Smart Materials

Due to their special properties (direct band gap, piezoelectric effect) III/V compound semiconductors are of high potential for the realization of monolithic micro-opto-electro-mechanical systems (MOEMS). Hetero-micromachining (HMM) is a novel technique for the fabrication of miniaturized sensors and actuators which is based on III/V compound semiconductor layers epitaxially grown on (001) silicon. Using this concept MOEMS may be realized in combination with well-established silicon microelectronics. In this contribution hetero-micromachining of indium phosphide, gallium arsenide and stacks of different III/V compound semiconductor layers is described. Mechanical structures (cantilevers, membranes) were realized exploiting the etching selectivity of these materials against silicon in KOH solution. Both etching and fracture properties of InP cantilevers are dependent on the concentration of silicon impurities in the layer. For GaAs a fracture limit in excess of 1.5 GPa was found which is close to figures known for standard silicon wafer material. Micromirrors of various designs were fabricated by hetero-micromachining of InP. Actuation is performed using the bimorph effect between the InP and a metallization deposited on top of the mirror suspensions. Depending on the design of the suspensions mirror deflections up to 0.07° per milliwatt of electrical input power could be achieved at excitation frequencies ranging from the quasistatic case to several kilohertz. For the higher frequency range a piezoelectric actuator was designed based on highly resistive III/V-semiconductors. Using iron doping a resistivity of 7.10⁶ Ωcm at 2 V could be obtained with InP which can be further improved by reducing the unintentional incorporation of silicon impurities during heteroepitaxy.

A model describing oscillations of nonlinear thin plates excited by patches made of a piezoelectric ceramic is considered. The specific of the model is that the mutual coupling between elastic deformations and electric fields is assumed. Partial differential equations describing the model are stated and their solvability is proved. The question of homogenization when the number of the piezoelectric patches goes to infinity whereas their dimension goes to zero is discussed.

Amorphous TbDyFe films show excellent soft-magnetic properties combined with giant magnetostriction. However, for technical applications the major drawback is the low Curie temperature which is typically around 400 K. To increase the Curie temperature and simultaneously achieve good soft-magnetic properties as well as giant magnetostriction the preparation of crystalline films with nanometer-sized grains is necessary. Terfenol-D-like films with additives of Zr, Nb, Mo were prepared by ion-beam sputtering and different heat treatments were applied to investigate the crystallization behaviour. These additives enhance the nucleation of REFe₂ grains and hinder the formation of REFe₃ which is assumed to be responsible for high coercivity values. Furthermore, nanometer-scaled multilayers with Nb interlayers were prepared. This multilayer structure is suitable to inhibit grain growth and hence further decreases the average grain size. The resulting nanocrystalline microstructure leads to small coercive fields and high Curie temperatures. In addition, protective layers were investigated in order to avoid oxidation during the heat treatments for crystallization. The results will be discussed with respect to possible applications in microsystem technology.

Electrorheological fluids are smart materials in so far as they almost instantaneously change their rheological properties, in particular their viscosity, under the influence of an outer electric field. Therefore, in applications electrorheological devices offer a more flexible control of power transmission than conventional tools. Appropriate models describing the electrorheological effect and efficient and robust numerical simulation tools provide the basis for an optimal design of such devices.

The paper presents a mathematical description of shape memory alloys within the framework of continuum mechanics, where the spatially multidimensional case is considered. A new simple model is introduced, which correctly describes not only all well-studied shape memory effects (e.g. one-way-effect, pseudoelasticity) but also the more complex behavior (e.g. reorientation of stress-induced martensite). The key idea is a new set of internal state variables, which are averaged values for a representative volume element of the polycrystalline material. A tensor-valued variable describes the state of orientation of martensite. The relative volume fraction of stress induced martensite is defined by taking a certain tensor norm of this internal variable. A free energy is chosen and thermodynamical forces are derived. These forces are sufficient to define the onset of the phase transitions, so that we do not need to introduce transition surfaces explicitly within the evolution equations. Finite element discretizations for the approximation of the field variables (strain, stress) and finite difference approximations for the time integration of local variables (internal variables) are explained.

In our global society, communication and information transfer is playing a role of exponentially increasing importance. This means that there is always a need for increased, faster information transfer. Since the velocity (the distance divided by time) of the information transfer is limited by the speed of light, the only real strategy for improved throughput is to make the distance and in turn the physical structures smaller. This need for ever increasing information processing speeds has driven the ever decreasing structure sizes applied in microelectronics. A new challenge exists, namely to exploit the very successful microelectronics technology to create and realize quite new devices and entirely new micro- and nanosystems.

The production of modern microchips requires a large number of fabrication steps. One important step is the creation of semiconducting areas, where charged dopants are implanted into a semiconducting material and under the influence of high temperatures the dopants penetrate into the layer. This redistribution of dopants and point defects can be described in terms of pair diffusion mechanisms. The resulting reaction-drift-diffusion equations include systems of nonlinear parabolic, ordinary and elliptic differential equations. We state a new existence result of strong solutions in Sobolev spaces. Moreover we present an adaptive algorithm for the numerical simulation of the doping process which is based on an error controlled mesh design in the course of time integration. The adaptation of the mesh is automatically done up to a prescribed tolerance.

The richness of phenomena observed in the load-deformation-temperature behaviour of shape memory alloys has provided a challenge for the physicist and mathematician as well as the engineer.

The magnetic properties of TbFe and SmFeB giant magnetostrictive films are measured under applied stress in a cantilever configuration. The 1μm thick films are produced on 200 μm thick Si (100) substrates by magnetron sputtering in Ar atmosphere. Magnetic characterization is obtained with complementary fluxmetric and magneto-optic Kerr effect methods. The role of stress-induced anisotropy is discussed, with reference to the intrinsic and thermally induced anisotropy which can lie in-plane or out-of-plane depending on composition and preparation conditions. Optimal film compositions are determined for sensors and actuators requiring magnetic softness and large stress sensitivity.

The present work is directed to the analysis of interface corner and crack configurations which occur in smart materials. It delivers a new technique for solving the corresponding piezoelectric boundary value problems by asymptotic eigenfunction expansions in connection with the conventional finite element method. This approach represents the extension to coupled electromechanical material behaviour of a method which was introduced in former times for geometrical and physical linear and non-linear solid mechanics [4,5].

Problems and prospects of linking inanimate structures to biological systems are outlined, using the development of a biosensor on the basis of intact insect antennae as an example. The optimisation of a bioelectronic interface between μ-electronics and insect antenna demands materials with well defined surface- and bulk- properties. First approaches to the development of biological robots “Biobots” emphasise the importance of the bioelectronic interface. Considering constraints from demands of the bioelectronic interface is crucial for a design of microsystems containing μ-pumps, μ-valves, and μ-motors supporting biological perceptive systems on a cellular level. So, miniaturised biosensor systems and Biobots might serve as smart devices in areas of obstructed accessibility. This enables applications as a fire extinction system in complex electrical environments like wiring harnesses or as a localisation system for persons buried alive after earthquakes and explosions. Elucidation of the olfactory signalling cascade in insects and mammals is in fast progress. First isolation and cloning work is accomplished already. Thus, synthesis of stabilised protein components of these molecular signal transduction cascades can give way to bioelectronic nanosystems mimicking biological perceptive systems on a molecular level. First approaches to the development of μ-compartments mimicking biological compartimentation in different cellular organelles will need synthetic membrane substitutes and n-electrodes, as well as n-pumps and n-valves. This kind of semi-synthetic receptor cells can yield highly miniaturised sensor systems with biological performance and intimate integration of actuator features leading to applications like single-cell targeted μ-surgery.

This paper presents an overview of the principles governing the formation of shape memory alloys (SMA) martensite in thin films SMA/Si composites and some examples. The transformation in these composites is constrained by the compatibility conditions at the martensite/austenite interface and as the martensite. Due to the difference of the thermal expansivities of the bimorph layers and due to the martensitic transformation in the active layer, internal stresses arise upon changing the temperature. These internal stresses together with the dual constraint determine the martensitic microstructure and, with it, the actuation characteristics of the bimorph. For tetragonal martensites bi- and uniaxial tensile and compressive stresses lead to larger or smaller hystereses. Details depend on the sign of the eigenstrain, c/a-1, and the texture of the film.