Mechanical Properties of Hydrocolloid and Rubber Impression Materials
Hampson' has studied the elasticity of alginates under compression and has observed that materials with smaller amounts of permanent deformation were more suitable for use in the presence of undercut areas. Schoonover and Dickson2 report the breaking stress, percent strain, and percent set of alginates. Cresson3 suggested the revision of the testing procedure for compressive strengths of dental elastic impression materials that would apply the load at the rate of 18,000 Gm. per minute. Skinner et al.4 and Phillips5 point out the importance of the rate of deformation on the technic of handling alginate hydrocolloids. Warner" also shows that the breaking strength and percent deformation of alginates are functions of the loading rate where the higher the rate, the greater is the strength qualities. Anderson7 shows the recovery from deformation of alginate to be a direct function of time from the start of mixing. Donnison and Docking8 have measured the permanent deformation, stiffness, and toughness of alginates, the latter being determined as a combination of compressive strength and the strain at rupture. The ADA Specification 189 requires the compressive strength to be determined at a rate of 10,000 -+ 2 000 Gm. per minute. Based on an average strength of 7,000 Gm./cm.2 and a specimen '2 inch in diameter, the time required for rupture would be approximately half a minute. Recently, Braden'0 has determined the energy of tearing of one each of mercaptan, silicone, and alginate impression materials as a function of the rate of tearing and the temperature of testing. As the rate of tearing increased and the temperature decreased,
Research in dental materials.
Work that will have the greatest impact on the practice of dentistry now and in the future has been chosen for review from the vast field of recent research in dental materials. The research ranges from adhesive restorative materials to resins for prosthetics.
Photoreflectance characterization of vanadium-doped GaAs layers grown by metalorganic vapor phase epitaxy
Abstract We characterized the vanadium-doped (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy. The conductivity of the samples is checked by the Hall effect measurements, and it is n type with electrons concentration ranging from 2×10 13 to 2×10 17  cm −3 . We used the photoreflectance technique (PR) to study the effect of vanadium doping on the optical properties of GaAs. Our experimental results show that the built-in electric field F and the broadening parameter Γ 0 increase with V-doping concentration. Furthermore, the temperature dependence of the built-in electric field and the broadening parameter of GaAs:V samples have been investigated by PR. It has been observed that these parameters increase with increasing temperature. The decrease of the built-in electric field with decreasing temperature is due to the photovoltage effect.We characterized the vanadium-doped GaAs (GaAs:V) layers grown on GaAs substrate by metalorganic vapor phase epitaxy.