Effects of synthesis conditions on the nanostructre of CexZr1-xO2 mesoporous ceramics.

CexZr1-xO2 (0.5 ≤ x ≤ 0.9) were synthesized with Zr and Ce chloride precursors, using the triblock copolymer Pluronic P123 and HCl (2 mol/L). The pH adjustment was performed in two ways: synthesis A used 11.4 mL of a NH4OH solution added at once to the initial mixture, composed by metal precursors and template in HCl; synthesis B was done by dripping slowly until the change of pH value (between 3 and 6). In this work, CexZr1-xO2 samples synthesized by these two processes are compared. The effects of pH values in materials characteristics were also evaluated. These samples were analysed by X-Ray Diffraction (XRD) with Rietveld refinement, and Nitrogen Adsorption/Desorption. In both processes, the studied materials presented two crystalline phases of CexZr1-xO2 solid solution: cubic and tetragonal. The synthesis A also presented a tetragonal phase of ZrO2. The average crystallite size and the Brunauer- Emmett-Teller (BET) surface area are bigger in process A. Both processes give samples with a mesoporous structure.

Local structure around Fe ions on multiferroic Pb(Fe1/2Nb1/2)O-3 ceramics probed by x-ray absorption spectroscopy

Local structure around Fe ions on Pb(Fe1/2Nb1/2)O-3 ceramics was probed by x-ray absorption spectroscopy in order to settle the controversies about its structure. It is observed that the shell structure around Fe atoms exhibits a monoclinic local symmetry at 130 and 230 K, tetragonal local symmetry at room temperature, and cubic local symmetry at 410 K. Independently of the coordination, temperature, or symmetry, Fe-O mean bond-length does not vary significantly. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4709490]

Relationship between elastic and mechanical properties of dental ceramics and their index of brittleness

The purpose of the study was to verify the effects of a number of materials' parameters (crystalline content; Young's modulus, E; biaxial flexure strength, sigma(i); Vickers hardness, VH; fracture toughness, K-Ic; fracture surface energy, gamma(f); and index of brittleness, B) on the brittleness of dental ceramics. Five commercial dental ceramics with different contents of glass phase and crystalline particles were studied: a vitreous porcelain (VM7/V), a porcelain with 16 vol% leucite particles (d.Sign/D), a glass-ceramic with 29 vol% leucite particles (Empress/E1), a glass-ceramic with 58 vol% lithium-disilicate needle-like particles (Empress 2/E2), and a glass-infiltrated alumina composite with 65 vol% crystals (In-Ceram Alumina/IC). Discs were constructed according to manufacturers' instructions, ground and polished to final dimensions (12 mm x 1.1 mm). Elastic constants were determined by ultrasonic pulse-echo method. sigma(i) was determined by piston-on-3-balls method in inert condition. VH was determined using 19.6 N load and K-Ic was determined by indentation strength method. gamma(f) was calculated from the Griffith-Irwin relation and B by the ratio of HV to K-Ic. IC and E2 showed higher values of sigma(i), E, K-Ic and gamma(f), and lower values of B compared to leucite-based glass-ceramic and porcelains. Positive correlations were observed for sigma(i) versus K-Ic, and K-Ic versus E-1/2, however, E did not show relationship with HV and B. The increase of crystalline phase content is beneficial to decrease the brittleness of dental ceramics by means of both an increase in fracture surface energy and a lowering in index of brittleness. (C) 2012 Elsevier Ltd and Techna Group Sri. All rights reserved.

Determination of relative in vivo osteoconductivity of modified potassium fluorrichterite glass-ceramics compared with 45S5 bioglass

Potassium fluorrichterite (KNaCaMg5Si8O22F2) glass-ceramics were modified by either increasing the concentration of calcium (GC5) or by the addition of P2O5 (GP2). Rods (2 x 4 mm) of stoichiometric fluorrichterite (GST), modified compositions (GC5 and GP2) and 45S5 bioglass, which was used as the reference material, were prepared using a conventional lost-wax technique. Osteoconductivity was investigated by implantation into healing defects in the midshaft of rabbit femora. Specimens were harvested at 4 and 12 weeks following implantation and tissue response was investigated using computed microtomography (mu CT) and histological analyses. The results showed greatest bone to implant contact in the 45S5 bioglass reference material at 4 and 12 weeks following implantation, however, GST, GC5 and GP2 all showed direct bone tissue contact with evidence of new bone formation and cell proliferation along the implant surface into the medullary space. There was no evidence of bone necrosis or fibrous tissue encapsulation around the test specimens. Of the modified potassium fluorrichterite compositions, GP2 showed the greatest promise as a bone substitute material due to its osteoconductive potential and superior mechanical properties.

Correlating phase and microstructure development versus dielectric properties in La3+ and Er3+ co-doped Bi4Ti3O12 ferroelectric ceramics

Bi3.25La0.75-xErxTi3O12 and Bi3.25La0.75Ti3-xErxO12-delta ceramics were prepared and studied in this work in terms of dopant-induced phase and microstructure development as well as dielectric response. The results show that introduction of Er3+ tends to reduce the materials' sintering temperature and average grain size. Moreover, it was noted that in these systems the substitution site of this dopant is controlled by valence state and ionic radii mismatch effects. In particular, even when a nominal substitution of Ti4+ is conceived, here it is found that Er3+ also incorporates at the (Bi,La)(3+) sites. These and other interesting concluding remarks from this work, including Er3+ tolerance, were possible only after comparing, especially, the X-ray diffraction results and the intrinsic ferroelectric characteristics extracted from the dielectric measurements. (C) 2011 Elsevier B.V. All rights reserved.

Pb0.90Ba0.10Zr0.40Ti0.60O3 Nanostructured Ferroelectric Ceramics Prepared by Spark Plasma Sintering

Nanostructured Pb0.90Ba0.10Zr0.40Ti0.60O3 dense ceramics presenting an average grain size of 62 +/- 5 nm was prepared by the polymeric precursor method and using the spark plasma sintering technique. The dielectric permittivity curves versus temperature exhibit broad anomaly, indicative of a diffuse phase transition. This result can be explained by the spread of Curie temperatures which are expected to depend on the degree of tetragonality related to the grain size distribution. A pronounced decrease in the maximum of the dielectric permittivity value is attributed to the existence of a large amount of grain boundaries which are non-ferroelectric regions.

Size-dependent phase transitions in nanostructured zirconia-scandia solid solutions

The dependences of phase stability and solid state phase transitions on the crystallite size in ZrO2-10, 12 and 14 mol% Sc2O3 nanopowders are investigated by X-ray powder diffraction using a synchrotron source (S-XPD). The average crystallite sizes lie within the range of 35 to 100 nm, approximately. At room temperature these solid solutions were previously characterised as mixtures of a cubic phase and one or two rhombohedral phases, beta and gamma, with their fractions depending on composition and average crystallite sizes. In this study, it is shown that at high temperatures these solid solutions become cubic single-phased. The size-dependent temperatures of the transitions from the rhombohedral phases to the cubic phase at high temperature are determined through the analyses of a number of S-XPD patterns. These transitions were studied on cooling and on heating, exhibiting hysteresis effects whose relevant features are size and composition dependent.

GREEN MACHINING ORIENTED TO DIMINISH DENSITY GRADIENT FOR MINIMIZATION OF DISTORTION IN ADVANCED CERAMICS

After sintering advanced ceramics, there are invariably distortions, caused in large part by the heterogeneous distribution of density gradients along the compacted piece. To correct distortions, machining is generally used to manufacture pieces within dimensional and geometric tolerances. Hence, narrow material removal limit conditions are applied, which minimize the generation of damage. Another alternative is machining the compacted piece before sintering, called the green ceramic stage, which allows machining without damage to mechanical strength. Since the greatest concentration of density gradients is located in the outer-most layers of the compacted piece, this study investigated the removal of different allowance values by means of green machining. The output variables are distortion after sintering, tool wear, cutting force, and the surface roughness of the green ceramics and the sintered ones. The following results have been noted: less distortion is verified in the sintered piece after 1mm allowance removal; and the higher the tool wear the worse the surface roughness of both green and sintered pieces.

Size-induced diffuse behavior in Pb0.89La0.11Zr0.40Ti0.60O3 nanocrystalline ferroelectric ceramics

The Pb1-xLaxZryTi1-yO3 system is a perovskite ABO(3) structured material which presents ferroelectric properties and has been used as capacitors, actuators, transducers and electro-optic devices. In this paper, we describe the synthesis and the characterization of Pb0.89La0.11Zr0.40Ti0.60O3 (PLZT11) nanostructured material. The precursor polymeric method and the spark plasma sintering technique were respectively used to prepare ceramic samples. In order to compare the effect of grain size, microcrystalline PLZT11 ceramic samples were also prepared. PLZT11 samples were characterized by X-ray diffraction technique which results show a reduction on the degree of tetragonality as the average grain size decreases. Moreover, the grain size decrease to a nanometer range induces a diffuse behavior on the dielectric permittivity curves as a function of the temperature and a reduction on the dielectric permittivity magnitude. Furthermore, the large number of grain boundaries due to the nanometer size gives rise to a dielectric anomaly. (C) 2012 Elsevier Masson SAS. All rights reserved.

Synthesis, characterisation, luminescence and defect centres in solution combustion synthesised CaZrO3:Tb3+ phosphor

Tb3+ doped CaZrO3 has been prepared by an easy solution combustion synthesis method. The combustion derived powder was investigated by X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscopy techniques. A room temperature photoluminescence study showed that the phosphors can be efficiently excited by 251 nm light with a weak emission in the blue and orange region and a strong emission in green light region. CaZrO3:Tb3+ exhibits three thermoluminescence (TL) glow peaks at 126 degrees C, 200 degrees C and 480 degrees C. Electron Spin Resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TL peaks. The room temperature ESR spectrum of irradiated phosphor appears to be a superposition of two distinct centres. One of the centres (centre I) with principal g-value 2.0233 is identified as an O- ion. Centre II with an axial symmetric g-tensor with principal values g(parallel to) = 1.9986 and g(perpendicular to) = 2.0023 is assigned to an F+ centre (singly ionised oxygen vacancy). An additional defect centre is observed during thermal annealing experiments and this centre (assigned to F+ centre) seems to originate from an F centre (oxygen vacancy with two electrons). The F centre and also the F+ centre appear to correlate with the observed high temperature TL peak in CaZrO3:Tb3+ phosphor. (c) 2012 Elsevier B.V. All rights reserved.

Densification and enhancement of the grain boundary conductivity of gadolinium-doped barium cerate by ultra fast flash grain welding

Doped barium cerate is a promising solid electrolyte for intermediate temperature fuel cells as a protonic conductor. However, it is difficult to sinter it to high density at a reasonable temperature. Moreover, it presents a high grain boundary resistivity at intermediate temperatures. Flash grain welding was applied to compacted samples, starting from a temperature of 910 degrees C and applying, for a short time, an ac electric polarization of 40 V, 1000 Hz. At that frequency, the resulting current flows through the grain boundaries promoting a welding via a local Joule heating. A large decrease of the grain boundary resistivity was observed by impedance spectroscopy. Scanning electron microscopy observations of polished and etched surfaces revealed highly sintered regions. Attempts were also made to combine flash grain welding with conventional sintering. (C) 2012 Elsevier Ltd. All rights reserved.

Surface Segregation in Chromium-Doped Nanocrystalline Tin Dioxide Pigments

Surface properties play an important role in understanding and controlling nanocrystalline materials. The accumulation of dopants on the surface, caused by surface segregation, can therefore significantly affect nanomaterials properties at low doping levels, offering a way to intentionally control nanoparticles features. In this work, we studied the distribution of chromium ions in SnO2 nanoparticles prepared by a liquid precursor route at moderate temperatures (500 degrees C). The powders were characterized by infrared spectroscopy, X-ray diffraction, (scanning) transmission electron microscopy, Electron Energy Loss Spectroscopy, and Mossbauer spectroscopy. We showed that this synthesis method induces a limited solid solution of chromium into SnO2 and a segregation of chromium to the surface. The s-electron density and symmetry of Sn located on the surface were significantly affected by the doping, while Sn located in the bulk remained unchanged. Chromium ions located on the surface and in the bulk showed distinct oxidation states, giving rise to the intense violet color of the nanoparticles suitable for pigment application.

Evaluation of Chemical Treatment on Zirconia Surface with Two Primer Agents and an Alkaline Solution on Bond Strength

Objectives: This study evaluated the effect of an alkaline solution and two 10-methacryloyloxydecyl dihydrogen phosphate (MDP)-based primer agents on bond strength to zirconia (yttria-stabilized tetragonal zirconium polycrystal [Y-TZP]) through the shear bond strength (SBS) test. Materials and Methods: Sixty square-shaped Y-TZP samples were embedded in an acrylic resin mold, polished, and randomly assigned to one of six groups (n=10) according to treatment surface: group CR, no treatment (control); group NaOH, 0.5 M NaOH; group AP, Alloy Primer; group ZP, Z-Primer Plus; group NaOH-AP, 0.5 M NaOH + Alloy Primer; and group NaOH-ZP, 0.5 M NaOH + Z-Primer Plus. The resin cement (Rely X U100) was applied inside a matrix directly onto the Y-TZP surface, and it was light-cured for 40 seconds. The samples were stored in distilled water at 37 C for 24 hours prior to the test, which was performed in a universal machine at a crosshead-speed of 0.5 mm/min. The data were analyzed by one-way analysis of variance and Tukey tests (p<0.05). Light stereomicroscopy and scanning electron microscopy were used to assess the surface topography and failure mode. Results: The SBS was significantly affected by the chemical treatment (p<0.0001). The AP group displayed the best results, and the use of NaOH did not improve SBS results relative to either AP or ZP. The samples treated with Alloy Primer displayed mainly mixed failures, whereas those conditioned with Z-Primer Plus or with 0.5 M NaOH presented a balanced distribution of adhesive and mixed failure modes. Conclusions: The use of a NaOH solution may have modified the reactivity of the Y-TZP surface, whereas the employment of a MDP/6-4-vinylbenzyl-n-propyl amino-1,3,5-triazine2,4-dithione-based primer enhanced the Y-TZP bond strength.

Effect of pH-induced nanopowder deagglomeration on sintering, microstructure and dielectric properties of Ba0.77Ca0.23TiO3 ceramics

Ba0.77Ca0.23TiO3 ceramics were produced in this work starting from nanopowders synthesized via a polymeric precursor method. By adjusting the pH values of the precursor solutions above 7, it was possible to prepare powders weakly aggregated and with a smaller particle size, both facts which traduced into an enhanced nanopowders' sintering process at comparatively lower temperatures. Irrespective of the initial pH value, highly-dense and second phase-free ceramics were obtained following optimal sintering parameters (temperature and time) extracted from dilatometric and density measurements. By considering these and other sintering conditions, moreover, polycrystalline materials with an average grain size varying from 0.35 to 8 mm were produced, the grain growth process involving liquid phase-assisted sintering for heat treatments achieved at 1320 °C. The study of grain size effects on the ferroelectric properties of these materials was conducted, the results being discussed in the light of previous debates, including grain size-dependent degree of tetragonal distortion in such materials, as verified in this work.

Size-dependent phase transitions in nanostructured zirconia-scandia solid solutions.

Size effects on phase stability and phase transitions in technologically relevant materials have received growing attention. Several works reported that metastable phases can be retained at room temperature in nanomaterials, these phases generally corresponding to the high-temperature polymorph of the same material in bulk state. Additionally, size-dependent shifts in solubility limits and/or in the transition temperatures for on heating or on cooling cycles have been observed. ZrO2-Sc2O3 (zirconia-scandia) solid solutions are known to exhibit very high oxygen ion conductivity provided their structure is composed of cubic and/or pseudocubic tetragonal phases. Unfortunately, for solid zirconia-scandia polycrystalline samples with typical micrometrical average crystal sizes, the high-conductivity cubic phase is only stable above 600°C. Depending on composition, three low-conductivity rhombo-hedral phases (β, γ and δ) are stable below 600°C down to room temperature, within the compositional range of interest for SOFCs. In previous investigations, we showed that the rhombohedral phases can be avoided in nanopowders with average crystallite size lower than 35 nm.