Phenomena Occurring upon the Sintering of a Mixture of Yttria–Zirconia Nanometric Powder and Sub-Micrometric Pure Zirconia Powder

Mixtures of powders essentially differing in their particle morphology and size were applied to prepare polycrystals in a Y2O3-ZrO2 system. An yttria–zirconia solid solution nanometric powder with a Y2O3 concentration of 3.5% was prepared by subjecting co-precipitated gels to hydrothermal treatment at 240 °C. The crystallization occurred in distilled water. The pure zirconia powders composed of elongated and sub-micrometer size particles were also manufactured through the hydrothermal treatment of pure zirconia gel, although in this case, the process took place in the NaOH solution. Mixtures of the two kinds of powder were prepared so as to produce a mean composition corresponding to an yttria concentration of 3 mol%. Compacts of this powder mixture were sintered, and changes in phase composition vs. temperature were studied using X-ray diffraction. The dilatometry measurements revealed the behavior of the powder compact during sintering. The polished surfaces revealed the microstructure of the resulting polycrystal. Additionally, the electron back scattering diffraction technique (EBSD) allowed us to identify symmetry between the observed grains. Hardness, fracture toughness, and mechanical strength measurements were also performed.

Characterization of Nanostructured Phases and Peculiar Phase Transitions in BNBT Lead-Free Piezoceramics

Submicron-structured (Bi0.5Na0.5)0.94Ba0.06TiO3 (BNBT6) dense ceramics, from nanometric powder synthesized by sol gel auto-combustion at 500°C and obtained by hot-pressing (800°C-2h) and subsequent recrystallization at moderate temperature (1000-1050°C-1h), have been studied. In-situ measurements at the shear mode of electromechanical resonance of non-standard thickness-poled shear plates as a function of the temperature show higher depolarization temperature than measurements at the radial mode of thin disks. Shear mode related material coefficients are measurable up to 160°C, being k15≈30% and d15≈250 pC.N-1 at 130°C. Depolarization is a complex phenomena caused by a ferroelectric (FE) macrodomains thermal randomization and a phase transition from the field-induced FE phase to a relaxor phase. The early stage of such a transition involves a non-negligible piezoelectricity arising most probably by the percolative coexistence of ferroelectric macrodomains in the resonator under the given stress field for each resonance mode.

Obtained of Diamond Nanometric Powders Using High Energy Milling for the Production of Alumina-Diamond Nanocomposites

The addition of nanometric particles of a second phase into ceramics matrix is one of the most recent alternatives in the development of materials with high mechanical properties and wear resistance. These nanostructured materials can be defined as systems that have at least one microstructural characteristic of nanometric dimensions (less 100nm). In this work aluminadiamond nanocomposites were produced using diamond nanometric powders obtained by high energy milling. Diamond powder was produced in the SPEX shaker/mill during 6h, with a ball-tomass ratio of 4:1. The crystallite size was 30nm. After the elimination of the Fe deriving of the contamination during the milling, and desaglomeration, this nanometric powder was added in the alumina matrix in the ratio of 5wt%. The powder densification was performed by hot pressing sintering. The obtained nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and microhardness, and they have promising characteristics regarding abrasion and wear resistance.

Segmented Thermistors Printed by NTC Nanometric Paste and Applied in Volume Air-Flow Sensors

An investigation path starting from (Mn,Ni,Co,Fe)3O4 nanometric powder preparation, NTC thick-film characterization, realization and optimization of new planar thermistor geometries to custom design thermistors and their application for temperature and volume airflow sensors was recently determined. The aim was to make custom designed segmented NTC thermistors printed and sintered on alumina. Two types of volume airflow sensors were formed: indirectly heated and self-heated. A pair of segmented thermistors was placed in an air guide and connected to differential branches of the Wheatstone bridge. A heater was placed in the middle between the thermistors for the indirectly heated type, while for the self-heated type the same thermistors were self-heated by a constant current source. First the thermistor responses vs. the airflow (calibration curves) were measured, and then thermistor responses in time (inertia) to fast airflow changes from the minimum to the maxi

Mixed valences in nanometric ferrites investigated by resonant powder diffraction

The dependence of the absorption-edge position on ion valence in resonant scattering can be used to analyse the ionic composition of complex oxides. As the diffraction spectra are more sensitive to f^{\,\prime} than to f^{\,\prime\prime}, the shift in f^{\,\prime} associated with the valence can be used to refine the cation distribution. Below the absorption edge the shape of f^{\,\prime} is quite insensitive to other site parameters. High-angle data are important to reach significant variation of the diffraction lines. The feasibility of this method for powder materials was tested using a nanometric powder of magnetite, Fe3O4. More complex ferrites were then investigated and the results compared with neutron diffraction experiments.