Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
1997-11-25
2001-03-27
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S719000
Reexamination Certificate
active
06208026
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a monolithic multi-layer actuator.
With piezoceramic materials use is made of the effect that they are charged when subject to a mechanical load or pulling force and, on the other hand, in the event of electrical charging, they expand or contract. In order to reinforce this effect, monolithic multi-layer actuators are used which consist of a sintered stack of thin films of piezoceramic material (for example lead zirconate titanate) with inserted metal internal electrodes. The internal electrodes lead out of the stack in alternate directions and are connected electrically in parallel by way of external electrodes. For this purpose a basic metallisation is applied to the contact sides of the stack, which metal is connected to the individual internal electrodes. The basic metallisation is reinforced by means of areal or partial covering of the basic metallisation with solder. This reinforcement produces, on the one hand, the necessary material cross section in order to carry the high currents which occur during operation of the actuator (about 20-80 amperes). On the other hand, the soldering-on of electrical supply leads is made possible.
If an electric voltage is applied to the external electrodes, the piezofilms expand in the field direction. The nominal expansion of all the piezoceramic material is achieved already at low electrical voltages by means of the mechanical series connection of the individual piezofilms.
The indicated monolithic multi-layer actuators are described in detail in DE 40 36 287 C2. Use in a flow throughput control valve is also indicated here.
Piezoceramic materials are naturally brittle and have only a low tensile strength (about 80.10
6
Pa). This is further reduced with multi-layer actuators by way of the laminar arrangement of the internal electrodes and the anisotropy of the strength which occurs during polarization. The maximum permitted tensile stress is often exceeded already during polarization with the result that cracking inevitably occurs.
However, there is no indication that this type of cracking leads to failure of the actuators under normal operating conditions.
The growth of cracking within the ceramic materials can additionally be influenced considerably by grain size, grain boundary composition and porosity. Under suitably set conditions cracks do not run in a transcrystalline manner and are rapidly stopped by energy sinks at grain boundaries and pores. Already after about 1000 loading cycles the crack growth has largely stopped and even after long operating times (10
9
cycles) only increases slightly.
However, with high dynamic loads of the multi-layer actuators, these cracks can become critical if the cracks in the ceramic material cut through the basic metallisation and the applied layer of solder. In the most favourable case only individual piezofilms are then separated. However, voltage flashovers at the crack edge occur substantially more frequently, which voltage flashovers lead to a destruction of the multi-layer actuator because the entire operating current flowing at this point is disconnected.
SUMMARY OF THE INVENTION
The object of the invention therefore is to improve a monolithic multi-layer actuator in such a way that even with high dynamic loads no destruction of the multi-layer actuator occurs.
In accordance with the invention the object is achieved by arranging a three-dimensionally structured electroconductive electrode between the basic metallisation and the connection elements, which electrode is connected to the basic metallisation by way of partial contact points and is constructed to be expandable between the contact points. The operating current of the actuator is divided into secondary currents by means of this arrangement. The secondary currents flow from the contact points by way of the basic metallisation to the metallic internal electrodes and typically amount to 0.5 amperes. A reinforcement of the basic metal lining is not necessary for these currents.
By means of this construction in accordance with the invention, allowance is made that during the dynamic operation of the actuator the base metallisation can receive cracks. However, these cracks cannot be propagated in the electrode which is in a three-dimensional structured because the electrode is only connected to the basic metallisation by way of partial contact points and is constructed to be expandable between the contact points. In this way the electrical contact is always maintained because the operating current flowing in the three-dimensional electrode is on no occasion interrupted. The cracks occurring in the basic metallisation only lead to a diverting of the secondary currents by way of the three-dimensional electrode.
A dynamic load is understood to mean the application of an alternating voltage. The level of the dynamic load depends on the edge steepness of the individual pulses and the frequency. With high dynamic loads the edge steepness lies typically at 10 to 500 &mgr;s, the frequency typically between 10 and 1000 Hz. Tests, even long term tests, have shown that with these dynamic loads no failures of the multi-layer actuators in accordance with the invention were to be recorded.
In a preferred embodiment the electrode between the contact points lifts from the basic metallisation and is advantageously a structured metal foil. These films with a thickness of about 50 &mgr;m are expandable and are excellently suitable for the electrode in accordance with the invention.
The lifting from the basic metallisation can be easily achieved if the electrode has a wave-like cross section. A herringbone pattern—seen from above—is also eminently suitable.
So that the electrode projects somewhat from the basic metallisation. The electrode is advantageously provided with knobs at the contact points. It is likewise advantageous if the electrode is provided with openings for washing processes of the base metallisation. Used fluxing material can be efficiently removed by means of the washing processes.
In a preferred embodiment the electrode is constructed as a cooling body. This reduces the thermal loading of the actuator.
The electrode can also advantageously be a knitted wire, wire mesh or open-pored metal sponge.
Advantageously the electrode is connected to the basic metallisation at the contact points by means of soldering, adhesion with conductive adhesives or welding, for example laser-welding.
Bronze or brass have proven to be particularly advantageous as the material for the electrodes.
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patent: 5438477 (1995-08-01), Pasch
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patent: 5597494 (1997-01-01), Kohno
patent: 0569235A1 (1993-11-01), None
Bindig Reiner
Gunther Andreas
Helke Gunther
Schmieder Jurgen
Antonelli Terry Stout & Kraus LLP
Cao Phat X.
CeramTec AG Innovative Ceramic Engineering
Chaudhuri Olik
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