Walking can generate electricity!Piezoelectric materials meet fictional experience

By Zhao Xinhua

You press some material and it generates electricity? Can the energy generate from your walking be turned into electricity to charge your phone?It sounds a bit magical,but that's exactly what the piezoelectric effect does.

Piezoelectric materials respond to applied stress and convert energy between mechanical and electrical domains.The piezoelectric effect occurs when a piezoelectric material accumulates electrical charges with applied mechanical stress,while the reverse piezoelectric effect occurs when the electric field is converted to the mechanical strain by a piezoelectric material.Piezoelectric ceramics and crystals commonly used as piezoelectric materi—als show good piezoelectric property,but they cannot be used in flexible ap—plications due to their rigidity.

Lightweight and flexible piezoelectric polymers like PVDF are good candidate materials for sensors because of their sustainable piezoelectricity and high piezoelectric constant.PVDF is a semi—crystalline and chemically stable piezoelectric polymer that exhibits the highest piezo,pyro,and ferro electric properties with great potentials for various device applications.PVDF is most widely used in transducers and sensors for its extraordinary properties including easy mold—ability,good toughness,flexibility,and particularly excellent electrical properties.This polymer has also shown wonderful growth in energy conversion applications between electri—cal and mechanical forms because of its high flexibility,low cost,and biocompatibility.

The crystallinity of the PVDF polymer will be a major factor on the piezoelectric constant of polymers.Depending on the processing conditions of PVDF,four different crystal structures of α,β,and σ can be obtained.The piezo—electricity of PVDF originates from the polar β—crystalline phase formation and is highly dependent on the orientation of dipoles in the β—phase.The β—crystalline phase can be formed from modification of the α—phase by different pro—cessing conditions such as mechanical stretching,the ap—plication of high electric field,and crystallization under high pressure.It has been shown that the higher β—phase por—tion of the PVDF film shows a higher piezoelectric constant as sensor material.

One of the main uses of the piezoelectric effect is to gen—erate electricity.Energy generated by mechanical vibrations and noise,for example,can be captured and converted into electricity.A new transducer developed at Pennsylvania State University captures energy from low—frequency movements of the human body to power smartphones,wearables,iPad and other electronic devices.

Researchers Anja Lund and Christian Muller from Chalmers University of Technology have developed a fabric that generates electricity when stretched or sub—jected to pressure.Piezoelectric yarn and conductive yarn can be made into fabric by deformation of piezoelectric material.The fabric is soft and more efficient when wet.A backpack strap made of this fabric can produce a continu—ous output of 4 milliwatts when carrying 3 kilograms of books,which is enough to power an LED light intermittently.By mak—ing this fabric into a complete bag,we can capture enough en—ergy to transmit radio signals.

In addition to mechanical stretching and electrical poling,the electrospinning process is another interesting method to achieve a high percentage of α—phase PVDF nanofibers.The electrospin—ning process is useful for the piezoelectric polymer PVDF because it can provide both effects of mechanical stretching and electri—cal poling simultaneously.Copolymers of PVDF such as PVDF—tetrafluoroethylene (PVDF—TrFE) show higher crystallinity due to its chemical structure,resulting in better piezoelectric response.

Electrospinning nanofibers can be used in water treatment,pro—tective clothing,masks and so on in filtration and personal protection.It can be used as resistance sensors,optical sensors,etc.in the field of sensors;it can be used for catalyst in the chemical industry;in the biomedical field,it can be used as wound dressing,tissue engineering stent,drug carrier and so on.

One of the materials used in piezoelectric textiles is nylon,which has a piezoelectric crystalline phase and,if tapped,produces a perfect accumulation of electric charges for pressure response and energy harvesting from environmental movement.

The bodyta fiber sensor developed by China Mianjie Technology Co.,Ltd.is a new kind of sensor which can make nylon have piezoelectricity properties by changing the in—ternal structure of fiber.The use of nylon as a base material also allows the sensor to have a wider range of applications and a more non—sensory application experience.

In 2021,Xiaomi 8H intelligent AI mattress was offi—cially launched.The smart mattress can be used to monitor vital signs and dangers during sleep,including breathing,quenching and time spent out of bed,through bodyta fi—ber sensors.At the same time,it monitors breathing,body movement and sleeping position in multiple dimensions,collects data,accurately finds out the specific frequency and time,finds out the healthy sleep rules,and helps users de—velop the habit of a good night's sleep.

Bodyta fiber sensor is different from other widely rec—ognized photoelectric sensors,pressure sensors,etc.Bodyta fiber sensor innovatively combines China's advanced textile technology with sensor technology.In the fiber preparation process,the principle of electronic equipment is added and successfully mass produced,which are the interdisciplinary technologies that are not yet available in Silicon Valley in the U.S.,Europe,and India.