Michio Ohta

Effect of Fluoride Concentration and pH on Corrosion Behavior of Titanium for Dental Use

Titanium is used as a metal for biocompatible materials such as dental implants or restorations because of its excellent chemical stability. However, the corrosion of Ti in the prophylactic fluoride-containing environment can become problematic. To clarify the effects of fluoride concentration and pH on the corrosion behavior of Ti, we conducted anodic polarization and immersion tests in NaF solution of various concentrations and pH values. The concentrations of dissolved Ti in the test solutions were analyzed by inductively coupled plasma mass spectroscopy. There were obvious limits of fluoride concentration and the pH value at which the corrosion behavior of Ti changed. The corrosion of Ti in the solution containing fluoride depended on the concentration of hydrofluoric acid (HF). When the HF concentration in the solution was higher than about 30 ppm, the passivation film of the Ti was destroyed. The results of this study revealed a relation between the fluoride concentrations and pH values at which Ti corrosion occurred and provided data on such corrosion in environments where the fluoride concentration and pH value are known.

IR and NMR Analyses of Hardening and Maturation of Glass-ionomer Cement

It has been reported that the silicate phase as well as the cross-linking of the polycarboxylic acid by aluminum and calcium ions played an important role in the hardening of glass-ionomer cement. The objective of this study was to investigate the structural change during hardening of the cements by means of infrared (IR) spectroscopy and solid-state nuclear magnetic resonance (NMR) spectroscopy and to confirm the role of the silica phase in the hardening of the cement. For that purpose, we measured the change in compressive strength of an experimental glass-ionomer cement, two commercial glass-ionomer cements, and a polycarboxylate cement and carried out 29Si and 27 Al NMR analyses of the cement samples after the strength measurement. In the IR spectra during hardening, a characteristic band of the silicate network around 1000 cm-1 shifted toward high frequency with time. The spectrum after hardening was similar to that for a hydrated amorphous silica structure. The 27Al NMR analysis showed that Al3+ ion was tetrahedrally coordinated by oxygen in the original glass, but a part of the Al3+ ion was octahedrally coordinated after hardening to form Al polyacrylate gel. The chemical shift of Si in the 29Si NMR spectra also changed during hardening. The variation in the chemical shift reflected the structural change in the silicate network. The initial increase in compressive strength of the cement was mainly caused by polycarboxylate gel formation. However, it was concluded that the reconstruction of the silicate network contributed to the increase in strength with time during the period after the gelation by cross-linking was completed.

Difference in Age-hardening Mechanism in Dental Gold Alloys

TEM and SAED studies were conducted to clarify a difference in age-hardening mechanism with changes of gold content in Au-Cu-Ag ternary alloys, 18K, 16K, and 14K gold alloys, for which the ratio of copper to silver remained at 65 to 35 in weight. Age-hardening in the 18K alloy was attributed to the formation of the AuCu I type ordered platelets on the matrix {100}. In the 16K and 14K alloys, a periodic antiphase domain structure of the AuCu II type ordered phase made a major contribution to age-hardening. An alternating coarse lamellar structure was formed by the mechanism of discontinuous precipitation in the 16K alloy, while a modulated structure by spinodal decomposition was observed in the 14K alloy.

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.

Effects of added bioactive glass on the setting and mechanical properties of resin-modified glass ionomer cement.

In this study, the effects of added bioactive glass on the basic setting properties of a commercially available resin-modified glass ionomer cement were investigated with respect to setting time, mechanical strength, and setting mechanism. It was found to be clinically acceptable whether the setting time was extended or shortened depending on the type of bioactive glass added. The compressive strength of the set cement containing the bioactive glass decreased and was much higher when compared with the conventional type glass ionomer cement containing bioactive glass. The Fourier-transform infrared and 13C CP/MAS-NMR spectroscopies revealed that the extent of the acid-base reaction was larger in the cements containing bioactive glass than in the commercial resin-modified glass ionomer cement because of its high basicity in the bioactive glass. The 27Al MAS-NMR showed that crosslinking of the carboxylates in the polymeric acid by Al proceeded less in the cement containing the bioactive glass.

Dental gold alloys with age-hardenability at intraoral temperature

Dental gold alloys with age-hardenability at intraoral temperature were developed. Either 3 or 6 at% Ga, Al, Zn, In, Ni or Pd were added to an equiatomic AuCu alloy and the effect of additives on the low-temperature age-hardenability was examined. Alloys containing Ga, Al or Zn exhibited excellent low-temperature age-hardenability. The hardness of an alloy containing 6 at% Ga or 6 at% Al was doubled in one to ten days by ageing at 37 °C. Pronounced hardening was not demonstrated in an as-cast alloy, but solution treatment for only a few minutes provided enough age-hardenability. The age-hardening rate at low temperature related closely with the melting temperature of the alloy. Experimental gold alloys exhibited electrochemical behaviour similar to that of a commercial Type IV gold alloy. It is therefore concluded that AuCu alloy with added Ga, Al or Zn is expected to have enough age-hardenability and corrosion resistance for clinical use in the oral environment.

Age-hardening characteristics of a commercial dental gold alloy☆

Abstract The age-hardening characteristics of a commercial dental gold alloy Au-11wt.%Pt-6wt.%Pd-6wt.%Ag-9wt.%Cu were studied by means of resistometric measurements, hardness tests, X-ray diffraction and electron microscopy. The resistometric and hardness studies showed that age hardening occurred in two stages. The first stage arose from the nucleation of the AuCu I type ordered structure and was characterized by a slow growth rate of the ordered platelets. In the second stage, simultaneous ordering and precipitation took place. An alternating lamellar structure composed of the AuCu I type ordered platelets and platinum-rich f.c.c. precipitates was observed in electron micrographs. The age hardening in this stage was considered to arise primarily from ordering and to be brought about with the aid of precipitation. The grain boundary precipitates did not contribute to the age hardening in the alloy we studied.

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.

Age-hardening behaviors and grain boundary discontinuous precipitation in a Pd-free gold alloy for porcelain bonding

Isothermal age-hardening behaviors at 400° and 450 °C and discontinuous precipitation reaction at 450 °C in a commercial Pd-free gold alloy for porcelain bonding were investigated by hardness testing, X-ray powder diffraction, and light microscopy. Variations of electrical resistivity during continuous heating and cooling processes were also measured. The alloy exhibited pronounced age-hardening in the early stage of aging and the maximum hardness exceeded twice that of the solution-treated sample. Precise lattice parameter measurements and investigations of full width at half maximum values for the X-ray Bragg reflections implied that nonuniform strains due to the pre-precipitation or zone formation was responsible for the quick and pronounced age-hardening at 450 °C. Discontinuous precipitation reaction, producing a mixture of a small amount of Pt3In-phase with the L12-type superstructure and a large amount of (Pt, In)-depleted solid solution, started at grain boundaries in the late stage of aging process at 450 °C. The growth of the grain boundary discontinuous precipitates toward the intragrain area led to a gradual decrease in hardness of the alloy.