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Mechanical behaviour of porous hydroxyapatite.

The aim of the study was to investigate the role of microstructure and porosity on the mechanical behaviour of sintered hydroxyapatite. Hydroxyapatite disks with four different porosities were used in this investigation. With a nanoindentation system, elastic modulus, hardness, contact stress-strain relationship, energy absorption and indentation creep behaviour were investigated. The elastic modulus and hardness of hydroxyapatite exhibited an exponential relationship (e(-bP)) with the porosity P, which is similar to Rices finding with the minimum solid area model. High porosity samples showed more substantial inelastic behaviour, including higher energy absorption, no linear elastic region in the contact stress-strain curve and some indentation creep behaviour. We conclude that porous microstructure endows hydroxyapatite with inelastic deformation properties, which are important in a material for bone substitution usage.

Epitaxial (001) BiFeO3 thin-films with excellent ferroelectric properties by chemical solution deposition-the role of gelation

High-quality phase-pure (001) epitaxial bismuth ferrite (BiFeO3; BFO) thin films have been realized by chemical solution deposition. A thorough chemical investigation of the precursor molecular changes during gelation reveals that control of the delicate balance between gelation and salt (metal nitrate) precipitation through solvent evaporation is the key to a homogenous gel, necessary to ultimately obtain high-quality films. Spin-coating at 3000 rpm for 30 seconds on a preheated STO(001) substrate (∼70 °C) and subsequent heating at 90 °C leads to a suitable gel, which is then heated to 650 °C for crystallization. Pure phase BFO thin films of 150 nm thickness prepared by this route on lanthanum strontium manganite (La0.67Sr0.33MnO3; LSMO) buffered (001)-SrTiO3 (STO) substrates are shown to have not only epitaxial nature, but also robust ferroelectric properties with low coercive field. Critically, we show that these films can be achieved using stoichiometric 0.25 M precursors (with no Bi excess), thus obviating complexities typically arising from secondary phases associated with precursors having excess Bi. Square hysteresis loops with a high remanent polarization of 2Pr = 97.8 μC cm−2 and a low coercive field of 2Ec = 203.5 kV cm−1 are obtained at room temperature. Frequency-dependent hysteresis loops reveal a switching mechanism that is nucleation dominated. In addition, polarization direction dependent resistive switching behavior is also observed. The findings here thus show it is possible to realize high-quality bismuth ferrite thin films via chemical process techniques.

Phase and microstructural development in alumina sol-gel coatings on CoCr alloy

Phase transformation of γ-Al2O3 to α-Al2O3 in alumina sol gel coatings on biomedical CoCr alloy was studied as function of heat treatment temperature and time. Transformation in unseeded coatings was significant only above ∼1200 °C. Addition of α-Al2O3 seed particles having an average size of approximately 40 nm lowered the phase transformation temperature to around 800 °C. These particles were considered to act as heterogeneous nucleation sites for epitaxial growth of the α-Al2O3 phase. The kinetics and activation energy (420 kJ/mol) for the phase transformation in the seeded coatings were similar to those reported for seeded monolithic alumina gels indicating that the transformation mechanism is the same in the two material configurations. Avrami growth parameters indicated that the mechanism was diffusion controlled and invariant over the temperature range studied but that growth was possibly constrained by the finite size of the seed particles and/or coating thickness. The phase transformation occurred by the growth of α-Al2O3 grains at the expense of the precursor fine-grained γ-Al2O3 matrix and near-complete transformation coincided with physical impingement of the growing grains. The grain size at impingement was ∼100 nm which agreed well with that predicted from the theoretical linear spacing of seed particles in the initial sol.

Optical properties of zirconia ceramics for esthetic dental restorations: A systematic review

Statement of problem. Yttria‐stabilized tetragonal zirconia polycrystal has been used as a dental biomaterial for several decades because the fracture toughness and bend strength are increased by a stress‐induced transformation‐toughening mechanism. However, its esthetics are compromised by its poor translucency and grayish‐white appearance. Purpose. The purpose of the present systematic review was to assess information on the mechanical, chemical, and optical requirements of monolithic zirconia dental restorations. Material and methods. The following databases (2010 to 2015) were electronically searched: ProQuest, EMBASE, SciFinder, MRS Online Proceedings Library, Medline, Compendex, and Journal of the American Ceramic Society. The search was limited to English‐language publications, in vitro studies, experimental reports, and modeling studies. Results. The data from 57 studies were considered in order to review the intrinsic and extrinsic characteristics of zirconia and their effects on the optical properties. Conclusions. The materials and microstructural issues relevant to the esthetics and long‐term stability of zirconia have been considered in terms of monolithic restorations, while there also are restorations specifically for esthetic applications. Although zirconia‐toughened lithium silicate offers the best esthetic outcomes, transformation‐toughened zirconia offers the best mechanical properties and long‐term stability; cubic stabilized zirconia offers a potential compromise. The properties of these materials can be altered to some extent through the appropriate application of intrinsic (such as, annealing) and extrinsic (such as, shade‐matching) parameters.

Chemical solution deposition derived (001)-oriented epitaxial BiFeO3 thin films with robust ferroelectric properties using stoichiometric precursors (invited)

Phase pure bismuth ferrite (BiFeO3) thin films with (001)-oriented epitaxial structure are realized on lanthanum strontium manganite (La0.67Sr0.33MnO3) buffered (001)-SrTiO3 substrates by chemical solution deposition. The annealing process is optimized such that a stoichiometric precursor can be used to accurately control the Bi:Fe ratio. Ferroelectric, dielectric, and resistive switching behaviours are investigated for 40 nm, 70 nm, and 150 nm BFO thin films. While the thinnest film (40 nm) shows very leaky loops, square and fully saturated polarization hysteresis loops are shown for the thicker films. The highest remanent polarization (2Pr = 100 μC/cm2) and relative dielectric constant (er = 613) are obtained in the 150 nm BFO thin film. High cycle fatigue tests show that the thick films are resistant to polarization fatigue. Piezoresponse force microscopy results show that the domain structure varies with thickness. Resistive switching and polarization mediated diode effects are also observed. These ro...

Increased bending rigidity of wool fabric imparted by hybrid organic-inorganic sol–gel coatings

Hybrid organic–inorganic coatings prepared by the sol–gel method can impart desirable properties to textiles, but may adversely affect properties such as bending rigidity. This study investigated the causes of increased bending rigidity. Woven wool fabric was pad coated with formulations of methyltriethoxysilane (MTES) and Hercosett polyamide resin, examined by scanning electron microscopy, and the bending rigidities were determined. MTES coatings of up to 3.0% solids on mass of wool did not impart unacceptable bending rigidity. The coatings were not uniform on the fiber surfaces, and the increases in fabric bending rigidity could be partially attributed to inter-fiber bonding. In addition, the coatings “pinned” the edges of the cuticle scales, making individual fibers harder to bend. These effects are only weakly dependent on the Youngs moduli of the coating materials.

Mixed-phase bismuth ferrite thin films by chemical solution deposition

Epitaxial mixed-phase bismuth ferrite (BiFeO3, BFO) thin films were successfully synthesized on (001) lanthanum aluminate (LaAlO3, LAO) substrates by a chemical solution deposition (CSD) technique. X-ray diffraction measurements confirm the co-existence of a completely relaxed rhombohedral-like (R′) phase and a strained tetragonal-like (T′) phase. Atomic resolution scanning transmission electron microscopy (STEM) measurements reveal that the T′ and R′ phases in our CSD derived BFO/LAO films are mixed homogeneously at the nanoscale. This is in stark contrast to the typical physical vapor deposition derived mixed-phase BFO thin films, which show R′ phase striations embedded in a T′ phase matrix. This phenomenon is attributed to the specific deposition-nucleation-crystallization-relaxation pathway characteristic of the CSD route. This homogenously mixed-phase still demonstrates the well-known morphotropic phase boundary effect, i.e. superior electromechanical properties compared to either the pure T′ phase or R′ phase constituents themselves. Moreover, the maximum piezoelectric coefficient measured by using nanoscale top electrodes shows surprising insensitivity to the clamping effect from the substrate, thereby offering considerable promise in thin film applications.

Phase boundaries in the ternary (Bi0.5Na0.5TiO3)x(BaTiO3)y(SrTiO3)1−x−y system

The phase boundaries within (Bi0.5Na0.5TiO3)x(BaTiO3)y(SrTiO3)1−x−y with x ≥ 0.68 have been outlined. This was achieved using a combinatorial sample fabrication method and scanning synchrotron X-ray diffraction to rapidly characterise crystallographic structures over a large region of phase space. A parametric refinement method was used to clearly outline the phase boundaries as a function of the composition. The pseudo-cubic structure from the high strain non-ergodic Bi0.5Na0.5TiO3-BaTiO3 composition extends into the phase diagram with doping of SrTiO3, with regions of tetragonal and rhombohedral in the BaTiO3 and Bi0.5Na0.5TiO3 ends, respectively. This information can be used in conjunction with further compositional modifications to develop high strain piezoceramics that make use of electric-field-induced phase transformations and further understand the mechanisms in ergodic vs non-ergodic relaxors.

Absence of Microwave Effect in Ceramics: Precise Temperature, Thermal Gradient, and Densification Determination in A Proportional-Power Microwave Furnace

Many potential advantages have been ascribed to the microwave heating of ceramics.1–3 From a processing point of view, the most important of these are rapid heating, uniform volumetric heating, and lowered firing temperatures. While the phenomenon of rapid heating is universally accepted, the latter two advantages have not been proved to exist.