Thomas Gietzelt

Micro Powder Injection Molding

The availability of manufacturing processes suitable for medium and large-scale production of microscale devices is an important requirement for the economic success of microsystems technology. Powder injection molding is on its way to become such an established and economically viable process for manufacturing complex shaped metal or ceramic parts in quantity.

Micro powder-injection moulding of metals and ceramics

Development of micro-MIM/-CIM was started at Forschungszentram Karlsrahe with the aim of creating a process suitable for a wide range of materials as well as for medium-scale and large-scale production of micro components. Using enhanced machine technology and special tempering procedures, this process enables the manufacturing of metal and ceramic devices with smallest wall thicknesses of 50 Μm and structural details of less than 3 Μm. Using ultrafine ceramic powders (e.g. zirconia) and high-quality LIGA mould inserts, surface qualities ofRa = 40 nm or Rmax ≤ 3 mm could be obtained. Possible practical applications are demonstrated by components of micro-annular gear pumps made of zirconia for future handling of very small volumes of dangerous fluids and micro samples (tensile and bending specimens) suitable for mechanical testing of metals (316L, 17-4PH) and ceramic materials (A12O3, ZrO2) in the micrometre range.

Mechanical Micromachining by Drilling, Milling and Slotting

Micromachining is not only a simple miniaturization of processes using macroscopic tools. As a matter of fact, a lot of specific concerns have to be met for successful fabrication of microstructures. This chapter will be focussed on micromachining using geometrically determined cutting edges, namely on techniques like drilling, milling and slotting. These methods are very flexible. Compared to EDM, ECM or lithographic processes like LIGA, they can be applied to a wide range of materials, like polymers, metals and alloys as well as to some kinds of ceramics, possess a high material removal rate and allow a great degree of freedom concerning design. There are nearly no geometrical limitations and also 3Dstructures can be manufactured easily.

Micro-structured heat exchanger for cryogenic mixed refrigerant cycles

Mixed refrigerant cycles (MRCs) offer a cost- and energy-efficient cooling method for the temperature range between 80 and 200 K. The performance of MRCs is strongly influenced by entropy production in the main heat exchanger. High efficiencies thus require small temperature gradients among the fluid streams, as well as limited pressure drop and axial conduction. As temperature gradients scale with heat flux, large heat transfer areas are necessary. This is best achieved with micro-structured heat exchangers, where high volumetric heat transfer areas can be realized. The reliable design of MRC heat exchangers is challenging, since two-phase heat transfer and pressure drop in both fluid streams have to be considered simultaneously. Furthermore, only few data on the convective boiling and condensation kinetics of zeotropic mixtures is available in literature. This paper presents a micro-structured heat exchanger designed with a newly developed numerical model, followed by experimental results on the single-phase pressure drop and their implications on the hydraulic diameter.

Diffusion Bonding: Influence of Process Parameters and Material Microstructure

Diffusion welding is a solid joining technique allowing for full cross-section welding. There is no heat-affected zone, but the whole part is subjected to a heat treatment. By diffusion of atoms across the bonding planes, a monolithic compound is generated. The process takes place in a vacuum or inert gas atmosphere at about 80% of the melting temperature and is run batch-wisely. Hence, it is rarely used despite its advantages to achieve holohedral joints and is widespread in the aerospace sector only. The quality of a diffusion-welded joint is determined by the three main parameters bonding temperature, time, and bearing pressure. The difficulty tailoring the process is that they are interconnected in a strong nonlinear way. Several additional factors may influence the result or may change the material, e.g. surface roughness and passivation layers, all kinds of lattice defects, polymorphic behaviour, and formation of precipitations at grain boundaries, design of the parts to be welded and its aspect ratio as well as mechanical issues of the welding equipment. Hence, experiments are necessary for almost each special part. In this chapter, an overview about the experience of diffusion welding is given. Influences are discussed in detail and conclusions are derived.