Paul Muralt

Introducing symmetry breaking in materials enables the emergence of functionalities. This can be microscopically and macroscopically driven by applying external stimuli such as mechanical stress, electric field, temperature, and chemical modification. For instance, non‐zero net dipole moments are formed in a material with the presence of local charged defects or their clusters, which can alter the crystal structure, charge states, and electrostatic potential across the material. Here, a conceptual approach is demonstrated to defects‐mediated symmetry breaking that allows for built‐in polarization in a nominally centrosymmetric defective oxide, Gd _x Ce _{1‐ x} O _{2‐ δ} (CGO) films by creating a macroscopic charge asymmetry. These results show that switchable and enduring polarization in CGO films is governed by the electric field‐driven redistribution of oxygen vacancies with a critical field strength of ≈0.5 MV cm ⁻¹ at room temperature. This leads to notable and persistent pyroelectric effect with a coefficient of ≈180 µC m ⁻² K ⁻¹ . These findings highlight the potential to develop high‐performance, sustainable, environmentally friendly polar film materials by manipulating ionic defects from their centrosymmetric ground states. This approach provides new opportunities to expand the range of polar materials in current and future energy and electronic applications.

Electrostriction is a property of dielectric materials whereby an applied electric field induces a mechanical deformation proportional to the square of that field. The magnitude of the effect is usually minuscule (–19 m² V^–2 for simple oxides). However, symmetry-breaking phenomena at the interfaces can offer an efficient strategy for the design of new properties^1,2. Here we report an engineered electrostrictive effect via the epitaxial deposition of alternating layers of Gd₂O₃-doped CeO₂ and Er₂O₃-stabilized δ-Bi₂O₃ with atomically controlled interfaces on NdGaO₃ substrates. The value of the electrostriction coefficient achieved is 2.38 × 10^–14 m² V^–2, exceeding the best known relaxor ferroelectrics by three orders of magnitude. Our theoretical calculations indicate that this greatly enhanced electrostriction arises from coherent strain imparted by interfacial lattice discontinuity. These artificial heterostructures open a new avenue for the design and manipulation of electrostrictive materials and devices for nano/micro actuation and cutting-edge sensors.

Piezoelectrics are materials that linearly deform in response to an applied electric field. As a fundamental prerequisite, piezoelectric materials must have a noncentrosymmetric crystal structure. For more than a century, this has remained a major obstacle for finding piezoelectric materials. We circumvented this limitation by breaking the crystallographic symmetry and inducing large and sustainable piezoelectric effects in centrosymmetric materials by the electric field–induced rearrangement of oxygen vacancies. Our results show the generation of extraordinarily large piezoelectric responses [with piezoelectric strain coefficients ( d ₃₃ ) of ~200,000 picometers per volt at millihertz frequencies] in cubic fluorite gadolinium-doped CeO _{2− x} films, which are two orders of magnitude larger than the responses observed in the presently best-known lead-based piezoelectric relaxor–ferroelectric oxide at kilohertz frequencies. These findings provide opportunities to design piezoelectric materials from environmentally friendly centrosymmetric ones.

Pyroelectric materials are very promising for thermal energy harvesting applications. To date, lead‐based systems are the foremost studied materials in this field. A facile and simple metal organic chemical vapor deposition route is applied for the fabrication of lead‐free, high quality, epitaxial Bi_(1−x)Dy_xFeO₃ (x = 0, 0.06, 0.08, 0.11) thin films deposited on conductive SrTiO₃:Nb (100) single crystal substrates. The films are studied by structural, morphological, compositional, and functional characterization. The correlation between the Dy‐doping amount and the dielectric properties is thoroughly investigated. Unipolar polarization–electric field loops and permittivity measurements show the important impact of Dy on ferroelectric, dielectric, and pyroelectric properties. Dy doping increases considerably the dielectric response, but much more the pyroelectric coefficient, up to a concentration of 8% Dy. The films are self‐poled, which is an ideal situation for pyroelectric applications. The best figure of merit for pyroelectric energy harvesting, F_E, is 82 J m⁻³ K⁻², showing a factor increase of 2.6 as compared to the undoped film of the sample series. It constitutes a factor 4.5 improvement as compared to previous results obtained on BiFeO₃ based thin films.

This paper presents a systematic approach to transduction optimization of resonant piezo-based MEMS devices. The method is based on inverse design approach using direct piezoelectric effect and exploits mechanical action of desired mode to calculate the polarization map of the piezoelectric ceramics. Optimal transduction is achieved by matching the excited electric field distribution to the inversely retrieved polarization map using the electrode segmentation and topology optimization technique. The proposed method was applied to improve the design of existing state-of-the-art piezoelectric MEMS mirrors. The optimized devices feature 70% increase in the optical scan angle ( θ _opt ) as well as 47 % reduction in power consumption compared to original design. At the same optical scan angle, the optimized devices required 2.28-fold less driving voltage amplitude. [2020-0342]

We present fabrication of 570-um-thick, millimeter-sized soda-lime-silicate float glass blocks with a 1-um-thick AlN-thin-film piezoelectric transducer sandwiched between thin metallic electrodes and deposited on the top surface. The electro-mechanical properties are characterized by electrical impedance measurements in the frequency range from 0.1 to 10 MHz with a peak-to-peak voltage of 0.5 V applied to the electrodes. We measured the electrical impedance spectra of 35 devices, all of width 2 mm, but with 9 different lengths ranging from 2 to 6 mm and with 2-7 copies of each individual geometry. Each impedance spectrum exhibits many resonance peaks, of which we carefully measured the 5 most prominent ones in each spectrum. We compare the resulting 173 experimental resonance frequencies with the simulation result of a finite-element-method model that we have developed. When using material parameters from the manufacturer, we obtain an average relative deviation of the 173 simulated resonance frequencies from the experimental ones of (-4.2 +/-0.04)%. When optimizing the values of the Young's modulus and the Poisson ratio of the float glass in the simulation, this relative deviation decreased to (-0.03 +/- 0.04)%. Our results suggest a method for an accurate in-situ determination of the acoustic parameters at ultrasound frequencies of any elastic solid onto which a thin-film transducer can be attached