Masaji Yamane

The Mechanism for Erosion of Glass-ionomer Cements in Organic-Acid Buffer Solutions

The behavior and mechanism for erosion of glass-ionomer cements in organic-acid buffer solutions were studied as a function of time, pH, and citric-acid concentration. In acidic solutions, the dissolution of the cement was controlled by the diffusion of the eluted species in the cement matrix, which depended on H+ ion concentration. In citric-acid solutions, the dissolution of the cement was controlled by both the diffusion and the surface reaction between the acid anion and the eluted species. Contribution of the latter reaction was larger with the increase in the acid concentration.

Age-hardening of Dental Ag-Pd-Cu-Au Alloys

Age-hardening mechanisms of dental Ag-Pd-Cu-Au alloys were investigated by means of electron microscopic observations. Hardening was due to the precipitation of the L1o type CuPd-ordered platelet in the grain interior and to the discontinuous precipitation at the grain boundary. The characteristics of age-hardening curves were determined by rates of continuous and discontinuous precipitation.

Age hardening of AgPdCu dental alloy

Abstract The age-hardening characteristics of Au-containing AgPdCu dental alloys were investigated. The composition ranges of the three commercial alloys tested were: Ag 49.3–52.5 at.%, Pd 24.5–29.3 at.%, Cu 15.3–21.0 at.% and Au 2.6–6.5 at.%. From the electron microscope study it is concluded that the cause of hardening in this alloy is the precipitation of a CuPd ordered phase, which has the AuCuI type face-centred tetragonal structure.

Mechanism for Erosion of Glass-ionomer Cements in an Acidic Buffer Solution

In order to clarify the mechanism for erosion of glass-ionomer cements, we immersed two commercial luting cements in an acidic buffer solution under various conditions. The amounts of F, Al, Si, and Ca eluted from the cement were (1) in proportion to the square root of immersion time, (2) unrelated to shape or volume of the sample, (3) dependent on its surface area, and (4) not affected by shaking of the solution. It was concluded that the dissolution was controlled by the diffusion of those species in the cement matrix, which was influenced by the structure of the matrix and the concentration of H+ ion at the cement surface. The unreacted glass particles near the cement surface were dissolved by the long immersion, and many pores were left in the surface region.

Aging Reactions in a Low Gold, White Dental Alloy

Aging behavior of a low gold, white dental alloy was investigated by electrical resistivity measurements, X-ray diffraction, and electron microscopy. It was characterized by both grain interior and grain boundary reactions. The coexistence of AuCu-I and CuPd in dental alloys was first confirmed.

Decomposition of Gypsum Bonded Investments

Decomposition of gypsum bonded investments was investigated by means of TG-DTA and X-ray diffraction. The decomposition began at about 900°C and led to the formation of CaSi03 as a major product and Ca2SiO4 as a minor one. CaO was not formed during the decomposition of any of the investments. The decomposition process followed Janders equation, [1-(1-α)1/3]2 = kt, and the Ca2+ ion diffusion in CaSi03 was concluded to be a rate-detennining step.

Corrosion evaluation of Au-Cu-Pd ternary alloys

The corrosion resistivity of single-phase Au-Cu-11 at% Pd alloys was evaluated by using a parameterQ which represented the total amount of anodic reaction in a potentiostatic polarization test. The result was compared with those for binary Au-Cu, ternary Au-Cu-11 at % Ag and some commercial alloys. The validity of usingQ as a corrosion parameter was confirmed by the good agreement between the analysed and calculated values of copper ion dissolved into the test solution. By replacing a part of the copper in Au-Cu alloys with palladium the corrosion resistivity can be greatly improved, but silver has no such significant effect. The value ofQ decreased by both treatments of homogenization and grain refinement of the alloy. One of the advantages of the alloy having a single-phase structure is that inhomogeneity in the distribution of the constituents is small even in the as-cast state, which results in a small galvanic effect.

Phase transformation and age-hardening of Au-Cu-Pd ternary alloys

Phase transformations in Au-Cu-Pd single-phase alloys Were studied. Age-hardening of the alloys examined vvas attributed to the formation of fine domainu of long-range ordered (LRO) AuCuI type lattice in the interior of the grain. Prolonged ageing caused formation of large LRO domains of single variant at the grain boundary or microtwinning of LRo in the interior of the grain, depending on the ordering rate of the alloy. The electron-atom ratio (e/a) of the alloy and the axial ratio (c/a) of the ordered lattice seemed to playa part in the phase transformation behaviour in this alloy system. The effectiveness of rhodium addition on the grain refining was proved experimentally. These resuRs will be helpful in developing low nobility and high corrosion resistant dental alloys.