J. Muñoz-Saldaña

Stress induced movement of ferroelastic domain walls in BaTiO3 single crystals evaluated by scanning force microscopy

Abstract We report on the quantitative investigation of lateral domain wall motion in BaTiO 3 single crystals subjected to a compressive unidirectional mechanical stress. Simultaneous to the mechanical testing, the single crystals prepared by the modified exaggerated growth method were characterized by scanning force microscopy and piezoresponse force microscopy (PFM) which allow both topographical details and the true three-dimensional ferroelectric domain configuration to be reproduced simultaneously. Stress induced domain formation is initiated at the sample surface followed by the forward- and lateral-domain growth both perpendicular and parallel to the direction of induced stress. Knowing the crystallographic orientation of the BaTiO 3 single crystal (from Kikuchi patterns) clearly associates our experimental observations with a 90° domain switching process, in accordance with a theoretical model. Additionally, 180° ferroelectric domain boundaries ( a -domains) were detected with PFM which are not visible from the sample topography. The formation of these newly formed domains is driven by the compensation of the positive surface charge arising from the ferroelastic growth of C + domains.

Fracture toughness from submicron derived indentation cracks

Indentation tests with loads between 0.5 and 10 mN were performed on fused quartz, (0001) oriented sapphire and (001) oriented barium titanate. The resulting submicron cracks were used to determine the fracture toughness KIC of the tested samples. The indentation crack length method was applicable, but a c/a dependency of the constant of proportionality was found. In addition, a very effective and simple approach—using the extra penetration of the indenter, due to the formation of cracks, so called pop-in—was used to determine KIC.

Mechanical properties and low-temperature aging of tetragonal zirconia polycrystals processed by hot isostatic pressing

The influence of grain size and density of yttria-tetragonal zirconia polycrystals (Y-TZPs) ceramics on mechanical properties and on low-temperature aging degradation (LTD) in air and in hot water was investigated. A TZP powder containing 3 mol% Y 2 O 3 was consolidated by slip casting and densified by the sintering/hot isostatic pressing (HIP) method. Only the presintered samples that contained less than 0.15% open porosity reached near full density after HIP. The best conditions to reach full density were found to be attained by presintering and HIP both at 1400 °C. At these conditions, some of the best mechanical properties such as modulus of rupture and Weibull modulus reached 1397 ′ 153 MPa and, 10.6, respectively. These values were clearly higher than those obtained from sintered bodies and samples hot isostatically pressed at 1600 °C. Aging degradation of 3Y-TZP materials can be avoided through microstructural design. Fully dense materials with a critical grain size <0.36 μm did not show any evidence of degradation after extreme aging conditions at pressurized autoclaving in hot water at 100, 200, and 260 °C for 8 h. We propose a criterion to predict degradation in air as well as in hot water for the characterized materials based on the microstructure and density control of the samples.

Domain rearrangement during nanoindentation in single-crystalline barium titanate measured by atomic force microscopy and piezoresponse force microscopy

Nanoindentation tests in an aa-in-plane domain area of an {001} oriented barium titanate single crystal were performed using a conical indenter with a tip radius of 800 nm. The topography and the polarization vectors of the area after indentation were imaged afterwards by both atomic force and piezoresponse force microscopy (PFM), respectively. Two perpendicular oriented cracks in the {110} planes were identified in the topographic image. An unexpected considerable uplift occurs inside the residual impression, which was correlated with a sharp pop-out-like behavior observed in the force-displacement curve just prior to unloading. Furthermore, PFM revealed an almost a twofold symmetric arrangement of the domains around the indent, which can be explained by residual circumferential tensile stresses around a residual impression and was unambiguously correlated to the crystal orientation.

Nanoindentation of BaTiO3: dislocation nucleation and mechanical twinning

This paper presents a study of the deformation mechanisms of barium titanate under nanoindentation. The (0 0 1) and (1 1 0) crystallographic orientations of BaTiO3 giant grains were indented and critical mean contact pressures for dislocation nucleation were extracted from the indentation curves. The orientation of the dislocation slip lines was identified by atomic force microscopy, showing that the 1 1 0{1 1 0} glide systems were activated. Twin bands, observed on both orientations, also occur on the {1 1 0} habit planes and actively participate in the accommodation of the plastic deformation. Furthermore, dislocation pile-ups introduced by spherical indentations have shown a close relation between dislocations and twinning formation.

Inhibition of the two-photon absorption response exhibited by a bilayer TiO2 film with embedded Au nanoparticles

We use two different synthesis approaches for the preparation of TiO(2) films in order to study their resulting third order optical nonlinearity, and its modification by the inclusion of Au nanoparticles in one of the samples. An ultrasonic spray pyrolysis method was used for preparing a TiO(2) film in which we found two-photon absorption as a dominant nonlinear effect for 532 nm and 26 ps pulses; and a purely electronic nonlinearity at 830 nm for 80 fs pulses. A strong optical Kerr effect and the inhibition of the nonlinear optical absorption in 532 nm can be obtained for the first sample if Au nanoparticles embedded in a second TiO(2) film prepared by a sol-gel technique are added to it. We used an optical Kerr gate, z-scan, a multi-wave mixing experiment and an input-output transmittance experiment for measuring the optical nonlinearities.

Ferroelectric domains in coarse-grained lead zirconate titanate ceramics characterized by scanning force microscopy

Ferroelectric domain configurations in silver- and lanthanum-doped lead zirconate titanate (PZT) ceramics were characterized by scanning force microscopy using contact as well as piezoelectric response force [i.e., piezoelectric force microscopy (PFM)] modes. Coarse crystallites of hard and soft PZT ceramics (12 m in Ag-PZT and 30 m in La-PZT average grain size, respectively) with surface oriented in the {001} planes were chosen to characterize the domain configuration. Results show the conventional right-angled domain structures, which correspond to the {110} twin-related 90° and 180° domains of homogeneous width from 50 to 150 nm. The ability of PFM to image the orientation of pure in-plane arrays of domains (containing 90°-aa- and 180°-aa-types of domain boundaries) is highlighted, and a more detailed notation for in-plane domains is proposed. In addition to such periodical domain arrays, other ordered domains were found, having a misfit of 26° with respect to the {110} domain walls and the {100} surface. This array of domain walls could not be predicted with a geometrical analysis of the intersection of domain walls at the surface according to the conventional spatial array of {110} crystallographic planes. It could be explained only with {210} planes being the domain walls. The reason for this unconventional domain configuration is explained with the clamped conditions of the investigated crystallites in the polycrystalline material.

Indentation size effect in barium titanate with spherical tipped nanoindenters

Nanoindentation tests in an 90°-ac-domain area of an {001} orientated barium titanate single crystal were performed using four different indenters (two with cube corner and two with spherical shape) with tip radii from 61nm to 1.9μm. Extensive calibrations of the tips on fused quartz and sapphire defined the penetration depth range for approximately spherical contact prior to indentation of barium titanate (BaTiO3). The measured elastic modulus is independent of the different indenters. The measurements showed plastic deformation after “pop-in”. The calculated mean pressure remained constant for each indenter, but clearly depends upon the indenter radius. The indenter radius dependence of the hardness support the concept of “geometrically necessary dislocations”, proposed by W. D. Nix and H. Gao [J. Mech. Phys. Sol., 46, 411 (1998)] and its extension to spherical tipped indenters [J. G. Swadener, E. P. George, G. M. Pharr, J. Mech. Phys. Solids, 50, 681 (2002)]. The results show this concept fits the data...

Preparation of neodymium-doped yttrium aluminum garnet powders and fibers

Abstract Using nitrate precursors, a novel spray-drying assisted citrate gel process for the preparation of neodymium-doped yttrium aluminum garnet (YAG) phase was developed. Synthesis of single-phase polycrystalline YAG was achieved at temperatures as low as 800 °C using the spray-drying methodology whilst conventional approaches currently available require 1000 °C. Initially, a solution was prepared by mixing aluminum and yttrium nitrates, citric acid, etilenglycol and neodymium oxide. This solution was dried by pulverization (spray dryer) to obtain aggregated precursor powders of the compound. These aggregates were calcined at 800, 850 and 900 °C to determine the phase evolution from amorphous to crystalline by X-ray diffraction (XRD). The morphology of aggregates was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Moreover, through XRD it was determined that the crystallization of YAG phase started at about 800 °C without any intermediate phases. The powders were composed of spherical aggregates with an average diameter of 1 μm. From these powders, ceramic fibers with additions of 2at.% and 5at.% Nd, were extracted from the melt with diameters ranging from 30 μm to 50 μm.

Hybrid natural-synthetic chitosan resin: thermal and mechanical behavior

Poly(methyl methacrylate) (PMMA) is one of the most commonly plastics used as dental-base material, due to its good biological compatibility and mechanical properties. Chitosan has wide application in chemical, biochemical and biomedical fields of research. In this work, chitosan (CTS) was functionalized with glycidyl methacrylate (GMA), to ease a further reaction with MMA. The resulting co-polymer was finally blended with PMMA and poly(butyl acrylate) PBA which works as a damper, the polymers were cured by UV to obtain the final resin. Characterization of UV-cured resins was carried out by thermal measurements, X-ray diffraction, atomic force microscopy (AFM), micro and nanoindentation, water absorption and elution in water. As a result a higher thermal stability of the final resin compared with the precursor co-polymer ((CTS-GMA)-g-PMMA) was obtained. The resin presented roughness in the nanometer scale and nanoparticles embedded in the acrylic matrix producing a tough material. However, XRD measurements show that all materials are in an amorphous state. Values of hardness and elastic modulus results were very near to those of the dentine. The results of elution in water of the tested resin samples show them as clinically acceptable as a dental base material.