1,721,004 research outputs found
Study of the refractive index of microscopic glass beads by light-refraction analysis
No full textThe refractive index of submillimeter glass beads has been measured by means of a novel, to our knowledge, procedure with reference liquids that does not require close index matching and therefore avoids the use of toxic compounds for high-index glasses (i.e., n ≥ 1.8). The method is based on the analysis of the light refracted by a monolayer of beads in comparison with ray-tracing simulations. For the three different types of glass beads investigated a satisfactory fit is achieved by the assumption of a radial variation of the refractive index inside the beads. This is ascribed to the tensile and compressive stresses originating inside the beads during rapid solidification of the glass. © 1997 Optical Society of America
Dimensional Accuracy of Pickup Implant Impression: An In Vitro Comparison of Novel Modular Versus Standard Custom Trays
No full textPurpose
The purpose of this in vitro study was to evaluate the dimensional accuracy of pick-up impression technique using a Modular Individual Tray (MIT), and to compare it to the Standard individual Tray (ST) in multiple internal connection implants, considering both the role of materials and the geometrical misfits.
Materials and Methods
Since MIT relies on stiffness and elasticity of an acrylic Pattern Resin, a preliminary investigation on the dynamics of the resin volume contraction of this material during curing and polymerization process was carried out. Two series of specimens were tested to compare the accuracy of MIT (test group) to the ST (control group). The specimens were analyzed in three different steps during the impression procedure. Linear and angular displacements of the transfer copings were measured and compared between two groups.
Experimental measurements were performed using a computerized coordinate measuring machine.
Results
Investigation of the curing dynamics of the acrylic Pattern Resin showed that the increase of the powder/liquid ratio speeds up the polymerizing process of the resin (less of 1/3 of the time required) and reduces the final volume contraction of 45%. This behavior showed a strong dependence by the physical properties of acrylic material and those linked to the powder/liquid ratio. However, the total shrinkage does not, in any case, exceed the elastic limits of the material hence it does not affect the coping’s stability.
Test group measurements showed the reduction of linear errors of 55% and the angular one of 65%.
The experimental investigation on the dimensional accuracy showed that the use of MIT technique could reduce the introduction of linear and angular misfits during all the phases of the implant impression procedure.
Conclusions
Linear and angular displacements of the transfer copings were significantly reduced in Modular Individual Tray group, which leads to a higher dimensional accuracy with respect to the Standard Tray group.
MIT approach, in combination with thin and uniform amount of LS Pattern Resin in the pick-up impression technique, doesn’t showed significant permanent distortions in multiple misalignment internal connection implants, compared to the Standard Tray
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Simultaneous photoacoustic measurements of specific heat and thermal conductivity critical behaviour at a smectic A–nematic phase transition
No full textSimultaneous measurements of the thermal conductivity and specific heat at the smectic A-nematic phase transition in 4-n-octyl-4′-cyanobiphenyl (8CB) have been carried out with the photoacoustic technique. A critical increase in the thermal conductivity is reported. A critical decrease in the thermal diffusivity data confirms that the transition in 8CB is fluctuation dominated as also shown by the critical behaviour of the specific heat. © 1989 Taylor & Francis Group, LLC
Effect of quenched didorder on the RI-RV and RII-RI, and Liquid-RII rotator phase transitions in alkanes
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