= Solution
A material exhibiting <piezoelectricity> converts cyclic mechanical stress into <electric charge>; its mechanical and electrical boundary conditions jointly set the extractable <energy>. <piezoelectric ceramics> such as <lead zirconate titanate> are stiff and often have large charge coefficients and electromechanical coupling, but are brittle and can be difficult to match to large-strain flexible motion. <piezoelectric polymers> such as <polyvinylidene fluoride> are compliant, light and compatible with flexible large-area films. Their lower charge coefficient is partly offset in voltage-sensitive applications by lower <permittivity>: the <voltage> coefficient is $g=d/\epsilon$.
\b[Ceramics often favor strong charge generation and coupling; polymers favor flexibility and strain accommodation.] The larger open-circuit <voltage> of a low-capacitance film does not itself imply more useful <power>. For a simple fixed-stress comparison the electrical <energy> scale involves $d^2/\epsilon$, while coupling also depends on mechanical compliance. At fixed imposed strain, different stiffnesses change the available mechanical <energy>. <resonance> tuning, fatigue, mechanical mounting, rectifier losses and load matching can reverse a simplistic material ranking; compare complete devices under the same actual vibration source.
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