly reduced overall heat transfer coefficients have been measured compared with the values to be anticipated for laminar flow. In some ranges higher heat transfer rates have been achieved with the stainless steel heat exchanger compared with the system made from copper. This leads to the conclusion that the heat transfer behavior for low gas flow rates is largely determined by heat conduction in the longitudinal direction within the wall and fin materials. With a homogeneous model the influence of longitudinal heat conduction in the stationary walls can be explained and is in fairly good agreement with the experimental data.

Fabrication of microlenses by plasmaless isotropic etching combined with plastic moulding

Abstract In appropriate mixtures of bromine and fluorine BrF3 is generated, which can then be used for the structuring of silicon under room conditions without plasma support. Direct reaction of the intermediary BrF3 with silicon results in the formation of SiF4 and bromine. By further addition of fluorine, the etching reaction can be started again. Bromine then acts as a catalyst. In spite of the high etching rate, the roughness of the etched surfaces remains small. By adding xenon to the etching gases, the roughness can be reduced to a minimum. Thermally produced SiO2 can be applied as the etching mask. Complete isotropy of the etching process allows underetching of closely adjacent LIGA structures. Moreover, optical application is possible due to the good quality of the etched surfaces. When underetching small circular holes in the SiO2 mask, spherical depressions are generated. After the SiO2 mask has been removed, these structures can be moulded in plastic and used as microlenses.

Fabrication of microlenses by combining silicon technology, mechanical micromachining and plastic molding

Silicon can be subjected to plasmaless isotropic etching in mixtures of elemental bromine and fluorine. BrF3 is generated in the etching process. This ensures a high etching rate on smooth surfaces. The addition of noble gases, e.g. xenon, allows extremely smooth surfaces to be etched. Thermally oxidized SiO2 layers are applied as the etching mask. Among other applications, this technique can be used to manufacture microlenses. As a consequence of the complete isotropy of the etching process, spherical depressions of 100 to 500 micrometers in diameter are produced in the silicon when small circular holes of 5 to 50 micrometers are underetched in the SiO2 mask. After removal of the SiO2 mask the silicon sample can be used as a mold insert for plastic molding. The molded microlenses have been checked dimensionally and verified optically. The microlenses are planned for technical use in a miniaturized endoscope. This requires further processing of the silicon sample. As no hemispherical recesses but calotte shells are needed, the silicon surface must be machine prior to molding. This is done by microgrinding with variable-grain diamond tools on CNC high- precision machines. To generate adjusting devices, stoppers, and holding structures, the ground silicon sample and a mechanically microstructured perforated plate are combined in a modular multi-level mold insert. The microlenses molded by hot embossing or injection molding are separated mechanically. They can then be integrated in the endoscope with a holding unit manufactured independently.

IR spectroscopic studies in microchannel structures

By means of the various microengineering methods available, microreaction systems can be produced among others. These microreactors consist of microchannels, where chemical reactions take place under defined conditions. For optimum process control, continuous online analytics is envisaged in the microchannels. For this purpose, a special analytical module has been developed. It may be applied for IR spectroscopic studies at any point of the microchannel.

FT-IR Studies Accompanying the Development of a Plasmaless Etching Process on the Basis of the Elemental Halogens Bromine and Fluorine

Plasmaless etching with a Br2/F2 mixture allows extremely rapid etching of silicon. By selection of the gas mixture the silicon etching rate can be controlled precisely. The microstructures obtained possess very smooth surfaces and an isotropic etching profile. They can be applied to generate moulding tools made of silicon with spherical depressions, for batchwise production of small plastic microlenses.

Fourier transform infrared spectroscopy as an analytical tool for etching investigations of silicon

Silicon and its compounds (SiO2, Si3N4) were etched for microelectronic or micromechanical applications using different methods such as plasma etching processes, laser induced etching, or plasmaless etching processes. In our experiments, halogen containing gases, e.g., molecular fluorine or chlorine and interhalogen compounds such as JF5 or ClF3 were used as etching agents. Some of the initial gases (JF5 ClF3) and most of the reaction products (SiF4, SiCl4, ClF3, ClF) are infrared active. For this reason, online FTIR spectroscopy was applied as the analytical method during our static investigations on plasmaless and laser induced etching.

Fourier transform infrared studies on optical components for the far infrared

For far infrared measurements, pre-detector filtering is necessary for special applications. Hexagonal close-packed arrays of precision drilled microholes can be used as high pass filters for the far infrared1. They are fabricated, for instance, using the mechanical microfabrication methods developed at the Karlsruhe Nuclear Research Center, i.e., microdrilling with hard metal drills of variable diameter down to 50 pm 2.

Qualification of mechanically fabricated IR filters using FTIR spectroscopy

A new method for the mechanical manufacturing of microstructured bodies has been developed by the Karisruhe Nuclear Research Center in cooperation with the MesserschmittBolkow-Blohm company, Munich-Ottobrunn. This technique is based on the surface structuring of foils by means of high-precision machining using, for example, rectangular profiled microdiamondsi. In Fig. 1, a scanning electron microscopy (SEM) of a 100 pm thick aluminum foil with a rectangular groove structure is shown. If copper foils, which have been microstructured in the same way, are stacked on top of each other and are subjected to diffusion welding, finely structured bodies are obtained which are covered by a multitude of microscopically small channels (Fig. 2). The copper body shown possesses approximately 8700 channels/cm2. The optical transparency is about 57 %. The length of the microstructure samples is freely optional within wide limits.