Japanese inventors have created improved piezoelectrics - transparent crystals that will be useful in the development of new generation technology.
Some crystalline materials have ways of changing their shape when they are struck by an electric shock. Scientists have used these so-called piezoelectrics in ultrasound medicine for decades: materials based on them are so sensitive that they can detect the movement of sound waves passing through tissues. Researchers recently came up with a new way to create powerful transparent piezoelectrics that could not only improve the quality of medical photographs, but also create invisible robots and touch screens that activate when touched without third-party batteries.
Piezoelectrics are composed of many tiny crystallites or single crystals of various materials, including ceramics and polymers. In both cases, the mixture of atoms turns into a simple crystalline unit - usually a few atoms in size - that repeats over and over. Inside each of these building blocks, atoms are located in what is called an electric dipole, with a lot of positive charges on one side and a lot of negative charges on the other.
Applying pressure to these materials can subtly change the position of the atoms, which is enough to rearrange charges and generate electrical voltage. Applying electrical voltage has the opposite effect, causing the material to expand in one direction and contract in the other.
This property makes piezoelectrics extremely useful in a wide variety of applications. Bioengineer Sri Rajasekhar Kotapalli notes that piezoelectric devices are part of everything from lighters and barbecue grill buttons to the precision systems of modern microscopes.
They are also required for photoacoustic imaging, in which a piezoelectric device called a transducer is used to detect ultrasonic waves emitted by soft tissue when light from a laser is absorbed. Different molecules - from hemoglobin to melanin - absorb different frequencies, so doctors can visualize different types of tissue to look for health problems. However, opaque transducers cast a small shadow, which means that the fabric directly below them cannot be displayed. To get around this problem, researchers have created transducers using transparent piezoelectrics, but so far these materials have been too weak and unreliable to definitively solve the problem.
Several years ago, researchers in Japan came up with an ingenious way to create transparent piezoelectrics. Their material of choice, a compound of lead niobate and lead titanate (PMN-PT), was a ferroelectric that naturally powers electric dipoles. Researchers have already converted these materials into piezoelectrics by exposing them to a DC electric current. But the Japanese team found that exposing them to alternating current - from those supplied to homes and businesses - generates a powerful charge of piezoelectricity. “It's like shaking the crystal back and forth,” explains Long-Qing Chen, a Pennsylvania-based computational scientist. Such a shake-up could double the piezoelectric properties of the crystal, as the Japanese team announced back in 2011.
PMN-PT is usually opaque because individual groups of dipoles scatter light in all directions. Using alternating current, the team flattened the dipoles, then heated and polished the material to transparent and piezoelectric properties 50 times more powerful than conventional transparent piezoelectric materials. The result of the work is presented in the journal Nature.
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Enhanced piezoelectrics can be used to make more sensitive photoacoustic imaging devices that can help clinicians with everything from detecting breast cancer and melanoma to monitoring blood flow to treat vascular disease. The researchers report that this progress may also inspire engineers to create transparent actuators for invisible robotics and screens that are activated by touch.
Vasily Makarov