Smart Thin Film TiNi/Pzt Heterostructures
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Introduction A material system is considered 'smart' if it has the ability to perform both sensing and actuating functions with some inherent control between these functions. Smart materials are categorized as either passive or active. Passive smart materials respond to external stimuli without any assistance, whereas active smart materials employ a feedback loop to initiate an appropriate response. A family of smart materials can be fabricated by depositing ferroelectric ceramics onto shape memory alloys (SMA). These hybrid structures couple the broad mechanical stress-strain hysteresis properties of SMA with the mechanical-electrical relationship associated with piezoelectric materials. These hetero-structures may have effective applications for active suppression of large acoustic waves. Many alloys exhibit the ability to be deformed at low temperatures and regain their original undeformed shape when heated to higher temperatures. This phenomenon is known as the shape memory effect (SME) and is associated with dramatic changes in temperature dependent mechanical properties. It is due to a first order martensitic phase transformation from a high symmetry and high temperature austenitic B2 phase to a low symmetry and low temperature martensitic B 19' phase. At the low temperature phase, the SMA is easily twinned into many crystallographic twin variants. Crystallographic twins grow at the expense of others with an applied stress. Macroscopically, the martensitic material is easily deformed through the growth of these twins, which can accommodate seemingly plastic strains (up to 8% for TiNi) that are recoverable upon heating. Unlike most materials which display first order phase transformations, martensitic transformations demonstrate a transformation temperature hysteresis with a start and finish temperature for each transformation. The austenitic to martensitic transformation starts at a temperature denoted as Ms and finishes at a temperature Mf. The reverse transformation to the austenitic phase starts at As and finishes at Af. In TiNi, the austenitic phase is a B2 bcc phase and the martensitic phase is a B19' monoclinic face-centered tetragonal structure. The transformation temperatures are very sensitive to the chemical stoichiometry of the alloy. 419
Mat. Res. Soc. Symp. Proc. Vol. 360 01995 Materials Research Society
Annealed TiNi can possess typical transformation temperatures of Af=100 'C, Mr= 30 °C, As=60 'C and Ms=50 'C, although these have to be characterized for each alloy. Compositional deviations of 1%Ni rich TiNi will dramatically lower the transformation temperature Af to -100 0C [1]. The piezoelectric effect is the linear interaction between the electrical and mechanical systems of a material. An applied mechanical stress on a crystal produces a proportional electrical field known as the direct piezoelectric effect, whereas an applied electrical field produces a strain on the crystal known as the converse effect. There are a large number of ferroelectric ceramics, but the most widely researched
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