Here we report on molten salt synthesis of four different compounds in the K 2 O-Na 2 O-Nb 2 O 5 system. The three compounds K 4 Nb 6 O 17 , K 2 Nb 4 O 11 and KNb 3 O 8 , with non-cubic crystal structures, were prepared as single crystalline particles with large aspect ratios. K 4 Nb 6 O 17 was prepared with a platelike morphology, K 2 Nb 4 O 11 a fibre-like and finally KNb 3 O 8 had a plate like morphology. K x Na 1Àx NbO 3 , with a cubic perovskite crystal structure at the synthesis conditions, was only obtained as cube-shaped crystals, which became larger with increasing synthesis temperature. A systematic study of the influence of the processing conditions on the morphology of the product compounds was conducted. The study demonstrated that the morphology of the product phase was controlled by the crystal structure and the connectivity of the NbO 6 -octahedra of the crystal structures. Finally, the crystal structure of K 2 Nb 4 O 11 was determined to be a tungsten bronze type structure with space group P4/mbm.

K0.5Na0.5NbO3 (KNN)-based ceramics are promising lead-free piezoelectrics, but processing of these materials is an ongoing challenge. Here, we present a spray pyrolysis route to highquality KNN-based powders. Fine-grained (<100 nm as-prepared, ~130 nm calcined at 800 °C) and phase-pure KNN and Li0.03K0.485Na0.485Nb0.8Ta0.2O3 (KNNLT) powders were obtained from aqueous precursor solutions. Ceramics with 95 % of theoretical density and with normalized strain of 333 pm/V were obtained by conventional sintering. The sintering of KNN remained challenging even for the fine grained powders, and the origins of these processing challenges were investigated by dilatometry and electron microscopy. Coarsening into cuboidal grains, which strongly reduced the sinterability, was observed in alkali-oxide excess KNN, caused by the formation of a liquid phase at ~650 °C. We propose that the reactivity of alkali oxide with moisture and carbon dioxide at lower temperatures cause the formation of the liquid phase at the surface even for stoichiometric KNN powders.Evaporation, mainly of potassium oxide, at high temperatures was shown to cause the formation of a Nb-rich secondary phase. The segregation of a secondary phase and inhomogeneous distribution of K/Na are discussed in relation to sintering above the solidus temperature of KNN.

Improved performance by texturing has become attractive in the field of lead-free ferroelectrics, but the effect depends heavily on the degree of texture, type of preferred orientation, and whether the material is a rotator or extender ferroelectric. Here we report on successful texturing of K0.5Na0.5NbO3 (KNN) ceramics by alignment of needle-like KNN templates in a matrix of KNN powder using tape casting. Homotemplated grain growth of the needles was confirmed during sintering, resulting in a high degree of texture parallel to the tape casting direction and the aligned needles. The texture significantly improved the piezoelectric response parallel to the tape cast direction, corresponding to the direction of the strongest <001>pc orientation, while the response normal to the tape cast plane was lower than for a non-textured KNN. In situ X-ray diffraction during electric field application revealed that non-180° domain reorientation was enhanced by an order of magnitude in the tape casting direction, compared to the direction normal to the tape cast plane and in the non-textured ceramic. The effect of texture in KNN is discussed with respect to possible rotator ferroelectric properties of KNN.3

KNbO 3 (KN) and K 1Àx Na x NbO 3 (KNN) anisometric crystals have been produced by a chemical conversion route. Crystals of the compounds K 4 Nb 6 O 17 , K 2 Nb 4 O 11 and KNb 3 O 8 with high aspect ratio were first grown in molten KCl. The non-perovskite compounds were further converted to KN or KNN perovskites by reaction with K 2 CO 3 or Na 2 CO 3 at 800 C with or without the presence of molten KCl. Through a dissolution-precipitation mechanism needle-or plate-like KNN and KN particles with high aspect ratios were formed with preferred orientation along the [011] or [100] directions. A short reaction time was observed to yield complete chemical conversion to polycrystalline KN/KNN, while single crystals with the morphology resembling the non-perovskite templates were obtained after longer reaction time by Ostwald ripening.

The exhaust system of the light-duty diesel engine has been evaluated as a potential environment for a mechanical energy recovery system for powering an IoT (Internet of Things) remote sensor. Temperature, pressure, gas speed, mass flow rate have been measured in order to characterize the exhaust gas. At any engine point explored, thermal energy is by far the most dominant portion of the exhaust energy, followed by the pressure energy and lastly kinetic energy is the smallest fraction of the exhaust energy. A piezoelectric flexible device has been tested as a possible candidate as an energy harvester converting the kinetic energy of the exhaust gas flow, with a promising amount of electrical energy generated in the order of microjoules for an urban or extra-urban circuit.