For over a hundred years, diamond held a fixed position in the field of materials science: remarkably hard, thermally conductive, and electrically insulating, yet not piezoelectric. Its crystal structure simply exhibited too much symmetry to convert an applied force into a net electrical polarization.<\/p>\n
A group spearheaded by researchers from the University of Hong Kong has now demonstrated that this principle has a specific exception. Ultrathin polycrystalline diamond membranes generated repeatable voltage signals when bent, with the most significant response occurring in films approximately five micrometres thick.<\/p>\n
This discovery does not imply that an engagement ring can charge a phone when squeezed. The samples were synthetic diamond films cultivated from numerous microscopic crystals, detached from a silicon substrate, and made thin enough to flex. Their internal grain boundaries offer something a flawless diamond crystal lacks: local asymmetry.<\/p>\n
The peer-reviewed research published in Science Advances reveals a true electromechanical response in this engineered variant of diamond. It also highlights a considerable gap between a laboratory membrane that produces millivolts and a viable commercial power source.<\/p>\n
Piezoelectricity requires charge not to neutralize<\/p>\n
The direct piezoelectric effect involves generating electrical charge when a material experiences mechanical stress. Quartz, certain ceramics, and substances like zinc oxide can slightly separate positive and negative charge centers when compressed, stretched, or bent.<\/p>\n
Such separation induces an electric polarization and a voltage that electrodes can capture. The inverse effect is also present: an applied electric field can deform a piezoelectric material. This two-way conversion renders piezoelectrics valuable in microphones, ultrasound transducers, precision actuators, vibration sensors, and small energy harvesters.<\/p>\n
Crystal symmetry dictates whether the microscopic alterations accumulate. In a material with a center of inversion, movements on one side of the structure mirror those on the other. Their electrical contributions cancel out, resulting in no net piezoelectric polarization.<\/p>\n
Ordinary cubic diamond has this centrosymmetric configuration. Each carbon atom forms four strong bonds in a tetrahedral shape, and a high-quality single crystal maintains the higher symmetry that renders its piezoelectric coefficient essentially zero.<\/p>\n
Thus, the century-old classification was not merely an oversight in experimentation. It accurately described ideal bulk diamond. The new finding modifies what qualifies as “diamond” in that context by analyzing a vastly different microstructure.<\/p>\n
The samples were flexible films cultivated in a lab<\/p>\n
The researchers did not cut membranes from natural gems. They deposited diamond onto silicon utilizing microwave plasma chemical vapor deposition, beginning with diamond seeds smaller than 10 nanometers. The seeds grew, fused, and developed upward into a continuous polycrystalline film.<\/p>\n
A previously established edge-exposed exfoliation method enabled the film to be detached from the growth substrate. The broader fabrication efforts had previously demonstrated that large, ultraflat polycrystalline diamond membranes could be produced and transferred without damage.<\/p>\n
Thinness alters the mechanical properties. While diamond remains hard, strongly resisting localized scratching and indentation, a sheet just a few micrometers thick can flex because flexural stiffness significantly decreases with thickness. Hardness and flexibility need not be opposites when the geometry changes to this extent.<\/p>\n
For the electrical assessments, the team applied a gold electrode coating to both surfaces and mounted the membrane on a flexible polyethylene terephthalate support. Insulating tape separated the film from the support to lessen the possibility that contact charge from the polymer would falsely represent a diamond signal.<\/p>\n
Bending and releasing triggered opposite voltage pulses<\/p>\n
When the membrane was bent, the electrodes detected a voltage pulse. Releasing it generated a pulse of the opposite sign. Repeating the mechanical cycle replicated the electrical pattern, the signature expected when polarization reversibly follows deformation.<\/p>\n
The team measured the intrinsic longitudinal piezoelectric coefficient, denoted d33. It indicates generated charge per unit of applied force and is typically reported in picocoulombs per newton. Commercial bulk single-crystal diamond has a d33 of zero.<\/p>\n
In the polycrystalline membranes, d33 increased from about 2 picocoulombs per newton at one micrometer thickness to 3 at 2.5 micrometers and 4 at five micrometers. Beyond seven micrometers, it gradually decreased from approximately 3 to about 1.<\/p>\n
The increase and decrease are as significant as the non-zero readings. A contaminant or a simple surface artifact would not necessarily be expected to peak at an intermediate thickness in the same manner as a response governed by evolving grain structure.<\/p>\n
The largest voltage measured was around 70 millivolts<\/p>\n
One-centimeter-square membranes experienced bending through strains ranging from 0.35 to 1.05 percent, with voltage amplitude rising as strain increased. The five-micrometer sample yielded the highest voltage and current across the assessed thicknesses.<\/p>\n
At the highest reported strain of 1.4 percent, its output attained approximately 70 millivolts. The authors calculated a piezoelectric voltage coefficient, g33, of roughly 82.2 millivolts per meter.<\/p>\n","protected":false},"excerpt":{"rendered":"
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