Dimensional Accuracy of Computer-Aided Design/Computer-Assisted Manufactured Orbital Prostheses

Purpose: The aim of this research was to assess the dimensional accuracy of orbital prostheses based on reversed images generated by computer-aided design/computer-assisted manufacturing (CAD/CAM) using computed tomography (CT) scans. Materials and Methods: CT scans of the faces of 15 adults, men and women older than 25 years of age not bearing any congenital or acquired craniofacial defects, were processed using CAD software to produce 30 reversed three-dimensional models of the orbital region. These models were then processed using the CAM system by means of selective laser sintering to generate surface prototypes of the volunteers` orbital regions. Two moulage impressions of the faces of each volunteer were taken to manufacture 15 pairs of casts. Orbital defects were created on the right or left side of each cast. The surface prototypes were adapted to the casts and then flasked to fabricate silicone prostheses. The establishment of anthropometric landmarks on the orbital region and facial midline allowed for the data collection of 31 linear measurements, used to assess the dimensional accuracy of the orbital prostheses and their location on the face. Results: The comparative analyses of the linear measurements taken from the orbital prostheses and the opposite sides that originated the surface prototypes demonstrated that the orbital prostheses presented similar vertical, transversal, and oblique dimensions, as well as similar depth. There was no transverse or oblique displacement of the prostheses. Conclusion: From a clinical perspective, the small differences observed after analyzing all 31 linear measurements did not indicate facial asymmetry. The dimensional accuracy of the orbital prostheses suggested that the CAD/CAM system assessed herein may be applicable for clinical purposes. Int J Prosthodont 2010;23:271-276.

Precursor scavenging of the resistive grain-boundary phase in 8 mol% yttria-stabilized zirconia: Effect of trace concentrations of SiO2

The influence that trace concentrations Of SiO2 have on improving grain-boundary conduction via precursor scavenging using additional heat treatment at 1200 degreesC for 40 h before sintering was investigated. At a SiO2-impurity level (SIL) less than or equal to 160 ppm by weight, the grain-boundary resistivity (p(gb)) decreased to 20% of its value, while no improvement in grain-boundary conduction was found at a SIL greater than or equal to 310 ppm. The correlation between the resistance per unit grain-boundary area, p(gb), and average grain size indicated that the inhomogeneous distribution of the siliceous phase in the sample with a SIL greater than or equal to 310 ppm. hampered the scavenging reaction.

Mechanical Testing of Indirect Composite Materials Directly Applied on Implant Abutments

Purpose: To test the strength to failure and fracture mode of three indirect composite materials directly applied onto Ti-6Al-4V implant abutments vs cemented standard porcelain-fused-to-metal (PFM) crowns. Materials and Methods: Sixty-four locking taper abutments were randomly allocated to four groups and were cleaned in ethanol in an ultrasonic bath for 5 min. After drying under ambient conditions, the abutments were grit blasted and a custom 4-cusp molar crown mold was utilized to produce identical crowns (n = 16 per group) of Tescera (Bisco), Ceramage (Shofu), and Diamond Crown (DRM) according to the manufacturer`s instructions. The porcelain-fused-to-metal crowns were fabricated by conventional means involving the construction and a wax pattern and casting of a metallic coping followed by sintering of increasing layers of porcelain. All crowns were loaded to failure by an indenter placed at one of the cusp tips at a 1 mm/min rate. Subsequently, fracture analysis was performed by means of stereomicroscopy and scanning electron microscopy. One-way ANOVA at 95% level of significance was utilized for statistical analysis. Results: The single load to failure (+/- SD) results were: Tescera (1130 +/- 239 N), Ceramage (1099 +/- 257 N), Diamond Crown (1155 +/- 284 N), and PFM (1081 +/- 243 N). Stereomicroscopy analysis showed two distinct failure modes, where the loaded cusp failed either with or without abutment/metallic coping exposure. SEM analysis of the fractures showed multiple crack propagation towards the cervical region of the crown below a region of plastic deformation at the indenter contact region. Conclusion: The three indirect composites and PFM systems fractured at loads higher than those typically associated with normal occlusal function. Although each material had a different composition and handling technique, no significant differences were found concerning their single load to fracture resistance among composite systems and PFM.

Free-radical bulk polymerization of styrene: ESR and near-infrared spectroscopic study of the entire conversion range

The free radical polymerization of styrene in bulk was monitored by ESR and FT near-infrared spectroscopy at 70°C for a series of concentrations of the initiator, dimethyl 2,2′-azobis(isobutyrate). In order to obtain detailed kinetic information over the intire conversion range, and the gel effect range in particular, conversion and free radical concentration data points were accumulated with exceptionally short time intervals. The polystyrene radical concentration ([St•]) went through a sharp maximum at the gel effect, a feature that has hitherto escaped observation due to the rapid concentration changes in the gel effect range relative to the data point time intervals of previous studies. Temperature measurements throughout the polymerization were employed to calculate that a temperature increase was not the cause of the [St•] maximum, which thus appeares to be a genuine feature of the gel effect of this system under isothermal conditions. The propagation rate constant (kp) as a function of monomer conversion exhibited a marked dependence on initiator concentration at high monomer conversion; the sharp decrease in kp with increasing conversion was shifted to higher conversions with increasing initiator concentration.

Scavenging of siliceous grain-boundary phase of 8-mol%-ytterbia-stabilized zirconia without additive

The grain-boundary conductivity (sigma (gb),) of 8-mol%-ytterbiastabilized zirconia increased markedly with heat treatment between 1000 degrees and 1300 degreesC with a slow heating rate (0.1 degreesC/min) before sintering. The extent of the sigma (gb) improvement was the same or larger than that via Al2O3 addition. The heat treatment did not affect the grain-interior conduction when sintered at 1600 degreesC, while Al2O3-derived scavenging significantly did, given the larger increment of total conductivity in the heat-treated sample. The formation of a silicon-containing phase in a discrete form was suggested as a possible route of scavenging the resistive phase from the correlation between average grain size and sigma (gb).

Investigations of growth processes in Y-Ba-Cu-O materials by microstructural examination of quenched samples

Y-Ba-Cu-O samples with additions of Y2O3 and CeO2 were quenched during seeded isothermal melt processing and examined by optical microscopy and scanning electron microscopy. Large YBa2Cu3O7-y (Y123) particles in the starting powder were found to form a distinct type of melt during heating, which was unaffected by the Y2O3 or CeO2 additives. This type of melt later formed regions with a low concentration of Y2BaCuO5 (Y211) particles in the Y123 matrix. The maximum growth rate of Y123 that could be sustained in the sample was found to be lower in the melt formed from large Y123 particles, and this may lead to growth accidents and subgrains in some samples.

Precursor scavenging of resistive grain-boundary phase in 8 mol % ytterbia-stabilized zirconia

The grain-boundary conduction of 8 mol % ytterbia-stabilized zirconia (8YbSZ) was improved markedly by precursor scavenging via the two-stage sintering process. The most significant increase in the grain-boundary conductivity was found when the sample, whose conductivity was higher than that via Al2O3-derived scavenging, was heat-treated at 1250degreesC for greater than or equal to 20 h. The formation of a stable Si-containing phase such as ZrSiO4 during the first-stage heat-treatment was suggested as one probable scavenging route from the optimal heat-treatment temperature (HTT), long duration time (>20 h) at HTT, and the stability of the formed phase up to sintering temperatures (1500degrees C). (C) 2002 The Electrochemical Society.

Calorimetric demonstration of the potential of molecular crowding to emulate the effect of an allosteric activator on pyruvate kinase kinetics

A method based on isothermal calorimetry is described for the direct kinetic assay of pyruvate kinase. In agreement with earlier findings based on the standard coupled assay system for this enzyme in the presence of a fixed ADP concentration, the essentially rectangular hyperbolic dependence of initial velocity upon phosphoenolpyruvate concentration is rendered sigmoidal by the allosteric inhibitor phenylalanine. This effect of phenylalanine can be countered by including a high concentration of a space- filling osmolyte such as proline in the reaction mixtures. This investigation thus affords a dramatic example that illustrates the need to consider potential consequences of thermodynamic nonideality on the kinetics of enzyme reactions in crowded molecular environments such as the cell cytoplasm.

Combined chemical separation of Lu, Hf, Sm, Nd, and REEs from a single rock digest: Precise and accurate isotope Determinations of Lu-Hf and Sm-Nd using multicollector-ICPMS

A combined procedure for separating Lu, Hf, Sm, Nd, and rare earth elements (REEs) from a single sample digest is presented. The procedure consists of the following five steps: (1) sample dissolution via sodium peroxide sintering; (2) separation of the high field strength elements from the REEs and other matrix elements by a HF-free anion-exchange column procedure; (3) purification of Hf on a cation-exchange resin; (4) separation of REEs from other matrix elements by cation exchange; (5) Lu, Sm, and Nd separation from the other REEs by reversed-phase ion chromatography. Analytical reproducibilities of Sm-Nd and Lu-Hf isotope systematics are demonstrated for standard solutions and international rock reference materials. Results show overall good reproducibilities for Sm-Nd systematics independent of the rock type analyzed. For the Lu-Hf systematics, the reproducibility of the parent/daughter ratio is much better for JB-1 (basalt) than for two analyzed felsic crustal rocks (DR-N and an Archaean granitoid). It is demonstrated that this poorer reproducibility of the Lu/Hf ratio is truly caused by sample heterogeneity; thus, results are geologically reasonable.

The prediction of sharkskin instability observed during film blowing

The characteristics of sharkskin surface instability for linear low density polyethylene are studied as a function of film blowing processing conditions. By means of scanning electron microscopy and surface profilometry, is it found that for the standard industrial die geometry studied, sharkskin only occurs on the inside of the film bubble. Previous work suggests that this instability may be due to critical extensional stress levels at the exit of the die. Isothermal integral viscoelastic simulations of the annular extrusion process are reported, and confirm that the extensional stress at the die exit is large enough to cause local melt rupture. However the extensional stress level at the outer die wall predicts melt rupture of the outside bubble surface also, which contradicts the experimental findings. A significant temperature gradient is expected to exist across the die gap at the exit of the die, due to the external heating of the die and the low conductivity, of the polymer melt. It is shown that a gradient of 20 degreesC is required to cause sharkskin to only appear on the inner bubble surface.

Binder treatment and lubricant system for aluminium P/M

Organic binders are used in premixes for powder metallurgy applications to prevent dusting and segregation. This is a particular problem for aluminium powder metallurgy because the dust is a potential safety hazard. The binder must also burn out completely at low temperatures in an inert environment and not react with the metal powders. It is demonstrated that cellulose acetate, polyvinyl acetate and polyvinyl alcohol are effective dedusting agents but they react with the metal powders during sintering and decrease the sintered density. Paraffin wan is an effect dedusting agent that provided die wall lubricity, does not interfere with sintering and increases tensile strength and ductility.

gamma-alumina nanofibers prepared from aluminum hydrate with poly(ethylene oxide) surfactant

Introducing poly(ethylene oxide) surfactant to aluminum hydrate colloids can effectively direct the crystal growth of boehmite and the crystal morphology of final gamma-alumina crystallites. Fibrous crystallites of gamma-alumina about 3-4 nm thick and 30-60 nm long are obtained. They stack randomly, resulting in a structure with a low contact area between the fibers but with a very large porosity. Such a structure exhibits strong resistance to sintering when heated to high temperatures. A sample retains a BET surface area of 68 m(2)/g, after being heated to 1473 K. The surfactant molecules form micelles that interact with the colloid particles of aluminum hydroxide through hydrogen bonding. This interaction is not sufficient to change the intrinsic crystal structure of boehmite, but induces profound changes in the morphology of boehmite crystallites and their growth. The surfactant-induced fiber formation (SIFF) process has distinct features from templated synthesis but shows similarities in some respects to biomineralization processes in which inorganic crystals with complex morphological shapes can be formed in biological systems. SIFF offers an effective approach to create new nanostructures of inorganic oxide from aqueous media.

Further evidence for the reliance of catalysis by rabbit muscle pyruvate kinase upon isomerization of the ternary complex between enzyme and products

Isothermal calorimetry has been used to examine the effect of thermodynamic non-ideality on the kinetics of catalysis by rabbit muscle pyruvate kinase as the result of molecular crowding by inert cosolutes. The investigation, designed to detect substrate-mediated isomerization of pyruvate kinase, has revealed a 15% enhancement of maximal velocity by supplementation of reaction mixtures with 0.1 M proline, glycine or sorbitol. This effect of thermodynamic non-ideality implicates the existence of a substrate-induced conformational change that is governed by a minor volume decrease and a very small isomerization constant; and hence, substantiates earlier inferences that the rate-determining step in pyruvate kinase kinetics is isomerization of the ternary enzyme product complex rather than the release of products. (C) 2003 Elsevier Science B.V. All rights reserved.

Improving the ionic conductivity of yttria-stabilised zirconia electrolyte materials

Low-temperature anneals (1200 degreesC for 40 h) of 8 mol% yttria-stabilised zirconia, prior to the samples being sintered at 1500 degreesC, had the effect of improving the ionic conductivity of the specimens. The presence Of SiO2 in the specimens was shown to be detrimental, however. Irrespective of the SiO2 content, this type of heat treatment also leads to improvements in conductivity. Extensive microstructural analysis provided indication of the mechanism of this phenomenon. This included selective formation of zircon, relief of sintering strain leading to the formation of coherent grain boundaries and segregation effects. (C) 2002 Elsevier Science B.V All rights reserved.

Polyethylene-layered silicate nanocomposites for rotational moulding

A series of polyethylene-layered silicate nanocomposites has been studied as possible new candidates for rotational moulding. Two organically treated layered silicates were melt-compounded into a maleated linear low-density polyethylene host polymer at loadings of 6 and 9%, by weight. The morphology and properties of the nanocomposites were assessed by using dynamic mechanical thermal analysis, parallel-plate rheometry, wide-angle X-ray diffraction and transmission electron microscopy. The sintering behaviour of the nanocomposites was qualitatively assessed via hot-stage microscopy, indicating that the choice of nanofiller will play an important role in terms of producing nanocomposite materials with acceptable processability for rotational moulding. (C) 2003 Society of Chemical Industry.