Final Shrinkage Research Articles

In Situ Monitoring of Structural Changes during Colloidal Self-Assembly

Reflectance spectroscopy is utilized to monitor structural changes during the self-assembly of a monodisperse colloidal system at the meniscus of a sessile drop on an inert substrate. Treating the ordered colloidal structure as a photonic crystal is equivalent to monitoring the changes in the photonic band gap (PBG) as the colloidal system self-assembles heterogeneously into a crystal through solvent evaporation in ambient conditions. Using a modified Bragg's law model of the photonic crystal, we can trace the structural evolution of the self-assembling colloidal system. After a certain induction period, a face-centered cubic (FCC) structure emerges, albeit with a lattice parameter larger than that of a true close-packed structure. This FCC structure is maintained while the lattice parameter shrinks continuously with further increase in the colloidal concentration due to drying. When the structure reaches a lattice parameter 1.09 times the size of that of a true close-packed structure, it undergoes an abrupt decrease in lattice spacing, apparently similar to those reported for lattice-distortive martensitic transformations. This abrupt final lattice shrinkage agrees well with the estimated Debye screening length of the electric double layer of charged colloids and could be the fundamental reason behind the cracking commonly seen in colloidal crystals.

Impact of Curing Protocol on Conversion and Shrinkage Stress

Since considerable shrinkage stress develops during the curing of dental composites, various soft-start photocuring protocols, aiming to lower stress but not compromise conversion, have been proposed. We hypothesized that utilizing soft-start photocuring will result in not only reduced stress, but also decreased conversion. We evaluated the impact of 3 protocols (soft-start, pulse, and standard) on the stress development and polymerization extent of an experimental composite. A novel set-up capable of simultaneous shrinkage stress, conversion, and temperature measurements on the same specimen was utilized. Analysis of the data shows that stress rises dramatically as a function of conversion in the vitrified state, and the utilization of soft-start or pulse curing results in specimens with reduced final conversion and shrinkage stress, compared with specimens cured according to the standard full-intensity protocol. Finally, this study demonstrates that the predominant reason for the reduced shrinkage stress attained with soft-start or pulse curing is a modest decrease in final conversion.

Three-Dimensional Model for Calculating Duration of Viscous-Flow Sintering

A method for estimating the duration of isothermal sintering of a powdered body and its final porosity and shrinkage is considered. The method, based on a three-dimensional model in the form of a cubic packing of spherical particles, uses sintering physics equations, the physicochemical properties of materials, and particle sizes. The model allows for describing not only the initial state (coalescence of particles) and the final state (compression of individual pores), but also the intermediate consolidation stage according to the general mass transfer mechanism. The sintering parameters for a sample of container glass powder are calculated using the viscous flow mechanism. The absolute discrepancy between the estimated results and experimental data is equal to 2.0% in porosity and 1.5% in shrinkage.

Shrinkage behaviour of crushed basaltic rock and residual clay mixture stabilized with cementitious binders

This laboratory investigation was focussed on shrinkage behaviour of stabilized crushed basaltic rock with varying quantities of fine-grained soil and cementitious binders drying under different conditions. The study is applicable to in-situ stabilization of degraded road pavements or construction of new pavements where the available material may have significant quantities of reactive fines. One conventional cementitious binder and two binders comprising industrial waste products were used as cementitious additives. Locally available basaltic crushed rocks and highly plastic basaltic clay were used as the host pavement material and the fine-grained soil, respectively. Shrinkage tests were carried out under two drying conditions, namely one- and three-dimensional drying. It was observed that inclusion of fine-grained soil had significant adverse effect on the shrinkage characteristics of stabilized pavement materials, particularly increasing the potential for cracking. One-dimensional drying featured slower rate of shrinkage development but higher final shrinkage in comparison to three-dimensional drying. This was validated by carrying out similar tests on clay soil only. Effects of different types and quantities of binders on shrinkage performance are presented in this paper.

Radiosensitivity of rat mammary tumors correlates with early vessel changes assessed by power Doppler sonography.

To investigate the changes occurring in the vascularization of tumors during irradiation, we used a model of autochthonous mammary tumors in rats and assessed early vascular changes after irradiation by power Doppler sonography. Mammary tumors were induced in 24 female Sprague Dawley rats by a single subcutaneous injection of N-nitroso N-methyl urea. After tumor areas reached 1 cm2, the animals received a single fraction of 18-Gy radiation or intraperitoneal saline injection. Power Doppler sonographic quantification of detected vessels was performed 1 day before irradiation and 7 days after the use of a power Doppler index of 5 different tumor imaging planes. Final tumor shrinkage was compared with early changes in the power Doppler index. Not all tumors regressed in a similar fashion. Radiosensitive tumors were defined as tumors with a greater than 50% decrease in baseline area 28 days after irradiation, whereas radioresistant tumors were tumors with a less than 50% decrease in baseline area. Statistical analysis was performed by the Mann-Whitney U test. Tumor area changes were similar in radioresistant and radiosensitive tumors 7 days after irradiation (-41% and -35%, respectively; P > .05, not significant), whereas reduction in the power Doppler index was significantly greater in radiosensitive tumors (mean value, -63%) than in radioresistant tumors (mean value, -12%) (P = .001). Late tumor regrowth was correlated with day 7 power Doppler index changes (P = .009). A 40% reduction in the power Doppler index at day 7 distinguished 8 of 9 radiosensitive tumors and 8 of 9 radioresistant tumors (P = .003). This study suggests that early changes in tumor perfusion as assessed by power Doppler sonography after tumor irradiation may precede the long-term tumor regression.

Terbutaline-induced triphasic changes in volume of rat alveolar type II cells: the role of cAMP.

Changes in the volume of rat alveolar type II cells (AT-II cells) induced by terbutaline, a beta(2)-agonist, were measured using video-enhanced contrast microscopy. The changes consisted of three phases: initial cell shrinkage, cell swelling, and gradual cell shrinkage. The initial cell shrinkage was Ca(2+)-dependent and was inhibited by quinine (a K+ channel blocker). The subsequent cell swelling was cAMP-dependent and was inhibited by amiloride (a Na+ channel blocker). The final cell shrinkage was cAMP-dependent and was inhibited by 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB, a Cl- channel blocker). Thus, terbutaline-induced cell volume changes were regulated by both Ca2+ and cAMP. Accumulation of cAMP alone, however, induced the Ca2+ -dependent cell shrinkage of AT-II cells and H-89 (a PKA inhibitor) inhibited terbutaline-induced cell volume changes. This suggests that cAMP accumulation stimulates the Ca2+ signal during terbutaline stimulation. In conclusion, terbutaline stimulates not only Na+ influx, but also K+ and Cl- release mediated via cAMP accumulation in rat AT-II cells, which induces the triphasic cell volume changes.

The effect of additives on sintering behavior and strength retention in silicon nitride with RE-disilicate

Abstract Four kinds of Si 3 N 4 –RE 2 Si 2 O 7 (RE: Nd, Sm, Y, Yb) ceramics have been fabricated by hot pressing at 1725–1750°C for 2 h in N 2 gas flow and post-annealing at 1450°C for 4 h. The effect of rare-earth oxide additions on the densification process was investigated by measuring the shrinkage of the compact. Results revealed that the rare-earth oxides affect the densification process of Si 3 N 4 –RE 2 Si 2 O 7 ceramics in which the shrinkage temperature varies with the types of rare-earth additives. The Si–RE–O–N liquid phase formation temperature can be estimated from the onset temperature of final shrinkage stage, and the temperature is in the order of Sm 2 gas flow. The high temperature strength of Si 3 N 4 –Yb 2 Si 2 O 7 or Si 3 N 4 –Y 2 Si 2 O 7 ceramics is better than that of Si 3 N 4 –Sm 2 Si 2 O 7 or Si 3 N 4 –Nd 2 Si 2 O 7 ceramics. There is a roughly linear relation between the strength retention and the liquid phase formation temperature. Present results indicate that Si 3 N 4 –RE 2 Si 2 O 7 ceramics with high strength retention can be fabricated by adopting additives with small ionic radius and high ionic charge.

Experimental and numerical analysis of the effects of process parameters on the properties of components in metal injection molding

*The paper mainly concerns the effects of process parameters on the final properties of components obtained by injection molding of metallic powders loaded by thermoplastic binders. The two main stages of the process are investigated. The injection stage is studied both from experiments and numerical modeling. The biphasic flow approach developed reveals well the effects of mould design and injection parameters on the final powder volume fraction distribution. The sintering stage is also investigated both from experiments and modeling. The viscoplastic like sintering model accounts well for the effect of initial powder volume fraction and sintering cycle on the final density and shrinkage of the component. Furthermore the final mechanical properties of the components obtained by metal injection molding are also investigated.

Coke shrinkage and coking pressure during carbonization in a coke oven

The relationship between coking pressure, lateral shrinkage and vertical shrinkage during carbonization in the coke oven was investigated. During the early and middle stage of the carbonization, no lateral shrinkage (gap between the coke cake and the oven wall) is observed since the outer coke layer is pressed against the oven walls by the coking pressure. It is not until the coking pressure peak ends in the later stage of the carbonization process that the lateral shrinkage starts to develop. The horizontal contraction of coke across the oven width is distributed between the lateral shrinkage and the gap at the oven centre. The final measured lateral shrinkage is 2–5 mm and increases only by 1.6 mm per 10% increase in volatile matter. The coke cake is prevented from shrinking vertically when the coking pressure is high. The coke cake starts to shrink vertically when the downward force (the weight of coke cake) becomes greater than the upward frictional force between the coke cake and the oven wall, which is caused by the coking pressure. The later the coke cake starts to shrink vertically, the smaller the final vertical coke shrinkage.

Capsule contraction after continuous curvilinear capsulorhexis: Poly(methyl methacrylate) versus silicone intraocular lenses

Purpose To evaluate the progressive contraction of the anterior capsule opening after in-the-bag implantation of 2 types of intraocular lenses (IOLs). Setting Department of Ophthalmology, University of Brest, Brest, France. Methods In this prospective study, 32 single-piece poly(methyl methacrylate) (PMMA) (Pharmacia 812 C) and 30 3-piece silicone IOLs with PMMA haptics (Allergan SI-40NB) were implanted in the bag after standardized phacoemulsification performed by the same surgeon. All patients were older than 70 years, and none had zonular weakness. The surface of continuous curvilinear capsulorhexis (CCC) was measured 1, 30, and 150 days postoperatively using a 3 charge-coupled device camera and a digitized computer analysis system. Results A significant progressive constriction was observed at 150 days in 70% in the silicone group and 32% in the PMMA group. In addition, CCC contraction was greater in silicone group ( P < .05). The mean surface decrease was 4.20 mm 2 in the silicone group and 1.53 mm 2 in the PMMA group. There was no correlation between sex, age, initial capsulorhexis area, and final capsule shrinkage. In some eyes, especially in those with a single-piece PMMA IOL (41%), there was no constriction, but fine changes in the capsule opening were seen. Conclusion Evolutive anterior capsulorhexis modifications were observed in all patients; however, the contraction rate was statistically higher in the silicone group. These results suggest that silicone IOL implantation should be avoided in eyes at risk for CCC constriction.

Structure and properties of gels

A brief review is presented of the structure and properties of inorganic gels. The mechanism of formation of gels and precipitates involves the aggregation of polymers or particles. If bonds form irreversibly, fractal aggregates grow and eventually fill space, resulting in a continuous elastic gel network; if the bonding is reversible, then the aggregates reorganize and become more compact, leading to precipitates. The existence of capillary pores in cement paste indicates that C-S-H is a precipitate, rather than a gel. Chemical reactions continue after gelation, leading to increasing rigidity and, in some cases, to spontaneous shrinkage (called syneresis). During drying, capillary pressure on the order of 10 to 100 MPa develops and produces huge shrinkage of compliant gels. As the gel shrinks, its modulus increases by several orders of magnitude; shrinkage stops when the network is stiff enough to resist the capillary forces. For gels, the modulus increases in proportion to the volume fraction of solids to a power between ∼3 and 4; consequently, the evolution of the modulus during drying and the final shrinkage can be estimated. Warping and cracking do not depend on the magnitude of the capillary pressure, but only on the gradient in pressure within the body. Slow drying reduces the gradients and prevents damage. The stresses and strains during drying can be calculated in terms of the permeability, physical dimensions, and viscoelastic properties of the gel. A variety of techniques have been developed to measure the latter properties, the simplest of which is a beam-bending measurement. When this method is applied to a rod of cement paste, the permeability and modulus are obtained in a matter of minutes.

A high-temperature furnace for X-ray diffraction with directly machined α-Al2O3ceramic parts

A complex high-temperature furnace has been produced from presintered α-Al2O3precompacts. The furnace is designed for the Mar345 imaging-plate detector system. Up to now, the design complexity of ceramic parts has been limited by the difficulty of machining hard ceramic materials or by the moulds used in hot-pressing and casting. However, the machining of soft presintered ceramic materials enables the realisation of much more demanding designs, provided that the final sintering shrinkage of the ceramic is homogeneous and predictable. The design of the furnace and its realisation and application in an X-ray diffraction study with synchrotron radiation are presented.

Polymerization shrinkage and elasticity of flowable composites and filled adhesives

Objectives: The magnitude and kinetics of polymerization shrinkage, together with elastic modulus, may be potential predictors of bond failure of adhesive restorations. This study examined these properties in visible-light-cured resins, in particular new flowable composites and filled adhesives. Methods: Polymerization shrinkage values were obtained by digital video imaging before and after light-curing; shrinkage kinetics were obtained by the “deflecting disk” method and the elastic modulus by analysis of the fundamental period of vibration. Results: Flowable composites generally showed higher shrinkage than traditional non-flowable composites, while more densely filled adhesives presented lower shrinkage than lightly filled or unfilled resins. The elastic moduli of flowable composites were in the low-medium range, whilst the hybrid composites showed the highest values and the microfilled the lowest. More densely filled adhesives were more rigid than lightly filled and unfilled adhesives. The kinetics behavior was material dependent, mainly characterized by the coefficient of near-linear contraction between 10 and 40% of the final shrinkage and the time to reach 75% of the final shrinkage. Significance: The higher shrinkage of flowable composites over that of hybrids may indicate a potential for higher interfacial stresses. However, their lower rigidity may be a counteracting factor. The microfilled composite showed low shrinkage and low rigidity, a combination that may prove less damaging to the interface. As the kinetics parameters tended to be material specific, no specific class of materials should be seen as more stress inducing until studies determine the relative importance of each examined parameter. The performance of adhesive resins as stress buffers also remains unpredictable.

As‐molded shrinkage measurements on polystyrene injection molded products

AbstractAn experimental study of shrinkage in injection molded products is presented in this paper and documented with all information needed for any further analysis. In particular, the effect of holding pressure, holding time and mold geometry on product shrinkage was investigated for amorphous polystyrene. It turned out with increasing holding pressure the in‐plane shrinkages varied from 0.6% to 0.1%, while the product thickness increased from about 1% to 10%. The holding time only affected product shrinkage for settings less than gate freeze‐off time. Shrinkage usually increases along the flow path unless back flow sets in at the end of the holding stage. It was also found that if a constraint prevents in‐mold shrinkage to take place, final shrinkage may decrease if holding pressure and time are small. The effects of all the variables analyzed were described by a thermomechanical model recently proposed.

In-Mould Shrinkage Measurements of PS Samples with Strain Gages

Abstract The problem of shrinkage in injection moulding has been studied by means of a new technique based on strain gages placed on mould surface before injection. The local shrinkage development from the moment it starts inside the mould to soon after ejection can be followed by means of this method. With reference to a simple rectangular cavity, the effects of different holding pressures, of geometrical constraints placed inside the mould, of position in the cavity and of total cavity length on local shrinkage of injected PS samples are analysed. Shrinkage inside the mould is, indeed, registered: it starts later if higher holding pressures are applied; furthermore, any factor which produces an enhancement of shrinkage evolution before complete solidification gives rise to a corresponding increase of final shrinkage.

Effect of drying on properties of silica gel

Viscoelastic properties and permeability of a silica gel have been followed during the course of drying. The modulus and viscosity of the network rise by several orders of magnitude, and exhibit power-law dependence on the density of the gel. An aerogel was prepared from an undried gel, and its modulus was found by hydrostatic compression in a mercury porosimeter. Pore size distribution of the aerogel was determined by nitrogen sorption before and after compression. The bulk modulus of the aerogel was very close to that of the wet gel at low densities, but at higher densities the wet gels were more rigid that the compressed aerogels. This difference is attributed to aging of the wet gels in the pore liquid (water). The final shrinkage of the wet gels was greater than expected, based on the viscoelastic properties and the calculated capillary pressure, and the difference is attributed to accelerated viscoelastic relaxation of the network under high capillary stresses. The permeability decreases by almost four orders of magnitude as the gel contracts and pore size inferred from the permeability ( r w) decreases in direct proportion to the pore volume. The pore size measured by nitrogen desorption ( r BT) agrees closely with r w, as expected on theoretical grounds. For compliant aerogels, the nitrogen desorption process causes substantial compression of the gel, because of the capillary pressure exerted by liquid nitrogen. In such cases, the true pore size and pore volume can be estimated, given knowledge of the elastic modulus of the network; the corrected value of r BT agrees well with r w. At high densities r w was about 0.5 nm smaller than r BT, and this discrepancy is probably caused by neglect of the existence of a relatively immobile layer of water of that thickness on the surface of the network.

水熱ホットプレス法によるガラスビーズの固化に対する電解質の影響

Soda lime glass beads were solidified by hydrothermal hot-pressing at 300°C for a period of 1 hour with NaOH, KOH, NaCl, KCl, Na2SO4, HCl solutions and deionized water, respectively. Shrinkage was monitored during experiment, and the solidified products were examined by SEM-EDX, XRD, FT-IR, TG and Shore hardness tester. The cross-section of the reacted glass bead showed three distinct layers of varying thickness that have undergone different degrees of alteration. The degree of alteration was higher approaching the surface of the glass beads. The surfaces of the glass beads were highly altered, deformed and cemented with each other. The final shrinkage increased in the order of acidic, neutral and alkali solution. The Shore hardness was found to increase with any increase in shrinkage for solutions containing same cation but was higher for Na+ solution than K+ solution. Thermogravimetric results indicated that the water contents in solidified products with K+ solutions were higher than the ones with Na+ solutions. This suggests that the drop in the hardness was caused by the presence of larger amount of water in solidified products with K+ than Na+.

Animal tissue-polypyrrole hybrid biomaterials: shrinkage temperature evaluation

The shrinkage temperature of animal tissue-polypyrrole hybrids have been evaluated. The results indicate that the tissue's shrinkage temperature behaviour is retained. The tissues do not appear to have been affected by surface layering with polypyrrole or poly(sodium 3-pyrrolyl-butanesulphonate) (PSPBS). The main influence on the shrinkage temperature appears to be the reaction conditions in producing the hybrids i.e. the solvent and the presence of FeCl 3. Hybrids obtained from reactions where the solvent was acetic acid demonstrated irreversible denaturation giving a final lower shrinkage temperature than the original value. All tissues exposed to FeCl 3 were found to have higher shrinkage temperatures. This has been attributed to transient coordination bonding between the metal ion and side groups on the amino acid. The single peak of PSPBS-tissue hybrids in acetic acid is probably associated with the effective penetration of the PSPBS into the tissue, permitting some interaction between the functionalities of the conducting polymer and collagen.

VOID INITIATION AND GROWTH IN UNIDIRECTIONAL FREEZING: THE INFLUENCE OF NATURAL CONVECTION

Experiments were conducted to observe the solidification sequence and final shrinkage void distribution more precisely than was previously possible for liquids frozen from above in a rigid, constant-volume container. Physical models have been developed to give quantitative predictions about the different regimes of void initiation and why voids ultimately cease to grow downward with the freezing front. In both models, natural convection is of paramount importance for understanding the heat flows within the solidification chamber. Agreement of the results with experimental data is encouraging. We feel that these results will eventually lead to better void management in phase-change materials for storing thermal energy.

VOID FORMATION IN UNIDIRECTIONAL FREEZING FROM ABOVE

Abstract Experiments were conducted to observe the solidification sequence and final shrinkage void distribution for liquids solidified from above in a rigid, constant-volume container. The overall void distribution appears to depend strongly on the initial liquid temperature and to a lesser extent on the cooling rate. A thermodynamic model has been developed that follows the liquid, vapor, and solid phases throughout the cooling process. Several void-formation phenomena are then examined qualitatively with reference to the thermodynamic model. We feel that these results will lead eventually to better void management in phase-change materials for storing thermal energy.