Foot-powered radio tags or ultra-sensitive vibration sensors are just two of the applications under discussion for piezoelectric nanowires that yield electricity when bent or stretched. Scientists have already demonstrated that nanowires made from zinc oxide (ZnO) and gallium nitride (GaN) generate electricity when under strain, but the materials' low piezoelectric charge constant means that the process is relatively inefficient.One option is to make the nanowires from barium titanate (BaTiO3), which has a charge constant of 85 pC/N compared with 12 pC/N for ZnO and just 3 pC/N for GaN. As Min-Feng Yu from the University of Illinois at Urbana-Champaign, US, points out, the chemical synthesis is more complicated and harder to control, but it should lead to better performing devices. To find out, Yu and his colleagues put a freshly grown BaTiO3 nanowire to the test on their custom loading platform. The nanowire was anchored at each of its ends across a split substrate by two deposits of platinum located using an electron beam (see photo). One part of the substrate was fixed and the other was driven using a single axis positioning stage. This allowed the team to load and unload its BaTiO3 sample with great precision. To measure the response, the nanowire's piezoelectric output was routed through a highly sensitive charge amplifier and then captured using data acquisition software.
As well as performing experiments, Yu and his team modelled the energy-harvesting process in detail. The researchers came up with an expression that describes the electricity generated by the nanowire during each loading cycle and begins the process of preparing design guidelines.
"Devices may require an array of BaTiO3 nanowires in order to output sufficient energy or to be sensitive enough to ambient perturbations such as mechanical vibrations and acoustic waves," Yu told nanotechweb.org. "It also looks like the nanowires need to be even thinner and longer."
The group's test structure had a diameter of 280 nm and measured 15 µm in length, but still appeared to out-perform the competition. According to the team's analysis, a BaTiO3 nanowire generates more than 16 times the output of a ZnO nanowire operating under the same conditions.
Next, the researchers plan to optimize their circuit configuration by lowering the parasitic capacitance and increasing the parallel load resistance. The scientists believe that these changes could improve energy conversion by a factor of 10.
As well as performing experiments, Yu and his team modelled the energy-harvesting process in detail. The researchers came up with an expression that describes the electricity generated by the nanowire during each loading cycle and begins the process of preparing design guidelines.
"Devices may require an array of BaTiO3 nanowires in order to output sufficient energy or to be sensitive enough to ambient perturbations such as mechanical vibrations and acoustic waves," Yu told nanotechweb.org. "It also looks like the nanowires need to be even thinner and longer."
The group's test structure had a diameter of 280 nm and measured 15 µm in length, but still appeared to out-perform the competition. According to the team's analysis, a BaTiO3 nanowire generates more than 16 times the output of a ZnO nanowire operating under the same conditions.
Next, the researchers plan to optimize their circuit configuration by lowering the parasitic capacitance and increasing the parallel load resistance. The scientists believe that these changes could improve energy conversion by a factor of 10.
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