In-Situ Temperature Measurement on CMOS Integrated Micro-Hotplates for Gas Sensing Devices.

Metal oxide gas sensors generally need to be operated at elevated temperatures, up to and above 400 °C. Following the need for miniaturization of gas sensors and implementation into smart devices such as smartphones or wireless sensor nodes, recently complementary metal-oxide-semiconductor (CMOS) process-based micro electromechanical system (MEMS) platforms (micro-hotplates, µhps) have been developed to provide Joule heating of metal oxide sensing structures on the microscale. Heating precision and possible spatial temperature distributions over the µhp are key issues potentially affecting the performance of the overall gas sensor device. In this work, we use Raman spectroscopy to directly (in-situ and in-operando) measure the temperature of CMOS-based µhps during the application of electric current for Joule heating. By monitoring the position of the Raman mode of silicon and applying the theoretical framework of anharmonic phonon softening, we demonstrate that state-of-the-art µhps are able to reach the set temperature with an error below 10%, albeit with significant spatial temperature variations on the hotplate. This work demonstrates the potential of Raman spectroscopy for in-situ and in-operando temperature measurements on Si-based devices, an aspect of high relevance for micro- and nano-electronic device producers, opening new possibilities in process and device control.

Atomic scale symmetry and polar nanoclusters in the paraelectric phase of ferroelectric materials.

The nature of the "forbidden" local- and long-range polar order in nominally non-polar paraelectric phases of ferroelectric materials has been an open question since the discovery of ferroelectricity in oxide perovskites, ABO3. A currently considered model suggests locally correlated displacements of B-site atoms along a subset of <111> cubic directions. Such off-site displacements have been confirmed experimentally; however, being essentially dynamic in nature they cannot account for the static nature of the symmetry-forbidden polarization implied by the macroscopic experiments. Here, in an atomically resolved study by aberration-corrected scanning transmission electron microscopy complemented by Raman spectroscopy, we reveal, directly visualize and quantitatively describe static, 2-4 nm large polar nanoclusters in the nominally non-polar cubic phases of (Ba,Sr)TiO3 and BaTiO3. These results have implications on understanding of the atomic-scale structure of disordered materials, the origin of precursor states in ferroelectrics, and may help answering ambiguities on the dynamic-versus-static nature of nano-sized clusters.

Strategies to Improve the Energy Storage Properties of Perovskite Lead-Free Relaxor Ferroelectrics: A Review.

Electrical energy storage systems (EESSs) with high energy density and power density are essential for the effective miniaturization of future electronic devices. Among different EESSs available in the market, dielectric capacitors relying on swift electronic and ionic polarization-based mechanisms to store and deliver energy already demonstrate high power densities. However, different intrinsic and extrinsic contributions to energy dissipations prevent ceramic-based dielectric capacitors from reaching high recoverable energy density levels. Interestingly, relaxor ferroelectric-based dielectric capacitors, because of their low remnant polarization, show relatively high energy density and thus display great potential for applications requiring high energy density properties. In this study, some of the main strategies to improve the energy density properties of perovskite lead-free relaxor systems are reviewed, including (i) chemical modification at different crystallographic sites, (ii) chemical additives that do not target lattice sites, and (iii) novel processing approaches dedicated to bulk ceramics, thick and thin films, respectively. Recent advancements are summarized concerning the search for relaxor materials with superior energy density properties and the appropriate choice of both composition and processing routes to match various applications' needs. Finally, future trends in computationally-aided materials design are presented.

Mechanical properties of zirconia ceramics biomimetically coated with calcium deficient hydroxyapatite.

Mechanical properties and stability of porous tetragonal yttria-stabilised zirconia (Y-TZ) ceramics, biomimetically coated with calcium deficient hydroxyapatite (CaDHA) to obtain a bioactive material, were investigated. The 5.7 mol% yttria-stabilised tetragonal zirconia was obtained by sol-gel process and sintered at different temperatures to obtain a homogeneous and porous structure whose strength would match that of human bone. Sufficient strength was achieved by sintering at 1400 °C. The CaDHA coating was obtained at room temperature by a simplified preparation method consisting of immersion of the Y-TZ ceramics into a calcifying solution, after a short surface pretreatment in HCl. Although HAP or ß-TCP are more frequently used, CaDHA was chosen due to its structural similarity to the bone mineral and ability to support bone ingrowth to a greater extent than biphasic calcium phosphates. To verify the applicability CaDHA coatings, we tested their adherence to Y-TZ ceramics for the first time to the best of our knowledge. Vickers hardness (3.8 ± 0.2 GPa) reflected the hardness of underlying ceramic. The tensile strength (269 ± 52 MPa) and Weibull modulus (5) of the obtained biomaterials matched or exceeded those of bone. There was no statistical difference in the tensile strength between the coated (269 ± 52 MPa) and the uncoated (239 ± 46 MPa) ceramics. The Y-TZ-CaDHA coating system presented adequate structural integrity under scratch test with critical load for coating cracking of 18 ± 2 N. These results indicate the potential of the prepared bioceramic to be used as bone implants.

Mechanosynthesis of the Whole Y1-xBixMn1-xFexO3 Perovskite System: Structural Characterization and Study of Phase Transitions.

Perovskite BiFeO3 and YMnO3 are both multiferroic materials with distinctive magnetoelectric coupling phenomena. Owing to this, the Y1-xBix Mn1-xFexO3 solid solution seems to be a promising system, though poorly studied. This is due to the metastable nature of the orthorhombic perovskite phase of YMnO3 at ambient pressure, and to the complexity of obtaining pure rhombohedral phases for BiFeO3-rich compositions. In this work, nanocrystalline powders across the whole perovskite system were prepared for the first time by mechanosynthesis in a high-energy planetary mill, avoiding high pressure and temperature routes. Thermal decomposition temperatures were determined, and structural characterization was carried out by X-ray powder diffraction and Raman spectroscopy on thermally treated samples of enhanced crystallinity. Two polymorphic phases with orthorhombic Pnma and rhombohedral R3c h symmetries, and their coexistence over a wide compositional range were found. A gradual evolution of the lattice parameters with the composition was revealed for both phases, which suggests the existence of two continuous solid solutions. Following bibliographic data for BiFeO3, first order ferroic phase transitions were located by differential thermal analysis in compositions with x ≥ 0.9. Furthermore, an orthorhombic-rhombohedral structural evolution across the ferroelectric transition was characterized with temperature-dependent X-ray diffraction.

First-order transverse phonon deformation potentials of tetragonal perovskites.

A theoretical formalism is put forward with the aim of describing the softening of first-order transverse optical phonons in a strained tetragonal perovskitic lattice. On the basis of the dynamical equation for nondegenerate polar modes, the influence of oblique phonons could be first described by assuming a prevalence of short-range interatomic forces; then, the softening effect arising from external stress could be explicitly expressed as a function of orientation of the crystallographic texture. As a further step in the adopted formalism, the microstructure of a perovskitic polycrystal has been ideally modeled as an ensemble of mesocrystals, whose individual crystallographic directions corresponded to an average orientation over the unit volume of the probe. An experimental confirmation of the theoretical formalism is concurrently carried out, and phonon deformation potentials (PDP) have been directly measured for the first-order transverse phonon of a tetragonal PbZrTiO3 perovskite lattice.

Reconstruction of the domain orientation distribution function of polycrystalline PZT ceramics using vector piezoresponse force microscopy.

Lead zirconate titanate (PZT) is one of the prominent materials used in polycrystalline piezoelectric devices. Since the ferroelectric domain orientation is the most important parameter affecting the electromechanical performance, analyzing the domain orientation distribution is of great importance for the development and understanding of improved piezoceramic devices. Here, vector piezoresponse force microscopy (vector-PFM) has been applied in order to reconstruct the ferroelectric domain orientation distribution function of polished sections of device-ready polycrystalline lead zirconate titanate (PZT) material. A measurement procedure and a computer program based on the software Mathematica have been developed to automatically evaluate the vector-PFM data for reconstructing the domain orientation function. The method is tested on differently in-plane and out-of-plane poled PZT samples, and the results reveal the expected domain patterns and allow determination of the polarization orientation distribution function at high accuracy.

Average and local atomic-scale structure in BaZrxTi(1-x)O3 (x = 0. 10, 0.20, 0.40) ceramics by high-energy x-ray diffraction and Raman spectroscopy.

High-resolution x-ray diffraction (XRD), Raman spectroscopy and total scattering XRD coupled to atomic pair distribution function (PDF) analysis studies of the atomic-scale structure of archetypal BaZrxTi(1-x)O3 (x = 0.10, 0.20, 0.40) ceramics are presented over a wide temperature range (100-450 K). For x = 0.1 and 0.2 the results reveal, well above the Curie temperature, the presence of Ti-rich polar clusters which are precursors of a long-range ferroelectric order observed below TC. Polar nanoregions (PNRs) and relaxor behaviour are observed over the whole temperature range for x = 0.4. Irrespective of ceramic composition, the polar clusters are due to locally correlated off-centre displacement of Zr/Ti cations compatible with local rhombohedral symmetry. Formation of Zr-rich clusters is indicated by Raman spectroscopy for all compositions. Considering the isovalent substitution of Ti with Zr in BaZrxTi1-xO3, the mechanism of formation and growth of the PNRs is not due to charge ordering and random fields, but rather to a reduction of the local strain promoted by the large difference in ion size between Zr(4+) and Ti(4+). As a result, non-polar or weakly polar Zr-rich clusters and polar Ti-rich clusters are randomly distributed in a paraelectric lattice and the long-range ferroelectric order is disrupted with increasing Zr concentration.