Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-03-09
2002-10-08
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06462462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a product made of a non-linear piezoelectric material in which a plurality of reference points are defined and which can be exposed to a distribution of an electrical field. Moreover, the invention relates to a method for determining a distribution of characteristic moduli in such a product.
The present invention relates in particular to a product that is to be used as a piezoelectric actuator or piezoelectric drive element. Such a product may be used as a driving component of an injection valve for supplying fuel to an internal-combustion engine. An injection valve of this sort is of particular interest in what is known as a “common rail” injection apparatus for an internal-combustion engine. In this injection apparatus, there is a reservoir, commonly called a “common rail,” in which the fuel is provided under high pressure. Several injection valves are connected to the reservoir, through which valves the fuel reaches the internal-combustion engine.
2. Description of Related Art
A material that may be used for such an application is a ceramic made of a mixed oxide containing lead, zirconium and titanium, known under the designation “PZT.” This typically polycrystalline-textured material exhibits a significant non-linear characteristic with respect to its electrical and mechanical characteristics. This is especially true when the electrical field strength to which the material is exposed exceeds 1 kV/mm, or the mechanical tension to which the material is exposed exceeds 25 MPa. This characteristic is due to the fact that under a corresponding stress the material exhibits ferroelectricity and ferroelasticity, in addition to piezoelectricity. Ferroelectricity and ferroelasticity are both characterized by marked non-linearity and also hysteresis. Microscopically, ferroelectricity or, respectively, ferroelasticity are expressed in that regions of uniform polarization or uniform expansion form in the material, called domains. Changes in the stress have the effect that these domains become larger or smaller. Polarization and expansion in such domains are both strongly anisotropic and also strongly coupled with one another, in a way that differs from the conventional linear piezoelectric effect observed in a monocrystal comprising a single domain.
An essential step in the manufacture of a product from a non-linear piezoelectric material is what is called polarization, in which the product is exposed for the first time to an electrical field. This leads to a distribution of a lasting polarization in the material and first gives rise to its piezoelectric characteristic. For this purpose, the product is exposed to an electrical field with a field strength of, typically approx. 2 kV/mm. This results in an expansion of the product in the direction of the electrical field of, typically 0.3%. Regarded microscopically, the polarization has the effect of orienting the domains in the product, and is essentially determined by the mobility of the surfaces that divide adjacent domains from one another, called “walls.” The behavior of the product in the context of later operation for its intended purpose is also essentially influenced by the mobility of the walls. An expansion of such a product typically required during operation is on the order of magnitude of 0.15%. If the product is non-homogenously structured, e.g. as a stack with internal electrodes which each traverse the product only incompletely, then as a rule no homogenous polarization of the material in the product is achieved. Regions of the product that are not exposed to the electrical field during polarization, or are exposed only partially (non-active regions), are then subject to mechanical tensile stress if the regions of the product that are exposed to the electrical field (active regions) lengthen under the effect of the electrical field. In some circumstances, this can lead to cracks in the material. If such cracks arise during polarization, they are typically called “polarization cracks.” Such polarization cracks can grow during the later operation of the product, in particular when fatigue phenomena of the material occur. Additional cracks can also occur during operation.
In order to enable construction of a product of the type named above with assurance of suitable operational reliability, it is necessary to be able to describe the behavior of the product under the expected electrical and mechanical loadings. A corresponding description is also useful as an instrument for quality assurance and quality control, as well as for proof of operation, as required by corresponding standards (ISO-9000). The description can also be a valuable means for meeting the information requirements of the automobile industry regarding design, function and quality of products to be supplied.
Previously known methods for describing a product made of a piezoelectric material are limited to conditions in which a linear characteristic of the material and a piezoelectric effect not influenced by ferroelectricity and ferroelasticity can be assumed. Such a method is for example available in the context of the software package ANSYS, which uses finite element analysis; see “ANSYS Users Manual for Revision 5.0,” vol. 1, Swanson, Analysis Systems, Inc., Houston, Pennsylvania, 1992. Of particular interest in this publication is chapter 8, “Coupled-Field Analysis,” pp. 8-1 ff. The treatment of the piezoelectric effect is explained in the section “Piezoelectric Analysis,” pages 8-11 to 8-13.
A product made of a piezoelectric material with internal electrodes is found in the article “Internal Electrode Ceramic Actuator,” Jap. Journal of Applied Physics 22 (1983) 157. This article also mentions the possibility for taking into account the tensile stress that may occur in the product, in order to ensure a sufficient operational reliability of the product. A method for investigating a linear piezoelectric material with the methods of linear electroelastic fracture mechanics can be found in an article by Y. E. Pak, International Journal of Fracture 54 (1992) 79. This method makes it possible to describe the environment of a crack in a linear piezoelectric material. DE 44 26 814 C1 describes an arrangement for dynamic force-path measurement that can be used to carry out measurements in a product of the type presently under consideration.
SUMMARY OF THE INVENTION
One object of the invention is to characterize a piezoelectric product in such a way that a determination of the loading of the product to be expected under a given stress is possible. Both a correspondingly characterized product and a method for characterization of a product are provided.
With respect to the product, a non-linear piezoelectric material in which a multiplicity of reference points is defined and which can be exposed to a distribution of an electrical field. A distribution of characteristic linear moduli is allocated to this product, whereby each linear modulus is determined for each reference point as an increase in a secant reaching, in an associated characteristic curve, from a zero point to a point corresponding to the reference point. The characteristic curve is projected from a system of characteristic curves that describes relations between the electrical field and further state variables in the material.
The method, for determination of a distribution of characteristic linear moduli in a product made of a non-linear piezoelectric material, in which a multiplicity of reference points is defined and which is exposed to a distribution of an electrical field, comprises: determining a system of characteristic curves that describe relations between the electrical field and further state variables in the material; for each modulus, projecting an associated characteristic curve from the system of characteristic curves; at each reference point, determining each modulus as an increase of a secant reaching from a zero point of the associated characteristic curve to a point of the characteristic curve corresp
BakerBotts LLP
Medley Peter
Ramirez Nestor
Siemens Aktiengesellschaft AG
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