Electrical generator or motor structure – Dynamoelectric – Reciprocating
Patent
1998-01-12
2000-12-05
Dougherty, Thomas M.
Electrical generator or motor structure
Dynamoelectric
Reciprocating
H01L 4120, H01L 4126, H01F 147
Patent
active
061571016
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method for controlling the twin orientation by the magnetic field in a material having such a structure. The aim is to produce shape changes, motion and force by using actuators based on this method.
BACKGROUND OF THE INVENTION
Control of motion and force is one of the basic elements in mechanical engineering. Development of new materials has made it possible to produce motion and force using special functional materials called actuator materials. The most important groups of actuator materials available are piezoelectric ceramics, magnetostrictive intermetallics, and shape memory alloys. Piezoelectric ceramics develop strains when subjected to an electric field. Frequency response of these materials is fast, but the strain amplitudes are very small, which limits their applicability. Magnetostrictive materials are strained when a magnetic field is imposed on them. Certain high-magnetostrictive intermetallics (e.g., Terfenol-D, Etrema Products, Inc., Ames, IA, USA) offer strains up to 0.17%, which is an order of magnitude higher than those of the current piezolectrics. The frequency response of the magnetostrictive intermetallics is lower than that of the piezoelectrics.
Shape memory metals are materials which, when plastically deformed at one temperature, can recover their original undeformed state upon raising their temperature above an alloy-specific transformation temperature. In these materials, crystal structure undergoes a phase transformation into, and out of, a martensite phase when subjected to mechanical loads or temperature. The process when a mechanically deformed shape memory material returns to its original form after heating is called a one-way shape memory effect. Cooling the material subsequently will not reverse the shape change. The one-way shape memory effect is utilized in fastening, tightening and prestressing devices. Strains of several percent can be completely recovered, and recovery stresses of over 900 MPa have been attained. In the case of the two-way effect, no deformation is required, and the material "remembers" two configurations that are obtained by heating and cooling to alloy-specific temperatures. The temperature difference between the two configurations can be as small as 1 to 2 K. Materials that exhibit a two-way shape memory effect are used to develop forces and displacements in actuators. Those actuators are applied in machinery, robotics and biomedical engineering. The most extensively used shape memory materials are Ni--Ti and Cu-based alloys. A drawback of the shape memory actuators is their slow response due to the thermal control (especially in cooling) and low efficiency (energy conversion), which in many alloys is only about one percent.
In order for the shape memory effect to occur, the material must exhibit a twinned substructure. The shape change of the shape memory material is based on the reorientation of the twin structure in the external stress field. A two-dimensional illustration of the twin reorientation is presented in FIG. 1. FIG. 1(a) shows two twin variants, denoted by 1 and 2, with equal proportions in the absence of the external stress. When the stress is applied, FIG. 1(b), the twin boundaries move and variant 2 grows at the expense of variant 1, producing the shape which better accommodates the applied stress. The result of moving a twin boundary is thus to convert one twin variant into another. The variants which are most favorably oriented to the applied stress will grow. Ultimately, a single variant of martensite can be produced by straining a sufficient amount, as illustrated in FIG. 1(c). In the martensite phase, the variants are usually oriented in several crystallographic directions. Therefore, complex shape changes of the material can be produced by the reorientation of the twin structure, and a full shape recovery will be obtained. Crystallographic analysis has shown that the boundaries between the martensite plates also behave as twin boundaries, i.e., the individual plat
REFERENCES:
patent: 5750272 (1998-05-01), Jardine
patent: 5958154 (1999-09-01), O'Handley et al.
Somerday, et al.; "A Systematic Anaylsis of Transformation Stress Anisotropy in Shape Memory Alloys"; Philosophical Magazine A (from internet); 1-16, Sep. 1996.
K. Ullakko, "Journal of Materials Engineering and Performance," vol. 5, No. 3, pp. 405-409, Jun. 1996.
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